ADVANCED THERMODYNAMICS ENGINEERING phần 7

Ghi chú Khi nhiệt độ tăng, giá trị của Pksat cho một tăng chất. Do đó XK, l giảm. Sự ấm lên của nước sông làm giảm nồng độ O2 và N2 trong đó. j. Ví dụ 10: Một khối lượng 20 lít cứng nhắc bao gồm 80% chất lỏng và hơi 20% theo khối lượng 111,4 º C và 1,5 bar. | In air X N X O . Furthermore X X X O X N 1 T PO . T C O2 l 2 l O2 1 2 1 2 O2 Since Psal T - P_sal T Since PN2 T PO2 T Remarks As the temperature rises the value of Pksal for a substance increases. Hence Xk 2 decreases. The warming of river water decreases the O2 and N2 concentrations in it. j. Example 10 A 20-liter rigid volume consists of 80 liquid and 20 vapor by mass at C and bar. A pin is placed on piston to prevent its motion. Gaseous nitrogen is isother-mally injected into the volume until the pressure reaches 2 bar. What is the nitrogen mole fraction in the gas phase Assume that N2 does not dissolve in the liquid. What happens if there is no pin during the injection of N2. Instead of adding N2 discuss the effects with salt addition at C. Solution Psat C bar. The total volume V mg vg mf vf m x vg 1-x vf A m m . Therefore m kg Using the ideal gas law for the vapor phase m Vf vf V - Vf RT Po B Vf m - Po V RT 1 vf - Po RT vf m - Po V RT . C The pressure increases as additional gas is injected thereby increasing the Gibbs energy of the liquid and vapor phases. In case of liquid water g T P gf T Psat vf P - Psat . For an ideal gas mixture in the vapor phase g H2O T P Xh2o g h2O T p h2O gH20 T Psat J vH2O g dP g g T Psat R T ln p h2o Psat . D Equating Eq. C with Eq. D v P - Psat RT ln p h2o Psat or ln p H2O Psat vf P - Psat RT E This relation is known as the Kelvin-Helmholtz formula which shows the effect of total pressure on partial pressure of vapor. Note that the partial pressure of H2O in the vapor phase is not the same as saturation pressure at T. For water vf m3 kmole-1 Psat bar and for this case P 2 bar and T K. Therefore the partial pressure of H2O in vapor phase p HO bar. a value close to saturation pressure at T since vf is small. Further Figure 7 The variation of the mole fraction of .