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Distillation is one of the important [[unit operations]] of [[chemical engineering]]. If the feed contains more than two components, it is referred to as ''multi-component distillation'' and, if it contains only two components, it is referred to as ''binary distillation''. | Distillation is one of the important [[unit operations]] of [[chemical engineering]]. If the feed contains more than two components, it is referred to as ''multi-component distillation'' and, if it contains only two components, it is referred to as ''binary distillation''. | ||
== | == The process == | ||
When a liquid mixture is heated, the evolved vapor will have a higher concentration of the more [[Relative volatility|volatile]] (i.e., lower [[boiling point]]) components than the liquid from which it evolved. Conversely, | When a liquid mixture is heated so that it boils, the evolved vapor will have a higher concentration of the more [[Relative volatility|volatile]] (i.e., lower [[boiling point]]) components than the liquid mixture from which it evolved. Conversely, when a vapor mixture is cooled, the less volatile components tend to condense in a greater proportion than the more volatile components. | ||
That is what happens in a distillation column. A liquid mixture is fed into the distillation column. On entering the column, the heated feed is partially vaporized and rises up the column. However, as it rises, it cools and partially condenses so that while part of it continues up as vapor, the condensed portion is enriched in the less volatile component(s) and descends. Thus, as the vapor continues to flow upward, it becomes ''richer'' in the more volatile component(s). | |||
The part of the feed that did not vaporize on entering the column, flows downward and is heated until it is partially vaporized. The resulting vapor flows upward and the residual liquid becomes richer in the less volatile components. | |||
Thus, the vapor that exits the top of the column is rich in the more volatile component(s) of the column feed and the liquid which exits the bottom of the column is rich in the less volatile component(s) of the column feed. | |||
Image 3 depicts a simple continuous fractional distillation tower for separating a feed stream into two fractions, an overhead distillate product and a bottoms product. The "lightest" products (those with the lowest boiling point or highest volatility) exit from the top of the columns and the "heaviest" products (the bottoms, those with the highest boiling point) exit from the bottom of the column. The overhead stream may be cooled and condensed using a water-cooled or air-cooled [[Condenser (heat transfer)|condenser]]. The [[reboiler|bottoms reboiler]] may be a steam-heated or hot oil-heated [[heat exchanger]], or even a gas or oil-fired [[furnace]]. | Image 3 depicts a simple continuous fractional distillation tower for separating a feed stream into two fractions, an overhead distillate product and a bottoms product. The "lightest" products (those with the lowest boiling point or highest volatility) exit from the top of the columns and the "heaviest" products (the bottoms, those with the highest boiling point) exit from the bottom of the column. The overhead stream may be cooled and condensed using a water-cooled or air-cooled [[Condenser (heat transfer)|condenser]]. The [[reboiler|bottoms reboiler]] may be a steam-heated or hot oil-heated [[heat exchanger]], or even a gas or oil-fired [[furnace]]. | ||
Revision as of 16:02, 19 July 2008
Continuous distillation is an ongoing process in which a liquid mixture of two or more miscible components is continuously fed into the process and continuously separated into two or more products by preferentially boiling the more volatile components out of the mixture. [1][2][3][4]
Large-scale, continuous distillation is used widely in the chemical process industries where large quantities of liquids have to be distilled, as in petroleum refining, natural gas processing, petrochemical production, coal tar processing and the liquefaction of gases such as hydrogen, oxygen, nitrogen, and helium).
Industrial distillation is typically performed in large, vertical cylindrical columns (see adjacent photograph) known as "distillation towers" or "distillation columns" with diameters ranging from about 65 centimeters to 11 meters and heights ranging from about 6 meters to 60 meters or more.
Distillation is one of the important unit operations of chemical engineering. If the feed contains more than two components, it is referred to as multi-component distillation and, if it contains only two components, it is referred to as binary distillation.
The process
When a liquid mixture is heated so that it boils, the evolved vapor will have a higher concentration of the more volatile (i.e., lower boiling point) components than the liquid mixture from which it evolved. Conversely, when a vapor mixture is cooled, the less volatile components tend to condense in a greater proportion than the more volatile components.
That is what happens in a distillation column. A liquid mixture is fed into the distillation column. On entering the column, the heated feed is partially vaporized and rises up the column. However, as it rises, it cools and partially condenses so that while part of it continues up as vapor, the condensed portion is enriched in the less volatile component(s) and descends. Thus, as the vapor continues to flow upward, it becomes richer in the more volatile component(s).
The part of the feed that did not vaporize on entering the column, flows downward and is heated until it is partially vaporized. The resulting vapor flows upward and the residual liquid becomes richer in the less volatile components.
Thus, the vapor that exits the top of the column is rich in the more volatile component(s) of the column feed and the liquid which exits the bottom of the column is rich in the less volatile component(s) of the column feed.
Image 3 depicts a simple continuous fractional distillation tower for separating a feed stream into two fractions, an overhead distillate product and a bottoms product. The "lightest" products (those with the lowest boiling point or highest volatility) exit from the top of the columns and the "heaviest" products (the bottoms, those with the highest boiling point) exit from the bottom of the column. The overhead stream may be cooled and condensed using a water-cooled or air-cooled condenser. The bottoms reboiler may be a steam-heated or hot oil-heated heat exchanger, or even a gas or oil-fired furnace.
In a continuous distillation, the system is kept in a steady state or approximate steady state. Steady state means that quantities related to the process do not change as time passes during operation. Such constant quantities include feed input rate, output stream rates, heating and cooling rates, reflux ratio, and temperatures, pressures, and compositions at every point (location). Unless the process is disturbed due to changes in feed, heating, ambient temperature, or condensing, steady state is normally maintained. This is also the main attraction of continuous distillation, apart from the minimum amount of (easily instrumentable) surveillance; if the feed rate and feed composition are kept constant, product rate and quality are also constant. Even when a variation in conditions occurs, modern process control methods are commonly able to gradually return the continuous process to another steady state again.
Since a continuous distillation unit is fed constantly with a feed mixture and not filled all at once like a batch distillation, a continuous distillation unit does not need a sizable distillation pot, vessel, or reservoir for a batch fill. Instead, the mixture can be fed directly into the column, where the actual separation occurs. The height of the feed point along the column can vary on the situation and is designed so as to provide optimal results. See McCabe-Thiele method.
- ↑ Kister, Henry Z. (1992). Distillation Design, 1st Edition. McGraw-Hill. ISBN 0-07-034909-6.
- ↑ King, C.J. (1980). Separation Processes. McGraw Hill. 0-07-034612-7.
- ↑ Perry, Robert H. and Green, Don W. (2007). Perry's Chemical Engineers' Handbook, 8th Edition. McGraw-Hill. ISBN 0-07-142294-3.
- ↑ McCabe, W., Smith, J. and Harriott, P. (2004). Unit Operations of Chemical Engineering, 7th Edition. McGraw Hill. ISBN 0-07-284823-5.