Adiabatic screening tests

Non-adiabatic screening tests such as Carius tube111 and the Accelerating Rate Calorimeter (ARC ), corrected for sample heat losses due to thermal inertia, can also be used for screening. If it is known that the reaction is a vapour pressure system, DSC may be used. 

Most screening tests are likely to lump all reactions that generate gas together. Tempered hybrid systems will not be distinguished but these will require a smaller relief area than a gassy system with the same gas generation rate. If the worst case is subsequently-found to be a tempered hybrid reaction, rather than a gassy system, then some reiteration to check that it is still the worst case may be required. 

If all the identified relief scenarios give rise to the same system type, then the worst case is likely to be that which gives the highest rate of reaction, at the appropriate temperatures, in the screening test, as follows  

However, it must be recognised that these maximum rates will be underestimates due to the heat losses from thej sample in the tests. Careful interpretation is needed when using such non-adiabatic screening tests to find the worst case, and this is best done by skilled and experienced analysts. In addition, for gas-generating systems, it may be difficult to contain the peak pressure using a screening test such as the Carius tube. The use of an adiabatic screening test (see 3.5.3 below) may therefore be preferred. -  

Non-adiabatic screening tests can be used to narrow down the range of relief scenarios which may be the worst case, but in many cases two or three possibilities may still remain.  

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Other screening tests, including shock sensitivity and flammability tests, and thermodynamic computations raised no specific concerns. After the explosion, the material was tested in an ARC. Such testing showed that a typical batch of the compound could self-heat to full decomposition if held under adiabatic conditions at 120 to 125°C for 24 hours. These tests were run within the normal processing temperature range, and the ARC test results were hailed as demonstrating the likely cause of the accident. 

Adiabatic Screening A UNDESIRED onsef A aDIAB P, 1, t, dP/dt, dT/dt Simple kinetics EA, A Sample — a few grams Reasonably quick to test (— /> day) Poor/moderate sample agitation Not reliable for scale-up (high cefaclor)... 

The main disadvantage of these screening tests is that the test conditions tend towards being isothermal (whereas the conditions in a reactor runaway are nearer adiabatic). This can mean that the tests are not always sufficiently sensitive, and that the measured onset temperature for thermal decomposition is a function of the sample heating rate. Also, the small sample size may lead to it being unrepresentative of plant materials, and evaporation losses can lead to errors unless sealed test cells are used. 

Some indication of the rates of heat and gas production can be obtained from the results of the screening tests (Section 3.4, page 32), but getting accurate data requires the use of adiabatic calorimeters (Sections 3.6.2 and 3.6.3). A different type of calorimeter, developed specially to provide data for the design of venting systems, uses a pressure equalization system and a weak test cell with a low thermal mass (Section 3.6.3). 

Screening tests (see Section 4.3, page 52) can give guidance on when secondary reactions or decompositions may occur. Once this has been established as a possible hazard, adiabatic calorimetry is needed to give a reliable value for the minimum temperature at which self-heating will stan under plant conditions. 

The results of the hazardous chemical evaluation are used to determine to what extent detailed thermal stability, runaway reaction, and gas evolution testing is needed. The evaluation may include reaction calorimetry, adiabatic calorimetry, and temperature ramp screening using accelerating rate calorimetry, a reactive system screening tool, isoperibolic calorimetry, isothermal storage tests, and adiabatic storage tests. 

The RSST (reactive system screening tool) is a laboratory device used to characterize the reactive nature of liquid materials. It is essentially an adiabatic calorimeter, with the test sample heated at a constant temperature rate until an exothermic reaction is encountered. 

The test is primarily a screening tool relative to reactivity of substances and reaction mixtures and is highly useful for that purpose. The determined initiation temperature is approximate. The energy calculations based on temperature increase and heat capacities are semi-quantitative because of the quasi-adiabatic mode of the system operation. The method of insulating the test cell results in moderate reproducibility of temperature rise and related pressure rise. Another disadvantage is the relatively small sample quantity with respect to full scale quantities thus, there could be a problem in that the sample may not be truly representative. 

Five of nine respondents to the CSB survey frequently use both screening and more sophisticated approaches, including adiabatic calorimetry, to determine the thermal stability or compatibility of process materials. Seven of nine respondents use screening alone for chemical reactivity testing. The most often used testing objectives are ... 

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