The current trend in fluidized catalytic cracking (FCC) operations in the USA is to produce more C4 unsaturates for downstream processing to supply motor fuel oxygenates.1 The same chemistry that produces greater volumes of C4 unsaturates also increases yields of lighter unsaturates. Environmental trends in other major world markets will force much of the world FCC operating capacity to follow suite.
Figures illustrate the importance of this trend on the C3 yield by showing the relative production from four catalyst systems. The four cases shown are: historical operation with an octane catalyst at moderate conversion, high conversion operation with catalyst systems for basic operation for maximum octane FCC naphtha, a modified catalyst system for increased yields of C4 olefins for oxygenate production, and maximum oxygenate production with a modified catalyst, FCC riser design, and over-cracking.
The progression shows bulk propylene purity increasing from 62 per cent to 84 per cent. At the same time, propylene yield on FCC charge rises from 2.4vol% to 11.7vol%. This shift emphasizes the importance of the current trends in FCC operation to downstream handling of propylene. A 7,940 M3PD (50,000 BPD) charge FCC unit will produce up to 285,000 TPY of contained propylene on the high yield systems. This uses a basis of 95 per cent recovery of propylene from the FCC gas plant.
At the same time as FCC propylene production increases with total light unsaturate yield, propylene production for petrochemical consumption has failed to keep pace with increasing demand. Propylene production from steam cracking is highly dependent on the overall economics driving steam cracking for ethylene production. Recent trends have been for steam cracking unsaturate production to follow the general gross national produce (GNP) growth pattern. Concurrently, polypropylene demand has been increasing at approximately 2 per cent per year above GNP growth. This has tightened high-purity propylene prices and created considerable interest in polypropylene manufacturers for alternate propylene suppliers.
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