646 'Separation of Miscible Liquids by Distillation. 



The total evnporation necessary in the separation of a mass 

 of the more volatile constituent is the same as that required 

 in the discontinuous separation of the same mass from a large 

 quantity of the mixture. 



The heat required for the separation of one gram is 



Q I i 



^T)} ^ 



\ T «(£-l) 



The separations afforded by continuous and by discon- 

 tinuous distillation may be compared. From equation (2), 

 assuming k to be constant and equal to 2'5 for mixtures of 

 benzene and toluene and Q to be constant and equal to 

 90 calories, the heat required to separate by discontinuous 

 distillation p grams of benzene from a mixture of one gram 

 of benzene with one gram of toluene, leaving- a residue of 

 (1— p) gram of benzene and one gram of toluene, is 



Qt P » 1+ h 2 wf v?> dx 

 Jo * U -x){k — l) j 



Q r 



k-i \ 



.k- 



log e (l-p)} (6) 



Equation (5) shows that 21,000 calories are theoretically 

 capable of separating 100 grams of benzene from 100 grams 

 of toluene in continuous distillation. The same quantity of 

 heat in discontinuous distillation will only separate from 

 this mixture 78 grams of benzene, leaving 22 grams of 

 benzene with the 100 grams of toluene. In order to 

 separate 99 grams of benzene and leave 1 gram with the 

 toluene, twice this amount of heat is required. 



This comparison indicates that continuous distillation is 

 much more economical than discontinuous. In continuous 

 working, moreover, a column can be employed at its maxi- 

 mum efficiency throughout, while the length must be variable 

 if the highest efficiency in discontinuous distillation is to be 

 obtained. - 



The experimental portion of this investigation was carried 

 out in Cambridge at the Cavendish Laboratory «nd one of 

 the still-heads used was kindly lent by Dr. S. h\ Dufton, to 

 whom I am much indebted for encouragement and advice. 



