Study on Solar Dryer with Rotating Rack for Cocoa Beans

Abstract

This study discusses the engineering aspect of sokw-assisted drying for cocoa beans using GHE type dryer with rotating rack. This dryer has a capacity of 60.5 kg/m3. Solar irradiation and charcoal combustion were used as the thermal energy sources. Thin layer drying was performed in order to obtain drying lcinetics, thermophysical properties related to drying process as wei as the benchmark for fiek:t expeJinent. The air temperature influenced drying rate in all periods. while air velocity as well as absolute humidity did mainly just in the beginning period. The equations of the thermo physical properties related to air conditions have multiple po/ynorTlkll Individually. the validation of the properties. which were substituted to the moisture content - tine equation shQYoled a good cooelation. in which a~ of the COOS were above ~. Performance test of the solar dryer was carried out under the weather condition of Bogor. in cxder to obtain the pafameters. which involved the changes of temperature. hurridlty and moisture content during drying. In addition. measurement of the torque and power requirements to rotate the rack as wei as quality evaluation of the dried J)(oduct were also conducted. The result showed that the average drying air temperatures of the solar dryer were 39.SOC - 4O.SOC. while the drying time ranged from 34.7 to 46.6 hours. During the tempering period. desorption process occurred. In the final stage of test II. hQYolever. moisture readsorption occurred in the tempering period due to the quite low moisture content of the beans. The torque requirement was relatively small. i.e .. 6.4 - 10.2 Nm with equivalent poyter of 18.6 - 29.9 W. The quality parameters of dried beans of the experiment were in the required general conditions for trading purposes. Measurement of solar irradiation. fuel consumption and electricol power were intended to calclAote energy effICiency. The specific energy consumptions were 7.63 - 9.68 kJ/kg of water evaporated. These vetoes were much IQYoIer than those resulted from the J)(evious study on troy drying for cocoa beans (Nelwan. 1997) and were apprOximately equal to the result of Manalu (1999) using bed dryer. Excluding solar energy. the thermal efficiency was between 30.6 - 44.n.. Development of mathematical modeling was based on both spatial and lump aPJ)(ooches. The first used finite volume method for three-dimensional dIStribution. whle the second employed finite difference of Adams Moulton method. The solutions were arranged in Visual Basic 6.0 c..odes. Using this approach. change of temperature. RH and moisture content during the drying process were successfully predicted. The COOS between measured and calculated temperature. RH and moisture content were 0.91.0.75 and 0.99. respectively. Simulation was performed by using lump approach. fuzzy logic controlling was applied to obtain the effective change of the combustion rate and 0irlI0w rate. By using the logic controlling. the drying fine can be shortened with considerable low energy consumption. In addition, this method could provide low drying cost. i.e" Rp. 297 per kg wet beans. Keywords: s%r-ossisted dtying. rotating rack. cocoa beans. drying kinetics. mathematical modeling