Studies on controllability of integrated process systems

• In response to stricter requirements on increased efficiencyand reduced environmental impact, production plants in theprocess industries are becoming tightly integrated withextensive recycling of material and energy. This thesisconsiders some consequences of such recycling for the dynamicbehavior of process systems. The focus is on the dynamics ofmain importance for the ability to handle process systems usingfeedback control, i.e. the plant controllability.It is shown that material and energy recycling imposepartial feedback mechanisms in a plant, and that such partialfeedback can induce unstable zero dynamics in control relevanttransfer-functions. The presence of unstable zero dynamics willimply a hard limitation in the process controllability.Necessary and sufficient conditions for the existence ofunstable zero dynamics as a consequence of partial feedback arederived. The conditions are well suited for incorporation in aprocess design framework for dealing with controllabilitythrough process design. The derived results are of relevancealso to other systems with partial feedback, e.g. decentralizedfeedback control systems.The disturbance sensitivity of an integrated plant is to alarge extent caused by interactions between various processunits, imposed by recycle flows. In this thesis, a systematicmethod is proposed for modifying these interactions, so as toreduce the disturbance sensitivity to a desired level, usingstorage capacities integrated in the plant. It is shown thatthe minimal required capacity for a given disturbancesensitivity reduction is obtained with a plug flow (delay) tankintegrated in the recycle path, combined with a mixed tankcascaded with the plant. The delay tank serves to modify thefeedback properties imposed by the recycle flow, while themixed tank acts as a low-pass filter. It is shown that the sizeof capacities required with the proposed method can besignificantly smaller than that required by a traditionalcascaded buffer system. The use of integrated buffers with theaim of stabilizing unstable process systems is alsoaddressed.The last part of the thesis considers the effect of processintegration on the controllability of a bleach plant in thepulp and paper industry. A flexible dynamic model, allowing forvarious degrees of integration to be studied, is constructedand calibrated against available data. A controllabilityanalysis reveals that, while the low-frequency disturbancesensitivity is significantly increased by the integration, thecontrollability is only slightly affected. A systems analysis,based on decomposing the overall model, is used to explain theresults of the controllability analysis.

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