Thermal balance is the heart of the fluid catalytic cracking process. An innovative approach to the thermal balance is disclosed, based on the cooling of a slip stream of regenerated catalyst to the riser under controlled conditions and its mixture with a major stream of hotter regenerated catalyst prior to the contact with the feedstock. As a result, the control of the catalyst circulation is rendered independent from the heat balance of the unit, with reduction of the undesirable thermal cracking, and therefore lower coke and fuel gas products with increasing gasoline yield and quality.
The new proprietary process, the so-called IsoCat, is mainly suited for residue cracking and its first commercial application will be a 44,000 BPD grass roots residue FCC unit. The basic engineering is concluded and the EPC phase is underway.
Fluid catalytic cracking - FCC - is effected by contacting in a conversion zone - riser - heavy hydrocarbons and fluidised catalyst made up of a fine particulate porous material containing crystals of zeolite in its structure.
During the cracking reactions, a substantial amount of coke, a high molecular weight stuff, byproduct of the reaction, is deposited on the catalyst. The coke-covered catalyst - spent catalyst - is continuously removed from the riser and replaced by catalyst essentially free of coke from the regeneration zone. In the regenerator it occurs the burning of the coke deposited on the surface and in the pores of the catalyst.
The catalyst, besides its essential function, which is to promote the catalysis of the chemical reactions, is also the means for transporting heat from the regenerator to the riser. Riser is a sink of heat. It requires feed heating/vaporisation. There is also a predominance of endothermic reactions.
Nowadays the heavy FCC feedstocks being processed by PETROBRAS deposit too much coke over the catalyst. This fact associated with the operation of regenerators in complete combustion, lead to the use of catalyst coolers, to keep regenerator temperature at a reasonable level sufficient just to assure good regeneration. Otherwise, catalyst would be deactivated, mechanical problems would occur and the catalyst circulation, which is fundamental to assure high conversion and good selectivity to noble products, would adjust itself to a very low value.
These coolers usually remove heat from a catalyst stream withdrawn from the regenerator and return as cooled stream to the regenerator. Heat is recovered through generation of high-pressure steam.
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One of the main goals of the FCC technology is to reduce the thermal cracking in the riser bottoms, favouring the catalytic cra