Dear Washim Akram,

From my view, I think the modelling for both of them are different. For example, the drying model for convective drying especially, hot air drying the simple model is (x - xe)/(xo - xe) = kt ==> this is Newton model. The term of xe (equibrium moisture content) comes from desorption isotherm of the product. The isotherm that I usaully use is a relationship between three main parameter such as drying temperature, relative humidity and moisture content. So, in the case of microwave drying, it will lack of drying temperture term that it will effect to xe. Then, we don’t have xe to substitute in the drying model. However, there are another way to use the drying model for microwave drying in same model as convective drying. It is the assumption of xe in the microwave drying of the sample reach or become to zero (it will depend on the drying characteristics of you r sample as well). Then, the model for microwave drying will be MR or (x)/(xo) = kt. That’s an example.

Hope it will useful for you

Traiphop Phahom answer has focussed on the equilibrium moisture content and how it can be accounted for in modeling microwave heating. Let me instead give a more physical interpretation of the drying process.

In conventional drying, energy in the form of heat is transferred to the surface of the material(e.g., food) by either conduction, convection or radiation, or in various combinations thereof. The heat then penetrates the material by thermal conduction. The surface moisture is flashed off from the surface, causing the interior moisture to diffuse to the surface. This is often the slow step in conventional drying, because it is diffusion rate limited.Thus in drying, there are two main periods: the constant drying rate period (CDRP) and the falling drying rate period (FDRP). It is important to note that these two periods of drying are characterized by different mechanisms of moisture transport. The drying rate during CDRP is independent of time, and assumes that the surface water present is loosely bound, or located near the surface. Of course, for food products this is a poor approximation. So the duration of the CDRP is quite limited.

in conventional drying, the FDRP begins when the body surface dries in some places so that liquid starts to redistribute with in the body by capillary action- a wetting effect. This bound water is then held within the pore structure by capillary forces as well as by chemical forces, which characterizes the FDRP. In short during the FDRP there is a weak driving force to bring water to the surface.

In microwave heating, the absorption of energy depends primarily on the amount of water content in the food product, or other molecules that have polar character. The amount of heat generated in microwave heating is equal to the product of electric conductivity \[Alpha] and the square of the electric field intensity E, which can be expressed as R=\[Alpha] E^2/2. The electric field E decays exponentially with thickness E=Subscript[E, 0]Exp[-2\[Delta] x\[CenterDot]n], where \[Delta] is the dialectic constant of the material. Hence the amount of microwave energy absorbed by drying material decays exponentially with distance, as determined by the parameter \[Delta], which depends on the dielectric properties of the medium.

Microwave heating is most efficient on liquid water, and less so on fats/sugars. Depending on the water content, the depth of initial heat deposition by microwave heating can be several centimeters or more , in contrast to oven grilling, which relies on infrared radiation, or the thermal convection of a convection oven.

Hence the key advantage of microwave drying is the volumetric heating that occurs leading to internal vapor generation. The internal vapor pressure then drives the moisture out of the product, resulting in a significant reduction in drying time, and thereby bypassing the FRDP.

There is no unambiguous answer to the question. Everything depends on the assumptions made. Drying is a process where both mass transport and energy transport takes place. The structure of the dried material is also important. Considering only the method of energy supply: in the case of microwaves to the entire volume, and in the case of convective drying to the surface, it is difficult to expect that in both cases the temperature distribution was the same. It is also worth remembering that microwave drying causes less drying shrinkage than convection drying. From the point of view of the moving boundary, modeling will be different in both cases. Of course, if one considers the simplest approach, where it is assumed that the rate is proportional to the difference between the current and the limit humidity of body (the so-called second period of drying), then there will be no differences. Regards,