# Study on Low Temperature Drying Characteristics of Chicken Manure in a Humid Environment
## 3 Results and Discussion
### Fitting of the Drying Curves  
The dehydration characteristics of chicken manure were finally shown as the speed of the drying rate. It was of great significance to establish a drying model to explore the low-temperature drying mechanism in a humid environment and select reasonable drying parameters to ensure that chicken manure can be dehydrated within a predetermined drying period. In this study, the data on moisture content obtained under different conditions were converted into the moisture ratio (\(MR\)) and fitted to the common thin-layer drying models ([PERSON] model, [PERSON] model, [PERSON] and Pabis model, [PERSON] and Singh model, and Exponential model) using Origin 8.5 software [21; 22; 23]. The common thin-layer drying models are listed in Table 4.  
The goodness of fit of each model was evaluated using the coefficient of determination (\(R^{2}\)), the root means square error (\(RMSE\)), and the reduced chi-square (\(\chi^{2}\)). The higher the value of \(R^{2}\) and the lower the values of \(RMSE\) and \(\chi^{2}\), the better the goodness of the fit was. These parameters can be calculated using the following equations [20]:  
\[RMSE=\left[\frac{1}{N}{\sum_{i=1}^{N}}\big{(}MR_{exp,i}-MR_{pre,i}\big{)}^{2} \right]^{1/2} \tag{7}\]  
\[\chi^{2}=\frac{1}{N-P}{\sum_{i=1}^{N}}\big{(}MR_{exp,i}-MR_{pre,i}\big{)}^{2} \tag{8}\]  
where \(MR_{exp,i}\) was the experimental observation value, which corresponds to the model fitted value \(MR_{pre,i}\); \(N\) was the number of observations; and \(P\) was the number of constants.  
The above five models were used to fit the low-temperature drying curve of chicken manure, and the fitting parameters were calculated by the above equations. The fitting parameters obtained from the above five models at different experimental groups are shown in Table 5. As shown in Table 5, the highest value of \(R^{2}\) and the lowest values of \(RMSE\) and \(\chi^{2}\) were obtained from the Exponential drying model. Accordingly, the Exponential model was selected as the suitable model to represent the low-temperature drying process in a humid environment. The drying model can be used to predict the drying rate and select reasonable drying parameters in actual production.  
In order to verify the accuracy of the Exponential model to describe the low-temperature drying process in a humid environment, the experimental values of moisture ratio at different temperatures were compared with the predicted values given by the model. Figure 6 indicates the comparison of these values at different drying temperatures with a manure depth of 40 mm, wind speed of 1.2 m/s, and relative humidity of 85%. It shows that the Exponential model had good agreement between the predicted and experimental moisture ratio values, which were banded around at 45\({}^{\circ}\) in a straight line.  
\begin{table}
\begin{tabular}{c c} \hline
**Drying Model** & **Model Expression** \\ \hline [PERSON] & \(MR=exp(-Kt)\) \\ Page & \(MR=exp(-Kt^{n})\) \\ [PERSON] and singh & \(MR=1+At+Bt^{2}\) \\ Henderson and Pabis model & \(MR=Aexp(-Kt)\) \\ Exponential & \(MR=Aexp(-Kt)+C\) \\ \hline \end{tabular}
\end{table}
Table 4: Common thin-layer drying models.