Compressibility Factors for Carbon Dioxide

Figure A2 Compressibility Factor for Carbon Dioxide as a Function of the Reduced Pressure (Abscissa) and the Reduced Temperature (Third Parameter). See Text for Definitions of the Three Quantities.

Figure 4.14 Comparison of the van der Waals (vdW), Redlich-Kwong (RK), and modified Redlich-Kwong (mRK) equations for predicting the compressibility factor of carbon dioxide along the supercritical isotherm T = 350 K. For each equation the parameters a and b were computed from expressions in Table 4.4, using Tc = 304.2 K and = 73.82 bar. Points are exp>erimen-tal values taken from Vargaftik [19].

It would be convenient if the compressibility factor at a single temperature and pressure were the same for all gases, so that a single chart or table of z T, P) could be used for all PVT calculations. Nature is not that accommodating, unfortunately for example, z for nitrogen at 0°C and 100 atm is 0.9848 while z for carbon dioxide at the same temperature and pressure is... 

We now show predictions of the compressibility factor for pure carbon dioxide along two isotherms, one supercritical and the other subcritical. All results shown here used values of a and b computed from and P. Figure 4.14 shows the results for the supercritical isotherm, T = 350 K. Up to about 75 bar, the three equations are all in good agreement with experiment, indicating that, at least at this temperature, all three satisfactorily estimate the second virial coefficient. However, for P > 100 bar, errors in... 

Comments In selecting the compressibility factor, the phase of the pure components must be taken into consideration For carbon dioxide we select the largest root and for pentane the smallest. 

However, the presence of hydrogen sulfide and carbon dioxide causes large errors in compressibility factors obtained by the methods previously discussed. The remedy to this problem is to adjust the pseudocritical properties to account for the unusual behavior of these acid gases.10 The equations used for this adjustment are... 

A new pressure-explicit equation of state suitable for calculating gas and liquid properties of nonpolar compounds was proposed. In its development, the conditions at the critical point and the Maxwell relationship at saturation were met, and PVT data of carbon dioxide and Pitzers table were used as guides for evaluating the values of the parameters. Furthermore, the parameters were generalized. Therefore, for pure compounds, only Tc, Pc, and o> were required for the calculation. The proposed equation successfully predicted the compressibility factors, the liquid fugacity coefficients, and the enthalpy departures for several arbitrarily chosen pure compounds. 

Most enantiomer separations depend, at least in part, on multiple polar interactions between the enantiomers and chiral selector to form diastereomer complexes. These complexes are often too stable for the enantiomers to be eluted by carbon dioxide in the absence of polar modifiers. Optimum resolution is usually observed at low temperatures, frequently subcritical. These conditions typically result in increased peak separations accompanied by increased band broadening. The observed change in resolution depends on which factor is dominant. Mobile phase composition and temperature are the two most important parameters for optimizing resolution in the minimum separation time pressure is often less important as the modified mobile phases are not very compressible. 

Using the saturated vapor compressibility factor data for the system n-butane -carbon dioxide (Olds et al) presented in Table 11.E.7 ... 

Table 11.E.7 Compressibility Factors and Interaction Coefficients for n-Butane - Carbon Dioxide...

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