مطالعات تجربی و شبیه سازی فرآیند خشک کردن خلاء اولیه مواد جامد تصادفی در گرمای مایکروویو

This paper concerns experimental and theoretical studies of freeze-drying process at microwave heating. Two kinds of random solids were dried: material which are assumed to have no internal porosity (ground glass), as well as one containing internal porosity (Sorbonorit 4 activated carbon). Formulated one-dimensional two-region model of freeze-drying process at microwave heating takes into account unknown a priori sublimation temperature Ts(t) and mass concentration of water vapor Cs(t) at moving ice front. Steady capacity of internal heat source is correlated with electric field strength E and dissipation coefficient K(T) in both regions of the material to be dried. Linear temperature dependency of dissipation coefficient is assumed and described by two regression parameters: μ1i and μ2i for dry (i = I) and frozen (i = II) bed, respectively. A correlation between both measured and calculated temperatures of the sample and actual electric field strength was observed. Fairly good agreement between experimental and simulated results was stated.

Application of microwaves in freeze-drying process results in energy generation directly in dried material. The ability of electromagnetic energy to selective heat affects process kinetics and considerably enhances mass and energy transfer in freeze-dried sample. For specific microwave system of constant frequency f, microwaves absorption and dissipation mechanism depends on dielectric constant ɛ′ and loss factor ɛ″ of material. Electric field strength E around a sample reflects heating intensity and is the parameter that can be controlled [1], [2] and [3]. In freeze-dried sample there exist two different dielectrics corresponding to frozen and dried layer of a bed separated by ice sublimation front being moving boundary. Both layers can be assumed to have constant composition. During the process the loss factor ɛ″ of material varies with its temperature therefore empirical correlation ɛ″ = f(T) should be applied for both sample regions [4]. In this paper temperature dependence of layer loss factor is directly incorporated into source term in equations of energy balance and assumed to have linear character. Freeze-drying is divided into three stages: pre-freezing, primary drying when ice sublimation takes place under vacuum, followed by desorption of residual, unfreezable bounded water during secondary stage (residual moisture about the few percent by dry product weight). In freeze-drying of materials with no internal porosity (e.g. ground glass) only primary drying stage exists but for materials with internal porosity (e.g. activated carbon) additionally secondary drying takes place. Formulated mathematical model in this paper concerns the primary freeze-drying at microwave heating.

The mathematical model of primary freeze-drying of random solids at microwave heating was developed. Experimental and theoretical investigations of the freeze-drying at microwave heating were presented for two chosen materials: ground glass and Sorbonorit 4 activated carbon. For dried materials such as ground glass, it was assumed to have no internal porosity, primary freeze-drying stage is sufficient to remove entire moisture contained in the dried bed. Theoretical analysis shows that in this case there is no significant influence of sample thickness on microwave freeze-drying time. During primary freeze-drying at microwave heating of Sorbonorit 4, which represents material with internal porosity, moisture contained in intergranular volume and unbounded moisture in pores is removed. Despite of grater mass transfer resistance of water vapor in dried layer of the material, process kinetics is enhanced in comparison with freeze-drying at conventional heating.