Chemical processes for the production of polypropylene. There are three types of chemical processes for the production of polypropylene: suspension process, bulk process with liquid-phase monomer, and bulk process with gas-phase monomer.
For the purposes of this paper, we will explain the bulk process known as Spheripol:
- The Spheripol process is a continuous process based on a superactive catalyst (Ti/MgCl₂), an alkylaluminum, and an electron donor.
- There are two reactors in series:
The first is designed for bulk processing, and the second for the gas phase.
The first reactor is a loop-type reactor, in which the catalyst and monomer are circulated at high speed so that they remain in suspension in the diluent.
The diluent is actually the liquid propylene itself, which, given the operating conditions, facilitates the removal of heat generated by the reaction and at the same time increases the efficiency of the catalytic system. In this loop reactor, the reactant mixture is driven by an axial pump (no external agitation is required); it reaches high velocity, which prevents polymer deposition on the reactor walls; the heat transfer area per unit volume is higher than in an agitated reactor, allowing for better temperature control and higher productivity. The residence time in the reactor can be adjusted by varying the recirculation rate.
Propylene, which can be diluted with propane (up to 40%), is fed into the reactor in the presence of a catalyst and hydrogen. The reaction temperature ranges from 65 to 80 Cº, and the pressure is 33 atm. The reaction mixture circulates rapidly and contains up to 50% solids.
- The polymer and unreacted monomer are separated in a flash tank at 16 atm. The monomer is recirculated to the reactor, and the excess propane is removed.
- In the second reactor—the gas-phase reactor—the polymer produced in the loop reactor is fed in. In this phase, copolymers with special properties are prepared by adding a comonomer (such as ethylene) in addition to the monomer.
- Polymer particles and water vapor are fed into the fluidized-bed reactor to deactivate the catalyst. Drying is performed with nitrogen. The polymer is obtained in the form of spherical particles with a diameter between 1.5 and 2 mm and a narrow size distribution. The productivity is on the order of 20 kg per gram of supported catalyst (1,000 kg per gram of Ti). The isotacticity index is 97%.
- The molecular weight is controlled by adding hydrogen during the reaction, as well as by using organometallic compounds derived from zinc or cadmium.