Cooling ethylene, polymerization

Eor all these reasons heat removal and reliable temperature control are key factors in all technical ethylene polymerization processes to ensure an economical and safe process. The different processes may chose different ways to limit or remove the reaction heat (e.g., by limited conversion per reactor pass, cooling of unreacted monomer, large surface area for heat exchange) in all concepts heat management is a key aspect of the reactor design. 

In 1988 Howard W. Turner reported the preparation of a solid catalyst component based on the reaction of (BuCpl ZrCl and methylalumoxane at room temperature. The solid was isolated by cooling the reaction vessel to -30°C for one hour and then decanting the liquid layer and washing the solid material with pentane to yield a glassy solid. The solid with an Al/ Zr ratio of 20 was examined as a homogeneous ethylene polymerization catalyst using toluene as a solvent. Catalyst activity was 164 g PE/g catalyst at 80°C, 35 psi ethylene in 10 minutes [45]. 

The tower reactors similar to the ethylene polymerization reactors are used in other free-radical vinyl polymerization processes. Figure 4a shows a schematic of the tower reactor for bulk styrene polymerization developed by Farben in the 1930s [4]. The prepolymers prepared in batch prepolymerization reactors to about 33-35% conversion are transferred to a tower reactor whose temperature profile is controlled from 100°C to 200°C by jackets and internal cooling coils. There is no agitation device in the tower reactor. The product is then discharged from the bottom of the tower by an extruder, cooled, and pelletized. 

Continuous polymerization systems offer the possibiUty of several advantages including better heat transfer and cooling capacity, reduction in downtime, more uniform products, and less raw material handling (59,60). In some continuous emulsion homopolymerization processes, materials are added continuously to a first ketde and partially polymerized, then passed into a second reactor where, with additional initiator, the reaction is concluded. Continuous emulsion copolymerizations of vinyl acetate with ethylene have been described (61—64). Recirculating loop reactors which have high heat-transfer rates have found use for the manufacture of latexes for paint appHcations (59). 

In addition to the desired polymerization reaction, the dialcohol reactants can participate in deleterious side reactions. Ethylene glycol, used in the manufacture of polyethylene terephthalate, can react with itself to form a dialcohol ether and water as shown in Fig. 24.4a). This dialcohol ether can incorporate into the growing polymer chain because it contains terminal alcohol units. Unfortunately, this incorporation lowers the crystallinity of the polyester on cooling which alters the polymer s physical properties. 1,4 butanediol, the dialcohol used to manufacture polybutylene terephthalate, can form tetrahydrofuran and water as shown in Fig. 24.4b). Both the tetrahydrofuran and water can be easily removed from the melt but this reaction reduces the efficiency of the process since reactants are lost. 

The process in Figure 23-2 shows the compression of the ethylene in two stages. (There are more.) Ethylene will start to polymerize on its own in an uncontrolled fashion at 212°F, so in between compressors, the gas needs to be cooled. (Compression always makes the gas temperature rise. Thats why the bottom of your bicycle pump is hot after youVe filled your tire.)... 

MPa was obtained. The temperature was further elevated to 85°C and maintained at this temperature for 2 hours. At the end of polymerization the ethylene feed was stopped and the reactor quickly cooled, vented, and polyethylene powder isolated from hexane. Polymerization scoping reactions studies are provided in Table 1. 

The PEO salt complexes are generally prepared by direct interaction in solution for soluble systems or by immersion method, soaking the network cross-linked PEO in the appropriate salt solution [52-57]. Besides PEO, poly(propylene)oxide, poly(ethylene)suceinate, poly(epichlorohydrin), and polyethylene imine) have also been explored as base polymers for solid electrolytes [58]. Polyethylene imine) (PEI) is prepared by the ring-opening polymerization of 2-methyloxazoline. Solid solutions of PEI and Nal are obtained by dissolving both in acetonitrile (80 °C) followed by cooling to room temperature and solvent evaporation in vacuo. Polyethyleneimine-NaCF3S03 complexes have also been explored [59],... 

Because of the extremely high pressures (15,000 to 45,000 psig), ethylene exists in the liquid phase and polymerization occurs in solution. Owing to high temperatures (typically >200 °C), polyethylene is also dissolved in monomer and the reaction system is homogeneous. LDPE precipitates only after the reaction mass is cooled in post-reactor separation vessels. Relative to other processes, reactor residence times are very short (<30 seconds for the autoclave process and <3 min for the tubular process) (7). 

Free radical polymerization of neat monomer in the absence of solvent and with only initiator present is called bulk or mass polymerization. Monomer in the liquid or vapor state is well mixed with initiator in a heated or cooled reactor as appropriate. The advantages of this method are that it is simple, and because of the few interacting components present, there is less possibility for contamination. However, vinyl-type polymerizations are highly exothermic so that control of the temperature of bulk polymerization may be difficult. Also, in the absence of a solvent viscosities may become very high toward the end of a polymerization, which could make stirring difficult, and add to the difficulty of heat removal from the system. The advantages of this system, however, are sufficiently attractive for this to be used commercially for the free radical polymerization of styrene, methyl methacrylate, vinyl chloride, and also for some of the polymerization processes of ethylene [7]. 

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