Che 10405
Short Description
none...
Description
Engineering Encyclopedia Saudi Aramco DeskTop Standards
Dehydration And Hydrate Inhibition
Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.
Chapter : Process File Reference: CHE10405
For additional information on this subject, contact R.A. Al-Husseini on 874-2792
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Contents
Pages
PRINCIPLES OF DEHYDRATION ....................................................................... 1 Background .................................................................................................. 1 Water Content of Hydrocarbons................................................................... 2 Water Content Measurement for Natural Gas .............................................. 3 Hydrate Formation ....................................................................................... 3 Hydrate Inhibition ........................................................................................ 4 WATER REMOVAL PROCESSES ........................................................................ 6 Liquid/Solid Desiccants................................................................................ 6 Glycol Dehydration ...................................................................................... 6 Background....................................................................................... 6 Process/Design Variables.................................................................. 9 Solid Desiccant Dehydration ...................................................................... 12 Background..................................................................................... 12 Adsorption Calculations ............................................................................. 17 Regeneration Calculations.......................................................................... 20 Dehydrating Liquids................................................................................... 21 Process Variables ............................................................................ 22 Other Dehydration Processes.......................................................... 26 OPTIMIZING AND TROUBLESHOOTING DEHYDRATOR ........................... 28 Operations .................................................................................................. 28 Glycol Maintenance ................................................................................... 28 Methanol ......................................................................................... 28 Oxidation ........................................................................................ 28 Thermal Decomposition.................................................................. 28 pH Control ...................................................................................... 29 Salt Contamination.......................................................................... 29 Saudi Aramco DeskTop Standards
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Hydrocarbons.................................................................................. 29 Sludge ............................................................................................. 29 Foaming .......................................................................................... 30 Analysis and Control of Glycol .................................................................. 30 Glycol Loss Prevention .............................................................................. 31 Glycol Filtration ......................................................................................... 32 Optimizing Adsorption-Type Dehydrators................................................. 32 Desiccant Performance ................................................................... 32 Equipment Items ............................................................................. 34 WORK AID 1: SOLUBILITY OF WATER IN LIQUID HYDROCARBONS .. 35 WORK AID 2A:WATER CONTENT OF HYDROCARBON GAS .................... 36 WORK AID 2B: EFFECTIVE WATER CONTENT FOR CO2............................ 37 WORK AID 2C: EFFECTIVE WATER CONTENT FOR H2S ............................ 38 WORK AID 3: PHYSICAL PROPERTIES OF CHEMICAL INHIBITORS ..... 39 WORK AID 4: USEFUL EQUATIONS FOR DEHYDRATION CALCULATIONS...................................................................... 40 WORK AID 5: TYPICAL DESICCANT PROPERTIES .................................... 43 GLOSSARY .......................................................................................................... 46 APPENDIX A - SAUDI ARAMCO SOLID DESICCANT DEHYDRATION UNITS ............................................................ 48 APPENDIX B - REPRESENTATIVE VENDORS OF SOLID DESICCANT EQUIPMENT ....................................................... 49 APPENDIX C - VENDOR INFORMATION........................................................ 50
Saudi Aramco DeskTop Standards
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
PRINCIPLES OF DEHYDRATION Background Liquid water and/or water vapor are removed from natural gas to: • • •
Prevent formation of hydrates in transmission lines. Meet a water dew point requirement of a sales gas contract. Prevent corrosion.
Many sweetening agents used in gas treating utilize an aqueous solution. Therefore, dehydration usually follows gas treating. Techniques for dehydrating natural gas include: • • •
Absorption using liquid desiccants. Adsorption using solid desiccants. Dehydration by refrigeration.
Through absorption, the water in a gas stream is dissolved in a relatively pure liquid solvent stream. The reverse process, in which the water in the solvent is transferred into the gas phase, is known as stripping. The term regeneration is also used to describe stripping (or purification) because the solvent is usually recovered for reuse in the absorption step. Absorption and stripping are frequently used in gas processing and most gas sweetening operations, as well as in glycol dehydration. The second major process by which water vapor is removed from a gas stream is called adsorption. Adsorption is a physical phenomenon that occurs when molecules of a gas are brought into contact with a solid surface and some of them condense on the surface. Dehydration of a gas (or a liquid hydrocarbon) with a dry desiccant is an adsorption process in which water molecules are preferentially held by the desiccant and removed from the stream.
Saudi Aramco DeskTop Standards
1
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Water Content of Hydrocarbons Work Aid 1, based on experimental data, shows the solubility of water in sweet hydrocarbon liquids. In sour hydrocarbon liquids, water solubility can be substantially higher. For sour liquids an equation of state may be used to estimate water solubility. The water content of a gas depends on pressure, temperature, and composition. The effect of composition increases with pressure. For lean, sweet natural gases containing small amounts of "heavy ends," pressure-temperature correlations are suitable for many applications. Work Aid 2A is an example of one such correlation that has been widely used for many years in the design of natural gas dehydrators. When the gas contains more than about 5% CO2 and/or H2S, correction for the acid gas components should be made, particularly above 700 psia. Below 40% acid gas components, one method of estimating the water content is to use Equation 1 (Work Aid 4) and Work Aids 2A, 2B, and 2C.
Saudi Aramco DeskTop Standards
2
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Water Content Measurement for Natural Gas Specifications are given in GPA Publication 2140 in the GPA Technical Standards Book. These include the valve freeze method, the Bureau of Mines Dew Point Tester, and the cobalt bromide method. Cobalt bromide color change occurs at about 25-30 ppm. Absolute water content can be determined by titration with Karl Fischer reagents. There are several commercial instruments available for monitoring water content based on other principles. Measuring water content less than 10 wppm, or determinations at less than 40°F can be very difficult. Hydrate Formation A hydrate is a physical combination of water and other small molecules to produce a solid that has an "ice-like" appearance, but possesses a different structure than ice. There are two crystalline structures for gas hydrates. Limiting hydrate numbers (ratio of water molecules to molecules of included gaseous component) are calculated using the size of the gas molecules and the size of the holes in the H2O lattice. Smaller molecules (CH4, C2H6, H2S) form a body centered cubic (Structure I) with a limiting hydrate number of 5 3/4 for CH4 and 7 2/3 for C2H6. Larger molecules (C3H8, i-C4H10) form a diamond lattice (Structure II) with a limiting hydrate number of 17. Mixed gases will form Structure II. The conditions that promote hydrate formation are: Primary Considerations • • •
Gas must be at or below its water dew point with "free" water present. Low temperature. High pressure.
Secondary Considerations • • • •
High velocities. Pressure pulsations. Introduction of a small crystal of the hydrate. Physical site for crystal formation such as a pipe elbow, an orifice, thermowell, or line scale.
All of these primary and secondary considerations should be minimized when forced to operate near a possible hydrate region. Conditions for hydrate formation can be calculated using Hyprotech's HYSIM, SimSci's PROCESS or PRO/II simulation programs. These calculations are based on the gas composition and vapor-solid equilibrium constants.
Saudi Aramco DeskTop Standards
3
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Hydrate Inhibition The formation of hydrates can be prevented by dehydrating to prevent a free water phase, or by inhibiting hydrate formation in the free water phase. Dehydration is usually preferable, but inhibition can often be satisfactory. Inhibition utilizes injection of one of the glycols or methanol to lower the hydrate formation temperature at a given pressure. Both glycol and methanol can be recovered. For continuous injection in non-cryogenic conditions, one of the glycols usually offers an economic advantage. At cryogenic conditions, methanol usually is preferred because glycol's higher viscosity makes effective separation very difficult. Ethylene, diethylene, and triethylene glycols have been used for glycol injection. The most popular has been ethylene glycol because of its lower cost, lower viscosity, and lower solubility in liquid hydrocarbons. Physical properties of these glycols are tabulated in Work Aid 3. The inhibitor must be present at the very point where the wet gas is cooled to its hydrate temperature. Therefore, the inhibitor is sprayed upon the face of the feed gas chiller tube sheet where free water is present. Injection must provide good distribution to every tube in chillers and heat exchangers operating below the gas hydrate temperature. Glycol and its absorbed water are separated from the gas stream, possibly along with hydrocarbons. The glycol-water solution and liquid hydrocarbons can emulsify when agitated, or when let down together from a high pressure to a lower pressure. Careful separator design will allow nearly complete recovery of the glycol for regeneration and recycle. The regenerator in a glycol injection system should be operated to produce a regenerated glycol solution that will have a freezing point below the minimum temperature encountered in the system. Figure 1 shows the freezing point of various concentrations of glycol water solutions. To use this plot, locate the glycol concentration, read up to the glycol type, and then read across to find the freezing point temperature. Glycol concentrations less than 70-75 wt% are typically used. The minimum inhibitor concentration in the free water phase may be approximated by Hammerschmidt's equation (Equation 2 in Work Aid 4). The quantity of inhibitor required to prevent hydrate formation can also be calculated using Hyprotech's HYSIM, SimSci's PROCESS or PRO/II simulation programs.
Saudi Aramco DeskTop Standards
4
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
The amount of inhibitor to be injected not only must be sufficient to prevent freezing of the inhibitor water phase, but also must be sufficient to allow for some vaporization and the solubility of the inhibitor in any liquid hydrocarbon. The vapor pressure of methanol is high enough that significant quantities will vaporize. The total injection rate required may be about three times that needed to maintain the water concentration desired. The amount of methanol that will vaporize can be estimated using the above computer programs. No allowance for glycol vaporization is necessary. Inhibitors can cause problems in downstream process units. In these cases efficient inhibitor separation should be provided. FREEZING POINTS OF AQUEOUS GLYCOL SOLUTIONS
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book. FIGURE 1
Saudi Aramco DeskTop Standards
5
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
WATER REMOVAL PROCESSES Liquid/Solid Desiccants In those situations where inhibition is not feasible or practical, dehydration must be used. Both liquid and solid desiccants may be used, but economics favor liquid desiccant dehydration when it will meet the required dehydration specification. Liquid desiccant dehydration equipment is simple to operate and maintain. It can easily be automated for unattended operation; for example, glycol dehydration at a remote production well. Liquid desiccants can be used for sour gases, but additional precautions in the design are needed due to the solubility of the acid gases in the desiccant solution. Solid desiccants are normally used for extremely low dew point specifications as required to recover liquid hydrocarbons. Glycol Dehydration Background The more common liquids in use for dehydrating natural gas are diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (TREG). In general, glycols are used for applications where dew point depressions of the order of 60°F to 120°F are required. DEG was the first glycol to be used commercially in natural gas dehydration and can provide reasonable dew point control. With the exception of TEG, DEG is the best liquid available. However, with normal field equipment, DEG can be concentrated to only 95% purity, whereas TEG concentrations can reach 98 to 98.5% without special equipment. Although both glycols perform sufficient dehydration in many situations, TEG is used more commonly because it requires lower circulation rates for a comparable dew point depression than DEG does and can reach lower dew points. It is not advisable to use triethylene glycol for dehydration at low temperatures (approximately 50°F), due to its high viscosity. TREG is primarily used when dehydration conditions fall between those encountered in normal TEG operations, and those in which gas stripping or vacuum distillation becomes necessary. The properties of these glycols are compared in detail in Work Aid 3. A process flow diagram of a glycol dehydration unit is shown in Figure 2. Good practice dictates installing an inlet gas scrubber, even if the dehydrator is near a production separator. The inlet gas scrubber will prevent accidental dumping of large quantities of water, hydrocarbons, and/or salt water into the glycol contractor. Even small quantities of these materials can result in excessive glycol losses due to foaming, reduced efficiency, and increased maintenance.
Saudi Aramco DeskTop Standards
6
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
PROCESS FLOW DIAGRAM FOR GLYCOL DEHYDRATION UNIT
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book. FIGURE 2 In the glycol dehydration process, regenerated glycol is pumped to the top tray of the contactor (absorber). The glycol flows down through the contactor countercurrent to the gas flow. Water rich glycol is removed from the bottom of the contactor, passes through the condenser coil, flashes off gas in a flash drum, and flows through the glycol-glycol heat exchanger to the regenerator. In the regenerator, absorbed water is removed from the glycol at atmospheric pressure by heating. The regenerated glycol is cooled in the glycol heat exchangers and is recirculated to the contactor by the glycol pump. TEG will absorb about 1 SCF of natural gas per gal at 1000 psig absorber pressure. There will be more absorption if aromatic hydrocarbons are present. A three to five minute residence time in the flash drum is required for degassing. Excessive hydrocarbons in the glycol may cause high glycol losses and foaming. The overhead vent from the glycol regenerator may contain hydrocarbons and should be piped to a safe location. The separation of TEG and water in the regenerator is accomplished easily with only internal reflux. The separation of DEG and water is more difficult due to DEG's higher vapor pressure.
Saudi Aramco DeskTop Standards
7
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
EQUILIBRIUM WATER DEW POINTS FOR GASES IN CONTACT WITH VARIOUS CONCENTRATIONS OF TEG
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book. FIGURE 3 To obtain the high glycol concentrations required for high dew point depressions, stripping gas or vacuum distillation must be used in the reboiler portion of the regeneration unit. The amount of stripping gas required to reconcentrate the glycol to a high purity ranges from 2 to 10 ft3 per gallon of glycol circulated. If stripping gas is used, a recovery system may be justified. The dew point depression obtainable with triethylene glycol can be estimated from Figure 3 based on the contact temperature and the concentration of the reconcentrated glycol that is used. Figure 3 shows the equilibrium water dew point at different temperatures for gases in contact with various concentrations of glycol. To use this plot, locate the contact temperature, read up to the glycol concentration, and then read across to find the equilibrium water dew point. In practice it is seldom economical for actual gas dew points to approach equilibrium dew points closer than 20°F. Saudi Aramco DeskTop Standards
8
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Process/Design Variables Several process and design variables have an important effect on the successful operation of a glycol dehydration system. Gas Temperature Plant performance is especially sensitive to the temperature of the incoming gas. At constant pressure, the water content of the inlet gas increases as this temperature is raised. Glycol vaporization losses are also increased at the the higher temperature. Furthermore, problems can result from too low a temperature (below 50°F) because glycol becomes very viscous. Lean Glycol Temperature The temperature of lean glycol entering the absorber has a significant effect on the gas dew point depression, and should be held to a minimum to achieve the best operation. However, it should be kept at least 10°F above the inlet gas temperature to minimize hydrocarbon condensation in the absorber and subsequent foaming. Glycol Reboiler Temperature The reboiler temperature controls the concentration of the water in the glycol. With a constant pressure, the glycol concentration increases with higher reboiler temperatures. The reboiler temperature should never be allowed to remain at or above the glycol degradation temperatures (see Work Aid 3) for any period of time. When higher glycol concentrations are required, stripping gas can be added to the reboiler. Regenerator Top Temperature The temperature in the top of the regenerator is also important. A high temperature can increase glycol losses due to excessive vaporization. The recommended temperature in the top of the column is about 225°F. If the temperature in the top of the column drops too low, too much water can be condensed and washed back into the regenerator to flood the column and fill the reboiler with excessive liquids. Contactor Pressure At constant temperature, the water content of the inlet gas decreases with increasing pressure. Therefore, less glycol circulation is required at higher pressures. However, if carbon dioxide is present, at a certain point a higher pressure will actually increase the water content. If not otherwise fixed, optimum dehydration pressure is typically in the range of 700 to 1100 psig. Reboiler Pressure Reducing the pressure in the reboiler at a constant temperature results in higher glycol purity. This pressure reduction lowers the water partial pressure in the vapor, increasing the driving force under which water leaves the glycol solution.
Saudi Aramco DeskTop Standards
9
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Glycol Concentration The water content of the dehydrated gas depends primarily on the lean glycol concentration. The dry gas leaves the contactor approaching equilibrium with the lean glycol. The leaner the glycol flowing to the absorber, the more efficient the dehydration. Figure 3 shows the effect of glycol concentration on gas dew point. Glycol Circulation Rate When the number of absorber trays and glycol concentration are fixed, the dew point depression of a saturated gas is a function of the glycol circulation rate. Whereas the glycol concentration mainly affects the dew point of dry gas, the glycol rate controls the total amount of water that can be removed. A typical glycol circulation rate is about three gallons of glycol per pound of water removed (seven maximum). The minimum circulation rate to assure good glycol-gas contacting is about two gallons of glycol for each pound of water removed. A greater dew point depression is easier to achieve by increasing the glycol concentration rather than by increasing the glycol circulation rate (see Figure 4). To use this plot, locate the glycol circulation rate, read up to the glycol concentration, and then read across to find the dew point depression. An excessive circulation rate, especially above the design capacity, overloads the reboiler and prevents good glycol regeneration. It also prevents adequate glycol-gas contacting in the absorber, increases pump maintenance problems, and can increase glycol losses. EFFECT OF TEG CIRCULATION RATE AND CONCENTRATION ON DEW POINT DEPRESSION
From non-proprietary information from Exxon Production Research Company, Production Operations Division, "Dehydration and Hydrate Inhibition," July, 1986, from Surface Facilities School, Vol. II. FIGURE 4
Saudi Aramco DeskTop Standards
10
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Number of Absorber Trays For a new design, the number of theoretical trays required for dehydration in the absorber or contactor is determined by constructing a McCabe-Thiele diagram. A tray efficiency of 25% to 40% is typically used. Generally, the number of actual trays in a design ranges from 4 to 12. Bubble cap trays are normally used in glycol dehydrators to facilitate low liquid loadings and to allow significant turndown. For high-performance units, specifying more than four trays in a new design can achieve fuel savings because a lower circulation rate, reboiler temperature, and/or stripping gas rate are required to achieve the same dew point depression. Figure 5 shows that specifying a few additional trays in the contactor is much more effective than increasing the glycol circulation rate. To use this plot, locate the glycol circulation rate, read up to the number of actual trays, and then read across to find the dew point depression. The additional investment for additional trays and a taller contactor is often justified by fuel savings. EFFECT OF NUMBER OF ABSORBER TRAYS ON DEW POINT DEPRESSION
From non-proprietary information from Exxon Production Research Company, Production Operations Division, "Dehydration and Hydrate Inhibition," July, 1986, from Surface Facilities School, Vol. II. FIGURE 5
Saudi Aramco DeskTop Standards
11
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Solid Desiccant Dehydration Background Since solid desiccant units cost more than glycol units, their use is usually limited to applications such as very sour gases, very low water dew point requirements, simultaneous control of water and hydrocarbon dew points, and special cases such as oxygen containing gases, etc. In cryogenic plants, solid desiccant dehydration usually is preferred over methanol injection to prevent hydrate and ice formation. Solid desiccants are also often used for the drying and sweetening of NGL liquids. Appendix A is a listing of Saudi Aramco's solid desiccant dehydration units. Desiccants in common commercial use fall into one of three categories: •
Alumina - Regenerable aluminum oxide base desiccant.
•
Silica Gel - Regenerable silicon oxide adsorbent.
•
Molecular Sieves - Regenerable solid desiccants composed of crystalline metal aluminosilicates (zeolites).
Each desiccant category offers advantages in different services. The best choice is not routine. A listing of representative vendors of solid desiccants is presented in Appendix B. Activated alumina has a strong affinity for water and high internal adsorption area due to the presence of pores or very fine capillaries. Alumina condenses and holds the water in the pores by surface adsorption and capillary attraction. Activated alumina desiccant can be used for drying liquids which do not contain unsaturates such as olefins or diolefins. It is less costly than molecular sieve desiccant but its capacity for absorbing water also tends to be lower, particularly when attempting to reach very low water levels, e.g. 5 wppm in the product. Silica gel has a higher equilibrium adsorption capacity (see Figure 6) than alumina because its available surface is greater. Due to silica gel's higher price per pound, alumina is generally the economic choice. Silica gel is not used where free water can be present, because free water destroys silica gel. Free water over long-term operation, either as droplets or slugs, will also damage molecular sieve and activated alumina by mechanical attrition and should be avoided. Molecular sieves have a high water equilibrium capacity at low relative humidities (see Figure 6). Molecular sieves also have the feature of uniform pore size, which allows them to exclude molecules based on size. Because different pore size molecular sieves are produced, selection of proper type of sieve can alleviate the problem of undesirable coadsorption. Molecular
Saudi Aramco DeskTop Standards
12
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
sieves have a higher design adsorption capacity than the other regenerable desiccants, but this is often offset by their considerably higher price per pound.
Saudi Aramco DeskTop Standards
13
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Molecular sieve dehydrators are commonly used ahead of NGL recovery plants where extremely dry gas is required. Cryogenic NGL plants designed to recover ethane produce very cold temperatures and require very dry feed gas to prevent formation of hydrates. Dehydration to approximately 1 ppmw is possible with molecular sieves. Two types of molecular sieves, Type 3A and Type 4A, are commonly used for drying hydrocarbon liquids. Type 4A sieves are less costly than Type 3A sieves and are used for distillates which do not contain unsaturates. When unsaturates are present in the feed, Type 3A are used to assure good regeneration. WATER VAPOR ADSORPTION AT 60°F
From non-proprietary information from Exxon Production Research Company, Production Operations Division, "Dehydration and Hydrate Inhibition," July, 1986, from Surface Facilities School, Vol. II. FIGURE 6
Saudi Aramco DeskTop Standards
14
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Solid desiccants are used in gas dehydrators containing two or more towers. Figure 7 is a simple two-tower system. One tower is onstream adsorbing water from the gas, while the other tower is being regenerated and cooled. Figure 8 shows a typical molecular sieve dehydrator vessel. Hot gas removes the adsorbed water, after which the tower is cooled. The towers are switched before the onstream tower becomes water saturated. Generally a bed is designed to be on line for 8 to 24 hours. When the bed is taken off-line, the water is removed by heating the bed to 450-600°F. The regeneration gas used to heat the bed is usually a slipstream of dry process gas. The regeneration gas is returned to the process after it has been cooled and the free water removed. Since heat is a major operating cost, this is a major design consideration. SOLID DESICCANT DEHYDRATOR TWIN TOWER SYSTEM
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book. FIGURE 7
Saudi Aramco DeskTop Standards
15
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
TYPICAL MOLECULAR SIEVE GAS DEHYDRATION VESSEL
From non-proprietary information from Exxon Production Research Company, Production Operations Division, "Dehydration and Hydrate Inhibition," July, 1986, from Surface Facilities School, Vol. II. FIGURE 8
Saudi Aramco DeskTop Standards
16
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Adsorption Calculations Adsorption calculations for a molecular sieve dehydrator are discussed below. The allowable superficial vapor velocity through the bed is the first parameter that must be estimated using Figure 9. To use this plot, locate the operating pressure, read up to the type sieve, then read across to find the allowable superficial velocity. Once the allowable superficial velocity is estimated, the bed diameter can be calculated for a design vapor rate. The design pressure drop through the bed is calculated using Equation 4 in Work Aid 4 and should be about five psi. A design pressure drop higher than eight psi is not recommended. ALLOWABLE VELOCITY FOR MOLE SIEVE DEHYDRATOR
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book. FIGURE 9 The next step is to choose a cycle time and calculate the pounds of sieve required. Eight to twelve hour cycles are common. Cycles greater than 12 hours may be justified, especially if the gas is not water saturated. Long cycles mean fewer regenerations and longer sieve life, but larger beds and additional capital investment are required.
Saudi Aramco DeskTop Standards
17
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
During the adsorption cycle, the bed operates with three zones The top zone is called the saturation zone. The molecular sieve in this zone is in equilibrium with the wet inlet gas. The middle or mass transfer zone (MTZ) is where the water content of the gas is reduced from saturation to < 1 ppm. Normally a system is designed so that there is a moisture analyzer to indicate when the mass transfer zone is likely to break through the end of the bed. A guard bed zone (typically one to two feet deep) is provided after this point to prevent actual breakthrough before the system has a chance to change to the regenerated bed. Unfortunately, both the water capacity and the rate at which the molecular sieves adsorb water change as the molecular sieves age. The object of the design is to install enough sieve so that three to five years into the life of the sieve, the mass transfer zone will be at the bottom of the bed at the end of the adsorption cycle. In the saturation zone, the molecular sieve is expected to hold approximately 13 pounds of water per 100 pounds of sieve. This capacity needs to be adjusted when the gas is not water saturated or when the temperature is above 75°F. See Figures 10 and 11 for the correction factors. To determine the pounds of molecular sieve required in the saturation zone, calculate the amount of water to be removed during the cycle and divide by the sieve capacity (use Equations 5 and 6 in Work Aid 4). Even though the MTZ will contain some water, the saturation zone is calculated assuming it will contain all the water to be removed. The length of the mass transfer zone can be calculated using Equation 7 from Work Aid 4. The total bed height is the summation of the saturation zone, mass transfer zone, and guard bed zone heights. Approximately six feet free space above and below the bed is needed.
Saudi Aramco DeskTop Standards
18
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
MOLE SIEVE CAPACITY CORRECTION FOR UNSATURATED INLET GAS
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book. FIGURE 10
Saudi Aramco DeskTop Standards
19
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
MOLE SIEVE CAPACITY CORRECTION FOR TEMPERATURE
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book. FIGURE 11 Regeneration Calculations Regeneration calculations for a molecular sieve dehydrator are discussed below. The first step in the regeneration calculations is to calculate the total heat required to desorb the water and heat the sieve and vessel. A 10% heat loss is assumed. For the entire regeneration cycle, only about 1/2 of the heat put into the regeneration gas is utilized. This is because by the end of the cycle the gas is leaving the bed at about the same temperature at which it enters. The heating time is usually 1/2 to 5/8 of the total regeneration time, which must include a cooling period. For 8 hour adsorption cycles, the regeneration normally consists of 4-1/2 hours of heating, 3 hours of cooling and 1/2 hour for standby and switching. For longer cycles, the heating time can be lengthened as long as a minimum pressure drop of 0.1 psi/ft is maintained. Figure 12 can be used to estimate the required minimum velocity to meet 0.10 psi /ft. To use this plot, locate the operating pressure, read up to the type sieve, then read across to find the minimum superficial velocity. The regeneration cycle frequently includes depressuring/repressuring to match the regeneration gas pressure and/or to maximize the regeneration gas volume to meet the velocity criterion. Some applications, termed pressure swing adsorption, regenerate the bed only with depressurization and sweeping the bed with gas just above atmospheric pressure.
Saudi Aramco DeskTop Standards
20
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
MINIMUM REGENERATION VELOCITY FOR MOLE SIEVE DEHYDRATOR
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book. FIGURE 12 Dehydrating Liquids Typical liquid dehydration systems have filtering equipment ahead of the dehydration unit to remove particulates that may clog the solid desiccant. There are usually two beds containing the desiccant, although it is sometimes necessary to have additional beds to handle very large streams. The desiccant beds are regenerated on an alternating basis. Downstream from the desiccant beds, another filter must be used to capture small particles of desiccant which may be attrited during the drying step. The diameter of desiccant beds is usually determined by the volumetric flow rate. For activated aluminas, a maximum of 30 gallons per minute per square foot of cross-sectional area is recommended. Bed depth determines the time required for the drying step. In liquid dehydration, drying times are typically 12 to 120 hours before regeneration. Liquids that cannot be dried with activated alumina are those exhibiting complete miscibility with water, and highly acidic and caustic compounds. The latter types can cause disintegration of the activated alumina in addition to being corrosive to equipment.
Saudi Aramco DeskTop Standards
21
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Desiccant regeneration is performed in one of two basic ways. At times the only available medium for heated regeneration is a slipstream of the process stream which must be vaporized. An alternative scheme uses vapor or gas other than the vaporized process stream to regenerate the desiccant. The heat requirement for regeneration include the sensible heats for the desiccant and equipment plus the heat required to desorb the water and hydrocarbons. The latter consists of the sensible heats, the latent heats of vaporization, and the heats of wetting. These values may be calculated individually and added together or may be estimated at 1,600 Btu/lb water and 200 - 800 Btu/lb hydrocarbons. The amount of water for this calculation is assumed to be the water content of the inlet stream over the drying period. The hydrocarbon liquids present are estimated to be the amount contained in the total pore volume of 0.048 gallons for hydrocarbon liquid per pound of adsorbent. The temperature necessary for regeneration is that needed to assure the desired effluent water content in the next drying step. For activated aluminas, temperatures of approximately 400°F at the exit end of the bed during regeneration result in effluent water levels down to about 10 ppmw. Increasing the temperatures to about 550°F results in effluent water contents of 1 ppmw or below. The extent of removal of hydrocarbons depends primarily on the boiling points of the hydrocarbons. The regeneration temperature should be that which gives as complete removal of the hydrocarbons as possible within the economics of the system. Drying of hydrocarbon liquids (NGL, liquid propane and butane) is done at Saudi Aramco's Shedgum, Uthmaniyah, Ju'aymah, and Yanbu facilities (see Appendix A). Process Variables Several process variables can have a major effect on solid desiccant bed sizing and operating efficiency. Quality of Inlet Gas The most important variable in sizing a desiccant bed is the relative saturation of the inlet gas. This variable is the driving force that affects the transfer of water to the adsorbent. The performance of a desiccant bed designed to remove water is adversely affected by gas containing quantities of carbon dioxide, heavy hydrocarbons (even in vapor phase), and sulfur-bearing compounds. The greater the molecular weight of a compound, the greater its adsorption potential.
Saudi Aramco DeskTop Standards
22
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Temperature The following are important temperature-related factors: •
Molecular sieves and most other adsorbents have significantly higher adsorptive capacity at low temperatures.
•
Temperature of the regeneration gas that mixes with the incoming wet gas ahead of the dehydrators is also important. If the temperatures of these two gas streams differ more than 15 to 20°F, liquid water and hydrocarbons may condense as the hotter gas stream cools. Condensed liquids that strike the bed can shorten the desiccant's life.
•
The temperature of the hot gas entering and leaving a desiccant tower during the heating cycle can significantly affect plant efficiency and the desiccant life. To assure good desorption of the water and contaminants, a high regeneration gas temperature is needed. The maximum hot gas temperature needed depends on the type of contaminants and the "holding power" or affinity of the desiccant for the contaminants.
•
The desiccant bed temperature reached during the cooling cycle is important. If wet gas is used to cool the desiccant, the cooling cycle should be terminated when the desiccant bed reaches a temperature of about 125°F. Additional cooling may cause water to be adsorbed from the wet gas stream and presaturate or preload the desiccant bed, before the next adsorption cycle begins. If dry gas is used for cooling, the desiccant should be cooled within 10 to 20°F of the incoming gas temperature during the adsorption cycle. This will keep the desiccant's adsorption efficiency high.
•
The temperature of the regeneration gas going through the regeneration gas scrubber or water knockout should be held low enough to condense and remove as much of the water and hydrocarbons from the gas as possible.
Saudi Aramco DeskTop Standards
23
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Pressure Generally, the adsorption capacity of a desiccant bed decreases as the pressure is lowered. At constant temperature, the water content of the inlet gas increases as the separation pressure is lowered. If the dehydrators are operated well below design pressure, the desiccant will have to work harder to remove the additional water and to maintain the desired effluent dew point. With the same volume of incoming gas, the increased gas velocity occurring at the lower pressures could affect the effluent moisture content and damage the desiccant. At pressures above 1300 to 1400 psia, the co-adsorption effects of hydrocarbons are very significant. Cycle Time Most adsorbers operate on a fixed drying cycle time, and that time is frequently set for the worst conditions. However, the adsorbent capacity is not a fixed value; it declines with usage. For the first few months of operation, a new desiccant normally has a high capacity for water removal. If a moisture analyzer is used on the effluent gas, a much longer drying cycle can typically be achieved. As the desiccant ages, the cycle time can be shortened to save regeneration fuel costs and improve the desiccant life. Gas Velocity As the gas velocity during the drying cycle decreases, both lower effluent moisture contents and longer drying cycle times to the breakthrough point are usually obtained. However, low linear velocities require towers with large cross-sectional areas to handle a given gas flow. In selecting the linear flow rate, therefore, a compromise must be made between the tower diameter and the maximum utilization of the desiccant. At lower velocities, the gas may channel through the desiccant bed without being properly dehydrated. A lower adsorption efficiency and desiccant damage may occur at higher velocities. The regeneration gas velocity is important, especially when effluent moisture contents below 0.1 ppm are needed. A minimum heating gas velocity of 10 ft/min may be required to achieve this superdehydration. At lower velocities, the hot gas may channel through the desiccant bed, tending to leave excess water in the bed after regeneration and resulting in poor dehydration. Sources of Regeneration Gas The source of gas for heating and cooling desiccant beds depends on plant requirements and, possibly, on the availability of a suitable gas stream. When low effluent moisture contents (in the range of 0.1 ppm) are required, the regeneration stream should be dry. Plant tail gas can normally be used for this purpose. If only moderate drying is required, a portion of the wet feed gas can be used.
Saudi Aramco DeskTop Standards
24
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Direction of Gas Flow The flow direction influences effluent purity, regeneration gas requirements, and desiccant life. The direction of flow during the drying cycle is normally downward. This direction permits higher velocities without lifting or fluidizing the desiccant bed. Fluidization can severely damage the desiccant. The direction of heating gas flow is generally countercurrent (opposite) to the direction of the adsorption flow. This flow permits better reactivation of the lower portion of the desiccant bed, which must perform the superdehydration during the drying cycle, especially in cryogenic plants. Bed support and screens on top of the bed consist of three to five layers in graduated sizes. Since the flow is both directions through the bed, both ends must be protected. Desiccant Selection No desiccant is suitable for all applications. In some cases, the choice is determined primarily by economics. Sometimes process conditions control the choice. Many times desiccants are interchangeable, and the equipment designed for one desiccant can often operate effectively with another. The desirable characteristics of a solid desiccant are listed below: •
High adsorptive capacity (lb/lb), which reduces contactor size.
•
Easy regeneration, for simplicity and economics of operation.
•
High rate of adsorption, which allows higher gas velocities and thereby reduces contactor size.
•
High adsorptive capacity retained after repeated regeneration, allowing smaller initial charge and longer service before replacement.
•
Low resistance to gas flow, to minimize gas pressure drop through the unit.
•
High mechanical strength, to resist crushing and dust formation.
•
Inert chemicals, to prevent chemical reactions during adsorption and regeneration.
•
Volume unchanged when product is wet, which would otherwise necessitate costly allowance for expansion.
•
Noncorrosive and nontoxic properties, avoiding the need for special alloys and costly measures to protect the operator's safety.
•
Low cost, to reduce initial and replacement costs.
Saudi Aramco DeskTop Standards
25
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Desiccant Selection (Cont'd) Certain physical characteristics of the more common solid desiccants are given in Work Aid 5 and Appendix C. Molecular sieves have the highest adsorptive capacity of all the desiccants, when the feed gas is at very high temperatures or at low relative saturation. If the feed gas entering the dehydrators is saturated with water vapor, however, silica gel or alumina may be a better selection. For water-saturated gases, these desiccants can adsorb twice as much water as molecule sieves and at lower first cost. Silica gel has another advantage in that it can be regenerated to a lower water content than molecular sieves and at much lower temperatures (400°F for silica gel versus 500 to 600°F for sieves). Effect of Regeneration Gas on Outlet Gas Quality Regeneration gas desorbs molecular sieve beds in the reverse order of the adsorption bond. For example, adsorbed methane and ethane would be desorbed first, then propanes and heavier hydrocarbons, then carbon dioxide, followed by any hydrogen sulfide that might have been in the inlet gas, and, last of all, the water. The effect of the concentration of these impurities in the regeneration gas stream may be significant when regeneration gas is 10 to 15% of the net inlet gas. Other Dehydration Processes Two other general categories of dehydrators are briefly discussed below. The first of these is the nonregenerable dehydrator, of which the calcium chloride brine unit will be described. The second is refrigeration, in which dehydration is not necessarily the prime purpose; but the availability of dehydration as a side benefit further justifies its installation. Nonregenerable Dehydrator Calcium chloride is used as a consumable desiccant. Solid calcium chloride combines with water vapor to form a brine solution. The drying tower must periodically be recharged with fresh calcium chloride and the brine frequently removed from the bottom of the tower (see Figure 13). A tray section in the tower is often used in conjunction with a dry bed of calcium chloride to take advantage of the brine's capability to combine with additional water vapor. Up to 3.5 lb H2O/lb CaCl2 can be absorbed with the addition of trays versus 1.1 lb H2O/lb CaCl2 for a dry bed type. This type of installation is efficient for remote, small-capacity gas wells without heat or fuel. It is reported that units of this type can lower the dew point to 7°F with a bed depth as low as 2 feet and a gas temperature of 127°F. Refrigeration Whether cooling is obtained by gas expansion or mechanical refrigeration, its net effect is the condensation of water from saturated inlet gas when the gas is cooled. The removal of water from the system achieves dehydration. Of course, the water condensation and water removal steps require an environment resistant to hydrate formation, either by use of inhibitors or by spot heating. Saudi Aramco DeskTop Standards
26
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
CALCIUM CHLORIDE (NONREGENERABLE) DEHYDRATOR
From non-proprietary information from Exxon Production Research Company, Production Operations Division, "Dehydration and Hydrate Inhibition," July, 1986, from Surface Facilities School, Vol. II. FIGURE 13 Saudi Aramco DeskTop Standards
27
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
OPTIMIZING AND TROUBLESHOOTING DEHYDRATOR Operations Glycol Maintenance Operating and corrosion problems usually occur when the circulating glycol gets dirty. Therefore, to achieve a long, trouble-free life from the glycol, it is necessary to recognize these problems and know how to prevent them. Some of the major areas are discussed below: Methanol Methanol in the feed gas to a glycol dehydrator will be absorbed by the glycol. This results in the following problems: •
Methanol will add additional heat duty on the reboiler and additional vapor load on the regenerator. High methanol injection rates and slug carryover can cause flooding.
•
Aqueous methanol causes rust in carbon steel, so corrosion can occur in the regenerator and reboiler vapor space.
Most of the methanol absorbed in the rich glycol solution can be removed by flashing in the regenerator. Activated carbon filters are used to adsorb methanol from the lean glycol solution to avoid these problems. Oxidation Oxygen enters the system with the incoming gas, through unblanketed storage tanks and sumps, or through the pump packing glands. Sometimes glycol will oxidize in the presence of oxygen and form corrosive acids. To prevent oxidation, bulk storage tanks should have a gas blanket to keep air out of the system. Oxidation inhibitors can also be used to minimize corrosion. Thermal Decomposition Excessive heat, a result of one of the following conditions, will decompose glycol and form corrosive products: •
High reboiler temperature above the glycol decomposition level.
•
High heat-flux.
•
Localized overheating, caused by deposits of salt or tarry products on the reboiler fired tubes or by poor flame direction on the fired tubes.
Saudi Aramco DeskTop Standards
28
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
pH Control New glycol has a neutral pH of approximately seven. As it is used, however, the pH always decreases and the glycol becomes acidic and corrosive, unless pH neutralizers or buffers are used. The equipment corrosion rate increases rapidly with a decrease in the glycol pH. Acids created by glycol oxidation, thermal decomposition products, or acid gases picked up from the gas stream are the most troublesome of corrosive contaminants. A low pH accelerates the decomposition of glycol. Ideally, the glycol pH should be held at a level of 7.0 to 7.5. A value above 8.0 to 8.5 tends to make glycol foam and emulsify. Borax, ethanolamines (usually triethanolamine), or other alkaline neutralizers can be used to control the pH. These neutralizers should be added with great care -- slowly and continuously -- for best results. An overdose of neutralizer will usually precipitate a suspension of black sludge in the glycol. The sludge could settle and restrict glycol circulation. Frequent filterelement changes should be made while pH neutralizers are added. Salt Contamination Salt deposits accelerate equipment corrosion, reduce heat transfer in the reboiler tubes, and alter specific gravity readings when a hydrometer is used to measure glycol-water concentrations. This troublesome contaminant cannot be removed with normal regeneration. Therefore, an efficient scrubber upstream of the glycol plant should be used to prevent salt carryover with the incoming gas. In areas where large quantities of brine are produced, some salt contamination will occur. The removal of salt from the glycol solution is then necessary. Salt contaminated glycol may be reclaimed by several methods. Scraped-surface heat exchangers in conjunction with centrifuges are used in cases of extreme contamination. Other reclamation methods are vacuum distillation or ion exchange. Hydrocarbons Liquid hydrocarbons, a result of carryover with the incoming gas or condensation in the absorber, increase glycol foaming, degradation, and losses. They must be removed with a glycol-gas separator, hydrocarbon liquid skimmer, or activated carbon beds. Sludge An accumulation of solid particles and tarry hydrocarbons very often forms in the glycol. This sludge is suspended in the circulating glycol; over a period of time, the accumulation becomes large enough to settle out. This action results in the formation of black, sticky, abrasive gum that can cause trouble in pumps, valves, and other equipment, usually when the glycol pH is low. The gummy substance becomes hard and brittle when deposited on the absorber trays, stripper packing, and other places in the circulating system. Good solution filtration prevents a buildup of sludge.
Saudi Aramco DeskTop Standards
29
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Foaming Foaming can increase glycol losses and reduce plant capacity. Entrained glycol will be carried over the top of the absorber with the sales gas when stable foam builds up on the trays. Foaming also causes poor contacting between the gas and glycol, decreasing the drying efficiency. Some foam promoters are: • • • •
Hydrocarbon liquids. Field corrosion inhibitors. Salt. Finely divided suspended solids.
Excessive turbulence and high liquid-to-vapor contacting velocities usually cause the glycol to foam. This condition can be caused by mechanical or chemical problems. The best way to prevent foaming is proper care of the glycol. This involves effective gas cleaning ahead of the glycol system and good filtration of the circulating solution. The use of defoamers does not solve the basic problem, and serves only as a temporary control until the conditions generating foam can be identified and removed. Analysis and Control of Glycol Analysis of glycol is essential to good plant operation. Meaningful analytical information helps pinpoint high glycol losses, foaming, corrosion, and other operating problems. Analyses enable the operator to evaluate plant performance and make operating changes to obtain maximum drying efficiency. A glycol sample should first be visually inspected to identify some of the contaminants: •
A finely divided black precipitate may indicate the presence of iron corrosion products.
•
A black, viscous solution may contain heavy hydrocarbons.
•
The characteristic odor of decomposed glycol (a sweet aromatic odor) usually indicates thermal degradation.
•
A two-phase liquid sample usually indicates the glycol is heavily contaminated with hydrocarbons.
Saudi Aramco DeskTop Standards
30
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Analysis and Control of Glycol (Cont'd) The visual inspections should next be supported by chemical analysis. Samples of the lean and rich glycol should be taken and routine tests performed: salt analysis, solids content, pH, iron content, foam test, and titration procedure (to determine the amount of neutralizer necessary to raise the pH to a safe level). These analyses usually provide sufficient information to determine the condition of the glycol. Glycol Loss Prevention Glycol losses can be defined as liquid carryover from the contactor (normally 0.10 gal/ MSCF with a standard mist eliminator) plus vaporization from the contactor and regenerator, and spillage. Glycol losses, exclusive of spillage, range from 0.05 gal/MSCF for high pressure, low temperature gases to as much as 0.30 gal/MSCF for low pressure, high temperature gases. There are several ways to reduce glycol losses. •
A certain amount of glycol always vaporizes in the sales gas stream. Adequate cooling of the lean glycol before it enters the absorber minimizes these losses.
•
Normally, most of the glycol entrainment is removed by a mist eliminator in the top of the absorber. Excessive gas velocities and glycol foaming in the absorber sharply increase the glycol carryover. A downstream gas scrubber can pay for itself quickly and save much money by trapping the carryover and recovering the excess glycol. This gas scrubber also helps prevent problems downstream of the glycol plant.
•
Vaporization losses in the stripper can be held to a minimum with good glycol condensation and control of the tower top temperature. Glycol entrainment, or mechanical carryover, can be reduced with proper maintenance of the stripper and reboiler.
•
Mechanical leaks can be reduced by keeping the pump, valves, and other fittings in good order. The glycol from these leaks should be collected and reprocessed.
•
Excessive entrainment losses may be the result of foaming in the absorber and/or regenerator. Defoamers are sometimes used.
Saudi Aramco DeskTop Standards
31
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Glycol Filtration Filters extend the life of the glycol pumps, and prevent an accumulation of solids in the absorber and regeneration equipment. Solids that settle on metal surfaces frequently set up cell corrosion. Filters also remove the solids that contribute to fouling, foaming, and plugging. Sock-type filters are preferred, although fine screens and cartridge filters are also suitable. The filters should be designed to remove all solid particles over 5 microns in size. They should be able to operate up to pressure drops of 20 to 25 psi. For best results, filters should be placed in the rich glycol line, but the lean glycol can also be filtered to help keep the glycol clean. Frequent filter changes may be needed during plant start-up, or when neutralizers are added to control the glycol pH. Activated carbon filters can eliminate most problems caused by hydrocarbons, well-treating chemicals, compressor oils, and other troublesome impurities in the glycol. Use of carbon filters increases glycol efficiency and life. The carbon filter should be placed on the lean stream since it has the lowest hydrocarbon load (a large portion of entrained hydrocarbons are flashed off in the regenerator) and it should be placed downstream of the solids filter. The carbon filter is generally sized for 1 to 5 gpm per ft2 of carbon bed cross-sectional area with a bed depth of 3 to 10 feet. A slipstream of lean glycol equal to 10% of the circulation rate is typically fed to the carbon filter. A full stream filter would require such a large vessel that it would not be economical or practical to use. Optimizing Adsorption-Type Dehydrators Desiccant Performance Operating data should be monitored to try to prevent permanent damage to the desiccant. Performance tests are frequently scheduled on a routine basis, ranging from monthly during early operations, to six months or longer. The size of the unit and the quantity of the desiccant also affect the frequency of performance tests. Desiccants decline in adsorptive capacity at different rates under varying operating conditions. Markedly different capacity-decline rates may be experienced for the same desiccant under similar conditions of gas flow, temperature, pressure, water removal requirements, cycle times, and regeneration temperatures. Desiccant aging is a function of many factors, including the number of cycles experienced and exposure to any harmful contaminants present in the inlet stream. Many of these contaminants are not completely removed during normal reactivation. Contaminants may be the cause of 90% of unsatisfactory solid desiccant operations. Therefore, the single most important variable affecting the decline rate of desiccant capacity is the chemical composition of the gas or liquid to be dried. Feed stream composition should always include the contaminants.
Saudi Aramco DeskTop Standards
32
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
The capacity of a new desiccant will decline slowly during the first few months in service because of cyclic heating, cooling, and wetting. Desiccant capacity usually stabilizes at about 55 to 70% of the initial capacity. To get maximum use out of the desiccant, a moisture analyzer can be used to optimize the drying cycle time. That time can be shortened as the desiccant ages. Both inlet and outlet moisture-analyzer probes should be used. Moisture analyzers for very low water contents require care to prevent damage to the probes. Sample probes and temperature probes must be installed to reach the center of the gas phase. Proper conditioning of the inlet gas is important. Compressor oils, corrosion inhibitors, glycols, amines, and other high-boiling contaminants present in the feed gas cause a further decline in desiccant capacity, because normal reactivation temperatures will not vaporize the heavy materials. The residual contaminants slowly build up on the desiccant's surface, reducing the area available for adsorption. Many corrosion inhibitors chemically attack certain desiccants, permanently destroying their usefulness. A layer of less expensive desiccant can be installed on the top of the bed to catch these contaminants. Although gases rich in heavier hydrocarbons may be dried satisfactorily with molecular sieves, the use of this same rich gas in a 550 to 600°F regeneration service aggravates coking problems. Lean dry gas is always preferable for regeneration, if it is available. Methanol in the inlet gas is a major contributor to the coking of molecular sieves where regeneration is carried out at temperatures above 550°F. Polymerization of methanol during regeneration may produce dimethyl ether and other intermediates that will cause coking of the beds. Monitoring bed differential pressure is important. An increase in differential pressure can indicate desiccant problems such as excessive coking or the formation of fines. The differential pressure along with the bed run length should also be recorded when doing a performance test on a desiccant bed. The useful life of most desiccants ranges from one to four years in normal service. A longer life is possible if the feed gas is kept clean. The effectiveness of reactivation can also play a major role in slowing the decline of a desiccant's adsorptive capacity and in prolonging its useful life. Obviously, if all the water is not removed from the desiccant during each regeneration, its usefulness will sharply decrease.
Saudi Aramco DeskTop Standards
33
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Performance data are used for monitoring desiccant life and planning for desiccant change out. The steps involved are as follows: • • •
Plot plant capacity and cycle time versus number of cycles. Extrapolate to determine when shortest cycle possible with existing regeneration equipment will be reached. At that point, or during the nearest regular plant turnaround preceding that point, an adsorbent change must be planned.
Regeneration gas not only supplies heat but also acts as a carrier to remove water vapor from the desiccant bed. Insufficient reactivation can occur if the regeneration gas temperature or velocity is too low. The desiccant manufacturer will generally recommend the optimum regeneration temperature and velocity for the product. Velocity should be high enough to remove the water and other contaminants quickly. This measure will minimize the amount of residual water and protect the desiccant. To maximize desiccant capacity and to ensure the minimum effluent moisture content, a higher reactivation temperature or a drier reactivation gas, or both, may be needed. Higher reactivation temperatures may also be used to remove volatile contaminants before they can form coke on the desiccant. The final effluent hot gas temperature should be held one or two hours to achieve effective desiccant reactivation. Equipment Items In addition to the above process variables, engineers can optimize solid desiccant dehydration equipment by considering the following: •
An accurate estimation of bed sizes in order to realistically evaluate competitive bids from desiccant vendors.
•
Optimal design of adsorber internals (inlet gas distributor, internal insulation and bed supports), switching valves, and control systems.
•
Proper design of regeneration gas systems.
•
Since mole sieve can produce dust, filters are frequently installed downstream to protect subsequent equipment.
Saudi Aramco DeskTop Standards
34
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
WORK AID 1: SOLUBILITY OF WATER IN LIQUID HYDROCARBONS
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book.
Saudi Aramco DeskTop Standards
35
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
WORK AID 2A: WATER CONTENT OF HYDROCARBON GAS
Use Photostat
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book.
Saudi Aramco DeskTop Standards
36
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
WORK AID 2B: EFFECTIVE WATER CONTENT FOR CO2
Use Photostat
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book.
Saudi Aramco DeskTop Standards
37
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
WORK AID 2C: EFFECTIVE WATER CONTENT FOR H2S
Use Photostat
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book.
Saudi Aramco DeskTop Standards
38
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
WORK AID 3: PHYSICAL PROPERTIES OF CHEMICAL INHIBITORS
PROPERTY
METHANO L
MOLECULAR WEIGHT
ETHYLEN DIETHYLEN E GLYCOL E GLYCOL
TRIETHYLEN TETRAETHYLE E GLYCOL NE GLYCOL
32.04
62.10
106.10
150.20
194.23
148.10
387.10
427.60
532.90
597.2
94
0.12
< 0.01
< 0.01
< 0.01
0.7868
1.110
1.113
1.119
1.120
--
1.085
1.088
1.092
1.092
6.55
9.26
9.29
9.34
9.34
FREEZING POINT,°F
-144
8
17
19
22
POUR POINT, °F
---
< -75
-65
-73
-42
ABSOLUTE VISCOSITY IN CENTIPOISES AT 77°F
0.55
16.5
28.2
37.3
39.9
ABSOLUTE VISCOSITY IN CENTIPOISES AT 140°F
0.36
5.1
7.6
9.6
10.2
22
47
44
45
45
SPECIFIC HEAT AT
0.27
0.58
0.55
0.53
0.52
FLASH POINT, °F
---
240
280
320
365
FIRE POINT, °F
---
245
290
330
375
DECOMPOSITION TEMPERATURE, °F
---
329
328
404
460
HEAT OF VAPORIZATION AT 14.65 psi, Btu/lb
473
364
232
179
---
BOILING POINT AT 760 mm Hg, °F VAPOR PRESSURE AT SPECIFIC GRAVITY AT 77°F SPECIFIC GRAVITY AT 140°F POUNDS PER GALLON AT 77°F
SURFACE TENSION AT
From non-proprietary information from Exxon Production Research Company, Production Operations Division, "Dehydration and Hydrate Inhibition," July, 1986, from Surface Facilities School, Vol. II.
Saudi Aramco DeskTop Standards
39
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
WORK AID 4: USEFUL EQUATIONS FOR DEHYDRATION CALCULATIONS •
Water content of natural gas (with < 40% acid gas). W = yHC WHC + yCO2 WCO2 + yH2S WH2S where: y= WHC = WCO2 = WH2S = W=
•
Mole fraction. Water content for hydrocarbon components, lb/MSCF. Water content for CO2, lb/MSCF. Water content for H2S, lb/MSCF. Water content for natural gas stream, lb/MSCF.
Minimum inhibitor concentration in free water phase. d= where:
d I MWI KH
•
(Eqn. 1)
= = = =
KH I 100 MWI - MWI I
(Eqn. 2)
Depression of the gas hydrate freezing point, °F. Weight percent inhibitor in the liquid phase. Molecular weight of inhibitor (see Work Aid 3). 4000 for glycols; 2335 for methanol.
Water removal rate by dehydration process. Wr = where: Wr Wi Wo G
= = = =
Wi - Wo G 24
(Eqn. 3)
Water removed, lb/hr. Water at inlet, lb/MSCF. Water at outlet, lb/MSCF. Gas flow rate, MSCF/D.
Saudi Aramco DeskTop Standards
40
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
•
Molecular sieve pressure drop. ∆ P = BµV + Cρ V2 L where:
∆P = L= V= µ= ρ=
(Eqn. 4)
Pressure drop, psi. Length of packed bed, ft. Vapor velocity, ft/min. Vapor viscosity, cP. Vapor density, lb/ft3.
Constants: Desiccant Type
B
1/8 in. bead 1/8 in. extrudate 1/16 in. bead 1/16 in. extrudate •
0.0560 0.0722 0.152 0.238
C 0.0000889 0.000124 0.000136 0.000210
Molecular sieve requirement in adsorber saturation zone SS =
Wr 0.13 CSS CT
(Eqn. 5)
where: SS = Amount of molecular sieve required in saturation zone, lb. CSS = Saturation correction factor for sieve (see Figure 10). CT = Temperature correction factor (see Figure 11). •
Length of molecular sieve packed bed saturation zone. LS =
SS ρ bd 4 3.14 D2
(Eqn. 6)
where: ρbd = Bulk density (see Work Aid 5). LS = Length of packed bed saturation zone, ft. D = Bed diameter, ft.
Saudi Aramco DeskTop Standards
41
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
•
Length of molecular sieve packed bed mass transfer zone. LMTZ = (V/35)0.3 (Z)
(Eqn. 7)
where: LMTZ =Length of packed bed mass transfer zone, ft. V = Vapor velocity, ft/min. Z = 1.70 for 1/8 in. sieve. 0.85 for 1/16 in. sieve.
Saudi Aramco DeskTop Standards
42
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
WORK AID 5: TYPICAL DESICCANT PROPERTIES
DESICCANT
SHAPE
BULK DENSITY lb/ft3
PARTICLE SIZE
APPROXIMATE MINIMUM MOISTURE CONTENT OF EFFLUENT GAS (wppm)
Alumina Gel
Spherical
52
1/4 in.
5-10
Activated Alumina
Granular
52
1/4 in.-8 mesh
0.1
Activated Alumina
Spherical
47-48
1/4 in.-8 mesh
0.1
Silica Gel
Spherical
50
4-8 mesh
5-10
Silica Gel
Granular
45
3-8 mesh
5-10
Mole Sieve
Spherical
42-45
4-8 mesh or 8-12 mesh
0.1
Mole Sieve
Extruded Cylinder
40-44
1/8 in. or 1/16 in.
0.1
With permission from Gas Processors Suppliers Association. Source: GPSA Engineering Data Book.
BULK DENSITY (lb/ft3)
SPECIFIC HEAT (Btu/lb/°F)
NORMAL SIZES USED
DESIGN ADSORPTIVE CAPACITY (wt%)
Activated alumina
51
0.24
1/4 in.-8 mesh
7
Mobil SOR beads
48
0.25
4-8 mesh
7
Florite
50
0.24
4-8 mesh
4-5
Alumina gel (H-151)
52
0.24
1/8 in.-1/4 in.
7
Silica gel
45
0.22
4-8 mesh
7
Molecular sieves (4A)
45
0.25
1/8 in.
14
Saudi Aramco DeskTop Standards
43
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
Saudi Aramco DeskTop Standards
44
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
From non-proprietary information from Exxon Production Research Company, Production Operations Division, "Dehydration and Hydrate Inhibition," July, 1986, from Surface Facilities School, Vol. II.
Saudi Aramco DeskTop Standards
45
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
GLOSSARY absorption
The assimilation of one material into another. In natural gas dehydration, the use of an absorptive liquid to selectively remove water vapor from a gas stream.
adsorption
Adhesion of molecules of gases, liquids, or dissolved substances to a solid surface, resulting in relatively high concentration of the molecules at the place of contact.
alumina
A regenerable aluminum oxide base desiccant.
calcium chloride
A type of consumable desiccant.
DEG
Diethylene glycol.
dehydration
The act or process of removing equilibrium water from gases or liquids.
desiccant
A substance used in a dehydrator to remove water and moisture.
EG
Ethylene glycol.
hydrate
A solid material resulting from the combination of a hydrocarbon with water under pressure.
molecular sieves
Regenerable solid desiccants composed of crystalline metal aluminosilicates (zeolites).
MSCF
Million standard cubic feet.
MTZ
Mass transfer zone for an adsorption bed.
NGL
Natural gas liquids are those hydrocarbons liquefied at the surface in field facilities or in gas processing plants. Natural gas liquids include propane, butanes and natural gasoline.
regeneration
A process by which a catalyst or a chemical reagent is returned close to its original reactiveness.
Saudi Aramco DeskTop Standards
46
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
silica gel
A regenerable silicon oxide adsorbent.
stripping
Substantially complete removal of the more volatile components from a mixture. It is usually accomplished by passing the hot bottoms from a flash drum or tower through a stripping vessel through which steam or inert gas is passed, to sweep out the volatile components.
TEG
Triethylene glycol.
TREG
Tetraethylene glycol.
water dew point
The temperature at which water vapor starts to condense from a gas mixture.
Saudi Aramco DeskTop Standards
47
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
APPENDIX A - SAUDI ARAMCO SOLID DESICCANT DEHYDRATION UNITS
LOCATION/PLT. # BERRI
DEHYDRATION SERVICE
FLOW RATE MOLE/H R
INSTALLED
BED
TYPE
VENDOR
G.P. #470 SWEET NGL GAS
46279
MOLECULAR SIEVE,
SHEDGUM SWEET NGL GAS G.P. #R42
30400
MOLECULAR SIEVE,
SWEET NGL LIQUID
UTHMA. G.P. #R33 SWEET NGL GAS SWEET NGL LIQUID
JU'AYMAH LIQUID PROPANE G.P. #R84 LIQUID BUTANE
YANBU
G.P. #V84 LIQUID PROPANE LIQUID BUTANE
R.T. REF.
ABQ.
PLT #25 SOUR GAS
PLANT 462 SOUR GAS
Saudi Aramco DeskTop Standards
4A UNION CARBIDE OR GRACE
NO. OF BEDS 3+1 ON REGEN
4A UN.CARBID E OR GRACE ACTIVATED ALUMINA,A RHONEF-200 POULENC, ALCOA
2+1 ON REGEN
MOLECULAR SIEVE,
4A UN.CARBID E OR GRACE ACTIVATED ALUMINA, A RHONEF-200 POULENC, ALCOA
2+1 ON REGEN
14416
MOLECULAR SIEVE,
1+1 ON REGEN
5976
MOLECULAR SIEVE,
4A UN.CARBID E OR GRACE 4A UN.CARBID E OR GRACE
9676
MOLECULAR SIEVE,
1+1 ON REGEN
3964
MOLECULAR SIEVE,
4A UN.CARBID E OR GRACE 4A UN.CARBID E OR GRACE
2398
32908 2314
4840
MOLECULAR SIEVE, 564C GRACE
14293
MOLECULAR SIEVE, 50% 564C GRACE, AND 50% SF1087 UN.CARBID E
1+1 ON REGEN
1+1 ON REGEN
1+1 ON REGEN
1+1 ON REGEN
2+2 ON REGEN
2+1 ON REGEN
48
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
APPENDIX B - REPRESENTATIVE VENDORS OF SOLID DESICCANT EQUIPMENT Activated Alumina Alcoa-Aluminum Company of America Pittsburgh, Pennsylvania 15219 Telephone: (800) 533-4511 Rhone-Poulenc Chimie Minérale Fine 18 Avenue d'Alsace Cedex 29, 92097 Paris La Defense Courbevoie, France Telephone: (1) 47 68 1234 Telex: 610500F Molecular Sieve W. R. Grace & Co. Davison Chemical Division Dept. TR P.O. Box 2117 Baltimore, Maryland 21203 Telephone: (301) 659-9000 Telex: 87834 UOP (Union Carbide/Allied Signal) Molecular Sieve Adsorbents Dubai International Trade Centre, Floor 25 P.O. Box 9248 Dubai, United Arab Emirates Telephone: (971) 4-376846 Silica Gel W. R. Grace & Co. Davison Chemical Division Dept. TR P.O. Box 2117 Baltimore, Maryland 21203 Telephone: (301) 659-9000 Telex: 87834
Saudi Aramco DeskTop Standards
49
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
APPENDIX C - VENDOR INFORMATION TYPICAL PROPERTIES OF RHONE-POULENC ALUMINAS
Saudi Aramco DeskTop Standards
50
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
ALCOA F-200 ACTIVATED ALUMINA FOR ADSORPTION APPLICATIONS
Saudi Aramco DeskTop Standards
51
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
ALCOA F-200 ACTIVATED ALUMINA FOR ADSORPTION APPLICATIONS (CONT'D)
Saudi Aramco DeskTop Standards
52
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
DAVISON MOLECULAR SIEVES – ESTABLISHED APPLICATIONS
Saudi Aramco DeskTop Standards
53
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
DAVISON MOLECULAR SIEVES – ESTABLISHED APPLICATIONS (CONT'D)
Saudi Aramco DeskTop Standards
54
Engineering Encyclopedia
Process Dehydration and Hydrate Inhibition
BASIC TYPES OF UNION CARBIDE MOLECULAR SIEVES
Saudi Aramco DeskTop Standards
55
View more...
Comments
