A Novel Method for Gas to Particulate Mass Transfer Measurements in Fluidized Beds

Mass transfer between the gas and the particulate phase is a key process in fluidized bed applications, in which the particulate phase can be to a greater or lesser extent active: while for example chemical looping combustion applies solely active particles, the particles in combustion and gasification are inert or less active. Despite the mass transfer between the gas and particulate phase being a decisive phenomenon for the modelling and design of fluidized bed reactors, the difficulty to accurately measure this phenomenon has resulted in little experimental data available in literature. Experimental studies providing such data have typically been carried out based on a mass balance closure by means of gas measurements, which implies uncertainties related to the gas measurements (with respect to for example accuracy of the gas analyzer and reproducibility, convolution and possible post-reaction of the gas sample).

This paper presents a novel method to measure the mass transfer between the gas and particulate phase in laboratory fluidized beds, which is based on the bed resting on a high-precision scale (±0.01 g). The scale can monitor the mass gain of the bed, initially consisting of dry silica gel particles, while fluidized by humified air. From the sampled signal of mass gained by the bed due to moisture adsorption by the silica gel particles, mass transfer coefficients can be calculated with different spatial resolution and compared for different operational parameters. This represents a novel, accurate and non-intrusive method to measure mass transfer in fluidized beds, which makes it possible to determine mass transfer from the gas phase to beds of active particles (e.g. oxygen carrier particles in a fuel reactor of a CLC unit) and to active particles (fuel particles) dispersed in an inert bed.

Application of the method proposed in this work shows that with an increase in fluidization velocity, the mass transfer between the gas and particulate increases, reaches a maximum and then decrease. This is due to the change in fluidization behavior with increased velocity where the bed enters different bubble regimes, which will be discussed in the paper. Further, the mass transfer is reduced with increased bed height.