Performance of ZSM-5 as a fluid catalytic cracking catalyst additive: Effect of the total number of acid sites and particle size

The performance of ZSM-5 as a fluid catalytic cracking (FCC) catalyst addit ive has been tested in a microactivity test unit for the cracking of gas oi l. Laboratory-synthesized ZSM-5 samples, with various Si/Al ratios, fresh o r hydrothermally dealuminated were tested. A 2 wt % ZSM-5, on total catalys t weight, found by previous investigators to be the optimum additive concen tration, was used for all of the experiments. It has been found that a dire ct and smooth correlation exists between the product yields and the total n umber of ZSM-5 acid sites measured by ammonia temperature-programmed desorp tion tests. This is a unique function of the aluminum content of fresh samp les as well as the temperature of the hydrothermal deactivation of steamed ZSM-5 zeolite samples. Previous works have been qualitatively compared to t he results of this work and have been classified on the basis of different total acidity regions of the ZSM-5 used in each case. Higher total acidity results in gasoline loss, liquified petroleum gases (LPG) and ethylene incr eases, and research octane number gain. The C-5 aliphatics are increased in the low acidity region because of cracking and isomerization of larger alk enes, while an increase of the total ZSM-5 acidity resulted in a monotonic decrease of all of the gasoline range hydrocarbons, except of n-C-5 alkane, C-6-C-7 aromatics, and C-5 naphthenes. The branched/linear (B/L) ratios fo r C-5-C-6 alkenes were found to increase with acidity, while the B/L ratios for C-5-C-9 alkanes decrease. The number and strength of the additive's ac id sites control the contribution of the different reaction paths, through which cracking, isomerization, and aromatization occur, to the final produc t distribution. The sizes of the ZSM-5 particles have a strong effect on th e changes in product yields and gas and gasoline compositions. Smaller part icles favor a decrease in gasoline and an increase in LPG yield more than l arger particles. The effect is more pronounced in the high acidity region w here the decrease in the yields of C7+ isoalkanes, naphthenes, and aromatic s is favored by small particle additives, while the effect of particle size on gasoline range hydrocarbons is clearly evident on the yields of gaseous products.