Molecular or short-path distillation is one form of high vacuum distillation, and offers a means of separating heat-sensitive, high-boiling materials which cannot be separated by conventional methods. Many of the so-called "undistillables," which include the natural fats and waxes, sugar derivatives, petroleum residues, plasticizers, dyes, and a host of other substances now fall within the scope of molecular distillation. The use or the process is limited, however, due to the high costs involved and the poor separatoey powers which are still characteristic of this method or distillation. Although there have been many mechanical improvements in the constructional designs of the molecular stills since the early beginnings of the crude pot stills, they still remain too inflexible in design and too compact in construction to allow the proper study or the fundamental factors affecting their performance. One of the problems yet to be solved satisfactorily is the prevention of leaks through the bearings and packings of the rotor shaft. It was the purpose of this investigation to redesign and reconstruct a five-inch magnetically-driven centrifugal molecular still, and by so doing to eliminate leaks around the rotor shaft by enclosing the rotor within the still head, and to gain flexibility by the use of a high-form bell jar, and a water-cooled condenser. The still head assembly consisted of a number of integrated units, many of which were of the same design and construction as the units used in other stills by previous investigators. Some of the more important changes were; a magnetic drive, consisting or an inner flange of magnets connected to the rotor shaft, and an outer flange of magnets connected to the drive shaft of the motor, a nitralloy rotary gear feed pump, especially fitted and constructed as a vacuum tight unit, and a water-cooled copper condenser, so constructed that its distance from the rotor surface could be varied. The feed, residue, and distillate tanks, as well as all flow lines for the feed, were constructed of aluminum or aluminum tubing; the vacuum lines were constructed of copper tubing, and all joints on the vacuum lines were either bronzed or silver-soldered to minimize the possibilities of leaks. All other constructional features of the still, such as electrical connections, vacuum connections to the base plate, bell jar seal, and thermocouple construction, were the same as used by previous investigators. / Master of Science

The vacuum distillation of heat-sensitive substances such as organic oils, dyes, and drugs requires special equipment from that available for atmospheric distillation. Study in the resulting field, high vacuum distillation, has subsequently led to the development of such equipment. Three types of high vacuum distillation and distillation equipment are recognized: conventional distillation, unobstructed-path distillation, and molecular distillation. Conventional vacuum distillation employs standard equipment under relatively high vacuum conditions. For unobstructed-path distillation, the equipment is modified so that the vapor path between the evaporating and condensing surfaces is clear. When the vapor path is unobstructed and the condenser is separated from the evaporator by a distance less than the mean free path of the evaporating molecules, the phenomenon is called molecular distillation. Molecular distillation is the limiting type of high vacuum distillation with the absolute pressure range approximately 1 to 7 microns of mercury. The use of the centrifugal molecular still permits the distillation of organic substances which might suffer thermal decomposition even under the conditions of the conventional high vacuum still. Against the advantages of prevention of thermal decomposition and shorter time requirements must be placed the poor separatory power of distillation and the necessity of numerous redistillations. The molecular still is valuable not because it gives good separation but because distillation is accomplished 50 to 250 degrees centigrade below temperatures of conventional atmospheric distillation. In ideal molecular distillation, equilibrium does not exist between the vapor and the liquid, no molecules re-enter the distilland after once being vaporized. Under these ideal conditions, Langmuir's equation predicts that the quantity of a given material distilling at a given temperature is proportional to P/√M where P is the vapor pressure and M is the molecular weight. Previous investigations at the Virginia Polytechnic Institute and other laboratories substantiate the importance of molecular weight and structure. Since most of these investigations have been only exploratory, insufficient work has been done to permit any conclusions of major importance. The purpose of this investigation was to determine the effects of various molecular structures of fatty acids on their distillation characteristics when distilled in a laboratory-size centrifugal molecular still. Saturated (C₁₅, C₁₆, C₁₇, C₁₈, C₁₉, and C₂₀) and unsaturated (C₁₈ with one, two, and three double bonds) fatty acids were used in the study. Such effects as the chain length, presence of double bonds and their position in the molecule, and cis-trans and positional isomerism were studied. / Ph. D.

The three basic requirements for unobstructed, short path distillation are: a surface over which a think layer of distilland is distributed, a cooler condensing surface in close proximity to the evaporating surface, and a suitable system for evacuating the space between the two surfaces. When the distance of transfer between the evaporating and condensing surfaces is comparable to the mean free path of the vapor molecules in the residual gas, the process is known as molecular distillation. [Continued in document] / Ph. D.

Molecular distillation is that process of free transfer of molecules under high vacuum from an evaporator to a condenser, where the distance of travel is within the limits of the mean free path of the vapor molecules in the residual gas. Under these conditions, distillations can be made at temperatures 100 to 150°C. lower than ordinary distillations. Many of the so-called "undistillables“ can be distilled under conditions existing in the molecular distillation process. The natural fats and waxes, sugar derivatives, petroleum residues, plastics, and plasticizers now fall within the scope of molecular distillation. The use of the molecular distillation process is now limited by the high costs involved. Thermal efficiency is low, only 2% of the total input heat is utilized for distillation. The separating powers of molecular stills are low, never more than that of one ”theoretical plate“. Only those materials that can bear the high process costs are now being produced commercially, the production of vitamins being the most prominent. Considerable work has been done relative to the development of molecular stills. Advancement to the present day centrifugal stills has been accomplished relatively recently. Many problems are yet unsolved. Much work must be done before molecular distillation can be placed on a level with other methods of distillation. An increase in thermal efficiency, an increase in separating powers, a decrease in film thickness, and a lowering of the costs of the process are necessary. The purpose of this investigation was to design and construct a five inch centrifugal molecular still. Four interchangeable rotors were employed, each of different thickness, different diameter, and having a different angle of inclination. The effect of angle of inclination on film thickness can then be studied. Although no data has been published relative to still performance of five inch commercial stills, data obtained from the operation of the still constructed in this investigation can be compared when such data is published. / Master of Science