Method for purifying a crude ethylene glycol stream

Abstract

Disclosed is a method for refining a crude ethylene glycol stream comprising monoethylene glycol and at least one acid contaminant. The method includes reacting the acid contaminant with the monoethylene glycol in at least one reaction zone to form an ester, and removing the ester in a separation step.

Description

Technical Field

[0001] The present invention relates to a method for refining a crude ethylene glycol stream. Specifically but not exclusively, the present invention relates to a method for refining a crude ethylene glycol stream which is prepared by hydrogenating a stream comprising methyl glycolate. The stream comprising methyl glycolate is preferably prepared by esterifying a stream comprising glycolic acid, and the stream comprising glycolic acid is preferably prepared by reacting formaldehyde and carbon monoxide. Background Art

[0002] Monoethylene glycol (MEG) is typically produced industrially from ethylene via the ethylene oxide intermediate. Ethylene oxide reacts with water to produce MEG. However, other routes are also available, including the hydrogenolysis of dimethyl oxalate, for example by the oxidation of carbon monoxide with methyl nitrate, or the hydrogenation of methyl glycolate. Methyl glycolate can be obtained, for example, by reacting 1,2 - diols or 1,2 - diols and primary alcohols with oxygen. Methyl glycolate can also be obtained by the esterification of glycolic acid with methanol. The method for preparing MEG provided by Johnson Matthey Davy Technologies involves reacting formaldehyde and carbon monoxide to prepare glycolic acid. Esterifying glycolic acid with methanol to prepare methyl glycolate, which is hydrogenated to form MEG. The methods for preparing MEG generally also produce diethylene glycol (DEG), so the unrefined product of such methods is a crude ethylene glycol stream comprising MEG and DEG. Some methods will contain very little DEG. For example, the selectivity for MEG is 90% or even 99%.

[0003] The specifications of the final MEG product may depend on the use of the MEG product. However, these specifications generally require a minimum level of MEG and a maximum level of DEG. For example, ASTM E2470 - 09 (reapproved in 2015), "Standard Specification for Polyester Grade Ethylene Glycol", requires a minimum MEG content of 99.9% (m / m) and a maximum DEG content of 0.05% (m / m). Therefore, the crude ethylene glycol stream generally needs to be refined to prepare a refined MEG product stream. Note that "% (m / m)" represents mass percentage.

[0004] These specifications also provide upper limits for other contaminants. For example, the ASTM E2470-09 standard has a maximum acidity (as acetic acid) of 0.002% (m / m) determined by ASTM test method E2679. In China, the Chinese national standard GB / T 14571.1-2016 "Test methods for industrial use monoethylene glycol - Part 1: Determination of acidity - Titrimetric method" specifies a maximum acid content of 10 ppmw (i.e., 0.001% (m / m)). Depending on the preparation method used, it may be necessary to reduce the level of acid contaminants in the crude ethylene glycol stream in order to meet this specification. The refining of the crude ethylene glycol stream is typically carried out by distillation. However, removing acid contaminants by distillation can be difficult and / or uneconomical, for example if the acid contaminant has a boiling point close to that of the desired MEG or DEG product. To assist in removing acid contaminants, it is known to add, for example, a base to neutralize the acid component. However, adding a base may be undesirable as it adds additional components to the system. This may, for example, require a stronger purge of the recycle stream to prevent contaminant accumulation, which is accompanied by a related loss of potential product.

[0005] Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art. Specifically, preferred embodiments of the present invention seek to provide an improved method for refining a crude ethylene glycol stream. Summary of the Invention

[0006] According to a first aspect of the present invention, there is provided a method for refining a crude ethylene glycol stream comprising monoethylene glycol and at least one acid contaminant, the method comprising reacting the acid contaminant with the monoethylene glycol in at least one reaction zone to form an ester and removing the ester in a separation step.

[0007] Preferably, the crude ethylene glycol further comprises diethylene glycol, and the method comprises reacting the acid contaminant with at least one of the monoethylene glycol or the diethylene glycol.

[0008] The crude ethylene glycol stream preferably comprises at least 30% (m / m) of monoethylene glycol (MEG), more preferably at least 40% (m / m) of MEG, and still more preferably at least 50% (m / m) of monoethylene glycol (MEG). The crude ethylene glycol stream preferably comprises at least 0.5% (m / m) of diethylene glycol (DEG), more preferably at least 1% (m / m) of DEG.

[0009] By reacting acid contaminants with MEG or DEG, esters are formed that are more readily separable subsequently (e.g., by distillation) than the original acid contaminants. By forming esters from the existing components of the crude ethylene glycol stream, no new components are added to the system. The separated esters can be recycled back to an upstream point of the process, such as hydrogenation, and thus there may be no product loss. Even if the separated esters are not recycled, since the level of acid contaminants is usually low, e.g., less than 1% (m / m), the loss of MEG or DEG is also low. The large excess of MEG and DEG compared to the acid contaminants also helps to drive the esterification reaction at a good rate, making the process a cost-effective way to remove acid contaminants. Thus, preferably, the crude ethylene glycol stream contains less than 5% (m / m), more preferably less than 2% (m / m), and most preferably less than 1% (m / m) acid contaminants. The crude ethylene glycol stream preferably contains at least 0.01% (m / m), more preferably at least 0.05% (m / m) acid contaminants.

[0010] The esterification reaction is typically an equilibrium-limited reaction, and thus it may be necessary to remove reaction products such as water to help drive the equilibrium and convert a sufficient amount of acid contaminants to meet the maximum acid content of the product specifications. Thus, the process preferably includes one or more water removal steps.

[0011] If water is present in the crude ethylene glycol stream, it may be desirable to have a water removal step, e.g., in a distillation column, before the reaction zone. For ease of reference, such a water removal step may be referred to in this application as light ends separation, and when the light ends separation is carried out in a distillation column, it may be referred to as a light ends distillation column. In addition to water, light ends separation may also remove other "light ends". For example, light ends separation may remove alkanols such as methanol. In a particularly preferred embodiment, MEG and DEG (if present) are prepared by hydrogenation of alkyl glycolates, which in turn are prepared by esterification of glycolic acid with an alkanol. Some alkanol may remain in the crude ethylene glycol stream after hydrogenation, and this alkanol may be removed in light ends separation. Preferably, the alkyl glycolate is methyl glycolate and the alkanol is methanol. Thus, the crude ethylene glycol stream may contain MEG, optionally DEG, acid contaminants, water, and an alkanol, preferably methanol, and may be fed to a light ends distillation column where separation occurs such that the alkanol and water are recovered at the top of the column and a stream containing MEG, DEG (if present), and acid contaminants is transferred to the reaction zone or the first reaction zone. In this way, water is removed from the crude ethylene glycol stream such that the equilibrium of the esterification reaction can be further pushed towards complete conversion of the acid contaminants, and the maximum level of acid contaminants required by the specifications can be met. Removing other "light ends" such as methanol may be beneficial in reducing the flow rate through the reaction zone and subsequent separation, thereby reducing equipment size, lowering costs, and facilitating an increase in the reaction rate by removing potential diluents from the stream.

[0012] Then it may also be desirable to remove water from the reaction zone, for example by using a reaction zone that allows for continuous water removal, or by removing water in an intermediate step between two reaction zones. The latter may be particularly desirable because a separation step that will be provided anyway, such as a distillation column for removing components lighter than MEG, can be used as an intermediate step for removing water. Thus, the method preferably comprises reacting the acid contaminants with MEG or with at least one of MEG and DEG to form esters in two reaction zones, wherein water is removed between the two reaction zones. The water removal is preferably carried out in a distillation column. Preferably, the distillation column also removes at least one component having a boiling point lower than MEG in addition to water. For ease of reference, such a separation step may be referred to in this application as crude MEG separation, and when the crude MEG separation is carried out in a distillation column, it may be referred to as a crude MEG distillation column. In a particularly preferred embodiment, the crude ethylene glycol stream contains light esters, i.e., esters having a boiling point lower than MEG, and the crude MEG separation removes the light esters and water from MEG and DEG (if present). For example, the crude MEG distillation column may receive a stream containing MEG, DEG (if present), unreacted acid contaminants, esters, light esters, and water from the first reaction zone, and may separate the light esters and water into an overhead stream, and separate MEG, DEG (if present), unreacted acid contaminants, and esters into a crude MEG stream. Then the crude MEG stream is preferably transferred to the second reaction zone, where the unreacted acid contaminants react with MEG or DEG to form more esters. In this way, the equilibrium of the esterification reaction is further pushed towards complete conversion of the acid contaminants, and the acid contaminant level can be sufficiently reduced to meet the maximum level specified in the specification.

[0013] In some embodiments, water can be directly removed from the reaction zone. The water removal is preferably continuous. The water removal will drive the equilibrium towards a high conversion of the acid contaminants. Such a reaction zone may include, for example, a stirred tank reactor having an overhead vapor removal function for continuous water removal. In such cases, it may be beneficial to combine the functions of the low-boiling component separation and the crude MEG separation discussed above into a single separation. This is because there is no longer a need for separate water removal upstream of and between the reaction zones, and all components having a boiling point lower than MEG, such as water, alkanols, and light esters, can be removed in the low-boiling component separation upstream of the reaction zone. Then water is continuously removed from the reaction zone. In such embodiments, although multiple reaction zones can be used, it is preferred to have a single reaction zone with a water removal function. Such embodiments can save costs in terms of combining the low-boiling component separation and the crude MEG separation into a single separation, but may involve greater costs in designing the reaction zone for water removal.

[0014] The at least one reaction zone preferably contains a catalyst. Although the esterification reaction of the acid contaminants with MEG or DEG may be autocatalyzed by the acid contaminants themselves, the amount of acid contaminants present is usually too low for effective autocatalysis. The catalyst is preferably selected to promote the esterification reaction of the acid contaminants with MEG or DEG while not promoting or only promoting to a lesser extent the formation of ethers, such as by the reaction of MEG with MEG. Although some ether formation, such as some DEG formation, may be tolerated, especially if DEG is present in the crude ethylene glycol stream and is a product of the process, the formation of long-chain ethers or the formation of excessive DEG leading to diketone formation will result in product loss. Etherification is generally promoted by stronger acids and higher temperatures than esterification, so the catalyst is preferably an acid catalyst strong enough to promote esterification but not strong enough to promote significant etherification. Similarly, the temperature is preferably high enough to promote esterification at an acceptable rate but not so high as to promote significant etherification.

[0015] Preferred catalysts include silica, resins, zeolites, and silica-alumina. Preferably, the resin catalyst is a partially surface-sulfonated resin, such as sulfonated divinylbenzene-styrene copolymer or PTFE resin. Such resins can promote esterification. However, sulfur-free catalysts such as silica, zeolites, and silica-alumina may also be advantageous because sulfur does not leach from such catalysts. Since sulfur contaminants may affect the performance of the hydrogenation catalyst, it may be necessary to remove the leached sulfur from the recycle stream, especially for hydrogenation. If a sulfur-free catalyst is used, such a sulfur removal step is avoided. Heteropolyacids can also be used as catalysts. The catalyst is preferably a heterogeneous catalyst, preferably in a fixed bed. Heterogeneous catalysts are advantageous because no separation step for removing the catalyst is required.

[0016] Preferably, the temperature in the reaction zone is from 50 °C to 200 °C, more preferably from 50 °C to 160 °C.

[0017] In some embodiments, the temperature can be from 50 °C to 90 °C, preferably from 60 °C to 80 °C. Such temperature ranges are particularly suitable for resin catalysts.

[0018] In some embodiments, the temperature can be from 140 °C to 170 °C, preferably from 150 °C to 160 °C. Such temperature ranges are particularly suitable for catalysts such as silica or silica / alumina or high-temperature resins, such as the WK10 or WK11 series from DIAION. The advantage of using those temperatures is that those temperatures can match the temperature of the crude MEG distillation column, and thus no heating or cooling of the stream between the reaction zone and the crude MEG distillation column is required.

[0019] Preferably, a crude ethylene glycol stream is prepared by hydrogenating a stream comprising methyl glycolate. The present invention is particularly advantageous in treating streams from such hydrogenations. This is because the crude ethylene glycol stream from such hydrogenations may contain methoxyacetic acid (MAA), which has a boiling point similar to that of MEG and is thus difficult to remove economically by conventional separation techniques such as distillation. The present invention esterifies MAA to prepare ethylene glycol methoxyacetate (EGMA), which has a higher boiling point than MAA and is more easily separated from MEG by distillation. Preferably, EGMA is recycled to the hydrogenation. Therein it will be converted to MEG and 2-methoxyethanol. In this way, the yield of MEG from the process is not reduced and may even be increased, since material that would otherwise be lost as MAA is now converted back to MEG.

[0020] Preferably, a stream comprising methyl glycolate is prepared by esterifying glycolic acid and methanol. The present invention is particularly advantageous in such processes. The esterification of glycolic acid with methanol generally does not proceed to 100% conversion and may be uneconomical even if such conversion is achieved, so some unreacted glycolic acid typically slips from the esterification reaction to the hydrogenation reaction, i.e., passes downstream. If glycolic acid is not removed in the hydrogenation reaction, it will be present in the crude ethylene glycol stream and will need to be removed during the refining process if the acid specifications of the product are to be met. However, glycolic acid has a boiling point similar to that of DEG and may thus be difficult to separate using conventional separation techniques such as distillation. While it is possible to operate the hydrogenation to remove unreacted glycolic acid by hydrogenating it to MEG, doing so may place stress on the operating conditions of the hydrogenation, requiring it to operate at conditions close to those where the hydrogenation catalyst is damaged. Careful control of the hydrogenation conditions may thus be required, with the conditions being changed over a very small range to achieve controlled variation. In contrast, the present invention can remove glycolic acid during the refining process and thus does not require the hydrogenation to be operated to remove glycolic acid. In the process of the present invention, the hydrogenation can be operated to convert no more than 99%, preferably no more than 98%, of the glycolic acid in the stream comprising methyl glycolate to MEG, and the remaining unreacted glycolic acid is removed during the refining process. Operating with such a conversion method can allow for more benign operating conditions during the hydrogenation process, thereby increasing catalyst stability and life and providing a greater range of controlled variation without damaging the hydrogenation catalyst. Preferably, the hydrogenation is operated to convert at least 96% of the glycolic acid in the stream comprising methyl glycolate to MEG. Converting the glycolic acid in the stream comprising methyl glycolate in the range of 96% to 99% to MEG may be a particularly advantageous operating window, since it achieves a good MEG yield while allowing for a range of controlled variation without damaging the catalyst.

[0021] Thus, the acid contaminant preferably comprises one or more of methoxyacetic acid or glycolic acid. The acid contaminant is preferably methoxyacetic acid.

[0022] Preferably, the glycolic acid used for preparing methyl glycolate by esterification of glycolic acid and methanol is prepared by hydrocarboxylation reaction between formaldehyde and carbon monoxide.

[0023] Preferably, the method further comprises collecting a low-acid feed stream from at least one reaction zone and sending it to at least one additional separation, where MEG and DEG (if present) are separated from the esters. The low-acid feed stream is so called because it has a reduced acid content compared to the crude ethylene glycol feed stream. This is because acid contaminants have reacted in at least one reaction zone to form esters. Preferably, the at least one additional separation separates the low-acid feed stream into an MEG product stream comprising at least MEG and a recycle stream comprising esters. When DEG is present, preferably, the at least one additional separation separates the low-acid feed stream into an MEG product stream comprising at least MEG, a DEG product stream comprising DEG, and a recycle stream comprising esters. The MEG product stream preferably comprises at least 99% (m / m), more preferably at least 99.5% (m / m) and most preferably at least 99.9% (m / m) of MEG. The DEG product stream preferably comprises at least 99% (m / m), more preferably at least 99.5% (m / m) and most preferably at least 99.9% (m / m) of DEG. Preferably, the DEG product stream comprises no more than 0.2% (m / m) of water, no more than 0.5% (m / m) of MEG and no more than 1.0% (m / m) of triethylene glycol (TEG). The recycle stream comprising esters is preferably recycled to an upstream point in the process (preferably hydrogenation) to convert the esters at least partially into MEG. The recycle stream comprising esters may also contain sulfur contaminants, for example if a sulfonated resin catalyst is used. Thus, the method may comprise subjecting the recycle stream to a sulfur removal process. This may be particularly beneficial when recycling the recycle stream to hydrogenation, as sulfur can affect the performance of the hydrogenation catalyst. In some embodiments, all or part of the recycle stream may be sent for purge. Preferably, the low-acid feed stream is sent to a first separation, which for reference purposes may be referred to herein as MEG product separation, where the MEG product stream is separated. Preferably, the MEG product separation is a distillation column, which for reference purposes is referred to herein as the MEG product distillation column, and the MEG product stream is preferably recovered as a side stream from the MEG product distillation column. It is also preferred to recover a stream comprising esters and DEG (if present) from the MEG product separation, preferably from the bottom of the MEG product distillation column. When DEG is present, preferably the stream comprising DEG and esters is sent to an additional separation zone where the DEG product stream is recovered. It is also preferred to recover the recycle stream from the additional separation zone. A stream comprising components having a boiling point lower than that of DEG may also be recovered from the additional separation zone. High DEG purity can be achieved in the DEG product stream by allowing some DEG to slip into the recycle stream and the stream comprising components having a boiling point lower than that of DEG. The stream comprising components having a boiling point lower than that of DEG will typically contain at least some MEG, as MEG is typically allowed to slip into the stream comprising DEG and esters in the MEG product separation in order to achieve high MEG purity in the MEG product stream.Accordingly, a stream comprising components having a boiling point lower than DEG is preferably recycled to the MEG product separation. In this way, MEG is not lost. This additional separation zone preferably comprises two separations, which are preferably two distillation columns. The two separations can, for example, be combined into a single distillation column, such as a dividing wall distillation column. The first of the two separations can remove a stream comprising components having a boiling point lower than DEG, which is preferably removed from the distillation column as an overhead stream. The second of the two separations can remove a recycle stream, which is preferably removed from the distillation column as a bottoms stream. The first and the second of the two separations can be in either order. The DEG product stream is preferably recovered downstream of the two separations. Thus, if the first of the two separations is upstream of the second of the two separations, the DEG product stream is preferably removed from the second of the two separations, which is preferably removed as an overhead stream. If the second of the two separations is upstream of the first of the two separations, the DEG product stream is preferably removed from the first of the two separations, which is preferably removed as a bottoms stream.

[0024] A stream comprising components having a boiling point lower than MEG is preferably recovered from the MEG product separation and preferably recycled to be added to the crude ethylene glycol stream. The stream comprising components having a boiling point lower than MEG is preferably recovered as an overhead stream from the MEG product distillation column. Preferably, some MEG is allowed to slip into the stream comprising components having a boiling point lower than MEG and into the stream comprising DEG (if present) and esters. In this way, high-purity MEG can be achieved in the MEG product stream.

[0025] In some embodiments, an alkali can be added downstream of the reaction zone to at least partially neutralize unreacted acid contaminants. For example, an alkali can be added in the MEG product separation. The advantage of adding an alkali downstream of the reaction zone is that less alkali may be required compared to the case where adding an alkali is the only way to remove acid contaminants, so that operation can be carried out with a smaller purge while still avoiding the accumulation of the alkali or its derivatives in the recycle loop. Preferably, the alkali is sodium hydroxide.

[0026] Another advantage of the present invention is the removal of acid contaminants upstream of a separation such as a distillation column used for separating and purifying MEG and DEG (if present), which means that more economical construction materials can be used in those separations. The presence of acid contaminants may require the use of expensive corrosion-resistant materials. By converting the acid contaminants into esters upstream of the separation, the use of such materials can be avoided.

[0027] According to a second aspect of the present invention, there is provided a method for producing monoethylene glycol, the method comprising hydrogenating a stream comprising methyl glycolate to prepare a crude ethylene glycol stream comprising monoethylene glycol and acid contaminants, and sending the crude ethylene glycol stream to the method according to the first aspect of the present invention.

[0028] The acid contaminant is preferably selected from methoxyacetic acid, glycolic acid, and combinations thereof.

[0029] According to a third aspect of the present invention, there is provided a method for producing monoethylene glycol and diethylene glycol, the method comprising hydrogenating a stream comprising methyl glycolate to prepare a crude ethylene glycol stream comprising monoethylene glycol, diethylene glycol, and an acid contaminant, and sending the crude ethylene glycol stream to the method according to the first aspect of the present invention.

[0030] The acid contaminant is preferably selected from methoxyacetic acid, glycolic acid, and combinations thereof.

[0031] According to a fourth aspect of the present invention, there is provided a method for producing monoethylene glycol, the method comprising esterifying glycolic acid with methanol to prepare a stream comprising methyl glycolate, and sending the stream comprising methyl glycolate to the method according to the second aspect of the present invention.

[0032] According to a fifth aspect of the present invention, there is provided a method for producing monoethylene glycol and diethylene glycol, the method comprising esterifying glycolic acid with methanol to prepare a stream comprising methyl glycolate, and sending the stream comprising methyl glycolate to the method according to the third aspect of the present invention.

[0033] According to a sixth aspect of the present invention, there is provided a method for producing monoethylene glycol, the method comprising reacting formaldehyde with carbon monoxide to prepare a stream comprising glycolic acid, and sending the stream comprising glycolic acid to the method according to the fourth aspect of the present invention.

[0034] According to a seventh aspect of the present invention, there is provided a method for producing monoethylene glycol and diethylene glycol, the method comprising reacting formaldehyde with carbon monoxide to prepare a stream comprising glycolic acid, and sending the stream comprising glycolic acid to the method according to the fifth aspect of the present invention.

[0035] It should be understood that features described with respect to one aspect of the present invention may equally apply to another aspect of the present invention. For example, features described with respect to the first aspect of the present invention may equally apply to the second, third, fourth, fifth, sixth, and seventh aspects of the present invention, and vice versa. Some features may not apply to a particular aspect of the present invention and may be excluded from a particular aspect of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the present invention will now be described by way of example and not in any limiting sense with reference to the accompanying drawings, in which:

[0037] Figure 1 is a schematic diagram of the method according to the present invention;

[0038] Figure 2 is a schematic diagram of the method according to the present invention;

[0039] Figure 3 is a schematic diagram of the method according to the present invention;

[0040] Figure 4 is a schematic diagram of the method according to the present invention; and

[0041] Figure 5 is a schematic diagram of the method according to the present invention, which includes Figure 4 the method of, and has an additional separation. Detailed Embodiment

[0042] In Figure 1 , carbon monoxide and formaldehyde are fed separately or together 5 to the carbonylation reaction 1, where carbon monoxide and formaldehyde react to form glycolic acid. The resulting glycolic acid is fed 6 to the esterification 2 with methanol, where the glycolic acid is esterified to methyl glycolate. The methyl glycolate is fed 7 to the hydrogenation 3, where the methyl glycolate reacts with hydrogen to form MEG. DEG can also be formed. Acid pollutants, including MAA and glycolic acid, are also produced. The product from the hydrogenation 3 is fed 8 to the purification step 4, where the acid pollutants are esterified by and separated from one or both of MEG and DEG. A product stream 9 containing purified MEG is produced. An additional product stream containing purified DEG can also be produced.

[0043] Figure 2 shows a purification system. The crude ethylene glycol stream 10 contains MEG and acid pollutants, including MAA and glycolic acid. The crude ethylene glycol stream 10 can also contain DEG. In this embodiment, the crude ethylene glycol stream 10 contains 900 ppm MAA and 6500 ppm glycolic acid. The crude ethylene glycol stream 10 is fed to the esterification reactor 16, where the acid pollutants are esterified by reaction with MEG. In this embodiment, the esterification reactor 16 is operated with a resin catalyst at 60°C to 80°C. The crude ethylene glycol stream 10 can be passed through a cooler upstream of the esterification reactor 16 to reduce the temperature of the crude ethylene glycol stream 10 to a suitable inlet temperature for the esterification reactor 16. To maximize the energy efficiency of the method, the cooler can include two stages. In the first stage, the crude ethylene glycol stream 10 can be passed through an exchanger where it is cooled by heat exchange with the cold stream 11 leaving the esterification reactor. In the second stage, the partially cooled stream leaving the exchanger is further cooled to the suitable inlet temperature for the esterification reactor by heat transfer to a cooling medium (such as water or air). The stream 11 leaving the esterification reactor is reheated as it passes through the exchanger.

[0044] Resin catalysts and conditions are selected to limit the formation of DEG. Water can be removed from the esterification reactor 16 to increase the conversion of acid contaminants. The outlet stream 11 from the esterification reactor 16 is fed to a distillation column 17. The MEG product stream 14 is taken as a side cut from the distillation column 17. In this embodiment, the level of acid contaminants in the MEG product stream 14 is less than 10 ppm. The overhead stream 15 from the distillation column 17 contains components lighter than MEG. To obtain a high MEG purity in the MEG product stream 14, some MEG can be allowed to slip into the overhead stream 15. Therefore, preferably the overhead stream 15 is recycled, optionally purged, and fed back into the crude ethylene glycol stream 10. In this way, the MEG in the overhead stream 15 is not lost. The bottoms stream 12 contains components heavier than MEG, including esters produced by the esterification of acid contaminants. The bottoms stream 12 can be sent for further processing to recover valuable components such as DEG, or can be recycled to upstream hydrogenation, or otherwise removed from the process. The recycle stream can optionally be passed through a desulfurization system, for example if the resin catalyst is sulfonated and leaches sulfur.

[0045] Figure 3A purification system is shown. The crude ethylene glycol stream 20 contains MEG and acid contaminants, including MAA and glycolic acid. The crude ethylene glycol stream 20 may also contain DEG. In this embodiment, the crude ethylene glycol stream 20 contains 900 ppm MAA and 6500 ppm glycolic acid. The crude ethylene glycol stream 20 is fed to an esterification reactor 26 where the acid contaminants are esterified by reaction with MEG. In this embodiment, the esterification reactor 26 is operated with a resin catalyst at 60 °C to 80 °C. Approximately 95% (m / m) of the MAA is converted in the esterification reactor 26. The crude ethylene glycol stream 20 may be passed through a cooler upstream of the esterification reactor 26 to reduce the temperature of the crude ethylene glycol stream 20 to a suitable inlet temperature for the esterification reactor 26. The resin catalyst and conditions are selected to limit the formation of DEG. The outlet stream 32 from the esterification reactor 26 is fed to a distillation column 28. In the distillation column 28, water and other light components are removed in the overhead stream 29. The bottoms stream 30 from the distillation column 28 is fed to a further esterification reactor 31 where the remaining acid contaminants are esterified by reaction with MEG. The further esterification reactor 31 contains a resin catalyst and is operated at approximately 60 °C to 80 °C. Approximately 95% of the remaining MAA is converted in the further esterification reactor 31. The resin catalyst and temperature range are again selected to reduce the production of DEG. DEG production involves the production of water, which is detrimental to the level of conversion of the acid contaminants. Providing two esterification reactors 26 and 31 with water removal in between in the distillation column 28 preferably means that there is no need to remove water from the esterification reactors 26 and 31. This simplifies the design and cost of the esterification reactors 26 and 31, and the distillation column 28 can be used for other useful purposes in the separation by removing other light components together with the water. The separation method of the prior art may already include a column suitable as the distillation column 28, so the method of the present invention can be achieved by inserting the esterification reactor 26 upstream of the column and the further esterification reactor 31 downstream of the column. The outlet stream 21 from the further esterification reactor 31 is fed to a distillation column 27. The MEG product stream 24 is taken as a side stream from the distillation column 27. In this embodiment, the level of acid contaminants in the MEG product stream 24 is less than 10 ppm. The overhead stream 25 from the distillation column 27 contains components lighter than MEG. To obtain a high MEG purity in the MEG product stream 24, some MEG may be allowed to slip into the overhead stream 25. Therefore, preferably the overhead stream 25 is recycled, optionally with a purge, and fed back into the crude ethylene glycol stream 20. In this way, the MEG in the overhead stream 25 is not lost. The bottoms stream 22 contains components heavier than MEG, including esters produced by the esterification of the acid contaminants. The bottoms stream 22 may be sent for further processing to recover valuable components such as DEG, or may be recycled to upstream hydrogenation, or otherwise removed from the process.

[0046] Figure 4A purification system is shown. The crude ethylene glycol stream 50 contains MEG and acid contaminants, including MAA and glycolic acid. The crude ethylene glycol stream 50 may also contain DEG. In this embodiment, the crude ethylene glycol stream 50 contains 900 ppm MAA and 6500 ppm glycolic acid. The crude ethylene glycol stream 50 is fed to a water removal distillation column 63. The overhead stream 64 from the water removal distillation column 64 contains water and, if methanol is present in the crude ethylene glycol stream from, for example, methyl glycolate production, contains methanol. The bottoms stream 65 from the water removal distillation column 63 contains MEG, DEG (if present), and acid contaminants. The bottoms stream 65 is fed to an esterification reactor 56 where the acid contaminants are esterified by reaction with MEG. In this embodiment, the esterification reactor 56 is operated with a silica catalyst at 150 °C to 160 °C. Approximately 95% (m / m) of the MAA is converted in the esterification reactor 56. The bottoms stream 65 may be passed through a cooler upstream of the esterification reactor 56 in order to reduce the temperature of the bottoms stream 65 to a suitable inlet temperature for the esterification reactor 56. The silica catalyst and conditions are selected to limit the formation of DEG. The silica catalyst also does not leach any sulfur. The effluent stream 62 from the esterification reactor 56 is fed to a distillation column 58. In the distillation column 58, water and other light components are removed in the overhead stream 59. The bottoms stream 60 from the distillation column 58 is fed to a further esterification reactor 61 where the remaining acid contaminants are esterified by reaction with MEG. The further esterification reactor 61 contains a silica catalyst and is operated at approximately 150 °C to 160 °C. Approximately 95% of the remaining MAA is converted in the further esterification reactor 61. The resin catalyst and temperature range are again selected to reduce the production of DEG. DEG production involves the production of water, which is detrimental to the level of conversion of the acid contaminants. Providing two esterification reactors 56 and 61 with water removal in between in the distillation column 58 preferably means that there is no need to remove water from the esterification reactors 56 and 61. This simplifies the design and cost of the esterification reactors 56 and 61, and the distillation column 58 can be used for other useful purposes in the separation by removing other light components together with the water. The separation method of the prior art may already include a column suitable as the distillation column 58, so the method of the present invention can be achieved by inserting the esterification reactor 56 upstream of the column and the further esterification reactor 61 downstream of the column. The effluent stream 51 from the further esterification reactor 61 is fed to a distillation column 57. The MEG product stream 54 is taken as a side stream from the distillation column 57. In this embodiment, the level of acid contaminants in the MEG product stream 54 is less than 10 ppm. The overhead stream 55 from the distillation column 57 contains components lighter than MEG. In order to obtain a high MEG purity in the MEG product stream 54, some MEG may be allowed to slip into the overhead stream 55. Accordingly, the overhead stream 25 is recycled, optionally purged, and fed back into the crude ethylene glycol stream 50.In this way, there is no loss of MEG in the overhead stream 55. The bottoms stream 52 contains components heavier than MEG, including esters produced by the esterification of acid contaminants. The bottoms stream 52 can be sent for further processing to recover valuable components such as DEG, or can be recycled to upstream hydrogenation, or otherwise removed from the process.

[0047] In Figure 5 the Figure 4 method is repeated, where like numbered items have like meanings and are not repeated here. In this embodiment, the crude ethylene glycol stream 50 contains DEG as well as MEG, and thus the DEG is contained in the bottoms stream 52 from distillation column 57 together with esters from the esterification of acid contaminants, some slippage of MEG, and other heavy contaminants. The bottoms stream 52 from distillation column 57 is sent to distillation column 66. The overhead stream 67 from distillation column 66 is recycled to feed into the effluent stream 51 from additional esterification reactor 61. The overhead stream 67 contains MEG, and thus any MEG that has slipped into the bottoms stream 52 is recycled to distillation column 57 and there is no loss in the process. The bottoms stream 68 from distillation column 66 contains DEG, esters from the esterification of acid contaminants, and other heavy contaminants. The bottoms stream 68 is fed to distillation column 71, and a DEG product stream 70 containing DEG is taken overhead from this distillation column. The DEG product stream 70 can be taken as a side stream additionally or alternatively. The bottoms stream 69 containing esters from the esterification of acid contaminants, some slippage of DEG to ensure high purity of DEG in the DEG product stream 70, and other heavy contaminants is removed from the bottom of distillation column 71. The bottoms stream 69 can be recycled to upstream hydrogenation or otherwise removed from the process.

[0048] Those skilled in the art will appreciate that the above embodiments are described by way of example only and not in any limiting way, and that changes and modifications can be made without departing from the scope of the invention as defined by the appended claims. For example, an embodiment described as using a resin catalyst can alternatively be operated using a silica catalyst, and vice versa. Other catalysts such as silica-alumina or zeolite can also be used. The Figure 5 method can be rearranged such that the esters from the esterification of acid contaminants are removed as a bottoms stream in a first distillation column, where a stream containing MEG and DEG is taken overhead and sent to a second column where MEG and DEG are separated. The columns can also be combined, for example as a dividing wall column.

Claims

1. A method for refining a crude ethylene glycol stream containing monoethylene glycol and at least one acid contaminant, the method comprising reacting the acid contaminant with the monoethylene glycol in at least one reaction zone to form an ester and removing the ester in a separation step, wherein the at least one reaction zone contains a catalyst to promote the esterification reaction, wherein the catalyst of the at least one reaction zone is selected from silica, sulfonated divinylbenzene-styrene copolymer or PTFE resin, zeolite, and silica-alumina, the acid contaminant includes one or more of methoxyacetic acid or glycolic acid, and the temperature in the reaction zone is 50°C to 200°C.

2. The method according to claim 1, wherein the crude ethylene glycol further contains diethylene glycol, and the method comprises reacting the acid contaminant with the diethylene glycol.

3. The method according to claim 1 or claim 2, wherein the crude ethylene glycol stream further contains water, and the method comprises a water removal step before the reaction zone.

4. The method according to claim 3, wherein the monoethylene glycol and, if present, the diethylene glycol are prepared by hydrogenation of an alkyl glycolate, and the alkyl glycolate is in turn prepared by esterification of glycolic acid with an alkanol.

5. The method according to claim 4, wherein the crude ethylene glycol stream further contains the alkanol, and the alkanol and water are removed in the water removal step.

6. The method according to claim 1 or claim 2, wherein water is removed from the reaction zone.

7. The method according to claim 1 or claim 2, wherein the method comprises reacting the acid contaminant with the monoethylene glycol, or with at least one of the monoethylene glycol or the diethylene glycol if diethylene glycol is present, to form the ester in two reaction zones, wherein water is removed between the two reaction zones.

8. The method according to claim 7, wherein the crude ethylene glycol stream further contains a light ester having a boiling point lower than that of monoethylene glycol, and the water removal also removes the light ester.

9. The method according to claim 1 or claim 2, wherein the catalyst is a sulfonated divinylbenzene-styrene copolymer or PTFE resin, and the temperature in the reaction zone is 50°C to 90°C.

10. The method according to claim 1 or claim 2, wherein the catalyst is a silica or silica-alumina catalyst, and the temperature in the reaction zone is 140°C to 170°C.

11. The method according to claim 1 or claim 2, wherein the crude ethylene glycol stream is prepared by hydrogenating a stream containing methyl glycolate.

12. The method according to claim 11, wherein the stream containing methyl glycolate is prepared by esterification of glycolic acid and methanol.

13. The method according to claim 12, wherein the stream containing methyl glycolate further contains glycolic acid, and the hydrogenation is operated so as to convert no more than 99% of the glycolic acid into monoethylene glycol.

14. The method according to claim 12 or 13, wherein the glycolic acid used for preparing methyl glycolate by esterification of glycolic acid and methanol is prepared by a hydrocarboxylation reaction between formaldehyde and carbon monoxide.

15. The method according to claim 1 or claim 2, wherein the acid contaminant is methoxyacetic acid.

16. The method according to claim 1 or claim 2, wherein the method further comprises collecting a low-acid feed stream from the at least one reaction zone and sending it to at least one additional separation that separates the low-acid feed stream into at least a monoethylene glycol product stream containing monoethylene glycol and a recycle stream containing the ester.

17. The method according to claim 16, wherein the monoethylene glycol product stream contains at least 99.9% m / m of monoethylene glycol.

18. The method according to claim 16, wherein the at least one additional separation separates the low-acid feed stream into at least the monoethylene glycol product stream containing monoethylene glycol, a diethylene glycol product stream containing diethylene glycol, and the recycle stream containing the ester.

19. The method according to claim 18, wherein the diethylene glycol product stream contains at least 99.9% m / m of diethylene glycol.

20. A method for preparing monoethylene glycol, the method comprising hydrogenating a feed stream containing methyl glycolate to prepare a crude ethylene glycol feed stream containing monoethylene glycol and an acid contaminant, and sending the crude ethylene glycol feed stream to the method according to any one of the preceding claims.

21. A method for preparing monoethylene glycol and diethylene glycol, the method comprising hydrogenating a feed stream containing methyl glycolate to prepare a crude ethylene glycol feed stream containing monoethylene glycol, diethylene glycol, and an acid contaminant, and sending the crude ethylene glycol feed stream to the method according to any one of claims 1 to 20.

22. A method for preparing monoethylene glycol, the method comprising esterifying glycolic acid with methanol to prepare a feed stream containing methyl glycolate, and sending the feed stream containing methyl glycolate to the method according to claim 20.

23. A method for preparing monoethylene glycol and diethylene glycol, the method comprising esterifying glycolic acid with methanol to prepare a feed stream containing methyl glycolate, and sending the feed stream containing methyl glycolate to the method according to claim 21.

24. A method for preparing monoethylene glycol, the method comprising reacting formaldehyde with carbon monoxide to prepare a feed stream containing glycolic acid, and sending the feed stream containing glycolic acid to the method according to claim 22.

25. A method for preparing monoethylene glycol and diethylene glycol, the method comprising reacting formaldehyde with carbon monoxide to prepare a feed stream containing glycolic acid, and sending the feed stream containing glycolic acid to the method according to claim 23.

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