1. FLASH CALCULATION
A flash is a single-equilibrium stage distillation in witch a feed is partially vaporized to give a vapor richer in the most volatile components than the remaining liquid.
a) Adiabatic flash
As shown in fig. (4.10, Seader), a liquid feed is heated under pressure and flashed adiabatically across a valve to lower pressure, resulting in the creation of a vapor phase that is separated from the remaining liquid in a flash drum.
ChE 334: Separation Processes
Dr Saad Al-Shahrani

2. FLASH CALCULATION b) Isothermal flash
If the valve is omitted, a low-pressure liquid can be partially vaporized in the heater and then separated into two phases in the flash drum (when TV is specified).
ChE 334: Separation Processes
Dr Saad Al-Shahrani

3. FLASH CALCULATION
Alternatively, a vapor can be cooled and partially condensed, with phase separation in a flash drum as in figure (4.10b, Seader), to give a liquid that is richer in the less volatile components.
In both cases, if the equipment is properly designed, the vapor and liquid leaving the drum are in equilibrium
ChE 334: Separation Processes
Dr Saad Al-Shahrani

4. FLASH CALCULATION Isothermal flash calculations
In this process an entering stream (a liquid at high pressure) flash as it comes into a vessel that is at lower pressure. Significant quantities of both liquid and vapor phases are formed.
V= vapor flow rate (moles/ time)
L= Liquid flow rate (moles/ time)
F= Feed flow rate (moles/ time)
zi,xi,yi= mole fraction of component i in the feed, liquid, and vapor streams respectively.
TV=TL
PV=PL
ChE 334: Separation Processes
Dr Saad Al-Shahrani

5. FLASH CALCULATION In a typical isothermal flash calculation,
Given: F , zi , T and P in the vessel.
unknown: V , L, yi , and xi.
We have (C) component balances, C more equations are required. They are obtained from phase equilibrium.
Total mass balance (1 eqn)
ChE 334: Separation Processes
Dr Saad Al-Shahrani

6. FLASH CALCULATION Component mass balance (C) equation:
Phase equilibrium (C) equation:
(3) and (1) are substituted in (2):
Solving for xi:
ChE 334: Separation Processes
Dr Saad Al-Shahrani

7. FLASH CALCULATION Dividing numerator and denominator by F yields
If the system is ideal, iL= 1.0, Ki =Pisat/P
Equations 6,7 can be written for each of the (C) components in the system
ChE 334: Separation Processes
Dr Saad Al-Shahrani

8. FLASH CALCULATION If and Then
A flash calculation is an iterative trial and error procedure where the problem is to guess (V/F) ratio until:
ChE 334: Separation Processes
Dr Saad Al-Shahrani

9. FLASH CALCULATION Flash calculation iteration for ideal systems
The first thing to do in any flash calculation is to make sure that something will flash (there is liquid and vapor phases).
If we are in a region where nothing flashes P of the system must higher than the bubble point P of the feed at the specific T.
x,y
Sub-cooled liquid
1.0
Superheated Vapor y z
Pressure x x1 y1 P1
yBP
xDP
Saturated
vapor line
Saturated liquid line
PBP
PDP
T= constant
If P>PBUB, nothing will flash, V/F=0 and xi=zi
ChE 334: Separation Processes
Dr Saad Al-Shahrani

10. FLASH CALCULATION Dew point
If P< PDew, every thing flashes, V/F=1 and yi=zi
Once we know that two phases will be present, equation (7) must be solved for (V/F).
ChE 334: Separation Processes
Dr Saad Al-Shahrani

11. FLASH CALCULATION
Example: A liquid feed stream containing 40 mole perercent benzene, 35 mole percent toluene and 25 mole percent o-xylene is flashed to 110°C and 760 mm Hg (14.7psia). Determine the flow rates and compositions of the vapor and liquid streams after the flash for 100 kg-mole/hr of feed. The VLE relationships can be assumed to be ideal. i.e the system obeys Raoult's Law.
Solution
The first thing to do in any flash calculation is to check whether the actual pressure P is between the bubble point and dew point pressures of the feed at the operating temperature.
The vapor pressures at 110°C (230°F) are read from the Cox chart.
ChE 334: Separation Processes
Dr Saad Al-Shahrani

12. FLASH CALCULATION Pisat at 110 oC from Cox chart.
Benzene (1) = P1sat = 33 psia
Toluene (2)= P2sat = 14 psia .
O-Xylene (3)= P3sat = 5.6 psia F
100 kgmole/hr
z1= 0.4
z2=0.35
z3=0.25
V=?
y1= ?
y2=?
y3=?
L=?
x1= ?
x2=?
x3=?
TV=TL=110 oC
PV=PL= 1 atm
PDew < P=14.7 psia < PBUB
So there are two phases in the drum (vap. and liq. phases
ChE 334: Separation Processes
Dr Saad Al-Shahrani

13. ChE 334: Separation Processes
FLASH CALCULATION
Guess: V/F = 0.5, 0.7 and 0.65
V/F=0.5
V/F=0.7
V/F=0.65 yi
(at V/F=0.65)
Component zi
Pjsat xi
Benzene
0.4 33
0.247
0.214
0.22
0.496
Toluene
0.35 14
0.359
0.362
0.36
0.344
O-xylene
0.25
5.6
0.441
0.42
0.159
Total
1.000
0.968
1.017
1.00
0.999
ChE 334: Separation Processes
Dr Saad Al-Shahrani

14. ChE 334: Separation Processes
FLASH CALCULATION
V =(V/F)* F =0.65 * 100= 65 kg mole/ hr
L= F - V = = 35 kg mole/hr
ChE 334: Separation Processes
Dr Saad Al-Shahrani

15. FLASH CALCULATION
Flash calculations in VLE relationship = f(T,P) only and not f(composition), the isothermal flash calculation is essentially the same as if the system were ideal.
a) Binary system (T,P are specified)
ChE 334: Separation Processes
Dr Saad Al-Shahrani

16. FLASH CALCULATION y1 and y2 can be obtained from yi=Kixi
The fraction vaporization (V/F) can be obtained from
ChE 334: Separation Processes
Dr Saad Al-Shahrani

17. FLASH CALCULATION
ChE 334: Separation Processes
Dr Saad Al-Shahrani

18. FLASH CALCULATION b) Multi-components systems: divide by L divide by L
ChE 334: Separation Processes
Dr Saad Al-Shahrani

19. FLASH CALCULATION
ChE 334: Separation Processes
Dr Saad Al-Shahrani

20. FLASH CALCULATION
ChE 334: Separation Processes
Dr Saad Al-Shahrani

21. FLASH CALCULATION Case #1 If T and ( )of one component i is specified
Guess P and find 
If equation (2) is divided equation. (1)
ChE 334: Separation Processes
Dr Saad Al-Shahrani

22. FLASH CALCULATION or if equation (1) is divided by equation (2)
ChE 334: Separation Processes
Dr Saad Al-Shahrani

23. FLASH CALCULATION Case # 2 P and T are specified
If f(V/F) is positive at V/F =0 and negative at V/F =1.0, then two phases are present . If f(V/F) is negative at V/F =0, the system is sub-cooled. If f(V/F) is positive at V/F =1, the system is superheated vapor.
It is called Rachford-Rice function
ChE 334: Separation Processes
Dr Saad Al-Shahrani

24. FLASH CALCULATION Case # 3 Guess T P and V/F specified Like case # 2
P and ( )of one component are specified
Guess T
Like case # 1
ChE 334: Separation Processes
Dr Saad Al-Shahrani