Custom Columns

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Custom Columns

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ADV 7_1.pdf


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WorkshopMost 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 prebuilt 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.

Learning ObjectivesAfter completing this module, you will be able to:

Build custom columns in HYSYS

Replace generic reboilers with sizable heat exchangers

PrerequisitesBefore 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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Process Overview


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Column Overview


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Custom ColumnsThe 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 onthe top stage that allows for a refluxing stream in the column.

Reboiled Absorber- an absorber tower with a reboiler at itsbase, and no condenser at the top.

Distillation Column- an absorber tower with both a

condenser and a reboiler.

Customizing 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 column will be modified.


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The Column SubFlowsheetWhenever a column is added to a HYSYS simulation, a Column

Subflowsheetis created. The Subflowsheet is essentially another layer

in the HYSYS simulation. It is located under the Main layer, and can be

seen by selecting Tools/PFDsin the Main Menu Bar, or by pressing the

hot-key

.The nature of the layering scheme can be seen in

the displayed list as the Columns PFD is indented under the Main PFD.

Selecting the desired PFD and pressing the Viewbutton will open that

particular layer for viewing.

In order to edit the column, i.e. to add and modify operations, it is

necessary to enter the Column Environment. The Column

Environment is accessed through the Column Property Viewer. Withthe Column Property Viewer as the active view, press the Column

Environmentbutton. This will take the user into the Column

Environment, allowing them to add and/or modify operations. The

operations can be deleted, modified, replaced, and controlled just like

ordinary unit operations that most users are familiar with when in the

Main Environment.

When in the Column Environment, returning to the Main Environment

is as easy as pressing the Parent Simulation button 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 themethod of installation is identical.

Parent Simulation button


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Building the SimulationThis module will continue with the column built in the Column Sizing

Module. This column is a depropanizer for use in a gas plant. Any

molecule less volatile than propane leaves the column in the Liquid

Bottom Product and any molecule more volatile than i-butane leaves

the column in the Overhead Vapour Product.

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.


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Replacing the ReboilerIt is desired to replace the generic reboiler 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 theColumn Solving Method to Modified HYSIM Inside-Out. TheSolving method is accessed through the Solver page (on theParameters tab) of the Column Property View.

2. Press the Runbutton to reconverge to column.

3. In the Basis Environment, addWaterto the list of components.

4. Enter the Column Environment and delete the existing reboilerand energy stream from the PFD.

Modified HYSIM Inside-Outis a general purpose solverthat allows Heat Exchangersand other operations in theColumn Environment.


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5. Because we are replacing a generic reboiler that had no pressuredrop with a heat exchanger that will have a pressure drop, weneed to add a pump to the simulation that will "push" the fluidthrough the exchanger. Add a Pumpto the Column Environmentwith the following information.

6. After the newly created pump, add a Heat Exchangerto theColumns PFD, with the following information:

7. Define the Steam In streamas saturated at 200 oC (400 oF). Themass flow rate is 1000 kg/hr (2,200 lb/hr).

The pressure of the bottom stage in the column must be set at 320 kPa

(45 psia). In order to solve the column initially, the pressure in the

reboiler was set; however, the reboiler was deleted. Therefore, the

pressure on this stage needs to be fixed.

8. On the Pressurespage of the Column Property Viewer within theColumn Environment, set the pressure of the bottom stage(10_Main TS)to be 320 kPa (45 psia).

In This Cell... Enter...

Name Reboiler Pump

Inlet To Reboiler

Outlet Pump Out

Energy Pump Duty

Delta P (on Parameterspage) 75 kPa (11 psi)


Name Reboiler

Heat Exchanger Model Weighted

Tube Side Inlet Steam In

Tube Side Outlet Steam Out

Shell Side Inlet Pump Out

Shell Side Outlet Reboiler Out

Tube Side Pressure Drop 25 kPa (4 psi)

Shell Side Pressure Drop 75 kPa (10 psi)


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9. Add aV/L Separatorafter the Heat Exchanger, with the vapourproduct returning to the bottom stage of the column, and theliquid product leaving the Column Environment as the BttmLiquid Product.

10. Ensure that the three active specifications for the column are:Heavy Key, Light Key, and Heat Balance.

Because the Heat Exchanger was installed in the Column Environment

its 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.

11. Run the column; after it has converged, answer these questions.

What is the UA of the Heat Exchanger? __________

What is the LMTD for this exchanger? __________

Save your case!


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Connecting Streams Between Flowsheets1. Open the Main PFD. Notice the streams Steam In and Steam Out

are not visible here. This is because they were created in theColumn Subflowsheet, and have not been connected to the MainFlowsheet.

2. Streams are connected between the Main and Subflowsheets onthe Connectionspage. Notice that there are two types of streamslisted here, Internaland External. Internal streams are thosestreams that appear only in the Subflowsheet. Steam In andSteam Outare both internal streams.

3. Internal streams are connected to the Main Flowsheet by typing aname in the External Stream column. Add the appropriate namesto the External Streams column, and check to make sure that

these streams appear in the Main PFD.

Save your case!


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Optional ExerciseAdjust the Steam Feed Rate

Once both steam streams are installed on the Main PFD, an Adjust

Operation can be installed to ensure that the proper amount of steam is

supplied so that complete condensation of the steam occurs in the

reboiler.

1. The Steam Inflow rate was defined in the Column Environment.Therefore, it is not a specifiable parameter in the Main

Environment. Delete the flow rate of Steam Inin the ColumnEnvironment, and enter the same value as a flow rate for SteamIn in the Main Environment.

2. Add anAdjustUnit Operation to the Main PFD. Supply thefollowing information:

3. Press the Startbutton to activate the operation, and an error willappear.


Adjusted Variable Steam In - Mass Flow

Target Variable Steam Out - Vapour Fraction

Specified Target Value 0.0

Step Size 10 kg/hr (20 lb/hr)

Maximum Iterations 50

Why does this error occur? ___________________________

What can be done to prevent this from happening?Remember that the adjusted variable for an Adjustoperation must be specifiable.

Make the necessary corrections to the simulation andrecalculate the Adjust operation.


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4. Calculate the operation, and answer the following question:

Of course, any steam flow rate less than this value will also give

complete condensation, with some subcooling. However, the Adjust

operation stops calculation once the specified target value is reached.

The correct value was calculated because the calculation was started at

a higher vapour fraction value, and the value decreased until a vapour

fraction of zero first appeared. It can be assumed, due to the small stepsize, that this flow will lead to complete condensation with little, or no,

subcooling.

What is the required mass flow rate of steam? __________

A subcooled liquid always hasa vapour fraction of zero, andan superheated vapour has avapour fraction of 1.

Save your case!


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Custom Columns

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