In the ’50s, the company Ethyl Corporation (the predecessor of the current Afton Chemical Corporation) developed a product to be used as antiknocking additive for car engines. The purpose was to replace additives with lead, which were extensively used in the world until that time, for a new additive containing manganese, which was a bit more environment-friendly than lead, but not that much, as will be seen below.
The new product, the Ecotane, based on the methylcyclopentadienyl manganese tricarbonyl compound, also known as MCMT or MMT, promised to be the safest and most economic and effective antiknocking non-lead additive in the market. It also helped reducing carbon dioxide emissions when used in the industries, as well as nitrous oxide and carbon monoxide emissions when used in gasoline for cars. Throughout time, it was used in about 20 countries of the 5 continents, including our country, favored, in particular by the end of the ’80s, by measures aiming at restricting and eliminating lead from fuel that were implemented throughout the world(1). In Argentina, for instance, the maximum allowed lead content in gasoline for cars is 13 mg/l, while the maximum manganese content is 18 mg/l. (2)
These are some of Ecotane's characteristics:
- Sability at high temperatures
- Low steam pressure
- Low melting point
However, despite the practical advantages announced during more than 20 years of use, T2, the manufacturer itself acknowledges in the Safety Data Sheet of Ecotane risks related to the product in pure state(3). This is clearly illustrated by the transport classification assigned by T2:
- UN Number: UN 1992.
- Proper Shipping Name: Flammable liquids, toxic, n.o.s. (petroleum distillates and methylcyclopentadienyl manganese tricarbonyl, mixture).
- Risk Class: 3 (Flammable liquid
- Secondary Risk Class: 6.1 (toxic substances)
- Packing Group: III
- Additional Risk: Marine pollutant
According to this information, the product is dangerous for transport and is subject to two risk classes: Class 3 (flammable liquid) and Class 6.1 (toxic substance), and to an additional risk as it is considered dangerous for the environment.
The product safety sheet specifies the risks mentioned above as well as MMT physical-chemical properties, but no objective parameters of acute toxicity, such as oral Lethal Dose 50. Even the technical-commercial sheet(3) mentions MMT as a dangerous substance for transport, although such classification is not complete as it is not declared a “marine pollutant”. On the other hand, a technical report prepared by the manufacturer in 2006(1) states that MMT is safe, economic and effective, and that it is also beneficial for the environment as it replaces lead by manganese. The report further states that “it is not a risk for aquatic organisms.” This information is not consistent with the one included in the Safety Data Sheet prepared by the same company.
As regards the assessment of risks to health, the report above makes an analysis considering the exposure to combustion products of gasoline containing MMT, but the analysis does neither contain objective data on product toxicity, which will be more useful for users preparing mixtures, nor explain anything with respect to the risk of liquid flammability.
The main risks to health evaluated by T2 are those related to the inhalation of emissions with manganese. This metal may cause irreversible neurological damages if inhaled at high concentrations for a long time, causing mental and emotional disturbances in human beings, in addition to lack of coordination and slow body movements. This disease is called “manganese disease”(4). Public concern for MMT as an antiknocking additive for gasoline arose in this context. After making in 2004 an assessment of the risks involved in the use of MMT, the United States Environment Protection Agency (EPA) indicated that, with the information available at that time, it was impossible to determine the existence of risks to public health caused by exposures to emissions of fuels containing that additive. Currently, the company Afton Chemical, the only manufacturer of the product in the United States, is carrying out, as required by the EPA, studies to complete the assessment of risks to public health related to the use of MMT(5).
In Europe, on the other hand, MMT is not included in the list of Priorities for the Evaluation and Control of the Risks of Existing Substances, according to Regulation EEC793/03.
Risk Assessments
So far, we have mentioned the different risk assessments taken into account with respect to the MMT.
On the one hand, an assessment of risks in transport including a classification, with a United Nations number, a main class of risk and a secondary class of risk, a dispatch name and a Packing Group. It could be noted that this information may be inconsistent with other data presented by T2 in technical-commercial sheets and documents, but the information is there and it somehow considered that there are certain risks involved in MMT transport.
On the other hand, assessments of risks to public health prepared jointly by another manufacturer, Afton Chemical, and the US EPA in connection mainly with the risks derived from the inhalation of gaseous emissions from cars using gasoline containing MMT as additive, may be highlighted. Such studies are still made until today and their progress may be followed by visiting the EPA Web page.
However, it is also necessary to consider the risks involved in production, which also entail an analysis of the reactive agents used and the processes involved. This means, an analysis of the risks inherent to processes, which may be made through the so called HAZOP study. This was precisely one of the weak points of the company, as it failed to carry out such analysis, according to the Chemical Safety and Hazard Investigation Board of the United States (CSB)(7).
The Accident
At the midday of December 19, 2007, the City Jacksonville, in the United States, was rocked by a terrible explosion which resulted in four deaths and 32 injured people.
The cause of the accident was a self-accelerated exothermic reaction in a batch reactor of the company T2 during the manufacturing of the product used as antiknocking additive for gasoline: methylcyclopentadienyl manganese tricarbonyl, also known as MCMT or MMT.
Chemical reactions may absorb or release heat. When there is heat absorption, the reaction is said to be endothermic. When heat is released as a result of the reaction, the reaction is said to be exothermic.
However, such heat absorption or release does not necessarily mean that it will be followed by a decrease or an increase in temperature, respectively. If the system (the reactor) allows heat exchange with the environment, temperature may be maintained at a constant value. The problems of these processes arise when heat exchange with the environment is not enough or when the heat generated by the reaction starts to be higher than the heat released to the environment, thus, increasing the system temperature.
When temperature starts to increase, the rate of reaction becomes more important. Such speed depends on two key factors: concentration of reactive agents and temperature.
When temperature increases, the rate of reaction increases exponentially and, consequently, there is a larger heat release (therefore, its increase is also exponential with respect to temperature increase), which is increasingly difficult to dissipate out of the system (given that the rate at which heat generated by the reaction is eliminated increases lineally with temperature, rather than exponentially), and this causes a continued temperature increase. Control of the reaction is thus lost until reaching a state of self-acceleration that would give rise, for instance, to other secondary effects which would not occur at lower temperatures, such as boiling over, or to a pressure increase such that would cause an explosion.
This type of reactions which are absolutely out of control is called runaway reactions and may be considered one of the chemical reaction risks that may exist in industrial processes, together with the reactions between incompatible products and chemical decomposition reactions which are typical of self-reactive substances.
These are some of the causes of self-acceleration of exothermic chemical reactions acknowledged by the Health and Safety Executive of England:
- Mistakes during the handling of reactive agent when added to the reaction.
- Inappropriate stirring in the reactor.
- Failures in the temperature control systems.
But above all, the underlying causes of most accidents involving a chemical reaction are:
- Poor knowledge of the chemical reactions involved in the process.
- Inappropriate heat transfer systems
- Inappropriate training of the staff operating the process
- Human errors related to the failure to comply with the Company Operative Procedures(6).
As regards the T2 accident, the United States Chemical Safety Board determined that the direct causes were errors in the size of the rupture disk and failures in the cooling system.
The rupture disk was sized considering the maximum generation of hydrogen gas expected during a normal operation and not the overpressure that a self-accelerated chemical reaction could generate.
The cooling system used was designed in a laboratory scale. However, T2 not only took that cooling system to an industrial scale but then, in 2005, it increased the reactor capacity in one third without taking into account that the increase in the amount of raw materials could increment the heat released during the reaction. Obviously, T2 overlooked the fact that the change of scale without an appropriate risk assessment could lead to an underestimation of certain factors which, although irrelevant in a laboratory, are determinant in an industrial chemical reactor.
Specifically, the change from the laboratory to the plant scale has a direct impact on the ratio of generated/released heat, and the designers of the processes involved in the accident should have taken that into account. The highest the volume of the reactor is, the highest the increase in heat generation and release is. However, heat generated by the reaction increases faster because that depends on the cube of the vessel diameter, while heat release depends on the surface through which heat is transferred to the environment, i.e., the square of the vessel diameter. As a conclusion, applying safety measures for a laboratory or a pilot plant to an industrial plant may not be effective, as heat release at this latter scale may be insufficient, thus, causing a temperature increase favoring a self-accelerated reaction, as well as the unwanted secondary reactions mentioned above.
Importance of Risk Identification and Description
However, the root cause, as determined by the research carried out by the CSB, was that product developers failed to recognize the risk of self-accelerated reaction during the production of MMT. The CSB has not found any evidence that T2 had performed the relevant risk assessment while designing the processes and/or to carry out the scale changes made. A typical analysis, HAZOP, could have been applied to the processes involved.
This has been a repeated factor throughout the years in other companies. The identification and description of the risks related to chemical reactions of substances is one of the essential elements to be taken into account at the time of designing or redesigning processes. All decisions regarding process safety must be taken at the time of identifying and describing such risks; consequently, a wrong assignation and assessment of risks is usually mentioned as the root cause of the accidents associated to chemical reactions. According to the Chemical Safety Board, about 25% of industry incidents, out of a total of 167 accidents recorded in that Entity from 1980 to 2001, were attributed to the wrong identification of the chemical risks involved in the processes(8); this entails a deficient knowledge of the chemical aspects of the process. The other causes may be grouped in inappropriate plant design, inadequate plant control and safety systems, and inadequate operation procedures and instructions(9).
35% of those 167 accidents recorded which involved risks associated to chemical reactions were attributed to risks of self-accelerated exothermic reaction (or runaway reactions)(8), and the others were attributed to accidents related to chemical incompatibilities and chemical decomposition caused by heat or impacts (something very usual in self-reactive substances).
These statistics show the importance of an appropriate identification and description of risks, in particular those involving chemical reactions, for an appropriate design of processes including safe operations. In the case of T2, the responsible individuals took into account the aspects of safe use and transport and identified the risks related to those activities, but they never took into account, or failed to detect, the risks associated to chemical reactions of the substances involved in the production of MMT.
It should be noted that this type of accidents has been still occurring in the United States for the last years with different consequences. The accidents in the plants of Morton International Inc (Paterson, New Jersey, 1998), Concept Sciences Inc (Allentown, Pennsylvania, 1999), and Synthron, LLC (Morganton, North Carolina, 2006) are examples of that(7).
In Argentina, there is no public information on accidents of this kind in the industry, and there is no such a huge industrial activity as that in the United States either. However, that does not guaranty that an uncontrolled chemical reaction may not occur during the manufacturing of a product.
1. “Technical Paper. Introduction to Ecotane ® (methilcyclopentadiene manganese tricarbonyl),” R. S. Gallagher, M.F.Wyatt, T2 Laboratories Inc., 2006.
2 Resolution No. 1283/06 of the Secretary of Energy.
4 Agency for Toxic Substances and Disease Registry (ATSDR), 2001. Toxicological Profile of Manganese, Atlanta, GA, United States Department of Health and Human Services, Public Health Service.
5. Comments on the Gasoline Additive MMT (methylcyclopentadienyl manganese tricarbonyl), EPA, http://www.epa.gov/otaq/regs/fuels/additive/mmt_cmts.htm
6. “Designing and operating safe chemical reaction processes,” Health and Safety Executive, UK.
7. “Investigation Report, T2 Laboratories Inc. – Runaway Reaction.” Chemical Safety Board –Report No. 2008-3-I-FL, September 2009.
8. “Hazard Investigation, Improving Reactive Hazard Management”, US Chemical Safety Board, October 2002.
9. “Chemical Reactions Hazards: A Guide to Safety.” Barton and Rogers, 1997.
Translated by Camila Rufino, Acredited Translator
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