Enhanced Digital Control of a Mechanical Recompression Evaporator in a Modern Fructose Refinery

Fixed‐speed, motor‐driven recompression evaporators are inherently more difficult to bring up from a cold start than variable speed turbine‐driven ones. They also need more careful operator attention during throughout changes. These difficulties are mostly caused by the choking action of the motor driven compressor's main control device the variable turbine‐inlet guide vanes. Compression of fugitive water vapors so far suffers from another problem not encountered during turbo‐compression of gas or air: no reliable simple flow element for compressor discharge is in general use. Thus standard compressor surge control schemes involving measured flow will not work. The method presented involves a two‐tier hierarchial control scheme. The usual flow, pressure, temperature and density loops are directly controlled by electronic analogues of P‐I‐ D ‐controllers, some even having conventional cascade linkages. Those P‐I‐ D ‐controllers, in turn, have their setpoints coordinated by a supervisory master computer as though they were all cascaded. The results of the control scheme are intended to the following:Increased safety during start‐up because not only guide ‐vane opening but also heat‐up and purge‐out are monitored via a consistent algorithm, not left to an operator's whim. Reliable and safe turn‐down capability of the evaporator over a range of from full to about 1/3 design capacity. Reduced need for operator attention since the control scheme will handle most plant upsets by itself. It was mostly the latter of the hoped‐for advantages which prompted development of the method, since the user wanted to keep down the number of highly trained process operators needed to man the centrally‐located control room of the syrup refinery.