ABSTRACT

The Gas Absorption experiment was conducted in order to examine the air pressure drop

across the absorption column as a function of air flow rate with a different rates of water

flow. The result obtained is to be compared between theoretical values that has been

calculated. The experiment was run three times with different water flow rate which are 1.0

L/min, 2.0 L/min and 3.0 L/min. For every water flow rate, it was run for different air flow

rate of 20 L/min, 40 L/min, 60 L/min, 80 L/min, 100 L/min, 120 L/min, 140 L/min, 160

L/min and 180 L/min. Graph of pressure drop against the air flow rate was plotted and it

showed an increasing pattern. The flooding point was recorded during the water flow rate of

3.0 L/min and air flow rate of 80 L/min. The value of pressure drop taken was 14 mm H2O

with 3 mm H2O as compared in Appendix. Basically the pressure drop is increasing when the

air flow rate increased. The flooding happened when the air pressure from the bottom is too

high and pushed the water up. The percentage different between the pressure drop gained

from Appendix and the one recorded is 78.57%.

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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 air 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 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. 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.

2

Figure 1: Process flow diagram for gas absorption unit.

3

OBJECTIVE

To examine the air pressure drop across the column as a function of air flow rate for

different water flow rates through the column.

THEORY

In an absorption process, two immiscible phases (gas and liquid) are present in which the

solute will diffuse from one phase to the other through an interface between the two phases.

For a solute A to diffuse from the gas phase V into the liquid phase L, it must first pass

through phase V, the interface, and then into phase L in series. A concentration gradient has

to exist to allow the mass transfer to take place through resistances in each phase, as

illustrated below.

The concentration in the bulk gas phase, yL decreases to yi at the interface, while the liquid

concentration starts at xi at the interface and drops to the bulk liquid phase concentration xL.

There is usually negligible resistance to mass transfer across the interface, thus xi and yi are

in equilibrium with each other and are related in an equilibrium relationship.

There are typically two types of diffusions in an absorption process :

i) Equi molar counter- diffusion – two components diffusing across the interface, one

from the gas to liquid phase, while the other from the liquid to gas phase.

ii) diffusion through stagnant or non-diffusing phase – only one component diffuses

across the interface through stagnant gas and liquid phases.

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For a gas absorption process, it is common that only one solute component diffuses through

stagnant gas and liquid phases. Thus, the rates of mass transfer in a packed absorption

column for air (gas) can be quantified by this equation :

NA = KG (PA1 – PA2)

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APPARATUS

The type of gas absorption unit used as figure below was SOLTEQ-QVF Gas Absorption

Unit with a glass type made of borosilicate 3.3 glasses with PTFE gaskets.

PROCEDURE

1. General start-up procedure of gas absorption unit was performed by laboratory

assistance.

2. Firstly, the valve V11 is opens slowly and adjusted to get the water flow rate of

around 1 L/min. Water are allowed to enter at the top of the column K1, and then flow

down the column and accumulated at the bottom until it overflows back into vessel

B1.

3. After that, valve V1 is open and adjusted to get an air flow rate of 20 L/min into

column K1.

4. After 2 minutes, the liquid and gas flow in column K1 is observed, and the pressure

drop across the column at dPT-201 is recorded.

5. Repeat step 3 and 4 with different values of air flow rate until the flooding in the

column K1 occurs while the water flow rate is maintained.

6. Step 2 to 5 was repeated with different values of water flow rate by adjusted the valve

V11.

6

RESULTS

a)

Flow rate

(L/min)

Pressure drop (mm H2O)

Ai

r

Water

20 40 60 80 100 120 140 160 180

0 - - - - - - - - -

1.0 - 1 2 4 8 12 18 56 flooding

2.0 - 2 5 10 19 47 floodin

g

flooding flooding

3.0 1 4 7 14 floodin

g

flooding floodin

g

flooding flooding

b) Data used to plot graph of column pressure drop against the air flow rate for every

different water flow rate

Flowrate

(L/min)

Pressure drop (mm H2O)

Log V, Air

Water

1.3010 1.6021 1.7782 1.9031 2 2.0792 2.1461 2.2041 2.2552

0 - - - - - - - - -

1.0 - 0 0.3010 0.6021 0.9031 1.079 1.2553 1.7481 -

2.0 - 0.3010 0.6990 1 1.2788 1.6721 - - -

3.0 0 0.6021 0.8451 1.1461 - - - - -

Table 1 (a) and (b) : Table of pressure drop values experimentally.

*At flow rate 3.0 l/min and the air 160 l/min, it experienced the flooding point.

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Flow

Rates

(L/min)

Pressure Drop (mm H20)

Air

Water

0.004

9

0.019

4

0.043

5

0.077

6

0.120

9

0.174

1

0.236

9

0.309

4

0.391

6

0 0 0 0 0 0 0 0 0 0

0.7290 0 0 2 3 3.5 5 7 8 10

0.9720 0 0 2 3 4 7 10 21 29

1.0935 0 0 0 3 4 7 19 42 -

Table 4 : Pressure – drop taken from pressure – drop correlation graph.

(Based on Appendix D)

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DISCUSSION

In this experiment, the data was tabulated based on the necessary formula given. As shown in

the calculation part, the pressure drop based on the equipment was stated. The following are

the graph showing the pattern on the pressure drop with different value of water flow rate; 1.0

l/m , 2.0 l/m and 3.0 l/m.

All the graphs plotted are based on the pressure drop versus the flow rate of air.

Graph of pressure drop against air flow rate of three different water flow rate

0 20 40 60 80 100 120 140 160 1800

10

20

30

40

50

60

1.0 l/min2.0 l/min3.0 l/min

As can be observed from the graph above, as the flow rate of air increase, the pressure drop

values are also increased. The pattern of pressure drop, increased proportionally with air flow

rate. This is because; the gas starts to hinder the liquid downflow, and local accumulation or

pools of liquid starts to appear in the packing. As the air flow rate increased, the liquid

holdup increases (C. H. Geankoplis, 2003). Also, shown is the plotted graph of column

pressure drop, log P against the air flow rate, log V.

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Air flow rate

Pressure drop

1.301 1.6021 1.7782 1.9031 2 2.0792 2.1461 2.2041 2.25520

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

column pressure drop, log P against air flow rate, log V (air flow rate)

1.0 L/min2.0 L/min3.0 L/min

log

P (P

RESS

URE

DRO

P)

In table 4, the data obtained was from appendix D, which is calculation of theoretical value of

flooding point. In this section, it is to be compared of what have been recorded from the

instrument itself. Below are the comparisons of the value of pressure drop.

Flow

Rates

(L/min

)

Pressure Drop (mm H20)

Air

Water

20 40 60 80 100 120 140 160 180

A B A B A B A B A B A B A B A B A B

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 0 1 0 2 2 4 3 4 3.5 8 5 1

2

7 1

8

8 5

6

10 -

2 0 0 0 2 2 5 3 1

0

4 1

9

7 4

7

10 - 21 - 29 -

3 0 1 0 4 0 7 3 1 4 - 7 - 19 - 42 - - -

10

4

A – Theoretical value of flooding point ( from Appendix )

B – Data from experiment

To be discussed here is the flooding point, at the point where the flow rate of air is 160 l/min

and flow rate of water is 3 l/min the pressure drop has achieved the flooding point in this

experiment. At that point, the water level has reached the limit on the warning line on the

instrument itself in the tower.

The different percentage of the two values of the flooding point above is as below :

Error=¿❑|Theoreticalvalue−Actual valueActual value |x 100 %¿

Where theoretical value is the value from the appendix D meanwhile actual is what recorded

during the experiment.

Error=¿❑|14−314 |x 100 %¿

Error=¿❑78.6 %¿

Flooding is very significant terms with both gas absorption and distillation of it involves the

use of packed towers. Flooding means that the gas velocity is very high, therefore, does not

allow the flow of the liquid from the top of the tower, and flooding occurs on the top of it

(ergo, the gas phase is not completely mixed by the water phase). The best gas velocity,

should be half of the flooding velocity. 

A key aspect in an absorption system is the contact between the gas and liquid phase.

Although other contactor exist such as tray column, spray column, bubble column and

membrane contactor, this research work focuses more on packed column which operates with

either random or structured packing. A packed column like many other industrial processes

encounters certain problems, of which flooding is a major concern. Flooding is a

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phenomenon by which gas moving in one direction in the packed column entrains liquid

moving in the opposite direction in the packed column.

Flooding is undesirable because it can cause a large pressure drop across the packed column

as well as other effects that are detrimental to the performance and stability of the absorption

process. Hence, in the design of the absorption packed column, many parameters need to be

considered for efficiency to be attained and also avoid flooding problem which will be

preceded with setting the limits for the experiment.

The air pressure drop across the column shows increment values when the air flow rate

increases. At high air velocity the friction encountered by the down-flowing liquid is greater

which make the pressure drop increases. Thus at higher air velocities the diffusion rate of

liquid is increases. More liquid fill the column which decreasing the cross-sectional area

available for gas flow. As liquid start to build up in the column, the pressure drop increased

until a point flooding happened in the column

There are few aspects in term have to be considered in conducting this experiment in order to

achieve the objective. Human error, which tends to make mistake during the experiment has

to be considered when the time adjusting the valve might not be exactly 100% perfect to get

best value. However, the nearest value is set during the experiment. Apart from that the

vapour which is consider as moist content that still exist in the tower. This will also affect the

reading during the experiment been conducted. Also, there should be gap of time from one to

another during the changes of the flow rate of water so that the system been stabilise.

As for the equipment error, it may happen due to packed bed is not been set up properly

before the experiment. Other than that, condition of the compressor also should be checked

before conducting the experiment just to ensure that the compressor operates normally. Other

than compressor, the air valve also should be checked just to make sure that it can adjusted to

desired flow rates because without getting right air flow rates, the result cannot be recorded.

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CONCLUSION

The flooding point for the experiment has been observed. The importance of why the

flooding point has to be figure out is to know what is the flooding point of the instrument

been used, as the user will notified by the limit at which level the instrument will faced up the

flooding point. The flooding point was recorded during the water flow rate of 3.0 L/min and

air flow rate of 80 L/min. The value of pressure drop taken was 14 mm H2O with 3 mm H2O

as compared in Appendix. The percentage different between the pressure drop gained from

Appendix and the one recorded is 78.57%.

RECOMMENDATIONS

Use inert gases such as nitrogen gas or carbon dioxide gas to replace air in order to

observe different behavior of pressure drop.

Replacing water with triethylene glycol, TEG. When TEG flows from top to bottom,

it will absorb water vapor in air. Increasing TEG flow rate increases water absorption

rate thus affecting its pressure drop.

Use structured packing rather than random packing. Example of structured packing is

Flexipac and Mellapak. It has high surface area which allows vapour-liquid contact

area, having a good efficiency. It also has a large void space per unit column volume

which will minimize the resistance for gas up flow thereby enhancing packing

efficiency.

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REFERENCES

Absorption. (n.d.). Gas Absorption & Desorption. Retrieved on 29th of October

from http://www.separationprocesses.com/Absorption/GA_Chp03.htm

Balogun K.A (2010) Optimization of Flooding In An Absorption Desorption

Unit. Retrieved from

http://publications.theseus.fi/bitstream/handle/10024/20671/Balogun_Kamorudeen.pd

f?sequence=1 on the 24th April 2013.

Dixon, D., Higgins, K., Fox, B. (2012). Gas Absorption Into a Liquid in a

Packed Column. Oklahoma State University. Retrieved in 20th April, 2013.

Dr. Rami Jumah (2002). Unit Operation Laboratory. Jordan University of

Science and Technology. Retrieved on 20th April 2013

Geankoplis, C. J. (2003). Transport Processes and Separation Process

Principle. Fourth edition, page 659.

Giles, R.V. et. al. (1994), Outline Series Theory and Problems of Fluid

Mechanics and Hydraulic, McGraw Hill Intl.

J. M Coulson et. al., Fluid Flow, Heat Transfer and Mass Transfer, Volume 1,

6th Edition, Coulson & Richardson.

Richardson, J. F. and Harker, J. H. (2002). Chemical Engineering. Fifth

Edition. Page 655.

Sakshat Virtual Lab. (n.d.). Gas Liquid Absorption. Retrieved on 29th of

October 2014 from http://iitb.vlab.co.in/?sub=8&brch=116&sim=951&cnt=1

Yunus A. Cengel et. al., Fluid Mechanics Fundamentals and Applications, 2nd

Edition, McGraw Hill.

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APPENDIX

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