Electrostatics Charging Behavior of a Metallocene Catalyst in Gas-Solid Fluidization of Polyethylene

Contact electrification is a surface phenomenon in which materials acquire electrostatic charge after contact and separation, potentially resulting in undesired occurrences of solids agglomeration and fouling. For instance, in the commercial gas-phase polyethylene (PE) fluidized bed reactors electrostatics could cause reactor wall fouling and sheeting, in turn necessitating reactor shut down. In contact charging, the amount of transferred charge and the generated polarity largely depend on solids surface chemistry. In this study, in relation to the polyethylene process, for the first time the role of presence of a commonly used catalyst, and the charging behaviour of components present on the catalyst surface were investigated in an atmospheric gas-solid fluidized bed under cold flow conditions.

A 0.1 m in diameter stainless-steel fluidized bed was used to fluidize a linear low density polyethylene (LLDPE) resin received from commercial reactors. A metallocene catalyst in its active and inactive form was tested along with a bottom-up approach in which the catalyst base and the catalyst activator were examined separately. Fluidization using nitrogen at 1.5 minimum fluidization velocity was carried out for a period of 1 hr where the powders were added to the bed at the onset of fluidization. The column contained two Faraday cups; one below the distributor plate to measure the particles charge inside the column ( “Bulk”) or adhered on the wall “Fouling”, and one above the freeboard to measure the entrained particles charge during fluidization.

Results showed that catalyst base alone induced a strong negative polarity into the bed of polyethylene and increased the amount of Fouling. Adding a co-catalyst changed the Bulk polarity and thus the amount of Fouling declined. The active catalyst, however, did not change the polarity or the magnitude of Bulk charge, in turn did not result in significant changes in the amount of Fouling.