TW202500249A - Cleaning of co2-containing feed gases - Google Patents

Abstract

A process is provided for cleaning a CO ₂-rich gas stream from one or more impurities selected from sulfur-containing compounds, higher hydrocarbons and aromatic hydrocarbons. The process comprises the step of passing the CO ₂-rich gas stream together with a steam feed over a guard material, comprising a catalyst active in the conversion of higher hydrocarbons into methane in the presence of said steam feed (i.e. a pre-reforming catalyst).

Description

Purification of feed gas containing carbon dioxide

The present invention relates to a process for purifying a gas stream rich in carbon dioxide, in particular for removing impurities such as hydrocarbons and residual sulfur-containing impurities after a hydrodesulphurization (HDS) stage.

CO2 is commercially available in different grades. Typically, "food grade" or "beverage grade" CO2 has a purity of 99.9%. However, in processes involving the catalytic conversion of CO2 to other chemical products (e.g. Power-to-X technologies), impurities such as sulfur compounds and oxygen in the CO2 stream at concentrations of even 50-100 ppbV (0.000005 – 0.00001%) can poison the synthesis catalyst. The content of hydrocarbons, especially _C2 +, can also lead to undesirable reactions on the CO2 conversion catalyst together with the content of nitrogen species.

Although some carbon dioxide sources are of high purity, further purification has been found to be necessary to avoid catalyst poisoning of downstream synthesis catalysts. It would be advantageous if multiple impurities of different types (e.g., sulfur compounds, higher hydrocarbons, and aromatics) could be removed in a single process without the addition of oxygen or air, which would result in downstream hydrogen losses.

Systems and methods for purifying carbon dioxide streams are known, for example, from EP2457636, CN112999843 and CN112957872.

The inventors have discovered that the introduction of a catalytic protector after the HDS stage can protect the downstream synthesis/CO2 conversion catalyst from impurities and unwanted side reactions that may limit the catalyst life. The protector is a nickel-based catalyst, a noble metal, or a combination thereof, which is capable of converting hydrocarbons such as wax, aromatic hydrocarbons, and oxygen-containing compounds contained in the CO2 stream passing through the HDS stage after mixing with the steam stream. Nitrogen species other than molecular nitrogen, such as nitrogen oxides and ammonia, also participate in the reaction, and the catalytic protector adsorbs sulfur on the nickel surface, thereby reducing the active sites of the protector, preventing catalyst activity loss and shortening the life of the protector downstream catalyst.

The inventors have discovered that a pre-reforming catalyst can be used as a shield for the carbon dioxide feed. This will ensure that sulfur slip from the upstream sulfur purification process is captured and can remove hydrocarbons and convert them to methane.

Thus, a first aspect of the invention relates to a method for purifying a gas stream rich in carbon dioxide, the gas stream rich in carbon dioxide comprising at least 80% by weight of carbon dioxide and one or more impurities selected from: -   sulfur compounds; -   higher hydrocarbons; and -   aromatic hydrocarbons, -   nitrogen substances, wherein the method comprises the following steps: -   passing the gas stream rich in carbon dioxide together with a steam feed through a protective material, the protective material comprising a catalyst having an activity of converting higher hydrocarbons to methane in the presence of the steam feed, and adsorbing one or more of the impurities on the protective material to provide a purified gas stream rich in carbon dioxide.

This provides additional sulfur protection to handle low concentrations (ppm levels) of hydrocarbon content that may be present in the CO2 feed.

The invention also relates to a method for producing a chemical or fuel stream, the method comprising purifying a carbon dioxide-rich gas stream in the method described herein, then feeding the purified carbon dioxide-rich gas stream to a synthesis stage, optionally mixing with a hydrogen feed, and outputting a chemical or fuel stream from the synthesis stage. Chemicals and fuels produced by this method include but are not limited to _H2 , CO, MeOH, formaldehyde, DME, FT-based fuels, gasoline, and synthetic aviation fuels.

Additional aspects are described in detail in the following description, drawings, and claims.

Unless otherwise stated, any given percentage of gas content is by volume. All feeds are preheated as necessary. Unless otherwise stated, concentrations are calculated on a dry basis, i.e. without regard to the water content.

Higher hydrocarbons are all hydrocarbons that have more than one carbon atom in their molecule, which means almost all hydrocarbons except methane.

Synthesis gas, as a reference to synthesis gas, is a gaseous mixture containing hydrogen, carbon monoxide, carbon dioxide, and water and methane, usually in the form of steam. It is called synthesis gas/synthesis gas because it is the feed required for downstream catalytic synthesis, which ultimately produces the desired products. In some applications, the downstream feed after the above-mentioned purification treatment can be mixed with hydrogen and used as synthesis gas, for example, for methanol synthesis in other applications. The purified gas, after mixing with hydrogen and optionally steam, may need to be converted in a reverse water gas shift reactor (RWGS) or a combined RWGS and methanation reactor to form the final synthesis gas for the synthesis of the final product.

A purified CO2 stream is defined as an outlet stream from a CO2 purification process in which a minimum of 95% of the sulfur-containing impurities in the feed are removed, or the sum of the sulfur-containing impurities in the purified CO2 stream is less than 50 ppbV (parts per billion by volume), preferably less than 1 ppbV.

The sum of sulfur-containing impurities is to be understood as sulfur equivalent, i.e. 10 ppbV _SO2 corresponds to 10 ppbV sulfur and 10 ppbV _CS2 corresponds to 20 ppbV sulfur.

The proposed CO2 purification solution ensures that any feed gas used for downstream conversion to syngas and synthesis of chemicals such as MeOH (methanol), DME (dimethyl ether), FT (Fischer-Tropsch), synthetic fuels, etc. will not cause problems due to the by-products of higher hydrocarbons and can be prepared without poisoning the downstream synthesis catalysts with sulfur. This will ensure long-term operation and catalyst life in line with the expectations of industrial catalysts.

Thus, a first aspect provides a method for purifying a gas stream rich in carbon dioxide. Suitably, the gas stream rich in carbon dioxide entering the method has been treated in a HDS stage to remove a substantial portion of the sulphur and react with any oxygen content.

The carbon dioxide-rich gas stream provided to the process comprises at least 80 wt. %, such as at least 90 wt. % carbon dioxide, such as at least 95 wt. % carbon dioxide, such as at least 99.0 wt. % carbon dioxide, preferably at least 99.5 wt. % carbon dioxide, more preferably at least 99.9 wt. % carbon dioxide. Thus, before the process of the invention, the carbon dioxide-rich gas stream already has a high purity.

Suitably, the CO2-rich gas stream originates from a renewable source, such as: -   The combustion or gasification of woody cellulosic biomass such as wood products, algae, grasses, forestry waste and/or agricultural residues; -   The combustion or gasification of municipal waste, in particular the organic fraction thereof, wherein municipal waste is defined as feedstock containing public waste material, such as mixed municipal waste as defined in EU Directive 2018/2001 (RED II), Annex IX, Part A; -   The microbial conversion of nitrogen-rich renewable feedstocks such as faeces or sewage sludge; -   The fermentation of hydrocarbon (sugar)-rich feed streams such as corn, sugar cane and sugar beet; -  Carbon dioxide recovery units in chemical production, such as hydrogen or ammonia production plants, where carbon dioxide is removed from the product gases.

Carbon dioxide-rich gas streams can also be obtained from direct air capture processes, metallurgical processes, cement production or fossil fuel combustion.

The carbon dioxide concentration in some of the above-mentioned gas streams may generally be too low for further chemical processing and a concentration step may be required to increase the carbon dioxide concentration to the desired value as described above.

The carbon dioxide-rich gas stream contains one or more impurities selected from sulfur compounds, higher hydrocarbons, aromatic hydrocarbons and nitrogen species.

The CO2-rich gas is mixed with a steam stream and directed to the catalytic guard reactor. The amount of steam added to the CO2-rich gas depends on the hydrocarbon content to be converted and the hydrogen content in the CO2-rich gas after the HDS stage. The total molar amount of steam added should not be higher than 100 times the molar flow of hydrogen coming with the CO2-rich gas, preferably not higher than 40 times the molar flow of hydrogen. If this amount of steam is not sufficient to ensure the conversion of higher hydrocarbons, additional hydrogen should be added to the CO2-rich gas. The amount of higher hydrocarbons can be increased by adding a separate recycle stream from the product gas obtained downstream after the light end of the product gas obtained by catalytic synthesis recycle. The composition of this recycle stream depends on the downstream synthesis.

However, the amount of hydrogen added is limited by the desire not to obtain a significant temperature increase due to the exothermic methanation reaction that occurs when a carbon dioxide-rich gas mixed with a large amount of hydrogen reacts over a nickel or noble metal catalyst. In practice, the outlet temperature of the catalytic guard reactor can be controlled by adding additional hydrogen to the carbon dioxide-rich gas.

The carbon dioxide-rich gas containing steam (and possibly hydrogen) is passed through a guard material comprising a catalyst active in the presence of the steam feed for the conversion of higher hydrocarbons to methane. The "catalyst active in the conversion of higher hydrocarbons to methane" is typically a pre-reforming catalyst.

The catalyst may be a nickel-based catalyst, a noble metal-based catalyst or a combination of nickel and a noble metal. This may be a catalyst having an alumina, calcium aluminate, magnesium aluminate, a promoter alumina or a spinel carrier material, wherein the promoter metal may be Ti, La, Ce, Zr or Y. The nickel content may be 15-60% by weight, preferably 25-55% by weight. The noble metal content will be less than 0.5 – 10% by weight, and when combined with nickel the content is 0.1 – 2% by weight. The catalyst may be an impregnated catalyst or a co-precipitated catalyst. The catalyst may be pre-reduced before placement, or may be reduced in an activation reactor according to the reduction requirements to form an active metallic nickel or noble metal phase from the respective oxides.

The catalyst is able to convert higher hydrocarbons into methane, carbon monoxide, hydrogen and water via the following reactions which also involve carbon dioxide. First the reforming reaction: C _x H _y + x H ₂ O => x CO + (y/2+x) H ₂ followed by methanation: CO + 3 H ₂ ＜=＞ CH ₄ + H ₂ O High levels of carbon dioxide also lead to a reverse water gas shift to balance the concentrations of CO ₂ , CO, H ₂ and H ₂ O: CO ₂ + H ₂ ＜=＞ CO + H ₂ O

With the exception of methane, these products are normal components of syngas.

Another characteristic of nickel catalysts is the ability to adsorb sulfur on the nickel surface. At low temperatures (less than 500°C), almost all sulfur will be adsorbed, thus preventing sulfur from escaping to downstream catalysts. The adsorbed sulfur eliminates the catalytic activity of the above catalytic reactions, so it is necessary to ensure that the catalyst protectant has sufficient adsorption capacity and catalytic activity during its service life.

The catalyst also has the ability to convert nitrogen species such as nitrogen oxides and ammonia. Nitrogen oxides will be reduced to ammonia, and ammonia will reach equilibrium according to the following catalytic reaction: 2 NH ₃ ó N ₂ + 3 H ₂

The ammonia content leaving the catalytic guard reactor will depend on the nitrogen content of the CO2-rich gas, the hydrogen concentration, the pressure and the temperature. This ammonia decomposition reaction produces nitrogen and hydrogen, and the equilibrium will shift towards the products as the temperature increases and the pressure decreases. If the downstream catalyst is sensitive to ammonia, the nitrogen ( _N2 ) concentration in the CO2-rich gas is lower, and guarding at high temperature and low pressure can achieve low ammonia slip.

The guard reactor will be insulated with an inlet temperature between 250–500°C. Care should be taken not to allow significant temperature differences in the reactor, especially temperature rise. For this reason, the amount of hydrogen added should be limited to prevent significant methanation from occurring.

Suitably, the CO2-rich gas stream comprises one or more impurities selected from _H2S , DMS (dimethyl sulfide) and COS. For example, the total _SO2 content in the CO2-rich gas stream at the reactor vessel inlet is 0.1-100 ppm _SO2 , such as 1-50 ppm _SO2 .

Thus, in general, the method comprises the steps of passing a carbon dioxide-rich gas stream together with a steam feed through a guard material comprising a catalyst active in converting higher hydrocarbons to methane in the presence of the steam feed (i.e., a pre-reforming catalyst), and adsorbing one or more of the impurities on the guard material to provide a purified carbon dioxide-rich gas stream.

The steam feed is typically a high purity steam feed, ie it comprises at least 99 wt % H ₂ O, such as at least 99.5 wt % H ₂ O.

The operating pressure of the catalyst and adsorption system will be in the range of 1 to 90 barg, depending on the feed pressures of CO2 and other feed gases and the pressure of the downstream synthesis unit. Generally speaking, lower pressures are more favorable to minimize the formation of unwanted by-products, but with the attendant cost of larger equipment.

In one aspect of the invention, the carbon dioxide rich gas stream is passed through a guard material along with (e.g., mixed with) the first hydrogen rich feed to ensure that sufficient hydrogen is available for conversion of higher hydrocarbons.

The higher hydrocarbons removed in the process of the present invention may be selected from C2-C6 alkanes, C2-C6 oxygenates (i.e. hydrocarbons containing one or more oxygen atoms) and combinations thereof.

The higher aromatic hydrocarbons may be selected from C6-C8 aromatic hydrocarbons, C9-C12 aromatic hydrocarbons and combinations thereof.

In the method, the carbon dioxide-rich gas stream can be subjected to one or more desulfurization steps, preferably two or more desulfurization steps, before passing through the protective material. Desulfurization can be performed by the methods described in the co-applied DK patent applications PA202300339 and PA202300340, which are incorporated herein by reference.

According to one aspect, desulfurization can be a two-step process. According to this aspect, the method comprises: -    passing the carbon dioxide-rich gas stream through a first sulfur protective material, and then -    passing the carbon dioxide-rich gas stream through a second sulfur protective material before the carbon dioxide-rich gas stream passes through the protective material, and adsorbing one or more sulfur-containing compounds on the first and second sulfur protective materials.

Preferably, the first sulfur barrier material is characterized by being susceptible to H ₂ S evolution and the second sulfur barrier material is characterized by not exhibiting H ₂ S evolution. Optionally, water may be removed from the carbon dioxide rich gas stream between the first barrier material and the second barrier material.

The first and second sulfur guard materials may have the same composition, with the second sulfur guard material being at a different process condition that allows for operation without _H2S evolution. This may be accomplished by installing a water removal unit between the first and second sulfur guard materials and/or by installing a heat exchange unit to reduce the temperature between the first and second sulfur guard materials.

The second sulfur barrier material may have a different chemical composition than the first sulfur barrier material, for example, a ZnO-based first barrier material and a Cu-Zn-Al-based second sulfur barrier material. Even so, as described above, it may be beneficial to operate the two barrier materials under different process conditions.

In one aspect, the carbon dioxide-rich gas stream undergoes a preheating step prior to passing through the protective material.

In one embodiment, the purified carbon dioxide-rich gas stream contains less than 50 ppbV, preferably less than 10 ppbV, and most preferably less than 5 ppbV of sulfur. In another embodiment, the purified carbon dioxide-rich gas stream contains less than 100 ppm, preferably less than 50 ppm, and more preferably less than 10 ppm of higher hydrocarbons. In another embodiment, the purified carbon dioxide-rich gas stream contains less than 10 ppm, preferably less than 5 ppm, and most preferably less than 1 ppm of aromatic hydrocarbons. In another embodiment, the purified carbon dioxide-rich gas stream contains less than 10 ppm, preferably less than 1 ppm, and most preferably less than 0.1 ppm of alcohol.

In the process described herein, the step of passing the carbon dioxide-rich gas stream together with the steam feed through the barrier material can be carried out at a temperature between 250° C. and 550° C., preferably between 300° C. and 400° C. The operating pressure can vary from 5 barg to 80 barg depending on the optimal design of the overall process.

The amount of catalyst can also vary, with feed rates of CO2- and steam-rich gas typically ranging from 1,000 ^Nm3 /h/ ^m3catalyst to 10,000 ^Nm3 /h/ ^m3catalyst .

Also provided is a process for producing a chemical or fuel stream comprising purifying a carbon dioxide-rich gas stream in the process described herein, subsequently feeding the purified carbon dioxide-rich gas stream to a synthesis stage, optionally in admixture with a hydrogen feed, and outputting a chemical or fuel stream from the synthesis stage.

Specific embodiments of the present invention

Figure 1: A CO2-rich gas stream (1) (e.g. from a HDS stage) can optionally be mixed with a hydrogen stream (2) before being mixed with a steam stream (3). The CO2-rich gas stream is fed to a catalytic guard reactor (11) containing a guard material (10). The purified CO2-rich gas stream (50) leaving the catalytic guard reactor (11) is sent to downstream conversion.

Figure 2: CO2-rich gas (1) is mixed with a hydrogen stream (2) and passed through a hydrodesulfurization stage consisting of a first reactor (20) containing a hydrogenation catalyst and a subsequent second reactor (30) containing a sulfur adsorbent or a sulfur adsorption/chemisorption guard. The hydrodesulfurized CO2-rich gas can then be mixed with a second hydrogen stream (5) as required and/or a recycle stream (R) of unconverted synthesis gas or light-boiling hydrocarbons from downstream synthesis as required. The recycle stream (R) is typically a mixture of light hydrocarbons, CO, _CO2 and _H2 .

The carbon dioxide rich gas mixed with hydrogen and steam or mixed with steam (3) alone can be preheated in the preheater (10) to the desired inlet temperature. The heater (40) can be used to heat the carbon dioxide rich gas before or after mixing with the second hydrogen stream (5) or (2) and/or the recycle stream (recycle). The steam stream (3) is mixed with this stream before entering the catalytic guard reactor (50). The purified carbon dioxide rich gas (50) leaves the chemical guard reactor (10) and is sent to the downstream catalytic process. The second hydrogen stream (5) or (2) as required will be the hydrogen required to produce synthesis gas for the downstream catalytic process. This hydrogen stream can also be mixed with the purified carbon dioxide rich gas (50) after the catalytic guard reactor (10) as required.

The present invention has been described with reference to several aspects and drawings. However, a skilled person is able to select and combine various aspects within the scope of the invention as defined by the appended patent claims. All documents cited herein are incorporated by reference.

1: CO2-rich gas/stream 2: Hydrogen stream/first hydrogen-rich feed 3: Steam feed/stream 5: Second hydrogen stream 10: Preheater/protective material 11: Catalytic protective reactor 20: First reactor 30: Second reactor 40: Heater 50: Purified CO2-rich gas stream R: Recycle stream

[Figure 1] shows a simple layout of a specific example of the method of the present invention. [Figure 2] shows a more advanced layout of a specific example of the method of the present invention.

1: Carbon dioxide-rich airflow

2: The first hydrogen-rich feed

3: Steam feeding

10: Protective materials

11: Catalytic protection reactor

50: Purification of carbon dioxide-rich airflow

Claims (15)

A method for purifying a carbon dioxide-rich gas stream (1), the carbon dioxide-rich gas stream (1) comprising at least 80% by weight of carbon dioxide and one or more impurities selected from the following: -    sulfur compounds; -    higher hydrocarbons; and -    aromatic hydrocarbons, -    nitrogen substances, wherein the method comprises the following steps: -    passing the carbon dioxide-rich gas stream (1) together with a steam feed (3) through a protective material (10), the protective material (10) comprising a catalyst having an activity of converting higher hydrocarbons into methane in the presence of the steam feed (3), and adsorbing one or more of the impurities on the protective material (10) to provide a purified carbon dioxide-rich gas stream (50). The method of claim 1, wherein the carbon dioxide-rich gas stream (1) comprises at least 90 wt% carbon dioxide, such as at least 95.0 wt% carbon dioxide, preferably at least 99 wt% carbon dioxide, and more preferably at least 99.5 wt% carbon dioxide. A method as claimed in any preceding claim, wherein the carbon dioxide rich gas stream (1) passes through the protective material (10) together with a first hydrogen rich feed (2). A method as claimed in any of the preceding claims, wherein the catalyst is a pre-reforming catalyst, in particular a nickel-supported catalyst, such as nickel supported on an alumina support or a spinel support, for example nickel supported on an activated magnesium-aluminium spinel support. The method of any of the preceding claims, wherein the sulfur-containing compound is one or more compounds selected from COS, DMS and H ₂ S, preferably H ₂ S. The method of any of the preceding claims, wherein the higher hydrocarbon is selected from C2-C6 alkanes, C2-C6 alkenes, C2-C6 alkynes and combinations thereof. The method of any of the preceding claims, wherein the higher aromatic hydrocarbon is selected from C6-C8 aromatic hydrocarbons, C9-C12 aromatic hydrocarbons, and combinations thereof. A method as claimed in any of the preceding claims, wherein the carbon dioxide-rich gas stream (1) undergoes one or more desulfurization steps (20, 30), preferably two or more desulfurization steps, before passing through the protective material (10). A method as claimed in any of the preceding claims, wherein the carbon dioxide-enriched gas stream (1) undergoes a preheating step before passing through the protective material (10). A method as claimed in any of the preceding claims, wherein the purified carbon dioxide-rich gas stream (50) comprises less than 50 ppbV, preferably less than 10 ppbV and most preferably less than 5 ppbV sulfur. A method as claimed in any of the preceding claims, wherein the purified carbon dioxide-rich gas stream (50) contains less than 100 ppm, preferably less than 50 ppm, more preferably less than 10 ppm of higher hydrocarbons. A method as claimed in any of the preceding claims, wherein the purified carbon dioxide-rich gas stream (50) contains less than 10 ppm, preferably less than 5 ppm and most preferably less than 1 ppm aromatic hydrocarbons. A method as claimed in any of the preceding claims, wherein the purified carbon dioxide-rich gas stream (50) contains less than 10 ppm, preferably less than 1 ppm and most preferably less than 0.1 ppm alcohol. A method as claimed in any of the preceding claims, wherein the step of passing the carbon dioxide enriched gas stream (1) together with the steam feed (3) through a protective material (10) is carried out at a temperature between 250 and 550°C, preferably between 300 and 400°C. A method for producing a chemical or fuel stream, the method comprising purifying a carbon dioxide rich gas stream (1) in a method as claimed in any one of claims 1 to 14, subsequently feeding the purified carbon dioxide rich gas stream (50) to a synthesis stage, optionally in admixture with a hydrogen feed, and outputting the chemical or fuel stream from the synthesis stage.