Self-consistent inventory control

Inventory or material balance control is an important part of process co ntrol. In the literature, many rules have been proposed to held in designing such systems, bu t their justification is often unclear. The main contribution of this paper is to proposed the more ge neral selfconsistency rule for evaluating inventory control systems. Consistency me ans that the steadystate mass balances (total, component and phase) for the individual units a nd the overall plant are satisfied. In addition, self-consistency is a desired property, meanin g 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 sa y if 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 inve ntory 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 contro l up loose ends, and to provide some good illustrative example s. The main result (self-consistency rule) can be regarded as obvious, but nevertheless we have no t s en it presented in this way before. The main result is a simple rule to check whether an inventory c ntrol system isconsistent . Here, consistency means that the mass balances for the entir 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 nee d to rely on control loops outside the unit. Consistency is a required property, because the mass ba lances must be satisfied in a plant, whereas self-consistency is a desired property of an invent ory control system. In practice, an inconsistent control structure will lead to a situation wit h a fully open or closed control valve and the associated control loop cannot fulfill or attain the cont r l set point. 2 In most plants, we want the inventory control system to use si mple PI controllers and be part of the basic (regulatory) control layer. This is because it is g enerally desirable to separate the tasks of regulatory (stabilizing) control and supervisory (econom ic) control. From this it follows that the structure of the inventory control system is usually difficu lt to change later. The importance of consistency of inventory control structu res is often overlooked. Our work is partly inspired by the many examples of Kida, who has given in dustrial courses in Japan on control structures for many years. In a personal communication 2 he states that “most process engineers, and even academic people, do not understand the serious prob lem of inconsistency of plantwide control configurations. When writing a paper, you have to clea rly explain this point and make them convinced at the very outset. Otherwise they will not list en to or read through your detailed statements, but skip them all” . A very good early reference on inventory control in a plantwi de setting is Buckley . 3 He states that material balance control must be in the direction of flow d nstream a given flow and opposite the direction of flow upstream a given flow. Price et. al 1,4 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 requiri ng the inventory control system to be self-consistent.