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.