Mathematical Modeling of Thin layer Solar Drying of of Gum of Acacia senegal and Acacia seyal

Thin layer solar drying of gum from two tree species namely; Acacia senegal (Hashab) and Acacia seyal (Talh) was carried out at the Workshop of the Department of Agricultural Engineering, Faculty of Agriculture, University of Khartoum during March 2007. The main objective of this research work was to study the drying behavior of gum from the two tree species. The specific objectives were: 1) to investigate thin layer solar drying characteristics of gum from the two tree species and 2) to test three semi-theoretical drying models to simulate thin layer solar drying of gum from two tree species. A natural convective solar dryer previously constructed for a research work was used to conduct the thin layer solar drying experiment for the gum from the two tree species. Gum samples from the two tree species in the form of nodules and picked from healthy trees were collected from Kordofan and Gadarif areas. The initial moisture contents of the samples from the two tree species were 26.30% (wet basis) and 20.86% (wet basis) for Acacia senegal and Acacia seyal, respectively. Dry and wet bulb temperatures of ambient air and air at the solar inlet and outlet of the dryer plus the dry bulb temperature of air inside the solar dryer were recorded using thermocouples connected to a data logger. The data logger was configured to record air temperatures at intervals of one hour. Weights of the thin layer solar dried gum samples from the two tree species were recorded at intervals of one hour using a sensitive balance. The thin layer solar drying experiment for gum was carried out by xi placing the samples from the two tree species in the drying chamber of the solar dryer at the same time. The experiment was started at 09:00 till 18:00 and continued for seven days until reaching constant weights for the dried gum samples. Three semi-theoretical drying models namely; Lewis model, Henderson and Pabis model and Page model were used to simulate thin layer solar drying of the gum from the two tree species. The models constants and coefficients were obtained by linear regression technique using Microsoft Excel of Microsoft Office software. A computer program for each the three tested drying models was written using the programming language of Turbo Pascal for Windows, Version 1.5 and the programs were run on a personal computer (PC). Results indicated that the air inside the solar dryer was heated adequately as shown by the maximum obtained temperature and the maximum obtained relative humidity differences between the heated air and the ambient air which were 63.2°C and 20.46%, respectively. The thin layer solar drying of the gum from the two species showed that the drying characteristics of gum samples such as; moisture content, moisture ratio and drying rate decreased with the increase of the drying time and the drying process occurred effectively during the first day. Also the drying process took place during the falling rate period. The three tested drying models predicted the moisture contents of the thin layer solar drying of the gum from the two tree species accurately. Statistical validation for the three tested drying models showed that the coefficient of determination (R2) values, obtained by plotting the predicted moisture contents versus the measured moisture contents, for the gum from the two tree species ranged from 0.96 to 0.98. The average model error (AME), average absolute difference (AAD) and standard error of estimate (SEE) regarding Acacia senegal gum and Lewis model were - 0.0106, 0.014 and 0.0198, respectively. The same values with respect to xii Henderson and Pabis model were 0.00005, 0.00459 and 0.0073, respectively. Also the same values regarding Page model were -0.0001, 0.0044 and 0.0093, respectively. For Acacia seyal and Lewis model, the same values were -0.0073, 0.0094 and 0.01433, respectively. The same values for Henderson and Pabis model were 0.0006, 0.0042 and 0.00634, respectively. With regard to Page model the values were 0.0004, 0.0077 and 0.0089, respectively. The two samples independent t-test showed that since the null hypothesis holds true i.e. ⎢tcal ⎢〈 ttab there is insufficient evidence to reject the null hypothesis. Also this confirmed that the three tested drying models predicted the moisture contents of the thin layer solar dying of the gum from the two tree species accurately