Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Reexamination Certificate
2002-10-29
2004-04-27
Cheung, William (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in tubular or loop reactor
C526S065000, C526S068000, C526S348000, C526S352000
Reexamination Certificate
active
06727326
ABSTRACT:
The present invention relates to a process for the continuous preparation of ethylene homopolymers and ethylene copolymers in the presence of free-radical polymerization initiators and in the presence or absence of molecular weight regulators in a tube reactor having a hot water jacket and one or more reaction zones at pressures in the range from 1000 to 4000 bar and temperatures of from 120° C. to 350° C., where the hot water jacket of each reaction zone is divided into at least two independently regulatable, separate longitudinal sections.
The present invention further provides novel ethylene homopolymers and copolymers and provides for their use.
The homopolymerization and copolymerization of the ethylene by the high-pressure bulk process is generally known. The reactors used are usually operated continuously at pressures of from 150 to 350 MPa and temperatures of from 150° C. to 350° C. using mean residence times of from 30 to 120 seconds (Ullmann's Encyclopädie der technischen Chemie, 4th Edition, Vol. 19/1980/pp. 169-178).
The quality and conversion attained of the polymers produced is determined largely by the temperature conditions in the reactor, with the achievable conversion being proportional to the quantity of heat removed. Part of the heat generated in the polymerization is removed via the reactor walls and can there be, for example, passed to a cooling medium. A problem which frequently occurs is that even at high temperatures and high pressures, high molecular weight polymer depositions are formed on the cooled interior walls of the reactor. Since polyethylene is a poor conductor of heat, this drastically reduces heat transfer. If the removal of heat is insufficient, the ethylene can decompose explosively as a result of the temperature increase. To avoid such polymer deposits, various methods of operating the reactor have been developed.
DE 4102808-A describes the temperature profile in a high-pressure multizone reactor, where the temperature of the hot water leaving the jacket directly upstream of the peroxide introduction point is controlled so that the temperature difference between introduction point and a second temperature measurement point is from 8 to 30° C. The hot water outlet temperature is from 207° C. to 225° C.
In DD 146 298 polyethylene having improved properties is obtained by means of specific temperatures in the cooling water circuit. The heat transfer medium fed into the cooling jacket is set to temperatures of from 215° C. to 235° C. along the reactor. The temperature of the heat transfer medium must at the same time be no more than 70° C.-95° C. below the maximum temperature of the reaction mixture in the respective reaction zone.
A further specific temperature profile for optimizing the polymer quality is described in DD 137 591. The temperature difference between the interior wall of the reactor and the stream in the middle of the reactor is maintained in a defined region, where this temperature difference within a reaction zone between the temperature at the introduction point for the initiator and the temperature maximum is smaller than that between the temperature maximum and the temperature at the location of renewed introduction of the monomer. In addition, the patent teaches that this temperature difference, particularly in the first reaction zone up to the first temperature maximum, should be kept particularly small. The cooling water used is at from 237° C. to 247° C.
The abovementioned processes are all relatively complicated.
It is an object of the present invention to achieve a good balance between conversion and product quality using a simple temperature profile of the cooling medium.
We have found that this object is achieved by a process for the continuous preparation of ethylene homopolymers and ethylene copolymers in the presence of free-radical polymerization initiators and in the presence or absence of molecular weight regulators in a tube reactor having a hot water jacket and one or more reaction zones at pressures in the range from 1000 to 4000 bar and temperatures of from 120° C. to 350° C., where the hot water jacket of each reaction zone is divided into at least two independently regulatable, separate longitudinal sections, wherein the hot water outlet temperature from the jacket of the first longitudinal section of each reaction zone, which extends over the region in between the initiator introduction point and from 20 to 50% of the reaction zone length, is set to from 180° C. to 210° C. and the hot water outlet temperature from the jacket in the subsequent, second longitudinal section of this reaction zone is set to from 140° C. to 180° C. Preference is given to hot water outlet temperatures from the jacket of from 190° C. to 210° C. in the first longitudinal section and from 150° C. to 180° C. in the subsequent longitudinal section. The hot water jacket of each reaction zone is preferably divided into two independently regulatable, separate longitudinal sections.
In the following, a reaction zone is in each case the zone commencing at an initiator introduction point up to before the next initiator introduction point. It usually has a temperature maximum beyond which the polymer/monomer mixture becomes cooler again.
According to the present invention, it has been found that the region between the initiator introduction point and from 20 to 50% of the reaction zone length after the initiator introduction point is particularly susceptible to polymer deposits. These deposits are prevented by an increased temperature in the hot water jacket in this region. It may be assumed, without being tied to this theory, that mixing of the initiator with the polymerization stream does not occur quickly enough in this region, thus leading to polymer deposits. A hot water outlet temperature of from 180° C. to 210° C., preferably from 190° C. to 210° C., is preferably set in the hot water jacket between the initiator introduction point and from 20 to 40% of the reaction zone length, very particularly preferably between the initiator introduction point and from 20 to 30% of the reaction zone length, in order to prevent the polymer deposits. The hot water inlet temperature is usually from 1° C. to 30° C., preferably from 2° C. to 20° C. and particularly preferably from 2° C. to 10° C., lower than the respective hot water outlet temperature.
The process of the present invention can be employed either for the homopolymerization of ethylene or for the copolymerization of ethylene with other monomers, provided that these monomers copolymerize with ethylene by a free radical mechanism under high pressure. Examples of suitable copolymerizable monomers are &agr;,&bgr;-ethylenically unsaturated C
3
-C
8
-carboxylic acids, in particular maleic acid, fumaric acid, itaconic acid, acrylic acid, methacrylic acid and crotonic acid, &agr;,&bgr;-ethylenically unsaturated C
4
-C
15
-carboxylic esters or anhydrides, in particular methyl methacrylate, ethyl acrylate, n-butyl acrylate, methacrylic anhydride, maleic anhydride and itaconic anhydride, and &agr;,&bgr;-olefins such as propene, 1-butene, 1-pentene or 1-hexene. Vinyl acetate can also be used as comonomer. The proportion of comonomer or comonomers in the reaction mixture is preferably from 1 to 45% by weight, particularly preferably from 5 to 30% by weight. In these cases, the remainder of the reaction mixture is made up by ethylene.
The polymerization is carried out at pressures of from 500 to 5000 bar, preferably from 1500 to 4000 bar and particularly preferably from 2000 to 3300 bar. The reaction temperatures are above 40° C. It is advantageous for the reaction temperatures to be in the range from 120 to 350° C., preferably from 150 to 330° C.
In the process of the present invention, the molar mass of the polymers to be prepared can be controlled in a customary fashion by addition of molecular weight regulators. Examples of suitable regulators are aliphatic and olefinic hydrocarbons, e.g. pentane, hexane, cyclohexane, propene, pentene or hexene, ketones such as acetone, diethyl ketone o
Daiss Andreas
Littmann Dieter
Mähling Frank-Olaf
Basell Polyolefine GmbH
Cheung William
Keil & Weinkauf
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