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EGR 334 Thermodynamics Chapter 3: Section 11. Lecture 09: Generalized Compressibility Chart. Quiz Today?. Today’s main concepts:. Universal Gas Constant, R Compressibility Factor, Z. Be able to use the Generalized Compressibility to solve problems
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EGR 334 ThermodynamicsChapter 3: Section 11 Lecture 09: Generalized Compressibility Chart Quiz Today?
Today’s main concepts: • Universal Gas Constant, R • Compressibility Factor, Z. • Be able to use the Generalized Compressibility to solve problems • Be able to use Z to determine if a gas can be considered to be an ideal gas. • Be able to explain Equation of State Reading Assignment: • Read Chap 3: Sections 12-14 Homework Assignment: From Chap 3: 92, 93, 96, 99
Limitation: Like cp and cv, today’s topic is about compressible gases….This method does not work for two phase mixtures such as water/steam. It only applies to gases. Compressibility Factor, Z where and
R can also be expresses on a per mole basis: Universal Gas Constant where M is the molecular weight (see Tables A-1 and A-1E)
Sec 3.11 : Compressibility The constant R is called the Universal Gas Constant. Where does this constant come from? For low pressure gases it was noted from experiment that there was a linear behavior between volume and pressure at constant temperature. then and the limit as P0 The ideal gas model assumes low P molecules are elastic spheres no forces between molecules
Sec 3.11 : Compressibility To compensate for non-ideal behavior we can use other equations of state (EOS) or use compressibility Define the compressibility factor Z, Z1 when ideal gas near critical point T >> Tc or (T > 2Tc) Step 1: Thus, analyze Z by first looking at the reduced variables Pc = Critical Pressure Tc = Critical Pressure
Step 2: Using the reduced pressure, pr and reduced temperature, Tr determine Z from the Generalized compressibility charts. (see Figures A-1, A-2, and A-3 in appendix). Fig03_12
Step 3: Use Z to a) state whether the substance behaves as an ideal gas, if Z ≈ 1 b) calculate the specific volume of the gas using where The figures also let’s you directly read reduced specific volume where
Sec 3.11 : Compressibility Summarize: 1) from given information, calculate any two of these: (Note: pc and Tc can be found on Tables A-1 and A-1E) 2) Using Figures A-1, A-2, and A-3, read the value of Z 3) Calculate the missing property using where or (Note: M for different gases can be found on Table 3.1 on page 123.)
Sec 3.11 : Compressibility Example: (3.95) A tank contains 2 m3 of air at -93°C and a gage pressure of 1.4 MPa. Determine the mass of air, in kg. The local atmospheric pressure is 1 atm. • V = 2 m3 • T = -93°C • pgage= 1.4 MPa • patm= 0.101 MPa
Sec 3.11 : Compressibility • Example: (3.95) Determine the mass of air, in kg • V = 2 m3 • T = -93°C = 180 K • p = pgauge + patm = 1.4 MPa + 0.101 MPa = 1.5 MPa = 15 bar From Table A-1 (p. 816): For Air: 16) Tc = 133 K pc = 37.7 bar View Compressibility Figure Z=0.95
Sec 3.11.4 : Equations of State & Sec 3.12 : Ideal Gas Model Ideal Gas Alternate Expressions Equations of State: Relate the state variables T, p, V When the gas follows the ideal gas law, Z = 1 p << pcand / or T >> Tc and
Sec 3.11.4 : Equations of State & Sec 3.12 : Ideal Gas Model Ideal Gas Van der Waals b volume of particles Equations of State: Relate the state variables T, P, V a attraction between particles Redlich–Kwong Peng-Robinson virial B Two molecule interactions C Three molecule interactions
Example: (3.105) A tank contains 10 lb of air at 70°F with a pressure of 30 psi. Determine the volume of the air, in ft3. Verify that ideal gas behavior can be assumed for air under these conditions. • m = 10 lb • T = 70°F • p= 30 psi
Sec 3.12 : Ideal Gas Example: (3.105) Determine the volume of the air, in ft3. Verify that ideal gas behavior can be assumed for air under these conditions. For Air, (Table A-1E, p 864) Tc = 239 °R and pc = 37.2 atm • m = 10 lb • T = 70°F = 530°R • p = 30 psi= 2.04 atm View Compressibility Figure Z= 1.0 (Figure A-1)
Example 3: Nitrogen gas is originally at p= 200 atm, T = 252.4 K. It is cooled at constant volume to T = 189.3 K. What is the pressure at the lower temperature? SOLUTION: From Table A-1 for Nitrogen pcr= 33.5 atm, Tcr= 126.2 K At State 1, pr,1= 200/33.5 = 5.97 and Tr,1 = 252.4/126.2 = 2. According to compressibility factor chart , Z = 0.95 vr' = 0.34. Following the constant vr' line until it intersects with the line at Tr,2= 189.3/126.2 = 1.5 gives Pr,2 = 3.55. Thus P2 = 3.55 x 33.5 = 119 atm. Since the chart shows Z drops down to around 0.8 at State 2, so it would not be appropriate to treat it as an ideal gas law for this model.