Patent Application
20110275868
The present invention relates to a process for preparing low molecular weight aromatic materials of value from a lignin-comprising starting material produced from biomass.
Aromatic compounds having a low molecular weight and specifically phenolic compounds have found wide use as intermediates and products of value. They serve, for example, as precursor for various resins, surface-active compounds, specialty chemicals, etc. It is known that such compounds can be prepared from lignin-comprising starting materials. However, there continues to be a need for a simple, inexpensive process which makes it possible to provide many different aromatic products for various fields of use.
Subjecting streams from various processes for digesting lignin- or lignocellulose-comprising materials to an after-treatment to obtain materials of value is known.
U.S. Pat. No. 2,057,117 describes a process for preparing vanillin, in which a starting material selected from lignocellulose, a crude lignin extract and ligninsulfonic acid is heated with an aqueous alkali metal hydroxide solution under superatmospheric pressure and the reaction mixture obtained is admixed with sulfuric acid in order to precipitate organic constituents and convert the vanillin into a soluble form.
WO 99/10450 describes a process for converting lignin into a hydrocarbon fuel. Here, lignin is subjected to a base-catalyzed depolymerization and subsequently hydroprocessing. This hydroprocessing comprises a hydrodeoxygenation and mild hydrocracking. The latter is carried out under conditions under which partial hydrogenation of the aromatic rings occurs.
WO 2008/027699 A2 describes a process in which lignin originating from a pyrolysis of biomass is, after water-soluble constituents have been separated off, decarboxylated and hydrodeoxygenated and the organic products from this process step are subsequently subjected to hydrocracking.
WO 2010/026244 describes an integrated process for producing pulp and at least one low molecular weight material of value, in which
In an embodiment of the process, a cellulose-enriched fraction and a lignin-enriched fraction are isolated from the digested material, the lignin-enriched fraction is subjected to a depolymerization and an aromatics composition is isolated from the depolymerization product.
WO 2009/108601 describes a process for producing a starting material for biorefinery processes for producing a biofuel from a lignin-comprising starting material. Here, lignin from a black liquor of the pulping process or the black liquor itself is subjected to hydroprocessing in the presence of a hydrogen-comprising gas and a catalyst on an amorphous or crystalline oxidic support. Specifically, a heterogeneous molybdenum sulfide catalyst is used. When black liquor is used, the hydroprocessing can also be carried out in two stages. The process can either be carried out at a refinery site to which the lignin or black liquor is transported or directly on the site of a paper mill. The biorefinery process following hydroprocessing is not described in more detail.
WO 2009/108599 has a disclosure content comparable to WO 2009/108601, with the focus on paper production.
In Angew. Chem. 2008, 120, 9340-9351, M. Stöcker describes the catalytic conversion of lignocellulose-rich biomass to produce BTL (biomass-to-liquid) fuels in biorefineries. The use of a lignin material obtained from the biomass in a pyrolysis to produce biooil and a further work-up to give phenolic resins, synthesis gas, etc., is also shown schematically.
US 2009/0227823 describes a process for producing at least one liquid hydrocarbon product from a solid hydrocarbon starting material (e.g. a lignocellulose material), in which the starting material is subjected to catalytic pyrolysis and the pyrolysis products are subjected to a catalyzed after-reaction to give liquid products.
In Chem. Rev. 2006, 106, 4044-4098, G. W. Huber et al. describe synthesis of fuels from biomass. According to that article, lignocellulose materials can in principle be converted into liquid fuels by means of three routes which differ in their primary step: gasification to synthesis gas, pyrolysis to biooil, hydrolysis to give sugars and lignin. The biooils obtained in the pyrolysis can subsequently be subjected to hydrodeoxygenation in the presence of hydrogen or to steam reforming. However, this document does not teach subjecting a lignin-comprising starting material firstly to pyrolysis and then directly, essentially without a component being separated off and in particular without condensation, to a dealkylation process with the primary purpose of obtaining low molecular weight aromatic materials of value.
It has surprisingly been found that the pyrolysis products obtained in the pyrolysis of lignin-comprising materials can advantageously be subjected to a dealkylation process (or comparable processes to reduce the molecular weight further) without a component being separated off and in particular without preceding condensation with at least partial gas/aromatics separation.
The invention therefore firstly provides a process for preparing an aromatics composition having a high content of single-ring aromatics which are unalkylated or have a low degree of alkylation from a lignin-comprising starting material, wherein
For the purposes of the present invention, an aromatics composition having a high content of single-ring aromatics which are unalkylated or have a low degree of alkylation is a composition which, based on its total weight, comprises at least 50% by weight of single-ring aromatics. The total content of unalkylated, unalkoxylated, of at most monohydroxylated and of monoalkylated aromatics is, based on the total weight of the aromatics composition, at least 50% by weight.
For the purposes of the invention, pyrolysis is a thermal treatment of the lignin-comprising starting material, with molecular oxygen not being introduced or introduced only in a small amount. Here, a small amount is an amount which is significantly smaller than the amount necessary for complete oxidation of the carbon comprised in the lignin-comprising starting material to give CO2. The amount of molecular oxygen introduced in the pyrolysis is preferably at least 50 mol % below, particularly preferably at least 75 mol % below, in particular at least 90 mol % below, the amount necessary for complete oxidation of the carbon comprised in the biomass starting material to give CO2. The pyrolysis generally occurs endothermically.
For the purposes of the invention, “dealkylation” refers to a reaction of the substituted and/or polycyclic aromatic compounds comprised in an aromatics composition in the presence of hydrogen and/or water vapor, with these being at least partly transformed in such a way that substituents are replaced by hydrogen and/or compounds comprising a plurality of aromatic rings are cleaved to give compounds having a lower number of rings. The substituents replaced by hydrogen are selected from alkyl groups, hydroxy groups, alkoxy groups, aryloxy groups, etc. For the purposes of the present invention, the term “dealkylation” also refers to reactions different therefrom which are associated with a decrease in the molecular weight, e.g. dehydroxylation, dealkoxylation, aromatics cleavage. The term aromatics cleavage refers to a reaction in which essentially the number of aromatic rings per molecule is reduced without the aromatic rings themselves being destroyed.
It has been found that carrying out pyrolysis and dealkylation separately as per the invention is particularly advantageous even when the same reactions may partly occur in the two steps.
According to the invention, a lignin-comprising starting material is provided in step a).
Suitable lignin-comprising starting materials are pure lignin and lignin-comprising compositions. Here, the lignin content of the compositions is not critical within a wide range, although at excessively low lignin contents the process can no longer be carried out economically.
Preference is given to a lignin-comprising starting material comprising at least 10% by weight, preferably at least 15% by weight, based on the dry mass of the material, of lignin being provided in step a). Preferred lignin-comprising compositions are compositions comprising from 10 to 100% by weight, particularly preferably from 15 to 95% by weight, based on the dry mass of the material, of lignin. For the purposes of the present invention, the term dry mass is as defined in the standard ISO 11465.
Lignocellulose-comprising materials are also suitable for providing a lignin-comprising starting material for use in the process of the invention. Lignocellulose forms the structural skeleton of the cell walls of plants and comprises, as main constituents, lignin, hemicelluloses and cellulose. Further constituents of the cell walls of plants and therefore of lignocellulose-comprising materials obtained therefrom are, for example, silicates, extractable low molecular weight organic compounds (known as extractables, e.g. terpenes, resins, fats), polymers such as proteins, nucleic acids and plant gum (known as exsudate), etc.
Lignin is a biopolymer whose basic unit is essentially phenylpropane which, depending on the natural source, may be substituted by one or more methoxy groups on the phenyl rings and by a hydroxy group on the propyl units. Typical structural units of lignin are therefore p-hydroxyphenylpropane, guaiacylpropane and syringylpropane which are joined to one another by ether bonds and carbon-carbon bonds.
Suitable starting materials for the process of the invention include both lignocellulose-comprising materials which are used with their natural composition without further chemical treatment, e.g. wood or straw, and lignin-comprising streams from the processing of lignocellulose, e.g. from processes for cellulose production (pulp processes).
The lignocellulose materials which can be used according to the invention can be obtained, for example, from wood fibers and plant fibers as starting material. Preferred lignocellulose materials are those from wood and residues of the wood-processing industry. They include, for example, the various types of wood, e.g. broadleaved tree wood such as maple, birch, pear tree, oak, alder, ash, eucalyptus, common beech, cherry tree, lime, nut tree, poplar, willow, etc. and conifer timbers such as Douglas fir, spruce, yew, hemlock, pine, larch, fir, cedar, etc. Wood cannot be divided only into wood from broadleaved trees and that from conifers but also into “hardwoods” and “softwoods”, which are not synonymous with the terms broadleaved tree timbers and conifer timbers. In contrast to hardwood, the term softwood refers to lighter wood, (i.e. wood having a dried density below 0.55 g/cm3, for example willows, poplars, limes and virtually all conifers). All hardwoods and all softwoods are in principle suitable for use in the process of the invention. The wood used in the process of the invention can also be present in manufactured form, e.g. in the form of pellets. Suitable residues from the wood-processing industry are not only scrap wood but also sawdust, parquetry grinding dust, etc. Further suitable lignocellulose materials are natural fibers such as flax, hemp, sisal, jute, straw, coconut fibers, switchgrass (Panicum virgatum) and other natural fibers. Suitable lignocellulose materials are also obtained as residues in agriculture, e.g. in the harvesting of cereal (wheat straw, maize straw, etc.), maize, sugarcane (bagasse), etc. Suitable lignocellulose materials are also obtained as residue in forestry, e.g. in the form of branches, bark, wood chips, etc. Another good source of lignocellulose materials is short rotation crops which allow high biomass production on a relatively small area.
A lignin-comprising stream from the digestion of a lignocellulose material for producing cellulose (pulp) is preferably used for providing the lignin-comprising starting material. The digestion makes possible an at least partial separation of the lignocellulose-comprising starting material into cellulose and materials accompanying cellulose, with lignin being among the latter.
In a preferred embodiment, a lignocellulose-comprising material is provided in step a), subjected to digestion and a cellulose-enriched fraction and a lignin-enriched (and at the same time cellulose-depleted) fraction are isolated from the digested material.
Processes for digesting lignocellulose-comprising materials to produce cellulose are known in principle. Lignin-comprising streams from all digestion processes known to those skilled in the art are suitable in principle for use in the process of the invention. These processes can basically be divided, on the basis of the treatment medium used, into aqueous-alkaline processes, aqueous-acidic processes and organic processes. An overview of these processes and the digestion conditions may be found, for example, in WO 2010/026244.
The treatment medium used for digesting the lignocellulose-comprising materials is capable of solubilizing at least part of the lignin. The cellulose comprised in the lignocellulose-comprising material, on the other hand, is generally not solubilized or solubilized to only a minor extent in the treatment medium. A cellulose-enriched fraction is then preferably separated off by filtration or centrifugation.
A lignin-comprising (cellulose-depleted) fraction which comprises, in addition to lignin, at least one further component selected from hemicellulose, cellulose, degradation products of the abovementioned components, digestion chemicals and mixtures thereof is preferably isolated from the digested material.
In many cases it is not critical to the pyrolysis in step b) if a lignin-comprising starting material comprising at least one further component in addition to lignin is used.
If a lignin-comprising fraction which comprises at least one further component in addition to lignin is used for providing the lignin-comprising starting material, at least part of the compounds other than lignin can be removed before the pyrolysis in step b). The components removed from the lignin-comprising fraction (organic components and/or inorganic process chemicals) are preferably passed to a further work-up and/or thermal utilization, preferably within the process for cellulose production from which the lignin-comprising fraction was obtained.
To remove at least part of the compounds other than lignin, the pH of the lignin-comprising fraction can firstly be adjusted to a desired value. For adjusting the pH, lignin-comprising fractions from aqueous-alkali processes (such as the Kraft process) can be admixed with an acid. Suitable acids are, for example, CO2, mineral acids such as hydrochloric acid, sulfuric acid and phosphoric acid. Particular preference as acid is given to CO2 (or the carbonic acid resulting therefrom with water). Preference is given to using CO2 from an offgas stream of the process of the invention or of a pulp process coupled to the process of the invention. For example, the offgas from a black liquor combustion (recovery boiler) or a lime kiln is suitable. Here, the offgas can either be introduced directly or after removal of the other components (e.g. by means of a scrubbing process such as a Benfield scrub) into the lignin-comprising fraction. The carbonates and/or hydrogen carbonates formed as a result of the addition of CO2 can generally be recirculated in a simple fashion to the coupled pulp process, e.g. into a black liquor previously taken off to obtain lignin. Use of CO2 for adjusting the pH of the lignin-comprising fraction is thus associated with lower costs than the use of other acids and also generally allows good integration into a pulp process.
Lignin-comprising fractions from aqueous-acidic processes can be admixed with a base to adjust the pH. Suitable bases are, for example, alkali metal bases such as sodium hydroxide or potassium hydroxide, alkali metal carbonates such as sodium carbonate or potassium carbonate, alkali metal hydrogencarbonates such as sodium hydrogencarbonate or potassium hydrogencarbonate and alkaline earth metal bases such as calcium hydroxide, calcium oxide, magnesium hydroxide or magnesium carbonate and also ammonia or amines.
The removal of at least part of the compounds other than lignin from the lignin-comprising fraction in step a) is preferably carried out by filtration, centrifugation, extraction, precipitation, distillation, stripping or a combination thereof. A person skilled in the art can control the composition of the lignin-comprising fraction and thus of the lignin-comprising starting material for the pyrolysis in step b) via the separation process. The at least partial removal of the components other than lignin can be carried out in one or more stages. Customary filtration processes are, for example, cake filtration and deep bed filtration (e.g. as described in A. Rushton, A. S. Ward, R. G. Holdich: Solid-Liquid Filtration and Separation Technology, VCH Verlagsgesellschaft, Weinheim 1996, pages 177ff., K. J. Ives, in A. Rushton (editor): Mathematical Models and Design Methods in Solid-Liquid Separation, NATO ASI Series E No. 88, Martinus Nijhoff, Dordrecht 1985, pages 90ff.) and crossflow filtrations (e.g. as described in J. Altmann, S. Ripperger, J. Membrane Sci. 124 (1997), pages 119-128). Customary centrifugation processes are described, for example, in G. Hultsch, H. Wilkesmann, “Filtering Centrifuges” in D. B. Purchas, Solid-Liquid Separation, Upland Press, Croydon 1977, pages 493-559, and in H. Trawinski, Die äquivalente Klärfläche von Zentrifugen, Chem. Ztg. 83 (1959), pages 606-612. Extraction can be carried out using, for example, a solvent which is not miscible with the treatment medium from pulp production or at least one solvent which has a miscibility gap and in which lignin and optionally further desired components are soluble in a sufficient amount. The removal of components which can be vaporized without decomposition from the lignin-comprising fraction can be carried out by customary distillation processes known to those skilled in the art. Suitable apparatuses for the work-up by distillation comprise distillation columns such as tray columns equipped with bubble caps, sieve plates, sieve trays, ordered packing, random packing elements, valves, side offtakes, etc., evaporators such as thin film evaporators, falling film evaporators, forced circulation evaporators, Sambay evaporators, etc., and combinations thereof.
In a specific embodiment, a lignin-comprising stream from the digestion of a lignocellulose material which still comprises part of the liquid treatment medium from the digestion is used to provide the lignin-comprising starting material in step a). The lignin-comprising stream is then preferably subjected to a precipitation of a lignin-comprising fraction followed by partial or complete removal of the liquid components to provide the lignin-comprising starting material for the pyrolysis carried out in step b).
The provision of the lignin-comprising starting material is preferably carried out in a process for producing cellulose (pulp) into which the preparation according to the invention of low molecular weight aromatic materials of value is integrated.
In a specific embodiment, the removal of at least part of the liquid compounds is then carried out in the process for producing pulp. Thus, for example, a black liquor which is taken off before or during the individual evaporation steps of the parent pulp process can be used for providing the lignin-comprising starting material.
Preference is given to a lignin-comprising stream from the digestion of a lignocellulose material by means of an alkaline treatment medium being used for providing the lignin-comprising starting material in step a). Particular preference is given to using a black liquor, in particular a black liquor from sulfate digestion (kraft digestion). To provide a lignin-comprising material, a black liquor from the kraft digestion can firstly be acidified to precipitate at least part of the lignin comprised and the precipitated lignin can subsequently be isolated. The abovementioned acids are suitable for acidification. In particular, CO2 is used. The pH of the black liquor is preferably reduced to a value of not more than 10.5. The isolation of the precipitated lignin is preferably carried out by means of a filtration process. Suitable filtration processes are those mentioned above. If desired, the isolated lignin can be subjected to at least one further work-up step. Such steps include, for example, further purification, preferably washing with a suitable washing medium. Suitable washing media are, for example, mineral acids such as sulfuric acid, preferably in aqueous solution. In a specific embodiment, a black liquor from the kraft digestion is then firstly acidified by means of CO2 to precipitate at least part of the lignin comprised, the precipitated lignin is subsequently isolated by filtration and the filtrate is subjected to washing with sulfuric acid to provide a lignin-comprising material. A process for isolating lignin from a black liquor by precipitation using CO2 is described in WO 2008/079072, which is hereby incorporated by reference. The “lignoboost” process described in WO 2006/038863 (EP 1797236 A1) and WO 2006/031175 (EP 1794363 A1), which are likewise incorporated by reference, is also particularly suitable.
In step b) of the process of the invention, the lignin-comprising starting material is subjected to a pyrolysis to give a pyrolysis gas stream.
The pyrolysis can be carried out batchwise or continuously. Continuous pyrolysis is preferred.
The pyrolysis is carried out in at least one pyrolysis zone. The lignin-comprising starting material, for example as a moist or predried solid, can be introduced into a pyrolysis zone by means of suitable transport devices, e.g. screw conveyors or pneumatic transport.
The pyrolysis zone can have various embodiments, e.g. as rotary tube furnace or fluidized bed. Both stationary and circulating fluidized beds are suitable. Furthermore, in the embodiment of the pyrolysis zone as fluidized bed, a fluidizing gas (preferably steam or a gas mixture from one of the subsequent process steps) and as fluidized material a particulate material which is inert under the prevailing conditions are introduced. A particularly suitable material is silica sand. Such a fluidized-bed process is described, for example, in U.S. Pat. No. 4,409,416 A. In an alternative embodiment, the pyrolysis zone comprises at least one fixed bed. The fixed beds can comprise at least one inert fixed bed and/or at least one catalytically active fixed bed. If the process of the invention is operated using at least one fixed bed as pyrolysis zone, operation in cycles, where a pyrolysis phase is followed by a burning-off phase in order to remove relatively nonvolatile components from the fixed bed, can be advantageous.
For the pyrolysis, a fluidizing gas can be fed into the pyrolysis zone. Preferred fluidizing gases are steam, carbon dioxide, nitrogen, etc., or mixtures of these gases. Preference is given to using a diluent gas which under the conditions of the pyrolysis undergoes essentially no reaction with the lignin-comprising starting material or the pyrolysis products resulting therefrom. In a first preferred embodiment, the pyrolysis is not carried out with addition of hydrogen and/or hydrogen-comprising gases and/or hydrogen-donating compounds. In this embodiment, the hydrogenating reaction is carried out essentially exclusively in the dealkylation step c). In a second suitable embodiment, the pyrolysis is carried out with addition of hydrogen and/or hydrogen-comprising gases and/or hydrogen-donating compounds. This embodiment of the pyrolysis can also be referred to as hydrocracking.
The pyrolysis can, if desired, be carried out in the presence of at least one pyrolysis catalyst. Suitable pyrolysis catalysts are, for example, silica, alumina, aluminosilicates, aluminosilicates having sheet structures and zeolites such as mordenite, faujasite, zeolite X, zeolite Y and ZSM-5, zirconium oxide or titanium dioxide.
The temperature in the pyrolysis is preferably in the range from 200 to 1500° C., particularly preferably from 250 to 1000° C., in particular from 300 to 800° C.
The pressure in the pyrolysis is preferably in the range from 0.5 to 250 bar (absolute), preferably from 1.0 to 40 bar (absolute).
The residence time at the pyrolysis temperature can be from a few seconds to a number of days. In a specific embodiment, the residence time at the pyrolysis temperature is from 0.5 second to 5 minutes, especially from 2 seconds to 3 minutes. The residence time, especially in a fluidized-bed reactor, is given by the total volume of the reactor divided by the volume flow of the fluidizing gas under the pyrolysis conditions.
Suitable processes for the catalyzed or uncatalyzed pyrolysis of lignin are also described, for example, in WO 96/09350 (Midwest Research Institute, 1996) or U.S. Pat. No. 4,409,416 (Hydrocarbon Research Institute, 1983), which are hereby incorporated by reference.
In the pyrolysis zone, the lignin is converted into a mixture of components which under the conditions of the pyrolysis are partly present in gaseous form (“pyrolysis gas”) and partly in solid and/or liquid form (e.g. tar-like or as “carbonaceous deposits”).
A discharge which can comprise not only the pyrolysis gases but also proportions of solid and/or liquid components is taken off from the pyrolysis zone. These solid and/or liquid components are, for example, relatively nonvolatile components (carbonaceous deposits) formed in the pyrolysis. If at least one solid inert material is used for the pyrolysis in step b), the discharge from the pyrolysis zone can also comprise proportions of the inert material. These solid and/or liquids components can be separated off from the pyrolysis gas by means of a suitable apparatus, e.g. a cyclone. Solid inert material which has been separated off is preferably recycled to the pyrolysis zone. Components separated off other than inert material are fed to another use, e.g. combustion for generating heat which is preferably used again in the process of the invention or an integrated process. The combustion gas obtained, which comprises predominantly CO2 and also water and optionally O2, can likewise be fed to a use. It is also possible to bring a discharge from the pyrolysis zone which comprises at least one inert material and components which are relatively nonvolatile under the pyrolysis conditions into contact with an oxygen-comprising gas, preferably air, in a burning-off zone which is physically separate from the pyrolysis zone, leading to burning-off of relatively nonvolatile components (“carbonaceous deposits”) formed in the pyrolysis. The material is then separated off from the combustion gas formed by means of a suitable separation apparatus and returned to the pyrolysis zone by means of a suitable transport device.
The components separated off directly from the discharge from the pyrolysis zone, i.e. solid and/or liquid components which are not vaporizable (=gaseous) under the pyrolysis conditions, are according to the invention not counted as part of the pyrolysis gas and for the purposes of the invention are not considered to be material components separated off from the pyrolysis gas.
The pyrolysis gas stream obtained in step b) comprises substituted aromatics and/or polycyclic aromatics. The pyrolysis gas stream can, in addition to aromatics, comprise further components selected from water vapor, inert gas (e.g. nitrogen), nonaromatic hydrocarbons, H2, CO, CO2, sulfur-comprising compounds such as H2S, etc., and mixtures thereof. The nonaromatic hydrocarbons are preferably degradation products such as methane.
In the dealkylation, the aromatic lignin degradation products formed in the pyrolysis in step b) are at least partly transformed by action of hydrogen and/or water vapor so that substituents are replaced by hydrogen and/or compounds comprising a plurality of aromatic rings are cleaved to form compounds having a smaller number of rings. As indicated above, the term “dealkylation” also refers to reactions in which no alkyl substituent is replaced by hydrogen, e.g. dehydroxylation, dealkoxylation, aromatics cleavage, etc. The substituents replaced by hydrogen are preferably selected from alkyl groups, hydroxy groups, alkoxy groups and aryloxy groups.
Dealkylation processes suitable for use in step c) comprise hydrodealkylation, steam dealkylation or mixed forms derived therefrom. In the case of a pure hydrodealkylation in the context of the invention, molecular hydrogen (in pure form or in admixture with other components, such as CO), but no water is fed into the dealkylation zone in addition to the pyrolysis gas stream. In the case of a pure steam dealkylation in the context of the invention, water (in pure form or in admixture with other components), but no molecular hydrogen is fed into the dealkylation zone in addition to the pyrolysis gas stream. The dealkylation process in step c) can also be configured as a mixed form of hydrodealkylation and steam dealkylation. Both water and molecular hydrogen are then fed into the dealkylation zone in addition to the pyrolysis gas stream. Suitable and preferred process parameters, partly specific to hydrodealkylation and partly to steam dealkylation, are indicated below. Using this information, a person skilled in the art will be able to determine suitable and preferred process parameters for a mixed form of hydrodealkylation and steam dealkylation. The reaction gas used for the dealkylation preferably then has a mixing ratio of H2 to H2O in the range from about 0.1:99.9 to 99.9:0.1. An especially suitable mixing ratio of H2 to H2O is in the range from about 40:60 to 60:40.
The hydrogen required for the reaction is, in the case of the steam dealkylation, formed in-situ by reaction of water with (mainly organic) components which are either comprised in the starting mixture for the steam dealkylation or are formed during the steam dealkylation. An example which may be mentioned here is the formation of hydrogen from methane and water according to the equation CH4+H2O→CO+3 H2.
It is an important feature of the process of the invention that the pyrolysis gas stream from step b) is fed into the dealkylation zone essentially without a material component being separated off. Here, components which are relatively nonvolatile under the conditions of the pyrolysis in step b) and are not present in gaseous form but instead in solid or liquid form in the discharge from the pyrolysis zone are not counted as part of the pyrolysis gas stream.
For the purposes of the present invention, “essentially without a material component being separated off” means that not more than 10% by volume is separated off from the pyrolysis gas stream obtained in step b) before the pyrolysis gas stream is fed into the dealkylation zone. Preference is given to not more than 1% by volume, particularly preferably not more than 0.5% by volume, being separated off from the pyrolysis gas stream obtained in step b) before the pyrolysis gas stream is fed into the dealkylation zone.
In particular, essentially no components comprised in the pyrolysis gas stream obtained in step b) are condensed out from this pyrolysis gas stream before it enters the dealkylation zone. In particular, not more than 10% by weight, particularly preferably not more than 5% by weight, in particular not more than 0.5% by weight, especially not more than 0.1% by weight, of the condensable components comprised in the pyrolysis gas stream obtained in step b) are condensed out from this pyrolysis gas stream before the pyrolysis gas stream is fed into the dealkylation zone. The pyrolysis gas stream is maintained under suitable conditions which essentially rule out condensation of components comprised therein.
The temperature of the pyrolysis gas stream before it enters the dealkylation zone is preferably never more than 200° C. below, particularly preferably never more than 100° C. below, the exit temperature from the pyrolysis zone.
The temperature in the dealkylation zone is preferably in the range from 400 to 900° C., particularly preferably from 500 to 800° C.
The absolute pressure in the dealkylation zone is preferably in the range from 1 to 100 bar, particularly preferably from 1 to 20 bar, in particular from 1 to 10 bar.
In a first preferred embodiment, the pyrolysis gas stream is subjected to a hydrodealkylation in step c). For this purpose, the reaction in step c) is carried out in the presence of hydrogen.
The temperature in the dealkylation zone for the hydrodealkylation is preferably in the range from 500 to 900° C., particularly preferably from 600 to 800° C.
The absolute pressure in the dealkylation zone for the hydrodealkylation is preferably in the range from 1 to 100 bar, particularly preferably from 1 to 20 bar, in particular from 1 to 10 bar.
The ratio of the amount of H2 used to H2 (stoichiometric) in the hydrodealkylation is preferably in the range from 0.02 to 50, particularly preferably from 0.2 to 10. Here, H2 (stoichiometric) is the amount of H2 is theoretically required for complete conversion of the aromatics fed into the dealkylation zone into benzene, with the assumption that 1 mol of H2 reacts per ring substituent.
The residence time in the dealkylation zone for the hydrodealkylation is preferably in the range from 0.1 to 500 s, particularly preferably from 0.5 to 200 s.
In a second preferred embodiment, the pyrolysis gas stream is subjected to a steam dealkylation in step c). For this purpose, the reaction in step c) is carried out in the presence of water vapor.
The temperature in the dealkylation zone for the steam dealkylation is preferably in the range from 400 to 800° C., particularly preferably from 475 to 600° C., in particular from 525 to 600° C.
The absolute pressure in the dealkylation zone for the steam dealkylation is preferably in the range from 1 to 100 bar, particularly preferably from 1 to 20 bar, in particular from 1 to 10 bar.
The ratio of the amount of H2O used to C* in the steam dealkylation is preferably in the range from 0.1 to 20 mol/mol, particularly preferably from 0.5 to 2 mol/mol. C* is the molar amount of carbon determined by carbon-based balancing of the pyrolysis or by determination of the amounts of product discharged from the steam dealkylation by means of methods known to those skilled in the art.
The molar ratio of H2 to CH4 in the dealkylation zone for the steam dealkylation is preferably <50:1, particularly preferably <25:1.
In a steam dealkylation in the absence of a dealkylation catalyst, the molar ratio of OR (where R=H, alkyl) to Ctotal in the dealkylation zone is preferably >0.05:1, particularly preferably in the range from 0.1:1 to 0.2:1.
In a steam dealkylation in the absence of a dealkylation catalyst, the ratio of OR (where R=H, alkyl) to Celiminatable in the dealkylation zone is preferably >0.5:1, particularly preferably in the range from 1:1 to 10:1, in particular from 1:1 to 2:1.
The WHSV in the steam dealkylation is preferably in the range from 0.05 to 10 kg/l*h, particularly preferably from 0.1 to 2 kg/l*h.
The steam dealkylation can be carried out in the presence or absence of a catalyst. In a specific embodiment, the steam dealkylation is carried out in the absence of a catalyst. A catalyzed process for steam dealkylation is described in WO 2008/148807 A1. This document and the references cited therein in respect of suitable catalysts are hereby fully incorporated by reference. Further information on catalyst types and process steps for steam dealkylation may be found in WO 2007/051852 A1, WO 2007/051851 A1, WO 2007/051855 A2, WO 2007/051856, WO 2008/135581 A1 and WO 2008/135582 A1 (EP 2008055585), without being restricted thereby. U.S. Pat. No. 3,775,504 states that a steam dealkylation actually comprises a combination of steam dealkylation and hydrodealkylation, since it is inherent in the system that at least part of the hydrogen produced is immediately reacted again.
At least one low molecular weight aromatic material of value is formed as target product of the process of the invention in the dealkylation step c). The low molecular weight aromatic materials of value are preferably selected from benzene and phenolic compounds such as phenol and/or dihydroxybenzenes.
They have, in particular, smaller proportions of the following components than the pyrolysis discharge before introduction into the dealkylation step c): monoalkylated, dialkylated and polyalkylated phenols; alkoxyphenols such as methoxyphenols; polyalkylated benzenes; compounds comprising two or more aromatic rings. These components will hereinafter be referred to as “aromatics which are not dealkylated or have a low degree of dealkylation”.
Separation of the Discharge from the Dealkylation Zone (Step d)
A discharge is taken from the dealkylation zone and subjected to a separation to obtain the aromatics composition having a high content of single-ring aromatics which are unalkylated or have a low degree of alkylation which is wanted as target product.
The discharge from the dealkylation zone is preferably subjected to a separation to give the following three streams:
The discharge from the dealkylation zone can optionally be subjected to a separation to give further streams, e.g. a water-comprising stream.
Stream D1) is the aromatics composition having a high content of single-ring aromatics which are unalkylated or have a low degree of alkylation which is prepared in the process of the invention. In addition, stream D1) can be subjected to a further work-up to give the aromatics composition prepared according to the invention.
Stream D1) comprises, based on the total amount of D1), preferably at least 70% by weight, particularly preferably at least 80% by weight, in particular at least 90% by weight, of single-ring aromatics.
Stream D1) comprises, based on the total amount of D1), preferably not more than 30% by weight, particularly preferably not more than 20% by weight, in particular not more than 10% by weight, of aromatics which are not dealkylated or have a low degree of dealkylation.
As mentioned above, the term “dealkylation” also refers, for the purposes of the invention, to the replacement of substituents other than alkyl groups (e.g. alkoxy groups, aryloxy groups, hydroxy groups, etc.) by hydrogen. Depending on the composition of the lignin-comprising starting material provided in step a) and/or of the pyrolysis gas stream obtained in step c), stream D1) then also has a high content of aromatics in which a substituent other than alkyl groups has been replaced by hydrogen. In particular, stream D1) has a high content of aromatics which are unalkoxylated or have a low degree of alkoxylation.
Stream D2) comprises, based on the total amount of D2), preferably at least 70% by weight, particularly preferably at least 80% by weight, in particular at least 90% by weight, of aromatics which are not dealkylated or have a low degree of dealkylation.
Stream D3) comprises components which are, for example, selected from nonaromatic hydrocarbons, especially methane, hydrogen, carbon monoxide, carbon dioxide and mixtures thereof. Depending on the lignin-comprising starting material provided in step a), the stream D3) can comprise further components. When a lignin-comprising starting material from the kraft process is used, these include sulfur-comprising by-products, especially H2S.
A gaseous discharge is preferably taken from the dealkylation zone and subjected to a separation in step d).
As process for the separation, it is possible to use the generally known thermal separation processes.
The separation of the discharge from the dealkylation zone in step d) preferably comprises an absorption. In the absorption, the gaseous discharge from the dealkylation zone is brought into contact with a solvent (absorption medium), with part of the components comprised in the gas stream being absorbed and thus separated off.
The absorption is carried out in a suitable apparatus, e.g. a countercurrent column, bubble column, etc. The absorption is preferably carried out in a countercurrent column.
The absorption can have one or more stages.
The absorption is preferably carried out using a solvent (not loaded: absorbent, loaded: absorbate) in which the aromatics obtained in the dealkylation are soluble in a sufficient amount and the volatile by-products different therefrom are essentially insoluble. The aromatics which are not dealkylated or have a low degree of dealkylation are at least partly absorbed together with the single-ring aromatics which are unalkylated or have a low degree of alkylation (=target product).
The absorption thus firstly comprises an absorbate loaded with aromatics. The aromatic components comprised in the absorbate correspond in terms of their composition to the sum of the aromatics in streams D1) and D2) plus optionally aromatics comprised in the absorption medium. The components comprised in the remaining gas stream correspond in terms of their composition to stream D3). If desired, the gas stream can be subjected to an additional purification step to remove aromatics. These can then once again be combined with the aromatics comprised in the solvent separated off for joint work-up. However, such an isolation of aromatics from the gas stream separated off is generally not necessary.
In a preferred embodiment, the separation of the discharge from the dealkylation zone in step d) comprises the following substeps:
The absorption medium preferably has a boiling point which is above the boiling point of the components of the stream D1. Furthermore, the absorption medium preferably has a high solvent capability for the aromatics formed in the dealkylation step. Suitable solvents are, for example, aliphatic, cycloaliphatic and aromatic hydrocarbons, aliphatic, cycloaliphatic and aromatic alcohols, amides such as N-methylpyrrolidone or dimethylformamide. Aliphatic, cycloaliphatic and aromatic hydrocarbons preferably have at least 6 carbon atoms. Aliphatic, cycloaliphatic and aromatic alcohols preferably have at least 4 carbon atoms.
In a particularly preferred variant, an aromatics composition which can be obtained by the process of the invention is used as solvent. This is in particular a mixture of aromatics which were not reacted or were incompletely reacted in the dealkylation. In a particularly preferred variant, a substream of stream D2 or a mixture of D1 and D2 is used as solvent. In this variant, the solvent can be obtained by partial condensation of the stream from the dealkylation or of a gas stream from a preliminary removal of high boilers following the dealkylation. Here, it can be advantageous to insert a further partial condensation in which, in particular, water is condensed out between the abovementioned partial condensation and the absorption.
In this variant too, an at least partial absorption of the unreacted or incompletely reacted aromatics takes place together with the absorption of product of value, i.e. in this variant too, the composition of the aromatic components comprised in the absorbate corresponds to the sum of the aromatics of the streams D1) and D2).
In step d2), the aromatics-enriched absorbate is preferably separated by distillation. The solvent recovered here is, optionally after removal of absorbed water, recycled to the absorption (step d1). The aromatics are processed further as described above and below.
In step d2), the aromatics-enriched absorbate is preferably separated by distillation in at least one column (“regeneration column”). The distillation conditions are preferably selected so that essentially aromatics which are unalkylated or have a low degree of alkylation and, if present, water are obtained as overhead product and essentially the aromatics which are not dealkylated or have a low degree of dealkylation are obtained as bottom product.
It goes without saying that the temperature at the bottom in the separation by distillation in step d2) is selected at such a low value that undesirable secondary reactions of the bottom product are essentially avoided. This can, be achieved in particular, by setting a suitable column pressure and/or via the low boiler content in the bottoms (the low boiler content can be reduced further by downstream stripping).
The overhead product obtained in the distillation in step d2) comprises the target product of the process of the invention. It can either be taken off directly as stream D1) or be subjected to a further work-up. Water comprised in the overhead product can be separated off by known methods. The overhead product after condensation of the vapor from the distillation can for this purpose be fed to a phase separator to separate off the water. The resulting water is discharged as a further stream from the process. The organic phase from the phase separator can either be taken off at least partly as stream D1) or be subjected to a further work-up. The organic phase from the phase separator can partly be recycled as runback to the column and/or be subjected to a further work-up by distillation. This preferably serves to remove water still present and/or undesirable organic components.
The bottom product obtained in the distillation in step d2) comprises the aromatics which have not been reacted or not been sufficiently reacted in the dealkylation, i.e. it is enriched in aromatics which are not dealkylated or dealkylated to only a small extent. It can either be taken off directly as stream D2) or be subjected to a further work-up. The bottom product obtained in the distillation in step d2) is preferably divided into at least two substreams. A first substream is preferably recycled to the absorptive separation of the discharge from the dealkylation zone in step d), e.g. as solvent. For this purpose, this substream is, if necessary, cooled to a suitable temperature. A second substream is taken off as stream D2). This stream D2) is preferably at least partly recycled to the dealkylation zone of step c). The stream D2 can be subjected to removal of constituents which do not belong to stream D2 before it is recycled to the dealkylation zone of step c). This is advantageous, for example, when an absorption solvent which is not obtained as intermediate in the process of the invention is used. It is also advantageous to take off a purge stream from stream D2) at this point and, for example, pass it to a combustion apparatus in order to reduce the accumulation of components which do not react or react only slowly under the conditions of the dealkylation.
The stream D2) is preferably subjected to vaporization before being fed into the dealkylation. A preferred variant is shown in
The stream D3) obtained step d), which is depleted in aromatics and enriched in volatile by-products, can be passed to various uses. These include, firstly, combustion. If the process of the invention is in the physical vicinity of a pulp process, it can be advantageous to feed stream D3) into an apparatus of the pulp process. The stream D3) is particularly preferably fed into the waste liquor combustion (recovery boiler). This embodiment has the advantage that no additional apparatuses for steam or power generation or for flue gas desulfurization in the case of combustion of the stream D3) are required. In another variant, the combustion of stream D3) is preceded by a desulfurization, e.g. in the form of a gas scrub to remove hydrogen sulfide, followed by conversion of the H2S formed into elemental sulfur. The formation of sulfur can be effected by known methods, e.g. the Claus process.
Preference is given to using at least part of the stream D3) obtained in step d) for producing synthesis gas. If, according to the above-described preferred embodiment of the process of the invention, the separation of the discharge from the dealkylation zone in step d) comprises an absorption, the gas stream leaving the absorption apparatus (stream D3) is, optionally after a purification step to remove absorption medium and/or aromatics, preferably at least partly used for producing synthesis gas.
For the purposes of the invention, the term “synthesis gas” refers to a gas mixture comprising carbon monoxide and hydrogen. This gas mixture can additionally comprise further gases such as CO2, CH4, etc. The process of the invention advantageously makes it possible to produce synthesis gas having a high content of carbon monoxide and hydrogen.
At least one further stream comprising, for example, water vapor and/or oxygen can optionally be used for synthesis gas production. It is also possible to use an offgas stream from the pyrolysis in step b) and/or the dealkylation in step c) in the production of synthesis gas. This can be, for example, a burning-off gas from the combustion of relatively nonvolatile components. Introduction of such an offgas stream enables the H2/CO ratio of the synthesis gas to be reduced.
The production of synthesis gas preferably comprises the following stages:
The way in which the production of synthesis gas is carried out corresponds to the prior art as described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, article “Gas Production”, DOI: 10.1002/14356007.a12—169.pub2.
In a preferred variant, all or part of the synthesis gas stream 11 produced in the process of the invention is, (if necessary after further purification steps known per se to remove water, sulfur-comprising components, CO2, etc.) used in at least one process which consumes hydrogen, CO or mixtures of the two. Such a process can be, for example, a hydrogenation, hydroformylation, carbonylation, methanol synthesis, synthesis of hydrocarbons by the Fischer-Tropsch process, etc.
In a preferred embodiment of the process of the invention, a synthesis gas-comprising stream produced in the process or a hydrogen-enriched stream produced from the synthesis gas is fed into the pyrolysis in step b) and/or the dealkylation in step c). Enrichment of the synthesis gas with hydrogen can, as described above, be effected by means of the water gas shift reaction.
Preference is given a stream comprising synthesis gas produced in the process or a hydrogen-enriched stream produced from the synthesis gas being fed into the dealkylation in step c). The particular advantage of this variant is that the proportion of phenol(s) in the products of the dealkylation is higher than in the pure steam dealkylation, i.e. without introduction of hydrogen. This advantage is obtained overall in the dealkylation in the presence of hydrogen. The higher phenol formation represents an economic advantage since phenol is a more valuable material than oxygen-free aromatics such as benzene. In addition, hydrogen which is not produced in the process of the invention is more expensive and in many cases unavailable or obtainable only with a high degree of difficulty, especially when the dealkylation is to be carried out away from an integrated chemical site.
A preferred embodiment of the process of the invention is shown in
A lignin-comprising starting material (1) is subjected to a pyrolysis.
The pyrolysis gas (2) is fed together with a hydrogenation gas stream (5) to a dealkylation unit, essentially without a condensation or other separation of the stream being carried out.
The discharge from the dealkylation zone (6) is subjected to a separation into the following three streams:
A water stream is optionally separated off from the discharge from the dealkylation zone (6) and discharged.
Stream (7) is, optionally after a further work-up, taken off as product stream.
The stream (8) enriched in aromatics which have not been dealkylated or been dealkylated to only a small extent is conveyed via a vaporization back to the dealkylation. A preferred embodiment of the vaporization is depicted in
The stream (9) comprising the volatile by-products from the separation can be passed to various uses, e.g. a suitable combustion; in the case of close physical proximity to a pulp process it is advantageous to feed stream (9) to an apparatus of this pulp process, particularly preferably to the waste liquor combustion (recovery boiler). This embodiment has the advantage that no additional apparatuses for steam or power generation or flue gas desulfurization are required. In another variant, the combustion is preceded by a desulfurization, e.g. in the form of a gas scrub to remove hydrogen sulfide, followed by conversion of the H2S into elemental sulfur (e.g. Claus process).
In a particularly preferred embodiment, stream (9) is fed to a reforming unit in which the organic components comprised are converted, optionally with introduction of an optional stream (10) comprising water or oxygen, into a synthesis gas (11) comprising CO and H2.
If the dealkylation is a hydrodealkylation, a hydrogen-comprising stream (12) obtained from the production of synthesis gas can be fed into the dealkylation.
If hydrogen is used for the pyrolysis, a hydrogen-comprising stream (13) obtained from the production of synthesis gas can be fed into the pyrolysis.
The aromatics stream (8) is preheated in apparatus A to a temperature at which no appreciable decomposition yet occurs in the liquid phase. This preheated stream (stream 100) is fed together with a gaseous stream (stream 200) whose amount, temperature and composition are selected so that the stream 100 is partially or fully vaporized into an apparatus B. The stream 200 comprises reactants for the dealkylation, i.e. in the case of steam dealkylation water vapor and in the case of hydrodealkylation a hydrogen-comprising gas (stream 5 in
Apparatus B is a liquid-gas contact apparatus according to the prior art, e.g. a vessel having a jet nozzle or a column, with stream 100 being introduced from the top. Liquid and gas are conveyed in cocurrent or countercurrent and a relatively nonvolatile residue (stream 250) can optionally be taken off in the lower part. As an alternative, apparatus B can also be configured as a fluidized bed. Additional energy can be introduced efficiently into the stream 100 via the externally heated fluidized material.
In a preferred variant, stream 300 is divided into streams 400 and 500, with stream 400 being recycled to the dealkylation and stream 500 being recycled via a heat exchanger C to apparatus B. This variant allows the temperatures of the streams 100, 200 and 500 (downstream of the heat exchanger) to be limited to limiting values determined by the availability of the heat sources, the thermal stability of the materials and the stability of the materials of construction. The pressure drop which naturally occurs along the streams 300, 400 and 500 is compensated by means of a suitable apparatus for compression. Here, it is possible to use generally known compressors or blowers. However, it is also possible to configure the apparatus B completely or partially as a liquid jet blower, with stream 100 being used as driving medium. In this case it is possible, if the amount of stream 100 is not sufficient for the compressing power required, to circulate liquid via apparatus B.
| Number | Date | Country | |
|---|---|---|---|
| 61332200 | May 2010 | US |
