Drying
Barry Fox,
Pellegrin i
Giovanni Bellini, and Laura
SECTION I: INDIRECT DRYING (bu Giovanni Bellini and Laura
Pellegrini)
1.0 INTRODUCTION
The drying operation is often the final step of a manufacturing process.
Indirect drying will be discussed in this section; it is the process of removing
liquid by conductive heat transfer.
Sometimes drymg is apart ofthe manufacturing process itself, as in the
case of seasoning oftimber or in paper making, but generally, the reasons for
carrying out a drying operation are:
To reduce the cost of transport
To ensure a prolonged storage life
To make a material more suitable for handling
To avoid presence of moisture that may lead to corrosion
To provide the product with definite properties
The type of raw material is of extreme importance in the drying
process; for instance, to retain the viability and the activity of biological
materials such as blood plasma and fermentation products, the operation is
carried out at very low temperatures, while more severe conditions can be
applied to foodstuffs.
706
Drying 707
If it is possible to remove moisture mechanically, this will always be
more economical than removing it by evaporation. However, it will be
assumed in the following that, for the type of raw material and its final use,
the removal of volatile substances is camed out by heat.
2.0 THEORY
Drying Definition Drying is a unit operation in which a solvent,
generally water, is separated from a solution, semisolid material or cakeholid
pastes by evaporation.
In the drymg process, the heat is transferred simultaneously with the
mass, but in the opposite direction.
Drying Process Description. The moisture content of a material is
usually expressed as a weight percentage on a dry basis. The moisture may
be present as:
Free moisfure. This is the liquid in excess ofthe equilibrium
moisture content for the specific temperature and humidity
condition of the dryer. Practically, it is the liquid content
removable at a given temperature and humidity.
Bound moisture. This is the amount of liquid in the solids that
exhibits a vapor pressure less than normal for the pure liquid.
In the drying of materials it is necessary to remove free moisture from
the surface as well as bound moisture from the interior. The drying
characteristics of wet solids can be described by plotting the rate of drying
against the corresponding moisture content. A typical drying curve is shown in
Fig. 1 and it can easily be seen that this is subdivided into four distinct sections:
The curved portion, AB, is representative of the unsteady state period
during which the solid temperature reaches its steady state value, ts. AB may
occur at decreasing rate as well as at the increasing rate shown.
The critical moisture content is thus identified as the average moisture
content of the solid at the instant the first increment of dry area appears on
the surface of solid.
The critical moisture content depends upon the ease of moisture
movement through the solid, and hence, upon the pore structure of the solid,
sample thickness and drying rate. Segment BC is the constant-rate period.
During this period, the drying is controlled simultaneously by heat and mass
transfer applied to a liquid-gas interface in dynamic equilibrium with a bulk
gas phase.
708 Fermentation and Biochemical Engineering Handbook
T
A
I
PON
XI 6
IC
Y
M
E
A
f
Mass of liquid / mass of dry solid
Figure 1. Drying rate curve.
Moisture flow from within the material to the surface is fast enough to
maintain a completely wet surface. The surface temperature reaches the wet-
bulb temperature. The rate of drying can be expressed as:
Eq. 1
where dW/i+ is the rate of drying, Le., change in moisture with time; Kp is
the mass transfer coefficient, ps is the saturation vapor pressure of the liquid
at the surface temperature, fs; and pa is the partial pressure of water vapor.
In addition, the following equation also applies:
Eq. 2
(fa - ts) = Kp (ps - pa)
dW ha
dO A
-=-
Drying 709
where A is the latent heat of vaporization, ha is the heat transfer coefficient,
ta is the dry bulb temperature ofthe air and ts is the temperature ofthe product
surface.
By integrating Eq. 2, it is possible to derive the drying time in the
constant rate period. Equation 2 is derived for heat transfer to the material
being dried by circulating air. When large metal sheets or trays are close to
the product, it is not possible to ignore the conduction and radiation
contribution to heat transfer. In this case, the solid temperature is raised
above the air wet-bulb temperature and Eq. 2 becomes:
dW A1 hc A2 FA3 E6
- = ha-(ta- ts)+ -(tc- ts)+ -(Tr4 - Ts4)
Eq. 3
do a a a
where Al, A2, A3 are the solid surfaces, respectively, for convection,
conduction and radiation heat-transfer, tc is the temperature of the heat
surface for conductive transfer, Fis a view factor, depending on thegeometry,
E is the emissivity of the surface, S is the Stefan-Boltztnann constant, Tr is
the absolute temperature of the radiating surface and Ts is the absolute
temperature of the product surface. The increase in Ts allows the drying at
an increased rate, both during the constant rate and the first falling rate period.
At the end of the constant-rate period, the movement of the liquid to the solid
surface becomes insufficient to replace the liquid being evaporated. The
critical moisture content is thus identified as the average moisture content of
the solid at the instant the first increment of dry area appears on the surface
of the solid. The critical moisture content depends upon the ease of moisture
movement through the solid and, hence, upon the port structure of the solid,
sample thickness and drying rate.
Segment CD is the first falling-rate drying period. It is the period
between the appearance of the first dry area on the material surface and the
disappearance of the last liquid-wet area; drying occurs at a gradually
reduced rate. At point D, there is no significant area of liquid saturated
surface.
During the phase CD, Eq. 2 is still applicable to the moisture removal
rate, provided that ts andps are suitably modified and account is taken of the
partial dryness of the surface.
Segment DE is the second falling-rate. The moisture content continues
to fall until it reaches the equilibrium moisture content, E. The equilibrium
moisture content is reached when the vapor pressure over the solid is equal
to the partial pressure of vapor in the atmosphere. This equilibrium condition
is independent of drying rate. It is a material property. Only hygroscopic
materials have an equilibrium moisture content.
710 Fermentation and Biochemical Engineering Handbook
For non-hygroscopic materials, the equilibrium moisture content is
essentially zero at all temperatures and humidities. Equilibrium moisture
content is particularly important in drying because it represents the limiting
moisture content for given conditions of humidity and temperature. The
mechanisms of drymg during this phase are not completely understood, but
two ideas can be considered to explain the physical nature of this process-
one is the diffusion theory and the other the capillary theory.
Diffusion Mechanism. In relatively homogeneous solids, such as
wood, starch, textiles, paper, glue, soap, gelatin and clay, the movement of
moisture towards the surface is mainly governed by molecular diffusion and,
therefore, follows Ficks' Law.
Sherwood and Newman gave the solution of this equation in the
hypothesis of an initial uniform moisture distribution and that the surface is
dry; the following expression is derived (for long drying times):
Eq. 4
where dW/dq+is the rate ofdrying during the falling rate period, D is the liquid
difisivity of the solid material, L is the total thickness of the solid layer
thickness through which the liquid is diffusing, W is the moisture content of
the material at time, 0, and We is the equilibrium moisture content under the
prevailing drying conditions. Equation 4 neglects capillary and gravitational
forces.
Capillary Model. In substances with a large open-pore structure and
in beds of particulate material, the liquid flows from regions of low
concentration to those of high concentration by capillary action. Based on
this mechanism, the instantaneous drying rate is given by:
Eq. 5
dW h (ta- ts) (W- We)
D0 2p L 1 (Wo- We)
--
-
where 9 is the density of the dry solid and Wo is the moisture content when
diffusion begins to control.
Most biological materials obey Eq. 4, while coarsegranular solids such
as sand, minerals, pigments, paint, etc., obey Eq. 5.
Shrinkage and Case Hardening. When bound moisture is removed
from rigid, porous or nonporous solids they do not shrink appreciably, but
colloidal nonporous solids often undergo severe shrinkage during drying.
This may lead to serious product difficulties; when the surface shrinks against
Drying 711
a constant volume core, it causes the material to warp, check, crack or
otherwise change its structure. Moreover, the reduced moisturecontent in the
hardened outer layer increases the resistance to diffusion. In the end, the
superficial hardening, combined with the decrease in diffusive movement,
make the layer on the surface practically impervious to the flow of moisture,
either as liquid or vapor. This is called case hardening.
All these problems can be minimized by reducing the drylng rate, thereby
flattening the moisture gradient into the solid. Since the drying behavior
presents different characteristics in the two periods-constant-rate and
falling-rate-the design of the dryer should recognize these differences, Le.,
substances that exhibit predominantly a constant-rate drying are subject to
different design criteria than substances that exhibit a long falling-rate period.
Since it is more expensive to remove moisture during the falling-rate
period than during the constant-rate one, it is desirable to extend as long as
possible the latter with respect to the former. Particle size reduction is a
practical way to accomplish this because more drying area is created.
An analysis of the laws governing drying is essential for a good dryer
design, therefore, it is important to note that, due to the complex nature of
solid phase transport properties, only in a few simple cases can the drying rate
(and drying time) be predicted with confidence by the mathematical expres-
sions reported above. In these cases, one usually deals with substances that
exhibit only, or primarily, constant-rate drying.
For materials that present a non-negligible falling-rate period, the use
of specific mathematical equations is subject to a high number of uncertain-
ties and simplifying assumptions are generally required.
It is clear that the purely mathematical approach for designing a drying
plant is not possible, given the present state of knowledge.
3.0 EQUIPMENT SELECTION
Several methods of heat transfer are used in the dryers. Where all the
heat for vaporizing the solvent is supplied by direct contact with hot gases and
heat transfer by conduction from contact with hot boundaries or by radiation
from solid walls is negligible, the process is calledadiabah'c, or direct drying.
In indirect or nonadiabatic drying, the heat is transferred by conduc-
tion from a hot surface, first to the material surface and then into the bulk.
This chapter discusses only indirect drying.
The problem of equipment selection can be very complex; different
factors must be taken into consideration, for example, working capacity, ease
of cleaning, hazardous material, dryer location and capital cost (see Fig. 2).
712 Fermentation and Biochemical Engineering Handbook
The first step refers to the choice of continuous versus batch drying and
depends on the nature of the equipment preceding and following the dryer as
well as on the production capacity required. In general, only batch dryers will
be considered in the following.
Batch dryers include:
Fluidized-bed dryers. These may be used when the average
particle diameter is I 0.1 mm. (The equipment required to
handle smaller particles may be too large to be feasible.)
Inert gas may be used if there is the possibility of explosion
of either the vapor or dust in the air.
It is easy to carry out tests in a small fluid-bed dryer.
Shelf dryers. Theys are usually employed of? small capacities
and when the solvent doesn’t present particular problems.
Vacuum dryers. These are the most-used batch dryers.
Vacuum dryers are usually considered when:
Low solids temperature (< 40°C) must be maintained to
prevent heat causing damage to the product or changing its
nature
When toxic or valuable solvent recovery is required
When air combines with the product, during heating, causing
Before starting work on selecting a dryer, it is good practice to collect
all the data outlined in Table 1.
In vacuum drying, the objective is to create a temperature difference or
“driving force” between the heated jacket and the material to be dried. To
accomplish this with a lowjacket temperature, it becomes necessary to reduce
the internal pressure of the dryer to remove the liquidsolvent at a lower vapor
pressure. Decreasing the pressure creates large vapor volumes, Economic
considerations arising from concerns of leakage, ability to condense the
solvent, size of vapor line and vacuum pump, affect the selection of the
operating pressure. Materials handled in vacuum dryers may range from
slurries to solid shapes and from granular, crystalline product to fibrous
solids. The characteristics of each type of vacuum dryer is discussed below
to help make a proper choice.
Vertical Vacuum Pan Dryers. The agitated vertical dryer (Fig. 3.)
has been designed for drying many different products which may come from
centrifuges or filters. Generally, the body is formed by a vertical cylindrical
casing with a flat bottom flanged to the top cover head. The unit is fully heated
by an outside half-pipe jacket welded on the cylindrical wall, the bottom and
the top head.
oxidation or an explosive condition
Drying 713
nuximum product
temperature <40T 7
solvent 7
air oxidation ?
no
r, average paate sire
>.1 nirn 7
flarnniable vapour 7
/q fkr1d:bed
'01 LtationrequiredlM gentle agitation 712
?I shelf dryer
medium agitation 7
,Ino * G3 ,
vacuum
tumbler/
paddle
Figure 2. Flowchart for selection of a batch dryer.
Table 1. Data To Be Assessed Before Attempting Drying Selection
- Production capacity (kgh)
- Initial moisture content
- Particle size distribution
- Drying curve
- Maximum allowable product temperature
- Explosion characteristics (vapodair and dust/air)
- Toxicological properties
- Experience already gained
- Moisture isotherms
- Contamination by the drying gas
- Corrosion aspects
- Physical data of the relevant materials
71 4 Fermentation and Biochemical Engineering Handbook
tator
i'
I!
Figure 3. Multidry-EV Pan Dryer (Courtesy of COGEIMSpA)
Drying 715
Dim. mm
700
900
1200
1400
1600
1800
The dished head is provided with the appropriate nozzles for feed inlet,
instrumentation, heating or cooling medium, vapor outlet, lamp and rupture
disk. The dished head and the cylindrical body are separated by means of a
hydraulic system to provide easy access to the vessel for inspection or
cleaning. A high powered agitator having two crossed arms located at
different heights, is designed for processing products that go through a
viscous transition phase (high viscosity). The same dryer can be provided
with a different agitator, high speed, which is applicable for low to medium
viscous products. The agitator can be totally heated. To eliminate possible
agglomerates or lumps formed during the drying process, and discharge
problems, a chopper device is supplied. The shaft sealing can be either a
stuffing-box or a mechanical seal. A bottom discharge valve for the dry
product is hydraulically driven and located in a closed hatch. The geometri-
cal volume ofthese vertical dryers ranges from a few liters to approximately
500 liters (see Table 3).
Table 2. Standard Pan Dryers - Multidry-EV*
Cylindrical Geometrical
height, mm volume, m3
500 0.3
600 0.6
700 1.2
95 0 2.0
1100 3.0
1400 5.0
Agitator
speed rpm
10-80
5-55
5-40
3-35
2-3 0
2-28
Installed
power kW
11
15
18
30
45
75
Materials having average-low density (1 00-500 kg/m3) and low-
medium viscosities, which require perfect mixing of the dried product, could
require another type of vertical dryer.
Here, a dryer having a truncated-cone casing is used (Fig. 4). The
agitator is supplied combining:
716 Fermentation and Biochemical Engineering Handbook
- A screw feeder which propels the product upwards.
- An orbital rotation, of the same screw feeder following the
geometry of the cone, providing efficient circumferential
mixing. The screw is totally supported by the rotating shaft.
The shaft and the screw design guarantee containment ofthe
lubricants. A special detector indicates any possible leak
before it contacts the product. The geometrical volume of
these dryers ranges from a few liters to approximately
12,000-15,000 liters (Table 3).
Figure 4. MIXODRY-EMV Conical Pan Dryer. (CourteJy of COGEIMSpA)
Drying 717
Table 3. Standard Conical Dryers - Mixodry-EMV*
Installed power
Screw rotation, kW
2.2
2.2
5.5
11.0
22.0
22.0
Diam. mm
1050
1500
2000
2700
3 150
3600
Installed power
screw revolu., kW
0.37
0.37
0.75
1.1
1.5
1.5
Useful
Volume, m3
0.3
1 .o
2.0
5.0
8.0
12.0
*Courtesy of COGEIM SPA
Horizontal Vacuum Paddle Dryers. For products having high
specific weight (1000-1500 kg/m3), middle-high viscosity, and requiring
high process temperature and a very large working capacity, the use of the
horizontal dryer (Fig. 5) is recommended. Generally the horizontal dryer is
constructed as a jacketed horizontal cylinder with two heads provided with
an outside jacket. Nozzles, for product feeding and discharge, for the bag
filter, nitrogen inlet, bursting disc are provided. A heated horizontal shaft
with radial paddles which scrape the wall, performs the mixing operation and
supplies the majority of the total heat flux when compared to the heated
outside vessel walls. This is due to the design of the shaft and the agitator
blades, which allow the forced circulation ofthe heating fluid; the design and
working conditions are the same as for the outside jacket system.
The rotating agitator blades prevent deposits which would reduce the
total heat-exchange in a short period of time. The scraping blades are
designed and installed to facilitate the discharge of the dried product through
the product discharge nozzle. This is achieved by reversing rotation. The
shaft sealing can be either by a stuffing-box or amechanical seal. The agitator
drive consists ofagear box coupled to a reversiblehariable speed motor. The
heating fluid inside the agitator both enters and exits the system through a
rotating joint. A discharge plug valve is normally installed in the central part
ofthe vessel and is activated by a pneumatic piston. A jacketed dust filter can
be installed with a reverse jet cleaning system. The dryer capacity normally
ranges from a few liters to approximately 20,000 liters (Table 4).
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Drying 719
Diam. mm
500
1000
1200
1400
1800
2200
Table 4. Standard Horizontal Dryers - Chemidry-EO*
Geometrical Agitator
Length, mm volume, m3 speed rpm
1000 0.2 6-80
2000 1.6 6-12
3000 3.4 6-12
4200 6.4 6-12
5000 13.0 5-10
6000 22.0 3-6
Installed
power kW
4-5.5
11-15
18-22
30-37
55-75
75-90
*Courtesy of COGEIM SpA
For products which need a very high standard quality level, a different
version of the horizontal paddle dryer (Fig. 6) has been designed. The
construction differences can be summarized as follows:
- The shaft is supported on only one side to allow the opening
- The design of the paddles provides for self-cleaning of the
- All surfaces are consistent with GMP norms
- The sealing system is a special double mechanical seal with
- All surfaces in contact with the product are mirror polished.
The geometrical volume ranges from a few liters to approximately
5000 liters (Table 5).
Vacuum Shelf Dryer. It is the simplest and oldest vacuum dryer
known. It can be used for drying a wide range of materials like solids, free
flowing powders, fibrous solids having special forms and shape, practically
any material that can be contained in a tray. It finds application where the
material is sensitive to heat and so valuable that labor costs are insignificant.
At the same time, it is normally used when the powder production is very low.
of the opposite head for cleaning and inspection
cylindrical body and heads
flushing system
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au
Om UCI m
Id
OM
au
I
Drying 721
500
1000
1150
Table 5. Standard Horizontal Dryers - Steridry-EO*
700 0.14
1500 1.18
1650 1.71
Geometrical
Diam. mm 1 Length, mm 1 volume,m3
1300
1500
1700
1800
1800 2.39 5-20 15-18.5
1900 3.36 5-20 18.5-22
2000 4.53 5-15 22-3 0
2250 5.72 5-15 22-37
Agitator
speed rpm
5-40
5-25
5-25
Installed
power kW
4-5.5
5.5-1 1
11-15
The dryer consists of a vacuum tight cylindrical or rectangular
chamber containing a number of heated shelves on which trays are heated. A
quick opening permits easy loading, unloading and during maintenance, easy
cleaning. Different heating mediums are used, i.e., steam, hot water or hot
oil. There are no moving parts inside the vessel, that means no sealing
problem and consequently, a good vacuum can easily be maintained. The
disadvantages are mainly due to its lower heat transfer rate (long drying time)
and impossibility of safely handling toxic products because of the hazards
involved in charging and discharging trays.
Tumbler Vacuum Dryer. This dryer is designed for drying of
chemical, pharmaceutical products which are not sticky. Its double cone
rotating shape ensures direct contact between the material and the heated
surface, resulting in uniform heat transfer.
For optimum drying results approximately 50% to 60% of the total
volume is required. Any greater percent fill would greatly restrict the product
movement and retard the evaporation rate. A frequent condition that occurs
with some sticky materials is the formations of balls which can be broken by
addition of an intensifier bar with the rotating vessel or by intermittent
rotation of the vessel. The unit is completely jacketed and designed for
circulation of a heating medium. The tumbler is normally gentle in action and
the absence of internal moving parts assures against disintegration ofcrystals
or abrasion.
722 Fermentation and Biochemical Engineering Handbook
3.1 Testing and Scale-up
Generally, it is possible to carry-out some tests in the manufacturer’s
small scale units, but it is necessary to remember that during shipping the
material may have changed its property due to chemical or physical modifi-
cations, because the quantity of sample is limited, it is not possible to check
long-run performance. It is also impossible to evaluate the behavior of the
dried material in the plant’s solids-handling equipment. If the pilot test is
positive, it is good practice, before designing a production unit, to install a
small-scale dryer in the plant and investigate what is possible under actual
process conditions. It is essential in this test that a representative sample of
the wet feed is used and the test conditions simulate as closely as possible the
conditions characteristic of the commercial size dryer. The experimental
method for measuring product moisture content should be clearly defined and
consistent with that used in the industrial plant. It should be noted that a heat
transfer coefficient is the main product of the test and, based on this, a scale-
up to the final heating surface can be done.
The heat transfer coefficient combines the surface coefficient for the
condensing steam, the resistance of the metal wall and the surface coefficient
on the working side. Because conditions vary with the type of material
involved, the amount of moisture it contains, the thickness of the layer in
contact with the surface, the structure of this layer, and many other factors,
it is impossible to construct an overall heat transfer coefficient without
experimental data.
Scaleup of laboratory data is a critical step and requires considerable
experience. Since scaleup is subject to many factors that are not quantifiable,
it is based primarily on experience and is a function of the specific dryer.
When the heating surface is known, it is easy to calculate the working volume
and the dryer’s geometrical volume (Fig. 7).
For a pan dryer, the percent of the total volume occupied by the batch
is called the working volume, which is another critical consideration. As the
working volume approaches 100% ofthe total volume, there is less void space
available for material movement and contacting of the heated surface.
In the vacuum batch dryer, approximately 60% total volume is
required for optimum drying result. A very simple andapproximate equation
can be used for the scaleup of vertical pan and horizontal paddle dryers:
tb (AIV)u
tu (AIV)b
-=
Drying 723
where
t = Dryingtime
A = Heat transfer surface, m2
V = Vessel working volume, m3
a = Pilot plant
b = Industrial size plant
Figure 7. Total dryer heating surface versus geometrical volume. (Chemidy-EO)
724 Fermentation and Biochemical Engin eeting Handbook
3.2 Cost Estimation
Capital investment is the total amount of money needed to supply the
plant and manufacturing facilities plus the amount of money required as
working capital for operation of the facilities. To estimate a fixed-capital
investment it is necessary to consider the following costs:
- Purchased equipment
- Instrumentation
- Electrical
- Piping
- Service facilities
- Building
The cost of purchased equipment is the basis for estimating the capital
investment. The various types of equipment can be divided conveniently into:
- Process equipment
- Raw materials handling and storage equipment
- Finished products handling and storage equipment
Of course, the most accurate method of determining process equipment
cost is to obtain bids from the supplier. When a dryer unit must be evaluated,
the following have to be considered.
Dryer Type and Size. If a vacuum pan dryer is selected, it is then
necessary to choose its configuration and size. The size is dependent on the
capacity needed and this is based on pilot tests. The configuration is
dependent on the property of the material to be dried and the pollution
specifications. For instance, the system blades-agitator can be heated or not,
and the sealing between agitator and dryer body can be accomplished either
by a stuffing-box or a mechanical seal. The latter can either be double-
pressurized or simple. The chopper can be installed or not, the agitator
rotation can be electricaVfrequency converter or hydraulic. All of the
hydraulic system has to be considered including all piping connections, etc.
Construction Materials. Normally, the dryers are made of stainless
steel. The most common stainless steels used are the type 304 and 316
generally having low carbon content. They contain chromium and nickel at
different percents. The addition of molybdenum to the alloy, as in type 3 16,
increases the corrosion resistance at high temperature strength. The presence
of chromium increases its resistance to oxidizing agents. The price for the
type 304 and 3 16 is quite similar. Ifvery highly corrosion resistant materials
are required then Hastelloy C276 or C22 can be used.
Drying 725
Hastelloy is used where structural strength and good corrosion resis-
tance are necessary under conditions of high temperatures. Compared to
stainless steel, the price of a Hastelloy dryer is approximately double. Other
less expensive alloys can be used, such as Inconel, 77 percent nickel and 15
percent chromium. Nickel exhibits high corrosion resistance to most alkalies.
Internal Finishing (GMP). For pharmaceutical purpose the internal
finishing must be at least 220 grit and the dryer manufactured according to
GMP standards. This makes the price of the dryer some 20 to 30 percent (%)
higher than the standard design.
Installation. The installation involves costs for labor, foundations,
platforms, construction expenses, etc. The installation cost may be taken as
a percentage of the dryer cost, approximately 20 to 50 percent, depending
upon its sophistication.
If no cost data are available for the specific dryer selected, a good
estimate can be obtained by using the logarithmic relationship known as the
six-tenths-factors rule. A price for a similar one, but having different
capacity, is the sole requirement.
Cost dryer A
This relation
information.
I”
(Capacity dryer A)
=
dryer (Capacity dryer B)
should be only used in the absence of any other
3.3 Installation Concerns
The dryer performance is effected by the auxiliary equipment.
Heating System, Depending on the maximum temperature allowed
inside the dryer, water S98OC or steam low/medium pressure 3-6 bar can be
used as heating medium in the jacket. Due to the relatively low temperature
required to dry fermentation products, the heating medium is generally
circulating pressurized hot water. The water can be heated by either an
electric immersion heater or steam in a shell and tube heat exchanger. The
recirculating pump should always be pumping into the heater so that its
suction is from the outlet ofthe dryer. In addition, the suction side ofthe pump
should always have an air separator to prevent cavitation. The entrapment
of air is inevitable in a hot water heating system.
Cooling System. Where cooling of the product is absolutely neces-
sary, a cooling exchanger can be mounted in parallel with the heating
exchanger and used at the end of the drying cycle. By turning a couple of
726 Fermentation and Biochemical Engineering Handbook
valves to direct the flow through the cooling exchanger, the recirculated water
will then remove the heat from the product and transfer it to a cooling medium
in the cooling exchanger.
Vacuum system. A well designed system should include:
1. Dust collector-which is installed on the top of the dryer
and made of a vertical cylindrical casing, complete with
an outside jacket. Generally the filter elements are bags
fixed on the upper side to a plate and closed on the lower
part. The filter bags (Fig. 8) are supported through an
internal metal cage. Cleaning of the bags is obtained by
a mechanical shaking device or by nitrogen pressure.
Design and working conditions are the same as for the
vacuum dryer.
2. Condenser-designed according to the scaled-up pilot
test evaporation rate. Normally it is a shell and tube unit.
It should also be self-draining into a vacuum-receiver,
which collects the solvent as well as maintains the vacuum
integrity of the entire system. The condensate receiver
should have a sight glass so that visual inspection will
indicate when it needs emptying. Obviously the receiver
should be large enough to contain all of the condensate
from one batch of product.
3, Vacuum pump-whose flow-rate depends largely upon
the in take of air at the various fittings, connections, etc.
Different kinds of vacuum pumps can be used; e.g.,
rotary-water/oil sealed, reciprocating dry vacuum pump.
If a water sealed-vacuum pump is used, the liquid ring may permit
scrubbing the effluents (non-condensed vapors) and removal ofthe pollution
load by controlling the vapor emission. Obviously, when the liquid ring
becomes saturated, it must be discharged. This type of device should always
be considered where low boiling solvents and hazardous or toxic vapors are
involved; better if a closed circuit is considered. This type of pump is simple
to operate and requires little maintenance. Depending on service liquid
temperature, a single-stage pump will allow a vacuum of 100 to 150 torr.
However, it is more usual to employ a two-stage liquid ring pump which will
attain 25 torr, and below 10 torr when used in combination with an ejector.
The ejectors consist essentially of a steam nozzle, which discharges a high-
velocity jet across a suction chamber connected to the equipment to be
evacuated. The gas is entrained by the steam and carried into a venturi-shaped
diffiser which converts the velocity energy ofthe steam into pressure energy.
Drying 727
Figure 8. Standard bag filter. (Courtey of COGEIMSpA)
728 Fermentation and Bioch cmical Engineering Handbook
Where it is necessary to operate at the end of the drying cycle below
5 torr, different types of oil sealed rotary pumps can be supplied. Such
applications might occur where there is a need to operate at very low drying
temperatures.
Hydraulic System. Generally, for vertical pan dryers an hydraulic
system is provided for the agitator rotation, the opening-closing of the dryer
by a rapid device as bayonet and/or TRI-CLAMP and the lowering of the
vessel for maintenance or cleaning purposes. The hydraulic components
positioned on the dryer are normally:
1. One hydraulic motor for agitator rotation
2. Three hydraulic cylinders for lowering, raising the vessel
3. Hydraulic cylinder for a rapid opening (the number is
dependent upon the dimensions of the vessel)
The hydraulic system consists of: oil reservoir, electric motor, hydrau-
lic pump, heat exchange for oil cooling, oil filters, oil level indicator, electric
valves and flow distributors. An hydraulic plant which has been properly
installed and care has been taken during the start-up phase, should enjoy long
life and not need much maintenance.
A cardinal principle in the operation of a trouble-free hydraulic system,
on which all manufacturers agree, is that the operator continuously monitors
the quality as well as the condition ofthe hydraulic fluid to make certain there
are no impurities, The reliability of the hydraulic system is directly related
to the integrity of the fluid.
The following periodic checks are recommended:
1. Monthly external cleaning and inspection. This will
2. Monthly air filter checking and replacement of the dirty
3. Weekly oil filter checking.
4. Weekly oil level check, each time the level falls to the
5. Oil replacement on the average every 2,000-3,000 hours.
6. Heat exchanger must be cleaned semiannually.
uncover any leaks which can then be repaired.
cartridge.
minimum, oil must be added
Drying 729
3.4 Safety Considerations
Where the handling of materials containing highly flammable solvents
is concerned, the dryer must be located in a classified area and the electrical
parts designed according to the standards specified for this level.
The mechanical, electrical and instrument specification should also
include requirements for:
1. Explosion protection-a vent should be considered for a
safe relief of a positive pressure.
2. Avoidance of ignition-potential ignition sources may be
electrical equipment, discharge of static electricity or
mechanical friction (associated with the agitator). The
dryer must be grounded.
3. Facilitating safe operation-ventilation should be pro-
vided during loading; a supply of inert gas is required for
breaking the vacuum.
Most hazards are listed below:
Ignition of dust cloud can occur during unloading of a
dusty flammable product from the dryer
Ignition of bulk powder can occur if a dryer is opened to
atmosphere while still hot
Ignition of flammable vapor can occur when loading
solvent-wet material into the dryer, and also when unload-
ing the product ifthe dryer has not previously been purged
with nitrogen
Exothermic decomposition-some heat-sensitive materi-
als may decompose with evolution of large volumes ofgas
if they are overheated during drying
The danger from an explosion can be reduced in two different ways:
1. The dryer can be designed according to pressure vessel
code and consequently be able to contain any possible
explosion
2. The process/operating conditions should be altered to
insure a higher level of safety
730 Fermentation and Biochemical Engineering Handbook
At the same time, the following start-up and shutdown procedures are
recommended:
Start-up:
1. Inspect the plant and remove any deposits, check position
of valves and settings of temperature and vacuum
regulators
2. Purge the dryer with nitrogen
3. Load the wet material in the dryer
4. Start cooling water to the condenser
5, Start the vacuum pump
6. Start the agitator
7. Apply heat to the jacket
1. Switch off the heating medium
2. Wait till the product has cooled for sale discharge
3. Close the vacuum line
4. Stop the agitator
5. Fill the vessel with nitrogen to atmospheric pressure
6. Open the dryer and remove the product
7. Clean the dryer
Shutdown
4.0 EQUIPMENT MANUFACTURERS
1. Vertical vacuum dryers:
Bolz, GmbH
COGEIM SpA
GLATT GmbH
Hosokawa Micron Europe
Moritz
Patterson Kelley Co.
2. Paddle dryers:
Buss
COGEIM
List
Drying 731
3. Filters or Filter-Dryer Products
COGEIM, Charlotte, NC
Jaygo, Mahwah, NJ
Krauss-Maffei, Florence, KY
Micro Powder Systems, Summit, NJ
Rosenmund, Charlotte, NC
Sparkler Filter, Conroe, TX
Steri-Technologies (Zwag), Bohemia, NY
4. Dryers, Spray
APVKrepaco, Tonawanda, NY
Niro Atomizer, Columbia, MD
5. DryerEIlenders
GEMCO, Middlesex, NJ
J.H. Day, Cincinnati, Ohio
Micron Powder Systems, Summit, NJ
Niro-Fielder, Columbia, MD
Patterson-Kelly, East Stroudsburg, PA
Processall, Cincinnati, OH
6. Dryers, Freeze
Edward High Vacuum, Grand Island, NY
Finn-Aqua, Windsor Locks, CT
Hull, Hatboro, PA
Stokes, Warminster, PA
Virtis, Gardner, NY
5.0 DIRECTORY OF MANUFACTURERS
BOLZ GmbH
P. 0. Box 1153
7988 Wangen IM Allgau
Fed. Rep. of Germany
Buss AG
4 133 Pratteln, 1
Base1
Switzerland
732 Fermentation and Biochemical Engineering Handbook
COGEIM SpA
Compagnia Generale Impianti
Via Friuli, 19
24044 Dalmine (Bergamo)
Italy
GLATT GmbH
Process Technology
P. 0. Box 42
7852 BinzedLorrach
Federal Republic of Germany
GUEDU
21 140 Semur-En-Auxois
France
Hosokawa Micron Europe
P. 0. Box 773
2003 Rt Haarlem
The Netherlands
List AG
4 133 Pratteln
Switzerland
Moritz
7, Avenue de Pommerots
B. P. 37
78400 Chatov
France
Pattrerson Kelley Co.
Division of Harsco Corporation
101 Burson St.
P. 0. Box 458
E. Stroudsburg, PA 18301
United States
Drying 733
REFERENCES (for Section I: Indirect Drying)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Badger, W. L. and Banchero, J. T., Introduction to Chemical Engineer-
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Coulson, J. M., Richardson, J. F., Backhurst, J. R., and Harker, J. H.,
Chemical Engineering, Vol. 2, 3rd. Ed., Pergamon Press (1978)
Forrest, J. C., Drying Processes, Biochemical and Biological Engineer-
ing Science, (N. Blakebrough, ed.), Academic Press, London and New
York (1967)
Foust, A. S. and Wenzel, L. A., Principles of Unit Operations, 2nd. Ed.,
Wiley
McCabe, W. L. and Smith, J. C., Unit Operations of Chemical Engineer-
ing, 3rd. ed., McGraw-Hill.
McCormick, P. Y., The Key to Drying Solids, Chemical Engineering
(Aug. 15, 1988)
Peters, M. S. and Timmerhaus, K. D., Plant Design andEconomics for
Chemical Engineers, McGraw-Hill Kogakusha (1 980)
Spotts, M. R. and Waltrich, P. F., VacuumDryers, ChemicalEngineering
(Jan. 17, 1977)
Van't Land, C. M., Selection of Industrial Dryers, Chemical Engineering
(March 5, 1984)
Wentz, T. H. and Thygeson, J. R., Jr., Drying of Wet Solids, Handbook
of Separation Techniquesfor Chemical Engineers, (P. A. Schweitzer,
ed.), McGraw-Hill, New York (1979)