Thermophysical Properties of Partially Characterized Systems

M.L. Huber, D.G. Friend, C.D. Holcomb, S.L. Outcalt, and J.R. Elliott (Univ. of Akron)

Objective: To provide industry with models for the thermophysical properties of partially characterized systems such as petroleum fractions and lubricants.

Problem: One may encounter fluid systems that cannot be easily characterized in terms of known compositions of pure fluids. An example is a petroleum fraction, where often the fraction is characterized by an average boiling point and a density or specific gravity, and the exact composition of the fluid is not known. Another example of a partially characterized system is a lubricant. These systems are typically proprietary mixtures generally characterized by their viscosity and density. Currently there is a lack of models for the thermophysical properties of these types of systems. In the refrigeration industry, the lack of adequate models for mixtures of alternative refrigerants and lubricants prevents optimal equipment design.

Approach: We are pursuing two different approaches that build upon existing work performed at NIST. For petroleum fractions, we are developing models based on the theory of extended corresponding states that has been shown to be reliable for nonpolar hydrocarbons of low-to-moderate molar masses. For synthetic lubricant systems, we are investigating the use of various equations of state such as statistical associating fluid theory (SAFT) and Elliott, Suresh and Donahue (ESD). In addition to these models, we plan to model the mixture with a Helmholtz-energy-based mixture model that has been very successful in modeling the thermophysical properties of refrigerants and refrigerant mixtures. This allows us to build upon and enhance existing NIST databases such as NIST4 (Thermophysical Properties of Hydrocarbon Mixtures) and NIST23 (REFPROP: Thermodynamic Properties of Refrigerants and Refrigerant Mixtures).

Results and Future Plans: We have developed an upgrade to the NIST4 (SUPERTRAPP) database that allows computation of the thermophysical properties of undefined petroleum fractions (only API gravity and an average boiling point are required). The new version is currently undergoing review by Standard Reference Data, and we anticipate the public release of this new version in the very near future.

We began our refrigerant/lubricant work with some preliminary bubble-point measurements on the R134a/lubricant system. We applied the ESD equation of state and made comparisons with our own data and with literature data. The results of the work so far are promising, but it is only a first step in the analysis of refrigerant/lubricant systems. As more data become available for the lubricant, we will further develop our models for the lubricant. Future work on the mixture model will focus on developing mixing and combining rules. The eventual goal is to incorporate lubricants into the REFPROP database.

Publications:

Huber, M.L., Holcomb, C.D., Outcalt, S.L., and Elliot, J.R., "Vapor-Liquid Equilibria for a R134a/Lubricant Mixture: Measurements and Equation-of-State Modeling," ASHRAE Trans. (in press).