Drying characteristics and model of cowpea in tunnel hot air dryer

Abstract: Cowpea, also known as beans, black-eyed peas, is an important leguminous vegetable, which is extensively grown in China. Fresh cowpeas having relatively high moisture content must be distributed to customers or processed as quickly as possible in order to prevent microbial fermentation and thermal degradation. Cowpeas resources have great losses as which are putrescible at normal temperatures and pressures, and this severely limits the transportation, storage and process of cowpea, therefore the development of cowpea deep processing is necessary. As Asian people have the habit of eating dried vegetables such as dried cowpea, drying can not only solve the serious problem of huge losses, but also create more economic benefits for farmers; and because most of deep processing technology needs dried cowpea, drying is of great importance as a preprocessing method.　　The tunnel type of hot air drying technology has been widely utilized in the drying of carrots, preserved fruits, and plums etc. In this paper, hot air drying characteristics and a drying model of tunnel type of hot air drying technology was studied in drying Cowpea, in order to establish the best model to simulate and predict the water ratio and to provide a technical basis for the industrial application of the control and prediction of the drying process and the tunnel type of hot air drying for cowpea.　　The tunnel dryer was made by the Institute of Agricultural Product Processing, Chinese Academy of Agricultural Engineering and College of Mechanical Engineering, Tianjin University of Science and Technology. According to the pre-test results, the drying characteristics of Cowpea was investigated under different air temperatures (70℃、80℃ and 90℃), air velocities (0.3、0.4 and 0.5 m/s) and material thickness (6 mm、18 mm and 30 mm). The empirical relationships between Cowpea moisture then natural logarithmic lnMR and drying time, water effective diffusion coefficient Deff and drying air temperature was established. And then using a MATLAB software system to process a large number of experimental data, afterwards the results were analyzed through nonlinear regression in order to get the function between seven mathematical model equations and the experimental data.　　The results show that the drying time can be shortened by increasing air temperature and air velocity and reducing material thickness, and the effect of air temperature and material thickness is more effective than that of air velocity. At different dying conditions, the drying rate always changed from constant-rate drying or quasi-constant-rate drying to falling-rate drying stage at 2.0 g/g dry basis moisture content of cowpea. The moisture effective diffusivity was increased with the addition of the air temperature and air velocity, and decreased with the addition of the material thickness, and the activation energy was 33.9 kJ/mol, which was determined from the Arrhenius equation. According to the statistical parameters, as the correlation coefficient(R2)、root mean square error (RMSE) and sum of squared error (SSE), the Page model was the best suitable to descript the relationship of Cowpea moisture and drying time by the tunnel type of hot air drying, which could be used in production to predict the moisture content on a certain air temperature, air velocities and layer-thickness using tunnel dryer for drying cowpea.