Gas Absorption Theory,Apparatus,Procedure

1.0 ABSTRACT The experiment is based on pressure drop, the air flow rate, the water flow rate and also the packed column. The pressure drop is increased when the water flow rate and air flow rate is increased. This experiment is to examine the air pressure drop across the column as a function of air flow rate for different water flow rates through the column. The graph of log pressure drop against log of air flow rate is plotted. The graph of generalized theoretical pressure drop correlation chart for random packing is also plotted. Both of the graph have same principle where high flow rate parameter is meant for high liquid flow and high pressure drop while low flow rate parameter is meant for low liquid flow and low pressure drop. In conclusion, the air pressure drop across the column increases as the air flow rate increases as well as the water flow rate through the column. From the experiment, the value of experimental pressure drop is higher compared to the correlated values for packed column. For packed column of water flowrate of 1 LPM, there is no error since it flooded according to the theory, followed by that of water flowrate of 2 LPM which is 14.28 %. At water flowrate of 3 LPM, the error involved is 20%. These percentage errors between theoretical and correlated calculations of flooding point are slightly high due to some error in reading the flowrate. Hence, the reading should be taken twice to make sure accurate reading is taken.

2.0 INTRODUCTION Gas absorption (also known as scrubbing) is an operation in which a gas mixture is contacted with a liquid for the purpose of preferentially dissolving one or more components of the gas mixture and to provide a solution of them in the liquid. Therefore we can see that there is a mass transfer of the component of the gas from the gas phase to the liquid phase. The solute so transferred is said to be absorbed by the liquid. In gas desorption (or stripping), the mass transfer is in the opposite direction, i.e. from the liquid phase to the gas phase. The principles for both systems are the same. There are 2 types of absorption processes: physical absorption and chemical absorption, depending on whether there is any chemical reaction between the solute and the solvent (absorbent). When water and hydrocarbon oils are used as absorbents, no significant chemical reactions occur between the absorbent and the solute, and the process is commonly referred to as physical absorption. When aqueous sodium hydroxide (a strong base) is used as the absorbent to dissolve an acid gas, absorption is accompanied by a rapid and irreversible in the liquid phase and the process is referred to as chemical absorption or reactive absorption. The absorption process requires the following steps: 1. Diffusion of the solute gas molecules through the host gas to the liquid boundary layer based on a concentration gradient 2. Solvation of the solute gas in the host liquid based on gas-liquid solubility 3. Diffusion of the solute gas based on concentration gradient, thus depleting the liquid boundary layer and permitting further solvation

Method of Operation: A. Counter-current Operation 

It was widely used in the industry.



The gas enters the column or tower from below as leaves at the top, while liquid enters from the top and flows in opposite direction and exits from the bottom.

B. Co-current Gas Absorption 

This mode of operation is seldom used in practice.



It is less efficient than counter-current operation.

ABSORPTION EQUIPMENT 

Tray (or plate or stage) Columns- The types of trays used in absorption include: sieve tray, valve tray and bubble-cap trays. These internals are the same as those covered in "Distillation"



Packed Column- Both random and structured packing had been used.



Spray Column- The gas flows upward continuously through an open chamber in which scrubbing liquid droplets falls from spray nozzles through the gas. The gas pressure drop is small, but separation is not as good as the bubble column. This column is widely used for its simplicity, low pressure drop, and resistance to scale deposition and plugging.



Bubble Column- The gas is forced under pressure through perforated pipes submerged in the scrubbing liquid. As such the gas phase is dispersed and the liquid phase is continuous. As the bubbles rise through the liquid, absorption of the gas occurs. This type of device suffers from the high pressure drop due to the liquid hydrostatic head.

3.0 OBJECTIVES 

To examine the air pressure drop across the column as a function of air flow rate for different water flow rates through the column.



To determine the pressure drop across the dry column as a function of air flow rate.



To study the operation of Gas absorption.

4.0 THEORY Deals with the mass transfer operation known as gas absorption in which a soluble gas is absorbed from its mixture with an inert gas by means of a liquid in which the solute gas is more or less soluble. A common apparatus used in gas absorption is packed tower, consists of a cylindrical column, or tower, equipped with a gas inlet and distributing space at the bottom, a liquid inlet and a distributor at top, gas and liquid outlets at the top and bottom respectively, and a supported mass of inert solid shapes, called tower packing. In a gas absorption column, a component of the gas stream is absorbed into the liquid stream. The absorption may be purely physical, or it may involve solution of the gas into the liquid followed by chemical reaction. There are typically two types of diffusions in an absorption process: i)

Equimolar 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.

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: PA 1−PA 2 NA=KG ¿ ) Where NA is the molar flux of species A, KG is gas phase mass transfer coefficient, PA 1−PA 2 are the pressure drop. This experiment required to plot graph of pressure drop against air flow rate in graph. The flow parameter shows the ratio of liquid kinetic energy to vapour kinetic energy and parameter of K4 or y-axis needs and x-axis or FLV can be calculated by using these formulae: G2y F P μ0.1 Gx x g c ( ρ x −ρ y ) ρ y G y

√

ρy ρx −ρ y

Gas absorption is a process where mixture of gas is in contact with liquid and becomes dissolve. Therefore, there is mass transfer occurs in the component that changes from gas phase to liquid phase. The solutes are absorbed by liquid. Inside this experiment, only the mass transfer between air and liquid are concerned. Gas absorption is widely use in industries to control the air pollution and to separate acidic impurities out of mixed gas streams. The pressure drop values are observed from the manometer. The graph of pressure

correlation for different flow rate of water is plotted in order to find the relationship between K4 and FLV. The steps on how to obtain K4 and FLV is shown below: Density of air, ρG = 1.175 kg/m3 Density of water, ρL = 996 kg/m3 Column diameter, Dc = 80 mm Area of packed diameter,

A c=

π 2 D 4

Packing Factor: Fp = 900 m-1 Water viscosity, µwater = 0.001 Ns/m2 Theoretical Flooding Point 1. Gy must be in m3/h 2. To calculate gas flow rate, GG (kg/m2s) G G=

3.

Gy × ρ Ac

To calculate capacity parameter, K4, 2

13.1 ( GG ) F p K4=

μL ρL

0.1

( )

ρG ( ρ L− ρG )

4. To calculate liquid flow rate, GL (kg/m2) (1 LPM, 2 LPM, 3 LPM) GL =

G× ρ Ac

5. To calculate flow parameter, FLV (1 LPM) FLV =

GL GG

(√ ) ρG ρL

Where: Gy

= Air flow rate (m3/h)

5.0 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.

Column K1

Vessel B1 Receiving vessel B2 Air Flow rate Pump P1

6.0 PROCEDURE / METHODOLOGY 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.

Flow rate (l/min)

Pressure drop (mm H2O)

Water Air 20

40

60

80

100

120

140

160

180

0

-

-

-

-

-

-

-

-

-

1

0.0

0.0

1.0

3.0

5.0

8.0

1.2

20.0

45.0

2

0.0

1.0

3.0

7.0

16.0

29.0

65.0

-

-

3

-17.0

-13.0

-6.0

3.0

40.0

-

-

-

-

RESULTS

Figure below show the graph for Pressure drop against air flow rate

7.0

8.0 CALCULATION Sample Calculations Data: Density of air = 1.175kg/m3 Density of water= 996kg/m3 Column diameter = 80mm Area of packed column diameter = 0.005027m2 Packing Factor = 900 m-1 Water viscosity = 0.001 Ns/m2 Theoretical Flooding Point: GG, gas flow rate (kg/m2s) GG = GyXp / A

=

20 L 1 min 1.175kg 1m 3    3 min 60 sec m 1000 L  0.0050207

=0.0779kg/m2s

Capacity parameter, y-axis

L ) L G ( PL  PG )

0.1

13.1(GG ) 2 Fp ( =

2

13.1(0.0779) 900( =

0.001

996 1.175(996  1.175)

0. 1

)

= 0.0154 GL, liquid flowrate per unit column cross-sectional area GL X p =

A

=

1L 1 min 996kg 1m 3    min 60 sec 1000 L m3 0.005027

= 3.3022 Flow parameter , x- axis

x-axis =

GL  G ( ) GG  L

=1.456 Water Flow Rate (L/min)

GL (kg/m2s)

1.0

3.3022

2.0

6.6043

3.0

9.9065

Table 2 : Water Flowrate and GL , Liquid Flowrate per Unit Column Cross-sectional Area

Air flow rate (L/min)

GG (kg/m2s)

Capacity Parameter (y-axis)

Flow parameter (x-axis)

1.0 LPM

2.0LPM

3.0LPM

3.3022

6.6043

9.9065

20

0.0779

0.0154

1.456

2.910

4.368

40

0.1557

0.0614

0.729

1.456

2.186

60

0.2336

0.1383

0.486

0.971

1.457

80

0.3115

0.2459

0.364

0.728

1.092

100

0.3893

0.3841

0.291

0.583

0.874

120

0.4672

0.5532

0.243

0.486

0.728

140

0.5453

0.7531

0.208

0.416

0.624

160

0.6232

0.9832

0.182

0.359

0.546

Table 3: Air Flowrate, gas flow rate (kg/m2s) abrv. GG,capacity parameter and flow parameter.

Figure 2 : Theoretical Pressure Drop Correlation Chart For Random Packings

Water Flow Rate

Theoretical Flooding Air Flow rate (L/min)

Experimental Flooding Air Flow rate (L/min)

Error (%)

1.0

180

180

0.0

2.0

140

160

14.28

3.0

100

120

20.00

(L/min)

Table 4: comparison of theoretical and correlation of flooding point

9.0 CONCLUSION In conclusion, the air pressure drop across the column increases as the air flow rate increases as well as the water flow rate through the column. From the experiment, the value of experimental pressure drop is higher compared to the correlated values for packed column. For packed column of water flowrate of 1 LPM, there is no error since it flooded according to the theory, followed by that of water flowrate of 2 LPM which is 14.28 %. At water flowrate of 3 LPM, the error involved is 20%. These percentage errors between theoretical and correlated calculations of flooding point are slightly high due to some error in reading the flowrate. Hence, the reading should be taken twice to make sure accurate reading is taken.

10.0 DISCUSSION The objective this experiment is to examine the air pressure drop across the column as a function of air flow rate for different water flow rates through the column. The experiment based on the flow rate of liquid and gas in the packed. Firstly the water flow rate is kept constant to 1 L/min and the air flow rate is then recorded after a 1 minute interval. Air flow rate is kept rising at constant by 20 L/min by each 5 minutes. All reading of pressure drop are then recorded until the flooding point is reached. The pressure drop for flow rate of air are 0, 1,3,5,8,12,20 and 45 mm H20 respectively to 20,40,60,80,100,120,140,160 and 180 L/min of air. The flow rate of water is then adjusted to 2 L/min, the data recorded are 0, 1,3,7,16,29 and 65 mm H20 respectively to 20,40,60,80,100,120,140 L/min of air. Next the experiment continue by 3 L/Min by each 5 minutes. All reading of pressure drop are then recorded until the flooding point is reached. The pressure drop for flow rate of air are -17,13,-6, 3, 40 mm H20 respectively to 20,40,60,80 and 100 L/min of air. At 2L/min and 3L/min cannot reach 180 L/min of air flow rate as the water will sprayed out from the column due to the high flow rate. The graph of column Pressure Drop vs. Air Flow Rate is plotted and in which the results from the plotted graph shown the higher the gas flow rate, the higher the pressure drop. For correlated value of the pressure drop, calculations has been made. The data from simple calculation that had been made density of air is 1.175kg/m 3, density of water is 996kg/m3, column diameter is 80mm, area of packed column diameter is 0.005027m 2, packing Factor is 900 m-1 and water viscosity is 0.001 Ns/m2. For Theoretical Flooding Point, GG, gas flow rate (kg/m2s) and Flow parameter, x- axis were calculated. Lastly, a graph of capacity parameter against flow rate parameter was plotted. The capacity parameter is indirectly proportional to flow rate parameter. From the calculation, the experiment for 1 LPM, there is no error, followed by that of water flow rate of 2 LPM which is 14.28 %. At water flow rate of 3 LPM, the error involved is 20% .

11.0 RECOMMENDATIONS

Some suggestion in improving the safety are to always check and rectify any leak and all operating instructions supplied with the unit must be carefully read and understood before attempting to operate the unit. Next, be extremely careful when handling hazardous, flammable or polluting materials such as CO2. Make sure the system is sufficiently ventilated when working at atmospheric pressure. The flow rate of air and water should be on the right amount, since this will affect the pressure drop. Do not proceed with different phases of the experiment until you understand how each piece of apparatus works. Do not be afraid to ask for help, for this experiment is rather complex and requires attention to detail to get good results. Other than that, when starting up the system, always use low initial air and water velocities. Be sure the recycle valve to the sump pump is always at least partially open to prevent build-up of liquid and flooding. An extension has been added to the top of the column to help prevent spillage of caustic. The gas cylinder regulator handle should be “loose” (easy to turn) before opening the tank. See safety instructions in the auxiliary section notebook. Open the tank valve slowly. Remember to plug in the gas heater 5 minutes before turning on the gas. Turn off the gas at the end of the day, or else you will not be able to operate during the next lab period. Relieve the spring pressure on the regulator diaphragm by backing out the regulator handle to its original “loose” position.

12.0 REFERENCES / APPENDICES Sakshat Virtual Lab. (n.d.). Gas Liquid Absorption. Retrieved on 10th of November 2015 from http://iitb.vlab.co.in/?sub=8&brch=116&sim=951&cnt=1 Books: 1. R. Treybal, Mass Transfer Operations, 2nd ed. McGraw-Hill, 1980. 2. J. H. Perry, Ed., Chemical Engineer's Handbook, 5th or 6th ed., p. 14.2 - 14.40, McGraw-Hill Publishing Co., New York, NY, 1973. 3. W. L. McCabe and J. C. Smith, Unit Operations of Chemical Engineering, 4th ed., p. 617-631, McGraw-Hill Publishing Co., New York, NY, 1985. 4. Dixon, D., Higgins, K., Fox, B. (2012). Gas Absorption into a Liquid in a Packed Column. Oklahoma State University. Retrieved in 20th April, 2013. 5. Dr. Rami Jumah (2002). Unit Operation Laboratory. Jordan University of Science and Technology. Retrieved on 20th April 2013 6. J. M Coulson et. al., Fluid Flow, Heat Transfer and Mass Transfer, Volume 1, 6th Edition, Coulson & Richardson. 7. Richardson, J. F. and Harker, J. H. (2002). Chemical Engineering. Fifth Edition. Page 655.