Relative Volatility

The key separation factor in distillation is the relative volatility, defined by eqn [1]. As the value of relative volatility increases, the easier it is for components to be separated by distillation.

The number or theoretical stages required to separate two species to a desired degree is strongly dependent on the value of a.

The variation of this parameter with pressure is shown in Figure 1 for the system ethane-propane. As seen, a is greater at low pressure than at high pressure. Therefore, at low pressure (e.g. 1 atm), for

Figure 1 Variation of relative volatility with pressure.

Figure 1 Variation of relative volatility with pressure.

The design or sizing of distillation equipment requires the calculation of diameter and height of the column. Diameter depends on volumetric flow rates of liquid and vapour inside the column, and these are functions of the total amount of the mixture feed to the column and the reflux or boil-up ratios. The desired purity of the components at the top and bottom of the tower dictates the height of the column. First, the theoretical or ideal plates are calculated, then efficiency is estimated to convert from ideal to real stages. By specifying the distance between plates (30-60 cm), and providing space for about four plates at the top of the column and six for the bottom for disengagement of the phases, the total height of the shell is determined. Figure 4 shows a block diagram for a typical design of a distillation column.

Distillation at high and low pressures involves special characteristics, summarized in Table 2.

Solar Panel Basics

Solar Panel Basics

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