Gas Absorption Lab Report 5636r

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 . 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 ing 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 ed 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 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.

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Figure 1: Process flow diagram for gas absorption unit.

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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 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

ii)

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

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a) Flow rate

Pressure drop (mm H2O)

(L/min) Ai r

20

40

60

80

100

120

140

160

180

1

1 2 4

2 5 7

4 10 14

8 19 flooding

12 47 flooding

18 flooding flooding

56 flooding flooding

flooding flooding flooding

Water 0 1.0 2.0 3.0

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

Pressure drop (mm H2O)

(L/min) Log V, Air

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

-

-

-

-

-

Water

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.

Flow

Pressure Drop (mm H20)

Rates (L/min) Air Water 0 0.7290 0.9720 1.0935

0.004

0.019

0.043

0.077

0.120

0.174

0.236

0.309

0.391

9

4

5

6

9

1

9

4

6

0 0 0 0

0 0 0 0

0 2 2 0

0 3 3 3

0 3.5 4 4

0 5 7 7

0 7 10 19

0 8 21 42

0 10 29 -

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Table 4

: Pressure – drop taken from pressure – drop correlation graph. (Based on Appendix D)

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 Pressure drop

8

60

50

40 1.0 l/min

30

2.0 l/min 3.0 l/min

20

10

0 0

20

40

60

80

100

120

140

160

180

Air flow rate

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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column pressure drop, log P against air flow rate, log V (air flow rate) 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

log P (PRESSURE DROP)

1.0 L/min 2.0 L/min 3.0 L/min

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

Pressure Drop (mm H20)

Rates (L/min ) Air 20

40

60

80

100

120

140

160

180

Water 0 1 2 3

A B A B A B A 0 0 0 0 0 0 0 0 1 0 2 2 4 3 0 0

0 1

0 0

2 4

2 0

5 7

B 0 4

3

1

3

0 1

A 0 3.5

B 0 8

4

1

4

9 -

A 0 5

B 0 1

A 0 7

B 0 1

A 0 8

B 0 5

A 0 10

B 0 -

7

2 4

10

8 -

21

6 -

29

-

7

7 -

19

-

42

-

-

-

4 A – Theoretical value of flooding point ( from Appendix ) B – Data from experiment 10

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 : value− Actual value |TheoreticalActual |x 100 value

Error=¿❑

¿ Where theoretical value is the value from the appendix D meanwhile actual is what recorded during the experiment. x 100 |14−3 14 |

Error=¿❑

¿ Error=¿❑ 78.6 ¿

Flooding is very significant 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 between the gas and liquid phase. Although other or exist such as tray column, spray column, bubble column and membrane or, 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 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 11

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.

CONCLUSION

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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 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 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.

REFERENCES 13

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.

APPENDIX 14

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