Fluidization properties

The PSD is an indicator of the fluidization properties of the catalyst. In general, fluidization improves as the fraction of the 0-40 micron particles is increased however, a higher percentage of 0-40 micron particles will also result in greater catalyst losses. 

Catalyst circulation is largely influenced by the physical layout of the unit and the fluidization properties of the catalyst. Some cat crackers circulate with ease regardless of the catalyst s physical properties. However, in other designs, the unit can experience circulation difficulties with minor changes in catalyst properties. 

The improvements in the catalyst s binder properties will reduce the catalyst attrition rate thus, lowering the flue gas stack opacit This improvement allows refiners to use a harder catalyst without adversely affecting the catalyst s fluidization properties. 

Few models include the effects of in situ gas formation on the fluidization properties of the reactors this improvement, along with improvements in other areas, such as inclusion of improved structured models of microbial kinetics or inclusion of maintenance energy requirements or the effects of suspended cells on the reaction rate, might produce more accurate models, though it is unclear at this point whether the increased complexity would be justified. 

Utilize a denser, coarser grade PSD fresh catalyst while ensuring that the fluidization properties remain acceptable... 

Geldart (1973) classified powders into four groups according to their fluidization properties by air at ambient conditions. This classification is now used widely in all fields of powder technology. 

Solids of group C are very fine-grained, cohesive powders (e.g. flour, fines from cyclones, and electrostatic filters) that virtually cannot be fluidized without fluidization aids. The adhesion forces between particles are stronger than the forces that the fluid can exert on the particles. Gas flow through the bed forms channels extending from the grid to the top of the bed, and the pressure drop across the bed is lower than the value from cq 1. Fluidization properties can be improved by the use of mechanical equipment (agitators, vibrators) or flowability additives such as Aerosil. 

Dual particle or separate traps such as RV4+ must have attrition and fluidization properties similar to FCC catalyst. Their advantages are that they do not change the selectivity of the base catalyst and theoretically have a higher capacity for vanadium capture. Performance evaluation of dual particle traps is usually simpler. They can often be isolated from equilibrium catalyst and analyzed for vanadium capture. Confirmation of preferential pick up on integral traps tends to be a bit more qualitative. A disadvantage may be that they are more dependent on vanadium mobility than integral traps. 

The cracking catalyst was selected as a bed material due to its ideal fluidizing properties and not for its catalytic properties. The nitrogen gas was preheated to furnace temperature and was fed to the reactor through an inlet on the bottom segment. The reactor was heated by a tubular Lindberg furnace and the temperature was controlled by a thermocouple feedback mechanism connected to the oven control unit. 

Resin manufacturers often express the fluidization properties of thejr resins in terms of the percentage expension of the bed related to its pseked depth. This expansion cannot be calculatad directly from the above equation since a range of bead sizes is present. Typical data are shown in Fig. 13.3-1, taken from a Rohm and Haas pamphlet. These curves can be correlated by an equation giving the expension as a fouction of the 1.5 power of the flow rale. An equation of almost identical form can be derived from the Richardson-Zaki equation by expressing the expansion in cerma of the void fraction and using appropriate values for n. 

The procedure for the scale-up of an expanded-bed-adsorption process is relatively straightforward and the principles are similar to those used for a packed-bed process. It is important that the length of the laboratory column be equal to the pilot-plant column. If the pilot-plant equipment is not specifically designed for expanded-bed-adsorption procedures, it should be modified as described in the previous section on laboratory equipment. To verify that the expanded-bed flow patterns are similar for the lab and pilot-plant columns, pulse tests using NaCl solution should be carried out. The adsorbent used, whole-broth-solvent ratio, bed height, and linear velocity, should not be changed on scale-up. The volumetric flow should be increased m proportion to the mcrease in the cross-sectional area of the two columns. Thus, the superficial velocity will be maintained and the adsorption and the fluidization properties will be constant. 

Powders in group A have the most desirable fluidization properties. They expand significantly when fluidized, and take a long time to de-aerate after the gas supply is cut off. This makes them easy to circulate in pneumatic and fluidized systems, but also makes them liable to flood on discharge from hoppers. Interparticle forces are present in group A powders, but are smaller than the hydrodynamic forces. Gas bubbles are limited in size and may break up at high fluidizing velocities. 

Figure 12.8 Classification of fluidization properties according to the size and density of powders (adapted from Geldart, 1973).

The fluidization properties of a powder in air may be predicted by establishing in which group it lies. It is important to note that at operating temperatures and pressures above ambient a powder may appear in a different group from that which it occupies at ambient conditions. This is due to the effect of gas properties on the grouping and may have serious implications as far as the operation of the fluidized bed is concerned. Table 7.1 presents a summary of the typical properties of the different powder classes. 

The above equation provides a correlation for estimating Shbed (or A)g bed) based on particle and fluidization properties, which then can be used to determine the rate of mass transfer employing Eq. (7) based on the homogeneous bed approach. In the application, the following equations provided by Kunii and Levenspiel (1991) are needed for calculating fluidization properties appearing in Eq. (53)... 

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