DE102019008016A1 - Method and device for the production of carbon monoxide by reverse water gas shift - Google Patents

Abstract

The invention relates to a method and a device for generating a product gas (15) containing carbon monoxide, a feed stream (2) containing hydrogen and carbon dioxide being converted into a carbon monoxide-containing intermediate product (4) in a reactor (R) by reverse water gas shift which the product gas (15) is separated. A characteristic feature here is that the reverse water-gas shift reaction (R) is carried out non-catalytically and homogeneously in the gas phase at temperatures of more than 1250 ° C and the heat required for this is generated by combustion of the hydrogen contained in the feed stream (2).

Description

The invention relates to a process for producing a product gas containing carbon monoxide, a feed stream containing hydrogen and carbon dioxide being converted in a reactor by reverse water gas shift into an intermediate product containing carbon monoxide, from which the product gas is separated off.

The invention also relates to a device for carrying out the method according to the invention.

Product gases containing carbon monoxide, such as oxo or synthesis gases, are usually obtained in a thermo-chemical conversion process from fossil carbon-containing inputs such as natural gas by allo- or autothermal steam reforming or partial oxidation. All of these processes create carbon dioxide and release it into the atmosphere, where it accumulates and increases the greenhouse effect. The aim is therefore to reduce the consumption of fossil fuels in the production of carbon monoxide and to use more renewable energy sources.

The easiest way to achieve this is to replace fossil fuels with agriculturally produced biomass. However, this reduces the acreage available for food production, which in view of the increasing world population is likely to lead to conflicts in the medium to long term.

Another possibility is to split water into hydrogen and oxygen in an electrolyser from renewable electrical energy - e.g. through wind power or photovoltaics - and to split the hydrogen obtained in this way with carbon dioxide, which is a by-product in many industrial processes, through catalytically supported reverse water gas shift (rWGS for short) according to the equation CO ₂ + H ₂ ⇒ CO + H ₂ O to convert to carbon monoxide in an endothermic reaction.

The catalysts used here, arranged in a fixed bed, are selective for the rWGS reaction and suppress the methanation, which is also thermodynamically favored (CO ₂ + 4H ₂ => CH ₄ + 2H ₂ O). The thermodynamically very limited conversion of the rWGS increases with increasing reaction temperature. The mass action constant Kp, which has a value of only 0.1 at 400 ° C, reaches a value of approx. 1 at 800 ° C. The transport of the required heat of reaction is limited by the properties of the catalyst bed and the design-related limited possibilities of heat transfer, so that, depending on the type of catalyst used, the reaction can only be carried out in a temperature range between 400-700 ° C.

To compensate for the conversion limitations, according to the prior art, carbon dioxide and / or hydrogen are introduced in a large excess into the rWGS reactor used to carry out the rWGS reaction. In order to obtain the product gas in the desired composition (for example with H2 / CO = 1), unconverted carbon dioxide and excess hydrogen are separated from the product stream with considerable effort and returned to the rWGS reactor.

A third possibility to reduce the climate-damaging effects of carbon monoxide production is given by the electrolytic decomposition of carbon dioxide and water using regenerative electrical energy. However, processes of this type are not yet suitable for large-scale use, since they are currently still in the development stage.

The object of the present invention is therefore to provide a method and a device of the generic type by means of which the limitations of the prior art are overcome.

In terms of the process, this object is achieved according to the invention in that the reverse water-gas shift reaction is carried out non-catalytically and homogeneously in the gas phase at temperatures of more than 1250 ° C. and the heat required for this is generated by combustion of hydrogen contained in the feed stream.

At temperatures of more than 1250 ° C, the rWGS reaction proceeds sufficiently quickly in the gas phase without the use of a catalyst. At the same time, the constant of mass action Kp assumes a value of approx. 2.8, so that sufficiently high conversions are achieved with comparatively low unconverted and recirculated amounts of carbon dioxide and hydrogen. Furthermore, methane is only formed in small quantities, since methanation is only slightly thermodynamically favored under these conditions.

The reactor used for the rWGS reaction is supplied with oxygen in an amount which may not be sufficient to burn all of the hydrogen present in the feed stream, but which is sufficient to raise the temperature in the gas phase to over 1250 ° C. The rWGS is preferably carried out autothermally and all of the heat required for this is obtained from the combustion of im Feed stream contained hydrogen generated. The oxygen can be metered into the feed stream containing hydrogen and carbon dioxide upstream of the rWGS reactor and mixed with it homogeneously. Preferably, however, the feed stream containing hydrogen and carbon dioxide is introduced separately from the oxygen via a burner into the reaction chamber of the rWGS reactor and only mixed there with it there.

The heat of combustion generated in the reaction of hydrogen and oxygen is generated in the gas phase, and thus directly where it is required as heat of reaction for the rWGS reaction. In contrast to an allothermal process management according to the prior art, the transport of the heat of reaction therefore has no or only a very minor limiting effect on the rWGS reaction.

To carry out the rWGS reaction, it is expedient to use a reactor similar to that known from the prior art for the production of synthesis gas by partial oxidation. The carbon monoxide-containing intermediate product available in the rWGS reactor is then cooled, for example in a quench device in direct contact with water or in a special heat exchanger (synthesis gas cooler) by evaporation of water, before the product gas containing carbon monoxide is separated from it.

To separate the product gas containing carbon monoxide, it is proposed that the intermediate product obtained in the rWGS reactor be subjected to acid gas scrubbing after cooling, in which carbon dioxide is washed out with the aid of a suitable scrubbing agent, such as methanol, with a largely consisting of carbon monoxide and possibly hydrogen Gas mixture and a detergent laden with carbon dioxide can be obtained. Depending on its composition, the gas mixture consisting largely of carbon monoxide and possibly hydrogen is released as product gas or, in order to set the desired composition for the product gas, subjected to a further separation step in which, for example, excess hydrogen is returned to the rWGS reactor in a cryogenic Process that is separated by membranes or by pressure swing adsorption.

A preferred variant of the process according to the invention provides for the scrubbing agent loaded with carbon dioxide in the acid gas scrubber to be regenerated by stripping, with hydrogen used as the stripping gas for use in the rWGS reactor. The hydrogen stream loaded with carbon dioxide during the stripping is expediently used to provide the feed stream containing hydrogen and carbon dioxide, for which purpose carbon dioxide from an external source and possibly also hydrogen are added.

Particularly preferably, at least part of the hydrogen required in the process according to the invention is obtained from water by electrolysis, the electrical current used being sensibly drawn from one or more renewable energy sources. Part of the electricity required for the electrolysis can be generated by a generator coupled to a steam turbine, with the steam produced during the cooling of the carbon monoxide-containing intermediate product being expanded in the steam turbine.

The oxygen required for the combustion of hydrogen in the rWGS reactor is preferably also obtained from water by electrolysis, which is expediently the same electrolysis in which hydrogen is also generated for the rWGS reaction. Some or all of the oxygen produced during electrolysis is used in the rWGS reactor to provide heat for the rWGS reaction through the combustion of hydrogen. Oxygen that arises in excess during electrolysis is expediently given off as an additional product stream. Water formed during the rWGS reaction is expediently condensed out of the carbon monoxide-containing intermediate and, after an optional purification, fed to the electrolysis for decomposition.

The invention also relates to a device for generating a product gas containing carbon monoxide, with a reactor in which a feed stream containing hydrogen and carbon dioxide can be converted into an intermediate product containing carbon monoxide by reverse water gas shift, and a separating device for separating the product gas from the intermediate product containing carbon monoxide.

The object is achieved in terms of the device according to the invention in that the reactor is designed with a fireproof thermal insulation which encloses an essentially empty reaction space in which the reverse water-gas shift reaction at temperatures of more than 1250 ° C non-catalytically and homogeneously in the Gas phase carried out and the heat required for this can be generated by combustion of hydrogen contained in the feed stream.

The fact that the reaction space of the rWGS reactor is essentially empty means that, in particular, it does not contain any catalyst. On the other hand, devices for feeding input materials and measuring devices into the Extend interior of the reaction space. The rWGS reactor is preferably constructed similarly to a reactor known from the prior art which can be used for the generation of synthesis gas by partial oxidation. It has a steel, essentially cylindrical and upwardly tapering pressure vessel with a vertical axis, which is lined with a fireproof insulation which encloses the reaction space. At the highest point of the reactor there is a process burner which fires vertically downwards and via which the feed stream containing hydrogen and carbon dioxide can be fed to the reaction chamber. The carbon monoxide-containing intermediate product produced can be withdrawn from the reaction space via an opening at the lower end of the reactor, for example in order to be cooled in direct contact with water in a quenching device.

The separating device arranged downstream of the rWGS reactor for separating the product gas from the carbon monoxide-containing intermediate preferably comprises an acid gas scrubber in which carbon dioxide can be washed out with the aid of a suitable washing agent such as methanol and a gas mixture consisting largely of carbon monoxide and possibly hydrogen as well as a Carbon dioxide loaded detergents are available. The acid gas scrubbing is sensibly connected to the rWGS reactor in such a way that the carbon dioxide produced during the regeneration of the loaded scrubbing agent can be fed back as an input upstream of the rWGS reactor.

Depending on the composition of the intermediate product and the desired quality of the product gas, the separation device can comprise one or more further devices for gas separation. For example, the acid gas scrubber can be followed by a cryogenic gas separator with which excess hydrogen can be separated from the gas mixture, which largely consists of carbon monoxide and hydrogen, for recycling upstream of the rWGS reactor.

In one embodiment of the device according to the invention, the acid gas scrubber for regenerating the washing agent loaded with carbon dioxide comprises a stripping device in which hydrogen intended for use in the rWGS reactor can be used as stripping gas. The stripping device is expediently connected to the rWGS reactor in such a way that the hydrogen loaded with carbon dioxide during the stripping can be used to provide the feed stream containing hydrogen and carbon dioxide.

In a preferred embodiment, the device according to the invention comprises at least one electrolyzer for the electrochemical decomposition of water into hydrogen and oxygen. The electrolyser is usefully connected to the rWGS reactor in such a way that the hydrogen that can be generated can be fed to the rWGS reactor as part of the feed stream containing hydrogen and carbon dioxide. Furthermore, the connection between this or another electrolyser and the rWGS reactor can be designed in such a way that all or part of the oxygen produced during the electrolysis can be used in the rWGS reactor to heat the reaction for the rWGS reaction through the combustion of hydrogen deliver.

The electrolyser is preferably connected to the rWGS reactor via the acid gas scrubber, so that at least some of the hydrogen obtainable through the electrochemical water decomposition can be used as stripping gas in the stripping device of the regeneration part.

In the following, the invention is based on one in the 1 schematically illustrated embodiment will be explained in more detail.

The 1 shows a preferred embodiment of the method according to the invention, in which hydrogen for the rWGS reaction is obtained by electrochemical decomposition of water.

A feed stream consisting largely of hydrogen and carbon dioxide 1 is sucked in by the compressor V and with increased pressure via the line 2 into the rWGS reactor R. initiated, where it is converted by non-catalytic homogeneous reverse water gas shift at temperatures of more than 1250 ° C in the gas phase. The heat required for the overall endothermic reverse water-gas shift is generated entirely by burning part of the hydrogen used, including the rWGS reactor R. oxygen 3 is fed. The carbon monoxide intermediate 4th , which also includes hydrogen, carbon dioxide and water and leaves the rWGS reactor at a high temperature, is cooled in the cooling device K to such an extent that water 5 condenses out and a cooled gas mixture consisting largely of carbon dioxide, hydrogen and carbon monoxide 6th is obtained, which can be introduced to the separation of carbon dioxide in the acid gas scrubber S, which is, for example, a methanol scrubber.

That from the intermediate product containing carbon monoxide 4th condensed water 5 is fed to the electrolyser E after a possibly required treatment (not shown), where it is fed together with further water 7th using electric current 8th in hydrogen 9 and oxygen 10 is disassembled, of which a first part 3 the rWGS reactor R. fed and a second part 11 is sold as a product. The entire amount of electricity used in the electrolyser E is preferably obtained from a regenerative energy source (not shown).

In the acid gas scrubber S, the gas mixture consisting largely of carbon dioxide, hydrogen and carbon monoxide is used 6th in countercurrent to regenerated detergent 12th through the absorber column A. directed to largely selectively wash out carbon dioxide. A gas mixture that has been cleaned of carbon dioxide and consists primarily of carbon monoxide and hydrogen 13th leaves the acid gas scrubber S while the detergent loaded with carbon dioxide 14th from the bottom of the absorber column A. is deducted. In the separation device C, the gas mixture 13th in a cryogenic process, via membranes, by pressure swing adsorption or a combination of these processes into a product gas containing carbon monoxide 15th as well as raw hydrogen 16 Cut.

The loaded detergent is used to regenerate it 14th in the heat exchanger W against detergent that has already been regenerated 12th warmed and via the throttle element a on the head of the stripping column B. relaxed, with part of the absorbed carbon dioxide going into the gas phase. In order to separate further carbon dioxide, the loaded detergent comes into intensive contact with hydrogen on its way down 9 , 16 brought, which acts as a stripping gas and the stripping column B. is abandoned in its lower area. If necessary, stripping steam can also be used in the bottom space of the stripping column B. arranged reboiler (not shown) are generated. From the over line 17th from the acid gas scrubber S withdrawn, hydrogen and carbon dioxide-containing gas mixture is obtained by metering in carbon dioxide obtained from an external source 18th the feed stream consisting largely of hydrogen and carbon dioxide 1 receive.

Claims (10)

Process for generating a product gas (15) containing carbon monoxide, a feed stream (2) containing hydrogen and carbon dioxide being converted in a reactor (R) by reverse water gas shift into an intermediate product (4) containing carbon monoxide, from which the product gas (15) is separated is, characterized in that the reverse water gas shift reaction (R) is carried out non-catalytically and homogeneously in the gas phase at temperatures of more than 1250 ° C and the heat required for this is generated by combustion of the hydrogen contained in the feed stream (2). Procedure according to Claim 1 , characterized in that the reverse water-gas shift reaction (R) is carried out autothermally and all the heat required for this is generated by the combustion of hydrogen contained in the feed stream (2). Method according to one of the Claims 1 or 2 , characterized in that the hydrogen (9) supplied to the reactor (R) is obtained from water by electrolysis (E), the electrical current (8) required for the electrolysis (E) being supplied from one or more renewable energy sources. Method according to one of the Claims 1 to 3 , characterized in that the oxygen (3) required for the combustion of hydrogen in the reactor (R) is obtained from water by electrolysis (E), the electrical current (8) required for the electrolysis (E) being supplied from renewable energy sources . Method according to one of the Claims 1 to 4th , characterized in that an acid gas scrubber is used to separate carbon dioxide from the carbon monoxide-containing intermediate product (4). Procedure according to Claim 5 , characterized in that hydrogen intended for use in the reactor (R) is used as stripping gas (9, 16) in the acid gas scrubber (S) during the regeneration (B) of the scrubbing agent (14) loaded with carbon dioxide. Apparatus for generating a product gas (15) containing carbon monoxide, with a reactor (R) in which a feed stream (2) containing hydrogen and carbon dioxide can be converted into an intermediate product (4) containing carbon monoxide by reverse water gas shift, as well as a separation device (K , S, C) for separating the product gas (15) from the carbon monoxide-containing intermediate product (4), characterized in that the reactor (R) is designed with a fireproof thermal insulation which encloses an essentially empty reaction space in which the reverse water gas Shift reaction carried out non-catalytically and homogeneously in the gas phase at temperatures of more than 1250 ° C and the heat required for this can be generated by combustion of hydrogen contained in the feed stream (2). Device according to Claim 7 , characterized in that it comprises an electrolyser (E) for the electrochemical decomposition of water (7) into hydrogen (9) and oxygen (10), which is connected to the reactor (R) in such a way that the hydrogen (9) that can be generated is the Reactor (R) can be fed as part of the feed stream (2) containing hydrogen and carbon dioxide. Device according to one of the Claims 7 or 8th , characterized in that it comprises an electrolyser (E) for the electrochemical decomposition of water (7) into hydrogen (9) and oxygen (10), which is connected to the reactor (R) in such a way that the oxygen (10) that can be generated is completely or can be used in part in the reactor (R) in order to supply heat of reaction for the reverse water gas shift reaction through the combustion of hydrogen. Device according to one of the Claims 7 to 9 , characterized in that the separating device comprises an acid gas scrubber (S) for separating carbon dioxide from the carbon monoxide-containing intermediate product (4), in which hydrogen intended for use in the reactor (R) is used during the regeneration (B) of the scrubbing agent (14 ) can be used as stripping gas (9, 16).

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