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