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Thesis Summary: Development and Study of Low-Pressure Superheated Steam Drying System for a Heat-Sensitive Material

At present, there is limited experience with superheated steam drying (SSD), especially for a process operated at reduced pressure. One of the objectives of the thesis was thus to study low-pressure superheated steam drying (LPSSD), especially from the dried product quality point of view. Using carrot cubes as a model heat-sensitive material, experimental investigations were conducted to examine the drying kinetics and various quality parameters of the dried product undergoing low-pressure superheated steam, vacuum, and hot air drying. Effects of operating parameters, i.e., pressure and temperature, on the drying characteristics as well as physical quality attributes, i.e., volume, shrinkage, apparent density, color, and rehydration behavior, of the dried product undergoing two drying processes were also evaluated and compared. Although low-pressure steam drying required a longer dwell time to achieve the same final moisture content as vacuum drying, some of the quality attributes were superior to those obtained in vacuum drying. The effects of LPSSD and other drying methods on the chemical quality attribute of a model material, i.e., b-carotene in carrot, were also evaluated. LPSSD and vacuum drying led to less degradation of b-carotene in carrot than in the case of hot air drying. The empirical models, which can describe the experimental data of b-carotene degradation in carrot undergoing different drying techniques, were also proposed. bCarotene degradation in carrot depended more on the carrot temperature than its moisture content in all cases. In the second part of the study, an inversion phenomenon, which is an interesting phenomenon occurring when comparing superheated steam drying and air drying, of the LPSSD system was studied to investigate the effect of operating pressure on the inversion temperature of the system. This part of the study was also conducted to point out the fundamental differences of the values of inversion temperature obtained using only the rates of drying in the constant drying rate period and those obtained from the combined rates of drying in the constant and falling rates of drying. Molecular sieve beads, which were used as the model material in this section, were dried as a single layer in low-pressure superheated steam and vacuum dryers. The differences between the values of the inversion temperature calculated only from the rates of drying in the constant rate period and those calculated from the whole drying period were pointed out and discussed. Page’s equations and a single-term exponential equation were found to satisfactorily describe the kinetics of low-pressure superheated steam drying and vacuum drying systems, respectively. Finally, a simple three-dimensional liquid diffusion– based model was developed to predict the evolution of the product moisture content and temperature of a product undergoing LPSSD. The effect of the product shrinkage was included directly in the model and the effect of this inclusion on the predictability of the model was shown. The model was found to be able to predict the heat and mass transfer behavior reasonably well over some range of moisture content if accurate values of heat transfer coefficient were used in the model.