SCRUBBING OF GAS TO RECOVER HYDROCARBONS

Abstract: For scrubbing C₂+ or C₃+ hydrocarbons from gaseous mixture containing hydrocarbons and/or inerts like H₂, N₂, CO and possibly acid gases, e.g. CO₂H₂S, COS e.g., natural gas and the like, the scrubbing medium consists essentially of compounds having one to two rings formed by 5 or 6 carbon atoms, wherein the compounds comprise, in total, at least 9 and at most 17 carbon atoms, and the rings are saturated un­ saturated or partially saturated especially alkylated ring com­ pounds of 9 or 10 carbon atoms, e.g. isomeric forms of tri­ methylbenzene, propylbenzene, propylcyclohexane, tetra­ hydronaphthalene, decahydronaphthalene.

Description: Background of the Invention The present invention relates to a process for the recovery of C₃+ hydrocarbons together with acid sulfur compounds from natural gas mixtures containing hydrocarbons, sulfur compounds and inerts like H₂ or N₂ by scrubbing with a physical scrubbing medium. It is conventional to separate C₂+ hydrocarbons from gaseous mixtures, such as natural gas, by the so-called Expander process. In this process, the precooled gas, after separation of condensate, is isentropically expanded in an expansion turbine. During this step, the cooling requirements for the condensation of C₂+ hydrocarbons from the gaseous mixture are satisfied at least by the cold-producing expansion of the gas. However, if unfavorable marginal conditions occur, such as, for example, low concentrations of C₂+ hydrocarbons or simultaneously a higher content of CO₂ in the crude gas, then alternatively, physical scrubbing procedures are also suitable for recovering the C₂+ fraction. Examples of known scrubbing media for such scrubbing operations are various polyethylene glycol dialkyl ethers (PGE), N-methylpyrrolidone (NMP), dimethylformamide (DMF), propylene carbonate, or sulfolane. Disadvantageous to the use of these known scrubbing media is that, besides the hydrocarbons, CO₂ and further sour gases (e.g. H₂S, COS) are likewise dissolved with relative ease, which gases are in most cases present in the crude gas. At the same time the solubility of C₂+ hydrocarbons is very low. Under practical conditions, this means that, besides C₂+ hydrocarbons, a considerable portion of the CO₂ present in the gas is also scrubbed out and the amount of the circulated solvent ist relatively large. In order to produce a C₂+-fraction suitable for further processig, two different ways for the recovery of this fraction from the loaded solvent are possible. The first way, normally used in oil scrubbing systems, was distillation, thereby separating the solvent as bottoms product and the adsorbed gases as the overhead stream. Thereafter, this overhead stream had to be separated into a C₂+-fraction and a gas containing CO₂, CH₄ and lighter components like H₂, N₂ and CO. The second way is the expansion of the loaded solvent to low pressure. The flash gas is compressed and fed to a rectification column for the further recovery and purification of the C₂+ product. In the rectification of the desorbed gas into methane as the overhead product and residual C₂+ hydrocarbons as the bottoms product, problems are also encountered due to the tendency to from an azeotrope between CO₂ and ethane. DE-OS 14 43 692 describes a process for the production of butadiene, wherein the reaction product is subjected to extraction with a scrubbing liquid which preferentially dissolves butadiene and butenes, with the butadiene being recovered from the liquid practically free of acetylene. This process however, deals only with unsaturated hydrocarbons. US 3,349,145 discloses a process of purifying a paraffins containing crude H₂ stream by scrubbing with a C₉+ aromatic lean oil for a following hydrodealkylation of alkylaromatic compounds in the presence of hydrogen. The process is restricted to the treatment of a hydrogen-rich fraction to recover pure hydrogen. The scrubbing medium absorbes methane, ethane, propane, N-butane, isobutane, pentanes and C₆+ paraffins including hexanes, heptanes and octanes. A removing of sour gases like H₂S and/or CO₂ is not teached. Summary of the Invention An object of this invention is to provide an improved scrubbing medium for removing light hydrocarbons, especially C₃+ hydrocarbons, from natural gas mixtures, containing CO₂ and only saturated hydrocarbons. Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparant to those skilled in the art. To attain these objects, there are employed as the scrubbing media: compounds having one or two rings formed by 5 or 6 carbon atoms, respectively, wherein the compounds, in total, contain at least 9 and at most 17 carbon atoms, and the rings are saturated, unsaturated, or partially saturated. Consequently, the optimum scrubbing medium is constituted by an organic ring compound with 5 or 6 carbon atoms in the ring and, in total, between 9 and 17 carbon atoms wherein the ring(s) can have aliphatic, olefinic or aromatic character. The fusion of several rings is possible in this connection, but it is preferred that the melting point of the compound be below 0°C and the boiling point be above 130°C, especially from 130°C to 250°C. The compounds utilized according to this invention exhibit, besides stability, other excellent properties rendering them particularly suitable for use as scrubbing media for C₂+ or C₃+ hydrocarbons. The compounds are highly selective between ethane/propane on the one hand, and the light components in the crude gas on the other hand, such as, for example, methane, N₂ and H₂. Also, the absolute solubility of ethane/propane in the compounds is relatively high. The compounds to be used according to this invention exhibit a C₂ or C₃ solubility which in particular, is higher than that of Co₂. Accordingly, the CO₂ can remain at least in part in the scrubbed gas. Morever, the vapor pressure of the compounds employed is so low, so that losses of scrubbing medium are minor, and at the same time satisfactory purities of the resultant products can be realized. All of the aforementioned advantageous properties of the compounds to be utilized as scrubbing agents according to this invention have not been simultaneously associated with the organic liquids heretofore suggested as scrubbing media for the scrubbing of natural gas. In the case that the natural gas mixture also contains sulfur compounds, as e.g. H₂S. COS, mercaptanes, these sulfur compounds are co-absorbed together with C₃+ hydrocarbons. This is achieved, because the compounds to be used according to this invention also exhibit a solubility for sulfur compounds which in particular is higher than that of CO₂. According to a preferred embodiment of the process of this invention, compounds to be employed as scrubbing media have the formulae I wherein in the positions 1 to 6 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1-5 carbon atoms. Accordingly, examples for suitable scrubbing media, include, but are not limited to alkylated ring-compounds of 9 or 10 carbon atoms. These relatively heavy ring compounds exhibit a very high selectivity between ethane/propane and methane and a high solubility of ethane/propane. Accordingly, the following compounds are particularly suitable as scrubbing media: Formula I A: Isomeric forms of trimethylcyclohexane, propylcyclohexane, butylcyclohexane Formula I B: Isomeric forms of trimethylcyclohexene, especially tetrahydromesitylene, butylcyclohexene, propylcyclohexene Formula I C: Isomeric forms of trimethylcyclohexadiene, butylcyclohexadiene, propylcyclohexadiene Formula I D: Isomeric forms of methylethylbenzene, trimethylbenzene, butylbenzene, propylbenzene, methylpropylbenzene, diethylbenzene, dimethylethylbenzene, tetramethylbenzene. All isomeric forms of trimethylbenzene or propylbenzene are utilized herein as the especially preferred scrubbing media. Likewise, the scrubbing agents of this invention also include, but are not limited to the following groups of compounds and particular species thereof. Compounds to be employed as scrubbing medium have the formulae II wherein in the positions 1 to 5 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1-5 carbon atoms. Accordingly, the following compounds are particularly suitable as scrubbing media. Formula II A: Isomeric forms of diethylcyclopentane, tetramethylcyclopentane, butylcyclopentane Formula II B: Isomeric forms of pentylcyclopentene Formula II C: Isomeric forms of methylpropylcyclopentadiene. Further compounds to be employed as scrubbing media have the formulae III wherein in the positions 1 to 8 , 9 or 10 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1 to 5 carbon atoms. Accordingly, the following compounds are particularly suitable as scrubbing media: Formula III A: decahydronaphthalene (decaline) Formula III B: Isomeric forms of bicyclodecene Formula III C: Isomeric forms of bicyclodecadiene Formula III D: 1,2,3,4-tetrahydronaphthalene (tetraline) Formula III E: Isomeric forms of dihydronaphthalene Formual III F: 1-methylnaphthalene. Further compounds to be employed as scrubbing media have the formulae IV wherein in the positions 1 to 7 or 9 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1 to 5 carbon atoms. Accordingly, the following compounds are particularly suitable as scrubbing media: Formula IV A: hydrindane Formula IV B: Isomeric forms of tetrahydroindane Formula IV C: Isomeric forms of bicyclononadiene Formula IV D: indane (hydrindene) Formula IV E: Isomeric forms of indene Formula IV F: Isomeric forms of hexahydroindene. Further compounds to be employed as scrubbing media have the formulae V wherein in the positions 1 to 9, 10 or 11 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1 to 5 carbon atoms. Accordingly, the following compounds are particular suitable as scrubbing media: Formula V A: cyclopentylbenzene Formula V B: Isomeric forms of phenylcyclopentene Formula V C: Isomeric forms of cyclohexylcyclopentene. Further compounds to be employed as scrubbing media have the formulae VI wherein in the positions 1 to 8 or 10 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1 to 5 carbon atoms. Accordingly, the following compounds are particularly suitable as scrubbing media: Formula VI A: bicyclopentyl Formula VI B: Isomeric forms of cyclopentencyclopentyl Formula VI C: Isomeric forms of bicyclopentenyl Formula VI D: cyclopentylidene-cyclopentadiene. According to a further development of the idea of this invention, the provision is made to use as the scrubbing media also halogen-substituted compounds. In particular, the compounds can be chlorine-substituted. For example, 1-chloronaphthalene. It is also contemplated in this invention that the compounds can be modified by the substitution, addition or interposition of other atoms which do not materially affect the basic and novel aspects to this invention, and such modified compounds are embraced herein. The compounds herein can be utilized in the pure form as well as in a mixture with one another. In addition to the use of these compounds, it is also possible to use as the scrubbing agent, mixtures of said compounds with solvents such as benzene, toluene or xylene, or polar, organic, water-soluble physical solvents. In particular, those added solvents suitable for this invention are those conventionally employed for sour gas scrubbing operations, such as alcohols, ketones, ethers, polyethers, amides, esters, ethylene glycols, polyethylene glycol ethers, morpholine, alkylated morpholines, caprolactams and acetates. Typical solvents for physical absorption methods for removal of sour gases are cited, for example, in A.L. Kohl, F.C. Riesenfeld, "Gas Purification" 3rd edition, 1979; as well as R.N. Maddox, "Gas Conditioning and Processing"vol. 4, 1982, incorporated by reference herein. By the addition of toluene or xylene, the viscosity of the scrubbing medium is lowered. Moreover, the ring character of the scrubbing medium is enhanced, and these additives are also selective in favor of ethane/propane/sulfur compounds as compared to CO₂. The additives are admixed to the compounds of this invention in such amounts that do not completely mask the benefits of this invention which would in turn materially alter the basic and novel characteristics of this invention. Thus, the scrubbing agent will preferably consist essentially of the above described ring compounds of this invention containing 9-17 carbon atoms, with 5 or 6 atoms in a ring. In any case, the added solvents are preferably used in minor amounts, advantageously up to 25 % by weight, and preferably in the range of 3 to 20 % by weight relative to the weight of the compounds of this invention. With such low amounts of sour gas solvents being added, the CO₂ solubility in the scrubbing medium is slightly increased, but on the other hand, water can also be scrubbed out of the crude gas to such an extent that the drying of crude gas can in some cases be advantageously omitted. The addition of water dissolving solvents to the recommended solvents is especially preferred in those cases, where a low concentration of C₂+ or C₃+ is present in the feed gas. In these cases, the methane rich gas can be dried at least to a water dewpoint suitable for pipeline transportation (usually 7 lbs H₂O per MMSCF) and only the small part of the absorbed gas, which has to be handled in the distillation column has to be dried. Without the addition of the water dissolving solvent, the entire feed gas must be dried by adsorbers or a glycol wash. It is known to use water dissolving additives with special solvents, e.g., methanol as additive to toluene for selective sulfur removal or to N-methylpyrrolidone, if C₂H₂ is removed. In the special case of recovery of heavier hydrocarbons, the additive must have a much higher boiling point than methanol. Preferred additions are therefore polyethyleneglycoldialkyl ethers or similar components with a high boiling temperature, e.g. NMP. The scrubbing step is advantageously performed at temperatures of between -20° and +40° C. The pressure at which scrubbing takes place is not significant so that the gaseous mixture can be treated at crude gas pressure. Especially preferred scrubbing media consist essentially of trimethylbenzene and, in this connection, particularly 1,2,4-trimethylbenzene (pseudocumene) or 1,3,5-trimethylbenzene (mesitylene) and propylbenzene, in particular n-propylbenzene, decaline, or tetraline. The loaded scrubbing medium is preferably recirculated by any conventional regeneration technique, e.g. thermal, pressure reduction and/or stripping, with the preferred technique being pressure reduction and compression of the flashgas. The raw gases that are especially advantageously treated by this invention comprise natural gas and all associated oil gases, but also gases from refinery or gases produced by gasification of coal or another organic feedstock. Tables 1 to 4, respectively, set forth below, indicate the absolute solubilities for C₂H₆ and C₃H₈ and selectivities for C₂H₆ and C₃H₈ with respect to CH₄ and for C₂H₆ and C₃H₈ with respect to CO₂ at 20 °C for several conventional scrubbing media (Tables 1 and 3) and scrubbing media according to this invention (Tables 2 and 4). In this context, λ indicates the technical solubility coefficient in Nm³/ton. ata, (cubic meters under standard conditions per ton of scrubbing medium and per atmosphere partial pressure at a given temperature). Table 4a indicates the absolute solubilities for CO₂, H₂S, COS and mercaptanes, e.g. CH₃SH at 20°C for scrubbing media according to this invention. Table 4b indicates the absolute solubilities for ethane, methane propane, CO₂, H₂S, COS and mercaptanes, e.g. CH₃SH at 20°C for further scrubbing media according to this invention. In this context, λ again indicates the technical solubility coefficient in Nm³/ton ata. As can be seen from the tables, absolute C₂H₆ and C₃H₈ solubility as well as selectivity are more favorable for the compound of this invention than for those scrubbing media proposed heretofore. Since the amount of scrubbing medium circulated in a physical scrubbing operation is inversely proportional to the solubility of the key component in the gas to be scrubbed, and the dissolved amount of CH₄ and/or CO₂ and lighter gases is proportional to the quantity of scrubbing medium and the solubility of CH₄ and CO₂ and lighter gases it can be seen that, in the present instance, the pumping energy as well as the compression energy for compressing the methane and/or CO₂, and lighter gases dissolved in the scrubbing column are both lower, when using the compounds of this invention,e.g., trimethylbenzene, than in the case of the conventional solvents. Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following specific embodiment is, therefore, to be construed as merely illustrative for the comparison of different solvents, and not limitative of the remainder of the disclosure in any way whatsover. Brief Description of the Drawing Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in connection with the accompanying drawing , in which like reference characters designate the same or similar parts, and wherein: Figure 1 shows a schematic flow diagram for the recovery of C₃+ hydrocarbons together with sulfur compounds. Description of the Drawing Figure 1 In this embodiment of the invention an example is given for the simultaneous separation and recovery of C₃+ hydrocarbons and H₂S from a CO₂ rich natural gas The natural gas is to be treated in such a way that a sulfur-depleted CO₂ fraction containing at most 0.2% hydrocarbons is recovered. The absorbed H₂S is to be converted to elemental sulfur, the C₃+ hydrocarbons are to be recovered as LPG (liquid petroleum gas) fraction. The recovery of CO₂ from the pre-purified gas takes place in a cryogenic unit by means of rectification and subsequent final purification by means of membrane technology or chemical absorption. Since the feed gas, which is scrubbed in a pre-absorption zone, is further treated in a cryogenic unit to recover liquid CO₂, the necessary drying can be carried out prior to the scrubbing. Thus, the scrubbing can also be carried out without the addition of a polar solvent at temperatures below 0°C. 100.000 kmol/h of the dried feed gas consisting of N₂ 25 mol% CH₄ 27 mol% C₂H₆ 3 mol% C₃+ 3 mol% CO₂ 41 mol% H₂S 1 mol% are passed via conduit 41 under a pressure of 40 bar to a heat exchanger 42, where the feed gas is cooled down from 30°C to about -3°C in heat exchange with scrubbed gas, which thereby warms up from -11°C to +25°C. The feed gas is thereafter passed into scrubbing colum 43, where it is scrubbed counter-currently with tetraline (200 tons/h). The solvent, i.e. tetraline, was cooled prior to its use in scrubbing column 43 in a cooler 45 in conduit 44 to -15°C. The quantity of the solvent is large enough to scrub out H₂S and the C₃+ hydrocarbons in the scrubbing column 43 so that the required residiual contents of 1 ppm H₂S is achieved. Thereby a residual contents of at most 5 ppm C₃+ appears. The scrubbed gas leaves the column under a pressure of 39 bar via conduit 46 and is warmed up in heat exchanger 42. In this embodiment of the invention 95.445 kmol/h leave as product gas via conduit 46 with the following composition: N₂ 26,19 mol% CH₄ 28,28 mol% C₂H₆ 3,09 mol% C₃+ 10 ppm CO₂ 42,44 mol% H₂S 1 ppm It is also possible to separate the scrubbed gas immediately in a cryogenic unit and to warm up the recovered separation product streams against the feed gas. In the scrubbing column there are dissolved besides H₂S and C₃+ hydrocarbons also the other components of the feed gas according to the partial pressure and the solubility. To recover a concentrated H₂S/C₃+ fraction the loaded solvent is withdrawn from the bottom of column 43 via conduit 47 and flashed to a pressure of 16 bar into a column 48, whereby a large portion of the less soluble lighter components flashes. To reach the conditions of purity of the C₃+ fraction, the loaded solvent in column 48 is warmed up via reboiler 49. Thus the still dissolved inerts degas, while the major portion of the heavier hydrocarbons and the H₂S is re-washed in the column 48. Since the gaseous mixture, which is withdrawn from the top of column 48 via conduit 49, still contains absorbable components, like H₂S and C₃+ according to its temperature, pressure and liquid composition, this gaseous mixture is compressed in compressor 50 to the pressure of the feed gas and is conveyed to the feed gas after intermediate cooling in cooler 51. The solvent still containing H₂S and C₃+ is withdrawn from the bottom of column 48 via conduit 52 at a temperature of 170°C and is passed after warming in heat exchanger 53 against regenerated solvent into rectifying column 54, where it is separated by means of distillation in H₂S and C₃+ as head products and regenerated solvent as a bottoms. The pressure in column 54 is such as to liquify the head products of column 54 in conduit 55 at least partially in cooler 56. The cooled product stream is conveyed to separator 57. Part of the recoverd condensate is recycled to column 54 via conduit 58, while via 59 4.555 kmol/h of the H₂S/C₃+ fraction are withdrawn under a pressure of 15 bar, a temperature of 25°C and the following composition: CH₄ 0.11 mol% C₂H₆ 1.10 mol% C₃+ 65.86 mol% CO₂ 10.98 mol% H₂S 21,95 mol% This H₂S/C₃+ fraction may be further treated e.g. in an amine washing zone, whereby the H₂S is absorbed and, after relaese in a regeneration step, passed into a Claus plant for the recovery of elemental sulfur. The regenerated solvent is withdrawn from the bottom of column 54 via conduit 60 under a pressure of 15 bar and a temperature of 305°C. The regenerated solvent still contains a small quantity of heavier hydrocarbons (C₅+). The regenerated solvent is cooled down in heat exchanger 53 to 180°C and is passed after further cooling into column 43 via conduit 44. The amount of heat necessary for the warming of the solvent and for rectification is produced in reboiler 61 by means of steam or a heat carrier. From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

Claims: A process for the recovery of C₃+ hydrocarbons together with acid sulfur compounds from natural gas mixtures containing hydrocarbons, sulfur compounds, CO₂ and inerts like H₂ or N₂ by scrubbing with a physical scrubbing medium, the improvement wherein the scrubbing medium comprises a compound having one to two rings formed by 5 or 6 carbon atoms, respectively, wherein the compound comprises in total at least 9 and at the most 17 carbon atoms, and the rings are saturated, unsaturated, or partially saturated. A process according to claim 1, said compounds being of the formulae I wherein in the positions 1 to 6 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1-5 carbon atoms. A process according to claim 1 wherein the compound is an alkylated mononuclear ring of 9 or 10 carbon atoms in total. A process according to claim 1, said compound being isomeric trimethylbenzenes or propylbenzenes. A process according to claim 1, said compound being of the formulae II wherein in the positions 1 to 5 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1-5 caron atoms. A process according to claim 1, said compound being trimethylcyclohexane, propylcyclohexane, or butylcyclohexane. A process according to claim 1, said compound being butylcyclopentane. A process according to claim 1, said compound being of the formulae III wherein in the positions 1 to 8, 9 or 10 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1 to 5 carbon atoms. A process according to claim 1, said compounds being decahydronaphthalene, tetrahydronaphthalene, or 1-methylnaphthalene. A process according to claim 1, said compound being of the formulae IV wherein in the positions 1 to 7 or 9, there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1 to 5 carbon atoms. A process according to claim 1, said compound being indan or indene. A process according to claim 1, said compound being of the formulae V wherein in the positions 1 to 9, 10 or 11 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1 to 5 carbon atoms. A process according to claim 1, said compound being of the formulae VI wherein in the positions 1 to 8 or 10 there may be one or two substituted groups, being identical or different, representing a hydrogen atom or an alkyl radical of 1 to 5 carbon atoms. A process according to claim 1, said compound being halogen-substituted. A process according to claim 14, said compound being chlorine-substituted. A process according to claim 1, where the scrubbing medium comprises a mixture of at least two of said compounds. A process according to claim 1, said scrubbing medium further comprising toluene or xylene. A process according to claim 1, said scrubbing medium further comprising a minor amount of a polar, organic, water-dissolving, physical solvent. A process according to claim 17, said toluene, or xylenes being present in a amount of up to 25% by weight of said compound. A process according to claim 1, characterized by conducting the scrubbing operation at temperatures of between -20°C and +40°C. A process according to claim 1, wherein the gaseous mixture contains a major amount of methane.

Inventors: LANDECK, HEINER, DR. DIPL.-PHYS., RANKE, GERHARD, DIPL.-ING.
Applicants: LINDE AKTIENGESELLSCHAFT
Application year: 1986
Main classification: C07C7
IPC codes: C10G5, C07C7