Choosing an Appropriate Form in Applications

Here we can only begin to address general elements that should be included in reaching a correct decision. More specific elements involve considerations of the behaviors and limitations of particular models that might apply to the problem. We divide the general considerations into three parts. 

Assess the risks. What use is to be made of the calculated property Do we need an exact value (accuracy) or are we only trying to avoid major blunders (reliability) What is the impact if the computed property is in error by 1%, 10%, or 100% How accurate and reliable are the data that will be used as inputs to the calculation Remember, no property is ever measured or computed exactly. This means we must understand the problem weU enough to be able to determine the desired accuracy. 

Make reasonable assumptions. By considering the state of the system and the nature of its components, we can introduce sensible approximations that may vastly simplify the analysis but do little harm to the accuracy of the calculation. A consideration of the state would include identification of the phases present (solid, liquid, gas), estimates for temperature and pressure, and rough estimates for the composition. (For example, is a mixture dominated by one component or are any components present in very small amounts ). In general, it is helpful to locate known state points on phase diagrams, or at least to find where a mixture temperature lies relative to the pure-component melting and critical temperatures. By nature of the components we mean the kinds of intermolecular forces, such as simple van der Waals interactions, hydro- 

Identify the resources available. What computational methods can be applied and what parameters and data are needed to implement a particular method Critical properties Heat capacities Vapor pressures Parameters for a PvTx equation of state Parameters in models for excess properties When available data are sparse (the usual situation) or unreliable or conflicting, then set upper and lower bounds on the property and do a sensitivity analysis (which input data have the largest impact on the calculated property ). Considerations should also be given to the resources needed to set up the calculation (pencil and paper, calculator commands, computer software, original computer codes) and the hardware needed to carry them out (brain, fingers, calculator, PC, workstation). 

All phase and reaction equilibrium computations require expressions for the fugaci-ties of all components. The possible expressions are presented in 6.4 as the five famous fugacity formulae (FFF). Here are some general guidelines for choosing from those possibilities. 

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