Development of an on-line entrainment measurement device for a bubbling fluidized bed

Abstract

A selective combination of the principles of a thermal mass flow meter and constant temperature anemometry was used to develop a solid mass flow meter that improves significantly on the flow meter developed by De Vos et al (2010). The flow meter has a measurement plate that is kept at a constant temperature. Due to conductive heat transfer between the entrained solids and the measurement plate, additional power is needed to maintain the plate at this setpoint temperature value. This additional power was correlated against the average solids flow rate. The calibration curve shows a linear relationship between the power measurement and the entrainment flux for entrainment fluxes between 3.4 x 10-4 kg/(m2.s) and 7.5 x 10-3 kg/(m2.s). Deviation from a linear response at lower entrainment fluxes may be caused by a longer residence time of fine particles on the measurement plate due to lower shear forces. At higher entrainment fluxes the power measurements were unreliable due to poor temperature control. The turndown ratio of the linear section of the calibration curve is approximately 3 times that of the linear part of the calibration curve of the flow meter developed by De Vos et al (2010). Even further improvement is possible with better temperature control. In a case study to test the applicability of the flow meter to measure changes in entrainment rate associated with hydrodynamic properties other than a change in gas superficial velocity, small amounts of ethanol were dosed to the inlet air. The continuously measured entrainment rate increased at lower ethanol dosing rates but decreased as the dosing rate of ethanol was increased. The increase in entrainment rate may be explained by a reduction in static electricity in the bed, while the decrease at higher dosing rates may be as a result of increased powder cohesivity.