Microwave Drying Characteristics Combined Hot Air And Heat With Mass Transfer Mechanism Of Crushed Cotton Stalks

Preperation of reconsolidated cotton stalk is one of the new approaches of utilizing the cotton stalk resources. As the substitute of man-made board, reconsolidated cotton-stalk has a large potential. In this dissertation, one of the most important pretreatment process about reconsolidated cotton-stalk, drying technology, i. e., the drying characteristics of using microwave and microwave combined with hot air were studied. And the microwave drying technology combined hot air was also investigated. This study discussed the influence on crushed cotton stalks and reconsolidated cotton-stalks by using different drying methods. The heat and mass transfer mechanism of crushed cotton stalks in the drying process were also analyzed. In general, the major work and conclusions of this dissertation are as follows.(1) The microwave drying characteristics of cotton stalks were studied. During the experiments, microwave power was fixed to 1000 W, and loading of crushed cotton stalks was set to be 34 g~200 g. The drying time was about 10~20 min to reduce the dry basis moisture content from 110% to 6%. There were two processes of microwave drying process of crushed cotton stalks, the drying process of accelerating rate and falling rate. During the drying process, there was no constant rate in the drying process, and the process during reduction had a longer duration. The effective diffusion coefficient value of microwave drying of crushed cotton stalks was between 4.1997×10-8 to 1.8078×10-8 m2/s。(2)The Midilli model was used to describe the microwave drying behavior of crushed cotton stalks. Seven mathematical models were selected and studied from classic drying kinetics models. Three evaluation criteria, R2, χ2, and RMSE were used for nonlinear fitting. The maximum value of R2, the minimum value of χ2, and the value of RMSE were obtained from the Midilli model. Linear relationship between the error of the predicted values and the measured values is less than 1%.(3) The influence of drying methods on the drying time of crushed cotton stalks was investigated, and the value range of single factor was determined. Results showed that with the single drying condition of 80℃ hot-air, the processing time of drying the target to the moisture content was longer than 60 min. The drying time had a reduction trend with the increase of moisture content. When the moisture content of conversion point was 85%, 60%, and 40%, the drying time was about 26 min, 34 min, and 42 min, respectively. The range of three factors was determined to be 80℃~100℃ of hot-air temperature, 50%~60% moisture content of conversion point, and 60 W to 700 W of microwave power.(4) Optimized the technology parameters of microwave drying combined hot air for crushed cotton stalks. Single factor and orthogonal experiment were conducted, the average drying rate and unit energy consumption of dehydration were considered as response values. The optimal parameters of microwave drying combined hot-air were determined to be 95℃ hot-air temperature, 57% moisture content of conversion point, and 700 W microwave power. Under this drying condition, the unit energy consumption of dehydration in the microwave drying process combined hot air reduces 106.7% and 10.4%, respectively, compared with single hot-air and microwave drying.(5) The modulus of rupture and modulus of elasticity value of crushed cotton stalks were tested using different drying methods. The electronic universal testing machine was used to determine the bending properties of the crushed cotton stalk which dried under the hot air temperature of 60℃, 80℃, 100℃ and the microwave power of 350 W, 500 W, 700 W, respectively. The results showed that the bending properties of crushed cotton stalks dried by microwave were better than that of hot air. The modulus of rupture(MOR) and the modulus of elasticity(MOE) values of crushed cotton stalks dried by hot air were 11.34~23.82 MPa, and 0.51~0.96 GPa, respectively. The values of crushed cotton stalks dried by microwave were 24.26~33.57 MPa, and 0.91~1.28 GPa. In the hot-air drying, there was a positive correlation between the MOR and MOE values and hot-air temperature, and there was no clear correlation of that in the microwave drying. The MOR and MOE values of crushed cotton stalks dried by 700 W microwave were greater than that of 1000 W.(6) The electronic universal testing machine was used to test the bending properties of reconsolidated cotton-stalk prepared by different pretreatments. The results showed that among the three different drying methods, the MOR and MOE of reconsolidated cotton-stalk pretreated by microwave drying combined hot air had the maximum values, which were respectively 52.07 MPa, 2.01 Gpa, and that of reconsolidated cotton-stalk pretreated by microwave drying and hot-air drying decreased systematically, which were respectively 40.86 MPa, 1.83 GPa and 24.73 MPa, 1.19 GPa.(7) Simulation and validation of heat and mass transfer process of crushed cotton stalks were conducted. According to the distribution of temperature and moisture in different time, the software of ANSYS was used to obtain the distribution of temperature and moisture during the drying process. Verification results showed that the measured value basically agreed well with calculated value, which indicated that numerical simulation can be used for the simulation of the drying temperature and moisture content changes.(8)Analyzed the organization and composition changes of crushed cotton after drying using scanning electron microscopy(SEM) and infrared spectrum. Results showed that the drying time, hot-air temperature and microwave power influenced the mechanical property of the crushed cotton stalks. Compared with hot-air drying, microwave drying was more suitable for crushed cotton stalks drying. Microwave drying can improve the mechanical properties of crushed cotton stalks by increasing the number of pores internal crushed cotton stalks and the drying rate meanwhile reducing the growth stress.