Introduction to Chemical Engineering Calculations
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
Lecture 6.
Real Gas Relationships
31
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE
2
PVT Relationship for a Real Gas
For a real gas,
PV nRT
The best method for obtaining the PVT relationship for a real gas is thru experimentation. In the absence of an experimental data, the following methods may be used:
1. Compressibility Chart
2. Equations of State
3. Estimated Properties
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE
3
Compressibility Factor, Z
The compressibility factor is a factor that compensates for the nonideality of the gas.
Using this factor, the ideal gas equation becomes a real gas equation:
PV = nZRT
This is called the generalized equation of state.
The value of Z can be obtained from compressibility charts.
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE
4
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE
5
The Critical and Reduced Properties
Substances have critical properties (critical temperature critical pressure, and critical volume). Values of critical properties for common substances are available.
Reduced properties are PVT conditions normalized by their respective critical conditions:
r r rC C C
T P VT P VT P V
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE
6
Example 6-1. Use of the Compressibility Factor
After liquid ammonia (NH3) fertilizer has been injected into the soil, there is still some ammonia left in the source tank (V = 120 ft3), but in the form of a gas.
Determine the weight of NH3 remaining in the tank if the pressure is 292 psig and temperature of the tank is 1250F.
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE
7
Example 6-1. Use of the Compressibility Factor
Solve for n using the generalized equation of state,
PVnZRT
As given, values of the P-V-T are
P = 292 psi + 14.7 psi = 306.7 psiT = 12560F + 460 = 5850RV = 120 ft3
And the value of R:
3
0
ft atmR 10.73
lbmol R
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE
8
Example 6-1. Use of the Compressibility Factor
The value of Z is obtained from the generalized compressibility chart if Tr and Pr are known,
The critical properties for ammonia are:
TC = 405.5K 729.90R
PC = 11.28 Mpa 1636 psia
Solving for the reduced parameters:
0
r r0c c
T 585 R P 306.7psiaT 0.801 and P 0.187
T P 1636psia729.9 R
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE
9
Example 6-1. Use of the Compressibility Factor
From the compressibility chart,
Z 0.855
Solving for n and m:
3
330
0
3 33
306.7 psia 120ftn 6.858lbmol NH
ft atm0.855 10.73 585 Rlbmol R
17lbmm 6.858lbmol NH 116.58lbm NH1lbmol NH
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE10
Example 6-2. Use of the Compressibility Factor
Liquid oxygen is used in the steel industry, in the chemical industry, in hospitals, as rocket fuel, and for wastewater treatment as well as many other applications.
In a hospital a tank of 0.0284 m3 volume is filled with 3.500 kg of liquid O2 that vaporized at -250C.
Determine whether the pressure in the tank exceed the safety limit of the tank of 104 kPa.
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE11
Example 6-2. Use of the Compressibility Factor
Using the generalized equation of state:
nZRTPV
Given the following values:
n = 3.500 kg O2 (1/32) = 0.109 kmol
T = - 250C + 273 = 248 K
V = 0.0284 m3
R = 8.314 (kPa)(m3)/(kmol)(K)
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE12
Example 6-2. Use of the Compressibility Factor
To determine the value of Z, values of Tr and Pr must be known.
Critical properties of oxygen:
TC = 154.8 K
PC = 5.08 MPa = 5080 kPa
And the reduced temperature is
rc
T 248KT 1.602
T 154.8K
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE13
Example 6-2. Use of the Compressibility Factor
The pressure is determined by iteration
Step P (kPa) Pr Z Pcalc (kPa) e = |P – Pcalc|
1 10000.00 1.97 0.87 6845.20 3154.80
2 6845.20 1.35 0.91 7201.31 356.11
3 7201.31 1.42 0.88 6983.90 237.41
4 6983.90 1.37 0.89 7043.04 79.14
5 7043.04 1.39 0.90 7122.17 79.14
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE14
Equations of State – Van der Waals Equation
2
2 2
2 2C
C
C
C
a n aˆP V b RT or P V nb nRTV̂ V
ˆwhere V specific molar volume(V/n)
R T27 a =64 P
RT1b8 P
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE15
Equations of State – Redlich-Kwong Equation
1/2
2 2.5C
C
C
C
RT aP ˆ ˆ ˆV b T V V b
R Twhere a = 0.42748P
RTb 0.08664P
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE16
Equations of State – Peng-Robinson Equation
2 2C
C
C
C21/2
r
2
RT aP ˆ ˆ ˆ ˆV b V V b b V b
R Twhere a =0.45724P
RTb 0.07780P
1 1 T
0.37464 1.5422 0.26992accentricfactor
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE17
Example 6-3. Use of Van der Waals’ Equation
A cylinder 0.150 m3 in volume containing 22.7 kg of propane (C3H8) stands in the hot sun. A pressure gauge shows that the pressure is 4790 kPa gauge.
Using the Van der Waals equation of state, determine the temperature of the propane in the cylinder.
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE18
Example 6-3. Use of Van der Waals’ Equation
Solving for T using Van der Waals equation
2
2n aP V nbVT
nR
Given the following values:
n = 22.7 kg (1/44) = 0.516 kmol = 516 gmol
V = 0.150 m3 = 0.150 106 cm3
P = 4790 kPa + 101 kPa = 4891 kPa = 48.3 atm
R = 82.06 (atm)(cm3)/(gmol)(K)
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE19
Example 6-3. Use of Van der Waals’ Equation
The values of a and b can be computed using values for the critical properties or obtained from handbook.
23 36 cm cma 9.24 10 atm and b 90.7
gmol gmol
Solving for T:
2 66
26
516 9.24 1048.3 0.150 10 516 90.7
0.150 10T
516 82.06T 384K
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE20
Example 6-4. Solution of Van der Waals Equation for V
Given the values in a vessel of
Solve for the volume of the vessel.
234
3
0
30
ftP 679.7 psia a 3.49 10 psialbmol
psia ftn 1.136 lbmol R 10.73lbmol RftT 683 R b 1.45
lbmol
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE21
Example 6-4. Solution of Van der Waals Equation for V
Since the Van der Waals equation cannot be easily solved explicitly for volume, it is written in cubic form:
Obtaining the coefficients:
2 33 2
3 23 2 1 0
nRT n a n abV nb V V 0P P P
(a x a x a x a 0)
2 3nRT n a n abnb 13.896 ; 66.262; 109.147P P P
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE22
Example 6-4. Solution of Van der Waals Equation for V
Rewriting the equation:
f(V) = V3 – 13.896V2 + 66.262V – 109.147 = 0
Solve for the roots using any suitable technique such as the Newton’s method which is an iterative procedure. Starting with a guess value for V, a new value is calculated using the following formula:
kk 1 k
k
f (V )V Vf '(V )
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE23
Example 6-4. Solution of Van der Waals Equation for V
f’(V) is obtained by differentiating f(V):
f’(V) = 3V2 – 27.792V + 66.262
For the initial guess value, the volume is calculate using the ideal gas law:
30
1.136 10.73 683nRTV 12.26ftP 679.7
Real Gas Relationships6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños SLIDE24
Example 6-4. Solution of Van der Waals Equation for V
Step VK(ft3) f(V) f'(V) VK+1(ft3) E
0 12.26 457.32 176.45 9.67 2.59
1 9.67 136.30 77.99 7.92 1.75
2 7.92 40.81 34.34 6.73 1.19
3 6.73 12.26 15.13 5.92 0.81
4 5.92 3.61 6.89 5.40 0.52
5 5.40 0.92 3.66 5.15 0.25
6 5.15 0.13 2.69 5.10 0.05
7 5.10 0.00 2.55 5.10 0.00