Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Reexamination Certificate
2002-01-24
2003-02-11
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in tubular or loop reactor
C526S068000, C526S348200, C526S348500, C526S348600
Reexamination Certificate
active
06518372
ABSTRACT:
The present invention relates to a process for the gas-phase polymerization of &agr;-olefins at from 20 to 130° C. and pressures of from 1 to 100 bar. The invention also relates to a tubular reactor for the gas-phase polymerization of &agr;-olefins having a length:diameter ratio of >100.
Gas-phase polymerization processes have been found to be particularly economical processes for the polymerization of &agr;-olefins, in particular for the homopolymerization and copolymerization of ethylene and propylene and for the copolymerization of these olefins with higher &agr;-olefins. Particularly, for the preparation of homopolymers and copolymers of ethylene, these gas-phase polymerizations are usually carried out in fluidized-bed reactors. Examples of such gas-phase fluidized-bed processes are described in U.S. Pat. No. 5,208,109 and in U.S. Pat. No. 5,041,473.
In the known gas-phase fluidized-bed processes, the reaction gas, which may, if desired, comprise regulators and inert gases in addition to the monomers, is circulated and utilized for mixing the fluidized bed which comprises small polymer particles. During the course of the polymerization process, the polymer particles grow and are discharged from the reactor either continuously or intermittently. As a result of the virtually perfect mixing of the fluidized bed and the discharge of only a small part of the reactor contents, the polymer particles in the reactor have very different residence times. Thus, some of the particles discharged have been subjected to the polymerization process for only a very short time and consequently have small diameters. On the other hand, there are particles which have spent a long time in the polymerization reactor before being discharged. The result is that the polymer particles have a very broad residence time distribution.
To achieve a defined structural inhomogeneity of the polymer, e.g. in the production of bimodal polyethylene or high impact polypropylene, the polymerization can be carried out either in a plurality of apparatuses or in different reaction zones of the same apparatus. In either case, the polymer particles have to be subjected to various reaction conditions. These different reaction conditions can comprise, for example, different temperature, different pressure, different monomer concentrations or different concentrations of the regulator, for example hydrogen, or combinations of these. However, if the residence time distribution of the polymer particles in the different reaction zones or in the different reactors is broad, as is the case in the abovementioned gas-phase fluidized-bed polymerization, the polymer properties resulting from the different process parameters become blurred and the polymer product has a broad, random distribution of different polymer particles.
To reduce the influence of a broad residence time distribution and the associated width of the distribution of the particle properties, methods involving an increase in the number of reactors connected in series have been pursued. Thus, U.S. Pat. No. 5,504,166 describes a horizontal reactor whose volume is divided into chambers so that the polymer powder can only flow forward from chamber to chamber. The chambers themselves can be regarded as virtually ideally mixed. The polymer powder in the individual chambers is mixed by mechanical stirrers.
A similar gas-phase polymerization process is described in U.S. Pat. No. 5,378,434. In this process, the individual reaction chambers contain fluidized beds of polymer and different gas compositions can be set in the various chambers so that the preparation of bimodal or multimodal polymers is possible. However, as a result of the virtually ideal mixing of the polymer particles in the individual reactor chambers, these processes too have relatively large product inhomogeneities owing to the broad residence time distribution of the polymer particles in the individual reactors.
The differences in the structure of the polymer from particle to particle and thus the inhomogeneity of the polymer product increase as the residence time distribution broadens. From the fundamentals of chemical engineering, it is known that a single well-mixed reactor, e.g. a stirred tank or a fluidized-bed reactor, has the broadest residence time distribution and a tube reactor with plug flow has the narrowest residence time distribution. In the theoretical, ideal case, the plug-flow tube reactor corresponds to a reactor cascade having an infinite number of mixing cells.
WO-97/04015 describes a gas-phase polymerization process which is carried out in a flow tube. However, this flow tube is arranged as a loop, so that the polymer particles are circulated during the polymerization process. Since the particle circulation times in this loop reactor are very short in order to achieve intensive mixing of the particles and these particle circulation times are far below the mean residence time, this process too has a particle residence time distribution which is not significantly different from a customary fluidized-bed reactor.
It is an object of the present invention to provide a gas-phase polymerization process for the polymerization of &agr;-olefins which has a narrow residence time distribution of the polymer particles and is therefore suitable for preparing, in particular, bimodal and multimodal polymers of excellent homogeneity.
We have found that this object is achieved by a process for the gas-phase polymerization of &agr;-olefins at from 20 to 130° C. and pressures of from 1 to 100 bar, wherein the polymerization is carried out in a tubular reactor having a length:diameter ratio of >100 and the growing polymer particles pass through the tubular reactor in its longitudinal direction without a significant part of the polymer particle stream being circulated. Furthermore, we have found a tubular reactor for the gas-phase polymerization of &agr;-olefins having a length:diameter ratio of >100, comprising at least one facility for feeding in reaction gas, at least one facility for feeding in catalyst, a polymer discharge system and at least one facility for separating the reaction gas from the polymer particles and recirculating the reaction gas to the inlet region of the reactor or for feeding in the reaction gas at a point upstream of the separation position.
The temperature and pressure conditions in the process of the present invention generally correspond to those in known gas-phase fluidized-bed processes, although the process offers the opportunity of varying these temperatures within the customary ranges in various parts of the reactor. The process can be carried out at from 20 to 130° C., in particular from 70 to 120° C. and particularly advantageously from 80 to 110° C. The reaction pressures can also be within the ranges which are customary for gas-phase fluidized-bed polymerizations. Thus, the process can advantageously be carried out at pressures of from 5 to 50 bar, particularly preferably at pressures of from 15 to 30 bar.
An important feature of the reactor of the present invention is its length:diameter ratio. The greater this length:diameter ratio, the narrower is, in general, the residence time distribution of the polymer particles. In the case of extremely long and thin reactors, either the pressure drop in the direction of the longitudinal co-ordinate is uneconomically high or the throughput achieved is too small, so that the reactor geometry is limited by these considerations. Good flow of the polymer particles with approximately plug flow and also narrow residence time distributions of the polymer particles are obtained in polymerization reactors having a length:diameter ratio of >100; the tubular reactors preferably have a length:diameter ratio of >300, particularly preferably from 300 to 1000.
A preferred geometry of a reactor according to the present invention for the industrial, commercial scale has a tube diameter in the range from 10 to 100 cm and a length of from 50 to 2000 m.
In contrast to the gas-phase polymerization process described in WO-97/04015, w
Basell Polyolefine GmbH
Cheung William
Connolly Bove & Lodge & Hutz LLP
Wu David W.
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