634 Mr. A. F. Dufton on the Separation 



The invention in 1918 by Dr. S. F. Dufton * of a labora- 

 tory still-head of small working volume and not inconvenient 

 length, giving perfect separation of simple binary mixtures, 

 e.g. benzene and toluene, renders possible the examination in 

 the laboratory of the rationale of separation by distillation. 



This examination is the object of the present research. 

 The problems presented by close isomers and by the recently 

 discovered isotopes render desirable every possible refinement 

 in methods of separation, and a knowledge of the physical 

 processess taking place in a still-head should render 

 possible both the application of distillation to more difficult 

 separations and the elimination of the empirical element in 

 design. 



To minimise the empirical element in the design of still- 

 heads by determining more clearly the physical processes 

 involved, calculation has been made in the following pages 

 of the requisite flow of liquid and of vapour at each point in 

 a theoretically perfect column, and indication has been given 

 of the corresponding supply or withdrawal of heat involved. 

 A quantity, termed the Thermal Efficiency, has been 

 defined to afford a comparison of the performance of columns 

 yielding a pure distillate and a measure of the approach 

 towards theoretical perfection. 



2. F. D. Brown | has pointed out that when a mixture of 

 two liquids which mingle in all proportions is heated to 

 ebullition, it evolves a mixed vapour containing a certain 

 proportion of each of the two substances, the liquid mixture 

 and the gaseous mixture being mutually related and existing* 

 together in a state of equilibrium. 



It is upon this equilibrium between liquid mixture and 

 gaseous mixture of different composition that separation by 

 distillation depends. Ideal discontinuous distillation consists 

 in the evaporation of liquid, in the removal of the vapour 

 produced and in the return to the still, in liquid of the 

 same composition as that in the still, of the whole of the less 

 volatile constituent. The evaporation of 2*32 grams from 

 a large mass of a mixture of equal masses of benzene 

 and toluene, for example, yields as vapour 0'66 gram of 

 toluene and 1*66 gram of benzene, and the return of the 

 0*66 gram of toluene with 0'66 gram of benzene as liquid 

 involves the separation of 1*00 gram of benzene. 



The amount of evaporation necessarv and the corresponding 

 quantity of heat required may be calculated mathematically. 

 If in a system of two substances A and B, which mingle in 



* S. F. Dufton. J. Soc. Chem. Industry, 1919, p. 45. 

 t Brown, Journ. Chem. Sue. 1881, p. 531. 



