Improved water gas shift catalyst

Abstract

The present disclosure relates to an improved water gas shift catalyst, in particular an improved high temperature shift catalyst and process using the catalyst. The water gas shift catalyst includes Zn, Al, optionally Cu, and an alkali metal or alkali metal compound, wherein the content of alkali metal, preferably K, is in the range 1-6 wt %, such as 1-5 wt % or 2.5-5 wt % based on the weight of oxidized catalyst, and wherein the water gas shift catalyst has a pore volume, as determined by mercury intrusion, of 240 ml/kg or higher, such as 250 ml/kg or higher. A process for enriching a synthesis gas in hydrogen by contacting the synthesis gas in a water gas shift reactor with the water gas shift catalyst.

Claims

1. Water gas shift catalyst comprising Zn, Al, optionally Cu, and an alkali metal or alkali metal compound, wherein the water gas shift catalyst is a Zn/Al-based catalyst comprising in its active form a mixture of zinc aluminum spinel and optionally zinc oxide in combination with an alkali metal compound selected from K, Rb, Cs, Na, Li and mixtures thereof, in which the Zn/Al molar ratio is in the range 0.3-1.5 and the content of alkali metal is in the range 1-6 wt % based on the weight of oxidized catalyst, and wherein the water gas shift catalyst has a pore volume, as determined by mercury intrusion, of 240 ml/kg or higher.

2. The water gas shift catalyst according to claim 1, having a pore volume, as determined by mercury intrusion, of 240-380 ml/kg.

3. The water gas shift catalyst according to claim 1, comprising only Zn, Al, optionally Cu, and an alkali metal or alkali metal compound.

4. The water gas shift catalyst of claim 1, wherein the Zn/Al molar ratio is in the range 0.5-1.0.

5. The water gas shift catalyst of claim 1, wherein the content of Cu is in the range 0.1-10 wt % based on the weight of oxidized catalyst.

6. The water gas shift catalyst of claim 1, wherein the catalyst is in the form of a pellets, extrudate, or tablet, and wherein the density is 1.2-1.9 g/cm3, as measured by dividing the weight of the catalyst by its geometrical volume

7. The water gas shift catalyst of claim 1, wherein the catalyst is in the form of pellets, extrudates or tablets, and wherein the mechanical strength is in the range ACS: 30-750 kp/cm2, or SCS: 4-100 kp/cm, wherein ACS and SCS are measured in the oxidized form of the catalyst, and according to ASTM D4179-11

8. Process for enriching a synthesis gas in hydrogen by contacting said synthesis gas in a water gas shift reactor with a water gas shift catalyst according to claim 1.

9. The process of claim 8, wherein the water gas shift reactor is a high temperature shift (HTS) reactor.

10. The process of claim 8, wherein the water gas shift reactor is a HTS reactor operating at a temperature in the range of 300-550 C., and optionally also at a pressure in the range 2.0-6.5 MPa.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] FIG. 1 shows the increase in temperature and thereby catalytic activity when feeding gas mixture after a number of start-ups in a HTS reactor, as a function of reactor length, in accordance with Example 1.

[0070] FIG. 2 shows the pore volume (PV) and mechanical strength (ACS, SCS) of catalysts according to Example 2.

[0071] FIG. 3 shows the conversion of carbon monoxide with a HTS catalyst according to the invention with respect to different alkali metals (promoters), according to Example 3. For comparison, an unpromoted catalyst essentially containing no alkali metal compounds, is included.

[0072] FIG. 4 shows the conversion of carbon monoxide with a HTS catalyst according to the invention with respect to the weight of potassium as the alkali metal (promoter) in the catalyst, according to Example 3.

DETAILED DESCRIPTION

Example 1

[0073] A start-up of a HTS reactor under condensing steam carried out at 11.85 atm abs (i.e. about 12 atm abs) with a dew point of T.sub.sat=188 C. (i.e. about 190 C.) is representative of an industrial case. The amount of condensate depends on the mass of steel, i.e. the reactor vessel, and the mass of catalyst contained in the reactor as well as on the initial temperature which is usually between 0 C. and 50 C. Table 1 shows typical volumes of liquid (water) that would form in industrial HTS units of small i.e. internal diameter of about 1 m, and big size i.e. internal diameter of about 5 m. It is apparent that the pore volume of the water gas shift catalyst, which by the invention is for instance in the interval 240-380 ml/kg, 250 -800 ml/kg, is sufficient to contain the total volume of liquid that condenses during the heating process.

TABLE-US-00001 TABLE 1 Water Water Total Internal Wall Bed Catalyst Initial condensed condensed condensate per diameter thickness length mass temperature at wall at catalyst catalyst mass [m] [m] [m] [ton] [ C.] [m3] [m3] [ml/kg] 5.186 0.098 3.295 67.9 50 1.52 3.27 70.6* 2 2.04 4.40 95.0 1.189 0.023 2.965 3.2 50 0.08 0.17 76.8 2 0.10 0.22 101.3 *Calculated as (1.52 + 3.27)/(67.9) 1000

[0074] The catalyst pellets or tablets that are in close proximity to the reactor wall are exposed to water that condenses to heat up the reactor vessel and the catalyst mass. This means that there is a region of the catalyst bed, which is confined to the periphery of the reactor, whose entire pore volume is utilized to contain the liquid that condenses at the reactor wall. The width of this region depends on the pore volume of the catalysts, and it was found that close to the reactor wall, the larger pore volume enables taking up the additional water being condensed at the wall.

Catalyst A

[0075] A HTS catalyst of the potassium-promoted Zn/Al-type is the catalyst according to example 1 of applicant's patents U.S. Pat. Nos. 7,998,897 or 8,404,156, and where the powder of ZnAl.sub.2O.sub.4 (spinel) and ZnO includes Cu by co-precipitation with a copper salt. The pore volume, as determined by mercury intrusion measurements, tablet density (as measured by simpy dividing the weight of the tablet by its geometrical volume), and potassium content, as measured by the ICP method, as well as copper content, is as follows: pore volume 229 ml/kg, tablet density 1.8 g/cm.sup.3, K content: 1.97 wt %, Cu content: 2.71 wt %, based on weight of oxidized catalyst.

[0076] A series of start-ups under condensing steam were carried out in a pilot plant. In the beginning of the tests and after each start-up, the catalyst was exposed to HTS conditions with a gas mixture containing 35 vol % H.sub.2O, 16 vol. % CO, 4 vol. % CO.sub.2, balance H.sub.2, with the reactor operating in (pseudo) adiabatic mode. The increase in the temperature along the reactor length, corresponding to the fraction of the catalyst bed in %, of the accompanying figure, is a direct indication of the catalytic activity. FIG. 1 shows that there is only a marginal loss of activity after the first start-up under condensing steam, and the activity remained unaffected in the subsequent tests.

[0077] The pilot studies have also shown that start-up procedures in condensing steam with conditions that replicate those of an industrial condensing start-up provided about 50 start-ups without substantial loss of activity.

Example 2

Catalyst B, C

[0078] Improved HTS catalysts also of the potassium-promoted Zn/Al-type, in accordance with the present invention were also tested. Accordingly, two catalysts were prepared according to Example 1 of applicant's patents U.S. Pat. Nos. 7,998,897 or 8,404,156, and where the powder of ZnAl.sub.2O.sub.4 (spinel) and ZnO included Cu which was incorporated by co-precipitation of a copper salt. Further, the pore volume of the particles was tailored to 240, 250 ml/kg or higher, for instance in the range 240-380 ml/kg in accordance with the present invention, by compactifying e.g. pelletizing the particles (e.g. tablets) from a powder starting catalyst material, for instance after calcining the catalyst, impregnating it with a solution comprising an alkali compound such as a solution of K.sub.2CO.sub.3, and final mixing with a lubricant such as graphite, as disclosed in Example 1 of the above U.S. Pat. No. 7,998,897 or the above U.S. Pat. No. 8,404,156, yet prior to pelletizing. Thus, instead of compactifying the powder to a catalyst having a density of 1.8 or 2.1 g/cm.sup.3 as in example 1 of U.S. Pat. Nos. 7,998,897 or 8,404,156, respectively, the compactifying of the present invention intentionally and surprisingly is conducted to form less dense tablets. The pore volume, as determined by mercury intrusion measurements, tablet density as measured by simply dividing the weight of the tablet by its geometrical volume, and potassium content, as measured by the ICP method, as well as copper content, is as follows:

[0079] Catalyst B: pore volume 451 ml/kg, tablet density 1.4 g/cm.sup.3, K content: 1.66 wt %, Cu content: 3.81 wt %, based on weight of oxidized catalyst.

[0080] Catalyst C: pore volume 320 ml/kg, tablet density 1.7 g/cm.sup.3, K content: 3.80 wt %, Cu content: 3.56 wt %, based on weight of oxidized catalyst.

[0081] Despite Catalysts B and C being made less dense than Catalyst A, the mechanical strength of the former catalysts is maintained so as not impairing catalyst performance. Catalyst B and C show that start-up procedures in condensing steam with conditions that replicate those of an industrial condensing start-up, result for these catalysts in over 100 start-ups without substantial leaching and thereby without substantial loss of activity.

[0082] Additional samples D-1 in below Table 2 were prepared by compactifying in a small, hand-fed tableting machine (so-called Manesty machine) a single batch of powder made according to Example 1 of applicant's U.S. Pat. No. 7,998,897 and with a Zn/Al molar ratio of 0.6. Higher mechanical strengths for the same densities are achievable by conducting the tableting with automated full-scale devices such as a Kilian RX machine with rotary press. For tablet densities of 1.45-1.75 g/cm.sup.3, we achieved SCS in the range 50-100 kp/cm and ACS 300-750 kp/cm.sup.2 using such device, with values of PV in the range 450-300 ml/kg, thus similar to what was obtained for samples made on the Manesty machine with similar tablet densities. ACS and SCS are measured in the oxidized form of the catalyst. Further, the above mechanical strengths are measured in compliance with ASTM D4179-11.

TABLE-US-00002 TABLE 2 Mechan- Mechan- ical ical Pore Den- Den- strength, strength, volume, Tablet D H sity sity, SCS ACS PV sample mm mm g/cm.sup.3 g/cm.sup.3 kp/cm kp/cm.sup.2 ml/kg D 5.89 3.88 1.8 1.8 35 319 279 0.05 E 5.87 3.85 1.65 1.65 26 217 331 0.05 F 5.86 3.85 1.55 1.55 14 133 400 0.05 G 5.86 5.76 1.5 1.5 10 101 426 0.05 H 5.86 3.91 1.4 1.4 6 57 499 0.05 I 5.86 3.72 1.25 1.25 5 39 548 0.05

[0083] FIG. 2 shows the pore volume (upper curve) and mechanical strength (ACS kp/cm.sup.2 or SCS kp/cm) of the data of Table 2. FIG. 2 shows clearly, that it is possible to increase the pore volume (PV) by lowering the tablet density and yet still enabling that the mechanical strength is sufficiently high (both ACS and SCS) even for low densities. For instance, even at the lower density of 1.25 g/cm.sup.3, the SCS is 5 kp/cm or alternatively ACS is 39 kp/cm.sup.2, which is sufficient mechanical strength for operation with the high temperature catalyst.

Example 3

[0084] A HTS catalyst of the potassium-promoted Zn/Al-type is the catalyst without copper according to e.g. example 1 of applicant's patents U.S. Pat. No. 7,998,897. FIG. 3 shows the effect of the alkali metals on catalytic activity in terms of CO-conversion at 380 C., in particular the high promoting effect of K, Rb and Cs. The conversion was measured on an aged catalyst. The aging was done by exposing the catalyst to increasing temperature from 330 to 480 C. within a period of 36 hours. For instance, K presents an increase in activity of about 4.5 times with respect to the non-promoted catalyst, while Rb and Cs result in a catalytic activity about 4 times higher with respect to the non-promoted catalyst.

[0085] FIG. 4 shows the CO-conversion for potassium as the alkali metal, which surprisingly shows a high promoting effect in particularly the range 1-6 wt % or 1-5 wt %. By operating with a catalyst having more potassium, e.g. about 6 wt %, any leaching of K will actually result in an increase of catalytic activity. If the content of potassium is e.g. 2.5-5 wt %, any leaching of K will still maintain or increase the catalytic activity. The catalyst acts as an alkali-buffer and thereby the catalytic activity is not impaired significantly. For instance, leaching of, say 10% (relative) of the potassium, would decrease the K content from, say 4 wt % K, to 3.6 wt % K, which will not decrease the catalyst activity. In fact, if the initial K content is 5 wt % K, the activity wouldon a 10% (relative) leachingincrease, since a catalyst with 4.5 wt % K has higher activity than a catalyst with 5 wt % K. This feature compounded with the provision of a higher pore volume in accordance with the present invention, results in a surprisingly robust water gas shift catalyst with significant mechanical strength and no substantial, if any, loss of catalytic activity.