Production process of chloroacetic acid

Currently, there are more than a dozen methods for producing chloroacetic acid, including those based on vinyl chloride, hydrolysis of chloroacetyl chloride, hydrolysis of trichloroethylene, chloroacetylene, tetrachloroethylene, glycerol, dichloroacetic acid, trichloroacetaldehyde, oxidation of chloroethanol, co-oxidation with ethylene, and catalytic chlorination of acetic acid. The main industrial methods are hydrolysis of trichloroethylene, the method using chloroacetyl chloride, and catalytic chlorination of acetic acid. The trichloroethylene hydrolysis method involves the hydrolysis of trichloroethylene to produce chloroacetic acid, using 93% sulfuric acid as a catalyst at a reaction temperature of 160–180°C, with the ratio of trichloroethylene to water being controlled. This method can produce chloroacetic acid of high purity, with a yield of up to 90%. However, its drawbacks include the generation of large amounts of hydrochloric acid as a by-product (2.57 tons of hydrochloric acid are produced per ton of product), high production costs, a long process flow, and elevated operational expenses. Currently, this method is mainly adopted by some chloroacetic acid manufacturers in Europe. The chloroacetyl chloride method involves the hydrolysis of chloroacetyl chloride under alkaline conditions to produce chloroacetic acid; due to limitations in the available raw materials, few manufacturers use this method these days. The acetic acid-catalyzed chlorination method is currently the most common approach for producing chloroacetic acid both domestically and internationally. It can be further divided into batch production processes and continuous production processes. The batch production process uses sulfur powder as a catalyst, with its amount being controlled at approximately 3% of the total mass of acetic acid. The reaction involves two-stage sequential chlorination: chlorine gas is introduced into the main reactor at 90°C, with the reaction temperature maintained between 96–100°C; the reaction temperature in the secondary reactor is kept between 85–90°C. The reaction is complete when the density reaches 1.35. After maintaining the reaction for 1 hour, the circulating mother liquor is added to facilitate cooling and crystallization. Seeds are added at a temperature 1–2°C above the freezing point, and the mixture is cooled slowly to around 25°C; the product is then obtained through filtration or centrifugation. Hydrogen chloride from the exhaust gases is sent to a packing absorption tower to recover by-product hydrochloric acid. At present, this method has been phased out abroad, while manufacturers in our country mainly use it for production. Although this production method is relatively simple, it results in high consumption, low yields, poor quality of the products obtained, long production cycles, high production costs, and severe pollution from waste gases, wastes, and wastewater. The sulfur powder used as a catalyst not only contaminates the main product but also the by-product hydrochloric acid; it can sometimes cause blockages in piping equipment, disrupting production and limiting the range of applications for the products. The continuous production process for chloroacetic acid uses acetic acid and liquid chlorine as raw materials, with acetic anhydride and sulfuric acid as catalysts, to produce chloroacetic acid through processes such as acetic acid chlorination, distillation, crystallization, separation, and drying. The product obtained by this method has high quality, requires less raw material, and is not overly demanding regarding the chlorine content of the raw materials; it can be produced using liquid chlorine exhaust or gaseous chlorine. The downside is that the reaction conversion rate is low, at around 45%, which increases steam and electricity consumption. Currently, this method is the primary approach used worldwide for the production of chloroacetic acid. Large-scale manufacturers of chloroacetic acid in countries such as the United States, Japan, Germany, the Netherlands, and Canada all employ this method for production.