Heterogeneous azeotropic distillation

When a multicomponent fluid mixture is nonideal, its separation by a sequence of ordinaiy distillation columns will not be technically and/or economically feasible if relative volatiK-ties between key components drop below 1.05 and, particularly, if azeotropes are formed. For such mixtures, separation is most commonly achieved by sequences comprised of ordinary distillation columns, enhanced distillation columns, and/or liquid-liquid extraction equipment. Membrane and adsorption separations can also be incorporated into separation sequences, but their use is much less common. Enhanced distillation operations include extractive distillation, homogeneous azeotropic distillation, heterogeneous azeotropic distillation, pressure-swing distillation, and reactive distillation. These operations are considered in detail in Perry s Chemical Engineers Handbook (Perry and Green, 1997) and by Seader... 

The first successful appHcation of heterogeneous azeotropic distillation was in 1902 (87) and involved using benzene to produce absolute alcohol from a binary mixture of ethanol and water. This batch process was patented in 1903 (88) and later converted to a continuous process (89). Good reviews of the early development and widespread appHcation of continuous azeotropic distillation in the prewar chemical industry are available (90). 

Historically azeotropic distillation processes were developed on an individual basis using experimentation to guide the design. The use of residue curve maps as a vehicle to explain the behavior of entire sequences of heterogeneous azeotropic distillation columns as weU as the individual columns that make up the sequence provides a unifying framework for design. This process can be appHed rapidly, and produces an exceUent starting point for detailed simulations and experiments. 

Figure 12.33 Separation of isopropyl alcohol (IPA) and water mixture using di-isopropyl ether (DIPE) as entrainer in heterogeneous azeotropic distillation.

Some systems form two-liquid phases for certain compositions and this can be exploited in heterogeneous azeotropic distillation. The use of liquid-liquid separation in a decanter can be extremely effective and can be used to cross distillation boundaries. 

If the system forms azeotropes, then the azeotropic mixtures can be separated by exploiting the change in azeotropic composition with pressure, or the introduction of an entrainer or membrane to change the relative volatility in a favorable way. If an entrainer is used, then efficient recycle of the entrainer material is necessary for an acceptable design. In some cases, the formation of two liquid phases can be exploited in heterogeneous azeotropic distillation. 

Heterocyclic sulfides, 23 645 Heteroepitaxial layers, for compound semiconductors, 22 145 Heteroepitaxy, on lattice mismatched substrates, 22 160 Heterofullerenes, 12 231—232 chemistry of, 12 252—253 Heterogeneous azeotropic distillation, 8 819-845... 

Keywords Batch distillation Heterogeneous entrainer Azeotropic distillation... 

Figure 3.22 Separating ethanol from water by heterogeneous azeotropic distillation.

Table 10.4 presents basic physical properties of the key components. By boiling point the acetic acid is the heaviest. Vinyl acetate is a light species with a normal boiling point at 72.6 °C. Of major interest is the low-boiler heterogeneous azeotrope vinyl acetate/water with 25 mol% water and nbp at 65.5 °C. The very low solubility of vinyl acetate in water, less than 1 wt%, is to be noted. Low reciprocal solubility can be exploited for separating the mixtures vinyl acetate/water by azeotropic distillation. In addition the densities of water and vinyl acetate are sufficiently distinct to ensure good liquid-liquid decanting. 

Ethanol-water Minimum-boiling azeotrope None Alternative to extractive distillation, salt extractive distillation, heterogeneous azeotropic distillation must reduce pressure to less than 11.5 kPa for azeotrope to disappear... 

FIG. 13-89 Separation of butanol-water with heterogeneous azeotropic distillation. 

For many years water has been separated from many liquids by azeotropic distillation. Water is substantially insoluble not only in the oils but also in the diluents or solvents. Thus, the vapor pressures of the water and diluent are additive as in the familiar steam distillations. A heterogeneous azeotrope is formed this always boils below the boiling point of water because pressure must be reduced for water to boil at the vapor pressure of the hydrocarbon. 

The principles of azeotropic distillation have often been described (1,2,3,4,5,6, 7), and applying them gives a simple method of separating water from its oil emulsions. The low-boiling constituents of an oil—or the diluent used—would be the entrainer (withdrawing agent) which forms the heterogeneous azeotrope or steam distillation with the water. 

Eresh aqueous ethanol feed is first preconcentrated to nearly the azeotropic composition in column C3, while producing a water bottoms product. The distillate from C3 is sent to column Cl, which is refluxed with the entire organic (entrainer-rich) layer, recycled from a decanter. Mixing of these two streams is the key to this sequence as it allows the overall feed composition to cross the distillation boundary into Region II. column Cl is operated to recover pure high-boihng node ethanol as a bottoms product and to produce a distillate close to the ternary azeotrope. If the ternary azeotrope is heterogeneous (as it... 

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