Bubble Point and Dew Point Calculations

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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 Newtons
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
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Multicomponent Flash Calculations
For this system there are 3C10 variables F, V,
L, T, P3, Q, xi , yi ,ziC and C5 degrees
of freedom. We specify the C3 variables F, zi,
TF, PF and two additional variables
Common Specifications TV,PV Isothermal
Flash V/F0, PL Bubble-Point Temperature V/F1,
PV Dew-Point Temperature V/F0, TL Bubble-Point
Pressure V/F1, TV Dew-Point Pressure Q0,
PV Adiabatic Flash Q, PV Nonadiabatic flash V/F,
PV Percent Vaporization Flash
Last lecture
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Isothermal Flash Calculations
If we specify F, zi, TF, PF and TV ,PV then we
apply the Rachford-Rice procedure
Flash Drum
Vapor out
Liquid Feed
V, yi, TV, PV
F, zi, TF, PF
Liquid out
Q
L, xi, TL, PL
Steps 12
Step 3 Solve Rachford-Rice for V/F.
Step 4
Determine V
Steps 5 and 6
Step 7
Determine L
Determine Q
Step 8
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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
Vapor out
Liquid Feed
V0, yi, TV, PV
F, zi, TF, PF
Liquid out
Q
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.
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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
Vapor out
Liquid Feed
V0, yi, TV, PV
F, zi, TF, PF
Liquid out
Q
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.
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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
Vapor out
Liquid Feed
V, yi, TV, PV
F, zi, TF, PF
Liquid out
Q
L0, 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.
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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
Vapor out
Liquid Feed
V, yi, TV, PV
F, zi, TF, PF
Liquid out
Q
L0, 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.
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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 fromthe
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 fromthe 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.
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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
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
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
V/F0.207 V207kg/hr L793kg/hr
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Depriester Determination of K-Values
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Example Dew Point Temperature
A flash chamber operating 400 kPa pressure is
producing a top product of 35 n-butane, 30
n-pentane, 15 mole n-hexane, 20 mole
heptane. What is the temperature of the flash
drum to operate at the dew point?
From the Depriester chart
T K1 (n-butane) K2 (n-pentane) K3 (n-hexane)
K4 (heptane)
T K1 (n-butane) K2 (n-pentane) K3 (n-hexane)
K4 (heptane)
T K1 (n-butane) K2 (n-pentane) K3 (n-hexane)
K4 (heptane)
T K1 (n-butane) K2 (n-pentane) K3 (n-hexane)
K4 (heptane)
y1 (n-butane) 0.35 y2 (n-pentane) 0.3 y3
(n-hexane) 0.15 y4 (heptane) 0.2
x1 (n-butane) x2 (n-pentane) x3 (n-hexane)
x4 (heptane)
V/F T
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K-Values Iterations for Dew Point Temperature
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Example Bubble Point Pressure
A flash chamber operating 80 degrees C is
producing a bottoms product of 15 n-butane, 20
n-pentane, 25 n-hexane, 40 heptane. What
is the pressure of the flash drum to operate at
the bubble point?
From the Depriester chart
P K1 (n-butane) K2 (n-pentane) K3 (n-hexane)
K4 (heptane)
P K1 (n-butane) K2 (n-pentane) K3 (n-hexane)
K4 (heptane)
P K1 (n-butane) K2 (n-pentane) K3 (n-hexane)
K4 (heptane)
P K1 (n-butane) K2 (n-pentane) K3 (n-hexane)
K4 (heptane)
y1 (n-butane) y2 (n-pentane) y3 (n-hexane)
y4 (heptane)
x1 (n-butane) 0.15 x2 (n-pentane) 0.20 x3
(n-hexane) 0.25 x4 (heptane) 0.4
V/F P
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Depriester Determination of K-Values
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Summary
This lecture covered Using a simple
spreadsheet to apply the Rachford Rice
Procedure Bubble point and Dew Point
temperature and pressure calculations An
example of a dew point temperature calculation
Next Lecture will cover Ternary Liquid-Liquid
extractions. Ternary phase diagrams. A
procedure to determine the product compositions
and flow rates of a liquid-liquid extraction
separation.