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Part I1 of this paper deals with the analysis of continuous separation of an unstable emulsion in a gravity settler of a miser settler unit. When the emulsion passes from the mixing vessel into the settler, droplets of the dispersed phase approach one another to form a heterogeneous zone between the two liquid phases. The formation of this zone is deteimined by the local velocity gradients and the density difference between the phases. In the mixing zone, a high level of turbulence is generated to optimize mass transfer between two liquid phases, but in the settler all turbulence must be rapidly damped out, and the velocities of the phases in the settling tank must be low and controlled only by the throughput rates. The position of the heterogeneous layer with respect to the final interface between the settled phases will be deteimined by the density of the dispersed phase, and the liquid droplets in this layer tend to attain some stable packing arrangement determined by the diameter of the drops. It is important in any settling tank to prevent extract phase from being entrained with raffinate leaving the settler and vice versa. This difficulty can be overcome by suitable design and positioning of off-take lines and by operating the unit in such a way that the heterogeneous layer does not occupy the total available cross-sectional area between the phases in the settler. Under such conditions, the size of the heterogeneous wedge formed is determined by the throughput rates and by the physical properties affecting coalescence of the droplets, such as interfacial tension and viscosity of the liquids. MODELConsider the behavior of the wedge of drops in more detail. Let the dispersed phase consist of drops of the more dense liquid; then the wedge is formed above the interface. At the lower surface of the wedge, droplets are coalescing with the bulk liquid phase by a drop-interface coalescence mechanism. Inside the wedge and at the upper surface, drops are coalescing together by a drop-drop coalescence mechanism. The residence time or life of the droplets in the wedge is controlled by these two processes.Consider a settler of unit width operating under steady state conditions with the dispersed phase made up of the more dense liquid when a wedge of droplets will be LESS DENSE PHASE I Fig. 1. Parameters of the differential model. formed above the interface, as shown in Figure 1. Consider ;I small increment 61 in the wedge at a distance 2 from the inlet. Let the depth of the wedge at this position be h, let the number of drops entering the element per second be n, and let the mean diameter of the drops be 4.In practice, a distribution of drop sizes will exist at any position in the wedge. If uniform sized droplets enter the wedge, variation in sizes is brought about by variations in coalescence times. This will be discussed later in more detail. However, in this analysis a mean dro diameter ing per second is equal to the volume of dispersed phase leaving the element, plus the volume of dispersed phase coalesced with the ...
Part I1 of this paper deals with the analysis of continuous separation of an unstable emulsion in a gravity settler of a miser settler unit. When the emulsion passes from the mixing vessel into the settler, droplets of the dispersed phase approach one another to form a heterogeneous zone between the two liquid phases. The formation of this zone is deteimined by the local velocity gradients and the density difference between the phases. In the mixing zone, a high level of turbulence is generated to optimize mass transfer between two liquid phases, but in the settler all turbulence must be rapidly damped out, and the velocities of the phases in the settling tank must be low and controlled only by the throughput rates. The position of the heterogeneous layer with respect to the final interface between the settled phases will be deteimined by the density of the dispersed phase, and the liquid droplets in this layer tend to attain some stable packing arrangement determined by the diameter of the drops. It is important in any settling tank to prevent extract phase from being entrained with raffinate leaving the settler and vice versa. This difficulty can be overcome by suitable design and positioning of off-take lines and by operating the unit in such a way that the heterogeneous layer does not occupy the total available cross-sectional area between the phases in the settler. Under such conditions, the size of the heterogeneous wedge formed is determined by the throughput rates and by the physical properties affecting coalescence of the droplets, such as interfacial tension and viscosity of the liquids. MODELConsider the behavior of the wedge of drops in more detail. Let the dispersed phase consist of drops of the more dense liquid; then the wedge is formed above the interface. At the lower surface of the wedge, droplets are coalescing with the bulk liquid phase by a drop-interface coalescence mechanism. Inside the wedge and at the upper surface, drops are coalescing together by a drop-drop coalescence mechanism. The residence time or life of the droplets in the wedge is controlled by these two processes.Consider a settler of unit width operating under steady state conditions with the dispersed phase made up of the more dense liquid when a wedge of droplets will be LESS DENSE PHASE I Fig. 1. Parameters of the differential model. formed above the interface, as shown in Figure 1. Consider ;I small increment 61 in the wedge at a distance 2 from the inlet. Let the depth of the wedge at this position be h, let the number of drops entering the element per second be n, and let the mean diameter of the drops be 4.In practice, a distribution of drop sizes will exist at any position in the wedge. If uniform sized droplets enter the wedge, variation in sizes is brought about by variations in coalescence times. This will be discussed later in more detail. However, in this analysis a mean dro diameter ing per second is equal to the volume of dispersed phase leaving the element, plus the volume of dispersed phase coalesced with the ...
A coalescence model in the context of the ReDrop concept (Representative Drops) is proposed to design technical equipment for separating liquid-liquid dispersions in settlers or extraction columns. A fundamental study of drop interactions has been performed to obtain the complete picture of coalescence. The model proposed accounts for collision frequency of the drops, bouncing probability, and coalescence probability, for which the film-drainage approach is applied. The model proposed allows considering the appropriate formulation for different types of equipment. Especially noteworthy is that the coalescence probability fundamentally differs from the expression of Coulaloglou and Tavlarides, which is frequently used, but which shows inconsistencies at fundamental level.Batch settling experiments have been performed to validate the numerical approach and thus the coalescence model. The
A detailed literature review to predict drop/drop and drop/interface rest‐times is presented. The difficulties encountered in usefully employing these data in design is demonstrated and a new method of approach where the relative contributions of the two coalescence modes are utilized, is described. The validity of this approach is illustrated by considering coalescence occurring in a laboratory single‐stage settler unit.
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