Applicability of the axial dispersion model to coiled flow inverters containing single liquid phase and segmented liquid-liquid flows

Residence time distribution (RTD) curves for coiled flow inverters (CFIs), helically coiled tubes (HCT), and straight tubes (ST) with single phase and segmented liquid-liquid flows were studied with pulse injection employing laser optical and conductivity detection. Reactor design and volume rates were chosen to cover conditions relevant for practical laboratory investigations, such as channel diameters between 0.8 mm and 3.2 mm, volume rates between 1 and 360 ml/min, 14 ≤ Re ≤ 2414, 4 ≤ De ≤ 451 and a number of up to 60 flow inversions for CFIs. RTD curves were examined in terms of moment analysis and time-domain least squares fitting. For single phase flows deviations from the axial dispersion model (ADM) were observed for ST and HCT and rationalized in terms of Fo number. For the CFIs consistency of the RTD curves with the ADM was observed over the entire range of experimental parameters due to strong chaotic mixing. For segmented liquid-liquid flow simultaneous measurements of RTD curves and slug lengths were performed. The length of the slugs decreased with increasing flow rate. The RTD curves showed good consistency with ADM throughout the experiments for all reactors investigated. Observed Bo numbers decreased at relatively low flow rates in the order CFI > HCT > ST while at relatively high flow rates Bo for HCT and CFI became similar but still higher than ST. Ca numbers indicated that mixing between the vortex and film regions probably dominated the dispersion at low flow rates, while at high flow rates dispersion was likely to be dominated by the film thickness.