Absorption final report

1. Introduction

Absorption or gas absorption is a unit operation used

in chemical industry and increasingly in environmental

applications. It 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 is more or less soluble. An example is

the absorption of the solute ammonia from air.

Figure 1. Packed tower

A common apparatus used in gas absorption is the packed

tower (Figure 1). The device consists of a cylindrical

column, or tower, equipped with a gas inlet and distributing

space at the bottom, a liquid inlet and distributing device

at the top, gas and liquid outlet at the top and bottom

respectively, and supported mass of inert solid shapes,

called packing. Packed columns are relatively simple devices

compared to plate columns. They can be categorized according

to the type of flow used in the operation: counter current,

co-current, and crosscurrent modes. The column used in this

experiment is operated in countercurrent flow, which is the

most frequently used type operation.

In a given packed tower with a given type and size of

packing and with definite flow of liquid, the upper limit to

the rate of gas flow is called the flooding velocity. The

tower cannot operate above this gas velocity. In a

countercurrent packed gas-liquid tower, the gas phase will

pass upward through the column. In order to let the gas

phase flow, there must be a sufficient pressure drop to

overcome the friction and form drag caused by the packing

and the falling liquid. The liquid must fall down against

this pressure drop by means of gravitational force. Once the

liquid is distributed over the top of the packing, the

liquid ideally flows in thin films over the entire packing

surface, but what really happens is the films tend to grow

thicker in some places and thinner in others, so that the

liquid collects into small rivulets and flows along

localized paths through the packing, especially at low

liquid rates. This is known as channeling. Loading point is

the point where the liquid will start to accumulate in the

column and when the gas flow increases further, flooding

point is reached. It is the point when the liquid in the

column will overflow.

2. Objectives of the experiment

Determine experimentally the pressured drop across a

wet packed column as a function of the air flow rate

and compare the results with theoretically calculated

values

Determine through visual observation and by graphical

methods the loading and flooding points of the packed

column at preset values of water flow rates

Construct form experimental data the loading and

flooding curves of the packed column based on the

generalized correlations proposed by Sherwood, Shipley

and Holloway

3. Methodology

3.1 Materials

Water

3.2 Equipment and Apparatus

Packed Absorption Column (with Raschig ring random

packing)

Stopwatch

Thermometer

Figure 1. Packed Absorption Column

Parts of a Packed Absorption Column

(1) Sump tank

(2) Valve in the discharge pipe to the sump tank

(3) Water(left) and mercury(right) manometers

(4) Air Flow meter

(5) Gas analyzing equipment

(6) Air flow valve

(7) Upper stopcock (S2)

(8) Water flow valve

(9) Air flow meter

(10) CO2flow meter

(11) Lower Stop cock (S3)

(12) Air pump

(13) Water pump

(14) Down-coming tube

3.3 Procedures

3.3-1. Preliminary Preparations

The valve in the water discharge tube was opened while

the drains under the tank and down-coming tube were closed.

The tank was filled with distilled water up to three fourth

its volume. The three 3-way cocks between the column and

manometer were positioned in such a way that only the water

manometer is used to measure the pressure drop.

3.3-2. Pressure Drop in a Wetted Column

In this part of the experiment, the pressure drop in a

wetted column was determined using conditions number two (2)

from Table 422-5.1 of the manual. The gas and water flow

meters and the stopcocks were closed, while C4 was fully

opened. The water pump was switched on and the flow rate was

set to 3-4 liters/minutes. The pump ran for two to three

minutes before the pump was turned off. The column was

drained for five minutes. With S2 and S3 open, manometer

readings of pressure differences across the column were

taken for airflow rates ranging from 20 to 170 liters/minute

starting with low rates. Another trial was done but this

time starting with the highest flow rate which is from 170

down to 20 liters/minute. At least ten flow rates were

tested for each trial. The operational temperatures of the

liquid and the gas were recorded at the start and end of the

experiment. The pressure drop across the wet column were

calculated from using the manometer readings obtained. These

experimental values were plotted against air flow rates and

were compared with theoretical values.

3.3-3. Identifying the Loading and Flooding Points

In this part of the experiment the loading and flooding

points of the packed column were identified using conditions

number two (2) for Part C from Table 422-5.1 of the manual.

The operational temperatures of the liquid and the gas were

recorded at the start and end of this experiment. For each

assigned value of water flow rate, ten air flow rates were

applied ranging from 20 to 170 liters/minute. Visual

observations of the column at each setting were noted and

through this the loading and flooding points of the column

were identified. The loading point is the point when water

started to accumulate the column while the flooding point is

the point when the water level has reached the top of the

packing in the column. Manometer readings of pressure

difference across the column were recorded per air flow

rate. Pressure drop was calculated and plotted against gas

mass velocity to determine the loading and flooding points

graphically. Using experimental data the loading and

flooding curves of the packed column were constructed based

on the generalized correlations proposed by Sherwood,

Shipley and Holloway.

3.3-4. Shut Down Operations

After the experiment, the water tank and the water from

the down-coming tube were drained and then their valves were

closed.

4. Results and Discussions

4.1 Pressure Drop in a Wetted Column

Table 1. Pressure Drop in a Wetted Column with Increasing

Air Flow Rate

Air Flow Rate

(L/min)

∆H

(m)

∆P (Pa)velocity

(m/s)∆P/HActual Theoret

ical

20 0.0019.7710543

25.09789

0.075451232

13.38501

35 0.00329.3131629

43.10072

0.132039656

40.15502

50 0.00548.8552715

65.84833

0.188628081

66.92503

65 0.00768.3973801

93.34113

0.245216505

93.69504

80 0.0197.710543

125.5781

0.301804929

133.8501

95 0.014136.7947602

162.5597

0.358393353

187.3901

110 0.018175.8789774

204.2851

0.414981778

240.9301

125 0.022214.9631946

250.7554

0.471570202

294.4701

140 0.034332.2158462

301.9668

0.528158626

455.0902

155 0.039

371.3000

634357.9249

0.58474705

522.0152

170 0.044

390.8421

72418.6293

0.641335474

588.9403

* Twater = 29°C, Tair = 26°C

Table 2. Pressure Drop in a Wetted Column with Decreasing

Air Flow Rate

Air Flow Rate

(L/min)

∆H

(m)

∆P (Pa)velocity

(m/s)∆P/HActual Theoreti

cal

170 0.048

469.0765

296 93.62101

0.6413354

74

642.57

06

155 0.041

400.6695

357 79.26914

0.5847470

5

548.86

24

140 0.035

342.0349

695 66.24251

0.5281586

26

468.54

11

125 0.028

273.6279

756 54.5416

0.4715702

02

374.83

28

110 0.021

205.2209

817 44.16075

0.4149817

78

281.12

46

95 0.016

156.3588

432 35.10377

0.3583933

53

214.19

02

80 0.012

117.2691

324 27.36952

0.3018049

29

160.64

26

65 0.008

78.17942

16 20.95889

0.2452165

05

107.09

5150 0.006 58.63456 15.8714 0.1886280 80.321

62 81 32

35 0.003

29.31728

31 12.10808

0.1320396

56

40.160

66

20 0.003

29.31728

31 9.668494

0.0754512

32

40.160

66* Twater = 29°C, Tair = 26°C

The experimental and theoretical pressure drops in a

wetted column at increasing and decreasing air flow rates

are shown in Tables 1 and 2, respectively. As evident in the

tables presented above, the pressure loss increases with

increasing gas flow rate. A greater manometer reading is

obtained when the air flow rate is increased thus a greater

pressure drop. The pressure drop will increase because of

the drag force against the packing and the falling water.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

100

200

300

400

500

600

700

Air Flow Rates

Theoretical Pressure DropDecreasing Air Flow RateIncreasing Air Flow RateDry Column

v (m/s)

∆P/H

Figure 2. Comparison between the experimental and

theoretical pressure drop in a wetted column

As shown in Figure 2, the experimental pressure drop

increases as the air flow rate is increased and the reverse

can be observed for the decreasing air flow rate. It can

also be observed that the theoretical pressure drop is

lesser than the actual. This is because of the assumptions

made in the calculation of the theoretical value. A smooth

pipe was assumed

2.2 Visual and Graphical Determination of the Loading and

Flooding Points of the Packed Bed at Pre-set Water Flow

Rates

Flooding and loading points were determined both

graphically and through visual observation. Loading point

was observed when the water started to accumulate at the

base of the column whereas the flooding point was observed

when the water level reached the topmost part of the packing

in the column.

The loading and flooding point was determined

graphically by plotting log (∆P/∆L) against log vo which is

shown in figure 3. The points where there is a drastic

change in the slope of the curve are the loading and

flooding points.

Table 3. Loading and Flooding Points obtained graphically

and visually

Water

flow

rate

(L/min)

Air flow rate (L/min)Loading point Flooding point

visual graphical Visual graphical

1.5 140 135.187 - -2.0 125 115.041 155 -2.5 110 94.101 135 130.1503.0 95 80.052 110 95.1613.5 70 73.160 110 91.9804.5 65 58.581 80 72.3655.0 50 53.014 75 64.678

Based on the table above, it is observed that as the

liquid water flow rate is increased, the air flow rates at

which loading and flooding are observed decrease. This

occurs because the higher the water flow rate, the easier it

is for the liquid to accumulate due to the significant

decrease in the area of path of the gas to flow. The area is

decreased by the resisting liquid flow countercurrent to the

flow of gas.

For the graphical method, the flooding point was not

reached for water flow rates 1.5 L/min and 2.0 L/min because

it was observed that the height of the water almost reached

the topmost part of the column therefore no further readings

were made.

2.2Loading and Flooding Curves of the Packed Column From

Experimental Data Based on the Generalized Correlations

Proposed by Sherwood, Shipley and Holloway

Empirical correlations for various random packing based

on experimental data are used to predict the pressure drop

in the gas flow. Sherwood, Shipley and Holloway proposed the

first generalized correlation for flooding of packed columns

based on tests done with air-water systems.

0.3 3

0.005

Loading Based on Visual Observation

Power (Loading Based on Visual Observation)

Flooding Based on Visual Observation

Power (Flooding Based on Visual Observation)

Loading Based on Graphical Method

Power (Loading Based on Graphical Method)

Flooding Based on Graphical Method

Power (Flooding Based on Graphical Method)

Figure 4. Generalized Pressure Drop Correlation

Figure 4 shows the flooding line and loading line based

on both visual observation and graphical method. The

flooding and the loading points obtained through visual

observation are slightly greater than the one obtained

graphically, this is because the increments of the air flow

rates experimentally are big which makes it difficult to

determine the exact flooding and loading point, unlike

graphically wherein the trend of the curve can be clearly

seen.

The Y-axis represents the capacity parameter and the

X-axis is the flow parameter. The flow parameter corresponds

to the liquid-to-gas kinetic energy ratio while the capacity

parameter is a function of the square of the actual gas

velocity [Seader, 2006]. The figure above shows that the

capacity parameter decreases with increasing flow parameter.

This is because an increase in water flow rate increases the

liquid-to-gas kinetic energy. A decrease in this ratio

causes a decrease in the capacity parameter.

5. Conclusion

Absorption is a unit operation that contacts two phases

—usually a gas and a liquid—where the more gas soluble

solute is absorbed from the liquid. To increase contact

between the two and increase efficiency as a consequence,

packed beds are used. The presence of the packed beds

requires greater pressure for the gas to flow past it and

this result in pressure loss. As the air flow rate is

increased, the resulting pressure drops increase as well.

This is in agreement with the results if the pressure losses

are to be calculated theoretically. The use of Erguns

equation for the pressure drop in a packed bed, as well as

Hagen-Poisuille equation to account for the parts of the

absorption column free of the packings, gives an increasing

pressure loss with increasing air flow rates.

One major problem that has to be given attention when

it comes to absorption is that of flooding. Flooding is the

point at which the liquid overflows the column as a result

of a high air flow rate. The prelude to this point is the

loading point which can be observed as the start of

accumulation of the liquid in the packings. Aside from these

visual observations, the flooding and loading points can be

determined using the graphical method as well. These help in

the designing of the absorption tower as most equipment run

on 50-70% only of the flooding velocity; that is why it is

important to know these two velocities.

Sherwood, Shipley and Holloway were some of the first

people that proposed a generalized correlation for flooding

and loading of packed columns. They plotted the ratio of the

kinetic energy of the gas to the potential energy in the

liquid versus the flow parameter—a dimensionless number that

measures the relative kinetic energy of the system. This

gives a downward slope curve resulting from tests using a

broad range of air and water velocities. In the experiment,

only a part of this curve was achieved.

6. References

Geankoplis, C.J. (2009). Transport Processes and Unit

Operations.4th Edition.

Prentice Hall,New Jersey.

Henley, E.J., Seader, J.D. (2006).Separation Process

Principles.2nd Edition. John

Wiley & Sons., New Jersey.

McCabe,W.L. et.al. (2001). Unit Operations of Chemical

Engineering.6th Edition,

McGraw-Hill., New York.

VII. Appendices

Appendix A. Sample Calculations

Solving for Actual Pressure Drop:

ΔP=ρg(h2−h1)

where ρ = density of water (kg/cb. m) taken at

average temperature

Ex: ΔP=996.03×9.81× (10.6−10.4 )

ΔP=19.5421PaSolving for the Velocity:

v=ϕA× (conversionfactors )÷(crosssectionalareaofcolumn)

where ϕA = volumetric flow rate of air (L/min)

Ex: v=65×( 1min60sec )( 1m3

1000L )( 10.00442m2 )v=0.245m/s

Solving for Theoretical Pressure Drop:

ΔP=150μaapv∆L

ρa(1−ε)2

ε2+1.75v2ap∆L

ρa

(1−ε)

ε3+ρgH+4fρ (H−∆L)

Dv2

2

where μa = viscosity of air (kg/ms)

ε = porosityρa = density of air (kg/cb. m)

ap = total packing area

v = velocity of air flow∆L = height of packingsH = height of columnD = diameter of column

ΔP=150×1.849×10−5×420×0.2452×0.54

1.1833(1−0.63)2

0.632 +1.75×0.24522×420×0.54

1.1833(1−.63)

.633 +1.1833×9.81×0.73+4×0.0142×1.1833 (0.73−0.54)0.075

0.24522

2

ΔP=43.848Pa

Appendix B. Tables

Table 4. Graphical determination of loading and flooding

points (φw= 1.5 L/min)AirFlowRate(L/min)

h1 h2 ∆H(m) ∆P (Pa) velocity

(m/s) ∆P/H log v logdP/H

20 0.0 0.60.00

658.6304

460.0754512

3240.157

84

-1.1223

31.6037

7

35-

0.2 0.90.01

1107.489

150.1320396

5673.622

71-

0.87931.8670

12

50-

0.5 1.20.01

7166.119

60.1886280

81113.78

05

-0.7243

92.0560

68

65-

1.0 1.90.02

9283.380

490.2452165

05194.09

62

-0.6104

52.2880

17

80-

1.8 3.50.05

3517.902

270.3018049

29354.72

76

-0.5202

72.5498

95

95-

3.1 4.70.07

8762.195

80.3583933

53522.05

19

-0.4456

42.7177

14

110-

4.2 4.90.09

1889.228

430.4149817

78609.06

06

-0.3819

72.7846

6

125-

5.4 7.00.12

41211.69

590.4715702

02829.92

87

-0.3264

52.9190

41

140-

9.0 9.80.18

81837.08

730.5281586

261258.2

79

-0.2772

43.0997

77

155

-12.

614.4 0.27

2638.3701

0.58474705

1807.103

-0.2330

33.2569

83

170

-21.

022.4

0.434

4240.9356

0.641335474

2904.75

-0.1929

13.4631

09


points (φw= 2 L/min)

Air Flow Rate(L/min) h1 h2 ∆H

(m) ∆P (Pa) velocity(m/s) ∆P/H

20 0.0 0.60.00

658.63456

6 0.07545123240.1606

6

35 -0.2 0.8 0.0197.72427

7 0.13203965666.9344

4

50 -0.6 1.20.01

8 175.9037 0.188628081 120.482

65 -1.1 2.70.03

8371.3522

5 0.245216505254.350

9

80 -2.2 3.90.06

1596.1180

9 0.301804929408.300

1

95 -3.3 4.00.07

3713.3872

2 0.358393353488.621

4

110 -5.1 5.60.10

71045.649

8 0.414981778716.198

5

125 -7.8 10.00.17

81739.492

1 0.4715702021191.43

3

140-

15.0 16.40.31

43068.542

3 0.5281586262101.74

1

155-

24.6 26.80.51

45023.027

8 0.58474705 3440.43


points (φw= 2.5 L/min)



2010.

4 10.80.004

39.092458 0.075451232

26.77566

3510.

1 11.10.0

197.73114

4 0.13203965666.9391

4

50 9.6 11.60.0

2195.4622

9 0.188628081133.878

3

65 8.6 12.50.039

381.15146 0.245216505

261.0626

80 7.4 13.80.064

625.47932 0.301804929

428.4105

95 6.2 15.10.089

869.80718 0.358393353

595.7583

110 0.0 20.40.204

1993.7153 0.414981778

1365.558

130-

6.0 27.00.3

33225.127

8 0.490433012208.99

2

135

-24.

0 46.0 0.76841.180

1 0.509295818 4685.74





20 10.4 10.9 0.00548.8655

720.0754512

3233.469

57

35 10.2 11.2 0.0197.7311

440.1320396

5666.939

14

50 9.4 11.8 0.024234.554

750.1886280

81160.65

39

65 8.2 13.2 0.05488.655

720.2452165

05334.69

57

80 6.8 14.6 0.078762.302

920.3018049

29522.12

53

95 2.0 19.8 0.1781739.61

440.3583933

531191.5

17

100 -3.4 26.8 0.3022951.48

050.3772561

612021.5

62

105 -17.0 40.0 0.575570.67

520.3961189

693815.5

31

110 -22.0 44.0 0.666450.25

550.4149817

784417.9

83





20 10.6 10.8 0.00219.5448

550.0754512

3213.386

89

35 10.2 11.2 0.0197.7242

770.1320396

5666.934

44

50 9.4 12.2 0.028273.627

980.1886280

81187.41

64

60 8.8 12.8 0.04390.897

110.2263536

97267.73

77

70 7.4 14.0 0.066644.980

230.2640793

13441.76

73

80 5.4 17.2 0.1181153.14

650.3018049

29789.82

63

90 1.2 22.4 0.2122071.75

470.3395305

451419.0

1

100 -7.2 31.4 0.3863772.15

710.3772561

612583.6

69

110 -20.2 44.8 0.656352.07

80.4149817

784350.7

38





20 0.3 0.6 0.00329.3172

830.0754512

3220.080

33

35 -0.1 1.1 0.012117.269

130.1320396

5680.321

32

50 -1.7 2.6 0.043420.214

390.1886280

81287.81

81

65 -4.2 5.6 0.098957.697

910.2452165

05655.95

75

70 -6.0 7.6 0.1361329.05

020.2640793

13910.30

83

75 -11.4 13.8 0.2522462.65

180.2829421

211686.7

48

80 -21.0 23.8 0.4484378.04

760.3018049

292998.6

63

81 -34.0 35.0 0.696742.97

510.3055774

914618.4

76





20 0.0 0.6 0.00658.6304

460.0754512

3240.157

84

30 -0.3 0.9 0.012117.260

890.1131768

4880.315

68

40 -0.9 1.5 0.024234.521

780.1509024

65160.63

14

50 -1.2 2.8 0.04390.869

640.1886280

81267.71

89

53 -2.6 3.2 0.058566.760

980.1999457

66388.19

25

60 -4.0 4.4 0.084820.826

240.2263536

97562.20

98

65 -6.0 6.0 0.121172.60

890.2452165

05803.15

68

70 -14.0 15.8 0.2982911.97

880.2640793

131994.5

06

75 -22.0 32.0 0.545276.74

010.2829421

213614.2

06

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Absorption final report