Process Simulation – Part 4

In my March 14, 21, and 28th postings I summarized some of the basics associated with design and implementation of process simulation that are addressed in the book Control Loop Foundation – Batch and Continuous Processes. Simulations based on duplicating the process step response may be effectively used for operator training and control system check. However, this simulation technique assumes that the process behaves in a linear fashion. When the simulation is used over a wide operating range, the step response may not accurately show the impact of process non-linearity. However, the changes needed to compensate for process non-linearity may be easily added to a process simulation based on process step response.

The necessary corrections in process gain that are needed to account for process non-linearity are based on input/output relationships determined by doing an energy and/or mass balance for steadystate operation. From the simulation diagram, the inputs and outputs of a process are known. A process energy or mass balance is based on the fact that under steady-state operation, what goes into the process must come out. For example, when blending two flow streams using a mixer, the concentration of material dissolved or suspended in the liquid (expressed as the weight percent of the liquid steam) of the mixer outlet stream varies in a non-linear fashion with the inlet flow rates and inlet stream concentrations. The simulation diagram for a mixer process is shown below.

Mixer Process - Simulation Diagram.jpg

The outlet stream concentration may be calculated based on a mass balance around the mixer, and blocks may be added to provide a dynamic response to input changes. The outlet concentration may be calculated as follows:

As this shows, the process gain (i.e., the change in mixer outlet concentration for a change in the flow rate of inlet flow to the mixer) depends on the flow rate and the concentration of both streams. This nonlinear response can be included in the process simulation by calculating the outlet concentration based on the steady-state mass balance. Any transport delay or lag due to mixing may be accounted for using a deadtime and filter block in combination with this calculation as is illustrated in the mixer simulation composite as illustrated below.

The 19 workshops included on the Control Loop Foundation web site illustrate the fidelity of the simulated process response that may be achieved over a wide range of operation using these techniques.