White Paper Separation of Industrial Multi-Component Mixtures

1. The Separation of Industrial Multi-
Component Mixtures
WHITE PAPER
© 2017 AGC Refining & Filtration LLC

2. AGC REFINING & FILTRATION
THE SEPARATION OF INDUSTRIAL MULTI-COMPONENT MIXTURES 2
Contents
The Separation of Substances 2
Vapor Pressure 3
Types of Mixtures 5
Boiling Point 5
Distillation 6
The Nomograph 7
Raschig Super Rings 8
References 9

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THE SEPARATION OF INDUSTRIAL MULTI-COMPONENT MIXTURES 3
The Separation of Substances
A pure compound consists of only one type of molecule. A mixture of compounds may contain a multitude
of different types of molecules.
Figure 1: A Model of a Complex Molecular Structure
To isolate one more compounds from a mixture requires separation. Separation is purification. The basis
for separation is the different properties of the components in the mixture. Some of the characteristics by
which compounds can be separated are boiling points, refractive index, infrared spectrum or by
comparison with a known pure sample.
Distillation is a method of separation based on the differences in boiling points of components of a
mixture. There are two types of distillation. Simple or atmospheric distillation and fractional distillation.
Figure 2
A form of simple atmospheric distillation. The moisture from the food in the cooking pot evaporates in response to the
heat supplied by the wood fire.
Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in sepa-
rating chemical compounds by their boiling point by heating them to a temperature at which several frac-
tions of the compound will evaporate. It is a special type of distillation. Generally the component parts boil
at less than 25 °C from each other under a pressure of one atmosphere. If the difference in boiling points
is greater than 25 °C, a simple distillation is used.
A third method of separation is Vacuum Distillation. Vacuum distillation is a method of distillation whereby
the pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure (usually
less than atmospheric pressure) causing evaporation of the most volatile liquid(s) (those with the lowest
boiling points).
This distillation method works on the principle that boiling under vacuum occurs at a lower temperature
than under atmospheric conditions.

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THE SEPARATION OF INDUSTRIAL MULTI-COMPONENT MIXTURES 4
Figure 3: A Typical Large Scale Industrial Fractional Distillation Column
Courtesy Pfaudler
Allen Science & Engineering designs and manufactures small to mid-scale industrial vacuum distillation
systems that are used to separate and recover valuable components of mixtures based on their behavior
under carefully controlled heat and vacuum conditions.
Vapor Pressure
In distillation, a liquid is heated to the temperature at which it changes to a vapor. The vapor is then con-
densed and recovered or simply evaporated in the case of water.
If a liquid is placed in an empty container, some of the liquid will vaporize and the pressure in the
container will rise until it reached a constant value. This is the equilibrium vapor pressure. It increases
with temperature.
The existence of vapor pressure can be explained in the molecular level, by molecules of liquid escaping
the attractive forces of the liquid. At higher temperatures, the kinetic energy transferred to the molecules
results in a greater rate of escape.
Equilibrium is then re-established at a higher temperature and with a corresponding higher vapor
pressure. If there are other compounds in the mixture, the total vapor pressure will be the sum of the
equilibrium pressure (partial pressure) in addition to the vapor pressure of the other compounds, i.e. the
sum of their partial pressures.

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THE SEPARATION OF INDUSTRIAL MULTI-COMPONENT MIXTURES 5
Types of Mixtures
There are three types of liquid mixtures; miscible, partially miscible and non-miscible.
Miscible mixtures are those in which the individual components mix completely and no interface can be
detected. The mixture is homogeneous. Pouring grain alcohol into water results in a single liquid phase.
No meniscus forms between the alcohol and the water, and the two liquids are considered “miscible”.
Tow liquids of like polarity will form a miscible mixture through hydrogen bonding.
Many liquid mixtures fall between these two extremes. Two liquids are “partially miscible” if shaking equal
volumes of the liquids together results in a meniscus visible between two layers of liquid, but the volumes
of the layers are not identical to the volumes of the liquids originally mixed. For example, shaking water
with certain organic acids results in two clearly separate layers, but each layer contains water and acid
(with one layer mostly water and the other, rich in acid).
Liquids tend to be immiscible when attractions between like molecules are much stronger than attractions
between mixed pairs. Oil and water don’t mix. Pouring 10 ml of olive oil into 10 ml of water results in two
distinct layers, clearly separated by a curved meniscus. Each layer has the same volume and essentially
the same composition as the original liquids. Because very little mixing has apparently occurred, the
liquids are called “immiscible” or unmixable. Actually, extremely low concentrations of oil can be found in
the water and the oil layer contains detectable amounts of water. Complete immiscibility is rare. Non-polar
and polar components are immiscible because no hydrogen bonds exist between them.
Boiling Point
The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the atmo-
spheric pressure surrounding the liquid. The normal boiling point (also called the atmospheric boiling
point or the atmospheric pressure boiling point) of a liquid is the special case in which the vapor pressure
of the liquid equals the defined atmospheric pressure at sea level, 1 atmosphere. At that temperature, the
vapor pressure of the liquid becomes sufficient to overcome atmospheric pressure and lift the liquid to
form bubbles inside the bulk of the liquid. The standard boiling point is now defined as the temperature at
which boiling occurs under a pressure of 1 bar.
The heat of vaporization is the amount of energy required to convert or vaporize a saturated liquid (i.e., a
liquid at its boiling point) into a vapor.
Liquids may change to a vapor at temperatures below their boiling points through the process of evapora-
tion. Evaporation is a surface phenomenon in which enough heat is applied for the molecules located
near the vapor/liquid surface escape into the vapor phase. On the other hand, boiling is a process in
which molecules anywhere in the liquid escape, resulting in the formation of vapor bubbles within the
liquid.
The Relationship Between The Normal Boiling Point And Vapor Pressures Of A
Liquid
The higher the vapor pressures of a liquid at a given temperature, the lower the normal boiling point (i.e.,
the boiling point at atmospheric pressure) of the liquid.
In terms of intermolecular interactions, the boiling point represents the point at which the liquid molecules
possess enough thermal energy to overcome the various intermolecular attractions binding the molecules
as liquid (e.g. the instantaneous-dipole induced-dipole attractions, and hydrogen bonds) and therefore un-
dergo a phase change into the next phase (vapor). Thus the boiling point of a liquid is also an indicator of
the strength of the attractive forces between the liquid’s molecules.

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THE SEPARATION OF INDUSTRIAL MULTI-COMPONENT MIXTURES 6
The Relationship Between Liquid And Vapor Phases
When two liquids are placed together in a closed, evacuated space, the vapor pressure and the boiling
When two liquids are placed together in closed, evacuated space, the vapor pressure and the boiling
point can only be determined if:
1. The liquids are immiscible
2. They are miscible and show no change in temperature or volume when mixed
Immiscible Liquids
If the components are immiscible, the composition of the vapor and the vapor pressure and the boiling
point, is independent of the relative quantities of the components, if both are present in sufficient quanti-
ties and that evaporation takes place unhindered. The composition of the vapor can then be calculated if
the vapor pressures and densities of the immiscible liquids are known.
Partially Miscible Liquid
If two liquids are partially miscible, the observed vapor pressure, composition and boiling point will be dif-
ferent that those of immiscible liquids. However, the difference will be small if the miscibility is slight.
Miscible Liquids
In the case of miscible liquids, the vapor pressures and boiling points can be accurately calculated, if no
change occurs when the liquids are mixed.
Distillation
The chosen method for separation of liquid mixture into their respective individual component is distil-
lation. Allen Filters, Inc.’s Separation Technology Group has specialized in phase separation through both
simple and vacuum distillation.
Atmospheric (Simple) Distillation
Separation by atmospheric distillation thus is based on gas-liquid equilibrium, in which the phases consist
of the components themselves. The ease of separation is based on the differences in the boiling points of
the substances; because boiling point is related, to a first approximation, to the molecular weight of the
substance, distillation separates on the basis of weight (or size) of molecules. If the boiling points are
close together, a multistage distillation, which can be achieved by placing a column above the boiling
liquid solution, is required. This glass column contains some randomly packed material, and the hot
vapors from the boiling solution partially condense on the surfaces. The condensed liquid flows back
toward the mixture until it meets rising hot vapors. The more volatile portion of the returning liquid re-
vaporizes, and the less volatile part of the rising vapor condenses. Thus what occurs in the column
amounts to a multistage operation. The result is that the component of lower boiling point concentrates at
the top of the column and that of higher boiling point in the bottom part. Condensation of the vapor at the
top of the column provides material much richer in the component having the lowest boiling point. This
vapor is then drawn off, condensed and recovered in a pure state.

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THE SEPARATION OF INDUSTRIAL MULTI-COMPONENT MIXTURES 7
Fractional Distillation
It is practiced for those mixtures, in which the boiling point of the components differ by less than 25 °C at
an atmospheric pressure of one.
The component with the lowest boiling point is separated first, while the component with the highest boil-
ing point is drawn off the last. If the boiling point of the constituents differ by more than 25 °C, then simple
atmospheric distillation procedure is implemented for separation. Fractional distillation is also practiced in
large scale alcoholic fermentation in order to purify alcoholic beverages. Beer and wine produced after
fermentation of grains and grapes contain less than 15 percent alcohol. It is due to the fact that a high
concentration of alcohol kills the yeast cultures that are responsible for fermentation. In such a condition,
fractional distillation is practiced to purify alcohol and make stronger alcoholic drinks. In addition,
fractional distillation is also used for solvent recycling, extraction of essential oils and purification of
fragrances in perfume industries.
Reduced Pressure (Vacuum) Distillation
Because many compounds undergo thermal decomposition at higher temperatures, distillation is required
for those substances to thermal prevent degradation. Generally if the normal boiling point of a substance
is higher than 125–175 °C, distillation is carried out under reduced pressure. In general, the boiling point
of a compound will decrease by about 20–30 °C, each time the external pressure is reduced by a factor of
two. Thus, if the compound is distilled under a vacuum of 45–50 torr, its boiling point will be 80–100 °C
lower than its normal atmospheric boiling point.
The Nomograph
The nomograph shown below is useful for estimating both the expected reduced-pressure boiling points
with respect to normal boiling points and normal boiling points from observed reduced-pressure boiling
points. The nomograph applies to liquids that are nor associated in the liquid phase. The variation of
boiling point with pressure for associated fluids such as alcohols is 10–20% less than for non-associated
liquids.
Figure 4: Nomograph For Determining Boiling Points Under Vacuum
Courtesy Sigma-Aldrich

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THE SEPARATION OF INDUSTRIAL MULTI-COMPONENT MIXTURES 8
When a mixture of two compounds is heated, the vapor will be richer than the liquid, in the more volatile
component. If we would distill the mixture further, the distillate will be richer still in the more volatile com-
ponent. The amount of the distillate would however become smaller with each distillation. Thus, in order
to obtain a substantial quantity of distillate, systematic fractional distillation is necessary.
Raschig Super Rings
The Raschig Super Ring is one of many so-called dumped types of packing, meaning they are just
dumped into the fractionation columns. The difference between other packing types is that in the design,
consideration has been given to the liquid flow behavior as a result of fluid dynamics studies.
The Raschig Super Ring (RSR) achieves an even distribution of liquid in the packed bed, which in
addition to short diffusion pathways between the rising gas and the falling liquid in the column. This leads
to a highly homogeneous distribution of gas and liquid over the column cross-section. The liquid film
trickles down through the very open geometry. The very even distribution of the material and liquid make
possible the very large ratios of column diameter to nominal packing diameter that can be achieved. The
design of the distributor thus becomes a critical factor. Allen used proprietary distributor designs based on
computational hydro-dynamic modeling of optimum spray patterns.

9. AGC REFINING & FILTRATION
THE SEPARATION OF INDUSTRIAL MULTI-COMPONENT MIXTURES 9
Figure 5: The Specific Pressure Drop Of RSR No. 0.3 In Metal
For various gas capacity factors and liquid load.
Figure 5 shows the pressure drop curves of various RSR for the system of air/water and the mass
transfer efficiency curves for the absorption of ammonia from air in water and for the desorption of carbon
dioxide from water in air. The no. 0.3 RSR is an Allen standard for vacuum distillation processes,
although no. 0.5 are also used for the larger system.
References
1. Billet, R. Packed Towers in Processing and Environmental Technology. (1995).
2. Billet, R. Industrielle Destillation. Verlag Chemie Gmbh, Weinheim Germany (1973).
3. Schultes, M. Raschig Super Rings: A Fourth Generation Random Packing. AIChE (2001).
4. Simon, R. The Separation of N-Butanol From Water. AIChE (2009).
5. Simon, R. Separation of Azeotrope Mixtures by Fractional Distillation Methods. AIChE (2009).

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