Dense phase transport

As pointed out in the introduction to this chapter, there are many different definitions of dense phase transport and of the transition point between dilute phase and dense phase transport. For the purpose of this section dense phase transport is described as the condition in which solids are conveyed such that they are not entirely suspended in the gas. Thus, the transition point between dilute and dense phase transport is saltation for horizontal transport and choking for vertical transport. 

The continuous dense phase flow pattern, in which the solids occupy the entire pipe, is virtually extrusion. Transport in this form requires very high gas pressures and is limited to short straight pipe lengths and granular materials (which have a high permeability). 

Discontinuous dense phase flow can be divided into three fairly distinct flow patterns discrete plug flow in which discrete plugs of solids occupy the full pipe cross-section dune flow in which a layer of solids settled at the bottom of 

Saltating flow is encountered at gas velocities just below the saltation velocity. Particles are conveyed in suspension above a layer of settled solids. Particles may be deposited and re-entrained from this layer. As the gas velocity is decreased the thickness of the layer of settled solids increases and eventually we have dune flow. 

It should be noted, first, that not all powders exhibit all these flow patterns and, secondly, that within any transport line it is possible to encounter more than one regime. 

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For dense phase transport in vertical pipes of small diameter, see... 

There are three basic modes of transport which are employed. The first, and most common, is termed dilute phase or lean phase transport in which the volume fraction of solids in this suspension does not exceed about 0.05 and a high proportion of the particles spend most of their time in suspension. The second is transport which takes place largely in the form of a moving bed in which the solids volume fraction may be as high as 0.6 this is relevant only for horizontal or slightly inclined pipelines. The third form is dense phase transport in which fairly close packed slugs of particles, with volume fractions of up to... 

Although lots of information is available on dilute phase transport that is useful for designing such systems, transport in the dense phase is much more difficult and more sensitive to detailed properties of the specific solids. Thus, because operating experimental data on the particular materials of interest are usually needed for dense phase transport, we will limit our treatment here to the dilute phase. 

Figure 16. Molerus dense phase transport system.

A starting model can be selected depending on the type of dense phase transport. Modeling a homogeneous dense phase would use the same approach as dilute phase with a new frictional term. This approach would have two contributions due to the gas alone and the linear combination with the solids contribution. 

Tsuji et al. (1990) have modeled the flow of plastic pellets in the plug mode with discrete dynamics following the behavior of each particle. The use of a dash pot/spring arrangement to account for the friction was employed. Their results show remarkable agreement with the actual behavior of real systems. Figure 28 shows these flow patterns. Using models to account for turbulent gas-solid mixtures, Sinclair (1994) has developed a technique that could have promise for the dense phase transport. 

The use of recirculating fluid beds has caused considerable interest in dense phase vertical conveying. These units are indeed dense phase transport systems with a significant amount of recirculation taking place. 

Borzone, L. A., and Klinzing, G. E., Dense Phase Transport Vertical Plug Flow, Powder Tech., 53 273 (1987)... 

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. 

The relatively sparse data on dense phase transport is described by Klinzing (1981) and Teo and Leung (1984). Here only the more important category of dilute phase transport will be treated. 

For dense phase transport in vertical pipes of small diameter, see Sandy, Daubert, and Jones Chem. Eng. Prog., 66, Symp. Ser., 105, 133—142 [1970]). 

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. 

Hydraulic design aims at the realization of an intensive heat and mass transfer. For two-phase gas-liquid or gas-solid systems, the choice is between different regimes, such as dispersed bubbly flow, slug flow, churn-turbulent flow, dense-phase transport, dilute-phase transport, etc. 

Kwauk, M., Particulate Fluidization in Chemical Metallurgy, Sdentia Sinica 16,407 (1973). Kwauk, M., Dense-Phase Transport Downcomers (in Chinese, unpublished), Inst. Chem. Metall., 1974-1-9, rev. 1977-2-13 (1974a). 

In this chapter the choking and saltation velocities will be used to mark the boundaries between dilute phase transport and dense phase transport in vertical and horizontal pipelines, respectively. These terms are defined below in considering the relationships between gas velocity, solids mass flow rate and pressure drop per unit length of transport line in both horizontal and vertical transport. 

The main advantages of dense phase transport arise from the low gas requirements and low solids velocities. Low gas volume requirements generally mean low energy requirements per kilogram of product conveyed, and also mean that smaller pipelines and recovery and solids-gas separation are required. Indeed in some cases, since the solids are not suspended in the transport gas, it may be possible to operate without a filter at the receiving end of the pipeline. Low solids velocities means that abrasive and friable materials may be conveyed without major pipeline erosion or product degradation. 

Whatever the mechanism used to tackle the plug problem, all commercial dense phase transport systems employ a blow tank which may be with fluidizing element (Figure 8.13) or without (Figure 8.14). 

Figure 8.13 Dense phase transport blow tank with fluidizing element...

The blow tank is automatically taken through repeated cycles of filling, pressurizing and discharging. Since one third of the cycle time is used for filling the blow tank, a system required to give a mean delivery rate of 20 t/h must be able to deliver a peak rate of over 30 t/h. Dense phase transport is thus a batch operation because of the high pressures involved, whereas dilute phase transport can be continuous because of the relatively low pressures and the use of rotary valves. The dense phase system can be made to operate in semi-continuous mode by the use of two blow tanks in parallel. 

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