Recovery of diols from mixtures

Abstract

The present invention relates to a method for purifying diols obtained by fermentation, in particular to improving the recovery of diols during distillation operations.

Description

[0001] The project that gave rise to this invention was funded by the Bio Based Industries Joint Undertaking (JU) under grant agreement No. 837866. JU received support from the European Union's Horizon 2020 research and innovation program and the Bio Based Industries Joint Undertaking.

[0002] This invention relates to a method for purifying diols obtained by fermentation, and particularly to improving the recovery rate of diols during distillation operations.

[0003] Examples of diols are short-chain saturated aliphatic diols, such as 1,3-butanediol and 1,4-butanediol.

[0004] 1,3-Butanediol (commonly known as BG, 1,3-BG, 1,3-BDO, or 1,3-butyleneglycol) is a tetracarbon diol with two stereoisomers: R-1,3-BDO and S-1,3-BDO. Racemic mixtures are commonly used in many industrial processes, such as as organic solvents for food flavorings or as reagents in the production of polyurethane resins and polyesters. Due to its low toxicity and high tolerability, 1,3-Butanediol is also increasingly used in the cosmetics industry for personal care products, such as in the formulation of shampoos and bath products, eye and face makeup, perfumes, personal hygiene products, shaving products, and skin care products. Optically active 1,3-BDO is also a widely used component in antibiotics, pheromones, fragrances, and pesticides.

[0005] On the other hand, 1,4-Butanediol (commonly known as 1,4-BDO, 1,4-BD, or 1,4-butyleneglycol) is widely used, for example, as a monomer for the production of various types of products (e.g., diacid-diol type polyesters) or as an intermediate for the synthesis of compounds such as γ-butyrolactone and tetrahydrofuran. Polyesters containing repeating units derived from dicarboxylic acids and diols are now widely used in all areas where thermoplastic polymer materials (e.g., films, molded and blow-molded articles, and fibers) are applied due to their mechanical and processing properties. In particular, according to EN 13432, it is also preferred that the polyester obtained therefrom is biodegradable.

[0006] The chemical production of C2 to C4 short-chain diols from fossil sources has been developed and optimized for decades. 1,3-BG is traditionally produced by a chemical method that involves hydrating acetylene to form acetaldehyde, then converting acetaldehyde to 3-hydroxybutyraldehyde and reducing it to form 1,3-BG.

[0007] 1,4-BDO can be synthesized from petrochemical feedstocks through various chemical methods: acetylene via acetylation with formaldehyde; butadiene via acetylation or halogenation; propylene via epoxidation or oxyacetylation; and n-butane via the formation of maleic anhydride through various pathways followed by hydrogenation.

[0008] 1,3-Propanediol is primarily produced via the hydration of acrolein. Alternative routes involve the hydroformylation of ethylene oxide to obtain 3-hydroxypropanal, which is then hydrogenated to yield 1,3-propanediol.

[0009] Due to the dwindling fossil fuel sources, the volatility of oil prices, and the increasing environmental concerns, the production of C2 to C4 diols from renewable sources via biological methods has attracted considerable attention. Indeed, 1,4-BDO can be produced by: directly from renewable sources (e.g., carbohydrates such as sugars and lignocellulosic biomass) or from syngas (CO, CO2, and / or H2) via fermentation (WO 2015 / 158716); via the formation of biosuccinic acid (WO 2011 / 063055) and its subsequent hydrogenation via fermentation; or via the formation of polyhydroxyalkanoates (WO 2011 / 100601).

[0010] Patent application WO 2015 / 158716 describes a method for producing 1,4-BDO, comprising fermentation in a culture medium by microorganisms having at least one metabolic pathway for synthesizing 1,4-BDO, wherein the culture medium comprises a mixture of glucose and sucrose. Similarly, WO 2010 / 127319 describes a fermentation process for producing 1,3-BG from a renewable source.

[0011] 1,3-Propanediol can also be produced by, for example, by fermentation of glucose using a transgenic strain of Escherichia coli as described in U.S. Patent 2008 / 176302, or by fermentation of glycerol using bacteria belonging to the genus Clostridium (WO 2020 / 030775).

[0012] Methods for producing diols from both chemical and renewable sources are typically followed by a purification process to remove unwanted impurities.

[0013] These impurities must be removed to ensure that the diol can be used in the synthesis of, for example, cosmetics and diacid-diol polyesters. The higher the purity level, the more sought-after the monomer is in these fields.

[0014] In fermentation methods for producing 1,4-BDO, such as those described in the aforementioned patent application WO 2015 / 158716, the diol is synthesized from sugars, preferably glucose, and optionally one or more sugars other than glucose, by microorganisms having at least one metabolic pathway for the synthesis of 1,4-BDO. However, the conversion rate of sugar to 1,4-BDO during fermentation is typically less than 100% because, in addition to the diol, intermediates (so-called byproducts) of the microbial metabolic pathway for the production of 1,4-BDO are generated, and these intermediates may accumulate in the fermentation broth as impurities.

[0015] Furthermore, at the end of fermentation, the organisms or cells that constitute the cellular biomass in the fermentation broth may be inactivated or killed, for example, by heating, resulting in the release of impurities such as residues and cell metabolites into the fermentation broth.

[0016] Many organic compounds, such as sugars, protein hydrolysates, proteins, amino acids, organic acids, and yeast extracts, can also be found as impurities in diols produced by fermentation. These are often supplied at a rate exceeding the needs of the microorganisms at the start of fermentation or during fermentation and are therefore retained in the fermentation broth at the end of fermentation. These compounds, along with released cellular residues and metabolites, may be degraded, especially during purification processes (which typically involve operations at high temperatures under dehydration conditions).

[0017] Among the byproducts, the aforementioned cell residues and metabolites, as well as organic compounds, are known as "heavy compounds." Heavy compounds are defined as mixtures of compounds with higher boiling points than diols (i.e., compounds with boiling points higher than pure diols). Examples of heavy compounds when the diol includes 1,4-BDO include 2-pyrrolidone, 1,6-hexanediol, cell residues and metabolites, and organic compounds that remain in the fermentation broth at the end of fermentation and may have undergone degradation.

[0018] Methods for purifying diols typically involve distillation operations to remove these heavy compounds, characterized by high viscosity and usually originating from the bottom of the distillation column, which constitute the so-called "heavy fraction." This heavy fraction is typically disposed of in landfills or incinerated, or fed to biogas digesters for biogas production to reduce the environmental impact of the method.

[0019] For example, patent application CN 105597351 describes an apparatus for recovering 1,4-BDO from a fraction containing compounds with high boiling points (the so-called "heavy fraction"). Such an apparatus comprises a storage tank, a rising film evaporator, a falling film evaporator, and a scraped film evaporator connected in sequence. The recovered BDO is then reintroduced into the purification process while the heavy fraction is incinerated.

[0020] However, the distillation process described above may result in the removal of some of the diols themselves from the heavy fraction, leading to product loss and reduced equipment productivity.

[0021] In fact, heavy fractions can contain as much as 10% to 60% by weight, typically 30% to 60% by weight, of glycols. Such high glycol content will not only cause serious product loss, but will also make it difficult to feed heavy fractions into biodigesters. High glycol concentrations in biodigesters can lead to sludge deactivation and biodigester equipment failure.

[0022] To overcome the aforementioned problems, it has now been unexpectedly discovered that diols can be recovered from mixtures containing these diols and compounds with higher boiling points, thereby limiting product loss while obtaining a heavy fraction suitable for feeding into a biodigester.

[0023] In fact, a method has been established that allows the recovery of diols from a mixture containing at least one diol and a compound having a boiling point higher than that diol, through one or more distillation operations to remove fractions rich in the compounds with higher boiling points, followed by dilution and filtration of the fractions. Due to the reduced diol content, the resulting filtrate can be advantageously fed into a biogas digester for biogas production.

[0024] Furthermore, it is advantageous to reintroduce the diol obtained as a permeate upstream of the purification process.

[0025] Therefore, the present invention relates to a method for recovering a diol from a mixture comprising at least one diol and a higher boiling point compound (i.e., a compound having a boiling point higher than that of the diol), the method comprising the following steps:

[0026] (a) subject the mixture to distillation to separate fractions rich in such compounds having boiling points higher than those of diols;

[0027] (b) Dilute the enriched fraction separated in step (a) with water;

[0028] (c) subject the diluted fraction from step (b) to one or more filtration operations by separating the permeate containing diol from the residue containing a compound having a boiling point higher than that of the diol.

[0029] Figure 1 A diagram showing step c) of the method according to Embodiment 2 is shown.

[0030] The fraction enriched in step a) may also contain 10% to 60% by weight, preferably 20% to 50% by weight, of the compound having a higher boiling point than the diol, relative to the total weight of the enriched fraction.

[0031] By means of the method according to the invention, up to 80% by weight, advantageously up to 90% by weight, of diols present in a mixture comprising diols and compounds having a higher boiling point than diols can be recovered.

[0032] The mixture according to the invention comprises at least one diol selected from the following: 1,2-ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,11-undecanediol. Alkanediols, 1,12-dodecanediol, 1,13-tetrazanediol, 1,4-cyclohexanediol, neopentyl glycol, 2-methyl-1,3-propanediol, 3-methyl-1,5-pentanediol, 2-methyl-1,8-octanediol, 2,2-diethyl-1,3-propanediol, bis(sorbitol), bis(mannitol), bis(idylol), cyclohexanediol, cyclohexanemethylenediol, dialkylene glycols, polyalkylene glycols, and mixtures thereof.

[0033] More preferably, the mixture contains at least one linear C2 to C6 aliphatic diol, preferably a linear C2 to C4 aliphatic diol.

[0034] Preferably, the mixture comprises at least one diol selected from the group consisting of 1,2-ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2-methyl-1,3-propanediol, and mixtures thereof.

[0035] Even more preferably, the mixture comprises a diol selected from 1,4-BDO, 1,3-BDO, and mixtures thereof. Even more preferably, the mixture comprises 1,4-BDO.

[0036] The method according to the invention is particularly advantageous because, through a simple operation aimed at removing the solvent, preferably water, and any residual impurities present in the permeate, high-purity diols particularly suitable for use in polymerization processes (where higher purity levels of monomers are highly desirable) can be obtained.

[0037] The method according to the present invention will be described in more detail below.

[0038] In step a) of the method according to the invention, a mixture comprising at least one diol and a compound having a boiling point higher than that of the diol (so-called heavy compound) is subjected to one or more evaporation and / or distillation operations to separate it from the solvent and / or a compound having a boiling point lower than that of the diol.

[0039] To effectively purify diols with varying impurity contents, distillation operations can be performed by appropriately determining the dimensions of the distillation system.

[0040] There are no particular restrictions on the number of distillation operations or the number of columns per operation.

[0041] Each distillation operation can be performed independently using different types and configurations of distillation columns according to techniques known in the art. For example, distillation columns may include random filling, structured filling, flat filling, random and structured filling, random and flat filling, or a combination of structured and flat filling. Filled columns are preferred, and structured filled columns are advantageous.

[0042] Each distillation operation can be performed using a single column or a group of columns, or through a more integrated configuration that allows more than two streams to be obtained from each column, such as by side extraction of the product or by inserting vertical baffles to minimize the number of columns and auxiliary equipment.

[0043] The distillation operation according to the invention is preferably carried out by reducing or minimizing the exposure of the compound to high temperatures. In fact, both the product and its impurities may undergo thermal or chemical degradation due to heating during distillation. It is preferable to operate the distillation column under reduced pressure (below atmospheric pressure) or vacuum, as this lowers the boiling temperature of the mixture in the distillation column and allows the column to operate at lower temperatures.

[0044] Those skilled in the art will be able to adjust the operating conditions for each step of the method depending on the type of one or more columns used. A standard vacuum system can be used with some or all of the distillation columns to achieve reduced pressure, or each column can have its own vacuum system.

[0045] The pressure in the distillation column can be measured anywhere, at the top or in the condenser, at the bottom or in between. Different distillation columns according to the method of the invention can operate at different pressures.

[0046] The evaporation and / or distillation operations in step a) are carried out at a temperature typically between 100°C and 200°C, preferably between 120°C and 170°C. The bottom temperature typically corresponds to the boiling point of the most commonly found diol plus 40°C, preferably plus 30°C, and even more preferably plus 20°C. The applied pressure head is preferably between 10 mbar and 300 mbar, more preferably between 20 mbar and 100 mbar. In this application, the operating pressure of the distillation column should be understood as measured in absolute millibars (mbar). 1 mbar corresponds to 100 Pascals.

[0047] Step a) of the method yields a fraction rich in heavy compounds. "Enriched fraction" means a fraction having a heavy compound content of greater than 40% by weight, preferably greater than 50% by weight, relative to the total weight of the enriched fraction. Advantageously, it has a diol content of greater than 10% by weight, preferably greater than 20% by weight, relative to the total weight of the enriched fraction.

[0048] This fraction can typically have a high viscosity, usually above 500 cP.

[0049] In step b) of the method according to the invention, the fraction rich in heavy compounds separated in step a) is diluted with water in order to reduce its viscosity and osmotic pressure and allow for subsequent separation by filtration.

[0050] Those skilled in the art will be able to assess the most suitable dilution based on the selected filtration technique. For example, in the case of separation by nanofiltration, the fraction is advantageously diluted to a concentration of less than 10% by weight of heavy compounds and 1% to 10% by weight of diols relative to the total weight of the diluted fraction.

[0051] The fraction diluted in step b) may undergo an optional solid / liquid separation step to remove any solid material contained therein before undergoing filtration in step c) of the method according to the invention.

[0052] The solid / liquid separation can be carried out by utilizing the different sizes of the particles present, and includes one or more operations selected from pressing, extrusion, decanting, sedimentation, centrifugation, microfiltration, and any other suitable techniques and combinations thereof for solid-liquid separation.

[0053] Preferably, the optional separation is performed by microfiltration.

[0054] Following the optional solid / liquid separation operation, the fraction obtained in step b) is subjected to filtration in step c).

[0055] The filtration operation in step c) can be performed in one or more stages and advantageously includes ultrafiltration and / or nanofiltration. Preferably, the filtration operation includes at least one stage, preferably multiple stages, of nanofiltration, with at least one percolation stage. If multiple stages are present, countercurrent percolation may be more effective.

[0056] Based on the characteristics of the fractions undergoing the filtration operation, those skilled in the art will be able to select the type of membrane to be used, taking into account the materials used to make them, their electrochemical properties, and their porosity. Based on the characteristics of the selected materials, those skilled in the art will also be able to readily select the optimal pH conditions and operating pressures during each separation operation, as well as assess the desirability of performing one or more percolation steps (i.e., diluting the percolate by adding water and repeating the separation operation).

[0057] For example, filtration can be effectively achieved using both naturally derived organic membranes (e.g., rubber, polysaccharides) and synthetically derived organic membranes (e.g., polymer membranes) and inorganic membranes (e.g., ceramic, metal, or glass membranes). Among organic membranes, polyamides, polyimides, polyalkylene oxides, polyetherimides, polyaryl ethers, poly(ether ketone), polycarbonates, cellulose acetates, and their derivatives are preferred. Specific examples of suitable organic membranes include polysulfones, aromatic polyamides, polypiperazine amides, polyethylene, polytetrafluoroethylene (PTFE), polypropylene, polyvinyl alcohol, polystyrene, polybenzimidazole (PBI), polyphenylene oxide, polyphosphazene, polyvinylidene fluoride (PVDF), polyethersulfone (PES), polyacrylonitrile (PAN), and polyvinyl chloride (PVC). Isotropic (or symmetrical) membranes, anisotropic (or asymmetric) membranes, and composite membranes are all suitable. Preferably, anisotropic membranes are used.

[0058] Porous membranes (i.e., pore sizes of 1 nm to 10 μm, e.g., macropores > 50 nm, mesopores 2 nm to 50 nm, and micropores 1 nm to 2 nm) can be used in this method. Dense membranes (pore sizes < 1 nm) can also be advantageously used, particularly after at least one preliminary membrane separation operation.

[0059] The diaphragm used in step c) advantageously has an average pore size of 5 nm or less, and even more advantageously has an average pore size in the range of 2 nm (corresponding to a molecular cutoff or MWCO of about 1000 Da to 1200 Da) to 0.7 nm (corresponding to about 120 Da to 150 Da).

[0060] The diaphragm can be formed in different configurations, such as flat, tubular, capillary, or hollow fibers. Flat diaphragms can be used to increase the surface area to volume ratio when the diaphragm is in a filter press system, in a rotary system, or wound in a spiral module.

[0061] The filtration operation according to the invention can be carried out in an intermittent or continuous process; depending on the circumstances, filtration methods in orthogonal (vertical) flow or tangential flow are preferred. A separation operation through a diaphragm in a tangential flow state is preferred.

[0062] Nanofiltration is preferably performed using a membrane made of a material selected from polysulfone, polypiperazine amide, polyamide, and polyimide, according to the present invention.

[0063] During the filtration operation in step c), the mixture is preferably maintained at a temperature of room temperature (20°C to 25°C) to 50°C, more preferably 35°C to 45°C.

[0064] At the end of filtration step (c), the permeate containing diol is separated from the residue containing compounds with higher boiling points (i.e., compounds with boiling points higher than those of diol).

[0065] The diol obtained from the permeate in step c) of the method according to the invention can advantageously undergo a purification process, which includes further optional solid / liquid separation and / or concentration and / or distillation operations aimed at removing water and possible residual impurities. Such operations make it possible to obtain a high-purity diol suitable for use in the polymerization process and increase the overall yield of the production unit.

[0066] Other optional solid / liquid separation operations can be performed, for example, by one or more of decantation, centrifugation, filtration, microfiltration, nanofiltration, ultrafiltration, ion exchange, osmosis, other suitable solid / liquid separation techniques, and combinations thereof.

[0067] According to one aspect of the invention, compounds having a boiling point higher than that of diols include compounds having low molecular weight and low rejection rates, thus allowing them to pass through into the permeate of step c), thereby reducing the purity of the diol.

[0068] In this case, the method according to the invention advantageously includes an optional step d) following step c), in which the permeate is concentrated, for example, by evaporation or reverse osmosis; and a subsequent step e), in which the concentrated permeate is treated by ion exchange, adsorption, or other techniques that allow for the separation of impurities from the diol-containing stream. The permeate thus treated can advantageously be reintroduced upstream of the purification process and undergo further optional solid / liquid separation and / or concentration and / or distillation operations aimed at removing water and any possible residual impurities. This embodiment of the invention is particularly advantageous for recovering 1,4-BDO from mixtures containing compounds having low molecular weights and boiling points higher than 1,4-BDO (e.g., 2-pyrrolidone).

[0069] The mixture comprising diols and heavy compounds can originate from methods used to purify diols obtained from biomass, such as from previous distillation operations typically performed to remove solvents. Therefore, the method according to the invention is advantageously applicable to methods for purifying diols that include distillation operations.

[0070] The permeate obtained at the end of step c) or optional step e) may, for example, undergo one or more treatments with ion exchange resins, as described in patent application WO 2019 / 102030.

[0071] These resins can be cation exchange resins or anion exchange resins.

[0072] Cation exchange resins are typically selected from resins derived from strong acids (e.g., sulfonic acid groups) or weak acids (e.g., carboxylic acid groups), and preferably contain functional groups selected from sulfonic acid groups. Non-limiting examples of cation exchange resins include, for example, those marketed under trademarks... 88 or 88MB of commercially available resin.

[0073] Anion exchange resins are typically selected from resins derived from strong bases (e.g., quaternary ammonium groups) or weak bases (e.g., tertiary amine groups), and preferably contain functional groups selected from quaternary ammonium groups. Non-limiting examples of anion exchange resins include, for example, those marketed under trademarks... 22 Commercially available resins.

[0074] There is no particular restriction on the order in which the resins pass through the cation exchange resin and the anion exchange resin. One or more passes through the cation exchange resin may be performed before or after one or more passes through the anion exchange resin. Preferably, one or more passes through the cation exchange resin occur before one or more passes through the anion exchange resin.

[0075] The permeate obtained at the end of step c) or optional step e), or the solution obtained after treatment with ion exchange resins or between different passes through these resins, can undergo a concentration operation using techniques known to those skilled in the art.

[0076] Concentration operations can be selected, for example, from evaporation and / or reverse osmosis.

[0077] The permeate obtained at the end of step c) or optional step e), or the aqueous solution obtained after passing through an ion exchange resin or after concentration, may undergo a distillation operation.

[0078] According to one implementation scheme, the permeate obtained at the end of step c) is subjected to evaporation, ion exchange treatment and distillation.

[0079] The following examples illustrate the present invention, but do not limit the scope of the invention. Example:

[0080] Example 1

[0081] Step a)

[0082] A mixture containing 80 wt% 1,4-BDO, water, and impurities derived from the production process (containing 0.76 wt% of compounds with boiling points higher than 1,4-BDO, relative to the weight of the mixture) of biological properties is fed into a first distillation column operating at a head of approximately 110 mbar to remove water. The bottoms from the first column are then fed into a second separation column operating at a head of approximately 33 mbar.

[0083] The fraction separated from the bottom of the second column is fed into a membrane evaporator operating at 10 mbar to produce a fraction rich in heavy compounds, the fraction having a content of 70% by weight of such compounds having a boiling point higher than that of diols relative to the total enriched fraction, and a diol content of 30% by weight relative to the weight of the enriched fraction.

[0084] Step b)

[0085] The fractions of heavy compounds separated in step a) were diluted with water to a heavy compound concentration of 8.6% by weight (1:8) relative to the total weight of the diluted fractions.

[0086] Step c)

[0087] The fraction diluted in step b) is subjected to a first filtration on paper to retain any solid compounds, followed by a laboratory filtration module (labstack) equipped with a diaphragm (NF PET series). TM Nanofiltration is performed using M20.

[0088] Nanofiltration is carried out in batch mode at a pressure of 30 bar, in which the permeate is recycled back to the feed tank and the permeate is continuously extracted.

[0089] This process is continued until a VCF of 4 is achieved, resulting in a clear permeate. Throughout the nanofiltration process, the BDO rejection rate ranges from 0% to 10%.

[0090] Example 2

[0091] Step c) of the method was simulated on a spreadsheet using a mixture having the composition of the dilute solution prepared in step b) of Example 1.

[0092] After filtering with a bag filter to retain any solid deposits, according to Figure 1 The scheme described herein subjectes a dilute solution to a continuous nanofiltration process, comprising a first concentration step and a second percolation step. The first step proceeds to a VCF of 3, and the second step to 3.2. Percolation is performed by feeding water and percolate from the first step at a ratio of 2:1. The composition of the inlet and outlet flows in each step (1 to 11) is shown in Table 1.

[0093] As can be seen from Table 1, at the end of step (c), a residue (9) was obtained, which contained 26.9% by weight of a compound having a higher boiling point than the diol relative to the weight of the residue.

[0094] Through steps b) and c), more than 83% of the 1,4-BDO present in the enrichment fraction can be recovered in the permeate (11), thereby improving the recovery rate of the diol distilled in step a). The permeate, which is a clear, dilute solution of BDO, can be conveniently recycled upstream of the production process, offering significant economic advantages.

[0095]

Claims

1. A method for recovering a diol from a mixture comprising at least one linear C2 to C6 aliphatic diol and a compound having a boiling point higher than that of the linear C2 to C6 aliphatic diol, the method comprising the steps of: (a) subjecting the mixture to distillation to fractionate the compounds rich in the compounds having boiling points higher than those of the linear C2 to C6 aliphatic diols; (b) Dilute with water the fraction of the compound separated in step (a) that is rich in the compound having a boiling point higher than that of the linear C2 to C6 aliphatic diols; (c) subjecting the fraction diluted in step (b) to one or more filtration operations by separating the permeate containing the linear C2 to C6 aliphatic diol from the residue containing the compound having a boiling point higher than that of the linear C2 to C6 aliphatic diol.

2. The method according to claim 1, wherein the mixture comprises at least one linear C2 to C6 aliphatic diol selected from the group consisting of 1,2-ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 1,6-hexanediol, and mixtures thereof.

3. The method according to claim 2, wherein the linear C2 to C6 aliphatic diol is selected from 1,2-ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol and mixtures thereof.

4. The method according to claim 3, wherein the linear C2 to C6 aliphatic diol is 1,3-butanediol and / or 1,4-butanediol.

5. The method according to any one of claims 1 to 4, wherein, relative to the weight of the fraction rich in the compound having a boiling point higher than that of the linear C2 to C6 aliphatic diols, the fraction separated in step a) is rich in the compound having a boiling point higher than that of the linear C2 to C6 aliphatic diols containing more than 40% by weight of the compound having a boiling point higher than that of the linear C2 to C6 aliphatic diols.

6. The method according to any one of claims 1 to 4, wherein, relative to the total weight of the fraction rich in the compound having a boiling point higher than that of the linear C2 to C6 aliphatic diols, the diol content of the fraction separated in step a) rich in the compound having a boiling point higher than that of the linear C2 to C6 aliphatic diols is 10% to 60% by weight.

7. The method according to any one of claims 1 to 4, wherein the filtration operation in step c) comprises ultrafiltration and / or nanofiltration.

8. The method of claim 7, wherein the filtration operation in step c) comprises nanofiltration.

9. The method according to any one of claims 1 to 4, wherein the filtering operation in step c) is performed in multiple stages.

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