## 4 Process Design and Simulation  
### Process Description  
Based on our previous work [34], a flowsheet of the novel \(\mathrm{PODE_{n}}\) production process is depicted in Figure 4. There are two main differences between this work and the original process. One is that the column C3 separates methylal as a distillate product (stream 7) from mixtures containing formaldehyde, water, methanol, methylal, and \(\mathrm{PODE_{2}}\) (stream 8), which are recycled back to the reactor. It is well known that an azeotrope exists between methanol and methylal [51]. However, experimental results from our batch distillation experiments show that formaldehyde has effects on the methylal-methanol azeotrope, therefore, we can obtain high-purity methylal. The batch distillation experiments are given in supporting information. We will explore the reasons for the rigorous simulation in the following section. The other is water and \(\mathrm{PODE_{2}}\) is separated by heterogeneous azeotropic distillation. Water is obtained from column C6 as bottom product (stream 15), and \(\mathrm{PODE_{2}}\) is obtained from column C7 as bottom product (stream 19).  
The detailed description is as follows: the feed stream 1, containing methylal, formaldehyde and \(\mathrm{H_{2}O}\), is fed to the acidic catalyzed reactor R1 to produce \(\mathrm{PODE_{n}}\) after mixing with the recycle streams 6 and 7. The reactor outlet stream 2 comprising formaldehyde, \(\mathrm{H_{2}O}\), methanol, methylal, and \(\mathrm{PODE_{n}}\) of various chain lengths is fed to a rectifying sequence to separate main products of \(\mathrm{PODE_{3-5}}\) and recycle other components. The column C1 separates \(\mathrm{PODE_{n}}\) of chain lengths \(\mathrm{n}\geq 3\) as the bottom product (stream 4), which avoid formaldehyde polymerization when all other light components are distilled together as overhead products (stream 3). Column C2 separates target products of \(\mathrm{PODE_{3-5}}\) and recycle other components back to the reactor. Column C3 separates methylal as an overhead product (stream 7) from mixture formaldehyde, \(\mathrm{H_{2}O}\), methanol, methylal and \(\mathrm{PODE_{2}}\) (stream 8) which are recycled back to the reactor. Column C4 using water as entrainer can separate formaldehyde from the system (\(\mathrm{PODE_{2}}\) + formaldehyde + \(\mathrm{H_{2}O}\) + methanol). Thus, formalin can be segregated from the intricate mixture by stream 11. For column C5, it is proved that methanol is obtained in a sharp split as a top product (stream 12), and \(\mathrm{PODE_{2}}\) with water is obtained as the bottom product (stream 13). Finally, water is obtained from column C6 as the bottom product (stream 15), and \(\mathrm{PODE_{2}}\) is obtained from column C7 as the bottom product (stream 19) by heterogeneous azeotropic distillation. \(\mathrm{PODE_{2}}\) can be used as a product or be recycled to the reactor R1 by an additional pipeline.  
The principles of optimization are as follows: by controlling the same product quality through reflux ratio and quantity of distillate, the number of theoretical plates and feed plate is optimized. When increasing the number of theoretical plates has little influence on the energy consumption of the reboiler, the number of theoretical stages is determined. After determining the number of theoretical stages, it is possible to find the feeding plate with the lowest energy consumption of the reboiler. In this way, the separation conditions of the distillation column can be obtained.  
Figure 4: Optimized process design flowsheet (mass fraction).