Solved: Formaldehyde is produced from methanol in a

Chapter 8, Problem 8.81

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Formaldehyde is produced from methanol in a catalytic oxidation reactor. The following reactions take place: CH3OH ! HCHO H2 2H2 O2 ! 2H2O A stream of methanol is joined by a recycle stream, also methanol, and the combined stream is fed to the conversion reactor. Also entering the reactor are air (to oxidize some of the hydrogen produced in the methanol conversion reaction) and steam (to control the reactor temperature). The product gas from the reactor contains 19.9 mole% formaldehyde, 8.34% methanol, 30.3% nitrogen, 0.830% oxygen, 5.0% hydrogen, and 35.6% water vapor and is at 600C and 1 atm. The following process is used to separate the formaldehyde from the unreacted methanol and noncondensable gases. The gases leaving the reactor are fed to a waste-heat boiler, in which they are cooled to 145C, in the process generating steam at 3.1 bar from saturated liquid water (i.e., liquid water at its boiling point) at the same pressure. The gases are cooled further to 100C in a heat exchanger, where they come into thermal contact with cooling water fed at 30C. To reduce scaling on the heat exchanger tubes, the temperature increase of the cooling water is limited to 15C. The cooled gases are fed to an absorption column, where methanol and formaldehyde are absorbed in water. Pure water at 20C is fed to the top of the column. The gas leaving the absorber is saturated with water vapor at 27C and 1 atm and contains 200 parts of formaldehyde per million parts (by volume) of total gas. The aqueous solution leaving the bottom of the absorber at 88C is fed to a distillation column that operates at 1 atm. The final product solution, which contains 37 wt% formaldehyde, 1% methanol, and the balance water, is removed from the reboiler at the bottom of the column, while pure methanol vapor emerges as the overhead product and is condensed at 1 atm. A portion of the condensate is refluxed to the top of the column, and the rest is recycled to the methanol conversion reactor. The reflux ratio, or ratio of methanol refluxed to methanol recycled to the reactor, is 2.5:1. (a) Taking as a basis of calculation 100 mol of gas leaving the conversion reactor, draw and completely label a flowchart of this process. Then calculate the moles of fresh methanol feed, formaldehyde product solution, recycled methanol, and absorber off-gas, the kg of steam generated in the waste-heat boiler, and the kg of cooling water fed to the heat exchanger between the waste-heat boiler and the absorber. Finally, calculate the heat (kJ) that must be removed in the distillation column overhead condenser, assuming that methanol enters as a saturated vapor at 1 atm and leaves as a saturated liquid at the same pressure. (b) By what factor must all of the calculated quantities be multiplied to scale the flowchart to a production rate of 3.6 104 metric tons per year of formaldehyde solution, assuming that the process is on-stream 350 days per calendar year?