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Distillation is the most common unit operation for separating liquid mixtures into valuable and/or high purity products. It is also one of the most energyintensive operations. Hence, optimization of distillation-column design and operation should get high priority.
Numerous distillation heuristics (rules of thumb) for quick optimization have emerged over the years. For instance, heuristics on optimal reflux ratio as a certain multiple of the minimum reflux ratio have been widely used as quick tools to estimate optimum reflux ratio.
However, changes over time in the relative cost of equipment and energy (which affects operating cost) can affect the validity of such rules of thumb. Meanwhile, it has now become more feasible to assess their validity, as today's availability of commercial simulators and high-speed computers allows rigorous and thus more-accurate distillation calculations be carried out with relative ease.
This article assesses the validity of optimal-reflux-ratio and other heuristics in light of recent cost data, by considering seven binary and six multicomponent systems. Distillation columns for each of the 13 have been designed and optimized by both shortcut (heuristics-based) calculations and rigorous simulations. In addition to the reassessment, a key observation emerges: that the cost of a column designed by shortcut calculations can be reduced substantially by optimizing the location of the feed stage.
Laying the groundwork
The reflux ratio is a key variable, affecting both the capital cost and the operating cost of a column. As the reflux increases, the number of stages and the column height both decrease but the flowrates in the column and, consequently, its diameter increase.
Despite that diameter increase, the capital cost of the column generally decreases as the reflux increases, because the savings in tower height more than offset the cost of the increase in diameter. However this is not the case at very high reflux ratios. And as alternatives having successively higher reflux ratios are compared with each other, there is a particular, high ratio at which the capital cost of the column begins to rise again [7J. In addition, the capital as well as the operating costs for the reboiler and condenser will rise in proportion to the vapor rate in the column.
Column optimization, therefore, reflects a balance between (1) the capital cost, which decreases (to a...