Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Reexamination Certificate
2000-09-11
2002-03-12
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in tubular or loop reactor
C526S065000, C526S073000, C526S348200
Reexamination Certificate
active
06355741
ABSTRACT:
BACKGROUND TO THE INVENTION
The present invention relates to a process for the production of polyolefins, in particular polyethylene or polypropylene. In particular, the present invention relates to the production of a polyethylene having a multimodal molecular weight distribution, for example a bimodal molecular weight distribution.
DESCRIPTION OF THE PRIOR ART
It is known to produce polyethylene in liquid phase loop reactors in which ethylene monomer, and optionally an alpha-olefinic comonomer typically having from 3 to 10 carbon atoms, are circulated under pressure around a loop reactor by a circulation pump. The ethylene monomer and comonomer when present are present in a liquid diluent, such as an alkane, for example isobutane. Hydrogen may also be added to the reactor. A catalyst is also fed to the loop reactor. The catalyst for producing polyethylene may typically comprise a chromium-based catalyst, a Ziegler-Natta catalyst or a metallocene catalyst. The reactants in the diluent and the catalyst are circulated at an elevated polymerisation temperature around the loop reactor thereby producing polyethylene homopolymer or copolymer depending on whether or not a comonomer is present. Either periodically or continuously, part of the reaction mixture, including the polyethylene product suspended as slurry particles in the diluent, together with unreacted ethylene and comonomer, is removed from the loop reactor.
The reaction mixture when removed from the loop reactor may be processed to remove the polyethylene product from the diluent and the unreacted reactants, with the diluent and unreacted reactants typically being recycled back into the loop reactor.
Alternatively, the reaction mixture may be fed to a second loop reactor serially connected to the first loop reactor where a second polyethylene fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polyethylene product, which comprises a first polyethylene fraction produced in the first reactor and a second polyethylene fraction produced in the second reactor, has a bimodal molecular weight distribution.
It is known in the art to operate a loop reactor under conditions of high temperature and pressure such that the diluent is present under supercritical conditions. Thus the diluent is at a pressure greater than the critical pressure P
c
and at a temperature greater than the critical temperature T
c
. Under these conditions, there is no thermodynamic transition between the gas phase and the liquid phase and the homogeneous supercritical fluid has the properties of a dense gas and a low density liquid.
For example, WO-A-92/12181 discloses a method for homo- or copolymerising ethene in the presence of a Ziegler-Natta catalyst in a loop reactor under supercritical conditions. The diluent which is in the supercritical state is propane. It is disclosed that the use of a propane phase at a supercritical state provides some advantages, namely that the hydrogen content of the reactor may be adjusted within a wide range and no pressure-shock effects occur which would otherwise tend to damage the circulation pump for the diluent, as a result of the high compressibility of the supercritical fluid. This specification makes clear that propane should be used as a diluent rather than for example isobutane, because the use of propane enables more polymer types to be prepared in the reactor and also the solubility of polyethylene is lower in propane than in isobutane. The specification also discloses that since the boiling point of propane is low, the hydrocarbons may readily be separated from the polymer particles after the polymerisation. The specification discloses that two loop reactors in series may be employed for making ethylene polymers and/or copolymers having a wide or bimodal molecular weight distribution.
EP-B-0517868 also discloses a multi-stage process for producing polyethylene which employs supercritical conditions. It is disclosed that the inert hydrocarbon medium which is employed under supercritical conditions is propane. It is also disclosed that the polyethylene may have a bimodal molecular weight distribution.
WO-A-96/18662 discloses a process for preparing polyethylene which may have a multimodal molecular weight distribution by using supercritical conditions. Again, it is disclosed to be advantageous to use propane as the inert hydrocarbon medium under supercritical conditions.
WO-A-96/34895 discloses a process for manufacturing LLDPE polymers again using propane as a reaction medium under supercritical conditions. The LLDPE polymers are manufactured using a metallocene catalyst. It is disclosed that the excellent polymer morphology of the products produced with the metallocene catalysts together with the low polymer solubility into the diluent and relatively low diluent density, especially in the supercritical conditions, result in very good settling properties of the polymer and thus efficient reactor operation, (i.e. diluent flow into the reactor can be minimised). However, there is no disclosure of any specific reactor structure indicating how the operation of the reactor may be made more efficient.
WO-A-97/13790 discloses a process for making propylene homo- or copolymers in a loop reactor under supercritical conditions. It is disclosed that a polypropylene having a bimodal molecular weight distribution may be employed using two reactors in series.
While the above-specified patent specifications relating to supercritical conditions for the diluent provide advantages of higher hydrogen solubility in the diluent and easier hydrogen flashing if the reaction has been continued in the second reactor, combined with reduced swelling of the polymer in the supercritical diluent and the absence of pressure shocks as a result of the high compressibility of the supercritical diluent, nevertheless, the use of propane as a diluent tends to require the use of comonomers having low carbon numbers, for example butene which militates against the use of high carbon comonomers, for example hexene which would assist in the production of polymers having better properties than if butene were used. Moreover, the use of propane as a diluent tends to require a relatively high pressure to be employed above the critical pressure P
c
for propane. Moreover, the supercritical processes referred to hereinabove do not permit a particularly high comonomer concentration to be employed in the reactor, particularly for comonomer with high carbon number, e.g. hexene.
In the manufacture of polyolefins having a bimodal molecular weight distribution under supercritical conditions employed in serially connected reactors, the above-identified specifications suffer from the disadvantage that there is no specific disclosure as to how the reaction medium is transferred from the first reactor to the second reactor.
U.S. Pat. No. 4,754,007 discloses a process for copolymerising ethylene to form LLDPE copolymers in which liquid propane is used as a diluent in a slurry process. It is disclosed that the use of propane diluent provides more economical production of copolymers having more desirable physical properties than slurry processes using isobutane, hexane or other liquid diluents. There is no disclosure of the diluent being under supercritical conditions.
EP-A-0649860 discloses a process for the copolymerisation of ethylene in two liquid full loop reactors in series in which the average molecular weight is regulated. A comonomer is introduced into the first reactor and high and low average molecular weight polymers are produced respectively in the first and second reactors. One or more settling legs is provided for the first reactor in order to transfer the high average molecular weight polymer from the first reactor to the second reactor. The reaction is carried out in a diluent, for example isobutane, in a slurry process. This process suffers from the disadvantage that although the use of settling legs for concentrating the fluff between the first and second reactors allows preferential polymerisati
Cheung William K
Fina Research S.A.
Hitt Gaines & Boisbrun
Wu David W.
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