Editing Distillation (section)

==Industrial process==
[[File:Colonne distillazione.jpg|right|thumb|Typical industrial distillation towers]]
{{Main|Continuous distillation}}
Large scale industrial distillation applications include both batch and continuous fractional, vacuum, azeotropic, extractive, and steam distillation. The most widely used industrial applications of continuous, steady-state fractional distillation are in [[oil refinery|petroleum refineries]], [[petrochemical]] and [[chemical plant]]s and [[natural gas processing]] plants.

To control and optimize such industrial distillation, a standardized laboratory method, ASTM D86, is established. This test method extends to the atmospheric distillation of petroleum products using a laboratory batch distillation unit to quantitatively determine the boiling range characteristics of petroleum products.

Industrial distillation<ref name=Perry>{{cite book|author1=Perry, Robert H.  |author2=Green, Don W.|title=Perry's Chemical Engineers' Handbook|edition=6th| publisher=McGraw-Hill|year=1984|isbn=978-0-07-049479-4|title-link=Perry's Chemical Engineers' Handbook}}</ref><ref name=Kister>{{cite book|author=Kister, Henry Z.|title=Distillation Design|edition=1st |publisher=McGraw-Hill|year=1992|isbn=978-0-07-034909-4|title-link=Distillation Design}}</ref> is typically performed in large, vertical cylindrical columns known as distillation towers or distillation columns with diameters ranging from about {{convert|0.65|to|16|m}} and heights ranging from about {{convert|6|to|90|m}} or more. When the process feed has a diverse composition, as in distilling [[crude oil]], liquid outlets at intervals up the column allow for the withdrawal of different ''fractions'' or products having different [[boiling points]] or boiling ranges. The "lightest" products (those with the lowest boiling point) exit from the top of the columns and the "heaviest" products (those with the highest boiling point) exit from the bottom of the column and are often called the bottoms.

[[File:Continuous Binary Fractional Distillation.PNG|left|thumb|Diagram of a typical industrial distillation tower]]
Industrial towers use [[reflux]] to achieve a more complete separation of products. Reflux refers to the portion of the condensed overhead liquid product from a distillation or fractionation tower that is returned to the upper part of the tower as shown in the schematic diagram of a typical, large-scale industrial distillation tower. Inside the tower, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficiency of the distillation tower. The more reflux that is provided for a given number of [[theoretical plate]]s, the better the tower's separation of lower boiling materials from higher boiling materials. Alternatively, the more reflux that is provided for a given desired separation, the fewer the number of theoretical plates required. [[Chemical engineer]]s must choose what combination of reflux rate and number of plates is both economically and physically feasible for the products purified in the distillation column.

Such industrial fractionating towers are also used in [[cryogenic]] [[air separation]], producing [[liquid oxygen]], [[liquid nitrogen]], and high purity [[argon]]. Distillation of [[chlorosilane]]s also enables the production of high-purity [[silicon]] for use as a [[semiconductor]].

[[File:Bubble Cap Trays.PNG|frame|right|Section of an industrial distillation tower showing detail of trays with bubble caps]]
Design and operation of a distillation tower depends on the feed and desired products. Given a simple, binary component feed, analytical methods such as the [[McCabe–Thiele method]]<ref name=Perry/><ref name=SeaderHenley>{{cite book|author1=Seader, J. D.  |author2=Henley, Ernest J.|title=Separation Process Principles|publisher=Wiley|location=New York|year=1998|isbn=978-0-471-58626-5}}</ref> or the [[Fenske equation]]<ref name=Perry/> can be used. For a multi-component feed, [[simulation]] models are used both for design and operation. Moreover, the efficiencies of the vapor–liquid contact devices (referred to as "plates" or "trays") used in distillation towers are typically lower than that of a theoretical 100% efficient [[equilibrium stage]]. Hence, a distillation tower needs more trays than the number of theoretical vapor–liquid equilibrium stages. A variety of models have been postulated to estimate tray efficiencies.

In modern industrial uses, a packing material is used in the column instead of trays when low pressure drops across the column are required. Other factors that favor packing are: vacuum systems, smaller diameter columns, corrosive systems, systems prone to foaming, systems requiring low liquid holdup, and batch distillation. Conversely, factors that favor [[plate column]]s are: presence of solids in feed, high liquid rates, large column diameters, complex columns, columns with wide feed composition variation, columns with a chemical reaction, absorption columns, columns limited by foundation weight tolerance, low liquid rate, large turn-down ratio and those processes subject to process surges.
[[File:Vacuum Column.jpg|thumb|left|183px|Large-scale, industrial vacuum distillation column<ref>[http://resources.schoolscience.co.uk/SPE/knowl/4/2index.htm?vacuum.html Energy Institute website page] {{webarchive|url=https://web.archive.org/web/20071012072758/http://resources.schoolscience.co.uk/SPE/knowl/4/2index.htm?vacuum.html |date=12 October 2007 }}. Resources.schoolscience.co.uk. Retrieved on 2014-04-20.</ref>]]
This packing material can either be random or dumped packing ({{convert|1|-|3|in|order=flip}} wide) such as [[Raschig ring]]s or [[structured packing|structured sheet metal]]. Liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where [[mass transfer]] takes place. Unlike conventional tray distillation in which every tray represents a separate point of vapor–liquid equilibrium, the vapor–liquid equilibrium curve in a packed column is continuous. However, when modeling packed columns, it is useful to compute a number of "theoretical stages" to denote the separation efficiency of the packed column with respect to more traditional trays. Differently shaped packings have different surface areas and void space between packings. Both these factors affect packing performance.

Another factor in addition to the packing shape and surface area that affects the performance of random or structured packing is the liquid and vapor distribution entering the packed bed. The number of [[Theoretical plate|theoretical stages]] required to make a given separation is calculated using a specific vapor to liquid ratio. If the liquid and vapor are not evenly distributed across the superficial tower area as it enters the packed bed, the liquid to vapor ratio will not be correct in the packed bed and the required separation will not be achieved. The packing will appear to not be working properly. The [[Theoretical plate|height equivalent to a theoretical plate]] (HETP) will be greater than expected. The problem is not the packing itself but the mal-distribution of the fluids entering the packed bed. Liquid mal-distribution is more frequently the problem than vapor. The design of the liquid distributors used to introduce the feed and reflux to a packed bed is critical to making the packing perform to it maximum efficiency. Methods of evaluating the effectiveness of a liquid distributor to evenly distribute the liquid entering a packed bed can be found in references.<ref name=Moore>Moore, F., Rukovena, F. (August 1987) ''Random Packing, Vapor and Liquid Distribution: Liquid and gas distribution in commercial packed towers'', Chemical Plants & Processing, Edition Europe, pp. 11–15</ref><ref name=Spiegel>{{cite journal|last1=Spiegel|first1=L|title=A new method to assess liquid distributor quality|journal=Chemical Engineering and Processing|volume=45|page=1011|year=2006|doi=10.1016/j.cep.2006.05.003|issue=11|bibcode=2006CEPPI..45.1011S}}</ref> Considerable work has been done on this topic by Fractionation Research, Inc. (commonly known as FRI).<ref name=Kunesh>{{cite journal|last1=Kunesh|first1=John G.|last2=Lahm|first2=Lawrence|last3=Yanagi|first3=Takashi|title=Commercial scale experiments that provide insight on packed tower distributors|journal=Industrial & Engineering Chemistry Research|volume=26|page=1845|year=1987|doi=10.1021/ie00069a021|issue=9}}</ref>

===Multi-effect distillation===
The goal of multi-effect distillation is to increase the [[Efficient energy use|energy efficiency]] of the process, for use in desalination, or in some cases one stage in the production of [[ultrapure water]]. The number of effects is inversely proportional to the kW·h/m<sup>3</sup> of water recovered figure and refers to the volume of water recovered per unit of energy compared with single-effect distillation. One effect is roughly 636&nbsp;kW·h/m<sup>3</sup>:
* [[Multi-stage flash distillation]] can achieve more than 20 effects with thermal energy input, as mentioned in the article.
* [[Vapor compression evaporation]] – Commercial large-scale units can achieve around 72 effects with electrical energy input, according to manufacturers.

There are many other types of multi-effect distillation processes, including one referred to as simply multi-effect distillation (MED), in which multiple chambers, with intervening heat exchangers, are employed.