Dryers gravity

FIG. 12-34 Cross-flow of gas and solids in a cascade-type gravity dryer. (Link-Belt Co., Multi-Louvre piinciple.)... 

Indirect-Heat Rotary Steam-Tube Dryers Probably the most common type of indirec t-heat rotaiy diyer is the steam-tube diyer (Fig. 12-69). Steam-heated tubes running the full length of the cylinder are fastened symmetrically in one, two, or three concentric rows inside the cylinder and rotate with it. Tubes may be simple pipe with condensate draining by gravity into the discharge manifold or bayonet-type. Bayonet-type tubes are also employed when units are used... 

Another example in the polymers industry is illustrated in Figure 17, which is a process aimed at the batch drying of waste residue with solvent recovery. In this application liquid or viscous waste solutions are pumped into a batch dryer where they are dried under vacuum to a solid granular residue. Vaporized water and solvent are recovered by condensation and then separated by gravity. The process scheme is flexible, offering a range of temperatures and vacuum levels for treating... 

Gravity feed is to be used for charging additives to the reactor, for discharging the reactor to the hold tanks, and for feeding the dryer. 

Fig. 6, A multi-tier, multi-pass conveyor dryer. Due to space limitations il is sometimes necessary to tier dryer, and use gravity or transfer conveyors to carry the product from one tier to the other. The location of the discharge end will vary with the number of tiers. (The National Drying Machinery Company)...

The catalyst slurry is fed to the top of the spray dryer where it is atomized in the form of small micron sized droplets. These droplets fall through gravity in a flow of hot air following a spiral downward movement. The particles with the desired size are collected at the bottom of the spray dryer and the fines are recovered with the aid of cyclones and recycled. This contained process explains why recoveries of 99% are achieved. The height of a commercial spray dryer is measured in meters and such a process is difficult to downsize, therefore explaining partly the lack of academic research in the field of zeolite catalysts spray drying (6). 

Dilute This is a fully expanded condition in which the solids particles are so widely separated that they exert essentially no influence upon each other. Specifically, the solids phase is so fully dispersed in the gas that the density of the suspension is essentially that of the gas phase alone (Fig. 12-29). Commonly, this situation exists when the gas velocity at all points in the system exceeds the terminal settling velocity of the solids and the particles can be lifted and continuously conveyed by the gas however, this is not always true. Gravity settling chambers such as prilling towers and countercurrent-flow spray dryers are two exceptions in which gas velocity is insufficient to entrain the solids completely. 

A gravity dryer consists of a stationary vertical, usually cylindrical housing with openings for the introduction of solids (at the top) and removal of solids (at the bottom). Gas flow is through the solids bed and may be cocurrent or countercurrent and, in some instances, cross-flow. By definition, the rate of gas flow upward must be less than that required for fluidization. 

The palm oil exiting the low-pressure screw press has approximately 66% oil, 24% moisture, and 10% solids (18). The solid particles are typically separated from the oil using the traditional method of pumping the oil into a tank with approximately 2 hours of residence time to allow the heavier solid particles to settle and be continuously dredged from the base of the tank. After gravity separation, the oil is then pumped through a liquid cyclone to remove residual solids. Solid particles separated from the clean oil stream are saturated with oil and recycled back into the process. After the palm oil is cleaned of solid particles, it is heated and pumped through a vacuum oil-dryer to remove moisture. 

Convection is the transfer of heat from one point to another within a fluid by the mixing of one portion of the fluid with another. In natural convection, the motion of the fluid is caused by gradients of temperature and gravity. In forced convection, the motion is caused by mechanical means that enhance the rate of heat transfer over natural convection. An example of convection drying would include the use of hot air in tray dryers and fluid bed dryers. 

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