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11.3.2. Flash or equilibrium distillation

Flash or equilibrium distillation, frequently carried out as a continuous process, consists of vaporising a definite fraction of the liquid feed in such a way that the vapour evolved is in equilibrium with the residual liquid. The feed is usually pumped through a fired heater and enters the still through a valve where the pressure is reduced. The still is essentially a separator in which the liquid and vapour produced by the reduction in pressure have sufficient time to reach equilibrium. The vapour is removed from the top of the separator and is then usually condensed, while the liquid leaves from the bottom.

In a typical pipe still where, for example, a crude oil might enter at 440 K and at about 900 kN/m2, and leave at 520 K and 400 kN/m , some 15 per cent may be vaporised in the process. The vapour and liquid streams may contain many components in such an application, although the process may be analysed simply for a binary mixture of A and B as follows:

If F = moles per unit time of feed of mole fraction x/ of A,

V = moles per unit time of vapour formed with y the mole fraction of A, and S = moles per unit time of liquid with x the mole fraction of A, then an overall mass balance gives:

DISTILLATION and for the more volatile component:

Thus:

Equation 11.31 represents a straight line of slope:

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