Water gas shift process

Abstract

A process is described for increasing the hydrogen content of a synthesis gas mixture comprising hydrogen, carbon oxides and steam, comprising the steps of: (i) passing the synthesis gas mixture at an inlet temperature in the range 300-450 C. over a first water-gas shift catalyst disposed in a first shift vessel to form a first shifted gas mixture, and (ii) passing the first shifted gas mixture at an inlet temperature in the range 170-300 C. over a second water gas shift catalyst disposed in a second shift vessel to form a second shifted gas mixture, wherein the second water-gas shift catalyst comprises copper and the first shift vessel contains a sorbent material for capturing halogen contaminants disposed downstream of the first water gas shift catalyst.

Claims

1. A process for increasing the hydrogen content of a synthesis gas mixture comprising hydrogen, carbon oxides and steam, the process comprising: (i) passing the synthesis gas mixture through a first shift vessel having an inlet temperature in the range of 300-450 C., the first vessel comprising a first high temperature water-gas shift catalyst and a sorbent material for capturing halogen contaminants disposed downstream of the first high temperature water gas shift catalyst to form a first shifted gas mixture, and then (ii) passing the first shifted gas mixture at an inlet temperature in the range of 170-300 C. over a second low temperature water-gas shift catalyst comprising copper disposed in a second shift vessel to form a second shifted gas mixture, wherein the second shifted gas mixture has a higher hydrogen content than does the synthesis gas mixture.

2. The process according to claim 1, wherein the sorbent material comprises a solid material which is more basic than zinc oxide.

3. The process according to claim 2, wherein the sorbent material comprises a basic compound of any element of Group IA or Group IIA of the Periodic Table, other than beryllium.

4. The process according to claim 2, wherein the sorbent material comprises a solid material which is more basic than zinc oxide supported on a carrier material.

5. The process according to claim 1, wherein the sorbent material comprises at least one of sodium oxide, sodium carbonate, potassium oxide or potassium carbonate supported on a carrier material that is alumina, silica, titania, zirconia, ceria, magnesia or zinc oxide, or a mixture thereof, or a refractory cement.

6. The process according to claim 5, wherein the alkali concentration in the sorbent material is in the range of 0.1 to 10.0% by weight calculated as sodium oxide or potassium oxide.

7. The process according to claim 5, wherein the alkali concentration in the sorbent material is in the range of 0.5 to 5% by weight calculated as sodium oxide or potassium oxide.

8. The process according to claim 1, wherein the sorbent material is present in two or more different forms to enhance the flow of the first shifted gas through the sorbent material.

9. The process according to claim 8, wherein the sorbent material is provided in two or more zones.

10. The process according to claim 9, wherein the zones are provided as layers within the first shift vessel.

11. The process according to claim 8, wherein the sorbent material is provided as fixed bed of particles comprising one or more horizontal layers above one or more annular layers disposed around a gas collector situated adjacent an outlet from the vessel.

12. The process according to claim 11, wherein the one or more annular layers comprise either an inert ceramic bed support material, a sorbent material, or an inert ceramic bed support material and a sorbent material.

13. The process according to claim 8, wherein the sorbent material is provided in 2, 3 or 4 zones.

14. The process according to claim 1, wherein the high-temperature shift catalyst comprises an iron-containing catalyst.

15. The process according to claim 1, wherein the water gas shift process is performed adiabatically in the each of the first and second shift vessels.

Description

(1) The Invention will now be further described by reference to the drawings in which;

(2) FIG. 1 is a depiction of one process according to the invention; and

(3) FIG. 2 is a depiction of one arrangement of sorbent materials in a first shift vessel.

(4) In FIG. 1, a synthesis gas mixture 10 having a CO content in the range 5-30 mole % on a dry gas basis and containing steam at a steam:synthesis gas volume ratio in the range 1:1 to 2.5:1, if fed to a heat exchanger 12 where the temperature is adjusted to a temperature between 300 and 450 C. and passed via line 14 to an inlet at the top of a first shift vessel 16 containing a first water gas shift catalyst 18. The first water-gas shift catalyst 18 is in the form of a fixed bed of a particulate high temperature shift catalyst. The particulate high temperature shift catalyst is suitably an iron-containing high temperature shift catalyst in the form of cylindrical pellets, such as Katalco 71-5. The water-gas shift reaction occurs as the synthesis gas mixture is passed downwards through the bed 18 to convert a portion of the CO to CO.sub.2 and form hydrogen. The first shifted synthesis gas mixture passes from the bed 18 to a fixed bed of particulate sorbent material 20 disposed downstream of the first water-gas shift catalyst 18 and within the first shift vessel 16. The sorbent material is suitably an alkalised alumina in the form of 4-hole quadralobe pellets. The sorbent material is effective at capturing the halogen contaminants and reducing the chloride content of the first shifted gas mixture. In this embodiment, the sorbent material 20 supports the bed of first water-gas shift catalyst 18 with the vessel 16. The sorbent material 20 may be divided into two or more zones (not shown), each having a different voidage and different pressure drop, that enhance the flow of the shifted gas through the bed 20. The sorbent material 20 is prevented from leaking from the vessel 16 by means of a gas collector 22 disposed about the outlet of the vessel. The gas collector 22 comprises a perforate member, such as a perforate screen or mesh sized to prevent the particles of sorbent material from passing through. The halogen-depleted first shifted gas mixture is recovered from the outlet of the first shift vessel 16 and fed via line 24 to a heat exchanger 26 in which the temperature of the first shifted gas mixture is adjusted to 170-300 C. The temperature-adjusted, halogen depleted first shifted gas mixture is passed from the heat exchanger 26 via line 28 to an inlet at the top of a second shift vessel 30 containing a second water gas shift catalyst 32. The second water-gas shift catalyst 32 is in the form of a fixed bed of a particulate copper containing catalyst. The particulate copper-containing catalyst is suitably a copper-containing low temperature shift catalyst in the form of cylindrical pellets, such as Katalco 83-3X. The water-gas shift reaction occurs as the first shifted gas mixture is passed downwards through the bed 32 to convert at least a portion of the remaining CO to CO.sub.2 and form hydrogen. The second shifted gas mixture passes from the bed 32 though a supporting bed of inert ceramic balls, pellets or lumps 34 disposed beneath the second water-gas shift catalyst 32 within the second shift vessel 30. The ceramic support material may suitably be high purity alumina spheres such as Katalco 92-1, available from Johnson Matthey PLC. The ceramic support material may be divided into two or more zones (not shown), each having a different particle size and/or voidage, that enhance the flow of the shifted gas through the bed support material 34. The ceramic support material 34 is prevented from leaking from the vessel 30 by means of a gas collector 36 disposed about the outlet of the vessel. The gas collector 36 may be the same type as that used in the first shift vessel. The second shifted gas 38, enriched in hydrogen and further depleted in carbon monoxide, is recovered from an outlet of the second shift vessel 30 and used in downstream processes.

(5) In FIG. 2 the bed of sorbent material 20 under the bed of first water-gas shift catalyst 18 at the bottom of the first shift vessel 16 is divided into four zones 40, 42, 44 and 46. The first and second zones 40, 42 comprise horizontal cylindrical layers of particulate sorbent materials. The first zone 40 is disposed immediately beneath the first water gas shift catalyst 18. The second zone 42 is disposed immediately beneath the first zone 40. The third and fourth zones 44, 46 are disposed as annular beds beneath the second zone 42. The third zone 44 is disposed as an outer annular bed in contact with the vessel wall and the fourth zone 46 as an inner annular bed in contact with the gas collector 22. The first zone may be separated from the first water-gas shift catalyst 18 by a perforate mesh or screen (not shown). If desired, the first and second zones 40, 42 may be separated from each other by a perforate mesh or screen, but with a suitable grading of the particle size, this is not necessary. The particle size of the sorbent materials in the first and second zones may be the same, but in a preferred embodiment the particle size in the second zone 42 is larger than that of the particles in the first zone 40. The third zone 44 is separated from the fourth zone 46 by a perforate mesh or screen (not shown). Optionally, the second and third zones may also be separated by a perforate mesh or screen. The third zone 44 is filled with a particulate sorbent material or particles of an inert ceramic bed support material. The particles of sorbent material or inert ceramic bed support material may be the same size as the sorbent material in the second zone 42 but in a preferred embodiment the particle size in the third zone 44 is larger than that of the particles in the second zone 42. The fourth zone 46 may be filled with a particulate sorbent material or particles of an inert ceramic bed support material. The particles of sorbent material or inert ceramic bed support material may be the same size as the sorbent material in the third zone 44 but in a preferred embodiment the particle size in the fourth zone 46 are larger than that of the particles in the third zone 44. In one embodiment, the fourth zone 46 is empty such that there is an empty space around the gas collector 22 defined by the perforate mesh or screen, which may be suitably reinforced. This arrangement offers a reduced overall pressure drop though the bed of sorbent material.

(6) In use the first shifted gas mixture emerging from the first water gas shift catalyst 18 passes to the first zone 40 and then the second zone 42. The sorbent material in the first and second zones removes at least a portion of the halogen contaminants from the shifted gas mixture at the exit temperature of the first water-gas shift catalyst. The shifted gas mixture then passes through the third and fourth zones 44, 46 to the gas collector 22 and then to the outlet of the vessel 16.

(7) The invention is further illustrated by reference to the following Examples.

EXAMPLE 1

(8) A laboratory fixed bed reactor was charged with 50 ml of a potassium-doped alumina sorbent material containing 2% wt K.sub.2O. A gas mixture mimicking a first shifted gas mixture and comprising (on a dry gas basis) 55% vol hydrogen, 25% vol nitrogen, 4% vol carbon monoxide, and 16% vol carbon monoxide and steam was passed through the sorbent material for 25 days at a flowrate of 13001/hr, a reactor temperature of 430 C., a pressure of 30 barg and a steam to dry gas volume ratio of 0.5:1. The HCl concentration in the gas feed was 11.5 ppbv. No chloride was detected in the exit gas, using a Kitigawa gas test tube, throughout the test period. After the test was completed, the sorbent material was recovered and analysed for Cl content. The inlet portion contained 330 ppm Cl and the exit portion 130 ppm Cl. The condensed water was also collected and analysed for potassium content by ICP IES. No potassium in the condensate was observed. The results showed the effectiveness and stability of the sorbent at capturing the HCl under the exit conditions of a first shift vessel.

EXAMPLE 2

(9) Various arrangements according to FIG. 2 were modelled to assess the overall pressure drop (dP) through the first shift vessel containing different particulate ceramic support and sorbent materials. The relevant support and sorbent data is shown in the following tables:

(10) TABLE-US-00001 TABLE 1 Ceramic support data KATALCO 90-1 DYPOR 607 KATALCO 92-1 90-1E 90-1H 90-1J Code FB EC FD 92-1G 92-1K (Small) (Medium) (Large) Diameter/ 16.0 40.0 85.0 25.0 75.0 25-50 50-100 100-200 Width [mm] Length [mm] 16.0 40.0 80.0 No of through- 1 1 1 holes hole Diameter 7.0 14.0 35.0 [mm]

(11) TABLE-US-00002 TABLE 2 Sorbent material data Sorbent material Code Sorbent 1 Sorbent 2 Diameter [mm] 13.0 16.0 Length [mm] 17.0 20.0 no. of holes 4 4 hole Diameter 3.5 4.4 [mm]

(12) DYPOR 607 is in the form of cylindrical rings, KATALCO 92-1 is in the form of spheres and Katalco 90-1 in the form of irregular lumps. Since the KATALCO 90-1 support material is made of irregular lumps, their size is given as a range (90-1E=25-50 mm, 90-1H=50-100 mm, 90-1J=100-200 mm). The sorbent materials, Sorbent 1 and Sorbent 2, were in the form of 4-holed, 4-fluted cylinders.

(13) Shift Vessel (I)

(14) A computer model of a first shift vessel (I) was based on high temperature shift vessel containing a particulate iron-based high temperature shift catalyst (Katalco 71-5/Katalco 71-6). The conditions were as follows;

(15) TABLE-US-00003 Flow 224.6 te/hr Density 8.993 kg/m.sup.3 Viscosity 2.28 10.sup.2 cP End ratio 4.426 h1 depth of zone 1 (40) 76.2 mm h2 depth of zone 2 (42) 25.4 mm h3 depth of zone 3 (44) to top of collector (22) 100.0 mm h4 height of collector (22) 528.6 mm d1 diameter of collector (22) 1028.6 mm d2 diameter of zone 4 (46) 1428.6 mm d3 diameter of vessel (16) 4080.0 mm

(16) TABLE-US-00004 TABLE 3 Pressure drop results of First Shift Vessel (I) Example 2(a) Example 2(b) Example 2(c) Sorbent in Sorbent in Sorbent in Comparative zones 1 & 2 zones 1-3 zones 1-3 Ceramic and ceramic and ceramic and ceramic Example 2(d) Support in support in support in support in Sorbent in Zones 1-4 zones 3 & 4 zone 4 zone 4 zones 1-4 Zone 1 90-1E Sorbent 1 Sorbent 1 Sorbent 1 Sorbent 1 Zone 2 90-1E Sorbent 1 Sorbent 1 Sorbent 1 Sorbent 1 Zone 3 90-1H 90-1H Sorbent 1 Sorbent 1 Sorbent 1 Zone 4 90-1J 90-1J 90-1J FD Sorbent 1 dP_support [bar] 0.027 0.029 0.050 0.037 0.167 dP_bed [bar] 0.181 0.181 0.181 0.181 0.181 dP_tot [bar] 0.208 0.210 0.231 0.218 0.348 dP_support [%] 13% 14% 22% 17% 48%

(17) In the comparative configuration, the support material has a pressured drop of 0.027 bar, contributing 13% of the total pressured drop (catalyst bed+support).

(18) If zones 1 and 2 are replaced by the chloride guard Sorbent 1 (Examples 2(a)), the pressure drop increases by a negligible amount (0.027 to 0.029 bar), and its contribution to the total pressure drop goes from 13% to 14%.

(19) The amount of material that can be placed in zones 1 and 2 is limited; therefore Example 2(b) considers filling zone 3 as well with the chloride guard Sorbent 1. In this case the support pressure drop increases to 0.050 bar (22% of total), which is still deemed acceptable.

(20) When the material currently inside zone 4 is replaced by DYPOR 607 FD (Example 2(c)), then the pressure drop is improved at 0.37 bar (17% of total).

(21) Example 2(d) illustrates what would happen if the bottom of the first shift vessel was filled with the chloride guard Sorbent 1. In this case the pressure drop in the support would be higher than the other examples.

(22) This example demonstrates that zones 1 and 2 may readily be filled with the chloride guard, and in case these zones are not large enough, a large additional volume of zone 3 can be filled with the chloride guard in the curved vessel bottom. In this case it is preferred that the zone 4 material contains a ceramic support material of large particle size.

(23) Shift Vessel (II)

(24) A model of another first shift vessel (II) was based on high temperature shift vessel containing a particulate iron based high temperature shift catalyst (Katalco 71-5/Katalco 71-6). The conditions were as follows;

(25) TABLE-US-00005 Flow 213.7 te/hr Density 8.731 kg/m.sup.3 Viscosity 2.25 10.sup.2 cP End ratio 3.52 h1 depth of zone 1 (40) 214.9 mm h2 depth of zone 2 (42) 190.8 mm h3 depth of zone 3 (44) to top of collector (22) 251.5 mm h4 height of collector (22) 215.9 mm d1 diameter of collector (22) 939.8 mm d2 diameter of zone 4 (46) 1778.0 mm d3 diameter of vessel (16) 3886.2 mm

(26) TABLE-US-00006 TABLE 4 Pressure drop results of First Shift Vessel (II) Example Example Example (2e) 2(f) 2(h) Sorbent in Sorbent in Sorbent in Comparative Comparative Zones 1 & Zones 1 & Example zones 1-3; Ceramic Ceramic 2; ceramic 2; ceramic 2(g) ceramic Support in Support in support in support in Sorbent in support in Zones 1-4 Zones 1-4 zones 3 & 4 zones 3 & 4 zones 1-4 zone 4 Zone 1 FB 92-1G Sorbent 1 Sorbent 1 Sorbent 1 Sorbent 1 Zone 2 EC 92-1G Sorbent 2 Sorbent 2 Sorbent 1 Sorbent 1 Zone 3 FD 92-1K EC FD Sorbent 1 Sorbent 1 Zone 4 FD 92-1K EC FD Sorbent 1 FD dP_support [bar] 0.031 0.161 0.105 0.038 0.605 0.062 dP_bed [bar] 0.192 0.192 0.192 0.192 0.192 0.192 dP_tot [bar] 0.223 0.353 0.298 0.230 0.797 0.254 dP_support [%] 14% 46% 35% 17% 76% 24%

(27) The comparative examples feature just support systems, and it can be seen that the graded support system gives a lower pressure drop.

(28) Example 2(e) replaces zones 1 and 2 with the Sorbent 1 and Sorbent 2 chloride guards, and it uses DYPOR 607 EC in the bottom of the converter. The pressure drop has clearly increased from the first comparative examples but it is still lower than the solution with the second comparative example.

(29) If a mesh is placed between zones 2 and 3 (Example 2f), allowing the bottom of the vessel to be filled with larger material, then the pressure drop goes back to a value just slightly higher than the first comparative example.

(30) Example 2(g), where Sorbent 1 is placed in all 4 zones, produces a higher pressure drop than the comparative examples.

(31) If a mesh screen is fitted around the gas collector (22) to define a zone 4 (46) and zone 4 is filled with very large material (Example 2(h)), then the pressure drop goes back to acceptable levels, the rest of the vessel bottom still being completely filled with chloride guard.