“About” generally refers to a range of numbers that a person of ordinary skill in the art would consider a reasonable amount of deviation to the recited values. The lower limit is the minimum concentration of hydrogen sulfide suitable for a feed to a Claus plant. The process of claim 1, wherein the hydrogen sulfide-enriched gas has a concentration of hydrogen sulfide that is greater than 80 mol %. The high concentration of hydrogen sulfide in the hydrogen sulfide-rich gas allows the sulfur recovery unit to operate more efficiently. But even at 30 selectivity and with only 20 mol % hydrogen sulfide in the feed-gas stream 401, the single-pass retentate stream 411 still has a concentration of hydrogen sulfide (0.2 mol %) that is not suitable for venting to the atmosphere after catalytic or thermal oxidation. 4 were simulated to illustrate some of their features. 4b, feed-gas stream 401 can be combined with compressed second-stage permeate stream 433 to obtain mixed feed-gas stream 402. Nitric acid and hydrogen sulfide rection. These membrane separation units are capable of enriching hydrogen sulfide to at concentrations of 20-90 mol %. 18. The results show that the residual concentration of hydrogen sulfide in single-pass retentate stream 411 decreases as membrane selectivity increases from 5-30. TABLE 12 Three-stage configuration performance: H2S recovery using a feed-gas stream containing 10 mol % H2S Theoretical H2S in third compressor MS permeate Recovered Membrane area (m2) power (kW) stream 482 H2S MS MS MS Unit Unit Selectivity (mol %) (%)a 460 470 480 465 475 Fifty mole percent H2S in second MS retentate stream 5 1.16 90 64 34 45 129 56 10 0.367 97 55 29 44 112 45 20 0.114 99 52 26 43 105 40 30 0.057 99.5 51 25 43 103 39 Ninety mole percent H2S in second MS retentate stream 5 1.26 88 75 95 50 151 134 10 0.384 97 62 64 49 125 78 20 0.115 99 58 53 48 116 60 30 0.057 99.5 56 51 48 114 55 MS, membrane stage aPercent H2S from the feed-gas stream that is recovered in the second retentate-in-series retentate stream. 9. A few examples of suitable membrane module designs include spiral-wound modules, hollow fiber modules that use small polymeric capillaries or fibers that have the geometry of a shell and tube heat exchanger, and plate-and-frame modules. In these instances, the single-pass retentate stream 411 must be treated to reduce the concentration of hydrogen sulfide to an acceptable level, or it must be recycled to a convenient stage in the gas treatment plant. For a feed gas having 2 mol % hydrogen sulfide, a suitable configuration would be capable of producing a permeate stream with a molar concentration of hydrogen sulfide reduced by at least a factor of 5, preferably at least a factor of 10. Unless otherwise indicated, all numbers expressing quantities, percentages or proportions, and other numerical values used in the specification and claims should be understood as being modified by the term “about.” The term “about” applies to all numeric values, whether or not explicitly indicated. 16. 4. Today, 27(4), pp. According to at least one embodiment, the process can include processing the sour gas in the membrane separation unit and sending the hydrogen sulfide-stripped stream to a selective amine absorption unit to obtain overhead gas and recovered hydrogen sulfide. The description can use the phrases “in some embodiments,” “in various embodiments,” “in an embodiment,” or “in embodiments,” which can each refer to one or more of the same or different embodiments. The recovered hydrogen sulfide can be recycled to the membrane separation unit, and the overhead gas can be polished to remove remaining hydrogen sulfide, oxidized and vented to the atmosphere, or simply vented to the atmosphere. The process in this configuration was simulated to achieve 20, 50, and 90 mol % hydrogen sulfide concentration in second membrane-stage retentate stream 471 (Table 8). This configuration is suitable for producing a hydrogen sulfide-stripped stream that can be safely discharged or used in other applications without additional hydrogen sulfide separation when membranes having carbon dioxide-hydrogen sulfide selectivity of 20 or more are used; membranes with less selectivity may be unsuitable for this purpose. The two reactions that occur in the hydrogen generation pro-cess are reforming, which uses steam to convert the hydrocarbons (methane, ethane, propane, and butane) to carbon monoxide and hydrogen and shift, which combines steam (some leftover from the reforming reaction) with the carbon monoxide to form carbon dioxide and hydrogen. First permeate-in-series permeate stream 442 is compressed and then introduced to second permeate-in-series membrane stage 450 where it is separated using a carbon dioxide-selective membrane to obtain second permeate-in-series retentate stream 451 and second permeate-in-series permeate stream 452; the second permeate-in-series retentate stream 451 and the second permeate-in-series permeate stream 452 being enriched in hydrogen sulfide and carbon dioxide respectively. 4a, feed-gas stream 401 is fed to single-pass membrane stage 410 where it is introduced to a carbon dioxide-selective membrane to obtain single-pass retentate stream 411 and single-pass permeate stream 412; the single-pass retentate stream 411 and the single-pass permeate stream 412 being enriched in hydrogen sulfide and carbon dioxide respectively. The process of claim 1, wherein the carbon dioxide-selective membrane has carbon dioxide-hydrogen sulfide selectivity greater than 30. TABLE 6 Two-stage retentate-in-series configuration performance: H2S recovery using a feed-gas stream containing 2 mol % H2S H2S in first retentate-in- Membrane Theoretical series permeate Recovered area (m2) compressor stream 422 H2S MS MS power Selectivity (mol %) (%)a 420 430 (kW) Twenty mole percent H2S in second retentate-in-series retentate stream 5 0.65 70 51 20 38 10 0.31 86 50 19 35 20 0.15 93 49 18 34 30 0.10 96 49 18 33 Fifty mole percent H2S in second retentate-in-series retentate stream 5 0.71 65 53 33 59 10 0.32 85 53 27 48 20 0.15 93 52 25 43 30 0.10 95 52 24 41 Ninety mole percent H2S in second retentate-in-series retentate stream 5 0.82 59 54 61 109 10 0.35 83 54 40 68 20 0.16 92 54 33 53 30 0.10 95 53 31 49 MS, membrane stage aPercent H2S from the feed-gas stream that is recovered in the second retentate-in-series retentate stream. For instance, Copolymer 3 achieves much greater hydrogen-methane selectivity than Cytop® and Hyflon® AD60, while its hydrogen permeance is greater than that of Cytop® and less than that of Hyflon® AD60. The process is also capable of producing a hydrogen sulfide-stripped stream, second permeate-in-series permeate stream 452, that has a sufficiently low concentration of hydrogen sulfide to be safely discharged to the atmosphere without further treatment (other than by catalytic or thermal oxidation). Sulfur and nitric oxide (NO) are given as the products by the reaction of hydrogen sulfide and nitric oxide. In some embodiments, the membrane can have a carbon dioxide/hydrogen sulfide selectivity of at least 10 and a carbon dioxide permeance of at least 500 gpu. Various membrane stage configurations are shown in FIG. In this configuration, the process was simulated to achieve 20, 50, and 90 mol % hydrogen sulfide concentration in first permeate-in-series retentate stream 441 (Table 7). In FIG. Copolymers of Monomer D and Monomer H can achieve combinations of selectivity and permeance that are unattainable using commercially available perfluoropolymer materials (Table 3 and Table 4). 4a is the simplest configuration with a single membrane stage. This stream is sent to a sulfur recovery unit to be processed for sulfur recovery. 4c involves two membrane stages with permeate from the first stage being fed to the next stage. To illustrate the process of FIG. A membrane stage can include one or multiple membrane modules in various configurations. An embodiment of a process for enriching hydrogen sulfide in a sour gas for feed to a sulfur recovery unit can include the steps of: providing a sour gas having carbon dioxide and hydrogen sulfide, separating the sour gas in a membrane separation unit to obtain hydrogen sulfide-enriched gas and hydrogen sulfide-stripped gas, and processing the hydrogen sulfide-enriched gas in a sulfur recovery unit. By reducing the overall flow of gas to the sulfur recovery unit, relatively smaller equipment can be used in the sulfur recovery process and in subsequent tail gas treatment processes. Elimination of carbon dioxide from combustion gases. Adv. FIG. All of the polymers shown in Table 3 and Table 4 have carbon dioxide-hydrogen sulfide selectivities of more than 10. 4: a single-pass membrane configuration in FIG.