RUNAWAY REACTION – ACCIDENT IN T2

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.

3. http://t2labs.com/ecotane/ecotane.htm

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

SUMMARY OF REGULATIONS ON TRANSPORT OF DANGEROUS GOODS (*)

Transport of dangerous goods is a key link in the logistics chain, as there are several transport-related technical requirements which are sometimes difficult to determine and even to apply. Under Argentine law, these requirements are listed in Annex S of Transit Law [Ley de Tránsito] No. 24449, through Decree No. 779/95, and, in particular, in the technical provisions stated in Resolution No. 195/97 of the Argentine Secretary of Transport [Secretaría de Transporte de la Nación]. Each country has its own rules in place as regards this issue.

As stated in the first lines, there are many other enforcement authorities that regulate specific technical aspects, according to the kind of risk of the goods to be transported; for instance, explosives (Argentine Registry of Fireweapons [Registro Nacional de Armas, RENAR]), radioactive materials (Nuclear Regulatory Authority [Autoridad Regulatoria Nuclear, ARN]), etc.

At the MERCOSUR level, these regulatory provisions have been included in the Agreement for the Transport of Dangerous Goods by Road. Worldwide, the United Nations Recommendations on the Transport of Dangerous Goods prepared by its Committee of Experts on the Transport of Dangerous Goods establish a technical guide containing the requirements for the safe transport of all kinds of dangerous goods. In addition, radioactive goods are governed by the “Regulations for the Safe Transport of Radioactive Materials,” published by the International Atomic Energy Agency (IAEA). All international regulations governing the transport of dangerous goods are based on these UN recommendations.

The most significant international regulations are the International Maritime Dangerous Goods (IMDG) Code, of the International Maritime Organization (IMO), which governs maritime transport of dangerous goods; the Dangerous Goods Regulations (DGR), of the International Air Transport Association (IATA); the Technical Instructions (TI), of the international Civil Aviation Organisation (ICAO); and the European Agreement on Dangerous Goods by Road (ADR), of the European Union.

What does "dangerous good" mean?

Based on the criteria of dangerousness of the goods, “dangerous goods” may be defined as any article, substance or material capable of posing risks to health, safety and/or property of people, as well as the environment, when transported in commerce. In particular, Dangerous Goods are those expressly stated in the listings of transport regulations and/or complying with the criteria of some or several of the following classes of risks:

Class 1: Explosives.

Division 1.1: Articles and substances having a mass explosion hazard.

Division 1.2: Articles and substances having a projection hazard, but not a mass explosion hazard.

Division 1.3: Articles and substances having a fire hazard and either a minor blast hazard or a minor projection hazard or both, but not a mass explosion hazard.

Division 1.4: Articles and substances presenting no considerable hazard.

Division 1.5: Very insensitive substances having a mass explosion hazard.

Division 1.6: Extremely insensitive articles which do not have a mass explosion hazard.

Class 2: Gases

Division 2.1: Flammable gases

Division 2.2: Non-flammable, non-toxic gases

Division 2.3: Toxic gases

Class 3: Flammable liquids

Class 4: Flammable solids; substances liable to spontaneous combustion; substances which, in contact with water, emit flammable gases

Division 4.1: Flammable solids, self-reactive substances and solid desensitized explosives.

Division 4.2: Substances liable to spontaneous combustion.

Division 4.3: Substances which, in contact with water, emit flammable gases.

Class 5 Oxidizing substances and organic peroxides

Division 5.1: Oxidizing substances
Division 5.2 Organic peroxides.

Class 6 Toxic and infectious substances

Division 6.1 Toxic substances.

Division 6.2 Infectious substances.

Class 7 Radioactive materials

Class 8 Corrosive substances

Class 9 Miscellaneous dangerous substances and articles

Road Transport: Comparison between the ADR and Argentine Regulations

The main difference between the ADR and the Argentine law is the distribution of rules. While the European Rule includes all aspects of the processes related to the transport of dangerous goods, in Argentina, the applicable rules are scattered in different laws and Regulatory Decrees with their respective resolutions, issued by the applicable Enforcement Authority. To that regard, the most comprehensive Argentine rule regarding the technical aspects of road transport is Resolution No. 195/97 of Decree No. 779/95, regulating Transit Law No. 24449, issued by the Secretary of Transport.

However, radioactive goods and hazardous waste are also regulated by other rules and enforcement authorities. As the ADR provides a comprehensive treatment for dangerous goods, the application of those rules is preferred to the application of the Argentine rules.

Another aspect to be taken into account for the management of dangerous goods is the degree of update of the Argentine rules. Considering the constant adaptation and innovation of technologies and the daily experience with this type of goods, technical guides should evolve according to those changes and experiences.

Although the local rules of each country usually adopt the recommendations as the basis for their application, the Argentine rules have become obsolete, as they have not been reviewed to be consistent with the guidelines established in international rules. This gives rise to problems when considering packaging not mentioned in those rules or when it is necessary to dispatch goods with United Nations numbers not included in the regulation (e.g.: UN 3351, Pyrethroid pesticide, liquid, flammable, toxic.
Furthermore, the European Agreement for the Transport of Dangerous Goods by Road (ADR, 2005) has included, among other things, the Safety Adviser and regulations on informative statements of the cases involving dangerous goods. Both of them are important instruments in the Management of Dangerous Goods, which have not been included in our regulations yet.

Other very important instrument for risk management which is included in all regulations, are the Packaging Instructions specifically stated for each United Nations number. These instructions establish the types of packaging and packaging materials that may be used for each product, as well as other requirements such as marking, labelling, etc. This helps to better adapt the packaging to the product and, consequently, improves the management of the risk that the product may involve. The Argentine rules on transport of dangerous goods by road do not include the Packaging Instructions mentioned above.

There is a worldwide trend to unify both the technical criteria on classification, packaging, labelling, marking and documentation of dangerous goods, as well as the regulatory structures applicable to the different means of transport. In this context, it would be advisable to make a new revision of the provisions in force in the Mercosur in order to conform them to the international scenario.

(*) Article: "El difícil arte de la reglamentación" (The Difficult Art of Creating Regulations) prepared for the journal Enfasis Logística, published in July 2005. http://www.logistica.enfasis.com/notas/4059-el-dificil-arte-la-reglamentacion

Translated by Camila Rufino, Certified Translator

REACCION QUÍMICA FUERA DE CONTROL – ACCIDENTE EN PLANTA DE T2 (USA)

En la década del ’50, la empresa Ethyl Corporation (antecesora de la actual Afton Chemical Corporation) desarrolló un producto destinado al uso como antidetonante en motores de automóviles. El objetivo era reemplazar a los aditivos que contienen plomo, extensamente utilizados en el mundo hasta ese momento, por un nuevo aditivo que contiene manganeso, un poco más amigable con el medio ambiente que el plomo, aunque no tanto, como veremos más adelante.

El nuevo producto, el Ecotane, basado en el compuesto tricarbonil (metilciclopentadienil) manganeso, también conocido como MCMT o MMT, prometía ser el más seguro, económico y efectivo entre los aditivos antidetonantes sin plomo existentes en el mercado, ayudando también a la reducción de emisión de dióxido de carbono al ser aplicado en industrias, y a la reducción de emisión de oxidos de nitrógeno y de monóxido de carbono cuando es aplicado en gasolinas para automóviles. A lo largo del tiempo, su uso se ha extendido en aproximadamente 20 países de los 5 continentes inclusive en nuestro país, favorecido especialmente durante fines de la década del ’80, cuando comenzaron a ser implementadas en todo el mundo medidas tendientes a la limitación y eliminación de plomo en naftas (1). En Argentina, por ejemplo, el contenido máximo de plomo permitido en todas las naftas para automotores, es de 13 mg/l, en tanto que el contenido máximo de manganeso es de 18 mg/l. (2)

Algunas de las características del Ecotane son las siguientes:

• Estabilidad a altas temperaturas

• Baja presión de vapor

• Bajo punto de fusión.

Sin embargo, a pesar de las ventajas prácticas anunciadas durante más de 20 años de uso, el mismo fabricante T2 reconoce en la Hoja de Seguridad del Ecotane la existencia de riesgos relacionados con el producto puro (3). Un claro ejemplo es la clasificación para el transporte que T2 le asigna:

• Número UN: UN 1992,

• Nombre de expedición: Producto líquido, inflamable, tóxico, N.E.P (destilados de petróleo y tricarbonil (metilciclopentadienil) manganeso en mezcla),

• Clases de riesgo 3 (líquidos inflamables)

• Clase de riesgo secundaria: 6.1 ( sustancias tóxicas)

• Grupo de Embalaje: III

• Riesgo adicional: contaminante marino.

Según esta información el producto es peligroso para el transporte, con dos clases de riesgo: Clase 3 (líquido inflamable) y Clase 6.1 (sustancia tóxica), y un riesgo adicional al ser considerado como producto peligroso para el medio ambiente.

La hoja de seguridad de producto declara estos riesgos y las propiedades físicoquímicas del MMT, aunque no declara parámetros objetivos de toxicidad aguda, tales como Dosis Letal 50 oral. Inclusive la ficha técnica-comercial (3) también menciona al MMT como sustancia peligrosa para el transporte, aunque dicha clasificación no es completa porque no lo declara como “contaminante marino”. Por otro lado, un informe técnico elaborado por el fabricante en 2006 (1) indica que el MMT es seguro, económico y efectivo, además de beneficiar al medio ambiente por reemplazar al plomo por manganeso, agregando que “No implica un riesgo para organismos acuáticos”. Esta información no es coherente con la brindada en la Hoja de Seguridad elaborada por la misma empresa.

En cuanto a la evaluación de riesgos para la salud, el mencionado informe realiza un análisis considerando la exposición a los productos de combustión de gasolinas que contienen MMT, pero el análisis no incluye datos objetivos de toxicidad del producto, que son aquellos que más van a servir a los usuarios que preparan las mezclas, ni aclara nada respecto al riesgo de inflamabilidad del líquido.

Los principales riesgos a la salud evaluados por T2 corresponden a aquellos relacionados con la inhalación de las emisiones que contienen manganeso. Este metal puede causar daños neurológicos irreversibles si es inhalado a altas concentraciones durante mucho tiempo, dando lugar a perturbaciones mentales y emocionales en el ser humano, además de falta de coordinación y movimientos lentos del cuerpo. Esta enfermedad se denomina “manganismo” (4). En este marco nace la preocupación que generó el MMT en la opinión pública respecto a su uso como aditivo antidetonante para naftas. La Agencia de Protección Ambiental de los Estados Unidos (EPA), luego de realizar una evaluación de riesgos en 2004 en el uso del MMT, indicó que con la información disponible en aquella época era imposible determinar la existencia de riesgos a la salud pública debidos a la exposición de emisiones de naftas que contienen este aditivo. Actualmente la empresa Afton Chemical, único fabricante del producto en Estados Unidos, se encuentra realizando estudios que permitirán completar una evaluación de riesgos para la salud pública relacionados con el uso del MMT, por requerimiento de la EPA (5).

En Europa, por otra parte, el MMT no se encuentra en el listado de Prioridades para la Evaluación y Control de Riesgos de Sustancias Registradas, según la Regulación EEC793/03.

Evaluaciones de riesgos

Hasta aquí se mencionaron las distintas evaluaciones de riesgos que se han tenido en cuenta con respecto al MMT.

Por un lado, una evaluación de riesgos en el transporte que incluye una clasificación, con un número de Naciones Unidas, una clase de riesgo principal y una secundaria, un nombre de expedición y un Grupo de Embalaje. Se podría observar que esta información puede ser incoherente con otros datos presentados por T2 en fichas y documentos técnico-comerciales, pero la información está y de algún modo tuvieron en cuenta que el transporte del MMT implica riesgos.

Por otro lado se puede resaltar la evaluación de riesgos a la salud pública realizados en conjunto entre otro fabricante, Afton Chemical, y la EPA de Estados Unidos, refiriéndose principalmente a los riesgos derivados de la inhalación de las emisiones gaseosas de los automotores que contienen naftas aditivazas con MMT. Dichos estudios siguen siendo realizados hasta el día de hoy y puede seguirse su avance en la página de la EPA.

Pero también ha sido necesario tener en cuenta los riesgos en la producción, que implican además un análisis de los reactivos utilizados y los procesos involucrados. Es decir, un análisis de riesgos de procesos, que podría ser cumplido mediante un estudio HAZOP. Y aquí estuvo uno de los puntos débiles en la empresa, ya que dicho análisis no fue realizado, según el Panel de Seguridad Química de Estados Unidos (CSB) (7).

El accidente

Durante el mediodía del 19 de diciembre de 2007, la ciudad de Jacksonville, en Estados Unidos, se vio sacudida por una terrible explosión que dejó como resultado cuatro personas muertas y 32 heridas.

La causa del siniestro estuvo en una reacción exotérmica autoacelerada ocurrida en un reactor tipo batch de la empresa T2 durante la elaboración de aquel producto utilizado como aditivo antidetonante para gasolinas: el tricarbonil (metilciclopentadienil) manganeso, también conocido como MCMT o MMT.

Las reacciones químicas pueden absorber o liberar calor. Cuando ocurre una absorción de calor, se dice que la reacción es endotérmica. Cuando la reacción libera calor, se indica que la reacción es exotérmica. Pero el hecho de que la reacción implique una absorción o una liberación de calor no necesariamente quiere decir que sea acompañada de una disminución o de un aumento de temperatura, respectivamente. Si el sistema (el reactor) permite el intercambio de calor con el medio, la temperatura puede ser mantenida en un valor constante. Los problemas con estos procesos surgen cuando el intercambio de calor con el medio no es suficiente, cuando el calor generado por la reacción comienza a ser mayor que el calor liberado al medio, dando lugar el incremento de la temperatura en el sistema.

En el momento en que la temperatura comienza a ascender, comienza a tener cada vez más importancia la velocidad de reacción, la cual depende de dos factores primordiales: la concentración de los reactivos y, precisamente, la temperatura.

Al aumentar la temperatura, la velocidad de reacción aumenta en forma exponencial, y al aumentar la velocidad de reacción hay mayor liberación de calor (por lo que su aumento también es exponencial con respecto al aumento de temperatura), que cada vez cuesta más disipar fuera del sistema (ya que la velocidad de eliminación de calor generado por la reacción aumenta en forma lineal con la temperatura, y no en forma exponencial), y esto provoca que siga aumentando la temperatura. De esta forma se pierde el control de la reacción alcanzando un estado de autoaceleración que permitiría, por ejemplo, dar lugar a otros efectos secundarios que, a menores temperaturas, no ocurrirían, tales como el rebosamiento de la masa de reacción (boiling over), o un aumento de presión que de lugar a una explosión.

En la jerga angloparlante, este tipo de reacciones en las que se pierde totalmente el control se denomina runaway reactions, y pueden ser consideradas dentro de los riesgos de reactividad química en procesos industriales, junto con las reacciones entre productos incompatibles y junto con reacciones de descomposición química propias de sustancias autorreactivas.

Algunas de las causas de la autoaceleración de reacciones químicas exotérmicas reconocidas por el Health and Safety Executive de Inglaterra son las siguientes:

• Errores durante la manipulación de los reactivos en el momento de agregarlos a la reacción.

• Agitación inadecuada en el reactor

• Fallas en los sistemas de control de temperaturas.

Pero por sobre todas las cosas, las causas de fondo de la mayoría de los accidentes que envuelven una reacción química son:

• Conocimiento inadecuado de las reacciones químicas involucradas en el proceso.

• Sistemas transferencia de calor inadecuados

• Entrenamiento inadecuado del personal operador del proceso

• Errores humanos relacionados con el incumplimiento de Procedimientos Operativos de la Empresa (6).

Para el accidente de T2, el Chemical Safety Board de los Estados Unidos determinó como causas inmediatas a errores en el dimensionamiento del disco de ruptura y fallos en el sistema de enfriamiento.

El disco de ruptura fue dimensionado teniendo en cuenta la generación de gas hidrógeno máxima esperada durante una operación normal, y no teniendo en cuenta una posible sobrepresión generada por una reacción química autoacelerada.

El sistema de refrigeración utilizado, en tanto, fue diseñado teniendo en cuenta una escala de laboratorio. No obstante, T2 no solo llevó ese sistema de refrigeración a una escala industrial sino que posteriormente, en el año 2005, incrementó un tercio más la capacidad del reactor sin tener en cuenta que el incremento de la cantidad de materias primas podía aumentar el calor liberado durante la reacción. Evidentemente no se consideró que el cambio de escala sin una evaluación de riesgos apropiada podía hacer que se subestimaran algunos factores irrelevantes en un laboratorio, aunque determinantes en un reactor químico industrial.

Concretamente el cambio de escala de laboratorio a planta tiene implicancia directa en la relación calor generado / calor liberado, y esto es lo que deberían haber tenido en cuenta los diseñadores de los procesos involucrados en el accidente. Cuanto mayor es el volumen del reactor, se incrementan tanto la generación como la liberación de calor. Sin embargo la generación de calor por la reacción aumenta más rápido porque depende del cubo del diámetro del recipiente, en tanto que la liberación de calor depende de la superficie por la cual es realizada la transferencia de calor al medio, es decir, depende del cuadrado del diámetro del recipiente. Como conclusión, interpolar las medidas de seguridad en un laboratorio o en una planta piloto a una planta industrial puede no ser efectivo ya que a esta escala la liberación de calor puede no ser suficiente, dando lugar a un incremento de temperatura que favorezca la existencia de una reacción autoacelerada junto con sus indeseadas reacciones secundarias mencionadas líneas arriba.

Importancia de la identificación y caracterización de riesgos

Pero la causa raíz determinada durante la investigación llevada a cabo por el CSB fue la falta de reconocimiento del riesgo de reacción autoacelerada durante la producción del MMT por parte de los desarrolladores del producto. El CSB no ha encontrado evidencias de que T2 haya realizado un estudio de riesgos adecuado durante el diseño de los procesos y/o para los cambios de escala llevados a cabo. Un análisis típico que podría haberse aplicado para los procesos involucrados es el HAZOP.

Pero este es un factor que se ha ido repitiendo a lo largo de los años en otras empresas. La identificación y caracterización de los riesgos asociados a la reactividad química de las sustancias, es uno de los pilares en el momento del diseño o del rediseño de los procesos. Todas las decisiones que se toman en cuanto a la seguridad del proceso nacen en el momento de la identificación y caracterización de los mencionados riesgos, de esta forma una asignación y valoración de riesgos desacertada es comúnmente mencionada como causa raíz de los accidentes asociados a la reactividad química. Según el Chemical Safety Board, cerca del 25% de los incidentes en industrias, sobre un total de 167 accidentes registrados en ese Organismo desde 1980 hasta 2001, fueron atribuidos a una identificación inadecuada de los riesgos químicos en los procesos (8), lo que equivale a un conocimiento inacabado de la química del proceso. Las demás causas se podrían agrupar en diseños de planta inadecuados, sistemas de control y de seguridad en planta inadecuados, y en procedimientos e instructivos de operaciones inadecuados (9).

De esos 167 accidentes registrados que involucraron riesgos asociados con reactividades químicas, el 35% han sido atribuidos a riesgos de reacciones autoaceleradas exotérmicas (o runaway reactions) (8), el resto correspondió a accidentes relacionados con incompatibilidades químicas y con descomposiciones químicas debido a calor o impactos (muy común en sustancias autorreactivas).

Estas estadísticas demuestran la importancia de una adecuada identificación y caracterización de riesgos, especialmente de aquellos que involucran reacciones químicas, para un adecuado diseño de procesos que incluyan operaciones seguras. En el caso de T2, los responsables tuvieron en cuenta los aspectos de seguridad en el uso y en el transporte, identificando los riesgos asociados a dichas actividades, pero no tuvieron en cuenta, o no supieron ver, los riesgos asociados con la reactividad química de las sustancias involucradas en la producción del MMT.

Es de notar que este tipo de accidentes han seguido siendo registrados en los Estados Unidos durante los últimos años, con consecuencias de distintas magnitudes. Los accidentes en las plantas de Morton Internacional Inc (Paterson, New Jersey, 1998), Concept Sciences Inc (Allentown, Pennsylvania, 1999), y Synthron, LLC (Morganton, North Carolina, 2006) son ejemplos de ello (7).

En Argentina no existe información pública respecto a accidentes de este tipo en la industria, ni tampoco existe la monstruosa actividad industrial que sí existe en los Estados Unidos. Aunque esto no garantiza que en nuestro país no pueda llegar a ocurrir una reacción química descontrolada durante la fabricación de un producto.

1. “Technical Paper. Introduction to Ecotane ® (methilcyclopentadiene manganese tricarbonyl)”, R. S. Gallagher, M.F.Wyatt, T2 Laboratories Inc., 2006

2 Resolución S.E. 1283/06.

3. http://t2labs.com/ecotane/ecotane.htm

4 Agency for Toxic Substances and Disease Registry (ATSDR), 2001. Reseña Toxicológica del Manganeso (en inglés), Atlanta, GA, Departamento de Salud y Servicios Humanos de los Estados Unidos, Servicio de Salud Pública.

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, Octubre 2002.

9. “Chemical Reactions Hazards: A Guide to Safety”. Barton and Rogers, 1997.