Abstrict The invention resides in a process air dryer system for plastics
processing machines and method for drying process air comprising
two desiccant towers repeatedly alternating between process and
regeneration cycles. In addition to conventional bottom heaters,
each tower has an additional generally central heater so as to reduce
the time required to regenerate saturated desiccant. When a tower
is undergoing regeneration of its desiccant, generally centrally
heaters aid conventional bottom heaters in drying saturated desiccant
so as to reduce conventional regeneration cycle time by as much
as 25% to 30%.
Claims I claim:
1. A system of drying and heating process air to be circulated
for drying hygroscopic thermoplastic material contained in a drying
hopper before the material is introduced into a plastic processing
machine, the system including:
a drying hopper for containing hygroscopic thermoplastic material
before the thermoplastic material is introduced into a plastics
processing machine;
process air for drying said hygroscopic thermoplastic material
contained in said drying hopper;
first and second desiccant towers, each of said towers alternating
between a process cycle and a regeneration cycle while the other
tower is in an opposite cycle;
desiccant contained in said first and second desiccant towers for
adsorbing moisture from the process air;
regeneration air for driving moisture off saturated desiccant in
one of said towers in a regeneration cycle;
air conduit means providing a path for circulating the process
air between the drying hopper and the first and second desiccant
towers;
air circulating means for circulating the process air through the
air conduit means;
air flow directing means for directing the process air to the top
end of one of said desiccant towers thereby placing said one of
the towers in a process cycle while the other of the towers is placed
in a regeneration cycle, said air flow directing means also directing
the regeneration air exiting from a top end of said other of the
towers in a regeneration cycle to a vent;
means for regulating the time period and temperature of the process
and regeneration cycles;
first and second bottom heater means respectively housed adjacent
to a bottom end of the first and second desiccant heating towers
and extending in a plane traverse to the longitudinal direction
of the respective desiccant heating towers to heat the desiccant
so as to adsorb moisture from said process air circulating downwardly
through one of said towers in a process cycle, and for simultaneously
super-heating and driving moisture off the desiccant by means of
said regeneration air circulating upwardly through the other of
said towers in a regeneration cycle, said regeneration air carrying
by means of convection a generally bottom-originating hot convection
wave front propagating upwardly through the desiccant in said tower
in a regeneration cycle; and
first and second generally central heater means respectively housed
between top and bottom ends of said first and second desiccant towers
and extending in a plane transverse to the longitudinal direction
of the respective desiccant heating towers for super-heating and
driving moisture off the desiccant by means of said regeneration
air carrying a generally centrally-originating hot convection wave
front propagating upwardly through the desiccant of one of said
towers in a regeneration cycle, the generally centrally-originating
hot convection wave front coupled with the bottom-originating hot
convection wave front considerably reducing the time to completely
regenerate the saturated desiccant in said one of said towers in
a regeneration cycle.
2. A system of drying and heating process air according to claim
1 wherein said first generally central heater means is closer to
the top end than that of the bottom end of said first desiccant
tower, and said second generally central heater means is closer
to the top end than that of the bottom end of said second desiccant
tower.
3. A system of drying and heating process air according to claim
2 wherein the distance between said first generally central heater
means and the top end of said first desiccant tower is approximately
between sixty percent and seventy percent of the distance between
said first generally central heater means and the bottom end of
said first desiccant tower; and wherein
the distance between said second generally central heater means
and the top end of said second desiccant tower is approximately
between sixty percent and seventy percent of the distance between
said second generally central heater means and the bottom end of
said second desiccant tower.
4. A system of drying and heating process air according to claim
1 wherein a small portion of the process air exiting one of the
desiccant towers in a process cycle becomes regeneration air to
circulate upwardly through the other desiccant tower in a regeneration
cycle.
5. A system of drying and heating process air according to claim
1 wherein the adsorbent is a molecular sieve.
6. A system of drying and heating process air according to claim
1 wherein the first and second bottom heater means and the first
and second generally central heater means are heater coils.
7. A system of drying and heating process air according to claim
1 wherein the means for regulating the period and temperature of
the process and regeneration cycles is a microprocessor controller.
8. A system of drying and heating process air according to claim
1 wherein the air conduit means takes the form of hollow cylindrical
tubes.
9. A system of drying and heating process air to be circulated
for drying hygroscopic thermoplastic material contained in a drying
hopper before the material is introduced into a plastic processing
machine, the system including:
a drying hopper for containing hygroscopic thermoplastic material
before the thermoplastic material is introduced into a plastics
processing machine;
process air for drying said hygroscopic thermoplastic material
contained in said drying hopper;
first and second desiccant towers, each of said towers alternating
between a process cycle and a regeneration cycle while the other
tower is in an opposite cycle;
desiccant contained in said first and second desiccant towers for
adsorbing moisture from the process air;
regeneration air taken from a small portion of the process air
that is newly dried, the regeneration air driving moisture off saturated
desiccant in one of said towers in a regeneration cycle;
air tubes providing a path for circulating the process air between
the drying hopper and the first and second desiccant towers in a
virtual closed-loop path;
blower for circulating the process air through the air tubes;
valve means directing the process air to the top end of one of
said desiccant towers thereby placing said one of the towers in
a process cycle while the other of the towers is placed in a regeneration
cycle, said valve means also directing the regeneration air exiting
from a top end of said other of the towers in a regeneration cycle
to a vent;
control means for automatically regulating the time period and
temperature of the process and regeneration cycles;
first and second bottom heater coils respectively housed adjacent
to a bottom end of the first anti second desiccant heating towers
and extending in a plane transverse to the longitudinal direction
of the respective desiccant heating towers to heat the desiccant
so as to adsorb moisture from said process air circulating downwardly
through one of said towers in a process cycle, and for simultaneously
super-heating and driving moisture off the desiccant by means of
said regeneration air circulating upwardly through the other of
said towers in a regeneration cycle, said regeneration air carrying
by means of convection a generally bottom-originating hot convection
wave from propagating upwardly through the desiccant in said tower
in a regeneration cycle; and
first and second generally central heater coils respectively housed
generally centrally of top and bottom ends of said first and second
desiccant towers and extending in a plane transverse to the longitudinal
direction of the respective desiccant heating towers for super-heating
and driving moisture off the desiccant by means of said regeneration
air carrying a generally centrally-originating hot convection wave
front propagating upwardly through the adsorbent of one of said
towers in a regeneration cycle, the generally centrally-originating
hot convection wave front coupled with the bottom-originating hot
convection wave front considerably reducing the time to completely
regenerate the saturated desiccant in said one of said towers in
a regeneration cycle.
10. A system of drying and heating process air according to claim
9 wherein said first generally central heater coil is closer to
the top end than that of the bottom end of said first desiccant
tower, and said second generally central heater coil is closer to
the top end than that of the bottom end of said second desiccant
tower.
11. A system of drying and heating process air according to claim
10 wherein the distance between said first generally central heater
coil and the top end of said first desiccant tower is approximately
between sixty percent to seventy percent of the distance between
said first generally central heater coil and the bottom end of said
first desiccant tower; and wherein
the distance between said second generally central heater coil
and the top end of said second desiccant tower is approximately
between sixty percent to seventy percent of the distance between
said second generally central heater coil and the bottom end of
said second desiccant tower.
12. A system of drying and heating process air according to claim
9 wherein a small portion of the process air exiting one of the
desiccant towers in a process cycle becomes regeneration air to
circulate upwardly through the other desiccant tower in a regeneration
cycle.
13. A system of drying and heating process air according to claim
9 wherein the adsorbent is a molecular sieve.
14. A system of drying and heating process air according to claim
9 wherein the means for regulating the period and temperature of
the process and regeneration cycles is a microprocessor controller.
15. A system of drying and heating process air according to claim
9 wherein the air conduit means takes the form of hollow cylindrical
tubes.
16. A method of drying and heating process air to be circulated
for drying hygroscopic thermoplastic material contained in a drying
hopper before the material is introduced into a plastic processing
machine comprising the steps of:
upwardly circulating hot, dry process air so as to dry thermoplastic
material contained in a drying hopper;
providing two desiccant towers each containing desiccant for adsorbing
moisture from the process air;
directing moist process air leaving the drying hopper to one of
the towers thereby placing said one of the towers in a process cycle
while the other of the towers is simultaneously placed in a regeneration
cycle;
downwardly circulating the moist process air through said one of
the desiccant towers in a process cycle to adsorb moisture from
the moist process air and to heat the moist process air to a predetermined
process temperature;
returning the dried process air back to the drying hopper to further
dry the thermoplastic material contained therein;
simultaneously circulating hot, regeneration air upwardly through
said other of the desiccant towers in a regeneration cycle whereby
moisture is driven off saturated desiccant by means of upwardly
propagating hot convection wave fronts originating within said other
of the desiccant towers from the bottom and from a generally central
portion thereof;
redirecting the moist process air leaving the drying hopper to
said other of the desiccant towers after completion of the regeneration
cycle thereby placing said one of the desiccant towers in a regeneration
cycle and said other of the desiccant towers in a process cycle;
and
continuing to alternate the process and regeneration cycles between
said one and said other of the desiccant towers so as to form a
repeating sequence.
17. A method of drying and heating process air according to claim
16 wherein a small portion of the process air exiting one of said
desiccant towers in a process cycle is directed to become the regeneration
air to circulate upwardly through the other of said desiccant towers
in a regeneration cycle.
18. A method of drying and heating process air according to claim
16 wherein said upwardly propagating hot convection wave front of
said generally central portion of one of said desiccant towers in
a regeneration cycle originates from a location which is closer
to a top end of said desiccant tower than is a bottom end of said
desiccant tower.
19. A method of drying and heating process air according to claim
18 wherein said upwardly propagating hot convection wave front of
said generally central portion of one of said desiccant towers in
a regeneration cycle originates from a location whose distance to
a top end of said desiccant tower is approximately seventy percent
of the distance between said generally central portion and a bottom
end of said desiccant tower.
Description BACKGROUND OF THE INVENTION
The present invention relates to a system of drying and heating
process air to be circulated for drying hygroscopic thermoplastic
material contained in a drying hopper before the material is introduced
into a plastic processing machine. The invention is concerned more
particularly with improved desiccant towers used in a drying system
for effectively drying thermoplastics in areas of high ambient humidity
and for considerably reducing the time required to effectively and
completely regenerate or purge moisture from a saturated desiccant
tower.
The drying of thermoplastics before entering plastic processing
machines such as injection, and extrusion molding machines is highly
critical. Thermoplastic materials are generally hygroscopic. Moisture
in ambient air can harm the mechanical, electrical, and visual properties
of the finished plastic product. Hence, there has long been the
need for reliable and efficient dryer systems to dry thermoplastic
material contained in a plastics storage device such as a drying
hopper before the material is processed. Drying thermoplastic material
is usually accomplished by first drying process air which subsequently
circulates through and drives moisture from thermoplastics contained
in a drying hopper.
A standard implementation of a dryer system used with plastic processing
machines comprises a desiccant tower containing desiccant (adsorbent)
for adsorbing moisture from ambient or process air while the tower
is in a process cycle. Adsorbents used are molecular solids oppositely
ionized relative to that of water molecules. The water molecules
are thereby electrically attracted and absorbed into the molecular
solid. Some well-known types of solid desiccants used in dryer systems
include molecular sieves, silica gel, and activated alumina.
The reason for using such adsorbents is due to their high moisture-holding
capacity defined by the equation: equilibrium H.sub.2 O capacity=lb.
of adsorbed H.sub.2 O/100 lb. of adsorbent. At a typical thermoplastics
drying temperature of 150.degree. to 300.degree. Fahrenheit (F.),
some molecular sieves can adsorb as much as 20% of their dry weight
in moisture, such as water.
The desiccant, however, eventually becomes saturated, thereby losing
its effectiveness for drying process air. Consequently, the saturated
desiccant must be taken "off-stream" in order to regenerate
its moisture adsorbing capacity. A heater located at or near the
base of the saturated tower super-heats air circulating upwardly
through the tower. The super-heated air transfers its thermal energy
to the saturated desiccant by means of thermal convection. Moisture
evaporates and is driven off the hot desiccant, whereupon the hot
circulating air carries the evaporated moisture away from the saturated
tower to be vented.
Obviously, the time (usually hours) necessary to regenerate the
saturated desiccant cannot be used for drying thermoplastic material.
In response to the off-stream problem, several solutions have been
developed to ensure a constant supply of dry process air to the
drying hopper even when a saturated tower is under regeneration
(in a regeneration cycle).
As one solution, Conair dryer models CD-100 through CD-2400 employ
four desiccant towers. When a saturated desiccant tower needs to
be regenerated, a carrousel indexes a fresh desiccant tower to replace
the saturated tower. Hence, desiccant tower indexing assures an
uninterrupted supply of dry process air for drying thermoplastics.
However, the drying system is expensive and prone to mechanical
breakdown because it uses a multitude of moving parts. Other solutions
have been developed using fewer moving parts.
Dual fixed-bed desiccant towers have become an industry standard
as a simple solution for maintaining a constant flow of dry process
air. After an adsorbent in a desiccant tower is fully regenerated,
valves redirect the process air flow so that the newly regenerated
tower is subsequently placed in a process cycle, while simultaneously
the other tower previously in a process cycle is subsequently placed
in a regeneration cycle. The towers are, therefore, always in opposite
cycles (i.e. process vs. regeneration). Hence, desiccant tower switchover
also assures an uninterrupted supply of dry process air with the
advantage of few moving parts.
The above-mentioned equipment, however, may fail under high humidity
conditions. As the humidity of the ambient air increases, the effective
time period of the process cycle decreases because of a faster build-up
of moisture within the surface of the desiccant. Danger arises when
the tower in a process cycle becomes fully saturated before the
other tower is completely regenerated.
By enlarging the size of the tower to hold more desiccant, the
effective process cycle time period can be increased. This solution
however is generally undesirable because the time needed to completely
regenerate a fully saturated tower also increases.
Another solution is to decrease the regeneration cycle time period
by more rapidly heating the saturated desiccant. U.S. Pat. No. 2783547
discloses dual fixed-bed desiccant towers each containing a heater
coil helically wound within a central metal tube extending longitudinally
along a central axis of the tower. Metal fins extend outwardly into
surrounding silica gel to facilitate rapid heating of the desiccant
as regeneration air is forced through the gel. Because desiccant
does not have good thermal conductivity, the outwardly directed
heating of the surrounding gel may be undesirably localized near
the thermally conductive fins.
Similarly, U.S. Pat. No. 4601114 discloses a helically wound
heater coil extending along the central axis of a desiccant tower.
Regeneration air, however, is forced sideways from the central heater
in order to heat the surrounding desiccant. Again, heating of the
surrounding desiccant may tend to be undesirably localized and irregular
since convection heat flow from the sideways-forced air is counter
to the natural tendency of the regeneration air to rise. Hence,
a more effective and even heating of the desiccant can be achieved
by circulating regeneration air upwardly from below the desiccant.
In response to the above-mentioned difficulties, it is a general
object of the present invention to reduce the regeneration cycle
time period 25% to 30% from that of a standard desiccant tower having
a single heater.
It is another object of the present invention to substantially
increase the effective process cycle time period by doubling the
height of a standard desiccant tower so as to ensure an uninterrupted
supply of dry process air under high humidity conditions.
SUMMARY OF THE INVENTION
One aspect of the invention resides in a system of drying and heating
process air to be circulated for drying hygroscopic thermoplastic
material contained in a drying hopper before the material is introduced
into a plastic processing machine. A drying hopper is provided for
containing hygroscopic thermoplastic material dried by process air
before the thermoplastic material is introduced into a plastics
processing machine. First and second desiccant towers are also provided
wherein each of the towers alternates between a process cycle and
a regeneration cycle while the other tower is in an opposite cycle.
Desiccant is contained in the first and second desiccant towers
for adsorbing moisture from the process air. Some of the process
air is further used as regeneration air for driving moisture off
saturated desiccant in one of the towers in a regeneration cycle.
Air conduit means provides a path for circulating the process air
between the drying hopper and the first and second desiccant towers.
Air circulating means provides for circulating the process air through
the air conduit means. Air flow directing means provides for directing
the process air to the top end of one of the desiccant towers thereby
placing the tower in a process cycle while the other of the towers
is placed in a regeneration cycle. The air flow directing means
also directs the regeneration air exiting from a top end of the
other of the towers in a regeneration cycle to a vent. Means are
provided for regulating the time period and temperature of the process
and regeneration cycles. First and second bottom heater means are
respectively located adjacent to a bottom end of the first and second
desiccant heating towers to heat the desiccant so as to adsorb moisture
from the process air circulating downwardly thorough one of the
towers in a process cycle, and for simultaneously super-healing
and driving moisture off the adsorbent by means of the regeneration
air circulating upwardly through the other of the towers in a regeneration
cycle. The regeneration air carries by means of convection a generally
bottom-originating hot convection wave front propagating upwardly
through the desiccant in the tower in a regeneration cycle. The
first and second generally central heater means are respectively
located between top and bottom ends of the first and second desiccant
towers for super-heating and driving moisture off the desiccant
by means of the regeneration air carrying a generally centrally-originating
hot convection wave front propagating upwardly through the desiccant
of one of the towers in a regeneration cycle. The generally centrally-originating
hot convection wave front coupled with the bottom-originating hot
convection wave front considerably reduces the time to completely
regenerate the saturated desiccant in the one of the towers in a
regeneration cycle.
Another aspect of the present invention resides in a method of
drying and heating process air to be circulated for drying hygroscopic
thermoplastic material contained in a drying hopper before the material
is introduced into a plastic processing machine. The method comprises
the steps of upwardly circulating hot, dry process air so as to
dry thermoplastic material contained in a drying hopper. Two desiccant
towers are each provided containing desiccant for adsorbing moisture
from the process air. Moist process air leaves the drying hopper
and is directed to one of the towers thereby placing one of the
towers in a process cycle while the other of the towers is simultaneously
placed in a regeneration cycle. The moist process air is downwardly
circulated through the one of the desiccant towers in a process
cycle to adsorb moisture from the moist process air and to heat
the moist process air to a predetermined process temperature. The
dried process air is returned back to the drying hopper to further
dry the thermoplastic material contained therein. Simultaneously,
hot regeneration air is circulated upwardly through the other of
the desiccant towers in a regeneration cycle whereby moisture is
driven off saturated desiccant by means of upwardly propagating
hot convection wave fronts originating from the bottom and from
a generally central portion of the other of the desiccant towers.
The moist process air leaving the drying hopper is redirected to
the other of the desiccant towers after completion of the regeneration
cycle thereby placing the one of the desiccant towers in a regeneration
cycle and the other of the desiccant towers in a process cycle.
The process and regeneration cycles continue to alternate between
the one and the other of the desiccant towers so as to form a repeating
sequence.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a schematically illustrates a thermoplastics drying system
using improved fixed-bed desiccant towers in accordance with the
present invention.
FIG. 1b schematically illustrates a different process air flow
path from that of FIG. 1a after a desiccant bed changeover.
FIG. 2 is a graph including three curves labeled TOP, MIDDLE, and
BOTTOM illustrating the average temperature over time of desiccant
in a lower portion (BOTTOM), an upper portion (TOP), and a central
portion (MIDDLE) of an improved desiccant tower undergoing a regeneration
cycle.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1a and 1b (using the same reference numerals to designate
like parts) schematically illustrate fixed desiccant towers 20 and
30 employing generally central heater coils, 80 and 56 respectively
arranged in a novel way. The desiccant towers are employed in a
thermoplastics drying system 10 which dries and heats process air
to be circulated for drying hygroscopic thermoplastic material contained
in a drying hopper 100 before the material is introduced into a
plastic processing machine (not shown).
Desiccants (adsorbents) D1 and D2 forming packed desiccant beds
in the desiccant towers 20 and 30 respectively, are used to dry
the process air because of their high moisture adsorbing capacity
defined by the equation: equilibrium H.sub.2 O capacity=lb. of adsorbed
H.sub.2 O/100 lb. of adsorbent.
As mentioned, the adsorbents are molecular solids oppositely ionized
relative to that of water molecules. The water molecules are thereby
electrically attracted and bond to the surface of the adsorbent.
In this instance, Molecular Sieve Type 4A manufactured by Union
Carbide is chosen as an adsorbent because it can adsorb as much
as 20% of its dry weight in moisture at a typical thermoplastics
drying temperature of 150.degree. F. to 300.degree. F.
Referring now to FIGS. 1a and 1b in more detail, upwardly circulating
process air which has absorbed moisture from hygroscopic thermoplastic
material contained in the drying hopper 100 is directed (as shown
by arrows) via air conduits, such as hollow cylindrical tubes in
this instance, from an upper outlet of the drying hopper through
conduit 40 and through filter 42 which removes fines from the moist
process air. Air intake opening 41 formed in the conduit 40 allows
a relatively small amount of outside air to mix with the moist process
air in the conduit 40 in order to compensate for air exiting at
vent 88. The filtered, process air is then directed through conduit
44 toward and through blower 46 which circulates the process air
in a virtual closed-loop path, to and from the desiccant towers
2030 and the drying hopper 100. Process air leaving the blower
46 circulates through conduit 48 toward and through a four-way valve
50 directing the moist process air to one of the desiccant towers
20 and 30 to be explained more fully below.
During a first predetermined time interval shown in FIG. 1a, the
four-way valve 50 directs the moist process air through conduit
52 and toward the desiccant tower 30 which is in a process cycle
to adsorb moisture from the process air. The desiccant tower 30
comprises tower body 58 and top and bottom cones 54 and 62 respectively.
The cones 54 and 62 communicate with the tower body 58 which is
packed with the desiccant D2. The tower body 58 houses the central
heater coil 56 in a novel arrangement, preferably extended in a
plane transverse to the longitudinal direction of the tower body
58 and located generally centrally of the tower's top and bottom
ends 94 and 96 respectively. (In FIGS. 1a and 1b the central heater
coil 56 is shown slightly closer to the top end 94 than that of
the bottom end 96 for reasons to be explained hereinbelow.) The
bottom cone 62 houses bottom heater coil 60 also preferably extended
in a plane transverse to the longitudinal direction of the tower
body 58.
In FIG. 1a, the bottom heater coil 60 heats the process air to
an optimal thermoplastics drying temperature of about 150.degree.
F. to 300.degree. F. The process air circulates downwardly through
the desiccant tower 30 from a top portion 57 defined between the
top end 94 and the central heater coil 56 and continues through
a bottom portion 59 defined between the central heater coil 56 and
the bottom end 96. Meanwhile, moisture carried in the process air
is adsorbed by the surrounding desiccant D2 in the tower body 58.
The dried, heated process air then exits the desiccant tower 30
through the bottom cone 62 and enters into conduit 64.
As determined by air flow pressure through a standard desiccant-packed
tower and drying hopper, the greater portion (95% to 98%) of the
dried, process air typically returns to the drying hopper 100 through
conduit 68 to further dry the thermoplastic material.
A smaller portion (2% to 5%) of the dried, process air is used
as regeneration air and circulates through conduit 70 and into the
desiccant tower 20 (in a regeneration cycle) to regenerate or drive
moisture off its desiccant. The desiccant tower 20 comprises tower
body 76 and top and bottom cones 72 and 74 respectively. The cones
72 and 74 communicate with the tower body 76 which is packed with
the desiccant D1. The tower body 76 houses the central heater coil
80 preferably extended in a plane transverse to the longitudinal
direction of the tower body 76 and located generally centrally of
top and bottom ends, 90 and 92 respectively, of the tower body 76.
(As is the case with the central heater coil 56 of the tower 30
in FIGS. 1a and 1b the central heater coil 80 of the tower 20 is
shown slightly closer to the top end 90 than that of the bottom
end 92 for reasons to be explained hereinbelow.) The bottom cone
74 houses bottom heater coil 78 also preferably extended in a plane
transverse to the longitudinal direction of the tower body 76.
The generally central heater coil 80 and the bottom heater coil
78 simultaneously super-heat the regeneration air circulating upwardly
through the tower 20 to a predetermined regeneration temperature
of about 550.degree. F. to 600.degree. F. to completely drive moisture
off (regenerate) the saturated desiccant D1 in the tower body 76.
As the desiccant tower 20 is undergoing regeneration, the bottom
heater coil 78 produces a first, bottom-originating hot convection
wave front propagating upwardly with the process air through D1.
The wave front originates near the bottommost layers of D1 at 92
and flows upwardly through D1 in a bottom portion 82 of the tower
body 76 defined between the bottom end 92 and the central heater
coil 80. The first convection wave front continues to propagate
through a top portion 84 of the tower body 76 defined between the
top end 90 and the central heater coil 80 of the desiccant tower
20. Similarly, the central heater coil 80 produces a novel second,
generally centrally-originating hot convection wave-front propagating
upwardly through D1 in the top portion 84.
By means of the thermal convection, regeneration air circulating
upwardly through the tower body 76 super-heats D1 to a regeneration
temperature of about 550.degree. F. to 600.degree. F. so as to drive
moisture off the desiccant. The moisture-laden regeneration air
is then swept out of the tower 20 through the top cone 72. The saturated
regeneration air circulates through conduit 86 and toward the valve
50 which directs the air out the vent 88.
One advantage of adding the novel second generally central heater
coil is that the two hot convection wave fronts permit the standard
height of a desiccant tower as well as the amount of desiccant filling
the tower to be doubled. Doubling the amount of desiccant substantially
increases the effective length of the process cycle resulting in
the ability to maintain an uninterrupted supply of dry, process
air even under high humidity conditions. In addition, the two hot
convection wave fronts also reduce the time to completely regenerate
the saturated desiccant by about 25% to 30% over that of conventional
methods.
One acceptable trade-off of doubling the tower body height is that
after regeneration, the time required to cool the desiccant down
to process temperature before switching the tower to a process cycle
is approximately 25% more than that of using a conventional tower
height with a single heater.
During a second predetermined time period after the tower 20 is
completely regenerated, as illustrated in FIG. 1b, the four way
valve 50 effects a desiccant bed changeover by diverting the flow
of the moist process air (as shown by arrows) through the conduit
86 and to the desiccant tower 20 so as to remove the tower 20 from
its regeneration cycle and place it in an opposite (i.e., process)
cycle. The desiccant tower 30 is likewise removed from its process
cycle and placed in an opposite, (i.e., regeneration) cycle. Hence,
repeatedly alternating the cycles of the towers 20 and 30 between
process and regeneration always ensures a fresh tower in a process
cycle for delivering a constant, uninterrupted supply of dry process
air for the drying of thermoplastics.
Due to the symmetry of the operation of drying the process air
after the desiccant bed changeover by merely interchanging the towers
20 and 30 with respect to the process air flow path, no further
explanation for drying the process air as shown in FIG. 1b is deemed
necessary.
FIG. 2 graphically illustrates a preferred regeneration temperature
profile of, in this instance, the tower 20 shown in FIG. 1a over
time of its topmost layers of D1 at 90 shown by the curve labeled
(TOP), the bottommost layers of D1 at 92 shown by the curve labeled
(BOTTOM), and the middle layers of D1 at 91 immediately below the
central heater coil at 80 shown by the curve labeled (MIDDLE).
Referring to FIGS. 1a and 2 immediately prior to time zero, the
desiccant tower 20 is ending its process cycle and nearing the beginning
of its regeneration cycle. As seen by all three of the curves, i.e.,
TOP, BOTTOM, and MIDDLE, to the extreme left of FIG. 2 the bottom
heater coil 78 has been maintaining D1 at a process temperature
of about 150.degree. to 300.degree. F.
The regeneration cycle of the desiccant tower 20 begins at time
zero. A microprocessor controller 98 shown in FIGS. 1a and 1b may
be employed to automatically regulate the time period and temperature
of the process and regeneration cycles. The slopes of all three
curves start to increase as the central and bottom heater coils,
80 and 78 heat the desiccant from 150.degree. F. to 300 .degree.
F. to a regeneration temperature of about 550.degree. F. to 600.degree.
F. The temperature of D1 respectively located at the top and bottom
portions 84 and 82 of the tower 20 do not rise at the same rate
due to the differing distribution of moisture throughout the tower
20. When the process air circulated downwardly through D1 during
the previous process cycle, a greater portion of the moisture was
adsorbed in the top portion 84 of tower 20. Because more energy
is required to heat the adsorbed moisture in the top portion 84
less energy is transferred for directly heating the adjacent desiccant.
Hence the TOP curve representing the temperature of D1 in the topmost
layers of desiccant at 90 rises more slowly than does that of the
BOTTOM curve representing the temperature of D1 in the bottom portion
82 at 92.
However, the layer of desiccant at 81 just below the central heater
coil 80 rises more slowly than does the moist, topmost layers of
desiccant at 90. The reason is that the desiccant at 81 below the
central heater is only heated by the wave front originating from
the bottom heater coil 78 whereas the topmost layers of desiccant
at 90 are heated by the wave front originating from the bottom heater
coil 78 as well as the wave front originating from the central heater
coil 80.
In general, the temperature rise levels off between 210.degree.
F. to 250.degree. F. during which energy is expended solely to boil
off moisture from D1 (latent heat of vaporization). The leveling
off of temperature at the topmost layers of D1 at 90 (TOP) and middle
layers at 91(MIDDLE) are relatively longer than that of the bottommost
layers of D1 at 92 (BOTTOM) because of the greater moisture content
in the top portion 84. After most of the moisture is boiled off,
the temperature of the desiccant throughout the desiccant tower
20 resumes rising at relatively higher rates.
Because of the higher moisture content of the top portion 84 it
has been determined through experiment that placing the central
heater coil 80 closer to the topmost layers of D1 at 90 than that
of the bottommost layers of D1 at 92 results in a more effective
and efficient means for regenerating the desiccant. For example,
the central heater coil 80 may be placed so that the top portion
84 is about 60% to 70% of the length of the bottom portion 82.
It has also been determined through experiment that the best, fastest,
and most energy efficient results for regenerating the desiccant
D1 are attained by heating the bottom portion 92 of the tower 20
shown in the BOTTOM curve to about 550.degree. F. to 600.degree.
F. and maintaining the temperature until the topmost layers 90 shown
in the TOP curve reaches 350.degree. F. after about forty minutes
into the cycle. Thereafter, power to both the bottom heater coil
78 and the central heater coil 80 is stopped by the microprocessor
controller 98 for cooling the desiccant which is immediately evident
in FIG. 2 by the descent of the BOTTOM curve from a temperature
range 550.degree. F. to 600.degree. F. when the FOP curve temperature
is about 350.degree. F.
Meanwhile, the hot, bottom-originating convection wave front continues
to propagate upwardly into the top portion 84 of the desiccant tower
20. Consequently, the desiccant temperature shown in the TOP curve
continues to rise to the desired regeneration temperature of about
550.degree. F. to 600.degree. F. even though the temperature shown
in the BOTTOM curve is simultaneously rapidly dropping.
Eventually, the TOP and BOTTOM curves merge as the desiccant in
the topmost layers of D1 at 90 also drops down to and levels off
at the process temperature of about 150.degree. F. to 300.degree.
F. After about 20 minutes required to completely cool D1 to the
process temperature of 150.degree. F. to 300.degree. F., the four-way
valve 50 effects a cycle changeover of the towers 20 and 30.
It will be understood that numerous modifications and substitutions
may be made without departing from the spirit of the invention.
For example, different types of heaters or even multiple heaters
may be used either inside or outside along the length of the desiccant
tower. In addition, the generally central heaters may be placed
considerably off-center of the tower for obtaining the shortest
effective regeneration cycle time. Also, different types of desiccant
from the type mentioned may be substituted. Accordingly, the present
invention has been described in several preferred embodiments by
way of illustration, rather than limitation. |