Spray driers operating temperatures

Dryer Operating Parameters Having considered the formulation and preparation of the material to be spray dried, one must now consider the effect of spray drier operating variables on flavor retention. One needs to choose operating temperatures, air flow rates, inlet air dehumidiiication etc. 

For the preparation of spray-dried polyelectrolyte complexes, the polyanion was dissolved in dilute NH4HCO3 solution and mixed with the chitosan carbamate solution just before spray-drying. The excess NH4HCO3 decomposed thermally between 60 and 107 °C on the other hand, the carbamate function released carbon dioxide under the effect of the temperature at which the spray-drier was operated, thus regenerating chitosan at the moment of the polyelectrolyte microsphere formation (Fig. 5). 

All the driers discussed above are unsuitable for mixtures with a high liquid proportion (slurries). For such mixtures spray driers (see Fig. 7.2-11) or cyclone driers (see Fig. 7.2-12) are effective. In the former the sluri-y is injected into the drier by a nozzle that atomizes the suspension. Small droplets fall down countercurrently to the hot gas that rises in the conical drier zone. In cyclone driers the sluriy and the hot gas flow cocurrently. Driers of both kinds are characterized by short residence times. Therefore, they are particularly suitable for temperature sensitive products. A significant proportion of driers of all kinds, appropriately modified, can also be operated as solids mixers and/or granulators. 

One of the important stages for a successfiil operation of a spray drier scrubber is slurry preparation in lime processes. As lime cannot be simply dissolved in water it must be ground. Particle size and slaking temperature are important parameters. Sahar and Kehat (1991) give 1.46/im as recommended average particle size for the lime slurry to be fed to the system. Unless the slurry is in a homogeneous form, the extensive surface area required for a complete gas-solid contact cannot be achieved by atomization. Plugging problems may occur in pipes in which lime slurry flows. 

The kiln operator selects the appropriate sehedule and adjusts the temperature and humidity as necessary by manipulation of the heating eoils (dry heat) and spray valves (steam), with occasional adjustments to the air inlet and outlet vents, which exchange hot, moist air from the kiln for cold, drier air from outside. The steam line raises and maintains the humidity while the kiln warms up. It should be used only infrequently thereafter as economy in operation depends on making maximum use of the moisture extracted from the lumber to maintain the required humidity in the kiln. As moisture evaporates the humidity rises above the schedule value and the vents open a little to bleed off some of the hot damp air. 

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