For the prediction of the condensation behavior of natural gas,
one has to select an equation of
state (EoS) which will be accurate in the temperature and pressure
range of interest (10 <
P/bar < 70 bar and 250 < T/K < 310).
Another requirement of the selected EoS is that it
easily
can be adapted to a characterization procedure for the heavy-end
fraction. For that purpose,
two equations were tested: the Peng−Robinson (PR), which is one of
the most applied cubic
EoS, and the simplified-perturbed-hard-chain theory (SPHCT) equation,
which is one of the
simplest EoS based on sound statistical mechanical principles. In
the underlying study, their
predictive capabilities for the prediction of saturated vapor pressures
of pure compounds and
vapor/liquid equilibrium pressures for binary mixtures are compared.
Only components present
in natural gas are considered. In addition, new pure-component
parameters for the SPHCT
EoS for n-alkanes are evaluated. Also a method to find
the characteristic energy for non-n-alkane molecules is proposed in this study. This study revealed
that the PR EoS predicts more
accurately the liquid phase composition, whereas the SPHCT EoS is
superior for the gas phase
prediction, especially for asymmetric binary mixtures. It was
concluded that, with respect to
the purpose of this study, both EoS, when used with optimum binary
interaction parameters,
have an equivalent descriptive accuracy. Therefore, the simpler PR
EoS was preferred to describe
natural gas mixtures.
The occurrence of liquid dropout in natural gas pipelines may cause operational problems during storage, transport, and processing. Therefore, the availability of a model that accurately predicts the amount of liquid formed is of great importance for the natural gas industry. The objective of this study is to develop a thermodynamic model for the accurate prediction of the amount of liquid formed in natural gas pipelines at transportation conditions. As input, the model requires an accurate gas analysis. A modified Peng-Robinson equation of state was selected for the phase equilibrium calculations. Interaction parameters were optimized from experimental data at conditions of practical interest, i.e., at pressures 10 < p < 70 bar and at temperatures 250 < T< 290 K. For a number of "keysystems,'" the interaction parameters were calculated from new accurate solubility data of heavy hydrocarbons in some of the main constituents of natural gas like methane and nitrogen. Also, an extensive experimental program was carried out to study the influence of minute amounts of nitrogen, ethane and carbon dioxide in methane on the solubility behavior of decane in these gas mixtures. From a sensitivity analysis, it could be concluded that the liquid dropout is influenced mainly by the concentration and characterization of C7-C~3 fractions. In this work, two characterization procedures to represent these fractions are compared. For two types of lean natural gas, the model predictions are compared with field measurement data, recently supplied by the Dutch natural gas industry.
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