Computer estimation of thermodynamic properties of real gases

Abstract

A large number of papers dealing with the computer estimation of thermodynamic properties of real fluids have appeared recently (1-5). Generally; the predicting methods which yield accurate results over a wide range of conditions are quite complex to use and need lengthy calculations on large core memory computers. They are often limited to the prediction of the departure functions. Our purpose was to provide the students and the research staff of our school with a simple and efficient means to evaluate some properties of real gases for computer calculations in applied thermodynamics and chemical engineering. We set up on a small computer, a package of FUNCTION subroutines written in Fortran IV, which were able to calculate separately specific volumes, heat capacities, enthalpy and entropy of a large number of compounds fur a given pressure and temperature. The characteristic ptoperties of these compounds, used in these functions, are stored in a permanent file on disc memory. Principle of ComputationThe method we used involved, first the estimation of the value of the thermodynamic function for the compound in the ideal gas state, and second, the associated isothermal function departure which was derived from a generalized equation of state.Several types of correlations have been used (6-9,24) to obtain analytical expressions for the ideal gas heat capacity ( C p o ) , (9) used a set of thermodynamically consistent equations ((1)-(3)), based on a polynomial development of the fifth degree for the enthalpy H " = A + B T + C T 2 + D T 3 + E T 4 + F T 5 (1) CPD = B + 2CT + 3 D P + 4ET3 + 5 F P (7) So = BlnT + 2CT + 312DP + 413 E F + 514 FT4 + G (3) (Ho = 0 kJ kg-' a t O°K, S o = 0 kJ kg-lK-' at ODK and 1 atm) These equations produce values which fit well with the values calculated by statistical mechanical procedures for 146 gases. Among the numerous generalized equations of state (10-17) which we have tested, we selected the three parameters model of Lee and Edmister (16): This equation (4) was initially intended for the calculation of fugacity coefficients and enthalpy departure of gases. This model gives a good representation of PVT data in a wide pressure and temperature range. I t is very simple to use and can readily he extended to gas mixtures. enthalpy (Ho), and entropy (So). Passut and Danner (8) and Huang and Dauhertwhere T, = critical temperature (OK), PC = critical 'pressure (atm), w = acentric factor, and R = gas constant (82.05606 em3 atm mole-' K-'1. To obtain the specific volume, the compressibility factor (Z = PVIRT) is computed fur each value of P and T by solving the cubic eqn. (14) derived from eqn. (4). In all cases the largest real root was retained.Z3 -ZZ -Z[P2b2/R2T2 + PbIRTo P / R 2 P ] + (abe ) P2/R3T3 = 0 (14)

Developing and Using a Computer Simulation of Liquid–Vapor Transitions to Improve Students’ Assimilation of Concepts Related to the Behavior of Real Gases

“…To facilitate the transfer of knowledge, some authors have proposed performing theoretical calculations of the properties of real gases using software. , However, UCA-GAS has been specifically designed with a visual and intuitive environment that makes it easy to use, thus reducing the capacity for abstract thinking required by students that have to assimilate the concepts mentioned above. The software is intended for use with laboratory practice, comprising a series of activities of increasing difficulty.…”