PROCESS FOR MINIMISING THE LOSS OF ACTIVITY IN REACTION STEPS CARRIED OUT IN CIRCULATION

Abstract

A novel process can be used for preparing methacrylates, such as methacrylic acid and/or alkyl methacrylates, especially MMA. The process allows for prolonging of the catalyst service life and an increase in efficiency of the methacrylate preparation based on C2 or C4 raw materials, especially when proceeding from isobutylene or tort-butanol or ethylene as raw material. The process allows for performance for longer periods without disruption, with constant or even increased activities and selectivities. This gives rise to the possibility of performing such processes in a very simple, economically viable, and environmentally benign manner. In addition, it has been possible to minimize known safety risks that emanate from the methacrolein intermediate.

Claims

1: A process for preparing methacrylic acid and/or alkyl methacrylate, the process comprising: forming a first methacrolein-containing fraction from a C2 source or a C4 source in a first reactor, and reacting a second methacrolein-containing fraction, based on the first methacrolein-containing fraction, in at least one second reactor, wherein the second methacrolein-containing fraction is in liquid form and is kept in cooled storage in an intermediate vessel to a storage temperature between −30° C. and 50° C. with a dwell time of less than 48 h, and thence is guided into an evaporator or in liquid form is guided into the at least one second reactor for an oxidative esterification.

2: The process according to claim 1, wherein, proceeding from the C4 source in the first reactor, a methacrolein-containing process gas is formed as the first methacrolein-containing fraction.

3: The process according to claim 1, wherein, proceeding from the C2 source in the first reactor, a first propionaldehyde-containing stream is formed which is worked up by a purification comprising at least one distillation, to form a second propionaldehyde-containing stream, which is in turn converted in a downstream reactor and a further downstream purification to a third methacrolein-containing stream, and this third methacrolein-containing stream is guided into the intermediate vessel.

4: The process according to claim 3, wherein a liquid methacrolein-containing stream is guided from the intermediate vessel into the evaporator, where the liquid methacrolein-containing stream is evaporated to form a methacrolein-containing process gas.

5: The process according to claim 2, wherein the methacrolein-containing process gas is guided together with an oxygen- and water vapour-containing gas mixture, optionally premixed as a gas stream, into the at least one second reactor, which is an oxidation reactor, forming a methacrylic acid-containing process gas in the at least one second reactor, wherein a. the methacrylic acid-containing process gas is separated in a separation apparatus into a predominantly methacrylic acid-containing stream and a predominantly methacrolein-containing stream, b. the separation apparatus includes at least one quenching, one crystallization, and one distillation, c. the predominantly methacrolein-containing stream is condensed at the top of a fractional distillation column and a condensed methacrolein-containing stream is then guided into the intermediate vessel, and d. a thermostatted methacrolein-containing stream is then guided out of the intermediate vessel into the evaporator in order to convert the thermostatted methacrolein-containing stream to the methacrolein-containing gas stream therein, which is guided into the at least one second reactor together with the oxygen- and water vapour-containing gas mixture, optionally mixed to give the gas stream.

6: The process according to claim 5, wherein the predominantly methacrylic acid-containing stream is purified by distillation and/or extraction, and is optionally further reacted with an alcohol in a further reactor under acidic catalysis, to give the alkyl methacrylate.

7: The process according to claim 1, wherein a methacrolein-containing stream is taken in liquid form from the intermediate vessel and reacted in the at least one second reactor in the presence of an oxygen-containing gas, a catalyst, and an alkyl alcohol in the liquid phase in a direct oxidative esterification, to obtain an alkyl methacrylate-containing substance mixture.

8: The process according to claim 5, wherein, in the methacrolein-containing process gas, an isobutene content does not exceed a value of 2000 ppm by volume, and wherein workup, thermostatting and re-evaporation of the thermostatted methacrolein-containing stream arising from the methacrolein-containing process gas give rise to a methacrolein-containing stream which, based on methacrolein, contains 0.2% to 25% by weight of further carbonylic C.sub.1-C.sub.4 hydrocarbons and has a content of methacrolein dimers (di-MAL) of less than 1% by weight.

9: The process according to claim 5, wherein the predominantly methacrolein-containing stream, which is obtained after workup and removal of the predominantly methacrylic acid-containing stream, is condensed and stored in such a way that a concentration of methacrolein dimers in the thermostatted methacrolein-containing stream to the evaporator or to the at least one second reactor does not exceed a content of 1% by weight.

10: The process according to claim 5, wherein a condensation temperature of the methacrylic acid-containing process gas is higher than a storage temperature of the condensed methacrolein-containing stream in the intermediate vessel.

11: The process according to claim 1, wherein the dwell time of the second methacrolein-containing stream in the intermediate vessel is less than 12 h and the storage temperature is between −20° C. and 30° C.

12: The process according to claim 11, wherein the dwell time in the intermediate vessel is less than 6 h and the storage temperature is between −10° C. and 20° C.

Description

LIST OF REFERENCE SYMBOLS

Description of the Figures

[0087] FIG. 1 shows a process scheme of preferred embodiments of the process according to the invention for conversion of C2 or C4 sources to methacrylic acid or alkyl methacrylates.

[0088] FIG. 2 shows the progression of the di-MAL concentrations in MAL over time as a function of storage time and temperature.

[0089] FIG. 3 shows an enlarged section from FIG. 2.

[0090] FIG. 4 shows the summary of the oil bath temperatures observed in Examples 2 to 4 in oxidation of methacrolein (c[di-MAL.sub.fresh]=35 ppm) that was stored before the start of the experiment at temperature T.sub.L for duration of t.sub.L.

APPARATUSES

[0091] A First oxidation reactor [0092] B Second oxidation reactor [0093] C Separation apparatus [0094] D Intermediate vessel [0095] E Evaporator [0096] F Esterification reactor [0097] G Direct oxidative esterification [0098] H Propionaldehyde synthesis reactor [0099] I Propionaldehyde catalyst removal [0100] J Methacrolein synthesis reactor [0101] K Methacrolein catalyst removal [0102] L Mixing point 1 [0103] M Mixing point 2

STREAMS

[0104] 1 Methacrolein-containing process gas from the first oxidation reactor [0105] 2 Methacrylic acid-containing process gas from the second oxidation reactor [0106] 3a Predominantly methacrylic acid-containing stream from separation apparatus C [0107] 3b Predominantly methacrolein-containing stream from separation apparatus C [0108] 4 Cooled, predominantly methacrolein-containing stream in the feed stream to evaporator E [0109] 5 Methacrolein-containing gas stream [0110] 6 Oxygen- and water vapour-containing gas mixture [0111] 7 Methacrolein-containing process gas feed to the second oxidation reactor [0112] 8 Liquid, predominantly propionaldehyde-containing process stream [0113] 9 Liquid, predominantly hydroformylation catalyst-containing process stream [0114] 10 Purified liquid propionaldehyde-containing stream [0115] 11 Liquid, predominantly methacrolein-containing process stream [0116] 12 Liquid process stream containing predominantly the Mannich catalyst system [0117] 13 Purified liquid methacrolein-containing stream [0118] 14 Cooled, predominantly methacrolein-containing stream in the feed stream to reactor G [0119] f1 O.sub.2-containing gas in the feed stream to reactor A [0120] f2 C4 source in the feed stream to reactor A [0121] f3 O.sub.2-containing gas for mixing of the oxygen- and water vapour-containing gas mixture 6 [0122] f4 Water vapour for mixing of the oxygen- and water vapour-containing gas mixture 6 [0123] f5 Synthesis gas stream in the feed stream to reactor H [0124] f6 C2 source in the feed stream to reactor H [0125] f7 Formalin-containing stream in the feed stream to reactor J [0126] f8 Alcohol-containing stream in the feed stream to reactor F [0127] f9 Alcohol-containing stream in the feed stream to reactor G [0128] f10 O.sub.2-containing gas in the feed stream to reactor G [0129] p1 Optionally purified methacrylic acid product stream from reactor C [0130] p2 Optionally purified alkyl methacrylate product stream from reactor F [0131] p3 Optionally purified alkyl methacrylate product stream from reactor G

OTHER REFERENCE SYMBOLS

[0132] t(L) Storage time in intermediate vessel D [0133] T(L) Storage temperature in intermediate vessel D [0134] GC Gas chromatography [0135] MAL Methacrolein [0136] di-MAL Cyclic methacrolein dimer [0137] ppm Parts per million [0138] E.sub.A Activation energy [kJ/mol] [0139] A Impact factor [ppm/d] [0140] c(i) Concentration of component i [0141] GHSV Gas hourly space velocity [0142] X(i) Conversion of component i [%] [0143] S(i) Selectivity of component i [%] [0144] Y(i) Yield of component i [%] [0145] MAA Methacrylic acid [0146] MMA Methyl methacrylate [0147] T(oil) Oil bath temperature [0148] T(35 cm) Reactor temperature within the catalyst bed at a distance of 35 cm from the reactor inlet [0149] T(45 cm) Reactor temperature within the catalyst bed at a distance of 45 cm from the reactor inlet [0150] SAPT Self accelerating polymerization temperature [° C.] [0151] B.p. Boiling point [° C.] [0152] DOE Direct oxidative esterification [0153] PMMA Poly(methylmethacrylate) [0154] IBEN Isobutene [0155] TBA tert-Butyl alcohol [0156] MTBE Methyl tert-butyl ether

DESCRIPTION OF THE FIGURES

[0157] With regard to the figures, it should be noted that further components known to the person skilled in the art may additionally be present for performance of the process according to the invention. For example, in general, each of the columns listed will have a condenser. It should also be noted that not every preferred embodiment is included in the drawings. The position of the feed lines additionally does not indicate their real position, but merely illustrates the topological arrangement of the corresponding process steps.

[0158] FIG. 1 shows the general process scheme of the reaction of C2 or C4 sources with an oxygen- and water vapour-containing gas to give methacrylic acid or alkyl methacrylates. In one variant, a C4 source f2 is converted together with an oxygen-containing gas f1 in a reactor A to a methacrolein-containing process gas 1 which, after a further methacrolein-containing process gas 5 and optionally a gas stream 6 arising from an oxygen-containing stream f3 and a water vapour-containing stream f4 has been mixed in, gives rise to a methacrolein-containing process stream 7 which is fed to a reactor B. In reactor B, the methacrolein-containing process stream 7 is converted to a methacrylic acid-containing process stream 2 which, in the subsequent separation apparatus C, is separated into a preferably methacrylic acid-containing stream 3a and a preferably methacrolein-containing process stream 3b. After optional purification steps, a methacrylic acid-containing product stream p1 can be obtained directly from the preferably methacrylic acid-containing stream 3a. Alternatively, the preferably methacrylic acid-containing process stream 3a can be converted together with an alcohol-containing stream f8 in a reactor F to a product stream p2 containing alkyl methacrylates. The preferably methacrolein-containing process stream 3b is stored in liquid form in a cooled intermediate vessel D and fed as cooled process stream 4 to an evaporator E, where it is converted to the above-described methacrolein-containing process gas 5 which, as set out above, is fed back upstream of the inlet of reactor B.

[0159] Optionally, the liquid, preferably methacrolein-containing process stream 14 can be converted together with an oxygen-containing gas mixture f10 and an alcohol-containing stream f9 in a reactor G in a direct oxidative esterification reaction directly to a product stream p3 containing alkyl methacrylates.

[0160] In a further, alternative variant, a C2 source f6 is converted together with synthesis gas f5 and a catalyst in a hydroformylation reactor H to a propionaldehyde-containing process stream 8 which, in a separation apparatus I, is separated into a preferably catalyst-containing process stream 9 and a low-catalyst, propionaldehyde-containing stream 10. While the preferably catalyst-containing process stream 9 is fed back to the hydroformylation reactor H, the low-catalyst, propionaldehyde-containing stream 10 is converted together with a formalin-containing stream f7 and a further catalyst in reactor J to a methacrolein-containing stream 11 which, in the downstream separation apparatus K, is separated into a preferably catalyst-containing process stream 12 and a preferably methacrolein-containing liquid process stream 13. The preferably methacrolein-containing liquid process stream 13 may, in this process variant, be fed directly to the evaporator E already described, where it is converted together with the cooled, preferably methacrolein-containing process stream 4 to the methacrolein-containing process gas 5. Alternatively, the preferably methacrolein-containing liquid process stream 13 can first be stored together with the preferably methacrolein-containing process stream 3b already described in the cooled intermediate vessel D likewise already described to form the cooled process stream 4. The methacrolein-containing process gas 5 obtainable in both alternatives can be converted as described above to methacrylic acid or alkyl methacrylates.

EXAMPLES

Example 1 (Kinetics of Formation of di-MAL from MAL)

[0161] For the study of the formation kinetics of di-MAL from MAL, MAL was stored at different temperatures for 46 days, and the di-MAL concentration was monitored continuously by means of GC. The MAL used for the experiments was distilled beforehand in order to obtain a minimum di-MAL starting concentration (35 ppm here). The di-MAL concentrations ascertained, which are summarized in Table 2 and in FIG. 2 and FIG. 3, can be used to derive pseudo-zeroth-order kinetics of di-MAL formation with the following formation law (E.sub.A=90.25 kJ/mol, A=4.57*10.sup.18 ppm/d):

[00001]$c\left(t\right)=c_0+\left(A*e^{\frac{-E_A}{R*T}}\right)*t$

[0162] At a storage temperature of −5° C., formation of 12 ppm of MAL per day can be expected. At 25° C., by contrast, nearly 700 ppm of MAL per day is formed, and at 50° C. 1.2%/d.

TABLE-US-00003 TABLE 2 Concentration of di-MAL in MAL as a function of storage temperature T(L) and time t(L) c(di-Mal) (ppm) Sample T(L) [d] T(L) = −19.1° C. T(L) = 4.8° C. T(L) = 23.5° C. 0 0 35.35 35.35 40 1 1 38.47 56.79 900 2 4 41.64 138.06 3400 3 5 53.95 183.27 4500 4 7 64.81 — 6400 5 11 58.2 300.14 11400 6 13 58.05 370.8 11900 7 15 60.69 438.89 12900 8 18 64.8 484.05 14400 9 20 70.25 554.75 17600 10 26 104.33 749.8 21600 11 32 96.35 865.74 27400 12 46 106.27 1239.89 40500

Comparative Example 1 (Gas Phase Oxidation of MAL; c(Di-MAL)=10 000 ppm)

[0163] The gas phase oxidation of MAL to MAA was studied in a continuously operated tubular reactor. For monitoring of the reaction temperature, the reactor is equipped with 2 thermocouples at a distance of 35 cm and 45 cm from the reactor inlet. For the experiment, methacrolein having a di-MAL content of 35 ppm is first stored at 25° C. for 10 days, after which it has a di-MAL content of 10 000 ppm. The MAL is converted to the gas phase in an evaporator at 160° C. with the aid of a gas stream composed of air and nitrogen. The gas mixture that results after further addition of air and nitrogen (MAL/O.sub.2/H.sub.2O/N.sub.2=1:2.5:4.5:22.5) is passed over a molybdenum-bismuth mixed oxide catalyst at a GHSV of 1070 h.sup.−1, in the course of which the temperature of the oil bath is adjusted so as to result in an MAL conversion X(MAL) of 65.8%. The MAL conversion is determined by comparing the gas compositions ascertained by means of GC on entry to and exit from the reactor. Once the conversion has stabilized after an operating time of 12 h, the oil bath temperature is raised to such an extent that an MAL conversion X(MAL) of 68.8% is attained. In the same way, conversions of 74.9% and 75.3% are established by further increasing the oil bath temperature. The oil bath temperatures (T[oil]) required and the methacrylic acid selectivities (S[MAA]) and temperatures observed within the catalyst bed at a distance of 35 cm and 45 cm from the reactor inlet are compiled in Table 3.

TABLE-US-00004 TABLE 3 Temperatures T, conversions X(MAL) and methacrylic acid selectivities S(MAA) in the gas phase oxidation of methacrolein (di-MAL content 10 000 ppm) T(oil)/ T (35 cm)/ T (45 cm)/ Example X(MAL)/% S(MAA)/% ° C. ° C. ° C. 2-1 65.8 89.7 288.5 307.1 308 2-2 68.8 89.3 289.8 312 311.2 2-3 74.9 88.6 293.7 323.2 319.7 2-4 75.3 89 293.7 322 317.7

Example 2 (Gas Phase Oxidation of MAL; c(Di-MAL)=5000 ppm; Effect of Shorter Storage Time)

[0164] The gas phase oxidation of MAL to MAA was studied analogously to Comparative Example 1. For the experiment, methacrolein having a di-MAL content of 35 ppm is first stored at 25° C. for 5 days, after which it has a di-MAL content of 5000 ppm. The gas mixture that results after evaporation of MAL and mixing-in of nitrogen and air, as described in Comparative Example 1, is passed over the catalyst, in the course of which the temperature of the oil bath is adjusted so as to result in an MAL conversion of 66.3%. Once the conversion has stabilized after an operating time of 12 h, the oil bath temperature is raised to such an extent that an MAL conversion X(MAL) of 67.8% is attained. In the same way, conversions of 70.1% and 75.4% are established by further increasing the oil bath temperature. The oil bath temperatures (T[oil]) required and the methacrylic acid selectivities (S[MAA]) and temperatures observed within the catalyst bed at a distance of 35 cm and 45 cm from the reactor inlet are compiled in Table 4.

TABLE-US-00005 TABLE 4 Temperatures, conversions and methacrylic acid selectivities in the gas phase oxidation of methacrolein (di-MAL content 5000 ppm) T(oil)/ T (35 cm)/ T (45 cm)/ Example X(MAL)/% S(MAA)/% ° C. ° C. ° C. 3-1 66.3 88.5 287.4 311.4 317.1 3-2 67.8 87.8 287.4 311.4 317.1 3-3 70.1 87.7 289 315.6 320.5 3-4 75.4 87.6 291.7 321.6 319

Example 3 (Gas Phase Oxidation of MAL; Di-MAL Content 300 ppm; Effect of Lower Storage Temperature)

[0165] The gas phase oxidation of MAL to MAA was studied analogously to Comparative Example 1. For the experiment, methacrolein having a di-MAL content of 35 ppm is first stored at 5° C. for 10 days, after which it has a di-MAL content of 300 ppm. The gas mixture that results after evaporation of MAL and mixing-in of nitrogen and air, as described in Comparative Example 1, is passed over the catalyst, in the course of which the temperature of the oil bath is adjusted so as to result in an MAL conversion of 64.0%. Once the conversion has stabilized after an operating time of 12 h, the oil bath temperature is raised to such an extent that an MAL conversion of 65.5% is attained. In the same way, conversions of 69.0%, 71.5%, 74.3%, 74.9% and 76.4% are established by further increasing the oil bath temperature. The oil bath temperatures (T[oil]) required and the methacrylic acid selectivities (S[MAA]) and temperatures observed within the catalyst bed at a distance of 35 cm and 45 cm from the reactor inlet are compiled in Table 5.

TABLE-US-00006 TABLE 5 Temperatures, conversions and methacrylic acid selectivities in the gas phase oxidation of methacrolein (di-MAL content 300 ppm) T(oil)/ T (35 cm)/ T (45 cm)/ Example X(MAL)/% S(MAA)/% ° C. ° C. ° C. 4-1 64 89.6 285.1 303.4 318.4 4-2 65.5 88.5 285.2 308.6 312.7 4-3 69 86.8 287.4 308.4 323.3 4-4 71.5 87.3 287.4 316.3 318.5 4-5 74.3 84.2 290.4 321.4 324.4 4-6 74.9 83 290.3 314.3 336.6 4-7 76.4 88.6 290.9 320.7 313.4

[0166] The oil bath temperatures observed in Comparative Example 1 and in Examples 2 and 3 are summarized in FIG. 4. It becomes clear that, for the entire conversion range examined, that MAL which requires the highest oil bath temperature was stored at the highest temperature (25° C.) for the longest time (10 d). By halving the storage time, the oil bath temperature can be lowered by an average of 2 K. By reducing the storage temperature from 25° C. to 5° C. with the same storage time, the oil bath temperature can even be lowered by an average of 3 K. Bearing in mind the adverse effect of the bath temperature on the lifetime of a methacrolein oxidation catalyst, it is thus possible to achieve an extension in catalyst lifetime by storing intermediately stored MAL at minimum temperatures for minimum periods of time.

Comparative Example 2 (Direct Oxidative Esterification of MAL in the Liquid Phase in a Batch Test; c(Di-MAL)=10 000 ppm)

[0167] For the experiment, methacrolein having a di-MAL content of 35 ppm is first stored at 25° C. for 10 days, after which it has a di-MAL content of 10 000 ppm. The MAL pretreated in this way (1.2 g) is suspended together with an AuCoO/SiO.sub.2—Al.sub.2O.sub.3—MgO catalyst (384 mg) and methanol (9.48 g) in a 140 ml steel autoclave with a magnetic stirrer. The pH of the MAL was first adjusted to 7.0 with 1% NaOH in MeOH, and the MAL was stabilized with 100 ppm of Tempol. The autoclave was pressurized to 30 bar gauge with a gas mixture of 7% O.sub.2 in N.sub.2. The explosion limit of the mixture is 8% by volume of oxygen. The autoclave was heated to 80° C. for 2 hours, cooled down and degassed, and the suspension was filtered. The filtrate was analysed by means of GC. The conversion of MAL was 64.0%, the selectivity for MMA was 93.7%, and the space-time yield was 10.6 mol of MMA/kg of catalyst per hour. The filtrate also contained 500 ppm of di-MAL and 10 450 ppm of di-MAL ester. It is found that 95% of the di-MAL present in the feed is oxidatively esterified to di-MAL methyl ester, and about 5% is unchanged in the reaction.

Example 4 (Direct Oxidative Esterification of MAL in the Liquid Phase in a Batch Test; c(Di-MAL)=300 ppm; Effect of Low Storage Temperature)

[0168] The direct oxidative esterification of MAL to MMA was studied analogously to Comparative Example 2. For the experiment, methacrolein having a di-MAL content of 35 ppm is first stored at 5° C. for 10 days, after which it has a di-MAL content of 300 ppm. The MAL pretreated in this way (1.2 g) is suspended together with an AuCoO/SiO.sub.2—Al.sub.2O.sub.3—MgO catalyst (384 mg) and methanol (9.48 g) in a 140 ml steel autoclave with a magnetic stirrer. The pH of the MAL was first adjusted to 7.0 with 1% NaOH in MeOH, and the MAL was stabilized with 100 ppm of Tempol. The autoclave was pressurized to 30 bar gauge with a gas mixture of 7% O.sub.2 in N.sub.2. The explosion limit of the mixture is 8% by volume of oxygen. The autoclave was heated to 80° C. for 2 hours, cooled down and degassed, and the suspension was filtered. The filtrate was analysed by means of GC. The conversion of MAL was 67.0%, the selectivity for MMA was 93.7%, and the space-time yield was 11.1 mol of MMA/kg of catalyst per hour. The filtrate also contained 15 ppm of di-MAL and 315 ppm of di-MAL ester. It is found that 95% of the di-MAL present in the feed is oxidatively esterified to di-MAL methyl ester, and about 5% is unchanged in the reaction. Compared to Example 5, it is additionally found that the reduced di-MAL content in the feed as a result of the low storage temperature was able to increase the space-time yield of the target reaction.