TABLE OF CONTENTS As a mixture of substances is evaporated, the residue becomes relatively depleted in the more volatile constituents. A relation for binary mixtures due to Rayleigh is developed as follows: The differential material balance for a change dL in the amount of liquid remaining is
| (13.54) |  |
Upon rearrangement and integration, the result is
| (13.55) |  |
In terms of a constant relative volatility
| (13.56) |  |
the integral becomes
| (13.57) |  |
Simple distillation is not the same as flashing because the vapor is removed out of contact with the liquid as soon as it forms, but the process can be simulated by a succession of small flashes of residual liquid, say 1% of the original amount each time. After n intervals, the amount of residual liquid F is
| (13.58) |  |
and
| (13.59) |  |
Then the flash equation (13.41) becomes a function of temperature,
| (13.60) |  |
Here z i is the composition at the end of interval n and K i also may be taken at the temperature after interval n. The composition is found by material balance as
| (13.61) |  |
where each composition y ik of the flashed vapor is found from Eqs. (13.43) and (13.44)
| (13.62) |  |
and is obtained during the process of evaluating the temperature with Eq. (13.60). The VERs must be known as functions of temperature, say with Eq. (13.24).
