Adiabatic heat-pressure accumulation test

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Adiabatic heat-pressure accumulation test

1. Project definition

Identify the safety characteristics for safe handling of the test substance with respect to thermal stability.

2. Experimental analysis

This section describes the experimental methods. The test substance was tested in its as-delivered condition. No pre-treatment was performed.

2.1. Determining temperature development in the adiabatic heat-pressure accumulation test

The reaction behavior is determined in the adiabatic heat-pressure accumulation test performed on the basis of UN transport guidelines Test H.2, according to the test setup defined by Grewer and Klais[1] or the VDI guideline 2263, Sheet 1.

The setup of the measuring system consists of a pressure vessel with a volume of approx. 0.75 L. A Dewar flask made of glass with an internal volume of approx. 0.2 L is used as a reaction tank. The Dewar flask is thermally insulated from the environment by the vacuum in its double walls and its mirror coating. After sealing, the pressure vessel is placed in a furnace. After reaction start, the furnace temperature is adjusted to the sample temperature. Consequently, reactions or decompositions can be tested under quasi-adiabatic conditions above the starting temperature. The temperature is measured by a thermocouple, which is located in the sample in a protective glass sheath. In addition, the pressure in the gas space of the pressure vessel is measured.

The temperature profiles of the sample and the furnace, as well as the pressure profile in the pressure vessel during the entire experiment, are measured and documented.

The preparation of the sample and the following experiment were performed under a nitrogen atmosphere.

3. Test results – safety parameters

This chapter describes the results of the tests and the safety parameters derived from them.

3.1. Determining temperature and pressure development in adiabatic heat-pressure accumulation test

Test description

101 g of the test substance was put into a thin-walled Dewar flask at room temperature. The Dewar flask was installed in the pressure vessel, which was sealed and then placed in the furnace. The furnace temperature was set to a setpoint value of 80°C. The temperature and pressure were continuously recorded by a data logging system, Figure 1.

Figure 1: Temperature and pressure profile in the adiabatic heat-pressure accumulation test.

The temperature of the sample reached the set oven temperature of 80°C within approx. 49 hours. At this temperature, a pressure of about 0.3 barg was recorded. After reaching the furnace temperature, the sample continued to slowly self-heat. From a test time of about 57 hours, the furnace temperature was adjusted to the sample temperature. The self-heating of the sample transitioned to an exponential temperature rise. This led to a maximum temperature of 203°C after about 158 hours of testing and a maximum pressure of 14 barg. At this pressure the dewar burst and the sample came into contact with the autoclave wall. The test was ended and the cooling curve was recorded.

Figure 2 shows the test substance and the Dewar flask after the end of the test.

Figure 2: Decomposed product and broken glass Dewar flask after the end of the test.

Derived parameters

The application of pressure via the reciprocal temperature (Antoine application) shows that the pressure build-up is not a pure vapor pressure effect. After the test and cooling down to about 29°C, a residual pressure of about 4.8 barg remains in the autoclave which is due to the formation of permanent gas Figure 3. From the pressure after the end of the test and the corresponding temperature of 29°C, and taking into account the free gas volume of about 360 ml after the bursting of the Dewar flask (assumed density of the reaction mixture: 0.9 kg/L, filling level after the bursting of the Dewar flask ≈ 21%), a permanent gas quantity of 16 LN/kg of substance produced and related to standard conditions (0°C and 1.01325 barabs) can be calculated.

Figure 3: “Antoine application”: absolute pressure as a function of the reciprocal temperature.

The pressure rise rate in Figure 4 is derived from the temporal pressure profile. This pressure rise rate also includes the pressure rise caused by other effects, such as an increase in vapor pressure or expansion of the liquid. A maximum pressure rise rate of the test substance is specified at about 184°C of (dp/dt)max = 6 bar/min. From this pressure rise rate, a specific gas volume flow in a gas space can be calculated which is set at an initial filling level of 32% (corresponding to a free gas volume of 240 ml at the beginning of the test). The maximum specific gas volume flow is therefore V̇spec, max = 8 L/(min•kgsubstance).

Figure 4: Pressure rise rate and gas production rate in the adiabatic heat-pressure accumulation test.

The thick-walled Dewar flask used as the measuring vessel has a heat capacity of 49 J/K. Combined with the assumed specific heat capacity of Cp = 2000 J/(kg•K) for the test object, this results in a total heat capacity for the measuring system of 251 J/K and an a φ-factor[2] of φ = 1.2. This and the measured temperature increase of 123 K (temperature rise from 80°C to 203°C) results in a reaction heat of ∆HR = -310 J/gsubstance. Taking into account the φ factor, an adiabatic temperature rise of ΔTad = 153 K can be calculated from the measured temperature rise.

The temperature rise rate is determined from the temperature profile. Assuming a total heat capacity of the measuring system of 251 J/K, the heat production rate is determined from this and related with the weight of the sample taken, Figure 5. A maximum temperature rise rate of (dT/dt)max = 40 K/min and a maximum heat production rate of Q̇ = 1700 W/kgsubstance are identified. These occur at a temperature of about 190°C.

Figure 5: Rate of temperature rise and heat production rate in the adiabatic heat-pressure accumulation test.

From the plot of the temperature rise in the Arrhenius diagram (logarithmic plotting of the rate of temperature rise as a function of the reciprocal, absolute temperature), the activation energy of the decomposition reaction is obtained from the slope of the straight line by fitting a straight line to the measured values, assuming a simple model for a reaction of 0th order, Figure 6. A temperature range between 75°C and 150°C is used to determine the activation energy. The activation energy of the decomposition reaction is specified as EA = 135 kJ/molsubstance.

Figure 6: Kinetic analysis of the temperature profile in the Arrhenius diagram.

The lowest ambient temperature at which the heat loss of a container corresponds to the heat production rate of the test substance is called self-accelerating decomposition temperature, or SADT. It is identified in the same way as in UN Test H.2. For the assessment, the container’s heat dissipation capacity is assumed to be 63 mW(kg∙K) (corresponds to a 50 L 1A1-type steel drum). The point where the tangent of the heat dissipation rate intersects with the heat production rate corresponds to the equilibrium temperature set in the container. The underlying kinetics are obtained from the temperature rise at the beginning of decomposition in the temperature range between 90 and 150°C. The identified SADT is obtained at 87°C. A calculated SADT is rounded up to the next integer multiple of 5 as defined in the UN transport guidelines. The SADT calculated in this way results at 90°C, Figure 7.

Figure 7: Determining the SADT for a 50 L steel drum (type 1A1, heat dissipation: 63mW/(K∙kgsubstance)).

The adiabatic induction time indicates the time span within which, under adiabatic conditions, the maximum rate of temperature rise is reached. The adiabatic decomposition temperature for 24 hours (ADT24 h) describes the temperature at which the process requires 24 hours under adiabatic conditions to reach the maximum rate of temperature rise. In Figure 8 the range between 85°C and 120°C is adapted to the measured values by a straight line. 190°C is used as reference temperature and thus the maximum of the measured temperature rise rate.

Based on the real measured values, the resulting ADT24h is 91°C.

Figure 8: Induction time as a function of the reciprocal, absolute temperature to determine the ADT24h.

Summary of the main results of the adiabatic test in the heat-pressure accumulation test:

Weight of sample taken m 100  g
Initial filling level 32  %
Formation of permanent gas Yes Figure 3
Quantity of permanent gas produced under standard conditions Vpermanent gas 16  LN/kgsubstance
Maximum rate of pressure rise1* (dp/dt)max‘ ≥6  bar/min Figure 4
Maximum specific gas volume flow1* spec, max ≥8  L/(min•kgsubstance)
φ-factor φ 1,2
Adiabatic temperature rise2* ΔTad ≥153  K
Energy of the thermal effect2* ∆HR ≥-310  J/gsubstance
Maximum rate of temperature rise1* (dT/dt)max ≥40  K/min Figure 5
Maximum rate of heat production2* max ≥1700  W/kgsubstance
Activation energy2 EA 135  kJ/mol Figure 6
Self-accelerating decomposition temperature
(50 L steel drum; type: 1A1)2
SADT 90  °C Figure 7
Adiabatic decomposition temperature for 24 hours1 ADT24 h 91  °C Figure 8
1  Measured value

2 φ-factor corrected or based on values corrected accordingly

*  Due to the bursting of the dewar, the maximum values could not be completely recorded. The maximum detected values (or the values derived from them) are given. It cannot be ruled out that more critical values will be reached under actual adiabatic conditions.

4. Analysis

In this chapter the previous tests are assessed and statements are formulated regarding safe handling.

4.1. Thermal stability

Adiabatic heat-pressure accumulation test

The test substance was examined in an adiabatic heat-pressure accumulation test. Starting at 80°C, an adiabatic temperature rise of at least 153 K was detected. Assuming a specific heat capacity of the test substance of 1700 J/(kg•K), a reaction heat of approx. -310 J/g is indicated.

Based on the time of the maximum rate of temperature rise, the resulting ADT24 h is 91°C. In accordance with TRAS 410, the limit temperature for safe handling, taking into account a safety margin of 10 K from the ADT24 h, is h Texo = ADT24 h – 10 K = 81°C.

At 81°C, a specific gas volume flow of <10-3 L/(min•kgsubstance) is to be expected, which must be taken into account in a closed system.

Starting from the maximum temperature rate of temperature rise, adiabatic decomposition temperatures (ADT) were determined for various adiabatic induction times.

Compilation of adiabatic decomposition temperatures (ADT) for various induction times:

Induction time Adiabatic decomposition temperature (ADT) Spec. gas volume flow
[L/(min∙kgsubstance)]
Heat production rate
[W/kgsubstance]
24 h ADT24 h = 91 °C ≤ 10-3 0,2
12 h ADT12 h = 96 °C 1,5 ∙ 10-3 0,4
6 h ADT6 h = 102 °C 3,0 ∙ 10-3 1,5
2 h ADT2 h = 112 °C 10,0 ∙ 10-3 3,0

[1] Grewer Th., Klais, O.: Exothermic Decomposition: Investigation of the Characteristic. Properties, VDI Verlag Düsseldorf, 1988.

[2] Ratio of the total heat capacity of the measuring system (sample and heat capacity of the dewar to be considered) to the heat capacity of the sample.

Sources
UN transport guidelines:
UN Recommendations on the Transport of Dangerous Goods: Manual of Tests and Criteria, Rev. 7 (2019) and UN Model Regulations, Rev. 21 (2019)

VDI 2263 Sheet 1: “Test methods for the determination of the safety characteristics of dusts.”

DIN EN ISO 11357-1: “DSC: General principles”

TRAS 410: “Identify and control exothermic chemical reactions”