Fluid Catalytic Cracking With Supplemental Heat (ExxonMobil)

CATEGORY: FLUID CATALYTIC CRACKING
Fluid Catalytic Cracking With Supplemental Heat (ExxonMobil)
United States Patent Application 20160362613
Cunningham; Brian A. ;   et al.   December 15, 2016
Applicant: ExxonMobil Research and Engineering
Abstract
A potentially heat-deficient fluid catalytic cracking process for effecting a bulk boiling point conversion of a high boiling point petroleum feed to lower boiling products in which the overall enthalpy balance between the endothermic cracking and exothermic regeneration is maintained by combustion of a supplemental fuel in the middle or upper region of the dense bed in the regenerator (including the region immediately above the dense phase bed). There is a direct economic benefit from operation of the cracking process in this way since the preferred supplemental fuel is methane (natural gas) which is currently a low cost fuel while liquid products are higher value. Use of natural gas as a supplemental fuel will allow re-optimization of the catalyst and operations separately from the heat balance demand.
FIELD OF THE INVENTION
[0002] The present invention relates to the fluid catalytic cracking (FCC) process for converting high boiling point petroleum oils to lower boiling products.
BACKGROUND OF THE INVENTION
[0003] The fluid catalytic cracking (FCC) process has become the pre-eminent source for motor gasoline in the USA and also serves the petrochemical industry with light olefins as petrochemical feedstock. In the FCC process, a pre-heated high boiling point petroleum feed such as a vacuum gas oil or residual fraction is subjected to a bulk boiling range conversion by contact with a hot, acidic-function catalyst in a specialized process unit in which the feed comes into contact with the hot catalyst at the bottom of a tall vertical pipe or "riser" in which the essential cracking reactions take place to produce a range of cracked hydrocarbon fragments in the vapor phase. The mixture of catalyst, vaporous cracking products and unconverted residues then enters a disengaging zone in which the catalyst is separated from the hydrocarbons, usually by cyclones or other inertial devices; for reasons arising from the early history of the process, the disengaging zone is usually referred to as the "reactor" although the majority of the cracking reactions take place, as intended, in the riser and the intention is that cracking in the reactor itself should be minimized. The separated, spent catalyst is then stripped of occluded hydrocarbons with steam in a stripping zone at the bottom of the reactor and the stripped catalyst is sent to a regenerator in which the carbon (`coke) which accumulates on the catalyst as a result of the carbon rejection reactions taking place during the cracking process is oxidatively combusted to reactivate the catalyst and to supply the heat for the endothermic cracking reactions. The hot catalyst from the regenerator is then recirculated to the riser to participate in another round of cracking.
[0004] Many types of FCC unit exist with variations too numerous to described here but all rest upon a few simple principles; namely, that the cracking reactions which effectively reduce the boiling point range and molecular weight of the hydrocarbon species in the feed, are endothermic in nature are mediated by the acidic functioning catalyst which becomes deactivated as a result of the endothermic cracking by the deposition of rejected carbon onto the catalyst. This carbon ("coke") is then burned off in the regenerator to supply the heat required for the cracking reactions to take place and this heat is carried from the regenerator to the cracking riser by the circulating catalyst. In this way, an overall heat balance for the unit is maintained with only a minor portion of the total heat requirement being provided by the feed pre-heat. Thus, the operation of the unit depends on the balance between the heat consumed by the endothermic cracking reactions and the exothermic combustion of the coke on the spent catalyst.
[0005] For FCCs processing all but the heaviest feed (highest Conradson Carbon content), some of the coke which is burned in the regenerator is `discretionary`, to the extent that it is burned in the regenerator to supply the heat required for stable, heat-balanced operation. With lighter (lower boiling) feeds and hydroprocessed feeds, stable unit operation becomes problematical without operating under conditions which result in a greater amount of coke being generated during the cracking portion of the FCC cycle simply in order to supply the heat demands of the unit. Thus, economically valuable hydrocarbon liquids are turned to low value coke merely to sustain operations.
[0006] U.S. Pat. No. 8,753,502 (Sexton) describes a way to maintain the overall unit enthalpy balance by combusting a low carbon fuel in a FCC catalyst heater-fuel gas/catalyst combustion chamber of specialized design through which the catalyst is circulated. While noting that most conventional FCC feedstocks contain enough coke precursors in the form of multi-ring aromatics to deposit sufficient "catalytic coke" on the circulating catalyst to satisfy the overall unit enthalpy balance while achieving the desired level of conversion, it is also noted that FCC processes have continued to evolve with unit designs that offer greater processing flexibility with enhanced product yields via improved coke selectivity, i.e. less coke relative to liquid product volume. Catalyst improvements have also enabled coke yields to be reduced but in all cases, the unit enthalpy balance must be met via a certain amount of coke or coke yield on fresh feed regardless of the feedstock's quality. Thus, changes in catalyst, unit design and/or operations could be made to decrease the in-unit coke yield with consequent increases in liquid product yield but the heat balance then registers a deficit.
[0007] U.S. Pat. No. 8,354,065 (Sexton) presents a related conceptual approach using a combination of the FCC catalyst heater-fuel gas/catalyst combustion chamber with a catalyst cooler.
[0008] While the proposals in U.S. Pat. Nos. 8,753,502 and 8,354,065 appear in principle of maintaining the unit enthalpy balance, they fail to make the most effective use of the existing unit, requiring specialized unit modifications to provide the combustion chambers in which light hydrocarbon fuels are burned to supply the additional heat with this heat being transferred to the main body of circulating catalyst by means of a catalyst stream which is passed into or through the combustion chamber. With combustion chambers such as those described, necessarily operating at a high temperature, there exists the potential for catalyst hold-up in the chamber, excessive erosion by fast circulating catalyst flows, as well as inefficient heat transfer by a limited stream of catalyst.
SUMMARY OF THE INVENTION
[0009] We have now devised an improved scheme for redressing an inadequate heat supply to the cracking reactions in the FCC unit by the substitution of methane (natural gas) (or other light fuels such as fuel gas) for this `discretionary coke`. The combustion of this supplementary fuel in the regenerator itself will result in increased regenerator heat release for the same amount of air/oxygen consumption while decreasing CO.sub.2 emissions. The supplementary fuel is injected into a dense bed of the catalyst in the middle or upper region of the dense bed or even or just above the bed. Operation of the regenerator in this way will maximize the oxygen concentration at the bottom of regenerator for coke burning purposes which is a slower reaction while essentially eliminating excess oxygen in the dilute phase and the flue gas resulting in a more reducing atmosphere for minimizing NOx formation. The added heat of combustion is directly added to the body of catalyst being regenerated so avoiding problems of heat transfer and catalyst hang-ups in separate combustion chambers.
[0010] According to the present invention, a potentially heat-deficient fluid catalytic cracking process is modified to maintain an overall enthalpy balance between the endothermic cracking and exothermic regeneration by combustion of a supplemental fuel in the middle or upper region of the dense bed in the regenerator (including the region immediately above the dense phase bed).
[0011] The present fluid catalytic cracking process for effecting a bulk boiling point conversion of a high boiling point petroleum feed to lower boiling products contacts the feed with a hot cracking catalyst to effect endothermic cracking of the feed after which the spent catalyst is exothermically regenerated by oxidative combustion of coke deposited on the catalyst during the cracking in a dense bed of catalyst in a regeneration step; in this process, the overall enthalpy balance between the endothermic cracking and exothermic regeneration is maintained by combustion of a supplemental fuel in the middle or upper region of the regeneration dense bed (including the region immediately above the dense phase bed). There is a direct economic benefit from operation of the cracking process in this way since the preferred supplemental fuel is methane (natural gas) which is currently a low cost fuel in the USA while liquid products are higher value. Use of natural gas as a supplemental fuel (in a manner similar to torch oil in the regenerator) will allow re-optimization of the catalyst and operations separately from the heat balance demand.
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