Question

In gas absorption column experiment, the purpose of the experiment is to measure the absorption of carbon dioxide into water flowing down the tower, using the gas analysis equipment provided. The experiment procedure is as follows.

1. First fill the two globes of the absorption analysis equipment on the left of the panel with 1.0 Molar caustic soda. Adjust the level in the globes to the '0' mark on the sight tube, using drain valve Cv into a flask to do this.
2. Turn on the mains water supply to the sump tank. Fit the drain pipe to the water return from the column so that water is not returned to the sump tank.
3. With gas flow control valves C2 and C3 closed, start the liquid pump and adjust the water flow through the column to approximately 4 litres/minute on flowmeter F1 by adjusting control valve C1. 4. Start the compressor and adjust control valve C2 to give an airflow of approximately 25 litres/minute in flowmeter F2.
5. Carefully open the pressure regulating valve on the carbon dioxide cylinder, and adjust valve C3 to give a value on the flowmeter F3 approximately one half of the air flow F2. Ensure the liquid seal at the base of the absorption column is maintained by, if necessary, adjustment of control valve C4.
6. After 5 minutes or so of steady operation, take samples of gas from the top and middle sample points. Analyse these consecutively for carbon dioxide content in these gas samples as shown in the accompanying sketch and following notes.
7. Flush the sample lines by repeated sucking from the line, using the gas piston and expelling the contents of the cylinder to atmosphere. Note that the volume of the cylinder is about 100 cc. Three full volumes of the gas piston is sufficient.
8. With the absorption globe isolated and the vent to atmosphere closed, fill the cylinder from the selected line by drawing the piston out slowly. Note volume taken into cylinder V1, which should be approximately 20ml for this particular exercise. Wait at least two minutes to allow the gas to come to the temperature of the cylinder.
9. Isolate the cylinder from the column and the absorption globe and vent the cylinder to atmospheric pressure. Close after about 10 seconds.
10. Connect cylinder to absorption globe. The liquid level should not change. If it does change, briefly open to atmosphere again.
11. Wait until the level in the indicator tube is on zero showing that the pressure in the cylinder is atmospheric.
12. Slowly close the piston to empty the cylinder into the absorption globe. Slowly draw the piston out again.
13. Once the final value has been read from the scale, slowly close the piston to empty the cylinder into the absorption globe. Reposition the valves to allow the globe to equilibrate with atmosphere.

Write theoretical explanation about experiment.

Answer 1

In gas absorption a soluble vapor in a gas mixture is preferentially absorbed by a contacting liquid. For example, absorption columns and towers are used for removing gaseous pollutants from an air stream or recovering valuable solvents for reuse. The solute can subsequently be recovered from the liquid by distillation or in some cases by bringing the liquid into contact with an inert gas. Such an operation, the reverse of gas absorption, is desorption or gas stripping. In this experiment a packed tower will be used for the removal of a common gaseous byproduct, carbon dioxide, with water as the liquid medium.

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.

Tower packing is chosen to 1) provide for large interfacial area between liquid and gas; 2) possess desirable fluid-flow characteristics; 3) be chemically inert to fluids being possessed; 4) have structural strength to permit easy handling and installation; and 5) represent low cost. The packing may be random or regular in various shapes.

The liquid entering the top of the column may be a pure solvent or a dilute solution of solute in solvent. This liquid (sometimes called weak liquor) is distributed over the top of the packing by the distributor. In ideal operation the surfaces of the packing would be uniformly wetted. The solute-containing gas, or rich gas, enters the distributing space below the packing and flows upward through the interstices in the packing. This is called a countercurrent packed tower, the gas is flowing countercurrent to the liquid. The solute in the rich gas is absorbed by the liquid entering the tower and exits the top as a dilute or lean gas. The liquid is enriched in solute as it flows down the tower and becomes concentrated liquid, sometimes called strong liquor, and leaves the bottom of the tower through the liquid outlet.