Rotary dryer, control

Three examples of ordinaiy gas seals are shown in Fig. 12-61. On direct rotary dryers, few gas seals are intended to be completely gastight, but by careful control of the internal pressure, generally Between 0.25 and 2.5 mm of water below atmosphere, dusting to the outside is prevented and in-leakage of outside air is minimized. 

Alfalfa dehydration is carried out in a direct-fired rotary dryer. The dried product is transported pneumatically to an air cooler and then to a collecting cyclone. The collected particles are ground or pelletized and then packaged for shipment. The major atmospheric emission from the process is particulate matter, which is controlled by baghouses. Odors may also be a problem, but they disperse rapidly and are no longer a problem at distances of over 1 km. 

Figure 16.13. Indirectly heated rotary dryer. A-Firing door. B-Air regulator. C-Fumace. D-Control valves. E-Feed chute. F-Furnace flue. G-Feed screw. H-Fan. J-Driving gear. K-Discharge bowl. L-Duct lifters.

While a rotary dryer is shown, commonly used for grains and minerals, this control system has been successfully applied to fluid-bed drying of plastic pellets, air-lift drying of wood fibers, and spray drying of milk solids. The air may be steam-heated as shown or heated by direct combustion of fuel, provided that a representative measurement of inlet air temperature can be made. If it cannot, then evaporative load can be inferred from a measurement of fuel flow, which then would replace AT in the set-point calculation. 

Equipment commonly employed for the drying of solids is described both in this subsection in Sec. 12, where indirect heat transfer devices are discussed, and in Sec. 17 where fluidized beds are covered. Dryer control is discussed in Sec. 8. Excluding fluid beds this subsection contains mainly descriptions of direct-heat-transfer equipment. It also includes some indirect units e.g., vacuum dryers, furnaces, steam-tube dryers, and rotary calciners. 

One manner in which size may be computed, for estimating purposes, is by employing a volumetric heat-transfer concept as used for rotary dryers. If it is assumed that contacting efficiency is in the same order as that provided by efficient hfters in a rotary dryer and that the velocity difference between gas and solids controls, Eq. (12-52) may be employed to estimate a volumetric heat-transfer coefficient. By assuming a duct diameter of 0.3 m (D) and a gas velocity of 23 m/s, if the sohds velocity is taken as 80 percent of this speed, the velocity difference between the two would be 4.6 m/s. If the exit gas has a density of 1 kg/m, the relative mass flow rate of the gas G becomes 4.8 kg/(s m the volumetric heat-transfer coefficient is 2235 J/(m s K). This is not far different from many coefficients found in commercial installations however, it is usually not possible to predict accurately the actual difference in velocity between gas and solids. Furthermore, the coefficient is influenced by the sohds-to-gas loading and particle size, which control the total solids surface exposed to the gas. Therefore, the figure given is only an approximation. 

Motor-driven process compressors Turbine-driven process compressors Complex multi-unit packages TDC/PLC and batch controllers Rotary dryers / filters... 

Rotary dryers are usually controlled by heat transfer. Thus, Equations T9.8 through T9.10 in Table 4.9 are proposed in Ref. [131] for the estimation of the corresponding heat transfer coefficients. 

Canales ER, Borquez RM, Melo DL. Steady state modeling and simulation of an indirect rotary dryer. Food Control, 12, 77-83, 2001. 

The rotary dryers (direct-heat dryers and kilns) are controlled by indirect means, e.g., by measuring and controlling the gas temperatures in their two ends, whereas shell temperature is measured on indirect calciners, and steam pressure and temperature as well exit gas temperature and humidity are controlled on steam-tube dryers. It is not possible to achieve control by measuring the product temperature because not only this is difficult but also its changes are slowly detected, although the product temperature is used for secondary controls. 

Rather few papers deal with the kinetics of biofuel drying and most designs seem to be experience-based. Bagasse is reported to be easily dewatered with exit temperatures for commercial rotary dryers approaching the wet bulb temperature [7]. A study of drying rates of milled peats [21] in a fluid-bed bench-scale dryer showed no influence on the origin of the peat. On the other hand, different size fractions of the same peat gave quite different results. The intraparticular resistance is pronounced and, of course, controlled by the particle size. 

After beneficiation, the washed rock may contain 7%-20% moisture, which is reduced to between 1% and 2% moisture using direct-fired rotary dryers. Emissions expected from the dryer consist primarily of fine rock dust. Some sulfur dioxide may also be present in the dryer exhaust from the combustion of sulfur in the fuel. Phosphate rock dryers are usually equipped with dry cyclones, followed by wet scrubber systems for control of rock dust. The scrubber systems will operate at typical particulate collection efficiencies of 97%-98% with emissions ranging from 0.1 to 0.3 kg/t of phosphoric anhydride (P2O5) [77]. 

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