Polypropylene Process

Fig. 9. High yield Montedison polypropylene process (121). Courtesy of Gulf PubHshing Co.

In the polymer filed, new-generation metallocenes, which are currently used in many polyethylene and polypropylene processes, can polymerize proplylene in two different modes alternating blocks of rigid isotactic and flexible atactic. These new developments and other changes and approaches related to polymerization are noted in Chapters 11 and 12. 

Very active, not too expensive, catalysts may be left in the product if they are used in such small quantities that they are not detrimental to the properties of the product. An example is the gas-phase polypropylene process. When the desired product is formed as a solid, i.e. crystallizes from the homogeneous reaction mixture, the product can be removed in relatively simple way. 

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 The catalyst powder is continuously added to the bed and the granular product is continuously withdrawn.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. 

Glass fibre reinforced polypropylene processing scraps ... 

The new polypropylene processes do not require solvents because of a new and highly efficient catalyst [12]. Therefore, there are also no solvent emissions in the exhaust gases. Small amounts of gaseous hydrocarbons that are formed are incinerated. In the manufacture of the polymer the amount of aqueous wastewater accumulated is much smaller, since the amount of catalyst used can be reduced to such low levels that it no longer needs to be washed after the reaction. Comparable results have also been achieved in the manufacture of high-density polyethylene. 

Fig. 49. Montedison high-yield slurry polypropylene process 173). By permission of Gulf Publ. Co.

This function can be effectively illustrated with a catalyst synthesis used in an early commercial polypropylene process, now obsolete. The catalyst system employed ethylaluminum sesquichloride (EASC) for "prereduction" of TiCl in hexane (eq 4.5). EASC reduces the oxidation state of titanium and TiClj precipitates as the P (brown) form. Reduction is believed to proceed through an unstable alkylated TP" species (eq 4.5) which decomposes to TP (eq 4.6). Lower oxidation states (Ti+ ) may also be formed. These reactions are exothermic and very fast. 

Nucleating agents not only shorten the time of crystallization but also improve the mechanical properties of materials. Polypropylene processed with a nucleating agent (2% CaCOs) had its impact strength and modulus increased by 50%. ... 

The Spheripol process was a significant step change in the polypropylene process, allowing a broadening of product capabilities and a considerable saving in terms of investment costs as well as operating costs. 

Major Developments since 1980. During the 1980s, two fundamental changes in the polypropylene process led to a breakthrough in terms of improved process economics, energy efficiency, and reduced environmental impact. The first of these changes was the conversion from a diluent process to a process in which liquid propene acts as both reactant and diluent for the reactor slurry. This mode of operation, which is called LIPP (Liquid Propylene Process) leads to a considerable increase in the pressure at which such a process is operated. 

Figure 94. Flow diagram of the bulk polypropylene process (LIPP)...

R. Di Drusco, R. Rinaldi, "Polypropylene Process Selection Criteria" -Hydrocarb. Proc. 113-117, Nov. (1984). 

The ability of these energy quenchers to stabilize polypropylene fibers to weathering permitted the development of many new end-uses, but their capabilities have been surpassed by a new group of stabilizers that contain a hindered piperidine structure. As shown above, these HALS compounds can be very good long-term thermal stabilizers. Hindered piperidines react with hydroperoxides during polypropylene processing to form nitroxyl radicals (II) that arc effective polymer radical traps [134]. These nitroxyl radicals react with polymer free radicals to form the polymeric hydroxylamine (III). 

Polypropylene Processing Guide, (INEOS Olefins Polymers USA, Technical Center, LaPorte, TX, USA, 2007)... 

This may be exemplified, by so-called captodative lactones of the benzofiiranone type that are used as the polypropylene processing stabilizers and weak antioxidants (Scheme 12.9). 

Khare, N.P., Lucas, B., Seavey, K.C., Liu, Y.A. Steady-state and dynamic modeling of gas-phase polypropylene processes using stiired-bed reactors. Ind. Eng. Chem. Res. 43, 884-900 (2004)... 

Atactic (amorphous) polypropylene is a popnlar plasticizer in many applications, such as modified bitumen roofing, paper lamination, adhesives and sealants, asphalt pavement modification, wire and cable, carpet, tiles, films, and antomotive products. In some applications, plasticizers are nsed in polypropylene processing ... 

A very efficient alternative for heat removal is to use overhead condensers. This modification uses the latent heat of evaporation of the monomer to remove the heat of polymerization. Monomer is evaporated in the reactor, condensed in the overhead condenser, and the cooled liquid monomer is returned to the reactor. This design works well for propylene polymerization, but it is not a good option for ethylene because of its much lower boiling point. Overhead condensers are used in the El Paso bulk polypropylene process [72]. 

Corbin, G. and Lee, K.F. (1993) Simplified Gas Phase Polypropylene Process Technology. DeWitt Petrochemical Review, Houston, Texas, USA, 23-25 March. 

Li X, Chen M, Huang Y and Cong G (1999) In situ generation of polyamide-6 fibrils in polypropylene processed with a single screw extruder, Polym Eng Sci 39 881-886. 

Alcock B (2004) Single poljoner composites based on polypropylene processing and properties, PhD Thesis, Queen Mary, University of London, London, UK. 

Figure 3.14 Flow diagram of the Spheripol polypropylene process [15, IJIImann, 2001]...

The monomer consumption data of the HDPE process is slightly different for the polypropylene process due to variable purity of the monomer raw material used by polypropylene plants. 

There are some catalytic processes where a gas and a liquid are contacted with a solid catalyst. They are often carried out in slurry reactors or in three phase packed beds. Also in this case there may be a possibility of dissolving the gaseous reactant in the liquid before it enters the reactor, thus avoiding the problems inherent to gas/liquid mass transfer (example polypropylene process). 

The result is that the driving force at the wall must increase by a factor of about 10 when scaling with S = 512 and constant power per unit volume. This may be acceptable when the pilot unit operates with a AT of 2°C but becomes problematic when the pilot plant operates with a 20°C AT. There are many solutions. In a CSTR, use cold feed. Some processes for PMMA use a 40°C feed to control the reaction exotherm. Diluents and low per-pass conversions can also be used this approach is typical of solution polyolefin processes. Reflux boiling can be used it is common in styrenic polymerizations where the reflux solvent is normally returned as a liquid. In some polypropylene processes, the returning propylene is flashed into the first reaction vessel. Finally, the external loop shown in Figure... 

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