In this course we consider lumped capacities where there are no spatial variations in conditions. In real systems there are usually significant temperature and concentration profiles within pieces of equipment. Where the object of a control system is control a temperature or concentration to a particular value , it is usually necessary to control the maximum or minimum value within a piece of equipment. This is done by taking measurements at several different places and then taking the maximum or minimum value and passing this to the control system. This is called auctioneering, or selective, control.
An example of the application of an auctioneering control system is in the control of an exothermic tubular reactor:
In these reactors the temperature varies along the length of the reactor, and there is usually a clear 'hot spot' somewhere in the reactor. If the objective of a controller is to control this maximum temperature then a temperature probe needs to be inserted at the appropriate location in the reactor. Unfortunately, things are never simple and the position of the 'hot spot' can move up and down the length of the reactor depending on the flows and compositions of the various streams. To detect the position of and to measure the hotspot, reactors of this sort normally have multiple probes along the length of the reactor. The problem is deciding which of these signals should be used for control. Nowadays, the solution is rather simple and merely involves feeding the temperature signals into a 'greater than' unit which extracts the largest value which is supplied to it. This value is then passed to the temperature controller.
In VisSim the same thing can be achieved using the 'Max' and 'Min' blocks in the 'Non-linear' library. Do NOT use the '>' or '<' in the 'Boolean' library, as these blocks perform logical comparisons and output 1 or 0.