Drying rate data

Performances of dryers with simple flow patterns can be described with the aid of laboratory drying rate data. In other cases, theoretical principles and correlations of rate data are of value largely for appraisal of the effects of changes in some operating conditions when a basic operation is known. The essential required information is the residence time in the particular kind of dryer under consideration. Along with application of possible available rules for vessel proportions and internals to assure adequate contacting of solids and air, heat and material balances then complete a process design of a dryer. 

Garud s (G4) data on the drying of welding electrodes show agreement within 15%, (Fig. 12) although the critical moisture content was not known accurately. Whenever data were not sufficient to calculate heat-and mass-transfer coefficients by boundary-layer theory, initial drying rate data was used for the purpose. 

In summary, drying rate data do not ordinarily provide sufficient information to distinguish the mechanisms involved in moisture removal. Further, the mechanisms will usually change as drying proceeds. Drying rate data have been analyzed mathematically on the basis of particular mechanisms for moisture movement through the solid. The complex nature of the moisture movement process, however, makes it impossible to attach any fundamental significance to rate constants obtained from such analysis. The fact that experimental data do not conflict with a theory does not prove its validity. 

Numerous drying rate data have been obtained in laboratory drying experiments using thermo-gravimetric analyzers as shown in the previous chapters. However it was difficult to know the actual dr5dng rate or solvent concentration profiles in the pilot or production scale dryer from these experimental results. 

If the effective moisture diffusion coefficients are calculated from Eqs 3.2 and 3.3, it can be noted that the coefficient in the case of SSD would be higher when the medium temperature is higher than 140 °C. This corresponds to the experimental drying rate data shown in Fig. 3.7 it can be seen that the inversion temperature of the whole drying process was approximately 140 °C. 

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