Method for reducing content of 2, 3-pentanediol in propylene glycol

Abstract

A process for removing 2, 3-pentanediol from a propylene glycol-rich mixture, said process comprising an extractive distillation in which an entrainer having a Hansen solubility parameter [delta] H of between 5 and 15 is used, and said entrainer has a boiling point at atmospheric pressure of at least 200 DEG C.

Description

[0001] INTRODUCTION

[0002] The present invention relates to a method for reducing the content of 2,3-pentanediol in a mixture rich in propylene glycol, wherein the mixture comprises a small amount of 2,3-pentanediol. The method comprises extractive distillation, in which an entrainer having a Hansen solubility parameter δH between 5 and 15 is used, and the boiling point of said entrainer at atmospheric pressure is at least 200 °C. BACKGROUND

[0004] For the last decade, there has been an ongoing interest in developing methods for the manufacture of chemical building blocks from renewable resources. One such method of interest is the derivation of monoethylene glycol (in short: ethylene glycol, MEG or EG) from (renewable) carbohydrate sources such as sugars.

[0005] WO 2016 / 114661 discloses a continuous process for the preparation of ethylene glycol from a carbohydrate source. The process is carried out in a stirred tank reactor (CSTR) in the presence of a catalyst system. The catalyst system comprises a tungsten compound and at least one hydrogenolysis metal. The hydrogenolysis metal is preferably present in the form of a catalyst supported on a carrier. Such heterogeneous catalyst particles can be separated rather easily from the effluent stream and recycled.

[0006] The resulting liquid effluent is typically subjected to a series of separation steps, for example to separate the tungsten compound used as a co-catalyst (or formed by the process), and to separate the various (volatile) alcohols, polyols and diols produced. Typically, the target product in the above-mentioned reference and similar processes is ethylene glycol. Although the selectivity of such processes for ethylene glycol can be about 40% to 70%, a significant amount of other components is also produced, mainly mono-propylene glycol (MPG), but also polyols such as glycerol, sorbitol and erythritol. The resulting mixture typically contains, in addition to MEG and MPG, other diols and diols, such as 1,2-butanediol, 1,4-butanediol, 1,2-pentanediol and 2,3-pentanediol. Although the typical method for separating miscible liquids on an industrial scale is fractional distillation, this method has limitations in separating some of the diols, because the boiling points are often very close, and other diols or polyols to be separated tend to form azeotropes with ethylene glycol or propylene glycol, which makes separation by distillation more difficult.

[0007] Although the purification of ethylene glycol has been extensively studied in the context of carbohydrate conversion to ethylene glycol, there are fewer publications addressing the removal of secondary compounds from propylene glycol.

[0008] As mentioned above, when the hydrogenolysis of a carbohydrate is intended to produce ethylene glycol, a significant amount of monopropylene glycol (abbreviated: MPG, or propylene glycol, or PG) is also produced. Depending on the carbohydrate source and process conditions, the EG:PG weight ratio produced can range from 10:1 to 1:1. Propylene glycol is also a valuable product, although its market is smaller than that of ethylene glycol. Since it is valuable, particularly in pure form, and since the production is significant, it is desirable to be able to obtain propylene glycol in a reasonably pure form. However, one of the byproducts of carbohydrate hydrogenolysis is 2,3-pentanediol. Depending on the carbohydrate source and process conditions, the amount of 2,3-pentanediol produced can be 0.2 to 3 wt% of the propylene glycol. In fact, since there are two chiral centers (2 and 3 positions), there are four different stereoisomers of 2,3-pentanediol. The four stereoisomers are: (2R,3R)-2,3-pentanediol, (2S,3S)-2,3-pentanediol, (2R,3S)-2,3-pentanediol, and (2S,3R)-2,3-pentanediol. To the best of the inventors’ knowledge, all four forms are produced by the reaction. Of the four isomers, the (2R,3R) and (2S,3S) enantiomers have a boiling point (at atmospheric pressure) of about 175°C, which can be reasonably easily separated from monopropylene glycol, which has a boiling point of about 188.2°C (at atmospheric pressure). However, the (2R,3S) and (2S,3R) enantiomers have a boiling point of about 187.5-189.5°C, which makes it very difficult, if not impossible, to separate these components from monopropylene glycol (boiling point 188.2°C) by direct fractional distillation to a reasonable degree. For monopropylene glycol produced by carbohydrate hydrogenolysis, it is desirable not only to remove the (2R,3R) and (2S,3S) stereoisomers to a significant degree, but also to achieve a reduction in the (2R,3S) and (2S,3R) stereoisomers.

[0009] WO 2017 / 050847 discloses a process for removing 1,2-butanediol from ethylene glycol by extractive distillation, using a C3-C6 sugar alcohol (e.g. glycerol) as the extractant. In the process of this reference, extractive distillation is such that ethylene glycol is extracted by the extractant: 1,2-butanediol and propylene glycol (as well as the byproducts 2,3 butanediol, 1,2 pentanediol, 2,3-pentanediol, 1,2-hexanol and 1,2-heptanol) are removed at the top of the extractive distillation column, and a mixture of ethylene glycol and extractant is obtained at the bottom of the column. The bottom mixture is then subjected to separation to produce ethylene glycol free of 1,2-butanediol and extractant. It should be noted that this reference is exemplified by a model, not by actual experiments. The specification and Figure 2It is indicated that the overhead stream can be separated by (fractional) distillation into a purified propylene glycol stream and the by-product stream. The examples do not provide any evidence for this and considering that the boiling points of 2,3-pentanediol (i.e. a mixture of (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol) and propylene glycol are very close (187-189.5 °C and 188.2 °C, respectively), this is not possible in practice. Another disadvantage of the WO 2017 / 050847 process is that the extractant needs to be separated from the main product (ethylene glycol), which means that the volume of fluid handled in the extractant recovery column is large: all of the extractant and all of the ethylene glycol. This has a negative impact from an investment (larger equipment) and operating cost (large volume that needs to be heated and cooled) point of view and is therefore undesirable.

[0010] WO 2023 / 041842 discloses a method to reduce the content of MEG and butanediols (1,2- and 2,3-) in a composition containing MPG. The model example contains about 53% MPG, 21% MEG, 4% butanediols and about 0.6% organic impurities that form azeotropes with MPG. The method relies on distillation with the addition of a solvent. The solvent disclosed in the examples is triethylene glycol (TEG). The solvent:feed ratio in the experiments is high (e.g. >12 in experiments 9 and 10) and the reflux ratio is also high (between 5 and 50 in the experiments), which leads to high energy costs.

[0011] CN 112920021 discloses a method to purify 1,3-propanediol by liquid-liquid extraction.

[0012] Therefore, there is a need for a method that allows for a reduction of the content of 2,3-pentanediol present in a mixture comprising at least 80 wt% (preferably at least 90 wt%) of propylene glycol and 0.05 to 5 wt% of 2,3-pentanediol (in particular (2R,3S)- and (2S,3R)-2,3-pentanediol). In this respect, "reduction" means that less of these pentanediols are present compared to the diol mixture before the application's separation process is applied and preferably should refer to (2R,3S)- and (2S,3R)-2,3-pentanediol. Preferably, the reduction of 2.3-pentanediol should be such that the content of 2,3-pentanediol in any ethylene glycol produced is 50% or less (preferably 30% or less, more preferably 1% or less, even more preferably 0.2% or less) of the amount of 2,3-pentanediol in the product feed (calculated as propylene glycol).

[0013] Regarding the propylene glycol produced, it is desirable to provide a purification method that can reduce the content of (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol (hereinafter referred to as: (2R,3S), (2S,3R)-2,3-pentanediol) in a stream mainly containing propylene glycol by at least 30% by weight, preferably by at least 50% by weight. The reduction percentage is the percentage reduction in the amount of the combination of enantiomers (2R,3S) and (2S,3R) in the purified stream compared to the amount of the corresponding combination in the stream before purification. Furthermore, it is desirable that this purification method can be integrated with a separation process that can be used to separate the product stream from carbohydrate hydrogenolysis into a fraction rich in ethylene glycol and a fraction rich in propylene glycol, wherein the fraction rich in ethylene glycol is also subjected to a method to reduce the content of 1,2-pentanediol. All of this is done in a practical manner, requiring as few compounds as possible to achieve the three separation / purification steps (the three steps being: EG / PG separation, removal of 1,2-pentanediol from EG, and removal of (2R,3S) and (2S,3R)-2,3-pentanediol from PG).

[0014] It is also preferred that the method provides propylene glycol containing less than 1% by weight (based on propylene glycol) of a combination of (2R,3S)-2,3-pentanediol and (2S,3R)-pentanediol, preferably less than 0.5% by weight (based on propylene glycol).

[0015] This method should preferably minimize the amount of auxiliary compounds (e.g., extractants or solvents), as this is beneficial for operation, and preferably minimize the volume of the stream to be processed. Furthermore, it is desirable to minimize the number of unit operations required and energy consumption is preferably minimized, and this method preferably does not have the disadvantages of prior art. Summary of the Invention

[0016] It has now been found that the above objective can be achieved, at least in part, by a method of reducing the 2,3-pentanediol content in a mixture by extractive distillation, the mixture comprising 90-99.9% by weight of propylene glycol and 0.05-5% by weight of 2,3-pentanediol, wherein an entrainer is used in the extractive distillation, the entrainer having a Hansen solubility parameter δH between 5 and 15, and the entrainer having a boiling point of at least 200°C at atmospheric pressure.

[0017] Preferably, the Hansen solubility parameter δP of the entrainer is between 2 and 12, more preferably between 3 and 11, more preferably between 3.7 and 10, and most preferably between 3.8 and 9. Even more preferably, the Hansen solubility parameter δD of the entrainer is between 15 and 20, more preferably between 15 and 18. Regarding the Hansen solubility parameter δH, preferably the Hansen solubility parameter δH of the entrainer is between 6 and 14, more preferably between 7 and 13.

[0018] Suitable entrainers are preferably selected from: C6-C10 glycol ethers, or C9-C18 unbranched aliphatic primary or secondary alcohols, and mixtures thereof.

[0019] Since 2,3-pentanediol is inevitably formed by the hydrogenolysis of carbohydrates with hydrogen in the presence of a catalyst, and (2R,3R)-2,3-pentanediol and (2S,3S)-2,3-pentanediol can be readily removed by conventional fractionation, but until now, the method claimed in this invention has only achieved sufficient removal of [(2R,3S)-2,3-pentanediol + (2S,3R)-2,3-pentanediol]. Therefore, this invention further relates to a propylene glycol composition comprising at least 99.5% by weight of propylene glycol and further comprising less than 0.5% by weight of 2,3-pentanediol, wherein the weight ratio of 2,3-pentanediol [(2R,3R)-2,3-pentanediol + (2S,3S)-2,3-pentanediol] : [(2R,3S)-2,3-pentanediol + ... [(2S,3R)-2,3-pentanediol] is between 1:1 and 1:20, preferably between 1:1 and 1:10. Detailed Implementation

[0020] The term "entrainer" in this article includes pure compounds as well as mixtures of compounds having the claimed Hansen solubility parameter and boiling point.

[0021] The method according to the invention involves two enantiomers, (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol. Two stereoisomers, (2R,3R)-2,3-pentanediol and (2S,3S)-2,3-pentanediol, also exist. Without being bound by theory, it is assumed that all four stereoisomers are formed during the hydrogenolysis of carbohydrates, although their amounts may vary. The enantiomers (2R,3R)-2,3-pentanediol and (2S,3S)-2,3-pentanediol have boiling points of about 175°C and can therefore be sufficiently separated from propylene glycol by fractional distillation. However, the boiling points of the (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol enantiomers are very close to those of propylene glycol, thus requiring a different technique for their separation from propylene glycol, as is now provided. Although the claimed extractive distillation may remove some or all of (2R,3R)-2,3-pentanediol and (2S,3S)-2,3-pentanediol, these components with boiling points sufficiently lower than propylene glycol are usually removed before the claimed extractive distillation, for example by fractionation.

[0022] The inventors of this invention discovered that a common feature of entrainers suitable for this purpose is that their Hansen solubility parameter δH is between 5 and 15 (and preferably between 2 and 12, and between 15 and 20). Prior to this invention, reducing the content of (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol in propylene glycol was problematic, especially when present in small amounts (e.g., 0.5-2%), because (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol have very close boiling points to propylene glycol. With this method, it has been found that the content of these 2,3-pentanediols present in compositions primarily comprising monopropylene glycol can be significantly reduced. For practical reasons (considering the temperatures involved), the entrainers used in this method have a boiling point of at least 200°C at atmospheric pressure.

[0023] Surprisingly, it was also found that using the same entrainer for extractive distillation could reduce the content of 1,2-pentanediol in the composition, which mainly comprises (e.g., at least 90% by weight, preferably at least 95%) ethylene glycol.

[0024] Therefore, the inventors of this case discovered that the specified entrainer can be used as an entrainer in the extractive distillation of ethylene glycol as well as in the extractive distillation of propylene glycol. That is, the same entrainer can be used for two different separations, which is very convenient for methods such as carbohydrate hydrogenolysis, which, in addition to primarily producing ethylene glycol, also produces a large amount of propylene glycol, and obtains both ethylene glycol and propylene glycol in relatively pure form. Byproducts are obtained in such hydrogenolysis methods, which contain 1,2-pentanediol and 2,3-pentanediol. 1,2-pentanediol typically ends up in the ethylene glycol stream and is difficult to remove adequately from the desired ethylene glycol, while 2,3-pentanediol (especially the enantiomers (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol) ends up in the propylene glycol stream and is also difficult to remove from the desired propylene glycol.

[0025] To identify suitable entrainers, a variety of chemical components were screened in the laboratory, specifically the relative volatility of propylene glycol and 2,3-pentanediol, and how the presence of an entrainer altered this relative volatility. This provided a prediction of the suitability of an entrainer for this purpose. Details are set forth in Example 2. Several glycol ethers and some fatty alcohols performed promisingly in this test, and all were found to meet the Hansen solubility parameters currently claimed. Therefore, preferably, the entrainers of the present invention are selected from: C6-C10 glycol ethers, or C9-C18 (preferably C10-C12) unbranched aliphatic primary or secondary alcohols, and mixtures thereof. "C6-C10 glycol ethers" herein refers to glycol ethers having 6 to 10 (inclusive) carbon atoms in their molecular formula. Similarly, "C9-C18 unbranched aliphatic primary or secondary alcohols" herein refers to unbranched aliphatic primary or secondary alcohols containing 9 to 18 carbon atoms.

[0026] Based on this (and the requirement that the boiling point should be at least 200°C under atmospheric pressure), preferably, in the method of the present invention, when the entrainer is a glycol ether, it is selected from the compounds and mixtures thereof appearing in Table 1. Table 1 also gives the Hansen solubility parameters of each compound.

[0027] Table 1

[0028] Therefore, these are preferred glycol ethers in this invention. Based on factors such as cost, availability, safety, impurities, and thermal stability, when using glycol ethers as entrainers, the glycol ethers are preferably selected from the group consisting of triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, and mixtures thereof.

[0029] As an alternative to diols, the entrainer can also be a C9-C18 unbranched aliphatic primary or secondary alcohol, preferably a C10-C12 unbranched aliphatic primary or secondary alcohol. Based on this (and the requirement that the boiling point should be at least 200°C under atmospheric pressure), preferably, in the method of the present invention, when the entrainer is an alcohol, it is selected from the compounds and mixtures thereof appearing in Table 2.

[0030] Table 2

[0031] Another suitable entrainer for the purposes of this invention is triethyl phosphate. This compound has a δD of 16.7, a δP of 11.4, and a δH of 9.2.

[0032] For the purposes of this invention, potential entrainers used industrially for similar purposes were tested, and it was found that these entrainers appear unsatisfactory in the context of this invention. These entrainers are listed in Table 3.

[0033] Table 3

[0034] Due to factors such as cost, availability, safety, impurities, and thermal stability, when the entrainer is an unbranched aliphatic primary or secondary alcohol, the preferred alcohols are 1-dodecanol and 2-decanol. Preferred glycol ethers in this invention are triethylene glycol monoethyl ether and triethylene glycol monobutyl ether. Therefore, preferably, the entrainer in this invention is selected from triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, 1-dodecanol, and 2-decanol, and mixtures thereof.

[0035] The present invention also relates to a method, wherein the method comprises: a) Feed the mixture into distillation column 1. b) Feed the entrainer into the distillation column 1. c) Remove propylene glycol from the top portion of the tower. d) Remove the mixture from the bottom portion of the distillation column 1, the mixture comprising an entrainer and 2,3-pentanediol (preferably (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol).

[0036] Clearly, it is necessary to release the entrainer from 2,3-pentanediol for reuse. This method is a method for regenerating the entrainer. Therefore, the present invention also relates to a method comprising: - The mixture containing the entrainer and 2,3-pentanediol obtained in step d) is fed into column 2. The regenerated entrainer is produced at the bottom of column 2, and a mixture containing the entrainer and 2,3-pentanediol (preferably (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol) is produced at the top of column 2. - Combine the regenerated entrainer with the entrainer fed into tower 1 in b).

[0037] Distillation in column 1 can be carried out in a distillation column known in the art, into which is fed (crude) propylene glycol to be purified [therefore containing some 2,3-pentanediol (preferably: (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol), for example, 0.05-5% by weight of crude propylene glycol, preferably 0.1 to 3% by weight].

[0038] Regarding the extractive distillation column 1, for efficient extractive distillation, it is generally preferred that the feed to be purified is added neither near the top nor near the bottom of column 1. Preferably, the feed is added in the middle half of the column (between the top quarter and the bottom quarter). This should be understood as follows: if column 1 has 100 theoretical stages, the feed to be purified is preferably added between stages 25 and 75 of the column. Furthermore, it is generally preferred that the entrainer is added above the feed. Thus, in the method according to the invention, preferably, in a) the position where the mixture comprising propylene glycol and 2,3-pentanediol is fed into column 1 is located at stage N of the column, and in b) the position where the entrainer is fed into column 1 is located at stage M of the column, wherein stage M is above stage N.

[0039] Since the entrainer is preferably fed into the column at a point above the crude propylene glycol feed point, the distillation column 1 preferably has two inlets. The distillation column 1 is connected to a reboiler at the bottom and a condenser at the top.

[0040] In the method according to the invention, for good operation, the weight ratio of the entrainer to the mixture fed into column 1 is preferably 15:1 to 1:1, depending, for example, on the amount of 1,2-pentanediol to be removed and the entrainer used. More preferably, this ratio is between 12:1 and 2:1.

[0041] By employing the method of the present invention, preferably, the content of 2,3-pentanediol (especially (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol) in the propylene glycol obtained in c) is less than 50% by weight (preferably less than 30% by weight) of the content of 2,3-pentanediol (preferably (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol) in the mixture fed into distillation column 1 in a).

[0042] Compared to existing methods, the method of the present invention has the following advantages: extractive distillation of two different products (ethylene glycol and propylene glycol) can be achieved using a single entrainer. In the case of ethylene glycol, it is obtained as a relatively pure component (containing less 1,2-pentanediol than the feed), unlike some existing methods which mix it with an entrainer. Instead, the entrainer contains the component to be removed: 1,2-pentanediol.

[0043] The separation process according to the invention is preferably carried out downstream of a hydrogenolysis reactor, in which carbohydrates are converted to ethylene glycol with hydrogen in the presence of a catalyst. Propylene glycol is generally a byproduct, in addition to 1,2-pentanediol, but in greater quantities. Preferably, in the method according to the invention, one or more other separation processes precede the currently claimed separation process. It is preferable to first remove volatiles and water downstream of the reactor and before the currently claimed separation process. Furthermore, it is preferable to separate high-boiling byproducts (boiling points higher than ethylene glycol and propylene glycol), such as glycerol and erythritol. Additionally, it is preferable that the separation of ethylene glycol and propylene glycol has already been performed before the currently claimed purification method. All of the above implies a smaller propylene glycol stream to be purified. Therefore, preferably, in the invention, the mixture fed to column 1 in a) contains less than 1% by weight of ethylene glycol.

[0044] To achieve the desired extractive distillation, column 1 is preferably operated under the following conditions: the reboiler operating pressure of column 1 is between 20 and 100 mbar, preferably between 40 and 90 mbar; and the condenser operating temperature of column 1 is between 70 and 140°C, preferably between 90 and 130°C; and the reboiler operating temperature of column 1 is between 150 and 240°C, preferably between 170 and 210°C. An advantage of this invention is the possibility of using a low reflux ratio. Therefore, preferably, in this invention, column 1 is operated with a reflux ratio of 0.5 to 3, more preferably between 0.7 and 2.4.

[0045] As for column 2 used for regenerating the entrainer, it is desirable that the reboiler operating temperature of column 2 is between 150 and 220°C, preferably between 165 and 210°C, and the pressure is between 20 and 400 mbar, preferably between 30 and 200 mbar; and wherein in c) the condenser operating temperature of column 2 is between 90 and 180°C, preferably between 100 and 160°C.

[0046] In the method of the present invention, the preferred propylene glycol mixture comprises propylene glycol and (2R,3S)-2,3-pentanediol and / or (2S,3R)-2,3-pentanediol, with a weight ratio of propylene glycol:(2R,3S)-2,3-pentanediol and / or (2S,3R)-2,3-pentanediol between 200:1 and 50:1.

[0047] In step b), the weight ratio of entrainer to propylene glycol is between 15:1 and 4:1, more preferably between 10:1 and 5:1. In step c), the weight ratio of entrainer to propylene glycol is between 15:1 and 4:1, more preferably between 10:1 and 5:1.

[0048] Using the method of the present invention, propylene glycol with reduced (2R,3S)-2,3-pentanediol and / or (2S,3R)-2,3-pentanediol content (when measured on a propylene glycol basis) can be obtained compared to the process feed. Therefore, the method of the present invention can produce the top stream of column 1 in b) comprising at least 98 wt% (based on top stream weight) propylene glycol and less than 2 wt% (2R,3S)-2,3-pentanediol and / or (2S,3R)-2,3-pentanediol. Preferably, this purified propylene glycol top stream of column 1 comprises at least 99 wt%, more preferably at least 99.5 wt% propylene glycol, and preferably comprises less than 1 wt%, more preferably less than 0.5 wt% (2R,3S)-2,3-pentanediol and / or (2S,3R)-2,3-pentanediol combination.

[0049] Example

[0050] Example 1

[0051] Example 1 is a laboratory screening of the relative volatility of ethylene glycol and 1,2-pentanediol, and how the presence of an entrainer alters this relative volatility. This provides a prediction of the suitability of an entrainer for this purpose. Relative volatility was determined by headspace gas chromatography.

[0052] The mixture of components (ethylene glycol, 1,2-pentanediol, and entrainers) was prepared in the proportions shown in Table 4 below. A 5 mL volume of the mixture was injected into a 10 mL vial. The vial was intentionally left half-empty to ensure adequate headspace volume for liquid-vapor equilibrium. The entrainers tested were: triethylene glycol monobutyl ether, propylene glycol monophenyl ether (1-phenoxy-2-propanol), dodecyl alcohol, triethyl phosphate, diethylene glycol monoethyl ether acetate, 2-decyl alcohol, diethylene glycol monobutyl ether, di(ethylene glycol)hexyl ether, tripropylene glycol methyl ether, dipropylene glycol methyl ether, di(ethylene glycol)divinyl ether, and ethylene glycol monobenzyl ether.

[0053] Comparative entrainers include: dipropylene glycol, glycerin, triethylene glycol, diethylene glycol, octadecane, decane, and dodecane.

[0054] Table 4

[0055] After preparing the vial, the contents are thoroughly mixed to ensure uniform distribution in the liquid phase. The vial is then placed in a headspace gas chromatograph (HS-GC) autosampler and heated to 130°C. After 60 minutes, the headspace sample containing the gaseous contents is collected and analyzed by the machine. This injection produces a chromatogram, where each separated peak represents a component in the mixture, and the area under the peak indicates its corresponding concentration in the gas phase.

[0056] The resulting area is proportional to the partial pressure of the component, and the relative volatility (α) is calculated as follows:

[0057] in: A n HS-GCMS area of ​​component n with added entrainer A n,0 It is the HS-GCMS area of ​​component n in the reference sample (without entrainer). P n,sat It is the saturation pressure of component n x n It is the mole fraction of component n in the liquid phase. 1 refers to component 1 (here: ethylene glycol). 2 refers to component 2 (here: 1,2-pentanediol). Relative volatility during distillation is an indicator of the feasibility of separating components in a mixture. Higher relative volatility indicates that the relative concentration of component 1 relative to component 2 in the gas phase is higher than its relative concentration in the liquid phase. Therefore, in single-stage separation tests, such as the test for triethylene glycol monobutyl ether in this study, it shows that ethylene glycol is more volatile than 1,2-pentanediol in the presence of triethylene glycol monobutyl ether. The same applies to other entrainers.

[0058] As a reference point, the relative volatility of the mixture without the entrainer indicates that it is almost impossible to separate the two components (relative volatility = ~1). The results for the relative volatility α of the compounds according to the invention are listed below. Figure 1 (EG refers to ethylene glycol) The results of the relative volatility α of the comparative compounds are listed in Figure 2 middle.

[0059] In this test, the higher the relative volatility of a given compound, the better its suitability as an entrainer for extractive distillation. There is no hard limit, but in this test, lower relative volatility means that more entrainer (relative to ethylene glycol) is needed to remove 1,2-pentanediol from ethylene glycol, or that 1,2-pentanediol cannot be removed to the same extent. For operational and economic reasons, lower entrainer dosages are desirable.

[0060] Example 2

[0061] In the same manner as in Example 1, the suitability of the same glycol ether for removing 2,3-pentanediol from monopropylene glycol was now tested. This included screening the relative volatility of propylene glycol and 2,3-pentanediol, and how the presence of an entrainer altered this relative volatility. This provides a prediction of the suitability of an entrainer for this purpose. A 10 mL volume of the mixture was injected into a 20 mL vial. The vial was intentionally left half-empty to ensure adequate headspace volume for liquid-vapor equilibrium. The relative volatility was determined by headspace gas chromatography (as described in Example 1).

[0062] The tested mixtures differed because sufficiently pure 2,3-pentanediol could not be obtained (only 95% at most), thus the results were unreliable. Therefore, 2,3-pentanediol could not be tested as in Experiment 2 of Example 1. Instead, a reference sample was taken from the hydrogenolysis experiment to obtain propylene glycol (“crude MPG”) containing approximately 0.72% by weight of 2,3-pentanediol (based on analysis), and the amount of 2,3-pentanediol was calculated, assuming it to be a mixture of all four stereoisomers. This combination is denoted as “crude MPG” in Table 5 below.

[0063] The mixture of components was prepared in the proportions shown in Table 5 below. Thereafter, the same method as in Example 1 was followed.

[0064] Table 5

[0065] The relative volatility α can be calculated in a manner similar to that in Example 1, and the results are listed below. Figure 3 It shows that among the tested compounds, glycol ethers are the most suitable as extractive distillation entrainers for removing 2,3-pentanediol from propylene glycol.

[0066] Example 3

[0067] For this experiment, triethylene glycol monobutyl ether (BTEG) was used as an entrainer in a distillation column with a maximum of 100 theoretical stages to test the model feed.

[0068] The model feed consists of a mixture of 97.72 wt% ethylene glycol, 1.68 wt% 1,2-pentanediol and 0.54 wt% 1,2-hexanediol, and some minor components (other impurities).

[0069] In continuous operation mode, a feed mixture containing ethylene glycol (at 78-85°C and ambient pressure) and a triethylene glycol monobutyl ether entrainer (at 111-112°C and ambient pressure) are introduced into a column with up to 100 theoretical stages (the entrainer is positioned above the ethylene glycol feed mixture), where the purified ethylene glycol is separated in the distillate, and impurities in the feed leave the column as a bottom stream along with the entrainer.

[0070] The column operated under condenser conditions of 116°C and 50 mbar, with a pressure drop of 13.5–15 mbar. A reflux ratio of 2 was used throughout the experiment.

[0071] Three experiments were conducted, and the changes in the entrainer-to-feed ratio are shown in Table 6 below. The results are also listed in this table. The content of 1,2-pentanediol in the top stream was below the detection limit (0.001 wt%).

[0072] Table 6

[0073] Note: "Separation mass % in bottom stream" refers to the percentage of the component present in the bottom stream, based on the amount of the component in the EG feed stream.

[0074] Example 4

[0075] In an experiment similar to the foregoing examples, a model propylene glycol containing commercially available 2,3-pentanediol was prepared. The model feed comprised: 98.684 wt% propylene glycol, a mixture of 0.758 wt% (2R,3R)- and (2S,3S)-2,3-pentanediol, and a mixture of 0.558 wt% (2R,3S)- and (2S,3R)-2,3-pentanediol.

[0076] In continuous operation mode, a feed mixture containing propylene glycol (at 78-85°C and ambient pressure) and a triethylene glycol monobutyl ether (BTEG) entrainer (at 111-113°C and ambient pressure) are introduced into a column with up to 100 theoretical stages (the entrainer is positioned above the propylene glycol feed mixture), where the purified propylene glycol is separated in the distillate, and impurities in the feed leave the column as bottom stream along with the entrainer.

[0077] The column operated under condenser conditions of 108-109°C and 50 mbar, with a pressure drop of 12.8-18.1 mbar. A reflux ratio of 2 was used throughout the experiment.

[0078] The results are listed in Table 7.

[0079] Table 7

[0080] Note: If the model MPG composition does not contain (2R,3R)- and (2S,3S)-2,3-pentanediol, the purity of the MPG in the headstream will be higher than the percentage shown. The model composition contains (2R,3R)- and (2S,3S)-2,3-pentanediol because commercially available compositions containing only (2R,3S)- and (2S,3R)-2,3-pentanediol (without these stereoisomers) are not readily available experimentally, and the method of this invention is more suitable for removing (2R,3S)- and (2S,3R)-2,3-pentanediol (this is the objective) rather than removing (2R,3R)- and (2S,3S)-2,3-pentanediol.

[0081] In actual commercial methods for producing MPG from carbohydrates, most of the (2R,3R)- and (2S,3S)-2,3-pentanediols are removed before the extractive distillation of this invention, because the boiling points of these stereoisomers are sufficiently lower than the boiling point of MPG to allow for fractionation. Therefore, in such a setup, the purity of MPG in the extractive distillation topstream will be higher than the figures shown here.

Claims

1. A process for reducing the content of 2,3-pentanediol in a mixture comprising 90 to 99.9 wt% propylene glycol and 0.05 to 5 wt% 2,3-pentanediol by extractive distillation, wherein an entrainer is used in said extractive distillation, said entrainer having a Hansen solubility parameter δΗ between 5 and 15 and a boiling point at atmospheric pressure of at least 200°C.

2. The process according to claim 1, wherein the Hansen solubility parameter δΡ of the entrainer is between 2 and 12, preferably between 3 and 11, more preferably between 3.7 and 10, most preferably between 3.8 and 9.

3. The process according to any one of the preceding claims, wherein the Hansen solubility parameter δD of the entrainer is between 15 and 20, preferably between 15 and 18.

4. The process according to any one of the preceding claims, wherein the Hansen solubility parameter δΗ of the entrainer is between 6 and 14, preferably between 7 and 13.

5. The process according to any one of the preceding claims, wherein the entrainer is selected from the group consisting of C6-C10 glycol ethers, or C9-C18 unbranched aliphatic primary or secondary alcohols, and mixtures thereof.

6. The process according to claim 5, wherein the entrainer is selected from the group consisting of triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, 1-dodecanol and 2-decanol, and mixtures thereof.

7. The process according to any one of the preceding claims, wherein the 2,3-pentanediol comprises at least one of (2R,3S)-2,3-pentanediol and (2S,3R)-2,3-pentanediol.

8. The process according to any one of the preceding claims, wherein the process comprises: a) feeding the mixture to a distillation column 1, b) feeding the entrainer to the distillation column 1, c) removing propylene glycol from the top section of the column, d) removing a mixture comprising entrainer and 2,3-pentanediol from the bottom section of the distillation column 1.

9. The process according to claim 8, - feeding the mixture comprising entrainer and 2,3-pentanediol obtained in step d) to a column 2, producing a regenerated entrainer at the bottom section of column 2 and a mixture comprising entrainer and 2,3-pentanediol at the top section of column 2, - combining the regenerated entrainer with the entrainer fed to column 1 in b).

10. The process according to claim 8 or 9, wherein the 2,3-pentanediol content of the propylene glycol obtained in c) is between 5 and 60 wt% of the 2,3-pentanediol content of the mixture fed to the distillation column 1 in a).

11. The process according to any one of claims 8 to 10, wherein the mixture comprising propylene glycol and 2,3-pentanediol is fed to column 1 in a) at a position N of the column, and wherein the entrainer is fed to column 1 in b) at a position M of the column, wherein position M is located above position N.

12. The process according to any one of claims 8 to 11, wherein the weight ratio of entrainer to mixture in b) to a) is between 15:1 and 1:1, preferably between 12:1 and 2:

1.

13. The process of any one of claims 8 to 12, wherein the reboiler of column 1 is operated at a pressure between 20 and 100 mbar, preferably between 40 and 90 mbar, and wherein the condenser of column 1 is operated at a temperature between 70 and 140 °C, preferably between 90 and 130 °C; and wherein the reboiler of column 1 is operated at a temperature between 150 and 240 °C, preferably between 170 and 210 °C.

14. The process of any one of claims 8 to 15, wherein column 1 is operated at a reflux ratio of 0.5 to 3, preferably 0.7 to 2.

4.

15. A propylene glycol composition comprising at least 99.5 wt.% propylene glycol, said composition further comprising less than 0.5 wt.% 2,3-pentanediol, and the weight ratio of [(2R,3R)-2,3-pentanediol + (2S,3S)-2,3-pentanediol]: [(2R,3S)-2,3-pentanediol + (2S,3R)-2,3-pentanediol] of said 2,3-pentanediol is between 1 : 1 and 1 :20, preferably between 1 : 1 and 1 : 10.

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