Measuring the Energy Required to Dry Rice in Commercial Rice Dryers

The objective of this research was to quantify and assess the energy use and efficiency of commercial cross-flow dryers when drying rice using a range of drying and ambient conditions. First, equations that predict the theoretical energy required to dry rice from any given initial moisture content to a desired final moisture content were developed for several rice cultivars using a semi-theoretical approach to obtain a basis for comparison to calculate energy efficiency. Theoretical energy requirements, expressed as the energy required per unit mass of water removed, increased exponentially as initial moisture content decreased. Additionally, medium-grains required more energy to be dried than long-grains; non-parboiled rice required more energy to be dried than parboiled rice. Second, a two-year study was performed to measure energy requirements of both an on-farm, cross-flow dryer and a commercial, cross-flow dryer. In 2011 for the on-farm dryer, energy requirements ranged from 2,840 to 5,310 kJ/kg water removed and in 2012 from 3,730 to 5,840 kJ/kg water removed. Energy efficiencies, which were calculated as the ratio of theoretical energy requirements to the measured energy requirements, ranged from 47 to 90% in 2011 and from 44 to 69% in 2012. Thermal energy requirements of the commercial dryer ranged from 6,900 to 9,670 kJ/kg water removed in 2011 and from 8,800 to 9,620 in 2012. Electrical energy use, which ranged from 300 to 400 kJ/kg water removed in 2011 and from 410 to 630 in 2012. Energy efficiency ranged from 26 to 36% in 2011 and from 27 to 29% in 2012. It was found for both dryers that thermal energy requirements were linearly correlated to the difference between drying air temperature and ambient temperature and linearly and inversely correlated to the amount of water removed per mass dry matter. Equations were developed to predict energy use and efficiency as a function of these two parameters