Effect of Fluidization Temperature and Catalyst Presence on the Extent of Wall Fouling and Charge Distribution in Polyethylene Gas-Solid Fluidized Beds

Abstract

Electrostatic charge transfer among solids can pose significant operational challenges in various industries dealing with solid handling and processing including pneumatic conveying, gas-solid fluidization, etc. This phenomenon, known as triboelectrification or contact electrification, has been notably observed to negatively affect the gas-phase polyethylene production process. In this process, polyethylene resin is produced in a gas-solid fluidized bed reactor, while the catalyst is injected into the reactor to initiate polymerization reaction. The contact electrification among particles and particles with the reactor wall prompts the adhesion of polyethylene and catalyst particles to the reactor wall, forming a thick layer, an occurrence known as sheeting. Sheeting can eventually necessitate reactor shutdown, and in turn, resulting in significant economical loss due to production downtime and maintenance costs. Therefore, it is essential to investigate the mechanisms and parameters that potentially influence the polarity and magnitude of the generated charge in such reactors in order to determine mitigation techniques. Given that the gas-phase polyethylene reactors temperature typically ranges from 80°C to 110°C, studying the triboelectrification of polyethylene at these temperatures is important. Furthermore, contact electrification is primarily a surface phenomenon, and thus, particles with various surface chemistry are able to alter electrostatic charge generation magnitude or charge transfer direction. Consequently, catalyst particles with diverse and complex chemistries could potentially be a key player influencing sheet formation in commercial polyethylene reactors. This thesis attempts to provide information concerning the influence of temperature and the presence of catalyst on triboelectrification and fouling formation in gas-phase polyethylene fluidized bed reactors. The first investigation of this thesis focuses on assessing the effect of temperature, which spanned from 24°C to 58°C, on the electrostatic charging characteristics of a commercially manufactured linear low-density polyethylene (LLDPE) resin. This examination was conducted within a pressurized pilot-plant fluidized bed operating at 2600 kPa. Through these experiments, the amount of wall fouling and the charge of particles in the bed, including those fouled on the wall and those entrained, were examined. Results indicated a significant reduction in wall fouling as the fluidization temperature increased to 58°C from 24°C, corresponding to a reduction in net specific charge of the fluidized particles. To delve deeper into the mechanism behind the influence of temperature on solid's charging, shake tests were performed to also differentiate the effects of types of contacts within the fluidized bed, namely interactions between polyethylene particles and between particles and the stainless-steel column wall. No variation in charge transfer behaviour between particles and a stainless-steel cup was observed with increasing temperature, whereas charge transfer for insulator-insulator contacts reduced. A volume resistivity measurement cell was constructed in-house in accordance with ASTM D257 standards, and the volume resistivity of various insulator films (nylon, LLDPE and LDPE) was measured at 23°C and 60°C. Volume resistivity measurements confirmed the decline in materials' volume resistivity as temperature increased, in turn validating the contribution of this parameter to the reduction in charge transfer in the fluidized bed. In the subsequent investigation, the triboelectric consequences which are originated from chemistry of catalysts employed in polyethylene production, as well as their components including the catalyst support and cocatalyst, are investigated. Silica as the catalyst base, methylaluminoxane as the cocatalyst, and a metallocene catalyst in both active and deactivated forms were studied at two mass loadings. These samples were introduced to a bed of LLDPE resin through a pneumatic conveying line, and the electrostatic charge generation of each sample in the conveying tube and in the fluidized bed was examined. Low powder concentration of 100 ppm did not affect the resin charging; however, 1000 ppm concentrations led to notable changes. Methylaluminoxane was believed to be ionized in the bed, and in turn, significantly mitigated the amount of fouling as well as charge generation. Metallocene catalysts, without altering the bed charge magnitude and polarity, caused a substantial augmentation in the mass of fouling by retaining fine particles in the bed. Adding static control agents to the catalyst helped in suppressing the resulted fouling augmentation. Furthermore, powders with an acidic property such as silica and deactivated metallocene catalyst induced a negative polarity to the bed. The concluding investigation concentrates on the impact of electrostatic charge on particle's pneumatic conveying velocity. This influence was investigated by measuring particle velocity through a new measurement technique developed in-house to measure particle velocity based on the electrostatic charge of particles in pulse pneumatic injections inside a conductive tube. Results from this measurement were in good agreement with a video imaging technique, and it was revealed that particle velocity is influenced by the turbulency of the conveying gas as well as the presence of electrostatic charge on particles, a parameter that is not included in the existing empirical and semi-empirical models proposed to predict particles conveying velocity.