Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 84

exposed to the elements, or may store it in large sheds. Thus, hulls will absorb some
moisture and become dry if sunlight is available and this cycle is repeated. The net result
is that if a producer decides to avail himself of the hulls in this condition for whatever
reason (e.g., close proximity to his/her plant, easy transport, economic reasons),
drying of the material in bulk will have to be carried out before the final extraction of
moisture to achieve the desired moisture levels before compounding with the polymer
matrix or additives. For these two operations, different drying systems must be used.
7.2.1 Bulk Drying Systems
There are many drying systems and, for the benefit of the reader, a wide spectrum of
drying systems is presented here in general terms. There are three basic methods of
removing moisture from materials such as: air dryers, infrared (IR) dryers and spray
dryers. Air drying brings the material to be dried into direct contact with hot or cold
air. It is the most widespread method and is used by flash dryers, freeze dryers, fluid
bed dryers and rotary dryers for applications for food processing, paper production,
pharmaceutical, pollution control and agricultural industries. IR dryers use energy
from IR wavelengths to remove moisture from the inside out, without overheating
the material. In spray dryers, after any solid material is separated it is directed into
a spray of hot steam that quickly causes moisture to evaporate. Drying can be done
in batches or in a constant stream in continuous dryers, which are frequently also
conveyor dryers. Conveyor dryers use a conveyor belt to move the material through
a dryer. Both are commonly used in air-drying systems.
Most drying units used in small operations (e.g., at home) are simple air dryers. Air
warmed by gas or electric heating elements is dispensed into a machine and interacts
with the material or any items inside. The moisture evaporates and the resulting
vapour leaves through an exhaust vent. Rotary dryers (sometimes referred to as
'drum dryers') use hot air to heat a rotating drum. Wet materials are placed inside
and may pass through multiple chambers before completing the process. Air may
come in direct contact with each compartment or may be held in a separate chamber
whose radiating heat will be adequate to dry the material. Many powders are dried
this way, including industrial chemicals. Flash dryers can be classified as air dryers so
long as air is the heat source used for drying. A short burst of high heat can be used
to dry the material quickly as it passes through the drying duct.
Fluid bed dryers use a permeable surface to support the materials. Air or gas is heated
and released into the dryer until all particles begin to vibrate slightly. The vibration
indicates that the velocity of the air slightly exceeds the velocity of gravity. This type
of drying is often used for materials that are mostly uniform in size. Technically, freeze
dryers are air dryers except that, in their case, the air reaches such a low temperature