Gas Absorbtion

ABSTRACT

The experiment conducted was called the gas absorption experiment. The purpose of 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. Besides that, it is also to examine the relationship between the experimental pressure drop values and correlated values for a packed column and also calculate its percentage error. Pressure drop was determined by testing each of the water flow rate which is 1LPM, 2LPM and 3LPM while increasing air flow from 10 L/min to 180 L/min in manometer. The y-axis and x-axis are calculated by using the formula of:

In order to get the flooding point, pressure drop chart and graph of log pressure drop against log gas flow rate from result must be generalized. In the end of experiment, it can be concluded that at constant water flow rate from 1 LPM to 3 LPM, as pressure drop across the packed column increase, the air flow rate increases.INTRODUCTION

Gas absorption can be defined as the unit operation in which one or more soluble components of a gas mixture are dissolved in a liquid. The liquid phase very slightly vaporize in gas phase because of liquid phase is immiscible in gas phase while the gas phase is inert gas. Mass transfer is involved in this process from the gas phase to liquid phase. Gas absorption usually used in controlling industrial air pollution and to separate acidic impurities from mixed gas stream. Most common mass transfer devices used for air pollution is packed towers. Packed tower devices operates in counter-current flow which is involved the liquid enters the system from the top and the gas stream mixed with discharged will enter from bottom. These will results transferred from the gas to the liquid. In absorption process, packing materials play an important role as it provide large surface area between the liquid flows down and gas containing solute that enters from bottom.

OBJECTIVE

1. To examine the air pressure drop across the column as a function of air flow rate for different water flow rates through the column.2. To examine the relationship between the experimental pressure drop values and correlated values for a packed column3. To calculate its percentage error between the experimental values and the correlated values

THEORY

Pressure drop is result of fluid friction between liquid flow and the packing. In figure 1 show a straight line is plotted and wet column three curvy lines are plotted. The point where the slope is starting to change is shows that the points where liquid hold up to increases and also can be known as loading point. As the gas flow rate is increases, pressure drop increased until the line plotted almost vertical. The liquid in this point is continuous phase and this point also known as flooding point and it occur when liquid accumulates because of high gas flow rate and continues until it completely filled with liquid.

Figure 1 : Graph of relationship between pressure drop and gas flow rateThe formulae used for the calculation of theoretical flooding point and values to plot the graphs are:

, where Gy is the gas mass flow rate per unit column cross-sectional area, kg/m2s,, where Gx is the liquid mass flow rate per unit column cross-sectional area, kg/m2s

= 0.005027 m2

APPARATUS

Figure 2: Gas Absorption Unit

PROCEDURES

General Start-Up Procedures:1.All valves except for the ventilation valve V13 are ensured to be closed. 2.All gas connection was checked so that it is properly fitted.3.The valve on the compressed air supply line was opened. The supplied pressure was set between 2 to 3 bar by turning the regulator knob clockwise.4.The shut-off valve on CO2 gas cylinder was opened and checks whether it is sufficient.5.The power for control panel was turned on.General Shut-Down Procedures:1.The pump P1 was switched off.2.The valves V1, V2, and V12 were closed.3.The valve on the compressed air supply line was closed and the supply pressure was exhausted by turning the regulator knob counter clockwise all the way.4.The shut-off valve was closed on the CO2 gas cylinder.5.All liquid in the column K1 was drained by opening valves V4 and V5.6.All liquid from the receiving vessels B1 and B2 was drained by opening valves V7 and V8.7.All liquid from the pump P1 was drained by opening valve V10.8.The power for the control was turned off.

Experiment: Hydrodynamics of a Packed Column ( Wet Column Pressure Drop)Procedures:1.The general start-up procedure was performed.2.The receiving vessel B2 was filled trough the charge port with 50 L of water by opening valves V3 and V5.3.Valve V3 was closed.4.Valve V10 and valve V9 was opened slightly. The flow of water was observed from vessel B1 through pump P1.5.Pump P1 was switched on, then valve V11 was slowly opened and adjusted to give water flow rate of around 1 L/min. The water was allowed to enter the top of column K1, flow down the column and accumulate at the bottom until it overflows back into vessel B1.6.Valve V11 was opened and adjusted to give a water flow rate of 0.5 L/min into column K1.7.Valve V1 was opened and adjusted to give an air flow rate 40 L/min into column K1.8.The liquid and gas flow in the column K1 was observed and then the pressure drop across the column was recorded at dPT-201.9.The steps 5 and 6 was repeated with different values of air flow rate, each time increasing by 40 L/min while maintaining the same water flow rate.10.Steps 4 to 7 was repeated with different values of water flow rate, each time increasing by 0.5 L/min by adjusting valve V11.RESULT

Flow rate (L/min)Pressure drop (mm H20)

Air 20406080100120140160180

Water

10135710142057

202691644---

31492071----

Figure 3: Graph of log pressure drop against log gas flow rateCALCULATION

THEORETICAL FLOODING POINTFor 1LPMAt 20L/min water flow rateTo calculate Gy, gas flow rate,

To calculate y-axis ,

To calculate Gx, liquid flow rate,

To calculate x-axis,

At 40L/min water flow rateTo calculate Gy, gas flow rate,
































For 2LPMAt 20L/min water flow rateTo calculate Gy, gas flow rate,
























For 3LPMAt 20L/min water flow rateTo calculate Gy, gas flow rate,




















Air flow rate(m3/h)K4 (y-axis)FLV(x-axis) in FLV(x-axis) in FLV(x-axis) in Pressure drop correlated in mm H20

1LPM2LPM3LPM1LPM2LPM3LPM

200.0021.4542.8824.2341.525.0812.7

400.0080.7271.4112.1175.0810.1625.4

600.0180.4850.9411.4118.8925.438.1

800.0320.3630.7061.05812.740.6450.8

1000.050.290.5640.84725.445.7253.34

1200.0720.2430.471-40.6450.8-

1400.0980.208--43.18--

1600.1280.182--50.80--

1800.1620.363--55.89--

Figure 4: Graph of log correlated pressure drop vs log gas flow rate

PERCENTAGE ERROR %1LPM Total correlated pressure drop = 243.38 mm H20Total pressure drop = 117 mm H20

2LPM Total correlated pressure drop = 177.8 mm H20Total pressure drop = 77 mm H20

3LPM Total correlated pressure drop = 180.3mm H20Total pressure drop = 104 mm H20

DISCUSSION

Based on the results obtained, the pressure drop in the dry packed column increases as the air flow rate increased. These occur due to the air flow rate increased results of increasing in resistance for the water to flows down the column and give high pressure drop across the packing. From the results also shows the pressure drop in the wet column is much higher than pressure drop in dry column. This is because of the amount of space for gas flow is hindered by the liquid flowing down the column. Gravitational pull forced water flows down and gas flows in counter-current direction. The packing column functions as providing contact for air and water. In 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:

Then those values can use to generalize correlation for pressure drop in packed column in chart shows in Appendix. Theoretical generalized correlation charts show that the high flow parameters are typical of high liquid rates and high pressure drop. However, by looking at both of graph, it shows difference of value pressure drop in theoretical and experimental. Percentage error of pressure drop in 1LPM is 52%, 2LPM is 56% and 3LPM is 42%. These errors 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 gas flow rate increased, pressure drop increased and some of the water will trapped in packing. Later, the water from bottom will increase until the highest level and this will results in flooding. Flooding point is the highest point for each line in the graph of pressure against gas flow rate. When this happen, the process can be no longer be conducted because there is too much liquid entrainment. The flooding points occur at 120 L/min in 2 LPM and 100 L/min in 3LPM.CONCLUSION

In conclusion, the pressure drop will increase when the gas flow increased at constant water flow rate of 1LPM to 3LPM. The resistance to of water flows down the column due to the increasing water flow rate. The same principle is applied for the theoretical value based on the pressure drop correlation charts as well as the experimental value. However, there are some errors made while conducting the experiment which leads to different value of pressure drops.

RECOMMENDATION

There are some recommendations that should be taken account into to ensure the experiment to become more accurate: The valve controlling the level of water flowing back to the water reservoir should be constantly checked so that we can get a better reading Make sure all the valves are closed before using the column so that the experiment runs smoothly Make sure the gas and liquid flow rates were constant at that particular flow rate Give the experiment some more time before the results are taken

REFERENCES

Gas Liquid Absorption retrieved http://iitb.vlab.co.in/?sub=8&brch=116&sim=951&cnt=1 Gas Absorption Lab Manual Principle of Gas Absorption retrieved from http://pubs.acs.org/doi/abs/10.1021/ie50180a002 Packed column-column diameter retrieved fromhttp://www.separationprocesses.com/Absorption/GA_Chp04a.htm Figure and table for mass transfer image , retrieved from http://www.reviewpe.com/penotes/distil/R_distil.htm

APPENDIX

Gas Absorbtion

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