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Coal slurry: Piloting and flow simulation (normalized shear rate at the wall) for reliable scale-up
Mining and processing of coals and ores usually take place in remote and less developed regions. Mines and the respective processing plants are often located several hundred miles apart so that slurry transport through pipelines often represents the only economically viable alternative. In order to keep the plant dimensions and costs as low as possible, solids transport is carried out in the form of concentrated suspensions that often reach the limits of the pumpability. As an undesirable side-effect these slurries often exhibit a non-Newtonian behavior including a flow limit.
To remedy this situation, a customized mixing technology is used in front of the pump stations and at the end of the pipeline to liquefy the suspensions in vessels up to10,000 m3. Applying this technique, highly concentrated solids suspensions can be transported trouble-free with relatively low investment and operating costs.
A yet further requirement in the downstream processing is to achieve a complete mixing of the various feed streams so that they can be transported smoothly and at a constant discharge rate. To provide a pumpable liquefaction of the suspension within the entire vessel and to overcome the flow limits – in particular close to the wall and on the surface – a sufficient shear stress must be guaranteed in the media during the vessel design phase. The specifically designed methodology that EKATO uses permits a fast and safe scale-up based on standardized tests with original slurry. At the same time, a numerical model (CFD) that is in a position to represent the different flow conditions for this application has been developed and validated: turbulent flow near the impeller, passing a transition phase, laminar close to the vessel wall and up to the flow limit. CFD (Computational Fluid Dynamics) permits quick parameter studies with which it is possible to examine different vessel geometries and thus minimize the need for experimental tests.