The multizone circulating reactor technology

Since the early stage of the polyolefins development, properties have been expanded by improving catalyst chemistry, morphology and by conducting poly merization in two successive steps, generating in each step a different pol ymer in ten-ns of molecular weight, crystallinity, composition to match the designed properties balance. Typical examples are heterophasic PP copolyme rs and broad MWD polyethylene. The two step technology has the typical limi tation related to the final polymer unhomogeneity related to difficulty of achieving a good phase distribution when in synthesis too large domains of different materials are generated. Recently a new technology concept, the m ultizone circulating reactor has conceptually and practically moved to a hi gher level the phase distribution, the homogeneity and therefore the proper ties. Through this new principle, the growing polymeric granule is kept con tinuously circulating between two interrelated zones, where two distinct an d different fluodynamic regimes are realized. In the first zone, the polyme r is kept in a "fast fluidization"; leaving said zone, the transport gas is separated and the polymer crosses the second zone in the "packed bed mode" and is then reintroduced in the first zone. A complete and massive circula ting is obtained between the two zones, managed by pressure balance. The fl uodynamic. peculiar regime of the second zone, where the polymer enters as dense phase in "plug flow", offers the opportunity to alter, through simple but substantial means, the monomeric composition with respect to the chain terminator (hydrogen) and to the comonomer. Whenever required, by the same route and on the same growing granule, a multiple, alternate and cyclic as well as continuous polymerization can be obtained which attains the most c omplete and intimate mixing of different polymers, giving a substantial "ho mogeneity" of the final polymer. With a proper dimensioning of the two zone s, it is thus possible to get a broad polymer molecular weight distribution , even starting from a catalyst able to develop only a limited molecular we ight distribution. Moreover, intimate mixing of polymers that are different and even mutually incompatible is obtained, not only because they have gro wn in the same granule but also because the polymers are generated through short cyclic steps. (C) 2001 Elsevier Science Ltd. All rights reserved.