Wet air containing 4.0 mole% water vapor is passed through a column of calcium c

Question

Wet air containing 4.0 mole% water vapor is passed through a column of calcium chloride pellets. The pellets adsorb 97.0% of the water and none of the other constituents of the air. The column packing was initially dry and had a mass of 3.40kg. Following 5.0 hours of operation, the pellets are reweighed and found to have a mass of3.54kg.

(a)Calculate the molar flow rate (mol/h) of the feed gas and the mole fraction of water vapor in the product gas.

(b)The mole fraction of water in the product gas is monitored and found to have the value calculated in part (a) for the first 10hours of operation, but then it begins to increase. What is the most likely cause of the increase? If the process continues to run,what will the mole fraction of water in the product gas eventually be?

Explanation / Answer

a) 3.54kg - 3.4 kg = 0.14 kg = 140 grams 140 grams /18g/mole = 7.78 moles H2O 7.78 moles/5 hours = 1.56 moles/hour b) the most likely cause of the increase is that the column is now SATURATED with water. Calcium chloride forms a hydrate that can adsorb/absorb a specific number of water molecules per unit of calcium chloride. Once all of the calcium chloride has reacted, no more water will be absorbed. if the process continues, the mole fraction of product gas will eventually reach the mole fraction of intake gas, which is 4.0%

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