Proton Transfer Reaction-Mass Spectrometry (PTR-MS) as a tool for the determination of mass transfer coefficients

• Approach for measuring mass transfer coefficients with PTR-MS from the gas phase.
• Measurements online with direct injection into the mass spectrometer.
• Measurements on isoprene, farnesene and 6 monoterpenes.
• We report overall volumetric, liquid-side and gas-side mass transfer coefficients.
• We report plausibility considerations based on known correlations.

We present an alternative technique for the measurement of mass transfer coefficients for bubble columns from the gas phase with the mass spectrometric technique of Proton Transfer Reaction-Mass Spectrometry (PTR-MS). Instead of liquid phase measurements, we apply gas phase measurements of the effluent gas phase with the PTR-mass spectrometer. The method is based on the inert gas stripping approach for the determination of Henry׳s law constant. Therein gas is bubbled through a liquid-water-compound mix, and the declining concentration of the compound in the gas phase is measured over time. For the determination of the mass transfer coefficients, the concentration over time profile is used. Experiments for determination of mass transfer coefficients were carried out in small lab scale under controlled isothermal (thermostated) conditions and with a glass frit sparger. In principle this setup could be performed offline with any detector that has a linear signal dependence over a wide concentration range. PTR-MS has the advantages of low limit of detection (ppt(v)–ppq(v)), wide linear range, and split second detection. We review the formulas for the determination of mass transfer coefficients. The relation with the equilibrium Henry׳s law constant is pointed out (extrapolation to origin considering gas holdup). We determine the overall volumetric mass transfer coefficient KLa, and moreover apply a method for the additional estimation of the liquid-side and gas-side mass transfer coefficients from KLa. Correlations of the superficial gas velocity to the gas holdup ε, the overall volumetric mass transfer coefficient KLa, the liquid side and gas phase volumetric side mass transfer coefficients kLa and kGa, as well as the ration kLa /ε are reported. The results demonstrate the suitability of PTR-MS to the determination of mass transfer coefficient for bubble columns.

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