DISTILLATION OF ETHANOL LAB OVERVIEW Level: Grades 9-16 Estimated Time to Completion: 90 Minutes Prior Knowledge: Background Provided In this lab investigation, students will become familiar with the distillation process using a simple distillation column under the context of ethanol purification. A fermentation broth will be created or sampled from a previous lab. The broth will then undergo distillation in a simple distillation column. You will then analyze the purity of the product in terms of ethanol concentration. Upon completion, students will be able to: • Understand the relations of volatility and boiling points of substances. • Purify a compound by separating its components based on their volatility. • Apply the practice of distillation in the context of ethanol production. • Identify and discuss the costs and benefits of ethanol distillation as a method of biofuel production. MATERIALS REQUIRED Yeast Glucose Ethanol Beaker, 250mL Weigh Boats Pipet, 10mL Graduated Cylinder, 100mL Brix RI Meter Ethanol Probe
Round Bottom Flask, 500mL (distillation flask) Round Bottom Flask, 250mL (receiving flask) 3-Way Adapter (distilling adapter, distillation head) Water-Jacketed Condenser Receiving Adapter Keck Clips Stand and Clamps
NOTES TO INSTRUCTOR • It is recommended that several batches of fermentation broth are prepared ahead of time and distributed to students so the solution is consistent among groups. A fermentation broth may be produced by following the same procedure as the fermentation lab. Alternatively, students may prepare the broth during the “down time” of the fermentation lab. • A distillation glassware kit may be purchased alternatively to the above materials. • A detailed description of the distillation apparatus can be found in the “Supporting Materials” folder of the downloadable lab package. • Additionally, a movie tutorial of recommended procedure can also be found at: http://youtu.be/K8jXmOvNA9Q
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DISTILLATION OF ETHANOL INTRODUCTION Distillation is a method of purification in which a liquid is vaporized and successively condensed in order to separate its components. Distillation operates on the property of a substance’s volatility, or its ease of vaporization. When a liquid is heated, the more volatile substances vaporize, leaving the less volatile substances behind. Thus, liquids containing substances with different levels of volatility can easily be separated by distillation. Ethanol is one such substance, capable of being purified from a fermentation broth to 95% purity. A 5% impurity remains from a small portion of co-distilled water. In industry practice, molecular sieves (materials with small, uniform pores) are used to further distill ethanol into an acceptably purer form. In this lab you will become familiar with the distillation process using a simple distillation column under the context of ethanol purification. A fermentation broth will be created or sampled from a previous lab. The broth will then undergo distillation using a simple distillation apparatus. You will then analyze the purity of the product in terms of ethanol concentration.
PROCEDURE STEP 1: PREPARING A FERMENTATION BROTH 1. Prepare a yeast suspension by dissolving 4.67g yeast into 100mL of deionized water. 2. Heat the yeast suspension to 30째C for at least 30 minutes. Stir while heating at about 250rpm. 3. Prepare a 200g/L glucose solution in water by dissolving 200g anhydrous dextrose in 1.0L deionized water. 4. Using a refractometer, measure the dissolved solids contents of the glucose solution. You should measure around 20.0% Brix. 5. Obtain a 250mL Erlenmeyer flask with stopper size #6. 6. Using a graduated cylinder, pour 10.0mL of the yeast suspension into the Erlenmeyer flask. 7. Using a graduated cylinder, add 200mL of the glucose solution into the Erlenmeyer flask with yeast suspension. 8. Seal the Erlenmeyer flask with a one-hole #6 stopper. In the hole place either a bubble trap, or a cotton or glass wool packed column. STEP 2: MEASURING THE FERMENTED SOLUTION 1. Calibrate the ethanol probe using your ethanol solutions. 2. Record the ethanol content of the fermented solution as described in the fermentation lab. STEP 3: CONSTRUCTING THE DISTILLATION APPARATUS 1. Set up your distillation apparatus as shown in the figure on the next page.
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PROCEDURE (cont.)
STEP 4: DISTILLATION OF ETHANOL 1. Quantitatively transfer the 200mL of fermented solution into the 500mL distillation flask. Make sure when washing you do not overfill the 500mL flask. The flask should ideally be half full. 2. Heat the distillation flask using either a Meker burner (keeping flame at least 2 inches away from flask), a hot plate, or a heating mantle. A heating mantle is recommended as the safest method. 3. Slowly adjust the temperature of your heat source until the temperature within the distillation flask reaches 79째C. At this point you should notice condensation in the condenser. 4. Pay close attention to the temperature within the distillation flask. A lowering temperature is an indicator that ethanol is no longer vaporizing into the condensation tube. When this happens, remove the heat source and let the condensed product cool.
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PROCEDURE (cont.) STEP 5: ANALYSIS 1. Measure the mass of ethanol collected by either weighing the sample or measuring the volume in a 25mL graduated cylinder. Record your observation. 2. Measure the concentration of ethanol in both the condensed product and the leftover fermentation broth. You will likely need to dilute the ethanol sample with distilled water by a factor of 10 to 1 to allow the probe to accurately measure its concentration. Record your observations.
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DISCUSSION Compare the measurements of the probe with your mass measurements for ethanol. Which is more accurate?
What are the limitations of the simple distillation column used in this experiment? Why can ethanol only be purified to 95%?
What could the remaining distillation broth be used for? Hint: was there remaining glucose in the broth?
Is the distillation process a sustainable way to manufacture biofuels? How could it be improved? Hint: consider the energy of the heat source.
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DISTILLATION APPARATUS INFORMATION
Distillation Flask (pot) The distillation flask, or pot, is a round bottom flask containing the liquid to be distilled . Size of the distillation flask should be carefully considered when assembling the distillation apparatus. If the flask is too small, the liquid is likely to bump or foam over into the receiving flask without vaporization occurring. If the flask is too large, a substantial amount of the liquid may be lost as vapor filling the flask. The liquid should ideally fill the distillation flask from 1/2 to 2/3 of its capacity. To promote boiling of a liquid, pieces of porous clay plate or silicon carbide chips may be added to the liquid before it is heated. The regular chips provide additional surface for bubbles of vapor to form. An alternative method for promoting even boiling is to agitate the liquid with a magnetic stirrer as it is being heated. CAUTION: Never add boiling chips or a stir bar to a hot liquid. This may cause a seemingly calm liquid to boil suddenly and violently. Distilling Adapter (3-way adapter, distillation head) The distilling adapter, or 3-way adapter, connects the distillation flask, condenser, and the thermometer. This type of adapter is also often referred to as the distillation head. Ground glass joints used in the apparatus must be lined up and mated tightly to avoid leakage of the vaporization liquid. Leakage will result in loss of liquid and pollution of the laboratory environment. In order for reliable temperature readings to be taken in the distilling adapter, vapors of the heated liquid must totally surround the thermometer bulb. To ensure proper contact, position the thermometer so that the bulb is below the adapter sidearm connected to the condenser. DISTILLATION OF ETHANOL
Condenser (water-jacketed condenser) The condenser cools the vapor, causing it to reliquify. The condenser then directs the cool, condensed liquid, the condensate, to the receiving flask. The most common type of condenser is the water-jacketed condenser. With a water-jacketed condenser, a water supply is connected to the condenser with rubber hoses. The water flows into the condenser through the lower hose connection (the connection farthest from the distillation flask) and out of the condenser through the upper hose connection. Water flow should be adjusted so there is a slow, constant flow through the condenser. When using a water-jacketed condenser, extra precaution is needed to ensure the security of hose connections so they do not dislocate when the water is turned on. Wire may be twisted around the hoses at the site of connection for additional security. For the distillation of liquids with very high boiling points, it is common practice to cool the condenser with air instead of water. Thermal shock to glassware from a large temperature difference between cold water and hot vapor can crack the glassware, making air a more suitable cooling substance. Receiving Flask (receiver) The container to collect the liquefied vapor is called the receiver. A round-bottomed flask, Erlenmeyer flask, bottle, or graduated cylinder are appropriate for use as the receiver. Connected to the receiver with an adapter is the condenser. The adapter servers to direct the condensate form the condenser to the receiver. Use of an adapter exposes the distillation apparatus to the atmosphere. If the liquid being collected has a low boiling point, it is good practice to cool the receiving flask with a cold water bath so that the condensed liquid will not evaporate and be lost to the atmosphere. Heat Source (heating mantle, hot plate, or bunsen burner) If available a heating mantle may be used to provide the heat energy to drive the distillation. A lower cost alternative that may be considered is to use a hot plate. If a hot plate is used, position the hot plate under the flask leaving a small space (~1/4 inch) between the glass bottom and the plate. The gap prevents too much direct heat going into the flask. Shroud the bottom half of the bottle to the mantle surface with aluminum foil to direct the heat more effectively into the bottle. It is recommended that the heat settings on the plate are gradually increased over 15 minutes into ethanol distillation begins. An alternative arrangement is to place an oil bath on top of the hot plate and place the round bottom flask into the oil bath to transfer heat into the round bottom. An even lower cost alternative method to a hot plate as the heat source is a bunsen burner. A burner may be placed under the round bottom flask, with the temperature is controlled to the distillation flask through use of spacing between the flame tip and glass bottom. Due to the open flame hazards of the bunsen burner and danger of fire, it is recommended to use a heating mantle or hot plate when possible.
DISTILLATION OF ETHANOL