Methane purification by pressure swing adsorption

Claim 2

A pressure swing adsorption process for the separation of ethane from a feedstream comprising mixtures thereof with methane, said process comprising: (a) passing the feedstream at an upper adsorption pressure through an adsorption zone containing a zeolitic molecular sieve of the faujasite type having a framework SiO 2 /Al 2 O 3 molar ratio of from 2 to 100 and containing at least 20 equivalent percent of zinc cations and containing not more than 80 equivalent percent of alkali or alkaline earth metal cations or mixtures thereof, wherein ethane and methane are adsorbed and recovering an adsorption effluent stream enriched in methane relative to the feedstream; (b) regenerating the adsorption zone by depressurizing the adsorption zone to a desorption pressure, said desorption pressure being lower than said adsorption pressure, and recovering a desorption effluent stream comprising ethane and methane; and (c) repressurizing the adsorption zone to the adsorption pressure.

Claim 3

The process of claim 1 wherein said depressurizing is performed in a direction countercurrent to said passing of said feedstream through said adsorption zone.

Claim 4

The process of claim 2 which further includes at least one cocurrent depressurization step performed prior to said regenerating step comprising cocurrently depressurizing the adsorption zone to a pressure intermediately between the adsorption pressure and the desorption pressure and recovering a cocurrent depressurization effluent stream comprising methane.

Claim 5

The process of claim 3 which includes three or more cocurrent depressurization steps.

Claim 6

The process of claim 2 which further includes a countercurrent purge step performed either prior to, simultaneously with, or subsequently to said regenerating step, comprising passing a methane-containing purge gas countercurrently through said adsorption zone and recovering a countercurrent purge effluent stream comprising ethane.

Claim 7

The process of claim 2 wherein said process includes a first and second adsorption zone and said repressurizing step comprises passing at least a portion of one of the adsorption effluent stream or the cocurrent depressurization effluent streams or both from said first adsorption zone to said second adsorption zone.

Claim 8

The process of claim 1 wherein the framework SiO 2 /Al 2 O 3 ratio of the zeolitic molecular sieve is from 2 to 20.

Claim 9

A process according to claim 7 wherein the zeolitic molecular sieve contains at least 40 equivalent percent of zinc cations and less than 40 equivalent percent of alkali and alkaline earth cations.

Claim 10

The process of claim 1 wherein the feedstream additionally comprises at least one component selected from the group consisting of C 3 + hydrocarbons, CO 2 , H 2 O and mixtures thereof.

Claim 11

The process of claim 9 wherein the feedstream comprises from about 70 to 98 mol. % methane, from about 0.1 to 10 mol. % ethane, from about 0.1 to 10 mol. % C 3 + hydrocarbons, and from about 0.1 to 20 mol. % CO 2 .

Claim 12

The process of claim 9 wherein said components are adsorbed on the zeolitic molecular sieve.

Claim 13

The process of claim 9 wherein the adsorption zone further comprises at least two adsorbents wherein one adsorbent is selected from the group consisting of silica gel, alumina, and activated carbon.

Claim 14

The process of claim 1 wherein the adsorption pressure is from about 100 to 500 psia.

Claim 15

The process of claim 1 wherein the desorption pressure is from about 14.7 to 50 psia.

Claim 16

A pressure swing adsorption process for the separation of ethane from a feedstream comprising mixtures thereof with methane, said process comprising: (a) passing the feedstream at an adsorption pressure of from about 100 to 500 psia to one of at least four adsorption zones containing a zeolitic molecular sieve of the faujasite type having a framework SiO 2 /Al 2 O 3 molar ratio of from 2 to 100 and containing at least 20 equivalent percent of zinc cations and containing not more than 80 equivalent percent of alkali or alkaline earth metal cations or mixtures thereof; wherein ethane and methane are adsorbed and recovering an adsorption effluent stream enriched in methane relative to the feedstream, (b) cocurrently depressurizing said one adsorption zone and passing the effluent stream therefrom to another adsorption zone undergoing a repressurizing step as hereinafter described and equalizing the pressures therebetween; (c) further cocurrently depressurizing said one adsorption zone and passing the effluent stream therefrom as purge gas to another adsorption zone undergoing a purging step as hereinafter described; (d) countercurrently depressurizing said one adsorption zone to a pressure of from about 14.7 to 50 psia and recovering a desorption effluent stream comprising ethane and methane; (e) countercurrently purging said one adsorption zone with a purge gas comprising the cocurrent depressurization effluent stream of step (c) from another adsorption zone and recovering a countercurrent purge effluent stream comprising ethane; (f) repressurizing said one adsorption zone by passing the cocurrent depressurization effluent stream of step (b) from another adsorption zone to the adsorption zone to equalize the pressures therebetween; and (g) further repressurizing said one adsorption zone by passing a portion of the adsorption effluent stream from another adsorption zone thereto.

Claim 17

A pressure swing adsorption process for the separation of ethane from a feedstream comprising mixtures thereof with methane, said process comprising: (a) passing the feedstream at an adsorption pressure of from about 100 to 500 psia to one of at least four adsorption zones containing a zeolitic molecular sieve of the faujasite type having a framework SiO 2 /Al 2 O 3 molar ratio of from 2 to 100 and containing at least 20 equivalent percent of zinc cations and containing not more than 80 equivalent percent of alkali or alkaline earth metal cations or mixtures thereof; wherein ethane and methane are adsorbed and recovering an adsorption effluent stream enriched in methane relative to the feedstream, (b) cocurrently depressurizing said one adsorption zone and passing the effluent stream therefrom to another adsorption zone undergoing a repressurizing step as hereinafter described and equalizing the pressures therebetween; (c) further cocurrently depressurizing said one adsorption zone and passing the effluent stream therefrom as purge gas to another adsorption zone undergoing a purging step as hereinafter described; (d) countercurrently depressurizing said one adsorption zone to a pressure of from about 14.7 to 50 psia and recovering a desorption effluent stream comprising ethane and methane; (e) countercurrently purging said one adsorption zone with a purge gas comprising the cocurrent depressurization effluent stream of step (c) from another adsorption zone and recovering a countercurrent purge effluent stream comprising ethane; (f) repressurizing said one adsorption zone by passing the cocurrent depressurization effluent stream of step (b) from another adsorption zone to the adsorption zone to equalize the pressures therebetween; and (g) further repressurizing said one adsorption zone by passing a portion of the adsorption effluent stream from another adsorption zone thereto.

Claim 18

The process of claim 15 wherein the feedstream comprises from about 75 to 95 mol. % methane, from about 0.5 to 10 mol. % ethane, from about 0.1 to 10 mol. % CO 2 and from about 0.1 to 5 mol. % C 3 + hydrocarbons.

Claim 19

The process of claim 16 wherein the adsorption effluent stream comprises less than about 2000 ppm ethane.

Claim 20

The process of claim 17 wherein the adsorption effluent stream comprises less than about 300 ppm ethane.