Polymerization high pressure

An independent development of a high pressure polymerization technology has led to the use of molten polymer as a medium for catalytic ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization at a high pressure (see Olefin polymers, low density polyethylene) have been converted to accommodate catalytic polymerization, both stirred-tank and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C (57,83,84). CdF Chimie uses a three-zone high pressure autoclave at zone temperatures of 215, 250, and 260°C (85). Residence times in all these reactors are short, typically less than one minute. 

PE produced by a high-pressure polymerization process (pressure 1000-3000 atm) using a free radical initiator is a highly branched material that contains both LCBs and SCBs. The polymer so produced is a low-density material (density up to about 0.925 g/cc) and is known as high-pressure low-density PE (HP LDPE). The LCBs are formed via intermolecular hydrogen transfer [19], whereas SCBs are formed by intramolecular hydrogen abstraction [16]. 

High-pressure polymerization of ethylene was introduced in the 1930s. The discovery of a new titanium catalyst hy Karl Ziegler in 1953 revolutionized the production of linear unhranched polyethylene at lower pressures. The two most widely used grades of polyethylene are low-density polyethylene (LDPE) and high-density polyethylene (HDPE). Currently,... 

For convenience, we will use the abbreviation S for high-pressure polymeric sulfur since the spectra are analogous to the STP polymer which is often called Sj,... 

Figure 5.11 Example of backbiting reaction to form a butyl branch during the high pressure polymerization of polyethylene...

Thomas, P. W., "Multi-Stage Over-Pressure Protection and Product Containment on High Pressure Polymerization Reactors," I. Chem. Eng. Symposium Series, 85, 229 (1984). 

Polymerization of methyl methacrylate to Plexiglas is done in the bulk process. High pressure polymerization of ethylene is done this way also. But other addition polymerizations frequently become too exothermic and without adequate heat removal system, the reaction tends to run away from optimum conditions. 

The English experience eventually developed into the high-pressure polymerization route to Low Density Polyethylene (LDPE). The German experiment was the forerunner of the low-pressure route to High Density Polyethylene (HDPE). 

Because of their lower level of crystallinity these copolymeis soften at lower temperatures than polyethylene. Table I compares the torsional stiffness at various temperatures of an 18% ethyl acrylate-82 % ethylene copolymer with that of a 21,000 molecular weight, 0.918 density polyethylene made by high pressure polymerization (Bakelite DYNH, Union Carbide Corp.). 

Rees and Vaughn (26) state that this type of ionomer is prepared by high pressure polymerization using free radical initiators. They are presumably random copolymers. The crystallinity as determined by x-ray diffraction is about 10% (26). 

The radical polymerization of ethylene, in practice, is initiated by free-radical initiators, although radiation-induced,155 210 photoinduced,211-213 and thermal213,214 initiations are also possible. The temperature of high-pressure polymerization should not exceed 350°C since above this temperature a rapid exothermic (AH = —30.4kcal/mol) thermal decomposition of ethylene can take place leading to a runaway reaction ... 

Figure 5.1 -1. Flow sheet of the industrial high-pressure polymerization of ethylene.

The autoclave reactors used today in the high-pressure polymerization of ethylene are single stirred-tank reactors, cascades of stirred autoclaves, and multi-chamber autoclaves. 

The safety requirements are then explained regarding high-pressure polymerization and extraction. 

Co-monomers can reduce or increase the critical temperature for runaway and decomposition. As an example, the influence of vinyl acetate, which is often used as a comonomer in the high-pressure polymerization of ethylene, is shown in Fig. 7.2-4. For this purpose, the critical temperature for decomposition during runaway from polymerization is... 

As in any other process, so also in the high-pressure polymerization of ethylene, do capital costs, utilities, maintenance, manpower, and costs of raw materials contribute to the production costs of low-density polyethylene (LDPE). The cost structure is typical for the production of bulk chemicals but is strongly influenced by the requirements of a high-pressure process. 

In many cases, the linear low-density polyethylene (LLDPE) produced in low-pressure processes competes for the same market as LDPE. For this reason, in Figure 8.2-7 capital- and operation costs of the high-pressure polymerization are compared with those of a low-pressure solution process having the same capacity. Also, the production costs of the low-pressure process are dominated by the costs of the monomer, but some differences can be noted which are typical for the economics of low- and high-pressure processes. 

Above all, the total costs of the low-pressure solution process are 1,393 DM/t compared to 1,491 DM/t of the high-pressure polymerization, which is a difference of 7%. However, it must be mentioned that the average European sales prices of standard film-grades LDPE in recent years have been higher by 100 to 200 DM/t than those of LLDPE [3]. 

High-pressure polymerization with metallocene catalysts G. Luft... 

Advantages of high-pressure polymerization with metallocenes... 

The advantages known from the production of low-density polyethylene (LDPE) become obvious also when metallocene catalysts are used under high-pressure conditions. The compressed monomer can dissolve the polymer which is formed during polymerization, which means that no additional solvent is required for the polymer. The high-pressure polymerization proceeds with a high rate, which requires a short residence time and small reactor volume. Established technology, with stirred autoclaves as well as tubular reactors, can be applied. 

In order to estimate the quantity of catalyst which is required to produce a given amount of polyethylene, the productivity can be considered. The productivity of the high-pressure polymerization of ethylene with a zirconocene catalysts is in the range of 400 - 6,000 kg PE/g Zr. It depends steeply on the ratio of co-catalyst to catalyst, as well as on pressure and temperature. 

The electronic properties are also modified by polymerization. Experimentally, the band gap decreases to less than 1.2 eV in the low-pressure orthorhombic phase [65], and experiments [66,88,108] and calculations [80,109-111] agree that the band gap should decrease with an increasing number of intermolecular bonds. (We note the possible exception of the high-pressure polymerized orthorhombic phase, as discussed above.) Calculations [85, 111] show that the rhombohedral phase should have a more three-dimensional band structure than the orthorhombic phase but still be a semiconductor. However, recent measurements by Makarova et al. [88] showed that oriented samples of the rhombohedral phase had an extremely large electrical anisotropy, larger than that of single-crys-... 

CSTR for most reactions. These conditions are best met for short residence times where velocity profiles in the tubes can be maintained in the turbulent flow regime. In an empty tube this requires high flow rates for packed columns the flow rates need not be as high. Noncatalytic reactions performed in PFRs include high-pressure polymerization of ethylene and naphtha conversion to ethylene. A gas-liquid noncatalytic PFR is used for adipinic nitrile production. A gas-solid PFR is a packed-bed reactor (Section IV). An example of a noncatalytic gas-solid PFR is the convertor for steel production. Catalytic PFRs are used for sulfur dioxide combustion and ammonia synthesis. 

Imai Y (1996) High pressure polymerization processing. In Salamone JC (ed) Polymeric materials encyclopedia. CRC Press, Boca Raton, 2994... 

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