AbstractThe objective of this experiment is to analyse the
absorption of liquid in gas flow. It is also to determine a
relationship between the flowrate of the absorbent and the
absorbed. Loading and flooding of the water is also being determine
in this experiment. The pressure drop as a function of gas and
liquid mass velocities (m3/hour) using flexi glass column packed
with Raschig Ring. Air will be use as the function of gas while
water will be the function of liquid. The relationship allows
future users of the column to determine the possible conditions to
achieve the absorption of gas that being desire. The relationship
between the absorbent flowrate and concentration change was
expected to be linear and have a significant effect on the change
in concentration; however, the correlation deviated from the
anticipated trend. The data contained outlier points, which when
excluded improved the fit of seen correlations. With water as the
absorbent, a linear relationship was observed. The water had a
higher average of gas concentration change, a lower average percent
error, and a lower standard deviation among the calculated k
values. Data, good or bad, proved difficult to obtain mostly due to
gas analyser. Calibration of the analyser took a majority of the
time spent in lab, and in the final lab session the analyser was
never able to be calibrated.
IntroductionGas absorption is a widely unit operation in
Chemical Engineering. 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. Gas absorption operation is
widely used in controlling industrial air pollution and to separate
acidic impurities from lots of mixed gas streams. Packed tower had
been used as mass transfer devices used for both pollution control
and recovery process gases. Packed tower is made of piece of pipe
set on its end and is being filled with inert materials or packing.
It is usually operate in counter current flow, which the liquid
enters the system via the top and flows down the packing and will
wetted the surface of the packing and this will make the gas stream
mixed with discharge and lead to the bottom which entered from the
bottom. Liquid and gas will be contacted and the effluent will be
transferred from the gas to the liquid. There are numerous process
applications where particular components need to be selectively
removed from the gas stream. This process (absorption process) has
been developed by Chemical Engineers as a process that shows high
selectivity and capacity as compared to absorbents which do not
chemically react with the absorbing series. Examples are the
removal of ammonia in the Haber process of ammonia synthesis,
separations of acid such as CO2 and H2S from natural gas, and CO
separation from gas mixtures with H2.
AimsThis 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. It is also to determine the Loading and
Flooding Points in the column and to model the pressure drop as a
function of gas (air) and liquid (water) using flexi glass column
packed with Raschig Ring.
TheoryPacked columns are used for the efficient gas-liquid
contact processes contact interface into the bulk of the liquid.
The absorption involves a concentration gradient across the
gas-liquid interface. Figure I is the example of the visual gas
liquid contact interface. Packed columns is commonly being used for
efficient gas-liquid processes. Being used in a processes like gas
absorption, desorption (stripping), distillation and others.
Consisting of a cylindrical column filled with packing, liquid
inlet and distributor at the top, gas inlet at the bottom, liquid
and gas outlets at the bottom and top respectively. Dumped and
structured had been categorised which are from the column packing.
Distributor also consists of several pipe used for spreading the
liquid uniformly throughout the cross-section of column.
Figure I: Gas liquid interface LoadingFlooding
1. Loading is the when liquid is accumulate in side packed
column that generate the pressure drop.2. Flooding amount of liquid
flood at the top of column with increasing pressure drop due to
accumulation of liquid in side pack column
Apparatus and Material
Solteq-QVF absorption column (model: BP 751-B)
ProcedureGeneral 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 counter clockwise 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 counter clockwise 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
General Procedure
1) U-tube manometer will be filled with water by arranging the
values according U-tube arrangement.2) All the values is being set
up to operating arrangement before the operation is started.3) The
valves is need to be checked carefully before the column is to be
use.4) Fill the receiving vessel B2 through the charge port with 50
L of water by open the valve which are V3 and V5. Later close valve
V35) Valve V10 and Valve V9 should be open slightly. Observe the
flow of water from vessel B1 through pump P1.6) Pump P1 will be
switched on and carefully and slowly open and adjust the V11 valve
to give a water flow rate around 1L/min. By allowing the water to
enter the top of column K1, flow down the column and accumulate at
the bottom until it overflows back into vessel B1.7) At valve V11,
open and adjust it to give a water flow rate of 1.0 L/min into
column K1.8) Open and adjust valve V1 to give an air flow rate of
1.0 L/min into column K1.9) One should observe the liquid and gas
flow in the column K1, and the pressure drop should be recorded
across the column at Dpt-201.10) Repeats step 6 7 with a different
values of air flow rate, each time increasing by 1.0 L/min while
flow rate is being maintained.11) Steps 5 8 will also be repeated
with a different values of water flow rate, each time by increasing
by 1.0 L/min by adjusting valve V11.12) The flow rate is only
observe till 3.0 L/min.
Results
Flow rate L/minPressure Drop (mm H20)
Air Water 20406080100120140160180
1.021824262629344877
2.00147121946--
3.010111550-----
Table A: Data obtained from the experiment
Figure 1.0
Flow rate L/minPressure Drop (mm H20)
Air Water 1.31.61.81.92.02.12.152.22.3
1.00.31.31.41.41.41.51.51.71.9
2.0000.60.81.11.31.7--
3.011.01.21.7-----
Table B: Log table
Figure 2.0
Water flow rate(L/min)GL (kg/m2s)
1.03.3002
2.06.6004
3.09.9006
Calculations
Data: Density of air = 1.175 kg/m3density of water = 996
kg/m3Column diameter =80 mmArea of packed column diameter =
0.005027m2Packing Factor = 900 m-1Water viscosity = 0.001Ns/m2
Theoretical flooding point:GG, gas flow rate (kg/m2s)GG = =
=0.0779kg/m2sCapacity parameter, y-axisy-axis = = = 0.0154GL,
liquid flow rate per unit column cross-sectional areaGL = ==3.3002
kg/m2sFlow parameter, x-axisx-axis = = = 1.4551
Air flow rate(L/min)GG(kg/m2s)Capacity parameter(y-axis)Flow
parameter (x-axis)
1 LPM2 LPM3 LPM
200.07790.01541.45512.91024.3653
400.15570.06140.72801.45602.1840
600.23360.13830.48520.97051.4557
800.31150.24590.36390.72781.0917
1000.38930.38410.29120.58230.8735
1200.46720.55320.24280.48520.7279
1400.62290.75310.20790.41590.6238
1600.62290.98320.18200.36390.5459
1800.64490.99830.19300.38950.5955
Table 3.0
Figure 3.0: Correlation data
Discussion
This experiment used packed tower that has a Raschig Rings. We
need to determine the pressure drop across the dry column as a
function of air flow rate and the air pressure drop across the
column as a function of air flow rate for different flow rates
through the column. This experiment were based on the flow rate of
liquid and gas in the packedBased on the Table A, when the flowrate
is set to 1 L/min the flow of the air is at 20, the pressure drop
is at 2. After that, the air is at 20, 40, 60, 80, 100, 140, 160
and 180 the values for the pressure drop is increase. Meanwhile for
the flow rate set at 2 L/min, the air is at 20, the pressure drop
is at 0 mmH20. After that, at 40, 60, 70, 80, 100, 120 and 140, the
values just increasing. But, there is nothing happen at the 160 and
180.For the flow rate that had been set at 3 L/min, the values of
the pressure drop that had appear is only at air 20, 40, 60 and 80.
There is nothing happen at 100, 120, 140, 160 and 180.After all,
the flooding point is happen at only two of the flow rate that had
been set up which are at 2 L/min and 3 L/min. All reading of the
pressure drop is recorded until flooding point is reached. The
flooding point happen at 2 L/min is after the air is at 160
meanwhile for the flow rate of 3 L/min. When flooding occurs, the
process can no longer be run because there is too much liquid
entrainment. The flooding point is happen because flooding point is
in a packed or tray column where it have vapour flowing up and
liquid flowing down, there is an upper limit to how fast the liquid
can drain downwards. The point at which liquid cannot flow down as
fast as it is coming into the column. The actual flooding point is
partly dependent on how fast the liquid can flow down with no
vapour flowing upwards and the rate at which vapour is trying to
flow upwards. Cross sections of the column occupied by vapour are
not available for liquid flow - effectively reducing the
cross-section for downward flow of the liquid. Also get entrainment
of liquid in the upward flowing vapour and drag on the liquid as it
fights the direction of the vapour flow - the vapour wants to go up
while the liquid wants to go down. This additional drag also slows
down the flow of liquid trying to drain downward in the columnThe
data that had been recorded, which is the graph of log pressure
drop against the log air flow rate is plotted. From this, the log
pressure drop is directly proportional log gas to flow rate. Hence,
the higher the log of gas flow rate the higher the log of pressure
drop. But the higher the water flow rate, the lower is the log gas
flow rate.The experiment is been repeated three times as we get
error by accidently not controlling the water flow rate. This
happened and the hissing sound can be heard as the water is fill up
the column and evaporated. There are precautions that have to be
concerned. Firstly, the gas-liquids absorption column should be
check carefully to avoid any accident from occurring. Next, we need
to ensure that all the valves are free from air bubble so that the
reading at the manometer is free from parallax error. In addition,
the manometer calibration is in the right order. Last but certainly
not least, the level of water at the bottom VR 4 should always be
adjusted. This is to avoid air from trapped in the line.
ConclusionIn conclusion, the aim of experiment which is to
determine the pressure drop across the column as a function of air
flow rate for different water flow rate through the column. At 1
L/min of flow rate, there is nothing happened (no flooding point).
But at 2 L/min and 3 L/min, there is flooding point which at 2
L/min, the flooding point is happened at 160 of air while at 3
L/min it is happened at 100 of air. However, before the flooding
point is occurred, the pressure drop will moderately increase and
sometimes decrease due to the error that had been occurred during
experiment. But in the end, we managed to visualize the pressure
drop as a function of gas (air) and liquid (water) using the flexi
glass column packed with Rashig Ring. However, there are some
errors made when the experiment is being conducted resulting in a
slight inaccuracy of the experimental chart plotted.
RecommendationsThose who used the absorption column must ensure
that the gas analyser is properly calibrated to improve the data.
This can be done by asking the lab assistant. The data that being
recorded must be consistent throughout the experiment for this will
getting the better results. One should also know how does each of
the apparatus works. Do not proceed the experiment without asking
the labs assistant help. Those who control the flow of the water
should be ALERT and not daydream otherwise the experiment will be
repeated again and again. Remember to turn of the gas too!
Referencehttp://en.wikipedia.org/wiki/Liquid%E2%80%93liquid_extractionhttp://www.academia.edu/3641270/Liquid-Liquid_Extraction_Basic_Principleshttp://www.chem.ualberta.ca/~orglabs/Interactive%20Tutorials/separation/Theory/theory1_1.htmhttp://courses.chem.psu.edu/chem36/Experiments/PDF's_for_techniques/Liquid_Liquid.pdfLab
ManualYaminah 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
Appendix