Fluidised catalytic cracking reactor - where cyclone dip-legs extend from reactor vessel into stripping vessel through vent lines

A fluidized catalytic cracking unit has a vertical cylindrical reactor vessel for maintaining a dilute phase of catalyst suspended in hydrocarbon vapour above a fluidized dense phase bed of spent catalyst. A cylindrical stripping vessel is vertically positioned below the reactor. A spent catalyst collector communicates with the reactor interior through its bottom to provide a static head of catalyst which will exceed the pressure differential between the reactor vessel and the stripping vessel. A spent catalyst transfer arrangement is in communication with the lower interior of the spent catalyst collector and with the stripping vessel interior for transferring catalyst from the reactor bottom into the stripping vessel. Stripper vent lines communicate between the top of the stripping vessel and the upper portion of the reactor to allow stripping vapours from the stripping vessel to the dilute phase region of the reactor. Cyclone separator catalyst diplegs extend from cyclone separators in the reactor, through the stripper vent lines into the stripping vessel. They provide a path for separated catalyst from the cyclone separator into the stripping vessel. A column of catalyst is maintained in the diplegs to prevent blow-by of either hydrocarbon vapours from the cyclone separator or stripping vapours from the stripping vessel through the legs. The invention reduces the required regenerator vessel height. This results in a redn. in the cost of such a cracking unit. Also, the depth of dense phase catalyst bed in the reactor vessel may be reduced to the minimum required to properly seal the reactor vessel slide valve. This eliminates overcracking of hydrocarbons in the dense phase bed.