z Estimating values
The book has 4 general compressibility charts. Figure 5.4-1 (shown on previous slide) is the most general Figures 5.4-2, 5.4-3, and 5.4-4 are expansions of various regions of 5.41 To fully understand these charts we must introduce the parameter
1/27/11
Estimating z with compressibility charts
1.
Look up the critical temperature and pressure If the gas is either hydrogen or helium, determine the adjusted critical constants with Newtons correction(empirically derived) Tac = Tc + 8K Pac = Pc + 8 atm Calculate reduced values of the two
2.
3. /27/11 1
Example
One hundred gram-moles of nitrogen are contained in a 5-liter vessel at -20.6C. Estimate the pressure in the cylinder using the compressibility factor EOS.
1/27/11
1/27/11
Nonideal Gas Mixtures
We have seen earlier that it is easy deal to with mixtures of ideal gases The cubic EOS we saw earlier become very difficult to deal with for nonideal gas mixtures
1/27/11
Kays rule
Easy way to deal with mixtures of nonideal gases _____________________________of mixtures by averaging pure component critical constants Tc = yATcA + yBTcB + yCTcC + Pc = yAPcA + yBPcB + yCPcC +
yi is the ___________of species i in the
1/27/11
Kays rule
The pseudocritical propertiescan be used to calculate the pseudocritical temperatureand pressure Tr=T/Tc Pr=P/Pc
We can know use the compressibility charts to estimate zmthe compressibility factor for the mixture
1/27/11
Example
A mixture of 75 % H2 and 25% N2 (molar basis) is contained in a tank at 800 atm and -70C. Estimate the specific volume of the mixture in L/mol using Kays rule.
1/27/11
1/27/11