Gas phase polymerization process

Details Gas phase polymerization process Gas phase polymerization process

1999-04-02

2001-10-23

Wilson, Donald R. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Removing and recycling removed material from an ongoing...

C526S070000, C526S901000, C422S132000, C422S135000, C422S138000, C422S140000, C422S146000

Reexamination Certificate

active

06306981

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a continuous process for the gas-phase polymerization of monomers in a fluidized bed or in a stirred bed reactor, and in particular to a process having improved levels of productivity.
Gas phase processes for the homo-polymerization and co-polimerization of monomers, especially olefin monomers are well known in the art. Such processes can be conducted for example by introducing the gaseous monomer into a stirred and/or fluidized bed comprising pre-formed resin particles and a catalyst for the polymerization.
In the gas fluidized bed polymerization of olefins, the polymerization is conducted in a fluidized bed reactor wherein a bed of polymer particles are maintained in a fluidized state by means of an ascending gas stream comprising the gaseous reaction monomer. The polymerization of olefins in a stirred bed reactor differs from polymerization in a gas fluidized bed reactor by the action of a mechanical stirrer within the reaction zone which contributes to fluidization of the bed. The start-up of such a polymerization process generally employs a bed of pre-formed polymer particles similar to the polymer which it is desired to manufacture. During the course of polymerization, fresh polymer is generated by the catalytic polymerization of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favored process employs a fluidization grid to distribute the fluidizing gas to the bed, and also to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidization grid. The fluidized bed comprises a bed of growing polymer particles, polymer product particles and catalyst particles. This reaction mixture is maintained in a fluidized condition by the continuous upward flow from the base of the reactor of a fluidizing gas which comprises recycle gas drawn from the top of the reactor, together with added make-up monomer.
The fluidizing gas enters the bottom of the reactor and is passed, preferably through a fluidization grid, upwardly through the fluidized bed.
The polymerization of olefins is an exothermic reaction and it is therefore necessary to provide means for cooling the bed to remove the heat of polymerization. In the absence of such cooling the bed would increase in temperature until, for example, the catalyst became inactive or the bed commenced to fuse.
In the fluidized bed polymerization of olefins, the preferred method for removing the heat of polymerization, is by passing a cooling gas, preferably the fluidizing gas, which is at a temperature lower than the desired polymerization temperature, through the fluidized bed to conduct away the heat of polymerization. The gas is removed from the reactor, cooled by passage through an external heat exchanger and then recycled to the bed.
The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidized bed at the desired polymerization temperature. In this method of polymerizing alpha olefins, the recycle gas generally comprises one or more monomeric olefins, optionally together with, for example, an inert diluent gas or a gaseous chain transfer agent such as hydrogen. The recycle gas thus serves to supply monomer to the bed to fluidize the bed and to maintain the bed within a desired temperature range. Monomers consumed by conversion into polymer in the course of the polymerization reaction are normally replaced by adding make-up monomer to the recycle gas stream.
It is well known that the production rate (i.e. the space time yield in terms of weight of polymer produced per unit volume of reactor space per unit of time) in commercial gas fluidized bed reactors of the above mentioned type, is limited by the maximum rate at which heat can be removed from the reactor. The rate of heat removal can be increased for example, by increasing the velocity of the recycle gas and/or reducing the temperature of the recycle gas. However, there is a limit to the velocity of the recycle gas which can be used. Above this limit the bed can become unstable or even lift out of the reactor into the gas stream, leading to blockage of the recycle line and damage to the recycle gas compressor or blower. Even at velocities below this level, it is important to keep the velocity of the exiting gases safely below the level at which excessive amounts of polymer fines are carried out of the top of the reactor. There is also a practical limit on the extent to which the recycle gas can be cooled. This is primarily determined by economic considerations and is normally determined by the temperature of the industrial cooling water available on site. Refrigeration can be employed if desired, but this adds to the production costs. Thus, in commercial practice, the use of cooled recycle gas as the sole means of removing the heat of polymerization from the gas fluidized bed polymerization of olefins has the disadvantage of limiting the maximum production rates obtainable.
BACKGROUND OF THE INVENTION
The prior art discloses a number of methods for removing heat from gas fluidized bed polymerization processes.
GB 1415442 relates to the gas phase polymerization of vinyl chloride in a stirred or fluidized bed reactor, the polymerization being carried out in the presence of at least one gaseous diluent having a boiling point below that of vinyl chloride. Example 1 of this reference describes the control of the temperature of polymerization by the intermittent addition of liquid vinyl chloride to fluidized polyvinyl chloride material. The liquid vinyl chloride evaporates immediately in the bed, resulting in the removal of the heat of polymerization.
U.S. Pat. No. 3,625,932 describes a process for polymerization of vinyl chloride wherein beds of polyvinyl chloride particles within a multiple stage fluidized bed reactor are kept fluidized by the introduction of gaseous vinyl chloride monomer at the bottom of the reactor. Cooling of each of the beds to remove heat of polymerization generated therein is provided by spraying liquid vinyl chloride monomer into the ascending gas stream beneath the trays on which the beds are fluidized.
GB 1398965 discloses the fluidized bed polymerization of ethylenically unsaturated monomers, especially vinyl chloride, wherein thermal control of the polymerization is effected by injecting liquid monomer into the bed using one or more spray nozzles situated at a height between 0% and 75% of that of the fluidized material in the reactor.
U.S. Pat. No. 4,390,669 relates to homo- or co-polimerization of olefins by a multi-step gas phase process which can be carried out in stirred bed reactors, fluidized bed reactors, stirred fluidized bed reactors or tubular reactors. In this process polymer obtained from a first polymerization zone is suspended in an intermediate zone in an easily volatilized liquid hydrocarbon. The suspension, so obtained, is fed to a second polymerization zone where the liquid hydrocarbon evaporates. In Examples 1 to 5, gas from the second polymerization zone is conveyed through a cooler (heat exchanger) wherein some of the liquid hydrocarbon condenses (with co-monomer if this is employed). The volatile liquid condensate is partly sent in the liquid state to the polymerization vessel where it is vaporized and serves to remove some the heat of polymerization. This reference is ambiguous as to how or where the liquid is introduced into the polymerization vessel.
U.S. Pat. No. 4,543,399 relates to a process for increasing the space time yield in continuous gas fluidized bed processes for the polymerization of fluid monomers, the process comprising cooling part or all of the unreacted fluids to form a two phase mixture of gas and entrained liquid below the dew point and reintroducing said two phase mixture into the reactor. This technique is referred to as operation in the “condensing mode”. U.S. Pat. No. 4,543,399 also discloses that it is poss

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