42 2 - - CATALYTIC PROCESSES IN INDUSTRY
Catalyst and Adsorbent
Mordenite is a versatile zeolite, the Si/A1 ratio of which can easily be changed from 5 to higher values. Its parallel channels having a somewhat elliptical cross section (0.65x0.7 nm) allow all Cs and C6 isomers to enter and to leave. Platinum 2+ Competitive exchange with NH~
is introduced by ion exchange with Pt(NH3) 4 9
(excess) is recommended in order to obtain a high Pt dispersion. A careful calcination follows in which local high temperatures in the zeolite crystal have to be prevented. Finally the Pt catalyst is reduced with hydrogen.
The selective adsorbent CaA is prepared in the sodium form (the detergent zeolite NaA) and subsequently partially exchanged with Ca(II).
2.4 ISOTACTIC POLYPROPYLENE
2 E C A T A L Y T I C P R O C E S S E S I N I N D U S T R Y 43 organic and inorganic chemicals may be added, such as antioxidants, UV stabili- zers, fillers, reinforcing fibres, dyes, etc. Processing may involve injection mould- ing, film extrusion, and pipe extrusion. The processing parameters in the second transformation~temperature, cooling rate, etc.~determine the mechanical properties of the consumer product. The product has a crystallinity of 60%. The crystalline spherulites may have different morphological forms which strongly depend on the processing conditions.
Scale
Twenty million tons (1996) of polypropylene are produced annually. In 1973 this was only 3 million tons.
Process Description
The performance of the catalyst is of crucial importance to the design of the plant.
Figure 2.11 shows the scheme of a high-yield liquid process for which the catalyst was pioneered by Montedison and Mitsui. The catalyst is a titanium tetra- chloride supported on magnesium dichloride which, in the presence of various modifying donor compounds, is reduced by triethylaluminium. This leads to a highly active catalyst, a product with a high stereoregularity, and a high bulk density. The propene feed has to be thoroughly purified. Traces of contaminants such as water, oxygen, carbon monoxide and sulphur compounds would de- stroy the catalyst. The components are fed to the reactor (60-70~ 10 bar), a CSTR (see Section 8.3.3). The reaction is highly exothermic (2500 kJ/kg, from gaseous C3H6). Cooling coils are used in slurry processes and the feed may be additionally cooled. In the second step the unreacted propene is removed by
solvent._ ___ s o l v e n t r e c y c l e ~ _
p r o p e n e r e c y c l e
= T ~__ /
r
reactor propene centrifuge dryer
flosh
Fig. 2.11. Slurry process for polypropylene with high activity catalyst.
44 2 - - C A T A L Y T I C PROCESSES IN I N D U S T R Y
flashing and recycled. The solid isotactic product is separated by centrifugation.
Depending on the catalyst, the liquid from the centrifugation stage may contain dissolved atactic polypropylene which is collected by removal of the solvent with steam. The isotactic product from the centrifugation is dried and the solvent is recycled after thorough drying. In this process the catalyst concentrations are low and no catalyst removal ('de-ashing') is needed. The fluffy powder is very sensitive to oxidation and a stabilization package is added to the product before it is exposed to air. The dried powder is pelletized in an extruder.
Alternatives
The similar, older slurry process uses a less active catalyst. The monomer is dissolved in isooctane, the titanium catalyst and aluminium cocatalyst are added and this mixture is fed to the reactor which is maintained at 70~ The inorganic corrosive (C1) residues are removed in a washing step with alcohols. The atactic material is removed by extraction. A third process employs propene as the liquid in combination with a high activity catalyst. The Himont 'Spheripol' process, which uses spherical catalyst particles, gives spherical polymer beads of milli- metre size that need no extrusion for certain purposes. A more recent develop- ment is the gas-phase polymerization using an agitated bed. All processes are continuous processes, where the product is continuously removed from the reactor. Over the years we have seen a reduction of the number of process steps.
The process costs are very low nowadays, propene feed costs amounting to more than 60% of the total cost.
The
Catalyst
The catalyst was discovered by Ziegler who mixed TIC14 and aluminium alkyls and thus polymerized ethene. Natta recognized the peculiar structural proper- ties that a substituted alkene might give to the polymer. Commercial production of both polymers with the new catalysts started very soon after its discovery in 1954. Titanium is reduced to the trivalent state and it is generally accepted that propene insertion into the titanium alkyl bond leads to the polymer. The stereo- regularity is determined by the geometry of the site of the catalytic centre. This is known as the Cossee-Arlman mechanism. The solid TiC13 catalysts contain different sites, some of which give atactic and some of which give isotactic polymer. The major improvement in the second generation catalysts was in the preparation of a high surface area TiC13, free of cocrystallized A1C13. The modern high-yield catalysts consist of very small crystallites of MgCI2 with supported TIC14 and a Lewis base, e.g. dimethyl phthalate. They are reduced by A1Et3.
The productivity of the catalyst has been the topic of continuous research. Table 2.2 gives a review of the development of the catalyst in the last four decades.
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TABLE 2.2
Development of PP catalyst activity
45
PP catalysts Year Yield (kg/g) Isotacticity (%)
TIC13, 1st generation 1954 4 92
TIC13, 2nd 1971 16 96
MgC12, TIC14, 3rd 1975 325 92
MgC12, TIC14, 3rd 1981 1300 96
MgC12, TIC14, 3rd 1998 5000 98
In addition to the molecular properties, the macroscopic growth of the poly- mer particles is important; they should be sufficiently large and dense, otherwise there is no means of collecting and processing the product. What is more, the ca- pacity of the plant also increases with the bulk density of the product. The theo- retical bulk density for ideal uniform spheres is 540 k g / m 3. Values obtained in modern plants are of the order of 500 k g / m 3. The morphology is also determined by the solid catalyst preparation. The morphology of the macroscopic catalyst particles is 'reproduced' by the growing polymer particles.