Fisicoquimica

PETROLEUM SOCIETY
CANADIAN INSTITUTE OF MINING, METALLURGY & PETROLEUM

PAPER 2005-152

Application of Nasrifar-Moshfeghian EoS for Prediction of Enthalpy Departures of Natural Gas and LNG Systems
H. DASHTI, M. MOSHFEGHIAN
Kuwait Institute for Scientific Research
This paper is to be presented at the Petroleum Society’s 6th Canadian International Petroleum Conference (56th Annual TechnicalMeeting), Calgary, Alberta, Canada, June 7 – 9, 2005. Discussion of this paper is invited and may be presented at the meeting if filed in writing with the technical program chairman prior to the conclusion of the meeting. This paper and any discussion filed will be considered for publication in Petroleum Society journals. Publication rights are reserved. This is a pre-print and subject tocorrection.

Abstract
In this work, a data bank consisted of the experimental enthalpy departure data from ideal gas state for several pure and binary, and ternary systems was used to evaluate the accuracy of the modified Nasrifar-Moshfeghian (MNM) EoS. The impact of binary interaction parameters which were optimized based on vapor-liquid-equilibrium (VLE) data on enthalpy departure was also studied.In each case, the accuracy of MNM EoS has been compared with the Peng-Robinson and Soave-Redlich-Kowng EoSs and for each cases, the average absolute deviations are reported. The overall analysis of the results indicates that the accuracy of MNM EoS in predicting enthalpy departures is in the same order of magnitude as for the SRK and PR EoSs for light hydrocarbon systems such as natural gas andLNG.

Introduction
Many equations of state (EoSs) are developed based on the pressure-volume-temperature (PVT) behavior and properties for pure compounds. Once the EoS was developed, expressions for enthalpy and entropy departures from ideal gas state, i.e., isothermal effect of pressure on enthalpy and entropy, can be used for calculation of these two properties. Accurate prediction of enthalpyand entropy is very important for design and simulation of unit operations such as compressor, turbine, expansion valve, distillation column, refrigeration cycles, and etc. When these EoSs are used to predict thermodynamic properties and vapor-liquid-equilibrium (VLE) behavior of binary and multicomponent systems, their accuracy deteriorates due to the dissimilarities in size, shape andinteraction forces among the different molecules. In order to overcome this

1

deficiency, a common practice is to introduce binary interaction parameters, kij., which are normally determined from the binary experimental VLE data Therefore, the industrial application of any cubic EoS with better accuracy requires the kij values. This work was undertaken to evaluate the accuracy of theModified-Nasrifar-Moshfeghian (MNM) EoS [1] for prediction of enthalpy departure from ideal gas state of pure compounds, binary, ternary and multicomponent systems. The effect of kij values on prediction of enthalpy departure prediction was investigated. The optimized values of binary interaction parameters reported by Moshfeghian and Nasrifar [2] were used for this evaluation. In addition, the accuracy of theMNM EoS is compared with the two widely used SRK [3] and the PR [4] EoSs. The same data bank of experimental enthalpy data was used for the MNM, SRK and the PR EoSs. In addition, for each case, the average absolute deviations are calculated and reported.

For numerical calculation, Eq (1) is transformed into a dimensionless form:

Z3 −(1− B)Z2 + A− 4B2 − 2B Z − AB− 2B2 − 2B3 = 0
with

(

)(

)

(12)

A=

aP

( RT )

2

,

B=

bP RT

and

Z=

Pv RT

(13)

An important application of a cubic EoS is the calculation of phase equilibria for multicomponent systems. In order to extend the application of an EoS to mixtures, mixing rules are used. For mixtures consisting of nonpolar or slightly polar compounds, the one-fluid van der Waals mixing rules with one...