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Ammonia Gas Absorption
by
Oscar D. Crisalle
Professor
Chemical Engineering Department
University of Florida
Revision 12: September 24, 2013
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Ammonia Absorption Rev 08 - 04/15/2013 Page 1
CONTENTS
1 Introduction 4
2 Experiment 1: Absorption of Ammonia (NH3) 5
3 Operational Information 6
4 Thermodynamic Phase-Equilibrium 9
5 Henry's Law 11
6 Gas Densities 12
7 Colburn's NTU Equation 137.1 Number of transfer units for gas-phase controlled transfer . . . . . . . . . . . . . . . . . . . . . . 137.2 Height of a transfer unit for gas-phase controlled transfer . . . . . . . . . . . . . . . . . . . . . . . 147.3 Remarks on the NTU Equation (7) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8 Overall Mass Transfer Coe�cient 15
9 Interpretation of the Absorption Factor 16
10 Interpretation of NTU and HTU 17
11 Characteristics of Flow in the Column 1811.1 Hold-up time (also called residence time) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1811.2 Number of hold-ups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
12 Scrubbing E�ectiveness 19
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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13 Theoretical Expectations 21
14 Measurement of NH3 Gas Compositions 22
15 Rotameter: Water Flow Measurement 2315.1 Liquid solvent rotameter (RTM): water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2315.2 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
16 Rotameters: Gas Flow Measurements 2516.1 Gas feed-line rotameters (RTM): (NH3 +N2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2516.2 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
17 Experimental Details 27
18 Experimental Procedures 3018.1 Start-up and normal-operation procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3018.2 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 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
l The 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 (NH3)
Counter-current absorption packed tower
Ammonia
Sensor
Rotameter
Rotameter
Solvent (W)Raffinate (N2 + NH3)
Feed (N2 + NH3) Extract (W + NH3)
Lout, xout
Lin, xinVout, yout
Vin, yin
h
Nomenclature
l Solute: Ammonia (NH3)
l Feed Carrier Gas: Nitrogen (N2)
l Solvent: Water (W )
Thermodynamic Equilibrium
y = mx*
Equilibrium N2 + NH3
W + NH3
y
x*
Feed-solvent Phase
(or raffinate phase)
Extract-solvent Phase
(or extract phase)
N2 + NH3
W
Assumption: Nitrogen is insoluble in W. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 OPERATIONAL INFORMATION
l Column available
m Height of the column : 800 mm
m Internal diameter: 100 mm
m Type of packing: Standard 6 mm Raschig rings
l Density
m The liquid water stream has a higher density than the N2 +NH3 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 are making the assumption that the solvent (W ) is nonvolatile at the temperature of
the experimental conditions
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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OPERATIONAL INFORMATION
l Continuous and dispersed phases
m Two 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 de�nes
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 su�cient 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 at 0% �ooding
m Start gas �ow
m Set 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 NH3 from the gas phase (Nitrogen + NH3) absorbs into the liquid phase (Water +
NH3) establishing a phase equilibrium after a su�ciently long time
x∗ mole fraction of NH3 in the liquid phase at equilibrium
y mole fraction of NH3 in the gas phase at equilibrium
l The equilibrium mole-fraction (x∗) of absorbed NH3 is known as the solubility of NH3 in
water
N2 + NH3
W + NH3
y
x*
Gas Phase
Liquid Phase
P
T
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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THERMODYNAMIC PHASE-EQUILIBRIUM
l The solubility of NH3 in water is high at room temperature and 1 atm of pressure
m The solubility increases with pressure and decreases with temperature
l It is possible to derive a relationship relating x∗ to y, but we are mostly interested in cases
of low values of y (use of dilute mixture of NH3 and nitrogen)
m Focus: 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 (x∗)
is proportional to the partial pressure (yP ) of the solute gas in the gas phase: i.e,
yP = H(T )x∗ (1)
or
y = mx∗ (2)
where
m =H (T )
P(3)
where P is the operating pressure of the column and
H (298.15) = 0.885 atm
l Valid only for dilute solutions and when the solute (NH3) does not react with the solvent
(W )
l Temperature dependence is given by the van't Ho� equation
H (T ) = H(T ref
)exp
[−C
(1
T− 1
T ref
)](4)
and for NH3/water 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 = 20C = 68F = 293.15K P = 1atm = 1.01325 bar = 14.696 psi
m IUIPAC-STP: International Union of Pure and Applied Chemistry
T = 0C = 32 = 273.15K P = 0.9869 atm = 1bar = 14.504 psi
m Gas rotameter manufactures usually use di�erent standards. Refer the instrument manual
for details.
l Density Models
m Density of dry air (model using the speci�c air constant)
ρAir =P
RSpecificAir T
(5)
where P is the feed gas pressure and
RSpecificAir = 286.689 J/ (kg ·K) = 2.829× 10−3m3 · atm/ (kg ·K)
m Density of ammonia gas
ρNH3≈ MWNH3
MWAirρAir =
17.031
29ρAir = 0.587ρAir (6)
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7 COLBURN'S NTU EQUATION
7.1 Number of transfer units for gas-phase controlled transfer
NOG =A
1− Aln
mxin − youtmxin − yin
1− 1
A
(1− mxin − yout
mxin − yin
) (7)
and
A =L′inmV ′in
(8)
where
yout solute mole fraction in the ra�nate [dimensionless]
yin solute mole fraction in the feed [dimensionless]
xin solute mole fraction in the solvent [dimensionless]
A absorption factor [dimensionless]
V ′in = Vin/Across feed molar super�cial velocity[lbmole/
(min · ft2
)]L′in = Lin/Across solvent molar super�cial velocity
[lbmole/
(min · ft2
)]m equilibrium constant for dilute solution [dimensionless]
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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COLBURN'S NTU EQUATION
7.2 Height of a transfer unit for gas-phase controlled transfer
HOG =h
NOG(9)
l h is the height of the packed bed [ft]
7.3 Remarks on the NTU Equation (7)
l Only valid for dilute feed streams
l It is assumed that the solute mole fraction in the solvent is zero, i.e., xin = 0
l Number of transfer units is expressed in terms of concentration in the gas phase
m Solubility of ammonia in water is high
l As a result, the dominant resistance to di�usion (mass transfer) resides within the gas
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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8 OVERALL MASS TRANSFER COEFFICIENT
l Overall mass transfer coe�cient on a gas-phase basis
Kya =Vin
HOGAcross=
1
HOGV ′in (10)
m Interfacial area per unit volume of packing, a, is normally left lumped with the mass
transfer coe�cient (De�nition: Ainterfacial = aAcross)
l Mass-transfer resistance: inverse of the mass-transfer coe�cient
Resistance =1
Kya(11)
l Correlation (Solve using least-squares regression)
Kya = c1Lc2inV
c3in =⇒ ln (Kya) = c4 + c2 lnLin + c3 lnVin (12)
where c4 = ln c1
m See 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 Veri�cation of correlation
n Carry out at least one additional experimental run
Error =
∣∣(Kya)correlation − (Kya)run∣∣
(Kya)run100% (13)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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9 INTERPRETATION OF THE ABSORPTION FACTOR
l The Absorption Factor A is de�ned 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=
(L′in/V′in)
m=
L′inmV ′in
l For the transfer of NH3 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− mxin − youtmxin − yin
m Hence, the absorption of NH3 from the gas phase into the liquid phase occurs only when
the above condition on A is met.
l Observations
m When A < 1 − mxin − youtmxin − yin
mass transfer occurs from the liquid phase into the gas phase
(desorption or stripping)
m When A = 1− mxin − youtmxin − 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 yout desired for a given yin
m Measure of the di�culty of separation
m If 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 coe�cient and the gas �ow rate
m Measure of the separation e�ectiveness of the packing for the species being absorbed
m HTU is proportional to the resistance to mass transfer
HOG =1
Kya
VinAcross
(14)
m HTU is small (lower resistance) when
n There is a high rate of interface mass transfer
n There 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 =VPackQWater
(15)
thold−up hold-up time (residence time) (min)
QWater water (solvent) �ow rate (GPM)
VPack packed volume (gal)
11.2 Number of hold-ups
Nhold−up =tSS
thold−up(16)
Nhold−up number of hold-ups (dimensionless)
tSS time to steady-state (min)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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12 SCRUBBING EFFECTIVENESS
l De�nition of Scrubbing E�ectiveness
ε :=Overall rate of NH3absorption into the liquid solvent
Rate of NH3entering via the feed stream(17)
l Formula Derivation: De�ne
Y =youtyin
(18)
and use the mass-balance result Vout =1− yin1− yout
Vin:
ε =yinVin − youtVout
yinVin=
yinVin − yout(
1− yin1− yout
Vin
)yinVin
=yin (1− yout)− yout (1− yin)
yin (1− yout)=
yin − youtyin (1− yout)
=
1− youtyin
1− yinyoutyin
=1− Y
1− yinY
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ammonia Absorption Rev 08 - 04/15/2013 Page 19
l Calculation
m 1. From experimental data
Information needed: yin and youtProcedure:
n Calculate
Y exp =youtyin
(19)
n Calculate
εexp =1− Y exp
1− yinY exp(20)
m 2. From NTU predictions
Information needed: yin, m, Vin, Lin, and NOG
Procedure
n Calculate A and �nd the value of Y by solving (graphically or numerically) from
NOG =A
A− 1ln
1 +1
Y(A− 1)
A(21)
n Calculate
εpred =1− Y
1− yinY(22)
m 3. Prediction error
n Calculate the prediction error PE
PE =
∣∣εpred − εexp∣∣εexp
100% (23)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ammonia Absorption Rev 08 - 04/15/2013 Page 20
13 THEORETICAL EXPECTATIONS
l The mass transfer should increase for larger Lin/Vin ratios
l The mass transfer should more strongly a�ected by the gas-feed �ow rate (Vin) than by the
solvent �ow rate (Lin)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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14 MEASUREMENT OF NH3 GAS COMPOSITIONS
l BACHARACH Ammonia gas 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 Accuracy: ±10 ppm ± 10% of reading
from 0− 1000 ppm
l Response Time: 9 to 30 seconds, depend-
ing on tube length and gas concentration
l Operating Temperature: 32 to 122°F (0
to 50°C)
l Operating Humidity: 5 to 90% RH, non-
condensing
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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15 ROTAMETER: WATER FLOW MEASUREMENT
15.1 Liquid solvent rotameter (RTM):
water
l Dwyer Rate-Master Flowmeter: Model
RMC
l 2 rotameters (coarse and �ne adjust-
ments)
l Measurement units
m Coarse: Gallons per Minute (GPM)
m Fine: Gallons per Hour (GPH)
Coarse Fine
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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ROTAMETER: WATER FLOW MEASUREMENT
15.2 Measurements
l Reading: QRTM,solvent (graduation mark on the scale)
l For �ne rotameter
Qsolvent(GPH) = QRTM,solvent (24)
l For coarse rotameter
Qsolvent(GPH) =60 min
1 hrQRTM,solvent (25)
l Mass �ow rate
Qsolvent(lb/hr) = ρsolventQsolvent(GPH) (26)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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16 ROTAMETERS: GAS FLOW MEASUREMENTS
16.1 Gas feed-line rotameters (RTM):
(NH3 + N2)
l Dwyer Rate-Master Flowmeter: Model
RMB
l 2 rotameters (coarse and �ne adjust-
ments)
l Measurement units: �Standard� Cubic
Feet per Hour (SCFH)
Coarse Fine
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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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: RRTM (�oat position on the scale)
Qfeed(SCFH) = RRTM
√PfeedPref
Tref(K)
Tfeed(K)(27)
Qfeed(lb/hr) = fSCFH→CFH ρfeed Qfeed(SCFH) (28)
where the conversion factor fSCFH→CFH is
fSCFH→CFH =PrefPfeed
Tfeed(K)
Tref(K)(29)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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17 EXPERIMENTAL DETAILS
l Because NH3 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 the NH3 composition in the feed and ra�nate stream using the sensor.
m Take 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 rotame-
ters 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 the column at various values of the absorption coe�cient.
l Note also that at steady state the NH3 composition in the extract stream is estimated from
the following expression (obtained from a mass balance)
xout =yinVin − youtVout
Lout(30)
where Vout is obtained from yet another mass-balance calculation as
Vout =1− yin1− yout
Vin (31)
l Assumptions
m The NH3/N2 mixture behaves as an ideal-gas mixture
m The solvent stream contains no absorbed NH3 on inlet to the column
m The extract stream contains no absorbed nitrogen
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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EXPERIMENTAL DETAILS
l Example of a data record
m Consider recording your data in a table similar to the one shown below
Run T QRTM,feed QRTM,solvent 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 NH3 sensor into the outlet.
2. Open the valve for the water outlet (extract) line.
3. Open fully the feed-gas cylinder (N2 + NH3). 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 NH3 sensor.
6. Switch on the di�erential pressure gauge to measure the pressure drop across the column.
7. Open the appropriate sensor gas-valves to measure the concentration of NH3 in either the
feed stream or the ra�nate 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 + NH3).
Important note: DO NOT turn o� the feed gas before turning o� the water.
3. 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 start-up sequence (creates the wrong dispersed phase)
l Not waiting su�ciently for steady-state conditions
l Experiments may not have been carried out at isothermal conditions
l The 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 identi�ed 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 A
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 A.
l Objective 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 the mass transfer coe�cient
n Find a correlation for the mass transfer coe�cient and verify the correlation using
additional experimental test
n Establish the dependence of the mass transfer coe�cient on the absorption factor A: (a)
Plot the mass transfer coe�cient as a function of A, (b) Plot the natural logarithm of
mass transfer coe�cient as a function of A. Superimpose on these plots the correlation
curve
l Objective 5
m Plot the the scrubbing e�ectiveness as a function of A as a function of the NTUs.
l Objective 6
m Plot the ra�nate and the extract compositions as a function of A
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ammonia Absorption Rev 08 - 04/15/2013 Page 34
REFERENCES
[1] Hodgman, C. D., Weast R. C., and Selby, S. M., editors, CRC Handbook of Chemistry and
Physics, 42nd edition. CRC Press, Cleveland Ohio, (1961).
[2] Geankoplis, C. J., Transport Processes and Unit Operations, Third Edition. Prentice-Hall Inc.,
Englewood Cli�s, 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, Principles of Unit
Operations. John Wiley & Sons, New York, 1960. page 552.
[5] Onda, K., Takeuchi, H., and Okumoto, Y, Mass transfer coe�cients between gas and liquid
phases in packed columns, Journal of Chemical Engineering of Japan, Vol 1, pp. 56�62 (1968).
[6] Treybal, R. E., Mass Transfer Operations, 2nd. ed., McGraw-Hill, New York (1968).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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