Gas separation: processes – With control responsive to sensed condition – Pressure sensed
Reexamination Certificate
2002-03-08
2002-11-12
Simmons, David A. (Department: 1724)
Gas separation: processes
With control responsive to sensed condition
Pressure sensed
C095S098000, C095S130000, C096S115000
Reexamination Certificate
active
06478850
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of gas concentrators, and in particular to a miniaturized, portable gas concentrator and method of miniaturized gas concentration.
BACKGROUND OF THE INVENTION
The pressure swing adsorption cycle was developed by Charles Skarstrom.
FIGS. 1A and 1B
describe the operation of the Skarstrom “Heatless Dryer”. In particular, ambient humid air is drawn into the system from an intake port, by a compressor. The pressurized air flows from the compressor through conduit
9
to a switching valve
4
. With the valve in the shown position in
FIG. 1A
, pressurized air passes through conduit
5
a
to a pressure vessel
6
a.
The air feeds into the pressure vessel to a flow-restrictive orifice
1
a.
The effect of the restrictive orifice is to restrict the flow of gas escaping the pressure vessel. As the pressure builds up in the pressure vessel, water vapour condenses on the sieve material
8
. Air with reduced humidity passes through orifice
1
a
to conduit
12
. At conduit junction
11
, some of the air is extracted for use from gas extraction port
2
while the remainder passes through conduit
13
to restrictive orifice
1
b.
The less humid air that passes through orifice
1
b
is used to blow humid air out of the unpressurized vessel
6
b,
through conduit
5
b,
through valve
4
, to a vent port
7
. When valve
4
switches to the position as shown in
FIG. 1B
, the opposite cycle occurs.
Thus, as valve
4
cycles from the position of
FIG. 1A
to the position of
FIG. 1B
, cyclically, there is a gradual reduction of humidity in the air as sampled at port
2
. Likewise gases can be separated by adsorbing components of the gas on selective molecular sieves.
From laboratory observations, employing the Skarstrom cycle in the context of an oxygen separator or concentrator, wherein nitrogen is absorbed by molecular sieve beds to incrementally produce oxygen-enriched air, and using a precursor to the concentrator
1
arrangement of
FIG. 1
, it was observed that miniaturized (in this case nominal ¾ inch NPT pipe ×6 inch long) molecular sieve beds
12
and
14
could only reach a maximum of 30% concentrated or enriched oxygen detected at the gas extraction ports
11
. It was thought that this was because the control valve of the laboratory arrangement was switching before all the nitrogen could be vented out of the molecular sieve beds and the exhaust lines. However, measurements showed that the oxygen concentration was higher than normal. Therefore this was not the problem.
It was also observed that there was a lot of airflow coming out of the molecular sieve bed before the molecular sieve bed was completely pressurized. It seemed that the molecular sieve bed was saturated with nitrogen before the bed was finished pressurizing.
FIG. 2
diagrammatically represents such a molecular sieve bed
16
. Compressed air enters the bed in direction A through inlet passage
16
a.
A volume of air B is contained within the bed cavity. A proportion of the volume of air C escapes out through an outflow needle valve
18
while the molecular sieve bed pressurizes. It was thought that the volume of air C escaping could be a much larger volume than the volume of air B inside the bed
16
. Thus the question became, what happens when the volume of the molecular sieve bed is decreased during miniaturization, but everything else stays the same?
Poiseauille's Law was used in comparing the old bed volume B to the miniaturized bed volume to calculate the flow of a fluid that passes through a small hole such as needle valve
18
under a pressure difference.
1
)
Q
=
r
4
(
p
InsideBed
-
p
OutsideBed
)
8
η
L
Where “Q” is the fluid flow in meters cubed per second. “r” is the radius of the small hole. “P
InsideBed
−P
OutsideBed
” is equal to the pressure difference between inside the molecular sieve bed and outside the molecular sieve bed. “&eegr;” is the fluid viscosity, and “L” is the depth of the small hole.
The flow rate, Q, in meters per second multiplied by the time the flow rate occurred is equal to the volume of flow in meters cubed.
V=Qt
2)
The variable for Q in equation
1
in this case is constant so
V=Kt
3)
where K is some constant value.
Using this information to create a comparison of the Flows and Volumes of the original oxygen concentrator's bed volume to the new bed volume may be described as:
4
)
R
=
V
FlowNew
V
BedVolum
eNew
V
FlowOld
V
BedVolum
eOld
Since the time to pressurize the molecular sieve bed can be accurately timed using a programmable logic controller (PLC) timer, the following can be stated:
5
)
R
=
Kt
New
V
BedVolum
eNew
Kt
Old
V
BedVolum
eOld
or
6
)
R
=
Kt
New
V
BedVolumeOld
Kt
Old
V
BedVolumeNew
=
t
New
V
BedVolumeOld
t
Old
V
BedVolumeNew
The ratio may then be calculated by inserting values using representative values for a prior art bed and a miniaturized bed (in this case ¾ inch NPT×6 inch long). Thus, for example:
7
)
R
=
(
1
)
(
0.001885741
)
(
7
)
(
0.0000434375
)
=
6.2
From this it was concluded that the molecular sieve material of a nominal ¾ inch NPT pipe×6 inch long molecular sieve bed (the example used in equation 7) has approximately 6.2 times the air passing through it during its pressurization cycle than the molecular sieve material of a prior art oxygen concentrator during its pressurization cycle.
As a consequence of the findings of this analysis it was found to be advantageous to pressurize and vent the molecular sieve beds in a different way than the prior art pressure swing adsorption (PSA) technique. In the method of the present invention the bed is mechanically evacuated after being substantially fully pressurized, hereinafter referred to as a gas packet system or method.
SUMMARY OF THE INVENTION
The gas, such as oxygen, concentrator of the present invention for enriching a target component gas concentration, such as the oxygen concentration, in a gas flow, includes an air compressor and vacuum pump, an air-tight first container containing a molecular sieve material for adsorbing a waste component gas such as nitrogen, and a second air-tight container containing molecular sieve material for adsorbing the waste component gas. The first container is in fluid communication with the compressor and vacuum pump through a first gas conduit, and the second container is in fluid communication with the compressor and vacuum pump through a second gas conduit. A third gas conduit connects the first and second molecular sieve containers in fluid communication with each other. A fourth gas conduit branches or “tees” off or otherwise cooperates, by means of a flow controller, with the third gas conduit to facilitate delivery of the target gas to the end use. For example, the flow controller may be mounted between two valves on the third conduit. A gas flow controller such as PLC or other dedicated electronic circuit controls actuation of valves mounted to the gas conduits. The electronically controlled valves may also work in co-operation with two passive one-way valves to regulate gas flow through the conduits so as to, in repeating cycles:
(a) prevent gas flow between the first and second containers and to allow compressed gas from the compressor into the first container during a first gas pressurization phase, whereby the first container is pressurized to a threshold pressure level to create a gas packet having an incrementally enriched target component gas concentration such as incrementally enriched oxygen-enriched air, while simultaneously evacuating the second container to a threshold vacuum level during a first evacuation phase whereby the second container is evacuated to the threshold evacuation level to remove a vacuum packet wherein a target waste gas such as nitrogen is removed from the molecular sieve of the second container and expelled to atmosphere,
(b) prevent gas flow between e
Edwards Antony C.
Lawrence Frank M.
Simmons David A.
Wearair Oxygen Inc.
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