Catalyst entrainment

The distance above the catalyst bed in which the flue gas velocity has stabilized is refened to as the transport disengaging height (TDH). At this distance, there is no further gravitation of catalyst. The center-line of the first-stage cyclone inlets should be at TDH or higher otherwise, excessive catalyst entrainment will cause extreme catalyst losses. 

Figure 4-100. Monitoring of catalyst entrainment (Elliott "Laser Sentry ...

Catalysts developed in the titanium-aluminum alkyl family are highly reactive and stereoselective. Very small amounts of the catalyst are needed to achieve polymerization (one gram catalyst/300,000 grams polymer). Consequently, the catalyst entrained in the polymer is very small, and the catalyst removal step is eliminated in many new processes. Amoco has introduced a new gas-phase process called absolute gas-phase in which polymerization of olefins (ethylene, propylene) occurs in the total absence of inert solvents such as liquefied propylene in the reactor. Titanium residues resulting from the catalyst are less than 1 ppm, and aluminum residues are less than those from previous catalysts used in this application. 

The Effect of Cohesive Forces on Catalyst Entrainment in Fluidized Bed Regenerators... 

ECC catalyst is subject to hydrothermal deactivation. This occurs when the A1 atom in the zeolitic cage is removed in the presence of water vapor and temperature. The result is a loss of activity and unit conversion. The effect of temperature on this process is nonlinear. The deactivation rate increases exponentially with temperature. Units that experience high afterburn have attributed high rates of catalyst deactivation on the higher dilute phase temperatures. This phenomenon is more apparent on units with high combustion air superficial velocities. The high velocity not only increases afterburn, but also increases catalyst entrainment to the cyclones and dilute area. COP is used to decrease afterburn and minimize catalyst deactivation. 

Leading characteristics of five main kinds of reactors are described following. Stirred tanks, fixed beds, slurries, and three-phase fluidized beds are used. Catalyst particle sizes are a compromise between pressure drop, ease of separation from the fluids, and ease of fluidization. For particles above about 0.04 mm dia, diffusion of liquid into the pores and, consequently, accessibility of the internal surface of the catalyst have a minor effect on the overall conversion rate, so that catalysts with small specific surfaces, of the order of 1 m2/g, are adequate with liquid systems. Except in trickle beds the gas phase is the discontinuous one. Except in some operations of bubble towers, the catalyst remains in the vessel, although minor amounts of catalyst entrainment may occur. 

The problem of catalyst recovery is materially reduced because catalyst entrained in the exit gases from the reactor or regenerator amounts to less than 1% of the catalyst-circulation rate, as compared with 100% in the upflow design. 

Dave and Saraf (2002) described regenerator as two-region (the dense bed and the dilute phase) model, essentially following the scheme of Krishna et al. (1985) with some modification. Bulk of the coke combustion reaction occurs in the dense bed and the dilute phase is the region above the dense bed, after burning of carbon monoxide and catalyst entrainment are the main effects of this section. The dense bed is modelled as a... 

Other than altering the catalyst level, no operational changes pinpoint a particular type of cyclone failure. However, the troubleshooter can differentiate between catalyst attrition and cyclone damage as a cause of excessive catalyst losses. By collecting a sample of catalyst entrained in the regenerator flue gas, the amount of catalyst fines can be measured. This value is then compared to the percentage of fines in a sample of regenerated catalyst. 

In FBMRs, the membranes are inserted inside the fluidized catalyst bed, serving as a product extractor or a reactant distributor. Figure 7.1 shows a typical FBMR structure for selective removal of a product (hydrogen) [4,5]. Pd-membrane tubes are placed vertically in the FBMR.The reactant gas is fed through the gas distribution plate at the bottom of the reactor to fluidize the fine particulate catalysts. Entrained solids are separated from the reaction product gas stream by internal cyclone separator and then returned to the reactor catalyst bed. 

Figures 1.3.3 a, b show two commercially available cyclones designed for light industrial use. An example of a much larger scale cyclone installation is presented in Fig. 1.3.4. This is a good example of a complete system— including cyclone, blower, rotary airlock valves and ducting—all supplied by the same manufacturer. Fig. 1.3.5 illustrates a huge spent catalyst regenerator cyclone system typical of today s modern FCCU installations. Such cyclones are used to capture and return the catalyst entrained off the vessel s fluidized bed. Fig. 1.3.6 illustrates where these and other cyclones are typically used in a commercial FCCU refinery process. The left- and right-hand frames in...

Fig. 16.2.9. A large 2-stage cyclone system typical of that used to collect catalyst entrained off of the fluidized bed of a fluid catalytic cracking unit (FCCU). Courtesy Emtrol LLC...

---