CRYOGENIC PROCESS FOR THE REMOVAL OF ACIDIC GASES FROM MIXTURE OF GASES BY SOLVENTS

Abstract: A cryogenic process is disclosed for the removal of acidic gases from natural gases or from synthesis gases by absorption and preliminary cooling of the natural gas or of the synthesis gas to the purpose of condensing a portion of the acidic gases contained in the same gas, which is characterized in that it uses a mixture constituted by one or more selective solvents selected among low molecular weight esters, alcohols and ethers of the following classes: - esters of alcohols of general formula R₁COOR₂, wherein R₁ and R₂ are alkyl groups of from 1 to 4 carbon atoms, equal to or different from each other, wherein one or more hydrogen atoms can be substituted by alcoholic groups; - esters of glycols of general formula wherein R₁ and R₂ are alkyl groups having from 1 to 3 carbon atoms, equal to or different from each other, R₃, R₄, R₅, R₆, equal to or different from each other, are either alkyl groups containing from 1 to 3 carbon atoms or hydrogen atoms, m and n are integers which can have the value 0 or 1; - cyclic esters (lactones) of formula wherein R₂, R₃, R₄, R₅, equal to or different from each other, are alkylene groups wherein one or more hydrogen atoms can also be substituted by alkylic, alcoholic or ether groups; - alcohols of general formula wherein R₁, R₂, R₃, R₄, R₅, R₆, equal to or different from each other, are alkyl groups having from 1 to 3 carbon atoms or hydroxy groups or hydrogen atoms, m and n are integers which can assume the values 0 and 1; - cyclic ethers such as wherein R₂, R₅, R₆, equal to or different from each other, are alkyl groups wherein hydrogen can also be substituted by alkyl or methoxy groups, R₃ is either an oxygen atom or an alkylene group wherein hydrogen can also be substituted by alkyl or methoxy groups, R₄ is either equal to R₃ or can be missing in case of a pentaatomic ring; - ethers of general formula R₁-O-CH₂-(R₃) n -CH₂-O-R₂ wherein R₁ is an alkyl group of from 1 to 4 carbon atoms, R₂ is either an alkyl group of from 1 to 4 carbon atoms or a hydrogen atom , R₃ is either an alkylene group or (CH₂-O-CH₂), n is an integer which can have the value 0 or 1; - ethers of general formulal R₁-O-R₂ wherein R₁ and R₂, either equal to or different from each other, are alkyl groups of from 1 to 4 carbon atoms, wherein one or more hydrogen atoms can be substituted by alcoholic groups; - ester-ethers, compounds containing both the functions, of formula: (R₄-O) n - R₁ - COOR₂ (OR₃) m wherein R₃ and R₄, equal to or different from each other, are alkyl groups having from 1 to 4 carbon atoms, R₂ is an alkylene group having from 1 to 4 carbon atoms, R₁ is either equal to R₂ or equal to R₃, m and n are integers which can have the value 0 or 1, and by one or more organic compounds selected from: - heptane, - fractions of natural gasolines containing hydrocarbons having a number of carbon atoms comprised within the range of from 5 to 8; - monoaromatic compounds having a number of paraffinic carbon atoms comprised within the range of from 2 to 4, bound to the benzene ring either separately or as one or more linear or branched side chains; - dimethylethers of polyglycols having the glycolic unit -CH₂-CH-O- repeated from once to five times; - methoxy- and/or ethoxyalcohols, wherein the alcoholic group is a radical having from 1 to 4 carbon atoms in either linear or branched chain.

Description: The present invention relates to a process for the removal of acidic gases, such as hydrogen sulphide and carbon dioxide from mixtures of gases, specifically natural gases or synthesis gases, which contain them, particularly suitable for the processing of gaseous mixtures with even very high concentrations of acidic gases. To start up the production of gas fields with gases having a high content of acidic gases, or to the purpose of purifying the synthesis gases produced by starting from fuel oil or from coal, treatment processes are presently needed suitable to handle gases with high and very high contents of acidic components, able to yield products with even very strict specification. The treatment of such gases requires the adoption of mixed cryogenic and solvent-using technologies, so as to combine the advantages of both technologies, a good purification of the gases with acceptable costs being obtained. The same Applicant has already claimed a process of such a type by the U.K. Patent Nr. 1.555.068 granted on January 9, 1980 . In such patent application, the purification is reported of a raw gas containing more than 70% of acidic gases, by the combined use of a low-temperature distillation, and of absorption by solvent. The solvents described are dimethylether polyethyleneglycol and propylene carbonate. Another approach to the problem of withdrawing acid gas from natural gas or synthesis gas streams is disclosed in GB-A-2 133 717, by the same Applicants hereof. That prior art document teaches a process for removing acid gases from natural or synthesis gas by absorption and comprises (a) cooling the impure natural or synthesis gas so that part of the C0₂ contained in said gas condenses; (b) feeding the cooled and partly condensed gas to an absorption column to reduce the acid gas content to the required value; and (c) regenerating the solvent or solvents used in the absorption column. The solvent or solvents used are selected from particular esters, alcohols and ethers of low molecular weight: they are listed in the preamble of claim 1 hereof. This invention, therefore, provides: A process for removing acid gas from natural or synthesis gas by absorption, comprising the stages of (a) cooling the natural or synthesis gas to be purified to condense part of the acid gas contained therein;(b) feeding the cooled and partly condensed gas to an absorption column in order to reduce the acid gas content thereof; and (c) regenerating at least some of the solvent or solvents used in the acid gas absorption stage, the solvent or solvents comprising one or more of the following low-molecular-weight esters, alcohols and ethers: esters of alcohols of general formula R 1 COOR 2 , wherein R₁ and R₂ are alkyl groups of from 1 to 4 carbon atoms, equal to or different from each other, wherein one or more hydrogen atoms can be substituted by alcoholic groups; esters of glycols of general formula wherein R₁ and R₂ are alkyl groups having from 1 to 3 carbon atoms, equal to or different from each other, R₃, R₄, R₅, R₆, equal to or different from each other, are either alkyl groups containing from 1 to 3 carbon atoms or hydrogen atoms, m and n are integers which can have the value 0 or 1; cyclic esters (lactones) of formula wherein R 2 , R 3 , R 4 , R 5 , equal to or different from each other, are alkylene groups wherein one or more hydrogen atoms can also be substituted by alkylic, alcoholic or ether groups ; alcohols of general formula wherein R₁, R₂, R 3 , R₄, R₅, R₆, equal to or different from each other, are alkyl groups having from 1 to 3 carbon atoms or hydroxy groups or hydrogen atoms, m and n are integers which can assume the values 0 and 1; cyclic ethers such as wherein R₂, R₅, R₆, equal to or different from each other, are alkylene groups wherein hydrogen can also be substituted by alkyl or methoxy groups, R₃ is either an oxygen atom or an alkylene group wherein hydrogen can also be substituted by alkyl or methoxy groups, R 4 is either equal to R₃ or can be missing in case of a pentaatomic ring; ethers of general formula R 1 -O-CH 2 -(R 3 ) n -CH 2 -O-R 2 wherein R₁ is an alkyl group of from 1 to 4 carbon atoms, R 2 is either an alkyl group of from 1 to 4 carbon atoms or a hydrogen atom, R 3 is either an alkylene group or (CH 2 -O-CH 2 ), n is an integer which can have the value 0 or 1; ethers of general formula R₁-0-R₂ wherein R₁ and R 2 either equal to or different from each other, are alkyl groups of from 1 to 4 carbon atoms, wherein one or more hydrogen atoms can be substituted by alcoholic groups; ester-ethers, compounds containing both the functions, of formula: (R 4 -O) n - R 1 - COOR 2 (OR 3 ) m wherein R₃ and R₄, equal to or different from each other, are alkyl groups having from 1 to 4 carbon atoms, R₂ is an alkylene group having from 1 to 4 carbon atoms, R₁ is either equal to R₂ or equal to R₃, m and n are integers which can have the value 0 or 1, wherein to the above defined selective solvent mixture, at least one organic compound is added, in an amount of from 0,3% to 40%, by weight, relative to the weight of the resulting mixture, characterized in that said added organic compound is selected from: fractions of natural gasolines, containing hydrocarbons having from 5 to 8 carbon atoms; monoaromatic compounds having from 2 to 4 paraffinic carbon atoms bound to the benzene ring either separately, or as one or more linear or branched side chains; dimethylethers of polyglycols having a glycolic unit -CH₂-CH-0 repeated from 1 to 5 times, and methoxy- and/or ethoxy alcohols, wherein the alcoholic group has from 1 to 4 carbon atoms, in either a linear or a branched chain. The above reported solvents have a combination of several properties particularly favourable for their use as selective solvents. They have indeed a high stability under the use conditions, high solvent power for the acidic gases, high selectivity for H₂S and C0₂ relatively to hydrocarbons, low molecular weight and lou melting point. This last characteristic is essential for the application in a cryogenic process. In case of treatment of natural gas, after the condensation at low temperature and before the end absorption by solvent, the gas is available at low temperatures, considerably lower than 0°C. During the end absorption, it is useful to be able to reach temperatures considerably lower than the gas temperature, which are very favourable, because by so doing the absorption power of the solvent and its selectivity increase. The solvents of the process according to the invention show low melting point and are hence particularly suitable for use in a cryogenic process. The organic compound can be added in proportions comprised within the range of from 0.3 to 40% by weight relatively to the weight of the resulting mixture. The above described mixture can be used in a two-step system, such steps comprising: a) An absorption in tower of the previously cooled and partly condensed gas, to the purpose of reducing its content of acidic gases to the desired value; b) a regeneration of the mixture used in the absorption of the acidic gases. The cooling of the natural gas or of the synthesis gas can take place in a heat exchanger by vapourizing a portion of the acidic gases contained in the CO₂-rich mixture in a suitable point of the regeneration. It is preferable that the acidic gases not condensed after the cooling be at a concentration not higher than 30% by mol in the gas phase, and it is still more preferable that they are comprised within the range of from 15 to 30% by mol. The cooling of the gas can also take place inside the absorber tower. By means of such a cooling, which causes part of the acidic gases to condense, the distillation tower used in the prior processes is eliminated. The mixture used for the absorption of the acidic gases in the absorber tower can be regenerated first by means of one or more expansion steps (at maximum, three steps) from which above all the useful components co-absorbed in step a) are recovered, and then by means of one or more further expansion steps (at maximum, four steps), from which mainly the acidic gases are evolved. The mixture thus regenerated is recycled to the absorber tower. The regeneration of the mixture must be completed by a distillation tower if in the acidic gases also H₂S is contained, because the specifications as for residual H₂S in the treated gas are much more limitative than for CO₂ only; whilst if in the acidic gases only CO₂ is contained, using or not using a distillative regeneration tower depends on the maximum allowed CO₂ content in the purified gas. Always in case the regeneration tower is present, a portion of the mixture leaving the expansion steps can be sent to the regeneration tower, whilst the other portion, not completely regenerated, can be recycled to the absorber tower. The useful components which evolve from the expansions of the solvent rich in acidic gases are compressed, cooled and recycled to the absorber tower. The expansions of the mixture rich in acidic gases can be carried out in an expansion valve, or, at least partly, in a turbine. The regeneration by expansion of the mixture rich in acidic gases can be integrated with the heating of the same mixture, in order to promote the removal of the acidic gases by evaporation and recover cooling power to be used in the process. The expansion steps from which mainly acidic gases evolve can be from 1 to 4, producing streams of acidic gases under progressively lower pressures, of which, one or two can be kept under vacuum, in which case the acidic gases evolved must be compressed again. In some cases, on the contrary, operating under vacuum is not necessary, because the end pressure is a function of the temperature reached and of the purification level desired. The mainly acidic gases containing streams produced under high pressures can be expanded by means of an expansion valve, or of a turbine, to the delivery pressure, to the purpose of producing work and refrigerating effect. The absorber tower operates under a pressure comprised within the range of from 20 to 110 kg/cm² and at a temperature comprised within the range of from -100°C to 10°C. In case the distillation tower for the regeneration of the mixture is present, said tower shall operate under a pressure comprised within the range of from 0.1 to 5 kg/cm² and at a head temperature comprised within the range of from -60°C to 10°C and at a bottom temperature comprised within the range of from 10°C to 200°C. A further possible contrivance consists in adding the mixture to the natural gas or synthetic gas to be purified before said gas to be cooled by heat exchangers or expansion in valve or turbine, to the purpose of preventing CO₂ from crystallizing. The absorbing mixture of absorber tower can be drawn from an intermediate point of the same absorber tower, cooled by using at least a portion of the residual cooling power of the gas processed and/or by using at least a portion of the residual cooling power of the acidic gases, and fed to the tower immediately under the drawing point. The exhausted mixture leaving the absorber tower can be mixed to the natural gas or synthesis gas and cooled to the purpose of carrying out a preliminary absorption and discharging the absorber. At last, a further contrivance consists in mixing at least once the regenerated mixture with the gas outcoming from the absorber tower. The mixture thus formed is then cooled to the purpose of separating the regenerated mixture from the same gas. The mixing or the mixings between the gas outcoming from the absorber tower and the absorption mixture are preferably carried out in a mixer by feeding such streams in countercurrent to each other. Said mixings are preferably two. In case the gas outcoming from the tower undergoes two mixings, such mixings can be carried out as followss: first, a mixing of the treated gas with the regenerated mixture takes place, and then to said mixing a cooling and a separation is carried out of the mixture obtained, thus separating from the gas the regenerated mixture, which is in its turn mixed in a second mixing step with the gas leaving the absorber tower, to which second mixing step a cooling and a separation follows of the mixture obtained in said second mixing, from the gas the regenerated mixture being separated, which is fed to the absorber tower. The two coolings following the two mixings can be carried out at the same temperature or at different temperatures, preferably within a temperature range between -50 and -100°C. The invention shall be now better described by referring to the flow sheet of attached Figs. 1 and 2 which show preferred forms of embodiment, which however must not be considered as limitative of the same invention. Referring to Fig. 1, the raw gas is supplied to the plant by the piping 1, is cooled in 2 and is expanded by the valve 3, and is washed in countercurrent flow by the absorber mixture in the absorber 4 to the purpose of removing the acidic gas. The gas leaving the absorber 4 is mixed with the mixture completely purified and cooled in the heat exchanger 5, is separated from the mixture in 6 and sent to the recovery of the refrigerating power in the heat exchangers 7 and 8, and is then supplied to the network by means of the piping 9. The cooled mixture separated from the gas in 6 is pumped into the absorber 4 by the pump 10. Further absorber mixture, not completely purified, is introduced into the absorber at an intermediate level. In order to reduce the average absorption temperature, the absorber mixture is extracted from an intermediate tray of the absorber, pumped in 11 and cooled in 7 by the processed gas. The mixture rich in acidic gases leaving the absorber 4 is regenerated by expansion. It is fed to the separator 12, through the valve 13, wherein a methane-rich gas evolves, which is recycled to the absorber 4 by the compressor 14, after cooling by the treated gas in 8. The mixture outcoming from 12 is expanded by the valve 15, is heated in 16 and sent to the separator 17, from which the acidic gases evolve; they are heated in 18 and in 19 and are discharged from the plant. The mixture outcoming from 17 is expanded under vacuum by means of the valve 20, is heated in the heat exchanger 21 and is then sent to the separator 22, from which the acidic gases evolve, which are compressed to nearly atmospheric pressure in the compressor 23 and are discharged from the plant. A portion of the mixture outcoming from 22, not yet completely purified, is sent by the pump 24 and through the valve 25 to the absorber 4 at an intermediate level (26). The residual portion of the mixture is fed by the pump 27 and through the valve 28 to the regeneration tower 29 after having been treated in 30. The mixture is regenerated from the acidic gases in the regenerating tower 29, equipped with condenser 31, reflux accumulator 32, reflux pumps 33, reboiler 34; this latter is heated by a whatever heat source. The acidic gases outcoming from 32 are combined with those outcoming from the exchangers 18 and 19 and with those outcoming from the compressor 23; they are then removed by the piping 35. The regenerated mixture is cooled by outer refrigerating means (either air or water, and/or a suitable refrigerating cycle) in 36 and by the mixture to be regenerated in 30 and, by means of the pump 37, is sent to the refrigerator 5 after having been cooled in the heat exchanger 38. In Fig. 2 a flow sheet is schematically shown, in which two mixings are provided between the regenerated mixture and the gas outcoming from the absorber tower. The raw gas is supplied to the plant by the piping 1, is cooled and partly condensed in 2, is then fed to the separator 39, the liquid 40 which, through the valve 41, is then fed to the absorber tower 4, being separated from the not condensed gas 42. The not condensed gas 42 is cooled and partly condensed in 43 and sent to the separator 44, from which the gas 45 not condensed in 43 evolves, which is fed, after being cooled in 46, to the absorber tower 4, through the valve 47, whilst the liquid 48 is fed in its turn through the valve 49 to the absorber tower 4. In the exchangers 2, 43 and 46, the heat exchange can be carried out by using the refrigerating power of the process streams. From the bottom of the absorber tower 4 a liquid 50 rich in acidic gases, which is regenerated by expansion, is obtained. It is expanded in two steps by means of the valves 13 and 15 in the separators 12 and 17, from which the vapours 51 and 52, essentially constituted by methane and carbon dioxide, are collected and recycled to the tower 4 after being compressed in 53 and 54. From the bottom of the separator 17 a liquid 55 is obtained, whch is essentially constituted by the mixture and by the separated CO₂, this latter being vapourized by expansion by the valves 56 and 57 and heating in the heat exchange unit 58. Said heat exchange unit has been represented for simpleness' sake as one single exchanger, but in reality it supplies the cooling power necessary to the process (for example, in the heat exchangers 2, 43, 46). Vapourized CO₂ is separated from the mixture in the separators 59 and 60, the first one being under a slight over-pressure, the second under vacuum, maintained by the vacuum pump 61, and is discharged from the plant in 62. The regenerated mixture 63 outcoming from the bottom of the separator 60 is pumped by the pump 64 and mixed in 65 with the gas outcoming from the absorber tower. The mixture obtained is cooled in 66 and separated in 67, the processed gas 68 being separated from the absorber mixture 69. The mixture 69, after being pumped in 70, is mixed in 71 with the gas outcoming from the absorber tower 4; the mixture obtained is cooled in 72 and separated in 73, the treated gas 74, which is subsequently mixed in 65 with the regenerated mixture 69, being separated from the solvent 75, which is pumped by the pump 76 to the absorber tower 4.

Claims: Process for removing acid gas from natural or synthesis gas by absorption, comprising the stages of (a) cooling the natural or synthesis gas to be purified to condense part of the acid gas contained therein; (b) feeding the cooled and partly condensed gas to an absorption column in order to reduce the acid gas content thereof; and (c) regenerating at least some of the solvent or solvents used in the acid gas absorption stage, the solvent or solvents comprising one or more of the following low-molecular-weight esters, alcohols and ethers: esters of alcohols of general formula R 1 COOR 2 , wherein R₁ and R₂ are alkyl groups of from 1 to 4 carbon atoms, equal to or different from each other, wherein one or more hydrogen atoms can be substituted by alcoholic groups; esters of glycols of general formula wherein R₁ and R₂ are alkyl groups having from 1 to 3 carbon atoms, equal to or different from each other, R₃, R₄, R₅, R₆, equal to or different from each other, are either alkyl groups containing from 1 to 3 carbon atoms or hydrogen atoms, m and n are integers which can have the value 0 or 1; cyclic esters (lactones) of formula wherein R₂, R₃, R₄, R₅, equals to or different from each other, are alkylene groups wherein one or more hydrogen atoms can also be substituted by alkylic, alcoholic or ether groups; alcohols of general formula wherein R₁, R₂, R 3 , R 4 , R 5 , R 6 , equal to or different from each other, are alkyl groups having from 1 to 3 carbon atoms or hydroxy groups or hydrogen atoms, m and n are integers which can assume the values 0 and 1; cyclic ethers such as wherein R₂, R₅, R₆, equal to or different from each other, are alkylene groups wherein hydrogen can also be substituted by alkyl or methoxy groups, R₃ is either an oxygen atom or an alkylene group wherein hydrogen can also be substituted by alkyl or methoxy groups, R₄ is either equal to R₃ or can be missing in case of a pentaatomic ring; ethers of general formula R 1 -O-CH 2 -(R 3 ) n -CH 2 -O-R 2 wherein R 1 is an alkyl group of from 1 to 4 carbon atoms, R₂ is either an alkyl group of from 1 to 4 carbon atoms or a hydrogen atom , R₃ is either an alkylene group or (CH₂-O-CH₂), n is an integer which can have the value 0 or 1; ethers of general formula R₁-O-R₂ wherein R₁ and R₂, either equal to or different from each other, are alkyl groups of from 1 to 4 carbon atoms, wherein one or more hydrogen atoms can be substituted by alcoholic groups; ester-ethers, compounds containing both the functions, of formula: (R 4 -O) n - R 1 - COOR 2 (OR 3 ) m wherein R₃ and R₄, equal to or different from each other, are alkyl groups having from 1 to 4 carbon atoms, R₂ is an alkylene group having from 1 to 4 carbon atoms, R₁ is either equal to R₂ or equal to R₃, m and n are integers which can have the value 0 or 1, wherein, to the above defined selective solvent mixture, at least one organic compound is added, in an amount of from 0,3% to 40%, by weight, relative to the weight of the resulting mixture, characterized in that said added organic compound is selected from: -fractions of natural gasolines, containing hydrocarbons having from 5 to 8 carbon atoms; -monoaromatic compounds having from 2 to 4 paraffinic carbon atoms bound to the benzene ring either separately, or as one or more linear or branched side chains; -dimethylethers of polyglycols having a glycolic unit -CH 2 -CH-O repeated from 1 to 5 times, and -methoxy- and/or ethoxy alcohols, wherein the alcoholic group has from 1 to 4 carbon atoms, in either a linear or a branched chain. Process according to claim 1, wherein the absorbing mixture used in the absorbing stage is regenerated by at least one expansion step to recover the absorbed compounds, whereafter additional expansion steps are carried out to set the acidic gases free and the regenerated solvent mixture is recycled to the absorption stage. Process according to claim 1, wherein the regeneration of the mixture is completed by a distillation step. Process according to claim 1, wherein the regeneration of the acid-gas-laden solvent mixture is completed by a heating step to vaporize the dissolved acid gas in order to produce a refrigerating effect. Process according to claim 1, wherein the absorbing stage is carried out under a pressure of from 19,613 bar to 107,873 bar (20 to 110 kg/cm 2 ) and at a temperature of from -100°C to +10°C. Process according to claim 3, wherein the distillation step is carried out under a pressure of from 0,098066 bar to 4,9033 bar (0,1 to 5 kg/cm 2 ), the head temperature being from -60°C to +10°C, and the bottom temperature being from +10°C to +200°C. Process according to claim 1, wherein a portion of the solvent mixture is added to the gas to be purified before the cooling stage, to prevent carbon dioxide crystallization. Process according to claim 1, wherein the gas is cooled and partially condensed prior to the absorption stage and the acid-gas-laden solvent mixture is withdrawn at an intermediate point of the absorbing stage, cooled and fed back to the absorbing stage at a point close to the withdrawal point and below same. Process according to claim 1, wherein the regenerated solvent mixture is admixed with the gaseous stream exiting the absorbing stage in two steps, the first step in which said gaseous stream is mixed with the regenerated solvent mixture, cooled, and stripped of the regenerated solvent mixture, and the second step in which the stripped regenerated mixture is mixed with the gaseous stream exiting the absorbing stage, cooled, and stripped of the regenerated solvent mixture, the stripped gas stream is fed back to the first blending stage and the regenerated mixture is fed back to the absorbing stage. Process according to claim 9, wherein each cooling stage of the admixture of the gas and the regenerated solvent mixture is carried out at a temperature of from -50°C to -100°C.

Inventors: GAZZI, LUIGI, RESCALLI, CARLO
Applicants: SNAMPROGETTI S.P.A.
Application year: 1986
Main classification: F25J3
IPC codes: B01D53, C10L3, C10K1, F25J3