Investigation of electrostatic charge distribution within the reactor wall fouling and bulk regions of a gas–solid fluidized bed

The distribution of charge within the wall fouling region and bulk of a fluidized bed reactor was investigated. Experiments were conducted in a 0.1 m in diameter carbon steel fluidization column under atmospheric conditions. Polyethylene particles were fluidized with extra dry air at 1.5 the minimum fluidization velocity (bubbling flow regime) for 1 h. Using an online Faraday cup measurement technique, the net charge-to-mass ratio (q/m), as well as the size distribution of all particles adhered to the column wall and those in the bulk of the bed was determined. The wall particles were found to be predominantly negatively charged while those which did not adhere to the wall were predominantly positively charged. The charge distribution within each region was then investigated by a custom made charged particle separator that separated the particles according to their charge magnitude and polarity. It was determined that although the net charge of the wall layer particles was negative, a significant amount of positively charged particles existed within each sample and therefore the entire wall particle layer. This suggests that the wall layer was formed through layering between positively and negatively charged particles. Particles in the bulk of the bed also consisted of bipolarly charged particles.     

Synthesis and complexing properties of 1,5:3,7-doubly bridged calix[8]arenes with mixed spanning elements

Two different bridges were introduced at the 1,5:3,7-positions of p-tert-butylcalix[8]arene 1 using a two-step alkylation procedure. A probable cation template effect in the introduction of the second bridge was evidenced. The obtained bis-bridged derivatives 3 possess encapsulating properties toward alkali cations modulated by the length and nature of the bridges.     

Comparison of influence of fluidization time on electrostatic charge build-up in the bubbling vs. slugging flow regimes in gas–solid fluidized beds

Electrostatic charge generation poses significant problems in some commercial gas–solid fluidized bed reactors such as those in gas-phase polyolefin production. Understanding the contributing factors to charge generation is important in determining the charge generation mechanisms, leading to the development of methods to reduce or prevent this phenomenon. This work focused on determining the effect of fluidization time on particle charging and the amount of particle adhesion on the fluidization column wall in both the bubbling and slugging flow regimes. The charging effect was investigated for particles in three regions of the fluidized bed: elutriated fines, bulk particles inside the bed, and particles adhered to the column wall. The particles size distribution, mass and charge were measured for all three regions. Fluidization was carried out with polyethylene resins from an industrial reactor; times of 15, 30, 60, 120, 180, and 360 min were evaluated. Increased fluidization time decreased the amount of particles mass collected in the bulk region and increased those adhered to the column wall during the velocities tested in the bubbling flow regime. Whereas the quantity of particles in each region was not affected by fluidization time for the velocities examined in the slugging flow regime. Bipolar charging was observed with relatively smaller particles becoming predominately positively charged and larger particles becoming predominately negatively charged. Each region of the bed affected the magnitude of net q/m, with elutriated fines having the largest magnitude, followed by those adhered to the column wall, and finally those in the bulk of the bed. Charge saturation was attained for fluidization times greater than 60 min for particles in the bulk and along the column wall for all gas velocities. However, extended fluidization times were required with the entrained fines in bubbling flow; whereas charge saturation of fines in slugging flow occurred shortly after the onset of fluidization. Mean particle diameter for each measurement region was not impacted by the fluidization time for any of the gas velocities tested. The bed hydrodynamics was found to definitely have an impact on the particle–wall fouling where the particle layer continued to develop on the inner column wall as fluidization time increased for those velocities in the bubbling regime while comparatively less impact on particle layer growth was observed in the slugging flow regime. In addition, the bubbling flow regime resulted in particle layers formed on the column wall to be longer and thinner whereas those formed in the slugging flow regime were shorter and thicker.     

New technique for electrostatic charge measurement in gas–solid fluidized beds

Electrostatic charge generation within gas–solid fluidized bed reactors has been a concern to industry for many years. Over the years, numerous methods for measuring this phenomenon within a fluidization column have been proposed. This paper focuses on the design of a new method that minimizes effects such as extra charging due to particle handling and bed hydrodynamic disruption due to the location of the measurement device. In addition, the new method provides the bulk charge of the bed particles rather than a local measurement. The device is flexible and can be adapted to a range of fluidization columns. The new method developed consisted of a Faraday cup placed within the windbox of a fluidization column. The distributor plate was designed in such a way that it can be automatically opened to drop the charged fluidizing particles into the Faraday cup below. The new measurement technique was validated by conducting fluidization experiments in a system consisting of a 0.10 m in diameter carbon steel column with glass beads as the fluidizing particles. The technique was proven to be suitable for measuring the total net electrostatic charges in gas–solid fluidized beds.