Chloride in coal tar is divided into inorganic chlorides and organic chlorides. Sodium chloride in inorganic chlorides is not easy to be hydrolyzed, but magnesium chloride and calcium chloride are very easy to be hydrolyzed by heat. Their hydrolysis temperatures are 120℃ and 175℃ respectively. After hydrolysis, hydrogen chloride is generated as a corrosive medium. When organic chlorides exist alone, they do not cause corrosion to equipment. However, under conditions of high temperature, high pressure, and the presence of hydrogen gas, hydrogen chloride is generated. When water is present, it has strong corrosiveness to equipment. In addition, due to the unbound lone pair electrons and great electron affinity of fluorine and chlorine, it is easy to react with metal ions, and the chloride ions also have high mobility, which often migrate downstream with the process, resulting in full bed catalyst poisoning.