Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR MONITORING
CATALYTIC ABATOR EFFICIENCY
FIELD OF THE INVENTION
The present invention relates to a gas treatment process wherein the residual
treatment gas is vented to the atmosphere. More specifically, there is
provided a
method and apparatus for monitoring emissions of a catalytic abator to
determine the
efficiency of the abator.
BACKGROUND OF INVENTION
A catalytic abator is used to convert airborne volatile organic compounds such
as
ethylene oxide into harmless carbon dioxide and water vapor. The abator offers
controlled purification with a high destruction efficiency for use in
sterilization
processes.
Sterilizers commonly employ a combination of dichlorodifluoromethane (CFC-12)
and ethylene oxide (Et0). Et0 is the actual sterilant whereas CFC- 12 is used
as a
diluent to form a non-flammable blend. A typical combination is 12% Et0 mixed
with
88% CFC-12, referred to as "12/88". The CFC-12 is a Class I ozone depleting
substance and has been phased out and replaced by a 100% Et0 sterilant or a
blend of Et0 and carbon dioxide. Et0 is a toxic air pollutant and is under
proposal
for federal regulation. Used Et0 is fed into a heated air stream where it is
diluted
and catalytically converted into carbon dioxide and water vapor to be released
into
the ambient air. Of course, abator efficiency is crucial given its potential
to release a
human carcinogen.
Sterlizers, or abators, are commonly used, particularly in health care
facilities or any
field in which products are routinely sterilized, and generally includes an
enclosed
catalyst bed, an air heater, a fan and the controls necessary to complete the
operation.
The conventionally known techniques for measuring abator efficiency are
typically
costly and/or lengthy processes. For example, one method involves the
determination of the mass of compound at both the inlet and the outlet of the
control
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equipment. The mass at the inlet may be either estimated or determined from an
actual measurement of the flow and concentration at the inlet to the abator
while the
outlet mass is determined by measuring the flow and concentration of the
exhaust at
the outlet. Several drawbacks to this methodology include the potential for
the
sample at the inlet to be dangerous and the handling can expose the sampling
personnel to one or more toxic compounds. For example, Et0 at room temperature
is flammable and potentially explosive. Further, the samples must be collected
in
bags and must be analyzed either at an on-site lab or, more typically, at an
external
lab within a specified period of time causing the potential for condensation
during the
collection of the samples at the outlet resulting in Et0 losses. Obviously,
the losses
due to condensation and prolonged storage of the bag samples prior to analysis
will
result in the overestimation of the control efficiency. In addition, the known
methods
also require that the collection of the bags be timed precisely to include
large
concentrations at the beginning of the abator cycle. A failure to include the
large
concentrations can result in inaccuracies in the data collected.
Consequently, there is a need for a method and apparatus to achieve a safer,
instantaneous, cheaper and more accurate determination and control of the
efficiency of the abator which is independent of the inlet mass to the abator,
the
dilution and excess air in the system, and which can be programmed to
determine
the control efficiency for any portion of the cycle.
SUMMARY OF THE INVENTION
The present invention provides for a method for measuring exhaust stream
emissions of a catalytic abator for determining the efficiency of the removal
of a
treatment gas, the steps comprising:
a) providing a calibrated sensor to measure a selected range of the treatment
gas concentration;
b) providing an exhaust stream from the abator after the treatment gas has
been treated by the abator;
c) withdrawing at least a portion of the exhaust stream from the abator and
passing the withdrawn portion of the exhaust stream into the sensor;
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d) measuring the concentration of treatment gas in the withdrawn portion of
the exhaust stream in the selected range; and,
e) verifying the abator efficiency based on the measurement.
Preferably, there is provided the step of diluting the portion of an exhaust
stream with
cleaned and dried ambient air, utilizing an exhaust fan in the abator to mix
the
exhaust from a sterilizer with ambient air, controlling the amount of the
exhaust
stream withdrawn from the abator exhaust and subjecting the withdrawn exhaust
to a
cleaning step after sensing has been completed.
Further, it is preferred there is provided the step of controlling the flow of
the
withdrawn exhaust stream to the sensor and the sensing step includes the steps
of
sensing at least one of carbon dioxide or ethylene oxide.
It is also desirable the sensor is a photo acoustic sensor, an electrochemical
sensor
or a non-dispersive infrared sensor, the withdrawn portion of the exhaust
stream
from the abator is dehumidified and the abator is automatically adjusted for
optimum
efficiency based on the efficiency measurements.
In another embodiment of the present invention there is provided a device for
monitoring the efficiency of a sterilizer treatment gas abator, comprising:
a monitoring device having an inlet for receiving a treatment gas from an
outlet of the abator and an outlet, the treatment gas having been treated by
the abator to remove the treatment gas;
the monitoring device including a calibrated treatment gas sensor and means
for withdrawing at least a portion of the exhausted gas stream from the abator
outlet;
the sensor being positioned and adapted to measure the treatment gas in the
withdrawn stream to permit verification of the efficiency of the abator; and
output means for verifying the efficiency measurements.
The abator preferably includes an exhaust fan and a catalyst together with a
means
for heating the catalyst, the monitoring device includes control means for
controlling
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the amount of exhaust withdrawn from the abator and the monitoring device
includes
cleaning means for cleaning the withdrawn exhaust after sensing has been
completed.
It is also preferable the monitoring device includes at least one flow
controller for
controlling the flow of gas, the sensor includes a function for sensing at
least one of
carbon dioxide or ethylene oxide, the sensor is a photo acoustic sensor, an
electrochemical sensor or a non-dispersive infrared sensor and the abator
includes
dilution means for diluting the withdrawn gas.
Moreover, it is desirable the abator includes cleaning means, the abator
includes
drying means for dehumidifying the treatment gas, the monitoring device
includes a
monitor for displaying the efficiency values and the efficiency of the abator
is
adjusted automatically based on said efficiency measurements.
From the above, it is found that the present invention allows the real-time
determination of the abator control efficiency that can be used to provide
real-time
continuous control of the abator operation. Further, the feedback to the
abator
control module can be used to optimize the heat input (electrical heater) and
throttling pattern of the feed from the sterilizer chamber to optimize the
control
efficiency of the abator on a continuous basis.
BRIEF DESCRIPTION OF THE DRAWINGS
Having generally defined the invention reference will now be made to the
accompanying drawings with respect to the preferred embodiments.
Fig. 1 shows a schematic view of a sterilization process including an abator
in
use with an efficiency monitoring apparatus of the present invention;
Fig. 2 shows a schematic view of the abator and monitoring device of Fig. 1;
Fig. 3 shows the theoretical effect of dilution air;
Fig. 4 shows two graphs illustrating the relationship between the
concentrations of COZ/Et0 and the abator efficiency; and,
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Fig. 5 shows the effect of dilution on the determination of control
efficiencies.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring now to Figure 1, there is illustrated a system for carrying out a
method of
monitoring a gas treatment process. .The system includes a monitoring
apparatus 10
5 which is provided for use with an abator 20 and a sterilizer 30. The
monitoring
apparatus 10 is in fluid communication with the abator 20 by way of conduit
line 40
and an outlet 50 of the abator 20. The abator 20 has an inlet 60 in fluid
communication with the sterilizer chamber 30 by way of outlet 70 from which
the
sterilizer 30 exhausts the treatment gas.
The sterilizer 30 is of the conventional type typically used in hospitals,
research
facilities and other similar institutions requiring sterilization processes
using
potentially hazardous treatment gases. Similarly, the abator 20 is of the
conventional type and typically includes elements common to such devices such
as
a dilution inlet 80, an exhaust fan 90, a throttling valve 100 and a heated
catalyst
110, all of which are illustrated for the purposes of example in Figure 1.
However,
the common elements outlined above and as shown in Fig. 1 are not intended to
be
limiting as any suitable abator device can be readily substituted as would be
understood by those skilled in the art.
Monitoring apparatus 10 includes a control means 120 to control the amount of
exhaust sampled from the abator 20, a sensing means 130 for measuring trace
components in the exhaust and output means 140 for output of the control
efficiency
results. An air cleaning means 150 suitable for cleaning the sampled exhaust,
after
sensing has been completed, is provided to release the exhaust into the
ambient air
is also provided along with a pump 160 for drawing the sampled exhaust into
the
monitoring apparatus 10.
Referring to Fig. 1 and Fig. 2, the sensing means 130 will be calibrated prior
to
operation of the apparatus to ensure accurate quantification of the treatment
gas. A
calibration means is provided and can include a source 170 of the treatment
gas and
a source control means 180 such as a valve. The source 170 can be any
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conventional method such as a zero or span gas bag. Obviously, the type of gas
used in the source 170 for completing the calibration of the sensor is similar
to that
used in the sterilization process. The gas passes into the monitoring
apparatus 10
through conduit 40 and through one or more flow controllers 190 (for example a
rotameter) before entering the sensing means 130 to make the necessary
adjustments.
Although the calibration means set out above is preferable, it would be
readily
understood that any conventionally known method of calibrating the sensing
means
130 can be used. Once the sensing means 130 has been appropriately calibrated
the monitoring apparatus may then be placed into operation.
During operation of the present invention, an exhaust of a gas or fluid having
a gas
to be treated and having exited the sterilizer chamber 30 by way of the outlet
70
enters the abator 20 via inlet 60. In turn, the treated gas in abator 20 is
exhausted
by way of outlet 50. To effect monitoring of the abator 20, a portion of the
exhaust
gas or fluid is drawn from outlet 50 in a controlled manner by way of, for
example, a
valve 120. As the exhaust portion is being drawn into the monitoring apparatus
10
and controlled throughout by suitable control means 190, before subsequently
entering sensing means 130 for sensing traces of treatment gas components
present
in the exhaust from abator 20.
It is to be understood that the sensing means 130 can include one or more
sensing
devices for sensing the trace treatment gas as well as other gases such as,
for
example C02, to carry out the analysis of the reading. Further, the sensing
means
100 can operate singularly or in parallel as shown in Figure 2 depending on
the
required application. For example, the different types of sensors contemplated
by the
present invention include but are not limited to photo acoustic devices,
electrochemical devices and non-dispersive infrared devices, etc.
It has been found that some environments having a high humidity value
introduce
condensation in the apparatus 10 that results in inaccurate readings of the
treatment
gas. As the exhaust typically includes trace amounts of the treatment gas
which are
effected by condensation there is optionally to be provided a dilution means
200 for
diluting the withdrawn sample with ambient air. The dilution means 200 can
also
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include a cleaning means 150 to remove unwanted impurities from the ambient
air.
The cleaning means 150 can further include a dryer (not shown) to further
inhibit
condensation of the treatment gas prior to entry in the sensing means 130.
It is important to note that the determination of control efficiency is
independent of
the dilution of the exhaust gases. A theoretical example using the readings at
11:40
from Table 1 is shown in Figure 3. In the example, as both the C02 and Et0 are
diluted by the same amount, there is no change in the control efficiency. This
is
applicable for all dilution ratios. Of course, it is readily understood that
excessive
dilution will render the target concentration below the sensitivity of the
instrument
and is outside the parameters of the present invention.
The results of the detection of the treatment gas by sensing means 130 having
been
sensed and calculated by calculation means can then be output by a suitable
output
means 140. The output means can include a monitor to display results for
verification purposes or manual adjustments. This invention also contemplates
the
use of automatic adjustments of the abator 20, through suitable means for the
detected treatment gas to remain below the predetermined range.
The calculation means (not shown) provides an output of control efficiency
determined by using the following unique relationship which utilizes the
carbon
dioxide in the flue gas as a surrogate for the destroyed gas, for example
ethylene
oxide. The amount of carbon dioxide formed by way of.this example follows a
stoichiometric quantity based on the equation below. The concurrent
measurement
of C02 and Et0 provides both the inlet and outlet components for the control
efficiency calculation. The stoichiometric relationship and the determination
of
control efficiency are detailed below.
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CZH40 + 2.5(Oz + 3.78NZ) --~ 2C02 + 2H20 + 2.5(3.78 N2)
Control E~ciency = CC02 for pure Et0 sterili~erc
CC02 +2Cgt0
where Et0 and C02 are measured at the outlet
or
I
CC02 _-Y Cy0
Control Efficiency = y far Et0/COZ mixtttre.e
CCO= +2Cgt0
where yi.s thevolame,Jraction of EtOin thesterili_ationmixture
This relationship between the above calculation and the method and apparatus
of
the present invention is now shown by way of Example 1.
EXAMPLE 1
The determination of control efficiencies are given in both Figure 4 and in
Table 1
below. Table 1 illustrates data collected at the exhaust of a Donaldson abator
treating sterilizer gases from a 3M sterilizer. The flue gas sample was
analyzed with
a Bruel & Kjaer Model 1302 analyzer which uses the photoacoustical (PA)
detection
of infra red active gases such as EtO, C02 and H20. This instrument operates
on
measurement cycles of slightly greater than one (1 ) minute. Prior to the
field testing,
the instrument was calibrated with separate gas standards of each compound,
and
the analyzer created response corrections for the compounds of interest, so as
to
minimize cross sensitivity.
The abator (catalytic oxidizer) requires a large volume of ambient air for the
catalytic
oxidation of Et0 that is far in excess of that required for stoichiometric
oxidation.
One reason for the additional volume is to keep the catalyst in the abator
from
overheating. The flow from the sterilizer chamber represents a minor fraction
(approximately one percent) of the total flow exiting the abator.
As ambient air contains carbon dioxide, the concentrations of carbon dioxide
from
the PA analyzer are corrected for this background level prior to calculating
the
control efficiency.
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As shown by way of example in the data in Table 1 below, the concentrations of
C02
and Et0 are cyclical. Traces of the exhaust components exhibit a sinusoidal
pattern
with a frequency of four (4) minutes. This pattern is independent of the
levels of C02
and EtO. On the basis of the efficiency, the increasing variations of the
abator
efficiency are measured in the first 16 minutes of the abator cycle. This is
followed
by decreasing variations with an upward trend in efficiency towards the end of
the
cycle.
Note that the one-hour cycle shown in Figure 4 corresponds to the evacuation
phase
of the sterilizer chamber. This is followed by a much longer period of
aeration of the
sterilizer chamber. Opposite trends in the concentration of C02 and Et0 denote
either increases or decreases in efficiency. Two examples are noted in Figure
4 with
vertical dashed lines. In the case of the first dashed line (at 05:11, left
line), the Et0
concentration increased and the C02 dropped. This corresponds to a drop in
efficiency. On the other hand, a rise in C02 and corresponding drop in Et0
denotes
an increase in efficiency as is evidenced in the second dashed line (at 15:29,
right
line). The majority of the Et0 releases notably occur in the first hour of the
abator
cycle.
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Table 1 Temporal Variation of Efficiency Using Ethylene Oxide and
Carbon Dioxide Concentrations Using a Photoacoustic Analyzer
Time COi" Et0 Efficiency
(mina) (ppm) (ppm) (%)
01:15 1982 150.6 86.81
02:38 3792 132.6 93.46
03:54 3612 78.4 95.84
05:11 2572 170.6 gg,2g
06:26 3092 84.5 94.82
07:41 4982 106.6 95.90
09:07 2242 112.6 gp, g7
10:23 2512 87.6 93.48
11:40 4052 107.6 94.96
12:56 1862 71.9 g2, 83
14:13 2182 123.6 gg, g3
15:29 3182 44.2 97.30
16:48 1512 78.0 90.65
18:09 5322 224.6 92.22
19:59 4062 167.6 92.38
21:14 2452 152.6 gg,g3
22:30 4692 169.6 93.26
23:46 3392 131.6 92.80
25:02 1762 112.6 88,67
26:17 3082 113.6 93.14
27:33 2522 98.3 g2, 77
29:08 1502 89.1 89.40
30:27 2482 73.9 94.38
31:45 1302 61.8 91.33
33:04 992 50.0 90.85
34:20 1532 38.3 95.24
35:38 832 33.9 92.47
36:54 662 28.6 92.05
38:09 1042 25.0 95.42
39:37 642 21.2 93.81
40:52 482 18.5 82,88
42:08 692 15.5 95.72
43:23 762 14.4 96.36
44:38 355 11.9 93.73
45:54 392 10.1 95.11
47:10 562 8.8 96.95
48:57 320 6.7 95.99
50:13 273 5.8 95.96
51:30 380 5.1 97.37
52:47 409 4.7 g7,78
54:02 205 4.2 96.04
55:17 230 4.2 96.52
56:34 296 3.4 g7, 7g
57:49 341 3.0 98.26
Average 93.65
* C02 in ambient air (518 ppm) correction included for each value
An example of an actual dilution on the exhaust of an abator is illustrated in
Figure 5.
The example provides for two types of instruments used in parallel for the
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measurement of C02 and EtO. A PA instrument was used for the undiluted sample
whereas paired sensors using electrochemical sensing (EC for Et0) and non
dispersive infra red (NDIR for C02) were utilized for the diluted abator
exhaust. Air
was used to dilute the exhaust gas with the dilution ratio being estimated
based on
the concurrent levels of COz from the PA and NDIR. For this example the
dilution
ratio was estimated at 6.3 times.
The two types of sensing technologies (PA versus EC/NDIR) used were different
in
terms of sampling frequency and response characteristics. The difference is
reflected in the efficiency values for both methods reflected in the whole
hour and is
particularly true for the second half of the cycle. The difference in the
efficiencies in
the second half (right side) of the cycle are a reflection of the two sampling
technologies rather than the control efficiency determination technique. The
shallow
hump for the PA analyzer at 11 or 12 minutes was due to a skipped analysis
cycle
on the instrument.
Figure 5 is illustrative that despite the two measurement technologies being
used,
and the wide range of C02 and Et0 concentrations between the PA and
Electrochemical/ non-dispersive infra red levels (EC/NDIR), the effect of
dilution did
not preclude the calculation of control efficiency using the method in this
application.
Both traces showed the same pattern. Efficiencies decreased towards the middle
of
the cycle and showed an increasing trend to full efficiency at the end of the
hour. The
overall control efficiencies were 93.5 and 94.4% for the EC/NDIR and PA
techniques
respectively, a difference of one percent.
On-line Monitoringi versus Integrated Sampling
The reliability of an on-line method to represent an abator cycle was
addressed by
collecting an integrated sample of the abator exhaust gases (not shown) for
the one
hour evacuation period. In this method, a sample is continuously drawn from
the
exhaust flue at a constant rate and stored in an enclosed container such as a
flexible
bag or canister. The integrated sample, containing a representative portion of
the
exhaust gases, was analyzed using the same instrumentation as the on-line
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measurement. This approach minimizes any difference in response due to
different
sensing methods.
The results shown in Table 2 indicate that the average of the periodic
efficiency
readings (in this case, every minute) is identical to a single integrated
efficiency over
the sample period.
Table 2 Comparison of On-line Monitoring Versus an
Integrated Sample Using a Photoacoustic Analyzer
On-line* Inte
rated**
Unit Et0 COZ EfficiencyEtO COZ Efficiency
(ppm) (ppm) (%) (ppm) (ppml (%)
0.64 1963 99.93 0.78 2066 99.92
#1
0.69 1920 99.93 0.77 1806 99.91
6.15 876 98.62 6.40 911 98.61
#2
,.
n I
~
6.61 941 .bi 6.13 900 '9
~~ 66
* Et0 and COZ readings represent an average of 45 to 50 readings. Efficiency
based on
average of 45 to 50 efficiencies.
** Integrated sample collected in a 6-Litre pacivated stainless steel
canister. Efficiency
based on a single Et0 and COz reading.
The result of the application of an on-line method being an accurate
indication of
overall cycle efficiency.
While the above relates to sterilizer abatement devices, it is understood to
those
skilled in the art the above method and apparatus may be adapted to other
systems
having a gas treatment process wherein the residual treatment gas is vented to
the
atmosphere. For example, this system may be applied to control equipment with
a
well-defined inlet gas and catalytic oxidizer with unique products of
combustion such
as hydrocarbons (carbon tetrachloride, Freons, volatile organic compounds
(VOC's),
acid gases and carbon dioxide etc.)
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The above clearly provides for a monitoring of catalytic abator efficiency in
a safer,
instantaneous and more efficient manner than previously known methodologies.
