Distillation of liquid mixtures

Bcrthelot, On the Distillation of Liquid Mixtures, Compt. rend.,... 

CH2 CH CH0. a colourless, volatile liquid, with characteristic odour. The vapour is poisonous, and intensely irritating to eyes and nose b.p. 53"C. It is prepared by the distillation of a mixture of glycerin, potassium sulphate and potassium hydrogen sulphate. It is manufactured by direct oxidation of propene or cross-condensation of ethanal with meth-anal. 

The process of distillation is aimed at the separation of components from a liquid mixture. This process depends on the differences in boiling points of the individual components. Also, depending on the concentrations of the components present, the liquid mixture will have different boiling point characteristics. This means that distillation processes depends on the vapor pressure characteristics of liquid mixtures. 

Distillation is probably Ihe most widely used separation (mass transfer) process in the chemical and allied industries. Its applications range from the rectification of alcohol, which has been practiced since antiquity, lo the fractionation of crude oil. The separation of liquid mixtures by distillation is based on differences in volatility between the components. The greater the... 

FIGURE 8.38 Some of the steps that represent fractional distillation of a mixture of two volatile liquids (benzene and toluene). The original mixture boils at A and its vapor has composition B. After condensation of the vapor, the resulting liquid boils at C and the vapor has composition D, and so on. 

The separation of liquid mixtures by distillation depends on differences in volatility between the components. The greater the relative volatilities, the easier the separation. The basic equipment required for continuous distillation is shown in Figure 11.1. Vapour flows up the column and liquid counter-currently down the column. The vapour and liquid are brought into contact on plates, or packing. Part of the condensate from the condenser is returned to the top of the column to provide liquid flow above the feed point (reflux), and part of the liquid from the base of the column is vaporised in the reboiler and returned to provide the vapour flow. 

A packed column, 1.2 m in diameter and 9 m tall, is packed with 25 mm Raschig rings, and used for the vacuum distillation of a mixture of isomers of molecular mass 155 kg/kmol. The mean temperature is 373 K, the pressure at the top of the column is maintained at 0.13 kN/m2 and the still pressure is 1.3-3.3 kN/m2. Obtain an expression for the pressure drop on the assumption that this is not appreciably affected by the liquid flow and may be calculated using a modified form of Carman s equation. Show that, over the range of operating pressures used, the pressure drop is approximately directly proportional to the mass rate of flow rate of vapour, and calculate the pressure drop at a vapour rate of 0.125 kg/m2. The specific surface of packing, S = 190 m2/m3, the mean voidage of bed, e = 0.71, the viscosity of vapour, // = 0.018 mN s/m2 and the molecular volume = 22.4 m3/kmol. 

Gillespie equilibrium still analychem A recirculating equilibrium distillation apparatus used to establish azeotropic properties of liquid mixtures. ga Ies-pe, e-kw3 llbre-om, stil ... 

The dominance of distillation-based methods for the separation of liquid mixtures makes a number of points about RCM and DRD significant. Residue curves trace the liquid-phase composition of a simple single-stage batch stillpot as a function of time. Residue curves also approximate the liquid composition profiles in continuous staged or packed distillation columns operating at infinite reflux and reboil ratios, and are also indicative of many aspects of the behavior of continuous columns operating at practical reflux ratios (12). 

The nitrogen required is obtained by fractional distillation of liquid air.16 The hydrogen used to be obtained by electrolysis of liquid water if inexpensive surplus electrical capacity becomes available in the future, this method may well be reintroduced. Catalytic photolysis of water using sunlight is another possible future source of H2. The Haber-Bosch process of 1916 used water-gas, which is a mixture of H2, CO, and CO2 made by alternating blasts of steam and air over coke at red heat ... 

The fractional distillation of a mixture of water and ethanol (Figure 2.22, p. 21) illustrates that a pure liquid boils at one temperature but a mixture of liquids boils over a range of temperatures. 

Fractional distillation of a mixture of ethanol and water will separate two miscible liquids and illustrates that an impure liquid does not boil at a specific temperature but over a range of temperatures (p. 21). 

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