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The design of two-phase liquid-liquid separation systems

Michael Minotti, University of Massachusetts Amherst


The conceptual design procedure is used to quickly and efficiently develop and evaluate chemical process flowsheets. For two-phase liquid-liquid systems where extraction may be a process alternative, there are no quick and efficient design techniques which can be applied to a wide range of design problems. In this dissertation, we describe such techniques for the design of two-phase liquid-liquid systems. A method for the quick screening of conceptual design alternatives for liquid extraction processes is demonstrated. As an example, we consider the extraction of acetic acid from water using diethyl ether as a solvent. We begin by generating two feasible extraction flowsheets based on an analysis of the phase behavior. Design estimates are made using recently developed geometric methods. An economic analysis shows a significant incentive for both flowsheets as an alternative to conventional binary distillation. Using a rank order analysis, we find that the fractional recovery of solvent is the most important design variable in the extraction flowsheets. Surprisingly, we find that the more complex flowsheet is the more economically attractive, because it avoids the difficult separation of small amounts of acetic acid from water by distillation. We design the liquid extractors using a new method which exploits the geometric properties of the composition profiles in a multistage countercurrent arrangement. These profiles are found by solving the extractor model equations in the form of an initial value problem. An analysis of the fixed points or pinches in the profiles gives the minimum solvent flow requirement. Profiles at solvent flows above the minimum are guaranteed to be feasible and are to be used to find the equilibrium stage requirements and product distributions. The fixed points of the extractor model equations can be determined without performing stage-to-stage balances, and a comprehensive picture of the solutions and tradeoffs in extractor design can thus be determined efficiently. This approach differs from the more classical techniques in that it is applicable to mixtures with more than three components as illustrated in an example. A general method for the design of liquid extraction-reaction cascades is described. Composition profiles are determined by solving the reactor-extractor model equations, and reactive-phase holdups are determined as a function of reactant conversions, number of reactor-extractors and solvent flowrate. These methods are intended for conceptual design, e.g. to determine the incentive for using a solvent in a reactive system to extract product from the reactive phase. The production of butyl acetate by the esterification of acetic acid with butanol is examined to demonstrate the method. For this reaction system, a cocurrent cascade arrangement is more suitable than the countercurrent arrangement. The example shows that the solvent selected for a reaction/extraction system must preferentially extract products and not reactants from the reactive phase in order to achieve good performance.

Subject Area

Chemical engineering

Recommended Citation

Minotti, Michael, "The design of two-phase liquid-liquid separation systems" (1998). Doctoral Dissertations Available from Proquest. AAI9823757.