Gas phase polymerisation

In gas phase polymerisation, the monomer is introduced in the gaseous phase and put in contact with a catalyst deposited on a solid structure. Gas phase processes allow an easy removal of the reaction heat, they are low in emissions and waste and no additional solvents are needed. Gas phase processes are not applicable for all end-products and the investment costs are relatively high, partially caused by the high pressure equipment needed for most of the processes. 

Currently, gas phase processes are only applied to the polyolefins  

This process is often used, e.g. in Ziegler-Natta type polymerisations of ethylene and propylene where the catalyst is supported on inert silica particles so the reaction therefore takes place at the surface. This helps control the stereochemistry (especially for isotactic polypropylene). 

A single reactor system is used to make olefin homopolymers and random copolymers. Two reactors are operated in series for the production of block copolymers (impact copolymers). An inert conveying gas (nitrogen) is used to maintain the fluidised bed in the reactor for impact copolymerisation [43,51]. 

The development of the gas-phase technology represents a major advance for the commercial production of polyolefins. A gas-phase process avoids the problem of the high cost remaining in the high-mileage slurry and solution processes (associated with recycling of diluent or solvent and drying of the polymer). 

The gas-phase polymerisation of ethylene is typically carried out at 85-100 °C under a pressure of 20-25 atm. The ethylene monomer circulates, thus removing the heat of polymerisation and fluidising the catalyst bed. In order to keep the temperature of the process at values below 100 °C, gas conversion is maintained at 2-3 per pass [37], In the production of polypropylene by the gas-phase process, usually the polymerisation temperature is lower and ranges from about 50 to 85 °C, and the pressure range from ca 15 to 40 atm [553]. 

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Gas phase polymerisation is known in case of a very few olefinic polymers. The methods used in gas phase polymerisation are... 

Examples of gas phase polymerisation are the polymerisation of ethylene and /7-xylene. 

In the early 1990s supported metallocenes were introduced to enable gas phase polymerisation. Also ethene/a-olefin copolymers with high comonomer content, cycloolefin copolymers and ethene-styrene interpolymers became available. In 1990 Stevens at Dow [22] discovered that titanium cy-clopentadienyl amido compounds (constrained geometry catalysts) are very beneficial for the copolymerisation of ethene and long-chain a-olefins. 

The technologies that have been developed for the production of polyolefins, olefin homopolymers and copolymers are slurry, solution and gas-phase polymerisation bulk polymerisation of propylene in the liquid monomer as a special case of the slurry process has also emerged. The fundamental differences in the various olefin polymerisation processes reflect the different approaches that have been devised to remove the substantial heat of polymerisation. In addition, processes can be operated in a batch or a continuous mode. In the batch process the reagents are loaded into a polymerisation vessel, the polymer forms and the vessel is emptied before a new charge of reagents is introduced. In the continuous process, the catalyst precursor, activator and other necessary... 

Figure 3.58 Flow scheme of polypropylene production using the gas-phase polymerisation process...

The high pressure gas phase polymerisation of ethene to form Polythene involves a free radical mechanism. Thus, when ethylene is heated under great pressure in the presence of a suitable catalyst a large number of molecules of ethene combine together to form a much smaller number of molecules of... 

Figure 2.1 2 Union Carbide process for gas phase polymerisation of ethylene.

A new method is described in [91] for the stndy of the nonnniformity of AC. It is based on the mass-spectrometry control of temperature-programmed desorption of prodncts, from the catalyst surface, at the initial stage of the gas-phase polymerisation of olefins. Polymerisation conditions have been selected in a way to favonr the formation of low MW products (up to 14 monomer links in a chain). The anthors report two definite maximums in the areas of 180-210 and 280-320 °C in the process of desorption from the Si02/TiCl4-Al(C2H5)2Cl surface. Therefore, the catalyst contains at least two types of AC with different activation energies of thermal destruction of Ti-C bonds. This publication also contains calculations of the activation energy distribution of thermal Ti-C bond destruction for various types of AC. 

Gas Phase Polymerisation of Propylene with MgCk ipported Catalysts 35... 

One particular development was the commercialization of the Spheripol process by Himont (now licensed by Basell) in 1982. This process combines bulk phase polymerization in loop reactors with gas phase polymerisation and has become the most dominant licensed process. More recently. Borealis has taken a similar route with the adaptation of its Bor-star bimodal PE process to produce PP. 

Gas-phase polymerisations occur by composite mechanisms that may be of a molecular type or of a radical type. In liquid solutions and emulsions, again both ionic and radical polymerisation mechanisms are important, depending upon whether one is dealing with condensation or addition polymerisation. In this section we will consider the formation of macromolecules M made by a repetition of a certain number of identical monomers M by... 

Results are presented of studies of the continuous mixing in a twin-screw extrader of granular free-flowing EPDM obtained by gas phase polymerisation. 

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