Downflow fluid catalytic cracking process and apparatus

A process for fluid catalytic cracking of a hydrocarbon charge in a reaction zone in which the charge and the catalyst circulate in an overall co-current top to bottom manner, comprising a stage of supplying at least partly regenerated catalyst to the upper end of the reaction zone, a stage of introducing and atomizing the charge in an introduction zone located below the catalyst supply zone into the upper part of said reaction zone, a stage of contacting said catalyst and said charge in the upper part of said reaction zone, a stage of circulating the catalyst and the charge in the reaction zone during which the cracking of said charge takes place under appropriate cracking conditions and the catalyst is at least partly deactivated by depositing coke thereon, a stage of separating and stripping the deactivated catalyst from the cracked hydrocarbon charge in a separating zone at the lower end of the reaction zone, a stage of regenerating at least part of the deactivated catalyst in at least one regenerating zone and a stage of recycling at least partly regenerated catalyst in a recycling zone upstream of the upper end of the reaction zone, wherein

• a) during the recycling stage, the at least partly regenerated catalyst is sampled from a dense phase,
• b) the stage of supplying catalyst to the upper end of the reaction zone takes place in the presence of at least one fluidization gas whereby suspension of catalyst and gas is obtained,
• c) the density of said suspension is measured by measuring means upstream of the charge introduction zone and
• d) adjustment and regulation is made of the flow rate of the fluidization gas upstream of the charge introduction zone under conditions such that the density of the gas-solid suspension formed in this way is between 80 and 500 kg/m.sup.3 prior to its contacting with the charge.