1 research outputs found
Predicting Binary-Interaction Parameters of Cubic Equations of State for Petroleum Fluids Containing Pseudo-components
Cubic
equations of state (EoS) are widely used for the prediction
of thermodynamic properties of petroleum fluids containing both well-defined
and <i>pseudo</i>-components. Such EoS require as input
parameters the critical temperature (<i>T</i><sub>c</sub>), the critical pressure (<i>P</i><sub>c</sub>), and the
acentric factor (ω) of each compound. For well-defined components,
such properties are known from experiments and easily obtained. For
pseudo-components they are routinely estimated using one of the numerous
characterization methods (CM) available in the open literature. A
CM is nothing more than a set of correlations which makes it possible
to estimate <i>T</i><sub>c</sub>, <i>P</i><sub>c</sub>, and ω of a pseudo-component (PC) from the knowledge
of its normal boiling point (NBP), molecular weight (MW), or specific
gravity (SG). Regarding the binary-interaction parameters (BIP) <i>k</i><sub><i>ij</i></sub> (where <i>i</i> and/or <i>j</i> are/is a pseudo-component(s)) which appear
in classical mixing rules, they are either set to zero or estimated
by a specific correlation. Most of the proposed correlations are however
purely empirical and usually only make possible the estimation of
the <i>k</i><sub><i>ij</i></sub> between light
components (H<sub>2</sub>S, CO<sub>2</sub>, N<sub>2</sub>, C<sub>1</sub>, C<sub>2</sub>, and C<sub>3</sub>) and a pseudo-component. The full <i>k</i><sub><i>ij</i></sub> matrix is thus beyond reach
and the BIP are usually temperature-independent. In this work, the
PPR78 model is used to predict BIP suitable for the Peng–Robinson
EoS whereas the PR2SRK model is used to predict BIP suitable for any
other cubic EoS. Since these models can be seen as group-contribution
methods (GCM) to estimate the <i>k</i><sub><i>ij</i></sub>, one needs to access the chemical structure of each PC. The
chemical structure of PC is however too complex to be precisely determined.
For this reason, it was assumed that each PC was made of only three
groups: C<sub>PAR</sub>, C<sub>NAP</sub>, and C<sub>ARO</sub> in order
to take into account their paraffinic, naphthenic, and aromatic characters,
respectively. The occurrences (<b>N</b>) of the three aforementioned
groups are determined from the knowledge of <i>T</i><sub>c,CM</sub>, <i>P</i><sub>c,CM</sub>, and ω<sub>CM</sub> (issuing from a CM). To reach this goal, GC methods aimed at estimating <i>T</i><sub>c</sub>, <i>P</i><sub>c</sub>, and ω
of hydrocarbons were developed. Such methods have the ability to consider
only three elementary groups: C<sub>PAR</sub>, C<sub>NAP</sub>, and
C<sub>ARO</sub>. In the end, the three known properties (<i>T</i><sub>c,CM</sub>, <i>P</i><sub>c,CM</sub>, and ω<sub>CM</sub>) can be expressed as functions of <i>N</i><sub>PAR</sub>, <i>N</i><sub>NAP</sub>, and <i>N</i><sub>ARO</sub> (the occurrences of the groups) and we thus only need
to solve a system of three equations with three unknowns. To check
its validity, the present approach is applied to the prediction of
the phase behavior of real petroleum fluids containing pseudo-components.
The test results show the pertinence of the proposed method to predict
the <i>k</i><sub><i>ij</i></sub> when <i>i</i> and/or <i>j</i> is a pseudo-component