## 4 Process Design and Simulation
### Effects of Water Content on Reboiler Duty per Unit Mass of \(\text{PODE}_{3-5}\)  
Generally, the production process of \(\text{PODE}_{\text{n}}\) consists of raw material preparation section and \(\text{PODE}_{\text{n}}\) synthesis section. To explore the effects of water content on energy consumption per unit mass of \(\text{PODE}_{3-5}\), the energy of the chain-group providers with different water content should also be considered. The chain-group providers we chose was trioxane. Trioxane is a very important raw material in the production of \(\text{PODE}_{\text{n}}\). The analysis of trioxane is representative, which helps us understand why the route DMM/TOX costs more. The separation process with different water content is different. However, producing \(\text{PODE}_{\text{n}}\) in different processes need to meet the same constraints. These constraints are given in Table 1. To simplify issues, the energy consumption we discuss is just reboiler duty, which basically reflects the level of energy expenditure. Power for the transfer of material is not considered. According to trioxane preparation process [33; 52] given in Figure 8, we now separate chain-group providers containing formaldehyde, trioxane and water as the bottom product from column T3 by changing the bottoms rate. For reboiler duty per unit mass of the chain-group providers containing formaldehyde, trioxane, and water, the reboiler duty of each equipment in the process is shown in Figure 9. It can be seen that allowing water and formaldehyde in feedstock can reduce reboiler duty. As the allowable water content and formaldehyde increase, the circulation does not have to be increased to improve the quality of the product, which has a significant impact on column T2. For the anhydrous case, the process of \(\text{PODE}_{\text{n}}\) produced from trioxane and methylal developed by [PERSON] et al. [29] is applied (see Figure 10). For other water content, the process of \(\text{PODE}_{\text{n}}\) produced from methylal, trioxane, formaldehyde, and water by from our work is applied (see Figure 4). \begin{table}
\begin{tabular}{c c} \hline \hline
**Constraints** & **Goal** \\ \hline Overall mass fraction of \(\text{PODE}_{3-5}\) in stream 5 & 0.99 g/g \\ Overall mass fraction of methylal in stream 7 & 0.99 g/g \\ Overall mass fraction of formaldehyde in stream 10 & 1 ppm \\ Overall mass fraction of methanol in stream 13 & 0.001 g/g \\ Overall mass fraction of \(\text{PODE}_{2}\) in stream 15 & 100 ppm \\ Overall mass fraction of \(\text{PODE}_{2}\) in stream 19 & 0.99 g/g \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Constraints used for the optimization of the process. Figure 8: Flowsheet of the highly concentrated chain-group providers containing formaldehyde, trioxane, and water production process. The reboiler duty of each equipment in the process is shown in Figure 11. As can be seen from Figure 11, the reboiler duty of column C1 and C2 increases with the increase of water content. The role of the column C1 is the separation of \(\mathrm{PODE_{n\geq 3}}\) as the bottom product from mixtures of formaldehyde, water, methanol, methylal, and \(\mathrm{PODE_{n\geq 2}}\). The role of column C5 is separating methanol as a top product from mixtures of water, methanol, and \(\mathrm{PODE_{2}}\). This means that the reasons for the increase of reboiler duty are the increase of methanol in the reactor by decomposition and the decrease in the conversion rate of \(\mathrm{PODE_{n}}\). It should be noted that since only the reboiler duty is considered, the accurate cost of the product needs further analysis required to decide which \(\mathrm{PODE_{n}}\) synthesis process is economic. Total energy expenditure per unit mass of \(\mathrm{PODE_{3-5}}\) with different water content is shown in Figure 12. The results showed an impressive and rapid impact on reboiler duty per unit quality product. With the increase in water content, total energy expenditure decreases.