User:Milton Beychok/Sandbox
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.
Principle
When a liquid mixture is heated, the evolved vapor will have a higher concentration of the more volatile (i.e., lower boiling point) components than the liquid from which it evolved. Conversely, if a vapor mixture is cooled, the less volatile components will tend to condense in a higher proportion than the more volatile components.
The principle for continuous distillation is the same as for normal distillation: when a liquid mixture is heated so that it boils, the composition of the vapor above the liquid differs from the liquid composition. If this vapor is then separated and condensed into a liquid, it becomes richer in the lower boiling component(s) of the original mixture.
This is what happens in a continuous distillation column. A mixture is heated up, and routed into the distillation column. On entering the column, the feed starts flowing down but part of it, the component(s) with lower boiling point(s), vaporizes and rises. However, as it rises, it cools and while part of it continues up as vapor, some of it (enriched in the less volatile component) begins to descend again.
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.