Phase separation, gravitational effects

In liquid systems, we can suppress the effect of the Froude number by using baffles to eliminate vortexing and hence the gravitational effects. The Weber number is of some importance only when separate physical phases are present in the liquid mixing system. 

There are a large number of processes in the chemical industries that handle a variety of suspensions of solid particles in liquids. The application of filtration techniques for the separation of these heterogeneous systems is sometimes very costly. If, however, the discrete phase of the suspension largely contains settleable particles, the separation can be effected by the operation of sedimentation. The process of sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening,... 

This is 2500 times faster than with gravity alone, but the residence time in the centrifuge would have to be about 20 minutes, which is not practical. To speed up the separation, naphtha is added to the level of 25%. This lowers the viscosity to about 4.5 mPa-s and lowers the density of the continuous phase to 0.88 g/mL. Note that now the water drops would sediment rather than cream under gravitational force, and while the emulsion density is much reduced, the absolute value of the density difference changes very little Ap = -0.07 g/mL originally, and becomes Ap = +0.09 g/mL The overall effect is to lower the viscosity by about two orders of magnitude. The droplet velocity now becomes (dx/dt)" = 1.1 cm/s, which yields a satisfactory residence time of about 8 seconds. 

Reactions were quenched after 4 hr. In cases of treatment with resin, simple gravitation filtering was sufficient to isolate the resin from the sample. Subsequent assays for aldehyde (or peroxide) and boron were effected. When NaBH4 or (C2H5)4 NBH4 were used, samples were neutralized with excess aqueous hydrochloric acid. Organic and aqueous phases were separated when possible and both were subsequently analyzed for aldehyde (or peroxide). Weights of analyzed samples were noted percent reduction was calculated. 

Thermodynamic discussion of real systems usually involves certain approximations which are made for the sake of convenience and are not always stated explicitly. For example, in dealing with vapour-liquid equilibrium, in addition to neglecting the interfacial layer, it is customary to assume that each phase is unifbrm throughout its depth, despite the incipient separation of the components due to the gravitational field. However, the latter effect can itself be treated thermodynamically, whenever it is of interest. 

Water soluble polymers can influence the stability of emulsions to gravitational separation in a variety of ways. Non-adsorbed polymers may either increase or decrease stability depending on their effective size and concentration in solution. Increasing the concentration of a polymer in solution causes an increase in continuous phase viscosity (and may even lead to gelation), which should slow down droplet movement. On the other hand, the presence of non-adsorbed polymer also increases the magnitude of the depletion attraction between droplets, which may cause flocculation and therefore accelerate droplet movement. The presence of an adsorbed polymer may also influence stability to gravitational separation in a number of ways. For example, the size of the droplets produced... 

In devices with cross parallel flow, there is a local unidirectional movement of streams, as a rule, to the subsequent centrifugal— gravitational separation. Speed limit of an easy phase in devices of this type makes 6-lOmps. They are expedient for using, for example, in processes with significant thermal effects (Karpenkov, 1970 and Vazovkin, 1972). 

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