Separation of Liquids by Fractional Distillation and Analysis by Gas Chromatography PDF

Preview

Summary

Lecture reading from TA...

Description

Separation of Liquids by Fractional Distillation and Analysis by Gas Chromatography O H3 C

O O

CH3

Ethyl acetate (C4H8O2) B.P. 77°C, 88.11 g/mol

H3C

O

CH3

Butyl acetate (C6H12O2) B.P. 126.3°C, 116.16 g/mol

Methods and Background The objective of this lab is to set up a fractional distillation apparatus in order to separate a 1:1 mixture of ethyl acetate and butyl acetate. The goal was to analyze the fractions collected in fractional distillation through gas chromatography to determine the % composition of ethyl acetate and butyl acetate in each fraction. Finally the last one was to create a graph of volume vs. temperature from the fraction distillation and compare it to the graph of simple distillation to determine which technique is effective in separating the components of a mixture. The boiling point of a substance is the temperature at which equilibrium vapor pressure of a liquid equals to the atmospheric pressure. Boiling point is an important physical property of each compound which determines the purity and identity of each compound. It is process when liquid bubbles and there is a spontaneous vaporization Compounds with higher equilibrium vapor pressure have lower boiling points whereas compounds with lower equilibrium vapor pressure have higher boiling points. The importance of boiling points is crucial in fractional distillation to separating the components of a mixture. The liquid boiling in a closed system increases the number of gas molecules until the rate of the molecules entering the gas phase and liquid phase equal out. And reach the dynamic equilibrium. This process of molecules in gas phase in a rapid motion collides against the walls of the vessel exerting pressure. The pressure is called equilibrium vapor pressure which is dependent on the temperature. As the temperature increases, the vapor pressure above the boiling liquid increases. Whereas when a liquid is boiled in an open system, the vapor above the liquid is mixed with air and the total pressure. This process is denoted by Dalton’s Law of partial pressures, Ptotal = Psample + Pair. So, the total pressure above the liquid is calculated by adding the partial pressure of each component The partial pressure of the sample is equal to its equilibrium vapor pressure at a given temperature. At a higher temperature, the rate of evaporation increases and the equilibrium vapor pressure equals the total pressure which is referred to as the boiling point of the liquid. More volatile liquids can readily vaporize at very low temperatures and they have low boiling points. Relatively, volatile liquids have higher equilibrium vapor pressure at low temperatures. But the impurities in a nonvolatile liquid decrease the vapor pressure at a given temperature. This relationship od vapor pressure and volatile components in a liquid is denoted by Raoult’s law, PX=PºXNX where PX is the partial pressure of component X, PºX is the vapor pressure of pure X at a given temperature, and NX is the mole fraction of each component. The mole fraction of each component X and Y in mixture is calculated from equation, NX= PX/ (PX+PY+…). Thus, this relationship between temperature, composition of the liquid and the vapor phase is important in determining which component will be distilled first.

In this lab, two compounds will be examined. Their structures contain an ester group but due to more number of carbon chains in butyl acetate than ethyl acetate, they differ in their boiling points and equilibrium vapor pressures. The boiling point of ethyl acetate and butyl acetate is 77ºC and 126ºC respectively. Ethyl acetate is more volatile than butyl acetate. Fractional distillation separates two or more volatile components present in the mixture. As shown in Figure 1, the liquid mixture is placed in a still pot along with a stir bar and the distillation flask is clamped. The Hempel column or distillation column with raschig rings is then attached. Then the west condenser with two water tubes is attached to the column. Sometimes the vapor from the still pot rises up the column and some of it condenses in the column to the still pot. If the temperature of the lower part of the column is higher than the temperature of the upper part of the column, the condensate will be partially revaporized as it flows down the column. The uncondensed vapor along with that produced by revaporization of the condensate in the column rises higher and higher in the column and undergoes a repeated series of condensation and revaporization. This cycle of condensation and vaporization is equivalent to performing simple distillation and is termed as theoretical plates in the column. The number of theoretical plates determines the efficiency and separating power in fractional distillation. The height equivalent to a theoretical plate (HETP) is the vertical length of the column which increase the efficiency of theoretical plate. The number of theoretical plates increases as the surface area and the length of the vertical column increases. During this process, three fractions will be collected depending upon the temperatures. The first fraction remains close to the boiling point of ethyl acetate. The second fraction temperature begins to increase and the third fraction stabilizes at or near the boiling point of butyl acetate. These fractions are then used for gas chromatography. In Part I of fractional distillation, three fractions were collected: 13mL of fraction one was collected between the temperature range of 75-76.5° C, 3mL of second fraction was collected between 93-122° C and 6mL of third fraction was collected between 122-122.1°C. The table below illustrates the data observed for the three fractions. In the second part of gas chromatography analysis, the first fraction produced 100 % of ethyl acetate and the third fraction produced 100% of butyl acetate. Whereas in second fraction there were both components of mixture found.

Figure 1. Simple Distillation and Fractional Distillation Apparatus Gas chromatography is used to separate a mixture of two volatile liquids. Gas chromatography separates the components of a mixture between two immiscible phases: mobile phase and stationary phase. The mobile phase in gas chromatography is a gas which is called a “carrier gas”; gases that include nitrogen and helium. A stationary phase is a very high boiling, carbowax. In gas chromatography, the components that get adsorbed to the mobile phase move through the column more quickly, whereas those that show high affinity to the stationary phase migrate more slowly. More volatile substances such as ethyl acetate in this lab would elute out from the column faster and have lower retention time than less volatile substances such as butyl acetate which have higher retention time. The time required for the compound to pass from the point of injection to the detector is called as the retention time of the component. The retention time of a component is not affected by the presence or absence of other mixture components. However, experimental factors affect the retention time of a compound are nature of the stationary phase, length of the column, temperature of the column, and flow rate of the carrier gas. Thus, for a particular column, the retention time will be the same for a specific compound. In gas chromatography, the column’s efficiency increases with increasing length and decreasing diameter. Increasing the length of the column increases the difference in retention time between bands, whereas decreasing the diameter results in narrower bands. Fractions inserted into the gas chromatography instrument are analyzed by observing the peaks on the graph. The first peak observed in the graph is of a less polar compound, which has a short retention time. The second peak observed in the graph is of a more polar compound, which has a larger retention time. Area under peak is also calculated which is proportional to the moles of the compound eluted in the column. For this lab, the ideal % composition is calculated to figure out the % composition of each compound in each fraction. However, the thermal conductivity of substances is slightly different.

So, a correction factor must be used in order to figure out the corrected % composition of substances in a given fraction. Experimental Procedures For the fractional distillation, the apparatus was set up according to Figure 1. Attach a 100 mL still pot equipped with a Hempel column with raschig rings, still head, west condenser (thick column), bend vacuum adapter, thermometer, and thermometer adapter in a vertical position. Then, to ensure the stability, the apparatus was clipped with Keck clips to prevent any leakage of vapor. A thermometer was placed into the still head with the support of thermometer adapter. The thermometer was kept below the entrance of the condenser to ensure the correct temperature of the vapors. Also, to keep the whole apparatus from bending another stand was used. Along with that to avoid the air flow in the Hempel column two balloons were used to secure the holes in the column. Then, two water rubber tubes were connected with west condenser, one allowing the water to enter in the condenser and other one to drain the water out of the condenser. Graduated cylinder was by the open end of bent vacuum adapter to collect three fractions. After everything was set up, the hot plate was placed underneath the still pot, while 30 mL of 1:1 mixture of ethyl acetate and butyl acetate was added to the still pot. The temperature was set to 45ºC. The vapors were collecting less than 1 drop/ sec so the temperature was increased to 55ºC. Three fractions were transferred in a beaker with a watch glass containing 10 mL, 10 mL, and 8 mL for fractions 1, 2 and 3 respectively. The temperature was recorded for every 1 ml of liquid distilled. For the gas chromatography instrument, 5 micro liters of the liquid samples were taken into syringes and were inserted into GC instrument individually in port A of carbowax. All of the fractions were taken to run and analyzed by the gas chromatography instrument. The graph indicating peaks and area of each fraction was printed out to further analyze. Data Acquisition/Calculations: I. Equations  Boiling Point (mixture) B.P= Ptot= Patm o Ptot- Total pressure above the liquid o Patm- Atmospheric pressure  Dalton's Law: Ptot = Px + Py + Pz..., where: o Ptot = sum of all the partial pressure of the components in a mixture 

Mole Fraction: Nx=nx/(nx+ny) o Nx – Mole fraction o nx – Moles of one compound in a compound o nx+ny – Total moles of compound



Ideal % Composition Mol % (of compound A) =



Area of compound A x 100 [(Area of compound A) + (Area of compound B)]

Correction % Composition Mol % (of compound A) = Area of compound A x Mf (A) x 100 [ [(Area of compound A x Mf) +(Area of compound B x Mf)]

Part I: Fractional Distillation In Part I, three fractions were collected: 13mL of fraction one was collected between the temperature range of 75-76.5° C, 3mL of second fraction was collected between 93-122° C and 6mL of third fraction was collected between 122-122.1°C. The table below illustrates the data observed for the three fractions. 1. Ethyl Acetate: Mole fraction = 0.89 Molecular Weight = 88 g/mol Boiling Point = 77° C 2. Butyl Acetate: Mole Fraction = 0.74 Molecular Weight = 116 g/mol Boiling Point = 126° C Fraction One: Volume (mL) Temperature ºC 1 mL 75 2 mL 75 3 mL 75 4 mL 75.1 5 mL 75.5 6 mL 75.6 7 mL 75.8 8 mL 75.8 9 mL 75.8 10 mL 75.8 11 mL 75.8 12 mL 76.5 13 mL 93 Fraction Two: Volume (mL) Temperature ºC 14 mL 119 15 mL 122 16 mL 122 Fraction Three: Volume (mL) Temperature ºC 17 mL 122.1 18 mL 122.1 19 mL 122.1 20 mL 122.1 21 mL 120

22 mL

122

Part II: Analysis of Distillation Fractions by GC The peaks from the GC in the graph were analyzed and Mole percentages of the compound were calculated. See attached copies of the three fractions from gas chromatography. Ideal % Composition Mol % (of compound A) =

Correction % Composition Mol % (of compound A) =

Area of compound A [(Area of compound A) + (Area of compound B)]

x 100

Area of compound A x Mf (A) x 100 [(Area of compound A x Mf) + (Area of compound B x Mf)]

Table 1: Analyzing Fractions from Fractional Distillation and Gas Chromatography Fractions Fraction 1 Fraction 2 Fraction 3 Peaks Peak 1 Peak 2 Peak 1 Peak 2 Peak 1 Peak 2 Area (s*mV) 5766 1371 4254 5844 Retention Time (s) 25 15 24.5 39 Ethyl Butyl Ethyl Butyl Ethyl Butyl Identity Acetate Acetate Acetate Acetate Acetate Acetate Ideal % Composition 100 0 24.4 75.6 0 100 Corrected % Composition 100 0 27.9 72.1 0 100 Table 2: Analyzing Fractions from Simple Distillation and Gas Chromatography Fractions Fraction 1 Fraction 2 Fraction 3 Peaks Peak 1 Peak 2 Peak 1 Peak 2 Peak 1 Peak 2 Area (s*mV) 3484 329.2 2337 1154 240.7 3227 Retention Time (s) 14 8 14 11 9 13 Ethyl Butyl Ethyl Butyl Ethyl Butyl Identity Acetate Acetate Acetate Acetate Acetate Acetate Ideal % Composition 91.3 8.63 66.9 32.9 6.9 93 Corrected % Composition 92.7 7.28 70.9 29.1 8.23 91.8

Table 3: Comparison of Simple Distillation and Fractional Distillation

Simple Distillation

Fractional Distillation

Volume (mL) of Fraction 1

10

13

Volume (mL) of Fraction 2

10

3

Volume (mL) of Fraction 3

10

6

Mol % EtOAc in Fraction 1

92.7

100

Mol % BuOAc in Fraction 1

7.28

0

Mol % EtOAc in Fraction 2

70.9

27.9

Mol % BuOAc in Fraction 2

29.1

72.1

Mol % EtOAc in Fraction 3

8.23

0

Mol % BuOAc in Fraction 3

91.8

100

Graph 1. Temperature (°C) at Each Volume in Fractional Distillation of 1:1 mixture of EtOAc/BuOAc (1 drop /sec) 130 120 110

V o lu m e(m l)

100 90 80 70 60 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Temperature (°C)

The Volume vs. Temperature graph shows that in fraction one, the temperature is relatively constant between 75-76.5° C. In fraction two, between 13-15mL, there is a sudden rise in temperature. And in Fraction three between 17-22mL, the temperature again remained constant around 122.1ºC.

V o lu m e(m l)

Graph 2. Temperature (°C) at Each Volume in Simple Distillation of 1:1 mixture of EtOAc/BuOAc (1 drop /sec) 130 125 120 115 110 105 100 95 90 85 80 0

5

10

15

20

25

30

35

Temperature (°C)

The Volume vs. Temperature graph shows that in fraction one, the temperature is relatively constant between 86.8-99° C. In fraction two, between 11-20mL, there is a sudden rise in temperature. And in Fraction three between 21-28mL, the temperature again remained constant around 120ºC. Part II: Analysis of Distillation Fractions by GC The peaks from the GC in the graph were analyzed and Mole percentages of the compound were calculated. See attached copies of the three fractions from gas chromatography. Ideal % Composition Mol % (of compound A) = Area of compound A x 100 [(Area of compound A) + (Area of compound B)] Correction % Composition Mol % (of compound A) =

I.

Area of compound A x Mf (A) x 100 [(Area of compound A x Mf) + (Area of compound B x Mf)]

Calculation for Mole percent ideal:

Fraction one: Peak one: Mol % = {(5766 s*mV)/ [(5766 s*mV) + (0 s*mV)]} x 100 = 100% Peak two: Mol % = {(0 s*mV)/ [(5766 s*mV) + (0 s*mV)]} x 100 = 0 % Fraction two: Peak one: Mol % = {(1371 s*mV)/ [(1371 s*mV) + (4254 s*mV)]} x 100= 24.4% Peak two: Mol % = {(4254 s*mV)/ [(1371 s*mV) + (4254 s*mV)]} x 100 = 75.6 % Fraction three: Peak one: Mol % = {(0 s*mV)/ [(0 s*mV) + (5844 s*mV)]} x 100 = 0% Peak two: Mol % = {(5844 s*mV)/ [(0 s*mV) + (5844 s*mV)]} x 100 = 100% Calculation for Mole percent corrected: Fraction one: Peak one: Mol % = {(5766 s*mV x 0.89)/ [(5766 s*mV x 0.89) + (0 s*mV x 0.74)]} x 100

= 100 % Peak two: Mol % = {(0 s*mV x 0.74)/ [(5766 s*mV x 0.89) + (0 s*mV x 0.74)]} x 100 = 0% Fraction two: Peak one: Mol % = {(1371 s*mV x 0.89)/ [(1371 s*mV x 0.89) + (4254 s*mV x 0.74)]} x 100 = 27.9% Peak two: Mol % = {(4254 s*mV x 0.74)/ [(1371 s*mV x 0.89) + (4254 s*mV x 0.74)]} x 100 = 72.1 % Fraction three: Peak one: Mol % = {(0 s*mV x 0.89)/ [(0 s*mV x 0.89) + (5844 s*mV x 0.74)]} x 100 = 0% Peak two: Mol % = {(5844 s*mV x 0.74)/ [(0 s*mV x 0.89) + (5844 s*mV x 0.74)]} x 100 = 100% Conclusion The objective of this lab was to separate a 1:1 mixture of ethyl acetate and butyl acetate using fractional distillation and then analyzing it by gas chromatography. For the first fraction, the temperature it was collected was 75-75.1°C which is close to the boiling point of ethyl acetate (77°C). This means the first fraction had higher moles of ethyl acetate than butyl acetate. This is true because the results from gas chromatography graph of fraction on showed that there was only one of ethyl acetate with shorter retention time and no second peak of butyl acetate. In the first fraction, the mole percent of ethyl acetate was 100% whereas there was not any amount of butyl acetate being at 0%. The second fraction was collected at rapid increasing temperature of 93-122° C which indicates it contained about equal proportions of both ethyl acetate and butyl acetate. From the peaks by GC, for fraction two, the mole percent of ethyl acetate and butyl acetate were 27.9% and 72.1% respectively. The results from gas chromatography graph of fraction two showed that the first peak is ethyl acetate with shorter retention time and the second peak is butyl acetate with larger retention time. For the third fraction, the temperature it was collected was 122-122.1°C which is close to the boiling point of butyl acetate (126.3°C). This means the third fraction had larger portion of butyl acetate than ethyl acetate. This is true because the results from gas chromatography graph of fraction on didn’t show the first peak is ethyl acetate with shorter retention time rather the second peak of butyl acetate with larger retention time and larger area was seen. In the third fraction, the mole percent of ethyl acetate was 0% which was much smaller than the mole percent of butyl acetate which was 100%. Comparing the corrected data with the ideal data for all the fraction, the fractional distillation is efficient because there was no significant difference between the ideal composition and the corrected composition. Since butyl acetate has higher boiling point and is less volatile, it eluted out from the column last and had a larger retention time. After analyzing the volume vs. temperature graphs of simple distillation and fractional distillation, it would conclude that the fractional distillation separates the liquids more efficiently than simple distillation. From the observations and graphs, the first fraction collected in simple distillation didn’t have constant temperature if compared to fractional distillation. Similarly there was sharp rise in temperature of second fraction for fractional distillation. At the same time, in fraction second the graph of simple distillation the temperature increased gradually over a larger

volume. Similar pattern was seen in third fraction where the temperature remained constant for a long period of time in fractional distillation indicating the boiling point of Butyl acetate. Whereas in simple distillation the temperature took significant amount of time to make the temperature constant. Overall, as the comparison the fractional distillation yielded 100 % of ethyl acetate in first fraction in compare to simple distillation having both mixtures yielding in the first fraction. For the third fraction, the simple distillation again yielded both components of mix...