UNIPOL® process

Unipol PP process Unipol process Uniprocessor computers UNIQUAC UNISITALE process... 

Reaction conditions are generally mild, but they differ from one process to another. In the newer Unipol process (Eigure 12-1) used to produce both HDPE and LLDPE, the reaction occurs in the gas phase. Ethylene and the comonomers (propene, 1-butene, etc.) are fed to the reactor containing a fluidized bed of growing polymer particles. Operation temperature and pressure are approximately 100°C and 20 atmospheres. A single-stage centrifugal compressor circulates unreacted ethylene. The circulated gas fluidizes the bed and removes some of the exothermic reaction heat. The product from the reactor is mixed with additives and then pelletized. New modifications for gas-phase processes have been reviewed by Sinclair. ... 

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

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. 

In the Unipol process of olefine polymerization 0.1 ixm particles of Ziegler Natta catalyst are... 

Chapter 7 is the climax of the book Here the educated student is asked to apply all that he/she has learned thus far to deal with many common practical industrial units. In Chapter 7 we start with a simple illustrative example in Section 7.1 and introduce five important industrial processes, namely fluid catalytic cracking in FCC units in Section 7.2, the UNIPOL process in Section 7.3, industrial steam reformers and methanators in Section 7.4, the production of styrene in Section 7.5, and the production of bioethanol in Section 7.6. 

A major accomplishment of this period was the development of the Unipol process for the production of linear, low-density polyethylene, a new process for an established commodity polymer that enabled exquisite manipulation and control over the materials properties of the product [2], This achievement was made through the efforts of scientists to develop new catalysts and of chemical engineers armed with new means of rational analysis and design of polymerization processes. 

Figure 19. Gas-phase polymerization of ethylene (Unipol process) [2] (a) compressor, (b) cooler, (c) catalyst feed hopper, (d) reactor, (e) separator...

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). 

The Unipol process employs a fluidized bed reactor (see Section 3.1.2) for the preparation of polyethylene and polypropylene. A gas-liquid fluid solid reactor, where both liquid and gas fluidize the solids, is used for Ziegler-Natta catalyzed ethylene polymerization. Hoechst, Mitsui, Montedison, Solvay et Cie, and a number of other producers use a Ziegler-type catalyst for the manufacture of LLDPE by slurry polymerization in hexane solvent (Fig. 6.11). The system consists of a series of continuous stirred tank reactors to achieve the desired residence time. 1-Butene is used a comonomer, and hydrogen is used for controlling molecular weight. The polymer beads are separated from the liquid by centrifugation followed by steam stripping. 

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. 

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. 

In addition, modern reactors now operate in condensed mode, which greatly increases their throughput. The Unipol process for LLDPE is now licensed by Univation. 

Both HDPE and LLDPE can be made using the Unipol process, although this process has found broader acceptance for the production of LLDPE. Some Unipol plants were designed and operated in the swing mode between HDPE and LLDPE, but most plants are designed only for LLPDE production. 

The Unipol process, initially developed for polyethylene production, was later extended to polypropylene manufacture. The process consists of a large fluidized-bed gas-phase reactor for homopolymer and random copolymer production, and a second smaller reactor for impact copolymer production. The second reactor is smaller than the first one because only 20% of the production comes from the second reactor. This reactor typically has a lower pressure rating as copolymerization is usually carried out at lower temperatures and pressures. Condensed mode operation is used in the homopolymer reactor but an inert diluent is not required because propylene is partially fed as a liquid. The copolymerization reactor is operated purely in the gas phase. The Unipol process has a unique and complex product discharge system that allows for very efficient recovery of unreacted monomer, but this does add complexity and capital cost to the process. 

The Novolen process [78], now licensed by Novolen Technology Holdings (NTH), uses the vertically stirred reactor for propylene polymerization. The basic process is similar to the Unipol process but with a simpler polymer discharge system. Novolen claims this process requires the lowest capital investment of its class and has low operating costs. 

Figure 2.6. Simplified flow diagram of Unipol process.

The Unipol process was first introduced by Union Carbide in its Seadrift, Texas, facility in 1968. Other companies such as Amoco and British Petroleum developed the technology further. Today over a 100 reactors are in operation (or in constmction) worldwide with an annual capacity of 19 billion pounds of HOPE or LLDPE resin. 

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