Polyolefins such as polyethylene and polypropylene may be prepared by particle form polymerization, such as slurry polymerization or gas phase polymerization.
Olefin polymerizations are frequently carried out using monomer, diluent and catalyst and optionally co-monomers and hydrogen in a reactor. The polymerization is usually performed under slurry conditions, wherein the product consists usually of solid particles and is in suspension in a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. The product is discharged by means of settling legs, which operate on a batch principle to recover the product. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product is further discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers are flashed off and recycled. Alternatively, the product slurry may be fed to a second loop reactor serially connected to the first loop reactor where a second polymer fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polymer product, which comprises a first polymer fraction produced in the first reactor and a second polymer fraction produced in the second reactor, has a bimodal molecular weight distribution.
Slurry polymerization in a loop reaction zone has proven commercially successful. The slurry polymerization technique has enjoyed international success with billions of pounds of polyolefins being so produced annually.
A variety of equipment and operations within a polyolefin manufacturing process may consume energy. Noteworthy consumers of electricity within a polyolefin plant, for example, may include the pumps that circulate the liquid reaction mixture in the polymerization reactors (e.g., loop slurry reactors), the pumps that circulate the cooling medium (e.g., treated water) through the polymerization reactor jackets, the compressors that pressurize and return recycled diluent (and/or monomer) to the polymerization reactor, the blowers used to convey fluff and pellets, and the extruders that convert the polyolefin fluff to polyolefin pellets. Significant users of steam in a typical polyolefin plant may include heaters that flash liquid in the effluent of the polymerization reactor, and fractionation columns that process recovered diluent and/or monomer. Relatively large consumers of fuel gas may include activation processes (which may utilize high heat) of the polymerization catalyst, and operations that maintain adequate combustible content in the plant flare header (in the feed to the flare). In general, extensive energy is required to polymerize the monomer and comonomer to polyolefin fluff, to process recycled effluent from the reactor, and to convert the polyolefin fluff to pellets.
Therefore, the production of polyolefin is an energy-intensive process, consuming electricity, steam, fuel gas, and so on. Such energy consumption generally contributes significant cost to the production of polyolefins, as well as to the downstream products constructed of polyolefins distributed to the customer.
WO 2006/026493 describes a manufacturing process for producing polyolefin configured to consume less than about 445 kilowatt-hours of electricity per metric ton of polyolefin produced.
However, there is still a need to improve the energy efficiency of polyolefin production processes.