Using Gas Flows to Probe Segregation in Moving Granular Beds

Abstract

We study the segregation of spherical particles of different diameters in moving beds i.e. steady-state dense-phase flows of granular solids. The transients typical of, for instance, hopper discharge are absent. Such flows often occur in vessels in which a fluid flows, either co-currently or counter-currently to the particles, which themselves move vertically downwards under gravity. They are widely used in industry in applications such as continuous catalyst regeneration reactors and blast furnaces.We have used three different “gas probes”. The first is the pressure drop measured when known steady flows of air are passed through moving beds. This lets us identify how to arrest the granular flow while preserving the structure of the bed, permitting most subsequent experiments to be conducted more conveniently on such “frozen” beds. Our second consists of injecting a pulse of CO2 tracer into a steady inlet flow of air; the CO2 concentration in the outlet air is measured using a mass spectrometer. We thence infer gas residence time distributions (RTDs). Our third “gas probe” consists of injecting the CO2 tracer at a particular position within the bed: the response at a position vertically above the injection point is measured. Radial traverses then reveal the radial profile of tracer axial velocity and so the radial variation in resistance to gas flow.The probes were proven by experiments using monosized glass spheres, or “ballotini”, of diameter 1, 3 and 5 mm, revealing significant maldistribution of gas flow, dependent on the frictional properties of the vessel wall. By adopting walls of the type which cause little maldistribution when enclosing mono-sized particles, we have been able to focus on the dramatic maldistribution caused by segregation in binary mixtures. For example, with a mixture of 40% (by mass) 1 mm spheres with 3 mm spheres, the RTD may display two peaks separated by a factor of 4 in time, consistent with a variation over the cross section of the gas axial velocity by a factor of four.The particle segregation which imposes the radial variation in resistance to gas flow and so causes the maldistribution is found to be strongly influenced by the “free fall gap” i.e. the distance between a control orifice in the particle inlet tube and the base of the free surface of the moving bed. Under suitable conditions, the smaller particles accumulate preferentially near the central axis; under other conditions they accumulate instead in an outer annulus near the vessel wall; under yet further conditions they spread approximately uniformly over the cross-section. Visual observation discloses the segregation mechanisms at work.