Distillation

Distillation

The process of distillation is one of the most important separation techniques practiced worldwide and is used in petroleum refining, liquid gas production, desalinization of sea water for drinking, and in other industrial and research processes. Distillation is a practical application of a rather simple principle of physical chemistry. When a liquid is heated, its molecules begin to escape from its surface. If enough heat is applied, the liquid boils away. If the vapor which escapes from a boiling vessel is collected and then cooled or condensed so that it becomes a liquid, the process is known as distillation.

Around 350 b.c., the Greek philosopher Aristotle described a method of evaporating sea water to collect drinking water. Both Romans and Egyptians in Alexandria distilled oil of turpentine from pine resin. These early procedures used air-cooled outlets from the boiling vessel to condense the liquid product. Most likely the Arabians were the first to use a water-cooled condenser to collect the more volatile (easily evaporated) components which would have escaped with the earlier method. With their new method, the Arabians were able to discover various essential oils from plants which were then used for flavorings and perfumes. By 800 a.d., the Islamic scholar Al-Jabiz was producing acetic acid from vinegar, the only acid known at the time. Later, medieval alchemists were able to produce other acids from minerals such as sulfuric acid from sulfur. Distillation of alcohol for medicine was established by the early twelfth century in Italy, where herbs and spices were added, and it is believed that this practice led to the development of liqueurs such as Benedictine and Chartreuse. Later, other organic mixtures were fermented and distilled to create various alcoholic beverages. For example, wine is distilled from brandy, whisky from fermented grain mash, rum from fermented sugar mash, and vodka from fermented potato mash. By the 1800s a large-scale distilled spirits industry had been established.

Several principal methods of distillation are in use today including straight distillation, the simplest method; fractional distillation, the heart of petroleum refining; steam distillation, used to distill organic compounds; reduced pressure distillation, also called vacuum distillation; and molecular distillation, which is used for materials sensitive to heat or easily attacked by oxygen. Straight distillation involves a vessel called a still, or retort, in which the substance is heated and a condenser in which the vapor is returned to a liquid state and transported to a collection vessel. This method is useful for separating liquids from solids or separating liquids which have different boiling points such as alcohol and water. In Kuwait, where there is abundant gas for fuel, sea water is distilled for drinking. In laboratories, a common apparatus is the Liebig condenser, in which the condensing tube is surrounded by another tube called a water jacket through which cold water is circulated to produce faster condensation. Fractional distillation is used where boiling points of liquids are close. When petroleum or crude oil is refined into its components (propane and butane, gasoline, kerosene and jet fuel, diesel, fuel oils, and heavier products like asphalt), each component boils and condenses at a different temperature, the lightest vaporizing first. If the still is in the shape of a column, the vapor cools and condenses as it rises and the less volatile elements fall back into the boiling vessel or pot, a process called refluxing. Gradually the remaining vapor becomes more concentrated in the remaining volatile components. In one design of a fractional still, horizontal plates are spaced like shelves up the column and holes allow the rising vapor to pass through until it condenses and is drawn off at various levels. Fractional distillation is also used to separate liquefied air into oxygen, nitrogen, and even the inert gases such as neon and argon. Steam distillation is most useful to obtain organic compounds, such as aniline, which are immiscible (do not mix) in water, and which might decompose or oxidize at its own boiling point which is higher than water. Steam is passed through the water and organic mixture and, as they boil, the components are distilled out.

Compounds with large molecules generally boil at higher temperatures but as the temperature goes above 662° F (350° C), the compound is likely to decompose. If the pressure in the vessel is reduced, the boiling point can be lowered to where the molecules do not break down. This method is known as vacuum distillation.

Complex organic compounds of high molecular weight (500-1000) which are very heat sensitive or readily oxidized can be collected by molecular distillation. At very low pressures and with low temperature, molecules can get enough energy to leave the surface of the liquid and move to a condensing surface. This is actually evaporation at very low pressure.