Simple Distillation

The simplest batch distillation apparatus is shown in Figure 1. The operation is known as simple distillation or differential distillation. This is the oldest manifestation of distillation itself; it was used by many ancient cultures as a way of producing alcoholic beverages, essential oils and perfumes.

With regard to the separation of a binary mixture, the batch apparatus is run as follows. The pot is continuously heated, so that a vapour rich in the more volatile component is produced, condensed and collected in the external receiver. Indicating with xB and y the instantaneous mole fractions of the more volatile component in the liquid and vapour phases in equilibrium, and with HB the molar amount of boiling liquid remaining in the pot, the following material balances hold during any small time interval:

where dHV is the molar amount of vapour produced during the same time interval. Combining [1] and [2], and integrating from the original feed conditions (HF moles of boiling liquid of mole fraction z) to the present conditions, leads to the so-called Rayleigh equation:

Eqn [3] can be integrated numerically or graphically. It relates the amount of liquid remaining in the pot to the composition of this liquid. The molar amount HD and average composition xD of the liquid accumulated in the receiver can be determined from the following material balances:

and:

the definition of b„ it follows that:

If the system displays a constant relative volatility (a), the integration of eqn [3] can be performed analytically, leading to:

Figure 2 Simple distillation: fraction of the initial charge recovered after the purification of three different binary feeds from a light impurity.

ZNCibi

A graphical representation eqn [6] is presented in Figure 2. It is shown that, for a given fractional purification xB/z of the feed, a larger amount of liquid can be recovered at the end of the batch when the feed is leaner in the more volatile component. It is also clear that it is not possible to obtain a finite amount of the pure heavy component, whatever the feed composition.

For a mixture of Nc components, Nc Rayleigh equations of the form [3] can be derived, and an iterative procedure is usually employed in order to determine the composition profile of each component as a function of the amount of liquid remaining in the still. In the case of constant relative volatility mixtures, direct calculation of the composition profiles can be accomplished by defining the amount bi of the z'th component remaining in the pot (bi = HBxB;i). It can be shown that:

where j is an arbitrarily chosen reference component, f = HFz, and a, j is the relative volatility of the zth component with respect to the reference one. From

Figure 2 Simple distillation: fraction of the initial charge recovered after the purification of three different binary feeds from a light impurity.

Differential distillation is a very simple way of separating a mixture of several components into cuts of different purities, for example for laboratory applications. However, it permits only rough separations, even with large energy consumption (it is a one-theoretical-stage operation). This is the reason why multistaged columns with reflux are employed whenever a sharper separation is needed.

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Solar Panel Basics

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