Hydrogen and carbon monoxide production by pressure swing adsorption purification

Abstract: 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: The present invention is directed to a method of producing merchant grade hydrogen and carbon monoxide from a steam reformed hydrocarbon feed mixture. More particularly, the present invention is directed to a method of producing hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane. Various methods are known for separating gaseous mixtures produced by the steam reforming of hydrocarbons. Steam reforming to produce hydrogen consists of treating a hydrocarbon feed mixture with steam in a catalytic steam reactor (reformer) which consists of a number of tubes placed in a furnace at a temperature in the range from about 676.7°C (1250°F) to about 926.7°C (1700°F). The reversible reforming reactions which occur when methane is used as the hydrocarbon feed mixture are set out below. CH 4 + H 2 O = CO + 3H 2 CH 4 + 2H 2 O = CO 2 + 4H 2 CO + H 2 O = CO 2 + H 2 Carbon monoxide and carbon dioxide are generally removed by shift conversion (reaction of carbon monoxide with steam to form additional hydrogen and carbon dioxide), absorption in amines or other alkaline solvents (carbon dioxide removal), and methanation (conversion of trace carbon monoxide and carbon dioxide to methane). When carbon monoxide is a desired product, the shift conversion and methanation steps are not employed. The hydrogen-rich gas mixture exiting the steam reformer consists of an equilibrium 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 reactions and hydrogen separation processes are disclosed in more detail in "Ammonia and Synthesis Gas: Recent and Energy Saving Processes", Edited by F.J. Brykowski, Chemical Technology Review No. 193, Energy Technology Review No. 68, Published by Noyes Data Corporation, Park Ridge, New Jersey, 1981. Conventional methods for recovering hydrogen and carbon monoxide from a hydrocarbon steam reformed feed mixture have generally focused on cryogenic distillation processes to separate and purify hydrogen and carbon monoxide in the mixture after carbon dioxide is removed. Cryogenic separation processes tend to have a high capital cost especially when more than one pure product is required. Methods for separating hydrogen and carbon monoxide by pressure swing adsorption processes are disclosed in EP-A-0 317 235 which relates to a method for forming hydrogen and carbon monoxide from a feed mixture exiting a hydrocarbon steam reformer comprising hydrogen, carbon monoxide, and carbon dioxide. The method comprises the steps of passing the feed mixture through a sorptive separation to separate a hydrogen product, a carbon monoxide-rich product, and a carbon dioxide-rich product. The carbon monoxide-rich product is further purified in a two stage pressure swing adsorption system. The first stage comprises an activated carbon adsorbent which removes carbon monoxide and methane as the strongly adsorbed waste stream. The second stage comprises a zeolite adsorbent and produces a pure carbon monoxide stream as an adsorbed product. US-A-4,917,711, discloses an adsorbent for carbon monoxide and unsaturated hydrocarbons which comprises a high surface area support, such as a zeolite, alumina, silica gel, aluminosilicate, or aluminophosphate, and cuprous or cupric compound. The adsorbent may be used to separate carbon monoxide and unsaturated hydrocarbons from a gaseous mixture containing hydrogen, nitrogen, argon, helium, methane, ethane, propane, and carbon dioxide by passing the mixture 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 swing adsorption system for separating carbon monoxide from a gaseous mixture. The columns contain an adsorbent containing copper to adsorb carbon monoxide gas. The term "adsorption bed" refers either to a single bed or a serial arrangement of two beds. The inlet end of a single bed system is the inlet end of the single bed while the inlet end of the two bed system (arranged in series) is the inlet end of the first bed in the system. The outlet end of a single bed system is the outlet end of the single bed and the outlet end of the two bed system (arranged in series) is the outlet end of the second 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. An aim of the present invention is to provide an improved method of producing hydrogen and carbon monoxide from a hydrocarbon steam reformed feed mixture employing a novel combination of pressure swing adsorption methods which minimizes capital cost requirements and increases the recovery of carbon monoxide. According to the present invention there is provided a method of producing by pressure swing adsorption hydrogen and carbon monoxide products from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane, characterised by repeating the steps of: (a) providing a pressure swing adsorption system (M) 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 inlet end of the first stage of the pressure swing adsorption system (M) 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 (M) to separate carbon dioxide and methane as an adsorbed fraction and hydrogen as a non-adsorbed substantially pure product; (d) stopping the flow of feed mixture into the inlet end of the first stage; (e) desorbing carbon dioxide and methane from a location intermediate the first and second stages of the pressure swing adsorption system (M) to form a carbon dioxide-rich fraction; (f) 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 and withdrawing the purge effluent gases from a location intermediate the first and second stages of the pressure swing adsorption system (M); and (g) desorbing carbon monoxide from the first stage of the pressure swing adsorption system (M) to form a substantially pure carbon monoxide product. Embodiments of the invention will now be described, by way of example, reference being made to the Figures of the accompanying diagrammatic drawings in which:- FIGURE 1 is a schematic process flow diagram illustrating a novel two stage pressure swing adsorption system according to the present invention to separate hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane; and FIGURE 2 is a schematic process flow diagram illustrating a first stage of a pressure swing adsorption system for separating carbon monoxide as an adsorbed product and a second stage of a pressure swing adsorption system for separating carbon dioxide as an adsorbed fraction and hydrogen as a non-adsorbed product, according to the present invention. In the embodiment illustrated in the drawings, a novel two stage pressure swing adsorption system is utilized which efficiently and economically yields enriched hydrogen and carbon monoxide. The first stage of the pressure swing adsorption system separates carbon monoxide as an adsorbed fraction and hydrogen, carbon dioxide, and methane as a non-adsorbed fraction. The second stage of the pressure swing adsorption system separates carbon dioxide and methane as an adsorbed fraction and hydrogen as a non-adsorbed pure product. The novel combination of pressure swing adsorption separation methods of the present invention provides significant savings in capital and operating expense over completely cryogenic methods. The steps in the present method may be integrated into steps in the hydrocarbon steam reformer method to enhance the reforming process. For example, the carbon dioxide-rich fraction from the second 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 feed mixture (exhaust gas, effluent gas, exit gas, feed gas) in the present invention is a mixture comprising 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, silica, or zeolite. Desorption of the water from the desiccant may be accomplished by purging the desiccant with a dry waste purge gas (such as the carbon dioxide-rich fraction or nitrogen gas). Any water remaining in the feed mixture is 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 swing adsorption system. 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 mixture, the hydrocarbon steam reformed feed mixture will by-pass the shift converter. The adsorbent material in the adsorbent bed in the second stage is an adsorbent having a greater affinity for carbon dioxide, methane, and carbon monoxide than for hydrogen. The adsorbent material may be a molecular sieve or activated carbon, and preferably is a combination of molecular sieves and activated carbon. Both calcium and sodium aluminosilicate zeolites may be employed. Carbon molecular sieves and silica molecular sieves are also useful. Suitable zeolite sieves include, but are not limited to, the type 5A, 10X, 13X zeolite molecular sieves, and mordenites. Preferred zeolite sieves are the type 5A zeolite sieves and molecular sieves with comparable pore size and molecular attraction. The adsorbent material in the adsorbent bed in the first stage is an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane. In general, suitable adsorbent materials are copper exchanged substrates such as those 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. In a preferred embodiment, the adsorbent material is copper aluminosilicate zeolite molecular sieve material, available under the tradename NKK type adsorbent in a package from Nippon Kokan K. K., Tokyo, Japan. Copper aluminosilicate zeolite molecular sieves can be prepared by exchanging sodium in sodium aluminosilicate zeolite molecular sieves 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. see, for example, US-A-4,917,711, US-A-4,914,076, US-A-4,783,433,, and US-A-4,743,276. In accord with the present invention, a 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%. Referring to FIGURE 1, gaseous feed mixture is fed through feed conduit 70 to two stage pressure swing adsorption system 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 pressure in the range from about 1.034 MPa (150 psia) to about 4.136 MPa (600 psia), preferably from about 1.034 MPa (150 psia) to about 2.758 MPa (400 psia), and more preferably from about 1.034 Mpa (150 psia) to about 2.068 MPa (300 psia). After being cooled, the feed mixture entering pressure swing adsorption system M will be at ambient temperature. During the hydrogen product production step, feed mixture is fed into and hydrogen product is withdrawn from two stage pressure swing adsorption system M. Hydrogen product is separated as a pure non-adsorbed product and carbon monoxide, carbon dioxide, methane, and water vapor is separated as an adsorbed fraction. Hydrogen product (merchant grade, less than about 10 vpm impurities) is withdrawn from two stage pressure swing adsorption system M through feed conduit 71 and passed to hydrogen reservoir B. After the hydrogen product production step, pressure swing adsorption system M undergoes a pressure equalization step, an intermediate depressurization step, and a second stage purge/first stage purge step (carbon dioxide-rich fraction production steps). During the intermediate depressurization step, carbon dioxide-rich gas is collected as secondary product, compressed, and recycled to the reformer feed gas. During the second stage purge/first stage purge step, the second stage is purged with hydrogen from another stage and the first stage is purged with carbon monoxide product gas from the reservoir Q. 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 reformer feed gas via feed conduit 74. In general, carbon dioxide compressor 0 will compress the carbon dioxide-rich fraction to a pressure in the range from about 1.034 MPa (150 psia) to about 4.136 MPa (600 psia), preferably from about 1.034 MPa (150 psia) to about 3.102 MPa (450 psia), and more preferably from about 1.034 MPa (150 psia) to about 2.413 MPa (350 psia). After the carbon dioxide-rich fraction production steps, pressure swing adsorption system M undergoes a second stage purge/first stage evacuation step (carbon monoxide production step). In the second stage purge/first stage evacuation step, the second stage is purged with hydrogen gas and the first stage is evacuated using vacuum pump P to remove carbon monoxide product gas. The carbon monoxide product gas is withdrawn through feed conduits 77 and 78 and passed to carbon monoxide reservoir Q. After the carbon monoxide production step, pressure swing adsorption system M undergoes a pressure equalization step (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 system. The carbon monoxide product (merchant grade, less than about 1500 vpm impurities) is passed through feed conduits 77, 78, and 75 to carbon monoxide reservoir Q. FIGURE 2 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 second embodiment of the present invention. As set out in FIGURE 4, the feed mixture from the hydrocarbon steam reformer is passed through feed conduit 70 to pressure swing adsorption system M. In FIGURE 2, the two stage pressure swing adsorption system M comprises adsorption beds A1'', A2''; B1'', B2''; C1'', C2''; and D1'', D2'', carbon monoxide storage vessel Q, carbon dioxide-rich gas buffer vessel N, hydrogen product reservoir B, hydrogen product pressure control valve PCV1, carbon monoxide-rich product gas pressure control valve PCV2, carbon dioxide product pressure control valve PCV3, repressurization flow control valve FCV1, hydrogen product purge gas flow control valve FCV2, stop valves 221 to 248 and 251 to 259, and non-return valves 249 and 250. Adsorption beds A'' through D'' are connected in parallel. Each of the adsorption beds, A'' through D'', is physically divided into two stages, a first (bottom) stage and a second (top) stage, A1''/A2'', B1''/B2'', C1''/C2'', and D1''/D2'', respectively, which are connected in series. Each stage contains an inlet (feed) end and a outlet (discharge) end. The first stage and second stage of each bed are isolated by two stop valves for sequential depressurization and carbon monoxide production steps (i.e., first stage A1'' and second stage A2'' are isolated by stop valves 252 and 256). The two part stages facilitate 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 0.172 Mpa (25 psia), and passed to carbon dioxide storage vessel N. The first adsorption bed stage (for example, first stage A1'') comprises an adsorbent having a greater affinity for carbon monoxide than for hydrogen, carbon dioxide, and methane and may be selected from 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, second 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 is a combination of molecular sieves and activated carbon. The two stage pressure swing adsorption system is operated in accordance with the full cycle sequence shown in Table 1. The sequence is described below in detail using stages A1''/A2''. Stages B1''/B2'', C1''/C2'', and D1''/D2'' are employed in the same sequence but at an offset as shown in Table 1. All stop valves may be controlled automatically on a predetermined schedule. Four Bed Two Stage Pressure Swing Adsorption Cycle Sequence Step No Bed A'' Bed B'' Bed C'' Bed D'' Valves Open 1 Feed gas Product gas Eq.Press. (repress.) Top Purge Bottom Purge Eq.Press. (depress.) 221,237,252,256, 242,248,253,257, 255,259,251,244, 258,254,235,228 2 Feed gas Product gas Repressurize H 2 Product gas Top Purge CO Production Top/Bottom Depressurize 221,237,252,256, 242,257,253,251, 244,258,235,229, 259,255,236 3 Eq.Press. (depress.) Feed gas Product gas Eq.Press. (repress.) Top Purge Bottom Purge 224,240,253,257, 239,245, 252,256, 258,254,251,247, 259,236,231,255 4 Top/Bottom Depressurize Feed gas Product gas Repressurize H 2 Product gas Top Purge CO Production 224,240,253,257, 256,252,233,245, 258,254,251,247, 259,236,232 5 Top Purge Bottom Purge Eq.Press. (depress.) Feed gas Product gas Eq.Press. (repress.) 227,243,254,258, 251,238, 256,233, 222,252,242,248, 253,257,259,255 6 Top Purge CO Production Top/Bottom Depressurize Feed gas Product gas Repressurize H 2 Product gas 227,243,254,258, 251,238,256,233, 223,257,253,234, 248,259,255 7 Eq.Press. (repress.) Top Purge Bottom Purge Eq.Press. (depress.) Feed gas Product gas 230,246,255,259, 239,245, 252,256, 251,241,258,254, 257,234,225,253 8 Repressurize H 2 Product gas Top Purge CO Production Top/Bottom Depressurize Feed gas Product gas 230,246,255,259, 239,256,252,251, 241,257,234,226, 258,254,235 Feed gas/Product gas Feed gas admitted to bottom of bed. Product gas released from top of bed. Eq.Press. (depress.) Bed pressure equalized to another bed at lower pressure through top ends of beds. Top/Bottom Depressurize Top and bottom beds depressurized from intermediate location. Depressurized gas recycled to reformer feed gas. Top Purge/Bottom Purge Top bed purged with hydrogen. Bottom bed purged with carbon monoxide. Purge effluent removed from intermediate location. Top Purge/CO Production Top bed purged with hydrogen. Bottom bed evacuated to remove carbon monoxide product gas. Eq.Press. (repress.) Bed pressure equalized with another bed at higher pressure through top ends of beds. Repressurize H 2 Product gas Repressurization of bed to adsorption pressure using hydrogen product gas from pressure swing adsorption system. At the start of the pressure swing adsorption cycle, first stage A1'' and second stage A2'' are in the hydrogen production step. Feed mixture from the hydrocarbon steam reformer is passed to the inlet end of first stage A1'' via open stop valve 221 at a pressure typically in the range from about 1.034 MPa (150 psia) to about 4.136 MPa (600 psia). The feed mixture passes from the outlet end of the first stage A1'' through stop valves 252 and 256 to the inlet end of the second stage A2''. The feed mixture is adsorbed in first stage A1'' and second stage A2'' to selectively sieve 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 second stage 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 substrate in first stage A1'' preferentially adsorbs carbon monoxide. The activated carbon/molecular sieve adsorbent material in second stage A2'' preferentially adsorbs 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 stage A1'', carbon dioxide, water vapor, and methane become concentrated in second stage A2'', and the mixture exiting the bed stages becomes enriched in hydrogen. The zeolite molecular 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 stage and flow of product gas from the outlet end of the second stage are stopped just before the breakthrough point of non-hydrogen components from the outlet end of the second stage. A typical feed and production cycle is conducted for a period of about two to about six minutes. 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 first stage A1''/A2'' is stopped. Stop valves 221 and 237 are closed stopping the production of hydrogen. First stage A1'' and second stage A2'' are then depressurized (treated as one bed with stop valves 252 and 256 open) and stage C1''/C2'' is repressurized (treated as one bed with stop valves 254 and 258 open) by pressure equalization of the beds through the outlet (top, discharge) ends of the beds. Stop valves 239 and 245 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 pressures in beds A1''/A2'' and beds C1''/C2''. This pressure equalization step typically is conducted for a period of about twenty to about forty seconds. During the pressure equalization step, void gas containing carbon dioxide and methane is passed to the repressurized bed and the pressure in first stage A1''/A2'' decreases. Optionally, first stage A1''/A2'' may be pressure equalized with an equalization tank through the outlet end of second stage A2''. The gas collected in the equalization tank is subsequently used to repressurize a bed in the pressure swing adsorption system. Ater the pressure equalization step is complete, first stage A1''/A2'' begins the intermediate depressurization step (a first carbon dioxide-rich fraction production step). During the intermediate depressurization step, first stage A1'' and second stage A2'' are depressurized from an intermediate location to withdraw and produce a carbon dioxide-rich fraction. Stop valves 239 and 245 are closed and stop valves 233, 252, and 256 are opened to withdraw the carbon dioxide-rich fraction from a position intermediate between first stage A1'' and second stage A2''. The carbon dioxide-rich fraction is drawn at an intermediate pressure, for example at about 0.172 MPa (25 psia), and passed to carbon dioxide storage vessel N. The carbon dioxide-rich fraction from carbon dioxide storage vessel N is then passed to the reformer feed gas via compressor 0. During the intermediate depressurization step, void gas in second stage A2'' which is predominantly carbon dioxide, is withdrawn from the inlet (bottom) end of the bed. Withdrawal of carbon dioxide-rich void gas from the outlet (top) end of first stage 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 intermediate depressurization step is typically about two minutes. When the intermediate depressurization carbon monoxide production step is complete, first stage A1''/A2'' undergoes a second stage purge/first stage purge step (a second carbon dioxide-rich fraction production step). During the second stage purge/first stage purge step, second stage A2''is purged with hydrogen gas and first stage A1'' is purged with carbon monoxide product gas from carbon monoxide reservoir Q. During the purge of second stage A2'' with hydrogen gas, stop valves 251, 238, and 233 are opened. Hydrogen from hydrogen reservoir B is then passed through open flow control valve FCV2, open non-return valve 250, and open stop valves 251 and 238, through second stage A2'', and open stop valves 256 and 233 to carbon dioxide reservoir N. During the purge of first stage A1'' with carbon monoxide gas, stop valves 222, 252, 233 are opened. Carbon monoxide from carbon monoxide reservoir Q is then passed through open flow control valve FCV3, open stop valve 222, through first stage A1'', and open stop valves 252 and 233 to carbon dioxide reservoir N. The flow of hydrogen through second stage A2'' is in a direction countercurrent and the flow of carbon monoxide is in a direction cocurrent to the flow of the feed mixture during the hydrogen production step. Generally, the purge steps are carried out 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 monoxide, and small amounts of methane, with the balance being hydrogen. When the second stage purge/first stage purge step is complete, stages A1'' and A2'' undergo a second stage purge/first stage 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 A1'' is evacuated using vacuum pump R to withdraw carbon monoxide product gas from the inlet end of first stage A1'' for passage to carbon monoxide reservoir Q. During the production of carbon monoxide, stop valves 252 and 222 are closed. Carbon monoxide is 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. The resulting carbon monoxide-rich fraction, which is produced at a pressure at about 0.17 MPa (about 25 psi), generally contains carbon monoxide having a purity exceeding 98%. After the carbon monoxide production step is complete, first stage 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 252 is opened. Void gas is passed from the outlet end of second stage C2'' to the outlet end of second stage A2'' to substantially equalize the pressure of bed A1''/A2'' and bed C1''/C2''. After the pressure equalization step (repressurization step) is complete, first stage A1'' and second stage A2'' are backfilled with hydrogen product gas. 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 A1''/A2'' through open stop valve 239. When the backfill step is complete, first stage A1'' and second stage A2'' are ready to again begin the hydrogen production step. Repressurization flow control valve FCV1 and stop valve 239 are closed and stop valves 221 and 237 are opened to admit feed mixture to the inlet end of first stage A1''. The hydrogen production step in bed A1''/A2'' is begun and the cycle is repeated. Beds A1''/A2'', B1''/B2'', C1''/C2'', and D1''/D2'' operate in the sequence set out in Table 3. 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 seconds. The selectivity of the adsorbent material in the bed of the pressure swing adsorption system for a gaseous component is 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 adsorbent are adsorbed and retained in the adsorbent while gaseous molecules with a diameter larger than the pore size of the adsorbent pass through the adsorbent. The adsorbent thus sieves the gaseous molecules according to their molecular size, The adsorbent may also separate molecules according to their different rates of diffusion in the pores of the adsorbent. Zeolite molecular adsorbents adsorb gaseous molecules with some dependence upon crystalline size. In general, adsorption into zeolite is fast and equilibrium is reached typically in a few seconds. The sieving action of zeolite is generally dependent upon the difference in the equilibrium adsorption of the different components of the gaseous mixture. When air is separated by a zeolite adsorbent, nitrogen is preferentially adsorbed over oxygen and the pressure swing adsorption method may be employed to produce an oxygen enriched product. When hydrogen, carbon monoxide, carbon dioxide, and methane are separated by a zeolite adsorbent, carbon dioxide, carbon monoxide, and methane are the adsorbed 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 equalization step. A certain amount of carbon monoxide is lost with the vent gas. This 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 equalization step. This vent gas containing carbon monoxide is recycled to the pressure swing adsorption system as feed gas during the carbon monoxide cocurrent purge step.

Claims: A method of producing by pressure swing adsorption hydrogen and carbon monoxide products from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane, characterised by repeating the steps of: (a) providing a pressure swing adsorption system (M) 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 inlet end of the first stage of the pressure swing adsorption system (M) 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 (M) to separate carbon dioxide and methane as an adsorbed fraction and hydrogen as a non-adsorbed substantially pure product; (d) stopping the flow of feed mixture into the inlet end of the first stage ; (e) desorbing carbon dioxide and methane from a location intermediate the first and second stages of the pressure swing adsorption system (M) to form a carbon dioxide-rich fraction; (f) 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 and withdrawing the purge effluent gases from a location intermediate the first and second stages of the pressure swing adsorption system (M); and (g) desorbing carbon monoxide from the first stage of the pressure swing adsorption system (M) to form a substantially pure carbon monoxide product. The method according to claim 1, characterised in that the adsorbent in the adsorption bed in the first stage (A1'') of the pressure swing adsorption system (M) in step (b) is selected from the group consisting of copper exchanged Y-type aluminosilicate zeolite molecular sieves, copper supported on alumina, and copper supported on activated carbon, and mixtures thereof. The method according to claim 1, characterised in that the adsorbent is copper exchanged Y-type aluminosilicate zeolite molecular sieves. The method according any one of to claims 1 to 3, characterised in that the adsorbent in the adsorption bed in the second stage (A2'') of the pressure swing adsorption system in step (c) comprises activated carbon and zeolite molecular sieves. The method according to claim 4, characterised in that the zeolite molecular sieves are an aluminosilicate zeolite selected from the group consisting of type 5A, 10X, 13X zeolite molecular sieves, and mordenites.

Inventors: KRISHNAMURTHY RAMACHANDRAN, KRISHNAMURTHY, RAMACHANDRAN
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1991
Main classification: B01D53
IPC codes: C01B3, B01D53