Riser temperature control catalyst

Most FCC units only have a few independent variables. Typically, these independent variables are the feed rate, feed preheat temperature, reactor/riser temperature, air flow rate to the regenerator, and catalyst activity. The feed rate and air flow rate to the regenerator are set by flow controllers. The feed temperature is set by the feed temperature controller. Catalyst activity is set by catalyst selection and fresh catalyst addition rate. Reactor temperature is controlled by the regenerator slide valve that regulates the catalyst circulation rate. The catalyst circulation rate is not directly measured or controlled. Instead, the unit relies on the heat balance to estimate the catalyst circulation rate. Except for these independent variables, other variables, such as regenerator temperature, degree of conversion, and carbon-on-catalyst, etc., will vary accordingly to keep the FCC unit in heat balance. These variables are dependent variables. 

In current industrial practice, reactor (or riser) temperature is usually controlled by the flowrate of hot catalyst fed to the reactor from the regenerator. A slide valve in the... 

Until recently only a few papers were available on moving beds in cross flow [11-18]. This type of reactor is sometimes a favorable process solution for a selective catalytic process with a moderate catalyst rcsidence time and with a short gas residence time, especially when the process is accompanied by a continuous catalyst regeneration. The use of conventional short-contact-time reactors like fluidized-bed reactors, risers, and fixed-bed reactors does not always yield satisfactory results. This may be explained by problems connected with gas back-mixing, channeling of gas, low catalyst holdup, attrition of the solid catalyst, or difficulties in temperature control. 

Control of the FCC is achieved by the two slide valves. The regenerated catalyst standpipe slide valve controls the riser reactor outlet temperature by controlling the flow of hot regenerated catalyst supplied to the riser. The spent catalyst standpipe slide valve controls stripper level by adjusting the efflux of catalyst from the stripper. 

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. 

The flow rate of the regenerated catalyst to the riser is commonly regulated by either a slide or plug valve. The operation of a slide valve is similar to that of a variable orifice. Slide valve operation is often controlled by the reactor temperature. Its main function is to supply... 

The latest model is known as Exxon Flexicracker (Figure 22) (56). In this model, the U-tubes are replaced by a standpipe that is followed by upwardly sloped laterals, referred to as J-bends. This model takes advantage of riser cracking and spent catalyst in the J-tube is controlled by a slide valve. Reactor temperature is controlled by pressure differential between the regenerator and reactor. 

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