2. Cyanoethanol method: this method uses chloroethanol and sodium cyanide as raw materials to react to generate cyanoethanol, which is hydrolyzed at 175℃ in the presence of sulfuric acid to generate acrylic acid: if the hydrolysis reaction is carried out in methanol, methyl acrylate is generated;
3. β -propiolactone method: The raw material of this method is ketene, so it is also called ketene method. The reaction is as follows: firstly, acetic acid is cracked into ketene, and then it reacts with anhydrous formaldehyde to generate β -propiolactone; As a catalyst, propiolactone was contacted with hot 100% phosphoric acid at 140 ~ 180℃ and 2.5 ~ 25 MPa for isomerization to acrylic acid. The production of acrylic acid from β -propiolactone has high purity and high yield, and the by-products and unreacted materials can be recycled, which is suitable for continuous production, but acetic acid is needed as raw material, especially because propiolactone is considered as a carcinogen, so this method is not used in industry.
4. High pressure Raphael method: Acetylene dissolved in tetrahydrofuran reacts with carbon monoxide and water to prepare acrylic acid in the presence of catalysts consisting of nickel bromide and copper bromide. The characteristics of this method are: using tetrahydrofuran as solvent can reduce the danger of acetylene high pressure treatment; At the same time, the catalyst does not use nickel carbonyl used in the original Rapey method, but only uses nickel salt. Propylene, air and steam were mixed in a certain molar ratio, and in the presence of composite catalysts such as molybdenum and bismuth, the reaction temperature was 3 10-470℃, and acrolein was prepared at atmospheric pressure, with a yield of 90%. Then acrolein is mixed with air and water vapor according to a certain molar ratio, and in the presence of composite catalysts such as molybdenum and vanadium, the reaction temperature is 300-470℃, and the acrylic acid is prepared by atmospheric oxidation, and the yield can reach 98%. This method has two steps. One-step method is that propylene is oxidized in a reactor to produce acrylic acid; The two-step method is that propylene is oxidized in the first reactor to produce acrolein, and acrolein is oxidized in the second reactor to produce acrylic acid. According to the reactor structure, the two-step process can be divided into two types: fixed bed process and fluidized bed process. Among the industrial production methods of acrylic acid, cyanoethanol method and high pressure Raphael method have been basically eliminated. Previously, acetic acid was cracked into ketene, then reacted with anhydrous formaldehyde to produce propiolactone, and then contacted with hot phosphoric acid to form acrylic acid. The ketene process or β -propiolactone process is basically eliminated, and the acrylonitrile process is only adopted by a few old plants. The improved Raphael process and propylene oxidation process are mainly used in industry, the latter is more common and has the most development prospect. In the patent report, there is also a production method using propionic acid as raw material;
5. Propylene oxidation method: propylene is mixed with air and steam according to a certain molar ratio, oxidized in the presence of Mo-Bi composite catalyst to prepare acrolein, then acrolein is mixed with air and steam according to a certain molar ratio, and oxidized in the presence of Mo-V-W composite catalyst to prepare acrylic acid. According to the reactor structure, this method can be divided into fixed bed method and fluidized bed method. Except for the fluidized bed used in Soho process in the United States, all other processes use tubular fixed beds.
① Fixed bed method. The preparation method is as follows: the propylene content in the first reactor is 4% ~ 7%, steam is 20% ~ 50%, and the rest is air, with airspeed 1300 ~ 2600h- 1, reaction temperature of 320 ~ 340℃ and pressure of 0.1~ 0.3 MPa; The space velocity of the second reactor is 1800 ~ 3600h- 1, the reaction temperature is 280 ~ 300℃, and the pressure is 0. 1 ~ 0.2 MPa. The conversion rate of propylene and acrolein is above 95%, and the selectivity of acrylic acid is 85% ~ 90% based on propylene. The process is as follows: propylene and steam are mixed with preheated air and enter the first reactor. Propylene is oxidized to acrolein. And then enter a second reactor for reaction to obtain acrylic acid. The first and second reactors are tubular reactors, and molten salt is used as a heat carrier. The reaction gas from the second reactor enters the quench tower after exchanging heat with the raw air, and reversely contacts with the water added at the top of the tower to obtain an acrylic acid aqueous solution with a content of 20% ~ 30%. The aqueous solution enters an extraction tower, and butyl acetate or xylene is used as an extractant to separate water from acrylic acid. The extract rich in water comes out from the top of the extraction tower, enters the solvent recovery tower, evaporates the extractant from the top of the tower, and sends it back to the extraction tower for recycling. Waste water is discharged from the bottom of the tower. The raffinate in the extraction tower enters the solvent distillation tower. The solvent (extractant) is distilled from the top of the tower and sent back to the extraction tower for recycling; Crude acrylic acid was obtained at the bottom of the tower, and acrylic acid products were obtained after removing light components and heavy components. In the process of producing acrylic acid from propylene by gas-phase contact oxidation, there are traces of acrolein, acetic acid, valeric acid, formic acid and other aldehyde impurities besides acrylic acid. Aldehyde is the by-product of propylene oxidation or the oxidation of impurities contained in propylene raw materials, such as acetaldehyde, formaldehyde, benzaldehyde, furfural and acrolein. The reaction gas containing these by-products remains in the acrylic acid product after cooling and extractive distillation. Acrylic acid products refined by conventional methods still contain about (50 ~ 500) × 10-6 aldehydes. In order to meet the demand of high-purity acrylic acid, Beijing Oriental Chemical Plant developed a method to prepare high-purity acrylic acid from polymer-grade acrylic acid, which made the total aldehyde content less than 5× 10-6, which reached or exceeded the data requirements reported in foreign literature (less than 10× 10-6 abroad). The experimental method is as follows: a certain amount of polymer-grade acrylic acid is added to the flask, and reagent DL is added. After treatment in the range of atmospheric pressure 10 ~ 80℃, distillation is carried out in a packed tower, with the bottom temperature of 60 ~ 80℃, the top temperature of 50 ~ 70℃ and the vacuum degree of 93.33~99.99KPa, and polymerization inhibitor and gas-phase polymerization inhibitor are added. The distillate collected by this method is high purity acrylic acid;
② Fluidized bed method. The preparation method comprises the following steps: propylene, air and water pass through a first fluidized bed reactor to generate acrolein, then enter a second fluidized bed reactor to generate acrylic acid, then spray, cool, extract and distill, and then remove acetic acid in a vacuum tower to obtain acrylic acid. The ratio of oxidation mixture is propylene: air: water = 1: 12: 8 (molar ratio). The temperature of the first fluidized bed reactor is 370℃ and the contact time is 2s. The temperature of the second fluidized bed reactor was 260℃ and the contact time was 2.25 seconds. The conversion of propylene is 75% ~ 80%, the total yield is 40%, the acrylic acid content is 97%, and the average content is 93%. In China, propylene was oxidized to acrolein using a seven-component catalyst (Mo-V-P-Fe-Co-Ni-K) in the first reactor. In the second reactor, acrolein is oxidized to acrylic acid by using a three-way catalyst (molybdenum-vanadium-tungsten), when the ratio of propylene: air: water is 1: 10: 6, the contact time is 5.5s, the linear velocity is 0.6m /s, the reaction temperature in the first stage is 370-390℃, and the reaction temperature in the second stage. The yield of acrolein is 5 1.9% ~ 57.2%, the conversion of secondary propylene is 79.3% ~ 89.4%, the yield of acrylic acid is 48.3% ~ 49.8%, and the space-time yield of acrylic acid is 55 ~ 60 kg/(m3 catalyst h).