Sublimation flux

The MTM-based methods, that also can provide information about the sublimation flux, will be discussed in the next section. 

At a generic time t, given the values of l heif (known from the measured, or modeled, distribution of shelf temperature) and of (known from CFD calculations), the sublimation flux (and the residual water content and the product temperature) at time t + At are calculated by integrating the detailed one-dimensional model. This assumes that Tsheif and Pc do not vary significantly in the time interval At, that is, that the variation of the sublimation flow rate is small this assumption is reasonable if a small time interval At is used, for example, 30 min. 

At time t -F At, the new value of Pc is calculated using the CFD model (either carrying out new CFD simulations, or using correlations obtained from a previous CFD campaign of simulations) and the previously calculated sublimation flux as boundary condition. 

Fig. 5.5 (a) Design space forthe primary drying stage of a coffee extract freeze-drying process calculated for different values of dried cake thickness (b) Values ofthe sublimation flux as a function ofthe operating conditions when Lj/... 

Equations 5.11-5.13 can be used to calculate the interface temperature, once the operating conditions (temperature and humidity of the air stream) have been selected, and the values of the heat and mass transfer coefficients have been determined (see, among others, Boeh-Ocansej, 1988 Krokida et al., 2002). It is then possible to calculate the sublimation flux and, finally, the variation of the dried layer thickness with time ... 

Fine particle fraction SubUmation flow rate Immunoglobulin G Sublimation flux... 

Thereby, it is now possible to derive the sublimation flux /w(r) in connection with (10.12) and eventually the sublimation flow rate G from the sample particle surface to the drying chamber ... 

Two limiting cases for gasification at the fuel surface were considered. In case 1, the fuel concentration was assumed constant and independent of time, i.e., f(Cf) = Cf and in case 2, it was assumed that the fuel mass flux was constant and independent of time or pressure, i.e.,/(Cy) = — D 8Cf/ dx = rfi. Case 1 was identified with a condensed phase behaving as a boiling liquid or subliming solid, and case 2 with a polymer undergoing irreversible decomposition at constant temperature. 

A technique for the determination of Tc amounts as little as 4 x 10 g by neutron activation analysis has been described by Foti et al. . Tc in triply distilled water is irradiated in a thermal neutron flux of 5 x 10 neutrons per cm and per second to produce °°Tc. Other radionuclides are removed by co-precipi-tation with Fe(OH)j. Then, °°Tc is co-precipitated twice with tetraphenylarsonium perrhenate which can be removed by sublimation. The chemical purification of °°Tc requires 40-45 s and the technetium yield is about 53%. 

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