Designing of the dryer section of nutritional yeast production plant – Industrial sector review – Part 21

Designing of the dryer section of nutritional yeast production plant – Industrial sector review – Part 21

Drying of Spray

When the spray comes into contact with the drying air, evaporation takes place from the ‘saturated vapor’ film which is established at the droplet surface.  The surface temperature of the droplet approaches the wet bulb temperature of the air (air becomes
saturated at the surface of the droplet).

The complete drying of the droplets takes place in two stages. At the first stage, the evaporation takes place at the saturated surface of the droplet (Rose, 1971). This moisture removal can be constant with near constant droplet surface temperature (saturated
wet bulb temperature of air) and constant partial pressure of vapor at surface. Diffusion of moisture within the droplet maintains the saturated surface condition (as long as this lasts). 

The second stage of drying occurs when the moisture content decrease to a level where it can no longer maintain the saturated condition (saturated surface layer), which is termed as ‘Critical moisture content’. Dried layer is then produced at the surface
of the droplet and the thickness of the dried shell increases with time through moisture diffusion in dried cell, causing a decrease in evaporation rate.  The generic drying rate curve for the dryer with is represented in the following diagram.

The air which is in contact with the boundary of the droplet remains no mores saturated during the 2^nd stage of drying and hence the temperature of the droplet starts increasing at this point (droplet temperature was previously limited by the
saturated wet bulb temperature of the air)

Continued in part 22

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Posted by Ahmed Tariq on Feb 29 2012. Filed under Sci-Tech. You can follow any responses to this entry through the RSS 2.0. You can leave a response or trackback to this entry