Amine Scrubbing with Aspen HYSYS
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| ABSORBER AND STRIPPER COLUMN |
Welcome to our blog discussing
one of the most interesting aspects of chemistry and chemical engineering:
absorption simulation using the powerful software, Aspen HYSYS. In this discussion,
we will understand the basic principles of absorption, how Aspen HYSYS serves
as an important tool in modeling this process, and why a deep understanding of
this simulation can help design efficient solutions in the industry.
In an ever-evolving world,
industry strives to optimize production processes, efficiency, and
environmental friendliness. One technique that has proven to be very useful in
achieving these goals is the absorption process, where specific components of a
gas mixture can be taken up and absorbed by an absorbent liquid. However,
designing and understanding this process traditionally may be a complicated challenge. This is where Aspen
HYSYS comes in as a tool that enables accurate simulation of absorption processes
with multiple variables.
Let's dive into the concept of
absorption, the practical steps in creating a simulation model using Aspen
HYSYS, and how simulation results can be analyzed and interpreted for better
process development. Not only that, we will present practical tips and relevant
case study examples to give you a deeper understanding of the real applications
of absorption simulation using Aspen HYSYS.
So, if you are a student or
professional in chemistry, chemical engineering, or industry interested in
exploring the world of absorption process simulation and optimization, you are
in the right place. Get ready to embark on this exciting journey and broaden
your horizons on how Aspen HYSYS can help design more efficient and innovative
solutions in modern industry.
Let's go ahead and dive into the
exciting world of absorption simulation using Aspen HYSYS!
Absorption can occur when an
absorbate (gas component) diffuses into an absorbent (liquid) and forms a
solution. The basic concept of the absorption process is to utilize the
diffusivity of gas molecules in a particular solution. When the adsorbate (gas mixture)
is contacted with an adsorbent that can dissolve one of the components in the gas and this contact
process lasts for a long time at a fixed temperature, an equilibrium will occur
which causes no more mass transfer. The driving force in this mass transfer is
the level of dissolved gas concentration (partial pressure) in the total gas
exceeding the equilibrium concentration with the liquid at any time. When the
concentration of dissolved gas (expressed in partial pressure) in the gas phase
exceeds the equilibrium concentration in the absorbing liquid, there will be a
concentration difference that creates a tendency for the gas to dissolve or be
absorbed by the liquid.
In more detail, if the partial
pressure of the dissolved gas in the gas phase is higher than the equilibrium
concentration in the liquid at any given time, then the gas molecules will move
from the gas phase to the liquid phase. This creates mass transfer from the gas
to the liquid, which is the main objective of the absorption process. This concept is very important in
understanding how the absorption process takes place and how various factors
such as pressure, temperature, and composition contribute to the efficiency of
the process.
There has been a recent surge of
interest in the recovery of carbon dioxide from flue gas. The recovery of
carbon dioxide gas will reduce greenhouse gas emissions and the captured carbon
dioxide can be sold. In addition, the captured CO2 can be used in the process
of improving oil capture. In this method, CO2 is injected into underground oil
wells. When CO2 is dissolved in the oil, it reduces the viscosity (thickness)
of the oil and its surface tension. This effect makes the oil more fluid and
easier to flow through the well, thereby increasing the oil recovery rate from
the reservoir. Generally, this is known as an EOR (enhanced oil recovery)
method.
Gas-liquid absorption, also known
as gas stream scrubbing, can be used to remove CO2 from flue gas streams using
MEA (monoethanolamine) as a solvent. MEA acts as a weak base and neutralizes
acidic compounds such as CO2. This will cause the CO2 to ionize into HCO3-
which will prevent the CO2 from leaving the solvent, resulting in a gas stream
that is largely free of carbon dioxide.
Case Study Example
It is known that the amine
solvent has a theoretical loading capability of 0.5 moles of CO2 for each mole
of amine. Calculate the amount of MEA required to remove CO2 from a flue gas
stream containing 10mol% carbon dioxide with a total flow rate of 1000 tons/day.
Use Aspen HYSYS to simulate this process and confirm the results. Assume a
dilute solvent stream with a mass fraction of 0.25 MEA and a 20 stages absorber column.
Aspen HYSYS Setup
Open Aspen HYSYS and create a new
simulation.
Enter the components involved in
this simulation. In the Component List folder select Add. Then
add water, carbon dioxide, nitrogen, monoethanolamine, and oxygen
to the component list.
Define the fluid package that
will be used by clicking the Fluid Packages folder and selecting Add.
Select Acid Gas-Chemical
Solvents as the property package.
Next, we can start the simulation by
clicking Simulation on the bottom left of the screen.
Looking at the FLUE GAS stream,
we can see that it contains ~150 kg mol/hr of carbon dioxide. This means that we will need a
minimum of ~300 kg mole/hour
of MEA to remove all the carbon dioxide. In our simulation, we will create a
feed with slightly more MEA than the calculated minimum to ensure successful
carbon dioxide removal. Since our solvent feed has an MEA mass fraction of
0.25, this means that our solvent stream will require a total mass flow of
about 80,000 kg/hr.
Add a second material stream to the flowsheet. This will be the solvent stream. Double-click on the stream and rename it to Solvent. Enter Temperature 25°C, Pressure 1 bar, and Mass Flow 80,000 kg/hr. In the Composition form, enter a Mass Fraction of 0.25 for MEA and 0.75 for water. The flow must be solved.
Add Absorber Column
Sub-Flowsheet from the Model
palette
Double-click on the column (T-100). This will open the Absorber Column Input Expert.
On the first page of Column Input Expert select Solvent stream as Top Stage Inlet and Flue Gas stream as Bottom Stage Inlet.
Create an Ovhd Vapor Outlet called CLEAN AIR and a Bottom
Liquid Outlet called SOLUTION.
Enter 20 for # Stages.
Click Next when finished.
On the second page of the Column
Input Expert enter a Top Stage Pressure of 1 bar and a Bottom
Stage Pressure of 1.2 bar. Click Next when done
On the last page of the input
expert, enter an approximate Top and Bottom Stage Temperature of 50°C.
This does not have to be very accurate, but it will help the troubleshooter to
find a solution. Click Done to configure the column
The column properties window will
now appear. Click Run to start the calculation. The absorbent column
should converge
Check the results. Double-click on the Clean Airstream. Open the Composition
form under the Worksheet tab. You will see that there is no carbon
dioxide left in the stream.
Conclusion
This simulation has confirmed the calculated amount of MEA required to remove carbon dioxide from the flue gas stream. It was found that a solvent stream of 80,000 kg/hr is sufficient to remove carbon dioxide from a flue gas stream of 1000 tons/day. The clean air stream can now be released to the atmosphere and the captured carbon dioxide can be removed from the solvent and sold or used for various applications