Acetic acid corrosivity reduction

Abstract

Disclosed is a method for reducing the corrosiveness of acetic acid recovered from a process of subjecting wood to reaction with an acetylation fluid comprising acetic anhydride. It has been found that such corrosiveness can be reduced by removing chlorinated terpenes and chlorinated terpenoids from the acid. A method to accomplish this is, inter alia, by cooling crystallization.

Description

[0001] Title: ACETIC ACID CORROSIVITY REDUCTION

[0002] Field of the Invention

[0003] The invention relates to a method to reduce the corrosiveness of impure acetic acid. Particularly this concerns acetic acid recovered from acetylation fluid used in the acetylation of wood.

[0004] Background

[0005] Acetic acid is an important chemical reagent and industrial chemical. Its production and properties have been well-established.

[0006] One process to produce acetic acid is by reacting wood with an acetylation fluid comprising acetic anhydride. The products of this reaction are acetylated wood and acetic acid. Reference is made to, e.g., WO 2009 / 095687. Herein a process is described for the acetylation of wood, by allowing wood to react with an acetylation fluid typically comprising acetic anhydride.

[0007] The used 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.

[0008] The reaction involved, can be represented according to the equation: [wood]OH + CH3C(O)OC(O)CH3[wood]OC(O)CHs + CH3C(O)OH. This process of acetylating wood thus effectively presents a source of acetic acid. The resulting acetic acid contains impurities, mainly wood extractives. As discussed in WO 2009 / 095687, it is known to sell this acetic acid as technical grade acid.

[0009] One particular problem with acetic acid produced from wood acetylation, is its corrosiveness. Steel subjected to durable contact with the acetic acid, exhibits an undesirably high corrosion rate. This corrosiveness is a limiting factor in potential uses of the acetic acid, and in the art, this has hitherto not been remedied.

[0010] The present invention seeks to address this particular problem.

[0011] Summary of the Invention

[0012] The invention, in one aspect, presents a method for reducing the corrosiveness of acetic acid recovered from a process of subjecting wood to reaction with an acetylation fluid comprising acetic anhydride, the method comprising removing chlorinated terpenes and terpenoids from the acid.

[0013] In another aspect, the invention resides in the use of crystallized acetic acid formed in a process comprising subjecting acetic acid recovered from wood acetylation to cooling crystallization, thereby separating the crystallized acetic acid from its corresponding mother liquor, for the purpose of reducing the corrosiveness of the acetic acid.

[0014] In a further aspect, the invention presents a process of producing acetic acid, the process comprising reacting an acetylation fluid comprising acetic anhydride with wood so as to form esterified wood and a used acetylation fluid comprising acetic acid, subjecting the used acetylation fluid to a step of separating acetic acid from acetic anhydride, and subjecting the acetic acid to a purifying step of removing chlorinated terpenes and terpenoids so as to form purified acetic acid, determining, preferably by means of gas chromatography, the level of chlorinated terpenes and terpenoids in the purified acid, and repeating the purifying step and the determining step until the level of chlorinated terpenes and terpenoids is non- detectable.

[0015] In still another aspect, the invention provides the use of acetic acid seed crystals for reducing the corrosiveness of acetic acid recovered from wood acetylation.

[0016] Detailed description

[0017] The invention is based on the judicious insight that chlorinated terpenes and terpenoids contribute substantially to the corrosiveness of acetic acid recovered from wood acetylation. This insight goes against the conventional explanation of the causes of said corrosiveness, which particularly is based on the presence of residual acetic anhydride in the recovered acetic acid. Additionally, it has been suggested in the field of steel alloys, that corrosiveness of acetic acid relates to the presence of ionic chlorides. This, however, is not specific for acetic acid recovered from wood acetylation, and cannot therefore serve to explain the particular corrosiveness thereof.

[0018] Corrosiveness in accordance with this disclosure is determined as the corrosivity, measured in terms of the corrosion rate, to stainless steel 316L at 115°C. This particularly refers to the corrosion rate measured using a standard method in which steel coupons are tested under an environment of air, in 300 g of a test liquid in a IL three-necked flask. The flask connections are to a reflux cooler, a thermocouple tube, and stopper. The test liquid is stirred by a stirring bead, at 70 rpm. The flask is heated by an oil bath, with the temperature being measured at approximately 1 cm above the coupon in the test liquid. The set-up is insulated, Tested coupons are Stainless Steel 316L. Test temperature is 115 °C. Test duration is at least 100 hours, with longer durations, up to two weeks, being applied to estimate the corrosivity of liquids that are barely corrosive.

[0019] The foregoing insight represents a novel and unexpected finding, which allows to better remedy the corrosiveness of acetic acid recovered from wood acetylation. Accordingly, in a broad sense, the invention puts to use this finding by a step of removing chlorinated terpenes and terpenoids from acetic acid recovered from wood acetylation, thereby reducing the corrosivity of said acid.

[0020] Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, including trees. Terpenes are hydrocarbons. Terpenoids are modified terpenes, containing additional functional groups, usually oxygen-containing. Terpenes exist as monoterpenes as well oligoterpenes. In the present disclosure, the term “chlorinated terpenes” indicates terpenes that have one or more chlorine substituents. The term “chlorinated terpenoids” indicates modified terpenes that, in addition, have one or more chlorine substituents.

[0021] Terpenes may be classified by the number of isoprene units in the molecule; a prefix in the name indicates the number of terpene units needed to assemble the molecule. Hemiterpenes consist of a single isoprene unit. Isoprene 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 C10H16. Examples of monoterpenes and monoterpenoids include geraniol, terpineol, limonene, myrcene, linalool or pinene. Iridoids derive from monoterpenes. Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24. Examples of sesquiterpenes and sesquiterpenoids include humulene, farnesenes, farnesol. Diterpenes are composed of four isoprene units and have the molecular formula C20H32. Examples of diterpenes and diterpenoids are cafestol, kahweol, cembrene and taxadiene. Sesterterpenes, terpenes having 25 carbons and five isoprene units, are rare relative to the other sizes. An example of a sesterterpenoid is geranylfarnesol. Triterpenes consist of six isoprene units and have the molecular formula C30H48. Sesquarterpenes are composed of seven isoprene units and have the molecular formula C35H56. Examples of sesquarterpenoids are ferrugicadiol and tetraprenylcurcumene.

[0022] Tetraterpenes contain eight isoprene units and have the molecular formula C40H64.

[0023] Typical chlorinated terpenes are bornyl chloride, fenchyl chloride, isobornyl chloride, pinene hydrochloride, dipentene hydrochloride, their isomers or their derivatives.

[0024] The acetic acid of which the process of the invention serves to reduce corrosivity, is used acetylation fluid recovered from any acetylation process of wood that contains terpenes. With any such wood, hydrochlorinated terpenes are possibly formed under the harsh conditions that are applicable during acetylation. Without wishing to be bound by theory, the inventors believe that, e.g., bornyl and fenchyl chloride form easily from beta-pinene, and to a lesser extent from alpha-pinene.

[0025] Such 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. The acetylation fluid can be acetic acid, acetic anhydride, or combinations thereof. Generally, the used acetylation fluid will comprise by-products from wood acetylation, extracted components from wood, such as terpenes and / or terpenoids, and excess acetylation fluid. Typically, when including acetic anhydride in the acetylation fluid, a by-product is acetic acid. Excess acetylation fluid will generally be acetic anhydride, acetic acid, or both.

[0026] Frequently, in the event of acetylation processes yielding a combination of acetic anhydride and acetic acid as a used acetylation fluid, it is desired to first separate the acetic acid from the acetic anhydride. This is generally done by distillation. As noted in the art, such distillation will result in acetic acid from which wood-derived impurities such as terpenes and / or terpenoids are not, or at least not sufficiently, removed.

[0027] In an embodiment, acetic anhydride is present in addition to the acetic acid in the used acetylation fluid that is subjected to the step of removing chlorinated terpenes and terpenoids. The presence of acetic anhydride specifically in conjunction with chlorinated terpenes, results in a synergistically increased corrosivity.

[0028] In an embodiment, the used acetylation fluid has been subjected to a step of removing acetic anhydride prior to being subjected to the step of removing chlorinated terpenes and terpenoids. It will be understood that the removal of acetic anhydride suitably is accomplished by distillation.

[0029] In an interesting embodiment, the step of removing chlorinated terpenes comprises subjecting the acetic acid to solvent extraction in order to remove chlorinated terpenes. Such solvent extraction can be carried out, e.g., by a method as disclosed in W02009 / 114070. This process comprises forming a separating composite extraction medium, by adding water to the acetic acid, as well as an organic solvent substantially immiscible with acetic acid. This results in that impurities are concentrated in the organic phase, whilst acetic acid can be recovered from the aqueous phase.

[0030] A preferred method to remove chlorinated terpenes from acetic acid containing these impurities, is by subjecting the acetic acid to cooling to below the melting point of acetic acid, under the formation of crystals, and separating the crystals from the fluid. Reference is made to WO 2022 / 23452.

[0031] In this method, a fluid comprising the recovered acetic acid, i.e., a used acetylation fluid, or acetic acid separated off from such fluid, is cooled to below the melting point of acetic acid. Said melting point being 16.6°C, cooling will generally be to below this temperature, such as to a temperature in a range of from 0°C to 16 °C, such as 10°C to 15°C. Lower temperatures can generally be applied, and will be applied in the event that the acetylation fluid has a lower freezing point, e.g. in the event of the presence of substantial amounts of lower melting compounds, such as water and / or acetic anhydride. The skilled person will be able to determine the freezing point of any fluid, such as used acetylation fluid, without difficulty.

[0032] In conducting the cooling crystallization method of this embodiment, it is preferred to recirculate the liquid that remains after crystallization, i.e., the mother liquor, rather than discard it after each round of crystallization. The recirculated fluid will thereby be replenished with fresh recovered acetylation fluid. This has the advantage of reducing waste, and it makes the process more economical. In preferred embodiments, such mother liquor is recirculated at least 20 times, such as at least 50 times, e.g., 20 to 200 times, preferably 50 to 100 times. Upon such recirculation, as a result of repeated crystallization of acetic acid, the concentration of possibly present liquids other than acetic acid will increase. E.g., if water is present in 0.1% in fresh recovered acetylation fluid, it will rise by more than 5% after 60 times recycling in the recirculated mother liquor. In fact, water can be regarded as an impurity, which is present in relatively high content as compared to the other impurities, in the acetic acid, and will build up in the mother liquor (as will do all the other molecules that do not freeze). Mixing of such fresh recovered acetylation fluid with the recirculating mother liquor can occur batch-wise from a collecting vessel for used acetylation fluid. The crystallization process preferably is conducted as a continuous process. Thereby the fresh recovered used acetylation fluid can be still added in batches, or also as a continuous feed.

[0033] The additional liquids typically are acetic anhydride or water. The former will be present as a result of the original composition of the acetylation fluid, the amount dependent on the extent at which such anhydride is optionally removed before subjecting the used acetylation fluid to the crystallization method of the invention. Water can be present depending on the concentration of the acetic acid and / or acetic anhydride used as the wood acetylation fluid. Water can be added to quench the anhydride. This reduces the risk of anhydride-based corrosion in the crystallization equipment, as such quenching will effectively remove anhydride. A catalyst, as known to the skilled person, can be added to speed up the reaction between water and acetic anhydride.

[0034] The cooling can be conducted in any vessel or tube suitable for allowing crystals to be formed. The skilled person is familiar with suitable equipment, such as a scraped-wall crystalliser. It will be understood that, in order to carry out the present method, the crystallization equipment will allow cooling. Typically, crystals formed will be subjected to washing, generally in a wash column, e.g. to remove a liquid film of the mother liquor that typically remains in melt crystallization. The skilled person is well- knowledgeable on how to operate crystallization equipment.

[0035] In order to bring about crystallization, no special measures need to be taken. If desired, the process can be aided by adding a small amount, such as less than 10 wt.%, such as less than 5 wt.%, e.g. 1 wt.% to 3 wt.%, of a suitable contaminant, e.g. water or acetic anhydride, in order to aid initiating crystallization. The skilled person will be aware of techniques to facilitate the occurrence of crystallization, such as scratching the wall of the crystallizer, e.g., with a spatula.

[0036] It can be advantageous to boost crystallization by having one or more seed crystals of acetic acid present. Such seed crystals can be added to the fluid when the temperature thereof is about or below the melting point of acetic acid. Seed crystals will generally be obtained and stored in advance. Advantageously, they can also be obtained in situ, in the process of the invention.

[0037] In an interesting embodiment, the crystallization is conducted in two stages. Thereby, in a first step, the recovered acetylation fluid is subjected to cooling to below the melting point of acetic acid under the formation of crystals. This step can be conducted as above, with or without seed crystals. Subsequently, in a second step, a first portion of the crystals so obtained, generally after washing off any film of residual feed liquid, is subjected to melting so as to obtain an acetic acid melt. Since not all of the crystals are melted, a second portion thereof is retained. The acetic acid melt is recirculated and again subjected to cooling, to below the melting point of acetic acid. This cooling being done in the presence of at least part of the retained crystals, the second crystallization step is thus conducted in the presence of seeds. The recirculated melt can be subjected to cooling as such, or after being combined with a further amount of used acetylation fluid recovered from a wood acetylation process (either from the same process run, or from a different run thereof, or - e.g., if multiple acetylation reactors operate in parallel, from a different process). The aforementioned process can be repeated so as to provide multiple washing and recrystallization steps, generally leading to a further degree of purity.

[0038] In the two-staged embodiment, said first portion (i.e., the crystals to be re-melted) comprises generally more than 50% of the crystals formed in the first step Preferably said first portion comprises 60% to 99% of the crystals formed in the first step, and more preferably 85% to 95% thereof. If desired, any third and subsequent stages can be conducted: after the crystallization of the second step, a portion of the crystals then formed can be re-melted again, and the process continued as above. Thus, wood acetylation and recovery of acetylation fluid with the purification process of this embodiment of the invention, can be an ongoing continuous or semi- continuous operation.

[0039] In another interesting embodiment, seed crystals are separately added, such as provided from storage, not requiring re-melting crystals obtained from the same acetylation fluid being crystallized. An advantage of this embodiment is that it more easily facilitates conducting the crystallization in a continuous process. It will be understood that also the continuous process may advantageously involve re-melting and recrystallization.

[0040] A combination of the foregoing embodiments is also conceivable. Hereby, in any stage, seed crystals can be chosen to be from storage, or from in situ crystal formation. E.g., in a first stage seed crystals can be provided from storage, and one or more subsequent stages, seed crystals are obtained by leaving part of the formed crystals out of re-melting.

[0041] In the preferred method of cooling crystallization, the formed acetic acid crystals play a pivotal role in obtaining acetic acid from which chlorinated terpenes and terpenoids are removed. This is reflected in a further aspect of the invention, related to the use of crystallized acetic acid. Herein the acetic acid crystals are formed in a process comprising subjecting acetic acid recovered from wood acetylation to cooling crystallization. These crystals consist of pure acetic acid and are solids obtained from a liquid phase (i.e., the mother liquor from which the crystals are obtained). The crystals are then used as a vehicle to separate pure acetic acid from the mother liquor, containing impurities. In the present invention this use serves a purpose not hitherto known, viz. for reducing the corrosiveness of the acetic acid. Particularly, this use is for the purpose of reducing the corrosiveness of the acetic acid by separating it off from chlorinated terpenes and terpenoids.

[0042] In an embodiment of forming the aforementioned acetic acid crystals, as mentioned above, acetic acid seed crystals can be added in order to initiate and / or boost crystallization. Accordingly, such seed crystals are applied in order to remove chlorinated terpenes and terpenoids from acetic acid, thereby rendering the latter less corrosive. This reflects a use of acetic acid crystals by adding these to a fluid comprising acetic acid recovered from wood acetylation, for the purpose of reducing the corrosiveness of the acetic acid. In another aspect, the invention presents a process of producing acetic acid. This process comprises reacting an acetylation fluid comprising acetic anhydride with wood. 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:

[0043] [wood]-OH + CH3-C(=O)-O-C(=O)-CH3[wood]-O-C(=O)-CHs + CHs-C(=O)-OH

[0044] Accordingly, this process of acetylating wood, effectively produces acetic acid. This acetic acid is purified, as discussed hereinbefore, in the sense that it is subjected, possibly after separating acetic acid from any residual acetic anhydride, to a purifying step so as to form purified acetic acid. According to the invention, this process is carried in such a way that it ensures the removal of the substances that the inventors have judiciously found as being causes of corrosivity, viz. chlorinated terpenes and terpenoids. This is reflected in a process step comprising determining the level of chlorinated terpenes and terpenoids in the purified acid. In the event that detectable levels are present, the process of the invention requires repeating the purifying step. Thereafter, the process comprises again determining step the level of chlorinated terpenes and terpenoids. This sequence of steps is repeated until the level of chlorinated terpenes and terpenoids is non-detectable.

[0045] Generally, the level of chlorinated terpenes and terpenoids is determined by means of a technique, preferably gas chromatography, having a detection limit of at most 1 ppm, preferably at most 0.5 ppm.

[0046] Preferably, the level of chlorinated terpenes and terpenoids is determined by gas chromatography, more preferably using dodecane as an internal standard and a flame ionization detector.

[0047] The process of reacting wood with an acetylation fluid comprising acetic anhydride, is generally conducted in accordance with optimized acetylation processes as are known in the field. Preferred processes comprise the following steps:

[0048] Providing wood (solid wood or wood elements);

[0049] Controlling, and if necessary adjusting, the moisture content of the wood or wood elements;

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

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

[0052] 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.

[0053] Acetylation reactions are generally conducted at temperatures of from 120°C to 200°C, such as 160°C to 180°C. The duration of the acetylation treatment generally ranges from 30 minutes to 3 hours. The skilled person will be able, for a given reactor equipment and depending on the wood species to be acetylated, to optimize the time and temperature conditions.

[0054] The wood to be acetylated is either in the form of wood elements or solid wood, and includes also wood veneers. The wood elements can preferably be, e.g., wood chips, wood strands, wood particles. The wood preferably belongs to non-durable wood species such as soft woods, for example, coniferous trees, typically spruce, pine or fir, or to non-durable hardwoods. Non-limiting examples of suitable types of wood are spruce, sitka spruce, maritime pine, scots pine, radiata pine, eucalyptus, red alder, European alder, beech, birch, loblolly pine, lodgepole pine, pitch pine, red pine, Southern yellow pine, Japanese cedar (sugi), and hemlock. Also suitable are monocots, such as palm, and other hardwoods, such as Paulownia, teak, maple, oak, white oak, and the like.

[0055] Typically, the wood to be acetylated is not wood pulp. Particularly, wood acetylation processes are distinguished from processes in which woodbased 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 shape, and only change the wood 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.

[0056] Typical dimensions of wood elements subjected to acetylation are given in the following table.

[0057] Table 1 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.

[0058] 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.

[0059] In sum, a method is disclosed for reducing the corrosiveness of acetic acid recovered from a process of subjecting wood to reaction with an acetylation fluid comprising acetic anhydride. It has been found that such corrosiveness can be reduced by removing chlorinated terpenes and chlorinated terpenoids from the acid. A method to accomplish this is, inter alia, by cooling crystallization.

[0060] The invention is illustrated with reference to the following, nonlimiting test example.

[0061] Example

[0062] The corrosivity of several test liquids is determined on coupons of stainless steel according to the following set-up:

[0063] Coupon in IL three-necked flask

[0064] Flask connections: o Reflux cooler o Thermocouple tube o Stopper

[0065] Test liquid is stirred by stirring bead

[0066] - Flask is heated by oil bath

[0067] Temperature is measured ~1 cm above coupon, in test liquid Set up is insulated Coupons = Stainless Steel 316L

[0068] Temperature = 115 °C

[0069] Duration = 100 hrs (or longer, up to two weeks, to estimate the corrosivity liquids that are barely corrosive)

[0070] Stirring = 70 rpm

[0071] Environment = Air

[0072] - Test Liquid = 300 g

[0073] Test liquids:

[0074] (A) Impure acetic acid recovered from the acetylation of radiata pine;

[0075] (B)Test liquid (A) with added 0.5% of water;

[0076] (C) Commercial glacial acetic acid;

[0077] (D) Commercial glacial acetic acid, with added chlorinated terpenoids;

[0078] (E) Commercial acetic acid, with added pinene;

[0079] (F) Commercial glacial acetic acid with added terpenes and acetic anhydride;

[0080] (G) Commercial glacial acetic acid with added chlorinated terpenes and acetic anhydride;

[0081] (H)Commercial glacial acetic acid with added chlorinated terpenoids in a reduced amount as compared to test liquid (D);

[0082] (I) Commercial glacial acetic acid with added sodium chloride;

[0083] (J) Commercial glacial acetic acid with added acetic anhydride;

[0084] (K)Test liquid (A), subjected to purification by melt crystallization;

[0085] (L) Test liquid (A), subjected to azeotropic distillation with water;

[0086] Results are given in Table 2 below (“nd” = non- detectable) Table 2

Claims

Claims1. A method for reducing the corrosiveness of acetic acid recovered from a process of subjecting wood to reaction with an acetylation fluid comprising acetic anhydride, the method comprising removing chlorinated terpenes and terpenoids from the acid.

2. A method according to claim 1, comprising subjecting the acetic acid to cooling to below the melting point of acetic acid, under the formation of crystals, and separating the crystals from the fluid.

3. The use of acetic acid seed crystals for reducing the corrosiveness of acetic acid recovered from wood acetylation.

4. The use of crystallized acetic acid formed in a process comprising subjecting acetic acid recovered from wood acetylation to cooling crystallization, thereby separating the crystallized acetic acid from its corresponding mother liquor, for the purpose of reducing the corrosiveness of the acetic acid.

5. The use of acetic acid crystals by adding these to a fluid comprising acetic acid recovered from wood acetylation, for the purpose of reducing the corrosiveness of the acetic acid.

6. A process of producing acetic acid, the process comprising reacting an acetylation fluid comprising acetic anhydride with wood so as to form esterified wood and a used acetylation fluid comprising acetic acid, subjecting the used acetylation fluid to a step of separating acetic acid from acetic anhydride, and subjecting the acetic acid to a purifying step of removing chlorinated terpenes and terpenoids so as to form purified acetic acid, determining the level of chlorinated terpenes and terpenoids in the purified acid, and repeating the purifying step and the determining step until the level of chlorinated terpenes and terpenoids is non-detectable.

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