In many current slurry polymerization processes, the slurry exits the polymerization reactor(s) and undergoes additional final (downstream) processing steps which may including heating the solids, removal of residual unreacted carrier fluid and/or catalyst system deactivation. As used herein, “slurry” shall mean a combination of polymer solids suspended in unreacted carrier fluid. The carrier fluid may be unreacted monomer and/or an inert diluent. However, prior to the final processing steps, the excess unreacted carrier fluid must be separated from the polymer. Typically, the separated, unreacted carrier fluid is recovered and recycled back to the reactor(s). The polymer and residual unreacted carrier fluid then continue downstream for the final processing steps where the polymer is ultimately sold or fed to an extruder for pelletization prior to sale.
Many current processes for the separation of a polymer slurry include the use of a series of heated jacketed pipes followed by a first separator, usually operating at high pressure, and finally a second separator, generally operating under low pressure conditions. The heated jacketed pipes are used to increase the temperature of the slurry. These pipes may be heated by steam, hot water or other similar methods commonly known in the art. The slurry is heated by the pipes to a temperature sufficient to vaporize at least a portion of the unreacted carrier fluid and is then fed into the first separator. The first separation of the vaporized portion of the unreacted carrier fluid from the polymer generally occurs at a pressure of at least 15 atmospheres (1.52 megapascals (MPa)) in the first separator. Separation may be achieved by methods including cyclone separators, gravity separators, bag filters or other methods commonly known in the art. Preferably, the vaporized, unreacted carrier fluid is recycled to the reactor(s) by methods commonly known in the art.
After the first separation, the slurry is enriched in polymer, but significant amounts of unreacted carrier fluid may still be present. In many current methods, the next processing step is the separation of the enriched slurry. The additional separation generally includes feeding the enriched slurry to a second separator. The second separator generally operates at a pressure of about 1 atmosphere (0.10 MPa) or less. Separation in this step may be achieved by use of a bag filter, though other methods commonly known in the art may also be employed. In the second separation step, the reduced pressure of the second separator causes an expansion of the remaining unreacted carrier fluid and a significant reduction in the temperature of the enriched slurry. Preferably, the additional unreacted carrier fluid recovered during the second separation is compressed and recycled to the reactor(s) by methods commonly known in the art.
Current processes for separation of a slurry, such as described above, are limited as to the temperature to which the slurry may be heated, particularly prior to entering the high pressure first separator, as the catalyst system has not been deactivated at that point and increased temperatures may cause unfavorable additional reactions to occur. This temperature limitation may, in turn, limit the amount of unreacted carrier fluid extracted from the slurry in both the first separator and the second separator, thereby burdening downstream processing equipment with the final removal of residual unreacted carrier fluid. Additionally, the expansion of the unreacted carrier fluid in the second separator due to reduced pressure may result in limitations on current operating rates and thus, reduced processing capacity. As used in this specification and in the claims “operating rate(s)” shall mean the quantity of polymer processed through a slurry polymerization unit over a specified period of time.
The equipment commonly used to perform the final, downstream processing steps is generally very large in size and often located in elevated structures to facilitate polymer particle movement. As such, replacement of currently installed equipment to accommodate increased operating rates and capacities may be difficult and costly.
Opportunities exist in the art for a method and apparatus to improve the efficiency of separating polymer from a slurry, including increasing recovery of unreacted carrier fluid, thereby increasing the overall operating rate and resulting capacity of a slurry polymerization unit. The present invention meets these and other needs.