Dense-phase risers

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. 

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. ... 

Riser Inlet Riser Outlet Blower Discharge Rcgen. Dense Phase Regen. Flue Gas Ambient... 

Riser Cracking—Applied to fluid catalytic cracking units where the mixture of feed oil and hot catalyst is continuously fed into one end of a pipe (riser) and discharges at the other end where catalyst separation is accomplished after the discharge from the pipe. There is no dense phase bed through which the oil must pass because all the cracking occurs in the inlet pipe (riser). 

A two-stage stripper is utilized to remove hydrocarbons from the catalyst. Hot catalyst flows at low velocity in dense phase through the catalyst cooler and returns to the regenerator. Regenerated catalyst flows to the bottom of the riser to meet the feed. 

The transport velocity can also be evaluated from the variations of the local pressure drop per unit length (Ap/Az) with respect to the gas velocity and the solids circulation rate, Jp. An example of such a relationship is shown in Fig. 10.4. It is seen in the figure that, along the curve AB, the solids circulation rates are lower than the saturation carrying capacity of the flow. Particles with low particle terminal velocities are carried over from the riser, while others remain at the bottom of the riser. With increasing solids circulation rate, more particles accumulate at the bottom. At point B in the curve, the solids fed into the riser are balanced by the saturated carrying capacity. A slight increase in the solids circulation rate yields a sharp increase in the pressure drop (see curve BC in Fig. 10.4). This behavior reflects the collapse of the solid particles into a dense-phase fluidized bed. When the gas... 

Consider a riser where the voidage in the bottom dense region is uniform and the top dilute region behaves as the freeboard of a dense-phase fluidized bed. The axial profile of the voidage in the top dilute region can be expressed by [Kunii and Levenspiel, 1990] (see 10.4.1)... 

A schematic diagram of the reactor system of the 100 B/D plant is shown in Figure 13. There are three major vessels reactor, regenerator, and external cooler. The reactor consists of a dense fluid-bed section (60 cm ID x 13.2 m height) located above a dilute phase riser. Two modes will be studied to remove reaction heat ... 

The fast fluidization regime is represented by a dense region at the bottom of the riser and a dilute region above it. The inter-relationship of the fast fluidization regime with other fluidization regimes in dense-phase fluidization and with the dilute transport regime is... 

We often approximate the riser radial flow structure by assuming it consists of two characteristic regions a dilute gas-solid suspension preferentially traveling upward in the center (core) and a dense phase of particle clusters or strands descending near the wall (annulus), as shown in Figure 8. 

In summary, a single model has not been developed that can fully characterize riser gas phase hydrodynamics. The studies indicate that under dense phase conditions, typical of commercial FCC riser operation, a simple axial dispersion model may be adequate to characterize gas mixing. Under dilute conditions, a two-phase core-annular model is a good first approximation to the flow structure. However, both radial dispersion and radial gas velocity profiles must be accounted for to provide a realistic and reliable interpretation. The model suggested by Martin et al. should be further developed and applied to risers of different geometry operating with different powders [83]. However, contact efficiency may provide the simplest means from which scale-up criteria can be developed. 

The riser model effluent is connected to the reactor vessel models. These include the reactor dilute phase, the dense phase, and the cyclone system, which represents the segregation of effluent vapors and catalyst. The cyclone model performs a two-phase, loading-based AP calculation, for which cyclone inlet and body diameters are used. The cyclone model can be configured with one or two riser inlet ports. 

Krypton at 77 K is a subcritical vapour, the adsorption of which on crystalline surfaces is known to give rise to stepped isotherms, classified as type VI by lUPAC [5]. For such vapours, when a given pressure is reached the intermolecular forces between adsorbed molecules overwhelms their thermal energy, by which a phase transition occurs between and adsorbed gas-like and adsorbed dense phases [6, 7]. This phenomenon leads to a riser in the isotherms that corresponds to the complete coverage, at a given pressure, of the surface by a 2D dense phase. Such isotherms have been reported for the adsorption of several subcritical vapours carbon nanotubes [8, 9]. As type VI isotherms are never observed for amorphous solids, even with so-called subcritical vapours [7], we propose to exploit this... 

Dense phase transport lines should have gradual curvatures to prevent rough operation. Aeration should be provided to ease the flow of solids in dense phase transport lines and in risers. Blast connections should be available to clear them in case of plugging. Technique should be developed to weld patches on eroded lines during operation. 

Despite the dilution, high-activity zeolite catalysts achieved almost 100% riser cracking compared with only 15-20% with silica/alumina catalysts. This made it possible to redesign FCC units with full riser cracking and so avoid the usual overcracking in the dense phase of the reactor. Increased conversion and higher throughput led to a reduction in the volume of heavy cycle oil produced and recycle rates could be decreased. Thus, production capacity was further increased with httle capital expenditure. 

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