Abstrict An apparatus and method of use for effectively purging desiccant
air dryers of moisture load by introducing heated purge air into
the containment vessel holding the desiccant, directly into those
areas of the desiccant bed where the moisture loads are most heavily
saturated. The improved heat regenerated desiccant gas dryer utilizes
parallel ports for introducing heated air or an internal heat distributor,
the former automatically controlled by valves selectively operated
to allow purge air to quickly cause water vapor adsorbed by the
desiccant to be released and discharged from the vessel, allowing
the dryer to be returned to service in the gas drying mode with
greater confidence that undesirable dew point spikes have been eliminated.
More efficient use of heated purging air enables shorter turn-around
times for the dryer and energy savings.
Claims We claim:
1. A desiccant gas dryer, which comprises
a vessel having an interior chamber defined by a vessel side wall
comprising one or more purge inlet ports directly communicating
with the interior chamber of said vessel, a first end wall having
a moist gas inlet, and a second end wall opposite the first end
wall having a dry gas outlet;
a bed comprising a regenerable desiccant agent disposed within
said interior chamber of said vessel;
means for injecting a stream of moist gas through said moist gas
inlet of said vessel for diffusion through said bed and for discharging
dried gas through said dry gas outlet during a moisture sorption
phase, and
means for injecting a stream of heated air directly into said bed
through said one or more purge inlet ports in said vessel side wall
for removing moisture from said bed.
2. The desiccant gas dryer of claim 1 comprising a plurality of
parallel purge inlet ports in the side wall of said vessel.
3. The desiccant gas dryer of claim 2 including means for simultaneous
injection of a stream of heated air into said bed through said parallel
purge inlet ports in said vessel side wall and said dry gas outlet
in the second end wall of said vessel.
4. The desiccant gas dryer of claim 2 which is a device for drying
a gas selected from the group consisting of compressed air, natural
gas and nitrogen.
5. The desiccant gas dryer of claim 2 wherein said parallel purge
inlet ports are positioned adjacent to the bed in proximity to the
highest concentration of moisture.
6. The desiccant gas dryer of claim 2 wherein said parallel purge
inlet ports have at least one port positioned in proximity to said
moist gas inlet.
7. The desiccant gas dryer of claim 4 wherein said parallel purge
inlet ports are positioned along the vessel side wall and spaced
from one another at distances sufficient to achieve substantially
uniform temperature balance during desiccant regeneration.
8. The desiccant gas dryer of claim 1 wherein said controls for
said valve means comprise multiple thermal sensors and a microprocessor,
said thermal sensors generating signals to said microprocessor for
regulating the flow of heated air to said bed for achieving balanced
temperature in said dryer during desiccant regeneration.
9. The desiccant gas dryer of claim 8 wherein the thermal sensors
comprise at least first and second sensors and the bed comprising
the desiccant contains a moisture gradient, said first sensor and
said second sensor being positioned in areas of the bed having low
and high moisture levels, respectively.
10. The desiccant gas dryer of claim 4 including means for heating
the stream of air injected into the parallel purge inlet ports during
desiccant regeneration. interior chamber of said vessel;
a bed comprising a regenerable desiccant agent disposed within
the interior chamber of said vessel adjacent to said porous heated
air distributor;
means for injecting a stream of moist gas through said moist gas
inlet of said vessel for diffusion through said bed and for discharging
dried gas through said dry gas outlet during a moisture sorption
phase, and
means for injecting a stream of heated air into said bed through
said porous heated air distributor for removing water from said
bed, said desiccant gas dryer including exhaust means for discharging
moisture and air during desiccant regeneration.
11. A desiccant gas dryer, which comprises
a vessel having an interior chamber defined by a vessel side wall,
a first end wall having a moist gas inlet, and a second end wall
having a dry gas outlet;
a porous heated air distributor disposed within the interior chamber
of said vessel;
a bed comprising a regenerable desiccant agent disposed within
the interior chamber of said vessel adjacent to said porous heated
air distributor, said distributor having greater outlet capacity
in the region of the bed where moisture stratification is more concentrated;
means for injecting a stream of moist gas through said moist gas
inlet of said vessel for diffusion through said bed and for discharging
dried gas through said dry gas outlet during a moisture sorption
phase, and
means for injecting a stream of heated air into said bed through
said porous heated air distributor for removing water from said
bed.
12. The desiccant gas dryer of claim 11 which is a device for drying
a gas selected from the group consisting of compressed air, natural
gas and nitrogen.
13. The desiccant gas dryer of claim 12 wherein the porous heated
air distributor is a tubular member running substantially the length
of the bed with spaced outlets directing the flow of heated air
to layers of the bed.
14. The desiccant gas dryer of claim 13 wherein the outlets of
the porous heated air distributor are in the greatest number in
the region of the bed in proximity to the moist gas inlet of said
pressure vessel.
15. The desiccant gas dryer of claim 13 wherein the outlets of
said porous heated air distributor are positioned at distances from
one another sufficient to achieve substantially uniform temperature
balance during desiccant regeneration.
16. A method of regenerating a desiccant gas dryer, which comprises
the steps of:
providing a bed comprising a regenerable desiccant agent disposed
within an interior chamber of a vessel, said bed having moisture
adsorbed thereon;
(i) regenerating the desiccant by directly injecting multiple streams
of heated air into the interior of said bed through multiple heated
air discharge locations within said bed, said heated air discharge
locations being more concentrated in those regions of the bed where
moisture stratification is greater, or (ii) impinging multiple streams
of heated air directly against the bed exterior wherein at least
one stream directly engages the bed through a port in an exterior
side wall of said vessel, and
purging moist air from said vessel.
17. The method of regenerating a desiccant gas dryer according
to claim 16 wherein multiple streams of heated air from entry points
from the exterior of said vessel are introduced into said bed at
locations containing the highest concentration of moisture.
18. The method of regenerating a desiccant gas dryer according
to claim 16 wherein multiple streams of heated air are injected
into the bed spaced from one another at distances sufficient to
achieve substantially uniform temperature balance during desiccant
regeneration.
19. The method of regenerating a desiccant gas dryer according
to claim 16 wherein said multiple streams of heated air are directly
injected into said bed from parallel ports.
20. A desiccant gas dryer for drying a gas selected from the group
consisting of compressed air, natural gas and nitrogen, which comprises
a vessel having an interior chamber defined by a vessel side wall
comprising a plurality of parallel purge inlet ports, a first end
wall having a moist gas inlet, and a second end wall having a dry
gas outlet;
a bed comprising a regenerable desiccant agent disposed within
the interior chamber of said vessel, said parallel purge inlet ports
positioned along the vessel side wall and spaced from one another
at distances sufficient to achieve substantially uniform temperature
balance during desiccant regeneration;
means for injecting a stream of moist gas through said moist gas
inlet of said vessel for diffusion through said bed and for discharging
dried gas through said dry gas outlet during a moisture sorption
phase, and
valve means and controls therefor for automatically regulating
the injection of heated air into said bed during desiccant regeneration.
21. The desiccant air dryer of claim 1 including exhaust means
for discharging moisture and air during desiccant regeneration.
Description TECHNICAL FIELD
The present invention relates generally to devices for drying gas,
and more specifically, to highly efficient multiport injection,
heat regenerated desiccant air dryers and methods of use.
BACKGROUND OF THE INVENTION
In industry, air and pressurized (compressed) air are widely employed
in operating equipment and machinery in manufacturing, product fabrication,
and in countless other applications and scenarios. Air and compressed
air in particular, is often laden with moisture which negatively
impacts on the performance and life expectancy of equipment, machinery,
applications and processes, ultimately resulting in costly equipment
failure, downtime and befouled product.
One desirable method employed in removing moisture from air and
other gases has been through the use of a drying agent or desiccant
(adsorbent). Desiccant dryers have been one of the premiere means
for successfully removing substantially all moisture from air for
industrial applications thereby reducing equipment failures and
improving product quality. Desiccant air dryers typically are comprised
of a pressure vessel filled with the desiccant material, piping
and valves as means for controlling air flow throughput and means
for purging moisture from the drying agent. Generally, these systems
can have other useful features, such as filters to remove oil and
dirt, screens to contain the desiccant within the vessel, electronic
controls, liquid draining devices, and so on. In addition, many
configurations of desiccant air dryers are constructed of dual components,
so that the apparatus may be on-line using one set of components
while the second set is regenerating itself of captured (adsorbed)
moisture. Such systems are known as "Twin Tower" dryers.
Because desiccant loses its ability to effectively remove moisture
after a period of use, desiccant filled gas dryers are required
to operate in a regenerative mode with a desorption half-cycle to
remove water from the drying agent to reactivate it for another
adsorption half-cycle. This regeneration process represents a significant
amount of unproductive downtime and direct costs in operation of
heaters and/or blowers, or in the case of dry purge methods, to
extract the collected moisture from the desiccant material. Regeneration
of the desiccant and purging moisture from the dryer can take several
hours to complete, and represents typically fifty percent of the
duty cycle of the dryer, e.g., 4 hours on-line for removing water
from an air stream, and 4 hours off-line for regenerating, cooling
and repressurizing, depending of the capacity/size of the system
and air volume.
As previously stated, one aspect of the problem of desiccant regeneration
stems from the high energy requirements in the form of heat and/or
electrical power needed to remove moisture and dry the desiccant.
Heretofore, in the regeneration phase it has been the practice to
inject heated purging air into the pressure vessel at one end and
force it through the moisture laden desiccant bed and out the other
end of the vessel. Moisture adsorbed by the desiccant is routinely
more concentrated at the first end or air entry end of the vessel
and is less concentrated in the region of the second end or air
exit end of the vessel. In an "up-flow dryer" the bottom
or first end of a vessel holding the desiccant bed is the air entry
end during the air drying phase of a cycle (adsorption half-cycle)
while the second end or top end of the pressure vessel is the air
entry end during the regeneration phase of a cycle (desorption half-cycle)
or moisture purge mode in removing water from the adsorbent. Conversely,
in a "down-flow dryer" the top or first end is the air
entry end during the air drying mode of the cycle while the bottom
or second end is the air entry end during the regeneration mode
of the cycle.
Irrespective of the type desiccant air dryer, either up-flow dryer
or down-flow dryer, the practice of injecting hot air through a
single port at one end of the vessel in the regeneration mode results
in an undesirable temperature imbalance when purging moisture from
the desiccant. Conventional regeneration processes typically overheat
the vessel output end because of the prolonged time required to
sufficiently reach a proper temperature at the vessel input at the
opposite end, where most of the moisture resides. That is, the time
required for the flow of heated air to purge or desorb the stratified
portions of the desiccant most heavily saturated with moisture at
the vessel input end results in excessive heating of the vessel
and desiccant, wasting energy and time to allow these overly heated
areas to sufficiently cool before returning the unit to drying air
or other gases.
To illustrate further, in the desorption half-cycle (regeneration
mode) it has been found that the temperature can range from about
350.degree. F. to about 650.degree. F. or more, at the heated air
entry end while only 200.degree. F. at the air exit end of the vessel
during regeneration. This imbalance leads to wasted energy and higher
operating costs through higher temperatures in vessel heating and
longer operation of blower motors for the period of time required
to purge substantially all the moisture from throughout the bed.
Other consequences of this temperature imbalance include overly
(or unevenly) dried desiccant at one end of the bed; protracted
downtimes due to longer cool down periods before the system can
be returned to service (adsorption cycle); and dew point "bump"
or spikes where moisture laden air remains in the desiccant bed
due to a temperature hot spot. Such moist air, if forced out of
the dryer as dry air can result in down stream technical problems
in the application of the use of dried air
Accordingly, there is a need for an improved heat regenerated desiccant
gas dryer and method of use which will consume less energy and use
it more efficiently, permit shorter turn around times in regenerating
the desiccant bed in removing water during the desorption half-cycle,
avoiding temperature imbalances and over heating the apparatus,
and elimination of hot spots and spikes or dew point bumps.
SUMMARY OF THE INVENTION
In accordance with the invention an improved heat regenerated desiccant
gas dryer and method of regeneration are provided which effectively
balances the temperature of the dryer during the regeneration mode
by more effective, direct injection of heated air within the dryer
for more efficient thermal drying of the desiccant. Heat for drying
the desiccant is distributed more closely in accordance within the
normal stratification of moisture in a desiccant bed, so the heat
is delivered directly to those regions more heavily saturated with
moisture, which means that less heat needs to be applied to those
areas having lower moisture levels. As a result, the turn around
time for the air dryer from off-line to on-line service to the air
drying mode of operation (adsorption half-cycle) is shortened. Advantageously,
the improved desiccant gas dryers of the invention are more economic
to operate, and more productive, as well. In sum, the heat regenerated
gas dryers and methods of use of the present invention avoid temperature
imbalances, over heating and hot spots with lower to zero spikes
in dew point (bumps), and reduced energy consumption without negatively
affecting the performance of the dryer in the removal of moisture
from a process stream.
It is therefore a principal object of the invention to provide
a desiccant gas dryer, which comprises
a pressure vessel having an interior chamber defined by a vessel
side wall comprising one or more purge inlet ports, a first end
wall having a moist gas inlet, and a second end wall having a dry
gas outlet;
a bed comprising a regenerable desiccant agent disposed within
the interior chamber of the pressure vessel;
means for injecting a stream of moist gas through said moist gas
inlet of the pressure vessel for diffusion through the bed and for
discharging dried gas through the dry gas outlet during a moisture
sorption phase, and
means for directly injecting a stream of heated air into the bed
through the one or more purge inlet ports in the pressure vessel
side wall for removing moisture from the bed. The desiccant gas
dryer includes exhaust means for discharging moisture and air during
desiccant regeneration.
The purge inlet ports are preferably parallel ports with at least
one port positioned off-center in the region proximate to the moist
gas inlet. The objective is to position the parallel ports along
the side wall of the vessel so they are appropriately spaced from
one another to achieve substantially uniform temperature balance
during desiccant regeneration.
It is still a further object of the invention to provide heat regenerated
desiccant gas dryers including valve means and controls therefor
for automatically regulating the injection of heated air to the
bed during desiccant regeneration. Appropriate controls may include,
for instance, multiple thermal sensors for generating signals to
a programmable controller or microprocessor which responds by automatically
regulating the flow of heated air during the regeneration phase.
For purposes of this invention the term "gas" as appearing
in the specification and claims is intended to include any of air,
and particularly compressed air, natural gas, nitrogen and mixtures
thereof. The term is intended to include other gases in addition
to those specifically named above, and contain moisture which can
be safely and suitably purged from the desiccant during regeneration.
In describing the invention, the specification may refer to the
heat regenerated desiccant gas dryers as "desiccant air dryers"
or simply as "air dryers" or variations of these expressions.
The reference to "air" type dryers is for purposes of
convenience only, and is not intended as a limitation or restriction
with regard to the type of gas which may be suitably dried with
the devices and methods of the invention. Accordingly,
it should be understood the expression "desiccant air dryer"
also includes the utility of drying other gases, such as nitrogen,
natural gas, and so on.
The term "desiccant" as used herein is intended to have
its ordinary understood meaning. It comprises drying agents which
are solids, rather than liquids, and are regenerable, i.e., can
be reactivated through the application of heat or air flow passing
over and around the desiccant. Representative examples include such
members as activated alumina, silica gel and molecular sieves, e.g.,
naturally occurring or synthetic zeolites characterized by the ability
to undergo dehydration with little or no change in crystal structure
thereby providing a very high surface area for adsorption of foreign
molecules.
The term "sorbed" or "sorption" or "sorb"
and variations thereof, e.g., desorb, as appearing in the specification
and claims herein are intended to mean to take up water by any mechanism,
usually either adsorption or absorption of moisture or molecules
of the water by desiccants, or the release of the same by the desiccant,
i.e., desorb.
It is yet a further object of the invention to provide a desiccant
gas dryer, which comprises
a pressure vessel having an interior chamber defined by a vessel
side wall, a first end wall having a moist gas inlet, and a second
end wall having a dry gas outlet;
a porous heated air distributor disposed within the interior chamber
of the pressure vessel;
a bed comprising a regenerable desiccant agent disposed within
the interior chamber of the pressure vessel adjacent to the porous
heated air distributor;
means for injecting a stream of moist gas through the moist gas
inlet of the pressure vessel for diffusion through the bed and for
discharging through the dry gas outlet during a moisture sorption
phase, and
means for injecting a stream of heated air into the bed through
the porous heated air distributor for removing water from the bed.
The desiccant gas dryer includes an exhaust means for discharging
moisture and air during desiccant regeneration.
Generally, the porous heated air distributor is a tubular member
which performs substantially as a sparger. It usually runs the length
of the bed with spaced outlets discharging heated air internally,
directly within the interior of the bed. The population of outlets
in the distributor is greater in the region of the bed where the
concentration of moisture in the desiccant is normally greater,
i.e.,in the area of the moist gas inlet of the pressure vessel,
and fewer in number in the areas of bed having less moisture, i.e.,
dry gas outlet. The objective is to have outlets positioned at distances
relative to one another sufficient to achieve substantially uniform
temperature balance in accordance with the moisture distribution
in the desiccant.
It is still a further object of the invention to provide a method
of regenerating a desiccant gas dryer, which comprises the steps
of:
providing a bed comprising a regenerable desiccant agent disposed
within an interior chamber of a pressure vessel, the bed having
moisture adsorbed thereon;
(i) regenerating the desiccant by directly injecting multiple streams
of heated air into the interior of the bed through multiple heated
air discharge locations positioned internally within the bed or
(ii) impinging multiple streams of heated air directly against the
bed exterior wherein at least one stream engages a side of the bed
through a port in a side wall of the pressure vessel, and
purging moist heated air from the pressure vessel.
According to the method of the invention it is an object to regenerate
a desiccant gas dryer and do so by thermal balancing through the
introduction of multiple streams of heated air from points other
than the top or bottom of the pressure vessel and by concentrating
heat entry points where the moisture stratification in the bed is
more concentrated.
BRIEF DESCRIPTION OF THE DRAWINGS
For a further understanding of the invention and its characterizing
features reference should now be made to the accompanying drawings
wherein:
FIG. 1 is a side elevational view of a desiccant air dyer representative
of the prior art, illustrated in a drying mode, adsorption half-cycle
phase;
FIG. 2 is a side elevational view of the prior art desiccant air
dyer of FIG. 1 configured in a regeneration mode, desorption half-cycle;
FIG. 3 is a side elevational view of a first embodiment of a heat
regenerated gas dryer according to the invention configured in a
regeneration mode and illustrated with multiple external injection
ports;
FIG. 4 is a side elevational view of the heat regenerated gas dryer
of FIG. 3 but configured in a drying mode;
FIG. 5 is a side elevational view of a second embodiment of a heat
regenerated gas dryer according to the invention shown with thermal
sensors and solenoid actuated control valves, wherein the dryer
is configured in regeneration, desorption half-cycle mode;
FIG. 6 is a side elevational view of a third embodiment of a heat
regenerated gas dryer according to the invention illustrated with
a multiple outlet distributor mean for injecting heated air directly
into the interior of the desiccant, and configured in regeneration
mode, and
FIG. 7 is a side elevational view of the heat regenerated gas dryer
of FIG. 6 but configured in a gas drying mode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning first to FIG. 1 there is shown a general arrangement of
components found in prior heat regenerated adsorbent air dryers
of conventional design, illustrated in drying or in adsorption half-cycle
mode of operation. The dryer typically comprises a containment vessel
5 with an air inlet 10 a dry air outlet 15 and a dry mode control
valve set 20. The vessel is filled with a desiccant 25. When operating
in the drying mode, air flow passes through the bed containing desiccant
25 entering the system from an inlet 10 and exiting through outlet
15 as indicated by air flow arrows 30.
Air 35 entering the dryer through inlet 10 is high in moisture
(water) content. As it makes its way through containment vessel
5 the moisture is sorbed or captured by the desiccant 25. The purpose
of the apparatus is to remove moisture from the air to provide dry
or substantially dry air 40 at outlet 15 so it may be used in industry
for any number of applications, a few of which were described hereinabove.
The prior art air dryer of FIG. 1 is shown in a regeneration or
desorption half-cycle mode in FIG. 2 wherein the desiccant, after
a period of operation in the drying mode, is reactivated/renewed
by removing or purging water therefrom. In the regeneration mode
of operation, the dry mode control valves 20 are closed and purge
mode control valve set 45 are opened. Other components, namely heater
elements 50 heater element containment 55 and muffler 60 are utilized
in the purge process to heat incoming purge air and to discharge
exhaust air and moisture desorbed from the desiccant bed. Purge
air source 65 enters heater containment 55 and heated by heater
elements 50 for example, to 500.degree. or 600.degree. F. The flow
of heated air shown by arrows 70 indicate the usual path of heated
purge air 65 and entry into vessel 5 as arrows 75 at a temperature
of about 450.degree. F., for example. It is significant to note,
in this prior art device the path of the heated purge air is directed
into the top of vessel 5 where the moisture content of the desiccant
is relatively low. This is exactly opposite to the direction of
the drying mode air flow path 30 within the vessel 5 as illustrated
by FIG. 1. The purge air flow 65 passes through the bed containing
desiccant 25 to the opposite end of vessel 5 to exit the desiccant
air dryer via muffler 60. Arrows 80 85 90 and 95 represent the
purge air flow, cooling as it progresses through the desiccant bed
to exit the vessel and finally the device. These cooling temperatures
could be, for example, 350.degree. F., 250.degree. F., 200.degree.
F., 150.degree. F., respectively as the purge air flow 65 exits
the vessel, and through the muffler 60 into the ambient air as exhausted
purged air 98.
It is important to appreciate the negative result of the heating
curve indicated above, where the hotter purge air first entering
the vessel needs to be present for a sufficient time period to traverse
the desiccant bed to the exiting end. If the exiting end of the
vessel is not heated to a sufficiently high temperature, e.g., 200.degree.
F., or exposed to air flow for sufficient time period to release
the moisture from the desiccant, the desiccant remains moisture
laden and not properly dried. However, to assure a proper purge
temperature at the exiting end, the above mentioned problem of over
heating is created at the entering end. Hence, the heat regenerated
gas dryer of the prior art operates inefficiently with a temperature
imbalance.
The prior art has been detailed in the FIGS. 1 and 2 so there
is a full understanding of the present invention, i.e., an improved
heat regenerated gas dryer and process for regeneration. FIGS. 1
and 2 represent an up-flow configuration. As mentioned above, in
an up-flow dryer in the air dryer mode of operation moist air enters
the vessel at the bottom and flows up through the bed of desiccant
and exits out the top, while in the regeneration mode the heated
air flow enters the top of the vessel and passes through the desiccant
bed to exit out the bottom. The opposite configuration, namely the
down-flow dryer, in the air drying mode moist air enters the vessel
at the top, traverses the desiccant bed and exits at the bottom,
while in the regeneration mode heated air for drying the bed enters
the bottom of the vessel and exits at the top. A further dryer with
concurrent flow operates so both drying and regeneration modes enter
and exit the vessel in the same direction. This configuration leaves
an undesirable potential dew point bump or spike condition within
the desiccant at the system exit end upon changeover to the drying
mode of operation.
The remaining figures detailing the present invention are shown
in their preferred embodiment in up-flow configuration, but are
equally suited for down-flow and concurrent flow configurations.
FIG. 3 illustrates a first embodiment of the improved heat regenerated
desiccant gas dryer 100 configured in an up-flow layout, but in
desiccant regeneration mode of operation with the injection of heated
air for drying in a downward direction of flow. The vessel 5 is
shown with moisture load 112. A desiccant bed, not shown for clarity
of presentation, is the element which sorbes water vapor from the
moist gas during the drying mode of operation. The moisture load
112 is stratified within the desiccant bed with the more heavily
saturated region 114 at the vessel's lower moist gas input end 116
than the vessels dry gas output end 118 where less moisture, i.e.,
water vapor 120 is sorbed.
The improved heat regenerated desiccant gas dryer 100 of the present
invention utilizes at least one injection port comprising a directed
purge inlet port 122 through the side wall of the vessel, and a
purge control valve 124. It will be understood that a multiport
injection configuration comprising a plurality of parallel purge
inlet ports 122 and 126 with parallel purge control valves 124 and
128 are fabricated on the vessel as shown by FIG. 3. To fully understand
the benefits of the multiport purge injection concept, a discussion
of how the moisture load 112 is formed is presented in the detailed
discussion of FIG. 4.
In operating the air dryer of FIG. 3 in desiccant regeneration
mode, parallel purge control valves 124 (and 128 if more than one)
and the main purge control valve 45 are opened to allow heated air
from heater elements 50 at temperatures, e.g., 450.degree. F., to
be simultaneously injected directly into the lower side wall of
the vessel housing the desiccant having the heavier stratified moisture.
This is balanced with the delivery of heated air through the output
end 118 of the vessel via main purge control valve 45 into the upper
regions of the desiccant bed less heavily stratified with adsorbed
moisture. The drying effect of heated air 75 is more balanced, both
at the vessel output end 118 and at the multi purge injection ports
122 and 126 for more optimal temperature balance, and more efficient
heating and drying of the bed. As the flow of heated air passes
through the desiccant bed the air cools to 350.degree. F. at the
location of flow arrows 80 then to 250.degree. F. at the location
of flow arrows 85 as it nears the vessel input end 116. Purged moisture
and air 98 are discharged through muffler 60. Valves 20 opened during
drying mode are closed during the desiccant regeneration mode.
In operation, the improved heat regenerated dryer 100 has a more
thermally balanced drying effect on the desiccant bed in that the
heated purge air is directed to the areas of the desiccant bed which
most need the drying. The result of directly injecting air flow
75 to the heavily saturated moisture layers 114 allows the sorbed
moisture to be released (vaporized away) and purged from the desiccant
bed in a more efficient manner. Hot air passes through the entire
bed of desiccant quickly so as to not over heat the desiccant, vessel
and associated ports and piping; as is the case in conventional
purge processes where hot spots exist, creating undesirable dew
point spikes in the drying mode of operation.
FIG. 4 illustrates heat regenerated desiccant air dryer 100 of
FIG. 3 but in the air drying mode of operation. Desiccant air dryer
100 is quiescent, and when operating in drying mode air flows are
unaffected because the parallel purge control valve 124 (and 128
if more then one--See FIG. 3) are closed. Also, coincidentally the
main purge mode control valves 45 are closed during the drying mode.
In the adsorption half-cycle drying mode moist air 35 is received
for drying via air inlet 10; passes through lower drying mode control
valve 20 and into vessel 5 through input end 116. Air flow 30 passes
over and through the desiccant bed within vessel 5 and the moisture
in the air is sorbed by the desiccant in the process. Over time,
for example four hours, the desiccant bed becomes laden with moisture
load 112 resulting in stratified weight layers of moisture forming
a moisture gradient in the desiccant bed. The greatest weighted
layers of moisture load are located at the vessel input end 116.
The moist gas first entering the desiccant bed captures the largest
number of water molecules resulting in heavily saturated stratification
114. As air flow 30 continues through to the vessel output end 118
there are fewer and fewer water vapor molecules to be attached or
sorbed by the desiccant, resulting in a less saturated stratification
120 of moisture load 112.
The present invention is shown alternatively in FIG. 5 with electronically
controlled purge valves 124 128 and 130 each with a solenoid 132
135 and 134 respectively. A first thermal sensor 136 is disposed
within the less saturated area 120 of the moisture load 112 and
a second thermal sensor 138 disposed within the heavily saturated
area 114. This embodiment of the invention utilizes a conventional
PLC (Programmable Logic Module), not shown. The PLC is programmed
to accept the thermal sensors as signal inputs and respond logically
to activate and deactivate the solenoids 132 135 and 134 for opening
and closing purge valves 124 128 and 130.
FIG. 5 illustrates one scenario where thermal sensor 136 has signaled
a sufficiently high temperature, indicating substantially all moisture
in the desiccant bed has been evaporated, and effect the PLC to
signal the solenoid 134 to close control valve 130. The directed
purging is now being injected exactly where the remaining moisture
load is, within the heavily saturated area 114 of the vessel. Once
thermal sensor 138 reaches a sufficiently high temperature it signals
the PLC the desiccant has achieved proper purging temperature and
provides an output to solenoids 132 and 135 to close control valves
124 and 128. Optionally, within a sequence of events the PLC may
turn off heater element 50 and open all control valves 130 124
and 128 for purging any residual heat remaining in the desiccant.
Thermal sensors 136 and 138 provide even greater capability to the
invention by timely removing the heated purge air flow from vessel
5 allowing faster turn around times from off-line regeneration
mode to on-line service gas drying mode.
FIGS. 6-7 disclose a further embodiment of the invention. FIG.
6 illustrates the air dryer in regeneration mode, whereas FIG. 7
shows the device in a working mode of drying gas. This third embodiment
of the
improved heat regenerated desiccant air dryer 100 comprises as
a key element a porous heated air distributor 140 for injecting
purge air directly into the interior of the desiccant bed located
in vessel 5 for more efficient drying of the desiccant (FIG. 6).
The distributor 140 in the form of tubular member surrounded by
the desiccant material, contains a multiplicity of air outlets 142
and 144 running substantially the length of the member. Outlets
142 are less populated in the region proximate to the vessel output
end 118 and more densely populated 144 in the region proximate
to the vessel input end 116. Thus, the population of air outlets
142 and 144 may increase gradually towards the vessel input end
where the moisture in the desiccant is more heavily stratified.
Distributor 140 stops short as it nears the vessel input end 116
which may have appropriate screening to contain the desiccant material.
To prevent a by-pass effect of air flow 30 (FIG. 7), porous heated
air distributor 140 is filled with the same drying material used
in the desiccant bed in vessel 5. Filling distributor tube 140 with
desiccant may be performed through access port 146.
In the regeneration mode of operation (FIG. 6), directed purge
air is injected into the vessel areas in a balanced manner, i.e.,
a greater volume of drying air 70 to the more heavily saturated
layers 114 of stratified moisture load 112 within the desiccant
bed, and less heated air to the less heavily saturated areas 120
within the desiccant bed.
To more fully appreciate the present invention, the improved heat
regenerated desiccant dryer 100 effectively balances the air flow
and thermal drying effect on the desiccant 25 within containment
vessel 5. While the improved dryer 100 fully has no negative consequence
on the air drying mode of operation, it greatly enhances purging
moisture from the desiccant bed in the regeneration mode of operation
by balancing the distribution of heated purge air through the device.
While the invention has been described in conjunction with various
embodiments, they are illustrative only. Accordingly, many alternatives,
modifications and variations will be apparent to persons skilled
in the art in light of the foregoing detailed description, and it
is therefore intended to embrace all such alternatives, modifications
and variations as to fall within the spirit and broad scope of the
appended claims. |