Pneumatic transport dense-phase

Fig. 15-4, except that transport is possible in the dense phase in which the pressure gradient, though quite large, is still usually not as large as for hydraulic transport. The entire curve shifts up and to the right as the solids mass flux increases. A comparison of typical operating conditions for dilute and dense phase pneumatic transport is shown in Table 15-1. 

Table 15-1 Dilute vs. Dense Phase Pneumatic Transport...

Wypych, P. W., and Arnold, P. C., The Use of Powder and Pipe Properties in the Prediction of Dense-Phase Pneumatic Transport Behavior, Pneumatech 2, Canterbury, England, Organised by the Powder Advisory Centre, London (1984)... 

T. Ramakrishnan, K. Ramakoteswara Rao, M.A. Parameswaran, S. Sivakumar, Experimental investigation of a dense-phase pneumatic transport system, Chem. Eng. Process. 32 (1993) 141-147. 

Blasco et al. [12] proposed two-dimensional mathematical model for the drying process of dense phase pneumatic conveying. However, heat and mass transfer were not considered and therefore their model may be used for dense phase pneumatic transport only. In their paper, both experimental and numerical predictions for axial and radial profiles for gas and solid velocity, axial profiles for solid concentration and pressure drop were presented. 

In a dense phase pneumatic transport system, why is it necessary to limit plug length in some cases Describe three ways in which the plug length might be limited in practice ... 

In his treatment of clouds of particles, Soo (1967) considered most systems having intimate contact all the way from packed beds through fluidization to dense-phase pneumatic transport. His analysis of clouds by individual particle bombardments between themselves and the confining wall relies on impact mechanics. This approach has served as a basis for both heat and electrostatic charge transfer in particulate systems. The elasticity of the particles and contact times became vital parameters in the analysis. 

S. Laouar and Y. Molodtsof, Ejqjerimental characterization of the pressure drop in dense phase pneumatic transport at very low velocity. Powder Technology 95, pp. 165-173 (1998). 

Fluidized Bed Tests. These tests have direct relevance to all applications where particles are subjected to conditions of fluidization. Some authors believe that these tests can also to some extent simulate the stress of pneumatic transport. Coppingeretal. (1992) found at least a good correlation with the attrition resistance in dense-phase pneumatic conveying when they tested various powders in a slugging fluidized bed. 

Knowlton has cautioned on the difference between small diameter and large diameter systems for pressure losses. The difference between these systems is especially apparent for dense phase flow where recirculation occurs and wall friction differs considerably. Li and Kwauk (1989, 1989) have also studied the dense phase vertical transport in their analysis and approach to recirculating fluid beds. Li and Kwauk s analysis included the dynamics of a vertical pneumatic moving bed upward transport using the basic solid mechanics formulation. Some noncircular geometries were treated including experimental verification. The flows have been characterized into packed and transition flows. Accurate prediction of the discharge rates from these systems has been obtained. 

Viscosity. Dense phase solid-gas mixtures may be required to flow in transfer line catalytic crackers, between reactors and regenerators and to circulate in dryers such as Figures 9.13(e), (f). In dilute phase pneumatic transport the effective viscosity is nearly that of the fluid, but that of dense phase mixtures is very much... 

The spouted beds (SBs) can also be used to dry polymer beads. It is an efficient solid-gas contactor. In the conventional SB there is dilute-phase pneumatic transport of particles entrained by the spouting jet in the central core region and dense-phase downward motion of the particles along the... 

Knowlton and Hirsan (1978) established that the best position for the injection of the aeration gas is in the vertical leg (sometimes called the downcomer or standpipe) about 1.5 pipe diameters above the centreline of the horizontal leg. This is because gas is needed to reduce friction between the particles and the pip>e at the inside comer of the elbow, and if the gas is injected on or below the centreline, less is available to perform this function. This gas then streams along the top of the bed of powder lying in the horizontal leg carrying particles with it in a mode of transport that resembles dense phase pneumatic conve5ung. As the aeration gas flow is increased the depth of the flowing stream of powder increases and the mass flow rate of solids increases. Eventually the whole depth of powder may become active with the solids velocity increasing from bottom to top of the horizontal leg. 

As the gas velocity is increased beyond 17 a point is reached at which the particles are transported out of the bed altogether either in dense-phase or in dilute-phase pneumatic transport the velocity at which this occurs is called the transport velocity, C/tr- In order to maintain a constant inventory of particles in the bed at velocities in excess of 7tr it is necessary to recycle them via external cyclones and a standpipe, a geometry known as a circulating fluidized bed. A comprehensive review of the literature on circulating systems has been given by Grace et al. (1997), and the subject is also dealt with in Chapter 19 of this handbook. 

K. Konrad, D. Harrison, R.M. Nedderman and J.F. Davidson, Prediction of the pressure drop for horizontal dense phase pneumatic conveying of particles, Proc. 5th Int. Conf. on Pneumatic Transport of Solids in Pipes, Pneumotransport 5, pp.225-244 (1980). 

One major difference between pneumatic transport and hydraulic transport is that the gas-solid interaction for pneumatic transport is generally much smaller than the particle-particle and particle-wall interaction. There are two primary modes of pneumatic transport dense phase and dilute phase. In the former, the transport occurs below the saltation velocity (which is roughly equivalent to the minimum deposit velocity) in plug flow, dune flow, or sliding bed flow. Dilute phase transport occurs above the saltation velocity in suspended flow. The saltation velocity is not the same as the entrainment or pickup velocity, however, which is approximately 50% greater than the saltation velocity. The pressure gradient-velocity relationship is similar to the one for hydraulic transport, as shown in... 

The usual prescription for controlling triboelectrification in pneumatic transport is to limit the flow rate, but this solution conflicts with the tendency to increase plant production levels. One alternate proposal for the control of tribocharging is to exploit the so-called dense-phase transport mode (G. Butters, 1985) however, there seems to be some dispute about the efficacy ofthis scheme (Konrad, 1986). 

Although the phenomena are not clearcut, partial settling out of solids from the gas stream and other instabilities may develop below certain linear velocities of the gas called choking velocities. Normal pneumatic transport of solids accordingly is conducted above such a calculated rate by a factor of 2 or more because the best correlations are not more accurate. Above choking velocities the process is called dilute phase transport and, below, dense phase transport. 

Pneumatic conveying systems can be classified on the basis of the angle of inclination of pipelines, operational modes (i.e., negative- or positive-pressure operation), and flow characteristics (i.e., dilute or dense phase transport steady or unsteady transport). A practical pneumatic conveying system is often composed of several vertical, horizontal, and inclined pipelines. Multiple flow regimes may coexist in a given operational system. 

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