Phillips Particle Form Process

Figure 5 Phillips Particle-forming process a) catalyst hopper b) double loop reactor c) flash tank d) purge drier e) powder-fed extruder f) impeller g) sedimentation leg.

Stripped for solvent removal. The later Phillips "particle form" process is a slurry process in which the polymer precipitates as it forms. This process uses a circulating-loop reactor. Because of improved catalyst use efficiency, catalyst removal from the polymer is unnecessary. 

Figure 6.20.4 Schematic view of the loop reactor process (Phillips Particle Form process) for the suspension polymerization of ethene. Adapted from Whiteiey (2012).

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. 

Phillips catalysts were originally operated in solution or particle form (slurry) processes to produce HDPE, although the particle form process was preferred in 1983. In the 1960s, following a lead by Philhps, licensees began to develop gas phase processes using mechanical agitation (BASF) or fluidized beds (Un-... 

Phillips Par tide-forming process (Figure 5) In a double-loop reactor, constructed from wide-bore jaeketed pipe, the catalyst and growing polymer particles are suspended in a slurry and kept in rapid circulation to avoid polymer deposits on the reaetor walls. Due to its high surface-to-volume ratio, this reactor facilitates heat removal and allows short residence times. Typical reaction conditions are 100°C and 30-40 bar. Isobutane, a poor solvent for polyethylene, is used as a diluent and as a vehicle to introduce the catalyst into the reactor. The solid polymer is collected from a sedimentation leg and passed to a flash tank where the monomer and isobutane diluent are separated by evaporation and subsequently recondensed and recycled, while the polymer powder is fed into an extruder and formed into pellets. 

Polymerizations that use supported chromium (Phillips) catalysts are conducted predominantly in slurry processes (though a small portion employs the gas phase process, see below). The historical development of the Phillips process has been expertly reviewed by Hogan (5, 6) and McDaniel (7-9). The slurry process originally developed by Phillips Petroleum (now Chevron Phillips) has been called the "particle form loop slurry process" and the "slurry loop reactor process" for production of HDPE and LLDPE (10). Hexene-1 is most often used as comonomer for LLDPE in the Phillips process. A simplified process flow diagram for the Phillips loop-slurry reactor process is shown in Figure 7.3 and key operating features are summarized in Table 7.4. 

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 ethylene polymerization of this catalyst was carried out in an autoclave reactor at 221°F in isopentane as the slurry solvent in the presence of triisobutylaluminum as cocatalyst and 50 psig of hydrogen and sufficient ethylene to achieve a total reactor pressure of 550 psig. The catalyst activity was 10,540 g of PE/g of catalyst/ hr, which corresponded to an activity of 146,000 g PE/g Ti/hr. The granular polyethylene product obtained was considered suitable for a particle-form slurry process such as the Phillips slurry process. The polyethylene sample displayed a Melt Index (I value of 0.70 and a High Load Melt Index ) value (HLMI) of 3 1 with a HLMI/MI ratio of 45, which indicates tfiat the polyethylene molecular weight distribution was of an intermediate value. 

The selection and treatment of the support is fundamental to the process, and a plant may use catalysts made from a variety of supports to produce a whole range of products. Catalyst productivities are of the order of 5 kg of polyethylene per gram of catalyst or higher, with a corresponding chromium content of 2 ppm or less. The percentage of Cr atoms that form active polymerisation centres has been estimated as 12% [43]. Typically, commercial Phillips catalysts contain ca 1 % total Cr and have particle sizes of 30-150 pm [224]. 

The quality of the carbon black powder (Figure 6) product was evaluated as it impacts the overall economics of the process. A transmission electron photomicrograh (TEM) of a carbon black powder produced at a power level of 8.5 kW 2000 K) indicates a primary particle size of between 20 and 40 nanometers. In addition, the solar-thermally produced carbon black is compared to Shawinigan carbon black (tradename of Chevron-Phillips Corp.), the high quality world standard specialty carbon black. A comparative TEM image indicates that the carbon particles being formed have a similar chainlike structure as compared to the Shawinigan black. An X-ray diffraction pattern of the carbon... 

This technology accoxmts for an annual production of 4-5 million tons of HDPE. The process involves polymerization of ethylene at temperatures below the melting point of the polymer using a solid catalyst to form solid polymer particles suspended in an inert hydrocarbon diluent. Recovery of polymer (by filtration, centrifugation, or flashing) is economic. The chromium oxide-on-silica catalyst developed by Phillips yields polymers which can be easily extruded and blow-moulded. The process is unable to produce copolymers of density below 0.937 g/cm. For a density of 0.92 g/ cm or below, the polymer swells, becomes sticky, and starts to dissolve in the reaction diluent. 

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