Table of Content

No. Title Page No

1. Objective 2

2. Introduction 2-3

3. Materials and Apparatus 4

4. Methodology 5-6

5. Results 6

6. Discussion 7-9

7. Conclusion 9

8. Limitation and Precautionary Steps 10

9. Reference 10

10. Appendix 11

1

Objectives

To demonstrate absorption of CO2 into water using a packed column.

To calculate the rate of absorption of CO2 into water.

To find out the saturation of CO2 in water.

To investigate the effect of the gas flow rate, gas inlet composition and liquid flow rate on

the rate of CO2 absorption.

Introduction

Gas absorption is one of the most commonly used unit operation in chemical engineering. The

gas absorption unit is mainly designed to demonstrate the principles of gas absorption and allows

the students to investigate the principles of the packed absorption processes and hydrodynamics.

Besides that, it also helps to provide practical training in the operation of the gas absorption

plant. One of the most important uses of the absorption column is to absorb carbon dioxide from

air into the water which flows down the column.

Carbon dioxide changes the pH of the water when it is dissolved in water. The mechanism on

how it works can be shown as below:

Carbon dioxide dissolves slightly in water to form a weak acid called carbonic acid, H2CO3 :

CO2 + H2O H2CO3

After that, the carbonic acid reacts slightly and reversibly in water in order to form

hydronium cation, H3O+ and bicarbonate ions, HCO3- which can be shown below:

H2CO3 + H2O H3O+ + HCO3-

This chemical behavior as shown above can be shown to explain when carbon dioxide dissolves

in water which normally has the pH of 7 will be changed to have a pH of 5.5 when it is exposed

in air.

2

Operation of Gas Absorption Column

The diagram above shows the diagram on how the gas absorption column operates. The gas is

pumped into the absorption column in which the gas will be dissolved in the water. The content

of the carbon dioxide (concentration of the carbon dioxide) will be different at each time

collected based on the amount of carbon dioxide dissolved.

Therefore in this experiment, the concentration of the carbon dioxide dissolved in water at

different times can be determined to estimate the amount of carbon dioxide dissolved in water.

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Materials and Apparatus

Absorption column, 0.05M sodium hydroxide solution, NaOH, pipette, a beaker, a stand with

clamp, burette and phenolphthalein indicator.

Methodology

General Start-Up Procedure:

1. It was ensured that all the valves were initially closed.

2. All the gas connections were checked to ensure that they were properly fitted.

3. The air compressor was switched on and the pressure was allowed to build up until 5 bars.

4. The valve which was located at CO2 compressor was opened. The supply pressure was set to

be between 4 to 5 bars by turning the air filter regulator knob clockwise.

5. The valve at the air gas supply was opened. The supply of pressure was set to be between 2

to 3 bars by turning the gas regulator knob in the clockwise direction.

6. The unit is now prepared for the experiment.

Experimental Procedure:

1. The general start-up procedures as mentioned above were performed prior to the experiment.

2. The sump tank B1 was then filled with 40L of fresh water. The valve V4 was then opened.

3. The circulation pump P1 was then switched on and the water which was filled up early was

allowed to by-pass into the sump tank B1 via valve V4.

4. Both the air and CO2 was then introduced into the absorption column K1 as according to the

following steps:

a) The appropriate air and CO2 gas flow rate was determined in order to achieve the desired

inlet gas composition for the absorption process.

b) The valve V1 was opened and adjusted set the flow rate of gas CO2, FI-101.

c) The valve V2 was opened and adjusted to set the flow rate of air, FI-102.

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5. The initial concentration of CO2 in the sump tank B1 was recorded as the feed composition

value.

6. The liquid was then introduced into the absorption column K1 as follows:

a) The valve V3 was slowly opened.

b) The liquid which enters the top of the absorption column, flowing down the column and

back into the sump tank B1 was observed.

c) The valve V3 was adjusted to fix the desired inlet liquid flow rate at FI-103.

7. At every 5 minutes interval, a sample from the outlet liquid of the absorption column was

collected and analyzed to determine the concentration of CO2 in the water. The procedure for

this process in given below.

Analysis of dissolved CO2 in water:

a) Exactly 100ml of sample was prepared in a conical flask.

b) About 5-10 drops of phenolphthalein was added to the sample prepared. If the sample

turns pink immediately, no free CO2 is present. If the sample remains colorless, it sample

was then titrated with the standard 0.05M of sodium hydroxide, NaOH solution.

c) The volume of 0.05M NaOH required to reach the end point of titration was recorded.

The end point is said to occur when there is a definite pink color persists in the solution

for more than 30seconds.

d) In order to obtain a higher accuracy results, a color comparison standard was used.

Exactly 100ml of 0.01M sodium bicarbonate, NaHCO3 solution was prepared in a conical

flask and 5-10 drops of phenolphthalein indicator was added.

8. The data was continued to be recorded and the liquid sample was taken until the CO2

composition of the outlet has a steady value.

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Results

Time

(min)

Initial Volume of

NaOH (ml)

Final Volume of

NaOH (ml)

Volume of

NaOH used (ml)

Concentration of

CO2 (M)

0 25 17.5 7.5 0.00375

5 25 19.5 5.5 0.00275

10 25 19.2 5.8 0.00290

15 25 19.2 5.8 0.00290

20 25 19.2 5.8 0.00290

Discussions

Sample Calculations:

1. TO FIND THE CONCENTRATION OF CO2 IN WATER

Using V2 = 7.5ml, M2 = 0.05M, V1= 100ml

M1V1 = M2V2

Where M1 is the concentration of CO2 in water

V1 the volume of sample used for titration

M2 is the concentration of NaOH used

V2 the volume of NaOH used for titration

(M1)(100) = (0.05)(7.5)

M1 = 0.00375 M

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0 5 10 15 200

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.0040; 0.00375

5; 0.00275 10; 0.0029 15; 0.0029 20; 0.0029

Graph of Time Vs. Concentration of CO2

Time, (minutes)

Con

cent

ratio

n O

f CO

2 (M

)

As mentioned in the introduction, when carbon dioxide dissolves in water it forms carbonic acid

which decreases the pH of the water from neutral to slightly acidic which is from pH 7.0 to about

pH 5.5. The equation given below is to demonstrate the process which occurs when the carbon

dioxide dissolves in water:

CO2 + H2O H2CO3 (formation of carbonic acid)

H2CO3 + H2O H3O+ + HCO3- (formation of hydronium cation and bicarbonate ions)

The concentration of the carbon dioxide which is dissolved in water is to be determined in this

experiment. The concentration of carbon dioxide in water can be determined by performing

titration with the sample solution collected with a fixed concentration of NaOH solution. The

volume of NaOH solution needed to neutralize the carbonic acid will be the indicator to show the

concentration of carbon dioxide present in the water.

The indicator which was used in this experiment will be the phenolphthalein indicator. The color

of the indicator in each neutral, acid and alkali state:

Alkaline – Pink

Acid – Colorless

Neutral – Colorless (first faint pink color)

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The color change noted in this experiment will be from colorless to faint pink. The solution used

in the conical flask will be sample and the NaOH solution will be inside the burette.

Based from the results obtained, the concentration of the CO2 which was dissolved in water was

calculated using the formula as shown in the sample calculations. By using the values calculated,

the graph of concentration of the CO2 against time was plotted. By analyzing the graph drawn,

the general trend of the graph that can be seen is that the concentration of CO 2 decreased as the

duration of the experiment increased. After a certain period of time, the concentration of the CO2

does not increase anymore and stays constant with increasing time duration.

By plotting the graph of concentration of carbon dioxide against time, it can be seen that the

slope of the graph plotted will represent the rate of absorption of CO2 into water. Based from the

graph plotted, there were two main phases which can be seen from the graph plotted:

a) Decreasing Gradient

During this phase, the concentration of the carbon dioxide which was dissolved in the fresh

water decreases with time. This suggests to us that as the time passes, lesser carbon dioxide

gas was dissolved into the water compared to that at the start of the experiment. Therefore as

the time passed, the concentration of the carbon dioxide gas is dissolved decreased

suggesting a negative relationship between the concentration of carbon dioxide and time.

This also means that the rate of carbon dioxide absorption into water was decreasing.

b) Constant Gradient

During this constant gradient, the concentration of the carbon dioxide does not increase any

more with the increase of time duration. This can be seen from the graph that, after 20

minutes into the experiment, the value of CO2 concentration remains constant. This shows to

us, that the CO2 gas which dissolves into the water has reached it saturated level. The

dissolving of CO2 into the water has reached its limit and increasing the gas flow rate or the

inlet composition does not affect the concentration of the CO2 dissolved in water. This also

indicates that the rate of carbon dioxide absorption into water was stable.

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Conclusion

The main objective of this experiment which was to demonstrate the absorption of carbon

dioxide into water using a packed column was proven to be successful. In this experiment, we

were able to determine the concentration of carbon dioxide collected in water at intervals of 5

minutes. By looking at our results it can be seen that, as the time increased, the concentration of

carbon dioxide dissolved in water decreased up to a certain point after which it started to

stabilize. This shows as the time increased, lesser carbon dioxide was being dissolved in water.

Once the maximum concentration of carbon dioxide dissolves in water was reached, it did not

increase the concentration of carbon dioxide any more.

Limitation and Precautionary Steps

1. It is important to ensure that the observer view the level of NaOH solution in the burette

below the meniscus line. If the eye level of the observer is not at the correct position the

volume of the NaOH used may not be exactly accurate.

2. The color change for the phenolphthalein from colorless to pink occurs almost drastically,

therefore is it important for the person conducting the titration to ensure that the NaOH from

the burette is released slowly into the conical flask.

3. The color change that occur when the sample to turn from colorless to pink is almost pale in

color. Therefore, it is advisable for the student to place a small white tile below the conical

flask so that the pale color change can be seen clearly.

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References

1. Carbon Dioxide [Online]

http://www. lenntech.com/carbon-dioxide.htm

(Data Accessed on 6th Feb 2016)

2. Carbon Dioxide and Carbonic Acid [Online]

http://ion.chem.usu.edu/~sbialkow/Classes/3650/Carbonate/Carbonic%20Acid.html

(Data Accessed on 6th Feb 2016)

3. Absorption of Carbon Dioxide in water

http://www.scribd.com/doc/151016680/EXPERIMENT-2-Absorption-of-Carbon-Dioxide-in-

Water

(Data Accessed on 6th Feb 2016)

4. Gas Absorption [Online]

http://www.slideshare.net/dp93/gas-absorption-experiment

(Data Accessed on 6th Feb 2016)

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