Not applicable.
Not applicable.
The present invention pertains to production of hydrogen from a hydrogen/carbon oxide synthesis gas and/or a hydrogen feedstock.
Conventional hydrogen production facilities have a first part or a front end of the process that produces a synthesis gas consisting essentially of hydrogen, carbon monoxide, carbon dioxide and other gases such as nitrogen, argon, methane and water. Steam reformation of hydrocarbons is a well known process for production of the synthesis gas. In the basic process a hydrocarbon or a mixture of hydrocarbons is treated to remove trace contaminants such as sulfur and olefins which would adversely affect the reformer catalyst. Methane is the preferred starting material since it contains a higher proportion of hydrogen than other hydrocarbons. The hydrocarbon, after being treated for removal of adverse components, is combined with steam and the mixture is injected into a reformer furnace. The reformer furnace operates at an elevated temperature and is necessary in order to drive the reaction to completion, the reaction being endothermic.
As stated above the effluent from the reformer furnace is principally hydrogen, carbon monoxide and carbon dioxide with minor amounts of methane and other gases. In the conventional process, the effluent from the reformer furnace is introduced into a single or multi-stage shift reactor to form additional hydrogen and carbon dioxide by the conversion of carbon monoxide. In the shift reactor carbon monoxide is converted to carbon dioxide with the liberation of additional hydrogen by reaction in the presence of steam.
In one process the effluent from the reformer is subjected, first a high temperature shift reaction which takes place at temperatures of between about 600-850xc2x0 F. utilizing an iron-chrome catalyst. After the high temperature shift reaction, the effluent is cooled with heat recovery and passed through a low temperature shift reaction where additional amounts of carbon monoxide are converted to carbon dioxide at a temperature of about 385xc2x0 F. to 500xc2x0 F. in the presence of a copper-zinc catalyst. The effluent from the low temperature shift reactor is sent to a heat recovery station so the effluent can be subjected to further processing. In the event the desired product is hydrogen, the effluent from the low temperature shift reactor is passed to an acid gas removal unit so that carbon dioxide can be removed by any of the well known carbon dioxide removal techniques such as using methyl ethyl amine (MEA) or methyl diethyl amine (MDEA). The effluent from the CO2 removal unit is subjected to a heat integration step whereby the temperature is adjusted so that the effluent, consisting mainly of hydrogen with minor amounts of carbon monoxide and carbon dioxide, as well as any trace gases present in the original synthesis gas such as argon and nitrogen, is sent to a methanation unit for conversion of the carbon monoxide and carbon dioxide.
In an attempt to improve on the process described above it has been proposed to eliminate the CO2 removal and the methanation steps and subject the effluent, after either a high temperature shift followed by a low temperature shift or a high temperature shift and heat recovery, to a pressure swing adsorption (PSA) unit operation for purification.
Use of a pressure swing adsorption step in production of hydrogen and carbon dioxide is disclosed in U.S. Pat. Nos. 4,963,339 and 5,000,925 wherein patentees disclose integrating a hydrogen PSA, carbon dioxide PSA and various recycle streams into a reformer process. Patentees discuss and disclose purification and recovery of the hydrogen PSA purge gas using pressure swing adsorption, distillation, compression and recycle. There is no discussion of the optimization of the hydrogen PSA unit operation through manipulation of the product hydrogen stream.
J. R. Phillips of the Mississippi Chemical Corporation, Yazoo City, Miss. is noted as the compiler of a section on gas purification in a text book titled Fetitizer, Science and Technology Services, No., 2, (1974) pages 311-319 section II-B entitled Methanation. In the section identified as II-B xe2x80x9cmethanationxe2x80x9d the author discusses the need for methanation of carbon oxide streams in ammonia synthesis. The author explains the need to remove carbon monoxide from a synthesis gas stream to prevent damage to various ammonia and hydrogenation catalysts.
In an article titled Methanator Design And Operation which appeared in Volume 69 No. 1, pages 75-79 of Chemical Engineering Progress, the authors provide a general overview of the design and operation of methanation units and the potential hazards of nickel carbonyl.
An article entitled Separation Of Hydrogen Mixtures by a Two-Bed Pressure Swing Adsorption Process Using Zeolite 5A, which appeared in Ind. Eng. Chem. Res. Volume 36 No. 7, pages 2789-2798, discusses experimental and theoretical development regarding the bulk separation of hydrogen/carbon monoxide (H2/CO) and hydrogen/methane (H2/CH4) mixtures using pressure swing adsorption technology. The effects of various process parameters and a mathematical model are disclosed.
U.S. Pat. No. 4,318,711 discloses a process to upgrade low BTU feed gas formed by gasification of carbonaceous materials to a high BTU gas. The process disclosed includes carbon dioxide removal, carbon monoxide removal, hydrogen production via the well known steam-iron process, drying and methanation. There is no disclosure of the utilization of pressure swing adsorption in such a process. Patentees claim it will be unnecessary to purify the hydrogen gas or any of the other gases, except for water removal, before introduction of the gas into the methanation zone to produce the higher BTU product gas.
There is no teaching or suggestion in the art that the production of hydrogen from a hydrogen/carbon oxides synthesis gas can be improved by the use of a combination of the well known methanation and pressure swing adsorption processes.
The main objective of the present invention is to increase hydrogen recovery from synthesis gas using a pressure swing adsorption (PSA) unit. The present invention utilizes a methanator down stream of the PSA unit to treat the entire product hydrogen stream or some portion thereof to remove trace amounts of carbon oxides, e.g. CO and CO2, which are allowed to escape or breakthrough during a normal PSA cycle. According to the present invention the process permits an increase in the cycle time beyond carbon oxide impurity breakthrough by methanating residual carbon oxides to methane.
Therefore, in one aspect, the present invention is a method for increasing the recovery of hydrogen from hydrogen/carbon oxide synthesis gas comprising the steps of: (a) passing the synthesis gas stream through one of a high temperature shift reactor, a medium temperature shift reactor, a low temperature shift reactor or a combination thereof to convert carbon monoxide and water vapor in the stream to carbon dioxide and hydrogen; (b) passing the synthesis gas stream exiting the shift reactor(s), after heat recovery, through a pressure swing adsorption unit adapted to remove carbon oxides from the stream; the pressure swing adsorption unit operated for a cycle time beyond which carbon oxide impurities breakthrough the pressure swing adsorption unit and are contained in an effluent from said pressure swing adsorption unit; and (c) passing said effluent from step (b) through a methanator to convert residual carbon oxides in said effluent to methane to produce a purified hydrogen stream with a volumetric increase in hydrogen recovery from the pressure swing adsorption unit.
In another aspect, the present invention is a process for recovering hydrogen from a hydrogen/carbon oxide synthesis gas wherein the synthesis gas is subjected to shift reaction to convert carbon monoxide to carbon dioxide followed by treating an effluent from the shift reaction in a pressure swing adsorption unit to yield a purified hydrogen stream containing no carbon oxides, the improvement comprising: operating the pressure swing adsorption unit under a cycle time wherein carbon oxides breakthrough and are present in an effluent from the pressure swing adsorption unit; and passing the effluent from the pressure swing adsorption unit through a methanator to convert residual carbon oxides to methane to produce a purified hydrogen stream with a volumetric increase in hydrogen recovery from the pressure swing adsorption unit.
According to yet another aspect, the present invention is a process, wherein the effluent from the pressure swing adsorption step is divided into at least two streams, a first stream being subjected to heat recovery followed by methanation and a second stream split into two sub-streams, a first sub-stream being subjected to heat recovery and methanation and a second sub- stream recovered without further treatment as hydrogen product.
In a further aspect, the present invention is a process for recovering hydrogen from a hydrogen/carbon oxide synthesis gas wherein the synthesis gas is subjected to shift reaction to convert carbon monoxide and steam to carbon dioxide and hydrogen followed by treating an effluent from the shift reaction, after heat recovery, in a pressure swing adsorption unit to yield a purified hydrogen stream containing essentially no carbon oxides the improvement comprising operating the pressure swing adsorption unit under a cycle time wherein carbon oxides breakthrough are present in an effluent from the pressure swing adsorption unit; and passing the effluent from the pressure swing adsorption unit through a methanator to convert residual carbon oxides to methane to produce a purified hydrogen stream, whereby there is a volumetric increase in hydrogen recovery from the pressure swing adsorption unit.
In a still further aspect, the present invention is a process for recovering hydrogen from a hydrogen/carbon oxide synthesis gas wherein said synthesis gas is subjected to shift reaction to convert carbon monoxide and steam to carbon dioxide and hydrogen followed by treating an effluent from the shift reaction, after heat recovery, in a pressure swing adsorption unit to yield a purified hydrogen stream containing essentially no carbon oxides the improvement comprising operating the pressure swing adsorption unit under a cycle time wherein carbon oxides breakthrough are present in an effluent from the pressure swing adsorption unit; and passing the effluent from the pressure swing adsorption unit through a methanator to convert residual carbon oxides to methane to produce a purified hydrogen stream, whereby there is a volumetric increase in hydrogen recovery from the pressure swing adsorption unit.