Distillationof：蒸馏.doc

Whats Distilling?(Developed with the assistance of Dr. Paige Stevenson, Virginia Tech)IntroductionYou have probably heard the term distillation and distilled many times during your life. For example, you can buy distilled water at the grocery store. Crude oil is separated into its many components (methane, propane, butane, gasoline, kerosene, oil, paraffin wax, and major components of tar/asphalt) by distillation. The term distillation refers to a technique used to separate one or more volatile components in a liquid/liquid or liquid/solid mixture by taking advantage of the different vapor pressures (or boiling points) of the components in the mixture. (A substance that exhibits a significant vapor pressure at a given temperature is called volatile. To smell a substance, it must be volatile because some of it must be in the vapor phase.)In the first example above, distilled water is water that has been boiled (vaporized) and then cooled and condensed into another vessel. Because the other substances in the impure water tend to be non-volatile (e.g. salts, bacteria), they remain behind, giving pure water.Crude oil contains many volatile substances, making its distillation much more complex. Crude oil is separated by a technique called fractional distillation. When you have more than one volatile substance, both substances have a significant vapor pressure at the boiling point of the more volatile (lower boiling) substance. The gas mixture being condensed therefore contains both substances, but the lower boiling substance is in higher concentration than the higher boiling substance. To isolate a pure substance from the mixture, multiple distillations must be completed in sequence. Fractional distillation allows you to perform a bunch of “mini-distillations” in one set-up, ultimately yielding better separation. When performing a simple or fractional distillation to separate volatile components of a mixture, the distilled liquid is typically collected in small increments called fractions. Each individual fraction is then tested for purity and combined with other pure fractions or redistilled (if not pure) to eventually separate the components of a mixture. Goals:In this lab, students will:1. compare two different distillation set-ups (simple and fractional) for separating mixtures.2. use the distillation set-ups to distill a 50/50 mixture of cyclohexane and toluene.3. monitor and graph the change in temperature with amount collected for each set-up.4. collect and analyze fractions from each set-up using a GC.Prelab:1. Draw the structures of cyclohexane and toluene below.2. Look up the boiling points of toluene and cyclohexane and record them below.Boiling Point Toluene: _Boiling Point Cyclohexane: _3. Predict what a plot of volume of distilled solution vs. temperature will look like for distilling 20 mL of a 50% cyclohexane:50% toluene solution.4. Predict what a plot of % cyclohexane in the distilled fraction vs. volume solution distilled will look like for distilling 20 mL of a 50% cyclohexane:50% toluene solution. 5. When a mixture of cyclohexane and toluene is injected into a GC, which substance do you expect to come off first? Why? Safety: Wear safety goggles. Toluene and cyclohexane are flammable. Make sure there are no open flames during this procedure. Wash with soap and water if you get any of the solutions on you. The GC injection ports can get VERY HOT. DO NOT TOUCH. The syringes are sharp. Do not inject yourself.ProcedureThis experiment involves a group effort. The class should be divided into 6 teams (A through F). Three teams will focus on the simple distillation column (teams A, B, C) and three will focus on the fractional distillation column (teams D, E, F). For each column, distilled fractions will be collected in 2 mL increments and analyzed by GC. The exact fractions that each team will analyze and GC they should use are indicated below. The members of each team are responsible for filling in the portions of the data table pertaining to their work.TeamStillGC NumberFractions to GCASimple1Starting Mixture and 1st and 2nd fractionsBSimple2Starting Mixture and 3rd and 4th fractionsCSimple15th and 6th fractions and remaining distillateDFractional3Starting Mixture and 1st and 2nd fractionsEFractional4Starting Mixture and 3rd and 4th fractionsFFractional35th and 6th fractions and remaining distillateDistillation1. The stills are set-up and maintained by the MCL staff. Fractions of distilled solution will be collected in 2 mL increments. During your shift at the distillation set-up, record the volume and temperature of all fractions collected in Table 2 (simple) or 3 (fractional).2. Sketch your distillation set-up below.3. Label the following parts in your sketcha) distilling flaskb) distilling column (fractional or simple)c) thermometerd) condenser (label and put arrows describing the movement of the water through the condenser)e) receiving flaskf) side arm4. Describe what is happening to the distilling mixture.5. Describe how the simple and the fractional distillation columns differ. How do you expect the different columns to affect the distillation?6. What is the correct placement of the thermometer?7. Why is it necessary that the water run through the condenser?8. How would a longer distillation column affect the distillation?9. Why is it important that the side-arm be open to air?GC Analysis1. Situate one team member by the computer and one by the GC to inject the sample. One team member should click collect. At the same instant the word stop appears in place of collect, the other team member should use the syringe to injects 0.5 mL of the distillation fraction into injection port A. *CAUTION* THE INJECTION PORT AND THE TOP OF THE CHROMATOGRAPH WILL BE VERY HOTDO NOT TOUCH! GUIDE TO PROPER HANDLING OF THE SYRINGE Draw liquid into the syringe and discard into a waste container. Wipe the needle with a Kimwipe Again draw up the liquid into the syringe to the 0.5 mL mark. Insert the needle of the syringe completely into the injection port, guiding the needle with your fingers to prevent it from bending. *CAUTION* The injection knobs get hot, so do not touch. Depress the plunger to inject the contents and withdraw the needle. This process must be done as quickly and smoothly as possible. *CAUTION* The needle is sharp and fragile. Do not play with the syringe. 2. The software will collect data for five minutes. 3. At the completion of the five minutes, determine the area under the cyclohexane peak by first holding down the left mouse button and dragging the cursor so that the cyclohexane peak is highlighted. Then click Analyze and Integral. The window that appears tells the integral (or area) under the curve. 4. Record the sample identity and the cyclohexane peak area in your data table.5. Record the area of the toluene peak by repeating procedures 3 and 4 for the toluene peak. You can move the integral window for the toluene by clicking on the colored bar and dragging it to another part of the graph.6. Label the graph by clicking Analyze and Make Annotation. Enter important identifying information in the window and click OK. 7. Print the file by clicking in your graph window and then choosing File and Print.8. Inject your next fraction and repeat procedures 2-7. When you hit the collect button, click yes to the message ok to and inject the sample at the instant collect changes to stop.9. During slow times, fill in Table 1 (with the help of your instructor or MCL staff).10. Inject your 3rd fraction and repeat procedures 2-7.Table 1: GC SettingsGC numberColumnFlow RateColumn TempInjector TempCurrentDetector TempRet. Time, AirAttenuationRet. Time, CyclohexaneVolume InjectedRet. Time, tolueneTable 2: Simple Distillation ColumnTo be filled in during labPostlab CalculationsFractionGC Total Vol. collected (mL)Temp (C)Area (cyclohex)Area (toluene)Corrected Area(toluene x scale factor)% cyclohex% toluenestarting A mixturestarting B mixtureA1A2B3B4C5C6RemainingCDistillateTable 3: Fractional Distillation Column To be filled in during labPostlab CalculationsFractionGC Total Vol. collected (mL)Temp (C)Area (cyclohex)Area (toluene)Corrected Area(toluene x scale factor)% cyclohex% toluenestarting D mixturestarting E mixtureD1D2E3E4F5F6RemainingFDistillatePostlab:1. Get the remaining data from each group so volume, temperature, and area columns of Tables 2 and 3 are filled in.2. The peak areas can be used to calculate the percentage of the different components in a solution. However, the areas usually do not translate directly to % compositionsome compounds are more sensitive than other compounds. For example, your starting solution is 50% cyclohexane: 50% toluene, but the peak areas are not 1:1a scale factor must be used. For each instrument, calculate what value you must multiply your toluene area by to get the same area for toluene as you got for cyclohexane. This is your scale factor. (Many scale factors should really be calculated, but due to lack of time, we are only having you collect one on each instrument.)Table of Scale Factors for 50