Purification of wood acetylating fluid

Abstract

A method for purifying a post-consumer acetylated fluid recovered from a wood acetylation process is disclosed, which includes subjecting the recovered acetylated fluid to cold crystallization, which removes terpene and terpenoid impurities as well as other impurities. Also disclosed is a method for acetylating wood, which provides acetylated wood and a post-consumer acetylated fluid containing acetic acid, and which purifies the post-consumer acetylated fluid by cold crystallization.

Description

Technical Field

[0001] The present invention relates to the purification of acetic acid recovered from a wood acetylation process, which leads to, for example, the removal of by-products and / or lignocellulosic impurities. In particular, the present invention relates to the removal of terpene impurities and terpenoid impurities from said acid.

Background Art

[0002] The acetylation of lignocellulosic materials (hereinafter referred to as wood) is a well-known method that can improve various properties of these materials. This is particularly related to durability and dimensional stability.

[0003] The wood acetylation process generally has in common that the wood is brought into contact with an acetylation fluid under appropriate acetylation conditions. The background art document is WO 2009 / 095687. Here, a method for the acetylation of wood is described, which includes the steps of immersing the wood in an acetylation liquid in a reaction pressure vessel, performing an impregnation procedure, removing excess acetylation fluid, introducing an inert fluid (typically nitrogen gas, which may contain non-inert acetic anhydride and / or acetic acid) into the vessel, circulating and heating the inert fluid according to a heating method to effect proper acetylation of the wood, and removing the circulating fluid and cooling the acetylated wood.

[0004] The acetylation fluid is generally selected from acetic acid, acetic anhydride, and mixtures thereof. After the wood acetylation process, the used acetylation fluid is removed from the wood. Thus, for example, it is desirable to avoid wasting the removed acetylation medium by recycling the removed acetylation medium and reusing it in wood acetylation.

[0005] In many cases, the acetylated fluid after use contains excess acetic acid because acetic acid is formed as a by-product of acetylation. It is desirable to separate such acetic acid and use it separately, sell it as a chemical, and / or use it in the production of ketene.

[0006] However, such a specific source of acetic acid, namely from wood acetylation, has limitations inherent in its further use due to the presence of terpene impurities and terpenoid impurities in the wood. Other impurities that are desirable to remove typically include by-products such as acetonitrile and acetic acid esters such as methyl acetate and ethyl acetate. Wood acetylation can also result in the presence of chlorides in the acetylated fluid after use. The chloride concentration in the acetylated fluid after use depends on the chloride content, generally the chloride content of the chemical composition of the wood before acetylation. The level of chloride impurities present in the wood may also depend on the wood species and the origin of the wood. For example, chlorides are particularly prominent in rubberwood, especially when grown in coastal areas.

[0007] Terpenes and terpenoids include both natural wood impurities and conversion products resulting from the heat that the natural wood terpenes and terpenoids undergo during wood acetylation.

[0008] Such impurities, especially terpenes and terpenoids, are very difficult to remove. This is particularly undesirable when recycling the acetylated fluid because a controlled purity of the acetylated fluid is required to obtain the desired high degree of acetylation in a controlled manner. Furthermore, the presence of impurities, especially terpene and terpenoid impurities, limits not only the sale of acetic acid recovered from wood acetylation alone but also further uses. For example, it is not desirable to use the terpene impurities and terpenoid impurities in a ketene furnace because the high temperature provided in the ketene furnace makes it easier for coke to form in the furnace.

[0009] The purification of acetic acid recovered from the acetylation of wood is addressed in WO 2009 / 120257. Here, acetic acid containing the impurities is fed to a distillation column together with water. The necessary addition of the amount of water generally required for such distillation results in the drawback of producing more dilute acetic acid than desired. Adding water to the distillation column also reduces the economic viability of the process in other respects, such as heat energy consumption. Therefore, it is desirable to remove terpenes and terpenoids without distilling them with water.

Summary of the Invention

Means for Solving the Problems

[0010] In one aspect, the present invention is a method for purifying a spent acetylated fluid recovered from a process of acetylating wood, wherein the recovered acetylated fluid contains acetic acid, and the method includes cooling the recovered acetylated fluid to a temperature below the melting point of acetic acid under the formation of crystals, and separating the crystals from the fluid.

[0011] In another aspect, the present invention provides a method for removing one or more impurities selected from the group consisting of chlorides, acetonitrile, acetic acid esters such as methyl acetate and ethyl acetate, terpenes, terpenoids, and combinations thereof from a spent acetylated fluid recovered from a process of acetylating wood, wherein the recovered acetylated fluid contains acetic acid, and the method includes cooling the recovered acetylated fluid to a temperature below the melting point of acetic acid under the formation of crystals, and separating the crystals from the fluid.

[0012] Specifically, in yet another aspect, the present invention provides a method for removing one or more impurities selected from the group consisting of terpenes, terpenoids, and combinations thereof from a spent acetylated fluid recovered from a process of acetylating a lignocellulosic material, wherein the recovered acetylated fluid contains acetic acid, and the method includes cooling the recovered acetylated fluid to a temperature below the melting point of acetic acid under crystal formation, and separating the crystals from the fluid.

[0013] In yet another aspect, the present invention provides a method for acetylating wood, the method including contacting the wood with an acetylating fluid containing acetic anhydride and / or acetic acid under wood acetylation conditions to obtain acetylated wood and a spent acetylated fluid containing acetic acid, and purifying the spent acetylated fluid by the method described in any of the preceding paragraphs of this summary.

[0014] In a further aspect, the present invention provides the use of acetic acid obtained by the purification method described herein as a reactant in the production of ketene from acetic acid and / or in the production of acetic anhydride from acetic acid and ketene, and provides an integrated method for the acetylation of wood and the production of acetic anhydride.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention utilizes careful insights for subjecting a spent acetylated fluid to cooling crystallization. As a result, it has been found that acetic acid purified from terpenes can be obtained not only when the temperature is lower than the melting point of acetic acid but also when the temperature is higher than the melting point of acetic acid. Such terpenes are expected to crystallize or co-crystallize with acetic acid, so this discovery is surprising. Also, considering that it is very difficult to remove terpene impurities and terpenoid impurities from a spent acetylated fluid, it was unexpected that yet another method different from azeotropic distillation was found.

[0016] As an unexpected further discovery, it has been found that cooling crystallization of the spent acetylated fluid is applied to remove one or more impurities selected from the group consisting of chlorides, acetonitrile, water, methyl acetate, and acetate esters such as ethyl acetate. Chlorides are typically ionic chlorides present in acetic acid after quenching with water after separation of acetic acid / acetic anhydride. Without wishing to be bound by theory, the inventors believe that the detected chlorides contain hydrogen chloride and acetyl chloride (generally hydrolyzed). Other chlorides in the process may include stable organic chlorides and non-volatile chlorides that remain with the anhydride in the separation of anhydride / acetic acid.

[0017] The present invention relates preferably to the removal of impurities, which are preferably terpenes, terpenoids, or combinations thereof, from the spent acetylated fluid. Terpenes are a wide variety of organic compounds produced by various plants, including trees. Terpenes are hydrocarbons. Terpenoids are modified terpenes that usually contain additional oxygen-containing functional groups. Terpenes exist as monoterpenes and oligoterpenes.

[0018] Terpenes may be classified by the number of isoprene units in the molecule, and the prefix in the name indicates the number of terpene units required to assemble the molecule. Hemiterpenes consist of a single isoprene unit. Sopropene itself is considered the only hemiterpene, but oxygen-containing derivatives such as prenol and isovaleric acid are hemiterpenoids. Monoterpenes consist of two isoprene units and have the molecular formula C 10 H 16 Examples of monoterpenes and monoterpenoids include geraniol, terpineol, limonene, myrcene, linalool, or pinene. Iridoids are derived from monoterpenes. Sesquiterpenes consist of three isoprene units and have the molecular formula C 15 H 24 Examples of sesquiterpenes and sesquiterpenoids include humulene, farnesene, farnesol. Diterpenes are composed of four isoprene units and have the molecular formula C 20 H 32It has. Examples of diterpenes and diterpenoids are cafestol, carvol, cembrene, and taxadiene. Sesterterpenes, which are terpenes having 25 carbons and 5 isoprene units, are rare compared to other sizes. An example of a sesterterpenoid is geranyl farnesol. Triterpenes consist of 6 isoprene units and have the molecular formula C 30 H 48 Sesquiterpenes are composed of 7 isoprene units and have the molecular formula C 35 H 56 Examples of sesquiterpenoids are ferrugicadiol and tetraterpenyl curcumen. Tetraterpenes contain 8 isoprene units and have the molecular formula C 40 H 64 It has.

[0019] Preferably, the method of the present invention purifies the acetylated fluid from the main lignaceous terpenes. Such lignaceous terpenes include, among others, camphene, α-terpinolene, myrcene, α-pinene, β-pinene, p-cymene, 3-carene, and limonene.

[0020] The method of the present invention is carried out with the used acetylated fluid recovered from any wood acetylation treatment. Such wood acetylation treatments include liquid-phase processes, gas-phase processes, and combinations thereof. Generally, the wood to be acetylated is impregnated with the acetylated fluid and generally subjected to one or more heating steps under high pressure. The acetylated fluid can be acetic acid, acetic anhydride, or a combination thereof. Generally, the used acetylated fluid contains by-products from wood acetylation, components extracted from wood such as terpenes and / or terpenoids, and excess acetylated fluid. Typically, when acetic anhydride is included in the acetylated fluid, the by-product is acetic acid. The excess acetylated fluid is generally acetic anhydride, acetic acid, or both.

[0021] Often, in the case of an acetylation process that produces a combination of acetic anhydride and acetic acid as the post-use acetylated fluid, it is first desirable to separate acetic acid from acetic anhydride. This is generally done by distillation. As mentioned in the art, such distillation results in acetic acid in which wood-derived impurities such as terpenes and / or terpenoids have not been removed or at least not sufficiently removed.

[0022] In one embodiment, in the post-use acetylated fluid to be cooled and crystallized, in addition to acetic acid, acetic anhydride is present.

[0023] In one embodiment, the post-use acetylated fluid is subjected to a step of removing acetic anhydride before being subjected to the cooling crystallization method of the present invention. It is understood that the removal of acetic anhydride is preferably achieved by distillation.

[0024] The method of the present invention is generally carried out according to an optimized acetylation process as known in the art. Preferred processes include the following steps: · Providing wood (solid wood or wood elements); · Controlling and optionally adjusting the water content of the wood or wood elements; · Impregnating the wood or wood elements with an acetylated fluid; · Subjecting the impregnated wood or wood elements to one or more heating steps to achieve acetylation of the wood elements; · Separating the acetylated wood or wood elements from the excess acetylated fluid.

[0025] Accordingly, the present invention also relates to a method for the acetylation of wood, the method comprising contacting the wood with an acetylated fluid comprising acetic anhydride and / or acetic acid under wood acetylation conditions to obtain acetylated wood and a post-use acetylated fluid containing acetic acid. In this method, the post-use acetylated fluid is purified by a cooling crystallization method according to any one or more of the embodiments described above and below.

[0026] The acetylation process itself can be carried out using a liquid and / or gaseous acetylating fluid, as is known in the art. Typical acetylating fluids are acetic acid, acetic anhydride, and mixtures thereof. Preferably, the initial acetylating fluid used is acetic anhydride (since acetic acid is formed as a result of the acetylation reaction, the composition of the acetylating fluid changes during the process).

[0027] In an interesting embodiment, the acetylation is carried out according to any of the acetylation processes described in WO 2009 / 095687, WO 2011 / 95824, WO 2013 / 117641, WO 2013 / 139937, or WO 2016 / 008995, the disclosures of which are incorporated herein by reference.

[0028] The acetylation reaction is generally carried out at a temperature of 120°C to 200°C, for example 160°C to 180°C. The duration of the acetylation treatment is generally in the range of 30 minutes to 3 hours. Those skilled in the art can optimize the time and temperature conditions according to a given reactor and the wood species to be acetylated.

[0029] In the method of the present invention, the used acetylating fluid is cooled and crystallized. For this purpose, the recovered fluid is cooled to below the melting point of the used acetylating fluid. In embodiments where substantial removal of acetic anhydride from the acetylating fluid is carried out, the cooling is to below the melting point of acetic acid. The melting point is 16.6°C, and the cooling is generally carried out to a temperature lower than this temperature, for example, in the range of 0° to 16°C, for example, in the range of 10°C to 15°C. Generally, lower temperatures can be applied, and lower temperatures are applied when the freezing point of the acetylating fluid is low (for example, when a significant amount of low melting point compounds such as water and / or acetic anhydride are present). Those skilled in the art can readily determine the freezing point of any fluid such as the used acetylating fluid.

[0030] When implementing the cooling crystallization method of the present invention, it is preferable to recycle the liquid remaining after crystallization, i.e., the mother liquor, rather than discarding it after each round of crystallization. Thereby, the recycled fluid is replenished with fresh recovered acetylated fluid. This has the advantage of reducing waste and making the process more economical. In a preferred embodiment, such mother liquor is recycled at least 20 times, for example at least 50 times, for example 20 to 200 times, preferably 50 to 100 times. During such recycling, as a result of the repeated crystallization of acetic acid, the concentration of liquids that may be present other than acetic acid increases. For example, if 0.1% water is present in the fresh recovered acetylated fluid, it rises above 5% after 60 recycles in the recycled mother liquor.

[0031] In practice, water can be regarded as an impurity present in a relatively high content compared to other impurities in acetic acid and accumulates in the mother liquor (like all other molecules that do not freeze). Such mixing of the fresh recovered acetylated fluid and the recycled mother liquor can occur batchwise from a recovery vessel for the used acetylated fluid. The crystallization process is preferably carried out as a continuous process. Thereby, the fresh recovered used acetylated fluid can still be added batchwise or as a continuous feed.

[0032] The additional liquid is typically acetic anhydride or water. The former is present as a result of the original composition of the acetylated fluid, and the amount depends on the extent to which such anhydride is optionally removed before subjecting the used acetylated fluid to the crystallization method of the present invention. Depending on the concentration of acetic acid and / or acetic anhydride used as the wood acetylating fluid, water may be present. Water can be added to quench the anhydride. This reduces the risk of corrosion in the crystallization apparatus, as such quenching effectively removes the anhydride and makes the resulting treatment liquid (the used acetylated liquid sent to crystallization) less corrosive. The method of the present invention advantageously functions even in situations where the amount of acetic anhydride and / or water present in the used acetylated fluid has increased. This is unexpected considering that such additional liquid results in a decrease in the freezing point (or, alternatively, melting point) of acetic acid, especially when water is present. For example, when 10% water is present, the freezing point of acetic acid decreases from 16°C to 5°C.

[0033] Cooling can be carried out in any container or tube suitable for forming crystals. Those skilled in the art are familiar with appropriate equipment such as a scraped-wall crystalliser. It is understood that the crystallization apparatus enables cooling in order to implement this method. Typically, the crystals formed are generally washed in a washing column, for example, to remove the liquid film of the mother liquor that typically remains during melt crystallization. Those skilled in the art are familiar with the operation method of the crystallization apparatus.

[0034] No special means are required to effect crystallization. If desired, the process can be assisted by adding a small amount of a suitable contaminating substance (such as water or acetic anhydride), for example, less than 10 wt%, less than 5 wt%, for example 1 wt% to 3 wt%, etc., to help initiate crystallization. Those skilled in the art will recognize techniques for facilitating the occurrence of crystallization, such as scraping the walls of the crystallizer with a spatula.

[0035] It may be advantageous to promote crystallization by the presence of one or more seed crystals of acetic acid. Such seed crystals can be added to the fluid when the temperature of the fluid is near or below the melting point of acetic acid. The seed crystals are generally obtained and stored in advance. Advantageously, the seed crystals can also be obtained in situ in the process of the present invention.

[0036] In an interesting embodiment, the crystallization is carried out in two stages. Thus, in the first step, the recovered acetylated fluid is cooled below the melting point of acetic acid under crystal formation. This step can be carried out as described above, with or without seed crystals. Next, in the second step, generally after washing off the film of the residual raw material liquid, the first portion of the obtained crystals is melted to obtain an acetic acid melt. Since not all of the crystals melt, the second portion thereof is retained. The acetic acid melt is recycled and cooled again below the melting point of acetic acid. This cooling is carried out in the presence of at least a part of the retained crystals, and thus the second crystallization step is carried out in the presence of seeds. The recycled melt can be cooled as it is, or after being combined with a further amount of the used acetylated fluid recovered from the wood acetylation process (from the same process run, or from a different run thereof, or from different processes, for example, when a plurality of acetylation reactors operate in parallel). The process can be repeated to provide a plurality of washing and recrystallization steps, generally resulting in further purity.

[0037] In the two-stage embodiment, the first portion (i.e., the crystals to be remelted) generally contains more than 50% of the crystals formed in the first step. Preferably, the first portion contains 60% to 99%, more preferably 85% to 95% of the crystals formed in the first step. If desired, any third and subsequent steps can be carried out. After the crystallization in the second step, a part of the crystals formed at that time can be remelted again, and the above treatment can be continued. Therefore, the wood acetylation and the recovery of the acetylated fluid involving the purification process of the present invention can be a continuous or semi-continuous operation.

[0038] In another interesting embodiment, the seed crystal is added separately, such as provided from storage, and does not require remelted crystals obtained from the same acetylated fluid to be crystallized. The advantage of this embodiment is that it makes it easier to carry out crystallization in a continuous process. It is understood that a continuous process may also advantageously include remelting and recrystallization.

[0039] Also, combinations of the above embodiments are conceivable. Thereby, at any stage, the seed crystal can be selected from storage or from in situ crystal formation. For example, the seed crystal of the first stage can be provided from storage, and in one or more subsequent stages, the seed crystal is obtained by excluding a part of the formed crystals from remelting.

[0040] The wood to be acetylated is in the form of a wood element or a solid wood material, and also includes veneer. The wood element can preferably be wood chips, wood strands, or wood particles. The wood preferably belongs to non-durable wood species such as softwoods, typically conifers such as spruce, pine, or fir, or non-durable hardwoods such as rubberwood. Non-limiting examples of suitable types of wood are spruce, Sitka spruce, Japanese cedar, Scots pine, radiata pine, eucalyptus, red alder, European alder, beech, basswood, loblolly pine, lodgepole pine, pitch pine, red pine, southern yellow pine, Japanese cedar (sugi), and hemlock. Monocotyledons such as palm, and other hardwoods such as paulownia, rubberwood, teak, maple, oak, and white oak are also suitable.

[0041] Typically, the wood to be acetylated is not wood pulp. In particular, the wood acetylation process is distinguished from processes in which lignocellulosic starting materials, such as pulp, undergo chemical reactions involving the formation of new materials and / or shapes, such as the production of nanocellulose from cellulosic starting materials. Essentially, the wood acetylation process only retains the wood (sawn timber, veneer, wood element) in its original shape and changes it to the extent of making the wood acetylated. In particular, except in the case of nanocellulose, acetylated wood contains hemicellulose and especially lignin in addition to cellulose. The effect of acetylating wood is to acetylate these wood components, resulting in the presence of acetylated cellulose, acetylated hemicellulose, and acetylated lignin.

[0042] Typical dimensions of the acetylated wood elements are shown in the following table.

[0043]

Table 1

[0044] In some embodiments, the wood element has a length of 1.0 - 75 mm, a width of 0.05 - 75 mm, and a thickness of 0.05 - 15 mm.

[0045] In alternative embodiments, the wood is sawn timber or a veneer of wood, preferably having a length or width of at least 8 cm. The thickness is preferably at least 1 mm. In some embodiments, the wood has a width of 2 cm - 30 cm, a thickness of 2 cm - 16 cm, and a length of 1.5 - 6.0 m. In other embodiments, the wood has a thickness of at least 1 mm, a width of 20 cm - 2.5 m, and a length of 20 cm - 6 m.

[0046] In short, the present disclosure provides a method for purifying a post-use acetylated fluid recovered from a wood acetylation process. The method includes cooling and crystallizing the recovered acetylated fluid. By this method, terpene and terpenoid impurities are removed, and other impurities are also removed. Also disclosed is a method for acetylating wood, which obtains acetylated wood and a post-use acetylated fluid containing acetic acid, and purifies the post-use acetylated fluid by cooling crystallization.

[0047] It is understood that the product obtained from the above purification method is purified acetic acid. This obtained purified acetic acid can be used for separate uses and can be sold as a chemical. In particular, it can be used in the production of acetic anhydride, ketene, or more generally, acetylating agents, acetylating agent components, intermediates or components of acetylated fluids.

[0048] Those skilled in the art are well-versed in the methods for producing ketene from acetic acid and the method for producing acetic anhydride from acetic acid and ketene. The ketene production process generally involves heating acetic acid to dehydrate it. This typically occurs in a ketene furnace operated at a temperature in the range of, for example, 700°C to 750°C. Acetic anhydride can be formed in an exothermic reaction by reacting ketene with acetic acid. The present invention includes the use of acetic acid obtained by the purification method presented in this disclosure, and in all embodiments, it includes using it as a reactant in the production of ketene from acetic acid, as a reactant in the production of acetic anhydride from acetic acid and ketene, or as a reactant in both. The ketene used in the production of acetic anhydride can be supplied from other sources, but preferably, it is obtained from a ketene production process combined in the acetic anhydride production process. More preferably, either or both of the ketene production process and the acetic anhydride process are combined with a wood acetylation process in which the acetylated fluid is recovered and purified according to the crystallization method described herein. This combination is such that the resulting purified acetic acid is directed to either or both of the ketene production process and the acetic anhydride production process. This can be achieved, for example, according to the general process scheme of Figure 1 in WO 2016 / 09060. This includes sending the acetic acid obtained after being separated from acetic anhydride in the spent acetylated fluid recovered from wood acetylation (ACA / ANH separation) to either or both of the ketene furnace (decomposition furnace) and the reactor for producing acetic anhydride (ANH reaction). In this embodiment of the present invention, the cooling crystallization process as described above is carried out downstream of the ACA / ANH separation and upstream of both the decomposition furnace and the ANH reaction. Preferably, this enables an integrated process for the acetylation of wood and the production of acetic anhydride. Among them, the method for the acetylation of wood includes contacting wood with an acetylated fluid containing acetic anhydride and / or acetic acid under wood acetylation conditions. As a result, acetylated wood and a spent acetylated fluid containing acetic acid are obtained, and thereby, the method includes, in all embodiments, purifying the spent acetylated fluid by cooling crystallization substantially as described above.The crystallization process of the present invention leads to the production of purified acetic acid, which may be mixed with fresh acetic acid and used, as described above, in the production of ketene and acetic anhydride. The anhydride produced can then be further used as, or as a component of, the acetylating fluid in the said process for the acetylation of wood.

[0049] It will be understood that variations of the combined processes are fully possible. For example, the purified acetic acid can be used in the production of ketene, while the production of acetic anhydride can use fresh acetic acid or vice versa. Or, for example, the acetic anhydride produced is not used in the acetylation of wood and is used or sold outside the integrated process. These and other variations will be apparent to those skilled in the art.

[0050] Other uses of the purified acetic acid obtained are for the production of vinyl acetate monomer which is polymerized to poly(vinyl acetate) and other polymers, for the production of esters used as solvents in inks, paints, and coatings, and for food applications.

[0051] The present invention is illustrated with reference to the following non-limiting examples.

Examples

[0052] Unless otherwise stated, all parts per million (ppm), percentages (%), and ratio values correspond, respectively, to milligrams per kilogram, weight percent, and weight ratio values. The density of all solutions described in these examples is considered equal.

[0053] Unless otherwise stated, the experiments disclosed in this specification were carried out using impure acids produced by distilling the acetylated fluid after utilization of an industrial wood acetylation plant for manufacturing Accoya® wood. This impure acetic acid contained about 1% acetic anhydride, some terpenes in concentrations ranging from less than 10 to over 100 ppm, about 1,000 ppm methyl acetate, and about 50 ppm ethyl acetate and acetonitrile. Impurities are understood as terpenes, terpenoids, acetic esters, acetonitrile, or combinations thereof.

[0054] Impure acetic acid is understood as a solution containing acetic acid as the main component and impurities removed by the present invention.

[0055] Unless otherwise stated, when water was added to impure acetic acid containing acetic anhydride, sufficient time was maintained for the hydrolysis of acetic anhydride to be completed.

[0056] To determine the concentrations in the organic impurities, impure acetic acid, purified acetic acid, and mother liquor were analyzed by gas chromatography. To determine the concentrations in the chlorides, quenched impure acetic acid, quenched purified acetic acid, and quenched mother liquor can be analyzed by silver titration. The results are given in ppm and % of the concentrations in the original impure acetic acid. The experimental and analytical accuracy was evaluated by mass balance control of each impurity.

[0057] Unless otherwise stated, the experimental crystallization setup consisted of a tubular crystallization device having a jacket equipped with a circulating cooler. A peristaltic pump was used to continuously recirculate acetic acid through the inside of the tube. Before the start of crystallization, when the residence time in the crystallization device was measured, it was 1.5 minutes. Cooling water was pumped through the jacket of the tube at a flow rate such that the temperature in the cooling jacket was uniform at all points. When the temperature of the acetic acid exiting the crystallization setup did not change by more than 1 °C per minute, it was considered that temperature equilibrium had been reached.

[0058] Unless otherwise stated, the crystals produced were washed with a minimum amount of glacial acetic acid at room temperature before melting.

[0059] In the examples, the following impurities are mentioned. 3C = 3-carene αP = α-pinene βP = β-pinene αTA = α-terpinyl acetate fen = fencon lim = limonene cym = p-cymene cam = camphene ACN = acetonitrile EAc = ethyl acetate MAc = methyl acetate αT = α-terpinene

[0060] [Example 1] The experiment was carried out using acetic acid obtained by distilling the spent acetylated fluid obtained from an industrial wood acetylation plant that manufactures Accoya® acetylated wood.

[0061] Prior to the experiment, a sample of acetic acid was analyzed on a gas chromatography column to determine the amount of impurities.

[0062] The experimental setup consisted of a glass tube with a jacket. Acetic acid was continuously recirculated through the inside of the tube. Cold water (14 °C) was pumped into the jacket of the tube and then discharged. Two rotary pumps were used, one to recirculate acetic acid in a closed loop and the other to pump cold water into the jacket.

[0063] To cause crystallization, after cooling for 30 - 40 minutes, the acetic acid loop was stopped and the bottom of the tube was closed. When a thermometer was gently inserted into the upper part of the tube, movement occurred and the surface area inside the system increased. As a result, acetic acid crystallized almost instantaneously throughout the tube.

[0064] Next, acetic acid was melted to about 90% (judged visually), leaving about 10% of the crystals as seeds for subsequent crystallization. After the melting, the acetic acid loop was started again. At this time, crystallization was extremely slow, and most of the inner wall and inside of the tube were covered with acetic acid crystals.

[0065] On average, impurities (mainly terpenes) were removed to the extent of about 40% - 50%. This removal was consistent for all identified terpene impurities, for example, regardless of the melting point of the terpene. It was unexpected that terpenes with a freezing point higher than that of acetic acid, such as camphene (freezing point 50°C - 51°C), were removed. This also applies to embodiments where the melting point of acetic acid actually decreases as a result of the presence of water and / or anhydrides. For example, terpene phencone (melting point 6°C) is effectively removed from acetic acid whether water is absent or present.

[0066] Removal of impurities is significantly improved by washing the crystals with purified acetic acid. Furthermore, an improved removal rate is obtained by redissolving and recrystallizing the washed acetic acid crystals.

[0067] [Example 2] In the crystallization setup, 100 mL of impure acetic acid labeled with Ba was circulated. The cooling water was set at 13.0°C. The acid and cold water were circulated until temperature equilibrium was reached. Then, 50 mg of crystalline acetic acid seeds were placed in the crystallization zone of the setup to cause crystallization of the impure acetic acid. After 15 minutes, the mother liquor labeled with Bb was drained from the setup. 77 mL of the mother liquor was recovered. The cooling of the setup was stopped, and the crystals were melted to obtain 18 mL of purified acetic acid labeled with Bd.

[0068] The analysis results showed an increase in the mother liquor and a decrease in the purified acetic acid for all impurities. The ppm concentrations and the concentrations relative to the initial impure acetic acid are shown in Table 2 and Table 3, respectively.

[0069]

Table 2

[0070]

Table 3

[0071] [Example 3] In this example, the purification of acetic acid is carried out in the presence of a relatively large amount of impurities (e.g., those arising from repeated recycling). For this purpose, 80 mL of impure acetic acid was further contaminated with industrial-grade limonene, 3-carene, α-pinene, camphene, para-cymene, β-pinene, and methyl acetate. The resulting highly impure acetic acid was labeled with Ca.

[0072] Ca was circulated within the crystallization setup. The cooling water was set at 14.0 °C. The acid and cold water were circulated until temperature equilibrium was reached. Then, 50 mg of crystalline acetic acid seeds were placed in the crystallization zone of the setup, causing the crystallization of the highly impure acetic acid. After 1 hour, the mother liquor labeled with Cb was drained from the setup. 73 mL of mother liquor was recovered. The cooling of the setup was stopped, and the crystals were melted to obtain 10 mL of purified acetic acid labeled with Cd.

[0073] The analysis results show an increase in the mother liquor for almost all impurities and a decrease in the purified acetic acid for all impurities. The ppm concentrations and the concentrations relative to the initial impure acetic acid are shown in Table 4 and Table 5, respectively.

[0074]

Table 4

[0075]

Table 5

[0076] [Example 4] In the crystallization setup, a mixture consisting of 95 mL of impure acetic acid labeled with Da and 5 mL of water was circulated. The cooling water was set at 7.0 °C. The mixture of Da and cold water was circulated until temperature equilibrium was reached. Thereafter, 50 mg of crystal acetic acid seeds were placed in the crystallization zone of the setup to initiate crystallization of the impure acetic acid. The cooling water was set at 5.0 °C. After 20 minutes, the mother liquor labeled with Db was discharged from the setup. 88 mL of the mother liquor was recovered. The cooling of the setup was stopped, and the crystals were melted to obtain 12 mL of purified acetic acid labeled with Dd.

[0077] The analysis results show an increase in the mother liquor for most impurities and a decrease in the purified acetic acid for all impurities. The ppm concentrations and the concentrations relative to the initial impure acetic acid are shown in Table 6 and Table 7, respectively.

[0078]

Table 6

[0079]

Table 7

[10] The method according to any one of [1] to [9] above, comprising subjecting the recovered acetylated fluid to removal of acetic anhydride by distillation or the like before the fluid is cooled.

[11] The method according to

[10] above, wherein the cooling is carried out to a temperature below the melting point of acetic acid.

[12] The method according to

[11] above, wherein the cooling is carried out to a temperature in the range of 0° to 16°C, preferably 10°C to 15°C.

[13] The method according to any one of [1] to

[12] above, comprising washing the formed crystals.

[14] The method according to any one of [1] to

[13] , comprising a plurality of subsequent melting steps and recrystallization steps.

[15] The method according to any one of [1] to

[14] , wherein the fluid remaining after separating the crystals is recycled, and the cooling step and the crystallization step are carried out again, and the recycling is carried out 20 to 200 times, preferably 50 to 100 times.

[16] A method for the acetylation of wood, comprising contacting the wood with an acetylation fluid containing acetic anhydride and / or acetic acid under wood acetylation conditions to obtain acetylated wood and a spent acetylation fluid containing acetic acid, and purifying the spent acetylation fluid by the method according to any one of [1] to

[15] .

[17] Use of the purified acetic acid obtained by the method according to any one of [1] to

[15] as an acetic acid reactant in the production of ketene by dehydrating acetic acid.

[18] Use of the purified acetic acid obtained by the method according to any one of [1] to

[15] as an acetic acid reactant in the production of acetic anhydride by reacting acetic acid with ketene.

[19] The use according to

[17] or

[18] in a method in which the production of ketene and the production of acetic anhydride are combined such that the produced ketene is used as a reactant in the production of acetic anhydride, wherein the purified acetic acid, which may be mixed with acetic acid from another source, is used in the production of ketene, the production of acetic anhydride, or both.

[20] An integrated method for the acetylation of wood and the production of acetic anhydride, wherein the method for the acetylation of wood is the method according to

[16] , whereby purified acetic acid is obtained, and the purified acetic acid, which may be mixed with fresh acetic acid, is used in the production of ketene and acetic anhydride according to

[19] , and the acetic anhydride thus obtained is provided to the acetylation fluid used in the method for the acetylation of wood.

Claims

**Claim 1** A method for purifying a used acetylated fluid recovered from a process of acetylating wood, wherein the recovered acetylated fluid contains acetic acid, the method comprising removing acetic anhydride from the recovered acetylated fluid, then cooling the fluid to below the melting point of acetic acid to form crystals of acetic acid, and separating the crystals from the fluid. **Claim 2** The method according to claim 1, comprising removing impurities selected from the group consisting of chlorides, acetonitrile, acetate esters, terpenes, terpenoids, and combinations thereof. **Claim 3** The method according to claim 1 or 2, wherein the wood is selected from the group consisting of solid wood, veneer, and wood elements. **Claim 4** The method according to any one of claims 1 to 3, wherein the process of acetylating the wood comprises subjecting the wood to an acetylated fluid containing acetic anhydride. **Claim 5** The method according to any one of claims 1 to 4, comprising cooling the recovered acetylated fluid in the presence of one or more seed crystals of acetic acid. **Claim 6** (a) cooling the recovered acetylated fluid to below the melting point of acetic acid to form crystals of acetic acid; (b) melting a first portion of the crystals to obtain a molten acetic acid product and retaining a second portion of the crystals; (c) recycling the molten acetic acid product; (d) cooling the molten product to below the melting point of acetic acid in the presence of at least a portion of the retained crystals; The method according to claim 5, comprising the steps of. **Claim 7** The method according to claim 6, wherein the first portion comprises 60% to 99% of the crystals formed in step (a). **Claim 8** The method according to claim 7, wherein the first portion comprises 85% to 95% of the crystals formed in step (a). **Claim 9** The method according to any one of claims 5 to 8, comprising adding seed crystals separately. **Claim 10** The method according to any one of claims 1 to 9, wherein the recovered acetylated fluid contains acetic anhydride and acetic acid. **Claim 11** The method according to any one of claims 1 to 10, wherein the removal of acetic anhydride is by distillation. **Claim 12** The method according to any one of claims 1 to 11, wherein the cooling is carried out to a temperature below the melting point of acetic acid. **Claim 13** The method according to claim 12, wherein the cooling is carried out to a temperature in the range of 0° to 16°C. **Claim 14** The method according to claim 13, wherein the cooling is carried out to a temperature in the range of 10°C to 15°C.

15. The method according to any one of claims 1 to 14, comprising washing the formed crystals.

16. The method according to any one of claims 1 to 15, comprising a plurality of subsequent melting steps and recrystallization steps.

17. The method according to any one of claims 1 to 16, wherein the fluid remaining after separating the crystals is recycled, and the cooling step and the crystallization step are carried out again, and the recycling is carried out 20 to 200 times.

18. The method according to claim 17, wherein the fluid remaining after separating the crystals is recycled, and the cooling step and the crystallization step are carried out again, and the recycling is carried out 50 to 100 times.

19. The method according to any one of claims 1 to 18, wherein the treatment of acetylating the wood comprises subjecting the wood to an acetylating fluid consisting of acetic anhydride.

20. A method for acetylating wood, comprising contacting the wood with an acetylating fluid containing acetic anhydride and / or acetic acid under wood acetylation conditions to obtain acetylated wood and a spent acetylating fluid containing acetic acid, and purifying the spent acetylating fluid by the method according to any one of claims 1 to 19.

21. A method for producing ketene, comprising purifying the spent acetylating fluid by the method according to any one of claims 1 to 19 to obtain acetic acid, and dehydrating the acetic acid to produce ketene.

22. A method for producing acetic anhydride, comprising purifying the spent acetylating fluid by the method according to any one of claims 1 to 19 to obtain acetic acid, and reacting the acetic acid with ketene to produce acetic anhydride.

23. The method for producing ketene and the method for producing acetic anhydride are combined such that the produced ketene is used as a reactant in the production of acetic anhydride, The method according to claim 21 or 22, wherein the purified acetic acid, which may be mixed with acetic acid from another source, is used in the production of ketene, the production of acetic anhydride, or both.

24. An integrated process for the acetylation of wood and the production of acetic anhydride, wherein the process for the acetylation of the wood is the process according to claim 20, whereby purified acetic acid is obtained, and the purified acetic acid, which may be mixed with fresh acetic acid, is used in the production of ketene and acetic anhydride according to claim 23, and the acetic anhydride thus obtained is provided to the acetylation fluid used in the said process for the acetylation of wood.

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