Introduction to the Phosgenation Process for Amine--Basic Principles

The reaction between amine and phosgene (carbonyl chloride) to synthesize isocyanate is the most important method for producing isocyanates. Currently, large-scale isocyanate plants generally use the phosgenation production process.

Phosgene Generation

Carbon monoxide and chlorine gas are purified and dried before being mixed at a molar ratio of CO= 1.1:1 and introduced into a tubular reactor. Using activated carbon as a catalyst at a temperature of about 100°C, the conversion rate can reach as high as 98% to 100%. The reaction heat is removed by circulating hot water or oil.

Initially, a one-step high-temperature phosgenation method was used to prepare isocyanates. Due to numerous side reactions and low yields, this process was eliminated. It was later developed into a two-step phosgenation process: cold phosgenation followed by hot phosgenation.

Cold Phosgenation

Cold phosgenation takes place in the stable range of 0~70°C, primarily involving the following reactions:

R-NH2 + COCl2 → R-NHCOCl + HCl

R-NH2 + HCl → R-NH2·HCl (The above 2 are the main reactions)

R-NH2 + R-NHCOCl → R-NH2·HCl + R-NCO

R-NH2 + R-NCO → R-NHCONH-R

Hot Phosgenation

Hot phosgenation involves further reacting the cold phosgenation reaction liquid with phosgene in the stable range of 80-200°C. The primary reactions that occur during this process are as follows:

R-NHCOCl → R-NCO + HCl

R-NH2·HCl + COCl2 → R-NCO + 3HCl (The above 2 are the main reactions)

R-NH2·HCl + R-NCO → R-NHCONH-R + HCl

R-NHCONH-R + COCl2 → 2R-NCO + 2HCl + tar-like substances

From the phosgenation reaction process, it is evident that urea compounds formed from side reactions further consume phosgene, producing tar-like waste and reducing the yield of isocyanates. Therefore, to reduce the formation of urea compounds during cold phosgenation and increase the yield of isocyanate products, amine compounds are generally first reacted with hydrochloric acid or carbon dioxide to form amine hydrochloride or carbonate salts, especially for aliphatic compounds.

Industrial Production Methods for Aromatic Isocyanates

Despite the differences in equipment levels adopted by various companies, the industrial production methods for aromatic isocyanates generally follow these steps:

Amine compounds are dissolved in inert aromatic solvents such as aniline, o-dichlorobenzene, or toluene, and phosgene is also prepared in the same solvent solution (or in gaseous form). Both are reacted in a cold phosgenation reactor at temperatures below 70°C.

The slurry from the cold phosgenation reactor is transported to 1-3 series-connected hot phosgenation reactors. The temperature in each reactor is gradually increased, generally controlled within the range of 100~200°C (the upper temperature limit is mainly determined by the boiling point of the selected solvent), and additional phosgene is supplied for several hours.

The reaction mixture from the last hot phosgenation reactor is blown with inert gas such as nitrogen or methane for several hours to remove the by-product HCl and any unreacted residual phosgene. The solvent, isocyanate products, and residues are then distilled out. The yield of isocyanates is generally between 85% and 95%.

Phosgenation Production Processes

There are five main phosgenation production processes:

1. Batch reactor production process

2. Continuous reactor production process

3. High-temperature one-step production process

4. Pressurized production process

5. Tower continuous production process