**3. ASPEN Flow Sheets**

Based on real time industrial data, simulations are performed for the both processes using ASPEN PLUS® V8.8. Methanol, oxygen, nitrogen, silver, and molybdenum were selected as the components for simulation. For both the process, non-random two liquids (NRTL) was selected as the fluid package. The mixture of air and methanol is preheated and then sent to the reactor. The reaction mechanism that produces the formaldehyde depends upon the type of catalyst used. The product stream is sent to the adsorption column where it is treated with water to separate the desired product. The simulation models are explained in Sections 3.1 and 3.2.

#### *3.1. Simulation Model of Molybdenum-Based Formaldehyde Process*

Figure 1 presents the simulation of Molybdenum-based process on ASPEN PLUS. Non-random two-liquid model (NRTL) is used as the fluid package. The air and methanol are used as starting materials. Methanol and air both are initially mixed and passed to pre-heater where the temperature of the mixture increases from 27 ◦C to 107 ◦C. The mixture is passed to the reactor. The reactor is a shell and tube type reactor. The reaction mixture enters the reactor in catalyst-filled tube sides at 107 ◦C. The reaction occurs here and water and formaldehyde are produced. The shell side contains DTH (Dowtherm heat transfer media), which is used to extract the excess heat of the reaction i.e., the reaction is exothermic producing 159 KJ/mol of the energy. The boiling point of DTH is around 260 ◦C and same is the temperature inside the reactor, so the DTH leaves the reactor shell side as vapors. DTH goes to the condenser where the DTH vapors condense back to liquid state and accumulated in DTH tank. Upon requirement, DTH again goes to the reactor and the cycle continues. The mixture (product and unreacted reactants) leaving the tube side is at 280 ◦C. Since this mixture is at a very high temperature, so the heat of this mixture can be utilized. This mixture goes to the shell side of the pre-heater to heat the incoming reaction mixture. After pre-heating the incoming mixture, it leaves the shell side of preheater at 150 ◦C and goes to the absorption column from bottom side. Water at 37 ◦C is showered from the top of the absorption column. The amount of water showered is very critical as it produces the required concentration of the final product. The formalin is removed from the bottom. The temperature of the exiting product is 27 ◦C. The unreacted reaction mixture i.e., CH3OH, O2, N2 is removed as off gas from top of absorption column at 23 ◦C.

**Figure 1.** Simulation model of Molybdenum-based formaldehyde process.

#### *3.2. Simulation Model of Silver-Based Formaldehyde Process*

Figure 2 presents the ASPEN PLUS simulation of silver-based formaldehyde process. NRTL was selected as the fluid package. The reaction temperature for this process is around 600–650 ◦C. This process does not incorporate the DTH cycle. The air is passed through a compressor and then mixed with methanol using a mixer. The reacting mixer goes directly into the reactor from the mixer as a pre-heater is not used in this process. The reactor is a fixed bed catalyst type of reactor incorporating the bed of catalyst. The reaction occurs at the catalytic bed and quenching water is used at the bottom of reactor to cool down the product. The mixture of products and unreacted reactants enters the absorption column from bottom where water is showered from top. The unreacted reaction mixture is removed from the top as off gas, air is recycled from this off gas and again passed to the mixer while the product is removed from bottom. The product goes to the distillation column where separation takes place and we ge<sup>t</sup> formalin as a bottom product and methanol as a top product, which is then recycled.

**Figure 2.** Simulation model of silver-based formaldehyde process.
