Evaluate Chemical Reactivity Concerns with Experimental Screening

In response to recommendations by the U.S. Chemical Safety and Hazard Investigation Board, in June 2007, OSHA had launched a Process Safety Management National Emphasis Program (PSM NEP) aimed at reducing or eliminating workplace hazards associated with catastrophic release of HHCs at petroleum refineries. By the end of 2009, the agency had completed comprehensive inspections of 81 refineries.

As part of the PSM NEP, OSHA emphasized chemical reactivity hazards. The chemical reactivity portion of process safety information (PSI) was of key area of interest for inspectors. Compliance was evaluated by determining whether the organization had a system in place for identifying chemical reactivity hazards and whether it had prepared and maintained sufficient documentation of that system. In particular, inspectors wanted to see that the plant had established safe operating limits associated with the process chemistry and the related energy balances.

Compliance officers also sought documentation of stability information for chemical storage. In its investigation of chemical reactivity hazards, the U.S. Chemical Safety and Hazard Investigation Board found that storage facilities experienced the second-highest number of reactive chemical incidents, second only to reactors.

Chemical facilities were expected to have learned from the PSM NEP experience and implemented robust, systematic methods of maintaining full compliance with the PSM standard. Understanding the chemical reactivity potential of our systems is integral to managing process safety. In this article we take a look at relatively quick and inexpensive tests that can be completed to evaluate any reactivity concerns.

Process Safety & Chemical Reactivity

Compliance requires that each reactive chemical undergo testing to determine the extent of the potential hazard. Testing is key to preventing harm to people, property, or the environment.

Types of chemical reactivity hazards:

• Runaway reactions
• Unanticipated decompositions
• Smoldering fires
• Phase changes

Where screening methods suggest thermal instability, experimental screening tests are relatively quick and inexpensive tests that can be completed to further evaluate any reactivity concerns. Coupling screening results with process operating information (e.g., temperature and pressure ranges, composition, order of addition) will determine if more comprehensive tests are necessary.

An experimental screening is recommended if either there is no literature available for the chemical reactivity of a sample or if a similar compound or mixture has a computed heat of reaction >100 cal/g, and is expected to be of high hazard (>100 cal/g in combination with CART value >700K) or oxygen balance between -80 and +120.

Experimental screening involves measuring data to identify the existence of chemical reactivity hazards using commonly available screening tools. These tools include combustibility tests, drop-weight or hammer sensitivity test, differential scanning calorimeter (DSC), and thermogravimetric analysis (TGA).

Experimental Screening Techniques

Differential Screening Calorimetry (DSC)

The DSC instrument is used primarily to determine glass transition temperatures, melting and boiling points, heat of fusion, specific heat, heats of reaction, and onset temperatures of reactions.

• Typically employ milligram scale samples (e.g., 1-20 mg)
• Highly accurate temperature measurement control, but relatively low thermal detection sensitivity due to inertial effects
• Experimental options include open and closed sample containers, system pressurization, and noble metal containers (to avoid catalysis/inhibition effects)
• Heating options include scanning, isothermal, and isoperibolic
• Good for thermal characterization and heat of reaction

Properties measured with DSC:

• Transitions like melt, glass transition, phase change, and curing
• Existence of endotherm/exotherm Heat of reaction (ΔHr)
• Heat of decomposition (ΔHd)
• Heat of combustion (ΔHc)
• Onset temperature (To) End temperature of reaction (Tf)
• Peak reaction temperature (Tp)

Thermogravimetric Analysis (TGA)

The TGA instrument is used primarily to determine the thermal and/or oxidative stabilities of materials as well as their compositional properties.

• Typically employ milligram scale samples (e.g., 1-20 mg)
• Highly accurate temperature measurement/control, but relatively low thermal detection sensitivity due to inertial effects
• Heating options include scanning and isothermal
• Good for samples that lose weight when heated

Properties measured with TGA

• Onset temperature of reaction (To)
• Peak reaction temperature (Tp)
• End of reaction temperature (Tf)

Experimental Screening Case Study

The example in Figure 1 below shows TGA and DSC scans for 100% dicumyl peroxide. In the figure, the TGA scan is blue and the DSC is green. Looking only at the TGA scan, the weight loss curve reveals two degradation reactions: the first major one starting at 74 °C and the second one at 213 °C. Looking at the DSC scan, two (2) major reactions were observed, beginning at 40 °C and 140 °C.

Figure 1 - Comparison of TGA and DSC Test Results for Dicumyl Peroxide

In this example, the DSC was able to detect the change in physical state occurring at 40 °C (melting) due to its measurement of heat flow. Alternatively, TGA was able to detect two reactions within the temperature range of 70 °C to 450 °C due to its ability to measure weight changes very accurately. The second reaction had a small heat flow and runs the risk of being confused with a slow baseline change in the DSC. The difference in information shows the benefit of using both DSC and TGA instruments in experimental screening.

Table 1 below summarizes the chemical and physical characteristics measured by DSC and TGA, which can provide insight into the chemical reactivity potential of a chemical or mixture.

Table 1 – Results of TGA and DSC Test Results for Dicumyl Peroxide

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