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Improved Benfield Process for Ammonia Plants by
S.K. Furukawa and R.K. Bartoo UOP Des Plaines, Illinois, U.S.A.
Abstract Several process improvements have recently been incorporated into this Hot Potassium Carbonate process for CO2 removal. Of interest to ammonia plant operators are an improved activator to replace DEA, new packing evaluations for high-efficiency packings, and changes in process flow sheets to reduce energy consumption. The result is a process design for CO2 removal that is competitive in today’s new plants in all aspects: capital costs, operating costs, and ease of operation. Comparative cost and energy values are included. Introduction The BenfieldSM process is well known around the world for CO2 removal. Operating Benfield units in India include some 25 units installed in ammonia plants plus 10 in other industries. Because the majority of the Indian units were designed prior to 1985, the casual observer may think that this process is fully mature and not capable of further improvements. As this paper shows, continuing improvements have been made in several areas for both proposed newly designed units and for revamping older units. These improvements have been fully proven in units operating elsewhere, such as China, Indonesia, Malaysia, and the United States. The specific Benfield improvements to be discussed that are of interest to the ammonia industry include the following. • • •
New highly effective ACT-1TM activator, which shows substantial benefits over DEA Newly evaluated high-efficiency random packings for process designs Proven energy integration in the Benfield Hybrid LoHeat design to further reduce regeneration steam compared with designs of 10 years ago
The New Activator The Benfield process uses a chemical absorbing solvent based on 30% potassium carbonate (K2CO3) in water plus an activator and corrosion inhibitor. An activator is a low-concentration additive put into the carbonate solution to improve the absorption rate of CO2. For many years, DEA (diethanolamine) has been the standard activator and is still used today in many operating plants.
However, like most other organic chemicals, DEA is subject to degradation. • DEA will break down from overheating (thermal degradation). • DEA reacts with oxygen from air contact or from over use of reoxidation agents such as potassium nitrite (KNO2), used to regenerate the corrosion inhibitor (vanadium). • By absorbing CO2, a secondary amine activator such as DEA forms a carbamate chemical that normally is easily regenerated. However, because further reactions can occur, some by-products are formed that are not regenerable, and thus a degradation compound is formed. Typically, these compounds are large-molecular-weight polymer-type chemicals. Evidence of the degradation of DEA can be seen in solutions from operating potassium carbonate plants. Visually, the solution samples appear black and opaque as liquid coal. Interference occurs with analytical procedures such as carbonate titrations and vanadium valence determinations. Foaming upsets are frequent and the constant addition of antifoam may be required. Often there is a rapid reduction of valence state of the corrosion inhibitor (vanadium) from the active V +5 to passive V+4 material. Potassium formate and a few other carboxylic acid salts are one result of the breakdown of the DEA molecule, and these can be analyzed for. These salts are benign at low concentrations. However, when they are found at concentrations of 5% or more, they interfere with operations by altering the physical properties of the carbonate solution. However, most of the other known and unknown DEA degradation compounds are notoriously difficult to analyze for because they are large polymer-type compounds that are still reactive and only a few have been identified chemically. Some amine degradation compounds are even considered to be corrosion accelerators in that they may solubilize iron, keeping it in solution and preventing it from reprecipitating as in formation of the passivation coating. UOP has found an alternative to DEA in its recently commercialized new activator called ACT-1 activator. This material, a proprietary chemical from UOP, is also an amine but with a more-stable molecule that is considerably more resistant to degradation. Side-byside accelerated laboratory degradation tests were performed to compare a carbonate solution with DEA and with ACT-1 activator. The first test was to heat samples of both solutions to 75o C (167o F) and expose them to oxygen by continuously injecting air. The DEA was 15% destroyed (degraded) within 45 days, but the ACT-1 activator was still 100% available. See Table 1. In another test, both solutions were heated to 121o to 132o C (250o to 270o F) and saturated with CO2 at autoclave pressures of 9 to 14 bar (135 to 200 psi). After 15 days, only 25% of the DEA remained; 100% of the ACT-1 activator remained and was reactive after 50 days. See Table 2. The ACT-1 activator is currently in use in at least 15 units worldwide, including several ammonia plants, and is being tested in a few units in India. It has been used in new units
where no DEA was present and in existing units that have used DEA for more than 20 years. Concentrations most effectively used in plant solutions are 0.3 to 1.0 wt.% ACT-1 Table 1 Effect of Oxygen on Benfield Activators % of Starting Material * Days of Test
0 100 100 3 100 8 100 10 97 14 100 18 93 21 100 37 87 43 100 46 86 67 100 ___________ * Lab test conditions: CO2 saturated, constant air injection, temperature at 75o C
Table 2 Effect of Temperature and CO2 on Benfield Activators % of Starting Material * Days of Test
0 100 100 100 3 100 75 70 8 100 50 50 10 100 42 46 15 100 26 40 18 100 33 20 100 30 30 100 50 100 __________ * Lab test conditions: 250 to 270o F and continuously exposed to 135 to 200 psi CO2 in vapor.
activator compared to about 3% for DEA. When an ammonia plant operator or process designer considers the quantities of replacement chemicals for losses of DEA by degradation plus the higher concentrations needed, the overall costs for using ACT-1 activator can be equal or lower than for DEA. When further cost savings are included for a considerably lower antifoam consumption, a typically much reduced consumption of reoxidizing agent, and the improved smoother process operation (less foaming upsets), ACT-1 activator appears attractive for operations of new and old Benfield units. Even more important, the CO2 absorption rate or activation with ACT-1 activator is far superior to that of DEA. In all comparisons, the ACT-1 activator in Benfield solutions substantially reduces the CO2 slippage to methanation typically to about 50% of the levels achieved by DEA activation. This improved absorption of CO2 is at no added costs for energy to regenerate the solvent and at no added pumping rates; in fact, plants frequently find slight reductions in regeneration heat or solution pumping are achievable compared with the requirements for the same units operating with DEA additive. Table 3 compares the features of a Benfield unit designed for activation with DEA or with ACT-1 activator. Table 3 also compares designs using high-efficiency packings, as discussed below. In summary, UOP believe the most important benefits of the ACT-1 activator are a reduction in the CO2 slip to about half of that achieved with DEA; plus either • Potential for capacity increases of up to 10% in the CO2 removal unit; or • Potential reduction in regeneration heat by up to 10%, or • Potential reduced solution pumping requirements by up to 10% Much improved operations with better solution condition is another important benefit. These benefits are fully achievable in new green-field units, and the same benefits can be achieved in units fully converted from DEA to ACT-1 activator. A UOP technical paper that covers additional information specific to the ACT-1 activator was presented at the AIChE annual ammonia symposium, September 1994, by T.M. Gemborys.
High-Efficiency Packings Designs for the Benfield process using random packings have always been based on experimental data generated in-house by Benfield and UOP. Over the years, this data has required extensive experimental work using the same carbonate-plus-activator solution composition as used in operating Benfield plants: 30% carbonate plus up to 3% activator. The original testing was with DEA; some testing has now been done with the ACT-1 activator. Packing efficiency is a measure of the degree of gas and liquid contact, and so any comparison of the hydraulic efficiency remains the same regardless of which activator is used.
Table 3 Process Design Comparisons Case 1 Packing Activator Major Equipment Sizes Absorber Tower Top Diameter Packed Height Packing Volume 3 ft Packing Type IMTP
Pall Ring DEA
8.75 ft 2 x 27 ft 3247 ft3
7.75 ft 3 x 22 ft 3113 ft3
Case 3 IMTP ACT-1
7.75 ft 2 x 24 ft 2264
2 in Metal Pall
#50 Metal IMTP
Absorber Tower Bottom Diameter Packed Height Packing Volume Packing Type IMTP
14.00 ft 2 x 21 ft 6465 ft3 2 in Metal Pall
12.50 ft 2 x 26 ft 6381 ft3 #50 Metal IMTP
12.50 ft 2 x 22 ft 5400 ft3 #50 Metal
Regenerator Tower Diameter Packed Height Packing Volume Packing Type IMTP
17.75 ft 3 x 27 ft 20,043 ft3 2 in Metal Pall
15.75 ft 4 x 25 ft 19,483 ft3 #50 Metal IMTP
15.50 ft 3 x 26 ft 14,718 ft3 #50 Metal
Utilities Net Heating Duty: MM Btu/hr Btu/lb mol CO2 Lean Pump, U.S. gpm Electricity, kWh Cooling Water, U.S. gpm Capital Investment (± 30%) Purchased Equip. Cost, U.S. $ MM Installed Equip. Cost, U.S. $ MM
130.3 33400 5935 1920 12180
130.6 33500 5935 1920 12180
123.6 31,700 5890 1830 11640
UOP’s standard for many years has been the steel Pall Ring or equivalent-type random packing. This type of packing was widely available from several manufacturers, the efficiency was fully adequate for industrial use, and it came in several sizes and materials. Typically, 1.5 inch, 2 inch, and 3.5 inch ring sizes in stainless and mild steels were used for Benfield ammonia plant designs. As a result of the latest evaluations, UOP has identified several newly commercialized packings available that are super-efficient for CO2 absorption in hot carbonate service. In particular, UOP now has a new standard for designs proposed for new and revamped units based on UOP laboratory data and confirmed in the first new units using these packings. Evaluation has shown that the following packings have higher efficiency than Pall Rings for Benfield service. • • • •
Norton Co. IMTP packings Glitsch Co. Mini-Ring packings Nutter Engineering Co. Nutter Ring packings Koch Engineering Co. Fleximax packing
UOP is also looking at designing with structured packings, and three Benfield units are already operating using a Sulzer Flexipac brand of structured packings. By combining the use of the new ACT-1 activator with the high-efficiency packings, UOP is able to design for reduced tower diameters for new units by 0.2 to 0.5 meters (1/2 to 1.0 ft) and reduce the packed heights by 3 to 4 meters per column for some designs. Table 3 compares a unit designed for Pall Ring packings or for IMTP Packing, with DEA or with ACT-1 activator. This comparison is for CO2 removal designs for a 1,500 MTD ammonia plant. Comparing Case 1, Pall Rings, vs. Case 2, IMTP, the absorber diameter is reduced with IMTP by 1.0 ft in the top section and by 1.5 ft in the bottom, and the regenerator tower is reduced by 2.0 ft with DEA activator. Even more reduction in diameter and packed height is found when both the new activator and high-efficiency packings are used. In comparing Case 1 with Case 3 (ACT-1 in combination with IMTP) in Table 3, the packed volume is reduced by 26.7% in the absorber and 36% in the regenerator tower. Conservatively, this result translates to at least a 10% reduction in capital cost for building a new unit. Reference lists of installed units using these new packings and for units using ACT-1 activator are available from UOP.
Benfield Hybrid LoHeat Process The UOP Benfield low-energy CO2 removal process integrates well into the newest designs for high-energy-efficient ammonia plants. When optimized, this newly commercialized process can achieve a net thermal energy consumption of only 650 kcal/Nm3 of CO2 removed or less. Four ammonia plants of 1,350 MTD capacity were constructed in China using this low-energy hybrid concept; three were designed by CF
Braun and one by Uhde. All are operating now for 1 to 3 years. Figure 1 shows the principal features of this process design. The Benfield Hybrid LoHeat process, as it is called, is similar in design to the standard 4-stage ejector, lean solution flash tank design of the Benfield LoHeat process, which is used in several Indian ammonia plants today. The conventional LoHeat process reduces energy consumption from about 1,200 kcal/Nm3 for Benfield units without LoHeat, down to about 800 kcal/Nm3 of CO2 removed for conventional Loheat units. A form of internal energy recovery and recycling, this energy recovery is accomplished by flashing the reboiled solution to generate steam, and then using steam ejectors to compress the flashed steam for injection back to the regenerator column. To make further improvements on the LoHeat process, UOP has added a fifth stage of solution flashing, with mechanical vapor recompression (MVR) to boost the fifth-stage flashed steam back to the pressure of the regenerator column. The combination of ejectors plus MVR for multistage heat recovery is referred to as the Benfield Hybrid LoHeat process. This improvement by itself allows energy consumption to be reduced to 650 kcal/Nm3, and can be combined with benefits of the ACT-1 activator to further reduce to as low as 600 kcal/Nm3 of CO2 removed. Figure 1 illustrates the overall Hybrid LoHeat process flow scheme with synthesis gas heat integration for the internal generation of motive steam and for heating boiler feed water. Data from a recent design of a 1,500 MTD ammonia plant show the following. • • • •
Purified gas would contain not more than 500 ppm CO2. The removed by-product of 1,650 kg/hr CO2 is fully recovered at a purity expected to be 99.3% CO2 (dry basis). Net thermal energy is calculated at 23.9 MM kcal/hr. Net electrical power for the Benfield Hybrid LoHeat unit is 1,826 kWh distributed as follows: - 1,635 kWh for process pumps - 730 kWh for the MVR compressor - 539 kWh reduction by incorporating hydraulic power recovery from the letdown of enriched solution.
Net thermal energy consumption is 647 kcal/Nm3 of CO2 removed, and net electrical power is 1,826 kWh. Converting electrical energy to thermal energy, this electrical consumption is equivalent to 6.23 MM kcal/hr so that the net total energy consumed is only 30.1 MM kcal/hr, or 815 kcal/Nm3 of CO2 removed. Table 4 shows UOP’s estimates of installed costs for this Benfield Hybrid LoHeat flow sheet.
Comparison of Energy Consumption Table 5 compares the energy consumption for the Benfield Hybrid LoHeat process with the conventional Benfield Loheat process and with older Benfield units not using the Loheat process. The Hybrid LoHeat process was designed to achieve a thermal regeneration energy consumption of less than 650 kcal/Nm3 of CO2 removed, compared with typically about 1,200 kcal/Nm3 for units without LoHeat and about 850 kcal/Nm3 for the conventional ejector LoHeat process.
Table 4 Installed Costs for Benfield Hybrid Process Equipment Cost, MM U.S. $ (*) Towers & Vessels 2.820 Tower Packings 0.343 Flashtank 0.455 Pumps 1.037 Compressor 0.618 Turbine 0.462 Exchangers 2.428 Total Installed Costs $ 8.163 ___________ * Cost based on 1,500 MTD ammonia plant
Table 5. Comparison of Energy Consumption
Energy Consumption __
________ Benfield Process *_________ Not LoHeat LoHeat Hybrid Loheat
Thermal Energy, MM kcal/hr 47.91 34.01 kcal / Nm3 of CO2 1297 920 Electrical Power, kWh 1826 1826 MM kcal/hr equivalent 6.23 6.23 Net Total Energy, MM kcal/hr 54.14 40.24 kcal/Nm3 of CO2 1465 1089 __________ * Energy consumption based on 1,500 MTD ammonia plant
23.90 647 2365 8.07 31.97 865
Adding a Mechanical Compressor In the past, several potential objections have been raised to the use of a mechanical compressor in the CO2 removal unit of a typical ammonia plant: • • •
What do you do when the compressor is down? Do you have to shut down the ammonia plant? How reliable are these compressors? How frequently are they out of service? How big are the motors, relative to other motors in the plant? Using electricity to operate a compressor increases the required generation of electricity by more than just the pure energy equivalent. For units that generate their own electric power, for example, what is the true energy consumption, including the inefficiencies of generating electricity?
Although these concerns are valid, actual experience gained over several years of operating time with MVRs in some 10 to 15 Benfield units show that the compressor size is really smaller than expected, the reliability is much better than expected (well over 98% on-stream time for most units), and for the few hours per year when the compressor is out of service for routine maintenance, the unit can run satisfactory with a temporarily higher energy consumption by using extra steam from increased reboiling or by using plant steam injected directly into the base of the Benfield regenerator. For the 1,500 MTD ammonia plant Benfield Hybrid LoHeat example discussed previously, the motor for the compressor requires only 730 kWh, less than half that of the lean Benfield solution pumps (which consume 1,630 kWh electricity). The compressor cost is estimated at U.S. $618,000 installed. The estimated cost for lean solution pumps is U.S. $975,000. Finally, as shown in Table 5, the electricity to operate the compressor adds only 1.84 MM kcal/hr to the net total equivalent energy consumption, and the Hybrid system saves 10.11 MM kcal thermal energy compared with the ejector LoHeat system. So the net savings is 8.27 MM kcal/hr over a conventional LoHeat process unit.
Hybrid LoHeat and MVR Experience The Arcadian ammonia plant in Memphis, Tennessee, was constructed more than 20 years ago with a one-stage lean solution flashtank using ejector Loheat in their Benfield CO2 removal unit. In about 1985, they added a stage 2 of lean solution flash using MVR and thus became a Hybrid LoHeat unit. Operating for 10 years now, the plant operators comment that the reliability of their compressor has been excellent: typically, they had only one or two shutdowns per year in the first few years, usually because their operators would accidentally surge the machine. On-stream time has been better than 98%, and during the few times that the MVR was out of operation, the ammonia plant still operated at full capacity but with added steam for regeneration. The steam-to-carbon ratio normally is 3.52, the CO2 slippage to methanation was 800 ppm (designed for 1,000), and capacities of 102 to 105% of design were achieved with the ammonia loop being the limitation to further capacity.
Similar comments have been received from the operators of Hybrid LoHeat units in China. One stated they have achieved an on-stream time of more than 8000 hours per annum. The four units there have run for a cumulative total of about 9 years. Some 12 to 15 other Benfield units use mechanical compression, either MVR alone or in combination with ejector LoHeat flash.
Summary These three basic improvements to the Benfield process have each been fully proven by several years of operating experience in ammonia plants and are now available for designing into newly proposed units. Further, UOP can retrofit most existing units for achieving many of the same features and benefits. The IFFCO plant at Phulphur, India, has just added a four-stage ejector LoHeat to improve their energy efficiency. The RCF Thal units have recently changed packings to gain the benefit of added capacity of the higher efficiency. At least three units in India have begun testing the ACT-1 activator. The Hybrid LoHeat system has been demonstrated in China, but not yet in India. The net energy consumption of the Hybrid LoHeat process is extremely competitive when compared with any other CO2 removal process. The excellent energy efficiency plus a lower solution circulation than used by many competing processes makes this process the choice for modern ammonia plant operators who want the lowest net operating costs for CO2 removal. Also, the UOP standard design uses two towers compared with three for some other processes, and the Benfield columns are smaller by design because they use high-efficiency commercial packings and the new ACT-1 activator. Thus, the Benfield process also has the lowest capital costs. The new, improved Benfield process is still the number-one choice worldwide for CO2 removal in new ammonia plants.
RKB, 1 / 97 UOP’s Benfield process and other gas processing technology is represented in India by Mr. Youg Ganju and Mr. J.P. Roy. They can be contacted at offices of UOP Asia Limited. UOP Asia Limited 3rd Floor, Vaitaklik Building, USO Road A-8 Qutab Institutional Area New Delhi 110 067 INDIA Telephone: 011-65-18919 Telefax: 011-65-17814 Telex: 031-65167 UOPA IN