Reference product: methacrylic acid [kg]
Location: RER - Europe
The process “methacrylic acid, at plant, RER” is modelled for the production of methacrylic acid from acetone in Europe. Raw materials are modelled with a stoechiometric calculation. Emissions are estimated. Energy consumptions, infrastructure and transports are calculated with standard values.
Methacrylic acid (CH2C(CH3)COOH, CAS 79-41-4, α-methylacrylic acid, 2-methylpropenoic acid) is a colourless, moderately volatile, corrosive liquid with a strongly acrid odour. Methacrylic Acid and Methyl Methacrylate from Acetone Cyanohydrin
The most common approach to methacrylic acid synthesis is the hydrolysis of methacrylamide sulfate, obtained from acetone cyanohydrin. Methyl methacrylate may be prepared directly in a similar way by adding methanol in the final reaction step.
Dry acetone and hydrogen cyanide react in the presence of a basic catalyst to give the cyanohydrin, which is then reacted with excess concentrated sulfuric acid (1.4 – 1.8 mol per mole of cyanohydrin) to form methacrylamide acid sulfate:
The sulfuric acid serves both as a specific reactant and as a solvent for the reaction, which appears to involve an α-sulfatoamide intermediate. If insufficient sulfuric acid is used, the reaction mass becomes a slurry or solid that is difficult or impossible to cool and pump. Both the sulfuric acid and the acetone cyanohydrin must be anhydrous in order to minimize hydrolysis of the sulfato derivative to α-hydroxyisobutyramide. The initial reaction is carried out continuously in a series of stirred tank reactors. Good heat transfer is required to assure removal of the heat of reaction. Thorough mixing is also necessary to avoid decomposition of the cyanohydrin to acetone and hydrogen cyanide, which can react to form byproduct acetone sulfonates and formamide sulfate. After initial reaction is completed at 80 –110 °C, the mixture is subjected to brief thermal cracking at ca. 125 – 160 °C to convert most of the α-hydroxyisobutyramide byproduct to methacrylamide sulfate, along with some acetone, carbon monoxide, and water. Total residence time at this stage of the process is about 1 h.
In a second stage, the methacrylamide sulfate stream is either hydrolyzed with excess water to give methacrylic acid and ammonium acid sulfate, or it is treated with aqueous methanol in a combined hydrolysis – esterification step to produce a mixture of methyl methacrylate and methacrylic acid.
Several modifications are possible in the hydrolysis – esterification step. For example, the methacrylamide sulfate, water, methanol, and recycle streams can be led through a series of continuous reactors at 80 – 110 °C with a 2 – 4 h residence time. The reactor effluent then passes to a stripping column where crude methyl methacrylate, water, and excess methanol are removed overhead. The waste acid ammonium sulfate residue can be either treated with ammonia for conversion to fertilizer or burned to regenerate sulfuric acid. Crude methyl methacrylate is extracted with water to recover methanol, which is concentrated and recycled to the esterification reactor. Washed ester is then purified by further distillation. The overall yield based on acetone cyanohydrin is in the range of 80 – 90 %.
In the manufacture of methacrylic acid, methacrylamide sulfate is reacted with water under conditions similar to those used for formation of the ester. The reactor effluent separates into two phases. The upper organic layer is distilled to provide pure methacrylic acid. The lower layer is steam stripped to recover dilute aqueous methacrylic acid, which is recycled to the hydrolysis reactor. The waste acid stream is treated as in the manufacture of the ester.
In an alternative process, the methanolysis reaction is carried out at a pressure of ca. 800 kPa in one or more reactors operated at 100 – 150 °C. The reactor effluent is separated into two layers while still under pressure. The lower layer is steam stripped to recover methacrylic acid for recycle to the esterification reactor, and acid waste is treated as described previously. The upper layer is passed to a distillation column where light ends (primarily dimethyl ether) are removed. The bottoms from this column are washed with aqueous ammonia to recover methanol and methacrylic acid for recycle to the esterification reactor. The crude, washed methyl methacrylate is then dehydrated in a downstream distillation column and distilled in a product column to provide pure methyl methacrylate.
A sulfuric acid regeneration plant is usually operated in conjunction with the methyl methacrylate plant, because approximately 1.6 kg of sulfuric acid is required to produce each kilogram of methyl methacrylate. The presence of a regeneration facility avoids the need to dispose of large quantities of ammonium sulfate contaminated with organic material.
One of the driving forces for development of the alternative routes discussed in Sections Methacrylic Acid from Isobutene, Methacrylic Acid from Isobutyric Acid, Methacrylic Acid From Ethylene has been the desire to eliminate the need for sulfuric acid regeneration. An additional concern is the hazard associated with transporting hydrogen cyanide, which is not always generated at the methacrylate plant site. On the other hand, both acetone (from phenol manufacture) and hydrogen cyanide (from acrylonitrile manufacture) have the economic advantage of being themselves industrial byproducts.
Asahi Chemical manufactures methyl methacrylate by the sulfuric acid hydrolysis of methacrylonitrile, using a plant originally designed for the acetone cyanohydrin process. The requisite methacrylonitrile is obtained by ammoxidation of isobutene, avoiding the need for hydrogen cyanide.
Methacrylic Acid from Isobutene
In recent years many companies have investigated the manufacture of methacrylic acid by two-stage catalytic oxidation of isobutene or tert-butanol. Nihon Methacryl Monomer (a joint venture of Sumitomo and Nippon Shokubai) and Mitsubishi Rayon have both constructed commercial plants using this technology.
In the first stage of the process, isobutene is oxidized to methacrolein ( Acrolein and Methacrolein – Methacrolein), and in a second stage the methacrolein is oxidized to methacrylic acid.
A published account of process and catalyst developments contrasts methacrylic acid production from isobutene with a similar process for preparing acrylic acid from propene. Selectivity of the second-stage catalysts is best at modest conversions (65 – 85 %). In the Sumitomo–Nippon Shokubai process, the first-stage reactor is operated at high conversion, and its effluent passes directly to the second oxidation reactor. Conversion in the second stage is kept low in order to ensure good catalyst selectivity and increase catalyst life. Unreacted methacrolein from the second-stage reactor effluent is separated and recycled. The overall yield of methacrylic acid from isobutene is about 65 – 70 %. Typical catalysts for the first stage are multicomponent metal oxides containing bismuth, molybdenum, and several other metals to promote activity and modify selectivity. Second-stage catalysts are based on phosphomolybdic acid, but they usually contain an alkali metal to control the acidity. Other elements such as copper and vanadium may also be present.
Reactor effluent from the second-stage oxidation passes to a quencher where crude aqueous methacrylic acid is obtained. The gaseous effluent from the quencher is passed to an absorber the unreacted methacrolein is absorbed, usually in aqueous carboxylic acid. Absorber off-gases are sent to a combustion unit before being discharged to the atmosphere. A portion of the incinerated gases may be recycled to the first-stage reactor to provide inert gas diluent for the feed. The methacrolein absorbate is transferred to a methacrolein recovery tower from which methacrolein is recycled to the second-stage oxidation reactor; recovered absorbent solution is returned to the absorber.
The crude, aqueous methacrylic acid is sent to a solvent extraction unit for methacrylic acid recovery. Next, a solvent recovery/dehydration tower affords crude methacrylic acid as a bottoms product. The overhead organic solvent layer is recycled to the extraction step, whereas the overhead aqueous layer is combined with extractor raffinate and sent to a solvent stripping tower before being subjected to wastewater treatment.
Dry, crude methacrylic acid from the extract stripper can be further purified if methacrylic acid is the desired end product, or it can be sent directly to an esterification reactor where catalyst and methanol are added for conversion to methyl methacrylate. Crude ester is extracted with water (j) to recover excess methanol, which is removed by distillation and recycled to the esterification reactor. The washed, crude ester is sent to a light ends stripper for removal of light byproducts (e.g., methyl acetate) and then distilled to provide pure methyl methacrylate. The bottoms from the final distillation column are recycled to the first extraction step with the exception of a small bleed for removal of inhibitor residues.
Plants based on the C-4 route were introduced in Japan by Nippon Shokubai in 1982 and by Mitsubishi Rayon in 1983. A joint venture plant operated by Sumitomo and Nippon Shokubai came on stream in 1984. Mitsui Toatsu and Kyowa Gas have also announced plans to construct a plant of this type. Several firms, including Nippon Kayaku, Mitsui Toatsu, Rohm and Haas, and Oxirane have carried out extensive research on the isobutene process. In 1987 ARCO acquired exclusive worldwide rights to Halcon SD technology using C4-feedstocks.
Halcon has proposed a variation of this process that commences with dehydrogenation of isobutane.
Methacrylic Acid from Isobutyric Acid
Acid-catalyzed carbonylation of propene to isobutyric acid, followed by oxidative dehydrogenation, presents still another route to methacrylic acid. In this case the starting material is propene itself rather than the oxydized derivative acetone, as in the acetone cyanohydrin route. Although the propene – isobutyric acid – methacrylic acid route is not currently in commercial use, several major methyl methacrylate manufacturers have research efforts aimed at its commercialization.
In the first stage of the process, propene, carbon monoxide, and water are reacted in the presence of a strong acid catalyst to produce isobutyric acid. Sulfuric acid, hydrogen fluoride, and boron fluoride have all been reported to be effective catalysts. Patents to Röhm and Ashland Oil describe variations that involve preliminary preparation of isobutyroyl fluoride, which is then hydrolyzed to isobutyric acid. Alternatively, isobutyric acid may be synthesized directly by including carefully controlled amounts of water in the carbonylation step.
Hydrogen fluoride (which acts as both solvent and catalyst), carbon monoxide, and propene (14 – 40 : 1.5 : 1) are reacted in the presence of a slight stoichiometric deficiency of water relative to propene to generate isobutyric acid. Reaction conditions are included in the patent literature; these range from about 30 °C at 20 MPa to 120 °C at 14 MPa. Depending upon the temperature and pressure, residence time during the carbonylation step varies from about 5 to 30 min. Reactor effluent is passed to staged flash tanks, in the first of which excess carbon monoxide can be recovered for recycle to the carbonylation reactor. The second tank permits removal of inert gases, which can be passed to the atmosphere after scrubbing with caustic solution to remove any hydrogen fluoride or isopropyl fluoride. The bulk of the hydrogen fluoride is separated overhead in a distillation tower for recycle to the carbonylation reactor. The bottoms from this tower pass to a hydrolysis stripper where any remaining fluorine-containing materials are reacted with water; hydrogen fluoride is stripped off for recycle. A final distillation step provides isobutyric acid overhead for feed to the second part of the process. Bottoms from this distillation contain small amounts of C7 and C10 carboxylic acids resulting from multiple condensations of propene prior to reaction with carbon monoxide. The overall selectivity with respect to propene is reported to be 95 – 97 %. The preceding steps must be carried out in a facility carefully designed to prevent fugitive fluoride emissions.
In the second stage of the process, isobutyric acid, steam, and air are passed over a fixed-bed catalyst in a multitubular reactor, causing oxidative dehydrogenation to methacrylic acid. The reactor effluent is sent to a quench tower from which an aqueous methacrylic acid stream is obtained. This part of the process is similar to the methacrolein oxidation step in the C-4 process described in Section Methacrylic Acid from Isobutene. In addition to carbon monoxide and carbon dioxide, which are incinerated with other noncondensable gases, the crude methacrylic acid stream contains acetone and acetic acid as byproducts. If desired, and with proper choice of quench tower conditions, the acetone can be directed to the incinerator along with the noncondensables.
If methacrylic acid is to be isolated, the crude product may be extracted into a solvent and dehydrated in a distillation tower. Isobutyric and acetic acids are then separated by distillation as light ends prior to final distillation of the methacrylic acid. The distillative separation of isobutyric acid from methacrylic acid is very difficult; normal boiling points of the two materials are 155 and 162 °C, respectively.
Catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid are of two general types. The first series, often referred to as Mo – P – V mixed oxide catalysts, were developed by Mitsubishi Chemical Industries, Röhm, and others, and they are similar in composition to catalysts used in the oxidation of methacrolein. Most are phosphomolybdic acid derivatives, usually with some replacement of molybdenum by vanadium or tungsten. The better catalysts frequently contain at least some copper, and they are partially neutralized by cesium, rubidium, or potassium. Some catalysts are reported to achieve conversions of 99.8 % with selectivities above 74 %.
A second type of catalyst has been intensively studied by Ashland Oil. These include iron phosphate materials and give selectivities of about 84 – 85 % at conversions in the 85 – 95 % range. Such catalysts are used at about 400 °C, in contrast to ca. 300 °C for the phosphomolybdate catalysts. Iron phosphate catalysts require high levels of steam in the reactor feed for optimum selectivity and life.
According to patents issued to Röhm, crude reactor product from the oxidative dehydrogenation step can be sent directly to the esterification reactor, which constitutes the starting point for the third section of the process. In this case, the product methyl methacrylate must be separated from methyl acetate and methyl isobutyrate. Separation of methyl isobutyrate from methyl methacrylate is difficult because its boiling point (92 °C) is close to that of methyl methacrylate (101 °C). The esterification step is similar to that described in Section Methacrylic Acid from Isobutene for the C-4 process.
Norsolor has an exclusive European license to the Ashland Oil technology, which Norsolor has further developed in a pilot plant at St. Avold (France). Röhm has developed its own process for this route in a pilot plant at Darmstadt.
An alternative process involves the hydroformylation of propene to give isobutyraldehyde, followed by oxidation of the aldehyde to isobutyric acid.
Methacrylic Acid From Ethylene
Other routes to methacrylic acid include the condensation of formaldehyde with propionic acid to generate methacrylic acid and the condensation of formaldehyde with propanal to give methacrolein.
BASF has developed such a process based on ethylene, synthesis gas, and formaldehyde. A plant with a production capacity of 40 000 t/a came on stream at Ludwigshafen in 1990.
Ethylene is first hydroformylated to give propanal, which is then condensed with formaldehyde to produce methacrolein. Catalytic air oxidation of methacrolein to methacrylic acid completes the synthesis, a step that is common to the C-2 and C-4 routes:
An alternative process would entail oxidation of propanal to propionic acid, condensation of which with formaldehyde would give methacrylic acid directly.The fracture toughness of boron carbide can be improved by addition of yttria or of yttria in combination with other oxides. The fracture toughness of B4C can be increased to 3.9 MPa•m1/2 by vacuum sintering at 1900 – 1975°C of powder compacts of composition 97.5 wt % B4C and 2.5 wt % carbon packed in a yttria grit of 0.15 – 1.4 mm grain size. The vacuum allows yttria vapor to penetrate the powder compact and promote reaction sintering of carbon-doped B4C to full density (2.62 g/cm3). X-ray diffraction showed that yttrium boride and yttrium borocarbide were coexistent with B4C. The method disclosed in uses liquid-phase sintering under vacuum or streaming argon of powder compacts comprising B4C powder (average particle size 0.6 – 3.5 μm) and 1 – 28 wt % additions of Y2O3 in combination with Al2O3 or aluminum nitride (AlN) and La2O3 or CeO2.
Methacrylic acid and methacrylate esters are used to prepare a wide range of polymers. Poly(methyl methacrylate) is the primary polymer in this category, and it provides water-clear, tough plastics that are used in sheet form in glazing, signs, displays, and lighting panels. Automotive lighting lenses and similar products can be prepared from molding pellets. Methyl methacrylate incorporated into copolymers forms the basis for durable coatings and inks. Higher methacrylate polymers are useful in the manufacture of oil additives, solventless inks and coatings, and binders for xerography. Salts of poly(methacrylic acid) can serve as the basis for water-soluble thickeners and detergent additives.
Frischknecht R., Jungbluth N., Althaus H.-J., Doka G., Dones R., Heck T., Hellweg S., Hischier R., Nemecek T., Rebitzer G. and Spielmann M. (2007) Overview and Methodology. Final report ecoinvent v2.0 No. 1. Swiss Centre for Life Cycle Inventories, Dübendorf, CH, retrieved from: www.ecoinvent.org.
Gendorf (2000) Umwelterklärung 2000, Werk Gendorf. Werk Gendorf, Burgkirchen as pdf-File under: http://www.gendorf.de/pdf/umwelterklaerung2000.pdf
William Bauer Jr.: Methacrylic Acid and Derivatives. Published online: 2000. In: Ullmann's Encyclopedia of Industrial Chemistry, Seventh Edition, 2004 Electronic Release (ed. Fiedler E., Grossmann G., Kersebohm D., Weiss G. and Witte C.). 7 th Electronic Release Edition. Wiley InterScience, New York, Online-Version under: DOI: 10.1002/14356007.a16_441
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hydrolysis of methacrylamide sulfate