A process and plant for converting carbon dioxide into carbon monoxide using in situ generated carbon

Abstract

The present invention relates to a process for converting carbon dioxide into carbon monoxide using in situ generated carbon comprising the step of reacting a starting composition containing carbon dioxide and dibromomethane in a reactor at a temperature of 900 to 2,000°C so as to produce a carbon monoxide and hydrogen bromide containing gaseous reaction mixture.

Description

[0001] A process and plant for converting carbon dioxide into carbon monoxide using in situ generated carbon

[0002] The present invention relates to a process for converting carbon dioxide into carbon monoxide using in situ generated carbon as well as to a respective plant for performing the process.

[0003] Carbon dioxide is the most important greenhouse gas in the atmosphere of the earth and thus drastically contributes to the natural warming of the earth surface by the so-called greenhouse effect, which means trapping and reflecting infrared radiation. Therefore, a reduction of carbon dioxide emissions and / or a reduction of the carbon dioxide content in the atmosphere of the earth is a crucial factor to mitigate the environmental impacts of climate change and protecting the long-term health of the earth. On account of these reasons, processes, which consume carbon dioxide, are more and more desired. An industrially interesting way of consuming carbon dioxide is its conversion to carbon monoxide, which is an important industrial raw material. For instance, carbon monoxide is used in the steel industry as reducing agent, in the ammonia synthesis as catalyst and in admixture with hydrogen as so-called synthesis gas as educt of hydrocarbons, such as methanol, acetic acid, aldehydes and Fischer-Tropsch hydrocarbons, the latter being a mixture consisting primarily of long-chain normal paraffins, which may be converted by cracking and isomerization to synthetic fuels.

[0004] A prominent process for consuming carbon dioxide and for generating synthesis gas is dry reforming of methane, which is the reaction of carbon dioxide with methane to a synthesis gas containing carbon monoxide and hydrogen. However, dry reforming of methane is a highly endothermic reaction occurring at high temperatures so that a high temperature is required during the reaction in order to obtain satisfyingly high conversion rates. Consequently, the operational costs of dry reforming of methane are comparable high. Another disadvantage of the dry reforming of methane is that it requires a catalyst and that the catalyst is deactivated after comparably short operation times due to metal sintering and due to coke deposition being the consequence of the carbon being produced during the dry reforming of methane as side-product.

[0005] Recently a two-step process for obtaining synthesis gas from a mixture containing carbon dioxide and methane has been proposed, which is called CARGEN or CARbon GENerator process, respectively. This kind of process, which is for instance disclosed in US 2020 / 0109050 A1 , comprises two subsequent steps. While in the first step a mixture containing carbon dioxide and methane is reacted to solid carbon and a gas stream containing carbon monoxide, hydrogen, water, unconverted methane and unconverted carbon dioxide, in a second step the gas stream containing carbon monoxide, hydrogen, water, unconverted methane and unconverted carbon dioxide is separated from the solid carbon and is directly processed in a reformer reactor so as to produce a high concentration mixture of carbon monoxide and hydrogen in a ratio that meets downstream applications. The second step may be performed as dry reforming, steam reforming and / or partial oxidation reforming. However, the aforementioned process has the disadvantages of requiring a catalyst for the formation of carbon from methane, wherein the catalyst is a consumable, since the catalyst cannot be recovered from the produced carbon. In addition, there is requirement to provide energy for both very endothermic reactions in the first and in the second reaction steps, wherein the only feasible option therefore is firing which, however, creates additional emissions.

[0006] In view of this, the object underlying the present invention is to provide a process and plant for converting carbon dioxide into carbon monoxide, which are characterized by low operational and capital expenditures, are characterized by a high conversion rate and are robust in the sense that a catalyst deterioration after comparable short operational time does not occur. In accordance with the present invention, this object is solved by providing a process for converting carbon dioxide into carbon monoxide using in situ generated carbon comprising the step of reacting a starting composition containing carbon dioxide and dibromomethane in a reactor at a temperature of 900 to 2,000°C so as to produce a carbon monoxide and hydrogen bromide containing gaseous reaction mixture.

[0007] It has been found in the present invention that by subjecting a starting composition containing carbon dioxide and dibromomethane in a reactor to a temperature of 600 to 2,000°C, dibromomethane is pyrolyzed to solid carbon and hydrogen bromide, wherein the generated solid carbon reacts upon generation, i.e. in situ, with the carbon dioxide to carbon monoxide, in particular at 900 to 2,000°C for full conversion of CO2. Thus, in total the carbon dioxide and dibromomethane react to a mixture of carbon monoxide and hydrogen bromide according to the following formula: CH2Br2+ CO2— 2 CO + 2 HBr, wherein the following partial reactions occur: CH2Br2— > C + 2 HBr and C + CO2— 2-CO. In other words, the process in accordance with the present invention exploits that both aforementioned partial reactions occur with a high conversion ratio at the same temperature within the temperature range of 900 to 2,000°C. In addition to the gaseous reaction mixture containing carbon monoxide and hydrogen bromide, also solid carbon may be produced during the reaction, if - for instance due to a molar excess of dibromomethane in comparison to carbon dioxide in the starting composition - in not all of the generated solid carbon reacts with carbon dioxide to carbon monoxide. By using a reaction temperature of at least 900°C, the presence of cold spots in the reactor, which would result in a significant heterogeneity of the quality of the gaseous reaction mixture, is reliably avoided. It is of further advantage that the process in accordance with the present invention is autothermal, which is due to the fact that the pyrolysis of dibromomethane is exothermic and thus compensates the energy demand for the endothermic reaction of solid carbon and carbon diox- ide to carbon monoxide. Another important advantage of the process in accordance with the present invention is that it may be performed without catalyst or with a catalyst being neither sensitive to metal sintering nor to coke deposition. All in all, the process in accordance with the present invention leads to a conversion of carbon dioxide into carbon monoxide with a high conversion rate, wherein the process is further characterized by low operational and capital expenditures and is robust in the sense that a catalyst deterioration after comparable short operational time does not occur. Another important advantage of the process in accordance with the present invention is that the obtained hydrogen bromide may be decomposed by electrolysis to bromine and hydrogen, wherein the so produced hydrogen may then be mixed with the produced carbon monoxide in a desired ratio. By adjusting the molar ratio of carbon dioxide and dibromomethane in the starting composition, the process may be easily optimized so that the requested ratio of carbon monoxide and hydrogen in the intended synthesis gas is achieved. The bromine being coproduced in the electrolysis may be used for generating the di- bromomethane required in the starting composition by reacting the bromine with methane. Then, in fact the total process comprises the reaction of methane and carbon dioxide to synthesis gas.

[0008] In accordance with a particular preferred embodiment of the present invention, the starting composition comprises less than 5% by mole of hydrogen donor. By using a starting composition containing less than 5% by mole of hydrogen donor, the generation of side products during the pyrolysis of the dibromomethane is minimized, such as the generation of transbromination products and the generation of hydrogenation products, such as of monobromomethane, of tribromomethane and of aromatic hydrocarbons, which then needs to be separated from the gaseous reaction mixture. Such side products may also lead to the generation of soot as pyrolysis product, which does not readily and completely react with the carbon dioxide to carbon monoxide, thus reducing the conversion rate of the total reaction. A hydrogen donor means in accordance with the present invention, with the exception of the educt dibromomethane and with the exception of hydrogen bromide as the product of the pyrolysis of dibromomethane, any compound, i.e. molecule, which contains an abstractable or transferable hydrogen. A molecule containing an abstractable or transferable hydrogen is meant in accordance with the present invention to be a molecule in which a hydrogen atom is bound, in particular covalently bound, to an atom having a the same or a higher electronegativity than hydrogen, i.e. a higher electronegativity than 2.20, which is the Pauling electronegativity of hydrogen. A hydrogen donor is in particular hydrogen or an inorganic or organic molecule, in which hydrogen is covalently bound to an atom, for example a carbon atom, having a Pauling electronegativity of 2.55, to an oxygen atom having a Pauling electronegativity of 3.44, to a halogen atom, such as to a chlorine atom having a Pauling electronegativity of 3.16 or to a fluorine atom having a Pauling electronegativity of 3.98. Examples for such a hydrogen donor are hydrogen (H2), aliphatic hydrocarbons, such as methane, ethane, propane, olefinic hydrocarbons, cycloalkanes and cycloalkenes, aromatic hydrocarbons, such as nitrogencontaining hydrocarbons, water, oxygenated hydrocarbons - for instance alcohols, ethers, diols, triols, aldehydes, ketones and carboxylic acids - hydrogen sulfide, sulfur-containing hydrocarbons, such as thiols and mercaptans, or monohalogeno hydrocarbons, in particular monobromo hydrocarbons, such as monobromomethane. Since the dibromomethane included in the starting and hydrogen bromide, which is one of the products of the reaction step, are in fact hydrogen donors, dibromomethane and hydrogen bromide are excluded from the definition of the term hydrogen donor according to the present invention. In other words, the step of exothermic pyrolysis is performed in accordance with the present invention with a starting composition containing less than 5% by mole of hydrogen donor being different to dibromomethane and being different to hydrogen bromide. Good results are in particular obtained, when the starting composition contains, based on 100% by mole of the starting composition, less than 3% by mole, preferably less than 1 % by mole, more preferably less than 0.5% by mole, still more preferably less than 0.25% by mole and most preferably less than 0.1 % by mole of hydrogen donor.

[0009] In accordance with another particular preferred embodiment of the present invention, the starting composition comprises less than 0.5% by mole of oxygen. Thereby, an oxidation of hydrogen bromide being generated during the reaction to bromine and water, which would result in a contamination of the gaseous reaction mixture with bromine and water, is reliably avoided. Good results are in particular obtained, when the starting composition being exothermically pyrolyzed contains, based on 100% by mole of the starting composition, less than 0.25% by mole, preferably less than 0.1 % by mole and most preferably less than 0.05% by mole of oxygen.

[0010] In accordance with a further preferred embodiment of the present invention, the starting composition is also substantially free of trihalogeno hydrocarbons (in particular tribromo hydrocarbons) and tetrahalogeno hydrocarbons (in particular tetrabromo hydrocarbons). The concentration of these compounds in the starting composition is preferably below 2% by weight. These compounds do not specifically impact the formation of carbon from dibromomethane during the pyrolysis, but they are thermally not very stable and already start to decompose at 400°C resulting in a fouling and in the formation carbon species with high bromine content. The product resulting from a starting composition containing 2% by weight or more of tribromo hydrocarbons and tetrabromo hydrocarbons requires additional hydrotreatment capacities and may leads to different kinds of operability issues.

[0011] The present invention is not particularly restricted concerning the total amounts and relative ratios of the carbon dioxide and of the dibromomethane in the starting composition. As noted above, the process in accordance with the present invention is flexible and allows to adjust the total amounts and relative ratios of the carbon dioxide and of the dibromomethane in the starting composition so that a desired ratio of carbon monoxide and of hydrogen bromide, which may be later decomposed by electrolysis to hydrogen and bromine so that the ratio of carbon monoxide and of hydrogen bromide corresponds to the ratio of carbon monoxide and hydrogen, is obtained. Good results are in particular obtained, when the starting composition contains, based on 100% by mole of the starting composition, 0.1 to 50% by mole of carbon dioxide and 99.9 to 50% by mole dibromomethane. More preferably, the starting composition contains 5 to 20% by mole of carbon dioxide and 95 to 80% by mole dibromomethane. Optionally, the starting composition may contain some hydrogen bromide, for instance, based on 100% by mole of the starting composition, in a concentration of 0.1 to 50% by mole. Hydrogen bromide may provide the heat to launch or light on, respectively, the decomposition of dichloromethane.

[0012] Even if the starting composition may consist of carbon dioxide and dibromomethane and the optional hydrogen bromide, i.e. contains, based on 100% by mole of the starting composition, in sum 100% by mole of carbon dioxide and dibromomethane and the optional hydrogen bromide, it is suggested in a further development of the idea of the present invention that the starting composition being reacted in the reactor contains, based on 100% by mole of the starting composition, 1 to 50% by mole, preferably 1 to 40% by mole, more preferably 1 to 30% by mole and most preferably 1 to 20% by mole of an inert gas. The addition of an inert gas helps to maintain the temperature within the reactor or reaction zone, respectively, at a predetermined numeric value and thus maintains the optimal hydrodynamic conditions for the pyrolysis of dibromomethane occurring during the reaction. Suitable examples for inert gases are inert gases being selected from the group consisting of hydrogen bromide, carbon monoxide, nitrogen, helium, argon and any arbitrary combination of two or more of the aforementioned inert gases. In accordance with the present invention, the starting composition is reacted in the reactor at a temperature of 900 to 2,000°C so as to produce the gaseous reaction mixture and optionally solid carbon. Good results are in particular obtained, when the starting composition is reacted in the reactor at a temperature of 950 to 2,000°C, preferably at a temperature of 1 ,000 to 2,000°C, more preferably at a temperature of 1 ,100 to 2,000°C, even more preferably at a temperature of 1 ,200 to 2,000°C. By performing the reaction comprising a pyrolysis of dibromomethane at such a temperature and preferably in an at least substantially hydrogen donor free and more preferably also in an at least substantially oxygen free atmosphere, it is achieved that the pyrolysis rate of dibromomethane during the reaction is at least 90%, preferably at least 95% and more preferably at least 99%, such as 98.9 or even 100%. In addition, the selectivity on carbon basis to carbon formation is at least 90%, preferably at least 95% and most preferably at least 99%, such as 92.5% or 99.6%.

[0013] In accordance with a further preferred embodiment of the present invention, the starting composition is heated to a temperature of at least 96.95°C, which is the boiling point of dibromomethane, and preferably to a temperature of 100 to less than 450°C, before the heated starting composition is fed into the reactor and reacted therein. Thereby, a gaseous starting composition is fed into the reactor, in which undesired side reactions do not take place, before the starting composition is heated in the pyrolysis reactor to at least 900°C and then decomposed by pyrolysis.

[0014] The present invention is not particularly limited concerning the pressure, at which the reaction is performed in the reactor. Preferably, the step of reacting the starting composition is performed at a pressure of 0.01 to 2 MPa and more preferably of 0.1 to 0.5 MPa. Good results are in particular obtained, when the pressure within pyrolysis reactor and the content of the dibromomethane in the starting composition are selected so that the partial pressure of dibromomethane during the reaction is 10 kPa to 2 MPa.

[0015] On account of the fact that the total reaction is autothermal, the reaction does not require an external heat supply. However, a temperature management is preferred so that the temperature within the reactor or reactor zone, respectively, where the dibromomethane is pyrolyzed or decomposed, respectively, is precisely controlled during the reaction. In view of this, it is preferred that the step of exothermically pyrolyzing the starting composition is performed in an isothermal reactor or in an adiabatic reactor, because both, an isothermal reactor as well as an adiabatic reactor allows to carefully control the temperature within the reactor or reactor zone, respectively, in which the pyrolysis reaction takes place.

[0016] For instance, the temperature control may be achieved in an adiabatic reactor by cooling the reactor with water, by quenching the cold mixture of carbon monoxide, carbon dioxide and hydrogen bromide of gases or by running an additional heat absorbing process in the same reaction zone, such as a benzene pyrolysis, or by utilizing a heat vector, i.e. by diluting the starting composition to be reacted with an inert gas so as to quickly evacuate the heat. For instance, hydrogen bromide as inert gas may be added to the starting composition by separating hydrogen bromide from the gaseous reaction mixture and by partially recycling the separated hydrogen bromide into the starting composition. In another embodiment, benzene is injected into the reactor in a concentration below 3% by mole, in order to handle the exothermicity of the dibromoethane decomposition, because benzene pyrolysis is a strongly endothermic reaction with a very little hydrogen donor potential. In another embodiment, the dibromomethane is evaporated before subjecting it to the reaction, wherein the evaporation of dibromoethane may be performed with a hot hydrogen bromide stream. In accordance with still another embodiment, the dibromomethane may be injected as a liquid into the pyrolysis reactor.

[0017] Independent from the type of reactor used, i.e. in the case of using an adiabatic reactor and in the case of using an isothermal reactor, it is preferred that the reactor contains a solid in order to maintain the temperature gradient homogeneous across the reactor. Suitable materials, from which the fouling resistant packing or conductive solid may be made, are compounds being selected from the group consisting of silicon carbide, graphite, carbon black, alumina, silica, aluminosilicates, clays, alumophosphates, polycrystalline silicon, zirconia, molybdenum disulfide, molybdenum disilicide, metallic carbides, transition metal nitrides, metallic phosphides and arbitrary combinations of two or more of the aforementioned compounds. Preferably, the solid is formed from particles having a median particle size of 50 pm or more and more preferably of 100 pm or more.

[0018] As set out above, the reaction may be performed without any catalyst. However, it is also possible to add a catalyst, such as an alkali metal salt solutions. Examples for preferred alkali metal salt solutions are those being selected from the group consisting of lithium salts, sodium salts, potassium salts and cesium salts, such as respective carbonates, acetates, formiates, oxalates or other salts with organic anions.

[0019] The present invention is not particularly restricted concerning the kind, with which the dibromomethane contained in the starting composition is prepared. For instance, the dibromomethane may be prepared by brominating methane with bromine, by transbrominating dichloromethane with hydrogen bromide, by brominating monobromomethane with bromine, by transbrominating tribromomethane or the like with hydrocarbons or mono bromo methane, by reduction of tribro- momethane with hydrogen. In accordance with a particular preferred embodiment of the present invention, the dibromomethane contained in the starting composition is prepared by brominating methane. Thus, it is preferred that the process further comprises a step of producing the starting composition, which comprises a sub-step of reacting a methane containing composition with bromine to a dibromomethane containing composition and a further sub-step of mixing the dibromomethane containing composition with a carbon dioxide containing composition. Preferably, the sub-step of reacting a methane containing composition with bromine is performed at a temperature of below 450°C, more preferably of 300 to less than 450°C and still more preferably of 370 to less than 420°C. The reason for reacting methane and bromine preferably at a temperature of below 450°C is that the reaction between methane and bromine at a temperature of 450°C or more would result in lower yield of dibromomethane. The reduced yield at or above 450°C is due to a partial disproportionation of dibromomethane resulting in an uncontrolled formation of soot, i.e. of carbon having a low quality, which may lead to a fouling of the reactor. Moreover, it is preferred that the reaction is performed at a pressure of 0.4 to 1 .5 MPa, because under this condition, the yield of dibromomethane is the highest.

[0020] In a further embodiment of the present invention, it is proposed that the methane containing composition is reacted with bromine in an isothermal reactor, such as a fluidized bed reactor or multitubular reactor.

[0021] Preferably, the methane containing composition contains at least 80% by mole, preferably at least 90% by mole, more preferably at least 95% by mole, still more preferably at least 99% by mole and most preferably 100% by mole methane. This methane containing composition is preferably mixed with bromine in a molar ratio of 2:1 to 1 :2, preferably of 1.5:1 to 1 :1.5, more preferably of 1.2:1 to 1 :1.2, even more preferably of 1 .2:1 to 1 :1 .1 and most preferably of 1 :1 , before the so obtained mixture is reacted preferably at a temperature of below 450°C. The reaction of methane and bromine leads to a mixture of monobromometane and dibromometane with hydrogen bromide and unreacted methane and with at most low contents of remaining bromine, such as typically about less than 0.1 % by mole of bromine. In accordance with a further preferred embodiment of the present invention, the dibromomethane containing composition is subjected to a separation step so as to separate the dibromomethane containing composition into a dibromomethane enriched composition and into a monobromomethane enriched composition containing also hydrogen bromide, unreacted methane and potentially traces of bromine. Good results are in particular obtained, when the bromomethane enriched composition is further subjected to an absorption step with water as absorbent to remove hydrogen bromide from the composition. Preferably, the resulting monobromomethane enriched composition and the unreacted methane are at least partially recycled into the sub-step of reacting the methane containing composition with bromine, where the recycled monobromomethane reacts with bromine to dibromomethane according to the following formula: CH3Br + Br2— CH2Br2+ HBr. The dibromomethane enriched composition can be further subjected to a separation step to remove tribromomethane and tetrabromomethane species, while producing dibromomethane compositions containing less than 5% by weight of hydrogen donors and less than 0.5% by weight of oxygen.

[0022] As set out above, it is preferred that the starting composition contains inert gas so as to maintain the optimal hydrodynamic conditions for the pyrolysis of dibromomethane, wherein suitable examples for inert gases are inert gases being selected from the group consisting of hydrogen bromide, carbon monoxide, nitrogen, helium, argon and any arbitrary combination of two or more of the aforementioned inert gases. In view of this, it is preferred that inert gas is added to the dibromomethane enriched composition in addition to carbon dioxide for preparing the starting composition. In addition to inert gas, other components may be added to the dibromomethane enriched composition. Alternatively, the dibromomethane enriched composition being produced with the aforementioned embodiment may be fed without addition of an inert gas or other compound(s) except carbon dioxide as starting composition into the reaction step.

[0023] In accordance with an alternative embodiment of the present invention, the dibromomethane is prepared by brominating dichloromethane with hydrogen bromide. Thus, it is preferred that the process further comprises a step of producing the starting composition to be reacted comprising a sub-step of reacting dichloromethane containing composition with hydrogen bromide at a temperature of 100 to 450°C and preferably of 250 to 420°C to a dibromomethane containing composition, and a further sub-step of mixing the dibromomethane containing composition with a carbon dioxide containing composition. Good results are in particular obtained, when the dichloromethane containing composition contains at least 80% by mole, preferably at least 90% by mole, more preferably at least 95% by mole, still more preferably at least 99% by mole and most preferably 100% by mole di- chloromethane.

[0024] Also in this embodiment, it is preferred that the obtained dibromomethane containing composition is subjected to a separation step so as to separate the dibromomethane containing composition into a dibromomethane enriched composition and into a dibromomethane depleted composition. Good results are in particular obtained, when the separation step is performed by distillation. Preferably, the unreacted dichloromethane enriched composition containing also chloromethane is at least partially recycled into the sub-step of reacting a dichloromethane containing composition with hydrogen bromide. In turn, the dibromomethane enriched composition is admixed at least with carbon dioxide and optionally also with one or more compounds, such as an inert gas, before the so obtained mixture is fed as starting composition into the reaction step.

[0025] The gaseous reaction mixture obtained during the step of reacting the starting composition is withdrawn from the reactor and then processed as desired. In accordance with a particularly preferred embodiment of the present invention, the gaseous reaction mixture obtained during the step of reacting the starting composition is withdrawn from the reactor, then hydrogen bromide is separated therefrom so as to obtain a carbon monoxide enriched composition and a hydrogen bromide composition, before the hydrogen bromide composition is subjected to an electrolysis to bromine and hydrogen. Preferably, the hydrogen bromide is separated from the gaseous reaction mixture by absorption, for instance, in water or in acetic acid. Thereafter, remaining unreacted carbon dioxide may be separated from the carbon monoxide, for example, by an amine wash and recycled back into the starting composition. By electrolyzing the hydrogen bromide being obtained during the pyrolysis to bromine and hydrogen, not only valuable hydrogen is produced as co-product allowing to produce synthesis gas, but also bromine, which may be and is preferably recycled into the step of producing dibromomethane by reacting methane, monobromomethane or another hydrocarbon with bromine, which is then used for preparing starting composition. Thus, this embodiment is in fact a process of reacting methane and carbon dioxide into a synthesis gas containing carbon monoxide and hydrogen in accordance with the following reaction equation:

[0026] CH4+ CO22 CO + 2 H2.

[0027] This reaction equation is the sum of the following reactions:

[0028] CH4+ 2 Br2CH2Br2+ 2 HBr

[0029] CH2Br2+ CO22 CO + 2 HBr

[0030] 4 HBr — » 2 H? + 2 Br?

[0031] CH4+ CO22 CO + 2 H2 Good results are in particular obtained, when the electrolysis is performed by using an electrolytic cell comprising an anode, a cathode and a membrane sandwiched between the anode and the cathode, wherein the hydrogen bromide containing composition is fed to the cathode, and the electrolytic cell is operated to produce hydrogen at the cathode, wherein the bromine containing composition is produced at the anode.

[0032] In a further development of the idea of the present invention, it is proposed that the electrolytic cell comprises a membrane made of a fluoropolymer membrane having a glass transition temperature of at least 110°C.

[0033] Good results are in particular obtained, when the electrolysis is performed by operating the electrolysis cell at an operational temperature of at least 70°C and preferably at an operational temperature of 70°C to 130°C.

[0034] In accordance with a further preferred embodiment of the present invention, the electrolysis is performed by operating the electrolytic cell at an operational pressure, which increases from the anode to the cathode.

[0035] The hydrogen being produced during the electrolysis may be mixed with the carbon monoxide enriched composition being obtained after separating hydrogen bromide from the reaction mixture so as to produce synthesis gas containing carbon monoxide and hydrogen. In this embodiment, the process in accordance of the present invention is in fact a process for producing synthesis gas using carbon dioxide and dibromomethane.

[0036] In a further development of the idea of the present invention, it is suggested that the synthesis gas is converted to one or more hydrocarbons and preferably to one or more hydrocarbons being selected from the group consisting of Fischer- Tropsch hydrocarbons, methanol, acetic acid, heavy alcohols and arbitrary combi- nations of two or more of the aforementioned hydrocarbons. In this embodiment, the process in accordance of the present invention is in fact a process for producing one or more hydrocarbons, such as synthetic jet fuel, using carbon dioxide and dibromomethane.

[0037] In accordance with an alternative embodiment of the present invention, the gaseous reaction mixture obtained during the step of reacting the starting composition is withdrawn from the reactor, then hydrogen bromide is separated therefrom so as to obtain a carbon monoxide enriched composition and a hydrogen bromide composition, before the hydrogen bromide composition is subjected to a thermal oxidation step. Preferably, in the thermal oxidation step at least a portion of the hydrogen bromide is mixed with oxygen or an oxygen containing gas, such as air, before the so obtained mixture is reacted or “burnt”, respectively at a temperature of at least 700°C to bromine and water according to the formula 2 Br2+ 2 H2O. This reaction is exothermic and accomplished with a conversion level of 50 to 95%. The gaseous mixture obtained during the thermal oxidation mainly comprises bromine and water, but in addition non-reacted hydrogen bromide, nitrogen, a small amount of excess-oxygen or excess-air, respectively, and carbon dioxide from the oxidation of traces of hydrocarbons contained in the gaseous reaction mixture. The produced bromine may be separated from the mixture and recycled into the optional sub-step of brominating methane.

[0038] In accordance with a further preferred embodiment of the present invention, the oxidation of hydrogen bromide in a thermal oxidizer is performed with at least 50% by weight of stoichiometric oxygen excess relative to the ratio, which is required by reaction according to the formula 4-HBr + O2 2 Br2+ 2-H2O. Advantageously, the effluent stream containing non-converted hydrogen bromide from the thermal oxidizer is further directed to a catalytic converter at temperature below 700°C. The catalytic converter is preferably loaded with a catalyst selected from cerium dioxide, titanium dioxide, zirconium dioxide, magnesium oxide, aluminum oxide or a mixture thereof. The catalyst may also contain 0.5 to 10% by weight of copper, cobalt, ruthenium, iron, manganese, palladium elements as promoters. The use of a combination of the thermal and the catalytic converters allows to achieve hydrogen bromide conversion of at least 98% by weight in a single pass. An effective heat management may be achieved, when the catalytic hydrogen bromide oxidation process is carried out in presence of steam. Preferably, the steam generated by the oxidation of hydrogen bromide is as energy, for example in a turbine to generate electricity.

[0039] In turn, optional remaining solid carbon, which is generated during the reaction by pyrolysis of dibromomethane, is withdrawn from the reactor as solid carbon. Optionally, the solid carbon may be withdrawn from the reactor in admixture with the gaseous reaction mixture containing carbon monoxide, hydrogen bromide gas and optionally inert gas and traces of non-converted dibromomethane. Preferably, the withdrawn mixture is cooled down to the temperature below 300°C in a one or a series of heat exchanger, before the solid carbon is separated from the gaseous reaction mixture preferably in one or a series of filters, i.e. bag filters or jet filters. Then, the gaseous reaction mixture may be optionally humidified, before it is optionally further processed as described above.

[0040] In order to remove residual bromide from the solid carbon, it is proposed in a further development of the idea of the present invention to subject the solid carbon to a stripping step with hydrogen at a temperature of at least 300°C and preferably at a temperature of at least 400°C so as to remove hydrogen bromide from the solid carbon. Good results are in particular obtained, when the hydrogen used in the stripping step has a temperature of 450 to 800°C and preferably of 500 to 650°C.

[0041] Moreover, the solid carbon may be subjected to at least one heating step in a hydrogen bromide free atmosphere at a temperature of 800 to 3,000°C so as to remove bromine from the carbon. This heating step may be performed in addition to and after the aforementioned stripping step, or this heating step may be performed as alternative to the aforementioned stripping step. Hydrogen bromide free atmosphere means in this connection an atmosphere, which contains less than 1 ,000 ppm and preferably less than 100 ppm hydrogen bromide. The separated hydrogen bromide may be recycled into the step of brominating methane so as to form the starting composition.

[0042] In accordance with a further aspect, the present invention relates to a plant for converting carbon dioxide into carbon monoxide comprising: a) a first reactor for converting carbon dioxide into carbon monoxide, the converting of carbon dioxide into carbon monoxide in particular comprising the step of reacting a starting composition containing carbon dioxide and dibromomethane in the reactor at a temperature of 900 to 2,000°C, wherein the reactor comprises one or more inlet lines and an outlet line for a gaseous reaction mixture, wherein one inlet line is for feeding the starting composition containing carbon dioxide and dibromomethane into the reactor, b) a first separation device for separating hydrogen bromide from the gaseous reaction mixture comprising an inlet line being directly or indirectly connected with the outlet line for the gaseous reaction mixture of the first reactor, an outlet line for hydrogen bromide and an outlet line for hydrogen bromide depleted gaseous reaction mixture, and c) a second separation device for separating carbon dioxide from the depleted gaseous reaction mixture comprising an inlet line being connected with the outlet line for the hydrogen bromide depleted gaseous reaction mixture of the first separation device, an outlet line for carbon dioxide and an outlet line for carbon monoxide enriched gas, wherein the outlet line for carbon dioxide of the second separation device is connected with an inlet line of the first reactor. Preferably, the first separation device for separating hydrogen bromide from the gaseous reaction mixture is an absorption column, such as one being operated with water or with acetic acid as absorption agent.

[0043] In a further development of the idea of the present invention, it is suggested that the second separation device for separating carbon dioxide from the hydrogen bromide depleted gaseous reaction mixture is a washing column, such as one being operated with amine as washing agent.

[0044] In a further preferred embodiment of the present invention, the plant comprises a third separation device for separating carbon from the gaseous reaction mixture. Good results are in particular obtained when the third separation device is a filter, such as a bag filter or jet filter. Preferably, the third separation device is arranged between the first reactor and the first separation device, i.e. the third separation device comprises an inlet line being connected with the outlet line for the gaseous reaction mixture of the first reactor, an outlet line for carbon and an outlet line for carbon depleted gaseous reaction mixture, wherein the outlet line for carbon depleted gaseous reaction mixture of the third separation device is connected with the inlet line of the first separation device. The denotations as first, second and third separation device is here not intended to specify the relative arrangement of the single separation devices, but are chosen in accordance with the row of mentioning in the description.

[0045] In accordance with a further particularly preferred embodiment of the present invention, the plant further comprises: d) an electrolysis cell comprising an anode, a cathode, a membrane sandwiched between the anode and the cathode, an inlet line being connected with the outlet line for hydrogen bromide of the first separation device, an outlet line for hydrogen and an outlet line for bromine, and e) a second reactor for brominating methane to dibromomethane comprising an inlet line for methane, an inlet line for bromine being connected with the outlet line for bromine of the electrolysis cell and an outlet line for a dibromomethane enriched composition being connected with an inlet line of the first reactor.

[0046] Specific embodiments in accordance with the present invention are subsequently described with reference to the appended drawings and by examples.

[0047] Fig. 1 is a schematic view of a plant for converting carbon dioxide into carbon monoxide in accordance with one embodiment of the present invention.

[0048] The plant 10 for converting carbon dioxide into carbon monoxide shown in figure 1 comprises a first reactor 12 for converting carbon dioxide into carbon monoxide, a separation device 14 (being denoted above as third separation device) for separating carbon from the gaseous reaction mixture being obtained in the first reactor 12, a separation device 16 (being denoted above as first separation device) for separating hydrogen bromide from the gaseous reaction mixture, a separation device 18 for separating carbon dioxide from the hydrogen bromide depleted gaseous reaction mixture (being denoted above as second separation device), an electrolysis cell 20 and a second reactor 22 for brominating methane to dibromomethane. More specifically, the first reactor 12 comprises an inlet line 24 for carbon dioxide being connected with a feed line 26 for fresh carbon dioxide and a line 42 carbon dioxide being recycled via line 42, and an outlet line 28 for a gaseous reaction mixture, wherein the outlet line 28 is connected via a heat exchanger 30 with an inlet line 32 of the separation device 14. The separation device 14 comprises an outlet line 33 for solid carbon and an outlet line 34 leading into the separation device 16, which in turn comprises an outlet line 36 for hydrogen bromide and an outlet line 38 for hydrogen bromide depleted gaseous reaction mixture. While the outlet line 36 of the separation device 16 is connected with an inlet for hydrogen bromide of the electrolysis cell 20, the outlet line 38 for hydrogen bromide depleted gaseous reaction mixture of the separation device 16 is connected with an inlet of the separation device 18, which in turn comprises an outlet line 40 for carbon monoxide enriched gas as well as an outlet line 42 leading via the heat exchanger 30 into the inlet line 24 for carbon dioxide of the first reactor 12. Furthermore, the electrolysis cell 20 comprises an outlet line 44 for a mixture of water and hydrogen as well as an outlet line 46 for bromine leading into an inlet of the second reactor 22 for brominating methane to dibromomethane. The second reactor 22 further comprises an inlet line 48 for methane and an outlet line 50 being connected with an inlet of the first reactor 12. For instance the separation device 14 comprises one or more jet filters, the separation device 16 is an absorption column being operated with water as absorption agent and the separation device 18 is an amine washer.

[0049] During the operation of the plant 10, fresh carbon dioxide is fed via the feed line 26 together with carbon dioxide being recycled via line 42 through the inlet line 24 and dibromomethane is fed through line 50 into the first reactor 12, in which the carbon dioxide reacts with the dibromomethane via in situ generated carbon to carbon monoxide, hydrogen bromide and a small amount of solid carbon. The so obtained reaction mixture is led through lines 28, 32 via the heat exchanger 30 into the separation device 14, in which solid carbon is separated from the gaseous reaction mixture. While the solid carbon is removed via line 33, the carbon depleted gaseous reaction mixture is led via line 34 into the separation device 16, in which hydrogen bromide is separated from the carbon depleted gaseous reaction mixture. While the hydrogen bromide is led via line 36 into the electrolysis cell 20, the hydrogen bromide depleted gas mixture is led via line 38 into the separation device 18. In the separation device 18, carbon dioxide is separated from the gas mixtur so that a carbon monoxide enriched gas is obtained and withdrawn via line 40, whereas the separated carbon dioxide is recycled via lines 42, 24 back into the first reactor 12. During the electrolysis in the electrolysis cell 20, hydrogen bromide is converted into hydrogen and bromine. While the hydrogen is withdrawn together with water via line 44, the bromine is fed via line 46 into the second reactor 22, in which the bromine reacts with methane being supplied via line 48 to dibromomethane, which is led via line 50 into the first reactor 12.

[0050] Reference Numeral List

[0051] Plant

[0052] First reactor

[0053] Separation device for separating carbon from the gaseous reaction mixture

[0054] Separation device for separating hydrogen bromide from the gaseous reaction mixture

[0055] Separation device for separating carbon dioxide from the hydrogen bromide depleted gaseous reaction mixture

[0056] Electrolysis cell

[0057] Second reactor

[0058] Inlet line for carbon dioxide

[0059] Feed line for fresh carbon dioxide

[0060] Outlet line of the first reactor

[0061] Heat exchanger

[0062] Inlet line of the separation device for separating carbon

[0063] Outlet line for carbon

[0064] Outlet line of the separation device for separating carbon

[0065] Outlet line for hydrogen bromide

[0066] Outlet line for hydrogen bromide depleted gas

[0067] Outlet line for carbon monoxide enriched gas

[0068] Outlet line for carbon dioxide

[0069] Outlet line for a mixture of water and hydrogen

[0070] Outlet line for bromine

[0071] Inlet line for methane

[0072] Outlet line for dibromomethane

Claims

Claims:1 . A process for converting carbon dioxide into carbon monoxide using in situ generated carbon comprising the step of reacting a starting composition containing carbon dioxide and dibromomethane in a reactor at a temperature of 900 to 2,000°C so as to produce a carbon monoxide and hydrogen bromide containing gaseous reaction mixture.

2. The process in accordance with claim 1 , wherein the starting composition comprises less than 5% by mole of a hydrogen donor and preferably less than 0.5% by mole of oxygen, wherein the hydrogen donor is a compound comprising a molecule with an abstractable or transferable hydrogen, the hydrogen being bound to another atom in the molecule, and the other atom having the same or a higher electronegativity than hydrogen.

3. The process in accordance with claim 2, wherein the starting composition contains less than 3% by mole, preferably less than 1 % by mole, more preferably less than 0.5% by mole, still more preferably less than 0.25% by mole and most preferably less than 0.1% by mole of hydrogen donor.

4. The process in accordance with claim 2 or 3, wherein the starting composition contains less than 0.25% by mole, preferably less than 0.1 % by mole and most preferably less than 0.05% by mole of oxygen.

5. The process in accordance with any of the preceding claims, wherein the starting composition contains 0.1 to 50% by mole of carbon dioxide and 99.9 to 50% by mole dibromomethane and preferably 5 to 20% by mole of carbon dioxide and 95 to 80% by mole dibromomethane.

6. The process in accordance with any of the preceding claims, wherein the starting composition contains 1 to 50% by mole, preferably 1 to 40% by mole, more preferably 1 to 30% by mole and most preferably 1 to 20% by mole of an inert gas, wherein the inert gas is preferably selected from the group consisting of hydrogen bromide, carbon monoxide, nitrogen, helium, argon and any arbitrary combination of two or more of the aforementioned inert gases.

7. The process in accordance with any of the preceding claims, wherein the reaction of the starting composition is performed in the reactor at a temperature of 950 to 2,000°C and at a pressure of 0.01 to 2 MPa.

8. The process in accordance with any of the preceding claims, wherein the process further comprises a step of producing the starting composition comprising a sub-step of reacting a methane containing composition with bromine at a temperature of below 450°C, preferably of 300 to less than 450°C and more preferably of 370 to less than 420°C to a dibromomethane containing composition, and a further sub-step of mixing the dibromomethane containing composition with a carbon dioxide containing composition.

9. The process in accordance with any of claims 1 to 7, wherein the process further comprises a step of producing the starting composition comprising a sub-step of reacting dichloromethane containing composition with hydrogen bromide at a temperature of 100 to 450°C and preferably of 250 to 420°C to a dibromomethane containing composition, and a further sub-step of mixing the dibromomethane containing composition with a carbon dioxide containing composition.

10. The process in accordance with any of the preceding claims, wherein the gaseous reaction mixture obtained during the step of reacting the starting composition is withdrawn from the reactor, then hydrogen bromide is separated therefrom so as to obtain a carbon monoxide enriched composition and a hydrogen bromide composition, before the hydrogen bromide composition is subjected to an electrolysis to bromine and hydrogen.11 . The process in accordance with claim 10, wherein the hydrogen produced during the electrolysis is mixed with the carbon monoxide enriched composition so as to produce synthesis gas containing carbon monoxide and hydrogen.

12. The process in accordance with claim 11 , wherein the synthesis gas is converted to one or more hydrocarbons and preferably to one or more hydrocarbons being selected from the group consisting of Fischer-Tropsch hydrocarbons, methanol, acetic acid, heavy alcohols and arbitrary combinations of two or more of the aforementioned hydrocarbons.

13. The process in accordance with any of claims 1 to 9, wherein the gaseous reaction mixture obtained during the step of reacting the starting composition is withdrawn from the reactor, then hydrogen bromide is separated therefrom so as to obtain a carbon monoxide enriched composition and a hydrogen bromide composition, before the hydrogen bromide composition is subjected to a thermal oxidation step.

14. A plant for converting carbon dioxide into carbon monoxide comprising: a) a first reactor for converting carbon dioxide into carbon monoxide comprising the step of reacting a starting composition containing carbon dioxide and dibromomethane in the reactor at a temperature of 900 to 2,000°C, wherein the reactor comprises one or more inlet linesand an outlet line for a gaseous reaction mixture, wherein one inlet line is for feeding the starting composition containing carbon dioxide and dibromomethane into the reactor, b) a first separation device for separating hydrogen bromide from the gaseous reaction mixture comprising an inlet line being directly or indirectly connected with the outlet line for the gaseous reaction mixture of the first reactor, an outlet line for hydrogen bromide and an outlet line for hydrogen bromide depleted gaseous reaction mixture, and c) a second separation device for separating carbon dioxide from the hydrogen bromide depleted gaseous reaction mixture comprising an inlet line being connected with the outlet line for the hydrogen bromide depleted gaseous reaction mixture of the first separation device, an outlet line for carbon dioxide and an outlet line for carbon monoxide enriched gas, wherein the outlet line for carbon dioxide of the second separation device is connected with an inlet of the first reactor.

15. The plant in accordance with claim 14, which further comprises: d) an electrolysis cell comprising an anode, a cathode, a membrane sandwiched between the anode and the cathode, an inlet line being connected with the outlet line for hydrogen bromide of the first separation device, an outlet line for hydrogen and an outlet line for bromine, and e) a second reactor for brominating methane to dibromomethane comprising an inlet line for methane, an inlet line for bromine being connected with the outlet line for bromine of the electrolysis cell and an outlet line for a dibromomethane enriched composition being connected with an inlet line of the first reactor.

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