TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES363 P. CASAL ST., QUIAPO, MANILACOLLEGE OF ENGINEERING AND ARCHITECTURECHEMICAL ENGINEERING DEPARTMENT

EXPERIMENT NO. 3: GAS ABSORPTION

SUBMITTED BY:

SUBMITTED TO:

DECEMBER 4, 2014

EXPERIMENT NO. 2DRYING

Discussion:

Gas absorption is the unit operation in which one or more soluble components of a gas mixture are dissolved in a liquid. The gas phase or gas mixture is inert gas while the liquid phase is immiscible in the gas phase. The liquid will therefore vaporize slightly in gas phase. The process involves mass transfer of the component of the gas from the gas phase to the liquid phase. The process involves the transfer of solute fromgas to liquid phase. Where the mass transfer occur in the opposite direction known asdesorption or stripping. An example of gas absorption is the removal of ammonia from air by contact with liquid water. Ammonia is very soluble in water whereas air is only slightly soluble.

Gas absorption operation is widely used in controlling industrial air pollution, andto separate acidic impurities from mixed gas streams. Packed towers are the mostcommon mass transfer devices used for both air pollution control and recovery of processgases. Generally, a packed tower is made up of a piece of pipe set on its end and is filled with inert materials or packings. The packed tower usually operates in counter- currentflow, whereby the liquid enters the system via the top, flows down across the packingsand wets the surface of the packings, and the gas stream mixed with discharge will enterfrom the bottom. When the liquid and gas are in contact with one another, effluent will betransferred from the gas to the liquid.

In considering the design of equipment to achieve gas absorption, the main requirement is that the gas should be brought into intimate contact with the liquid, and the effectiveness of the equipment will largely be determined by the success with which it promotes contact between the two phases. In absorption, the feed is a gas introduced at the bottom of the column, and the solvent is fed to the top, as a liquid; the absorbed gas and solvent leave at the bottom, and the unabsorbed components leave as gas from the top. The essential difference between distillation and absorption is that in the former the vapour has to be produced in each stage by partial vaporisation of the liquid which is therefore at its boiling point, whereas in absorption the liquid is well below its boiling point.

Materials:

Gas Absorption Column Carbon Dioxide Cylinder 300 mL of 0.1 M NaOH Solution Fresh Tap Water

Procedure:

1. First, fill the gloves of the absorption analysis equipment to the left of the panel with 0.1 M caustic soda. Wear gloves and goggles while doing this. Adjust the level in the gloves to the 0 mark on the right tube using drain valve into a flask to do this.2. Fill the liquid reservoir tank to the three-quarters full with fresh tap water.3. With gas flow control valve C2 and C3 closed, start the liquid pump and adjust the water flow through the cloumn to approximately 6 liters per minute on the flow meter F1 by adjusting flow control valve C1.4. Start the compressor and adjust the control valve C2 to give air flow to approximately 10% of full scale on flow meter F2.5. Carefully open the pressure regulating valve on the carbon dioxide cylinder, and adjust valve to give a C3 on the flow meter F3 approximately one half of the airflow F2. Ensure the liquid seal at the base of the absorption column is maintained by, if necessary, adjust of control valve C4.6. After 15 mins, or so of steady operation, take samples of gas simultaneously from sample points S1 and S2. Analyze this consecutively for CO2 content in these gas samples.7. Flash the sample lines by repeated sucking from the line, using the gas piston and expelling the contents of the cylinder to the atmosphere. Note that the volume of the cylinder is about 100 mL. Estimate the volume of the tube leading to the device. Then decide how many times you need to suck and expel.8. With the absorption glove isolated and vent to the atmosphere closed, fill the cylinder from the selected line by drawing the piston cut slowly. Note the volume taken into cylinder V1, which should be approximately 20 mL for this particular experiment. Wait at least 2 minutes to allow the gas to come to the atmosphere/temperature of the cylinder. If the concentration of CO2 in the gas sampled is greater than 8%, it is possible to suck liquid into the cylinder. This will ruin your experiment and takes time to correct. Stop it at a particular mark, for example, Y1 = 20 on the coarse scale and read the fine scale.9. Isolate the cylinder from the column and the absorption glove and vent the cylinder to atmosphere pressure. Close after 10 seconds.10. Connect cylinder to absorption glove, the liquid level should not change. If it does not change, briefly open to atmosphere again.11. Wait until the level in the indicator is on zero showing that the pressure in the cylinder.12. Slowly close the piston to empty the cylinder into the absorption glove. Slowly draw the piston out agin. Note the level in the indicator tube. Repeat steps 5 and 6 until no significant change in level occurs. Read the indicator tube marking, V, this represent the volume of the gas sampled.13. Repeat the experiment for trials 2 and 3.

Experminental Set-Up:

Data and Results:

MW of NaOH = 204.2 g/molMW of CO2 = 44 g/mol

Determination of Molarity of NaOH before titration:Volume (L)Mass of NaOH (g)

Trial 10.00270.1681

Trial 20.00240.1579

After titration:Volume (mL)

Trial 1Trial 2Trial 3

Blank0.40.10.2

Run 10.20.40.1

Run 20.30.20.2

Run 30.10.30.3

Run 40.20.20.2

Run 50.10.20.2

Determination of the moles of NaOH:Moles of NaOH (mol)

Trial 1Trial 2Trial 3

Blank Sample0.0001254173.13543E-056.2709E-05

1st run6.27086E-050.0001254173.1354E-05

2nd run9.40629E-056.27086E-056.2709E-05

3rd run3.13543E-059.40629E-059.4063E-05

4th run6.27086E-056.27086E-056.2709E-05

5th run3.13543E-056.27086E-056.2709E-05

Determination of the Concentration of carbon dioxide absorbed:Moles of CO2 (mol)

Trial 1Trial 2Trial 3

Blank Sample6.27086E-051.56771E-053.13543E-05

1st run3.13543E-056.27086E-051.56771E-05

2nd run4.70314E-053.13543E-053.13543E-05

3rd run1.56771E-054.70314E-054.70314E-05

4th run3.13543E-053.13543E-053.13543E-05

5th run1.56771E-053.13543E-053.13543E-05

Concentration of CO2 absorbed(ppm)

Trial 1Trial 2Trial 3

Blank Sample68979.44168979.44168979.441

1st run68979.44168979.44168979.441

2nd run68979.44168979.44168979.441

3rd run68979.44168979.44168979.441

4th run68979.44168979.44168979.441

5th run68979.44168979.44168979.441

Sample Calculations:

Reaction: (1) (2)

Determination of Molarity of NaOH: = 0.1681 g x = = 0.1579 g x = = = 0.304893532 = = 0.322192295 = = = 0.313542914

Determination of the moles of NaOH:a. Blank SampleTrial 1:

Determination of the Concentration of carbon dioxide absorbed:a. Blank SampleTrial 1: = Concentration of carbon dioxide absorbed = = = 68979.441

Analysis and Interpretation:

The data and the graph shows that the CO2 absorbed is in minimal amount. It also shows that Trials 2 and 3 absorbed more CO2 than Trial 1.

Conclusions and Recommendations:

The CO2 absorbed by the water is very minimal. This may be due to errors. Getting the concentration of the CO2 in the column was not accurate because it took some to transport the sample to the instrumentation room to titrate it, which causes the CO2 to evaporate. Also, the compressor also stops constantly when overheated, so it affects the data.

It is recommended that the titration apparatus is near the gas absorber equipment as to quickly titrate it, which therefore, results to minimal CO2 losses. It is recommended that future users of the absorption column ensure that the carbon dioxide analyzer is properly calibrated. It is also recommended to change or at least repair the compressor.

Questions:1. Discuss the following:a. Loading pointb. Flooding pointc. Hydronamicsd. Channeling2. Differentiate packed columns and tray columns.3. Dry gas containing 75% air and 25% NH3 vapor enters the bottom of cylindrical packed absorption tower that is 2 ft in diameter. Nozzles in the top of the tower distribute water over packing. A soluttion of NH3 in water is drawn from the bottom of the column, and scrubbed gas leaves the top. The gas enters 80F and 730 mmHg. The leaving gas contains 1% NH3.a. If the entering gas flows through the empty the mepty column at an average velocity of 1.5 ft/s, how many ft3 of entering gas are treated per hour?b. How many pounds of ammonia are absorbed per hour?1. 2. 3. 4. A gas mixture containing 1% acetone and 99% dry air by volume is contacted with pure water in a plate gas absorber in which 90% of the acetone is recovered. The inlet gas flow rate is 30 kgmol/hr and the inlet water flow rate is 90 kmol/hr. The absorber operates at 27C and 1 atm. The equilibrium curve is given by y* = 2.53xa. What is the concentration of the strong liquor?b. What is the number of theoretical plates required?5. A mixture of benzene vapor and flue gas contains 12.7 mole % benzene and is to be scrubbed continuously in a packed tower operated at tmospheric pressure at 43C. The tower is to treat 36000 ft3 per hour of entering gas and the exit gas is to contain 1.5 mole % benzene and will be supplied at the rate of 28 lbmol/hr. the solution of benzene in oil may be assumed to follow Raoults law and the vapor pressure of pure solvent at 43C is 0.263 atm. It is agreed that the maximum allowable superficial velocity of the gas is 1 ft/sec. calculate the height of the tower required, assuming that the height of a transfer unit is 2 ft.

Answers to Questions:1. Loading point the liquid flow is reduced due to the increased gas flow. The liquid is held in the void space between packing.Flooding point the liquid stops flowing altogether nd collects in the top of the column due to very high gas flow.Hydronamics the study of liquids in motion.Channleing the gas or liquid is much greater at some point than at others.2. The most striking difference between trays and packings is the percent opening of each phase-contacting device. A tray typically has an opening ranging from 8 to 15% of the tower cross-section area, while the projected opening of a typical packing design is usually more thatn 5-% of tower cross-section. The void fraction of a packed column is even higher, usually more than 90% of the tower volume.