Drying mass transfer resistance

The techniques of measuring drying of stationary products, as on trays, are relatively straightforward. Details may be found in the references made with the data of Figure 9.3. Mass transfer resistances were eliminated by Wenzel through use of superheated steam as the drying medium. 

A two-parameter model is obtained if wetting is incomplete (tice < 1) > but the inactively wetted surface is assumed to have negligible mass transfer resistance (Bi - °°). This latter condition was used by Mata and Smith (13) and physically corresponds to the inactively wetted area being dry, or to the presence of stagnant liquid film which is at equilibrium with the gaseous reactant. The expression for the conversion given by Equation 10 reduces to ... 

Mass transfer resistance, product temperature, and drying rate... 

Since the thimble and the freeze-dried mass could hinder mass transfer, another set of experiments were conducted at 24 and 31 MPa In the cups of an aluminum sample holder with six cups (0.5 cm x 0.5 cm). It was reasoned that If the thimble was eliminated from the extraction, and If only the lipids were exposed to the SC CO2, there would be negligible mass transfer resistance to extraction. Solvent extracted lipids were placed In the cups, and extracted with SC CO2. 

The kinetics of partial oxidation, ATR, and dry reforming of liquid hydrocarbons have also been reported recently.103,155 Pacheco et al.155 developed and validated a pseudo-homogeneous mathematical model for the ATR of isooctane and the subsequent WGS reaction, based on the reaction kinetics and intraparticle mass transfer resistance. They regressed the kinetic expressions from the literature for partial oxidation and steam reforming reactions to determine the kinetics parameters for the ATR of isooctane on Pt/ceria catalyst. The rate expressions used in the reformer modeling and the parameters of these rate expressions are given in Tables 2.19 and 2.20, respectively. 

The value ofTj is not easily determined, since it depends on the mass transfer resistances in the dry layer and gas film as well as on the heat transfer resistances. 

Q4. What is the driving force that causes each of these four forms of potential flow fluid flow electric current heat transfer drying (mass transfer) Identify the resistance for each. 

Figure 6 Simulations of (A) drying rate and (B) ice interface temperature as a function of the product mass transfer resistance and shelf temperature 2mL fill in a 5 mL vial, 26.6 Pa (200 mT) chamber pressure. Note the change from heat to mass transfer control over the dr5fing rate and temperature.

If the freeze-drying process is controlled by water vapor mass transfer through the freeze-dried layer - for example in the case of dried cake of very low permeability or for very favorable heat transfer conditions - the drying rate for one vial is proportional to the mean driving force P - Pc) - that is, to the water vapor pressure difference between the sublimation front interface and the drying chamber - and controlled by the freeze-dried cake mass transfer resistance, denoted J p, which is inversely proportional to the dried layer permeability defined by Darcy s law. Inthiscaseitholds ... 

Several techniques to determine experimentally the values of the freeze-dried cake mass transfer resistance, Rp, have been reported in the literature (Pikal, 1985 Patapoff et al., 1999). They were all based on the gravimetric method used either for a single vial or for a limited number of vials. In contrast to such methods, the estimation of Rp values from the P RA model allows the determination of the mean freeze-dried product mass transfer resistance for the whole vial batch, averaging the heterogeneities of the cake structures between individual vials generated by the random nucleation phenomena. As shown in Fig. 3.6, a typical set of Rp values identified by the PRA model by... 

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