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Title:
METHOD FOR THE PURIFICATION OF RAW GAS
Document Type and Number:
WIPO Patent Application WO/2012/130258
Kind Code:
A1
Abstract:
A method for the purification of a raw gas comprising hydrogen, carbon oxides and nitrogen, which method comprises the steps of a) subjecting the raw gas to a methanation reaction and converting carbon monoxide and carbon dioxide contained in the raw gas to methane; b) drying the methanated raw gas; d) subjecting the dried methanated raw gas to an ammonia reaction and converting nitrogen in the gas to ammonia; e) removing ammonia from the raw gas from step d); and f) withdrawing a purified gas with a reduced content of carbon oxides and nitrogen.

Inventors:
NIELSEN POUL ERIK HOEJLUND (DK)
CENNI ROBERTA (DK)
WIX CHRISTIAN (DK)
Application Number:
PCT/EP2011/001572
Publication Date:
October 04, 2012
Filing Date:
March 29, 2011
Export Citation:
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Assignee:
HALDOR TOPSOE AS (DK)
NIELSEN POUL ERIK HOEJLUND (DK)
CENNI ROBERTA (DK)
WIX CHRISTIAN (DK)
International Classes:
B01D53/75; C01C1/04; C10K3/04; C10L3/08
Domestic Patent References:
WO2002072243A12002-09-19
Foreign References:
EP0011404A11980-05-28
US4148866A1979-04-10
EP0093502A11983-11-09
US3441393A1969-04-29
EP0574285A11993-12-15
EP0194765A21986-09-17
EP1375422A12004-01-02
Attorney, Agent or Firm:
HALDOR TOPSØE A/S (Kgs. Lyngby, DK)
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Claims:
Claims :

1. A method for the purification of a raw gas comprising hydrogen, carbon oxides and nitrogen, which method com- prises the steps of

a) subjecting the raw gas to a methanation reaction and converting carbon monoxide and carbon dioxide contained in the raw gas to methane;

b) drying the methanated raw gas;

d) subjecting the dried methanated raw gas to an ammonia reaction and converting nitrogen in the gas to ammonia; e) removing ammonia from the raw gas from step d) ; and f) withdrawing a purified gas with a reduced content of carbon oxides and nitrogen.

2. The method of claim 1, comprising a further step of removing a part of carbon monoxide contained in the raw gas by subjecting the gas to a water gas shift reaction prior to step a) .

3. The method of claim 1 or 2 , comprising the further step of subjecting the raw gas to a water removal step prior to the methanation in step a) . 4. The method of claim 1,2 or 3, further comprising an acid gas removal step prior to the methanation in step a) .

5. The method of claim 1,2,3 or 4, wherein the ammonia is removed in step e) by treating the raw gas in an aqueous ammonia wash step.

6. The method of claim 1,2,3,4 or 5, comprising the further step of withdrawing a split stream from the raw gas; treating the split stream in an acid gas removal step; and admixing at least a part of the thus treated split stream with the purified gas.

7. The method of claim 1,2,3,4,5 or 6, wherein the purified gas is further converted to substitute natural gas. 8. The method of claim 4,6 or 7, wherein at least part of the carbon dioxide being removed in the acid gas wash is recovered from the acid gas wash step and introduced into the purified gas prior to the conversion to the substitute natural gas .

9. The method of claim 1,2,3,4,5,6,7 or 8, wherein the raw gas is a tail gas from a Fischer-Tropsch process, a recycle gas from an oxygenate synthesis or oxygenate to gasoline process .

10. The method according to anyone of the preceding claims, wherein the ammonia being removed from the gas in step e) is utilized for the production of ammonium thiosul- phate .

Description:
Title: Method for the Purification of Raw Gas

The present invention is directed to purification of raw gas. In particular, the invention concerns removal of car- bon oxides and nitrogen from the raw gas.

Industrial raw gasses arise typically from gasification of coal, oil petroleum coke, biomass and the like, or as coke oven gas .

As used herein in the following, "raw gas" shall comprise any gas containing hydrogen, carbon oxides and nitrogen.

Typically, such raw gas is obtained by the above mention gasification process and off gas from the production of coke, the so called coke oven gas.

These gases contain hydrogen, which is inter alia a valuable reactant for use in the preparation of a number of bulk chemicals or for use as alternative fuel.

As an example, gasifier gas and coke oven gas may be employed in the preparation of substitute natural gas (SNG) . SNG must have a high content of methane and residual amounts of impurities arising during preparation of SNG must be low if a high calorific value of SNG is required, when using SNG as replacement of natural gas. Further raw gases being useful when purified by means of the inventive method are hydrogen and nitrogen containing purge gases and gases being recycled to hydrogen consuming processes .

Although the concentration of nitrogen in the raw gas from gasification may be minimised by operating the gasifier with high purity oxygen, nitrogen is still a component of coal, biomass, pet coke and other feed stocks being employed in the gasification process and will thus be released from the feed-stock and transferred to the raw gas produced by gasification.

Similar considerations apply for the coking process .

Carbon dioxide and optionally hydrogen sulphide, if present in the raw gas may be removed by means of a conventionally acid gas removal process, wherein the gas is treated with a chemical or physical carbon dioxide and hydrogen sulphide sorbent, like the known "Rectisol" or "Selexol" process. Common solutions to reduce the content of nitrogen in the raw gas are cryogenic processes, in which the gas is cooled below the boiling point of nitrogen.

Cryogenic processes are expensive to establish and to oper- ate. Thus, this invention suggests a method for the reduction of undesired impurities from a raw gas, including nitrogen, by conversion of nitrogen to ammonia, which is more convenient to remove from the raw gas than in the known methods and which at the same time may be used as feed- stock for the production of valuable chemicals, e.g. ammonium thiosulphate . The invention is in its broadest embodiment, a method for the purification of a raw gas comprising hydrogen, carbon oxides and nitrogen, which method comprises the steps of: a) subjecting the raw gas to a methanation reaction and con- verting carbon monoxide and carbon dioxide contained in the raw gas to methane;

b) drying the methanated raw gas;

d) subjecting the dried methanated raw gas to an ammonia reaction and converting nitrogen in the gas to ammonia;

e) removing ammonia from the raw gas from step d) ; and f ) withdrawing a purified gas with a reduced content of carbon oxides and nitrogen.

Nitrogen is converted to ammonia by catalytic reaction with hydrogen further contained in the raw gas by means of the known catalytic ammonia conversion process. Depending on the concentration of nitrogen in the gas, the ammonia conversion reaction may be carried out in a once-through operation .

Produced ammonia is preferably removed from the raw gas by conventional aqueous ammonia wash.

In order to allow the catalytic conversion of nitrogen to ammonia, it is necessary to remove essentially all amounts of carbon monoxide and carbon dioxide (carbon oxides) and water from the raw gas prior to introduction into the ammonia conversion reaction. To this end the raw gas is treated in a methanation step, wherein carbon monoxide and carbon dioxide react with hydrogen to methane in presence of a methanation catalyst. The methanated raw gas is subsequently dried by e.g. cooling and separation of condensed water and/or by contact with molecular sieves being able to remove water molecules from a gas stream. Those molecular sieves are known in the art and conventionally employed in the drying of moist gases .

It is preferred to convert part of the carbon monoxide in the raw gas by reaction with water in the water gas shift reaction and thereby increase the concentration of hydrogen in the gas and to decrease the carbon monoxide content. The main part of the carbon monoxide content will be removed from the shifted gas in the subsequent methanation step, as mentioned hereinbefore.

It is furthermore preferred to treat the raw gas from e.g. a gasifier in an acid gas removal step, wherein the amount of carbon dioxide and hydrogen sulphide, if further present in the raw gas, is reduced by chemical or physical absorption or adsorption according to known methods as already mentioned above.

The method of the invention is in particular useful in the preparation of SNG from raw gas being produced by gasifying of carbonaceous materials .

SNG is produced by catalytic methanation of carbon monoxide with hydrogen. As already discussed hereinbefore, carbon oxides are in the method according to the invention preferably converted to methane during purification of the raw gas, in case the purified gas is utilized for the prepara- tion of SNG. If a certain content of nitrogen can be tolerated in the final SNG product, it is according to a further embodiment of the invention possible to increase methane production by use of the purified raw gas, by withdrawing a split stream from the raw gas prior to its purification and treating the split stream solely in an acid gas removal step to reduce or remove the content of carbon dioxide from the split stream. In the acid gas removal, hydrogen sulphide which also may be present in the raw is also adsorbed in the chemical or physical wash. Spent washing solution is regenerated by desorbing carbon dioxide and optionally hydrogen sulphide. A part of the desorbed carbon dioxide can be recycled to the purified raw gas for use in the SNG synthesis.

Desorbed hydrogen sulphide may be utilized for the preparation of ammonium thiosulphate by reaction with ammonia be- ing obtained in the purification of the raw gas as e.g described in European patent no.1 375 422 .

The method of the invention is also useful to reduce the nitrogen concentration in recycle streams, thus affording a reduction of the recycle and, possibly, of the volume to purge, particularly for processes in which the additional amount of methane being formed in the methanation step is beneficial or can be handled downstream.