K y rtffiiPF

In comparison with Eq. (4), for any pressure,

The notation meaning in Eq. (11) refers to chapter 1 (Eqs. (30) - (32)). It is evident that under lower and middle pressures Eq. (11) can be further simplified and the simplification procedure has already been made in chapter 1 (1.1. The equilibrium ratio).

Since relative volatility is an important index for evaluating the possible entrainers, it can be seen from Eq. (1) that activity coefficient models of both liquid-liquid equilibrium and vapor-liquid equilibrium should be known beforehand in order to predict relative volatility. The liquid composition and activity coefficient in the isothermal liquid-liquid equilibrium are solved by numerical iteration from the equations (Eqs. (5) - (8)) in order to look for the compositions for which the activities arc equal in the two phases for cach individual component (Eq. (5)). On this basis, the vapor composition in the isothermal vapor-liquid equilibrium is calculated by using Eq. (4). However, one probicm arises, that is, both one activity coefficient model suitable for VLE and another activity coefficient model suitable for

LLE should be utilized in determining the same yi and jr.. In general, one model caivt solve this problem, due to accuracy limitation of activity coefficient models under different conditions. Thus, it adds to some difficulty in calculation. But this contributes to one way to select the potential entrainers, herein, called calculation method.

Of course, experiment method is the most reliable in selecting the entrainers. but apparently much time and money will be spent. Only until a limited few of possible entrainers are found by means of calculation method, should the experiments be made.

Besides, simple rules using maps of distillation lines and residue curves (see what follows) have been suggested for screening many possible entrainers. Since drawing distillation lines and residue curves may be a very tedious thing, it is advisable to straightforwardly apply the conclusions generalized by the researchers from many maps. Stichlmair and Herguijuela [38] enumerate some experience rules for entraîner selection, which are given in Table I.

Table I

Experience rules for entraîner selection Mixtures with a minimum-boiling azeotrope:

• Low boiler (lower than the original azeotrope).

• Medium boiler that forms a minimum-boiling azeotrope with the low boiling species.

• High boiler that forms minimum-boiling azeolropes with both species. At least one of ihe new minimum-boiling azeotropes has a lower boiling temperature than the original azeotrope. Mixtures with a maximum-boiling azeotrope:

• Low boiler (lower than the original azeotrope).

• Medium boiler that forms a minimum-boiling azeotrope with the low boiling species.

• High boiler thai forms minimum-bo ilittg azeotropes with both species. At least one of the new maximum-boiling azeotropes has a higher boiling temperature than the original azeotrope._

Some cases using azeotropic distillation as the separation method are listed in fable 2. The interested readers can compare the entrainers used in practice with the rules of selecting entrainers.

Based on computer-aided design, an advanced knowledge integrating system for the selection of solvents for azeotropic distillation is developed by Thomas and Hans [34], The mass separating agents are searched in the system SOLPERT (Solvent Selecting Kxpert System) by means of combining databases, heuristic and numerical methods. Several inherent limitations of solvent screening methods, e.g. database search, empirical methods and group contribution methods, are circumvented and minimized to gain an maximum of usable information for solvent selection. The solvent selection of SOI-PERT consists of four steps, as listed in Table 3. Table 2

Some azeotropic distillation cases No. Components to be separated Entrainers

1 Ethanol / water; isopropanol / water, Benzene, toluene, hexane, cyclohexane, tert-butanol / water methanol, diethylether, methyl-ethyl-ketone (MEK) [2, 4, 5, 13, 16,21,23]

2 Acetone / n-heptane Toluene [28]

3 Acetic acid / water n-butyl acetate [17]

4 Isopropanol / toluene Acetone [20]

2 Acetone / n-heptane Toluene [28]

3 Acetic acid / water n-butyl acetate [17]

4 Isopropanol / toluene Acetone [20]

Table 3

Was this article helpful?

0 0

Post a comment