Molecular sieve abstract
Water contamination is purged from molecular sieve material used
in oxygen concentrators by heating the molecular sieve material
to a temperature in the range of approximately 450.degree. to 950.degree.
F., and subjecting the molecular sieve material to a stream of dry
sweep gas having a dew point in the range of approximately -80.degree.
to -100.degree. F. In the preferred embodiment, the molecular sieve
material is contained in a vessel and heated by means of a number
of heating elements. In addition, a flow of dry sweep gas is produced
by at least two drying chambers containing a desiccating material.
A first valve assembly alternately cycles the sweep gas through
the first drying chamber and then through said second drying chamber.
A second valve assembly routes a portion of the sweep gas leaving
the active drying chamber to the vessel, and routes the remainder
of the sweep gas through the remaining drying chamber(s) to purge
water from the desiccating material contained therein during the
temporal portions of the cycle when each drying chamber is not supplying
sweep gas to the vessel.
Molecular sieve claims
We claim:
1. An apparatus for thermal regeneration of molecular sieve material
contaminated with water comprising:
a vessel for containing a quantity of said molecular sieve material;
means for heating said molecular sieve material within said vessel
to a temperature in the range of approximately 450.degree. to 950.degree.
F.
a first drying chamber containing a desiccating material;
at least a second drying chamber containing a desiccating material;
means for alternately cycling a flow of sweep gas through said
first drying chamber to said vessel, and then through said second
drying chamber to said vessel, said sweep gas entering said vessel
having a dew point in the range of approximately -80.degree. to
-100.degree. F.; and
means for alternately purging water from said desiccating material
in said drying chambers during that portion of said cycle when each
drying chamber is not supplying sweep gas to said vessel.
2. The apparatus of claim 1 wherein said vessel is heated to a
temperature of approximately 550.degree. F.
3. The apparatus of claim 1 wherein said sweep gas comprises air.
4. The apparatus of claim 1 wherein said sweep gas comprises nitrogen.
5. The apparatus of claim 1 wherein said means for purging water
from said desiccating material in said drying chambers comprises:
means for routing a first portion of said sweep gas leaving the
drying chamber to said vessel;
means for routing a second portion of said sweep gas through the
remaining drying chambers to purge water from said desiccating material
contained therein; and
means for alternately cycling the flow of said second portion of
said sweep gas through said drying chambers so that water is purged
from said desiccating material in said drying chambers during that
portion of said cycle when each drying chamber is not supplying
sweep gas to said vessel.
6. An apparatus for thermal regeneration of molecular sieve material
contaminated with water comprising:
a vessel for containing a quantity of said molecular sieve material;
means for heating said molecular sieve material within said vessel
to a temperature in the range of approximately 450.degree. to 950.degree.
F.;
a compressor for compressing a flow of sweep gas;
a condenser for cooling said compressed sweep gas to cause condensation
of water from said sweep gas;
a water separator for separating said water from said sweep gas;
a first drying chamber containing a desiccating material;
at least a second drying chamber containing a desiccating material;
a first valve for alternately cycling a flow of sweep gas from
said water separator through said first drying chamber and then
through said second drying chamber; and
a second valve for routing a first portion of said sweep gas leaving
the drying chamber to said vessel, and routing a second portion
of said sweep gas through the remaining drying chambers to purge
water from said desiccating material contained therein during that
portion of said cycle when each drying chamber is not supplying
sweep gas to said vessel, said sweep gas entering said vessel having
a dew point in the range of approximately -80.degree. to -100.degree.
F.
7. The apparatus of claim 6 wherein said sweep gas comprises air.
8. The apparatus of claim 6 wherein said sweep gas comprises nitrogen.
9. The apparatus of claim 6 wherein said vessel is heated to a
temperature of approximately 550.degree. F.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of thermal
regeneration of water-contaminated molecular sieve material used
in oxygen concentrators. More specifically, the present invention
discloses a method and apparatus for regenerating molecular sieve
material using a combination of heat and a dry sweep gas.
2. Statement of the Problem
Oxygen concentrators are widely used for medical purposes to produce
concentrated oxygen by removing nitrogen from air. The pressure
swing adsorption process used in virtually all present day oxygen
concentrators is capable of producing up to 95.6% oxygen. There
are currently estimated to be in excess of 500000 of these systems
in use.
A molecular sieve material, such as zeolite, is employed by the
oxygen concentrator to capture the nitrogen molecules from the air.
A typical oxygen concentrator contains up to ten pounds of molecular
sieve material, at a cost of approximately $4.00 to $10.00 per pound.
However, molecular sieves are also capable of preferentially adsorbing
other constituents found in air, such as water vapor and carbon
dioxide. In particular, they have an extremely high affinity for
holding water molecules. When water vapor is allowed to accumulate
in the molecular sieve material, the pores and capture surfaces
are blocked for nitrogen removal. The concentrator thus fails to
perform its intended purpose.
In commercial drying applications, the molecular sieve material
must have a water content below 10% by weight to produce air with
a -40.degree. F. dew point, and to below 3% by weight to produce
air with a -100.degree. F. dew point. In contrast, in order for
an oxygen concentrator to function efficiently, the molecular sieve
material should have a water content below 1.5% by weight, and preferably
below 1% by weight. A water content of 2% by weight will severely
hamper concentrator performance. A 5% water content will render
it useless. For example, a sieve exposed to 20% relative humidity
at 80.degree. F. will quickly accumulate a water content equal to
20% of its total weight and render it useless as an oxygen concentrator.
Even commercially dry air having a dew point of -40.degree. F. is
capable of contaminating molecular sieve material to 10% of its
weight in water. Thus, water contamination is a major problem in
molecular sieve materials used for concentrating oxygen to a much
greater degree than in other commercial drying applications.
During normal operation of an oxygen concentrator, most atmospheric
water vapor is ejected back into the atmosphere by the pressure
swing operation of the concentrator. Water contamination of the
molecular sieve material within the concentrator can result from
valve failure, or other leakage, or if the molecular sieve material
is accidentally exposed to atmospheric air for a period of time.
The prior art contains other examples of thermal regeneration of
molecular sieve materials, including the following:
______________________________________ Inventor U.S. Pat. No. Issue
Date ______________________________________ Sircar, et al. 4971606
Nov. 20 1990 Sircar 4784672 Nov. 15 1988 Francis, et al. 3756961
Sept. 4 1973 ______________________________________
Sircar, et al. U.S. Pat. No. 4971606 discloses a thermal regeneration
system using a combination of high temperature and dry sweep gases
(e.g. air or nitrogen) to remove water absorbed by zeolite adsorbents.
A hot regeneration gas is passed through a bed of the adsorbent
at a sufficiently high flow rate such that the residence time and
reaction of the desorbed components in the adsorbent bed are minimized.
Residence times of less than one second are discussed.
Sircar U.S. Pat. No. 4784672 discloses a process for regeneration
of adsorbents used in pretreatment of landfill gas. One of the pretreatment
sections is a layer of molecular sieve zeolite that is regenerated
by a flow of hot regeneration gas.
Francis, et al., disclose a process for regenerating a bed of coke-containing
zeolitic molecular sieves by continuously passing a closed-loop
flow of hot, oxygen-containing inert gas through the molecular sieve
bed. Water in the circulating gas stream is maintained below a predetermined
concentration.
3. Solution to the Problem
None of the prior art references uncovered in the search show a
method and apparatus for thermal regeneration of molecular sieve
materials using a combination of heat and a dry sweep gas to achieve
the extremely low water-content tolerances required by medical oxygen
concentrators. In addition, the dual-chamber desiccant air dryer
employed in the present invention is neither taught nor suggested
by the prior art for this field of use.
SUMMARY OF THE INVENTION
This invention provides a method and apparatus for purging water
contamination from molecular sieve material used in oxygen concentrators.
The molecular sieve material is heated to a temperature in the range
of approximately 450.degree. to 950.degree. F. and subjected to
a stream of dry sweep gas having a dew point in the range of approximately
-80.degree. to -100.degree. F. In the preferred embodiment, the
molecular sieve material is contained in a vessel and heated by
means of a number of heating elements. In addition, a flow of dry
sweep gas is produced by at least two drying chambers containing
a desiccating material. A first valve assembly alternately cycles
the sweep gas through the first drying chamber and then through
said second drying chamber. A second valve assembly routes a portion
of the sweep gas leaving the active drying chamber to the vessel,
and routes the remainder of the sweep gas through the remaining
drying chamber(s) to purge water from the desiccating material contained
therein during the temporal portions of the cycle when each drying
chamber is not supplying sweep gas to the vessel.
A primary object of the present invention is to provide a method
and apparatus for purging water contamination from molecular sieve
materials to the tolerances required for effective operation of
oxygen concentrators.
Another object of the present invention is to provide a method
and apparatus for thermal regeneration of molecular sieve materials
that is economical and reliable.
These and other advantages, features, and objects of the present
invention will be more readily understood in view of the following
detailed description and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention can be more readily understood in conjunction
with the accompanying drawings, in which:
FIG. 1 is an overall schematic diagram of the thermal regeneration
apparatus.
FIG. 2 is a simplified schematic diagram of the dual drying chambers
configured to use the right drying chamber for drying, and to purge
moisture from the left drying chamber.
FIG. 3 is a simplified schematic diagram of the switch valve assembly
for the drying chambers during change-over from the right drying
chamber to the left drying chamber.
FIG. 4 is a simplified schematic diagram of the switch valve assembly
for the drying chambers configured to use the left drying chamber
for drying, and to purge moisture from the right drying chamber.
FIG. 5 is a simplified schematic diagram of the switch valve assembly
for the drying chambers during change-over from the left drying
chamber to the right drying chamber to complete the cycle.
DETAILED DESCRIPTION OF THE INVENTION
Turning to FIG. 1 a schematic diagram is provided of the entire
thermal regeneration apparatus. The molecular sieve material to
be treated is first placed in a vessel 40 through a fill door 42.
In the preferred embodiment, the vessel 40 is made of stainless
steel and is sized to hold approximately 43 pounds of molecular
sieve material. The vessel 40 contains a number of heating elements
44 capable of heating the vessel and molecular sieve material to
the desired operating temperature in the range of approximately
450.degree. to 950.degree. F. In the preferred embodiment, the operating
temperature is maintained at approximately 550.degree. F.
The remainder of the apparatus is used to produce a flow of dry
sweep gas to purge moisture from the molecular sieve material within
the vessel 40. For example, air can be used as the sweep gas, or
alternatively nitrogen can be employed for this purpose. The sweep
gas entering the vessel 40 must have an extremely low dew point
in order to purge water contamination from the molecular sieve material
to the tolerances required for use in an oxygen separator. For example,
the dew point of the sweep gas should be on the order of approximately
-80.degree. to -100.degree. F. In the preferred embodiment, the
sweep gas is introduced at the top of the vessel 40 gradually flows
downward through the molecular sieve material, and exits through
small openings in the drain door 46 in the bottom of the vessel.
The sweep gas first passes through an air filter 10 (e.g. a HEPA
filter) to remove particulates as shown at the left side of FIG.
1. The sweep gas is then compressed by a compressor 12 to produce
adiabatic heating. In the preferred embodiment the compressor 12
produces an output flow of approximately 180 CFH at a pressure of
approximately 50 psig. The compressed gas passes through a condenser
14 cooled by a fan 16 to cause some of the moisture in the gas to
condense into liquid. This liquid-phase water is removed from the
sweep gas by a water separator 20. An unloader valve 18 is included
to allow manual release of pressure when the apparatus is being
unloaded after use.
Next, the sweep gas is delivered to a switch valve assembly 22
shown in greater detail in FIGS. 2-5. The switch valve assembly
22 consists of two multi-port spindle valves, both of which can
be electrically switched by solenoids between two positions. The
switch valve assembly 22 alternately directs the flow of sweep gas
to two drying chambers 24 and 26 containing a desiccating material.
In the preferred embodiment, each drying chamber 24 26 is a cylinder
approximately 22 inches in length, with a radius of approximately
3.5 inches. These dimensions allow each drying chamber 24 26 to
contain approximately 5 pounds of zeolite as the desiccating material.
Additional drying chambers could be added if desired.
As previously noted, the switch valve assembly 22 alternately routes
the sweep gas through the left drying chamber 24 and then through
the right drying chamber 26. (The drying chamber drying the sweep
gas at any given moment is referred to as the "active"
drying chamber. The other drying chamber is referred to as the "inactive"
drying chamber.) In the preferred embodiment, each drying chamber
dries the sweep gas for approximately 4 minutes per cycle. A series
of check valves 30 31 32 and 33 direct the flow of sweep gas
exiting the active drying chamber. These check valves 30-33 and
the purge flowrator 28 direct a portion of the sweep gas leaving
the active drying chamber to the vessel through a pressure reducing
valve 35 and route the remainder of the sweep gas through the inactive
drying chamber during the temporal portions of the cycle when each
drying chamber is not supplying sweep gas to the vessel. The sweep
gas purges water from the desiccating material contained in the
inactive drying chamber and is vented to the atmosphere through
the switch valve assembly 22 as depicted in FIG. 2. In the preferred
embodiment, approximately 60 CFH of the sweep gas is delivered to
the vessel 40 and approximately 120 CFH is used to purge the inactive
drying chamber.
The cycle may be better understood by considering each of the steps
in the cycle. For example, FIG. 2 shows the right drying chamber
26 being used to dry the sweep gas (i.e. active) and the left drying
chamber 24 is being purged of moisture (i.e. inactive). At this
step in the cycle, the sweep gas is supplied into the switch valve
assembly from the left. The solenoid has positioned the upper spindle
value to route the flow of sweep gas to the right drying chamber
26. The desiccating material in the right drying chamber 26 removes
water vapor from the sweep gas, as previously described. The sweep
gas exiting the right drying chamber 26 flows through the check
valve 31 but does not pass the check valves 33 and 30. The flow
splits downstream from the check valve 31 with a portion flowing
through the pressure reducing valve 35 to the vessel 40 and the
remainder flowing through the purge flowrator 28 and the check valve
32 to the left drying chamber 24. The respective flow rates through
the left drying chamber and the vessel are determined by the relative
flow resistances of these paths. These flow rates can be controlled
to a degree by adjusting the purge flowrator 28.
FIG. 3 shows the second step in the cycle. The solenoid of the
lower spindle valve in the switch valve assembly 22 changes position
to prevent sweep gas from venting from the left drying chamber to
the atmosphere. This raises the pressure in the left drying chamber
(i.e., to approximately 50 psig) to equalize the pressure in both
drying chambers prior to the third step in the cycle.
FIG. 4 shows the third step in the cycle. The solenoid of the upper
spindle valve in the switch valve assembly 22 changes position to
direct the flow of sweep gas from the condenser 14 to the left drying
chamber 24. This step essentially reverses the order of the drying
chambers. The left drying chamber 24 dries the sweep gas delivered
to the vessel 40 and the right drying chamber 26 is purged of moisture.
In particular, the desiccating material in the left drying chamber
24 removes water vapor from the sweep gas. The sweep gas exiting
the left drying chamber 24 flows through the check valve 30 but
does not pass the check valves 31 and 32. The flow splits downstream
from the check valve 30. A portion flows through the pressure reducing
valve 35 to the vessel 40. The remainder flows through the purge
flowrator 28 and the check valve 33 to the right drying chamber
26. The sweep gas and moisture purged from the right drying chamber
26 is vented to the atmosphere through the switch valve assemble
22 as shown in FIG. 4.
FIG. 5 completes the cycle. The solenoid of the lower spindle valve
in the switch valve assembly 22 changes position to prevent sweep
gas from venting from the right drying chamber 26 to the atmosphere.
This raises the pressure in the right drying chamber 26 to equalize
the pressure in both drying chambers prior to repeating the cycle.
This cycle is repeated until water contamination of the molecular
sieve material has been reduced to the desired level. In the preferred
embodiment, this process can take two to three hours depending upon
the quantity of molecular sieve material in the vessel and the initial
degree of water contamination. At the end of the process, the molecular
sieve material is emptied from the vessel 40 through a drain door
46.
In one alternative embodiment, a thermostat is included in the
lower portion of the vessel 40 above the drain door 46. Since the
dry sweep gas is introduced from the top of the vessel, the top
layers of the molecular sieve material tend to be the first to be
purged of moisture. The middle layers and then the bottom layers
of the molecular sieve material are progressively purged as the
regeneration process continues. A slight temperature rise occurs
at each layer as it is purged, due to the fact that thermal energy
is no longer being absorbed by vaporization of the water contamination
at that layer. The thermostat detects when this temperature increase
reaches to the bottom of the vessel 40 and then terminates the regeneration
process.
The above disclosure sets forth a number of embodiments of the
present invention. Other arrangements or embodiments, not precisely
set forth, could be practiced under the teachings of the present
invention and as set forth in the following claims. |