636 



Mr. A. F. Dufton on the Separation 



The Thekmal Efficiency of a still-head for such a binary 

 mixture may be defined as the ratio of the mass actually 

 separated by this amount of heat to unit mass, or, equiva- 

 lently, as the ratio of the theoretical quantity of heat 

 required for a separation to the amount actually expended. 

 It is expressed conveniently as a percentage. 



The value of k — that is, the relative composition of vapour 

 and liquid phases in equilibrium — must be determined experi- 

 mentally. Attempts have been made to express k in terms 

 of the physical constants of the components, and some success 

 has been attained ; thus, for alcohol and water, a mixture of 

 considerable industrial interest, Duclaux * has established 

 the relation 



A a 



■P 



E 



a + e' 



where p is a constant, 

 and where 



a represents alcohol per cent, by vol. in the liquid, 



e „ water „ „ 



A ,, alcohol „ ,, condensed 



E 



water 



r apoui 



„ „ condensed 



vapour. 



For substances which are chemically closely related to 

 each other a simpler relation exists, viz. : 



where P 2 and P 2 are the vapour-pressures of the pure com- 

 ponents at the temperature considered f. 



Fig. 1 shows for mixtures of benzene and toluene the 

 composition of liquid J and vapour phases in equilibrium at 

 atmospheric pressure, the composition of the vapour being 

 calculated from that of the liquid by means of this relation. 



As in the design of an efficient column a knowledge of 

 the flow of vapour and of liquid is desirable, the minimum 

 flow in an ideal column must be calculated. If in the column 



* Duclaux, Ann. de Phys. et Chim. 1878. 



t Brown, Journ. Chem. Soc. 1881, p. 304. Zawidski, Zeit. f. Phys. 

 Chem. xxv. p. 129 (1900) ; cf. also Young/ Stoichiometry,' p. 256 (1918). 

 Ostwald, Lehrbuch d. Allyem. Chem. (2 Aufl.), iii. p. 613. 



X Spielman & Wheeler, Tables of Chem. and Phys. Constants. 

 H.M.S.O. 1919. ' 



