101
Increasing the cracking temperature increases the catalyst circulation rate. This
increases the LPG yield, thereby increasing the yield of propylene. Higher temperature with shorter residence time promotes the endothermic cracking reactions and
limits the exothermic hydrogen transfer reactions [5]. This increases the selectivity
toward propylene (Figs. 24 and 25).
Higher temperatures also favor thermal cracking reactions, which increase the
coke and dry gas yields; hence, a catalyst with good coke selectivity is applicable.
An increase in reaction temperature by 10 °C increases propylene yield by 0.9 wt%
[26]. The increase in reaction temperature is limited by the wet gas compressor or
the main air blower’s capacity as well as the coke make and metallurgy of the unit.
Higher catalyst circulation means higher catalyst to oil ratio; this results in the unit
Fig. 22 Changes in gasoline yield with temperature [88]
Fig. 23 Changes in LPG yield with temperature [88]
Recent Developments in FCC Process and Catalysts