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The thermal decomposition of hydrated calcium hypochlorite (UN 2880)
Fire Safety Journal 35 (2000) 223}239
The thermal decomposition of hydrated calcium hypochlorite (UN 2880) Brian F. Gray*, Brendan Halliburton Department of Chemistry, Macquarie University, Sydney, NSW 2109, Australia Received 23 March 2000; received in revised form 27 May 2000; accepted 30 May 2000
Abstract Although this material has been generally regarded as prone to exothermic self-decomposition with very rapid release of heat, oxygen and gaseous chlorine derivatives it has not been the subject of an orthodox thermal ignition study such as that performed on anhydrous calcium hypochlorite (UN 1748), studied in the standard manner in 1978. This paper describes such tests on samples varying in mass from 5 g up to 200 kg in cylindrical containers of varying aspect ratio and various materials including stainless steel gauze, high-density polyethylene and coated "bre. The heat transfer coe$cients of all the containers were measured accurately by both steady-state and cooling curve procedures and the thermal conductivity of the hypochlorite was measured independently. These measurements revealed that the Biot numbers of the samples tested were in a range where the critical ambient temperatures were sensitive to the values and thus sensitive to the convective air#ow around the test bodies. With these factors taken into account the usual Frank}Kamenetskii (F}K) plot gave a good straight line in the range 1153C upwards. However, below this temperature, i.e. in the range 40}1153C very strong deviations occur. Nevertheless, the F}K theory still holds as the low-temperature points also fall on a good straight line with activation energy 48.5 kJ/mol which compares with the high-temperature activation energy much greater than this. These results indicate a sharp change in the rate determining step for heat generation at temperatures around 100}1203C. In temperature ranges both above and below this area the temperature}time traces for the samples behave classically as would be expected from the F}K theory but inside this range the traces show much more complex behaviour with some oscillatory characteristics, also some evidence of endothermic behaviour. The consequences of this unexpected behaviour for safety (particularly in the bulk marine shipping context) are serious. The deviations predicted from simple extrapolation of the high-temperature results indicate much lower critical ambient temperatures for large quantities of this material than previously thought. Our low-temperature results
* Corresponding author. E-mail address: [email protected] (B.F. Gray). 0379-7112/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 3 7 9 - 7 1 1 2 ( 0 0 ) 0 0 0 3 0 - 8
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obtained by orthodox ignition methods are entirely consistent with independent direct isothermal calorimetric measurements already present in the literature but not used quantitatively in this context. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The most recent study [1] of these materials (anhydrous and hydrated hypochlorite), employing di!erential scanning calorimetry and thermogravimetric techniques concluded that the thermal stability data obtained showed less stability than reported in the literature. It was concluded that storage conditions were crucial for the stability of the product. It was determined that decomposition started at 603C and that the decomposition pro"le was highly dependent on the moisture content. The more water present, they concluded, the lower the temperature at which decomposition commenced and the greater the amount of heat given out. This is in marked contrast to the conclusion drawn by Mandell [2] in 1971 on the basis of a single DSC trace showing an endotherm at 503C for the hydrated material which was not present for the anhydrous material. From this trace and other entirely inappropriate tests involving cigarettes and matches on 100 g samples Mandell concluded quite unjusti"ably `It has been demonstrated that a new kind of high strength calcium hypochlorite has been made with heat sink characteristics such that it will not spontaneously propagate exothermic decompositiona. A few years later, in 1974, Clancey [3] referred to a number of serious and costly shipboard "res involving the anhydrous material over the period 1967}1973. It appears that the appearance of the &new product' and the unsubstantiated claims on its behalf turned the spotlight o! fundamental scienti"c investigation of this material although Bibby and Milestone [4] in 1984 did extensive isothermal calorimetric work on both hydrated and anhydrous materials. The rates of heat production in the temperature range 20}703C were carefully measured and the results showed very clearly that after drying over P O under vacuum for 6 d the rates of heat generation were considerably reduced. Conversely, exposure of the samples to 80% RH for 6 d signi"cantly increased the heat output. They concluded `Clearly the removal of H O from calcium hypochlorite increases its stability while adding H O decreases ita, totally contradicting the conclusions of Mandell. These workers also concluded that in DSC and similar tests the heating rate a!ected the way in which hypochlorite decomposes to a very considerable extent making such results di$cult if not impossible to interpret. The above results and the re-emergence of major losses due to marine "res involving various forms of calcium hypochlorite over the past three years clearly indicate a need for investigation of hydrated calcium hypochlorite with a view to characterising its &ignition' properties. Strictly speaking, it does not ignite on its own but it does show subcritical, critical and supercritical behaviour, i.e. runaway reaction. The adiabatic temperature rise is not large by combustion standards (a few hundred degrees) but this with the presence of free liberated oxygen is su$cient to cause
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a major and "ercely burning "re if it contacts any organic material such as wood or plastic. We will use the term ignition for convenience on this understanding.
2. Experimental For samples up to 10 kg the hypochlorite used was the product of US, Japanese and Chinese manufacturers (PPG, Toya Soda and Shanghai Chloralkali, respectively) and all were tested. No signi"cant di!erences were found between these samples. For the larger scale tests ('20 kg) only US material (PPG) was used. However, all samples showed the break in slope of the Frank}Kamenetskii (F}K) plot (see [9]) around 1203C. The laboratory oven used was a Heraeus design with forced air#ow capable of producing a spatially uniform temperature within the oven with a variation of $0.53C. This level of constancy could be maintained for inde"nite periods of time even during tests when ignition occurred for sample quantities of 10 kg or less. The tests of samples of 40 kg upwards were carried out in an oven of internal dimensions 1.2 m;1.8 m;1.8 m constructed of kiln "red concrete &Besser' blocks inserted over a welded steel frame running through their internal spaces. The latter were "lled with insulating "breglass material. The internal walls were lined with a "re retardant lining and the door consisted of a 150 mm thick steel tubular frame, thin external and internal "re retardant lining, all packed with thermal insulating material. This door was designed to vent in the event of rapid pressurisation. A 600 mm stirring fan was located at the back of the oven. Air inlet and outlet pipes together with internal heater elements of 6;850 W as well as air preheating facilities allowed accurate thermostatting of the air inside the oven even during supercritical behaviour of large samples. The spent air was taken through chlorine scrubbers charged with 50 kg slaked lime. Temperature control was achieved using a CAL controls C3200 PID ($0.13C) controller in early tests and a Brainchild BTC 9200 controller ($0.13C) in later tests. Chromel}alumel thermocouples were placed at various points in the oven to act as sensors for the control units. Data were recorded using Advantech Adam 4018 "eld point module and a TOA intelligent chart recorder. As some of the tests needed two weeks or more to complete, precautions were taken to have available generator power and custom software to monitor the mains supply and coordinate switching to generator power when necessary. The mains supply to the controlling computer and logging equipment was "ltered and stabilised. Two uninterrupted power supplies, connected in series, were used to power the equipment in the event of mains failure and before the generator was energised. Temperatures were checked with a &Comark KM 45' hand-held digital thermometer and referenced to type K KM-PRO6 sensor (National Association of Testing Authorities calibration number 5961N, traceable to Australian National Measurement Standards via Primary Standard Platinum Resistance Thermometer (SPRT) serial number 213832 report number 40562).
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The samples were equipped with a number of chromel}alumel thermocouples. Every run had a thermocouple placed at the centre of the contents and a second touching the inside wall in the case of the larger containers (40 and 200 kg). Other runs were instrumented with thermocouples placed so as to give information on spatial temperature pro"les within the material as required. The tests in the Heraeus oven were carried out with the sample placed in a central position on a thin metal stand. Those in the larger oven were placed on a wooden board, itself supported on two wood planks thus allowing air circulation underneath. Some of the 40 kg tests and all the 200 kg tests were carried out with the sample keg enclosed inside a thin-walled (2 mm) steel box (without bottom) which itself was placed in the oven. This was in order to examine the e!ect of the variation of heat transfer coe$cient from keg surface to air in nominally &still' air inside the box compared with forced convection taking place directly in the circulated oven air. The larger oven was designed to comfortably exceed the requirements for determination of the SADT by UN protocol [5].
3. Procedures The thermal conductivity of the hydrated calcium hypochlorite (PPG manufacture) was measured according to ASTM C518 and ASTM C687 at a mean temperature of 35.13C and was found to be 0.147$0.015 W/mK. This measurement was performed on the material in its standard form as delivered, i.e. granulated. A similar work carried out by Uehara et al. [6] on anhydrous material apparently used pulverised material for a similar measurement and obtained the value 0.438 W/mK. The higher resultant bulk density could account for some of this di!erence as could the hydration. The moisture content measurements were carried out in three ways, drying to constant weight over P O in vacuo, drying to constant weight over P O in air and drying to constant weight in air over silica gel. In all cases the "nal percentage (on a wet basis) was the same although the relaxation time to constant weight was much shorter for the vacuum technique. Values obtained were 8.45% by weight on a wet basis. The actual test runs were all carried out in standard fashion, i.e. the samples were placed in a preheated oven set at the required test temperature. Invariably, the sample centre temperature was lower than the test temperature giving a variable time for the sample centre to reach ambient (oven) temperature. The various temperatures of points in the sample and various points in the oven were recorded continuously by the dataloggers and recorders and complete thermal records were obtained. After each run, whether supercritical or subcritical, the sample material was discarded and new material used for the next test at the new set temperature. All the containers were cylindrical and all the gauze ones were equicylinders. The commercial containers of HDPE (40 kg) and "bre (200 kg) were slightly longer in comparison to width but this does not present any problems since the critical parameters for "nite cylinders of any height/diameter ratio are known.
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In view of the rather high value of the thermal conductivity (0.147 W/mK) the heat transfer coe$cients s of the containers were measured since the Biot number (sr/i), where i is the thermal conductivity and r the characteristic dimension, was not large enough for the F}K (in"nite Biot number) approximation to be used [9]. s was measured by two methods, "rstly by following a cooling curve in a constant temperature atmosphere and secondly (and more accurately) by using the accurately calibrated electrical power input necessary to maintain a constant temperature di!erence with the environment itself controlled to $0.13C. The power input was calculated from simultaneous measurement of the rms AC voltage and current #ow using a clamp current meter (resolution 0.01 A) and digital multitester (resolution 0.01 V).
4. Results The critical ambient temperatures (CATs) of various size samples were determined using the above procedures. Each group of tests involved determination of the highest ambient temperature at which ignition did not occur and also determination of the lowest ambient temperature at which it did occur. The arithmetic mean of these two "gures was then taken as the CAT for that particular size container under the speci"ed conditions of heat transfer. With smaller samples, in the ambient temperature range above 1203C, the supercritical reaction produces a large volume increase in the material which has the appearance of a &sou%e' and is a highly porous solid. In contrast, for the lower temperature tests involving 10}200 kg of material this was not evident. The remaining solid after ignition more or less retained the shape and volume of the original container even in the (HDPE, 40 kg) case where the container caught "re and was completely consumed. In all cases the remaining solid was extremely hygroscopic and corrosive consisting largely of calcium chloride. Table 1 gives the CATs measured for samples of Japanese manufacture (Sample A) and US manufacture (Sample B). Each critical temperature is the result of at least four, usually "ve, runs at various ambient temperatures. All results are for equicylinders except the last four. For the HDPE kegs h"0.44 m and d"2.53. For the "bre drum h"0.83 m and d"2.37. For the thermos #ask, h"0.19 m and d"2.28. In many cases in this work these values of d have to be corrected for "nite Biot number before testing the applicability of the F}K ignition theory. The standard test of the F}K theory, i.e. plot of 2 ln+¹ /r, vs. 1/¹ is shown in Fig. 1. Examination of the "gure suggests that at least two reactions are occurring and that each "ts the standard F}K theory quite well in its own range. Least-squares "tting gives the equation of the higher temperature line as
ln
2¹ (18.97$1.85)1000 "(66.12$4.55)! r ¹
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Table 1 Critical ambient temperatures Sample
Radius (m)
Critical ambient temperature (3C)
A A A A A A B B B B B B B B B B B
0.00925 0.01875 0.02875 0.055 0.075 0.105 0.00925 0.01875 0.02875 0.055 0.075 0.105 0.175 0.175 (HDPE) 0.175 (HDPE, still air in box) 0.275 ("bre, still air in box) 0.055 (thermos)
154.2$2.5 141.5$2.5 128.5$2.5 123.0$2.5 101.0#2.5 90.6$2.5 149.5$2.5 135.5$2.5 126.5#2.5 120.5$2.5 102.5$2.5 87.5$2.0 64.0$3.0 60.1$0.5 55.2$1.3 43.4$1.3 57.3$2.0
Fig. 1. F}K plot of raw data.
giving an apparent activation energy of 158$15 kJ/mol. Of rather more interest from the practical point of view is the lower temperature region corresponding to larger size containers, up to and including 200 kg. The least-squares "t for this section is given by
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Table 2 Heat transfer characteristics Keg type
s (natural) (W/m K)
s (forced) (W/m K)
Bi (natural)
Bi (forced)
40 kg HDPE 200 kg "bre
10.3$0.4 7.9$0.4
16.4$0.4 *
12.3$0.5 14.8$0.5
19.6$0.5 *
the equation
ln
2¹ (4.9$0.5)1000 "(29.3$1.5)! r ¹
giving an apparent activation energy of only 41$4 kJ/mol. For the containers in this temperature range we have directly measured the heat transfer coe$cients by both steady-state methods (with a constant electrical input and measurement of the steady temperature di!erential set up against ambient) and cooling curve methods. Results from both methods agree well and are given in Table 2. The Biot numbers are calculated from the heat transfer coe$cients using the standard formula sr Bi" , i where i is the thermal conductivity of the hypochlorite and r is the radius of the keg. The value of i is 0.147 W/mK at 353C and 0.142 W/mK at 233C. Throughout this work we will use the higher value to calculate Biot numbers since we are most interested in ambient temperatures in the 30}403C range. These measurements allow recalculation of the data to include correction for the "nite Biot numbers encountered using the formula obtained by Barzykin and presented by Bowes [9]. The corresponding values of d are of course signi"cantly smaller than the values of d but more importantly they are quite sensitive to container size and hence a!ect the apparent activation energy obtained from the F}K plot. Gri$ths et al. [7] and more recently Jones [8] have measured the heat transfer coe$cients and Biot numbers directly for stainless steel gauze containers in laboratory ovens with forced convection and from this work the most reliable value for s is 14.0 W/m K. If we calculate the critical parameters using this value and the data in Table 2 for the larger containers we can re-plot according to the equation >"ln d #2 ln(¹ /r)"A!E/R¹ using the data given in Table 3. The last two rows in Table 3 represent kegs standing inside thin sheet metal steel boxes (heat transfer coe$cients measured as 4.5 W/m K) which themselves are placed
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Table 3 Biot number corrected data Radius (m)
1000/¹
0.00925 0.01875 0.02875 0.055 0.075 0.105 0.175 (gauze) 0.175 (HDPE, forced) 0.175 (HDPE, nat) 0.275 ("bre, nat)
2.366 2.448 2.503 2.541 2.663 2.774 2.967 3.002 3.047 3.160
ln d
Y
!0.27 0.17 0.40 0.67 0.75 0.825 0.899 0.829 0.54 0.60
21.19 20.15 19.48 18.41 17.79 17.11 15.88 16.08 15.61 14.69
inside the oven on a wooden base. The overall Biot numbers for each assembly of keg and box were calculated using the normal assumption that the two thermal resistances were in series and therefore additive. Temperature di!erences across the two surfaces involved were su$ciently small for the assumption of conductive heat transfer to be valid. The practical reason for this arrangement for the larger bodies was that this con"guration resembles closely the actual situation of kegs inside a shipping container where they will stand in reasonably quiescent air. The container itself will normally be in a ventilated hold. Of course in the latter case the kegs inside the container will interact with each other as they are all dumping heat into the same airspace and this e!ect will be signi"cant. Such interactions are presently being studied and further results on this topic will be presented later. A plot of this data is shown in Fig. 2. Least-squares "tting of these data gives the equation
2¹ (5.85$0.22)1000 #ln d "! #(33.33$0.64) r ¹ which shows considerably reduced deviations compared with the best "t for the raw data. The corresponding quantities for the high-temperature plot are 15.10$2.03 and 57.02$5.01. Also, the apparent low-range activation energy is increased to 48.5$ 1.8 kJ/mol. From this plot we can also deduce the pre-exponential heat production rate ln
QZ"7.48;10 W/m, where Q is the heat of reaction and Z is the Arrhenius pre-exponential factor. With the above value for E/R we can calculate the heat production rate at any temperature that is implied by the CATs. For example at 503C we obtain the value of 120 W/m which compares satisfactorily with the results of Bibby and Milestone [4] whose graphical results indicate a heat output in the region 100}200 W/m measured by a completely di!erent method (isothermal calorimetry). In contrast, extrapolation
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Fig. 2. F}K plot of the data corrected for "nite Biot number.
of the high-temperature results down to the low-temperature regime implies a "gure around 0.01 W/m at 503C, so this reaction is completely negligible as a thermogenerator in the range of temperatures likely to be important in practical situations. The high-temperature plot corrected for "nite Biot numbers (using the heat transfer coe$cients of the gauze baskets as measured in [7, 8]) satis"es the F}K equation
2 ln
¹
(15.10$2.03)1000 #ln d "! #(57.02$5.00). r ¹
This translates to an activation energy of 125 kJ/mol compared with the uncorrected value of 152 kJ/mol. Gri$ths et al. [7] found a similar correction to high-temperature data for bagasse and wood#our when this correction is taken into account. Subcritical and supercritical temperature time traces are shown in Fig. 3 for the 35 kg stainless steel gauze basket, the largest of the orthodox containers used. The usual divergence between sub- and supercritical heating is clear here but it is not possible to predict supercritical behaviour until 45 h after the start of the run even when the initial temperature of the hypochlorite is 253C. At an initial temperature of 153C, 60 h elapse before it is clear whether runaway will occur. Two supercritical runs in 40 HDPE kegs are shown in Fig. 4. In the HDPE plastic kegs there are two &critical conditions' in the sense that the "rst transition from hypochlorite self-heating of around 20 to '1503C, the one we are
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Fig. 3. 35 kg gauze basket, ambient temperatures of 68 and 603C.
Fig. 4. At 60.13C temperature rise is '9503C due to ignition of HDPE keg. At 59.83C temperature rise is 1503C without plastic ignition, but supercritical for hypochlorite.
studying in detail here, may or may not ignite the keg itself. In the run at an ambient temperature of 60.13C the hypochlorite self-heated to around 1753C and then remained almost stationary for 30 min before the extremely rapid temperature jump up to approximately 10003C occurred. This was accompanied by visible #ame and complete destruction of the keg (see Fig. 4).
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Fig. 5. 40 kg keg * subcritical and supercritical temperature traces.
In the run at an ambient temperature of 59.83C the temperature rose to approximately 1503C and levelled out. This did not produce ignition of the keg as before. However, inspection of the keg after cooling revealed that considerable charring and melting had occurred. Centre temperatures for three runs spanning criticality for a 40 kg HDPE keg inside a steel box, with a wooden #oor (dimensions 0.41;0.41;0.48 m) are shown in Fig. 5. In these runs the samples were in a condition of natural convection only inside the box, which was itself subject to the forced convection inside the oven. The critical temperature in this case turned out to be 55.23C. In this case the resistance to heat loss from the sample is considerably increased by virtue of the decreased heat transfer coe$cient from the keg to naturally convecting air inside the box and the series thermal resistance represented by the heat transfer through the box wall to the ambient air in the oven. In Fig. 6 a pro"le of the supercritical run is shown. Whilst the centre temperature reached 1903C, the half-radius temperature reached only 1303C in this case. The keg itself did not ignite although it was distorted and partially charred. In Fig. 7 a run performed with no internal thermocouples at all, is shown. It was instrumented with thermocouples at the top and bottom, externally mounted in the centre of the two faces. The keg was tested with the manufacturer's seal unbroken; thus there was no possibility of either contamination or catalysis by the thermocouple junction. This keg was supercritical and ignited in the usual way with, as would be expected, a larger temperature rise on the top face than on the bottom face (1263C cf. 803C). Ambient temperature in this case was 58.13C and the HDPE was badly damaged.
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Fig. 6. 40 kg keg * centre, half-radius and ambient temperature traces for ¹ "56.63C.
Fig. 7. Unopened 40 kg keg in box with external thermocouples only, as shipped, ¹ "58.13C.
In Fig. 8 the centre temperatures for two subcritical runs and one supercritical run are shown for a 200 kg "bre container, also inside a steel box with a wooden #oor of dimensions 0.612;0.612;0.952 m. The centre attained nearly 2003C in the supercritical run with an ambient temperature of 44.73C and the "bre keg was destroyed, falling to pieces when attempts were made to remove it from the oven.
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Fig. 8. Supercritical and subcritical runs for a 200 kg drum in a box. Time to ignition from an initial temperature of 36.43C was 7.5 d (see text).
The time to reach 36.43C, the temperature at the start of Fig. 8, from the storage temperature of 183C was a further 200 h (8.3 d). Thus, the &time to ignition' for a single 200 kg "bre drum containing hypochlorite at 183C would be of the order of two weeks when subject to an ambient temperature of 44.73C. Fig. 9 shows a supercritical pro"le across the drum. It also shows the response of the temperature controller for the oven to the ignition of 200 kg inside. The slight increase in ambient temperature was quickly corrected even with this large extra heat release inside the box. Drying of samples of the hypochlorite under vacuum with phosphorus pentoxide, under air with phosphorus pentoxide and under air with silica gel gave moisture content of 8.45% by weight on a wet basis in all cases although the vacuum treatment produced equilibration much more quickly than the other two methods. The silica gel treatment took almost three weeks to reach equilibrium. These "gures are within the range required for UN 2880 speci"cation.
5. Discussion Clearly, the reaction mechanism for hypochlorite decomposition is complex but nevertheless exhibits apparently simple behaviour over each of two limited temperature ranges where the F}K theory "ts the experimental results rather well. Two parallel reactions, both exothermic, appear to be taking place. One dominates at higher temperatures (the higher activation energy reaction) but the higher activation energy e!ectively cuts out this reaction in the lower temperature range where the
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Fig. 9. Temperature pro"le for 200 kg drum in a box, ¹ "44.73C.
low-temperature low-activation energy reaction dominates. Cases such as this have been known for many years and are discussed in Bowes [9] where a bilinear F}K plot similar to Fig. 1 is displayed. The relatively high thermal conductivity of the substrate in this case (0.147 W/mK compared with typical agricultural materials of 0.05 W/mK) leads to an interesting situation with respect to the sensitivity of critical ambient temperatures to the nature of the air#ow in the nearby vicinity. As is well known, critical conditions are relatively insensitive to Biot number when the latter is greater than 20 and the latter is a measure of the relative resistance to heat #ow within the body and from the body to the surroundings. It follows that for smaller Biot numbers the critical parameters will be sensitive to the heat transfer characteristics of the surroundings e.g. air #ow velocity, degree of turbulence, presence of natural convection, etc. This is clearly shown at the experimental level in Table 1. The CAT for a 40 kg HDPE keg in the forced convection of the oven at 60.13C is signi"cantly higher than that for a similar keg in still air inside a steel box (55.23C). The change in CAT can be calculated in such cases by using the simple theory of heat transfer where thermal resistance in series is taken to be additive. The heat transfer coe$cients for the keg to still air and from internal to external air through the wall of the steel box have been measured experimentally. The calculated change in CAT agrees quite well with the measured value (4.93C). For reasons of cost and time we did not carry out a similar comparison for 200 kg drums, restricting measurements to drums inside a steel box. A similar calculation of
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the total thermal resistance assuming series resistance gives an e!ective Biot number for this case also and from this e!ective Biot number it is simple to calculate the critical value of d using the equation of Barzykin quoted in detail in Bowes [9]. From this Fig. a &corrected' point for the F}K plot can be obtained. Such points are shown in Fig. 2 and the standard deviations for the &corrected' straight line are considerably smaller than for the raw data, again indicating that this simple interpretation is reliable. Of course corrections for "nite Biot number are larger for smaller samples; so the data obtained by Uehara et al. [6] which were uncorrected, are questionable as they did not measure any heat transfer coe$cients and quoted a very high value of 0.44 W/mK for the conductivity of anhydrous hypochlorite, which also may have been pulverised for that particular measurement. Similar comments apply to our high-temperature results. We have not calculated corrections for these as we do not use any of the data for predictions as to lowtemperature large-size CATs as the reaction mechanism switch totally invalidates any extrapolation from this region to lower temperatures. The highest degree of subcritical self-heating measured was in the gauze 35 kg equicylinder with a Biot number of 10.2 with a "gure of 243C at 60.63C. At the critical temperature the in"nite Biot number for this critical temperature rise is 1.77R¹/E with the value 333C, probably around 10% smaller including the Biot correction. Allowing for the fact that we were a few degrees below criticality at 60.63C, this agreement is satisfactory. Times to ignition are seen in the cases of larger samples to be determined to a considerable extent by heat transfer rates, particularly with initial sample temperatures below 203C which is common in commercial practice. The largest (200 kg) sample took close to two weeks to ignite starting from 183C and one week after reaching ambient temperature. However, we would be very hesitant to apply any of the semi-empirical formulae for times to ignition derived on the basis of thermal ignition theory as in this case there are indications that radical chain reactions are involved, particularly at low temperatures. The sensitivity to small amounts of impurities and water points in this direction, as does the obvious change in mechanism occurring at intermediate temperatures. Such features need not invalidate the application of the F}K theory to the critical condition itself which will still re#ect a limiting heat balance condition however complex the chemical kinetics may be as shown by Gray as long ago as 1969 [10]. At the same time, the latter can have a profound and complex e!ect on time to ignition factors which are much more di$cult to predict than the critical condition itself.
6. Conclusions (1) The thermal decomposition of hydrated calcium hypochlorite involves two distinct reactions (at least) each of which separately obeys the F}K relationship for thermal ignition.
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(2) As must be the case, the reaction dominating in the low-temperature range ((903C) has a rather low-activation energy of 48.5$1.8 kJ/mol. (3) The pre-exponential heat release rate for this reaction is 7.48;10 W/m. (4) The thermal conductivity of the hydrated calcium hypochlorite is 0.147 W/mK. (5) The relatively high thermal conductivity places the Biot numbers for most container materials and container sizes in the region where the CATS are sensitive to heat transfer characteristics to the surrounding medium. (6) For the low-temperature reaction, correction for relatively small Biot numbers gives an F}K plot which has smaller standard deviations than the raw data plot. (7) For prediction of CATs of practical containers of 40 kg or greater traditional high-temperature measurements give extremely dangerous predictions when extrapolated to lower temperatures due to the bilinear nature of the F}K plot. (8) A 40 kg HDPE keg has a CAT of 60.13C when immersed in forced convective air in a thermostatted oven. (9) A similar 40 kg keg when inside a steel box, itself immersed in the same oven, has a CAT of 55.23C. (10) A 200 kg "bre drum when inside a steel box itself in contact with forced convective air on the outside, has a CAT of 43.43C. (11) These results in conjunction with the low activation energy indicate that the transport of commercial quantities of 13}18 tonnes of hypochlorite kegs or drums inside shipping containers will result in assembly CATs well below the above "gure and probably in the region of likely temperatures in ships holds particularly in tropical waters. (12) Times to ignition, which are of the order of two weeks for a single 200 kg drum, may be somewhat longer than this for such a commercial assembly and further study in this area is required. The time to ignition in these cases is for practical purposes measured from the initial temperature of the hypochlorite and not from when it reaches ambient. The crucial comparison is between this "gure and the length of time the assembly is exposed to hold temperatures on the voyage. Supercritical conditions may well not be a danger in some circumstances but virtually no research has been published on these topics. References [1] Cardillo P, Nebuloni M. Reactivity and thermal stability of calcium hypochlorite. Rev Combust 1994;48:300}5. [2] Mandell Jr HC. A new calcium hypochlorite and a discriminatory. Test Fire Technol 1971;7(2):157}61. [3] Clancey VJ. Fire hazards of calcium hypochlorite. J Hazardous Mater 1975/1976;1:83}94. [4] Bibby DM, Milestone NB. The decomposition of high grade bleaching powder. J Chem Tech Biotech 1984;34A:423}30. [5] Transport of dangerous goods and materials, Manual of tests and criteria. New York and Geneva: United Nations, 1995. [6] Uehara Y, Uematsu H, Saito Y. Thermal ignition of calcium hypochlorite. Combust Flame 1978;32:85}94.
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[7] Gri$ths JF, Hasko SM, Tong AW. Thermal ignition in packed particulate solids: criticality under conditions of variable Biot number. Combust Flame 1985;59:1}9. [8] Jones JC, Wade M. Direct determination of the Biot number in oven heating tests. J Fire Sci 1999;17:421}9. [9] Bowes PC. Self-heating: evaluating and controlling the hazards. London: HMSO, 1984. [10] Gray BF. Uni"ed theory of explosions, cool #ames and two-stage ignitions * I. Trans Faraday Soc 1969;65:1603}13.
The thermal decomposition of hydrated calcium hypochlorite (UN 2880) Brian F. Gray*, Brendan Halliburton Department of Chemistry, Macquarie University, Sydney, NSW 2109, Australia Received 23 March 2000; received in revised form 27 May 2000; accepted 30 May 2000
Abstract Although this material has been generally regarded as prone to exothermic self-decomposition with very rapid release of heat, oxygen and gaseous chlorine derivatives it has not been the subject of an orthodox thermal ignition study such as that performed on anhydrous calcium hypochlorite (UN 1748), studied in the standard manner in 1978. This paper describes such tests on samples varying in mass from 5 g up to 200 kg in cylindrical containers of varying aspect ratio and various materials including stainless steel gauze, high-density polyethylene and coated "bre. The heat transfer coe$cients of all the containers were measured accurately by both steady-state and cooling curve procedures and the thermal conductivity of the hypochlorite was measured independently. These measurements revealed that the Biot numbers of the samples tested were in a range where the critical ambient temperatures were sensitive to the values and thus sensitive to the convective air#ow around the test bodies. With these factors taken into account the usual Frank}Kamenetskii (F}K) plot gave a good straight line in the range 1153C upwards. However, below this temperature, i.e. in the range 40}1153C very strong deviations occur. Nevertheless, the F}K theory still holds as the low-temperature points also fall on a good straight line with activation energy 48.5 kJ/mol which compares with the high-temperature activation energy much greater than this. These results indicate a sharp change in the rate determining step for heat generation at temperatures around 100}1203C. In temperature ranges both above and below this area the temperature}time traces for the samples behave classically as would be expected from the F}K theory but inside this range the traces show much more complex behaviour with some oscillatory characteristics, also some evidence of endothermic behaviour. The consequences of this unexpected behaviour for safety (particularly in the bulk marine shipping context) are serious. The deviations predicted from simple extrapolation of the high-temperature results indicate much lower critical ambient temperatures for large quantities of this material than previously thought. Our low-temperature results
* Corresponding author. E-mail address: [email protected] (B.F. Gray). 0379-7112/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 3 7 9 - 7 1 1 2 ( 0 0 ) 0 0 0 3 0 - 8
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obtained by orthodox ignition methods are entirely consistent with independent direct isothermal calorimetric measurements already present in the literature but not used quantitatively in this context. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The most recent study [1] of these materials (anhydrous and hydrated hypochlorite), employing di!erential scanning calorimetry and thermogravimetric techniques concluded that the thermal stability data obtained showed less stability than reported in the literature. It was concluded that storage conditions were crucial for the stability of the product. It was determined that decomposition started at 603C and that the decomposition pro"le was highly dependent on the moisture content. The more water present, they concluded, the lower the temperature at which decomposition commenced and the greater the amount of heat given out. This is in marked contrast to the conclusion drawn by Mandell [2] in 1971 on the basis of a single DSC trace showing an endotherm at 503C for the hydrated material which was not present for the anhydrous material. From this trace and other entirely inappropriate tests involving cigarettes and matches on 100 g samples Mandell concluded quite unjusti"ably `It has been demonstrated that a new kind of high strength calcium hypochlorite has been made with heat sink characteristics such that it will not spontaneously propagate exothermic decompositiona. A few years later, in 1974, Clancey [3] referred to a number of serious and costly shipboard "res involving the anhydrous material over the period 1967}1973. It appears that the appearance of the &new product' and the unsubstantiated claims on its behalf turned the spotlight o! fundamental scienti"c investigation of this material although Bibby and Milestone [4] in 1984 did extensive isothermal calorimetric work on both hydrated and anhydrous materials. The rates of heat production in the temperature range 20}703C were carefully measured and the results showed very clearly that after drying over P O under vacuum for 6 d the rates of heat generation were considerably reduced. Conversely, exposure of the samples to 80% RH for 6 d signi"cantly increased the heat output. They concluded `Clearly the removal of H O from calcium hypochlorite increases its stability while adding H O decreases ita, totally contradicting the conclusions of Mandell. These workers also concluded that in DSC and similar tests the heating rate a!ected the way in which hypochlorite decomposes to a very considerable extent making such results di$cult if not impossible to interpret. The above results and the re-emergence of major losses due to marine "res involving various forms of calcium hypochlorite over the past three years clearly indicate a need for investigation of hydrated calcium hypochlorite with a view to characterising its &ignition' properties. Strictly speaking, it does not ignite on its own but it does show subcritical, critical and supercritical behaviour, i.e. runaway reaction. The adiabatic temperature rise is not large by combustion standards (a few hundred degrees) but this with the presence of free liberated oxygen is su$cient to cause
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a major and "ercely burning "re if it contacts any organic material such as wood or plastic. We will use the term ignition for convenience on this understanding.
2. Experimental For samples up to 10 kg the hypochlorite used was the product of US, Japanese and Chinese manufacturers (PPG, Toya Soda and Shanghai Chloralkali, respectively) and all were tested. No signi"cant di!erences were found between these samples. For the larger scale tests ('20 kg) only US material (PPG) was used. However, all samples showed the break in slope of the Frank}Kamenetskii (F}K) plot (see [9]) around 1203C. The laboratory oven used was a Heraeus design with forced air#ow capable of producing a spatially uniform temperature within the oven with a variation of $0.53C. This level of constancy could be maintained for inde"nite periods of time even during tests when ignition occurred for sample quantities of 10 kg or less. The tests of samples of 40 kg upwards were carried out in an oven of internal dimensions 1.2 m;1.8 m;1.8 m constructed of kiln "red concrete &Besser' blocks inserted over a welded steel frame running through their internal spaces. The latter were "lled with insulating "breglass material. The internal walls were lined with a "re retardant lining and the door consisted of a 150 mm thick steel tubular frame, thin external and internal "re retardant lining, all packed with thermal insulating material. This door was designed to vent in the event of rapid pressurisation. A 600 mm stirring fan was located at the back of the oven. Air inlet and outlet pipes together with internal heater elements of 6;850 W as well as air preheating facilities allowed accurate thermostatting of the air inside the oven even during supercritical behaviour of large samples. The spent air was taken through chlorine scrubbers charged with 50 kg slaked lime. Temperature control was achieved using a CAL controls C3200 PID ($0.13C) controller in early tests and a Brainchild BTC 9200 controller ($0.13C) in later tests. Chromel}alumel thermocouples were placed at various points in the oven to act as sensors for the control units. Data were recorded using Advantech Adam 4018 "eld point module and a TOA intelligent chart recorder. As some of the tests needed two weeks or more to complete, precautions were taken to have available generator power and custom software to monitor the mains supply and coordinate switching to generator power when necessary. The mains supply to the controlling computer and logging equipment was "ltered and stabilised. Two uninterrupted power supplies, connected in series, were used to power the equipment in the event of mains failure and before the generator was energised. Temperatures were checked with a &Comark KM 45' hand-held digital thermometer and referenced to type K KM-PRO6 sensor (National Association of Testing Authorities calibration number 5961N, traceable to Australian National Measurement Standards via Primary Standard Platinum Resistance Thermometer (SPRT) serial number 213832 report number 40562).
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The samples were equipped with a number of chromel}alumel thermocouples. Every run had a thermocouple placed at the centre of the contents and a second touching the inside wall in the case of the larger containers (40 and 200 kg). Other runs were instrumented with thermocouples placed so as to give information on spatial temperature pro"les within the material as required. The tests in the Heraeus oven were carried out with the sample placed in a central position on a thin metal stand. Those in the larger oven were placed on a wooden board, itself supported on two wood planks thus allowing air circulation underneath. Some of the 40 kg tests and all the 200 kg tests were carried out with the sample keg enclosed inside a thin-walled (2 mm) steel box (without bottom) which itself was placed in the oven. This was in order to examine the e!ect of the variation of heat transfer coe$cient from keg surface to air in nominally &still' air inside the box compared with forced convection taking place directly in the circulated oven air. The larger oven was designed to comfortably exceed the requirements for determination of the SADT by UN protocol [5].
3. Procedures The thermal conductivity of the hydrated calcium hypochlorite (PPG manufacture) was measured according to ASTM C518 and ASTM C687 at a mean temperature of 35.13C and was found to be 0.147$0.015 W/mK. This measurement was performed on the material in its standard form as delivered, i.e. granulated. A similar work carried out by Uehara et al. [6] on anhydrous material apparently used pulverised material for a similar measurement and obtained the value 0.438 W/mK. The higher resultant bulk density could account for some of this di!erence as could the hydration. The moisture content measurements were carried out in three ways, drying to constant weight over P O in vacuo, drying to constant weight over P O in air and drying to constant weight in air over silica gel. In all cases the "nal percentage (on a wet basis) was the same although the relaxation time to constant weight was much shorter for the vacuum technique. Values obtained were 8.45% by weight on a wet basis. The actual test runs were all carried out in standard fashion, i.e. the samples were placed in a preheated oven set at the required test temperature. Invariably, the sample centre temperature was lower than the test temperature giving a variable time for the sample centre to reach ambient (oven) temperature. The various temperatures of points in the sample and various points in the oven were recorded continuously by the dataloggers and recorders and complete thermal records were obtained. After each run, whether supercritical or subcritical, the sample material was discarded and new material used for the next test at the new set temperature. All the containers were cylindrical and all the gauze ones were equicylinders. The commercial containers of HDPE (40 kg) and "bre (200 kg) were slightly longer in comparison to width but this does not present any problems since the critical parameters for "nite cylinders of any height/diameter ratio are known.
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In view of the rather high value of the thermal conductivity (0.147 W/mK) the heat transfer coe$cients s of the containers were measured since the Biot number (sr/i), where i is the thermal conductivity and r the characteristic dimension, was not large enough for the F}K (in"nite Biot number) approximation to be used [9]. s was measured by two methods, "rstly by following a cooling curve in a constant temperature atmosphere and secondly (and more accurately) by using the accurately calibrated electrical power input necessary to maintain a constant temperature di!erence with the environment itself controlled to $0.13C. The power input was calculated from simultaneous measurement of the rms AC voltage and current #ow using a clamp current meter (resolution 0.01 A) and digital multitester (resolution 0.01 V).
4. Results The critical ambient temperatures (CATs) of various size samples were determined using the above procedures. Each group of tests involved determination of the highest ambient temperature at which ignition did not occur and also determination of the lowest ambient temperature at which it did occur. The arithmetic mean of these two "gures was then taken as the CAT for that particular size container under the speci"ed conditions of heat transfer. With smaller samples, in the ambient temperature range above 1203C, the supercritical reaction produces a large volume increase in the material which has the appearance of a &sou%e' and is a highly porous solid. In contrast, for the lower temperature tests involving 10}200 kg of material this was not evident. The remaining solid after ignition more or less retained the shape and volume of the original container even in the (HDPE, 40 kg) case where the container caught "re and was completely consumed. In all cases the remaining solid was extremely hygroscopic and corrosive consisting largely of calcium chloride. Table 1 gives the CATs measured for samples of Japanese manufacture (Sample A) and US manufacture (Sample B). Each critical temperature is the result of at least four, usually "ve, runs at various ambient temperatures. All results are for equicylinders except the last four. For the HDPE kegs h"0.44 m and d"2.53. For the "bre drum h"0.83 m and d"2.37. For the thermos #ask, h"0.19 m and d"2.28. In many cases in this work these values of d have to be corrected for "nite Biot number before testing the applicability of the F}K ignition theory. The standard test of the F}K theory, i.e. plot of 2 ln+¹ /r, vs. 1/¹ is shown in Fig. 1. Examination of the "gure suggests that at least two reactions are occurring and that each "ts the standard F}K theory quite well in its own range. Least-squares "tting gives the equation of the higher temperature line as
ln
2¹ (18.97$1.85)1000 "(66.12$4.55)! r ¹
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Table 1 Critical ambient temperatures Sample
Radius (m)
Critical ambient temperature (3C)
A A A A A A B B B B B B B B B B B
0.00925 0.01875 0.02875 0.055 0.075 0.105 0.00925 0.01875 0.02875 0.055 0.075 0.105 0.175 0.175 (HDPE) 0.175 (HDPE, still air in box) 0.275 ("bre, still air in box) 0.055 (thermos)
154.2$2.5 141.5$2.5 128.5$2.5 123.0$2.5 101.0#2.5 90.6$2.5 149.5$2.5 135.5$2.5 126.5#2.5 120.5$2.5 102.5$2.5 87.5$2.0 64.0$3.0 60.1$0.5 55.2$1.3 43.4$1.3 57.3$2.0
Fig. 1. F}K plot of raw data.
giving an apparent activation energy of 158$15 kJ/mol. Of rather more interest from the practical point of view is the lower temperature region corresponding to larger size containers, up to and including 200 kg. The least-squares "t for this section is given by
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Table 2 Heat transfer characteristics Keg type
s (natural) (W/m K)
s (forced) (W/m K)
Bi (natural)
Bi (forced)
40 kg HDPE 200 kg "bre
10.3$0.4 7.9$0.4
16.4$0.4 *
12.3$0.5 14.8$0.5
19.6$0.5 *
the equation
ln
2¹ (4.9$0.5)1000 "(29.3$1.5)! r ¹
giving an apparent activation energy of only 41$4 kJ/mol. For the containers in this temperature range we have directly measured the heat transfer coe$cients by both steady-state methods (with a constant electrical input and measurement of the steady temperature di!erential set up against ambient) and cooling curve methods. Results from both methods agree well and are given in Table 2. The Biot numbers are calculated from the heat transfer coe$cients using the standard formula sr Bi" , i where i is the thermal conductivity of the hypochlorite and r is the radius of the keg. The value of i is 0.147 W/mK at 353C and 0.142 W/mK at 233C. Throughout this work we will use the higher value to calculate Biot numbers since we are most interested in ambient temperatures in the 30}403C range. These measurements allow recalculation of the data to include correction for the "nite Biot numbers encountered using the formula obtained by Barzykin and presented by Bowes [9]. The corresponding values of d are of course signi"cantly smaller than the values of d but more importantly they are quite sensitive to container size and hence a!ect the apparent activation energy obtained from the F}K plot. Gri$ths et al. [7] and more recently Jones [8] have measured the heat transfer coe$cients and Biot numbers directly for stainless steel gauze containers in laboratory ovens with forced convection and from this work the most reliable value for s is 14.0 W/m K. If we calculate the critical parameters using this value and the data in Table 2 for the larger containers we can re-plot according to the equation >"ln d #2 ln(¹ /r)"A!E/R¹ using the data given in Table 3. The last two rows in Table 3 represent kegs standing inside thin sheet metal steel boxes (heat transfer coe$cients measured as 4.5 W/m K) which themselves are placed
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Table 3 Biot number corrected data Radius (m)
1000/¹
0.00925 0.01875 0.02875 0.055 0.075 0.105 0.175 (gauze) 0.175 (HDPE, forced) 0.175 (HDPE, nat) 0.275 ("bre, nat)
2.366 2.448 2.503 2.541 2.663 2.774 2.967 3.002 3.047 3.160
ln d
Y
!0.27 0.17 0.40 0.67 0.75 0.825 0.899 0.829 0.54 0.60
21.19 20.15 19.48 18.41 17.79 17.11 15.88 16.08 15.61 14.69
inside the oven on a wooden base. The overall Biot numbers for each assembly of keg and box were calculated using the normal assumption that the two thermal resistances were in series and therefore additive. Temperature di!erences across the two surfaces involved were su$ciently small for the assumption of conductive heat transfer to be valid. The practical reason for this arrangement for the larger bodies was that this con"guration resembles closely the actual situation of kegs inside a shipping container where they will stand in reasonably quiescent air. The container itself will normally be in a ventilated hold. Of course in the latter case the kegs inside the container will interact with each other as they are all dumping heat into the same airspace and this e!ect will be signi"cant. Such interactions are presently being studied and further results on this topic will be presented later. A plot of this data is shown in Fig. 2. Least-squares "tting of these data gives the equation
2¹ (5.85$0.22)1000 #ln d "! #(33.33$0.64) r ¹ which shows considerably reduced deviations compared with the best "t for the raw data. The corresponding quantities for the high-temperature plot are 15.10$2.03 and 57.02$5.01. Also, the apparent low-range activation energy is increased to 48.5$ 1.8 kJ/mol. From this plot we can also deduce the pre-exponential heat production rate ln
QZ"7.48;10 W/m, where Q is the heat of reaction and Z is the Arrhenius pre-exponential factor. With the above value for E/R we can calculate the heat production rate at any temperature that is implied by the CATs. For example at 503C we obtain the value of 120 W/m which compares satisfactorily with the results of Bibby and Milestone [4] whose graphical results indicate a heat output in the region 100}200 W/m measured by a completely di!erent method (isothermal calorimetry). In contrast, extrapolation
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Fig. 2. F}K plot of the data corrected for "nite Biot number.
of the high-temperature results down to the low-temperature regime implies a "gure around 0.01 W/m at 503C, so this reaction is completely negligible as a thermogenerator in the range of temperatures likely to be important in practical situations. The high-temperature plot corrected for "nite Biot numbers (using the heat transfer coe$cients of the gauze baskets as measured in [7, 8]) satis"es the F}K equation
2 ln
¹
(15.10$2.03)1000 #ln d "! #(57.02$5.00). r ¹
This translates to an activation energy of 125 kJ/mol compared with the uncorrected value of 152 kJ/mol. Gri$ths et al. [7] found a similar correction to high-temperature data for bagasse and wood#our when this correction is taken into account. Subcritical and supercritical temperature time traces are shown in Fig. 3 for the 35 kg stainless steel gauze basket, the largest of the orthodox containers used. The usual divergence between sub- and supercritical heating is clear here but it is not possible to predict supercritical behaviour until 45 h after the start of the run even when the initial temperature of the hypochlorite is 253C. At an initial temperature of 153C, 60 h elapse before it is clear whether runaway will occur. Two supercritical runs in 40 HDPE kegs are shown in Fig. 4. In the HDPE plastic kegs there are two &critical conditions' in the sense that the "rst transition from hypochlorite self-heating of around 20 to '1503C, the one we are
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Fig. 3. 35 kg gauze basket, ambient temperatures of 68 and 603C.
Fig. 4. At 60.13C temperature rise is '9503C due to ignition of HDPE keg. At 59.83C temperature rise is 1503C without plastic ignition, but supercritical for hypochlorite.
studying in detail here, may or may not ignite the keg itself. In the run at an ambient temperature of 60.13C the hypochlorite self-heated to around 1753C and then remained almost stationary for 30 min before the extremely rapid temperature jump up to approximately 10003C occurred. This was accompanied by visible #ame and complete destruction of the keg (see Fig. 4).
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Fig. 5. 40 kg keg * subcritical and supercritical temperature traces.
In the run at an ambient temperature of 59.83C the temperature rose to approximately 1503C and levelled out. This did not produce ignition of the keg as before. However, inspection of the keg after cooling revealed that considerable charring and melting had occurred. Centre temperatures for three runs spanning criticality for a 40 kg HDPE keg inside a steel box, with a wooden #oor (dimensions 0.41;0.41;0.48 m) are shown in Fig. 5. In these runs the samples were in a condition of natural convection only inside the box, which was itself subject to the forced convection inside the oven. The critical temperature in this case turned out to be 55.23C. In this case the resistance to heat loss from the sample is considerably increased by virtue of the decreased heat transfer coe$cient from the keg to naturally convecting air inside the box and the series thermal resistance represented by the heat transfer through the box wall to the ambient air in the oven. In Fig. 6 a pro"le of the supercritical run is shown. Whilst the centre temperature reached 1903C, the half-radius temperature reached only 1303C in this case. The keg itself did not ignite although it was distorted and partially charred. In Fig. 7 a run performed with no internal thermocouples at all, is shown. It was instrumented with thermocouples at the top and bottom, externally mounted in the centre of the two faces. The keg was tested with the manufacturer's seal unbroken; thus there was no possibility of either contamination or catalysis by the thermocouple junction. This keg was supercritical and ignited in the usual way with, as would be expected, a larger temperature rise on the top face than on the bottom face (1263C cf. 803C). Ambient temperature in this case was 58.13C and the HDPE was badly damaged.
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Fig. 6. 40 kg keg * centre, half-radius and ambient temperature traces for ¹ "56.63C.
Fig. 7. Unopened 40 kg keg in box with external thermocouples only, as shipped, ¹ "58.13C.
In Fig. 8 the centre temperatures for two subcritical runs and one supercritical run are shown for a 200 kg "bre container, also inside a steel box with a wooden #oor of dimensions 0.612;0.612;0.952 m. The centre attained nearly 2003C in the supercritical run with an ambient temperature of 44.73C and the "bre keg was destroyed, falling to pieces when attempts were made to remove it from the oven.
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Fig. 8. Supercritical and subcritical runs for a 200 kg drum in a box. Time to ignition from an initial temperature of 36.43C was 7.5 d (see text).
The time to reach 36.43C, the temperature at the start of Fig. 8, from the storage temperature of 183C was a further 200 h (8.3 d). Thus, the &time to ignition' for a single 200 kg "bre drum containing hypochlorite at 183C would be of the order of two weeks when subject to an ambient temperature of 44.73C. Fig. 9 shows a supercritical pro"le across the drum. It also shows the response of the temperature controller for the oven to the ignition of 200 kg inside. The slight increase in ambient temperature was quickly corrected even with this large extra heat release inside the box. Drying of samples of the hypochlorite under vacuum with phosphorus pentoxide, under air with phosphorus pentoxide and under air with silica gel gave moisture content of 8.45% by weight on a wet basis in all cases although the vacuum treatment produced equilibration much more quickly than the other two methods. The silica gel treatment took almost three weeks to reach equilibrium. These "gures are within the range required for UN 2880 speci"cation.
5. Discussion Clearly, the reaction mechanism for hypochlorite decomposition is complex but nevertheless exhibits apparently simple behaviour over each of two limited temperature ranges where the F}K theory "ts the experimental results rather well. Two parallel reactions, both exothermic, appear to be taking place. One dominates at higher temperatures (the higher activation energy reaction) but the higher activation energy e!ectively cuts out this reaction in the lower temperature range where the
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Fig. 9. Temperature pro"le for 200 kg drum in a box, ¹ "44.73C.
low-temperature low-activation energy reaction dominates. Cases such as this have been known for many years and are discussed in Bowes [9] where a bilinear F}K plot similar to Fig. 1 is displayed. The relatively high thermal conductivity of the substrate in this case (0.147 W/mK compared with typical agricultural materials of 0.05 W/mK) leads to an interesting situation with respect to the sensitivity of critical ambient temperatures to the nature of the air#ow in the nearby vicinity. As is well known, critical conditions are relatively insensitive to Biot number when the latter is greater than 20 and the latter is a measure of the relative resistance to heat #ow within the body and from the body to the surroundings. It follows that for smaller Biot numbers the critical parameters will be sensitive to the heat transfer characteristics of the surroundings e.g. air #ow velocity, degree of turbulence, presence of natural convection, etc. This is clearly shown at the experimental level in Table 1. The CAT for a 40 kg HDPE keg in the forced convection of the oven at 60.13C is signi"cantly higher than that for a similar keg in still air inside a steel box (55.23C). The change in CAT can be calculated in such cases by using the simple theory of heat transfer where thermal resistance in series is taken to be additive. The heat transfer coe$cients for the keg to still air and from internal to external air through the wall of the steel box have been measured experimentally. The calculated change in CAT agrees quite well with the measured value (4.93C). For reasons of cost and time we did not carry out a similar comparison for 200 kg drums, restricting measurements to drums inside a steel box. A similar calculation of
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the total thermal resistance assuming series resistance gives an e!ective Biot number for this case also and from this e!ective Biot number it is simple to calculate the critical value of d using the equation of Barzykin quoted in detail in Bowes [9]. From this Fig. a &corrected' point for the F}K plot can be obtained. Such points are shown in Fig. 2 and the standard deviations for the &corrected' straight line are considerably smaller than for the raw data, again indicating that this simple interpretation is reliable. Of course corrections for "nite Biot number are larger for smaller samples; so the data obtained by Uehara et al. [6] which were uncorrected, are questionable as they did not measure any heat transfer coe$cients and quoted a very high value of 0.44 W/mK for the conductivity of anhydrous hypochlorite, which also may have been pulverised for that particular measurement. Similar comments apply to our high-temperature results. We have not calculated corrections for these as we do not use any of the data for predictions as to lowtemperature large-size CATs as the reaction mechanism switch totally invalidates any extrapolation from this region to lower temperatures. The highest degree of subcritical self-heating measured was in the gauze 35 kg equicylinder with a Biot number of 10.2 with a "gure of 243C at 60.63C. At the critical temperature the in"nite Biot number for this critical temperature rise is 1.77R¹/E with the value 333C, probably around 10% smaller including the Biot correction. Allowing for the fact that we were a few degrees below criticality at 60.63C, this agreement is satisfactory. Times to ignition are seen in the cases of larger samples to be determined to a considerable extent by heat transfer rates, particularly with initial sample temperatures below 203C which is common in commercial practice. The largest (200 kg) sample took close to two weeks to ignite starting from 183C and one week after reaching ambient temperature. However, we would be very hesitant to apply any of the semi-empirical formulae for times to ignition derived on the basis of thermal ignition theory as in this case there are indications that radical chain reactions are involved, particularly at low temperatures. The sensitivity to small amounts of impurities and water points in this direction, as does the obvious change in mechanism occurring at intermediate temperatures. Such features need not invalidate the application of the F}K theory to the critical condition itself which will still re#ect a limiting heat balance condition however complex the chemical kinetics may be as shown by Gray as long ago as 1969 [10]. At the same time, the latter can have a profound and complex e!ect on time to ignition factors which are much more di$cult to predict than the critical condition itself.
6. Conclusions (1) The thermal decomposition of hydrated calcium hypochlorite involves two distinct reactions (at least) each of which separately obeys the F}K relationship for thermal ignition.
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(2) As must be the case, the reaction dominating in the low-temperature range ((903C) has a rather low-activation energy of 48.5$1.8 kJ/mol. (3) The pre-exponential heat release rate for this reaction is 7.48;10 W/m. (4) The thermal conductivity of the hydrated calcium hypochlorite is 0.147 W/mK. (5) The relatively high thermal conductivity places the Biot numbers for most container materials and container sizes in the region where the CATS are sensitive to heat transfer characteristics to the surrounding medium. (6) For the low-temperature reaction, correction for relatively small Biot numbers gives an F}K plot which has smaller standard deviations than the raw data plot. (7) For prediction of CATs of practical containers of 40 kg or greater traditional high-temperature measurements give extremely dangerous predictions when extrapolated to lower temperatures due to the bilinear nature of the F}K plot. (8) A 40 kg HDPE keg has a CAT of 60.13C when immersed in forced convective air in a thermostatted oven. (9) A similar 40 kg keg when inside a steel box, itself immersed in the same oven, has a CAT of 55.23C. (10) A 200 kg "bre drum when inside a steel box itself in contact with forced convective air on the outside, has a CAT of 43.43C. (11) These results in conjunction with the low activation energy indicate that the transport of commercial quantities of 13}18 tonnes of hypochlorite kegs or drums inside shipping containers will result in assembly CATs well below the above "gure and probably in the region of likely temperatures in ships holds particularly in tropical waters. (12) Times to ignition, which are of the order of two weeks for a single 200 kg drum, may be somewhat longer than this for such a commercial assembly and further study in this area is required. The time to ignition in these cases is for practical purposes measured from the initial temperature of the hypochlorite and not from when it reaches ambient. The crucial comparison is between this "gure and the length of time the assembly is exposed to hold temperatures on the voyage. Supercritical conditions may well not be a danger in some circumstances but virtually no research has been published on these topics. References [1] Cardillo P, Nebuloni M. Reactivity and thermal stability of calcium hypochlorite. Rev Combust 1994;48:300}5. [2] Mandell Jr HC. A new calcium hypochlorite and a discriminatory. Test Fire Technol 1971;7(2):157}61. [3] Clancey VJ. Fire hazards of calcium hypochlorite. J Hazardous Mater 1975/1976;1:83}94. [4] Bibby DM, Milestone NB. The decomposition of high grade bleaching powder. J Chem Tech Biotech 1984;34A:423}30. [5] Transport of dangerous goods and materials, Manual of tests and criteria. New York and Geneva: United Nations, 1995. [6] Uehara Y, Uematsu H, Saito Y. Thermal ignition of calcium hypochlorite. Combust Flame 1978;32:85}94.
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[7] Gri$ths JF, Hasko SM, Tong AW. Thermal ignition in packed particulate solids: criticality under conditions of variable Biot number. Combust Flame 1985;59:1}9. [8] Jones JC, Wade M. Direct determination of the Biot number in oven heating tests. J Fire Sci 1999;17:421}9. [9] Bowes PC. Self-heating: evaluating and controlling the hazards. London: HMSO, 1984. [10] Gray BF. Uni"ed theory of explosions, cool #ames and two-stage ignitions * I. Trans Faraday Soc 1969;65:1603}13.