Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Patent
1993-05-25
1995-02-21
Schofer, Joseph L.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in tubular or loop reactor
526905, 526352, C08F 206
Patent
active
053916549
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for homo- or copolymerizing ethene in the form of solid polymer particles.
The polymerization of ethene in the form of solid particles in a liquid phase is known. In such processes, the polymerization is typically performed in a reaction medium or a diluent, which is formed by isobutane, pentane, hexane or some other saturated aliphatic hydrocarbon. The polymerization is often performed in the presence of a Ziegler-Natta catalyst at an elevated temperature. In addition to monomers, hydrogen is often used as a modifier in the polymerization, by means of which the molecular weight of the polymer to be prepared and its other properties may be affected.
Processes are also known, in which liquid propane is used as a reaction medium. For example the U.S. Pat. No. 4,754,007 describes a process for preparing an ethene copolymer in the form of particles by using liquid propane as a liquid phase. It is postulated that certain problems related to the usage of the abovementioned hydrocarbons may be avoided in this way. These disadvantages include for example that the usable temperature is limited, since polyethene dissolves in the reaction medium at high temperatures. Therefore, the separation of comonomers and the reaction medium from each other is difficult. Furthermore, the copolymers to be prepared have a relatively wide molecular-weight distribution.
In accordance with the U.S. Pat. No. 4,754,007, liquid propane is used as a reaction medium. In addition to the cheapness of propane, the advantages to be achieved in this way include that propane has a lower vaporization heat and it evaporates more easily that e.g. isobutane. The use of propane thus makes a more effective separation of the reaction medium from the polymer possible. Furthermore, the use of propane helps to keep the reactor clean when polymerizing ethene in the form of particles.
The U.S. patent especially emphasizes that the polymerization should be performed below the critical temperature of propane, which is 96.8.degree. C. According to the patent, the process is performed in a batch reactor, in which a considerable gas volume prevails above the liquid phase. If a continuous loop reactor is preferred, the situation is completely different. The reactor has then to be completely filled with the liquid phase, since the gas bubbles in the reaction medium would cause cavitation and thereby wearing in the circulation pumps for the reaction medium. The situation worsens, if it is desirable to prepare in the loop reactor a product with a higher melt index by adding hydrogen to the reaction medium. When operating below the critical point, as is the case in the U.S. Pat. No. 4,754,007, separation of phases occurs as hydrogen is bubbling, and pressure shocks arise, which means that the process cannot be applied to a loop reactor.
It may be indicated that when the reaction mixture of a loop reactor contains 93 mol-% of propane and 7 mol-% of ethene, the critical point of the mixture is T.sub.c =92.9.degree. C., P.sub.c =44.1 bar. If the reaction medium contains 7 mol-% of ethene and 2 mol-% of hydrogen (the remainder being propane), the maximum operable temperature of the reactor is 75.degree. C., which is far too low a temperature from the point of view of the polymerization. In the case of a mixture containing 7 mol-% of ethene and 2.5 mol-% of hydrogen (the remainder being propane), the maximum operable temperature is only 60.degree. C. It is thus apparent that the process according to said US patent cannot be realized in a loop reactor. However, it has been observed in accordance with the invention that a loop reactor may be used, if the polymerization is performed above the critical point. More specifically, this relates to a critical point of a fluid mixture, which is formed by ethene acting as a monomer (+ possible comonomers), propane and possibly hydrogen acting as a modifier. Ethene and hydrogen decrease said critical point to some extent and the critical point may be determined experimentally, when the concentration
REFERENCES:
patent: 3242150 (1960-03-01), Scoggin
patent: 3324093 (1963-10-01), Alleman
patent: 4007321 (1977-02-01), Scholz et al.
patent: 4582816 (1986-04-01), Miro
patent: 4754007 (1988-01-01), Pullukat et al.
Ahvenainen Antero
Andtsjo Henrik
Sarantila Kari
Neste Oy
Schofer Joseph L.
Wu David
LandOfFree
Method for homo- or copolymerizing ethene does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for homo- or copolymerizing ethene, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for homo- or copolymerizing ethene will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1930791