A chemical plant produces an ammonia-rich waste gas that cannot be released as-i

Question

A chemical plant produces an ammonia-rich waste gas that cannot be released as-is into the environment One method to reduce the ammonia concentration in the waste gas is to bubble the waste gas through a liquid solvent. Components of the gas that are highly soluble, like ammonia, will dissolve into the liquid phase. Water is an appropriate liquid solvent for this process. The chemical plant produces 135.0 m^3/hr of waste gas (rho = 0.0407 mol/L) initially having a mole fraction of 0.160 NH_3, and wishes to remove 90.0% of the initial amount of NH_3. The maximum concentration of ammonia in water at this temperature is 0.2500 mol NH_3/mol water. Neglect the absorption of other waste gas components into the water, and the evaporation of water into the waste gas stream. At what rate is NH_3 being removed from the feed gas? What is the minimum flow rate of water (rho = 0.990 g/mL) required to scrub out 90.0% of the incoming NH_3? What is the mole fraction of NH_3 in the scrubbed waste gas stream?

Explanation / Answer

waste gas flow rate = 135 m3/hr = 135000L/hr

density of waste gas = 0.0407 mol/L

molar flow rate of waste gas = 135000 * 0.0407 = 5494.5 mol/hr

mole fraction of NH3 in waste gas = 0.16

molar flow rate of NH3 in waste gas = 0.16 * 5494.5 = 879.12 mol/hr

90% of NH3 is being removed from waste gas

Hence, NH3 is being removed from waste gas = 0.9 * 879.12 = 791.208 mol/hr

NH3 is being removed from waste gas is 791.208 mol/hr

maximum concentration of ammonia in water is 0.25 mol NH3 / mol water

molar flow rate of water required to remove 90% NH3 is = 0.791.208 / 0.25 = 3164.832 mol/hr

density of water = 0.99g/mL = 990 g/L

molecular weight of water = 18 g/mol

water flow rate = (3164.832 mol/hr) * (18 g/mol) / (990 g/L) = 57.5424 L/hr

Minimum flow rate of water is 57.5424 L/hr

Moles of waste gas =  5494.5 mol/hr

NH3 remained in scrubbed waste gas = 10% of original NH3 = 879.12 *0.1 = 87.912 mol/hr

mole fraction of NH3 remained in scrubbed waste gas = 87.912 / 5494.5 = 0.016

mole fraction of NH3 remained in scrubbed waste gas is 0.016

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