METHOD AND SYSTEM FOR OBTAINING A HYDROGEN RICH GAS

Abstract

The present invention relates to a method for obtaining a hydrogen rich gas from a natural gas comprising gas stream. The present invention relates to a system for obtaining a hydrogen rich gas from a gas stream comprising natural gas. The invention can be used in a chemical plant for hydrocarbon synthesis.

Claims

1. A method for obtaining a hydrogen rich gas from a natural gas comprising gas stream, said method comprising the following steps: (1) feeding said natural gas comprising gas and an appropriate amount of steam to a reforming unit comprising at least a steam methane reformer (SMR) and optionally a pre-reforming reactor up stream of the SMR, obtaining a first effluent; (2) feeding said first effluent and optionally an appropriate amount of steam through a high, medium or low temperature shift reactor(s) or a combination thereof to convert at least part of the carbon monoxide and water into hydrogen and carbon dioxide, to obtain a second effluent; (3) optionally, removing bulk water from the second effluent obtained in steps (1) or (2); (4) feeding the second effluent of step (2) and/or (3) through a pressure swing adsorption (PSA) unit operated such that a hydrogen rich gas stream is obtained wherein an off gas is added to the natural gas comprising gas stream and/or the first effluent obtained in step (1), wherein the off gas provided upstream of the reforming unit is mixed with steam prior to being added to the natural gas comprising gas stream.

2. The method according to claim 1 wherein step 4 comprises the following steps: (A) feeding the second effluent obtained in step (2) and/or (3) through one or more columns in the PSA unit, said one or more columns comprising an adsorbent bed, wherein the adsorbent bed comprises alumina, a carbon molecular sieve, silicalite, activated carbon, a zeolite, or mixtures thereof, with upon commencement of said feeding, the bed and column being pre-saturated and pre-pressurized to a pressure in the range of 20 to 80 bar absolute (bar a), and discharging a third effluent from the other end of said bed, and continuing said feeding and said discharging until a nitrogen and/or argon comprising gas has reached at least 45% of the length of the bed and has reached at most 80% of the length of the bed, calculated from the end of the bed at which the second effluent is being fed; (B) ceasing the feeding of the second effluent, and reducing the pressure in the column and the bed by about 2 to 25 bar a; and (C) further reducing the pressure of the column and adsorbent bed to a pressure in the range of 1 to 5 bar a; and (D) rinsing the column and adsorbent bed by feeding a gas, through the column and adsorbent bed: the column and bed being at a pressure in the range of 1 to 5 bar a; and (E) pressurizing the column and adsorbent bed to a pressure in the range of 15 to 75 bar or comprising 80 to 99.9 volume % hydrogen.

3. The method according to claim 1 wherein said off gas is generated by a synthesis reaction of hydrocarbons from synthesis gas, comprising gas stream and the first effluent obtained in step (1).

4. The method according to claim 1 wherein the off gas is fed through the steam methane reforming reactor in step (1) and/or through the high, medium or low temperature shift reactor(s) in step (2).

5. The method according to claim 3 wherein the off gas comprises (in volume percentage based on the total volume of the off gas): TABLE-US-00004 Methane 1-50 vol %; Carbon Monoxide 10-45 vol %; Carbon Dioxide 10-65 vol %; Hydrogen 5-80 vol %; Nitrogen 0.5-55 vol %; Argon 0-55 vol %.

6. The method according to claim 3 wherein the gas fed to the high, medium or low temperature shift reactor(s) or a combination thereof comprises (in volume percentage based on the total volume of the gas fed): TABLE-US-00005 Methane 1-50 vol %; Carbon Monoxide 5-45 vol %; Carbon Dioxide 5-65 vol %; Hydrogen 5-80 vol %; Nitrogen 0.001-55 vol %; Argon 0-55 vol %.

7. The method according to claim 3 wherein the second effluent comprises (in volume percentage based on the total volume of the second effluent): TABLE-US-00006 Methane 4-20 vol %; Carbon Monoxide 1-10 vol %; Carbon Dioxide 10-40 vol %; Hydrogen 40-95 vol %; Nitrogen 0.001-10 vol %; Argon 0.0001-5 vol %.

8. A system for obtaining a hydrogen rich gas from a gas stream comprising natural gas, comprising, connected in series: one or more reforming units, each unit comprising at least a pre-reforming reactor and a steam methane reforming reactor and optionally a pre reforming reactor; one or more high, medium or low temperature shift reactor(s) or a combination thereof to convert at least part of the carbon monoxide and steam into hydrogen and carbon dioxide; and one or more pressure swing adsorption units.

9. The system according to claim 8 wherein the pressure swing adsorption unit comprises: one or more columns, comprising an adsorbent bed, wherein the adsorbent bed comprises alumina, a carbon molecular sieve, silicalite, activated carbon, a zeolite, or mixtures thereof.

10. The system according to claim 8 wherein the system comprises upstream of the one or more steam methane reforming reactors an inlet for adding off gas to the natural gas stream wherein the off gas originates from a hydrocarbon synthesis reactor such as a Fischer-Tropsch reactor.

11. The system according to claim 8 wherein the system comprises upstream of one or more high, medium or low temperature shift reactor(s) or a combination thereof, an inlet for adding off gas to the first effluent wherein the off gas originates from a hydrocarbon synthesis reactor such as a Fischer-Tropsch reactor.

12. The system according to claim 8 wherein the system comprises: a further PSA unit comprising one or more columns provided down-stream of the first PSA unit, said one or more columns comprising an adsorbent bed, the adsorbent bed comprising alumina, a carbon molecular sieve, silicalite, activated carbon, a zeolite, or mixtures thereof.

Description

[0085] The invention will be further illustrated by the figures. The figures represent preferred embodiments of the invention and are not intended to limit the present invention.

[0086] FIG. 1 schematically depicts a system according to the present invention with no off gas added.

[0087] FIG. 2 schematically depicts a system according to the present invention with off gas addition upstream of an SMR reactor.

[0088] FIG. 3 schematically depicts a system according to the present invention with off gas addition downstream of the SMR reactor only.

[0089] FIGS. 4, 5 and 6 schematically depicts a system according to the present invention with off gas addition both upstream and downstream of the SMR reactor.

[0090] In the figures systems according to the present invention are depicted. In these Figures 1 represents an SMR reactor, 2 CO shift reactor and 3 a PSA unit. Item 4 indicates the natural gas comprising gas stream and 6 the enriched hydrogen gas stream. Item 7 indicates the gas stream comprising the remainder of the constituents (waste stream of the PSA unit). Item 8 depicts the steam stream. In FIGS. 1-4 this stream is added to the natural gas comprising gas stream (4) and in FIG. 5 the steam is added to the off gas stream (5). In addition in FIG. 6 steam is added to the off gas stream downstream of the SMR reactor.

[0091] Besides the systems depicted in the figures other options of adding steam are possible, such as adding steam directly to and separately from the off gas, to the first effluent exiting the SMR reactor.