Self-consistent inventory control

Inventory or material balance control is an important part of process control. In the literature, many rules have been proposed to held in designing such systems, but their justification is often unclear. The main contribution of this paper is to proposed the more general selfconsistency rule for evaluating inventory control systems. Consistency means that the steadystate mass balances (total, component and phase) for the individual units and the overall plant are satisfied. In addition, self-consistency is a desired property, meaning that the mass balances are satisfied locally with local inventory loops only. Introduction One of the more elusive parts of process control education is inventory or material balance control. An engineer with some experience can usually immediately say if a proposed inventory control system is workable. However, for a student or newcomer to the field it is not obvious, and even for an experienced engineer there may be cases where experience and intuition are not sufficient. The objective of this paper is to present concise results on inventory control, relate to previous work, tie †Norwegian University of Science and Technology ‡Current address: StatoilHydro Research Center, Trondheim, Norway 1 Elvira Marie B. Aske et al. Self-consistent inventory control up loose ends, and to provide some good illustrative examples. The main result (self-consistency rule) can be regarded as obvious, but nevertheless we have not seen it presented in this way before. The main result is a simple rule to check whether an inventory control system is consistent. Here, consistency means that the mass balances for the entire plant and units are satisfied.1 In addition, we usually want the inventory control system to be self-consistent. Self-consistency means that consistency is satisfied locally, without the need to rely on control loops outside the unit. Consistency is a required property, because the mass balances must be satisfied in a plant, whereas self-consistency is a desired property of an inventory control system. In practice, an inconsistent control structure will lead to a situation with a fully open or closed control valve and the associated control loop cannot fulfill or attain the control set point.2 In most plants, we want the inventory control system to use simple PI controllers and be part of the basic (regulatory) control layer. This is because it is generally desirable to separate the tasks of regulatory (stabilizing) control and supervisory (economic) control. From this it follows that the structure of the inventory control system is usually difficult to change later. The importance of consistency of inventory control structures is often overlooked. Our work is partly inspired by the many examples of Kida, who has given industrial courses in Japan on control structures for many years. In a personal communication2 he states that “most process engineers, and even academic people, do not understand the serious problem of inconsistency of plantwide control configurations. When writing a paper, you have to clearly explain this point and make them convinced at the very outset. Otherwise they will not listen to or read through your detailed statements, but skip them all”. A very good early reference on inventory control in a plantwide setting is Buckley .3 He states that material balance control must be in the direction of flow downstream a given flow and opposite the direction of flow upstream a given flow. Price et. al1,4 extended this and state that the inventory control must “radiate” outwards from the point of a given flow (throughput manipulator). As shown in this paper, these statements are a consequence of requiring the inventory control system to be self-consistent.