Flow fluid catalytic cracking

The alkylation unit in a petroleum refinery is situated downstream of the fluid catalytic cracking (FCC) units. The C4 cut from the FCC unit contains linear butenes, isobutylene, n-butane, and isobutane. In some refineries, isobutylene is converted with methanol into MTBE. A typical modern refinery flow scheme showing the position of the alkylation together with an acid regeneration unit is displayed in Fig. 1. 

Fluidized catalytic processes, in which the finely powdered catalyst is handled as a fluid, have largely replaced the fixed-bed and moving-bed processes, which use a beaded or pelleted catalyst. A schematic flow diagram of fluid catalytic cracking (FCC) is shown in Fig. 4. 

In the S W fluid catalytic cracking process (Figure 8-15), the heavy feedstock is injected into a stabilized, upward flowing catalyst stream whereupon the feedstock-steam-catalyst mixture travels up the riser and is separated by a high efficiency inertial separator. The product vapor goes overhead to the main fractionator (Long, 1987). 

Riser the part of the bubble-plate assembly which channels the vapor and causes it to flow downward to escape through the liquid also the vertical pipe where fluid catalytic cracking reactions occur. 

Figure 13 The apparent flow regime diagram calculated with EMMS-based multiscale CFD and the intrinsic flow regime diagram for the air-FCC system (fluid catalytic cracking particle, dp = 54 m, pp = 930 kg/m3) calculated by using the EMMS model without CFD. The intrinsic flow regime diagram is independent of the riser height (Wang et al., 2008).

Figure 6. Flow diagram refining of SRC-II oil by hydrotreating and fluid catalytic cracking, case 3...

Several reactor types have been described [5, 7, 11, 12, 24-26]. They depend mainly on the type of reaction system that is investigated gas-solid (GS), liquid-solid (LS), gas-liquid-solid (GLS), liquid (L) and gas-liquid (GL) systems. The first three arc intended for solid or immobilized catalysts, whereas the last two refer to homogeneously catalyzed reactions. Unless unavoidable, the presence of two reaction phases (gas and liquid) should be avoided as far as possible for the case of data interpretation and experimentation. Premixing and saturation of the liquid phase with gas can be an alternative in this case. In homogenously catalyzed reactions continuous flow systems arc rarely encountered, since the catalyst also leaves the reactor with the product flow. So, fresh catalyst has to be fed in continuously, unless it has been immobilized somehow. One must be sure that in the analysis samples taken from the reactor contents or product stream that the catalyst docs not further affect the composition. Solid catalysts arc also to be fed continuously in rapidly deactivating systems, as in fluid catalytic cracking (FCC). 

Fluid catalytic cracking (FCC) (Fig. 13.5) was first introduced in 1942 and uses a fluidized bed of catalyst with continuous feedstock flow. The catalyst is usually a synthetic alumina or zeolite used as a catalyst. Compared to thermal cracking, the catalytic cracking process (1) uses a lower temperature, (2) uses a lower pressure, (3) is more flexible, (4) and the reaction mechanism is controlled by the catalysts. Feedstocks for catalytic cracking include straight-run gas oil, vacuum gas oil, atmospheric residuum, deasphalted oil, and vacuum residuum. Coke inevitably builds up on the catalyst over time and the issue can be circumvented by continuous replacement of the catalyst or the feedstock pretreated before it is used by deasphalting (removes coke precursors), demetallation (removes nickel and vanadium and prevents catalyst deactivation), or by feedstock hydrotreating (that also prevents excessive coke formation). 

The entrained-flow reactor (Fig. 8.5) is used when very short contact times are required, as in the case of highly active catalysts that deactivate fast. In fluid catalytic cracking (FCC) the circulating catalyst also supplies part of the heat for the endothermic reaction. Depending on the catalyst loading one can distinguish dilute and dense phase risers. ... 

The catalysts used in Fluid Catalytic Cracking (FCC) are reversibly deactivated by the deposition of coke. Results obtained in a laboratory scale entrained flow reactor with a hydrowax feedstock show that coke formation mainly takes place within a time frame of milliseconds. In the same time interval conversions of 30-50% are found. After this initial coke formation, only at higher catalyst-to-oil ratios some additional coke formation was observed. In order to model the whole process properly, the coke deposition and catalyst deactivation have to be divided in an initial process (typically within 0.15 s) and a process at a larger time scale. When the initial effects were excluded from the modeling, the measured data could be described satisfactory with a constant catalytic activity. 

Fluid Catalytic Cracking (FCC) is one of the main industrial catalytic processes, in which heavy hydrocarbons are converted into lighter hydrocarbons. The main products are gasoline and light cycle oil. The reactions are carried out in an entrained flow reactor, in which the... 

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