Adiabatic self-heat rate

This test can be used to give early detection of the initial exothermicity. It is possible to estimate thermokinetic parameters (e.g., the activation energy and the adiabatic self-heat rate) and to estimate how the initial temperature for self-sustaining reactions will vary with the quantity of material present. 

For a zero order reaction proceeding under adiabatic conditions the rate of reaction can be related to the adiabatic self-heat rate by ... 

The adiabatic self heat rate can be calculated from the experimentally measured self heat rate after correction for the thermal inertia Phi. 

Liquid ethylene oxide under adiabatic conditions requires about 200°C before a self-heating rate of 0.02°C/min is observed (190,191). However, in the presence of contaminants such as acids and bases, or reactants possessing a labile hydrogen atom, the self-heating temperature can be much lower (190). In large containers, mnaway reaction can occur from ambient temperature, and destmctive explosions may occur (268,269). 

Accelerating Rate Calorimeter (ARC) The ARC can provide extremely useful and valuable data. This equipment determines the self-heating rate of a chemical under near-adiabatic conditions. It usu-aUy gives a conservative estimate of the conditions for and consequences of a runaway reaction. Pressure and rate data from the ARC may sometimes be used for pressure vessel emergency relief design. Activation energy, heat of reaction, and approximate reaction order can usually be determined. For multiphase reactions, agitation can be provided. 

In the ARC (Figure 12-9), the sample of approximately 5 g or 4 ml is placed in a one-inch diameter metal sphere (bomb) and situated in a heated oven under adiabatic conditions. Tliese conditions are achieved by heating the chamber surrounding the bomb to the same temperature as the bomb. The thermocouple attached to the sample bomb is used to measure the sample temperature. A heat-wait-search mode of operation is used to detect an exotherm. If the temperature of the bomb increases due to an exotherm, the temperature of the surrounding chamber increases accordingly. The rate of temperature increase (selfheat rate) and bomb pressure are also tracked. Adiabatic conditions of the sample and the bomb are both maintained for self-heat rates up to 10°C/min. If the self-heat rate exceeds a predetermined value ( 0.02°C/min), an exotherm is registered. Figure 12-10 shows the temperature versus time curve of a reaction sample in the ARC test. 

Accelerating Rate Calorimetry. This is a heat-wait-search technique (see Fig. 5.4-62). A sample is heated by a pre-selected temperature step of, typically, 5 C, and then the temperature of the sample is recorded for some time. If the self-heating rate is less than the calorimeter detectability (typically 0.02 "C) the ARC will proceed automatically to the next step. If the change of the sample temj)erature is greater than 0.02 °C, the sample is no longer heated from outside and an adiabatic process starts. The adiabatic run is continued until the process has been completed. ARC is usually carried out at elevated pressure. 

De Haven [127] gives an overview of the results of accelerating rate calorimeter (ARC) experiments. The ARC was described in Section 2.3.2.3. As mentioned in the previous description, care must be taken in scale-up of results from experiments with relatively high phi-factors. For direct simulation of plant operating conditions, a phi-factor of 1.0 to 1.05 is required. As stated in [127], a decrease in the phi-factor from 2.0 to 1.0 increases the adiabatic temperature rise by a factor of 2, but the maximum self-heat rate increases by a factor of 20. Later in Chapter 3 (Section 3.3.4.6), an example of scale-up of ARC results is given. 

Special Studies High Sensitivity Calorimetry A- DESIREEV UNDESIRED dT/dt ATadiab Kinetics, EA, A Sample size 1- 50 ml, pW/g sensitivity Shelf life studies by accelerated aging Combine with low adiabatic to confirm solids low self-heating rate studies... 

Adiabatic calorimetry Chemical testing technique that determines the self-heating rate and pressure data of a chemical under near-adiabatic conditions. ( Adiabatic refers to any change in which there is no gain or loss of heat.) This measurement technique conservatively estimates the conditions for, and consequences of, a runaway reaction. 

A series of 11 nitrobenzaldehydes was examined by TGA, DSC and ARC techniques. Only 5-hydroxy-2-nitrobenzaldehyde decomposed exothermally in an unsealed container, but all did so in sealed capsules, under dynamic, isothermal or adiabatic conditions, with evolution of much gas. Initial decomposition temperatures in °C (compound, ARC value, and DSC value at 10°/min, respectively, followed by ARC energy of decomposition in kJ/g) were - 2-nitro-, 176, 220, 1.44 3-nitro-, 166, 218, 1.94 4-nitro-, 226, 260, 1.27 2-chloro-5-nitro-, 156, 226,. 697 2-chloro-6-nitro-, 146, 220,. 832 4-chloro-3-nitro-, 116, 165, 1.42 5-chloro-2-nitro-, -, 240, - 3-hydroxy-4-nitro-, -, 200, - 4-hydroxy-3-nitro-, -, 200, - 5-hydroxy-2-nitro-, -, 175, - 3-methoxy-4-nitrobenzaldehyde, -, 245°C, -. 4-Nitrobenzaldehyde showed by far the highest self-heating rate in ARC tests (approaching 100°/min at 240°C) anf the final pressure exceeded 170 bar when the pressure relief operated. The results are compared with those from various nitrobenzyl derivatives. 

By this technique, the temperature directly tracks the exothermal process under pseudo-adiabatic conditions. Pseudo, because a part of the heat released in the sample serves to heat the cell itself. Nevertheless, essentially in the USA, it became a very popular method as a screening technique. Concerning its sensitivity, for a well-tuned instrument, able to detect a self heating rate of 0.01 Kmin"1, with a sample mass of 2g, the sensitivity is as low as 0.5 W kg"1. 

The ( )-factor does not account for the heat loss to the environment. It is used to adjust the self-heating rates as well as the observed adiabatic temperature rise. 

An adiabatic method represents the most adequate technique for determining the relative tendencies of certain coals to heat spontaneously since it simulates most closely the real phenomenon. Conceivably, a field system would be similar to the adiabatic system but with appropriate modifications to hasten the oxidation process and increase the effluent gas concentrations within a reasonable test period. This could involve a more versatile system which would allow either the study of self-heating rates, similar to a method used by Guney (10) or which may be used for adiabatic calculations of a liability index through incorporation of a constant heat input. In the latter case, the heat might be supplied exclusively from the oxidizing air stream. 

Then, according to the result of the calculation given in Table 2, at the time when the adiabatic self-heating test started from the T,. is interrupted, i.e., at the time when the temperature of the peroxide has increased by 1.25 K from the the rate of increase in temperature of the peroxide will be about 1.346 (= 1.15 X 1.1701) K/h. 

When the rate of increase in temperature of 2 cm of the chemical subjected to the adiabatic self-heating test started from a T, has varied or has accelerated from 1.15 up to 1.346 K/h after the lapse of one hour, we may assume that the rate has remained, in fact, at a mean, and almost constant, value of 1.25 (1.15 + 1.346)/2 K/h during this one hour. 

Both liquid and powdery chemicals of the TD type are, however, the same to the effect that their exothermic decomposition reactions are accompanied with no phase transition. Therefore, when charged in the open-cup cell, or confined in the closed cell, in accordance with the self-heating property of the chemical, and subjected to the adiabatic self-heating test started from a r, 2 cm each of a liquid chemical, or a powdery chemical, of the TD type continues to self-heat over the at a very slow, but virtually constant, rate depending on the value of Ts in accordance with the Arrhenius equation, after its having been warmed up to the Ts. 

The temperature of the air bath, i.e., the nominal T, of the run, is set at a proper value by means of the temperature dial on the air bath on the basis of the self-heating property of the chemical tested to give estimated rates of increase in temperature of 1.25 K/h in the adiabatic self-heating test. 

In the meantime, the A Tdijf pen runs almost parallel with the time axis on the strip chart of the two-pen strip-chart recorder after the start of the adiabatic control. This denotes that the state, A - T - Tqiiii 0, IS rcslizcd. in 8.nd around 2 cm of the chemical confined in the closed cell and subjected to the adiabatic self-heating test. The A Tdiff pen, however, comes to drift toward the plus side on the strip chart in the course of time. This denotes that the state, 7 > Tam, comes to occur in and around 2 cm of the chemical confined in the closed cell, in spite of the adiabatic control, because the rate of increase in temperature, i.e., the rate of the exothermic decomposition reaction, of 2 cm of the chemical confined in the closed cell and subjected to the adiabatic self-heating test... 

The second reason is that it is observed usually in the adiabatic self-heating test performed for 2 cm of a chemical of the TD type that the temperature of the chemical continues to increase at a virtually constant rate, while it increases by 50 yV, in the case of the CA thermocouple, from a 7, as shown diagrammatically in Fig. 4 in Section 2.5, and as shown by a digital record of the digital D.C. millivolt recorder presented in Table 5 in Section 4.7. That is, so long as the increment in temperature of 2 cm of a chemical of the TD type subjected to the adiabatic self-heating test started from a 7, are within 50 juV... 

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