Process and plant for producing very pure carbon from a dibromomethane feedstock

Abstract

The present invention relates to a process for producing carbon comprising the step of exothermically pyrolyzing a starting composition containing at least 50% by mole of dibromomethane, less than 5% by mole of hydrogen donor and less than 0.5% by mole of oxygen in a pyrolysis reactor at a temperature of 600 to 2,000°C to carbon and hydrogen bromide.

Description

[0001] Process and plant for producing very pure carbon from a dibromomethane feedstock

[0002] The present invention relates to a process for producing very pure carbon from a dibromomethane feedstock as well as to a respective plant for performing the process.

[0003] Carbon is not only essential to all known living systems, but also an important raw material for industrial processes. For instance, carbon is used as ingredient of alloys, for instance in carbon steels, as lubricant, as pigment, in carbon fibers, in fuel cells, in electrodes of batteries, in electrodes of electrolyzers, in capacitors, in wires, in cables, in belts, in hoses, in floors, in shoes, in rollers, in heaters, in paints and others. Carbon exists in different modifications, for example amorphous carbon black, graphite, diamond and graphene, the latter of which being a single layer of graphite. At least some of the aforementioned applications require the use of carbon having a high degree of purity and in particular having a very low hydrogen content.

[0004] Carbon in the form of graphite, carbon black or graphene is often produced by pyrolysis of a carbon precursor, such as of methane, acetylene, heavy oil residue or another hydrocarbon-containing feedstock. However, none of the known processes leads with reasonable operational and capital expenditures to sufficiently pure carbon being characterized, among others, by a particularly low hydrogen content, low sulfur content, low metal content and low oxygen content.

[0005] In view of this, the object underlying the present invention is to provide a process and a plant for producing pure carbon, which are characterized by low operational and capital expenditures and lead to very pure carbon, which is in particular char- acterized by a very low amount of hydrogen and other impurities, such as in particular of sulfur, metals and oxygen.

[0006] In accordance with the present invention, this object is solved by providing a process for producing carbon comprising the step of exothermically pyrolyzing a starting composition containing at least 50% by mole of dibromomethane, less than 5% by mole of hydrogen donor and less than 0.5% by mole of oxygen in a pyrolysis reactor at a temperature of 600 to 2,000°C to carbon and hydrogen bromide.

[0007] It has been found in the present invention that the pyrolysis of dibromomethane is a suitable technique for producing very pure carbon having a low hydrogen content with low operational and capital expenditures, among others since the pyrolysis of dibromomethane produces in addition to carbon also hydrogen bromide, which may be used in a subsequent step to produce valuable hydrogen by electrolysis. In particular, it has been found that very pure carbon with a particular low amount of hydrogen and other impurities can be obtained with the process in accordance with the present invention, because a dibromomethane containing starting composition is pyrolyzed, which contains at least 50% by mole of dibromomethane, but less than 5% by mole of hydrogen donor and less than 0.5% by mole of oxygen. The content of at least 50% by mole of dibromomethane in the dibromomethane containing starting composition allows to maintain a self- sustaining pyrolysis process using the heat released from the pyrolysis of dibromo methane, without need for any external heating source, as it would be required in the case of using a starting composition containing less than 50% by mole of dibromomethane. By using a starting composition containing less than 5% by mole of hydrogen donor and less than 0.5% by mole of oxygen, the generation of side products is minimized and thereby not only the quality of the produced carbon is increased, but also the yield of the carbon is increased and an increase of energy consumption due to endothermic side reactions with oxygen and hydrogen is avoided. Thus, undesired side reactions of the dibromomethane contained in the starting composition to be pyrolyzed leading to undesired impurities in the carbon are reliably avoided or at least significantly reduced in the process in accordance with the present invention. Furthermore, undesired side reactions of the hydrogen bromide, which is in addition to carbon one of the products of the pyrolysis reaction, are avoided or at least significantly reduced due to the limitation of the oxygen content in the starting composition to less than 0.5% by mole. More specifically, by reducing the content of oxygen or by eliminating oxygen in the starting composition to be pyrolyzed, an oxidation of hydrogen bromide under formation of bromine according to the formula HBr + O2— > H2O + Br2, which would contaminate the carbon product, as well as a partial carbon gasification according to the formula 2 C + O2—> 2 CO and C + H2O CO + H2, are reliably avoided.

[0008] A hydrogen donor means in accordance with the present invention, with the exception of the educt and one product of the pyrolysis reaction, i.e. with the exception of dibromomethane and of hydrogen bromide, 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 hydrocrbons, such as thiols and mercaptans, or monohalogeno hydrocarbons, in particular monobromo hydrocarbons, such as monobromomethane. Since the dibromomethane included in the starting composition to be pyrolyzed and hydrogen bromide, which is one of the products of the exothermic pyrolysis 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.

[0009] In accordance with a preferred embodiment of the present invention, the dibromomethane containing 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.

[0010] Pyrolysis reactor means in accordance with the present invention any vessel or reactor, respectively, which is suitable to resist to the pyrolysis temperature of up to 2,000°C. In accordance with the present invention, the starting composition containing at least 50% by mole of dibromomethane, less than 5% by mole of hydrogen donor and less than 0.5% by mole of oxygen is subjected in a pyrolysis reactor to an exothermic pyrolysis, i.e. to a reaction producing heat or energy, respectively, but not consuming heat or energy, respectively. More specifically, the exothermic pyrolysis follows the following formula: CH2Br2C + 2 HBr, AH298K= -79 kJ / mol. Consequently, the exothermic pyrolysis is self-sustainable and does not require any external heat supply. Therefore, it is preferred that during the step of exothermically pyrolyzing the starting composition no electrical energy and preferably no energy at all is supplied to the pyrolysis reactor.

[0011] In accordance with the present invention, the starting composition contains, based on 100% by mole of the starting composition, at least 50% by mole of dibromomethane. Good results are in particular obtained, when the starting composition being exothermically pyrolyzed contains, based on 100% by mole of the starting composition, at least 60% by mole, more preferably at least 70% by mole and most preferably at least 80% by mole of dibromomethane. Furthermore, the starting composition may contain, based on 100% by mole of the starting composition, at least 90% by mole or at least 95% by mole or even at least 100% by mole of dibromomethane.

[0012] As further set out above, the starting composition contains, based on 100% by mole of the starting composition, less than 5% by mole of hydrogen donor. 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 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. As further set out above, the starting composition contains, based on 100% by mole of the starting composition, less than 0.5% by mole of oxygen. 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.

[0013] Even if the starting composition may consist of dibromomethane, i.e. contains, based on 100% by mole of the starting composition, 100% by mole of dibromomethane, it is suggested in a further development of the idea of the present invention that the starting composition being exothermically pyrolyzed in the pyrolysis 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. As set out below, the addition of an inert gas helps to maintain the temperature within the pyrolysis reactor or reaction zone, respectively, at a predetermined numeric value and thus maintains the optimal hydrodynamic conditions for the pyrolysis of dibromomethane. 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.

[0014] Even if the pyrolysis of dibromomethane to carbon and hydrogen bromide is exothermic, a certain temperature is required in order to achieve a fast and complete pyrolysis of the dibromomethane within a reasonable short period of time. In view of this, the exothermic pyrolysis of the starting composition containing at least 50% by mole of dibromomethane is performed in accordance with the present invention at a temperature of 600 to 2,000°C and preferably at a temperature of 650 to 1 ,500°C. By performing the pyrolysis at such a temperature and in an at least substantially hydrogen donor free and oxygen free atmosphere, it is achieved that the conversion rate of dibromomethane during the pyrolysis 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%.

[0015] 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 pyrolyzed. Thereby, a gaseous starting composition is fed into the pyrolysis reactor, but a composition in which undesired side reactions do not take place, before the starting composition is heated in the pyrolysis reactor to at least 600°C and then decomposed by pyrolysis.

[0016] The present invention is not particularly limited concerning the pressure, at which the pyrolysis is performed in the pyrolysis reactor. Preferably, the step of exothermically pyrolyzing the starting composition is performed at a pressure of 0.01 to 2 MPa and more preferably of 0.1 to 0.5 MPa.

[0017] 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 exothermic pyrolysis is 20 kPa to 0.5 MPa.

[0018] On account of the fact that the pyrolysis of dibromomethane is exothermic, heat or energy, respectively, is generated during the pyrolysis reaction. On account thereof, a temperature management is preferred so that the temperature within the pyrolysis reactor is precisely controlled during the pyrolysis. Such a temperature control is advantageous, in order to avoid that the reaction heat increases the temperature within the pyrolysis reactor above a desired value and in order to produce carbon product having a very homogeneous quality. 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.

[0019] For instance, the temperature control may be achieved in an adiabatic reactor by cooling the reactor with water, 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 pyrolyzed 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 to be pyrolyzed by separating hydrogen bromide from the pyrolyzed composition and by partially recycling the separated hydrogen bromide into the starting composition to be pyrolyzed. The aforementioned alternative, namely running of an additional heat absorbing process in the same reaction zone, such as a benzene pyrolysis, leads to an absorption of energy in the form of heat by the injected benzene, thus consuming the heat generated by the dibromomethane pyrolysis and thus keeping the pyrolysis of the starting composition autothermal, since benzene pyrolysis is strongly endothermic.

[0020] In accordance with an alternative embodiment, the step of exothermically pyrolyzing the starting composition is performed in an isothermal reactor. Good results are in particular obtained, when a fluidized bed is used as isothermal reactor.

[0021] Independent from the type of pyrolysis 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 pyrolysis 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.

[0022] 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 tribromomethane with hydrogen.

[0023] 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 to be pyrolyzed and containing at least 50% by mole of dibromomethane, less than 5% by mole of hydrogen donor and less than 0.5% by mole of oxygen, wherein the step comprises a sub-step of reacting a methane containing composition with bromine at a temperature of below 450°C. The reason for reacting methane and bromine 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 pyrolysis reactor. More specifically, at or above 450°C dibromomethane reacts with methane in accordance with the following formulae:

[0024] 2CH2Br2— > CH3Br + CHBr3— > soot (which is (CHx)yBrz-polyaromatics) + HBr. CH2Br2+ CH42 CH3Br soot (which is (CHx)yBrz-polyaromatics) + HBr.

[0025] Good results are in particular obtained, when the starting composition to be pyrolyzed is produced by reacting a methane containing composition with bromine at a temperature of 300 to less than 450°C and more preferably at a temperature of 370 to less than 420°C to a dibromomethane containing composition. Moreover, it is preferred that this reaction is performed at a pressure of 0.4 to 1 .5 MPa, because under this condition, the yield of dibromomethane is the highest.

[0026] 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.

[0027] 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 at a temperature of below 450°C.

[0028] 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 separa- tion 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

[0029] 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.

[0030] 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 being produced with the aforementioned embodiment, before the so obtained mixture is fed as starting composition into the step of exothermically pyrolyzing 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) as starting composition into the step of exothermically pyrolyzing the starting composition or one or more compounds. 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 pyrolyzed and containing at least 50% by mole of dibromomethane, less than 5% by mole of hydrogen donor and less than 0.5% by mole of oxygen, wherein the step comprises 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. 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 dichloromethane.

[0031] 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 fed as starting composition into the step of exothermically pyrolyzing the starting composition or one or more compounds, such as an inert gas, is / are added to the dibromomethane enriched composition, before the so obtained mixture is fed as starting composition into the step of exothermically pyrolyzing the starting composition.

[0032] The exothermic pyrolysis of dibromomethane leads to (solid) carbon and (gaseous) hydrogen bromide. The hydrogen bromide is withdrawn as gas from the pyrolysis reactor and is preferably subjected to an electrolysis to bromine and hydrogen. By electrolysing the hydrogen bromide being obtained during the pyrolysis to bromine and hydrogen, not only valuable hydrogen is produced as coproduct, 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 fed as starting composition into the pyrolysis reactor and is pyrolyzed therein to carbon and hydrogen bromide. Thus, this embodiment is in fact a process of splitting methane into carbon and hydrogen in accordance with the following formula: CH4— > C + 2 H2. This is the sum of the following reactions:

[0033] CH4+ 2 Br2CH2Br2+ 2 HBr

[0034] CH2Br2C + 2 HBr

[0035] 4 HBr 2 H? + 2 Br?

[0036] CH4C + 2 H2

[0037] 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.

[0038] 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.

[0039] 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. 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.

[0040] In accordance with an alternative embodiment of the present invention, the hydrogen bromide obtained during the pyrolysis is withdrawn from the pyrolysis reactor as hydrogen bromide containing composition and 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 4 HBr + O2- 2 Br2+ 2 H2O. This reaction is exothermic and accomplished with a conversion level of 50 to 95%. The reaction 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 reaction mixture. The produced bromine may be separated from the mixture and recycled into the step of brominating methane.

[0041] 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 hydro- gen 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.

[0042] In turn, the carbon, which is generated during the exothermic pyrolysis of dibromomethane, is withdrawn from the pyrolysis reactor as solid carbon in admixture with hydrogen bromide gas and optionally with some inert gases and traces of non-converted dibromomethane. Preferably, the carbon containing composition is cooled down to the temperature below 300°C in a one or a series of heat exchanger before separation. Then, the gas may be optionally humidified. Afterwards, the solid is preferably separated from the hydrogen bromide containing gas in one or a series of filters, i.e. bag filters or jet filters. Then, the carbon handling may be performed with pneumatics conveys driven by an inert gas, preferable nitrogen. Preferably, the so obtained carbon is subjected to one or more purification steps.

[0043] In order to remove residual bromide from the carbon, it is proposed in a further development of the idea of the present invention to subject the 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 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.

[0044] The released hydrogen bromide may be combined with the stream being withdrawn from the pyrolysis reactor as hydrogen bromide containing composition and subjected to an absorption step so as to separate hydrogen bromide from the inert gas and the residual dibromomethane for recycling. Moreover, the 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.

[0045] In accordance with a further aspect, the present invention is related to a plant for producing carbon comprising: a) a reactor comprising one or more inlets and an outlet for a reaction mixture effluent, b) a first distillation column comprising an inlet being connected with the outlet for the reaction mixture effluent of the reactor, wherein the first distillation column comprises an overhead outlet and a bottom outlet, c) a second distillation column comprising an inlet being connected with the bottom outlet of the first distillation column, an overhead outlet and a bottom outlet, d) a pyrolysis reactor comprising an inlet being connected with the overhead outlet of the second distillation column and an outlet, e) a solid-gas-separation unit comprising an inlet being connected with the outlet of the pyrolysis reactor, a gas outlet and a solid outlet and f) an electrolysis cell comprising an anode, a cathode and a membrane sandwiched between the anode and the cathode as well as an inlet for a hydrogen bromide containing composition, an outlet for hydrogen and an outlet for a bromine containing composition, wherein the inlet of the electrolysis cell is connected with the gas outlet of the solid-gas-separation unit e) either directly fi) or f2) with an absorption column arranged therebetween compris- ing a first outlet for a hydrogen bromide enriched aqueous liquid and a second gas outlet, with the first outlet for a hydrogen bromide enriched aqueous liquid being connected with the inlet of the electrolysis cell, and wherein the outlet for the bromine containing composition of the electrolysis cell is directly or indirectly connected with an inlet of the reactor a).

[0046] Preferably, the plant further comprises: g) a third distillation column comprising an inlet being connected with the overhead outlet of the first distillation column, an overhead outlet and a bottom outlet and h) an absorption column being connected with the overhead outlet of the third distillation column, a liquid outlet and a gas outlet, wherein the gas outlet is directly or indirectly connected with the reactor a).

[0047] Specific embodiments in accordance with the present invention are subsequently described with reference to the appended drawings and by examples.

[0048] Fig. 1 is a schematic view of a plant for producing carbon in accordance with one embodiment of the present invention.

[0049] Fig. 2 are microscopic (SEM) images of the carbon produced in example 1 .

[0050] Fig. 3 are microscopic (SEM) images of the carbon produced in example 2.

[0051] The plant 10 for producing carbon shown in figure 1 comprises a bromination reactor 12, a filter 14, a first distillation column 16, a second distillation column 18, a first absorption column 20, a third distillation column 22, a pyrolysis reactor 24, a stripping column 26, a second absorption column 28 and an electrolysis cell 30. More specifically, the bromination reactor 12 comprises an inlet line 32 for starting composition and an outlet line 34 for a reaction mixture effluent, which leads into the filter 14 for separating solid soot from the gaseous reaction mixture effluent, wherein the filter 14 comprises a gas outlet line 36 and a solid outlet line 38. While the solid outlet line 38 leads back into to the inlet line 32 for starting composition, the gas outlet line 36 of the filter 14 is connected with the inlet line 40 of the first distillation column 16. The first distillation column 16 further comprises an overhead outlet line 42 and a bottom outlet line 44. While the bottom outlet line 44 leads into the third distillation column 22, the overhead outlet line 42 of the first distillation column 16 leads into the second distillation column 18. In turn, the second distillation column 18 comprises an overhead outlet line 46 and a bottom outlet line 48, wherein the overhead outlet line 46 leads into the first absorption column 20, whereas the bottom outlet line 48 of the second distillation column 18 leads back into the inlet line 32 for starting composition of the bromination reactor 12. The first absorption column 20 comprises a gas outlet line 50 leading via a dryer 52 into a nitrogen rejection unit 54 being further connected via line 56 with a pretreatment unit 58 comprising an inlet line 60 for natural gas, wherein the nitrogen rejection unit 54 further comprises an outlet line 62 leading into the inlet line 32 for starting composition of the bromination reactor 12. In addition, the first absorption column 20 comprises an inlet line 64 for water and a liquid outlet line 66. In turn, the third distillation column 22 comprises an overhead outlet line 68 and a bottom outlet line 70, wherein the overhead outlet line 68 of the third distillation column 22 leads into the pyrolysis reactor 24, which comprises an outlet line 72 leading into a filter 74 for separating the pyrolysis composition into solid carbon and a gaseous, hydrogen bromide enriched composition. While the solid carbon is led via line 76 into the stripping column, the gaseous, hydrogen bromide enriched composition is led via line 78 into the second absorption column 28, which further comprises in inlet line 80 for water a gas outlet line 82 and an outlet line 84 for hydrogen bromide enriched aqueous liquid.

[0052] Furthermore, the plant 10 comprises a thermal oxidizer 86, a third absorption column 88, a fourth distillation column 90, a condenser 92, a vessel 94 and a dryer column 96. The thermal oxidizer 86 is connected with the solid outlet line 38 of the filter 14, with an inlet line 98 for air, with the liquid outlet line 66 of the first absorption column 66 and with an outlet line 100 leading into the third absorption column 88. In turn, the third absorption column 88 comprises an outlet line 102 for depleted air and an outlet line 104, which combines with the outlet line 84 for hydrogen bromide enriched aqueous liquid of the second absorption column 28 to the inlet line 106 of the electrolysis cell 30. The electrolysis cell 30 comprises an outlet line 108 for a bromine containing composition as well as an outlet line 110 for hydrogen, which is connected with a compressor 112 and a hydrogen removal line 114. In turn, the fourth distillation column 90 comprises an inlet line being connected with the outlet line 108 for the bromine containing composition of the electrolysis cell 30, a bottom outlet line and recycle line 116 for a mixture of water and hydrogen bromide and an overhead outlet line 118 for hydrogen. The overhead outlet line 118 leads into the condenser 92 and from there into the vessel 94, from which a return line 120 leads back into the fourth distillation column 90 and an outlet line 122 leads into the dryer column 96. The dryer column 96 comprises a bottom outlet line 124 for bromine, which is connected with the inlet line 32 for starting composition, and an overhead outlet line 126, which leads back into the overhead outlet line 118 of the fourth distillation column 90.

[0053] During the operation of the plant 10, natural gas is led via line 60 into the pretreatment unit 58, from there via line 56 into the nitrogen rejection unit 54, in which it is mixed with the gaseous outlet composition being obtained in the first absorption column, before the so obtained mixture is led via line 62 to line 124 and mixed so with the bromine being recycled via line 124, which is then mixed with the bottom composition of the second distillation column 18 being led through line 48, before the so obtained mixture is led as starting composition via inlet line 32 into the bromination reactor 12. In the bromination reactor 12, the methane reacts with the bromine being contained in the starting mixture, which is withdrawn from the bromination reactor 12 and led into the filter 14, to a reaction mixture effluent. In the filter 14, from the reaction mixture effluent obtained in the bromination reactor 12 soot is separated off, which is then led via the solid outlet line 38 into the thermal oxidizer 86. In turn, the remaining gaseous phase of the reaction mixture effluent is led into the first distillation column 16, in which it is distilled into a bottom composition containing dibromomethane and higher brominated methane, such as tribromomethane, and into an overhead composition containing nitrogen, hydrogen bromide, non-reacted methane and monobromomethane. While the bottom composition is led via line 44 into the third distillation column 22, the overhead composition is led via line 42 into the second distillation column 18, in which the overhead composition is distilled into a bottom composition containing monobromomethane and into an overhead composition containing nitrogen, hydrogen bromide and non-reacted methane and monobromomethane. While the bottom composition is recycled via lines 48, 32 into bromination reactor 12, the overhead composition is led via line 46 into the first absorption column 20, in which the overhead composition of the second distillation column 18 is subjected to an absorption with water, which is led into the first absorption column 20 via inlet line 64, into a gaseous composition containing nitrogen and non-reacted methane, which is recycled into the nitrogen rejection unit 54 via line 50, as well as into a liquid composition comprising water and hydrogen bromide, which is led via line 66 into the third absorption column 88 and from there via lines 104, 106 into the electrolysis cell 30. non-reacted methane and monobromomethane. In turn, the bottom composition of the first distillation column 16 is distilled in the third distillation column 22 into a bottom composition comprising tribromo- and higher brominated methane and into an overhead composition containing dibromomethane. The dibromomethane containing overhead composition of the third distillation column 22 is led via line 68 into the pyrolysis reactor 24 and decomposed therein into carbon and hydrogen bromide. The so obtained reaction mixture is withdrawn from the pyrolysis reactor 24 via outlet line 72 and separated in the filter 74 into a gaseous composition and into a solid carbon composition being led via line 76 into the stripping column 26, in which the carbon composition is heated to a tempera- ture of about 400°C so as to remove the hydrogen bromide therefrom, before the so obtained pure solid carbon is removed from the plant via the carbon removal line 128. In turn, the gaseous composition being obtained in the stripping column 26, which contains hydrogen bromide and various bromomethanes, is led via line 78 into the second washing column 28, in which the gaseous composition is subjected to an absorption with water. While the hydrogen bromide enriched aqueous liquid line obtained in the second absorption column 26 is led via lines 84, 104 into the electrolysis cell 30, the remaining gaseous composition containing various bromomethanes is led via lines 82, 38 into the thermal oxidizer 86.

[0054] The mixture obtained by combining the compositions led through lines 38, 70, 82 into the thermal oxidizer 86 is oxidized in the thermal oxidizer 86 mainly to bromine, which is then together with the hydrogen bromide enriched composition being withdrawn from the first absorption column 20 via line 66 treated in the third absorption column 88, from which via line 104 hydrogen bromide enriched composition (which is in fact an aqueous solution containing hydrogen bromide) is withdrawn and led together with the hydrogen bromide enriched composition being withdrawn from the second absorption column 28 via line 84 into the electrolysis cell 30. The electrolysis cell 30 produces from this composition at the cathode hydrogen and at the anode bromine. While the hydrogen is withdrawn from the electrolysis cell 30 via line 110, the bromine containing composition, which is an aqueous solution containing bromine and hydrogen bromide, is withdrawn from the electrolysis cell 30 via line 108. Afterwards, the bromine containing composition is separated in the fourth distillation column 90 into an aqueous solution containing hydrogen bromide, which is withdrawn via line 116, and into a bromine enriched composition. The latter is withdrawn from the distillation column 90 via line 118 and dewatered in the dryer column 96 so as to obtain pure bromine, which is led via lines 124, 32 into the bromination reactor 12.

[0055] Examples Example 1

[0056] A quartz reactor with an inner diameter of 20 mm was loaded with 9.15 grams of a graphite fraction with a particle size of 0.8 to 1 .2 mm. The reactor was placed in a furnace and heated up in downflow argon flow of 10 ml / min to 650°C for 2 hours. Dibromomethane was evaporated at 150°C and preheated to 250°C, before it was sent to the quartz reactor with a flow rate of 10 g / h corresponding to 21 ml / min gas in admixture with 10 ml / min of argon. The gas at the exit of the reactor was bubbled through a water solution, before it is introduced with the molar concentration of dibromomethane at the entrance to the pyrolysis reactor being 67.7% by weight into the pyrolysis reactor. The pyrolysis was performed for 4.5 hours.

[0057] 3.0 g of carbon were collected corresponding to about 95% of the theoretical yield.

[0058] A portion of the produced carbon was treated at 1 ,000°C in argon flow of 10 ml / min for 4 hours. Another portion was hydrotreated in hydrogen at 600°C for 4 hours. A third portion one was kept as it was, i.e. as produced. All portions were subjected to microscopic (SEM) and elemental analysis (SEM EDX). According to the SEM EDX, the content of bromine in the “as produced” portion was about 1 .36% by weight. A treatment could significantly reduce the content of bromine to about 0.1% by weight. SEM images of the produced and the treated portions are shown in figure 2. Figure 2a shows the carbon being kept as it was, whereas figure 2b shows the carbon being treated at 1 ,000°C in argon flow for 4 hours and figure 2c shows the carbon being hydrotreated in hydrogen at 600°C for 4 hours.

[0059] The content of hydrogen in the portions was obtained as the difference between the total hydrogen measured by CHN analysis and the hydrogen content in the physiosorbed water measured by thermogravimetric analysis at 150°C. Accordingly, the hydrogen in adsorbed water was subtracted from the hydrogen measured by CHN analysis. CHN data corrected to water content at 150°C showed that the content of carbon was 99.5% by weight in the portions having been pretreated in argon at 1,000°C or in hydrogen at 600°C for 4 hours.

[0060] Example 2

[0061] A quartz reactor with an inner diameter of 20 mm was loaded with 30 grams of a graphite petroleum coke fraction with a particle size of 2 to 4 mm. The reactor was placed in a furnace and heated up in downflow argon flow of 10 ml / min to 650°C for 2 hours. Dibromomethane was evaporated at 150°C and preheated to 250°C, before it was sent to the quartz reactor with a flow rate of 10 g / h corresponding to 21 ml / min gas in admixture with 10 ml / min of argon. The gas at the exit of the reactor was bubbled through a water solution, before it is introduced with the molar concentration of dibromomethane at the entrance to the pyrolysis reactor being 67.7% by weight into the pyrolysis reactor. The pyrolysis was performed for 18 hours.

[0062] 11 .77 g of carbon were collected corresponding to about 92.1 % of the theoretical yield.

[0063] A portion of the produced carbon was treated at 1 ,000°C in argon flow of 10 ml / min for 4 hours. Another portion was hydrotreated in hydrogen at 600°C for 4 hours. A third portion one was kept as it was, i.e. as produced. All portions were subjected to microscopic (SEM) and elemental analysis (SEM EDX). According to the SEM EDX, the content of bromine in the “as produced” portion was about 1 .58% by weight. A treatment could significantly reduce the content of bromine to about 0.01% by weight. SEM images of the produced and the treated portions are shown in figure 3. Figure 3a shows the carbon being kept as it was, whereas figure 3b shows the carbon being treated at 1,000°C in argon flow for 4 hours and figure 3c shows the carbon being hydrotreated in hydrogen at 600°C for 4 hours. The CHN data corrected to water content (TG at 150°C as in example 1) showed the content of carbon of 99.7 wt% in the portions having been pretreated in argon at 1,000°C or in hydrogen at 600°C for 4 hours.

[0064] Reference Numeral List

[0065] 10 Plant

[0066] 12 Bromination reactor

[0067] 14 Filter

[0068] 16 First distillation column

[0069] 18 Second distillation column

[0070] 20 First absorption column

[0071] 22 Third distillation column

[0072] 24 Pyrolysis reactor

[0073] 26 Stripping column

[0074] 28 Second absorption column

[0075] 30 Electrolysis cell

[0076] 32 Inlet line for starting composition

[0077] 34 Outlet line for reaction mixture effluent

[0078] 36 Gas outlet line of the filter

[0079] 38 Solid outlet line of the filter

[0080] 40 Inlet line of the first distillation column

[0081] 42 Overhead outlet line of the first distillation column

[0082] 44 Bottom outlet line of the first distillation column

[0083] 46 Overhead outlet line of the second distillation column

[0084] 48 Bottom outlet line of the second distillation column

[0085] 50 Gas outlet line of the first absorption column

[0086] 52 Dryer

[0087] 54 Nitrogen rejection unit

[0088] 56 Line

[0089] 58 Pretreatment unit

[0090] 60 Inlet line for natural gas 62 Outlet line of nitrogen rejection unit

[0091] 64 Inlet line

[0092] 68 Overhead outlet line of the third distillation column

[0093] 70 Bottom outlet line of the third distillation column

[0094] 72 Outlet line of pyrolysis reactor

[0095] 74 Filter

[0096] 76 Line to stripping column

[0097] 78 Line to the second absorption column

[0098] 80 Inlet line for water

[0099] 82 Gas outlet line of the second absorption column

[0100] 84 Outlet line for hydrogen bromide enriched aqueous liquid line of the second absorption column

[0101] 86 Thermal oxidizer

[0102] 88 Third absorption column

[0103] 90 Fourth distillation column

[0104] 92 Condenser

[0105] 94 Vessel

[0106] 96 Dryer column

[0107] 98 Inlet line for air

[0108] 100 Outlet line of thermal oxidizer

[0109] 102 Outlet line of the third absorption column

[0110] 104 Outlet line of the third absorption column

[0111] 106 Inlet line to electrolysis cell

[0112] 108 Outlet line of electrolysis cell

[0113] 110 Outlet line for hydrogen

[0114] 112 Compressor

[0115] 114 Hydrogen removal line

[0116] 116 Recycle line

[0117] 118 Overhead outlet line of fourth distillation column

[0118] 120 Return line Outlet line of the vessel Bottom outlet line of the dryer column Overhead outlet line of the dryer column Carbon removal line

Claims

Claims:1 . A process for producing carbon comprising the step of exothermically pyrolyzing a starting composition containing at least 50% by mole of dibromomethane, less than 5% by mole of hydrogen donor and less than 0.5% by mole of oxygen in a pyrolysis reactor at a temperature of 600 to 2,000°C to carbon and hydrogen bromide.

2. The process in accordance with claim 1, wherein during the step of exothermically pyrolyzing the starting composition no electrical energy and preferably no energy at all is supplied to the pyrolysis reactor.

3. The process in accordance with claim 1 or 2, wherein the starting composition being exothermically pyrolyzed contains at least 60% by mole, preferably at least 70% by mole and more preferably at least 80% by mole of dibromomethane.

4. The process in accordance with any of the preceding claims, wherein the starting composition being exothermically pyrolyzed 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.

5. The process in accordance with any of the preceding claims, wherein the starting composition being exothermically pyrolyzed 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.

6. The process in accordance with any of the preceding claims, wherein the starting composition being exothermically pyrolyzed 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 or a protective gas, wherein the inert gas or protective 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 or protective gases.

7. The process in accordance with any of the preceding claims, wherein the step of exothermically pyrolyzing the starting composition is performed at a temperature of 650 to 1 ,500°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 to be pyrolyzed and containing at least 50% by mole of dibromomethane, less than 5% by mole of hydrogen donor and less than 0.5% by mole of oxygen, wherein the step comprises 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.

9. The process in accordance with claim 8, wherein the dibromomethane enriched composition is fed as starting composition into the step of exothermically pyrolyzing the starting composition or one or more compounds, such as an inert gas, is / are added to the dibromomethane enriched composition, before the so obtained mixture is fed as starting composition into the step of exothermically pyrolyzing the starting composition.

10. The process in accordance with any of claims 1 to 7, wherein the process further comprises a step of producing the starting composition to be pyrolyzed and containing at least 50% by mole of dibromomethane, less than 5% by mole of hydrogen donor and less than 0.5% by mole of oxygen, wherein the step comprises 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.11 . The process in accordance with any of the preceding claims, wherein the hydrogen bromide obtained during the pyrolysis is withdrawn as hydrogen bromide containing composition from the pyrolysis reactor and is subjected to an electrolysis to bromine and hydrogen.

12. The process in accordance with any of claims 1 to 9, wherein the hydrogen bromide obtained during the pyrolysis is withdrawn from the pyrolysis reactor as hydrogen bromide containing composition and is subjected to a thermal oxidation step.

13. The process in accordance with any of the preceding claims, wherein the carbon obtained during the pyrolysis is withdrawn from the pyrolysis reactor and then subjected to one or more purification steps, wherein the carbon is i) subjected to at least one 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 carbon, and / or ii) wherein the carbon is 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.

14. A plant for producing carbon comprising:a) a reactor comprising one or more inlets and an outlet for a reaction mixture effluent, b) a first distillation column comprising an inlet being connected with the outlet for the reaction mixture effluent of the reactor, wherein the first distillation column comprises an overhead outlet and a bottom outlet, c) a second distillation column comprising an inlet being connected with the bottom outlet of the first distillation column, an overhead outlet and a bottom outlet, d) a pyrolysis reactor comprising an inlet being connected with the overhead outlet of the second distillation column and an outlet, e) a solid-gas-separation unit comprising an inlet being connected with the outlet of the pyrolysis reactor, a gas outlet and a solid outlet and f) an electrolysis cell comprising an anode, a cathode and a membrane sandwiched between the anode and the cathode as well as an inlet for a hydrogen bromide containing composition, an outlet for hydrogen and an outlet for a bromine containing composition, wherein the inlet of the electrolysis cell is connected with the gas outlet of the solid-gas- separation unit e) either directly f-i) or f2) with an absorption column arranged therebetween comprising a first outlet for a hydrogen bromide enriched aqueous liquid and a second gas outlet, with the first outlet for a hydrogen bromide enriched aqueous liquid being connected with the inlet of the electrolysis cell, and wherein the outlet for the bromine containing composition of the electrolysis cell is directly or indirectly connected with an inlet of the reactor a).

15. The plant in accordance with claim 14, which further comprises: g) a third distillation column comprising an inlet being connected with the overhead outlet of the first distillation column, an overhead outlet and a bottom outlet andh) an absorption column being connected with the overhead outlet of the third distillation column, a liquid outlet and a gas outlet, wherein the gas outlet is directly or indirectly connected with the reactor a).

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