GAS ABSORPTION
Kenneth M. Soriano, EIT, AMIChemE
GAS ABSORPTION
Gas Absorption (Gas Scrubbing, Gas Washing) – a
gas mixture is contacted w/ a liquid (absorbent or
solvent) to selectively dissolve on or more
components (absorbate or solute) by mass
transfer from the gas to the liquid.
Stripping (Gas Desorption) – a liquid mixture is
contacted w/a gas to selectively remove
components by mass transfer from liquid to the
gas.
HENRY’S LAW
• At a constant T, the amount of a given gas that
dissolves in a given type and volume of liquid
is directly proportional to the partial pressure
of that gas in equilibrium with that liquid.
• The solubility of a gas is directly proportional
to the pressure of the gas.
• Has since been shown to apply for a wide
range of DILUTE solutions.
APPLICATION
• Separate gas mixture
• Remove impurities, contaminants, pollutants
or catalyst poison
• Recover valuable chemicals
TWO TYPES OF ABSORPTION/STRIPPING
Physical Absorption – no significant chemical
reactions occur between the absorbent and the
solute.
Ex.
Solute
Absorbent
Acetone
Water
Ammonia
Water
Ethanol
Water
HCl
Water
TWO TYPES OF ABSORPTION/STRIPPING
Chemical Absorption (Reactive Absorption) – absorption
is accompanied by a rapid and irreversible (sometimes
reversible) reaction in liquid phase.
Ex.
Solute
CO2
HCl
NOX
Cl2
Absorbent
Aq. NaOH
Aq. NaOH
Water
Water
Irreversible
Irreversible
Reversible
Reversible
EQUIPMENT
•
•
•
•
•
Trayed Towers (Plate Columns)
Packed Columns
Spray Towers
Bubble Columns
Centrifugal Contactors
Trayed Towers & Packed Columns
Spray Towers & Bubble Columns
Centrifugal Contactors
TRAY TOWERS
• Vertical, cylindrical pressure vessels in w/c
vapor and liquid, w/c flows countercurrently,
are contacted on a series of trays or plates.
• Liquid flows across each tray, over an outlet
weir, and into a downcomer, w/c takes the
liquid by gravity to the tray below.
• Gas flows upward through the openings in
each tray, bubbling through the liquid on the
tray.
TRAY TYPES
• Sieve Tray (Perforated Tray) – trays w/ 1/8 to
½ in. – diameter holes
TRAY TYPES
Valve Tray – has holes commonly from 1 to 2
in. in diameter. Each hole is filled w/ a valve
that consists of a cap, w/c overlaps the hole,
w/ legs or a cage to limit the vertical rise while
maintaining the horizontal location of the
valve. With no vapor flow, each valve sits on
the tray, over a hole. As the vapor rate is
increased, the valve rises, providing a larger
and larger peripheral opening for vapor to
flow into the liquid.
TRAY TYPES
• Valve Tray
TRAY TYPES
Bubble-Cap Tray – has bubble caps that
consist of a fixed cap, 3 to 6 in. in diameter,
mounted over and above a concentric riser of
2 to 3 in. in diameter. The cap has rectangular
or triangular slots cut around its side. The
vapor flows up through the tray opening into
the riser, turns around, and passes out
through the slots of the cap and into the
liquid.
TRAY TYPES
• Bubble-Cap Tray
Parts of Tray Column
Flow Patterns
Multipass Crossflow Trays
Size Comparison
An 11- ft diameter tray might have
500
1000
50,000
4 -in. diameter bubble caps
2 - in. diameter valves
3/16 - in. diameter holes
COMPARISON
Sieve
Valve
Bubble
Relative Cost
1.0
1.2
2.0
Pressure Drop
Lowest
Intermediate
Highest
Efficiency
Lowest
Highest
Highest
Vapor Capacity
Highest
Highest
Lowest
2
4
5
Turndown Ratio
(ratio of maximum
& minimum Vapor
Capacity)
Weeping and Flooding
• Weeping – low vapor flow rate, high liquid
flow rate
Weeping and Flooding
• Flooding – high vapor flow rate, low liquid
flow rate
GENERAL DESIGN CONSIDERATIONS
Design or analysis of an absorber/stripper requires
consideration of a number of factors.
A) Entering gas (liquid) flow rate, composition,
temp & pressure
B) Desired degree of recovery of one or more
solutes
C) Choice of absorbent
D) Operating T,P and allowable gas pressure drop
E) Minimum absorbent flow rate and actual
absorbent flow rate as a multiple of the minimum
rate needed to make the separation (usually from
1.4 - 1.5 of the minimum)
GENERAL DESIGN CONSIDERATIONS
F) Number of equilibrium stages & stage
efficiency
G) Heat effects & need for cooling/heating
H) Type of absorber/stripping equipment
I) Height of absorber/stripping equipment
J) Diameter of absorber/stripper
Choice of Absorbent
Should
• Have high solubility for the solute
• Have low solubility
• Be Stable
• Non-corrosive
• Low Viscosity
• Non Foaming
• Be non toxic and nonflammable
• Be available
Calculation Procedures for
Trayed Towers
• Graphical
• Analytical
MATERIAL BALANCE
Convert to
WET BASIS
Convert to
DRY BASIS
Convert to
DRY BASIS
Convert to
WET BASIS
Definition of Terms
Definition of Terms
Definition of Terms
Overall Material Balance
Example 1
• The gas mixture enters the absorber at 25%
SO2. What is the composition of the mixture
on solute free basis?
a) 0.3333
b) 0.6667
c) 0.25
d) 0.3906
Example 2
The mole fraction of solute entering a gas
absorber is 0.25. If the percentage recovery is
75%, then the mole fraction of the solute
leaving the tower will be
a) 0.0833
b) 0.0625
c) 0.0769
d) 0.157
Example 3
In a carbon dioxide absorber using carbonate
solution as absorbent, 500 kgmoles/hr of flue gas
containing 45% carbon dioxide by volume, the
balance being an inert gas, is fed into the column.
The lean gas leaves the absorber with 3.5%
carbon dioxide by volume. The quantity of carbon
dioxide absorbed by the carbonate solution in
kgmol/hr is
a) 251
c) 215
b) 125
d) 152
Example 4
A continuous absorption tower is used for absorbing
HCl in water. Two hundred lbmoles per hour of a
gaseous mixture containing 5 mol of HCl per 2 mol of
air enters the bottom of the tower, and 15000 lb of
pure water enters the top of the tower per hour. The
aqueous HCl solution leaving the bottom of the tower
contains 1 mol of HCl for every 7 mol of water.
Assuming no water is vaporized in the tower, how
many moles of HCl per mole of air in the exit gas
stream are present?
a) 0.103
c) 0.417
b) 0.310
d) 0.184
Graphical Method
• There are three components. Since graphical
solution is only for binary systems, we have to
neglect the presence of one of the three
components (solute)
Operating Line
• OMB:
• Operating Line:
Gas Absorber
Gas Desorber (Stripper)
Equilibrium Curve
• Experimental Data = Solubility Data
• Raoult’s Law
0
0.1
0.2
…
…
…
1
Equilibrium Curve
• Henry’s Law
0
0.1
0.2
…
…
…
1
Equilibrium Curve
• K-Value
0
0.1
0.2
…
…
…
1
Equilibrium Curve
• K-Value (DePriester Chart Fig. 13-9 ChE HB 8th
ed.)
Gas Absorber
Limiting Liquid to Gas Ratio
Gas Absorber
Limiting Liquid to Gas Ratio
Example 5
• A 1000 lb/hr mixture of acetone in air (2.5 mole %
acetone) is to be treated in a countercurrent gas
absorber, which is 1.4 ft in diameter and packed with
Raschig Rings. The equilibrium equation is ya = 2.53xa
(xa is mole % acetone in water and ya is mole % acetone
in air). Assume mass rate of lean gas mixture remains
unchanged. What is the minimum rate (lb/hr) of water
flow (acetone free) for 98% recovery of acetone?
a) 1128.33
c) 1894.14
b) 1233.82
d) 1486.6
Example 6
Example 6
Tiller-Tour Equation
Analytical Solution (Kremser Equation)
• Absorber
Kremser Equation
• CMB:
Kremser Equation
Kremser Equation
• CMB:
Kremser Equation
• CMB:
Kremser Equation
Kremser Equation
Analytical Solution (Kremser Equation)
• Stripper
Kremser Equation
Kremser Equation
• For Absorption
If
• For Stripping
If
Example 7
In the figure below, the heavier components in a
superheated hydrocarbon gas are removed by
absorption at 400 psia with a high molecular
weight oil. Estimate exit vapor and liquid flow
rates and compositions by the Kremser method,
using estimated component absorption and
stripping factors from the entering values of L
and V, and the component K-values below based
on an average entering temperature of (90 +
105)/2 = 97.5˚F.
Example 7
Packed Column
Vertical, cylindrical pressure vessel containing
one or more sections of a packing material
over whose surface the liquid flows by gravity,
as a film or as droplets between packing
materials. The vapor flows upward through
the wetted packing, containing the liquid.
Channeling
• Liquid flows down the column mainly near the
wall and the gas flow mainly up the center of
the column
Packing Materials
• Random Packing (Dumped Packing)
Ex. Raschig rings & berl saddles – OLD
Pall rings, Bialecki rings, Intalox saddles – NEW
Intalox IMTP, cascade mini-rings-lattice-work
design
• - Size ranges from 1 in. to 3.5 in.
(as the size of the packing material increases, the
mass transfer and pressure drop decreases)
Packing Materials
• Structure Packing (Stacked packing material)
Packing Materials
Packing Materials
Comparison
Design Limitation
• The nature of countercurrent flow imposes an
operational constraint known as FLOODING.
• This situation occurs when the liquid stream is
unable to freely flow down the packed column
by gravity because the upward gas stream
produces a pressure drop sufficient to hold up
the liquid.
Design Limitation
• When the liquid flow rate cannot equal the
feed rate because layer of liquids accumulated
above the packing, then the column becomes
unstable and eventually becomes flooded,
rendering the packed tower to inoperable.
• To ensure proper operating conditions, an
empirical flooding correlation is extensively
made for gas-liquid continuous contacting in
packed column.
Design Limitation
• The maximum allowable gas velocity per unit
cross sectional area of the tower
corresponding to the incipient flooding
characteristics depends on a number of
factors:
a) Packing geometry
b) Physical properties of fluid streams
c) gas-liquid flow ratio (or pressure drop)
CHANNELING
• Channeling – the condition that occurs
especially at low liquid flow rates, when much
of the packing surface is dry or, at best,
covered by a stagnant film of liquid.
- this is the chief reason for the poor
performance of large packed towers.
* In towers of moderate size, channeling can be
minimized by having the diameter of the tower
at least 8 times the packing diameter
FLOODING and LOADING POINT
• Flooding – the condition that occurs mostly at
high gas flows and the liquid can no longer flow
down through the packing and is blown out with
gas.
• Loading Point – the point at which the gas flow
rate starts to hinder the liquid down flow and
local accumulations or pools of liquids start to
appear in the packing (liquid holdup)
* The optimum economic gas velocity is about
one half of the flooding velocity.
Diameter of the Column
For rapid mass transfer between phases,
intimate contact between the gas and the liquid
is desirable. This is made possible if the gas and
the liquid flow through a narrow range inside
the tower.
Height of the Column
The solute inside the column must be allowed
sufficient time to diffuse from the gas into the
liquid. This time depends on the height of the
column.
Pressure Drop in a Packed Tower
Correlations
Generalized Correlation
Generalized Correlation
Alternate Generalized dP Correlation
Example
1700
0.35
0.23 in. H2O/ft
0.0343
0.0346
Equation of Tie-Line
• Equation of Tie-Line
Fixed Cost vs. Direct Cost
• Large L’, good for mass transfer
- Fixed Cost (FC) is small (equipment – Short
Tower)
- Direct Cost (DC) is large (material – amount
of solvent used)
• Small L’, not good for mass transfer
- FC is large (equipment – Tall Tower)
- DC is small (material)
Height of the Absorber, Z
• Gas Phase
• Liquid Phase
Simplifications
• Case 1: For very soluble solute: kxa >>> kya
Slope of Tie-Line:
Tie-Line is VERTICAL
- gas phase resistance is CONTROLLING
- liquid phase resistance is NEGLIGIBLE
Simplifications
• In terms of overall mass transfer coefficient,
Kya and Kxa
Simplifications
• If kxa >>> kya, then Kya
kya
Simplifications
• Case 2: For very slightly soluble solute: kya
>>> kxa; Slope of Tie-Line is 0 and Tie-Line is
HORIZONTAL
- liquid phase resistance is CONTROLLING
- gas phase resistance is NEGLIGIBLE
- Kxa kxa
Simplifications
• Case 3: If the gas mixture is DILUTE (or LEAN),
then (1-y) 1, then
Simplifications
• And if E.C. is linear, then
Simplifications
• Case 4: If the gas mixture is concentrated (y is
large) and if both E.C. and O.L. are linear w/in
limits.
Rate Based Method for Packed Columns
Rate Based Method for Packed Columns
Rate Based Method for Packed Columns
To avoid the need to determine the composition at the
interface between the 2 phases, overall, volumetric
mass transfer coefficients can be defined in terms of
overall driving forces for either the gas of liquid phase.
HEIGHT OF PACKING MATERIAL
Usually based on Kya because liquid usually has
a strong affinity for the solute so that resistance
to mass transfer is mostly in gas
HEIGHT OF PACKING MATERIAL
HEIGHT OF PACKING MATERIAL
• If
HEIGHT OF PACKING MATERIAL
HEIGHT OF PACKING MATERIAL
EXAMPLE 8
Repeat the ethyl alcohol absorption example.
This time use packed tower filled with 1.5 in.
metal pall rings. If HOG = 2.0 ft, compute the
required packed height.
EXAMPLE 9
Experimental data have been obtained for air
containing 1.6% by volume SO2 being scrubbed with
pure water in a packed column of 1.5 m2 in cross
sectional area and 3.5 m in packed height. Entering
gas and liquid flow rates are 0.062 and 2.2 kmol/s,
respectively. If the liquid outlet mole fraction of SO2
in the gas is 0.004 and column temperature is near
ambient with KSO2 = 40, calculate from data:
a) NOG for absorption of SO2
b) HOG in meters
EXAMPLE 10
A gaseous reactor effluent consisting of 2 mole %
ethylene oxide in an inert gas is scrubbed with
water at 30 deg C and 20 atm. The total gas feed
rate is 2500 lbmol/h. The column, with a diameter
of 4 ft, is packed in two 12 ft. high sections with 1.5
in. metal pall rings. A liquid redistributor is located
between the two packed sections. Under the
operating conditions for the scrubber, the K-value
for ethylene oxide is 0.85 and estimated values of
kya and kxa are 200 lbmol/h-ft2-∆y and 165
lbmol/h-ft2-∆x respectively. Calculate Kya and HOG.
Number 1
For moderately soluble gases with relatively
little intersection between the gas and liquid
molecules, the law which applies most to
establish equilibrium relationship is
A) Raoult’s Law
B) Henry’s Law
C) Dalton’s Law
D) None of these
Number 2
In the choice of solvent for gas absorption, one
of the properties to be considered is
A) Solvent should have a low vapor pressure
B) Solvent should have low boiling point
C) Solvent should be highly viscous
D) Solvent should be unusual
Number 3
In the case of a straight or concave downward operating
line and a straight or concave upward equilibrium line,
the minimum liquid rate is that at which
A) The operating line just touches the equilibrium line at
the bottom of the tower.
B) The operating line just touches the equilibrium line at
the top of the tower.
C) The operating line intersects the equilibrium line at
the middle of the tower.
D) The slope of the operating line is smaller than the
equilibrium line
Number 4
The use of a liquid rate far above the minimum
value will result in
A) Infinite number of plates
B) Small number of plates
C) Minimum number of plates
D) None of these
Number 5
When dilute solutions are involved in gas
absorption, one of the following is not applicable
A) Henry’s Law
B)
C) Equilibrium and operating line are straight
D)
may be used
Number 6
In gas absorption, packings are used
A) To reinforce the absorption tower due to drag
created by the liquid and gas
B) To disseminate the heat generated to
maintain isothermal conditions
C) To increase the area of contact between the
gas and the liquid
D) To reduce the pressure drop created by the
flowing gas.
Number 7
When the liquid flow rate is fixed and the
composition of the liquid and gas at the bottom
of the absorption tower are fixed, the limiting
flow rate to give infinitely tall tower becomes
A) The maximum flooding velocity of the vapor
B) The minimum loading velocity of the liquid
C) The maximum flow rate of the vapor
D) The minimum flow rate of the vapor
Number 8
The rate equation in terms of the gas-phase mass
transfer coefficient is used in design calculations
instead of that in terms of the liquid phase when
A) The solubility of the solute in the solvent is very
low
B) The controlling resistant is that of the liquid
phase
C) The controlling resistant is that of the gas phase
D) The tie line is horizontal
Number 9
It is not very necessary to correct the rate
equation for the phase drift encountered during
gas absorption, since the height calculated will
be
A) Slightly over-designed anyway
B) Slightly under designed anyway
C) Based on equimolal counter diffusion anyway
D) Based on the same percentage recovery
anyway
Number 10
The overall height of a transfer unit based on
the gas phase will equal the gas phase individual
HTU, Hy when
A) The equilibrium curve is linear
B) The operating line is linear
C) The gas phase resistance is very small
compared with the liquid phase
D) The liquid phase resistance is very small
compared with the gas phase
Number 11
In mass transfer, a transfer unit means that
A) A mole of solute is transferred in a unit
height
B) The mass transfer coefficient will approach
unit
C) The average driving force is equal to unity
D) The concentration range is equal to the
average driving force
Number 12
The mole fraction of solute entering a gas
absorber is 0.25. If the percentage recovery is
75%, then the mole fraction of the solute
leaving the tower will be
A) 0.0833
B) 0.0625
C) 0.0769
D) 0.157
Number 13
Assuming that the Penetration Theory applies in
an interfacial mass transfer contact, the liquid
phase mass transfer coefficient will increase by a
factor of ______ if the diffusivity of the solute
becomes twice and the contact time becomes
one-half.
A) 2
C) 1/2
B) 1
D) 4
Number 14
In stripping or desorption operation, the
equilibrium curve is
A) Above the operating line
B) Below the operating line
C) Perpendicular with the tie line
D) Linear
Number 15
Co-current gas-liquid up flow contacting device
is highly characterized by
A) low-gas liquid interaction regime
B) Disperse bubble flow regime
C) Liquid-rich and gas-rich regime
D) Pulsing flow regime
Number 16
Given the individual mass transfer coefficients
and equilibrium equation at 25C, kxa = 60, kya =
15 mol/hr-ft3-mole fraction and x* = 0.3226y,
the overall MTC Kya is ______ mol/hr-ft3-mole
fraction
A) 8.45
C) 7.35
B) 38.62
D) none of these
Number 17
The total weight of a 13-mm ceramic Raschig
rings that can be contained in a cylindrical gas
absorption tower 5 m high and 0.55 m in
diameter is approximately_____ kg.
A) 350
B) 940
C) 1045
D) 200
Number 18
The pressure drop is one of the most important
parameters in the design of two-phase co-current
bed because
A) Pumping cost could be a significant portion of
the total operating cost
B) It may help characterize the hydrodynamics of
the system
C) It may affect the retention of the liquid in the
packed bed
D) All of these
Number 19
Flooding condition is the only consideration that
dictates the minimum column diameter that can
be used for a specified gas absorption in a
packed column.
A) True
B) False
C) Cannot be determined
Number 20
In gas absorption, if the concentrations are
expressed in make ratio, that is, moles solute
per mole of solute-free medium, it is necessary
to assume that the operating line is straight
when the coordinates used are X and Y.
A) True
B) False
Number 21
In packed columns, the pressure drop for
irrigated packings is always greater than the
pressure drop of dry packing because
A) The surface of dry packing is smoother
B) The flow area of dry packing is greater
C) The drag friction is lesser than the skin
friction for dry packing
D) None of these
Number 22
For an absorber (mass transfer from gas to
liquid) the operating lines are always
A) Lies below the equilibrium – solubility curve
B) Lies above the equilibrium – solubility curve
C) Coincides with the equilibrium curve
D) Intersects at one-end on the equilibrium
curve
Number 23
If the solute-free gas flow rate is 500 kg-mole/hr
of 0.10 moles ratio of solute entering the plate
absorber at 1 atm, what is the minimum water
rate, in kgmole/hr, for a 90% recovery if the
vapor pressure of solute is 228 mmHg.
Number 24
The mechanism of mass transfer in a gas
absorber is best described by the
A) Penetration Theory
B) Two-Film Theory
C) Chilton-Colburn Analogy
D) Surface-Renewal Theory
Number 25
A wetted-wall column is usually used to
determine
mass
transfer
coefficient
experimentally because ________ is accurately
known.
A) Concentration gradient of A
B) Mass transfer area
C) Thickness of the laminar films adjacent to the
interface
D) The flow rate of gas and liquid
Number 26
The analogy between mass and heat transfer is
complete when
A) The temperature gradient and concentration
gradient are equal
B) No convective flux or phase drift is involved
C) The resistance to mass and heat transfer are
negligible
D) The momentum transfer rate is negligible
Number 27
In gas absorption packed tower, the most
economical gas velocity is considered to be
________ the flooding velocity.
A) Equal to
B) Twice
C) About half
D) Greater than
Number 28
In gas absorption, when the conditions at the
bottom of the tower are fixed and the gas flow
rate is given, the limiting flow rate based on
equilibrium conditions that will require infinitely
tall tower is
A) Minimum liquid flow rate
B) Reducing vapor flow rate
C) Increasing the diameter of the column
D) Increasing the height of the column
Number 29
When the gas flow rate induces flooding in a
packed column gas absorber, this is best
remedied if the percent recovery of the solute is
to be maintained by
A) Reducing the liquid flow rate
B) Reducing the vapor flow rate
C) Increasing the diameter of the column
D) Increasing the height of the column
Number 30
When the Two-Film theory is applied to gas
absorption, the over-all resistance can be expressed
in terms of the individual resistances in the form
1/Kya = 1/kya + m/kxa. This is possible because of
an important assumption. This assumption is
A) The operating line is linear
B) The tie line has a slope between 0 and 1.0
C) The slope of the equilibrium curve near the tie
line is linear
D) The solute forms an ideal solution with the
solvent
Number 31
It has been determined that the equivalent HTU
and NTU of a packed absorption tower are 0.85
ft and 15, respectively. If the same separation
can be done in a 10-plate absorption tower,
then the HETP is ____ ft.
A) 12.75
B) 1.275
C) 0.567
D) None of these
Number 32
In absorption tower packings are placed inside
to
A) Provide a bigger mass transfer area
B) Strengthen the tower
C) Fill up void spaces
D) Reduce pressure drop across tower
Number 33
In a gas absorption tower, mass transfer occurs
primarily
A) From the liquid phase to the gas phase
B) From the liquid phase to the packing
C) From the gas phase to the liquid phase
D) From the gas phase to the packing
Number 34
It is a unit operation which involves_____ the
mass transfer of soluble materials from the
liquid phase to the gaseous phase.
A) Absorption
B) Drying
C) Desorption
D) Humidification
Number 35
In a gas absorption column, the pressure of the
lean gas is expected to be________ entering the
column
A) Lower than
B) Higher than
C) The same as
D) Double the pressure of the rich gas
Number 36
The method of separating a particular gaseous
component from a mixture of gases due to their
differences in solubility in a liquid phase is
A) Adsorption
B) Absorption
C) Extraction
D) Distillation
Number 37
The mass transfer rate of a material from one phase
to another across an interface is increased by
A) Increasing
B) Decreasing
C) Keeping constant
D) Making zero
the difference between the bulk concentration and
the concentration at the interface between two
phases
Number 38
• The pressure drop along a packed tower is
A) Inversely proportional to the mass flow rate
B) Inversely proportional to the square of the
mass flow rate
C) directly proportional to the mass flow rate
D) directly proportional to the square of the
mass flow rate
Number 39
The interfacial concentration Yi, in a gas absorber
corresponding to y = 0.05 mole fraction if the slope
of operating line id 1.3 with the mole ratios Y1 =
0.08, Y2 = 0.03 and X2 = 0 is ______ mole fraction.
The equation of the equilibrium curve is y = 0.8 x,
the M.T.Cs are kxa = 60 and kya = 15 lbmoles/hr-ft3mole fraction concentrate difference
A) 0.03
C) 0.023
B) 0.064
D) none of these
Number 40
A 25 mm ID wetted wall column is used to
absorb dilute SO2 m/s. If Dv = 0.455 ft2/hr, the
gas film coefficient for the absorption is
A) 0.0134
B) 0.0265
C) 0.0373
D) None of these