CA2246915A1

CA2246915A1 - Process for the removal of water, co2, ethane and c3+ hydrocarbons from a gas stream

Info

Publication number: CA2246915A1
Application number: CA002246915A
Authority: CA
Inventors: Wilbur Clymer Kratz; Timothy Christopher Golden; Mohammad Ali Kalbassi
Original assignee: Air Products and Chemicals Inc
Current assignee: Air Products and Chemicals Inc
Priority date: 1997-09-16
Filing date: 1998-09-09
Publication date: 1999-03-16
Also published as: MY133549A; US5840099A; EP0908508A1; AU702778B1; NO984188L; NO984188D0

Abstract

A process is set forth for the selective removal of water, CO2, ethane and C3+ hydrocarbons from gas streams, particularly a natural gas stream comprising primarily methane. The process comprises contacting the gas stream with an adsorbent material consisting exclusively of one or more compounds which are basic (i.e. compounds which, when contacted with a pH neutral aqueous solution, cause such solution to have a pH greater than 7.0) and which are mesoporous (i.e. compounds which have moderately small pores providing a surface area less than 500m2/g). A key to the present invention is that the adsorbent material is selective for the removal of all the impurities noted above, thereby eliminating the need for a separate impurity-specific adsorbent such as an adsorbent specific for the removal of CO2 and/or an adsorbent specific for the removal of water. Typical mesoporous adsorbents which are useful in the present invention include zinc oxide, magnesium oxide and, in particular, activated alumina.

Description

CA 0224691~ 1998-09-09 TITLE OF THE INVENTION:
Process for the removal of water, CO2, ethane and C3+ h yu"~ anJUI 1~ from a gas stream BACKGROUND OF THE INVENTION
Pipeline natural sas Is commonly liquefied for storage to smooth the peaks In 10 demand between winter and summer. Water and CO2 removal is required in this application to prevent freeze out during the low temperature liquefaction step. More recently, a market has developed for methane as a ll dl)~ ll i '' ) fuel. Again, the plpeline nature gas Is liquefied for storage, but In this case C3+ I.Jdlucdl b ons must be removed in addltion to the water and C02 to maintain a uniform fuel c~", .r ~ "' 1 15 Water and C02 are typlcally removed by Themmal Swing Adsorption (TSA3 while some fomm of cryogenic dlstillation Is used for the C3+ separation. This Is a relatlvely complex operatlon. As the level of C02 In the plpeline Increases to between 1.5 and 2%, cu,, J~,., mal TSA systems become capaclty limited and therefore run on short cycle times. The short cycle Umes Increase the amount of ~ayd,~e~_ ) gas (produce 20 methane) requlred whlch lowers the product methane recovery to the low 80'5%. For higher C02 levels, the ~.o" . ~ ,t' ., M technology would be chemical absorption followed by a drier. Absorption systems are undeslrable because of O~al _ ~ ,al and CA 0224691~ 1998-09-09 Illdilllenclllce problems and the resulting cc..li,in ' ~ of absorption, drying and cryogenic distillation is both , " ' ' and expensive. A one step process that simultaneously removes water, CO2 and C3+ h ,'~ UWI i o"s would be much simpler and cost effective but such a process is not available at this time. The process of the 5 present invention addresses this need. The process comprises contacting the gas stream with an adsorbent material consisting exclusively of one or more compounds which are basic (i e. compounds which, when contacted with a pH neutral aqueous solution, cause such solution to have a pH greater than 7.0) and which are mesoporous (i e. compounds which have moderately small pores providing a surface area less than 10 500m2/g). Typical basic, mesoporous adsorbents which are useful in the present invention include zinc oxide, magnesium oxide and, in particular, activated alumina.
The closest prior art to the present invention is ~ e,ul e~el Ited by US Patents5,013,334 and 5,171,333, both by Maurer and assigned to UOP (hereafter 4Maurer~).
Maurer teaches microporous adsorbents consisting of the zeolitic molecular sieves ZnX
15 and CaY for separating ethane from methane-containing feed streams. These a iso, iJdl ~ts were shown to be more effective for ethane removal than the 1, d il~iUn_ ,~
used 13X zeolite. Maurer also teaches that it may be desirable to employ an additionai adsorbent such as silica gel, activated carbon, or activated alumina where the removal of water and C3+ h ydl OCCl(iJOns is also required. Maurer also discusses the problem of 20 methane coadso.~" n in his separation. Methane uoadsu" :' n occurs because microporous adsorbents like the zeolites and carbons taught in Maurer coadsorb significant qUanUtieS of methane owing to the high inlet partial pressure of methane which in turn causes thermal problems dufing processing in a pressure swing adsorption (PSA) system. In particular, methane coad~o.~", causes heating during25 the adsorption step and cooling dufing the desorpUon step. This themmal behavior nuns CA 022469l~ l998-09-09 counter to what is desired in PSA systems since the equilibrium loading of many a~ s such as eUhane is reduced at higher temperatures.
The present invention has found Uhat, in addition to providing for water and C3+removal, basic a i~u, be"l~ such as activated alumina can also provide for C02 removal down to low (less than 50 ppm) levels where the CO2 is present in dilute (1-5%) CullCdl,tl ,~. This is due to the interaction of basic sites on the activated alumina with Uhe acidic CO2 molecule. In addition, the present invenUon has also found that acUvated alumina can provide for ethane removal down to modest (1-2%) levels where the ethane is present in dilute (1-1 OYo) col1cél IU aliOI ~s. Thus, the selective removal of 10 water, C~2~ ethane and C3+ hJdlu.,dlbOI)S from gas streams such as methane is possible with a single adsorbent without sacrificing pei fu""a, Ice Furthermore, the mesoporous nature of adsorbents such as activated alumina also provides other key adva, Ita~jeS vis-a-vis microporous adsorbents like the zeolites and carbons taught in Maurer. In general, as the pore size of an adsorbent increases, 15 adsorption capacity for non-polar ad~, bdtes such as methane and C3+ h, dl UWI bo, 1:~
decreases. Hence, methane and C3+ I,,~ ,dli ons are more weakly adsorbed on activated alumlna vis-a-vis microporous adsorbents. This provides for a reduction in the thermal problems associated with the coa i~u" ~ of methane as noted above. This also provides for easier desorption of the C3+ 1 ~dlu~,dl bù"s when ~ e9el ,e, 'i~ ,g the adsorbent while still providing for sufflclent adsorption of dilute C3+ h ,dl U'_dl i,vns during the adsorptlon step. (Although activated alumlna's affinlty for C3+ h Jdl O.,dl i,o"s is less than the afiinity of Maurer's microporous adsu, bel ~ts for C3+ h~ /Lal bOI 1~, activated alumina's aflinity Is n-)l letl ,aldss a more optimum affinity where the C3+ h J il uw, i,ons are present In dilute ~1-2%)concdntl l and where the adsorbent must be continually l~aenél ~;

CA 0224691~ 1998-09-09 BRIEF SUM~L9RY OF THE INVENTION
The present invention is a process for the selective removal of water, CO2, ethane and C3+ h JdlU~ from gas streams, particularly a natural gas stream comprising primarily methane. The process comprises contacting the gas stream with an adsorbent material consisting exclusively of one or more ~ , ' which are basic (i.e. ~ r which, when contacted with a pH neutral aqueous solution, cause such solution to have a pH greater than 7.0) and which are r UU~ (i e. ,- r ~ which have moderately small pores providing a surface area less than 500m2/g). A key to the present invention is that the adsorbent material is selective for the removal of all the impurities noted above, thereby eliminating the need for a separate impurity-specific adsorbent such as an adsorbent specific for the removal of CO2 and/or an adsorbent specific for the removal of water.
Typical basic, ~ uu~ adsorbents wbich are useful in the present invention include zinc oxide, magnesium oxide and, in particular, activated alumina.

BRIEF DESCRIPTION OF THE SEVERAL VIEW OF THE DRAWINGS
Not applicable.

DETAILED DESCRIPTION OF THE INVENTION
The present invention is a process for the selective removal of water, CO2, ethane and C3+ h,.~)~ from gas streams, particularly natural gas streams comprisingprimarily methane. The process comprises contacting the gas stream with an adsorbent material consisting exclusively of one or more , ' which are basic (i.e. c which, when contacted with a pH neutral aqueous solution, cause such solution to have a pH greater than 7.0) and which are . . uu~ (i.e. , ' which have moderately small pores providing a surface area less than 500m2/g) In CA 022469l~ l998-09-09 .

contrast to microporous compounds which have very small pores and thus provide very large surface areas for Uhe, ,,c~,h dl li:~lll of adsoprtion to take place on, mesoporous compounds have only ll,odc,l~.t~,!y small pores and thus provide only ",ode,.h~,ly larse surface areas. Typical basic, mesoporous aclsG, I,ants which are useful in the present 5 invention inciude zinc oxide, magnesium oxtde and, in particular, activated alumina.
The skilled ~., . ' ler will appredate that the pH limitation of the present invention would commonly be referred to in the art as the adsorbent having a "zero point of charge~ greater than 7.0 The skilled ,u~ " ' )e r will further appreciate that the surface area llmitation of the present invention can be measured by the BET method 10 which is based on the multilayer adsorpUon theory developed in 1938 by Brunauer, Emmett and Teller.
The process of the present invention is particularly designed to treat a typical natural gas feed stream which Is at high pressure (about 500 psig for naturai sas pipeiines), saturated with water (about 700 ppm for a gas stream at ambient 15 temperature and 500 psig) and further contains about 1-3% C~2 ~ about 1-10% ethane and about 1-2%C~+I IJJl-~.dl Lolls with the remaining 85-97% being methane. The process of the present invention is desi5ned to simultaneousiy remove water, CO2, some ethane and ali C3+ i lJd~ d~ bOIl:~ from this feed stream in order to produce a purified stream containins about 98-99% methane, about 1-2% ethane and less than 20 about 50 ppm CO2 wherein about 90% of the methane in the feed gas stream is recovered in the purified stream.
Preferably, the step of contacting of the natural gas stream with the adsorbent Is performed within a pressure swing adsorption (PSA) vessel containing said adsorbent and is foilowed by an adsorbent ,t,~ r_'i 1 seqùence comprising the steps of 25 depressu, kln~ve"" ,g the adsorpUon vessel down to low pressure followed by ..

CA 0224691~ 1998-09-09 repressufizing Uhe adsorbent-containing vessel wiUh a portion of the purified gas stream back to the pressure level at which the gas stream was iniaally contacted with the adsorbent.
Also preferably, the depressufization is partly performed via one or more 5 pressure eq~ ~ 1 steps with other PSA vessels undergoing said repressufizing, The skilled r "" ~e( will appreciate that this will recover some of the void methane and thus improve the recovery of the methane, albeit at the expense of additional PSA
vessels operating sequentially in paraliel with one another.
Also preferably, the depressufizing Is performed down to vacuum pressure 10 levels by connecting the adsorption vessel to a vacuum pump (i.e. vacuum swing adsorption or VSA). The skilled ~-, ,er will appreciate that this will improve the d isu,Le"ts rejecUon of the adsorbed impuritles during the depressurization step, albeit at the expense of power.
Also preferably, the adsorbent is purged or rlnsed with a porUon of the purified 15 gas stream subsequent to said depressufization step and pfior to said repressufization step. The skilled p,e. ~titiOI ,e( will appreciate that this will further improve the c,d~o,l,e ut s rejectlon of Uhe adsorbed impuriUes, albeit at the expense of methane recovery. Note however that In a typical PSA process, the purge step is performed at 1 atm pressure.
By lowefing the purge pressure to 0.1 atm, one can obtain the same degree of purging 20 wlUh about 10% of the gas required at 1 atm Thus the methane recovery penalty associated with purging is much less severe in a VSA process as opposed to a PSA
process.
Also preferably, the steps of the process are perfonned as a conUnually repeating cycie of steps in a system comprising a plurality of adsorpUon vessels which each undergo 25 their respecUve cycie of steps while collecUvely operated sequenUally In parallel with one CA 0224691~ 1998-09-09 another. The cycie time depends on the specific design but typically might be one minute per step.
Example 1 A natural gas feed co,nr ~ ' ~ 1 of 0.5% N2, 1.3% CO2, 94.7~/0 methane, 2.2%
ethane, 1% C3, 0.11% C4, 0.05% C5, 0.02% C6, 0.029/o C7 and 0.003~/0 C8 was processed in the above described preferred s...L ~ ll of the present invention. The adsorbent was activated alumina (Alcan AA-300 made by The Aluminum Company of Canada) having a BET surface area equal to 325 m2/g. Three pressure eql ~
steps and six adsorption vessels were utilized. The final evacuation and purge pressure was 1.5 psia. The process was run for over 10,000 cycles to del llurlall ate that the C3+ hJdlu~.albolls were not accumulating in the adsorbent and degrading the pelrullnall~ The ptoduct methane co",~,ûaitiun was 0.5% N2, 0.0029% C02, 98.3%
methane and 1.2% ethane. The methane recovery was 90~/0. This example d~,. "o"al(ales the use of a single adsorbent in a methane purification process while still achieving pe,ro""a".,e objectives Including (i) less than 50 ppm CO2, (ii) no C3~
hJdlu~,alLol)s, (iii) less than 2Yo ethane and (iv) high methane recovery. Convenbonal ~, ' , rur the separation in this example might have suggested a bed of activated carbon followed by 13X molecular sieve for this application. This adsorbent allanu-lllent Is suggested by Maurer in US Patent 5,013,334 and is the catalyst al, al ~y_., ll in Example 2 below.

Example 2 The operaUng conditions are roughly the same as In Example 1 except the adsorbent al lan_ It is a 38-52% carbon-1 3X spllL The result was a rapld decline in pel r~" ,nance. Even with fresh adsorbent, there was difficulty holdlng the product CO2 -7.

CA 0224691~ 1998-09-09 level to 50 ppm and after only a few cycles (about 20 to 30) the pe,ro""a".,e had degraded si~u,lliri~ l,lly. In parlticular, the C02 level increased to 130 ppm and the evacuation gas per cycle declined by about 25%, Similar results were seen with an all 1 3X bed. The BET surface area of the activated carbon was 1000 m2/g and that fot the 1 3X was 825 m2/g. The key negative results noted here are (i) zeolites, despite high CO2 capacity and selectivity, could not produce a low impunty level C02 product and (ii) adivated carbon, often used for l,ydlu.,a,i o,) removal, was ineffective in ~self-cleaning~
heavy I ,y il eca~i uns from the feed stream under VSA operating conditions as noted by degrading process pe, ru" lldl l~,è over time. Conttast these results with the activated alumina used In Example 1 which une,~pe-,leu'y provided a good CO"li in ~n of good working C02 capacity, high purity CH4 product (low C02 impurity) and the ability to reject heavy l,,l~u~,.,,i uns at the operating conditions of the process.

Claims

1. A process for the selective removal of water, CO2, ethane and C3+
hydrocarbons from a gas stream in order to produce a purified gas stream, said process comprising the step of contacting the gas stream with an adsorbent material consisting exclusively of one or more compounds which are basic (i.e. compounds which, when contacted with a pH neutral aqueous solution, cause such solution to have a pH greater than 7.0) and which are mesoporous (i.e. compounds which have moderately small pores providing a surface area less than 500m2/g).

2. The process of Claim 1 wherein the adsorbent is one or more compounds selected from the group consisting of activated alumina, zinc oxide and magnesium oxide.

3. The process of Claim 2 wherein the gas stream is a natural gas stream comprising primarily methane.

4. The process of Claim 3 wherein the natural gas stream is at about 500 psig pressure and contains about 700 ppm water, about 1-3% CO2, about 1-10%
ethane and about 1-2% C3+ hydrocarbons and about 85-97% methane.

5. The process of Claim 4 wherein the purified gas stream contains about 98-99% methane, about 1-2% ethane and less than about 50 ppm CO2 and wherein about 90% of the methane in the natural gas stream is recovered in the purified gas stream.

6. The process of Claim 5 wherein said contacting of the gas stream with the adsorbent is performed at about 500 psig within a pressure swing adsorption (PSA) vessel containing said adsorbent.

7. The process of Claim 6 wherein following the step of contacting the gas stream with the adsorbent, said process further comprises an adsorbent regeneration sequence comprising the steps of depressurizing the adsorption vessel down to low pressure followed by repressurizing the adsorbent-containing vessel with a portion of the purified gas stream back to the pressure level at which the gas stream was initially contacted with the adsorbent.

8 The process of Claim 7 wherein:
(i) said depressurizing is partly performed via one or more pressure equalization steps with other PSA vessels undergoing said repressurizing;
(ii) said depressurizing is performed down to vacuum pressure levels;
and (iii) the adsorbent is purged with a portion of the purified gas stream subsequent to said depressurization step and prior to said repressurization step.

9. The process of Claim 8 wherein the steps of the process are performed as a continually repeating cycle of steps in a system comprising a plurality of adsorption vessels which each undergo their respective cycle of steps while collectively operated sequentially in parallel with one another.

10. The process as defined in any of Claims 1 through 9, wherein said step of contacting the gas stream with an adsorbent material is carried out at an elevated pressure.

11. The process as defined in Claim 10, wherein said elevated pressure is approximately 500 psig.