Fractional Distillation

PURPOSE:
To expose the student to new equipment and lab procedures. And to determine the percent of ethanol in a fermented mixture.

INTRODUCTION:
Fractional distillation is a method of separating and collecting the different liquids in a solution. This process will work if the components of the solution boil at different temperatures. When a sugar and water solution is allowed to ferment, an ethanol and water solution forms. Since water boils at 100 C and ethanol boils at about 78 C, this fermented solution can be separated by fractional distillation. A typical laboratory fractional distillation set-up is shown below

A. Round bottom flask
B. Vigreux column

C. Distillation head
D. Liebig condenser
E. Vacuum tube (delivery tube)

Heat is applied to the bottom of the larger round bottom flask that holds the fermented mixture. As the vapors rise in the Vigreux column, the higher boiling point water condenses and falls back into the round bottom flask. The ethanol continues to rise slowly and reaches the distillation head where a thermometer registers its temperature. If the temperature is kept at about 78 C, by adjusting the heat, ethanol will flow over to the Liebig condenser and turned back into a liquid. The Liebig condenser has a cold water jacket wrapped around a central tube and when the hot ethanol vapor comes in contact with the walls of the inner tube, it loses heat and returns to a liquid state. The condensed ethanol drips down and is collected in the smaller round bottom flask.

Even under the best conditions only 95% ethanol (5% water) can be collected. This is due to the fact that some water is actually carried over to the Liebig condenser by the energy of the ethanol molecules. The distilling properties of ethanol and water are best described as an azeotropic mixture.

During this lab you will prepare a fermentation solution, filter and then fractionally distill the solution and collect the ethanol.

In the first room you will use a spoon of sugar kept in an evaporating dish, a small amount of brewers yeast kept in a paper boat and a quantity of water kept in a beaker. These items will be added to a flask and then stoppered with an air trap. This mixture will be allowed to set for some time while fermentation will take place.

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In the second room you will filter your fermentation mixture through a funnel with a cotton layer. The filtered ethanol - water solution will be measured for volume in a graduated cylinder and then poured into a round bottom flask.

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A burner is supplied as a source of heat and the level of flame can be adjusted by right and left arrows located near the gas valve.

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If you get into trouble and perform some procedure that causes the lab to fail(lab equipment will no longer operate), you can press the "Reset" button and the simulation will return to the starting position.

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PROCEDURE:
Room 1: preparation of the fermentation mixture

1) You can adjust the background shading by clicking on the "Special" button to the right and selecting "Background". Click on the "Special" button and select "Print Blank Report" to obtain a web page that can be printed and used as a lab report. (the program will not be interrupted)

2) Wear your goggles. Pick up the spoon of sugar found in the evaporating dish to the left and hold the bowl, of the spoon, centered and above the beaker of water. Press "p" to dump the contents of the spoon. Pick up the paper boat containing a small amount of brewers yeast. Hold the boat over the beaker of water so that the left lip of the boat is centered and above the beaker. Press "p" once again to pour the yeast into the beaker.

3) Pick up the beaker and hold the spout (left lip) centered above the flask. Press "p" to pour the mixture into the flask. Pick up the black rubber stopper with glass air trap and drop it into the mouth of the flask. The mixture is now complete and fermentation will start to take place producing carbon dioxide and ethanol. The air trap in the top of the flask will allow the carbon dioxide gas to escape and will not allow outside air to enter. If certain bacteria enter the flask before fermentation is done, it may cause the mixture to produce vinegar instead of ethanol. Move to the next room.

Room 2: filtration and measurement

4) Place the funnel, with cotton layer, in the mouth of the graduated cylinder. Pick up the fermentation flask and center the left lip over and above the center of the funnel. Press "p" to pour the mixture through the funnel. Note that the flask has a yellowish layer in the bottom. This layer consists of many yeast cells that produced the ethanol from sugar and settled to the bottom. Record the volume of fermentation solution collected.

5) Pour the contents of the graduated cylinder into the round bottom flask. Move to the next room.

Room 3: fractional distillation

6) Fill the Liebig condenser with cold running water by clicking on the faucet handle. Click and hold on the "red" right arrow near the gas valve for a short time, you should hear gas escaping. Pick up the match found on its box and light the top of the burner. Dispose of the burnt match in the evaporating dish on the table. Note that the flame ignited and is causing the ethanol - water solution to boil. Also note that a vapor column (light blue haze) has risen up from the round bottom flask and may have entered the Vigreux column. If you press more on the red arrow, the vapor column will continue to rise. In reverse fashion, pressing the "blue" arrow will cause the flame to shrink and the vapor column to fall. Notice that much of the vapor condenses and drips back into the flask.

7) Slowly adjust the heat so that the vapor column just reaches the thermometer in the distillation head. When it does the thermometer reading will start to climb rapidly due to the vapors heat content and stop at the boiling temperature of ethanol, 78 C. If you have the flame set too high, the vapor column will rise all the way to the top of the distillation head and the temperature will climb past 78 C. DON'T ALLOW THIS TO HAPPEN! At a temperature above 78 C, water will pour over into the Liebig condenser and ruin your collected sample of ethanol.

8) As the temperature rests on 78 C, the ethanol will flow over into the Liebig condenser and be turned back into liquid. This liquid will then run down into the small round bottom flask and collected. WATCH CAREFULLY because the ethanol will finally run out and the temperature will climb past 78 C (the water starts to reach the top). At this moment press and hold on the blue arrow and turn off the flame, your distillation is done.

9) Once the flame is off, grab the smaller round bottom flask and hold the right lip centered above the small funnel in the graduated cylinder (the flask rotates clockwise when poured). Press "p" to pour the ethanol into the cylinder. Reattach the small round bottom flask to its original position. Record the volume of ethanol you collected.

If you think that you collected some water in your sample, click on "Reset" and try again.

10) Calculate the requested values asked for on the lab sheet and any given by your teacher. For help on these values click on the "Special" button and select "View Data & Hints". Select "File Report" to send a copy to be viewed by your teacher.