Transportation, density segregation

Transportation (by belt, rail, or truck) can initiate (due to movement of the coal) processes that result in size and density segregation. Thus, variations from one side of a conveyor belt to the other, from side-to-side, end-to-end, and top-to-bottom locations in individual cars or trucks, and between one location and another in a coal pile, must be anticipated (ASTM D-346 ASTM D-2234 ASTM D-4182 ASTM D-4702 ASTM D-4915 ASTM D-4916 ASTM D-6315 ASTM D-6518 ASTM D-6543 ISO 1988). Therefore, the challenge in sampling coal from a source or shipment is to collect a relatively small portion of the coal that accurately represents the composition of the coal. This requires that sample increments be collected such that no piece, regardless of position (or size) relative to the sampling position and implement, is collected or rejected selectively. Thus, the coal sample must be representative of the composition of the whole coal (i.e., coal in a pile or coal in a railcar or truck) as represented by the properties or quality of the sample. 

Thus the addition of an inert gas which does not intervene chemically in the transport reaction but adds to the density of the gas, reduces the segregation due to thermal diffusion. An example of this is the reduction of thermal separation in a mixture of H2 and H20 by the addition of Hg vapour (Dastur and Chipman, 1948). 

Transport properties of hydrated PFSA membranes strongly depend on nanophase-segregated morphology, water content, and state of water. In an operational fuel cell, these characteristics are indirectly determined by the humidity level of the reactant streams and Faradaic current densities generated in electrodes, as well as the transport properhes of catalyst layers, gas diffusion layers, and flow... 

CA 70, ll6782f(1969) [A stable, easily transportable mixt suitable as a mining expl or a flare proplnt consists of an intimate mixt of finely divided Mg and S in a 1.25— 1.33 1 Mg-S ratio. Densities of Mg S should be substantially identical to assure lack of segregation and, preferably, are bound together with water-sol silicates. 

Particle mixing is caused by the bubbles, partly be shear displacement or drift but also by the bulk transport of particles in the bubble wake. Bubbles may also cause segregation if there are different kinds of particles present. Unlike other kinds of mixers, segregation is insensitive to particle size difference but particularly sensitive to density difference. In a binary system of particles segregation increases approximately as particle density ratio to the power 5/2 but with particle size ratio only to the power 1/5 (11). This can cause problems in, for example, coal combustion where char has a markedly lower density than ash and also in some ore reduction processes using coke. 

For laminar flow, the characteristic time of the fluid phase Tf can be deflned as the ratio between a characteristic velocity Uf and a characteristic dimension L. For example, in the case of channel flows confined within two parallel plates, L can be taken equal to the distance between the plates, whereas Uf can be the friction velocity. Another common choice is to base this calculation on the viscous scale, by dividing the kinematic viscosity of the fluid phase by the friction velocity squared. For turbulent flow, Tf is usually assumed to be the Kolmogorov time scale in the fluid phase. The dusty-gas model can be applied only when the particle relaxation time tends to zero (i.e. Stp 1). Under these conditions, Eq. (5.105) yields fluid flow. This typically happens when particles are very small and/or the continuous phase is highly viscous and/or the disperse-to-primary-phase density ratio is very small. The dusty-gas model assumes that there is only one particle velocity field, which is identical to that of the fluid. With this approach, preferential accumulation and segregation effects are clearly not predicted since particles are transported as scalars in the continuous phase. If the system is very dilute (one-way coupling), the properties of the continuous phase (i.e. density and viscosity) are assumed to be equal to those of the fluid. If the solid-particle concentration starts to have an influence on the fluid phase (two-way coupling), a modified density and viscosity for the continuous phase are generally introduced in Eq. (4.92). 

The transportation of sludges and slurries in pipelines is advantageous, but poses more problems arising from high viscosity, nonhomogenity of the fluid system and the tendency of suspended materials to segregate and settle. The tendency to settle varies with the particular flow condition. Particle density, shape and size as well as size distribution, concentration and composition influence the settling characteristics. 

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