In these systems, the growing polymer particles (containing the catalyst) are suspended directly in a gaseous medium consisting essentially of the monomer.
Strictly speaking, polymerization does not occur in the gas phase; in fact, only the fluid medium is gaseous. The chemical reaction continues to take place in the microreactors that make up each of the support particles, the catalyst, and the growing polymer.
Naturally, the direct contact between the growing particles and the monomer eliminates the need for solvents or carriers—and, therefore, for solvent recycling and recovery units—resulting in savings in both capital and operating costs. As one might expect, these GPP processes must place greater emphasis on the issue of reaction heat removal, since—unlike SP processes—they do not have a liquid intermediate or carrier with a higher specific heat capacity and particle-fluid heat transfer coefficient.
Specifically, the differences between the various GPP processes lie in the way in which the extraction of reaction energy is carried out.
In BASF’s GPP process,a vertical, stirred tank reactor is used to promote gas-solid contact and prevent the formation of hot spots, using a patented mixing device consisting essentially of a helical screw with a large surface area.
Reaction heat is removed through the evaporation of liquid propylene, which is continuously added to the reactor. The vaporized monomer is extracted from the reactor and, via an external water cooling loop, is then condensed and returned to the reactor.
To better control the reaction temperature, liquid propylene is continuously sprayed onto the system of particles suspended in gaseous monomer.
The mixture of polymerized particles and gas from the reactor is periodically withdrawn and sent to a cyclone to separate the gas from the particles. The gas is recompressed and recycled to the reactor. Depending on their size and size distribution, the particles can be sold directly or pelletized beforehand via extrusion. The low velocity of the vaporized monomer being recycled (1 to 2 cm/s) prevents both fluidization and the entrainment of the reacting particles into the external cooling and condensation zone.
In AMOCO’s GPP process, the reactor is a horizontal stirred tank, similar to the one used in the company’s PE technology. The various compartments allow for different temperature and gas composition values in each one. Reaction heat is removed by the evaporation of liquid propylene and/or other light diluents such as isobutane or isopentane. The temperature is controlled by adjusting the injection of the liquid to be evaporated—which is sprayed onto the particle bed—and the catalyst feed.
The final product is sent for processing to have additives added and to be pelletized.
In Union Carbide’s UNIPOL-PP process, a fluidized-bed reactor is used—similar to the one already described for this company’s LLDPE and HDPE technologies—with some modifications related to the catalysts used to polymerize PP.