Dynamics Fundamental Principles
This documentation is incomplete and at an experimental stage. Back to Dynamics Documentation
Introduction
This documentation will help you understand some of the basic principles of dynamics models.
Steady State versus Dynamics
It is important to understand the difference between a steady state Petro-SIM model and a dynamics mode model. A steady state model generally solves one unit operations at a time, sequentially. Dynamics mode employees a hybrid approach, where certain aspects of the entire simulation model are solved simultaneously.
This difference can be significant. For example, consider a pipeline and a valve in series:
Assume the pressures of the Feed and Product streams are known. Because steady state solves one unit operation at a time, it is unable to solve for the flow rate without the use of iteration. But because dynamics mode solves the hydraulics portions of the flow sheet simultaneously, it can easily solve this problem.
In dynamics mode, pressures are usually only specified on boundary streams (feed and product streams) with the pressures and flow rates inside the model then a function of the equipment and contents of that equipment over time. Flow rates can be specified as well, but that is only occasionally done for a feed stream.
Material generally flows in a certain direction at a certain rate because there is a pressure gradient (driving force) and some sort of resistance (e.g. a pipe). It never flows against a pressure gradient and there is usually some sort of resistance.
A dynamics model tends to adhere closely to these concepts. It is possible to set up a model where these fundamentals are ignored for simplicity sake (e.g. flow with no pressure gradient, or excluding valves and such from a model, commonly done in steady state), but such details can be important if realistic behavior and responses are desired.
Another difference is that a dynamics model has additional degrees of freedom that are not present in steady state. In steady state the flow rate into a vessel usually equals the flow rate leaving it, but in dynamics that is not true, because the level and pressure of the vessel can also change as a function of the flows.
Vessel
To further illustrate these concepts and introduce additional ones, consider this case:
In steady state the feed to the vessel would be fully specified, and assuming that the pipe and valves are configured, that would determine the flows and pressures throughout the model.
In dynamics model, the situation is far more complex. Assume that the vessel feed flow rate is specified, its vent stream pressure is specified, and the valve outlet pressure is also specified. The flow rate out of the vessel through the pipe will be determined by the pressure gradient between the vessel and the valve outlet stream. That pressure gradient may also include static head contributions that depend on the level inside the vessel. If the feed flow rate is increased or the valve is closed, then the level inside the vessel will increase. Eventually liquid will start flowing out of the vent stream, as it would in real life. If the flow is changed to be a small amount of vapour feed only, then eventually all the liquid may drain out of the vessel and vapour will start flowing through the pipe.
Note that in steady state one typically specifies the pressure drop across a valve, but such a specification is generally unrealistic in dynamics mode. Typically a valve is sized, which establishes a relationship between the flow rate and pressure gradient across the valve. The valve can be viewed as a flow resistor. Many pieces of equipment (heat exchangers, pipes, valves etc.) all model flow resistors. Equipment such as vessels or tray sections also model material holdup (contents).
Nozzles and Holdup
Consider the vessel in the case above. What flows in or out of the streams connected to it?