**2. Materials and Methods**

### *2.1. Material and Energy Balance*

The steady-state material balance calculation is done for both processes. It was assumed that there was no accumulation in the system and there was no change with respect to time. The total amount of material in and out is almost the same. The basis for calculations were considered to be one day of operation i.e., 90 tons of formalin is produced in one day.

The energy balance calculations involve calculation of ΔH at both inlet and outlet sides. The reference temperature was 25 ◦C. Since the entering and leaving streams are mixtures of components, average heat capacity (Cp,avg) was calculated by the product of mole fraction (xi) of the component in the mixture and its individual heat capacity (Cp). The change in temperature was calculated by the subtraction of the stream temperature with the reference temperature. The overall enthalpy content was found by taking the di fference of the total heat content at the outlet and the inlet. The heat of the reaction was incorporated in the energy balance of the reactor and heat of condensation of formaldehyde was included in the energy balance around the absorption column.

$$
\Delta \mathbf{H} = \mathbf{m} \mathbf{C} \mathbf{p}\_{\text{avg}} \Delta \mathbf{T} \tag{3}
$$

## *2.2. Cost and Payback Period*

The equipment and utility costs were calculated from Aspen Plus®. Then, the different incorporating factors like piping, instrumentation, buildings, and total physical plant cost were calculated. Then, total fixed capital was determined by incorporating some other factors like design and engineering, contractor's fee, and contingency. After that, working cost was calculated, which is 5% of total fixed capital. Then, the total investment was determined by adding working cost and total fixed capital.

$$\text{Total Investment} = \text{Working Capital} + \text{Total Fixed Cost} \tag{4}$$

Revenue was calculated by multiplying the amount of formalin produced by price of formalin.

$$\text{Revenue} = \text{Price of Formula (kg)} \times \text{Formula} \text{ produced } (\text{kg/year}) \tag{5}$$

The cost of raw material and catalyst was determined by multiplying the quantities (units) consumed per year by price per unit.

Then, profit was calculated by subtracting all the expenses from the revenue and payback period was determined.

$$\text{Payback Period} = \frac{\text{Total Investment}}{\text{Profit}} \tag{6}$$
