Particle size reduction equipment generally

Comminution, or particle size reduction of solids, is considerably different from that of the breakup of one liquid by dispersal as small droplets in another. Particle size reduction is generally achieved by one of four mechanisms (1) compression, (2) impact, (3) attrition and (4) cutting or shear. Equipment for particle size reduction or milling includes crushers (which operate by compression, e.g., crushing rolls), grinders (which operate principally by impact and attrition, although some compression may be involved, e.g.,... 

Figure 5-5X. Type R-500. Very high shear radial flow impeller for particle size reduction and uniform dispersion in liquids. By permission, Lightnin, (Formerly Mixing Equipment Co.) a unit of General Signal.

If the particle size of the biomass fuel or feedstock is not predetermined by its history, as is the case for sawdusts, nutshells, and a few other waste biomass materials, size reduction is usually carried out with one or more units that make up the front end of the total processing system. Many different kinds of machines are employed. Generally, the size of the feed is reduced by grinding, cutting, or impact mechanisms. Not all of the designs are suitable for biomass energy applications because the equipment is customized for certain uses or the cost of size reduction is excessive. Agricultural crops and woody biomass are also usually processed by different types of machines. A brief review of the basic types of machines that are or have been used for biomass follows to illustrate the variety of size-reduction equipment and their biomass applications. 

Breaking is the term applied to size operations on large material (say +75 mm) and crushing to particle size reduction below 75 mm the term grinding covers the size reduction of material to below about 6 mm. However, these terms are loosely employed. A general term for all equipment is size reduction equipment, and because the term comminution means size reduction, another general term for the equipment is comminution equipment. 

Particle strength Zones of high shear in equipment (e.g. centrifuges) will fracture brittle particles, leading to a broadening of the particle size distribution and (generally) a reduction in cake porosity. 

Crushers and Roller Mills. In this equipment group, stress is applied by either cmshing single particles or a bed of particles between two sohd surfaces. In general, most machines are used for coarse and medium-size reduction, with the exception of the high pressure roUer mill which can achieve extremely fine particle distributions. 

A reduction in the magma density will generally increase nucleation and decrease the particle size. This technique has the disadvantage that crystal formation on the equipment surfaces increases because lower shiny densities create higher levels of supersaturation within the equipment, particularly at the critical boiling surface in a vaporization-type ciystaUizer. 

The desirability of using a number of size reduction units when the particle size is to be considerably reduced arises from the fact that it is not generally economical to effect a large reduction ratio in a single machine. The equipment used is usually divided into classes as given in Table 2.1, according to the size of the feed and the product. 

The generally accepted definition of micronization is the size reduction of particles down to 1-30 m in diameter. Although it is possible to use some of the previously discussed equipment to reduce the particle size distribution of a product down to this range they are not ideally suited to the application as the distribution curve can be fairly wide spread and possibly even bimodal where as a tight PSD and single node curve is the goal of most manufacturers (Fig. 21). The equipment commonly used are pin mills, hammermills, fine grind, and jet mills. 

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