Performance Comparison of Industrially Produced Formaldehyde Using Two Di ﬀ erent Catalysts

: Formaldehyde is an important industrial chemical that is a strong-smelling and colorless gas. It is used in a number of processes such as making household products and building materials, glues and adhesives, resins, certain insulation materials, etc. Formaldehyde can be produced industrially using air and methanol as raw materials in the presence of metal oxide catalyst or silver-based catalyst. The operating conditions and requirements of the process depend on the type of catalyst used. Therefore, a comparative study of both processes was conducted, and the results were compared. It was observed that the silver-based catalyst process has a compact plant size since the amount of air required is halved as compared to the metal oxide process. Thus, it appears that the silver-based catalyst process is more suitable for small-scale production due to its compact size and reduced utility cost.


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][4][5][6][7]. The extensive range of applications of formaldehyde makes it a valuable chemical. It may be used in different industries such as domestic, medical, cosmetics, and the textile industry [8][9][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 effectiveness of products against different 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 different 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].
The second method involves the production of formaldehyde in the presence of silver oxide catalyst present in fixed catalytic bed reactor [16].
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.
Lefferts et al. studied the production process of formaldehyde through oxidative hydrogenation of methanol in the presence of silver catalyst [18]. They studied the effect 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 N 2 O and O 2 used as oxidants. They observed that N 2 O 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 different 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 different catalysts based on material and energy balances, the size of the plant, the installation, and utility cost.

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 ∆H 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 (C p,avg ) was calculated by the product of mole fraction (x i ) of the component in the mixture and its individual heat capacity (C p ). 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 difference 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.

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.
Total Investement = Working Capital + Total Fixed Cost (4) Revenue was calculated by multiplying the amount of formalin produced by price of formalin.
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.

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. 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., CH 3 OH, O 2 , N 2 is removed as off gas from top of absorption column at 23 • C. 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 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 get formalin as a bottom product and methanol as a top product, which is then recycled.

Catalyst Properties
The properties of the molybdenum oxide and silver oxide catalyst used in the production process are shown in Table 1. It can be observed that the conversion of the molybdenum oxide-based plant is 99% and that of the silver oxide-based plant is 85%. The life of the molybdenum oxide catalyst is 12-18 months and that of the silver oxide-based process is 3-8 months. The porosity of the molybdenum oxide catalyst is 0.7 and that of the silver oxide catalyst is 0.5.
The life of the silver catalyst is highly dependent upon the operating conditions of the formaldehyde reactor. Silver catalysts in the formaldehyde plant are typically used for a period of a few months to a year depending on the reaction temperature and pressure [22]. Generally, sintering  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 get formalin as a bottom product and methanol as a top product, which is then recycled. 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 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 get formalin as a bottom product and methanol as a top product, which is then recycled.

Catalyst Properties
The properties of the molybdenum oxide and silver oxide catalyst used in the production process are shown in Table 1. It can be observed that the conversion of the molybdenum oxide-based plant is 99% and that of the silver oxide-based plant is 85%. The life of the molybdenum oxide catalyst is 12-18 months and that of the silver oxide-based process is 3-8 months. The porosity of the molybdenum oxide catalyst is 0.7 and that of the silver oxide catalyst is 0.5.
The life of the silver catalyst is highly dependent upon the operating conditions of the formaldehyde reactor. Silver catalysts in the formaldehyde plant are typically used for a period of a few months to a year depending on the reaction temperature and pressure [22]. Generally, sintering

Catalyst Properties
The properties of the molybdenum oxide and silver oxide catalyst used in the production process are shown in Table 1. It can be observed that the conversion of the molybdenum oxide-based plant is 99% and that of the silver oxide-based plant is 85%. The life of the molybdenum oxide catalyst is 12-18 months and that of the silver oxide-based process is 3-8 months. The porosity of the molybdenum oxide catalyst is 0.7 and that of the silver oxide catalyst is 0.5. The life of the silver catalyst is highly dependent upon the operating conditions of the formaldehyde reactor. Silver catalysts in the formaldehyde plant are typically used for a period of a few months to a year depending on the reaction temperature and pressure [22]. Generally, sintering occurs in the silver catalyst due to reaction temperatures, which results in high pressure drop over the bed, which decreases the performance of catalyst. It is observed that the water ballast process prolongs the life of catalyst by introducing water with its associated high heat capacity that equally distributes the heat over the catalyst bed resulting in minimizing coke formation and sintering [23,24]. Our objective is the performance comparison of both processes based on the plant size, installation and utility cost, and material and energy balance.

Results and Discussion
The results in this section include the materials and energy balance of both plants, costing, payback period, and comparison on the basis of size for the two catalytic processes. Section 5.3 presents the discussions on the results.

Material and Energy Balance
The materials and energy balance sheets that were obtained from Aspen Plus ® are given in Sections 5.1 and 5.2.

Material Balance of Molybdenum Oxide
The material balance of Molybdenum Oxide catalyzed process is shown in Table 2. The table shows the flow rates and densities of all of the components of the process. The methanol in inlet stream is 46.25 kmol/h. The flow rate of oxygen in the inlet air stream is 27.56 kmol/h. The formaldehyde in the product stream is 45.7875 kmol/h.

Energy Balance of Molybdenum Oxide
The energy balance of Molybdenum oxide process is shown in Table 3. The table shows the temperatures, pressures, and enthalpies of all the components of the process. The pressure varies from 1 bar to 1.8 bar during the process and temperature varies from 25 to 246.69 • C during the process. The input air has a temperature of 25 • C and the temperature of DTH going out of condenser is 246.69 • C. The enthalpy in reactor is −18,685.22 cal/mol. The highest pressure during the process is inside the absorption column, which is 1.8 bar.

Material Balance of Silver Oxide
The materials and energy balance of the silver oxide catalyzed process obtained from aspen plus is shown in Table 4. The table shows the material balance for silver oxide plant. The molar flow rate of oxygen in the air stream is 14.28 kmol/h. The molar flow rate of methanol in the inlet stream is 49.5 kmol/h. The formaldehyde in the product stream is 46.728 kmol/h.

Energy Balance of Silver Oxide
The energy balance of the Molybdenum oxide process is shown in Table 5. The table shows the temperatures, pressures, and enthalpies of all the components of the process. The pressure varies from 0.5 bar to 1.8 bar during the process and temperature varies from 25 • C to 600 • C during the process. The inlet gas is at 25 • C, whereas the reactor has a temperature of 600 • C. The enthalpy in the reactor is −19,619.55 cal/mol. The highest pressure during the process is in the absorption column, which is 1.8 bar.

Catalyst Performance
The comparison of both catalytic processes' performance is given in Table 6. It can be observed that the size of the Molybdenum Oxide plant is larger due to the greater amount of air requirement, whereas the size of silver oxide plant is lesser.

Comparsion of Molybdenum-and Silver-Based Processes
It can be observed from the results that the amount of methanol used was slightly different, but the amount of air varied significantly in the two processes. In the molybdenum oxide plant, the methanol was utilized at a rate of 46.25 kmol/h whereas it was utilized at a rate of 49.9 kmol/h in the silver-based plant. The amount of oxygen consumed in the silver-based plant was 27.56 kmol/h whereas the amount of oxygen required was at a rate of 15.12 kmol/h. Therefore, the lesser requirement oxygen in the silver-based plant lowers the utilities cost as well as decreasing the size of the equipment used. The capital cost in the case of silver is greater due to presence of an additional distillation column, however the size of overall plant is smaller though. The payback period of the silver-based plant is 2.8 years while it is 3.5 years for the molybdenum-based plant. The lesser payback period in the silver-based plant is due to a lower utility cost in the case of the silver-based process. The regeneration of silver catalyst is also possible. Therefore, from our results we can see that the silver plant has lower utility cost, shorter payback period, lower amount of oxygen required, and compact size as compared to molybdenum. Figures 3 and 4 show the comparison of the outcomes of the two processes for the formaldehyde formation i.e., Molybdenum-based and silver-based processes.

Catalyst Performance
The comparison of both catalytic processes' performance is given in Table 6. It can be observed that the size of the Molybdenum Oxide plant is larger due to the greater amount of air requirement, whereas the size of silver oxide plant is lesser.

Comparsion of Molybdenum-and Silver-Based Processes
It can be observed from the results that the amount of methanol used was slightly different, but the amount of air varied significantly in the two processes. In the molybdenum oxide plant, the methanol was utilized at a rate of 46.25 kmol/h whereas it was utilized at a rate of 49.9 kmol/h in the silver-based plant. The amount of oxygen consumed in the silver-based plant was 27.56 kmol/h whereas the amount of oxygen required was at a rate of 15.12 kmol/h. Therefore, the lesser requirement oxygen in the silver-based plant lowers the utilities cost as well as decreasing the size of the equipment used. The capital cost in the case of silver is greater due to presence of an additional distillation column, however the size of overall plant is smaller though. The payback period of the silver-based plant is 2.8 years while it is 3.5 years for the molybdenum-based plant. The lesser payback period in the silver-based plant is due to a lower utility cost in the case of the silver-based process. The regeneration of silver catalyst is also possible. Therefore., from our results we can see that the silver plant has lower utility cost, shorter payback period, lower amount of oxygen required, and compact size as compared to molybdenum. Figures 3 and 4 show the comparison of the outcomes of the two processes for the formaldehyde formation i.e., Molybdenum-based and silver-based processes.

Conclusions
The performance of two catalysts was studied simultaneously and the results were compared. The molybdenum-based plant has a larger plant size, high utility cost, but low cost of installation. Moreover, it has higher conversion of methanol to product i.e., 99%. The silver-based plant requires half the amount of air as compared to the molybdenum-based process, so it has compact plant size, the installation cost is high, but utility cost is low. However, the conversion is less in this process i.e., 75%-85% as compared to the molybdenum-based process. Based on the comparative study and calculations, the silver-based process for formaldehyde production is better on an industrial scale.

Conclusions
The performance of two catalysts was studied simultaneously and the results were compared. The molybdenum-based plant has a larger plant size, high utility cost, but low cost of installation. Moreover, it has higher conversion of methanol to product i.e., 99%. The silver-based plant requires half the amount of air as compared to the molybdenum-based process, so it has compact plant size, the installation cost is high, but utility cost is low. However, the conversion is less in this process i.e., 75-85% as compared to the molybdenum-based process. Based on the comparative study and calculations, the silver-based process for formaldehyde production is better on an industrial scale.