Fractional and Simple Distillation

1. EXPERIMENT # 2 FRACTIONAL AND SIMPLE DISTILLATION 2. PURPOSE AND INTRODUCTION A mixture of cyclohexane and toluene (mixture X) was separated via fractional and simple distillation. The effect of fractionation on increasing the efficiency of separation was assessed by comparing the stillhead temperature in fractional and simple distillation as a function of distillate produced. The three fractions collected during fractional distillation were also analyzed using gas liquid chromatography (GC) to determine the extent of fractionation. The peak areas in the GC printout were also calculated using manual methods (i.e. triangulation and cut and weigh) to compare their accuracy against the computer’s integration of the peak area. 3. EXPERIMENTAL PROCEDURES: Table of Reagents Compound Cyclohexane, C6H12 Molecular Weight 84.1608 g/ mol Physical Properties flammable b.p. = 80.70C Toluene, C6H5CH3 92.1402 g/ mol flammable b.p. = 110.60C flash pt. = -180C flash pt. = 40C Fractional Distillation: 0.5-mL of the mixture X was reserved for GC analysis and 26.0-mL of the mixture was placed in the florence flask of the fractional distillation setup shown in Figure 1. A B C D A: Distilling adapter (stillhead) B: Fractionating distillation column C: Florence flask (stillpot) D: Condenser The fractionating distillation column was packed loosely with steel sponge, and the distillation column and the florence flask neck were insulated with aluminum foil to maintain the proper temperature gradient. The heat source, a Thermowell heater, was adjusted to obtain a distillation rate of about 1 drop/ second. Figure 1. Fractional distillation setup. The stillhead temperature and volume were recorded for every 2-mL of distillate produced. Three fractions were collected. The first fraction, containing mostly cyclohexane, was collected at 79830C. The second fraction, containing a mixture of cyclohexane and toluene, was collected as the 1 temperature rose sharply to ~1040C. The last fraction, containing mostly toluene, was collected at ~1040C. Samples of each fraction were analyzed by GC to determine the extent of fractionation and the relative amounts of cyclohexane and toluene present in the original mixture. Simple Distillation: The distillate from the fractional distillation and the pot residue were recombined and distilled using a simple distillation setup (i.e. the same as the fractional distillation setup, but without the fractionating distillation column). The stillhead temperature and volume were recorded for every 2-mL of distillate produced. 4. RESULTS AND OBSERVATIONS Fractional Distillation and Simple Distillation Table 1. Fractional Distillation, Head Temperature With Increasing Distillate Volume Vol Vol Temp (deg Vol Temp (deg (mL) C) (mL) Temp (deg C) (mL) C) 0.0 79.0 17.0 87.0 20.0 104.0 A B C 2.0 79.7 17.4 97.0 22.0 104.3 4.0 80.1 17.6 99.5 22.8 102.0 6.0 81.0 17.8 100.8 8.0 81.2 18.0 101.5 10.0 82.0 18.2 102.0 12.0 83.0 18.4 102.5 14.0 85.0 18.6 103.0 16.0 82.5 19.0 103.7 19.4 104.0 Graph 1. Fractional Distillation versus Simple Distillation Head Temperature With Increasing Distillate Volume Temperature (degrees Celsius) Table 2. Simple Distillation Head Temperature With Increasing Distillate Volume Vol Temp (deg (mL) C) 0.0 80.5 2.5 82.0 4.0 82.8 6.0 83.8 9.0 85.5 10.0 86.1 12.0 88.0 14.0 90.0 16.0 93.0 18.0 97.1 20.0 103.7 21.5 105.0 110 105 100 95 90 85 80 75 0 5 10 15 20 25 Volume (mL) fractional distillation siimple distillation 2 Gas Liquid Chromatography Analysis of Mixture X and Fractions Table 3. Gas liquid chromatography analysis of fractions from fractional distillation: Percentages and volumes of cyclohexane and toluene in each fraction and in the total distillate % % mL total vol cyclohexane toluene cyclohexane mL toluene (mL) Fraction A 81.582% 18.418% 13.05 2.95 16 Fraction B 26.213% 73.787% 0.89 2.51 3.4 Fraction C 1.457% 98.543% 0.05 3.35 3.4 Fraction A+B+C 61.404% 38.596% 14.0 8.8 22.8 Mixture X, expected 56.472% 43.528% 12.9 9.9 22.8 Figure 2. GC chart for fraction A, B, and C from the fractional distillation. The first peak for each fraction represents cyclohexane, the second peak represents toluene. 3 Table 4. Peak Area Determination for Mixture X Computer integration % cyclohexane 56.472% % toluene 43.528% Cut & Weigh mass (g) % % error cyclohexane 0.0695 57.5% 1.8% toluene 0.0513 42.5% 2.4% *weighed with the Mettler-Toledo balance in Rm510 Triangulation height (mm) cyclohexane toluene width at halfheight (mm) area (mm squared) % % error 131.5 3.2 420.8 54.5% 3.5% 69 5.1 351.9 45.5% 4.6% Figure 3. GC chart for Mixture X. The first peak represents cyclohexane, and the second peak represents toluene. 4 5. CONCLUSIONS A comparison of the fractional distillation and simple distillation curve showed that fractional distillation was the more efficient technique for separating cyclohexane and toluene. The fractional distillation curve showed a sharper, steeper temperature change separating the two boiling point plateaus of cyclohexane and toluene. Since the slope (corresponding to fraction B) was steeper, this indicates that there was less distillate collected which was an even mixture of both cyclohexane and toluene. Also, since the plateaus (corresponding to fraction A and C) for the boiling points of cyclohexane and toluene were flatter, the distillate collected for each component was more highly enriched (i.e. purer) than what was obtained through just simple distillation. The extent of fractionation was shown by the high percentage of cyclohexane and low percentage of toluene collected in fraction A, and the high percentage of toluene and low percentage of cyclohexane collected in fraction C. The large percentage of toluene collected in fraction B and the low volume of distillate collected in fraction C suggest that the collection of fraction C’s distillate should have occurred earlier than it did. A comparison of the manual methods for peak area determination for the GC and the computerintegration method showed that the manual methods, while prone to human error, are reliable and accurate. 6. ANSWERS TO ASSIGNED PROBLEMS #11. At 500C, nmethanol = 0.1 mol nethanol = 0.2 mol Pvap, methanol0 = 406 torr Pvap, ethanol0 = 222 torr 0.1mol   Pvap, methanol in soln =  406torr = 135torr  0.1mol + 0.2mol  0.2mol   Pvap, ethanol in soln =  222torr = 148torr  0.1mol + 0.2mol  Ptotal soln = Pvap, methanol in soln + Pvap, ethanol in soln = 135 torr + 148 torr = 283 torr 5