**1. Introduction**

The extensively used formaldehyde is produced by using air and methanol as the raw materials. The reaction occurs in the reactor in the presence of a catalyst. The resulting products of the reaction are formaldehyde and water [1]. Then the mixture of products and unreacted reactants goes to the absorption column where water is showered from the top. The bottom product is formalin i.e., a 37% aqueous solution of formaldehyde [2]. The unreacted reaction mixture is removed from the top [1,3–7]. The extensive range of applications of formaldehyde makes it a valuable chemical. It may be used in di fferent industries such as domestic, medical, cosmetics, and the textile industry [8–10].

The consumption and demand of formaldehyde is increasing. Formaldehyde is the principal component for the production of resins, phenols, urea, and melamine [11]. It is used for weather resistance i.e., in adhesives and wood coatings [12]. In addition, it has a disinfectant property; it is present in soaps as a disinfectant. In medical fields, formaldehyde is used for the sterilization of the surgical instruments. It imparts the resistance to fabric against crumples. In cosmetic products, formaldehyde is used as a preservative since it enhances the e ffectiveness of products against di fferent microorganisms. It is used in glue production for household use. Formaldehyde is used in the manufacturing of plastics, carpets, and vaccines, etc. In plastic utensils industry, it is the major component [13].

Commercially, formaldehyde is produced mostly from air and methanol as raw materials using three di fferent methods. In the first method, formaldehyde is produced using air and methanol in the presence of molybdenum oxide catalyst present inside the tubes of shell and tube reactor [14]. The reacting mixture enters at tube side to interact with catalyst forming the product [15].

$$\text{CH}\_3\text{OH} + \frac{1}{2}\text{O}\_2 \rightarrow \text{HCHO} + \text{H}\_2\text{O} \ \Delta\text{H} = -156 \text{ kJ} \tag{1}$$

The second method involves the production of formaldehyde in the presence of silver oxide catalyst present in fixed catalytic bed reactor [16].

$$\begin{array}{c} \text{CH}\_3\text{OH} + \frac{1}{2}\text{O}\_2 \rightarrow \text{HCHO} + \text{H}\_2\text{O} \ \Delta\text{H}\_1 = -156 \text{ kJ} \\ \text{CH}\_3\text{OH} \rightarrow \text{HCHO} + \text{H}\_2 \ \Delta\text{H}\_2 = 85 \text{ kJ} \end{array} \tag{2}$$

In third method, formaldehyde is produced using oxidation of methane and other hydrocarbons [17]. The separation processes and reaction mechanism in the above three methods are almost the same. For commercial production of formaldehyde, process optimization is required.

Le fferts et al. studied the production process of formaldehyde through oxidative hydrogenation of methanol in the presence of silver catalyst [18]. They studied the e ffect of temperature, gas velocity, and concentration of both reactants on the production process. They developed the reaction model based on the experimental data and explained the impact of form and composition of silver catalyst over methanol conversion. Yang et al. used molybdenum oxide catalyst supported over silica for the oxidation of methanol to formaldehyde [19]. Their study was based on the selectivity and activity of N2O and O2 used as oxidants. They observed that N2O is responsible for the oxidation of carbon monoxide. Moreover, the supported molybdenum catalyst has higher activity than the non-supported catalyst. Qian et al. explained the formaldehyde synthesis process using polycrystalline silver catalyst [20]. They compared the water ballast process with the methanol ballast process and observed an increased selectivity of formaldehyde in the absence of water. Moreover, the selectivity of the product is highly temperature dependent. Moreover, Waterhouse et al. used SEM techniques to determine the relationship between morphology of silver catalyst and its performance [21].

In this study, a performance comparison of the industrially produced formaldehyde using two di fferent catalysts is presented. Real-time industrial data are collected from a local industry in Pakistan, and material and energy balances, simulations, and cost analysis are executed. We have compared the two di fferent catalysts based on material and energy balances, the size of the plant, the installation, and utility cost.
