Polymer yield, slurry process

Montedison and Mitsui Petrochemical iatroduced MgCl2-supported high yield catalysts ia 1975 (7). These third-generation catalyst systems reduced the level of corrosive catalyst residues to the extent that neutralization or removal from the polymer was not required. Stereospecificity, however, was iasufficient to eliminate the requirement for removal of the atactic polymer fraction. These catalysts are used ia the Montedison high yield slurry process (Fig. 9), which demonstrates the process simplification achieved when the sections for polymer de-ashing and separation and purification of the hydrocarbon diluent and alcohol are eliminated (121). These catalysts have also been used ia retrofitted RexaH (El Paso) Hquid monomer processes, eliminating the de-ashing sections of the plant (Fig. 10) (129). 

The slurry process is the oldest and still widely used method for manufacturing polymers of ethylene, propylene and higher a-olefins. In this process, the monomer dissolves in the polymerisation medium (hydrocarbon diluent) and forms a solid polymer as a suspension containing ca 40 wt-% of the polymer the polymerisation occurs below the melting point of the polymer. In slurry polymerisation, the temperature ranges from 70 to 90 °C, with the ethylene pressure varying between 7 and 30 atm. The polymerisation time is 1-4 h and the polymer yield is 95-98 %. The polymer is obtained in the form of fine particles in the diluent and can be separated by filtration. Removal of the catalyst residues from the polymer can be achieved by the addition of alcohol (isopropanol, methanol), followed by recovery and extraction of the catalyst residues. The polymer is freed from diluent by centrifuging and then dried. In the case of polypropylene manufacture, the atactic fraction remains in the diluent [28,37]. 

Synthesis. The early PP plants used a slurry process adopted from polyethylene technology. An inert liquid hydrocarbon diluent, such as hexane, was stirred in an autoclave at temperatures and pressures sufficient to keep 10-20 percent of the propylene monomer concentrated in the liquid phase. The traditional catalyst system was the crystalline, violet form ofTiCl3 and A1C1(C2H5)2. Isotactic polymer particles that were formed remained in suspension and were removed as a 20-40 percent solid slurry while the atactic portion remained as a solution in the liquid hydrocarbon. The catalyst was deactivated and solubilized by adding HC1 and alcohol. The iPP was removed by centrifuging, filtration, or aqueous extraction, and the atactic portion was recovered by evaporation of the solvent. The first plants were inefficient because of low catalyst productivity and low crystalline yields. With some modifications to the catalyst system, basically the same process is in use today. 

FIGURE 94 LCB content of PE made in the slurry process at varying polymer yields (e.g., varying reaction times). As catalyst productivity (yield) increases, the polymer elasticity drops, indicated here by falling JC-a and a broadening of the relaxation time distribution (rising CY-a). 

Three grades of polymer were produced, differing in MI (or MW), which also influences the response to shear stress. In each case, however, this response declined as the polymer yield increased, just as it did in the slurry process. This result indicates that although all sites become active simultaneously, there is still a dependence of elasticity on polymer yield. 

The average activity is also shown. This is the polymer yield at any specified time, divided by that time. Average activity is the value that is most often cited in comparison between catalysts. As the data of Figures 168 and 169 show, it depends on time. The kinetics observed with Cr/silica produces an average activity that rises with time, whereas the average activity of Cr/AlPC>4 drops with time. These two curves illustrate the difficulty of quoting one value to summarize the activity of any catalyst. In industrial operations, the residence time is usually about 1 h for slurry processes. Thus, as stated above, the polymer yield obtained in 1 h is the standard definition of activity used in this review. 

Another difference that is sometimes apparent is a lower LCB level in polymers produced in fluidized-bed reactors. As noted in Sections 9 and 10, lower LCB is the result of use of a lower chromium loading on the catalyst, and of longer residence times, which improves the polymer yield. For example in blow-molding applications, resins produced in the fluidized-bed process are likely to exhibit higher die swell than resins made in the slurry process, when a similar catalyst is used. 

Although the Phillips and Standard Oil processes can be used to prepare polypropylene, the polymer yields tend to be low and it appears that these processes have not been used for commercial production of polypropylene. Until about 1980, polypropylene has been produced commercially only by the use of Ziegler-Natta catalysts. Commonly a slurry process is used and is carried out in much the same manner as described previously for the preparation of polyethylene (see section 2.3.2(b)). In the case of polypropylene, some atactic polymer is formed besides the required isotactic polymer but much of this atactic material is soluble in the diluent (commonly heptane) so that the product isolated is largely isotactic polymer. Recently, there has been a marked shift towards processes involving gas phase polymerization and liquid phase polymerization. Few details of these newer processes have been published. Gas phase processes resemble those described previously for the preparation of polyethylene (see section (2.3.2(b)) and swing plants are now feasible. In liquid phase processes polymerization is conducted in liquid propylene, typically at 2 MPa (20 atmospheres) and 55°C. Concurrently with these developments, new catalyst systems have been introduced. These materials have very high activity and the reduced levels that are required make it unnecessary to remove catalyst from the final polymer. Also, the new catalyst systems lead to polypropylene with higher proportions of isotactic polymer and removal of atactic polymer is not necessary. 

Eig. 1. The key steps for the Phillips PPS process are (/) production of aqueous sodium sulfide from aqueous sodium hydrogen sulfide (or hydrogen sulfide) and aqueous sodium hydroxide 2) dehydration of the aqueous sodium sulfide and NMP feedstocks 5) polymerization of the dehydrated sulfur source with -dichlorobenzene to yield a slurry of PPS and by-product sodium chloride in the solvent (4) polymer recovery (5) polymer washing for the removal of by-product salt and residual solvent (6) polymer drying (7) optional curing, depending on the appHcation and (< ) packaging. 

Starch conversion refers to the process of converting starch into other products. It involves gelatinization, liquefaction, and saccharification. Liquefaction refers to the acid-or enzyme-catalyzed conversion of starch into maltodextrin. Starch, usually from wet milling of com, is pumped in a slurry to the conversion plant, where it undergoes one or more hydrolytic processes to yield mixtures of various carbohydrates in the form of syrups. The kind and amount of the various carbohydrates obtained depend upon the type of hydrolysis system used (acid, acid-enzyme, or enzyme-enzyme), the extent to which the hydrolytic reaction is allowed to proceed, and the type of enzyme(s) used. The fact that most starches consist of two different kinds of polymers... 

The three processes differ by the operating conditions as reaction temperature, type of heat control and degree of system heterophasicity. Moreover, different processes could bring about also different thermal effects inside the polymer particle The solution process mainly yields polyethylene with a medium to very narrow (Q values of 2 or 3) MWD, while both slurry and vapor phase processes give... 

The typical process would yield a slurry of 15 to 18% by weight of polymer solid particles in a hexane dispersion. An operating temperature of at least 80° C. would be required to keep the side products of the reaction—atactic polymer, for example—in solution. 

More commonly used and widely researched is microencapsulation. In this context, the prefix micro refers to the dimension of the encapsulated product, which is typically 1-2 mm in size or, increasingly, 10-100 pm range. If a slurry containing a solution, emulsion, or suspension of polymer is dispersed into small particles and dried in a spray dryer microencapsulated particles of the type described above are formed. Such a process yields a dry, free-flowing powder which, in most cases, satisfies the criteria defined for instant products. 

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