Applications of Tribo-Electric Probes in Fluidized Beds

When fluidized bed particles carried by a bubble wake hit a metal probe, they strip electrons from its surface, inducing an electrical current that can be monitored. The main advantage of tribo-probes, when compared to optical fibers or capacitance probes, is that they can be used at high temperature and in turbulent fluidized beds, where high gas velocities are combined with high solids concentrations. By using multiple tribo-probes at various bed locations, one can gather local and global fluidization information simultaneously. This study shows how tribo-probes can be used to characterize the hydrodynamics of a fluidized bed.

This study was conducted at room temperature in a column with a rectangular cross-section (0.5 m x 0.1 m x 2.28 m) filled with silica sand (group B powder). Compressed air provided the fluidization gas. The gas distributor consisted of 20 tuyeres whose flow could be individually controlled: by opening 10 valves at different locations, bubbles could be concentrated to the western side, eastern side or center of the bed. Two baffle configurations modified the bed hydrodynamics: in the first configuration, a baffle with an open-ended triangular shape with an internal angle of 45 ° of the same width as the bed (0.18 m x 0.18 m x 0.1 m) was placed on one side of the bed, while in the second configuration two smaller baffles (0.125 m x0.125 m x 0.1 m) with the same geometry were attached on opposite sides of the bed. Gas and gas-liquid horizontal jets were formed with scaled-down versions of commercial nozzles. Experiments were conducted with a superficial gas velocity ranging from 0.1 to 1.0 m/s.

Each tribo-probe was a simple metal rod with a diameter of 6.35 mm that was connected to the electrical ground. The probe diameter was selected to avoid deformation at high gas velocities. A Teflon fitting isolated each probe from the grounded bed wall. An array of tribo-probes (5 rows × 9 lateral probe locations) inserted into the bed to half of the bed width (0.05 m) provided information over the whole bed. A new system was developed to measure the instantaneous current from each probe to the ground accurately and reliability, with a reasonable cost. Transient-voltage-suppression (TVS) diodes protect the current measurement system from damage caused by high-voltage discharges from the fluidized bed. Since conventional optical fiber or capacitance probes could not be used at high gas velocities, a radioactive non-invasive transmission method determined the local bed voidage to verify the lateral bubble gas distributions obtained from the tribo-probes. A radioactive particle was located outside the column, on one side of the bed and a radiation detector was placed just outside the opposite bed wall.

Tribo-probes were successfully applied to obtain the following information:

• The gas bubbles never spanned the distance between the two opposite bed walls. This was verified by using cross-correlation of signals from tribo-probes on opposite walls of the bed.
• The local bubble velocity and its evolution with bed height, gas distribution and gas velocity. This was obtained from cross-correlation of signals from probes located at different heights.
• The lateral distribution of the bubble gas, its evolution with bed height, superficial gas velocity, initial gas distribution, and the use of baffles. Results were validated with the radiation transmission method.

Three gas distributor configurations were studied. By opening 10 nozzles evenly distributed over the bottom of the fluidized bed, an even gas distribution is formed. When opening 10 nozzles close to the western side bed wall, in the eastern case, the bubbles concentrate on the eastern side bed wall.

A single baffle extending 18 cm from one wall (L=18 cm) greatly concentrates the bubbles to the baffle tip region, whichever gas distributor configuration is employed. However, when applying smaller symmetrical baffles (L=12.5 cm), the impact of the baffle on gas bubble profile dissipates with the increase of distance to baffle tip. The pure gas jet would have a minor impact on lower level bubble profiles.

Cross-correlation coefficient between two rows of tribo-probes provides the gas bubble velocity. The results show that the bubble velocity would increase with the increasing distance to the gas distributor, which applies to all 3 cases. The highest gas bubble velocity could reach around 3 m/s, which is validated with a standard correlation (Hilligardt & Werther et al., 1986).

• The penetration of horizontal gas and gas-liquid jets. The results obtained with the tribo-probes for the gas-liquid jets were validated with a separate method using a cold liquid and thermocouples. The results also agreed well with predictions from a standard correlation (Ariyapadi et al. 2005).