ABSTRACT

The objectives of gas absorption experiment are to determine the loading and flooding

point in the column as well as to determine the pressure drop (∆P)as a function of gas (air)

and liquid (water) mass velocities (L/min) using flexi glass packed with Raschig rings.

Before the actual experiment started, the equipment was set-up first using set-up procedure

where the valves are arranged according to U-tube (LEFT) arrangement. Water is then filled

into the monotube using VT-3 and the water level is adjusted to 20 mm H2O for both left and

right monotube and the pump is then switched on. Next, the valve arrangement is set

according to operating arrangement. After the set-up is done, valve VR-3 and VR-4 are

opened and the water flow rate is adjusted to 1.0 (L/min). The level of water returning to the

water reservoir is controlled using VR-4 so that it always higher than the bottom of the

reservoir. Then, the gas flow rate is adjusted to 20 (L/min) and after two minutes, the

pressure at the left and right monotube is taken. The experiment is continued by varying the

gas flowrate until 180 (L/min) and the experiment is then repeated with volume flow rate of

2.0 (L/min) and 30 (L/min) respectively. At the end of the experiment, we had managed to

determine the loading and flooding point where the loading point is from the volume water

flow rate of 1.0 (L/min), 2.0 (L/min) and 3.0 (L/min). When the volume water flow rate at 3.0

(L/min), the flooding point started at gas flow rate of 140 (L/min). Other than that, we also

had determined the pressure drop (∆P) as a function of gas (air) and liquid (water) mass

velocities (L/min) using flexi glass packed with Raschig rings. The pressure drop for the

volume water flow rate 1.0 (L/min), the range is between 0.0 mm H2O until 15.0 mm H2O

where 15 mm H2O is the highest pressure drop at gas flow rate of 180 (L/min). While the

pressure drop for the volume water flow rate 2.0 (L/min), the range is between 0.0 mm H 2O

until 59 mm H2O where 59 mm H2O is the highest pressure drop at gas flow rate of 180

(L/min). For volume water flow rate 3.0 (L/min), the pressure drop range is between 0.0 mm

H2O until flooding point at gas flow rate 140 (L/min). We can conclude that as the gas flow

rate increasing, the pressure drop (∆P) will also increase. Thus, we can conclude that all the

objectives of the experiment had been reached.

1

INTRODUCTION

Absorption is a mass transfer process in which a vapor solute A in a gas mixture is

absorbed by means of a liquid in which the solute more or less soluble. The gas mixture consists

mainly of an inert gas and the soluble. An example of gas is the absorption of the solute ammonia

from an air-ammonia mixture by water. A major application of absorption is the removal of CO2

and H2S from nature gas or synthesis gas by absorption in solution of amines or alkaline salts.

A common apparatus used in gas absorption and certain other operations is the packed

tower, shown in Figure 1 below. The device consists of a cylindrical column, or tower, equipped

with a gas inlet an 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.

Their common dumped packing, Ceramic Berl saddles and Raschig rings are older

types of packing that are not much used now, although there were big improvements over ceramic

spheres or crushed stone when first introduced. The shape prevents pieces from nesting closely

together, and this increasing the bed porosity. As for this experiment we used the column packed

with Raschig rings.

In given packed tower with a given type and size of packing and with defined flow of

liquid, there is an upper limit to the rate of gas flow, called the flooding velocity or flooding point.

Above this gas velocity the tower cannot operate due to high pressure. At the flow rate called the

loading point, the gas start to hander the liquid down flow, and local accumulations or pools of

liquid start to appear in the packing.

2

Figure 1: Gas Absorption Packing Column

OBJECTIVES

To determine the loading and flooding points in the column.

To determine the pressure drop (∆P) as a function of gas (air) and liquid (water) mass

velocities (L/min) using flexi glass packed with Raschig rings.

THEORY

3

A common instrument used in gas absorption or stripping is a packed tower. A packed

tower consists of the following: a cylindrical tube with inert packing material, a gas inlet at

the bottom with an exit out the top, and a liquid inlet at the top with its exit out the bottom. In

an ideal operation the liquid will descend through the packed column and distribute

uniformly over the packing surface in films. The gas will enter the column from below the

packed section and rise upward countercurrent to the liquid flow through the small spaces

between the packing materials. The large amount of intimate contact between the liquid and

gas streams allows for pressure drop in the packing column.

The pressure drop along the packed column is calculated by using the formula below:

Pressure drop , ∆P (mmH2O) = High pressure of monotube (mmH2O) – Low pressure of

monotube (mmH2O)

In this experiment, the graph of Ln (V) versus Ln (∆P/m packing) is needed in order to

investigate and observed the relationship between the gas flow rate and the pressure drop in

the packing column. Ln (V) can be easily calculated using scientific calculator where:

V = gas flow rate in (L/min)

Moreover, to calculate Ln (∆P/m packing), we must first calculate the pressure drop using the

formula above then divided it with the packings value and then multiplied it with Ln using

scientific calculator.

For this experiment, we used Raschig rings packing column. It is given that the packings

value is:

Packings = 8 mm glass Raschig Rings

After we had calculated the values for Ln (V) and Ln (∆P/m packing), directly plot the graph.

APPARATUS

Gas- Liquid Absorption Column

PROCEDURE

4

R

B

R B

R

B

B

R

R

B

R

B

R B

R

B

B

R

R

B

R

B

R

B

R

B

R B

B

R

R B

B

R

Manometer calibration (see diagram)

For calibration of manometers and during operation of the column, the following valves must

be in the positions stated below:

a) U-tube (left)

VT-1

VT-2

VT-3

VT-4

VT-5

V-2 (OPEN)

b) U-tube (right)

VT-1

VT-2

VT-3

VT-4

VT-5

V-3 (OPEN)

c) Sphere ball

V-1 (OPEN)

VT-1

VT-2

VT-3

d) Operation

VT-1

VT-2

VT-4

VT-5

Operation

5

1. The manometer U-tube is filled with water by arranging the values according to the U-

tube arrangement. The pump is switched on. For this experiment, we used the left U-tube

arrangement only.

2. The values are set to operating arrangement before the operation is started.

3. All valves are checked carefully (closed) before the column is safe to use.

4. Valve VR-3 and VR-4 are opened such that the liquid flow rate is set at 10 m3/h.

Note: The level of liquid returning to the water reservoir must always be higher than the

bottom of the reservoir. This is to avoid air being trapped in line. Valve VR-4 is adjusted

accordingly to avoid this phenomena.

5. Valve VR-1 is opened and the airflow rate is set to be 10 m3/h. Wait for 2 minutes and

during this time the flow rate of air and water is make sure to be constant. The pressure

drop (∆P) mmH2O is read in the monotube.

6. The gas flow rate is increased by adding an extra of 5 m3/h to the column. Wait for 2

minutes and the pressure drop is read again.

7. Part 4 is repeated until Flooding Point is reached.

8. The curve of Ln (V) versus Ln(∆P / m packing) is plotted.

9. Steps 2 to 6 are repeated with different kind of liquid flow rate.

RESULTS

6

Flow rate

(L/min)

Presssure drop (mm H2O)

Air

Water

20 40 60 80 100 12 140 160 180

1.0 0 0 1 3 5 6 9 12 15

2.0 0 1 3 5 9 13 20 39 59

3.0 2 0 2 9 23 52 Floodin

g point

Flooding

point

Flooding

point

CALCULATIONS

In order to plot the graph Ln (V) versus Ln (∆P / m packing), we need to calculate Ln (V)

first. Where V = Gas flow rate in (L/min)

Gas Flow Rate(L/min)

Ln (V)

20 Ln (20) = 2.996

40 Ln (40) = 3.689

60 Ln (60) = 4.094

80 Ln (80) = 4.382

100 Ln (100) = 4.605

120 Ln (120) = 4.787

140 Ln (140) = 4.942

160 Ln (160) = 5.075

180 Ln (180) = 5.193

Then we need to calculate Ln (∆P / m packing) where packings = 8 mm glass Raschig Rings.

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Volume Flow Rate

(L/min)

Gas Flow Rate(L/min)

Pressure Drop,∆P(mmH2O)

Ln (∆P / m packing)

1.0

20 0 Ln (0/0.008) = Math Error

40 0 Ln (0/0.008) = Math Error

60 1 Ln (1/0.008) = 4.83

80 3 Ln (3/0.008) = 5.93

100 5 Ln (5/0.008) = 6.44

120 6 Ln (6/0.008) = 6.62

140 9 Ln (9/0.008) = 7.03

160 12 Ln (12/0.008) = 7.31

180 15 Ln (15/0.008) = 7.54

2.0

20 0 Ln (0/0.008) = Math Error

40 1 Ln (1/0.008) = 4.83

60 3 Ln (3/0.008) = 5.93

80 5 Ln (5/0.008) = 6.44

100 9 Ln (9/0.008) = 7.03

120 13 Ln (13/0.008) = 7.39

140 20 Ln (20/0.008) = 7.82

160 39 Ln (39/0.008) = 8.49

180 59 Ln (59/0.008) = 8.91

3.0

20 2 Ln (2/0.008) = 5.52

40 0 Ln (0/0.008) = Math Error

60 2 Ln (2/0.008) = 5.52

80 9 Ln (9/0.008) = 7.03

100 23 Ln (23/0.008) = 7.96

120 52 Ln (52/0.008) = 8.78

140 Flooding point Flooding point

160 Flooding point Flooding point

180 Flooding point Flooding point

So, the calculated results are as below;

Volume Flow Gas Flow Rate Pressure Drop, Ln (V) Ln (∆P / m packing)

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Rate(L/min)

(L/min)∆P

(mmH2O)

1.0

20 0 2.996 Math Error

40 0 3.689 Math Error

60 1 4.094 4.83

80 3 4.382 5.93

100 5 4.605 6.44

120 6 4.787 6.62

140 9 4.942 7.03

160 12 5.075 7.31

180 15 5.193 7.54

2.0

20 0 2.996 Math Error

40 1 3.689 4.83

60 3 4.094 5.93

80 5 4.382 6.44

100 9 4.605 7.03

120 13 4.787 7.39

140 20 4.942 7.82

160 39 5.075 8.49

180 59 5.193 8.91

3.0

20 2 2.996 5.52

40 0 3.689 Math Error

60 2 4.094 5.52

80 9 4.382 7.03

100 23 4.605 7.96

120 52 4.787 8.78

140 Flooding point 4.942 Flooding point

160 Flooding point 5.075 Flooding point

180 Flooding point 5.193 Flooding point

The curve of Ln (V) versus Ln (∆P / m packing),

For volume flow rate = 1.0 (L/min)

9

2.5 3 3.5 4 4.5 5 5.50

1

2

3

4

5

6

7

8

(V) versus Ln (∆P / m packing)

Ln (∆P / m packing)

Ln (V

)

For volume flow rate = 2.0 (L/min)

3.6 3.8 4 4.2 4.4 4.6 4.8 5 5.2 5.40

1

2

3

4

5

6

7

8

9

10

Ln (V) versus Ln (∆P / m packing)

Ln (∆P / m packing)

Ln (V

)

For volume flow rate = 3.0 (L/min)

10

2.5 3 3.5 4 4.5 5 5.50

1

2

3

4

5

6

7

8

9

10

Ln(V) versus Ln (∆P / m packing)

Ln (∆P / m packing)

Ln (V

)

ERROR CALCULATIONS

There are some errors when calculating the value of Ln (∆P / m packing). Notice that for the

volume flow rate of 1.0 (L/min), 2.0 (L/min) and 3.0 (L/min) the values for gas flow rate are

20 (L/min), and 40 (L/min), the value for Ln (∆P / m packing) are all math errors.

Volume Flow Rate

(L/min)

Gas Flow Rate(L/min)

Ln (∆P / m packing)

1.020 Ln (0.0/0.008) = Math Error

40 Ln (0.0/0.008) = Math Error

2.0 20 Ln (0.0/0.008) = Math Error

3.0 40 Ln (0.0/0.008) = Math Error

This value affects the graph of Ln (V) versus Ln (∆P / m packing).

DISCUSSION

11

The important things that we want to find out from the experiment are to determine

where is the flooding and loading point of the gas absorption as well as to determine the

pressure drop (∆P)as a function of gas (air) and liquid (water)mass velocities (m3/h) using

flexi glass packed with Raschig rings.

Loading and flooding point

At the flow rate called the loading point, the gas start to hander the liquid down flow and

local accumulations or pools of liquid start to appear in the packing. Flooding is an

undesirable state that occurs when the gas or liquid flow rate is too high for a given system

operation. At the point of flooding, the liquid begins to hold up in the column impeding the

flow of air which causes the pressure drop to rise. The hold up of water causes a decrease in

surface contact area between the gas and liquid streams which in turn decreases the rate of

mass transfer. Above the flooding velocity, the tower cannot operate. In this experiment, the

flooding point is noticed when the water in the packing column suddenly shooting up to the

column with high speed and the monometer reading starts to be unstable. This situation is

being discovered when the water flow rate is 40 (L/min) and the gas flow rate at 35 (L/min).

Notice that flooding point occurred at higher flow rate of water and also gas. At high gas flow

rate, the liquid is prevented from draining down the tower by the frictional drag of the gas on

the liquid. The loading point is the rest of the situation from 10 (L/min) until 30 (L/min) of

volume water flow rate.

Pressure drop (∆P)as a function of gas (air) and liquid (water) mass velocities (L/min)

using flexi glass packed with Raschig rings

There is a pressure drop occurs in the packed tower due to differences in pressure at the top

and bottom of the column due to intimate contact between the liquid and gas flow streams in

the packed column so that mass transfer yields. For this packed column, we used raschig

rings. Raschig rings are pieces of tube (approximately equal in length and diameter) used in

large numbers as a packed bed within columns for distillations and other chemical

engineering processes. The pressure drop for the volume water flow rate 1.0 (L/min), the

range is between 0.0 mm H2O until 15 mm H2O where 15 mm H2O is the highest pressure

drop at gas flow rate of 180 (L/min). While the pressure drop for the volume water flow rate

2.0 (L/min), the range is between 0.0 mm H2O until 59 mm H2O where 59 mm H2O is the

highest pressure drop at gas flow rate of 180 (L/min). For volume water flow rate 3.0

(L/min), the pressure drop range is between 0.0 mm H2O until flooding point at gas flow rate

12

of 140 (L/min). We noticed that as the gas flow rate increases, the pressure drop will also

increases. This is because packed tower used for continuous counter current contact of liquid

and gas is vertical columns which have been filled by packings. The liquid is distributed over

and trickles down to the packed bed thus exposing a large surface to contact the gas. The

frictional losses increase as the gas flow rate is increased. Since both the gas and liquid are

competing for the free cross-sectional area left by packing, an increase in liquid flow rate will

result in an increase in the frictional losses thus producing an increase in the pressure drop

also. The graph Ln (V) versus Ln (∆P / m packing) for every volume water flow rate also

clearly shows that the pressure drop increases linearly to the gas flow rate.

While undergoing the experiment, there are few mistakes that we made. One of them

that we do not control the valve VR-4 well so that the level of liquid returning to the water

reservoir must always be higher than the bottom of the reservoir. As a result, the pressure

reading on the monotube is not stable that make it harder for us to give an accurate value.

Furthermore, we also happened to have parallax error in reading the pressure value.

Figure 1 : packed tower of Raschig Rings

CONCLUSION

13

At the end of the experiment, we had managed to determine the loading and flooding

point where the loading point is from the volume water flow rate of 1.0 (L/min), 2.0 (L/min)

and 3.0 (L/min). When the volume water flow rate at 3.0 (L/min), the flooding point started at

gas flow rate of 140 (L/min). Other than that, we also had determined the pressure drop (∆P)

as a function of gas (air) and liquid (water) mass velocities ((L/min)) using flexi glass packed

with Raschig rings. The pressure drop for the volume water flow rate 1.0 (L/min), the range

is between 0.0 mm H2O until 15 mm H2O where 15 mm H2O is the highest pressure drop at

gas flow rate of 180 (L/min). While the pressure drop for the volume water flow rate 2.0

(L/min), the range is between 0.0 mm H2O until 59 mm H2O where 59 mm H2O is the highest

pressure drop at gas flow rate of 180 (L/min). For volume water flow rate 3.0 (L/min), the

pressure drop range is between 0.0 mm H2O until flooding point at gas flow rate of 140

(L/min). We can conclude that as the gas flow rate increasing, the pressure drop (∆P) will

also increase. Thus, we can conclude that all the objectives of the experiment had been

reached.

RECOMMENDATIONS

During the experiments, there are some mistakes that have been made. Those

mistakes had affected the result of experiment which is the pressure drop (∆P)as a function of

gas (air) and liquid (water)mass velocities ((L/min)) using flexi glass packed with Raschig

rings. The mistakes are caused by parallax error when reading the pressure on the monotube

and also due to the lack of control on the water pump. So, there are some recommendations in

order to improve our results in getting an accurate pressure drop. First, we need to make sure

that we set-up the arrangement of valves accordingly. For this experiment, we need to set up

the arrangement for valve for U-tube (Left) only. Then, we need to set-up the operating

arrangement. The U-tube (Right) and Sphere ball arrangement are not used. Make sure that,

the valve V-3 is closed. If not the water could not be pumped into the monotube. Other than

that, we need to make sure that the level of liquid returning to the water reservoir must

always be higher than the bottom of the reservoir. This is to avoid air being trapped in line. In

order to do that, valve VR-4 must be well controlled and conducted. If not, the water level in

the monotube would be unstable and hard for us to read the pressure. Moreover, we must

observed that there are no air bubble in the gas flow so it will not affect our results such as

the gas pressure is too low to pump the water up to the column. To avoid parallax error when

reading the pressure on the monotube, we must put a white paper behind the glass of the

monotube and with a ruler, we read the pressure with our eyes directly straight the scale. In

14

this experiment, there is calibration error in the scale of the liquid water flow rate

measurement where the scale begins at 11 (L/min). So, to begin with 10 (L/min), the volume

flow rate must be set up at 21 (L/min). Lastly, we have to constantly adjust the volume flow

rate when we change the gas flow rate. This is because the volume flow rate will

automatically change slightly from its initial position as we change the gas flow rate.

REFERENCES

15

CAG Gas Absorption Column. Retrieved June 1, 2015 from

http://www.edibon.com/products/catalogues/en/units/chemicalengineering/

chemicalengineeringbasic/CAG.pdf.

Gas Absorption handouts. Retrieved June 1, 2015 from

http://www.engr.uconn.edu/~ewanders/CHEG237W/Gas-Absorption.pdf.

Gas-liquid Absorption Column. Retrieved June 1, 2015 from

http://www.eng.utoledo.edu/polymer/info/Courses/LabIIHandouts/Gas_absorption.pd

f

Solution for Gas Absorption Experiment. Retrieved June 1, 2015 from

http://www.solution.com.my/pdf/BP05(A4).pdf.

Gas absorption Experiment. June 1, 2015 from

http://www.slideshare.net/dp93/gasabsorption-experiment.

Chemical Engineering: Gas Absorption Column. June 1, 2015 from

http://chem.engr.utc.edu/Webres/435F/ABS_COL/abs_col.html.

C.J. Geankoplis.Transport Processes and Separation Process Principles(includes unit

operations). Fourth edition. Page 119.

APPENDICES

16

Figure 1 Control unit

Figure 2 Water flow rate controller (LPM)

17

Figure 3 Gas absorption unit

Figure 4 Schematic diagram of gas absorption column

18