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Characterizing the ignition process of activated carbon
Carbon 37 (1999) 335–346
Characterizing the ignition process of activated carbon Y. Suzin, L.C. Buettner *, C.A. LeDuc ERDEC, Aberdeen Proving Ground, MD 21010, USA Received 21 November 1996; accepted 20 June 1997
Abstract Properties of activated carbons at elevated temperature, specifically the spontaneous ignition temperature (SIT ) and the point of initial oxidation (PIO), are of great significance in a number of unit operations. Although the SIT and PIO should be evaluated for each individual use, a standard method is required to compare products, insure quality, and optimize processes. Different methods for determining the SIT and PIO are compared using base (BPL, Coconut Shell, and MaxSorb) and impregnated carbons (ASC, ASZM, and ASZM-T ). The SIT was determined by the ASTM and thermal analysis (TG and DSC ) methods. The PIO was determined by effluent CO concentration 2 analysis, thermal analysis (TG and DSC ) and temperature profiling. These comparisons show that not all of these methods have general utility, some are not easily correlated, and that there is evidence of the validity of two reaction regimes of interest. The simplest method that provides the most consistent conservative estimate of reaction commencement is measuring the PIO with the temperature profiling technique. © 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; B. impregnation; B. oxidation; C. thermal analysis
1. Introduction Activated carbons are widely used in heterogeneous catalysis, filtration of hazardous industrial effluents, and personal protection within contaminated atmospheres. External heating, exothermic chemical reactions, and adsorption may raise the carbon temperature well above ambient, thus making the understanding of its properties at elevated temperatures of great operational and safety significance. Two such regions of interest are the temperature that causes the carbon to start significantly oxidizing (the PIO [1]) and the temperature that causes the bed to combust in a self-sustaining manner (the SIT [2,3]). The start of significant surface reaction is an important parameter in several testing procedures (i.e. insuring the surface is not altered by the process of drying a sample), in medium-temperature processes (i.e. insuring that potential chemical loading does not decrease with completed surface reactions), and in direct effluent breathing environments (dangerous CO or CO 2 * Corresponding author. E-mail: [email protected]
levels possibly generated). The temperature that causes spontaneous ignition is of special concern in processes of high operating temperature (i.e. incinerators or nuclear reactors), in adsorption units with high loading of organics (i.e. solvent recovery), or in operations involving impregnated carbons with low ignition points (i.e. filtration of low molecular weight toxic gases). The SIT and PIO are not exclusively intrinsic properties of the carbon. The SIT and PIO change with the system operating conditions (i.e. oxygen availability, air flow rate, flow regime, relative humidity, bed and air temperatures, heating rate, bed dimensions, and thermal insulation), physical carbon conditions (i.e. particle size and bulk density) as well as intrinsic properties of the carbon (i.e. surface area and impregnants). Therefore, it is necessary to uniquely determine the SIT and PIO under application-specific conditions for each adsorbent of interest. Despite this caveat, a standard test method is required for analysis and optimization of processes, carbon comparison, and quality control purposes. The current methods available for SIT evaluation are: 1) ASTM [2]. A sample of carbon is exposed to a
0008-6223/99/$ — see front matter © 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0 0 0 8- 6 2 23 ( 9 9 ) 0 01 3 8 -3
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Table 1 Activated carbon properties
Type
Precursor
Ash(%)
Shape
Size
Bulk density (g cm−3)
Surface area (m2 g−1)
Pore volume (cm3 g−1)
BPL MaxSorb Coconut Shell
Bituminous coal Petroleum coke Coconut shells
8 30a 4
Granular Cylinder Granular
12×30 1.5 mm, 3 mm 8×16
0.48 0.37 2060 0.48
>1100 1.1 1200
0.7 – –
aMaxSorb is chemically activated with KOH causing the high ash content. Table 2 Composition of impregnation in activated carbons Impregnating ingredients (wt%) Type
Copper
Chromium oxide
Silver
Zinc
Molybdenum
TEDA
ASC ASZM ASZM-T
8 4–6 4–6
3 – –
<0.1 <0.1 <0.1
– 4–6 4–6
– 1–3 1–3
– – 2–4
Table 3 Experimental parameters used to simulate the ASTM method and military filters operation
Bed diameter (mm) Bed length (mm) Superficial velocity (cm s−1) Heating rate (°C min−1)
ASTM
Military filters
25 25 50 2
31 52 22.9 2
stream of heated air. Sample and air temperatures are increased at a constant rate of 2 or 3°C min−1 until the carbon ignites. The slopes of the sample bed or the outlet temperature profiles from pre- and post-ignition are linearly extrapolated and the intersection of the two curves is defined as the SIT. 2) Thermal analysis methods [4,5]. The carbon SIT can be determined by thermal gravimetry ( TG – weight change as a function of temperature) or differential scanning calorimetry (DSC – heat flux emitted with increasing temperature). The SIT for each of these methods is usually defined as the intersection of the baseline and the slope at the inflection point of the sample mass or power density as a function of temperature curves, respectively. The SIT is the temperature at which the oxidation reaction has gotten to the point that is self-sustaining. In contrast, the PIO is the temperature at which the surface properties have started to change due to the oxidation reactions reaching an arbitrary level of significance. We propose four approaches to defining the PIO: 1) Temperature comparison ( TC ) [1]. A bed and the air flowing into it are heated at a constant rate and the inlet and outlet bed temperatures are monitored. For an
inert bed, the outlet will lag the inlet due to the heat capacity of the system. Once the carbon begins to oxidize, the outlet temperature will increase because of the heat released by the exothermic reaction. The PIO is defined as the point where the two temperature profiles intersect. 2) CO emissions (CO P). The effluent CO concen2 2 2 tration is monitored and modeled with an Arrhenius equation. The PIO is defined as the lowest temperature where the natural log of CO concentration can be 2 linearly correlated with the inverse temperature using the Arrhenius equation. 3) and 4) Thermal analysis methods. The carbon PIO can be determined by TG and DSC by defining it as the point of deviation from the baseline values due to the exothermic processes. The intent of this study is to compare and correlate the SIT and the PIO and the methods that generate this information. In addition, we investigate the behavior of the carbons in both the ASTM bed and in a bed configuration that more closely mimics the intended filter application (in this case using military filter dimensions and flow rates). In this study we test three unimpregnated (BPL, MaxSorb, and Coconut Shell ) and three commercially impregnated activated carbons (ASC, ASZM, and ASZM-T ).
2. Methods and materials 2.1. Activated carbons and impregnation The unimpregnated carbons investigated were BPL (Calgon, Pittsburgh, PA), MaxSorb (Tokyo Zairyo,
Y. Suzin et al. / Carbon 37 (1999) 335–346
White Plains, NY ), and Coconut Shell (Sorb-Tech, Woodlands, TX ). The physical properties of these carbons, according to the suppliers’ literature, are listed in Table 1. All impregnated carbons were prepared [1] by the manufacturer (Calgon) from the base BPL activated carbon. The composition of these impregnants is summarized in Table 2. 2.2. Measurement of temperature and concentration profiles The apparatus used in the current work is based on the ASTM method # D3466-76 and is extended to include CO and CO concentration analysis of the 2 effluent air stream. A carbon bed held inside a glass tube is exposed to a gas mixture of ultra high purity N (80%) and O (20%) (both from Matheson) flowing 2 2 at a constant superficial velocity. Using a temperaturecontrolled oven and gas heat exchanger, the temperature of both the sample and the inlet air is raised at a rate
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of 5° min−1 to at least 40°C below the expected PIO. For determination of the PIO, after equilibration at the initial temperature, the temperature is increased at a constant rate of 2°C min−1 to the point at which the exit temperature of the carbon bed exceeds the inlet temperature by 5–10°C. The CO and CO concen2 trations in the bed effluent are recorded in one-minute intervals using a MIRAN-80 Infrared Analyzer. Each sample analysis was repeated 4–6 consecutive times for each sample, quenching the oxidation reaction between each iteration by cooling the bed to the initial temperature. For SIT determination, the samples were heated at 2°C min−1 until the bed ignited and, after ignition, the reaction was quenched by stopping air flow over the bed and turning off the oven. Two different bed configurations were used in this study: one is defined by the ASTM specifications, and the other is sized to mimic a typical military filter. The SIT and PIO were measured for each carbon in both bed configurations (ASTM, CO P, and TC methods 2
Fig. 1. Inlet and outlet temperatures are shown as a function of time online. Two separate experiments are superimposed on this graph. Two tubes were packed with ASZM carbon to either the ASTM or the end user (military filter) specifications. The method of PIO determination is described in the text. The application specific bed temperature profiles are shown on the same figure by shifting the graph 1000 seconds forward in time. Note that, although the ignition temperature is a sharp peak, considerable reaction occurs before the onset of ignition.
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Table 4 Ignition temperatures (°C ) evaluated by the various methods Carbon SIT Standard method (ASTM bed specs.) Standard method (military filter specs.) DSC TG PIO Temp. profiles ( TC ) (ASTM bed specs.) Temp. profiles ( TC ) (military filter specs.) CO profiles (CO P) (ASTM bed specs.) 2 2 CO profiles (CO P) (military filter specs.) 2 2 DSC TG
ASZM-T
ASC
ASZM
Coconut Shell
300 275 365 370
265 230 330 335
300 260 365 370
150 155 165 150 165 215
195 190 195 200 220 290
225 225 – 225 260 315
with either the ASTM specified bed and superficial velocity or the alternative military sizing). Table 3 is a side-by-side comparison of the two bed configurations highlighting the differences in operating parameters.
MaxSorb
BPL
365 330 475 480
>370 340 410 455
>370 >370 525 565
210 180 215 185 215 325
265 230 – 255 295 385
360 315 – 340 470 500
2.3. Thermal analysis DSC and TG measurements for each sample were simultaneously performed on a SETARAM instrument
Fig. 2. DSC thermogram for MaxSorb and ASZM showing power density emitted as a function of sample temperature. Samples (3–5 mg) are tested under dry air flow in cylindrical platinum crucibles with the reference side of the TG balanced with an identical weight test sample in flowing N . The samples are held at 150°C to dry off residual water and then heated from 150 to 600°C at a 2 heating rate of 5°C min−1 with a gas flow rate of 16 cm3 min−1. The SIT is defined as the temperature where the tangent to the point of inflection intersects the baseline DSC value. The SIT determination is shown on the figure.
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(model TG–DSC 111). The samples (3–5 mg obtained from a 50 mg sample that was crushed and uniformly mixed), were tested under dry air flow in cylindrical platinum crucibles. A reference of identical mass to the test sample was balanced on the reference side of the TG. The temperature range was 150–600°C, with a heating rate of 5°C min−1. The gas flow rate through the sample and reference cell was 16 cm3 min−1. Prior to each experiment, carbon samples were kept for 30 minutes at the initial temperature of 150°C to evaporate any water vapor collected during handling. The DSC and TG thermograms were computed by subtracting the reference curves obtained with the same type of carbon at identical conditions under nitrogen.
3. Results 3.1. Determination of the SIT 3.1.1. Standard method mode (SM). In the standard method as applied in this study, the SIT is found experimentally by the intersection of the linear extrapolations of the outlet temperature profile before and after ignition. The bed conditions (sizing and
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flow parameters) greatly affect the SIT that is measured. This study looked at two bed conditions, those defined by the ASTM, and bed properties that mimicked an end use of interest to the authors (a military bed configuration). To show the effect of bed operating parameters, Fig. 1 indicates the determination of the SIT for ASZM carbon using both bed configurations. The SIT is defined as the intersection of the two linear extrapolations that are shown. Because the exiting temperature is not truly linear prior to ignition, there are possible deviations inherent in the determination of this line. In addition, the inlet temperature that corresponds to the ignition can be significantly overestimated. Because of the potentially life-threatening nature of a failure of this type, a more conservative estimate of the ignition temperature could be necessary. Table 4 compares the SIT values obtained with both bed configurations for each of the carbons. The effect of using the application-specific bed parameters as opposed to the ASTM bed parameters was, in every case, to lower the SIT by as much as 40°C. 3.1.2. DSC analysis. Fig. 2 shows DSC thermograms obtained with MaxSorb and ASZM carbons. Each thermogram describes a continuous exothermic carbon oxidation
Fig. 3. Shown are TG thermograms for Coconut Shell and ASZM-T showing sample mass as a function of sample temperature. Experimental apparatus is the same as for the DSC determination. The SIT is defined as the point where the tangent to the point of inflection intersects the baseline (100% weight retained ). The SIT determination is indicated on the figure.
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terminated by complete combustion of the carbon. The SIT of an individual carbon was extracted from its thermogram by determining the temperature that corresponds to the intersection of the baseline with the tangent of the slope at the point of inflection for each exotherm. The SIT, determined by this method, is indicated on the figure for each carbon. The results for all carbons studied are summarized in Table 4. 3.1.3. TG analysis. Similar to the previous method, the SIT was defined as the temperature where the intersection of the baseline (100% relative weight) and the tangent to the slope at the point of inflection for the TG thermogram occurs. Fig. 3 shows the TG results and determination of the SIT by this method for Coconut Shell and ASZM-T carbons. The SIT’s for each carbon determined by this method are summarized in Table 4.
3.2. Determination of the PIO 3.2.1. Temperature profiles (TC). The PIO is determined by heating the bed and the air inlet at a constant rate; the PIO is defined as the temperature where the inlet and outlet temperature profiles intersect. As with the SIT, this temperature is a function of bed parameters and PIO’s were determined under bed conditions consistent with the ASTM and the end use application. Fig. 1 indicates the determination of the PIO under the two bed operating parameters studied. The air inlet and bed temperature were raised to 5 to 10°C above the PIO and then cooled to the starting temperature. The bed inlet and outlet temperatures and effluent CO and CO concentrations were monitored. 2 Fig. 4 indicates, for 4–6 consecutive cycles, both the inlet and outlet temperatures (right scale) and the effluent
Fig. 4. The CO and CO effluent concentrations as well as the inlet and outlet temperatures are shown as functions of time. The 2 temperature of the inlet and the bed is increased by 5°C min−1 until the PIO is exceeded by 10°C, the inlet air and oven are then rapidly cooled to the equilibration temperature. This process is repeated for 4 to 6 consecutive iterations. The PIO is defined as the temperature where the inlet and outlet temperatures are equal. Coconut Shell carbon data is shown.
Y. Suzin et al. / Carbon 37 (1999) 335–346
CO and CO concentrations ( left scale) for Coconut 2 Shell carbon. The PIO, indicated by an arrow for each cycle, is the intersection of the inlet and outlet temperature profiles. To illustrate the precision of this method, Fig. 5 shows an expanded view of the time interval from 40–55 minutes in the first cycle of ASZM-T. Figs. 4 and 5 show that, (i) during the temperature ramp to the oxidation point, the outlet temperature is lower than that at the inlet (due to the heat capacity of the carbon bed); (ii) at the PIO, the two profiles intersect; and (iii) after the PIO, the outlet is greater than the inlet (caused by heat generated by carbon combustion). It is of interest to note the continuous rise of the PIO with consecutive iterations found in all experiments (illustrated in Fig. 4). 3.2.2. CO concentration profiles (CO P). 2 2 CO profiles in Fig. 4 show several similarities 2 common to all samples tested: (1) a concentration increase of up to 350 ppm during sample heating to the pre-ignition initial temperature; (2) a sharp increase (oxidation) to 1000 ppm or more, followed by a steep decrease (quenching) to negligi-
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ble values, occurs for each of the 4–6 consecutive repetitions; and (3) an initial large CO emission, indicated by a wider 2 concentration peak for the first repetition compared to the peaks that follow. After the initial temperature cycle, CO effluent concentrations are fairly 2 consistent from cycle to cycle. CO formation at temperatures below the PIO has 2 been discussed in previous studies [4,6 ]. It is postulated that this pre-ignition CO liberation is caused by thermal 2 decomposition of oxygen-containing complexes. Although these complexes are largely decomposed by the initial heating stage, they still contribute to the CO emission during the first temperature increase. Since 2 the concentration profiles thereafter are reproducible, the second run was used for PIO evaluations based on CO emission. 2 The location of the PIO based on CO emission 2 assumes that at a point of significant change in the kinetic profile of carbon oxidation takes place. The (natural ) log of CO concentration is shown as a func2 tion of the inverse absolute temperature in Figs. 6 and 7. In all of the carbon samples tested there was a region of linearity at the high CO emission region. These lines 2
Fig. 5. Expanded scale plot of inlet and outlet air temperatures as functions of time during the first iteration in the heating cycle for ASZM-T is shown. The PIO is defined as the intersection of the inlet and outlet temperature profiles, shown as a sharp point. This figure demonstrates the precision of the proposed method.
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Fig. 6. ASZM-T results by the CO P method. An Arrhenius plot of the natural log of CO concentration as a function of the inverse 2 2 absolute temperature is shown for six successive temperature cycles. Each successive iteration emits a lower concentration of CO at 2 a given temperature and has a slightly higher PIO, but each iteration has the same activation energy (slope). The numbers above the curves correspond to the iteration number. The activation energies of the curves are as follows (in cycle order): n E =−34.7 kcal mol−1; & E =−35.7 kcal mol−1; + E =−35.2 kcal mol−1; × E =−34.4 kcal mol−1; 0 E =−33.8 kcal mol−1; a a a a a $ E =−33.5 kcal mol−1. The PIO was defined as the lowest temperature that fits the Arrhenius plot to within 1% of the a expected value.
were regressed, and a best fit was found. The PIO was defined as the lowest temperature that was within 1% of the regressed line. The PIO’s for each carbon, as determined by this method, were summarized in Table 4. Figs. 6 and 7 present Arrhenius plots of the ASZM-T and Coconut Shell carbons, respectively. In every iteration after the first, the activation energies (slopes of the lines) were almost constant for each carbon sample tested. The activation energies of the impregnated carbons were all approximately 40 kcal mol−1 and the activation energies of the non-impregnated carbons were all approximately 25 kcal mol−1 as shown in Table 5. 3.2.3. DSC analysis. Expanded scale DSC thermograms of MaxSorb and BPL carbons are shown in Fig. 8. Each thermogram represents a continuous exothermic carbon oxidation terminated by consumption of the sample. The PIO that is determined by this method is intrinsic to the carbon and represents the point at which the exothermic reaction is significant. For comparison with the other methods
used in this work, the PIO of an individual carbon was extracted from its thermogram by locating the initial point of monotonic deviation from the baseline (the PIO was defined as the first temperature that deviated by more than 2% from the running average of the prior 5 points). These points of deviation are indicated in Fig. 2 by arrows. This definition is quite arbitrary and the PIO can change significantly with slight changes in the deviation required or the number of prior points that create the running average. The primary advantage of this approach is that it makes the definition consistently applied from one carbon to the next. Table 4 summarizes the PIO’s that were determined by the DSC method. 3.2.4. TG analysis. Similar to the previous method, the PIO was defined as the point where the TG thermogram deviates continuously from the baseline (here, PIO is defined as the first temperature that is 0.1% smaller than the running average of the prior 5 readings). Expanded scale TG thermograms for ASC and ASZM carbons are shown
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Fig. 7. Coconut Shell carbon results by the CO P method. An Arrhenius plot of the natural log of CO concentration as a function 2 2 of inverse absolute temperature is shown for six successive temperature cycles. Each successive iteration emits a lower concentration of CO at a given temperature and has a slightly higher PIO, but each iteration, after the first, has the same activation energy (within 2 experimental error). During the first heating cycle it is postulated that surface adsorbents are being decomposed. The numbers above the curves correspond to the iteration number. The activation energies of the curves are as follows (in cycle order): n E =−8.8 kcal mol−1; & E =−21.1 kcal mol−1; + E =−24.2 kcal mol−1; × E =−26.1 kcal mol−1. The PIO was defined as the a a a a lowest temperature that fits the Arrhenius plot to within 1% of the expected value.
Table 5 Activation energy from the CO P method (kcal mol−1) 2 Carbon
1st cycle
2nd cycle
3rd cycle
4th cycle
5th cycle
6th cycle
ASZM-T ASC ASZM MaxSorb Coconut shell BPL
35 21 19 11 9 12
36 38 37 22 21 24
35 40 40 26 24 26
34 42 40 – 26 –
34 43 – – – –
34 43 – – – –
in Fig. 9. The PIO’s for all of the carbons obtained, using the above criteria, are summarized in Table 4.
4. Discussion The SIT and PIO values tested in this paper by the various methods indicate that they represent two
different phenomena affecting the carbons. Some carbons (especially those impregnated with TEDA in this study) can have surprisingly low PIO’s when compared to their SIT values. The standard method to define the SIT can lead to significant overestimations of the temperatures at which reactions commence, and caution needs to be exercised in all cases in which the exiting stream will be breathed.
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Fig. 8. Expanded scale for DSC measurements of power density as a function of sample temperature for MaxSorb and BPL carbons to show the placement of the PIO. The PIO is defined as the temperature at which the DSC reading is 2% greater than the running average of the five prior data points.
Another surprising finding is that the order of the SIT’s and the PIO’s is not consistent. A carbon can have a relatively high SIT (ASZM-T or Coconut Shell carbon) and have a corresponding relatively low PIO. In all instances the application must be considered when looking at carbon ignition properties. The TC and CO P methods give PIO values that are 2 comparable within experimental error. These are also the two methods that consistently yield lower values. The highest PIO values, on the other hand, are obtained using the TG method. The TG method gives significantly higher values for both the SIT and the PIO (by 5–110°C ) compared to the DSC analysis which is performed simultaneously. In all cases, the SIT was lowered significantly when the application-specific filter bed dimensions and flow rates, as opposed to the ASTM parameters, were used. This reinforces the fact that each filter application must be tested with a specific carbon as opposed to exclusively relying on the ASTM results. The deviation in SIT values caused by changing the bed parameters is as large as 40°C. The PIO was not uniformly affected by the
change in the bed operating parameters, possibly indicating that this parameter has less dependence on bed operating conditions. One of the more interesting results is that the CO P 2 method yielded good consistency with the TC method, even though there appear to be two unique reactions occurring, one on the impregnated and the other on the unimpregnated carbons.
4.1. Applicability of the specific methods 4.1.1. PIO measurement. The CO P method appears to be of significant use. 2 However, due to uncertainty into the exact location of the PIO using the deviation from the Arrhenius plot and due to the relative difficulty in completing this measurement, it may have limited practical applicability. Thermal analysis methods, despite yielding good reproducibility in thermogram measurement, are also hardly useful as a standard technique due to the uncertainty in locating the point on the exotherm where
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Fig. 9. TG thermograms of weight retained as a function of temperature for ASC and ASZM carbons. An expanded scale is shown to indicate the location of the PIO. The PIO is defined as the temperature at which the TG reading is 0.1% smaller than the running average of the five prior data points.
baseline deviation commences and difficulty in making measurements. The TC method is simple to set up, gives reproducible results, and involves simple calculations. This method appears to be ‘‘forgiving’’ in that the bed operating parameters do not appear to have a radical effect on the PIO determination. 4.1.2. SIT measurement. The ASTM test results in SIT’s that are consistent for one filter sizing, but are greatly dependent on the physical construction of the filter and the flow rates over the bed. In addition, in some cases, it greatly overestimates the temperature where oxidation commences. Caution must be exercised when using this method to get a bed sizing and linear flow velocity that are consistent with the intended application-specific filter. Thermal analysis methods have the distinct advantage that they are intensive properties of the carbon; however, they generally give values of the SIT’s that are too high for practical use.
We strongly recommend that all carbon used in potential high temperature filtration be tested in filter dimensions consistent with the specific application using both the TC method to determine the PIO and the ASTM technique to determine the SIT. To ensure that the surface characteristics are not altered we recommend that all filter applications be operated below this PIO temperature. If the filter effluent is directly breathable, it is absolutely essential that PIO determination be made and that the filter operating temperature is maintained at a safe level below this value.
5. Conclusions This paper serves as a warning to those that use or condition activated carbon filters in applications that involve insulated beds and/or elevated temperatures. The ASTM method for determining the spontaneous ignition temperature does not necessarily reflect the minimum temperature at which filter ignition can occur.
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This ignition temperature is dependent on the physical properties of the filter, flow rates of the filtrate, and on the impregnants on the carbon. Four methods were examined (temperature profiling, effluent CO monitoring, DSC, and TG) that demon2 strate the existence of a temperature where oxidation reactions commence (the so-called PIO [1]). Each of these independent methods indicate that reactions occur on the carbon at temperatures significantly below those temperatures that would be deemed safe by the ASTM defined SIT. The effect of these reactions is, at this point, unknown on the adsorptive properties of the carbon. We suggest that when conditioning beds, the PIO is not exceeded. In addition, the SIT was determined using DSC, TG, and the ASTM method (standard and modified for larger bed size). These measurements indicate that ignition is not exclusively an intensive property of the carbon; the method that produced the lowest temperature of ignition was the method that mimicked the current application of the filter in the military setting. To ensure a safe operatoration, the ignition temperature should be determined in the application settings that it is intended to be used.
Acknowledgements This work was performed while one of the authors ( Y. S.) held a National Research Council – ERDEC Research Associateship.
References [1] Suzin Y, Buettner LC, LeDuc CA. Behavior of impregnated activated carbons heated to the point of oxidation. Carbon (submitted ). [2] Standard test method for ignition temperature of granular activated carbon. ASTM D 3466-76 (reapproved 1993). [3] Hardman JS, Lawn CJ, Street PJ, Further studies of the spontaneous ignition behavior of activated carbon. Fuel 1983;62:632. [4] Akubuiro EC, Wagner NJ, Assessment of activated carbon stability towards adsorbed organics. Ind Eng Chem Res 31339–346.1992 [5] Liang SHC, Cameron LE. Differential scanning calorimetry (DSC ) for the analysis of activated carbon ( U ). Report no. 1098, Defense Research Establishment, Ottawa, 1991. [6 ] The reaction of oxygen–nitrogen mixtures with granular activated carbons below the spontaneous ignition temperature. Prepared for the US Naval Research Laboratory under Contract No. N00014-83-C-2184, GC-TR-84-385, 1984.
Characterizing the ignition process of activated carbon Y. Suzin, L.C. Buettner *, C.A. LeDuc ERDEC, Aberdeen Proving Ground, MD 21010, USA Received 21 November 1996; accepted 20 June 1997
Abstract Properties of activated carbons at elevated temperature, specifically the spontaneous ignition temperature (SIT ) and the point of initial oxidation (PIO), are of great significance in a number of unit operations. Although the SIT and PIO should be evaluated for each individual use, a standard method is required to compare products, insure quality, and optimize processes. Different methods for determining the SIT and PIO are compared using base (BPL, Coconut Shell, and MaxSorb) and impregnated carbons (ASC, ASZM, and ASZM-T ). The SIT was determined by the ASTM and thermal analysis (TG and DSC ) methods. The PIO was determined by effluent CO concentration 2 analysis, thermal analysis (TG and DSC ) and temperature profiling. These comparisons show that not all of these methods have general utility, some are not easily correlated, and that there is evidence of the validity of two reaction regimes of interest. The simplest method that provides the most consistent conservative estimate of reaction commencement is measuring the PIO with the temperature profiling technique. © 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; B. impregnation; B. oxidation; C. thermal analysis
1. Introduction Activated carbons are widely used in heterogeneous catalysis, filtration of hazardous industrial effluents, and personal protection within contaminated atmospheres. External heating, exothermic chemical reactions, and adsorption may raise the carbon temperature well above ambient, thus making the understanding of its properties at elevated temperatures of great operational and safety significance. Two such regions of interest are the temperature that causes the carbon to start significantly oxidizing (the PIO [1]) and the temperature that causes the bed to combust in a self-sustaining manner (the SIT [2,3]). The start of significant surface reaction is an important parameter in several testing procedures (i.e. insuring the surface is not altered by the process of drying a sample), in medium-temperature processes (i.e. insuring that potential chemical loading does not decrease with completed surface reactions), and in direct effluent breathing environments (dangerous CO or CO 2 * Corresponding author. E-mail: [email protected]
levels possibly generated). The temperature that causes spontaneous ignition is of special concern in processes of high operating temperature (i.e. incinerators or nuclear reactors), in adsorption units with high loading of organics (i.e. solvent recovery), or in operations involving impregnated carbons with low ignition points (i.e. filtration of low molecular weight toxic gases). The SIT and PIO are not exclusively intrinsic properties of the carbon. The SIT and PIO change with the system operating conditions (i.e. oxygen availability, air flow rate, flow regime, relative humidity, bed and air temperatures, heating rate, bed dimensions, and thermal insulation), physical carbon conditions (i.e. particle size and bulk density) as well as intrinsic properties of the carbon (i.e. surface area and impregnants). Therefore, it is necessary to uniquely determine the SIT and PIO under application-specific conditions for each adsorbent of interest. Despite this caveat, a standard test method is required for analysis and optimization of processes, carbon comparison, and quality control purposes. The current methods available for SIT evaluation are: 1) ASTM [2]. A sample of carbon is exposed to a
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Table 1 Activated carbon properties
Type
Precursor
Ash(%)
Shape
Size
Bulk density (g cm−3)
Surface area (m2 g−1)
Pore volume (cm3 g−1)
BPL MaxSorb Coconut Shell
Bituminous coal Petroleum coke Coconut shells
8 30a 4
Granular Cylinder Granular
12×30 1.5 mm, 3 mm 8×16
0.48 0.37 2060 0.48
>1100 1.1 1200
0.7 – –
aMaxSorb is chemically activated with KOH causing the high ash content. Table 2 Composition of impregnation in activated carbons Impregnating ingredients (wt%) Type
Copper
Chromium oxide
Silver
Zinc
Molybdenum
TEDA
ASC ASZM ASZM-T
8 4–6 4–6
3 – –
<0.1 <0.1 <0.1
– 4–6 4–6
– 1–3 1–3
– – 2–4
Table 3 Experimental parameters used to simulate the ASTM method and military filters operation
Bed diameter (mm) Bed length (mm) Superficial velocity (cm s−1) Heating rate (°C min−1)
ASTM
Military filters
25 25 50 2
31 52 22.9 2
stream of heated air. Sample and air temperatures are increased at a constant rate of 2 or 3°C min−1 until the carbon ignites. The slopes of the sample bed or the outlet temperature profiles from pre- and post-ignition are linearly extrapolated and the intersection of the two curves is defined as the SIT. 2) Thermal analysis methods [4,5]. The carbon SIT can be determined by thermal gravimetry ( TG – weight change as a function of temperature) or differential scanning calorimetry (DSC – heat flux emitted with increasing temperature). The SIT for each of these methods is usually defined as the intersection of the baseline and the slope at the inflection point of the sample mass or power density as a function of temperature curves, respectively. The SIT is the temperature at which the oxidation reaction has gotten to the point that is self-sustaining. In contrast, the PIO is the temperature at which the surface properties have started to change due to the oxidation reactions reaching an arbitrary level of significance. We propose four approaches to defining the PIO: 1) Temperature comparison ( TC ) [1]. A bed and the air flowing into it are heated at a constant rate and the inlet and outlet bed temperatures are monitored. For an
inert bed, the outlet will lag the inlet due to the heat capacity of the system. Once the carbon begins to oxidize, the outlet temperature will increase because of the heat released by the exothermic reaction. The PIO is defined as the point where the two temperature profiles intersect. 2) CO emissions (CO P). The effluent CO concen2 2 2 tration is monitored and modeled with an Arrhenius equation. The PIO is defined as the lowest temperature where the natural log of CO concentration can be 2 linearly correlated with the inverse temperature using the Arrhenius equation. 3) and 4) Thermal analysis methods. The carbon PIO can be determined by TG and DSC by defining it as the point of deviation from the baseline values due to the exothermic processes. The intent of this study is to compare and correlate the SIT and the PIO and the methods that generate this information. In addition, we investigate the behavior of the carbons in both the ASTM bed and in a bed configuration that more closely mimics the intended filter application (in this case using military filter dimensions and flow rates). In this study we test three unimpregnated (BPL, MaxSorb, and Coconut Shell ) and three commercially impregnated activated carbons (ASC, ASZM, and ASZM-T ).
2. Methods and materials 2.1. Activated carbons and impregnation The unimpregnated carbons investigated were BPL (Calgon, Pittsburgh, PA), MaxSorb (Tokyo Zairyo,
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White Plains, NY ), and Coconut Shell (Sorb-Tech, Woodlands, TX ). The physical properties of these carbons, according to the suppliers’ literature, are listed in Table 1. All impregnated carbons were prepared [1] by the manufacturer (Calgon) from the base BPL activated carbon. The composition of these impregnants is summarized in Table 2. 2.2. Measurement of temperature and concentration profiles The apparatus used in the current work is based on the ASTM method # D3466-76 and is extended to include CO and CO concentration analysis of the 2 effluent air stream. A carbon bed held inside a glass tube is exposed to a gas mixture of ultra high purity N (80%) and O (20%) (both from Matheson) flowing 2 2 at a constant superficial velocity. Using a temperaturecontrolled oven and gas heat exchanger, the temperature of both the sample and the inlet air is raised at a rate
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of 5° min−1 to at least 40°C below the expected PIO. For determination of the PIO, after equilibration at the initial temperature, the temperature is increased at a constant rate of 2°C min−1 to the point at which the exit temperature of the carbon bed exceeds the inlet temperature by 5–10°C. The CO and CO concen2 trations in the bed effluent are recorded in one-minute intervals using a MIRAN-80 Infrared Analyzer. Each sample analysis was repeated 4–6 consecutive times for each sample, quenching the oxidation reaction between each iteration by cooling the bed to the initial temperature. For SIT determination, the samples were heated at 2°C min−1 until the bed ignited and, after ignition, the reaction was quenched by stopping air flow over the bed and turning off the oven. Two different bed configurations were used in this study: one is defined by the ASTM specifications, and the other is sized to mimic a typical military filter. The SIT and PIO were measured for each carbon in both bed configurations (ASTM, CO P, and TC methods 2
Fig. 1. Inlet and outlet temperatures are shown as a function of time online. Two separate experiments are superimposed on this graph. Two tubes were packed with ASZM carbon to either the ASTM or the end user (military filter) specifications. The method of PIO determination is described in the text. The application specific bed temperature profiles are shown on the same figure by shifting the graph 1000 seconds forward in time. Note that, although the ignition temperature is a sharp peak, considerable reaction occurs before the onset of ignition.
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Table 4 Ignition temperatures (°C ) evaluated by the various methods Carbon SIT Standard method (ASTM bed specs.) Standard method (military filter specs.) DSC TG PIO Temp. profiles ( TC ) (ASTM bed specs.) Temp. profiles ( TC ) (military filter specs.) CO profiles (CO P) (ASTM bed specs.) 2 2 CO profiles (CO P) (military filter specs.) 2 2 DSC TG
ASZM-T
ASC
ASZM
Coconut Shell
300 275 365 370
265 230 330 335
300 260 365 370
150 155 165 150 165 215
195 190 195 200 220 290
225 225 – 225 260 315
with either the ASTM specified bed and superficial velocity or the alternative military sizing). Table 3 is a side-by-side comparison of the two bed configurations highlighting the differences in operating parameters.
MaxSorb
BPL
365 330 475 480
>370 340 410 455
>370 >370 525 565
210 180 215 185 215 325
265 230 – 255 295 385
360 315 – 340 470 500
2.3. Thermal analysis DSC and TG measurements for each sample were simultaneously performed on a SETARAM instrument
Fig. 2. DSC thermogram for MaxSorb and ASZM showing power density emitted as a function of sample temperature. Samples (3–5 mg) are tested under dry air flow in cylindrical platinum crucibles with the reference side of the TG balanced with an identical weight test sample in flowing N . The samples are held at 150°C to dry off residual water and then heated from 150 to 600°C at a 2 heating rate of 5°C min−1 with a gas flow rate of 16 cm3 min−1. The SIT is defined as the temperature where the tangent to the point of inflection intersects the baseline DSC value. The SIT determination is shown on the figure.
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(model TG–DSC 111). The samples (3–5 mg obtained from a 50 mg sample that was crushed and uniformly mixed), were tested under dry air flow in cylindrical platinum crucibles. A reference of identical mass to the test sample was balanced on the reference side of the TG. The temperature range was 150–600°C, with a heating rate of 5°C min−1. The gas flow rate through the sample and reference cell was 16 cm3 min−1. Prior to each experiment, carbon samples were kept for 30 minutes at the initial temperature of 150°C to evaporate any water vapor collected during handling. The DSC and TG thermograms were computed by subtracting the reference curves obtained with the same type of carbon at identical conditions under nitrogen.
3. Results 3.1. Determination of the SIT 3.1.1. Standard method mode (SM). In the standard method as applied in this study, the SIT is found experimentally by the intersection of the linear extrapolations of the outlet temperature profile before and after ignition. The bed conditions (sizing and
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flow parameters) greatly affect the SIT that is measured. This study looked at two bed conditions, those defined by the ASTM, and bed properties that mimicked an end use of interest to the authors (a military bed configuration). To show the effect of bed operating parameters, Fig. 1 indicates the determination of the SIT for ASZM carbon using both bed configurations. The SIT is defined as the intersection of the two linear extrapolations that are shown. Because the exiting temperature is not truly linear prior to ignition, there are possible deviations inherent in the determination of this line. In addition, the inlet temperature that corresponds to the ignition can be significantly overestimated. Because of the potentially life-threatening nature of a failure of this type, a more conservative estimate of the ignition temperature could be necessary. Table 4 compares the SIT values obtained with both bed configurations for each of the carbons. The effect of using the application-specific bed parameters as opposed to the ASTM bed parameters was, in every case, to lower the SIT by as much as 40°C. 3.1.2. DSC analysis. Fig. 2 shows DSC thermograms obtained with MaxSorb and ASZM carbons. Each thermogram describes a continuous exothermic carbon oxidation
Fig. 3. Shown are TG thermograms for Coconut Shell and ASZM-T showing sample mass as a function of sample temperature. Experimental apparatus is the same as for the DSC determination. The SIT is defined as the point where the tangent to the point of inflection intersects the baseline (100% weight retained ). The SIT determination is indicated on the figure.
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terminated by complete combustion of the carbon. The SIT of an individual carbon was extracted from its thermogram by determining the temperature that corresponds to the intersection of the baseline with the tangent of the slope at the point of inflection for each exotherm. The SIT, determined by this method, is indicated on the figure for each carbon. The results for all carbons studied are summarized in Table 4. 3.1.3. TG analysis. Similar to the previous method, the SIT was defined as the temperature where the intersection of the baseline (100% relative weight) and the tangent to the slope at the point of inflection for the TG thermogram occurs. Fig. 3 shows the TG results and determination of the SIT by this method for Coconut Shell and ASZM-T carbons. The SIT’s for each carbon determined by this method are summarized in Table 4.
3.2. Determination of the PIO 3.2.1. Temperature profiles (TC). The PIO is determined by heating the bed and the air inlet at a constant rate; the PIO is defined as the temperature where the inlet and outlet temperature profiles intersect. As with the SIT, this temperature is a function of bed parameters and PIO’s were determined under bed conditions consistent with the ASTM and the end use application. Fig. 1 indicates the determination of the PIO under the two bed operating parameters studied. The air inlet and bed temperature were raised to 5 to 10°C above the PIO and then cooled to the starting temperature. The bed inlet and outlet temperatures and effluent CO and CO concentrations were monitored. 2 Fig. 4 indicates, for 4–6 consecutive cycles, both the inlet and outlet temperatures (right scale) and the effluent
Fig. 4. The CO and CO effluent concentrations as well as the inlet and outlet temperatures are shown as functions of time. The 2 temperature of the inlet and the bed is increased by 5°C min−1 until the PIO is exceeded by 10°C, the inlet air and oven are then rapidly cooled to the equilibration temperature. This process is repeated for 4 to 6 consecutive iterations. The PIO is defined as the temperature where the inlet and outlet temperatures are equal. Coconut Shell carbon data is shown.
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CO and CO concentrations ( left scale) for Coconut 2 Shell carbon. The PIO, indicated by an arrow for each cycle, is the intersection of the inlet and outlet temperature profiles. To illustrate the precision of this method, Fig. 5 shows an expanded view of the time interval from 40–55 minutes in the first cycle of ASZM-T. Figs. 4 and 5 show that, (i) during the temperature ramp to the oxidation point, the outlet temperature is lower than that at the inlet (due to the heat capacity of the carbon bed); (ii) at the PIO, the two profiles intersect; and (iii) after the PIO, the outlet is greater than the inlet (caused by heat generated by carbon combustion). It is of interest to note the continuous rise of the PIO with consecutive iterations found in all experiments (illustrated in Fig. 4). 3.2.2. CO concentration profiles (CO P). 2 2 CO profiles in Fig. 4 show several similarities 2 common to all samples tested: (1) a concentration increase of up to 350 ppm during sample heating to the pre-ignition initial temperature; (2) a sharp increase (oxidation) to 1000 ppm or more, followed by a steep decrease (quenching) to negligi-
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ble values, occurs for each of the 4–6 consecutive repetitions; and (3) an initial large CO emission, indicated by a wider 2 concentration peak for the first repetition compared to the peaks that follow. After the initial temperature cycle, CO effluent concentrations are fairly 2 consistent from cycle to cycle. CO formation at temperatures below the PIO has 2 been discussed in previous studies [4,6 ]. It is postulated that this pre-ignition CO liberation is caused by thermal 2 decomposition of oxygen-containing complexes. Although these complexes are largely decomposed by the initial heating stage, they still contribute to the CO emission during the first temperature increase. Since 2 the concentration profiles thereafter are reproducible, the second run was used for PIO evaluations based on CO emission. 2 The location of the PIO based on CO emission 2 assumes that at a point of significant change in the kinetic profile of carbon oxidation takes place. The (natural ) log of CO concentration is shown as a func2 tion of the inverse absolute temperature in Figs. 6 and 7. In all of the carbon samples tested there was a region of linearity at the high CO emission region. These lines 2
Fig. 5. Expanded scale plot of inlet and outlet air temperatures as functions of time during the first iteration in the heating cycle for ASZM-T is shown. The PIO is defined as the intersection of the inlet and outlet temperature profiles, shown as a sharp point. This figure demonstrates the precision of the proposed method.
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Fig. 6. ASZM-T results by the CO P method. An Arrhenius plot of the natural log of CO concentration as a function of the inverse 2 2 absolute temperature is shown for six successive temperature cycles. Each successive iteration emits a lower concentration of CO at 2 a given temperature and has a slightly higher PIO, but each iteration has the same activation energy (slope). The numbers above the curves correspond to the iteration number. The activation energies of the curves are as follows (in cycle order): n E =−34.7 kcal mol−1; & E =−35.7 kcal mol−1; + E =−35.2 kcal mol−1; × E =−34.4 kcal mol−1; 0 E =−33.8 kcal mol−1; a a a a a $ E =−33.5 kcal mol−1. The PIO was defined as the lowest temperature that fits the Arrhenius plot to within 1% of the a expected value.
were regressed, and a best fit was found. The PIO was defined as the lowest temperature that was within 1% of the regressed line. The PIO’s for each carbon, as determined by this method, were summarized in Table 4. Figs. 6 and 7 present Arrhenius plots of the ASZM-T and Coconut Shell carbons, respectively. In every iteration after the first, the activation energies (slopes of the lines) were almost constant for each carbon sample tested. The activation energies of the impregnated carbons were all approximately 40 kcal mol−1 and the activation energies of the non-impregnated carbons were all approximately 25 kcal mol−1 as shown in Table 5. 3.2.3. DSC analysis. Expanded scale DSC thermograms of MaxSorb and BPL carbons are shown in Fig. 8. Each thermogram represents a continuous exothermic carbon oxidation terminated by consumption of the sample. The PIO that is determined by this method is intrinsic to the carbon and represents the point at which the exothermic reaction is significant. For comparison with the other methods
used in this work, the PIO of an individual carbon was extracted from its thermogram by locating the initial point of monotonic deviation from the baseline (the PIO was defined as the first temperature that deviated by more than 2% from the running average of the prior 5 points). These points of deviation are indicated in Fig. 2 by arrows. This definition is quite arbitrary and the PIO can change significantly with slight changes in the deviation required or the number of prior points that create the running average. The primary advantage of this approach is that it makes the definition consistently applied from one carbon to the next. Table 4 summarizes the PIO’s that were determined by the DSC method. 3.2.4. TG analysis. Similar to the previous method, the PIO was defined as the point where the TG thermogram deviates continuously from the baseline (here, PIO is defined as the first temperature that is 0.1% smaller than the running average of the prior 5 readings). Expanded scale TG thermograms for ASC and ASZM carbons are shown
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Fig. 7. Coconut Shell carbon results by the CO P method. An Arrhenius plot of the natural log of CO concentration as a function 2 2 of inverse absolute temperature is shown for six successive temperature cycles. Each successive iteration emits a lower concentration of CO at a given temperature and has a slightly higher PIO, but each iteration, after the first, has the same activation energy (within 2 experimental error). During the first heating cycle it is postulated that surface adsorbents are being decomposed. The numbers above the curves correspond to the iteration number. The activation energies of the curves are as follows (in cycle order): n E =−8.8 kcal mol−1; & E =−21.1 kcal mol−1; + E =−24.2 kcal mol−1; × E =−26.1 kcal mol−1. The PIO was defined as the a a a a lowest temperature that fits the Arrhenius plot to within 1% of the expected value.
Table 5 Activation energy from the CO P method (kcal mol−1) 2 Carbon
1st cycle
2nd cycle
3rd cycle
4th cycle
5th cycle
6th cycle
ASZM-T ASC ASZM MaxSorb Coconut shell BPL
35 21 19 11 9 12
36 38 37 22 21 24
35 40 40 26 24 26
34 42 40 – 26 –
34 43 – – – –
34 43 – – – –
in Fig. 9. The PIO’s for all of the carbons obtained, using the above criteria, are summarized in Table 4.
4. Discussion The SIT and PIO values tested in this paper by the various methods indicate that they represent two
different phenomena affecting the carbons. Some carbons (especially those impregnated with TEDA in this study) can have surprisingly low PIO’s when compared to their SIT values. The standard method to define the SIT can lead to significant overestimations of the temperatures at which reactions commence, and caution needs to be exercised in all cases in which the exiting stream will be breathed.
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Fig. 8. Expanded scale for DSC measurements of power density as a function of sample temperature for MaxSorb and BPL carbons to show the placement of the PIO. The PIO is defined as the temperature at which the DSC reading is 2% greater than the running average of the five prior data points.
Another surprising finding is that the order of the SIT’s and the PIO’s is not consistent. A carbon can have a relatively high SIT (ASZM-T or Coconut Shell carbon) and have a corresponding relatively low PIO. In all instances the application must be considered when looking at carbon ignition properties. The TC and CO P methods give PIO values that are 2 comparable within experimental error. These are also the two methods that consistently yield lower values. The highest PIO values, on the other hand, are obtained using the TG method. The TG method gives significantly higher values for both the SIT and the PIO (by 5–110°C ) compared to the DSC analysis which is performed simultaneously. In all cases, the SIT was lowered significantly when the application-specific filter bed dimensions and flow rates, as opposed to the ASTM parameters, were used. This reinforces the fact that each filter application must be tested with a specific carbon as opposed to exclusively relying on the ASTM results. The deviation in SIT values caused by changing the bed parameters is as large as 40°C. The PIO was not uniformly affected by the
change in the bed operating parameters, possibly indicating that this parameter has less dependence on bed operating conditions. One of the more interesting results is that the CO P 2 method yielded good consistency with the TC method, even though there appear to be two unique reactions occurring, one on the impregnated and the other on the unimpregnated carbons.
4.1. Applicability of the specific methods 4.1.1. PIO measurement. The CO P method appears to be of significant use. 2 However, due to uncertainty into the exact location of the PIO using the deviation from the Arrhenius plot and due to the relative difficulty in completing this measurement, it may have limited practical applicability. Thermal analysis methods, despite yielding good reproducibility in thermogram measurement, are also hardly useful as a standard technique due to the uncertainty in locating the point on the exotherm where
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Fig. 9. TG thermograms of weight retained as a function of temperature for ASC and ASZM carbons. An expanded scale is shown to indicate the location of the PIO. The PIO is defined as the temperature at which the TG reading is 0.1% smaller than the running average of the five prior data points.
baseline deviation commences and difficulty in making measurements. The TC method is simple to set up, gives reproducible results, and involves simple calculations. This method appears to be ‘‘forgiving’’ in that the bed operating parameters do not appear to have a radical effect on the PIO determination. 4.1.2. SIT measurement. The ASTM test results in SIT’s that are consistent for one filter sizing, but are greatly dependent on the physical construction of the filter and the flow rates over the bed. In addition, in some cases, it greatly overestimates the temperature where oxidation commences. Caution must be exercised when using this method to get a bed sizing and linear flow velocity that are consistent with the intended application-specific filter. Thermal analysis methods have the distinct advantage that they are intensive properties of the carbon; however, they generally give values of the SIT’s that are too high for practical use.
We strongly recommend that all carbon used in potential high temperature filtration be tested in filter dimensions consistent with the specific application using both the TC method to determine the PIO and the ASTM technique to determine the SIT. To ensure that the surface characteristics are not altered we recommend that all filter applications be operated below this PIO temperature. If the filter effluent is directly breathable, it is absolutely essential that PIO determination be made and that the filter operating temperature is maintained at a safe level below this value.
5. Conclusions This paper serves as a warning to those that use or condition activated carbon filters in applications that involve insulated beds and/or elevated temperatures. The ASTM method for determining the spontaneous ignition temperature does not necessarily reflect the minimum temperature at which filter ignition can occur.
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This ignition temperature is dependent on the physical properties of the filter, flow rates of the filtrate, and on the impregnants on the carbon. Four methods were examined (temperature profiling, effluent CO monitoring, DSC, and TG) that demon2 strate the existence of a temperature where oxidation reactions commence (the so-called PIO [1]). Each of these independent methods indicate that reactions occur on the carbon at temperatures significantly below those temperatures that would be deemed safe by the ASTM defined SIT. The effect of these reactions is, at this point, unknown on the adsorptive properties of the carbon. We suggest that when conditioning beds, the PIO is not exceeded. In addition, the SIT was determined using DSC, TG, and the ASTM method (standard and modified for larger bed size). These measurements indicate that ignition is not exclusively an intensive property of the carbon; the method that produced the lowest temperature of ignition was the method that mimicked the current application of the filter in the military setting. To ensure a safe operatoration, the ignition temperature should be determined in the application settings that it is intended to be used.
Acknowledgements This work was performed while one of the authors ( Y. S.) held a National Research Council – ERDEC Research Associateship.
References [1] Suzin Y, Buettner LC, LeDuc CA. Behavior of impregnated activated carbons heated to the point of oxidation. Carbon (submitted ). [2] Standard test method for ignition temperature of granular activated carbon. ASTM D 3466-76 (reapproved 1993). [3] Hardman JS, Lawn CJ, Street PJ, Further studies of the spontaneous ignition behavior of activated carbon. Fuel 1983;62:632. [4] Akubuiro EC, Wagner NJ, Assessment of activated carbon stability towards adsorbed organics. Ind Eng Chem Res 31339–346.1992 [5] Liang SHC, Cameron LE. Differential scanning calorimetry (DSC ) for the analysis of activated carbon ( U ). Report no. 1098, Defense Research Establishment, Ottawa, 1991. [6 ] The reaction of oxygen–nitrogen mixtures with granular activated carbons below the spontaneous ignition temperature. Prepared for the US Naval Research Laboratory under Contract No. N00014-83-C-2184, GC-TR-84-385, 1984.