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Problem Set 2

This document contains 4 homework problems related to mass transfer for a chemical engineering course. Problem 1 involves gas diffusion and a catalytic reaction. Problem 2 calculates the concentration of benzoic acid absorbed in water flowing through a pipe. Problem 3 determines mass transfer coefficients and flux given gas and liquid compositions across an interface. Problem 4 involves chlorine absorption and calculates interfacial concentrations.

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170 views2 pages

Problem Set 2

This document contains 4 homework problems related to mass transfer for a chemical engineering course. Problem 1 involves gas diffusion and a catalytic reaction. Problem 2 calculates the concentration of benzoic acid absorbed in water flowing through a pipe. Problem 3 determines mass transfer coefficients and flux given gas and liquid compositions across an interface. Problem 4 involves chlorine absorption and calculates interfacial concentrations.

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Hanoi University of Mining and Geology

Advanced Program in Chemical Engineering


Mass Transfer

Homework Problem Set # 2 (Due 08/20)

Problem 1: The following sketch illustrates the gas-phase diffusion in the neighborhood of
a catalytic surface:

Z=0

A B

Z=

Component A diffuses through a stagnant film to the catalytic surface, where it is


instantaneously converted into species B by the reaction A  B. When species B diffuses into
the stagnant film, it begins to decompose by the first-order reaction B  A. The rate of
formation of component A within the film is equal to RA = k1yB, where yB is the concentration
of B expressed in mole fraction. Determine the rate at which A enters the gas film if this is a
steady-state process.

Problem 2: Water at 25°C flows through a 6 ft long 2 in ID (inner diameter) pipe at a velocity
of 5 ft/s. The pipe is coated on the inside with a thin layer of benzoic acid. Calculate the
average concentration of benzoic acid (gmol/L) in the exit stream. State and justify all
necessary assumptions. The solubility of benzoic acid in water (at 25°C) is 0.029 gmol/L, and
the Schmidt number is 942. Viscosity of water (at 25°C) is 0.9 centipoise. For turbulent flow
in smooth tubes, the following empirical correlation can be used: j D =0.023*Re-0.2, where jD is
the Chilton-Colburn factor.

Problem 3: In the absorption of component A from an airstream into an aqueous stream,


the bulk composition of the two adjacent streams were analyzed to be PA,G = 0.1 atm and CA,L
= 0.25 lbmol/ft3. The Henry's constant for this system is 0.265 atm/(lbmol/ft3 of solution).
The overall gas-side mass transfer coefficient KOG was determined to be 0.055 lbmol
A/hr/ft2/atm. If 57% of the total resistance to mass transfer is in the gas film, determine:

(a) The gas-side mass transfer coefficient.


(b) The concentration on the liquid side of the interface, C Ai.
(c) The liquid-side mass transfer coefficient, kL.
(d) The molar flux of A.

Problem 4: Chlorine water for pulp bleaching is being prepared by absorbing chlorine gas
in water within a packed tower operating at 293 K and 1.013 x 105 Pa pressure. At one point
in the tower, the chlorine pressure in the gas is 4 x 104 Pa and the concentration in the liquid
is 1.0 kg/m3. Data on the solubility of chlorine in water at 293 K are given below. If 80% of
the resistance to mass transfer lies in the liquid phase, what are the interfacial
concentrations?

PA (mm Hg) 0 5 10 30 50 100 150


Dissolved 0 0.438 0.575 0.937 1.21 1.773 2.27
Cl2 (g Cl2/L)

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