Process for obtaining ethylene glycol and propylene glycol with reduced amount of pentanediols

Abstract

A process for the isolation of ethylene glycol and propylene glycol from a glycol mixture, which ethylene glycol and propylene glycol have a reduced amount of pentanediol. The process comprises fractional distillation of the glycol mixture to separate the majority of ethylene glycol and propylene glycol, with subsequent extractive distillation using an entrainer to remove 1,2-pentanediol from ethylene glycol and (2R,3S) 2,3-pentanediol and / or (2S,3R) 2,3-pentanediol from propylene glycol.

Description

[0001] PROCESS FOR OBTAINING ETHYLENE GLYCOL AND PROPYLENE GLYCOL WITH REDUCED AMOUNT OF PENTANEDIOLS

[0002] Introduction

[0003] The present invention relates to a process for obtaining ethylene glycol and propylene glycol from a mixture of glycols. Such mixture of glycols can be produced by hydrogenolysis of carbohydrates with hydrogen in presence of catalysts. More specifically, the present invention relates to obtaining such ethylene glycol and propylene glycol with reduced amounts of pentanediols (notably 1,2-pentanediol and 2,3-pentanediol), which pentanediols are generally produced as side products in said hydrogenolysis.

[0004] Background of the invention

[0005] Since about a decade, there is ongoing research in developing processes for manufacturing chemical building blocks from renewable sources. One such process of interest is obtaining monoethylene glycol (in short: ethylene glycol, MEG, or EG) from a (renewable) carbohydrate source such as sugar.

[0006] WO 2016 / 114661 discloses a continuous process for preparation of ethylene glycol from a carbohydrate source. Said process is carried out in a stirred tank reactor (CSTR) in which a catalyst system is present. Said 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 fairly easily be separated from the effluent stream and added back.

[0007] The liquid effluent obtained is usually subjected to a series of separation steps, e.g. to separate the tungsten compound used as co-catalyst (or a tungsten compound that is formed by the process), but also to separate the various (volatile) alcohols, polyols and glycols produced. Generally, the target product in the processes like the reference above and similar processes is ethylene glycol. Whilst such processes may have a selectivity for ethylene glycol of about 40 to 70%, considerable amounts of other components are also produced, mainly monopropylene glycol (MPG), but also polyols like glycerol, sorbitol and erythritol. The mixture produced usually contains also other glycols and diols next to MEG and MPG, such as 1,2- butanediol, 1,4-butanediol, 1,2-pentanediol and 2,3-pentanediol. Although the typical way to separate miscible liquids on an industrial scale would be fractional distillation, such method has its limits in separating some of these glycols, as boiling points are often close together, and other glycols or polyols to be separated tend to form azeotropes with either ethylene glycol or propylene glycol, which makes separation by distillation even more difficult.

[0008] Whilst the separation of e.g. 1,2-butanediol from ethylene glycol in the context of the conversion of carbohydrates to ethylene glycol has been widely researched, less work seems to have been done on removing 1,2-pentanediol from mixtures which are rich in ethylene glycol. In the processes such as in WO 2016 / 114661 and similar 1,2-pentanediol is formed in amounts of about 1 to 2% (by weight, based on ethylene glycol produced). The problem with removing 1,2-pentanediol from ethylene glycol to a high degree, preferably such that the ethylene glycol has sufficient purity for use in the manufacturing of polyester (in short: polyester-grade EG) is that 1,2-pentanediol forms an azeotrope with ethylene glycol. Hence, removal of 1,2-pentanediol from EG to obtain EP with a limited amount of 1,2-pentanediol by conventional fractional distillation is difficult or leads to loss of large amounts of EG.

[0009] As mentioned, there are processes developed for removing 1,2-butanediol from a mixture rich in ethylene glycol, yet the separation of 1,2, -pentanediol seems less well researched. Only CN104370696 and CN104230658 refer to separation of EG and 1,2-pentanediol. In these references removal of 1,2- pentanediol from a mixture rich in ethylene glycol using azeotropic distillation is disclosed. These references do not give details as to the entrainer used in these references. For the azeotropic distillation of these references, an entrainer is added and the ethylene glycol / entrainer azeotrope is obtained at the top. This azeotropic mixture is then separated by the addition of an extractant and by filtration. The filtrate is a mixed solution of ethylene glycol and an extractant, and the filter cake is an azeotropic agent. The entrainer can be recycled after it has been recovered. The separated ethylene glycol and extractant mixture is subjected to atmospheric or vacuum distillation in a rectification column to enable efficient separation of ethylene glycol from the extractant, to obtain an ethylene glycol product, and to circulate the extractant. This requires both an entrainer and an extractant, and several unit operations (and associated equipment) to operate this process. Also, this requires that all of the ethylene glycol is processed through the entrainer recovery column, which is undesired as it is a large volume and as it risks thermal damage to the ethylene glycol.

[0010] As mentioned, mono propylene glycol (for short: MPG, or propylene glycol, or PG, or propane-1, 2-diol) is also produced in substantial quantities when aiming for producing ethylene glycol from carbohydrates with hydrogenolysis. Depending on the source of carbohydrates and processing conditions, the weight ratio of EG : PG produced may range from 10 : 1 to 1 : 1. Propylene glycol is a valuable product as well, although the market is smaller than ethylene glycol. Since it has value, in particular in pure form, and since it is produced in substantial amounts, it is desirable to be able to obtain the propylene glycol in a fairly pure form. However, among the side products produced by the hydrogenolysis of carbohydrates is 2,3- pentanediol. Depending on the source of carbohydrates and processing conditions, the amount of 2,3- pentanediol produced may be between 0.2 and 3% by weight on propylene glycol. In fact, four different stereoisomers of 2,3-pentanediol exist due to the presence of two chiral centres (2 and 3 position). These four stereoisomers are: (2R,3R) 2,3-pentanediol, (2S,3S) 2,3-pentanediol, (2R,3S) 2,3-pentanediol, and (2S,3R) 2,3-pentanediol. As far as is known to the inventors, all four forms are produced by said reaction. Of these four, the (2R,3R) and (2S,3S) enantiomers have a boilingpoint (at atmospheric conditions) of about 175°C, and these can be separated fairly easy from monopropylene glycol, which has a boilingpoint of about 188.2°C (at atmospheric conditions). However, the (2R,3S) and (2S,3R) enantiomers have a boilingpoint of about 187.5-189.5°C, which makes it very difficult if not impossible to separate these components from monopropylene glycol (bp 188.2°C) by straightforward fractional distillation to a reasonable degree. For a monopropylene glycol produced by hydrogenolysis of carbohydrates it is desired that not only the (2R,3R) and (2S,3S) stereoisomers are removed to a substantial degree, but also a reduction in the (2R,3S) and (2S,3R) stereoisomers is achieved.

[0011] WO 2017 / 050847 discloses a process for extractive distillation to remove 1,2-butanediol from ethylene glycol, using as extractant C3-C6 sugar alcohols, e.g. glycerol. In the process of this reference, the extractive distillation is such that the ethylene glycol is extracted by the extractant: 1,2-butanediol and propylene glycol (and side products 2,3 butanediol, 1,2 pentanediol, 2,3-pentanediol, 1,2-hexanol and 1,2- heptanol) are removed at the top of the extraction distillation column, and at the bottom the mixture of ethylene glycol and extractant are obtained. Said bottom mixture is then subjected to a separation to yield ethylene glycol without 1,2-butanediol and extractant. It should be noted that this is exemplified in this reference by modelling only, not by experiments actually carried out. The description and figure 2 suggest that the top stream may be separated by (fractional) distillation in a stream of purified propylene glycol and a stream of the mentioned side products. The examples do not give any evidence, and given the closeness of the boiling points of 2,3-pentanediol (that is, the mixture of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol) and propylene glycol (187-189.5°C and 188.2°C, respectively) this is not possible in practice. Another disadvantage of the method of WO 2017 / 050847 is that the extractant is to be separated from the main product (ethylene glycol) which means that the stream that is processed in the extractant recovery column has to handle a big volume: all extractant and all ethylene glycol. This has consequences from an investment point of view (larger equipment) and operational-cost point of view (large volumes which have to be heated and cooled) and hence is undesired.

[0012] Similar to WO 2017 / 050847 referred to above, also WO 2022 / 073923 discloses extractive distillation to remove 1,2-butanediol from MEG, using extractive distillation using a C3-C6 sugar alcohol or C4-C6 polyol as extractant. Similar to the previous reference W02017 / 050847, the desired diol (MEG) is obtained at the bottom stream together with the extractant. The diol to be removed (1,2-butanediol) is removed at the top. This case suffers from the same disadvantages as WO 2017 / 050847. Both WO 2017 / 050847 and WO 2022 / 073923 deal with removing 1,2-butanediol from MEG. However, there is no evidence of the methods of these two references being able to remove 1,2-pentanediol from MEG.

[0013] WO2023 / 041842 discloses a process for reducing the amount of MEG and butanediols (1,2- and 2,3-) from a composition containing MPG. The model example contains about 53% MPG, 21% MEG, 4% butanediols, and about 0.6% of an organic impurity forming an azeotrope with MPG. The method relies on distillation in which a solvent is added. The solvent disclosed in the examples is tri-ethylene glycol (TEG). The ratio solvent : feed is high in the experiments (e.g. > 12 in experiments 9 and 10) and also the reflux ratio is high (between 5 and 50 in the experiments), which leads to high energy costs.

[0014] WO2012 / 125276 discloses a process for distilling a polyol product mixture, notably to obtain bio-derived propylene glycol. More in particular, the method aims to limit formation and removal of undesired epoxides which form by particular methods of distilling.

[0015] CN112920021 discloses a process for purification of 1,3-propanediol, by liquid-liquid extraction.

[0016] Hence, there is a need for a process that allows obtaining ethylene glycol and propylene glycol from the reaction product of hydrogenolysis of carbohydrates (a glycol mixture) in good yield, which ethylene glycol has a reduced amount of 1,2-pentanediol, and which propylene glycol has a reduced amount of 2,3- pentanediol (in particular (2R,3S)- and (2S,3R) 2,3-pentanediol). "Reduced" in this connection means: less of these pentanediols than were present in the glycol mixture prior to applying the separation process of the present invention. Preferably the reduction of 1,2-pentanediol should be such that the amount of 1,2- pentanediol in any ethylene glycol produced is 10% or less (preferably 5% or less, more preferably 1% or less, even more preferably 0.2% or less) of the amount of 1,2-pentanediol that was in the feed of the product (when taken on ethylene glycol). Preferably, the process should be such that it yields ethylene glycol comprising less than 0.1% by weight (on ethylene glycol) of 1,2-pentanediol, preferably such is less than 0.05%, more preferably less than 0.01% by weight on ethylene glycol.

[0017] As to the propylene glycol produced, it is desired that a purification process is provided that can achieve a reduction in the amount of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol (hereinafter called: (2R,3S), (2S,3R) 2,3-pentanediol) in a stream comprising predominantly propylene glycol of at least 30% by weight, preferably a reduction of at least 50% by weight. The percent reduction is then the amount of the enantiomers (2R,3S) and (2S,3R) combined, in the stream subjected to the purification after said purification, compared to the amount in such stream prior to said purification. It is furthermore desired that such purification method can be integrated with a separation process that can be used to separate a product stream from hydrogenolysis of carbohydrates that separates into an ethylene glycol-rich fraction and a propylene glycol rich fraction, wherein also the ethylene glycol rich fraction is subjected to a process that reduces the amount of 1,2-pentanediol, all in a practical manner with only a minimal number of chemical compounds needed to achieve the three separation / purification step (three steps as in: separation EG / PG, removal of 1,2-pentanediol from EG, and removal of (2R,3S), (2S,3R) 2,3-pentanediol from PG).

[0018] It is also preferred that the process will provide propylene glycol comprising less than 1% by weight (on propylene glycol) of the combination of (2R,3S) 2,3-pentanediol and (2S,3R) pentanediol, preferably such is less than 0.5%, by weight on propylene glycol.

[0019] The process should preferably such that the number of auxiliary compounds (e.g. extractants) is minimized, as this facilitates operation, and preferably the volume of streams to be processed should be minimized. It is furthermore desired that the number of unit operations required is as low as possible and that the energy consumption is preferably minimized and that the process preferably does not have the disadvantages of the prior art.

[0020] Summary of the invention

[0021] It has now been found objective that the above may be achieved, at least in part, by a process for obtaining ethylene glycol and propylene glycol (propane-1, 2-diol) from a glycol mixture, which glycol mixture comprises ethylene glycol and propylene glycol in a weight ratio ethylene glycol : propylene glycol of between 20 : 1 and 1 : 1, and which glycol mixture further comprises 1,2-pentanediol, and at least one of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol, in a weight ratio ethylene glycol : 1,2-pentanediol of between 200 : 1 and 20 : 1 and a weight ratio propylene glycol : (2R,3S) 2,3-pentanediol and (2S,3R) 2,3- pentanediol of between 200 : 1 and 20 : 1, which process comprises the steps of: a. subjecting said glycol mixture to a distillation column 1 which produces a top stream comprising propylene glycol, which comprises at least 90% by weight of propylene glycol that was present in said glycol mixture, and a bottom stream comprising ethylene glycol, which comprises at least 90% by weight of the ethylene glycol present in said glycol mixture, b. subjecting the top stream comprising propylene glycol obtained under a) to an extractive distillation by feeding said top stream to a distillation column 2, to which is also fed an entrainer in a weight ratio entrainer : propylene glycol of between 20 : 1 and 2 : 1, thereby obtaining a top stream comprising at least 99% by weight of propylene glycol, which top stream comprises (2R,3S)

[0022] 2.3-pentanediol and / or (2S,3R) 2,3-pentanediol in a weight ratio propylene glycol : (2R,3S) 2,3- pentanediol and (2S,3R) 2,3-pentanediol which is higher than said ratio in the glycol mixture prior to step a), and a bottom stream comprising entrainer and (2R,3S) 2,3-pentanediol and / or (2S,3R)

[0023] 2.3-pentanediol, c. subjecting the bottom stream comprising ethylene glycol obtained under a) to an extractive distillation by feeding said bottom stream to a distillation column 3, to which is also fed an entrainer in a weight ratio entrainer : ethylene glycol of between 20 : 1 and 2 : 1, thereby obtaining a top stream comprising at least 99% by weight of ethylene glycol, which top stream comprises 1,2-pentanediol in a weight ratio ethylene glycol : 1,2-pentanediol which is higher than said ratio in the glycol mixture prior to step a), and a bottom stream comprising entrainer and 1,2- pentanediol, d. feeding both bottom stream obtained under b) and the bottom stream obtained under c) to a regeneration column 4, which produces a stream comprising regenerated entrainer and a stream comprising entrainer, 1,2-pentanediol, and at least one of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3- pentanediol, wherein the entrainer fed to column 2 in b) is the same compound as the entrainer fed to column 3 in step c).

[0024] In this process according to the present invention, the top stream comprising propylene glycol obtained from column 1 in step a) may be fed to column 2 as it was obtained, or may be first subjected to further purification steps such as (fractional) distillation prior to being fed to column 2 in step b). Likewise, the bottom stream comprising ethylene glycol obtained from column 1 in step a) may be fed to column 3 as it was obtained, or may be first subjected to further purification steps such as (fractional) distillation prior to being fed to column 3 in step c).

[0025] Preferably, in the process of this invention, said entrainer has a Hansen solubility parameter 6H of between 5 and 15, preferably of between 6 and 14, more preferably of between 7 and 13, and the entrainer having a boiling point at atmospheric pressure of at least 200°C. Further, it is preferred that said entrainer further has a Hansen solubility parameter 6P of between 2 and 12, preferably of between 3 and 11, more preferably between 3.7 and 10, most preferably between 3.8 and 9. Also further, it is preferred that said entrainer further has a Hansen solubility parameter 6D of between 15 and 20, preferably of between 15 and 18.

[0026] Detailed description of the invention

[0027] The word "entrainer" herein encompasses pure compounds but also mixtures of compounds having the Hansen solubility parameters and boiling point claimed.

[0028] In the process according to the invention, it is referred to the two enantiomers (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol. Also exist the two stereoisomers (2R,3R) 2,3-pentanediol and (2S,3S) 2,3- pentanediol. Without wishing to be bound by theory, it is believed that all four stereoisomers are formed in the hydrogenolysis of carbohydrates, although perhaps in unequal amounts. The enantiomers (2R,3R) 2,3-pentanediol and (2S,3S) 2,3-pentanediol have a boiling point of about 175°C, and thus can be separated from propylene glycol with fractional distillation to a sufficient degree. However, the (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol enantiomers have a boiling point that is very close to that of propylene glycol, and hence separation of these from propylene glycol requires a different technique, as now provided. Although the extractive distillation claimed also can remove part or all of the (2R,3R) 2,3- pentanediol and (2S,3S) 2,3-pentanediol, generally such components with a boiling point sufficiently lower than that of propylene glycol will have been removed already prior to the extractive distillation now claimed, e.g. by fractional distillation. Inventors of the present case found that entrainers suitable for the purpose have in common that they have a Hansen solubility parameter 6H of between 5 and 15 (and preferably a Hansen solubility parameter 6P of between 2 and 12, and a Hansen solubility parameter 6D of between 15 and 20). Reducing the amount of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol) in propylene glycol was problematic, in particular when present in small amounts of e.g. 0.5-2%, due to the closeness of the boiling points of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol and propylene glycol. With the present method, it was found that the amount of these 2,3-pentanediols present in a composition mainly comprising monopropylene glycol can be reduced to a substantial degree. For practical reasons (in view of the temperatures involved), the entrainer used in the present method has a boiling point of at least 200°C, at atmospheric pressure.

[0029] Hence, inventors of the present case found that the entrainers specified can be used as an entrainer in extractive distillation of ethylene glycol (to reduce the content of 1,2-pentanediol) as well as in extractive distillation of propylene glycol. That the same entrainer can be used for two different separations is very convenient in a process such as the hydrogenolysis of carbohydrates that yields, next to predominantly ethylene glycol also propylene glycol in substantial amounts, and which ethylene glycol and propylene glycol are to be obtained in a relatively pure form. In such hydrogenolysis process side products are obtained, amongst which are 1,2-pentanediol and 2,3-pentanediol. The first is difficult to remove to a sufficient degree from the desired ethylene glycol, whereas the latter (notably the enantiomers (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol), can be difficult to remove from the desired propylene glycol.

[0030] The downstream processing of glycols like MEG and MPG produced in hydrogenolysis usually involves removing volatile compounds and water, and at some point the stream is to be separated into a fraction rich in ethylene glycol, and a fraction rich in propylene glycol. It appears that the 1,2-pentanediol fraction almost completely ends up in the ethylene glycol-rich fraction, as these compounds form an azeotrope. It also appears that the 2,3-pentanediol fraction almost completely ends up in the propylene glycol-rich fraction. The latter in particular applies to the enantiomers (2R,3S) 2,3-pentanediol and (2S,3R) 2,3- pentanediol, as the boiling points of propylene glycol and these 2,3-pentanediols are very close.

[0031] Hence, the now-claimed process can reduce the amount of 1,2-pentanediol form the ethylene glycol fraction and it also can reduce the amount of the (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol from the propylene glycol fraction in a convenient way: using extractive distillation for both using the same entrainer.

[0032] In order to identify suitable entrainers, numerous chemical components were subjected to laboratory screening of the relative volatility of ethylene glycol and 1,2-pentanediol, and how such is modified by the presence of an entrainer. This provides a prediction of the suitability of an entrainer for the purpose. The details are set out in example 1. Compounds that came out promising in this test were several glycol ethers, and some aliphatic alcohols, and all were found to fit the Hansen solubility parameters now claimed. Hence, it is preferred that the entrainer for the present invention is selected from a C6-C10 glycol ether, or a C9-C18 (preferably C10-C12) primary or secondary aliphatic unbranched alcohol, and mixtures thereof. A "C6-C10 glycol ether" herein means a glycol ether having from 6 to 10 (including 6 and 10) carbon atoms in its formula. Similarly, a "C9-C18 primary or secondary aliphatic unbranched alcohol" herein means a primary or secondary aliphatic unbranched alcohol containing from 9 to 18 carbon atoms.

[0033] Following this (and the requirement that the boiling point at atmospheric conditions should be at least 200°C), it is preferred that in the process of the present invention, the entrainer, when such is a glycol ether, such is selected from the group consisting of as appears in table 1, and mixtures thereof. Table 1 also gives the Hansen solubility parameters of the individual compounds.

[0034] Table 1

[0035] Hence, these are preferred glycol ethers in the present invention. Depending e.g. on cost, availability and safety, impurities, thermal stability, in case a glycol ether is used as entrainer, said glycol ether is preferably selected from the group consisting of triethylene glycol monoethyl ether, triethyleneglycol monobutyl ether, and mixtures thereof.

[0036] As alternative to glycols, the entrainers may also be a C9-C18 (preferably C10-C12) primary or secondary aliphatic unbranched alcohol. Following this (and the requirement that the boiling point at atmospheric conditions should be at least 200°C), it is preferred that in the process of the present invention, the entrainer, when such is a alcohol such is selected from the group consisting of as appears in table 2, and mixtures thereof.

[0037] Table 2

[0038] Yet a further suitable entrainer for the purpose of the invention is triethylphosphate. This compound has a 6D of 16.7, a 6P of 11.4, and a 6H of 9.2. Potential entrainers that are used in industry for similar purposes have been tested for the purpose of the invention, and these appeared to be unsatisfactory in the context of the invention. These are listed in table 3.

[0039] Table 3

[0040] For reasons of e.g. cost, availability and safety, impurities, thermal stability, in case the entrainer is a primary or secondary aliphatic unbranched alcohol, preferred alcohols for such are 1-dodecanol and 2- decanol. Preferred glycol ethers in this invention are: triethyleneglycol monoethyl ether, triethyleneglycol monobutyl ether. Hence, it is preferred that in the present invention the entrainer is selected from triethyleneglycol monoethyl ether, triethyleneglycol monobutyl ether, 1-dodecanol and 2-decanol, and mixtures thereof. The process according to the invention comprises at least four distillation columns. These columns aim for: column 1: an ethylene glycol / propylene glycol split column 2: extractive distillation of propylene glycol column 3: extractive distillation of ethylene glycol column 4: regeneration of the entrainer

[0041] In more detail, column 1 is to split a feed containing a mixture of glycols (at least ethylene glycol, propylene glycol, 1,2-pentanediol and at least one of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol) into a fraction rich in ethylene glycol (bottom stream, which further comprises 1,2-pentanediol) and a fraction rich in propylene glycol (top stream, which further comprises at least one of (2R,3S) 2,3- pentanediol and (2S,3R) 2,3-pentanediol). Column 2 is to obtain a fraction of propylene glycol with a reduced amount of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol, by extractive distillation using the specified entrainer. Likewise, column 3 is to obtain a fraction of ethylene glycol with a reduced amount of

[0042] 1.2-pentanediol, by extractive distillation using the specified entrainer. And column 4 is used to regenerate the entrainer by removing the extracted 1,2-pentanediol and 2,3-pentanediol(s) from it, so that the entrainer can be used again in columns 2 and 3.

[0043] The desired split in column 1 of the feed comprising ethylene glycol and propylene glycol in a weight ratio ethylene glycol : propylene glycol of between 20 : 1 and 1 : 1, and which glycol mixture further comprises

[0044] 1.2-pentanediol and (2R,3S) 2,3-pentanediol and / or (2S,3R) 2,3-pentanediol, in a weight ratio ethylene glycol : 1,2-pentanediol of between 200 : 1 and 20 : 1 and a weight ratio propylene glycol : (2R,3S) 2,3- pentanediol and / or (2S,3R) 2,3-pentanediol of between 200 : 1 and 20 : 1, to yield a fraction rich in ethylene glycol and a fraction rich in propylene glycol can be achieved with a distillation column in which the reboiler is operated at a temperature of between 100 and 160°C, preferably between 110 and 150°C, and at a pressure of between 50 and 170 mbara, preferably at a pressure of between 70 and 110 mbara, and wherein the condenser of column 1 in a) is operated at a temperature of between 70 and 130°C, preferably between 80 and 120°C. The distillation column itself can be designed by the skilled person without undue burden.

[0045] The extractive distillation column 2 can be designed by the skilled person without undue burden. To effect the desired extractive distillation, it may be preferred to operate such column at such conditions that the reboiler is operated at a temperature of between 150 and 240°C, preferably between 170 and 210°C, and at a pressure of between 30 and 100 mbara, preferably at a pressure of between 40 and 90 mbara, and wherein the condenser of column 2 in b) is operated at a temperature of between 70 and 140°C, preferably between 90 and 130°C.

[0046] Regarding the feed to the extractive distillation column 1, it is generally preferred for effective extractive distillation that the feed of the stream to be purified is neither added near the top, nor near the bottom section. Preferably, the feed is added in the middle half of the column (between the top quarter and the bottom quarter). This is to be understood as: if column 2 has 100 theoretical stages, the feed to be purified is preferably added between stages 25 and 75 of this column. Also, it is generally preferred that the entrainer is added above this feed. Hence, in the process according to the present invention, it is preferred that in step b) the entrainer is fed to column 2 at a stage above the feed to column 2 of the top stream of a).

[0047] Extractive distillation column 3 can be designed by the skilled person without undue burden. To effect the desired extractive distillation, it may be preferred to operate such column at such conditions that the reboiler is operated at a temperature of between 160 and 220°, preferably between 165 and 210°C, and at a pressure of between 20 and 500 mbara, preferably at a pressure of between 40 and 250 mbara, and wherein the condenser of column 3 in c) is operated at a temperature of between 80 and 160°C, preferably between 100 and 140°C.

[0048] Regarding the feed to the extractive distillation column 3, it is generally preferred for effective extractive distillation that the feed of the stream to be purified (here: the ethylene glycol-rich stream) is neither added near the top, nor near the bottom section. Preferably, the feed is added in the middle half of the column (between the top quarter and the bottom quarter). This is to be understood as: if column 3 has 100 theoretical stages, the feed to be purified is preferably added between stages 25 and 75 of this column. Also, it is generally preferred that the entrainer is added above this feed. Hence, in the process according to the present invention, it is preferred that in step c) the entrainer is fed to column 3 at a stage above the feed to column 3 of the bottom stream of a).

[0049] Regeneration of entrainer that contains 1,2-pentanediol and / or 2,3-pentanediol(s) such that the entrainer is in a condition that it can be used in both extractive distillation columns 2 and 3 can be carried out in a column 4 that can be designed by the skilled person without undue burden. Preferably, such process can be carried out in a process wherein of column 4 in d) the reboiler is operated at a temperature of between 150 and 220°, preferably between 165 and 210°C, and at a pressure of between 20 and 400 mbara, preferably at a pressure of between 30 and 200 mbara, and wherein the condenser of column 3 in c) is operated at a temperature of between 90 and 180°C, preferably between 100 and 160°C.

[0050] The process of the invention can cope with different ratios of ethylene glycol to propylene glycol.

[0051] However, it is preferred that the glycol mixture comprises ethylene glycol and propylene glycol in a weight ratio ethylene glycol : propylene glycol of between 10 : 1 and 2 : 1, preferably 5: 1, and which glycol mixture further comprises 1,2-pentanediol and (2R,3S) 2,3-pentanediol and / or (2S,3R) 2,3-pentanediol. The latter components are preferably present in a weight ratio ethylene glycol : 1,2-pentanediol of between 150 : 1 and 30 : 1 (preferably a weight ratio of between 100 : 50) and a weight ratio propylene glycol : (2R,3S) 2,3-pentanediol and / or (2S,3R) 2,3-pentanediol of between 200 : 1 and 50 : 1.

[0052] In step b), the weight ratio entrainer : propylene glycol of between 15 : 1 and 4 : 1, more preferably between 10 : 1 and 5 : 1. In step c), the weight ratio entrainer : ethylene glycol of between 15 : 1 and 4 : 1, more preferably between 10 : 1 and 5 : 1.

[0053] With the process according to the present invention, it is possible to obtain a propylene glycol which has a reduced amount of (2R,3S) 2,3-pentanediol and / or (2S,3R) 2,3-pentanediol when compared with the feed to the process (when considered on the basis of propylene glycol). The process according to the invention thus may yield a top stream of column 2 in b) which comprises at least 98% by weight (based on the weight of the top stream) of propylene glycol and less than 2% by weight of (2R,3S) 2,3-pentanediol and / or (2S,3R) 2,3-pentanediol. Preferably, this purified propylene glycol top stream of column 2 comprises at least 99 % by weight, more preferably at least 99.5% by weight of propylene glycol, and such stream comprises preferably less than 1%, more preferably less than 0.5% by weight of (2R,3S) 2,3- pentanediol and / or (2S,3R) 2,3-pentanediol combined.

[0054] Likewise, the process according to the present invention, it is possible to obtain an ethylene glycol which has a reduced amount of 1,2-pentanediol when compared with the feed to the process (when considered on the basis of ethylene glycol). The process according to the invention thus may yield a top stream of column 3 in b) which comprises at least 99% by weight (based on the weight of the top stream) of ethylene glycol and less than 0.1% by weight of 1,2-pentanediol. Preferably, this purified ethylene glycol top stream of column 3 comprises at least 99.5 % by weight, more preferably at least 99.8% by weight of propylene glycol, and such stream comprises preferably less than 0.05%, more preferably less than 0.01% by weight of 1,2-pentanediol. Additionally, it was found that when treating the ethylene glycol in column 3 in the process of the present invention, some other diol components may be removed as well from the ethylene glycol. For example, it was found that the use of the entrainer according to the present invention also removes part of the 1,2- butanediol from ethylene glycol. This is an added benefit of the process of the present invention.

[0055] The above is the preferred basic set of columns required for the present invention. When using this for processing a product stream of glycols from hydrogenolysis of carbohydrates some further processing of the effluent is required. This further processing can include removal of volatile components, removal of water, removal of high-boiling polyols, removal of other side products than pentanediols, such as butanediol (both 1,2- as well as 2,3 butanediol), to name the most important ones. These further processing steps can be combined with the four columns of the present invention in several ways, but a preferred way is the following.

[0056] Preferably, upstream to column 1 (the EG / PG split) the glycol mixture is treated in a column 0, which column is designed to remove high-boiling side products obtained by the hydrogenolysis, such as one or more of glycerol, erythritol and sorbitol. Such column preferably operated such that the reboiler operates at a temperature of between 100 and 180°C and a pressure of between 60 and 400 mbar, and of which the condenser preferably operates at a temperature of between 90 and 170°C. The fraction rich in ethylene glycol and propylene glycol (and impurities like the pentanediols) is obtained as a top stream, and the high-boiling polyols is then obtained at a bottom stream.

[0057] Still further upstream from column 0 above, can be means to remove volatiles from the product mixture. Such volatiles may be methanol and ethanol, and other components with a boiling point (at atmospheric pressure) of below 100°C, Such removal of volatiles may be achieved by means known to the skilled person. Also, given the pressures and temperatures employed by column 1 and optional column 0, it is preferred that prior to column 1 (and if a column 0 is present as described above then preferably also prior to such column 0) there is means for removal of water, as water is generally employed as (part of) the solvent for the reaction. This water-removal means can be designed by the skilled person without undue burden, incorporating e.g. one or more columns, flashing vessels or other.

[0058] It may also be desired (in order to remove some other impurities from propylene glycol than (2R,3S) 2,3- pentanediol and / or (2S,3R) 2,3-pentanediol) that prior to processing the extractive distillation of propylene glycol (so downstream from column 1 and upstream from column 2) there is means for removing components having a lower boiling point than propylene glycol, which are still present. Such components may be one or more of water, 2,3-butanediol, tetrahydrofurfuryl alcohol or other. Components that also may be removed to a large extent by such means is (2R,3R) 2,3-pentanediol and / or (2S,3S) 2,3-pentanediol. These two enantiomers (2R,3R)- and (2S,3S) 2,3-pentanediol (which are diastereomers of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol), have a boiling point (at atmospheric conditions) different from propylene glycol and thus can be removed to a large extent in such column downstream from column 1 and upstream from column 2. Hence, it may be preferred that downstream of column 1 and upstream of column 2 there is an additional column 12, of which the reboiler operates at a temperature of between 120 and 210°C and a pressure of between 200 and 700 mbar, and of which the condenser preferably operates at a temperature of between 70 and 140°C, to remove at least part of one or more of water, 2,3-butanediol, tetrahydrofurfuryl alcohol, (2R,3R) 2,3-pentanediol and / or (2S,3S) 2,3-pentanediol (which then come off at the top of such column).

[0059] Likewise, it may be preferred to subject the ethylene glycol-rich fraction obtained from column 1 to some form of purification prior to the extractive distillation in column 3, to remove some other impurities than 1,2-pentanediol that may be present in in ethylene glycol. Such impurity may be 1,2-butanediol. Since thus butanediol forms an azeotrope with ethylene glycol, such azeotrope may be removed as top product from a column, whilst the bottom product may be ethylene glycol which has much less 1,2-butanediol. Hence, it may be preferred that downstream of column 1 and upstream of column 3 there is an additional column 13, of which the reboiler operates at a temperature of between 130 and 220°C and a pressure of between 200 and 700 mbar, and of which the condenser preferably operates at a temperature of between 120 and 200°C, to remove at least part of the azeotrope of ethylene glycol and 1,2-butylene glycol.

[0060] EXAMPLES

[0061] Example 1

[0062] Example 1 is a laboratory screening of the relative volatility of ethylene glycol and 1,2-pentanediol, and how such is modified by the presence of an entrainer. This provides a prediction of the suitability of an entrainer for the purpose. The relative volatility is determined by gas chromatography of the headspace.

[0063] Mixtures of components (ethylene glycol, 1,2-pentanediol and entrainer) were prepared in ratios presented in table 4 below. A 5 mL volume of the mixture was inserted into a 10 mL vial. The vial was purposefully left half empty to ensure proper volume of headspace available for achieving liquid-vapor equilibrium. The entrainers tested were: Triethylene glycol monobutyl ether, Propylene glycol monophenyl ether (l-Phenoxy-2-propanol), Dodecanol, Triethylphosphate, Diethylene Glycol Monoethyl Ether Acetate, 2-Decanol, diethylene glycol monobutyl ether, Di(ethylene glycol) hexyl ether, Tripropylene glycol methyl ether, Dipropylene glycol methyl ether, Di(ethylene glycol) divinyl ether, Ethylene glycol monobenzyl ether.

[0064] As comparative entrainers: Dipropylene glycol, Glycerol, Triethylene glycol, Diethylene glycol, Octadecane, Decane, Dodecane.

[0065] Table 4

[0066] After preparing the vial, the content of the vial was thoroughly mixed to ensure homogeneous distribution of content in the liquid phase. Subsequently, the vials were placed in a Headspace-Gas chromatography (HS-GC) autosampler where the vials were heated to 130°C. After 60 minutes, the vial's headspace which contains the gas content was sampled and analyzed by the machine. This injection resulted in a chromatogram where the separated peaks each represent one of the mixtures' components, and the area under the peak indicates their respective concentration in the gas phase.

[0067] The obtained areas are proportional to the components partial pressure and the relative volatility (a) was calculated as: wherein:

[0068] Anis HS-GCMS area of component n with entrainer addition

[0069] An,o is HS-GCMS area of component n in reference sample (no entrainer addition)

[0070] Pn,sat is saturation pressure of component n xnis molar fraction of component n in liquid phase

[0071] 1 is index for component 1 (here: ethylene glycol)

[0072] 2 is index for component 2 (here: 1,2-pentanediol)

[0073] The relative volatility in distillation is an indication of feasible separation of the components in a mixture. Higher relative volatility shows that the relative concentration of component 1 to 2 is higher in the gas phase than in liquid phase. Hence, in a single stage separation such as in this test for triethylene glycol monobutyl ether, the ethylene glycol is shown to be more volatile than the 1,2-pentanediol in the presence of triethylene glycol monobutyl ether. And so on for other entrainers.

[0074] As a point of reference, the relative volatility of the mixture without the entrainer present shows that almost no separation of the two components is possible (relative volatility =~1). The results of the relative volatility a are set out in figure 1 (EG means ethylene glycol) for the compounds according to the invention and figure 2 for comparatives.

[0075] The higher the relative volatility of a given compound in this test the better the suitability of this compound as entrainer in extractive distillation. There is no hard cut-off, but the lower the relative volatility in this test the more entrainer (in relation to the ethylene glycol) would be needed to remove 1,2-pentanediol from ethylene glycol, or the 1,2-pentanediol is not removed to the same extent. A lower amount of entrainer is desired for operational and economic reasons.

[0076] Example 2

[0077] In the same way as in example 1, now the suitability of the same glycol ethers for removing 2,3- pentanediol from mono propylene glycol was tested. This included screening the relative volatility of propylene glycol and 2,3-pentanediol, and how such is modified by the presence of an entrainer. This provides a prediction of the suitability of an entrainer for the purpose. A 10 mL volume of the mixture was inserted into a 20 mL vial. The vial was purposefully left half empty to ensure proper volume of headspace available for achieving liquid-vapor equilibrium. The relative volatility is determined by gas chromatography of the headspace as in example 1.

[0078] There was a difference in which mixtures were tested, as 2,3-pentanediol was not available in a pure enough form (only up to 95%) for results to be reliable. Hence, an experiment like that of experiment 2 of example 1 could not be performed for 2,3-pentanediol. For this reason, a reference sample was taken from a hydrogenolysis experiment from which the propylene glycol was obtained ("crude MPG"), which contained (as appeared from analysis) about 0.72 wt% of 2,3-pentanediol (and from this the amount of 2,3-pentanediol was calculated, which is assumed to be a mixture of all four stereoisomers). This combination is denoted "crude MPG" in the table 5 below.

[0079] Mixtures of components were prepared in rations presented in table 5 below. Thereafter, the same methodology as in example 1 was followed.

[0080] Table 5

[0081] The relative volatility a can be calculated in a similar way as for example 1, and the results of this is set out in figure 3. It shows that of the compounds tested, the glycol ethers are the most suitable compounds (of the compounds tested) as entrainer for extractive distillation to remove 2,3-pentanediol from propylene glycol.

[0082] Example 3

[0083] For this experiment, a trial was conducted in a distillation column having maximum 100 theoretical stages, with triethylene glycol monobutyl ether (BTEG) as entrainer, on a model feed.

[0084] The composition of the model feed was: 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).

[0085] In continuous operation mode, the ethylene glycol-containing feed mixture (at 78-85°C and ambient pressure) and triethylene glycol monobutyl ether entrainer (at 111-112°C and ambient pressure) were introduced (the entrainer higher than the ethylene glycol feed mixture) into a maximum 100 theoretical stage column where the purified ethylene glycol is separated in the distillate and the impurities of the feed leave the column with the entrainer as bottom stream.

[0086] The column was operated at 116°C and 50 mbara condenser conditions, with a column pressure drop of 13.5-15 mbara. A reflux ratio of 2 was used throughout the experiment.

[0087] Three experiments were performed where the entrainer to feed ratio was varied, according to the data in table 6 below. The table also lists the results. The amount of 1,2-pentanediol in the top stream was below the detection limit of 0.001 wt%.

[0088] Table 6

[0089]

[0090] Note: "sep mass % in bottom stream" refers the percentage of the stated component that is present in the bottom stream, based on the amount of said component in the EG feed stream.

[0091] Example 4 In an experiment similar as in the previous example, a model propylene glycol was made containing a commercial 2,3-pentanediol. The model feed contained: 98.684 wt% propylene glycol, 0.758 wt% mixture of (2R,3R)- and (2S,3S) 2,3-pentanediol, and 0.558 wt% mixture of (2R,3S)- and (2S,3R) 2,3-pentanediol. In continuous operation mode, the propylene glycol-containing feed mixture (at 78-85°C and ambient pressure) and triethylene glycol monobutyl ether (BTEG) entrainer (at 111-113°C and ambient pressure) was introduced (the entrainer higher than the propylene glycol feed mixture) into a maximum 100 theoretical stage column where the purified propylene glycol was separated in the distillate and the impurities of the feed leave the column with the entrainer as bottom stream.

[0092] The column was operated at 108-109°C and 50 mbara condenser conditions, with a column pressure drop of 12.8-18.1 mbara. A reflux ratio of 2 was used throughout the experiment. The results are set out in table 7.

[0093] Table 7 Note: the purity for the MPG in the top stream would be higher than the percentage indicated if no (2R,3R)- and (2S,3S) 2,3-pentanediol had been present in the model MPG composition. The (2R,3R)- and (2S,3S) 2,3-pentanediol were present in the model composition as no commercial composition of 2,3- pentanediol was easily available at the time of the experiments that was free of these stereoisomers (and would only contain (2R,3S)- and (2S,3R) 2,3-pentanediol), and the process of this invention is better suited to remove the (2R,3S)- and (2S,3R) 2,3-pentanediol (which is the aim) than to remove the (2R,3R)- and (2S,3S) 2,3-pentanediol.

[0094] In an actual commercial process of manufacturing MPG from carbohydrates most of the (2R,3R)- and (2S,3S) 2,3-pentanediol would have been removed before performing the extractive distillation of this invention, as these stereoisomers have a boiling point which is sufficiently lower than that for MPG to allow fractional distillation. Hence, in such set up the purity of MPG in the top stream of this extractive distillation would be higher than the numbers here suggest.

Claims

CLAIMS1. Process for obtaining ethylene glycol and propylene glycol from a glycol mixture, which glycol mixture comprises ethylene glycol and propylene glycol in a weight ratio ethylene glycol : propylene glycol of between 20 : 1 and 1 : 1, and which glycol mixture further comprises 1,2- pentanediol, and at least one of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol, in a weight ratio ethylene glycol : 1,2-pentanediol of between 200 : 1 and 20 : 1 and a weight ratio propylene glycol : (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol of between 200 : 1 and 20 : 1, which process comprises the steps of: a) subjecting said glycol mixture to a distillation column 1 which produces a top stream comprising propylene glycol, which comprises at least 90% by weight of propylene glycol that was present in said glycol mixture, and a bottom stream comprising ethylene glycol, which comprises at least 90% by weight of the ethylene glycol present in said glycol mixture, b) subjecting the top stream comprising propylene glycol obtained under a) to an extractive distillation by feeding said top stream to a distillation column 2, to which is also fed an entrainer in a weight ratio entrainer : propylene glycol of between 20 : 1 and 2 : 1, thereby obtaining a top stream comprising at least 99% by weight of propylene glycol, which top stream comprises (2R,3S) 2,3-pentanediol and / or (2S,3R) 2,3-pentanediol in a weight ratio propylene glycol : (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol which is higher than said ratio in the glycol mixture prior to step a), and a bottom stream comprising entrainer and (2R,3S) 2,3-pentanediol and / or (2S,3R) 2,3-pentanediol, c) subjecting the bottom stream comprising ethylene glycol obtained under a) to an extractive distillation by feeding said bottom stream to a distillation column 3, to which is also fed an entrainer in a weight ratio entrainer : ethylene glycol of between 20 : 1 and 2 : 1, thereby obtaining a top stream comprising at least 99% by weight of ethylene glycol, which top stream comprises 1,2-pentanediol in a weight ratio ethylene glycol : 1,2-pentanediol which is higher than said ratio in the glycol mixture prior to step a), and a bottom stream comprising entrainer and 1,2-pentanediol, d) feeding both bottom stream obtained under b) and the bottom stream obtained under c) to a regeneration column 4, which produces a stream comprising regenerated entrainer and a stream comprising entrainer, 1,2-pentanediol, and at least one of (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol,wherein the entrainer fed to column 2 in b) is the same compound as the entrainer fed to column 3 in step c).

2. Process according to claim 1, and wherein said entrainer has a Hansen solubility parameter 6H of between 5 and 15, preferably of between 6 and 14, more preferably of between 7 and 13, and the entrainer having a boiling point at atmospheric pressure of at least 200°C.

3. Process according to claim 1 or claim 2, wherein said entrainer further has a Hansen solubility parameter 6P of between 2 and 12, preferably of between 3 and 11, more preferably between 3.7 and 10, most preferably between 3.8 and 9.

4. Process according to any of the preceding claims, wherein said entrainer further has a Hansen solubility parameter 6D of between 15 and 20, preferably of between 15 and 18.

5. Process according to any of the preceding claims, wherein the entrainer is selected from a C6-C10 glycol ether, or a C9-C18 primary or secondary aliphatic unbranched alcohol, and mixtures thereof.

6. Process according to claim 5, wherein the entrainer is selected from triethyleneglycol monoethyl ether, triethyleneglycol monobutyl ether, 1-dodecanol and 2-decanol, and mixtures thereof .

7. Process according to any of the preceding claims, wherein in step b) the entrainer is fed to column2 at a stage above the feed to column 2 of the top stream of a).

8. Process according to any of the preceding claims, wherein in step c) the entrainer is fed to column3 at a stage above the feed to column 3 of the bottom stream of a).

9. Process according to any of the preceding claims, wherein downstream of column 1 and upstream of column 2 there is an additional column 12, of which the reboiler operates at a temperature of between 120 and 210°C and a pressure of between 200 and 700 mbar, and of which the condenser preferably operates at a temperature of between 70 and 140°C, to remove at least part of one or more of water, 2,3-butanediol, (2S,3S) 2,3-pentanediol, and (2R,3R) 2,3-pentanediol.

10. Process according to any of the preceding claims, wherein downstream of column 1 and upstream of column 3 there is an additional column 13, of which the reboiler operates at a temperature ofbetween 130 and 220°C and a pressure of between 200 and 700 mbar, and of which the condenser preferably operates at a temperature of between 120 and 200°C, to remove at least part of the azeotrope of ethylene glycol and 1,2-butylene glycol.

11. Process according to any of the preceding claims, wherein the top stream of column 2 in b) comprises at least 98% by weight of propylene glycol and less than 2% by weight of (2R,3S) 2,3- pentanediol and (2S,3R) 2,3-pentanediol combined, preferably said top stream comprises at least 99% by weight of propylene glycol and less than 1% (2R,3S) 2,3-pentanediol and (2S,3R) 2,3- pentanediol combined, more preferably said top stream comprises at least 99.5% by weight of propylene glycol and less than 0.5% (2R,3S) 2,3-pentanediol and (2S,3R) 2,3-pentanediol combined.

12. Process according to any of the preceding claims, wherein the top stream of column 3 in c) comprises at least 99% by weight of ethylene glycol and less than 0.1% by weight of 1,2- pentanediol, preferably at least 99.5% by weight of ethylene glycol and less than 0.05% by weight of 1,2-pentanediol.

13. Process according to any of the preceding claims, wherein of column 2 in b) the reboiler is operated at a temperature of between 150 and 240°, preferably between 170 and 210°C, and at a pressure of between 30 and 100 mbara, preferably at a pressure of between 40 and 90 mbara, and wherein the condenser of column 2 in b) is operated at a temperature of between 70 and 140°C, preferably between 90 and 130°C.

14. Process according to any of the preceding claims, wherein of column 3 in c) the reboiler is operated at a temperature of between 160 and 220°, preferably between 165 and 210°C, and at a pressure of between 20 and 500 mbara, preferably at a pressure of between 40 and 250 mbara, and wherein the condenser of column 3 in c) is operated at a temperature of between 80 and 160°C, preferably between 100 and 140°C.

15. Process according to any of the preceding claims, wherein of column 4 in d) the reboiler is operated at a temperature of between 150 and 220°, preferably between 165 and 210°C, and at a pressure of between 20 and 400 mbara, preferably at a pressure of between 30 and 200 mbara, and wherein the condenser of column 3 in c) is operated at a temperature of between 90 and 180°C, preferably between 100 and 160°C.

Images