The composition of the vapor will also change from the initial ratio we just calculated to a new ratio to reflect the new composition of the pot. The consequences of these changes are that the temperature of both the pot and the distillate will slowly increase from the initial value to a value approaching the boiling point and composition of the less volatile component. If we are interested in separating our mixture into components, we are left with the task of deciding how much material to collect in each receiver and how many receivers to use.
Obviously this will depend on the quality of separation we are interested in achieving. Generally, the more receivers we use, the less material we will have in each. It is possible to combine fractions that differ very little in composition but this requires us to analyze each mixture. While it is possible to do this, in general, we really want to end with three receivers, one each enriched in the two components of our mixture and a third that contains a composition close to the initial composition.
It is difficult to describe how much material to collect in each receiver since the volume collected will depend on the differences in the boiling points of the components. Each fraction collected can be analyzed and those with compositions similar to the initial composition can be combined. The main fractions collected can then be fractionated a second time if necessary.
The experiment we have just discussed is called a simple distillation. It is an experiment that involves a single equilibration between the liquid and vapor. This distillation is referred to as involving one theoretical plate. As you will see, it is possible to design more efficient distillation columns that provide separations on the basis of many theoretical plates.
Before discussing these columns and the advantages offered by such fractionating columns, it is important to understand the basis of the advantages offered by columns with many theoretical plates.
The following is a simplified discussion of the process just described involving a column with more than one theoretical plate. We have just seen that starting with a composition of , cyclohexane: methylcyclohexane, the composition of the vapor was enriched in the more volatile component.
Suppose we were to collect and condense the vapor and then allow the resulting liquid to reach equilibrium with its vapor. The properties of liquid 2 will differ from the original composition in two ways. First, since the composition of liquid 2 is higher in cyclohexane than the initial one; the temperature at which liquid 2 will boil will be lower than before what is the approximate boiling point of a 1.
In addition, the composition of the vapor, vapor 2, in equilibrium with liquid 2 will again be enriched in the more volatile component. This is exactly what happened in the first equilibration first theoretical plate and this process will be repeated with each new equilibration. If this process is repeated many times, the vapor will approach the composition of the most volatile component, in this case pure cyclohexane, and the liquid in the pot will begin to approach the composition of the less volatile component, methylcyclohexane.
In order for this distillation to be successful, it is important to allow the condensed liquid which is enriched in the less volatile component relative to its vapor, to return to the pot. In a fractional distillation, the best separation is achieved when the system is kept as close to equilibrium as possible. This means that the cyclohexane should be removed from the distillation apparatus very slowly.
Most fractional distillation apparati are designed in such a way as to permit control of the amount of distillate that is removed from the system.
Initially the apparatus is set up for total reflux, i. Once the distillation system reaches equilibrium, a reflux to takeoff ratio of about is often used about 1 out of every drops reaching the condenser is collected in the receiver.
A column which allows for multiple equilibrations is called a fractionating column and the process is called fractional distillation. An example of a fractionating column is shown in Figure 4. Each theoretical plate is easy to visualize in this column. The column contains a total of 4 theoretical plates and including the first equilibration between the pot and chamber 1 accounts for a total of 5 from pot to receiver. As you might expect, a problem with this column is the amount of liquid that is retained by the column.
Many other column designs have been developed that offer the advantages of multiple theoretical plates with low solvent retention. Typical spinning band columns often used in research laboratories offer fractionating capabilities in the thousand of theoretical plates with solute retention of less than one mL.
Commercial distillation columns have. Figure 4. A fractionating column which contains four chambers, each with a center opening into the chamber directly above. The vapor entering the first chamber cools slightly and condenses, filling the lower chamber with liquid. At equilibrium, all chambers are filled with distillate. A portion of the liquid condensing in the first chamber is allowed to return to the pot. The remaining liquid will volatilize and travel up the column condensing in the second chamber and so on.
As discussed in the text, the composition of the vapor at each equilibration is enriched in the more volatile component. The heat necessary to volatilize the liquid in each chamber is obtained from the heat released from the condensing vapors replacing the liquid that has been removed. The vacuum jacket that surrounds the column ensures a minimum of heat loss.
In addition to performing a fractional distillation at one atmosphere pressure, it is also possible to conduct fractional distillations at other pressures. This is often avoided when possible because of the increased difficulty and expense in maintaining the vacuum system leak free. The concentration and isolation of an essential oil from a natural product has had a dramatic impact on the development of medicine and food chemistry.
The ability to characterize the structure of the active ingredient from a natural product has permitted synthesis of this material from other chemicals, resulting in a reliable and often cheaper sources of the essential oil. The process often used in this isolation is called steam distillation. Steam distillation is an important technique that has significant commercial applications.
Many compounds, both solids and liquids, are separated from otherwise complex mixtures by taking advantage of their volatility in steam. A compound must satisfy three conditions to be successfully separated by steam distillation. It must be stable and relatively insoluble in boiling water, and it must have a vapor pressure in boiling water that is of the order of 1 kPa 0.
If two or more compounds satisfy these three conditions, they will generally not separate from each other but will be separated from everything else. The following example, expressed as a problem, illustrates the application of steam distillation:. Suppose we have 1 g of an organic compound present in g of plant material composed mainly of macromolecular material such cellulose and related substances.
Let's assume that the volatile organic material has a molecular weight of Daltons, a vapor pressure of 1 kPa and is not soluble in water to an appreciable extent. Examples of such materials characterized by these properties include eugenol from cloves, cinnamaldehyde from cinnamon bark or cuminaldehyde from cumin seeds.
How much water must we collect to be assured we have isolated all of the natural oil from the bulk of the remaining material? We can simplify this problem by pointing out that the organic material is not appreciably soluble in water. We know from previous discussions that boiling will occur when the total pressure of our system equals atmospheric pressure. We can also simplify the problem by assuming that the essential oil in not appreciably soluble in the macromolecular material.
Figure 1. Distillation apparatus. A distillation flask with a thermometer is placed in a heating mantle and is connected to a condenser. Figure 2. The tubes on the condenser are attached to a water source, with the water flowing in the low end and flowing out the high end of the condenser. The condensed vapor drips into the collection receiver. Figure 3. The thermometer is inserted in the distillation flask through a hole in the cork stopper. The arm of the flask is inserted through a hole in the stopper of the condenser.
Make sure these stoppers are airtight, or the vapor will escape. Figure 5. The collection receiver The vapors condense and drip from the condenser into the flask.
Simple distillation is effective only when separating a volatile liquid from a nonvolatile substance or when separating two liquids that differ in boiling point by 50 degrees or more. If the liquids comprising the mixture that is being distilled have boiling points that are closer than 50 degrees to one another, the distillate collected will be richer in the more volatile compound but not to the degree necessary for complete separation of the individual compounds.
The basic idea behind fractional distillation is the same as simple distillation only the process is repeated many times. Thus, if the applied pressure is reduced, the boiling point of the liquid decreases.
This behavior occurs because a lower vapor pressure is necessary for boiling, which can be achieved at a lower temperature. Experimentally the setups are arranged more or less the same, with small differences being how the steam is added to the flask: either indirectly if a steam line is available in the building, or directly by boiling water in the flask.
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