REMOVAL OF ACIDIC GASES IN A GASIFICATION POWER SYSTEM WITH PRODUCTION OF HYDROGEN

Abstract: This invention is an integrated process which removes acidic gases such as hydrogen sulfide, carbonyl sulfide and carbon dioxide from raw synthesis gas (4). The hydrogen sulfide and carbonyl sulfide is concentrated and separately recovered. The separately recovered carbon dioxide (42) is used as a temperature moderator with the purified syngas (8) in a combustion turbine. The process comprises separating hydrogen sulfide and carbonyl sulfide from a raw synthesis gas (4) by absorption with a liquid solvent (37) in a hydrogen sulfide stripper (2), removing coabsorbed carbon dioxide from the solvent by stripping the solvent (12) with nitrogen (28) in a carbon dioxide stripper (20), separating hydrogen sulfide and carbonyl sulfide from the solvent (48) in a stripper (46) and recovering sulfur from the hydrogen sulfide and carbonyl sulfide. The energy value of the carbon dioxide and its value as a diluent in reducing nitrogen oxides is recovered by using carbon dioxide as a moderator during combustion of the purified syngas in a gas turbine.

Description: REMOVAL OF ACIDIC GASES IN A GASIFICATION POWER SYSTEM WITH PRODUCTION OF HYDROGEN BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to gasification power generating systems wherein a hydrocarbonaceous fuel is partially oxidized in a gasifier to produce a synthesis gas or syngas which can be used as a fuel in a combustion turbine to produce power, and more specifically to a process for the selective removal of H2S and COS from the syngas while retaining the values associated with CO2 and other gases for power production. It also relates to a process for removing CO2 from the hydrogen content of the syngas. 2. Description of the Prior Art H2S and COS are generally removed from the syngas produced by the partial oxidation of a hydrocarbonaceous fuel before the syngas is burned as a high pressure fuel gas in a combustion turbine to drive a generator and produce power. One technique utilizes physical or chemical absoφtion with a liquid solvent, as disclosed in U.S. Patent No. 5,345,756 to Jahnke et al. In the process of removing the H 2 S and COS from the syngas, other acid gases, such as CO2, are undesirably removed with the H2S and COS. The retention of CO2 is desirable because its presence in the high pressure fuel gas generates power when it is expanded in the gas turbine. Hydrogen is a component of the synthesis gas produced by the partial oxidation of a hydrocarbonaceous fuel. The synthesis gas is purified before use. See U.S. Patent No. 5,152,975 to Fong et al. Procedures for such purification would be more efficient if the CO2 present in the hydrogen stream could be removed from the hydrogen prior to purification. To achieve desired SO 2 emission limits, approximately 97% of H 2 S + COS must be removed and recovered, typically as elemental sulfur in a Claus plant. Early attempts to use N 2 to strip coabsorbed CO 2 from physical solvents were either at high pressure, at essentially the same pressure as the H 2 S absorption, or at essentially atmospheric pressure. Because the stripping gas requirement increases in direct proportion to pressure, compressing the increased quantity of N 2 to the higher pressure was considered to be practicable only in the case where NH 3 is being produced and the N 2 compression is already required. A N 2 stripping stage to enrich the Claus plant acid gas feed has been used for some time. In one process, it operates at essentially atmospheric pressure and includes a bottom CO 2 stripping section surmounted by an H 2 S reabsorber section. In operation it uses N 2 to strip some CO 2 from the solvent. H 2 S in the stripped CO 2 is reabsorbed with an H 2 S solvent to give an N 2 plus CO 2 vent stream containing an acceptable amount of about 10 ppm of H 2 S. While operation at low pressure minimizes the N 2 stripping gas requirement which is directly proportional to pressure, it increases the solvent flow required to reabsorb the H 2 S which is inversely proportional to operating pressure. Because the reabsorbtion solvent flow is generally added to the main absorber solvent for regeneration, it increases the regeneration steam requirement. As a result, N 2 stripping at low pressure to obtain a concentrated Claus H 2 S stream is unattractive because regeneration solvent flows are excessive and result in prohibitive solvent regeneration steam and refrigeration requirements. Also, the stripped carbon dioxide along with the nitrogen strip gas is vented to the atmosphere and does not contribute to power production in the combustion turbine. A selective acid gas removal process is required to absorb essentially all the H 2 S while coabsorbing a minimum amount of CO 2 . Minimum CO 2 removal is required to obtain a concentrated H 2 S Claus plant feed to minimize the capital and operating costs of the Claus plant. Co-absorption of CO 2 not only dilutes the Claus H 2 S feed, it also decreases the integrated gasification combined cycle (IGCC) power generation thermal efficiency. Since the CO 2 in the high pressure fuel gas generates power when it is expanded in the gas turbine, its removal with the H 2 S loses that power generation potential. The problem is that available acid gas removal processes are not sufficiently selective and co-absorb significant CO 2 . The most selective physical solvents, such as mixed dialkylethers of polyethylene glycol and N methyl pyrrolidone coabsorb over 15% of the CO 2 when solvent flow is set to remove essentially all of the H 2 S. This results in a very dilute acid gas which cannot be processed in a conventional Claus plant. In commercial practice an expensive H 2 S selective amine preconcentration is used to increase the Claus feed to 25% H 2 S. Even at this concentration the purification is very expensive. U.S. Patent No. 4,242,108 to Nicholas et al solves the problem of obtaining a concentrated Claus H 2 S feed gas by a process utilizing an H 2 S absorber, a CO 2 absorber, a H 2 S stripper and two CO 2 strippers. The process involves heating the H 2 S absorber bottoms solvent and feeding it to a high pressure CO 2 stripping column operating at essentially the same pressure as the H 2 S absorber and stripping the coabsorbed CO 2 with a high pressure CO 2 -free inert gas. Nicholas et al notes the possibility of using high pressure N 2 from an air separation unit, however, this disclosure of N 2 use appears limited to NH 3 applications where N 2 has to be compressed and added to the H 2 after acid gas removal to make NH 3 synthesis gas. This application merely routes a portion of the required N 2 through the stripper for beneficial effects and appears limited to situations where CO 2 is rejected from the product gas as in NH 3 synthesis. A major problem with this process is the loss of CO 2 which is vented after being flashed off and the loss of CO 2 and N 2 from the second CO 2 stripper. U.S. Patent No. 4,568,364 to Galstaun et al discloses the advantage of adding carbon dioxide to a fuel gas for a gas turbine to decrease excess air compression with resultant increase in turbine net power. Also discussed is the advantage in low sulfur coal gasification applications of using nitrogen to strip coabsorbed carbon dioxide from hydrogen sulfide loaded solvent to obtain, after final hydrogen sulfide stripping, an acceptably concentrated hydrogen sulfide Claus feed gas. Galstaun's process, however, depends on using the selective hydrogen sulfide/carbon dioxide physical solvent acid gas removal system of an adjacent operation producing hydrogen to get the combined advantages of carbon dioxide addition to the fuel gas and the use of nitrogen stripping to obtain a concentrated Claus hydrogen sulfide feed gas. Galstaun imports carbon dioxide into the fuel gas stream by using carbon dioxide loaded solvent from the adjacent hydrogen plant carbon dioxide removal step. Galstaun does not recover coabsorbed carbon dioxide flashed or stripped with nitrogen from the hydrogen sulfide loaded solvent into the fuel gas. Nor does Galstaun recover nitrogen used for stripping into the fuel gas to produce the same advantages in the turbine operation that the carbon dioxide does. Because Galstaun's nitrogen stripper effluent is inevitably contaminated with hydrogen sulfide, it cannot be vented to the atmosphere. Therefore, the gas is sent to the adjacent carbon dioxide stripper where the contained hydrogen sulfide is reabsorbed for recovery. U.S. Patent Nos. 4,957,515 and 5,240,476, both to Hegarty, offer a solution to the problem of obtaining a concentrated H 2 S feed to a Claus unit while retaining the CO 2 content of the syngas as feed to the gas turbine to maximize power recovery. Hegarty uses a small amount of N 2 under pressure to strip coabsorbed CO 2 from the rich physical solvent for recycle to the fuel gas, free of H?S. In both Hegarty patents, the H 2 S and CO 2 rich solvent from the H 2 S absorber bottoms, at about 500 psia, is used to drive a turbine to reduce the pressure to about 90 psia, after which the solvent is stripped of CO 2 using N 2 in a CO 2 stripper operated at 78 psia. The gases from the stripper are recycled while the H 2 S laden solvent is sent to an H 2 S stripper. In the '476 Hegarty patent the CO 2 rich gases are recompressed and sent directly to the single H 2 S absorber. In the '515 Hegarty patent the recompression step is replaced by reabsorption of H 2 S in a secondary H S absorber. The CO 2 values are absorbed in a solvent and the solvent recycled to the H 2 S absorber; the H 2 S contaminated nitrogen stripping gas is vented. Nitrogen used in the CO 2 stripper is also vented. These processes respectively suffer from the energy need to recompress the CO 2 rich recycle to the H 2 S absorber from 78 psia to 500 psia and from the venting of N 2 to the atmosphere. What is needed is a purification process that yields a concentrated H 2 S Claus feed, that retains the value of CO 2 , and that does not require excessive pressure changes, or process heating or refrigeration. SUMMARY OF THE INVENTION This invention is an integrated process which removes acidic gases such as H 2 S, COS and CO 2 from raw synthesis gas. The H 2 S and COS is concentrated and separately recovered. The separately recovered CO 2 is used as a moderator with the purified syngas in a combustion turbine. The process comprises separating H 2 S and COS from a raw synthesis gas by absorption with a liquid solvent, removing coabsorbed CO 2 by stripping the solvent with nitrogen, separating the H 2 S and COS from the solvent and recovering sulfur from the H 2 S and COS. The energy value of the CO 2 and its value as a diluent in reducing NO is recovered by using the CO as a moderator during combustion of the purified synthesis gas in a gas turbine. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram showing the removal of H2S from the synthesis gas product from a gasifier. Figure 2 is a schematic diagram showing improved H2S gas removal using a high pressure flash drum. Figure 3 is a schematic diagram showing the removal of H 2 S in an integrated acid gas removal unit. Corresponding reference numbers indicate corresponding parts in each of the figures. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention comprises a process for removing acid gases such as H2S and COS from synthesis gas before the synthesis gas is burned as a fuel in a combustion turbine to drive a generator and produce power. The H 2 S and COS are removed as undesirable contaminants from the synthesis gas while other acid gases, such as CO2 are retained in the fuel streams fed to the combustion turbine. In one embodiment, the CO2 loss is reduced to a very low level by the use of a solvent for acid gases to remove acid gases from the syngas followed by the use of N2 at an intermediate pressure of 300 psig, ± 100 psig, preferably 300 psig ± 50 psig. to strip the solvent of absorbed CO2. Clean solvent is regenerated by removal of H2S by steam reboiling and recycled. The nitrogen stream, containing CO2 and some H2S is washed with clean solvent. The H2S-free stream of nitrogen and CO2 is utilized in the combustion turbine. The H2S is sent to a Claus unit for further processing. This invention also comprises operative steps for removing CO2 from hydrogen produced by partial oxidation of a hydrocarbonaceous fuel and subsequent shift conversion to form shifted syngas. In a gasification power system wherein a hydrocarbonaceous fuel is partially oxidized to produce a synthesis gas or syngas from a gasification reactor or gasifier at a pressure of about 1000 psig ± 300 psig, preferably about 1000 psig = 150 psig, the raw synthesis gas exiting the gasifier primarily comprises H 2 , CO, CO 2 . H 2 O, and to a lesser extent N 2 , Ar, H 2 S, COS, CH 4 , NH 3 , HCN, HCOOH. Environmental concerns require the removal of H 2 S and COS from syngas that will be burned in a combustion turbine. In removing H 2 S and COS from syngas, it is desirable to minimize removal of other acidic gases, such as CO 2 to avoid dilution of the H 2 S stream sent to the Claus unit and to maximize the amount of CO 2 sent to the combustion turbine. Increasing the CO 2 to the combustion turbine increases the power produced as the CO 2 is expanded in the combustion turbine and at the same time, minimizes oxides of nitrogen (NO x ) formation by lowering the combustion flame temperature. This separation is accomplished by sending the syngas to an acid gas recovery unit where it is treated in a first H 2 S absorber which uses a liquid solvent for the removal of H 2 S. Significant amounts of CO? are also removed by the H 2 S solvent in the first H 2 S absorber, even though the high pressure in the absorber reduces solvent circulation. To recover the CO 2 absorbed in the acid gas solvent, also referred to as the "rich solvent," the rich solvent is heated and the pressure reduced to about 300 psig to desorb the CO 2 . The flashed solvent is sent to a N 2 stripper to remove additional CO 2 . At 300 psig, the desorbed CO and stripping nitrogen can be fed to the combustion turbine without further compression as a diluent to control NO N and to increase power output. Diluent N 2 is normally produced from an air separation unit for that purpose and is readily available for stripping at the required pressure. Because a small amount of H 2 S is stripped with the CO 2 in the N 2 stripper, the stripped H 2 S is reabsorbed from the N 2 in a secondary H 2 S absorber. It is to be understood throughout this disclosure that the removal of H 2 S also encompasses the removal of COS unless otherwise specified. In accordance with one embodiment of the present invention, CO 2 removal is reduced to a very low level by the use of N 2 at intermediate pressure to strip the acid gas solvent of absorbed CO 2 . With reference to Fig. 1, a sour unshifted synthesis gas or syngas 4 principally comprising H 2 , CO, CO 2 and H 2 S at a temperature of 200 °F ± 125 °F, typically 150 °F ± 50 °F, a pressure of 1500 psig ± 1000 psig, typically 1 100 psig ± 400 psig, enters heat exchanger 5 where it is cooled to a temperature of about 90°F ± 50 °F, typically 110 °F ± 10 °F, and exits as stream 6. The term "sour" refers to a synthesis gas containing sulfur, whereas the term "sweet" refers to a synthesis gas wherein the sulfur content has been removed. The cooled unshifted sour syngas stream 6 enters the first H 2 S absorber 2 at a pressure of about 1000 psig wherein it contacts a chemical or physical solvent for acid gases, preferably a physical solvent such as methanol and N- methyl pyrrolidone, and most preferably dimethyl ether of polyethylene glycol, available commercially as Selexol® (Union Carbide). The temperatures and pressures shown are based on Selexol ® and may vary significantly for other solvents. The sulfur-containing gases such as H2S and COS are removed. The cleaned, unshifted, sweet syngas 8, at a temperature of about 60 °F to about 130 °F exits H 2 S absorber 2 and enters heat exchanger 5 in indirect heat exchange with the sour unshifted syngas 4. The warmed clean unshifted syngas 10 comprising H 2 , CO and some CO 2 at a pressure of about 1000 psig exits heat exchanger 5 and is directed to the combustion turbine (not shown) after being saturated with water, heated and expanded to the proper conditions for combustion in the turbine. The liquid stream of H2S rich solvent, including some absorbed CO2 5 exits the first H2S absorber 2 via line 12 and can be optionally combined with recycle solvent 16 exiting the second H2S absorber 14. The combined solvent 17 is preheated in heat exchanger 18 before entering the CO2 stripper 20 through line 22. Solvent stream 16 exits the second H2S absorber 14. A portion or all of the solvent stream represented by line 24 can be separated and combined with the stream 22 of H2S rich solvent entering the CO 2 stripper 20. A pressure reduction device 26 reduces the pressure to about 300 psig, which are the pressure conditions at which the stripper 20 operates. CO2 removal is accomplished by nitrogen stripping. Nitrogen gas stream 28 at a pressure of about 300 psig enters the CO2 stripper 20 and strips or desorbs the CO2 and a small amount of H2S from the solvent before exiting the stripper 20 through line 30. Because a small amount of H2S is stripped with the CO2, the CO 2 and H2S containing nitrogen stream 30 from the CO2 stripper 20 is sent to the second H2S absorber 14 where the H2S content is reabsorbed in a solvent. The CO 2 and H 2 S containing N 2 stream 30 can be fed directly to the second H 2 S absorber 14, or a portion or all of the CO 2 and H 2 S containing N 2 stream represented by line 36 can be separated and cooled in heat exchanger 32 and combined with the uncooled portion 34 before entering the second H 2 S absorber 14. The second H2S absorber 14 and the CO2 stripper 20 operate at the same pressure and can be combined into one vessel. Solvent 38 enters the secondary H2S absorber 14 and removes the H2S from the entering CO 2 and H 2 S containing nitrogen stream. Nitrogen stream 40, removed of the H 2 S and containing CO2 exits via line 40, and is passed through heat exchanger 41. The CO2-rich nitrogen stream exits via line 42 at a pressure of about 300 psig ± 100 psig, preferably about 300 psig ± 50 psig, and can be fed to the combustion turbine without further compression as a diluent to control NO x and increase power output. Diluent N2 is normally produced from an air separation unit (not shown) for that purpose and is readily available for stripping at the required pressure. Semi-rich solvent 16 containing H2S, exits the second H2S absorber 14 and can be recycled totally or in part to either the first H2S absorber 2 via line 44, or combined with the solvent 12 exiting the first H 2 S absorber 2 to form stream 17, or separated via line 24 and combined with the preheated solvent 22 entering CO2 stripper 20. Pump 15 increases the pressure of the solvent stream 16 exiting H 2 S absorber 14 from about 300 psig to about 1000 psig. All or a portion of the solvent stream enters the first H 2 S absorber 2 via line 44 or it can be combined via line 19 with the solvent 12 exiting the first H 2 S absorber. The H2S-containing solvent 48 exiting the CO2 stripper 20 is then passed to the H2S stripper 46. The H 2 S-containing solvent 48 is heated in heat exchanger 50 and enters the H2S stripper 46 via line 52. Because N2 is only slightly absorbed in the solvent, the N2 content of the H 2 S-containing solvent 52 is minimal. Thus, a highly concentrated H2S product stream 64 for a Claus or other sulfur processing unit is produced. The solvent 58 stripped of H2S exits H2S stripper 46, and after passing through pump 54 and heat exchangers 18 and 60 can be recycled to the first H2S absorber via line 37 and to the second H2S absorber via line 38. The temperature within the CO2 stripper 20 may be controlled at its optimum level of about 150 °F to about 250 °F by recovering part of the heat from the solvent 58 exiting H2S stripper 46 and passing through heat exchanger 18. Another option is for all or a portion of solvent stream 58 to pass through heat exchanger 50 in counter-current exchange with the solvent 48 exiting the CO 2 stripper 20. before entering heat exchanger 18. In H?S stripper 46 the solvent is reboiled with steam in indirect heat exchanger 80 via line 78 to strip H 2 S. The H 2 S exits overhead via line 64 where it is cooled in heat exchanger 66 to condense water. The mixed liquid vapor stream enters separator 70 via line 68 where a portion of the liquid H 2 O leaves via line 72 and the H 2 S rich product leaves via line 74 to the Claus unit (not shown). A portion of the H 2 O is recycled via line 76 to maintain the desired H 2 O-solvent balance. Figure 2 shows an alternative configuration for improved gas removal using a high pressure flash drum operating at a pressure of 1000 psig ± 300 psig, preferably about 1000 psig ± 150 psig. The solvent is flashed at a temperature of about 150 °F to about 250 °F. This embodiment recovers more CO2 at higher pressure and will reduce the size of the secondary H2S absorber 14. In this embodiment solvent 12 with acidic gases exits the first H2S absorber 2 through the pump 11 and exits heat exchanger 18 as preheated solvent stream 22. Instead of flowing directly to the CO2 stripper, stream 22 is diverted to a flash drum 82 where about 5% to 25 % of the H2S and about 10% to 70% of the CO2 are flashed off. The acid gas depleted solvent flows to the CO2 stripper 20 through line 56. The flashed gases 85 are returned to the first H2S absorber 2 and combined with the sour unshifted gas 4. after being cooled in exchanger 84 and exiting via line 86. It is sometimes desired to produce large quantities of hydrogen along with power from a gasification unit. In such instances a portion of the syngas from the gasifier is shifted to hydrogen in a reactor according to the reaction CO + H2O - CO2 + H2- See for example U.S. Patent No. 5,152,975 to Fong et al., incoφorated herein by reference. The remainder of the syngas is cooled without shifting and. after further processing, sent to a combustion turbine. The shifted gas is purified by a number of conventional means. One of the most efficient techniques utilized to purify the shifted gas is by means of a pressure swing absoφtion (PSA) process which removes impurities by use of a pressure change on the adsorbent beds. The shifted gas unfortunately contains a large quantity of CO2. This is undesirable since the CO2 reduces the recovery of the hydrogen in the PSA. Furthermore, since CO2 has no heating value, its presence in the PSA tail gas diminishes the heating value of the tail gas. Other techniques of H2 purification, such as methanation. also operate more efficiently when there is full removal of CO2 from the shifted gas. For production of power, it is desirable to have CO2 in the syngas since it helps reduce NO x formation by lowering the combustion flame temperature and also provides power as it runs through the expander side of the combustion turbine. A novel and effective technique of accomplishing CO removal and concentration is to combine the use of a physical solvent or other suitable solvent for acid gases to remove the CO2 from the shifted gas to be fed to the PSA or other purification process while maximizing the content of CO2 in the syngas used to fuel the combustion turbine. This is accomplished by combining the above features in the processing steps shown in Figure 3. Referring to Fig. 3. H2S and CO2 depleted shifted gas. consisting primarily of hydrogen, is produced from sour shifted syngas which has been subjected to H2S removal in shifted gas H2S absorber 90 and CO2 removal in CO2 absorber 104. The sour shifted gas 109 from the gasifier (not shown) enters shifted gas H2S absorber 90 through heat exchanger 1 10 and line 112. The H2S depleted gas leaves the H2S absorber 90 via line 1 14. is combined with solvent stream 1 16 from pump 1 15 and the CO2 absorber 104. is cooled in heat exchanger 1 18 and enters CO2 absorber 104 via line 120. In CO2 absorber 104 the syngas 120 is contacted with clean solvent recycle from the sweet CO2 stripper 100 via pump 101 and line 92 and cooled clean solvent from the H2S stripper 46 via lines 58 and 121. The H2S and CO2 depleted product gas 11 1. containing mostly hydrogen, is sent to the PSA or other purification procedure via line 122 and heat exchanger 110. A CO2 rich nitrogen stream 108 suitable as a diluent feed to the combustion turbine (not shown) is produced from shifted syngas by removing H2S in a shift gas H2S absorber 90, followed by solvent absoφtion of CO2 in the H 2 S depleted syngas from exit stream 114 in the CO2 absorber 104 and nitrogen stripping of the CO2 rich solvent 106 in the sweet CO2 stripper 100. A portion 128 of the CO2 rich solvent 124 exiting the CO2 absorber 104 is recycled to the first H2S absorber 2 and a portion 130 is recycled to the shift gas H2S absorber 90 where it absorbs H2S. There are three distinctive features to the combined process. The first feature takes the Cθ2/Η2S-rich solvent from the bottom of the shifted gas H2S absorber 90 and introduces it via line 88 into the lower part of the first H2S absorber 2, which is also referred to as unshifted gas H2S absorber 2. Because the unshifted gas has a much lower CO2 content and partial pressure than the CO2 H2S-rich solvent, the unshifted gas H2S absorber 2 strips the CO2 from the CO2 H2S-rich solvent. The second feature heats the CO2 H2S-rich solvent 12 from the unshifted gas absorber 2 and strips the rich solvent 22 with a sufficient amount of nitrogen or other suitable stripping gas to desorb the CO2 from the CO2 stripper 20 operated at about 1000 ± 150 psig. The CO2 H2S-rich solvent leaves the first H2S absorber 2 via pump 1 1 and line 12, is preheated in heat exchanger 18 and enters the CO2 stripper 20 via line 22. Nitrogen at about 1000 psig enters line 28. The CO2 stripper 20 reduces the CO2 content in the CO2 H2S-rich solvent prior to sending the solvent 48 to the H2S stripper 46 through optional heat exchanger 50 and line 52. The CO2 containing nitrogen stream 94 is recycled to the first H2S absorber 2 through heat exchanger 96 and line 98 where it is combined with the raw, unshifted syngas feed 4 and is ultimately recovered as part of the syngas product 10 passing to the combustion turbine (not shown) wherein the CO2/N 2 gas mixture functions as a moderator. The third feature utilizes a CO2 absorber 104 on the sweet shifted gas 122 which eventually passes to the PSA (not shown) or other H2 purification process as stream 111. Normally such a CO 2 absorber would rely primarily on pressure differential to regenerate the solvent and vent the CO2 at atmospheric pressure. However, in this invention the CO 2 rich solvent 106 exiting the CO 2 absorber 104 is directed to the sweet CO2 stripper 100 where the CO2 is stripped from the CO2-rich solvent by N 2 stream 102. The sweet CO2 stripper pressure is about 300 psig ±100 psig, preferably about 300 psig ± 50 psig. The CO2 and N2 product stream 108 exits from the sweet CO2 stripper 100 and is sent to the combustion turbine (not shown). Depending upon the quantity of nitrogen available and level of CO2 desired in the H2, it may be desirable to enhance the stripping of the CO2 from the solvent 106 by heating the solvent prior to entering the sweet CO2 stripper 100 and/or by flashing the stripped solvent 92 at atmospheric pressure after exiting the sweet CO2 stripper 100. However, it is preferred that these options not be used since heating requires additional equipment and cooling of the solvent. Flashing at atmospheric pressure vents the CO2 and makes it unavailable to generate power in the gas turbine. Nevertheless, depending upon economic optimizations, these options may be attractive for specific cases. This process also has the benefit of producing a concentrated stream 74 of H2S. The utilization of a CO2 stripper using nitrogen or some other non-soluble gas, such as H2 after preheating the rich solvent is extremely effective in minimizing the CO2 in the gas going to the Claus unit (not shown) to recover the sulfur values. An H2S purity of greater than 50% can be obtained using this process. This high concentration of H2S eliminates the need for special handling of the sour gas in the Claus unit and helps reduce the Claus unit size and cost. A flash drum (not shown) between the sour CO2 stripper 20 and the H2S stripper 46 can be used to eliminate additional CO2 and stripping gas. This can produce an H2S concentration greater than 95%. However, the relatively small amounts of CO2 remaining in the solvent after stripping provide a flash gas relatively rich in H2S and lean in CO2 and this flash gas requires a compressor to recover the sour gas flashed at the lower pressure along with the CO2. Alternatively, a lower pressure sour CO2 stripper can also be used in place of or in addition to the high pressure (1000 psig) sour CO2 stripper 20 described in Figure 3. The gas coming from the low pressure stripper is cleaned with a secondary absorber and then routed at about 300 psig ±100 psig, preferably about 300 psig ± 50 psig to the combustion turbine (not shown) as described in Figures 1 and 2. The advantage of having a second sour CO2 stripper operating at approximately 300 psig is the reduced gas flow which would need to be cleaned in the secondary absorber. In another embodiment, the high pressure sour CO2 stripper can be replaced with a flash drum at approximately 1000 ±300 psig, preferably about 1000 psig ±150 psig followed by an approximately 300 psig ±100 psig, preferably about 300 psig ±50 psig stripper as shown in Figure 2.

Claims: CLAIMS What is claimed is: 1. A process for the separation, recovery and utilization of acidic gases comprising H 2 S, COS and CO 2 contained in a raw synthesis gas produced from the partial oxidation of a hydrocarbonaceous reactant, comprising: (a) separating the acidic gases from the raw synthesis gas by contacting the synthesis gas with a liquid solvent to selectively absorb and remove at least a portion of the acidic gas from the synthesis gas and produce a purified synthesis gas; (b) selectively removing CO 2 from the acidic gases contained in the liquid solvent by stripping the liquid solvent with N 2 in a CO 2 stripper to selectively remove the CO 2 and form a gaseous mixture comprising N 2 and CO 2 , and a solvent residue containing H 2 S and COS; (c) purifying the solvent residue containing H 2 S and COS to recover the sulfur values. (d) contacting the gaseous mixture of N 2 and CO 2 with the purified synthesis gas to form a combined N 2 /CO 2 -purifιed synthesis gas mixture; and (e) combusting the N 2 /CO 2 -purified synthesis gas mixture in a combustion turbine wherein the N 2 /CO 2 component serves as a moderator during the combustion of said synthesis gas. 2. The process of claim 1, wherein the raw synthesis gas is selected from the group consisting of unshifted synthesis gas, shifted synthesis gas, and separate streams of unshifted synthesis gas and shifted synthesis gas. 3. The process of claim 2, wherein the raw synthesis gas comprises a separate stream of unshifted synthesis gas and a separate stream of shifted synthesis gas. 4. The process of claim 2. wherein the raw unshifted synthesis gas is contacted with a first liquid solvent in a first H 2 S absorber to produce a purified unshifted synthesis gas and a first acid-rich solvent containing the COS, H 2 S and CO 2 removed from the raw unshifted synthesis gas. 5. The process of claim 4. wherein the first acid-rich solvent is contacted with nitrogen gas in a first CO 2 stripper to form a first acid gas mixture comprising CO 2 , N 2 and a small amount of H 2 S and COS, and a residual second acid-rich solvent containing the major amount of H 2 S and COS present in the first acid-rich solvent. 6. The process of claim 5, wherein the first acid gas mixture is contacted with a second liquid solvent in a second H 2 S absorber to selectively remove the COS and H 2 S content of the first acid gas mixture and to produce a first gaseous mixture comprising N 2 and CO 2 , and a third acid-rich solvent containing the removed H 2 S and COS. 7. The process of claim 5, wherein at least a portion of the second acid rich solvent is purified to recover sulfur values. 8. The process of claim 5, wherein at least a portion of the second acid- rich solvent is recycled to the first H 2 S absorber. 9. The process of claim 6, wherein at least a portion of the third acid rich solvent is combined with the first acid rich solvent. 10. The process of claim 2. wherein the shifted synthesis gas is contacted with a fourth liquid solvent in a first unshifted gas absorber to produce a treated shifted synthesis gas containing most of the CO 2 and a fourth acid-rich liquid solvent containing H 2 S, COS and a minor amount of CO 2 . 11. The process of claim 9, wherein the treated shifted synthesis gas is contacted with a fifth liquid solvent in a first CO 2 absorber to remove most of the CO in a first CO 2 -rich liquid solvent and to produce a purified shifted synthesis gas. 12. The process of claim 11, wherein the purified shifted synthesis gas is passed to a pressure swing absoφtion unit. 13. The process of claim 10, where the fourth acid-rich liquid solvent is passed to the first H 2 S absorber. 14. The process of claim 1 1, wherein at least a portion of the first CO 2 - rich solvent is passed to a second CO 2 stripper and contacted with nitrogen gas to form a first CO 2 -stripped solvent and a second gaseous mixture comprising N 2 and CO 2 . 15. The process of claim 14, wherein the second gaseous mixture comprising CO 2 and N 2 is passed to a combustion turbine to serve as a moderator. 16. The process of claim 4. wherein the raw unshifted synthesis gas is processed to remove acid gases at the same time that a raw shifted synthesis gas is processed to remove acid gases.

Inventors: JAHNKE FREDERICK C, VOLK WILLIAM P, JAHNKE, FREDERICK, C., VOLK, WILLIAM, P.
Applicants: JAHNKE FREDERICK C, TEXACO DEVELOPMENT CORP, VOLK WILLIAM P, JAHNKE, FREDERICK, C., TEXACO DEVELOPMENT CORPORATION, VOLK, WILLIAM, P.
Application year: 1998
Main classification: C01B3
IPC codes: C01B3, C01B31, C10K1, C10L3