Temperature hazard evaluation

Figure 12-23. In(k seconds) versus -1,000/Absolute temperature plot of 20 w/w% di-tertiary butyl peroxide <(i = 1.09, (() = 1.75, and (() = 2.00). (Source Hazard Evaluation Laboratory Ltd.)...

Fig ure 12-27. Temperature versus time plot from a semi-batoh heat flow experiment. (Source Hazard Evaluation Laboratory Ltd.)... 

In addition to the extra hardware required for these experimental runs, the ARC was operated differently than under standard hazard evaluation conditions. Instead of heating, searching and waiting, the samples were heated to a specified temperature and were then maintained isothermally at that temperature for extended periods of time. Pressure and temperature data were then monitored and stored in the microcomputer at a rate of 1 Hz. It should be noted that the apparatus reverts back to normal operation (i.e., tracking an exotherm), if a heat rise rate greater than 0.02 °C/min is detected. 

In the hazard evaluation of the process, it was found that exotherms occurred with MNB-H2SO4 mixtures at temperatures above 150°C. The initiation temperature and extent of the exotherm depend on the acid concentration. During normal operation, the temperatures in die continuous stirred tank reactors and in the continuously operated separator are between 135 and 148°C. However, operating simulation showed that for certain feed rates well out of the normal operating range, the temperature could reach 180°C and a runaway is thus possible. 

A thermal scan showed that the exotherm of the principal reaction can be significant if the system is neither controlled nor vented. From isothermal studies (i.e., experiments at constant temperature), time-to-maximum rate was determined which was comparable to that obtained from the DCS data. The larger scale data showed, not surprisingly, more rapid reactions at elevated temperatures. Thus, it was decided to use the DSC data at lower temperatures, and the larger scale test data at higher temperatures for hazard evaluation. 

Evaporation Rate The rate at which a material is converted to vapor (evaporates) at a given temperature and pressure when compared to the evaporation rate of a given substance. Health and fire hazard evaluations of materials involve consideration of evaporation rates as one aspect of the evaluation. 

The rates of reaction (energy release in the form of heat or pressure) which depend on the temperature, pressure, and concentrations. In any hazard evaluation process, the rates of reaction during normal and abnormal operations (including the worst credible case) must be considered in order to design an inherently safe process. 

We think it is valuable to consider the techniques and approaches used in the rocket propulsion areas since many of the concerns, and the devices and approaches developed to cope with these concerns, can be used by the chemical industry. This is perhaps particularly true when one considers hazard evaluation. The procedures developed in the propellant industry to assess explosive and fire hazards are directly applicable to the chemical process industry. In addition, some of the techniques developed for measuring the properties of liquid propellants, particularly at elevated temperatures and pressures (and frequently on materials which can decompose or even detonate), can be used in similar studies with actual commercial chemicals. 

The Hazard Evaluation Chemist quantifies the actual potential hazards involved in an operation - heats of reaction, gas evolution rates, minimum decomposition temperatures, etc. 

When testing on a particular process is completed, the Hazard Evaluation Chemist prepares a report identifying any reactive problems which were observed. He also reports the actual values of any physical parameters (heat of reaction, decomposition temperature) which were determined and makes recommendations as to what areas (if any) should be addressed by the Process Engineer. 

There are several points to be kept in mind when using physical testing as part of process hazard evaluation. First, the limitations of the test method should always be kept in mind. For example, it has been pointed out that different thermal stability tests give different exotherm detection temperatures. In most cases it is not possible to define an exact exotherm onset because the decomposition reaction s rate does not go to zero as the temperature is lowered. Overconfidence in test results can be just as much of a hazard as no knowledge at all if the limitations of the tests are forgotten. 

The qnantity of gas or vapor in a container is a function of (in descending order of importance) the container volnme, inclnding the connected piping and other nonisolated equipment, the pressure, the molecular weight of the gas or vapor, the temperature, and the compressibility factor for the gas or vapor. The following eqnation can be used with sufficient accuracy for hazards evaluations ... 

Adiabatic calorimetry is an important technique for the study of self-propagating and thermally-sensitive reactions. As a thermal hazard evaluation technique, ARC is particularly suitable for large samples, including cells. ARC consists of a container that maintains the test sample under adiabatic conditions with respect to its environment. The inner temperature is... 

In the case of samples taken to very high temperature or pressure, capsules which hold internal pressures up to 150 bar should be used. Disposable high-pressure capsules which avoid the need for cleaning are available. For hazard evaluation, gold-plated high-pressure pans should be used, since these should be inert towards the sample. 

Explosibility and Fire Control. As in the case of many other reactive chemicals, the fire and explosion hazards of ethylene oxide are system-dependent. Each system should be evaluated for its particular hazards including start-up, shut-down, and failure modes. Storage of more than a threshold quantity of 5000 lb (- 2300 kg) of the material makes ethylene oxide subject to the provisions of OSHA 29 CER 1910 for "Highly Hazardous Chemicals." Table 15 summarizes relevant fire and explosion data for ethylene oxide, which are at standard temperature and pressure (STP) conditions except where otherwise noted. 

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