GB2165552A - Methanating synthesis gas - Google Patents

Abstract

Synthesis gases, to be methanated, are contacted with a carbon dioxide rich liquor prior to methanation and after methanation are treated with a CO2 lean liquor to selectively absorb the CO2, the liquor containing absorbed CO2 is recycled for stripping with synthesis gas to be methanated.

Description

SPECIFICATION Upgrading synthesis gases This invention relates to the production of fuel gases, and more particularly to the upgrading of gases derived from the partial oxidation of carbonaceous materials such as oil and coal.

Synthesis gas, for example those produced by the ash-slagging gasification of coal are not directly suitable for use as fuel gas e.g. as substitute natural gas, but require to be upgraded by, for example reaction of carbon monoxide (present in the synthesis gas) and steam to give carbon dioxide and hydrogen and thereafter reacting the carbon dioxide and hydrogen in the presence of a methanation catalyst to yield a methane-rich product gas.

A number of processes for upgrading the primary synthesis gas are known, for example, the HICOM process described in our UK Patent Specification No. 1544245 (the word "HICOM" is a registered Trade Mark). In the HICOM process, control over the methanation reactions is achieved by recycling a portion of the methanation reaction product. Since a pressure drop occurs through the methanation catalyst bed, the recycle gas at temperatures of the order of 450"C has to be pumped back to the inlet pressure. The energy and handling costs of recycling product gas only serve as a load in the overall construction and operating costs.

We now propose a process whereby the methanation reactions can be readily controlled and without recourse to the expensive recycling of hot product gas.

In accordance with the present invention there is provided a process for the production of methanecontaining gases, wherein a primary synthesis gas containing hydrogen and oxides of carbon is treated to remove acidic gases therefrom and thereafter is subjected to at least one methanation reaction characterised in that prior to methanation, a liquor containing dissolved or absorbed carbon dioxide is contacted with purified synthesis gas and stripped of its carbon dioxide and that after said methanation, a substantially carbon dioxide-free solvent for carbon dioxide is contacted with the methanated gas thereby to physically absorb carbon dioxide present in said methanated gas and to provide a carbon dioxide-rich liquor for contact with purified synthesis gas.

The primary synthesis gas may be any gas which can be methanated viz. it contains hydrogen and oxides of carbon. The method is particularly suitable for upgrading synthesis gas produced by the gasification of coal, especially those gases produced by ash-slagging gasification.

The synthesis gas is first purified to remove acid gases therefrom, including hydrogen sulphide, carbonyl sulphide and carbon dioxide. This process may be a non-selective physical absorbtion process.

The purified gas is then contacted, in a suitable vessel, for example a stripper, with a CO2-rich liquor. This rich liquor will have come from a down-stream absorber where the carbon dioxide has been picked up after contact with methanated gas.

Thus, it will be seen that carbon dioxide moves around a loop, and can be recycled time and time again so that a high level of carbon dioxide builds up in the gas stream to be methanated. This level may be as high as 50%. The presence of carbon dioxide suppresses excessive methanation reactor temperature risers. This is highly advantageous when applied to gases produced by ash-slagging gasification which do not contain large excesses of steam. It is conventional technology to use steam to moderate methanation reactions in addition to recycling product gas and in the case of upgrading ash-slagging synthesis gases, the cost of raising such high quality steam would add an economic burden to the process.

The methanation process chosen may be any of those known conventionally, for example, those disclosed in UK Patent Specification No. 1544245 and UK Patent Specification No. 2116581. Included in any such methanation processes are any necessary or desirable shift conversion stages.

In addition to economic advantages conferred on the methanation process, the process of the invention has a further economic advantage over processes requiring hot product gas recycle in that since the carbon dioxide is recycled (absorbed) in a liquid solvent pumping procedures are simpler and far less expensive than those for hot gas recirculation.

After methanation the product gas is contacted with a physical solvent which removes carbon dioxide specifically. Carbon dioxide absorption is known technology and examples of known processes are the Selexol Process and BASF MPE. The presence of high concentrations of carbon dioxide in the product increases the efficiency of the CO2 absorption process since the absorption power for carbon dioxide increases in direct proportion to the partial pressure of the carbon dioxide in the gas to be heated.

The absorption process removes carbon dioxide to meet the specification required for the product gas.

The CO2 rich absorbent is then recycled back to the stripper stage where the CO2 is yielded to fresh synthesis gas. Any adjustment in the CO2 content of the rich liquor, i.e. to prevent build up of carbon dioxide in the loop, can be met by flashing the rich liquor at pressure to produce a carbon dioxide-containing gas having a preferred pressure of at least 10 bar. The flashed carbon dioxide can be passed through a membrane separator, for example, to recover methane which is added back to the product gas. The high pressure carbon dioxide stream (e.g. at 15 bar) recovered from such separation can be turbo-expanded and used for generating "cold" e.g. for use with cryogenic separating processes, or may be used for generating power.

For example, the carbon dioxide may be admixed with an oxygen-containing gas, e.g. air and passed over a catalyst to promote low temperature combustion after which the combustion gases are turbo-expanded.

Alternatively, the carbon dioxide may be used for enhanced oil recovery techniques.

The present invention will be illustrated by the following Example and with reference to the accompanying drawings in which Figure 1 is a general schematic block diagram of a process train in accordance with the present invention and Figure 2 is a block diagram showing an embodiment of the invention.

Referring to Figure 1 a synthesis gas is subjected to non-selective purification in unit 1 and passed to the base of a stripping tower 3. The off-gases (acid) from 1 may be further treated in 2. The gas stream A entering stripper 3 has the composition shown in the Table (Column A), and, in ascending counter currently strips carbon dioxide from a descending stream of liquor. Exiting from the stripper are a CO2 rich gas stream 4 and a lean-liquor/solvent stream 5 which is cooled to about -3 C. Stream 4 is heated to about 300"C and mixed with steam prior to reaction in the methanation train 6.Typically, the methanation train will comprise 3 shift/methanation reactors (not shown), wherein the gas is cooled between stages from about 450"C to about 300"C prior to reaction in the next methanation stage. The product exiting the final methanation will typically have a composition as shown in Column B of the Table.

The product gas is then passed into the base of an absorber column 7 and ascends through a descending stream of absorbent liquor from Line 5. Exiting from absorber 7 is a methane-rich, substantially CO2-free stream 8 having the composition shown in Column C of the Table and a CO2-rich liquor stream 9.

As shown in Figure 1, stream 9 may be divided into two streams 10 and 11, stream 10 being recycled back to stripper 2, and stream 11 being flashed in unit 12 to give carbon dioxide 13 at medium pressure e.g. at 10 bar or more and a lean liquor stream 14 which is combined with stream 10. Alternatively the lean liquor stream may be fed separately (not shown) to stripper 3 at a point below the feed point for stream 10.

In an alternative embodiment shown in Figure 2, all of the rich liquor is throttled or turbo-expanded (in unit 15) and flashed in unit 16 to off gas 17 and a liquor stream 18. Although stream 18 is leaner in CO2 than stream 11, it is still a C02 rich liquor.

Stream 18 is then pumped to stripper 3 (Figure 1) whilst stream 17 is subjected to membrane separation in prism 19. The product gas from 19 is a methane rich stream 21 which after compression 20 can be admixed with the methanation product stream B, and a high pressure carbon dioxide stream 22 eg. at 15 bar.

The high pressure carbon dioxide recovered from unit 12 or unit 19 (via line 22) may be used for power generation, for example by combustion followed by turbo-expansion, for "cold" generation or for use in enhanced oil recovery techniques.

Alternatively the recovered carbon dioxide may be employed as a reactant in the primary gasification process. In partial oxidation processes the carbonaceous material is reacted with steam and oxygen according to the reactions; C+O2 = CO2 C+CO2 = 2CO C+H2O = H2+CO In applications where it is desired to produce synthesis gases containing a high proportion of carbon-monoxide, at least a proportion of the process steam may be replaced by the recovered carbon dioxide as a gasifying agent which reacts with the carbonaceous feedstock according to the reaction; C+CO2 = 2CO.

The processes for the absorption of carbon dioxide are well known and are described, for example, in "Gas Processing Handbook", Hydrocarbon Processing, April 1979.

TABLE Stream A B C Componenf(molo/ol CH4 10.7 32.1 96.2 CO 65.2 0.02 0.05 C 2 - 66.7 0.1 C2Hs 0.8 C2H4 0.1 ~ H2 22.5 0.63 1.9 N2 0.6 0.55 1.75

Claims (9)

1. A process for the production of methane-containing gases, wherein a primary synthesis gas containing hydrogen, and carbon oxides is treated to remove acid gases therefrom and thereafter is subjected to at least one methanation reaction characterised in that, prior to methanation, a liquor containing absorbed carbon dioxide is contacted with purified synthesis gas and stripped of its carbon dioxide and that, after methanation, the methanation reaction product is contacted with a substantially carbon dioxide-free physical solvent for carbon dioxide thereby to absorb carbon dioxide present in said methanated product and to provide a carbon dioxide-rich liquor for contact with purified synthesis gas.

2. A process as claimed in Claim 1 wherein a portion of said carbon dioxide rich liquor is flashed to produce a gas containing carbon dioxide and methane.

3. A process as claimed in Claim 2 wherein the gas is subjected to separation to recover methane and separate out carbon dioxide.

4. A process as claimed in Claim 2 or Claim 3 wherein gas containing carbon dioxide is at a pressure of at least 10 bar.

5. A process as claimed in any one of Claims 2 to 4 wherein gas containing carbon dioxide to turbo-expanded.

6. A process as claimed in any of Claims 2 to 4 wherein gas containing carbon dioxide is combusted.

7. A process as claimed in Claim 6 wherein the combusted gas is turbo-expanded.

8. A process as claimed in any one of Claims 2 to 4 wherein gas containing carbon dioxide is used as a feedstock reactant for the primary synthesis reaction.

9. A process for producing methane-containing gases according to Claim 1 and substantially as herein described with reference to the accompanying drawings.