Cascade without reflux

Handbook of Industrial Membrane Technology Cascades Without Reflux... 

Such cascades are only sensible in cases where the retentate is practically worthless. Norsk-Hydro, for example, is operating an electrolysis-heavy-water-production-plant where the deuterium is separated in a cascade without reflux (at least in the lower part of the cascade). Figure 6.14 illustrates the principle of a cascade without reflux. 

Analyze countercurrent extraction cascades without reflux. 

Whereas in ordinary countercurrent operation the richest possible extract product leaving the plant is at best only in equilibrium with the feed solution, the use of reflux at the extract end of the cascade can provide a product even richer, as in the rectifying section of a distillation column. Reflux is not needed at the raffinate end of the cascade since, unlike distillation, where heat must be carried in from the reboiler by a vapor reflux, in extraction the solvent (the analog of heat) can enter without a carrier stream. 

In Process Design and Optimization, R. Rautenbach explores the uses of cascade operations, without and with reflux, and of operations as a continuum. These and other embodiments, and their modes of operation and calculation, are detailed in subsequent chapters. 

If distillate flow is to be manipulated for control of composition, reflux must be a dependent variable in all cases except that shown in Fig. 11.10. In operation without a flooded condenser, then, liquid level in the overhead accumulator must manipulate reflux. As mentioned in Chap. 3, control of the level of boiling and condensing liquids is complicated by transport and thermal problems. Furthermore, the need for heavy damping requires a wide proportional band, and subsequently, reset. It is therefore absolutely essential to close a loop around the control valve otherwise hysteresis will promote an intolerable limit cycle. Either a valve positioner or a cascade flow loop will suffice for this purpose. 

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