HEAT, MOMENTUM, AND MASS
TRANSFER MEASUREMENTS IN
INDIRECT AGITATED SLUDGE DRYER
J. H. Ferrasse, P. Arlabosse,
*
and D. Lecomte
E
´
cole des Mines d’Albi Carmaux, Campus Jarlard Rte
de Teillet, 81013 Albi CT Cedex 09, France
ABSTRACT
Whereas indirect agitated drying has been extensively studied
for granular materials, little is known in the case of pasty
products. We describe an experimental set up specially
designed for the investigation of drying kinetics, of heat trans-
fer coefficient evolution, and of the mechanical torque neces-
sary for stirring. This device was applied to municipal sewage
sludge. Preliminary experiments were performed to investigate
the influence of ageing of sludge on the drying kinetics. It
appears that ageing does have no effect except for the first
two days. The influences of the wall temperature, the stirrer
speed, the dryer load and the location of the stirrer against the
heated wall were studied. Thr ee different rheological behav-
iors were observed during sludge drying. In particular, the
sludge goes through a ‘‘glue’’ phase, and high levels of
mechanical strain are recorded when the compact mass
begins to break up. A critical stirrer speed is found within
the range 40 60 rpm. To shorten the drying time, it seems
better to adjust the wall tempe rature in accordance with the
moisture content of the sludge.
Key Words: Agitation; Contact drying; Drying kinetics;
Heat transfer coefficient; Mechanical strain
INTRODUCTION
Sludge is the unavoidable result of wastewater treatment, and has to
be eliminated or, if possible, used as a resource. In France, up to 850,000
tons of dry matter were generated in 1998 from municipal sewage sludge
(Chassande, 1998). An increase to some 1,300,000 tons of dry matter per
year is expected in the year 2005 due to the higher collection rates, the
reliability and efficiency of wastewater treatment plants, and introduction
of new treatment methods to the water purification process, such as deni-
trification and deph osphatation.
In addition to this increase, European legislation is being strengthened
in regard to sludge incineration and its use in agriculture. Furthermore, in
the near future and after a transition period, the deposition of sludge at
dump sites will be permitted only if the organic carbon content is lower than
5% w/w; however dumping is still practiced for 25% w/w of sludge
collected.
In this context, drying of sewage sludge represents an interesting inter-
mediate stage common to all disposal methods, as it makes it possible to
stabilize the sludge, reduce its volume (the water content of a mechanically
dewatered sludge is close to 5 kg of water per kg of dry matter) and hygien-
ize the product, if the residence time and the temperature are sufficient.
Thermal drying of industrial sludge has been practiced for many decades
but the developments for municipal sludge were rather confidential. In the
late 1980s, interest was renewed and manufacturers, who have successfully
applied their technologies to other applications (chemicals and food indus-
try), tried to transfer existing technologies to sewage sludge. This explai ns
why most sludge dryers are issued from standard equipment. But, adapta-
tion of existing technologies is not straightforward, and equipment design is
more a question of know-how than scientific knowledge. Among the most
frequently encountered problems, the security issue is the one occurring
mainly in direct dryers whereas problems linked to sludge stickiness are
crucial for some indirect dryers like paddle, disk, or fluidized bed dryers.
In association with stickiness , wear problems are also observed when
moving parts are in contact with the sludge. However, as emphasized by
Lowe (1995), successful experience in thermal processing of sewage sludge
can be credited to few companies though only a limited number of these can
claim to have dryers designed specifically for this purpose.
In Europe, technological problems and market tensions have reduced
the offer of drying technologies during the past two years. For instance, the
fluidized bed technology was almost discarded for sewage sludge treatment.
Carre
`
re Ge
´
e (1999) presents an inventory and a classification of drying
technologies available on the market for sludge treatment. Four subgroups
of indirect dryers are identified: disc dryers, paddle dryers, thin film dryers
and drum dryers. Disc dryers are the most refere nced, far beyond padd le
dryers and thin film dryers. There are still few references for drum dryers
with indirect heating systems but an increasing demand was observed in the
past years probably due to their simpler design. Among direct dryers, four
subgroups are also identified: rotary dryers, conveyor belt dryers, pne umatic
transport dryers and other types of dryers including various types of batch
dryers as well as solar dryers. Rotary dryers are the most widely used direct
dryers far beyond conveyor belt dryers. The average specific energy con-
sumption seems to be higher than that of indirect dryers. For batch dryers,
smaller energy consumption is observed because the sludge is partially dried
and energy recovery is high. The sludge composition, the rheological prop-
erties, and the amount of sludge play a key role in dryer selection.
Nevertheless, the average residence time in direct dryers is shorter than
that in the indirect ones, and the processing plants have larger capacity.
We focus our attention on contact drying technologies with agitators.
They present the following advantages for sludge treatment: no pollution of
the heat carrying medium, steam and odor confinement, VOC concentration
and reduction of fire a nd explosion risks as the oxygen level is low in the
dryer. Mechanical agitation continuously renews the contact surface area
between the heated wall and the sludge, which is necessary to keep high heat
transfer coefficient at the heated wall. Neverthele ss, at a dry solids content of
around 55% w/w, the sludge goes through a ‘‘glue’’ phase and gets very
sticky. When this glue phase appears, a strong increase of the torque needed
for the stirring has been reported in industrial applications, which could
damage the dryers. Therefore, the operation parameters of the pre-dryers
are often set below the ones leading to this glue pha se. A single stage drying
to the final solids content above 90% w/w is performed using back-mixing
of about 85 90% of the dry product. Thus, in a ddition to the heat transfer
coefficient between the heated wall of the dryer and the sludge, which is
necessary for thermal design, the measurement of the torque needed for the
stirring is required to complete mechanical design of the dryer.
Although extensive studies are published on drying, little is devoted to
sewage sludge drying. This can stem from the fact that sewage sludge is quite
different from other products, as it is a complex of inorganic and organic
matter bound together with a high percentage of water. Most of the pub-
lished work on sludge drying focuses on process kinetics during convective
drying with the drying air either crossing the bed of extrude d sludge
(Le
´
onard, 2000), or flowing above the sludge surface (Vaxelaire, 2000).
The other scientific contributions concern the development and the optimi-
zation of new processes, like impulse dryers (Beckley and Banerjee, 1999),
jet spouted beds of inert particles (Amazouz and Benali, 2000) or pulsed
fluidized bed dryers (Poirier and Kudra, 1999). For indirect agitated drying
of sludge, the literature is still scarce. Yamahata and Izawa (1985) studied
experimentally the drying kinetics of two different types of sludge with a
paddle dryer and determined the heat transfer coefficien ts. It was observed
that polymer-conditioned sludge was easier to dry than sludge conditioned
with inorganic chemicals. The state of the sludge changed from a viscous
fluid to a powder with the decrease of the moisture content. Thus, a con-
tinuous dryer could be divided in two parts. In the first one, called the paste
zone, the sludge is packed fully (this geometrically corresponds to the first
third of the dryer) whereas, in the second part, the sludge is packed to only
about half of the full volume (this geometrically corresponds to the two
thirds of the dryer). An average overall heat transfer coefficient, on the
order of 200 260 W/(m
2
K), was obtained. But a much higher value, near
600 W/(m
2
K), was measured in the paste zone.
The bibliographic review reveals that little is known about sludge
drying in general and about contact drying in particular. In order to deter-
mine some fundamental parameters for the mechanical and thermal designs
of a contact dryer, we developed an experimental device to investigate the
drying kinetics of municipal sewage sludge in an agitated batch dryer,
the heat transfer coefficient at the heated wall, and the torque required for
agitation. In this paper, we describe this experimental tool and present the
preliminary results.
EXPERIME NTAL SET UP
The experimental device shown in Figure 1 consis ts of a 0.2 m diameter
Teflon
!
cylindrical vessel screwed onto the top of some flat metallic plates,
which simulate the wall of the dryer and its heating system.
In sludge contact dryers, steam or hot oil flowing inside the heating
jacket around the dryer wall supplies the energy necessary for drying.
Therefore, from the point of view of boundary conditions, a constant tem-
perature is imposed. The difficulty for a suitable simulation of the industrial
process is the determination of the heat transfer coe fficient between steam
and the interior of the wall. The system used in the laboratory scale-model
consists of (Figure 2a):
a heating device made of 10 electric resistance heaters embedded
into metallic plates. Each heater can supply up to 250 W,
a 0.01 m thick buffer copper plate for temperature homogenization,
a 0.01 m thick stainless steel plate, which represents the wall of the
dryer.
Thermal grease is used to arrange for better contact between the
plates. As the tempe rature of the buffer plate can be regulated, this heating
system (with a constant temperature on the rear side of the wall) is quite
similar to the industrial configuration. The thermal resistance between the
stainless steel and copper plates could be adjusted by changing the thickness
of the grease layer for instance, if the heat transfer coefficient between the
wall of the dryer and the vapor or oil flowing inside is known. The Teflon
vessel and the heating device lay on an iso-silicate insulator to reduce heat
losses.
Figure 1. Batch contact dryer with agitation experimental set up.
