An
analytical apparatus with improved recirculating and sampling mechanisms
was presented to investigate phase behavior of binary and multicomponent
systems, which effectively avoided undesirable pressure gradients
across the cell and serious disturbance of equilibrium during sampling
procedure. Two one-channel magnetic recirculation pumps were designed
and built to provide better mass transfer between the coexisting phases.
Two six-port switching valves were used for sampling, quantitative
fluid injection, and online compositional analysis by gas chromatography.
The apparatus was validated by means of isothermal vapor–liquid
equilibrium data for CO2 + hexane, and the results were
found to be in good agreement with the literature data. High pressure
phase equilibria data of CO2 + n-alkanes
and CO2 + crude oil + n-alkanes were measured
from 353.15 to 373.15 K. Three thermodynamic models of RR, PRSV, and
YQE, based on Peng–Robinson and a modified quartic equation
of state coupled with various mixing rules, were suggested to represent
phase equilibrium. Calculations were compared with experimental values
for evaluating the predictive ability of these proposed equations.
Analysis of the results confirmed that the YQE model was superior
to all the other equations examined and worked well in describing
phase behavior of complex real fluids.