On the feasibility of gas-phase catalytic cracking of 1,2-dichloroethane to produce vinyl chloride via hydrogen chloride removal

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:Lol 1,2-Dichloroethane gas-phase catalytic cracking for hydrogen chloride removal to produce vinyl chloride – Feasibility study Overview Vinyl chloride (H2C=CHCl) is an important monomer used in the polymer industry, and it can be produced from ethylene or acetylene. It is a colorless, easily liquefiable gas with a boiling point of -13.9°C, a critical temperature of 142°C, and a critical pressure of 5.22 MPa. It is mainly used in the production of polyvinyl chloride, and can be copolymerized with vinyl acetate, acrylonitrile, acrylates, vinylidene chloride (1,1-dichloroethylene), etc., to produce resins with various properties. Furthermore, it can also be used to synthesize 1,1,2-trichloroethane and 1,1-dichloroethylene, among others. In 1835, the Frenchman V. Leniot first obtained vinyl chloride by treating dichloroethane with potassium hydroxide in an ethanol solution. In the 1930s, the German company Grisham Elektronik achieved the industrial production of vinyl chloride for the first time, based on the addition of hydrogen chloride to acetylene. In the early days, vinyl chloride was produced by the catalytic addition of calcium carbide, acetylene, and hydrogen chloride, a method commonly referred to as the acetylene process. Later, with the development of the petrochemical industry, the synthesis of vinyl chloride rapidly shifted to a process route using ethylene as a raw material. In 1940, United Carbon Corporation developed the dichloroethane method. To balance the use of chlorine, Nippon Wuho Chemical Industry Company developed a combined method that integrates the acetylene process and the dichloroethane process for the production of vinyl chloride. In 1960, the American company Dow Chemical developed a method for synthesizing vinyl chloride from ethylene through oxychlorination, and by combining it with the dichloroethane method, created a complete process for producing vinyl chloride from ethylene; this method experienced rapid development. Other methods such as the acetylene method and the mixed alkyne method are gradually being phased out due to their high energy consumption. The characteristics of the production methods using ethylene, acetylene, and mixed olefins/acetylenes are as follows: Ethylene oxychlorination method: This is the main method used in industrial production of vinyl chloride today. It is carried out in three steps: the first step is the chlorination of ethylene to produce dichloroethane ； The second step involves the thermal cracking of dichloroethane to produce vinyl chloride and hydrogen chloride ； In the third step, ethylene, hydrogen chloride, and oxygen undergo an oxychlorination reaction to produce dichloroethane. 　　①Ethylene chlorination: The addition reaction between ethylene and chlorine takes place in a liquid phase:　　　　　　 CH2=CH2+Cl2→CH2ClCH2Cl. Catalysts such as ferric trichloride or copper chloride are used, with dichloroethane serving as the reaction medium. The reaction heat can be removed by cooling water or the vaporization of the product dichloroethane. The reaction temperature ranges from 40 to 110°C, and the pressure is between 0.15 and 0.30 MPa; both the conversion and selectivity of ethylene exceed 99%. 　　 ②The reaction equation for the thermal cracking of dichloroethane to produce vinyl chloride is: ClCH2CH2Cl → CH2=CHCl + HCl. This is a highly endothermic reaction that takes place in a tubular cracking furnace, at a temperature of 500–550°C and a pressure of 0.6–1.5 MPa ； Control the one-way conversion rate of dichloroethane at 50%–70% to suppress the occurrence of side reactions. The main side reactions are:　　　　　　　 CH2 =CHCl → HC=CH + HCl　　　　　 CH2 =CHCl + HCl → ClCH3CHCl　　　　　　　 ClCH2CH2Cl → 2C + H2 + 2HCl The decomposition products enter a quenching tower, where they are cooled with recycled dichloroethane to prevent further side reactions. After the product temperature is cooled to 50–150°C, it enters the dehydrochlorination tower. The bottom of the tower contains a mixture of vinyl chloride and dichloroethane; this mixture is distilled in a vinyl chloride distillation column, yielding high-purity vinyl chloride at the top of the column. The heavy components at the bottom are mainly unreacted crude dichloroethane, which, after having impurities removed through distillation, is still used as a feedstock for thermal cracking. 　　 ③Oxychlorination reaction: Copper chloride supported on γ-alumina is used as a catalyst, with alkali or alkaline earth metal salts serving as co-catalysts. The main reaction is: 　　　　 H2C=CH2 + 2HCl + ½O2 → ClCH2CH2Cl + H2O. The main side reactions are the complete oxidation of ethylene (yielding carbon monoxide, carbon dioxide, and water) and the oxychlorination of vinyl chloride (yielding various chlorides of ethane). The reaction temperature is 200–230°C, the pressure is 0.2–1 MPa, and the molar ratio of the feedstocks ethylene, hydrogen chloride, and oxygen is 1.05:2:0.75–0.85. Reactors come in two forms: fixed-bed and fluidized-bed. Fixed-bed reactors typically use tubular types, with granular catalyst filled inside the tubes. The raw materials, ethylene, hydrogen chloride, and air, pass through the catalyst bed from top to bottom. Pressured hot water is used as a heat carrier between the tubes to remove the heat generated by the reaction, producing steam at a pressure of 1 MPa as a by-product. The temperature in a fixed-bed reactor is difficult to control; in order to achieve a more reasonable temperature distribution, large amounts of inert gas are often used as a diluent, or solid substances are incorporated into the catalyst. The selectivity for dichloroethane can exceed 98%. 　 During the ethylene oxychlorination reaction in a fluidized bed reactor, fine-particle catalysts are used. The raw materials – ethylene, hydrogen chloride, and air – enter the reactor from the bottom; after thorough mixing, they are passed through the catalyst layer, keeping the catalyst in a fluidized state. A heat exchanger is installed within the reactor to effectively remove the heat generated by the reaction. The reaction temperature in this type of reactor is uniform and easy to control, making it suitable for large-scale production; however, the reactor structure is complex and the catalyst suffers significant wear. 　　The reaction product exiting the reactor is quenched with water and then condensed into liquid crude dichloroethane. The uncondensed portion of dichloroethane in the condenser, along with unconverted ethylene, inert gases, etc., are processed through steps such as solvent absorption to recover the dichloroethane. The resulting crude dichloroethane is refined before being fed into a pyrolyzer for cracking. 　　The main advantage of the ethylene oxychlorination method is the use of hydrogen chloride produced by the thermal cracking of dichloroethane as a chlorinating agent, thereby enabling complete utilization of chlorine. Acetylene method: In the presence of a mercury chloride catalyst, acetylene reacts with hydrogen chloride to directly produce vinyl chloride: CH=CH + HCl → CH2=CHCl. The process can be divided into three stages: the production and purification of acetylene, the synthesis of vinyl chloride, and the purification of the product. In the acetylene generator, calcium carbide reacts with water to produce acetylene, which is then refined, mixed with hydrogen chloride, dried, and fed into a tubular reactor. The tube is filled with a mercury chloride catalyst using activated carbon as a carrier (the content is generally 10% of the carrier’s mass). The reaction takes place at atmospheric pressure, with pressurized circulating hot water (97–105°C) used outside the tube to remove the heat of reaction and maintain the bed temperature between 180–200°C. The acetylene conversion rate reaches 99%, and the vinyl chloride yield is over 95%. The byproduct is 1,1-dichloroethane (about 1%), with small amounts of vinyl acetylene, dichloroethylene, trichloroethane, etc. as well. This method features simple processes and equipment, low investment, and high yield ； However, it has high energy consumption, high raw material costs, and the mercury salts used as catalysts are highly toxic; in addition, it is subject to restrictions related to safe production and environmental protection, making large-scale production unsuitable. 　　 Gas-phase catalytic dehydrochlorination method: This process utilizes a specific catalyst for gas-phase catalytic dehydrochlorination, thereby producing vinyl chloride monomer and hydrogen chloride gas. The simple process flow is shown in the figure below. A comparison between the high-temperature pyrolysis process and the gas-phase catalytic pyrolysis process is provided below. From the above data, we can conclude that: 1. The gas-phase catalytic pyrolysis method can increase the conversion rate of the product, with a conversion rate of around 95%. 2: The gas-phase catalytic cracking method can reduce costs by about 200 yuan per ton compared to high-temperature cracking. 3: The gas-phase catalytic cracking method operates at low temperatures, resulting in a mild reaction process that meets the current requirements for energy conservation and emission reduction. 4: The project requires minimal investment, allowing for extensive use of existing equipment and processes in the modifications.