Fluidized bed reactor with asymmetric gas inlet

Details Fluidized bed reactor with asymmetric gas inlet Fluidized bed reactor with asymmetric gas inlet

2000-11-17

2003-01-14

Wu, David W. (Department: 1713)

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

Synthetic resins

Removing and recycling removed material from an ongoing...

C526S348000, C422S139000

Reexamination Certificate

active

06506855

ABSTRACT:

The invention relates to a process for the gas-phase polymerisation of one or more olefinic monomers in a fluid-bed reactor which is bounded at the bottom by a gas-distributing plate containing apertures, comprising the supply of a gas to the reactor through the apertures.
The gas-phase polymerisation of olefines in a fluid bed is known per se, e.g. from WO-A-94/28032 and U.S. Pat. No. 4,543,399 and is effected in an elongated, generally vertically installed, reactor in which a bed of polymer particles is maintained in a fluidised state by means of a rising stream of gas which contains at least the gaseous monomers to be polymerised. The gas stream is supplied via a gas-distributing plate which separates the bottom part of the reactor from the reaction zone proper. This plate contains apertures which distribute the supplied gas stream across the area of the reaction zone as required, in the known reactors as homogeneously as possible. Structures for distributing the incoming gas stream or for preventing the risk of polymer dropping down from above may be present above the apertures. Such structures are known per se, as are suitable ways of distributing the apertures across the gas-distributing plate and the suitable sizes of the apertures. So as to nevertheless be able to realize a certain drop in pressure at a lower gas flow rate through the plate, the bottom part of the reactor may have a conical cross-section above the gas-distributing plate, or a peripheral part of the gas-distributing plate may have a closed design. To prevent the risk of accumulation of polymer particles on such a peripheral part, the sealing is preferably in the form of an oblique wall extending from the gas-distributing plate to the wall of the reactor. The angle between the oblique wall or the conical part of the wall and the plate must exceed the angle of repose of the polymer particles in the reactor, and is furthermore at least 30°, preferably at least 40°, and more preferably lies between 45° and 85°. If such an oblique wall is present, the ‘gas-distributing plate’ will hereinafter be understood to be the part of the floor of the reactor containing the apertures for the fluidising gas.
The rising gas stream may optionally also contain one or more inert gases and for example hydrogen as a chain length regulator. An important objective of the addition of inert gases is to control the gas mixture's dew-point. Suitable inert gases are for example inert hydrocarbons such as (iso)butane, (iso)pentane and (iso)hexane, but also nitrogen. They may be added to the gas stream either as a gas or in a condensed form as a liquid.
The gas stream is discharged via the top of the reactor and, after it has undergone certain treatments, new monomer is added to it to replace the monomer consumed in the polymerisation, after which it is returned to the reactor as (part of) the rising gas stream to maintain the bed. In addition, a catalyst is added to the bed. During the process, new polymer is constantly formed under the influence of the catalyst present, and at the same time some of the polymer formed is withdrawn from the bed, which ensures that the volume of the bed remains more or less constant. New monomer to replace the converted monomer is generally supplied in the form of a gas or liquid via the gas stream that maintains the bed.
The process applied in the known reactors shows the disadvantage that at a certain diameter of the fluid bed the height of the bed is restricted to at most 3 to 5 times that diameter. This seriously restricts the reactor capacity.
Object of the invention is to provide a process in which a fluid bed having a larger height/diameter ratio than in the known reactors can be stably maintained.
This aim is achieved because the average rate at which the gas is supplied to the reactor in a first part of the gas-distributing plate whose area equals half that of the gas-distributing plate exceeds the average rate at which the gas is supplied to a second part comprising the area of the gas-distributing plate outside the first part.
Surprisingly it was found that the height of the fluid bed may be greater thanks to the new distribution of the supplied gas across the gas-distributing plate. In the known process the ratio of the height and diameter of the fluid bed, is limited to at most a factor of 3 to 5. In the process according to the invention this ratio may be chosen to be up to no less than 50% higher than in the known process. When the same amount of polymer formed is withdrawn from the reactor, the residence time of catalyst then consequently increases by the same percentage, so that it is used more efficiently. The yield per unit of weight of the catalyst consequently increases, and the catalyst content of the polymer formed is consequently lower. This results in lower catalyst costs and a purer polymer.
Moreover this advantage allows applying slimmer reaction vessels, which implies major engineering advantages for polymerization reactors since these are pressure vessels.
A further advantage associated with the process according to the invention is the diminished entrainment of small polymer particles (‘fines’) in the gas stream leaving the reactor over its top.
Further it was found that it is possible in the process according to the invention to maintain in fluidised state larger particles, up to 2 and even 3 mm in diameter, than in the known process with its homogeneous gas distribution.
The polymerisation is an exothermal reaction. Heat must constantly be dissipated to maintain the temperature in the reactor at the required level. This is effected via the gas stream, whose temperature when it leaves the reactor is higher than that at which it was supplied to the reactor. The flow rate of the gas in the reactor cannot be chosen arbitrarily high, so it is not possible to dissipate any desired amount of heat. The minimum local flow rate of the gas is limited by the requirement that all parts of the bed must be kept in a fluid state. The risk of polymer powder being retained on the gas-distributing plate must be prevented. Therefore, the apertures are equally distributed across the entire area of the gas-distributing plate. On the other hand, the average flow rate of the gas must not be so great as to cause the polymer particles to be blown out of the top of the reactor. The aforementioned limits are to a great extent dictated by the dimensions and density of the polymer particles present in the bed and by the density and viscosity of the gas stream through the bed, and can be experimentally determined. 5 cm/sec is generally taken as the lower limit of the flow rate of the gas at any point in the fluid bed and 100 cm/sec as the upper limit of the average flow rate of the gas across the entire fluid bed. The latter value may be exceeded locally. In practice, average flow rates of between 20 and 80 cm/sec are often used. The flow or supply rate of the gas in a certain area of the gas-distributing plate is defined as the quotient of the amount of gas per second, expressed in m
3
/s, that is supplied to that area, expressed in m
2
. It is added that the rate at which the gas leaves the apertures may be substantially higher than the rate at which it is supplied as defined above. This is connected with the fact that the total effective area of the apertures will generally be less than 10% or even less than 5% of the area of the gas-distributing plate.
The aforementioned requirements limit the maximum flow rate of the gas at the given dimensions of the reactor, and hence the maximum amount of heat that can be dissipated. It also limits the maximum amount of reaction heat that may be produced, and hence also the maximum amount of polymer to be produced.
The detailed design and operation of fluid-bed reactors for polymerising olefinic monomers and the associated suitable process conditions are known per se and are for example described in detail in WO-A-94/28032 and in U.S. Pat. No. 4,543,399.
From this same U.S. Pat. No. 4,543,399 it is known to add new monomer to the gas stream discharged fr

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