Droplet Counter-Current Liquid

Otsuka, K. Yamakawa, T. The apphcation of droplet counter-current chromatography (DCC) forthe separation of acidic glycolipids. J. Biochem. (Tokyo) 1981,90,247-254. Kim, Y. Wang, T.C. Ma, Y.C. Liquid chromatography/ mass spectrometry of phosphohpids using electrospray ionization. Anal. Chem. 1994, 66, 3977. 

Spray Towers A spray tower consists of an empty shell into the top of which the liquid is sprayed by means of nozzles of various kinds the droplets thus formed are then allowed to fall to the bottom of the tower through a stream of gas flowing upwards. The use of sprays appears to offer an easy way of greatly increasing the surface area exposed to the gas, but the effectiveness of the m.ethod depends on the production of fine droplets. These are difficult to produce and suffer from the disadvantage that they are liable to entrainment by the gas even at low gas velocities. The surface area may also be reduced as a result of the coalescence of the droplets first formed. As a consequence of these effects, the large increase in surface area expected may not be achieved, or if achieved m.ay be accompanied by serious entrainment and internal circulation of the liquid so that true counter-current flow is not obtained. A single spray tower is suitable for easy absorption duties. For difficult duties, a number of towers in series can be used. 

In wet scrubbing the dust is removed by counter-current washing with a liquid, usually water, and the solids are removed as a slurry. The principal mechanism involved is the impact (impingement) of the dust particles and the water droplets. Particle sizes down to 0.5 /i.m can be removed in suitably designed scrubbers. In addition to removing solids, wet scrubbers can be used to simultaneously cool the gas and neutralise any corrosive constituents. 

In liquid-liquid extraction one or more components are removed from a liquid mixture by intimate contact with a second liquid that is itself nearly insoluble in the first liquid and dissolves the impurities and not the substance that is to be purified. In other cases, the second liquid may dissolve i.e., extract from the first liquid, the component that is to be purified, and leave associated impurities in die first liquid. Liquid-liquid extraction may be earned out by simply mixing die two liquids widi agitation and dien allowing diem to separate by standing. It is often economical to use counter-current extraction, in which the two immiscible liquids are caused to flow past or dirough one another in opposite directions. Thus fine droplets of heavier liquid can be caused to pass downward through the higher liquid in a vertical tube or tower,... 

Consider now the absorption extraction of heavy hydrocarbons under the conditions of counter-current flow in a column absorber presented schematically in Fig. 20.7. Gas of a given composition yo = (yoi, yo2, , yon), where yoi is the molar fraction of i-th component, with the flow rate Qgo enters the bottom part of the column. At the same time, an absorbent with composition xo = (xoi 5 02, , on) and flow rate qo enters the top part of the column. The number of contact stages is equal to N. Each stage is equipped with a perforated plate operating in the ablation regime. This means that the liquid is not collected on the plate, but exists in a dispersed state in the inter-plate space. Each contact stage contains a separation device, for example, a mesh droplet catcher, in which the exhausted absorbent is separated from the gas and directed toward the next plate. 

One of the most common reactors employed to treat continuously three phase systems (gas-liquid-solid) is the Trickle Bed Reactor. It consists of a column with a fixed bed of catalyst particles through which liquid flows in the form of films, droplets and rivulets. Gas moves cocurrently sometimes counter-current flows are also used. Usually one reactant is introduced in the liquid phase and the other in the gas phase. The cocurrent type of TBR is schematically shown in Fig. 1. 

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