A shortcut procedure to design sidestream distillation columns Is presented. Simple expressions are obtained for the number of trays, the minimum vapor rate, and for the bounds Imposed by thermodynamics on the sidestream composition. Degrees of freedom and feasible design specifications are also discussed. Three types of sidestream columns are examined for a ternary mixture. We also suggest a procedure for quaternaries and extensions to more than four-component ideal mixtures.
A shortcut procedure to evaluate the minimum vapor flows in distillation columns with a sidestream stripping section is presented. The method Is simple and highly accurate for relatively ideal mixtures. The case of a ternary mixture is discussed, but the algorithm readily extends to mixtures with any number of components. Also, the approach can be generalized to other types of complex columns as well.
A quantitative criterion for the selection of simple distillation sequences is derived for ideal mixtures. A simple cost model, along with a shortcut solution of Underwood's equations, gives an analytical form for the total vapor rate, which is the key design variable. The results for column sequencing that are based on the analytical criterion agree well with more exact solutions, but they indicate that in numerous situations the commonly accepted heuristics are incorrect.
SCOPEThe synthesis of a distillation separation sequence is often a key step in the invention of a process flowsheet. Heuristics are normally used for this step in the process synthesis. However, some heuristics, such as "remove the lightest component first," depend only on volatility, whereas other heuristics, such as "remove the most plentiful component first," depend only on feed composition. Thus, it should not be surprising that the previously published heuristics can be contradiktory.Since we expect that the selection of the best separation sequence must depend both on relative volatility and feed composition, we need to have a criterion available that includes both quantities. A few functions of this type have been proposed (Lu and Motard, 1982; Tedder and Rudd, 1978), but the functional forms chosen are based on intuition. As an alternate approach, we show how an appropriate functional form can be derived that is based on approximate solutions of the design equations and an order of magnitude analysis of the cost equations. The results indicate that the new analytical approach gives better predictions than the qualitative heuristics.
CONCLUSIONS AND SIGNIFICANCEConflicts and inaccuracies in heuristic or rank-order approaches to the selection of a distillation sequence can be eliminated using the analytical criterion developed here. The approximate, but accurate, solution of the design equations, along with a simple cost model, are sufficient for process synthesis. For ternary mixtures separated in a simple column sequence, the optimal sequence and, more importantly, the sensitivity of this decision to the composition and thermodynamic information are shown by Eq. 23. Mixtures containing more components can be easily investigated with similar expressions.
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