Multiple plantwide control

Chen, R., McAvoy, T.J., and Zafiriou, E. (2004). Plantwide control system design extension to multiple-forcing and multiple-steady-state operation. Ind. Eng. Chem. Res., 43, 3685-3694. 

Developing a plantwide control system for the Eastman process is fairly straightforward. There are only five unit operations and one gas recycle stream. No energy7 integration is present. So the major feature of this process from a plantwide viewpoint is the problem of accounting for the multiple component inventories. 

Plantwide control problems arise in the context of plants with recycles of mass and energy. Positive feedback effects complicate the dynamics because of interactions and non-linear phenomena, as multiple steady states and chaotic behaviour. 

This article considers processes involving two reactants and two reactions. It is demonstrated that plantwide control relying on self-regulation results in regions of state multiplicity or unfeasibility, even if the stand-alone reactor has a unique, stable operating point. Moreover, when selectivity reasons require low per-pass conversion, instability is very likely. 

One of the most basic issues in plantwide control is flow/inventory control. If a train of continuous processing units (reactors, columns, etc.) is considered, where should the production rate be controlled It can be controlled at the exit of the fine (e.g. a series of unit operations (as in Fig. G.la)), at the beginning of the line (Fig. G.lb), or at any point in between. In these figures, the sensors/transmitters have been omitted for clarity. It might seem logical to use a feed flow rate into each unit to control the inventory (level) in that unit as illustrated in the downstream method of Fig. Gla. However, as discussed below, adjusting each unit s effluent flow rate may be an easier way to control inventories if the flow rates of multiple streams into a unit are ratioed (see the upstream method in Fig. G.lb),... 

The most effective control structure is one in which the column feed is used to control temperature while recycling products back to the upstream feed unit. This strategy can be applied to multiple column sequences in a plantwide environment. 

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