July 14, 2009
Experiment 6: Fractional Distillation.
Separation of Petroleum Hydrocarbons
A mixture of cyclohexane and toluene were separate from one another by fractional distillation in order to measure the composition of each fraction and prepare the sample for gas chromatography.
Distillation has been used since antiquity to separate the components of mixtures. This method has been used to manufacture different organic chemicals but the most recent application is the refining of petroleum in order to produce fuels, lubricants and petrochemicals. In this experiments, the separation of 2 distilates were carried out using the differences in boiling points and the data was used for a gas chromatography test.
Separation of an equimolar mixture of cyclohexane and toluene by fractional distillation
Equimolar mixture: cyclohexane/toluene
40ml of an equimolar mixture of cyclohexane and toluene was obtained and transferred into a 100ml round bottomed boiling flask which contained boiling chips. The distilling column was packed with metal sponge and the height was measured in centimeters and recorded. The distillation apparatus was assembled and a heat mantle was set. Several vials were used as fraction receivers. The vials were labeled, 1-4, and weighed prior to adding the liquid mixture and after the liquid was added. The temperature was recorded before adding heat in order to begin distillation (20º C). The temperature was recorded prior to each distillation fraction and after each 2ml of distillate which was recorded in table 1. The system was turned off and cooled down letting the remaining condensed vapor drain into the round bottomed flask. This sample was transferred by pipette into a vial labeled HETP (height equivalent to a theoretical plate). Using a micro syringe, 2 to 3 micro milliliters was obtained from each vial. Each sample was placed into a gas chromatography machine which recorded areas of peaks and the data was printed out for later use. The triangulation method (height x width @ half the height), was used to measure the area of each peak. (The results for each vial was printed out and attached to the end of the report.)
|Vial |Temp. Range (C ) |Without cap |With cap |With liquid | | | | | | | |Total mass | |1 |79-85 |7.125 |8.034 |9.896 |1.862 | |2 |85-97 |7.104 |8.005 |13.935 |5.93 | |3 |97-107 |7.767 |8.69 |12.685 |3.995 | |4 |107-111 |7.124 |8.038 |13.669 |5.586 | |HEPT |111-20 |7.108 |8.016 |8.697 |.681 |
Height: 18 cm of metal sponge in distillation column.
Flow Chart of Process
Discussion and Conclusion:
The percent composition of the sample was obtained by the gas chromatography analysis and the “triangulation method”. The area under a component’s peak is used to determine its mass percentage in the sample by multiplying its height (mm), measured from the baseline, by its width at a point halfway between the top of the peak and the baseline (width at half-height, w1/2). This data was attached to the end of the report in order to compare the electronic method to the manual method. The electronic GC method was better because of the number of measuring errors made with a ruler. In the percent composition graph, we can see that...
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