Continuously Operated High-Pressure Polymerization Reactors

3 Continuously Operated High-Pressure Polymerization Reactors 

In addition to the above investigations, free-radical high-pressure polymerizations should also be studied in continuously operated devices for three reasons. (1) Because of the wealth of kinetic information contained in the polymer properties, product characterization is mandatory. Sufficient quantities of polymer, produced under well defined conditions of temperature, pressure, and monomer conversion, are best provided by continuous polymerization, preferably in a continuously stirred tank reactor (CSTR). (2) Copolymerization of monomers that have rather dissimilar reactivity ratios, such as in ethene-acry-late systems, will yield chemically inhomogeneous material if the reaction is carried out in a batch-type reactor up to moderate conversion. To obtain larger quantities of copolymer of analytical value, the copolymerization has to be performed in a CSTR. (3) Technical polymerizations are exclusively run as continuous processes. Thus, in order to stay sufficiently close to the application and to investigate aspects of technical polymerizations, such as testing initiators and initiation strategies, fundamental research into these processes should, at least in part, be carried out in continuously operated devices. 

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A schematic of a continuous bulk SAN polymerization process is shown in Figure 4 (90). The monomers are continuously fed into a screw reactor where copolymerization is carried out at 150°C to 73% conversion in 55 min. Heat of polymerization is removed through cooling of both the screw and the barrel walls. The polymeric melt is removed and fed to the devolatilizer to remove unreacted monomers under reduced pressure (4 kPa or 30 mm Hg) and high temperature (220°C). The final product is claimed to contain less than 0.7% volatiles. Two devolatilizers in series are found to yield a better quaUty product as well as better operational control (91,92). 

Rexene Co. and Philips Petroleum Co. first developed the bulk polymerization process with the first-generation TiCU catalyst [8,11,70]. It was then commercialized by Dart Industries in 1964. The reactor feed contains 10-30% propylene in the liquid phase. A mixture of hexane and isopropanol was employed for the removal of catalyst residue as well as the amorphous polypropylene. The process step of removing residual catalyst was later eliminated after the high-efficiency catalyst was adopted, constituting the so-called liquid pool process. Subsequently, Philips and Sumitomo companies further developed the liquid-phase polymerization process. This process enhances the reaction rate, catalyst efficiency, monomer conversion, and therefore results in high productivity. It also eliminates the need for solvent recovery and reduces environmental pollution. However, the process is somewhat complicated by the unreacted monomer, which has to be first vaporized and then liquefied before it is reused. The reaction vessel must be designed to operate under high pressures. In most cases, this process employs autoclaves for batch operation and tubular reactors for continuous operation. 

In most experimental research studies, polymerizations are carried out batchwise. For operations on a commercial scale, continuous polymerization may be attractive. This applies specifically to the free radical polymerization of ethylene under very high pressure. Of course, compressors can only operate continuously, and the very short residence time in the reactor (on the order of one minute) is only practical in continuous operation. 

The conventional stirred-tank reactor is an agitated vessel, typically a jacketed pressure vessel, and often with provisions for reflux of a solvent or monomer. The continuous-feed version is the CSTR. Continuous operation is typical of high-volume polymers but large batch and fed-batch stirred-tank reactors are occasionally used. Reactors other than stirred tanks may be functionally equivalent to stirred tanks. Loop reactors are widely used in the polymer industry, especially for solution and slurry olefin polymerizations the agitator in the stirred tank is replaced with a circulation pump. The loop many consist of jacketed pipe or there may be heat exchangers and even flash vessels in the loop. The loop may consist of many legs for space considerations, but the legs are connected in series and there is only one circulation pump. 

Polypropylene is made by polymerizing high-purity propylene gas recovered from cracked gas streams in olefin plants and oil refineries. The polymerization reaction is a low-pressure process that utilizes Ziegler-Natta catalysts (aluminum alkyls and titanium halides). The catalyst may be slumed in a hydrocarbon mixture to facihtate heat transfer. The reaction is carried out in batch or continuous reactors operating at temperatures between 50 and 80 C and pressure in the range of 5 to 25 atm. 

A control technique based on high-frequency pressure measurements was developed and implemented to avoid hydrodynamic instabilities in continuous olefin slurry-loop reactors [ 186]. The obtained high-frequency pressure patterns are compared to typical process responses and then used to classify the status of the plant operation. The idea is that pressure fluctuations that do not follow the standard pattern indicate some sort of process instabiUty. When hydrodynamic instabilities are detected, monomer flow rates and/or reactor temperatures are manipulated to reduce the polymer density and the reaction rates and reduce the risks of plant shutdown. Similar procedures can be used for detection and correction of abnormal plant operation in suspension [ 187] and emulsion [188] polymerizations with the help of Raman and near infrared spectroscopy techniques. 

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