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Unformatted text preview: cen84959_ch13.qxd 4/6/05 9:35 AM Page 681 Chapter 13
GAS MIXTURES
p to this point, we have limited our consideration to thermodynamic systems that involve a single pure substance such as water. Many important thermodynamic applications, however, involve mixtures of several pure substances rather than a single pure substance. Therefore, it is important to develop an understanding of mixtures and learn how to handle them. In this chapter, we deal with nonreacting gas mixtures. A nonreacting gas mixture can be treated as a pure substance since it is usually a homogeneous mixture of different gases. The properties of a gas mixture obviously depend on the properties of the individual gases (called components or constituents) as well as on the amount of each gas in the mixture. Therefore, it is possible to prepare tables of properties for mixtures. This has been done for common mixtures such as air. It is not practical to prepare property tables for every conceivable mixture composition, however, since the number of possible compositions is endless. Therefore, we need to develop rules for determining mixture properties from a knowledge of mixture composition and the properties of the individual components. We do this first for idealgas mixtures and then for realgas mixtures. The basic principles involved are also applicable to liquid or solid mixtures, called solutions. U Objectives
The objectives of Chapter 13 are to: • Develop rules for determining nonreacting gas mixture properties from knowledge of mixture composition and the properties of the individual components. • Define the quantities used to describe the composition of a mixture, such as mass fraction, mole fraction, and volume fraction. • Apply the rules for determining mixture properties to idealgas mixtures and realgas mixtures. • Predict the PvT behavior of gas mixtures based on Dalton’s law of additive pressures and Amagat’s law of additive volumes. • Perform energy and exergy analysis of mixing processes.  681 cen84959_ch13.qxd 4/6/05 9:35 AM Page 682 682
H2 6 kg 
+ Thermodynamics
O2 32 kg H2 + O2 38 kg 13–1 COMPOSITION OF A GAS MIXTURE: MASS AND MOLE FRACTIONS FIGURE 13–1 The mass of a mixture is equal to the sum of the masses of its components. H2 3 kmol + O2 1 kmol H2 + O2 4 kmol To determine the properties of a mixture, we need to know the composition of the mixture as well as the properties of the individual components. There are two ways to describe the composition of a mixture: either by specifying the number of moles of each component, called molar analysis, or by specifying the mass of each component, called gravimetric analysis. Consider a gas mixture composed of k components. The mass of the mixture mm is the sum of the masses of the individual components, and the mole number of the mixture Nm is the sum of the mole numbers of the individual components* (Figs. 13–1 and 13–2). That is,
k k mm a mi
i1 and Nm a Ni
i1 (13–1a, b) FIGURE 13–2 The number of moles of a nonreacting mixture is equal to the sum of the number of moles of its components. The ratio of the mass of a component to the mass of the mixture is called the mass fraction mf, and the ratio of the mole number of a component to the mole number of the mixture is called the mole fraction y:
mfi mi mm and yi Ni Nm
(13–2a, b) H2 + O2 yH2 = 0.75 yO2 = 0.25 1.00 Dividing Eq. 13–1a by mm or Eq. 13–1b by Nm, we can easily show that the sum of the mass fractions or mole fractions for a mixture is equal to 1 (Fig. 13–3):
k k a mfi
i1 1 and a yi
i1 1 FIGURE 13–3 The sum of the mole fractions of a mixture is equal to 1. The mass of a substance can be expressed in terms of the mole number N and molar mass M of the substance as m NM. Then the apparent (or average) molar mass and the gas constant of a mixture can be expressed as
Mm mm Nm a mi Nm a Ni Mi Nm
k a yi Mi
i1 and Rm Ru (13–3a, b) Mm The molar mass of a mixture can also be expressed as
Mm mm Nm a mi > M i mm 1 a mi > 1 mm M i 2 1 mfi aM i1 i
k (13–4) Mass and mole fractions of a mixture are related by
mfi mi mm Ni Mi Nm Mm yi Mi Mm
(13–5) *Throughout this chapter, the subscript m denotes the gas mixture and the subscript i denotes any single component of the mixture. cen84959_ch13.qxd 4/6/05 9:35 AM Page 683 Chapter 13
EXAMPLE 13–1 Mass and Mole Fractions of a Gas Mixture
3 kg O2 5 kg N2 12 kg CH4  683 Consider a gas mixture that consists of 3 kg of O2, 5 kg of N2, and 12 kg of CH4, as shown in Fig. 13–4. Determine (a) the mass fraction of each component, (b) the mole fraction of each component, and (c) the average molar mass and gas constant of the mixture. Solution The masses of components of a gas mixture are given. The mass
fractions, the mole fractions, the molar mass, and the gas constant of the mixture are to be determined. Analysis (a) The total mass of the mixture is FIGURE 13–4 Schematic for Example 13–1. mm mO2 mN2 mCH4 mO2 mm mN2 mm mCH4 mm 3 5 12 20 kg Then the mass fraction of each component becomes mfO2 mfN2 mfCH4 3 kg 20 kg 5 kg 20 kg 12 kg 20 kg 0.15 0.25 0.60 (b) To find the mole fractions, we need to determine the mole numbers of each component first: NO2 NN2 NCH4
Thus, mO2 MO2 mN2 MN2 mCH4 MCH4 NCH4 N O2 Nm N N2 Nm N CH4 Nm 3 kg 32 kg> kmol 5 kg 28 kg> kmol 12 kg 16 kg> kmol 0.094 0.179 0.094 kmol 0.179 kmol 0.750 kmol Nm
and NO2 NN2 0.750 1.023 kmol y O2 y N2 y CH4 0.094 kmol 1.023 kmol 0.179 kmol 1.023 kmol 0.750 kmol 1.023 kmol 0.092 0.175 0.733 (c) The average molar mass and gas constant of the mixture are determined from their definitions, Mm mm Nm 20 kg 1.023 kmol 19.6 kg/kmol cen84959_ch13.qxd 4/6/05 9:35 AM Page 684 684  Thermodynamics
or Mm 19.6 kg> kmol
Also, a yi Mi yO2MO2 yN2MN2 1 0.092 2 1 32 2 1 0.175 2 1 28 2 yCH4MCH4 1 0.733 2 1 16 2 Rm Ru Mm 8.314 kJ> 1 kmol # K 2 19.6 kg> kmol 0.424 kJ/kg # K Discussion When mass fractions are available, the molar mass and mole fractions could also be determined directly from Eqs. 13–4 and 13–5. 13–2 PvT BEHAVIOR OF GAS MIXTURES: IDEAL AND REAL GASES Gas A V, T PA + Gas B V, T PB Gas mixture A+B V, T PA + PB FIGURE 13–5 Dalton’s law of additive pressures for a mixture of two ideal gases. An ideal gas is defined as a gas whose molecules are spaced far apart so that the behavior of a molecule is not influenced by the presence of other molecules—a situation encountered at low densities. We also mentioned that real gases approximate this behavior closely when they are at a low pressure or high temperature relative to their criticalpoint values. The PvT behavior of an ideal gas is expressed by the simple relation Pv RT, which is called the idealgas equation of state. The PvT behavior of real gases is expressed by more complex equations of state or by Pv ZRT, where Z is the compressibility factor. When two or more ideal gases are mixed, the behavior of a molecule normally is not influenced by the presence of other similar or dissimilar molecules, and therefore a nonreacting mixture of ideal gases also behaves as an ideal gas. Air, for example, is conveniently treated as an ideal gas in the range where nitrogen and oxygen behave as ideal gases. When a gas mixture consists of real (nonideal) gases, however, the prediction of the PvT behavior of the mixture becomes rather involved. The prediction of the PvT behavior of gas mixtures is usually based on two models: Dalton’s law of additive pressures and Amagat’s law of additive volumes. Both models are described and discussed below. Dalton’s law of additive pressures: The pressure of a gas mixture is equal to the sum of the pressures each gas would exert if it existed alone at the mixture temperature and volume (Fig. 13–5). Amagat’s law of additive volumes: The volume of a gas mixture is equal to the sum of the volumes each gas would occupy if it existed alone at the mixture temperature and pressure (Fig. 13–6). Dalton’s and Amagat’s laws hold exactly for idealgas mixtures, but only approximately for realgas mixtures. This is due to intermolecular forces that may be significant for real gases at high densities. For ideal gases, these two laws are identical and give identical results. Gas A P, T + Gas B P, T VA VB Gas mixture A+B P,T VA + VB FIGURE 13–6 Amagat’s law of additive volumes for a mixture of two ideal gases. cen84959_ch13.qxd 4/6/05 9:35 AM Page 685 Chapter 13
Dalton’s and Amagat’s laws can be expressed as follows:
Dalton’s law: Amagat’s law: Pm Vm a Pi 1 Tm, Vm 2
k i1 k  685
N2 100 kPa 400 K 0.7 m3 a Vi 1 Tm, Pm 2
i1 exact for ideal gases, (13–6) ∂ approximate for real gases
(13–7) O2 + N2 100 kPa 400 K 1 m3 O2 100 kPa 400 K 0.3 m3 In these relations, Pi is called the component pressure and Vi is called the component volume (Fig. 13–7). Note that Vi is the volume a component would occupy if it existed alone at Tm and Pm, not the actual volume occupied by the component in the mixture. (In a vessel that holds a gas mixture, each component fills the entire volume of the vessel. Therefore, the volume of each component is equal to the volume of the vessel.) Also, the ratio Pi /Pm is called the pressure fraction and the ratio Vi /Vm is called the volume fraction of component i. FIGURE 13–7 The volume a component would occupy if it existed alone at the mixture T and P is called the component volume (for ideal gases, it is equal to the partial volume yiVm). IdealGas Mixtures
For ideal gases, Pi and Vi can be related to yi by using the idealgas relation for both the components and the gas mixture:
Pi 1 Tm, Vm 2 Vi 1 Tm, Pm 2 Vm Pm NmRuTm> Vm NmRuTm> Pm Vi Vm Ni Nm NiRuTm> Pm NiRuTm> Vm Ni Nm Ni Nm yi yi Therefore,
Pi Pm yi
(13–8) Equation 13–8 is strictly valid for idealgas mixtures since it is derived by assuming idealgas behavior for the gas mixture and each of its components. The quantity yi Pm is called the partial pressure (identical to the component pressure for ideal gases), and the quantity yiVm is called the partial volume (identical to the component volume for ideal gases). Note that for an idealgas mixture, the mole fraction, the pressure fraction, and the volume fraction of a component are identical. The composition of an idealgas mixture (such as the exhaust gases leaving a combustion chamber) is frequently determined by a volumetric analysis (called the Orsat Analysis) and Eq. 13–8. A sample gas at a known volume, pressure, and temperature is passed into a vessel containing reagents that absorb one of the gases. The volume of the remaining gas is then measured at the original pressure and temperature. The ratio of the reduction in volume to the original volume (volume fraction) represents the mole fraction of that particular gas. RealGas Mixtures
Dalton’s law of additive pressures and Amagat’s law of additive volumes can also be used for real gases, often with reasonable accuracy. This time, however, the component pressures or component volumes should be evaluated from relations that take into account the deviation of each component cen84959_ch13.qxd 4/6/05 9:35 AM Page 686 686  Thermodynamics
from idealgas behavior. One way of doing that is to use more exact equations of state (van der Waals, Beattie–Bridgeman, Benedict–Webb–Rubin, etc.) instead of the idealgas equation of state. Another way is to use the compressibility factor (Fig. 13–8) as
PV ZNRuT
(13–9) Pm Vm = Zm Nm Ru Tm Zm = i=1 ∑ k yi Zi The compressibility factor of the mixture Zm can be expressed in terms of the compressibility factors of the individual gases Zi by applying Eq. 13–9 to both sides of Dalton’s law or Amagat’s law expression and simplifying. We obtain
k FIGURE 13–8 One way of predicting the PvT behavior of a realgas mixture is to use compressibility factor. Zm a yiZi
i1 (13–10) Pseudopure substance k 'cr,m = yi Pcr,i P i=1 ∑
k T 'cr,m = i=1 ∑ yi Tcr,i FIGURE 13–9 Another way of predicting the PvT behavior of a realgas mixture is to treat it as a pseudopure substance with critical properties P cr and T cr. where Zi is determined either at Tm and Vm (Dalton’s law) or at Tm and Pm (Amagat’s law) for each individual gas. It may seem that using either law gives the same result, but it does not. The compressibilityfactor approach, in general, gives more accurate results when the Zi’s in Eq. 13–10 are evaluated by using Amagat’s law instead of Dalton’s law. This is because Amagat’s law involves the use of mixture pressure Pm, which accounts for the influence of intermolecular forces between the molecules of different gases. Dalton’s law disregards the influence of dissimilar molecules in a mixture on each other. As a result, it tends to underpredict the pressure of a gas mixture for a given Vm and Tm. Therefore, Dalton’s law is more appropriate for gas mixtures at low pressures. Amagat’s law is more appropriate at high pressures. Note that there is a significant difference between using the compressibility factor for a single gas and for a mixture of gases. The compressibility factor predicts the PvT behavior of single gases rather accurately, as discussed in Chapter 3, but not for mixtures of gases. When we use compressibility factors for the components of a gas mixture, we account for the influence of like molecules on each other; the influence of dissimilar molecules remains largely unaccounted for. Consequently, a property value predicted by this approach may be considerably different from the experimentally determined value. Another approach for predicting the PvT behavior of a gas mixture is to treat the gas mixture as a pseudopure substance (Fig. 13–9). One such method, proposed by W. B. Kay in 1936 and called Kay’s rule, involves the use of a pseudocritical pressure Pcr,m and pseudocritical temperature Tcr,m for the mixture, defined in terms of the critical pressures and temperatures of the mixture components as
k k P cr,m ¿ a y i Pcr,i
i1 and T cr,m ¿ a y i Tcr,i
i1 (13–11a, b) The compressibility factor of the mixture Zm is then easily determined by using these pseudocritical properties. The result obtained by using Kay’s rule is accurate to within about 10 percent over a wide range of temperatures and pressures, which is acceptable for most engineering purposes. Another way of treating a gas mixture as a pseudopure substance is to use a more accurate equation of state such as the van der Waals, Beattie– Bridgeman, or Benedict–Webb–Rubin equation for the mixture, and to determine the constant coefficients in terms of the coefficients of the components. cen84959_ch13.qxd 4/6/05 9:35 AM Page 687 Chapter 13
In the van der Waals equation, for example, the two constants for the mixture are determined from
am a a y i a 1>2 b i
k i1 2 k  687 and bm a y i bi
i1 (13–12a, b) where expressions for ai and bi are given in Chapter 3. EXAMPLE 13–2 P  v  T Behavior of Nonideal Gas Mixtures
2 kmol N2 6 kmol CO2 300 K 15 MPa Vm = ? A rigid tank contains 2 kmol of N2 and 6 kmol of CO2 gases at 300 K and 15 MPa (Fig. 13–10). Estimate the volume of the tank on the basis of (a) the idealgas equation of state, (b) Kay’s rule, (c) compressibility factors and Amagat’s law, and (d ) compressibility factors and Dalton’s law. Solution The composition of a mixture in a rigid tank is given. The volume
of the tank is to be determined using four different approaches. Assumptions Stated in each section. Analysis (a) When the mixture is assumed to behave as an ideal gas, the volume of the mixture is easily determined from the idealgas relation for the mixture: FIGURE 13–10 Schematic for Example 13–2. Vm
since N m R u Tm Pm 1 8 kmol 2 1 8.314 kPa # m3> kmol # K 2 1 300 K 2 15,000 kPa 2 6 1.330 m3 Nm NN2 NCO2 8 kmol (b) To use Kay’s rule, we need to determine the pseudocritical temperature and pseudocritical pressure of the mixture by using the criticalpoint properties of N2 and CO2 from Table A–1. However, first we need to determine the mole fraction of each component: y N2 T cr,m ¿ N N2 Nm 2 kmol 8 kmol 0.25 and y CO2 N CO2 Nm 6 kmol 8 kmol 0.75 a y i Tcr,i 1 0.25 2 1 126.2 K 2 y N2 Tcr,N2 1 0.75 2 1 304.2 K 2 1 0.75 2 1 7.39 MPa 2 y CO2 Pcr,CO2 y CO2 Tcr,CO2 259.7 K P ¿cr,m a y i Pcr,i 1 0.25 2 1 3.39 MPa 2 y N2 Pcr,N2 6.39 MPa Then, TR PR
Thus, Tm T cr,m ¿ Pm P¿ m cr, 300 K 1.16 259.7 K ∂ Zm 15 MPa 2.35 6.39 MPa 0.49 1 Fig. A–15b 2 Vm Z m N m R u Tm Pm Z mVideal 1 0.49 2 1 1.330 m3 2 0.652 m3 cen84959_ch13.qxd 4/6/05 9:35 AM Page 688 688  Thermodynamics
(c) When Amagat’s law is used in conjunction with compressibility factors, Zm is determined from Eq. 13–10. But first we need to determine the Z of each component on the basis of Amagat’s law: N2: TR,N2 PR,N2 Tm Tcr,N2 Pm Pcr,N2 Tm Tcr,CO2 Pm Pcr,CO2 Zm 300 K 2.38 126.2 K ∂ Z N2 15 MPa 4.42 3.39 MPa 300 K 0.99 304.2 K ∂ Z CO2 15 MPa 2.03 7.39 MPa 1.02 1 Fig. A–15b 2 CO2: TR,CO2 PR,CO2 0.30 1 Fig. A–15b 2 Mixture: a yi Zi yN2 ZN2 yCO2 ZCO2 1 0.25 2 1 1.02 2 1 0.75 2 1 0.30 2 0.48 Thus, Vm Z m N m R u Tm Pm Z mVideal 1 0.48 2 1 1.330 m3 2 0.638 m3 The compressibility factor in this case turned out to be almost the same as the one determined by using Kay’s rule. (d) When Dalton’s law is used in conjunction with compressibility factors, Zm is again determined from Eq. 13–10. However, this time the Z of each component is to be determined at the mixture temperature and volume, which is not known. Therefore, an iterative solution is required. We start the calculations by assuming that the volume of the gas mixture is 1.330 m3, the value determined by assuming idealgas behavior. The TR values in this case are identical to those obtained in part (c) and remain constant. The pseudoreduced volume is determined from its definition in Chap. 3: vR,N2 R uTcr,N2 > Pcr,N2 v N2 1 8.314 kPa # m3> kmol # K 2 1 126.2 K 2> 1 3390 kPa 2
Similarly, 1 1.33 m3 2 > 1 2 kmol 2 R uTcr,N2 > Pcr,N2 Vm > N N2 2.15 vR,CO2 1 8.314 kPa # m3> kmol # K 2 1 304.2 K 2 > 1 7390 kPa 2
0.99 and Z CO2 0.56. Thus, 1 1.33 m3 2 > 1 6 kmol 2 0.648 From Fig. A–15, we read Z N2 Zm
and yN2ZN2 yCO2ZCO2 1 0.25 2 1 0.99 2 1 0.75 2 1 0.56 2 0.67 Vm Zm Nm RTm Pm ZmVideal 1 0.67 2 1 1.330 m3 2 0.891 m3 cen84959_ch13.qxd 4/20/05 2:22 PM Page 689 Chapter 13
This is 33 percent lower than the assumed value. Therefore, we should repeat the calculations, using the new value of Vm. When the calculations are repeated we obtain 0.738 m3 after the second iteration, 0.678 m3 after the third iteration, and 0.648 m3 after the fourth iteration. This value does not change with more iterations. Therefore,  689 Vm 0.648 m3 Discussion Notice that the results obtained in parts (b), (c), and (d ) are very close. But they are very different from the idealgas values. Therefore, treating a mixture of gases as an ideal gas may yield unacceptable errors at high pressures. 13–3 PROPERTIES OF GAS MIXTURES: IDEAL AND REAL GASES Consider a gas mixture that consists of 2 kg of N2 and 3 kg of CO2. The total mass (an extensive property) of this mixture is 5 kg. How did we do it? Well, we simply added the mass of each component. This example suggests a simple way of evaluating the extensive properties of a nonreacting idealor realgas mixture: Just add the contributions of each component of the mixture (Fig. 13–11). Then the total internal energy, enthalpy, and entropy of a gas mixture can be expressed, respectively, as
k k k Um Hm Sm a Ui
i1 k a miui
i1 k a Ni u i
i1 k 1 kJ 2 1 kJ 2 1 kJ> K 2 2 kmol A 6 kmol B UA=1000 kJ UB=1800 kJ Um=2800 kJ (13–13) a Hi
i1 k a mihi
i1 k k a Nihi
i1 (13–14) a Si
i1 a misi
i1 a Ni s i
i1 (13–15) By following a similar logic, the changes in internal energy, enthalpy, and entropy of a gas mixture during a process can be expressed, respectively, as
k k k FIGURE 13–11 The extensive properties of a mixture are determined by simply adding the properties of the components. ¢ Um ¢ Hm ¢Sm a ¢ Ui
i1 k a mi ¢ u i
i1 k a Ni ¢ u i
i1 k 1 kJ 2 1 kJ 2 1 kJ> K 2 (13–16) a ¢ Hi
i1 k a mi ¢ hi
i1 k a N i ¢ hi
i1 k (13–17) 2 kmol A 3 kmol B uA=500 kJ/kmol uB=600 kJ/kmol um=560 kJ/kmol a ¢Si
i1 a m i ¢ si
i1 a Ni ¢ s i
i1 (13–18) Now reconsider the same mixture, and assume that both N2 and CO2 are at 25°C. The temperature (an intensive property) of the mixture is, as you would expect, also 25°C. Notice that we did not add the component temperatures to determine the mixture temperature. Instead, we used some kind of averaging scheme, a characteristic approach for determining the intensive properties of a mixture. The internal energy, enthalpy, and entropy of a mixture per unit mass or per unit mole of the mixture can be determined by dividing the equations above by the mass or the mole number of the mixture (mm or Nm). We obtain (Fig. 13–12) FIGURE 13–12 The intensive properties of a mixture are determined by weighted averaging. cen84959_ch13.qxd 4/6/05 9:35 AM Page 690 690  Thermodynamics
k um hm sm a mfi u i
i1 k 1 kJ> kg 2 1 kJ> kg 2 1 kJ> kg # K 2 k and and and um hm sm a yiu i
i1 k 1 kJ> kmol 2 1 kJ> kmol 2 1 kJ> kmol # K 2 (13–19) a mfi h i
i1 k a y i hi
i1 k (13–20) a mfi si
i1 a yi s i
i1 (13–21) Similarly, the specific heats of a gas mixture can be expressed as
k cv,m cp,m a mfi cv,i
i1 k 1 kJ> kg # K 2 1 kJ> kg # K 2 k and and c v,m c p,m a y i c v,i
i1 k 1 kJ> kmol # K 2 1 kJ> kmol # K 2 (13–22) a mfi cp,i
i1 a y i c p,i
i1 (13–23) Notice that properties per unit mass involve mass fractions (mfi) and properties per unit mole involve mole fractions (yi). The relations given above are exact for idealgas mixtures, and approximate for realgas mixtures. (In fact, they are also applicable to nonreacting liquid and solid solutions especially when they form an “ideal solution.”) The only major difficulty associated with these relations is the determination of properties for each individual gas in the mixture. The analysis can be simplified greatly, however, by treating the individual gases as ideal gases, if doing so does not introduce a significant error. IdealGas Mixtures
The gases that comprise a mixture are often at a high temperature and low pressure relative to the criticalpoint values of individual gases. In such cases, the gas mixture and its components can be treated as ideal gases with negligible error. Under the idealgas approximation, the properties of a gas are not influenced by the presence of other gases, and each gas component in the mixture behaves as if it exists alone at the mixture temperature Tm and mixture volume Vm. This principle is known as the Gibbs–Dalton law, which is an extension of Dalton’s law of additive pressures. Also, the h, u, cv, and cp of an ideal gas depend on temperature only and are independent of the pressure or the volume of the idealgas mixture. The partial pressure of a component in an idealgas mixture is simply Pi yi Pm, where Pm is the mixture pressure. Evaluation of u or h of the components of an idealgas mixture during a process is relatively easy since it requires only a knowledge of the initial and final temperatures. Care should be exercised, however, in evaluating the s of the components since the entropy of an ideal gas depends on the pressure or volume of the component as well as on its temperature. The entropy change of individual gases in an idealgas mixture during a process can be determined from
¢ si s°,2 i s°,1 i R i ln Pi,2 Pi,1 Pi,2 Pi,1 cp,i ln Ti,2 Ti,1 Ti,2 Ti,1 R i ln Pi,2 Pi,1 Pi,2 Pi,1
(13–24) or
¢ si s °,2 i s °,1 i R u ln c p,i ln R u ln
(13–25) cen84959_ch13.qxd 4/6/05 9:35 AM Page 691 Chapter 13
where Pi,2 yi,2Pm,2 and Pi,1 yi,1Pm,1. Notice that the partial pressure Pi of each component is used in the evaluation of the entropy change, not the mixture pressure Pm (Fig. 13–13).  691 Partial pressure Partial pressure of component of component i at state 2 at state EXAMPLE 13–3 Mixing Two Ideal Gases in a Tank ∆ si° = si°,2 – si°,1 – Ri ln
Partial pressure artial pressure of component of component i at state 1 at state —–– —–– Pi,,2 i Pi,,1 i An insulated rigid tank is divided into two compartments by a partition, as shown in Fig. 13–14. One compartment contains 7 kg of oxygen gas at 40°C and 100 kPa, and the other compartment contains 4 kg of nitrogen gas at 20°C and 150 kPa. Now the partition is removed, and the two gases are allowed to mix. Determine (a) the mixture temperature and (b) the mixture pressure after equilibrium has been established. Solution A rigid tank contains two gases separated by a partition. The pressure and temperature of the mixture are to be determined after the partition is removed. Assumptions 1 We assume both gases to be ideal gases, and their mixture to be an idealgas mixture. This assumption is reasonable since both the oxygen and nitrogen are well above their critical temperatures and well below their critical pressures. 2 The tank is insulated and thus there is no heat transfer. 3 There are no other forms of work involved. Properties The constantvolume specific heats of N2 and O2 at room temperature are cv,N 0.743 kJ/kg · K and cv,O 0.658 kJ/kg · K (Table A–2a). Analysis We take the entire contents of the tank (both compartments) as the system. This is a closed system since no mass crosses the boundary during the process. We note that the volume of a rigid tank is constant and thus there is no boundary work done.
2 2 FIGURE 13–13 Partial pressures (not the mixture pressure) are used in the evaluation of entropy changes of idealgas mixtures. O2 7 kg 40°C 100 kPa N2 4 kg 20°C 150 kPa Partition (a) Noting that there is no energy transfer to or from the tank, the energy balance for the system can be expressed as FIGURE 13–14 Schematic for Example 13–3. Ein 3 mcv 1 Tm
1 4 kg 2 1 0.743 kJ> kg # K 2 1 Tm Eout 0 T1 2 4 N2
20°C 2 ¢ Esystem ¢U 3 mcv 1 Tm ¢ UN2 T1 2 4 O2 ¢ UO2 0 By using cv values at room temperature, the final temperature of the mixture is determined to be 1 7 kg 2 1 0.658 kJ> kg # K 2 1 Tm 40°C 2 0 Tm PmVm
where 32.2°C (b) The final pressure of the mixture is determined from the idealgas relation Nm Ru Tm NO2 NN2 Nm mO2 MO2 mN2 MN2 NO2 7 kg 32 kg> kmol 4 kg 28 kg> kmol NN2 0.219 0.219 kmol 0.143 kmol 0.143 0.362 kmol cen84959_ch13.qxd 4/6/05 9:35 AM Page 692 692  Thermodynamics
and VO2 VN2 Vm Thus, a a NRuT1 b P1 O2 NRuT1 b P1 N2 VN2 1 0.219 kmol 2 1 8.314 kPa # m3> kmol # K 2 1 313 K 2 1 0.143 kmol 2 1 8.314 kPa # m3> kmol # K 2 1 293 K 2 150 kPa 5.70 2.32 8.02 m3 100 kPa 5.70 m3 2.32 m3 VO2 Pm Nm Ru Tm Vm 1 0.362 kmol 2 1 8.314 kPa # m3> kmol # K 2 1 305.2 K 2 8.02 m3 114.5 kPa Discussion We could also determine the mixture pressure by using PmVm mmRmTm, where Rm is the apparent gas constant of the mixture. This would require a knowledge of mixture composition in terms of mass or mole fractions. EXAMPLE 13–4
O2 25°C 200 kPa CO2 25°C 200 kPa Exergy Destruction during Mixing of Ideal Gases FIGURE 13–15 Schematic for Example 13–4. An insulated rigid tank is divided into two compartments by a partition, as shown in Fig. 13–15. One compartment contains 3 kmol of O2, and the other compartment contains 5 kmol of CO2. Both gases are initially at 25°C and 200 kPa. Now the partition is removed, and the two gases are allowed to mix. Assuming the surroundings are at 25°C and both gases behave as ideal gases, determine the entropy change and exergy destruction associated with this process. Solution A rigid tank contains two gases separated by a partition. The
entropy change and exergy destroyed after the partition is removed are to be determined. Assumptions Both gases and their mixture are ideal gases. Analysis We take the entire contents of the tank (both compartments) as the system. This is a closed system since no mass crosses the boundary during the process. We note that the volume of a rigid tank is constant, and there is no energy transfer as heat or work. Also, both gases are initially at the same temperature and pressure. When two ideal gases initially at the same temperature and pressure are mixed by removing a partition between them, the mixture will also be at the same temperature and pressure. (Can you prove it? Will this be true for nonideal gases?) Therefore, the temperature and pressure in the tank will still be 25°C and 200 kPa, respectively, after the mixing. The entropy change of each component gas can be determined from Eqs. 13–18 and 13–25: ¢Sm a ¢Si a Ni ¢ s i y i,2Pm,2 Pi,1 a N i a c p,i ln T 0 Ti,2→
i,1 R u ln Pi,2 Pi,1 b R u a N i ln R u a N i ln y i,2 since Pm,2 Pi,1 200 kPa. It is obvious that the entropy change is independent of the composition of the mixture in this case and depends on only cen84959_ch13.qxd 4/20/05 2:22 PM Page 693 Chapter 13
the mole fraction of the gases in the mixture. What is not so obvious is that if the same gas in two different chambers is mixed at constant temperature and pressure, the entropy change is zero. Substituting the known values, the entropy change becomes  693 Nm y O2 y CO2 ¢Sm N O2 N O2 Nm N CO2 Nm N CO2 3 kmol 8 kmol 5 kmol 8 kmol 13 5 2 kmol 8 kmol 0.375 0.625 N CO2 ln y CO2 2 R u 1 N O2 ln y O2 1 8.314 kJ> kmol # K 2 3 1 3 kmol 2 1 ln 0.375 2 1 5 kmol 2 1 ln 0.625 2 4 44.0 kJ/K
The exergy destruction associated with this mixing process is determined from X destroyed T0S gen 1 298 K 2 1 44.0 kJ> K 2 T0 ¢ S sys 13.1 MJ
Discussion This large value of exergy destruction shows that mixing processes are highly irreversible. RealGas Mixtures
When the components of a gas mixture do not behave as ideal gases, the analysis becomes more complex because the properties of real (nonideal) gases such as u, h, cv , and cp depend on the pressure (or specific volume) as well as on the temperature. In such cases, the effects of deviation from idealgas behavior on the mixture properties should be accounted for. Consider two nonideal gases contained in two separate compartments of an adiabatic rigid tank at 100 kPa and 25°C. The partition separating the two gases is removed, and the two gases are allowed to mix. What do you think the final pressure in the tank will be? You are probably tempted to say 100 kPa, which would be true for ideal gases. However, this is not true for nonideal gases because of the influence of the molecules of different gases on each other (deviation from Dalton’s law, Fig. 13–16). When realgas mixtures are involved, it may be necessary to account for the effect of nonideal behavior on the mixture properties such as enthalpy and entropy. One way of doing that is to use compressibility factors in conjunction with generalized equations and charts developed in Chapter 12 for real gases. Consider the following T ds relation for a gas mixture:
dhm Tm dsm vm dPm a a mfi vi b dPm Real gas A 25°C 0.4 m3 100 kPa Real gas B 25°C 0.6 m3 100 kPa Real gas mixture A+B 25°C 1 m3 102 kPa ? It can also be expressed as
d a a mfihi b Tm d a a mfi si b FIGURE 13–16 It is difficult to predict the behavior of nonidealgas mixtures because of the influence of dissimilar molecules on each other. cen84959_ch13.qxd 4/6/05 9:35 AM Page 694 694  Thermodynamics
or
a mfi 1 dhi Tm dsi vi dPm 2 vi dPm 0 which yields
dh i Tm dsi
(13–26) This is an important result because Eq. 13–26 is the starting equation in the development of the generalized relations and charts for enthalpy and entropy. It suggests that the generalized property relations and charts for real gases developed in Chapter 12 can also be used for the components of realgas mixtures. But the reduced temperature TR and reduced pressure PR for each component should be evaluated by using the mixture temperature Tm and mixture pressure Pm. This is because Eq. 13–26 involves the mixture pressure Pm, not the component pressure Pi. The approach described above is somewhat analogous to Amagat’s law of additive volumes (evaluating mixture properties at the mixture pressure and temperature), which holds exactly for idealgas mixtures and approximately for realgas mixtures. Therefore, the mixture properties determined with this approach are not exact, but they are sufficiently accurate. What if the mixture volume and temperature are specified instead of the mixture pressure and temperature? Well, there is no need to panic. Just evaluate the mixture pressure, using Dalton’s law of additive pressures, and then use this value (which is only approximate) as the mixture pressure. Another way of evaluating the properties of a realgas mixture is to treat the mixture as a pseudopure substance having pseudocritical properties, determined in terms of the critical properties of the component gases by using Kay’s rule. The approach is quite simple, and the accuracy is usually acceptable. EXAMPLE 13–5
Heat T1 = 220 K P1 = 10 MPa AIR 79% N2 21% O2 T2 = 160 K P2 = 10 MPa Cooling of a Nonideal Gas Mixture Air is a mixture of N2, O2, and small amounts of other gases, and it can be approximated as 79 percent N2 and 21 percent O2 on mole basis. During a steadyflow process, air is cooled from 220 to 160 K at a constant pressure of 10 MPa (Fig. 13–17). Determine the heat transfer during this process per kmol of air, using (a) the idealgas approximation, (b) Kay’s rule, and (c) Amagat’s law. FIGURE 13–17 Schematic for Example 13–5. Solution Air at a low temperature and high pressure is cooled at constant
pressure. The heat transfer is to be determined using three different approaches. Assumptions 1 This is a steadyflow process since there is no change with time at any point and thus mCV 0 and ECV 0. 2 The kinetic and potential energy changes are negligible. Analysis We take the cooling section as the system. This is a control volume since mass crosses the system boundary during the process. We note that heat is transferred out of the system. The critical properties are Tcr 126.2 K and Pcr 3.39 MPa for N2 and Tcr 154.8 K and Pcr 5.08 MPa for O2. Both gases remain above their cen84959_ch13.qxd 4/6/05 9:35 AM Page 695 Chapter 13
critical temperatures, but they are also above their critical pressures. Therefore, air will probably deviate from idealgas behavior, and thus it should be treated as a realgas mixture. The energy balance for this steadyflow system can be expressed on a unit mole basis as  695 ein eout q out 0 → ¢ esystem h1 h2 0 S ein y N2 1 h 1 eout S h 1 h 2 2 N2 y O2 1 h 1 h2 h 2 2 O2 Z h2 2 q out where the enthalpy change for either component can be determined from the generalized enthalpy departure chart (Fig. A–29) and Eq. 12–58: h1 h2 h 1,ideal h 2,ideal R uTcr 1 Z h1 The first two terms on the righthand side of this equation represent the idealgas enthalpy change of the component. The terms in parentheses represent the deviation from the idealgas behavior, and their evaluation requires a knowledge of reduced pressure PR and reduced temperature TR, which are calculated at the mixture temperature Tm and mixture pressure Pm. (a) If the N2 and O2 mixture is assumed to behave as an ideal gas, the enthalpy of the mixture will depend on temperature only, and the enthalpy values at the initial and the final temperatures can be determined from the idealgas tables of N2 and O2 (Tables A–18 and A–19): T1 T2 220 K S h 1,ideal,N2 h 1,ideal,O2 160 K S h 2,ideal,N2 h2,ideal,O2 y N2 1 h1 h 2 2 N2 6391 kJ> kmol 4648 kJ> kmol 6404 kJ> kmol 4657 kJ> kmol h 2 2 O2 1 0.21 2 1 6404 4657 2 kJ> kmol q out 1744 kJ/kmol 1 0.79 2 1 6391 4648 2 kJ> kmol y O2 1 h1 (b) Kay’s rule is based on treating a gas mixture as a pseudopure substance whose critical temperature and pressure are T ¿ r,m c
and a y iTcr,i y N2Tcr,N2 y O2Tcr,O2 1 0.79 2 1 126.2 K 2 1 0.21 2 1 154.8 K 2 a y i Pcr,i y N2 Pcr,N2 y O2 Pcr,O2 1 0.79 2 1 3.39 MPa 2 1 0.21 2 1 5.08 MPa 2 TR,1 PR Tm,1 Tcr,m Pm Pcr,m Tm,2 Tcr,m 220 K 132.2 K 10 MPa 3.74 MPa 160 K 132.2 K 1.66 2.67 132.2 K P ¿ r,m c
Then, 3.74 MPa f Z h1,m f Z h2,m 1.0 2.6 TR,2 1.21 cen84959_ch13.qxd 4/20/05 2:22 PM Page 696 696  Thermodynamics
Also, h m1,ideal y N2h1,ideal,N2 6394 kJ> kmol h m2,ideal y N2h2,ideal,N2 1 0.79 2 1 6391 kJ> kmol 2 y O2h1,ideal,O2 1 0.21 2 1 6404 kJ> kmol 2 4650 kJ> kmol
Therefore, 1 0.79 2 1 4648 kJ> kmol 2 y O2h2,ideal,O2 1 0.21 2 1 4657 kJ> kmol 2 q out 3 1 6394 1 h m1,ideal 4650 2 kJ> kmol 4 h m2,ideal 2 R uTcr 1 Z h1 1 8.314 kJ> kmol # K 2 1 132.2 K 2 1 1.0 Z h2 2 m 2.6 2 3503 kJ/kmol
(c) The reduced temperatures and pressures for both N2 and O2 at the initial and final states and the corresponding enthalpy departure factors are, from Fig. A–29, N2: TR1,N2 PR,N2 TR2,N2 Tm,1 Tcr,N2 Pm Pcr,N2 Tm,2 Tcr,N2 Tm,1 Tcr,O2 Pm Pcr,O2 Tm,2 Tcr,O2 R uTcr 1 Z h1 220 K 126.2 K 10 MPa 3.39 MPa 160 K 126.2 K 220 K 154.8 K 10 MPa 5.08 MPa 160 K 154.8 K Z h2 2 N2 1.74 2.95 1.27 f f Z h1,N2 0.9 Z h2,N2 2.4 O2: TR1,O2 PR,O2 TR1,O2 1.42 1.97 1.03 f f Z h1,O2 1.3 Z h2,O2 4.0 From Eq. 12–58, 1 h1 h 2 2 N2 1 h1 h 2 2 O2 3317 kJ> kmol 3 1 6404 1 h 1,ideal 3 1 6391 1 h 1,ideal h 2,ideal 2 N2 4648 2 kJ> kmol 4 1 8.314 kJ> kmol # K 2 1 126.2 K 2 1 0.9 Z h2 2 O2 2.4 2 h 2,ideal 2 O2 Therefore, 5222 kJ> kmol q out 4657 2 kJ> kmol 4 R uTcr 1 Z h1 1 8.314 kJ> kmol # K 2 1 154.8 K 2 1 1.3 4.0 2 y N2 1 h1 1 0.79 2 1 3317 kJ> kmol 2 h 2 2 N2 y O2 1 h1 1 0.21 2 1 5222 kJ> kmol 2 h 2 2 O2 3717 kJ/kmol cen84959_ch13.qxd 4/15/05 12:07 PM Page 697 Chapter 13
Discussion This result is about 6 percent greater than the result obtained in part (b) by using Kay’s rule. But it is more than twice the result obtained by assuming the mixture to be an ideal gas.  697 TOPIC OF SPECIAL INTEREST* Chemical Potential and the Separation Work of Mixtures When two gases or two miscible liquids are brought into contact, they mix and form a homogeneous mixture or solution without requiring any work input. That is, the natural tendency of miscible substances brought into contact is to mix with each other. As such, these are irreversible processes, and thus it is impossible for the reverse process of separation to occur spontaneously. For example, pure nitrogen and oxygen gases readily mix when brought into contact, but a mixture of nitrogen and oxygen (such as air) never separates into pure nitrogen and oxygen when left unattended. Mixing and separation processes are commonly used in practice. Separation processes require a work (or, more generally, exergy) input, and minimizing this required work input is an important part of the design process of separation plants. The presence of dissimilar molecules in a mixture affect each other, and therefore the influence of composition on the properties must be taken into consideration in any thermodynamic analysis. In this section we analyze the general mixing processes, with particular emphasis on ideal solutions, and determine the entropy generation and exergy destruction. We then consider the reverse process of separation, and determine the minimum (or reversible) work input needed for separation. The specific Gibbs function (or Gibbs free energy) g is defined as the combination property g h Ts. Using the relation dh v dP T ds, the differential change of the Gibbs function of a pure substance is obtained by differentiation to be
dg v dP s dT or dG V dP S dT 1 pure substance 2
(13–27) For a mixture, the total Gibbs function is a function of two independent intensive properties as well as the composition, and thus it can be expressed as G G(P, T, N1, N2, . . . , Ni). Its differential is
dG a 0G b dP 0 P T,N a 0G b dT 0 T P,N 0G dN i a a 0N b i i P,T,Nj 1 mixture 2
(13–28) where the subscript Nj indicates that the mole numbers of all components in the mixture other than component i are to be held constant during differentiation. For a pure substance, the last term drops out since the composition is fixed, and the equation above must reduce to the one for a pure substance. Comparing Eqs. 13–27 and 13–28 gives
dG V dP S dT a m i dN i
i or dg v dP s dT a m i dy i
i (13–29) *This section can be skipped without a loss in continuity. cen84959_ch13.qxd 4/7/05 10:51 AM Page 698 698  Thermodynamics
where yi Ni /Nm is the mole fraction of component i (Nm is the total number of moles of the mixture) and Mixture:
i mi
hi,mixture gi,mixture Tsi,mixture a 0G b 0 N i P,T,Nj gi hi T si 1 for component i of a mixture 2
(13–30) Pure substance: h Ts g is the chemical potential, which is the change in the Gibbs function of the mixture in a specified phase when a unit amount of component i in the same phase is added as pressure, temperature, and the amounts of all other components are held constant. The symbol tilde (as in v , h , and s ) is used to denote the partial molar properties of the components. Note that the summation term in Eq. 13–29 is zero for a single component system and thus the chemical potential of a pure system in a given phase is equivalent to the molar Gibbs function (Fig. 13–18) since G Ng Nm, where
m a 0G b 0 N P,T g h Ts 1 pure substance 2
(13–31) FIGURE 13–18 For a pure substance, the chemical potential is equivalent to the Gibbs function. Therefore, the difference between the chemical potential and the Gibbs function is due to the effect of dissimilar molecules in a mixture on each other. It is because of this molecular effect that the volume of the mixture of two miscible liquids may be more or less than the sum of the initial volumes of the individual liquids. Likewise, the total enthalpy of the mixture of two components at the same pressure and temperature, in general, is not equal to the sum of the total enthalpies of the individual components before mixing, the difference being the enthalpy (or heat) of mixing, which is the heat released or absorbed as two or more components are mixed isothermally. For example, the volume of an ethyl alcohol–water mixture is a few percent less than the sum of the volumes of the individual liquids before mixing. Also, when water and flour are mixed to make dough, the temperature of the dough rises noticeably due to the enthalpy of mixing released. For reasons explained above, the partial molar properties of the components (denoted by an tilde) should be used in the evaluation of the extensive properties of a mixture instead of the specific properties of the pure components. For example, the total volume, enthalpy, and entropy of a mixture should be determined from, respectively,
V a N ivi
i H a Ni hi
i and S a Ni s i
i 1 mixture 2 (13–32) A yA B yB instead of
Mixing chamber A+B mixture V* a Nivi
i H* a N i hi
i and S* a Ni s i
i (13–33) Then the changes in these extensive properties during mixing become
¢ Vmixing a N i 1 vi
i FIGURE 13–19 The amount of heat released or absorbed during a mixing process is called the enthalpy (or heat) of mixing, which is zero for ideal solutions. v i 2 , ¢ Hmixing a Ni 1 hi
i hi 2 , ¢ S mixing a Ni 1 s i
i si2
(13–34) where Hmixing is the enthalpy of mixing and Smixing is the entropy of mixing (Fig. 13–19). The enthalpy of mixing is positive for exothermic mix cen84959_ch13.qxd 4/20/05 2:22 PM Page 699 Chapter 13
ing processes, negative for endothermic mixing processes, and zero for isothermal mixing processes during which no heat is absorbed or released. Note that mixing is an irreversible process, and thus the entropy of mixing must be a positive quantity during an adiabatic process. The specific volume, enthalpy, and entropy of a mixture are determined from
v a y ivi
i  699 h a y ih i
i and s a yi s i
i (13–35) where yi is the mole fraction of component i in the mixture. Reconsider Eq. 13–29 for dG. Recall that properties are point functions, and they have exact differentials. Therefore, the test of exactness can be applied to the righthand side of Eq. 13–29 to obtain some important relations. For the differential dz M dx N dy of a function z(x, y), the test of exactness is expressed as ( M/ y)x ( N/ x)y. When the amount of component i in a mixture is varied at constant pressure or temperature while other components (indicated by j ) are held constant, Eq. 13–29 simplifies to
dG dG S dT V dP m i dN i m i dN i 1 for T 1 for P constant and N j constant and N j constant 2 a constant 2
(13–36) (13–37) Applying the test of exactness to both of these relations gives
a 0 mi b 0 T P,N a 0S b 0 N i T,P,Nj si and a 0 mi b 0 P T,N 0V b 0 N i T,P,Nj vi
(13–38) where the subscript N indicates that the mole numbers of all components (and thus the composition of the mixture) is to remain constant. Taking the chemical potential of a component to be a function of temperature, pressure, and composition and thus mi mi (P, T, y1, y2, . . . , yj . . .), its total differential can be expressed as
d mi dgi a 0 mi b dP 0 P T,y a 0 mi b dT 0 T P,y 0 mi dy i (13–39) a a 0y b i i P,T,yj where the subscript y indicates that the mole fractions of all components (and thus the composition of the mixture) is to remain constant. Substituting Eqs. 13–38 into the above relation gives
d mi vi dP s i dT 0 mi dy i a a 0y b i i P,T,yj
(13–40) For a mixture of fixed composition undergoing an isothermal process, it simplifies to
dmi vi dP 1T constant, y i constant 2
(13–41) IdealGas Mixtures and Ideal Solutions
When the effect of dissimilar molecules in a mixture on each other is negligible, the mixture is said to be an ideal mixture or ideal solution and the chemical potential of a component in such a mixture equals the Gibbs function cen84959_ch13.qxd 4/20/05 2:22 PM Page 700 700  Thermodynamics
of the pure component. Many liquid solutions encountered in practice, especially dilute ones, satisfy this condition very closely and can be considered to be ideal solutions with negligible error. As expected, the ideal solution approximation greatly simplifies the thermodynamic analysis of mixtures. In an ideal solution, a molecule treats the molecules of all components in the mixture the same way—no extra attraction or repulsion for the molecules of other components. This is usually the case for mixtures of similar substances such as those of petroleum products. Very dissimilar substances such as water and oil won’t even mix at all to form a solution. For an idealgas mixture at temperature T and total pressure P, the partial molar volume of a component i is v i vi RuT/P. Substituting this relation into Eq. 13–41 gives
dmi R uT P dP R uTd ln P R uTd ln Pi 1 T constant, y i constant, ideal gas 2 (13–42) since, from Dalton’s law of additive pressures, Pi mixture and
d ln Pi d ln 1 yiP 2 d 1 ln yi ln P 2 d ln P yi P for an ideal gas
1 yi constant 2
(13–43) for constant yi. Integrating Eq. 13–42 at constant temperature from the total mixture pressure P to the component pressure Pi of component i gives
m i 1 T, Pi 2 m i 1 T, P 2 R uT ln Pi P m i 1 T, P 2 R uT ln y i 1 ideal gas 2
(13–44) For yi 1 (i.e., a pure substance of component i alone), the last term in the above equation drops out and we end up with mi (T, Pi ) mi (T, P), which is the value for the pure substance i. Therefore, the term mi (T, P) is simply the chemical potential of the pure substance i when it exists alone at total mixture pressure and temperature, which is equivalent to the Gibbs function since the chemical potential and the Gibbs function are identical for pure substances. The term mi (T, P) is independent of mixture composition and mole fractions, and its value can be determined from the property tables of pure substances. Then Eq. 13–44 can be rewritten more explicitly as
mi,mixture,ideal 1 T, Pi 2 mi,pure 1 T, P 2 R uT ln y i
(13–45) Note that the chemical potential of a component of an ideal gas mixture depends on the mole fraction of the components as well as the mixture temperature and pressure, and is independent of the identity of the other constituent gases. This is not surprising since the molecules of an ideal gas behave like they exist alone and are not influenced by the presence of other molecules. Eq. 13–45 is developed for an idealgas mixture, but it is also applicable to mixtures or solutions that behave the same way—that is, mixtures or solutions in which the effects of molecules of different components on each other are negligible. The class of such mixtures is called ideal solutions (or ideal mixtures), as discussed before. The idealgas mixture described is just one cate cen84959_ch13.qxd 4/20/05 2:22 PM Page 701 Chapter 13
gory of ideal solutions. Another major category of ideal solutions is the dilute liquid solutions, such as the saline water. It can be shown that the enthalpy of mixing and the volume change due to mixing are zero for ideal solutions (see Wark, 1995). That is,
¢ Vmixing,ideal a N i 1 vi
i  701 vi 2 0 and ¢ Hmixing,ideal a Ni 1 hi
i hi 2 0
v V mixing,ideal 0 vi, mixture
mixture vi, pure
i (13–46) – – Then it follows that v i v i and h i hi. That is, the partial molar volume and the partial molar enthalpy of a component in a solution equal the specific volume and enthalpy of that component when it existed alone as a pure substance at the mixture temperature and pressure. Therefore, the specific volume and enthalpy of individual components do not change during mixing if they form an ideal solution. Then the specific volume and enthalpy of an ideal solution can be expressed as (Fig. 13–20) v mixing,ideal a yi v i
i y i vi, pure H mixing,ideal 0 hi, mixing h
mixture hi, pure
i y i hi, pure a y i v i,pure
i and hmixture,ideal a y ih i
i a y ihi,pure (13–47)
i Note that this is not the case for entropy and the properties that involve entropy such as the Gibbs function, even for ideal solutions. To obtain a relation for the entropy of a mixture, we differentiate Eq. 13–45 with respect to temperature at constant pressure and mole fraction,
a 0 m i,mixing 1 T, Pi 2 0T b a 0 m i,pure 1 T, P 2 0T b R u ln y i
P,y (13–48) P,y We note from Eq. 13–38 that the two partial derivatives above are simply the negative of the partial molar entropies. Substituting,
s i,mixture,ideal 1 T, Pi 2 s i,pure 1 T, P 2 R u ln y 1 1 ideal solution 2
(13–49) FIGURE 13–20 The specific volume and enthalpy of individual components do not change during mixing if they form an ideal solution (this is not the case for entropy). Note that ln yi is a negative quantity since yi 1, and thus Ru ln yi is always positive. Therefore, the entropy of a component in a mixture is always greater than the entropy of that component when it exists alone at the mixture temperature and pressure. Then the entropy of mixing of an ideal solution is determined by substituting Eq. 13–49 into Eq. 13–34 to be
¢ S mixing,ideal a Ni 1 s i
i si2 R u a N i ln y i
i 1 ideal solution 2 (13–50a) or, dividing by the total number of moles of the mixture Nm,
¢ s mixing,ideal a yi 1 s i
i si2 R u a y i ln y i
i 1 per unit mole of mixture 2 (13–50b) Minimum Work of Separation of Mixtures
The entropy balance for a steadyflow system simplifies to Sin Sout Sgen 0. Noting that entropy can be transferred by heat and mass only, the cen84959_ch13.qxd 4/6/05 9:35 AM Page 702 702  Thermodynamics
entropy generation during an adiabatic mixing process that forms an ideal solution becomes
S gen S out S in ¢ S mixing R u a N i ln y i
i 1 ideal solution 2 (13–51a) 1 per unit mole of mixture 2
(13–51b) or
s gen s out s in ¢ smixing R u a y i ln y i
i Also noting that Xdestroyed T0 Sgen, the exergy destroyed during this (and any other) process is obtained by multiplying the entropy generation by the temperature of the environment T0. It gives
X destroyed T0 S gen R uT0 a N i ln y i
i 1 ideal soluton 2 (13–52a) or
x destroyed
A Mixing chamber T0 A+B mixture T0 s gen R uT0 a y i ln y i
i 1 per unit mole of mixture 2 (13–52b) B Wrev Xdestruction T0 Sgen FIGURE 13–21 For a naturally occurring process during which no work is produced or consumed, the reversible work is equal to the exergy destruction. Exergy destroyed represents the wasted work potential—the work that would be produced if the mixing process occurred reversibly. For a reversible or “thermodynamically perfect” process, the entropy generation and thus the exergy destroyed is zero. Also, for reversible processes, the work output is a maximum (or, the work input is a minimum if the process does not occur naturally and requires input). The difference between the reversible work and the actual useful work is due to irreversibilities and is equal to the exergy destruction. Therefore, Xdestroyed Wrev Wactual. Then it follows that for a naturally occurring process during which no work is produced, the reversible work is equal to the exergy destruction (Fig. 13–21). Therefore, for the adiabatic mixing process that forms an ideal solution, the reversible work (total and per unit mole of mixture) is, from Eq. 13–52,
Wrev R uT0 a N i ln y i
i and wrev R uT0 a y i ln y i
i (13–53) A reversible process, by definition, is a process that can be reversed without leaving a net effect on the surroundings. This requires that the direction of all interactions be reversed while their magnitudes remain the same when the process is reversed. Therefore, the work input during a reversible separation process must be equal to the work output during the reverse process of mixing. A violation of this requirement will be a violation of the second law of thermodynamics. The required work input for a reversible separation process is the minimum work input required to accomplish that separation since the work input for reversible processes is always less than the work input of corresponding irreversible processes. Then the minimum work input required for the separation process can be expressed as
Wmin,in R uT0 a N i ln y i
i and wmin,in R uT0 a y i ln y i
i (13–54) cen84959_ch13.qxd 4/6/05 9:35 AM Page 703 Chapter 13
It can also be expressed in the rate form as
# Wmin,in # R uT0 a N i ln y i
i  703 # N mR uT0 a y i ln y i
i 1 kW 2 (13–55) . where Wmin,in is the minimum power input required to separate a solution that # # # NmMm kg> s) into its compoapproaches at a rate of Nm kmol> s (or mm nents. The work of separation per unit mass of mixture can be determined from wmin,in wmin,in> M m , where Mm is the apparent molar mass of the mixture. The minimum work relations above are for complete separation of the components in the mixture. The required work input will be less if the exiting streams are not pure. The reversible work for incomplete separation can be determined by calculating the minimum separation work for the incoming mixture and the minimum separation works for the outgoing mixtures, and then taking their difference. Reversible Mixing Processes
The mixing processes that occur naturally are irreversible, and all the work potential is wasted during such processes. For example, when the fresh water from a river mixes with the saline water in an ocean, an opportunity to produce work is lost. If this mixing is done reversibly (through the use of semipermeable membranes, for example) some work can be produced. The maximum amount of work that can be produced during a mixing process is equal to the minimum amount of work input needed for the corresponding separation process (Fig. 13–22). That is,
Wmax,out,mixing Wmin,in,separation
(13–56) A yA B yB Wmax,out = 5 kJ/kg mixture Mixing chamber A+B mixture (a) Mixing Therefore, the minimum work input relations given above for separation can also be used to determine the maximum work output for mixing. The minimum work input relations are independent of any hardware or process. Therefore, the relations developed above are applicable to any separation process regardless of actual hardware, system, or process, and can be used for a wide range of separation processes including the desalination of sea or brackish water. Wmax,in = 5 kJ/kg mixture A yA B yB (b) Separation Separation unit A+B mixture SecondLaw Efficiency
The secondlaw efficiency is a measure of how closely a process approximates a corresponding reversible process, and it indicates the range available for potential improvements. Noting that the secondlaw efficiency ranges from 0 for a totally irreversible process to 100 percent for a totally reversible process, the secondlaw efficiency for separation and mixing processes can be defined as
hII,separation # Wmin,in # Wact,in wmin,in wact,in and hII,mixing # Wact,out # Wmax,out wact,out wmax,out
(13–57) FIGURE 13–22 Under reversible conditions, the work consumed during separation is equal to the work produced during the reverse process of mixing. . where Wact,in is the actual power input (or exergy consumption) of the separa. tion plant and Wact,out is the actual power produced during mixing. Note that cen84959_ch13.qxd 4/20/05 2:22 PM Page 704 704  Thermodynamics
the secondlaw efficiency is always less than 1 since the actual separation process requires a greater amount of work input because of irreversibilities. Therefore, the minimum work input and the secondlaw efficiency provide a basis for comparison of actual separation processes to the “idealized” ones and for assessing the thermodynamic performance of separation plants. A secondlaw efficiency for mixing processes can also be defined as the actual work produced during mixing divided by the maximum work potential available. This definition does not have much practical value, however, since no effort is done to produce work during most mixing processes and thus the secondlaw efficiency is zero. Special Case: Separation of a Two Component Mixture
Consider a mixture of two components A and B whose mole fractions are yA and yB. Noting that yB 1 yA, the minimum work input required to separate 1 kmol of this mixture at temperature T0 completely into pure A and pure B is, from Eq. 13–54,
wmin,in
wmin,in = –RuT0 ln yA (kJ/kmol A) R uT0 1 y A ln y A Wmin,in y B ln y B 2 1 kJ> kmol mixture 2 1 kJ 2 (13–58a) or
pure A (1 kmol) A+B yA, yB Separation unit R uT0 1 N A ln y A N B ln y B 2 (13–58b) or, from Eq. 13–55,
A+B (a) Separating 1 kmol of A from a large body of mixture # Wmin,in # NmR uT0 1 y A ln y A y B ln y B 2 # mmR mT0 1 y A ln y A y B ln y B 2 1 kW 2 (13–58c) wmin,in = –RuT0 ( yA ln yA + yB ln yB) (kJ/kmol mixture) pure A A+B 1 kmol yA, yB Separation unit pure B (b) Complete separation of 1 kmol mixture into its components A and B Some separation processes involve the extraction of just one of the components from a large amount of mixture so that the composition of the remaining mixture remains practically the same. Consider a mixture of two components A and B whose mole fractions are yA and yB, respectively. The minimum work required to separate 1 kmol of pure component A from the mixture of Nm NA NB kmol (with NA 1) is determined by subtracting the minimum work required to separate the remaining mixture RuT0[(NA 1)ln yA NB ln yB] from the minimum work required to separate the initial mixture Wmin,in RuT0(NA ln yA NB ln yB). It gives (Fig. 13–23)
wmin,in R uT0 ln y A R uT0 ln 1 1> y A 2 1 kJ> kmol A 2
(13–59) FIGURE 13–23 The minimum work required to separate a twocomponent mixture for the two limiting cases. The minimum work needed to separate a unit mass (1 kg) of component A is determined from the above relation by replacing Ru by RA (or by dividing the relation above by the molar mass of component A) since RA Ru/MA. Eq. 13–59 also gives the maximum amount of work that can be done as one unit of pure component A mixes with a large amount of A B mixture. An Application: Desalination Processes
The potable water needs of the world is increasing steadily due to population growth, rising living standards, industrialization, and irrigation in agriculture. There are over 10,000 desalination plants in the world, with a total desalted cen84959_ch13.qxd 4/20/05 2:22 PM Page 705 Chapter 13
water capacity of over 5 billion gallons a day. Saudi Arabia is the largest user of desalination with about 25 percent of the world capacity, and the United States is the second largest user with 10 percent. The major desalination methods are distillation and reverse osmosis. The relations can be used directly for desalination processes, by taking the water (the solvent) to be component A and the dissolved salts (the solute) to be component B. Then the minimum work needed to produce 1 kg of pure water from a large reservoir of brackish or seawater at temperature T0 in an environment at T0 is, from Eq. 13–59,
Desalination: wmin,in R wT0 ln 1 1> y w 2 1 kJ> kg pure water 2
(13–60)  705 where Rw 0.4615 kJ/kg K is the gas constant of water and yw is the mole fraction of water in brackish or seawater. The relation above also gives the maximum amount of work that can be produced as 1 kg of fresh water (from a river, for example) mixes with seawater whose water mole fraction is yw. The reversible work associated with liquid flow can also be expressed in terms of pressure difference P and elevation difference z (potential energy) as wmin,in P/r g z where r is the density of the liquid. Combining these relations with Eq. 13–60 gives
¢ Pmin rwmin,in wmin,in> g rR wT0 ln 1 1> y w 2 1 kPa 2 1m 2
(13–61) and
¢ z min R wT0 ln 1 1> y w 2> g (13–62) where Pmin is the osmotic pressure, which represents the pressure difference across a semipermeable membrane that separates fresh water from the saline water under equilibrium conditions, r is the density of saline water, and zmin is the osmotic rise, which represents the vertical distance the saline water would rise when separated from the fresh water by a membrane that is permeable to water molecules alone (again at equilibrium). For desalination processes, Pmin represents the minimum pressure that the saline water must be compressed in order to force the water molecules in saline water through the membrane to the fresh water side during a reverse osmosis desalination process. Alternately, zmin represents the minimum height above the fresh water level that the saline water must be raised to produce the required osmotic pressure difference across the membrane to produce fresh water. The zmin also represents the height that the water with dissolved organic matter inside the roots will rise through a tree when the roots are surrounded by fresh water with the roots acting as semipermeable membranes. The reverse osmosis process with semipermeable membranes is also used in dialysis machines to purify the blood of patients with failed kidneys. EXAMPLE 13–6 Obtaining Fresh Water from Seawater Fresh water is to be obtained from seawater at 15°C with a salinity of 3.48 percent on mass basis (or TDS 34,800 ppm). Determine (a) the mole fractions of the water and the salts in the seawater, (b) the minimum work input required to separate 1 kg of seawater completely into pure water and pure salts, (c) the minimum work input required to obtain 1 kg of fresh cen84959_ch13.qxd 4/20/05 2:22 PM Page 706 706  Thermodynamics
water from the sea, and (d ) the minimum gauge pressure that the seawater must be raised if fresh water is to be obtained by reverse osmosis using semipermeable membranes. Solution Fresh water is to be obtained from seawater. The mole fractions of
seawater, the minimum works of separation needed for two limiting cases, and the required pressurization of seawater for reverse osmosis are to be determined. Assumptions 1 The seawater is an ideal solution since it is dilute. 2 The total dissolved solids in water can be treated as table salt (NaCl). 3 The environment temperature is also 15°C. Properties The molar masses of water and salt are Mw 18.0 kg/kmol and Ms 58.44 kg/kmol. The gas constant of pure water is Rw 0.4615 kJ/kg · K (Table A–1). The density of seawater is 1028 kg/m3. Analysis (a) Noting that the mass fractions of salts and water in seawater are mfs 0.0348 and mfw 1 mfs 0.9652, the mole fractions are determined from Eqs. 13–4 and 13–5 to be Mm 1 mfi aM i mfw 1 Mm Mw yw 1 mfs Ms 0.9652 1 mfw Mw 0.0348 58.44 1 0.9652 18.0 0.9888 1.12% 18.44 kg> kmol yw ys 18.44 kg> kmol 18.0 kg> kmol 0.0112 0.9888 (b) The minimum work input required to separate 1 kg of seawater completely into pure water and pure salts is wmin,in 147.2 kJ> kmol wmin,in wmin,in Mm 18.314 kJ>kmol # K 2 1288.15 K 2 10.9888 ln 0.9888 147.2 kJ> kmol R uT0 1 y A ln y A y B ln y B 2 R uT0 1 y w ln y w y s ln y s 2 0.0112 ln 0.0112 2 18.44 kg> kmol 7.98 kJ/kg seawater Therefore, it takes a minimum of 7.98 kJ of work input to separate 1 kg of seawater into 0.0348 kg of salt and 0.9652 kg (nearly 1 kg) of fresh water. (c) The minimum work input required to produce 1 kg of fresh water from seawater is wmin,in R wT0 ln 1 1> y w 2 1 0.4615 kJ> kg # K 2 1 288.15 K 2 ln 1 1> 0.9888 2 1.50 kJ/kg fresh water
Note that it takes about 5 times more work to separate 1 kg of seawater completely into fresh water and salt than it does to produce 1 kg of fresh water from a large amount of seawater. cen84959_ch13.qxd 4/20/05 2:22 PM Page 707 Chapter 13
(d ) The osmotic pressure in this case is  707 ¢ Pmin r m R w T0 ln 1 1> y w 2 Saline water 1 1028 kg> m3 2 1 0.4615 kPa # m3> kg # K 2 1 288.15 K 2 ln 1 1> 0.9888 2 ∆z Pure water P2 ∆P P2 P1 P1 1540 kPa
which is equal to the minimum gauge pressure to which seawater must be compressed if the fresh water is to be discharged at the local atmospheric pressure. As an alternative to pressurizing, the minimum height above the fresh water level that the seawater must be raised to produce fresh water is (Fig. 13–24)
Membrane ¢ z min wmin,in g 1.50 kJ> kg 9.81 m> s2 a 1 kg # m> s2 1N 1000 N # m ba b 1 kJ 153 m Discussion The minimum separation works determined above also represent the maximum works that can be produced during the reverse process of mixing. Therefore, 7.98 kJ of work can be produced when 0.0348 kg of salt is mixed with 0.9652 kg of water reversibly to produce 1 kg of saline water, and 1.50 kJ of work can be produced as 1 kg of fresh water is mixed with seawater reversibly. Therefore, the power that can be generated as a river with a flow rate of 106 m3/s mixes reversibly with seawater through semipermeable membranes is (Fig. 13–25) FIGURE 13–24 The osmotic pressure and the osmotic rise of saline water. # Wmax,out # rVwmax,out 1.5 1 1000 kg> m3 2 1 10 6 m3> s 2 1 1.50 kJ> kg 2 a 1 MW b 10 3 kJ> s 10 6 MW which shows the tremendous amount of power potential wasted as the rivers discharge into the seas. Fresh and saline water mixing irreversibly Fresh river water Sea water salinity 3.48% z 153 m Fresh and saline water mixing reversibly through semipermeable membranes, and producing power FIGURE 13–25 Power can be produced by mixing solutions of different concentrations reversibly. cen84959_ch13.qxd 4/6/05 9:35 AM Page 708 708  Thermodynamics SUMMARY
A mixture of two or more gases of fixed chemical composition is called a nonreacting gas mixture The composition of a gas mixture is described by specifying either the mole fraction or the mass fraction of each component, defined as mfi where
k k compressibility charts. The compressibility factor of the mixture can be expressed in terms of the compressibility factors of the individual gases as
k mi mm and yi Ni Nm Zm a yi Zi
i1 mm a mi
i1 and Nm a Ni
i1 where Zi is determined either at Tm and Vm (Dalton’s law) or at Tm and Pm (Amagat’s law) for each individual gas. The PvT behavior of a gas mixture can also be predicted approximately by Kay’s rule, which involves treating a gas mixture as a pure substance with pseudocritical properties determined from
k k The apparent (or average) molar mass and gas constant of a mixture are expressed as Mm Also, mfi yi Mi Mm and Mm 1 mfi aM i1 i
k P ¿cr,m a y i Pcr,i
i1 and T ¿ r,m c a y i Tcr,i
i1 mm Nm k a yiMi
i1 and Rm Ru Mm The extensive properties of a gas mixture, in general, can be determined by summing the contributions of each component of the mixture. The evaluation of intensive properties of a gas mixture, however, involves averaging in terms of mass or mole fractions:
k k k Um Hm Sm and
k a Ui
i1 k a miu i
i1 k a Niu i
i1 k Dalton’s law of additive pressures states that the pressure of a gas mixture is equal to the sum of the pressures each gas would exert if it existed alone at the mixture temperature and volume. Amagat’s law of additive volumes states that the volume of a gas mixture is equal to the sum of the volumes each gas would occupy if it existed alone at the mixture temperature and pressure. Dalton’s and Amagat’s laws hold exactly for idealgas mixtures, but only approximately for realgas mixtures. They can be expressed as Dalton’s law: Amagat’s law: Pm Vm a Pi 1 Tm, Vm 2
k i1 k a Hi
i1 k a mi hi
i1 k a N i hi
i1 k a Si
i1 a m i si
i1 a Ni s i
i1 k um hm sm cv,m cp,m a mfiu i
i1 k and and and and and um hm sm c v,m c p,m a yiu i
i1 k a Vi 1 Tm, Pm 2
i1 a mfih i
i1 k a y ihi
i1 k Here Pi is called the component pressure and Vi is called the component volume. Also, the ratio Pi /Pm is called the pressure fraction and the ratio Vi /Vm is called the volume fraction of component i. For ideal gases, Pi and Vi can be related to yi by Pi Pm Vi Vm Ni Nm yi a mfisi
i1 k a yi s i
i1 k a mficv,i
i1 k a y i c v,i
i1 k a mficp,i
i1 a y i c p,i
i1 The quantity yi Pm is called the partial pressure and the quantity yiVm is called the partial volume. The PvT behavior of realgas mixtures can be predicted by using generalized These relations are exact for idealgas mixtures and approximate for realgas mixtures. The properties or property changes of individual components can be determined by using idealgas or realgas relations developed in earlier chapters. cen84959_ch13.qxd 4/6/05 9:35 AM Page 709 Chapter 13 REFERENCES AND SUGGESTED READING
1. A. Bejan. Advanced Engineering Thermodynamics. 2nd ed. New York: Wiley Interscience, 1997. 2. Y. A. Çengel, Y. Cerci, and B. Wood, “Second Law Analysis of Separation Processes of Mixtures,” ASME International Mechanical Engineering Congress and Exposition, Nashville, Tennessee, 1999. 3. Y. Cerci, Y. A. Çengel, and B. Wood, “The Minimum Separation Work for Desalination Processes,” ASME  709 International Mechanical Engineering Congress and Exposition, Nashville, Tennessee, 1999. 4. J. P. Holman. Thermodynamics. 3rd ed. New York: McGrawHill, 1980. 5. K. Wark, Jr. Advanced Thermodynamics for Engineers. New York: McGrawHill, 1995. PROBLEMS*
Composition of Gas Mixtures
13–1C What is the apparent gas constant for a gas mixture? Can it be larger than the largest gas constant in the mixture? 13–2C Consider a mixture of two gases. Can the apparent molar mass of this mixture be determined by simply taking the arithmetic average of the molar masses of the individual gases? When will this be the case? 13–3C What is the apparent molar mass for a gas mixture? Does the mass of every molecule in the mixture equal the apparent molar mass? 13–4C Consider a mixture of several gases of identical masses. Will all the mass fractions be identical? How about the mole fractions? 13–5C The sum of the mole fractions for an idealgas mixture is equal to 1. Is this also true for a realgas mixture? 13–6C What are mass and mole fractions? 13–7C Using the definitions of mass and mole fractions, derive a relation between them. 13–8C Somebody claims that the mass and mole fractions for a mixture of CO2 and N2O gases are identical. Is this true? Why? 13–9C Consider a mixture of two gases A and B. Show that when the mass fractions mfA and mfB are known, the mole fractions can be determined from yA MA 1 1> mfA MB 12 MB and yB 1 yA 13–10 The composition of moist air is given on a molar basis to be 78 percent N2, 20 percent O2, and 2 percent water vapor. Determine the mass fractions of the constituents of air. 13–11 A gas mixture has the following composition on a mole basis: 60 percent N2 and 40 percent CO2. Determine the gravimetric analysis of the mixture, its molar mass, and gas constant. 13–12 Repeat Prob. 13–11 by replacing N2 by O2. 13–13 A gas mixture consists of 5 kg of O2, 8 kg of N2, and 10 kg of CO2. Determine (a) the mass fraction of each component, (b) the mole fraction of each component, and (c) the average molar mass and gas constant of the mixture. 13–14 Determine the mole fractions of a gas mixture that consists of 75 percent CH4 and 25 percent CO2 by mass. Also, determine the gas constant of the mixture. 13–15 A gas mixture consists of 8 kmol of H2 and 2 kmol of N2. Determine the mass of each gas and the apparent gas constant of the mixture. Answers: 16 kg, 56 kg, 1.155 kJ/kg · K 13–16E A gas mixture consists of 5 lbmol of H2 and 4 lbmol of N2. Determine the mass of each gas and the apparent gas constant of the mixture. 13–17 A gas mixture consists of 20 percent O2, 30 percent N2, and 50 percent CO2 on mass basis. Determine the volumetric analysis of the mixture and the apparent gas constant. PvT Behavior of Gas Mixtures
13–18C Is a mixture of ideal gases also an ideal gas? Give an example. 13–19C Express Dalton’s law of additive pressures. Does this law hold exactly for idealgas mixtures? How about nonidealgas mixtures? 13–20C Express Amagat’s law of additive volumes. Does this law hold exactly for idealgas mixtures? How about nonidealgas mixtures? where MA and MB are the molar masses of A and B.
*Problems designated by a “C” are concept questions, and students are encouraged to answer them all. Problems designated by an “E” are in English units, and the SI users can ignore them. Problems with a CDEES icon are solved using EES, and complete solutions together with parametric studies are included on the enclosed DVD. Problems with a computerEES icon are comprehensive in nature, and are intended to be solved with a computer, preferably using the EES software that accompanies this text. cen84959_ch13.qxd 4/6/05 9:35 AM Page 710 710  Thermodynamics
and the total pressure of the mixture. Answers: 178.1 kPa,
103.9 kPa, 282.0 kPa 13–21C How is the PvT behavior of a component in an idealgas mixture expressed? How is the PvT behavior of a component in a realgas mixture expressed? 13–22C What is the difference between the component pressure and the partial pressure? When are these two equivalent? 13–23C What is the difference between the component volume and the partial volume? When are these two equivalent? 13–24C In a gas mixture, which component will have the higher partial pressure—the one with the higher mole number or the one with the larger molar mass? 13–25C Consider a rigid tank that contains a mixture of two ideal gases. A valve is opened and some gas escapes. As a result, the pressure in the tank drops. Will the partial pressure of each component change? How about the pressure fraction of each component? 13–26C Consider a rigid tank that contains a mixture of two ideal gases. The gas mixture is heated, and the pressure and temperature in the tank rise. Will the partial pressure of each component change? How about the pressure fraction of each component? 13–27C Is this statement correct? The volume of an idealgas mixture is equal to the sum of the volumes of each individual gas in the mixture. If not, how would you correct it? 13–28C Is this statement correct? The temperature of an idealgas mixture is equal to the sum of the temperatures of each individual gas in the mixture. If not, how would you correct it? 13–29C Is this statement correct? The pressure of an idealgas mixture is equal to the sum of the partial pressures of each individual gas in the mixture. If not, how would you correct it? 13–30C Explain how a realgas mixture can be treated as a pseudopure substance using Kay’s rule. 13–31 A rigid tank contains 8 kmol of O2 and 10 kmol of CO2 gases at 290 K and 150 kPa. Estimate the volume of the tank. Answer: 289 m3 13–32 Repeat Prob. 13–31 for a temperature of 400 K. 13–33 A rigid tank contains 0.5 kmol of Ar and 2 kmol of N2 at 250 kPa and 280 K. The mixture is now heated to 400 K. Determine the volume of the tank and the final pressure of the mixture. 13–34 A gas mixture at 300 K and 200 kPa consists of 1 kg of CO2 and 3 kg of CH4. Determine the partial pressure of each gas and the apparent molar mass of the gas mixture. 13–35E A gas mixture at 600 R and 20 psia consists of 1 lbm of CO2 and 3 lbm of CH4. Determine the partial pressure of each gas and the apparent molar mass of the gas mixture. 13–36 A 0.3m3 rigid tank contains 0.6 kg of N2 and 0.4 kg of O2 at 300 K. Determine the partial pressure of each gas 13–37 A gas mixture at 350 K and 300 kPa has the following volumetric analysis: 65 percent N2, 20 percent O2, and 15 percent CO2. Determine the mass fraction and partial pressure of each gas. 13–38 A rigid tank that contains 1 kg of N2 at 25°C and 300 kPa is connected to another rigid tank that contains 3 kg of O2 at 25°C and 500 kPa. The valve connecting the two tanks is opened, and the two gases are allowed to mix. If the final mixture temperature is 25°C, determine the volume of each tank and the final mixture pressure. Answers: 0.295 m3,
0.465 m3, 422 kPa N2 1 kg 25°C 300 kPa O2 3 kg 25°C 500 kPa FIGURE P13–38
13–39 A volume of 0.3 m3 of O2 at 200 K and 8 MPa is mixed with 0.5 m3 of N2 at the same temperature and pressure, forming a mixture at 200 K and 8 MPa. Determine the volume of the mixture, using (a) the idealgas equation of state, (b) Kay’s rule, and (c) the compressibility chart and Amagat’s law. Answers: (a) 0.8 m3, (b) 0.79 m3, (c) 0.80 m3 A rigid tank contains 1 kmol of Ar gas at 220 K and 5 MPa. A valve is now opened, and 3 kmol of N2 gas is allowed to enter the tank at 190 K and 8 MPa. The final mixture temperature is 200 K. Determine the pressure of the mixture, using (a) the idealgas equation of state and (b) the compressibility chart and Dalton’s law. 13–40 Ar 1 kmol 220 K 5 MPa N2 3 kmol 190 K 8 MPa FIGURE P13–40
Reconsider Prob. 13–40. Using EES (or other) software, study the effect of varying the moles of nitrogen supplied to the tank over the range of 1 to 10 kmol of N2. Plot the final pressure of the mixture as a function of the amount of nitrogen supplied using the idealgas equation of state and the compressibility chart with Dalton’s law. 13–41 cen84959_ch13.qxd 4/20/05 2:22 PM Page 711 Chapter 13
13–42E A rigid tank contains 1 lbmol of argon gas at 400 R and 750 psia. A valve is now opened, and 3 lbmol of N2 gas is allowed to enter the tank at 340 R and 1200 psia. The final mixture temperature is 360 R. Determine the pressure of the mixture, using (a) the idealgas equation of state and (b) the compressibility chart and Dalton’s law. Answers: (a) 2700
psia, (b) 2507 psia  711 CO2 at 27°C and 200 kPa, and the other compartment contains 7.5 kmol of H2 gas at 40°C and 400 kPa. Now the partition is removed, and the two gases are allowed to mix. Determine (a) the mixture temperature and (b) the mixture pressure after equilibrium has been established. Assume constant specific heats at room temperature for both gases. Properties of Gas Mixtures
13–43C Is the total internal energy of an idealgas mixture equal to the sum of the internal energies of each individual gas in the mixture? Answer the same question for a realgas mixture. 13–44C Is the specific internal energy of a gas mixture equal to the sum of the specific internal energies of each individual gas in the mixture? 13–45C Answer Prob. 13–43C and 13–44C for entropy. 13–46C Is the total internal energy change of an idealgas mixture equal to the sum of the internal energy changes of each individual gas in the mixture? Answer the same question for a realgas mixture. 13–47C When evaluating the entropy change of the components of an idealgas mixture, do we have to use the partial pressure of each component or the total pressure of the mixture? 13–48C Suppose we want to determine the enthalpy change of a realgas mixture undergoing a process. The enthalpy change of each individual gas is determined by using the generalized enthalpy chart, and the enthalpy change of the mixture is determined by summing them. Is this an exact approach? Explain. 13–49 A process requires a mixture that is 21 percent oxygen, 78 percent nitrogen, and 1 percent argon by volume. All three gases are supplied from separate tanks to an adiabatic, constantpressure mixing chamber at 200 kPa but at different temperatures. The oxygen enters at 10°C, the nitrogen at 60°C, and the argon at 200°C. Determine the total entropy change for the mixing process per unit mass of mixture. 13–50 A mixture that is 15 percent carbon dioxide, 5 percent carbon monoxide, 10 percent oxygen, and 70 percent nitrogen by volume undergoes an adiabatic compression process having a compression ratio of 8:1. If the initial state of the mixture is 300 K and 100 kPa, determine the makeup of the mixture on a mass basis and the internal energy change per unit mass of mixture. 13–51 Propane and air are supplied to an internal combustion engine such that the airfuel ratio is 16:1 when the pressure is 95 kPa and the temperature is 30°C. The compression ratio of the engine is 9.5:1. If the compression process is isentropic, determine the required work input for this compression process, in kJ/kg of mixture. 13–52 An insulated rigid tank is divided into two compartments by a partition. One compartment contains 2.5 kmol of CO2 2.5 kmol 27°C 200 kPa H2 7.5 kmol 40°C 400 kPa FIGURE P13–52
13–53 A 0.9m3 rigid tank is divided into two equal compartments by a partition. One compartment contains Ne at 20°C and 100 kPa, and the other compartment contains Ar at 50°C and 200 kPa. Now the partition is removed, and the two gases are allowed to mix. Heat is lost to the surrounding air during this process in the amount of 15 kJ. Determine (a) the final mixture temperature and (b) the final mixture pressure.
Answers: (a) 16.2°C, (b) 138.9 kPa 13–54 13–55 Repeat Prob. 13–53 for a heat loss of 8 kJ. Ethane (C2H6) at 20°C and 200 kPa and methane (CH4) at 45°C and 200 kPa enter an adiabatic mixing chamber. The mass flow rate of ethane is 9 kg/s, which is twice the mass flow rate of methane. Determine (a) the mixture temperature and (b) the rate of entropy generation during this process, in kW/K. Take T0 25°C. 13–56 Reconsider Prob. 13–55. Using EES (or other) software, determine the effect of the mass fraction of methane in the mixture on the mixture temperature and the rate of exergy destruction. The total mass flow rate is maintained constant at 13.5 kg/s, and the mass fraction of methane is varied from 0 to 1. Plot the mixture temperature and the rate of exergy destruction against the mass fraction, and discuss the results. 13–57 An equimolar mixture of helium and argon gases is to be used as the working fluid in a closedloop gasturbine cycle.
2.5 MPa 1300 K He  Ar turbine 200 kPa FIGURE P13–57 cen84959_ch13.qxd 4/20/05 2:22 PM Page 712 712  Thermodynamics
during this process by treating the mixture (a) as an ideal gas and (b) as a nonideal gas and using Amagat’s law.
Answers: (a) 4273 kJ, (b) 4745 kJ The mixture enters the turbine at 2.5 MPa and 1300 K and expands isentropically to a pressure of 200 kPa. Determine the work output of the turbine per unit mass of the mixture. A mixture of 80 percent N2 and 20 percent CO2 gases (on a mass basis) enters the nozzle of a turbojet engine at 90 psia and 1800 R with a low velocity, and it expands to a pressure of 12 psia. If the isentropic efficiency of the nozzle is 92 percent, determine (a) the exit temperature and (b) the exit velocity of the mixture. Assume constant specific heats at room temperature. 13–59E Reconsider Prob. 13–58E. Using EES (or other) software, first solve the stated problem and then, for all other conditions being the same, resolve the problem to determine the composition of the nitrogen and carbon dioxide that is required to have an exit velocity of 2600 ft /s at the nozzle exit. 13–58E Heat 6 kg H2 21 kg N2 160 K 5 MPa FIGURE P13–63
13–64 Determine the total entropy change and exergy destruction associated with the process described in Prob. 13–63 by treating the mixture (a) as an ideal gas and (b) as a nonideal gas and using Amagat’s law. Assume constant specific heats at room temperature and take T0 30°C. 13–65 Air, which may be considered as a mixture of 79 percent N2 and 21 percent O2 by mole numbers, is compressed isothermally at 200 K from 4 to 8 MPa in a steadyflow device. The compression process is internally reversible, and the mass flow rate of air is 2.9 kg/s. Determine the power input to the compressor and the rate of heat rejection by treating the mixture (a) as an ideal gas and (b) as a nonideal gas and using Amagat’s law. Answers: (a) 115.3 kW, 115.3 kW,
(b) 143.6 kW, 94.2 kW 200 K 8 MPa · W 79% N2 21% O2 13–60 A piston–cylinder device contains a mixture of 0.5 kg of H2 and 1.6 kg of N2 at 100 kPa and 300 K. Heat is now transferred to the mixture at constant pressure until the volume is doubled. Assuming constant specific heats at the average temperature, determine (a) the heat transfer and (b) the entropy change of the mixture. 13–61 An insulated tank that contains 1 kg of O2 at 15°C and 300 kPa is connected to a 2m3 uninsulated tank that contains N2 at 50°C and 500 kPa. The valve connecting the two tanks is opened, and the two gases form a homogeneous mixture at 25°C. Determine (a) the final pressure in the tank, (b) the heat transfer, and (c) the entropy generated during this process. Assume T0 25°C.
Answers: (a) 444.6 kPa, (b) 187.2 kJ, (c) 0.962 kJ/K O2 1 kg 15°C 300 kPa N2 2 m3 50°C 500 kPa FIGURE P13–61
200 K 4 MPa 13–62 Reconsider Prob. 13–61. Using EES (or other) software, compare the results obtained assuming idealgas behavior with constant specific heats at the average temperature, and using realgas data obtained from EES by assuming variable specific heats over the temperature range. FIGURE P13–65
13–66 Reconsider Prob. 13–65. Using EES (or other) software, compare the results obtained by assuming ideal behavior, real gas behavior with Amagat’s law, and real gas behavior with EES data. 13–63 A piston–cylinder device contains 6 kg of H2 and 21 kg of N2 at 160 K and 5 MPa. Heat is now transferred to the device, and the mixture expands at constant pressure until the temperature rises to 200 K. Determine the heat transfer 13–67 The combustion of a hydrocarbon fuel with air results in a mixture of products of combustion having the composition on a volume basis as follows: 4.89 percent carbon dioxide, cen84959_ch13.qxd 4/6/05 9:35 AM Page 713 Chapter 13
6.50 percent water vapor, 12.20 percent oxygen, and 76.41 percent nitrogen. Determine the average molar mass of the mixture, the average specific heat at constant pressure of the mixture at 600 K, in kJ/kmol K, and the partial pressure of the water vapor in the mixture for a mixture pressure of 200 kPa. 13–68 A mixture that is 20 percent carbon dioxide, 10 percent oxygen, and 70 percent nitrogen by volume undergoes a process from 300 K and 100 kPa to 500 K and 400 kPa. Determine the makeup of the mixture on a mass basis and the enthalpy change per unit mass of mixture.  713 13–76E Fresh water is to be obtained from brackish water at 65°F with a salinity of 0.12 percent on mass basis (or TDS 1200 ppm). Determine (a) the mole fractions of the water and the salts in the brackish water, (b) the minimum work input required to separate 1 lbm of brackish water completely into pure water and pure salts, and (c) the minimum work input required to obtain 1 lbm of fresh water. 13–77 A desalination plant produces fresh water from seawater at 10°C with a salinity of 3.2 percent on mass basis at a rate of 1.4 m3/s while consuming 8.5 MW of power. The salt content of the fresh water is negligible, and the amount of fresh water produced is a small fraction of the seawater used. Determine the secondlaw efficiency of this plant. 13–78 Fresh water is obtained from seawater at a rate of 0.5 m3/s by a desalination plant that consumes 3.3 MW of power and has a secondlaw efficiency of 18 percent. Determine the power that can be produced if the fresh water produced is mixed with the seawater reversibly. Special Topic: Chemical Potential and the Separation Work of Mixtures
13–69C It is common experience that two gases brought into contact mix by themselves. In the future, could it be possible to invent a process that will enable a mixture to separate into its components by itself without any work (or exergy) input? 13–70C A 2L liquid is mixed with 3 L of another liquid, forming a homogeneous liquid solution at the same temperature and pressure. Can the volume of the solution be more or less than the 5 L? Explain. 13–71C A 2L liquid at 20°C is mixed with 3 L of another liquid at the same temperature and pressure in an adiabatic container, forming a homogeneous liquid solution. Someone claims that the temperature of the mixture rose to 22°C after mixing. Another person refutes the claim, saying that this would be a violation of the first law of thermodynamics. Who do you think is right? 13–72C What is an ideal solution? Comment on the volume change, enthalpy change, entropy change, and chemical potential change during the formation of ideal and nonideal solutions. 13–73 Brackish water at 12°C with total dissolved solid content of TDS 780 ppm (a salinity of 0.078 percent on mass basis) is to be used to produce fresh water with negligible salt content at a rate of 280 L/s. Determine the minimum power input required. Also, determine the minimum height to which the brackish water must be pumped if fresh water is to be obtained by reverse osmosis using semipermeable membranes. 13–74 A river is discharging into the ocean at a rate of 400,000 m3/s. Determine the amount of power that can be generated if the river water mixes with the ocean water reversibly. Take the salinity of the ocean to be 3.5 percent on mass basis, and assume both the river and the ocean are at 15°C. 13–75 Reconsider Prob. 13–74. Using EES (or other) software, investigate the effect of the salinity of the ocean on the maximum power generated. Let the salinity vary from 0 to 5 percent. Plot the power produced versus the salinity of the ocean, and discuss the results. Review Problems
13–79 Air has the following composition on a mole basis: 21 percent O2, 78 percent N2, and 1 percent Ar. Determine the gravimetric analysis of air and its molar mass. Answers:
23.2 percent O2, 75.4 percent N2, 1.4 percent Ar, 28.96 kg/kmol 13–80 Using Amagat’s law, show that
k Zm a yi Zi
i1 for a realgas mixture of k gases, where Z is the compressibility factor. 13–81 Using Dalton’s law, show that
k Zm a yi Zi
i1 for a realgas mixture of k gases, where Z is the compressibility factor. 13–82 A mixture of carbon dioxide and nitrogen flows through a converging nozzle. The mixture leaves the nozzle at a temperature of 500 K with a velocity of 360 m/s. If the velocity is equal to the speed of sound at the exit temperature, determine the required makeup of the mixture on a mass basis. 13–83 A piston–cylinder device contains products of combustion from the combustion of a hydrocarbon fuel with air. The combustion process results in a mixture that has the composition on a volume basis as follows: 4.89 percent carbon dioxide, 6.50 percent water vapor, 12.20 percent oxygen, and 76.41 percent nitrogen. This mixture is initially at 1800 K and 1 MPa and expands in an adiabatic, reversible process to 200 kPa. Determine the work done on the piston by the gas, in kJ/kg of mixture. Treat the water vapor as an ideal gas. cen84959_ch13.qxd 4/20/05 2:22 PM Page 714 714  Thermodynamics
13–93 Using EES (or other) software, write a program to determine the apparent gas constant, constant volume specific heat, and internal energy of a mixture of three ideal gases when the mass fractions and other properties of the constituent gases are given. Run the program for a sample case, and give the results. Using EES (or other) software, write a program to determine the entropy change of a mixture of three ideal gases when the mass fractions and other properties of the constituent gases are given. Run the program for a sample case, and give the results. 13–84 A rigid tank contains 2 kmol of N2 and 6 kmol of CH4 gases at 200 K and 12 MPa. Estimate the volume of the tank, using (a) the idealgas equation of state, (b) Kay’s rule, and (c) the compressibility chart and Amagat’s law. 13–85 A steady stream of equimolar N2 and CO2 mixture at 100 kPa and 18°C is to be separated into N2 and CO2 gases at 100 kPa and 18°C. Determine the minimum work required per unit mass of mixture to accomplish this separation process. Assume T0 18°C. 13–86 A gas mixture consists of O2 and N2. The ratio of the mole numbers of N2 to O2 is 3:1. This mixture is heated during a steadyflow process from 180 to 210 K at a constant pressure of 8 MPa. Determine the heat transfer during this process per mole of the mixture, using (a) the idealgas approximation and (b) Kay’s rule. 13–87 Reconsider Prob. 13–86. Using EES (or other) software, investigate the effect of the mole fraction of oxygen in the mixture on heat transfer using realgas behavior with EES data. Let the mole fraction of oxygen vary from 0 to 1. Plot the heat transfer against the mole fraction, and discuss the results. 13–94 Fundamentals of Engineering (FE) Exam Problems
13–95 An idealgas mixture whose apparent molar mass is 36 kg/kmol consists of N2 and three other gases. If the mole fraction of nitrogen is 0.30, its mass fraction is (a) 0.15 (b) 0.23 (c) 0.30 (d ) 0.39 (e) 0.70 13–96 An idealgas mixture consists of 2 kmol of N2 and 6 kmol of CO2. The mass fraction of CO2 in the mixture is (a) 0.175 (b) 0.250 (c) 0.500 (d ) 0.750 (e) 0.875 13–97 An idealgas mixture consists of 2 kmol of N2 and 4 kmol of CO2. The apparent gas constant of the mixture is (a) 0.215 kJ/kg K (b) 0.225 kJ/kg K (c) 0.243 kJ/kg (d) 0.875 kJ/kg K (e) 1.24 kJ/kg K K 13–88 Determine the total entropy change and exergy destruction associated with the process described in Prob. 13–86, using (a) the idealgas approximation and (b) Kay’s rule. Assume constant specific heats and T0 30°C. 13–89 A rigid tank contains a mixture of 4 kg of He and 8 kg of O2 at 170 K and 7 MPa. Heat is now transferred to the tank, and the mixture temperature rises to 220 K. Treating the He as an ideal gas and the O2 as a nonideal gas, determine (a) the final pressure of the mixture and (b) the heat transfer. 13–90 A mixture of 60 percent carbon dioxide and 40 percent methane on a mole basis expands through a turbine from 1600 K and 800 kPa to 100 kPa. The volume flow rate at the turbine entrance is 10 L/s. Determine the rate of work done by the mixture using (a) idealgas approximation and (b) Kay’s rule. 13–91 A pipe fitted with a closed valve connects two tanks. One tank contains a 5kg mixture of 62.5 percent CO2 and 37.5 percent O2 on a mole basis at 30°C and 125 kPa. The second tank contains 10 kg of N2 at 15°C and 200 kPa. The valve in the pipe is opened and the gases are allowed to mix. During the mixing process 100 kJ of heat energy is supplied to the combined tanks. Determine the final pressure and temperature of the mixture and the total volume of the mixture. Using EES (or other) software, write a program to determine the mole fractions of the components of a mixture of three gases with known molar masses when the mass fractions are given, and to determine the mass fractions of the components when the mole fractions are given. Run the program for a sample case, and give the results. 13–92 13–98 A rigid tank is divided into two compartments by a partition. One compartment contains 3 kmol of N2 at 600 kPa and the other compartment contains 7 kmol of CO2 at 200 kPa. Now the partition is removed, and the two gases form a homogeneous mixture at 300 kPa. The partial pressure of N2 in the mixture is (a) 75 kPa (b) 90 kPa (c) 150 kPa (d ) 175 kPa (e) 225 kPa 13–99 An 80L rigid tank contains an idealgas mixture of 5 g of N2 and 5 g of CO2 at a specified pressure and temperature. If N2 were separated from the mixture and stored at mixture temperature and pressure, its volume would be (a) 32 L (b) 36 L (c) 40 L (d ) 49 L (e) 80 L 13–100 An idealgas mixture consists of 3 kg of Ar and 6 kg of CO2 gases. The mixture is now heated at constant volume from 250 K to 350 K. The amount of heat transfer is (a) 374 kJ (d ) 525 kJ (b) 436 kJ (e) 664 kJ (c) 488 kJ 13–101 An idealgas mixture consists of 30 percent helium and 70 percent argon gases by mass. The mixture is now expanded isentropically in a turbine from 400°C and 1.2 MPa to a pressure of 200 kPa. The mixture temperature at turbine exit is (a) 195°C (d ) 130°C (b) 56°C (e) 400°C (c) 112°C cen84959_ch13.qxd 4/6/05 9:35 AM Page 715 Chapter 13
13–102 One compartment of an insulated rigid tank contains 2 kmol of CO2 at 20°C and 150 kPa while the other compartment contains 5 kmol of H2 gas at 35°C and 300 kPa. Now the partition between the two gases is removed, and the two gases form a homogeneous idealgas mixture. The temperature of the mixture is (a) 25°C (b) 29°C (c) 22°C (d ) 32°C (e) 34°C 13–103 A piston–cylinder device contains an idealgas mixture of 3 kmol of He gas and 7 kmol of Ar gas at 50°C and 400 kPa. Now the gas expands at constant pressure until its volume doubles. The amount of heat transfer to the gas mixture is (a) 6.2 MJ (d ) 10 MJ (b) 4.2 MJ (e) 67 MJ (c) 27 MJ  715 Design and Essay Problem
13–105 Prolonged exposure to mercury even at relatively low but toxic concentrations in the air is known to cause permanent mental disorders, insomnia, and pain and numbness in the hands and the feet, among other things. Therefore, the maximum allowable concentration of mercury vapor in the air at work places is regulated by federal agencies. These regulations require that the average level of mercury concentration in the air does not exceed 0.1 mg/m3. Consider a mercury spill that occurs in an airtight storage room at 20°C in San Francisco during an earthquake. Calculate the highest level of mercury concentration in the air that can occur in the storage room, in mg/m3, and determine if it is within the safe level. The vapor pressure of mercury at 20°C is 0.173 Pa. Propose some guidelines to safeguard against the formation of toxic concentrations of mercury vapor in air in storage rooms and laboratories. 13–104 An idealgas mixture of helium and argon gases with identical mass fractions enters a turbine at 1200 K and 1 MPa at a rate of 0.3 kg/s, and expands isentropically to 100 kPa. The power output of the turbine is (a) 478 kW (d ) 729 kW (b) 619 kW (e) 564 kW (c) 926 KW cen84959_ch13.qxd 4/6/05 9:35 AM Page 716 ...
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