Fractional Distillation Lab

FRACTIONAL DISTILLATION

October 29, 2012
CHEM 210 ME01

INTRODUCTION
The process of separating the components of a mixture by distillation into relatively pure fractions is referred to as fractional distillation. Simple distillation, a process with similar goals, is noted for being a satisfactory attempt at separating two components in an ideal mixture, but not as accurate as fractional distillation. To explore these statements further, there needs to be an in depth look at the theory that supports them and the differences in their construction. The distillation process is a classic technique that is commonly used in chemistry studies to identify and purify organic substances. The physical process of boiling a mixture allows vapors of individual components to be distinguished through their released vapors at their specific boiling points. In theory, when the mixture heats up, the temperature will rise until the lowest boiling point of the substance is reached and then that individual substance will become a vapor, leaving the other substance in the mixture until its boiling point is reached. The result of this process is a hot vapor that passes into a condenser where it is converted to the liquid, which is then collected in a receiver flask. While this procedure would suffice for simple distillation methods, fractional distillation includes multiple further levels that are take to ensure that the vapor re-liquefying is homozygous in its make-up. Many fractionations are made to occur within a distilling column which provides a large surface area on which many evaporation-condensation cycles can take place. The process requires that a temperature gradient be formed in the column so that as the height up the column increases, the temperature decreases to the right degree for vaporizing condensate as it becomes continually enriched in the lower boiling component. With the addition of a fractioning column full of wire mesh, the surface area inside the column is greatly increased. In order to assure the most accurate separation, there needs to be maximum possible contact between the liquid trickling down and the hot vapor rising to ensure the cyclic process. If you didn't have the packing, the liquid would all be on the sides of the condenser, while most of the vapor would be going up the middle and minimally come into contact with it, as in the case of simple distillation. (Clark 2005) The purpose of this lab was to test these theories and see if the addition of a long, wire packed fractioning column to a simple distillation set-up would experimentally provide a more accurate separation of the two components making up the mixture as the theories would claim.
SAFETY
Ethanol N-pentanol
Molecular Formula C2H6O C5H12O
Mass 46.07 g/mol 88.15 g/mol
Boiling Point 78.4oC 137.5oC
Structure  

Ethanol and n-pentanol are both chemical substances that require precautions to be taken when being used in lab. Ethanol and n-pentanol are flammable and therefore there needed to be caution taken that the mixture of these substances did not come in to contact with any open flame. This lab did not utilize a Bunsen burner therefore it was easy to ensure that there were not any complications. Ethanol can cause severe irritation if it comes into contact with the eye. For this reason, eye protection was worn at all times throughout the lab. Ethanol can also be dangerous if consumed, causing damage to the liver and kidneys with repeated exposure. There was no ingestion of any of the mixture for this reason. Lastly, ethanol may cause nerve damage with repeated inhalation so there was precaution taken to keep the solution away from breathing range. N-pentanol can cause skin dryness and scratching with repeated exposure. To avoid this from happening, long sleeves and pants were worn to minimize exposed skin area. Gloves and lab coats were worn to avoid any direct contact when dealing with the mixed solution. If ingested, it can cause irritation to the stomach and mucous membranes along with symptoms of dizziness and nausea. The solution was kept away from breathing and ingestion range.
EXPERIMENTAL OVERVIEW
To observe the boiling points of n-pentanol and ethanol in a mixed solution a factional distillation apparatus was set up. To do this, the following mechanism seen in the picture to the side was constructed.
To set this up, a thermometer was used to record the temperature and a thermometer adapter to block the distillate from leaking out of the cylindrical flask that contained 100 mL of the 50% ethanol-50% pentanol mixture and seven boiling stones, more than the three that the lab called for, to keep it from boiling over. After the system was set up, the distiller was set to a heating level six instead of five, as the instructor requested. This was changed in order to preserve time. This level was not surpassed though in fear that if the distillate was heated too quickly it would overheat and give skewed results. The distillation process began when the first drops of distillate liquefied back from being a vapor into the graduated cylinder. This process took an extended period of time due to the fractional distillation properties of a mixture where the vapor had to fill the fractioning column full of steel wool prior to re-liquefying. The fractionating column is packed with steel wool to give the maximum possible surface area for the vapor to condense on prior to reaching the top. When the volume reached 5mL, the temperature was recorded for every subsequent 5mL of distillate that accumulated in the graduated cylinder until reaching 90mL. (Lab Manual)
ANALYSIS OF RESULTS AND CALCULATIONS
Simple Distillation Data Set


Fractional Distillation Data Set

 For a steady period of time of simple distillation, the solution in the cylindrical flask came to a boil, but there was no rapid change in temperature during the period in which the first 5mL to 40mL of distillate was being collected in the graduated cylinder. During this period the temperature of the vapor slowly inclined from 84oC to 96oC. Also, from the period of 55mL to 95mL there was not a rapid increase in temperature. During this period the temperature of the vapor slowly inclined from 133oC to 142oC. However, between the collection of 40mL to 50mL, there was a rapid spike in temperature of the vapor being collected in the cylindrical flask as it spiked from 96oC to 133oC. In the case of fractional distillation, the solution in the cylindrical flask came to a boil, but there was even less of a temperature increase during the period in which the first 5mL to 45mL of distillate was being collected in the graduated cylinder. During this period the temperature of the vapor slowly inclined from 81oC to 84oC. Also, from the period of 50mL to 90mL there was not a rapid increase in temperature. During this period the temperature of the vapor slowly inclined from 136oC to 141oC. However, between the collection of 45mL to 50mL, there was a rapid spike in temperature of the vapor being collected in the cylindrical flask as it spiked from 84oC to 136oC.
DISCUSSIONS
The main difference between these two methods, simple and fractional distillation, is accuracy of the separation process of the same sample. Even though both distillation methods showed minimal temperature change between the collection of 5ml to 45ml, the temperature change during this period in fractional distillation only ranged by a few degrees Celsius, while in simple distillation it ranged approximately 18oC. This greater span in temperature during simple distillation shows that there was a higher percentage of n-pentanol, with a higher boiling point, in the vapor that was then re-condensing then compared to the vapor re-condensing during fractional distillation. The same can be said when looking at the ranges of 50mL to 90mL. Fractional distillation varied only 5oC during this range, where simple distillation ranged 26oC. This shows that there was less of a distinct separation in what was composed in the vapor being re-condensed during simple distillation, as the range of boiling points was steadily on an incline. In the case of two separate components in the mixture, ethanol and pentanol, with two different boiling points, 78.4oC and 137.5oC respectively, only fractional distillation showed consistent boiling point ranges which most reflected those of the published literature. This proves that during these two ranges during fractional distillation, the components were being re-condensed separately. This spike in temperature for both systems is explained by what we know about the properties of distillation. For simple distillation, the first slow incline in the temperature of the vapor, ranging from 84oC to 96oC, is due to the presence of mostly ethanol in the vapor. The second slow incline in the temperature of the vapor, ranging from 133oC to 142oC, is due to the presence of mostly n-pentanol in the vapor. Same can be said for fractional distillation with ranges of 81oC to 84oC and 136oC to 141oC, respectively. The spike in temperature is explained by the new presence of n-pentanol in the vapor as its total pressure then equaled the atmospheric pressure in the cylindrical flask. This spike can be visibly seen in the scatter plot shown above in the results section. For fractional distillation, the spike is much more drastic, occurring between 5mL of sample being collected. There was also a long pause in sample that was dripping into the graduated cylinder during the period that the temperature spiked. This is because the composition of the vapor in the distilling column was phasing over to n-pentanol from ethanol. There must also be a discussion as to why factional distillation separates the two components of an ideal mixture better than the process of simple distillation. The key to collecting a more purely separated component from a mixture of liquids is to do a succession of processes including boiling, condensing, and then re-boiling. This is why fractional distillation, with a greater surface area provided by the metal wires and longer distilling column provides the most accurate separation as compared to simple distillation. The increased surface area and longer distilling column required a greater level of pressure and concentration of vapor to be present in order to reach the level where it could finally re-condense and re-liquefy into the condenser. It also provides more time and attraction for the vapor to boil, condense, and then re-boil; each time yielding a more concentrated, homozygous vapor. For this reason, the process of fractional distillation takes much more time to conduct versus simple distillation. In simple distillation, there is no wire packed condenser, so there is less surface area and time for the vapor to boil, condense, and then re-boil. However, the vapor may still be more mixed then desired when it reached the condenser and re-liquefies. There is also something to be said about where the spike in temperature occurred when compared to the amount of distillate being collected. The temperature spike occurred between 45mL to 50mL. Knowing that the mixture was an equal 50% ethanol-50% pentanol mixture, it made sense that this spike in temperature would be occurring as the distillate was half way boiled. Half of the mixture in the beginning had already boiled off all of the ethanol, leaving the n-pentanol to burn for the second half.
CONCLUSIONS
Overall, the data collected from both simple and factional distillation experiments shows that fractional distillation yields a much more accurate separation of the distillate. During the collection of 5mL to 45mL, temperatures slightly fluctuated from 81oC to 84oC and during 50mL to 90 ml from 136oC to 141oC. This shows that the temperature of the vapor was not changing because there was a highly percentage of only ethanol or n-pentanol in the vapor at each given time. The drastic spike in temperature demonstrates a clear shift of concentration of vapor from ethanol to n-pentanol. The key to collecting a more purely separated component from a mixture of liquids is to do a cycle of processes including boiling, condensing, and then re-boiling. This is why fractional distillation, with a greater surface area provided by the metal wires and longer distilling column provides the most accurate separation as compared to simple distillation. These two factors provide the maximum possible contact between the liquid trickling down and the hot vapor rising. This cyclic process refines the vapor with each climb up the fractioning column. Therefore, in the future, if there is ever a need to separate components in a liquid mixture with a need for accuracy, fractional distillation would be the recommendation course of action.