PROCESS FOR REMOVING CO2 FROM A METHANE-CONTAINING GAS

Abstract

A process for removing CO.sub.2 from a methane-containing gas, having the steps of providing a methane-containing gas containing at least CO.sub.2 as an impurity, cooling the gas to remove CO.sub.2 from the methane-containing gas by freezing out same, and additionally reducing the CO.sub.2 concentration of the gas using a pressure temperature swing adsorption apparatus (PTSA), whereby a methane-enriched product gas is obtained. At least a part of the product gas is then used as treatment gas and is passed through the PTSA for treatment of the PTSA, whereby CO.sub.2 is absorbed by the treatment gas and is removed from the PTSA as a CO.sub.2-enriched treatment gas. The treatment gas is then recycled and admixed with the methane containing gas.

Claims

1. A process for removing CO.sub.2 from a methane-containing gas comprising the steps of: a) providing a methane-containing gas containing at least CO.sub.2 as an impurity; b) cooling the methane-containing gas to remove CO.sub.2 from the methane-containing gas from step a) by freezing-out; c) further reducing the CO.sub.2 concentration of the methane-containing gas from step b) using a pressure temperature swing adsorption apparatus (PTSA) to obtain a methane-enriched product gas; d) using at least a portion of the product gas from step c) as treatment gas which is passed through the PTSA for treatment of the PTSA, whereby CO.sub.2 is absorbed by the treatment gas and removed from the PTSA as a CO.sub.2-enriched treatment gas; and e) recycling the CO.sub.2-enriched treatment gas that has been passed through the PTSA and admixing same with the methane-containing gas in step a).

2. The process as claimed in claim 1, wherein before step d) the product gas is entirely or at least partially liquefied by cooling to obtain a liquefied product gas.

3. The process as claimed in claim 2, wherein to achieve at least partial liquefaction the product gas is cooled to 140 C. to 100 C.

4. The process as claimed in claim 2, wherein the liquefied product gas is decompressed to obtain a flash gas and a decompressed liquefied product gas, wherein the flash gas is passed through the PTSA as treatment gas.

5. The process as claimed in claim 1, wherein in step b) the CO.sub.2 concentration of the methane-containing gas is reduced to below 6000 ppm.

6. The process as claimed in claim 1, wherein in step c) the CO.sub.2 concentration of the product gas is reduced to below 5200 ppm.

7. The process as claimed claim 1, wherein the treatment gas is decompressed and preferably heated before it is passed through the PTSA in step d) as decompressed and optionally heated treatment gas.

8. The process as claimed in claim 7, wherein after being passed through the PTSA in step d) and before being recycled and admixed with the methane-containing gas in step e), the decompressed and optionally heated treatment gas is compressed to the pressure of the methane-containing gas.

9. The process as claimed in claim 1, wherein before being supplied to the PTSA in step d), at least a portion of the product gas from step c) respectively at least a portion of the treatment gas is used for cooling the provided methane-containing gas.

10. The process as claimed in claim 7, wherein at least a portion of the product gas from step c) is used for cooling the provided methane-containing gas by passing the product gas and the methane-containing gas through a heat exchanger, and in that at least a portion of the heated product gas exiting the heat exchanger is decompressed and used as treatment gas for the regeneration of the PTSA.

11. The process as claimed in claim 1, wherein at least a portion of the product gas from step c) is used to cool the PTSA after regeneration thereof.

12. The process as claimed in claim 1, wherein the methane-containing gas is supplied to the PTSA at a pressure of 5-25 bar.

13. The process as claimed in claim 12, wherein the methane-containing gas is supplied to the PTSA at a temperature of 130 C. to 80 C.

14. The process as claimed in claim 1, wherein the product gas respectively the liquefied product gas is decompressed before diversion of the treatment gas.

15. The process as claimed in claim 1, wherein the methane-containing gas in step a) has a CO.sub.2 concentration of not more than 60%.

16. The process as claimed in claim 4, wherein the decompressed liquefied product gas is passed through a particulate filter, which removes solids from the decompressed liquefied product gas.

17. An apparatus for performing a process for removing CO.sub.2 from a methane-enriched gas according to claim 1 comprising a gas introduction conduit, a compression unit, at least one cooling unit for freezing out CO.sub.2, a PTSA, a conduit for transporting away a methane-enriched product gas and a treatment gas conduit which connects the PTSA and the gas introduction conduit.

Description

[0054] The invention will now be more particularly elucidated hereinbelow with reference to one of the accompanying exemplary embodiments. Purely schematically in each case:

[0055] FIG. 1 shows a schematic representation of a preferred embodiment of the process according to the invention for removing CO.sub.2 from a methane-containing gas with a PTSA.

[0056] In the preferred embodiment of the process according to the invention shown schematically in FIG. 1, in a first step a) a methane-containing gas 103 containing at least CO.sub.2 as an impurity is provided from a gas introduction conduit 101.

[0057] The methane-containing gas 103 is provided at a pressure of 15 bar and in a second step b) cooled to a temperature TG (gaseous temperature) of 32 C. in a first heat exchanger 105. The compressed and cooled methane-containing gas 103 is moreover further cooled in a second heat exchanger 107 to a temperature T.sub.F (freeze temperature) of 114 C., at which the CO.sub.2 present in the methane-containing gas 103 freezes out on a surface (not shown) in a valve 109 of the heat exchanger and at regular intervals is scraped from said surface or removed in an alternative fashion. The freezing-out allows rapid removal of large amounts of CO.sub.2 from the methane-containing gas 103, thus allowing the CO.sub.2 concentration in the gas 103 to be reduced to below 2100 ppm. The methane-containing (and now CO.sub.2-reduced) gas 103 is subsequently supplied to a PTSA 111. The PTSA 111 generally comprises a plurality of adsorber containers 115 (only two are shown here), which are operated in parallel, overlapping or mutually alternating fashion.

[0058] After the freezing-out of the CO.sub.2 and the supplying of the methane-containing gas 103 to the PTSA 111 in a third step c) the residual CO.sub.2 is removed from the methane-containing gas 103 by means of an adsorber 115 present in the PTSA 111, thus affording a methane-enriched product gas 117. After purification by the PTSA 111 the methane-enriched product gas 117 has a CO.sub.2 content of preferably not more than 215 ppm. The methane-enriched product gas 117 is transported to a third heat exchanger 119 and liquefied by cooling to 120 C. Said gas is then passed via a valve 121 into a flash vessel 123 and decompressed to 1 bar, preferably atmospheric pressure, whereby the product gas is cooled to preferably 162 C. and a portion thereof is simultaneously regasified. In addition to decompressed liquefied product gas 126 a flash gas 127 is generally formed, i.e. a portion of the product gas 117 remains gaseous or a portion of the decompressed liquefied product gas 126 is regasified. In the process shown here the flash gas 127 is used as treatment gas 129 (dashed line). The treatment gas 129 from the flash vessel 123 has a CO.sub.2 content of preferably below 215 ppm and a temperature of preferably 162 C. At least a portion of the treatment gas 129 (corresponding in its composition to the product gas 117) is initially recycled to the first heat exchanger 105 to cool the methane-containing gas 103. This heats the treatment gas 129 to preferably 0 C. The now-heated treatment gas 131 is then passed to the PTSA 111 where it heats and flows through the CO.sub.2-saturated adsorbers 115 to detach the CO.sub.2 from the adsorbers 115. A now CO.sub.2-enriched treatment gas 133 (dotted line) is transported from the PTSA 111 to a compressor 135 where it is compressed to 15 bari.e. the starting pressure of the methane-containing gas 103.

[0059] The compressed and CO.sub.2-enriched treatment gas 133 is subsequently transported through a treatment gas conduit 134 to the gas introduction conduit 101 and is there mixed with the methane-containing gas 103. Once the adsorbers 115 of the PTSA 113 have been reconditioned they are cooled back down to operating temperature to allow efficient adsorption of CO.sub.2 from the methane-containing gas 103. Cooling of the adsorbers is effected either with product gas 117 or treatment gas 129 having a temperature of 162 C.

[0060] The decompressed liquefied product gas 126 is passed through a particulate filter 138 having a pore size of <10 m to remove solids in the product gas. The purified product gas is then supplied to the consumer using a pump 139. Since the two shown adsorbers 115 of the PTSA 111 can be operated in mutually alternating fashion, one adsorber 115 can be treated while the other adsorber 115 adsorbs CO.sub.2 from the methane-containing gas 103. This makes it possible to ensure a constant purification process.