Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Organic polymerization
Reexamination Certificate
1997-10-03
2002-07-02
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Organic polymerization
C422S132000, C422S139000, C422S145000, C422S146000, C422S147000, C422S235000
Reexamination Certificate
active
06413477
ABSTRACT:
The present invention relates to a process for the gas-phase polymerization of olefins carried out in two interconnected polymerization zones, to which one or more &agr;-olefins CH
2
═CHR are fed in the presence of a catalyst under polymerization conditions and from which the produced polymer is discharged. In the process of the present invention the growing polymer particles flow through a first polymerization zone under fast fluidization conditions, leave said first zone and enter a second polymerization zone through which they flow in a densified form under the action of gravity, leave said second zone and are reintroduced into the first polymerization zone, thus establishing a circulation of polymer between the two polymerization zones.
The development of catalysts with high activity and selectivity of the Ziegler-Natta type and, in more and more applications, of the metallocene type, has led to the widespread use on an industrial scale of processes, in which the polymerization of the olefins is carried out in a gaseous medium in the presence of a solid catalyst. Compared with the more conventional technology in liquid suspension (of monomer or of monomer/solvent mixtures), this technology has the following advantages:
a) operational flexibility: the reaction parameters can be optimized on the basis of the characteristics of the catalyst and of the product and are not limited by the physico-chemical properties of the liquid mixtures of the reaction components (generally including hydrogen as a chain transfer agent);
b) widening of the product range: the effects of swelling of the growing polymer particle and of solubilization of polymer fractions in a liquid medium greatly reduce the range of production of all the kinds of copolymers;
c) minimization of the operations downstream of the polymerization: the polymer is obtained directly from the reactor in the form of dry solid and requires simple operations for removing dissolved monomers and deactivating the catalyst.
All the technologies devised hitherto for the gas-phase polymerization of &agr;-olefins provide for maintaining a bed of polymer, through which the reaction gases flow; this bed is maintained in suspension either by mechanical stirring (stirred-bed reactor) or by fluidization obtained by recycling the reaction gases themselves (fluidized-bed reactor). In both the reactor types, the monomer composition around the polymer particle in the reaction is maintained sufficiently constant owing to the induced stirring. Said reactors approximate very closely the ideal behaviour of the “continuous stirred-tank reactor” (CSTR), making it relatively easy to control the reaction and thereby ensuring consistency of quality of the product when operating under steady-state conditions. What is by now the most widely established industrial technology is that of the fluidized reactor operating under “bubbling” conditions. The polymer is confined in a vertical cylindrical zone. The reaction gases exiting the reactor are taken up by a centrifugal compressor, cooled and sent back, together with make-up monomers and appropriate quantities of hydrogen, to the bottom of the bed through a distributor. Entrainment of solid in the gas is limited by an appropriate dimensioning of the upper part of the reactor (freeboard, i.e. the space between the bed surface and the gas offtake), where the gas velocity is reduced, and, in some designs, by the interposition of cyclones in the exit gas line.
The flow rate of the circulating gas is set so as to assure a fluidization velocity within an adequate range above the minimum fluidization velocity and below the “transport velocity”. The heat of reaction is removed exclusively by cooling the circulating gas. The catalyst components are fed in continuously. The composition of the gas-phase controls the composition of the polymer. The reactor is operated at constant pressure, normally in the range 1-3 MPa. The reaction kinetics are controlled by the addition of inert gases.
A significant contribution to the reliability of the fluidized-bed reactor technology in the polymerization of &agr;-olefins was made by the introduction of suitably pretreated spheroidal catalyst of controlled dimensions and by the use of propane as diluent (see WO 92/21706). Fluidized-bed technology has limits, some of which are discussed in detail below.
A) Removal of the Heat of Reaction
The maximum fluidization velocity is subject to quite narrow limits (which already entail reactor volumes for disengagement which are equal to or greater than those filled by the fluidized bed). Depending on the heat of the reaction, the polymer dimensions and the gas density, a limit to the productivity of the reactor (expressed as hourly output per unit reactor cross-section) is inevitably reached, where operation with gas inlet temperatures higher than the dew point of the mixture of the gases is desired. This limit can lead to reductions in the plant output, in particular in the copolymerization of ethylene with higher &agr;-olefins (hexene, octene), which is carried out with conventional Ziegler-Natta catalysts, requiring gas compositions rich in such olefins. Many ways of overcoming the limits, in terms of heat removal, of the traditional technology have been proposed, based on partial condensation of the circulating gases and on the use of the latent heat of evaporation of the condensates for controlling the temperature in the interior of the reactor (see EP-89691, U.S. Pat. No. 5,352,749, WO 94/28032). Although technically worthy of consideration, all the systems proposed for implementing the principle render the operation of the fluidized reactors critical.
In particular (and apart from problems associated with the distribution of wet solids in the plenum below the distribution grid), the technology used in patents EP-89691 and U.S. Pat. No. 5,352,749 relies on the turbulence generated by the grid to distribute the liquid over the polymer. Possible coalescence phenomena in the plenum can give rise to uncontrollable phenomena of poor distribution of liquid with formation of agglomerates which can not be redispersed, in particular in the case of polymers which have a tendency to stick. The discrimination criterion given in U.S. Pat. No. 5,352,749 reflects situations under steady-state conditions, but offers no feasible guide for situations of even a transient “reaction runaway”, which can lead to irreversible loss of fluidization, with a consequent collapse of the reactor.
The method described in patent WO 94/28032 involves separation of the condensates and their distribution above the grid by means of special, suitably located nozzles. In fact, the condensates inevitably contain solids in reactive conditions, whose concentration can become very high at low condensate amounts. Moreover, the inherent difficulty of uniformly distributing a suspension over a number of nozzles can compromise the operability of some of them and a blocking in one nozzle adversely affects the distribution of the liquid evaporating in the relevant section of the reactor. It is also clear that the efficiency of the operation depends upon a vigorous circulation of solids in the reactor and, below the injection points, this is reduced by an unbalancing of the gas flow rates caused by large quantities of condensates. Furthermore, any need for maintenance on one nozzle requires a complete shut-down of the reactor.
B) Molecular Weight Distribution
As already stated, a fluidized bed shows a behaviour directly comparable with an ideally mixed reactor (CSTR). It is generally known that, in the continuous polymerization of &agr;-olefins in a single stirred stage (which also involves steady composition of the monomers and of the chain transfer agent, normally hydrogen) with Ti catalysts of the Ziegler-Natta type, polyolefins having a relatively narrow molecular weight distribution are obtained. This characteristic is even more emphasized when metallocene catalysts are used. The breadth of the molecular weight distribution has an influence both on the rheological behavi
Covezzi Massimo
Galli Paolo
Govoni Gabriele
Rinaldi Roberto
Basell Technology Company BV
Bryan Cave LLP
Stiefel Maurice B.
Tran Hien
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