Process for recovering 2-methoxyethanol from a wastewater stream

Abstract

A method for recovering 2-methoxyethanol from a wastewater stream comprising water and 2-methoxyethanol is disclosed. The method comprises: passing the wastewater stream and a solvent to an extractive distillation zone; recovering from the extractive distillation zone an intermediate 2-methoxyethanol stream comprising 2-methoxyethanol and solvent and having a lower water concentration than the wastewater stream; passing the intermediate 2-methoxyethanol stream to a solvent recovery zone; recovering from the solvent recovery zone a crude 2-methoxyethanol stream having a higher 2-methoxyethanol concentration than the wastewater stream and a lower solvent concentration than the intermediate 2-methoxyethanol stream. The method is characterized in that the method further comprises: passing the crude 2-methoxyethanol stream to a fractionator; recovering from the fractionator a purified 2-methoxyethanol stream having a higher 2-methoxyethanol concentration than the crude 2-methoxyethanol stream in the form of a side draw; and recovering from the fractionator an overhead stream comprising water and 2-methoxyethanol.

Description

TECHNICAL FIELD

[0001] The present invention relates to a process for the recovery of 2-methoxyethanol from a wastewater stream comprising water and 2-methoxyethanol. In particular, but not exclusively, aspects of the invention relate to a process for the recovery of 2-methoxyethanol from a wastewater stream comprising water, methyl methoxyacetate and 2-methoxyethanol. In particular, but not exclusively, the invention relates to a process for the recovery of 2-methoxyethanol from a wastewater stream from a process for the production of monoethylene glycol. BACKGROUND

[0002] The production of monoethylene glycol, and in particular the production of monoethylene glycol by hydrogenation of methyl glycolate, results in wastewater which can be contaminated with various organic contaminants, such as methanol. The wastewater will also contain 2-methoxyethanol, which can be produced, for example, by the hydrogenation of methoxyacetic acid and related esters, such as methyl methoxyacetate, which themselves can be by-products of the monoethylene glycol production process. 2-Methoxyethanol can be present in sufficient quantities to be undesirable in a typical wastewater treatment plant. 2-Methoxyethanol can also be useful, for example as a solvent in some countries. It is therefore desirable to separate 2-methoxyethanol from the wastewater and to recover 2-methoxyethanol. It is known that 2-methoxyethanol forms an azeotrope with water, and so it is not feasible to separate 2-methoxyethanol from the wastewater by conventional distillation.

[0003] Extractive distillation is a well-known technique for breaking azeotropes, and has been applied to the separation of 2-methoxyethanol from water. For example, US 5151160 describes the use of solvents such as dimethyl sulfoxide, sulfolane, dimethylformamide and 1,4-butanediol for extractive distillation to separate 2-methoxyethanol from water.

[0004] Other techniques have also been used to extract 2-methoxyethanol from water. For example, CN 208829317 and CN 109052522 describe the use of a liquid-liquid extraction system to remove 2-methoxyethanol from wastewater in an ethylene glycol production process.

[0005] There is an ongoing need to prevent waste and to further improve the sustainability of processes, including processes for the production of monoethylene glycol, and so there remains a need for improved processes for treating wastewater and for recovering components such as 2-methoxyethanol. SUMMARY

[0006] According to a first aspect of the invention, there is provided a process for the recovery of 2-methoxyethanol from a wastewater stream comprising water and 2-methoxyethanol, the process comprising:

[0007] a. passing the wastewater stream and a solvent to an extractive distillation zone;

[0008] b. recovering from the extractive distillation zone an intermediate 2-methoxyethanol stream comprising 2-methoxyethanol and solvent and having a lower water concentration than the wastewater stream;

[0009] c. passing the intermediate 2-methoxyethanol stream to a solvent recovery zone;

[0010] d. recovering from the solvent recovery zone a crude 2-methoxyethanol stream having a higher 2-methoxyethanol concentration than the wastewater stream and a lower solvent concentration than the intermediate 2-methoxyethanol stream;

[0011] characterized in that the process further comprises:

[0012] e. passing the crude 2-methoxyethanol stream to a fractionator;

[0013] f. recovering from the fractionator a purified 2-methoxyethanol stream having a higher 2-methoxyethanol concentration than the crude 2-methoxyethanol stream in the form of a side draw; and

[0014] g. recovering from the fractionator an overhead stream comprising water and 2-methoxyethanol.

[0015] The inclusion of a fractionator downstream of the extractive distillation zone and the solvent recovery zone and the recovery of an overhead stream comprising water and 2-methoxyethanol from the fractionator advantageously allows some water to slip from the extractive distillation zone into the intermediate 2-methoxyethanol stream, since the water can be subsequently removed in the overhead stream from the fractionator. Thus, the extractive distillation zone of the present invention can advantageously be smaller and operated under milder conditions, resulting in lower heat duty and lower capital and operating costs compared to when such a fractionator is not present. The presence of the fractionator thus results in a more economical process. Furthermore, the fractionator can be designed such that the absolute value of 2-methoxyethanol slip from the top of the fractionator is less than the absolute value of 2-methoxyethanol slip from the top of the extractive distillation zone to achieve the same water content in the purified 2-methoxyethanol stream in the absence of the fractionator. This can be because the smaller flow in the fractionator allows for improved separation without incurring excessive operating costs. As a result, the 2-methoxyethanol yield of the overall process can be improved.

[0016] Preferably, at least a portion of the overhead stream is recycled back to the extractive distillation zone. In this way, the process can advantageously achieve a more complete separation of water and 2-methoxyethanol, with both water and 2-methoxyethanol having a higher yield. As mentioned above, the presence of the fractionator advantageously allows some water to slip into the crude 2-methoxyethanol stream, ensuring good removal of 2-methoxyethanol from the waste water stream. This can advantageously reduce or eliminate the need to treat 2-methoxyethanol in any downstream waste water treatment plant. Recycling at least a portion of the overhead stream back to the extractive distillation zone advantageously means that water that slips into the crude 2-methoxyethanol stream is recovered in the overhead stream from the fractionator and returned to the extractive distillation zone, and so is not lost from the process. Some 2-methoxyethanol can slip into the overhead stream, such that the purified 2-methoxyethanol stream recovered in the form of a side draw can have a low water content. As mentioned above, a satisfactorily low water content can be achieved in the purified 2-methoxyethanol stream, with less absolute overhead slip of 2-methoxyethanol compared to that required in the absence of a fractionator. Furthermore, in the case where at least a portion of the overhead stream is recycled to the extractive distillation zone, the slipped 2-methoxyethanol is not all lost from the process, as it is recycled back to the extractive distillation zone. Thus, the presence of the fractionator and the overhead stream recycled to the extractive distillation zone allows for optimisation of the separation of 2-methoxyethanol from waste water in the extractive distillation zone and the purity of the purified 2-methoxyethanol stream in the fractionator, while minimising the loss of both water and 2-methoxyethanol.

[0017] Preferably, the process further comprises recovering a bottoms organic waste stream comprising organic contaminants from the bottoms stream of the fractionator. The withdrawal of the purified 2-methoxyethanol stream as a side draw advantageously means that other organic contaminants in the crude 2-methoxyethanol stream can be separated and used, for example, as a waste liquor fuel stream. Preferably, in the case where the waste water stream comprises waste water from a process for preparing monoethylene glycol, the waste liquor fuel stream is preferably used as fuel in the process for preparing monoethylene glycol, for example in a boiler for generating steam for process heating. In this way, the overall economics and sustainability of the process are improved. It is possible that the bottoms organic waste stream comprises methyl glycolate. In this case, the bottoms organic waste stream can advantageously be recycled to the process for preparing monoethylene glycol from which the waste water stream is generated. Advantageously, the bottoms organic waste stream can be recycled to a hydrogenation reactor in which the methyl glycolate is hydrogenated to monoethylene glycol. In this way, the yield of monoethylene glycol production can be improved.

[0018] The overhead stream can be purified. Even in embodiments in which the overhead stream is recycled, it is preferred that a portion of the overhead stream is purified before the overhead stream is recycled to the extractive distillation zone. Purification prevents the build-up of contaminants in the recycle. Preferably, the purification is combined into a waste liquor fuel stream.

[0019] In addition to obtaining a purified 2-methoxyethanol stream (which can then be used as product 2-methoxyethanol), it is advantageous to remove other organic contaminants from the intermediate wastewater stream, so that those organic contaminants do not have to be treated by any downstream wastewater treatment plant. Although the extractive distillation zone is operated to avoid 2-methoxyethanol (preferably from the top of the extractive distillation zone) slipping into the intermediate wastewater stream leaving the extractive distillation zone, other organic contaminants can still be present in the intermediate wastewater stream. Therefore, the process preferably further comprises:

[0020] i. recovering from the extractive distillation zone an intermediate wastewater stream having a lower concentration of 2-methoxyethanol than the wastewater stream;

[0021] j. passing the intermediate wastewater stream to a separator in which organic contaminants are separated from the intermediate wastewater stream;

[0022] k. recovering from the separator a purified wastewater stream having a higher water concentration than the intermediate wastewater stream and an overhead organic waste stream comprising organic contaminants.

[0023] The separator is preferably a distillation column, and is preferably operated at an absolute pressure of 0.12 MPa to 0.25 MPa. Therefore, the separator is preferably a distillation column operated at pressure; i.e. above atmospheric pressure. When the separator is a distillation column, the distillation column is preferably operated at a column bottom temperature of 110 °C to 130 °C. The distillation column is preferably operated at a column top temperature of 100 °C to 120 °C.

[0024] The purified wastewater stream is preferably transferred to a wastewater treatment plant for further treatment. The overhead organic waste stream is preferably used as a slop fuel, preferably by combining it with other slop fuel streams (such as slop fuel streams obtained from the purification of the bottoms stream and / or the overhead stream of the fractionator). In the case that the wastewater stream comprises wastewater from a process for producing monoethylene glycol, the slop fuel stream is preferably used as fuel in that process.

[0025] The extractive distillation zone is preferably an extractive distillation column. The extractive distillation zone is preferably operated under vacuum. The solvent recovery zone is preferably operated under vacuum. Most preferably, both the extractive distillation zone and the solvent recovery zone are operated under vacuum. Preferably, the extractive distillation zone is operated at an absolute pressure of 10 kPa to 50 kPa. Preferably, the extractive distillation zone is operated at a bottom temperature of 140 °C to 185 °C. Preferably, the extractive distillation zone is operated at a top temperature of 60 °C to 80 °C. Preferably, the solvent recovery zone is operated at an absolute pressure of 10 kPa to 50 kPa. Preferably, the solvent recovery zone is operated at a bottom temperature of 150 °C to 185 °C. Preferably, the solvent recovery zone is operated at a top temperature of 80 °C to 110 °C. Operating at such pressures and temperatures can provide optimal separation of 2-methoxyethanol and water, thereby improving the yield of 2-methoxyethanol and the quality of the purified wastewater stream sent to downstream wastewater treatment. It can also be advantageous to avoid higher temperatures, which can lead to solvent degradation and thus increased solvent consumption.

[0026] While the wastewater stream can be fed to the extractive distillation zone in liquid form, it is preferred that it is vaporized before being fed to the extractive distillation zone. Vaporizing the wastewater stream before feeding it to the extractive distillation zone can advantageously improve the operation of the extractive distillation zone and reduce its size, and thus its cost. For example, the vaporized wastewater stream can be fed to the extractive distillation zone below the solvent. Thus, the vaporized water and 2-methoxyethanol from the wastewater stream flows upwards to the zone, which is preferably a distillation column, and contacts the solvent flowing downwards. Preferably, the extractive distillation zone comprises a reboiler, and the wastewater stream is vaporized in a vaporizer having a similar heat load to the heat load of the reboiler. For example, the heat loads can be within 20% of each other. In this way, about half (e.g., 40-60%) of the heat injected into the extractive distillation zone is injected by vaporizing the wastewater stream feed. In some embodiments, at least 30% (e.g., 30-70%) of the heat injected into the extractive distillation zone is injected by vaporizing the wastewater stream feed. The temperature of the vaporizer is preferably lower than the temperature of the reboiler, which can advantageously reduce the total heat transfer surface area and thus the cost.

[0027] It will be appreciated that the solvent is not consumed in the extractive distillation zone or solvent recovery zone except for some loss due to degradation or reaction over time as will be appreciated by those skilled in the art. Rather, solvent can be recycled from the solvent recovery zone to the extractive distillation zone, typically purified to prevent build-up of unwanted components resulting from the above degradation or reaction, and supplemented with fresh solvent typically provided from a solvent tank accordingly. Thus, the process preferably comprises recovering a solvent recycle stream having a higher solvent concentration than the intermediate 2-methoxyethanol stream from the solvent recovery zone, and recycling the solvent recycle stream to the extractive distillation zone. Preferably, a purge is taken from the solvent recycle stream and passed to a solvent recovery apparatus to recover at least some of the solvent in the purge. The purge is then preferably used as a waste liquor fuel, preferably by combining it with one or more other waste liquor fuel streams from the process. For example, the purge can be combined with a purge from the overhead stream of the fractionator and / or a waste liquor fuel stream taken from the bottoms stream of the fractionator. The recovery apparatus is preferably operated, for example, at a pressure of 0.2 bara to 0.5 bara, for example at 0.35 bara. The recovery apparatus is preferably operated, for example, at a temperature of 160 °C to 180 °C.

[0028] Preferably, the fractionator is operated at an absolute pressure of 0.12 MPa to 0.2 MPa. Thus, the fractionator is preferably operated under pressure; i.e. above atmospheric pressure. The fractionator is preferably operated at a bottoms temperature of 145 °C to 175 °C. The fractionator is preferably operated at an overhead temperature of 120 °C to 140 °C. Such temperature and pressure conditions can result in advantageous purity in the purified 2-methoxyethanol stream. In a particularly preferred set of conditions, the extractive distillation zone is operated at an absolute pressure of 10 kPa to 50 kPa, a bottoms temperature of 140 °C to 185 °C, and an overhead temperature of 60 °C to 80 °C; the solvent recovery zone is operated at an absolute pressure of 10 kPa to 50 kPa, a bottoms temperature of 150 °C to 185 °C, and an overhead temperature of 80 °C to 110 °C; and the fractionator is operated at an absolute pressure of 0.12 MPa to 0.2 MPa, a bottoms temperature of 145 °C to 175 °C, and an overhead temperature of 120 °C to 140 °C. In such cases, the separator is preferably a distillation column operated at an absolute pressure of 0.12 MPa to 0.25 MPa, a bottoms temperature of 110 °C to 130 °C, and an overhead temperature of 100 °C to 120 °C. This combination can advantageously result in maximum extraction of 2-methoxyethanol, high 2-methoxyethanol purity in the purified 2-methoxyethanol stream, and reduced loss of 2-methoxyethanol to waste (e.g. waste liquor fuel streams) and increased waste water quality, particularly in terms of reduced organic concentration in the purified waste water stream for transfer to a waste water treatment plant.

[0029] Solvents effective for extractive distillation of 2-methoxyethanol and water are known. In some embodiments of the present invention, the solvent is preferably a linear or branched alkanediol, preferably having 3 to 8 carbons, with 2-methyl-l,3-propanediol, 2-methyl-2,4-pentanediol, and 1,2-butanediol being preferred. 2-Methyl-l,3-propanediol and 1,2-butanediol are particularly preferred, and 1,2-butanediol is most preferred. In other embodiments of the present invention, the solvent is preferably selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, sulfolane, ethylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, hexanediol, dipropylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, tripropylene glycol, 2-methyl-l,3-propanediol, 2-methyl-2,4-pentanediol, and polyethylene glycol 200. Again, 2-methyl-l,3-propanediol, 2-methyl-2,4-pentanediol, and 1,2-butanediol are preferred. 2-Methyl-l,3-propanediol and 1,2-butanediol are particularly preferred, and 1,2-butanediol is most preferred. 2-Methyl-l,3-propanediol and 1,2-butanediol can be particularly preferred when methyl methoxyacetate is to be separated from 2-methoxyethanol. Again, 1,2-butanediol can be most preferred. Preferably, the solvent is 1,2-butanediol.

[0030] The present invention can be particularly advantageous when used in a plant for the production of monoethylene glycol. In particular, the present invention can be advantageous in a process for the production of monoethylene glycol in which formaldehyde is subjected to hydrocarboxylation to glycolic acid, which is esterified to an alkyl glycolate, preferably methyl glycolate, wherein the alkyl glycolate is purified and then hydrogenated to monoethylene glycol. Such a process can provide an economically advantageous route to the production of monoethylene glycol. In such a process, methoxyacetic acid can form as a by-product of the hydrocarboxylation of formaldehyde to glycolic acid. Upon subsequent esterification of the glycolic acid to an alkyl glycolate, the methoxyacetic acid can be esterified to methyl methoxyacetate, which can then be hydrogenated to 2-methoxyethanol in the course of the hydrogenation of the alkyl glycolate to monoethylene glycol. In such a process, water and an alkanol, typically methanol, are present in the crude monoethylene glycol produced and separated therefrom, in particular when the alkyl glycolate is methyl glycolate. The alkanol is then preferably separated from the water, the alkanol being reused in the esterification step and the water being waste water. 2-methoxyethanol forms an azeotrope with water and is thus separated together with the water. Removal of 2-methoxyethanol from the waste water in a standard water treatment plant can be undesirable and it can be desirable to recover the 2-methoxyethanol in a product stream. The present invention can be particularly suitable for those circumstances. The waste water of such a process preferably comprises at least 85 wt.% water. In addition to 2-methoxyethanol, the waste water typically also comprises organic contaminants, such as formaldehyde, methanol, methyl glycolate and methyl methoxyacetate. The present invention advantageously separates those components so that they can be sent to, for example, waste liquid fuel and do not contaminate the purified waste water stream or the purified 2-methoxyethanol stream. Thus, preferably, the waste water stream comprises waste water produced in a process for the production of monoethylene glycol. The process for the production of monoethylene glycol preferably comprises the hydrocarboxylation of formaldehyde to form glycolic acid, the esterification of the glycolic acid to an alkyl glycolate, preferably methyl glycolate, and the hydrogenation of the alkyl glycolate to monoethylene glycol. Preferably, the process comprises collecting waste water from the process for the production of monoethylene glycol and storing it in a buffer tank from which the waste water stream is drawn. The buffer tank advantageously allows the process of the present invention to be controlled independently of the process for the production of monoethylene glycol. In this way, the advantages of the present invention are achieved without imposing unnecessary restrictions on the operation of the process for the production of monoethylene glycol, for example in terms of the treatment of organic contaminants, such as formaldehyde, methanol, methyl glycolate and methyl methoxyacetate, and in terms of achieving high yields and purity in the purified 2-methoxyethanol stream and the desired cleanliness of the purified waste water stream. Such a buffer tank can also allow waste water from more than one, preferably co-located, monoethylene glycol plants to be collected and treated together in the process of the present invention.

[0031] It is possible that the wastewater stream also comprises methyl methoxyacetate. Methyl methoxyacetate can also form an azeotrope with 2-methoxyethanol, thus meaning that they cannot be separated by conventional distillation. The present invention can also be advantageously used to break this azeotrope and separate methyl methoxyacetate from 2-methoxyethanol. Thus, when the wastewater stream comprises methyl methoxyacetate, the intermediate 2-methoxyethanol stream recovered from the extractive distillation zone preferably has a lower concentration of methyl methoxyacetate than the wastewater stream. In other words, the extractive distillation zone removes methyl methoxyacetate in addition to water from the intermediate 2-methoxyethanol stream.

[0032] It will be appreciated that breaking the azeotrope between 2-methoxyethanol and methyl methoxyacetate is advantageous in itself, regardless of whether the additional advantages described above in relation to the fractionator are also achieved. Thus, in one aspect of the present invention, there is provided a method for recovering 2-methoxyethanol from a wastewater stream comprising water, methyl methoxyacetate and 2-methoxyethanol, characterised in that the method comprises:

[0033] a. passing the wastewater stream and a solvent to an extractive distillation zone;

[0034] b. recovering from the extractive distillation zone an intermediate 2-methoxyethanol stream comprising 2-methoxyethanol and solvent and having a lower concentration of water and methyl methoxyacetate than the wastewater stream; and

[0035] c. recovering from the extractive distillation zone an intermediate wastewater stream comprising water and methyl methoxyacetate.

[0036] Preferably, the method further comprises:

[0037] d. passing the intermediate 2-methoxyethanol stream to a solvent recovery zone; and

[0038] e. recovering from the solvent recovery zone a crude 2-methoxyethanol stream having a higher concentration of 2-methoxyethanol than the wastewater stream and a lower concentration of solvent than the intermediate 2-methoxyethanol stream.

[0039] By removing methyl methoxyacetate as well as water from the extractive distillation zone, a higher purity 2-methoxyethanol product can be obtained. In this aspect of the application, the solvent preferably comprises 2-methyl-1,3-propanediol or 1,2-butanediol. 1,2-butanediol is particularly preferred. The applicant has found that 2-methyl-1,3-propanediol or 1,2-butanediol, particularly 1,2-butanediol, can be particularly suitable for breaking the azeotrope between 2-methoxyethanol and methyl methoxyacetate. Preferably, the concentration of solvent in the extractive distillation zone is from 15 mole% to 80 mole%, more preferably from 20 mole% to 60 mole%. When the solvent is 1,2-butanediol, the concentration of solvent in the extractive distillation zone is preferably from 15 mole% to 65 mole%, and most preferably from 20 mole% to 40 mole%. When the solvent is 2-methyl-1,3-propanediol, the concentration of solvent in the extractive distillation zone is preferably from 30 mole% to 80 mole%, and most preferably from 40 mole% to 60 mole%. The skilled person will appreciate that there can be some variation in the concentration of solvent throughout the extractive distillation zone. Typically, the concentration can vary relatively across the majority of the zone, with some variation at either end of the zone. It will therefore be appreciated that the concentration of solvent can be within a given range across the majority (e.g. the central 80%) of the extractive distillation zone. Preferably, the extractive distillation zone is operated at an absolute pressure of from 10 kPa to 50 kPa, a column bottom temperature of from 140 °C to 185 °C and a column top temperature of from 60 °C to 80 °C. This aspect of the application can alternatively or additionally further comprise any of the features described above in relation to other aspects of the application. In particular, the fractionator can still be used to obtain the advantages described above. The separator can also be used to remove organic contaminants, including methyl methoxyacetate, from the intermediate waste water stream. BRIEF DESCRIPTION OF DRAWINGS

[0040] Embodiments of the application will now be described, by way of example and not by way of limitation, with reference to the accompanying drawings, in which:

[0041] Figure 1 is a schematic representation of the process according to the application. DETAILED DESCRIPTION

[0042] In Figure 1From the distillation train of the process for producing monoethylene glycol, wastewater is collected and fed 1 to a buffer tank 2. The process for producing monoethylene glycol comprises hydrocarboxylation of formaldehyde to form glycolic acid, esterification of the glycolic acid to an alkyl glycolate, preferably methyl glycolate, and hydrogenation of the alkyl glycolate to monoethylene glycol. The distillation train for purifying the monoethylene glycol preferably comprises a low boiler column in which the alkanol, preferably methanol, and water are removed and fed to another distillation column in which the alkanol and water are separated. The alkanol is reused, and water containing 2-methoxyethanol and other organic contaminants, such as formaldehyde, methanol, methyl glycolate, and methyl methoxyacetate, form the wastewater that is fed 1 to the buffer tank 2. From the buffer tank 2, a wastewater stream 15 is withdrawn and fed to an extractive distillation zone 5. In this embodiment, the wastewater stream 15 is vaporized in a vaporizer 3 before being fed to the extractive distillation zone. A solvent 16 is also fed to the extractive distillation zone 5. The extractive distillation zone 5 is operated at an absolute pressure of 30 kPa, a column bottom temperature of 152°C, and a column top temperature of 69°C to effect separation of water and, in some embodiments, methyl methoxyacetate into an intermediate wastewater stream 17 and to effect separation of the solvent and 2-methoxyethanol into an intermediate 2-methoxyethanol stream 18. The intermediate 2-methoxyethanol stream 18 is fed to a solvent recovery zone 6, which is operated at an absolute pressure of 30 kPa, a column bottom temperature of 163°C, and a column top temperature of 92°C. In the solvent recovery zone 6, 2-methoxyethanol and solvent are separated. The 2-methoxyethanol is withdrawn from the top of the column in a crude 2-methoxyethanol stream 19, and the solvent is withdrawn from the bottom of the column in a solvent recycle stream 20. The solvent recycle stream 20 is returned to the extractive distillation zone 5, a purge 21 is withdrawn to prevent build-up in the recycle loop, and a corresponding amount of fresh solvent is added from a fresh solvent tank 4. In this embodiment, the solvent is 1,2-butanediol. The purge 21 passes through a solvent recovery device 14 in which some solvent is recovered and, in this embodiment, returned to the solvent recovery zone 6. The purge leaving the solvent recovery device 14 is fed to the waste liquid fuel stream 12.

[0043] The crude 2-methoxyethanol stream 19 is then passed to a fractionator 9. The fractionator is operated at an absolute pressure of 0.135 MPa, a column bottom temperature of 159°C and a column top temperature of 132°C. A purified 2-methoxyethanol stream 22 is withdrawn as a side draw from the fractionator 9. The purified 2-methoxyethanol stream 22 is fed to a 2-methoxyethanol storage tank 10 from which 2-methoxyethanol can be withdrawn for downstream use or for removal from site. An overhead stream 23 is withdrawn from the top of the fractionator 9 and is recycled to the extractive distillation zone 5. The overhead stream 23 comprises water and 2-methoxyethanol and provides a means of water slip to the intermediate 2-methoxyethanol stream 18, thereby keeping the 2-methoxyethanol content of the intermediate waste water stream 17 low, which can be recovered and not lost from the process. The presence of 2-methoxyethanol in the overhead stream 23 keeps the water content of the purified 2-methoxyethanol stream 22 low. Because the overhead stream 23 is recycled to the extractive distillation zone 6, loss of 2-methoxyethanol is also avoided. A purge 24 is withdrawn to prevent build-up in the overhead stream 23 and is passed to the waste liquid fuel stream 12. A bottoms waste organic stream 26 is collected from the bottom of the fractionator 9 and is passed to the waste liquid fuel stream 12.

[0044] The intermediate waste water stream 17 has had most of the 2-methoxyethanol removed, but can still contain other organic components, particularly light (i.e. having a lower boiling point than 2-methoxyethanol) organic components such as formaldehyde and methanol. It can also contain methyl methoxyacetate. In order to reduce the load on downstream waste water treatment, it is desirable to remove those organic components. This is achieved in a separator 7, which in this embodiment is a distillation column operated at an absolute pressure of 0.15 MPa, a column bottom temperature of 118°C and a column top temperature of 109°C. A purified waste water stream 8 is collected from the bottom of the separator 7 and an overhead organic waste stream 25 is collected from the top. The overhead organic waste stream 25 is passed to the waste liquid fuel stream 12, while the purified waste water stream 8 is passed to further downstream treatment, for example in a waste water treatment plant.

[0045] The above embodiment effectively recovers a high yield of high purity 2-methoxyethanol from the waste water stream 15, while reducing the loss of both water and 2-methoxyethanol and effectively cleaning the waste water stream 15 to reduce the load on downstream waste water treatment plants. The recovered 2-methoxyethanol can be further used, while also collecting other organics present in the waste water stream 15 for use, for example as a waste liquid fuel in the process from which the waste water is generated. The embodiment is particularly effective for treating effluent from a process for making monoethylene glycol, as it achieves a high yield and purity of 2-methoxyethanol and also achieves a high degree of removal of 2-methoxyethanol and other organic components generated in the monoethylene glycol process from the waste water stream that is passed downstream for waste water treatment.

[0046] Those skilled in the art will understand that the above embodiments have been described by way of example only, and not in any limitative sense, and that modifications and alterations are possible without departing from the scope of the present application as defined in the appended claims. For example, the waste liquid fuel stream need not be combined, but can be delivered separately from the process.

Claims

1. A method for recovering 2-methoxyethanol from a wastewater stream containing water and 2-methoxyethanol, the method comprising: a. The wastewater stream and solvent are conveyed to the extraction distillation zone; b. Recover an intermediate 2-methoxyethanol stream from the extraction distillation zone, which contains 2-methoxyethanol and the solvent and has a lower water concentration than the wastewater stream; c. The intermediate 2-methoxyethanol stream is transferred to the solvent recovery area; d. Recover crude 2-methoxyethanol stream from the solvent recovery zone, the crude 2-methoxyethanol stream having a higher 2-methoxyethanol concentration than the wastewater stream and a lower solvent concentration than the intermediate 2-methoxyethanol stream; The method is characterized by further comprising: e. The crude 2-methoxyethanol stream is fed to a fractionator; f. Recovering a purified 2-methoxyethanol stream with a higher 2-methoxyethanol concentration than the crude 2-methoxyethanol stream from the fractionator as a side fraction; and g. Recover the overhead stream containing water and 2-methoxyethanol from the fractionator.

2. The method of claim 1, wherein the method further comprises recirculating at least a portion of the overhead stream to the extractive distillation zone.

3. The method according to claim 1 or claim 2, wherein the method further comprises recovering a bottom organic waste stream containing organic pollutants from the bottom stream of the fractionator.

4. The method according to claim 1, further comprising purifying at least a portion of the overhead stream.

5. The method according to claim 1 or claim 2, wherein the method further comprises: h. Recover an intermediate wastewater stream from the extraction distillation zone having a lower concentration of 2-methoxyethanol than the wastewater stream; i. The intermediate wastewater stream is conveyed to a separator, in which organic pollutants are separated from the intermediate wastewater stream; j. Recover from the separator a purified wastewater stream having a higher water concentration than the intermediate wastewater stream and a top organic waste stream containing organic pollutants.

6. The method according to claim 5, wherein the separator is a distillation column operating at a gauge pressure of 0.12 MPa to 0.25 MPa, a bottom temperature of 110°C to 130°C, and a top temperature of 100°C to 120°C.

7. The method according to claim 1 or claim 2, wherein the extraction distillation zone operates at an absolute pressure of 10 kPa to 50 kPa, a bottom temperature of 140°C to 185°C, and a top temperature of 60°C to 80°C.

8. The method according to claim 1 or claim 2, wherein the solvent recovery zone operates at an absolute pressure of 10 kPa to 50 kPa, a bottom temperature of 150°C to 185°C, and a top temperature of 80°C to 110°C.

9. The method of claim 1, wherein the method comprises recovering a solvent recirculation stream having a higher solvent concentration than the intermediate 2-methoxyethanol stream from the solvent recovery zone, and recirculating the solvent recirculation stream to the extractive distillation zone.

10. The method of claim 9, wherein the purified material is removed from the solvent recirculation stream and conveyed to a solvent recovery device to recover at least some of the solvents in the purified material.

11. The method according to claim 1 or claim 2, wherein the fractionator operates at a gauge pressure of 0.12 MPa to 0.2 MPa, a bottom temperature of 145°C to 175°C, and a top temperature of 120°C to 140°C.

12. The method according to claim 1, wherein the solvent is a straight-chain or branched-chain alkanediol.

13. The method according to claim 1, wherein the solvent is selected from the group consisting of: dimethyl sulfoxide, dimethylformamide, dimethylacetamide, sulfolane, ethylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, hexanediol, dipropylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, tripropylene glycol, 2-methyl-1,3-propanediol, 2-methyl-2,4-pentanediol, and polyethylene glycol 200.

14. The method according to claim 12, wherein the solvent is 2-methyl-1,3-propanediol or 1,2-butanediol.

15. The method of claim 1, wherein the wastewater stream comprises wastewater generated in the method for preparing monoethylene glycol.

16. The method of claim 15, wherein the method for preparing monoethylene glycol comprises the hydrocarboxylation of formaldehyde to form glycolic acid, the glycolic acid being esterified to an alkyl glycolate, and the alkyl glycolate being hydrogenated to monoethylene glycol.

17. The method of claim 16, wherein the alkyl glycolate is methyl glycolate.

18. The method of claim 15, the method comprising collecting wastewater from the method for preparing monoethylene glycol and storing it in a buffer tank, and extracting the wastewater stream from the buffer tank.

19. A method for recovering 2-methoxyethanol from a wastewater stream containing water, methyl methoxyacetate, and 2-methoxyethanol, characterized in that... The method includes: a. The wastewater stream and solvent are conveyed to the extraction distillation zone; b. Recover an intermediate 2-methoxyethanol stream from the extraction distillation zone, containing 2-methoxyethanol and the solvent, and having a lower concentration of water and methyl methoxyacetate than the wastewater stream; c. Recover the intermediate wastewater stream containing water and methyl methoxyacetate from the extraction distillation zone; d. The intermediate 2-methoxyethanol stream is transferred to the solvent recovery zone; and e. Recover crude 2-methoxyethanol stream from the solvent recovery zone, the crude 2-methoxyethanol stream having a higher 2-methoxyethanol concentration than the wastewater stream and a lower solvent concentration than the intermediate 2-methoxyethanol stream.

20. The method of claim 19, wherein the solvent comprises 2-methyl-1,3-propanediol or 1,2-butanediol.

21. The method according to claim 19 or 20, wherein the concentration of the solvent in the extraction distillation zone is from 15 mol% to 80 mol%.

22. The method according to claim 19 or claim 20, wherein the extraction distillation zone is operated at an absolute pressure of 10 kPa to 50 kPa, a bottom temperature of 140°C to 185°C, and a top temperature of 60°C to 80°C.

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