PROCESS FOR REMOVING AND RECOVERING H2S FROM A GAS STREAM BY CYCLIC ADSORPTION

Abstract

A process for altering the composition of a feed gas containing H.sub.2S equivalents is disclosed. The process comprises (a) contacting the feed gas with a solid adsorbent at a temperature of 250-500° C., to obtain a loaded adsorbent, (b) purging the loaded adsorbent with a purge gas comprising steam, thus producing a product stream which typically contains substantially equal levels of CO.sub.2 and H.sub.2S. The process further comprises a step (c) of regenerating the purged adsorbent by removal of water. The adsorbent comprises alumina and one or more alkali metals, such as potassium oxides, hydroxide or the like.

Claims

1.-18. (canceled)

19. A process for altering the composition of a gas comprising H.sub.2S equivalents and CO.sub.2, comprising the steps of: (a) contacting a feed gas containing H.sub.2S equivalents, CO.sub.2 and optionally H.sub.2O, wherein the molar ratio of H.sub.2O to H.sub.2S equivalents is within the range of 0-(5+X), with a solid adsorbent at a temperature of 250-500° C., to obtain a loaded adsorbent and a first product gas; (b) contacting the loaded adsorbent with a purge gas containing H.sub.2O, to obtain a second product gas; and (c) regenerating the adsorbent after step (b) by removal of H.sub.2O, wherein the process is performed in cycles of steps (a) to (c), and wherein the feed gas and/or the purge gas contains a reducing agent and the adsorbent comprises alumina and one or more alkali metals, and wherein X is defined as: $X=.Math.\frac{n_i{\left[H_2.Math.S.Math..Math.\mathrm{equivalent}\right]}_i}{\left[H_2.Math.S.Math..Math..Math.\mathrm{equivalents}\right]}$ wherein [H.sub.2S equivalents] indicates the total concentration of H.sub.2S equivalents, [H.sub.2S equivalent].sub.i indicates the concentration of a particular H.sub.2S equivalent i and n.sub.i indicates the amount of water molecules n consumed when said H.sub.2S equivalent i is converted to H.sub.2S.

20. The process according to claim 19, wherein the H.sub.2S equivalents comprise H.sub.2S, COS and/or CS.sub.2.

21. The process according to claim 19, wherein the molar ratio of H.sub.2S equivalents to CO.sub.2 in the feed gas is below 1, preferably in the range of 0.001-0.1.

22. The process according to claim 19, wherein the molar ratio of H.sub.2S equivalents to CO.sub.2 in the feed gas is in the range of 0.001-0.1.

23. The process according to claim 19, wherein the feed gas contains 0.1-20% H.sub.2 as the reducing agent.

24. The process according to claim 19, wherein the adsorbent further comprises one or more divalent metals, preferably as their oxides, hydroxides, carbonates, sulphides and/or hydrosulphides, preferably the adsorbent further comprises MgO.

25. The process according to claim 19, wherein the divalent metals are oxides, hydroxides, carbonates, sulphides and/or hydrosulphides.

26. The process according to claim 19, wherein the adsorbent further comprises MgO.

27. The process according to claim 23, wherein the alkali metal is K and the adsorbent is K-promoted alumina, or is based on a K-promoted hydrotalcite.

28. The process according to claim 19, wherein the process is continued with step (a) after the regeneration of step (c).

29. The process according to claim 19, wherein step (b) is performed counter-currently with respect to step (a).

30. The process according to claim 19, wherein the purge gas comprises at least 75% H.sub.2O.

31. The process according to claim 19, wherein the first product gas contains less than 10 ppm of H.sub.2S equivalents, and/or the first product gas contains less than 0.1 times the level of H.sub.2S equivalents of the feed gas, and/or the first product gas has a molar ratio of H.sub.2S equivalents to CO.sub.2 of less than 0.005.

32. The process according to claim 19, wherein the second product gas has a molar ratio of H.sub.2S equivalents to CO.sub.2 of at least 0.5.

33. The process according to claim 19, wherein the feed gas is an optionally pre-dried syngas further containing H.sub.2 and CO.

34. The process according to claim 19, wherein the feed gas is an optionally pre-treated Claus tail gas further containing N.sub.2.

35. A process for the conversion of H.sub.2S to elemental sulphur, comprising the step of subjecting the second product gas as obtained in the process according to claim 19, optionally after pre-drying, to a Claus process to obtain elemental sulphur and a tail gas comprising H.sub.2S equivalents and CO.sub.2.

36. The process according to claim 19, wherein the second product gas is subjected, optionally after pre-drying, to a Claus process to obtain elemental sulphur and a tail gas comprising H.sub.2S equivalents and CO.sub.2, and the tail gas is used as feed gas in step (a) of the process according to claim 19, optionally after pre-drying.

37. A system for performing the process according to claim 19, comprising: (A) a Claus unit comprising: (a1) a first inlet for receiving the second product gas; (a5) a first outlet for discharging elemental sulphur; and (a6) a second outlet for discharging a Claus tail gas; and (B) an adsorption module comprising: (b1) a reactor bed comprising the adsorbent as defined in any one of claims 19, 23 and 24; (b2) a first inlet for receiving the Claus tail gas; and (b4) a first outlet for discharging the second product gas, wherein outlet (a6) is in fluid connectivity with inlet (b2) and outlet (b4) is in fluid connectivity with inlet (a1).

38. A method for production of elemental sulphur comprising subjecting a H.sub.2S-enriched gas obtainable in step (b) of the process according to claim 19 to a Claus process.

Description

DESCRIPTION OF THE FIGURES

[0080] A preferred embodiment of the system according to the invention is depicted in FIG. 1. Claus unit (A) may be any Claus unit or Claus plant as known in the art. It comprises a first inlet (a1) for receiving a combined feed gas originating from means (a3) for combining the second product gas and a further feed gas. Unit (A) further comprises a first outlet (a5) for discharging elemental sulphur and a second outlet (a6) for discharging a Claus tail gas. Second outlet (a6) is in fluid connectivity via steam removal unit (C1) with inlet (b2) of the adsorption module (B). Adsorption module (B) comprises a bed (b1) containing the adsorbent according to the invention as bed material, a first inlet (b2) for receiving the Claus tail originating from unit (C1) and a second inlet (b3) for receiving a purge gas. Module (B) further comprises a first outlet (b4) for discharging the second product gas and a second outlet (b5) for discharging the first product gas. Module (B) is designed as such that incoming gases from inlets (b2) and (b3) are led through or over the bed towards outlets (b4) and (b5). First outlet (b4) is in fluid connectivity via steam removal unit (C2) with means (a3). Means (a3) is designed to combine the second product gas originating from unit (C2) and a further feed gas.

[0081] FIGS. 2-8 depict compositions of the tail gases obtained in examples 1-3.

EXAMPLES

Example 1

[0082] A feed gas containing 10% CO.sub.2, 10% H.sub.2 and 500 ppm H.sub.2S (balanced with N.sub.2) was subjected to adsorption in a packed bed placed in a cylindrical reactor containing 1 g adsorbent. The feed flow was 150 Nml/min, and the bed operated at a temperature of 400° C. and a pressure of 3 bar(a). The process according to the invention was operated in a cyclic co-current mode. Cycles consisted of an adsorption stage, a flushing stage, a purging stage and a regeneration stage. The adsorption stage was continued until full breakthrough of CO.sub.2 and H.sub.2S was reached. Subsequently, the loaded adsorbent was flushed with 10% Ar in N.sub.2 (flow=150 Nml/min) and then purged with a purging gas containing 30% H.sub.2O (balanced with Ar and N.sub.2; flow=150 Nml/min). As last step in the cycle, the adsorbent loaded with H.sub.2O was regenerated by flushing with a dry inert gas (10% Ar in N.sub.2; flow=150 Nml/min). The adsorbents used were K-promoted hydrotalcite MG30 (KMG30), K-promoted alumina (20 wt % K.sub.2CO.sub.2 on alumina) and unpromoted MG30 (control). A similar experiment was conducted with 0.5 g Na-promoted MG30 as adsorbent, which operated at 350° C. and 1 bar(a) and wherein the gas flows (feed, purge and flushes) were 100 Nml/min.

[0083] FIGS. 2-5 depict the tail gas (effluent) composition of a cycle of each of the four experiments: FIG. 2 shows the results for KMG30 as adsorbent, FIG. 3 for K-promoted alumina, FIG. 4 for Na-promoted MG30 and FIG. 5 for unpromoted MG30. Ar levels were also determined (data not shown), to visualise the switches between the different stages. These stages are indicated with A, D, F1 and F2, wherein “A” denotes the adsorption stage (feed gas), “D” the desorption or purging stage (purging gas), and “F1” and “F2” the first inert flush and second inert flush (regeneration), respectively. On the y-axis, the mass spectrometer (MS) response in arbitrary units is shown.

[0084] In all experiments, fast breakthrough of CO.sub.2 was observed after the adsorption period commenced. Because of the high sorbent capacity for H.sub.2S equivalents, breakthrough of H.sub.2S (and COS) was observed at a later time, indicating saturation of the adsorbent with H.sub.2S and COS at that time. For the control unpromoted adsorbent, breakthrough times for CO.sub.2, H.sub.2S and COS were similar (FIG. 5), indicating that significantly less H.sub.2S (and COS) is adsorbed during the adsorption phase. For the experimental adsorbents, the H.sub.2S+COS slip level before breakthrough as observed in the first effluent (tail gas of the adsorption phase) was less than 5 ppm, i.e. >2 orders of magnitude decrease with respect to the feed gas. It should be noted that no COS was present in the feed gas, meaning that the adsorbent promotes the H.sub.2S+CO.sub.2⇄COS+H.sub.2O equilibrium reaction at the operating conditions. In view of the simultaneous breakthrough of H.sub.2S and COS, those species are both adsorbed. Upon steam regeneration, CO.sub.2 was released swiftly from the adsorbent, while desorption of H.sub.2S is spread over a longer period of time. The second effluent (tail gas of the desorption phase) contained H.sub.2S, CO.sub.2, H.sub.2O and inert gases. No desorption of COS was observed, indicating that all adsorbed sulphur species are released as H.sub.2S. For the control unpromoted adsorbent, hardly any H.sub.2S desorption was observed (FIG. 5), reflecting the small amount of H.sub.2S adsorbed in the adsorption period.

Example 2

[0085] Two distinct feed gases containing 10% CO.sub.2, 10% H.sub.2 and 500 ppm or 900 ppm H.sub.2S (balanced with N.sub.2) were subjected to adsorption in a packed bed placed in a cylindrical reactor containing 0.5 g K-promoted hydrotalcite MG30 (KMG30) as adsorbent. The feed flow was 200 Nml/min, and the bed operated at a temperature of 350° C. and a pressure of 1 bar(a). The process according to the invention was operated in a cyclic co-current mode. Cycles consisted of an adsorption stage, a flushing stage, a purging stage and a regeneration stage. The adsorption stage was continued until full breakthrough of CO.sub.2 and H.sub.2S was reached. Subsequently, the loaded adsorbent was flushed with 10% Ar in N.sub.2 (flow=200 Nml/min) and then purged with a purging gas containing 30% H.sub.2O (balanced with Ar and N.sub.2; flow=200 Nml/min). As last step in the cycle, the adsorbent loaded with H.sub.2O was regenerated by flushing with a dry inert gas (10% Ar in N.sub.2; flow=200 Nml/min).

[0086] FIG. 6 depicts the tail gas compositions with respect to H.sub.2S and COS for the adsorption stage of a cycle of each of the two experiments: FIG. 6a shows the results for the feed gas comprising 500 ppm H.sub.2S and FIG. 6b for the feed gas comprising 900 ppm H.sub.2S. Levels (in ppm) of H.sub.2S, COS and “total S” (i.e. H.sub.2S+COS) are depicted. The start of breakthrough is observed at about 75 min in FIG. 6a and at about 50 min in FIG. 6b. Before start of breakthrough, the level of total S in the tail gas (slip level) was below 5 ppm. Both H.sub.2S and COS were observed at breakthrough, while only H.sub.2S was fed. At about t=130 min (FIG. 6a) or t=80 min (FIG. 6b), the adsorbent reached full capacity for the H.sub.2S equivalents, and full breakthrough was reached.

[0087] FIG. 7 depicts a more detailed analysis of the tail gas composition obtained with the feed gas comprising 500 ppm H.sub.2S. Levels (in ppm) of H.sub.2S, COS and “total S” (i.e. H.sub.2S+COS) are depicted. The results of a different cycle as the one presented in FIG. 6a are presented. In the cycle of FIG. 7, the slip level of total S was below 1 ppm (t=840-875 min). At full breakthrough, about 500 ppm of sulphur species (H.sub.2S to COS ratio of about 1) was observed in the tail gas, at which point the loaded adsorbent was briefly flushed (around t=950) and the purging stage commenced. During purging, a peak in the H.sub.2S level of the tail gas was observed, with initial H.sub.2S levels well above 600 ppm, while COS was absent in the tail gas from the start of the purging phase. The second product gas obtained during the purging phase thus contained high levels of H.sub.2S as sole H.sub.2S equivalent.

Example 3

[0088] A feed gas containing 10% CO.sub.2, 10% H.sub.2 and 100 ppm CS.sub.2 (balanced with N.sub.2) was subjected to adsorption in a packed bed placed in a cylindrical reactor containing 0.5 g K-promoted hydrotalcite MG30 (KMG30) as adsorbent. The feed flow was 200 Nml/min, and the bed operated at a temperature of 350° C. and a pressure of 1 bar(a). The process according to the invention was operated in a cyclic co-current mode. Cycles consisted of an adsorption stage, a flushing stage, a purging stage and a regeneration stage. The adsorption stage was continued until full breakthrough of CO.sub.2 and H.sub.2S was reached. Subsequently, the loaded adsorbent was flushed with 10% Ar in N.sub.2 (flow=200 Nml/min) and then purged with a purging gas containing 30% H.sub.2O (balanced with Ar and N.sub.2; flow=200 Nml/min). As last step in the cycle, the adsorbent loaded with H.sub.2O was regenerated by flushing with a dry inert gas (10% Ar in N.sub.2; flow=200 Nml/min).

[0089] FIG. 8 depicts the tail gas composition with respect to H.sub.2S equivalents for a cycle of the experiment. Levels (in ppm) of H.sub.2S, COS and “total S” (i.e. H.sub.2S+COS+CS.sub.2) are depicted. In the cycle of FIG. 8, the slip level of total S was below 1 ppm (t=24770-24830 min). At full breakthrough, about 200 ppm of sulphur species (H.sub.2S to COS ratio of about 7) was observed in the tail gas, while no CS.sub.2 was completely absent in the tail gas (H.sub.2S+COS=total S). The loaded adsorbent was briefly flushed (around t=24910) and the purging stage commenced. During purging, a peak in the H.sub.2S level of the tail gas was observed, with initial H.sub.2S levels well above 250 ppm, while both COS and CS.sub.2 were completely absent in the tail gas from the start of the purging phase. The second product gas obtained during the purging phase thus contained high levels of H.sub.2S as sole H.sub.2S equivalent, while CS.sub.2 was present as sole H.sub.2S equivalent in the feed gas.

Example 4

[0090] Seven distinct feed gases containing 10% CO.sub.2, 10% H.sub.2, and varying amounts of H.sub.2S and H.sub.2O (see Table 2, balanced with N.sub.2) were subjected to adsorption in a packed bed placed in a cylindrical reactor containing 0.5 g K-promoted hydrotalcite MG30 (KMG30) as adsorbent. The feed flow was 200 Nml/min, and the bed operated at a temperature of 350° C. and a pressure of 1 bar(a). The process according to the invention was operated in a cyclic co-current mode. Cycles consisted of an adsorption stage, a flushing stage, a purging stage and a regeneration stage. The adsorption stage was continued until full breakthrough of CO.sub.2 and H.sub.2S was reached. Subsequently, the loaded adsorbent was flushed with 10% Ar in N.sub.2 (flow=200 Nml/min) and then purged with a purging gas containing 30% H.sub.2O (balanced with Ar and N.sub.2; flow=200 Nml/min). As last step in the cycle, the adsorbent loaded with H.sub.2O was regenerated by flushing with a dry inert gas (10% Ar in N.sub.2; flow=200 Nml/min). During cyclic steady state, both the breakthrough adsorption capacity at and the total adsorption capacity of the adsorbent for H.sub.2S equivalents was determined, the results of which are presented in table 2. Breakthrough adsorption capacity refers to the capacity of the adsorbent during the adsorption phase until start of breakthrough, wherein start of breakthrough is defined as the point in time when the total slip level of sulphur species (H.sub.2S+COS) in the tail gas reaches a level of 10 ppm. Total adsorption capacity refers to the capacity of the adsorbent during the adsorption phase until total breakthrough is reached, i.e. when the content of sulphur species (H.sub.2S+COS) in the tail gas is equal to the content of sulphur species in the feed gas.

TABLE-US-00002 TABLE 2 Feed gas compositions and adsorption capacities for H.sub.2S Feed gas composition (ppm) Adsorption capacity (mol/kg) Entry H.sub.2S H.sub.2O H.sub.2O/H.sub.2S breakthrough total 1 500 0 0 0.57 0.841 2 500 575 1.15 0.40 0.727 3 500 900 1.80 0.31 0.617 4 900 0 0 0.62 1.124 5 900 750 0.83 0.50 1.053 6 900 2100 2.33 0.33 0.816 7 25000 117000 4.68 n.d. 0.14

[0091] For both the feed gases comprising 500 ppm H.sub.2S and the feed gases comprising 900 ppm H.sub.2S, the adsorption capacity of the adsorbent decreased with increasing H.sub.2O content of the feed gas. The adsorption capacity for H.sub.2S decreased by about a factor 2 when the H.sub.2O/H.sub.2S ratio increased to above 2. Extrapolating the results in Table 2, the adsorption capacity for H.sub.2S decreased to unacceptable levels in case the H.sub.2O/H.sub.2S ratio increases to above 5, while the best results are obtained with a H.sub.2O/H.sub.2S ratio of at most 2. It should be noted that since only H.sub.2S was used as H.sub.2S equivalent, X amounts to zero for the feed gases tested here.