Loop slurry process, high-density

Linear polyethylenes are produced in solution, slurry, and increasingly, gas-phase low-pressure processes. The Phillips process developed during the mid 1950s used supported chromium trioxide catalysts in a continuous slurry process (or particle-form process) carried out in loop reactors. Earlier, Standard Oil of Indiana patented a process using a supported molybdenum oxide catalyst. The polyethylenes made by both these processes are HDPE with densities of 0.950-0.965 g/cm and they are linear with very few side-chain branches and have a high degree of crystallinity. 

The process, set up with a loop reactor part and a gas phase reactor part, is similar to corresponding single reactor processes. The catalyst is mixed with propane diluent and fed into a pre-polymerisation reactor. Cocatalyst, ethylene, comonomer and hydrogen are also fed into this reactor. The pre-polymerised slurry, together with the main feeds, is then fed into the slurry loop reactor, designed for supercritical conditions and typically operated at 85 - 100 C and 5.5 - 6.5 MPa. This reactor produces a low molecular weight, high density product. The reactor content is sent to a flash tank where diluent and unreacted components are separated from the polymer produced in the loop reactor. The diluent is condensed and recycled back into the loop reactor. 

Slurry-phase polymerization involves a solid from the beginning of the polymerization process. An important example is the production of high-density polyethylene (HOPE) using immobilized, solid Ziegler-Natta catalysts in a solvent (typically liquid alkanes). Slurry phase polymerization is carried out in stirred tank reactors or loop reactors where the three-phase system is pumped with high flow velodties to reduce the probability of reactor fouling (for details see Section 6.20). 

Chevron Phillips Chemical Co., LP LPE process from Phillips Petroleum Co., isobutane slurry, loop reactor, very high activity proprietary catalysts comonomers butene-1 hexene-1, 1,4 methyl-1 pentene, and octene-1, no waxes and other by-products, minimum environmental emissions 82 reactor lines, 34% of worldwide capacity slurry-loop reactor. LPE homo- and co-polymers (density 920-970 kg/m ) for films, blow moulding, injection moulding, rotomoulding, pipes, sheets and thermoforming, and wire and cables. 

The overall conversion in the loop polymerization reactor is very high for a polyolefin manufacturing process, and the conversion per pass is limited by the amount of monomer that is needed to carry the polymer out of the reactor. It is desirable to operate the reactor at as high a slurry density as possible to get the highest throughput and the highest conversion. Typically, loop reactors operate at slurry concentrations of about 45-50 wt% polymer. 

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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