Purification process of crude methyl methacrylate

Abstract

A process is described for purifying a crude methyl methacrylate (MMA) stream, typically obtained from one or more depolymerized (co)polymers containing methyl methacrylate (MMA). The stream contains MMA at a level of at least 80% by weight and ethyl acrylate (EA). The process comprises the steps of: fractional crystallizing said MMA stream to provide a fractionally crystallized MMA stream having a reduced EA content compared to the MMA stream immediately prior to fractional crystallization.

Description

[Technical field]

[0001] The present invention relates to a process for purifying crude methyl methacrylate from one or more depolymerized (co)polymers, typically including methyl methacrylate (MMA). The impurities may include other monomers or depolymerization by-products. The present invention is particularly concerned with the removal of one such impurity, namely ethyl acrylate. Ethyl acrylate (EA) may be present as an impurity resulting from the depolymerization of copolymers containing ethyl acrylate residues and / or as a by-product of the depolymerization process. Typically, it is present due to the depolymerization of copolymers of MMA and ethyl acrylate. Ethyl acrylate is known to be a "close boiler" of methyl methacrylate, i.e., the boiling point of ethyl acrylate is close to that of methyl methacrylate. This makes it difficult to completely separate ethyl acrylate and methyl methacrylate using conventional distillation. In particular, MMA is susceptible to polymerization, and to avoid this, a distillation column with a large number of stages, high reflux ratios, and high pressures is undesirable. [Background technology]

[0002] For purification of products on an industrial scale, alternative purification methods are known to those skilled in the art. These include, but are not limited to, fractional distillation, reactive distillation, dividing wall distillation and spinning band distillation, reactive crystallization, evaporative crystallization, cooling crystallization, evaporation, vapor compression evaporation, membrane filtration, reverse osmosis, ultrafiltration, gas-liquid chromatography, high pressure liquid chromatography (HPLC), gel permeation chromatography, ion exchange chromatography, adsorption, sublimation and specialized distillation techniques such as liquid-liquid extraction. US Pat. No. 5,399,411, US Pat. No. 5,499,423 and US Pat. No. 5,499,435 all disclose various separation methods that can be used to remove targeted molecules from product streams such as methacrylic acid. US Pat. No. 5,499,425 discloses various methods for the separation of biologically derived compounds from other components in the culture.

[0003] Furthermore, purification of crude methyl methacrylate often requires the removal of multiple impurities depending on the source of the crude stream. Although EA can be separated from MMA by chromatography, it is known that such processes can be problematic on an industrial scale due to the associated cleaning, downtime, and operating costs.

[0004] US Pat. No. 5,399,633 (Eastman Kodak) describes a purification process for alkyl methacrylates that involves fractional crystallization to remove methyl butyrate, which has a lower freezing point than MMA.

[0005] US Patent No. 6,299,333 (Parten) discloses that close boilers with higher freezing points than methyl methacrylate can be separated from crude MMA in a manufacturing process by fractional crystallization. Parten also shows that other process impurities such as methyl isobutyrate (MiB), which actually has a lower freezing point, can also be separated from the methyl methacrylate.

[0006] Parten notes a reduction in MiB levels after fractional crystallization. The freezing point of MiB is -85°C. However, Parten's MiB levels in the crude stream and MMA crystals are reduced from 2300 ppm to 1300 ppm, only 56% of their original levels.

[0007] EA levels in recycle streams would generally need to be reduced to much lower levels to be effective: some copolymer sources for recycle streams have relatively high percentages of EA, with levels of 5% and 10% by weight being not uncommon, and even if lower levels such as 2500 ppm are present in some streams, reducing it to 1300 ppm would still not be a sufficient level in purified MMA monomer streams.

[0008] Surprisingly, of the available purification processes, it was found that EA can be removed from MMA crude streams to sufficiently low levels by fractional crystallization. Despite the fact that EA is more similar in chemical structure to MMA than to MiB, and that EA has a "higher" freezing point (-71 °C) closer to that of MMA (48 °C) than to MiB (-85 °C), this process of removing EA from MMA by fractional crystallization is surprisingly more effective than the previously reported removal of MiB. Due to such similarity in structure, it was expected that EA would co-crystallize with MMA and become trapped in the crystal lattice of the latter, resulting in correspondingly lower levels of EA removal, but the opposite effect was found. This effect is particularly evident at low levels of EA already below 2 wt. %. It is surprising that even at such low levels EA can be effectively removed to a fraction of its previous levels, typically below 100 ppm.

[0009] Furthermore, fractional crystallization, or a combination of fractional crystallization with pre- or post-fractional distillation purification steps, provides a monomer stream of MMA that is sufficiently purified to be under current reach regulations. The purity levels of MMA produced by this invention are much higher, and the levels of EA are very low with this technology. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] US Patent Publication No. 20150119541 [Patent Document 2] U.S. Patent No. 6,380,427 [Patent Document 3] International Publication No. 2020006058 [Patent Document 4] U.S. Pat. No. 10,808,262 [Patent Document 5] GB 1235208 [Patent Document 6] U.S. Patent No. 6,670,501 Summary of the Invention [Means for solving the problem]

[0011] According to the present invention there is provided a process for purifying crude MMA according to the claims. Typically, the ratio of EA compared to the fractionally crystallized MMA stream is less than 1:5, more typically less than 1:10, and most typically less than 1:50. Advantageously, it has been found that the process of the present invention allows for the removal of more than 90% w / w of EA from the crude MMA stream, more typically more than 95% w / w, and most typically more than 97% w / w.

[0012] Advantageously, the MMA purity in the fractionally crystallized MMA stream may be greater than 98 wt%, such as greater than 98.5 wt%, typically greater than 99 wt%, for example greater than 99.5 wt%, 99.6 wt%, 99.7 wt%, 99.8 wt%, or 99.9 wt%.

[0013] The process of the invention can provide a yield (total end product / total feed) from fractional crystallization of 80%, such as greater than 85%, greater than 90%, or greater than 95%. The fractional crystallisation process of the present invention can use any form of fractional crystallisation known to those skilled in the art, such as suspension crystallisation or layer crystallisation, e.g. static crystallisation or falling film crystallisation.

[0014] A typical process for fractional crystallization according to the invention comprises a first stage including a first cooling phase of the crude stream producing MMA crystals and residue, an optional dissolution phase in which the crystals formed in the first cooling phase are heated and partially remelted to produce dissolved crystals and a liquid of the sweating phase, and a crystal melting phase to produce a purified liquid therefrom. The dissolution phase is utilized to remove residual EA and other impurities from the impure portion of the crystals that melt at a lower temperature than the MMA. The dissolution phase may include a single heating and remelting step, or multiple heating and remelting steps, such as 1, 2, 3, 4 or 5 steps, as necessary to achieve the desired purity of the crystals. Residual liquid is removed after the cooling phase and the dissolution phase, or after each heating and remelting step of the dissolution phase. The residual liquid can be recycled to extract further MMA crystals, optionally after mixing with a further stream of crude MMA feed.

[0015] Typically, at least one further crystallization stage is carried out, in which the purified liquid stream produced from the first stage is typically recrystallized according to the protocol of the first stage. Optionally, two or more further crystallizations of the liquid product are carried out in succession to produce increasingly pure liquids. Depending on the required purity of the final product, up to six or seven successive purification stages can be carried out. However, it has been found that advantageously, sufficient purification and EA removal can be achieved after one or two purification stages.

[0016] After the first cooling phase, the remaining liquid can be removed or recycled. In addition, any dissolution phase liquid can also be recycled for further crystallization. This can improve the yield of the process.

[0017] Optionally, an initial nucleation step can be performed in which the temperature is temporarily lowered to initiate crystal formation and then raised to a higher temperature for slower crystal formation.

[0018] The cooling phase therefore optionally includes an initial nucleation phase where the temperature of the liquid being purified is temporarily reduced to initiate crystal formation, and a crystal formation phase where the temperature initially increases and optionally decreases again slowly for slow crystal formation during the remainder of the cooling phase.

[0019] Typically, the liquid product stream to be purified is cooled to about −45° C. to about −75° C., resulting in a portion of the crude liquid product stream freezing to form solid methyl methacrylate crystals and a residual liquid, or supernatant, which is the portion of the liquid product stream that remains unfrozen.

[0020] The level of impurities in the methyl methacrylate crystals can be affected by the rate at which the crude liquid product stream is cooled. The rate at which the liquid product stream is cooled can be controlled to optimize the separation of methyl methacrylate from the impurities by minimizing the amount of impurities contained in the crystals. It has been found that relatively slow cooling rates produce methyl methacrylate crystals that contain a lower percentage of impurities than crystals formed as a result of faster cooling of the liquid product stream. The cooling rate of the liquid product stream is preferably less than 30°C / min, more preferably less than 20°C / min, and most preferably less than 10°C / min. Even slower cooling rates can be used, for example, less than 5 or 4 or 3 or 2 or 1 or 0.5 or 0.1°C / min.

[0021] A suitable temperature range for crystal formation is the saturation point of MMA in the liquid, for example -48 to -70°C, more typically -50 to -69°C, most typically -52 to -69°C. Suitable temperatures for nucleation are below the freezing point of MMA, for example in the range of -53 to -75°C, more typically -55 to -72°C, and most typically -58 to -62°C.

[0022] Therefore, a suitable protocol for crystallization is a nucleation cooling step in the range described above until crystals begin to form, heating step above the nucleation temperature to the crystallization range described above, and slow cooling in the same temperature range.

[0023] Typically, the heating step involves raising the temperature to -48 to -63°C, optionally followed by slow cooling to a temperature range of less than -48°C to -70°C. Cooling to effect crystal formation may be carried out slowly, for example over a period of 1 to 20 hours, typically 4 to 10 hours, and most typically 6 to 8 hours, to optimize crystal growth.

[0024] definition As used herein, (co)polymer means a homopolymer or a copolymer. The term copolymer includes polymers having more than one type of monomer residue, and thus includes terpolymers and the like.

[0025] By crude MMA is meant any MMA having impurities therein, whether or not some of the impurities have been removed. Thus, crude MMA includes streams of MMA that have been purified prior to fractional crystallization.

[0026] By yield is meant total final product / total feed. The yield is based on the final product, which may have undergone one or more than one fractional crystallization stage. Typically, one or two stages are sufficient, but one to seven stages, more commonly one to three stages of purified product streams can be carried out to produce the final product stream.

[0027] The invention will now be described, by way of example only, with reference to the following examples and drawings in which: [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic diagram of fractional crystallization. [Diagram 2] 1 is a graph of temperature versus time for the fractional crystallization of Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Example 1 The results are based on a synthetic crude with an MMA purity of about 94 wt.% in addition to several impurities. Tests showed that purification by crystallization could significantly reduce the EA concentration and improve the MMA purity in the presence of several other impurities. Two successive fractional crystallizations were performed.

[0030] The fractional crystallization was a static crystallization and was carried out on a crude mixed feed having the components detailed in Table 1 below.

[0031] [Table 1]

[0032] As can be seen from the above results, the removal of EA is significantly greater in fractional crystallization than other common impurities such as MiB and MA, both in single-stage and subsequent crystallizations. The product stream of the first stage was also recrystallized and recollected by melting in a second stage to further purify the product by removal of the equally valuable EA.

[0033] In both stages it was also possible to recycle uncrystallized impure melt from the uncrystallized residual liquid or from the dissolved crystals for crystallization in a later stage. The general process is shown diagrammatically in Figure 1. In the feed stage, the impure feed liquid is rapidly cooled to a temperature below the freezing point of pure MMA, such as -60°C, to allow nucleation of MMA crystals, and then heated to -48°C, the freezing point of MMA, where MMA crystals begin to slowly crystallize. Over the course of several hours, the liquid is gradually cooled to -65°C to allow gradual crystallization from the mother liquor. Once a sufficient amount of the mother liquor has crystallized, the process is stopped. In the purification stage, the remaining uncrystallized mother liquor is removed from the crystallizer, and the crystals are partially remelted or "dissolved" by slowly increasing the temperature until the required crystal purity is reached. The remelted liquid is then also removed from the crystallizer, and the remaining crystals of the required purity are completely melted, and the purified liquid is then collected and analyzed. This first stage purified liquid is then recrystallized in a second stage, and the above process repeated. In Table 1, only two stages are exemplified, but multiple stages can be performed if necessary.

[0034] Although not explained, both the original mother liquor residue and the dissolved crystal residue can be recycled and, if desired, further crystallized to improve yield. Example 2 A jacketed vessel with an internal volume of 6 liters was used to carry out fractional crystallization of binary mixtures of MMA-EA (5 wt% EA, 2 wt% EA, and 1 wt% EA in MMA). These represent the levels of EA that may be present in the processed crude MMA. The crystallization vessel was cooled using an external Unistat 705 refrigeration unit (Huber Offenburg / Germany) connected by an insulated hose. The system used a heat transfer fluid (Huber Thermal Fluid (HTF) DW-Therm M90.200.02). The temperature of the crystallizer system was controlled by the built-in temperature control of the Unistat 705 unit. The outlet HTF temperature was controlled in the range of 0 to -60 °C (Figure 2).

[0035] The fractional crystallization process consisted of nucleation, growth, multiple "melt" and melting phases of the final purified crystalline product over a 23-hour period. As can be seen in Figure 2, nucleation is achieved by lowering the temperature relatively quickly below the freezing point of MMA, followed by a relatively rapid increase in temperature to a temperature at or near the freezing point of MMA. Crystal growth is then achieved by lowering the temperature slowly as the freezing point decreases depending on the purity of the supernatant. The supernatant is then removed and the temperature is raised slightly towards the melting point of MMA to initiate crystal dissolution. In Figure 2, a single dissolution step is shown. However, multiple dissolution steps can also be performed. After each dissolution phase, a liquid process fraction rich in EA was removed from the crystallizer vessel to reduce the level of this impurity in the crystallized MMA. The temperature was then raised to -20°C to melt the crystals and remove the purified MMA as a liquid. The separation efficiency was determined from the initial binary MMA-EA concentrations and the concentrations of the final molten product. Ethyl acrylate concentrations were determined by GC-FID analysis from two separate calibration curves: levels of 10–1 wt% EA in MMA and levels of 500–5 ppmw EA in MMA.

[0036] The results are shown in Table 2.

[0037] [Table 2]

[0038] As can be seen from Table 2, the separation efficiency for removing EA from the MMA product stream is very high. Attention is directed to all publications and documents filed contemporaneously or prior to this application related to this application and published herewith, the contents of all such publications and documents being incorporated herein by reference.

[0039] All features disclosed in this specification (including any accompanying claims, abstract, and drawings), and / or all steps of any method or process so disclosed, may be combined in any combination, except where at least some of such features and / or steps are mutually exclusive.

[0040] Each feature disclosed in this specification (including any accompanying claims, abstract, and drawings), unless expressly stated otherwise, may be replaced by alternative features serving the same, equivalent, or similar purpose. Thus, unless otherwise stated, each feature disclosed is merely an example of a generic series of equivalent or similar features.

[0041] The invention is not limited to the details of the foregoing embodiments, but extends to any novel one, or any novel combination, of features disclosed in this specification (including any accompanying claims, abstract and drawings), or any novel one, or any novel combination of steps of any method or process so disclosed.

Claims

1. 1. A process for purifying a crude methyl methacrylate (MMA) stream containing MMA at a level of at least 80 wt.% and ethyl acrylate (EA), comprising: (i) fractionally crystallizing said crude MMA stream to provide a fractionally crystallized MMA stream having a reduced EA content compared to said crude MMA stream immediately prior to fractional crystallization.

2. 10. The process of claim 1, wherein one, two or more fractional crystallizations of the crude MMA stream are carried out sequentially to gradually remove the EA from the crude MMA stream.

3. 2. The process of claim 1, wherein the crude MMA stream is subjected to a pre-fractional crystallization purification step by a technique other than fractional crystallization to produce a pre-fractional crystallization purified crude MMA stream having an MMA level of at least 92.5 wt.%.

4. 4. The process of claim 3, wherein the crude MMA stream also contains impurities other than EA, and the crude MMA stream containing EA and impurities other than EA is pre-purified by a technique other than fractional crystallization to obtain, prior to fractional crystallization of the crude MMA stream in step (i), a pre-fractional crystallization purified crude MMA stream comprising at least 92.5 wt. % MMA and a reduced content of said impurities other than EA compared to the unpurified crude MMA stream.

5. 4. The process according to claim 1 or 3, wherein the fractionally crystallized MMA stream contains impurities other than EA, and the fractionally crystallized MMA stream is subjected to a post-fractional crystallization purification step by a technique other than fractional crystallization to reduce the content of said impurities other than EA compared to the content of the fractionally crystallized stream before the post-fractional crystallization purification step.

6. 2. The process of claim 1, wherein the crude MMA stream comprises 80 to 99 wt. % MMA, typically 90 to 99 wt. % MMA.

7. 2. The process of claim 1, wherein the EA is present in the crude MMA stream at a level of 0.01 to 10 wt %, typically 0.01 to 7.5 wt %, more typically 0.05 to 5 wt %, and most typically 0.1 to 1.5 wt %.

8. 2. The process of claim 1, wherein the crude MMA stream immediately prior to fractional crystallization comprises at least 92.5 wt.%, typically at least 97.5 wt.%, more typically at least 99 wt.% MMA.

9. 2. The process of claim 1, wherein the ratio of EA compared to the fractionally crystallized MMA stream to the crude MMA stream is less than 1:2, typically less than 1:10, and more typically less than 1:

50.

10. 2. The process of claim 1, wherein the crude MMA stream is obtained from depolymerized (co)polymers comprising MMA residues, typically more than 80% MMA residues, such as more than 85%, more than 90% or more than 95% MMA residues.

11. 11. The process of claim 10, wherein the crude MMA stream is obtained from depolymerized (co)polymers comprising copolymers of MMA and EA residues, typically greater than 1% EA residues, such as greater than 2, 3, 4, or 5% EA residues, for example 1-20%, 1-15% or 1-10% EA residues.

12. 10. The process of claim 1, wherein the fractionated crystallized MMA stream contains less than 5000 ppm EA, typically less than 1000 ppm EA, more typically less than 500 ppm EA, and most typically less than 100 ppm EA.

13. 10. The process of claim 1, wherein the fractionated crystallized MMA stream contains less than 350 ppm MiB, typically less than 200 ppm MiB, more typically less than 100 ppm MiB.

14. 2. The process of claim 1, wherein the fractional crystallization process of step (i) is selected from suspension crystallization or layer crystallization, such as static crystallization or falling film crystallization.

15. 2. The process of claim 1, wherein the fractional crystallization comprises a first stage including a first cooling phase of the crude MMA stream to produce crystals and a residue, an optional dissolution phase to heat and partially remelt the crystals formed in the first cooling phase to produce dissolved crystals and a dissolved phase liquid, and a crystal melting phase to produce a purified liquid therefrom.

16. 16. The process of claim 15, wherein the cooling phase comprises an initial nucleation phase in which the temperature of the crude MMA stream to be purified is temporarily reduced to initiate crystal formation, and a crystal formation phase in which the temperature is initially increased and optionally reduced again slowly for slow crystal formation during the remainder of the cooling phase.

17. 2. The process of claim 1, wherein the fractional crystallization includes a crystal growth phase, typically wherein the temperature is first increased to effect nucleation and then optionally slowly decreased again to allow for slow crystal formation.

18. 10. The process of claim 1, wherein the crystals formed during the fractional crystallization are subjected to one or more dissolution phases to heat and partially remelt the formed crystals.

19. 17. The process of claim 16, wherein the protocol for fractional crystallization comprises gradually cooling the stream of crude MMA from an initial temperature during the crystal formation phase over a crystal formation period.

20. 20. The process of claim 19, wherein the crystal formation period is from 1 to 20 hours.

21. The process of claim 5, which is dependent on claim 3, wherein the purification steps before and after fractional crystallization are independently selected from specialized distillation techniques such as fractional distillation, reactive distillation, dividing wall distillation and spinning band distillation, reactive crystallization, evaporative crystallization, cooling crystallization, evaporation, vapor compression evaporation, membrane filtration, reverse osmosis, ultrafiltration, gas-liquid chromatography, high pressure liquid chromatography (HPLC), gel permeation chromatography, ion exchange chromatography, adsorption, sublimation, and liquid-liquid extraction.

22. 5. The process according to claim 3 or 4, wherein prior to fractional crystallization of the crude MMA stream, one or more purification steps other than fractional crystallization are carried out on the crude MMA stream.

23. 6. The process of claim 5, wherein one or more purification steps other than fractional crystallization are performed on the fractionally crystallized MMA stream.

24. A stream of MMA produced by the process of claim 1, having a purity of at least 99% by weight and containing less than 5000 ppm EA, typically less than 1000 ppm EA, more typically less than 500 ppm EA, and most typically less than 100 ppm EA.

25. MMA produced by the process of claim 1, having a purity of at least 99% by weight and containing less than 5000 ppm EA, typically less than 1000 ppm EA, more typically less than 500 ppm EA, and most typically less than 100 ppm EA.

26. Methyl methacrylate (MMA) having a purity of at least 99% by weight and containing less than 5000 ppm ethyl acrylate (EA), typically less than 1000 ppm EA, more typically less than 500 ppm EA, and most typically less than 100 ppm EA.

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