Abstract
This paper introduces a novel MILP approach for the design of distillation columns sequences of zeotropic mixtures that explicitly include from conventional to fully thermally coupled sequences and divided wall columns with a single wall. The model is based on the use of two superstructure levels. In the upper level a superstructure that includes all the basic sequences of separation tasks is postulated. The lower level is an extended tree that explicitly includes different thermal states and compositions of the feed to a given separation task. In that way, it is possible to a priori optimize all the possible separation tasks involved in the superstructure. A set of logical relationships relates the feasible sequences with the optimized tasks in the extended tree resulting in a MILP to select the optimal sequence. The performance of the model in terms of robustness and computational time is illustrated with several examples.
Highlights
Distillation is the most widely used method in modern chemical industries to separate liquid mixtures into pure components
The thermally coupled distillation (TCD), in which heat is directly transferred between columns through two streams that substitute a condenser or a reboiler, has received considerable attention
The goal of this paper is to present an MILP approach for synthesizing TCD sequences, maintaining the rigorous of existing MINLP models
Summary
The model of thermally coupled distillation systems lead to complex nonlinear and non-convex MINLP models. For each separation task we obtain the minimum and actual number of trays, the number of trays in each section (feed tray location), the actual vapor and liquid flows in both the rectifying and stripping sections, the relative volatilities, the diameter of each column section, the heat load in condenser and reboiler (if necessary), the equivalent thermal state of distillate and bottoms if a thermal couple appears (equations (1) and (2)), the area of the reboilers and condensers, the investment cost of columns and heat exchangers and the operating costs. It is necessary to calculate the investment and operating conditions of the extra heat exchangers introduced in the connection points to correct the imbalance in vapor and liquid flows of different column section (figure 6). The interested reader is referred the original work for further details
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