1) A flat bottomed test tube of 200mm height and 10mm internal diameter stands on its base. It initially contains a residue of a liquid solvent of depth 2mm, which is gradually evaporating. The test tube is in an air filled chamber at atmospheric pressure and 27C . The chamber is large enough such that the concentration of the solvent in the chamber is negligible. The molecular weight of the solvent is 32 . At 27C the solvent's vapour pressure is 19kPa , its liquid density is 780kg /m3 and its diffusivity in air is D = 1.2 x10-5 m2 / s .
The mass transfer through the vapour column can be approximated by j''=(CD/l)* ln[ (1-x1)/(1-x0)]= N'/A where N is the molar flow rate across area A , C is the total molar concentration, l is height of the vapour column and x0 and xl are the solvent mole fractions at height 0 and l
above the liquid surface.
i) Neglecting variations in the height of the vapour column in the tube during evaporation, determine how long it will take for the solvent to evaporate completely.
ii) The air pressure in the chamber is now doubled, with everything else remaining as in part i). Determine how long it will now take for the solvent to evaporate completely.
iii) The temperature of the entire system is now raised significantly relative to the conditions in part ii). In a brief paragraph, state what should happen to the solvent's evaporation time, and explain how this occurs by making use of the equation above.