ABSTRACT
Thermal runaway reactions were involved in the Bhopal disaster of 1984, in which methyl isocyanate was vented from a storage tank, and occur in liquid peroxide explosions; yet to date there have been few investigations into the mechanism of thermal runaway in such liquid thermoreactive systems. Consequently protocols for storing thermally unstable liquids and deactivating liquid peroxide bombs may be suboptimal. This contribution presents simulation results for hydrolysis of methyl isocyanate and thermal decomposition of triacetonetriperoxide, using a gradientless, continuous-flow reactor paradigm. This approximation enabled stability analyses of steady-state solutions of the dynamical mass and enthalpy equations. Results show that thermal runaway in both systems is due to the onset of a
large amplitude, hard thermal oscillation initiated at a subcritical Hopf bifurcation. This type of thermal misbehavior cannot be predicted using classical ignition theory and may be typical of liquid thermoreactive systems. The mechanism of oscillatory thermal instability on the nanoscale is elucidated, and the potential for exploiting this instability in nanoscale thermal analysis for detection of peroxide explosives is explored. 9.1 IntroductionThe causes of the world’s worst industrial disaster at Bhopal, India, on December 2-3, 1984, are still being debated in the international media, more than 25 years after the event. Was it caused by neglect, parsimony, or procrastination by Union Carbide on training, safety, and maintenance? Corruption, sabotage, and cover-up? Inadequate government regulation? Any or all of the above may well have helped set up the worst possible scenario-for it could not have been any worse-but they are contributing factors rather than causes. (A brief account of the disaster is given in the appendix.) The primary cause of the thermal runaway that led to the venting of a lethal mist of methyl isocyanate (MIC) over Bhopal City was physicochemical. In this chapter, I present a stability analysis of a simple dynamical model for the MIC-H2O reacting system, revealing oscillatory thermal misbehavior that cannot be predicted using classical ignition theory. A similar instability is shown to govern the explosive thermal decomposition of triacetonetriperoxide (TATP) in liquid solution. The provenance of oscillatory thermal instability on the nanoscale is elucidated and shown to lie in the ability of the reactant molecules to store energy in the internal molecular motions-in other words, the heat capacity. The significant potential for exploiting oscillatory thermal instability is explored as a diagnostic in nanoscale thermal analysis for the detection of explosives. Despite the enormity of the Bhopal disaster little or no research has been published that elucidates the fundamental physicochemical cause of thermal runaway in liquid reactive systems such as MIC hydrolysis. In terms of achieving the millennium development goals (MDGs) (United Nations, 2011) it is important that thermoreactive
processes in liquids be thoroughly investigated at the nanoscale and the knowledge disseminated widely. Given the horrific legacy of the disaster, the long-term adverse health effects in children and adverse reproductive effects in women of MIC exposure that have been well documented (Mishra, Samarth, Pathak, Jain, Banerjee, and Maudar, 2009), such knowledge is relevant to the MDGs of child health and maternal health. More generally people have a right to expect that thermally unstable and hazardous liquids be stored safely. The Millennium Development Goals Report (United Nations, 2011) highlights the challenges posed by conflicts and armed violence to human security and MDG achievements. A growing threat to security is the use of liquid peroxide explosives by terrorists. The ingredients for making such bombs are cheap and widely available, and they cannot be detected by metal detectors and nitrogenous explosives detectors or distinguished from hand lotion by X-ray machines. TATP and other peroxide-based explosives were used in the suicide attacks on the London transit system in 2005, which killed 56 people, and the terrorists convicted of the 2006 transatlantic aircraft conspiracy had planned to blow up a number of planes using TATP and other peroxide explosives. (Many more accounts of peroxide misuse incidents are easily found on the web.) To this day there are severe restrictions on carrying liquids through security barriers at most airports. It seems grimly inevitable that the use of peroxide explosives as mass murder weapons will increase. Knowledge of their fundamental mechanism of action at the nanoscale may help to counter their use by informing recent efforts to develop nanoscale thermal analysis for explosives’ detection (Zuck, Greenblatt, Zifman et al., 2008; Carreto-Vazquez, Wojcik, Liu, Bukur, and Mannan, 2010). The literature on thermal runaway criteria and parametric sensitivity in batch reactors and storage tanks was summarized by Velo, Bosch, and Recasens (1996). In defining critical conditions they, along with other authors cited therein, begin with the assumption that storage tanks can be modeled as well-stirred batch reactors with linear thermal coupling to the environment. However, batch reactors have no nontrivial steady states, and there is no general theory for determining whether a thermal excursion will grow or decay. It is shown in this work that a simple model with nonequilibrium steady states that is also spatially homogeneousthe continuous-flow stirred tank reactor (CSTR) paradigm-can
provide great insight into thermoreactive instabilities in liquid systems and provide fundamental causative information that cannot easily be extracted from detailed numerical simulations that include convective motions. In Section 9.2, the chemical reactions are described and the relevant data for the physical properties of the reactants and thermodynamic and kinetic parameters is provided. The CSTR paradigm is described in Section 9.3, and the dynamical equations are given. In Section 9.4 the results of numerical stability analyses of the equations are shown graphically and discussed where values of the parameters for MIC hydrolysis and for TATP thermal decomposition in solution were used in the equations. Some points regarding the applicability of the CSTR paradigm are discussed in Section 9.5; the nanoscale aspects of oscillatory thermal runaway are elucidated through examining the behavior in the limits of the two timescales of the relaxation oscillation, and the potential these results offer for improved microthermal analysis in explosives detection is discussed. A summary of the conclusions is given in Section 9.6. 9.2 Chemistry and Data
9.2.1 MIC HydrolysisIsocyanates hydrolyze exothermically to the corresponding amine and carbon dioxide. In excess water isocyanates react exothermical-ly with the hydrolysis product amine to form the disubstituted urea (Saunders and Slocombe, 1948; D’Silva, Lopes, Jones, Singhawangcha, and Chan, 1986). With MIC the product is N,N-dimethyl urea, and the reaction sequence is as follows: CH3NCO(l) + H2O(l)K T1( )æ Æææ CH3NH2(aq) + CO2(aq) (R9.1) CH3NCO(l) + CH3NH2(aq)K T2( )æ Æææ CH3NHCONHCH3(aq) (R9.2) For reaction R9.2 DH2(298 K) = –174.6 kJ/mol, and for the sequence overall DHtot(298 K) = –255 kJ/mol (Lide, 2008-2009). A chemical analysis of the residue in the MIC storage tank (Tank 610) at the Union Carbide plant at Bhopal, sampled 17 days after the event, found a variety of MIC condensation products, mainly the cyclic trimer (D’Silva, Lopes, Jones, Singhawangcha, and Chan,
1986). However, experiments indicated that these condensations must have been initiated at temperatures and pressures well above the normal boiling point of MIC. So for modeling the initial thermal runaway these reactions need not be considered. No kinetic data are available for reaction R9.2. So only reaction R9.1 is used in the model. It will be seen from the results that reaction R9.1 alone is sufficient to induce thermal runaway. Relevant physicochemical data is given in Table 9.1. Table 9.1 Physical, kinetic, and thermochemical parameters for MIC hydrolysisMolecular weight MIC 57.05 g/mol
Specific heat capacity Cp° (298) MIC 1959 J/(kg K) Poling, Thomson, Friend, Rowley, and Wilding (2008) Specific heat capacity Cp° (298) H2O 4181 J/(kgK)Boiling point MIC at 1 atm 38.3°C Lide (2008-2009)Density MIC at 25°C 0.9588 g/cm3 Lide (2008-2009)R1 reaction enthalpy 80.4 kJ/mol Lide (2008-2009)�R1 activation energy 65.4 kJ/mol Castro, Moodie, and Sansom (1985) R1 pseudo first-order frequency factor 4.13 × 108/s Castro, Moodie, and Sansom (1985)
�From standard enthalpies of formation at 298 K 9.2.2 TATP Thermal DecompositionTATP, a cyclic trimer, is an explosive made by mixing acetone and hydrogen peroxide, both of which are legal, cheap, and readily available over the counter. Pure TATP is a white crystalline powder that looks like sugar and is soluble in organic solvents. The thermal decomposition of TATP does not involve combustion; the main reaction products are acetone, some carbon dioxide, and ozone (Eyler, Mateo, Alvarez, and Canizo, 2000; Oxley, Smith, and Chen, 2002). Its high explosive power is in part due to the large entropy increase of the formation of four gas molecules from one condensed-phase molecule (Dubnikova, Kosloff, Almog et al., 2005). Relevant
parameters for the thermal decomposition of TATP in toluene are given in Table 9.2. Table 9.2 Physical, kinetic, and thermochemical parameters for thermal decomposition of TATP in toluene Molecular weight 222.2356 g/mol
Specific heat capacity Cp° (298) toluene 1698.25 J/(kgK) Poling, Thomson, Friend, Rowley, and Wilding (2008)Boiling point toluene at 1 atm 110.8°CDensity of toluene at 298 K 866.9 kg/m3Reaction enthalpy 330-420 kJ/mol� Dubnikova, Kosloff, Almog et al. (2005)Activation energy 178.52 kJ/mol Eyler, Mateo, Alvarez, and Canizo (2000)Frequency factor 9.57 × 1016/s Eyler, Mateo, Alvarez, and Canizo (2000)Feed concentration of TATP 2 mol/kg
The spatially homogeneous flow reactor, or reacting mass or volume, in which a reactant undergoes a first-order, exothermic conversion is a simple but elucidatory model for thermoreactive systems when it is appropriate to ignore convection, because as a dynamical system it has nontrivial steady states that can be analyzed for stability. The dynamical mass and enthalpy equations may be written as follows: M
c t
dd = MAe-E/RTc + F(cf – c) (9.1) MC
T tr
d
d = (–DH) MAe-E/RTc + F(CfTa – CrT) – L(T – Ta). (9.2)
Notation and quantities are defined in Table 9.3. For numerical and comparative reasons it is more convenient to work with the following dimensionless system corresponding to Eqs. 9.1 and 9.2:
ddxt = –xe-1/u + f(1 – x) (9.3)e t
ddu = xe-1/u + e f(gua – u) – l(u – ua), (9.4) where the dimensionless groups are defined in Table 9.3.
