Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Reexamination Certificate
2000-04-27
2003-05-13
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, C526S351000, C526S901000, C526S905000, C526S124100, C526S124200, C525S053000, C525S270000, C525S322000
Reexamination Certificate
active
06562914
ABSTRACT:
FIELD OF THE INVENTION
The invention is related in process for making propylene polymers or copolymers in propylene medium.
BACKGROUND OF THE INVENTION
The loop reactor bulk polymerization process for making propylene polymers is a well-known process, in which polymerization takes place in loop reactors with the polymer forming as solid particles suspended in a liquid comprising mainly propylene monomer. The reactor content is maintained in a highly agitated state by circulating the reaction mixture containing reactants and polymer at comparatively high velocity around the reactor loop by means of a circulation pump. The heat of polymerization is removed by a water cooling jacket enclosing each reactor leg. The polymer is removed from the reactor by opening and closing continuously or periodically a discharge valve to release both liquid and solid reaction mixture.
There are at least three major problems in the conventional loop reactor polymerization. The reactor temperature and pressure must be such that the entire reactor is completely filled with a reaction mixture containing reactants and polymer and no vapor bubbles will cause cavitation in the circulation pump. The reactor diluent must be selected to provide the minimum polymer solubility to prevent reduction of heat removal by fouling, especially with copolymers. The reactor diluent must be volatile to minimize the requirement for heat addition in order to separate diluent from the finished polymer powder.
In ethylene polymerization these problems can be avoided if supercritical conditions are used. The normally used diluent, isobutane, is replaced by propane and no bubble formation and cavitation in the loop reactor takes place, when the operating temperature and pressure are in the supercritical region. Further, the polymer is less soluble in supercritical propane and the flashing of monomer is easier, because the fluid contains more heat from the polymerization reactor. (see WO 92/12181). However, in ethylene polymerization with a slurry process the monomer concentration in the reaction medium is usually rather low (e.g. 5-15 wt. %).
Another known process for manufacturing propylene polymers is the gas phase process, where the propylene monomer is polymerized in a fluidized bed of forming polymer particles. The polymerization bed is cooled by circulating gaseous mixture of propylene and optional comonomers and hydrogen from the top of the reactor via cooler and returning it back to the bottom of the reactor. Make-up propylene is added to the circulation.
Known are also multistage processes for manufacturing polypropylene, where more than one reactor are used in series.
All known processes have some limitations. Therefore a need exists for a flexible process, where it is possible to produce various kinds of propylene polymers having a molecular weight range from high to low and molecular weight distribution from narrow to broad.
Propylene has a critical temperature, which is even lower than propane (91.4° C./96.8° C.). The polymerization of propylene under supercritical conditions is not novel. In U.S. Pat. No. 4,740,550 there is disclosed a multistage copolymerization process for making propylene-ethylene copolymers. In this process propylene is homopolymerized in one or more pipe-loop reactors in the presence of magnesium and titanium containing catalyst, resulting in a homopolymer reaction suspension, transferring said homopolymer reaction suspension to a separator and separating a stream of reduced-fine homopolymer particles, transferring said stream of reduced-fine homopolymer particles to a gas-phase fluidized bed homopolymer reactor and homopolymerizing additional propylene, removing a stream of propylene homopolymer particles and transferring it to a gas-phase fluidized bed copolymer reactor, and copolymerizing ethylene and propylene resulting in a propylene-ethylene impact copolymer.
In said patent, the homopolymerization of propylene under supercritical state in the pipe-loop reactor is mentioned as one alternative. The patent is theoretical and contains no real examples. The only calculated examples refer to the polymerization under subcritical conditions. A six-inch diameter pipe-loop reactor having a considerable length of about 400 m is described. The reactor has a total volume of 6.78 m
3
, which is a typical size of a prepolymerization reactor, and small when compared to ordinary loop reactors (35 m
3
) . In such a reactor the residence time is only 10 minutes. Otherwise, it would be impossible to circulate the polymer reaction medium in the pipe reactor. Moreover, the process described in the patent is rather complex, comprising three reactors and is intended to produce propylene-ethylene impact copolymers.
SUMMARY OF THE INVENTION
The present invention concerns a multistage process for homo or copolymerizing propylene, wherein propylene is polymerized in the presence of a catalyst at an elevated temperature in a reaction medium, in which a major part is propylene. This invention does not concern so called solution polymerization where polymer is dissolved in the reaction medium, but rather bulk polymerization where polymerization takes place essentially among the monomer reactants and catalysts. According to the invention, the polymerization is carried in at least one continuous stirred-type reactor (CSTR) or loop reactor, where the polymerization is carried out at a temperature and a pressure above the corresponding critical temperature and pressure of the reaction medium and where the residence time is at least 15 minutes.
The term “supercritical” condition or state in the specification means that both the temperature and the pressure in the reactor are above the corresponding supercritical temperature and the pressure of the reaction medium.
In super critical operation mode with appropriate catalyst one can achieve higher conversions, e.g. 57% instead of conversion of 50% in a normal 70° C. loop. This has great impact on the savings of the catalyst and possibility to have less unconverted monomer to the recovery section. The same can put also in the terms of higher reactor productivity allowing a smaller reactor volume to be used and thereafter lower investment costs. However, that advantage is reduced by higher investment costs of higher pressure reactors.
The reactor concept, in which at least one reactor is a CSTR reactor or loop reactor and operates under supercritical conditions is new and gives possibility to a more flexible production of different polypropylene grades. In the reactor it can be produced a very high molecular weight polymer up to 2,000,000 or more by adding no hydrogen or a minor amount of hydrogen. Because the temperature is high, the molecular weight distribution will be narrow. On the other hand, hydrogen can be added in very great amounts to the reactor without risk to cause cavitation in the blades of the reactor pump circulating the reaction mixture. Thereby manufacturing polymers of low molecular weight or high melt index comes possible. Similarly comonomers, such as ethylene can be added into the reactor in any desired amount.
According to one preferable embodiment of the invention, the process comprises two loop reactors connected in series, whereby both reactors are operated at the supercritical pressure and the first reactor is operated either below or above the critical temperature of the reaction medium and the second reactor is operated above the supercritical temperature. If in the first loop reactor a lower temperature is used and no or a minor amount of hydrogen is fed to the reactor, a very high molecular weight polymer having a relatively broad molecular weight distribution is produced. If a higher temperature is used, the molecular weight distribution will be narrower. The second reactor operates above the supercritical temperature of the reaction medium and hydrogen is added in greater amounts, which gives a final polymer having a bimodal molecular weight distribution.
According to the still another embodiment of the invention, the first reactor is a lo
Andtsjö Henrik
Harlin Ali
Pentti Ismo
Birch & Stewart Kolasch & Birch, LLP
Borealis A/S
Choi Ling-Siu
Wu David W.
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