NOVELTY - Catalytically cracking a heavy hydrocarbon feedstock and obtaining selected hydrocarbon fractions, comprises: contacting a heavy hydrocarbon feedstock with a hydrocarbon cracking catalyst in a fluidized reactor zone (214) to produce a hydrocarbon effluent comprising hydrocarbon products including light olefins; separating the hydrocarbon effluent to form at least one separator liquid stream and a separator vapor stream; and deethanizing, demethanizing, splitting and depropanizing.    USE - The process is useful for catalytically cracking a heavy hydrocarbon feedstock and obtaining selected hydrocarbon fractions i.e. light olefins such as ethylene and propylene.    DETAILED DESCRIPTION - Catalytically cracking a heavy hydrocarbon feedstock and obtaining selected hydrocarbon fractions, comprises: contacting a heavy hydrocarbon feedstock with a hydrocarbon cracking catalyst in a fluidized reactor zone (214) to produce a hydrocarbon effluent comprising hydrocarbon products including light olefins, where the hydrocarbon cracking catalyst has a catalyst composition including a first component comprising a large pore molecular sieve and a second component comprising a zeolite with no greater than medium pore size, and the zeolite with no greater than medium pore size comprises at least 1 wt.% of the catalyst composition; separating the hydrocarbon effluent in a separation system (220) comprising a main column section (222) and a compression section (224) to form at least one separator liquid stream and a separator vapor stream, where the main column section provides one or more fractions comprising gasoline, light cycle oil, heavy cycle oil and clarified oil, the separator liquid stream comprises C4+ hydrocarbons, and the separator vapor stream comprises C4- hydrocarbons; deethanizing at least a portion of the separator vapor stream to form a first deethanizer process stream comprising C2- hydrocarbons including ethylene and a second deethanizer process stream comprising C3+ hydrocarbons including propylene; demethanizing the first deethanizer process stream to form a first demethanizer process stream comprising hydrogen and methane and a second demethanizer process stream comprising C2 hydrocarbons including at least a portion of ethylene; splitting the second demethanizer product stream in a C2 hydrocarbon splitter to form a first C2 hydrocarbon splitter process stream comprising ethylene and a second C2 hydrocarbon splitter process stream comprising ethane; depropanizing the second deethanizer process stream to form a first depropanizer process stream comprising C3- hydrocarbons including propylene and a second depropanizer process stream comprising C4+ hydrocarbons; and treating at least a portion of the at least one separator liquid stream and at least a portion of the second depropanizer process stream in at least a single downstream processing unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of a system for catalytic cracking a heavy hydrocarbon feedstock and obtaining selected hydrocarbon fractions.    Line; (212, 216)    Fluidized reactor zone; (214)    Hydrocarbon separation system; (220)    Main column section; (222)    Staged compression section. (224)