The Distillation Method utilizes an ITT estimator to examine the treatment effect on utilization- and cost-based measures for a subset of the randomized sample that is predicted to be more responsive.
This allows us to turn one of the most common threats to validity in observational evaluation analysis into a power advantage.
Temperature
Once a distillation vessel has been heated, it is important to ensure that the temperature remains constant throughout the distillation process. This will help to ensure that all of the compounds are separated at their proper boiling points and that the vapors do not condense before reaching their corresponding liquid phases. For this reason, it is best to use a thermometer that can be attached to the top of the distillation flask and kept directly above it (Figure 5.26).
A round-bottomed flask with a three-way adapter is usually used for this purpose. A plastic clip should never be used to connect the adapter to the flask, as this is one of the hottest parts of the apparatus and could melt the plastic (especially if a compound with a high boiling point is being distilled). It is also preferable to use a metal extension clamp instead of a ring clamp in order to ensure that the extension can be easily removed when it is necessary to lower the heat source.
As a compound is distilled, it should be collected into a separate receiver as soon as condensation can be seen in the three-way adapter (Figure 5.26). It is important to collect distillate at a rate of no more than one drop per second in order to allow adequate time for equilibration between the liquid and gas phases. Distillation at too fast a rate prevents the vapors from being sufficiently heated, and therefore the separation is incomplete.
It is often necessary to distill a mixture of compounds that boil at different temperatures in order to obtain pure samples of each substance. This is particularly true for organic compounds such as plant oils and petrochemicals such as crude oil and gasoline. These compounds are often distilled using steam distillation. This is because the higher pressure of the steam allows the boiling point of a mixture to be reduced without requiring it to be heated at a very high temperature.
Heating an ideal mixture of two volatile substances, A and B with the ratio of A to B being equal in both the liquid and vapor phase will result in a vapor above the liquid that is enriched in A. The vapor will then rise through the condenser and be removed from the system. Repeating this process will result in successively more pure fractions of the original mixture. It is worth pointing out that this scenario described above does not happen in practice, even in an idealized system such as the one depicted above. Even the most carefully formulated chemical mixtures do not attain perfect vapor-liquid equilibria, and as a result, distillation cannot be relied upon to produce completely pure samples of any compound.
Pressure
The pressure of the vapor and liquid mixture can vary significantly depending on the temperature. As the liquid is heated, it can vaporize into a gas and the pressure drops. A lower pressure means that the molecules have a shorter distance to travel and therefore evaporate faster. The lower the pressure, the higher the rate of evaporation and the closer to a pure state it will be.
This is a generalization that holds true for most distillation processes, but there are exceptions. The azeotrope that forms between water and alcohol (ethyl alcohol with 4 percent water) is an example. At a low enough temperature, the azeotrope breaks and distillation can occur all the way to 100 percent alcohol. But the energy required to heat the system and achieve this temperature is high, so this process is not economical for most purposes.
In addition to temperature, the other major variable in the distillation process is the ratio of liquid to vapor. A liquid that is richer in one component than the other will have a lower boiling point, which will result in a slower vaporization rate. The vapor will then have less energy per molecular weight and be more likely to condense back into the liquid. This is what we call a countercurrent flow, which is the opposite of a continuous column process, as shown in Figure 2.
It is a common misconception that once a given pressure is applied to a liquid mixture all the components will boil at their corresponding boiling points and thereby separate into vapor and liquid. This is not the case, even in an idealized system.
There are many different ways to distill a substance, and they can be used on a laboratory scale or on a large industrial level to produce various chemicals, such as petroleum products. They are also used in the production of food, pharmaceuticals, perfumery, and herbal preparations.
A simple experimental apparatus can be constructed by filling a glass or plastic jug with a mixture of two substances that have different boiling points. The container is then sealed except for a small hole to which a balloon or rubber band is attached to keep air out. This vessel can then be connected to a vacuum pump and the vapor from the upper portion of the container can be collected under a reduced pressure while the liquid remains at atmospheric pressure. A more sophisticated apparatus, called a Perkin triangle, has means via a series of taps to allow fractions to be isolated, the vapor redirected and the liquid connected back to the distillation system for another round of collection at a different pressure.