Dynamics Assistant

Table of Contents

Introduction

The dynamics assistant can be activated by pressing its button on the button bar or by clicking Tools, Dynamics Assistant.

The main purpose of the dynamics assistant is to ensure that a sensible set of pressure flow specifications is selected and that new users are educated. After analyzing the case, it will recommend a list of changes along with an explanation of each group of changes. In addition, the assistant will also identify potential trouble spots in the case.

The recommendations of the assistant are intended to ensure that the case does not have too many or too few or incompatible stream pressure and flow or unit operation specifications. It looks at specifications which contribute equations to the simultaneous solution only, to ensure that the solver is able to solve and integrate.

For example, if you have a case with a valve and a feed and product streams, and you place a pressure specification on both streams and change the valve to use a pressure drop specification, the resulting equation set is not solvable. The dynamics assist would recommend changing the valve to use a pressure flow equation rather than a pressure drop equation.

The recommendations of the assistant may not always apply or be appropriate for every case. It serves as a learning tool for new users. When you no longer need the assistant, you can disable it via the preferences. The assistant is usually disabled by advanced users since it then no longer serves any purpose.

View

The General tab of the assist view looks as follows:

It shows a list of issues that have been identified in the current case. You can review what the assistant reports without making any changes. To have the assist make its recommended changes, click the "Make Changes" button. You can also select what groups of changes you want made. Click on the green check mark to enable or disable a group of changes.

You can double click on an item on the first page to see more details about it:

On the more detailed view you see exactly what the assistant wants to change. You can double click on a name to open the view for the affected unit operation. You can also uncheck the OK option for individual items if you do not want the assistant to change those items. You can also click on "Tell my why..." to get an explanation of why the assistant recommends the change.

Approach

The approach the dynamic assistant takes is loosely based on this philosophy:

Pressure specifications should be made on terminal streams only where the stream is exposed to a constant pressure source such as the atmosphere or infinite feed tank. The pressures of internal streams should be determined by the surrounding equipment. (One exception is the component splitter as explained in its section.)
Flow rates should only be determined by pressure differences or by equipment such as pumps or compressors, so flow rate specifications are generally discouraged. The assistant may select flow specifications for simple feed streams (vessel or column feeds) instead of adding equipment to keep the case simple.
Valves are inserted at the end of some lines to establish a link between pressure and flow rate. This is done in some situations where a terminal line has a pressure specification, but it may result in a singular equation set.
Unit operations set to use pressure drop specifications are changed to use flow equations, which provides a natural link between flow and a pressure gradient.
Valves with unknown sizes or equipment such as heat exchangers with unknown constants for the pressure flow relationship are sized.
Unit operations with unknown volumes are sized. Tray sections are also roughly sized.
Specifications inside pumps, compressors, valve etc. You may have too many, too few, or incompatible specifications selected.
The assistant also handles special cases as the component splitter, and warns about other incompatible issues detected in the case such as multiple stream connections (which are unrealistic in real life and hence not supportable in dynamics), unit operations that are not properly connected, unit operations that are part of a system but ignored which may result in the simultaneous equation set being unsolvable.

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