Extractive workup of a sodium-salt-containing MMA-methanol mixture
The present invention relates to a process for preparing methyl methacrylate by direct oxidative esterification of methacrolein. Methyl methacrylate is used in large amounts for preparing polymers and copolymers with other polymerizable compounds. In addition, methyl methacrylate is an important synthesis unit for a variety of specialty esters based on methacrylic acid (MAA) which can be prepared by transesterification with the appropriate alcohol. There is consequently a great interest in very simple, economic and environmentally friendly processes for preparing this starting material. In particular the present invention relates to an optimized workup of the reactor effluent from the oxidative esterification of methacrolein by means of which process waste streams can be minimized and process water can be recycled in optimal fashion. The methacrylic acid generated in the process is also optimally recovered and isolated or converted into the commodity alkyl methacrylate. This process moreover has the advantage that fewer demands than described in the prior art are placed on plant apparatus configuration.
1. A process for producing methyl methacrylate, comprising:
producing methacrolein in a reactor I;
oxidatively esterifying the methacrolein with methanol to produce a reaction product comprising methyl methacrylate in a reactor II;
releasing methacrolein and, partially, methanol from the reaction product comprising methyl methacrylate in a distillation column III, to obtain a stream comprising methyl methacrylate, water, an alkali metal methacrylate and/or methacrylic acid and methanol;
mixing the stream with a strong acid;
separating the stream comprising the strong acid in a phase separation and/or an extraction into a light phase comprising methyl methacrylate and methacrylic acid and a heavy phase comprising water, methanol, at least one alkali metal salt, methacrylic acid, and the strong acid;
passing the heavy phase directly or indirectly into a distillation column IV,
obtaining a low-boiling fraction from the heavy phase in the distillation column IV, the low-boiling fraction comprising mainly methanol; and
reusing the methanol in the low-boiling fraction for producing a methyl methacrylate.
2. The process according to claim 1 , wherein the heavy phase is separated in the distillation column IV into the low-boiling fraction, a sidestream fraction comprising water and methacrylic acid, and a bottoms fraction comprising water, the strong acid and the at least one alkali metal salt.
3. The process according to claim 1 , wherein the stream from the distillation column III comprising methyl methacrylate is admixed with the strong acid and passed into a phase separation, and an organic phase is passed into the extraction and an aqueous phase is passed into the distillation column IV from the phase separation.
4. The process according to claim 1 , wherein the stream from distillation column III comprising methyl methacrylate is passed into a mixer into which the strong acid and optionally water are sent, and the strong acid is mixed with the stream comprising methyl methacrylate to obtain a mixture, which is subsequently passed into the phase separator and/or into the extraction.
5. The process according to claim 1 , wherein a feeding of the strong acid into the heavy phase from the extraction is effected directly upstream of the distillation column IV.
6. The process according to claim 1 , wherein the alkali metal salt is sodium sulfate, the alkali metal methacrylate is sodium methacrylate, and the inorganic acid is sulfuric acid.
7. The process according to claim 1 , wherein the light phase from the extraction is fed into at least two further distillation stages and/or a crystallization, and a fraction comprised predominantly of methacrylic acid and a fraction comprised-predominantly of methyl methacrylate are thus obtained.
8. The process according to claim 7 , wherein the fraction comprised predominantly of methacrylic acid is esterified with methanol to produce methyl methacrylate.
9. The process according to claim 1 , wherein the strong acid is added to a stream downstream of the distillation column III or into the extraction in an amount sufficient to ensure that a pH of the extraction is 3 or more.
10. The process according to claim 1 , wherein the low-boiling fractions from the distillation column III and/or the distillation column IV are passed into the reactor II.
11. The process according to claim 2 , wherein the low-boiling fraction from the distillation column IV comprises more than 60 wt % of methanol and less than 20 wt % of water, the bottoms fraction comprises more than 60 wt % of water, and the sidestream fraction from the distillation column IV comprises more than 80 wt % of water and less than 5 wt % of methanol.
12. The process according to claim 2 , wherein a lower part of the distillation column IV comprises a dividing wall, and wherein column feed is supplied at an upper end of the dividing wall and a draw of the side stream is taken off in liquid form at a different side of the dividing wall where a concentration of the strong acid is less than 0.1 wt % while a content of methanol is less than 1 wt %.
13. The process according to claim 2 , wherein a stream from the bottoms fraction from the distillation column IV is sent to a further separating stage to recover water and methacrylic acid and to concentrate the salt-containing waste stream.