Senin, 19 Juni 2017
In chemistry, azeotropic
distillation is any of a range of techniques used to
break an azeotrope in distillation.
In chemical engineering, azeotropic
distillation usually refers to the specific technique of adding another
component to generate a new, lower-boiling azeotrope that is heterogeneous
(e.g. producing two, immiscible liquid phases), such as the example below with
the addition of benzene to water and ethanol. This practice of adding an
entrainer which forms a separate phase is a specific sub-set of (industrial) azeotropic distillation methods,
or combination thereof. In some senses, adding an entrainer is similar to extractive distillation.
Material
Separation Agent
The
addition of a Material Separation Agent, such as benzene to
an ethanol/water mixture, changes the molecular interactions and eliminates the
azeotrope. Added in the liquid phase, the new component can alter the activity
coefficient of various compounds in different ways thus altering a mixture's
relative volatility. Greater deviations from Raoult's
law
make it easier to achieve significant changes in relative volatility with the
addition of another component. In azeotropic distillation the volatility of the
added component is the same as the mixture, and a new azeotrope is formed with
one or more of the components based on differences in polarity.[2] If
the material separation agent is selected to form azeotropes with more than one
component in the feed then it is referred to as an entrainer. The added
entrainer should be recovered by distillation, decantation, or another
separation method and returned near the top of the original column.
Distillation
of ethanol/water
A
common historical example of azeotropic distillation is its use in dehydrating ethanol
and water mixtures. For this, a near
azeotropic mixture is sent to the final column where azeotropic distillation
takes place. Several entrainers can be used for this specific process: benzene, pentane, cyclohexane, hexane, heptane, isooctane, acetone,
and diethyl ether
are all options as the mixture.[2] Of
these benzene and cyclohexane have been used the most extensively. However,
because benzene has been discovered to be a carcinogenic compound, its use has
declined. While this method was the standard for dehydrating ethanol in the
past, it has lost favor due to the high capital and energy costs associated
with it. Another favorable method and less toxic than using benzene to break
the azeotrope of the ethanol-water system is to use toluene instead
