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7- Catalytic Cracking
to increase the octane # of gasoline, while maintaining yield from high-boiling stocks using catalysts.
1). Batch reactor catalytic cracking to produce light distillates (1915) Catalyst: AlCl3 - A Lewis acid, electron acceptor Alkane - electron (abstracted by AlCl3 )-> a carbocation(+) -> ionic chain reactions to crack long chains 2).Houdry (1936) - a commercial process Continuous feedstock flow with multiple fixed-bed reactors Cracking/catalyst regeneration cycles Catalyst: clays, natural alumina/silica particles 3). Thermoform Catalytic Cracking (TCC) (1942) Continuous feedstock flow with moving-bed catalysts Catalyst: synthetic alumina/silica particles Higher thermal efficiency by process integration 4). Fluid Catalytic Cracking (FCC) (1942) Continuous feedstock flow with fluidized-bed catalysts Catalyst: synthetic alumina/silica +zeolites (1965) 4. Process diagram
5. Feedstock and Products Feedstocks: straight run gas oil vacuum gas oil atmospheric residue heavy ends (coker gas oil, DAO) vacuum residue Pretreatment: deasphalting to prevent excessive coking on catalyst surfaces demetallation (Ni, V) prevent catalyst deactivation hydrocracking to prevent excessive coking
Products from Catalytic Cracking n-paraffins, i-paraffins, aromatics, naphthenes, and olefins. I-paraffins come from ionic reaction, is branchesd chain, have high Oct.# C3 C4 gases C5 C10 isoparaffins (gasoline), olefins, aromatics, napthenes > C10 light cycle oil > C14 decant oil 6. Comparison between thermal and catalytic cracking Catalytic cracking is differences wrt Thermal Cracking uses a catalyst lower temperature lower pressure more flexible different reaction mechanisms - ionic vs. free radical High thermal efficiency Good integration of cracking and regeneration High yields of gasoline and other distillates Low gas yields High product selectivity Low n-alkane yields High octane number Chain-branching and high yield of C4 olefins High yields of aromatics
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