Vinyl chloride is one of the most important monomers, which is mainly used to produce polyvinyl chloride. In terms of output, PVC ranks second only to polyethylene in ethylene series polymers. Vinyl chloride can also be polymerized with 1- 1- dichloroethylene, vinyl acetate, methyl acrylate, butadiene and acrylonitrile. In addition, vinyl chloride is also used as a refrigerant.
Summary of production methods of vinyl chloride 1.
In the history of vinyl chloride production, there are the following four production methods.
(1) acetylene method This is the main production method of vinyl chloride before 1950s, and some domestic chemical enterprises still use this method to produce vinyl chloride.
The conversion of acetylene is 97% ~ 98%, and the yield of vinyl chloride is 80% ~ 95%. The main by-product is 1- 1- dichloroethane, which is formed by the addition reaction of vinyl chloride and excess hydrogen chloride. In the reaction, in order to ensure that the catalyst HgCl2 is not reduced to low-valent mercury salt Hg2Cl2 or metallic mercury by acetylene, the excess of hydrogen chloride shall not exceed 15%. Acetylene process has mature technology, mild reaction conditions, simple equipment, few by-products and high yield. Because hydrogen chloride is used as raw material, it is suitable for enterprises (such as electrochemical plants) that use hydrogen chloride as by-product to organize production. The main disadvantages of this method are that acetylene is expensive and the catalyst contains mercury, which not only harms workers' health, but also pollutes the environment.
(2) Ethylene method This is a production method developed after the 1950s. Ethylene and chlorine undergo an addition reaction to generate dichloroethane;
Dichloroethane is pyrolyzed at 500 ~ 550℃ or alkali decomposed at 1.0 MPa, 140 ~ 145℃ to produce vinyl chloride;
Petroleum hydrocarbon pyrolysis can produce ethylene in large quantities, which is cheaper than acetylene and less toxic than mercuric chloride. However, the utilization rate of chlorine is only 50%. After the other half of chlorine gas is separated from the cracking gas in the form of hydrogen chloride, its color and purity can not meet the national standards, so its sale and utilization have become technical and economic problems that factories must solve. Although hydrogen chloride can be oxidized by air or oxygen to chlorine for reuse, the equipment cost and operation cost are high, which leads to the increase of vinyl chloride production cost.
(3) The combination method is an improvement of the above two methods. The purpose is to use acetylene to consume hydrogen chloride, a by-product of ethylene process. This method is equivalent to establishing two sets of devices for producing vinyl chloride in parallel in the factory, which will obviously increase the capital investment and operating expenses. Half of the hydrocarbon feed is expensive acetylene, which will increase the total production cost, and the introduction of acetylene method will still bring mercury pollution problems. So this rule is not ideal.
(4) oxychlorination. This is a good way for 1 to use only ethylene as raw material and consume by-product hydrogen chloride. Now it has become the main method to produce vinyl chloride in the world.
The conversion rate of ethylene is about 95%, and the yield of dichloroethane is above 90%. High-pressure steam can also be used as a by-product for equipment related to the process or for power generation. This method is also called ethylene balance method because the balance of hydrogen chloride should always be considered in equipment design and factory production to prevent excessive or insufficient hydrogen chloride. Obviously, this method has the advantages of cheap and easily available raw materials, low production cost and environmental friendliness. However, there are still many shortcomings such as more equipment and long process route, which need further improvement.
2. Process principle of oxychlorination method
1928, Raschig Company of Germany first developed the gas phase oxychlorination method for preparing chlorobenzene from benzene;
This is the earliest oxychlorination method used for hydrocarbon substitution chlorination. Subsequently, the liquid-phase oxychlorination method was successfully developed, and the conversion rate and selectivity were greatly improved. Since the process of producing phenol from chlorobenzene was gradually replaced by the automatic oxidation of cumene after the 1930s, this method could not be further developed. Few factories produce chlorobenzene by this method.
Oxychlorination is also used for the substitution chlorination of lower alkanes and ethylene chlorination to trichloroethylene and tetrachloroethylene, but the production scale is not very large.
The most successful application of oxychlorination in industry is the production of dichloroethane from ethylene and hydrogen chloride, which shows a good prospect for the application of oxychlorination in other chlorination fields.
(1) chemical reaction ethylene oxychlorination to produce vinyl chloride includes three processes: ethylene chlorination, ethylene oxychlorination and dichloroethane cracking. Only oxychlorination of ethylene is discussed here.
The main reactions of oxychlorination are:
There are three main side effects of oxychlorination.
① Deep oxidation of ethylene?
C2H4+2O2→2CO+2H2O？
C2H4+3O2→2CO2+2H2O？
② By-products 1. 1.2- trichloroethane and chloroethane are produced.
In addition, there are a few saturated or unsaturated monochloro or polychlorinated derivatives, such as chloroform, carbon tetrachloride, vinyl chloride, 1, 1- trichloroethane, cis-1, 2- dichloroethylene, etc. , but the total amount is not much, only 1, 2- dichloroethylene.
(3) Reaction Mechanism Although a lot of research work has been done on the mechanism of ethylene oxychlorination at home and abroad, it is still inconclusive. There are two main mechanisms:
① redox mechanism? Japanese scholars such as Fujitani and Kanuchi believe that chlorine is transported to ethylene through the valence change of copper chloride in oxychlorination reaction. The reaction is divided into the following three steps:
C2H4+2CuCl2→C2H4Cl2+Cu2Cl2
Cu2Cl2+ 1/2O2→CuCl2 CuO
Copper chloride CuO+2HCl→2CuCl2+H2O
In step 1, the adsorbed ethylene reacts with copper chloride to generate dichloroethane, and the copper chloride is reduced to cuprous chloride. This step is the control step of the reaction; Secondly, cuprous chloride is oxidized into a complex of copper chloride and copper oxide; Thirdly, the complex reacts with hydrogen chloride and decomposes into copper chloride and water. The mechanism is based on: a) when ethylene passes through copper chloride catalyst alone, dichloroethane and cuprous chloride are formed; (b) When air or oxygen passes through the reduced cuprous chloride, it can be completely converted into cuprous chloride; (c) The concentration of ethylene has the greatest influence on the reaction rate.
Therefore, the chlorinating agent for converting ethylene into dichloroethane is not chlorine, but copper chloride, which continuously transports chlorine to ethylene through redox mechanism.
② Mechanism of ethylene oxidation? According to the fact that the reaction speed of oxychlorination increases with the increase of the partial pressure of ethylene and oxygen, but has nothing to do with the partial pressure of hydrogen chloride, the American scholar R.V.Carrubba proposed the following mechanism:
Where a represents the adsorption center of the catalyst surface; HCl (a), O (a) and C2H4 (a) represent adsorbed species of HCl, O and C2H4; The control step of this reaction is the reaction of adsorbed ethylene and adsorbed oxygen.
In the early study of oxychlorination, it was also suggested that hydrogen chloride was oxidized to chlorine gas under the catalysis of copper chloride, and then chlorine gas reacted with ethylene to produce dichloroethane.
(3) Reaction Kinetics According to the above reaction mechanism, when copper chloride is used as catalyst, the kinetic method measured by experiment is as follows:
Where: PC, pH and PO represent the partial pressures of ethylene, hydrogen chloride and oxygen respectively.
It can be seen from the above two kinetic methods that the partial pressure of ethylene has the greatest influence on the reaction rate, and increasing the partial pressure of ethylene can effectively increase the formation rate of 1, 2- dichloroethane. In contrast, the change of hydrogen chloride partial pressure has much less influence on the reaction rate. When the oxygen partial pressure exceeds a certain value, it has no effect on the reaction rate. At a lower value, the change of oxygen partial pressure has obvious influence on the reaction rate. These two kinetic equations are basically consistent with the above two reaction mechanisms.
(4) The early studies of catalysts show that metal chlorides can be used as oxychlorination catalysts, among which copper chloride has the highest activity, and copper chloride supported on γ-al2o 3 and aluminum silicate is widely used in industry. The content of copper on the catalyst affects the conversion and selectivity of the reaction. With the increase of copper content, the conversion rate increases, but the amount of CO2 produced by deep oxidation increases. The copper content is determined to be 5% ~ 6% through experiments. At this time, the conversion rate of hydrogen chloride can be close to 100%, and the amount of CO2 produced is not much. Although this one-component catalyst has good selectivity, copper chloride is volatile. The higher the reaction temperature, the greater the volatilization loss of copper chloride, the faster the catalyst activity decreases and the shorter the service life. In order to prevent or reduce the loss of active components of copper chloride catalyst, potassium chloride, the second component, was added to the catalyst to become a two-component catalyst. Although the reaction activity decreased, the thermal stability of the catalyst was obviously improved. This is probably because potassium chloride and copper chloride form a nonvolatile double salt or low melting point mixture, thus preventing the loss of copper chloride. In order to improve the activity of two-component catalyst, rare earth metal chlorides, such as cerium chloride and lanthanum chloride, were added to the catalyst, which not only improved the catalytic activity, but also prolonged the service life of the catalyst, and the catalyst changed from two-component to multi-component.
Fig. 5-2-0 1 Effect of temperature on reaction rate
Figure 5-2-02
Effect of temperature on selectivity (in the case of chlorine)
Figure 5-2-03
Effect of temperature on ethylene combustion reaction
There are two kinds of oxychlorination reactors: fixed bed and fluidized bed. When the fixed bed is used, the formed γAl2O3 carrier is impregnated with active components, dried and activated by air, and can be put into use. For fluidized bed catalyst, γ-al2o 3 microspheres were used to impregnate the active components. Alternatively, the aluminosilicate sol can be mixed with the active component, then the gelling agent is added, and the fluidized bed microsphere catalyst can be prepared by spray drying.
(5) Selection of process conditions
① reaction temperature? The effects of reaction temperature, reaction speed, selectivity and ethylene combustion reaction on CuCl _ 2/γ Al _ 2O _ 3 catalyst with copper content of 12%(w) were studied. The results are shown in figure 5-2-0 1, figure 5-2-02 and figure 5-2-03. As can be seen from Figure 5-2-0 1, the reaction speed rises rapidly with the increase of temperature in the initial stage, then gradually slows down after 250 degrees Celsius, and then begins to decline after 300 degrees Celsius. Therefore, the higher the reaction temperature, the better, but there is a suitable range. As can be seen from Figure 5-2-02, the reaction selectivity also increases with the increase of temperature in the initial stage, and then gradually decreases after reaching the maximum at about 250 degrees Celsius, which shows that there is also a suitable range in selectivity. Figure 5-2-03 shows the relationship between the side reaction of ethylene deep oxidation and the reaction temperature. The curve in the figure shows that the rate of side reaction of ethylene deep oxidation increases slowly with the increase of reaction temperature before 270℃, and increases rapidly after 270℃. From the point of view of catalyst use, with the increase of reaction temperature, the amount of CuCl2 _ 2, the active component of the catalyst lost by volatilization, increases, the deactivation speed of the catalyst is accelerated and the service life is shortened. From the point of view of operation safety, because ethylene oxychlorination is a strong exothermic reaction, the heat of reaction can reach 25 1 kJ/mol, and the heat released by the main and side reactions, especially the side reaction of ethylene deep oxidation, increases when the reaction temperature is too high. If it cannot be removed from the reaction system in time, the reaction temperature will rise further due to the accumulation of heat in the system. Such a vicious circle leads to explosion or burning accident. Therefore, the lower the reaction temperature, the better under the premise of satisfying the reaction activity and selectivity. The specific reaction temperature is determined by the selected catalyst. For CuCl _ 2 KCl/γ Al _ 2O _ 3 catalyst, the fluidized bed temperature is 205 ~ 235℃ and the fixed bed temperature is 230 ~ 290℃.
② Reaction pressure? High pressure has an adverse effect on the reaction speed and selectivity of oxychlorination, but at the actual operating temperature below 65438±0.0 MPa, the pressure has little effect on the reaction speed and selectivity. Therefore, you can choose normal pressure or low pressure operation. Considering that pressurization can improve the utilization rate of equipment and is beneficial to subsequent absorption and separation operations, low-pressure operation is generally adopted in industry.
③ Proportion of ingredients? The ratio of ethylene, hydrogen chloride and air must ensure that the excess of ethylene is 3% ~ 5%. Oxygen should also be slightly excessive to ensure the normal redox process of the catalyst, but hydrogen chloride should not be excessive, because excessive hydrogen chloride will be adsorbed on the surface of the catalyst, which will expand the catalyst particles and reduce the apparent density. If a fluidized bed reactor is used, the bed will rise sharply due to the large expansion of catalyst particles, and even the phenomenon of slugging will occur. Too much ethylene should not be used, otherwise the deep oxidation reaction of ethylene will be intensified, CO and CO2 in the tail gas will increase, the reaction selectivity will decrease, and too much oxygen will also promote the deep oxidation reaction of ethylene. The composition of feed gas is also required to be outside the explosion limit range of raw material ratio to ensure safe production. The industrial ratio is: ethylene: hydrogen chloride: oxygen = 1: 2: 0.5 (molar ratio).
④ Raw gas purity? The adopted air can only be used after filtering, washing and drying to remove a small amount of solid impurities, SO2, H2S and moisture. Hydrogen chloride gas comes from the cracking process of dichloroethane and usually contains acetylene. Therefore, after mixing with hydrogen, hydrogen chloride gas is dealkylated in the hydrogenation reactor before entering the oxychlorination reactor. The contents of acetylene, propylene and C4 olefins in raw ethylene must be strictly controlled, because they are more active than ethylene, and oxychlorination will occur, resulting in polychlorinated salts such as tetrachloroethylene, trichloroethylene, 1, 2- dichloropropane, which increases the difficulty of product purification. At the same time, they are more prone to deep oxidation reaction, and the released heat will promote the reaction temperature and bring adverse effects to the reaction. Generally, the content of ethylene in raw material ethylene is required to be above 99.95%(m). Table 5-2-02 shows the specifications of vinyl chloride raw material ethylene in China.
Table 5-2-02
Specification of vinyl chloride raw material ethylene in China
group
minute
point to
mark
group
minute
point to
mark
C2H4
CH4+C2H6
C2H2
99.95%
500 ppm
10 ppm
C2S
S (calculated by H2S)
H2O
100 ppm
5 ppm
15 ppm
⑤ Stay time? Residence time has an effect on HCl conversion. The experiment shows that when the residence time reaches 10 s, the conversion rate of hydrogen chloride can be close to 100%, but if the residence time is too long, the conversion rate will decrease slightly, because 1, 2- dichloroethane is cracked to produce hydrogen chloride and vinyl chloride. Too long residence time not only reduces the production capacity of the equipment, but also intensifies the side reaction, leading to the increase of by-products and the decrease of reaction selectivity.
Fig. 5-2-04 Process Flow of Chloroethylene Production by Oxychlorination in PPG Chemical Industry Company
1. Direct chlorination reactor; 2. Gas-liquid separator; 3. Oxychlorination reactor; 4. Delimiter; 5. Light fraction tower; 6. Heavy fraction tower; 7. Cracking furnace; 8. Quenching tower; 9. Hydrogen chloride recovery tower; 10. vinyl chloride distillation column
3. Balanced vinyl chloride production process
Figure 5-2-04 shows the process of producing vinyl chloride by oxychlorination in PPG Chemical Industry Company. Because all the hydrogen chloride produced by thermal cracking of dichloroethane is consumed in oxychlorination reaction, it is also called balanced vinyl chloride production process. The process consists of three processes: ethylene liquid phase addition chlorination production 1, 2- dichloroethane; Gas-phase oxychlorination of ethylene produces 1, 2- dichloroethane; Production of vinyl chloride by pyrolysis of 1, 2- dichloroethane.
The reaction conditions of liquid-phase addition chlorination of ethylene are as follows: the reaction temperature is about 50 degrees Celsius, the catalyst is FeCl3 _ 3 _ 3, its concentration in the chlorination solution is maintained at 250 ~ 300 ppm (0.025% ~ 0.03%), and the molar ratio of ethylene to chlorine gas is 1. 1: 1, that is, the ethylene is excessive.
The reaction conditions of ethylene gas-phase oxychlorination are as follows: the reaction temperature is 225 ~ 290℃, the pressure is 1.0MPa, the catalyst is cuc L2/γal2o 3 or modified CuCl2-KCl/γAl2O3, and the copper content in the catalyst is 5% ~ 6% (converted into CuCl2, it is11.
Crude dichloroethane generated by addition chlorination and oxychlorination enters the light component removal tower and heavy component removal tower. The light component contains a small amount of hydrogen chloride gas and can only be used after washing. The heavy component contains more dichloroethane, which needs to be recovered by vacuum distillation for further treatment. The purity of dichloroethane obtained is very high, which can reach about 99%. Entering the thermal cracking furnace, the operating conditions are: the temperature is 430 ~ 530 degrees Celsius, the pressure is 2.7MPa, and the catalyst is pumice or activated carbon. The reaction conversion rate can reach 50% ~ 60%, and the selectivity of vinyl chloride can reach 95%. Hydrogen chloride with purity of 99.8% was distilled from pyrolysis products in a hydrogen chloride fractionator, which contained alkynes. If necessary, it can be used as raw material for oxychlorination after hydrodeacetylene. In the vinyl chloride fractionator, the finished vinyl chloride with purity of 99.9% distilled from the top of the tower and the dichloroethane at the bottom of the tower contain heavy components produced by thermal cracking, which are sent to the dichloroethane refining process for treatment.
The process uses oxygen instead of air as oxidant, which has the advantages that after the reaction, excess ethylene can still be recycled in the oxychlorination reactor after cooling, condensation and separation, and the utilization rate of ethylene is higher than that when air is used as oxidant; When air is used as oxidant, the concentration of ethylene in tail gas is low, only about 65438 0%. Combustion consumes fuel. When oxygen is used as oxidant, the tail gas emission is small, but the ethylene concentration is high, so the combustion method does not need additional fuel. Because the prepared feed gas does not contain nitrogen, the concentration of ethylene in the feed gas is improved, which is beneficial to improving the reaction speed and the production capacity of the catalyst, and the reactor can be made smaller, saving the equipment manufacturing cost; When oxygen is used as oxidant, due to the small amount of tail gas, it is not necessary to recover a small amount of dichloroethane from the tail gas through solvent absorption and cryogenic, which simplifies the process and reduces the equipment investment cost. For the fixed bed reactor, when oxygen is used as oxidant, the hot spot is not obvious, so the selectivity of 1, 2- dichloroethane is high, and the conversion rate of hydrogen chloride is also high, while when air is used as oxidant, the opposite is true. Table 5-2-03 lists the comparison results of the two.
Table 5-2-03
Comparison of ethylene oxychlorination results in fixed bed
Selectivity of converting ethylene into various substances,%
Air oxychlorination method
Oxychlorination method
1, 2- dichloroethane
chloroethane
Carbon monoxide+carbon dioxide
1, 1, 2- trichloroethane
Other chlorine derivatives
95. 1 1
1.73
1.78
0.88
0.50
97.28
1.50
0.68
0.08
0.46
Conversion rate of HCl,%
99. 13
99.83
At present, many large chemical enterprises have built air separation units, and there is no problem in oxygen supply, which provides a good opportunity for the development of ethylene oxychloride process with oxygen as oxidant. The consumption quota of oxygen oxychlorination method (based on the production of 1 t dichloroethane) is: 287kg of ethylene (100%), 742kg of hydrogen chloride (100%) and 0/77kg of oxygen (100%).
Figure 5-2-05 is the material balance diagram of vinyl chloride production by oxychlorination.
Figure 5-2-05 Balanced Production Organization of Vinyl Chloride *
The figures in the figure are the actual weight proportions of various materials.
4. Oxychlorination Reactor
Whether air oxychlorination or oxygen oxychlorination, fixed bed or fluidized bed reactor can be used.
(1) fixed bed oxychlorination reactor
Figure 5-2-06
Schematic diagram of structure of fluidized bed ethylene oxychlorination reactor
1. ethylene and HCl inlets; 2. Air inlet; 3. Plate distributor; 4. Tubular reactor; 5. Gas catalyst inlet; 6. Reactor shell; 7. Cooling pipe group; 8. Pressurized hot water inlet; 9, 1 1, 12. Cyclone separator; 10. Reaction gas outlet; 13. Manhole; 14. high pressure steam outlet
The structure of this reactor is basically the same as that of the ordinary fixed bed reactor, with multiple tubes and granular catalyst inside, and the feed gas flows through the catalyst layer from top to bottom for catalytic reaction. Pressurized hot water is used as the carrier between pipelines, and a certain amount of medium-pressure steam is by-produced.
There are hot spots in the fixed bed reactor tubes. Excessive local temperature will reduce the reaction selectivity, accelerate the loss of active components and shorten the service life of the catalyst. In order to make the bed temperature distribution more uniform and reduce the hot spot temperature, three fixed bed reactors are often used in series in industry: oxidant air or oxygen is introduced into the three reactors respectively according to a certain proportion. In this way, the oxygen concentration in the materials of each reactor is low, so that the reaction will not be too violent, the amount of CO and CO2 generated by deep oxidation can also be reduced, and the oxygen concentration in the mixed gas is also guaranteed to be outside the flammable range, which is conducive to safe operation.
(2) Fluidized bed oxychlorination reactor The reaction temperature of the fluidized bed reactor is uniform, and there is no hot spot. The feeding speed can be controlled by an automatic control device, so that the reactor temperature can be controlled within a suitable range. Therefore, it is of great benefit to improve the reaction selectivity. The heat generated by the reaction can be discharged in time through the built-in heat exchanger. The structure of fluidized bed oxychlorination reactor is shown in Figure 5-2-06.
Air (or oxygen) enters from the bottom, and the air (or oxygen) is evenly distributed on the whole section through the multi-nozzle plate distributor. An air inlet pipe of C2H4 and HCl mixed gas is arranged above the plate distributor, and the air inlet pipe is connected with the distributor with the same number of nozzles as the air distributor, and the nozzles are just inserted into the nozzles of the air distributor. In this way, the two feed gases can be evenly mixed in the nozzle before entering the catalytic bed.
A certain number of vertical cooling pipes are arranged in the reaction section, and pressurized hot water is introduced into the pipes, and the reaction heat is taken away through the vaporization of water, and medium-pressure steam is produced as a by-product. Three groups of three-stage cyclone separators are arranged at the upper part of the reactor to recover the catalyst entrained by the reaction gas. The wear of catalyst is about 0. 1% every day, and the catalyst to be replenished is sent to the reaction section by compressed air from the upper part of the gas distributor.
Because water is produced by oxychlorination (and deep oxidation of ethylene), if some parts of the reactor are not well insulated and the temperature is too low, when the dew point temperature is reached, water will condense out and dissolve into hydrogen chloride gas to generate hydrochloric acid, which will cause serious corrosion to the equipment. Therefore, the insulation of the reactor is very important. In addition, if iron oxide adheres to the surface of the catalyst, it will be transformed into ferric chloride, which can catalyze the addition chlorination reaction of ethylene and generate by-product chloroethane (CH3CH2Cl). Therefore, the storage and transportation equipment and pipelines of catalyst cannot use iron materials.