RISK ASSESSMENT continued
investigated. A comparative results evaluation indicates which signals correspond to desired and undesired reactions.
Secondary reaction heat evolution dynamics can be obtained from iso- thermally or dynamically measured heat evolution rates, typically using DSC techniques or adiabatic calorimetry. (Adiabatic techniques require careful experiment selection and are less adequate for reactions with complex kinetics.)
Physical properties such as boiling points, vapor heat/pressure, and data related to process equipment are then explored to adequately assess thermal runaway consequences.
Evaluating process risk and criticality Process risk depends on the severity and probability of occurrence. Process
criticality, therefore, can be evaluated using relative levels of different temperatures attained if the desired reaction and decomposition reac- tions proceed under adiabatic conditions. Probability can be estimated using a time scale: If there is enough time left to take emergency measures before the runaway becomes too fast after cooling failure, runaway prob- ability will remain low.
Figure 5 – RC1e Reaction Calorimeter and iC Safety software.
The criticality of a reaction presenting thermal potential overall can be estimated by looking at process temperature (Tp
), MTSR, TD24
(time at which time-to-maximum rate, or TMR, is 24 hr), and MTT (maximum technical temperature, e.g., boiling point, maximum al- lowable pressure material, etc.). A graphical representation of these temperatures (see Figure 4) allows process classification from noncritical to highly critical.
Depending on the allocated criticality class, a process might be safe and not require any modifications at all. Higher criticality processes may re- quire considerable modification or a rework of the entire process.
Arriving at intrinsic safety Data obtained while running the smaller-scale experiments to answer
the questions that determine runaway potential (above) can then be used to guide necessary redesign, including changing reaction media, reordering additions, or making systematic variations of concentration, temperature, or feed profiles. Any changes undertaken will give rise to a new process with distinctly different hazard potential that will also need to be addressed with their own sets of experiments. In each case, arriving at a safe and optimized process requires the utmost attention to all sources of available information describing the process from raw materials to intermediates to final products.
While modeling runaway scenarios, using tools such as the RC1e reac- tion calorimeter along with appropriate software (see Figure 5) involves additional experiment steps. These actions help provide information to create not just an intrinsically safe process, but one that is ideal in terms of raw materials use, time-to-reaction, and overall manufacturing costs.
Figure 4 – Criticality graph from iC Safety software.
Urs Groth is Product Manager, Reaction Calorimetry, and Market Manager, Reaction Engineering, METTLER-TOLEDO AG, Business Unit AutoChem, Son- nenbergstrasse 74 8603 Schwerzenbach, Switzerland; tel.: +41-44-806 7379; e-mail:
urs.groth@
mt.com;
www.mt.com
AMERICAN LABORATORY • 20 • JUNE/JULY 2014
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