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Improved Benfield Process for Ammonia Plants

by

S.K. Furukawa and R.K. Bartoo

UOPDes Plaines, Illinois, U.S.A.

AbstractSeveral process improvements have recently been incorporated into this HotPotassium Carbonate process for CO2 removal. Of interest to ammonia plantoperators are an improved activator to replace DEA, new packing evaluations forhigh-efficiency packings, and changes in process flow sheets to reduce energyconsumption. The result is a process design for CO2 removal that is competitivein today’s new plants in all aspects: capital costs, operating costs, and ease ofoperation. Comparative cost and energy values are included.

IntroductionThe BenfieldSM process is well known around the world for CO2 removal. OperatingBenfield units in India include some 25 units installed in ammonia plants plus 10 inother 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 furtherimprovements. As this paper shows, continuing improvements have been made in severalareas 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 bediscussed 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 ActivatorThe Benfield process uses a chemical absorbing solvent based on 30% potassiumcarbonate (K2CO3) in water plus an activator and corrosion inhibitor. An activator is alow-concentration additive put into the carbonate solution to improve the absorption rateof CO2. For many years, DEA (diethanolamine) has been the standard activator and isstill used today in many operating plants.

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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 canoccur, some by-products are formed that are not regenerable, and thus a degradationcompound is formed. Typically, these compounds are large-molecular-weightpolymer-type chemicals.

Evidence of the degradation of DEA can be seen in solutions from operating potassiumcarbonate plants. Visually, the solution samples appear black and opaque as liquid coal.Interference occurs with analytical procedures such as carbonate titrations and vanadiumvalence determinations. Foaming upsets are frequent and the constant addition ofantifoam may be required. Often there is a rapid reduction of valence state of thecorrosion 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 breakdownof the DEA molecule, and these can be analyzed for. These salts are benign at lowconcentrations. However, when they are found at concentrations of 5% or more, theyinterfere with operations by altering the physical properties of the carbonate solution.However, most of the other known and unknown DEA degradation compounds arenotoriously difficult to analyze for because they are large polymer-type compounds thatare still reactive and only a few have been identified chemically. Some aminedegradation compounds are even considered to be corrosion accelerators in that theymay solubilize iron, keeping it in solution and preventing it from reprecipitating as information of the passivation coating.

UOP has found an alternative to DEA in its recently commercialized new activator calledACT-1 activator. This material, a proprietary chemical from UOP, is also an amine butwith a more-stable molecule that is considerably more resistant to degradation. Side-by-side accelerated laboratory degradation tests were performed to compare a carbonatesolution with DEA and with ACT-1 activator. The first test was to heat samples of bothsolutions to 75o C (167o F) and expose them to oxygen by continuously injecting air. TheDEA was 15% destroyed (degraded) within 45 days, but the ACT-1 activator was still100% available. See Table 1.

In another test, both solutions were heated to 121o to 132o C (250o to 270o F) andsaturated with CO2 at autoclave pressures of 9 to 14 bar (135 to 200 psi). After 15days, only 25% of the DEA remained; 100% of the ACT-1 activator remained and wasreactive after 50 days. See Table 2.

The ACT-1 activator is currently in use in at least 15 units worldwide, including severalammonia plants, and is being tested in a few units in India. It has been used in new units

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where no DEA was present and in existing units that have used DEA for more than 20years. Concentrations most effectively used in plant solutions are 0.3 to 1.0 wt.% ACT-1

Table 1Effect of Oxygen on Benfield Activators

% of Starting Material *

Days of Test ACT-1 DEA

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 2Effect of Temperature and CO2 on Benfield Activators

% of Starting Material *

Days of Test ACT-1 DEA MMEA

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.

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activator compared to about 3% for DEA. When an ammonia plant operator or processdesigner considers the quantities of replacement chemicals for losses of DEA bydegradation plus the higher concentrations needed, the overall costs for using ACT-1activator can be equal or lower than for DEA.

When further cost savings are included for a considerably lower antifoam consumption, atypically much reduced consumption of reoxidizing agent, and the improved smootherprocess operation (less foaming upsets), ACT-1 activator appears attractive foroperations of new and old Benfield units.

Even more important, the CO2 absorption rate or activation with ACT-1 activator is farsuperior to that of DEA. In all comparisons, the ACT-1 activator in Benfield solutionssubstantially reduces the CO2 slippage to methanation typically to about 50% of thelevels achieved by DEA activation. This improved absorption of CO2 is at no added costsfor energy to regenerate the solvent and at no added pumping rates; in fact, plantsfrequently find slight reductions in regeneration heat or solution pumping are achievablecompared 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 orwith ACT-1 activator. Table 3 also compares designs using high-efficiency packings, asdiscussed below.

In summary, UOP believe the most important benefits of the ACT-1 activator are areduction 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 beachieved in units fully converted from DEA to ACT-1 activator. A UOP technical paperthat covers additional information specific to the ACT-1 activator was presented at theAIChE annual ammonia symposium, September 1994, by T.M. Gemborys.

High-Efficiency PackingsDesigns for the Benfield process using random packings have always been based onexperimental data generated in-house by Benfield and UOP. Over the years, this data hasrequired extensive experimental work using the same carbonate-plus-activator solutioncomposition 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 theACT-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 whichactivator is used.

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Table 3Process Design Comparisons

Case 1 Case 2 Case 3

Packing Pall Ring IMTP IMTPActivator DEA DEA ACT-1

Major Equipment SizesAbsorber Tower Top Diameter 8.75 ft 7.75 ft 7.75 ft Packed Height 2 x 27 ft 3 x 22 ft 2 x 24 ft Packing Volume 3247 ft3 3113 ft3 2264ft3

Packing Type 2 in Metal Pall #50 Metal IMTP #50 MetalIMTP

Absorber Tower Bottom Diameter 14.00 ft 12.50 ft 12.50 ft Packed Height 2 x 21 ft 2 x 26 ft 2 x 22 ft Packing Volume 6465 ft3 6381 ft3 5400 ft3

Packing Type 2 in Metal Pall #50 Metal IMTP #50 MetalIMTP

Regenerator Tower Diameter 17.75 ft 15.75 ft 15.50 ft Packed Height 3 x 27 ft 4 x 25 ft 3 x 26 ft Packing Volume 20,043 ft3 19,483 ft3 14,718 ft3

Packing Type 2 in Metal Pall #50 Metal IMTP #50 MetalIMTP

UtilitiesNet Heating Duty: MM Btu/hr 130.3 130.6 123.6 Btu/lb mol CO2 33400 33500 31,700Lean Pump, U.S. gpm 5935 5935 5890Electricity, kWh 1920 1920 1830Cooling Water, U.S. gpm 12180 12180 11640

Capital Investment (± 30%)Purchased Equip. Cost, U.S. $ 4.1 MM 4.0 MM 3.6MMInstalled Equip. Cost, U.S. $ 8.3 MM 8.2 MM 7.3MM

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UOP’s standard for many years has been the steel Pall Ring or equivalent-type randompacking. This type of packing was widely available from several manufacturers, theefficiency was fully adequate for industrial use, and it came in several sizes andmaterials. Typically, 1.5 inch, 2 inch, and 3.5 inch ring sizes in stainless and mild steelswere used for Benfield ammonia plant designs.

As a result of the latest evaluations, UOP has identified several newly commercializedpackings 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 revampedunits based on UOP laboratory data and confirmed in the first new units using thesepackings. Evaluation has shown that the following packings have higher efficiency thanPall 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 arealready 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 (1/2to 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 DEAor with ACT-1 activator. This comparison is for CO2 removal designs for a 1,500 MTDammonia plant. Comparing Case 1, Pall Rings, vs. Case 2, IMTP, the absorber diameteris reduced with IMTP by 1.0 ft in the top section and by 1.5 ft in the bottom, and theregenerator tower is reduced by 2.0 ft with DEA activator. Even more reduction indiameter and packed height is found when both the new activator and high-efficiencypackings 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 theregenerator tower. Conservatively, this result translates to at least a 10% reduction incapital cost for building a new unit.

Reference lists of installed units using these new packings and for units using ACT-1activator are available from UOP.

Benfield Hybrid LoHeat ProcessThe UOP Benfield low-energy CO2 removal process integrates well into the newestdesigns for high-energy-efficient ammonia plants. When optimized, this newlycommercialized process can achieve a net thermal energy consumption of only 650kcal/Nm3 of CO2 removed or less. Four ammonia plants of 1,350 MTD capacity wereconstructed in China using this low-energy hybrid concept; three were designed by CF

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Braun and one by Uhde. All are operating now for 1 to 3 years. Figure 1 shows theprincipal features of this process design.

The Benfield Hybrid LoHeat process, as it is called, is similar in design to the standard4-stage ejector, lean solution flash tank design of the Benfield LoHeat process, which isused in several Indian ammonia plants today. The conventional LoHeat process reducesenergy 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 ofinternal energy recovery and recycling, this energy recovery is accomplished by flashingthe reboiled solution to generate steam, and then using steam ejectors to compress theflashed steam for injection back to the regenerator column.

To make further improvements on the LoHeat process, UOP has added a fifth stage ofsolution flashing, with mechanical vapor recompression (MVR) to boost the fifth-stageflashed steam back to the pressure of the regenerator column. The combination ofejectors plus MVR for multistage heat recovery is referred to as the Benfield HybridLoHeat process. This improvement by itself allows energy consumption to be reduced to650 kcal/Nm3, and can be combined with benefits of the ACT-1 activator to furtherreduce to as low as 600 kcal/Nm3 of CO2 removed.

Figure 1 illustrates the overall Hybrid LoHeat process flow scheme with synthesis gasheat integration for the internal generation of motive steam and for heating boiler feedwater. 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 tobe 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 let- down of enriched solution. Net thermal energy consumption is 647 kcal/Nm3 of CO2 removed, and net electricalpower is 1,826 kWh. Converting electrical energy to thermal energy, this electricalconsumption is equivalent to 6.23 MM kcal/hr so that the net total energy consumed isonly 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 flowsheet.

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Comparison of Energy ConsumptionTable 5 compares the energy consumption for the Benfield Hybrid LoHeat process withthe conventional Benfield Loheat process and with older Benfield units not using theLoheat process. The Hybrid LoHeat process was designed to achieve a thermalregeneration energy consumption of less than 650 kcal/Nm3 of CO2 removed, comparedwith typically about 1,200 kcal/Nm3 for units without LoHeat and about 850 kcal/Nm3

for the conventional ejector LoHeat process.

Table 4Installed Costs for Benfield Hybrid Process

Equipment Cost, MM U.S. $ (*)Towers & Vessels 2.820Tower Packings 0.343Flashtank 0.455Pumps 1.037Compressor 0.618Turbine 0.462

Exchangers 2.428 Total Installed Costs $ 8.163

___________ * Cost based on 1,500 MTD ammonia plant

Table 5.Comparison of Energy Consumption

________ Benfield Process *_________Energy Consumption __ Not LoHeat LoHeat Hybrid Loheat

Thermal Energy, MM kcal/hr 47.91 34.01 23.90 kcal / Nm3 of CO2 1297 920 647Electrical Power, kWh 1826 1826 2365 MM kcal/hr equivalent 6.23 6.23 8.07Net Total Energy, MM kcal/hr 54.14 40.24 31.97 kcal/Nm3 of CO2 1465 1089 865__________ * Energy consumption based on 1,500 MTD ammonia plant

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Adding a Mechanical CompressorIn the past, several potential objections have been raised to the use of a mechanicalcompressor 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 theammonia plant?

• How reliable are these compressors? How frequently are they out of service? How bigare the motors, relative to other motors in the plant?

• Using electricity to operate a compressor increases the required generation ofelectricity by more than just the pure energy equivalent. For units that generate theirown electric power, for example, what is the true energy consumption, including theinefficiencies of generating electricity?

Although these concerns are valid, actual experience gained over several years ofoperating time with MVRs in some 10 to 15 Benfield units show that the compressorsize is really smaller than expected, the reliability is much better than expected (well over98% on-stream time for most units), and for the few hours per year when the compressoris out of service for routine maintenance, the unit can run satisfactory with a temporarilyhigher energy consumption by using extra steam from increased reboiling or by usingplant steam injected directly into the base of the Benfield regenerator.

For the 1,500 MTD ammonia plant Benfield Hybrid LoHeat example discussedpreviously, the motor for the compressor requires only 730 kWh, less than half that of thelean Benfield solution pumps (which consume 1,630 kWh electricity). The compressorcost is estimated at U.S. $618,000 installed. The estimated cost for lean solution pumpsis U.S. $975,000. Finally, as shown in Table 5, the electricity to operate the compressoradds only 1.84 MM kcal/hr to the net total equivalent energy consumption, and theHybrid system saves 10.11 MM kcal thermal energy compared with the ejector LoHeatsystem. So the net savings is 8.27 MM kcal/hr over a conventional LoHeat process unit.

Hybrid LoHeat and MVR ExperienceThe Arcadian ammonia plant in Memphis, Tennessee, was constructed more than 20years ago with a one-stage lean solution flashtank using ejector Loheat in their BenfieldCO2 removal unit. In about 1985, they added a stage 2 of lean solution flash using MVRand thus became a Hybrid LoHeat unit. Operating for 10 years now, the plant operatorscomment that the reliability of their compressor has been excellent: typically, they hadonly one or two shutdowns per year in the first few years, usually because their operatorswould accidentally surge the machine. On-stream time has been better than 98%, andduring the few times that the MVR was out of operation, the ammonia plant stilloperated at full capacity but with added steam for regeneration. The steam-to-carbonratio normally is 3.52, the CO2 slippage to methanation was 800 ppm (designed for1,000), and capacities of 102 to 105% of design were achieved with the ammonia loopbeing the limitation to further capacity.

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Similar comments have been received from the operators of Hybrid LoHeat units inChina. One stated they have achieved an on-stream time of more than 8000 hours perannum. The four units there have run for a cumulative total of about 9 years. Some 12 to15 other Benfield units use mechanical compression, either MVR alone or incombination with ejector LoHeat flash.

SummaryThese three basic improvements to the Benfield process have each been fully proven byseveral years of operating experience in ammonia plants and are now available fordesigning into newly proposed units. Further, UOP can retrofit most existing units forachieving 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 RCFThal units have recently changed packings to gain the benefit of added capacity of thehigher 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 competitivewhen compared with any other CO2 removal process. The excellent energy efficiencyplus a lower solution circulation than used by many competing processes makes thisprocess the choice for modern ammonia plant operators who want the lowest netoperating costs for CO2 removal. Also, the UOP standard design uses two towerscompared with three for some other processes, and the Benfield columns are smaller bydesign because they use high-efficiency commercial packings and the new ACT-1activator. 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 innew ammonia plants.

RKB, 1 / 97

UOP’s Benfield process and other gas processing technology is represented in India byMr. Youg Ganju and Mr. J.P. Roy. They can be contacted at offices of UOP Asia Limited.

UOP Asia Limited3rd Floor, Vaitaklik Building, USO RoadA-8 Qutab Institutional AreaNew Delhi 110 067 INDIA

Telephone: 011-65-18919Telefax: 011-65-17814Telex: 031-65167 UOPA IN