Counter-current gravity separators

Counter-current gravity separators (dust settling chambers with vertical gas flow) the particles are precipitated by the action of gravity, i.e., they descend in the rising stream of gas, which is thus relieved of its dust burden... 

Current American practice entails continuous treatment of molten TNT (> 80 ) with an aq soir containing about 0.1 part anhyd Na sulfite per part TNT. This corresponds to about four times the theoretical requirement, according to the equation given above. The liqs are counter-currently contacted in two washers, each of which is followed by a gravity separator (Ref 9). See Fig 1 in the section on Preparation for a general view, and Fig 5 for more detail... 

The counter-current pattern of adsorption and desorption favors high removal efficiencies. Desorption of the adsorbed solvents starts after the delay required to heat the activated carbon bed. The specific steam consumption increases as the residual load of the activated carbon decreases (Figure 22.1.6). For cost reasons, desorption is not run to completion. The desorption time is optimized to obtain the acceptable residual load with a minimum specific steam consumption. The amount of steam required depends on the interaction forces between the solvent and the activated carbon. The mixture of steam and solvent vapor from the adsorber is condensed in a condenser. If the solvent is immiscible with water the condensate is led to a gravity separator (making use of the density differential) where it is separated into a aqueous and solvent fraction. 

The rest of this chapter, however, is concerned with continuous counter-current washing. The separators used may be sedimenting centrifuges, gravity thickeners or filters but the preferred choice is often hydrocyclones. The inherent advantages of hydrocyclones are compactness, small hold-up of liquid, high shear forces and turbulence in the flow, the latter two of which improve mass transfer and dispersion. 

Note that hydrocyclones are not the only suitable separators for counter-current washing in a large number of stages. Where enough space is available, gravity thickeners can also be used. 

As the models of counter-current separations drawn up so far [1,2] have proven unsuitable or too complex [12] for the evaluation of the multistage cross-flow gravity separation studied here. In order to describe the process efficiency, the separation tapping model of a turbulent cross-flow hydroclassification developed by Schubert [8] and Neesse [5] was supplemented with a model for multistage turbulent cross-flow aeroseparation [16],... 

Low shear process will produce larger liquid droplets (>30 microns), and heavy/light liquids can be separated using gravity settling method. Examples of low shear process are solvent extraction, mixer settlers, steam stripping, washing process, and counter-current tower. Coalescer pad can be used to speed up the liquid/liquid separation and reduce the separator size. 

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