- Relative volatility
Relative volatility is a measure comparing the
vapor pressures of the components in a liquid mixture of chemicals. This quantity is widely used in designing large industrial distillationprocesses.cite book|author=Kister, Henry Z.|title= Distillation Design|edition=1st Edition|publisher=McGraw-hill|year=1992|id=ISBN 0-07-034909-6] cite book|author=Perry, R.H. and Green, D.W. (Editors)|title= Perry's Chemical Engineers' Handbook|edition=7th Edition|publisher=McGraw-hill|year=1997|id=ISBN 0-07-049841-5] cite book|author= Seader, J. D., and Henley, Ernest J.|title=Separation Process Principles|publisher=Wiley| location=New York|year=1998|id=ISBN 0-471-58626-9] In effect, it indicates the ease or difficulty of using distillation to separate the more volatile components from the less volatile components in a mixture. By convention, relative volatility is usually denoted as .
Relative volatilities are used in the design of all types of distillation processes as well as other separation or absorption processes that involve the contacting of
vaporand liquidphases in a series of equilibrium stages.
Relative volatilities are not used in separation or absorption processes that involve components reacting with each other (for example, the absorption of gaseous
carbon dioxidein aqueous solutions of sodium hydroxide).
For a liquid mixture of two components (called a "binary mixture") at a given
temperatureand pressure, the relative volatility is defined as
When their liquid concentrations are equal, more volatile components have higher vapor pressures than less volatile components. Thus, a value (= ) for a more volatile component is larger than a value for a less volatile component. That means that ≥ 1 since the larger value of the more volatile component is in the numerator and the smaller of the less volatile component is in the denominator.
is a unitless quantity. When the volatilities of both key components are equal, = 1 and separation of the two by distillation would be impossible under the given conditions. As the value of increases above 1, separation by distillation becomes progressively easier.
A liquid mixture containing two components is called a binary mixture. When a binary mixture is distilled, complete separation of the two components is rarely achieved. Typically, the overhead fraction from the distillation column consists predominantly of the more volatile component and some small amount of the less volatile component and the bottoms fraction consists predominantly of the less volatile component and some small amount of the more volatile component.
A liquid mixture containing many components is called a multi-component mixture. When a multi-component mixture is is distilled, the overhead fraction and the bottoms fraction typically contain much more than one or two components. For example, some intermediate products in an
oil refineryare multi-component liquid mixtures that may contain the alkane, alkeneand alkyne hydrocarbons ranging from methanehaving one carbon atomto decanes having ten carbon atoms. For distilling such a mixture, the distillation column may be designed (for example) to produce:
* An overhead fraction containing predominantly the more volatile components ranging from methane (having one carbon atom) to
propane(having three carbon atoms)
* A bottoms fraction containing predominantly the less volatile components ranging from
isobutane(having four carbon atoms) to decanes (ten carbon atoms).
Such a distillation column is typically called a depropanizer. The designer would designate the key components governing the separation design to be propane as the so-called light key (LK) and isobutane as the so-called heavy key (HK). In that context, a lighter component means a component with a lower
boiling point(or a higher vapor pressure) and a heavier component means a component with a higher boiling point (or a lower vapor pressure).
Thus, for the distillation of any multi-component mixture, the relative volatility is often defined as
Large-scale industrial distillation is rarely undertaken if the relative volatility is less than 1.05.
The values of have been correlated empirically or theoretically in terms of temperature, pressure and phase compositions in the form of equations, tables or graph such as the well-known DePriester charts. [DePriester, C.L. (1953), "Chem. Eng. Prog. Symposium Series", 7, 49, pages 1-43] [ [http://www.utwired.engr.utexas.edu/che363/docs/abs/abs1a_doc.pdf DePriester Charts] ]
values are widely used in the design of large-scale distillation columns for distilling multi-component mixtures in oil refineries,
petrochemicaland chemical plants, natural gas processingplants and other industries.
*Equilibrium flash of a multi-component liquid
* [http://www.nt.ntnu.no/users/skoge/publications/1999/DistillationTheory/original/distillationtheory.pdf Distillation Theory] by Ivar J. Halvorsen and Sigurd Skogestad,
Norwegian University of Science and Technology(scroll down to: 2.2.3 K-values and Relative Volatility)
* [http://lorien.ncl.ac.uk/ming/distil/distilpri.htm Distillation Principals] by Ming T. Tham,
University of Newcastle upon Tyne(scroll down to Relative Volatility)
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