Molecular distillation apparatus

The efficiency of molecular distillation apparatus is less than one theoretical plate (see Chapter 5). It is used to remove very volatile sub-... 

Micro-form of molecular distillation apparatus according to Glould et al. (cold-finger method)... 

Micro-form of molecular distillation apparatus after Breger (falling film principle) for less than 4 g charge... 

Molecular distillation apparatus of Hickmann with arrangement for recycling... 

In a previous review of this topic (5), it was suggested that molecular distillation drying followed by resin embedding needed more work before it could be evaluated as a preparative technique for microanaly-tical work as there had only been one investigation that had used it (59). Unfortunately, as far as this author is aware, there has only been one more study (12) that has used the technique to prepare material for micro-analysis. It is possible that workers have been put off using it by the apparent fall from grace of the related freeze substitution technique or, more likely, by the expense of molecular distillation drying apparatus. 

Number-average molar masses were determined using a vapor pressure osmometer (VPO) (Hitachi 117 Molecular Weight Apparatus) at 54.8 0.1°C in toluene (Fisher Scientific, certified A.C.S.) which was distilled from freshly crushed CaH2. The VPO apparatus was calibrated with pentaerythritol tetrastearate (Pressure Chemical). Gel permeation chromatographic (GPC) analyses were performed in tetrahydrofuran by HPLC (Perkin-Elmer 601 HPLC) using six y-Styragel columns (106, 105, 10l, 103, 500, and 100 A) after calibration with standard polystyrene samples. 

The residue from distillation is a viscous oil consisting of high-molecular-weight compounds. Further cyclic oligomers, especially trimer and tetramer, can be obtained by heating this oil to about 400°C.To do this, a new receiver is fitted to the distillation apparatus and the flask heated to 400-450°C.The more volatile products are carried over in a slow stream of nitrogen and the distillate is then fractionated as before. 

Into a 3-L three-neck round-bottom flask equipped with a thermometer, a KPG stirrer and a 20-cm Vigreux column linked to a distillation apparatus with three cooled traps (MeOH/dry ice) were placed rw-4.5-(thiocarbonyldioxy)spiro[2.3]hexane (50 98.2 g, 0.63 mol) and freshly distilled tributyl phosphite (472 g, 1.89 mol). The mixture was heated at 115 120 0C/17 Torr for 44 h. The condensed distillates in the traps were combined, dried over 4 A molecular sieves and fractionally distilled through a 40-cm Vigreux column to give the product 51 yield 40 g (80%) bp 66.5 nC (760 Torr). 

Not all liquids form ideal solutions and conform to Raoult s law. Ethanol and water are such liquids. Because of molecular interaction, a mixture of 95.5% (by weight) of ethanol and 4.5% of water boils below (78.15°C) the boiling point of pure ethanol (78.3°C). Thus, no matter how efficient the distilling apparatus, 100% ethanol cannot be obtained by distillation of a mixture of, say, 75% water and 25% ethanol. A mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition is called an azeotrope 95% ethanol is such an azeotrope. The boiling point-composition curve for the ethanol-water mixture is seen in Fig. 4. To prepare 100% ethanol the water can be removed chemically (reaction with calcium oxide) or by removal of the water as an azeotrope (with still another liquid). An azeotropic mixture of 32.4% ethanol and 67.6% benzene (bp 80.1 °C) boils at 68.2°C. A ternary azeotrope (bp 64.9°C) contains 74.1% benzene, 18.5% ethanol, and 7.4% water. Absolute alcohol (100% ethanol) is made by addition of benzene to 95% alcohol and removal of the water in the volatile benzene-water-alcohol azeotrope. 

Figure E9-1. Apparatus used for the molecular distillation of vitamin E from margarine.

Molecular distillation (section 5.4.4) on a micro-scale may be performed by both the cold-finger method and that of the falling film or thin layer. A practical example of a cold-finger apparatus is illustrated in Fig. 131 [10]. The mixture to be separated is heated in A, and condenses on the cold finger B the distillate builds up at C, from... 

Apparatus for molecular distillation on a micro-scale for 0.5 g charge (Paschke et al.)... 

The process going on in molecular distillation is not the normal ebullition it might be called molar evajioration . The equilibrium between evaporated molecules and the liquid is continually disturbed by condensation so that, in accordance with physical laws, equilibrium has to be re-established. This means, however, that more molecules will evaporate from the liquid surface. Thus, we have a true example of a simple distillation which is also termed one-way evaporation [141]. The whole field of molecular distillation is covered by the books of Burrows and Holo et al. [139]. Ridgway-Watt [140] presents an introductory survey of apparatus from the micro to the technical scale. In addition, the reader is referred to chap. 1.5 of [122] and some more review articles [108, 131, 145, 156, 157]. 

The apparatus employed for molecular distillation should comply with the following requirements ... 

It should be observed that recycling can be practised in all these forms of apparatus in order to sej>arate low-boiling components effectively this is often necessary in view of the fact that molecular distillation effectively corresponds to a single equilibrium stage. We must not forget, however, that in a distillation from a flask entrainment may constitute a complication in molecular distillation, on the other hand, evaporation takes place at the surface only, so that the molecules can leave it selectively without mechanical disturbance [147]. 

Micro-cascade apparatus for short-path and molecular distillation... 

Multi-stage apparatus for molecular distillation (Malyusov and Shavoronkov [150])... 

Important factors in molecular distillation are the character of the evaporating surface and, of course, the distance between this latter and the condensing surface. For falling-film apparatus a good wetting of the evaporator surface and effective circulation of the liquid film are achieved by means of the same constructional measures as those described on p. 286 [122]. 

As an example of a spiral still Fig. 210 represents apparatus for molecular distillation manufactured by VEB Jenaer Glaswerk Schott Gen., Jena. The glass spiral. 3 rotates around the heater 2. Thus a film thickness of about 0.1 mm and a good circn-... 

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