Process for preparation of methacrylic acid and methacrylic acid esters

Abstract

The invention relates to a process for preparation of at least one of methacrylic acid and a methacrylic acid ester, comprising the process stepsgas phase oxidation of at least one C.sub.4 compound, quenching of the reaction phase, separation and purification of the obtained methacrylic acid and optionally esterification, wherein the C.sub.4 compound is a methacrolein comprising mixture, originating from at least two different methacrolein sources, a first methacrolein source being a feed stream obtained by the heterogeneously catalyzed gas phase oxidation of isobutylene or tert-butyl alcohol or isobutylaldehyde or a mixture of two or more thereof, a second methacrolein source being a feed stream obtained by the reaction of propionaldehyde with a C.sub.1 extending agent, preferably formaldehyde, and where said methacrolein can be obtained either completely from the first methacrolein source, or completely from the second methacrolein source or from any mixture of both.

Claims

1. A process for preparing at least one of methacrylic acid and a methacrylic acid ester, the process comprising: a1) oxidizing a C.sub.4 compound in a gas phase to obtain a reaction phase comprising methacrylic acid; a2) quenching the reaction phase to obtain a crude aqueous phase comprising methacrylic acid; a3) separating at least a part of the methacrylic acid from the aqueous phase comprising methacrylic acid to obtain at least one crude methacrylic acid-comprising phase; a4) separating and optionally purifying at least a part of the methacrylic acid from the crude methacrylic acid-comprising phase obtained in a3) via a thermal separation process; a5) optionally esterifying at least a part of the methacrylic acid obtained in a4); wherein the C.sub.4 compound oxidised in a1) originates from a mixture of at least two different methacrolein comprising feed streams, and the mixture comprises 1 to 99 percent by weight of a first methacrolein comprising feed stream obtained by a heterogeneously catalysed gas phase oxidation of isobutylene or tert-butyl alcohol or isobutylaldehyde or a mixture of two or more thereof, and 99 to 1 percent by weight of a second methacrolein comprising feed stream obtained by reacting propionaldehyde with a C.sub.1 extending agent.

2. The process according to claim 1, wherein the mixture comprises 5 to 95 percent by weight of the first methacrolein and 95 to 5 percent by weight of the second methacrolein.

3. The process according to claim 1, wherein the mixture comprises 20 to 80 percent by weight of the first methacrolein and 80 to 20 percent by weight of the second methacrolein.

4. The process according to claim 1, wherein a3) comprises extracting at least a part of the methacrylic acid from the crude aqueous phase comprising methacrylic acid into an organic solvent to obtain a crude organic phase comprising methacrylic acid and a first aqueous phase, wherein the first aqueous phase comprises components (i) at least 65 wt. % of water, based on a total weight of the first aqueous phase, and (ii) not more than 35 wt. % of at least one organic compound, based on the total weight of the first aqueous phase, wherein a total weight of (i) and (ii) is 100 wt. %.

5. The process according to claim 1, wherein the C.sub.4 compound of the first methacrolein comprising feed stream is obtained by the heterogeneously catalysed gas phase oxidation of isobutylene, derived from splitting of methyl tert-butyl ether (MTBE) or ethyl tert-butyl ether (ETBE).

6. The process according to claim 1, wherein the methacrolein of the second methacrolein comprising feed stream is obtained by reacting propionaldehyde with formaldehyde.

7. The process according to claim 6, wherein the methacrolein of the second methacrolein comprising feed stream is obtained by reacting propionaldehyde with formaldehyde in the presence of a secondary amine and/or an acid.

8. The process according to claim 6, wherein the formaldehyde is obtained by oxidizing methanol in the presence of a molybdenum oxide or silver or silver oxide catalyst.

9. The process according to claim 1, wherein the propionaldehyde is obtained from ethylene and a synthesis gas in the presence of a rhodium and phosphorus comprising catalyst.

10. The process according to claim 1, wherein a quench liquid in a2) is water or at least a portion of a condensate formed in a2).

11. The process according to claim 1, wherein in a4) the methacrylic acid is purified via rectification to obtain a pure methacrylic acid, and the pure methacrylic acid is removed in a side outlet from a column used for the rectification.

12. The process according to claim 11, wherein the rectification in a4) is carried out at a bottom pressure in a range from 1 to 100 mbar.

13. The process according to claim 11, wherein the rectification in a4) is carried out at a bottom temperature in a range from 40 to 200° C.

14. The process according to claim 3, further comprising: b) separating at least a part of the water comprised in the first aqueous phase obtained in a3) from at least a part of at least one component (ii) to obtain a second aqueous phase and an organic phase, wherein the organic phase comprises the at least one component (ii), and wherein the second aqueous phase is depleted in the at least one component (ii) compared to the first aqueous phase; c) optionally separating at least a part of at least one organic compound from the second aqueous phase obtained in b) to obtain a third aqueous phase; and d) optionally separating at least a part of the at least one component (ii) from the organic phase obtained in b).

15. The process according to claim 1, comprising a5) esterifying at least a part of the methacrylic acid obtained in a4), to obtain a methacrylic acid ester.

16. The process according to claim 1, wherein the mixture comprises 22 to 79 percent by weight of the first methacrolein and 78 to 21 percent by weight of the second methacrolein.

17. The process according to claim 1, wherein the mixture comprises 50 to 79 percent by weight of the first methacrolein and 50 to 21 percent by weight of the second methacrolein.

Description

(1) The invention is more closely illustrated by the following figure and non-limiting examples.

(2) FIG. 1 shows schematically a preferred embodiment of the process according to the invention in the form of a flow diagram.

(3) FIG. 2 shows schematically an embodiment of the device according to the invention in which second separation unit B is an extraction unit.

(4) FIG. 3 shows schematically an embodiment of the device according to the invention in which second separation unit B is a crystallisation unit.

(5) FIG. 4 shows schematically the combination of the first aqueous phase 23 from unit A3 with the high boiler phase 24 from unit A4 in the combination unit R.

(6) FIG. 5 shows schematically the process according to the invention, having both a C.sub.4 and a C.sub.2 based methacrolein synthesis branch, in the form of a flow diagram.

(7) According to the embodiment of FIG. 2, a C.sub.4 compound is introduced into gas phase oxidation unit A1 where it is oxidised in a one- or two-stage catalytic gas phase oxidation to methacrylic acid. Inlets into gas phase oxidation unit A1 for C.sub.4 compound, oxygen, steam and inert diluent gas are not shown. The C.sub.4 compound can be provided from an MTBE splitting unit AA1 (not shown), via an isobutylene separating unit S1 (not shown). The gaseous methacrylic acid phase obtained in gas phase oxidation unit A1 is conducted via line 1 to quench unit A2, where it is cooled and absorbed into water or an aqueous phase to form an aqueous methacrylic acid-comprising phase. An inlet for the quench liquid into quench unit A2 is not shown. The aqueous methacrylic acid phase is conducted via line 2 to first extraction unit A3, where it is extracted with an organic solvent as extraction agent to form an organic phase and an aqueous phase (the first aqueous phase of the process according to the invention). These two phases are separated in first extraction unit A3.

(8) The organic phase from first extraction unit A3 is conducted via line 3 to first separation unit A4, where it is distilled to separate methacrylic acid and extraction agent, as well as a high boiler phase. The extraction agent can be recycled via line 6 to first extraction unit A3. The methacrylic acid can be collected via line 5 and optionally purified in downstream purification unit or units (not shown), or it can be conducted via line 4 to first esterification unit A5, optionally via a purification (not shown). In first esterification unit A5, the methacrylic acid can be esterified, for example with methanol, for example methanol separated from an MTBE splitting phase in separating unit S1 (not shown), to form methyl methacrylate. It is also possible to esterify methacrylic acid in first esterification unit A5 with other alcohols as mentioned above. The ester produced in first esterification unit A5 is collected via line 7 and can be optionally polymerised in polymerisation unit A6 (not shown), optionally with intermediate and/or downstream purification. The high boiler phase collected in first separation unit A4 is conducted to second separation unit B, optionally via combination unit R where it can be combined with the aqueous phase separated in first extraction unit A3 if so desired.

(9) The aqueous phase separated in first extraction unit A3 is conducted to second separation unit B (direct conduit not shown), optionally via line 24 and combination unit R, where it can be combined with the high boiler phase if so desired. Combination unit R may also be omitted, and the aqueous phase and the high boiler phase combined directly with each other in second separation unit B.

(10) The combined aqueous phase and high boiler phase is extracted in second separation unit B with an organic solvent as second extraction agent to form an aqueous phase (corresponding to the second aqueous phase of the inventive process) and an organic phase. The aqueous phase is conducted via line 9 to third separation unit C, where remaining extraction agent from the second extraction step can be at least partially separated and optionally recycled via line 25 to second separation unit B. The remaining aqueous phase, corresponding to the third aqueous phase of the inventive process, can be recycled, for example to gas phase oxidation unit A1 (conduit not shown), used as process water, conducted to a biological purification unit (not shown) or discharged, via line 20. The organic phase separated in second separation unit B can be conducted via line 10 to fourth separation unit D, where at least one component ii. can be separated. At least a part of the at least one component ii. separated in fourth separation unit D can be collected via line 11 and optionally purified (not shown). If a mixture of components ii. is separated in fourth separation unit D, this mixture can be conducted to a further separation unit for separation of components ii. from each other (not shown). If methacrylic acid or a methacrylic acid-comprising phase is separated in fourth separation unit D, this methacrylic acid or methacrylic acid-comprising phase can be conducted via line 15 to first extraction unit A3 or via line 16 to first separation unit A4. It is also possible that at least a part of the at least one component ii. separated in fourth separation unit D is conducted via line 14 to second esterification unit G. Either of the organic and aqueous phases separated in second separation unit B, or the aqueous phase separated in third separation unit C, may be conducted to second esterification unit G. In second esterification unit G at least one component ii. is esterified with an alcohol to form a corresponding ester. If the alcohol is methanol, this methanol can, for example, be introduced from MTBE splitter AA1 via separation unit S1, optionally with intermediate purification (not shown). If the ester phase obtained in second esterification unit G comprises more than one ester, at least one ester can be separated in ester separation unit H. At least one ester can be purified in downstream ester purification unit J (not shown). At least one ester obtained in one or more of the second esterification unit G, the ester separation unit H and the ester purification unit J can be conducted to second separation unit B for use as extraction agent.

(11) FIG. 3 shows another embodiment of the device according to the invention in which second separation unit B is a crystallisation unit. In this embodiment, the details concerning device components A1 to A6, R, G, H, J, AA1 and S1 are the same as in the embodiment of FIG. 2 and only the different aspects are described in the following. In the embodiment exemplified in FIG. 2, the combined aqueous phase and high boiler phase is generally cooled in the crystallisation unit B2a so that water at least partially crystallises out. If crystals form at least partially on cooled surfaces of the crystallisation unit B2a, these can be scraped off. The resulting slurry is then optionally conducted to a residence unit T1 (not shown), where the slurry is preferably stirred while more crystals grow and/or crystal size increases. From the crystallisation unit B2a and/or the residence unit T1 the slurry of crystals and mother liquor is then conducted via line 9 to the crystal separation unit B2b, where the solid crystals are at least partially separated from the mother liquor and optionally washed to at least partially remove impurities. A part of the crystals may be conducted back from crystal separation unit B2b to crystallisation unit B2a and/or to residence unit T1 to act as crystal seed (conduit not shown).

(12) At least a part of the optionally washed crystals can be melted and at least a part of the melted part can be recycled, for example to gas phase oxidation unit A1 (conduit not shown), used as process water, used as wash liquid for washing the crystals in the crystal separation unit B2a, conducted to a biological purification unit (not shown) or discharged, via line 20, The mother liquor separated in crystal separation unit B2b can be conducted via line 10 to fourth separation unit D, where at least one component ii. can be separated. Fourth separation unit D can comprise a dewatering unit D2a and/or a thermal separation unit D2b. If a mixture of components ii. is separated in fourth separation unit D, this mixture can be conducted to a further separation unit for separation of components ii. from each other (not shown). If methacrylic acid or a methacrylic acid-comprising phase is separated in fourth separation unit D, this methacrylic acid or methacrylic acid-comprising phase can be conducted via line 15 to first extraction unit A3 or via line 16 to first separation unit A4. At least a part of the at least one component ii. separated in fourth separation unit D can be collected via line 11 and optionally purified in a further purification unit (not shown). It is also possible that at least a part of the at least one component ii. separated in fourth separation unit D is conducted via line 14 to second esterification unit G. The mother liquor separated in crystal separation unit B2b may be conducted to second esterification unit G.

(13) FIG. 5 shows the process according to the invention, having both a C.sub.4 and a C.sub.2 based methacrolein synthesis branch. Crude C4 containing Isobutene is fed via Line 101 to MTBE synthesis (M1). Methanol (fresh) and recycled Methanol line 102 and 107 are also fed to M1. Treated C4 goes via line 103 to a Cracker or an olefin treating unit (not shown). MTBE as product goes via line 106 from M1 to the MTBE splitter M2. Highboilers (line 105) and methanol (line 107) are withdrawn from M2. Pure Isobutene is fed to Oxidation A1 via line 106.

(14) Methanol (via line 201) and air (via line 202) are fed to Formalin synthesis (F1). Tail gas is withdrawn from F1 via line 203 and has to be treated (not shown). Formalin is fed via line 204 to Methacrolein-synthesis (F3). Ethylene is fed via line 205 to Propionaldehyde-Synthesis (F2). Synthesis gas (a mixture from Hydrogen and Carbon monoxide) is fed via line 206 to Propionaldehyde synthesis. Tail gas is withdrawn via line 207 and has to be treated (not shown). High boilers are withdrawn from F2 via line 208. Propionaldehyde is fed to the Methacrolein synthesis (F3) via line 209. Carbonic acid (210) and secondary amine (e.g. Dimethyl amine) (211) are fed to F3. A waste water (212) is withdrawn from F3 and has to be treated. Methacrolein (213) is fed to Oxidation A1. M1: MTBE Synthesis M2: MTBE Splitter F1: Formalin Synthesis F2: Propionaldehyde Synthesis F3: Methacrolein Synthesis
Streams 101: Crude C4 with Isobutene 102: Methanol (Makeup) 103: Treated C4 104: MTBE 105: Highboilers 106: Isobuten 107: Methanol 201: Methanol to Formalin synthesis 202: Air to Formalin synthesis 203: Exhaust Air 204: Formalin 205: Ethylene 206: Carbon monoxide/Hydrogen 207: Exhaust-Air 208: Highboilers 209: Propionaldehyde 210: Carbonic Acid 211: Dimethyl amine 212: Waste water 213: Methacrolein to Oxidation
Test Methods
Measurement of Partition Coefficient (k Value)

(15) An aqueous phase comprising a pre-determined amount of acetic acid is combined with the same volume of an organic solvent (extraction agent). The two phases are shaken and/or stirred for 15-30 minutes at 50° C. to ensure that the equilibrium distribution of acetic acid over the aqueous and organic phases is achieved. The mixture is then allowed to separate back into organic and aqueous phases at 50° C. and these two phases are separated from each other. The amount of acetic acid present in the separated organic phase is measured by gas chromatography (GC) or high pressure liquid chromatography (HPLC).

(16) TABLE-US-00001 HPLC:    Agilent 1200 Pump:       Quaternary Pump    Eluent:       Acetonitrile KH.sub.2PO.sub.4 (0.02 mol/L) pH 2        Gradient    3 min     0% 100%    15 min 50%   50%    30 min 70%   30%    Flow:    1.0 ml/min    Stop-Time:    30 min    Post-Time:    5 min    Control pressure:    190 bar, max. 250 bar Autosampler:       Autosampler    Injection volume:    20 μL Column oven: including column switch control    Temperature: 30° C.    Columns:    Agilent SB-Aq        Maβe    Length 150 mm, d.sub.i 4.6 mm, 3.5 μm Material Detector    MWD or DAD    UV    210 nm, 241 nm, 254 nm, 265 nm (DAD preferred) GC:    Perkin Elmer Autosystem Autosampler: Perkin Elmer    Cleaning solvent    THF    Injection volume    1.0 μL Injektor:    Split    split ratio 100    Temperature program    200° C. Flow    constant Pressure 12.0 Column oven:    Column    J&W Scientific DB 225    Dimensions    Lenght 30 m, d.sub.i 0.25 mm, 0.25 μm Material    Temperature program    Rate  Temp. (° C.) Stop-Time (min)    Initial  40     5.0    15   180     .sup. 4.0    Running time: 18.3 min Detector    FID    Setpoint    260° C.

Example 1

(17) Example 1 describes part of the process as shown in FIG. 4. The high boiler phase as generated on first separation unit A 4 containing 82.1 wt % MAA, 14.3 wt % various and partly unknown high boilers (dimeric and oligomeric MAA, maleic acid, terephthalic acid, citraconic acid, polymers etc.), and 3.6 wt % inhibitors (mainly Hydrochinon) is combined with the first aqueous phase as isolated from first extraction unit A3 containing 0.6 wt % MAA and 5.0 wt % high boilers in a ratio of high boiler phase to first aqueous phase of 1:80 in combination unit R. The concentrations in the resulting combined phase are measured to 1.6 wt % MAA and 5.1 wt % high boilers. The combined phase is extracted with n-hexane in a second separating unit B. Yielding 4.9 wt % MAA in the organic phase vs. 3.9 wt % MAA in the comparative case when the first aqueous phase is not combined with the high boiler phase.

Example 2

(18) In a heatable two-step reactor (diameter: 16 mm) for the oxidation comprising an evaporator, a salt bath and a column of quenching following streams have been fed. A polyphosphoric molybdenum acid (composition: Mo(10)V(1)P(1)Cu(0.2)As(0.2)Ce(0.2)) was used as catalyst. The load of the catalyst in the second stage (oxidation to methacrylic acid) was 1580 h.sup.−1.

(19) Stream 1: Methacrolein (MAL) synthesized via an Aldol reaction with propionaldehyde and formaldehyde as educts, containing 0.7% by weight DIMAL (dimeric methacrolein), 1.5% by weight water and 0.1% propionaldehyde. This stream was evaporated and in a consecutive step to this stream oxygen, nitrogen and water in a ratio of 2.6 and 14 and 7 (referred to 1 part of MAL) were added.

(20) Stream 2: MAL synthesized via a gas phase oxidation of tert-butanol has been fed as gas together with oxygen, nitrogen and water to the reactor. The ratio of MAL to air to nitrogen to water was 1 and 2.6 and 14 and 7.

(21) For the examples corresponding to this invention the streams 1 and 2 have been brought together.

Comparative Example 2a) 100% Stream 1

(22) Content of DIMAL: about 7000 ppm,

(23) Temperature of the salt bath (for a conversion of 75%): 312.9° C.

(24) Selectivity to methacrylic acid: 82.0%

(25) Content of terephthalic acid (TPA) in the quenched liquid: 120 ppm

(26) Minor clogging in the column; no downtime for cleaning was necessary.

Comparative Example 2b) 100% Stream 2

(27) Content of DIMAL: about 110 ppm,

(28) Temperature of the salt bath (for a conversion of 75%): 308.8° C.

(29) Selectivity to methacrylic acid: 86.0%

(30) Content of TPA in the quenched liquid: 1000 ppm,

(31) Massive clogging (TPA) in the column; a downtime for cleaning was necessary after 10 days.

Example 2c) Mixture of Streams 1 and 2 in a Ratio (Referred to MAL) of 1 to 1

(32) Content of DIMAL: 3300 ppm

(33) Temperature of the salt bath (for a conversion of 75%): 311.5° C.

(34) Selectivity to methacrylic acid: 83.8%

(35) Content of TPA in the quenched liquid: about 400 ppm

(36) Minor clogging in the column; a downtime for cleaning was necessary after 25 days.

Example 2d) Mixture of Streams 1 and 2 in a Ratio (Referred to MAL) of 21 to 79

(37) Content of DIMAL: 300 ppm

(38) Temperature of the salt bath (for a conversion of 75%): 309.7° C.

(39) Selectivity to methacrylic acid: 85.5%

(40) Content of TPA in the quenched liquid: about 600 ppm

(41) Minor clogging in the column; a downtime for cleaning was necessary after 15 days.

Example 2e) Mixture of Streams 1 and 2 in a Ratio (Referred to MAL) of 78 to 22

(42) Content of DIMAL: 6000 ppm

(43) Temperature of the salt bath (for a conversion of 75%): 312.5° C.

(44) Selectivity to methacrylic acid: 82.7%

(45) Content of TPA in the quenched liquid: about 200 ppm

(46) Minor clogging in the column; a downtime for cleaning was necessary after 50 days.

(47) It is obvious for a person skilled in the art that a higher temperature of the salt-bath results in a shorter life-time of the catalyst. Therefore it was a surprising result of this invention that a combination of streams 1 and 2 affects a longer life-time of the catalyst in combination with less downtime.