Differential contacting

Liquid-liquid contacting equipment may be generally classified into two categories stagewise and continuous (differential) contact. 

Mass transfer processes involving two fluid streams are frequently carried out in a column countercurrent flow is usually employed although co-current flow may be advantageous in some circumstances. There are two principal ways in which the two streams may be brought into contact in a continuous process so as to permit mass transfer to take place between them, and these are termed stagewise processes and continuous differential contact processes. 

The main process variables in differential contacting devices vary continuously with respect to distance. Dynamic simulations therefore involve variations with respect to both time and position. Thus two independent variables, time and position, are now involved. Although the basic principles remain the same, the mathematical formulation, for the dynamic system, now results in the form of partial differential equations. As most digital simulation languages permit the use of only one independent variable, the second independent variable, either time or distance is normally eliminated by the use of a finite-differencing procedure. In this chapter, the approach is based very largely on that of Franks (1967), and the distance coordinate is treated by finite differencing. 

Differential centrifugation, 5 531 Differential contacting, 10 760-762 Differential display, 13 354 Differential equations, 11 736 of motion, 11 738-739 Differential interference contrast (DIC), 16 480-483... 

The three factors, the inducement of countercurrent flow, stage-wise or differential contacting and the means of effecting phase separation are the basis of a classification of contactors proposed by Hanson1 which is summarised in Table 13.1. 

A single-stage extraction using the same total volume of solvent achieves only 92% extraction, and the extract concentration is only 0.23, vs. nearly 0.25 for the cross-flow extraction. The use of four cross-flow extraction stages is clearly preferable to a single extraction. Equally, of course, the use of more than four extraction stages, each with a proportionately smaller volume, would improve the performance. In the limit, one would seek a differential contacting process similar to the Soxhlet extractor employed for extraction from solid phases, but such a contactor has not found use in solvent extraction. 

There followed a brief discussion of equipment for carrying out solvent extraction in industrial practice, both by stagewise and differential contact. Some of the first principles for the design of differential contactors were outlined and the part played by the efficiency of extraction in continuous equipment was discussed. Finally there was an outline of methods for the control of solvent loss which forms probably the most important environmental aspect of the application of solvent extraction. 

D scattering function from a crack reference signal in frequency domain signal in frequency domain reference signal in time domain signal in time domain differential contact stiffness... 

HETP is another quantity that is used to express the efficiency of a device for carrying out a separation, particularly in which mass is transferred by a stage-wise action rather than a differential contact. For example, in a tray column, the HETP value is the tray spacing divided by the fractional overall tray efficiency. 

Mixer-settler assemblies for extraction purposes often are preferable to differential contact towers in order to obtain very high extraction yields or to handle large flow rates or when phase... 

In differential contact theory k(t) monotonously decreases with time, starting from the maximum kinetic value (3.145) and approaching the minimal stationary value ... 

Horizontal vessels are normally used as settlers in order to provide a large area of phase interface. It is generally necessary to conduct experiments to ascertain the coalescence rate. The settling area can usually be reduced by the use of coalescing aids such as mesh and metal swarf. In recent years, attempts have been made to use an electrostatic field as a coalescence aid. Mumford and Thomas (1972) and Jeffreys and Mumford (1974) have described various methods for reducing the overall size of the settlers. In many instances, differential contact (DC) packings arc used that are claimed to act as excellent coalescence aids. 

Extraction equipment can be classified by function as providing discrete stages or continuous differential contact. Separation may be by gravity alone or by centrifugal force. Additional energy may be applied to control drop size, either by mechanical agitation or pulsation. This classification is shown in Table 1, along with major examples of available equipment. 

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