Study of hydrodynamics and mixing in an airlift reactor with an enlarged separator using magnetic tracer method

Abstract

The magnetic tracer technique was used to obtain essential information on hydrodynamics, mixing and reactor design in a multiphase airlift reactor with an enlarged separator. The tracer method involving the use of a magnetic particle as a neutrally buoyant flowfollower allowed measurement of the residence time of the tagged particle in all reactor sections (riser, downcomer, separator and bottom sections). Knowing the settling velocity of the tagged particle, the liquid circulation velocity was also determined. The effects of the design of the enlarged separator (HDT/HC), of the riser to downcomer crosssectional area ratio (AD/AR) and of the liquid level in the head zone (HT) were investigated. The hydrodynamics of the ALR was affected by all design parameters. The results of experiments with different heights of draft tube demonstrated how easily various operating flow patterns can be achieved ranging from typical flow patterns for an internal-loop ALR with high downcomer gas holdup to those of an external-loop with low or nil one. On the base of histograms of the residence time of the tagged particle in the separator zone and visual observations, three flow patterns of the particle were described. The RTDs of the tracer particle were also used for assessment the mixing efficiency of reactor sections. The results showed that the best option for the design of ALR with an enlarged separator concerning mixing intensity is the use of reactor configuration with a dual separator and an area ratio AD/AR around 4.