ENHANCED GAS SEPARATION PROCESS

Abstract: A process for the production of a desired product wherein a feed gas stream is treated to reduce its concentration of a component or components harmful to a permeable membrane prior to said feed gas stream being contacted with said permeable membrane for the separation of components useful in said production of said desired product from undesired impurities present therewith, characterised by; (a) passing said feed gas stream at an upper adsorption pressure to the feed end of an adsorbent bed in a pressure swing adsorption system capable of selectively adsorbing said harmful component or components present therein, with unadsorbed useful components of said feed gas stream being withdrawn from the product end of the bed at said upper pressure level as a treated gas stream depleted of said harmful component or components. (b) passing said treated gas stream substantially at said upper adsorption pressure to a separator zone containing a gas permeable membrane capable of selectively separating said useful components from said undesired impurities present therewith, the concentration of said harmful component or components in said treated gas stream being at a desired residual level; (c) separately withdrawing the non-permeate and the permeate portions of the treated gas stream from the separator zone, one such portion comprising a purified gas containing said useful components of the feed gas stream, the other such portion containing a substantial amount of the impurities content of said treated gas stream passed to the separator zone; (d) passing said purified gas withdrawn from the separator zone for use in the production of said desired product, said purified gas, or a portion thereof, being first recycled to an adsorbent bed in said pressure swing adsorption system at a pressure lower than said upper adsorption pressure as a purge gas to remove said harmful component or components from the bed where said harmful component or components comprises product gas and/or a desirable reactant or reactants in the production of desired product; (e) discharging said gas stream containing a substantial amount of impurities to waste or other use, with said gas stream being recycled to an adsorbent bed in said pressure swing adsorption system at a pressure lower than said upper adsorption pressure as a purge gas to remove said harmful component or components from the bed where said harmful component or components does not comprise a desirable reactant or reactants in the production of desired product, whereby undesired contact of the permeable membrane with harmful components of said feed gas stream is avoided.

Description: The invention relates to the separation of desired components of a gas stream from impurities contained therein. More particularly, it relates to the enhancing of the recovery of high purity product in such gas separation operations. Permeable membrane systems provide a commercially feasible means for separating and purifying a desired component or components from a feed gas mixture thereof with other less desired components or impurities. For such purposes, the feed gas mixture is passed to a gas permeable membrane capable of selectively permeating certain components of the mixture that are withdrawn from the discharge side of the membrane as a permeate gas at a reduced pressure. The non-permeate portion of the gas mixture is withdrawn from the feed side of the membrane substantially at the feed gas pressure. Typically, the component desired to be purified will be recovered as the permeate portion of the feed gas mixture, while the impurities associated therewith are withdrawn as the non-permeate portion of said feed gas mixture. In applications involving the purification of purge gases from the reaction loop of ammonia or methanol plants, such purge gases are commonly passed to a permeable membrane system at substantially the reaction pressure level. It is desirable that, in avoiding contact with the permeable membrane with feed gas components harmful thereto at particular concentrations, the pretreatment process be simplified and the need for employing water scrubbing and ammonia regeneration be avoided. At the same time, the enhanced recovery of the ammonia content of the purge gas is desirable so that the overall product recovery of the ammonia production operation can be enhanced. Those skilled in the art will appreciate that similar concerns and desires pertain to the field of methanol production. Similarly, a desire for such improvements exists with respect to other gas purification operations, not necessarily associated with a reaction synthesis loop, but wherein components of a feed stream harmful to a permeable membrane but otherwise valuable are desirably removed therefrom prior to contact with said membrane. In such operations, it is also generally desired to avoid the loss of such valuable components so that an enhanced recovery of desired, high purity product can be achieved. The use of permeable separation membranes in the treatment of ammonia synthesis purge streams to recover hydrogen is taught by Gardner et al, in "Hollow Fiber Permeator for Separating Gases", Chemical Engineering Progress, October, 1977, pages 76 to 78. This approach was affirmed by Perry, US-A-4,172,885, with reference to the known reaction of hydrogen and nitrogen at a superatmospheric pressure to produce ammonia. As stated therein, an ammonia-containing reaction effluent is withdrawn from the ammonia synthesis reaction zone. This reaction effluent, which contains substantial amounts of unreacted hydrogen and nitrogen, is recycled in a well-known ammonia synthesis loop to the ammonia synthesis reaction zone. Combined with this recycling reaction effluent in the ammonia synthesis loop is a synthesis feed gas stream of hydrogen, nitrogen and impurities such as methane and argon. In addition to the removal of product effluent from the reaction effluent in the ammonia synthesis loop, a purge stream is removed from the reaction effluent in an amount such as to maintain the proportion of impurities desirably low. In the process as described by Perry, the purge stream from the ammonia synthesis is chilled to condense and coalesce ammonia and is then scrubbed with water to provide a purge stream having less than about 0.5 volume % ammonia and water vapour therein. Alternatively, the ammonia content of the purge stream is reduced to less than 0.1 volume % by either chilling to coalesce ammonia, or adsorption or absorption of ammonia. The thus-treated purge stream, substantially at the synthesis reaction pressure level, is thereafter passed to the permeable membrane for permeation of hydrogen therethrough and reintroduction to the ammonia synthesis reaction zone. In DE-A- 2 248 501 there is disclosed a process for the separation, concentration and/or purification of components from gas mixtures, in which gas mixtures collected from thermal and/or catalytic transformation processes and at least containing hydrogen, methane, ethane, ethylene, acetylene, propane, propylene and C₄-hydrocarbons, or an industrial gas consisting of hydrocarbons and hydrogen, preferably a heating gas, a relief gas or an excess gas, are/is contacted in stages with membrane package segments or hollow fibre bundle segments having different selectivities to permeability, in permeation elements being functionally independent from one another and closed within themselves and successively arranged in any desired number, whereby at least one gas component of the gas mixture or of the industrial gas, such component preferably permeating through the membrane package segments or through the holow fibre bundle segments, is separated as permeate and/or admitted as the product charged in a following permeation element. An absorbent may be used either prior to or subsequent to the use of a membrane unit. In EP-A- 0 142 005 there is disclosed a method of separating a more easily diffusing component from a gas mixture which contains said component in addition to at least one more heavily diffusing component, by diffusion with the aid of semipermeable membranes, in which a gas is supplied on the permeable side of the membranes which is free of constituents of the more easily diffusing component. It has now been found possible to provide an improved process for the separation of desirable components of a gas stream from impurities associated therewith, particularly a process for the enhanced separation of impurities from a gas stream upon contact with a gas permeable membrane. It has also been found possible to provide a process for facilitating the recovery of desirable components removed from a gas stream prior to the separation of impurities from said gas stream by contact with a gas permeable membrane. By means of the present invention, components of a feed gas stream deemed harmful to a permeable separtion membrane can be removed therefrom prior to contact of said gas stream with the membrane. For this purpose, a pretreatment pressure swing adsorption system is employed. The purified gas stream from the membrane can be used as a purge gas for the pressure swing adsorption system to enhance recovery when said harmful components constitute valuable constituents of the feed gas stream. When said harmful components are not desired constituents of the fed gas stream, the waste stream from the membrane is similarly employed as a purge gas so as to facilitate the enhanced recovery of desired high purity product. The present invention involves a combination of pressure swing adsorption (PSA) processing and permeable membrane separation operation. Such a combination, as described and claimed herein, serves to obviate the need to water scrub from a feed gas stream various components that may be harmful to the permeable membrane. Regeneration of the PSA system is facilitated in a manner enhancing the recovery of desired high purity product from the PSA-permeable membrane combination. According to the present invention there is provided a process for the production of a desired product wherein a feed gas stream is treated to reduce its concentration of a component or components harmful to a permeable membrane prior to said feed gas stream being contacted with said permeable membrane for the separation of components useful in said production of said desired product from undesired impurities present therewith, characterised by: (a) passing said feed gas stream at an upper adsorption pressure to the feed end of an adsorbent bed in a pressure swing adsorption system capable of selectively adsorbing said harmful component or components present therein, with unadsorbed useful components of said feed gas stream being withdrawn from the product end of the bed at said upper pressure level as a treated gas stream depleted of said harmful component or components; (b) passing said treated gas stream substantially at said upper adsorption pressure to a separator zone containing a gas permeable membrane capable of selectively separating said useful components from said undesired impurities present therewith, the concentration of said harmful component or components in said treated gas stream being at a desired residual level; (c) separately withdrawing the non-permeate and the permeate portions of the treated gas stream from the separator zone, one such portion comprising a purified gas containing said useful components of the feed gas stream, the other such portion comprising an impurity containing gas stream containing a substantial amount of the impurities content of said treated gas stream passed to the separator zone; (d) recycling one of said purified gas stream or said impurity containing gas stream withdrawn from said separation zone to an adsorbent bed in said pressure swing adsorption system at a pressure lower than said upper adsorption pressure as a purge gas to remove said harmful component or components from said bed, such that said purified gas stream provides said purge gas when said harmful component or components comprises product gas and/or a desirable reactant or reactants in the production of desired product and said impurity containing gas stream provides said purge gas when said harmful component or components does not comprise product gas and/or a desirable reactant or reactants in the production of desired product; (e) passing said purified gas withdrawn from the separator zone for use in the production of said desired product; and (f) discharging said impurity containing gas stream to waste or other use whereby undesired contact of the permeable membrane with harmful components of said feed gas stream is avoided. The feed gas stream may comprise an ammonia loop purge gas removed from an ammonia synthesis loop, the ammonia loop purge gas being passed to the pressure swing adsorption system for the selective adsorption of ammonia therefrom, the ammonia being harmful to the permeable membrane but comprising valuable product gas. The treated gas stream in ammonia depleted form may be passed to the gas permeable membrane capable of selectively permeating the hydrogen and nitrogen components thereof at reduced pressure, with methane and argon impurities comprising non-permeate components thereof. The purified gas stream comprising substantially dry purge gas effluent discharged from the pressure swing adsorption system and containing hydrogen, nitrogen and desorbed ammonia may be recycled to the ammonia loop. The treated gas stream passed to the separator zone may have an ammonia concentration of less than 0.5 volume %. The purge gas effluent may be either recycled to the ammonia synthesis feed gas being passed to the ammonia loop or recycled directly to the ammonia synthesis loop. Preferably the ammonia concentration is from about 0.1% to about 0.5 volume %, more preferably less than 0.1 volume %, and most preferably from about 10 to about 1,000 ppm. In one embodiment of the present invention, the feed gas stream may comprise a methanol synthesis loop purge gas removed from a methanol loop, the methanol loop purge gas being passed to the pressure swing adsorption system for the selective adsorption of methanol therefrom, the methanol being harmful to the permeable membrane. In another embodiment of the present invention, the feed gas stream may comprise a natural gas stream containing H₂S and water as well as hydrocarbons that are harmful to the permeable membrane and impurities with respect to a desired natural gas processing operation, the harmful impurities being selectively adsorbed in the pressure swing adsorption system. The carbon dioxide-containing non-permeate portion of the treated gas stream may be recycled to the pressure swing adsorption system at reduced pressure as a purge gas to remove therefrom the components harmful to the permeable membrane and impurities with respect to the desired processing operation, the purge gas effluent discharged from the pressure swing adsorption system and containing the components being discharged to waste or other use. The purge gas is preferably heated prior to its recycle to an adsorbent bed to facilitate desorption and removal of harmful components from the bed.

Claims: A process for the production of a desired product wherein a feed gas stream is treated to reduce its concentration of a component or components harmful to a permeable membrane prior to said feed gas stream being contacted with said permeable membrane for the separation of components useful in said production of said desired product from undesired impurities present therewith, characterised by: (a) passing said feed gas stream at an upper adsorption pressure to the feed end of an adsorbent bed in a pressure swing adsorption system capable of selectively adsorbing said harmful component or components present therein, with unadsorbed useful components of said feed gas stream being withdrawn from the product end of the bed at said upper pressure level as a treated gas stream depleted of said harmful component or components; (b) passing said treated gas stream substantially at said upper adsorption pressure to a separator zone containing a gas permeable membrane capable of selectively separating said useful components from said undesired impurities present therewith, the concentration of said harmful component or components in said treated gas stream being at a desired residual level; (c) separately withdrawing the non-permeate and the permeate portions of the treated gas stream from the separator zone, one such portion comprising a purified gas containing said useful components of the feed gas stream, the other such portion comprising an impurity containing gas stream containing a substantial amount of the impurities content of said treated gas stream passed to the separator zone; (d) recycling one of said purified gas stream or said impurity containing gas stream withdrawn from said separation zone to an adsorbent bed in said pressure swing adsorption system at a pressure lower than said upper adsorption pressure as a purge gas to remove said harmful component or components from said bed, such that said purified gas stream provides said purge gas when said harmful component or components comprises product gas and/or a desirable reactant or reactants in the production of desired product and said impurity containing gas stream provides said purge gas when said harmful component or components does not comprise product gas and/or a desirable reactant or reactants in the production of desired product; (e) passing said purified gas withdrawn from the separator zone for use in the production of said desired product; and (f) discharging said impurity containing gas stream to waste or other use. A process according to claim 1, wherein said feed gas stream comprises an ammonia loop purge gas removed from an ammonia synthesis loop, said ammonia loop purge gas being passed to said pressure swing adsorption system for the selective adsorption of ammonia therefrom, said ammonia being harmful to the permeable membrane but comprising valuable product gas. A process acording to claim 2, wherein said treated gas stream in ammonia depleted form is passed to said gas permeable membrane capable of selectively permeating the hydrogen and nitrogen components thereof at reduced pressure, with methane and argon impurities comprising non-permeate components thereof. A process according to claim 3, wherein said purified gas stream comprising substantially dry purge gas effluent discharged from the pressure swing adsorption system and containing hydrogen, nitrogen and desorbed ammonia is recycled to said ammonia loop. A process according to claim 4, wherein said treated gas stream passed to the separator zone has an ammonia concentration of less than 0.5 volume %. A process according to claim 5, wherein said purge gas effluent is recycled to the ammonia synthesis feed gas being passed to the ammonia loop. A process according to claim 5, wherein said purge gas effluent is recycled directly to said ammonia synthesis loop. A process according to any of claims 5 to 7, wherein ammonia concentration is from about 0.1% to about 0.5 volume %. A process according to any of claims 5 tp 7, wherein ammonia concentration is less than 0.1 volume %. A process according to claim 9, wherein said ammonia concentration is from about 10 to about 1,000 ppm. A process according to claim 1, wherein said feed gas stream comprises a methanol synthesis loop purge gas removed from a methanol loop, said methanol loop purge gas being passed to said pressure swing adsorption system for the selective adsorption of methanol therefrom, said methanol being harmful to the permeable membrane. A process according to claim 1, wherein said feed gas stream comprises a natural gas stream containing H₂S and water as well as hydrocarbons that are harmful to the permeable membrane and impurities with respect to a desired natural gas processing operation, said harmful impurities being selectively adsorbed in the pressure swing adsorption system. A process according to claim 12, wherein said carbon dioxide-containing non-permeate portion of the treated gas stream is recycled to said pressure swing adsorption system at reduced pressure as a purge gas to remove therefrom said components harmful to the permeable membrane and impurities with respect to the desired processing operation, the purge gas effluent discharged from the pressure swing adsorption system and containing said components being discharged to waste or other use. A process according to any of claims 1 to 13, wherein the purge gas is heated prior to its recycle to an adsorbent bed to facilitate desorption and removal of harmful components from the bed.

Inventors: DOSHI, KISHORE JASRAJ
Applicants: UOP
Application year: 1986
Main classification: B01D53
IPC codes: B01D53