03-12-2012, 01:15 PM
ENTROPY AND EQUILIBRIUM
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Flash calculations
Flash calculations are used for processes with vapor/liquid-equilibrium (VLE). A
typical process that requires flash calculations, is when a feed stream (F) is separated
into a vapor (V ) and liquid (L) product; see Figure 7.4.
In principle, flash calculations are straightforward and involve combining the VLE-
equations with the component mass balances, and in some cases the energy balance.
Some flash calculations are (with a comment on their typical numerical solution or
usage):
1. Bubble point at given T (easy)
2. Bubble point at given p (need to iterate on T )
3. Dew point at given T (easy)
4. Dew point at given p (need to iterate on T )
5. Flash at given p and T (relatively easy)
6. Flash at given p and H (“standard” flash, e.g., for a flash tank after a valve)
7. Flash at given p and S (e.g., for condensing turbine)
8. Flash at given U and V (e.g., for dynamic simulation of an adiabatic flash drum)
The last three flashes are a bit more complicated as they require the use of the energy
balance and relationships for computing H, S, etc. The use of flash calculations is best
illustrated by some examples.
Bubble point calculations
Let us first consider bubble point calculations, In this case the liquid-phase
composition xi is given (it corresponds to the case where V is very small (V ? 0)
and xi = zi in Figure 7.4). The bubble point of a liquid is the point where the liquid
just starts to evaporate (boil), that is, when the first vapor bubble is formed. If the
temperature is given, then we must lower the pressure until the first bubble is formed.
If the pressure is given, then we must increase the temperature until the first bubble
is formed.
Dew point calculations
Let us next consider dew point calculations. In this case the vapor-phase composition
yi is given (it corresponds to the case where L is very small (L ? 0) and yi = zi in
Figure 7.4). The dew point of a vapor (gas) is the point where the vapor just begins to condense, that is, when the first liquid drop is formed. If the temperature is given,
then we must increase the pressure until the first liquid is formed. If the pressure is
given, then we must decrease the temperature until the first liquid is formed.