Relative volatility increase

Since relative volatilities increase in most distillation systems as pressure decreases, the optimum operation would be to minimize the pressure at all times. One way to do this is to just completely open the control valve on the cooling water. The pressure would then float up and down as cooling water temperatures changed. 

Lockett (12) and King (126) noted some theoretical sense in O Connell s correlation. Higher viscosity usually implies lower dif-fusivity, and therefore, greater liquid phase resistance and lower efficiency (12). Higher relative volatility increases the significance of the liquid phase resistance [Eq. (7.13)), thus reducing efficiency (126), Lockett expressed the O Connell plot in equation form... 

Solvent Loading. The solvent circulation rate is a function of the reflux ratio in the primary tower and the liquid-phase concentration of the solvent. For a given solvent selectivity, as the solvent concentration rises, the propane-propylene relative volatility increases and hence the required reflux rate falls. The increased relative volatility results in a decreased number of equilibrium stages required for the desired separation. Figure 4 shows the effect of solvent concentration on the number... 

Figure 3, taken from the data of Dobroserdov (2), gives another example of the substantial effect that a salt, even at reasonably moderate concentration, can have in certain systems. The key components are again ethanol and water, but here the salt is calcium chloride, present at a constant concentration of 10 grams/100 ml of alcohol-water solution. The azeotrope has been completely eliminated, and relative volatility increased substantially. 

Froth systems higher pressure slightly increases efficiency Low relative volatility increases efficiency Minimal effect... 

In contrast to the evaporation of solutions, where only the solvent vaporizes and the partial pressure of the soluted substance is negligible, in distillation processes all components found in the liquid phase are also present in the vapor phase. The fraction of each component in the vapor phase depends upon the effort required to escape from the liquid into the vapor phase. The relative volatility is a measure of this effort , the separability of a mixture by distillation (discussed in Chapter 1.5). Since the relative volatility increases with decreasing pressure, the separation efficiency of a distillation process is increased by decreasing the operating pressure. 

Because these constants differ for various compounds, the vapor pressures for different components usually do not change at the same rate with temperature variations. For an ideal system, the relative volatility between the two components, as expressed by Equation 7-6, varies with the boiling temperature. As an example, consider the separation of a system of toluene and ethyl benzene. Assume that the separation produces substantially pure toluene as distillate and equally pure ethyl benzene as bottoms. If the distillation column is operated at atmospheric pressure, the relative volatility between these two components is 2.1. If the distillation pressure is raised to 5 atmospheres absolute, the relative volatility is reduced to 1.8. However, if the pressure of distillation is lowered to 200 mm Hg absolute, the relative volatility increases to 2.3. Thus, separation becomes easier as the column pressure is reduced. 

As discussed in the preceding chapter, the relative volatility between components usually varies with the distillation temperature. Only a few systems, such as toluene and n-octane, exhibit a nearly constant relative volatility regardless of the distillation temperatures. Normally, the relative volatility increases as distillation pressure is lowered due to the accompanying reduction in boiling temperatures. For example, the relative volatility of a 17.6 mol% acetone/82.4 mol% water system increases from 19 at atmospheric pressure to 24 at 200 mm Hg absolute pressure [5]. Likewise, the relative volatility of a 72.4 mol% methanol/27.6 mol% water system increases from 2.8 at atmospheric pressure to 3.7 at 200 mm Hg absolute pressure. 

As an example of such an operation, consider the process of Fig. 9.54, The separation of toluene (bp 110.8 C) from paraffin hydrocarbons of approximately the same molecular weight is either very difficult or impossible, due to low relative volatility or azeotropism, yet such a separation is necessary in the recovery of toluene from certain petroleum hydrocarbon mixtures. Using isooctane (bp = 99.3°C) as an example of a paraffin hydrocarbon, Fig. 9.54a shows that isooctane in this mixture is the more volatile, but the separation is obviously difficult. In the presence of phenol (bp = 181.4 C), however, the isooctane relative volatility increases, so that, with as much as 83 mole percent phenol in the liquid, the separation from toluene is relatively easy. A flowsheet for accomplishing this is shown in Fig. 9.546, where the binary mixture is introduced more or less centrally into the extractive distillation tower (1), and phenol as the solvent is introduced near the top so as to be present in high concentration upon most of the trays in the tower. Under these conditions isooctane is readily distilled as an overhead product, while toluene and phenol are removed as a residue. Although phenol is relatively high-boiling, its vapor pressure is nevertheless sufficient for its appearance in the overhead product to be prevented. The solvent-recovery section of the tower, which may be relatively short, serves to separate the phenol from the isooctane. The residue from the tower must be rectified in the auxiliary tower (2) to separate toluene from the phenol, which is recycled, but this is a relatively easy separation. In practice, the paraffin hydrocarbon is a mixture rather than the pure substance isooctane, but the principle of the operation remains the same. 

In conventional distillation, column pressure is usually selected to be as low as possible while still being able to use cooling water in the condenser. This is because relative volatilities increase with decreasing temperamre in many chemical systems. The other situation... 

Results for different a39o cases are displayed in Table 3.8. These are the optimum designs in terms of the four design optimization variables column pressure and the number of stripping, rectifying, and reactive trays. Reducing the relative volatility increases both capital and energy costs. 

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