Use of a Decanter to Recover Solvent and Cross Distillation Boundaries with Aspen HYSYS
Liquid-liquid Equilibrium
The separation process is an important process to
obtain high-purity products. There are various types of separation processes but on this
occasion, we
will discuss the separation process based on the principle of liquid-liquid
equilibrium. We will also simulate cases related to the liquid-liquid
equilibrium in Aspen HYSYS. But first, we need to know what liquid-liquid
equilibrium is.
This liquid-liquid
equilibrium principle is used in liquid extraction. Solvent extraction or often
also called liquid extraction is a method of separating or retrieving solutes
in solution (usually in water) using another solvent (usually organic). The
basic principle of liquid-liquid extraction involves contacting a solution with another solvent that
does not dissolve each other (immiscible) with the original solvent which has a different density so that two
phases will form sometime after the addition of the solvent. This causes mass transfer from the
original solvent to the extracting solvent (solvent).
The transfer of
solute into the new solvent is caused by the driving force that occurs due to the difference in chemical potential between the two solvents.
Thus, the liquid-liquid extraction process is a diffusional mass transfer
process. For example, acetone is soluble in water and chloroform, but more
soluble in chloroform. Without using a separator, we can separate the mixture
of acetone and water by adding chloroform to the mixture. Chloroform and water cannot mix, so the mixture
will automatically separate.
Azeotropic distillation (AD)
Azeotropic
distillation (AD) is the process of breaking an azeotrope where another
volatile component, called an entrainer or solvent is added to form a new
heterogeneous lower boiling point azeotrope.
As illustrated in
the figure below, the AD process consists of two distillation columns: an
azeotropic column for dehydration of the 92.4 wt% ethanol solution from the pre-concentration step
with the help of an entrainer, and a stripping column for separation of the entrained from the product
stream. In the azeotropic column, the ethanol product (>99 wt%) leaves the
bottom. The formed ternary azeotrope containing vapor water, entrainer, and a
small amount of ethanol exits the top section, then enters a separator (called
a decanter), and is divided into an organic phase (ethanol-entrainer) and a
water phase (water-entrainer) stream. The former is refluxed back into the
first column, while the latter is processed in the stripping column for trainer and ethanol
recovery (Lee and Pahl, 1985; Kovach III and Seider, 1987; Chianese and
Zinnamosca, 1990; Luyben, 2006).
 |
Flow sheet of AD
system for ethanol dehydration (Chianese and Zinnamosca, 1990; Luyben, 2006). |
Case background
Cyclohexane is used
as a reservoir during separation to break the ethanol-water azeotrope in an
anhydrous ethanol production plant. The stream from the top of the first
distillation column is usually a mixture with a composition very close to the
ternary azeotrope. Since cyclohexane and water are insoluble, a decanter can be
used to separate cyclohexane from ethanol and water. The second role of this
liquid-liquid separation is to cross the distillation boundary. One of the two
streams out of the decanter is the
recovered solvent. The other stream has a composition in
the distillation region that is rich in ethanol and is fed to the second column.
Problem
Statement
A 100 kmol/h feed
stream consisting of 35 mol% ethanol, 6 mol% water, and 59 mol% cyclohexane is
fed to the decanter. Determine the composition of the two streams exiting the
decanter and their flow rates.
Solution with
Aspen HYSY
Open Aspen HSYSY
and create a new Simulation.
Enter the component
list, in the Component List folder, click Add then add Ethanol, Water,
and Cyclohexane to the component list.
Define the property
package that will be used. In the Fluid Packages folder select Add
and select UNIQUAC as the property package. Select Rk for the Vapor Model
used in the Activity Model Specification grid.
Add 3-Phase
Separator to Flowsheet
Double click
3-Phase Separator Vessel (V-10). Name the Inlet stream as FEED,
Outlet as VAPOR, LIQUID1, and LIQUID2.
Define the
operating conditions for the FEED stream. Go to the Worksheet tab, Enter Temperature
25oC, pressure 1 bar, and Molar Flow 100 kgmole/h.
In the Composition
section enter a Mole Fraction of 0.35 for ethanol, 0.06
for water, and 0.59 for cyclohexane. The separator will finish when the
compositions are finished.
Check the results.
You can see that the FEED stream is separated into two separate liquid streams due to the
density difference between the two liquid phases. The Liquid1 stream has
a flow rate of 54.6 kgmol/hr, while the heavier Liquid2 stream
has a flow rate of 45.4 kgmol/hr.
Check the
composition section on the Worksheet tab. Stream Liquid1 is
enriched in cyclohexane while stream Liquid2 is enriched in ethanol.
Conclusion
The decanter can be
used to concentrate cyclohexane from 59% to 78% so that it can be recycled or
reused. The other outlet (Liquid2) has a composition in the distillation region
that is different from the FEED stream, providing a product that crosses the distillation boundary.
This serves the role of the second decanter mentioned in the background
section.
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