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ExxonMobil Proprietary CONFIDENTIAL WATER POLLUTION CONTROL
Section
Page
FLOTATION UNITS
XIX-A2
1 of 53
DESIGN PRACTICES
November, 2004
CONTENTS Section
Page
1 SCOPE ....................................................................................................................................................... 3 2 REFERENCES............................................................................................................................................ 3 2.1
DESIGN GUIDES........................................................................................................................... 3
2.2
GLOBAL PRACTICES ................................................................................................................... 3
2.3
DESIGN PRACTICES .................................................................................................................... 3
2.4
OTHER REFERENCES ................................................................................................................. 3
3 DEFINITIONS ............................................................................................................................................. 5 4 BACKGROUND.......................................................................................................................................... 6 4.1 GENERAL ...................................................................................................................................... 6 4.1.1 Design for Contaminant Removal............................................................................................. 7 5 EQUIPMENT DESCRIPTION ..................................................................................................................... 7 5.1
DISSOLVED AIR FLOTATION (DAF) – PROCESS DESCRIPTION ............................................. 7
5.2
INDUCED GAS FLOTATION – PROCESS DESCRIPTION .......................................................... 8
5.3
COMPARISON OF DAF AND IGF ................................................................................................. 9
5.4
SLUDGE THICKENING WITH DAF ............................................................................................... 9
6 BASIC DESIGN CONSIDERATIONS......................................................................................................... 9 6.1
FACTORS INFLUENCING THE PERFORMANCE OF FLOTATION PROCESSES...................... 9
6.2
pH NEUTRALIZATION................................................................................................................. 11
6.3
EQUALIZATION ........................................................................................................................... 11
6.4
WINTERIZATION ......................................................................................................................... 11
6.5
HAZARD SPECIFICATION .......................................................................................................... 11
6.6
AIR EMISSIONS CONTROL ........................................................................................................ 11
VENDOR SPECIFIC INFORMATION ................................................................................................... 12 6.8
DISSOLVED AIR FLOTATION (DAF) DESIGN............................................................................ 12
6.9
INDUCED GAS FLOTATION (IGF) DESIGN ............................................................................... 15
SPARING / BACK-UP........................................................................................................................... 16 7 DESIGN PROCEDURES .......................................................................................................................... 16 7.1
TREATABILITY TESTING............................................................................................................ 16
7.2 DISSOLVED AIR FLOTATION (DAF) .......................................................................................... 17 7.2.1 Components of a Dissolved Air Flotation System................................................................... 17 7.2.2 Key Parts of the Design.......................................................................................................... 18 7.3 INDUCED GAS FLOTATION (IGF) .............................................................................................. 20 7.3.1 Components of an IGF System .............................................................................................. 20 7.3.2 Key Parts in the Design .......................................................................................................... 20 8 EXAMPLE DESIGN PROBLEMS............................................................................................................. 22 8.1
ADVANCED PRIMARY TREATMENT FOR OIL AND SUSPENDED SOLIDS ............................ 22
This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
ExxonMobil Proprietary CONFIDENTIAL WATER POLLUTION CONTROL
Section
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FLOTATION UNITS
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8.2
November, 2004
SLUDGE THICKENING ............................................................................................................... 25
9 OPERATING / CONTROL STRATEGY.................................................................................................... 27 TABLES Table 1A Table 1B Table 2 Table 3 Table 4
Characteristics of ExxonMobil Dissolved Air Flotation Units ............................................. 28 Characteristics of ExxonMobil Induced Gas (Air) Flotation Units...................................... 32 Comparison of IGF and DAF ............................................................................................ 35 Sample Duty Specification for Dissolved Air Flotation ...................................................... 36 Sample Duty Specification for Induced Gas Flotation....................................................... 39
FIGURES Figure 1 Flotation Unit Selection Guidelines ................................................................................... 41 Figure 2 Alternative Technologies in Typical Refinery WWTPs, Primary / Advanced Primary / Sludge Dewatering Treatments ........................................................................................ 42 Figure 3 Schematic Sketch of Dissolved Air Floation Unit .............................................................. 43 Figure 4 WEMCO DepuratorTM – Induced Gas Floation Unit – Mechanical (Impeller) Inductor .... 44 Figure 5 WEMCO Induced Static Flotation Unit – Hydraulic Inductor (Nozzle / Striker Plate) ........ 45 Figure 6 Schematic Sketch of Gas Inductors in Induced Air (Gas) Flotation Process .................... 46 Figure 7 Schematic Diagram of DAF in Thickening Application...................................................... 47 Figure 8 Bench-Scale Test Apparatus for Induced Air Floation (Flotation and Screening Test) ..... 48 Figure 9 Mechanism of Attachment of Gas Bubble to Solid or Oil .................................................. 49 Figure 10 Optimum Dosage Level of Polyelectrolytes (Induced Air Floation) ................................... 50 Figure 11 Solubility of Air in Water.................................................................................................... 51 Figure 12 Schematic Sketch of Float Skimmers in IGF..................................................................... 52 Figure 13 Typical Instrumentation Diagram for Air / Water Mix System............................................ 53
Revision Memo
12/99
1. 2. 3.
8/04
1. 2. 3. 4. 5.
Minor editorial changes. Added new Tables 1A and 1B on Characteristics of Exxon Dissolved Air Flotation and Induced Gas Flotation Units. Updated Figure 1 - Alternative Technologies in Typical Refinery WWTPs, Primary / Advanced Primary / Sludge Dewatering Treatments
Editorial changes & corrections Updated chemical addition recommendations. Modified Tables 1A and 1B to include h-Mobil Units. Added new Figure 1 - Flotation Unit Selection Guidelines Updated Figure 13 - Typical Instrumentation Diagram for Pressurized Air / Water Mix System.
This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
ExxonMobil Proprietary CONFIDENTIAL WATER POLLUTION CONTROL
Section
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FLOTATION UNITS
XIX-A2
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DESIGN PRACTICES
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1
SCOPE
This section presents recommended process design procedures and mechanical design considerations for facilities to remove undissolved oil and suspended matter from water by air (gas) flotation. It also presents process design procedures applicable for waste activated sludge thickening using a dissolved air flotation system. Two of the major flotation processes: dissolved-air and induced-gas (air or N2) flotation systems are covered. Vendor specific information on a particular equipment configuration is still needed for the final design of dissolved air flotation and induced gas flotation units, and pilot plant testing is still recommended where the performance criteria for the particular project are stringent. Three example problems are included: two example design problems comparing DAF and IGF applications in wastewater clarification, and one example design problem on waste activated sludge thickening. Recommendations on operating / control strategies are presented. For reference purposes only, Tables 1A and 1B provide a summary of the characteristics of ExxonMobil flotation units. 2 ç
2.1
REFERENCES
DESIGN GUIDES
1.
DG 11-6-3
Chemical Feeders for Wastewater Treating, Water and Wastewater Design Guide (TMEE 080).
2.
DG 11-7-1
Wastewater Dissolved Air Flotation System, Water and Wastewater Design Guide (TMEE 080).
3. DG 11-7-2 Wastewater Induced Gas Flotation Units, Water and Wastewater Design Guide (TMEE 080). 2.2 GLOBAL PRACTICES 4. ç
IP 16-1-1
2.3
Area Classification and Related Electrical Design
DESIGN PRACTICES
5.
DP XVI
Thermal Insulation
6.
DP XIX-A
Guidelines for Selecting Wastewater Treatment
7.
DP XIX-A1
Primary Oil / Water Separators
8.
DP XIX-A4
Clarification / Thickener / Flocculation
9.
DP XX-A2
Gravity Sludge Thickeners
2.4
OTHER REFERENCES
10. Bennett, G.F., The Removal of Oil from Wastewater by Air Flotation – A Review, CRC Critical Review in Environmental Control, Environmental Progress, 18 (18), 189–253, 1988. 11. Eckenfelder, W.W., Jr., Industrial Water Pollution Control, N.Y., N.Y., McGraw-Hill, 71–80, 1989. 12. The NALCO Water Handbook, N.Y., N.Y., McGraw-Hill, 9–15 to 9–20 and 11–6 to 11–9, 1988. 13. Robertaccio, F.L., Polyelectrolyte Guide, EE.20E.84, February, 1984. 14. Shah, P.S., Improving Dissolved Air Flotation (DAF) Operation by Substituting Polymer for Alum as a Primary Coagulant, EE.112E.79, October, 1979. 15. Cassaday, A.L., Advances in Flotation Unit Design for Produced Water Treatment, the Society of Petroleum Engineer, Production Operations Symposium, Oklahoma City, OK, 581–590, March 21,1993. 16. Leach, C.A., Oil Flotation Processes for Cleaning Oil Field Produced Water, Super Session in Petroleum in Ocean Environment, Oily Water Clean-up, AICHE Annual Meeting in Houston, Texas, April 1, 1987. 17. Water Treatment Handbook, Secaucus, NJ, Lavoiser, 171–177 and 679–686, 1991. 18. Burkhardt, C.W., Control Pollution by Air Flotation, Hydrocarbon Processing, 58–61, May, 1983. 19. Goodrich, R.R., et. al., Guide for Reducing Waste Treatment Cost, EE.48E.85, July, 1987. This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
ExxonMobil Proprietary CONFIDENTIAL
DESIGN PRACTICES
WATER POLLUTION CONTROL
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20. Fort, L.R., Induced Air Flotation & Dissolved Air Flotation Comparison for ISLA Refinery (in Curacao) Waste Water Treating (for a Flexicoker Project), 93 ECS2 007, February 3, 1993. 21. Altemoeller, P.H., Design Basis Memorandum-lnduced Air Flotation, Esso A.G., Karlsruhe, Germany, April 4, 1991. 22. Waste Water Treatment Plant-Start-up, Shut-down & Emergency Procedure Manual (Baton Rouge Refinery), Baton Rouge, Louisiana, November 1, 1985. 23. Altemoeller, P.H., and Goodrich, R.R., Guidelines for Sizing Gravity Thickeners-Wastewater Treatment Sludge Applications, EE.24E.89, October, 1989. 24. Sludge Thickening-Manual of Practice No. FD-1, Facilities Development, Water Pollution Control Federation, Washington, D.C., 1980. 25. Tchobanoglous, G., et. al., Wastewater Engineering-Treatment, Disposal and Reuse, NY, NY, McGraw-Hill, 1991.
This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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3
DEFINITIONS
Absorption – The physical attraction of a substance into a particulate material, such as the attraction of contaminant oil droplets into clay or suspended particles into a bubble. Absorption is an interstitial phenomenon. Adsorption – The physical or chemical attraction of a substance onto the surface of a particulate material, such as the attraction of an organic contaminant onto the surface of suspended solids, such as clay or carbon particles. Adsorption is a surface phenomenon. Alum – Common name for hydrated forms of aluminum sulfate; used as a primary coagulant. Coagulants – Low molecular weight (less than 300 milliequivalent weights) chemical additives that usually ionize in solution to produce positive charge species. They are added to wastewater to cause destabilization by charge neutralization and initial aggregation of colloidal and finely divided suspended contaminants, oil, or solids. The resulting aggregate (floc) has a large surface area, which traps and agglomerates the solid particles and oil globules. Examples of coagulants are metal salts (alum), polyaluminum chloride, clay (montmorillonite), and organic polyelectrolytes. Colloid – Discrete particles less than one micron (10-3 mm) in size which are dispersed or suspended in solution. Dispersed-Phase – The non-continuous phase in a mixture of non-miscible components, for example oil in an oil-in-water emulsion and gas bubbles in a froth. Dissolved Air Flotation (DAF)12 - A flotation process in which air is dissolved in the water under pressure and subsequently released to atmospheric pressure in the flotation unit. This air may be dissolved at 35 to 85 psig (240 to 590 kPa) in all or a portion of the wastewater flow or a portion of the clarified effluent. Minute air bubbles are formed (30 to 120mm) which attach to suspended particles floating them to the surface. Electroflotation10 – A flotation process in which gas bubbles (hydrogen and oxygen) are generated by the electrolysis of the water by means of appropriate electrodes and electrical energy. Electroflotation equipment is not considered in this section. Emulsion – A dispersion of a liquid phase in a continuous liquid phase which is usually stabilized with surfactant If the dispersed liquid phase is oil and the continuous liquid phase is water, it is called an oil-in-water (o/w) emulsion. When oil is the continuous phase and water is the dispersed phase, it is called a water-in-oil (w/o) emulsion. Flash Mix Tank – A tank in which coagulants are rapidly mixed with the wastewater. Hold up time is on the order of two to five minutes. Float – The heterogeneous mixture of gas bubbles, oil and suspended solids that rises to the surface in a flotation process. Also known as froth. ç
Flocculant – Materials, generally high molecular weight, added to enhance the action of coagulants, generally by linking suspended particles together into larger "flocs" that are quicker to separate. Polyelectrolytes are commonly used flocculants. Flocculation – The process of combining the coagulated particles into larger aggregated masses. Flotation – Any operation in which a dispersed solid particle or oil globule is separated from a liquid by gas bubbles which float it to the surface of the liquid. Flotation Aids – Materials added to enhance the flotation of suspended particles in flotation processes. These materials include coagulants and flocculants. Flotator – The flotation tank / basin, where separation takes place. Sufficient detention time is provided and turbulence is minimized to allow the bubbles with attached contaminants to rise to the surface for removal. Induced Air Flotation (IAF)/Induced Gas Flotation (IGF)13 – A flotation process involving the aggressive entrainment of dispersed gas or air into the water, either mechanically or hydraulically. Treatment occurs at a constant pressure. Bubbles (up to 1000 mm) are generated by such means as mechanical shear of a mechanical inductor, passing the gas through a porous diffuser, or homogenizing water streams with a nozzle / eductor. Surface interaction at the air-water interface between air bubbles and impurities causes separation of oil and solids from the water. When air is used as the gas, the flotation unit is called an induced air flotation unit or IAF. The acronym IGF will be used as a general term to mean either an IAF or IGF in this section. Inductor – Mechanical or hydraulic equipment used to entrain gas or air into the wastewater in IGF units. Neutralization – The process by which compounds having hydrogen ions (H+) or acids and hydroxyl (OH–) base ions react to form water and salts. This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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Polyelectrolyte – Long chain polymers especially designed to attract many small, discrete particles into a large, heavy (flocculant) mass. Recycle Rate – A measure of the amount of the flotation unit effluent stream returned to the feed end of a unit, expressed as a percentage of influent flowrate. Rise Rate (sometimes called overflow rate) – A measure of the throughput of a settler, obtained by dividing the volumetric flow rate (including recycle, when used) by the horizontal "footprint” area of the basin, usually expressed in units of gpm/ft2 (l/sec/m2). This is equivalent to the hydraulic loading rate. Sludge Volume Index (SVI) – A measure of compaction of sludge after gravity settling. SVI is the volume in milliliters occupied by one liter of activated sludge after 30 minutes of settling divided by the MLSS concentration. Values below 100 are generally indicative of good settling and compaction. Surface Active Agent (Surfactant)13 – A material that significantly alters the surface and interfacial properties of water or oil, such as viscosity and wettability, when adsorbed onto the surface or interfaces of the system, usually at low concentrations. The term interface indicates a boundary of two immiscible phases, such as the boundary between a discrete oil phase and continuous water phase in o/w emulsion. The term surface denotes an interface where one phase is a gas, usually air, such as in the suspension of an air bubble in water. Surface Tension13 – A temperature-dependent physical property of a liquid which characterizes the state of a liquid film subjected to pressure by air (gas) at the air-water interface. Flotation aids, which usually concentrate at the boundary between two immiscible phases, can modify this liquid property. 4 4.1 ç
BACKGROUND
GENERAL
Flotation systems are one of several alternative technologies to achieve advanced primary treatment of wastewater to meet the increasingly stringent regulations on effluent quality, air emissions, and solid wastes (Figures 1 & 2). A decision tree that gives general guidance that may lead to the selection of flotation systems is given in DP XIXA, Guidelines for Selecting Wastewater Treatment Systems. Generally, flotation systems are used at ExxonMobil sites to reduce the free oil content of wastewater below that attainable in an API separator to protect the performance of downstream treatment units such as activated sludge biological treatment. Dissolved air flotation units are also applied to thicken sludges from biological treatment processes, such as waste activated sludge. A flotation unit can also be applied downstream of the wastewater clarifier as a "low-cost" option for dealing with excess suspended solids (TSS) in constrained wastewater treatment operations in place of a final settling pond. Gross quantities of free oil in refinery or chemical plant wastewater are usually removed by gravity settling and skimming of separated oil in an API or other types of separators (see DP XIX-A1). The oil content of the effluent water from an API varies, but it is usually above 50 mg/L on average and can be > 200 mg/L during peak loads. Oil separation is limited mainly by the long separation times of small droplets or oil-laden colloids, and by the presence of heavy oil with a density approaching that of water. In addition, suspended solids in the water can hinder the separation of the oil by effectively increasing the density of associated oil. Flotation units contact gas bubbles with oil or suspended solids to reduce the apparent density, thus greatly accelerating rise velocity and separation. As a consequence, small droplet separation times are shorter, and heavy oil can be removed. There are five different types of flotation systems, their classification being based on the method of bubble formation10: 1. 1. Dissolved air (gas)– the gas precipitates from a supersaturated solution as a result of the reduction of pressure. 2. 2. Induced gas (air) – the gas and liquid are mixed, mechanically or hydraulically, to induce bubble formation. 3. 3. Froth – the gas is directly injected into the liquid by means of a sparger in the liquid. 4. 4. Electrolytic – the bubbles are generated by electrolysis of the water. 5. 5. Vacuum – the gas or air is extracted from a saturated solution by negative pressure.
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Only the two major flotation processes – dissolved air flotation (DAF) and induced gas flotation (IGF) – are covered in detail in this section. These two processes are commonly used within ExxonMobil for treating oily wastewater. DAF units are generally specified for clarification of wastewater streams that must meet a very stringent (less than 20 mg/L on average) effluent oil limit, and where there are no downstream facilities after the DAF unit. When DAF is used in thickening applications, float solids concentration is the main performance criteria. IGF units are recommended when there are downstream facilities, such as biological treatment when effluent oil and suspended solids removal requirements are less stringent, or when air emissions controls are needed. DAF is currently the most commonly used advanced-primary treatment in ExxonMobil; DAF is approximately twice as common as IGF in ExxonMobil. Electrolytic Flotation is used in one Chemical Plant for cobalt / oil removal. Several production affiliates use IGF for produced water treating. This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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Design for Contaminant Removal
A properly designed flotation system, which normally uses coagulant and/or flocculant, can be expected to have a free oil and grease removal efficiency of about 80 to 90 percent and suspended solids removal efficiency of about 80 percent. Some reduction of insoluble BOD5, phenols, and H2S may also be achieved. Effluent oil and solids concentrations in the range of 10 to 30 mg/L and 15 to 40 mg/L, respectively, are achievable when treating refinery or chemical plant API separator effluent. Under similar operating conditions, a DAF will achieve a more consistent and slightly better effluent quality than an IGF. Flotation units are not specifically designed to include removal of dissolved organic components in wastewater, although some stripping of volatile hydrocarbons and dissolved gases can occur. Appropriate handling of these volatile materials transferred to the gas phase is required (see later discussions on Air Emissions Control). In waste activated sludge thickening, DAF is expected to recover at least 95 percent of the suspended solids. Typically, thickened sludge produced from DAF thickening has suspended solids concentrations of about 4 to 10 percent9,11. ç
In applications where an IAF is used downstream of the clarifier for TSS removal, 50% removal is typical. The lower solids loading results in an overall lower removal efficiency. 5 5.1
EQUIPMENT DESCRIPTION
DISSOLVED AIR FLOTATION (DAF) – PROCESS DESCRIPTION
Figure 3 shows schematic sketches of DAF units in circular and rectangular configurations. The DAF process in either clarification or thickening applications consists of four steps: 1. Flocculation and coagulation 2. Bubble formation and contact with the floc 3. Flotation of the floc to the surface 4. Skimming of the floating materials (float) DAF is used to remove particulate materials using pressurized water supersaturated with air. When the pressure is released in the flotation tank, air comes out of solution, forming micro bubbles of about 30 to 120 micrometers in size. Flotation occurs due to the bubbles' attachment to suspended oil and solid particles to form a combined float material of relatively low density that separates rapidly. As the bubbles rise, they may also collide with other contaminant oil globules or solid particles and carry them to the water surface. The resulting float is removed with a mechanical skimmer for further treatment or disposal. To improve the flotation process, flotation aids are added to the water. The DAF influent is usually first treated with a coagulant to effect particle agglomeration (form floc) and then treated with a flocculant to enhance the agglomeration of the flocs. Since pH affects these two steps, upstream pH control may be necessary (see discussion on Neutralization). The coagulant treated mixture is retained in the flocculation tank for a period of time required to achieve proper formation and growth of floc. The coagulant can either be an inorganic material such as alum, polyaluminum chloride, or an organic polyelectrolyte, while the flocculant is usually a cationic or anionic polyelectrolyte. Inorganic coagulants increase the mass of float requiring treatment and disposal. Air can be dissolved in water in three ways:
·
Total feed pressurization.
· ·
Partial feed pressurization. Recycle (a portion of the effluent) pressurization.
Recycle pressurization is the preferred method; the other two methods can cause emulsification and shearing of the floc. The pressurization system (Figures 3 and 13) consists of a pump, an air supply system, a vessel for air / water contacting and disengaging of the excess air, and a pressure-letdown throttling valve. A portion of the clarified effluent from the flotation tank, usually amounting to 30 to 50 percent of the fresh influent, is pumped to the requisite pressure of 35 to 80 psig (240 to 550 kPa) and contacted with the compressed air. The mixed stream flows to the air-saturation drum for contacting and disengaging. Depending on the pressure and temperature, about 50 to 90 percent of the theoretical air-in-water saturation is typically achieved. Undissolved air is vented from the air-saturation drum. The pressurized, air-rich recycle is reduced in pressure by the letdown valve near the inlet to the flotation tank and is added to the coagulated feed before the flotation tank. At the lower pressure, air is released from solution and flotation takes place in the tank. The bubbles must be small for efficient flotation, so the equipment is designed to avoid their coalescence. To achieve this, throttling valves are specially designed to impart high shear, and the distance from the throttling valve to the bubble / floc contacting region is This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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minimized. The mechanical skimmer gently moves the separated float over the water surface to a collecting chamber. The clarified water flows under a float retention baffle, then over an adjustable weir, and out of the unit. Although different gases can be used in the flotation process, air is most commonly used in dissolved air flotation. The dissolved air may also cause oxidation and/or volatilization of some compounds in the wastewater. 5.2
INDUCED GAS FLOTATION – PROCESS DESCRIPTION
In an induced gas flotation process, oil and solids are separated from the oily wastewater by the attachment to the gas bubble surface. This is different from the separation mechanism in DAF where the flotation of impurities is achieved by bubbles being enmeshed into flocs of solids and oil (see the following section comparing DAF versus IGF). A typical four-cell, induced gas flotation unit disperses gas (air) into the water by action of an impeller (Figure 4) or a nozzle / eductor (Figure 5). Fine bubbles of gas, up to 1000 micrometer in size, are dispersed through the agitated suspension of solids and oil to form foam, which rises to the surface of the water. The surfacing foam is removed with an internally installed float skimmer. In most commercially available IGF units, the wastewater enters the unit via the feed box to the first of the flotation cells (active cells) where separation takes place. The treated wastewater goes into the other active cells (typically four in series) via underflow baffles located in the connecting bulkhead. The last of the active cells feeds the discharge box. Contaminant removal occurs in each of the active flotation cells. Contaminants will continue to float to the surface of the discharge box even though no active flotation mechanism is present, because of the gas bubbles retained in the wastewater. Oil that floats to the surface in the discharge box is removed by skimming equipment similar to each of the active flotation cells. From the discharge box, the water is passed forward for further treatment, discharge or storage, depending on the particular applications. For units with nozzle / eductor systems, a portion of the effluent water is drawn off and recirculated. The discharge box allows for the separation of gas from the process water, which prevents cavitation of the recirculation pump. 10
Details of two basic types of IGF :
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·
Mechanical-type inductor (Figure 6A) – Uses motor-driven rotor (impeller) to induce gas from the vapor phase in the top of the vessel directly into the water phase. The impeller diameter and impeller speed influence the oil removal performance as both relate to the liquid circulation rate, gas induction rate and the required power. The gas transfer rate is at least 70 percent more than hydraulic-type units16.
·
Hydraulic-type inductor (Figure 6B) – Uses a venturi (eductor) to create a low pressure with a recirculating liquid to induce gas (air) from an external or internal source into the water. There are two sub-types of hydraulic inductors; the first is a nozzle / eductor inductor, which induces gas into the water phase with dip tubes; and the second is a nozzle / striker plate inductor which induces gas into a bottom nozzle and striker plate to shear the bubbles. The latter type of hydraulic inductor is used in a pressurized, sealed cylindrical vessel, which eliminates the risk of gasket or seal vapor leakage to the environment.
A comparison of the advantages and disadvantages of the two types of IGF is given below16: IGF-MECHANICAL
IGF-HYDRAULIC ADVANTAGES
1. 2.
Gas availability and gas/water ratio at design flowrate is higher; however, decreases with increasing design throughput.
1.
Less expensive than IGF-mechanical impeller units.
2.
Less downtime when provided with spare recycle pump.
Moving parts are all accessible without shutdown, draining, and confined space entry.
3.
Fabricated with tight enclosure, thus eliminating air emissions.
4.
Low horsepower required.
DISADVANTAGES
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1.
Impeller speed not adjustable.
2.
May require significant maintenance.
3.
Potential for air emissions due to leakage around the seal and cover.
4.
Mechanical skimmer requires adjustment
1.
Gas transfer per unit volume water is significantly less than mechanical-type.
As in DAF, different gases can be used in IGF. Air is typically used where there is a minimal potential for explosive hydrocarbon / air mixture, and the vapors are vented to the atmosphere. Gases other than air are used in closed units, which vent to gas recovery or a flare or where the use of air will create significant safety and/or corrosion issues. Safety and air emissions issues must be addressed on a site by site basis. This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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5.3
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COMPARISON OF DAF AND IGF
Most recent ExxonMobil Refinery and Chemical Plant wastewater treatment upgrades in Europe and the U.S. have selected installation of IGF units instead of the more traditional DAF units for advanced primary treatment. The reasons for selecting IGF over DAF have included lower capital cost (including the IGF float thickener), less complex operation, smaller footprint, ease of emissions control, and acceptable effluent quality with proper flow and load surge protection. However, DAF still has the advantage of producing a more consistent, higher quality effluent than IGF at similar operating conditions, and can often handle more difficult waste streams due to the nature of the technology and the inclusion of a flash mix tank and flocculation tank in the chemical addition facilities. Due the high energy associated with IGF units the floc is easily sheared, so very difficult wastewaters may require processing by DAF. Also, DAF typically produces a higher concentration float and therefore less float. IFGs typically produce 3-10% of the design feed flow rate as float. DAF float concentration can be as high as 2-6 wt% vs. 1-2 wt% for IGF. Table 2 shows a more detailed comparison between IGF and DAF. In general, both processes should be considered for advanced primary wastewater treatment, and the choice should be based on achievable effluent quality, overall cost as well as other factors such as plot space and emissions control. The cost analysis must include float handling and associated facilities. 5.4
SLUDGE THICKENING WITH DAF
In addition to advanced primary treatment of wastewater, the DAF process is also used to thicken waste sludges. The process principles of a "DAF thickener" are the same as previously described. However, the higher solids loading in thickening service requires modification to several design parameters which are detailed in a later section of this Design Practice. A DAF thickener is an alternative to a gravity thickener for waste sludge thickening (Figure 2). DAF is generally smaller than a gravity thickener unit for similar service and produces a thickened sludge with a higher concentration of solids. The sludges thickened by DAF within ExxonMobil include clarifier underflow from a biological wastewater treatment system (waste activated sludge), and granular media filter backwash from secondary wastewater oil removal processes. Figure 7 shows a schematic flow diagram around a DAF thickener in a conventional activated sludge system. The waste, which is a slip stream of the recycle activated sludge with typical suspended solids concentration range of about 0.6 wt% to 2 wt% percent, is fed into the DAF. In the flotator tank, about 95 percent or more of the incoming suspended solids are removed. The flotation unit effluent is resumed to the head of the aeration basin. The thickened sludge, with a suspended solids concentration of 4 to 10 wt%, is collected in a holding tank. The collected thickened sludge must be effectively mixed with air to prevent stratification and septic conditions. 6 6.1 ç
BASIC DESIGN CONSIDERATIONS
FACTORS INFLUENCING THE PERFORMANCE OF FLOTATION PROCESSES
Design Strategy – The designer must size the unit and provide sufficient operating flexibility to allow effluent quality to be maintained with the expected variations in influent characteristics. A recommended approach is to design for some confidence level, typically 90 to 99 percent confidence, based on sufficient operating data for oil and suspended solids concentrations in the wastewater stream that will feed the system. For an existing refinery or chemical plant, historical flow and contaminant data are compiled on a probability plot and the design point determined based on the site specific performance requirements as dictated by regulatory requirements, downstream treatment, and operating philosophy. For grassroots facilities with no operating data, the design values should be selected based on maximum values expected during normal operation. These might occur during storm events or maximum loadings from upstream processes. Average values are rarely used for design without large contingency factors of 150 to 200 percent. If the operating strategy includes the potential to use heavy opportunity crudes where the crude slate API approaches that of water, separation will take longer and additional residence time may be necessary. Consult with EMRE Environmental Engineering Section for these applications. Influent Characteristics
·
Influent Flowrate – Hydraulic loading is a primary sizing criteria for both DAF and IGF. Wide variations and rapid changes in flow rate should be avoided. Upstream equalization is recommended.
·
Oil / Solids Concentrations – An upstream separator (e.g., API or CPS) is required to maintain normal feed concentrations of total oil and solids in the 50 to 200 mg/L range. Wide variation and rapid changes in feed quality should be avoided, especially for IGFs.
·
Oil Globule Size – This affects the rise rate of the oil. The larger the globule size, the faster the oil will surface. Flotation aids such as polymeric polyelectrolytes can significantly increase globule size and rise rate.
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·
pH – The pH of the solution affects the ability to form a floc when coagulants are used. pH is often adjusted to within proper range prior to the addition of coagulants.
·
Surface Activity – The surface activity affects the attraction between bubbles and the contaminants and the strength of the bond between them. Flotation aids such as polymeric polyelectrolytes may alter surface activity by stabilizing the interfaces.
·
Concentration and Types of Suspended Solids – For DAF, particularly in thickening applications, the air requirements are related to suspended solids concentration since there must be enough air bubbles present to float all the solids. Oil-wet solids, including scales, iron sulfide, catalyst fines, and clays in the < 40-micron size range can stabilize emulsions by creating almost neutral buoyancy. Water-wet solids can collapse foam. Both effects decrease flotation unit performance. Where the normal suspended solids concentration in the wastewater is high (> 100 mg/L), an upstream separation device should be considered. (See DP XIX-A1, API Separators.)
Flotation Aids – flotation aids are commonly used to improve oil and solids removal efficiency in both DAF and IGF units. They are added to ensure attraction between the particles and bubbles, since suspended particles generally have a negative surface charge that inhibits coagulation. Flotation aids can reduce the time required to achieve a given oil / solids removal significantly (by as much as a factor of 40)16. Their use is critical in ensuring a consistent removal performance of flotation units. However, the dosage rate of flotation aids can reach a maximum above which performance (Figure 10) may degrade. Since flotation aids are critical in terms of unit performance and operating costs, selection of the optimum flotation aid type and/or dosage rate is required for every wastewater, and is usually established using a bench-scale test followed by field test confirmation. ç
The following general classifications of flotation aids are those most commonly used in ExxonMobil:
·
Primary coagulants (aluminum, polyaluminum chloride, and iron salts) are used as coagulants with a dosage range of 10 to 100 mg/L.
·
Cationic polyelectrolytes (e.g., polyamines, polyquaternaries, copolymers of acrylamide) are used as flocculants with a dosage range of 1 to 20 mg/L
·
Anionic polyelectrolytes (copolymer of acrylate salts with acrylamide). 10 mg/L.
Commonly used dosage rate range is about 2 to
Primary coagulants can be used in DAF units to increase particle size by neutralizing surface charges and promoting flocculation of suspended solids. They are usually used at higher concentrations than cationic polymer flocculants and thus result in significant sludge production and associated disposal costs. Usually, a combination of cationic and anionic polyelectrolytes is recommended to reduce sludge quantities. For challenged crude operations, a dual chemical system that includes both primary coagulant and polymer should be specified. The types and dosage rates of chemicals required for best performance cannot be predicted and should be accurately determined by laboratory scale tests, if possible. For DAF application, jar tests can be used to determine the best combination of flotation aids and optimum dosages for a particular wastewater or sludge. Such testing is nominally done in conjunction with the chemical vendor. Instructions for conducting jar tests are provided in Reference 12. For IGF, as shown in Figure 8 and References 12 and 16, bench scale test units can be used to test flotation aids. For both DAF and IGF, pilot plant tests can also determine appropriate chemical dosages, and when possible, are preferred because they can be related better with the operating unit. Some DAF vendors have pilot-scale units available for rent that are specifically scaled to their full-size units. An example is KomlineSanderson's DAF Flotation Model No. 1 ImpinjairTM pilot plant, which has 1 ft2 (about 0.1 m2) of flotation surface. Details of facility designs for adding chemicals to flotation units are described in References 1, 2, 3, and 8. Flow proportional control of chemical injection rates is recommended. ç
Flotation aids are supplied in solution or in water-in-oil emulsions. Flotation aids can be mixed on line with an in-line mixer to disperse and dissolve the active component in water. Another method of injecting emulsion polymer is with a PolyBlendTM System21 or equivalent. These package blending systems provide good control of dosing rate, dilution level, and dwell time ("aging”). Operating Variables – Control of the flotation aid type and dosage rate is very important for performance and economics. The optimum dosage rate(s) should be checked periodically to minimize over treatment (see discussions in Flotation Aids and Operating Control Strategies). After start-up, the operator can do any of the following within the limits set by design to exercise some control over effluent quality: (1) alter pH; (2) adjust dose of flotation aids; (3) change flotation aids; (4) increase air pressure or rate; (5) increase recycle; and (6) adjust skim rate. Unit design should provide features to allow these types of operating adjustments.
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pH NEUTRALIZATION
The pH of the wastewater can have a significant effect on the degree of coagulation / flocculation accomplished. Each chemical has an optimum operating pH range. The effect of pH on coagulation / flocculation can be determined by jar tests. The preferred pH range is 6-9. In most cases, neutralization uses caustic soda (usually spent / recycled or oxidized sodium hydroxide), and sulfuric acid. The amount of sulfuric acid or sodium hydroxide that is needed to effect a specific pH change is determined by the chemical characteristics of the wastewater. An approximation of acid or alkali requirements can be made in the laboratory through titration tests relating chemical dosages and pH. A neutralization process may also be included in the plant design to provide an optimum pH for biological treatment (BIOX) and/or to meet regulatory agency criteria. An automated pH control system is generally specified. As a minimum, backmixing and feedback control are required. Consult EMRE Environmental Section for details. 6.3
EQUALIZATION
The effluent quality from a flotation process is adversely affected by sudden and large increases in flow, oil, and suspended solids loadings (concentration and mass). Since the hydraulic residence time in IGF is much shorter than in DAF, IGFs are more sensitive to these surges. Generally, flow surges or contaminant loading higher than the design conditions, will result in higher than design concentrations of oil and solids in the effluent. The need for equalization upstream of a flotation process is determined based on effluent requirements and historical flow and loading data, or predicted frequency and maximums during storms and process upsets. Equalization can be provided by stormwater surge ponds or tanks, or by a specially designed equalization tank in the treatment train. A minimum of 8 hours of retention time in an equalization tank is recommended with the typical range being 12 to 24 hours. During flow and load surges operator adjustments including air rate, recycle rate, chemical addition rate, and skimming rate are required to minimize upsets. Ratio control of chemical addition will also help minimize the upset. Cycling of feed pumps should be avoided. 6.4
WINTERIZATION
Winterization should be provided as required per specific site standards (Reference 5). The float end sludge handling system should receive particular attention. 6.5
HAZARD SPECIFICATION
Consider minimum safety requirements including the following:
·
Availability of appropriate safety equipment and MSDS for handling concentrated polymer flotation aids and acids / bases for pH control, since they can affect / damage body tissues.
·
Applicable electrical classification should be Class 1, Group D, Division 1 or 2 (Reference 4).
·
Containment of toxic or flammable fluids.
·
Safety shower with eye wash station within 50 ft from chemical truck unloading station, tanks, and pumps. Check site-specific requirements for distance.
·
Where applicable, monitors and alarms for light hydrocarbon and hydrogen sulfide.
6.6 ç
AIR EMISSIONS CONTROL
Local emission regulations may dictate the need for controlling air emissions. This can be done relatively easily in an IGF, since they are usually enclosed. Controlling air emissions in DAF may be more expensive and less effective than in IGF because of their large surface area and the skimming arm mechanisms that interfere with the installation of airtight seals. For grassroots units, an IGF is generally the better option when controlling air emissions is a requirement. A DAF can be covered, but the covers are generally inefficient, difficult to seal, and inhibit the ability to see into and access the unit to ensure correct operation and maintenance. Additionally, safety issues due to the potentially explosive vapor space should be addressed on a site by site basis. If covered, off-gas can be directed to a carbon canister, a flare, an incinerator, or a vapor-recovery system that is designed with the appropriate safety standards. Also consider the gas seal option on rotating shafts entering the equipment for additional air emissions control. This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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VENDOR SPECIFIC INFORMATION
Final sizing and economics are based on using the sizing methods described later in this section and vendor input. Information required by the vendor to assist in selection and sizing of flotation equipment and polymer screening may include:
· ·
Type of process stream and influent flow rates Raw water characteristics (total suspended solids, dissolved solids, specific gravity of solids, particle size distribution, free oil & grease, metals, temperature, pH)
·
Location of installation (plot space, configuration)
·
Process flow sheet
Selection of standard vendor design sizes will reduce cost. Often the incremental cost of choosing a slightly larger size flotation unit is not high and may be prudent to allow operating flexibility. Most IGF units and some smaller DAF units are available as vendor "skid mounted" package units which reduces field installation costs. 6.8
DISSOLVED AIR FLOTATION (DAF) DESIGN
Note: In general, the design parameters that will be presented in this section apply to DAF applications in both wastewater clarification and sludge thickening. Parameters that do not apply to both clarification and thickening are identified. ç
Air Solubility – In DAF, micron-size bubbles (30 – 120 mm) are produced by dissolving gas into water at elevated pressures followed by subsequent release to atmospheric pressure. The amount of air that can be dissolved in water is determined by Henry's law, which states that for non-ionizing gases of low solubility, the volume dissolved in water varies with absolute pressure. Air solubility is a function of temperature, pressure, and total dissolved solids (TDS) of the water stream. The quantity of gas, which theoretically will be released from solution when the pressure is reduced to atmospheric conditions, is calculated from the following equation10,11,12: s = sg (f P – 1) where: s sg P = p f
Eq. (1) = Gas released at atmospheric pressure, mg/L = Gas saturation or solubility at atmospheric conditions, mg/L (Figure 11) = Pressure in saturation drum, atm (p + 14.7) / 14.7 (Customary units) = (p + 101.35) / 101.35 (Metric units) = psig for Customary units and kPa for Metric units = Fraction of saturation achieved in the pressurization tank. This is influenced by mixing and detention time with typical values between 0.8 and 0.9. The actual fraction of saturation should be determined during pilot plant testing. In the absence of pilot plant data, a value of 0.8 can be used for a rough estimate of f.
Rearranging (1) gives an expression of the minimum operating pressure for saturation drum: P = [(s / sg) +1] / f
Eq. (1a)
Wastewater Temperature – Temperature has a significant effect on gas solubility. Therefore, adjustments to system operation, particularly on the pressurization drum pressure, in winter or summer may be needed to ensure that the proper air rate is supplied. Figure 11 shows the solubility of air in a temperature range of 60–120°F (16–50°C). The design should provide the flexibility to meet air saturation requirements at the maximum expected temperature. Coagulation-Flocculation – Coagulation-flocculation is usually accomplished in the following manner: The coagulant is added to the wastewater feed and mixed rapidly for approximately 1 to 5 minutes in a flash mix tank. The resulting treated wastewater is fed into a flocculation tank where a flocculant, usually a cationic polymer, is added and mixed slowly for 10 to 30 minutes to flocculate the suspended particles. Mixing, in the coagulation-flocculation process, is an important variable. The wastewater and the coagulant must be mixed completely and rapidly if efficient coagulation is to take place. For this reason, a rapid mix tank equipped with high-speed mixer should be provided. This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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To allow the microflocs to grow after the addition of a flocculant, gentle agitation is required. A flocculation tank with slow-speed paddle type mixer should be used. Too vigorous agitation during flocculation will tear the fragile floc and break the long chain polymer molecules. Key Design Parameters
·
Hydraulic Loading Rate (HLR)24 – HLR is defined as the sum of the fresh feed flow plus recycle flow divided by the net available flotation area. The net available area is defined as the surface area in the flotator tank that is available for separation. HLR is the limiting design parameter in clarification of wastewater containing less than 1000 mg/L of total suspended solids. The typical design range is 1.8 to 3.0 gpm/ft2. A typical design value for clarification (without pilot data) is 2.0 gpm/ft2 (1.4 l/sec/m2). For bulky sludge in waste activated sludge thickening, an HLR of 2.0 gpm/ft2 (1.4 l/sec/m2) is recommended. Additional turbulence may hinder the establishment of a stable float blanket and reduce the attainable float solids concentration. Deterioration in solids capture also may result as increased turbulence forces the flow regime away from plug flow and more toward mixed flow.
·
Solids Loading Rate (SLR) – SLR is computed by dividing the maximum amount of oil and solids in the fresh feed by the net available flotation area. This parameter, along with the air / solids ratio (A/S), controls the sizing of a DAF thickener. An SLR range of 0.5 to 2.0 Ibs/hr/ft2 (2.5 to 10 kg/hr/m2) is recommended for biological sludge application. For other types of sludges, a typical design loading is 1 to 1.5 Ib/hr/ft2 (5 to 7.5 kg/hr/m2). Float removal difficulties may arise when the SLR exceeds 2.0 Ib/hr/ft2 (10 kg/hr/m2). Any chemicals used to flocculate and coagulate the suspended solids should be included as solids in determining the surface loading, since the chemicals will be removed as float with the solids. Since many chemicals are added in dilute form, make sure to account for active chemical only. For thickeners, surface loading calculations are based on 100 percent removal regardless of actual efficiency as a more conservative approach in sizing the float handling facilities.
·
Air to Solids Ratio (A/S) – This is defined as the weight ratio of air available for flotation to the solids to be floated in the feed stream. In DAF, this is perhaps the single most important factor affecting DAF performance, so flexibility in the range of A/S is needed in the design. Float solids content increases with increasing A/S, up to a point where further increases in A/S results in little or no increase in float solids. The optimum A/S is related to the feed sludge type and characteristics. For example, activated sludge having a low sludge volume index (SVI) (settles well) requires a lower A/S than sludges having a high SVI. Typical design A/S ratios range from 0.02 to 0.1 Ibs air/lb of solid (0.1 to 0.5 kg air/kg solid). In clarification application, the term "solids" refers to "suspended solids plus oil and grease," if both are present. A/S sets the minimum recycle rate through the following equation12,24: A/S = 1.3 (Sg / Sa) (R / Q) (f P – 1) where: A/S Sg Sa R Q (f P–1)
= = = = = =
Eq. (2)
Air to solids ratio, mg (air)/ mg (solids) Air solubility at atmospheric conditions, mg/L (Figure 11) Influent suspended solids / oil or sludge solids, mg/L Pressurized recycle, Mgal/day (3790 m3/day) Wastewater influent flow, Mgal/day (3790 m3/day) See Eq. (1)
Summary of Key DAF Design Parameters
ç Parameter
Hydraulic Loading Rate (HLR), gpm/ft Solids Loading Rate (SLR), lbs/h/ft Air to Solids Ration (A/S), lb/lb
2
2
Wastewater Clarification
Thickening
1.8 - 3.0 (2.0 Typical)
1.8 - 3.0 (2.0 Typical)
-
0.5 - 2.0
0.02 - 0.1
0.02 - 0.1
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Additional Design Parameters
·
Pressure Level – Air pressure utilized in the air-water mix drum ranges from 45 to 85 psig (310 to 590 kPa). Maximum removal is achieved at about 45 psig (310 kPa)8 since oil removal efficiency may decrease at higher pressure due to excessive liquid disturbance in the flotation column. The optimum pressure is determined by pilot testing. In the absence of pilot data, the pressure is calculated from Eq. (1a) using 0.35 SCF per 100 gallons of fresh feed (26.2 mg/l) for s.
·
Optimum Recycle Rate – Eq. (2) should be used in obtaining the optimum recycle rate for a given fresh wastewater flowrate, temperature, saturation drum pressure, saturation fraction (f) and the A/S ratio. The minimum design recycle rate should not be less than 40 percent of the feed flow for wastewater clarification, nor less than 100 percent for thickening. Typically a range of 40 to 60 percent for wastewater clarification and 100 to 500 percent for waste activated sludge are specified for recycle pump design.
·
Retention Time – The flotator residence time should be based on the hydraulic loading and the flotator dimensions. Retention is normally not specified, but falls within 20 to 60 minutes for wastewater clarification (30 minutes typical) and 30 to 40 minutes for thickening.
·
Flotation Tank (Flotator) – The flotation tank provides the necessary retention time for the bubbles to float the impurities to the surface. The tank is provided with a means to remove the float. Tank sizing (area) for DAF is based on the following: HLR for wastewater clarification applications. SLR for sludge thickening applications. The HLR or SLR is used to specify the flotator area. These parameters are best determined from pilot plant tests. Units for grass roots installation should be designed with the values stated in the Design Considerations and Design Procedures Sections. Tank shape can either be circular or rectangular. In general, the circular design should be selected when space permits10. Advantages of circular configuration include: Economical circular construction. Low velocities maintained throughout the active flotation zone. Pivoted arm skimmer reduces maintenance and lubrication requirements. Bottom scraper can be added with little extra cost. Top, centrally mounted drive shaft eliminates sprockets, chains, and underwater bearings. Advantages of rectangular clarifiers include: Conservation of space in congested areas. Most standard sizes, less than 500 ft2 (less than 50 m2), can be shipped set up, thus minimizing field erection. Hopper bottom eliminates the need for bottom scraper. More conducive to gravity flow from API Separator.
·
Air Supply – Air must be reliably supplied to the air-water mix drum (saturation tank) at sufficient pressure and in sufficient volume for the proper operation of the DAF unit.
·
Air-Water Mix Drum (Saturation Tank) – A drum is provided to promote intimate contacting of the air with the water stream, to provide holdup time for the air to dissolve, and to allow excess air to disengage and vent. Recommended residence time is 1 to 2 minutes to achieve a minimum saturation of 80 percent. Undissolved air can cause turbulence, and can result in the dissolved air contacting undissolved air bubbles rather than the contaminants.
·
Back-Pressure Valve (Let-Down Valve) – A valve is provided to drop the pressure of the aerated water stream to the pressure in the flotation tank.
·
Inlet Distributor – The aerated-water and influent stream enters the flotation chamber through an inlet distributor. The distributor should promote contact of the super-saturated aerated stream with the incoming wastewater and, should evenly distribute the bubbles to effectively use the area of the flotation tank.
·
Float Handling – The float is normally skimmed, removed, and further settled elsewhere. It is important to pilot the DAF float handling equipment as well as the DAF itself. Often a float transfer tank is provided to allow time for the air to disengage from the float. The float from a wastewater DAF is usually sent to an air-disengaging drum (~ 8 hours hold-up), which contains an agitator and a steam sparger to de-aerate the float. From the disengaging drum, the float is discharged to a setting tank or to the API
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separator oil sump. Slop oil facilities usually have trouble with float unless equipped for it. Separate float dewatering facilities are normally specified and may include centrifugation, filtration (recessed plate or belt press), and drying. See DP XX-A, Guidelines and Considerations for Managing Solid and Liquid Wastes, for additional information on managing wastes. In waste activated sludge thickening, the holding tank requires aeration which is achieved by air sparging to avoid potential odor problem due to the sludge turning "septic" from oxygen deficiency. A typical air sparging rate is about 0.5 SCFM per 1000 gallons (2.24 sm3/hr per 10m3) tank capacity. 6.9
INDUCED GAS FLOTATION (IGF) DESIGN
Note: The two types of IGF that are used in ExxonMobil are the mechanical type (impeller) and hydraulic-type (nozzle / striker plate}. The methods of gas induction and float skimming primarily differentiate these two types of IGF units. The mechanical type uses mechanical impeller gas inductors (Figure 6A) and mechanical skimmers (Figure 12A). The hydraulic type uses nozzle / striker plate gas inductors (Figure 6B) and a static skim trough (Figure 12B), located at the liquid surface. A typical configuration for both types of flotation units consists of four active cells (where oil droplets / suspended solids are removed and float is skimmed), and a discharge box. The first active cell receives the feed wastewater, while the fourth feeds the discharge box. Upstream Surge Capacity IGF units require upstream surge capacity to disengage gas, remove large (>40 microns) oil droplets, remove suspended solids, and smooth flow surges. A typical feed for IGF has concentrations of oil and grease or suspended solids of less than 100 mg/L each, which is typical of API separator effluents. Key Design Parameters
· ç
Flotation Tank 1. Tank Size – The flotation tank in grass root applications should provide the necessary detention time to remove oil and suspended solids from the inlet water by a stream of fine bubbles which lift the contaminants to the liquid surface for removal. The flotation cells should be designed with a total residence time of 4 to 8 minutes for a four-cell (active) IGF. If challenged crude slates are anticipated (generally less than 20° API), the recommended total residence time increases to 8 to 20 minutes for a four-cell (active) IGF. 2. Other Flotation Tank Details – A typical tank configuration consists of flotation cells with baffles, degassing, and a skim storage compartment. Two parallel tanks with equal capacity are typically provided. The design should incorporate: a. Hydraulic design and piping capable of permitting the total flow to go through either unit. b. Adjustable float weirs or trough and positive float removal from all cells. c. Discharge by gravity or pressure of skimmed float to the float thickener. The discharge piping should be designed, in size and configuration, to prevent plugging. d. A level controller on the effluent line of each IGF unit. e. An optional on-line oil-in-water analyzer or turbidity analyzer to indicate, record, and alarm, the quality of the IGF effluent. This will alert the operator of the need to make an adjustment to polymer rate, skimming, and/or to upstream facilities, such as a corrugated plate separator. f. Air tight enclosure to prevent / minimize air emissions. Where practical, direct off gas to flare or vapor recovery system rather than using an activated carbon canister. g. Water sprays for cleaning windows and flushing trough of hydraulic-type IGFs. h. Leveling legs for the installation and to compensate for ground settling. In mechanical type units, a pressure / vacuum breather valve is required with normal operating pressure being 0.03 to 0.1 psig (0.2 to 0.7 kPa)15 since the vortex action created by the impeller generates a steady vacuum. 3. Gas Inductor – This device induces gas (air) into flotation units. a. Mechanical Type – This is a mechanical powered inductor with an impeller on the end of a vertical shaft. It also has a steel-bearing stand, a cast bearing housing and seals, and grease lubricated taper roller bearings that carry the vertical steel shaft. The bearing stand is equipped with a swing-type motor mount for easy adjustment of the V-belt tension. A steel dispenser surrounds the impeller and is attached to a standpipe. The steel standpipe is equipped with a gas intake opening beneath the gas-tight cover. Each cell has one inductor. See Figures 4 and 6. b. Hydraulic Type – Each cell has one inductor located at the center, bottom of the cell. Gas is induced from the top with a venturi valve (eductor) to the inductors. The eductors utilize clean treated water drawn from the discharge cell with a recirculating pump. The air and water are impinged on a striker plate to produce fine bubbles. See Figures 5 and 6.
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Float Skimmer – This device removes the float from the surface. a. Mechanical Type – Uses a motor-driven skimmer with speed control or a timer for on / off operation to scrape float over an adjustable weir. Float-collecting launders are located on one or both outboard sides. b. Hydraulic Type – Uses a centrally positioned skim-trough, which extends through the flotation chambers. Liquid level is maintained below the skim trough using a level sensor in the outlet chamber during normal operation. Float eventually rises above the lip of the trough and is captured as it falls in. Level in the float cells can also be adjusted on a timer to facilitate skimming. Contents of the trough are discharged by gravity to the float chamber or separate float thickener. An adjustable secondary skimmer located at the discharge box is used to remove residual float.
Additional Design Parameters
·
Float Handling –The float generated from IGF consists of oily solids, oil and water. Since float is watery (0.5 to 2 wt% oil and solids), further separation is needed before disposal. The float can be discharged directly into a thickener. This can either be a gravity thickener or DAF thickener. It is important to pilot the IGF float handling as well as the IGF unit itself. A gravity thickener should be sized according to the guidelines described in References 9 and 15. A typical design for the gravity thickener area uses an HLR = 0.2 gpm/ft2 (0.14 l/sec/m2). If DAF is used as a thickener, it should be sized according to the procedures described in this Design Practice. For a gravity thickener, the skimmers and scraper mechanisms must be capable of removing the total quantity of either float or bottom sludge to a float receiver tank.
·
Floated oil should be recycled with caution. Emulsions caused by surface active agents can upset upstream oil-water separating processes and solids can build-up and seriously degrade performance if not removed. Although in practice, oily concentrate can be discharged directly to the API separator oil sump, it is preferable to break the emulsion first using heat or chemicals. 6. Precautions should be taken in handling float because of the potential presence of H2S.
·
Float Transfer Pump – The float transfer pumps should be capable of transferring thickened float from the float receivers to the float storage tanks. Two pumps serve each float receiver tank, one operating and one spare. Each pump shall be capable of delivering 100 percent of the maximum design flow containing 4 percent oily solids.
·
Float Storage Tank – This tank accumulates thickened float for feeding to the next step of the sludge disposal process, which is usually dewatering equipment. Since dewatering equipment is typically operated intermittently, sufficient storage capacity should be provided. This can be achieved by designing the tank's capacity equal to 2 to 4 days of thickened sludge production at design conditions. The float storage tank should be equipped with a mixer, and heating coils to raise and maintain the float at 80°F (27°C). Where desirable, foam insulation (1 in.) can be specified.
6.10 ç
SPARING / BACK-UP
Within ExxonMobil, there are no formal general policies regarding whole unit sparing or backup. Sparing practices are influenced primarily by local regulations, critical service, and operating strategy. Because there is typically no opportunity to completely shutdown a wastewater treatment plant for turnaround, two parallel trains are typically specified. One train should handle normal dry weather flow at a minimum, or 50% of design capacity, whichever is greater. Recycle pumps are usually spared, as there is the potential for plugging by CaCO3 or salts in the water, which is a reliability concern. The chemical storage facilities and the flocculation tank are not spared, but the chemical addition pumps, which normally have a lower service factor, should be spared. The DAF air compressor is normally spared or backed up with the refinery air system. The air saturation tank is not normally spared, but one is provided per flotation tank. 7 7.1
DESIGN PROCEDURES
TREATABILITY TESTING
A flotation facility should be designed using criteria developed from a test program, particularly for special applications or when the effluent from the flotation units must meet specific quality limits. Testing can often reduce conservatism and cost in the design. Pilot plant tests are recommended, particularly if performance criteria are stringent. A test must incorporate all the wastewater process conditions representative of the final application, if the data necessary to scale-up to a full size unit is to be provided. In addition, the following should be incorporated:
·
Operation under continuous conditions, for a time period sufficient to evaluate the effect of variable feed conditions, such as flow and contaminant concentration, on the unit's performance.
·
Basic geometry design features of the full-scale unit.
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Non-operator controlled skimmer or scraper to eliminate operator related variable. Pilot testing should define the following design parameters as indicated by an (X).
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DAF – WASTEWATER CLARIFICATION
PARAMETER
DAF – WASTE ACTIVATED SLUDGE THICKENING
IGF – WASTEWATER CLARIFICATION
Effluent Quality
X
X
X
Hydraulic Loading Rate
X
¾
X
Cell Retention Time
¾
¾
X
Solids Loading Rate
¾
X
¾
Air / Solids Ratio
X
X
¾
Recycle Rate or Ratio
X
X
X (hydraulic)
Air/Water Mix Drum Pressure Level
X
X
–
1. Type and Dosage of Chemicals
X
X
X
2. Flash-mix Retention Time
X
¾
¾
3. Flocculation Retention Time
X
X
¾
Float Characteristics
X
X
X
Float Thickening
¾
¾
X
Type of Chemical Conditioning Required
Pilot plant testing should be conducted using a unit easily scaleable to commercial units. Vendor's scale-up factors specific to the wastewater shall then be used along with the pilot plant data in the final design. EMRE's Environmental Engineering Section of the Plant Engineering Division should be contacted for specifics of pilot testing and interpretation of pilot plant data. 7.2
DISSOLVED AIR FLOTATION (DAF)
Note: In general, the design parameters that will be presented in this section apply to both DAF applications in wastewater clarification and sludge thickening. Parameters that do not apply to both clarification and thickening are identified. 7.2.1
Components of a Dissolved Air Flotation System
A dissolved-air flotation system consists of the following key parts: ·
Chemicals facilities
·
Flocculation tank
·
Inlet distributor
·
Flotation tank
·
Air supply
·
Float handling
·
Recycle pump
·
Air-water mix drum
·
Appurtenances
·
Flash mix drum
·
Back-pressure valve
Details of these parts, some of which are composed of several components, are described below.
This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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Key Parts of the Design
·
Chemicals Facilities – When feasible, the types and quantities of chemicals to be added to a DAF feed shall be determined by tests before the chemicals facilities are designed. Details on chemical addition facilities and on the chemicals themselves are given in References 1 and 13. Chemical facilities are typically provided for at least two chemicals: a coagulant and a flocculant. Acid, base, or both may also be required for pH adjustment. Provide the necessary equipment for chemicals storage, injection, pH adjustment if required, metering, flash-mixing, and flocculation. Size and design facilities on the following basis: 1. Coagulant – Typical quantities are 10 to 100 wppm on fresh influent 2. Flocculant – Typical quantities of a polyelectrolyte are 1 to 20 wppm on fresh influent. 3. pH Adjusting Chemicals – Acid, caustic, and lime can be used to adjust the pH of the wastewater. Note that the use of lime will generate additional oily sludge for disposal and may not be desirable. The pH is typically controlled at the value at which the coagulant works best, which is usually in a narrow range but within 7 to 9. 4. Minimum turndown on chemical addition pumps is 10:1. 5. Flow proportional control ratio of chemical rate is strongly recommended.
·
Flotation Tank – This is where oil droplets / particulate materials are separated from the wastewater. The tank consists of several parts, which are typically designed as follows: 1. Settler Size – The settler is sized according to its particular application by using key design parameters – HLR for wastewater clarification and SLR for thickening. Typical settler side water depths are 6 to 12 ft (2 to 4 m). Use 10 ft (3 m) as the default value. Retention time in the flotation tank shall satisfy the surface loading, either hydraulically in wastewater clarification applications or in terms of solids loading rate in thickening applications. A typical hold-up time is 1/2 to 1 hour. 2. Other Flotation Tank Details – The flotation tank can be fabricated of concrete, coated steel, or other metal, depending on the corrosive properties of the water being treated. A cover can be provided for air pollution control. A means should be provided for draining the tank. 3. Float Skimmer – Flight scraper type skimmers are typically used in rectangular flotators, and radial skimmers in round flotators. In circular units, the skimmer can be supported either by a beam or by a center column. The number of skimmers required is determined by the float production rate and the skimming rate per linear ft (m) of skimmer. 4. Sludge Scrapers – Bottom sludge scrapers are included when required to move deposited solids to the collecting hopper(s). Unless data indicate otherwise, sludge scrapers should be specified.
·
Recycle Pumps – The recycle pumps are normally centrifugal. Air has been added variously to the suction, interstage, and discharge sides of the pump. When the air is added to the discharge side of the centrifugal pump, it is injected through a porous diffuser. An alternate arrangement, used by one vendor, is two horizontal, centrifugal pump stages. The first stage primes the second stage and raises the pressure to about 75 percent of the final pressure. Air is introduced into the suction of the second stage. When air is introduced into the suction or interstage side of a pump, the pump shall be self-venting. If air is added to the pumps, they must be capable of handling the air-water mixture without binding, must resist corrosion by the aerated water and must resist pitting from cavitation. For wastewater clarification service, the recycle rate is typically 40 to 60 percent of the fresh feed rate. The pump capacity is specified as 40 percent of design feed rate or the recycle rate calculated using Eq. (2), whichever is greater. For DAF thickening, the recycle rate is 100 to 500 percent of feed flow and is calculated using Eq. (2). Pump head is determined based on the air-mix drum pressure. A spare recycle pump is normally specified.
·
Flash-Mixing Zone/Drum – Design for a holdup time of 2 to 5 minutes, and provide rapid agitation at a power level of about 3 hp/1000 gallons (0.6 kW/m3). The flash-mix section can be a small drum attached to the outside of a circular flotation tank or a separate zone in a rectangular flotation unit. The pH adjustment can also be carried out in the flash-mixing drum. Design the drum with a length-to-diameter ratio of about 1.0. The floc must not be pumped, and lines carrying the floc should be sized to avoid both breakup and settlement of the floc. Design for a velocity of about 2 ft/s (0.6 m/s).
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·
Flocculation Tank – Design for a holdup time of approximately 22 minutes for primary coagulation, and provide gentle agitation. For a polymer flocculant, 5 minutes may be sufficient if vendor has data to show that shorter holdup time is acceptable. For challenged crude systems, a 2 chemical system may require a flash-mixing drum followed by some residence time for the primary coagulant followed by a flash-mixing drum followed by flocculation zone for the flocculant. For all flocculation zones, use a slow-speed paddle mixer with a maximum tip speed of 2 ft/s (0.6 m/s). Baffles can be provided.
·
Air Supply –The pressure in the air-water mix drum is typically 45 to 85 psig (310 to 590 kPa) with 55 to 65 psig (380 to 450 kPa) as a typical design range for wastewater clarification. Thickeners typically use a higher pressure in the range of 75 to 85 psig (520 to 590 kPa) due to higher percent solids in sludge to be thickened. An air supply rate of 1.0 SCF per 100 gallons (0.08 sm3 per m3) of fresh influent is normally provided. This amount of air exceeds the solubility, and much of the air is excess and is not used for flotation. Approximately 0.35 SCF of air per 100 gallons (0.026 sm3 per m3) of fresh influent is solubilized. About 0.65 SCF/100 gallons (0.053 sm3 per m3) vents from the air-water mix drum. Actual air requirements can be determined using Eq. (2) and 200 percent excess air. Air supply can be plant air, a packaged compressor installation at the DAF unit or vacuum eductor (venturi). A packaged compressor installation usually consists of a self-contained, motor-driven, reciprocating compressor provided with an air receiving drum, instrumentation, and automatic pressure control. An eductor installation uses an eductor operating on bypass from the pressurizing pump discharge to aspirate atmospheric air into the pressurizing pump suction.
·
Air-Water Mix Drum – The air-water mix drum is a vertical drum containing baffles or a small amount of packing or other internals to assure intimate mixing of the air and the water. Vendors have proprietary designs for these drums. The drum is sized for a total volume of two minutes equivalent liquid holdup time, and a normal liquid level of about 30 seconds equivalent liquid hold-up time. Three to four ft (0.9 to 1.2 m) of headspace should be provided between normal liquid level and the baffles (internals) or the air vent to allow for frothing and for the air to disengage. Oxygenated water is an aggressively corrosive fluid, and the air-water mix drum must be constructed of appropriate materials. A lined carbon steel or 316 stainless steel vessel may be used. The air-water mix drums nominally contain a level controller, pressure relief valve, pressure indicator, low pressure alarm, and safety valve. Figure 13 is an example of instrumentation for the air-water system. If air is injected into the recycle pump, the system should include a shutoff on the air to the pump and a delayed start to prevent air-binding of the pump. Good pressure control of the injection air is necessary and adequate venting of excess air is important to prevent carryover. A means to drain the drum should be provided. The mechanical design pressure of the air-water mix drum should be sufficiently high to permit future operation at a process pressure 20 percent higher than that initially anticipated.
·
Let-Down Valve (Back-Pressure Valve) – Vendors use let-down valves of various designs. The let-down valve should be located as close to the flotation tank and to the influent stream as possible. The valve must be suitable for the mechanical design pressure of the system, should not be easily plugged by solids, and should minimize turbulence. A manually set, springloaded, "mushroom” type valve works well in recycle service.
·
Inlet Distributor – As with the air-water mix drum and the let-down valve, vendors have various designs for inlet distributors. The purpose of an inlet distributor is to mix the feed with the recycled flow and diffuse the mixture into the flotation tank without excessive disturbance or formation of density currents. Typical design, which is dependent on the tank configuration, is as follows: 1. Rectangular Tank Configuration – The inlet distribution (inlet box) is a box provided at one end of the tank. The feed box is typically attached to or supported by the tank wall. The box should be designed to prevent sludge deposits at the bottom. The box should be fabricated of not less than 1/4 in. thick steel plate. 2. Circular Tank Configuration – The inlet distributor (feedwell) should be concentric to the center shaft to diffuse the combined feed into the tank without disturbance or formation of density currents. The feedwell should be supported from the tank walls and should be fabricated of not less than 1/4 in. thick steel plate with necessary alignment angles.
·
Float Handling – Float production varies with the type and the amount of impurities and added chemicals. Typical float production is 0.25 to 3.0 volume percent of the fresh feed. The oil plus solids content of DAF float can be expected to be 1 to 10 wt% when the DAF unit is fed API separator effluent. If the float is difficult to handle because it contains wax or heavy solids, a screw conveyor can be used to remove the float from the float collector box. Float handling capacity is typically calculated assuming 100 percent removal of the oil and solids in the feed stream and a 2 to 6 wt% float solids concentration. A float solids concentration of 2 wt% is a conservative design value for wastewater clarification and 4 wt% is a conservative design value for sludge thickening. Float handling can add significantly to plant costs because of dewatering and disposal. Determining the float characteristics and production are important and pilot plant trials are best for determining them.
This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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Appurtenances – (see References 2, 4, and 8): 1. Instrumentation (Figure 14 is one example) a. Flow meter / recorder for fresh influent b. Feed pH meter / controller. c. Ratio controllers to control chemical dosages. d. Recycle flow rate meter / control valve. e. Air flow rate meter (usually a rotameter). f. Pressure indicator and low pressure alarm on air-water mix drum. g. Level control on air-water mix drum. h. Oil-in-water detector or turbidity meter on effluent. i. Pressure control valve on line from air-water mix drum to DAF. A local or remote control panel may be specified. Such a control panel shall be rated for the appropriate electrical classification (see Reference 4). The control system should be fully automatic for unattended operation. 2. Recycle Line – The recycle line shall assure full flow so that air entrainment will not bind the pump (first-stage or the single-stage pump, if one is used). 3. Sludge Withdrawal Line – Provisions shall be included to remove settled sludge from the flotation tank. Generally, a number of hoppers or a trough at the inlet end of the tank is provided. Settled sludge is scraped into the hoppers or trough and periodically removed either by pumping or by allowing the hydrostatic head in the flotation tank to pressure the sludge out. Clean-out or flushing connections should be provided. A sample duty specification for a dissolved air flotation system is shown in Table 3. Example design calculations are provided in the Example Design Problem Section of this Design Practice.
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7.3
INDUCED GAS FLOTATION (IGF)
7.3.1
Components of an IGF System
An induced gas flotation system consists of the following key parts: ·
Chemicals Facilities
·
Float Handling / Thickening
·
Flotation Tank
·
Appurtenances
·
Recycle Pump (for hydraulic-type units)
·
Float Pump (Optional)
Details of these parts, some of which are composed of several components, are described below. IGF systems are available from several vendors, each with specific design features. Typically, the designer specifies design flow, design feed concentrations and effluent requirements. The vendor will then recommend a standard sized unit to meet the requirements. Design information is provided below to allow the designer to evaluate the equipment being offered by the various vendors. 7.3.2
·
Key Parts in the Design
Chemical Facilities – When feasible, the types and quantities of chemicals to be added to an IGF feed should be determined by tests before the chemical facilities are designed. Details on chemical addition facilities and on the chemicals themselves are given in References 1 and 13. Chemical facilities are typically provided for a single flocculant. Provide the necessary equipment for chemicals storage, injection, and metering. pH adjustment may also be required; however, most flocculants are effective over a wide pH range. Float thickeners should be provided with both surface and bottom sludge removal facilities, each designed for the maximum thickened float rate. 1. Cationic Polymer – Typical quantities of polyelectrolyte are 1 to 20 wppm on fresh influent. It is preferable to inject the flotation aid 30 to 50 ft (10 to 15 m) upstream of the inlet box to thoroughly mix the polymer and wastewater. An in-line static mixer may also be used, but may be subject to plugging. Injection at the recirculation pump inlet on hydraulic units is not recommended since excessive shear may degrade the polymer and make it ineffective.
This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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pH Adjusting Chemicals – Acid and caustic can be used to adjust the pH of the wastewater. The use of lime to control pH is not recommended because it will generate additional oily sludge for disposal. Minimum tumdown on chemical addition pumps is 10:1. Flow proportional control of chemical rate is recommended.
Flotation Tank (Flotator) – This is where oil droplets and particulate materials are removed. There are several critical parts, which are designed as follows: 1. Cell Size
2.
Design the total tank volume based on a residence time of 4 – 8 minutes at maximum design influent flow rate plus 20 percent recycle (if any). If there is an effluent zone (pressure units), this is additional volume. If challenged crude slates are anticipated (generally less than 20 API), the recommended total residence time increases to 8 to 20 minutes. Parallel units, each with a capacity of 50 to 60 percent of the maximum design flow are normally specified. Associated piping for each unit should handle the maximum design flow. Equal flow division over the total flow range should be provided. Airtight or totally enclosed tanks are recommended where necessary to minimize or eliminate, air emissions. Specify a unit with a degassing cell and internal skim cell reservoir for hydraulic type units. Vessel pressure requirements for emissions control and vapor handling should be specified. Gas Inductor Mechanical Type – The motor horsepower shall be at least 9 hp/1000 gal (1.8 kW/m3). The impeller is designed by the vendor. Mechanical seals are recommended to control gas (air) leakage through the seals or drive shaft. b. Hydraulic Type – This type of gas inductor has no internal moving parts. Maintenance is considerably less than the impeller-type unit. The risk of gas leakage from seals and gaskets associated with impeller inductors is eliminated. The eductors should be constructed of corrosion resistant materials suitable for intended service. Consider using 316 stainless steel for the nozzle / eductor. If IGF feed has the propensity to foul and/or plug the striker plate where the educted gas enters the IGF, (commonly caused by calcium sulfate, magnesium salts, iron, coke fines, and/or hydrocarbon), installation of a local flow meter on the recycle gas is necessary. The flow meter will indicate when gas flow is reduced due to fouling prior to losing gas flow completely. Also, it is important to have the capability to throttle water flow to each eductor with pressure gauges downstream of the throttle valve. At maximum pressure, high turbulence may not allow sludge to overflow the weir. Float Skimmer a. Mechanical Type – Two stainless steel skimmers are normally provided and operated by two 1/4 hp motors. Hatches to access the skimmers for adjustment are recommended. b. Hydraulic Type – Skim cycles, including float discharge, should be automatically initiated by a timer. An adjustable secondary skimmer located at the outlet chamber should be used to remove float that overflows from the cells. Sight glasses and handholes to observe and adjust the skim trough are recommended. a.
3.
·
Recycle Pump (Hydraulic Units Only) The recycle pumps should be centrifugal. Typical recycle rates are 20 to 30 percent with 20 percent a good screening design value. A pressure switch for pump protection should be provided. Operational experience has found that cleanouts at bottom end of the recycle water header and at recycle pump suction low point are necessary and should be specified when equipment is ordered. The actual recirculation rate will depend on the size of the flotation unit, and is specified by the vendor. The recycle pump is normally spared or if two units are provided a common spare can be specified.
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·
Float Handling / Thickening 1. Float Generation Rate – Float production varies with the type and amount of impurities, added chemicals, and the type of skimmer (mechanical or hydraulic). Mechanical skimming rates can be up to 10% of the fresh feed. If optimized, the float production can be as low as 2 to 5 percent of fresh feed. This needs to be worked with the vendor. The oil plus solids content of IGF float is typically 0.5 to 2.0 wt%. Float generation rate is typically calculated assuming 100 percent removal of the oil and solids in the feed stream and a 0.5 to 1.0 wt% float concentration. Float handling can add significantly to costs because of dewatering and disposal. Determining the float characteristics and production rates are important and pilot plant trials are best for determining them. 2. Float Thickening – The low oil and solids content of IGF float usually dictates thickening in a DAF thickener or a gravity thickener. DAF thickening of IGF float is effective; however, gravity thickening is normally specified due to simplicity of operation. The design procedures described in References 9 and 15 are recommended for sizing a gravity thickener. It is likely that the float settling velocity will be very low. To enhance settling velocity a cationic polymer can be used. Design the thickener to handle the float generation rate calculated above using an HLR of 0.2 gpm/ft2 (0.14 I/sec/m2). Thickener depth should be minimum of 10 ft (3 m) at the side wall. Assume 95 percent clarification. Usually, the thickened sludge
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suspended solids concentration is in the 4 to 8 percent range. Float thickeners should be provided with both surface and bottom sludge removal facilities, each designed for the maximum thickened float rate. Float Storage – A float storage tank with a working capacity of 2 to 4 days sludge production should be provided to feed the downstream dewatering process. Specify mixer and steam heating coil. This capacity can be built into the thickener instead. Dewatering – The dewatering steps required will be dependent on the final disposition of the sludge generated. Alternative technologies for dewatering include filtration (pressure or belt filter), centrifugation, and drying. See DP XX-A, Guidelines and Considerations for Managing Solid and Liquid Wastes, for additional information on managing wastes.
Appurtenances - (see References 3, 4, and 8): 1. Instrumentation – Typical instrumentation includes:
2. 3. 4.
a. Flow meter / recorder for fresh influent. b. Ratio controllers to control chemical dosages. c. Pressure switch and local indicators. d. Liquid level controller. e. Oil-water detector or turbidity meter on effluent. A local or remote control panel may be specified. Control panel should be rated for the appropriate electrical classification (see Reference 4). The control system should be fully automatic for unattended operation. Relief valve, if closed vessel. T-Bar tool for adjusting the secondary skimming control from the outside of the vessel in nozzle-gas inductors. Float Transfer Pumps – Pump head will be dependent on downstream equipment and IGF operating pressure. Recessed impeller centrifugal, progressive cavity, double diaphragm, as well as other types of pumps have been used in this service. For pressurized vessel, float pump may not be needed. A sample duty specification for a hydraulic type induced gas flotation system with nozzle / striker plate eductors is shown in Table 4. Example design calculations are provided in the Example Design Problems Section that follows. 8
8.1
·
EXAMPLE DESIGN PROBLEMS
ADVANCED PRIMARY TREATMENT FOR OIL AND SUSPENDED SOLIDS Example 1 – Wastewater Clarification by Removal of Oil and Suspended Solids with DAF: Design Steps 1. 2. 3. 4.
Define the problem. From Figure 11 obtain the solubility of air based on: 14.7 psia and influent feed temperature. Calculate the pressurization pressure in atm. Determine the recycle ratio using Eq. (2) and calculate the recycle flowrate. The design recycle rate should not be less than 40 percent of the fresh feed. 5. Calculate the air rate in SCFM (sm3/min) using as a multiplier 1.0 SCF/100 gallons (0.08 sm3 per m3). 6. Calculate the total flowrate as: fresh feed flowrate + recycle rate. 7. Calculate the flotator tank area using an HLR = 2.4 gpm/ft2 (1.7 I/sec/m2), which was derived from pilot plant data. In the 2 absence of pilot data, use 2.0 gpm/ft . 8. Calculate the volume and weight of sludge generated: a. Calculate the weight of sludge generated with the oil removed, suspended solids removed, and the alum added. b. Add up the individual weight to get the total dry weight of sludge generated oil removed + weight of suspended solids removed + weight of alum added. 9. Calculate the volumetric production rate of sludge, in gpm (Ipm), using the total dry weight of sludge generated in 8b, sludge bulk density of 8 Ib/gal (956 kg/m3) and 2 wt% sludge solids concentration. 10. Calculate the design area by multiplying the area in (7) by a safety factor of 1.1 to 1.25. 11. Specify two units, each with 50 to 75 percent of the design size, to allow for maintenance on one unit while the other unit is in service. Actual sparing strategy depends on the specific site's operating requirements. This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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Example Step 1 – Define the Problem The clarification of wastewater by the removal of oil and suspended solids with a dissolved air flotation unit is required. The 3 wastewater, with a design flowrate of 2500 gpm (9.46 m /min), contains 150 mg/L of oil and grease and 100 mg/L of suspended solids. Flow and concentration data represent 95 percent probability values based on historical data. The particular facility has good upstream equalization, raw water management, oil loss stewardship, and well designed and operated oil / water separators. The influent wastewater temperature is 105°F (40.5°C). An effluent concentration of 20 mg/L is required for both oil and grease and suspended solids. An alum [Al2 (SO4)3] dosage of 80 mg/L was found necessary. A typical alum generates sludge at 0.46 mg sludge/ mg alum. An A/S ratio of 0.04, an operating pressure of 65 psig (447 kPa) and an HLR of 2.4 gpm/ft2 (1.7 l/sec/m2) were found to be optimal through pilot plant testing. Step 2 – Determine the solubility of air at 40.5°C and 14.7 psia (1 atm) from Figure 11 sg = 16 mg/I Step 3 – Determine the pressure in atmospheres P = (65 + 14.7) / 14.7 = 5.42 atm. Step 4 – Determine the required recycle ratio (R/Q) with Eq. (2) and the recycle rate (R) using feed flowrate A/S
=
1.3 (sg / Sa) (f P – 1) (R/Q)
0.04
=
1.3 (16 mg/l/250 mg/L) [(0.8) (5.42) – 1] (R/Q)
0.04
=
1.3 (0.064) (3.34) (R/Q)
R/Q
=
0.143, this is less than 40 percent of the fresh feed; therefore, use 0.4
R
=
0.4 (2500 gpm) = 1000 gpm (3.7 m3/min)
Step 5 – Calculate the total air required in SCFM (m3/min) using the fresh wastewater flow rate of 2500 gpm and 1.0 SCF/100 gallons Air Rate = (1 ft3 /100 gal) (2500 gal/min) = 25 SCFM (0.07 m3/min) Step 6 – Calculate the total flow rate Fresh feed flowrate + recycle flowrate = 2500 + 1000 = 3500 gpm (13.3 m3/min) Step 7 – Determine the flotator area using the hydraulic loading ratio (HLR) of 2.4 gpm/ft2 (1.7 l/sec/m2) obtained from pilot plant testing. In the absence of pilot plant data, use a HLR = 2 gpm/ft2 (1.4 l/sec/m2). 3500 gpm / (2.4 gpm/ft2) = 1460 ft2 (133 m2) Step 8 – Calculate the SLR – based area a.
Calculate the weight of sludge generated from the oil removed, the suspended solids removed, and the alum added (1) Dry weight of sludge from the removed oil and grease (150 – 20) mg/L x 2500 gpm x 1,440 min/day x [8.34 10-6 Ib/gal/(mg/L)] = 3903 Ib/day (2) Dry weight of sludge from the removed suspended solids (SS) (100 – 20) mg/L x 2500 gpm x 1,440 min/day x [8.34 10-6 Ib/gal/(mg/L)] = 2402 Ib/day (3) Dry weight of sludge from the added alum [(0.46 mg sludge/mg alum) (80 mg alum/l) (2500 gpm) (1440 min/day)] x [8.34 10-6 lb/gal/(mg/L)] = 1105 Ib/day
b.
Add up the individual weights to get the total dry weight of sludge generated 3903 + 2402 + 1105 = 7410 Ib/day (3365 kg/day)
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Step 9 – Calculate the volumetric production rate of sludge, in gpm (Ipm), using the total dry weight of sludge generated in 8b, a sludge bulk density of 8 Ib/gal (956 kg/m3) and 2 wt% sludge oil and solids concentration. Use this rate for sizing float handling system. Sludge production rate = (7410 Ib/day) (gal/8 lb) (100 Ibs sludge/2 Ibs solids) (day/1440 min) = 32 gpm (0.12 m3/min) Step 10 – Calculate the design area by multiplying the area in (7) by a safety factor of 1.1 since pilot testing was done 1460 ft2 (1.1) = 1600 ft2 (146 m2) Step 11 – Specify two units with each sized equal to 60 percent of (11) 1600 ft2 (0.60) = 960 ft2 (88 m2) ·
Example 2 – Wastewater Clarification by the Removal of Oil and Suspended Solids with IGF: Design Steps
1. 2. 3. 4. 5.
Define the problem. To determine the size of the flotation unit, use 4 minutes hydraulic retention time at maximum design flow rate, in the absence pilot plant data. Calculate the design flowrate and volume by applying a safety factor of 1.1 to 1 25, depending on the quality of data and availability of pilot plant data. Specify two units at 50 to 75 percent of design size to allow for maintenance on one unit while the other unit is in service. Actual sparing strategy depends on the specific site operating requirements and standard vendor size units. Calculate the volumetric sludge production rate, in gpm (m3/min). In the absence of pilot plant data use a 100 percent removal efficiency, a sludge bulk density of 8 Ib/gal (956 kg/m3) and a float solids and oil concentration of 0.75 wt%. a. Calculate the dry weight contributions on the sludge generated from oil and grease, suspended solids and the cationic flocculant: (1) Dry weight of sludge from oil and grease. (2) Dry weight of sludge from suspended solids. (3) Dry weight of sludge from the added cationic flocculant. b. Add up the individual weights to get the total dry weight of the float generated. c. Calculate the volumetric production rate of sludge, in gpm (m3/min). 6. Calculate the float gravity thickener area based on an HLR of 0.2 gpm/ft2 (0.14 l/sec/m2). The minimum side water depth of the thickener should be 10 ft (3.0 m). 7. Calculate the design area by multiplying the area in (6) by a safety factor of 1.1 to 1 25. Calculation Step 1 – Define the problem The clarification of wastewater by the removal of oil and suspended solids with an induced air flotation unit is required. The wastewater, with a design flowrate of 2500 gpm (9450 lpm), contains 150 mg/L of oil and grease and 100 mg/L of suspended solids. Flow and concentration data represent 95 percent probability values based on historical data. The particular facility has good upstream equalization, raw water management, oil loss stewardship, and well designed and operated oil / water separators. The influent wastewater temperature is 105°F (40.5°C). An effluent concentration of 20 mg/L is required for both oil and grease and suspended solids. The IGF effluent goes into an activated sludge system. To ensure a consistent removal performance, a cationic flocculant is injected, at an average dosage level of 4 mg/L, into the pretreated oily wastewater feed prior to entering the flotation unit. No pilot data is available. Step 2 – Determine the volume of the flotation unit, use a total of 4 minutes hydraulic retention time at design maximum flow, in the absence of pilot plant data (4 min) (2500 gal/min) = 10,000 gal (38 m3)
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Step 3 – Apply safety factor of 1.2 (no pilot data) to design flowrate and calculated volume 2500 gpm (1.2) = 3000 gpm (11,360 Ipm) 10,000 gal (1.2) = 12,000 gal (46 m3) Step 4 – Specify two units with each sized equal to 60 percent of 3000 gpm and 12,000 gallons 3000 gpm (0.6) = 1800 gpm (6810 lpm) 12,000 gal (0.6) = 7,200 gal (27 m3) Note: Safety factor and sparing strategy used in Steps 3 and 4 should consider available standard vendor size units. Step 5 – Calculate the volumetric sludge production rate, in gpm (m3/min). In the absence of pilot plant data use a 100 percent removal efficiency, a sludge bulk density of 8 Ib/gal (956 kg/m3) and a float solids concentration of 0.75 percent a. Calculate the dry weight contributions on the sludge generated from the oil removed, the suspended solids removed, and the cationic flocculant added (1) Dry weight of sludge from oil and grease 150 mg/L x 2500 gal/min x 1440 min/day x [8.34 10-6 Ib/gal/(mg/L)] = 4504 Ib/day (2) Dry weight of sludge from suspended solids 100 mg/L x 2500 gal/min x 1440 min/day x [8.34 10-6 lb/gal/(mg/L)] = 3004 Ib/day (3) Dry weight of sludge from the added cationic flocculant 4 mg/L x 2500 gal/min x 1440 min/day x [8.34 10-6 Ib/gal/(mg/L)] = 120 Ib/day b.
Add up the individual weights to get the total dry weight of float generated 4504 + 3004 + 120 = 7,628 Ib/day (3463 kg/day)
c.
Calculate the volumetric production rate of sludge, in gpm (m3/min) Sludge production rate = (7628 Ib/day) (gal/8 Ib) (100 Ibs sludge / 0.75 Ibs solids) (day/1440 min) = 88 gpm (0.3 m3/min)
Step 6 – Calculate the float thickener area using an HLR of 0.2 gpm/ft2 (0.14 l/sec/m2) for a gravity thickener (88 gpm) (ft2/0.2 gpm) = 440 ft2 (40 m2) Note: The minimum side water depth of the thickener should be 10 ft (3.0 m) Step 7 – Calculate the design area by multiplying the calculated area in (6) by 1.2 (440 ft2) (1.2) = 530 ft2 (48 m2) 8.2 ·
SLUDGE THICKENING Example 3 – Waste Activated Sludge Thickening With DAF Design Steps 1. 2. 3. 4.
Define the problem. Calculate the total weight of sludge to be removed, assuming 100 percent removal of incoming solids. Calculate the flotator tank area using an SLR = 2 lb/hr/ft2 (10 kg/hr/m2), if pilot plant data is not available. Calculate the flotator tank area based on HLR: a. From Figure 11 obtain the solubility of air based on: 1 atm and influent feed temperature. b. Determine pressurization pressure in atm. c. Determine the recycle ratio using Eq. (2) and calculate recycle flowrate. The recycle rate must be at least 100 percent. d. Calculate the total flowrate as: fresh feed flowrate + recycle rate. e. Calculate the flotator tank area using an HLR = 2.0 gpm/ft2 (1.4 l/sec/m2), if pilot plant data is not available.
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5. 6. 7.
Select the larger of (3) and (4e). Calculate the design area by multiplying the area in (5) by a safety factor of 1.1 to 1.25. Specify a unit with the design size calculated in (6). Spare or parallel waste activated sludge thickeners are typically not specified. Example Step 1 – Define the problem A DAF with pressurized recycle is to be designed to thicken the solids in waste activated sludge from 8000 mg/L to about 4wt%. The sludge flowrate is 70 gpm (260 Ipm) at 68°F (20°C). The sludge settles very well with an SVl of about 60, therefore, alum and flocculant are not used. Pilot data is not available and a saturation drum pressure of 75 psig (500 kPa) is assumed. Step 2 – Calculate the solids loading to the DAF at 100 percent solids removal (70 gal/min) (3.785 liters/gal) (8000 mg/L) (1 gm/1000 mg) (lb/453.6 gms) (60 min/hr) = 280 Ib/hr (127 kg/hr) Step 3 – Calculate the flotator area using solids loading rate determined from pilot plant testing. In the absence of pilot plant data, use an SLR = 2 Ib/hr/ft2 (10 kg/hr/m2) (280 lb/hr) / (2 lb/hr/ft2) = 140 ft2 (13 m2) Step 4 – Calculate the area based on HLR: a.
From Figure 11 obtain the solubility of air at 14.7 psia and 20°C sg = 18.7 mg/l
b.
Determine saturation drum pressure in atm P = (75 + 14.7) / 14.7 = 6.1 atm
c.
Determine the recycle ratio using Eq. (2) and calculate recycle flowrate. The recycle rate must be at least 100 percent. Use A/S = 0.2 in absence of pilot data. A/S 0.02 1.7 R/Q R
d.
= = = = =
1.3 (sg / Sa) (f P - 1) (R/Q) 1.3 (18.7/8000) [0.8 (6.1) - 1] (R/Q) (R/Q) 1.7, which is 170 percent of fresh wastewater feed flow rate and greater than 100 percent (1.7) (70 gpm) = 120 gpm (0.45 m3/min)
Calculate the total flowrate as: fresh feed flowrate + recycle rate 70 + 120 = 190 gpm (0.72 m3/min)
e.
Calculate the flotator tank area using an HLR = 2.0 gpm/ft2 (1.4 l/sec/m2), since pilot plant data is not available (190 gpm) / (2 gpm/ft2) = 95 ft2 (8.8 m2)
Step 5 – Select the larger of (3) and (4e) 140 ft2 (13 m2) Step 6 – Calculate the design area by multiplying the area in (5) by 1.2 (no pilot data) (140 ft2) (1.2) = 168 ft2 (15.3 m2) Step 7 – Specify one unit with an area of 168 ft2 (15.3 m2)
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9
OPERATING / CONTROL STRATEGY
A cost-effective operating / control strategy should contain not only procedures for proper operation of the flotation equipment but also elements for load minimization. Methods for minimizing load to the flotation equipment should include keeping upstream equipment operating efficiently and minimizing ingress of oil and suspended solids to the sewers. These upstream controls will help ensure consistently good removal performance and also less solid waste generation from flotation units. Reference 19 provides guidelines for formulating improved control / operating strategies to reduce operating costs for flotation units and the overall WWTP. In addition, the following should be considered in the operating / control strategies:
ç
·
Periodic Optimization of Flotation Aids – Flotation aids are used to enhance the performance of a flotation system. To ensure that effluent quality is maintained, periodic testing should be conducted since the influent wastewater characteristics can fluctuate significantly and a change of flotation aid and/or dosage rate may be needed. Also, dosage rate adjustment may be needed when upstream separators are not performing as designed. Optimization also minimizes over treatment with relatively expensive flotation aids. Over treatment tends to degrade effluent quality and can be costly. If an inorganic coagulant, e.g., alum, is being used, testing with several chemical vendors is recommended to find an organic coagulant with equal performance to reduce sludge volume.
·
Periodic Cleaning of Nozzle / Striker Plate Gas Inductors – The bubble size generated by the nozzles in nozzle / striker plate gas inductors influences oil removal performance. The shear imparted by the nozzles on the bubbles causes the formation of minute bubbles. Although there is an optimum bubble size below which improvement becomes insignificant, the smaller the bubble size the better the oil removal performance will be. Maintenance of high nozzle shear is important. Therefore, periodic maintenance of the nozzles is required, since they tend to plug with scale or solid deposits. For cleaning the nozzles, one refinery has good experience with 1% EDTA (a chelating chemical). During scheduled maintenance, this solution is pumped around with the recycle pump. The nozzles are cleaned one at a time. Cleaning with inhibited acid is another option provided the acid is compatible with all internals and coatings.
·
Maintenance of Key Components – Vendor's maintenance recommendations for critical parts of the system, such as pumps, nozzles, skimmers, and saturation-drum, should be implemented.
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TABLE 1A CHARACTERISTICS OF EXXONMOBIL DISSOLVED AIR FLOTATION UNITS
ç
LOCATION
INGOLSTADT REFINERY
Pretreatment Type of Service
Equalization Tank Oil/solids removal upstream of BIOX
Number of Units OPERATIONAL PARAMETERS Size, ft (m) Shape
2
TRECATE REFINERY API Separator, Equalization Tank Oil/solids removal upstream of Biox 1
DAF 1 23' diameter x 8' depth (7 x 2.5)
DAF 2 16' diameter x 8' depth (5 x 2.5)
Circular
Circular
Circular
43' diameter x 7.5' depth (13 x 2.3)
Feed Rate, gpm (m3/hr)
110 (25)
66 (15)
Normal: 1100 (250) Design: 1980 (450)
Recycle Rate, gpm (m3/hr)
132 (30)
88 (20)
615 (140)
103
78
Normal: 48 Design: 31
Organic Polymer (Polyacryleamid)
Organic Polymer (Polyacryleamid)
Fe2(SO4)3 for S-2 removal, AlCl3 flocculant, cationic polymer 12 mg/l (AlCl3), 1 mg/l (polymer), Fe2(SO4)3 as needed
Retention Time, min (including recycle) Float Quality Oil + Suspended Solids, wt% Flotation Aid(s) Dosage, mg/L PERFORMANCE PARAMETERS Influent Quality, mg/l
· · ·
Oil
75
75
50
Suspended Solids
100
100
45
· · ·
Oil
10
10
15
Suspended Solids
30
30
22
8.5 – 9
8.5 – 9
8.5
Rise Rate (HLR), gpm/ft2 (m3/h/m2)
0.6 (1.4)
0.8 (1.9)
Normal: 1.2 (2.9) Design: 1.8 (4.4)
PRESSURIZATION DRUM Operating Pressure, Psig (barg)
70 (4.8)
58 (4)
41 (2.8)
77 (25) Normal Flow: 7
77 (25) Normal Flow: 4
1.7
None
None
None
Yes No
Yes No
Yes No
Filter Return to crude oil tankage
Filter Return to crude oil tankage
Plate and Frame Filter
pH Effluent Quality, mg/l
pH Air-to-Solids Ratio
Operating Temp, °F (°C) Retention Capacity, min Saturation efficiency, % Baffles (yes/no) FLASH MIX TANK Baffles (yes/no) Agitator Size, HP FLOCCULATION TANK Mixer (yes/no) Baffles (yes/no) FLOAT TREATMENT Dewatering Disposal
Land
This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobil Affiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants, contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates. ExxonMobil Research and Engineering Company
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TABLE 1A (Cont) CHARACTERISTICS OF EXXONMOBIL DISSOLVED AIR FLOTATION UNITS LOCATION Pretreatment
PORT JEROME REFINERY
FAWLEY REFINERY
API (~80%) or Biox (~20%)
None
Type of Service
Clarification
Thickening
Number of Units
4
1
BATON ROUGE REFINERY API, CPS, Pre-Biox ABT (Aggressive Biological Treatment) Downstream of ABT for clarification (previously secondary oil removal DAF) 2
OPERATIONAL PARAMETERS Size, ft (m)
20' x 59' x 13'
(6 x 18 x 4)
75' x 23'
(23 x 7)
75' diameter x 16' depth
(23 x 5)
Shape
Rectangular
Rectangular
Circular
Feed Rate, gpm (m3/hr)
4400 (1000)
400 (90)
10,000 (2270)
440 (100)
1800 – 2200 (410 – 500)
Recycle Rate, gpm (m3/hr) Retention Time, min (including recycle) Float Quality Oil + Suspended Solids, wt%
26
Flotation Aid(s)
Alum (wastewater from chemical plant) and polymer
Dosage, mg/L
Alum ~ 180 Polymer ~0.7 (dry)
44 10
4–6
None (but planned)
Cationic prior to flash mix tank, cationic emulsion polymer added in flocculation tank Cationic prior to flash mix: 80 cationic emulsion in floc tank: 1.5
PERFORMANCE PARAMETERS Influent Quality, mg/l
· Oil · Suspended Solids · pH
350 (as TOC)
100
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