1) Batch dryers are commonly used for small production runs or flexible operations due to their lower capital costs and flexibility. Forced convection batch dryers are widely used. 2) Fluidized bed dryers have replaced some forced convection dryers as they provide faster drying times, easier loading/unloading, and higher efficiencies. 3) Other batch dryers include double-cone, agitated pan, and rotary vacuum dryers which are suitable for heat-sensitive materials but have slower drying times.

2. EFFICIENT ENERGY UTI
SPRAYDRYING
newdevelopments,new economics
NEW DIRECTIONSIN SPRAYDRYIN
cingpowder from paste
"e3-
I
To meetthe widely varying needs of
thedairy,food,chemicalandceramic/
mineralmarkets forproductsdriedto
a specific percent of moisture, a full
range of dryers is available through
the experience and manufacturing
capabilities of companies within the
AF'F Group. Units which can be pro-
vidp include spray, Spin Flash, con-
veyor band, flash (Thermo-Venturi),
traf: vacuum, and atmospheric film
d d m dryers.

4. Three basic methods of heat transfer are used in industrial dryers in varying
degrees of prominence and combinations. These are convection, conduction and
radiation.
In the chemical processing industry, the majority of dryers employ forced
convection and continuous operation. With the exception of the indirectly heated
rotary dryer and the film drum dryer, units in which heat is transferred by
conduction are suitable only for batch use. This limitation effectively restricts
them to applications involving somewhat modest production runs.
Radiant or so-called “infra-red’’ heating is rarely used in drying materials such
as fine chemicals, pigments, clays or synthetic rubbers. Its main application is in
such operations as the drying of surface coatings on large plane surfaces since for
efficient utilization, it generally is true that the material being irradiated must
have a sight of the heat source or emitter. There is, however, in all the dryers
considered here a radiant component in the heat transfer mechanism.
Direct heating is used extensively in industrial drying equipment where much
higher thermal efficiencies are exhibited than with indirectly heated dryers. This
is due to the fact that there are no heat exchanger losses and the maximum heat
release from the fuel is available for the process. However, this method is not
always acceptable, especially where product contamination cannot be tolerated.
In such cases, indirect heating must be used.
With forced convection equipment, indirect heating employs a condensing vapor
such as steam in an extended surface tubular heat exchanger or in a steam jacket
where conduction is the method of heat transfer. Alternative systems which
employ proprietary heat transfer fluids also can be used. These enjoy the
advantage of obtaining elevated temperatures without the need for high pressure
operation as may be required with conventional steam heating. This may be
reflected in the design and manufacturing cost of the dryer. Furthermore, in
addition to the methods listed above, oil- or gas-fired indirect heat exchangers
also can be used.
In general, dryers are either suitable for batch or continuous operation.
A number of the more common types are listed in Table I where an application
rating based on practical considerations is given. In the following review, some
of the factors likely to influence selection of the various types are discussed for
particular applications.
batch dryers
It will be apparent that batch operated equipment usually is related to small
production runs or to operations requiring great flexibility. As a result, the batch
type forced-convectionunit certainly finds the widest possible application of any
dryer used today.
The majority of designs employ recirculatory air systems incorporating large
volume, low pressure fans which, with the use of properly insulated enclosures,
usually provide thermal efficiencies in the region of 50-60%.However, in special
applications of this type of dryer which call for total air rejection, this figure
is somewhat lower and is largely related to the volume and temperature of the
exhaust air. Capital investment is relatively low as are installation costs.
Furthermore, by using the fan systems, both power requirements and operating
costs also are minimal. Against these advantages, labor costs can be high.
4

5. In such a plant, the drying cycles
are extended with 24 to 45 hours
being quite common in certain
cases. This is a direct result of
the low evaporativerate which
normally is in the region of
0.15 to 0.25lb/ft2hr.
interest shown in preforming
feedstock and in particular with
regard to the design of extruding
and tray-fillingequipment for
dewatered cakes (Fig.2), it now is
possible to obtain the maximum benefit of enhanced evaporative rates by using
through-air circulation dryers when handling preformed materials. Figure 1
shows how a high performance dryer can produce 1950 pounds of dried material
in a 24 hour period at a terminal figure of 0.5% moisture when handling a
preformed filter cake having an initial moisture content of 58%. The very great
improvement in performance can readily be seen from the curve in which it is
clear that the corresponding number of conventional two-truck recirculatory
units would be between seven and eight for the same duty. The advantage is
more apparent when it is seen that respective floor areas occupied are 55 sq ft for
the Thruflo dryer pictured in Figure 3 and 245 sq ft in the case of conventional
units using transverse air flow.
Reference to the drying curves for the processing of materials in solid or filter
cake form or, in fact, in the case of wet powders, clearly indicates that the
ultimate rate-governing factor is the rate of diffusion of moisture from the wet
mass. This becomes increasingly so during the falling rate period of drying. This
situation, however, can be improved by preforming the product in order to
increase the effective surface area presented to heat and mass transfer. The
logical extension of this technique is to total dispersion drying, i.e., flash or
Followingthe recent trend and
FIG. 2 Extruder and tray filler for Fig. 3 Thruflo dryer
5

6. ITABLE 1 Productclassificationanddryertypesasanaidtoselection
6
'Note: Evaporation rates for rotary dryers are expressedin Ib/ft3hr
pneumatic dryers, fluid beds, etc. where discrete particles can be brought into
contact with the hot gas. This produces rapid heat transfer with correspondingly
short drying times.
Batch type fluidized bed dryers have, therefore, superseded forced convection
units in many cases, notably in the drying of pharmaceuticals and for the
processing of certain thermoplastics. These machines generally are available in a
range of standard sizes with batch capacities from 50 to 200 lbs, although much
larger units are made for special applications.
When considering this type of dryer, it is important to ensure that the feed
material can be fluidized, both in its initial and final condition. It also should be
remembered that standard fan arrangements are not equally suitable for a variety
of materials of different densities. Therefore, it is necessary to accurately
determine the minimum fluidizing velocity for each product.
If the feedstock is at an acceptable level of moisture content for fluidization,
this type of dryer provides many advantages over a batch type unit. Simplified
loading and unloading results in lower labor costs, high thermal efficiencies are
common, and the drying time is reduced to minutes as opposed to hours in
conventional units. Current developments of this type of equipment now include
techniques for the simultaneous evaporation of water and the granulation of
solids. This makes the units ideal for use in the pharmaceutical field.

7. The various batch dryers referred to operate by means of forced convection, the
transfer of thermal energy being designed to increase the vapor pressure of the
absorbed moisture while the circulated air scavenges the overlying vapor. Good
conditions thus are maintained for continued effective drying.
Alternatively and where the material is thermosensitive - implying low temper-
atures with consequently low evaporative rates - some improvement can be
effected by the use of sub-atmospheric dryers, Le., by reducing the vapor
pressure. Several different configurations are in use and all fall into the category
of conduction-type dryers. The most usual type of heating is by steam although
hot water or one of the proprietary heat transfer fluids can be used.
Two particular types are the double-cone dryer (Fig. 4)with capacities up to
400 ft3 and the agitated-pan dryer not normally larger than 8 ft diameter where
average evaporative rates per unit wetted area usually are in the region of
4 lb/ft2hr. These units are comparatively simple to operate and, when adequately
insulated, are thermally quite efficient although drying times can be extended.
They are especially suitable for applications involving solvent recovery and will
handle powders and granules moderately well. They do, however, suffer from the
disadvantage with some materials that the tumbling action in double-conedryers
and the action of the agitator in agitated-pan machines can produce a degree of
attrition in the dried product which may prove unacceptable.
Similarly, quite large rotary vacuum dryers are used for pigment pastes and
other such materials, especially where organic solvents present in the feedstock
have to be recovered. These units normally are jacketed and equipped with an
internal agitator which constantly lifts and turns the material. Heat transfer here
is entirely by conduction from the wall of the dryer and from the agitator. Owing
to the nature of their construction, initial cost is high relative to capacity. Instal-
lation costs also are considerable. In general, these dryers find only limited
application.
continuous dryers
For the drying of liquids or liquid suspensions, there are two types of dryers
which can be used: film drum dryers for duties in the region of 600 lbs/hr for
a large dryer of about 4’diameter by 10’face length or large spray dryers (as in
Fig. 5) with drying rates of approximately 22,000 to 23,000lbs/hr. Where tonnage
production is required, the drum dryer is at a disadvantage. However, the
thermal efficiency of the drum dryer is high in the region of 1.3to 1.5lb steam/lb
of water evaporated and for small to medium production runs, it does have
many applications.
Drum dryers usually are steam heated although work has been done involving
the development of units for direct gas or oil heating. Completely packaged and
capable of independent operation, these dryers can be divided into two broad
classifications,i.e., single drum and double drum.
Double drum machines normally employ a “nip” feed device with the space
between the drums capable of being adjusted to provide a means of controlling the
film thickness. Alternately, and in the case of the single drum types, a variety of
feeding methods can be used to apply material to the drum. The most usual is the
simple “dip” feed. With this arrangement, good liquor circulation in the trough is
desirable in order to avoid increasing the concentration of the feed by evaporation.
Again, for special applications, single drum dryers use top roller feed. While the
7

8. FIG. 5 Conical
section of a
large spray dryer
FIG. 4 Double cone vacuum dryer
8

9. number of rolls is related to the particular
application and the material being
handled, in general this method of feeding
is used for pasty materials such as starches.
devices such as spray feeds are used.
It will be seen from the Figure 6
drawings that there are a number of
different feedingarrangements for drum
dryers, all of which have a particular use.
In practice, these variants are necessary
Where the feed is very mobile, rotating DISCHARG DISCHARGE
owing to the differing characteristics of
the materials to be dried and to the fact
DISCHA
DAND
EADING
LERS
that no universally satisfactory feeding
device has yet been developed. This again
illustrates the need for testing, not only
in support of theoretical calculations for
DRYECl WITH BOTTOM
ROLLER FEED
the determination of the best dryer size but
also to establish whether a satisfactory
film can be formed.
It must be emphasized that the method of
feedingthe product to the dryer is of
paramount importance to selection or design.
which are temperature-sensitive to such a
degree that their handling would preclude
such cases, special sub-atmosphericequipment
may provide the answer although the capital
cost in relation to output generally would
restrict its use to premium grade products.
an excellent solution to many drying
problems.Many materials which would suffer
from thermal degradation if dried by other
methodsoften can be handled by spray drying
due to the rapid flash evaporation and its
accompanying cooling effect. The
continuous method of operation also lends
itself to largeoutputs and with
equipment, to low labor costs
as well.
DRYER WITH TOP
FEEDROLLERSThere are, of course, certain materials
the use of an atmospheric drum dryer. In ROLLER FEED
As an alternate, the spray dryer offers MACHINE WITH DIP FEED
SIDETROUGH FEED
DRYER WITH PLAIN
DIP FEEDthe correct application of control
DISCHARGE
DOUBLE DRUM DRYERWITH
CENTER FEED AND BOTTOM
DOUBLE DRUM DRYER DISCHARGE
WITH CENTER FEED
FIG. 6 Feedingarrangements for drum dryers
9

10. spray dryers
Fundamentally, the spray drying process is a simple one. However, the design of
an efficient spray drying plant requires considerable expertise along with access
to large scale test facilities, particularly where particle size and bulk density
requirements in the dried product are critical. The sizing of spray dryers on a
purely thermal basis is a comparatively simple matter since the evaporation is
entirely a function of the At across the dryer. Tests on pilot scale equipment are
not sufficient in the face of such imponderables as possible wall build-up, bulk
density and particle size predictions. Atomization of the feed is of prime
importance to efficient drying and three basic feed devices are used extensively:
(a) single fluid nozzle or pressure type, (b) two-fluid nozzle or pneumatic type,
and (c)centrifugal (spinning disc).
The single fluid nozzle produces a narrow spray of fine particles. While a
multiplicity of nozzles of this type are used in tonnage plants to obtain the
desired feedrate, due to the high pressures employed (up to 7000 psig) excessive
wear can result, particularly with abrasive products. As an alternative, the two-
fluid nozzle with external mixing is used for a variety of abrasive materials. This
system generally is limited to small capacity installations. Normally, the feed is
pumped at about 25 psig merely to induce mobility while the secondary fluid is
introduced at 50-100psig, thus producing the required atomization.
Centrifugal atomization achieves dispersion by centrifugal force, the feed liquor
being pumped to a spinning disc. This system is suitable for and generally used
on large productions. When stacked or multiple discs are employed, feed rates of
40,000-60,000lbs/hr are not uncommon.
Many spray dryer configurations are in current use along with a variety of air
flow patterns. The nature of the chamber geometry selected is strictly related to
the system of atomization used. An example of this is the tower configuration
designed to accommodate the inverted jet of the two-fluid nozzle whereas the
cylinder and cone of the more usual configuration is designed for the spray
pattern produced by a disc type atomizer (Fig. 7).
FIG. 7 Alternative configurations of spray dryers showing (A) tall form type and (B)conical
10

11. I I
Energycost/lb of dried product
L-
I Initial moisture content (w/w) I 50% I 50% I
$065 $.070
I
I
0.5%
I0.5%
I1 Final moisture content (W/W)'
1 Feed (Ib/hr) I 12000 I 12000 I
5970
11 Evaporation (Ib/hr) I i5970
I OutDut (Ib/hr) I 6030 I 6030 I
393
I1150
II Inleiair temperature (OF)
II Exhaust air temperature ( O F ) I 250 I 300 I
1 Direct gas tiring 1 gas CV = lo00Btu/ft2 1 gas CV = 1000Btu/ft2 I,
1 Hourly operating cost gas &electricity $39.00 I $42.26 I
I I
I76.0% 69.5%
I Thermal efficiency i1 Installed hp I 149 I 154 I
I I650 2100
II Floor areaoccupied (ftz)
1 Installed cost I $465,000 I $330,000 I
TABLE 2 Comparison between direct fired band dryer and spray dryer costs in the processing of Ti02
The product collection systems incorporated in spray drying installations are
many and varied and can constitute a substantial proportion of the total capital
investment. In some cases, this can be as high as 20-25'E of the installed plant
cost. It also must be remembered that to be suitable for spray drying, the feed
must be in a pumpable condition. Therefore, consideration must be given to the
up-stream process, Le., whether there is any need to re-slurry in order to make
the feed suitable for spray drying.
It generally is accepted that mechanical dewatering is less costly than thermal
methods and while the spray dryer exhibits quite high thermal efficiencies, it
often is at a disadvantage relative to other drying systems due to the greater
absolute weight of water to be evaporated owing to the nature of the feed. It is,
however, interesting to consider this point further. A classic case for comparison
is provided by a thixotropic material which may be handled either in a spray
dryer using a mechanical disperser doing work on a filtered cake to make it
amenable to spray drying or alternately, drying the same feedstock on a contin-
uous band dryer. In the latter instance, the cake is fed to an extruder and
suitably preformed prior to being deposited on the conveyor band.
The operating costs presented in Table 2 are based on requirements for
thermal and electrical energy only: No consideration is given to labor costs for
either type of plant since these are likely to be approximately the same. Probably
the most obvious figure emerging from this comparison is the 20%price differen-
tial in favor of the continuous band dryer. Furthermore, while energy costs favor
spray drying, they are not significantly lower.
It is, of course, impossible to generalize since the economic viability of a drying
process ultimately depends on the cost per pound of the dried product and, as
previously mentioned, the spray dryer usually has a greater amount of water to
11

12. remove by thermal methods than
other types. In the particularcaseillus-
trated, the spray dryer would have an
approximate diameter of 21 ft for the
evaporationof 6000lbshr.If,however,
the feedsolidswere reduced to30%by
dilution, the hourly evaporation rate
would increase to 14,000lbs and the
chamberdiameterwould beabout 30ft
with corresponding increases in ther-
mal input and air volume.Thesystem
would, as a result, also require larger
fans and product collection systems.
The overall thermal efficiency would
remain substantially constant at 76%
with reducing feed solids. Installed
plant costs, however,increase propor-
tionally with increasing dryer size
necessary for the higher evaporation
involved in producing a dried output
equivalent to that shown in Table 2.
Spray drying does have many
advantages, particularly with regard
to the final product form. This is
especially so where pressing grade
materials are required, i.e., in the
production of ceramics and dust-free
productssuchasdyestuffs. It iscertain
that with the introduction of new
geometries and techniques there will
be further development into areas
such as foods and in the production
of powders which may be easily
reconstituted.
rotary dryers
Another type of dryer which is very
much in evidence in the chemical and
process industries is the continuous
rotary dryer. This machine generally
isassociatedwith tonnage production
and as a result of its ability to handle
products having a considerable size
variation, can be used to dry a wide
range of materials. The principal
sources of thermal energy are oil,gas
and coal. While typical inlet tempera-
tures for direct-fired dryers using
12

13. these fuels is in the order of I2OOoF,in certain instances they may be as high as
1500-1600°Fdepending largely on the nature of the product handled. Where feed
materials are thermo-sensitive, steam heating from an indirect heat exchanger
also is used extensively. These dryers are available in a variety of designs but in
general, can be divided into two main types: those arranged for direct heating and
those designed for indirect heating. As seen from Fig. 8, certain variants do exist.
For example, the directhdirect dryer uses both systems simultaneously.
Where direct heating is used, the products of combustion are in intimate
contact with the material to be dried while in the case of the indirect system, the
hot gases are arranged to circulate around the dryer shell. Heat transfer then is
by conduction and radiation through the shell.
With the indirect-direct system, hot gases first pass down a central tube
coaxial with the dryer shell and return through the annular space between the
tube and shell. The material being cascaded in this annulus picks up heat from
the gases and also by conduction from direct contact with the central tube. This
design is thermally very efficient. Again, while there are a number of proprietary
designs employing different systems of air flow, in the main these dryers are of
two types, namely, parallel and countercurrent flow. With parallel flow, only high
moisture content material comes into contact with the hot gases and, as a result,
higher evaporative rates can be achieved than when using countercurrent flow.
In addition, many thermo-sensitive materials can be dried successfully by this
method. Such an arrangement lends itself to the handling of pasty materials
since the rapid flashing off of moisture and consequent surface drying limits the
possibility of wall build-up or agglomeration within the dryer. On the other hand,
countercurrent operation normally is used where a low terminal moisture
content is required. In this arrangement, the high temperature gases are brought
into contact with the product immediately prior to discharge where the final
traces of moisture in the product must be driven off.
FIG. 9 Fixed tube rotary dryer
13

14. In both these types, however, gas velocities can be sufficiently high to produce
product entrainment. They therefore would be unsuitable for low density or fine
particle materials such as carbon black. In such cases, the indirect-fired
conduction type dryer is more suitable since the dryer shell usually is enclosed
in a brick housing or outer steel jacket into which the hot gases are introduced.
As heat transfer is entirely by conduction, conventional flighting and cascading
of the material is not used. Rather, the inside of the shell is fitted with small
lifters designed to gently turn the product while at the same time maintaining
maximum contact with the heated shell.
Another type of indirectly heated dryer which is particularly useful for
fine particles or heat sensitive materials is the steam-tube unit. This dryer can
be of either the fixed tube variety equipped with conventional lifting flights
designed to cascade the product through a nest of square section tubes, or
alternately, a central rotating tube nest can be used. Figure 9 shows a fixed-tube
rotary dryer which normally has an electrical vibrator fitted to the tube nest in
order to eliminate the possibility of bridging of the product with consequent loss
of heat transfer surface. Since the heat exchanger is positioned within the
insulated shell in this type of dryer, the air rejection rate is extremely low and
thermal efficiencies are high. In general, this design is suitable only for free
flowing materials.
A considerable amount of work has been done on the development of various
types of lifting flights, all designed to produce a continuous curtain of material
over the cross section of the dryer shell. Other special configurations involve
cruciform arrangements to produce a labyrinth path. The object is to give longer
residence times where this is necessary. When the diffusional characteristics of
the material or other process considerations call for extended residence times,
these machines no doubt will continue to find application.
pneumatic dryers
Where total dispersion of the product in a heated gas stream can be achieved
with a significant increase in evaporative rates, pneumatic or continuous fluid
bed dryers are preferred. The capital cost of these alternatives generally is lower
and maintenance is limited to such components as circulating fans and rotary
valves. When considering these two types of dryers, it is convenient to examine
them together since both share similar characteristics. Both employ forced
convection with dispersion of the feedstock and as a result of the intimate contact
between the drying medium and the wet solids, both exhibit much higher drying
rates than any of the other dryers previously mentioned.
In a fluidized bed dryer, the degree of dispersion and agitation of the wet solids
is limited whereas in a pneumatic dryer, the degree of dispersion is total and the
material is completely entrained in the gas stream. This often ic lirned to
advantage as the drying medium is used as a vehicle for the pz~tiallydried
product. Other operations such as product classification also can be carried out
where required. A further feature of fluid bed and flash dryers is that the method
of operation allows many temperature-sensitive materials to be dried without
thermal degradation due to the rapid absorption of the latent heat of vaporization.
This generally permits high-rate drying whereas in other types of dryers, lower
temperatures would be necessary and correspondingly larger and more costly
equipment would be required.
14

15. EXHAUST 0
FAN -
FILTER
MAIN FAN
0 Q VENTURI
FIG. 10 (A) Multipass and (B)air recycle arrangements in flash dryers
A very good degree of temperature control can be achieved in fluid bed dryers
and the residence time of the material can be varied either by the adjustment of
the discharge weir or by the use of multi-stage units. Similarly, the residence
time in the flash dryer can be adjusted by the use of variable cross-sectionalarea
and therefore, variable velocity. In addition, multiple effect columns can be
incorporated to give an extended path length or continuous recirculatory systems
employing both air and product recycle can be used as illustrated in Fig. 10.
Generally speaking, the residence time in fluidized bed dryers is measured in
minutes and in the pneumatic dryer in seconds. Both dryers feature high thermal
efficiencies particularly where the moisture content of the wet feed is sufficiently
high to produce a significant drop between inlet and outlet temperatures. While
the condition of the feed in the pneumatic dryer is somewhat less critical than
that in the fluid bed dryer owing to the fact that it is completely entrained, it still
is necessary to use backmixing techniques on occasion in order to produce a suit-
able feed. A variety of feedingdevices are used with these machines.
In fluid bed dryers, special attention must be paid to the nature of the proposed
feed since one condition can militate against another. To some extent, this is
reflected in the range of variation in the figures given in Table 1for evaporative
rates. If a large or heavy particle is to be dried, the fluidizing velocities required
may be considerable and involve high power usage. In such circumstances, if the
moisture content is low and the surface/mass ratio also is low, the thermal
efficiency and evaporation would be low. This would make selection of a fluid bed
dryer completely unrealistic and probably would suggest a conventional rotary
dryer for the application.
Another case is that in which the minimum fluidizing velocity is so low that a
dryer of very large surface dimensions is necessary to obtain the required thermal
input. This also occurs in problems of fluid bed cooling and usually is overcomeby
the introduction or removal of thermal energy by additional heating or cooling
media through extended-surface heat exchangers immersed in the bed.
15

16. With both types of dispersion dryer many configurations are available. While
the power requirements of each usually is well in excess of other dryers due to
the use of high efficiency product recovery systems, the smaller size of the fluid
bed dryer compared with conventional rotaries and the fact that the flash dryer
can be arranged to fit in limited floor space makes them very attractive.
hand dryers
When selecting a dryer, it always is necessary to consider the final product form.
When the degree of product attrition common to pneumatic and fluidized bed dryer
operation is unacceptable, continuous band or apron dryers can provide an effective
solution. These are widely used where moderately high rates of throughput on a
continuous basis are called for. The most commonly used continuous band dryer is
the single pass machine employing through-air circulation. Alternatively, and
where there is limited floor space or a possible need for long residence time, multi-
pass units are used with the conveyors mounted one above the other. In similar
circumstances, another special type of multi-deck dryer can be used which employs
a system of tilting trays so that the product is supported on both the normal
working and the inside of the return run of the conveyor band. This arrangement
considerably increases the residence time within the dryer and is particularly
useful where the product has poor diffusional characteristics.
The method of airflow employed on these dryers is either vertically downward
through the material and the supporting band or alternatively upward. Sometimes
a combination of the two may be dictated by the nature of the wet feed. It
occasionally happens that extruded materials have a tendency to coalesce when
deposited on the band, in which case one or more sections at the wet end of the
dryer may be arranged for upward air flow to reduce the effect. Wherever possible,
through-air circulation is used as opposed to transverse air flow. This results in
greatly increased evaporative rates as may be seen from Table 1.
An illustration of the relatively high performance of a band dryer operating on
this system as compared with a unit having transverse air flow can be cited in a
case involving the processing of a 70% moisture content filter cake. When this
material is dried in a conventional unit, the cycle time is in the region of 28 hours.
This is reduced to 55 minutes in the through-circulation band dryer largely as a
result of using an extruder/preformer designed to produce a dimensionally stable
bed of sufficient porosity to permit air circulation through the feed.
In view of this, transverse air flow usually is used only where the type of
conveyor necessary to support the product does not allow through-flow or where
the product form is not suitable for this method of airflow. The most usual
method of heating is by steam through heat exchangers mounted in the side
plenums or above the band although direct oil and gas firing sometimes are used.
In such cases, the products of combustion normally are introduced to a hot well
or duct at an elevated temperature from where they are drawn off and mixed
with circulating air in each zone or section of the dryer.
Another alternative with direct firing is to use a series of small individual
burners positioned so that each serves one or more zones of the dryer. Typical
single pass dryers of modular construction are illustrated in Figs. 11,E and 13.
With this type and size of dryer, the average product throughput is about 5600
Ibs/hr and involves an evaporation of 1600lbs/hr moisture. It is not unusual, how-
16

17. FIG. 11 Continuous conveyor band dryer arrangement fordirectgas firing
ever, to find equipment with evaporative capacities of 3000 lbshr. Such outputs
involve quite a large band area with correspondingly large floor area requirements.
Various types of feeding arrangements are available to spread or distribute the wet
product over the width of the band. Here again, the nature of the feed is an
important prerequisite for efficient drying. Steam heated, finned drums have been
used as a means of producing a partially dried, preformed feed. While the amount
of pre-drying achieved is reflected in increased output for a given dryer size or,
alternatively, enables a smaller dryer to be used, these items are usually much
more costly than many of the mechanical extruders which are available.
Generally, these extruders operate with rubber covered rollers moving over a
perforated die plate with feed in the form of pressed cakes or more usually, as the
discharge from a rotary vacuum filter. Others of the pressure type employ a gear
pump arrangement with extrusion taking place through a series of individual
nozzles while some use screw feeds which usually are set up to oscillate in order
to obtain effective coverage of the band. Alternate designs include rotating cam
blades or conventional bar-type granulators although the latter often produce
a high proportion of fines because of the pronounced shearing effect. This makes
the product rather unsuitable due to the entrainment problems which can occur.
Each of the types available is designed to produce continuous/discontinuous
extrudates or granules, the grid perforations being spaced to meet product charac-
teristics. In selecting the proper type of extruder, it is essential to carry out tests
on semi-scaleequipment as no other valid assessment of suitability can be made.
As a further illustration of the desirability of using a preforming technique,
tests on a designated material exhibited a mean evaporative rate of 1.9 lbs/ft2hr
when processed in filter cake form without preforming. When extruded, however,
17

18. I
. .
FIG. 12 Multi-stageband dryer
FIG. 13 Rand dryer and extruder for dye stuffs
18

19. the same material being dried under identical conditions gave a mean evaporative
rate of 3.8 lbs/ft2hr. This indicates, of course, that the effective band area re-
quired when working on extruded material would be only 50?6of that required in
the initial test. Unfortunately, the capital cost is not halved as might be expected
since the feed and delivery ends of the machine housing the drive and terminals
remain the same and form an increased proportion of the cost of the smaller
dryer. While the cost of the extruder also must be taken into account in the
comparison, cost reduction still would be about 15%.Of course, there are other
advantages which result from the installation of the smaller dryer. These include
reduced radiation and convection losses and a savings of approximately 40'E in
the floor area occupied.
This type of plant does not involve high installation costs and both mainte-
nance and operating labor requirements are minimal. Since they generally are
built on a zonal principle with each zone having an integral heater and fan,
a good measure of process control can be achieved. Furthermore, they provide
a high degree of flexibility due to the provision of variable speed control on
the conveyor.
selection
The application ratings given in Table 1 in which are listed the approximate
mean evaporative rates for products under a generic classification are based on
the writer's experience over a number of years in the design and selection of
drying equipment. It should be appreciated, however, that drying rates vary
considerably in view of the variety of materials and their widely differing
chemical and physical characteristics. Furthermore, drying conditions such as
temperature and the moisture range over which the material is to be dried have
a very definite effect on the actual evaporative rate. It is important, therefore,
when using the figures quoted that an attempt is made to assess carefully the
nature of the product to be handled and the conditions to which it may be
subjected in order to achieve greater accuracy. With these factors in mind, it is
hoped that the foregoing observations on drying techniques along with the
appropriate tables and curves will provide a basis for making an assessment
of the type, size and cost of drying equipment.
In making a preliminary assessment for dryer selection, there are a number
of further points to consider:
1)What is the nature of the up-stream process? Is it feasible to modify
the physical properties of the feed, e.g., mechanical dewatering to
reduce the evaporative load?
2)Does the quantity to be handled per unit time suggest batch or
continuous operation?
3)From a knowledge of the product, select the type(s) of dryer which it
appears would handle both the wet feedstock and the dried product
satisfactorily and relate this to the equipment having the highest
application rating in Table 1.
4) From a knowledge of the required evaporative duty, i.e., the total mass
of water to be evaporated per unit time and by the application of the
approximate Ea" figure given in Table 1,estimate the size of the dryer.
5)Having established the size of the dryer on an area or volumetric
basis, refer to the appropriate curve in Figs. 14 and 15 and establish
an order of cost for the particular type of unit.
19

20. continuousfluidbed dryers
Although a great deal of funclamental work has been carried out into the
mechanics of drying which enables recommendations to be made, it is most
desirable for pilot plant testing to be done. This is not only necessary to support
theoretical calculations but also to establish whether a particular dryer will
handle the product satisfactorily. In the final analysis, it is essential to discuss
the drying application with the equipment manufacturer who has the necessary
test facilities to examine the alternatives objectively and has the correlated data
and experience from field trials to make the best recommendation.
20

21. Efficient energy
utilizationin
drying
It generally is necessary to employ thermal methods of drying in order to reach what
is termed a “commercially dry” condition. As a result, dryingforms an important
part of mostfood and chemical processes and accountsfor a significantproportion of
totalfuel consumption.
The rapid escalation offuel costs over thepast tenyears together with theprevailing
uncertainty of future availability, cost and possible supply limitations highlights the
continued need to actively engage in the practice of energy conservation. This paper
discusses some of thefactors affecting dryer efficiency and outlines certain techniques
designed to reduce the cost ofthe dryingoperation.
In making an appraisal of factors affectingdryer efficiency, it is useful to draw
up a checklist of those items which have a significant bearing both on operation
and economy. It also is appropriate prior to examining various options to
emphasize that in the final analysis, the primary concern is the “cost per unit
weight” of the dried product. This single fact must largely govern any approach
to dryer selection and operation. Additionally, there is an increasing necessity to
consider the unit operation of drying in concert with other upstream processes
such as mechanical de-watering and pre-forming techniques in a “total energy”
evaluation.
In considering which factors have a bearing on dryer efficiency and what can
be done to maximize that efficiency, the following aims should be kept in mind:
1)maximum temperature drop across the dryer system indicating
high energy utilization. This implies maximum outlet temperature,
2) employ maximum permissible air recirculation, Le., reduce to an
absolute minimum the quantity of dryer exhaust, having due
regard for humidity levels and possible condensation problems,
3) examine the possibility of counter-current drying, i.e., two-stage
operation with exhaust gases from a final dryer being passed to a
pre-dryer, or alternatively, preheating of incoming air through the
use of a heat exchanger located in the exhaust gases,
4)utilize ‘direct’ heating wherever possible in order to obtain max-
imum heat release from the fuel and eliminate heat exchanger loss,
5) reduce radiation and convection losses by means of efficient
thermal insulation.
21

22. While the above clearly are basic requirements, there are a number of other areas
where heat losses occur in practice, viz: sensible heat of solids, etc. Furthermore,
other opportunities exist for improving the overall efficiency of the process.
These are related to dryer types, methods of operation, or possibly the use of a
combination of drying approaches to obtain optimum conditions.
types of dryers
Considering the requirements for high inlet temperature, the "flash" or
pneumatic dryer offers great potential for economic drying. This stems from the
simultaneous flash cooling effect which results from the rapid absorption of the
latent heat of vaporization and enables high inlet temperatures to be used without
thermal degradation of the product. This type of dryer also exhibits extremely
high evaporative rate characteristics but the short gas/solids contact time can in
certain cases make it impossible to achieve a very low terminal moisture.
However, a pneumatic dryer working in conjunction with a rotary or a
continuous fluidized bed dryer provides sufficient residence time for diffusion of
moisture to take place. Such an arrangement combines the most desirable
features of two dryer types, provides a compact plant and conceivably, the
optimum solution.
As a further example, it is common practice in the process industries to carry
out pretreatment of filter cake materials using extruders or preformers prior to
drying. The primary object is to increase the surface area of the product in order
to produce enhanced rates of evaporation and smaller and more efficient drying
plants. 'It is interesting to examine the improvement in energy utilization in a
conveyor band dryer resulting from a reduction in overall size of the dryer simply
because of a change in the physical form of the feed material.
Consider a typical case. As a result of preforming a filter cake, the evaporative
rate per unit area increases by a factor of 2, viz: 1-0-3-8lb/ft2/h. This permits the
effective dryer size to be reduced to one half of that required for the non-
preformed material and, for a plant handling one ton/h of a particular product at
a solids content of 60 percent, the radiation and convection losses from the
smaller dryer enclosure show an overall reduction of some 140,000 Btu/h.
Although it is necessary to introduce another processing item into the line to
carry out the pretreatment, it can be shown that the reduction in the number of
dryer sections and savings in horsepower more than offset the power required for
the extruder. This differential is approximately 15 kW and the overall saving in
total energy is some 10percent, i.e., 466 kW compared with 518 kW. Furthermore,
the fact that the dryer has appreciably smaller overall dimensions provides an
added bonus in better utilization of factory floor space.
drying techniques
If, as stated, the prime concern is the cost per pound of dried product, then major
savings can be achieved by reducing the amount of water in the feed stock to a
minimum prior to applying thermal methods of drying. Again, since it generally
is accepted that the mechanical removal of water is less costly than thermal
drying, it follows that considerable economies can be made when there is a
substantial amount of water that can be readily removed by filtration or
centrifuging. This approach, however, may involve changing the drying tech-
nique. For example, whereas a liquid suspension or mobile slurry would require a
22

23. spray dryer for satisfactory handling of the
feed,the drying of a filter cake on the other
hand calls for a different type of dryer and
certainly presents a totally different mate-
rials handling situation.
In approaching a problem by two alter-
native methods, the overall savings in
energy usage may be considerable as illus-
trated in Fig. 1 which is a plot of feed
moisture content versus dryer heat load.
As may be seen, the difference in thermal
energy used in (A) drying from a moisture
content of 35 percent down to 0.1 percent,
or alternatively, (B) drying the same mate-
rial from 14 percent down to 0.1 percent, is
24.3x lo6Btu. “Route A ’ involves the use
of a spray dryer in which the absolute
weight of water iq the feed stock is con-
siderably greater than in “route B,” which
involves the use of a thermoventuri dryer.
Both types, incidentally, come under the
classification of dispersion dryers and gen-
erally operate over similar temperature
ranges. As a result, their efficiencies are
substantially the same.
It will be apparent from Table 1 that the spray dryer is at a considerable
disadvantage due to the requirements for a pumpable feed. This highlights the
TABLE 1 Comparison of the operating conditions and energy utilization of pneumatic and
spray dryers processing concentrates
Note An assumed volumetric flow of 3 ftVmin/ftz of filter area with approximately 1 hp/lO ft3/min have
been used as being typical of the flow rates and energy requirements for the filtration or mechanical
dewatering equipment
23

24. Large Thermo Venturi dryer processes various concentrates.
need to consider the upstream processes and, where a particular “route” to
produce a dry product requires slurried feed, to consider whether a better
alternative would be to de-water mechanically and use a different type of dryer.
Comparing these two alternative methods for drying a mineral concentrate,
the spray dryer uses a single step atomization of a pumpable slurry having an
initial moisture content of 35 percent and employs thermal drying techniques
alone down to a final figure of 0.1 percent. The alternate method commences
with the same feedstock at 35 percent moisture content but employs a rotary
vacuum filter to mechanically dewater to 14 percent. From that point, a
24

25. pneumatic dryer handles the cake as the second of two stages and thermally dries
product to the same final moisture figure.
While the difference in thermal requirements for the alternate “routes”
already has been noted, it also is necessary to take into account the energy used
for mechanical dewatering in accordance with assumptions outlined in the
footnote to Table 1. From reference to this chart, it will be evident that the
two-stage system requires approximately one-third of the energy needed by the
single stage operation but that the savings are not limited to operating costs
alone. There also are significant differences in the basic air volumes required for
the two dryers which means that ancillaries such as product collection/gas
cleaning equipment will be smaller and less costly in the case of the pneumatic
dryer. The same applies to combustion equipment, fans and similar items. In
actual fact, the chart shows that capital cost savings are in the nature of 50
25

26. TABLE 2 Comparisonof self-inertizingt.v. dryer versus total rejectionI.v.dryer
percent in favor of the filter and pneumatic drying system.
This illustration amply demonstrates the need for a more detailed consideration
of drying techniques than perhaps has been the case in the past. It also points to
the desirability of examining a problem on a “total energy” basis rather than
taking the drying operation in isolation. Such a full evaluation approach often
will prove it advantageous to change technology, i.e., to use a different type dryer
than the one which possibly has evolved on the basis of custom and practice.
operating economies
Looking at dryer operations from the point of improving efficiency, it is
interesting to see the savings which accrue if, for example, a pneumatic dryer is
used on a closed circuit basis, i.e., with the recycle of hot gases instead of total
rejection. Briefly, the “self-inertizing” pneumatic dryer consists of a closed loop
as shown in Fig. 2 with the duct system sealed to eliminate the ingress of
ambient air. This means that the hot gases are recycled with only a relatively
small quantity rejected at the exhaust and a correspondingly small amount of
fresh air admitted at the burner. In practice, therefore, oxygen levels of the order
of 5 percent by volume are maintained. This method of operation raises a number
of interesting possibilities. It permits the use of elevated temperatures and
provides a capability to dry products which under normal conditions would
oxidize. The result is an increase in thermal efficiency. Furthermore, the amount
of exhaust gas is only a fraction of the quantity exhausted by conventional
pneumatic dryers. This is clearly important wherever there may be a gaseous
effluent problem inherent in the drying operation.
A comparison of a self-inertizing versus a conventional pneumatic dryer is
detailed in Table 2 with the man.y advantages clearly apparent. Drying with
closed circuit operation is tending toward super heated vapor drying and in
practice, 40-50 percent of the gases in circulation are water vapor. This suggests
that the specific heat of the gas will be approximately 0.34 Btu/lb/”F compared
with about 0.24 Btu/lb/”F in a conventional total rejection dryer. Since the mass of
gas for a given thermal capacity is appreciably less than in a conventional dryer
and, as previously mentioned, oxygen levels are low, much higher operating
temperatures may be used. This permits a reduction in the size of the closed
circuit dryer. For the case given in Table 2 and comparing the two dryers on the
26

27. basis of their cross-sectional areas, the total rejection dryer would have rather
more than double the area of the closed circuit system. There are obvious
limitations to the use of such a technique but the advantages illustrated are very
apparent, especially the savings in thermal energy alone, viz: 695,000 Btu/h
representing some 29.2 percent of the total requirements of the conventional
drying system.
effect of changing feed rates
Since it has a significant bearing on effi-
ciency, another factor of major importance
which must be considered when designing
dryers of this type is the possible effect of
reducingfeed rate. What variation in quan-
tity of feed is likely to occur as a result of
operational changes in the plant upstream
of the dryer and as a result, what turndown
ratio is required of the dryer? With spray
dryers and rotary dryers, the mass airflow
can be varied facilitatingmodulation of the
dryer when operating at reduced through-
puts.
This, however, is not the case with
pneumatic and true fluidized bed dryers
since the gases perform a dual function of
providing the thermal input for drying and
acting as a vehicle for transporting the
material. Sincethe mass flow has to remain
constant,the only means open for modulat-
ing these dryers is to reduce the inlet tem-
perature. This clearly has an adverseeffect
on thermal efficiency.It therefore isof para-
mount importance to establish realistic
production requirements. This will avoid
the inclusion of excessive scale-up factors or oversizing of drying equipment and
thereby maximize operating efficiency.
Fig. 3 illustrates the effects on ther.ma1 efficiency of either increasing the
evaporative capacity by increased inlet temperatures or alternatively, reducing
the inlet temperature with the exhaust temperature remaining constant at the
level necessary to produce an acceptable dry product. While the figure refers to
the total rejection case of the previous illustration where design throughput
corresponds to an efficiency of 62.4 percent, the curve shows that if the unit is
used at only 60 percent of design, dryer efficiency falls to 50 percent. The
converse, of course, also is true.
In this brief presentation, an attempt has been made to highlight some of the
factors affecting efficiency in drying operations and promote an awareness of
where savings can be made by applying new techniques. While conditions differ
from one drying process to another, it is clear that economies can and should be
made.
27

28. newdevelopments,neweconomics
For many years, spray drying has been one of the most energy consuming of the
drying processes but nevertheless, one that is essential to the production of dairy
and food product powders. The problem since the energy shock of a dozen or so years
ago has been to reduce processing costs while maintaining product quality. The
solution has been twofold: the development of new technology to provide greater
dryer efficiency and the introduction of new approaches to the recovery and reuse of
energy from the primary process.
drying principle
Spray drying basically is accomplished by atomizing feed liquid into a drying
chamber where the small droplets are subjected to a stream of hot air and are con-
verted to powder particles. As the powder is discharged from the drying chamber, it
is passed through a powder/air separator and collected for packaging. Most spray
dryers are equipped for primary powder collection at an efficiency of about 99.5%
and may be supplied with secondary collection equipment if necessary.
It should be noted that spray dry-
ing systems as manufactured by
APV Crepaco Inc.,Dryer Division,
are available in a number of drying
chamber configurations and with
a choiceof feed atomizing devices.
As shown in Figures 1 and 2,
basic dryingchambers aredesigned
either with a flat or conical bottom.
The flat bottom plant obviously
takes up less space when height
restrictions are a factor. An added
advantage is the use of a rotating
pneumaticpowder dischargerwhich
continuously removes powder and
thereby functions with a fixed prod-
uct holdingtime. Dependingon the
FIG 1 Flat bottomed drying chamber incorporates rotating
powder discharger and can be equipped with an air broom
product involved, a pneumatic powder cooling system also may be installed.
Typically, this type of dryer is used for egg products, blood albumin, tanning agents,
ice cream powder and toppings. The flat bottom chamber, incidentally, may be
provided with an air broom which is indicated by the color section in Figure 1.By
blowing tempered air onto the chamber walls while rotating, this device blows away
loose powder deposits and cools the chamber walls to keep the temperature below
the stick point of certain products. Some items with which the air broom technique
has been successful are fruit and vegetable pulp and juices, meat extracts and blood.
28

29. r
FIG 2 Typical spray dryer with conical chamber Arranged with
high pressure atomizer and air broom
The Figure 2 sketch, meanwhile, shows a conical bottom chamber arrangement
with a side air outlet, high pressure nozzle atomization, and pneumatic product
transport beneath the chamber. This single stage dryer is very well suited to
making relatively large particles of dairy products or proteins for cattle feed. If the
product will not withstand pneumatic transport, it may be taken out unharmed
directly from the chamber bottom.
The dryer is also suitable for use as a spray cooling system. While the inlet air is
cooled or used at ambient temperature, a feed product such as wax is heated and
melted. By atomizing the melted product into the cold air stream, a hardening takes
place and powder particles build up. Encapsulation of components such as steroids
used to increase or stimulate animal growth may be carried out in this way. The
conical type chamber also may be provided with an air broom for use in the drying of
sticky and heat sensitive products like orange juices or various fruit and vegetable
pulps. These products, however, normally require the addition of maltodextrine,
gumarabic or sodium caseinate “carriers” in order to produce a non-sticking powder.
atomization techniques
Proper atomization of liquid as dictated by the properties of the feed and the desired
final product characteristics is essential for satisfactory drying and for producing a
powder of prime quality. To meet varying parameters, APV offers a selection of
atomizing systems: centrifugal disc as indicated in Figure 1,high pressure nozzle as
shown in Figure 2, and steam injection.
With centrifugal or spinning disc atomization, liquid feed is accelerated to a
velocity in excess of 300 ft/sec to produce fine droplets which mix with the drying
air. Particle size can be controlled by wheel speed, feed rate, liquid properties, and
atomizer design. There are no vibrations, little noise, and small risk of clogging.
Furthermore, the system operates with low power consumption and provides feed
rate capacities in excess of 200 tons/hr.
With the pressure nozzle system, liquid feed is atomized when forced under high
pressure through a narrow orifice. This approach offers great versatility in the
selection of the spray angle, direction of the spray, and positioning of the atomizer
within the chamber. It also allows co-current, mixed-current or counter-current
drying with the production of powders having particularly narrow particle size
29

30. FIG 3 Steam is added around
and into the atomizing disc to
minimize air in the atomized
liquid droplet
FIG. 4 Spray dried autolyzed yeast
(2000x magnification).Particle is
hollow and filled with crevices.
FIG 5 Spray dried autolyzed yeast with '
steam injection (2OOOx magnification)
Particle is solid and essentially void-free
distribution and/or coarse characteristics. Since the particle size is dependent on the
feed rate, dryers with pressure nozzles are somewhat limited relative to changing
product characteristics and operating rates.
These two types of atomization are well established, often are incorporated into the
same dryer, help produce a most acceptable product, and offer maximum flexibility
for drying a wide variety of products.
To produce an even better than average product with a significant increase in
bulk density and fewer fines, the steam injection technique has been refined to a
point that allows its use in large drying operations.
As feed droplets are subjected to heated air during conventional drying operations,
water evaporates and diffuses to the particle surface, forming a hard shell. Beneath
this shell, a small amount of residual moisture spreads to void areas, vaporizes,
mixes with air, expands and penetrates weak points. This creates porous particles
as shown in the Figure 4 microphoto of spray dried autolyzed yeast.
When steam is added at the right point and in the correct amount to a properly
designed atomizer, air encapsulated in the liquid droplet is eliminated. Drying
particles thereby collapse to produce a dense, void-free powder as illustrated in the
Figure 5 microphoto. Control of the amount of steam injected permits a precise
adjustment in powder bulk density. Furthermore, reduction of air-exposed surfaces
often reduces product oxidation and prolongs powder shelf life.
cutting drying costs
The best way to reduce energy usage in spray drying is,of course, to try and reduce
the specific energy consumption of the process. Experience has shown that for
many products, the dryer inlet temperature can be raised, the outlet temperature
30

31. I
FIG 6 FLUID BED INSTANTIZING/AGGLOMERATlONPRINCIPLE
Powder is conveyed to the inlet chute of the vibrating fluid bed by a
metering device equipped with a variable speed drive The fluid bed
is divided into three sections. each with an air inlet system
In the first section, powder may be humidified by a wetting
agent and then agglomerated to the required particle
size Agglomerates are dried to the desired
moisture content in the second section and
cooled in the third During the process,
air-entrained fines are recoveredin a
cyclone collector for return to the
wetting zone The residence time
and temperature can be varied for
optimum drying conditions
lowered, and a larger temperature differential thus created. While this procedure
substantially cuts energy needs and does not harm most heat sensitive products,
care must be taken and a proper balance struck. The nature of the product usually
defines the upper limit, ie., the denaturation of milk protein or discoloring of other
products. A higher inlet temperature requires close control of the air flow in the
spray drying chamber and particularly around the atomizer. Furthermore, it must
be noted that a lower outlet temperature will increase the humidity of the powder.
mu1t i-stagt dryers
Ten years ago, the typical in- and outlet temperatures of a milk dryer were
356/203"F. Today,the inlet temperature has been increased to about 428°For higher
and the outlet reduced to around 185°F.This is made possible by the development of
multi-stagedrying systems.
With an ultimate goal of obtaining optimum drying economies combined with
improved powder, many plants now are being equipped with an APV fluid bed
dryerkooler. Figure 6 pictures this unit and describes its principle of operation
31

32. AIR OUT
3
)DUCT
JUT
""
dFIG 7 Two-stage drying with external
fluid bed
while Figures 7and 8 show the fluid
bed mounted externally with two-
and three-stage dryers respectively.
A variation of fluid bed dryingkool-
ing also can be integrated within the
drying chamber as sketched in Fig-
ure 9.
Using the two-stage design as an
example, powder at approximately
7%moisture is discharged from the
primary drying chamber to the fluid
bed for final drying and cooling.This
prolongs the drying time from about
22 seconds in a typical single stage
dryer to morethan 3minutes,making
it possible to use lowerdryer temper-
atures and to reduce heating energy
usage by 15-20%.At the same time,
the slower and more gentle drying
processproduces moresolid particles
of improved density and solubility.
If desired, the fluid bed can be de-
signed for rewet instantizing or
powder agglomeration and for the
addition of a surface active agent
such as lecithin.
While the specific consumption of
energy in the fluid bed process may
be relatively high, the evaporation is
minor compared with the spray dry-
ing process and the total energy use
therefore is lower. Figure 10 shows
the specificenergycosts asa function
FLUID BED
UCT
T
OUT
FIG. 9 Fluid bed is integrated into
two-stage dryer.
FLUID BED EVAPORATION
FIG. 10 Specific energy cost as a function of the
water evaporated in the fluid bed
32

33. water evaporation, total
powder production with 3.5% H20
solids in feed
inlet temperature spray dryer
outlet temperature spray dryer
water content from spray dryer
heat consumption
electric consumption
cooling consumption
operating hours per year
heat costs (4.32/M Btu)
electric costs (5&/kwhr)
cooling costs (4.32/M Btu)
energy costs per year
capital investment
investment costs (deprec. 8 yrs.,
energy and investment costs
interest 10%per year)
unit
Ibs/hr
Ibs/hr
%
"F
"F
Q/O
Btu/hr
kw
Btu/hr
hrs
$/yr
$/yr
$/yr
$
$
$/yr
$/yr
drying
single
stage
4,400
4,365
48
410
212
3.5
8,432,000
167
196,000
6,000
219,000
50,000
5,000
274,000
790,000
144,000
418,000
ocess
two-
stage
4,400
4,365
48
446
185
6 5
7,132,000
151
86,000
6,000
185,000
45,000
2,000
232,000
922,000
167,000
399,000
TWO-STAGEDRIED
DAIRYPRODUCTS
skim milk powder:low,
medium and high heat
0 heat stable skim milk
powder
0 straight through instant
skim milk dustfree
powder
0 whole milk powder
0 high density skim or
whole milk powder made
with steam injected
atomization
skim and whole milk
powdersuitablefor
instantizing
0 cream powder
filled milk powderfrom
yogurt powder
0 non.hygroscopic whey
whey protein powder
ice cream powder
cheese powder
skim milk with added fat
powder
TABLEA Economic comparison: single, two-stage dryers.
of the water evaporated in the fluid bed or the residual moisture in the powder from
the spray drying to the fluid bed process. The curves are only shown qualitatively
because the absolute values depend on energy prices, inlet temperatures, required
residual moisture content and other such parameters. The total cost curve shows a
minimum that defines the quantity of water to be evaporated in the fluid bed to
minimize the energy costs.
An economiccomparison of typical single and twostage drying systems is charted
in Table A.
heat recovery equipment
Although it is possible to reduce direct power use by widening the gap between spray
dryer in-and outlet temperatures, optimum thermal conditionsgenerally require the
use of heat recuperators as well. A few approaches are shown in Figure 11.
Despite the many recuperators available for recovering heat from drying air, only
a few are suited for the spray drying process. This is due to the dust-ladenair which
is involved in the process and which tends to contaminate heat exchange surfaces.
To be effective, therefore, recuperators for spray dryer use should have the following
properties:
modular system large temperature range
high thermal efficiency stainless steel
low pressure drop large capacity
automatic cleaning low price
Two such recuperators are the air-to-airtubular and the air-to-liquidplate designs.
33

34. FIG. 12 Air to air tubular
heat recuperators.
WASTEAIR
air-to-airheatrecuperator:Foroptimum
flexibility, this heat exchanger isof modular
design, each module consisting of 304 tubes
welded to an end plate. Available in various
standard lengths, the modules can be used
for counterflow, cross flow or two-stage
counterflowas sketched in Figure 12.While
the first two designSare based on well
known principles, the two-stage counter-
flow (patent pending) system is an APV
Crepaco development. This recuperator
consists of two sections, a long dry element
and a shorter wet section. Duringoperation,
hot waste air is passed downward through
the dry module tubes and cooled to just
above dew point. It then goes to the wet
section where latent heat is recovered. The
inner surfaces of these shorter tubes are
kept wet by recirculating water. The sys-
tem, therefere, not only recovers free and
latent heat from the waste air but also
scrubs the air and reduces the powder emis-
sion by more than 80%.Effectiveness and
AIRCOOLER
FIG. 11 Spray drying
plant with (top) liquid
coupled recuperator,
(middle)single stage
recuperator and (bottom)
two-stage recuperator
34
INLET
WASTEAIR
.ER -FLOW
CROSS-FLOW
WASTE AIR
SECTION ,'
WET ,
SECTION
COUNTER-FLOW

35. WAIAIFR OUl L F l
WATER I
FIG 13Thermoplate for air
to liquid plate recuperator.
c
SNLET
economy are such that the system replaces a conventional recuperator/scrubber
combination.
air-to-liquidrecuperator:In cases where an air-to-airtubular recuperator is imprac-
tical or uneconomical because of space limitations or the length of the distance
between the outlet and inlet air to be preheated, a liquid heat recovery system is
available. This plate recuperator as shown in Figure 13uses various standard plates
as the base with waste air being cooled in counterflow by circulating a water/glycol
mixture. At high temperature, circulating thermo oils are used. Heat recovered by
this means generally is recycled to preheat the drying air but can be used for other
purposes such as heating water, CIP liquids or buildings.
other heat recovery methods
While many spray drying plants are equipped with bag filters to minimize emissions
to the environment and recover valuable powder, the filtration system can be
coupled with a finned tube recuperator for heat recovery purposes. This type of
recuperator is particularly effective when the air is not dust-loaded.It is compact,
very flexible and normally, very inexpensive.
Plants equipped with scrubbers also provide a very inexpensive possibility for
heat recovery. Since the scrubber itself is a heat exchanger, part of the heat from
waste air can be captured and recycled by passing the scrubber water through a
finned tube preheater. An added benefit is lower water consumption due to a reduc-
tion in scrubber liquid evaporation. However, if the scrubber liquid is cooled inten-
sively, such an efficient recovery of latent heat will occur that a net condensation in
the scrubber will follow. This will, of course, create another pollution problem if the
concentration of solids in the liquid is so low that it cannot be returned to the pro-
cess. To offset this effect, a recuperator may be installed immediately before the
scrubber. After passing through the recuperator,the air temperature will be near dew
point when the dimensioning is correct and practically only latent heat will be freed
in the scrubber. This double heat recovery forms the basis for the previously
mentioned APV two-stage recuperator.
35

36. new directions
In the field of spray drying,the last ten years havewitnessed many new developments
initiated mainly to meet two demands from food and dairy processors - better
energy efficiency and improved functional properties of the finished powder. There
also has been much progress made in the automation of drying systems and in
complying with environmental requirements.
While demands for energy efficient drying may have been somewhat relaxed
recently with the stabilizing of energy costs, the fact remains that spray dryers and
evaporatorsareamong the most energy intensive processing systems and the future
of energy still is uncertain. Government forecasters from the Energy Information
Administration state that dependence on foreign oil is likely to reach historic peaks
in the 1990s.Oil imports by 1994may reach 9.9-millionbarrels per day or 55percent
of supplies and by the end of the century, the figures could be up to 11-millionbarrels
per day or 59 percent. The highest dependency on record, incidentally, was in 1977
when 46 percent of oil supplies were imported.
Of equal importance has been an increased show of interest in the production of
high-value specialty dried products instead of straight commodity products. This
has been accentuated by the government’s Dairy Reduction Program whereby there
will be fewer milk solids available. Processors naturally are looking into ways to
better use these availablesupplies by producing products with improved functional,
nutritional and handling properties and coincidentally,products which can demand
higher profit margins.
This is particularly noticeable in the utilization of whey and whey product deriv-
atives. Alarger percentageof the total whey produced today ishandled and processed
so that it will be suitable for human consumption. It also is being refined into whey
protein concentrate and lactose through crystallization, hydrolyzation, and separa-
tion by ultrafiltration.
The extra functionality that may be required of the powders is to a large extent
determined by the drying conditions, including pre- and post-treatment of the
concentrateand final powder. Desirablecharacteristics aredust-freeand good recon-
stitution properties which involvecarefully controlled drying conditions, often with
added agglomeration and lecithination steps.
36

37. To handle the increase in plant size and complexity and to meet demands for
closer adherence to exact product specifications, dryer instrumentation has been
improved and operation gradually has evolved from manual to fully automated.
Today, it is normal for milk drying plants to be controlled by such microprocessor
based systemsas the APV ACCOSwith automation covering the increasingly complex
startup,shut down and CIP procedures.
Finally, more stringent demands governing environmental aspects of plant oper-
ations have resulted in better compliance with anti-pollution regulations. Virtually
all plants are equipped with bag collectors to ensure clean exhaust air. In the case
of such products as acid whey where the use of bag collectors is not practical, high
efficiency scrubbers are used. Somemulti-purposeplants have both plus an elaborate
system of ducts and dampers to switch between the devices as different products
are dried.
An added benefit from the use of sanitary bag collectors is an improvement in the
yield of salable product. Although this increase typically is not more than 0.5%,it
still can be of substantial value over a year of operation.
principle and appronchcs
Spraydryingbasically is accomplished by atomizing feed liquid intoa dryingchamber
where the small dropletsare subjected to a stream of hot air and areconverted todry
powder particles for subsequent collection and packaging. While dryers initially
were of the single stage design, new directions in recent years have tended toward
multiple stages with numerous variations.
SINGLE Sr:ZC,E DRYER
Defined as a process in which the product is dried to its final moisture content
within the spray drying chamber alone, a typical single stage dryer is shown in the
Figure 1 flow diagram. 'This type of design is well known throughout the industry
although some differences in air flow patterns and chamber design exist.
As illustrated, the drying air is drawn through filters, heated to the drying tem-
perature by means of a direct combustion natural gas heater, and enters the spray
drying chamber through an air distributor located at the top of the chamber. The
AIR HEATFR
f
FIG I
iprai tlryrr with conical
AIR OUT rhamher Arrangpd with
I~
1yp1caI 5 l n g l ~ ~stage.
I c'c'ntrifugal atomizcr
L?r
AIR OUT
)DUC
r
T
37

38. I
38
feed liquid mters the chamber through a device called an atomizer which disperses
the liquid into a well defined mist of very fine droplets. The drying air and droplets
are very intimately mixed, causing a rapid evaporation of water. As this happens,
the temperature of the air drops, the heat being transferred to the droplets and used
to supply the necessary heat of evaporation of the liquid. Each droplet thus is
transformed into a powder particle. During this operation, the temperature of the
particles will not increase much due to the evaporative cooling effect.
The rate of drying will be very high at first but then declines as the moisture
contcnt of the particles decreases. When the powder reaches the bottom of the
drying chamber, it has attained its final moisture and normally is picked up in a
pneumatic cooling system to be cooled down to a suitable bagging or storage tem-
perature. Powder that is carried with the air is separated in cyclone collectors.
As the powder is cooled in the pneumatic system, it also is subjected to attrition.
This results in a powder that is fine, dusting and has relatively poor redis-
solving propert ivs.
Single stage dryers are made in different configurations,the basic ones being:
Box Dryers:
A very common type of dryer in the past, the drying air enters from the side of a
box-like drying chamber. Atomization is from a large number of high pressure
nozzles also mountcd in the side of the chamber. Dried powder will collect on the
flatfloor of the unit from which it is removed by moving scrapers. Theexhaust air
is filtered through filter bags that also a,re mounted in the chamber. In some
instances, cyclones will be substituted for the filter bags.
These are very tall towers in which thr air flow is parallel to the chamber walls.
The atomization is by high pressure nozzles.
While the height requirement is less than for the tall form type,the dryingchamber
is considerably wider. The atomizing device can be either centrifugal type or
nozzles. The product is sprayed as an umbrella shaped cloud and the air flow
follows a spiral path inside the chamber.
Tall Form Dryers:
Wide Body Dryers:
MULTIPLE STAGE DRYERS
Generally speaking,the drying process can be divided into two phases. The first is
the constant rate drying period when drying proceeds quickly and when surface
moisture and moisture within the particle that can move by capillary action are
extracted. The second is the falling rate period when diffusion of water to the
particle surface becomes the determining factor of the drying rate. Since the rate of
diffusion decreases with the moisture content, the time required to remove the last
few percent of moisture in the case of single stage drying takes up the major part of
the residence time within the dryer. The residence time of the powder thus is
essentially the same as that of the air and is limited to between 15-25seconds.As the
rate of water removal is decreasing toward the end of the drying process, the outlet
air temperature has to be kept fairly high in order to provide enough driving force to
finish the drying process within the available air residence time in the chamber.
In multiple-stagedrying, the residence time is increased by separatingthe powder
from the main drying air and subjectingit to further dryingunder conditions where
the powder residence time can be varied independently of the air flow. Technically,

39. I
this is done either by suspension in a fluidized bed or by retention on a moving belt
(Filtermat). Since a longer residence time can be allowed during the falling rate
period of the drying, it also is possible and desirable to reduce the drying air outlet
temperature. Enough time to complete the drying process can be made available
under the more lenient operating conditions.
The introduction of this concept has led to higher thermal efficiencies. Fifteen
years ago, the typical inlet and outlet temperatures of a milk spray dryer were 3601
205°F. Today. the inlet temperature is often above 430°F with outlet temperatures
down to about 185°F.
two stage drying
In a typical two stage drying
process as shown in Figure 2,
powder is produced at about 7%
moisture from the spray drying
zone with final drying being
done in an external fluid bed
(Figure 3). The fact that the
powder leaves the spray drying
zone at relatively high moisture
content means that either the
outlet air temperature can be
lowered or the inlet air temper-
ature increased. Compared to
single stage drying, this will
result in better thermal effi-
ciency and higher capacity from
the same size drying chamber.
As the product is well protected
by its surrounding moisture in
the spray drying phase, there nor-
mally are no adverse effects on
product quality resulting from
the higher inlet air temperature.
The o u t l e t a i r from t h e
chamber leaves through a side
air outlet duct and the powder
is discharged at the bottom of
the chamber into the external
fluid bed. The total drying time
can be extended toabout 15min-
FIG. 2 Two-stagr drying with external vibrating fluid bed
FIG. 3 External vibrating fluid bed is sectioned for
powder agglomeratlon, drying, and cooling.
utes, thus allowing for low temperature use in the fluid bed.
In the development of this type of drying system, an initial difficulty was to
provide a means to reliably fluidize the semi-moist powders. This stems from the
fact that milk powder products, and especially whey-basedones,show thermoplastic
behavior. This makesthem difficulttofluidizewhen warm and having high moisture
content. This problem was largely overcome by the development of a new type of
vibrating fluid bed.
39

40. The vibrating fluid bed has a
well defined powder flow and typ-
ically is equipped with different
air supply sections,each allowing
a different temperature level for
an optimum temperature profile.
The last section normallyiswhere
the product is cooled to bagging
and storage temperature. While
the specific energy consumption
in the fluid bed can be relatively
high,the evaporation is minor com-
pared with the spray dryingstage.
The higher efficiency made possi-
ble in the spray drying stage typi-
cally means that total energy use
for two stage drying is 15-20%1
lower than for a single stage unit.. .
integrated statioiar; fluid bed with directibnd air flow,
and external vibrating fluid bed. Provides optimum condi-
tions for producing non-dusty, hygroscopic and high fat
powder agglomeration and forthe
addition Of a surface active agent
content products. such as lecithin.
three stage drying
The advanced three stage drying concept basically is an extension of two stage
drying in which the second drying stage is integrated into the spray drying
chamber with product drying being conducted in an external third stage as shown
in Figure 4. This technology allows a higher moisture content from the spray
drying zone than is possible in a two stage unit and results in an even lower outlet
air temperature than is possible with other techniques. An added advantage is that
the design improves the drying conditions available for handling several difficult
products. This is accomplished by spray drying the powder to high moisture
content but at the same time, avoiding any contact with hot metal surfaces by
processing the powder directly on a fluidized powder layer in the integrated fluid
bed. Moist powder particles thereby are surrounded by particles which already
have been dried and any tendency for powder to stick to the metal surfaces will
be minimized.
40

41. I
In this three stage drying concept, it was undesirable to vibrate the chamber
fluid bed but, as pointed out relative to two stage drying, most of the powders that
are processed prove difficult to fluidize because of their thermoplastic and hygro-
scopic characteristics. They therefore need some kind of assistance. This is done
through the use of a special type of perforated plate which provides directional air
flow. The stationary fluid bed operates with high fluidizing velocities and high bed
depths, both of which are optimized to accommodate the particular product that is
being processed.
The air outlet of the integrated fluid bed system is located in the middle of the
fluid bed at the bottom of the drying chamber. This forms an annulus around the
air outlet duct and results in the creation of a very clean design which completely
eliminates the mechanical obstructions that are found in older two-point discharge
designs. The dryer is equipped with tangential air inlets or Wall Sweeps to provide
for conditioned air. These are important in two ways to the operation of the dryer.
Not only do they cool the chamber wall and remove powder that may have a
tendency to accumulate but they also serve to stabilize the air flow within the
chamber. The dryer thus operates with a very steady and well defined air flow
pattern that minimizes the amount of wall buildup. Atomization can be either by
pressure nozzlesor by the use of a centrifugal type atomizer.
The three stage drying system is especially suitable for the production of
products which are non-dusty or hygroscopic or those which have a high fat
content. The dryer can be used to produce either a high bulk density powder by
returning the fines to the external fluid bed, or a powder which exhibits improved
wettability. The latter is produced by operating with straight through agglomera-
tion with the fines then being reintroduced into the atomizing zone. The dryer
concept also allows for the addition of liquid into the internal fluid bed, thereby
opening the way to the production of very sophisticated agglomerated and
instantized products, Le., excellent straight through lecithinated powders. Whey
powder produced on such a system shows improved quality because of the higher
moisture which is present when the powder is introduced into the integrated fluid
bed. This provides good conditions for lactose after crystallization within
the powder.
A three stage drying system also is able to provide a high production capacity in
a small equipment volume. The specific energy requirement for such a unit is
about 10%less than normally is expended during the operation of a two stage dryer
for a comparable duty.
SPKAY BED DRYER
While the concept of providing a-threestage drying system which is operated with
an integrated fluid bed evolved from traditional dryer technology, it also served to
spur the development of a modified technique that is referred to by APV Crepaco as
the “Spray Bed Dryer.” This unique machine not only is characterized by the use
of an integrated fluid bed located at the bottom of the drying zone but also by the
employment of drying air which both enters and exits at the top of the chamber.
Atomization can be done either through the use of nozzles or by a centrifugal
atomizer.
41

42. I I i
FIG. 5-6
Two variations of APV Crepaco
Spray Bed Dryer arrangements.
Top drawing illustrates basic
dryer with centrifugal
atomizer and integrated fluid
bed which is agitated by
high fluidization velocity.
Figure 6 shows spray bed
dryer with added external
fluid bed for final powder
drying and cooling.
The chamber fluid bed is
vigorously agitated by a high
fluidization velocity. Particles
from the spray drying zone enter
the fluid bed with a moisture
content as high as 10-15%depend-
ing on the type of product and
are dried in the bed to about 5%.
Final drying and cooling take
place in the external fluid bed.
The structure of the powder
produced in the Spray Bed
differs considerably from the
conventional. It is coarse, con-
sists of large agglomerates and
consequently, has low bulk den-
sity. The powder, however, has
excellent flowability. The dryer
is very suited to the processing
of complicated products having
high content of fat,carbohydrate,
and protein. Twoof the many pos-
siblevariationsof this typedryer
are shown in Figures 5and 6.
conclusion
I I
Spray drying systems divided into two or more stages undoubtedly will be a charac-
teristic of almost all future dryer installations. The advantages resulting from this
technology will provide dairy and foodprocessors with the necessary flexibility and
energy efficiency required to meet today’s uncertain market and whatever changes
will be called for in the future.
42

44. the importance of the dispersion and dissolution steps, the time for which may vary
considerably with different agglomeration methods.
For powders that are produced by spray drying, there are a number of ways in
which the agglomeration can be accomplished in the spray dryer itself. This often is
referred to as the ‘straight-through’process and is illustrated in Figure 1.Note that
fines powder from the cyclone is conveyed up to the atomizer where it is introduced
intothe wet zonesurrounding the spraycloud. Cluster formation will occur between
the semi-moistfreshly produced
particles and the recycled fines.
The agglomerated product then
is removed from the bottom of
the drying chamber, cooled and
packaged. This method pro-
duces a degree of agglomeration
that is sufficient for many
applications.
An alternative approach to
agglomeration is referred to
as the re-wet method. This is
characterized by processing an
already existing fine dry powder
into an agglomerateusing fluid-
ized bed technology.
the agglonicration mechanisni
Two particles can be made to agglomerate if they are brought into contact and at
least one of them has a sticky surface. This condition can be obtained by one or a
combination of the followingmeans:
FIG I Straight through agglomeration
1)Droplet humidification whereby the
surface of the particles is uniformly
wetted by the application of a finely
2) Steam humidification whereby satu-
rated steam injected into the powder
causes condensation on the particles.
4) Addition of binder media, i.e., a solu-
tion that can serve as an adhesive
between the particles.
150-
dispersed liquid. E
$ 100
2
3) Heating -for thermoplasticmaterials. + 50
0 4 8 12 16
The steam condensation method usually POWDER MOISTURE (%)
cannot provide enough wetting without
less frequently on newer systems.
After having been brought into a sticky state, the particles are contacted under
such conditions that a suitable, stable agglomerate structure can be formed. The
successof this formation will dependon such physical properties as product solubility
and surface tension as well as on the conditions that can be generated in the proc-
ess equipment.
For most products, possible combinations of moisture and temperature can be
established as shown in Figure 2. Usually, the window for operation is further
FIG 2 Typical combinations of conditions
adversely heating the material and is used for agglomeration
44

45. narrowed down by the specifications for product characteristics. Once the agglom-
erate structure is created, the added moisture is dried off and the powder cooled
below its thermoplastic point.
agglomeration equipment
While slightly different in equipment design and operation, most commercially
available agglomeration processes fundamentally are the same. Each relies on the
formation of agglomerates by the mechanism already described.This is followed by
final drying, cooling, and size classification to eliminate the particle agglomerates
that either are too small or too large. Generally, designs involve a re-wet chamber
followed by a belt or a fluid bed for moisture removal. Such a system is shown in
Figure 3.
It is obvious that this system is quite sensitive to even very minor variations in
powder or liquid rates. A very brief reduction in powder feed rate will result in
overwetting of the material with consequent deposit formation in the chamber.
Conversely, a temporary reduction in liquid rate will result in sufficiently wetted
powder and therefore, weak agglomerates. Many designs rely on the product impact-
ing the walls of the agglomeration chamber to build up agglomerate strength. Other
designs include equipment for breaking large lumps into suitably sized agglomerates
before the final drying. Obviously, deposit formation always will be a concern in
agglomeration equipment as the process depends upon the creation of conditions
where the material becomes sticky.
Typically, equipment de-
signs are very complicated, POWD
probably reflecting the fact
that agglomeration actually STEAM
is a complicated process.
Despite the complexity of
the process, however, it is
possible to carry out ag-
glomeration by means of
comparativelysimpleequip-
ment which involves the
use of a fluidized bed for the
re-wettingand particle con-
tact phase. This approach
advantages:
AGGLOMERATED
provides the following PRODUCT
l)There is sufficient FIG 3 Typical agglomeration system
agitation in the bed to
obtain a satisfactory distribution of the binder liquid on the particle surfaces
and to prevent lump formation.
2) Agglomeratecharacteristics can be influenced by varying operatingparameters
such as the fluidizingvelocity, re-wet binder rate, and temperature levels.
3) The system can accept some degree of variation of the feed rate of powder and
liquid as the product level in the fluid bed always is constant, controlled by an
overflowweir. Thus, the re-wettingsection will not be emptied of powder. Even
during a completeinterruption of powder flow,the fluidized material will remain
in the re-wet section as a stabilizing factor in the process.
45

46. 4) By using fluid bed drying and coolingof the formed agglomerates, it is possible
to combine the entire agglomeration process into one continuously operating
unit.
5) Start-up, shut-down and operation of the fluid bed agglomerator are greatly
simplified due to the stabilizing effect of the powder volume in the re-wet zone.
Proper implementation of a fluid bed agglomeration system requires a detailed
knowledge of the fluidization technology itself. Fluidization velocities, bed heights,
air flow patterns, residence time distribution and the mechanical design of vibrating
equipment must be known.
features of fluid bed agglomeration
Figure 4 shows a typical agglomerator system where the process is implemented
through the use of a vibrated, continuous fluid bed.
POWDER IN
FIG 3 Fluid bed re url
agylomerntion 5)itcm
The powder is fed into the agglomerator by a volumetric screw feeder.Due to the
previously mentioned stabilizing effect of the material already in the fluid bed, the
reproducibility of a volumetric feeder is satisfactory and there is no need for a
complicated feed system such as a loss-in-weightor similar type.
The fluid bed unit is constructed with several processing zones, each with a
separate air supply system.The first section isthe re-wetand agglomeration section
where agglomerates are formed. Here, the powder is fluidized with heated air in
order to utilize its thermoplastic characteristics.
The binder liquid almost always is water or a water-based solution whereas
steam, as already explained, rarely is used. The binder liquid is sprayed over the
fluidized layer using two fluid nozzles driven by compressed air. For large systems,
numerous nozzlesare used. Powder deposits are minimized by accurate selection of
spray nozzle angles and nozzle position patterns. Powder movement is enhanced by
the vibration of the fluid bed unit itself and by the use of a special perforated air
distribution plate with directional air slots.A proper detailed design is vital in order
to have a troublefree operation.
46

47. 1
From the agglomeration zone, the powder automatically will flow into the drying
area where the added moisture is removed by fluidization with heated air. In some
instances,more than onedrying section isrequired and in such cases,these sections
are operated at successively lower drying temperatures in order to reduce thermal
exposure of the more heat sensitive dry powder.
The final zone is for coolingwhere either ambient or cooled air is used to cool the
agglomerates to a suitable packaging temperature.
During processing,air velocities are adjusted sothat fine,unagglomerated powder
isblown off the fluidized layer.The exhaust air is passed through acyclone separator
for removal and return of entrained powder to the inlet of the agglomerator. When
there are high demands for a narrow particle size distribution, the agglomerated
powder is passed through a sifter where the desired fraction is removed while over-
and undersize material is recycled into the process.
As with all re-wet agglomeration equipment, the operation must be performed
within certain operating parameters. Overwetting will lead to poor product quality
while underwetted powder will produce fragile agglomerates and an excessive
amount of fines. However, fluid bed agglomeration does offer a great degree of
flexibility in controlling the final result of the process. The characteristics of the
formed agglomerate can be influenced by operating conditions such as binder liquid
rate, fluidizing velocity,and temperature. Typically, the fluid bed re-wetmethod will
produce agglomerated products with superior redispersion characteristics.
As indicated by this partial list, this method has been used successfully with a
number of products.
Dairy products
Baby formula
Calf milk replacer
Flavor compounds
Fruit extracts
Malto dextrines
Corn syrup solids
Sweeteners
Detergents
Enzymes
Herbicides
Egg albumin
Starch
Coca mixes
In most cases, the agglomeration can be accomplished using only water as a re-wet
media. This applies to most dairy products and to malto dextrine based flavor
formulations. For some products, increased agglomerate size and strength has been
obtained by using a solution of the material itself as the binder liquid. In the case of
relatively water insoluble materials, a separate binder material has been used but
it must be one that does not compromise the integrity of the final product. The
addition of the binder material may have a beneficial effect on the end product at
times. This is seen,for example,in flavor compounds when a pure solution of malto
dextrine or Gum Arabic may further encapsulate the volatile flavor essences and
create better shelf life. In other instances, the added binder can become part of the
final formulation as is the case with some detergents.
For some materials, the addition of a binder compound is an unavoidable incon-
venience.At such times, the selected binder must be as neutral as possible and must
be added in small quantities so that the main product is not unnecessarily diluted.
An example of this is herbicide formulations which often have a very well defined
level of active ingredients.
47

48. For products containingfat,
surface active material,usu-
ally lecithin. This is done
by mounting an extra set of
spray nozzles near the end
of the drying section where
the surfactant is applied.
Variations of the fluid bed
Malto Dextrines
Gum Arabic
Starch
Gelatin
Molasses
Sugar
the norma' process Often is TABLE A Frequently used binders
combined with a step by
which the agglomeratesare _ ~ _ _
coated with a thin layer of
_ _ _ _ _ ~ _ _ _ _
FOR FOOD
Lignosulfonates
Poly-Vinyl Alcohol (PVA)
Any of the Food
Product Binders
FIG 5 Spray Bed type
agglomerating
spray dryer
48

49. I
InsolubilityIndex
TABLE B Reconstitutability and physical structure of different types of skim milk powder.
4.w 4.10 <0.20
I I I I 1
Density,I W W
1 Dispersibility: O/O 1 60-80 I 92-98 1 92-98 1
40-43 28-34 28-31
IAverage particle size, micron I 400 I >250 I >400
I
During operation,the chamber fluid bed is vigorously agitated by a high fluidiza-
tion velocity and as the particles from the spray drying zoneenter the fluid bed with
a very high moisture content, they agglomerate with the powder in the bed. Fines
carried upwards in the dryer by the high fluidizingvelocity have topass through the
spray cloud, thus forming agglomerates at this point as well. Material from the
integrated fluid bed is taken to an external fluid bed for final drying and cooling.
The Spray Bed dryer is a highly specialized unit that can only produce agglomer-
ated powder. It is best suited for small to medium sized plants sincethe efficiency of
the agglomeration process unfortunately decreases as the plant increases in size.
This isdue to the fact that the spray zonebecomes toofarremovedfrom the fluid bed
zoneas the size of the dryer increases.
Agglomerates from the Spray Bed dryer exhibit excellent characteristics. They
are very compact and show high agglomerate strength and good flowability.
considerations & conclusions
While the agglomeration process improves the redispersion, flowability and non-
dustiness of most fine powders, it invariably decrease6 the bulk density. The com-
parisons in Table Bclearly show that agglomeration improves the powderwettability
and dispersibility. Individual powder particles with a mean diameter of less than 100
micron are converted into agglomerates ranging in size from 250to 400 micron with
the re-wet method being able to produce the coarser agglomerate. The powder bulk
density will decrease from about 43 lbs/ft3to approximately 28 lbs/ft3. Use of the re-
wet method will expose the product to one additional processing step which, in this
case, will somewhat affect the proteins and result in a slightly poorer solubility.
Since fluid bed agglomeration can be operated as an independent process, it can
be used with already existing power producing equipment. It offers great flexibility
and ease of operation, and provides a convenient way to add functionality, non-
dustiness and value to a number of products.
49

50. I
SPIN FLASH
DRmRS
for continuous powder production
frompastes and filtercakes
While mechanical dewatering of a feed slurry is significantly less expensive than
thermal drying, this process results in a paste orfilter cake that cannot be spray dried
and can be difficult to handle in other types of dryers. This article describes the Spin
Flash dryer as one option availablefor continuouspowder productionfrom pastes and
filter cakes without the needforgrinding.
Powders generally are produced by some form of drying operation. There are several
generic types of dryers but all must involve the evaporation of water which can
take anywhere from 1000to 2500 Btu per pound depending on dryer type. The most
common of these dryers probably is the spray dryer due to its ability to produce a
uniform powder at relatively low temperatures. However,by its definition, a spray
dryer requires a fluid feed material to allow its atomization device to be employed.
Generally, there is a maximum viscosity limitation in the range of 1000-1500SSU.
Figure 2 illustrates the amount of water which must be evaporated to produce
one pound of bone dry powder from a range of different feed solids. It can clearly be
seen that even a 5%increase in total solids will reduce the water evaporation and
hence, the dryer operating costs by about 20%.If this water removal can be done
mechanically by, for example, filtration or centrifuging, the cost will be infinitely
lower than that required to heat and evaporate the same water. The direct energy
cost can be calculated as equivalent to 3-8Btu/lb. compared to an average 1500
BtuAb. for evaporation. This increase in solids, however, inevitably will result in
an increase in viscosity which may exceed the limitations of a spray dryer.
Options available for drying these higher viscosity feed materials are listed in
Table 1. The subject of this paper is the Spin Flash dryer, which is among the
newest of the dryer options and has the capability of drying most materials ranging
from a dilatent fluid to a cohesive paste.
spin flash dryer
The Spin Flash dryer was developed and introduced in 1970 in response to a
demand by the chemical industry to produce a uniform powder on a continuous
basis from high viscosity fluids,cohesive pastes, and sludges.
process description
The Spin Flash dryer can be described as an agitated fluid bed. As shown in Fig. 3,
the unit consists primarily of a drying chamber (9)which is a vertical cylinder with
an inverted conical bottom, an annular air inlet (7)and an axially mounted rotor (8).
The drying air enters the air heater (4), is typically heated by a direct fired gas
burner (5)and enters the hot air inlet plenum (6)tangentially. This tangential inlet
together with the action of the rotor causes a turbulent whirling gas flow in the
drying chamber.
50

51. I
Dryingmedium Air
Inert gas
Low humidity waste gas
Inlettemperature Up to 18OOOF
Capacity Up to 10tons per hour
of final product
Product residence 5-500seconds
Itime
DRYER
CHARACTERIST1CS
Dryingmethod Directgas contact
Flow Co-current
Feed material Dilatent fluids
Cohesivepaste
Filter cake
Moist granules
FIG. 1Typical APV Anhydro spin flash dryer with general dryer characteristics
The wet feed material, typically filter cake, is dropped into the feed vat (1)where
the low speed agitator (2) breaks up the cake to a uniform consistency and gently
presses it down into the feed screw (3).Both agitator and feed screw are provided
with variable speed drives.
In the case of a dilatent fluid feed, the agitated vat and screw would be replaced
with a progressive cavity pump and several liquid injection ports at the same ele-
vation as the feed screw.
As the feed material is extruded off the end of the screw into the drying chamber,
it becomes coated in dried powder. The powder coated lumps then fall into the fluid
bed and are kept in motion by the rotor. As they dry, the friable surface material is
abraded by a combination of attrition in the bed and the mechanical action of the
rotor. Thus, a balanced fluidized bed is formed which contains all intermediate
phases between raw material and finished product.
51

52. I I
FIG. 2 Water evaporatiodtotal solids ratio
et
52

53. The dryer and lighter particles
become airborne in the drying air
stream and rise up the walls of the
drying chamber, passing the end of
the feed screw and providing, in
effect, a continuous back mixing
action within the heart of the dryer.
At the top of the chamber, they
must pass through the classifica-
tion orifice which can be sized to
prevent the larger particles from
passing on to the bag collector.
These larger lumps tend to fall back
into the fluid bed to continue
drying.
Air exiting from the bag collector
(10)passes through the exhaust fan
(E)and is clean enough for use in a
heat recovery system. Dried powder
is discharged continuously from reduction
the bottom of the bag collector through the discharge valve (11).
Two important features make the Spin Flash dryer suitable for products which
tend to be heat sensitive. First, the dry powder is carried away as soon as it becomes
light enough and therefore is not re-introducedinto the hot air zone. Second,the fluid
bed consists mainly of moist powder which constantly sweeps the bottom and lower
walls of the drying chamber and keeps them at a temperature lower than the dryer
air outlet temperature. In addition to this self-coolingcapacity, the lower edge of the
drying chamber directly above the hot air inlet can be provided with an auxiliary
cooling ring.
Fig. 4 illustrates the very rapid reduction in air temperature that occurs due to
the high heat transfer rate obtained in the fluid bed.
operatingparameters
Inlet temperature of the drying air introduced into the chamber is dependent on
the particular characteristics of the product being dried but generally would be
similar to that used on a spray dryer for the same product.
Outlet temperature is selected by test work to provide the desired powder mois-
ture and is controlled by the speed of the feed screw. Since the Spin Flash dryer
produces a finer particle size than does a spray dryer, it has been found that a
slightly lower outlet temperature may be used to obtain the same powder moisture.
This provides an increase in thermal efficiency.
Air velocity through the cross section of the drying chamber is an important
design factor and is determined in part by the final particle size that is required. A
lower velocity will tend to reduce the final dried particle size carried out of the
FIG. 4 Fluid bed provides rapid air temperature
,
53

54. chamber. The major factor, however, is the stability of the very complex bed which
must neither settle back into the air distributor nor blow out of the top of the
chamber. Once the maximum velocity has been determined by test work for a given
product, the diameter of a drying chamber can be selected to provide the desired
water evaporation rate.
Capacity can be adjusted to suit the output from the preceding process equipment
which may be hard to control and slow in its response time. This is achieved by a
cascade control from a feed vat level sensor to the inlet temperature controller set-
point. The feed vat sometimes can be oversized to accept the batch discharge from a
preceding filter press while allowing the dryer to operate continuously.
closed cycle drying
Once a decision has been made to increase feed solids prior to drying, the small size
and lower air flow requirements of the Spin Flash dryer make it practical to design
the system as a closedcycle dryer with nitrogen as the drying medium. This type of
system can be used to dry a solvent based powder, allowing complete recovery of
the solvent.
The simplified Fig. 5 schematic shows a possible configuration of a closed cycle
Spin Flash dryer. The operating process as described earlier, is extended, with
exhaust gas from the baghouse being scrubbed and cooled in a condenser (13)using
cooled solvent from an external plate heat exchanger (15)as the scrubbing medium.
Recovered solvent is bled off at (16),downstream from the scrubber recirculation
pump (14)at a controlled rate based on scrubber liquid level.
The drying chamber would be maintained at a pressure slightly higher than
ambient using a pressurized nitrogen purge (17).The heater (5)would use either a
steam coil or a thermal fluid system with an external heater.
A less expensive alternative to the closed cycle approach is a “Lo-Ox” system
where a low excess air burner is used in a direct fired heater. The products of
combustion are recirculated through the condenser and the surplus gas is vented to
atmosphere. The oxygen level in such a system can be controlled to less than 5%.
FIG. 5 Spin flash dryer in closed cycle arrangement
54

55. TABLE 2
FIG. 6 Spin flash dryer being
prepared for shipment
I INLET AIRTEMPERATURE
OUTLETAIRTEMPERATURE F 185 293
TOTAL SOLIDS IN FEED
RESIDUAL MOISTURE
MEAN PARTICLE SIZE pm 40
BULK DENSITY gr/cmJ 0.8 0.45
-
W
P
0
z
83
sw
-
617
212
55
35
0.6
5
0.3
-
-
~
-
-
-
-
a
z
3
-
482
194
65
29
12.5
70
0.4
-
-
~
-
-
-
-
T
10
-
w
2W
[I)
I
2
5
0
212
126
70
43
0.32
16
0.14
-
-
-
-
-
-
-
-
I
914 662 1112
@
0.76 0.32 1 6 0 6
spin flashdried products
While Table 2 charts some typical products being dried on Spin Flash equipment, it
should be noted that in some cases the inlet temperatures are limited by the heat
source used and not by the Spin Flash process.
Other products test dried on a laboratory scale Spin Flash dryer include:
Product
iron oxide
effluent treatment sludge
lignin
gum
chitosan gel
crab meat paste
dutched cocoa cake
Percent Percent
Feed Solids Powder Moisture
70
22
49
40
6
30
70
2
40
15
13
40
9
2
cost benefits
In comparison to a spray dryer, the Spin Flash dryer has a much shorter residence
time and consequently is considerably smaller and requires less building space. Its
ability to dry to even higher solids than a spray dryer results in operating cost
savings. Table 3 shows a detailed size and cost comparison based on actual test
drying of yellow iron oxide.
55

56. ITABLE 3
SPACE REQUIREMENTS spray dryer
type Ill-K
No. 70/71
direct fired
14'0
650
46'0"
24,700
spin flash
No. 59
direct fired
2'7'h"
325
16'6
5,300
type 111
It can be seen that the investment in a spin flash dryer plant is some 32%lower than a spray dryer
for the same capacity and that spin flash operating costs are approximately 29?hlower. These
figures,however, do not include the capital investment in filtration equipment.
conclusion
Despite its obvious size and cost advantages, there are many instances when a Spin
Flash dryer cannot replace a spray dryer. Typically, such cases occur when a free
flowing spherical particle of a particular size range is required or when agglomera-
tion is needed.
There are, however, many situations both in the food and chemical industries
where the particular capabilities of the Spin Flash dryer to produce powders from
paste warrant careful consideration of its use.
56

60. I I
6APV
DIRECT INQUIRIESTO:
APV Crepaco Inc
Drver Division
APV Crepaco Inc. 165John L. Dietsch Square
Executiveand InternationalHeadquarters Attleboro Falls, MA 02763
9525 West BrynMawrAve., Rosemont, IL60018 Phone: (508) 695-7014
(708)678-4300 Fax:(708)678-4407 Fax: (508)695-7018
Engineeringand Manufacturing
395 FillmoreAve., Tonawanda,NY14150/(716)692-3000
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165John L.Dietsch Sq., Attleboro Falls,MA 02763/(508)695-7014
RegionalSales Offices:
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Sales Offices:
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790