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080172Bridge the Gap

080172Bridge the Gap - Reactions and Separations Gap with...

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72 www.cepmagazine.org August 2001 CEP Reactions and Separations ith new high-value, small-demand, specialty chemicals continually com- ing to market, there is an increased de- sire to create adaptable processes that enable the manufacture of a broad array of chemi- cals. Semicontinuous distillation meets this need in an efficient manner. This technology enables the separation of a wide variety of mixtures that may contain a large number of chemical species and ex- hibit complex thermodynamic behavior. Traditionally, distillation processes have been designed to operate either in batch mode, where a still is charged and light to heavier species are suc- cessively removed in the distillate, or to run contin- uously, where a column or a series of columns are kept near steady state to separate a feed mixture. In the former approach, process efficiency is sacri- ficed for separation flexibility, whereas in the lat- ter, the reverse applies. Semicontinuous distillation represents a depar- ture from these traditional approaches. Consider, for example, the column and tank configuration in Figure 1. By using this arrangement with a semi- continuous policy, a wide variety of chemical mix- tures can be separated efficiently. Mixtures that can be split include binaries, near-ideal ternaries, low- boiling azeotropes, and, with the use of an addi- tional product tank, near-ideal quaternaries. In semicontinuous distillation, the column, re- boiler and condenser operate throughout the cam- This processing strategy combines the advantages of batch and continuous operation, providing flexibility and efficiency in a single design. Bridge Semicontinuous Distillation with Gap the W James R. Phimister and Warren D. Seider, University of Pennsylvania T 1 D 1 S 1 S 10 S 9 S 2 S 6 S 7 S 8 S 4 S 3 S 5 T 2 T 3 T 4 T 5 Figure 1. Column and tank configuration for semicontinuous distillation.
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CEP August 2001 www.cepmagazine.org 73 paign, with a feed stream added constantly, while several modes of operation are used cyclically. Each mode often involves different feed tanks, feed compositions or prod- uct vessels. These campaigns are, in principle, quite sim- ple, and many have been found to be effective with unex- pected consequences ( 1–5 ). Note that, prior to the publi- cation of these papers, to our knowledge, no literature ex- isted on semicontinuous distillation. Two developments led to our close examination of semicontinuous distillation. The first involved the intro- duction of cyclic, batch-distillation processes, as exempli- fied by Sørensen and Skogestad ( 6 ), who showed how to optimize a batch distillation process in which the reflux drum was charged and discharged cyclically to reduce the time to achieve high-purity product. The second development was the use of new batch col- umn configurations, including middle-vessel columns (MVCs) ( 7 ), multi-vessel columns ( 8 ), and heat-integrat- ed batch columns ( 9 ).
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080172Bridge the Gap - Reactions and Separations Gap with...

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