In Situ Measurement of Steady-State Hydrogen Concentrations during a Hydrogenation Reaction in a Gas-Inducing Stirred Slurry Reactor

Abstract

A new instrument, the Fugatron, is presented that was developed to make in situ measurements of dissolved hydrogen concentrations in laboratory reactors. The principle of measurement is based on a probe with a tip that consists of a small porous metal substrate coated with a dense layer of fluoropolymer. The probe is exposed to the liquid phase and is internally purged by a constant flow of carrier gas. Driven by the hydrogen concentration gradient between the external and internal side of the composite layer, the hydrogen molecules permeate into the carrier gas stream. The concentration of the permeate in the carrier gas stream is measured by a gas analyzer. The analyzer signal is proportional to the partial pressure of dissolved gas. The steady-state concentration of dissolved hydrogen in the liquid is calculated from the analyzer signal by using solubility data. Calibration methods are described for immersion probes used for in situ measurements in reactors which are operated in batch or continuous mode as well as for flow-through probes used for liquid stream analysis. The sensitivity of the instrument and the reliability of the measurements were tested in different solutions as a function of temperature and pressure. The hydrogenation of an alkyne with a suspended palladium catalyst was chosen as a model reaction for in situ measurements. The hydrogenation experiments were carried out in a 0.5-L stirred-tank reactor with a turbine stirrer in a semibatch mode. Initial hydrogen uptake rates and dissolved hydrogen concentrations during this model reaction were measured as a function of the amount of catalyst in the liquid. The solubility of hydrogen in the pure liquid was determined by a gas absorption measurement at reaction conditions. From these measurements, the volumetric liquid-side mass transfer coefficient kLa at the gas−liquid interface was calculated as a function of catalyst loading. In addition, the kLa value was also determined in the absence of solid particles by the dynamic pressure drop method. Discrepancies that may result from calculating steady-state dissolved hydrogen concentrations using kLa values obtained from measurements conducted either in pure liquids or in liquids that contain suspended catalyst particles are discussed and illustrated by these measurements. The new method allows direct measurement of the concentrations of dissolved gases during a reaction, which can be a decisive factor for the rate as well as for the selectivity of a process. Estimations of the gas concentrations via reaction rate data and cumbersome determinations of kLa values are, therefore, no longer necessary.