A N E W LABORATORY STILL-HEAD MIECZYSLAW WOJCIECHOWSKI Ethyl Corporation, Detroit, Michigan
THE
s m ~ ~ r h e aof d an e5cient distilling column should permit precise control of the material distilled by providing a means of accurately measuring the temperature of distillation, and should allow observation of the rate of distillation and control of the reflux ratio. It is also desirable that the still-head provide cold distillate, especially when fairly low boiling liquids are being distilled. The still-head described possesses all these requirements in addition to some other advantages. It consists of a tube, A (Figure I), fitted a t the lower end with a ground-glass joint for connecting it to the column. This tube should be thermally insulated. The upper end of the tube has a thermometer well, T, of dimensions suitable for using a Beckmann thermometer. Other means of measuring the temperature of distillation, such as a different type of mercury in glass thermometer, a platinum resistance thermometer, or a multijunction thermocouple can also he used. This is possible because the thermometer well is usually filled with mercury, providing good heat transfer to the measuring instrument; thus the relatively large size of the thermometer well does not affect the precision of the measurement. At the upper end of tube A there is a large diameter sidetube, K, through which the vapor passes to the condenser, C. The condenser is built in the form of a coil in a countercurrent cold water bath. The vapor enters the upper end of the coil, where it condenses, and the condensate cools and flowsdown to thedropcounter, D,. The diameter of the coil tube should he large enough to avoid obstructing the coil with condensed liquid. Thus there is always communication between the thermometer well area in the still-head and the tube, E, connecting the still with the surrounding atmosphere, or with a manostat adjusted to the desired pressure. Since the condenser is directed downward, the still-head does not require extra height. The two drop counters, Dland Dz, are hemispherical in shape and of the same dimensions, as shown in the figure. Therefore the size of drops of a given liquid from DIand from Dzis the same. Below the drop counter DIthere is a shallow liquid seal, P, in which a layer of the condensed liquid closes this part of the still-head, protecting it from contact with vapor which otherwise would flow through the connecting tube, L. The tube L serves for returning the reflux which overflows the seal to the center of the distilling column. P is connected to the stopcock, S, by means of a capillary tube in order to reduce hold-up in the still-head.
The stopcock serves to adjust the rate of withdrawal of the distillate which is measured with the drop-counter, Dz. From the number of drops per minute measured with the drop-counter, Dl, and the number of drops per minute measured with the drop-counter, D2, the reflux ratio may be computed. The side tube, E, is fitted with a removable funnel, F, through which material can be added to the column without interrupting the operation of the still. For rectification a t reduced pressure, connection is made to the vacuum system at point G and a t a suitable point in the receiving system. Regarding the performance of the still-head, it should be mentioned that the formation of droplets of condensate above the main line of condensation, frequently encountered with other still-heads, is eliminated with this apparatus. In the conventional still-head, these
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