Exp 4 Gas Absorption

ABSTRACT The objectives of gas absorption experiment are to determine the loading and flooding point in the column as well as to determine the pressure drop (∆P)as a function of gas (air) and liquid (water) mass velocities (L/min) using flexi glass packed with Raschig rings. Before the actual experiment started, the equipment was set-up first using set-up procedure where the valves are arranged according to U-tube (LEFT) arrangement. Water is then filled into the monotube using VT-3 and the water level is adjusted to 20 mm H 2O for both left and right monotube and the pump is then switched on. Next, the valve arrangement is set according to operating arrangement. After the set-up is done, valve VR-3 and VR-4 are opened and the water flow rate is adjusted to 1.0 (L/min). The level of water returning to the water reservoir is controlled using VR-4 so that it always higher than the bottom of the reservoir. Then, the gas flow rate is adjusted to 20 (L/min) and after two minutes, the pressure at the left and right monotube is taken. The experiment is continued by varying the gas flowrate until 180 (L/min) and the experiment is then repeated with volume flow rate of 2.0 (L/min) and 30 (L/min) respectively. At the end of the experiment, we had managed to determine the loading and flooding point where the loading point is from the volume water flow rate of 1.0 (L/min), 2.0 (L/min) and 3.0 (L/min). When the volume water flow rate at 3.0 (L/min), the flooding point started at gas flow rate of 140 (L/min). Other than that, we also had determined the pressure drop (∆P) as a function of gas (air) and liquid (water) mass velocities (L/min) using flexi glass packed with Raschig rings. The pressure drop for the volume water flow rate 1.0 (L/min), the range is between 0.0 mm H 2O until 15.0 mm H2O where 15 mm H2O is the highest pressure drop at gas flow rate of 180 (L/min). While the pressure drop for the volume water flow rate 2.0 (L/min), the range is between 0.0 mm H 2O until 59 mm H2O where 59 mm H2O is the highest pressure drop at gas flow rate of 180 (L/min). For volume water flow rate 3.0 (L/min), the pressure drop range is between 0.0 mm H2O until flooding point at gas flow rate 140 (L/min). We can conclude that as the gas flow rate increasing, the pressure drop (∆P) will also increase. Thus, we can conclude that all the objectives of the experiment had been reached.

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INTRODUCTION Absorption is 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 more or less soluble. The gas mixture consists mainly of an inert gas and the soluble. An example of gas is the absorption of the solute ammonia from an air-ammonia mixture by water. A major application of absorption is the removal of CO2 and H2S from nature gas or synthesis gas by absorption in solution of amines or alkaline salts. A common apparatus used in gas absorption and certain other operations is the packed tower, shown in Figure 1 below. The device consists of a cylindrical column, or 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. Their common dumped packing, Ceramic Berl saddles and Raschig rings are older types of packing that are not much used now, although there were big improvements over ceramic spheres or crushed stone when first introduced. The shape prevents pieces from nesting closely together, and this increasing the bed porosity. As for this experiment we used the column packed with Raschig rings. In given packed tower with a given type and size of packing and with defined flow of liquid, there is an upper limit to the rate of gas flow, called the flooding velocity or flooding point. Above this gas velocity the tower cannot operate due to high pressure. At the flow rate called the loading point, the gas start to hander the liquid down flow, and local accumulations or pools of liquid start to appear in the packing.

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Figure 1: Gas Absorption Packing Column

OBJECTIVES  

To determine the loading and flooding points in the column. To determine the pressure drop (∆P) as a function of gas (air) and liquid (water) mass velocities (L/min) using flexi glass packed with Raschig rings.

THEORY 3

A common instrument used in gas absorption or stripping is a packed tower. A packed tower consists of the following: a cylindrical tube with inert packing material, a gas inlet at the bottom with an exit out the top, and a liquid inlet at the top with its exit out the bottom. In an ideal operation the liquid will descend through the packed column and distribute uniformly over the packing surface in films. The gas will enter the column from below the packed section and rise upward countercurrent to the liquid flow through the small spaces between the packing materials. The large amount of intimate contact between the liquid and gas streams allows for pressure drop in the packing column. The pressure drop along the packed column is calculated by using the formula below: Pressure drop , ∆P (mmH2O) = High pressure of monotube (mmH2O) – Low pressure of monotube (mmH2O) In this experiment, the graph of Ln (V) versus Ln (∆P/m packing) is needed in order to investigate and observed the relationship between the gas flow rate and the pressure drop in the packing column. Ln (V) can be easily calculated using scientific calculator where: V = gas flow rate in (L/min) Moreover, to calculate Ln (∆P/m packing), we must first calculate the pressure drop using the formula above then divided it with the packings value and then multiplied it with Ln using scientific calculator. For this experiment, we used Raschig rings packing column. It is given that the packings value is: Packings = 8 mm glass Raschig Rings After we had calculated the values for Ln (V) and Ln (∆P/m packing), directly plot the graph. APPARATUS 

Gas- Liquid Absorption Column

PROCEDURE 4

Manometer calibration (see diagram) For calibration of manometers and during operation of the column, the following valves must be in the positions stated below: a) U-tube (left)

b) U-tube (right)

R

VT-1

R

VT-1

B

B

VT-2

R

VT-3

B

VT-2

R

B R

R

B

B

VT-3

B B

VT-4

VT-4

R

R R

R

VT-5 VT-5

B

B

V-2 (OPEN)

V-3 (OPEN)

c) Sphere ball

d) Operation

V-1 (OPEN) VT-1

R B

VT-2

R

VT-1

R

B B

VT-2 VT-4

R R

B

B

VT-3

R B

B

VT-5 R

Operation

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1. The manometer U-tube is filled with water by arranging the values according to the Utube arrangement. The pump is switched on. For this experiment, we used the left U-tube arrangement only. 2. The values are set to operating arrangement before the operation is started. 3. All valves are checked carefully (closed) before the column is safe to use. 4. Valve VR-3 and VR-4 are opened such that the liquid flow rate is set at 10 m3/h. Note: The level of liquid returning to the water reservoir must always be higher than the bottom of the reservoir. This is to avoid air being trapped in line. Valve VR-4 is adjusted accordingly to avoid this phenomena. 5. Valve VR-1 is opened and the airflow rate is set to be 10 m 3/h. Wait for 2 minutes and during this time the flow rate of air and water is make sure to be constant. The pressure drop (∆P) mmH2O is read in the monotube. 6. The gas flow rate is increased by adding an extra of 5 m 3/h to the column. Wait for 2 minutes and the pressure drop is read again. 7. Part 4 is repeated until Flooding Point is reached. 8. The curve of Ln (V) versus Ln(∆P / m packing) is plotted. 9. Steps 2 to 6 are repeated with different kind of liquid flow rate.

RESULTS Flow rate

Presssure drop (mm H2O)

(L/min) Air

20

40

60

80

100

12

140

160

180 6

Water 1.0 2.0 3.0

0 0 2

0 1 0

1 3 2

3 5 9

5 9 23

6 13 52

9 20 Floodin

12 39 Floodin

15 59 Flooding

g point

g point

point

CALCULATIONS In order to plot the graph Ln (V) versus Ln (∆P / m packing), we need to calculate Ln (V) first. Where V = Gas flow rate in (L/min) Gas Flow Rate (L/min) 20 40 60 80 100 120 140 160 180

Ln (V) Ln (20) = 2.996 Ln (40) = 3.689 Ln (60) = 4.094 Ln (80) = 4.382 Ln (100) = 4.605 Ln (120) = 4.787 Ln (140) = 4.942 Ln (160) = 5.075 Ln (180) = 5.193

Then we need to calculate Ln (∆P / m packing) where packings = 8 mm glass Raschig Rings. Volume Flow Rate (L/min) 1.0

Gas Flow Rate (L/min) 20 40 60 80 100 120 140

Pressure Drop,∆P (mmH2O) 0 0 1 3 5 6 9

Ln (∆P / m packing) Ln (0/0.008) = Math Error Ln (0/0.008) = Math Error Ln (1/0.008) = 4.83 Ln (3/0.008) = 5.93 Ln (5/0.008) = 6.44 Ln (6/0.008) = 6.62 Ln (9/0.008) = 7.03 7

2.0

3.0

160 180 20 40 60 80 100 120 140 160 180 20 40 60 80 100 120 140 160 180

12 15 0 1 3 5 9 13 20 39 59 2 0 2 9 23 52 Flooding point Flooding point Flooding point

Ln (12/0.008) = 7.31 Ln (15/0.008) = 7.54 Ln (0/0.008) = Math Error Ln (1/0.008) = 4.83 Ln (3/0.008) = 5.93 Ln (5/0.008) = 6.44 Ln (9/0.008) = 7.03 Ln (13/0.008) = 7.39 Ln (20/0.008) = 7.82 Ln (39/0.008) = 8.49 Ln (59/0.008) = 8.91 Ln (2/0.008) = 5.52 Ln (0/0.008) = Math Error Ln (2/0.008) = 5.52 Ln (9/0.008) = 7.03 Ln (23/0.008) = 7.96 Ln (52/0.008) = 8.78 Flooding point Flooding point Flooding point

So, the calculated results are as below; Volume Flow Rate (L/min)

1.0

2.0

3.0

Gas Flow Rate (L/min) 20 40 60 80 100 120 140 160 180 20 40 60 80 100 120 140 160 180 20

Pressure Drop, ∆P (mmH2O) 0 0 1 3 5 6 9 12 15 0 1 3 5 9 13 20 39 59 2

Ln (V)

Ln (∆P / m packing)

2.996 3.689 4.094 4.382 4.605 4.787 4.942 5.075 5.193 2.996 3.689 4.094 4.382 4.605 4.787 4.942 5.075 5.193 2.996

Math Error Math Error 4.83 5.93 6.44 6.62 7.03 7.31 7.54 Math Error 4.83 5.93 6.44 7.03 7.39 7.82 8.49 8.91 5.52 8

40 60 80 100 120 140 160 180

0 2 9 23 52 Flooding point Flooding point Flooding point

3.689 4.094 4.382 4.605 4.787 4.942 5.075 5.193

Math Error 5.52 7.03 7.96 8.78 Flooding point Flooding point Flooding point

The curve of Ln (V) versus Ln (∆P / m packing), For volume flow rate = 1.0 (L/min)

(V) versus Ln (∆P / m packing) 12 10 8

Ln (V)

6 4 2 0 2.5

3

3.5

4

4.5

5

5.5

Ln (∆P / m packing)

For volume flow rate = 2.0 (L/min)

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Ln (V) versus Ln (∆P / m packing) 10 9 8 7 6 Ln (V)

5 4 3 2 1 0 3.6

3.8

4

4.2

4.4

4.6

4.8

5

5.2

5.4

Ln (∆P / m packing)

For volume flow rate = 3.0 (L/min)

Ln(V) versus Ln (∆P / m packing) 12 10 8 Ln (V)

6 4 2 0 2.5

3

3.5

4

4.5

5

5.5

Ln (∆P / m packing)

ERROR CALCULATIONS

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There are some errors when calculating the value of Ln (∆P / m packing). Notice that for the volume flow rate of 1.0 (L/min), 2.0 (L/min) and 3.0 (L/min) the values for gas flow rate are 20 (L/min), and 40 (L/min), the value for Ln (∆P / m packing) are all math errors. Volume Flow Rate (L/min) 1.0 2.0 3.0

Gas Flow Rate (L/min)

Ln (∆P / m packing)

20 40 20 40

Ln (0.0/0.008) = Math Error Ln (0.0/0.008) = Math Error Ln (0.0/0.008) = Math Error Ln (0.0/0.008) = Math Error

This value affects the graph of Ln (V) versus Ln (∆P / m packing).

DISCUSSION The important things that we want to find out from the experiment are to determine where is the flooding and loading point of the gas absorption as well as to determine the pressure drop (∆P)as a function of gas (air) and liquid (water)mass velocities (m3/h) using flexi glass packed with Raschig rings. Loading and flooding point At the flow rate called the loading point, the gas start to hander the liquid down flow and local accumulations or pools of liquid start to appear in the packing. Flooding is an undesirable state that occurs when the gas or liquid flow rate is too high for a given system operation. At the point of flooding, the liquid begins to hold up in the column impeding the flow of air which causes the pressure drop to rise. The hold up of water causes a decrease in surface contact area between the gas and liquid streams which in turn decreases the rate of mass transfer. Above the flooding velocity, the tower cannot operate. In this experiment, the flooding point is noticed when the water in the packing column suddenly shooting up to the column with high speed and the monometer reading starts to be unstable. This situation is being discovered when the water flow rate is 40 (L/min) and the gas flow rate at 35 (L/min). Notice that flooding point occurred at higher flow rate of water and also gas. At high gas flow rate, the liquid is prevented from draining down the tower by the frictional drag of the gas on 11

the liquid. The loading point is the rest of the situation from 10 (L/min) until 30 (L/min) of volume water flow rate. Pressure drop (∆P)as a function of gas (air) and liquid (water) mass velocities (L/min) using flexi glass packed with Raschig rings There is a pressure drop occurs in the packed tower due to differences in pressure at the top and bottom of the column due to intimate contact between the liquid and gas flow streams in the packed column so that mass transfer yields. For this packed column, we used raschig rings. Raschig rings are pieces of tube (approximately equal in length and diameter) used in large numbers as a packed bed within columns for distillations and other chemical engineering processes. The pressure drop for the volume water flow rate 1.0 (L/min), the range is between 0.0 mm H2O until 15 mm H2O where 15 mm H2O is the highest pressure drop at gas flow rate of 180 (L/min). While the pressure drop for the volume water flow rate 2.0 (L/min), the range is between 0.0 mm H 2O until 59 mm H2O where 59 mm H2O is the highest pressure drop at gas flow rate of 180 (L/min). For volume water flow rate 3.0 (L/min), the pressure drop range is between 0.0 mm H 2O until flooding point at gas flow rate of 140 (L/min). We noticed that as the gas flow rate increases, the pressure drop will also increases. This is because packed tower used for continuous counter current contact of liquid and gas is vertical columns which have been filled by packings. The liquid is distributed over and trickles down to the packed bed thus exposing a large surface to contact the gas. The frictional losses increase as the gas flow rate is increased. Since both the gas and liquid are competing for the free cross-sectional area left by packing, an increase in liquid flow rate will result in an increase in the frictional losses thus producing an increase in the pressure drop also. The graph Ln (V) versus Ln (∆P / m packing) for every volume water flow rate also clearly shows that the pressure drop increases linearly to the gas flow rate. While undergoing the experiment, there are few mistakes that we made. One of them that we do not control the valve VR-4 well so that the level of liquid returning to the water reservoir must always be higher than the bottom of the reservoir. As a result, the pressure reading on the monotube is not stable that make it harder for us to give an accurate value. Furthermore, we also happened to have parallax error in reading the pressure value.

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Figure 1 : packed tower of Raschig Rings CONCLUSION At the end of the experiment, we had managed to determine the loading and flooding point where the loading point is from the volume water flow rate of 1.0 (L/min), 2.0 (L/min) and 3.0 (L/min). When the volume water flow rate at 3.0 (L/min), the flooding point started at gas flow rate of 140 (L/min). Other than that, we also had determined the pressure drop (∆P) as a function of gas (air) and liquid (water) mass velocities ((L/min)) using flexi glass packed with Raschig rings. The pressure drop for the volume water flow rate 1.0 (L/min), the range is between 0.0 mm H2O until 15 mm H2O where 15 mm H2O is the highest pressure drop at gas flow rate of 180 (L/min). While the pressure drop for the volume water flow rate 2.0 (L/min), the range is between 0.0 mm H 2O until 59 mm H2O where 59 mm H2O is the highest pressure drop at gas flow rate of 180 (L/min). For volume water flow rate 3.0 (L/min), the pressure drop range is between 0.0 mm H 2O until flooding point at gas flow rate of 140 (L/min). We can conclude that as the gas flow rate increasing, the pressure drop (∆P) will also increase. Thus, we can conclude that all the objectives of the experiment had been reached.

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RECOMMENDATIONS During the experiments, there are some mistakes that have been made. Those mistakes had affected the result of experiment which is the pressure drop (∆P)as a function of gas (air) and liquid (water)mass velocities ((L/min)) using flexi glass packed with Raschig rings. The mistakes are caused by parallax error when reading the pressure on the monotube and also due to the lack of control on the water pump. So, there are some recommendations in order to improve our results in getting an accurate pressure drop. First, we need to make sure that we set-up the arrangement of valves accordingly. For this experiment, we need to set up the arrangement for valve for U-tube (Left) only. Then, we need to set-up the operating arrangement. The U-tube (Right) and Sphere ball arrangement are not used. Make sure that, the valve V-3 is closed. If not the water could not be pumped into the monotube. Other than that, we need to make sure that the level of liquid returning to the water reservoir must always be higher than the bottom of the reservoir. This is to avoid air being trapped in line. In order to do that, valve VR-4 must be well controlled and conducted. If not, the water level in the monotube would be unstable and hard for us to read the pressure. Moreover, we must observed that there are no air bubble in the gas flow so it will not affect our results such as the gas pressure is too low to pump the water up to the column. To avoid parallax error when reading the pressure on the monotube, we must put a white paper behind the glass of the monotube and with a ruler, we read the pressure with our eyes directly straight the scale. In this experiment, there is calibration error in the scale of the liquid water flow rate measurement where the scale begins at 11 (L/min). So, to begin with 10 (L/min), the volume flow rate must be set up at 21 (L/min). Lastly, we have to constantly adjust the volume flow rate when we change the gas flow rate. This is because the volume flow rate will automatically change slightly from its initial position as we change the gas flow rate.

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REFERENCES CAG Gas Absorption Column. Retrieved June 1, 2015 from http://www.edibon.com/products/catalogues/en/units/chemicalengineering/chemicalen gineeringbasic/CAG.pdf. Gas Absorption handouts. Retrieved June 1, 2015 from http://www.engr.uconn.edu/~ewanders/CHEG237W/Gas-Absorption.pdf. Gas-liquid Absorption Column. Retrieved June 1, 2015 from http://www.eng.utoledo.edu/polymer/info/Courses/LabIIHandouts/Gas_absorption.pdf Solution for Gas Absorption Experiment. Retrieved June 1, 2015 from http://www.solution.com.my/pdf/BP05(A4).pdf. Gas absorption Experiment. June 1, 2015 from http://www.slideshare.net/dp93/gasabsorption-experiment.

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Chemical Engineering: Gas Absorption Column. June 1, 2015 from http://chem.engr.utc.edu/Webres/435F/ABS_COL/abs_col.html. C.J. Geankoplis.Transport Processes and Separation Process Principles(includes unit operations). Fourth edition. Page 119.

APPENDICES

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Figure 1

Figure 2

Control unit

Water flow rate controller (LPM)

17

Figure 3

Figure 4

Gas absorption unit

Schematic diagram of gas absorption column 18