Process Simulation using PRO/II's GUI

General Approach to Simulation

In the PRO/II Graphical User Interface, the fields for operations that you are required perform, and data fields where data entry is required, are always bordered in red. Data entry fields for items with supplied defaults are always bordered in green. After you have supplied information in a data entry field, the border color changes to blue. If you supply data that lies outside the normal range of values for an entry field, the value is marked with a yellow border.

Problem data may be supplied in almost any order, and PRO/II warns you when required data are missing. However, it is still best to follow a logical path when supplying simulation data. Therefore, the following general approach is recommended for building a simulation flowsheet:

1. Draw the flowsheet and connect the unit operations with streams

Select the unit operations needed for the flowsheet calculations; then lay them down and position them on the Main Window draw area. The streams connect the unit operations and pass information from one unit to another.

2. Define the components in your system

 It is best to order the components in volatility order, starting with the lightest component. While not a necessity, defining water as the first component in hydrocarbon/water systems makes it easy to see the break between the aqueous and non-aqueous phases. Petroleum components for which you supply data should be defined next. Pseudocomponents generated by PRO/II from petroleum stream assay data are the last components in the component list.

3. Select the thermodynamic and transport property methods

For many problems, a predefined method set may be selected. Guidelines for thermodynamic methods are provided in the PRO/II on-line help and Reference Manual (accessed via the Help Menu), and the hard copy PRO/II Reference Manual. Hot line help is also available from SimSci.

4. Supply data for the feed streams and recycle streams

You must supply thermal conditions, flowrates, and compositions for all external feed streams to the flowsheet. Optionally, estimated data may be supplied for recycle streams to speed convergence of recycle calculations.

5. Supply operating conditions for the unit operations

Double-click on the icon for each unit operation to access its data entry windows. The color codes tell you what data you must supply and what data have defaults. You may also use the on-line help (accessed via the Help button) to learn more about the calculation options, data entry items, etc.

6. Run the process simulation

PRO/II uses color codes to let you know when sufficient information has been supplied to perform the calculations. When all stream and unit labels and all the borders for the toolbar buttons have changed from red (indicating missing data) to green or blue, you are ready to run your simulation. At this point, you may push the Run button on the toolbar to begin flowsheet calculations.

7. Analyze the simulation results

Use the many convenient report and plotting features of PRO/II to analyze the simulation results. At this point some careful engineering analysis is in order. Are the calculated results reasonable? How do the results compare with the plant data? Can differences be reconciled? Are better data needed for the feed stocks? Is the model adequate for the intended purposes?

Now that we have presented an overall plan for simulating a flowsheet, let's look at some of the individual steps in more detail.

Building the Flowsheet

Adding Unit Operations to the Flowsheet

The icons and names for the unit operations appear in the PFD Palette which may be moved or resized as desired. To add a unit operation to the flowsheet, click the unit icon in the palette, move the cursor to the desired location on the draw area, and click the left mouse button. The icon for the unit will be placed on the flowsheet. PRO/II assigns a default unit identifier to each unit operation as it is added to the flowsheet.

Connecting the Flowsheet :

Push the Stream button at the top of the PFD Palette to its "down" state. The cursor changes its shape to and all possible stream ports for each unit operation are now marked. Required outlet ports are red; optional ports are green (according to the color convention).

To add a stream, click the left mouse button at the point of origin for the stream, either a unit's exit port or a vacant location on the draw area for an external feed stream. Move the cursor to the desired terminus of the stream and click the left mouse button. The terminus for the stream can be a unit's input port or a vacant portion of the draw area for a flowsheet exit stream. When each stream is being added to the flowsheet, all valid entry ports appear and are colored either red or green as per the requirement. PRO/II assigns a default stream identifier to each stream as it is added to the flowsheet.

PRO/II adds each stream to the flowsheet in an orthogonal fashion, following a rectangular grid pattern. Thus, it is only necessary to define the origin and terminus for a stream; the routing is determined automatically.

Revising Stream Connections :

You may find that you want or need to change a stream connection. This may be accomplished quite easily by placing the cursor on the origin or terminus of the desired stream, and depressing the left mouse button. Drag the stream to the desired location and release the mouse button to complete the revised connection.

As soon as a valid flowsheet has been built, i.e., all required inlet, outlet, and connector streams have been added for all the process units, the red border around the Stream button changes to black.

Supplying Required Data

After the flowsheet is built, you must supply all the required data for the calculations:

1. Components

 Access the Component Selection Main Window by selecting the menu bar item Input, and then selecting Component Selection from the Input menu. Alternatively, you may select this window by pushing the button on the toolbar. Note that this button has a red border, indicating that components have not yet been defined.

Library components for which you know the library access names may be typed directly into this window. A convenient search procedure is also provided which you may use to find library components, and is accessed by pushing the Search/Pick from List button. Petroleum components are defined in the Petroleum Components Window which is reached by pushing the Petroleum button.

The petroleum pseudocomponents defined by PRO/II from petroleum stream assay data do not appear in the Component Selection Main Window.

2. Thermodynamic Methods

Thermodynamic methods are defined in the Thermodynamic Data Main Window which you may reach by selecting the menu bar item Input, and then selecting Thermodynamic Data from the Input menu. Alternatively, you may push the button on the toolbar. Note that this button is initially outlined in red, indicating that thermodynamic methods must be defined for the problem.

For most problems, you may use a pre-defined set of thermodynamic methods for calculation of K-values, enthalpies, entropies, and densities. You may modify a predefined method set by selecting it in the Defined System list box and then pushing the Modify button to access the Thermodynamic System Modification Window. Transport property calculations are not included in the predefined method sets, and you must add them with the Modify option when they are needed for the calculations.

3. Stream Information

 The stream labels for feed streams for which you must supply information are marked with red borders. You may supply stream information in the Stream Main Data Entry Window which is displayed by double-clicking on the desired stream.

 Three types of information must be supplied: the thermal condition of the stream, the flow rate for the stream, and the composition for the stream. For streams with petroleum assay data provided in addition to or in place of composition data, the assay data are converted into petroleum pseudo-components that are used to define the stream composition for you.

4. Unit Operations

 Unit operation labels for which data entries are needed are marked with red borders. To enter information for a unit operation, double-click its icon to display its Main Data Entry Window. The various input options and numeric values are supplied via this parent window and its child windows. Required information is always bordered in red; data entry fields for items with supplied defaults are always bordered in green. After you have supplied information in a data entry field, the border color changes to blue. Information you have supplied which lies outside the normal range for the value is marked with a yellow border.

You may also furnish a longer, descriptive name for the unit operation. Notice that the unit identifier on the flowsheet has a black border instead of red when you return to the flowsheet, signifying that all data entry requirements are satisfied. If the border is still red, you must return to the Main Data Entry Window for the unit operation and supply the missing data.

5. Miscellaneous Data

All data entries in this category are optional because they all are provided default entries. The Miscellaneous data category includes problem descriptive information, the calculation sequence, recycle convergence options, flowsheet tolerances, and product stream scaling.

Problem descriptive information include project name, problem name, user name, date, site, and problem description. Supply this information in the Problem Descriptive Information Window, which is accessed by selecting the submenu item Problem Description under the menu bar item Input, or by pushing the button on the toolbar.

For most problems, the calculation order determined by PRO/II is satisfactory. To supply your own sequence, push the button on the toolbar.

Definition of recycle loops is automatic in PRO/II. To define your own loops, or use acceleration techniques, push the button on the toolbar to enter the Problem Recycle Convergence and Acceleration Options Window.

Flowsheet tolerances are used for convergence of unit operation specifications and may be changed in the Default Unit Specification Tolerances Window, which you may reach by choosing the submenu item Flowsheet Tolerances under the menu bar item Input.

All flowsheet results may be scaled such that a desired flow is obtained for a product stream. To use the scaling feature, select the menu item Output, then select Define Report, and finally, Miscellaneous Data. Push the Product Stream Scaling button on the Miscellaneous Report Options Window to access the Scale Stream Flowrate Window.

Data Supplied by Default:

To simplify the input process, default options and values are provided wherever it is practical or possible. Default values are displayed with a green border, or in the case of linked text, are printed in green. For example, the default number of iterations for an Inside-Out algorithm column unit operation is supplied as 15. You must supply data for entries that are required but have no default value; these are indicated with the color red.

While you do not need to replace a default entry to satisfy the input requirements, default data should be inspected carefully to ascertain that they meet your requirements for the calculations. When you replace a default value, the border color for the data entry field changes to blue, indicating that this is a value you have supplied. For linked text strings, the color of the linked text is also changed to blue, indicating that you have replaced the default value.

Optional Data:

Optional data are indicated with the color black. These are data or options not necessary for the unit operation calculations to proceed. For example, the Description entry is optional for all of the unit operations. A reboiler is optional for the column unit operation since the calculation requirements may also be satisfied by a vapor feed to the bottom tray of the column.

Data options which do not apply to a particular combination of input data are marked in the color gray, and not available for data entry. For example, when the kettle reboiler option is selected for a column reboiler, the data entry fields for a thermosiphon reboiler are colored gray and not available for data entry.

Executing the Simulation:

The simulation cannot be run until all flowsheet topology has been correctly defined, all required unit operation and stream input have been supplied, and the component data and thermodynamic methods have been defined. Completeness of these data is indicated visually by the absence of red push buttons on the toolbar and red stream or unit labels.

To execute the flowsheet, push the Run button on the Run toolbar. PRO/II first performs the Check Data cross checks on the simulation data and, if no errors are found, the flowsheet calculations are begun. If errors are found, they are viewed by pressing the Status button or by selecting the Status option on the View menu. When the errors have been corrected, push the button again to execute the flowsheet.

You may wish to interact with the flowsheet calculations. You may do this by stepping through the calculations one unit at a time with the Step button on the Run Toolbar, or you may set calculation "breakpoints" in the flowsheet. Calculations are halted at the "breakpoints" until you restart them by pushing the Run or Step buttons. Thus, you can halt the calculations, inspect the results, change unit and/or stream data, and then resume the calculations.

To set breakpoints, push the Set Breakpoints button on the Run Toolbar to enter Breakpoint mode. The Breakpoints Window will be displayed giving a list of the units in the flowsheet. To set a breakpoint at a unit, position the cursor on the unit icon and press the left mouse button. The selected unit is highlighted in the list box in the Breakpoints Window and the icon fill color is changed to purple, indicating a breakpoint. To delete a breakpoint, select the unit and click the left mouse button again.

You may add new units and/or streams to the flowsheet and then push the Run button to run the revised flowsheet. Note that if you restart the calculations by pushing the Run button, they are started from the beginning of the flowsheet by default.

You may wish to restart the calculations at a particular unit operation after you have made changes to the unit and/or stream data or to the flowsheet topology. To mark a unit operation as the first unit to recalculate, select the unit and then push the Goto button on the Run Toolbar. Push Run to restart the flowsheet calculations at the marked unit, or the Step button to calculate the marked unit only.

Calculations may also be halted by pushing the button on the Run Toolbar or by clicking the Pause/Break key. PRO/II will stop the calculations at an appropriate juncture and mark the current unit for calculation when calculations are restarted. However, if you make changes to the flowsheet topology or input data, the calculations are restarted at the beginning of the flowsheet unless the Goto feature is used as described above to mark a desired starting unit for the calculations.

PRO/II uses a color code to mark the calculation status of the unit operations:

Yellow : No calculations performed yet.

Green  : Unit is being calculated.

Blue    : Unit calculations were performed successfully.

Red     : Unit calculations failed.

Reviewing Simulation Results:

The Messages button on the Run Toolbar allows you to display a window that shows convergence information as the flowsheet calculations proceed. The information displayed in this window is updated continuously during the calculations. The Messages Window will remain visible until it is closed using the control menu box in the upper left hand corner of the window.

You may view selective flowsheet results at the completion of all calculations or at any halt or breakpoint in the calculations. To view the results for an individual unit or stream, place the cursor on the unit or stream and click the left mouse button. The unit or stream color changes to brown, indicating selection. Next push the View Text Results button on the Run Toolbar to generate a file containing a standard output report for the selected unit or stream. This file will be displayed in the default editor. Alternatively, select the desired unit operation and right-click. From the icon menu that is displayed, select the View Results command. For columns, the right-click action provides two View Results choices: "View Results (Molar Units)" and "View Results (Mass Units)".

Note: Currently this feature is available for columns and side columns only and it is not available for shortcut column, BATCHFRAC, and batch distillation column.

A report containing complete results for the flowsheet may be generated by pushing the button on the toolbar or by selecting menu bar item Output and then selecting the Generate Text Report option. The View Text Report option under Output menu may then be used to view the report with a text editor.