Characteristics Of Onion Slices In Hot-air Drying Process

The study on the drying characteristics of onion slices is of great significance to improve the drying quality and efficiency,and to reduce the drying energy consumption.Pretreatment process and hot-air temperature have an important effect on drying results.In this paper,the moisture migration and its effect on the drying characteristics of onion slices were studied.The main research contents and results are as follows:1.The moisture migration and its influence on drying characteristics of onion slices were studied.Firstly,equation of equilibrium moisture content,and equation of relative humidity of the surface material drying process were established.Secondly,the effect of moisture migration of onion slices on the drying characteristics was analyzed by the binding energy.Finally,numerical analysis of Fick differential equation describing moisture migration of onion slices was carried out by Matlab.The results are as follows:1)The equation of equilibrium moisture content can be used to predict the equilibrium point of onion slices with different moisture content.Air flow is laminar in drying process,and the Reynolds number Re is 9.75í103,and the convective mass transfer coefficient hm is 1.36í10-2 m/s.2)When the moisture content of onion slices is greater than 2 kg/kg,the binding energy of onion slices was 0 kJ/kg in equilibrium state,and the minimum of binding energy was 90.1395 kJ/kg in drying process.When the moisture content is less than 2 kg/kg,the binding energy in equilibrium state and drying process was inversely proportional to the moisture content.2.The effect of blanching pretreatment on the hot-air drying characteristics of onion slices was studied.Firstly,Blanching experiments were carried out under the conditions of blanching Temperature of 70?,80?,and 90?,and blaching time 1 min,2 min and 3 min.Secondly,The drying experiment of onion slices was carried out under the condition of hot-air of 55?,60? and 65?,and speed of 1.5 m/s.Thirdly,drying experiment of fresh onion slices and 5% NaCl pretreatment of onion slices were set in hot-air of 65?,Finally,the drying model describing drying characteristics of blanching onion slice was established.The results are as follows:1)Compared with the 5% NaCl pretreated and untreated,blanching pretreatment significantly improved drying rate of onion slices.The drying rate increases with increase of blanching temperature,and increase of hot-air temperature,but the effect of blanching time on the drying rate is not significant.2)The effective diffusion coefficient varies from 1.0457×10-9m2/s to 2.1049×10-9m2/.Among Newton model,Page model and Henderson,Page model is best drying model to describe characteristics of onion slices in hot-air drying process.Modified Page model could well describe drying characteristicthe of onion slices.3.Influence of hot air temperature on the drying process of heat pump was studied.Firstly,drying model of onion slice,and physical model and mathematical model of heat pump drying system were established.Secondly,the heat pump drying process was simulated by the EES.The results are as follows:1)Drying rate increase with the increase of hot-air temperature,Page model can be used to describe well the hot-air drying characteristics of onion slices,the effective diffusion coefficient varies from 9.11í10-10 m2/s to 1.82í10-9 m2/s.2)Hot-air temperature has a great influence on the performance of heat pump drying system.The total energy consumption of heat pump drying system is inversely proportional to the temperature of hot air.The optimum hot air temperature of heat pump drying onion slice is 50?.