Introduction to the dehydration process in the synthesis of vinyl chloride from hydrogen chloride and acetylene

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PVC production is divided into the ethylene route and the acetylene route, based on the raw materials used for producing vinyl chloride; the acetylene route can further be split into the calcium carbide acetylene method and the natural gas acetylene method. In our country, only a few companies such as Shanghai Chlor-Alkali, Qilu Petrochemical, Beijing Huaer II, Tianjin Dagu Chemical, and Jinhua Chlor-Alkali use the ethylene oxychlorination process; some enterprises along the eastern coast produce PVC using imported VCM or EDC. At present, the raw material route for vinyl chloride production in China is primarily based on the acetylene process using calcium carbide as a starting material; in this process, conversion is the key step. The conversion mixture gas is composed of acetylene gas and hydrogen chloride gas mixed in a ratio of 1:1.05–1.10, with the water content (on a weight percentage basis) required to be less than 0.06%. The sources of moisture in mixed gases are varied; for example, hydrogen chloride gas can carry moisture with it ； Acetylene gas contains moisture ； When both are present, the moisture level exceeds the limit, along with other issues related to moisture. If the moisture level is too high, it will directly affect the proper progress of the entire production process; in severe cases, it can even cause the entire production system to come to a halt. Poor mixing and dehydration effects will impact the production capacity as well as the service life of the mercury chloride catalyst. Therefore, to ensure long-term stable operation, it is essential to strictly control the moisture content in the mixed gas. In most of the processes in our country for producing vinyl chloride via the calcium carbide-acetylene method, a mixed dehydration approach is employed. The purified acetylene gas supplied from the acetylene plant is cooled to around 5°C using flame arresters and acetylene pre-coolers. This gas is then mixed with hydrogen chloride gas supplied from the hydrogen chloride plant; the flow rates of both gases are controlled separately so that the molecular ratio of acetylene to hydrogen chloride is 1:1.05–1.1. After thorough mixing in a mixer, the mixture enters a series of graphite coolers, where it is indirectly cooled using frozen brine at -35°C. As a result, the majority of the moisture in the gas is converted into 40% hydrochloric acid mist (in the form of an aerosol in the gas phase), which enters a series of acid mist filters. The silica oil and glass wool in these filters capture and separate the mist, which is then discharged from the bottom. The dry mixture is preheated in a preheater to reduce the relative humidity, and is fed into the converter under control of a flow meter. The temperature for mixed cryogenic dehydration of acetylene and hydrogen chloride is generally controlled at -14±2°C. During mixed freeze-drying, the moisture in the feed gas is absorbed by hydrogen chloride, resulting in the precipitation of a 40% hydrochloric acid mist. The moisture content of the mixture depends on the vapor pressure of the 40% hydrochloric acid solution at that temperature; in other words, the lower the freeze temperature in the graphite condenser and acid mist filter, the lower the moisture content. When the temperature drops below -20 to -25°C, hydrate crystals are likely to form, blocking the tubes of the graphite condenser; therefore, the temperature should not be controlled too low. Our company uses the natural gas-acetylene method, employing a process in which acetylene and hydrogen chloride are dehydrated and dried separately before being mixed together. That is, the acetylene gas is first washed with an alkali, then dried with sulfuric acid, and subsequently sent to the mixer. After passing through the hydrogen chloride adsorber (which removes free chlorine), hydrogen chloride enters the hydrogen chloride cooler, where it is cooled indirectly by -35°C chilled brine. Most of the moisture is removed from it, and then it passes through a hydrogen chloride acid mist filter to further remove hydrochloric acid mist. At this point, the volume fraction of water vapor in the gas stream is very low; the water exists mainly in the form of gaseous water molecules, which cannot be captured by the mist collector. Based on the process principle of \"cooling first, then drying,\" we employ a mature system of gas absorption and mass transfer to carry out the dehydration process, in which water vapor in the gas phase is absorbed by sulfuric acid. After drying and dehydration, the final water content in the gas phase generally depends on the vapor pressure of water above the sulfuric acid level in the last contact vessel; in other words, it depends on the concentration and temperature of the sulfuric acid used as a desiccant in the last mass transfer and absorption vessel. Therefore, we adopted a dehydration process in which the material is preheated using a sulfuric acid preheater (to prevent sulfuric acid crystallization), then dried in three stages of sulfuric acid dryers (with the sulfuric acid concentration increasing sequentially in each dryer), after which it passes through a sulfuric acid mist filter before entering the mixer.