CL 351 Homework # 5
Due date:
1.
When molasses is fermented to produce a liquor
containing ethyl alcohol, a CO2 rich vapour
containing a small amount of ethyl alcohol is evolved. The alcohol can be recovered by absorption
with water in a sieve tray tower. For
the following conditions, determine the number of equilibrium stages required
for countercurrent flow of liquid and gas, assuming isothermal, isobaric
conditions in the tower and neglecting mass transfer of all components except
ethyl alcohol.
Entering
gas: 180 kmol/hr;
98% CO2, 2% ethyl alcohol; 30oC, 110 kPa
Entering
liquid absorbent: 50% more than the minimum rate; 100% water, 30oC,
110 kPa.
Required recovert (absorption) of ethyl alcohol: 97%
Equilibrium
data can be approximated by Y = 0.57X, Y = moles of alcohol/moles of vapour; X = moles of alcohol/moles of water.
3.
SO2 is absorbed in dilute solutions of NaOH with which it reacts instantaneously. The reported values of gas side and liquid
side mass transfer coefficients are 1.7 cm/sec and 0.0038 cm/sec,
respectively. Henry’s law constant is 840 cm3 atm/gmol. If partial pressure of SO2 in air
is 10 mm Hg, find interfacial concentration of SO2 when
concentration of NaOH is 0.1, 0.2 and 0.3
mol/lit. At what critical concentration
of NaOH is the interfacial concentration of SO2is
zero? Which phase controls mass transfer
in solutions without NaOH and in solutions with NaOH? Diffusivity of
NaOH in water is 2.1 x 10-5 cm2/sec
and that of SO2 is 1.9 x 10-5 cm2/sec.
1.
You are studying mass transfer with and without
chemical reaction in a 100-cm tall, 4-cm diameter wetted column. Water is flowing down the column in a 3 mm
thick film at 5 cm/sec. In non reacting systems, water becomes 10% saturated by
the time it leaves the column. In
reacting systems, 500 cm3/sec air containing 10% Cl2 is
flowing up the column and 50 cm3/sec of water is flowing down. Cl2 reacts with water to produce HOCl. Assume that
the system conforms to fast regime where kinetics are
important. 10 ml of exiting water was
required 40 ml of 0.01 molar NaOH. Determine the individual liquid side mass
transfer coefficient in units of cm/sec both for reacting and non reacting
systems, if the liquid side controls mass transfer for both systems. Temperature is 25oC and pressure
is 1 atmosphere. Henry’s law constant
for Cl2 – water system is 610 bars.
For non-reacting systems, make balance of solute on liquid
side. For reacting systems, perform mass balance on gas side and follow the
same procedure that was used for gas absorption.