Flow diagram of continuous solid state polymerisation

The reactor is fed with amorphous low viscosity PET chips. The reactor starts rotating and is heated to 120- 170 °C and maintained at this temperature long enough to allow the chips to become semi-crystalline and dry. When the chips are semi crystalline, their tendency to stick together at temperatures above the glass transition temperature (TG) of PET is much reduced. As an alternative, the chips can be crystallised prior to feeding it into the rotating reactor. 

The temperature of the product and the reactor wall has to be kept about 20 °C below the melting temperature of the polyester to prevent sticking. The reaction products, glycol and water, are continuously removed from the reactor to keep the polycondensation going. Therefore, the reactor is equipped with a vacuum system, which reduces the pressure inside the reactor to 5 mbar. The vacuum systems used are either based on a series of steam ejectors (option in combination with a watering pump) or dry vacuum pumps. 

In addition to the vacuum, in some cases nitrogen is injected into the reactor to further reduce partial pressures of the reaction products. Some acetaldehyde will be liberated from the chips during the crystallisation and solid state pol5mierisation phase. 

When the degree of polymerisation has reached the set value, the reactor is cooled down and pressurised with nitrogen. The product is then discharged, e.g. into a silo. 

The product is normally stored under nitrogen or dried air as the PET chips are very hygroscopic and the presence of small traces of water would already decrease the degree of polymerisation, especially during subsequent processing at elevated temperatures (hydrolysis reaction). 

---

Figure 10.4 Flow diagram of continuous solid state polymerisation...

---