Production of gas mixture of hydrogen and carbon dioxide

Abstract

The invention relates to a method for producing a hydrogen-carbon dioxide gas mixture, comprising the following method steps: I) separating CO2 from a gas mixture having a CO2 volume fraction in the range of 0.01% to 20% using an adsorber; iI) desorbing CO2 in a desorber using the heated hydrogen-containing gas stream; and III) adjusting the CO2 volume fraction of the hydrogen-containing gas stream to a target content using a recycle gas system by means of which the heated hydrogen-CO2 gas mixture is passed through the desorber, thereby increasing the CO2 volume fraction in the hydrogen-CO2 gas mixture. The method step III may be followed by a method step IV in which the hydrogen-carbon dioxide gas mixture is used to produce methanol after adjusting the desired percentage by volume of CO2.

Description

[0001] The invention relates to a method for obtaining a hydrogen-carbon dioxide gas mixture and its use for producing methanol. Prior art

[0002] Separation of CO2 from process waste gases is an important aspect of climate-neutral industrial production of raw materials or energy. The prior art uses so-called amine scrubbing, in which CO2 is passed through an aqueous amine solution and reversibly separated from a gas stream by forming carbamic acid. The disadvantages of this technology include, in particular, that the effective separation of CO2 requires a relatively high CO2 partial pressure. That is, the process is not suitable for process waste gases with a low CO2 content or for separating CO2 directly from the atmosphere (direct air capture, DAC). This problem can be solved by adsorbing CO2 on a solid adsorbent. The problem here is that CO2 is desorbed from the adsorbent. For example, this requires a significant increase in the temperature in the adsorbent during a temperature change process. For example, the introduction of superheated steam is typical here. This means that CO2 requires energy-intensive drying after desorption. It is better to have such a gas mixture that is suitable for further processing directly after desorption without expensive and inconvenient processing. The invention proposed here makes this possible. The use of hydrogen as a desorber gas and simultaneously as a heat transfer medium makes it possible to effectively obtain a hydrogen-carbon dioxide gas mixture, which can then be used, for example, to produce methanol without expensive and complex processing.

[0003] The use of hydrogen to desorb carbon dioxide is not new. Thus, DE 10 2013 022 021 B4 claims a method for separating CO2 from a gas mixture, wherein a gas mixture rich in CO2 is passed through a gas chromatography column and CO2 physically diffuses more slowly than tail gases. After loading the column, the column is isothermally backwashed with hydrogen, i.e. the column is not heated during the desorption step. This is disadvantageous, in particular in the case of chemical adsorbents, because desorption is an endothermic process and desorption only occurs incompletely (if at all) without a temperature increase.

[0004] EP 3530640 A discloses a method for producing methane from CO2 and hydrogen reduction. CO2 is desorbed from the adsorbent with preheated hydrogen and the hydrogen is heated by waste heat from the reduction reactor. The desorbed H2 / CO2 mixture is also mixed with fresh hydrogen to establish the necessary H2 / CO2 ratio. The disadvantage is that in the case of a low-CO2 gas mixture, the CO2 partial pressure of the gas mixture cannot be increased.

[0005] It is therefore an object of the present invention to overcome the disadvantages of the prior art and in particular to find a simple and cost-effective method for adjusting the CO2 content in a hydrogen-carbon dioxide gas mixture over a wide range. DETAILED DESCRIPTION

[0006] The present invention provides a method for producing a hydrogen-carbon dioxide gas mixture, the method comprising the following method steps:

[0007] I) separating CO2 from a gas mixture having a CO2 volume fraction in the range of 0.01% to 20% using an adsorber;

[0008] II) desorbing the CO2 using a heated hydrogen-containing gas stream in a desorber; and

[0009] III) adjusting the CO 2 volume fraction of the hydrogen-containing gas stream to a target content using a circulating gas system, by means of which the heated hydrogen-CO 2 gas mixture is passed through a desorber, thereby increasing the CO 2 volume fraction in the hydrogen-CO 2 gas mixture.

[0010] Describe the individual method steps:

[0011] I) Separation:

[0012] Carbon dioxide is preferably separated from process waste gas or other industrial or natural gas mixture. The gas mixture preferably has a CO2 volume fraction of 0.01% to 20%, in particular 0.04% to 15%. It is preferred that the gas mixture be purified of harmful components such as reactive secondary gases, such as sulfur dioxide, nitrogen oxides and / or finely divided solids such as dust before the adsorption step. This can be accomplished using established technologies such as wet scrubbers, NOx removal systems (catalytic nitrogen oxide reduction systems, nitrogen oxide removal systems), electric filters, cyclone separators and dust filters, etc. The adsorption is preferably carried out in a chemical adsorption manner, i.e., a chemical reaction between carbon dioxide and the adsorbent occurs during the adsorption. Therefore, the moisture content of the gas mixture is adjusted before entering the adsorber. The optimal moisture content of the gas mixture depends on the chemical reaction in the chemical adsorption step. The separation of carbon dioxide is preferably carried out in an adsorber equipped with a solid adsorbent. The adsorption step can be carried out in an airborne or stirred fluidized bed or in a fixed bed, wherein a fixed bed adsorber is preferred. 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 in the range of 20-100° C., i.e. the process gas temperature upon entering the adsorber is preferably in the range of 0-150° C., particularly preferably in the range of 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).

[0013] II) Desorption:

[0014] It is preferred to separate the CO2-loaded adsorber from the CO2-rich process gas stream and connect it as a desorber. The desorber is therefore a regenerated adsorber. Desorption uses preheated hydrogen-rich gas. The hydrogen gas volume fraction in the desorbed gas can be in the range of 100% to 0.1%. Therefore, pure hydrogen or a gas mixture can be used, in which the second gas component is carbon dioxide with a volume fraction of 99.1% to 0%. The CO2 content in the desorbed gas is increased through the desorber. This is particularly suitable for adsorption and desorption that are independent of partial pressure. That is, the capacity is only / mainly limited by temperature conditions, such as is usually the case with chemical adsorption. Once physical adsorption plays a relevant role, the high CO2 content in the desorbed gas is unfavorable or needs to be overcompensated by further temperature increase. In addition to heating via a CO2-rich process gas, the adsorber can be heated, for example, via a heating jacket (e.g., using a heat transfer liquid or gas), or electrically heated from the outside. The adsorber can also be optionally heated via a heatable internal component (such as a heating coil, a heating plate, or a heating finger) in the adsorber.

[0015] The heating of the CO2-rich process gas is preferably carried out via a gas heater, such as, for example, a plate heat exchanger, a shell and tube heat exchanger, a fin heater, a heating recorder or a resistance heater. Suitable energy sources for operating the gas heater include electrical energy (resistance heating) or fluid heat transfer media such as hot oil, hot water / pressurized water and steam, preferably from a heat recovery system. When heat is generated in a spatially close process at a temperature at least 20 K above the adsorption temperature, for example heat flows from a combustion device upstream of the CO2 recovery device or a downstream process step for CO2 utilization (e.g. exothermic methanol synthesis, optional process step IV), energy coupling for heat recovery is possible.

[0016] III) Adjust the CO2 volume fraction:

[0017] The regulation of the CO2 volume fraction of the present invention is preferably carried out via a circulating gas loop, by means of which the desorbed gas is passed completely or partially through one or more desorbers until the CO2 content in the desorbed gas reaches, for example, a target value required for a subsequent process step (e.g., methanol synthesis). In a first inventive embodiment (IIIa), the circulating gas loop is configured so that the desorber that discharges the circulating gas is identical to the desorber that receives the circulating gas. In the inventive operating mode of the circulating gas system, in a first step, the adsorber loaded with CO2 is purged with cold fresh hydrogen to replace the residual amount of process waste gas from the adsorber. The purge gas amount can be discarded or recycled into a process gas stream rich in CO2. Once the purity of the purge gas downstream of the absorber reaches a value of preferably at least 99% by volume, in particular at least 99.5% by volume, the desorption step is started by the start of the circulating gas process. To this end, the adsorber to be regenerated (which therefore becomes a desorber) is purged with heated circulating gas. The circulating gas can be pure hydrogen or a hydrogen-carbon dioxide gas mixture. The circulating gas is circulated 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. Once the target value is reached, at least one substream is taken out of the circulating gas and optionally sent for further treatment, such as reduction methanol synthesis. The substream taken out from the circulating gas is preferably replaced by adding pure hydrogen to the circulating gas loop. When the CO2 content of the circulating gas no longer changes significantly during the desorber, the desorption step terminates. The circulating gas process is then terminated, and the residual amount of the circulating gas in the desorber is subsequently replaced with cold hydrogen or process gas (preferably process gas), while the desorber is cooled. The purge gas amount can be discarded or supplied to the circulating gas stream or the process gas stream. Once the CO2 content in the purge gas is less than or equal to the CO2 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 terminates and the desorber can be used as an adsorber again.

[0018] In a particularly preferred embodiment, the process according to the invention comprises at least two independent recycle gas systems for the production of a hydrogen-carbon dioxide gas mixture, in order to allow continuous operation of the gas mixture for its optional subsequent utilization, for example for reductive methanol synthesis.

[0019] In another embodiment (IIIb) according to the invention, the desorber that discharges the circulating gas and the desorber that receives the circulating gas are at least partially different, that is, the desorber that is completely loaded with CO2 is connected to the circulating gas process, in which at least one other desorber is already in the desorption process, wherein the desorbers can be connected in series or in parallel, preferably in parallel. Similar to embodiment IIIa, after the residual amount of process gas is replaced by purging with fresh hydrogen or circulating gas (preferably circulating gas), the connection of the desorber that is completely loaded with CO2 is carried out. The purge gas amount can be discarded or recycled into the process gas stream rich in CO2. In the case of fresh hydrogen as purge gas, once the purity of the purge gas downstream of the absorber reaches a value of at least 99% by volume, in particular at least 99.5% by volume, or in the case of circulating gas as purge gas, the composition of the circulating gas is reached, the desorption step is started by the start of the circulating gas process. For this purpose, the adsorber to be regenerated (which therefore becomes a desorber) is purged with heated circulating gas. The circulating gas can be pure hydrogen or a hydrogen-carbon dioxide gas mixture. The circulating gas is circulated through the desorber until the gas composition corresponds to at least the target value, i.e., the target value of the CO2 volume fraction. Once the target value is reached, at least one substream is taken out from the circulating gas and optionally sent for further treatment, such as reduction methanol synthesis. The substream taken out from the circulating gas is preferably replaced by adding pure hydrogen to the circulating gas loop. When the CO2 content of the circulating gas no longer changes significantly during the desorber, the desorption step of at least one desorber is terminated. The corresponding desorber is separated from the circulating gas system. The residue of the circulating gas in the desorber is then replaced with cold hydrogen or process gas (preferably process gas), while the desorber is cooled. The purge gas amount can be discarded or supplied to the circulating gas flow or process gas flow. Once the CO2 content in the purge gas is less than or equal to the CO2 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 terminated and the desorber can be used as an adsorber again.

[0020] The above-mentioned circulating gas system preferably includes not only suitable piping with connection options to different adsorbers / desorbers, but also suitable ventilators / compressors for conveying the gas and suitable heating systems for adjusting the required temperature of the circulating gas. The circulating gas system is preferably also equipped with suitable feed and discharge systems for the gas and measurement and control technology for controlled process management, such as CO2 concentration measuring instruments (e.g. mass spectrometry, process gas chromatography, Raman spectroscopy) and conventional measuring instruments for pressure, temperature and flow rate.

[0021] The temperature of the desorbed gas is preferably in the range of 20-250° C., particularly preferably in the range of 50-200° C., and the pressure is preferably in the range of 0-50 bar (absolute), particularly preferably in the range of 0.5-10 bar (absolute). The temperature of the desorbed gas in step II is preferably at least 5° C. higher than the temperature of the gas mixture in step I, particularly preferably at least 10° C. higher, in particular at least 20° C. higher. In addition to heating by the desorbed gas, the desorber can be heated, for example, via a heating jacket (for example by a heat transfer liquid or gas), or electrically heated from the outside. The desorber can also optionally be heated via heatable internal components in the desorber (such as heating coils, heating plates or heating fingers).

[0022] The heating of the desorbed gas is preferably carried out via a gas heater, such as, for example, a plate heat exchanger, a shell and tube heat exchanger, a fin heater, a heating register or a resistance heater. The gas heater is preferably used in a circulating gas system to allow repeated readjustment to the desired value of the entry temperature of the desorbed gas into the desorber. Suitable energy sources for operating the gas heater include electrical energy (resistance heating) or fluid heat transfer media such as hot oil, hot water / pressurized water and steam, preferably from a heat recovery system. When heat is generated at a temperature at least 20 K higher than the desorption temperature in a spatially close process, for example a heat flow from a combustion device upstream of the CO2 recovery device or a downstream process step for CO2 utilization (for example exothermic methanol synthesis, optional process step IV), energy coupling for heat recovery is possible.

[0023] It is preferred when the number of desorbers corresponds to the number of adsorbers. This is to be understood as meaning that while one adsorber is in operation, another adsorber which has reached its CO2 absorption capacity is regenerated by purging with desorption gas and is therefore used as a desorber during this time. This is to be understood as meaning that the method according to the invention preferably comprises at least two adsorbers. However, for reasons of the total capacity of the process, 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 is to be understood as meaning that at least one adsorber is in operation, at least one adsorber is regenerated and is therefore used as a desorber, and the remaining number of adsorbers can remain in standby mode in the regenerated state, i.e. without CO2 load.

[0024] In a preferred embodiment, process step III is followed by the use of the hydrogen-carbon dioxide gas mixture removed from the cycle gas loop for the production of methanol. In a preferred embodiment, the hydrogen-carbon dioxide gas mixture can be mixed with further fresh hydrogen after being removed from the cycle gas loop and before being introduced into the methanol production process, in particular in order to allow, in the case of an excessively high CO2 volume fraction of the hydrogen-carbon dioxide gas mixture, its adjustment to an exact target value for the methanol synthesis.

[0025] IV) Optional use of a hydrogen-carbon dioxide gas mixture:

[0026] Gas mixture with regulated carbon dioxide volume fraction can be used to produce methanol in the mode of synthesis gas subsequently.Preferably when the ratio of hydrogen in the gas mixture is 75 mol %, i.e., CO2 ratio is 25 mol %.In the reactor (e.g., catalytic fixed bed reactor) of establishment design, CO2 is reduced by hydrogen in the gas mixture using suitable catalyst.Hydrogen-carbon dioxide gas mixture optionally needs to be preheated to necessary reaction temperature before reaction.Precise temperature and pressure depend on the selected catalyst.Due to reducing CO2 with hydrogen to provide methanol is exothermic with -49.6kJ / mol (at 300K), therefore the reaction heat released thereby (e.g., using heat exchanger) can be used to heat hydrogen-carbon dioxide gas mixture, or heating desorption gas is to release carbon dioxide in desorption step II.

Claims

1. A method for producing a hydrogen-carbon dioxide gas mixture, the method comprising the following method steps: I) separating CO2 from a gas mixture having a CO2 volume fraction in the range of 0.01% to 20% using an adsorber; II) desorbing the CO2 using a heated hydrogen-containing gas stream in a desorber; and III) adjusting the CO 2 volume fraction of the hydrogen-containing gas stream to a target content using a circulating gas system, by means of which a heated hydrogen-CO 2 gas mixture is passed through the desorber, thereby increasing the CO 2 volume fraction in the hydrogen-CO 2 gas mixture.

2. The method according to claim 1, wherein in step I, the gas mixture preferably has a CO2 volume fraction in the range of 0.04% to 15%.

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

4. The process according to claim 1 , wherein the adsorber loaded with CO 2 in step I is separated from the CO 2 -rich process gas stream and connected as a desorber in step II.

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

6. The method according to any one of the preceding claims, wherein in step III, the cycle gas system is configured such that the desorber that discharges the cycle gas is the same as the desorber that receives the cycle gas.

7. The method according to any one of claims 1 to 6, wherein in step III, the desorber which discharges the cycle gas and the desorber which receives the cycle gas are different, wherein the desorber which is completely loaded with CO2 is connected to a cycle gas process in which at least one further desorber is already in a desorption process.

8. The method according to any of the preceding claims, wherein method step III is followed by method step IV, in which the hydrogen-carbon dioxide gas mixture is used for producing methanol after adjustment of the desired CO 2 volume fraction.