VCM production process and characteristics

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VCM Production Process and Characteristics VCM Production Process I. Classification of production processes: VCM production can be classified into the calcium carbide method (natural gas acetylene method), the combined method, the oxychlorination method, and the balanced oxychlorination method. Compared with several production processes, the calcium carbide method and the combined method have disadvantages such as high energy consumption, high costs, mercury pollution, and serious problems with calcium carbide slag. In the balanced oxychlorination process, hydrogen chloride is in balance during production and does not need to be supplied from outside. It offers advantages such as large scale, low energy consumption, good economic efficiency, and environmental friendliness, making it one of the more advanced and cost-effective production methods in the world today. The main patent holders for oxychlorination production include Japan’s Mitsui & Co., Germany’s HOECHST, EVC, SOLVAY, and the United States’ GEON, DOW, PPG, etc. Production of VCM in our country began in 1958. In 2001, the total capacity for producing PVC via the balanced oxychlorination process was 870,000 tons per year. Among these, Shanghai Tianyuan Chlor-Alkali Co., Ltd. (300,000 tons per year), Qilu Petrochemical Company (230,000 tons per year), Beijing Chemical Industry No. 2 Factory Co., Ltd. (160,000 tons per year), Bohai Chemical Group Dagu Chemical Plant (100,000 tons per year), and Jinhua Chemical Group Co., Ltd. (80,000 tons per year) became the key large-scale enterprises in China’s VCM and PVC production sector. II. Characteristics of the production process: This project uses ethylene and chlorine as raw materials, and the balanced oxygen chlorination process will be employed. The balanced oxychlorination process for producing VCM mainly consists of direct chlorination, oxychlorination, EDC cracking and distillation, as well as VCM distillation. 1. Direct chlorination unit: Ethylene reacts with chlorine in the presence of an iron(III) trichloride catalyst to produce dichloroethane (EDC). Depending on the reaction conditions and the method of collecting the EDC, this process is divided into high-temperature chlorination and low-temperature chlorination techniques. ⑴ High-temperature chlorination: Ethylene and chlorine undergo a direct chlorination reaction at low pressure and around 90°C. The characteristics of high-temperature chlorination production are that the product exiting the EDC vapor column does not carry away the catalyst; therefore, no catalyst needs to be added, nor is washing to remove iron or drying required, and no wastewater is generated ； The product has a high purity of EDC, allowing it to be subjected to pyrolysis directly without the need for purification ； The heat of reaction can be removed easily, with high thermal efficiency. The drawback is that all equipment in contact with liquids must be made of alloy steel or stainless steel, resulting in high investment costs. ⑵ Low-temperature chlorination: Ethylene and chlorine react at around 50°C; the EDC is obtained as a liquid product. The heat generated by the reaction is removed using a complex external circulation cooler. EDC must be washed with water to remove the catalyst, followed by alkali washing and drying, resulting in a lengthy process. The characteristic of low-temperature chlorination is high catalyst consumption, which requires frequent replenishment; however, new types of chlorination reactors do not have this issue of catalyst replacement. The reaction heat cannot be utilized, resulting in high energy consumption. There is some exhaust loss, and the oxygen contained in the exhaust must be diluted with nitrogen, resulting in increased nitrogen consumption. The advantage of this method is its simple equipment structure; it is made of carbon steel, which reduces equipment investment. Currently, traditional low-temperature chlorination techniques have been largely phased out. Table 1: Comparison between high-temperature chlorination and low-temperature chlorination
High-temperature chlorination Low-temperature chlorination
Reaction conditions: FeCl3 catalyst, 90°C; FeCl3 catalyst, 50°C
1. EDC from the top of the gas phase tower 1. EDC from the liquid phase
2. No need to replenish the catalyst 2. Reaction heat is cooled by an external circulation cooler
3. No need for water washing for iron removal; product can be dried directly, with no wastewater generated 3. EDC requires water washing, alkali washing, and drying
4. The resulting EDC has high purity; it can be dried directly, and the process is short; no further purification is needed before cracking 4. A large amount of catalyst is required, and it needs to be replenished regularly
5. Reaction heat can be easily removed, allowing for efficient heat management 5. Reaction heat cannot be utilized, resulting in higher energy consumption
6. All equipment in contact with liquids must be made of alloy steel; this increases investment costs. Oxygen in the gas mixture must be diluted with nitrogen, leading to increased nitrogen consumption 6. There is some waste gas loss; the gas mixture contains oxygen, which must be diluted with nitrogen
7. The equipment structure is simple, and it can be made of carbon steel, reducing investment costs 7.

Abroad: Mitsui East Asia, GEON, HOECHST
Domestically: Beijing Huahere Co., Ltd., Shanghai Tianyuan Group, Dagu Chemical Plant, and Qilu Petrochemical Company (Mitsui Toyo Engineering & Construction technology)
2. Oxidochlorination unit: The oxidochlorination reaction uses ethylene, hydrogen chloride, and oxygen as raw materials, with a copper chloride catalyst to produce EDC. The main technical keys of the oxychlorination unit are the type of reactor and the source of oxygen. ⑴ Types of reactors: There are two types of reactors, namely bubbling bed reactors and fixed bed reactors. ① Boiling bed reactor: The boiling bed reactor is equipped with internal cooling coils that enable heat removal; a cyclone separator is located at the top of the reactor to collect the catalyst entrained in the material. During operation, ethylene, oxygen, and hydrogen chloride are simultaneously added to the reactor in a certain ratio. ②Fixed-bed reactor: Fixed-bed reactors generally have a relatively low production capacity per unit; several such reactors are often connected in series. However, it is now possible to build reactors with much larger capacities, with one reactor being capable of handling 750,000 tons per year. Oxygen is introduced into the reactor in different proportions, with the aim of keeping the reaction away from the explosive range, as well as reducing the formation of CO and CO2 due to the peroxidation of ethylene. Fixed-bed and fluidized-bed oxychlorination reactors each have their advantages and disadvantages, which is why their market shares are roughly equal. Table 2: Comparison between bubbling bed reactors and fixed-bed reactors Bubbling bed reactor Fixed-bed reactor 1. Easy to operate, quick to start up, and simple to shut down. The operational load has high elasticity. 2 Low material backmixing, high conversion rate. 3. Due to the presence of reaction hot spots in the fixed-bed, there are relatively many side reactions. 4 Since the local temperature inside the reaction tube may be lower than the material’s dew point, the materials required for the reactor are of high quality, resulting in high manufacturing costs. There is high resistance, resulting in high kinetic energy consumption. But the reactor has a long service life. 5 The labor intensity for loading and unloading the catalyst is high, yet the interval between such operations is long. Advantages and disadvantages of Dagu Chemical Plant: 1. Each reactor has a large production capacity, high efficiency; catalyst loading and unloading are convenient, and the requirements for the material of the equipment are not high. The temperature distribution in the two bed layers is uniform, resulting in fewer side reactions. It features high heat transfer efficiency, allowing reaction heat to be removed promptly, and can produce steam at high pressure as a by-product. 3 Low raw material consumption due to the use of a circulation system ； It also increased the operational flexibility of the reactor. 4) The reactor is complex to manufacture, requires high operational standards, and is less convenient for starting up and shutting down compared to fixed-bed reactors. 5) There is significant backmixing of the materials, resulting in a conversion rate slightly lower than that of a fixed-bed reactor. Application at home and abroad ⑵ Pure oxygen oxidation and air oxidation: Depending on the source of oxygen, it is divided into pure oxygen oxidation and air oxidation. The comparison between the two is as follows: Table 3 Comparison between pure oxygen oxidation and air oxidation. Air method vs. Oxygen method: Source of oxygen – Abundant and inexpensive; High cost. Catalyst consumption – Large initial investment; Small initial investment with high reaction efficiency. Equipment investment – Lower one-time investment; no need for an air separation unit, but an additional unit is required, resulting in slightly more exhaust gas. Absorption/desorption units: Environmental pollution – High volume of exhaust gas, leading to significant environmental impact; Low volume of exhaust gas, allowing for combustion and thus benefiting the environment. Safety – Less safe than the pure oxygen method; operates under non-explosive gas conditions, making it relatively safer. Ethylene consumption – Slightly higher; Ethylene consumption can be reduced. Major manufacturers: Mitsui & Co., GEON, PPG, and HOECHST. 3. EDC cracking unit: The main equipment in this cracking unit is the EDC cracking furnace, and most companies use single or double row horizontal coil-type cracking furnaces. However, there are two types of feed conditions for EDC: gas-phase or liquid-phase feeding. Gas-phase feeding can extend the operating cycle of the cracking furnace, but it requires higher capital investment. The material is first vaporized and heated before entering the pyrolysis furnace; nowadays, a combination of an evaporator and heat recovery from pyrolysis is commonly used. Due to the extended operation cycle of the pyrolysis furnace and the fact that the benefits of efficient heat utilization outweigh the costs of investing in additional equipment, it has gradually been adopted by various manufacturers. The process technologies for each unit of VCM plants have now become mature and reliable. Although there are technical differences, they are also moving toward convergence. Companies that hold patents for VCM production continue to make new advances in areas such as extending equipment operational time, reducing raw material consumption, recovering thermal energy, and improving the quality of the monomer. China has introduced both high-temperature chlorination and low-temperature chlorination from the direct chlorination process ； Boiling bed and fixed-bed reactors for oxychlorination have been introduced, as well as oxygen-based and air-based processes ； The process with two feed methods—gas-phase and liquid-phase EDC—has also been introduced in the EDC cracking unit. The products of these manufacturing enterprises enjoy a strong market presence and good profitability in the domestic market. Based on the above analysis and comparison, high-temperature direct chlorination is recommended for this device ； Fluidized bed, pure oxygen oxychlorination ； EDC gas-phase feed cracking process. Companies that hold patent technologies for the aforementioned processes include GEON in the United States, Mitsui Toyo Engineering & Construction in Japan, Hoechst in Germany, and Equinor in Norway. A comparison of the process technologies of Mitsui & Co., GEON, Heston, and EVC is as follows: Table 4 Process Technology Comparison. Mitsui & Co., GEON, Heston: High-temperature chlorination; High-temperature chlorination; EVC: High-temperature chlorination, direct process, sequential chlorination. Product discharge method: Gas-phase discharge; Heat removal method: The material is discharged via gas phase and used as a heat source for the EDC high-boiling tower. Most of the reaction heat is utilized; no washing required for the product. Alkali washing is needed in some cases. Process: High-temperature chlorination + minor low-temperature chlorination; Gas-phase discharge; Flash vapor discharge. Most of the material is discharged via gas phase, while the rest is discharged along with the reaction mixture and used as a heat source for the EDC high-boiling tower. Water cooling is used for heat removal. Oxygen reactor: Bubble bed; Catalyst type: 304SS; Oxygen source: Pure oxygen; Catalyst consumption: 0.02 kg/t of VCM; Catalyst needs to be replenished regularly; Easy to load and unload. Chlorination-hydrogenation process: Cracking; Catalyst type: Nickel tubes/steel; Oxygen source: Pure oxygen; Catalyst consumption: 0.05 kg/t of VCM; Needs to be replenished regularly; Easy to load and unload; Hydrogenation is available. Fixed-bed reactors: Three reactors in series; Catalyst type: Nickel tubes/steel; Oxygen source: Pure oxygen; Catalyst needs to be replaced every 2–4 years; Difficult to load and unload; No hydrogenation. Horizontal multi-tube reactors: Reactor material: 347SUS; Gas-phase feeding; Heat recovery is optional; Efficiency: 60%. Three towers plus one evaporation tank; Products from direct chlorination go directly into the EDC high-boiling tower. Three towers; Products from the VCM stripping tower are sent back to the HCl tower. Products from direct chlorination go directly into the EDC high-boiling tower. Two towers; VCM products are dried using solid alkali. Both liquid and gas phases can be handled with heat recovery options available. Cracking efficiency: 55%. Two distillation towers or four towers (five towers when not balanced). Ethane is directly chlorinated; heat exchange is used in direct chlorination processes; Products are directly fed into the EDC high-boiling tower. Chlorine distillation towers: Three towers; VCM distillation towers: Three towers; VCM stripping tower. Products are washed with alkali and then dried. VCM products are treated using solid agents. Product quality: VCM purity ≥ 99.99 wt%; VCM purity ≥ 99.98 wt%. III. Source of technology: China has carried out extensive work on assimilating and adapting foreign technologies, while also making efforts to develop its own technologies in various areas. The VCM unit of this project utilizes purchased process packages, a small number of key equipment items, and a DCS control system, with the vast majority of the equipment being domestically produced. The process flow of the VCM plant mainly consists of several sections: direct chlorination, oxychlorination, EDC cracking, EDC distillation, and VCM distillation. Among them, two trains are used for the EDC cracking process, while one train is used for all other processes. . The VCM plant was engineered and equipped with equipment procured domestically. The technologies used abroad are fairly similar; for VCM, one may consider the technologies offered by GEON in the USA or HOECHST in Germany.