Producing gas mixtures of hydrogen and carbon dioxide

Abstract

The invention relates to a method for producing a gas mixture of hydrogen and carbon dioxide, comprising the following method steps: I) isolating CO2 from gas mixtures having a volume percentage of CO2 in the range of 0.01% to 20%, by means of an adsorber; II) desorbing the CO2 by means of a heated hydrogen-containing gas stream in a desorber; and III) setting the CO2 volume percentage of the hydrogen-containing gas stream to the target percentage by means of a recycle gas system, which conducts the heated gas mixture of hydrogen and CO2 through the desorber and thus raises the CO2 volume percentage in the gas mixture of hydrogen and CO2. After method step III, in a method step IV the gas mixture of hydrogen and carbon dioxide can be used, after the required CO2 volume percentage has been set, to produce methanol.

Description

[0001] Production of hydrogen-carbon dioxide gas mixtures

[0002] The invention relates to a process for obtaining a hydrogen-carbon dioxide gas mixture and its use for producing methanol.

[0003] State of the art

[0004] The isolation of CO2 from process exhaust gases is an important aspect for the climate-neutral industrial production of raw materials or energy. The state of the art is so-called amine scrubbing, in which CO2 is passed through an aqueous amine solution and reversibly isolated from the gas stream by carbohydrate formation. A disadvantage of this technology is, among other things, that a relatively high CO2 partial pressure is required for effective CO2 separation. This means that process exhaust gases with a low CO^2 content or the isolation of CO2 directly from the atmosphere (direct air capture; DAC) are not possible using this process. This problem can be solved by adsorbing CO2 on solid adsorbers. The problem here is the desorption of CO2 from the adsorber. For this to happen, for example in a temperature swing process, the temperature in the adsorber must be increased significantly. A typical example of this is the introduction of superheated steam.This means that after desorption, the CO2 must be dried, which is energy-intensive. It would be better to have a gas mixture that can be further processed directly after desorption without complex processing. This is made possible by the invention proposed here. By using hydrogen gas as both the desorber gas and the heat transfer medium, a hydrogen-carbon dioxide gas mixture can be effectively obtained, which can then be used, for example, to produce methanol without complex processing.

[0005] The use of hydrogen for the desorption of carbon dioxide is not new. DE 102013 022 021 B4, for example, claims a process for isolating CO2 from gas mixtures. The CO2-rich gas mixture is passed through a gas chromatography column, with the CO2 diffusing physically more slowly than the residual gases. After loading, the column is backflushed isothermally with hydrogen, meaning the column is not heated during the desorption step. This is particularly disadvantageous for chemical adsorbers, since desorption is an endothermic process, and desorption does not occur or occurs only incompletely without increasing the temperature.

[0006] EP 3530640 A discloses a process for the reductive production of methane from CO2 and hydrogen. In this process, CO2 is desorbed from the adsorber using preheated hydrogen, which is heated by the waste heat from the reduction reactor. Furthermore, fresh hydrogen is added to the desorbed H2 / CCh mixture to adjust the required H2 / CCh ratio. A disadvantage here is that, in the case of a condensed gas mixture, there is no way to increase the CO2 partial pressure of the gas mixture.

[0007] The object of the invention is to eliminate the disadvantages of the prior art and in particular to find a simple and cost-effective way to adjust the CCh content in a hydrogen-carbon dioxide gas mixture within wide limits.

[0008] Description of the invention

[0009] The invention relates to a process for producing a hydrogen-carbon dioxide gas mixture comprising the following process steps:

[0010] I) Isolation of CO2 from gas mixtures with a volume fraction of CO2 in the range of 0.01% to 20%, using an adsorber;

[0011] II) Desorption of the CO2 using a heated hydrogen-containing gas stream in a desorber; and

[0012] III) Adjusting the CCh volume fraction of the hydrogen-containing gas stream to the target fraction by means of a recycle gas system that passes the heated hydrogen-CCh gas mixture through the desorber and thus increases the CCb volume fraction in the hydrogen-CCb gas mixture.

[0013] Description of the individual process steps:

[0014] I ) I insulation :

[0015] The carbon dioxide is preferably isolated from process exhaust gases or other industrial or natural gas mixtures. The gas mixtures preferably have a CO2 volume fraction in the range of 0.01% to 20%, in particular 0.04% to 15%. Before the adsorption step, the gas mixture is preferably cleaned of harmful components such as reactive secondary gases such as sulfur dioxide, nitrogen oxides, and / or finely divided solids such as dust. This can be done with the help of established technologies such as wet scrubbers, DeNOx systems (catalytic nitrogen oxide reduction systems; denitrification systems), electrostatic precipitators, cyclones and dust filters, etc. Adsorption preferably takes place in the form of chemisorption, i.e. a chemical reaction occurs between the carbon dioxide and the adsorber during adsorption. The moist content of the gas mixture is therefore preferably adjusted before it enters the adsorber.The optimum moisture content of the gas mixture depends on the chemical reaction in the chemisorption step. The carbon dioxide is preferably isolated in adsorbers charged with solid adsorbents. The adsorption step can take place in a gas-borne or stirred fluidized bed or in a fixed bed, with preference being given to a fixed bed adsorber. The number of adsorbers is preferably between 2 and 10, in particular between 2 and 5. The adsorption temperature is preferably in the range of 0 - 150 °C, particularly preferably between 20 - 100 °C, i.e. the process gas temperature on entry into the adsorber is preferably in the range of 0 - 150 °C, particularly preferably between 20 - 100 °C. The pressure in the adsorber is preferably in the range of 0 - 50 bar (absolute), particularly preferably in the range of 0.5 - 10 bar (absolute).

[0016] II) Desorption: Preferably, the CO2-laden adsorber is then separated from the CO2-rich process gas stream and connected as a desorber. The desorber is then an adsorber undergoing regeneration. A preheated, hydrogen-rich gas is used for desorption. The hydrogen volume fraction in the desorber gas can be in the range from 100% to 0.1%. This means that pure hydrogen or a gas mixture can be used, with carbon dioxide as the second gas component, with a volume fraction of 99.1 to 0%. As the adsorber flows through the desorber, the CO2 content in the desorber gas increases. This applies in particular to partial pressure-independent adsorption and desorption. This means that the capacity is defined solely / primarily by the temperature conditions, as is usually the case with chemisorption. As soon as physisorption plays a relevant role, high CO2 contents in the desorption gas are disadvantageous orrequire overcompensation through further temperature increases. In addition to heating by the CO2-rich process gas, the adsorber can be heated, for example, via a heating jacket using thermal fluids or gases, or electrically from the outside. If necessary, the adsorber can also be heated via temperature-controlled internals in the adsorber such as heating coils, heating plates or heating fingers. The heating of the CO2-rich process gas is preferably carried out via a gas heater such as a plate heat exchanger, tube bundle heat exchanger, fin heater, heating register or electrical resistance heater. Suitable energy sources for operating the gas heater are electrical energy (resistance heating) or fluid heat transfer media such as thermal oil, hot water / pressurized water and steam, preferably from heat recovery. Energy coupling for.

[0017] Heat recovery is suitable when heat with a temperature at least 20 K higher than the adsorption temperature is generated in processes in close proximity. This can be, for example, heat flows from combustion plants upstream of the CO2 recovery plant or downstream process steps for CO2 utilization (e.g. exothermic methanol synthesis, optional process step IV).

[0018] III) Adjusting the CCh volume fraction: The adjustment of the CCh volume fraction according to the invention is preferably carried out by means of a recycle gas line, with which the desorber gas is passed completely or partially through one or more desorbers until the CC^ fraction in the desorber gas reaches the target value required, for example, for a subsequent process step, e.g., methanol synthesis. In a first embodiment of the invention (IIIa), the recycle gas line is designed such that the desorber releasing the recycle gas is identical to the desorber receiving the recycle gas. In an operation of the recycle gas system according to the invention, the CO2-laden adsorber is purged with cold fresh hydrogen in a first step in order to displace residues of the process exhaust gas from the adsorber. This amount of purge gas can either be discarded or returned to the CO2-rich process gas stream.As soon as the purity of the purge gas after the adsorber has reached a value of preferably at least 99 full, in particular at least 99.5 full, the desorption step begins by starting the cycle gas process. For this purpose, the adsorber to be regenerated (which thus becomes the desorber) is purged with heated cycle gas. The cycle gas can be pure hydrogen or a hydrogen-carbon dioxide gas mixture. The cycle gas is cycled through the desorber until the gas composition corresponds at least to the target value, i.e. at least the target value of the CO2 volume fraction. As soon as the target value is reached, at least a partial stream is taken from the cycle gas and optionally fed for further use, for example in reductive methanol synthesis. The partial quantity taken from the cycle gas is preferably replaced by adding pure hydrogen to the cycle gas line.The desorption step is finished when the CC^ content of the cycle gas no longer changes significantly as it passes through the desorber. The cycle gas process is then stopped and any residues of the cycle gas in the desorber are displaced with cold hydrogen gas or process gas, process gas is preferred, and the desorber is cooled at the same time. This amount of purge gas can either be discarded or fed to the cycle gas stream or the process gas stream. As soon as the CC^ content in the purge gas is less than or equal to the CC^ content in the process gas and / or the temperature in the desorber is less than or equal to the process gas temperature before entering the adsorber, the purge step is finished and the desorber can be used as an adsorber again.

[0019] In a particularly preferred embodiment, the process according to the invention for producing a hydrogen-carbon dioxide gas mixture comprises at least two independent cycle gas systems in order to be able to operate the subsequent optional utilization of the gas mixture, for example the reductive methanol synthesis, continuously.

[0020] In a further embodiment according to the invention (III 1b), the desorber releasing the cycle gas and the desorber receiving the cycle gas are at least partially different, i.e. the desorber completely loaded with CO2 is connected to a cycle gas process in which at least one further desorber is already in the desorption process, it being possible for the desorbers to be connected in series or in parallel, with parallel connection being preferred. Analogous to embodiment IIIa, the desorber completely loaded with CO2 is connected after the residual amount of process gas has been displaced by purging with fresh hydrogen or cycle gas, with cycle gas being preferred. The amount of purge gas can either be discarded or returned to the CC^2-rich process gas stream. As soon as the purity of the purge gas after the adsorber, in the case of fresh hydrogen as the purge gas, has a value of preferably at least 99%, in particular at least 100%. 99.5 full, or.In the case of cycle gas as purge gas, once the composition of the cycle gas has largely been reached, the desorption step begins by starting the cycle gas process. For this purpose, the adsorber to be regenerated (which thus becomes the desorber) is purged with heated cycle gas. The cycle gas can be pure hydrogen or a hydrogen-carbon dioxide gas mixture. The cycle gas is cycled through the desorber until the gas composition corresponds at least to the target value, i.e. the target value of the CO2 volume fraction. As soon as the target value is reached, at least a partial stream is taken from the cycle gas and optionally fed for further use, for example in reductive methanol synthesis. The partial quantity taken from the cycle gas is preferably replaced by adding pure hydrogen to the cycle gas line.The desorption step of at least one desorber is finished when the CC^ content of the cycle gas no longer changes significantly as it passes through the desorber(s). The corresponding desorber(s) are disconnected from the cycle gas system. Residues of the cycle gas in the desorber are then displaced with cold hydrogen gas or process gas, process gas is preferred, and at the same time the desorber is cooled. This amount of purge gas can either be discarded or fed to the cycle gas stream or the process gas stream. As soon as the CCh content in the purge gas is less than or equal to the CC^ content in the process gas and / or the temperature in the desorber is less than or equal to the process gas temperature before entering the adsorber, the purge step is finished and the desorber can be used as an adsorber again.

[0021] The aforementioned recycle gas systems preferably consist of suitable piping with interconnection options to the various adsorbers / desorbers, as well as suitable fans or compressors for conveying the gases and suitable temperature control systems for adjusting the required recycle gas temperatures. A recycle gas system is also preferably equipped with suitable gas addition and discharge systems, as well as measurement and control technology, such as CO2 concentration measuring devices (e.g., mass spectroscopy, process gas chromatography, Raman spectroscopy), as well as conventional measuring devices for pressure, temperature, and flow rates, for controlled process management.

[0022] The temperature of the desorber gas is preferably in the range of 20 - 250 °C, more preferably in the range of 50 - 200 °C, the pressure is preferably in the range of 0 - 50 bar (absolute), more preferably 0.5 - 10 bar (absolute). The temperature of the desorber gas in step II is preferably at least 5 °C higher, more preferably at least 10 °C higher, in particular at least 20 °C higher than the temperature of the gas mixture in step I. In addition to heating by the desorber gas, the desorber can be heated, for example, via a heating jacket, for example by means of thermal fluids or gases, or electrically from the outside. If necessary, the desorber can also be heated via temperature-controlled internals in the desorber such as heating coils, heating plates or heating fingers. The desorber gas is preferably heated using a gas heater such as a plate heat exchanger, tube bundle heat exchanger, fin heater, heating register or electrical resistance heater.The gas heater is preferably used in the recycle gas system so that the inlet temperature of the desorber gas into the desorber can always be adjusted to the desired value. Suitable energy sources for operating the gas heater are electrical energy (resistance heating) or fluid heat transfer media such as thermal oil, hot water / pressurized water and steam, preferably from heat recovery systems. Energy coupling for heat recovery is suitable when heat with a temperature at least 20 K higher than the desorption temperature is generated in processes in close proximity. This can be, for example, heat flows from combustion plants upstream of the CO2 recovery plant or downstream process steps for CO2 utilization (e.g. exothermic methanol synthesis, optional process step IV). The number of desorbers preferably corresponds to the number of adsorbers.This means that while one adsorber is active, another adsorber, the absorption capacity of which for CO2 gas has been reached, is regenerated by purging with desorber gas and thus serves as a desorber during this time. This means that the process according to the invention preferably has at least two adsorbers. For reasons of the overall capacity of the process, however, it is advantageous to increase the number of adsorbers. The number of adsorbers is preferably in the range of 2 - 10, particularly preferably in the range of 2 - 5. This means that at least one adsorber is active, at least one adsorber is regenerated and thus serves as a desorber, and the remaining number of adsorbers can be kept in the regenerated state, i.e. without CO2 loading in standby operation.

[0023] In a preferred embodiment, after process step III, the hydrogen-carbon dioxide gas mixture removed from the recycle gas line is used to produce methanol. In a preferred embodiment, additional fresh hydrogen can be added to the hydrogen-carbon dioxide gas mixture after removal from the recycle gas line and before feeding it into the methanol production process, particularly in order to be able to adjust the CCH volume fraction of the hydrogen-carbon dioxide gas mixture to the precise target value for methanol synthesis if the CCH volume fraction is too high.

[0024] IV) Optional use of the hydrogen-carbon dioxide gas mixture:

[0025] The gas mixture with the adjusted carbon dioxide volume fraction can subsequently be used as a synthesis gas for the production of methanol. The hydrogen content in this gas mixture is preferably 75 mol%, i.e. the CO2 content is 25 mol%. The reduction of CO2 by the hydrogen in the gas mixture takes place in reactors of established design (e.g. catalytic fixed-bed reactors) with the aid of suitable catalysts. If necessary, the hydrogen-carbon dioxide gas mixture must be preheated to the required reaction temperature before the reaction. The exact temperature and pressure depend on the selected catalyst.Since the reduction of CO2 with hydrogen to methanol is exothermic at -49.6 kJ / mol (at 300 K), the released reaction heat can be used, for example, with the help of heat exchangers to heat the hydrogen-carbon dioxide gas mixture, or to heat the desorber gas to release the carbon dioxide in desorption step II.

Claims

Patent claims 1. Process for producing a hydrogen-carbon dioxide gas mixture comprising the following process steps: I) Isolation of CO2 from gas mixtures with a volume fraction of CO2 in the range of 0.01% to 20%, using an adsorber; II) Desorption of the CO2 using a heated hydrogen-containing gas stream in a desorber; and III) Adjusting the CCh volume fraction of the hydrogen-containing gas stream to the target fraction using a recycle gas system that passes the heated hydrogen-CCb gas mixture through the desorber, thereby increasing the CCb volume fraction in the hydrogen-CCb gas mixture.

2. The process according to claim 1, wherein in step I the gas mixtures preferably have a COb volume fraction in the range of 0.04% to 15%.

3. A process according to any one of the preceding claims, wherein in step I the adsorber is a fixed bed adsorber.

4. Process according to one of the preceding claims, in which the adsorber loaded with CO2 in step I is separated from the CO2-rich gas stream in step II and connected as a desorber.

5. A process according to any one of the preceding claims, wherein the temperature of the desorber gas in step II is at least 5°C higher than the temperature of the gas mixture in step I.

6. A process according to any one of the preceding claims, wherein in step III the cycle gas system is designed such that the desorber delivering the cycle gas is identical to the desorber receiving the cycle gas. Process according to one of claims 1 to 6, wherein in step III the desorber delivering the cycle gas and the desorber receiving the cycle gas are different, wherein the desorber completely loaded with CO2 is connected to a cycle gas process in which at least one further desorber is already in the desorption process. Process according to one of the preceding claims, wherein after process step III in a process step IV the hydrogen-carbon dioxide gas mixture is adjusted to the required CCh volume fraction for used to produce methanol.