General Comments

The efficiency of any distillation column is dependent to some extent upon its being operated under adiabatic conditions. Thus, distillation columns are usually insulated, enclosed within a silvered vacuum jacket, or enclosed within a heated jacket the temper ature of which can be measured and controlled to within about 3°C of the vapour in the column. Too much heat will result in superheating of the vapour, causing insufficient condensation and reboiling as the vapours proceed up the column; too little heat can result in excessive condensation and flooding in the column.

Since maximum efficiency also depends upon operation at or near thermal equilibrium it is important to be able to change from one fraction to another without interrupting the distillation. A number of designs are available from scientific laboratory glass supply houses to accomplish this. In this connection, it will be found convenient to install some form of manostatic device in the vacuum line to prevent pressure fluctuations during the course of the distillation.

The source of heat input to the pot must be provided with various zones such that the heat input to the flask may be restricted to the area of the flask immersed in the distilland as the distillation proceeds. Failure to control the heat input in this way will result in serious superheating of the vapour as the distillation nears its end; driving superheated vapour into the column will result in significant loss of efficiency.

Finally, allusion was made to use of forced reflux during fractionation; it is exceedingly important to the maintenance of column equilibrium and thus fractionation efficiency. Even with a simple column such as the Vigreux, use of a partial takeoff head will result in increased ability of the system to furnish fractions of relatively high purity. In the case of packed columns control of reflux is absolutely essential to proper column performance; in its absence vapour-liquid equilibrium is never established resulting in loss of up to 90% of the fractionating efficiency of the column. Typically, reflux ratios (reflux ratio is defined as the ratio of the amount of condensate returned to the column to the amount of condensate collected as efflux) as high as 50:1 are not uncommon during difficult separations; ratios approaching 1: 3 are sometimes used during centre cuts or end fractions. Various means to effect reflux control are available from laboratory glass suppliers, from simple devices utilizing manual control by means of a stopcock to elaborate units in which a magnetically controlled value is used to divert the condensate stream either to the column or to the receiver.

Solar Panel Basics

Solar Panel Basics

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