Catalysts regeneration catalytic cracking unit

CATALYSTS - REGENERATION - FLUID CATALYTIC CRACKING UNITS] (Vol5)... 

Indicated in Fig. 2 is a representative fluid catalytic cracking unit, comprising ( )a reactor (2) a regenerator (3) the main fractionator (4) an air blower or compressor (3) a spent-catalyst stripper (6) catalyst recovery equipment, including cyclones internal in the reactor and regenerator and slurry settler, and possibly an electrostatic precipitator and (7) a gas-recovery unit. The catalyst used is essentially u specially prepared composite of silica and alumina. 

The temperature in the regenerator is conventionally controlled by the flowrate of air from the blower. However, many catalytic cracking units run at maximum airflow as limited by the air blower. In this situation catalyst cooling rates in the regenerator can be used to hold regenerator temperature. However, if air is at a maximum constraint, the amount of coke that can be burned is limited. Therefore, feed flowrate may have to be reduced or feed composition altered (feed more light gasoil and less heavy material). 

Fig. 1. A 1 MMTA fluid catalytic cracking unit with fast fluidized bed regenerator (courtesy Gaoqiao Petrochemical Company). 1, regenerator 2, first-stage FFB regenerator 3, second-stage FFB regenerator, 4, main fractionator, 5. FFB catalyst cooler 6, catalyst hoppers 7, cyclone...

Carbon monoxide is a potential emission problem. Fortunately, it is a valuable industrial fuel and may be burned in a CO boiler to recover energy as steam and discharge carbon dioxide. However, it can also arise from the catalyst regenerators of some recent models of catalytic cracking, units which ran at temperatures too low to obtain complete oxidation of carbon to carbon dioxide. The most recent designs of regenerators operate at higher temperatures to achieve complete conversion of carbon to carbon dioxide. 

Control of catalyst particle losses from both the cracker and regenerator of fluid catalytic cracking units is achieved by two cyclones operating in series right inside each unit. This is usually followed by an electrostatic precipitator for fine particle control, working on the exhaust side of the catalyst regenerator [62]. The metal content of spent catalysts may be recovered for reuse [63]. 

An important advantage of the fluidized-bed reactor over the fixed-bed type is that the catalyst can be regenerated without disturbing the operation of the reactor. In fluidized-bed catalytic cracking units a portion of the solid particles is continuously removed from the reactor and regenerated in a separate unit. The regeneration is accomplished by burning off the carbon with air, and the reactivated catalyst is continuously returned to the reactor proper. In the fixed-bed reactor the closest approach to... 

In order to regenerate the activity of the catalyst completely, a small amount of fresh catalyst is added to the system from time to time. Figure 6.11 depicts typical reactor-regenerator sections of a catalytic cracking unit used in petroleum refineries. 

In a typical thermofor catalytic cracking unit, the preheated feedstock flows by gravity through the catalytic reactor bed. The vapors are separated from the catalyst and sent to a fractionating tower. The spent catalyst is regenerated, cooled, and recycled. The flue gas from regeneration is sent to a carbon monoxide boiler for heat recovery. 

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