Investigating dry powder flow within large-scale, counter-rotational, intermeshingTwin-Screw Mixers (TSMs), via Digital Image Processing (DIP) and Positron Emission Particle Tracking (PEPT)

Abstract

Solids processing is a common procedure spanning a wide selection of different industries. Fields as diverse as Fast Moving Consumer Goods (FMCG), pharmaceuticals, fine chemicals and even mineral processing will adopt some kind of solids handling technique, at some stage in a products development. Common objectives during solids processing include: (1) ensuring adequate and consistent mixing of multiple ingredients, and/or (2) adjusting the physicochemical properties of the particles, in order to improve bulk handling and metering, or to otherwise customise the product to look and behave in some desirable fashion [1, 2]. Despite its prevalence throughout industry, these procedures are noted for being notoriously inefficient owing to the complex interplay between chemical (and/or physical) rate processes and system geometry. Continuous twin-screw processing however, shows great promise with regard to improving this state of affairs, due to their inherent modularity coupled with a relatively small flow channel diameter. These properties, it is believed, may be exploited to facilitate a precise and homogeneous application of parameters (such as shear) upon the bulk, as it propagates axially through the system, which in turn could reduce deviations from a set-point [3, 4, 5]. This document investigates dry powder transport phenomena within large-scale Twin-Screw Mixers (TSMs), via Positron Emission Particle Tracking (PEPT) and Digital Image Processing (DIP). Changes to flow, as a result of altering: TSM size, internal geometry, screw speed and powder feed rate, has been thoroughly examined, and key underlying mechanisms identified. Due to the vast knowledge gap within literature pertaining to such vessels the work presented is highly novel, and should be of great interest to both industrialists and academics alike. This document also established a highly novel and robust framework for efficiently characterising powder flow within a TSM. This has been used to good effect whilst investigating two geometrically similar TSMs, of two different sizes, in order to help inform a capital investment decision for purchasing a commercial scale version of the machine. The PEPT investigation contained within the report has established that the bulk flows within the TSM annular spaces at the same rate as the screw lead rotates. In the TSMs Nip Region however, the average bulk velocity is approximately twice this magnitude.