Chemical Engineering September 2015 - 30

ter exposed for heating and drying.
Agitation can reduce cycle time by
speeding evaporation operation and
provide better product quality, avoiding
biscuit-effect or crust to form on
the cake, says Parker.
" Combining the isolation and drying
steps has been one of the major
innovations of the last century for
chemical processors, " he continues.
The Agitated Nutsche Filter Dryer
(ANFD; Figure 2) is an example of
this technology. It allows the user to
control and optimize the isolation by
filtering out most of the solvent using
pressure and heated agitator blades
to bring down the moisture content.
Then, within the same vessel, drying
is achieved under vacuum via heat
transfer and direct contact with the
heated jacket, plate and, in some
cases, the agitator as well. " Agitation
is an important part of an efficient
drying process, " says Parker.
Quality and Reliability Every Time
High- and low-viscosity liquids and
heat-sensitive products have a different
set of concerns, according to SPX's
Knudsen. " These products are usually
spray dried because it is a continuous
process, converting a pumpable liquid
into a particulate matter (Figure 3) in a
single operation, " he says. The liquid
is atomized into very fine droplets in
a drying chamber, where evaporation
takes place by contact with hot air and
a powder is formed. The powder is
separated from the drying air in a cyclone
or bag filter system.
" At the heart of this process is the
atomizer, " says Knudsen. SPX offers
two basic types of atomizers. Centrifugal
atomizers accelerate and atomize
the liquid feed using centrifugal
force in a spinning disc, which is
suitable for many chemical applications.
Nozzle atomizers spray the liquid
feed under high pressure or using
compressed air, which is particularly
effective when a coarse powder with
narrow particle-size distribution and
high bulk density are required.
Similarly, biomass as a feedstock
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has its own special requirements,
which can be properly addressed
using rotary dryers, according to
Becky Long, dryer design engineer
with Thompson Dehydrating
Co. (Topeka, Kan.; www.thompson
dryers.com). A rotary dryer features a
rotating cylindrical drum, which acts
as a container for the uniform transfer
of thermal energy from a hot gas
stream to a moisture-laden product,
such as biomass, for the purpose of
moisture reduction in the product.
Wet product is introduced into the
inlet of the drum, where it is dried as
it is conveyed to the drum's outlet.
The drum is equipped with flighting
to disperse the product into the drying
gas stream as the drum rotates.
It is the flighting, and its ability to
Circle 23 on p. 94 or go to adlinks.chemengonline.com/56201-23
30
adapt to new purposes, according
to Long, that makes rotary dryers a
suitable choice for biomass processing.
Flighting is the primary material
handling mechanism in a drum. And
Thompson designs its flighting to be
easily changed to provide flexibility.
" In the biomass industry, there is such
a variety in the materials being processed
that it can change from year
to year, so we need the ability to alter
the flighting system if the feedstock
ChemiCal engineering www.Chemengonline.Com september 2015
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Chemical Engineering September 2015

Table of Contents for the Digital Edition of Chemical Engineering September 2015
