Solids, bulk flow properties

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 applied to a feeder by the bulk solid must be minimized. Accuracy and control over discharge rate are critical as well. Knowledge of the bulk solid s flow properties is essential. 

All these problems are the result of the interaction of solids, solids flow properties, and the design of the equipment. Johanson (2002) has identified seven indices that relate to the bulk flow properties of solids. 

As with bulk density, permeability is a function ofpacking voidage and its uniformity, and in practice, it is best measured. It can vary substantially with previous compaction of the sample. An example is the change in bulk density—and therefore interstitial voidage—that occurs with a material as it moves through a hopper. By applying a load to the upper surface of the bed, permeability may be also determined as a function of solids consolidation pressure (see Bulk Flow Properties ). Permeability is a decreasing function of applied solids pressure, and bulk density is often written in log form, or... 

To design reliable devices for the handling of bulk solids and to characterize bulk solids the flow properties of these bulk solids have to be known. For their measurement a great number of shear and other testers are available. The paper gives a review of existing testers and demonstrates which tester should and can be used for which application. 

Sorption/desorption is the key property for estimating the mobility of organic pollutants in solid phases. There is a real need to predict such mobility at different aqueous-solid phase interfaces. Solid phase sorption influences the extent of pollutant volatilization from the solid phase surface, its lateral or vertical transport, and biotic or abiotic processes (e.g., biodegradation, bioavailability, hydrolysis, and photolysis). For instance, transport through a soil phase includes several processes such as bulk flow, dispersive flow, diffusion through macropores, and molecular diffusion. The transport rate of an organic pollutant depends mainly on the partitioning between the vapor, liquid, and solid phase of an aqueous-solid phase system. 

Ciystallization from solution is an important separation and purification process in a wide variety of industries. These range from basic materials such as sucrose, sodium chloride and fertilizer chemicals to pharmaceuticals, catalysts and specialty chemicals. The major purpose of crystallization processes is the production of a pure product. In practice however, a number of additional product specifications are often made. They may include such properties as the ciystd size distribution (or average size), bulk density, filterability, slurry viscosity, and dry solids flow properties. These properties depend on the crystal size distribution and crystal shape. The goal of crystallization research therefore, is to develop theories and techniques to allow control of purity, size distribution and shape of crystals. 

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. 

Compressibility. The bulk density of a solid 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 are not sufficient. Bulk solids exhibit a range of densities that vary as a function of consolidating pressure. This range of densities, called the compressibility of the solid, can often be expressed on a log—log plot as a line or relationship. 

Johanson, J. R. Predicting segregation of bimodal particle mixtures using the flow properties of bulk solids. Pharm Tech 46-57 (May 1996). 

Schwedes J, Schulze D. 1990. Measurement of flow properties of bulk solids. Powder Technol. 61 59-68. 

These equipment items are used to store feed and, in some cases, process bulk solids. The design of economical hopper systems is dependent on the physical, chemical, and flow properties of the materials being stored. It is essential to provide bin, hopper, and feeder designs to enhance the flow of the material from the hopper and to minimize potential problems. 

In actual situations several processes occur simultaneously. The details of any particular dispersion processes are also affected by the viscosity of each phase, the shear in the system, the interfacial energy, the pressure of solid particles, and dissolved substances. In nonuniform shear flow (e.g., tubular Poiseuille flow), for example, droplet breakup can be related to the bulk rheological properties of the dispersed and continuous phases and the critical Weber number (We ) as shown in Figure 3 (3). The We is a dimensional group defined by... 

Factors that Affect Flaw Properties An overview of the primary factors that affect the bulk solid flow properties. 

In addition, the flowability of a bulk solid is a function of the bulk solids flow properties and the design parameters of the handling equipment. For example, poor flowing bulk solids can be handled reliably in properly designed equipment, and good flowing bulk solids may develop flow problems in improperly designed equipment. As such, a more accurate definition of flowability is the ability of powder to flow in a desired manner in a specific piece of equipment. 

Prior to beginning the discussion of the measurement of flow properties, it is beneficial to review how bulk solids are different than liquids. Since one of our primary concerns with bulk solids handling is flow, a term that is commonly associated with liquids, it is often assumed that the principles of fluid mechanics may be used to describe the behavior of... 

Therefore, when Jenike developed his methods to mathematically model the flow of bulk solids, he concluded that a bulk solid must be modeled as a plastic, and not a visco-elastic, continuum of solid particles (1). This approach included the postulation of a flow-no-flow criterion that states the bulk solid would flow from a bin when the stresses applied to the bulk solid exceed the strength of the bulk solid. The terms stress and strength are further discussed in this section on cohesive strength tests below. The flow properties test methods discussed are used to obtain the equipment parameters required to provide consistent, reliable flow. 

There are instances where a qualitative test for comparative or quality control (QC) purposes may be desired and the quantitative test methods used for equipment design or analysis purposes described in the preceding sections are not essential for the flow concerns being assessed. These non-scalable, qualitative tests may be used to measure cenain attributes/characteristics of the bulk solid within a pre-defined range. These attributes may include chemical composition, particle size, color, moisture, and often, flow properties. 

The key to implementing any corrective actions designed to reduce adverse flow-effects will be using a mathematical two-phase flow analysis to assess the effects on the bulks solids stresses and interstitial gas pressure. This analysis would need to use inputs such as key flow properties (permeability, compressibility) and equipment/process parameters (tableting rate, bin/hopper geometry, and gas pressure gradients) to assess the effect of the potential corrective actions outlined above. 

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