Union Carbide, UNIPOL process

Figure 12-1. The Union Carbide Unipol process for producing HDPE (1) reactor, (2) single-stage centrifugal compressor, (3) heat exchanger, (4) discharge tank.

Gas-Phase Process. The gas-phase process is considered to be the most versatile low pressure process for producing polyethylene because it can make the broadest product portfolio in terms of molecular weight and density. It had been used since the 1960s to make HDPE and in 1977, Union Carbide built the first gas-phase plant for LLDPE production. Subseqnently, British Petroleiun and Himont developed alternative gas-phase processes for producing LLDPE. As a result of its versatility, it is the most widely licensed technology worldwide for linear low density production. A simplified schematic of the Union Carbide Unipol process is shown in Figure 20. 

Figure 6.20.6 Schematic view of a process for gas-phase poiymerization of ethene in a fluidized bed reactor (Union Carbide Unipol process). Adapted from Whiteley (2012).

In early 1980, Union Carbide expanded its manufacturing facilities in Seadrift, Texas, to manufacture the new grades of polyethylene summarized in Table 2.1. In addition, Exxon Chemical Company and Mobil Chemical Company were early licensees of the Union Carbide UNIPOL process in order to construct polyethylene manufacturing complexes in Saudi Arabia (Jubail, Saudi Arabia, was an Exxon site and Yanbu, Saudi Arabia, was a Mobil site), which came on stream in the early 1980s. In addition, Exxon also expanded its polyethylene manufacturing facilities in Baytown, Texas, while Mobil expanded its polyethylene manufacturing complex in Beaumont, Texas. Most of the polyethylene manufactured in Saudi Arabia was used to service European markets. 

Fluidized-bed reactors were developed by Union Carbide (Unipol Process) -currently Univation - and British Petroleum (BP). Some details in their configuration may vary, but their main characteristics are the same. In fluidized-bed reactors, gaseous monomers, chain-transfer agent, inerts, and catalysts are fed continuously into the reactor. The polymerization temperature should be kept well below the melting point of the polymer to avoid particle agglomeration, loss of fluidization, and bed collapse. Since polymer particles are formed in the gas phase in absence of diluent, there is no need for further separation steps when the product is exiting the reactor (except for removal of unconverted monomer), which is a clear advantage of gas-phase processes over slurry and solution processes. 

Union Carbide/UNIPOL Union Carbide introduced the gas-phase, fluid-bed process under the tradename of UNIPOL commercially in the late 1960s using a G-1250 reactor (reactor volume 1250 cubic feet) for the manufacture of HDPE. The LLDPE was added to the product mix in about 1977. Union Carbide formed a joint venture with Exxon in 1996 to pursue the addition of single-site catalysts to the... 

Using UNIPOL II process technology. Union Carbide introduced easy flow LLDPE products that had better processability than LLDPE with a narrow MWD and LDPE with an intermediate MWD. Union Carbide reported processability and product physical property data for these new resins [25]. Some of the data is summarized in Table 6.7 which compares a UNIPOL II easyflow resin with a high-pressure LDPE. 

Union Carbide Corp. also uses a siUca-supported chromium catalyst in their extremely low cost Unipol gas-phase linear low density ethylene copolymer process, which revolutionized the industry when it was introduced in 1977 (86—88). The productivity of this catalyst is 10 —10 kg polymer/kg transition metal contained in the catalyst. By 1990, the capacity of Unipol linear low density polyethylene reactors was sufficient to supply 25% of the world s total demand for polyethylene. 

UNIPOL [Union Carbide Polymerization] A process for polymerizing ethylene to polyethylene, and propylene to polypropylene. It is a low-pressure, gas-phase, fluidized-bed process, in contrast to the Ziegler-Natta process, which is conducted in the liquid phase. The catalyst powder is continuously added to the bed and the granular product is continuously withdrawn. A co-monomer such as 1-butene is normally used. The polyethylene process was developed by F. J. Karol and his colleagues at Union Carbide Corporation the polypropylene process was developed jointly with the Shell Chemical Company. The development of the ethylene process started in the mid 1960s, the propylene process was first commercialized in 1983. It is currently used under license by 75 producers in 26 countries, in a total of 96 reactors with a combined capacity of over 12 million tonnes/y. It is now available through Univation, the joint licensing subsidiary of Union Carbide and Exxon Chemical. A supported metallocene catalyst is used today. 

One recent development in Ziegler Natta catalysts was in producing catalyst particles that expanded as the polymerization reaction occurred. In this polymer the catalyst remains dispersed throughout the polymer, retaining its activity. This led to the development of fluidized bed processes to make polyethylene and polypropylene in which a sphere of polymer formed around each initial catalyst particle, and the polymer remained sohd as the reaction proceeded, rather than requiring a liquid solution. A major class of these catalysts and fluidized bed reactor was developed by Union Carbide and by Shell Oil and called the Unipol process. In this process a very active solid catalyst is introduced into the reactor, and reaction occurs on the catalyst particles, which expand to maintain active sites on the growing polymer sphere. 

Gas-phase processes commercialized in the late 1960s offer much simpler operation. Since they eliminate solvent, solvent separation, recovery, and purification are unnecessary. Polymerization is carried out in stirred-vessel reactors or in fluidized beds. A very successful fluidized-bed process is Union Carbide s UNIPOL technology444 designed originally for producing HDPE and extended later to LLDPE. Ethylene is polymerized by injecting the fine catalyst powder (organotitanium or... 

Mitsui Petrochemical Industries has developed a process for the gas-phase fluidizcd-bcd polymerization of propene, a plant using the process came on stream in 1984 [94] The Unipol-Shell process was jointly developed by Union Carbide and Shell and commissioned in 1986... 

A major step forward was taken in the area of base thermoplastics with the application of Union Carbide s Unipol process. Variations of this were subsequently offered by other low-density polyethylene (LDPE) producers such as Dow and CdF Chimie (now ORKEM). 

In 1978 Union Carbide reported a special manufacturing process called Unipol that gave linear low-density polyethylene (LLDPE). Linear low-density polyethylene may contain small amounts of butene or octene as co-monomers. The structural differences between HDPE, LDPE, and LLDPE are shown schematically in Fig. 6.1. These structural features determine physical properties such as elasticity, crystallinity, melt-flow index, etc. of the resultant polymers. 

Licensor Union Carbide Corp., a subsidiary of The Dow Chemical Co. Univation Technologies is the licensor of the UNIPOL PE process. 

Application To produce homopolymer, random copolymer and impact copolymer polypropylene using the Union Carbide gas-phase UNIPOL PP process. 

Unipol gas phase process for MDPE and HOPE commercialized by Union Carbide... 

Supported chromium catalysts were developed by Union Carbide Corporation in the 1970s using different chromium precursors than are used in standard Phillips catalysts (6,11). The most important of these are based on chromocene and bis(triphenylsilyl)chromate, depicted in Figure 5.5. These catalysts are used in the Unipol gas phase process for LLDPE and HOPE and are different from standard Phillips catalysts in several respects ... 

During the 1970s Jersey Standard s chemical product portfolio had been reduced to resemble that in 1960. Its primary chemical business remained the production of ethylene, basic polymers (including LDPE, linear low-density polyethylene (LLDPE) using the Unipol process, licensed from Union Carbide, PP, and polyvinyl chloride (PVC), plasticizers (particularly for vinyl production), elastomers, and synthetic rubbers. Profits returned. The 1974 balance sheet listed earnings of 456 million on revenue of 3.3 billion, making Exxon Chemical one of the five most profitable chemical companies worldwide. 

The use of fluidized beds for gas-phase polymerization started in 1968 with the UNIPOL process, which was developed by Union Carbide to make high-density polyethylene. This process has now been adapted to produce other grades of polyethylene as well as polypropylene and various copolymers. The fluid bed is composed of porous particles, which are aggregates of polymer containing fine grains of titanium, chromium, or other metal catalyst. Polymerization takes place at the polymer-catalyst interface, and the particles grow larger over a period of several hours. Some of the polymer is withdrawn continuously or at intervals to maintain the bed... 

During the late 1970s, Union Carbide developed a low-pressure polymerization process (Unipol process) capable of producing polyethylene in the gas phase that required no solvents. The process employed a chromium based catalyst. In this process (Figure 4.1) ethylene gas and solid catalysts are fed continuously to a fluidized bed reactor. The fluidized material is polyethylene powder which is produced as a result of polymerization of the ethylene on the catalyst. The ethylene, which is recycled, supplies monomer for the reaction, fluidizes the solid, and serves as a heat-removal medium. The reaction is exothermic and is normally run at temperatures 25-50°C below the softening temperatures of the polyethylene powder in the bed. This operation requires very good heat transfer to avoid hot spots and means that the gas distribution and fluidization must be uniform. 

The Unipol low-pressure gas-phase fluidized-bed process, which was introduced by Union Carbide in 1977 for LLDPE, has also been adapted to the production of PP homopolymers and block copolymers using Shell Chemicals high-activity (Ziegler-type) catalyst technology. The Spheripol and Unipol... 

Unipol Union Carbide s trade name for its low-pressure process that made possible the start of the large-volume utilization of linear low-density polyethylene and other plastics. 

In January 1957, DuPont filed for a patent, based on the finding that the incorporation of higher a-olefins in PE (US Patent 4076698 Arthur William Anderson, Gelu Stoeff Stamatoff—Filed 4 January 1957 published 28 February 1978 DuPont, for what we now call LLDPE) improved the product, but for DuPont, it appears that this ethylene copolymer was not really a very attractive venture compared to their other, high-margin proprietary products, like nylon. Although Du Pont of Canada introduced such a process in 1960, worldwide the products remained a small volume specialty until 1978, when Union Carbide announced their Unipol process, and actually coined the name linear low-density polyethylene (LLDPE). As we see later, since 1980, LLDPE has continued to increase its importance in the evolution of the portfolio of polyethylene products, likely to approach 1/3 of the total PE market by the end of this decade. 

Union Carbide Corporation developed a gas-phase process to make HOPE. In the gas-phase process, ethylene is catalyzed into HOPE in a fluidized-bed reactor consisting of catalyst-polymer particles. The polymerization occurs at the interface between the solid catalyst and the polymer matrix, which is swollen with monomers during polymerization. Ethylene is then easily separated from the HOPE particles which are then converted into pellets using an extrusion step. The first commercial gas-phase polymerization plant for making HOPE using a fluidized-bed reactor was constructed by Union Carbide in 1968 at Seadrift, Texas. Union Carbide also developed the dual reactor UNIPOL-II gas phase process in the 1980s to make bi-modal HOPE resins with superior mechanical properties compared to UNIPOL single-reactor HOPE resins. A world scale UNIPOL-II plant was constructed in 1996 to make differentiated bi-modal HOPE resins. 

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