in as well -Different desiccants adsorb moisture in different volumes.
The remaining three factors -chemical
reactions, molecular size and co-adsorbable components -gain
more significance in removing moisture from gas streams than in
removing moisture from air streams to dry plastics, and are only
worth a brief mention here.
The two most commonly used desiccants are silica
gel and molecular sieves. When inlet concentrations of moisture in an
air stream are high, silica gel will remove more moisture by weight.
That is, a fixed amount of gel will be more effective than the same
amount of sieve. However, with lower inlet conditions, molecular sieve
is the desired adsorbent choice. This, plus the fact that the incoming
air stream to the desiccant bed in a plastics dryer is warm (generally
above 1000 F.), and silica gel begins to loose it's useful capacity at
higher temperatures, the choice is obvious for us in drying plastics -we
must use molecular sieves.
Molecular sieves are round spheres or cylindrical
extrusions, with internal pores of various diameters. A desiccant
"bed" is merely many of these individual spheres placed
together in a chamber. An often asked question is, how do we know how
much desiccant is required to effectively remove moisture from the air
Several factors go into sizing a desiccant bed in
dynamic adsorption. Obviously, enough desiccant must be used to meet the
moisture removal requirement! Also, it must be deep enough to allow
sufficient time for the air stream to pass over the sieve so moisture
can be adsorbed. Pressure drop must not be too great for the fan
selected. Actual bed design is a subject all in its own, and beyond the
scope of this paper -but you can see many criteria must be considered in
order to provide a machine capable of meeting the exact requirements.
Now, let's take a look at the desiccant bed and
see exactly what does occur as the air passes over it. Transfer of the
moisture to the desiccant begins as soon as the wet air contacts it.
Initially, the bed contains little moisture and as the air stream
enters, moisture is collected in the pores of the desiccant -actually
"transferred". This area where the exchange occurs is referred
to as a "Mass Transfer Zone". The mass transfer zone is a
length of packed adsorbent through which the water content in the air is
reduced from the inlet condition to the outlet condition. The length of
this mass transfer zone depends on the velocity of the air and the exit
concentration of moisture in the air stream.
As the air stream is fed into the desiccant bed,
this mass transfer zone is continuously displaced from the inlet to the
outlet of the bed, and eventually moves to the bottom of the bed as the
cycle time progresses. When the leading edge of the mass transfer zone
reaches the bottom portion of the bed, the bed becomes saturated. Before
saturation "break-through" occurs, the bed must be regenerated
to remove the moisture in preparation for the next adsorption step.
(Normally, the main process airflow will be shifted to a second tower or
bed for adsorption while the regeneration process goes on in this tower
There are several methods of accomplishing
regeneration, but the most convenient is the use of heat. Generally,
heat is applied to the bottom of the bed and excess moisture is driven
off. One can see that regeneration flow counter-current to the
adsorption flow is highly desirable for two reasons:
The portion of the bed with less moisture can
be heated more rapidly.
The relatively dry portion of the bed assists
in driving off moisture from the top of the bed.
Regeneration is a most important factor in the
adsorption process, and proper 'regeneration is a necessity in obtaining
satisfactory dryer performance. With other factors being equal, the dew
point produced by a molecular sieve dryer will be lower as the
regeneration temperature is increased. Generally, temperatures to 6000
F. are required for proper regeneration. Besides proper inlet
temperature, regeneration heat must be employed long enough to raise the
temperature of the desiccant sufficiently to vaporize the liquid and
drive off the excess moisture. Thus, the period of heating as
well as temperature of the bed must be considered in establishing
good regeneration. In proper machine design, the regeneration air
temperature is approximately 500 F. higher than that desired for the
This then, provides a very simple explanation of
how water is removed from an air stream. The desiccant "traps"
water in adsorption and releases it in regeneration.
Now let's take a look at a dryer and see how it is
From the above discussion, the most important part
of the machine is the desiccant bed. Once it is designed, the other
components are merely supplemental to it to enable it to function
satisfactorily between adsorption and regeneration. A fan is employed to
move the air across the surface of the desiccant at the proper velocity
to effect a satisfactory transfer of moisture. The inlet air stream must
be filtered to prevent contamination of the desiccant bed. In the case
of plastics, fines and exceptional amounts of dirt may be present and
contact the desiccant. Since the desiccant life can be greatly reduced
when the pores become clogged, a substantial filter must be
employed. (A two-stage filtration system that will remove particles down
to 1 micron in size with a 99.9% efficacy such as used on Novatec
dryers, is highly desirable.) Inlet moist air then enters the dryer
through the filter and passes over the desiccant bed where moisture is
removed. In the case of plastics, this dry air then is reheated to a
pre-determined temperature depending on the material in the hopper.
While this is occurring on one tower or chamber, the other is being
regenerated. Ambient air is drawn in through a filter, by a second fan,
into a single regeneration heater and up over the desiccant bed. Moist
ail: then is driven off to the outside. This can either be ducted
directly into ambient air or it can be ducted outside if desired.
Four-way valves transfer the air from one tower to the other. Total
cycle time of this machine is eight hours for one tower; that is, four
hours of adsorption of process drying and four hours of regeneration.
Once the bed is heated to the temperature required
for effective regeneration, it must be cooled
before it returns to the process air stream. The reason
"longer" cycles are used with Novatec dryers is to provide
sufficient cooling of the desiccant bed. When outside air is used to
remove regeneration heat, the bed eventually cools to a point where it
starts adsorbing moisture again. This creates a problem when the valves
shift and the tower goes on stream to begin it's adsorption cycle, since
moisture adsorbed during regeneration cooling is then driven off into
the process air.
A most effective way of cooling the bed is to turn
off the fan and allow the desiccant to cool by radiating the heat rather
than bringing moisture laden outside air across it. Since moisture is
not introduced to the desiccant, the bed remains dry to begin the next
adsorption cycle. Because of this cooling cycle, longer overall
adsorption cycles are employed, meaning less valve and component wear
due to less activity, as well as longer desiccant life.
In addition, regeneration heaters are on for only
25% of total machine operating time, so energy efficiency is realized.
The regeneration heater is external to the dryer tower for easy
inspection if necessary.
Just as construction of the dryer is important,
the plenum hopper must also be constructed with certain design
parameters in mind:
The air inlet from the dryer must be positioned
so air is deflected in such a manner as to contact all of the
A diffuser or spreader cone is utilized to
deflect the inlet air, and also to distribute the air evenly
throughout the material in the hopper. This is especially important
in the "critical drying region".
A material drain, separate from the slide gate
discharge, large sight glass, and large clean out door are important
features of any hopper.
How is the hopper sized to hold material? From
pre-determined tests proper drying temperatures and exposure times of
various resins to the dryer air are recommended.
If the particular requirement calls for an
exposure time of four hours, and a process rate of 100 pounds per hour
is anticipated, a hopper is selected which will hold 400 pounds of
material. Obviously then, if 100 pounds of material are processed per
hour, a dryer with 100 CFM is selected. These parameters are recommended
for virgin materials which are injection molded. Extruded resins or
heavy regrind ratios will alter these parameters.
Different processing rates or
"through-puts" of course determine the physical size of
machine for the application. For larger processing rates, models, which
are mounted on the floor, are utilized. Hoppers can be mounted on stands
on the floor, or placed directly on the process machines to eliminate
moisture contamination of highly hygroscopic resins. In situations where
relatively small, but sophisticated production is required,
automatically regenerated dryers may be affixed directly to the drying
hopper and the complete assembly mounted directly to the molding
Now that we've seen how a dryer is constructed,
let's examine some of the particulars that play an important role in
drying the resin itself. Temperatures. dew point, volume of air, initial
and final moisture content, as well as material configuration all affect
drying of materials.
Temperature is probably the most important factor
in drying plastic resin. Moisture content in ABS is noted after the
materials' exposure to air at different temperatures. It can be easily
identified that the higher temperature had a marked effect on removing
moisture from this material. It takes five times longer to dry ABS at
1600 F., as compared to 2000 F.
It has been noted previously that lower dew point
(drier) air will retain more moisture than wet air, so dew point
naturally has a significant impact on drying hygroscopic resins.
Measuring dew point impact on polycarbonate is noted on this chart and
it is evident the difference between -200 F. and -400 F. do not
extremely impact this material. However, a relatively low dew point is
required to remove moisture. Other resins which are highly hygroscopic,
such as polyethylene terephthalate (PET) absolutely require dew points
in the -400 to -500 F. range to be adequately dried.
In some production facilities airflow sometimes
becomes a minor factor in drying. Everyone assumes (rather incorrectly)
that if the temperature is correct for the product, and the dew point is
relatively low, the material will be dry. Most resin producers however,
agree the volume of air required to dry resins is approximately 1 CFM/lb./material/hour.
The results with polycarbonate indicate the effect of proper airflow.
Initial and final moisture content obviously have
an effect on the drying time. Nylon 66 was dried with two different
initial moisture contents and time required noted. You can observe this
has a definite effect. Eventually, however, the two curves would reach
the same final equilibrium moisture content.
Configuration of the material plays an important
role in drying. Large and small pellets dry differently, as well as thin
film vs. thick sheet. '
Moisture adsorption capability depends on the type
of material as well as the ambient in which it is placed. In some
instances, exposure of only minutes can be detrimental.
If the material is exposed to a certain
temperature and relative humidity for a period of time, it will reach an
equilibrium point, referred to as the equilibrium moisture content. This
chart shows this EMC for several plastics @80° F., and because this
affects the drying time of the product, it is important for the material
to be stored in sealed containers. Incidentally, the time to reach the
EMC @80° F. is approximately five to seven days.
You can see a variety of factors influence the
proper drying of materials. The material qualities themselves, as well
as their handling, are extremely important. I might add too, that proper
dryers may be designed and selected to adapt to a particular situation
by the manufacturer, but it is most important these machines be
maintained correctly. Filters ~ be changed regularly to insure longer
desiccant life and hopper and dryer hoses maintained leak tight.
In summary, I hope the above has provided some
insight on drying and how we, as dryer