Rectification—Binary Systems

Continuous rectification, or fractionation, is a multistage countercurrent distillation operation. For a binary solution, with certain exceptions it is ordinarily possible by this method to separate the solution into its components, recovering each in any state of purity desired. 

Rectification is probably the most frequently used separation method we have, although it is relatively new. While simple distillation was known in the first century, and perhaps earlier, it was not until about 1830 that Aeneas Coffey 

In order to understand how such an operation is carried out, recall the discussion of reboiled absorbers in Chap. 8 and Fig. 8.28. There, because the liquid leaving the bottom of an absorber is at best in equilibrium with the feed and may therefore contain substantial concentrations of volatile component, trays installed below the feed point were provided with vapor generated by a reboiler to strip out the volatile component from the liquid. This component then entered the vapor and left the tower at the top. The upper section of the tower served to wash the gas free of less volatile component, which entered the liquid to leave at the bottom. 

The purities obtained for the two withdrawn products will depend upon the liquid/gas ratios used and the number of ideal stages provided in the two sections of the tower, and the interrelation of these must now be established. The cross-sectional area of the tower, however, is governed entirely by the quantities of materials handled, in accordance with the principles of Chap. 6. 

In Fig. 9.17, the theoretical trays are numbered from the top down, and subscripts generally indicate the tray from which a stream originates for example, L is mol liquid/ time falling from the nth tray. A bar over the quantity indicates that it applies to the section of the column below the point of introduction of the feed. The distillate product may be liquid, vapor, or a mixture. The reflux, however, must be liquid. The molar ratio of reflux to 

---

For a binary system with a single equilibrium stage, i.e., a reboiled stillpot with no rectification column, the mass balance is given by the Rayleigh equation ... 

Fig. 2-33. Rectification unit for binary systems. EC Enriching column SC Stripping column C Condenser R Reboiler...

Sometimes the new azeotrope which is formed contains all three constituents. The dehydration of ethanol-water mixture with benzene as added substance is an example. Dilute ethanol-water solutions can be continuously rectified to give at best mixtures containing 89.4 mole percent ethanol at atmospheric pressure, since this is the composition of the minimum-boiling azeotrope in the binary system. By introducing benzene into the top of a column fed with an ethanol-water mixture, the ternary azeotrope containing benzene (53.9 mol %), water (23.3 mol %), ethanol (22.8 mol %), boiling at 64.9°C, is readily separated from the ethanol (bp — 78.4 C), which leaves as a residue product. In this case also the azeotropic overhead product separates into two liquid layers, one rich in benzene which is returned to the top of the column as reflux, the other rich in water which is withdrawn. Since the latter contains appreciable quantities of both benzene and ethanol, it must be rectified separately. The ternary azeotrope contains nearly equal molar proportions of ethanol and water, and consequently dilute ethanol-water solutions must be given a preliminary rectification to produce substantially the alcohol-rich binary azeotrope which is used as a feed. 

D. Rectification in vertical wetted wall column with turbulent vapor flow, Johnstone and Pigford correlation =0.0.328(Wi) Wi P>vP 3000 < NL < 40,000, 0.5 < Ns. < 3 N=, v,.gi = gas velocity relative to R. liquid film = — in film -1 2 " [E] Use logarithmic mean driving force at two ends of column. Based on four systems with gas-side resistance only, = logarithmic mean partial pressure of nondiffusing species B in binary mixture. p = total pressure Modified form is used for structured packings (See Table 5-28-H). 

IEC 36, 871-5(1944) (Azeotropic distn) (14 refs) 4)R.H. Ewell L.M. Welch, IEC 37, 1224-31(l945)(Rectification in ternary systems contg binary azeotropes) (8 refs) 5)L.H. Horsley, Anal Chem 19, 50 600(1947)... 

Using the system dichloroethane-toluene as an ejcample Wagner and Blass [262] describe a procedure for a hybrid analog computer which simulates stepwise rectification. This allows the determination of the stiU head or still pot concentration in the separation of binary mixtures for given operating parameters and equilibrium curves as a function of time. 

Separation of Binary Azeotropic Mixtures. A large number of two-component systems form azeotropic mixtures, and it is frequently necessary to separate them into their components. Regular fractional distillation will not separate such mixtures into the components in high purity, but by suitable modifications it is frequently possible to obtain the desired separation. At the azeotropic composition the relative volatility is unity, and rectification is not possible. The methods employed for separating such systems involve using either (1) distillation plus other separation processes to get past the azeotropic composition or (2) a modification of the relative volatility. 

---