The discovery of the fluidized bed process for the production of polymers provided a means for producing a diversity of polymers, e.g., polyolefins such as polyethylene, with a drastic reduction in capital investment and a dramatic reduction in energy requirements as compared to the conventional processes. However, a limiting factor in the use of a fluidized bed reactor for carrying out an exothermic polymerization process is the rate at which heat can be removed from the bed.
The most common and perhaps universal means of heat removal employed in conventional fluidized bed reactor processes is by compression and cooling of the recycle gas stream at a point external to the reactor. In commercial scale fluidized bed reaction systems for producing polymers such as polyethylene, the amount of fluid which can be circulated to remove the heat of polymerization is greater than the amount of fluid required for support of the fluidized bed and for adequate solids mixing in the fluidized bed. However, the fluid velocity in the reactor is limited due to the need to prevent excessive entrainment of solids in the fluidizing gas stream as it exits the reactor. Necessarily then, the amount of fluid which can be circulated to remove heat is similarly limited.
One method of increasing the rate of heat removal is to compress and cool the monomeric gases recycled to the reactor to a point where a portion thereof is condensed. The resulting liquid portion is entrained in the recycled monomeric gas stream and carried back to the reactor. Such operation has been referred to as the "condensing mode" of polymerization operation and is disclosed in prior U.S. application Ser. No. 361,547, filed March 24, 1982 and in contemporaneously filed U.S. patent application of J. M. Jenkins et al. Ser. No. 643,884 filed August 24, 1984, entitled "Improved Method for Fluidized Bed Polymerization", both of which are incorporated herein by reference. As there disclosed, the employment of the condensing mode of operation permits a reduction in the temperature of the recycle stream, which, in combination with the heat of vaporization of the liquid, results in a marked increase in space-time-yield over that obtainable in the "non-condensing mode" of operation where the temperature of the recycled gas stream is maintained above the dew point of the recycle stream, the dew point being the temperature at which liquid condensate begins to form in the gas stream.
Test results from a scale model bottom reactor head and experience with a commercial polymerization reactor have indicated that an open nozzle-type reactor inlet is satisfactory for successful operation of a fluidized bed reactor in the condensing mode while a standpipe/conical cap-type reactor inlet is satisfactory for a non-condensing mode of operation of the reactor. The standpipe/conical cap-type inlet is not satisfactory for a condensing mode of operation due to liquid flooding or frothing in the bottom head, a phenomenon experienced with commercial reactors at relatively low levels of liquid in the recycle stream. Conversely, the open nozzle-type inlet has been found to be unsatisfactory for a non-condensing mode of operation in a commercial reactor because of excessive build-up of resin solids in the bottom head, particularly around the inlet opening.
In the practical operation of large commercial production reactors it is sometimes desireable to switch from a non-condensing mode of operation to a condensing mode of operation and vice versa. To do this in the past, for the reasons given above it has been necessary to shut down the reactor to replace or alter the inlet to conform with the requirements of the new mode of operation. Reactor shutdown for the transition is undesirable not only because of the maintenance costs associated with the changeover, but because the downtime results in significant production losses. For some commercial reactors, the transition may be required frequently, depending upon production schedules. Consequently, it is highly desirable to have an all-purpose reactor inlet configuration which satisfies the requirements for both the condensing and non-condensing modes of reactor operation.
The objectives of the invention, then, are to (1) augment the production rates of fluidized bed reactors, (2) lower the cost for the maintenance and/or operation of such reactors, and (3) provide flexibility to enable such reactors to produce a variety of polymers, e.g., polymers of ethylene and heavier alpha olefins (copolymers and terpolymers) and polymers of propylene (homopolymers and block or random copolymers) at higher than conventional production rates without incurring downtime production losses. The flow deflector means of the present invention assists in meeting these objectives by providing a multi-purpose reactor inlet configuration which eliminates the need for reactor shutdown when converting from the condensing mode to the non-condensing mode of operation and vice versa.
USPTO Backgrounds
USPTO Backgrounds