Process Principles Assignment
Production of Formaldehyde
World formaldehyde production is estimated to be about 8 million metric tons per year, most of which is eventually used in the manufacture of rigid plastic objects such as telephones and dishes. Formaldehyde is also used as a raw material in the production of slow-release nitrogen fertilizers, dyes, cosmetics, and explosives. The principal method of producing formaldehyde is the reaction of methanol on a stationary silver or iron-molybdenum oxide catalyst in a fixed-bed reactor. Two reactions occur in series in the production process:
CH30HHCHO + H2 (1)
H2 + ½ 02 ---> H20 (2)
If all of the hydrogen formed in Reaction 1 is consumed in Reaction 2, the overall stoichiometry is
CH30H + ½ 02 ---> HCHO + H20 (3)
PROCESS DESCRIPTION
Fresh and recycled methanol are mixed and fed to a vaporizer operating at a pressure of 1 atm. Ambient air is drawn into a blower which increases its pressure to 76 cm of H20; the air is then bubbled into the vaporizer through a sparger and leaves the vaporizer saturated with methanol. The methanol-air mixture is then heated to 145°C.
Saturated steam at 4.1 bars is metered into the air-methanol stream, and the combined stream then flows to the reactor. Feed rates of air and steam are determined from specified ratios of air to methanol and steam to methanol. A primary consideration in the specification of the methanol-to-air ratio is that a mixture containing between 6.7 and 36.5 volume% methanol in air at 1 atm constitutes a severe explosion hazard.
After entering the reactor, the feed gas passes through the catalyst bed. The bed consists of pure silver crystals, 0.5 to 3 mm in diameter, packed to a depth of about 3 cm, and supported on a stainless-steel wire mesh. The ratio of steam to methanol is fixed so that the outlet temperature is 600°C. The reactor may be considered adiabatic.
Gases leaving the reactor are fed directly to a waste-heat boiler, where they are cooled to 145°C. Saturated steam at 3.1 bars is generated in the boiler from saturated liquid water at 3.1 bars. The gases are cooled further to slightly above their dew point (100°C) and fed to an absorption column, where methanol and formaldehyde are absorbed in water. Pure water at 30°C is fed to the top of the column. The gases that leave the absorber contain 200 parts of formaldehyde per million parts (by volume) of total off-gas.
Liquid streams are removed from the column at two locations, cooled in heat exchangers, and recycled to entry points higher in the column.
The aqueous solution leaving the bottom of the absorber is fed to a distillation column. The final product solution is removed from the reboiler at the bottom of the distillation column, while pure methanol is removed at the top of the column and condensed.
A portion of the condensate (the reflux) is fed back into the top of the column, and the rest is recycled to the reactor. The distillation column, reboiler, and condenser operate at approximately 1 atm.
A plant is to be constructed to produce 3.6 x l04 metric tons per year of formaldehyde solution. From experience with other plants, there will be about 350 operating days per calendar year. The product solution has an analytical specification of 37 wt% formaldehyde, 1 wt% methanol, and the remainder water. The methanol-to-air ratio in the feed stream to the reactor will be 42:58 (molar basis). Data from other plants indicate that the specified process conditions should yield a conversion of 70 percent of the methanol entering the reactor, and that the effluent from the reactor will be 5 vol% hydrogen. The off-gas from the absorber will be at 27°C and saturated with water, and the liquid stream leaving the absorber will be at 88°C. Process cooling water is available at 30°C. To reduce scaling on the heat exchange tubes, temperature increases of cooling water are limited to 15°C. The ratio of the overhead returned to the column to that recycled to the reactor (the reflux ratio) is 2.5. Ambient conditions may be assumed to be 27°C and 1 atm.
PHYSICAL PROPERTY DATA
The specific gravity and heat capacity of the product solution are 4.65 and 3.35J/g °C, respectively. The heat of solution of formaldehyde gas at 25°C in water or aqueous alcohol solutions is -62.8 kJ/mol HCHO(g).
Standard heat of reaction 1 = 85.3 kJ/gmol CH3OH
Standard heat of reaction 2 = - 241.83 kJ/gmol H2
PROBLEMS
1. What is the world production rate of formaldehyde in kg/h?
2. Construct a detailed flow chart showing known compositions, flow rates, and temperatures.
Use this chart to keep track of your calculations and as a basis for a final flow chart to be prepared when you have finished the case study.
3. Determine the feed rate of methanol to the process (kg/h), neglecting trace amounts of formaldehyde and methanol in the off-gas stream from the absorber.
4. What is the percentage conversion of methanol for the process?
5. At what rate is methanol recycled to the reactor?
6. Does the specified air flow rate produce a gas mixture with a composition outside the explosion limits?
7. What is the flow rate of air to the reactor in (a) kmol/h (b) standard cubic meters/ min?
8. At what temperature should the vaporizer operate? (Hint: You need vapour pressure data, use Antoine Equation)
9. Because of its high solubility, essentially no methanol leaves the absorber in the off-gas stream. Determine the flow rates of water and formaldehyde in this stream.
10. What is the feed rate of water to the absorber? By increasing the water flow, the size of the absorber could be decreased. Why, then, is it desirable to limit the water flow rate to this amount? (Hint. Consider the ultimate fate of the water fed to the absorber). You may need the solution to Question 14.
11. Examine the process flow sheet and description carefully and itemize the utility consumptionunits (those that require steam, cooling water, and electricity).
12. What is the purpose of the waste-heat boiler ? Give reasons why, in practice, the reactor and waste-heat boiler are combined in one unit.
13. What are the standard heats of Reactions 1, 2, and 3?
14. What is the rate at which steam is fed to the reactor?
15. What fraction of the hydrogen produced by Reaction 1 is consumed in Reaction 2? Would more or less steam be required if Reaction 2 proceeded to completion?
Explain. What would be the required process changes if Reaction 2 were suppressed completely?
16. At what rate is steam generated in the waste-heat boiler?
17. Recognizing that increased residence time in a reactor usually means a higher conversion,why might the length of the reactor be only 3 cm? (Hint. Consider the possibility of unwanted side reactions.) What problems in heat transfer are presented by such a small reactor?
18. Assuming ambient air and stored methanol to be at 25"C, how much heat must be supplied to the vaporizer? Assuming that a portion of the steam generated in the waste heat boiler could be used for this purpose, estimate the required flow rate.
19. What is the required flow rate of cooling water to the exchanger between the wasteheat boiler and the absorber?
20. How much heat must be removed by the two heat exchangers cooling the recycled absorber liquid? Why is there a need to remove this heat?