Summary:

The purpose of this experiment is to determine the pressure drop across a packed column as a function of air and water flow rates through the column and to examine the relationship between the experimental pressure drop values and correlated values for a packed column. The pressure drop values are observed from the manometer as each of the water flow rate from 0LPM to 3LPM are tested with increasing air flow rates. At first the calculation for each GG and GL values are calculated from air flow rate of 1m3/h to 9m3/h for dry column and 1m3/h to 6m3/h and water flow rate of 0LPM to 3LPM. The y-axis, K4 is calculated using equation K4= and the x-axis, FLV= is calculated. X-axis and y-axis are required to plot graph for pressure drop correlation. Generalised pressure drop chart and graph of log pressure drop against log gas flow rate is plotted to determine the flooding point. In conclusion, the pressure drop across the packed column increases when the air flow rate increases at constant water flow rate from 0LPM to 3LPM due to the resistance to water flows down the column as a result of increasing air flow rate. The experimental and theoretically are applying the 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. However, there are some errors made when conducting the experiment affecting the experimental pressure drop.

Introduction:

The topic of this experiment is gas absorption and the purposes of carrying out this experiment are to determine the pressure drop across a packed column as a function of air and water flow rates through the column and to investigate the relationship between experimental pressure drop values and the correlated values for a packed column. This experiment focuses on the absorption of carbon dioxide from air into water.

Gas absorption is the unit operation in which one or more soluble components of a gas mixture are dissolved in a liquid. The gas phase or gas mixture is inert gas while the liquid phase is immiscible in the gas phase. Therefore, the liquid phase will vaporize very slightly in gas phase. The process involves mass transfer of the component of the gas from the gas phase to the liquid phase. The process involves the transfer of solute from gas to liquid phase. Where the mass transfer occur in the opposite direction known as desorption or stripping.

Gas absorption operation is widely used in controlling industrial air pollution, and to separate acidic impurities from mixed gas streams. Packed towers are the most common mass transfer devices used for both air pollution control and recovery of process gases. Generally, a packed tower is made up of a piece of pipe set on its end and is filled with inert materials or packings. The packed tower usually operates in counter- current flow, whereby the liquid enters the system via the top, flows down across the packings and wets the surface of the packings, and the gas stream mixed with discharge will enter from the bottom. When the liquid and gas are in contact with one another, effluent will be transferred from the gas to the liquid.

Packing materials play a crucial role in the absorption process. These packing materials provide a large surface area of contact between the liquid flows down the column and gas containing solute that enters the column from the bottom. Two types of commonly used packing materials will be structure packing materials and random packing materials. In this experiment random packing is being used because it is cheap, easily available and the material is made of plastic.

Pressure drop is the result of fluid friction between liquid flow and the packings. The graph above shows the relationship between pressure drop and gas flow rate and for dry column, a straight line is plotted and wet column three curvy lines are plotted. The three curves are parallel to the straight line. The point where liquid holdup starts to increase is the point where the slope starts to change. This point is known as the loading point. When the gas flow rate is further increased, pressure drop rises tremendously until the lines plotted are almost vertical and at this point, liquid is of continuous phase. This point is known as the flooding point and happens when liquid accumulates due to high gas flow rate and this accumulation continues until the packed column is completely filled with liquid.

The formulae used for the calculations of theoretical flooding point and to plot the graphs of capacity parameter against flow rate parameter are:

x-axis:

y-axis:

GG= , where GG is the gas mass flow rate per unit column cross- sectional area, kg/m2s

GL= , where GL is the liquid mass flow rate per unit column cross-sectional area, kg/m2s

Fp= 900m-1 (packing factor)

Water viscosity, 0.001 Ns/m2

Densities: Density of water, = 996 kg/m3

Density of air, = 1.175 kg/m3

Area= A

Column Diameter: 80mm

Figure a: Generalized pressure drop correlation, adapted from a figure by the Norton Co. with permission

Materials and Apparatus:

6. Circulating Pump

7. Absorption Column

1. Column K1 Pressure Drop

8. Sump Tanks (B1 and B2)

5. Sump Tank B1 low level switch

4. Water flow rate

3. Air flow rate

2. CO2 flow rate

Gas Absorption Unit

No.

Tag

Labelling

Range

Description

1

dPT-201

Column K1 pressure drop

0-500 mmH2O

-

2

FT-101

CO2 flow rate

2-20 L/min

Portable gas analyzer with measuring range of 0-100 vol% of CO2 and an accuracy of + 2% of CO2.

3

FT-102

Air flow rate

1-12 CMPH

-

4

FT-103

Water flow rate

1-10 L/min

-

5

LS-01

Sump Tank B1 low level switch

-

-

6

P1

Circulating Pump

62LPM

Magnetic drive seal-less centrifugal pump having a maximum flow rate of 62L/min at 5.7m head. The output power is 65 W and made up of polypropylene (PP)

7

K1

Packed Column

-

Packed column filled with Raschig rings. The column is of a diameter of 80mm and 1.5m high. Materials used to make this will be Perspex and stainless steel. Packing is made up of 10mm glass Raschig rings. The column comes with ports for pressure drop measurement.

8

B1 and B2

Sump Tank

50L

Sump Tank B1: Rectangular tanks with removable top cover and level sight tube. The capacity is 50L and is made of stainless steel material. Low level switch for protection of centrifugal pump from dry run.

Sump Tank B2: Rectangular tank with removable top cover and level sight tube. The capacity is 50L and is made of stainless steel material.

Experimental Procedures:

General Operating Procedures:

1. Gas flow rate control:

Needle valves V1 and V2 were used to manually control the CO2 and air flow rates, and consequently the CO2 composition entering the absorption column K1.

2. Liquid flow rate control:

Valve V3 was used to manually adjust the liquid flow rate entering the absorption column. The valve was closed when the pump P1 was initially switched on to prevent liquid surge through the flow meter FT-03.

3. Column pressure drop measurements:

If the pressure drop readings are fluctuating or inaccurate, it means that there might be some liquid trapped in the tubing leading to the manometer. The tubing was removed from the manometer side and blown into them to clear off any trapped liquid along the lines. The air trapped in the tubing also contributes to inaccuracy of measurement.

General Start- Up Procedures:

1. All valves were ensured to be closed initially except by- pass valve V4.

2. All gas connections were checked to ensure that they are properly fitted.

3. The valve at the air compressor was opened. The supply pressure was set to between 2 to 3 bars by turning the air filter regulator knob clockwise.

4. The valve at the CO2 gas supply was opened. The supply pressure was set to between 0.2MPa to 3MPa by turning the gas regulator knob clockwise.

General Shut- Down Procedures:

1. The circulation pumps P1 and air compressor were switched off.

2. Valves V1, V2 and V3 were closed.

3. The valve on the air compressor was closed and the supply pressure was released by turning the air filter regulator knob counterclockwise all the way.

4. The valve at the CO2 gas supply was closed and the supply pressure was released by turning the gas regulator knob counterclockwise all the way.

5. All liquid in column K1 was drained by opening valve V5.

6. All liquid was drain from the sump tanks B1 and B2 by opening valves V7 and V9.

Experiment 2(a): Hydrodynamics of a Packed Column

Procedures:

1. The general start- up procedures was performed except step 4.

2. The sump tank B1 was filled with 40L of fresh water.

3. To run the experiment with dry column, valve V2 was opened to introduce air into the column. The air was allowed to flow through the column until all evidence of moisture in the packing has disappeared.

4. Valve V2 was adjusted to fix the desired air flow rate at FI-101, starting at a low value.

5. The pressure drop across the packed column dPT-201 was recorded.

6. Steps 4 and 5 were repeated with increasing values of air flow rate from 1m3/h to 12m3/h.

7. Valve V2 was closed.

8. To run the experiment with wet column, valve V6 was opened and Valve V4 was partially opened.

9. Circulation pump P1 was switched on and valve V3 was slowly opened.

10. Water entering the top of the packed column K1, flowing down the column and back into the sump tank B1 was observed. Valve V5 was adjusted to maintain the water level in the bottom of column at height of the bottom vessel.

11. Valve V3 was adjusted to fix the desired water flow rate at FI-103, starting at a low value.

12. Valve V2 opened and adjusted to fix the desired air flow rate, starting at a low value.

13. The pressure drop across the packed column at dPT-201 was recorded.

14. Steps 11 and 12 were repeated with increasing values of air flow rate from 1 m3/h to 12m3/h while the water flow rate was maintained. It was stopped when flooding started to occur in the column.

15. Valve V1 was closed. Steps 10 to 13 were repeated with increasing values of water flow rate from 1 LPM to 3 LPM.

16. The experiment ended by performing the general shut-down procedures except step 4.

Variables

Range

Air flow rate

12 m3/h

Liquid flow rate

0- 3.0 L/min

Results and Analysis:

Gas Flow rate (m3/h)

GG (kg/ms2)

K4

FLV (1LPM)

FLV (2LPM)

FLV (3LPM)

1

0.06493

0.01068

1.74688

3.49375

5.24063

2

0.12985

0.04274

0.87344

1.74688

2.62031

3

0.19478

0.09616

0.58229

1.16458

1.74688

4

0.25971

0.17095

0.43672

0.87344

1.31016

5

0.32464

0.26711

0.34938

0.69875

1.04813

6

0.38956

0.38464

0.29115

0.58229

0.87344

7

0.45449

0.52354

0.24955

0.49911

0.74866

8

0.51942

0.68381

0.21836

0.43672

0.65508

9

0.58434

0.86545

0.19410

0.38819

0.58229

10

0.64927

1.06846

0.17469

0.34938

0.52406

11

0.71420

1.29283

0.15881

0.31761

0.47642

12

0.77913

1.53858

0.14557

0.29115

0.43672

13

0.84405

1.80569

0.13438

0.26875

0.40313

14

0.90898

2.09417

0.12478

0.24955

0.37433

15

0.97391

2.40402

0.11646

0.23292

0.34938

16

1.03883

2.73525

0.10918

0.21836

0.32754

17

1.10376

3.08784

0.10276

0.20551

0.30827

18

1.16869

3.46179

0.09705

0.19410

0.29115

19

1.23362

3.85712

0.09194

0.18388

0.27582

20

1.29854

4.27382

0.08734

0.17469

0.26203

Table 1: Theoretical Flooding Point

Flowrate

Pressure Drop (mm H2O)

Air (m3/h)

1

2

3

4

5

6

7

8

9

10

11

12

Water (L/min)

0

4

5

8

10

13

15

21

25

53

1

7

8

12

13

26

65

2

10

13

14

18

30

45

3

8

12

21

28

37

43

Table 2: Pressure Drop for Dry and Wet Column

Figure 1: Graph of Log Pressure Drop against Log Gas Flow Rate

Figure 2: Pressure Drop Correlation Chart (1LPM)

Figure 3: Pressure Drop Correlation Chart (2LPM)

Figure 4: Pressure Drop Correlation Chart (3LPM)

Figure 5: Generalised Theoretical Pressure Drop Correlation Chart for Random Packings

Sample Calculations:

Data:

Density of air, = 1.175 kg/m3

Density of water, = 996 kg/m3

Column diameter, Dc= 80 mm

Area of packed diameter, Ac =

=

= 0.005027m2

Packing Factor, FP= 900m-1

Water Viscosity, 0.001Ns/m2

Theoretical Flooding Point

To calculate gas flow rate, GG (kg/m2s)

GG=

=

= 0.06493 kg/m2s

To calculate capacity parameter, K4

K4=

=

= 0.01068

To calculate liquid flow rate, GL (kg/m2s)

GL=

=

= 3.30217 kg/m2s

To calculate flow parameter, FLV

FLV=

=

= 1.74688

Discussion:

According to Figure 1, the pressure drop increases as the air flow rate increases in the dry packed column. As for wet column, at constant water flow rate, when the air flow rate increases, the pressure drop increases. When gas flow rate increases, the resistance for the water to flows down the column will increase, thus resulting in the higher pressure drop across the packings. The pressure drop of wet column is higher compared to the pressure drop in dry column. This is due to the amount of space for gas flow is hindered by the liquid flowing down the column. Water flows down the packed column due to gravitational pull and gas flows at counter- current direction across water. The packings in the column served in providing contact for both air and water and enabling the development of interfacial surface at which the mass transfer of components from gas to liquid takes place. Carbon dioxide present in the air will then be absorbed into the water through mass transfer. Theoretically, the dry column line displayed in the graph should be a straight line but in this experiment, the line plotted for 0L/min flow rate is a curve. As for the curves plotted for the results of wet column obtained, theoretically these curves should be parallel to the straight line and becomes steeper as the gas flow rate increases because gas enters from the bottom starts to impede the water flowing down the column. When water flow rate increases, the curve should be further away from the dry column straight line and the curve should be closer to the y-axis. However, the curve for 2LPM intersect 3LPM because the pressure difference obtained experimentally for 3LPM at 1m3/h is lower compared 2LPM which are 10mmH2O and 8mmH2O respectively. The curves for 1LPM, 2LPM and 3LPM of water flow rate intersect at 6m3/h because the experimental pressure drop recorded is highest for 1LPM compared to 2LPM and 3LPM. Theoretically, at 6m3/h, 3LPM should have the highest pressure drop followed by 2LPM and 1LPM. These errors made might be due to minor leaking when the experiment is being carried out. Minor leaking will affect the flow rate of both water and air thus affecting the pressure drop.

When the liquid flow rate is low with a moderately low gas flow rate, there will be no obvious trapping of water at the packings. As the gas flow rate is increased, pressure drop increases. Some of the water will be trapped in the packings. The water trapped at the bottom slowly becomes more significant and later on results in flooding. The flooding point is the highest point for each line in the graph of pressure drop against gas flow rate. At this point, water starts to fill the bottom part of the packed column which is the higher gas flow region and gas has to bubble through the water accumulated at the bottom of the column. When flooding occurs, the process can no longer be run because there is too much liquid entrainment. This experiment should be conducted from air flow rate of 1 m3/h to 12 m3/h, however flooding point has occurred for dry column after 9 m3/h and wet column after 6 m3/h.

The flow parameter shows the ratio of liquid kinetic energy to vapor kinetic energy. Theoretical generalized correlation charts show that the high flow parameters are typical of high liquid rates and high pressures drop which effectively decreases the available cross section for gas flow. Conversely, the low flow parameters are typical of vacuum and low liquid rate operation. The pressure drop is increasing with increasing of gas flow rate. In this experiment, the experimental pressure drop for 1LPM, 2LPM and 3LPM generally apply the same concept as theoretical generalized correlation chart where all these curves show that pressure drop increases with increasing gas flow rate. However, at 2LPM of water, the pressure drop is lower at 6m3/h of air flow rate compared to the pressure drop at 6m3/h of air flow rate at 1LPM which are 65mmH2O and 45mmH2O respectively. The pressure drop for flow rate 1 m3/h, 2 m3/h and 6 m3/h recorded for 3LPM are lower compared to the readings at 2LPM which are 8 mmH2O, 12 mmH2O and 43 mmH2O for 3LPM and 10 mmH2O, 13 mmH2O and 45 mmH2O for 2LPM respectively. Errors occur might be cause by leaking at the top of the column affecting both the flow rate of water flowing down the column and the flow rate of air rising as the column is no longer closed allowing gas to flow out of the column. When leaking happens, water flow rate and the air flow rate are slightly reduced causing less resistance between water flowing down the column and air bubbling up which results in a slightly lower pressure drop. Errors might also due to parallex error when adjusting the air and water flow rate and when reading the pressure drop at the manometer.

Conclusion:

In conclusion, the pressure drop across the packed column increases when the air flow rate increases at constant water flow rate from 0LPM to 3LPM due to the resistance to water flows down the column as a result of increasing air flow rate. The experimental pressure drop and theoretical generalised pressure drop correlation chart are applying the 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. However, there are some errors made when the experiment is being conducted resulting in a slight inaccuracy of the experimental chart plotted.

References:

1. Column Diameter and Pressure Drop. Retrieved from separation process website: http://www.separationprocesses.com/Absorption/GA_Chp04a.htm

2. Christie John Geankoplis, Transport Processes and Separation Process Principles, 4th edition, New Jersey, Pearson Prentice Hall, 2003

3. Lab Report Gas Absorption (2014, Jan 25). Function of gas absorption column and the absorption process occur in the column. Retrieved from the scribd website: http://www.scribd.com/doc/202066355/LAB-REPORT-Gas-Absorption

4. Lab Manual

5. Henry Z Kister, Jeffrey Scherfius, Khashayar Afshar, Emil Abkar (July 2007). Realistically Practical Capacity and Pressure Drop for Packed Columns. Retrieved from aiche website: http://people.clarkson.edu/~wwilcox/Design/PackdCol.pdf

6. Yaminah Z.Jackson (2014, April 28). Modelling Gas Absorption. Retrieved from the wpi, e-project website: https://www.wpi.edu/Pubs/E-project/Available/E-project-042408-133605/unrestricted/Modeling_Absorption.pdf

Pressure Drop Correlation Chart (1LPM)

1.74687563039991670.873437815199958330.582291876799972190.436718907599979170.349375126079983390.291145938399986090.24955366148570240.218359453799989580.194097292266657420.174687563039991690.158806875490901530.145572969199993050.134375048492301310.12477683074285120.116458375359994450.109179726899994790.102757390023524529.7048646133328711E-29.1940822652627208E-28.7343781519995847E-21.0671809782252222E-24.2687239129008889E-29.6046288040269989E-20.170748956516035550.266795244556305540.384185152161079960.522918679330358740.682995826064142220.864416592362429851.06718097822522221.2912889836525191.53674060864431981.80353585320062512.0916747173214352.40115720100674952.73198330425656893.08415302707089233.45766636944971943.85252333139305184.2687239129008887

FLV

K4

Pressure Drop Correlation Chart (2LPM)

3.49375126079983331.74687563039991671.16458375359994440.873437815199958330.698750252159966780.582291876799972190.499107322971404810.436718907599979170.388194584533314850.349375126079983390.317613750981803060.291145938399986090.268750096984602620.24955366148570240.232916750719988910.218359453799989580.205514780047049040.194097292266657420.183881645305254420.174687563039991691.0671809782252222E-24.2687239129008889E-29.6046288040269989E-20.170748956516035550.266795244556305540.384185152161079960.522918679330358740.682995826064142220.864416592362429851.06718097822522221.2912889836525191.53674060864431981.80353585320062512.0916747173214352.40115720100674952.73198330425656893.08415302707089233.45766636944971943.85252333139305184.2687239129008887

FLV

K4

Pressure Drop Correlation Chart (3LPM)

5.24062689119974932.62031344559987471.74687563039991691.31015672279993731.048125378239950.873437815199958440.748660984457107270.655078361399968670.58229187679997230.5240626891199750.476420626472704560.436718907599979220.403125145476903980.374330492228553640.349375126079983390.327539180699984330.30827217007057350.291145938399986150.275822467957881680.26203134455998751.0671809782252222E-24.2687239129008889E-29.6046288040269989E-20.170748956516035550.266795244556305540.384185152161079960.522918679330358740.682995826064142220.864416592362429851.06718097822522221.2912889836525191.53674060864431981.80353585320062512.0916747173214352.40115720100674952.73198330425656893.08415302707089233.45766636944971943.85252333139305184.2687239129008887

FLV

K4

Generalised Theoretical Pressure Drop Correlation Chart for Random Packings

1LPM1.74687563039991670.873437815199958330.582291876799972190.436718907599979170.349375126079983390.291145938399986090.24955366148570240.218359453799989580.194097292266657420.174687563039991690.158806875490901530.145572969199993050.134375048492301310.12477683074285120.116458375359994450.109179726899994790.102757390023524529.7048646133328711E-29.1940822652627208E-28.7343781519995847E-21.0671809782252222E-24.2687239129008889E-29.6046288040269989E-20.170748956516035550.266795244556305540.384185152161079960.522918679330358740.682995826064142220.864416592362429851.06718097822522221.2912889836525191.53674060864431981.80353585320062512.0916747173214352.40115720100674952.73198330425656893.08415302707089233.45766636944971943.85252333139305184.26872391290088872LPM3.49375126079983331.74687563039991671.16458375359994440.873437815199958330.698750252159966780.582291876799972190.499107322971404810.436718907599979170.388194584533314850.349375126079983390.317613750981803060.291145938399986090.268750096984602620.24955366148570240.232916750719988910.218359453799989580.205514780047049040.194097292266657420.183881645305254420.174687563039991691.0671809782252222E-24.2687239129008889E-29.6046288040269989E-20.170748956516035550.266795244556305540.384185152161079960.522918679330358740.682995826064142220.864416592362429851.06718097822522221.2912889836525191.53674060864431981.80353585320062512.0916747173214352.40115720100674952.73198330425656893.08415302707089233.45766636944971943.85252333139305184.26872391290088873LPM1.0671809782252222E-24.2687239129008889E-29.6046288040269989E-20.170748956516035550.266795244556305540.384185152161079960.522918679330358740.682995826064142220.864416592362429851.06718097822522221.2912889836525191.53674060864431981.80353585320062512.0916747173214352.40115720100674952.73198330425656893.08415302707089233.45766636944971943.85252333139305184.26872391290088875.24062689119974932.62031344559987471.74687563039991691.31015672279993731.048125378239950.873437815199958440.748660984457107270.655078361399968670.58229187679997230.5240626891199750.476420626472704560.436718907599979220.403125145476903980.374330492228553640.349375126079983390.327539180699984330.30827217007057350.291145938399986150.275822467957881680.2620313445599875

FLV

K4

Graph of Log Pressure Drop against Log Gas Flow Rate

1LPM0.845098040014256810.903089986991943541.07918124604762491.11394335230683671.4149733479708181.81291335664285552LPM11.11394335230683671.1461280356782381.2552725051033061.47712125471966241.65321251377534373LPM0.903089986991943541.07918124604762491.32221929473391931.44715803134221921.5682017240669951.63346845557958640 LPM0.60205999132796240.698970004336018860.9030899869919435411.11394335230683671.17609125905568131.32221929473391931.39794000867203771.7242758696007889

Log Gas Flow Rate, Gy

Log Pressure Drop, mmH2O