Liquid Density of Pure Alkanes and Halogenated Alkanes in a Corresponding States Format

The development of a three parameters Corresponding States (CS) model is here proposed aiming at the prediction of the saturated and compressed liquid density of pure fluids pertaining to the two conformal families of alkanes (A) and hydrofluorocarbons (HFC) which are widely used as refrigerants. Two fluids of the same family are chosen for both their acentric factor value and for the saturated and compressed liquid density dedicated equations availability and, on the basis of the Teja et al. three parameters CS model, the saturated and compressed liquid density of a fluid of interest is obtained in reduced variables. Assuming experimental data of saturated liquid density for several components of each of the two families of fluids an improvement is introduced substituting the acentric factor with a new constant scaling factor. As a final result both the saturated and the compressed liquid models are predictive methods. The reached prediction accuracy of the proposed method is similar to that of the dedicated equations for all the fluids of a family. The result is particularly satisfactory for the application requirements in refrigeration. INTRODUCTION In the study and design of components and systems for refrigeration and heat pumping the prediction of thermodynamic properties of pure refrigerants is required with an high accuracy level over a wide range of pressure and temperature. A very accurate volumetric equation for a pure fluid can be generally obtained from a wide number of precise experimental data regressing over them the parameters of the equation. Each fluid can then be represented only after a long and expensive experimental work. More often the study of the thermodynamic properties of pure fluids are limited to the components of a same family of compounds, so that the development of accurate thermodynamic models, valid only for the fluids of the same family, looks as a slightly simplifYing task. The utilization of modelling techniques in a corresponding-states format can yield interesting advantages and get a high level of accuracy when utilized inside the same family of fluids, producing new models more accurate than those currently available and with the minimum amount of experimental inputs. DENSITY MODELS Saturated Liquid Model Pitzer and co-workers [1,2] initially proposed to express the compressibility factor Z of a fluid of interest as a first-order perturbation, in form of a power series truncated at the first term, of the compressibility factor z(O) of a "simple" fluid in the acentric factor of the fluid of interest: [ oz()) Z=z(o) +m om (I) Since it is not possible to represent [ ~:)) directly through an EoS, Lee and Kesler [3] proposed to substitute the derivative in eq. (1) With the finite difference between the compressibility factors z(O) of a "simple" fluid and z(r) of a reference fluid~ Selecting a "simple" fluid having an m value very close to zero it comes: