Method of starting-up a water gas shift reactor

Abstract

The invention comprises a method of operating a water gas shift reactor in a transient state such as during reactor start-up, the method comprising: providing a water gas shift catalyst comprising an alkali metal or alkali metal compound; heating the water gas shift catalyst up to the reaction temperature of the water gas shift reaction under steam condensing conditions, by applying steam as a heat transfer medium for the water gas shift catalyst, and where the water gas shift catalyst has a total pore volume larger than the volume of liquid water that forms during the heating.

Claims

1. Method of operating a water gas shift reactor in a transient state, the method comprising: providing a water gas shift catalyst comprising an alkali metal or alkali metal compound, said water gas shift catalyst being free of chromium (Cr) and iron (Fe); heating the water gas shift catalyst up to the reaction temperature of the water gas shift reaction under steam condensing conditions by applying steam as a heat transfer medium for the water gas shift catalyst, and where the water gas shift catalyst has a pore volume, as determined by mercury intrusion, larger than the volume of liquid water that forms during the heating; and wherein the pore volume of the water gas shift catalyst is in the range 100-800 ml/kg, as measured by mercury intrusion.

2. Method according to claim 1, wherein the pore volume of the water gas shift catalyst is in the range 400-800 ml/kg.

3. Method according to claim 1, wherein the water gas shift reactor is a low temperature shift (LTS) reactor, a medium temperature shift (MTS) reactor, or a high temperature shift (HTS) reactor.

4. Method according to claim 1, wherein the water gas shift catalyst comprises 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 compound is in the range 0.3-10 wt % based on the weight of oxidized catalyst.

5. Method according to claim 4, comprising only Zn, Al, optionally Cu, and an alkali metal or alkali metal compound.

6. Method according to claim 4, wherein the Zn/Al molar ratio is in the range 0.5-1.0 and the content of alkali metal is in the range 0.4-8 wt % based on the weight of oxidized catalyst.

7. Method according to claim 4, wherein the content of the alkali metal is in the range 1-6 wt %.

8. Method according to claim 4, wherein the content of Cu is in the range 0.1-10 wt %.

9. Method according to claim 1, wherein the heating up to the reaction temperature is conducted in the temperature range 100 C. to 600 C.

10. Method according to claim 1, wherein the water gas shift catalyst is heated up to the the reaction temperature of the water gas shift reaction by means of steam only.

11. Method according to claim 1, wherein the method is conducted at the steam condensing conditions i.e. heating at temperatures where liquid water is formed, of: about 12 atm abs with a dew point (T.sub.sat) of about 190 C.; or about 4.5 atm abs with dew point (T.sub.sat) of about 148 C.

12. Use of a water gas shift catalyst which comprises an alkali metal or alkali metal compound for the starting-up of a water gas shift reactor, the starting-up comprising heating the water gas shift catalyst up to the reaction temperature of the water gas shift reaction under steam condensing conditions by applying steam as a heat transfer medium for the water gas shift catalyst; said water gas shift catalyst being free of chromium (Cr) and iron (Fe), and having a pore volume in the range 100-800 ml/kg, as measured by mercury intrusion.

13. Use according to claim 12, wherein the water gas shift catalyst is a high temperature shift catalyst and the water gas shift reactor is a high temperature shift reactor.

14. Use according to claim 13, wherein the high temperature shift (HTS) catalyst comprises 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 compound is in the range 0.3-10 wt % based on the weight of oxidized catalyst.

15. Use according to claim 14, wherein the HTS catalyst comprises only Zn, Al, optionally Cu, and an alkali metal or alkali metal compound.

16. Use according to claim 12, wherein the content of the alkali metal is in the range 1-6 wt %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] FIG. 1 shows the increase of 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.

[0071] FIG. 2 shows the conversion of carbon monoxide with a HTS catalyst according to the invention with respect to different alkali metals (promoters), according to Example 2. For comparison, an unpromoted catalyst essentially containing no alkali metal compounds, is included.

[0072] FIG. 3 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 2.

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 in the interval of 100-800 ml/kg, for instance in the interval 200-600 ml/kg, e.g. 240-380 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.

[0075] A HTS catalyst of the potassium-promoted Zn/Al-type, which is applicable to the method of the invention, 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 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: 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 with this catalyst. 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

[0078] 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. 2 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.

[0079] FIG. 3 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.

[0080] It was also found that this feature compounded with the provision of a higher pore volume, for instance 240 ml/kg or 250 ml/kg or higher, e.g. 240-380 ml/kg, results in a surprisingly robust water gas shift catalyst with significant mechanical strength and no substantial loss of catalytic activity.