OPTIMIZED PROCESS FOR PREPARING METHACRYLIC ACID

Abstract

The present invention relates to an optimized process for preparing methacrylic acid, wherein methacrolein is prepared in a first stage from propionaldehyde and formaldehyde by means of a Mannich reaction and oxidized in a second stage to methacrylic acid. More particularly, the present invention relates to the reduction in the amounts of catalyst to be used in the first stage, especially to the reduction in the amounts of acid to be used here, by virtue of the additional installation of recycling streams suitable for the purpose.

Claims

1. A process for continuously preparing methacrylic acid, comprising: preparing methacrolein in a first process step from formaldehyde and propionaldehyde with at least one acid and at least one organic base as catalysts in a reactor 1, then separating methacrolein from the catalyst-containing phase present and oxidizing the methacrolein in a second process step to methacrylic acid with a heterogeneous catalyst in the presence of oxygen and water in a reactor 2, then condensing and quenching the process gas formed to produce an aqueous crude methacrylic acid, and then separating the methacrylic acid from the aqueous phase with an extractant, wherein the aqueous phase after being extracted, comprising carboxylic acids, is passed wholly or partly, directly or indirectly, into reactor 1.

2. The process according to claim 1, wherein the organic base is a secondary amine, and the acid which is fed fresh to reactor 1 is sulphuric acid, formic acid, acetic acid, propionic acid and/or mixtures of these acids.

3. The process according to claim 1, wherein the carboxylic acids present in the aqueous phase passed from the second process step into reactor 1 are acetic acid, methacrylic acid, propionic acid, maleic acid, acrylic acid, terephthalic acid or mixtures comprising at least one of these acids.

4. The process according to claim 1, wherein at least 5% by weight, of the acid fed into reactor 1 as catalyst is the carboxylic acids from the second process step.

5. The process according to claim 1, wherein the aqueous phase from the second process step after being extracted and before being introduced into reactor 1 is concentrated by a distillation or a membrane separation stage.

6. The process according to claim 1, wherein the converted reaction solution from the first process step after being withdrawn at the reactor exit is distilled in a column and then the methacrolein is separated in a phase separation vessel from the aqueous phase that separates out, said aqueous phase being returned wholly or partly to the column.

7. The process according to claim 1, wherein the converted reaction solution from the first process step after being withdrawn at the reactor exit is distilled in a column, and the top stream from this distillation is subsequently passed directly into reactor 2.

8. The process according to claim 6, wherein at least 20% by weight of the aqueous phase is recycled from the column bottoms comprising acids and organic bases and the particular proportion of base in the salts with the acids and in the base-containing intermediates of the first process step is recycled into the inlet of reactor 1.

9. The process according to claim 1, wherein the aqueous phase from the second process step is passed directly into reactor 1.

10. The process according to claim 8, wherein the aqueous phase from the second process step is passed into the column and thence at least partly into reactor 1 together with the column bottoms.

11. The process according to claim 10, wherein the column bottoms before being introduced into reactor 1 are concentrated by distillation or a membrane separation stage.

12. The process according to claim 7, wherein the aqueous phase from the second process step, before being introduced into reactor 1, is mixed with at least a portion of the bottoms and the mixture is optionally concentrated by distillation or a membrane separation stage.

13. The process according to claim 1, wherein the feed into reactor 1 has a ratio of propionaldehyde to formaldehyde between 1:1.2 mol and 1:0.8 mol and, including the aqueous phase from the second process step and the optional recycling stream from the column, contains between 1 and 3 mol of acid based on one mole of organic base.

14. The process according to claim 1, wherein the water content in the overall feed to reactor 1 is greater than 50% by weight and not more than 85% by weight, the amount of organic base in the feed to the reactor, comprising the pure base and the particular proportion of base in the salts with the acids and in the base-containing intermediates of the first process step, based on propionaldehyde, is more than 2 mol % and the residence time of the reaction mixture in reactor 1 is between 1 and 30 s.

Description

EXAMPLES

[0048] A formalin solution having a formaldehyde content of 37% by weight and propionaldehyde were mixed (referred to below as aldehyde solution) and the mixture was subsequently heated to the desired temperature (see Tab. 1) in an oil-heated heat exchanger. A catalyst solution which contained acetic acid (or retentate or raffinate or mixtures) and dimethylamine (as a 40% by weight solution in water) was likewise preheated to the desired temperature of 160° C. The preheated aldehyde solution and the preheated catalyst solution were then mixed in a static mixer. This reactant mixture was then fed to an oil-heated tubular reactor (⅛″ coil, 6 m; ID 1.44, reactor volume 10.1 ml). The reaction was conducted at a pressure of 55 bar. The residence time in the reactor was between 9.4 and 9.7 s. The product mixture at the outflow of the tubular reactor was decompressed via a valve and passed into the product column for distillation. At the top of this column, after condensation and phase separation, a biphasic mixture of methacrolein and an aqueous phase was obtained. This mixture was analysed by means of GC analysis (Varian CP 3800, column: DB Wax, detectors: WLD and FID). Further analysis was effected by means of HPLC analysis; instrument: Agilent 1200, columns: Agilent SB-Aq, UV detector.

Example 1

[0049] For the catalyst solution, rather than acetic acid (comparative examples), the retentate from a two-stage membrane system was used. For this purpose, the quench liquid obtained after process step 2 was extracted and the wastewater thus obtained (composition determined by means of HPLC analysis: 2.7% by weight of acetic acid; 0.1% by weight of formic acid; 0.2% by weight of maleic acid; 0.007% by weight of acrylic acid; 0.6% by weight of methacrylic acid; 0.02% by weight of phthalic acid; 0.03% by weight of isophthalic acid; 0.004% by weight of methyl methacrylate; 0.02% by weight of benzoic acid; 0.005% by weight of DiMAL acid (oxidation product of dimeric methacrolein); 0.02% by weight of terephthalic acid) was concentrated in a two-stage membrane system (stage 1 membrane, Duplex NF, Filmtec SW30H, SuS251 L module; stage 2 membrane, PCS-NF, Filmtech SW 30, 240 cm.sup.2 flat membrane). As fresh feed, 0.8-0.9 kg/h of wastewater was metered in. In this case, a temperature of 30° C. was chosen and a pressure of 80 bar for the first stage was chosen. Stage 2 was likewise operated at 30° C., but at 50 bar. The retentate obtained by membrane had the following composition: 6.4% by weight of acetic acid; 0.5% by weight of formic acid; 0.6% by weight of maleic acid; 0.21% by weight of acrylic acid; 1.5% by weight of methacrylic acid; 0.05% by weight of phthalic acid; 0.07% by weight of isophthalic acid; 0.009% by weight of methyl methacrylate; 0.05% by weight of benzoic acid; 0.01% by weight of DiMAL acid (oxidation product of dimeric methacrolein); 0.05% by weight of terephthalic acid (determined by means of HPLC analysis). In this case, the different acids were regarded as acetic acid equivalents.

[0050] This retentate was mixed with DMA and used as catalyst solution according to the procedure described. The conditions and yields are listed in Table 1.

[0051] It was found that, surprisingly, the retentate is suitable as substitute for acetic acid. As a result, it was possible to achieve high yields.

Example 2

[0052] In a further experiment, a mixture of retentate (for composition see Example 1) and pure acetic acid was used. The starting weights were chosen such that the acetic acid was replaced by the retentate to an extent of 40 mol % acid equivalent. The experimental conditions were chosen as in the preceding example and the experiment was conducted analogously.

Example 3

[0053] Here, the raffinate from the extraction was used for the catalyst solution prepared. The wastewater obtained after the extraction was not treated and was used directly for this experiment. The experimental conditions were set analogously to the preceding examples and the experiment was conducted analogously.

Example 4

[0054] Finally, the quench liquid was used as catalyst substitute. This contains an even higher proportion of methacrylic acid. The exact composition of the quench liquid used was as follows:

[0055] 2.7% by weight of acetic acid; 0.15% by weight of formic acid; 0.26% by weight of maleic acid; 0.14% by weight of acrylic acid; 33.0% by weight of methacrylic acid; 0.02% by weight of phthalic acid; 0.03% by weight of isophthalic acid; 0.01% by weight of methyl methacrylate; 0.07% by weight of benzoic acid; 0.01% by weight of DiMAL acid; 0.05% by weight of terephthalic acid; 0.009% by weight of 4-methylbenzoic acid (determined by means of HPLC analysis).

[0056] The experimental conditions were chosen as in the preceding example and the experiment was conducted analogously.

Comparative Example 1

[0057] The catalyst solution was made up by means of pure acetic acid (glacial acetic acid). The experimental conditions were chosen as in Examples 1 to 4 and the experiment was conducted analogously.

Comparative Example 2

[0058] The catalyst solution was made up by means of pure acetic acid (glacial acetic acid). The experimental conditions were chosen as in Examples 1 to 4. Rather than a ⅛″ capillary, a plate heat exchanger from IMM having the same reactor volume (10.1 ml) was used.

[0059] A further increase in yield can be assumed, since methacrylic acid was detectable in methacrolein. The latter is fed to the oxidation and contributes to an increase in the overall yield therein.

TABLE-US-00001 TABLE 1 Conditions for the methacrolein synthesis; conversion and Dimethylamine/ Propionaldehyde/ Dimethylamine/ acetic acid Reaction Conversion Selectivity formaldehyde propionaldehyde (equivalents) temperature (propionaldehyde) (methacrolein) Yield selectivity [mol/mol] [mol/mol] [mol/mol] [° C.] [%] [%] [%] Example 1 1.00 0.054 0.91 160 98.9 98.5 97.4 Example 2 1.00 0.054 0.89 160 99.6 98.4 98.0 Example 3 1.00 0.053 0.91 160 99.0 98.5 97.5 Example 4 1.00 0.056 0.89 160 99.8 98.5 98.3 Comparative 1.00 0.052 0.90 160 99.7 98.2 98.0 Example 1 Comparative 1.00 0.042 0.91 160 99.5 97.4 96.9 Example 2