Flash Drum simulation using Aspen HYSYS to remove Hydrogen from Methane, Ethylene, and Ethane
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| Flash Drum |
Hi chemical
engineering colleagues, we are together again in the Aspen HYSYS tutorial
discussion. This time we will discuss separation equipment in the industry. Yep
... according to our title, the equipment is a Flash drum. As usual chemical engineering colleagues, before the
simulation,
we provide a brief theory of what will be simulated.
Introduction
Liquid vapor
equilibrium is very important in industry, for example, in distillation equipment.
Distillation is the process of separating liquid mixtures based on differences
in boiling points. Liquid vapor equilibrium data is used to determine how the
liquid mixture will separate. Flash drums are classified as single-stage
distillation that works based on vapor-liquid equilibrium. The liquid mixture
with high pressure enters the flash drum and experiences a pressure drop. This
pressure drop causes part of the liquid mixture to evaporate based on its liquid-vapor equilibrium.
The gas mixed with the liquid mixture will be released when experiencing a
pressure drop in the flash drum.
The expansion
process also occurs in the flash drum. Where at lower pressures, some
components that are more volatile than other components will change phase to gas while other components
remain in a liquid
phase. More about expansion theory and Aspen HYSYS simulations related to
expansion has
been discussed on this page.
Below we will
simulate a case related to the separation process using a flash drum. I remind
you again to colleagues, before further simulation, it is good to know first
what we are simulating. For that chemical engineering colleagues, let us learn
the basic theory of a process before using Aspen HYSYS Software.
Problem
Statement and Aspen HYSYS Solution
In an ethylene
plant, we have a feed stream containing hydrogen, methane, ethylene, and
ethane. Before this stream can be fed to the demethanizer, the hydrogen must be
removed so that there is less volumetric flow, which reduces the size required
for the demethanizer column. Since hydrogen has a much higher vapor pressure
than the other components, one or more flash drums can be used to remove the
hydrogen.
The feed stream is
a combination of 6,306 Ib/h hydrogen, 29,458 Ib/h methane, 26,049 Ib/h
ethylene, and 5,671 Ib/h ethane. In the feed stream the hydrogen mole fraction
is greater than 0.51, indicating there is a large volume of hydrogen in the
feed stream. There are two main objectives in this process:
- After some
hydrogen has been removed, the stream should contain less than 0.02 mole
fraction of hydrogen.
- Ethylene loss to the hydrogen stream is less than 1%.
Open Aspen HYSYS
software and create a new simulation.
Go to Component List and select Add. Add Hydrogen, Methane, Ethylene, and Ethane to the component list.
Define the property
package. In the Fluid Package folder select Add. Then select Peng-Robinson
as the property package
Next, go to the bottom left
screen and click Simulation.
From the Model Palette add Cooler to the flowsheet.
Double-click Cooler (E-10). Define the Inlet stream as FEED, Outlet stream as ToFlash, and Energy stream as Q-COOL.
In the Parameters
section, enter Delta P = 0 and Duty = 0 kcal/h. Later, we will make adjustments to the block to achieve the desired specifications.
Define the feed stream.
go to the Worksheet tab, and enter the value of Temperature 90oF (-67.78oC) and pressure
475 psia (32.75 bar).
Next, Open the FEED stream and go to Composition Form, and enter the Mass Flow rates in kg/h as
below.
Add Separator
to Flowsheet
Double click
Separator (V-10). Select ToFlash stream as Inlet stream, Define Outlet
stream as VAP and LIQ.
Open the Worksheet tab to view
the separation results. You can see that the liquid stream has a flow rate of 0.
We must now add an adjustment block and spreadsheet to find the cooler duty
required to limit the loss of ethylene to the vapor stream to less than 1%.
Add Spreadsheet
to flowsheet
Double-click on the spreadsheet (SPRDSHT-1).
In the Spreadsheet tab enter "Ethylene flow in FEED" in
cell A1 and "Ethylene flow in LIQ stream" in cell A2.
Right-click
on call B1 and select Import Variable. select the Master Comp
Molar Flow (Ethylene) in the FEED stream. Right-click on cell B2 and select Import
Variable. select the Master Comp Molar Flow (Ethylene) in the LIQ stream.
In cell A3
type "Fraction Ethylene Lost" and in cell B3 enter the formula
=(B1-B2)/B1
You can see that we
are currently losing 100% Ethylene to the vapor stream. We will now add
an adjust block to vary the cooler duty to limit the fraction lost to below
0.01. Add the Adjust block to the flow chart from the Model Palette.
Double click on
adjust block (ADJ-1). Specify the Adjusted Variable to be Duty
from the Cooler E-10 block. Specify the Target Variable to be B3
from SPRDSHT-1. Enter a Specified
Target Value of 0.01.
On the Parameter
tab, enter a Step Size value of 1e+005 kcal/h and change Maximum
Iteration to 100. Click Start to begin the calculation.
The fraction of
ethylene lost in the vapor stream will now be less than 1%. We must also ensure
that the Mole
Fraction of
Hydrogen in the liquid
stream is less than 0.02. Double-click the LIQ stream
and click on the Composition section under the Worksheet
tab.
The Mole
Fraction of hydrogen in the liquid stream is 0.0172, which means it is less than the specified
value of 0.02.
Conclusion
The vapor pressure
of hydrogen is much higher (6,600 times higher than methane at -150°C) than the
vapor pressure of other components in the feed stream. We used adjustment
blocks and spreadsheets to determine a good value for the heat duty of the
cooling block to remove most of the hydrogen from the feed stream through the
separator block.
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