CL 351                                   Homework # 5

 

Due date: 13 September 2005

 

 

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.

 

  1. Carotene (from carrot) is being extracted into an organic phase from water. The mass transfer coefficient of carotene in water is 1 x 10-3 moles/(cm2 sec). The organic phase contains an ion exchanger, which binds the carotene through a chemical reaction. The diffusivity of carotene in the organic phase is 1 x 10-4 cm2/sec, and the reaction rate constant for the binding of carotene is 400 sec-1. Determine the overall mass transfer coefficient based on aqueous side both with and without ion exchanger. The mass transfer coefficient in the organic phase (without the ion exchanger) is 5 x 10-4 moles/(cm2 sec). The slope of the equilibrium curve can be assumed to be 5.  Assume that the total concentration in the organic phase remains constant at 8 x 10-3 mol/cc.

 

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.