Liquid countercurrent cascade with

Plachco, F.P. and Lago, M.E. 1975. Solid-Liquid Extraction in Countercurrent Cascade with Retention of Liquid by the Solid-Slab Geometry-Constant Diffusivity, hid. J. Technol., 13 438 145. 

In our schematic of a countercurrent cascade, we had the streams moving horizontally. Generally, a pump would then be required between each stage to overcome friction. With vapor and liquid, the density difference is sufficiently large, that we can use gravity to move the fluids if we stack the stages vertically rather than horizontally. A vertical stack of stages is called a tray tower. 

Example 15.5. The separation of benzene B from n-heptane H by ordinary distillation is difficult. At atmospheric pressure, the boiling points differ by 18.3°C. However, because of liquid-phase nonideality, the relative volatility decreases to a value less than 1.15 at high benzene concentrations. An alternative method of separation is liquid-liquid extraction with a mixture of dimethylformamide (DMF) and water. The solvent is much more selective for benzene than for n-heptane at 20°C. For two different solvent compositions, calculate interstage flow rates and compositions by the rigorous ISR method for the countercurrent liquid-liquid extraction cascade, which contains five equilibrium stages and is shown schematically in Fig. 15.22. 

The process scheme of Fig. 5.2-2 is, in principle, identical with the scheme shown in Fig. 5.2-3. Here, the distillation stages are arranged in a vertical cascade with countercurrent flow of gas and liquid. From a material balance follows ... 

A countercurrent cascade for leaching is shown in Figure 14-3A. We will consider the (idealized) case where entrainment of liquid with the solid underflow can be ignored. The assumptions are ... 

Effective countercurrent contacting of two phases and good heat transfer are often difficult to combine. There are examples of gas/liquid absoiption columns with internal cooling (such as in nitric acid and sodium carbonate processes). In other cases one might opt for cross flow instead, and use a cascade of gasAiquid contactors. 

A stream returned to a cascade of stages, as represented by the trays in the absorber, for the purposes of obtaining an enrichment beyond that obtained by countercurrent contact with the feed to the cascade, is called reflux. This principle is used extensively in distillation, liquid extraction, and adsorption, but it is applicable to any countercurrent enrichment operation. 

An arrangement for this is shown in Fig. 10.27. The feed to be separated into its components is introduced at an appropriate place into the cascade, through which extract and raffinate liquids are passing countercurrently. The concentration of solute C is increased in the extract-enriching section by countercurrent contact with a raffinate liquid rich in C. This is provided by removing the solvent from extract to produce the solvent-free stream E part of which is removed as extract product and part returned as reflux Rq. The raffinate-stripping section of the cascade is the same as the countercurrent extractor of Fig. 10.18, and C is stripped from the raffinate by countercurrent contact with solvent. 

Liquid solutions, where the solute to be removed is adsorbed relatively strongly compared with the remainder of the solution, are treated in batch, semicontinuous, or continuous operations in a maimer analogous to the mixer-settler operations of liquid extraction (contact filtration). Continuous countercurrent cascades can be simulated or actually realized by use of such techniques as fluidized beds. 

For a better foam separation than that obtainable with the simple mode, the process operation can be cascaded by employing one of the countercurrent modes as shown in Figs. 6A and 6B. For the countercurrent modes of process operation, the number of transfer units (NTU) can be calculated in terms of an upflowing stream of interstitial liquid plus bubble surface, and a downflowing stream of just interstitial liquid. If the surfactant concentration in the upflowing interstitial liquid is L, and the solute surface excess at the bubble surface is E (which is taken to be equilibrium with L), then the effective concentration in the upflowing stream on a gas-free basis is generalized as... 

Gas extraction extends the possibilities of separation processes like distillation, absorption and liquid-liquid extraction to the isolation and purification of components of low volatility. Furthermore, it enables separation of components with very similar properties if used in the countercurrent mode. Process temperatures in gas extraction are determined by the critical temperature of the solvent and not, as is the case of distillation of any kind, by the liquid-vapor transition of the feed mixture. As compared to liquid-liquid extraction, gas extraction makes easily possible to operate a two cascade separation column, applying a stripping and an enriching section. Combined, these possibilites allow gas extraction to be operated at very moderate temperatures and as a separation process for difficult separations. 

---