Distillation, sometimes referred to as fractionation or rectification, is a process for the separating of two or more liquids. The process utilizes the difference of the vapor pressures to produce the separation.
Distillation is one of the oldest unit operations. While the first technical publication was developed in 1597, distillation already had been practiced for many
centuries — specifically, for the concentration of ethyl alcohol for beverages. Today, distillation is one of the most used unit operations and is the largest consumer of energy in the process industries.
APV has been conducting business in the field of distillation since 1929. A brief
history of APV in distillation is shown .
Today, APV mainly concentrates its marketing efforts in the area of solvent
recovery, waste water stripping, chemical production and specialized systems,
such as high vacuum systems for oils.
A HISTORY OF APV IN DISTILLATION
1929 First Distillation Columns Manufactured
1933 West Tray License Obtained
1935 First Major APV Designed and Manufactured
Distillation System
1935 Distillation Laboratory Established
1939 First Fuel Ethanol Distillation System
1939-45 Many Toluene/Benzene Systems Produced
1946 Acetic Acid Recovery System
The Largest Order APV Had Ever Received
1969 Acquired L.A. Mitchell Group and Glitsch License
for Valve Trays
1971 First Distillation System in USA
1990 The 100th U.S. Distillation System
B A S I C P R I N C I P L E S
O F D I S T I L L A T I O N
When a mixture of two or more liquids is heated and boiled, the vapor has a
different composition than the liquid. For example, if a10% mixture of ethanol in
water is boiled, the vapor will contain over 50% ethanol. The vapor can be
condensed and boiled again, which will result in an even higher concentration of
ethanol. Distillation operates on this principle.
Clearly, repeated boiling and condensing is a clumsy process, however, this can
be done as a continuous process in a distillation column. In the column, rising
vapors will strip out the more volatile component, which will be gradually
concentrated as the vapor climbs up the column.
The vapor/liquid equilibrium (VLE) relationship between ethanol and water is
shown in Figure 2. A similar relationship exists between all compounds. From this
type of data, it is a relatively simple task to calculate the design parameters using
one of the classical methods, such as McCabe-Thiele.
The key to this separation is the relative volatility between the compounds to
be separated. The higher the relative volatility, the easier the separation and
vice versa. For a binary system, the mole fraction y of component a in the
vapor in equilibrium with the mole fraction x in the liquid is calculated from
the following equation.
ya = a.xa
1 + (a-1).xa
Where xa is the mole fraction of a in the liquid and a is the relative
volatility.
The larger the relative volatility, the more easily the compound will strip out
of water. For ideal systems which follow Raoult’s law, the relative volatility is
calculated by
a = Pa/Pb
Where Pa and Pb are the vapor pressures of components a and b at a
given temperature.
The partial pressure p of component a above a binary ideal solution can
be calculated by
pa = Pa.xa
Where xa is the mole fraction of component a in the liquid.
Similarly in a binary mixture, for component b.
pb = Pb.xb
Notice that the sum of the partial pressures must equal the total system
pressure: P=pb+pa. For non ideal mixtures (usually the case with steam
stripping duties), the partial pressure is calculated from
Pa = gaPaxa
Pb = gbPbxb
Where g is the activity coefficient of the compound. The activity coefficient
essentially quantifies the deviation from ideality.
For multicomponent mixtures, the mathematical representation of the VLE
becomes more complex. It is necessary to use complex equations to predict
the performance. The simplification commonly used as a substitute for the
rigorous equations is K value. ya=Kxa. The ratio of the K value of different
components reflects the relative volatilities between those components.
It is not the intention of this publication to discuss methods for calculating a
distillation system. Classical graphical calculations have been the McCabe-Thiele
method, using the data shown in Figure 2, and the Ponchon Savarit method,
which is more accurate and uses an enthalpy diagram, as shown in Figure 3, as
well as the VLE data.
All these graphical methods have been rendered obsolete by the various process
simulation programs, such as SimSci. Even with these highly sophisticated
programs, there is still a need for test work on many systems. For ideal mixtures,
which are rare, the program will provide a theoretically correct solution.
For non ideal mixtures, the program can only make estimates by using
thermodynamic equations such as UNIFAC. Experimental data can be used
for more precise solutions. A considerable amount of experimental data,
however, is in the program database.
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