Controlled catalyst circulation

Controlled catalyst circulation is one of the most important prerequisites for trouble-free operation of the FCC unit. Uniform circulation is ensured by controlling the differential pressure between the reactor and regenerator. The differential pressure in the existing plant is controlled by a differential pressure governor adjusting the position of the double slide valve upstream of the orifice chamber. 

Model II Regenerator at higher elevation and lower pressure than reactor. Slide valves control catalyst circulation. 

This model cat cracker does not use slide valves to control catalyst circulation. Instead, the lift air (valve V ) is used to transport catalyst from the reactor into the regenerator. The flow rate of catalyst from the regenerator to the reactor is controlled by changing the pressure differential between the reactor and the regenerator using valve V2. Pressure in the reactor is controlled by valve V3 on the suction of the wet gas compressor. 

The catalyst standpipes create a standing head that drives catalyst circulation. Different FCC designs make more or less use of standpipe technology. Standpipes typically end at a slide valve, which is used to control catalyst circulation. Standpipe diameter and length are variables in the standpipe model. 

Thus the ECCU always operates in complete heat balance at any desired hydrocarbon feed rate and reactor temperature this heat balance is achieved in units such as the one shown in Eigure 1 by varying the catalyst circulation rate. Catalyst flow is controlled by a sHde valve located in the catalyst transfer line from the regenerator to the reactor and in the catalyst return line from the reactor to the regenerator. In some older style units of the Exxon Model IV-type, where catalyst flow is controlled by pressure balance between the reactor and regenerator, the heat-balance control is more often achieved by changing the temperature of the hydrocarbon feed entering the riser. 

Coke on the catalyst is often referred to as delta coke (AC), the coke content of the spent catalyst minus the coke content of the regenerated catalyst. Delta coke directly influences the regenerator temperature and controls the catalyst circulation rate in the FCCU, thereby controlling the ratio of catalyst hydrocarbon feed (cat-to-od ratio, or C/O). The coke yield as a fraction of feed Cpis related to delta coke through the C/O ratio as ... 

Model IV Regenerator and reactor at approximately equal elevation and pressure. Catalyst circulates through U-bends, controlled by pressure balance and variable dense-phase riser. 

One pass through this process at 400 psi results in 100% conversion of the benzene to cyclohexane with purity of about 99%. The economies compared to the traditional processing scheme come from energy savings and simple equipment. In addition, the catalyst circulation system lends itself to fine control since deactivated catalysts can easily be replaced on the fly without shutting down the system. 

Total unit heat dnty will typically be in the range of 500-1000 BTU per pound of feed to the unit. This set of process heat requirements establishes the amount of heat that must be supplied by combustion of coke. Because of the process control schemes that are normally employed in FCCUs, the unit operation will automatically adjust itself so that the energy produced via coke combustion equals the heat requirements of the process. If the balance is shifted by changes to the feed quality or operating conditions, shifts in catalyst circulation rate and regenerator temperature will occur until a new equilibrinm set of conditions is established. 

Use of special plug-type valves for control of catalyst circulation. 

Fresh or regenerated catalyst is added to the top of the first reactor to maintain a constant quantity of catalyst. Catalyst transport through the reactors and the regenerator is by gravity flow, whereas that from the last reactor to the top of the regenerator and back to the first reactor is by the gas-lift method. Catalyst circulation rate is controlled to prevent any decline in reformate yield or hydrogen production over time onstream. 

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