Detection of bubble and dew point using optical thin-film interference

Bubble and dew point data are essential for many practical applications, and particularly the safe pipeline transport of post-capture CO2 which contain impurities. These mixtures show highly composition-specific phase properties, necessitating much more experimental data and motivating more rapid and inexpensive measurement methods. Here we demonstrate a responsive small-scale pressure-volume-temperature (PVT) cell system enabled by thin-film interference, and its application to an industrially-relevant post-capture CO2 mixture stream. The small (5 mL) volume is one-to-two orders of magnitude faster to equilibrate than conventional PVT cells with viewing windows. Inside the cell, top and bottom optical fiber sensors detect bubble and dew points, respectively. At vapor-liquid transition points, the reflection spectrum from the optical fiber tips report clear interference patterns caused by a thin film on the sensor. In addition to sharply delineating the phase change condition, the sensor also reports the real time thickness of the film (accuracy on the order of 1 μm). The method is validated with the well characterized pure CO2 test case (average error of 2.8 bar as compared to NIST data), and applied to an industrially-relevant CO2 stream, characteristic of post-capture oxyfuel combustion - an important source for downstream CO2 utilization and storage.