CA2055520A1

CA2055520A1 - Hydrogen and carbon monoxide production by hydrocarbon steam reforming and pressure swing adsorption purification

Info

Publication number: CA2055520A1
Application number: CA002055520A
Authority: CA
Inventors: Ramachandran Krishnamurthy
Original assignee: Ramachandran Krishnamurthy; The Boc Group, Inc.
Current assignee: Linde LLC
Priority date: 1990-12-05
Filing date: 1991-11-14
Publication date: 1992-06-06
Also published as: DE69128701D1; AU8832191A; AU662267B2; DE69128701T2; TR27542A; MY116330A; US5096470A; ZA919195B; IE914210A1; AU646704B2; EP0489555A1; EP0489555B1; AU6451294A

Abstract

HYDROGEN AND CARBON MONOXIDE PRODUCTION
BY HYDROCARBON STEAM REFORMING AND
PRESSURE SWING ADSORPTION PURIFICATION

ABSTRACT OF THE DISCLOSURE
The present invention is directed to a method for producing hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane, which comprises the steps of (a) passing the feed mixture through a first pressure swing adsorption system comprising an adsorbent having a greater affinity for carbon dioxide, methane, and carbon monoxide than for hydrogen to separate hydrogen as a non-adsorbed product and carbon dioxide, methane, and carbon monoxide as an adsorbed fraction, (b) desorbing carbon monoxide, (c) desorbing carbon dioxide and methane, (d) passing the carbon monoxide to a second pressure swing adsorption system comprising an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane to separate carbon monoxide as an adsorbed fraction and hydrogen, carbon dioxide, and methane as a non-adsorbed fraction, and (e) desorbing carbon monoxide. In a second embodiment, the invention is directed to a method which comprises the steps of (a) providing a pressure swing adsorption system having a first stage comprising an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane, and a second stage comprising an adsorbent having a greater affinity for carbon dioxide, methane, and carbon monoxide than for hydrogen, (b) passing the feed mixture through the first stage to separate carbon monoxide as an adsorbed fraction and hydrogen, carbon dioxide, and methane as a non-adsorbed fraction, (c) passing the non-adsorbed fraction through the second stage to separate carbon dioxide and methane as an adsorbed fraction and hydrogen as a non-adsorbed pure product, (d) desorbing carbon dioxide and methane, and (e) desorbing carbon monoxide.

Description

2~5532~

PATENT
Patent Pro;ect No. 88A237 BOG 1-014.doc 26 November 1990 ~YDROGEN A~D CA~BON MONOSIDE P~ODUCTION
BY ~yDR~GaaBON 8~A~-ByFQR~y_A~P
P~E8gURE ~R~NG AD80R~TION ~5~gaG~a~ZLQ~

BAC~GRO~ND OF ~ EN~IO~

1. ~ield of the Inven~ion ~he present invention is directed to a method for producing merchant grade hydrogen and carbon monoxide from a ~team reformed hydrocarbon feed mixture. More particularly, the present invention is directed to a method for producing hydrogen and carbon monoxide from a feed mixture comprisinq hydrogen, carbon monoxide, carbon dioxide, and methane.

~. De~cr~ption of the Prior A~t Various methods are known for separating gaseous mixtures produced by the steam reforming of hydrocarbon-. Steam reforming to produce hydrogen consists ot treatlng a hydrocarbon f-ed mixture with steam in a catalytic ~team reactor (reformer) which consists of a number of tubes placed in a furnace at a temperature in the range from about 1250 F. to about 1700 F. The reversible reforming reactions which occur when methane i6 used as the hydrocarbon feed mixture are set out below.

CH4 ~ H20 C0 ~ 3H2 CH4 + 2h20--C02 ~ 4H2 CO I H20 ~ C2 ~ H2 Carbon monoxide and~ carbon dioxide are generally removed by 6hift conversion (reaction of carbon monoxide with steam to form additional hydrogen and carbon dioxide), absorption in ~mines or other alkaline solvents (carbon dioxide removal), and methanation (conversion of trace carbon monoxide ~nd carbon dioxide to methane). When carbon monoxide is a desired product, the 6hift conver6ion and methanation 6teps are not employed.

The hydrogen-rich ga6 mixture exitinq the 6team reformer consist6 of an eguilibrium mixture of hydrogen, carbon monoxide, carbon dioxide, water vapor, and unreacted methane. The reforming reactions are endothermic and therefore hydrocarbons and process waste gases are burned in the reformer furnace to provide the endothermic heat.

Hydrocarbon steam reforming reaction6 and hydrogen 6eparation proce6se~ are di6closed in more detail in "A~monia ~nd Synthesis Gas: Recent and Energy Saving Proces6es", Edited by F.J. Brykow~i, Chemical Technology Review No. 193, Energy Technology Review No.
68, Published by Noyes Data Corporation, Park Ridge, New Jersey, 1981.

2 ~ 2 ~

Conventional ~ethods for recovering hydrogen and carbon monoxide from a hydrocarbon steam reformed feed mixture have generally focused on cryogenic di6tillation proce6se6 to ~eparate and purify hydrogen and carbon monoxide in the ~ixture after carbon dioxide i~ removed. Cryogenic separation proce~ses tend to have a h$gh capital c06t e6pecially when more than one pure product is reguired.

United State6 patent no. 4,778,670, i66ued to Pin~o, disclo6e~ a pre~6ure swing ad60rption proce~s for producing technical hydrogen which compri6es pa~6$ng a raw gas containing a specific ratio of hydrogen, nitrogen, and carbon oxides to a pressure swing adsorbent and stopping the flow of feed ga6 in the cycle when the integrated nitrogen content of the unadsorbed product gas of the pressure ~wing adsorption stage ~6 in the range of 1% to 10% by volume.

~0 German patent application no. 3,427,804, to Liade A.G., di6close6 a proces6 for reforming a hydrocarbon with carbon dioxide to obtain a gas mixture comprising hydrogen, carbon monoxide, and carbon dioxide and separating the mixture into ~eparate 6treams of hydrogen, carbon ~onoxide, and carbon dioxide. The methods for purifying the hydrogen and carbon monoxide stream6 are not di6closed.

Methods for separating hydrogen and carbon monoxide by pressure ~wing adsorption proces6e6 are di~closed in European patent application no. 317,235A2, to Xrishnamurthy et al., and the reference~ cited therein. Kri6hnamurthy et a~. di6close6 a method for forming hydrogen and carbon monoxide from a feed mixture exiting a hydrocarbon ~team reformer comprising hydrogen, carbon ~onoxide, and carbon dioxide. The method compri~es the 6teps of passing the feed mixture through a sorptive 6eparation to separate a hydrogen product, a carbon monoxide-rich product, and a carbon dioxide-rich 2 ~ 3 3 3 2 fJ

product. The carbon monoxide-rich product i6 further purified in a two stage pressure 6wing adsorption system.
The first stage comprises an activated carbon adsorbent which removes carbon monoxide and methane a6 the strongly adsorbed waste ~tream. The second stage comprises a zeolite adsorbent and produces a pure carbon monoxide stream as an adsorbed product.

United States patent no. 4,917,711, is6ued to ~e et al., di6closes an adsorbent for carbon monoxide and unsaturated hydrocarbon~ which comprises a high 6urface area support, 6uch as a zeolite, alumina, ~ilica gel, aluminosilicate, or aluminophosphate, and cuprous or cupric compound. The adsorbent may be used to ~eparate carbon monoxide and unsaturated hydrocarbons from a gaseous mixture containing hydrogen, nitrogen, argon, helium, methane, ethane, propane, and carbon dioxide by passing the ~ixture through the adsorbent and releasing the adsorbed carbon monoxide by heating, or lowering the pressure of, the adsorbent.

Japanese patent JP01203019 discloses a four column pressure ~wing adsorption ~ystem for separating carbon monoxide from a gaseous mixture. The columns contain an adsorbent containing copper to adsorb carbon monoxide gas.

United States patent no. 4,914,076, issued to Tsuii et al., disclo~es a ~ethod for preparing an adsorbent for carbon monoxide which compri6es contacting an alumina or ~ilica-alumina 6upport with a mixed solution or dispersion of a copper (II) ~alt and a reducing agent, and then removing the 601vent.

United States patent no. 4,783,433, issued to ~aiima et al., di~closes an adsorbent for separating carbon monoxide from a gaseous mixture containing carbon dioxide which comprise~ a zeolite resin with a silica/alumina ratio of not more than 10, in which not 2 ~ 5 ~ ~ h ~

- 5 - .
less than 50S of the cation exchange ~ites have been replaced by Cu(I) ions, in the pores of which, one or more 6alts of the metals Cu(I), Fe, Zn, Ni, and or Mg are dispersed.

Japanese patent JF61242908 di6closes an adsorbent for carbon ~onoxide which i6 prepared by supporting a copper (I) compound on an activated carbon support ~herein the volume of pore~ having a diameter of ~0 under 10 angstroms ~6 under 0.33ml/g.

United States patent no. 4,743,276, is6ued to Nishida et al., discloses an adsorbent for carbon monoxide which comprises a zeolite resin with a ~ilica/alumina ratio of not more than 10, in which not less than 50% of the cation exchange 6ites have been replaced by Cu(I) ions, in the pores of which, one or more 6alts of the met~l6 Cu(I), Fe, Zn, Ni, and or Mg are dispersed.
In a pressure ~wing adsorption 6ystem (~SA), a gaseous mixture is passed at an elevated pressure through a bed of an adsorbent material which 6electively adsorbs one or more of the components of the gaseous mixture.
Product gas, enriched in the unadsorbed gaseous component(s), is then withdrawn from the bed.

The term ~qaseous mixture", a6 used herein, refer6 to a gaseous mixture, ~uch as air, primarily comprised of two or more components having different molecular 6ize. The term ~enriched gas" refers to a gas comprised of the component(s) of the gaceou6 mixture relatively unadsorbed after pa66age of the gaseous ~ixture through the adsorbent bed. The enriched gas generally must meet a predetermined purity level, for example, from about 90% to about 99%, in the unadsorbed component(s). The term ~lean gas" refers to a gas exiting from the adsorption bed that fail6 to meet the predetermined purity level set for the enriched gas.

2~a~

When the 6trongly adsorbed component is the desired product, a cocurrent depressurization 6tep and a cocurrent purge step of the 6trongly adsorbed component are added to the process.

The term ~adsorption bed" refers either to a single bed or a serial arrangement of two beds. The inlet end oS a ~ingle bed sy6tem is the inlet ond of the single bed while the inlet end of the two bed ~y~tem (arranged in series) i~ the inlet end of the f~r~t bed in the 6ystem. The outlet end of a ~ingle bed 6ystem is the outlet end of the 6ingle bed and the outlet end of the two bed system (arranged in 6eries) is the outlet end of the ~econd bed in the system. By using two adsorption beds in parallel in a system and by cycling (alternating) between the adsorption beds, product gas can be obtained continuously.

As a g~6eous mixture travels through a bed of adsorbent, the adsorbable gaseou6 co~ponents of the mixture enter and fill the pore6 of the adsorbent. ASter a period of time, the composition of the gas exiting the bed of adsorbent i8 essentially the ~ame as the composition entering the bed. This period of time is known as the breakthrough point. At some time prior to this breakthrough point, the adsorbent bed must be regenerated. Regeneration involves stopping the flow of gaseous ~ixture through the bed and purging the bed of the ad60rbed components generally by venting the bed to atmo6pheric or ~ubatmo6pheric pre6sure.

A pre6sure swing adsorption ~y~tem generally employs two adsorbent beds operated on cycles which are 6equenced to be out of phase with one ~nother by 180 60 that when one bed i6 in the ad~orption or production step, the other bed i6 in the regeneration ~tep. The two adsorption beds ~ay be connected in series or in parallel. In a ~erial arrangement, the ga6 exiting the outlet end of the fir6t bed enters the inlet end of the second bed. In a parallel arrangement, the ~a~eous mixture enters the inlet end of all beds comprising the system. Generally, a serial arrangement of beds is preferred for obtaining a high purity gas product and a parallel arrangement of beds i6 preferred for purifying a large quantity of a gaseous mixture in a short time cycle.

Between the adsorption step and the regeneration ~tep, the pressure in the two ad60rption beds is generally equalized by connecting the inlet ends of the two beds together and the outlet ends of the two beds together. During the pressure equalization step, the gas within the pores of the adsorption bed which has just completed its adsorption step (under high pressure) flows into the adsorption bed which has ~ust completed its regeneration step (under low pressure) because of the pressure differential which exists between the two beds.
The adsorption bed which completed it~ adsorption ~tep is depressurized and the adsorption bed which completed its regeneration ~tep i~ repressurized. Thiæ pressure egualization step improves the yield of the product gas because the gas within the pores of the bed which has just completed its adsorption step has already been enriched. When more than two beds are employed in the adsorption system, it is common to have a number of pressure equalizations steps.

Gas ~eparation by tbe pressure swing adsorption ~ethod is more fully described in, for example, ~Gas Separation by Adsorption Processes~, Ralph T. Yang, Ed., Chapter 7, "Pressure Swing Adsorption: Principles and Processes" Butterworth 1987, and in United States patents nos. 2,944,627, 3,801,513, - and 3,960,522.
Modifications and improvements in the pres~ure swing adsorption process and apparatus are described in detail in, for example, United States patents nos. 4,415,340 and /~:

4,340,398.

While the above methods disclose processes for separating carbon monoxide, none of the methods disclose ~atisfactory processes for recovering both hydrogen and carbon monoxide from a hydrocarbon ~team reformed feed miYture economically and in high purity. Methods for separating hydrogen and carbon monoxide from a hydrocarbon steam reformed feed mixture reguire multi-stage systemC to purify carbon ~onoxide. Methods for separating carbon monoxide using copper exchanged sieves have focused on the separation of waste gases from steel mills which contain nitrogen, carbon monoxide, and carbon dioxide but not hydrogen. Conventional cryogenic separation processes tend to have a high capit~l cost especially when more than one pure product is reguired.
The present invention provide6 an improved method for b~' producing hydrogen and carbon monoxids from a hydrocarbon 6team re~ormed feed mixture employing a novel combination of pressure swing adsorption methods which minimizes capital cost requirements and increases the recovery of carbon monoxide.

~MMARY OF ~ I~V~IO~

The present invention is directed to a method for producing hydrogen and carbon monoxide from a feed mixture compri6ing hydrogen, carbon monoxide, carbon dioxide, and methane, which comprise6 the ~teps of (a) passing the feed mixture through a first pres6ure 6wing adsorption system containing an adsorption bed comprising an adsorbent having a greater affinity for carbon dioxide, methane, and carbon monoxide than for hydrogen to 6eparate hydroqen as a pure non-adsorbed product and carbon dioxide, methane, and carbon monoxide as an _ adsorbed fraction, (b) desorbing carbon monoxide from the 2~3~2~

pressure swing adsorption ~ystem in step (a) to form a carbon monoxide-rich fraction, (c) desorbing carbon dioxide and methane from the pressure swing adsorption system in step (a) to form a carbon dioxide-rich fraction, (d) pa~sing the carbon ~onoxide-rich fraction from step (b) to a second pressure swing adsorption system containing an adsorption bed compri~ing an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane to separate carbon monoxide as an adsorbed fraction hnd hydrogen, carbon dioxide, and methane as a non-adsorbed fraction, and (e) desorbing carbon monoxide from the pressure swing adsorption system in step (d) to form a pure carbon monoxide product.
In a 6econd embodiment, the invention is directed to a method for producing hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane, which comprises the ~teps of (a) providing a pressure swing adsorption system having a first stage and a second stage, wherein the first stage contains an adsorption bed comprising an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane, the 6econd stage contains an adsorption bed comprising an adsorbent having a greater affinity for carbon dioxide, methane, and carbon monoxide than for hydrogen, and the first and second 6tages are connected in 6eries and each stage contains an inlet end and an outlet end, (b) passing the feed mixture through the first stage of the pressure swing adsorption ~ystem to separate carbon monoxide as an adsorbed fraction and hydrogen, carbon dioxide, and ~ethane as a non-adsorbed fraction, (c) passing the non-adsorbed fraction from ~tep ~b) through the second stage of the pressure ~wing adsorption system to separate carbon dioxide and methane as an adsorbed fraction and hydrogen as a non-adsorbed pure product, (d) desorbing carbon dioxide and methane from the first and second 6tages of the pressure swing adsorption system to lo - 2 ~ 2 ~
form a carbon dioxide-rich fraction, and (e) desorbing carbon monoxide from the first 6tage of the pressure swing adsorption system to forD a pure carbon monoxide product.

~RIFF D~CRIP~IO~ OF T~ ~IG~R~B

FIGURE 1 i~ a schematic process flow diagram illustrating a novel combination of pre6sure 6wing adsorption sy6tems according to the present invention to separate hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane.

FIGURE 2 i8 a ~chematic process flow diagram illustrating a first pres~ure swinq adsorption method for separating hydrogen a~ a non-adsorbed product and carbon monoxide, carbon d~oxide, and methane as an adsorbed fraction, according to the pre6ent invention.

FIGURE 3 is a 6chematic proce6s flow diagram illustrating a second pressure swing adsorption method for separating carbon monoxide as an adsorbed product and hydrogen, carbon dioxide, and methane as a non-adsorbed fraction, according to the present invention.

FIGURE 4 is a schematic process flow diagram illustrating ~ novel two ~tage pressure ~wing adsorption system according to the present invention to ~eparate hydrogen ~nd carbon monoxide from a feed mixture compri6ing hydrogen, carbon monoxide, carbon dioxide, and methane.
FIGURE 5 i6 a schematic process flow diagram illustrating a fir6t stage of a pressure swing adsorption 6ystem for separating carbon monoxide as an adsorbed product and a second 6tage of a pressure swing adsorption 2 ~ 2 ~

system for separating carbon dioxide as an adsorbed fraction and hydrogen as a non-adsorbed product, according to the present invention.

D~TAIL~D D~CRIPTIO~ 0~ T~E ~V~T~ON

Applicant has discovered a ~ethod for sepAratinq hydrogen and carbon ~onoxide from a feed mixture comprising hydrogen, carbon ~onoxide, carbon dioxide, and methane. In a preferred embodiment, a novel combination of non-cryogenic separation steps is utilized which efficiently and economically yields tenriched hydrogen and carbon monoxide in high purity and yield from a feed mixture exiting a hydrocarbon steam reformer.
In a first preferred embodiment, the combination of non-cryogenic steps comprises two pressure ~wing adsorption systems. The first pressure ~wing adsorption sy6tem separates hydrogen from the f0ed mixture a3 a pure non-adsorbed product and carbon dioxide, methane, and carbon monoxide as an adsorbed fraction. The adsorbed fraction i8 then desorbed and passed to a second pressure swing adsorption system. The second pressure swing adsorption system separates carbon monoxide as an adsorbed fraction and hydrogen, carbon dioxide, and methane as a non-adsorbed fraction.

In ~ ~econd embodi~ent, a novel two stage pressure swing adsorption system iB utilized which efficiently and economically yields enriched hydrogen and carbon monoxide. The first 6tage of the pressure swing adsorption 6ystem eparate~ carbon monoxide as an adsorbed fraction and hydrogen, car~on dioxide, and methane as a non-adsorbed fraction. The second 6tage of the pressure ~wing adsorption system separates carbon dioxide and methane as an adsorbed fraction and hydrogen as a non-adsorbed pure product.

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The novel co~bination of pressure swing adsorption separation methods of the present invention provides 6ignificant savings in capital and operating expense over completely cryogenic methods. The steps in the present method may be integrated into steps in the hydrocarbon ~team reformer method to enhance the reforming proce~. For example, the carbon d~oxide-rich fraction from the fir6t pres6ure 6wing adsorption 6ystem in the first embodiment or the ~econd stage in the second embodiment may be recycled and used as fuel in the hydrocarbon steam reformer, further increasing the concentration of carbon monoxide in the feed mixture.
The hydrogen-rich fraction from the second pressure swing adsorption system in the first embodiment may also be recycled into the first pressure swing adsorption system to separate additional carbon monoxide.

The feed mixture (exhaust gas, effluent gas, exit gas, feed gas) in the present invention is a mixture compri~ing hydrogen, carbon monoxide, carbon dioxide, and methane. Preferably, the feed mixture is an effluent gas from a hydrocarbon steam reformer. The feed mixture will in general comprise hydrogen in an amount up to about 80%, carbon monoxide in an amount up to about 20%, carbon dioxide in an amount up to about 30%, and methane in an amount up to about 3%.

The feed mixture is typically available in a saturated state and may be dried by passing the mixture through a condenser (drier) containing a desiccant such as alumina, 6ilica, or zeolite. Desorption of the water from the desiccant may be accompli6hed by purging the desiccant with a dry waste purge ga6 (~uch as the carbon dioxide-rich fraction or nitrogen ga~). Any water remaining in the feed mixture iB removed with the strongly adsorbed stream ~carbon dioxide-rich fraction).
After being dried, the feed mixture may be compressed prior to passage of the mixture into the pressure 6wing adsorption 6ystem.

2 ~ 2 {3 The feed mixture from the hydrocarbon steam reformer will first be passed through a process cooler to cool the gas and condense and remove water vapor. To maximize the carbon monoxide concentration and minimize the carbon dioxide concentration in the feed ~ixture, the hydrocarbon ~team reformed feed mixture will by-pass the shift converter.

A typical feed mixture from a hydrocarbon steam reformer will have a pres6ure in the range from about 150 psia to about 600 psia, preferably from about 150 psia to about 400 psia, and more preferably from about 150 psia to about 300 psia. Generally the feed mixture will be available at a pressure sufficiently high to be used directly in the first pressure swing adsorption system. Optionally, a compressor may be employed to compress the feed mixture to the required pressure for the pressure swing adsorption system.
The adsorbent material in the adsorbent bed in the hydrogen pressure swing adsorption system (first pres6ure swing adsorption ~y~tem in the first embodiment and second stage in the pressure ~wing adsorption system in the second embodiment~ is an adsorbent having a greater affinity for carbon dioxide, methane, and carbon monoxide than for hydrogen. m e adsorbent material may be a molecular sieve or activated carbon, and preferably i~ a combination of molecular sieve~ and activated carbon. Both calcium and sodium aluminosilicate zeolites may be employed. Carbon molecular sieves and ~ilica molecular sieves are also u~eful. Suitable zeolite sieve6 include, but are not limited to, the type 5A, lOX, 13X zeolite ~olecular sieves, and mordenitee. Preferred zeolite sieve6 are the type 5A zeolite sieve6 and molecular 6ieves with comparable pore size and molecular attraction.

~a~2~

The adsorbent material in the adsorbent bed in the carbon monoxide pressure swing adsorption system (second pressure swing adsorption system in the first embodiment and first stage in ths pressure ~wing adsorption system in the second embodiment) is an adsorbent having a greater affinity for carbon ~onoxide than for hydrogen, carbon dioxide, and methane. In general, suitable adsorbent materials ~re copper exchanged substrate~ 6uch as those selectQd from the group consi~ting of copper exchanged Y-type aluminosilicate zeolite Dolecular sieves, copper exchanged alumina, and copper exchanged activated carbon, and mixtures thereof. In a preferred embodiment, the adsorbent material is copper aluminosilicate zeolite molecular sieve material, available under the tradename NXX type adsorbent in a pac~age from Nippon RoXan X. X., Tokyo, Japan. Copper aluminosilicate zeolite molecular sieves can be prepared by exchanging sodium in sodium aluminosilicate zeolite molecular sieve~ with copper (2+) followed by a heating and reducing treatment to enhance the affinity of the adsorbent for carbon monoxide and reduce the affinity of the adsorbent for carbon dioxide.
Copper exchanged 6upports, and methods for preparing such supports, are described in detail in United States patent no. 4,917,711, Japanese patent JP01203019, United States patent no. 4,914,076, United States patent no. 4,783,433, Japanese patent JP61242908, and United States patent no. 4,743,276, which reference~ are described above and are incorporated herein by reference.
In accord with the present invention, hydrogen gas product can be prepared having a purity of greater than about 99%, preferably greater than about 99.99%, and more preferably greater than about 99.999%.
A carbon monoxide gas product can be prepared having a purity of greater than about 98%, preferably greater than about 99%, and more preferably greater than about 99.85%.

2 ~

The method for producing hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and ~ethane can be better underetood by reference to the FIGURES in which like numerals refer to like parts of the invention throughout the FIGURES. Although the present invention i8 described and illustrated in connection with preferred embodiments, applicant intends that modifications and variations may be used without departing from the spirit of the present invention.

FIGURE 1 illustrates a preferred first embodiment of the present invention for producing hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane by a combination of pressure Qwing a~sorption steps. In FIGURE 1, gaseous feed mixture is fed through feed conduits 1 and 2 to hydrogen (first) pressure swing adsorption system A to separate the mixture. ~ypically the feed mixture from the hydrocarbon ~team reformer will enter hydrogen pre6sure swing adsorption sy~tem A at a pressure in the range from about 150 psia ~o about 600 psia, preferably from about 150 psia to about 400 psia, and more preferably from about 150 psia to about 300 psia. Optionally a compressor may be employed to compress the feed mixture to the pressure swing adsorption pressure. After being cooled, the feed mixture sntering hydroqen pressure swing adsorption system A will be at ambient temperature.
During the hydrogen product production step, feed mixture iQ fed into and hydrogen product is withdrawn from hydrogen pressure 6winq adsorption system A. Hydrogen product i8 ~eparated as a pure non-adsorbed product and carbon monoxide, carbon dioxide,methane, and water vapor are separated as an adsorbed fraction. Hydrogen product (merchant grade, le~s than about 10 vpm impurities) is withdrawn from hydrogen pressure swing adsorption system A through feed conduit 3 and passed to hydrogen reservoir B.

After the hydrogen product production step, hydrogen pressure ~wing adsorption system A undergoes a pressure e~ualization step and an intermediate depressurization step (carbon monoxide-rich fraction production step). During the carbon monoxide-rich fraction production step, a carbon monoxide-rich fraction lo i6 desorbed and withdrawn from hydrogen pressure ~wing adsorption sy6tem A via feed conduit 4 and passed to carbon monoxide storage vessel C.

After the carbon monoxide-rich fraction production step, hydrogen pressure swing adsorption system A undergoes a depressurization 6tep and a hydrogen product gas purge ~tep (carbon dioxide-rich fraction production 6teps). In the fir6t carbon dioxide-rich fraction production 6tep, a carbon dioxide-rich fraction i6 withdrawn by depressurizing hydrogen pressure 6wing adsorption system A. In the second carbon dioxide-rich fraction production step, a carbon dioxide-rich fraction is withdrawn by purging hydrogen pressure 6wing adsorption system A with hydrogen product gas. The carbon dioxide-rich fractions are withdrawn via feed conduit 5 and passed to carbon dioxide compressor D.
Carbon dioxide compressor D compresse6 the carbon dioxide-rich fractions to the 6team reforming pressure.
The compressed carbon dioxide-rich fractions from carbon dioxide ~ompressor D are then pas6ed through feed conduit 6 to the hydrocarbon steam reformer for recycle a6 reformer feed ga6 to enrich the carbon monoxide content of the reformer product gas. In general, carbon dioxide compres60r D will compre6~ the carbon dioxide-3S rich fraction to a pre6sure in the range from about150 psia to about 600 psia, preferably from about 150 psia to about 450 psia, and more preferably from about 150 psia to about 350 p~ia.

2 ~

The carbon monoxide-rich fraction in carbon monoxide storage vessel C i8 passed to carbon monoxide compressor E via feed conduit 7. Carbon monoxide compressor E compresses the carbon monoxide-rich fraction S to the separation pressure. In general, carbon monoxide compressor E will compres~ the carbon monoxide-rich fraction to a pressure in the range from nbout 20 psia to about 600 psia, preferably from about 20 psia to about 200 psia, and more preferably from about 20 psia to about 100 psia. Compre6sed carbon monoxide-rich fraction is then pas6ed from carbon monoxide compre~sor E to carbon monoxide (second) pres~ure ~wing adsorption system F via feed conduit 8 to further separate the mixture. The feed gas in feed conduit 8 may be available at temperatures ranging from about ambient to about 150 F.

During the carbon monoxide adsorption step, carbon monoxide-rich feed mixture is fed into, and carbon dioxide, methane, water vapor, and any remaining hydrogen is ~ithdrawn from, carbon monoxide pressure ~wing adsorption 6y~tem F. Carbon monoxide is ~eparated as an adsorbed product and carbon dioxide, methane, any water vapor, and any hydrogen are separated as a non-adsorbed fraction. The carbon dioxide, methane, water vapor, and hydrogen non-adsorbed fraction is withdrawn from carbon monoxide pressure swing adsorption system F through feed conduit 9 to recycle compressor G. Recycle compressor G
compre6ses the non-adsorbed gases to the hydrogen pre66ure swing adsorption ~eparation pressure. In general, the recycle compres60r G will compress the non-adsorbed gases to a pressure in the range from about 150 p~ia to about 600 psia, preferably from about 150 psia to about 400 psia, and more preferably from about 150 psia to ~bout 310 psia. The compressed non-adsorbed gases from recycle compressor G are thenrecycled to hydrogen pressure ~wing adsorption 6ystem A
through feed conduits 10 and 2.

2~5 ï~

After the carbon monoxide adsorption step, carbon monoxide pressure swing adsorption ~ystem F
undergoes a pressure egualization step, a carbon monoxide product gas purge ~tep, and a carbon monoxide product production step (a vacuum desorption step). During the pressure equalization step, a portion of the void gas, which contains mainly non-adsorbed ga~eou~ impurities, iB
withdrawn from the outlet (top) end of the bed being depre~surized and pas~ed to the outlet (top) end of the bed beinq pressurized to enrich the depressurized bed in the adsorbed carbon monoxide component. During the carbon monoxide product gas purge ~tep, carbon monoxide is introduced into the inlet (bottom) end of the depressurized bed in a cocurrent direction to purge and force remaining non-adsorbed gaseous i~purities to the outlet (top) end of the bed and into the purge exhaust gas recycle tank L (see FIGURE 3) for recycle into the feed gas. During the carbon monoxide product production step, carbon monoxide i8 desorbed and withdrawn ~rom the inlet (bottom) end of carbon monoxide pressure swing adsorption system F via feed conduits 12, 13 and 14 by applying vacuum from vacuum pump H via feed conduit 12.
The carbon monoxide product (merchant grade, léss than about 1500 vpm impurities) is passed through feed conduits 12, 13, and 14 to carbon monoxide reservoir I.

FIGURE 2 illustrates a first (hydrogen separation) pressure swing adsorption system for separating hydrogen from ~ feed ~ixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane according to the present invention. A~ set out in FIGURE 1, the feed mixture from the hydrocarbon eteam reformer i5 passed through feed conduit~ 1 and 2 to hydrogen pressure swing adsorption system A.
In FIGURE 2, the hydrogen pressure swing adsorption system A comprises adsorption beds A, B, C, and D, carbon monoxide storage vessel C, carbon dioxide-rich gas buffer vessel J, hydrogen product reservoir B, hydrogen product pressure control valve PCV1, carbon dioxide-rich fuel gas pressure control valve PCV2, carbon monoxide product pressure control valve PCV3, repressuri2ation flow control valve FCV1, hydrogen product purge ga~ flow control valve FCV2, ~top valves 21 through 49, and non-return valves S0 and S1.

Ad60rption beds A through D are connected in parallel. Each of the adsorption bed6, A through D, i6 physically divided into two bed parts, a first adcorption bed and a second ad~orption bed, Al/A2, Bl/B2, Cl~C2, ~nd D1/D2, respectively, which are connected in 6eries. Each adsorption bed contains an inlet (feed, bottom) end and an outlet (discharge, top) end. The two part ad60rption bed6 facilitate removal of a carbon monoxide stream from an intermediate position in the bed. The carbon monoxide stream i8 drawn at an intermediate pressure, for example at about 25 psia, and passed to carbon monoxide ~torage vessel C. The adsorbent bed comprises an adsorbent having a greater a~finity for carbon dioxide, methane, and carbon monoxide than for hydrogen. For example, the first adsorption bed (for example, bed A1) comprises an activated carbon adsorbent material and the 6econd adsorption bed ~for example, bed A2) comprises a lower level of an activated carbon adsorbent material and an upper level of a zeolite molecular 6ieve adsorbent.

The fir6t (hydrogen separation) pressure 6wing adsorption sy6tem is operated in accordance with the full cycle 6equence ~hown in Table 1. The ~eguence is described below in detail using bed~ Al/A2. 8eds 81/82, Cl/C2, and Dl/D2 are employed in the same ~equence but at an offset as 6hown in Table 1. All stop valves may be controlled automatically on a predetermined schedule.

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2~3~ih At the start of the pressure swing adsorption cycle, bed Al/A2 is in the hydrogen production step.
Feed mixture from the hydrocarbon steam reformer is passed to the inlet end of bed A1 via feed conduits 1 and 2 (see FIGURE 1) and open ~top valve 21 at a pressure typically in the range from about 150 psia to about 600 psia. ~he feed mixture is adsorbed in adsorption bed A1/A2 to selectively sieve hydrogen as a non-ad~orbed product ~nd carbon dioxide, carbon monoxide, and Dethane as adsorbed products. Non-adsorbed hydrogen product gas is withdrawn from the outlet end of bed A2 via open stop valve 37 and passed to hydrogen product reservoir B via hydrogen product pressure control valve PCVl. The hydrogen product gas typically contains less than about 10 vpm impurities.

During the hydrogen production step, the activated carbon adsorbent material in bed hl/A2 adsorbs carbon dioxide and water vapor more strongly than carbon monoxide and methane, which in turn are more strongly adsorbed than hydrogen. As the feed mixture flows through the adsorbent bed, the non-adsorbed mixture becomes enriched in hydrogen. The zeolite ~olecular sieve adsorbent removes all but traces of other gases and yields a hydrogen product substantially free of impurities. The flow of feed mixture into the inlet end of the first adsorbent bed and flow of product gas from the outlet end of the second adsorbent bed are stopped just before the breakthrough point of non-hydrogen components fro~ the outlet end of the second adsorbent bed. A typical feed and production cycle iB conducted for a period of about two to about ~ix minute~.

When the non-hydrogen co~ponent~ in the feed mixture advance close to the outlet end of bsd A2, the hydrogen production step in bed Al/A2 is stopped. Stop valves 21 and 37 are closed stopping the production of hydrogen. Bed Al/A2 i8 then depressurized and bed Cl/C2 i~ repressurized by pressure equalization of the beds.

2 ~

Stop valves 38 and 44 are opened and lean gas i6 passed from the outlet end of bed ~2 to the outlet end of bed C2 to substantially equalize the pressure of bed Al/A2 and bed Cl/C2. This pressure egualization step typically is conducted ~or a period of about twenty to about forty ~econds.

During the presgure equalization ~tep, the pressure in bed Al/A2 decrease~ causing carbon monoxide to be desorbed from the adsorbent material in preference to methane, carbon dioxide, and water vapor. Optionally, bed A1/A2 may be pressure equalized with an equalization tank through the outlet end of bed A2. The gas collected in the equalization tank is subseguently used to repressurize a bed in the pressure swing adsorption ~ystem.

After the preesure egualization step is complete, bed A1/A2 begins the carbon monoxide production step. Bed Al/A2 is depressurized by an intermediate depressurization step to withdraw and produce a carbon monoxide-rich fraction. Stop valves 38 and 44 are closed and ~top valve 33 is opened to withdraw the carbon monoxide-rich fraction from a position intermediate between be~ Al and bed A2. The carbon monoxide-rich fraction i~ drawn at an intermediate pressure, for example at about 25 psia, and pas6ed to carbon monoxide storage vessel C. The carbon monoxide-rich fraction from carbon monoxide storage vessel C i8 then passed to carbon monoxide pressure swing ad~orption system F via feed conduits 7 ~nd 8 (see FIGURE 1).

During the intermediate depressurization step, carbon monoxide, desorbed into the void spaces during the pressure egualization 6tep, iB withdrawn. Withdrawal of carbon monoxide during the intermediate depressurization 6tep causes the pressure in the bed to further decrease resulting in ~dditional carbon monoxide desorption.
Withdrawal of carbon monoxide from a location 2 ~ 5 ~ ~ 2 ~

intermediate between bed A1 and bed A2 minimizes retention of carbon monoxide (and carbon dioxide) in bed A2 which could contaminate a subsequent hydrogen production ~tep. The time for the carbon monoxide production ~tep is typically about two minutes. The re~ulting carbon monoxide-rich fraction, which is produced at a pressure between about 25 psia and abou~
40 psia, generally contains carbon monoxide at leact at about a volume fraction of about 2.5 times that in the feed gas, the remainder being ~ainly hydrogen with up to about 2% of ~ethane and carbon dioxide.

When the intermediate depressurization carbon monoxide production 6tep is complete, bed Al/A2 undergoes carbon dioxide-rich fraction production steps ~a depressurization step and a hydrogen product gas purge step). In the first carbon dioxide-rich fraction production step, a carbon dioxide-rich fraction i8 withdrawn by depressurizing bed A1/A2. During the depressurization ~tep, ~top valve 33 i~ closed and stop valve 23 i~ opened. The carbon dioxide-rich fraction is withdrawn from the inlet end of Bed A1. The flow of the carbon dioxide-rich fraction ls countercurrent to the flow of the feed mixture during the hydrogen production step. In general, the carbon dioxide-rich fraction is produced at a pressure of about 20 psia. Ths reduction in pressure during the depressurization step and withdrawal of the carbon dioxide-rich fraction cause~
desorption of carbon dioxide from the adsorbent.
Generally, the withdrawal of the carbon d$oxide-rich fraction i8 conducted for a period of one to two minutes.
In the second carbon dioxide-rich fraction production stept ~ carbon dioxide-rich fraction i8 withdrawn by purging bed Al/A2 w$th ~ydrogen product ga~. Stop valve 38 ~6 opened. Hydrogen product purge flow control valve FCV2 is opened and bed Al/A2 is purged with hydrogen product gas from bed Cl~C2. The flow of hydrogen product purge gas iB countercurrent to the flow of the hydrogen gas during the hydrogen production step.

2~C~ 9 Generally, the product gas purge is conducted for a period of about three minutes. The resultinq carbon dioxide-ric~ fraction generally contains at least about 50% by volume carbon dioxide and less than 10% by volume carbon monoxide, traces of methane and water vapor, with the balance being hydrogen. All water vapor in the feed ga~ iB separated into the carbon dioxide-rich fraction.

After the carbon dioxide-rich fraction 10 production ~teps are complete, bed Al/A2 iB repressurized and bed C1/C2 iB depressurized by pressure equalization of the beds. Stop valve 23 and hydrogen product purge flow control valve FCV2 are closed and stop valves 44 is opened. Void gas is passed from the outlet end of bed C2 15 to the outlet end of bed A2 to substantially equalize the pressure of bed Al/A2 and bed Cl/C2.

After the pressure equalization 6tep (repressurization step) iB complete, bed Al/A2 i8 20 back~illed with hydrogen product gas. Stop valves 38 and 44 are clo~ed and bed Al/A2 iB repressurized by backfill with product gas. Product gas from producing bed Dl/D2 i5 passed through open repressurization flow control valve FCVl and into the outlet end of bed A2 to 25 backfill bed Al/A2 through open 6top valve 39.

When the backfill step is complete, bed A1/A2 is ready to again begin the hydrogen production ~tep.
Repressurization flow control valve FCVl and ~top 30 valve 39 are closed and 6top valves 21 and 37 are opened to admit feed mixture from feed conduit 2 to the inlet end of bed Al. The hydrogen production step in bed Al/A2 is begun and the cycle i8 repeated. Beds Al/A2, Bl/B2, C1/C2, and D1/D2 operate in the sequence set out in 35 Table 1. In general, the time to complete a cycle (cycle time) is in the range from about 60 seconds to about 1500 seconds, preferably from about 180 seconds to about 960 seconds, and more preferably from about 240 seconds to about 720 ~econds.

- 25 - ~a ~2 a FIGURE 3 illustrates a second (carbon monoxide separation) pressure swing adsorption system for separating carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane according to the present invent$on. As set out in FIGURES 1 and 2, the feed mixture from hydrogen pressure swing adsorption ~ystem A ie passed through carbon monoxide 6torage vessel C and carbon monoxide compressor E to carbon monoxide pressure swinq adsorption system F.

In FIGURE 3, the carbon monoxide pressure swing adsorption system F comprises adsorption beds A', B', C', and D', carbon monoxide compressor E, carbon monoxide storage vessel X, carbon monoxide product reservoir I, purge gas recycle vessel L, stop valves 101 through 131, and pressure control valves PCV101, PCV102, PCV103, and flow control valves 101 and 102. Adsorption beds A' through D' are connected in parallel and each adsorption bed contain~ an inlet ~feed, bottom) end and an outlet (discharge, top) end. The adsorbent bed comprises an adsorbent having a greater affinity for carbon monoxide than ~or hydrogen, carbon dioxide, and methane. For example, the adsorption beds may contain an adsorbent such a~ copper exchanged aluminosilicate zeolite molecular sieves, copper exchanged alumina, and copper exchanged activated carbon, and mixtures thereof.

The second (carbon monoxide) pressure swing adsorption system is operated in accordance with the full cycle sequence ~hown in Table 2. The sequence is described below in detail using bed A'. Beds B', C', and D' are employed in the ~ame eequence but at an offset as shown in Table 2. All ~top valves may be controlled automatically on a predetermined schedule.

- 26 ~ J;~

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- 27 - ~ 3 ~; 2 ~
At the 6tart of the pressure 6wing adsorption cycle, bed A' is in the carbon monoxide adsorption step.
Carbon monoxide-rich feed mixture from carbon monoxide compressor E is passed to the inlet end of bed A' via open stop valve 101 at a pressure typically in the range from about 25 psia to about 600 psia. The feed ~ixture i8 adsorbed in adsorption bed A' to ~electively sieve carbon monoxide as an adsorbed fraction~ Non-adsorbed carbon dioxide, hydro~en, ~ethane, ~nd any carbon monoxide are withdrawn from the outlet end of bed A' via open ~top valve 105 and passed to feed conduit 9 for recycle to hydrogen pressure ~wing adsorption ~ystem A
(see FIGURES 1 and 2).

During the carbon monoxide adsorption ~tep, the adsorbent bed of copper alu~inosilicate zeolite molecular sieves adsorbs carbon monoxide more strongly than carbon dioxide, hydrogen, and methane. As the feed mixture flows through the adsorbent bed, the bed becomes enriched in carbon monoxide. The flow of carbon monoxide-rich ieed mixture into adsorbent bed A' is stopped just before the breakthrough point of carbon monoxide from the outlet end of adsorbent bed A'. A typical feed and carbon monoxide adsorption cycle i8 conducted for a period of about two to about six minutes.

When carbon monoxide in the feed mixture advances close to the outlet end of bed A', the carbon monoxide ~dsorption step in bed A' is stopped. Stop valves 101 and 105 are closed stopping the carbon monoxide ~dsorption step. Bed A' is then depressurized and bed C' is repressurized by pressure equalization of the bed6. Stop valves 114, 130, and 125 ~re opened and lean gas is passed from the outlet end of bed A' to the outlet end of bed C' to 6ubstantially equalize the pressure of bed A' and bed C'. ~his pressure equalization ~tep typically i8 conducted for a period of about twenty to about forty seconds. Optionally, bed A' may be pressure equalized with an equalization tank ~ ~3 through the outlet end of the adsorption bed. The gas collected in the equalization tank i8 subsequently used to repressurize a bed in the pressure 6wing adsorption ~ystem. If the adsorption pres6ure i8 very low (for example, under 25 psia or lower), the pressure equalization step i~ not employed or a partial pressure egualization step i8 employed.

After the pressura egualization 6tep, a carbon monoxide product cocurrent purge step i~ employed to displace the impurities ~carbon ~onoxide, hydrogen, and methane] in the void gas left in the bed ~fter the adsorption and pressure equalization ~teps. Bed A' i8 purged from the inlet end with carbon monoxide product gas from carbon monoxide product gas reservoir I. Stop valve 130 is closed and stop valves 110, 128, and 139 and product purge flow control valve FCV102 are opened.
Product purge gas effluent is then passed to purge gas recycle vessel L. The flow of carbon monoxide product purge gas i5 cocurrent to the ~low of the carbon monoxide gas during the carbon monoxide adsorption ~tep. The product purge gas from purge gas recycle vessel L is then recycled to the carbon monoxide pressure swing adsorption 6ystem via carbon monoxide compressor E. Generally, the purge with product gas is conducted for a period of about three minutes.

After the cocurrent carbon monoxide purge 6tep, bed A' undergoe6 a carbon monoxide production step.
Stop valves 128, 110, 114, and 129 are closed and stop valve 119 is opened. The adsorbed carbon monoxide is removed from the inlet end of bed A' by applying a vacuum u6ing vacuum pump H and open 6top valve 119. The resulting carbon monoxide-rich fraction generally contain~ at least about 98% by volume carbon monoxide with the balance being trace6 of hydrogen, carbon dioxide, and methane. In general, the desorption pressure i~ from about 75 torr to about 300 torr, preferably from about 100 torr to about 150 torr.

. ~3 After the carbon monoxide production ~tep (vacuum regeneration step) iB complete, bed A' is repre~surized and bed C' i8 depressurized by pre6sure squalization of the beds. Stop valves 130 and 123 ~re opened and void gas iB passed from the outlet end of bed C' to tbe outlet end of bed A' to Qubstantially egualize the pre~sure of bed A~ and bed C'.

After the repressurization ~tep, bed A' is bac~filled with hydrogen-rich product gas. Stop valves 130 is closed and stop valves 131 is opened.
Product gas from producing bed D' is passed through repressurization flow control valve FCVlO1 into the lS outlet end of bed A' to backfill and repressurize bed A~.

When the backfill ~tep is complete, bed A' is ready to again begin the carbon monoxide adsorption 6tep.
Repre~surization flow control valve FCV101 and stop valves 131 and 123 are closed and stop valves 101 and 105 are opened to admit carbon monoxide-rich feed mixture from compressor E to the inlet ~nd of bed A'. The carbon monoxide adsorptiGn step in bed A' is begun and the cycle i6 repeated. Beds A', ~', C', and D' operate in the sequence set out in Table 2. In general, the time to complete a cycle (cycle time) is in the range from about 60 seconds to about 1500 ~econds, preferably from about 240 ~econds to about 960 seconds, and more preferably from about 240 seconds to about 720 ~econds.
In a preferred ~mbodiment, the present invention i~ directed at a method for producing hydrogen and carbon monoxide from ~ feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane, which comprises the steps of:
ta) passing the feed mixture through a first pressure swing adsorption ~ystem containing an adsorption bed comprising an adsorbent having a greater ~ffinity for carbon dioxide, methane, and carbon monoxide than for 2~3 hydrogen to separate hydrogen as a pure non-adsorbed product and carbon dioxide, methane, and carbon ~onoxide as an adsorbed fraction;
(b) desorbing carbon monoxide from the pre~ure ~wing adsorption ~y~tam in ~tep ~a) to form a carbon monoxide-rich fr~ction;
(c) desorbing carbon dioxide and methane from the pressure swing adsorption ~ystem in step (a) to form a carbon dioxide-rich fraction;
(d) passing the carbon monoxide-rich fraction from step (b) to a second pressure swing adsorption system containing an adsorption bed comprising an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane to separate carbon monoxide as an adsorbed fraction and hydrogen, carbon dioxide, and methane as a non-adsorbed fraction; and (e) desorbing carbon monoxide from the pressure swing adsorption system in step (d) to form a pure carbon monoxide product.

FIGURE 4 illu6trates a second embodiment of the present invention for producing hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane by using a two stage (first stage/second stage, top stage/bottom stage) pressure swing adsorption system. In this embodiment, hydrogen is the unadsorbed gas, carbon dioxide and methane are the first desorbed gases, and carbon monoxide 30 iB the strongly ~dsorbed gas. In FIGURE 4, ga~eous feed mixture is fed through feed conduit 70 to two 3tage pressure swing adsorption ~ystem M to separate the mixture. Typically the feed mixture from the hydrocarbon steam reformer will enter two stage pressure swing adsorption system M at a pres~ure in the range from about 150 psia to about 600 psia, preferably from about 150 psia to about 400 p6ia, and more preferably from about 150 psia to about 300 psia. After being cooled, 2 ~3 the feed mixture entering pressure swing adsorption system M will be at ambient temperature.

During the hydrogen product production ~tep, feed mixture i8 fed into and hydrogen product i8 withdrawn from two ~tage pressure swing adsorption system M. Hydrogen product is separated as a pure non-adsorbed product and carbon monoxide, carbon dioxide, methane, and water vapor i~ separated a~ an ~dsorbed fraction. Hydrogen product (merchant grade, less than about 10 vpm impurities) is withdrawn from two 6tage pressure swing adsorption system M through feed conduit 71 and passed to hydrogen reservoir B.

After the hydrogen product production step, pres~ure swing adsorption system ~ undergoes a pressure equalization step, an intermediate depressurization step, and a ~econd 6tage purge/first stage purge step (carbon dioxide-rich fractlon production 6teps). Durlng the intermediate depres~urization step, carbon dioxide-rich gas i8 collected as 6econdary product, compressed, and recycled to the reformer feed gas. Durinq the second stage purge/first stage purge step, the second 6tage is purged with hydrogen from another 6tage and the first stage is purged with carbon monoxide product gas from the receiver. The depressurization and purge effluent gases are passed to carbon dioxide reservoir N and collected as secondary product via feed conduit 72. The gases are then passed to compressor 0 via feed conduit 73 and compressed And recycled to the refGrmer feed gas v~a feed conduit 74. In general, carbon dioxide compres~or 0 will compress the carbon dioxide-rich fraction to a pressure in the range from about 150 psia to about 600 psia, preferably from about 150 psia to about 450 psia, and more preferably from about 150 psia to about 350 psia.

After the carbon dioxide-rich fraction production steps, pres6ure ~wing adsorption 6y6tem M
undergoes a ~econd stago purge/first 5tage evacuation - 32 - 2~
step (sarbon monoxide production step). In the ~econd stage purge/first stage evacuation step, the 6econd stage is purged with hydrogen ga~ and the first 6tage is evacuated using vacuum pump P to remove carbon monoxide S product gas. The carbon monoxide product gas i8 withdrawn through feed conduits 77 and 78 and pas6ed to carbon monoxide reservoir Q.

After the carbon monoxide production step, pressure swing adsorption system ~ undergoe~ ~ pre~sure equalization ~tep (repressurizAtion). During the pressure equalization step, the bed is repressurized by pressure equalization with another bed. The bed is then repressurized to adsorption pressure using hydrogen gas from the pressure swing adsorption 6ystem. The carbon monoxide product (merchant grade, le88 than about 1500 vpm impurities) is passed through feed conduits 77, 78, and 75 to carbon monoxide reservoir Q.

FIGURE S illustrates a two stage pressure swing adsorption method for separating carbon monoxide and hydrogen from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane in accord with the ~econd embodiment of the present invention. As set out in FIGURE 4, the feed mixture from the hydrocarbon 6team reformer is passed through feed conduit 70 to pressure 6wing adsorption system M.

In FIGURE 5, the two stage pressure swing adsorption ~ystem M comprises adsorption beds A'', ~3'', C'', and D'', carbon monoxide ~torage vessel Q, carbon dioxide-rich gas buffer ve~sel N, hydrogen product reser~oir B, hydrogen product pressure control valve PCVl, carbon monoxide-rich product gas pressure control valve PCV2, carbon dioxide product pressure control valve PCV3, repressurization flow control valve FCVl, hydrogen product purge gas flow control valve FCV2, 6top valves 221 through 259, and non-return valves 249 and 250.

- 33 - 2~3~

Adsorption beds A'' through D'' are connected in parallel. Each of the adsorption beds, A'' through D'', is physically divided into two stages, a S first (bottom) stage and a ~econd (top) stage, Al''/A2'', B1''/B2'', C1''/C2'', and D1''/D2'', respectively, which are connected in 6eries. Each stage contains an inlet (feed) end and a outlet (di6charge) end. The first stage and second staqe of each bed are i601ated by two stop valves for sequential depressurization and carbon monoxide production steps (i.~., first stage Al'' and 6econd stage A2'' are i601ated by 6top valves 252 and 256). The two part ~tages f~cilitate removal of a carbon dioxide stream from an intermediate position in the bed.
The carbon dioxide stream is drawn at an intermediate pressure, for example at about 25 psia, and passed to carbon dioxide 6torage vessel N. The first adsorption bed stage (for example, first 6tage Al'') compri6es an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane and may be ~lected fro~ the group consisting of copper exchanged Y-type aluminosilicate zeolite molecular sieves, copper exchanged alumina, and copper exchanged activated carbon.
The second adsorption bed stage (for example, 6econd stage A2'') comprises an adsorbent having a greater affinity for carbon dioxide, methane, and carbon monoxide than for hydrogen and may be a molecular sieve or activated carbon, and preferably i8 a combination of molecular 6ieves and activated carbon.
The two 6tage pressure swing adsorption system i8 operated in accordance with the full cycle sequence shown in Table 3. The sequence i~ described below in detail using stages Al''/A2''. Stages Bl''/B2'', Cl''/C2'', and Dl''/D2'' are employed in the 6ame sequence but at an offset as shown in Table 3. All stop valves may be controlled automatically on a predetermined schedule .

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At the start of the pressure ~wing adsorption cycle, first stage Al'' and 6econd fitage A2'' are in the hydrogen production step. Feed mixture from the hydrocarbon ~team reformer is passed to the inlet end of first 6tage Al'' via open stop valve 221 at a pressure typically in the range from about 150 psia to about 600 psia. The feed mixture pas6es from the outlet end of the first ~tage A1~' through stop valve~ 252 and 2S6 to the inlet end of the ~econd 6tage A2''. The feed mixture 0 iB adsorbed in first 6tage A1'' and ~econd ~tage A2'' to 6electively 6ieve hydrogen as an non-adsorbed fraction and carbon dioxide, carbon monoxide, and methane as adsorbed products. Non-adsorbed hydrogen product gas is withdrawn from the outlet end of 6econd ~tage A2'' via open stop valve 237 and passed to hydrogen product reservoir B via non-return valve 249 and hydrogen product pressure control valve PCV1. The hydrogen product gas typically contains less than about 10 vpm impurities.

During the hydrogen production step, the copper exchanged 6ubstrate in first stage Al'' preferentially ad60rbs carbon monoxide. The activated carbon/molecular 6ieve adsorbent material in second 6tage A2'' preferentially adsorb~ carbon dioxide, water vapor, and methane more strongly than hydrogen. As the feed mixture flows through the adsorbent stages, carbon monoxide becomes concentrated in first 6tage Al'', carbon dioxide, water vapor, and methane become concentrated in second stage A2'', and the mixture exiting the bed ~tages becomes enriched in hydrogen. The zeolite molecular sieve adsorbent removes all but traces of other gase6 and yields a hydrogen product substantially free of impuritie6. The flow of feed mixture into the inlet end of the first 6tage and flow of product ga~ from the outlet end of the 6econd stage are ~topped ~u~t before the breakthrough point of non-hydrogen components from the outlet end of the second 6tage. A typical feed and production cycle i~ conducted for a period of about two to about six minutes.

2 ~ 2 i~

When the non-hydrogen components (i.e., methane) in the feed mixture advance close to the outlet end of second stage A2'', the hydrogen production step in ~irst ~tag~ A1''/A2'' ia stopped. Stop valves 221 and 237 are clo~ed ~topping the production o~ hydrogen.
First 6tage Al'' and second ~tage A2'' are then depressurized (treated as one ~ed with ~top valves 252 and 256 open) and stage Cl''/C2'' i8 repressurized (treated as one bed with Etop valves 254 and 258 open) by pres6ure equalization of the beds through the outle~
(top, discharge) ends of the beds. Stop valves 238 and 244 are opened and void gas is passed from the outlet end (top) of second stage A2'' to the outlet end (top) of bed C2'' to substantially equalize the prçssures in beds Al''/A2'' and beds Cl''/C2''. This pressure equalization 6tep typically is conducted for a period of about twenty to about forty seconds.

During the pressure equalization 6tep, void gas containin~ carbon dioxide and methane i8 passed to the repressurized bed and the pressure in first stage A1''/A2'' decreases. Optionally, first stage Al''/A2'' may be pressure egualized with an equalization tank through the outlet end of second stage A2''. The gas collected in the equalization tank is ubsequently u~ed to repressurize a bed in the pressure ~wing adsorption ~ystem.

After the pressure equalization 6tep is comple~e, first ~tage A1''/A2'' begins the intermediate depressurizAtion ~tep (~ ~ir~t carbon dioxide-rich fraction production ~tep). During the intermediate depres~urization ~tep, ~ir~t stage Al'' ~nd 6econd 6tage A2'' ~re depressurized from an intermediate location to withdraw and produce a carbon dioxide-rich fraction. Stop valves 238 and 244 are closed and stop valves 233, 252, and 256 are opened to withdraw the carbon dioxide-rich fraction from a position intermediate - 37 - 2~3~
between first stage A1'' and second stage A2''. The carbon dioxide-rich fraction i8 drawn at an intermediate pressure, for example at about 25 psia, and passed to carbon dioxide storage vessel N. The carbon dioxide-rich fract~on ~rom carbon dioxide storage vessel N is then passed to the reformer feed gas via compressor 0.

During the intermediate depressurization 6tep, void gas in second stage A2'' which i~ predominantly carbon dioxide, i~ withdrawn from the inlet (bottom) end of the bed. Withdrawal of carbon dioxide-rich void gas from the outlet (top) end of first ~tage A1'' causes the carbon monoxide, adsorbed near the bottom of the bed, to be desorbed displacing additional void gas. Withdrawal of carbon dioxide from a location intermediate between first stage A1'' and second stage A2'' minimizes retention of carbon dioxide in the top region of second stage A2'' which could contaminate a subsequent carbon monoxide production step. The time for the intermedi~te depre~surization step i~ typ~cally about two minutes.

When the intermediate depressurization carbon monoxide production step i~ complete, first stage A1''/A2'' undergoes a second stage purge/first stage purge step (a second carbon dioxide-rich fraction production 6tep). During the second 6tage purge/first stage purge step, ~econd stage A2''i~ purged with hydrogen gas and first stage Al'' is purged with carbon monoxide product gas from carbon monoxide reservoir Q.
During the purge of ~econd stage A2'' with hydrogen gas, ~top valves 251, 138, ~nd 233 are opened. Hydrogen from hydrogen reservoir 8 is t~en passed through open flow control valve FCV2, open non-return valve 250, and open ~top valves 251 ~nd 238, through second stage A2'', and open stop valve~ 256 and 233 to carbon dioxide reservoir N. During the purge of first stage A1'' with carbon monoxide ga~, stop valves 222, 252, 230 are opened. Carbon monoxide from carbon monoxide reservoir Q
is then passed through open flow control valve FCV3, open - 38 - 2 ~3~h stop valve 222, through first ~tage Al'~, and open 6top valves 252 and 233 to carbon dioxide reservoir N. The flow of hydrogen through second 8tage A2'' i8 in a direction countercurrent and the flow of carbon monoxide $s in a direction cocurrent to the flow of the feed mixture during the hydrogen production step. Generally, the purge steps are carried ~ut simultaneously and are conducted for a period of one to two minutes. The resulting carbon dioxide-rich fraction generally contains at least about 50% by volume carbon dioxide and less than 10% by volume carbon ~onoxide, and small amount~ of methane, with the balance being hydrogen.

When the ~econd stage purge/first stage purge step i6 complete, stage6 A1'' and A2'' undergo a 6econd stage purge/first 6tage evacuation step ~carbon monoxide production step). During the second stage purge/first stage evacuation step, second stage A2''is purged with hydrogen gas and first stage Al'' i8 evacuated using vacuum pump R to withdraw carbon monoxide product gas from the inlet end o~ first ~tage A1'' for passage to carbon monoxide reservoir Q. During the production of carbon monoxide, stop valves 252 and 222 are closed.
Carbon monoxide iB then passed from first stage A1'' through open stop valve 223 to carbon monoxide reservoir Q. Generally, the purge and production steps are carried out simultaneously and are conducted for a period of one to two minutes.

~he resulting carbon Donoxide-rich fraction, which iB produced at a pressure at about 25 psia, generally contains carbon monoxide having a purity exceeding 98%.

After the carbon monoxide production step is complete, first ~tage A1'' and second stage A2'' is repressurized and bed C1''/C2'' is depressurized by pressure equalization of the beds. Stop valves 223, 233, and 251 are closed and stop valve 244 i~ opened. Void .~ ~ I r ga6 i6 passed from the outlet end of ~econd 6tage C2'' to the outlet end of second stage A2'' to 6ubstantially equalize the pressure of bed A1''/A2'' and bed C1''/C2''.

After the pressure equalization step ~repressurization step) iB complete, fir~t stage Al'' and second ~tage A2'' are back~illed with hydrogen product ga6. Stop valves 238 and 244 are closed and bed A1''/A2'' is repressurized by backfill with product gas. Product gas from producing bed D1''/D2'' is passed through open repressurization flow control valve FCV1 and into the outlet end of second stage A2'' to backfill bed Al''/A2'' through open stop valve 239.

When the backfill step is complete, first 6tage Al'' and second 6tage A2'' are ready to again begin the hydrogen production 6tep. Repressurization flow control valve FCVl and stop valve 239 are closed and stop valves 221 and 237 are opened to admit feed ~ixture to the inlet end of ~irst ~tage Al''. The hydrogen production step in bed Al''tA2'' ls begun and the cycle is repeated. Beds Al''/A2'', Bl''/B2'', Cl''/C2'', and Dl''/D2'' operate in the 6equence 6et out in Table 3. In general, the time to complete a cycle (cycle time) is in the range from about 60 ~econds to About 1500 seconds, preferably from about 180 6econds to about 960 6econds, and more preferably from about 240 ~econd6 to about 720 second6.

In a preferred embodiment, the present invention i~ directed at a method for producing hydrogen and carbon monoxide from ~ feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane, which comprises the Rteps of:
(~) providing a pre6sure swing adsorption system having a fir6t 6tage and a 6econd 6tage, wherein the first stage contains an adsorption bed comprisinq an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane, the 2 ~ 2 v second 6tage contains an adsorption bed compri6ing an adsorbent having a greater affinity for carbon dioxide, methane, and carbon monoxide than for hydrogen, and the first and second stages are connected in ~eries and each ~tage contains an inlet end and an outlet end;
(b) pa6~ing th~ Peed mixture through the fir6t stage of the pre6sure ~wing adsorption system to ~eparate carbon monoxide a6 an adsorbed fraction and hydrogen, carbon dioxide, and methane as a non-adsorbed fraction;
(c) passing the non-ad60rbed ~raction from step (b) through the second ~tage of the pre6~ure 6wing adsorption 6ystem to separate carbon dioxide and methane as an adsorbed fraction and hydrogen as a non-adsorbed pure product;
(d) desorbing carbon dioxide and methane from the first and ~econd 6tages of the pressure 6wing adsorption system to form a carbon dioxide-rich fraction;
and ~ e) de~orbing carbon monoxide from the first 6tags of the pressure swing adsorption sy6tem to form a pure carbon monoxide product.

The selectivity of the adsorbent material in the bed of the pressure swing adsorption system for a gaseous component i8 generally governed by the volume of the pore size and the distribution of that pore size in the adsorbent. Gaseous molecules with a kinetic diameter - less than, or equal to, the pore size of the ad60rbent are ad60rbed and retained in the adsorbent while gaseous ~olecules with a diameter larger than the pore size of the ad60rbent pass through the adsorbent. The adsorbent thus sieves the gaseous molecules according to their molecular ize, The adsorbent may also separate molecule~ according to their diffsrent r~tes of diffusion in the pores of the adsorbent.

Zeolite molecular ad60rbents adsorb gaseous molecules with some dependence upon crystalline 6ize. In general, adsorption into zeolite is fast and equilibrium ~a3~2~

is reached typically in a few 6econds. The ~ieving action of zeolite is generally dependent upon the difference in the equilibrium adsorption of the different components of the gaseous mixture. When air i8 separated by a zeolite adsorbent, nitrogen iB preferentially adsorbed over oxygen and the pre~sure swing adsorption method may be employed to produce an oxygen enriched product. When hydrogen, carbon ~onoxide, carbon dioxide, and methane are separated by a zeolite adsorbent, carbon dioxide, carbon monoxide, and methane are the adaorbed components, in the order indicated, and hydrogen is the unadsorbed component.

During the carbon monoxide pressure swing adsorption separation, carbon dioxide, hydrogen, and methane are removed from the feed mixture as vent gas during the pressure egualization step. A certain amount of carbon monoxide is lost with the vent gas. ~his loss of carbon monoxide results from carbon monoxide not adsorbed in the sieves at the pressure swing adsorption operation pressure, and carbon monoxide present in the bed voids and discharged during the pressure egualization step. This vent gas containing carbon monoxide is recycled to the pressure swing adsorption system as feed gas during the carbon monoxide cocurrent purge ~tep.

Although a particular carbon monoxide pressure swing adsorption cycle seguence was illustrated as a preferred embodiment (Table 1 and Table 2), other variations of pressure swing adsorption process cycle 6eguences may be employed. A simple pressure 6wing adsorption process cycle 6eguence may consist of the following 6teps: (i) adsorption wherein feed mixture enters the inlet end of the adsorbent bed and the product gas exits the outlet end of the adsorbent bed, (ii) bed pressure egualization through the outlet and inlet ends of the bed to depressurize the bed, (iii) countercurrent vent, (iv) vacuum regeneration to remove components strongly adsorbed in the bed, (v) bed pressure r~ r~

equalization to partially repressurize the regenerated bed, and (vi) repressurization using a product backfill.

The proces~ cycle sequence illustrated in Table 1 can increase the carbon monoxide yield to approximately 70S. The carbon monoxide product withdrawal step incorporated in the proce~s ~eguence referred to in Table 1 reduce6 carbon ~onoxide 106a by permitting the withdrawal of substantial amounts of ~0 carbon monoxide containing bed ~oid ga~ and gas weakly bound to the adsorbent. The selection of nn intermediate location for wlthdrawing the carbon monoxide-rich fraction increase6 the amount of carbon monoxide withdrawn without affecting the hydrogen product purity.
If the carbon monoxide-rich fraction i6 withdrawn from the outlet end of the adsorption bed, then the quantity of the product withdrawn, without affecting hydrogen product purity, will be limited and the net carbon monoxide recovery that can be achieved will only be between about 30% and about 50%. When the carbon monoxide-rich fraction is withdrawn from an intermediate position in the bed, the beds need not be separate vessels (first ~tage A1'' and second ~tage A2'', for example) but may be t~o regions inside a single vessel between which a side port i6 located for withdrawing the carbon monoxide product.

The carbon monoxide yield may be improved to 85% or greater by including a cocurrent purge during the carbon monoxide production step in the carbon monoxide pressure swing adsorption system. In this variation of the cycle, a portion of the carbon dioxide-rich fraction i~ compressed and fed as cocurrent purge gas to the inlet end of the adsorption bed (inlet end of bed A', for example) during the carbon monoxide production step. The cocurrent purge, also referred as 6weep or displacement gas, displace~ the carbon monoxide near the inlet end of the adsorption bed (inlet end of bed A', for example) 2 ~

further along and permits remo~al of more carbon monoxide.

In yet another process cycle variation, the carbon monoxide production step in the hydrogen pre~sure ~wing adsorption system iB 6plit into two parts. In the first part, the outlet end ~econd stage A2'', f or example) i5 opened to the carbon monoxide product line while cocurrent purge ga6 (carbon diox$de-rich fraction) 0 iB admitted to the lower region (first ~tage Al'', for example). In the ~econd part, the outlet region is isolated and the inlet region i8 opened to the carbon monoxide product line. To conduct this process cycle variation, two additional 6top valves must be provided to isolate the two regions from each other and from the carbon monoxide product line.

In a preferred embodiment, the carbon monoxide depressurization product from the pressure swing adsorption system is withdrawn from the outlet end o~ the bed in the pressure ~wing adsorption ~ystem. In another preferred embodiment, the carbon monoxide depressurization product from the pressure swing adsorption system is withdrawn from an intermediate location in the bed in the pressure swing ad~orption ~ystem.

The carbon monoxide product withdrawal location i8 preferably a6 close as possible to the outlet end of the adsorption bed. The volume of 6econd stage A2'', for example, must be as s~all a6 pos6ible compared to the volume of first stage A1''. The volume of second stage A2'' mu~t, however, be large enough co that the hydrogen enriched product purity 18 not ~ffected a8 a result of carbon monoxide product ~roduction. During the production cycle, concentration fronts are formed for each of the component~ in the feed. Components that are strongly adsorbed (e.g. carbon dioxide) exist at feed concentration in the gas phase near the entrance of the L~

bed. Over a length egual to the eguilibrium saturation zone, the gas phase concentration i~ constant. Beyond this length the concentration of the adsorbed component decreases sharply. In the present separation (hydrogen, carbon monoxide, carbon dioxide, and methane from the feed mixture), the production of hydrogen-rich primary product must be ~topped when the eguilibrium f ront ~6 well within the pres6ure swing adsorption bed. ~he outlet end of a pres6ure ~wing ad~orption bed at the completion of the product~on step thu~ contain6 predominant amounts of hydrogen which mainly account~ for the hydrogen losses with the vent. It i~ desirable to collect a carbon monoxide product in a direction cocurrent to feed by moderate pressure reduction of the pressure swing adsorption beds before carbon monoxide product vent is initiated. The carbon monoxide product contain~ a significant amount of hydrogen.

When a ~mall quantity of carbon monoxide product, for example 5-10% Or feed, is collected, it is pref~rable to withdraw the carbon monoxide product stream from the outlet of the bed. If greater than 10~ of the feed i~ collected, the carbon monoxide product stream should be withdrawn from an intermediate location on the bed. Thi~ method prevents contamination of the high pressure hydrogen-rich product.

The pressure swing adsorption unit mu6t be regenerated periodically. Suitable modes of regeneration include (i) regeneration at or below 25 psia coupled with product purge or purge from an extern~l ~ource, and ~ii) vacuum regeneration.

Throughout this application, various publications have been referenced. The disclosures in these publications are incorporated herein by reference in order to more fully describe the state of the art.

2 7~ a ~

The embodiments de6cribed herein are merely exemplary and a person 6killed in the art may make many variations and modificat~ons without departing from the 6pirit and scope of the invention. All 6uch S modifications and ~ariations are intended to be included within the scope o~ the invention as defined in the appended claims.

Claims

1. A method for producing hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane, which comprises the steps of:
(a) passing the feed mixture through a first pressure swing adsorption system containing an adsorption bed comprising an adsorbent having a greater affinity for carbon dioxide, methane, and carbon monoxide than for hydrogen to separate hydrogen as a pure non-adsorbed product and carbon dioxide, methane, and carbon monoxide as an adsorbed fraction;
(b) desorbing carbon monoxide from the pressure swing adsorption system in step (a) to form a carbon monoxide-rich fraction;
(c) desorbing carbon dioxide and methane from the pressure swing adsorption system in step (a) to form a carbon dioxide-rich fraction;
(d) passing the carbon monoxide-rich fraction from step (b) to a second pressure swing adsorption system containing an adsorption bed comprising an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane to separate carbon monoxide as an adsorbed fraction and hydrogen, carbon dioxide, and methane as a non-adsorbed fraction; and (e) desorbing carbon monoxide from the pressure swing adsorption system in step (d) to form a pure carbon monoxide product.

2. The method according to claim 1, wherein the feed mixture comprises hydrogen in an amount up to about 80%, carbon monoxide in an amount up to about 20%, carbon dioxide in an amount up to about 30%, and methane in an amount up to about 3%.

3. The method according to claim 1, wherein the adsorbent in the adsorption bed in step (a) comprises activated carbon and zeolite molecular sieves.

4. The method according to claim 3, wherein the zeolite molecular sieves are an aluminosilicate zeolite selected from the group consisting of type 5A, 10X, 13X zeolite molecular sieves, and mordenites.

5. The method according to claim 1, wherein the adsorption bed in step (a) is divided into a first adsorption bed and a second adsorption bed, wherein the first and second beds are connected in series and each adsorption bed contains an inlet end and an outlet end.

6. The method according to claim 5, further comprising the steps of passing the feed mixture in step (a) through the inlet end of the first adsorption bed, withdrawing the hydrogen product from the outlet end of the second adsorption bed, stopping the flow of feed mixture into the inlet end of the first adsorption bed, withdrawing the desorbed carbon monoxide-rich fraction in step (b) from a location intermediate between the first and second adsorption beds, and withdrawing the desorbed carbon dioxide-rich fraction in step (c) from the inlet end of the first adsorption bed.

7. The method according to claim 6, wherein the carbon monoxide-rich fraction in step (b) is first withdrawn from the inlet end of the second adsorption bed, then withdrawn from the outlet end of the first adsorption bed, and while the carbon monoxide-rich fraction is withdrawn from the inlet end of the second adsorption bed, a portion of the carbon dioxide-rich fraction in step (c) is introduced into the inlet end of the first adsorption bed.

8. The method according to claim l, wherein the carbon dioxide-rich fraction in step (c) is desorbed by a depressurization step and a hydrogen gas purge step.

9. The method according to claim 1, wherein the desorbed carbon dioxide-rich fraction in step (c) is recycled to the hydrocarbon steam reformer.

10. The method according to claim 1, wherein the adsorbent in the adsorption bed in step (d) is selected from the group consisting of copper exchanged Y-type aluminosilicate zeolite molecular sieves, copper exchanged alumina, and copper exchanged activated carbon, and mixtures thereof.

11. The method according to claim 10, wherein the adsorbent is copper exchanged Y-type aluminosilicate zeolite molecular sieves.

12. The method according to claim 1, wherein the pressure swing adsorption system in step (d) is operated at an adsorption pressure in the range from about 150 psia to about 600 psia.

13. The method according to claim 1, further comprising the step of passing vent gas in step (d) from the pressure swing adsorption system to an egualization tank to minimize loss of void gas.

14. The method according to claim 1, further comprising the step of recycling a hydrogen-rich fraction from the pressure swing adsorption system in step (d) into the feed mixture passing to the pressure swing adsorption system in step (a).

15. The method according to claim 1, wherein the carbon monoxide-rich fraction in step (e) is desorbed by a vacuum withdrawal step subsequent to a product gas purge step.

16. The method according to claim 1, wherein the purity of the carbon monoxide product is greater than about 99%.

17. A method for producing hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane, which comprises the steps of:
(a) providing a pressure swing adsorption system having a first stage and a second stage, wherein the first stage contains an adsorption bed comprising an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane, the second stage contains an adsorption bed comprising an adsorbent having a greater affinity for carbon dioxide, methane, and carbon monoxide than for hydrogen, and the first and second stages are connected in series and each stage contains an inlet end and an outlet end;
(b) passing the feed mixture through the first stage of the pressure swing adsorption system to separate carbon monoxide as an adsorbed fraction and hydrogen, carbon dioxide, and methane as a non-adsorbed fraction;

(c) passing the non-adsorbed fraction from step (b) through the second stage of the pressure swing adsorption system to separate carbon dioxide and methane as an adsorbed fraction and hydrogen as a non-adsorbed pure product;
(d) desorbing carbon dioxide and methane from the first and second stages of the pressure swing adsorption system to form a carbon dioxide-rich fraction;
and (e) desorbing carbon monoxide from the first stage of the pressure swing adsorption system to form a pure carbon monoxide product.

18. The method according to claim 17, wherein the feed mixture comprises hydrogen in an amount up to about 80%, carbon monoxide in an amount up to about 20%, carbon dioxide in an amount up to about 30%, and methane in an amount up to about 3%.

19. The method according to claim 17, wherein the adsorbent in the adsorption bed in the first stage of the pressure swing adsorption system in step (b) is selected from the group consisting of copper exchanged Y-type aluminosilicate zeolite molecular sieves, copper exchanged alumina, and copper exchanged activated carbon, and mixtures thereof.

20. The method according to claim 19, wherein the adsorbent is copper exchanged Y-type aluminosilicate zeolite molecular sieves.

21. The method according to claim 17, wherein the adsorbent in the adsorption bed in the second stage of the pressure swing adsorption system in step (c) comprises activated carbon and zeolite molecular sieves.

22. The method according to claim 21, wherein the zeolite molecular sieves are an aluminosilicate zeolite selected from the group consisting of type 5A, 10X, 13X zeolite molecular sieves, and mordenites.

23. The method according to claim 17, further comprising the steps of passing the feed mixture through the inlet end of the first stage in step (b), withdrawing the hydrogen product from the outlet end of the second stage in step (c), stopping the flow of feed mixture into the inlet end of the first stage, and desorbing the carbon dioxide and methane in step (d) from a location intermediate between the first and second stages of the pressure swing adsorption system to form a carbon dioxide-rich fraction.

24. The method according to claim 23, wherein the desorbed carbon dioxide-rich fraction is recycled to a hydrocarbon steam reformer.

25. The method according to claim 23, further comprising the step of desorbing the carbon dioxide and methane by purging the second stage with hydrogen gas in a countercurrent direction and purging the first stage with carbon monoxide gas in a cocurrent direction.

26. The method according to claim 25, wherein the purge effluent gases from the first and second stages are withdrawn from a location intermediate between the first and second stages of the pressure swing adsorption system.

27. The method according to claim 17, wherein carbon monoxide is desorbed from the inlet end of the first stage of the pressure swing adsorption system in step (e) under vacuum.

28. The method according to claim 17, wherein the pressure swing adsorption system in steps (b) and (c) are operated at an adsorption pressure in the range from about 150 psia to about 600 psia.

29. The method according to claim 17, wherein the purity of the carbon monoxide product is greater than about 99%.