Real Time Optimization

395 Previous chapters have considered the development of process models and the design of controllers from an unsteady-state point of view. Such an approach focuses on obtaining reasonable closed-loop responses for set-point changes and disturbances. Up to this point, we have only peripherally mentioned how set points should be specified for the process. The on-line calculation of optimal set points, also called real-time optimization (RTO), allows the profits from the process to be maximized (or costs to be minimized) while satisfying operating constraints. The appropriate optimization techniques are implemented in the computer control system. Steady-state models are normally used, rather than dynamic models, because the process is intended to be operated at steady state except when the set point is changed. This chapter first discusses basic RTO concepts and then describe typical applications to process control. Guidelines for determining when RTO can be advantageous are also presented. Subsequently, set-point selection is formulated as an optimization problem, involving economic information and a steady-state process model. Optimization techniques that are used in the process industries are briefly described. For more information, see textbooks on optimization methodology (Ravindran et al., 2006; Griva et al., 2008; Edgar et al., 2001). Figure 19.1 is a detailed version of Fig. 1.7, which shows the five levels in the process control hierarchy where various optimization, control, monitoring, and data acquisition activities are employed. The relative position of each block in Fig. 19.1 is intended to be conceptual, because there can be overlap in the functions carried out, and often several levels may utilize the same computing platform. The relative time scale for each level's activity is also shown. Process data (flows, temperatures, pressures, compositions, etc.) as well as enterprise data, consisting of commercial and financial information, are used with the methodologies shown to make decisions in a timely fashion. The highest level (planning and scheduling) sets production goals to meet supply and logistics constraints and addresses time-varying capacity and manpower utilization decisions. This enterprise resource planning (ERP) and the supply chain management in Level 5 refer to the links in a web of relationships involving retailing (sales), distribution, transportation, and manufacturing (Bryant, 1993). Planning and scheduling usually operate over relatively long time scales and tend to be decoupled from the rest of the activities in lower levels (Geddes and Kubera, 2000). For example, Baker (1993) and Shobrys and White (2002) indicate that all of the refineries owned …

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