Chemical production technology of acrylamide: Acrylamide is a monomer for the synthesis of polyacrylamide, which is produced by hydration of acrylonitrile. According to different catalysts, it is roughly divided into the following different methods. Polyacrylamide flocculant, Catalyst for hydration reaction: the sulfuric acid hydration method was adopted in the 1960s; the skeleton copper catalyst was used in the 1970s to reduce environmental pollution, which is called the catalytic hydration method.
1. Sulfate hydration
First use equal molar ratio of acrylonitrile and water, in H, SO. In the presence, it is hydrated at 80~100℃ to generate acrylamide sulfate first, and then neutralize with ammonia (or caustic soda, quicklime), and crystallize and separate the acrylamide product and by-product ammonium sulfate. The response is as follows:
CH,= CHCN+H,o>CH,= CHCONH,·H,SO.
CH,= CHCHNH,·H,SO+2NH,-——→CH,CHCONH,+ (NH),SO,
The advantage of this method is that it is easy to prepare crystalline monomers. The main disadvantage is that the raw material acrylonitrile is high in consumption quota, the product purity is low, the yield is low, and a large amount of acrylamide-containing sulfate and waste liquid are generated, which pollutes the environment.
2. Catalytic hydration. In the early 1970s, foreign countries developed a process of direct reaction between acrylonitrile and water under the action of a framework copper catalyst.
The catalytic hydration method has higher purity and higher yield than the sulfuric acid hydration method, basically no three wastes, and easy to realize industrialization. At present, the world mainly uses catalytic hydration to produce acrylamide.
The framework copper catalyst used is a binary or higher alloy, which dissolves part of it after alkali treatment, leaving the metal in a living state as framework copper. Framework copper catalysts include Cu-Cr alloy, Cu-Ni alloy and Cu-Al-Zn alloy. The reactor can be a suspended bed or a fixed bed. The Dow Chemical Company of the United States adopts fixed-bed continuous catalytic technology, using Cu-Cr as catalyst, and the product is 50% AM aqueous solution; Japan's Mitsui Toa Chemical Co., Ltd. and Japan Mitsubishi Chemical Company adopts suspended bed continuous catalytic hydration technology, using Cu-Ni as catalyst , The product is 50% AM aqueous solution and crystals; at present, my country's AM production usually uses Cu-Al-Zn framework copper catalysts, and uses a fixed bed continuous catalytic hydration process. (1) Acrylamide production process of Dow Chemical Company, USA. The acrylamide and polyacrylamide produced by Dow Chemical Company in the United States have a history of more than 30 years. It uses a fixed-bed hydration reactor. After the raw material acrylonitrile enters the plant, it is refined by an advanced packing refining tower. The reaction conditions are: acrylonitrile; ion-free water = 1:3 (molar ratio). The catalyst is a Cu-Cr alloy and is processed as required. The single-pass conversion rate of acrylonitrile is 45%~70%. With the prolonged production time, the catalyst activity gradually decreases, and it is regenerated every two months. The reactor is switched to use during regeneration. The dimensions of the Dow Chemical Company's fixed-bed reaction tower: diameter is about 0.46m, height is about 4.6m. Each production line has 4
The reaction towers are operated in parallel. Except for the flash distillation tower and the rectification tower, there is no ion exchange resin or complexing agent to remove metal ions in the monomer process, but it is processed when the polymer is manufactured. Monomer product specifications: concentration is 48%~52%, copper ion is 12~25ug/g, insoluble matter is less than 100ug/g, and the rest is water. It is emphasized that there should be a certain content of copper ions in the monomer, which can inhibit polymerization during storage and transportation. Consumption quota (calculated per ton of acrylamide): 750kg for acrylonitrile, 1000kg for deionized water, 1kg for catalyst
(2) Acrylamide production process of Mitsubishi Chemical Corporation.
Japan's Mitsubishi Chemical Corporation has a history of more than 40 years since it began producing acrylamide in 1962. Japan's Mitsubishi Chemical Corporation uses a suspended bed reactor device. In addition to the reactor, there are also catalyst recovery and manufacturing equipment. In 1993, Daqing Oilfield introduced the company's production technology. The characteristic of the suspended bed reaction is that the catalyst is required to be suspended in the reaction solution. The particle size of the suspended bed catalyst is small, and it is made smaller under the action of temperature and the friction between the catalyst particles. Therefore, the AM aqueous solution after the reaction needs to pass through an efficient trapping filter to remove the catalyst particles. Otherwise, the remaining fine powder will be carried into the AM solution, which will have a serious impact on the subsequent refining process and even the polymerization process. The process of the suspended bed reaction is to add a metal copper catalyst to the mixture of acrylonitrile and water, and continuously enter the catalytic hydration reactor through an infusion pump at 70~120°C for hydration reaction. After the reaction, AM with copper catalyst The aqueous solution is passed through a filter to separate the AM aqueous solution from the catalyst. The dilute AM aqueous solution enters the concentrator to be concentrated, and then the concentration is adjusted to produce 30%-50% AM aqueous solution, and then the product is refined by activated carbon ion exchange resin. AM concentrate can also be crystallized and dried to produce crystalline solid AM products. The unreacted acrylonitrile from the AM concentrator is condensed and recycled back to the reactor for cyclic reaction. The catalyst filtered by the catalyst filter is regenerated or sent together with new catalyst
Catalytic hydration reactor. This forms a closed loop.
Since the catalyst used in the suspended bed process of Mitsubishi Chemical Corporation of Japan is continuously circulating, the activity remains basically unchanged. The catalyst used in the production device is produced by its own catalyst recovery and manufacturing section, and only the metal copper and nickel are purchased from the market. The catalyst consumption is 1-2kg/tAM.
①Product performance. ② Process characteristics. Under the action of a catalyst, AM is directly and continuously produced from acrylonitrile without by-products such as ammonium sulfate and sodium sulfate. The process consists of the following sections: hydration, concentration, refining, catalyst production and regeneration. Hydration: The acrylonitrile and water are continuously sent to the hydration reactor, and the acrylonitrile is directly converted into AM under the action of the suspension catalyst. The AM solution generated by the reaction is separated from the catalyst, and the auxiliary chemical agent of [Water] [Bingmeixi] is sent to the concentration section. Concentration: Continuously concentrate AM aqueous solution to 50% by evaporation method (catalytic production and regeneration
(Weight), the evaporated water and unreacted acrylonitrile are recycled and returned to the hydrolysis section.
Refining: Continuously remove a small amount of impurities in the concentrated AM solution to obtain a pure AM solution.
The production and regeneration of the catalyst: A part of the catalyst used in the catalytic reactor is extracted from the reactor for regeneration.
CH2 = CHCN +H20一+ CH2 =CH- CNH2 (acrylamide) Obviously the C=N trivalent bond reaction, the product is acrylamide, and the C=C double bond reaction product is hydroxypropionitrile. Inhibition of the reaction, the orientation of the above two hydration reactions is determined by the selectivity of the catalyst. Therefore, the alloy ratio smelting method used in the preparation of the catalyst and the process conditions of the catalyst during use are both key factors that affect the selectivity of the catalyst. In short, catalyst activity and selectivity are the most important factors affecting the conversion rate of acrylamide and the yield of acrylamide.
① Thicken. In order to meet the needs of monomer products and dry powder production, the concentration of acrylamide must reach the polymerization concentration. During the concentration process, acrylonitrile and water must be evaporated, and acrylamide must not be polymerized during the concentration process. Therefore, it is necessary to add a polymerization inhibitor, reduce the operating temperature, and prevent polymerization while blowing air at the bottom of the tower. Generally, a forced circulation flash evaporation process is adopted. The catalytic liquid enters the evaporation chamber through the pressure reducing valve, and the acrylonitrile and water emitted from the gas are condensed and recycled. The production process is a closed loop system with basically no waste. The acrylamide aqueous solution after flash evaporation enters the Yanfa kettle for further evaporation and concentration to the required concentration.
② Refined. After flash evaporation, the refined acrylamide will have a large amount of copper, iron ions and a small amount of by-products such as nitrotripropionamide (NT-PA). If it is not removed, it will seriously affect the polyacrylamide in the subsequent process. Product quality, the purpose of the refining section is to remove these impurities. The raw material preparation system is unstable and the product quality fluctuates greatly. Therefore, it is necessary to use a new type of resin to remove the copper, iron ions and by-product NTPA from the hydration reaction. There are special requirements for the equipment of the refining system. The reason is that acrylamide has a tendency to polymerize. This tendency is affected by the material of the equipment.
The qualitative influence is greater. If iron enters the system, it will cause the polymerization of acrylamide. Therefore, the equipment of the refined system is lined with non-metallic materials to make it difficult to polymerize.
(5) Process overview.
① Preparation of catalyst. Catalyst: A copper-chromium catalyst whose main component is metallic copper. The copper-ming catalyst is the basis of the direct hydration process to produce acrylamide. In the preparation of the catalyst, sodium hypophosphite and copper sulfate react with sulfuric acid at a temperature of about 50 to form copper hydride, and then the copper hydride is decomposed by sodium hydroxide to form the catalyst Cu**. A large amount of hydrogen is produced when copper hydride is decomposed.
2CuSO, + 3NaH,PO2 +2H20一, 2CuH + 3NaHPO2 + 2H,SO,
2CuH +2H,SO, +4NaOH一+2Cu++ +Na.SO, +4H,O +H,
If the catalyst is only Cu**, the activity is low, so add chromium according to the ratio of chromium to copper atomic ratio of approximately Cr/Cu =0.05. Preparation and storage of the catalyst: Although the Gangming catalyst (hereinafter referred to as the catalyst) has high activity, as the acrylonitrile hydration reaction time elapses, the catalyst activity gradually decreases. Therefore, the catalyst should be constantly supplemented to compensate for the catalyst activity.
Sexual reduction. The spent catalyst should be recovered in 1.5 batches of intermittent reactions per day. However, because the amount of catalyst retained in the reactor is significantly greater than the production capacity of the catalyst, only the catalyst preparation section should be started in advance when the device is completed and started. Because the catalyst is easy to oxidize in the air to generate heat, it is necessary to pay attention to the storage conditions of the catalyst, such as temperature and oxygen control.
In the production of the catalyst, a large amount of hydrogen is generated due to the decomposition of copper hydride. Special attention should be paid to treating the catalyst in N2, and the catalyst should not be placed under dry conditions.
②Catalyst recovery section. In the production of acrylamide, the activity of the catalyst gradually decreases as the reaction time passes. Therefore, when the catalyst is discharged from the tertiary reactor to the spent catalyst tank, it is discharged approximately twice a day. In order to regenerate the spent catalyst, the spent catalyst should be converted into copper sulfate, the main chemical in the catalyst product. Thoroughly clean the spent catalyst (in 10W% catalyst drilling fluid, the content of acrylamide should be less than 300mg/L).
On the other hand, the filtered reactant is collected in the concentration section through the acrylamide recovery tank and becomes 10% slurry. After the separated catalyst is filtered and washed, it reacts with sulfuric acid and air in the spent catalyst dissolution tank. The copper solution is again used in the preparation of the catalyst.
Cu+HS0, +%02-→+CuSO, + H20
The small amount of chromium contained in the catalyst is also regenerated simultaneously. The copper and chromium salts consumed in each operation should be replenished in time. ③Reaction. Acrylonitrile and water react directly to form acrylamide under the action of a catalyst. The reaction is as follows: the initial concentration of acrylonitrile is 35% (65% is water), the catalyst concentration is 20%, the pressure is 0.35MPa (-stage reactor), and the temperature is 90 ~ 95C. The residence time of each reactor is 3h, and 3 reactors are connected in series. The reaction yield is high, reaching a conversion rate of 90% to 92%, and a selectivity of 99.6%. Acrylonitrile from the acrylonitrile plant is stored in the acrylonitrile tank of the acrylamide plant. The temperature of the acrylonitrile is adjusted by the acrylonitrile heater, and then sent to the deoxygenation stripper to remove the dissolved oxygen of the acrylonitrile. Then, acrylonitrile is sent to the primary reactor. Unreacted acrylonitrile is added in the concentration section, then mixed with new catalyst, and sent to R-311 circulation. The reactant passes through the candle filter installed inside each reactor. Under the action of the pressure difference between each reactor, only the filtrate passes through the candle filter. ④ Concentration section. The reactant is concentrated to a 50% aqueous solution and sent to the refining section; the unreacted acrylonitrile in the reactant (about 10% of the acrylonitrile entering the reactor) is recovered and recycled to the reaction section. In the flash tank, the water and acrylonitrile are flashed out under reduced pressure (about 6.27kPa) for concentration. The sensible heat of the reactant and 50C hot water are used as the heat source for concentration. During the concentration process, the amount of evaporation should be controlled to maintain the concentration of acrylamide at 50%. The acrylamide concentration is measured with an acrylamide concentration meter. On the other hand, a 50% aqueous solution of acrylamide is easily polymerized even at a low temperature of 209C in the absence of oxygen. Therefore, air is blown into the mixing tank to maintain the dissolved oxygen content, and unreacted acrylonitrile is recovered. Demineralized water is used for cooling, and then CW and liquid ammonia are used to cool the demineralized water. After the evaporated acrylonitrile is condensed with cooling water, it is sent to the recovery acrylonitrile water deoxygenation stripping tower, and the 02 in the acrylonitrile aqueous solution is stripped with N,. Together with the acrylonitrile recovered by the K-type filter, it is stored in the recovery acrylonitrile water tank and sent to the reaction section for circulation by a pump. ⑤ Refining section. The refining section can also be divided into the following two processes: One is the acrylonitrile stripping process-reducing the approximately 3000mg/L acrylonitrile stored in the 50% acrylonitrile aqueous solution to below 50mg/L; the other is the ion exchange resin refining Process one removes the Cu”* produced by the dissolution of a small amount of catalyst and the by-product NTPA produced by the reaction. Acrylonitrile stripping process: pass air in the acrylonitrile stripping tower, and strip the acrylonitrile in the concentrated liquid. Exhaust gas The content of acrylonitrile is about 3000mng/L, which exceeds the allowable value of the working environment (20mg/L). From the perspective of safety and sanitation, let the acrylonitrile exhaust gas pass through the K-type filter waste acrylonitrile gas adsorber. In the exhaust gas, acrylonitrile is K The activated carbon fiber is absorbed in the type filter. After steam desorption and cooling, the acrylonitrile is recovered in the form of acrylonitrile water. The concentration of acrylonitrile in the exhaust gas is reduced to below 20mg/L.
Ion exchange resin refining: The dissolved copper ions (Cu2*) and NTPA (by-products of hydration reaction) in the reaction section are absorbed and removed by the ion exchange resin tower. Two towers with the same specifications (one for operation and one for regeneration) use HCl and NaCl for regeneration, once every 5 days, according to NTPA. When NTPA is removed from the acrylamide solution, the pH of the solution drops to about 5.5. Even the presence of dissolved oxygen may cause polymerization. Therefore, the pH value is adjusted to 6-7 with NaOH at the outlet of the tower. Next, add 10mg/L (p-acrylamide solution) Thiourea as a polymerization inhibitor in the product section. Acrylamide solution has a tendency to polymerize easily, and this tendency is affected by the material of the container. Polyacrylamide flocculant For example, if the pH value is in the range of 6~7, it can be considered that it will not polymerize in a stainless steel container after long-term operation. But if the pH is low, in a stainless steel container, polymer will be formed within a few hours. Therefore, the surface is lined with a phenol resin that does not cause polymerization, making it difficult to polymerize. For the same reason, the outlet pipelines are coated with polytetrafluoroethylene. Product section: 50% acrylamide aqueous solution is temporarily stored in two inspection tanks, and 10mg/L (for acrylamide solution) thiourea (polymerization inhibitor) is added during the inspection.