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Residual Properties and Real Gases
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Gravity
Terms in this set (45)
Real Gas volume
finite volume
Real Gas intermolecular forces?
with real gas, there are IMF's
With ideal gas, volume?
no molecular volume
Ideal gas IMF's?
no IMF's
All gases behave ideally at..
high temperature and low pressure
Why do gases behave ideally at low pressure and high temperature?
High temp: give the molecules a higher average kinetic energy that exceeds the intermolecular forces
at low pressure: the molecules are far apart
Compressibility symbol?
Z
compressibility factor
measure of how close a gas is to ideal conditions
compressibility factor of an ideal gas
z=1
Compressibility factor of real gas
z not 1
Ideal Gas Law
PV=RT
compressibility factor
Z=PV/RT= Vreal/ Videal
What two things can we use to describe the behavior of a real gas?
compressibility charts and equations of state
Cubic equations of state
cubic volume (V^3) term
Tc and Pc
critical temperature and pressure
Tr and Pr
reduced temperature and pressure
w
acentric factor
What trend do we notice when we plot the pressure against the volume for constant values of temp
when T > Tc, we get on single root or V
When T< Tc, we get 3 roots
T< Tc
largest root is sat vapor volume
middle root= two phase region
smallest root= sat liquid volume
critical point
indicates the critical temperature and critical pressure
Below critical point
phase transition, we if decrease the volume the vapor condense to liquid
above critical point
the volume of the fluid decreases as we increases pressure, but there is no phase change
gases above the critical point are called?
supercritical
All gases have a critical point. Below the critical point there is a _____ phase region and above the critical point there is a ______ phase
2
1
Law of Corresponding States
a few properties are the same for all gases when expressed in terms of deviation from the critical point, We do this by using the "Reduced temperature, and reduced pressure"
All fluids when compared at the some reduced temperature and pressure, have approximately the same compressibility factor and they exhibit about the same deviation from ideal gas behavior
The law of corresponding states says the the compressibility of a gas is not really a function of ________ but on ________
composition
reduced temperature and pressure
reduced temperature
Tr; = T/Tcr
reduced pressure
Pr; = P/Pcr
reduced temperature for hydrogen and helium
T/Tc + 8K
reduced pressure for hydrogen and helium
P/Pc + 8atm
The deviation from ideal conditions is relatively small at what kind of values of Tr?
higher values
The Law of Corresponding states works well, but what is one drawback?
Law of Corresponding states doesn't work well for non-spherical molecules
Why does the law of corresponding states not work well for spherical molecules?
acentric factor (w)
acentric factor
measure of how complex or spherical a molecules is
acentric value is zero for...
monatomic species like noble gases
acentric factor is large for
complex, molecules with large IMF's
What is the purpose of w and Z0?
Z1 is the compressibility at a given reduced temperature and pressure
the wZ1 term is a correction factor, really important for molecules that are non-spherical in shape because they will deviate more from the law of corresponding states
Residual property?
the difference between the actual property (real gas) and the value for an ideal gas
General formula for residual property
M=Mig +MR
---> M---> internal energy, enthalpy, entropy
How do we calculate the change in enthalpy for an ideal gas?
Cp(T2-T1)
How do we calculate the change in entropy for an ideal gas?
Cpln(T2/T1) - Rln(P2/P1)
For a throttling valve, what quantity is zero?
change in enthalpy
heat
work
When the efficiency of a turbine or compressor is 1, what does that mean?
the change in enthalpy is equal to the isentropic change in enthalpy
change in entropy is zero
What are the phases in rankine cycle?
stream leaving condenser is sat liquid
stream leaving compressor is subcooled liquid
stream leaving boiler is superheated vapor
stream leaving turbine is wet steam
What are the phases in a refrigeration cycle
stream entering evaporator is a mix of sat l and v
stream leaving evaporator is sat v
stream leaving compressor is superheated vapor
stream leaving condenser is sat liq
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