Ammonia Gas Absorption
by
Oscar D. Crisalle
Professor
Chemical Engineering Department
University of Florida
crisalle@che.ufl.edu
Revision 12: September
24, 2013
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Ammonia Absorption
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CONTENTS
1 Introduction
4
2 Experiment 1: Absorption of Ammonia (N H3)
5
3 Operational Information
6
4 Thermodynamic Phase-Equilibrium
9
5 Henry's Law
11
6 Gas Densities
12
7 Colburn's NTU Equation
13
7.1
Number of transfer units for gas-phase controlled transfer
. . . . . . . . . . . . . . . . . . . . . .
13
7.2
Height of a transfer unit for gas-phase controlled transfer . . . . . . . . . . . . . . . . . . . . . . .
14
7.3
Remarks on the NTU Equation (7) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
8 Overall Mass Transfer Coecient
15
9 Interpretation of the Absorption Factor
16
10 Interpretation of NTU and HTU
17
11 Characteristics of Flow in the Column
18
11.1 Hold-up time (also called residence time)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
11.2 Number of hold-ups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
12 Scrubbing Eectiveness
19
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13 Theoretical Expectations
21
14 Measurement of N H3 Gas Compositions
22
15 Rotameter: Water Flow Measurement
23
15.1 Liquid solvent rotameter (RTM): water
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
15.2 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
16 Rotameters: Gas Flow Measurements
16.1 Gas feed-line rotameters (RTM):
25
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
16.2 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
(N H3 + N2 )
17 Experimental Details
27
18 Experimental Procedures
30
18.1 Start-up and normal-operation procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
18.2 Shut-down procedure
31
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19 Anticipated Experimental Problems
32
20 Objectives
33
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1
INTRODUCTION
l This
experiment investigates the properties of gas absorption equipment where a gaseous
solvent mixed with air or Nitrogen is absorbed by dissolution into a water stream
l There
l The
are two gas absorption experiments in the Unit Operations lab
Experiment 1
Absorption of ammonia in water
Experiment 2
Absorption of carbon dioxide in water
focus of this lab is the Experiment 1 which deals with the absorption of ammonia in
water.
l It is MANDATORY to read the chapter entitled Gas Absorption in reference [3]
before carrying out this experiment.
l Remark:
Gas absorption is also referred to as gas scrubbing, or gas washing.
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2
EXPERIMENT 1: ABSORPTION OF AMMONIA (N H3 )
Counter-current absorption packed tower
Nomenclature
l Solute:
Raffinate (N2 + NH3)
Vout, yout
Solvent (W)
Lin, xin
l Feed
Ammonia
Carrier Gas: Nitrogen
l Solvent:
Rotameter
(N H3)
Water
(N2)
(W )
Thermodynamic Equilibrium
Ammonia
Sensor
Feed-solvent Phase
(or raffinate phase)
h
N2 + NH3
W
Equilibrium
y = mx*
N2 + NH3
W + NH3
y
x*
Extract-solvent Phase
(or extract phase)
Vin, yin
Rotameter
Feed (N2 + NH3)
Lout, xout
Extract (W + NH3)
Assumption: Nitrogen is insoluble in
W
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OPERATIONAL INFORMATION
l Column
available
m Height
of the column :
m Internal
m Type
diameter:
800 mm
100 mm
of packing: Standard
6 mm
Raschig rings
l Density
m The liquid water stream has a higher density than the N2 + N H3
gas stream. That is why
the liquid stream is fed from the top.
l Insolubility
m We
are making the assumption that
N2
is insoluble in
W. This is only an approximation.
l Nonvolatility
m We
W ) is nonvolatile at the temperature of
are making the assumption that the solvent (
the experimental conditions
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OPERATIONAL INFORMATION
l Continuous
m Two
and dispersed phases
phases form inside the column:
a
CONTINUOUS
phase and a
DISPERSED
phase. When a column is started up, it is FIRST lled with ONLY the gas. This denes
the continuous phase.
SECOND, the liquid stream is introduced, and it becomes the
dispersed phase.
m Making the gas the continuous phase creates more interfacial area than when the liquid is
the continuous phase (because the gas is constrained to reside in bubbles)
l Flooding
by the water phase
m Occurs
when the upward force exerted by the gas is sucient to prevent the liquid from
owing downward
m The 100% ooding velocity
of the gas stream can be determined for a given inlet liquid
stream ow:
n Set
the gas feed ow to a value that oods the column (water level is at the top of the
packing surface)
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OPERATIONAL INFORMATION
l Operation
m Start
m Set
at 0% ooding
gas ow
100% ooding conditions
m Progressively reduce the gas ow rate until a value where zero ooding (packing base level)
occurs
m Space below packing base must be covered with water to prevent gas escape via the liquid
exit pipe
l In
the case of unpacked column
m For
0% ooding, the height of the column at which the inlet of the feed gas stream is
located should be considered as the base level
m For 100% ooding, the height of the column at which the inlet of the water solvent stream
is located should be considered as the top level
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4
THERMODYNAMIC PHASE-EQUILIBRIUM
l Some N H3 from the gas phase (Nitrogen + N H3) absorbs into the
N H3) establishing a phase equilibrium after a suciently long time
l The
liquid phase (Water +
x∗
mole fraction of
N H3
in the liquid phase at equilibrium
y
mole fraction of
N H3
in the gas phase at equilibrium
equilibrium mole-fraction
(x∗)
of absorbed
N H3
is known as the solubility of
N H3
in
water
y
N2 + NH3
P
Gas Phase
Liquid Phase
W + NH3
x*
T
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THERMODYNAMIC PHASE-EQUILIBRIUM
l The
solubility of
m The
l It
N H3
in water is high at room temperature and
1 atm
of pressure
solubility increases with pressure and decreases with temperature
x∗ to y , but we are
N H3 and nitrogen)
is possible to derive a relationship relating
of low values of
m Focus:
y
(use of dilute mixture of
mostly interested in cases
dilute gas-phase regime
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5
HENRY'S LAW
l At constant T
and at equilibrium, the amount of solute gas that dissolves into the liquid
is proportional to the partial pressure
(yP )
(x∗)
of the solute gas in the gas phase: i.e,
yP = H(T )x∗
(1)
y = mx∗
(2)
or
where
m=
where
P
H (T )
P
(3)
is the operating pressure of the column and
H (298.15) = 0.885 atm
l Valid
(
only for dilute solutions and when the solute
W)
l Temperature
does not react with the solvent
dependence is given by the van't Ho equation
H (T ) = H T
and for
(N H3)
N H3/water
ref
exp −C
1
1
− ref
T
T
(4)
solution
C = 3670 K
l Resource:
http://www.henrys-law.org
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6
GAS DENSITIES
l Standard
Temperature and Pressure
m NIST-STP:
National Institute of Standards and Technology
T = 20 C = 68 F = 293.15 K
m IUIPAC-STP:
P = 1 atm = 1.01325 bar = 14.696 psi
International Union of Pure and Applied Chemistry
T = 0 C = 32 = 273.15 K
P = 0.9869 atm = 1 bar = 14.504 psi
m Gas rotameter manufactures usually use dierent standards.
Refer the instrument manual
for details.
l Density Models
m Density of dry
air (model using the
specic air constant )
ρAir =
where
P
P
Specific
T
RAir
(5)
is the feed gas pressure and
Specific
RAir
= 286.689 J/ (kg · K) = 2.829 × 10−3 m3 · atm/ (kg · K)
m Density
of ammonia gas
ρN H3 ≈
M WN H3
17.031
ρAir =
ρAir = 0.587ρAir
M WAir
29
(6)
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7
COLBURN'S NTU EQUATION
7.1 Number of transfer units for gas-phase controlled transfer
NOG
mxin − yout
A
mx − yin
in
ln
=
1
mx
−
y
1−A
in
out
1−
1−
A
mxin − yin
(7)
and
L0in
A =
mVin0
(8)
where
yout
yin
xin
A
Vin0 = Vin/Across
L0in = Lin/Across
m
solute mole fraction in the ranate
[dimensionless]
solute mole fraction in the feed
[dimensionless]
solute mole fraction in the solvent
[dimensionless]
absorption factor
solvent molar supercial velocity
[dimensionless] 2
lbmole/ min · ft2
lbmole/ min · ft
equilibrium constant for dilute solution
[dimensionless]
feed molar supercial velocity
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COLBURN'S NTU EQUATION
7.2 Height of a transfer unit for gas-phase controlled transfer
HOG =
lh
(9)
is the height of the packed bed [ft]
7.3 Remarks on the NTU Equation
l Only
l It
h
NOG
(7)
valid for dilute feed streams
is assumed that the solute mole fraction in the solvent is zero, i.e.,
l Number
of transfer units is expressed in terms of concentration in the gas phase
m Solubility
l As
xin = 0
of ammonia in water is high
a result, the dominant resistance to diusion (mass transfer) resides within the gas
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8
OVERALL MASS TRANSFER COEFFICIENT
l Overall
mass transfer coecient on a gas-phase basis
Vin
1
=
Vin0
HOGAcross HOG
m Interfacial area per unit volume of packing, a, is normally
transfer coecient (Denition: Ainterf acial = aAcross )
Ky a =
l Mass-transfer
left lumped with the mass
resistance: inverse of the mass-transfer coecient
Resistance =
l Correlation
(10)
1
Ky a
(Solve using least-squares regression)
Ky a = c1Lcin2 Vinc3 =⇒ ln (Ky a) = c4 + c2 ln Lin + c3 ln Vin
where
m See
(11)
(12)
c4 = ln c1
tutorial on the Excel function LINEST ( Least-squares regression using LINEST in
Excel ) posted in the course web site. You can also use MATLAB or OCTAVE
m Verication of correlation
n Carry out at least one additional experimental run
(Ky a)
correlation − (Ky a)run
Error =
100%
(Ky a)run
(13)
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9
INTERPRETATION OF THE ABSORPTION FACTOR
l The Absorption Factor A
is dened as the ratio of the local slope of the operating curve
to that of the equilibrium curve
A=
Slope of operating curve
Slope of equlibrium curve
(L0in/Vin0 )
L0in
=
=
m
mVin0
l For the transfer of N H3 from the gas phase (V ) to the liquid phase (L), the driving force
y − y ∗ should be positive, which implies the operating line should be above the equilibrium
line.
m This
is possible when
A>1−
m Hence,
N H3 from
A is met.
the absorption of
the above condition on
mxin − yout
mxin − yin
the gas phase into the liquid phase occurs only when
l Observations
m When A < 1 −
desorption
(
or
m When A = 1 −
mxin − yout
mxin − yin
mass transfer occurs from the liquid phase into the gas phase
stripping )
mxin − yout
mxin − yin
there is no net mass transfer between the gas and liquid phases
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10
INTERPRETATION OF NTU AND HTU
l Number
of transfer units (NTU)
m Depend
on the value of
m Measure
m If
yout
desired for a given
yin
of the diculty of separation
a high-level of absorption (separation) is desired, then a larger number of NTUs is
needed
l Height
of a transfer units (HTU)
m Depend
on the mass transfer coecient and the gas ow rate
m Measure
m HTU
of the separation eectiveness of the packing for the species being absorbed
is proportional to the resistance to mass transfer
HOG =
m HTU
1 Vin
Ky a Across
(14)
is small (lower resistance) when
n There
n There
is a high rate of interface mass transfer
is a large amount of interfacial area (better contact)
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11
CHARACTERISTICS OF FLOW IN THE COLUMN
11.1 Hold-up time (also called residence time)
thold−up =
thold−up
QW ater
VP ack
VP ack
QW ater
(15)
hold-up time (residence time)
(min)
water (solvent) ow rate
(GPM)
packed volume
(gal)
11.2 Number of hold-ups
Nhold−up =
Nhold−up
tSS
tSS
thold−up
number of hold-ups
(dimensionless)
time to steady-state
(min)
(16)
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12
SCRUBBING EFFECTIVENESS
l Denition
of Scrubbing Eectiveness
:=
l Formula
N H3absorption into the liquid solvent
N H3entering via the feed stream
Overall rate of
Rate of
Derivation: Dene
Y =
and use the mass-balance result
Vout =
yout
yin
(18)
1 − yin
Vin:
1 − yout
=
(17)
yinVin − youtVout
yinVin
1 − yin
Vin
yinVin − yout
1 − yout
=
yinVin
yin (1 − yout) − yout (1 − yin)
yin − yout
=
yin (1 − yout)
yin (1 − yout)
yout
1−
1−Y
yin
=
=
yout
1 − yinY
1 − yin
yin
=
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l Calculation
m 1. From experimental data
Information needed: yin and yout
Procedure:
n Calculate
yout
yin
Y exp =
n Calculate
m 2. From NTU predictions
Information needed: yin , m, Vin , Lin ,
exp
(19)
1 − Y exp
=
1 − yinY exp
and
(20)
NOG
Procedure
n Calculate A
and nd the value of
Y
NOG =
by solving (graphically or numerically) from
A
ln
A−1
n Calculate
pred =
m 3. Prediction error
n Calculate the prediction
error
1+
1
(A − 1)
Y
A
1−Y
1 − yinY
(21)
(22)
PE
PE =
pred
− exp
exp
100%
(23)
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13
THEORETICAL EXPECTATIONS
l The
mass transfer should increase for larger
l The
mass transfer should more strongly aected by the gas-feed ow rate
solvent ow rate
Lin/Vin
ratios
(Vin)
than by the
(Lin)
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14
MEASUREMENT OF
l BACHARACH
Ammonia
N H3
gas
GAS COMPOSITIONS
monitor:
Model AGMSZ
l Measures ammonia gas in the range of 25
to 10, 000 ppm
l Detector
Type:
Single
pass,
non-
dispersive infrared
l Sensitivity:
25 ppm
l Operating
Temperature:
32 to 122°F (0
to 50°C)
l Accuracy: ±10 ppm ± 10%
from 0 − 1000 ppm
of reading
l Operating
Humidity: 5 to 90% RH, non-
condensing
l Response
Time:
9
to
30 seconds,
depend-
ing on tube length and gas concentration
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15
ROTAMETER: WATER FLOW MEASUREMENT
15.1 Liquid solvent rotameter (RTM):
water
l Dwyer
Rate-Master Flowmeter:
Coarse
Fine
Model
RMC
l2
rotameters
(coarse
and
ne
adjust-
ments)
l Measurement
m Coarse:
m Fine:
units
Gallons per Minute (GPM)
Gallons per Hour (GPH)
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ROTAMETER: WATER FLOW MEASUREMENT
15.2 Measurements
l Reading: QRT M,solvent
l For
(graduation mark on the scale)
ne rotameter
Qsolvent(GP H) = QRT M,solvent
l For
coarse rotameter
Qsolvent(GP H) =
l Mass
60 min
QRT M,solvent
1 hr
(24)
(25)
ow rate
Qsolvent(lb/hr) = ρsolventQsolvent(GP H)
(26)
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16
ROTAMETERS: GAS FLOW MEASUREMENTS
16.1 Gas feed-line rotameters (RTM):
Coarse
Fine
(N H3 + N2)
l Dwyer
Rate-Master Flowmeter:
Model
RMB
l2
rotameters
(coarse
and
ne
adjust-
ments)
l Measurement
units:
Standard
Cubic
Feet per Hour (SCFH)
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ROTAMETERS: GAS FLOW MEASUREMENTS
16.2 Measurements
l Reference
(from the instrument manual)
m Tref = 70 F = 21.111 C = 294.261 K
m Pref = 1 atm = 1.01325 bar = 14.696 psi
l Reading: RRT M
(oat position on the scale)
s
Qf eed(SCF H) = RRT M
Pf eed Tref (K)
Pref Tf eed(K)
Qf eed(lb/hr) = fSCF H→CF H ρf eed Qf eed(SCF H)
where the conversion factor
fSCF H→CF H
(27)
(28)
is
fSCF H→CF H =
Pref Tf eed(K)
Pf eed Tref (K)
(29)
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17
EXPERIMENTAL DETAILS
l Because N H3
is highly soluble in water, one must operate at low solvent-to-feed rations
(i.e., low L/V) to prevent complete mass transfer to the liquid (dominant resistance to mass
transfer is in the gas phase)
l Measure
m Take
the
N H3
composition in the feed and ranate stream using the sensor.
repeated measurements to obtain statistical averages.
m Report
concentration values at steady state (take great care of ensuring steady state is
attained)
l Measure the volumetric mass ow rates of the the feed and solvent streams using the rotameters and convert the readings to mass and molar ow rates [lbmol/hr].
m Then
calculate the corresponding uxes need in Colburn's equation by dividing by the
cross sectional area of the column.
l Determine
the ooding velocity of the feed stream for each solvent ow rate considered.
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EXPERIMENTAL DETAILS
l Run
l Note
the column at various values of the absorption coecient.
also that at steady state the
N H3
composition in the extract stream is estimated from
the following expression (obtained from a mass balance)
xout =
where
Vout
yinVin − youtVout
Lout
(30)
is obtained from yet another mass-balance calculation as
Vout =
1 − yin
Vin
1 − yout
(31)
l Assumptions
m The N H3/N2
mixture behaves as an ideal-gas mixture
m The
solvent stream contains no absorbed
m The
extract stream contains no absorbed nitrogen
N H3
on inlet to the column
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EXPERIMENTAL DETAILS
l Example
of a data record
m Consider
Run
recording your data in a table
T
similar
QRT M,f eed QRT M,solvent
to the one shown below
yin
yout
∆P
···
1
2
3
4
5
..
.
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18
EXPERIMENTAL PROCEDURES
18.1 Start-up and normal-operation procedures
1. Plug the power cable of the
N H3
sensor into the outlet.
2. Open the valve for the water outlet (extract) line.
3. Open fully the feed-gas cylinder (N2 +
N H3).
Set the regulator pressure to the desired
setting. (Note: the pressure should not exceed 40 psi)
4. Adjust the rotameters to allow the desired ow of feed gas ow into the column.
5. Open the water inlet valve and adjust the rotameters to obtain the desired solvent (water)
ow into the column. DO NOT allow water into the column when the feed ow rate is zero,
as water might enter into the feed gas line until it reaches and damages the
N H3
sensor.
6. Switch on the dierential pressure gauge to measure the pressure drop across the column.
7. Open the appropriate sensor gas-valves to measure the concentration of
N H3
in either the
feed stream or the ranate stream.
8. During operation always maintain the water level at the bottom of the column below the
feed-gas inlet to prevent feed gas escaping the column through the extract-stream opening.
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EXPERIMENTAL PROCEDURES
18.2 Shut-down procedure
1.
Turn o the solvent ow into the column by closing the water valve completely.
2.
Close completely the valve of the feed-gas cylinder (N2 +
Important note:
3.
N H3).
DO NOT turn o the feed gas before turning o the water.
Wait for the feed-gas and water ow into the column to go to zero on the rotameter
scales; then turn o the rotameters.
(Closing the inlet valves of water and feed gas before turning o the rotameters helps
to release the pressure in the inlet lines in shut-down mode)
4.
Switch o the pressure gauge.
5.
Unplug the Ammonia sensor from the power outlet.
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19
ANTICIPATED EXPERIMENTAL PROBLEMS
l Incorrect
l Not
start-up sequence (creates the wrong dispersed phase)
waiting suciently for steady-state conditions
l Experiments
l The
may not have been carried out at isothermal conditions
feed gas may escape through the extract outlet when a small amount of water level is
not maintained at the extract outlet
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20
OBJECTIVES
NOTE: Address ONLY the objectives identied by the instructor (ignore the rest)
l Objective
1
m Characterize the ooding condition of the column at each liquid ow rate by determining
the ooding gas ow rate. Plot the ooding gas ow rate as a function of (a) liquid ow
rate, (b) the liquid-to-gas molar ow ratios, and (c) the absorption factor
l Objective
2
m Determine
the hold-up time and the number of hold-up times needed to achieve steady-
state as a function of absorption factor
l Objective
A
A.
3
m Characterize the dependence of NTUs and HTUs on the absorption factor A:
(a) Plot the
NTU and HTU results as a function of A, (b) Plot the natural logarithm of the NTU and
HTU results as a function of
A.
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OBJECTIVES
l Objective
4
m Characterize
n Find
the mass transfer coecient
a correlation for the mass transfer coecient and verify the correlation using
additional experimental test
n Establish the dependence of the mass transfer coecient on the absorption factor A: (a)
Plot the mass transfer coecient as a function of A, (b) Plot the natural logarithm of
mass transfer coecient as a function of A. Superimpose on these plots the correlation
curve
l Objective
m Plot
the the scrubbing eectiveness as a function of A as a function of the NTUs.
l Objective
m Plot
5
6
the ranate and the extract compositions as a function of A
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REFERENCES
[1] Hodgman, C. D., Weast R. C., and Selby, S. M., editors,
CRC Handbook of Chemistry and
Physic s, 42nd edition. CRC Press, Cleveland Ohio, (1961).
[2] Geankoplis, C. J.,
Transport Processes and Unit Operations, Third Edition. Prentice-Hall Inc.,
Englewood Clis, NJ (1990). (Chapter 10)
[3] McCabe, W. L., J. C. Smith, and P. Harriet,
Unit Operations of Chemical Engineering, Fifth
Edition. McGraw-Hill, Inc., New York, NY (1993). (Chapter 22)
[4] Foust, A. S., L. A. Wenzel, C. V. Clump. L. Maus. and L. B. Anderson,
Operations. John Wiley & Sons, New York, 1960. page 552.
Principles of Unit
[5] Onda, K., Takeuchi, H., and Okumoto, Y, Mass transfer coecients between gas and liquid
phases in packed columns,
[6] Treybal, R. E.,
Journal of Chemical Engineering of Japan, Vol 1, pp. 5662 (1968).
Mass Transfer Operations, 2nd. ed., McGraw-Hill, New York (1968).
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