(21a) Particles Wall Coating Due to Electrostatic Charge Generation in Gas-Solid Fluidized Beds in Turbulent Versus Pre-Turbulent Flow Regimes

Continuous contacts amongst the particles and the particles and the fluidization vessel wall give rise to electrostatic charge generation in gas-solid fluidization processes. This might lead to significant operational challenges including fluidizing particles adhesion to reactor wall, particle agglomeration, and electrostatic discharges. One industry that has significantly suffered from negative effects of fluidized bed electrification is the petrochemical industry, where fluidized bed reactors are used for gas-phase ethylene polymerization to produce polyethylene. In such reactors, electrostatic charge generation results in catalyst and polyethylene particles to adhere to the reactor wall, causing a problem known as “sheeting”. The formation of particle sheets can require shutdown periods for clean-up which results in significant economic losses. To better understand the particles reactor wall coating due to electrostatics in such process, it is beneficial to conduct studies in relation to polyethylene reactor electrifications under industrially-relevant operating conditions of elevated pressures and turbulent flow regime. In this work, the effect of gas velocity on the degree of particles wall coating was studied with a specific focus on the transition to turbulent flow regime. Experiments were conducted in a pilot-scale fluidized bed operating under industrially-relevant conditions of a pressure of 2600 kPa (abs) and fluidizing gas velocities of 1.5, 3, and 5 times of U_mf(pre-turbulent regimes) and 7.5 times of U_mf (turbulent regime). Polyethylene particles directly received from a commercial reactor were fluidized in this work. Increasing the gas velocity and transitioning to the turbulent flow regime improved particle-wall contacts; and thus, augmented the extent of wall fouling, which indicates the increase in bed electrostatic charge generation. The amount of particles wall coating was approximately five times larger in turbulent flow regime (7.5 U_mf) in comparison to that for the lowest gas velocity examined in bubbling flow regime (1.5 U_mf). The particles coating on the column wall consisted of a thick bottom layer which extended to the expanded bed height, and a thin top layer which extended to the top of the column near the outlet. The particles net specific charge in the top and bottom layers did not vary with the increase in gas velocity. However, the net charge of these particles increased. The fine particles entrained from the bed had a net negative charge resulting in a net positive charge to be left behind in the bed contributing to the increase in the magnitude of wall fouling at higher gas velocities, especially in turbulent flow regime.