Unit Operations V
Tutorial No. 1
Unit 4
1. A gas mixture at 2.026 × 105 Pa total pressure containing air and SO2 is brought into
contact in a single-stage equilibrium mixer with pure water at 293K. The partial pressure
of SO2 in the original gas is 1.52 × 104 Pa. The inlet gas contains 5.70 total kg mol and
the inlet water 2.20 total kg mol. The exit gas and liquid leaving are in equilibrium.
Calculate the amounts and compositions of the outlet phases. Use the equilibrium data
from Fig. 10.2-1 below.
2. The gas stream from a chemical reactor contains 25 mol% ammonia and the rest inert
gases. The total flow is 181.4 kg mol/h to an absorption tower at 303K and 1.013 × 105
Pa pressure, where water containing 0.005 mol fraction ammonia is the scrubbing liquid.
The outlet gas concentration is to be 2.0 mol% ammonia. What is the minimum flow
𝐿′𝑚𝑖𝑛 ? Using 1.5 times the minimum, plot the equation the equilibrium and operating
lines.
3. A relatively non-volatile hydrocarbon oil contains 4.0 mol% propane and is being
stripped by direct superheated steam in a stripping tray tower to reduce the propane
content to 0.2%. The temperature is held constant at 422 K by internal heating in the
tower at 2.026 x 105 Pa. A total of 11.42 kg mol of direct steam is used for 300 kg mol of
total entering liquid. The vapour-liquid equilibria can be represented by 𝑦 = 25𝑥, where
y is mole fraction propane in the steam and x is mole fraction propane in the oil. Steam
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can be considered as an inert gas and will not condense. Plot the operating and
equilibrium lines and determine the number of theoretical trays needed.
4. A gas stream contains 4.0 mol% NH3 and its ammonia content is reduced to 0.5 mol% in
a packed absorption tower at 293K and 1.103 x 105 Pa. The inlet pure water flow is 68.0
kg mol/h and the inlet gas flow is 57.8 kg mol/h. The tower diameter is 0.747 m. The film
mass-transfer are 𝑘𝑦′ 𝑎 = 0.0739 kg mol/s.m3.mol fraction and 𝑘𝑥′ 𝑎 = 0.169 kg
mol/s.m3.mol fraction. Using the design methods for dilute gas mixtures, do as follows:
a. Calculate the tower height using 𝑘𝑦′ 𝑎
b. Calculate the tower height using 𝐾𝑦′ 𝑎
5. A packed tower 4.0 m tall is used to absorb ethyl alcohol from an inert gas by 90 kg
mol/h of pure water at 303K and 101.3 kPa. The total gas stream flow rate of 100 kg
mol/h contains 2.0 mol% alcohol and the exit concentration is 0.20 mol%. The
equilibrium relation is y = mx = 0.68x for this dilute stream. Using the graphical method,
determine:
a. the number of theoretical trays, N
b. the number of transfer units NOG, HOG, and HETP.
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