University of San Carlos

School of Engineering

Talamban, Cebu City, Philippines

CHE 512L

Chemical Engineering Laboratory 2

Absorption

Mass Transfer in an Absorption Column: Packed Absorption Column II

A laboratory report submitted to

Dr. Camila Flor Y. Lobarbio

CHE 512L Instructor

by

Kara Sheen P. Batayola

October 5,2015

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1. Introduction

Absorption is one major operation of chemical engineering which is based on mass transfer.

This process is the removal of one or more components in a gas phase by contacting it to a

liquid phase. This operation is used in several industries like feedstock preparation, bulk

chemical synthesis and gas purifications (Richardson, Harker, & Backhurst, 2002).

The contact of both phases is done countercurrently where the vapor is introduced at the

bottom of a column while the liquid is pumped to the top of the column. As the two phases

flow through the column they contact with each other and since concentration gradient exists

between the two phases mass transfer occurs but this process does not solely depends on the

concentration difference for the transfer to occur. Equilibrium relationship between the phases

also dictates the extent of absorption (Richardson, Harker, & Backhurst, 2002).

Gas-liquid relationship is expressed by many laws. For dilute solutions, Henry’s law best

describes the equilibrium relationship for the gas and the liquid phase. According to Henry’s

law, at a constant temperature, 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 (Geankoplis, 2012). Equation 1 shows the relationship between concentration of

solute in the gas phase and in the liquid phase.

𝑦𝐴 = 𝐻𝑥𝐴 Eqn. 1

The rate at which a solute is transferred to the liquid phase is controlled by the concentration

gradient present, the equilibrium between the phases and by the resistance offered by the gas

and the liquid. Resistance for this process is best understood by understanding the two-film

theory. According to this theory, transfer of solute in the bulk phases is by convection currents,

and concentration differences only becomes significant at the interface (Seader, Henley, &

Roper, 2011). Figure 1 shows the concentration profile of the solute according to the two-film

theory.

Figure 1. Concentration profile of the solute by the two film theory

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For the diffusion of solute through the gas phase, the rate of mass transfer in the gas phase is

expressed by equation 2 and in the liquid phase by equation 3

𝑁𝐴 = 𝑘′𝐺(𝑦𝐴𝐺 – 𝑦𝐴𝑖) Eqn. 2

𝑁𝐴 = 𝑘′𝐿(𝑥𝐴𝐿 – 𝑥𝐴𝑖) Eqn. 3

where NA is the mass transfer rate, yAG are yAi are the concentrations of solute in the bulk phase

and the interface of the gas phase, xAL and xAi are the concentrations of the solute in the bulk

phase and the interface of the liquid phase, 𝑘′𝐺 and k′L are the mass transfer coefficient at the

gas and the liquid film (Geankoplis, 2012). It is difficult to measure the film mass transfer

coefficients experimentally thus overall mass transfer coefficients (𝐾′𝐺 and 𝐾′𝐿) are rather

measured where it is based on the bulk concentrations instead of the interfacial concentrations

thus resulting to equation 4.

𝑁𝐴 = 𝐾′𝐿(xA

∗ − 𝑥𝐴𝐿 ) = 𝐾′𝐺𝐴(𝑦𝐴𝐺 – 𝑦∗) Eqn. 4

where y* is the concentration that would be in equilibrium with xAL and x* is the value that

would be in equilibrium with yAG. From these equations together with an equilibrium

relationship and a mass balance around a differential height we can obtain another set of

equations in order to measure the overall mass transfer coefficient experimentally (Geankoplis,

2012). These equations can be seen in the methodology of this report. Mass transfer

coefficients are the resistances for our rate equation thus higher values of the mass transfer

coefficients means that there is a higher resistance of transfer in that phase thus solute do not

tend to transfer to that phase and the opposite for mass transfer with lower values.

In this experiment, absorption of carbon dioxide with water as solvent is investigated. In

theory, carbon dioxide is not very soluble in water thus it is expected that the resistance is

greater in the liquid phase, which would correspond to a higher value of mass transfer

coefficient. Since the solution is dilute, equilibrium relationship of solute in both phases can

be approximated by Henry’s Law. Temperature and the pressure in this experiment was

assumed to be constant thus it does not affect the absorption experiment. However, the flow

rates are varied for both phases and with this we would see how the change in flow rate affects

the results of our experiment.

2. Objectives of the Experiment

a.) Determine the mass transfer coefficient for absorption of CO2 in water in a packed column.

b.) Carry out a mass balance over the packed absorption column.

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3. Methodology

3.1. Methodological Framework

To determine the mass transfer coefficient for the absorption of CO2 in water, a packed

column was used for the experiment. Known concentrations of CO2 in the inlet and outlet

streams were used as data to be used in equations in order to determine the overall mass

transfer coefficient. Figure 2 shows a diagram of the process for objective 1.

Figure 2. Determination of the mass transfer coefficient for the absorption experiment

For objective 2, a mass balance was done around the absorption packed column to determine

mass losses. A diagram in Figure 3 shows the inlet and outlet streams in a packed bed column

to be used in the mass balance.

Figure 3. Mass Balance over the Packed Absorption Column

3.2. Materials

The absorption experiment was carried out by absorbing CO2 in air to water. The initial

concentration of CO2 in air was regulated by a setting the flowrate of CO2 and of air. In

analyzing gas samples 0.1N NaOH was used in a HEMPL apparatus to obtain the

concentration of CO2 in the gas. For liquid samples, 0.029N NaOH was prepared and

standardized which was then used in titrating the samples to obtain the concentration of CO2

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present in the liquid. Phenolphthalein indicator was used to indicate the change in color of the

solution from colorless to pink as its pH was changed during titration.

3.3. Equipment

The packed absorption column used consists of two 75 mm diameter clear acrylic cylinders

joined and installed vertically. It is filled with glass Raschig rings. Liquid is fed to the top of

the column by a centrifugal pump from a 50.0 L sump tank. The liquid flows down through

the packings and returns to the sump tank. A flow meter is used to set the flowrate of the liquid.

Figure 4. Armfield Bench Scale Gas-Liquid Absorption Column

1

2

3

4

5

7

Packed Bed Column

Flow Meters

Sump Tank

CO2 gas cylinder

Downcoming tube

HEMPL Apparatus

Manometers

Main Switch

Air Compressor

1

2

3

4

5

6

7

8

9

8

9

6

5

The gas to be absorbed in the liquid is CO2 and is taken from a gas cylinder. Carbon dioxide

passes through a flow meter and is mixed with an air stream that is delivered by an air

compressor to a flow meter then mixes with the CO2 stream. This gas stream enters at the

bottom of the column then passes through the packed bed and contacts with the liquid stream

countercurrently.

Pressure tappings at the bottom, center and top of the column are placed to allow readings of

pressure drop in the column using manometers. Also, these tappings provide a means of

obtaining gas samples for the analysis for the concentration of CO2 in the gas outlet stream.

Analysis of these samples is done by using a HEMPL apparatus.

The said flow meters, manometers and gas analysis equipment are all mounted on a vertical

backboard for easy operation. The equipment is depicted on Figure 4.

3.4. Procedures

Before the experiment proper, preliminary preparations were made. The drain valve of the

HEMPL apparatus was closed and the HEMPL apparatus was then filled with 0.1N NaOH

until the zero mark. The two other valves of the apparatus were opened to vent out air in the

tubes. The CO2 concentration of the ambient air was analyzed. All drain valves of the

equipment was closed before filling the sump tank with 25-30L of distilled water. The initial

volume and temperature of the liquid inside the sump tank was recorded.

Two solutions of 0.01M NaHCO3 with a volume of 250 mL were titrated with a solution of

0.029N NaOH. Prior to titration, 2-3 drops of phenolphthalein indicator was added to the

solution to determine the endpoint. The titrated solutions served as a visual reference in

determining the endpoint of other titrations. Initial CO2 concentration of the water in the sump

tank was also determined by titration.

Main switch of the pump for the water was switched on. Water was introduced to the column

with a flowrate set to 30 L/min and it was allowed to run for about 10 minutes. This was done

to flush out CO2 present in the column before carrying out the experiment. The main switch

for the air was then switched on and the air, water and CO2 flowrate was set according to the

given settings. Table 1 shows the settings given for experiment 1 and 2.

Table 1. Experiment Settings

Expt. No. 𝜙𝑤𝑎𝑡𝑒𝑟 (L/min) 𝜙𝑎𝑖𝑟 (L/min) 𝜙𝐶𝑂2 (L/min)

1 5 20 1.5

2 3 20 1.5

After the CO2 flow was set, time zero started. Liquid samples of 10 mL each were alternately

collected from the downcoming tube and the sump tank every 5 minutes for 50 minutes. For

the downcoming tube, samples were collected at time 5, 15, 25, 35 and 45 while for the sump

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tank, samples were collected at time 10, 20, 30, 40 and 50. The samples were immediately

titrated with a solution of 0.029 M NaOH for the determination of CO2 concentration. Equation

5 was used for determination of CO2 concentration in the liquid samples. Simultaneously, gas

samples of 100 mL each were collected with the HEMPL apparatus for analysis of CO2

concentration every 5 minutes for 50 minutes. After the first run, CO2 stream was shut off

while air and water flow was allowed to run for 20 minutes at a flow rate of 5 L/min and 30

L/min., respectively. This is to remove CO2 absorbed during the first experiment.

𝑥 =(𝑉𝑁𝑎𝑂𝐻 𝑢𝑠𝑒𝑑)(

𝐶𝑁𝑎𝑂𝐻

1000𝑚𝑙𝐿⁄

)

(𝑉𝑠𝑎𝑚𝑝𝑙𝑒𝜌𝐻2𝑂

𝑀𝑀 𝐻2𝑂)+(𝑉𝑁𝑎𝑂𝐻 𝑢𝑠𝑒𝑑)(

𝐶𝑁𝑎𝑂𝐻

1000𝑚𝑙𝐿⁄

)

Eqn. 5

Ten liters of distilled water was then added to the sump tank and another trial was performed

at another set of flowrates, following the procedure from the first run. See Table 1 for the

settings of experiment 2.

In analyzing the gas samples, the HEMPL apparatus was used. Before gathering samples, the

apparatus was vent out to the atmosphere to balance pressures and to flush out air in the tubes

connecting the apparatus and the column. This was done three times. Then, 100 mL (V1) of

gas sample was gathered from the column to the cylinder of the apparatus. This sample was

allowed to react to 0.1N NaOH. This reaction causes a decrease of the volume of the gas

sample. The volume (V2) of the CO2 was determined using the meter attached above the gas

absorption globe of the apparatus. Equation 6 was used for the CO2 concentration of the gas

samples.

𝑦 =𝑉2

𝑉1 Eqn. 6

The CO2 concentrations obtained and the flowrate settings was then used in order to determine

the overall mass transfer coefficients for the absorption experiment. Two equations were used

in solving the overall mass transfer coefficients where derivations were based on the two-film

theory and a macro-balance around the system. Equations 7 and 8 for the two film theory and

equations 9 and 10 for the macro-balance were the equations used for the determination of the

overall mass transfer coefficients.

𝐾′𝑥𝑎 =𝐿

𝑙(

1

1 −𝐿

𝑚𝐺

) ln (𝑥 −

𝑦𝑚

𝑥0 −𝑦0

𝑚

) Eqn. 7

𝐾′𝑦𝑎 =𝐺

𝑙(

1

1 −𝑚𝐺

𝐿

) ln (𝑦2 − 𝑚𝑥2

𝑦1 − 𝑚𝑥1) Eqn.8

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𝐾′𝑥𝑎 =𝐿

𝑙[

𝑥1 − 𝑥2

(𝑥∗ − 𝑥)𝑀]

𝑤ℎ𝑒𝑟𝑒 (𝑥∗ − 𝑥)𝑀 = (𝑥∗ − 𝑥)1 − (𝑥∗ − 𝑥)2

ln [(𝑥∗ − 𝑥)1

(𝑦 − 𝑦∗)2]

Eqn. 9

𝐾′𝑦𝑎 =𝐺

𝑙[

𝑦1 − 𝑦2

(𝑦 − 𝑦∗)𝑀]

𝑤ℎ𝑒𝑟𝑒 (𝑦 − 𝑦∗)𝑀 = (𝑦 − 𝑦∗)1 − (𝑦 − 𝑦∗)2

ln [(𝑦 − 𝑦∗)1

(𝑦 − 𝑦∗)2]

Eqn. 10

Mass balance was then done around the absorption system. Figure 5 is the schematic diagram

for the system. Equation 11 shows the component mass balance for this system. Mass balances

was based on the inert flow of the liquid (L’) and gas phase (V’).

Figure 5. Schematic Diagram for the System

𝐿′𝑋2 + 𝑉′𝑌1 = 𝐿′𝑋1 + 𝑉′𝑌2

𝑤ℎ𝑒𝑟𝑒 𝑉′ = 𝑉(1 − 𝑦𝐶𝑂2)

𝐿′ = 𝐿(1 − 𝑥𝐶𝑂2)

𝑋𝑖 =𝐿′

1−𝑥𝑖 × 𝑥𝑖

𝑌𝑖 =𝐺′

1−𝑦𝑖 × 𝑦𝑖

Eqn. 11

4. Results and Discussions

Objective 1: Overall Mass Transfer Coefficients

The molar fractions of CO2 in each stream, as well as information about the stream flow rates

and tower dimensions, were used to determine the mass transfer coefficient for absorption of

CO2 in water in these experiments.

Equations from the two-film theory and macro balance were used to calculate overall mass

transfer coefficients. Table 2 summarizes the overall mass transfer coefficients corresponding

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to experimental parameters set, and these values are in fair agreement. Overall liquid mass

transfer coefficients, K’xa, for Experiment 1 are 1773.59 and 1822.84 mol/m3-min while

overall gas mass transfer coefficients, K’ya, are 10.61 and 11.44 mol/m3-min. For Experiment

2, K’xa values are 484.43 and 489.11 mol/m3-min while K’ya values are 1.44 and 0.93 mol/m3-

min.

Table 2. Overall Mass Transfer Coefficient

Expt No.

Overall Mass Transfer Coefficient

Two-film Equation Macro balance Equation

K’xa K’ya K’xa K’ya

1 1773.59 10.61 1822.84 11.44

2 484.43 1.44 489.11 0.93

It is observed that the values of the overall liquid mass transfer coefficient K’xa are larger than

those of the overall gas mass transfer coefficient K’ya. A large overall mass transfer coefficient

signifies that there is great resistance of a given phase to mass transfer. This implies that in the

absorption of CO2, the major resistance to mass transfer is in the liquid phase.

For the second set of flow rates, gas and air flow rates were kept constant while changing the

water flow rate from 5 L/min to 3 L/min. This new set of flow rates shows an observed decrease

in the overall mass transfer coefficients. A decrease in the water flow rate corresponds to lesser

turbulence created in the system. This results to lesser contact between the streams and lowers

the rates of mass transfer.

From figure 4 in Annex 3, it is shown that Experiment 2 with a higher water flowrate has higher

CO2 concentration in the leaving stream than in experiment 1 at a lower water flowrate. This

supports the overall gas mass transfer coefficient obtained for the two experiments. From the

results, experiment 2 has a lower mass transfer coefficient than for experiment 1. This means

that resistance to transfer is so low thus more CO2 has a higher tendency to transfer in the gas

phase resulting to a higher concentration in the gas phase.

Objective 2: Overall Mass Balance

The mass balance carried out was a component mass balance of the carbon dioxide in liquid

and gas streams entering and exiting the absorption column. Results are presented in Table 3,

calculations can be found in Annex 5.

Table 3. Moles CO2 input and output

Experiment

No.

Input Output

Percentage

loss (%) CO2 in Liquid

Stream

(mol/min)

CO2 in Gas

Stream

(mol/min)

CO2 in Liquid

Stream

(mol/min)

CO2 in Gas

Stream

(mol/min)

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1 0.0050 0.0597 0.0098 0.0433 17.89%

2 0.0024 0.0597 0.0068 0.0479 12.07%

From Table 3, it is observed for both experiments that moles CO2 leaving water streams

increased compared to liquid introduced, while moles leaving the gas stream decreased. This

indicates that a mass transfer occurred from the gas to the liquid streams.

It can also be observed that there is a difference in the amount in the overall inlet and outlet

gaseous streams. Percentage loss for Experiment 1 is 17.89% while it is 12.07% for Experiment

2. This difference indicates a percentage of CO2 mass loss unaccounted for. It may be attributed

to the factors such as the error margin of the HEMPL apparatus, and some CO2 possibly lost

during the collection and analysis of the liquid samples.

5. Conclusions

Overall mass transfer coefficients were calculated for each experiment from the two-film theory

and macro balance equations. For flow settings of 5 L water/min, 20 L air/min and 1.5 L CO2/min,

and using the equations from the two-film theory, overall liquid mass transfer coefficient, K’xa, is

1773.59 mol/m3-min while overall gas mass transfer coefficient, K’ya, is 10.61 mol/m3-min. Using

the macro balance equations for the same set of flow rates, K’xa is 1822.84 and mol/m3-min and

K’ya is 11.44 mol/m3-min.

A change in the water flow rate to 3 L/min decreases the K’xa and K’ya values. Using equations

from the two-film theory, K’xa and K’ya values for these new set of flow rates are 484.43 and 1.44

mol/m3-min, respectively. Using the macro balance equations for the overall mass transfer

coefficients, K’xa and K’ya values are 489.11 and 0.93 mol/m3-min, respectively.

Percentage loss of the streams fed and leaving the column for both Experiments 1 and 2 were

calculated. These are at 0.0121% and 0.0174%, respectively.

References

Cussler, E. (2009). Diffusion Mass Transfer in Fluid Systems (3rd ed.). New York: Cambridge

University Press.

Geankoplis, C. J. (2012). Principles of Transport Processes and Separation Processes. Pearson

Education, Inc.

Richardson, J. F., Harker, J. H., & Backhurst, J. R. (2002). Chemical Engineering: Particle

Technology and Separation Processes. Oxford: Butterworth-Heinemann.

Seader, J. D., Henley, E. J., & Roper, D. K. (2011). Separation Process Principles. Hoboken,

NJ: John Wiley & Sons, Inc.

ANNEX

Data Processing & Analysis Report