Process For Producing Hydrogen From CO-rich Gases

Abstract

The disclosure relates to a process for enriching a synthesis gas in hydrogen by contacting said synthesis gas with a water gas shift catalyst, the synthesis gas being a CO-rich synthesis gas including at least 15 vol % CO and at least 1 ppmv sulfur, and the water gas shift catalyst including Zn, Al, optionally Cu, and an alkali metal or alkali metal compound; the water gas shift catalyst is free of chromium (Cr) and iron (Fe), and has a pore volume, as determined by mercury intrusion, of 240 ml/kg or higher.

Claims

1. Process for enriching a synthesis gas in hydrogen by contacting said synthesis gas with a water gas shift catalyst in a water gas shift reactor, said synthesis gas being a CO-rich synthesis gas comprising at least 15 vol % CO and at least 1 ppmv sulfur, the water gas shift catalyst comprising Zn, Al, optionally Cu, and an alkali metal or alkali metal compound, said water gas shift catalyst being free of chromium (Cr) and iron (Fe), and wherein the water gas shift catalyst has a pore volume, as determined by mercury intrusion, of 240 ml/kg or higher.

2. Process according to claim 1, the water gas shift catalyst is a high temperature shift (HTS) catalyst and the water gas shift reactor is a HTS reactor operating at a temperature in the range of 300-570 C., and optionally also at a pressure in the range 2.0-6.5 MPa.

3. Process according to claim 2, wherein the HTS reactor is an adiabatic HTS-reactor without recycle.

4. Process according to claim 1, wherein the CO-rich synthesis gas comprises at least 20 vol % CO but no more than 60 vol % CO.

5. Process according to claim 4, the CO-rich synthesis gas comprises: CO 30-60 vol % H.sub.2O 30-50 vol % CO.sub.2 0-5 vol % H.sub.2 0-20 vol %.

6. Process according to claim 1, further comprising a step for producing said synthesis gas, said step being any of: steam reforming of a hydrocarbon feed gas such as natural gas or naphta; by partial oxidation of the hydrocarbon feed gas; autothermal reforming (ATR) of the hydrocarbon feed gas; thermal decomposition of a carbonaceous material including gasification or pyrolysis of a solid carbonaceous material; combinations thereof.

7. Process according to claim 1, 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.

8. Process according to claim 1, wherein the water gas shift catalyst comprises only Zn, Al, optionally Cu, and an alkali metal or alkali metal compound.

9. Process according to claim 1, 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.

10. Process according to, wherein the content of alkali metal, preferably K, is in the range 1-6 wt %.

11. Process according to claim 1, wherein the content of Cu is in the range 0.1-10 wt %, based on the weight of oxidized catalyst.

12. Process according to claim 1, wherein the water gas shift catalyst is in the form of pellets, extrudates, or tablets, and wherein the particle density is 1.25-1.75 g/cm.sup.3, as measured by dividing the weight of the catalyst by its volume.

13. Process according to 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/cm.sup.2, or SCS: 4-100 kp/cm, wherein ACS and SPS are measured in the oxidized form of the catalyst, and according to ASTM D4179-11.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0072] The accompanying sole FIGURE shows a plot of the thermal stability of Catalyst A during high shift operation of Example 2.

DETAILED DESCRIPTION

Examples

Example 1. Preparation of Catalyst AAccording to Invention Embodiment

[0073] The catalyst was prepared according to the procedure given in applicants patent U.S. Pat. No. 7,998,897 Example 1 by adjusting the composition. According to ICP analysis, Catalyst A contains 1.99 wt % K, 1.65 wt % Cu, 34.3 wt % Zn, 21.3 wt % Al. Accordingly, the Zn/Al molar ratio is 0.665. The catalyst was shaped as 66 mm tablets. Furthermore, there is provided a pore volume (PV) of about 320 ml/kg and tablet density, as measured by simply dividing the weight of the tablet by its geometrical volume, of 1.7 g/cm.sup.3.

Example 2. Thermal Stability of Catalyst A

[0074] The test was carried out in a tubular reactor (ID 19 mm) heated by three external electrical heaters. 40 g of tablets of catalyst A was loaded. The gas composition was 9.4 vol % CO, 37.6 vol % H.sub.2O, 6.1 vol % CO.sub.2, 45 vol % H.sub.2, 1.9 vol % Ar. The experiments were conducted at 2.35 MPa. The duty of the three external electrical heaters was adjusted, so as to obtain almost isothermal conditions. The catalyst bed temperature was measured by 10 internal thermoelements and the difference between the inlet temperature and the exit temperature was always less than 2 C. The concentration of all components was regularly measured in both inlet and dry exit gas by GC (calibrated towards a gas mixture of known composition). All measurements were carried out at 397 C. (exit temperature) at a gas hourly space velocity GHSV=20000 NI/kg/h. Catalyst ageing (in between measurements) was done by maintaining all operational parameters except the temperature, which was raised to 570 C. The activity at 397 C. expressed as space-time yield (STY) in mol/kg/h as a function of time on stream is shown in the accompanying FIGURE. It is clearly seen that after an initial decline in activity the catalyst stabilizes after 400-600 hours and is practically unchanged for the remaining duration of the test.

[0075] In this example the ageing temperature of 570 C. was obtained by external heating instead of by using a CO-rich gas, i.e. the example represents the thermal exposure which results from using a CO-rich gas. This was done because the experimental setup allowed for much better temperature control this way. A temperature of 570 C. would be reached in the exit of an adiabatic rector by equilibrating a CO-rich gas with the composition 35 vol % CO, 45 vol % H.sub.2O, 5 vol % CO.sub.2 and 15 vol % H.sub.2 with an inlet temperature of around 350 C.

Example 3. Tolerance Towards Dry Synthesis Gas

[0076] As a test for the tolerance towards low oxygen/carbon ratio, Catalyst A was exposed to dry synthesis gas for 1.4 hour. A dry synthesis gas is a highly reducing gas having no H.sub.2O and with a low molar O/C-ratio, i.e. 1.5 or lower. The dry synthesis gas according to the present example had the composition 47.5 vol. % H.sub.2, 45.7 vol. % CO, 4.8 vol. % CO.sub.2, 2.0 vol. % Ar, with an oxygen/carbon (0/C) ratio of 1.10. This exposure was induced after 49 hours of operation in a normal (wet) synthesis gas. The pressure drop over the reactor, P, was measured before and after the exposure. Before and after the exposure, 120 Nl/h of normal (wet) synthesis gas was fed, having the composition 29.7 vol % H.sub.2, 28.6 vol % CO, 3.0 vol % CO.sub.2, 1.3 vol % Ar and 37.5 vol % H.sub.2O, with an O/C ratio of 2.28. The pressure at the reactor outlet was controlled by a back-pressure regulator with a setpoint of 5.07 MPa. The evolution of the pressure difference P between the outlet and the inlet of the reactor, measured after exposure to the dry synthesis gas and again operating in the wet synthesis gas with O/C=2.28, was followed. It was found that the pressure drop is very small, less than 0.5 bar, and almost the same before and after exposure to the dry synthesis gas.

Example 4. Comparative

[0077] A Cu-promoted Fe/Cr catalyst (Catalyst B) was submitted to the same test as described in Example 3, the only difference being that the exposure to dry synthesis gas was induced 73 hours after normal operation. The increase in pressure drop after exposure to the dry synthesis gas was found to be substantial, approximately 15 bar.

[0078] Clearly, the tolerance towards the low O/C synthesis gas is very high for Catalyst A while it is very low for Catalyst B, the Cu-promoted Fe/Cr catalyst.