Countercurrent extraction difference points

The characteristic velocity k is a function of droplet size, density difference, viscosity, etc. Thus, the holdup tends to increase either as the superficial flow velocities Uc and Ud are increased or as the characteristic velocity is reduced (e.g., by increasing agitation). A point is eventually reached where the increase in holdup becomes unstable (typically when = 0.3-0.4). This phenomenon is known as flooding, and it imposes a limit on the flow rates and agitation levels that can be used in countercurrent extraction processes. 

Density. The difference in density between the two liquid phases in equilibrium affects the countercurrent flow rates that can be achieved in extraction equipment as well as the coalescence rates. The density difference decreases to zero at a plait point, but in some systems it can become zero at an intermediate solute concentration (isopycnic, or twin-density, tie line) and can invert the phases at higher concentrations. Differential types of extractors cannot cross such a solute concentration, but mixer-settlers can. 

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. 

Countercurrent distillation enables components to be separated having differences in boiling point of about 0.5 deg C, whilst this figure can be as low as 0.05 °C if extremely efficient columns are employed, as in the separation of isotopes. By the use of selective methods and, in difficult cases, by combination with other methods of separation such as extraction, countercurrent distribution and gas chromatography, separations have been performed with mixtures previously r arded as inseparable. 

Analysis of complex mixtures often requires separation and isolation of components, or classes of components. Examples in noninstrumental analysis include extraction, precipitation, and distillation. These procedures partition components between two phases based on differences in the components physical properties. In liquid-liquid extraction components are distributed between two immiscible liquids based on their similarity in polarity to the two liquids (i.e., like dissolves like ). In precipitation, the separation between solid and liquid phases depends on relative solubility in the liquid phase. In distillation the partition between the mixture liquid phase and its vapor (prior to recondensation of the separated vapor) is primarily governed by the relative vapor pressures of the components at different temperatures (i.e., differences in boiling points). When the relevant physical properties of the two components are very similar, their distribution between the phases at equilibrium will result in shght enrichment of each in one of the phases, rather than complete separation. To attain nearly complete separation the partition process must be repeated multiple times, and the partially separated fractions recombined and repartitioned multiple times in a carefully organized fashion. This is achieved in the laborious batch processes of countercurrent liquid—liquid extraction, fractional crystallization, and fractional distillation. The latter appears to operate continuously, as the vapors from a single equilibration chamber are drawn off and recondensed, but the equilibration in each of the chambers or plates of a fractional distillation tower represents a discrete equihbration at a characteristic temperature. 

The actual Sorbex process is shown schematically in Figure 12,12. The process operates with a fixed adsorbent bed rather than with a moving bed and the countercurrent process is simulated by moving the feed, desorbent, and product points continuously by means of a rotary valve. The column sketched is divided into 12 segments, each with appropriate flow distributors to allow the introduction of feed or removal of products. In the position indicated, lines 2 (desorbent), 5 (extract), 9 (feed), and 12 (raffinate) are operational and all other lines are closed. When the rotary valve is moved to its next position the desorbent enters at point 3, extract leaves at point 6, feed at point 10, and desorbent at point I. Functionally the bed has no top or bottom and is equivalent to an annulus. The same distance is always maintained between adjacent streams, but this distance may be different for the different segments of the column. 

---