1. Field of the Invention
This invention relates to a method and apparatus for the separation of a catalyst phase from a gas suspension phase in a fluidized catalytic cracking (FCC) unit. More particularly, it relates to an improved method and apparatus for separating the catalyst phase from the gas suspension phase in a closed cyclone process and system, to reduce the cyclone size requirements while maintaining the desired hydrocarbon and catalyst feed flowrate.
2. Discussion of the Prior Art
The field of catalytic cracking, particularly fluid catalytic cracking, has undergone significant development improvements due primarily to advances in catalyst technology and product distribution obtained thereform. With the advent of high activity catalysts, and particularly crystalline zeolite cracking catalysts, new areas of operating technology have been encountered requiring refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
By way of background, the hydrocarbon conversion catalyst usually employed in an FCC installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase condition with a hydrocarbon feed generally upwardly through one or more riser conversion zones (FCC cracking zones), providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occuring at temperatures of least 1000.degree. F. or higher and at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the riser, are desirable for some operations before initiating separation of vaporous hydrocarbon product materials from the catalyst.
Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon conversion time. During the hydrocarbon conversion step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion step. The entrained hydrocarbons are subjected to further contact with the catalyst until they are removed from the catalyst by a separator which could be a mechanical means, and/or stripping gas in a separate catalyst stripping zone. Hydrocarbon conversion products separated and materials stripped from the catalyst are combined and passed to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, is then passed to a catalyst regeneration operation.
Cyclonic equipment is typically used for efficient separation of fluidizable catalyst particles from the gas suspension stage. In U.S. patent application Ser. No. 612,277 filed May 21, 1984, entitled "Closed Cyclone FCC Catalyst Separation Method and Apparatus" by J. H. Haddad et al, a cyclone separation system is disclosed which reduces the undesirable extended residence time of product vapor within a large reactor vessel. The reduction in residence time is brought about by connecting a number of cyclone seperators in series directly to the output of the riser conversion zone with the gaseous output of the last cyclone conducting cracked hydrocarbons out of the reactor vessel. While extremely effective, the passage of virtually all of the hydrocarbon feed through the primary cyclone requires that it be designed for a relatively high volume flow of gas products as well as fluidized catalyst particles. This size requirement dictates that the reactor vessel, enclosing the riser conversion zone and separation cyclones, be relatively large.
U.S. Pat. No. 4,404,095 to Haddad et al, discloses an FCC reactor comprising a riser 10 with radially extending sidearms 12 as the first catalyst-hydrocarbon separation means, as seen in FIG. 1 of the present application. The sidearms 12 force the suspension of the catalyst and hydrocarbons to change direction of flow, from the vertical to the horizontal direction, when striking end 14. The catalyst particles fall out of sidearm openings 16 in a downward direction, to a respective stripping means 18 which includes baffles 20. The hydrocarbons, with some entrained catalyst particles, proceed through passages 22 into the inlets 24 of cyclones 26. The sidearms 12 and the secondary separation means are enclosed by a vertical conduit 28 to prevent random uncontrolled thermal cracking of the hydrocarbons, which would ordinarly be present for a substantial period of time in the reactor vessel 30. Catalyst particles separated in cyclones 26 pass through diplegs 32 through to the bottom of the reactor and are removed through conduit 34. Hydrocarbon gases pass through the cyclone exhaust 36 into plenum 38 and out conduit 40 to a fractionation unit (not shown).
It can be seen in FIG. 1 that all of the catalyst particles and gaseous hydrocarbon materials exiting riser 10 must pass through the radially extending sidearms 12, requiring that they be sized to accommodate such a flow. Furthermore, during operation, substantial radial and longitudinal thermal expansion is encountered in the connecting conduits and intakes, and radially extending sidearms which necessitate some thermal accommodation.