Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in reactor of specified material – or in reactor...
Reexamination Certificate
1998-12-31
2001-10-09
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in reactor of specified material, or in reactor...
C526S088000, C526S070000, C526S068000, C526S059000, C526S074000, C526S078000, C526S901000
Reexamination Certificate
active
06300429
ABSTRACT:
TECHNICAL FIELD
This invention relates to the gas phase, fluidized bed polymerization of olefins and other polymerizable monomers. It is a method of reducing the temperature near the inner wall of a gas phase polymerization reactor through the application of water to the outside of the reactor.
BACKGROUND OF THE INVENTION
The gas phase fluidized bed reactors for which this invention is most useful are those such as described in US Patents to Noshay et al U.S. Pat. No. 4,482,687, Goode et al U.S. Pat. No. 4,803,251, Chirillo et al U.S. Pat. No. 4,855,370, Painter et al U.S. Pat. No. 5,428,118, and Chinh et al U.S. Pat. No. 5,733,510. These and other reactors to which our invention is applicable are characterized by a straight cylindrical section topped by a bulbous expanded section and are widely used for polymerizing &agr;-olefins such as ethylene, propylene and others having up to about eight carbon atoms. They are large reactors having relatively thick steel walls, i.e. about two inches to about four inches thick.
The reactions conducted in such reactors generally call for polymerization of the monomer or monomers in the gas phase. A catalyst is also introduced, and, as polymerization proceeds, small particles of polymer product are formed, which are suspended in the gas as a fluid bed.
The polymerization process is exothermic, and accordingly it is common continuously to recycle a large portion of the gas to extract energy, i.e. the heat of reaction, from the system so the process can be controlled. As the amount of heat generated is directly related to the amount of polymer produced, the efficiency of heat removal is a prime determinant of productivity and yield.
Moreover, if the temperature in the reactor becomes too high, the particles will soften or even melt, and tend to stick together. In addition there is commonly a risk of a triboelectric effects among the particles and especially between the particles and the wall. Static charges are difficult to control and may be generated for reasons not completely understood, but their presence appears to be related to the temperature of the wall, the temperature of the gas in the fluidized bed, and the dew point of the gas. When the temperature of the interior wall becomes too high relative to the dew point of the gas, static charges are more likely to be present, leading to excess coating formation on the wall and the undesirable phenomenon of sheeting.
Since large gas phase reactors are commonly outdoors and exposed to the elements, they have often been rained upon, but no one, to our knowledge, has attempted to adapt the operation of the reactor to accommodate the effect of rain on the internal temperature of the reactor. Attempts to control the wall temperature include the cooling tube disclosed in Japanese patent application 9-302008, and the cooling jacket described in Japanese patent applications 9-249703 and 7062009 A. All three of these disclosures speak of cooling the inner wall to a temperature below the dew point of the gas phase by cooling the outside of the reactor, but such methods require expensive cooling equipment and circulating apparatus. Negative factors in the use of various types of cooling jackets include the difficulty of sealing them, and the common presence of manways and instrumentation which could be covered by the jacket. A major safety factor is that a reactor leak may go undetected, underneath the jacket, so that explosive gases are concentrated in the cooling jacket or delivered with the used water to a drain basin or the like.
Wall cooling has been used in fluidized bed combustion systems—see U.S. Pat. Nos. 5,025,755 and 4,944,150.
U.S. Pat. No. 3,254,070 illustrates an early polymerization reactor equipped with a cooling jacket
56
for removing heat of reaction.
Canadian patent 709,470 shows a water jacket for cooling a gas phase polymerization process reactor, as a supplement to recycling unreacted monomer through a heat exchanger to reduce the temperature of the gas.
The reader may also be interested in Hussein U.S. Pat. No. 4,981,929 in which the dew point of the gas phase is controlled through the addition of a nonreactant gas recirculated with the unreacted monomer. The reaction temperature and the dew point are caused to approach one another either by such an addition to elevate the dew point or by lowering the temperature of the reaction. In the preferred mode, the polymerization is conducted at a temperature 0.1 to 5.0 degrees Centigrade above the dew point of the cycle gas in the reactor.
SUMMARY OF THE INVENTION
Our invention provides a method of cooling the outside wall of a gas phase olefin polymerization reactor to effect a reduction of temperature on the inside wall and/or in the gas immediately adjacent to the interior wall. The method includes bathing the reactor with a flow of water around its periphery, or substantially around it, to create a sheet or film of falling water in contact with the surface of the reactor. As will be apparent in the further explanation below, it is not necessary to know the temperature of the water used to form the sheet or film of falling water. Our invention recognizes the complexities of the relationship between the dew point of the reactant gas and the polymerization temperature, and accordingly the application of water to the outside of the reactor may be regulated concurrently with the condensing of recycle gas as described in Jenkins U.S. Pat. Nos. 4,543,399 and 4,588,790, and the adjustment of the dew point through addition of diluent as in the above cited U.S. Pat. No. 4,981,929, to Hussein et al. All three of these patents (U.S. Pat. Nos. 4,453,399, 4,588,790, and 4,981,929) are incorporated herein by reference. The conditions of the polymerization reaction and processes recited in these patents are all compatible with our invention.
The film of falling water may originate at various heights. It may be applied initially in the expanded section or at any point below the expanded section on the straight (sometimes called the cylindrical) section. While it is preferred that the falling film of water encircle the reactor in order to stabilize conditions in the fluidized bed, one may apply more water to one area than another, or even apply it only on one portion of the outer surface of the reactor, or application may be moved from one area to another as temperature readings from inside the reactor may indicate to be desirable. However, we prefer that the film of water be applied around the entire, or substantially the entire, periphery.
The sheet of falling water is preferably assured by using sprays originating around the periphery of the reactor and directed at the reactor wall.
The application of water to the outer surface of the reactor may be intermittent or continuous. The water may be applied in response to electrostatic phenomena measured inside the reactor as a harbinger of possible sheet formation. The water may be applied in response to temperature readings from inside the reactor, particularly temperature readings at or close to the inner surface of the reactor wall, preferably {fraction (1/16)} inch to ½ inch from the inner surface of the wall
The application of water to the outer surface of the reactor may be in conjunction with condensing of the reactor recycle gas as described in Jenkins U.S. Pat. No. 4,543,399 and in U.S. Pat. No. 4,588,790, (incorporated by reference elsewhere herein) as well as in U.S. Pat. No. 5,804,677, 4,981,929 (also incorporated by reference because of its description of the introduction of nonreactant liquid to the recycle fluid), U.S. Pat. Nos. 4,933,148, 5,352,749, 5,405,922, 5,436,304, 5,453,471, 5,834,571, and 5,804,677. In a preferred embodiment, the dewpoint of the cycle gas is within about 15 degrees Centigrade of the reaction temperature, more preferably within about 10 degrees of the reaction temperature during condensing operation. In this mode, gases may condense along the internal surface of the reactor, and liquid is more easily distributed to the internal surface from the condens
Goode Mark Gregory
Madden James Daniel
Riley, Jr. James Laurence
Schwarz, Jr. George William
Spriggs Thomas Edward
Choi Ling-Siu
Union Carbide Chemicals & Plastics Technology Corporation
Wu David W.
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