Role of Catalyst Support and Continuity Additives on the Degree of Electrostatic Charging and Reactor Fouling in Polyethylene Gas-solid Fluidized Bed Reactors
Creators: name: Taghavivand, Milad
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2021
Subjects: Triboelectrification
Pneumatic conveying
Gas-solid fluidized bed
Ethylene gas-phase polymerization
Amorphous silica catalyst support
Continuity additive
Reactor sheeting
Polyethylene

description: Electrostatic charging occurs in many gas-solid processes due to interparticle and particle-process vessel wall contacts during particle handling, transport, and processing. A commercial process adversely affected by electrostatic charging is ethylene gas-phase polymerization. In such a process, catalyst particles, which are periodically fed into the fluidized bed reactor could become electrostatically charged along with the produced polyethylene (PE) resin, resulting in their adhesion to the reactor walls, causing an operational challenge known as sheeting. Sheets can break off the reactor walls and block the distributor plate resulting in a significant economic loss due to reactor shutdown for clean up. Electrostatic charge generation in PE fluidized bed reactors has been the topic of research for several years. However, to date, the majority of the works have only considered PE resin charging behavior and thus the influence of catalyst charging on the extent of reactor wall fouling has received minimal attention. Catalyst particles which have different chemistry and particle size from the PE resin, are expected to have different triboelectrification behaviour and thus contribute to the reactor electrification. In addition, dry feeding of the catalyst into the PE fluidized bed reactor through narrow tubes could make them electrostatically charged upon entering the reactor due to their collisions with the conveying tube wall. The catalyst particles' initial charge upon entering the reactor in addition to the generated charge due to their contacts with the PE resin inside the reactor could affect the degree of bed electrification and consequently the extent of particles migration towards the reactor walls. A widely used technique to mitigate the sheeting formation in commercial PE gas-phase fluidized bed reactors is dry feeding of compounds referred to as continuity additives (CAs). Although CAs are hypothesized to reduce the charge build-up in PE reactors, their exact role in mitigating or controlling the reactor sheeting is unknown. This in turn has made their selection, in particular for processes with a newer catalyst generation, challenging and only based on trial and error. This thesis aims at understanding the roles that catalyst and CAs play on the degree of PE fluidized bed electrification and wall fouling formation. First, a thorough investigation was carried out on particles' electrostatic charging behaviour during their pneumatic conveying by designing and building a pneumatic conveying apparatus equipped with two electrostatic charge measurement techniques, from which one was an online method. This apparatus was then attached to an atmospheric gas-solid fluidized bed enabling the direct pneumatic injection of the powders into a PE fluidized bed. The entire experimental system for the first time allowed the electrostatic charge measurement of a number of additives while being pneumatically conveyed into a PE fluidized bed. The experimental apparatus also allowed measurements of the electrostatic charge generation inside the fluidized bed as well as the amount of fluidized bed wall fouling. Since a big portion of the catalysts utilized in the PE process is occupied by the support, in this thesis the charging behavior of a typical catalyst support (i.e., amorphous silica) was studied as a preliminary step to study catalyst particles in the future. In addition, five commercially available CAs including three ionic (aluminum distearate, calcium stearate, and zinc stearate) and two non-ionic (AS-990 and Chimassorb 944) solids were tested. Ion and electron transfer were proposed as the charging mechanism of the ionic and non-ionic powders, respectively. The amorphous silica catalyst support was found to become highly charged due to pneumatic injection while the CAs gained a minimal amount of charge. The fluidized bed wall fouling magnitude was observed to be directly related to the bed bulk particles' specific charge magnitude. Among the tested combinations, those that reduced the net charge of the bulk of the bed to close to zero were successful in reducing the amount of wall fouling. The bed bulk charge and the wall fouling magnitude were observed to be influenced by the type of the powders added, while being independent of the powders’ charge upon entering the fluidized bed. The findings of this thesis clearly emphasize the significance of having knowledge of the solids’ surface chemistry involved in the process and its utilization as a predictive tool in determining the charging mechanism of the continuity additives in contact with the PE resin inside the reactor. Knowledge of the solids’ charging mechanism enables the prediction of the dominant charge polarity within the reactor and in turn resulting in effectively mitigating the reactor sheeting.