1. KANKESHWARI DEVI INSTITUTE OF TECHNOLOGY,JAMNAGAR a Presentation on “Bubble Point and Dew Point Calculations” 5th sem sub:- CET-I Prepared by suvagiya jaldip timbadiya akshay vaghasiya rahul

2. Bubble Point and Dew Point Calculations
In the last lecture we discussed:
• An isothermal flash separations
• The derivation and solution of the Rachford Rice equation
• Newton’s iterative procedure to solve for the roots of the RR equation
• A numerical example to demonstrate this approach.
This lecture will cover:
• An example of using the Rachford Rice Procedure with
a simple spreadsheet
• Bubble point and Dew Point temperature and pressure
calculations
• An example of a dew point temperature calculation

3. Multicomponent Flash Calculations
For this system there are 3C+10 variables: F, V, L, {T, P}3, Q, {xi , yi ,zi}C
and C+5 degrees of freedom.
We specify the C+3 variables F, zi, TF, PF and two additional variables
Liquid Feed
Vapor out
Flash Drum
F, zi, TF, PF
L, xi, TL, PL
Liquid out
V, yi, TV, PV Q
Common Specifications:
TV,PV Isothermal Flash
V/F=0, PL Bubble-Point Temperature
V/F=1, PV Dew-Point Temperature
V/F=0, TL Bubble-Point Pressure
V/F=1, TV Dew-Point Pressure
Q=0, PV Adiabatic Flash
Q, PV Nonadiabatic flash
V/F, PV Percent Vaporization Flash
Last lecture
Lecture 6: Bubble and Dew Point

4. Isothermal Flash Calculations
If we specify F, zi, TF, PF and TV ,PV then we apply the Rachford-Rice procedure:
Flash Drum
Liquid Feed
Vapor out
V, yi, TV, PV
F, zi, TF, PF Q
Liquid out
L, xi, TL, PL
Steps 1&2:
Step 3: Solve Rachford-Rice for V/F.
Step 4:
Determine V
Steps 5 and 6:
Step 7:
Determine L
Step 8:
Determine Q
Lecture 6: Bubble and Dew Point

5. Bubble Point Pressure Calculations
For a bubble point pressure calculation we specify F, zi, TF, PF, TL and that we are at
the bubble point.
Flash Drum
Liquid Feed
Vapor out
F, zi, TF, PF
V=0, yi, TV, PV Q
Liquid out
L, xi, TL, PL
At the bubble point there is equilibrium between the vapor and liquid phases, but the system
is completely liquid and thus the vapor fraction  is zero.
Lecture 6: Bubble and Dew Point

6. Bubble Point Temperature Calculations
For a bubble point temperature calculation we specify F, zi, TF, PF, PL and that we are at
the bubble point.
Flash Drum
Liquid Feed
Vapor out
F, zi, TF, PF
V=0, yi, TV, PV Q
Liquid out
L, xi, TL, PL
At the bubble point there is equilibrium between the vapor and liquid phases, but the system
is completely liquid and thus the vapor fraction  is zero.
Lecture 6: Bubble and Dew Point

7. Dew Point Temperature Calculations
For a Dew Point Temperature calculation we specify F, zi, TF, PF, PV and that we are at
the dew point.
Flash Drum
Liquid Feed
Vapor out
V, yi, TV, PV
F, zi, TF, PF Q
Liquid out
L=0, xi, TL, PL
At the dew point there is equilibrium between the vapor and liquid phases, but the system
is completely vapor and thus the vapor fraction  is one.
Lecture 6: Bubble and Dew Point

8. Dew Point Pressure Calculations
For a Dew Point Pressure calculation we specify F, zi, TF, PF, PV and that we are at
the dew point.
Flash Drum
Liquid Feed
Vapor out
F, zi, TF, PF
V, yi, TV, PV Q
Liquid out
L=0, xi, TL, PL
At the dew point there is equilibrium between the vapor and liquid phases, but the system
is completely vapor and thus the vapor fraction  is one.
Lecture 6: Bubble and Dew Point

9. Bubble Point and Dew Point Calculations
To solve the bubble point and dew point problems we must find the unspecified temperature
or pressure which satisfies the Rachford Rice expressions.
Bubble Point
Dew Point
Bubble Point or Dew Point Temperature Procedure:
Step 1: Guess a temperature.
Step 2: Use the given product pressure and guess temperature to determine the K-values from the DePriester chart.
Step 3: Calculate the Rachford Rice expression appropriate to the equilibrium condition.
Step 4: Guess a new higher temperature if K values must be higher. Otherwise, guess a lower temperature.
Step 5: Repeat Steps 3 and 4 until the Rachford Rice equation is satisfied.
Bubble Point or Dew Point Pressure Procedure:
Step 1: Guess a pressure.
Step 2: Use the given product temperature and guess pressure to determine the K-values from the DePriester chart.
Step 3: Calculate the Rachford Rice expression appropriate to the equilibrium condition.
Step 4: Guess a new lower pressure if K values must be higher. Otherwise, guess a
higher pressure.
Step 5: Repeat Steps 3 and 4 until the Rachford Rice equation is satisfied.
Lecture 6: Bubble and Dew Point

10. Example: Rachford-Rice
A flash chamber operating 80 degrees C and 500kPa is separating 1000 kg moles/hr of a feed that is 10 mole %ethane, 5 mole %propane, 15 % n-butane, 10 % n-pentane, 12 mole % isopentane, 8 mole %n-hexane, 30 mole %heptane and 10% nonane.
What are the product compositions and flow rates?
K1 (ethane) = 11.0
K2 (propane) = 4.6
K3 (n-butane) = 1.85
K4 (n-pentane) = 0.75
K5 (isopentane) = 0.9
K6 (n-hexane) = 0.32
K7 (heptane) = 0.14
K8 (nonane) = 0.026
From the Depriester chart:
V/F=0.207
V=207kg/hr
L=793kg/hr
x1 (ethane) = 0.033
x2 (propane) = 0.029
x3 (n-butane) = 0.128
x4 (n-pentane) = 0.123
x5 (isopentane) = 0.105
x6 (n-hexane) = 0.093
x7 (heptane) = 0.365
x8 (nonane) = 0.125
y1 (ethane) = 0.358
y2 (propane) = 0.132
y3 (n-butane) = 0.236
y4 (n-pentane) = 0.11
y5 (isopentane) = 0.079
y6 (n-hexane) = 0.03
y7 (heptane) = 0.051
y8 (nonane) = 0.003
Lecture 6: Bubble and Dew Point

11. THANK YOU