1. ABSTRACTGas absorption is the process of absorption of gas
into a liquid and it possible to breakdown or separate one or more
components of a gaseous mixture and producing a liquid that contain
a desired quantity of a gas. Process of packed column of gas
absorption is under gas-liquid counter current condition. This
process can be obtained the value in experiments depends on the
packing size, shape and material of construction. The objective in
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 methods in this experiment are where
step up the experiment to switch on the pump and valve to run it.
Then, the flow rate of water was constant at 1.0 L/min, 2.0 L/min
and 3.0 L/min and adjusted the valve that control air flow rate
used in different rates at 20, 40,60,80,100,120,140,160 and 180
L/min. These shown that the results of pressure drops reading are
increased at increasing air flow rates and water flow rates. For
example at 2.0 L/min the pressure drop starting point from 20L/min
increase highly at 11 mm H2O,at 40L/min is 22 mm H2O, at 60L/min is
25 mm H2O, at 80 is 38 mm H2O, at 100 is 49 mm H2O and at 120 is 63
mm H2O which It was achieved floading point. When the conditions
are achieved at maximum temperature and pressure, the pressure
drops are at flooding point.
1. INTRODUCTIONGas absorption processes are widely used in the
industry. It can be used for removing contaminants or impurities
from a gas stream. There are numerous applications of this approach
in the chemical industry. Common example of gas absorption are
removal or recovery of NH3 in fertilizer manufacturing, control of
SO2 from combustion source, control of odorous gases from rendering
plants and removal of CO2 from air. In addition, gas absorption can
controlled industrial air pollution and make separation of acidic
impurities from mixed gas streams which are including carbon
dioxide, hydrogen sulfide and organic sulfur compounds (Jackson,
2008).
There are numerous types of contactors that have been developed
to assure a good contact between the gas and liquid streams. This
is important to obtain an efficient separation process that
requires a minimal size for the absorber. Many of the contactors
are similar to those used in distillation. These include bubble cap
trays and columns filled with various packing.
Chemical engineers need to be able to design gas absorbers which
produce a treated gas of a desired purity with an optimal size and
liquid flow. This can be based on existing correlations and when
required, laboratory and or pilot plant data. (Cusler, 2009).
Gas absorption is a unit operation in which soluble components
of a gas mixture are dissolved in a liquid. The inverse operation,
called stripping or desorption, is employed when it is desired to
transfer volatile components from a liquid mixture into a gas. Both
absorption and stripping, in common with distillation, make use of
special equipment for bringing gas and liquid phases into intimate
contact. The apparatus consists of a cylindrical column or tower
with a gas inlet and a distribution space at the bottom, a liquid
inlet and distribution space at the top, gas and liquid outlets at
the top and bottom respectively; and a supported mass of tower
packing, known as raschig rings. A schematic diagram of the gas
absorption column is shown in Figure 1.
Figure 1: Gas Absorption Column System
1. OBJECTIVESThe objective 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.-To plot the
column pressure drops against the air flow rate for every different
water flow rate in the log-log graph paper and compare with the
value obtained from generalized correlation chart (APPENDIX).
1. THEORYIn 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. The rate of mass transfer is
governed by stream flow rates, interfacial contact area, component
diffusivities, temperature, and pressure.
The packed column design is involved the diameter of the column
and height of the packing which essential for specific separation.
The liquid in the column fills up with increasing flow which can
cause the pressure drop increased and the space of gas flow is
reduced from the column. However, the pressure drop increased
rapidly when gas flow rises and the liquid hold up in the column
also increased when the conditions is beyond the loading point.
Meanwhile, at up loading point, the pressure drop flow the same
relation as in dry run. Since the column is not analyzed for gas
absorption, the only equations used are the ones for calculating
the values used on the pressure drop correlation chart.X- axis =
equation (1)
Y axis = equation (2)
Where: Gx = water flow rate, in lbs/sec*in2Gy = air flow rate,
in lbs/sec*in2rx = density of water, in lbs/ft3ry = density of air,
in lbs/ft3mx = viscosity of water, in centistokes g = acceleration
due to gravity, 32.2 ft/sec2FP = packing factor1. APPARATUS AND
MATERIALS
5.1 APPARATUS1. Gas absorption column2. 50 mL buret and buret
clamp3. 50 mL graduated cylinder4. Stopwatch5. Erlenmeyer
flasks
5.2 MATERIALS1. Phenolphthalein indicator solution2. Sodium
hydroxide solution3. Carbon dioxide gas
1. PROCEDURES
General Start-Up Procedures 1. All valves were closed except the
ventilation valve V13. 2. All gas connections were checked which in
properly fit. 3. Valve on the compressed air supply line was
opened. The supply pressure was set between 2 to 3 bar by turning
the regulator knob clockwise. 4. The shut-off valve was opened on
the CO2 gas cylinder. CO2 cylinder pressure was checked either in
sufficient or not. 5. The power was turned on for the control
panel.
General Shut-Down Procedures 1. Pump P1 was switched off. 2.
Valves V1, V2 and V12 were closed. 3. Valve on the compressed air
supply line was closed and the supply pressure was exhausted by
turning the regulator knob counterclockwise all the way. 4. The
shut-off valve on the CO2 gas cylinder was closed. 5. All liquid in
the column K1 was drained by opening valve V4 and V5. 6. All liquid
from the receiving vessels B1 and B2 were drained by opening valves
V7 and V8. 7. All liquid from the pump P1 were drained by opening
valve V10. 8. The power for the control panel then was turned
off.
EXPERIMENT : Hydrodynamics of a Packed Column (Wet Column
Pressure Drop)1. The general start-up procedures were performed as
aboved.2. The receiving vessel B2 was filled through the charge
port with 50 L of water by opening valve V3 and V5. 4. Then valve
V3 was closed. 5. Valve V10 and valve V9 were opened slightly. The
flow of water from vessel B1 through pump P1 was observed. 6. Pump
P1 was switched on, then V11 was opened and adjusted slowly to give
a water flow rate of around 1 L/min. The water was allowed to enter
the top of column K1, flow down the column and accumulated at the
bottom until it overflows back into vessel B1. 7. Valve V11 was
opened and adjusted to give a water flow rate of 0.5 L/min into
column K1. 8. Valve V1 was opened and adjusted to give an air flow
rate of 40 L/min into column K1. 9. The liquid and gas flow in the
column K1 was observed, and the pressure drop across the column at
dPT-201 was recorded. 10. Steps 6 to 7 were repeated with different
values of air flow rate, each time increasing by 20 L/min while
maintaining the same water flow rate. 11. Steps 5 to 8 were
repeated with different values of water flow rate, each time
increasing by 1.0 L/min by adjusting valve V11.
7.0RESULTFlowrate(L/min)Pressure drop (mm H2O)
AirWater20406080100120140160180
1.0002357112142
2.0112225384963---
3.02527334751----
Table 1: Flowrate and Pressure drop
Log Flowrate(L/min)Log Pressure drop (mm H2O)
AirWater1.31.61.81.92.02.12.152.22.3
1.0000.30.50.7811.31.2
2.011.31.41.61.71.8---
3.01.41.41.51.71.7----
Table 2: Flowrate and pressure drop
Flow rate(L/min)Pressure drop(mm H2O)
AirWater20406080100120140160180
1.0002357112142
2.01112225384963--
3.02527334751----
Table 3 Pressure drop across the column
Air Flow rate (L/min)Pressure drop (mm H2O)
Graph 1 Pressure drop against air flow rate.
Flow rate(L/min)Pressure drop(mm H2O)
AirWater20406080100120140160180
1.004.39.225.739.9NANANANA
2.006.928.562.4NANANANANA
3.0021.176.2NANANANANANA
Table 4 Pressure drop (in inch H20/foot) across the column
(Theoretical data)
Air Flow rate (L/min)Pressure drop (mm H2O)
Graph 2 Pressure drop (in inch H20/foot) across the column
(Theoretical data)
Graph 3 Pressure drop of experimental data and theoretical data
against air flow rate 1.0 L/min
Graph 4 Pressure drop of experimental data and theoretical data
against air flow rate 2.0 L/min
Graph 5 Pressure drop of experimental data and theoretical data
against air flow rate 3.0 L/min
8.0Calculations
1. Calculation of column diameter = 75 =0.075
2. Conversion of flowrate, L/min to mass flowrate, kg/m2.s
, = = = 4.42 103 2
() = =
= 0.08/ (2) == =
=3.77 103 /
=1.2 /3,
= 1000 /3
= = (0.08 /) (1.2 /3) = 0.10 /2.
= = (3.77103 /) (1000 /3) =3.77 /2.
3. Computation of X-axis
=
=
=1.31
4. Computation of Y-axis Y =
=
=
= 0.047
Sample Calculation of Error At air =60 mmH2O, water = 2L/min
, % = 100 %
= 100% = 22.81 %
9.0 DISCUSSION
The experiment was done by conducting the different flow rate of
water and flow rate of gas. All the data of pressure drops were
tabulated in Table 1 which shows the increasing air flow rate
caused the rises of pressure drop. When the result obtained at
water flow rate 1.0 L/min, the air flow rates were increased to get
resulted of the pressure drop from 20 L/min until 180 L/min of gas
flow rate. The reading of pressure drop at 20 L/min of air flow
rate at constant water flow rate at 1.0 L/min shown in Table 1 is 0
mmH2O. However, the air flow rate increased at 60 L/min, the
reading pressure drop increased at 1 mmH2O and the results of
pressure drop were obtained through increasing the air flow rate.
Although the increasing of air flow rate from 20 L/min until 180
L/min give the resulted that pressure drop rises but the pressure
drop achieved at flooding point at 180 L/min of air flow rate.
Whereas this explained that the pressure drops at this point have
maximum pressure and achieved maximum temperature.
The table 1 result also shows that the 2.0 L/min of water flow
rate also gets the increasing of pressure drop when the flow rate
of air increased from 20 L/min to 180 L/min. The initial flow rate
of air at 20 L/min shows that resulted of pressure drop at 1 mmH2O.
However, the resulted at 2.0 L/min of water flow rate, the pressure
drop only can be calculated or read at point 20 L/min until 140
L/min of air flow rate and after 140 L/min of air flow rate, there
were flooding of pressure drop occurred and achieved the maximum
temperature and pressure.
The tabulated result show that at 3.0 L/min of water flow rate
and at initial of air flow rate was increased at 25 mmH2O compared
to 1.0 L/min and 2.0 L/min. At this 3.0 L/min of water flow rate
also shows that the increasing air flow rate at 20 L/min until 60
L/min caused to increase the pressure drops. Even though the air
flow rate increased highly at this water flow rate level, the
flooding point of pressure drop was achieved at point 100 L/min
until 180 L/min. The highest water flow rate and air flow rate
used, produced the highest result of pressure drop where it caused
the flooding point at maximum temperature and pressure.
From the experiment, there also having calculation of pressure
drop from the correlation graph for pressure drop in the packed
column. This pressure drops were compared with pressure drop that
recorded from the experiment. The value pressure drop was finding
from graph pressure-drop correlation for random packing by Strigle
as shown in appendix. The result of pressure drop calculation can
be shown in Table 4, Table 5 and Table 6 depending on the water
flow rate. Based on that result, there have quite different value
pressure drop get from experiment and calculation. Besides that,
for all different water flow rate, the pressure drop were most of
at flooding point.
Regarding to this result, there may be some errors or problems
during conducting the experiment. One of the problem is it is hard
to have accurate flow rate values for water and air. This is
because, it is difficult to control the valve manually. This
resulted that the reading of pressure drops also not stable. As a
result, the pressure drop is not accurate and the value quite
different from calculation.
10.0CONCLUSIONBased on these experiment, it can concluded that
the objective of the experiment were achieved which is to determine
the pressure drop across the dry column as a function of the air
flow rate for different water flow rates through the column. For
the graph of log pressure drop against log gas flow rate, it can
concluded that the log gas flow rate enters to the packed column is
increases, the log pressure drop of the packed column also
increases. While for the graph of for generalized theoretical
pressure drop correlation, it concluded that the value of the flow
parameter, x-axis is higher, the value of the capacity parameter,
y-axis is lower. Furthermore, we manage to visualize pressure drop
as a function of gas(air) and liquid(water) using packed column and
Rashing Ring.
11.0 RECOMMENDATIONS
1. Before starting the experiment, Material Safety Data Sheets
(MSDS) was reviewed on NaOH. The sheets can be found in the MSDS
notebook located in the laboratory.2. Personal protective equipment
should include goggles and mask. Disposable nitrile gloves should
be worn when handling NaOH solutions.3. Safety requirements should
be checked and needed to be aware of when using high pressure gas
cylinders.4. 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 buildup of liquid
and flooding. Open the tank valve slowly.5. Remember to plug in the
gas heater 5 minutes before turning on the gas.
12.0 REFERENCES
1. E. L. Cussler, (2009). Diffussion: Mass Transfer in Fluid
Systems,(3rd Ed.) .Cambridge University Press, New York.Page
993-9981. Jackson Y.Z. (2008). Modeling gas absorption. Project
number: WMC 4028, page 5-57. 1. McCabe, W. L., Smith, J. C., &
Marriott, P, (1985). "Unit Operations of Chemical Engineering", 4th
Edition, McGraw-Hill.1. Treybal, Robert E., (1980). "Mass-Transfer
Operations", McGraw-Hill Book Company, Inc., New York, N.Y.1.
Washburn, E. W., Editor, "International Critical Tables of
Numerical Data, Physics, Chemistry, and Technology", McGraw-Hill
Book Company, Inc., New York, N.Y.13.0Appendices
Figure 2: Gas absorption column
Figure 3: Generalized correlation for pressure drop in packed
columns (1 in.H2O/ft = 817 Pa/m)
20