Short Path and Molecular Distillation

Distillation is a thermal separation technique. It is used to separate liquid mixtures into their various fractions or components by taking advantage of the differences in vapour pressure. The process becomes impractical if the thermal load (temperature, exposure time) becomes too high resulting in chemical reactions and/or decomposition of one or more components. To reduce temperature the distillation is achieved under vacuum. To reduce exposure time at elevated temperature the hold-up in the apparatus is minimized. Nowadays short path and molecular distillation belong to the gentlest distillation technologies. 

The first successful attempt on continuous distillation and improved efficiency as well as profitability was the Coffey Still in 1831. It was used to produce Whisky. The Coffey Still consists of two columns called analyser and rectifier. The fermented feedstock is preheated against condensing vapours in the rectifier and fed at the top of the analyser. The liquid flows over a copper plate with openings and from there into a down-comer leading onto another, underlying plate, and so on. More than 10 perforated plates (called trays) are typically installed in the analyser. 

Vacuum Technology in the Chemical Industry, First Edition. Edited by Wolfgang Jorisch. 

Steam is injected at the bottom of the analyser and funnels through the openings upwards. In the created bubbhng areas an intensive heat and mass transfer occurs the less volatile components in the upstreaming vapour condense and the more volatile components of the hquid vaporise. As a result the alcohol content of the downflowing hquid decreases until it is completely stripped off in the bottom effluent. The alcohol content increases in the upstreaming vapours. 

The alcohol-rich vapours from the top of the analyser are guided through a vapour duct into the bottom of the rectifier where the alcohol is further concentrated. The top vapours of high proof are liquefied in a condenser and discharged. 

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

The areas of application for these vacuum pumps are all within the rough and medium vacuum range, the principal vacuum range for chemistry processes. Only the short path and molecular distillation processes rely also high-vacuum pumps like the diffusion pump or even the turbomolecular pump (kinetic gas pumps). High-vacuum pumps are not covered in this book. For these refer to [2, 3]. 

Short path and molecular distillation were developed in the 1930s and 1940s mainly in the UK and in the USA. Different types of apparatus were developed. A still with a disk, rotating at high speed, is named after Hickman, one of the pioneers [2]. In this machine the centrifugal force is used to distribute the feedstock as a thin film on a heated disk. Exposure time is less than a few seconds ... 

Short path and molecular distillation always apply in case a gentle treatment of the material is a must, for example in the chemical industry, in the production of pharmaceuticals and of food ingredients or in bio technology. The number of applications in the production of polymers, special chemicals, biofuels and in the recovery of valuables from wastes is increasing. 

The checkers found this fractional distillation to be simplified if the initial crude product was first subjected to a rapid short-path distillation under reduced pressure to remove the bulk of the dialkyl ated material and other high molecular weight components. 

A 2-L, three-necked flask fitted with a condenser, argon bubbler, thermometer, and addition funnel is charged with 700 mL of toluene (freshly distilled and dried over 4 A molecular sieves) and 20 mL (Note 4) of cyclohexene (freshly distilled). With magnetic stirring, the mixture is heated to 105°C and the azidofuranone solution prepared above is added over a period of 20 min (Note 5). Upon completion, the reaction mixture is heated for an additional 1.25 hr at 105°C. The solution is cooled and concentrated under reduced pressure to yield a yellow-brown residue which is distilled using a short-path apparatus to give 13.3 g (76%) (bp 85-90°C, 0.5 mm) of the cyclobutanone as a very pale yellow oil which solidifies (mp 34-35°C) upon standing at 4°C (Notes 6 and 7). 

Distillation processes exploit the low volatility of cholesterol compared to the major triacylglycerols of milk fat for removal of cholesterol. Vacuum and short-path molecular distillation processes can efficiently remove cholesterol but it may be achieved at the expense of removing some low-molecular weight triacylglycerols and flavor components of the milk fat. Vacuum steam distillation is commonly used for refining fats and can also be used to refine milk fat. Cholesterol-reduced milk fat, which was produced by steam distillation, has been used successfully to formulate butter, cream and ice cream (Schroder and Baer, 1991, Elling et al., 1995, 1996). If the flavor of milk fat is to be preserved, the flavors can be trapped and re-incorporated into the milk fat that has been stripped of cholesterol (Boudreau and Arul, 1993). 

There have been many cholesterol-reduction technologies developed all over the world because of high interest by the dairy industry. However, there are only a few technologies available for technology transfer. Fractionation by thermal crystallization, steam stripping, short-path molecular distillation, supercritical fluid extraction, selective absorption, and crystallization using solvents or enzymatic modification can achieve fat alterations of significance to the dairy industry. 

As desirable products have similar molecular weights (ethyl docosahexaenoate is 356, and ethyl eicosapentaenoate is 330), it is clear that this type of fish oil product cleanup is best done at the oil stage to avoid losses at the same time as contaminant removal, which is carried out as described above. However, dimers and/or other polymeric and/or colored materials may be left behind if wiped wall equipment is used in its capacity of a simple and inefficient short-path still for the ethyl esters only. This process will produce a water-white distillate product. Regrettably, it is not an efficient way to separate ethyl-EPA from ethyl-DHA. 

Short-path molecular distillation Separates molecules by their molecular weight Fractionation of MAG, DAG, TAG, and fatty acid esters from lipolysate or reaction medium... 

The soybean oil was stripped of tocopherols by short-path molecular distillation. The relative potency was based on inhibition of hydroperoxides measured by conjugated diene formation at optimal concentrations of about 100 ppm for a-tocopherol, about 300 ppm for y-tocopherol. The optimal concentration for 5-tocopherol was not clearly defined and the antioxidant activity increased with concentration up to -1900 ppm. 

The vacuum processes which are utihsed in the area of chemical engineering are generally performed in the range of one to several hundred millibars (Figure 5.1). An exception to this is the process of short path distillation, respectively molecular distillation, which operates at low pressures in the medium and the beginning high vacuum range and which here is separately covered in chapter 15 of this book. 

Essential larger quantities of material can be treated with a newly developed plate molecular still [5]. It is a combination of plate-falling film evaporator and short path still. In a vacuum chamber heated and cooled plates are fixed alternately Figure 15.12 shows the principle. The feed is equally distributed at the top of the heated plates. The produced vapours are condensed on the cold surfaces. At the bottom of the still distillates and residues are collected separately and discharged. Main advantages no rotating parts and a large evaporation area per volume vacuum chamber. 

Examination of the of the cyclic product from the polymerization in the presence of 15-crown-5, after short path very high vacuum distillation, showed that it was mostly (90%) a cyclotetrasilane. It shows a MW of 512 by mass spectmm, and the corresponding cyclic material prepared from propylmethyldichlorosilane, a molecular weight of 344. The latter material also gave a molecular weight of 355 ( 15) by vapour phase osmometry. 

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