## 1 Introduction  
Poly(oxymethylene) dimethyl ethers (PODE\({}_{\text{n}}\)) are environmentally benign diesel additives with the formula of H\({}_{3}\)C-O-(CH\({}_{2}\)O)\({}_{\text{n}}\)-CH\({}_{3}\)(n \(\geq\) 2) [1]. PODE\({}_{\text{n}}\) of chain lengths n = 3-5 have suitable flash points [2] and melting points [3], which could reduce the soot and also, indirectly, NOx during the combustion process in compression ignition engines [4; 5; 6]. Thus, higher combustion efficiency could be achieved by blending PODE\({}_{\text{n}}\) in diesel [7; 8]. Moreover, PODE\({}_{\text{n}}\) can be used as oxygenated fuels to direct oxidation fuel cells [9] and as green solvents for the chemical industry [10; 11]. Two kinds of synthesized educs are necessary for the synthesis of PODE\({}_{\text{n}}\): end-group (CH\({}_{3}\)O-) providers, such as methylal (DMM), methanol (MeOH), or dimethyl ethers (DME) and chain-group (-CH\({}_{2}\)O-) providers, such as trioxane (TOX), formaldehyde (FA) or paraformaldehyde (PF) [12; 13]. All these providers can be synthesized by methanol at the very start. Poly(oxymethylene) dimethyl ethers is a key research direction for coal-based methanol, facing serious oversupply in China [14]. In the published articles, there are possible synthesis routes and technologies for production of PODE\({}_{\text{n}}\): DMM/TOX route [15; 16; 17], DMM/PF route [14; 18], DME/TOX route [19; 20], and MeOH/FA solution route [21; 22; 23]. The routes of DMM/TOX, DME/TOX, and DMM/PF have a high yield of PODE\({}_{3-5}\)[14; 15], but these routes have high costs due to chain-group providers TOX and PF. In contrast, the route using MeOH/FA solution has a lower yield of PODE\({}_{3-5}\)[23; 24]. Methylal and PODE\({}_{2}\) are the main oligomers in the MeOH/FA solution route due to the Schulz\(-\)Flory distribution of PODE\({}_{\text{n}}\) products [25; 26; 27]. Water in the reactants leads to the decomposition of \(\mathrm{PODE_{n}}\), which is the main reason for the decrease of \(\mathrm{PODE_{3-5}}\)[4]. Therefore, the selectivity of \(\mathrm{PODE_{3-5}}\) can be improved by reducing water content in feedstock. Meanwhile, water not only affects the yield of \(\mathrm{PODE_{3-5}}\) but also makes the purification process more complicated [28]. Therefore, the effects of water on the \(\mathrm{PODE_{n}}\) process represent one of the most important research fields. Large scale green production of \(\mathrm{PODE_{n}}\) would be crucial if diesel-\(\mathrm{PODE_{n}}\) blends are applied to tackle the worldwide formidable air pollution by particulate matter. In the articles, [PERSON] et al. [29] conceptually designed a large scale production in which \(\mathrm{PODE_{n}}\) was formed from methylal and TOX. Tsinghua University [30] provided a technology synthesizing \(\mathrm{PODE_{n}}\) from methylal and PF, which was applied in practice in Shandong Yuhuang Chemical Co., Ltd. in China. [PERSON] et al. [31] designed a conceptual process for the production of \(\mathrm{PODE_{n}}\) by the route using the MeOH/FA solution. In addition to the above manufacturing procedure, there are also many articles about the \(\mathrm{PODE_{n}}\) process [32; 33]. In the articles, there is no detail on the effects of water content on the manufacturing procedure and it still needs to be explored. In the present work, we will focus on the effects of water content on the \(\mathrm{PODE_{n}}\) process, especially energetic efficiency. To attain this goal, a complete \(\mathrm{PODE_{n}}\) process with lower water content needs to be established, which our group has already provided [34]. We continue to develop this process in this work. We explored by rigorous simulation that methylal is separated as a distillate product from mixtures containing formaldehyde, water, methanol, methylal, and \(\mathrm{PODE_{2}}\). Furthermore, the production process consisting of raw material preparation section and \(\mathrm{PODE_{n}}\) synthesis section was evaluated by comparing it with the various water contents in the feedstock.