Fluidized coking is a well established petroleum refinery process in which a heavy petroleum feedstock, typically a non-distillable residue (resid) from atmospheric and/or vacuum fractionation, are converted to lighter, more valuable materials by thermal decomposition (coking) at temperatures from about 900° F. (482° C.) to about 1100° F. (593° C.). Conventional fluid coking is performed in a process unit comprised of a coking reactor and a heater or burner. A petroleum feedstock is injected into the reactor in a coking zone comprised of a fluidized bed of hot, fine, coke particles and is distributed relatively uniformly over the surfaces of the coke particles where it is cracked to vapors and coke. The vapors pass through a gas/solids separation apparatus, such as a cyclone, which removes most of the entrained coke particles. The vapor is then discharged into a scrubbing zone where the remaining coke particles are removed and the products cooled to condense the heavy liquids. The resulting slurry, which usually contains from about 1 to about 3 wt. % coke particles, is recycled to extinction to the coking zone. The balance of the vapors go to a fractionators for separation of the gases and the liquids into different boiling fractions.
Some of the coke particles in the coking zone flow downwardly to a stripping zone at the base of the reactor vessel where steam removes interstitial product vapors from, or between, the coke particles, and some adsorbed liquids from the coke particles. The coke particles then flow down a stand-pipe and into a riser that moves them to a burning, or heating zone, where sufficient air is injected to burn at least a portion of the coke and heating the remainder sufficiently to satisfy the heat requirements of the coking zone where the unburned hot coke is recycled. Net coke, above that consumed in the burner, is withdrawn as product coke.
Another type of fluid coking employs three vessels: a coking reactor, a heater, and a gasifier. Coke particles having carbonaceous material deposited thereon in the coking zone are passed to the heater where a portion of the volatile matter is removed. The coke is then passed to the gasifier where it reacts, at elevated temperatures, with air and steam to form a mixture of carbon monoxide, carbon dioxide, methane, hydrogen, nitrogen, water vapor, and hydrogen sulfide. The gas produced in the gasifier is passed to the heater to provide part of the reactor heat requirement. The remainder of the heat is supplied by circulating coke between the gasifier and the heater. Coke is also recycled from the heater to the coking reactor to supply the heat requirements of the reactor.
The rate of introduction of resid feedstock to a fluid coker is limited by the rate at which it can be converted to coke. The major reactions that produce coke involve cracking of aliphatic side chains from aromatic cores, demethylation of aromatic cores and aromatization. The rate of cracking of aliphatic side chains is relatively fast and results in the buildup of a sticky layer of methylated aromatic cores. This layer is relatively sticky at reaction temperature. The rate of de-methylation of the aromatic cores is relatively slow and limits the operation of the fluid coker. At the point of fluid bed bogging, the rate of sticky layer going to coke equals the rate of introduction of coke precursors from the resid feed. An acceleration of the reactions involved in converting the sticky material to dry coke would allow increased reactor throughput at a given temperature or coking at a lower temperature at constant throughput. Less gas and higher quality liquids are produced at lower coking temperatures. Sticky coke particles can agglomerate (become heavier) and be carried under into the stripper section and cause fouling. When carried under, much of the sticky coke is sent to the burner, where this incompletely demethylated coke evolves methylated and unsubstituted aromatics via thermal cracking reactions that ultimately cause foaming problems in the acid gas clean-up units.
Therefore, there remains a need in the art for improved fluid coking processes that are capable of overcoming the problems associated with the formation of sticky material.