Shape liquid plug

Liquid plug flow produces a horizontal (i.e., flat) flow profile (Fig. 7.7a), Channeling produces a U-shaped flow profile (Fig, 7.8a). The liquid moves fast at the tray center and slow near the walls. Wide stagnant zones, a steep U shape, and liquid recirculation in the stagnant zones (Fig. 7.35) signify a highly channeled flow profile. The... 

The concept of surface tension is a very old one. Reportedly, Leonardo da Vinci had already observed and recorded the spontaneous rise of liquids in narrow, wetted capillaries, bores and plugs 1). From this rising the phenomenon acquired its name capillus (lat.) = hair the bores should be as narrow as a hair. Nowadays the term capiUaiy phenomena is used more widely (in this book also) to Indicate not only capillary rise, involving curved interfaces, but also all phenomena determined by the tendency of interfaces to adopt a minimum curea, such as drop shapes, bubble shapes, liquid bridges and wetting. 

Their model showed good agreement with experimental results. Raimondi et al. (2008) carried out numerical simulations of the mass transfer during liquid-liquid plug flow in square microchannels, where it was assumed that mass transfer did not deform the interface, since the hydrodynamics were decoupled from the mass transfer. Kashid et al. (2010) performed dimensional analysis to obtain a relationship (Eq. 2.2.32) between the mass transfer coefiftcient and various independent variables. In their studies however, they did not take into account the effect of contacting geometry and microchannel shape. 

Figure 11.6 shows a schematic representation of a microcapillary cross section. The capillary shown here is a hydrophilic channel several micrometers in width and a height h of about 80 nm, with a thin (r 180 nm) Si02/Si3N4 membrane on top. When an array of those capillaries was Riled with a fluid, observation using an optical microscope showed the peculiar shapes of the menisci at the fluid-air interface at the front of the liquid plugs (see Fig. 11.4). 

Fig. 19. Schematic design of a flow injection analysis (FIA) system. A selection valve (top) allows a selection between sample stream and standard(s). The selected specimen is pumped through an injection loop. Repeatedly, the injection valve is switched for a short while so that the contents of the loop are transported by the carrier stream into the dispersion/reaction manifold. In this manifold, any type of chemical or physical reaction can be implemented (e.g. by addition of other chemicals, passing through an enzyme column, dilution by another injection, diffusion through a membrane, liquid-liquid extraction, etc. not shown). On its way through the manifold, the original plug undergoes axial dispersion which results in the typical shape of the finally detected signal peak...

Expansion behavior according to the Richardson-Zaki equation provides reassurance that the particles behave properly. The next factor to determine is dispersion. A pulse of a tracer is passed through the bed and monitored after leaving the expanded bed. The dispersion is calculated by comparing the shape of the elution profile with that of the original plug of liquid. Often acetone or phenol is used as a tracer and is monitored using UV. 

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