Molecular-distillation

Vacuum distillation is suitable for the separation of thermally sensitive liquids, being a gentle distillation process which operates at low pressure. Due to the reduced operating pressure, the boiling point of the mixture is also reduced. A low residence time and a narrow residence time distribution are maintained, by careful selection of the distillation unit to be used. 

The advantages of vacuum distillation over ordinary distillation at normal or higher pressure are listed in Table 2-1. 

Different vacuum ranges, including the vacuum forming device are discussed in Table 2-2. Common distillation units are also shown. 

Molecular distillation is used to separate very temperature sensitive liquid mixtures at absolute operating pressures in the range 0.1-0.001 Pa. 

Vacuum range Vacuum forming device Distillation process Distillation unit 

Molecular distillation (MD) is a kind of high vacuum distillation method which is suitable for the separation of high boiling point, heat sensitive and viscous products. Molecular distillation is a special liquid-liquid separation technology which is different from the separation principle of traditional distillation depending on boiling point difference. It is depending on the difference of molecular mean free path to separate different matters. Here, molecular mean free path (A) is referred to the path between a molecule strikes two times. 

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 rate of evaporation depends on the vapour pressure of the substance at the temperature of the evaporating surface T, and on the molecular weight M of the substance to be distilled. 

This relationship is expressed in Langmuir s equation for the rate of evaporation [142]  

This equation assumes that evaporation is not impeded by foreign gas molecules. Since it is inevitable that some evaporated molecules will collide with those of residual gas before arriving at the condensing surface, the value of D given by this formula is not normally attained. It is therefore necessary to correct D by multiplying it by a factor a, which approaches unity the more closely, the lower the pressure of the residual gas. This factor a can amount to 0.9 in modern industrial apparatus. 

The amounts of distillate theoretically obtainable are quite small, as will be apparent from the figures given as examples [108] in Table 47. 

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Molecular distillation is used in the separation and purification of vitamins and other natural products, and for the distillation of high-boiling synthetic organic compounds. 

The so-called hydro-vac pump, shown in Fig. 11, 22, 2 (the upper half of the mercury reservoir and the column above it are insulated by a layer of asbestos), is an inexpensive, all-glass, mercury diffusion pump, which can be used in series either with an oil pmnp or with a water Alter pmnp (compare Fig. 11,21, 1) capable of producing a vacuum of at least 2 mm. It is accordingly of particular value in the organic laboratory for vacuum distillations, fractionations, sublimations and pyrolyses as well as for molecular distillations (see Section 11,26). The hydro-vac... 

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 —... 

Rearrangement to an open chain imine (165) provides an intermediate whose acidity toward lithiomethylthiazole (162) is rather pronounced. Proton abstraction by 162 gives the dilithio intermediate (166) and regenerates 2-methylthiazole for further reaction. During the final hydrolysis, 166 affords the dimer (167) that could be isolated by molecular distillation (433). A proof in favor of this mechanism is that when a large excess of butyllithium is added to (161) at -78°C and the solution is allowed to warm to room temperature, the deuterolysis affords only dideuterated thiazole (170), with no evidence of any dimeric product. Under these conditions almost complete dianion formation results (169), and the concentration of nonmetalated thiazole is nil. (Scheme 79). This dimerization bears some similitude with the formation of 2-methylthia-zolium anhydrobase dealt with in Chapter DC. Meyers could confirm the independence of the formation of the benzyl-type (172) and the aryl-type... 

Although all four tocopherols have been synthesized as their all-rac forms, the commercially significant form of tocopherol is i7//-n7i a-tocopheryl acetate. The commercial processes ia use are based on the work reported by several groups ia 1938 (15—17). These processes utilize a Friedel-Crafts-type condensation of 2,3,5-trimethylhydroquinone with either phytol (16), a phytyl haUde (7,16,17), or phytadiene (7). The principal synthesis (Fig. 3) ia current commercial use iavolves condensation of 2,3,5-trimethylhydroquiQone (13) with synthetic isophytol (14) ia an iaert solvent, such as benzene or hexane, with an acid catalyst, such as ziac chloride, boron trifluoride, or orthoboric acid/oxaUc acid (7,8,18) to give the all-rac-acetate ester (15b) by reaction with acetic anhydride. Purification of tocopheryl acetate is readily accompHshed by high vacuum molecular distillation and rectification (<1 mm Hg) to achieve the required USP standard. 

C-21 dicarboxyhc acids are produced by Westvaco Corporation in Charleston, South Carolina in multimillion kg quantities. The process involves reaction of tall oil fatty acids (TOFA) (containing about 50% oleic acid and 50% hnoleic acid) with acryhc acid [79-10-7] and iodine at 220—250°C for about 2 hours (90). A yield of C-21 as high as 42% was reported. The function of the iodine is apparendy to conjugate the double bond in linoleic acid, after which the acryhc acid adds via a Diels-Alder type reaction to form the cycHc reaction product. Other catalysts have been described and include clay (91), palladium, and sulfur dioxide (92). After the reaction is complete, the unreacted oleic acid is removed by distillation, and the cmde C-21 diacid can be further purified by thin film distillation or molecular distillation. 

Molecular distillation occurs where the vapor path is unobstmcted and the condenser is separated from the evaporator by a distance less than the mean-free path of the evaporating molecules (86). This specialized branch of distillation is carried out at extremely low pressures ranging from 13—130 mPa (0.1—1.0 p.m Hg) (see Vacuum technology). Molecular distillation is confined to appHcations where it is necessary to minimize component degradation by distilling at the lowest possible temperatures. Commercial usage includes the distillation of vitamins (qv) and fatty acid dimers (see Dimeracids). 

The operating principle of steam-jet ejectors is explained in Volume 1, Chapter 8. Then-specification, sizing and operation are covered in a comprehensive series of papers by Power (1964). Diffusion pumps are used where very low pressures are required (hard vacuum) for processes such as molecular distillation. 

Molecular sieves, membrane permeation Molecular distillation Gaseous diffusion, thermal diffusion Filtration, sieves... 

Advantages High analysis rate 3-4 elements per hour Applicable to many more metals than voltammetric methods Superior to voltammetry for mercury and arsenic particularly in ultratrace range Disadvantages Nonspecific absorption Spectral interferences Element losses by molecular distillation before atomisation Limited dynamic range Contamination sensitivity Element specific (or one element per run) Not suitable for speciation studies in seawater Prior separation of sea salts from metals required Suspended particulates need prior digestion About three times as expensive as voltammetric equipment Inferior to voltammetry for cobalt and nickel... 

Examples of Emulsifiers. Distilled Monoglycerides E 471 -These are a high purity monoglycerides prepared by molecular distillation. 

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. 

Hajibagheri MA, Flowers TJ. Use of freeze-substitution and molecular distillation drying in the preparation of Dunaliella parva for ion localization studies by x-ray microanalysis. Microsc Res Technol 1993 24 395-399. 

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