Question

In the production of ethanol from corn the solids left after fermentation are dried in a continuous flow oven. This process generates a flow of 15 kg/s moist air, 90°C with 70% relative humidity, which contains some volatile organic compounds and some particles. To remove the organic gasses and the particles, the flow is send to a thermal oxidizer where natural gas flames brings the mixture to 800°C. Find the rate of heating by the natural gas burners.

Answer 1

vapor pressure of water vapor at 90 degree C = 525.8 torr

partial pressure of vapor = 0.7 * 525.8 = 368.06 torr

moles fraction of vapor = 368.05 /760 = 0.484

let mass of vapor in 15 kg moist air = x, therefre (x/18)/(x/18+(15000-x)/29) = 0.484 , or x= 5519.5g

where

• 18 is the molecular weight of water

• and 29 is the average molecular weight of air

cp of water = 4.18 J/g-K

and

cp of air = 1.06 J/g-K,

therefore

cp avg = 0.484 *4.18 + (1-.484)*1.06=2.57 j/g-K ,

therefore rate of heating = m*cp avg * delta T = 15000* 2.57*(800-90) = 27.37 MJ