Formaldehyde is produced by decomposing methanol over a silver catalyst:

To provide heat for this endothermic reaction, some oxygen is included in the feed to the reactor,leading to the partial combustion of the hydrogen produced in the methanol decomposition.

The feed to an adiabatic formaldehyde production reactor is obtained by bubbling a stream of air at 1 atm through liquid methanol. The air leaves the vaporizer saturated with methanol and contains 42% methanol by volume. The stream then passes through a heater in which its temperature is raised to 145°C. To avoid deactivating the catalyst, the maximum temperature attained in the reactor must be limited to 600°C. For this purpose, saturated steam at 145°C is metered into the air–methanol stream, and the combined stream enters the reactor. A fractional methanol conversion of 70.0% is achieved in the reactor, and the product gas contains 5.00 mole% hydrogen. The product gas is cooled to 145°C in a waste heat boiler in which saturated steam at 3.1 bar is generated from liquid water at 30°C. Several absorption and distillation units follow the waste heat boiler, and formaldehyde is ultimately recovered in an aqueous solution containing 37.0 wt% HCHO. The plant is designed to produce 36 metric kilotons of this solution per year, operating 350 days/yr.

(a) Draw the process flowchart and label it completely. Show the absorption/distillation train as a single unit with the reactor product gas and additional water entering and the formaldehyde solution and a gas stream containing methanol, oxygen, nitrogen, and hydrogen leaving.

(b) Calculate the operating temperature of the methanol vaporizer.

(c) Calculate the required feed rate of steam to the reactor (kg/h) and the molar flow rate and composition of the product gas.

(d) Calculate the rate (kg/h) at which steam is generated in the waste heat boiler.

(e) Enough saturated steam was added to the feed to the reactor to keep the reactor outlet temperature at 600°C. Explain in your own words (i) why adding steam lowers the outlet temperature, and (ii) the economic drawbacks of higher and lower outlet temperatures.

Transcribed Image Text:

CH3OH – HCHO + H2