The invention relates to a process for maintaining heat balance in a continuous fluidized bed catalytic cracking unit. More specifically, the invention relates to a combustion control method capable of maintaining or restoring heat balance by conducting, under appropriate conditions, fuel and an oxygen-containing gas to a transfer line. The transfer line conducts effluent including catalyst and combustion products to a zone where the catalyst is separated from the effluent and returned to the process.
In a continuous fluid solids based catalytic cracking unit such as a fluidized catalytic cracking (xe2x80x9cFCCxe2x80x9d) unit, flowing hot regenerated catalyst is conducted to the base of a feed riser. A feed such as naphtha, gas oil, resid, heavy oil, and mixtures thereof is injected into the feed riser at a point downstream of the riser""s base. Typically, the downstream end of the feed riser terminates in a reactor vessel. Cracked product is taken overhead from the reactor vessel, and spent catalyst containing adsorbed hydrocarbons such as coke passes through a stripping region in the reactor vessel and then through a transfer line to a regenerator vessel. Coke is burned off the spent catalyst in the regenerator""s oxygen rich environment in order to heat and re-activate the catalyst. When the heat supplied by the combustion of the coke in the regenerator is equal to the heat dissipated by reaction endotherm, sensible heat to process streams, latent heat of vaporization where liquid process streams are introduced, and heat losses, the unit is said to be in heat balance.
While coke is necessary in conventional FCC processes for catalyst heating during regeneration, the amount of coke formed on the catalyst may be limited by, for example, operational parameters and feed choice. Operationally, it may be desirable to limit the amount of coke produced in order to increase the amount of carbon available in the process for forming more valuable (generally lower molecular weight) products. Moreover, coke formed in the reaction process may contain undesirable sulfur and nitrogen species, leading to increased environmental regulation compliance costs.
Additionally, some FCC processes use feeds which lead to less coke formation on the catalyst. For example, where the unit""s feed contains naphtha or a higher boiling feed which has been severely hydrotreated, substantially less coke is formed on the catalyst resulting in less heat produced by burning the coke in the regenerator. Such feeds, therefore detrimentally affect the unit""s heat balance.
Added heat is required when factors such as operating conditions or feed choice result in insufficient coke combustion to maintain the unit in heat balance. Moreover, non-steady-state operations, particularly such as occur during start-up, require additional heat to restore or maintain heat balance, even in cases where sufficient coke is normally present during operation.
One conventional FCC method for providing additional heat to the catalyst involves injecting a fuel such as torch oil into the oxygen-rich environment inside the regenerator. Torch oil, which may be FCC feed or derived therefrom, bums in the regenerator under combustion conditions that are at least stoichiometric (or leaner). Unfortunately torch oil burning results in high localized regenerator temperatures, and may lead to, for example, mechanical damage to the FCC unit, catalyst deactivation, catalyst decomposition, and combinations thereof.
In another conventional process, heat is provided by contacting and mixing the spent catalyst with a liquid fuel before the spent catalyst enters the regenerator. The liquid fuel then bums on the catalyst in the regenerator. Unfortunately, excessive catalyst temperatures may result during regeneration, especially in the most oxygen-rich regions of the regenerator. Moreover, while it is sometimes desirable to produce a significant amount of CO during regeneration, such processes typically result in complete combustion of the fuel to CO2.
In yet another conventional process, spent catalyst, freshly regenerated catalyst, fuel, and air are conducted to a mixing zone leading to the regenerator in order to control catalyst circulation. While the process results in adding heat to the FCC unit, catalyst temperatures as high as 1600xc2x0 F. are encountered.
There is therefore a need for improved methods for maintaining or restoring heat balance in a fluidized bed catalytic cracking unit that do not result in excessive catalyst temperatures and that regulate the amount of CO in the regenerator.
In one embodiment, the invention is a fluidized bed catalytic cracking process comprising the continuous steps of:
(a) conducting a hydrocarbon-containing feedstream to a reaction zone where the feed contacts a source of hot, regenerated catalyst in order to form at least cracked products and spent catalyst;
(b) conducting the cracked products and the spent catalyst to a separation zone and separating the spent catalyst;
(c) conducting the spent catalyst to a transfer line;
(d) conducting a fuel and an oxygen-containing gas independently to one or more points along the transfer line and combusting the fuel and the oxygen in the transfer line in order to form an effluent containing the hot, regenerated catalyst;
(e) separating the hot, regenerated catalyst from the transfer line""s effluent and then;
(f) conducting the hot, regenerated catalyst to step (a).
Preferably, the spent catalyst has a temperature ranging from about 900 to about 1175xc2x0 F., more preferably from about 900 to about 1150xc2x0 F., and still more preferably from about 900 to about 1100xc2x0 F. Preferably, the hot, regenerated catalyst has a temperature ranging from about 1200 to about 1400xc2x0 F., more preferably from about 1200xc2x0 F. to about 1300xc2x0 F., and still more preferably from about 1250xc2x0 F. to about 1285xc2x0 F.
In one preferred embodiment, the transfer line is a zoned transfer line including at least a first zone, a third zone downstream of the first zone, and a second zone situated therebetween. Preferably, at least a portion of the oxygen-containing gas and the fuel are combusted in the first zone to form CO, and at least a portion of the CO in the second zone and the zone(s) downstream of the second zone is oxidized in order to form CO2. More preferably, at least a portion of the oxygen-containing gas and fuel are combusted under sub-stoichiometric conditions in the zones downstream of the first zone in order to form CO, and at least a portion of the CO in the zones downstream of the second zone is oxidized in order to form CO2.
In another preferred embodiment, the fuel is conducted to the first zone, and the oxygen-containing gas is conducted to at least the second and third zones. At least a portion of the oxygen-containing gas and the fuel are combusted under partial oxidation conditions in the zones downstream of the first zone in order to form CO, and at least a portion of the CO in the zone(s) downstream of the second zone is oxidized in order to form CO2.
In yet another preferred embodiment, the oxygen-containing gas is conducted to the first zone, and the fuel is conducted to the zones downstream of the first zone. The amount and distribution of the fuel is regulated to provide distributed combustion along the transfer line resulting in localized temperatures in the transfer line below the catalyst deactivation temperature.