Synthesis Gas Treating with Physical Solvent Process Using Selexol Process Technology

The Selexol process has proven to offer economic advantage, energy savings, and operating reliability for gas treating. The use of Selexol- II solvent lowers the ammonia production cost even further.

Vinod A. Shah Norton Company, Akron, OH 44309

T.L. Huurdeman DSM Fertilizers, 6160 McGleen, The Netherlands

The selection of a gas treating process SELEXOL SOLVENT PROCESS - This section today is more complex than in the past. describes the SELEXOL solvent, its The high cost of energy which has applications for synthesis gas treating prevailed since the 70's has brought many and an illustration of a plant and energy saving processes to the field of its performance and operating history; gas treating industry. Because ammonia and production is a high energy intensive process and because future energy cost NEW SOLVENT - This section presents a is uncertain and expected to be higher, new solvent which is specially formu- ammonia producers and process licensors lated for synthesis gas treatment. were forced to seek more energy- The solvent is named SELEXOL II efficient production methods. This solvent. The solvent and its economic brought many new synthesis gas treating benefits are discussed and compared with processes to the ammonia industry. the present SELEXOL solvent. Some of these processes may be more energy intensive, some have more SELEXOL SOLVENT PROCESS; maintenance, some have economics for removing carbon dioxide while some have General economics for removing hydrogen sulfide from coal and partial oxidation (POX) The solvent is a homologue of dimethyl gasified gas streams. ether of polyethylene glycol, and has been commercially proven in over 40 This paper describes the process that licensed facilities throughout the covers efficient treatment of all world. The process is owned and licensed varieties of synthesis gases. This by Norton Company Chemical Process process is a physical solvent process Products. The process and solvent called the SELEXOL(R) Solvent Process. characteristics of the solvent are The paper is presented into two sections : highlighted in Table 1.

216 TABLE 1 Table 2 summarizes the relative sol- ubilities of various gases in the SELEXOL solvent, process solvent. The actual solubility data is Characteristics proprietary, however the relative sol- ubilities can be compared for the * Non-toxic, non-polluting, non application (Basis: Ü2 = 1.0). reactive * Non-corrosive - biodegradable TABLE 2 * 151° C (304° F) flash point Relative Solubility of Gases in * 0.0007 mm Hg vapor pressure at 25° SELEXOL Solvent C (77° F) - low solvent losses * Selectively absorbs H2S, COS, RSH, Solubility CS2 in preference to C(>2 Component Ratio * Little or no heat required for solvent regeneration H2 (low solubility) 1.0 * Selectively removes heavy paraf- N2 1.5 fins, olefins, naphthenes and CO 2.2 aromatic byproducts of CH4 5 gasification C02 76 * no change of solvent is necessary COS 175 H2S 670 Its application and experience in the SO2 7000 synthesis gas treating area includes: 19,000 H20 55,000 C H 70,000 * Selective sulfur compound and car- 8 10 (high solubility) bon dioxide removal from gases originated from: As shown in Table 2, the H2S is much * natural gas more soluble than H2, N2, and CO2- If * partial oxidation H2S is present, the solubility differ- * coal gasification ences allows the selective removal of * coak gasification hydrogen sulfide from the gas stream * Removal of coal gas heavy hydro- leaving most of the carbon dioxide carbon byproduct unabsorbed. The SELEXOL Process was first In the case of carbon dioxide removal, applied to the treatment of ammonia because it is more soluble than hydrogen, synthesis gas in 1965. Since that time it allows selective removal of carbon the process has proven its applicability dioxide while leaving hydrogen and for synthesis gas treatment in 16 nitrogen essentially unabsorbed. installations, including a total of 10 ammonia plants. These facilities range In the case of physical solvents, in size from demonstration scale to the because the solubilities are enhanced at world-scale 1,360 t/d DSM ammonia higher pressures and lower tempera- facility. Several of these installa- tures, in order to keep the solvent tions are integral parts of "low energy" circulation rates low, the absorbers are ammonia plants. operated at feed pressures and cold -12 ° C (10°F) to ambient temperatures. Since Synthesis Gas Treating Operating the solubility decreases at lower Philosophy pressure, the desorption of the carbon dioxide from the solvent is achieved by SELEXOL Solvent is a physical solvent simply reducing the pressure of the which means that its capacity to absorb solvent. This characteristic eliminates gases is based on the physical solubi- the need of heat for solvent regenera- lities of gases in the solvent and not tion, and that is he primary reason why on any chemical reactions with the the process is an "Energy Saving solvent. Process".

217 Energy Saving Process - Highlights In the case of the SELEXOL Process, the temperature could be any temperature * Application: Steam reforming of in the range of refrigerated at -12" C natural gas or naphtha: (10° F) to water cooled at 49° C (120° F) . If the refrigeration route is - The heat requirement for solvent selected, the requirements are small and regeneration is essentially zero. usually supplied by the ammonia plant - Reformer steam-to-carbon ratio is refrigeration compression system. No independent of treating require- separate refrigeration unit is required. ment; allows minimum steam-to- carbon ratio to the primary Normally the height of the absorber reformer. is moderate in the range of +/- 36 m - Allows substantial heat recovery to 46 m (120 feet to 150 feet) . downstream of low temperature shift. Heat can be used for purposes Process other than solvent regeneration. H2S removal from POX and coal based - Product carbon dioxide is available synthesis gas. The process schematic at high pressures and at low water diagram is shown in Figure 1. content. - Moderate operating temperatures 10° F to ambient.

* Application: POX or coal based HjS REMOVAL synthesis gas: MODUCI H2S removal section ACID GAS - highly selective for H2S over carbon dioxide which results in sub- stantial capital and energy cost savings for sulfur production - lower heat of regeneration FEED - low solvent circulation HEAT - ambient temperatures operation - no solvent degradation CO2 removal section - The heat requirement for solvent Figure 1. regeneration is essentially zero. - Product carbon dioxide is available at higher pressures and at low water content. The process unit is a simple operating - Operating temperatures -12° C unit which consists of a conventional (10°F) to ambient. absorber, a recycle flash drum system High carbon dioxide product and a reboiled stripper column. recovery. - No solvent degradation. The higher solubility of hydrogen sulfide and other sulfur compounds in solvent allows selective removal of Process Variables these compounds from the synthesis gas stream. The resultant acid gas For process optimization, the main stream is sufficiently concentrated in process variable is solvent circulation. hydrogen sulfide for Claus processing. The solvent circulation, however is dependent upon CO2 Removal From Synthesis Gas. The process flow diagram and process - the temperature of the solvent, and description for CÛ2 removal unit is discussed in greater detail in the next - absorber height section.

218 Synthesis Sas Treating - A Case packed absorber column with the lean History solvent entering at the top of the absorber. The carbon dioxide is In order to best evaluate the process absorbed by the solvent and collected in application, reliability, and solvent the bottom. capabilities, it is best to evaluate a specific plant performance and its several years of operating history. Carbon dioxide-rich solvent from the The SELEXOL unit at DSM - Ammonia plant bottom of the absorber then flows (formerly UCAM plant) is selected for through a power recovery hydraulic this purpose. turbine, which converts the pressure energy of the solvent to mechanical The DSM Ammonia plant, a part of DSM energy through reduction of pressure. Fertilizers division, is a 1,360 tons This energy is utilized by the main per day ammonia plant located at Geleen, circulating pump and reduces its energy the Netherlands. As of July 1989, the requirement by approximately 50%. plant has completed five years of successful operation. Rich solvent from turbine flows into the recycle flash drum where The plant was designed and constructed by Kellogg Continental, b.v., based on essentially all of the co-absorbed M. W. Kellogg's reduced-energy ammonia hydrogen and nitrogen is flashed. technology for steam reforming of The flashed gas is separated, compressed natural gas. The plant has achieved an and recycled back to the absorber. actual energy consumption of 7.0 million kcal per metric ton of ammonia (25.2 The rich solvent from the recycle flash million Btu/s.ton) without purge gas drum V-l flows into the low pressure recovery. The plant was commissioned flash drum V-2 where more than 70% of in July 1984 and was producing ammonia the carbon dioxide is flashed and only 3 1/2 weeks after initial start-up. recovered. Process Description. The SELEXOL unit In order to achieve the additional schematic process flow diagram is shown carbon dioxide recovery required at DSM in Figure 2. Ammonia Plant (DSM), the solvent from the low pressure drum is further flashed HIGH CO2 RECOVERY in a vacuum flash drum V-3. Vacuum is FLOW DIAGRAM generated by the vacuum compressor which simultaneously compresses the vacuum PRODUCT VENT flash gases up to the desired carbon dioxide product pressure. Should higher carbon dioxied product recovery (up to 97%) be desired, the solvent could be flashed to lower FEED pressures. The actual operating pressure of the vacuum flash drum depends upon the desired carbon dioxide AIR recovery, i.e. the lower the flash drum pressure, the higher the carbon dioxide Figure 2. recovery. The solvent is very stable at a wide range of pressures, therefore, vacuum operation has no detrimental Raw synthesis gas from the low effect on the solvent or on the process. temperature shift effluent, following Furthermore, because the solvent vapor heat recovery and steam condensate pressure is extremely low (1. x 10~4 removal, enters the carbon dioxide kPa or 0.0007 mmHg at 25°C), the vapor absorber column at the bottom. The gas losses at vacuum pressure are minimal is counter-currently contacted in the and are not a concern.

219 It should be noted here that the Equipment. All of the equipment in SELEXOL solvent process is also capable the SELEXOL unit is conventional. No of essentially 100% carbon dioxide special designs or materials of con- recovery. A patented process scheme struction unique to SELEXOL service are which accomplishes this has been dis- required. Primarily all carbon steel cussed in references 1 and 3. construction is used in the plant, including absorber and stripper towers. Following the carbon dioxide removal These towers have Norton's high effi- ciency distributors and redistribu- and recovery, the solvent is regenerated tors. Use of stainless steel tower in an air stripping column. The air, packings were specified by DSM, however, supplied through an air blower, enters carbon steel packings have been used in the packed stripping column at the other SELEXOL units for similar services bottom. It is counter-currently with no reported corrosion problems. contacted with the solvent containing The pumps and vacuum compressors are a residual amount of carbon dioxide. spared, however, the recycle compressor The air with the stripped carbon dioxide is not spared. is vented to atmosphere. The regenerated solvent from the bottom of the stripper unit Startup. startup is conven- column is refrigerated and recycled back tional, very easy, and can generally to the top of the absorber. be accomplished in less than 30 hours. During start-up, because the refriger- To keep the unit in water balance, ation is not in service, the solvent a small amount of the solvent taken cooling can be achieved by a water cooled from the stripper bottom is continuously exchanger. At about 50% of the gas dehydrated. During winter, when the flow, the unit will produce the desired treated gas. Alternatively, the ammonia ambient stripping air is very dry, could be imported for initial startup dehydration is not required and the and refrigeration unit started prior dehydrator is taken off the service. to the plant start-up. DSM had selected this route. Since the SELEXOL solvent is regen- erated using air as a stripping medium, Operating Expérience. The SELEXOL the ikeat which is contained in the unit on-stream efficiency of the DSM process gas downstream of the low plant has been excellent - approaching temperature shift effluent is available 100%. The solvent losses have been for other in-plant uses. At DSM, more negligible - approximately 5,000 kg per than 90%^ of this heat energy is recovered year, which is only 0.01 kg per ton of and supplied to the following plant ammonia. services; The SELEXOL unit of the DSM ammonia plant is very easy to start and easy to High pressure boiler feedwater operate. There is no mixing of solvent preheat or composition control. Also there are no stringent process control require- - Low pressure steam generation ments, nor any need of additives such as corrosion inhibitors or promoters. As - Process condensate stripper reboiler a result, the unit does not require much of the operator's attention. The only - Low pressure boiler feedwater pre- solvent analysis required, which is heat. normally checked only once a week, is the water content of the solvent.

By sensible use of the energy, Operating History. During more than DSM/Kellogg has taken full advantage of five (5) years of operation, only three the energy saving feature of the SELEXOL SELEXOL unit plant shutdowns have process and has achieved high energy occurred. These shutdowns were caused by efficiency. This energy recovery is the mechanical problems not attributed substantial; it represents 0.862 million to the SELEXOL process itself. These kcal/ton of ammonia. mechanical problems were pump seal

220 failure, low oil pressure to the TABLE 4 hydraulic governor of the steam turbine driver, and a flow instrument malfunc- SELEXOL Unit Energy Requirement tion. Energy Consumption (for 1,442 t/d NH3) On rare occasions, the vacuum flash drum showed signs of froth/foaming CO2 removal unit*, GJ/hr resulting in an overflow of the solvent into the storage tank. The exact cause LTS effl. heat recovery, GJ/hr of the froth/foaming is not determined, however when it occurs, it is controlled Net energy surplus, GJ/hr by adding 0.5 liters of anti^foam agent (198.3) directly into the flash drum. An Energy surplus, GJ/t (3.3) installed slip-stream filter unit also helps suppress foaming by eliminating *Includes electric power, steam and accumulation of foreign materials. refrigeration • Plant Performance Data Table 3 summarizes DSM's SELEXOL unit design unit is established to be only 0.3 GJ/t. specifications and performance. If credit is taken for the upstream low temperature shift effluent heat recovery of 3.62 GJ/t, the net result is a surplus of 3.3 GJ/metric ton. TABLE 3 To summarize SELEXOL unit operation SELEXOL Unit Performance Data at DSM, it can be said that it has DSM Fertilizer Ammonia Plant performed to expectations. Besides being a low energy process, it also Spec. Operation proves to be a very reliable process. NH3 Capacity, t/d 1360 (106%) CO2 Removal Unit: NEW SOLVENT FOR SYNTHESIS GAS TREATING - Feed CO2, mol% 18.2 18.1 With the success of the SELEXOL process - Treated Gas CO2 1000 400 in the area of synthesis gas treating, Norton Company initiated a research - C02 Product: program to further enhance the SELEXOL - Recovery, % 81* 82* process economics. - Purity, mol% 99.0 99.4 For this solvent, our goals were: - Temperature, °C 17.0 6.3 - increase the carbon dioxide solu- max. bility - Pressure, kPa. 150 162 min. - reduce coabsorption of gases - This was the recovery desired. Much - decrease the solvent circulation higher recovery - to essentially 100% - requirement can be achieved with the SELEXOL process. - increase the mass transfer effi- ciency Energy Requirement - summary The result of our research is a solvent, named SELEXOL II solvent. Like Table 4 summarizes energy requirement SELEXOL, it is a glycol based solvent, for SELEXOL unit at DSM Fertilizer plant. but with much higher solubility for The energy efficiency at the DSM carbon dioxide. Table 5 summarizes and Fertilizer plant has been excellent. compares the property of the SELEXOL II As indicated earlier, the plant has solvent with present SELEXOL solvent. achieved an actual energy consumption The SELEXOL II solvent has been of 29.3 GJ per ton of ammonia (25.2 tested successfully in the pilot plant. million Btu per ton of ammonia) without The results are very encouraging. It purge gas recovery. In fact the portion proves to be a more energy efficient and attributed to the battery limit SELEXOL cost effective solvent.

221 TABLE 5 The use of SELEXOL II solvent will even surpass the economic advantage of Physical Properties SELEXOL and prove to be as reliable and flexible as the SELEXOL solvent process. SELEXOL SELEXOL II SOLVENT SOLVENT TABLE 6 Vapor Pressure @ 25"C (kPa) 9.3 x 10~5 1.2 x 10~3 Economic Comparison ; SEI SELEXOL II Density @ 25 °C, 1.05 0.88 SELEXOL SELEXOL II Vise. @ 25°C cp 5.9 1.5 Basis Boiling Pt. °c Treated Gas, CO2 500 500 215 209 ppm Freezing Pt. CO2 Recovery % 91 92 °C -22 -58 Odor mild mild Economics

The SELEXOL II solvent has the I i following applications for the synthesis gas treatment: Capital Cost 100 75 - Grass root ammonia plant Utilities 100 80 - SELEXOL unit expansion from present Absorber Height 100 45 SELEXOL solvent to new SELEXOL II solvent Solvent Circulation 100 93 - Revamp of amine units and hot carbonate unit for expansion and Recycle Gas energy savings. Volume 100 104 Process Description Solvent The process flow diagram and process Inventory 100 42 description for use of SELEXOL II solvent is the same as discussed pre- viously for (present) SELEXOL system. Economie Comparison References:

Table 6 summarizes and compares 1. V. A. Shah and J. McFarland, "Low economics of using SELEXOL II solvent Cost Ammonia and CO2 Recovery", Hydro- for the treatment of ammonia synthesis carbon Processing, March 1988 gas. CONCLUSION 2.R. J. Hernandez and T. L. Huurdeman, The physical solvent process has an "Solvent Unit Cleans Synthesis Gas", important place in the ammonia synthesis Chemical Engineering, P-154-156, Feb- gas treatment. The experience of the ruary 1989 SELEXOL process proves its economic advantage, energy savings, and oper- ating reliability. All this results in 3. Vinod A. Shah, "CO2 Removal from one thing - a lower ammonia production Ammonia Synthesis Gas", Energy Progress, cost. 8 (2), 67-70, (1988)

222 DISCUSSION MAX APPL, BASF: I understand that Selexol has quite approximately 17 to 18%, 18 times 150 will look very a high solubility for hydrocarbons too. How is the quality attractive. of COa in your process? Can you properly separate from CÛ2 such organic impurities as propane or butane to RAGHURAMAN: I assume that below that level it won't meet food-grade specification for liquid CÛ2? be very interesting. SHAH: Max, on the separation of carbon dioxide from SHAH: That's correct, since there will be a larger such absorbed hydrocarbons as propane and butanes, circulation. But remember the pumping requirements. I presented a paper at the Gas Processors Association Since it is not pumped all the way to 400-500 Ib (182- convention in March, 1989. This paper also provides 227 kg), more circulation can be allowed to pick up all economic comparison of this process with conventional the CO2. scheme. For the feed gas containing carbon dioxide and RAGHURAMAN: Thank you. What's the minimum heavy hydrocarbons such as benzene and toluene, and partial pressure needed for COz to be economical? requiring the recovery of high-purity carbon dioxide, we propose the installation of a Selexol prewash unit SHAH: We checked up to 150 Ib (68 kg), and it looks (upstream of CCfe removal unit) to remove these heavy like an attractive way of doing the job. hydrocarbons prior to carbon dioxide removal. Once RAGHURAMAN: Are you talking about the 18% CO2 these hydrocarbons are removed, CO2 can be extracted in the gas? in the COz removal unit. Max, did I answer your question? SHAH: Correct. APPL: Not exactly, but I understood your point. I was actually referring to traces of propane or butane and other RAGHURAMAN: You also mentioned that it could be hydrocarbons in the recovered CÛ2. Removing COz in a good alternative for revamping existing plants with other an ammonia plant from syngas hydrocarbon leaves some processes. Have you ever done that? Most of the unwieldy small traces of methane and higher hydrocarbons, mostly problems I see stem from taking the heat out and doing being formed in the high-temperature shift conversion. something else with the same plant. Have you worked These hydrocarbons will probably remain in the CÛ2; on any plant which was revamped with Selexol to properly however, to process the CÛ2 for liquid CÛ2 for food use available heat. grade, impurities like hydrocarbons should not be allowed. How can you remove them, or do you have problems SHAH: One plant in Sweden, designed and constructed with that? by Haldor Topsoe, had a lot of excess heat available since the conversion to the Selexol unit. The available SHAH: The recovered carbon dioxide from Selexol unit heat was converted to produce low-pressure steam, which has been liquified and used for food processing. If then was converted to high-pressure steam that was hydrocarbons are present in the CÛ2 stream, the best worth a lot of money. I believe this steam was compressed way is to inject oxygen over catalyst and oxidize these to approximately 150 Ib (68 kg). hydrocarbons to produce COs- l believe this is a very inexpensive way to handle the hydrocarbons. HARRY VAN PRAAG, CIL: You mentioned that for expansion, you could switch from existing Selexol and K.S. RAGHURAMAN, KTI: What's the minimum partial Selexol II. But could you gradually change over and install pressure of CÛ2 needed in the syngas of the feed gas Selexol II to improve performance? to the Selexol absorber for its economic operation, in light of the fact that it is operated with a pressure swing SHAH: The only new solvent we have used is Selexol and the partial pressure is being used as the motivating n solvent. To date we haven't operated a unit with a force? What's the minimum pressure for Selexol to be mixture of Selexol and Selexol II solvent. Until we perform economical? pilot testing, I would not know the answer to that question. SHAH: Normally, we like to have a minimum of 150- A. ILYAN, Asean Fertilizer, Indonesia: If the feed were psi (1,033-kPa) absorber pressure which for 18% CÜ2 natural gas containing COz and high sulfur (H2S) by in the synthesis gas feed represents a 27-psi (186-kPa) Selexol solvent, is it possible to produce pure COz gas? partial pressure. CÛ2 gas used for raw material of urea which contains maximum of l ppm H2S. RAGHURAMAN: Partial pressure? SHAH: In fact, we do this kind of application all the SHAH: No. As a feed pressure in ammonia will have time. In the EOR-type (enhanced oil recovery) application,

223 the CO2 content is always very, very high (95 - 97%) or they needed hydrogen purity with CÛ2 content around all the way to 99%. And there is always a trace amount 50-100 ppm. of sulfur (FbS) present. We circulate a small amount of Selexol solvent which will pick up I-feS out of the CÛ2- NIRULA: In hydrogen production, the reformer rich stream, and several Selexol plants do this. pressures are generally low, which means that the feed gas to the Selexol unit would be low. What effect does ILYAS: How do you separate CC»2 gas from PfeS gas, that have on the economics of the Selexol process, when both of the gases are diluted in the Selexol solvent? compared with chemical solvent processes? How many absorption towers and regeneration towers are needed to treat the gas? SHAH: It depends on the energy cost. There was a SHAH: The simplest towers you can buy. question raised earlier as to what partial pressure level Selexol unit would be justified. The hydrogen plant feed ILYAS: Are the two towers required? Is there one would usually be around 200 Ib (91 kg), and at this pressure absorber and one stripper? it proves to be economical. I'd like to indicate that we SHAH: One needs to have just H2S absorber and a frequently have a misconception about the physical stripper. solvent because we always think of a partial pressure. Yes, it's a factor. But, remember, pressure is a correction SATISH NIRULA, S.R.I. International: You illustrated to the circulation. The lower the feed pressure, the more the application of Selexol to ammonia production. Has circulation may be required. But the pressure differential it ever been applied to hydrogen production? of the pump is not that high for low feed pressure SHAH: Yes. We have a unit at Shell Carson Refinery operation. The 200 Ib (91 kg) operation may require twice in California for a hydrogen cleanup. Another hydrogen the circulation compared to the 400 Ib (182 kg) operation. cleanup unit was where Selexol was to remove and But, overall pump horsepower requirements are the recover a bulk amount of CÛ2 for CÛ2 production only — same. So even at low feed pressures, installation of Selexol the remaining COa was removed by the PSA unit, because unit may be justified.

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