A comparative study on intermittent heat pump drying process of Chinese cabbage (Brassica campestris L.ssp) seeds

The aim of this paper was to make a comparative simulation study on different intermittent heat pump dryings with respect to moisture diffusion, energy consumption and drying time on Chinese cabbage seeds. Various intermittent ratios in describing different intermittent drying characteristic were investigated. Experimental parameters for drying initial moisture content 30% (d.b.), temperature 40 °C, relative humidity 40%, airflow velocity 1.0 m/s, and drying time in a range from 0 to1200 s in a cycle. On the basis of unsteady state drying dynamics model, drying rate was analyzed, parameters of control equation in seeds drying process was proved. Advantages of energy consumption and moisture diffusion uniformity were discussed by using simulation of moisture diffusion process in intermittent drying. Results indicated that model of equilibrium moisture content and drying environment could be expressed by Nellist equation as xc=0.41821−0.05448   ln(1−ϕ)−0.0692   ln   Txc=0.41821−0.05448   ln(1−ϕ)−0.0692   ln   T. The best intermittent ratio was 1/3 during intermittent drying process, which drying technology was continuous ventilation drying air 400 s, then stop 800 s in a cycle at 40 °C. In this drying process, percent energy saving over continuous drying was 48.1%. So there was an obvious advantage of intermittent drying over continuous drying as regards energy efficiency.

► The Chinese cabbage seeds parameters of drying process control equation was gotten.
► Three kinds of intermittent drying methods with continuous drying process were compared on moisture diffusion, drying rate and energy consumption.
► Unsteady state drying dynamics model was built, advantages of energy consumption and moisture diffusion uniformity were discussed by using simulation of moisture diffusion process.
► The best intermittent drying method was gotten, which the intermittency ratio is 1/3.