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Flowsheeting

© 2000 AEA Technology plc - All Rights Reserved.Chem 3_4.pdf

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WorkshopIn evaporation, a solution consisting of a non-volatile solute and a volatile solvent is concentrated by the addition of heat. In multiple effect evaporation, the volatile solvent recovered from the first evaporator is condensed and used as a heat source for the next evaporator. This means that the second evaporator must operate at a lower temperature and pressure than the first evaporator.

In this module you will simulate a series of three evaporators to concentrate a solution of sucrose/water. Each evaporator is modelled using a flash tank. You will convert the completed simulation to a template, making it available to connect to other simulations.

On the next page, a Process Overview is shown. This represents the actual process. On the third page a Simulation PFD is shown. This represents the simulation as you will build it in this module. Building the simulation in this way allows more flexibility in the design.

Learning ObjectivesOnce you have completed this section, you will be able to:

• Add and connect operations to build a Flowsheet• Add and use logical operations, Sets and Adjusts• Use the graphical interface to manipulate flowsheets in HYSYS• Understand information propagation in HYSYS• Convert HYSYS flowsheet to templates

PrerequisitesBefore beginning this section you need to know how to:

• Define a Fluid Package• Define Streams• Navigate the Workbook interface

Process Overview

Simulation PFD

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Building the SimulationThe first step to building any simulation is defining a Fluid Package. A brief recap on how to define a Fluid Package and install streams is described below. For a complete description see: Defining the Simulation Basis, Module 1.

Defining the Simulation Basis1. Start a New Case and add a Fluid Package.

2. Use Wilson/Ideal as the Property Package with the components Sucrose and H2O.

3. Move to the Binary Coefficients page. Notice that the interaction parameters for Aij and Bij are empty.

The program warns you that the binary coefficients have not been determined and the model will assume values of zero. Answer OK to this message. Enter the Simulation Environment.

The Wilson equation cannot be used for problems involving liquid-liquid equilibrium.

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4. Add a stream with the following values.

5. Add a second stream with the following properties:

In this cell… Enter…

Name Feed

Vapour Fraction 0

Pressure 101.3 kPa (14.7 psia)

Flowrate 50 kg/h (110 lb/hr)

Mass Fraction Surcose 0.3

Mass Fraction H2O 0.7

Note that the composition values for this stream are in Mass fractions. Double-click on the Mass Flow cell to enter these values.

In this cell… Enter…

Name Steam

Vapour Fraction 1.0

Pressure 275 kPa (40 psia)

Mass fraction H2O 1.0

What is the temperature of stream Feed? __________

What is the temperature of stream Steam? __________

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Adding Unit Operations to a FlowsheetAs with streams, there are a variety of ways to add Unit Operations in HYSYS:

The Triple Effect Evaporator consists of six operations:

• A series of three evaporators modelled as flash tanks (2 Phase separators)

• Three coolers

In this exercise, you will add each operation using a different method of installation.

To use the… Do this…

Menu Bar Select Add Operation from the Flowsheet menu.

Or

Press the <F12> hot key.

The UnitOps window displays.

Workbook Open the Workbook and go to the UnitOps page, then click the Add UnitOp button.

The UnitOps window displays.

Object Palette Select Object Palette from the Flowsheet menu or press <F4> to open the Object Palette and double click the icon of the Unit Operation you want to add.

PFD/Object Palette Using the right mouse button, drag’n’drop the icon from the Object Palette to the PFD.

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Adding a Separator

The Evaporator is modelled using a Separator in HYSYS.

The Separator will be added using the <F12> hot key.

1. Press the <F12> hot key. The UnitOps window displays:

2. Select Separator from the Available Unit Operations list.

3. Press the Add button. The Separator property view displays.

4. On the Connections page enter the data as shown here:

Note: Drop down boxes, such as for Feed and Product streams, contain lists of available streams which can be connected to the operation.

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Adding a CoolerAdd the first Cooler using the same method.

1. Press the <F12> hot key

2. The UnitOps window displays. Click the Category Heat Transfer Equipment and select Cooler.

3. Press the Add button. The Cooler property view displays.

4. On the Connections page enter the information as shown below:

5. Go to the Parameters page.

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6. Enter a value of 0 kPa (0 psi) for the Pressure Drop.

7. Go to the Worksheet tab.

8. Specify a Vapour Fraction of 0 for the stream Condensate.

To completely define the separation we need to provide an energy flow.

9. On the Worksheet tab, enter a value of 2.42e4 kJ/h (2.29e4 Btu/hr) for the Energy stream q1.

What is the flowrate of Water in the stream L1? __________

What is the temperature of the stream V1? __________

What is the mass flow of steam through the Cooler? __________

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Add the Second CoolerThis procedure describes how to add Unit Operations using the UnitOps page of the Workbook.

1. Open the Workbook and click the UnitOps tab.

2. Click the Add UnitOp button. The UnitOps window displays:

3. Select Heat Transfer Equipment from the Categories group.

4. Select Cooler from the Available Unit Operations list.

5. Press the Add button. The Cooler property view displays.

6. On the Connections page enter the information as shown below:

7. On the Parameters page specify a Pressure Drop of 0 kPa (0 psi).

8. Go to the Worksheet tab and specify the Vapour Fraction of the stream C2 as 0. Close this view.

What is the Heat Flow for stream q2? __________

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Add Another SeparatorThis procedure describes how to add a Separator using the Object Palette. The Object Palette contains icons for all the Streams and Unit Operations in HYSYS.

1. Press the <F4> hot key. The Object Palette displays:

2. Double click the Separator button on the Object Palette. The Separator property view displays.

Separator button

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3. On the Connections page enter the stream information as shown here:

4. On the Parameters page, delete the pressure drop specification. The Separator should become unsolved; Unknown Delta P.

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Add a Set OperationThe Set Operation is a steady-state logical operation used to set the value of a specific Process Variable (PV) in relation to another PV. The relationship is between the same PV in two like objects -- for instance, the temperature of two streams, or the UA of two exchangers.

In order for the energy to flow from Cooler 2 to Effect 2, the Separator outlet temperature must be cooler then the condensate from the Cooler. A Set operation will be used to maintain this relationship.

1. Add a Set operation by double-clicking on the Set icon in the object palette.

2. Complete the Connections page as shown here:

3. Go to the Parameters tab. Complete the view as shown below, if using field units the value for the offset will be -5 oF:

What is the Delta P of Effect 2? __________

Set operation button

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Add the Third CoolerWorking with a graphical representation, you can build your flowsheet in the PFD using the mouse to install and connect objects. This procedure describes how to install and connect the Cooler using the Object Palette Drag’n’drop technique.

To Drag’n’Drop in the PFD:1. Press the Cooler button on the Object Palette.

2. Move the cursor to the PFD. The cursor will change to a special cursor, with a box and a plus (+) symbol attached to it. The box indicates the size and location of the cooler icon.

3. Click the left mouse button to “drop” the Cooler onto the PFD.

There are two ways to connect the operation to a stream on the PFD:

To connect using the … Do this…

Attach Mode toggle button

Insert Icon

Press the Attach Mode toggle button.

Place the cursor over the operation. The Feed stream connection point is highlighted in dark blue.

Move the cursor over the stream you want to connect.

Press and hold the left mouse button.

Move the cursor to the operation icon and release the mouse button.

<Ctrl> key Press and hold the <Ctrl> key and pass the cursor over the operation.

Place the cursor over the stream you want to connect.

Press and hold the left mouse button.

Move the cursor to the operation icon and release the mouse button and the <Ctrl> key.

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4. Double click on the Cooler icon on the PFD. The Cooler property view displays. Enter the data shown below:

5. On the Parameters page specify a Pressure Drop of 0 kPa (0 psi).

6. Go to the Worksheet tab and specify the Vapour Fraction of the stream C3 as 0. Close this view.

Add the Third Separator1. Drag ‘n’ drop the Separator onto the PFD. Connect the stream L2

as the Feed to the Separator.

2. Double click on the Separator. Make the following connections:

3. On the Parameters page delete the pressure drop specification.

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Set Operation1. Add a Set operation and complete the Connections page as

shown here:

2. On the Parameters tab enter a value of –3 °C (-5°F) as the Offset, and 1.0 for the Multiplier.

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Add an AdjustThe Adjust operation is a Logical Operation - a mathematical operation rather than a physical operation. It will vary the value of one stream variable (the independent variable) to meet a required value or specification (the dependant variable) in another stream or operation.

It is desired to reach 15 weight% water in stream L3. The only parameter we have to manipulate this variable is the energy supplied to the first Effect. To meet a target concentration in L3 we can use an Adjust operation.

1. Add the Adjust operation. The Adjust property view displays.

2. Press the Select Var… button in the Adjusted Variable group to open the Variable Navigator.

3. From the Object list select q1. From the Variable list which is now visible, select Heat Flow.

4. Press the OK cell to accept the variable and return to the Adjust property view.

5. Press the Select Var… button in the Target Variable group.

What is the weight percent of Water in stream L3? __________

Adjust button

The adjusted variable must always be a user specified value.

Always work left to right in the Variable Navigator. Don’t forget you can use the Object Filter when the Object list is large.

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6. Select L3 and Comp Mass Frac (H2O) as the target variable.

7. On the Connections page, enter a value of 0.15 in the Specified Target Value box.

The completed Connections page is shown below.

8. Switch to the Parameters tab, and enter 2000 kJ/h (1900 Btu/hr) as the Step Size.

9. Press the Start button to begin calculations. Note: once the case is solved (OK status), this button will disappear from the property view.

10. To view the progress of the Adjust, go to the Monitor tab.

When adjusting certain variables, it is often a good idea to provide a minimum or maximum which corresponds to a physical boundary, such as zero for pressure or flow.

Note the Tolerance and Step Size values. When considering step sizes, use larger rather than smaller sizes. The Secant method works best once the solution has been bracketed and by using a larger step size, you are more likely to bracket the solution quickly.

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If you enter a step size too large for the energy HYSYS will not calculate because all the liquid has been flashed. You need to decrease the step size, enter a new value for q1 and restart the simulation.

Note that HYSYS does not predict the formation of solids; this will have to be verified separately.

Manipulating the PFDThe PFD is designed around using the mouse and/or keyboard. There are a number of instances in which either the mouse or the keyboard can be used to perform the same function. One very important PFD function for which the keyboard cannot be used is Object Inspection.

You can perform many of the tasks and manipulations on the icons in the PFD by using Object Inspection. Place the mouse pointer over the icon you wish to inspect and press the secondary mouse button. An appropriate menu is produced depending upon the icon selected (Stream, Operation, Column, or Text Annotation).

A list of the objects which you can Object Inspect are shown below with

What is the energy required to achieve a concentration of 85 wt% of sucrose in the product stream? __________

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the corresponding menus.

Object... Object Inspection Menu...

PFD

Unit Operations

Streams

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Customize the PFD by performing the following:

1. Add a Title, Triple Effect Evaporator.

2. Add a Workbook Table for the Material Streams in the simulation.

3. Add a Table for stream L3.

Adding Unit Operation Information to the Workbook

Each WorkBook has a UnitOps page by default that displays all the Unit Operations and their connections in the simulation. You can add additional pages for specific Unit Operations to the WorkBook. For example, you can add a page to the WorkBook to contain only Coolers in the simulation.

To add a Unit Operation tab to the WorkBook:1. Open the Workbook.

2. In the Menu Bar, select Workbook, and then Setup.

3. In the Setup view, press the Add button in the UnitOps group.

4. From the New Object Type view, select Heat Transfer Equipment, then Cooler.

5. Click OK. A new page, Cooler, containing only Cooler information is added to the WorkBook.

Double clicking on a title with a "+" sign will open an expanded menu.

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Adding Unit Operation Information to the PFD

For each Unit Operation, you can display a Property Table on the PFD. The Property Table contains certain default information about the Unit Operation.

Add Unit Operation information to the PFD:1. Open the PFD.

2. Select the Separator Effect 1.

3. Object Inspect the Unit Operation.

4. Select Show Table from the menu.

5. The Vessel Temperature, Pressure, Liquid Molar Flow, and Duty are shown as defaults in the table. Object Inspect the table and insert the Vapour Mass Flow.

6. Create two tables for the streams Feed and L3 showing the Component Mass Fraction of Sucrose and the Mass Flow.

Remember you can Object Inspect an object by selecting it and then clicking on it with the right mouse button.

Save your case!

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Exploring with the SimulationExercise 1

Try running the case for different final sucrose concentrations. Can you find any cases in which the program does not solve?

Watch for cases when the Adjust block takes too large of a step in energy, causing all of the liquid to be flashed.

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Saving the Simulation as a TemplateA template is a complete Flowsheet that has been stored to disk with some additional information included that pertains to attaching the Flowsheet as a Sub-Flowsheet operation. Typically, a template is representative of a plant process module or portion of a process module. The stored template can subsequently be read from disk and efficiently installed as a complete Sub-Flowsheet operation any number of times into any number of different simulations.

Some of the advantages of using templates are:

• Provide the mechanism by which two or more cases can be linked together

• Can employ a different property package than the main case to which it is attached

• Provide a convenient method for breaking large simulations into smaller, easily managed components

• Can be created once and then installed in multiple cases

Before you convert a case to a template, it needs to be made generic so it can be used with gas plants of various flowrates.

1. Delete the Flow and Composition of stream Feed.

2. Choose Main Properties from the Simulation menu.

3. Press the Convert to Template button.

4. Press Yes to convert the simulation case to a template.

5. Answer No to the question “Do you want to save the simulation case”.

6. Save the template as 3-Effect-Evap.tpl.

Note that once a case has been saved as a template, it can not be re-converted back into a normal simulation case.

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