PVT Behavior of Fluids and the Theorem of Corresponding States Compressibility Factor The volumetric...
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Transcript of PVT Behavior of Fluids and the Theorem of Corresponding States Compressibility Factor The volumetric...
PVT Behavior of Fluids and the Theorem of Corresponding States
Compressibility Factor• The volumetric PVT behavior of a material is often characterized
by the compressibility factor Z
• Typical values: ideal gas, Z = 1.0; liquids, Z ~ 0.01 to 0.2; critical point, Zc ~ 0.27 to 0.29
Theorem of Corresponding States• This theorem suggests that values of thermodynamic properties of
different fluids can be compared when properties are normalized by their values at the critical point
RTPV
NRTVP
Z
rrc PTZfZ ,,
PVT Behavior of Fluids and the Theorem of Corresponding States
Theorem of Corresponding States• The reduced temperature and
pressure are simply
• If Zc is assumed to be a constant for all materials
• Experimental observationsprovide strong evidence to support this approach
• The relation above indicates that all fluids have the same reduced volume Vr (as well as other properties) at a specified Tr and Pr
• In Example 7.2 the principle of corresponding states was used to relate the a and b constants of the van der Waals equation of state to a fluids critical properties, specifically Tc and Pc
cr TTT cr PPP and
rr PTfZ ,
PVT Behavior of Fluids and the Theorem of Corresponding States
Generalized Coexistence Curve• The theorem of corresponding states can be used to obtain a
generalized phase diagram• “Law of rectilinear diameters” – states that the average density in
the L-V region varies linearly over a wide temperature range below the critical point
• Zeno line – the Z = 1.0 line on the phase diagram
• For states with Z > 1.0 (hard fluid), repulsive forces dominate and conversely, for states where Z < 1.0 (soft fluid) attractive forces are dominant
PVT Behavior of Fluids and the Theorem of Corresponding States
Acentric Factor• To improve the accuracy of property predictions, Pitzer and
coworkers introduced the acentric factor as a third correlating parameter
• The acentric factor was developed as a measure of the difference in structure between the material of interest and a spherically symmetric gas such as argon
• Therefore, the parameter is related to a molecular property
• The parameter is defined using the reduced vapor pressure
rrrr PTZPTZZ ,ω, 10
1logω7.0,10
rTrvpP
PVTN Equations of State for FluidsOverview
• Although the corresponding states correlations are useful, the data is displayed in charts and graphs are therefore cumbersome to use
• A more useful means to express a fluids PVT behavior is through an analytical expression (PVT equation of state)
• Two general approaches• Statistical mechanical models – based on rigorous theoretical
principles accounting for the interactions between molecules (preferred by physicists and physical chemists)
• Empirical expressions – models formulated by determining parameters in a mathematical expression through a fit to experimental data (preferred by chemical engineers)
PVTN Equations of State for FluidsRelating EOS Parameters to Experimental Data
• Empirical models have parameters that must be determined to calculate PVT data
• A common way to determine these parameters is to relate the model parameters to a fluid’s critical properties
• By adopting this approach we are utilizing the theorem of corresponding states
• The critical point criteria for a single-component system require
• Recall Example 7.2, where these criteria were used to determine the parameters for the van der Waals EOS
0
cTVP
02
2
cTV
Pand
PVTN Equations of State for FluidsCubic Equations of State
• Since the pioneering work of van der Waals, numerous modifications of the original vdW EOS have been published
• Most are pressure-explicit cubic forms, i.e.,
van der Waals EOS
2Va
bVRT
P
NVTfP ,,
c
c
P
TRa
64
27 22
c
c
P
RTb
8and
Zc = 0.375
PVTN Equations of State for FluidsRedlich-Kwong
bVVTa
bVRT
P
c
ca
P
TRa
5.22
c
cb
P
RTb
with 42748.0129131
a
with
08664.03
12 31
b
Redlich-Kwong-Soave• The attractive term is made temperature dependent and the acentric
factor is introduced
• with
211,ω rcrc TkaTfaa
2ω15613.0ω55171.148508.0 k
Zc = 0.333
Zc = 0.333
PVTN Equations of State for FluidsPeng-Robinson
bVbbVV
TabV
RTP r
,
TraTa cr ,,
c
c
P
RTb
07780.0
c
cc P
TRa
2245724.0
with
211,ω rr TT
2ω26992.0ω54226.137464.0
Zc = 0.307
PVTN Equations of State for FluidsHard-Sphere Equation of State
• A class of EOSs that incorporate knowledge of the specific molecular interactions with additional fitted parameters
• The repulsive term is theoretically based• The term bo below accounts for the excluded volume of a molecule• The attractive term is empirical• Here is one example of a hard-sphere EOS that uses the Carnahan-
Starling repulsive term and van der Waals attractive term
23
3223
Va
bV
bVbVbV
VRT
Po
ooo
6
3ob → molecular diameter
PVTN Equations of State for FluidsVirial-Type Equation of State
• An important EOS class that has a rigorous basis in molecular theory (may be derived from statistical mechanics)
• The compressibility is expressed as a polynomial in volume
• Or as a polynomial expansion in pressure
• Where
• The coefficients B, C, D, … are termed the second, third, fourth, … virial coefficients
22 11 CB
VC
VB
RTPV
Z
321 PDPCPBZ
3
3
2
2 23
RT
BBCDD
RT
BCC
RTB
B
PVTN Equations of State for FluidsMethod of Abbott
• The virial equation is truncated after the second virial coefficient term and the pressure is obtained
• The second virial coefficient is expressed in terms of two functions of temperature B(0) and B(1) as well as the acentric factor
• Where
RTBP
RTPV
Z 1
6.1
0 422.0083.0
rTB
BVRT
P
10 ωBBP
RTB
c
c
and 2.4
1 172.0139.0
rTB
PVTN Equations of State for FluidsBenedict-Webb-Rubin EOS
• Another example of a virial-type EOS• Has eight parameters• Frequently used for modeling the PVT behavior of light
hydrocarbons and gases in petroleum and natural gas applications
223
2
6
322
exp1
1
VTV
VcVa
VabRT
VT
CARTB
VRT
P ooo
Evaluating Changes in Properties Using Departure FunctionsOverview
• We now develop methods for determining changes in derived thermodynamic properties of a pure substance
• Because properties are state functions, we can choose any convenient path to calculate the change in the property of interest (e.g., H, S, U, A, or G)
• To perform the calculation, we must generally have access to two constitutive relationships
1. A PVT or PVTN equation of state (EOS)2. An ideal-gas state heat capacity equation
Evaluating Changes in Properties Using Departure FunctionsConvenient Path
• To calculate property changes between two well-defined states (T1,P1,V1) and (T2,P2,V2) we use the following three-step process
1. Isothermal expansion at T1 from P = P1 to 0 (or V = V1 to ∞2. Isobaric (or isochoric) heating from T1 to T2 in an attenuated
ideal gas state [P = 0 (or V = ∞)]3. Isothermal compression at T2 from P = 0 to P2
(or from V = ∞ to V2)
Evaluating Changes in Properties Using Departure FunctionsConvenient Path
• The total property change between states 1 and 2, B (where B is one of H, S, U, A, or G), is the sum over these three steps
• Step (2) occurs entirely within the ideal-gas state• Recall that only 2 of the 3 intensive variables T, P, and V are
independent• We select T as one independent variable and choose P or V based
on the PVT EOS we have• If we have a pressure-explicit EOS (P = f (T,V)), we choose V• If we have a volume-explicit EOS (V = f (T,P)), we choose P
VPTBVPTB
VPTBVPTB
VPTBVPTB
VPTBVPTBB
,0,,,
,0,,0,
,,,0,
,,,,
2222
12
1111
111222
step (1)
step (2)
step (3)
Evaluating Changes in Properties Using Departure FunctionsExample 8.3
Develop expressions for S between states 1 and 2 assuming you have analytic forms for (1) a volume-explicit EOS and (2) a pressure-explicit EOS and
Summary
32 dTcTbTaTC op
dPPR
TV
PP
RdTT
TC
dPPR
TV
S
TTP
P
T
T
op
TT
P
P
11
2
1
2
2
0
1
2
0
ln
dVVR
TP
VV
RdTT
TC
dVVR
TP
S
TTV
V
T
T
ov
TT
V
V
11
2
1
2
2
1
2ln
Volume-explicit EOS Pressure-explicit EOS
Evaluating Changes in Properties Using Departure FunctionsDeparture (Residual) Functions
• A departure function is the difference between the property of interest in its real state at a specified T, P, and V and in an ideal-gas state at the same temperature T and pressure P
• There are two equivalent forms of the departure function
• B is any derived property (H, S, U, A, or G) and Vo = RT / P(T,V)
ooogasideal
ogasideal
VTBVTBVTBVTB
PTBPTBPTBPTB
,,,,
,,,,
Evaluating Changes in Properties Using Departure FunctionsResidual Entropy
• Based on Example 8.3, the residual entropy is defined as
dPPR
TV
PTSPTSP
P
o ,,0
Evaluating Changes in Properties Using Departure FunctionsResidual Helmholtz Energy
• When working with pressure-explicit EOSs, the departure function for the Hemlholtz energy is particularly useful
• The differential expression for A is
• Integrating at constant T between A(T,V) and A(T,∞) gives
• To obtain the departure function, we must also account for the change from V → ∞ to V = Vo, which occurs in the ideal-gas state
• To avoid a singularity we also add and subtract
PdVSdTdA
, ,V
A T V A T PdV
, ,o
o o
V ideal gas
A T A T V PdV
V RTdV
V
Evaluating Changes in Properties Using Departure FunctionsResidual Helmholtz Energy
• The final result is
Residual Entropy• The entropy is obtained by differentiating A with respect to T
, , lno
Vo o RT VA T V A T V P dV RT
V V
, , lno
Vo o RT VS T V S T V P dV R
T V V
VTA
S
Evaluating Changes in Properties Using Departure FunctionsOther Departure Functions
• Now that A and S are known, other departure functions can be obtained through simple algebra
General Result• To determine the change in a thermodynamic property between two
states, we evaluate two isothermal departure functions (at T1 and T2) and a temperature change contribution in the ideal gas state
oooo
ooo
oooo
SSTHHVTGVTG
RTPVUUVTHVTH
SSTAAVTUVTU
,,
,,
,,
2 2 1 1 2 2 2 2
1 1 1 1
2 2 1 1
, , , ,
, ,
, ,
o o
o o
o o o o
B T V B T V B T V B T V
B T V B T V
B T V B T V
departure function at T1
departure function at T2
ideal-gas isotherm
Evaluating Changes in Properties Using Departure FunctionsDeparture Functions for Volume-Explicit PVT EOSs
• When volume-explicit PVT EOSs are available the following isothermal departure functions should be used
Po
PdP
ZRTPTGPTG0
1,,
P
P
o
PdP
TZ
RTPTHPTH0
2,,
PP
P
o
PdP
ZRP
dPTZ
RTPTSPTS00
1,,
Evaluating Changes in Properties Using Departure FunctionsUseful Ideal-Gas Property Relationships
Internal Energy:
Enthalpy:
Entropy:
2
12 2 1 1 2 2 1 1, , , ,
To o o o o o ovT
U T V U T V U T P U T P C dT
2
1
22 2 1 1
1
, , lno o
To o o o voT
C VS T V S T V dT R
T V
2
12 2 1 1 2 2 1 1, , , ,
To o o o o o opT
H T V H T V H T P H T P C dT
2
1
22 2 1 1
1
, , lno
T po o
T
C PS T P S T P dT R
T P
Evaluating Changes in Properties Using Departure FunctionsHeat Capacity Departure Functions (Pressure-Explicit PVT EOS)
• The heat capacities are related to differentials of the entropy with temperature, for example Cv is defined as
• Evaluating the derivative yields
• The constant pressure heat capacity departure function requires a bit more work to obtain
• Often times it is more convenient to obtain Cp – Cpo by first
determining Cv – Cvo and then using an expression that relates Cv
and Cp that will be introduced below
2
2, ,
Vo ov v
V
PC T V C T V T dV
T
V
v
TS
TC
22
2, ,
Vo o Vp p
V T
P TPC T V C T V T dV T R
T P V
Evaluating Changes in Properties Using Departure FunctionsHeat Capacity Departure Functions (Volume-Explicit PVT EOS)
• With a volume-explicit PVT EOS the constant pressure heat capacity departure function is relatively straightforward to obtain
• Evaluating the derivative yields
2
20, ,
Pop p
P
VC T P C T P T dP
T
P
p
TS
T
C
Evaluating Changes in Properties Using Departure FunctionsGeneral Relationships Between Cv and Cp
• Using the formalism introduced in Chapter 5 one can obtain the following relationship between Cv and Cp
• The above expression is completely general and therefore can be used to relate heat capacities for real or ideal gases as well as for liquids and solids
• Recalling the definitions of T and p, the expression above can equivalently be written as
T
pvp
TVCC
2
T
Vvp VP
TPTCC
2
Compressibility and Heat Capacity of SolidsIncompressible Substances
• Most solids and many liquids far below their critical temperatures have very low compressibilities
• In these situations one can assume perfectly or nearly incompressible behavior as a first-order approximation
• An incompressible substance is defined as having a constant volume (V = constant) independent of T and P
• From this definition, it follows that p = T = 0 for an incompressible substance
• For a nearly incompressible substance the following expressions are appropriate (for a perfectly incompressible substance, the approximation becomes a true equality)
dTCdU vvp CC
VdPdTCVdPdTCdH vp dTT
CdS v
Derived Property RepresentationsOverview
• Before the infusion of computers, charts or tubular data were used widely for engineering problem solving
• Although computer-generated representations of thermodynamic properties are preferred, many graphical forms of data are still used in engineering practice
• The temperature-entropy (T-S) diagram for water is a good example