THERMAL CRACKING AND CATALYTIC CRACKING IN FLUID CATALYTIC CRACKING PROCESS
Thermal cracking reactions lead to excessive light gas yield and lower overall product values during the present typical high temperature commercial FCC operation.Catalyst properties and operating parameters (reaction temperature,catalyst to oil ratio,contact time) have great influence on thermal and catalytic cracking reactions.There exist some points of reaction temperature inflexion for catalytic cracking reaction and thermal cracking reaction.Catalytic cracking reaction plays the main role below around 538?℃,and the ratio of (C 1+C 2)/ i -C 4 is less than 0.6.When the reaction temperature is over around 610?℃,thermal cracking reaction is the key process and the ratio of (C 1+C 2)/ i -C 4 is large than 1.2.Below the temperature of 550～610?℃,catalytic cracking reaction and thermal cracking reaction can occur simultaneously,and the latter becomes dominant one when the reaction temperature increases.The rate of catalytic cracking reaction can increase with increase of the ratio of catalyst to oil resulting in much more amounts of active sites involved.The deposition of product coke on the catalyst leads to more severe thermal cracking reaction by making the active sites inaccessible for future participation in the catalytic cracking cycle.The short contact time of oil and catalyst may suppress the detrimental thermal cracking reaction dramatically.A satisfactory result could be obtained through selection of proper catalyst and operating conditions (especially the very short contact time of gas and oil).