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Not just a phase

Many industrial processes involve exposing compounds to high temperatures and pressures that induce phase changes, in which the compounds shift from a liquid state to a gaseous state and back again. When this involves many different compounds, each with its own boiling point, the situation can quickly become quite complex. Furthermore, the combination of high temperatures and pressures can lead to the formation of supercritical fluids, which possess the properties of both liquids and gases, adding to the complexity.

Because these phase changes can affect the efficiency of an industrial process, researchers are interested in knowing what kind of phase changes are taking place. One way to do this is with what is known as a high pressure variable volume view cell, which is a miniature reactor with a couple of windows in the side where researchers can expose small quantities of different compounds to high temperatures and pressures.

The windows allow the researchers to see inside the cell, which is the main way they have of studying the phase changes taking place. But it’s not very objective and also means that some of the intricacies of the phase changes are easily missed. These intricacies include the bubble point, which is the temperature at which the first bubble forms in a heated liquid, and the dew point, which is the temperature at which a substance evaporates and condenses at the same rate.

So a team of Brazilian researchers led by Cláudio Dariva at the Núcleo de Estudos em Sistemas Coloidais decided to try studying the phase changes with NIR spectroscopy as well. To do this, they simply attached an NIR probe to the inside of the cell. After testing that this probe could withstand high temperatures and pressures, they then used it to monitor a mixture of carbon dioxide and a distilled petroleum fraction exposed to temperatures of 20–60°C and pressures of up to 20 MPa.

As reported in The Journal of Supercritical Fluids, they found that the phase changes could easily be identified in the NIR spectra without any kind of statistical treatment, allowing both the bubble point and dew point to be determined for the compounds. Following this success, Dariva and his team are now using NIR spectroscopy to monitor the phase changes in various other mixtures of compounds.

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