Influence of Argon on Static Charge Mitigation in Pressurized Gas-Solid Fluidized Bed Reactors at Varying Temperatures and Concentrations

Abstract

Electrostatic charge generation is inherent to gas-solid systems, including fluidized beds, due to repeated particle contacts with surrounding surfaces. In industrial solids processing operations, such as gas-phase polyethylene production, electrostatic charging is generated from particle interactions with each other and with the reactor wall, promoting particle agglomeration and wall adhesion of highly charged particles, leading to operational issues. These occurrences pose safety risks and necessitate reactor shutdowns and maintenance with substantial operational costs. This thesis investigates the impact of adding argon gas on mitigating electrostatic charge buildup in polyethylene gas-solid fluidized bed reactors under industrially relevant temperatures (up to 70°C) and pressures (up to 2600 kPa). Argon, which has a much lower dielectric strength than gases like nitrogen, was expected to help lower static charge buildup in the fluidized bed because of its ability to break down, leading to the dissipation of surface charge on polyethylene particles. A multi-scale experimental approach was employed, including gas dielectric strength measurement, bench-scale shake tests, and pilot-scale pressurized gas-solid fluidization experiments, to investigate the influence of argon gas breakdown behavior on the degree of static buildup and the resulting fluidized bed wall fouling for a commercially produced polyethylene resin at various concentrations and temperatures. Bench-scale single-particle and multiple-particle shake tests were employed to simulate charging arising from particle-wall and particle-particle interactions within the fluidized bed, respectively. Tests using the polyethylene resin under nitrogen and argon at 23 and 65°C (± 2.5°C) showed that argon consistently resulted in lower saturation charge levels than nitrogen, while increasing temperature significantly reduced charge accumulation only for nitrogen, indicating gas-dependent charge dissipation behavior with temperature. A breakdown voltage measurement device was built in-house, and the dielectric strength of pure argon and nitrogen, and their mixtures (20, 50, 75, and 90 vol.%), were measured from atmospheric pressure up to 2600 kPa and temperatures of 25, 70, and 110°C (± 2°C). For all gases, dielectric strength increased with increasing pressure up to 2600 kPa, with argon consistently exhibiting a lower dielectric strength than nitrogen across all testing conditions. Minimal variation in dielectric strength with temperature between 25 and 70°C was seen, followed by a pronounced decrease for the pure gases at 110°C. In addition, argon-nitrogen mixtures exhibited intermediate dielectric behavior, with even 25 vol.% argon showing a substantial reduction of approximately 40-45% in dielectric strength relative to pure nitrogen. Moreover, the dielectric strength data obtained in this work fill a critical gap, particularly at high temperature and pressure, and can be used as a reference for future studies. Gas-solid fluidization at 2600 kPa and 25 ± 2°C and 68 ± 2°C demonstrated that increasing argon concentration and temperature reduced electrostatic charge accumulation in the bulk of the bed and the fouled layer on the column wall, resulting in mitigation of column wall fouling, with the influence of temperature diminishing under argon-rich conditions. Pure argon reduced wall fouling by approximately 70% compared to nitrogen at 2600 kPa and 68°C. Even at a low argon concentration of 25 vol.%, an approximately 30% reduction in wall fouling at 68°C was obtained. Importantly, the observed reduction trend in fouling with argon concentration strongly correlated with the dielectric strength trends for the gases, indicating the role of gas ionization in facilitating surface charge neutralization of the fluidizing particles. Overall, the results obtained in this thesis confirmed that argon gas, even at low concentrations of approximately 25 vol.%, can be utilized in industrial gas-phase polyethylene fluidized bed reactors to mitigate static buildup and consequently reduce the wall sheeting occurrences. Beyond the specific system studied, the results of this thesis may also be relevant to other gas-solid handling and processing systems susceptible to static charge generation.