Temperature Control of the Continuous Peanut Drying Process Using Microwave Technology

Abstract

The relationship between dielectric properties of peanuts (Arachis hypogaea L.), thermal and moisture distribution during continuous microwave drying, and automated control of the drying process was investigated. Dielectric properties (ε', ε') of ground samples of peanut pods and kernels were measured for several densities, temperatures, and moisture contents, in the range of 300 to 3000 MHz. Dielectric mixture equations were used to correlate the dielectric properties with density. The coefficients of quadratic and linear dielectric mixture equations are tabulated for 915 and 2450 MHz, for different temperatures and moisture contents. The values of the dielectric constants (ε') and loss factors (ε') of bulk peanut pods and kernels were determined by extrapolation of the first and second-order polynomials that relate ε' and ε' with density. An equation that determines the dielectric properties of bulk peanut pods and kernels as a function of their temperature and moisture content was determined using multiple linear regression.

Peanut dielectric properties were used in transport phenomena equations previously developed for batch-type microwave drying. The equations were adapted to account for the spatial variation of the electric field inside a continuous microwave drying applicator. The theoretical equations developed, together with experimental methods, were used to determine the effect of microwave power level, initial moisture content and dielectric properties on the temperature profiles and the reduction in moisture content of peanuts.

The temperature profiles obtained from solution of these equations matched the experimental profiles determined using fiber optic temperature probes. The temperature profiles were determined to be dependent on both moisture content and microwave power level. Although the maximum temperature in the microwave applicator was a function of power level only, the rate at which that maximum was attained was a function of dielectric properties and moisture contents of the peanuts. An absolute theoretical determination of moisture content reduction during microwave drying was not possible due to the dependence of dielectric properties on the moisture content. When dielectric properties were assumed independent of moisture content, the theoretical estimations of moisture losses were always lower than the losses determined experimentally, although they were in the same range of values. The surface temperature distribution of the peanut bed measured using infrared pyrometry was well correlated with internal temperature profiles. Thermal imaging demonstrated that the temperature of the peanut bed surface at the exit of the microwave curing chamber was uniformly distributed.

The surface temperature determined as a function of power level was used to create a feedback control loop of the continuous microwave drying process of farmer stock peanuts of different varieties and various initial moisture contents. Process parameters were determined using process reaction curves and a PI (proportional, integral) controller was implemented in the software routine that controlled the microwave generator power level. The servo scenario (set point change) was simulated to determine the optimum tuning parameters of the PI controller. The potential for a combined feedback/feed forward control of the process based on complete surface temperature distribution measured with an infrared camera placed at the location of maximum surface temperature was evaluated.

This study quantifies the relationships between the various parameters that influence the continuous microwave drying process of peanuts. The results of this study may be used as a foundation for development of optimum process conditions in microwave drying of peanuts and other agricultural commodities, as well as for development of advanced process control methods in continuous microwave processing.