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: passing the synthesis gas mixture at an inlet temperature in the range 170-500° C. over a water-gas shift catalyst to form a hydrogen-enriched shifted gas mixture, wherein the water-gas shift catalyst is in the form of a cylindrical pellet having a length C and diameter D, wherein the surface of the cylindrical pellet has two or more flutes running along its length, said cylinder having no through-holes and domed ends of lengths A and B such that (A+B+C)/D is in the range 0.25 to 0.25, and (A+B)/C is in the range 0.03 to 0.30.

Claims

1. A process for increasing the hydrogen content of a synthesis gas comprising hydrogen, carbon monoxide, and carbon dioxide, the process comprising the step of passing a synthesis gas mixture comprising the synthesis gas and steam at an inlet temperature in the range of from 300° C. to 500° C. over a high temperature water-gas shift catalyst to form a hydrogen-enriched shifted gas mixture, wherein the high temperature water-gas shift catalyst is in the form of a cylindrical pellet having a cylindrical portion length C and diameter D, wherein the cylindrical pellet has two or more flutes running along its length, said cylinder having no through-holes and domed ends of lengths A and B such that (A+B+C)/D defines a ratio of overall length:diameter that is in the range 0.25 to 1.25, and (A+B)/C is in the range of from 0.05 to 0.25.

2. The process according to claim 1, wherein the synthesis gas is derived by catalytic steam reforming, autothermal reforming or secondary reforming a hydrocarbon or gasifying coal, petroleum coke or biomass.

3. The process according to claim 1, wherein the synthesis gas has a carbon monoxide content in a range of from 3 to 70 mole % on a dry-gas basis.

4. The process according to claim 1, wherein the volume ratio of the steam:the synthesis gas in the synthesis gas mixture is in a range of from 0.3:1 to 4:1.

5. The process according to claim 1, wherein the water gas shift catalyst is a high temperature water-gas shift catalyst and the inlet temperature is in a range of from 310° C. to 500° C.

6. The process according to claim 5, wherein the high temperature water-gas shift catalyst comprises one or more iron oxides stabilized with chromia and/or alumina and optionally zinc oxide and one or more copper compounds.

7. The process according to claim 6, wherein the high temperature water-gas shift catalyst is a chromia-promoted magnetite catalyst containing acicular iron oxide particles.

8. The process according to claim 1, wherein (A+B+C)/D is in a range of from 0.50 to 1.00.

9. The process according to claim 1, wherein (A+B)/C is in a range of from 0.10 to 0.25.

10. The process according to claim 1, wherein the cylindrical pellet has 3 to 12, flutes running axially along its length.

11. The process according to claim 1, wherein the flutes are equally spaced about the circumference of the cylindrical pellet.

12. The process according to claim 1, wherein the flutes are semi-circular, elliptical, or U shaped.

13. The process according to claim 12, wherein there are 3, 4 or 5 flutes present that have a width “d” in the range of 0.1 D to 0.4 D.

14. The process according to claim 1, wherein the two or more flutes running along the length of the cylindrical pellet have a cumulative flute width that does not exceed 35% of the circumference of the cylindrical pellet.

15. The process according to claim 1, wherein (A+B+C)/D is in a range of from 0.55 to 0.70.

16. The process according to claim 1, wherein (A+B+C)/D is in a range of from 0.55 to 0.66.

17. The process according to claim 1, wherein the cylindrical pellet has 3 to 7 flutes running axially along its length.

18. The process according to claim 1, wherein the cylindrical pellet has 3 to 5 flutes running axially along its length.

Description

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

(2) FIG. 1 is a side view, end view and isomeric depiction of a first catalyst pellet according to the present invention having three flutes,

(3) FIG. 2 is a side view, end view and isomeric depiction of a second catalyst pellet according to the present invention having four flutes, and

(4) FIG. 3 is a side view, end view and isomeric depiction of a third catalyst pellet according to the present invention having five flutes.

(5) FIGS. 1, 2 and 3 together depict water-gas shift catalyst pellets 10 in the form of solid cylinders 12 having a length C and diameter D, which have three, four or five flutes 14 along its length, equally-spaced around the circumferences of the pellets 10. The cylinders 12 have domed ends 16, 18 of lengths A and B. A and B are the same. The flutes 14 create equally sized lobes 20. The evenly spaced flutes are all semi-circular in cross section.

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

EXAMPLE 1

(7) Computer modelling of a series of high temperature shift catalysts catalyst was performed

(8) Examples 1a-1c relate to the 3-, 4- and 5-fluted domed cylindrical pellets depicted in FIGS. 1-3 respectively. Comparative example X is a commercially-available high temperature shift catalyst cylindrical pellet currently widely used. The dimensions of the pellets were as follows;

(9) TABLE-US-00001 (A + Flute size A B C D B + (A + Width/depth Example mm mm mm mm C)/D B)/C mm Comparative X 0 0 4.50 8.50 0.529 — — 1a 3 flutes 0.25 0.25 4.50 8.50 0.588 0.111 3.1/1.24 1b 4 flutes 0.25 0.25 4.50 8.50 0.588 0.111 2.3/0.93 1c 5 flutes 0.25 0.25 4.50 8.50 0.588 0.111 1.8/0.75

(10) Strength analysis: A COMSOL FEM software package produced simulations to assess the relative strengths of the shaped materials. A total of 10N load was applied vertically along the cross-section of the pellets. The shape was not allowed to be displaced by the applied force and the principle stress was reported along a line going through the centre of the pellet shape. (The reported values are those along the weakest plane if the shape has two directional planes). The results were normalised to the comparative example.

(11) Voidage analysis: A DigiPac™ software package was used to simulate the packing of material in a cylindrical bed. The dimensions of the packed bed were set to 170 mm ID and 240 mm length and the simulated voidage was noted at the centre of the bed length to avoid the impacts of the ‘end effects’. The resolution used was at 0.2 mm/pixel. The results were normalised to the comparative example.

(12) Simulation of the pellet strength and flow under the same conditions gave the following;

(13) TABLE-US-00002 Example Relative Crush Strength Relative Voidage X 1.00 1.00 1a 0.70 1.07 1b 1.00 1.07 1c 1.20 1.09

(14) The results show the catalyst units according to the invention have a higher voidage (and so improved pressure drop) and for 4 and 5 flutes, the same or better crush strength than the commercially available catalyst.

EXAMPLE 2

(15) A co-precipitated high temperature shift catalyst composition comprising a mixture of oxides of iron, chromium and aluminium and containing acicular iron oxide particles, was prepared according to U.S. Pat. No. 5,656,566. The powder composition was pelleted using a single punch press to the 5-fluted shape of Example 1c. The catalyst powder composition was doped with a small amount of graphite lubricant to aid pellet ejection from the pelleting die and pelleted to a typical product pellet density (1.8-2.0 g/cc) using normal production loads. The resulting fluted pellets had a strength equivalent to typical production cylindrical pellets of similar dimensions. A comparative cylindrical pellet was prepared from the same composition and pelleted in the same manner to the simple cylindrical shape of Comparative Example X.

(16) The pellets were tested for the water gas shift reaction on a typical hydrogen synthesis gas composition (comprising 15.4 vol % CO, 6.8 vol % CO.sub.2, 70.8 vol % Hz, and 7.0 vol % N.sub.2) at an inlet temperature of 300-450° C., a pressure of 27 barg, and a gas hourly space velocity (GHSV) of 85,000 hr.sup.−1. The % molar CO conversion was calculated by using an Emerson X-Stream 4 channel IR spectrometer to measure the CO concentration in the dry inlet and outlet gases and determine the volume of CO consumed during the reaction. The results were as follows;

(17) TABLE-US-00003 CO Conversion (mole %) Temperature (° C.) 300 325 350 375 400 425 450 Comparative X 3 5 12 20 28 33 35 Example 2 3 6 13 23 32 37 41

(18) The results indicate enhanced water gas shift conversion from the domed, fluted catalyst.

(19) The pressure drop through the bed of pellets was calculated based on the voidage numbers generated by the DigiPac™ software simulations and the use of the Ergun Equation. The results were as follows;

(20) TABLE-US-00004 Relative pressure drop Comparative X 1.0 Example 2 0.8

(21) The results indicate a reduced pressure drop from a bed of the domed, fluted catalyst. A reduced pressure drop in water gas shift offers considerable advantages in downstream processes in particular in hydrogen and ammonia plants.

EXAMPLE 3

(22) The comparative pellets and the domed, fluted pellets described in Example 2 were tested for the water gas shift reaction on a typical ammonia synthesis gas composition (comprising 14.0 vol % CO, 6.5 vol % CO.sub.2, 55.5 vol % Hz, 0.5 vol % CH.sub.4 and 23.5 vol % N.sub.2) at an inlet temperature of 300-450° C., a pressure of 27 barg, and a gas hourly space velocity (GHSV) of 85,000 hr.sup.−1. The % molar CO conversion was calculated by using an Emerson X-Stream 4 channel IR spectrometer to measure the CO concentration in the dry inlet and outlet gases and determine the volume of CO consumed during the reaction. The results were as follows;

(23) TABLE-US-00005 CO Conversion (mole %) Temperature (° C.) 300 325 350 375 400 425 450 Comparative X 3 5 12 20 28 33 35 Example 3 4 6 13 24 30 37 41

(24) The results, which are very similar to those observed for the hydrogen syngas in Example 2 indicate enhanced water gas shift conversion from domed, fluted catalyst.