ChE Exam

Chemical Engineering Calculations

Question 1 (10 points)

One thousand kilograms of a 30% solution of Na2CO3 in water is cooled slowly to 293 K. During cooling, a portion of water is evaporated and removed. 700 kg of Na2CO3·10H2O crystals are formed in the process. What percent of total water in the feed is evaporated? The solubility of anhydrous Na2CO3 at 293 K is 21.5 kg/100 kg of water.

Answer: 10.1%

Question 2 (10 points)

Air is dehumidified at a constant pressure of 101.3 kPa. The partial pressure of water in the air admitted to the dehumidifier is 7 kPa and the temperature is 350 K. The partial pressure of water in the air leaving is 1.5 kPa. How much water (in kilograms) is removed from 100 cubic meters of air admitted?

Answer: 3.457 kg

Question 3 (20 points)

A mixture of ethane (C2H6) and ethylene (C2H4) occupies 4.0 × 10-2 m3 at 1 atm and 400 K. The mixture reacts completely with 0.130 kg of O2 to produce CO2 and H2O. Assuming ideal gas behaviour, calculate the mole fractions of C2H4 and C2H6 in the mixture.

Answer: 0.333 ethylene, 0.667 ethane

Question 4 (10 points)

N2O4 decomposes to NO2 at high temperatures. 20 g of N2O4 when heated to 373 K at 96 kPa, is found to occupy a volume of 0.0125 m3. Assuming the applicability of ideal gas law, calculate the percent dissociation of N2O4 to NO2.

Answer: 78%

Question 5 (10 points)

Wet lumber (5% moisture) is dried to 1% moisture in a hot-air drier. Air fed to the drier contains 0.5% water. The moist air leaving the drier contains 2% (weight) water. How much air is required to dry 2000 kg/h of lumber?

Answer: 5279.5 kg of wet air/h

Question 6 (10 points)

Oilseeds containing 40% oil and the rest inert insolubles are extracted with hexane to recover oil. Oil is dissolved in the solvent and is removed as a clear solution. The underflow sludge analyzed 10.53% oil and 26.32% hexane. Identify the key component. Determine the percent recovery of oil.

Answer: 75%