In them, the growing polymer particles (containing the catalyst) are suspended directly in a gaseous medium consisting essentially of the monomer.
Polymerization does not occur, strictly speaking, in the gas phase, and in fact only the fluid medium is gaseous. The chemical reaction continues to take place in the micro reactors that make up each of the support particles, catalyst and growing polymer.
Naturally, the direct contact of the growing particles with the monomer avoids the need for solvents or carriers and, therefore, for solvent recycling and recovery units, with consequent savings in investment and operating costs. As it is logical to suppose, these GPP processes must give more importance to the problem of the extraction of the reaction heat, since they do not have, like SP, an intermediate or liquid carrier of higher specific heat and particle-fluid heat transfer coefficient.
Precisely, the differences between the different GPP processes lie in the way in which the extraction of the reaction energy is carried out.
In BASF's GPP process, a vertical, stirred tank reactor is used to promote gas-solid contact and to prevent the formation of hot zones by means of a patented mixing device consisting essentially of a large-area helical screw.
The reaction heat is removed by evaporation of liquid propylene which is permanently added to the reactor. The vaporized monomer is extracted from the reactor and, by means of an external water cooling loop, then condensed and returned to the reactor.
For better control of the reaction temperature, liquid propylene is continuously sprayed over the system of particles suspended in gaseous monomer.
The mixture of polymerized particles and reactor gas is periodically extracted and sent to a cyclone to separate the gas from the particles. The gas is recompressed and recycled back to the reactor. The particles, depending on the size and size distribution, can be sold directly or pelletized, via extrusion, beforehand. The low velocity of the vaporized monomer to be recycled (1 to 2 cm/s) allows neither fluidization nor entrainment of the reacting particles to the external cooling and condensation zone.
In AMOCO's GPP process, the reactor is a horizontal stirred tank, similar to the one used in AMOCO's PE technology. The different compartments allow for different values of temperature and gas composition in each compartment. The heat of reaction is extracted by evaporation of the liquid propylene and/or other light diluents such as isobutane or isopentane. The temperature is controlled by manipulating the injection of the liquid to be evaporated which is sprayed onto the particle bed and the catalyst feed.
The final product is sent to processing for the addition of additives and pelletizing.
In Union Carbide's UNIPOL-PP process, a fluidized bed reactor is used, similar to the one already described for the same company's technologies for LLDPE and HDPE, with some modifications associated to the catalysts to be used to polymerize PP.