The stacked type Fluidized Catalytic Cracking (FCC) units are designed to operate with only a single stage of regeneration and do not contain any provision for external heat removal from the catalyst. It is common to find stacked FCC units where the single stage of regeneration is typically operated for partial CO combustion. The regeneration vessel does not contain equipment that is unsuitable for higher temperatures and thus accompanies complete catalyst regeneration. In this paper, the orthoflow configuration of FCC is discussed and a comparison of the arrangement with the side-by-side configuration using the study of the complete design, costs, process, and catalysts with their selectivity and deactivation is presented.

The primary function of the Fluidized Catalytic Cracking (FCC) unit is to crack the gas oil fraction of the crude oil to produce more gasoline and distillate fuels. Two basic types of FCC units in use today are the `side-by-side' type and the orthoflow or `stacked' type. All fluidized catalytic cracking units have three major sections—the reactor-riser, the regenerator, and the fractionators.

In the side-by-side design, the reactor vessel is located next to the regenerator vessel. The spent catalyst standpipe and the regenerated catalyst standpipe connect these two vessels. Generally the side-by-side configuration of the reactor and regenerator is applicable to large scale units which can process 20,000 or more Barrels Per Stream Day (BPSD) of fresh feed . In such configuration, the riser is inside the steam stripper with the operating pressure of 8-30 psig, which is generally limited to 15 to 20 psig corresponding to the design and pressure balance of the reactor and regenerator. The riser temperature ranges from 950 °F to 1,025 °F. A short residence time of the gas in the riser, usually less than 5 s, is maintained in order to produce the desired product selectivity and amount of coke formation. Fresh catalyst is added to maintain the desired activity of the catalyst as the activity slowly declines from residual coke deposition on catalyst surface and thus its deactivation. The reactor serves as a disengaging vessel for the catalyst and product vapor, and as the housing for the cyclones. Operating pressure and temperature of the reactor are similar to those of the riser.

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