FREE-FLOWING

Tunnel dryers are shown in Fig. 3.15a. Wet material on trays or a conveyor belt is passed through a tunnel, and drying takes place by hot air. The airflow can be countercurrent, cocurrent, or a mixture of both. This method is usually used when the product is not free flowing. 

The computation of F is relatively straightforward. We simply consider the free flow of particles into and out of the region in time t. An expression for this flow in the v-direction, for example, can be obtained by considering two thin layers of size v 6t5r Sr that contain particles that move into or out of a cell with its centre at (x,y,z) in time 51 (see figure A3.1.6. ... 

Most of the remarks above refer to unconfined or free flows. Many industrial appHcations involve the use of confined jets. It is customary to consider a jet confined when the ratio of the confinement radius to the source radius Hes in the range 4—100. Below a ratio of 2, the jet does not develop its similarity profile before striking the wall, whereas above a ratio of 100 the jet itself may usually be considered free. Under certain conditions, flow in confined jets is accompanied by the existence of a recirculation 2one which significantly affects the jet behavior by returning material upstream (9). This recirculation can be particularly important in combustion processes. 

Fluidization refers to the condition in which soHd materials are given free-flowing, fluid-like behavior (29). As a gas is passed upward through a bed of sohd particles, the flow of gas produces forces which tend to separate the particles from one another. At low gas flows, the particles remain in contact with other soflds and tend to resist movement. This condition is referred to as a fixed bed. As the gas flow is increased, a point is reached at which the forces on the particles are just sufficient to cause separation. The bed then becomes fluidized. The gas cushion between the soflds allows the particles to move freely, giving the bed a Hquid-like characteristic. 

Lithium Acetylide. Lithium acetyhde—ethylenediamine complex [50475-76-8], LiCM7H -112X01120112X112, is obtained as colodess-to-light-tan, free-flowing crystals from the reaction of /V-lithoethylenediamine and acetylene in an appropriate solvent (131). The complex decomposes slowly above 40°O to lithium carbide and ethylenediamine. Lithium acetyhde—ethylenediamine is very soluble in primary amines, ethylenediamine, and dimethyl sulfoxide. It is slightly soluble in ether, THF, and secondary and tertiary amines, and is insoluble in hydrocarbons. 

USP-grade anhydrous magnesium carbonate is used as a flavor impression intensification vehicle in the processed food industry (see Flavors and spices). Basic magnesium carbonates are used as free flowing agents in the manufacture of table salt, as a hulking agent in powder and tablet pharmaceutical formulations, as an antacid, and in a variety of personal care products (see Pharmaceuticals). 

Flow. The free flow of a powder through an orifice depends on the orifice which is standardized for the testing of the powder (14). Flow, therefore, depends not only on friction between powder particles, but also on friction between the particles and the wall of the orifice. Flow is usually expressed by the time necessary for a specific amount of powder (usually 50 g) to flow through the orifice. 

The free flow of a powder is necessary for automatically filled compacting dies. Powders having low flow rates need vibratory filling in order to overcome friction. Powders that do not flow at all can be used only for manual filling of the die cavity. 

Determination of Apparent Density ofFree-FlowingMetal Powders Using the Hall Apparatus MPIF Standard No. 04, and Determination of Apparent Density of Non-Free Flowing Metal Powders Using the Camej Apparatus, No. 4, Metal Powder Industries Federation, Princeton, N.J., 1992. 

Complex Coacervation. This process occurs ia aqueous media and is used primarily to encapsulate water-iminiscible Hquids or water-iasoluble soHds (7). In the complex coacervation of gelatin with gum arabic (Eig. 2), a water-iasoluble core material is dispersed to a desired drop size ia a warm gelatin solution. After gum arabic and water are added to this emulsion, pH of the aqueous phase is typically adjusted to pH 4.0—4.5. This causes a Hquid complex coacervate of gelatin, gum arabic, and water to form. When the coacervate adsorbs on the surface of the core material, a Hquid complex coacervate film surrounds the dispersed core material thereby forming embryo microcapsules. The system is cooled, often below 10°C, ia order to gel the Hquid coacervate sheU. Glutaraldehyde is added and allowed to chemically cross-link the capsule sheU. After treatment with glutaraldehyde, the capsules are either coated onto a substrate or dried to a free-flow powder. 

Hydroxjiamine is used as a substitute for the ferrous sulfamate (26). These systems are called salt-free flow sheets. The main purpose is to ease the problems associated with the processing and storage of the Hquid waste streams (27). Another approach is to use an electropulse column to electrolyticaHy produce to reduce Pu to Pu on a continuous basis (28,29). The half reactions for the flow sheets are... 

When petroleum occurs in a reservoir that allows the cmde material to be recovered by pumping operations as a free-flowing dark-to-light colored hquid, it is often referred to as conventional petroleum. In some oil fields, the downhole pressure is sufficient for recovery without the need for pumping. Heavy oil differs from conventional petroleum in that its flow properties are reduced and it is much more difficult to recover from the subsurface reservoir. These materials have a much higher viscosity and lower API (American Petroleum Institute) gravity than conventional petroleum, and primary recovery of these petroleum types usually requires thermal stimulation of the reservoir. 

Anhydrous sodium tripolyphosphate is slow to hydrate in contact with the atmosphere under normal ambient conditions and generally remains free-flowing. If the relative humidity is below a critical relative humidity, which is different for both anhydrous forms of STP and dependent on temperature, hydration does not take place. For prolonged storage at room temperature, relative humidities above ca 60% in the air result in water absorption. For shorter periods, high levels of humidity can be tolerated. However, even at higher humidities, the amount of water absorbed is small. The heats evolved from vapor hydration of STP-I and -II have been estimated at 343 and 334 kj /mol (82.0 and 79.9 kcal/mol), respectively (25). 

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