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Select a batch closed container with wood stacked for good flow of air by
kiln drying in a chamber with temperature and humidity control.
(f) Pieces of pottery.
Select a continuous tunnel dryer.
(g) Temperature-insensitive inorganic crystals, where particle size is to be
maintained and only surface moisture is to be removed.
Select a tunnel dryer, a belt or band dryer, or an indirect-heat, steam-tube
(h) Orange juice to produce a powder.
Select a batch freeze dryer. Exercise 18.5
Subject: Solar drying
Find: Advantages and disadvantages of solar drying for organic materials. Other applicable
dryers for organic materials and for continuously drying beans.
Analysis: Advantage of solar drying is no cost for energy.
Disadvantage of solar drying is long drying time, variability of weather, lack of control.
Other dryers suitable for organic materials are tunnel dryer and belt or band dryer.
For the continuous drying of beans, can use steam-tube rotary dryer, roto-louvre dryer,
screw conveyor dryer, through-circulation or band dryer, and fluidized-bed dryer. Might
also consider a column dryer with solids moving down by gravity with countercurrent
flow of warm air. Exercise 18.6
Subject: Fluidized-bed dryer
Find: Advantages of fluidized-bed dryer for vegetables, including potato granules, peas, diced
carrots, and onion flakes.
Analysis: Advantage is control and uniformity of temperature of both solids and gas. Exercise 18.7
Subject: Three-stage process for producing powdered milk from liquid milk
Find: Reasons why the following 3-stage process is better than a single stage of spray drying:
Stage 1: Vacuum evaporation in a falling-film evaporator to a high-viscosity
liquid of less than 50 wt% water.
Stage 2: Spray drying to 7 wt% moisture.
Stage 3: Fluidized-bed drying to 3.6 wt% moisture.
Analysis: Since milk is very temperature sensitive, it is best to first concentrate the liquid
rapidly at a low temperature. The falling-film evaporator is ideal for this. Then the combination
of a spray dryer and fluidized-bed dryer offers efficiency, product control, and flexibility.
Because of the large amount of moisture that would have to be removed compared to the final
dry-milk powder, it would be difficult to achieve a uniform moisture content with a spray dryer
alone. Exercise 18.8
Subject: Drying pharmaceutical products.
Find: Reasons why a closed-cycle spray dryer using N2 is a good choice for the drying of
materials from a non-aqueous solvent, such as methanol, ethanol, acetone, etc.
Analysis: Closed-cycle spray dryers offer flexibility, scalability, reliability, and controllability.
The product from such a dryer is a powder of controlled particle size that is readily compressed
into tablets. Exercise 18.9
Subject: Drying of paper made from a suspension of fibers in water, followed by draining the
fibers to a water-to-fiber ratio of 6:1, followed by pressing to a 2:1 ratio.
Find: Type of dryer to produce a continuous sheet to an equilibrium-...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
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