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Because few separations are truly binary, the ramifications that are encountered in column design and operation are extensive. A third component leaves with the distillate if it is the lightest component, or with the bottoms if it is the heaviest; or it may be withdrawn as a sidestream if it boils at a temperature between those of the light and heavy
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components. When azeotropes are encountered, a third component may be intentionally introduced to facilitate the separation. T o w e r s are often linked together in a chain, occasionally with a stream being recycled to improve recovery. Some separnt>ions require so many trays that t,he tower is broken into two or more sections. There are also partial condensers, vacuum stills, and other peculiar arrangcmcnts, which make up a wide variety of distillation processes. As might be expected, each tower or combination of towers has its own particular requirements for cont8rol-loop arrangement,. Beyond this, even, there are various ways of controlling the same tower, some of which are more effective than others. Virtually any arrangement of loops will operate satisfactorily in the steady st ate-but in the steady stat e no control is really needed. It is the unst eady state that tests control performance. If feed rate or composition or enthalpy change, or steam pressure, or weather, what happens then
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Interaction between Heat and Material Balances
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Whenever a column is arranged so that both product streams are under level control, its material balance cannot be directly manipulated. Figure 11.7 shows just such a system. With reflux on flow control, variations in vapor rate and reflux rate will both affect the flow of distillate. Earlier it was demonstrated that the distillate to feed ratio had the greatest effect on product quality. Therefore, it is imperative that distillate flow be controlled directly, instead of being subject to variations in vapor and reflux rates. If the flow of vapor leaving the top of the tower is the same as that generated
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FIG 12.7. This arrangement of control loops is not recommended, because of interaction between the heat and material balances.
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in t he reboiler, distillate flow varies with reflux flow I, as D=V-I, (11.10)
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F,q, n
The fractional change in D resulting from fractional variation in V and I, is dD dV v dL I , -=----(11.11) D V D L D
FIG 11.8. The material balance at the feed tray is affected by feed enthalpy qF.
Because V/D and L/D are ordinarily much greater than 1, variations in the actual flow of reflux and heat input will cause augmented upset s in product quality. In the numerical example, V/D is 10; a 1 percent change in heat input would upset distillate flow by 10 percent. But t he flow of vapor leaving the top of the tower is a function of many factors. If all of the feed enters as a liquid at it,s boiling point, flow of vapor above and below the feed tray will be the same. But if part of the feed is vaporized, vapor flow above the feed tray will exceed that below by the amount of vapor in the feed. Figure 11.8 describes the material balance at the feed tray. Feed enthalpy is identified as the fraction of the feed that is vaporized. Liquid at its boiling point has an enthalpy of 0, while that of saturated vapor is 1.0. The enthalpy of subcooled liquid is negative, that of superheated vapor exceeds 1.0. If no heat were added to or removed from the vapor stream until it entered the condenser, its flow into the condenser would equal that immediately above the feed tray. In this case, D=V+q,F-L Because it has been made the dependent variable, distillate flow is subject to variation in both feed rate and v+Lq, enthalpy. Reflux may not return to the tower exactly at the boiling point. In fact, it will usually be subcooled to some extent. This subcooling causes conL,q, densation of some of the vapors approaching the top tray of the tower. LCI-q,) V 1 Figure 11.9 shows the material balance at the top tray. If feed is entering the tower at its FIG 11.9. The material balance boiling point, the vapor flow approachat the top of the tower is a funcing the top tray will be t,he same as tion of the enthalpy qL of the that at the reboiler. But if the reflux reflux stream.
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