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Process of acrylic acid production at home and abroad and its development prospects
This design reviews the process of acrylic acid production at home and abroad and its development prospects. Based on a detailed comparison of various acrylic acid production processes, a two-step propylene production process for acrylic acid was selected.
Firstly, the acrylic acid two-step oxidation process for the production of acrylic acid was designed. It was divided into three sections, namely the reaction section, the absorption section and the refining section. Secondly, using Aspen Plus to simulate and optimize the process flow and various equipments, the process parameters and equipment parameters of each section were obtained. At the same time, the material balance and heat balance were calculated. On this basis, the reactor and distillation were carried out. The design of the tower, absorption tower and heat exchanger and the selection of the pump, and the automatic control scheme of each equipment was designed, and the equipment assembly diagram of the reactor, tower, process flow chart and process flow chart with control points were drawn. . Finally, the process of producing acrylic acid by two-step oxidation of propylene was evaluated economically, environmentally and safely.
Key words: acrylic acid; reaction; refining; process design
Acrylic acid (English name: Acrylic acid), the molecular formula is C 3 H 4 O 2 , the relative molecular weight is 72.06, and the structural formula is CH 2 =CHCOOH. The specific gravity (20 ° C) is 1.051, the boiling point is 141.3 ° C, the melting point is 13.2 ° C (approximate), and the flash point is 54.5 ° C, which is a colorless, transparent, bitter, corrosive liquid with a pungent odor. Soluble in water, ethanol, ether and other solvents. Its hazard category is 8.1, which is a high flash point flammable liquid and acid corrosive, belonging to the acryl compound. Acrylic acid is an important unsaturated organic acid. Its chemical structure contains unsaturated carbon-carbon double bonds and carboxyl groups. It can synthesize a series of acrylic copolymers, such as acrylates and polyacrylic acids, by homopolymerization or polycondensation. .
The traditional use of acrylic acid is as a raw material for the synthesis of acrylates. At present, the yield and amount of n-butyl acrylate are the largest, followed by the relatively small amount of ethyl acrylate, 2-ethylhexyl acrylate and methyl acrylate. The largest use of acrylates is coatings, followed by adhesives and sealants. It is also used for the modification of textiles and fibers. It is also an important modification aid for plastics. In addition, paper products, leather, etc. also require acrylate as a base material. Secondly, polyacrylic acid (salt) can be used as a builder, a dispersant, a thickener, a flocculant, a scale inhibitor and a sizing agent. In the future, the most developed downstream product of acrylic acid is superabsorbent resin (SAP). The main raw material is high-purity acrylic acid. 1tSAP requires about 0.77t of high-purity acrylic acid. Its acidity can be absorbed by human skin. SAP has been widely used in baby diapers, feminine napkins and adult hygiene products in other areas such as agriculture, gardens and food.
Domestic and foreign acrylic acid production capacity and market analysis
1. Foreign production capacity and market
At present, nearly 30 companies in 15 countries or regions in the world produce acrylic acid, mainly in the United States, Western Europe, Asia, and South Africa. Among them, BASF, Dow Chemical, Arkema, and Japan Catalyst are the world's four major acrylic monomer producers, which account for 22%, 21%, 12%, and 11% of the world's total production capacity, respectively. In 2008, the global acrylic acid production capacity was 5.03 million tons, and in 2009 it reached 5.13 million tons. As of August 2010, the global acrylic acid production capacity was about 5.21 million tons. In 2005, the total consumption of acrylic acid in the world was 3.361 million tons, and the total consumption of acrylate was 3.145 million tons. The main consumer regions are the United States, Western Europe, Japan and China, which account for about 80% of the world's total acrylic acid consumption. From 2006 to 2009, the total demand for acrylic acid in the world increased at an annual rate of more than 200,000 tons, reaching 5.205 million tons in 2010. After decades of development in the world's developed countries, acrylic acid production has basically met the local market demand, and developing countries will become the main demand market for acrylic products in the future.
2. Domestic production capacity and market
China's acrylic acid industry is the fastest growing country in the world in the past 10 years. It has formed a relatively stable market competition pattern with state-owned enterprises as the mainstay and foreign investment and private enterprises. The number of production enterprises has grown from 3 to 11 companies, including Jiangsu Jurong. Shanghai Huayi and Zhejiang Satellite rank among the top 10 global producers. Acrylic capacity has developed extremely rapidly. In 2001, the output was 147,000 t/a. In 2010, it increased to 10.28 million t / a (22. 1% of the world's total capacity). The average compound growth rate in the past 10 years was as high as 23. 3%, the import volume decreased rapidly, and a small amount of exports. After more than ten years of development, the production of acrylic monomers in mainland China has already reached a considerable scale. After the expansion, China will become the world's largest producer and consumer of acrylics. Before and after 2015, China's acrylic capacity is expected to exceed 2.5 million tons per year, and soon become a net exporter of acrylic monomers.
The increase in domestic demand for acrylic acid is mainly due to the rapid development of infrastructure construction such as construction and transportation, which has increased the demand for acrylic ester products such as acrylic latex architectural coatings and sealants. Therefore, domestic enterprises have accelerated the pace of construction of acrylic devices, and in recent years, large-scale acrylic devices have been put into production.
Process design for acrylic acid production
3. Selection of process plan; acrylic acid production method
Acrylic acid was industrially produced in the 1930s, and its production method was subjected to the cyanide method, the Reppe method, the enone method, the acrylonitrile hydrolysis method, and the propylene oxidation method.
(1) Chloroethanol method
The chlorohydrin process was the earliest method of industrial production of acrylic acid. In 1927 and 1931, industrial production plants were established in Germany and the United States. The method uses chloroethanol and sodium hydride as raw materials to form cyanoethanol under the action of a catalyst, and the cyanoethanol is dehydrated in the presence of sulfuric acid to form acrylonitrile, and then hydrolyzed to form acrylic acid. The production process is as follows:
HOCH 2 CH 2 Cl+NaCN → HOCH 2 CH 2 CN → CH 2 =CHCOOH
(2) Cyanide ethanol method
This method was developed from the chlorohydrin process. With the development of the petrochemical industry, cyanoethanol was produced from ethylene oxide and hydrocyanic acid. The acrylic acid yield of the method is only 60% to 70%, and more polymer is formed during the reaction process, the cyanide is more toxic, and the investment and product production cost are higher.
(3) Reppe method
The high pressure Reppe method was developed in Germany in 1930, using acetylene and carbon monoxide as raw materials. The esterification grade acrylic acid is first formed from acetylene, carbon monoxide and water under the action of a nickel-based catalyst, and then reacted with an alcohol to form an acrylate. In 1956, BASF began to produce acrylic acid by this method. By 1977, a total of 300,000 tons/year of production equipment was built. In 1995 the law stopped industrial production. The reaction equation is as follows:
CH== CO + H 2 O CH 10Mpa CH 2=CH--- COOH
(4) Acrylonitrile hydrolysis
This method is also indirectly a propylene route since acrylonitrile is made from propylene. In the 1960s, the production of acrylonitrile by propylene ammoxidation was developed. Acrylonitrile is abundant in source, and therefore, under certain conditions, acrylic acid can be synthesized from an acrylonitrile route.
CH 2 =CH-CH 3 + NH 3 + O 2 → CH 2 =CH-CN + H 2 O
Acrylonitrile can be hydrolyzed to acrylic acid at a certain temperature (200 to 300 ° C).
CH 2 =CH-CN + H 2 O ? CH 2 =CH-CONH 2
CH 2 =CH-CONH 2 + H 2 O + H 2 SO 4 →CH 2 =CH-COOH + (NH 4 )SO 4
CH 2 =CH-CONH 2 + ROH + H 2 SO 4 → CH 2 =CH-COOR + (NH 4 )SO 4
The acrylonitrile hydrolysis process is relatively simple and easy to implement, and its investment is also less, but it is more toxic. At present, although there is no large-scale industrial production in the world, there are still small-scale devices to produce small amounts of acrylic acid and acrylate by this method. There are factories in Japan, the United Kingdom, China and Mexico, all of which are under 20,000 tons/year. Japan's Asahi Kasei Corporation's 18,000-ton/year installation terminated the law in mid-1990. Ciba Specialty Chemicals' 15,000 tonne/year installation in Bradford, England, was also discontinued in 1999. Celanese's plant in Mexico was also converted to propylene oxidation in 1993.
(5) Ethylene method
The reaction formula of synthesizing acrylic acid using ethylene or the like as a raw material and palladium as a catalyst is as follows.
CH 2 =CH 2 + CO + O 2 → CH 2 =CH-COOH
United Petroleum Corporation established an industrial plant in California in 1973. However, the acrylic acid selectivity of this method is only 75% to 85%. This method is still under development and the process is not yet mature.
(6) Propane oxidation method
The propane oxidation method uses propane as a raw material, a metal oxide as a catalyst (such as a metal oxide mixture such as Mo-Sb-V-Nb-K), and propane gas phase oxidation to prepare acrylic acid. The reaction equation is as follows:
CH 3 CH 2 CH 3 + O 2 catalyst CH 2 =CHCOOH
(7) Direct oxidation of propylene
The direct oxidation of propylene to acrylic acid has a one-step process and a two-step process. The one-step method has the advantages of simple reaction device, short process flow, only one catalyst, less investment, etc., but there are several outstanding shortcomings:
1 One-step method is to carry out two oxidation reactions in one reactor, forcing a catalyst to adapt to the requirements of two different reactions, affecting the effective function of the catalyst, and the yield of acrylic acid is low; 2 combining the two reactions into one step The reaction heat effect is large. To reduce the heat of reaction, it can only be achieved by reducing the concentration of propylene, so the production capacity is low; 3 the catalyst has a short life, which leads to economic unreasonable.
In view of the above reasons, the current industry mainly uses a two-step process, that is, the first step of propylene oxidation to acrolein, and the second step of acrolein oxidation to acrylic acid.
The main reaction of the first step is:
CH=CH-CH 3 + O 2 → CH 2 =CH-CHO
The side reaction of the first step is:
CH 2 =CHCH 3 +0.5O 2 → CH 2 =CHCOOH
2CH 2 =CHCH 3 +7.5O 2 → 3CO 2 +3CO+6H 2 O
2CH 2 =CHCH 3 +4O 2 → CH 3 COOH+2CO 2 +2H 2 O
The main reaction of the second step is:
CH 2 =CH-CHO + 1/2O 2 → CH 2 =CH-COOH
The side reaction of the second step is:
2CH 2 =CHCHO+5.5O 2 → 3CO2+3CO+4H 2 O
4CH 2 =CHCHO+2H 2 O+O 2 → 4CH 2 O+CH 3 CHO
In addition, several other side reactions occur, and by-products such as propionic acid, furfural, acetone, formic acid, and maleic acid are formed.
The reaction is a strong exothermic reaction, and the effective removal of the heat of reaction is a prominent problem in the reaction process. In addition to the main reaction, there are a large number of side reactions, and its by-products include deep oxides such as CO and CO 2 as well as acetaldehyde, acetic acid and acetone. Therefore, it is very important to increase the selectivity of the reaction and the yield of the target product. To achieve this, it is necessary to use a highly active, highly selective catalyst during the reaction. Since the production of acrylic acid is carried out in steps, the catalyst used in each reaction is also different. The first step is the oxidation of propylene to acrolein. Most of the catalysts used are Mo-Bi-Fe-Co systems, and a small amount of other elements are added to exhibit catalytic activity in the form of molybdate. The second step is the oxidation of acrolein to acrylic acid. The catalysts currently used are all Mo-V-Cu systems, and it is usually necessary to add a cocatalyst.
At present, the research on acrylic acid production methods is developing towards environmentally friendly microbial catalysis. The method of microbial direct fermentation of sugar to produce acrylic acid not only avoids the use of fossil products as raw materials, but also solves the problem of environmental pollution. It has mild fermentation conditions and product separation process. The advantages of simplicity, renewable raw materials and low cost are the research hotspots for the production of acrylic acid by biological methods in the future. Japan Catalytic Synthesis Corporation promotes glycerol-based processes to produce acrylic acid, a by-product of biodiesel from vegetable oils. The new technology uses a highly active catalyst to produce an acrolein that produces acrylic acid. This technology produces carbon-neutral acrylic acid from a source of renewable raw materials. The newly developed catalyst produces acrolein by gas phase dehydration of glycerol, which is then oxidized to acrylic acid by gas phase oxidation.
In the above technology, the chlorohydrin method, the cyanide method, the Reppe method, etc. have been gradually eliminated due to low efficiency, high consumption, and high cost. In addition, the ethylene process, the acetylene process, and the propane process which are being developed in recent years are not yet mature, and there is no large-scale production facility. Only the propylene oxidation process monopolizes a large-scale acrylic acid production plant. Today, all large acrylic production facilities in the world are produced by propylene oxidation.
4. Choice of process route
Comprehensive comparison of various production methods, we choose the direct oxidation of propylene to acrylic acid. There are currently four companies with this technology, namely Japan Catalyst, Mitsubishi Chemical, BASF, and Sohio. Nippon Catalyst uses an integrated oxidation reactor. This reactor has the advantage of a small footprint and a small investment, and shortens the distance between the catalyst bed and the second stage, and inhibits the deep oxidation of acrolein. The separation system uses an azeotropic distillation purification method. BASF uses a two-stage reactor. The gas product produced by the oxidation of propylene is not absorbed by water, but is absorbed by a high-boiling organic solvent. Whether it is in the selection of catalysts required for propylene oxidation production, Mitsubishi Chemical has made great improvements in the purification technology of dilute acid. The process features high-concentration propylene as raw material to reduce the amount of inert gas and water vapor. Humidifying air, using a miniaturized compressor, reduces the amount of water vapor, which means that the quenching tower increases the concentration of acrylic acid, thus greatly reducing the amount of wastewater in the refining unit system. Through comprehensive comparison, we use Mitsubishi Chemical Technology, and its process flow:
Japan's Mitsubishi Chemical Technology uses two reactors in series; the air feed of the oxidation reaction feed is in two stages, the total oxygen to olefin ratio of the two stages is 2.2:1, and the feed oxygen to olefin ratio is 1.703:1. The water vapor absorption tower exhaust gas circulation part supplements the water vapor required for the reaction, the water vapor concentration is about 5%, the second stage water vapor concentration is about 10%, and the reactor outlet temperature is 320 °C.
6. Simulation and optimization of process flow
Simulation of each section
The most important tasks of process simulation are three things:
(1) Whether it is feasible to judge each candidate process;
(2) Select the most suitable process plan;
(3) Optimize the design of the selected process plan to determine the optimal process conditions.
Throughout the design process, Aspen Plus is used to calculate the entire process accurately. Aspen Plus has a strong physical database, but the choice of specific physical methods needs to be selected according to their actual process. By consulting the literature, the NRTL-RK model was selected and the whole process was simulated according to the thermodynamic model selected above.
7. Simulation of the reactor
The reactor modules available in Aspen Plus are RStoic, RYield, REquil, RGibbs, RCSTR, RPlug, RBatch, where RStoic, RYield are productivity-type reactors that do not consider thermodynamic possibilities and kinetic feasibility, REquil, RGibbs are not Thermodynamic equilibrium reactors with kinetic feasibility, RCSTR, RPlug, and RBatch are chemical kinetic reactors based on chemical kinetics.
Due to the lack of kinetic data for this reaction, we chose the RStoic conversion rate reactor. The RStoic reactor is modeled by artificially setting the reactor outlet reactant conversion rate so that as long as the inlet quantity is determined, the amount of each component in the outlet product is determined, independent of the operating pressure and temperature within the reactor. So that we can not determine the optimal operating conditions of the reactor through simulation, the best operating conditions can only be determined through literature research.
The main process conditions of the first reactor of the first stage reactor are reaction temperature, reaction pressure and raw material composition. By consulting the literature and simulation results, the reaction temperature is finally determined to be 310 ° C, the reaction pressure is 0.1 MPa, and the raw material composition is: Propylene accounted for 12%, water vapor accounted for 10%, air accounted for 78%, water-ene molar ratio was 0.83, and oxyalkylene molar ratio was: 1.37. The simulated result temperature is 583.1K
The main process conditions of the second stage reactor of the second stage reactor are reaction temperature and reaction pressure. By reviewing the literature and simulation results, the reaction temperature is finally determined to be 240 ° C, the reaction pressure is 0.06 MPa, and the raw material is the outlet of a reactor. The material was replenished with air at the second stage inlet so that the molar ratio of oxyalkylene was 2.1 and the molar ratio of water to olefin was 2.0. The simulation result has a discharge temperature of 533.1K and a pressure of 0.59 atm.
8. Simulation of the absorption tower
The absorption tower separates the acrylic acid from the mixed gas generated by the reaction by using the difference in solubility of acrylic acid and nitrogen, oxygen, carbon monoxide, carbon dioxide and the like in water. The FEED is a raw material feed stream, which is fed from the bottom of the column. The logistics WATER is the absorbent pure water stream, which is fed from the top of the tower. Logistics OVERHEAD is the waste gas stream. The main components are nitrogen, oxygen, carbon dioxide and a small amount of acrylic acid, which are discharged from the top of the tower. The main components of the absorbed solution are acrylonitrile, water, acetic acid, etc., which are discharged from the bottom stream BOTTOW into the next process. Some of the acrylic acid solution is cooled by the condenser and then returned to the absorption tower ABSBR by the flow BYCL. The main purpose of the reflux is Increase the mass concentration of the bottom acrylic acid. I set the feed temperature of the absorbent pure water to 25 ° C, the feedstock feed temperature to 170 ° C, and the feed pressure to 101.325 Kpa.
9. Simulation of light component separation tower
Using the azeotropic nature of toluene and water, toluene and acetic acid, the top of the light component separation column is enriched in toluene, water, and acetic acid components, using the heterogeneous azeotropy of toluene and water, in the decanter at the top of the column. Cooling stratification, non-condensable gas further treatment, the liquid phase is divided into an aqueous phase and an oil phase, the aqueous phase contains most of the water and a small amount of acetic acid and other components, the main component of the oil phase is toluene, which can be refluxed as an azeotropic agent to light The component separation column is recycled. The tower is enriched with heavy components to form a higher purity acrylic acid solution.
The rigorous convergence method RadFrac tower module is used to simulate the light component separation column. Because there is steam and incompatible toluene and aqueous solution in the top discharge, a decanter is used to separate the gas phase, the toluene phase and the water phase. Consider the gas-liquid phase equilibrium. The azeotrope in the stream TOL is the entire stream from the stream TOL+ in the decanter and a small amount of fresh high purity toluene. The stream product from the quench tower flows into the light component separation column, and the TOL component of the stream is an azeotropic toluene which enters from the top of the column. The distillate in column T1 is discharged from stream 2 and enters decanter V1. A small amount of gas in decanter V1 is discharged from the stream GAS. The liquid is layered and divided into oil phase stream TOL+ and water phase stream WATER, and logistics TOL+ large. Part of it is toluene, which is refluxed as an entrainer into the column and the aqueous phase is further processed. The parameters of the column are 8 plates, the third plate feed, the top of the azeotropic agent, and the top pressure is 1 atm.
10. Simulation of acetate column
The acetic acid column is mainly for removing acetic acid. The product of the light component separation column enters the acetic acid separation column to separate acetic acid. The top stream is mainly acetic acid, acrylic acid and water, and the bottom stream is mainly acrylic acid and maleic acid. Firstly, the acetic acid column was simulated by the stabilizing distillation design, and the parameter of the column was obtained as the minimum reflux ratio of 7.3, the actual reflux ratio was 12.3, the theoretical plate number was 14, the actual number of plates was 20, and the feeding position was the sixth plate, reboiling. The required heat is 416,752.23 KJ/h, and the required cooling capacity of the condenser is 446,209.96 KJ/h. The RadFrac method is used for accounting and determination, and the recovery rate of light key components and heavy key components at the top of the tower is required.
11. Recycling tower simulation
The function of the recovery tower is to recover a portion of the acrylic acid solution distilled from the top of the acetic acid column to reduce product loss. Using the principle of ordinary rectification, acetic acid and water are distilled off from the top of the column, and the bottom of the column is enriched with acrylic acid of higher purity, and then refluxed to the acetic acid column for further separation. The product from the top of the acetic acid column is added to the recovery tower, and the light components are discharged from the overhead stream. The main components are water and acetic acid; the heavy components are discharged from the bottom stream and refluxed into the acetic acid column, and the main component is acrylic acid. The same as the acetic acid column, the three-dimensional distillation design is used to simulate the recovery tower. The parameters of the tower are shown in Table 2-3, and then the RadFrac method is used for accounting and determination. The recycling of light key components and heavy key components at the top of the tower is carried out. The rate has reached the requirements.
12. Simulation of the purification tower
The function of the purification tower is to remove the heavy components in the solution, and the acrylic acid is distilled off from the top of the column by the principle of ordinary rectification. The bottom product of the acetic acid column is added to the purification tower, the light component is mainly acrylic acid, and the overhead stream is discharged as a product. The heavy components are discharged from the bottoms stream for further processing. The same as the acetic acid column, firstly using the stripping rectification design to simulate the recovery tower, the parameters of the column are obtained as the minimum reflux ratio of 0.01, the actual reflux ratio is 0.1, the theoretical plate number is 4, the actual number of plates is 10, and the feed position is the fourth block. The heat required for the plate and the reboiler is 2,541,156.78 KJ/h, and the required cooling capacity of the condenser is 247,1874.6 KJ/h. The RadFrac method is used for accounting and determination, and the recovery rate of light key components and heavy key components at the top of the tower is required.
13. Process optimization
The ultimate goal of the simulation is to optimize the process to achieve the best benefits in some areas, such as the best economic benefits and the best energy savings. In the process of establishing a full-process simulation, the optimal parameters for local optimization have been sought. These parameters include the number of plates in each distillation column, reflux ratio, recovery rate, feed plate position, and extractant. Dosage and the like. Some of the reaction parameters are optimal operating conditions determined according to the literature, and these parameters need not be optimized. Below we will illustrate the process of optimizing some typical equipment operating parameters when building a process.
14. Optimization example of absorption tower
(1) Determination of the location of the logistics BYCL return plate With the increasing emphasis on the environment in the whole world, the state is becoming more and more strict with the environmental protection system, and the pollutants discharged into the air should be minimized. The objective function is the smelting of the top stream. The acid has the smallest mole fraction. Investigate the effect of the flow BYCL on the molar fraction of acrylic acid in the overhead stream D at different reflux feed plate locations. When the mole fraction of acrylic acid in the overhead stream D is the smallest, the feed plate position is the optimum return plate position.
Under a certain theoretical number of plates, change the position of the feed plate, and then move from the first plate at the top of the tower to the last plate at the bottom of the tower for simulation, and record the propylene in the top D of the different feed plate positions. Acid mole fraction. When the molar fraction of acrylic acid in the top stream D is the smallest, that is, the environmental pollution is the smallest and the product loss is the smallest, which is the optimum feeding plate position. The optimum return plate position of the total theoretical plate number is 15, 20, 25, 30 is determined by simulation, and the results are recorded and plotted. It can be seen from Fig. 2.4 that the optimum return plate position is near a certain position at the bottom of the tower. As the position of the return plate moves down from the top of the tower to the bottom of the column, the mole fraction of acrylic acid in the top stream D decreases first. The increasing trend shows a minimum value, which is the optimum return plate position. Taking 15 theoretical plates as an example, when the feed plate position is from the 1st to the 11th theoretical plates, the mole fraction of acrylic acid in the overhead stream D is reduced from 0.0005415 to 0. 000784, because when reflowing When the plate is in the first to eleventh theoretical plates, the reflux liquid and the mixed gas have sufficient contact time, the concentration of acrylic acid in the reflux liquid is high, and the position of the reflux feed plate moves down the plate, the mixed gas and the pure The time of contact of the water absorbent is increasing, that is, the concentration of the top acrylic is getting lower and lower. When the reflux feed plate position is from the 11th block to the 15th theoretical plate, the mass fraction of acrylic acid in the overhead stream D is increased from 0. 000784 to 0. 00134, which is due to the position of the return feed plate. As the pressure is continuously lowered, the contact time of the reflux liquid and the mixed gas is continuously reduced, and the total time of contact between the mixed gas and the reflux liquid and the pure water absorbent is continuously reduced, and the absorption effect is worse and worse, resulting in an increase in the molar concentration of the top acrylic acid. The more the number of theoretical plates, the better the absorption effect. That is, as the number of theoretical plates increases, the mass concentration of acrylic acid at the top of the tower decreases continuously, but after decreasing to a certain value, the change trend becomes slower and slower.
Simulation of main factors of absorption The main factors affecting the absorption of the absorption tower are the flow rate of the absorbent and the number of theoretical plates. Simulate these two factors to find the optimal combination and determine the best process parameters. First, the theoretical plate number is determined to be 15 pieces, and at the optimum return plate position of the bottom stream BYCL, that is, the 11th theoretical plate is fed back. Next, the molar flow rate of the absorbent stream WATER is set to 200 Kmol/h, and the mole fraction of acrylic acid in the bottom stream of the column is recorded. Then, the molar flow rate of the absorbent stream WATER was adjusted to 150 Kmol/h and 250 Kmol/h, respectively. Taking 15 theoretical boards as an example, the number of different theoretical plates is under the theoretical plates of 15, 20, 25, and 30, and the results are shown in Table 2-5. The molar concentration of the acrylic acid at the top of the column at the optimum reflux feed plate position of the different theoretical plates at 150Kmol/h, 200Kmol/h and 250Kmol/h, respectively.
The most important influence on the absorption effect is the amount of absorbent, and finally the number of theoretical plates. The smaller the molar flow rate of the spray water, the greater the molar concentration of acrylic acid at the top of the column. The more theoretical plates, the smaller the mass fraction of acrylic acid at the top of the column. The theoretical plate number of the absorption tower should be controlled at 25-30. When the theoretical plate is less than 25, the theoretical plate number is too small, the gas-liquid contact time is short, which is not conducive to absorption, and the acrylic acid concentration at the top of the column is difficult to meet the design requirements. When the number of theoretical plates exceeds 30, the mass fraction of acrylic acid at the top of the column does not change much with the increase of the number of plates, and the number of theoretical plates is too large, which will increase the equipment cost. In summary, considering the influence of the amount of absorbent, the dosage of the absorbent is determined to be 200-250Kmol/h, the absorbed dose is too small, the liquid is not easily distributed in the packed tower, the effect is affected, the amount of absorbent is too large, and the tower is increased. The bottom water content, which in turn increases the subsequent refining energy consumption. Therefore, the optimum process conditions are selected to be 25 theoretical plates, and the reflux feed plate position of the flow BYCL is the 22nd theoretical plate, and the molar flow rate of the absorbent is 200Kmol/h.
15. Material balance
The meaning of material balance
In chemical engineering, design or modify process and equipment, understand and control the production process, calculate the economic benefits of the production process, determine the raw material consumption quota, determine the loss of the production process, analyze the existing process, select the most Material balance is required for an efficient process route, optimal design of the equipment, and determination of optimal operating conditions. Moreover, the development and amplification of chemical engineering are based on material balance. Material balance is a form of expression of the law of conservation of mass. The composition, mass or volume ratio of the material introduced into a device is equal to the composition, mass or volume of the product obtained after the operation plus the loss of material.
16. Principle of material balance
The material balance of the system is based on the theory of mass conservation, and studies the changes in the amount and composition of the incoming and outgoing materials in a system, namely: the mass of the system = the quality of the input system - the mass of the output system + the quality of the reaction - the reaction consumption The quality hypothesis system has no leaks and has:
dF/dt=F IN -F OUT +G R -C R
When no chemical reaction occurs in the system, there are:
dF/dt=F IN -F OUT
In steady state, there are:
dF/dt=F IN -F OUT =0, F IN =F OUT (3-3)
F IN — the material flow rate into the system;
F OUT — the material flow rate of the outflow system;
G R — the rate at which the reaction produces the material;
C R - the rate at which the reaction consumes material.
3.1.3 Material Accounting Task
Through the detailed material balance of the whole system and some main units, the key economic and technical indicators such as the output of the main and by-products, the consumption of raw materials, the emissions of “three wastes” and the quality indicators of the final products are obtained. The design process is quantitatively reviewed to provide a basis for the design of the later stages.
17. System material balance - energy balance - the meaning of energy balance
In the whole plant heat balance calculation, the unit equipment is the basic unit, considering the change of heat caused by mechanical energy conversion chemical release and simple physical change. Finally, a system-level heat balance calculation is performed on the entire process section, which is used to guide the design of energy saving and consumption reduction.
18. Principle of energy balance
The energy balance of the system is the theoretical basis to study the changes of various types of energy in a system, namely: the energy of the input system = the energy of the output system + the energy accumulated by the system. For continuous systems, there are: Q+W=ΣH OUT -ΣH IN
Q—the thermal load of the equipment;
W—the mechanical energy of the input system;
ΣH OUT — the sum of the materials leaving the equipment;
ΣH IN — The sum of the materials entering the equipment.
The energy balance of this project is based on unit equipment and calculates the heat changes caused by mechanical energy conversion, chemical reaction release energy and simple physical changes.
19. Energy balance task
(1) Determine the power required by the machinery in the process to provide a basis for equipment design and selection.
(2) Determine the heat or cooling capacity and transfer rate required for the operation of each unit of the rectification, determine the amount of the heating agent and the refrigerant, and prepare for the subsequent heat exchange and utility design.
(3) Determine the amount of heat exchange during the reaction and guide the design and selection of the reactor.
(4) Finally calculate the required energy and cost to determine the economics of the process.
20. Equipment design and selection of acrylic acid production
The chemical reaction process and reactor are the central links in the chemical production process, and the design of the reaction often occupies an important position. The design of the reactor mainly includes: reactor selection; finding suitable process conditions; determining the technical measures required to achieve these process conditions; determining the structural size of the reactor; and determining the necessary control means.
21. Reactor selection
There are various types of reactors, which can be roughly classified into tubular type, kettle type, tower type, fixed bed and fluidized bed according to their structure. Each type of reactor has its own characteristics. When selecting the type, it is necessary to combine the characteristics of the reactor for comprehensive analysis and make a reasonable choice. For this subject, a tubular fixed bed reactor was selected. Therefore, the type of reactor is designated as a fixed bed reactor. The characteristics of the fixed bed reactor are: small back mixing, low catalyst dosage at high conversion rate, and difficult catalyst wear; since the fixed bed reactor is basically flat, the temperature distribution has a gradient, so the heat transfer temperature is not easy. In addition, the loading and unloading of the catalyst is troublesome, and continuous production requires the provision of a standby reactor.
22. Reaction principle
The propylene oxidation process uses propylene and oxygen in the air as raw materials to carry out a reaction through a catalyst bed in the presence of steam and at a reaction temperature of 250 to 400 °C. The reaction is carried out in two stages.
The main reaction of the first step is:
CH 2 =CHCH 3 +O 2 → CH 2 =CHCHO+H 2 O
The side reaction of the first step is:
CH 2 =CHCH 3 +0.5O 2 → CH 2 =CHCOOH
2CH 2 =CHCH 3 +7.5O 2 → 3CO 2 +3CO+6H 2 O
CH 2 =CHCH 3 +2.5O 2 → CH 3 COOH+3CO 2 +H 2 O
The main reaction of the second step is:
CH 2 =CHCHO+0.5O 2 → CH 2 =CHCOOH
The side reaction of the second step is:
2CH 2 =CHCHO+5.5O 2 → 3CO 2 +3CO+4H 2 O
4CH 2 =CHCHO+2H 2 O+O 2 → 4CH 2 O+CH 3 CHO
In addition, several other side reactions occur, and by-products such as propionic acid, furfural, acetone, formic acid, and maleic acid are formed.
23. Catalyst selection
The catalyst is the core of acrolein and acrylic acid produced by two-step gas phase catalytic oxidation of propylene. It has undergone changes in the types of Cu 2 O, CuSeO 3 and Sb-U. After the 1970s, American Standard Oil Company successfully developed a composite metal oxide catalyst based on Mo-Bi element, and gradually became the leading catalyst for the catalytic oxidation of propylene to acrolein. At present, companies with propylene vapor phase catalytic oxidation technology include Japan Catalyst Chemical Company, Mitsubishi Chemical Corporation and BASF Corporation. The widely used acrolein catalysts in the industry are mainly Mo-Bi elements, including W, Fe, alkali metals and A composite metal oxide catalyst of an alkaline earth metal element or the like. Japan's Mitsubishi Chemical's oxidation catalyst has a long service life and a high selectivity rate of up to four years. The second stage can be used for six years and is expected to be used for eight years. The conversion of propylene is over 98%, the conversion of second-stage acrolein is over 99%, the catalyst strength is high, and there is no chalking phenomenon. This design selects the catalyst technology of Mitsubishi Chemical.
24. Reaction conditions
The reactor inlet gas consists of raw material propylene, oxygen in the air, nitrogen carried in by air, and water vapor. The composition control targets are propylene concentration, volume fraction ratio of oxygen to propylene (oxygen to olefin ratio), and water to olefin ratio.
Catalysts with different propylene concentrations are selected to have different propylene concentrations, generally between 7.0% and 10.0%; for higher propylene inlet concentrations, higher oxygen concentrations are required to meet the reaction requirements, but due to propylene The mixture with oxygen exists in the explosion area, so from the perspective of safety and the pursuit of ideal reaction results, it is necessary to adopt the method of sectional oxygen supply, that is, to add air from two points of the reactor inlet and the two-stage reactor inlet respectively. All the oxygen required for the reaction. The propylene concentration of this design is 12%.
The ratio of oxyalkylene to oxyalkylene is generally controlled between 1.8:1 and 2. 0:1. The ratio of too low and too high oxyalkylene will affect the conversion and selectivity of propylene, and may also cause damage to the catalyst and The safety hazard of the operation of the device. The oxyalkylene ratio of this design is 1.37.
The presence of hydrene in the presence of water vapor is to increase the selectivity of the catalyst, so that the reaction product is easily resolved from the surface of the catalyst; the second is to reduce the oxygen concentration in the reaction gas in order to slow the deep oxidation of propylene; The large heat capacity is conducive to the thermal stability of the bed. The water-to-olefin ratio of the design is 0.83.
25. Reaction airspeed
Absolute airspeed (SV 0 ): The ratio of the volume of propylene gas added per hour to the reactor volume at the inlet of a reactor, ie SV0 = V c / V. It reflects the actual reaction load of the catalyst at around 120 h-1.
Relative airspeed (SV): The ratio of the hourly volume flux of the mixed gas to the catalyst bed volume, ie: SV = V gas / V bed. Generally between 800 ~ 2000h - 1.
The reaction temperature is generally from 300 to 400 ° C for one reaction and from 250 to 320 ° C for the second reaction.
26. Tower equipment design
The column equipment used for separation and refining includes: absorption tower T0201, azeotropic distillation column T0202, acetic acid column T0301, acrylic acid recovery tower T0302, and purification tower T0303. The project completed the design of the process parameters of the whole plant tower equipment, and selected the most representative acetic acid tower T0301 to give detailed calculation and selection instructions. For other equipment, see the equipment selection design list.
27. Selection principle of tower equipment
The types of towers used in industry are mainly packed towers and plate towers. Currently, the two can only be compared relatively. The factors to be considered when selecting are: material properties, operating conditions, tower equipment performance, tower manufacturing, installation, Operation, maintenance, etc.
(1) The packed tower is preferred in the following cases:
a. In the case of high separation requirements, because of the high mass transfer efficiency of some new fillers, new fillers can be used to reduce the height of the tower;
b. For the distillation separation of heat sensitive materials, because the new type of filler has a small liquid holding capacity and a small pressure drop, the packed column under vacuum operation can be preferred;
c. Corrosive materials, optional packed tower. Because the packed tower can be made of non-metallic materials such as ceramics, plastics, etc.;
d. For materials that are easy to foam, a packed tower should be used.
(2) The plate tower is preferred in the following cases:
a. The liquid in the tower has a large liquid stagnation amount, the operating load varies widely, and is insensitive to the change of the feed concentration, and the operation is easy to be stable;
b. The liquid phase load is small;
c. Containing solid particles, easy to scale, and crystallized materials, because the plate tower can use a tray with a larger liquid flow channel, and the risk of clogging is small;
d. During the operation accompanied by exothermic or heating materials, it is necessary to provide internal heat exchange components in the tower, such as heating coils, requiring multiple inlets or multiple side outlets. This is because on the one hand, the structure of the tray tower is easy to realize, and in addition, there is more liquid stagnation on the tray for efficient heat transfer with the heating or cooling tube;
e. The distillation column operating at higher pressure still uses a tray tower. Comprehensive consideration, this project uses a plate tower.
28. Safety, storage and transportation design and three waste treatment
Since the raw materials and products involved in the acrylic acid production process are mostly flammable and explosive materials, sufficient safety measures must be taken in the design and operation of the device. It is mainly reflected in the following aspects.
(1) The general plan is arranged in the layout of the general plan, taking into account the fire hazard of the physical sciences, in strict accordance with the corresponding national standards, in the areas such as safety technology distance, setting fire exits, diffusion of harmful gases, etc. .
(2) Tank area safety measures According to the nature of different materials, the storage tanks should be arranged, the necessary range and height of cofferdams should be set up, and safe import and export should be established. For the storage parts of high flash point flammable materials such as propylene, Automatic detection facility for combustible gases.
(3) Buildings and frames should be as open or semi-open as possible. The main load-bearing members should be non-combustible. The outer surface of the concrete or steel column load-bearing members should have a fireproof protective layer. The area of the doors and windows of the house that can be used as the pressure relief channel should be big enough.
(4) The safety fire protection system is equipped with fire extinguishing equipment of a certain density (such as dry powder fire extinguisher, etc.) in different parts of the installation. If necessary, a fixed fire protection system is installed in the installation area, which may include fire hydrants, fire water guns, fire foam generation and injection systems, and halon fire extinguishing devices in the control room.
(5) Lightning protection, anti-static design and facilities of other electrical equipment; uninterrupted power supply of key control systems; pressure monitoring (alarm) points in the process equipment, relief pressure relief valves, explosion-proof holes and automatic interlocking systems Settings.
29. Personal safety
The substances involved in the production of acrylic acid are mostly toxic. Therefore, in the production process, special attention should be paid to the protection of the operators. When engaging in the operation of toxic and hazardous materials, it is necessary to equip and wear the necessary protective equipment. Table 7-1 shows the hazards of the main materials involved in acrylic acid production and the treatment measures after accidental unprotected contact
30. Packaging and storage
For acrylic acid, it is generally used in stainless steel, plastic, glass, and other clean containers that do not contain initiators or contaminants. At the same time avoid the exposure of sunlight. For the case of using a storage tank to store a large amount of materials, the storage tank must have perfect insulation measures. On the one hand, the outer layer of the tank is provided with an insulation layer, and the coil is provided in the tank, and the warm water of 16 to 30 ° C is passed (no water vapor can be used). At the same time, a delivery pump is also provided to maintain uninterrupted circulation of material in the tank. The recommended storage temperature for acrylic acid is between 14 and 30 ° C. In principle, the lower the acrylic point above, the better. Especially for the acrylic acid product used to prepare the polymer, the storage temperature is relatively low in order to suppress the occurrence of the dimerization reaction. The content of the polymerization inhibitor in the acrylic product varies depending on the purity of the stored acrylic acid. 200 mg/kg of hydroquinone monoformaldehyde is added, and it will not be abnormal after storage for about two months. Air has a hindrance effect, so full tank storage should be avoided to allow air to remain in some of the tank space. In order to prevent freezing during storage, it is generally preferred to maintain the acrylic acid in an 80% aqueous solution (freezing point - 5. 5 ° C) or to dilute the solvent used in the polymerization. During the storage and transportation of acrylic acid, especially small packages (such as barrels). There is often a freezing phenomenon that occurs due to inadequate insulation measures. The freezing of acrylic acid causes uneven distribution of the polymerization inhibitor content in the material in the container, and it is easy to polymerize due to local high temperature during the melting process. Therefore, the freezing operation of the frozen acrylic acid should be carried out at 30 ° C and appropriate measures should be taken to fully stir it, and it should be used immediately after melting. To avoid repeated freezing and melting, excessive temperatures and long storage times can result in a large increase in dimer content and polymerization of materials. It is generally believed that the storage time of acrylic acid at a suitable temperature should not exceed three months.
31. Three waste treatmen
Treatment of spent acid: The term "waste acid" as used herein refers to the distillate or the still of the acetic acid separation column. The main component is acetic acid, and may also contain a small amount of acrylic acid, solvent and lighter fractions produced by oxidation reaction. By-product. These waste acids can be used to prepare higher purity acetic acid, and special separation devices are required. Otherwise, incineration can be considered for harmless treatment. The specific method is: pressurizing the waste acid and spraying it through the nozzle. Incinerator. The incinerator can be used as fuel for heavy oil, residual oil, diesel, and the like. In the furnace, the spent acid is burned at a high temperature of nearly 1000 ° C and finally converted into carbon dioxide and water vapor. 6.3.2 Treatment of waste gas The main source of waste gas from the acrylic acid production unit is the tail gas of the acrylic acid absorption tower. It contains a small amount of organic gases such as propylene, propane, acrolein, and solvent, and also contains a certain amount of oxygen. The most abundant is nitrogen and water vapor. For this part of the exhaust gas, one can be treated by direct incineration (there can be no waste gas incinerator alone, or can be treated with the same incinerator together with waste acid, waste oil and the waste water mentioned later), and the second is to use catalytic incineration. The way to handle it.
The specific method is that the exhaust gas is heated and mixed with the quantitative oxygen through a reactor equipped with a catalytic reaction (mostly adiabatic reaction). Under the action of the catalyst, the organic component in the exhaust gas undergoes a combustion reaction at a temperature of about 600 ° C to be converted into carbon dioxide and water. The advantage of catalytic incineration is that energy is saved (no additional fuel is needed). Furthermore, since the processing temperature is relatively low, the amount of new pollutants such as nitrogen oxides is small, so this is a good way to promote. 6.3.3 Treatment of Wastewater The sources of wastewater from the acrylic production plant are: wastewater produced by the process (reaction-forming water, added absorption water, tail gas spray water, etc.), equipment washing water, and irregular water discharge. These waters contain almost all substances (both organic and inorganic) produced and added by the acrylic acid production process. CODCr usually reaches more than 10,000. For the treatment of wastewater, the most basic and most common method is still “direct incineration”. The specific steps can be considered as follows.
(1) Neutralization Decomposition Add an appropriate amount of NaOH to the wastewater. To neutralize the acidic substances in them and to decompose some of them.
(2) De-lighting The lighter components (organic matter) in the wastewater are removed by a stripper and sent to an incinerator for incineration.
(3) Concentration of wastewater and recovery of water The water in the waste water from which the stripping column is removed by the double-effect evaporator is recovered as much as possible and reused as process water, and at the same time, the wastewater is concentrated. The heat of the double effect evaporator is provided by the incinerator exhaust.
(4) The incineration concentrated wastewater is sprayed into the incinerator through a nozzle, and the organic matter therein is converted into dioxide and water. After the above steps are processed, the CODCr of the wastewater can be reduced to below 100, thus meeting the national environmental discharge standards. The incinerator can use heavy oil, residual oil, diesel oil, etc. as a fuel, and can also treat the waste water, waste oil, and waste gas in the same furnace. In the industry, there are also devices that use wet oxidation to treat wastewater by introducing wastewater and metered air into a catalyst-containing reactor (generally an adiabatic reactor) at a pressure of about 7 MPa and at 300 °C. The combustion reaction is carried out at a temperature, and the organic matter in the wastewater is converted into carbon dioxide and water.
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