Particle size and density

The control of processes involving the treatment of solids generally requires means for careful samphng and analysis of solids and slurries at various points in an operation. Unlike liquids, particulate solids are not homogeneous. The composition of individual particles will vaiy with particle size and particle density. It follows that care must be... 

A number of models have been developed to attempt to predict respiratory deposition, especially in the lower airways—the alveolar region. Experimental studies have tended to confirm the validity of the models, recognizing that there is much individual variation and thus a great spread in the results. The results have been clear enough, however, to indicate that till else being equal, deposition of particles in the lungs is greatly influenced by particle size and particle density. 

When particles to be mixed have the same physical properties, then a random mixture will be obtained if the particles are mixed for long enough. However, if the properties are different (in particular, particle size and particle density), then the particles may demix or segregate where particles with same physical property separate out and collect in one part of the mixture. The major property difference causing segregation is difference in particle size, although differences in particle density can also cause segregation. 

Particle size and particle density measurements were made on partially burnt char particles which were collected in a cyclone separator at the exit of the reactor. Their burn-off was evaluated from a knowledge of char feedrate, gas flowrates and gas composition. The particle size of the collected material was determined by sieving particle density was derived from measuring the bulk density of a bed of particles in a manner described by Field ( ). 

SERS active structures can be prepared by a variety of chemical physical and electrochemical methods described in Sect. 4.1. The chemical preparation of colloidal nanoparticles is frequently used (Sect. 4.1.1). An interesting electrochemical preparation procedure is the so-called double-pulse technique. This method is an electrochemical tool for controlling the metal deposition with respect to particle size and particle density (Sect. 4.1.2). 

The general tendencies of the experimental results are as follows The apparent viscosity in an unfluidized bed is very high (perhaps infinite) but drops rapidly with increasing gas velocity. When particles are fully fluidized, the bed viscosity becomes rather independent of gas velocity and increases with increasing particle size and particle density i.e., it shows a gradual change from a displacement effect to a collisional momentum transfer. 

When designing a local exhaust ventilation system for a process that generates dust particles, it is important to consider the minimum air velocity. The minimum air velocity is the velocity required to prevent settling of dust particles in the air ducts. The minimum velocity is a function of dust particle size and particle density. Listed in the table below are the minimum air velocities recommended for the transport of various types of particulate contaminants. 

Particle Size and Particle Density. An introduction to the importance of particle size distribution for sludge has already been presented. 

The superficial fluid velocity at which the packed bed becomes a fluidized bed is known as the minimum fluidization velocity, Lfmf- This is also sometimes referred to as the velocity at incipient fluidization (incipient meaning beginning). Umi increases with particle size and particle density and is affected by fluid properties. It is possible to derive an expression for Lfmf by equating the expression for pressure loss in a fluidized bed [Equation (7.2)] with the expression for pressure loss across a packed bed. Thus recalling the Ergun equation [Equation (6.11)] ... 

Another approach to scale up is the use of simplified models with key parameters or overall coefficients found by experiments in large beds. For example. May (1959) used a large-scale cold reactor model during the scale-up of the fluid hydroforming process. This technique must be used with care. A large cold model may not directly simulate the hydrodynamics of a real process that operates at elevated pressure and temperature unless care is taken in the selection of flow rates, particle sizes, and particle densities for the cold tests. 

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