Disperse Systems in a Gravity Field

Here is the hydraulic diameter and ju is the tortuosity factor (compare later the section Adsorption ). 

Here the factor 2/3 of the Reynolds number with the parameter and is omitted (Empirical results show Ap, see later). In Fig. 3.5-2 the friction 

It is reasonable to present the flow in disperse systems in a general way to avoid repetitions of this topic. Such disperse systems are gas or liquid fluidized beds, bubble or drop columns, and spray columns. In all cases the solid or fluid particles are suspended or moving due to the density difference A/ = p - p ) and the acceleration of gravity. In Fig. 3.5-1 a fixed bed on the left side and several fluidized beds with different flow patterns are depicted. The fluid flow density V , in the fluidized beds is greater than the minimum flow density V(,f necessary to achieve fluidization. The volumetric holdup of the continuous phase increases for Vj Vjf with the fluid throughput. The relative velocity between the fluid and the suspended particles is inversely proportional to the volnmetric holdup Sg. With 

In Fig. 3.6-2 a bubble or drop column is shown on the right-hand side (bubbles or drops are rising in an immiscible liquid) whereas a spray column (drops are falling in a gas) is depicted on the left-hand side. The fluid particles are formed at holes drilled or punched in a plate or tube distributor. Bubbles and drops are rising for Pi p ov p / Pi I. Let us first assume that no continuous phase (gas in a spray column, liquid in a bubble or drop column) is entering the column. 

The volumetric holdup of the dispersed phase increases with the volumetric flow density (or superficial velocity) of the dispersed phase and is inversely 

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Sedimentation analysis is commonly used and rather simple method of determining the size and size distribution function, based on the difference in particle settling rates in a gravity field. In this method a pan is placed into a homogeneously mixed disperse system, and the weight of particles, P, that accumulate on the pan, is monitored as a function of time, t (Fig. V-30). 

Up to this point we have considered distributed dilute dispersions of colloidal size particles and macromolecules in continuous liquid media. Where the particles are uncharged and of finite size, they are always separated by a fluid layer irrespective of the nature of the hydrodynamic interactions that take place. In the absence of external body forces such as gravity or a centrifugal field or some type of pressure filtration process, the uncharged particles therefore remain essentially uniformly distributed throughout the solution sample. We have also considered the repulsive electrostatic forces that act between the dispersed particles in those instances where the particles are charged. These repulsive forces will tend to maintain the particles in a uniform distribution. The extent to which a dispersion remains uniformly distributed in the absence of applied external forces, such as those noted above, is described in colloid science by the term stability, whereas colloidal systems in which the dispersed material is virtually insoluble in the solvent are termed lyophobic colloids. 

An emulsion is a stable mixture of oil and water that does not separate by gravity alone. In the case of a crude oil or regular emulsion, it is a dispersion of water droplets in oil. Oil is the continuous phase and water is the dispersed phase. Normal, or regular, oil-field emulsions consist of an oil continuous or external phase and a water dispersed or internal phase. In some cases, where there are high water cuts, such as when a water-drive field has almost "watered out," it is possible to form reverse emulsions with water as the continuous phase and oil droplets as the internal phase. Complex or "mixed" emulsions have been reported in low-gravity, viscous crude oil. These mixed emulsions contain a water external phase and have an internal water phase mixed in the oil dispersed phase. A stable or "tight" emulsion occurs when the water droplets will not settle out of the oil phase due to their small size and smface tension. Stable emulsions always require some form of treatment. The vast majority of oil treating systems deal with normal emulsions, which is the focus of this chapter. 

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