PROCESS FOR REMOVAL OF CARBON OXIDES FROM GAS MIXTURES

Abstract: A process for simultaneous separation of carbon mono­ xide and carbon dioxide from a mixed gas stream compris­ ing contacting the gas stream with a liquid nitrile absorbent having the general formula R-CN wherein R is selected from alkyl radicals having from one to five carbon atoms and alkoxyalkyl radicals having from two to ten carbon atoms, maintaining the temperature of the gas stream during separation at a temperature below the decomposition tem­ perature of the nitrile absorbent and removing the nitrile absorbent, carbon monoxide and carbon dioxide mixture from contact with the mixed gas stream.

Description: PROCESS FOR REMOVAL OF CARBON OXIDES FROM GAS MIXTURES BACKGROUND OF THE INVENTION This invention relates to a process for purification of mixed gas streams, and, in particular, relates to the simultaneous removal of carbon monoxide and carbon dioxide from mixed gas streams by absorption with an alkyinitrile or alkoxyalkylnitrile absorbent. Many important industrial processes require the removal of carbon monoxide and carbon dioxide from mixed gas streams in which they occur. For example, in coal gasification large quantities of carbon dioxide and hydrogen sulfide must be removed from the raw gas before it is suitable for use. Substantial quantities of carbon monoxide and carbon dioxide are also often present in synthesis gas. In the past, carbon dioxide has been removed, either alone or in combination with other impurities, from mixed gas streams by a variety of methods. One of the earliest known methods of carbon dioxide removal from mixed gas streams involves the use of alkanolamines such as monoethanolamine or diethanolamine as disclosed in U.S. 1,783,901 to Bottoms. Later variations in the use of such alkanolamines for carbon dioxide removal included diglycolamines as in U.S. 2,712,Y78 to Blohm et al and amines dissolved in glycol as taught in U.S. 2,940,652 to Bloch. These alkanolamines are effective for removal of carbon dioxide where the mixed gas stream contains only one or two additional impurities to be removed such as hydrogen sulfide. But where other impurities are present, such as carbonyl sulfide, carbon disulfide, hydrogen cyanide, organic acids and mercaptans, competing reactions and recovery problems exist which significantly reduce the effectiveness of the alkanolamines in removing carbon dioxide. Other amine absorbents for carbon dioxide removal have since been developed as typified by U.S. 4,100,257 to Linden et al. Aqueous ammonia solutions and liquid ammonia have been used to extract carbon dioxide from natural gas as in U.S. 3,524,72E to Every et al. Corrosion is a problem here and the solid ammonium carbonate requires further processing to be usable. Various alkaline solutions such as alkali metal salts and alkali or alkaline earth metal hydroxides have been found useful in carbon dioxide removal. The alkali metal salts as in U.S. 2,91k,3U1 to Schreiner et al, include, for example, sodium and potassium carbonates. The absorption rate is low due to the slow chemical reaction between dissolved molecular carbon dioxide and hydroxyl Sons. Catalytic conversion methods, while effective in neutralizing carbon monoxide and carbon dioxide, convert the carbon oxides to other components rather than physically removing them from the gas stream and thus do not result in recovery of usable carbon oxides. Catalyst systems have also been used to oxidize carbon monoxide to carbon dioxide as in U.S 4,292,288 to Gladrow. Carbon monoxide removal from mixed gas streams can be accomplished by absorption in water or in the alkaline solutions disclosed above. Such methods, however, still leave up to 4-5% carbon monoxide in the gas. Removal of most of the remaining small quantity of carton monoxide is by various salts such as a copper-ammonium salt of a weak acid, such as a formate, carbonate or acetate, with formation of a cuprous-ammonium-carbon monoxide complex. Here also tne carbon monoxide is not available for use since it is bound up in the complex which must be heated to free the carbon monoxide. Sice reactions further complicate the process, such as the auto-oxidaticn of cuprous to cupric ions. It is also known in the art that carbon monoxide can be recovered from mixed gas streams by means of a cuprous salt in propionitrile such as cuprous chloride. In U.S. 3,415,615, a highly saturated solution of a cuprous salt, preferably cuprous chloride, in propionitrile is used in the substantial absence of water to selectively form a salt complex with the carbon monoxide and with the unsaturated hydrocarbons, such as propylene. The complex must then be separated from the uncomplexed gases and the non-chemically complexed gases and then subjected to solvent extraction and further processing to recover the carbon monoxide. The patent specifically states that carbon dioxide is not complexed by this process. The process of this invention avoids the problems of the prior art by providing a method for simultaneous removal of carbon monoxide and caroon dioxide from feed gas streams containing carbon monoxide and carbon dioxide and at least one additional gaseous component without formation of chemical complexes or the requirement of multi-stage removal procedures. SUMMARY OF THE INVENTION According to the invention a process is provided for a simultaneous separation of carbon dioxide and carbon monoxide from feed gas streams containing carbon monoxide and carbon dioxide and at least one additional gaseous component comprising contacting the gas stream with a liquid nitrile absorbent having the general formula R-CN wherein R is selected from alkyl radicals having from one to five carbon atoms and alkoxyalkyl radicals having from two to ten carbon atoms and mixtures thereof, maintaining the temperature of the gas stream during separation at a temperature below the decomposition temperature of the nitrile absorbent and removing the nitrile absorbent containing dissolved carbon dioxide and carbon monoxide from contact with the mixed gas stream. The invention also involves preferential separation of carbon monoxide from the carbon dioxide after these two carbon oxides have been removed from the mixed gas stream using specific nitrile absorbents within the above general formula having enhanced affinity for carbon dioxide DESCRIPTION OF PREFERRED EMBOOIMENTS The liquid nitrile absorbents which are used to extract and separate carbon monoxide and carbon dioxide from mixed gas streams are those nitriles encompassed within the general formula R-CN wherein R is selected from alkyl radicals having from one to five carbon atoms and alkoxyalkyl radicals having from two to ten preferably two to six carbon atoms. Suitable nitriles where R is alkyl include alkylnitriles such as acetonitrile, propionitrile, n-butyronitrile, isobutyronitrile, n-valeronitrile, isovaleronitrile and capronitrile. Of this group propionitrile is preferred. Where R is an alkoxyalkyl radical the nitriles useful in the practice of the invention include methoxyacetonitrile, ethoxyacetonitrile, propoxyacetonitrile, butoxyacetonitrile, pentoxyacetonitrile, pentoxyacetonitrile, 2-rnethoxypropionitrile, i-methoxypropionitrile, 3-ethoxypropionitrile, 3-propoxyproDionitrile, 3-butoxypropionitrile, 3-methoxy butyronitrile, 4-methoxybutyronitrile, 4-ethoxybutyronitrile, 5-meth oxyvaleronitrile and 6-methoxycapronitrile. The preferred alkoxyalkyl nitriles are the alkoxyprooionitriles such as Z-methoxypro- pionitrile, 3-ethoxypropionitrile and 3-methoxypropionitrile which is especially preferred. These nitrile absorbents, although capable of separating suo- stantial amounts of carbon dioxide from mixed gas streams, are of unique utility where the gas stream contains both carbon monoxide and carbon dioxide. Simultaneous removal of both of these carbon oxide gases is easily accomplished. From 50 up to 70 wt % or more of the carbon dioxide can be removed. In addition, the nitrile absorbent can be selected or two or more nitrile absorbents blended so as to preferentially remove carbon dioxide at a ratio of carbon dioxide to carbon monoxide of from about 15 to 1 up to about 40 to 1 where carbon dioxide is at a greater concentration than carbon monoxide in the gas stream. This high degree of flexibility is, furthermore, not affected by any of the other gases that may be present in the stream. In the separation of carbon monoxide and carbon dioxide from mixed gas streams there are a number of factors which affect the separation efficiency of the nitrile absorbents. T mixed gas stream will often be under pressure and at elevated temperature conditions at the time separation is required. A thermal swing can be used alone or in combination with a pressure swing in effecting the gas separation. Separation and absorption of the carbon monoxide and carbon dioxide can be at any desired and convenient temperature level below the decomposition temperature of the nitrile absorbent at a pressure where the vapor pressure of the nitrile absorbent is a minor fraction of the total system pressure. While the temperature may vary widely, temperatures from about 200C to about 2500C will include most thermal conditions that must be considered during separation. A pressure swing can be used also, for example, superatmospneric absorption wherein preferentially dissolved carbon dioxide is ultimately recoverec rpm the nizrile absorbent by a reduction in pressure. The pressure swing can also consist of absorption at substantially azmosonerc pressure wherein dissolved carbon dioxide is recovered rom the nitrile pv substmospheric stripping. Althougn gas feed pressures of from sub atmospheric up to about 3500 kPa may be encounterec expending on the type of mixed gas stream, most gas streams involve pressure of from about atmospheric to about 350 kPa. Any carbon monoxide and carbon dioxide containing mixed gas stream is applicable to the process of this invention, for example, coal gas, usually a mixture of hydrogen, methane, carbon monoxide, olefins, benzene, nitrogen, and carbon dioxide; natural gas, primarily hydrogen and methane with small amounts of propane and other low molecular weight hydrocarbons, olefins, hydrogen sulfide, carbon monoxide and carbon dioxide; and synthesis and refinery gases which can vary widely in composition depending on the manufacturing operation and may contain, in addition to any of the above gases, mercaptans, ammonia, carbonyl sulfide, carbon disulfide, hydrogen cyanide, water and the like. In some instances, various inert diluents as nitrogen or argon may also be present in the mixed gas stream. Since the separation process does not involve the formation of a complex or other chemical reaction apd is in essence a physical separation with reduction in pressure of the gas stream and/or increase in the temperature of the nitrile absorbent any conventional gas purification liquid absorption system may be used in the-practice of the invention, from simple extraction operations to complex, continuous multiple-step recycle systems in a manner well known in the art. The invention is further illustrated by the following examples in which all parts and percents are by weight unless otherwise indicated. The separation factors are determined by dividing the mol. ratio of the more soluble species by the less soluble species in the liquid phase by the same relative ratios in the vapor phase as shown below. Separation factor ss = CO2/CO (liquid phase) / CO2/CO (vapor phase) The solubility ratio is determined by dividing the weight percent of the more soluble species in the liquid phase by the weight percent of the less soluble species in the liquid phase. EXAMPLE I This example compares the carbon monoxide and carnon dicxide solubility in propionitrile (PN) and in 3-methoxyorooionitrile (MOPN) at various temperatures and mole perzents of saebon dioxide in solution. The separation factor is ne excess a rncn dioxide absorbed over that of carbon monoxide. The results were as follows: TABLE I PN MOPN C02/CO mixture 29.6/70.4 24./75.5 33/67 27.3/72.7 Temp. QC 24 0 24 0 Wt. % CO 6.44 8.36 4.29 6.5 loading in nitrile Separation factor 18.5 36.8 20.8 9.4 CO-/CO Mol % C02 d8.6 92.3 91.1 77.9 in solution This example indicates that PN and MOPN absoro carbon dioxide preferentially over carbon monoxide at ratios of around 10 to 1 up to about 4U to 1, depending on the temperature. EXAMPLE II In this example a series of 9 test runs were conducted on solubility and separation of C02/CO mixtures of varying mol. percent concentrations using MOPN. Run No. 9 also contained 17.3 mol. % hydrogen and 4.9 mol. X ethane in the feed gas stream. The results are shown in Table IT. TABLE II Run No. 1 2 3 4 5 6 7 8 9 GAS FEED Mol. % CO 52.4 68.7 52.4 52.4 52.7 100 -- -- 69.9 " CO2 47.6 29.0 47.6 47.6 47.3 -- 100 100 7.9 Grams PN 141 141 141 142 141 141 141 141 141 Temp. C PN 24 24 25#0 0 24 24 24 24 24 FINAL GAS PRESSURE kPa 2227 2020 1910 17383 1896 2075 1551 1551 2069 (psia) (323) (293) (277) (252) (275) (301) (225) (225) (300) PARTIAL PRESSURE -kPa -CO 1489 1634 1489 1248 1317 2075 1558 (psia) (216) (237) (216) (181) (191) (301) -- -- (226) -CO2 738 386 421 490 579 1551 1551 90 (psia) (107) (56) (61) (71) (84) -- (225) (225) (13) ...cont'd. TABLE II - Cont'd. Run No. 1 2 3 4 5 6 7 8 9 FREE SPACE Mol. % CO 67.0 81.0 72.7 71.7 69.3 100 -- -- 69.9 " CO2 33.0 19.0 27.3 28.3 30.7 -- 100 100 4.4 SOLUTION Mol. % CO 8.9 21.2 22.1 29.7 14.0 -- -- -- 16.1 " CO2 91.1 78.8 77.9 70.3 86.0 -- -- -- 77.9 SEPARATION FACTOR CO2/CO 20.8 15.8 9.4 6.0 13.9 -- -- -- 76.9 The "free space' mol. % CO and C02 is the % in the vapor phase not absorbed by the MOPN solution. Carbon dioxide selectivity for MOPN is shown by the separation factor. The procedure used in conducting these uns tor ure CO and COZ was carried out by charging pure CO or C02 to a stainless steel high pressure cell of 500 cc. capacity containing 150 cc of nitrile absorbent and equipped with an internal magnetic stirred and with appropriate valves to sample both liquid and vapor phase. Solubility of pure CO and C02 were determined separately by pumping up to cell pressure to 1378 kPa - 224OkPa with the appropriate gas with constant agitation of the absorbent until equilibrium was established (the point at which no more gas was required after 30 minutes of agitation, to maintain the desired pressure). After about 30 minutes agitation at either OOC (wet ice bath) or 23 to 250C the pressure was Slowly released and the solvent was heated to just below its boiling point to assure complete stripping of dissolved gas from the solvent. The total volume of expelled gas was corrected for the cell free space, temperature and ccressibi1i. The corrected gas volume difference between total gas volume and free space volume was that dissolved in the absorbent. Generally the preferred absorbents have such a low vapor pressure with respect to the total system pressure that the absorbent vapor pressure effect is insignificant and may be ignored. It was found, using the above described procedure, that the solubility of pure CO is only a small fraction of pure C02 solubility in nitrile absorbents. Into the high pressure cell described above for pure gas solubility characteristics, the binary or multi component mixed gases. were pumped to a pressure of about 2067kPa. After equilibrium was established oetween the absorbent and the gas (generally 30 minutes or more of vigorous agitation) a sample of gas in vapor space of tne cell was withdrawn and analyzed by G.C. From the volume analysis of the feed gas and the analysis of tne unoissoivec gas in the vapor soace above the absorbent one can calculate the comoosition of the gas in solution. This calculated composition can be verified by withdrawing a small sample of absorbent under pressure into a sampling bomb or hypodermic syringe which is then transferred to G.C. for analysis. The analysis was conducted by back calculating the composition of the solubilized gas and verifying material balance after metering the total gas volume involved in the test. EXAMPLE III Example II is repeated using PN instead of MOPN and with 7 test runs of various mol. percentages of carbon monoxide and carbon dioxide. Table III gives the results. TABLE III Run No. 1 2 3 4 5 6 7 GAS FEED Mol. % CO 100 ¯¯ 52.4 -- 100 100 52.4 " CO2 100 ¯¯ 52.4 -- 100 100 52.4 Grams PN 117 117 117 117 117 117 117 Temp. C PN 25 25 24-25 24-25 24-25 24-25 0 FINAL GAS PRESSURE kPa 2095 1551 2185 1378 2095 2075 2216 (psia) (304) (225) (317) (200) (304) (301) (307) PARTIAL PRESSURE -kPa -CO 2095 -- 1537 -- 2095 2075 1600 (psia) (304) (223) (304) (301) (232) -CO2 -- 1551 648 1378 -- -- 517 (psia) (225) (94) (200) (75) FREE SPACE Mol. % CO 100 ¯¯ 70.4 -- 100 100 75.5 " CO2 -- 100 29.6 100 -- -- 24.5 SOLUTION Mol. % CO 100 ¯¯ 11.4 -- 100 100 7.7 " CO2 -- 100 88.6 100 -- -- 92.3 SEPARATION FACTOR CO2/CO -- -- 18.5 -- -- -- 36.8 EXAMPLE IV Using a mixed gas feed of 52.4 mol. % UO and 4{.6 mol. % CO2, a stainless steel cell containing 150 cc. of nitrile absorbent as described heretofore was filled with the gas mixture while constantly agitating the absorbent until the pressure reached approximately 2067kPa at 24 to 250C. A sample of the gas in the vapor phase was analyzed. From this analysis and the volume of the gas feed, the absorbent composition was calculated. The results are shown in Table IV. TABLE IV Wt. % Gas Solubility Mol. Pressure in Solution Ratio Absorbent Wt. kPa CO2 CO CGZICO Acetonitrile 41.05 2172 7.08 0.58 12.2/1.0 Propionitrile* 55.08 2186 6.44 U.53 12.2/1.0 Valeronitrile 85.17 2103 5.15 0.42 12.3/1.0 Methoxyaceto- 71.07 2206 4.72 0.30 15.7/1.0 nitrile Methoxypropio- 85.09 2227 4.Z9 0.27 15.9/1.0 nitri le* Ethoxypropio- 89.l0 2227 3.86 0.24 16.1/1.0 nitrile *Data generated from these nitriles was used to calculate results for the other nitriles. This example shows that the molecular weight of the nitrile has no significant affect on the solubility ratio and that both low and high molecular weight nitriles exhibit the same range of solubility ratios. While the exact mechanism is not clearly understood, it is believed that as the carbon chain increases on the aliphatic ni- trile group, the electron itdraing power of the ni.rile cecreases rendering the nitrogen less basic. However, incorporation of the ether linKage in the aliphatic carbon chain with their ability to donate or act as electron donors increases the basicity of the nitrile group. From a physical and chemical viewooint of practicality a preferred absorbent structure should exhibit the optimum selectivity for separating C02 from gases containing the same while providing a vehicle relatively easy to strip by a change in pressure and/or temperature. Concurrent with this preferred absorbent selectivity and. stripping behavior, thermal and chemical stability of the solvent must be considered. Acetonitrile may exhibit higher electron withdrawing power than valeronitrile but the volatility of acetonitrile is such that significant quantities of its vapor could be included in a vaporized gas stream enriched in C02 stripped from acetonitrile. As a practical matter, the absorbent choice should be predicated on relative economics of the process where removal of entrained vapors from gas enriched in C03 must be achieved. A further consideration is the relative potential hazard for the solvent of choice. For example, methoxyacetonitrile is more hazardous than methoxypropionitrile since methoxyacetonitrile is absorbed through the skin The foregoing detailed description has been given primarily for clearness of understanding only, and unnecessary limitations are not to be construed therefrom. The invention is not to be limited to the exact details shown and described since obvious modifications will occur to those skilled in the art, and any departure from the description herein that conforms to the present invention is.intended to be included within the scope of the claims.

Claims: CLAIMS 1. A process for simultaneous separation of carbon monoxide and carbon dioxide from a feed gas stream containing carbon monoxide, carbon dioxide and at least one additional gaseous component characterized by contacting the feed gas stream with a liquid nitrile absorbent having the general formula R-CN wherein R is selected from alkyl groups having from one to five carbon atoms and alkoxyalkyl groups having from two to ten carbon atoms, and mixtures thereof to form a liquid nitrile absorbent carbon monoxidecarbon dioxide mixture, maintaining the temperature of the mixture during separation at a temperature below the decomposition tempera- ture of the liquid nitrile absorbent and removing the liquid nitrile absorbent, carbon monoxide-carbon dioxide mixture from contact with the feed gas stream; 2. The process of Claim I wherein R is an alkyl group having from one to five carbon atoms. 3. The process of Claim 2 wherein R is ethyl or propyl. 4. The process of Claim 1 wherein R is an alkoxyalkyl group having from two to ten carbon atoms. 5. The process of Claim 4 wherein R is methoxyethyl or methoxybutyl. 6. The process of Claim 1 wherein the feed gas stream is selected from coal gas, natural gas and synthesis gas. 7. The process of Claim 1 wherein the contacting of the feed gas stream with the liquid nitrile absorbent occurs at a tempera- ture of from about 200 to about 2500C. 8. The process of Claim 1 wherein the contacting of the feed gas stream with the liquid nitrile absorbent occurs at a pressure of from atmospheric (101.35 kPa) to about 350 kPa. 9. The process of Cairn 1 erein at least 70 mole percent of the carbon dioxide in the feed gas stream is absorbed by the liquid nitrile absorbent. 10. The process of Claim 9 wherein at least 7U mole percent of the carbon monoxide in the feed gas stream is absorbed by the liquid nitrile absorbent. 11. The process of Claim 1 characterized by a C02/CO separation factor of from 10 to 1 to 14 to 1. 12. A process for separation of carbon monoxide and carbon dioxide from a feed gas stream containing carbon monoxide, carbon dioxide and at least one additional gas characterized by contacting the feed gas stream with propionitrile wherein the temperature of the gas stream during separation is less than the decomposition temperature of the propionitrile and removing the propionitrile containing dissolved carbon monoxide and carbon dioxide from contact with the feed gas stream. 13. A process for separation of carbon monoxide and carbon dioxide from a feed gas stream containing carbon monoxide and carbon dioxide and at least one additional gas characterized by contacting the feed gas stream with 3-methoxypropionitrile wherein the temperature of the gas stream during separation is less than the decomposition temperature of the 3-methoxypropionitrile and removing the 3-methoxypropionitrile containing dissolved carbon monoxide and carbon dioxide from contact with the feed gas stream.

Inventors: MAKIN, EARLE CLEMENT
Applicants: MONSANTO COMPANY
Application year: 1983
Main classification: C10K1
IPC codes: B01D53, C10K1