Bulk solid flow types

An interesting question that arises is what happens when a thick adsorbed film (such as reported at for various liquids on glass [144] and for water on pyrolytic carbon [135]) is layered over with bulk liquid. That is, if the solid is immersed in the liquid adsorbate, is the same distinct and relatively thick interfacial film still present, forming some kind of discontinuity or interface with bulk liquid, or is there now a smooth gradation in properties from the surface to the bulk region This type of question seems not to have been studied, although the answer should be of importance in fluid flow problems and in formulating better models for adsorption phenomena from solution (see Section XI-1). 

The classification system introduced by Carr [29,30] was used to evaluate the flow properties of the sorbitol powders. In Carr s system, a flowable powder is defined as free flowing and will tend to flow steadily and consistently. This is to be contrasted with a floodable powder, which will exhibit an unstable, discontinuous, and gushing type of flow. The parameters in Carr s system include the angle of repose, angle of spatula, compressibility, cohesion, and dispersibility. Based on these parameters, flowability and floodability indices are calculated to determine the handling properties of bulk solids. 

The first and most important single feature of a flow pattern in a container is whether slip takes place on all contact surfaces between the contents and the container walls during a fiilly developed discharge condition. If it does, it is termed mass flow by virtue of the movement of the entire mass (see Fig. 5.1). If it does not mass flow, it is often termed fimnel flow after the characteristic shape this type of flow channel takes in some cases (see Fig. 5.2), or cote flow . The first two definitions were laid down by Jenike in 1960, in his fundamental work on the gravity flow of bulk solids. Arnold Redler, in his UK and USA patents of 1920 relating to chain-type extractors, had previously defined the latter flow mode, and his term core flow is common parlance in the UK. This form is sometimes referred to as internal flow . 

Most of the available models where extraction is considered as purely a physical phenomena, as it should be, are based on diirercntial mass balances. Local liquid/liquid and liquid/solid equilibrium, intraparticle mass transport, the transport of the extracted material from the particle surface to the bulk fluid, and the bulk fluid flow in a bed packed with particles are covered in a great number of these models. More specifically the shrinking core, and the desorptioii-dissoiiition-diflusion type models are all based on these fundamental concepts. However, some models, though rather few in luiniber have suggested other routes. 

As was mentioned earlier, the bed motion is a function of the drum dimensionless rotational speed, n. Any theoretical development has to take into account the type of bed motion taking place in the drum. For the region 0.01 < n < 0.1, where bed rolling occurs, Saeman [42], Vahl and Kingma [43], and Kramers and Croockewit [44] arrived at the general equation for the bulk volumetric flow of solids through any cross section of the rotating drum. The equation is... 

A feeder is a device used to control the flow of bulk solids from a bin. Any feeder must be selected to suit a particular bulk solid and the range of feed rates required. It is particularly important to design the hopper and feeder as an integral unit, in order to ensure that the flow from the hopper is fully developed with uniform draw of material from the entire hopper outlet. There are several types of feeders but the most common are the belt or apron feeder, the screw feeder, the vibratory feeder, and the star feeder. Careful considerations, such as those described above, should be taken in selecting a feeder for a particular application. 

These fundamental aspects of the flow of granular material or bulk solids, similar to the flow in rotary kilns, have been reviewed by Savage (1979). Many theories have been considered in the effort to establish appropriate constitutive relations for such flows. As mentioned earlier, these theories extend from traditional soil mechanics to all types of viscous flows. The most important conclusions that can be drawn from these works to describe granular flow include the following ... 

For measuring the flow of solids (particulate bulk materials) various types of... 

Compressibilities in excess of 20% tend to create particle bridges in hoppers while powders with a compressibility percentage greater than 40% are difficult to discharge (Table 1.6). At present the Hausner ratio is used more widely than compressibility, although Carr (1965c) used his parameter extensively to classify various types of powder into categories of excellent or very poor flow. The Carr approach to flow has now been subsumed into a standard test method for bulk solid flowability (ASTM D 6393-99 1999). 

The instrument and technique which may be chosen is also governed by the type of bulk solid (which may range from free flowing to very cohesive) undergoing investigation and also the frequency of information needed, cost of the device, time and skill allocated for the test together with the availability of equipment for the measurement of both particle and bulk solids characteristics. 

Feeders are used to control the gravity flow of bulk solids from bins and stockpiles. While there are several types of feeders commonly used, their selection depends on the particular bulk solid and feed rate required. It is important that feeders be used in conjunction with mass-flow hoppers to ensure both reliable flow and good control over the feeder loads and drive powers. 

When a bulk material is transported through a horizontal pipe in the moving-bed mode of dense phase flow, two layers may be observed. The upper layer is a gas-solids mixture with a low solids concentration, while the lower layer is a gas-solids mixture with a high solids concentration. Similar types of stratified flow have been observed in liquid-solids flows. 

Figure 11.3.1 shows typical pressure profiles from the paper. For the case where flow types 1, 2 and 3 prevail, the pressure profiles in the two lower sections (where there is negligible rotation, and on the average a uniform distribution of solids over the cross section) represent a hydrostatic pressure increase, which more or less agrees with what one would calculate from the bulk density of the solids and gas in the section. On the other hand, when a packed bed develops in the lower part of the dipleg, the material is supported off the side wall, like in a silo, and the pressure profile is no longer described by a hydrostatic type of linear pressure increase with depth. 

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