Chain Transfer Agent Removal Between Polyolefin Polymerization Reactors (Chevron Phillips)

CATEGORY: OLEFINS
Chain Transfer Agent Removal Between Polyolefin Polymerization Reactors (Chevron Phillips)
United States Patent Application 20150353652
December 10, 2015
Assignee: Chevron Phillips Chemical Company LP
Abstract
A system and method for polymerizing olefin in the presence of a chain transfer agent in a first reactor to form a first polyolefin, discharging from the first reactor a transfer slurry having the first polyolefin and the chain transfer agent, and processing the transfer slurry in a separator to remove chain transfer agent and to provide a fluff slurry having the first polyolefin and a lower content of chain transfer agent than in the transfer slurry. The system and method provide for feeding the fluff slurry to a second reactor, polymerizing olefin in the second reactor to form a second polyolefin, and discharging from the second reactor a slurry having the second polyolefin.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates generally to polyolefin production with multiple polymerization reactors and, more particularly, to removing or recycling a chain transfer agent such as hydrogen from a polyolefin slurry flowing between polymerization reactors in series.
[0004] 2. Description of the Related Art
[0005] This section is intended to introduce the reader to aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
[0006] As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into everyday items. Polyolefin polymers such as polyethylene, polypropylene, and their copolymers, are used for piping, retail and pharmaceutical packaging, food and beverage packaging, plastic bags, toys, carpeting, various industrial products, automobile components, appliances and other household items, and so forth.
[0007] Specific types of polyolefins, such as high-density polyethylene (HDPE), have particular applications in the manufacture of blow-molded and injection-molded goods, such as food and beverage containers, film, and plastic pipe. Other types of polyolefins, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), isotactic polypropylene (iPP), and syndiotactic polypropylene (sPP) are also suited for similar applications. The mechanical requirements of the application, such as tensile strength and density, and/or the chemical requirements, such thermal stability, molecular weight, and chemical reactivity, typically determine what type of polyolefin is suitable.
[0008] One benefit of polyolefin construction, as may be deduced from the list of uses above, is that it is generally non-reactive with goods or products with which it is in contact. This allows polyolefin products to be used in residential, commercial, and industrial contexts, including food and beverage storage and transportation, consumer electronics, agriculture, shipping, and vehicular construction. The wide variety of residential, commercial and industrial uses for polyolefins has translated into a substantial demand for raw polyolefin which can be extruded, injected, blown or otherwise formed into a final consumable product or component.
[0009] To satisfy this demand, various processes exist by which olefins may be polymerized to form polyolefins. These processes may be performed at or near petrochemical facilities, which provide ready access to the short-chain olefin molecules (monomers and comonomers), such as ethylene, propylene, butene, pentene, hexene, octene, decene, and other building blocks of the much longer polyolefin polymers. These monomers and comonomers may be polymerized in a liquid-phase polymerization reactor and/or gas-phase polymerization reactor. As polymer chains develop during polymerization in the reactor, solid particles known as "fluff" or "flake" or "powder" are produced in the reactor.
[0010] The fluff may possess one or more melt, physical, rheological, and/or mechanical properties of interest, such as density, melt index (MI), melt flow rate (MFR), comonomer content, molecular weight, crystallinity, and so on. Different properties for the fluff may be desirable depending on the application to which the polyolefin fluff or subsequently pelletized polylefin is to be applied. Selection and control of the reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, catalyst type, and so forth, may affect the fluff properties.
[0011] In addition to the one or more olefin monomers, a catalyst (e.g., Ziegler-Natta, metallocene, chromium-based, post-metallocene, nickel, etc.) for facilitating the polymerization of the monomers may be added to the reactor. For example, the catalyst may be a particle added via a reactor feed stream and, once added, suspended in the fluid medium within the reactor. Unlike the monomers, catalysts are generally not consumed in the polymerization reaction. Moreover, an inert hydrocarbon, such as isobutane, propane, n-pentane, i-pentane, neopentane, and/or n-hexane, may be added to the reactor and utilized as a diluent to carry the contents of the reactor. However, some polymerization processes may not employ a separate diluent, such as in the case of selected examples of polypropylene production where the propylene monomer itself acts as the diluent. In general, the diluent may facilitate circulation of the polymer slurry in the reactor, heat removal from the polymer slurry in the reactor, and so on.
[0012] The slurry discharge of the reactor typically includes the polymer fluff as well as non-polymer components such as unreacted olefin monomer (and comonomer), diluent, and so forth. This discharge stream is generally processed, such as by a diluent/monomer recovery system (e.g. flash vessel or separator vessel, purge column, etc.) to separate the non-polymer components from the polymer fluff. The recovered diluent, unreacted monomer, and other non-polymer components from the recovery system may be treated and recycled to the reactor, for example. As for the recovered polymer (solids), the polymer may be treated to deactivate residual catalyst, remove entrained or dissolved hydrocarbons, dry the polymer, and pelletize the polymer in an extruder, and so forth, before the polymer is sent to customer.
[0013] In some circumstances, to increase capacity of a polyolefin polymerization line or to achieve certain desired polymer characteristics, more than one polymerization reactor may be employed, with each reactor having its own set of conditions. In certain examples, the reactors (e.g., loop reactors) may be connected in series, such that the polymer slurry from one reactor may be transferred to a subsequent reactor, and so forth, until a polyolefin polymer is produced discharging from the final or terminal reactor with the desired set of characteristics. The respective reactor conditions including the polymerization recipe can be set and maintained such that the polyolefin (e.g., polyethylene, polypropylene) polymer product is monomodal, bimodal, or multimodal.
[0014] The competitive business of polyolefin production drives manufacturers in the continuous improvement of their processes in order to lower production costs, improve product quality, increase operating flexibility and capability, expand product slate variety and capability, and so on. In an industry where billions of pounds of polyolefins are produced per year, small incremental improvements, such as in product quality and variety can result in significant economic benefit, increased sales, larger market share, greater price margins and netback, and so forth.
SUMMARY OF THE INVENTION
[0015] An aspect of the invention relates to a polyolefin reactor system including: a first reactor configured to produce a first reactor discharge having a diluent and a first polyolefin; a diluent separator configured to receive at least a portion of the first reactor discharge and to form a diluent recycle stream comprising diluent and a separator product stream comprising diluent and the first polyolefin; a second reactor configured to receive the diluent separator product stream and to produce a second reactor discharge having a second polyolefin; and a diluent feed stream configured to introduce fresh diluent to at least one of the diluent separator, the diluent separator product stream, or the second reactor, such that the second reactor has a lower hydrogen content than the first reactor.
[0016] Another aspect of the invention relates to a polyolefin reactor system including: a first polymerization reactor configured to polymerize olefin monomer in the presence of a chain transfer agent, and to discharge continuously a transfer slurry comprising a first polyolefin and the chain transfer agent; a separation system configured to receive the transfer slurry and to discharge a recycle stream having the chain transfer agent, and to discharge continuously a fluff slurry having the first polyolefin; and a second polymerization reactor configured to receive the fluff slurry from the separation system and to discharge a product slurry having a second polyolefin.
[0017] Yet another aspect of the invention relates to a polymerization reactor system including: a first loop reactor configured to polymerize olefin monomer in the presence of a chain transfer agent to produce a first polyolefin, and to discharge continuously a transfer slurry having the first polyolefin and the chain transfer agent; a separation system configured to remove the chain transfer agent from the transfer slurry, and to discharge continuously a fluff slurry having the first polyolefin to a second loop reactor; and the second loop reactor configured to polymerize olefin monomer to produce a second polyolefin, and to discharge a product slurry comprising a product polyolefin having comprising the first polyolefin and the second polyolefin.
[0018] Yet another aspect of the invention relates to a method of operating a polyolefin reactor system, including: polymerizing olefin in the presence of a chain transfer agent in a first reactor to form an intermediate polyolefin; discharging continuously from the first reactor a transfer slurry having the intermediate polyolefin and the chain transfer agent; continuously processing the transfer slurry to remove chain transfer agent and to provide a fluff slurry having the intermediate polyolefin and a lower content of chain transfer agent than in the transfer slurry; feeding the fluff slurry to a second reactor; polymerizing olefin in a second reactor to form a product polyolefin; and discharging from the second reactor a product slurry having the product polyolefin.
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