Plant optimization hierarchy

Planning and scheduling. This module corresponds with the business part of the control hierarchy. It has a business model and based on plant data (current and past values), on external data (market data, external plant info, etc.) and using the company business goals derives a production plan for the plant. It gives capacity production values as well as quality values to the lower, optimization, level. The resolution time at this level is days or weeks. 

Base layer control can be complex, based on the specific process requirements, with both feed-forward and feedback loops, sequence control, advanced inventory control, balancing control and various types of constraint and override control. The scope of the regulatory control of the Pearl GTL plant is large as approximately 9000 manipulated variables , i.e. control valves, motors, fin-fans, etc. are used to maintain stable operation at target. With regulatory control only (i.e. without other elements in the control and automation hierarchy) the plant should be able to operate smoothly and reject disturbances, although not necessarily in the most optimal fashion. 

Real-time optimization can be employed to determine the optimum controller set points for current operating conditions and constraints. The highest level of the process control hierarchy is concerned with planning and scheduling operations for the entire plant. The different levels of process control activity in the hierarchy are related and should be carefully coordinated. 

Once a conceptual control structure has been developed and the plant has been decomposed into subsystems, the control design procedure reverts to a traditional bottom-up approach. However, there are good reasons to treat the different control activities in a multilevel hierarchy, as shown in Fig. H.l. The first task in Step III is to identify the essential controllers, those that are absolutely required. The safety and regulatory levels in Fig. H.l enable safe and stable operation of the plant. The advanced control functions are handled at Level 3 and keep the controlled variables close to their optimum set points through standard methods such as cascade, ratio, feedforward, and multivariable control. Level 4 in Fig. H.1 considers the real-time optimization of the process operations. The purpose of control at this level is to choose operating conditions that meet overall objectives in an economically optimum fashion. 

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