Volume
DRY III Part 1
Introduction.
This describes the
basic features of fluidised bed dryers and their advantages
and disadvantages relative to other types of dryer. It also
explains how the volume is laid out and how to use it to best
effect.
Volume
DRY III Part 2
State of the technology.
This
describes and illustrates current industrial fluidised bed
dryers, and covers the main equipment options. Fluidised bed
dryers can be categorised into well-mixed, plug-flow and multi-stage
continuous units and batch units.
A well-mixed
fluid bed dryer is usually of circular cross-section, and
takes its name from the fact that the particle residence time
distribution approaches the exponential perfect mixing law.
Because of this, material in the bed has a nearly uniform
temperature and a composition equal to that of the product
stream. The wet feed thus falls into a bed of almost dry particles.
Consequently, this type of fluid bed can handle wetter feedstocks
than other types. In addition, its thermal efficiency can
be comparatively high as the hot gas is utilised uniformly
over the entire bed area. However, the product moisture content
distribution can be quite broad.
In many applications
either a narrow distribution of product moisture contents
is required or the residence time of all the material must
be reasonably uniform to guard against thermal degradation.
"Plug flow" fluid bed dryers attempt to achieve
a close approach to uniformity of particle residence time
by having a high length-to-width ratio of the bed. The bed
is usually shallow with an adjustable outlet weir. Disadvantages
include difficulty in fluidising certain wet feeds, possible
saturation of the exhaust gas near the inlet, and potential
overheating of dry solids near the outlet. Multi-stage dryers
may be used to reduce these problems. The drying behaviour
in batch dryers is similar to that in plug-flow units and
a uniform product is again achieved.
This Part also
describes equipment and operational practice in the following
areas: feeding, discharge, coolers, submerged heating surfaces,
applications, operating problems, fire and explosion hazards,
energy utilisation, maintenance and control.
Volume
DRY III Part 3
State of the science.
This
covers the underlying science of fluidised bed drying in depth,
with the following main headings:
- Fluidisation: gas
flow, solids mixing patterns and residence time distributions,
distributor effects, elutriation, entrainment and attrition.
- Heat transfer: between hot
gas, particles, walls and submerged heating surfaces.
- Drying kinetics: mass transfer,
batch drying curves, normalisation and scaling of drying
curves to other conditions.
- Design procedures: the theoretical
basis of the SPS recommended procedures and the alternatives.
Volume
DRY III Part 4 Design guide.
This
part covers the design procedure for a new fluidised bed dryer
or the performance assessment of an existing unit. The layout
is designed to be compatible with the PC programs DRYCUR and
FLUBED, which can be used to perform the calculations. Extensive
worked examples are provided.
The design procedure
is developed from the methods described in DR1 and DR3, now
withdrawn. It uses data collected from a few simple batch
drying tests and fluidisation measurements to predict the
performance of a full-size unit under a wide variety of conditions.
These calculations permit rapid preliminary equipment sizing
and optimisation of operating conditions in advance of pilot
plant testing.
The basic pattern
of the design procedure is similar for all types of fluidised
bed dryer. However, for well-mixed dryers the drying curve
must be converted to isothermal bed conditions and further
modified to allow for the range of residence times. Likewise,
the plug-flow procedure includes an axial dispersion model
which characterises the deviation from plug flow. The design
procedure can be adapted to more complex cases, including
vibro-fluidised bed dryers, beds fitted with internal heaters
and beds with a rotating rake at the feed end.
The contents
of the Part are as follows:
- Formulation of the design
problem, collection and collation of data
- Preliminary process and equipment
selection
- Scoping design procedure (gives
rough estimate of bed size)
- Generation of fluidisation
and drying data
- Detailed design procedures
for well-mixed, plug-flow, multi-stage and batch units
- Pilot plant tests and scale-up
Ancillary design calculations:
distributor, plenum chamber, freeboard, heater duty (indirect
or direct-fired), gas recycle systems, fan and condenser.
Volume
DRY III Part 5 Fluidised bed granulation and coating.
A fluidised bed can be utilised
to produce hard, roughly spherical granules or loose agglomerates
from solutions and slurries. It can also be used to apply
a uniform coating of a second material over the particles.
The report guides the reader through the technological options
and applications to help him decide on the suitability of
a fluidised bed, the most appropriate type of operation, the
preliminary design of the plant and the solution of operating
problems. Additional practical topics covered include instrumentation
and control, feeding methods, solids classification and mixing,
control of residence time distribution, costs, safety and
environmental considerations. The considerable advances that
have been made over the past few years in the fundamental
science are critically reviewed. In particular, fluidisation,
particle growth and reduction mechanisms are discussed. Published
simulation models and the usefulness of small scale tests
for evaluating feasibility and optimisation are assessed.
Volume
DRY III Part 6
Vibro-fluidised bed
dryers.
This
part of Volume III covers the "State of the Technology"
and "State of the Science" for vibro-fluidised bed
dryers, as their behaviour is rather different to that of
conventional fluidised beds.
In vibro-fluidised
bed dryers, the bed of solids is fluidised and also subjected
to sinusoidal vibrations. This technique is particularly useful
for materials which are cohesive or pasty, have a wide size
distribution, or are of low mechanical strength. The common
and distinguishing features of the different units employed
in industry are described. Typical applications are reviewed,
together with the current practices relating to design and
operation. The science of fluid-particle mechanics of vibrated-beds
is discussed. Published theories and experimental studies
of the effect of vibration on heat transfer and drying kinetics
are compared and useful guidance on the probable effects of
operating variables on dryer performance is compiled.
