ProCell units

However, even more opportunities for product design result from combining manipulations of the gas distributor, the process chamber and the spray system with each other. In the following, three examples of such combined manipulations will be briefly discussed, which result in Wurster equipment, in so-called ProCell units, and in lengthy rectangular ( horizontal ) fluidized beds. 

A very simple, but also coarse, method of detertniriitig the minimally required gas volume flow rate is to define it as the flow rate creating conditions of minimal fluidization at the upper surface of a stagnant bed of solids placed in the equipment under consideratiort This derivation will be illustrated for the ProCell unit depicted schematically in Fig. 4.9. The superficial gas flow velocity at the free surface of the respective stagnant bed is the ratio of the gas volume flow rate Vg to the respective cross-sectional area, which is twofold the product of the width Wo and the length L of the apparatus, perpendicular to the plane of Fig. 4.9. By setting the superficial gas flow velocity equal to the minimal fluidization velocity u f and the gas volume... 

Fig. 4.9 Schematic of a symmetrical ProCell unit, with two gas inlet gaps (Mori eta/., 2000).

Fig. 4.11 Bed pressure drop as a function of gas volume flow rate for a ProCell unit according to measurements by C czka (2009).

The procedure is to vary the parameter G by variation of the hold-up and, thus, of Abed.o for a given piece of equipment and a certain particulate material (i.e., at Ar = constant) until the limits of the stable operation range have been identified and the respective values of G and Rejn have been plotted on the Re-G-Ar-diagram. The same determination is then repeated for another particulate material, that is, with another value of Ar. This procedure is illustrated in Fig. 4.12 on the basis of experimental results by Gryczka (2009) for a ProCell unit. 

Piskova, 2002) E to F Conical-cylindrical spouted beds (Olazar et o/., 1993) C to H Prismatic spouted bed with one gas entrance (Mitev, 1979) I toj Prismatic ProCell unit with two gas entrances (Cryczka, 2009), same data as in Fig. 4.12. 

This is a dear demonstration of a very significant enhancement of particle-to-fluid mass transfer in the ProCell unit, compared to a similarly operated conventional fluidized bed. 

The same conclusion can be attained via the PFR evaluation. The respective data from the ProCell unit (gray cycles) are located in Fig. 4.16 significantly lower than Sherwood numbers evaluated with the CSTR assumption, because back-mixing reduces the efficiency of interface heat and mass transfer and, thus, leads to lower values of transfer coefficients, if taken into account in such coeffidents instead of being considered in the balance equation of the model. For the reference case of the conventional fluidized bed, so-called apparent Sherwood numbers (Shapp) must be used in combination with the PFR model (Groenewold and Tsotsas, 1999). 

Based on their data, Hoffmann et al. (2011) proposed for the mass transfer in ProCell units empirical correlations of the type... 

In total, the analysis by Fries (2012) indicated that spouted beds - in this case, the ProCell unit - might possess the highest versatility for the wet formulation of various products (agglomerates, coated particles, layered granules), coupled with the highest potential for intensification of the respective processes by apparatus and flow design. 

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