Method of offgas treatment

Abstract

A method for treating a hydrocarbon-containing offgas from the carbonization of polyacrylnitrile fibers or oxidized polyacrylnitrile fibers, comprising the steps of introducing the offgas into a reaction chamber, separating the hydrocarbons into carbon and hydrogen by way of hydrocarbon pyrolysis to obtain a hydrogenous offgas, and cooling the hydrogenous offgas.

Description

[0001] Exhaust gas treatment processes

[0002] Description:

[0003] The invention relates to a process for purifying exhaust gases from the oxidation and / or carbonization of polyacrylonitrile fibers or the production of carbon fibers.

[0004] Carbon fibers have become a sought-after material over the past 150 years. This development began with the invention of the electric light bulb, whose filament was initially made of carbon fibers obtained by carbonizing bamboo or other plant fibers. Such filaments were only later replaced by high-temperature-resistant metal wires.

[0005] It was only in the period after the Second World War that carbon fibers were discovered as a construction material with outstanding properties. Their low density and high tensile strength are particularly noteworthy. This makes them ideal for the production of components made of high-strength fiber composites, in which carbon fibers, also known to experts as "carbon fibers" or simply "carbon," are combined with a stiffening matrix material, typically a polymer. The low density of such carbon fiber-reinforced plastics, known to experts by the abbreviation "CFRP," is now used in many technical fields. In addition to the production of smaller items such as golf or tennis racket handles, the hulls of entire sports yachts and, for some years now, even wide-body commercial aircraft are made entirely or partially from CFRP.The resulting weight savings have a positive effect on the energy requirements of aircraft, for example.

[0006] Carbon fibers are typically produced by controlled oxidation and carbonization—known as "carbonization"—of organic fiber materials, so-called precursors. Besides cellulose fibers or fibers derived from tars, the most important precursor material is polyacrylonitrile spun into fibers.

[0007] To produce carbon fibers, polyacrylonitrile fibers are subjected to a multi-stage process in which they are first oxidized at temperatures between 200 and 300°C. The resulting oxidized polyacrylonitrile fibers are then carbonized into carbon fibers at temperatures above 600°C in an airtight environment.

[0008] This process generates large quantities of exhaust gases. A common rule of thumb is that currently, about half of the polyacrylonitrile material mass used is converted into exhaust gases. In addition, heating the oxidation and carbonization furnaces requires large amounts of energy. The carbon fiber manufacturing process therefore offers great potential for saving both energy and resources.

[0009] The exhaust gases from the oxidation and carbonization processes are complex and ecologically problematic for various reasons, so they cannot be released into the atmosphere without further treatment. This is due in particular to their high content of sometimes highly toxic gases such as hydrogen cyanide, commonly known as "hydrogen cyanide," and carbon monoxide. These exhaust gases also contain chain- or ring-shaped, saturated, unsaturated, and aromatic hydrocarbons, some of which are toxic and / or carcinogenic and often contribute significantly to global warming. For this reason, the exhaust gases from the oxidation and carbonization of polyacrylonitrile fibers are generally rendered harmless by incineration using natural gas.

[0010] The aim of the invention is to improve the ecological balance of carbon fibers by neutralizing the exhaust gases in a less energy-intensive manner and, where possible, by economically recycling them. Ideally, the exhaust gases should be usable as raw materials. This approach should save both energy and resources, thus bringing the goal of a CO2-neutral, circular economy in the field of carbon fiber production closer.

[0011] The object is achieved by a method for purifying exhaust gases from the oxidation and / or carbonization of polyacrylonitrile fibers or oxidized polyacrylonitrile fibers, comprising the steps of cooling the exhaust gas to condense condensable components, separating the cooled exhaust gas into condensed components and residual gas using at least one electrostatic precipitator, wherein the condensed components may be solid or liquid, binding ammonia and / or hydrogen cyanide from the residual gas, and forming ammonium compounds and / or cyanide compounds. The exhaust gases from the oxidation and / or carbonization of polyacrylonitrile fibers (hereinafter also referred to as "exhaust gases" or "the exhaust gas") are complex mixtures of gases, vapors, liquid, and solid components, which, depending on the process, arise at temperatures between 200°C and well over 1000°C.Regardless of the physical state of the individual components at room temperature, we will initially refer to "exhaust gas" here because the majority of the components are gaseous at the temperatures at which the exhaust gas is generated. The composition of the exhaust gas is, as is usual for exhaust gases from thermal and carbonization processes, very complex and depends on various factors. These include the precursor material and the location where the exhaust gas originated. Depending on whether the exhaust gases originate from the inlets and outlets or from inside the carbonization or oxidation furnaces, experts refer to them as "hood extraction" (from the inlets and outlets) or "center furnace extraction." Exhaust gases from the hood extraction always contain air and therefore atmospheric oxygen. Exhaust gases from the center furnace extraction of the carbonization furnaces are typically oxygen-free.Exhaust gases from the central furnace extraction of oxidation furnaces always contain oxygen compounds and often also atmospheric oxygen.

[0012] The proportion of condensable components in exhaust gases from the central furnace extraction system is typically significantly higher than the proportion of condensable components in exhaust gases from the hood extraction system.

[0013] Due to the composition of the precursor material polyacrylonitrile, the exhaust gases are primarily expected to contain nitrogen compounds and hydrocarbons. Exhaust gases from the oxidation of polyacrylonitrile fibers contain not only atmospheric oxygen but also oxygen compounds such as carbon dioxide, carbon monoxide, and organic oxygen compounds.

[0014] Typical nitrogen compounds present in exhaust gases are ammonia, hydrogen cyanide, and heteroaromatics such as pyridine, pyrrole, and their derivatives. Particular attention should be paid to hydrogen cyanide. Due to the presence of a cyano group in polyacrylonitrile, it is preferentially formed during oxidation and / or carbonization. Along with carbon monoxide, its extreme toxicity contributes significantly to the hazardous nature of the exhaust gases, making post-treatment of some kind essential.

[0015] In terms of hydrocarbons, exhaust gases can contain a wide range of components. These include, in particular, saturated and unsaturated aliphatic hydrocarbons such as methane, ethane, propane, and butane; ethylene, propylene, and acetylene; hexane, heptane, octane, and higher alkanes and their isomers; as well as aromatics such as benzene, toluene, xylenes, naphthalene, and anthracene. Aromatic hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) may also be present, the latter being considered highly carcinogenic.

[0016] In addition to carbon dioxide and carbon monoxide, oxygen compounds include various alcohols, ethers, carboxylic acids and their derivatives, ketones, and aldehydes. Aromatic organic oxygen compounds such as phenol and its derivatives also occur. Furthermore, the exhaust gas may contain solid components, for example, in the form of soot, dust, or fiber residues. In addition to carbon, these solid components may also include infusible, so-called oxidized polyacrylonitrile (oPAN) and polycyclic aromatic hydrocarbons.

[0017] Due to the complexity of the composition described above, it is difficult to specify the contents of the individual components. With regard to the treatment of the exhaust gas, it has therefore proven useful to initially classify the exhaust gas emerging from the furnace into "solid," "condensable," and "non-condensable" components.

[0018] The solid components of the exhaust gas are understood to be all those components that are already present in a solid state upon exiting the furnace. These can include, for example, soot, dust, coke, or fiber residues. Carbon predominates in the composition of these components. This can be elemental carbon (soot, coke, carbon fibers) as well as infusible, oxidized polyacrylonitrile (oPAN) or polycyclic aromatic hydrocarbons, the latter of which are problematic due to their toxic and, in some cases, carcinogenic effects. Elemental carbon-based solid components predominate in the exhaust gases from the central exhaust system of carbonization furnaces.

[0019] The condensable components are components that are gaseous upon exiting the furnace, but which precipitate as liquids or solids and thus condense when the temperature of the exhaust gas drops to 298 K. It is important to emphasize that, in the context of the present application, the term "condensable components" is to be understood as a category designation. This does not mean that they must be present in condensed form at any point in the process. Since this entire group of substances is involved, it is also possible to condense only a portion of the condensable components of the exhaust gas and leave another portion in the gaseous state, in particular by cooling the exhaust gas to a temperature above 298 K.The term "condensed condensable constituents" should be viewed against this background; it refers only to those condensable constituents that have actually been condensed. The terms "condensed condensable constituents" and "tar" are used synonymously in this application. Related to this is the term "uncondensed constituents," which in this application refers to all those components of the exhaust gas that are present in gaseous form at a given time. The uncondensed constituents always contain the non-condensable constituents and may also—depending on the temperature—contain a proportion of condensable constituents that are not condensed.

[0020] The condensable components can include the entire range of hydrocarbons, nitrogen compounds, or oxygen compounds mentioned above. If the yarn to be oxidized or carbonized contains spinning oils or sizing agents, their residues may also be found in the exhaust gas. In this case, organosilicon compounds such as silicones or their combustion and / or decomposition products are particularly relevant. If the condensable components are fully or partially condensed, an oily mixture of substances separates out, known to those skilled in the art as "tar."

[0021] "Non-condensable" components are all those components that are gaseous at 298 K. This category includes, in addition to nitrogen, gaseous or highly volatile hydrocarbons such as methane, ethane, propane, or butane, and their unsaturated analogues, as well as ammonia and hydrogen cyanide, which originate from the nitrogen atoms of the oxidizing or carbonizing polyacrylonitrile. Exhaust gases from oxidation furnaces also contain oxygen and oxygen compounds such as carbon monoxide and carbon dioxide.

[0022] If necessary, the exhaust gas is expediently cooled using a heat exchanger, through which thermal energy is transferred from the exhaust gas to another medium, such as air, thermal oil, or water. The heat absorbed by the other medium, known to those skilled in the art as "waste heat," can subsequently be utilized. Examples of possible uses include preheating furnaces for the oxidation or carbonization of polyacrylonitrile fibers, heating production or other buildings, or other facilities such as swimming pools. Feeding into a district heating network is also possible. This applies equally to all cooling processes in the process according to the present application.

[0023] Both solid and condensed, condensable components can be separated from the exhaust gas using electrostatic precipitators. Electrostatic precipitators as such are familiar to those skilled in the art. These are devices in which electrical voltages in the range of several thousand volts, for example, 30 kilovolts, are applied between so-called discharge electrodes and separation plates. The exhaust gas is passed through the space between the discharge electrodes and the separation plates. The electrodes cause a strong static charge on particles contained in the exhaust gas, such as liquid droplets or dust particles. In the applied electric field, the thus charged particles migrate to the grounded separation plate, where they are discharged and can collect and be removed.

[0024] The separation of solid and condensed, condensable components in the exhaust gas can be carried out together or separately. If the components are to be separated separately, the exhaust gas is first fed to one or more electrostatic precipitators, which separate the solid components. The exhaust gas is then cooled to condense some or all of the condensable components. The exhaust gas is then fed to one or more further electrostatic precipitators, in which the condensed, condensable components are separated.

[0025] If solid and condensed condensable components are separated together, the exhaust gas is cooled and fed to one or more electrostatic precipitators. Cooling the exhaust gas may also be necessary to separate the solid components of the exhaust gas if a certain maximum temperature must be maintained for the operation of the electrostatic precipitator(s).

[0026] After the exhaust gas passes through the electrostatic precipitator(s), only uncondensed components remain in the exhaust gas, which in this application is referred to as "residual gas." This residual gas is still highly toxic, as it always contains hydrogen cyanide and ammonia. It may also contain carbon monoxide.

[0027] Ammonia and hydrogen cyanide are valuable starting materials for a wide range of chemical compounds. Ammonia and the ammonium salts derived from it play an important role in the production of fertilizers, explosives, and other nitrogen-containing compounds. Among these are ammonium carbonate and ammonium bicarbonate, which are used as leavening agents under the names "hart's horn salt" or "ABC leavening."

[0028] Ammonia can be removed from the exhaust gas by absorption using acidic reagents. For example, an acid such as hydrochloric, nitric, or sulfuric acid can be sprayed into a chamber through which the residual gas is passed. Passing the residual gas through hydrochloric, nitric, or sulfuric acid is also possible. This process is called "gas scrubbing," specifically "acidic gas scrubbing." Depending on the acid used, ammonium chloride, ammonium nitrate, or ammonium sulfate is obtained. Ammonium nitrate and ammonium sulfate are widely used as fertilizers, and ammonium nitrate is also used as an explosive.

[0029] Hydrogen cyanide is a gas that itself exhibits a slightly acidic reaction and therefore cannot be removed from the exhaust gas under the conditions of an acidic gas scrubber. Instead, alkaline solutions such as caustic soda or potassium hydroxide solution can be sprayed in a manner analogous to acidic gas scrubbers. These salts bind hydrogen cyanide in the form of sodium cyanide or potassium cyanide. These salts are used in electroplating but are just as toxic as hydrogen cyanide.

[0030] In addition to the absorption processes mentioned above, it is also possible to reversibly separate ammonia and hydrogen cyanide by adsorption on a suitable material.

[0031] Suitable adsorption materials are known to those skilled in the art. Examples include activated carbon with suitable grain and pore sizes, as well as zeolites with corresponding grain and pore sizes. Zeolites, in particular, can also have functionalizations that make them particularly suitable for the reversible adsorption of ammonia and / or hydrogen cyanide. The use of metal-organic network structures, also known to those skilled in the art as "metal-organic frameworks" or "MOFs," which have suitable pore sizes or functionalizations, is also possible.

[0032] It may be necessary to remove hydrogen cyanide from exhaust gases to prevent any risk to the environment or to the health of humans and animals. Hydrogen cyanide's high affinity for iron ions can be exploited for this purpose by spraying an alkaline iron(II) salt solution into the residual gas or passing the residual gas through an alkaline iron(II) salt solution. One possible iron(II) salt is iron(II) sulfate. Contact with cyanide in an alkaline solution produces ferrous sulfate, which, depending on the nature of the alkaline iron(II) salt solution, forms ferrous sulfate (hexacyanidoferrate(II). This sulfate is available as sodium ferrous sulfate, potassium ferrous sulfate, or ammonium ferrous sulfate (hexacyanidoferrate(II).These salts can be further processed by contact with iron(III) salts, such as iron(III) chloride, to produce iron(III) hexacyanidoferrate(II / III), which is known to experts as "Berlin Blue" and several other synonyms such as "Prussian Blue," "Iron Blue," or "Turnbull's Blue." "Berlin Blue" is a widely used blue pigment, but is also used as an antidote for certain poisonings and as an adsorbent for gases.

[0033] In this way, components of the residual gas can be converted into valuable materials and thus used for industrial purposes. The remaining residual gas still contains uncondensed hydrocarbons and may also contain carbon monoxide, making it extremely toxic.

[0034] Non-condensed hydrocarbons can be removed from the residual gas by scrubbing with organic solvents such as petroleum ether, toluene, cyclohexane or dichloromethane.

[0035] The residual gas can be mixed with air and burned in a combustion chamber, if necessary using or following a catalyst, which ensures the most complete combustion possible into non-toxic substances such as water and carbon dioxide, which can be released into the atmosphere safely.

[0036] It is clearly disclosed that all steps of the disclosed process—separation of solid components, cooling of the exhaust gas, separation of condensed condensable components, separation of hydrogen cyanide and / or ammonia, scrubbing of uncondensed hydrocarbons, and combustion of the residual gas—in their entirety represent optional features of the process according to the present application and can be combined in any order deemed reasonable by a person skilled in the art. Process steps can also be omitted or further steps added.

[0037] In one embodiment, for example, an oxygen-free exhaust gas from the central exhaust of a carbonization furnace can be treated according to the process. The exhaust gas can be fed directly and without further pretreatment to a hydrocarbon pyrolysis plant. In one embodiment, the exhaust gas can be cooled before hydrocarbon pyrolysis. In one embodiment, solid components can be separated from the exhaust gas before hydrocarbon pyrolysis, for example by means of one or more electrostatic precipitators. In one embodiment, condensable components in the exhaust gas before hydrocarbon pyrolysis can be condensed and separated, for example, by means of one or more electrostatic precipitators. In one embodiment, ammonia or hydrogen cyanide can be removed from the exhaust gas before hydrocarbon pyrolysis by acidic or basic exhaust gas scrubbing or by adsorption.

[0038] In one embodiment, for example, an oxygen-containing exhaust gas from the hood extraction of a carbonization furnace or from the hood or center furnace extraction of an oxidation furnace can be treated according to the process according to the present application. Condensable components in the exhaust gas are condensed by cooling and separated, for example, with the aid of one or more electrostatic precipitators. In one embodiment, prior to the condensation of the condensable components, solid components can be removed from the exhaust gas, for example by means of an electrostatic precipitator. In one embodiment, after the condensed condensable components have been separated from the exhaust gas, ammonia or hydrogen cyanide can be removed by acidic or basic exhaust gas scrubbing. In one embodiment, after the condensed condensable components have been separated and / or after the acidic and / or basic exhaust gas scrubbing, the exhaust gas can be mixed with air and burned.

[0039] In one embodiment, for example, an oxygen-containing exhaust gas from the hood exhaust of a carbonization furnace or from the hood or center furnace exhaust of an oxidation furnace can be treated according to the method according to the present application. Optionally, solid components are first removed from the exhaust gas using an electrostatic precipitator.

[0040] The oxygen content of the exhaust gas is then determined, and the exhaust gas is mixed with a determined amount of air and / or water vapor. The resulting gas mixture is fed into a reactor where it undergoes partial oxidation to produce a mixture of carbon monoxide and hydrogen from hydrocarbons, oxygen, and water, which can be fed into synthesis processes. Such synthesis processes can, for example, lead to methanol or various types of hydrocarbons and are known to those skilled in the art, for example, the "Fischer-Tropsch process." In one embodiment, the electrostatic precipitators can be wet electrostatic precipitators.In wet electrostatic precipitators, the separated particles are continuously washed off the separator plate by a liquid film. This allows for a consistent deposited layer thickness and thus more controllable process control than a dry electrostatic precipitator, in which a growing layer of particles forms on the separator plate, which must be removed by regular tapping. As the layer thickness increases, the electric field strength of the precipitator and thus its effectiveness decreases with increasing time between tapping processes. Furthermore, the particles are further disturbed by the tapping, and some of them must be separated again.

[0041] Depending on the exact composition of the exhaust gas, the liquid film on the separator plate can either be created by spraying a liquid such as water onto it, or it can form spontaneously through the precipitation of a portion of droplets contained in the cooled exhaust gas. It is also possible to create the liquid film by injecting a liquid such as water into the gas channel between the separator plate and the discharge electrodes, rather than onto the separator plate. Generally, liquids other than water can be used to create the liquid film in an electrostatic precipitator. Particularly advantageous here are low-flammability organic substances that act as electrical insulators, such as silicone oils.

[0042] Wet electrostatic precipitators allow a particularly well-controllable, uniform separation process due to the continuous removal of the separated particles.

[0043] Dry electrostatic precipitators have the advantage that they continue to function effectively even at high temperatures, where many liquids are gaseous.

[0044] The condensable components of the exhaust gas are very complex, but clearly focus on aliphatic and aromatic hydrocarbons. Depending on the origin of the exhaust gas, they may also contain analogous oxygen- and nitrogen-containing compounds such as amines, alcohols, phenols, aldehydes, ketones, and carboxylic acids, as well as heteroaromatics such as pyrroles, pyridines, and their derivatives. Since such substances are also industrially applicable, the condensed condensable components separated by the electrostatic precipitator(s) can be subjected, in whole or in part, to tar distillation, as is known, for example, from the processing of wood or coal tar. The tar itself can also be used, for example, as a wood impregnating agent.

[0045] Finally, it is also possible to subject the tar to a so-called cracking process, in which larger hydrocarbon molecules are split into smaller molecular units. The cracking process can be combined with a hydrogenation process, in which the hydrocarbons react with elemental hydrogen. In this way, important raw materials for industry, such as ethylene, propylene, or methane, can be extracted from the condensed, condensable components.

[0046] The separated tar is typically an oily, combustible mixture of substances that can be burned in whole or in part to generate energy—for example, to heat oxidation or carbonization furnaces for polyacrylonitrile fibers. This can save large quantities of fossil fuels and thus improve the carbon footprint of the produced carbon fibers. In some cases, burning the tar can be more cost-effective than tar distillation. Depending on its composition, the solid components can also be burned in whole or in part to generate energy. This can be done individually, for example, in furnaces or fluidized beds, or by suspending the liquid components in the tar, which is also burned to generate energy.

[0047] In one embodiment, ammonia and propylene recovered from the exhaust gas in the process according to the present application can be fed together with atmospheric oxygen to the industrial process of the Sohio process, in which acrylonitrile, the raw material for the production of polyacrylonitrile, is produced from the said raw materials.

[0048] After separation of the solid and condensed condensable components and, if necessary, the binding of hydrogen cyanide and / or ammonia, the residual gas contains, in addition to larger quantities of gaseous, non-condensable hydrocarbons such as methane, ethane, propane, butane, ethylene, propylene, or acetylene in varying proportions. These components, as well as all other condensable components, can be subjected to hydrocarbon pyrolysis. Hydrocarbon pyrolysis can be carried out at any point in the process described here. In hydrocarbon pyrolysis, gaseous or vaporous hydrocarbons are broken down into their elemental components in a plasma torch, thermally or catalytically, producing carbon (e.g., in the form of soot or activated carbon) and molecular hydrogen. In all cases, however, it takes place at temperatures above 600 K.Hydrocarbon pyrolysis makes it possible to generate hydrogen from exhaust gases, which can be used as a synthesis precursor, for example, for the production of fertilizers and synthetic fuels, or as a CO2-neutral fuel. The resulting carbon can be used as a filler material for rubber production, as an adsorption material for chemicals, or as a pigment. Furthermore, it can be landfilled to permanently remove it from the atmosphere. If the thermal energy for hydrocarbon pyrolysis is obtained from renewable energies, the hydrogen produced in this way is known to experts as "turquoise hydrogen."

[0049] Within the scope of the present application, hydrocarbon pyrolysis can be carried out by all processes known to those skilled in the art. In particular, the use of a plasma torch (also known to those skilled in the art as the Kvaerner process), purely thermal or thermal-catalytic processes can be used. In thermal processes, the exhaust gases are passed over a suitably heated surface, through a molten metal such as a tin melt, or through a suitably heated bed of, for example, carbon granules. In catalytic processes, the exhaust gas is brought into contact with a suitably heated catalyst. Graphitic carbon is particularly suitable as a catalyst. It has now surprisingly been found that carbon fibers, which are structurally very similar to graphite, are also capable of catalyzing such processes.But other modifications of elemental carbon such as graphite, coke, soot or activated carbon can also be used.

[0050] In one embodiment, the catalyst is immobilized on a surface. In one embodiment, the catalyst is present in a fixed bed through which the exhaust gas flows. In one embodiment, the catalyst is present in a fluidized bed through which the exhaust gas flows.

[0051] The carbon produced during hydrocarbon pyrolysis can deposit on the catalyst and, depending on the process, be in a form that allows it to perform a catalytic effect. In particular, the carbon can occur in the form of soot or coke.

[0052] Carbon fiber residues, some of which are already present in the exhaust gas and / or which arise during further processing, e.g., in the form of offcuts or abrasion, can be used as catalysts in the process according to the present application. In one embodiment, these are carbon fiber residues that are free of spinning oils and sizing agents that could impair the contact of the gas with the surface.

[0053] If the residual gas contains carbon monoxide in addition to hydrocarbons, it forms a mixture of carbon monoxide and hydrogen after hydrocarbon pyrolysis, which can be used in various catalytic industrial processes such as Fischer-Tropsch synthesis for liquid or gaseous hydrocarbons or the production of methanol. The exhaust gas obtained from hydrocarbon pyrolysis contains other gases besides hydrogen. If the treated exhaust gas is the central exhaust of a carbonization furnace, the gas contains large amounts of nitrogen, as carbonization usually takes place under a nitrogen atmosphere. The hydrogen can be separated from the resulting gas mixture, for example, by fractional condensation or with the help of gas-permeable membranes. In addition, the gas mixture can be further purified and, if necessary,with the addition of nitrogen or hydrogen, it can be used for ammonia synthesis according to the Haber-Bosch process.

[0054] Hydrocarbon pyrolysis is not possible with exhaust gases that contain significant amounts of oxygen, since under these conditions, hydrocarbons do not produce hydrogen and carbon, but rather water and carbon oxides such as carbon dioxide and carbon monoxide in varying amounts. However, hydrocarbon pyrolysis is unproblematic with exhaust gases that do not contain oxygen but rather nitrogen compounds such as hydrogen cyanide or ammonia, which decompose into gaseous nitrogen, gaseous hydrogen, and possibly carbon at the high temperatures of pyrolysis.

[0055] Against this background, it is important to emphasize that the removal of ammonia, hydrogen cyanide, or condensable components from the exhaust gas is not a prerequisite for hydrocarbon pyrolysis. Since the condensable components, particularly in the exhaust gases from the central furnace exhaust system of carbonization furnaces, which are free of oxygen and oxygen compounds, consist exclusively of hydrocarbons and organic nitrogen compounds, hydrocarbon pyrolysis can also be carried out with these components, possibly with a significantly higher yield of hydrogen and carbon than if it were carried out solely with the uncondensed components of the exhaust gas.

[0056] The present application further relates to a device for purifying exhaust gases from the oxidation and / or carbonization of polyacrylonitrile fibers or oxidized polyacrylonitrile fibers, comprising at least one heat exchanger, at least one electrostatic precipitator, at least one reactor, and at least one combustion chamber. The device according to the present application can be integrated into the exhaust system of an oxidation furnace for polyacrylonitrile fibers or into the exhaust system of a carbonization furnace for oxidized polyacrylonitrile fibers. It is also possible for the exhaust gas from several oxidation furnaces and / or carbonization furnaces to be fed to it.

[0057] In one embodiment, the device according to the present application contains one or more heat exchangers. The heat exchangers can be any form of heat exchanger known to those skilled in the art. Typically, a heat exchanger has a large contact surface that is in thermally conductive contact with both the exhaust gas and a heat transfer medium, without any physical contact between the exhaust gas and the heat transfer medium. The heat transfer medium is typically a gas such as air, nitrogen, or carbon dioxide, or a liquid such as water or a thermal oil.One advantage of using water is its easy availability and high heat capacity, while gases and thermal oils have the advantage that they can be heated to temperatures above the boiling point of water, and in the case of gases even to temperatures of several hundred degrees Celsius, and can also release these high temperatures to other media. The heat transport medium typically releases the heat absorbed from the exhaust gas elsewhere. Both release into the atmosphere and technical use of the heat are conceivable, for example, to preheat oxidation furnaces or pyrolysis furnaces, to heat buildings, industrial plants or facilities such as swimming pools, or to feed it into a district heating network. In one embodiment, the heat exchanger(s) are coupled to corresponding devices. It is also possible for the heat transport medium of the heat exchanger to transfer heat elsewhere, e.g.from a heating device and passes it on to the exhaust gas, thus heating the exhaust gas.

[0058] The heat exchangers can serve various functions in the present application. For example, they can cool the exhaust gas to a temperature at which it can be fed into an electrostatic precipitator. Furthermore, they can cool the exhaust gas so that condensable components condense.

[0059] The device can comprise at least one electrostatic precipitator for separating solid and / or condensed, condensable components from the exhaust gas. Electrostatic precipitators as such are known to those skilled in the art. These are devices in which electrical voltages in the range of several thousand volts, for example 30 kilovolts, are applied between so-called discharge electrodes and separation plates. The exhaust gas is passed through the space between the discharge electrodes and the separation plates. The electrodes cause a strong static charge on particles contained in the exhaust gas, such as liquid droplets or dust particles. In the applied electric field, the thus charged particles migrate to the grounded separation plate, where they are discharged and can collect and be removed.

[0060] After the exhaust gas has passed through the electrostatic precipitator(s), only non-condensed components remain in the exhaust gas, which in this state is referred to as “residual gas” in the present application.

[0061] In one embodiment, the electrostatic precipitators can be wet electrostatic precipitators. In wet electrostatic precipitators, the separated particles are continuously washed off the separation plate by a liquid film, which enables a consistent thickness of the deposited layer and thus better controllable process control than a dry electrostatic precipitator, in which a growing layer of particles forms on the separation plate with increasing thickness, which must be removed by regular tapping. Due to the increasing layer thickness, the electric field strength of the precipitator and thus its effectiveness decreases with increasing time interval between the tapping processes. In addition, the particles are further disturbed by the tapping and sometimes have to be separated again.Depending on the exact composition of the exhaust gas, the liquid film on the separator plate can either be created by spraying a liquid such as water onto it, or it can form spontaneously through the precipitation of a portion of droplets contained in the cooled exhaust gas. It is also possible to create the liquid film by injecting a liquid such as water into the gas channel between the separator plate and the discharge electrodes, rather than onto the separator plate. Generally, liquids other than water can be used to create the liquid film in an electrostatic precipitator. Particularly advantageous here are low-flammability organic substances that act as electrical insulators, such as silicone oils.

[0062] Wet electrostatic precipitators allow a particularly well-controllable, uniform separation process due to the continuous removal of the separated particles.

[0063] In one embodiment, the device contains at least one dry electrostatic precipitator. Dry electrostatic precipitators offer the advantage of being operable even at high temperatures, where most liquids are gaseous.

[0064] The device can comprise at least one reactor. This reactor can be any type of reactor suitable for gas-phase reactions known to those skilled in the art. In one embodiment, the reactor is a chamber in which conditions for gas-phase reactions can be adjusted, for example by setting a specific temperature or providing a reagent and / or a catalyst. A reagent can be introduced in liquid or gaseous form, for example, through inlet openings. In one embodiment, the reactor comprises at least one device for spraying liquids. This device can be, for example, nozzles.

[0065] A catalyst can be provided in the reactor, for example, in immobilized form on a permanently installed support material, where the support material can be porous and / or have a channel structure. The channel structure can also be achieved using a powdered or granular support material. The granular or granular support material loaded with the catalyst, or the granular or granular catalyst itself, can form a bed through which the exhaust gas flows without any significant movement of the granular or granular material. Such arrangements are known to those skilled in the art as fixed-bed reactors. Furthermore, the catalyst can also be immobilized on a mobile support material and, in this form, blown into the reactor, for example, or provided in the form of a fluidized bed.In this case, the lumpy or powdered support material with the catalyst, or the lumpy or powdered catalyst itself, is stirred by the flowing gas and thus kept in motion. Experts refer to this as a fluidized-bed reactor.

[0066] In one embodiment, the reactor contains at least one region in which solid or liquid aerosols can settle. This can be, for example, a cyclone system in which aerosols are forced into a circular path, causing them to precipitate on the walls due to centrifugal forces. The use of one or more electrostatic precipitators is also possible, which can be electrostatic precipitators such as those already described in the present application.

[0067] In one embodiment, the device contains at least two reactors, which may be of the same or different design and in which different reactions can take place.

[0068] In one embodiment, one of the reactors is designed as a combustion chamber. A combustion chamber, as defined in the present application, is defined as a space in which an exothermic oxidation reaction can take place in the gas phase, which may be accompanied by a fire. The combustion chamber may contain an inlet through which air and / or oxygen is added to the residual gas.

[0069] The reactor may contain a catalyst that catalyzes the reaction taking place in the reactor. The catalyst may be provided in the reactor, for example, in immobilized form on a fixed support material, which may be porous and / or have a channel structure. Furthermore, the catalyst may also be immobilized on a mobile support material and, in this form, be injected into the reactor, for example, or provided in the form of a fluidized bed.

[0070] When exothermic reactions are carried out, heat is released in the reactor. The reactor can therefore have heat-resistant materials such as firebricks in its lining. Furthermore, it can be equipped with a heat exchanger that cools the reactor itself or the gas released from the reactor. Those skilled in the art will appreciate that the heat exchanger can be designed in any manner similar to that disclosed for the heat exchanger for cooling the exhaust gas in the present application. The reactor's heat exchanger can also be configured so that the absorbed heat can be used to preheat oxidation or carbonization furnaces, to heat buildings or plants or facilities such as swimming pools, or to feed it into a district heating network.

[0071] In one embodiment, at least one of the reactors is equipped to carry out hydrocarbon pyrolysis. In one embodiment, the hydrocarbon pyrolysis is carried out thermally, catalytically, or in the plasma of an electric arc. For this purpose, the reactor can contain an electrical connection, a suitable transformer, and electrodes. An arc or plasma can be generated between the electrodes by applying electrical voltage, through which the exhaust gas can be passed through suitable means such as nozzles.

[0072] In the case of catalytic hydrocarbon pyrolysis, the reactor is at least one fixed-bed reactor or at least one fluidized-bed reactor within the meaning of the present application, in which the catalyst is stored. The reactor can be lined with high-temperature-resistant material that can withstand the temperatures of hydrocarbon pyrolysis in a plasma torch, which can reach up to 1600°C.

[0073] In one embodiment, the reactor configured to carry out hydrocarbon pyrolysis contains a heat exchanger that cools the gas after hydrocarbon pyrolysis.

[0074] The individual components of the device are connected to each other by pipes.

[0075] In one embodiment, the pipes have heating devices to keep the exhaust gas at a defined temperature during its passage through the device. This serves, in particular, to prevent condensable components from condensing at undesirable locations. It should be noted that all features of the device according to the present application—electrostatic precipitators, reactors, heatable pipes, and heat exchangers—are optional features and can be combined in any order.

[0076] The application also relates to the use of at least one electrostatic precipitator for purifying exhaust gases from the oxidation and / or carbonization of polyacrylonitrile fibers or oxidized polyacrylonitrile fibers. Electrostatic precipitators are devices in which electrical voltages in the range of several thousand volts, for example, 30 kilovolts, are applied between so-called discharge electrodes and separation plates. The exhaust gas is passed through the space between the discharge electrodes and the separation plates. The electrodes cause a strong static charge on particles contained in the exhaust gas, such as liquid droplets or dust particles. In the applied electric field, the thus charged particles migrate to the grounded separation plate, where they are discharged and can collect and be removed.

[0077] After the exhaust gas has passed through the electrostatic precipitator(s), only uncondensed components remain in the exhaust gas, which in this state is referred to as "residual gas" in the present application. In one embodiment, the electrostatic precipitators can be wet electrostatic precipitators. In wet electrostatic precipitators, the separated particles are continuously washed off the separation plate by a liquid film, which enables a consistent thickness of the separated layer and thus better controllable process management than a dry electrostatic precipitator, in which an increasingly thick layer of particles forms on the separation plate and must be removed by regular tapping. As the layer thickness increases, the electric field strength of the precipitator and thus its effectiveness decreases with increasing time intervals between the tapping processes.In addition, the particles are further stirred up by the tapping and some of them have to be separated again.

[0078] Depending on the exact composition of the exhaust gas, the liquid film on the separator plate can either be created by spraying a liquid such as water onto it, or it can form spontaneously through the precipitation of a portion of droplets contained in the cooled exhaust gas. It is also possible to create the liquid film by injecting a liquid such as water into the gas channel between the separator plate and the discharge electrodes, rather than onto the separator plate. Generally, liquids other than water can be used to create the liquid film in an electrostatic precipitator. Particularly advantageous here are low-flammability organic substances that act as electrical insulators, such as silicone oils.

[0079] In one embodiment, the electrostatic precipitator is at least a dry electrostatic precipitator. Dry electrostatic precipitators offer the advantage of being operable even at high temperatures, where most liquids are gaseous.

[0080] Furthermore, the present application relates to the use of carbon fibers as catalysts in hydrocarbon pyrolysis.

[0081] The application relates to the following aspects 1. A method for purifying exhaust gases from the oxidation and / or carbonization of polyacrylonitrile fibers or oxidized polyacrylonitrile fibers, comprising the steps

[0082] • Cooling the exhaust gas to condense condensable components,

[0083] • Separating the cooled exhaust gas into condensed components and residual gas using at least one electrostatic precipitator, whereby the condensed components can be solid or liquid,

[0084] • Binding of ammonia and / or hydrogen cyanide from the residual gas and formation of ammonium compounds and / or salt-like and / or complex cyanide compounds.

[0085] 2. The method according to aspect 1, wherein the electrostatic precipitators are wet electrostatic precipitators.

[0086] 3. Process according to aspect 1 or 2, wherein the separated liquid components are at least partially fed to a tar distillation.

[0087] 4. Method according to one or more of the preceding aspects, wherein the separated liquid and / or solid components are at least partially combusted to generate energy.

[0088] 5. Process according to one or more of the preceding aspects, wherein ammonium carbonate and / or ammonium bicarbonate is obtained from the residual gas.

[0089] 6. Process according to one or more of the preceding aspects, wherein iron(III) hexacyanidoferrate(II / III) is obtained from the residual gas.

[0090] 7. Process according to one or more of the preceding aspects, wherein hydrocarbons from the residual gas are subjected to hydrocarbon pyrolysis to obtain hydrogen and carbon.

[0091] 8. A method according to one or more of the preceding aspects, wherein the method additionally comprises a step in which the residual gas is mixed with air and / or oxygen, the mixture is combusted, and the resulting gas is released into the atmosphere. 9. A device for purifying exhaust gases from the oxidation and / or carbonization of polyacrylonitrile fibers or oxidized polyacrylonitrile fibers, comprising

[0092] • At least one heat exchanger,

[0093] • At least one electrostatic precipitator,

[0094] • At least one reactor

[0095] 10. The apparatus of aspect 9, wherein the electrostatic precipitator is a wet electrostatic precipitator.

[0096] 11.Device according to aspect 9 or 10, wherein the reactor contains at least one device for spraying liquids.

[0097] 12. Device according to one or more of aspects 9 to 11, wherein the reactor contains a region in which solid or liquid aerosols can settle.

[0098] 13. Device according to one or more of aspects 9 to 12, wherein at least two reactors are present.

[0099] 14. Device according to one or more of aspects 9 to 13, wherein at least one reactor is designed as a combustion chamber, wherein the combustion chamber preferably contains a catalyst.Use of at least one electrostatic precipitator for cleaning exhaust gases from the oxidation and / or carbonization of polyacrylonitrile fibers or oxidized polyacrylonitrile fibers.

[0100] Character description

[0101] Figures 1 to 3 show flow diagrams for possible exhaust gas treatment pathways, depending on the type of exhaust gases involved. It is important to emphasize that these illustrations are to be understood as possible embodiments and are not limiting in any way.

[0102] Figure 1 shows the flow diagram of the possible treatment of free exhaust gas from the furnace center exhaust of carbonization furnaces operated with a nitrogen atmosphere. Two different carbonization furnaces ("carbonization furnace 1" and "carbonization furnace 2") are shown. This is due to the fact that in practice, carbonization is often carried out in several furnaces successively at different temperatures. The exhaust gas is first cooled, and the energy from it is recovered and used for another purpose ("energy recovery"). At a temperature of 400°C, the exhaust gas is then fed to an electrostatic precipitator, in which solids containing already solid hydrocarbons, such as polycyclic aromatic hydrocarbons, are separated. Ammonia and hydrogen cyanide are then separated using chemical or physical methods ("gas separation").During the subsequent hydrocarbon pyrolysis (“pyrolysis”), any remaining hydrocarbons in the exhaust gas are split into carbon and hydrogen. The resulting exhaust gas has a temperature of approximately 850°C. The thermal energy contained in the exhaust gas can be recovered and used for other purposes.

[0103] Figure 2 shows a flow diagram for the possible treatment of an oxygen-containing exhaust gas from the furnace center exhaust of an oxidation furnace. The exhaust gas leaves the furnace at a temperature of 200-300°C and is fed into an electrostatic precipitator, where solids containing solid hydrocarbons, such as polycyclic aromatic hydrocarbons, are separated. Ammonia and hydrogen cyanide are then separated using chemical or physical methods (gas separation). The thus pretreated exhaust gas can be released into the atmosphere.

[0104] Figure 3 shows the flow diagram of the possible treatment of an oxygen-containing exhaust gas from the hood exhaust of either a carbonization furnace or an oxidation furnace.

[0105] The exhaust gas leaves the furnace at a temperature of 200-300°C and is fed into an electrostatic precipitator, where solids containing solid hydrocarbons such as polycyclic aromatic hydrocarbons are separated. Ammonia and hydrogen cyanide are then separated using chemical or physical methods (gas separation). The pretreated exhaust gas can be released into the atmosphere.

Claims

Exhaust gas treatment processes Claims: 1 . A process for treating a hydrocarbon-containing exhaust gas from the carbonization of polyacrylonitrile fibers or oxidized polyacrylonitrile fibers, comprising the steps • Introducing the exhaust gases into a reaction chamber, • Separation of hydrocarbons into carbon and hydrogen by means of hydrocarbon pyrolysis to obtain a hydrogen-containing exhaust gas, • Cooling of the hydrogen-containing exhaust gas.

2. Process according to claim 1, wherein the hydrocarbon pyrolysis is catalyzed by solid carbon, preferably soot, coke or carbon fibers.

3. The method according to claim 2, wherein the solid carbon is at least partially already contained in the exhaust gas.

4. Process according to one or more of the preceding claims, wherein solid and / or condensed components are separated prior to hydrocarbon pyrolysis, for example by means of an electrostatic precipitator.

5. Process according to one or more of the preceding claims, wherein the hydrocarbon pyrolysis takes place in a fluidized bed and / or a fixed bed.

6. Process according to one or more of the preceding claims, wherein the hydrocarbon pyrolysis takes place when the exhaust gas is passed through a molten metal.

7. Process according to one or more of the preceding claims, wherein the hydrocarbon pyrolysis takes place in a plasma.

8. Process according to one or more of the preceding claims, wherein the carbon is obtained in the form of soot or coke.

9. Process according to one or more of the preceding claims, wherein the hydrogen is separated from the hydrogen-containing exhaust gas by fractional condensation or by means of gas-permeable membranes.

10. Process according to one or more of the preceding claims, wherein ammonia and / or hydrogen cyanide is separated from the exhaust gas or the hydrogen-containing exhaust gas by acidic or basic gas scrubbing or by adsorption.

11. A process according to one or more of the preceding claims, wherein hydrocarbons condensed from the exhaust gas prior to hydrocarbon pyrolysis are separated as tar.

12. The process according to claim 11, wherein the tar is subjected to tar distillation.

13. Apparatus for carrying out a process according to one or more of claims 1 to 12, the apparatus comprising at least one fixed bed reactor or at least one fluidized bed reactor designed to carry out hydrocarbon pyrolysis.

14. Use of carbon fibers as catalysts in hydrocarbon pyrolysis.

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