115 research outputs found
文献目録
Confined
fluids such as oil and gas mixtures inside tight reservoirs
are systems that can experience high capillary pressure difference
between the liquid and gas phases. This capillary pressure difference
has an effect on the phase equilibrium and in some cases is considerably
high. We presented an algorithm which can reliably compute the whole
phase envelope for multicomponent mixtures when there is a capillary
pressure difference. It uses an equation of state for the phase equilibrium
and the Young–Laplace equation for the capillary pressure model.
The algorithm proves to be robust and efficient for test mixtures
with wide ranges of compositions at different capillary radii and
vapor fractions. The calculation results show that the phase envelope
changes everywhere except at the critical point. The bubble point
and the lower branch of the dew point show a decrease in the saturation
pressure, whereas the upper branch of the dew point shows an increase.
The cricondentherm is shifted to a higher temperature. We also presented
a mathematical analysis of the phase envelope shift due to capillary
pressure based on linear approximations. The resulting linear approximation
equations can predict the correct direction of the phase envelope
shift. Combined with the multicomponent Clapeyron equation, the equations
reveal why the shift changes direction for the saturation pressure
at the cricondentherm and for the saturation temperature at the cricondenbar.
The equations can be used to estimate the magnitude of shift, and
the approximation is close for the change in the bubble point pressure
Calculation of Multiphase Chemical Equilibrium by the Modified RAND Method
A robust
and efficient algorithm for simultaneous chemical and phase equilibrium
calculations is proposed. It combines two individual nonstoichiometric
solving procedures: a nested-loop method with successive substitution
for the first steps and final convergence with the second-order modified
RAND method. The modified RAND extends the classical RAND method from
single-phase chemical reaction equilibrium of ideal systems to multiphase
chemical equilibrium of nonideal systems. All components in all phases
are treated in the same manner and the system Gibbs energy can be
used to monitor convergence. This is the first time that modified
RAND was applied to multiphase chemical equilibrium systems. The combined
algorithm was tested using nine examples covering vapor–liquid
(VLE) and vapor–liquid–liquid equilibria (VLLE) of ideal
and nonideal reaction systems. Successive substitution provided good
initial estimates for the accelerated computation with modified RAND,
to ultimately converge to the equilibrium solution without failure
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