Bulk sohds

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

Several stabilizers are useful in minimizing oxidative degradation during thermoplastic processing or in the bulk soHd. Phenothiazine, hindered phenohc antioxidants such as butylated hydroxytoluene, butylatedhydroxyanisole, and secondary aromatic amines in concentrations of 0.01—0.5% based on the weight of polymer, are effective. 

Bulk sohds do not always discharge rehably. Unrehable flow, which can occur with some frequency, can be expensive in terms of inefficient processes, wasted product, and operational comphcations. Predictable flow is often impeded by the formation of an arch or rathole, or fine powders may flood uncontroUably. 

Flooding. When a stable rathole forms in a bin and fresh material is added, or when material falls into the channel from above, a flood can occur if the bulk sohd is a fine powder. As the powder falls into the channel, it becomes entrained in the air in the channel and becomes fluidized (aerated). When this fluidized material reaches the outlet, it is likely to flood from the bin, because most feeders are designed to handle sohds, not fluids (see Eluidization). Fimited Discharge Kate. Bulk sohds, especially fine powders, sometimes flow at a rate lower than required for a process. This flow rate limitation is often a function of the material s air or gas permeabihty. Simply increasing the speed of the feeder does not solve the problem. There is a limit to how fast material... 

Segregation. The problem of segregation occurs when a bulk soHd composed of different particle sizes or densities separates. The result can be quite serious if uniform density or mixed material is required for a process. 

Funnel flow bins are only suitable for bulk soHds that are coarse, free flowing, and do not degrade, and for use when segregation is not important. For such materials, the principal benefits of funnel flow bins are reduced headroom and lower initial cost for the bin (excluding feeders or dischargers). Examples of funnel flow bins are shown in Figure 2. 

Wall Friction Angles. Wall friction values, important when characterizing the dow properties of a bulk soHd, are expressed as the wall friction angle or coefficient of sliding friction. The lower the friction, the less steep the hopper walls need to be to achieve mass dow. 

The following variables can affect wall friction values of a bulk soHd. (/) Pressure as the pressure acting normal to the wall increases, the coefficient of sliding friction often decreases. (2) Moisture content as moisture increases, many bulk soHds become more frictional. (3) Particle size and shape typically, fine materials are somewhat more frictional than coarse materials. Angular particles tend to dig into a wall surface, thereby creating more friction. (4) Temperature for many materials, higher temperatures cause particles to become more frictional. (5) Time of storage at rest if allowed to remain in contact with a wall surface, many soHds experience an increase in friction between the particles and the wall surface. (6) Wall surface smoother wall surfaces are typically less frictional. Corrosion of the surface obviously can affect the abiUty of the material to sHde on it. 

In order to characterize this bonding tendency, the flow function of a material must be deterrnined. Data on flow function can be generated in a testing laboratory by measuring the cohesive strength of the bulk soHd as a function of consoHdation pressure appHed to it. Such strength is directly related to the abihty of the material to form arches and ratholes in bins and hoppers. 

A material s flow function is usually measured on the same tester as the wall friction angle, although the cell arrangement is somewhat different (Fig. 6). ConsoHdation values are easily controUed, and the cohesive strength of the bulk soHd is determined by measuring interparticle shear stresses while some predeterrnined normal pressure is being appHed. 

Compressibility. The bulk density of a soHd is an essential value used in the analysis of its flow properties, such as when calculating mass flow hopper angles, opening sizes, bin loads, etc. Loose and/or packed density values ate not sufficient. Bulk soHds exhibit a range of densities that vary as a function of consoHdating pressure. This range of densities, called the compressibiHty of the soHd, can often be expressed on a log—log plot as a line or relationship. 

The foUowing variables can affect a material s bulk density. (/) Moisture higher moisture content often makes a material mote compressible. (2) Particle size and shape often, the finer the bulk soHd, the mote compressible it is. The shape of the particles can affect how they fit together and thein tendency to break while being compacted. (3) Temperature some materials become mote compressible as thein temperature increases. This could be due, for example, to softening of the particles. (4) Particle elasticity elastic materials tend to deform significantly when they ate compressed. 

To be consistent with a mass flow pattern in the bin above it, a feeder must be designed to maintain uniform flow across the entire cross-sectional area of the hopper outlet. In addition, the loads appHed to a feeder by the bulk soHd must be minimised. Accuracy and control over discharge rate ate critical as well. Knowledge of the bulk soHd s flow properties is essential. 

The basis of all bulk conveyor engineering is the precise definition and accurate classification of materials according to individual characteristics under a specific combination of handling conditions (1). Since the late 1960s there has been an extraordinary growth in research into the fundamental properties and behavior of particulate soHds. However, as of this writing, it is not possible to predict the handling behavior of a bulk soHds material relevant to conditions in a specific conveyor, merely on the basis of the discrete particle properties. 

The bulk soHds conveying industry is interwoven with all aspects of the chemical industry, from mining of raw materials to in-process handling and to final product dehvery. The 1987 value of U.S. product shipments of bulk conveyors and parts was 2.11 biUion according to CEMA. Lists of conveyor manufacturers in the United States (49—51), and worldwide (52) are available. 

G. D. Dumbaugh, "A Comparative Review of Vibratory Drives for Bulk SoHds Handling Systems," Proceedings of the 10th Powder andBulk Solids Conference, Chicago, May 1985, pp. 425—470. 

M. V. Arastoopour, M. V. Modi, D. V. Punwani, and A. T. Talwalker,M Keview of Design Equations for Dilute Phase Gas-Solids Hori ntal Conveying Systems for Coal and Belated Material, Powder and Bulk SoHds Conference, Philadelphia, 1979. 

Permeability Bulk sohd permeability is important in the iron and steel industiy where gas-solid reactions occur in the sinter plant and blast furnace. It also strongly influences compac tion processes where entrapped gas can impede compaction, and solids-handling equipment where restricted gas flow can impede product flowabihty. The permeabihty of a granular bed is inferred from measured pressure drop under controlled gas-flow conditions. 

Originally confined to the shipment of crude raw materials and fuels, the term transportation of bulk sohds now apphes also to manufactured produc ts, which often become raw materials for other industries. In recent years, increasing tonnages of highly processed, finished chemical products have moved to customers in large bulk units. A useful definition of a bulk shipment is any unit greater than 2000 kg (4000 lb) or 2 m (70 fF). The containers available range from small portable hoppers of 2-m (70-fF) capacity to railroad cars of 255-m (9000-fF) capacity. 

Johanson, J.R. Theory of bulk solids flow a historical perspective. Int. J. Bulk Sohds Storage in Silos 1987, 3 (1), 1-15. 

Behres, M. Klasen, C.-J. Schulze, D. Development of a shear cell for measuring the wah friction of bulk sohds with a ring shear tester. Powder Handhng Process 1998, 10 (4), 405 9. 

Johanson, J.R. The johanson indicizer system vs. the jenike shear tester. Bulk Sohds Handhng 1992,2, 237-240. 

Guidelines for Safe Handhng of Powders and Bulk Sohds, G-95, 2004. 

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