I'm trying to dimension a flash drum for a separation between water and isobutane. I now the total inlet flow (F), the composition of the inlet flow ($z_1$ for isobutane and $z_2$ for water) and the total pressure (P). My supervisor told me that if my inlet flow to the flash was gas I would want my flash drum to operate at dew point temperature. And vice versa I would like my flash drum to operate at bubble point if my inlet flow was liquid. Therefore I have to do a dew or bubble point calculation for the temperature. Why is this true? According to her this is something you always want you flash drum to operate at.
If I had known the outlet molar flow of vapor (V) and liquid (L) I would have used the following iterative model:
- We know F, $z_i$, P, V and L
- Guess T
- Calculate $P_1°$ and $P_2°$ from a model (as an example Antoines equation)
- Calculate $x_i=\frac{Fz_i}{V\frac{P_i°}{P}+L}$
- Check if $\sum x_i=1$. If not true redo from point 2. If true you have found the flash temperature.
However this isn't possible since I don't know the outlet flows V and L.
So to recap my question is: Why does the flash need to operate at dew or bubble point temperature? Is it because it's most cost efficient to do so? Do I get the best separation at this temperature (and if so why)? Any other reasons why this temperature is the best to operate at?