COSTALD method

An alternate method with approximately the same accuracy as the Rackett method is the COSTALD method of Hankinson and Thomson. The critical temperature, a characteristic volume near the critical volume, and an acentric factor optimized for vapor pressure prediction by the Soave equation of state are required input parameters. The method is detailed in the Technical Data Book. 

For the estimation of liquid densities or specific volumes, respectively, the COSTALD method [16] has proved to be reliable. It can be applied in the range 0.25 < Tr < 1. The calculation equations are... 

Estimate the liquid density of n-hexane at T = 293.15 K using the COSTALD method. 

It is important to note that even when we choose an equation-of-state approach. Aspen HYSYS does not calculate all physical properties from the equation of state. For hydrocarbons, equations of state do not generally predict the equilibrium properties of very light components such as hydrogen. In addition, density predictions (especially in the heavy hydrocarbon range) can be quite poor. We almost always modify the equation of state to account for these deficiencies. For the FCC process, we choose the COSTALD method to predict the liquid density (Figure 4.46). 

In the first type of method, the density at saturation pressure is calculated, then this density is corrected for pressure. The COSTALD and Rackett methods belong to this category. Correction for pressure is done using Thompson s method. These methods are applicable only if the reduced temperature is less than 0.98. 

The COSTALD (Corresponding states liquid density) method was originally developed for calculating the densities of liquefied gases its use has become generally widespread. 

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