Bubble segmented flow

Since 1970, new analytical techniques, eg, ion chromatography, have been developed, and others, eg, atomic absorption and emission, have been improved (1—5). Detection limits for many chemicals have been dramatically lowered. Many wet chemical methods have been automated and are controlled by microprocessors which allow greater data output in a shorter time. Perhaps the best known continuous-flow analy2er for water analysis is the Autoanaly2er system manufactured by Technicon Instmments Corp. (Tarrytown, N.Y.) (6). Isolation of samples is maintained by pumping air bubbles into the flow line. Recently, flow-injection analysis has also become popular, and a theoretical comparison of it with the segmented flow analy2er has been made (7—9). 

In the design of optimal catalytic gas-Hquid reactors, hydrodynamics deserves special attention. Different flow regimes have been observed in co- and countercurrent operation. Segmented flow (often referred to as Taylor flow) with the gas bubbles having a diameter close to the tube diameter appeared to be the most advantageous as far as mass transfer and residence time distribution (RTD) is concerned. Many reviews on three-phase monolithic processes have been pubhshed [37-40]. 

Slug flow-segmented flow transition. Taylor bubbles will be formed when the gas flow rate is increased to such an extent that it forces bubbles to become closely packed and to agglomerate into Taylor bubbles. 

In segmented flow in cylindrical capillaries, Taylor bubbles consist of a cylindrical part and two caps at the front and the rear menisci. The form of the caps may be axisymmetric or nonaxisymmetric. In a balance over a long inviscid bubble surrounded by a moving incompressible and viscous fluid, capillary, viscous, inertial, and gravity forces are taken into account. The latter three, relative to capillary force, are expressed in the following dimensionless numbers ... 

In segmented flow, the subsequent liquid slugs are separated from each other by the Taylor bubbles. Thus, each liquid slug is enclosed by the two ends of the adjacent Taylor bubbles. 

In two-phase flow through narrow channels, the pressure drop depends on the type of flow. For example, it is expected that at the same conditions and for a fixed total flow rate, bubble flow will give rise to a higher pressure drop than segmented flow does. This is due to the fact that the liquid phase is the main contributor to the frictional pressure drop. The pressure drop in annular flow may be either higher or lower than in segmented flow. In annular flow, the liquid film substantially reduces the effective diameter of the... 

Here, a parallel between the inception of segmented flow and flow injection analysis (section 2.3) can be drawn. Skeggs did not want air bubbles and incidentally realised the improved performance of systems with segmentation on the other hand, Ruzicka Hansen did want air... 

In 1985, mono-segmented flow analysis was proposed [64] as a means of achieving extended sample incubation times without excessive sample dispersion. The sample was inserted between two air bubbles into an unsegmented carrier stream therefore the innovation combined the favourable characteristics of both segmented and unsegmented flow systems. Further development revealed other potential applications, especially with regard to relatively slow chemical reactions, flow titrations, sample introduction to atomic absorption spectrometers, liquid-liquid extraction and multi-site detection (Chapters 7 and 8). This innovation was also referred to as segmental flow injection analysis [65]. 

Pulsating flows are beneficial with regard to segmented flow analysis, as the bubble additions are synchronised with the flow pulsation. In order to enhance the pulsating nature of the confluent stream of air to be added, some peristaltic pumps (including the widely used Technicon AAII) incorporate a lifting bar to successively pinch the air delivery tubing. 

Regarding segmented flow analysis, a relatively large de-bubbler is usually required and this device may act as a mixing chamber, thus impairing system performance. This limitation is typical of older instruments. The bubble-gating approach [23] can be exploited to circumvent this drawback, but it has not been universally implemented. 

FIGURE 5.2 Flow pattern inherent to segmented flow analysis. 1 = tube wall of part of a coiled reactor 2 = thin liquid layer adhering to it 3 = vortices inside the liquid phase 4 = air bubble outer large arrow = overall displacement of the segmented flow. Adapted from P.J. Worsfold, A. Townshend, C.F. Poole (Eds), Encyclopedia of Analytical Science, 2nd Edn, Oxford, 2005, v.3, p.24, E.A.G. Zagatto, PJ. Worsfold, Flow Analysis Overview, with permission from Elsevier (Ref. [4]). 

FIGURE 5.6 The segments established in a segmented flow analyser. 1 = tube wall 2 — air bubble 3 = thin liquid film on the tube wall 4 and 5 = liquid segments with indication of the vortices (rounded arrows) large arrow = overall flow movement. 

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