Slurry Suspension Process

Polymerizations may be conducted in diluents in which polyethylene is insoluble at the process temperature. Such processes are termed slurry (or suspension) processes. 

Diluents must be inert toward the catalyst system and are usually saturated hydrocarbons such as propane, isobutane and hexane. Slurry processes typically operate at temperatures from about 80 to 110 °C and pressures of 200-500 psig. Polyethylene precipitates as formed resulting in a suspension of polymer in diluent. The catalysts most commonly used in slurry processes are chromium-on-silica or supported Ziegler-Natta catalysts. 

Another well-known slurry (suspension) process was developed by what was then Hoechst in Germany in the mid-1950s. Hoechst was the first licensee to use the catalyst and process developed by Karl Ziegler for producing low pressure 

Features - particles of growing polymer form as suspension in hydrocarbon diluent - catalyst residence time 1 hour for Phillips loop slurry process - morphology and psd of catalyst are important - wide range of comonomers may be used 

The Hoechst slurry process was improved over the years and has evolved into what is now called the Hostalen process. Hostalen is a slurry-cascade process that is capable of producing a wide range of molecular weight distributions of HOPE. The modern Hostalen process employs 2 continuous stirred tank reactors that can be run in series or in parallel to produce unimodal and bimodal HOPE (11). 

---

Table 7.4 Typical operating features of slurry/suspension processes for LLDPE and HOPE.

A solvent is needed as the carrier for catalyst or initiator feeds or as the reactor diluent for the solution and slurry suspension processes, while a comonomer is used to control the polymer density of the final product. In principle, the more volatile the hydrocarbon solvent and comonomer, the easier the separation from the polymer will be. However, there are some practical limitations ... 

HDPE slurry suspension process - the maximum viscosity of the slurry limits the maximum concentration of polymer solids in the hydrocarbon diluent. The slurry has to be maintained transportable. Depending on particle size distribution this means that typically the solid concentration has to be maintained between 30 and 35 vol-%. 

Slurry (Suspension) Polymerization. This polymerization technology is the oldest used for HDPE production and is widely employed because of process engineering refinement and flexibHity. In a slurry process, catalyst and polymer particles are suspended in an inert solvent, ie, a light or a... 

Freeze Crystallization. Freezing may be used to form pure ice crystals, which are then removed from the slurry by screens sized to pass the fine sohds but to catch the crystals and leave behind a more concentrated slurry. The process has been considered mostly for solutions, not suspensions. However, freeze crystallization has been tested for concentrating orange juice where sohds are present (see Fruit juices). Commercial apphcations include fmit juices, coffee, beer, wine (qv), and vinegar (qv). A test on milk was begun in 1989 (123). Freeze crystallization has concentrated pulp and paper black hquor from 6% to 30% dissolved sohds and showed energy savings of over 75% compared with multiple-effect evaporation. Only 35—46 kJ/kg (15—20 Btu/lb) of water removed was consumed in the process (124). 

In a typical batch suspension process (Figure 12-5), styrene is suspended in water by agitation and use of a stabilizer. The polymer forms beads. The bead/water slurry is separated by centrifugation, dried, and blended with additives. 

The process of thickening involves the concentration of a slurry, suspension, or sludge, usually by gravity settling. Because concentrated suspensions and/ or fine particle dispersions are often involved, the result is usually not a complete separation of the solids from the liquid but is instead a separation into a more concentrated (underflow) stream and a diluted (overflow) stream. Thickeners and clarifiers are essentially identical. The only difference is that the clarifier is designed to produce a clean liquid overflow with a specified purity, whereas the thickener is designed to produce a concentrated underflow product with a specified concentration (Christian, 1994 Tiller and Tarng, 1995 McCabe et al., 1993). 

Slurry phase (or suspension) process. The uniquedooldng equipment in Figure 23—5 is a loop reactor. This process also takes place in a solvent (in this case, normal hexane, isobutane, or isopentane) so that the mixture can be pumped continuously in a loop while the polymerization is taking place. Feeds (the solvent, comonomer if any, ethylene and Ziegler-Natta catalyst) are pumped into the loop and circulated. Polymerization rakes place continuously at temperatures below the melting point of the polyethylene allowing solid polymer particles to form enough to form slurry. The reaction takes place at 185—212°F and 75—150 psi. A slurry of HOPE in hexane is drawn off continuously or intermittently. 

Commerical polymerizations of ethylene, propene, and other a-olefins are carried out as slurry (suspension) and gas-phase processes [Beach and Kissin, 1986 Diedrich, 1975 Lieberman and Barbe, 1988 Magovern, 1979 Vandenberg and Repka, 1977 Weissermel et al., 1975]. Solution polymerization has been used in the past for ethylene polymerization at 140-150°C, pressures of up to 8 MPa (1 MPa = 145 psi = 9.869 atm), using a solvent such as cyclohexane. The solution process with its higher temperatures was employed for polymerization with the relatively low efficiency early Phillips initiators. (Polyethylene, but not the initiator, is soluble in the reaction medium under the process conditions.) The development of a variety of high-efficiency initiators has allowed their use in lower-temperature suspension and gas-phase processes, which are more advantageous from many... 

Crystallization A process in which a liquid solution is cooled, or solvent is evaporated, to an extent that solid crystals of solute form. The crystals in the slurry (suspension of solids in a liquid) leaving the crystallizer may subsequently be separated from the liquid in a filler or centrifuge. 

Suspension Process. In the suspension process, the catalyst is slurried with a small amount of the methyl ester and then fed into the reactor together with the rest of the ester. 

Most single site catalysts are homogeneous (1). Consequently, psd and morphological control of catalyst and polymer particles are not possible. For solution processes, this is of little concern. However, for slurry/suspension and... 

The downstream slurry tank process becomes fully continuous. The bead/water slurry is centrifuged (3) so that most of the "suspension water" is removed. The beads are conveyed to a pneumatic-type flash dryer (4) where surface moisture is removed. 

A systematic study has been performed on the protonation reactions of the hydrides HRu3(CO)9 C2R(OH)R (Fig. 21a) in organic solvents the yields of dehydrated products increase either with time or on increasing the acid concentration, thus indicating direct electrophilic attack of H" " on the alcoholic OH in a process comparable with the protonation mechanisms reported for alcohols on surfaces. Very low yields of dehydrated products were, however, obtained on the silica of the TLC plates the same occurred when hydration reactions of the HRu3(CO)9 C2C(=CH2)R derivatives were attempted either on TLC plates or in a slurry (suspension) of silica and water in hydrocarbon solvents. This indicates that surfaee materials play a minor role in these reactions. 

---