The present disclosure relates generally to polyolefin production, and more particularly, to membrane fractionation systems employed in polyolefin production to facilitate diluent recovery.
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present disclosure, 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 disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
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 various everyday items. For example, polyolefin polymers, such as polyethylene, polypropylene, and their copolymers, are used for retail and pharmaceutical packaging, food and beverage packaging (such as juice and soda bottles), household containers (such as pails and boxes), household items (such as appliances, furniture, carpeting, and toys), automobile components, pipes, conduits, and various industrial products.
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 polyolefin or type of polyolefin is suitable.
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.
To satisfy this demand, various processes exist by which olefins may be polymerized to form polyolefins. Typically, these processes are performed at or near petrochemical facilities, which have 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 to form polymer (polyolefin) solid particulates, typically called fluff or granules. 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), copolymer content, comonomer content, modulus, and crystallinity. The reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, and so forth, may be selected to achieve the desired fluff properties.
In addition to the one or more olefin monomers and/or comonomers, a catalyst for facilitating the polymerization may be added to the reactor. For example, the catalyst may include particles added to the reactor in a reactor feed stream to produce catalyst particles suspended in the fluid medium within the reactor. An example of such a catalyst is a chromium oxide containing hexavalent chromium on a silica support. Further, a diluent may be introduced into the reactor. The diluent may be an inert hydrocarbon, such as isobutane, propane, n-pentane, i-pentane, neopentane, and n-hexane that is liquid at reaction conditions. Further, 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.
The effluent discharged from the reactor typically includes the polymer fluff as well as non-polymer components, such as unreacted olefin monomer (and comonomer), diluent, inerts, other hydrocarbons, and so forth. In the case of polyethylene production in liquid phase reactors, such as loop slurry reactors, the non-polymer components primarily include diluent, such as isobutane, having a small amount of unreacted ethylene (e.g., 5 wt. %) and other entrained hydrocarbons. For polypropylene production, the non-polymer components primarily include unreacted propylene monomer having a small amount of other entrained hydrocarbons. The reactor effluent is generally processed, such as by an effluent treatment system, to separate the non-polymer components from the polymer fluff. The polymer fluff may then be treated to deactivate residual catalyst, remove entrained hydrocarbons, dry the polymer, and pelletize the polymer in an extruder, and so forth, before the polymer is sent to a customer.
The non-polymer components, such as the recovered diluent, unreacted monomer, and other non-polymer components from the effluent treatment system, may be treated within a fractionation system to separate most of the recovered diluent from the other non-polymer components. The recovered diluent may ultimately be returned as purified or treated feed to the reactor while the other non-polymer components may be flared or returned to the supplier, such as to an olefin manufacturing plant or petroleum refinery. Typically, the fractionation system may employ fractionation columns to separate the diluent from the other non-polymer components. One or more of the fractionation columns may employ cold temperatures to facilitate separation of some of the components, particularly those with lower boiling points than the diluent. To achieve the cold temperatures, refrigeration systems may be employed within the fractionation columns. However, it is now recognized that the refrigeration systems may be costly to operate, install, and/or maintain.