CHAPTER ONE
1.0 INTRODUCTION
Absorption is a mass transfer process in which a vapour solute in a gas mixture is
dissolved into a liquid phase which the solute is more or less soluble. An example of
absorption is absorption of the solute ammonia from an air- ammonia mixture by water.
Absorption, in common with distillation, usually use special equipment for bringing gas and
liquid phases into intimate contact.
The gas absorption unit in this experiment is meant to demonstrate the absorption of
CO2 into water in a packed column. The gas and liquid normally flow counter-currently, and
the packing serve to provide the contacting and development of interfacial surface through
which mass transfer takes place. The gas absorption is also designed to operate at
atmospheric pressure in a continuous operation.
A common apparatus used in gas absorption and certain other operations is the packed
tower. The device consists of a tower, equipped with a gas inlet and distributing space at the
bottom; a liquid inlet and distributor at the top; gas and liquid outlet at the top and bottom,
respectively; and a supported mass of inert solid shapes, called tower packing. There are
many types of random packing available, for example Ceramic Ball saddle and the most
common is the Raschig ring.
These packing are used to increase the surface area of contact between the gas and the
liquid absorbent. In a packed tower, there is a limit to the rate of gas flow which is called as
flooding velocity. The tower cannot operate if it exceeds this limit. At loading point, which is
the point in which the droplets of liquid are carried up with the gas in packed column, the gas
start to prevent the liquid from flowing down, and thus, pools of liquid start to appear in the
packing.
Fig 1.1 Diagram of a Gas Absorption Column
Page | 1
1.1 AIMS & OBJECTIVES
 To calculate the rate of absorption of carbon dioxide into water from analysis
of liquid solutions flowing down absorption column.
1.2 SIGNIFICANCE/IMPORTANCE OF GAS ABSORPTION COLUMN
 To remove contaminants from gas streams.

It aims at separation of acidic impurities from mixed gas streams.
1.3 LIMITATIONS OF GAS ABSORPTION COLUMN


We assume every other component in air is non-reactive.
We assume they non-volatile (it does not evaporates quickly).
Page | 2
CHAPTER TWO
2.0 BACKGROUND THEORY
Gas absorption is mass transfer operation where one or more species is removed from
a gaseous stream by dissolution in a liquid. The component that is extracted from the gaseous
stream is known as solute and the component that extracting the solute is known as solvent.
Carrier gas is the insoluble component present in the gas that is not absorbed by the solvent.
The transfer is based on the preferential solubility of solutes in the solvent (Gas Absorption
and Desorption, n.d.).
Packed towers are used for continuous counter-current contacting of gas and liquid in
absorption (Geankoplis, 1993).The mechanism in packed tower is the gas and liquid phase
flows counter – currently where they interact on the packings interface. The liquid flows in
downward direction, over the surface of the packing, whereas the gas flows through the space
or voids of the packings in upward direction. The gas flow is driven by pressure while the
liquid flow is driven by the gravity force. The gas undergoes pressure drop due to the liquid
occupied some part of the open space and voids of the packing. Thus, reducing the area
available for the gas to flow. If the packing is dry with no liquid feed, then maximum flow
gas is available. The pressure drop increases as the liquid flow rate into the tower increases.
High flux will resulting in flooding. This occurrence happen at the upper limit of the
gas flow rate called flooding velocity since the liquid is blown out with the gas at the
flooding point. The gas start to hinder the liquid flow at loading point where accumulation of
liquid start appearing in the packing. Low flux will resulting in channelling or weeping,
There are two types of packings types which is random and structured.
Fig 2.1 Typical Packed Tower Packings: (a) Raschig ring, (b) Lessing ring, (c) Berl Saddle,
(d) Pall Ring (Geankoplis C. J., 1993)
One of the oldest specially manufactured types of random packings are Raschig rings
and still in general use. (Separation Columns (Distillation, Absorption and Extraction)) They
provide a large surface area within the volume of the column for the interaction between liquid
and gas. They also enhance the contact time between liquid and gas. (iitb.vlab.co.in, 2011).
Page | 3
Fig 2.2 Generalized Correlation for Pressure Drop in Packed Columns
(Eckert, Chem. Eng. Prog., 66(3), 39 (1970)
y-axis =
x-axis =
𝑮𝟐𝒚 𝑭𝒑 𝑽𝟎.𝟏
𝒙
𝒈𝒄 (𝝆𝒙 −𝝆𝒚 )𝝆𝒚
𝑮𝒙
𝑮𝒚
√𝝆
𝝆𝒚
𝒙 −𝝆𝒚
Where,
2
Gy
= Gas Mass Velocity, kg/m s
2
Gx
= Liquid Mass Velocity, kg/m s
3
𝜌𝑦
= Density of Gas, kg/m
3
𝜌𝑥
= Density of Liquid, kg/m
-1
Fp
= Packing Factor, m
2
Vx
= Kinematic Viscosity, m /s
gC
= Gravitational
Constant,
In the packed column, the gas and liquid streams are counter-currently run, with
liquid running down between the packings and gas flowing up through the wetted packings,
making contact with the liquid along the way. The gas contains the solute, the component to
be absorbed by the liquid, the absorbent. One phenomena that may occur in tall towers is
called channelling, in which liquid flows down closer to the walls and gas flows up through
Page | 4
the centre. This reduces the efficiency of mass transfer because there is less contact between
the two phases, (Seader 209).
Under the assumptions that diffusion controls the mass transfer and that there is no
resistance to diffusion across the liquid-gas interface, the two-resistance theory describes how
mass transfer occurs in the column. The transfer of CO2 to water occurs in three steps: 1)
from air to the air-water interfacial surface, 2) across the interfacial surface into the liquid
phase, and finally 3) into the bulk liquid phase.
As mentioned, the optimal operating conditions for the tower had to be determined
before the absorption of CO2 could be studied. To do this, the loading zone had to be found.
The loading zone is the region between the loading and flooding points. As water flows down
a column, at a certain air flow rate, water will begin to accumulate in the packings. This is
called the loading point. As air velocity is increased, more water will be accumulated. At the
flooding point, the air velocity will be high enough such that entire liquid is entrained,
causing the whole column to be filled with water.
Column operation in the loading zone is unstable, and thus it is recommended to
operate at below the loading point in the preloading region. The two critical aforementioned
points can be determined both physically and graphically. Physically, the loading point can
be observed when surges of air can be seen moving up the column. The flooding point can be
seen when water has been pushed out of the top of the column by the air.
The mass transfer flux is proportional to the concentration difference, where k is
called the mass transfer coefficient, A is the area of the interface, N1 is the flux at the
interface, Cli and Ci are the concentration at the interface and the bulk solution, respectively.
N1=(𝐶li−𝐶1)
Page | 5
CHAPTER THREE
3.0 DESCRIPTION OF APPARATUS
Figure 3.1
Gas Absorption Column Machine: This is where the experiment is being carried out, It
contains different parts such as the packed column, pressure gauge, valves and so on.
Figure 3.2
Oilless Air Compressor: This was responsible for the pumping the CO2 from the cylinder
is then pressurized within the compressor. Once the compressor completes the pressurization,
it expels the gas (CO2) to the gas absorption column machine.
Page | 6
Figure 3.3
Carbon dioxide Cylinder: This is responsible for storing the Carbon dioxide gas in a liquid
and pressurised form in the cylinder.
Figure 3.4
Stop Watch: To record the time and time interval taken during the experiment.
Page | 7
Figure 3.5
Phenolphthalein: Acts as an indicator during titration.
Figure 3.6
Conical Flask: This is where the titration takes place.
Figure 3.7
Measuring Cylinder: This is used for measuring a certain volume of liquid.
Page | 8
3.1 PROCEDURE

Fill the liquid reservoir tank at the base of the column to approximately threequarter full with preferably deionized water. Note the volume added [22.5
liters].

With gas flow control valves C and C closed, start the liquid pump and adjust
the water flow through the column to approx. 10 liters/minute on the flow
meter F by adjusting flow control valve C.

Start the compressor and adjust control valve C, to give an air flow of approx.
10% of full scale on flow meter F.

Carefully open the pressure regulating valve on the carbon dioxide cylinder,
and adjust valve C3 to give a valve on the flow meter F, approx. One half of
the air flow F2. Ensure the liquid seal at the base of the absorption column is
maintained by, if necessary adjustment of control valve C.

After 5 minutes of steady operation, take samples at 5 minute intervals from S
and S. Take 150ml samples at known times in each case. Analyze the samples
according to the procedure detailed below.

Withdraw a sample of liquid S5 from the sump tank with the sampler
provided, approximate volume of 150ml, or from liquid outflow point S4.

Discharge the sample at the base of a 100ml graduated cylinder, flicking the
cylinder to throw off excess liquid above the 100ml mark.

Add 5-10 drops of phenolphthalein indicator solution (a) above; if the sample
turns red immediately, no free Co, is present. If the sample remains colourless,
titrate with standard alkali solution (b) above. Stir gently with a glass rod until
a definite pink colour persists for about 30 seconds. This colour change is the
end point; Note the volume VB of alkali solution added.

For best results, use a colour comparison standard, prepared by adding the
identical volume of phenolphthalein solution (a) to 100ml of sodium
bicarbonate solution (c) in a similar graduated cylinder.
Page | 9
CHAPTER FOUR
4.0 EXPERIMENTAL RESULT/CALCULATION
This are the data generated in a tabular form, calculation of result and discussion of result.
TABLE 4.1
ParametersConcentration of NaOH = 0.0277
Volume of samples = 100ml
VT (Volume of Water in System) = 22.5 litres
F1 (Flow rate) = 10 litre/min
Table of Raw Data
Time from start
(minutes)
5
10
15
From Sump Tank S5
(Corresponding to
conditions at the top of the
tower)
VB(ml)
16.8
37.3
49.0
From liquid Outflow Point
S4 (Corresponding to
conditions at the bottom of
the tower)
VB(ml)
12.6
46.2
49.8
To obtain the different concentrations of carbon-dioxide dissolved in water from the amount
of NaOH required to neutralize the water sample, the relation below is used –
𝑔𝑚𝑜𝑙𝑒
𝑉𝐵 × 0.0277
𝑜𝑓 𝑓𝑟𝑒𝑒 𝐶𝑂2 =
= 𝐶𝑑
𝐿𝑖𝑡𝑟𝑒
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑖𝑛 𝑚𝑙
At 5 minutes from start, volume of base required to neutralize 100ml of water sample from
the top of tower = 16.8ml, therefore
𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐶𝑂2 =
16.8×0.0277
100
= 4.65 × 10−3 𝑀
At 5 minutes from start, volume of base required to neutralize 100ml of water sample from
the bottom of tower = 12.6ml, therefore
𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐶𝑂2 =
12.6×0.0277
100
= 3.49 × 10−3 𝑀
At 10 minutes from start, volume of base required to neutralize 100ml of water sample from
the top of tower = 37.3ml, therefore
Page | 10
𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐶𝑂2 =
37.3×0.0277
100
= 1.033 × 10−2 𝑀
At 10 minutes from start, volume of base required to neutralize 100ml of water sample from
the bottom of tower = 46.2ml, therefore
𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐶𝑂2 =
46.2×0.0277
100
= 1.279 × 10−2 𝑀
At 15 minutes from start, volume of base required to neutralize 100ml of water sample from
the top of tower = 49.0ml, therefore
𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐶𝑂2 =
49.0×0.0277
100
= 1.357 × 10−2 𝑀
At 15 minutes from start, volume of base required to neutralize 100ml of water sample from
the bottom of tower = 49.8ml, therefore
𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐶𝑂2 =
49.8×0.0277
100
= 1.379 × 10−2 𝑀
TABLE 4.2
Completed Table of Results
Time from start
(minutes)
5
10
15
From Sump Tank S5
(Corresponding to conditions at
the top of the tower)
Cd in tank
VB(ml)
(gmol/Litre)
(×10-2)
16.8
0.465
37.3
1.033
49.0
1.357
Average Rate of Absorption =
Average Rate of Absorption =
𝐶𝑑𝑖 (𝑡=40)− 𝐶𝑑𝑖 (𝑡=10)×𝑉𝑇
From liquid Outflow Point S4
(Corresponding to conditions at
the bottom of the tower)
Cd at outlet
VB(ml)
(gmol/Litre)
(×10-2)
12.6
0.349
46.2
1.280
49.8
1.379
𝑔. 𝑚𝑜𝑙𝑒/𝑠𝑒𝑐𝑜𝑛𝑑
30×60
0.01357(𝑡=15)− 0.00465(𝑡=5)×22.5
30×60
𝑔. 𝑚𝑜𝑙𝑒/𝑠𝑒𝑐𝑜𝑛𝑑
Average Rate of Absorption = 0.00000496
4.2 DISCUSSION OF RESULT
It can also be seen from the results gotten, that the concentration of CO2 in water at
the outflow point S4 increases as the volume VB increases. Also it can be seen from the results
gotten the concentration of CO2 in water from the outflow point S4 is higher than the
concentration of CO2 from Sump Tank S5 which is directly proportional to volume VB from
Sump Tank and the absorption rate is gotten at a value of 0.00000496.
Page | 11
CHAPTER FIVE
5.0 CONCLUSION
Upon analysis of the experimental results it can be concluded that gotten the
concentration of CO2 in water from the outflow point S4 is higher than the concentration of
CO2 from Sump Tank S5 which is directly proportional to volume VB from Sump Tank. Gas
absorption is a unit operation in which a soluble component is absorbed by contact with a
liquid phase in which the component is soluble. In this experiment, water is used to remove
carbon dioxide from air. The performance of packed gas-liquid absorption tower is evaluated
to determine how the mass transfer rate is affected by gas flow rate, especially as the column
approaches its loading and flooding points.
From the data, it can be observed that if the volume of the standardized sodium
hydroxide solution increases, the concentration increases, and also the rate of absorption
increases. Also the aim of the experiment which is to calculate the rate of absorption of
carbon dioxide into water from analysis of liquid solutions flowing down absorption column
was achieved.
5.1 RECOMMENDATION


Provision of a new and standard Gas Absorption Column as the one in the
laboratory is not inline with the standard.
Provision of standby generation in case of power supply failure.
5.2 REFERENCES


Gas-absorption-pdf.
https://pdfcoffee.com-gas-absorption-pdf.com
Gas Absorption Colum-Mass Transfer Experiment.
https://439341397-gas-absorption-column-mass-transfer-experiment-adocx.com
POST LABORATORY ASSIGNMENT
Question – Practical application(s) of Gas Absorption Column
Answer(s) –
 Petroleum Industry
 Food Industries.
 Industrial waste treatment
 Metallurgical industry for extraction.
 Pharmaceutical industry for drug production.
Page | 12