Saturday, March 7, 2009


Integrating process and operational factors into an overall design strategy requires a deep insight into the multivariable functional relationships which characterize the system so that optimal decisions can be taken. Specifically, multicomponent separations processes highlight these issues because of the abstract nature of the problem which is difficult to interpret. These relate operational states to the associated phase equilibria in an operational phase space which projects an image of the relative enrichment so that the influence of operational and design variables can be assessed.The work reported here presents the development of an approach to interactive computer based design of multistage separation processes and analysis, using projections of the operating line and phase equilibrium relationship into operational subspaces. It is illustrated with reference to ternary systems and demonstrates that it can be used to overcome the intrinsic restrictions of the conventional triangular diagrams which do not emphasize the relationship between vapour and liquid compositions resulting in a loss of important information. Without this facility it is difficult to determine what design decisions are required to improve performance since it is not possible to see the overall fundamental pattern of interactions between the variables at a glance. The objective is to provide a framework which enable the variables in both liquid and vapour phase to be displayed simultaneously. It is shown that this provides a means of identifying features which are crucial in design, particularly the number of stages, feed position, sidestream position and reflux ratio. The general shape of the equilibrium surfaces and the relative positioning of the operating line trajectories in terms of design and operational variables gives very useful picture of the patterns of behaviour as optimal conditions are approached.A modified interpretation of minimum reflux ratio is proposed which takes account of the partial specification of product streams and which constrain the operational trajectories. Because product specifications of multicomponent systems only partially define acceptable compositions, the concept of minimum reflux ratio is therefore not well-defined and is constrained by operational and design factors. The interactive design environment makes it possible to develop heuristic rules which facilitate the rapid identification of characteristic local points in the operational phase space which define the principal factors governing the overall performance as well as determining flexibility and controllability.
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