Molecular distillation, liquid

Molecular distillation Liquid mixtures Heat and vacuum Liquid and vapor Difference in kinetic theory maximum rate of vaporization Separation of vitamin A esters and intermediates 18, 42... 

In molecular distillation, the permanent gas pressure is so low (less than 0 001 mm. of mercury) that it has very little influence upon the speed of the distillation. The distillation velocity at such low pressures is determined by the speed at which the vapour from the liquid being distilled can flow through the enclosed space connecting the still and condenser under the driving force of its own saturation pressure. If the distance from the surface of the evaporating liquid to the condenser is less than (or of the order of) the mean free path of a molecule of distillate vapour in the residual gas at the same density and pressure, most of the molecules which leave the surface will not return. The mean free path of air at various pressures is as follows —... 

In the removal of water vapor from liquids or in their distillation, particularly in degassing columns, vacuum filling, and resin-casting plants, as well as in molecular distillation, the production of as large a liquid surface as possible is important. In all wet processes the provision of the necessary heat for evaporation of the moisture is of great importance. 

Conditions sometimes exist that may make separations by distillation difficult or impractical or may require special techniques. Natural products such as petroleum or products derived from vegetable or animal matter are mixtures of very many chemically unidentified substances. Thermal instability sometimes is a problem. In other cases, vapor-liquid phase equilibria are unfavorable. It is true that distillations have been practiced successfully in some natural product industries, notably petroleum, long before a scientific basis was established, but the designs based on empirical rules are being improved by modern calculation techniques. Even unfavorable vapor-liquid equilibria sometimes can be ameliorated by changes of operating conditions or by chemical additives. Still, it must be recognized that there may be superior separation techniques in some cases, for instance, crystallization, liquid-liquid extraction, supercritical extraction, foam fractionation, dialysis, reverse osmosis, membrane separation, and others. The special distillations exemplified in this section are petroleum, azeotropic, extractive, and molecular distillations. 

Molecular distillation is characterized by short exposure of the distilled liquid to elevated temperatures, high vacuum in the distillation space, and a small distance between the evaporator and the condenser. The short residence time of the liquid on the evaporating cylinder, in the order of a... 

We do not discuss equilibrium because the molecular distillation is a nonequilibrium process. Molecular distillation belongs to the class of processes that uses the technique of separation under high vacuum, operation at reduced temperatures, and low exposition of the material at the operating temperature. It is a process in which vapor molecules escape from the evaporator in the direction of the condenser, where condensation occurs. Then, it is necessary that the vapor molecules generated find a free path between the evaporator and the condenser, the pressure be low, and the condenser be separated from the evaporator by a smaller distance than the mean free path of the evaporating molecules. In these conditions, theoretically, the return of the molecules of the vapor phase to the liquid phase should not occur, and the evaporation rate should only be governed by the rate of molecules that escape from the liquid surface therefore, phase equilibrium does not exist. 

Figures 8-10 show the curves of tocopherol concentration in the residue (% w/w) vs the percentage of the distance on the evaporator (from the feed point) for feed flow rate ranging from 0.5 to 1.0 kg/h for the falling film molecular distillation unit. The initial tocopherol concentration was 8.50% (w/w). For a feed flow rate of 0.5 kg/h (Fig. 8), it can be observed that at the end of the distillation, the tocopherol concentration in the residue will be higher, at 150°C (about 15% [w/w]). At 160°C, at 80% of the distillation, the tocopherol concentration reaches a maximum and then decreases, because the tocopherols are already recovered in the vapor phase. Figures 8-10 show that by increasing the feed flow rate at the same temperature (160°C), the tocopherol concentration can increase until it doubles the initial concentration (for a feed flow rate of 0.6 kg/h). From this point, it decreases, requiring an increase in the temperature to concentrate more (for a feed flow rate of 1.0 kg/h at 170°C). For all feed flow rates (Figs. 8-10), at 180°C, practically all the tocopherols are found in the vapor phase. With this study, it is possible to observe which temperature is the best in order to recover the fatty acids (first step = 125°C) and, then, recover the tocopherols in the vapor phase (distillate) and the phytosterols in the liquid phase (residue) (second step = 170°C). At the lowest temperature (120°C) the tocopherol recovery was minimum (about 5%). By increasing the feed flow rate from 0.5 to 1.0 kg/h (100%), the quantity of tocopherol in the residue at 170°C, e.g., increases, which means that the process performance has decreased.

Several methods have been proposed to produce polyunsaturated fatty acid (PUFA) concentrates particularly high in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Most PUFA enrichment methods are based upon a combination of techniques such as saponification, solvent extraction, urea fractionation, molecular distillation, fractionation distillation, liquid chromatography, and super critical carbondioxide extraction. Current evidence suggests that the physiological effects of omega-3 fatty acids are such that the annual world supply of fish oils will be grossly inadequate as a source of these materials, and alternative sources will be needed (Belarbi et al, 2000). 

The tocopherols can be separated by a number of methods one is to esterify them with a lower alcohol followed by washing, vacuum distillation and saponification alternatively, fractional liquid-liquid extraction may be used. The product can then be further purified by molecular distillation, extraction or crystallisation. This process produces a product high in y- and (5-tocopherols but these can be converted into the more useful a-tocopherol by methylation. If required a-toco-pherol acetate can be made by acetylating the a-tocopherol. 

---