Advanced Columns 1
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Advanced Columns
© 2004 AspenTech - All Rights Reserved.
EA 1000.32.02
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Workshop
Most users are familiar with the prebuilt columns that are available in
the main HYSYS Environment. This module will introduce the concept
of custom columns. HYSYS allows users to build columns without using
the prebuilt configurations. This is useful for simulating columns that
do not fit into the usual configurations.
Building custom columns allows users to replace reboilers with heat
exchangers; the heat exchangers can then be rated and sized. Likewise,
thermosyphon reboilers can be used in place of generic reboilers.
HYSYS also includes a Column Sizing utility that can size and rate
column tray sections.
Learning Objectives
After completing this module, you will be able to:
• Build custom columns in HYSYS
• Replace generic reboilers with sizable heat exchangers
• Perform Tray Sizing and Rating calculations
Prerequisites
Before beginning this module, you should be able to:
• Navigate the Main Simulation
• Add unit operations to the PFD
• Add, and converge, a generic prebuilt column
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Custom Columns
The most common way of adding a column to a simulation is to use the
prebuilt columns that HYSYS offers. There are four prebuilt columns
available:
• Absorber - the simplest of all towers, no reboiler or condenser.
• Refluxed Absorber - an absorber tower with a condenser on the
top stage that allows for a refluxing stream in the column.
• Reboiled Absorber - an absorber tower with a reboiler at its
base, and no condenser at the top.
• Distillation Column - an absorber tower with both a condenser
and a reboiler.
Customising a Column gives the user greater control over the
simulation. For example, replacing the reboiler with a Heat Exchanger
allows the user to use steam as a heating medium and size the
exchanger based on the required duty.
Custom Columns can be built in two ways, either by modifying a
prebuilt column, or by constructing a new column from the beginning.
In this module, a prebuilt LPG recovery column will be modified.
The Column Sub-Flowsheet
Whenever a column is added to a HYSYS simulation, a Column Sub-
flowsheet is created. The sub-flowsheet is essentially another layer in
the HYSYS simulation. It is located under the Main layer, and can be
seen by selecting Tools-PFDs in the main menu bar, or by pressing the
hot-key CTRL P. The nature of the layering scheme can be seen in the
displayed list as the Column's PFD is indented under the Main PFD.
Selecting the desired PFD and clicking the View button will open that
particular layer for viewing.
Alternatively the PFD of a particular column can be seen by right
clicking on the column PFD icon and choosing Open PFD.
In order to edit the column, i.e. to add and modify operations, it is best
to enter the Column Environment. The Column Environment is
accessed through the Column Property View. To bring up the Column
In HYSYS Version 3.1+ it is
not necessary to enter the
Column Environment to edit
it. Changes can be made by
opening the PFD and editing
it, although it is still
recommended to enter the
Column Environment.
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Property View, double click the column icon on the PFD. With the
Column Property View as the active view, click the Column Environment
button. This will make HYSYS enter the Column Environment. The
operations within the column can be deleted, modified, replaced, and
controlled just like ordinary unit operations in the Main Environment.
When in the Column Environment, returning to the Main Environment
can be accomplished by clicking the Enter Parent Simulation
Environment icon located on the Main Menu Bar.
Adding operations to the column is very similar to adding operations in
the Main Environment. The choice of operations is reduced, but the
method of installation is identical.
Building the Simulation
This module will continue with the column built in the Getting Started
module. This column is an LPG Recovery unit for a Turbo Expander
plant.
This column was constructed as a generic distillation column. The
condenser is a “Total Reflux” type; this means that there is no liquid
product from the condenser, rather all of the condensed liquid is sent
back into the column to provide a refluxing liquid stream.
The reboiler on this column is also of the generic type. An energy stream
is supplied and the liquid product from the last stage is boiled up. The
vapours return to the column and the liquid leaves the column as a
liquid product.
Enter Parent Environment
icon
Don’t worry if you haven’t
built the Turbo Expander
plant case. The file
“ADV1_GettingStarted_Soln
.hsc” contains this case.
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Replacing the Reboiler
The generic reboiler will be replaced with a shell and tube heat
exchanger. This will allow the user to supply steam to the column as the
energy stream, and size, or rate, the heat exchanger.
1. Before modifying the column, it is necessary to change the Column
Solving Method to Modified HYSIM Inside-Out. The Solving
method is accessed through the Solver page (on the Parameters tab)
of the Column Property View.
2. In the Basis Environment, add Water to the list of components.
3. If necessary click the Run button to recalculate the column.
4. Enter the Column Environment and delete the existing reboiler and
energy stream from the PFD.
Because the generic reboiler that had no pressure drop is being replaced
with a heat exchanger that will have a pressure drop, a pump must be
added to the simulation to “push” the fluid through the exchanger.
Figure 1
Modified HTSIM Inside-Out
is a general-purpose solver
that allows Heat
Exchangers and other
operations in the Column
Environment.
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5. Add a Pump to the Column Environment with the following
parameters:
6. After the newly created pump, add a Heat Exchanger to the
Column's PFD, with the following parameters:
7. Define the Steam In stream as saturated pure steam at 200°C
(392°F). The mass flow rate is 2500 kg/h (5511 lb/hr).
Normally when the reboiler is deleted from the column Sub-flowsheet
the pressure that was specified at the bottom will be lost, and hence
must be added back on the Parameters page in Profiles tab.
However in this case the pressure at the bottom of the column is set by
the pressure in the bottoms liquid product stream (10), which is linked
to the Turbo Expander outlet pressure.
8. Add a Separator after the Heat Exchanger, with the vapour product
returning to the bottom stage of the column, and the liquid product
leaving the Column Environment as stream 10. (See the Column
Overview at the beginning of the module for the column PFD).
In This Cell... Enter...
Name Reboiler Pump
Inlet To Reboiler
Outlet Pump Out
Energy Pump Duty
Delta P (on Design... Parameters page) 75 kPa (10.9 psia)
In This Cell... Enter
Name E-100
Heat Exchanger Model Calculated by Column (default)
Tube Side Inlet Steam In
Tube Side Outlet Steam Out
Shell Side Inlet Pump Out
Shell Side Outlet Reboiler Out
Tube Side Pressure Drop 50 kPa (7.25 psia)
Shell Side Pressure Drop 75 kPa (10.9 psia)
Heat Exchanger icon
Be sure to pick the
Separator operation from
the object palette, rather
than one of the Condensers
which have similar icons.
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9. Because the Reboiler was deleted, HYSYS removed the Reboiler
liquid RVP spec. Add this spec back:
Since the Heat Exchanger was installed in the Column Environment it's
specifications appear on the Monitor page along with the specifications
of the column. Heat Exchanger specifications can be activated and
deactivated just like Column specifications; they can also be added in
exactly the same manner as Column specifications.
10. Return to the Main Flowsheet and ensure that the three active
specifications for the column are: Ohvd Duty, Btms RVP, and E-100
Heat Balance.
RVP Spec...
Type Column Vapour Pressure Spec
Name Btms RVP
Stage V-100
Type Reid Vapour Pressure
Phase Liquid
Spec Value 200 psia
Figure 2
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11. Run the column; after it has converged, answer these questions:
Connecting Streams Between Flowsheets
1. Look at the Main PFD. Notice the streams Steam In and Steam Out
are not visible here. This is because they were created in the Column
Sub-flowsheet, and have not been connected to the Main Flowsheet.
2. Streams are connected between the Main and Sub-flowsheet on the
Connections page on the Design tab. Notice that there are two types
of streams listed here, Internal and External. Internal streams are
those on the Column Sub-flowsheet. External streams are the
corresponding streams on the main Flowsheet. Currently Steam In
and Steam Out do not have any linked external streams.
What is the UA of the Heat Exchanger?
What is the LMTD for this exchanger?
What is the vapour fraction of the outlet steam?
Save your case!
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3. Internal streams are connected to the Main Flowsheet by typing a
name in the External Stream column. Add the appropriate names to
the External Streams column, and check to make sure that these
streams appear in the Main PFD.
The Steam In flow rate was defined in the Column Environment,
therefore, it is not a specifiable parameter in the Main Environment.
Some users prefer to have all the feed streams visible and editable in the
Main Environment.
4. Delete the flow rate of Steam In in the Column Environment, and
enter the same value as a flow rate for Steam In in the Main
Environment. The column will automatically resolve.
5. Delete the other Steam In specifications on the column sub-
flowsheet (Vapour Fraction, Temperature and Composition) and
transfer them to the main flowsheet.
Figure 3
As this is now a
Custom Column, the
Connections page of
the Design tab no
longer shows a
Distillation Column.
Save your case!
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Exercise - Simulating the Reboiler on the Main
Flowsheet Using Internal Streams
Another method of simulating the reboiler with a Heat Exchanger is to
use the Internal Stream feature of the column to make a copy of the
reboiler on the main flowsheet. Here this will be added to the case where
a rigorous heat exchanger has already been added to the column sub-
flowsheet. However this technique works just as well with columns
using the standard Reboiler operation.
This way of modelling the reboiler is just as accurate as adding the heat
exchanger to the column sub-flowsheet although it does not look quite
as good.
An 'Internal Stream' is a special HYSYS stream that represents a fluid
inside the column, for example the vapour leaving the top tray, or the
liquid leaving the bottom tray. Internal Streams show with a cyan colour
on the column sub-flowsheet.
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1. Go to the Flowsheet-Internal Streams page of the column, press the
Add button and configure the table as below:
2. Re-run the column to calculate the internal stream.
3. Locate the stream on the main flowsheet, check that its properties
are the same as those for the liquid leaving the column.
4. Add a pump, heat exchanger and steam streams as earlier in the
module.
Figure 4
Pump Name Reboiler Pump
Inlet Btm Tray Liquid
Outlet Pump Out
Energy Pump Duty
Delta P __75_kPa
Exchanger Name Copy of Reboiler
Tube Side Inlet Steam In 2
Tube Side Outlet Steam Out 2
Shell Side Inlet Pump Out
Type the name of
the internal stream
Stage and Phase the
stream will represent
Only relevant for streams with external draws.
Net = exclude effect of external draws (consider flows in column only)
Total = include draws (i.e. total flow leaving the stage)
Check this box to make the stream
appear on the main flowsheet
Instead of manually adding
the streams and operations
you can copy & paste them
from the column sub-
flowsheet by using the right-
click copy/paste options.
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5. Use a Set operation to specify the Reboiler Out stream temperature
to be the same as the column bottom product stream.
Column Sizing
HYSYS contains a Tray Sizing utility that greatly simplifies the
mechanical design of a distillation column. A user can size full towers, or
sections of towers, by specifying information related to the trays, tower
internals, the downcomers, and the weirs.
The most common use of the Tray Sizing utility is to identify a tray
section, and then make HYSYS size the tower into sections based on
your input, then if desired, perform a rating analysis on the column.
While HYSYS is able to size and rate tray sections, the values that it
provides are only rough estimates and should be treated as such.
Column Sizing in Design Mode
1. Select Tools/Utilities from the Main Menu bar, or press the hot key
CTRL U.
Shell Side Outlet Reboiler Out
Heat Exchanger Model Exchanger Design (Weighted)
Tube Side Pressure Drop 50 kPa
Shell Side Pressure Drop 75 kPa
Steam In 2 conditions Saturated pure steam at 200°C. Mass
flow 2500 kg/h.
Pump Name Reboiler Pump
Save your case!
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2. Select Tray Sizing from the list of available utilities and click the Add
Utility button.
3. Click the Select TS... button, and select LPG Recovery as the
Flowsheet and Main TS as the Object.
HYSYS allows users to select Tray Sections instead of entire columns so
that users may size Side Strippers independently from the Main
Column.
4. Click the Add Section... button. Accept all of the default values that
are presented.
HYSYS will calculate the dimensions of the column using preset values
for the column internals and for the various parameters.
Figure 5
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On the Performance tab a summary of the calculations are presented. A
brief explanation of the terms follows:
• Number of Flow Paths. The number of times liquid crosses the
tray, most trays are single-pass or have NFP of 1.
• Maximum Downcomer Backup. Represents the maximum
amount of liquid hold-up in the downcomer that can be tolerated
by the column before flooding occurs.
• Maximum Weir Loading. Measures the amount of liquid flowing
over the weir.
• Pressure Drops. Estimates the total pressure drop over the
section and the maximum pressure drop per tray.
Column Sizing in Rating Mode
In rating mode, HYSYS allows you to perform rating calculations based
on a specified tower diameter and fixed tray configuration.
What is the maximum pressure drop per tray in the Distillation Column?
Over which tray does this pressure drop occur?
What is the diameter of the trays inside the column?
What is the total section height?
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Exercise
Head office is desperate to build a distillation column, but the
fabrication mill is working overtime and there are very lengthy delays for
special orders. The mill has the following trays in stock:
Obtain the flow parameters and pressure drops for a column that uses
the trays as given above. The following requirements must be met in
order for the column to be constructed.
• Maximum% Flood = 85
• Maximum weir loading = 80 m3/h-m (860 ft3/h-ft)
• Maximum downcomer backup = 50%
For each case, follow these steps:
1. On the Design tab, click on the Specs page and set the Mode to
Rating.
2. Set the diameter and number of flow paths, and move to the
Performance tab.
3. Leave all the other specs at the default values.
Diameter, m (ft.) NFP (Passes)
0.75 (2.5) 1
1.0 (3.3) 1
1.25 (4.1) 1
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4. Complete this table with the information provided by HYSYS.
Challenge
The Export Pressures button on the Tray Sizing Utility Performance tab
allows the calculated pressure drops to be exported to the column
pressure profile.
Use this feature to supply the LPG Recovery Column with the rigorously
calculated pressure profile.
Hint: Currently the condenser, reboiler, top and bottom tray pressures
appear as calculated (black) numbers. You will need to make them
specified (blue) before the Export Pressures function will work.
Case Number 1 2 3
Diameter, m (ft.) 0.75 (2.5) 1.0 (3.3) 1.25 (4.1)
NFP 1 1 1
Weir Load
Flood
DC Back Up
Total Delta P
Compare the table above with the specifications on the previous page; which
set of trays will best meet the restrictions? Remember that smaller trays will
be less expensive.
Save your case!
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