Ammonia is one of the chemical constituents of industrial waste that must be rem

Question

Ammonia is one of the chemical constituents of industrial waste that must be removed in a treatment plant before the waste can safely be discharged into a river or estuary. Ammonia is normally present in waste water as aqueous ammonium hydroxide (NH4OH-). A two-part process is frequently carried out to accomplish the removal. Lime (CaO) is first added to the wastewater, leading to the reaction

        CaO + H2O --> Ca2+ + 2(OH-)

The hydroxide ions produced in this reaction drive the following reaction to the right, resulting in the conversion of ammonium ions to dissolved ammonia:

        NH4+ + OH- <--> NH3(g) + H2O(l)

Air is then contacted with the wastewater, stripping out the ammonia.

A) One million gallons per day of alkaline wastewater containing 0.03 mol NH3/mole ammonia-free H2O is fed to a stripping tower that operates at 68oF. Air at 68oF and 21.3 psi contacts the wastewater countercurrently as it passes through the tower. The feed ratio is 300 ft3 air/gal wastewater, and 93% of the ammonia is stripped from the wastewater. Calculate the volumetric flow rate of the gas leaving the tower and the partial pressure of ammonia in this gas.

B) Briefly explain in terms a first-year chemistry student could understand how this process works. Include the equilibrium constant for the second reaction in your explanation.

Explanation / Answer

the design equation of the absorption column is

L/g= (yb-yt)/(Xb-Xt)

g/l = 300ft3/ gal of water

yb - yt = 0.03 mol of NH3/ free H2O

Xt = 0.07 mol/ mol of free air

then gas flow rate g = 0.0033ft3

The vapour pressure of the ammonia in tha air is = 0.07 mol

the total pressure of air = 23 PSI

According to Raoults law = partial pressure = mole fraction * Total pressure

partial pressure of ammonia = 23*0.07= 1.491PSI

vapour pressure of ammonia = 1.491 PSI

b) The dissociation of ammonium hydroxide in to ammonia and water is formed during high pH of 7 to 8 under high pressure. because it is very slow process in the atmospheric pressure . In the NH4OH has been dissociated in to NH3 + H+ OH- = NH3 + H2O. For creating high pH only they are adding lime in the water.

Dissocation constant (Kc) = concentration of product / concentration of reactant

Kc = [NH3] [H2O] / [NH4OH]

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