Density aerated bulk

The starting point in bag-size determination is the weight or volume of product to be packaged and its bulk density (aerated and settled). 

The physical properties of spray-dried materials are subject to considerable variation, depending on the direction of flow of the inlet gas and its temperature, the degree and uniformity of atomization, the solids content of the feed, the temperature of the feed, and the degree of aeration of the feed. The properties of the product usually of greatest interest are (1) particle size, (2) bulk density, and (3) dustiness. The particle size is a function of atomizer-operating conditions and also of the solids content, liquid viscosity, liquid density, and feed rate. In general, particle size increases with solids content, viscosity, density, and feed rate. 

In some cases, especially for very fine powders, researchers have looked at other properties to explain or classify product behavior. For example, Gel dart et al. (1984) have found that the ratio of tapped to aerated bulk density provides a good indication of the likely fluidization characteristics of fine and cohesive powders. 

Assuming a catalyst density at flowing conditions in the standpipe of about 90% of the catalyst bulk density, the amount of excess gas above minimum fluidization that is entrained with the catalyst into the standpipe may be calculated. Sufficient aeration should be added to sustain minimum fluidization along the length of the standpipe. 

A variety of feeding arrangements may be employed to ensure that particulate matter of appropriate bulk density is presented to the nip of the rolls. For smoothly flowing materials gravity feed may be used with a control device to adjust feed rate (Fig. 5.4). For fine powders which tend to bridge or stick and are of low bulk density, some form of forced feed (such as the tapered screw feeder shown in Fig. 5.5) must be used to de-aerate, pre-corn-... 

Ft is doubtful whether XRD can distinguish a mixture of tobermorite and C-S-H from a uniform material of intermediate crystallinity the situation may lie between these extremes (A30). This question is discussed further in Section 11.7.4. Crystallization is probably favoured by low bulk density its extent is apparently minimal in calcium silicate bricks (P49), but considerable in aerated concretes (A30). In cement-silica materials, substantially all the AljOj appears to enter the C-S-H, which as its Ca/Si ratio decreases can accommodate increasing amounts of tetrahedrally coordinated aluminium (S70). NMR results (K34) support an early conclusion (K62) that 1.1-nm tobermorite, too, can accommodate aluminium in tetrahedral sites. Small amounts of hydrogarnet have sometimes been detected, especially in products made from raw materials high in AljOj, such as pfa or slag. Minor amounts of tricalcium silicate hydrate (jaffeite C, S2H,) have sometimes been detected (A29,K61). 

Bulk density. This is the weight per unit of volume of a quantity of solid particles, usually expressed in kilograms per cubic meter (pounds per cubic foot). It is not a constant and can be decreased by aeration and increased by vibration or mechanical packing. 

High ratios of gas (or vapor) to catalyst volumes, and velocities of 15 to 40 ft./second, are used in carrier lines in order to maintain the catalyst in dilute suspension and to prevent accumulation of stagnant catalyst at any point (53). The net flow rates of catalyst and vapors in the carrier lines typically correspond to an aerated bulk density of the order of 5 Ib./cu. ft. (68). However, the actual density is about twice the calculated value because of slip factor (105). 

Wheels with radial vanes have one important drawback, i.e., their capacity for pumping large amounts of air through the wheel. This so-called air pumping effect causes unwanted product aeration, resulting in powders of low bulk density for some sensitive spray dried products. 

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