CH3030 Tutorial 2

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1. A gas mixture containing 10 mol% SO2 and 90 mol% air at 1 atm pressure and 30°C
is to be scrubbed with water to remove 97% of SO2 in a tower packed with 25mm
ceramic Raschig rings. The feed gas rate is 1500 kg/h.

Calculate
a) The minimum liquid flow rate
b) If the tower operates as 1.25 times the minimum liquid flow rate, the packed height of
the tower.

The liquid can be assumed to have properties like water. The volumetric mass transfer
coefficients at the given conditions are kxa=1.25 kmol/m3
sΔx, kya = 0.075 kmol/m3
sΔy.

Assume MTCs determined under UMD and EMD conditions are nearly the same. Cross
sectional area of the tower is 0.781 m2
.

Equilibrium data in mole fraction unit at 30°C and 1 atm total pressure is
104
x 0 0.562 1.403 2.8 4.22 8.42 14.03 19.65 27.9
103
y 0 0.792 2.23 6.19 10.65 25.9 47.3 68.5 104

2. A CS2-N2 mixture is to be scrubbed with an absorbent hydrocarbon oil which will be
subsequently steam stripped to recover the CS2. The CS2-N2 mixture has a partial
pressure of CS2 of 50 mm Hg at 24°C and is to be blown into the absorber at
atmospheric pressure at the flow rate of 0.4 m3
/s.

The CS2 content of the gas is to be
reduced to 0.5%. The absorbent oil has an average molecular weight of 180 and
viscosity 2cP, specific gravity 0.81 at 24°C. The oil enters the absorber, stripped of all
the CS2 and the solution of CS2 and oil follows Raoult’s law. The vapour pressure of
CS2 at 24°C =346 mmHg. Assume isothermal operation.

a) Determine minimum liquid/gas ratio

b) For liquid/gas ratio of 1.5 times the minimum, determine the kg/h oil flow rate to
enter the absorber.

c) Determine the number of theoretical trays required both graphically and
analytically.

d) Determine the overall tray efficiency of the bubble cap tray absorber using the
plot below and the real number of trays required for this efficiency.