Production of acetic acid anhydride

Abstract

Disclosed is a process for producing acetic acid by reacting wood with acetic anhydride, thereby forming acetylated wood and acetic acid. The wood is rubberwood, which allows producing acetic acid devoid of terpene and / or terpene-derived impurities.

Description

[0001] Title: PRODUCTION OF ACETIC ACID ANHYDRIDE

[0002] Field of the Invention

[0003] The invention relates to a process for the production of acetic anhydride from acetic acid recovered from a wood acetylation process. The invention also relates to the production of acetic acid, and to a wood acetylation process.

[0004] Background

[0005] Acetylation of lignocellulosic materials, hereinafter referred to as wood, is a well-recognized process by which various properties of these materials can be improved. This relates, inter alia, to durability and dimensional stability.

[0006] Wood acetylation processes generally have in common that wood is subjected to contact with an acetylation fluid, under suitable acetylation conditions. A background reference is WO 2009 / 095687. Herein a process is described for the acetylation of wood, comprising the steps of submerging wood into an acetylation liquid in a reaction pressure vessel, conducting an impregnation procedure, removing excess acetylation fluid, introducing an inert fluid (typically nitrogen gas, the inert fluid possibly comprising noninert acetic anhydride and / or acetic acid) into the vessel, circulating and heating the inert fluid following a heating regime so as to bring about suitable acetylation of the wood, and removing the circulating fluid and allowing the acetylated wood to cool.

[0007] Acetylation fluids generally are selected from acetic acid, acetic anhydride, and mixtures thereof. After the wood acetylation process, utilized acetylation fluid is removed from the wood. It is thereby desired to avoid wasting the removed acetylation medium, e.g., by recirculating it, and re-using it in wood acetylation. In the event that acetic anhydride, or a mixture of acetic anhydride and acetic acid, is used as the acetylation fluid, the recovered acetylation fluid includes unreacted acetic anhydride. Such acetic anhydride is desirably recirculated to the wood acetylation process.

[0008] The utilized acetylation fluid resulting from the acetylation of wood with acetic anhydride, comprises a substantial amount of acetic acid. Such acetic acid is a by-product of the acetylation of wood, in which the acetic anhydride undergoes an esterifying reaction with hydroxyl groups in the wood, thereby yielding acetic acid. This can be represented according to the equation:

[0009] [wood]-OH + CH3-C(=O)-O-C(=O)-CH3[wood]-O-C(=O)-CHs + CHs-C(=O)-OH

[0010] Accordingly, this process of acetylating wood, effectively presents a source of acetic acid. It is desired to separate off such acetic acid, and put this to separate use, sell it as a chemical, and / or use it in the production of ketene and, particularly, in the production of acetic anhydride. More particularly, the latter acetic anhydride is desirably used again in the acetylation of wood.

[0011] However, the source of acetic acid, viz., recovered from the acetylation of wood, comes with inherent limitations to their further use. This is due to the presence of terpene and terpenoid impurities from the wood. Such impurities, inter alia, are not acceptable in a ketene production process, as the high temperatures thereby involved are known to result in coke formation, causing clogging of the equipment (typically a ketene furnace) used. In the art, therefore, several methods have been proposed for the purification of acetic acid recovered from wood acetylation, with emphasis on finding purification steps that serve to effectively remove terpenes and / or terpene-derived impurities.

[0012] As such, a suitable method is disclosed in WO2022 / 023452. The method involves subjecting the recovered acetylation fluid to cooling to below the melting point of acetic acid under the formation of crystals, and separating the crystals from the fluid. Preferably, the process comprises a plurality of subsequent melting and re -crystallizing steps. Whilst the method results in a high quality purification, it requires installing suitable equipment to conduct a crystallisation process and retrieve crystals, and it requires additional energy input associated with re-melting and recrystallization.

[0013] Another purification method is addressed in WO 2009 / 120257. Herein acetic acid comprising the aforementioned impurities is subjected to azeotropic distillation with water in a multi-tray distillation column. The process as disclosed specifically requires employing a type of distillation equipment that is expensive in terms of building and equipment costs. Also, the process is described with reference to a relatively large amount of condensate being refluxed. This inevitably comes with additional energy costs, since such a reflux results in cooling down of the distillation column.

[0014] Yet another method is disclosed in WO 2009 / 114070, essentially involving solvent extraction of terpenes and terpenoids. Acetic acid containing terpene and terpenoid impurities is combined with water and an organic solvent, such as cyclohexane, which is immiscible with acetic acid. This serves to form a separating composite extraction medium, the components of which are separated into an organic phase and an aqueous acid phase. The terpene and terpenoid impurities are said to be concentrated in the organic phase. As a result, purified acetic acid is recovered with the aqueous phase. The use of a water-immiscible organic solvent generally presents an additional health and safety burden for the recovery of acetic acid, which typically occurs in a plant that would otherwise operate with aqueous-based processes.

[0015] It would be desired to provide a process for producing acetic acid by means of the acetylation of wood, without a need to conduct steps to remove terpene and / or terpene-derived impurities in order to produce suitable acetic acid. Particularly, it would be desired to provide a process for the production of ketene from acetic acid comprising acetic acid obtained from wood acetylation without subjecting the acetic acid to an additional purification step, to remove terpene and / or terpene-derived impurities.

[0016] Summary of the Invention

[0017] In order to better address one or more of the aforementioned issues, the invention presents, in one aspect, a process for the production of acetic acid, the process comprising reacting an acetylation fluid comprising acetic anhydride with wood so as to form esterified wood and a utilized acetylation fluid comprising acetic acid, recovering the utilized acetylation fluid from the wood, subjecting the utilized acetylation fluid to a step separating acetic acid from acetic anhydride, and obtaining the acetic acid, wherein the wood is rubberwood.

[0018] In another aspect, the invention is a process for the production of acetic acid, the process comprising reacting acetic anhydride with rubberwood so as to form acetylated rubberwood and acetic acid, recovering a post- acetylation fluid comprising acetic acid and residual acetic anhydride, and removing the residual acetic anhydride from the post-acetylation fluid to obtain a terpene-free acetic acid.

[0019] In a further aspect, the invention resides in a process for the preparation of ketene by subjecting acetic acid to a ketene-forming thermal treatment, the acetic acid comprising acetic acid recovered from a wood acetylation process, said wood acetylation process comprising subjecting wood to acetylation with an acetylation fluid selected from the group consisting of acetic anhydride, acetic acid, and mixtures thereof, wherein the wood is rubberwood.

[0020] In yet another aspect, the invention concerns the use of rubberwood in a process for preparing acetic acid by reacting the rubberwood with acetic anhydride, thereby forming acetylated rubberwood and acetic acid.

[0021] In a still further aspect, the invention presents the use of rubberwood in a process for preparing acetylated rubberwood by reacting the rubberwood with acetic anhydride, for the purpose of producing acetic acid devoid of terpene and / or terpene-derived impurities.

[0022] Detailed description

[0023] The invention is based on the judicious insight that the acetylation of rubberwood does not result in the presence of terpene and / or terpenederived impurities in acetic acid recovered from said acetylation. This opens up the possibility to put such acetic acid to use without conducting a step serving to remove terpene and / or terpene-derived compounds from the acetic acid.

[0024] Accordingly, the invention puts to use rubberwood in a process for the production of acetic acid. This process comprises reacting acetic anhydride with the rubberwood so as to form esterified rubberwood and acetic acid. The acetic acid is comprised in utilized acetylation fluid recovered from the wood acetylation. Generally, this acetylation fluid will also comprise unreacted acetic anhydride. With a view to producing acetic acid, the process comprises subjecting the utilized acetylation fluid to a step separating acetic acid from acetic anhydride, thereby obtaining the acetic acid.

[0025] In the rare event that the utilized acetylation fluid does not comprise any unreacted acetic anhydride, it will be understood that the step of separating acetic acid from acetic anhydride can be dispensed with.

[0026] Particularly, in another aspect, a process is provided for the production of acetic acid, the process comprising reacting acetic anhydride with rubberwood. This results in acetylated rubberwood and acetic acid. From the acetylation of rubberwood, an acetylation fluid can be recovered comprising acetic acid and residual acetic anhydride. Particularly, this recovered acetylation fluid consists essentially of acetic acid and acetic anhydride. More particularly, the recovered acetylation fluid consists of acetic acid and acetic anhydride. Since the acetylation fluid that is recovered from the acetylation of rubberwood does not contain terpenes and / or terpene-derived compounds, the step of removing the residual acetic anhydride from the acetylation fluid results in obtaining a terpene-free acetic acid.

[0027] This disclosure refers to “recovered acetylation fluid” and “recovered post-acetylation fluid.” Herein the term “recovered” indicates the fluid as is, i.e., as obtained from a rubberwood acetylation process as described, more specifically as “directly obtained.” In other words, the term “recovered” refers to physically obtaining such fluid as is, by separating it from the acetylated rubberwood, particularly without any treatment steps other than separating it off and transporting and / or collecting it.

[0028] The step of removing the acetic anhydride from the acetic acid, in accordance with the common general knowledge in the field, is generally done by distillation. The skilled person is familiar with the applicable conditions for such distillation, suitably, e.g., at 150 to 250 mbar, such as 200 mbar, at 70°C to 90°C, such as 80°C, as non-limiting examples. The acetic anhydride obtained from this distillation can be put to any suitable use, preferably again as wood acetylation fluid. More preferably, the thus recovered acetic anhydride is recirculated to the equipment for the process for the production of acetic acid by the acetylation of rubberwood from which it resulted.

[0029] The acetic acid obtained as a product from the acetylation of rubberwood with acetic anhydride, can be put to use in all applications in which acetic acid is generally used. Particularly, the produced acetic acid is not limited in any further use by the presence of terpene and / or terpenederived impurities, as is the case with other wood species, generally.

[0030] The produced acetic acid can be sold as a chemical. An advantage of acetic acid produced via the esterification reaction of acetic anhydride with wood, is that it does not contain water. As a result, the acetic acid is produced as dry acetic acid, more commonly known as glacial acetic acid, which is suitable for a wide range of applications.

[0031] With reference to potential organic and other trace impurities present, the produced acetic acid typically meets the desired purity limit of 99.85 wt.% for glacial acetic acid. Particularly, organic impurities can be as low as 300 ppm, chloride impurities can be as low as 1 ppm. Depending on the batch of rubberwood employed, organic and / or chloride impurities can be higher or lower. Exceptionally, such impurities sometimes exceed the general specifications for glacial acetic acid. Also in that event, however the present acetic acid beneficially has no terpene or terpene-derived impurities.

[0032] The produced acetic acid can also be put to use in connection with the acetylation of wood, and particularly in recirculating it to the acetylation of rubberwood. In doing so, the acetic acid can be recirculated in either or both of two ways. One is to use it in a reaction with ketene to produce acetic anhydride. This acetic anhydride, in turn, is preferably used as an acetylation agent in the acetylation of rubberwood. The other way of recirculating the acetic acid is to use it in a ketene-forming reaction. Hereby it is greatly advantageous that the acetic acid, obtained as a product of the reaction of acetic anhydride with rubberwood, can be used in a ketene furnace directly, without subjecting it to a step of removing terpene or terpene-derived impurities. Particularly, this is without such a step selected from the group consisting of azeotropic distillation with water, extraction with an organic solvent immiscible with acetic acid, and cooling crystallization. It will be understood that, in either or both of the steps of producing ketene and producing acetic anhydride, the acetic acid obtained by the present process, can be combined with fresh acetic acid to the extent needed.

[0033] Rubberwood is a wood species obtained from the Para rubber tree (Hevea brasiliensis), also known as just “rubber tree” or “rubber plant”. Rubberwood is also known as “plantation hardwood”, “parawood”, or "Hevea". It has also been marketed under the name "Malaysian Oak". Rubberwood is considered to be a non-durable hardwood species.

[0034] Wood acetylation processes include liquid phase processes, gas phase processes, and combinations thereof. Generally, wood to be acetylated will be impregnated with acetylation fluid, and subjected to one or more heating steps, generally under elevated pressure.

[0035] In the present process, the acetylation fluid comprises acetic anhydride or a combination of acetic anhydride and acetic acid. Preferably the initial acetylation fluid used is acetic anhydride, possibly in combination with acetic acid in a range of 40-100 wt.% of acetic anhydride and 0-60 wt.% of acetic acid. Preferably the initial acetylation fluid comprises a minor amount of acetic acid, such as 2 wt.% to 20% wt.%, preferably 5 wt.% to 10 wt.%.

[0036] Acetylation reactions are generally conducted at temperatures of from 120°C to 220°C, such as 130°C to 210°C. such as 160°C to 180°C. The duration of the acetylation treatment generally ranges from 10 minutes to 3 hours, such as 10-60 minutes. Suitable reaction pressures conditions include, e.g., 0-10 barg.

[0037] The skilled person will be able, for a given reactor equipment and depending on the shape and size of the wood to be acetylated, to optimize the time and temperature conditions to achieve a level of acetylation of the wood that would be desirable.

[0038] Preferably, the process comprises conducting the acetylation of rubberwood with the acetylation fluid being in the vapour phase. Reference is also made to WO 2020 / 157333, which discloses a process for producing acetylated small rubberwood elements that are used in composite wood boards, such as medium density fibre board (MDF).

[0039] Preferably, the rubberwood employed in the present process is selected from the group consisting of solid wood, wood elements, and wood veneers. The wood elements can preferably be, e.g., wood chips, wood strands, wood particles, and wood fibres.

[0040] Typically, the rubberwood to be acetylated is not wood pulp. Particularly, wood acetylation processes are distinguished from processes in which wood-based starting materials such as pulp are subjected to chemical reactions involving the formation of new materials and / or shapes, such as in making nanocellulose from cellulose-based starting materials. Essentially, wood acetylation processes serve to retain the wood (solid wood, wood veneers, wood elements), in its original physical shape, and only chemically change the wood, i.e. to the extent that it becomes acetylated. Particularly, other than in the case of e.g. nanocellulose, acetylated wood contains, besides cellulose, also hemicellulose and, notably, lignin. The effect of acetylating the wood is to acetylate these wood components, resulting in the presence of acetylated cellulose, acetylated hemicellulose and acetylated lignin.

[0041] Typical dimensions of wood elements subjected to acetylation are given in the following table. Table 1

[0042] In some embodiments, the wood elements have a length 1.0-75 mm, a width of 0.05-75mm and a thickness of 0.05-15 mm.

[0043] In alternative embodiments, the wood is solid wood or veneers of wood and preferably has a length or width of at least 8 cm. The thickness preferably is at least 1mm. In some embodiments, the wood has a width of 2 cm to 30 cm, a thickness of 2 cm to 16 cm and a length of from 1.5 to 6.0 m. In other embodiments, the wood has a thickness of at least 1mm, a width of 20 cm - 2.5 m and length of 20cm to 6m.

[0044] The wood acetylation process by which the acetic acid is produced can be a batch process, a semi-continuous process (such as a process combining batch-wise feeding and continuous acetylation) process, or a continuous process.

[0045] Preferably his holds for the entire set-up of combined wood acetylation, acetylation fluid recovery, and separation of acetic acid and acetic anhydride. More preferably this also holds for the production of ketene and acetic anhydride downstream of the acetic acid production, and for the, still further preferred, use of said acetic anhydride in rubberwood acetylation. The preference for conducting the foregoing production steps of acetic acid, ketene, and acetic anhydride in a continuous manner, also holds in the event of a batch-wise filling of the acetylation reaction vessel with the rubberwood to be subjected to acetylation.

[0046] The acetic acid production process of the invention is generally conducted in accordance with optimized acetylation processes as are known in the field. Preferred processes comprise the following steps:

[0047] Providing wood (solid wood or wood elements, such as chips, particles or fibres);

[0048] Controlling, and if necessary adjusting, the moisture content of the solid wood or wood elements;

[0049] - Impregnating the solid wood or wood elements with acetylation fluid;

[0050] Subjecting the impregnated solid wood or wood elements to one or more heating steps in order to effectuate acetylation of the solid wood or wood elements;

[0051] Separating the acetylated solid wood or wood elements from excess acetylation fluid.

[0052] The acetylation process itself can be conducted, as known in the art, using liquid and / or gaseous acetylation fluid. Typical acetylation fluids are acetic acid, acetic anhydride, and mixtures thereof. Preferably the initial acetylation fluid used is acetic anhydride, more preferably comprising a minor amount of acetic acid, generally 2 wt.% to 20% wt.%, such as 5 wt.% to 10 wt.%. As a result of the acetylation reaction, the composition of the acetylation fluid will change during the process, since acetic acid is thereby formed.

[0053] In interesting embodiments, the acetylation is conducted in accordance with any one of the acetylation processes as described in W02009 / 095687, WO2011 / 95824, WO2013 / 117641, WO2013 / 139937, or WO20 16 / 008995, the disclosures of which are herein incorporated by reference. Acetic anhydride can be produced, in a well-known manner by reacting acetic acid and ketene. This reaction is generally conducted by absorption of the ketene in the acetic acid, and does not require elucidation to the skilled person. Ketene can also be produced from acetic acid, also in a well-known manner. This reaction is generally conducted by subjecting the acetic acid to a ketene-forming thermal treatment. This is a pyrolysis reaction, usually referred to as “cracking” the acetic acid, generally occurring in a reactor indicated as a ketene furnace. Also this reaction does not require elucidation to the skilled person.

[0054] In putting to use acetic acid produced by rubberwood acetylation with acetic anhydride, as discussed above, several processes can advantageously be combined, if desired. Accordingly, the acetic acid can be reacted with ketene so as to form acetic anhydride. This acetic anhydride, optionally together with unreacted acetic anhydride recovered from the rubberwood acetylation, can in turn be employed as a wood acetylation fluid. Equally, the produced acetic acid can be subjected to cracking so as to form ketene, which ketene can be employed, if desired, to produce acetic anhydride by reaction with acetic acid. The latter preferably also is the acetic acid produced in accordance with the process described hereinbefore.

[0055] In addition to the aspects of the invention described hereinbefore, the invention can also be viewed, in another aspect, as reflecting a new use of rubberwood. Accordingly, this use is in a process for preparing acetic acid by reacting the rubberwood with acetic anhydride, thereby forming acetylated rubberwood and acetic acid, and obtaining acetic acid as a product. Particularly, this use is for preparing acetylated rubberwood by reacting the rubberwood with acetic anhydride, for the purpose of producing acetic acid devoid of terpene and / or terpene-derived impurities. Obtaining the acetic acid as a product involves subjecting the utilized acetylation fluid that results from the acetylation of wood with acetic anhydride to a step separating acetic acid from acetic anhydride. Here, too, it will be understood that removing residual acetic anhydride from said utilized acetylation fluid, results in obtaining a terpene-free acetic acid, by virtue of the absence of terpenes and / or terpene-derived compounds after the acetylation of rubberwood. The invention is illustrated with reference to the following, non-limiting examples.

[0056] Example 1

[0057] Two batches of rubberwood chips were subjected to acetylation as follows.

[0058] 50 g of oven dried chips having a moisture content lower than 4 wt% were placed in an autoclave pre-heated to 170°C. The oxygen in the autoclave was replaced by nitrogen. 125 g of an ANH / ACA (90% ANH, 10% ACA) at 150°C was sprayed on the chips in 5 minutes. The reactor was maintained at 2.5 bar by pressurization with nitrogen or by releasing the pressure via a vent. After 20 minutes the autoclave was fully depressurized. After 20 min, the content of the autoclave was stripped with N2 so that the utilized acetylation fluid was collected dropwise via a condenser cooled to 5°C in successive fractions of about 30 grams.

[0059] The fractions of utilized acetylation fluid were weighted and analysed by GC-FID. The results reported in table 2, showing the impurities present in the utilized acetylation fluid expressed in milligram per kilogram acetic acid produced during the acetylation reaction.

[0060] As can be seen, the recovered acetic acid does not have any detectable terpene or terpenoid impurities.

[0061] Example 2 (comparative)

[0062] Batches of wood chips of four different wood species (Stika spruce, Scots pine, Logepole pine, and Radiata pine were subjected to acetylation following the same procedure as in Example 1. The results are depicted in Table 3, showing the impurities present in the utilized acetylation fluid expressed in milligram per kilogram acetic acid produced during the acetylation reaction. As can be seen, the recovered acetic acid has substantial levels of terpene or terpenoid impurities

[0063] Table 2

[0064] Impurities (mg / kg acetic acid produced during the acetylation reaction)

[0065] *nd = not detectable Table 3

[0066] Impurities (mg / kg acetic acid produced during the acetylation reaction)

Claims

Claims1. A process for the production of acetic acid, the process comprising reacting an acetylation fluid comprising acetic anhydride with wood so as to form esterified wood and a utilized acetylation fluid comprising acetic acid, recovering the utilized acetylation fluid from the wood, subjecting the utilized acetylation fluid to a step separating acetic acid from acetic anhydride, and obtaining the acetic acid, wherein the wood is rubberwood.

2. A process for the production of acetic acid, the process comprising reacting acetic anhydride with rubberwood so as to form acetylated rubberwood and acetic acid, recovering a post- acetylation fluid comprising acetic acid and residual acetic anhydride, and removing the residual acetic anhydride from the post- acetylation fluid to obtain a terpene- free acetic acid.

3. A process according to claim 1 or 2, comprising impregnating the wood with the acetylation fluid, followed by one or more heating steps under elevated pressure, preferably up to 10 barg.

4. A process according to claim 3, wherein the impregnation is with an acetylation fluid comprising 2 wt.% to 20% wt.% of acetic acid, preferably 5 wt.% to 10 wt.%.

5. A process according to any one of the preceding claims, wherein the wood is selected from solid wood, wood veneers, wood chips, wood strands, wood particles, and wood fibres.

6. A process according to claim 5, wherein the wood is solid wood.

7. A process for the preparation of ketene by subjecting acetic acid to a ketene-forming thermal treatment, the acetic acid comprising acetic acid recovered from a wood acetylation process, said woodacetylation process comprising subjecting wood to acetylation with an acetylation fluid selected from the group consisting of acetic anhydride, acetic acid, and mixtures thereof, wherein the wood is rubberwood.

8. A process according to claim 7, wherein the acetic acid is subjected to the ketene-forming thermal treatment without any prior treatment step suitable to remove terpene or terpene -derived impurities selected from the group consisting of azeotropic distillation with water, extraction with an organic solvent immiscible with acetic acid, and cooling crystallization.

9. A process for the preparation of acetic anhydride, the process comprising reacting acetic acid with ketene, the process comprising producing the acetic acid in accordance with any one of the claims 1 to 6, and producing the ketene in accordance with the process of claim 7 or 8.

10. The use of rubberwood in a process for preparing acetic acid by reacting the rubberwood with acetic anhydride, thereby forming acetylated rubberwood and acetic acid, and obtaining acetic acid as a product.

11. The use of rubberwood in a process for preparing acetylated rubberwood by reacting the rubberwood with acetic anhydride, for the purpose of producing acetic acid devoid of terpene and / or terpenederived impurities.

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