Homogeneous flow, slurry

Figure 2-48. Critical velocity characteristics depend on whether slurry is heterogeneous or homogeneous. By permission, Deramme-laere, R. H. and Wasp, E. J., Fluid Flow, Slurry Systems and Pipelines, Encyclopedia of Chemical Processing and Design, J. McKetta, Ed., M. Dekker, vol. 22,1985 [25].

Coal slurry pipelines have been constructed in severaJ countries, including a 38-mile (61-kilometer) 12-inch (30.4-centimeter) diameter pipeline m Russia, a 51 -mile (82-kilometer) pipeline in Poland, as well as Olliers in Prance and other locations in Europe. The feasibility of slurry transportation depends upon Ihc resolution of a number of variables, rhe most important of which from a hydraulic standpoint are (I) Size consist (2) velocity and (3) concemrauon. The selection of a proper size eonsisi (gradation) is important in order that homogeneous flow can he achieved at prudent operating velocities. For coal slurry, such a consist is on the order of 8 mesh by 0 (approximately 0.1-inch (2.5-millimeter) particle size to dust) Homogeneous flow (solids evenly distributed across the pipe diameter) is important if excessive wear in the bottom of the pipe is to be avoided and stable operation achieved. 

These are, in turn, controlled by the size distribution and density of the coal particles. To maintain homogenous flow, which is necessary to minimize wear of the pipeline, a coal slurry should have the following properties, assuming a coal specific gravity of 1.4 ... 

A variety of criteria for the classification of suspensions into homogeneous or heterogeneous categories have been proposed. In general, however, it is agreed that suspensions which consist of very fine particles exhibit homogeneous flow behavior, whereas the flow of slurries of coarser particles is usually heterogeneous. 

Erlenmeyer flask) by adding the solid adsorbent, a little at a time, to a quantity of the solvent. This order of addition (adsorbent added to solvent) should be followed sfricfly because fhe adsorbent solvates and liberates heat. If the solvent is added to the adsorbent, it may boil away almost as fast as it is added due to heat evolved. This will be especially true if ether or another low-boiling solvent is used. When this happens, the final mixture will be uneven and lumpy. Enough adsorbent is added to the solvent, and mixed by swirling the container, to form a thick but flowing slurry. The container should be swirled until the mixture is homogenous and relatively free of entrapped air bubbles. 

What are the chief distinguishing characteristics of homogeneous flow and heterogeneous flow in slurries ... 

Strictly speaking, an emulsion is not slurry. An emulsion is a mixture of two phases at certain temperatures resulting in an essentially homogeneous flow. An example of an emulsion is a mixture of bitumen at 70% by volume with water at 30% by volume. If sur-... 

When absolute (or dynamic) viscosity is an important factor, such as in clayish slurries or homogeneous flows, another parameter, the viscous transition critical velocity Kj-must be determined. There are two regimes for flow of homogeneous mixtures. Flow at speeds less than Fj-is associated with laminar flows, whereas flows above Vj-are characteristic of turbulent flows. 

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