Bed crushing

Stressing between Two Solid Surfaces Crushing. Either single particles (Fig. 3a) or a bed of particles (Fig. 3b) are cmshed between two solid surfaces. The amount of stress that can be applied is governed by the force applied to the solid surfaces. 

FIG. 12-57 Special conveyor dryer with air jets impinging on surface of bed on first pass. Dried material is crushed and passed again through dryer, with air going through the now-permeable bed. [Chem. Eng., 192 (June 19, 1967).]... 

At the alumina plant, the bauxite ore is further crushed to the correct particle size for efficient extraction of the alumina through digestion by hot sodium hydroxide liquor. After removal of "red mud" (the insoluble part of the bauxite) and fine solids from the process liquor, aluminum trihydrate crystals are precipitated and calcined in rotary kilns or fluidized bed calciners to produce alumina (AljOj). Some alumina processes include a liquor purification step. 

Go to a standard handbook like Perry s Chemical Engineer s Handbook and obtain several hydraulic conductivities (say for sand, crushed stone, gravel, diatomaceous earth, other). Assume a constant head of fluid over a bed of each material, and apply Darcy s law to calculate flowrates and compare the results. Try several hydraulic head calculations and plot the results. Which of the materials studied shows the highest hydraulic resistance ... 

Absorption Field A system of properly sized and constructed narrow trenches partially filled with a bed of washed gravel or crushed stone into which perforated or open joint pipe is placed. The discharge from the septic tank is distributed through these pipes into trenches and surrounding soil. While seepage pits normally require less land area to install, they should be used only where absorption fields are not suitable and well-water supplies are not endangered. 

Fluidized bed combustion boilers are often conventional boilers but with unconventional, specially designed furnace beds. Fluidized bed combustion boilers are used to burn solid fuels, such as low-grade, high-sulfur crushed coal. The benefit of this process is its ability to provide 95 to 100% combustion (depending on the reactivity of the fuel) combined with reduced emissions of sulfur dioxide (S02) and nitrogen oxides (NOx). 

High mechanical strength with respect to crushing (e.g. under the weight of the catalyst bed or during the shaping process). 

The imdoubted advantage of mini fixed-bed micro reactors is that they foUow a widely accepted processing path and in principle can use all of the commercial catalysts, if they can be crushed to a size much below the micro-channel diameter. Hence catalyst material flexibility is a major driver. 

Air-cooled blast furnace slag. If the liquid slag is poured into beds and slowly cooled under ambient conditions, a crystalline structure is formed, and a hard, lump slag is generated, which can subsequently be crushed and screened. 

Trickling filter systems are classified under the aerobic attached growth systems (Figure 30.7). Crushed rock and stone, slag, wood, or synthetic media with higher permeability are used to fill the filter bed. The size of the media is in the range of 25-100 mm diameter. The depth of the filter... 

The packing itself may consist of spherical, cylindrical, or randomly shaped pellets, wire screens or gauzes, crushed particles, or a variety of other physical configurations. The particles usually are 0.25 to 1.0 cm in diameter. The structure of the catalyst pellets is such that the internal surface area far exceeds the superficial (external) surface area, so that the contact area is, in principle, independent of pellet size. To make effective use of the internal surface area, one must use a pellet size that minimizes diffusional resistance within the catalyst pellet but that also gives rise to an appropriate pressure drop across the catalyst bed. Some considerations which are important in the handling and use of catalysts for fixed bed operation in industrial situations are discussed in the Catalyst Handbook (1). 

A packed bed is composed of crushed rock with a density of 175 lbm/ft3 of such a size and shape that the average ratio of surface area to volume for the particles is 50 in.2/in.3. The bed is 6 ft deep, has a porosity of 0.3, and is covered by a 2 ft deep layer of water that drains by gravity through the bed. Calculate the flow rate of water through the bed in gpm/ft2, assuming it exits at 1 atm pressure. 

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