## 4 Process Design and Simulation
### Effects of FA/MeOH Ratio and Water Content on Methylal Separation  
Methylal needs to be returned to the reactor and separated from the mixture containing formaldehyde, water, methanol, methylal, and PODE\({}_{2}\). However, batch distillation experimental results show that the high-purity methylal, as distillate product, can be obtained from column C3. The reasons for this phenomenon were explored. However, this system involving formaldehyde, methanol and water is complicated, therefore, it is difficult to discuss the influences of each of these components. The FA/MeOH ratio and water content have effects on effluent from the top of the distillation column. In this work, we just focus on the influence of the FA/MeOH ratio and the water content in Stream 3 at normal pressure to simplify the problem. The result is as follows:  
The effect of FA/MeOH ratio is shown in Figure 5. The conditions are far enough to meet the separation requirements that the number of plates (N) is 40 and the reflux ratio (RR) is 2. With the increasing FA/MeOH ratio, the concentration of methylal from the top of the distillation column first increases and then decreases, and there is a peak value between 1.4 and 1.5. The reason is that methanol reacting with formaldehyde to polymers of HFn from the Equations (3) and (4) are withdrawn at the bottom of the distillation column. According to the results, in order to gain higher concentrations of methylal, it is beneficial to ensure that the FA/MeOH ratio is between 1.4 and 1.5. The effect of water content is shown in Figure 6a,b. Two orientations need to be discussed in the conditions (N = 40, RR = 2). One is when FA/MeOH ratio is less than 1.4, and the other is greater than 1.5. Therefore, FA/MeOH ratio was selected 1 and 2 respectively. FA/MeOH ratio of 1 is shown in Figure 6a. At this point, the main composition of mixture from the top of the distillation column is methylal, trace of methanol, trace of water and almost no formaldehyde. It is not necessary to increase the water content without restriction and limit the water content to 0.15, as higher concentrations of water lead to considerably decreased yields of PODE\({}_{\text{n}}\) in the reactor [31]. With the increase of water content, water appeared, methanol content decreased and methylal content increased. The results showed that the presence of water was beneficial to increase the concentration of methylal. When the FA/MeOH ratio is less than 1.4, formaldehyde is not enough to completely react with methanol. When water is added to the system, water-PODE\({}_{2}\) azeotrope is more stable than FA-MeOH-PODE\({}_{2}\) azeotrope which appears in physico-chemical model. Formaldehyde is released to form HFn to combine more methanol, so that the content of methylal increases and the content of methanol decreases. Figure 5: The effect of FA/MeOH ratio on effluent from the top of distillation column. (N = 40, RR = 2, no water). A FA/MeOH ratio of 2 is shown in Figure 6b. The results show that the content of methylal increases first and then decreases with the increase of water content, formaldehyde decreases first and then disappears, methanol appears first and then increases, and water also appears in the substances on the top of the distillation column. It can be concluded that when the FA/MeOH ratio is greater than 1.5, appropriate water content is helpful to increase the content of methylal. When the FA/MeOH ratio is higher than 1.5, excessive formaldehyde is enough to completely react with methanol. When water is added to the system, water reacts with formaldehyde to the polymers of MG\({}_{\text{n}}\) from Equations (1) and (2). Therefore, the concentration of formaldehyde at the top of the distillation column decreases and the concentration of methylal increases. When limits are exceeded, water competes with formaldehyde. The result is a decrease of methylal and an increase of methanol. The analysis of FA/MeOH ratio and water content helps us to understand this complex reactive multicomponent mixture.