of Miscible Liquids by Distillation. 639 



It will be seen that condensation of toluene takes place 

 throughout the length of the column. The lower portion 

 of the table shows that an increased amount of evaporation 

 and condensation must take place as the mixture becomes 

 poorer in benzene. It is of interest to observe that in the 

 removal of a trace of toluene from " pure " benzene by 

 distillation 1*64 gram must be evaporated for each gram 

 collected. 



The heat which must be lost in the column during the 

 distillation of one gram of benzene from a mixture may be 

 calculated from Table I. At the top of the column 0*64 gram 

 of benzene must be condensed at its boiling-point. This in- 

 volves a loss of 61 calories. It is convenient to divide the 

 remainder of the column into sections, each corresponding to 

 a temperature difference of two degrees, and to evaluate the 

 loss in each. For these sections the loss of heat is shown in 

 Table II. 



Table II. 



Section. 

 °C. 



80- 82 



Grams of Vapour 



condensed. 



Calories lost 

 (approx.). 



010 



9 



82- 84 



012 



10 



84- 86 



0-14 



12 



86- 88 



016 



14 



88- 90 



049 



17 



90- 92 



0-23 



20 



92- 94 



0-29 



25 



94- 96 



0-37 



32 



96- 98 



0-51 



44 



98-100 



07S 



86 



100-102 



1-18 



101 



102-104 



1-60 



136 



108-110-6 



00 



00 



It is important to observe that for perfect thermal efficiency 

 the liquid and vapour need not be in equilibrium at any point 

 other than the top and bottom of the column. 



The particular case in which the whole of the necessary 

 loss of heat is effected at the top of the column is of interest. 

 It gives the maximum How for perfect efficiency. This now 

 for the distillation of one gram of benzene from a 50-per- 

 cent, mixture of benzene and toluene is shown in Table III. 

 The loss of heat is 117*5 calories, i.e. the 208 supplied for 



