Patent ID: 8076525

Claim:
A process for the preparation of C 3 and/or C 4 olefins by: (a) contacting in a dense fluidised bed system a light hydrocarbon feedstock with a first catalyst inventory comprising up to an amount of 30 wt % large pore zeolite catalyst and more than 70 wt % based on the total mass of the catalyst inventory of a medium pore size zeolite catalyst, at least 50% of which is fresh medium pore size zeolite catalyst, wherein said dense fluidised bed system has a catalyst density of between 300 and 800 kg/m 3 , and wherein said dense fluidised bed system is operated under first reaction conditions including a first temperature in the range of from 400 to 700° C., a first pressure in the range of from 0.3 to 10 bar absolute, a first weight hourly space velocity (WHSV) in the range of from 5 to 50 hr −1 , and wherein the weight ratio of said first catalyst inventory to said light hydrocarbon feedstock is in the range of from 1:1 to 20:1, to thereby form a cracked product stream comprising at least one of C 3 and C 4 olefins; (b) combining at least part of the first catalyst inventory as used in step (a) with an equilibrium catalyst, a fresh catalyst and a regenerated catalyst to form a second catalyst inventory comprising medium pore size zeolite catalyst and large pore size zeolite catalyst; and (c) contacting a hydrocarbon feedstock with the second catalyst inventory in a riser reactor to form a second cracked product stream, wherein the catalyst density of said dense fluidised bed system is higher than the catalyst density in said riser reactor, and wherein said riser reactor is operated under second reaction conditions including a second temperature in the range of from 400 to 700° C., a second pressure in the range of from 1 to 10 bar absolute, and a vapor residence time in the range of from 0.1 to 10 seconds.