STOCK CHEST CONSISTENCY CONTROL: A STRATEGY FOR A WIDE DEMAND RANGE

摘要

Throughout pulp and paper processing, the single most important variable to measure and control is pulp or stock consistency. Much has been written and said about how to do it, and many of the suggested methods are based on what works rather than on an analysis of the problem and proposed solutions. One of the very demanding areas is the control of the output consistency of specialty stock chests, because there can be a large range of flow demand that makes many of the strategies unworkable or difficult. The chests involved include the high density (HD) chest and the low density (LD) chest for stocks such as recycle pulp which may be used in small or large proportions depending on the machine and the grade being run on the machine. In this paper, several different strategies are considered and analyzed using first principles models. What is of interest is the performance of the consistency control when the demand for stock is low resulting in excessive dead time and high gain hence making good control more difficult. For purposes of analysis, even though the zone dilution flow to the high density (HD) chest, which could be quite large (but varies mostly due to demand) was assumed to be zero, while there is no zone dilution on a low density (LD) chest. Four strategies were considered: conventional consistency control (with no recirculation), dilution flow ratio control (with no recirculation), consistency control with a recirculation flow back to the inlet of the stock pump that is controlled to maintain constant pressure, and consistency control with a recirculation flow back to the stock chest. Convention control wisdom suggests that the dynamics of the last two options are very difficult to control well. However, the investigation results support the practice of recirculating the stock back to the chest mixing zone. This strategy is also shown to be superior to conventional consistency control using an adaptive controller.