Introduction to Optical Chemical Processes - Tower Continuous Production Process

The tower continuous process is currently one of the most advanced production methods for preparing organic isocyanates. This process is characterized by low phosgene consumption, high yield, large production capacity, stable product quality, and the potential for automation and large-scale production.

In the tower continuous production process, the key equipment includes the phosgenation tower, stripping tower, and thin film evaporator. The height-to-diameter ratio of the phosgenation tower ranges from 5 to 30:1, ideally between 5 and 10:1. The tower is packed with Raschig rings or contact plates to enhance gas-liquid contact. The temperature distribution in the phosgenation tower gradually increases from 40°C at the top to 180°C at the bottom. The bottom temperature depends on the solvent's boiling point: for chlorobenzene, it is 130°C; for o-dichlorobenzene, it is 180°C. Generally, the tower's temperature is divided into two sections: the upper part at 40-100°C and the lower part at 100-180°C. The stripping tower temperature is controlled below the solvent's boiling point, with dry ammonia or methane gas introduced at the bottom to remove HCl and COCl₂ from the phosgenation tower's reaction material. The thin film evaporator, which can be vertical, horizontal, falling film, or rising film type, serves to vacuum-evaporate the solvent and low-boiling substances.

The tower continuous production process flow is shown in the following diagram.

New Process for Tower Continuous Phosgenation for Producing Polyisocyanates

The advantages of this process are as follows:

1. The polyamine chlorobenzene solution directly enters the tower for reaction, eliminating the need for polyamine salt equipment and overcoming the high viscosity and difficult transport issues of polyammonium chloride slurry.

2. The material flow in the phosgenation tower is continuous and convective, which helps to promptly remove the HCl gas generated in the reaction, making full use of phosgene without requiring additional phosgene.

3. The use of a scraper-type thin film evaporator results in a short exposure time to high temperatures, producing fewer polymers and ensuring low product viscosity.

4. The entire process pipeline is sealed with a dryer at the outlet, preventing moisture from entering the system and causing equipment corrosion or product crosslinking and polymerization.

5. All control points of the process are centralized, allowing for automated production.

Process conditions are as follows:

1. Concentration of polyamine chlorobenzene solution: 5% - 20%;

2. Phosgene/amine molar ratio: 1.1 - 1.2;

3. Pressure: atmospheric or slightly higher;

4. Residence time in the phosgenation tower: less than 2 hours;

5. Temperature distribution: tower top temperature 40-50°C; tower middle temperature 100-110°C; tower bottom temperature 120-130°C;

6. Solvent: chlorobenzene, water content <100mg/kg.

The process is roughly as follows: The organic polyamine solution is metered and pumped from the top of the tower, while phosgene enters from the bottom. The amount of phosgene used is the theoretical amount required for the organic amine or 10-20% excess. HCl gas and residual phosgene generated in the phosgenation tower are discharged after condensing and recycling chlorobenzene at the top of the tower, with reaction products flowing from the bottom to an intermediate storage tank. The crude reaction product, containing trace amounts of HCl and COCl, is metered and pumped into the stripping tower's upper part, with dry nitrogen gas introduced from the bottom to remove HCl and phosgene at 130-180°C. The isocyanate solvent from the stripping tower bottom is vacuum-evaporated to remove the solvent. The solvent is condensed and recycled. The product from the bottom of the vacuum thin film evaporator is crude isocyanate or PAPI. For high-purity MDI, further distillation is required.