Gibbs Phase Rule in a Distillation Column with Aspen HYSYS
Welcome to our blog that will cover an interesting topic in the world of chemistry and industrial processes: simulating distillation based on the Gibbs Phase Rule principle using the powerful software, Aspen HYSYS. In this journey, we will deepen our understanding of the distillation process, how the Gibbs Phase Rule principle plays a role in driving component separation, as well as how Aspen HYSYS is an important tool in effectively simulating and analyzing this process.
The separation of components in
mixtures is a classic challenge in the chemical and petrochemical industries.
One of the most common and effective methods for performing component
separation is distillation. However, distillation is not just a simple method;
it is a science involving deep thermochemical principles and complicated laws
of physics. This is where the Gibbs Phase Rule Principle emerges as an
important guide to understanding the relationship between the components in a
mixture, the phase, and the operational conditions.
Not only that, but this blog will also present a case
example of simulating distillation using Aspen HYSYS based on the Gibbs Phase
Rule Principle. We will explain the practical steps in creating an accurate
distillation model and how you can analyze the simulation results to understand
process efficiency and possible improvements.
If you are a student,
professional, or researcher in chemistry, chemical engineering, or industry
interested in understanding more about distillation and the Gibbs Phase Rule
Principle, you are in the right place. Let's begin our journey to delve deeper into
the fascinating world of distillation simulation using Aspen HYSYS and the
Gibbs Phase Rule Principle.
Without further ado, let's go ahead and understand how an
understanding of the Gibbs Phase Rule Principle can help unravel the secrets of
component separation in complex distillation processes!
Distillation is a commonly used liquid mixture separation tool to separate it into different components. The working principle of distillation is based on the difference in boiling points of the components in the mixture. Understanding the Gibbs Phase Rule is essential for designing and optimizing distillation column operations.
Definition and basic concepts
Josiah Williard Gibbs, as the Gibbs Phase Rule developer,
defines it as a fundamental equation in thermodynamics used to describe
phase equilibrium in multicomponent systems. In the context of a distillation
column, the Gibbs Phase Rule can estimate the number of phases present in the
system and determine the number of variables that can be changed to achieve the
applied phase equilibrium.
The Gibbs Phase Rule formula in
the context of a distillation column is as follows:
- F = C - P + 2 (unknown Pressure
or Temperature)
- F = C - P + 1 (one of the
Pressure or Temperatures
is known)
- F = C - P (known value of both
Pressure and Temperature)
Where :
F = number of degrees of freedom
C = number of components in the
system
P = number of phases in the
system
Here, F represents the number of
variables that we can change independently to achieve our desired phase
equilibrium.
For example, if we have a system
with only two components and a constant temperature, then two phases are
possible: liquid phase and vapor phase. In this case, we have one degree of
freedom (F=1), which we can set through changeable variables, such as flow rate
or pressure settings.
Application of Gibbs Phase Rule in Distillation Columns
In a distillation column, the Gibbs Phase Rule can be used to predict the number of phases in the system, depending on the components and operational conditions. Below is an example of the application of the Gibbs Phase Rule in a distillation column:
Binary Distillation Column (two components)
a. If the system consists of two
components with a fixed operating temperature, then there will be two possible
phases: liquid phase and vapor phase.
b. If the system consists of two
components with a fixed operational pressure, then there will be only one
possible phase: liquid phase or vapor phase.
c. If the system consists of two components and the temperature and operating pressure can be changed independently, then the number of possible phases is two or three: liquid phase, vapor phase, or liquid-vapor equilibrium.
Multicomponent Distillation Columns (more than two components)
a. The number of possible phases
in a multi-component
system will depend on the number of components, temperature, pressure, and
composition of the system.
b. In multicomponent systems, the
number of possible phases can be more than two or three, depending on the
complexity of the system and operational conditions.
In distillation column
simulation, the main focus is often on the composition of the resulting
product. Therefore, it is important to measure and control the composition.
However, the process of measuring composition tends to be slower and more
expensive than temperature measurement. When the pressure is fixed, temperature
and composition are related to each other (except for the case of azeotropes).
Therefore, measuring and controlling the temperature at the top/bottom stage is
the same as measuring and controlling the composition at the top/bottom stage.
In this example, we will conduct some case studies to show that the composition
for the upper and lower stages is constant when the temperatures at the upper
and lower stages are fixed regardless of changes in other operating conditions
and column configurations.
Example problem
In a distillation apparatus of a
binary mixture consisting of ethane and ethylene, where the pressure and
temperature for stage equilibrium are fixed, will the composition of vapor and
liquid leaving this stage change with other conditions of the column?
Aspen HYSYS Solution
Create a new case in Aspen HSYSY
Enter the components that will be
simulated. In the Component list folder select Add. Then add Ethane
and Ethylene. If it is difficult to find Ethylene, type
"ethene" in the Search For section.
Define the Property package that
we are using. In this case,
we use Peng-Robinson. In the Fluid Packages folder select Add.
Then select Peng-Robinson as the property package
Next, we enter the simulation stage, Click Simulation
at the bottom left of the screen
Add Distillation Column
Sub-Flowsheet in the Model Palette section to the worksheet
Double-click on the distillation column
(T-10). In the Distillation Column Input Expert window enter a
name for each stream, as below. Then click Next when finished
On the second page keep the default for Once-through,
Regular HYSYS reboiler. Then click Next
On the third page, define the Condenser
and reboiler
pressure which is 100 kpa. Click Next.
On the fourth and fifth pages, keep the defaults and
click Done to configure the column.
Our first step is to define the
feed stream. Double-click on the FEED stream. Enter the value of Vapor
Fraction 0.5, pressure 100 kpa, and Molar Flow 100 kgmole/h.
In composition enter Mole
Fraction 0.5 for both Ethylene and Ethane. The FEED stream
will be completed.
Double-click on Column (T-10) to
finalize the specification of the column. Open the Spacs form located
under the Design tab. We will specify the stage top and stage bottom
temperatures. Click the Add
button and select a Column Temperature specification type. Select Stage 1 and
enter a value of -104.193
oC. This temperature is adjusted to have a mole fraction of 0.99
ethylene in the distillate stream.
Add the second Temperature specification select stage 50 and enter a Spec Value of -88.971oC. this temperature
is adjusted to have a mole fraction of 0.99 ethane at the bottom.
In the Specs Summary
section, make sure that the only active specifications are the two temperatures
that were just created. Once both temperature specifications are made active,
the column will complete
Check the product composition
results. Go to the Performance tab, in the Summary section we can
see that the Ethylene mole fraction in the distillate is 0.9934 and the
ethane mole fraction at the bottom is 1.
Go to the Column Profiles section under the Performance tab to see the Reflux and Boilup Rations.
Now we will change the feed
location to the column while holding the top and bottom stage constant
temperature specifications. The product composition should not change as we are
keeping the temperature and pressure constant. Go to the Design tab in
the column window and change the FEED Stream Inlet Stage to 29. Click Run to
start the calculation. The column should converge.
To see the results if there is a
change in the product composition, go to the Performance tab and you
will also see a change in the Reflux and Boilup Rations.
Now we will change the
composition of the feed stream. Double-click
the FEED stream and go to the Composition section under
the Worksheet tab. Change the Mole Fraction to 0.6 for Ethylene
and 0.4 for Ethane. Once done, the column will automatically
update and fuse. Again, the composition product should not change as we are
still keeping the temperature and pressure of the top and bottom stages
constant.
To view the composition results
go to the Performance tab
Finally, we will add 10 stages to
the column and observe the effect on product purity. Go to the Design
tab on the column window and enter 60 for the Number of Stages. Press Run
when done. The column should come together.
Go to the Performance tab
to see the results. Once again you will see that the composition of the product
remains unchanged.
Conclusion
This example shows that for a
binary distillation column, fixing the top/bottom stage temperature can keep
the top/bottom composition constant regardless of changes in other things e.g.
feed condition and location or number of stages in the column). This behavior
can be exploited for control. For binary mixtures with azeotropes, this is
still true assuming that the composite feed remains within a certain region
shared with the azeotrope.
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