Calorimetric Thermal Hazards of tert-Butyl Hydroperoxide Solutions
Yih-Wen Wang
a* and Chi-Min Shu
baDepartment of Occupational Safety and Health, Jen-Teh Junior College of Medicine, Nursing and
Management, 79-9, Sha-Luen-Hu, Xi-Zhou-Li, Houlong, Miaoli, Taiwan 35664, ROC
bProcess Safety and Disaster Prevention Laboratory, Department of Safety, Health, and
Environmental Engineering, National Yunlin University of Science and Technology, 123, University Rd., Sec. 3, Douliou, Yunlin, Taiwan 64002, ROC
Hazards evaluation was performed to investigate the thermal instability and incompatibility of tert-butyl hydroperoxide (TBHP) solution with various diluents. Two calorimeters, differential scanning calorimetry (DSC) and vent sizing package 2 (VSP2), are frequently used to characterize the inherent thermal hazards and adiabatic runaway features of organic peroxides. Through the thermal dynamic and isothermal scanning experiments, the enthalpy of energy-rich TBHP solutions to normalize various heats of decomposition was elucidated. The self-reactive rating of a runaway reaction can be characterized by the thermokinetic parameters, such as initial exothermic temperature (T0),
self-heating rate (dT∙dt−1), pressure rise rate (dP∙dt−1), pressure-temperature profiles, non-condensable
pressure, etc. We also suggested using the isothermal tests of DSC combined with the kinetic data to evaluate the exothermic enthalpy and reaction aging time of aqueous TBHP. The novel finding was that aqueous TBHP with DSC and VSP2 was observed to possess intrinsic qualities leading to thermal explosion, with a sharp self-reactive rate and pressure rise under adiabatic conditions. In summary, the thermal hazards of aqueous TBHP may cause a catastrophe or large loss in operation, storing or transporting TBHP. This result also demonstrates that applying calorimetric methodology to classify the thermal hazards of organic peroxides is an alternative technology for research.
1. Introduction
Organic peroxides caused a severe accidental explosion on January 08, 2010 in Taiwan (Figure 1)1.
Organic peroxides are mostly used as catalysts and promoters that accelerate decomposition in the
petrochemical industries. In fact, two applications, initiation of free radical polymerization and use as
a curing agent, account for more than 90% of the total consumption of organic peroxides in Taiwan.
Hydroperoxides, such as hydrogen peroxide, cumene hydroperoxide (CHP) and tert-butyl
hydroperoxide (TBHP), are extremely sensitive and are incompatible with thermal sources or
impurities.2 The thermal decomposition of organic peroxides proceeds partly through the peroxy
group (−O−O−), which can be broken readily, leading to a tendency toward several kinds of
spontaneous decomposition. As a class, organic peroxides are exceptionally sensitive to violent decomposition induced by thermal sources, mechanical shock and incompatibility. The various
hazards of organic peroxides require that such chemicals be handled and stored with adequate safety
precautions. A reactive hazard classification system for organic peroxides is now proposed, based on
our previously developed calorimetric methodology and on several newly developed experiments that
will be discussed in detail.3,4
Figure 1. An explosion of organic peroxides on January 08, 2010, in Taiwan.
There are many organic peroxides with an extraordinarily broad range of safety-related properties,
and therefore, manufacturers and users are concerned with thermal hazard identification problems. TBHP is a typical alkyl hydroperoxide in industrial applications, used as a curing agent or a free
radical initiator over a wide temperature range under appropriate redox reaction conditions for the
polymerization of acrylates, acrylic resins, coatings, engineering plastics, fiberglass reinforced
unsaturated polyesters, vinyl ester resin curing and water-borne acrylics in the petrochemical industries. The homolytic decomposition of hydroperoxides has been discussed in terms of the
reaction mechanism, but few studies have fully explored the thermal decomposition hazards.5−8
Commercially, TBHP is used mostly as an aqueous solution of 70 mass % TBHP and 30 mass %
water. Solutions of TBHP in water and in most hydrocarbon solvents are widely prepared and in
prevailing use. Its solution status makes it a versatile oxidation agent for selective aqueous or
non-aqueous oxidations.
The measurement of storage stability for potentially unstable materials is the temperature at which
the uncontrolled decomposition reaction can be initiated under actual storage configurations. Organic
peroxides have thermally unstable properties and may undergo self-accelerating decomposition. The
U.S. Department of Transportation (DOT) has established regulations that an organic peroxide supplier must perform a precise test of SADT (self-accelerating decomposition temperature) in any
commercial package.9 The lowest temperature at which the self-accelerating decomposition of a
substance in the original packaging can occur is the SADT, given the conditions for shipment with
regard to packaging, labeling, maximum package size, type of carrier, etc. The DOT requires stability
at 54.4 oC (130 oF) for the duration of the shipment. However, it has not yet set a recommended
maximum temperature for normal operation or for an emergency relief system during upsets.
By the adoption of the National Fire Protection Association (NFPA) 432, the Code for the Storage
of Organic Peroxide Formulations, the manufactures have officially recognized the U.S. code for
grouping commercial organic peroxides by hazard class. NFPA 432, which defines safe storage and
fire fighting methods, has regulated the quantity, conditions for storage, solvents for dilution, and material for packaging and has described the types of hazards of commercial organic peroxides. The
description given by the NFPA suggests five classes of organic peroxides and provides rigorously
controlled parameters for temperature, contaminant, confinement and quantity of each class in
a Class II (severe reactivity and fire hazard) in di-tert-butylperoxide (DTBP, an organic peroxide produced by TBHP reacting with acid) and tert-butanol, or a Class IV (ordinary combustible)
formulation in water. This study examined the hazardous properties of TBHP solution with different
diluents under thermal decomposition by calorimetric methodology. The commercial formulation of
aqueous TBHP 70 mass % solution requires elevated temperatures and pressures for decomposition to
occur and is considered to undergo ordinary combustion. A 55-gallon drum is the maximum
individual container size for storing TBHP 70 mass % solution,10 which has a high potential for shock
sensitivity or thermal instability. An NFPA 325M reactive rating of 4 for TBHP signifies its highest
reactive category, indicating a material that in itself is readily capable of detonation or of explosive
decomposition or reaction at normal temperatures and pressures (NFPA 704).11
An SADT should be provided to decide if a self-reactive substance should be subjected to temperature control or other preventive measures during transportation. Thermally reactive materials
must be stable at 55 oC for one week under SADT test conditions to be acceptable for transport with
temperature control or other available means. The United Nations Committee of Experts on the
Transport of Dangerous Goods (UN TDG) provides the guidelines for the measurement of SADT
values for large amounts of product. For safe handling procedures, transportation and storage, a better
understanding of the basic properties of organic peroxides is required. Misinformation or inadequacy
in handling organic peroxides, particularly as applied to process safety, should be prudently taken into
account. Commercial organic peroxide formulations are classified as Types A to G, which identify the
hazards of organic peroxides in accordance with the classifiable principles suggested by Division 5.2
of the TDG, which has classified TBHP as Organic Peroxide Type F with an SADT of 55 oC; the
suggested storage temperature should be controlled at less than 30 oC during transportation. The TDG
has also suggested that the concentration of TBHP solution suitable for transportation in tank
containers should not exceed 72 mass % with water (suggesting that steps should be taken to achieve
receptacle), and the SADT of a 25 or 190 kg package is 80 oC.12
The identification and rating of the hazardous properties of hydroperoxides have been the subject of
only a few studies. Commercial organic peroxide formulations release a large amount of
decomposition energy and may cause explosive accidents without adequate safety precautions. The
thermodynamic, kinetic, and physical-chemical parameters of these thermal unstable organic
peroxides need to be clearly determined, and more effort is needed for reactivity, self-reactivity and
incompatibility studies. The aim of this study was to compare various commercial TBHP solutions
using calorimetric methodology and simplified analytical equations to rank the thermal hazards of
self-reactive decomposition. By understanding the different safety-related properties of TBHP
solution in various diluents, the reactive or self-reactive hazards of TBHP solutions might be used as
an additional consideration to the formulation of NFPA and TDG.
2. Materials and Methods
2.1. Samples. In this study, we selected commercial TBHP with various diluents, which are recommended in the NFPA 432 code, to recognize the thermal hazards rating of TBHP solutions in
the industry. TBHP solutions of 70 mass % in water, DTBP 98 mass %, and tert-butanol were
purchased from Merck Co. Solutions of 5.5 M TBHP in nonane and 5.5 M TBHP in decane were
directly acquired from Fluka, Sigma-Aldrich Co. All of these solutions were stored in a refrigerator at
4 oC.
2.2. DSC (Differential Scanning Calorimetry). The dynamic and isothermal temperature-programmed screening experiments were performed on a Mettler TA8000 system DSC821e apparatus
with a high-pressure gold-plated crucible (ME-26732), which was sealed manually by a special kit
thermoanalytical software was used to obtain thermal curves and to evaluate the chemical kinetics.13
In a typical DSC experiment, a small amount of sample (1−10 mg) was placed in a closed crucible,
and the heating rate for the temperature-programmed ramp was 4 oC per minute, from 30 to 300 oC, to
sustain better thermal equilibrium.
A series of isothermal experiments were performed at various temperatures in a DSC. The oven
was preheated to the setup temperature with the reference crucible in place. The sample crucible was
then placed into the oven, and the experiment began once thermal equilibrium was achieved. On each
thermal curve, the maximum heat flux, which represents the worst case, was therefore measured.
2.3. VSP2 (Vent Sizing Package 2). Vent sizing package 2, manufactured by Fauske & Associates, Inc., is a PC-controlled adiabatic calorimeter with a pressure and temperature system that balances internal and external pressure and temperature.14 VSP2 has been developed by the AIChE’s
Design Institute for Emergency Relief Systems (DIERS) user group to review the application and
further contribution to the safe design of process reactors, in particular the vent sizing of reactors. An
adiabatic calorimeter with a low heat capacity of the test cell essentially ensures that all of the
released reaction heat remains within the tested sample. The very thin-walled sample container of the
VSP2 has an inside volume of about 116 mL, and the thermal inertia factor ( ) of the test cells is
from about 1.05 to 1.20. It can be used to assess the thermal data and then be directly extrapolated to
the process conditions. For a reactive system, overpressure in a vessel is normally due to the heat of
the reaction, which obeys the overall energy balance of the reactants and products. Both pressure and
temperature equalization techniques are used. The former enables the test cell to maintain its integrity at elevated pressure, and the latter enables the temperature of the sample cell and outer guard
container to remain equal to the usual adiabatic temperature. The pressure is developed in the closed
test cell and is automatically compensated by pressure of equal magnitude in the enclosed pressure
20 mass % were deliberately selected as the concentrations for the VSP2 experiments. A 50 g sample was injected into a stainless steel 316 cylindrical test cell with a net weight of 26 g. A standard
heat-wait-search (HWS) procedure was followed to conduct the adiabatic runaway test in the temperature
range from 30 to 400 oC. If there was any prominent temperature or pressure increase with respect to
time, the program terminated the HWS step, switched off the main heater and turned on the guard
heater to closely track any runaway reaction.
For a runaway reaction, overpressure in a vessel is normally due to the heat of reaction, which
obeys the runaway of the energetic chemicals such as unstable organic peroxides. The severity of a
runaway reaction is usually ranked by the self-heating rate or pressure rise rate. These calorimetric
methods may be used to estimate the actual self-heating rate of organic peroxides because they
determine the heat of reaction ( H△ ), which is the exothermic heats of decomposition are usually represented as being negative, and the related Arrhenius parameters, such as activation energy (Ea),
order of reaction (n), heat capacity (Cp), rate constant (k) and frequency factor (A), etc. To determine
reliable thermal decomposition kinetic data, the heat of reaction must be measured by an excellent
adiabatic calorimeter or calculated from chemical kinetic parameters.15−17
3. Results and Discussion
3.1. Thermal Decomposition of TBHP. There is general agreement that organic peroxides are the critical autocatalytic intermediates formed in the oxidation reaction. Hydroperoxide is believed to be
more reactive than other organic peroxides. The hydroperoxy group is intrinsically unstable or
incompatible with thermal sources, acids, bases, and metal ions via the decomposition pathway of free radicals or ionic species. Alkenes, alcohols, amines, carboxylic acid, etc., also induce the
decomposition of hydroperoxides. Thermal decomposition of all typical hydroperoxides to alkoxy and
hydroxyl radicals by homolysis is induced by metal ions or other sources of free radicals. The reaction
hydroperoxides were too fast and the activation energies of the reaction were too low. The hemolytic thermal dissociation of the intermediate hydroperoxides is as follows:18
OH RO
ROOH (1)
Although the hydroperoxide peroxy bond is weak, the thermolysis of alkyl hydroperoxides is
usually complicated by induced decomposition. Decomposition in alkane at 170–180 oC appears to be
largely induced, even at the lowest initial concentrations of TBHP. In both alkylbenzenes and alkanes,
RO• and HO• radicals from homolysis of the peroxide readily generated solvent radicals. However,
for TBHP in alkanes, the alkyl radicals were too reactive to accumulate.19 The enthalpy and
temperature for the thermal decomposition of TBHP in n-octane were 43.0 kcal∙mol−1 and 120–160 oC, as determined by Hiatt et al.7
TBHP can be characterized as decomposing rapidly upon heating and under the influence of light, and it reacts violently with incompatible substances or ignition sources (acids, bases, reducing agents,
and metal ions) to cause fire and explosion hazard because of its hydroperoxide group (–O–O–H).
The presence of atmospheric oxygen could likely make the decomposition of TBHP more energetic. It
is made from the reaction of tert-butanol with hydrogen peroxide:19
O H OH O C CH O H H O C CH H 2 3 3 2 2 3 3) ( ) ( (2)
Furthermore, the production of DTBP by TBHP-acid mixtures can be described by the following
reactions:20 2 2 3 3 3 3 3 3 3 3 3) ( ) ( ) ( 3 CH COOHH CH COO CH CH COCH CH OHH O (3)
It should be kept away from reducing agents and incompatible substances. The reaction products
without additional oxygen and the heat of decomposition are as follows:
1 1 ) ( 2 ) ( ) ( 4 ) ( 2 3 3) 1.5 2.5 2 72.2 (CH COOH l CH g Cs H Og Hd kcalg mol (4)
Furthermore, the heat of combustion is calculated from the heats of formation of the material, and
1 1 ) ( 2 ) ( 2 ) ( 2 ) ( 3 3) 5.5 4 5 595 (CH COOH O CO H O H kcalg mol c g g g l (5)
Hydroperoxide is well known to follow a violent exothermic decomposition reaction with a
complex mechanism and initiator/catalyst.11 In particular, the self-reactive decomposition of
free-radical compounds releases heat because of rising temperature. The photolytic or thermal
decomposition of TBHP, either in liquid phase or in inert solvents, occurs with the liberation of
oxygen and the formation of tert-butyl alcohol. Based on the known heat and free energy of formation
in the reaction, the following hypothetical thermal or energetically reasonable decomposition reaction
has been proposed:19,21
OH CH COCH CH OH CO CH H O O C CH3)3 ( 3)3 3 3 3 ( (6) O H CO H C H O O C CH3)3 3 8 2 ( (7)
The energy value actually released during thermal decomposition demonstrates the severe hazards
of many peroxides containing stored chemical energy. The induced reactions depend mostly on
competitions between nonterminating and terminating interactions of peroxy radicals. Furthermore,
the competitions rely on the hydroperoxide, solvent and temperature.
3.2. Dynamic/Isothermal Experiments on DSC. The most important advantage of DSC is the ability to measure decomposition energies. The heat flux type of DSC was used to associate examine
the relationship of temperature with the decomposition of TBHP solutions by thermal dynamic
scanning. Thermal curves detected by DSC provide thermal stability data such as the initially
exothermic temperature (Ti, the temperature at which the thermal curve deflects from the base line),
peak temperature, maximum heat-releasing peak and heat of decomposition ( H△ d) of TBHP with
various diluents, which were simply acquired by repeated tests with DSC. Figure 2 shows typical
thermal flux curves versus temperature for the exothermic decomposition of TBHP with various
diluents: water, nonane, decane, DTBP and tert-butanol. By thermal analysis, the thermal curve and
with various diluents; typical trials via repeated tests are listed in Table 1.
Table 1. Thermoanalytical Data on TBHP Solutions in Various Diluents in DSC Trials
Sample Diluent Weight,
mg Ti, °C H △ d, J∙g−1 Peak temperature, °C TBHPa Water 4.4 75.0 −1622.4 161.7/234.6 TBHPb Nonane 8.2 75.0 −1438.8 172.9 TBHPb tert-Butanolc 7.4 100.0 −753.2 161.3 TBHPd Decane 7.9 60.0 −1618.2 167.8 TBHPd DTBPe 6.8 75.0 −1229.9 159.9 DTBP N/A 7.0 100.0 −1347.9 182.2
a TBHP 70 mass % in water. b TBHP 5.5 M in nonane (C
9H20). c TBHP in nonane: tert-butanol
((CH3)3COH) = 1:1. d TBHP 5.5 M in decane (C10H22). e TBHP in decane: DTBP (C8H18O2) = 1:1.
Figure 2. Thermal curves of TBHP solutions in various diluents: (a) water, (b) nonane, (c) decane, (d) DTBP and (e) tert-butanol in DSC trials.
The thermal analysis of various TBHP solutions must evaluate the initially exothermic temperature,
the heat of decomposition, which is the integral of the exothermic peak and describes how much
energy is liberated, and the outline of the exothermal curve. Therefore, the two parameters Ti and H△ d
could be used to classify the thermal decomposition of various TBHP solutions. From the
experimental data, the lowest initial exothermic temperature of decomposition of TBHP in decane
was ca. 60 oC. The initial exothermic temperatures of TBHP solutions in water, in nonane, in
tert-butanol, and in DTBP were 75, 75, 100 and 75 oC, respectively. The higher heats of decomposition of
TBHP/water and TBHP/decane solutions were integrated and normalized to be −1622.4 and −1618.2
J∙g−1, respectively. The heat of self-reactive decomposition of aqueous TBHP 70 mass % measured by
DSC in this study is in good agreement with similar values from the literature.19 The initial
exothermic temperature can be considered to be the melting point, and the thermal decomposition or
different sample cells studied by Ando et al.22 DTBP is also an alkyl peroxide, and NFPA has
suggested it as a solvent for dilution of TBHP solutions. However, we could not realize the NFPA
recommended concentration of DTBP as a solvent, and the safety-related properties of TBHP solution
with DTBP should be taken into account. TDG suggests TBHP + DTBP (~82 + 9 mass %) in water (7
mass %) solution for the OP5A packing method.12
It is well known that different additives can influence the self-reactive decomposition behaviors of
organic peroxides. The thermal curve showed that the exothermic patterns were variable, and the
highest heat of decomposition of aqueous TBHP demonstrated a complicated reaction mechanism of
two exothermic peaks, which indicates that it will result in the highest heat of exothermic reaction in
the case of unexpected events during operation, transportation or storing.
Furthermore, we were able to evaluate the thermal hazards of energetic chemicals by the analytical equation-related adiabatic calorimetry in our previous studies.17,23 Two parameters, T
i and H△ d of the
thermal curves in the DSC trials, combined with the simplified analytical equations and the related
physical-chemical properties, were proposed to establish an adequate emergency relief system.
The dynamic method described above does not lead to a full understanding of the complex
decomposition mechanism. On the other hand, isothermal testing does eliminate the thermal lag effect
and maintain sound thermal equilibrium using a thermodynamic model. In this study, the main stress
fell on the calorimetry analytical equations to demonstrate the self-reactive results that contradicated
the complexity of thermal decomposition, which were in good agreement with those acquired from
the DSC isothermal curves associated with its related intrinsic kinetics. Aqueous TBHP was selected
for the DSC isothermal test because of its high thermal instability in various TBHP solutions. The isothermal trials recorded the heat flux of aqueous TBHP 70 mass % at several temperatures (in 4 oC
steps) to find related kinetic information about the thermal reaction behaviors. Table 2 lists the
experimental data from the isothermal decomposition of aqueous TBHP, showing the maximum heat
TBHP depicted in Figure 3, the isothermal induction time of the thermal curves decreases as the temperature increases. Clearly, the curves show the autocatalytic progress that is typical for aqueous
TBHP thermal decomposition reactions.24
Table 2. Isothermal Analytical Data on Aqueous TBHP 70 mass % in DSC Trials
Sample Isothermal temperature, °C Weight, mg Peak height, mW △Hd,, J∙g −1 Time of thermal decomposition, min TBHP 120 6.63 9.43 −382.6 14 TBHP 130 6.93 14.22 −424.9 18 TBHP 140 5.79 14.96 −473.6 12 TBHP 180 5.78 43.75 −467.4 6
Figure 3. DSC isothermal measurements of heat flux versus time of aqueous TBHP 70 mass % at various temperatures.
3.3. Adiabatic Runaway Reaction by VSP2. Aside from the qualitative similarity detected in DSC dynamic thermal scanning, these adiabatic exothermic behaviors are much more quantitatively
similar in VSP2 trials, which provide time (t)-temperature (T)-pressure (P) profiles for runaway
reactions taking place under thermal adiabatic condition. The effects of various TBHP solutions were
extremely different from each other in onset temperature (T0), self-heating rate (dT∙dt−1), reaction
maximum pressure (Pmax) and temperature (Tmax), pressure rise rate (dP∙dt−1), and other of adiabatic
runaway behaviors. Organic peroxides typically form free radicals depending upon ready
decomposition by heat or chemical agents. Without sufficient heat removal, a runaway reaction can
occur, which may eventually be followed by auto-ignition or a thermal explosion. If organic peroxides
the process reactor or tank, the material accelerates itself toward a runaway reaction, the energetic substance belongs to the class of explosives. Runaway hazards can be recognized from the adiabatic
trajectories obtained in VSP2 calorimetric trials. The thermal stability of TBHP-acid mixtures was
studied using an accelerating rate calorimeter (ARC) under adiabatic conditions, as previously done
by Andreozzi et al.20 Incompatible decomposition occurred when the C−O or O−O bond broke via
attack by a H+ ion.
Where the potential exists for an adiabatic runaway reaction, the temperature and pressure
trajectories of the reaction, determined via VSP2 adiabatic calorimetric methodology, could be
recognized as one suitable measure of the magnitude of the thermal hazard. Hence, to provide for the
needs of commercial storage and transportation precautions and code recommendations, we
summarized the experimental data to assess the hazardous ranking of TBHP dissolved in five kinds of diluents: water, nonane, tert-butanol, decane, and DTBP. From quantitative and repeated trials, the
characteristic curves for the self-heating rate versus reciprocal temperature and pressure behaviors for
TBHP solutions with various diluents are shown in Figures 4−8, and the curves for increasing
temperature versus reciprocal time profile are shown in Figure 9. The adiabatic runaway system was
used to substantiate the different exothermic onset temperatures by recording these curves for TBHP
solutions. The induction time can be easily derived from these curves as being the time from the
beginning to the end of the curves, characterized by a steep temperature rise. The relatively trivial
statement can be derived that induction time decreases with increasing temperature. Within a shorter
interval, the adiabatic temperature and pressure of aqueous TBHP increase quickly.
Figure 4. Pressure rise rate and self-heating rate for thermal decomposition of aqueous TBHP 15 mass % solution.
nonane solution.
Figure 6. Pressure rise rate and self-heating rate for thermal decomposition of TBHP 15 mass % in tert-butanol solution.
Figure 7. Pressure rise rate and self-heating rate for thermal decomposition of TBHP 15 mass % in decane solution.
Figure 8. Pressure rise rate and self-heating rate for thermal decomposition of TBHP 15 mass % in DTBP solution.
Figure 9. Temperature-time curve in adiabatic runaway system of TBHP 15 mass % solutions.
The temperature-time curve can simply depict the exothermic patterns from the experimental data.17
Thus, the comparison of the adiabatic self-heating system is consistent with the thermal hazards of
various TBHP solutions. Temperature control is an important measure to forestall runaway reaction,
evolution of gases and mists (which may lead to vapor explosion), auto-ignition or loss of product
quality. According to the calorimetric experiments, TBHP will be most hazardous when it is dissolved
in water. This was a novel finding from our study. Thermal runaway of aqueous TBHP had the
maximum self-heating rate, highest final temperature, largest maximum pressure, and largest pressure
rise rate (Table 3). The maximum self-heating rate ((dT∙dt−1)
max) and the largest pressure rise rate
((dP∙dt−1)
max) of aqueous TBHP 15 mass % were measured to be 524 oC∙min−1 and 235 psi∙min−1,
respectively. The SADT of aqueous TBHP was about 87.8 oC, and the type of decomposition was
burning.25 However, the strength of the runaway hazard of aqueous TBHP is also increased by higher
should be pointed out that aqueous TBHP will result in more violent runaway behavior with increasing concentrations. The adiabatic runaway trajectories of aqueous TBHP 20 mass % are also
presented in Table 3 and Figure 10. The temperature and pressure excursions indicate that an
unexpected explosion may be caused in the case of thermal decomposition in reactor or storage
procedures. By comparison of TBHP solutions using all experimental data on adiabatic exothermic
behaviors, aqueous TBHP alone was the most hazardous of thermal runaway reaction, even more
hazardous than TBHP solutions in nonane, decane, DTBP or tert-butanol. If the storage temperature
exceeds the exothermic onset temperature, the runaway reaction of aqueous TBHP will inevitably
cause accidents. This reveals either that the decomposition pathways initiated with various diluents
are significantly different, or that there are nonequal branching ratios in the decomposition
mechanism. Such a result could provide new safety considerations in storage or transportation. Without alert consideration of safety, the thermal runaway of TBHP solutions will result in
unavoidable hazards.
Table 3. VSP2 Adiabatic Experimental Data on TBHP Solutions Sample/ diluent Conc., mass % e Weight , g T0, °C Tmax,, °C Pmax,, psig (dP/dt)max, psi∙min−1 (dT/dt)max, °C∙min−1 Pf, psig TBHPa/ water 15 ~1.1 50 105 336 472 235 524 109 TBHPa/ water 20 ~1.1 50 100 326 509 888 1703 155 TBHPb/ nonane 15 ~1.1 50 108 211 306 39 9 104 TBHPb/ tert-butanol 15 ~1.1 50 103 193 354 6 1 104 TBHPc/ decane 15 ~1.1 50 79 212 275 127 28 79 TBHPc/DTBP 15d ~1.1 50 110 211 264 87 18 73 a TBHP 70 mass % in water (H
2O). b TBHP 5.5 M in nonane (C9H20). c TBHP 5.5 M in decane
(C10H22). d DTBP 98 mass % and TBHP 5.5 M in decane as solvents in solution is 15 mass %. e Cp
of TBHP = 2.89 kJ∙kg−1∙°C−1, C
Figure 10. Pressure rise rate and self-heating rate for thermal decomposition of TBHP 20 mass % solutions.
The severity of a runaway reaction is usually ranked by the self-heating rate or pressure rise rate.
The DIERS program suggests the vent sizing methodology for industrial energetic chemicals or
processes in the case of emergency relief systems, based on the empirical formula proposed by Fauske
and Leung.3 The emergency relief area is directly proportional to the self-heating rate. For a reaction
that gives a later exothermic if allowed to remain at a pressure lower than 30 psig and with the
self-heating rate set at 8.5 oC∙min−1, it is suggested that the vent area be 0.0014 m2 per 1000 kg TBHP (10
mass %) at relief conditions of 100 psig and 172 oC under a tempered/vapor flow system. The data
from thermal analysis of the runaway reactions of TBHP solutions were proposed to establish an
emergency relief system by applying the DIERS methodology. The pressure behavior of aqueous
TBHP classifies it as a tempered system. As in our previous studies,2,3,4,26 we used the approach of
describing the reactive hazards of organic peroxides by calorimetric measurements, which is adopted
for thermal hazards evaluation.
If a large part of a liquid material is transformed into a gas or vapor at a very high pressure, orders
of magnitude higher than the explosion pressures experienced with gas explosions, a self-reactive
decomposition reaction can arise, which can be estimated easily via VSP2 calorimetric methodology.
It is necessary to discuss commercially available recommendations that regulate pressure rise during
adiabatic heating of hazardous materials. The sudden release of pressure constitutes an explosion, and
therefore, the pressure increasing straightway of aqueous TBHP solution displays the rapid generation
of gas during an uncontrollable runaway reaction. The pressure-temperature diagrams of various
TBHP solutions shown in Figure 11 display the typical thermal runaway behaviors. Whereas a thermal explosion would generate an increasing rate of pressure rise, the curve for aqueous TBHP
solution has the steepest ascent shortly after the beginning of the exothermic runaway reaction. Unfortunately, we still do not fully understand the reason for this phenomenon. A possible
explanation is that the thermal decomposition is retarded by the vaporization of the aqueous TBHP,
which soon arrives at the gaseous state. It is interesting, however, to measure the possible pressure
venting of a runaway reaction for this sort of TBHP solution. In Table 3, the results of VSP2
experimental data of various TBHP solutions are compiled. If an uncontrolled runaway reaction
generates sufficient overpressure inside a tank, other vessel, piping, or transport cargo to reach the
burst pressure of the confined area, a vessel rupture explosion will result.11
Figure 11. Temperature-pressure curve on adiabatic runaway system of TBHP 15 mass % solutions.
Based on the observations of this study, it was determined that the self-heating rate of aqueous
TBHP increased exponentially with both temperature and concentration. The results of the adiabatic
runaway reaction experiment agreed with those obtained by calorimetric methodology, by which the
characteristics of the self-accelerating reaction for TBHP in various diluents were identified, and its
accurate technique was proposed for the study of thermal decomposition. From our results, we can be
fairly certain that the hazards of aqueous TBHP can cause violent thermal decomposition in a
runaway reaction, and we must pay attention to precautions for the safe use of this kind of organic
peroxide.
4. Conclusions
Dilution of a self-reactive peroxide with an inert solvent may be used to reduce the reactant
concentration and even to lower the adiabatic temperature rise by vaporization of the diluent. From
the above experimental results, we ranked the thermal hazards of different TBHP solutions from high
self-heating reactions in an adiabatic runaway situation, which can be ranked as follows:
TBHP/water (Tmax = 336 oC) > TBHP/decane (Tmax = 212 oC) > TBHP/DTBP (Tmax = 211 oC) >
TBHP/nonane (Tmax = 211 oC) > TBHP/tert-butanol (Tmax = 193 oC)
Purely thermal decomposition of hydroperoxides by homolysis to alkyl and hydroxy radicals and
the induced reactions depend on competitions among terminating interactions of peroxy radicals,
competitions between two hydrogen abstractions by alkoxy radicals from hydroperoxides or reactive
solvents, and the cleavage of alkoxy radicals.7 The thermal hazards of TBHP competitions depend on
diluents, concentrations and temperature. When a diluent is used as the solvent for such an organic
peroxide, the choice of diluent and its purity must be taken into account. Furthermore, the
effectiveness of diluents was due in part to the reduction in energy level by dilution or energy
adsorption; the diluent may act as a stopper, because most decomposition proceeds by a mild chain reaction. Again, in this test, we found that the diluent played an important role, in this case resulting
in different thermal hazards. Dilution decreased the potential for violent decomposition, but
unsuitable diluents frequently increased the self-decomposition rate under storage conditions.
Aqueous TBHP resulted in the highest risk of thermal and self-reactive hazards. However, more
severe phenomena were discovered by calorimetric methodology, as aqueous TBHP will undergo
thermal explosion in case of adiabatic self-heating conditions or external fire. Adiabatic calorimetric
methodology provided an excellent tool for investigating the hazards of the specific chemical
structure of organic peroxides, making it not only useful but also reliable.
It is necessary to reconsider the classification of organic peroxide in the future from the viewpoint
of a proactive approach to an intrinsically safer design.27 NFPA 432 could be consulted for specific
details of organic peroxide storage arrangements. This recommendation also classifies organic
peroxide formulations relative to their decomposition and flammability hazards. In hydrocarbon
solution, these peroxides attack the solvent to generate the solvent radical, which then reacts with
from the temperature and pressure evaluation in our studies, the options to avoid industrial accidents are to design safer process operation conditions, type and material of storage tanks for transportation,
and fire fighting via temperature control and pressure relief systems .
Acknowledgments
The authors are grateful to Professor Yih-Shing Duh for the experimental suggestions and to the
National Science Council of Taiwan, ROC (NSC 98−2221−E−407−003) for the financial support of
this study.
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