Chemical Engineering Science, Vol.71, 552-563, 2012
Effect of fluidizing particle size on electrostatic charge generation and reactor wall fouling in gas-solid fluidized beds
Electrostatic charge generation from particle-particle and particle-wall contacts in gas-solid fluidized beds is virtually unavoidable and generally undesired. Charged particles are known to cause problems including particle agglomeration and particle-reactor wall adhesion. The polymerization industry has been plagued by reactor wall fouling for many years, but the actual mechanism behind this problem is not well understood. The wide particle size distribution present in polyolefin fluidized bed reactors is believed to play an important role. This work studied the effect of particle size of a polyethylene resin received directly from industrial reactors on electrostatic charge generation and reactor wall fouling. The resin was sieved into five different narrowed particle size fractions and fluidized at two different gas velocities; 1.5 and 4 times the minimum fluidization velocity (u(mf)) representing the bubbling and the slugging flow regimes. The fluidization was performed in a 0.1 m carbon steel fluidization column with the charge measurement technique described by Sowinski et al. (2010). In the bubbling flow regime significant particle wall adhesion was found for particles sizes up to 600 mu m, after which very little reactor wall fouling was observed. In the slugging flow regime, there was significant particle wall adhesion for all particle size fractions tested with the exception of the 600-710 mu m range, which at times resulted in significant particle-wall adhesion and in other times none was obtained. Overall, it was found that the smaller particles had a higher charge and resulted in more reactor wall fouling. With the non-sieved resin, the particles that adhered to the column wall were approximately of the same size as the smallest size fraction tested (300-425 mu m). This study found that the measurement of the net charge-to-mass ratio inside a fluidized bed is not an indication of the amount of reactor wall fouling. (C) 2011 Elsevier Ltd. All rights reserved.
Keywords:Gas-solid fluidization;Electrostatics;Particle charging;Fouling;Polymer processing;Powder technology