Method and apparatus for preparing polymers

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in two or more physically distinct zones

Reexamination Certificate

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C526S064000, C526S088000, C526S901000, C526S920000, C422S132000, C422S134000, C422S139000, C422S140000

Reexamination Certificate

active

06476161

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to polymerization of olefinic monomers. Generally, the present invention concerns a process for preparing homopolymers and copolymers of ethylene and propylene or other olefins in a reactor system comprising a combination of at least one slurry reactor and at least one gas phase reactor. In particular, the present invention concerns a method and an apparatus for introducing a polymer slurry comprising a multiphase flow into the fluidized bed of a gas phase polymerization reactor.
2. Description of Related Art
A large number of bulk and gas phase processes for preparing ethylene and propylene homo- and copolymers are known in the art. Also combined bulk and gas phase processes have already been suggested in the prior art, Thus, U.S. Pat. No. 4,740,550 discloses polymerization of propylene in a loop reactors, which can be operated in supercritical conditions. The product of the loop reactor is conducted to a gas phase reactor, wherein the reaction is continued. Before entering the gas phase, the fines fraction of the polymerization product of the loop reactor is removed and fully or partially circulated back to the loop reactor. Together with the fines, a part of the unreacted monomers from the gas phase reactor is recycled directly to the first stage loop reactor.
The main object of U.S. Pat. No. 4,740,550 is to provide a process for preparing a block copolymer of high quality by feeding homopolymer with narrow residence time distribution to the block copolymerization stage.
One problem with the process in U.S. Pat. No. 4,740,550 is that if all the fines removed from the first stage loop reactor outlet are circulated back to the loop reactor, there is a risk that eventually the loop reactor is filled with inactive dead catalyst or slightly polymerized dead fines. On the other hand if a portion of this fines stream is combined with the product from the last reactor, this might cause inhomogeneity problems in the final product. Still, further, if a portion of said fines stream is collected separately and blended with a separate homopolymer product as also suggested in U.S. Pat. No. 4,740,550, this leads to complicated control systems because the fines streams cannot be controlled. As a consequence, the production split of the reactors and thereafter the composition of the product is unpredictable without the rejection of most of the fines. These features will all contribute to an economically unacceptable operation.
As far as the relative amounts of polymer produced in each of the various polymerization stages are concerned, U.S. Pat. No. 4,740,550 states the following: Regarding the homopolymer portion of the copolymer, the following ranges are most preferred: first stage pipe-loop reactor 70 to 90%, second stage fluidized-bed reactor 30 to 10%. Assuming that a matrix for homopolymers is produced at a rate of 25,000 kg/h, 70% of this is 17,500 kg/h and 30% of this is 7,500 kg/h. Thus, the rate of homopolymerization is 17,500 kg/h in the first stage loop reactor and 7,500 kg/g in the second stage gas phase reactor. This means that if the product from first stage loop reactor is 40 wt-% slurry as stated in U.S. Pat. No. 4,740,550 (after the fines separation in this example calculation), 26,250 kg/h of the reaction liquid, i.e. propylene, enters the second stage with the polymer product from the first stage. As only 7,500 kg/h of propylene is consumed by polymerization in the second stage and as an estimated 2,500 kg/h leaves the second stage with the homopolymer product, this gives an excess of 16,250 kg/g of propylene entering the second stage (26,250 kg/g−(7,500 kg/h+2,500 kg/h)=16,250 kg/h). In the process disclosed in U.S. Pat. No. 4,740,550 this excess amount of propylene is circulated back to the first stage.
The recycling of large amounts of unreacted monomers from the second stage reactor back to the first stage slurry (bulk) reactor increases investment and production costs, prevents independent control of reaction medium composition in the two reactors and slows up grade transitions.
Generally, in the actual practice of multi-step gas phase processes, the known methods suffer from the disadvantage that it is difficult to adjust the molecular weight distribution and/or chemical composition distribution of the resulting olefin polymer composition to the desired values.
As a solution to this problem, it has been proposed to provide a suspension zone for forming a suspension of the catalyst-containing polymer in an easily volatile hydrocarbon medium. For example, a gaseous portion containing a large amount of hydrogen can be easily separated from solid portion containing the polymer. The separated gaseous phase can be advantageously recycled directly to the first gas phase polymerization zone. Moreover, the heat of polymerization can be advantageously removed by the vaporization of the easily volatile hydrocarbon, EP-A 0 040 992. However, the suggested solution suffers from the technical problem of how to transfer the polymer from the first gas phase into the suspension zone and form the suspension.
The operation of the solution process is difficult, since transportation of the polymer solution as the reaction medium and separation of the polymer contained therein is difficult. This requires special technical solutions making the process economically expensive.
A way of controlling the MWD and the chemical composition is to use suspension polymerisation, wherein a gaseous portion is recycled to the first gas phase reactor. Typically, the reaction medium from the first reactor is recycled to the first reactor.
It should also be noted that in U.S. Pat. No. 4,740,550 discussed above, no particular attention has been attached to the introduction of the slurry from the bulk polymerization into the gas phase reactor.
The present invention aims at eliminating the problems related to the prior art and to improve the operation of combined bulk and gas phase processes by controlling the introduction of the slurry into the gas phase reactor. Before addressing this matter in more detail, some comments on conventional gas phase technology should be made:
As is well known in the art, conventional fluidized gas phase reactors comprise an elongated reactor body generally having a central axis which is vertical. The monomers are polymerized in a fluidized bed supported above a fluidization grid in the lower part of the reactor body. A gaseous stream containing unreacted monomers is recovered from the top of the reactor body and recycled to the fluidized bed, whereas the polymer product is withdrawn from the lower part of the reactor.
The polymerization system of a gas phase reactor used for polymerization of &agr;-olefins typically comprises the polymer together with a high yield (Ziegler-Natta) catalyst and a gaseous reaction medium. It can be maintained in the fluidized state by mechanically mixing or stirring the contents of the reaction and additionally or alternatively by blowing the monomer (i.e. the olefin) and/or an easily volatile hydrocarbon (the reaction medium) into it in the gaseous state. A reaction medium in the liquid state can be introduced into the polymerization system and the polymerization can be carried out while gasifying said reaction medium. The unreacted reaction medium can be partly or wholly liquefied and recycled in liquid state into the polymerization system, as disclosed in EP-A1 0 024 933.
Conventional gas phase technology is hampered by some considerable problems. Thus, in a gas phase reactor the production rate is. limited due to low heat transfer from polymer to gas. Furthermore, when a highly active catalyst is used for polymerization in the gas phase, it is difficult to provide a uniform mixture of polymer particles in a gas phase polymerization bed.
Based on the operation conditions of gas phase reactors typical of those reported in the literature (as shown in simulations), the gas is not ideal at the pressure and temperature used, Ray et.al.,
Chein.En

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