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Screening Analysis for Hazard Assessment of Peroxides Decomposition V. Casson and G. Maschio* Dipartimento di Principi e Impianti di Ingegneria Chimica, Universita di Padova, Via F. Marzolo, 935131 Padova, Italy ABSTRACT: In this study, the analysis of the decomposition of four different peroxides (including hydrogen peroxide) by screening calorimetry is proposed. The objective is to analyze the decomposition reactions and evaluate the consequences in particular when the process undergoes to thermal explosion and may be the cause of incidents. Screening calorimetry data allow us to define the conditions and ranges of temperature and pressure evolved during the reaction. In the experimental apparatus used (a screening calorimeter), the experiments have been carried out safely, even when there is a rapid and large increase in temperature and pressure. Scanning and isothermal conditions has been investigated. An Early Warning Detection System (EWDS) for thermal runaway, based on the divergence criterion, has also been applied off-line to the experimental isothermal data, in order to evaluate the sensitivity of the method applied to both temperature and pressure profiles. The results of the application of the EWDS have been compared to those obtained using the Hub and Jones criterion.
’ INTRODUCTION In chemical reactors, for several reasons, the rate of heat generation may be faster than the rate of heat removal by the cooling system, leading to an incremental increase of heat accumulated in the system, with a consequent acceleration of the reaction kinetics.1 According to Semenov2 theory, the reagent mass accumulates heat (self-heating) until a point is reached in which the temperature increases uncontrollably, as does the reaction rate; this phenomenon is known as a runaway reaction or thermal explosion, which means the loss of thermal control of the reacting system. Other causes leading to thermal explosions have been summarized in the past by statistical analysis of several industrial incidents.3,4 The main critical points identified have been as follows: • lack of understanding in the chemistry and thermodynamics of the process or inadequate reactor design (during the design and scale-up steps); • inadequate control and safety devices and of procedures/ operator training; and • self-heating behavior during storage and transport. These studies underlined that process safety is primarily based on an accurate and detailed knowledge of the thermochemistry of the reaction and, afterward, on correct scaleup and management. Runaway reactions have the potential to inflict considerable damage if appropriate preventive and protective measures are not in place: they may cause toxic or flammable releases or, more simply, a pressure increase in the reactor, because of the vapor pressure of products or the formation of gaseous decomposition products, leading to the collapse of the vessel.5 Studies conducted about the blast effect of runaway reactions concluded that the pressure reached inside the vessel is seldom above its failure pressure and the energy of the reaction after the vessel ruptures hardly contributes to the blast effect. The fraction of energy that contributes to the blast after the decomposition of an energetic material is often partial because the entire content of the vessel r 2011 American Chemical Society
does not decompose or decomposes very slow. The main effect, instead, can be the explosiveness or autoignition of the material formed by the reaction.6 Organic peroxides are unstable chemicals: they are liable to decompose exothermically at ambient or high temperatures.7,8 Because of their instability, they are involved in many radical chemical processes, such as polymerization reactions and organic synthesis, which are already among the most statistically dangerous processes; in the past, such reactions have caused most of the incidents, which have been due to runaway reactions.3 According to the Major Accident Reporting System (MARS) databank, in the last 30 years, there have been several major accidents involving peroxides.9 In particular, • 60% of these accidents have been caused by explosive decompositions/unexpected reactions and 40% wre caused by human error; • 30% of the accidents caused at least one fatality; • in 30% of the accident cases, the accident led to fires and pool fires. • 70% of the accidents were caused by hydrogen peroxide, and the other 30% involved different organic peroxides. This is the main reason why we focused our study on peroxides— in particular, hydrogen peroxide, even if the other peroxides tested also are widely employed in chemical industry. There are different approaches for handling the consequences of a runaway reaction: some are completely protective, other are partially preventive. Venting, containment, and venting with containment are passive protective measures (containment is an inherently safer measure) that are aimed at loss prevention. Special Issue: Russo Issue Received: August 1, 2011 Accepted: November 21, 2011 Revised: November 18, 2011 Published: November 21, 2011 7526
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Industrial & Engineering Chemistry Research Reaction inhibition is an active protective measure that involves the injection of small quantities of a particular substance into the reactor at an early stage of the runaway. Despite their apparent suitability, inhibition systems are rarely used in industrial processes and have been not studied widely, because of the intrinsic danger in the experimental activity.10 The main disadvantages include problems regarding the mixing, position, and number of the injectors, as well as the choice of the method for the early detection of the onset of runaway.11 In order to apply this prevention system, a robust on-line Early Warning Detection System (EWDS) must be developed to identify situations that can lead to runaway reactions. The EWDS should be supported by theoretical criteria for the prediction of the onset of the thermal explosion. Literature shows several criteria for the prediction of the onset of runaway phenomena.1,12 The existing different approaches can be classified as geometry-based and sensitivity-based. In the first case, thermal explosion is related to some geometrical features of the profile of a system variable (for example, the criteria of Semenov, Bowes, and Thomas, Adler and Enig, van Welsenaere, and Froment and Barkelew); the second approach is related to the parametric sensitivity of the system and is very suitable for practical application (see, for example, the criteria of Hub and Jones and Strozzi and Zaldívar). In this work, two different sensitivity-based criteria are applied to experimental temperature and pressure data: the first one is the Hub and Jones13 criterion, and the second is an EWDS based on divergence criterion1417 applied off-line to the data regarding the decomposition of hydrogen peroxide, in order to test the suitability of the criterion when applied to temperature or pressure decomposition profiles. Different studies18 have proved that temperature is a more-sensitive parameter in runaway detection; however, in the case of the formation of noncondensable gases, pressure increase is also high for quasi-isothermal profiles. Since overpressure is a very dangerous condition to be reached in a vessel, pressure monitoring can be used also as a parameter for early detection of anomalous behavior of the system. The aim of this application is to understand to which parameter the algorithm is more sensitive and detects the runaway reaction with greater advance notice. The EWDS consists of hardware and software sensors that permit, by the direct measurement of temperature in the reacting system, the early detection of the onset of a runaway situation. It is is composed of the following: • an interface with the process for the acquisition of the test data (monitoring); • a criterion that distinguishs the thermal explosion by the isothermal reactions (takeover); and • a procedure to start the alarms (detection and assessment). EWDSs can be classified into three categories: (1) those that use only experimental data measured by the system during the reactions, such as temperature, derivative with respect to time, or both, such as the Hub and Jones’s criteria; (2) those that use a model of the system, a model of the experimental measures, and a Kalman filter to evaluate the immeasurable variables; and (3) those that do not use a mathematical model, but apply space phase reconstruction using the measurement of system variables (such approaches are derived from chaos theory, in which the characterization of system comes from the divergence criterion).
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The Strozzi and Zaldívar criterion integrated in a EWDS has been used during the European AWARD Project. The final report of the AWARD Project19 illustrated that EWDS was applied to several reacting systems, either in laboratory- and industrial-scale tests. In the studied processes, the operating conditions were selected in order to guarantee safety and, as a consequence, the temperature and pressure increases during the runaway reaction were limited, with respect to the potential increase during a real incident of the same reacting system in a chemical plant. More recently, the EWDS has been applied in the esterification of acetic anhydride: because of the modest reaction enthalpy and low activation energy, this reacting system provides a severe test to the runaway criteria.12 In all of the investigated cases, the application of the EWDS permits early detection of the onset of runaway in the absence of false alarms. In this paper, the EWDS based on the Strozzi and Zaldívar criterion is applied to experimental tests carried out in the TSu, which is a pseudoadiabatic calorimeter. These tests permit a careful analysis of the EWDS under operating conditions very similar to a real runaway course of the process.
’ THERMAL RUNAWAY THEORY AND REACTOR STABILITY CRITERIA Thermal runaway is eventually caused by the lack of heat removal from the vessel where the process is taking place. In this perspective, the analysis of the energy balance of the system becomes of primary importance for the comprehension of the dynamics of heat production and removal. Many authors approached the thermal explosion from a mathematical point of view.1 However, let us consider first of all the simplest geometry-based criterion that was formulated by Semenov: an homogeneous distribution of temperature is assumed in the reactive mass, homogeneous self-heating and the heat transfer resistance is considered to be all concentrated in the wall of the vessel; also, the consumption of reagents is not taken into consideration.5 This is a good approximation, fundamentally correct and synthetic for some real systems, when runaway occurs at a very early stage of the process and the conversion of reagents is nearly zero. When the consumption of the reagent must be taken into consideration, the Semenov criterion becomes too conservative, because the system dynamics become more complex and the rate of temperature increase decreases as the conversion advances.1 To overcome these problems, other geometry-based criteria have been proposed, such as those of Bowes and Thomas, Adler and Enig, van Welsenaere, and Froment and Barkelew, which can still be applied just to systems where there is a temperature profile in time or conversion; nevertheless, geometry-based criteria do not give a measure of the extent of the runaway or even of its intensity. This limitation can be overcome with sensitivity-based criteria that identify the parametrically sensitive region of the system to define the runaway and nonrunaway behavior. The sensitivity-based criteria of Hub and Jones and Strozzi and Zaldívar will be described. Both criteria can be applied on-line, so they do not require a model for the reaction, which is different from the previously mentioned criteria. Historically, the first on-line criterion is that of Hub and Jones (1986).13 It is sensitivity-based and independent from a model for the process, so it has been widely used in industrial applications. This criterion states that the runaway reaction starts when both the first and second derivatives of the reactor temperature, 7527
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Figure 1. Photograph (left) and schematic diagram right) of the experimental apparatus (TSu).
with respect to time, become positive: dðT Ta Þ >0 dt
and
d2 T >0 dt 2
Let us consider a system of ODEs (i.e., the mass and energy balance): ð1Þ
This criterion is simply derived from the energy balance for the reactor, derived with respect to time: FV R cP
dT ¼ ð ΔHR ÞkðTÞcn V R SUðT Tα Þ dt
ð2Þ
where c is the reactant concentration [mol/m3]; t the time [s]; k(T) the reaction rate constant, as a function of temperature [(mol/m3)1n/s]; n the reaction order; F the density of the fluid mixture [kg/m3]; V R the reagent volume [m3]; cP the specific heat capacity of the reaction mixture [J/(K kg)]; T the temperature [K]; ΔH R the heat of reaction [J/mol]; U the overall heat-transfer coefficient [J/(m 2 s K)]; S the external surface area per unit of volume [m 2 /m 3 ]; and T a the ambient temperature [K]. In eq 2, it is easy to notice that the accumulation of heat in the system becomes ever increasing when the conditions given for eq 1 are verified. The Hub and Jones criterion is very simple to apply and is based just on temperature measurements. The main problem is the presence of noise in the temperature signals, which can produce errors, especially in the evaluation of the values of the first and second derivative. Usually, digital filters of high order and algorithms for data smoothing are applied to the monitored signals in order to avoid this problem, but noise cannot be totally removed. The EWDS here considered is based on the divergence criterion by Strozzi and Zaldívar (1994),15 which uses the Lyapunov exponents to define sensitivity.1 This criterion states that if the system of ordinary differential equations (ODEs) that describes the process (the mass and energy balances) exhibits positive divergence at some point on the temperature/pressure vs time trajectory, the process operates under runaway conditions. The divergence is a scalar quantity defined at each point as the sum of the partial derivatives of the energy and mass balances, with respect to temperature and conversion.
dxðtÞ ¼ F½xðtÞ dt
ð3Þ
where x(t) = [x1(t), ..., xd(t)] in Rd (d is the dimension of the system) and F = [F1, ..., Fd] is a nonlinear continuous function in x (usually known). For t > 0, the initial condition x(0) moves to a new point x(t). Similarly, all points placed initially in the region called Γ(0) will move to Γ(t). Let us define V(t) as the volume of the Γ(t) region; this can be expressed by Liouville’s theorem: dV ðtÞ ¼ dt
Z ΓðtÞ
div F½xðtÞ dxðtÞ1 ::: dxðtÞd
ð4Þ
where div F½xðtÞ ¼
∂F1 ½xðtÞ ∂F2 ½xðtÞ ∂Fd ½xðtÞ þ þ 333 ∂x1 ðtÞ ∂x2 ðtÞ ∂xd ðtÞ
ð5Þ
Assuming that this d-dimensional volume is small enough that the divergence can be considered constant on it, then dV ðtÞ ¼ V ðtÞ div F½xðtÞ dt
ð6Þ
and, hence, Z
t dV ðtÞ 0
V ðtÞ
¼
Z t 0
div F½xðtÞ dt
ð7Þ
which means that the initial phase-space volume V(0) shrinks or grows with time, according to the following expression: V ðtÞ ¼ V ð0Þ exp
Z t 0
div F½xðtÞ dt
ð8Þ
Therefore, for a system such as that described in eq 2, the rate of change of an infinitesimal volume V(t) that follows the orbit x(t) is given by the divergence of the flow, which is locally equivalent to the trace of the Jacobian of F. Since the integral of a strictly positive (or negative) function is itself strictly positive (or negative) and the integral of an identically zero function is still zero, it follows that, if div F(x) < 0, the flow of the trajectories shrinks, demonstrating that V(t) is contracting (dissipative 7528
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system); conversely, if div F(x) > 0, the system is expanding (or, if div F(x) = 0, the system is conservative). As for state, because a volume is an always positive quantity, it is possible to deduce that div FðxÞ > 0
is equivalent to
ΔV ðtÞ > 0
A chemical reactor is a system in which the dimension is the sum of the reactor temperature and the different concentrations that collapse to a point of zero dimension (the reactants are consumed and the temperature is that of the jacket). To detect the possibility of a runaway, we must study the dynamics of collapse of the orbits. In the practice, the application of the criterion consists of considering that, when the variation of phase space volume ΔV(t) is greater than a threshold value, the system can undergo a runaway reaction.20 Therefore, when ΔV ðtÞ > ΔVlim
ð9Þ
the EWDS algorithm emits an alarm. ΔVlim is determined by experimental data on the system considered, chosen when the variation of the phase space volume profile starts to destabilize; the choice of a correct value is a critical point to avoid false alarms.
’ EXPERIMENTAL SECTION Thermal screening of components and reaction mixtures is useful to identify conditions under which a thermal explosion can occur and the temperature and pressure ranges developed. With this approach, a preliminary risk analysis of the process can be made: onset temperature, maximum rate of self-heating, heat of reaction, kinetics parameters, and other parameters can be evaluated. The experimental runs here described are carried out in a Thermal Screening Unit (TSu), which is a pseudo-adiabatic and nondifferential thermal analysis instrument commercialized by HEL: during the tests, the temperature and pressure profile of the sample are observed. The apparatus used in this work can be seen in Figure 1. The advantages of this technique are that (i) it requires a short time for analysis and small quantities, and (ii) it is suitable for analyzing unstable substances, reaction intermediates, new compounds, or contaminants. The disadvantages in the utilization of this instrument are (i) the nonadiabaticity of the obtained experimental data, (ii) the difficulty in mixing the sample, and (iii) the impossibility of simulating the heat transfer with a jacket. Because of this, the instrument can be used as a first step in laboratory scaleup, but further experimental determinations must be done with more-complex calorimeters, such as reaction calorimeters and adiabatic calorimeters. The sample is contained in a pressure-tight metal (stainless steel, Hastelloy, titanium) or glass test cell, suspended in the middle of an oven, that consists of a metal cylinder with a heating coil wrapped around the outer surface that is heated at a userdefined rate. The oven is not under pressure and acts as a containment chamber in case of explosion of the sample holder.21 The instrument is interfaced with specific software that allow the operator to set a scanning or isothermal test (or even a combination of the two). The TSu has been designed to work up to temperatures of 500 °C and pressures of 250 bar.22 Upon performing a scanning test, the user controls the ramp rate of the oven or the set temperature for an isothermal run. After an initial delay that is due to thermal inertia, the sample
Figure 2. Structural formulas of organic peroxides: di(4-tertbutylcyclohexyl) peroxydicarbonate (PCARB), dibenzoyl peroxide (BPO), di-tert-butyl peroxide (DTBP), and hydrogen peroxide (HP).
temperature will be found to follow the oven ramp at the same rate with a slight “offset” (which will be dependent on the physical characteristics of the test material, such as specific heat). When an exothermic or endothermic process is detected, the sample temperature will be found to deviate from the background heating rate that identifies the detected “onset temperature”. The rate of rise in sample temperature (dT/dt) and the maximum value reached (TMAX) before returning to the background heating rate reflects important characteristics of the thermal event. In addition to temperature data, the thermal screening unit TSU is also equipped with a pressure transducer that records changes in sample pressure as the reaction proceeds. This provides the operator with a second method by which sample activity can be identified. This alternative method of sample analysis is particularly useful, because it provides a measure of the total pressure generated in the reaction (PMAX) and the rate of pressure rise (dP/dt). The pressure data also enables very mild exothermic decomposition reactions, which result in the production of noncondensable gas to be detected, even if the associated temperature rise is too low to be reliably detected.21
’ RESULTS OF THE SCANNING TESTS ON DIFFERENT TYPES OF PEROXIDES In this study, di(4-tert-butylcyclohexyl) peroxydicarbonate (PCARB), dibenzoyl peroxide (BPO), di-tert-butyl peroxide (DTBP), and hydrogen peroxide (HP) were analyzed (see Figure 2), because they are employed widely in the chemical industry (particularly in pharmaceutical and polymeric processes). For the first three peroxides, 5 g of solution of peroxide in toluene (20 wt %) have been tested under heating ramp conditions (2 °C/min). Hastelloy sample holders have been used. In Figure 3, the sample temperature, pressure, and rate of temperature increase are shown for the three peroxides. The experimental profiles show that, according to the Hub and Jones criterion, all the peroxides decompose with runaway behavior under the experimental conditions tested. The criterion has been adapted to our experimental data, by considering the runaway reaction to start when the rate of temperature of the sample is greater than the ramp value set by the operator, At such a point, a self-heating phenomenon is starting (see the dashed 7529
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line in Figure 3C): this temperature is the detected onset temperature (Tonset). The maximum rate of temperature increase can be easily determined: this indicates the rate of energy release and the intensity of the decomposition.
Both the peaks of temperature and pressure are visible on-line during the test. From the experimental profile, the rate of increase of these parameters can be calculated: this is also very important, for example, in the design of venting and other protection systems. Table 1 shows the data discussed above.
Figure 3. Experimental online profiles: [A] temperature, [B] pressure, and [C] rate of temperature ((1) PCARB (dashed curve), (2) BPO (solid curve), and (3) DTBP (dash-dotted curve)). Samples: 5 g, 20 wt % in toluene. Scanning rate: 2 °C/min. Hastelloy sample holders.
Figure 4. Experimental online profiles: [A] temperature, [B] pressure, and [C] rate of temperature ((1) 25%, (2) 30%, and (3) 35%). Sample conditions: 2 g, HP in distilled water. Scanning rate: 2 °C/min. Stainless steel sample holders.
Table 1. Data Obtained from the Experimental Profiles in the Scanning Tests of the Organic Peroxides TONSET [°C]
t [min]
TMAX [°C]
t [min]
dT/dtMAX [°C/min]
t [min]
PMAX [bar]
t [min]
dP/dtMAX [bar/min]
t [min] 45
PCARB
65
38
111
46
17
45
10
46
8
BPO
90
53
163
63
29
62
14
63
12
62
144
90
219
100
35
100
37
100
41
100
DTBP
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Table 2. Data Obtained from the Experimental Profiles in the Scanning Tests of the Hydrogen Peroxide in Different Concentrations dT/dtMAX
concentration of
dP/dtMAX
HP [wt %]
TONSET [°C]
t [min]
TMAX [°C]
t [min]
[°C/min]
t [min]
PMAX [bar]
t [min]
[bar/min]
t [min]
25 30
109 100
61 59
204 208
72 70
63 68
72 70
43 55
72 70
75 75
72 70
35
96
54
239
66
137
66
79
66
331
66
and pressure measurement, in order to determine which parameter makes it more sensitive to runaway reaction. Finally, it is interesting to emphasize that DTBP, which decomposes at higher temperatures, compared to BPO and PCARB, shows both higher temperature and pressure peaks, which is also due to the longer induction times: its runaway behavior can be considered more violent than that for the other two compounds. As mentioned previously, in this study, the investigation of hydrogen peroxide cannot be neglected, because it is involved in the majority of the accidents that have occurred in the last 30 years. This is one reason that makes it a very interesting system to analyze. HP decomposes (disproportionates) exothermically to water and oxygen gas spontaneously, according to the following reaction: 2H2 O2 f 2H2 O þ O2
Figure 5. Experimental online profiles: [A] temperature, [B] pressure, and [C] rate of temperature ((1) 90 °C test, (2) 95 °C test, (3) 100 °C test, and (4) 110 °C test). Samples: 2 g, 35 wt % HP. Stainless steel cells.
Predictably, the temperature and pressure peaks occur at the same time. That is one reason why, in the next section, the results of the application of the EDWS will be compared for temperature
ð10Þ
This process is thermodynamically favorable. It has a ΔH0 value of 98.2 kJ/mol and a ΔS0 value of 70.5 J mol1 K1. The rate of decomposition is dependent on the temperature (a cool environment slows decomposition; therefore, HP is often stored in refrigerators) and concentration of the peroxide, as well as the pH and the presence of impurities and stabilizers. HP should be stored in a cool, dry, well-ventilated area and away from any flammable or combustible substances. It should be stored in a container that is composed of nonreactive materials such as stainless steel or glass (other materials, including some plastics and aluminum alloys, may also be suitable). Figure 4 shows the sample temperature, pressure, and rate of temperature increase for three different solutions of HP: 25 wt %, 30 wt %, and 35 wt % (2 g). Solutions have been tested, first of all, in stainless steel sample holders, because this material is the one usually employed in the chemical plants and in transport and storage tanks. The tests have been conducted in scanning mode (2 °C/min). Table 2 summarizes the main results derived from the experimental profiles of Figure 4. Experimental data shows that the effects in terms of temperature and pressure of HP decomposition do not increase linearly, with respect to its concentration. The main effect is visible in the rate of pressure rise; that is, it the same for the first two solutions and it is four times bigger for 35 wt % HP. This evidence confirms the elevated risk in handling and storage a concentrated solution of HP and the limitation of the maximum concentration of the commercial solution of this substance. Commercial grades with >35% HP are also available; however, because of the potential risk of detonation for solutions with a HP concentration of >68%, higher grades are potentially far more hazardous and require special care in dedicated storage areas. 7531
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Table 3. Results Obtained from the Experimental Profiles of the Isothermal Tests on HP
a
test
TONSET [°C]
90 °C
t [min]
TMAX [°C]
t [min]
dT/dt MAX [°C/min]
t [min]
PMAX [bar]
t [min]
NDa
92
53
0.063
51
37
999
0.7
45
95 °C
NDa
105
78
0.685
232
41
279
0.5
73
100 °C
122
47
210
51
74
51
63
51
82
51
110 °C
126
43
217
47
83
47
65
47
99
47
dP/dt MAX [bar/min]
t [min]
Not detected using the Hub and Jones criterion.
Figure 6. dT/dt and dT2/d2t profiles for the 100 °C isothermal test on 35 wt % HP.
Figure 7. dT/dt and dT2/d2t profiles for the 110 °C isothermal test on 35 wt % HP.
These preliminary scanning tests allowed us to choose a sample system to be studied in the next section (2 g of a 35 wt % HP solution). Isothermal tests at temperatures higher and lower than the onset detected by the scanning test for the sample (i.e., 96 °C) have been performed, in order to analyze the decomposition behavior (thermally controlled vs runaway).
’ RESULTS OF THE ISOTHERMAL TESTS ON HYDROGEN PEROXIDE As mentioned previously, when the rate of heat generation is faster than the rate of heat loss by the system, the temperature of the reagent mass starts to increase and assume a self-accelerating behavior. The consequences are a very rapid, ever-increasing heat accumulation in the mass and the possibility of a violent thermal explosion. Therefore, it is important to have a control system that can detect an anomalous development of the reaction path from the very early stages of it, in particular when a very exothermic process is taking place in the reactor.3 Early warning detection systems have been created with this objective, They must be sensitive enough to detect the thermal explosion with adequate anticipation, in order to make some corrective measure possible, and they must be reliable, with respect to false alarms. The objective of performing isothermal tests is to analyze the performance of the EWDS applied to temperature or pressure data, especially when the HP decomposition is between runaway behavior and non-runaway (thermally controlled) behavior. The results of the application of the Strozzi and Zaldívar criterion have been compared to those obtained using the Hub and Jones criterion. Two-gram (2 g) samples of 2 g of a 35 wt % HP solution have been analyzed in four different isothermal tests: 90 °C, 95 °C, 100 °C, and 110 °C. Stainless steel sample holders have been used. As mentioned in the previous section, the choice of the set of temperatures is due to the results of a scanning test: two
Figure 8. Temperature (T) and ΔV(t) profile for the 100 °C isothermal test on 35 wt % HP.
temperatures under the onset and two temperature above the onset have been analyzed. The experimental profiles are shown in Figure 5, and the main experimental data are summarized in Table 3. As shown in Figure 5B, the samples in all four cases eventually reach the same final value of pressure (∼37 bar). According to these data, the final conversion in HP ((ni,HP nf,HP)/ni,HP) has been calculated for the isothermal experiments, considering the initial and final values of the pressure (after cooling the sample to ambient temperature). In all four cases, the same final conversion was reached (∼70%). The differences between the four experiments are seen in the decomposition behavior: in the tests run at 90 and 95 °C, HP decomposes isothermally very slowly (∼17 h for the test performed at 90 °C and 5 h for the 95 °C isothermal test), while in the higher-temperature tests, it decomposes rapidly with runaway behavior (2 h for the test run at 100 and 110 °C). Applying the Hub and Jones criterion to the temperature profiles of experimental data reported in Table 3, we found that, 7532
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for the test run at 90 and 95 °C, there is no detection of the onset temperature. In fact, we can say that, under these conditions, HP decomposes in a thermally controlled way. Instead, for the data from the tests performed at 100 and 110 °C, there is detection of the values of the onset temperature, congruent with the applied criterion, at 122 and 126 °C, respectively, in both cases higher, with respect to the set point for the test. The profiles of the first and second derivative of temperature, with respect to time, are reported in Figures 6 and 7 for both experiments. As already mentioned, while explaining the disadvantages in applying this criterion, the efficiency is very conditioned by the presence of noises in the signal of temperature, which is amplified by the derivatives. This makes the prediction of the onset subject to misinterpretation and, therefore, false alarms. To these data, the EWDS, congruent to the Strozzi and Zaldívar criterion previously described, has been applied; with this method, it is possible to detect runaway reactions before they start: when the variation of phase space volume ΔV(t), which is related to the divergence of the system, is greater than a threshold
Figure 9. Pressure (P) and ΔV(t) profile for the 100 °C isothermal test on 35 wt % HP.
value ΔVlim. The limit is chosen when the variation of phase space volume profile starts to destabilize and increases progressively until a peak, representing the system undergoing to a runaway reaction, is reached. Figures 8 and 9 show the profiles of ΔV(t) of the 100 °C isothermal test for temperature and pressure, respectively. This test allowed us to find a threshold value for the variation of phase space volume ΔVlim that can be considered the limit to define the runaway boundary for the decomposition for the tests run at higher temperature. In Figure 10, the determination of ΔVlim is shown: when the profile of ΔV(t) starts to destabilize and become progressively greater than zero, the value, denoted by a dot in the graphs in Figure 10, is chosen as a threshold for the alarm, detecting the start of possible thermal explosion. In Figure 10, the y-axes of ΔV(t) are expanded for better visualization of the destabilization of the profile and in order to distinguish ΔVlim. Figure 10 shows that the pressure profile becomes destabilized after the temperature profile. The ΔVlim values chosen are reported in Table 4. Figures 11 and 12 show the results of the application of the values of ΔVlim just discussed to the 110 °C isothermal test in order to detect the thermal explosion. The y-axes of ΔV(t) are expanded in order to visualize where the value of ΔV(t) of this test reaches the ΔVlim; using the difference between the time at which this happens and the time at which there is the maximum of temperature (or pressure), it is possible to evaluate the advance in the runaway detection (see Table 4). Comparing this value with the time at which the onset temperature is detected by the application of the Hub and Jones criterion is possible to make a comparison of efficiency between the two criteria. The results show that EWDS works better with temperature data, compared to pressure data, giving an earlier warning. With respect to the Hub and Jones criterion, comparing the results of
Figure 10. The choice of ΔVlim for the (a) temperature and (b) pressure profiles for the application of the EWDS. Isothermal test at 100 °C on HP.
Table 4. Results of the Application of the EWDS to the Isothermal Tests Performed at 100 and 110 °C Δ(tMAX tΔVlim)
parameter
maximum
peak value
time, tMAX [min]
ΔVlim
tΔVlim [min]
[min]
210 °C
51
1.75 104
40
11
63 bar
51
1.03 104
44
7
temperature, T
217 °C
47
1.75 104
39
8
pressure, P
65 bar
47
1.03 104
43
4
100 °C temperature, T pressure, P 110 °C
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Figure 11. Temperature (T) and ΔV(t) profile for the 110 °C isothermal test on 35 wt % HP.
ARTICLE
compared to pressure data. The experimental data obtained for the hydrogen peroxide decomposition show that the pressure always must be monitored carefully, because, under certain operating conditions, some substances are liable to completely decompose isothermally. The experimental results confirm the reliability of the EWDS also under operating conditions close to a real runaway reaction. The main advantages in the use of an EWDS based on the Strozzi and Zaldívar criterion is its operability on-line, using direct measurement of the temperature and pressure in the vessel. With respect to the use of the Hub and Jones criterion, the Strozzi and Zaldívar criterion presents significant advantages, including lower noise in the signal and a robustness, with respect to false alarms.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We wish to thank the Universities of Padua for financial support. Valeria Casson thanks the University of Messina for a Ph.D. grant, as part of the doctoral program in Nuclear and Industrial Safety coordinated by the University of Pisa.
Figure 12. Pressure (P) and ΔV(t) profile for the 110 °C isothermal test on 35 wt % HP.
Tables 3 and 4, the advance detection of the runaway reaction is higher for both 100 and 110 °C tests: by applying the Strozzi and Zaldívar criterion, the alarm would sound 7 and 4 min, respectively, before runaway.
’ CONCLUSIONS This study is focused on runaway decompositions of organic peroxides and especially of hydrogen peroxide, because they are very interesting in process safety and risk analysis research fields, because, in the past, they have been the cause of many severe accidents. Screening calorimetry has allowed us to analyze these processes in safety and gain useful results, such as onset temperatures, ranges of temperature and pressure evolved by reactions, and other results. Screening technique is an efficient instrument for hazard assessment in highly reactive systems, and it represents a first step in risk analysis studies and process scaleup. The data collected could be important support with regard to the sizing of venting systems to protect reactors and storage and transport tanks. An Early Warning Detection System (EWDS) has been applied off-line to the experimental data: this method requires only temperature or pressure measurements to detect a runaway reaction in the system. Because of this, it can be easily applied on-line, after an appropriate calibration, to industrial reactors and large storage vessels. The warning of anomalous behavior would allow the operator to adopt some corrective measures, such as quenching, blowing down, or inhibiting the reacting mass. The results show that EWDS works better with temperature data,
’ NOMENCLATURE BPO = dibenzoyl peroxide c = reactant concentration [mol/m3] cP = specific heat capacity of the reaction mixture [J/(K mol)] or [J/(K kg)] d2T/dt2 = second derivative of temperature, with respect to time [°C/min] dP/dt = derivative of pressure, with respect to time [bar/min] (dP/dt)MAX = maximum rate of pressure [bar/min] dT/dt = derivative of temperature, with respect to time or rate of temperature [°C/min] (dT/dt)MAX = maximum rate of temperature [°C/min] DTBP = di-tert-butyl peroxide HP = hydrogen peroxide k(T) = reaction rate constant as a function of temperature [(mol/ m3)1n/s] n = reaction order nf,HP = final quantity of hydrogen peroxide in liquid [mol] ni,HP = initial quantity of hydrogen peroxide in liquid [mol] PCARB = (4-tert-butylcyclohexyl) peroxydicarbonate PMAX = maximum pressure [bar] S = external surface area per unit of volume [m2/m3] t = time [s] T = temperature [K] Ta = ambient temperature [K] TJ = jacket temperature [°C] TMAX = maximum temperature [°C] Tonset = onset temperature [°C] TR = temperature inside the reactor [°C] U = overall heat-transfer coefficient [J/(m2 s K)] V = volume of a phase space element V R = reagent volume [m3] ΔH0 = standard enthalpy of formation [kJ/mol] ΔS0 = standard entropy of formation [J mol1 K1] ΔHR = heat of reaction [J/mol] F = density of the fluid mixture [kg/m3] 7534
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