Isochoric pVT and Phase Equilibrium Measurements for Carbon Dioxide + Nitrogen

Abstract

We have designed and constructed an isochoric apparatus for fast and accurate pressure-volumetemperature ipVT) and phase equilibrium measurements. The apparatus allows isochoric (p, Tat constant V) measurements for noncorrosive fluids over a wide range of pressures (7-105 MPa) and temperatures (100-450 K). Densities can be derived from densities determined at one isotherm (p, V at constant T) with another apparatus, such as a pycnometer or a Burnett apparatus. Temperatures are accurate to ±10 mK and precise to ±1 mK on ITS-90. Pressure measurements are accurate to ±10 kPa and precise to ±2 kPa. This apparatus requires about 20 min for temperature equilibration for measurements 2 K apart and about 50 min for measurements 10 K apart. We have measured 23 phase boundary points for three mole fractions, x\ = 0.3991, 0.4459, and 0.5037, for CO2 (1) + N2 (2) at temperatures from 207 to 268 K and pressures from 10.8 to 21.5 MPa. The estimated accuracy is ±40 mK in temperature and ±17 kPa in pressure. The isochoric measurements used to obtain these phase boundary points extend from 205 to 300 K and from 8 to 80 MPa. We have determined the saturation density for 20 of the phase boundary points. The saturation densities and the densities along the isochores were derived from density measurements at 300 and 285 K measured with a pycnometer estimated to be accurate to ±0.1%. The behavior of the solid + fluid phase boundary is discussed.

Continuous production of biofuel from refined and used palm olein oil with supercritical methanol at a low molar ratio

“…This method has been used by several researchers for obtaining accurate densities and phase transition [22][23][24][25]. The isochoric method was first proposed for biodiesel production from vegetable oils with supercritical alcohols by Velez et al [26,27].…”

“…The main operating parameters that influence the transesterification under supercritical conditions are temperature, pressure, alcohol to oil molar ratio, and reaction time. As shown in Table 1, the supercritical reaction of a variety of refined vegetable oils has been investigated over a wide range of operating temperatures (280-350°C), pressures (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35), and reaction times (4-30 min), depending on the types of feedstock and reactor.…”

Method and uncertainties to determine phase boundaries from isochoric data

“…Our first example is an equimolar mixture of CO 2 + N 2 . Duarte-Garza et al [6] report the isochoric measurements for this mixture. The data consist of 19 isochores, but the last four isochores have only two points in the two-phase region.…”

“…Consequently, it is not possible to apply the procedure for these data because we cannot use any function other than a straight line in the two-phase regions, and the error can be high. Duarte-Garza et al [6] did not calculate those phase boundary points. Table 1 shows the calculated values for the phase boundary with the uncertainty at each point.…”

“…Duarte-Garza et al [6] determined the phase boundary by first fitting a linear or second-order temperature function to the single-phase isochoric data and then fit another polynomial to the two-phase isochoric data. The intersection of these two functions determined the phase boundary.…”