Recovery of chloroacetic acid

Many chloroacetic acid plants are now recycling chloroacetic acid, but I don’t know the process.

The composition of chloroacetic acid mother liquor is generally as follows: acetic acid 15-20%; monochloroacetic acid 30-45% ； Dichloroacetic acid 30-40% ； A small amount of free chlorine and acetyl chloride. At present, many chloroacetic acid manufacturers generally use crude steaming to separate the acetic acid first, and then use azeotropic vacuum distillation to steam out most of the monochloroacetic acid, causing it to crystallize and precipitate at cold temperature, and then further react the mother liquor to generate by-products to further recover monochloroacetic acid and dichloroacetic acid.

Chloroacetic acid was first discovered in 1841. In an experiment in 1857, acetic acid was subjected to a chlorination reaction under direct sunlight to produce chloroacetic acid. At present, there are more than ten production methods for chloroacetic acid, including vinyl chloride, chloroacetyl chloride hydrolysis, trichlorethylene hydrolysis, chloroacetylene, tetrachloroethylene, glycerol, dichloroacetic acid, trichloroacetaldehyde, chloroethanol oxidation, ethylene co-oxidation and acetic acid catalytic chlorination. There are three main industrial production methods: trichlorethylene hydrolysis, chloroacetyl chloride and acetic acid catalytic chlorination. The trichlorethylene hydrolysis method is a hydrolysis reaction of trichlorethylene using 93% sulfuric acid as a catalyst and a reaction temperature of 160-180°C to generate chloroacetic acid by controlling the ratio of trichlorethylene and water. This method can achieve high-purity chloroacetic acid, with a yield of 90%. The disadvantage is that there is a lot of by-product hydrochloric acid (each ton of product can produce 2.57 tons of 30% hydrochloric acid by-product), the production consumption quota is high, the process is long, and the production cost is high. At present, this method is mainly used by some chloroacetic acid manufacturers in Europe. The chloroacetyl chloride method is the hydrolysis of chloroacetyl chloride under alkaline conditions to obtain chloroacetic acid. Due to limitations of raw materials, this method is currently used by few manufacturers. The catalytic chlorination of acetic acid is currently the most important method for producing chloroacetic acid at home and abroad. It can be divided into two types: intermittent production technology and continuous production technology. In the intermittent production process, sulfur powder is generally used as a catalyst, and its dosage is controlled to be about 3% (mass percentage) of the total amount of acetic acid. The reaction adopts two-stage series chlorination. The main pot is vented with chlorine gas at 90°C, and the reaction temperature is controlled to 96-100°C. The reaction temperature of the auxiliary pot is 85-90°C. The reaction end point is when the reaction end point density is 1.35. After 1 hour of heat preservation reaction, add circulating mother liquor to cool and crystallize, add seed crystals at 1-2°C above the freezing point, slowly cool to about 25°C, and obtain the product through suction filtration or centrifugal separation. The tail gas hydrogen chloride is sent to the packed absorption tower to recover the by-product hydrochloric acid. At present, this method has been eliminated abroad, and manufacturers in my country mainly use this method for production. Although the production process of this method is relatively simple, it has high consumption, low output, poor product quality, long production cycle, high production costs, and serious waste pollution. The catalyst sulfur powder not only pollutes the main product, but also pollutes the by-product hydrochloric acid. Sometimes it also causes blockage of pipeline equipment, causing abnormal production and limiting the application scope of the product. The continuous chloroacetic acid production process uses acetic acid and liquid chlorine as raw materials, acetic anhydride and sulfuric acid as catalysts, and produces chloroacetic acid through acetic acid chlorination, distillation, crystallization, separation, drying and other processes. The product obtained by this method has high quality, low raw material consumption, and is not too demanding on the raw material chlorine. It can be produced with liquid chlorine tail gas or gas chlorine. The disadvantage is that the reaction conversion rate is low, only about 45%, which increases steam consumption and electricity consumption. At present, this method is the main method for producing chloroacetic acid in the world. Large-scale chloroacetic acid production enterprises in the United States, Japan, Germany, the Netherlands, Canada and other countries all use this method for production.