Advanced Interactions of Hazardous Materials

¶ … Interactions of Hazardous Materials

Chemistry of the process

MCMT, methylcyclopentadienyl manganese tricarbonyl, was manufactured in a three-step process by T2 Laboratories, Inc. The first step of the chemical reaction necessitated the use of heating to activate or initiate the reaction. All three of the steps that followed were heat producing (exothermic) and in turn required some form of cooling. The first step (known as metalation), involves molten metallic sodium being reacted together with methylcyclopentadiene (MCPD) using diglyme (diethylene glycol dimethyl ether) as a catalyst in order to produce two MCPD molecules; these then reacted with sodium. The T2 firm released the hydrogen gas by-product produced by this reaction into the atmosphere.

The second step, referred to as substitution, involved the addition of dry manganese chloride powder to the reactor by the T2 firm employees. The manganese chloride then reacts with sodium MCPD within the reactor, resulting in the formation of sodium chloride as a by-product and manganese dimethylcyclopentadiene.

The third and final step, referred to as carbonylation, involves the injection of Carbon monoxide gas (CO) at the rear end of the reactor by the T2 chemists. The gas is bubbled, under pressure, through manganese dimethylcyclopentadiene. This final step also involves each of the two methylcyclopentadiene molecules on the manganese complex being replaced by 3 carbon monoxide molecules, thus forming MCMT.

After the carbonylation phase, the process operator at T2 then distilled the mixture to eliminate diglyme and MCMT. The remaining sodium chloride and methlycyclopentadiene were eliminated as solid wastes. Diglyme was however recovered, and later reused in the three step process (United States (U.S.) Chemical Safety and Hazard Investigation Board, 2009).

What the company and other organizations should have done to prevent the incident from occurring.

The T2 chemists failed to recognize the reaction runaway hazard linked to the MCMT that they were producing. Assuming that they had been aware of the reaction hazard, they could have incorporated additional safety mechanisms or measures. The cooling system utilized by the T2 firm was vulnerable to point failures, because of the absence of design redundancy. Thus a backup cooling mechanism could have stopped the disaster from occurring. The MCMT process relief system was not capable of relieving a runaway reaction pressure (U.S. Chemical Safety and Hazard Investigation Board, 2009).

The risks of an overheating reaction mixture were overlooked or underestimated by the T2 laboratory chemists. Adequate backup mechanisms were not put in place to offer additional quenching or cooling of the reaction mixture, and/or to relieve the pressure if overheating occurred. The most crucial aspect was the ability to foresee, expect, or at the very least, test for the likelihood of any unwanted exothermic reaction(s) occurring in the accessible temperature range of the experimental conditions utilized. It is sensible to make sure that a safety allowance, or margin of 100 oC or more, in the range of the set reaction temperature and the thermal runaway starting temperature should be planned. The scientists at T2 did provide a safety allowance of 199 "C -- 177 "C = 22 "C, but this was insufficient. The firms should have assessed their temperature safety margin utilizing exothermic runaway onset calculation calorimetric instruments such as the Advanced Reactive System Screening Tool (ARSST) or differential scanning calorimetry (DSC) (Levin, 2014).

Exothermic reactions

A chemical reaction that involves the release or emission of energy such as heat or light is referred to as an exothermic reaction. This can be illustrated using a chemical equation: reactants ? products + energy. Thermal runaway is a situation whereby an increase in temperature alters the conditions in such a manner that it results in a further increase in temperature, often causing destructive results. In other words, a thermal runaway can be thought of as uncontrolled positive feedback. This describes a process which is made even faster by increased temperature, and thus releases energy that results in even further temperature increases (Wikipedia, 2015). The safety problem associated with the T2 firm's preparation of MCMT was caused by the failure to recognize that the set reaction temperature for the process was quite close to the start-temperature for a runaway exothermic decomposition reaction. The metallation and de-dimerization of methycyclopentadiene using sodium metal in the initial step requires heating the mixture in MeOCH2CH2OCH2CH2OMe (diglyme) to achieve the required reaction temperature of 177 oC. However, even a slight further temperature increase to 199oC was adequate to achieve the activation energy threshold for an unwanted secondary reaction between the diglyme solvent and the sodium metal. Thus the cooling system of the reactor did not work as expected; the exothermic reaction between the diglyme solvent and sodium metal resulted in an over 100 degrees Celsius per minute temperature rise; and more than 2000 atm of pressure per minute was produced, which rapidly surpassed the pressure containment capacities and the relief capacities of the reactor, thus causing an explosion (Levin, 2014).The United States Chemical Safety and Hazard Investigation Board (CSB) approximated that the energy released by the explosion was equal to the kind of energy released by detonating about 1,400 pounds of trinitrotoluene (TNT). The reactor, even though equipped by a pressure relief vent, was only designed to release pressure when 400 psi above the atmospheric pressure was generated in the system. However, by the time this pressure threshold had been reached, the rate of pressure increase was so rapid that the release of pressure through the vent was insufficient to prevent rupture and the subsequent explosion of the reactor. CSB also estimated that a lower set pressure releasing vent design, set at only 75 psi above the atmospheric pressure, would have been enough to stop temperature increase above the reaction runaway temperature. That release would have allowed the heat of the boiling diglyme solvent to absorb the reaction heat and stop the temperature from reaching the thermal runaway start-temperature (Levin, 2014).