1.3 Dynamic Simulation
1.3.3 Using the Dynamics Assistant
Before you can run the simulation case in Dynamic mode, the degrees of freedom for the flowsheet must be reduced to zero by setting the pressure-flow specifications. It is also necessary to size the existing valves, vessels, coolers, and heat exchangers in the Main Flowsheet and the Column Sub-flowsheet. The following sizing parameters must be specified for these unit operations:
The Dynamics Assistant makes recommendations as to how the
flowsheet topology should change and what pressure-flow specifications are required in order to run a case in Dynamic mode. In addition, it automatically sets the sizing parameters of the equipment in the simulation flowsheet. Not all the suggestions that the Dynamics Assistant offers need to be followed.
The Dynamics Assistant will be used to do the following:
Unit Operation Sizing Parameter
Valves Cv value
Vessels Volume
1. Click the Dynamics Assistant icon in the toolbar.
Green checkmarks appear in the Make Changes column beside all recommendations by default. You can choose which
recommendations will be executed by the Assistant by activating or deactivating the checkboxes beside each recommendation.
Browse through each tab in the Dynamics Assistant view to inspect the recommendations.
2. Click the Streams tab.
Figure 1.144
Figure 1.145
Dynamics Assistant icon
The Streams tab contains a list of recommendations regarding the setting or removing of pressure- flow specifications in the flowsheet.
3. For each page in the Streams tab, set the following
recommendations as active or inactive according to the table shown below:
4. Click the Pressure Flow Specs tab.
This tab contains a list of unit operations which can use a Pressure Flow or Pressure Drop (DeltaP) specification. Typically, all unit operations in Dynamic mode should use the Pressure Flow specification.
5. Ensure that all the recommendations in this page are active:
Tab [Page] Recommendation Stream OK Checkbox
Streams
[Pressure Specs] Remove Pressure Specifications Feed 1 Active Set Pressure
Specifications LiquidProdSalesGas ActiveActive Streams [Flow
Specs] Remove Flow Specifications Feed 1Feed 2 InactiveInactive Streams [Insert
Valves] Insert Valves Feed 1Feed 2 InactiveInactive
Ovhd Inactive
Streams [Int.
Flow Spec] Set Internal Flow Specification Reflux Active
Figure 1.146
Tab [Page] Recommendation Unit Operation OK Checkbox
Pressure Flow Specs Pressure Flow Spec Chiller Active If some of the columns
or rows on the pages are not visible, use the scroll bars beside or under the information area to bring the columns or rows into view. An active recommendation will be implemented by the Dynamics Assistant. An inactive recommendation will be ignored by the Dynamics Assistant.
6. Click the Unknown Sizes tab and select the Volumes page.
You can modify any of the default sizing parameters in the Unknown Sizes tab. Once you modify a sizing parameter, the piece of
equipment is automatically sized and the volume, Cv, or k-value displayed.
7. For each page in the Unknown Sizes tab, ensure that all the recommendations are active:
8. Click the Tray sections tab.
The Tray sections tab identifies tray sections and streams whose total steady state pressure drops are inconsistent with the total pressure drop calculated according to the dynamics rating model. For the purpose of this tutorial, recommendations on this tab will be ignored.
Figure 1.147
Tab [Page] Recommendation Unit Operation OK Checkbox
Unknown Sizes [Volumes]
Vessel Sizing Chiller Active Gas/Gas (Tube) 1 Active Gas/Gas (Shell) 2 Active
Unknown Sizes [k values]
Heat Exchanger
Sizing ChillerGas/Gas (Tube) ActiveActive
Gas/Gas (Shell) Active The Unknown Sizes tab
contains a list of the unit operations in the flowsheet that require sizing.
• The Valve operations are sized based on the current flow rate and pressure drop across the valve. The valves are sized with a 50% valve opening.
• The Vessel operation volumes are determined based on the liquid exit volumetric flow rate and a 10-second residence time. • The Heat Exchanger
operations are sized based on the current flow rate and pressure drop across the equipment.
9. Click the Other tab.
10. Activate the following recommendations:
11. Click the Make Changes button once.
All the active suggestions in the Dynamics Assistant are implemented.
12. Close the Dynamics Assistant view.
13. Switch to Dynamic mode by clicking the Dynamic mode icon. When asked “Are you sure you want to switch to dynamics?”, click the Yes button.
Since you deactivated the suggestion to insert a valve on the Ovhd stream, you must set a pressure-flow specification on this stream.
14. In the PFD, double-click the Ovhd stream icon to open stream property view.
15. Click the Dynamics tab, then select the Specs page.
16. Activate the Pressure specification by selecting the appropriate checkbox.
The Pressure specification should be the only specification active.
Figure 1.148
Tab [Page] Recommendation Unit Operation OK Checkbox
Other [Misc
Specs] Set Equalize Option Mixers Mixer-101 Active
You can enter the Ovhd stream pressure specification in either the Main Flowsheet environment or the Column Sub- Flowsheet.
The Other tab contains a list of miscellaneous changes that should to be made in order for the Dynamic simulation case to run effectively.
Ensure that the Ovhd Molar Flow specification is inactive.
17. Close the Ovhd property view.
18. In the PFD, double-click the Heater icon to access the property view.
19. Click the Dynamics tab, then select the Specs page.
20. In the Model Details group, select the Product Temp Spec radio button.
21. Close the view.
22. Save the case as DynTUT1-3.usc.
Figure 1.149
Figure 1.150
You can specify the exit temperature of the Heater operation in Dynamic mode. The duty of the heater is back- calculated to make the temperature specification.