Dangerous Goods Classification

Classifying goods for transport is not the same as establishing its customs clearance number, as they have different purposes. This concept is still deeply rooted in most people involved in foreign trade transactions.

Customs regulations have almost no relationship with safety conditions in the transport of dangerous goods. And one of the most important issues to determine such safety conditions is the allocation of a UN number and a proper shipping name, as well as the determination of the relevant type of risk and, if applicable, the packaging group (which indicates how dangerous the relevant goods may be). This is classification for transport. A mistake in the classification for transport may entail mistakes in the determination of conditions for transport and, obviously, it may also have very serious consequences if those conditions trigger an accident.

A UN Number and a proper shipping name are not good for customs clearance. Similarly, a clearance customs number is not enough to determine if certain goods are dangerous for transport.

The main responsible for classifying a product for transport is the shipper, who defines if the goods are dangerous or not for transport. If the goods to be transported are dangerous, shipper must prepare and sign a Shipper's Declaration stating that the contents of the shipment have been duly classified, packaged, marked and labeled, and that all requirements under transport regulations have been met.

In fact, a customs officer or agent may represent the Shipper and act as such when the goods are sent and he/she may sign the Declaration, with the same effects stated above. In fact, this situation is expressly contemplated in the Dangerous Goods Regulations of the International Air Transport Association (IATA), which further states that the individual signing on behalf of the shipper must be duly trained regarding this issue. Thus, IATA implemented training through well-known schools or IATA Schools, which are endorsed by such institution.

Meanwhile, regarding sea transport, training of land staff is mandatory since January 2010, though there is no certification system for courses on sea transport of dangerous goods. This new requirement was imposed by the International Maritime Organization after the fire in vessel Hyundai Fortune in March 2006, allegedly due to dangerous cargo not declared, i.e., cargo classified as not dangerous by shipper.

We always highlight the importance of a proper classification of goods for their transport.

Customs clearance will be correct if a customs clearance number is assigned which truly reflects the goods to be exported. However, this does not ensure that the goods will be safe for transportation, even less, in the case of dangerous goods.

 

 

Translated by Camila Rufino, accredited translator

Hydrofluoric Acid Alkylation

 In the past, it was thought that almost no chemical compound caused alkanes to react. These hydrocarbons were even called paraffins, from “parum affinis” or “little affinity.”

Thereafter, it was discovered that, in fact, their reactivity depends on the reagents used. When in contact with an alkene (which is also a hydrocarbon, but which may be called “olefin,” with carbon-to-carbon double bonds, unlike alkanes, which have simple bonds), an interesting addition reaction of the alkane to the alkene, a combination of two molecules to create a single one, may occur.

Addition reactions need multiple bond molecules, such as alkenes (C=C double bonds). This allows for the formation of a stable carbocation, an essential step for the reaction to develop. In this case, the carbocation is the alkene to which a proton contributed by a strong acid is transferred. Protons are electron deficient and they have affinity to those electrons of one of the two C=C bonds, specifically, the π bond electrons, which bond is the most labile one. Thus, one of the double bond carbons losses the π electrons to the incoming proton, which results in a positive charge density of the double bond carbons due to the resulting electronic deficiency and in the formation of an electron-avid carbocation.

Thereafter, the carbocation will attack the double bond of another olefin and will take the relevant π bond electrons, thus, creating a bigger carbocation out of which used to be two olefins. This second carbocation will take a hydrogen atom from another alkane together with its electron pair (i.e., with negative charge, a hydride ion), thus neutralizing the positive charge density and ending its reaction. However, this step results in the formation of a new carbocation that will repeat the process (¹).

In brief, olefins behave as a weak base that may accept a proton by giving electrons from the π orbitals originally used in the double C=C bond, which finally breaks.
All this leads us to a specific addition reaction between hydrocarbons: alkylation.

From Theory to Practice

In practice, it is important to make a paraffin, such as isobutane, react with a light olefin, such as propylene or butylene. However, they are not the only characters of this story. We stated above that the carbocation was the essential intermediary in the alkylation reaction, and that this reaction is caused by the presence of a strong acid which allows for the formation of that carbocation by giving it positively charged protons which seek to attract the alkene double bond electrons.

The strong acids generally used to cause the reaction are sulfuric acid and hydrofluoric acid.

This reaction has been applied to several industrial processes. One of the most valuable applications has been in refineries, for the manufacturing of high-octane hydrocarbons based on low molecular weight olefins and paraffins. In specialized literature these hydrocarbons are called “alkylates,” which are added to fuel for octane enhancement.

Alkylate production was first developed during the Second World War in 1940, searching for high-octane airplane fuels and petrochemical charges to manufacture explosives and synthetic rubber (2). That same year, not only alkylation was developed, but also paraffins that would become part of the reaction were manufactured. This process was called isomerization.

On the other hand, olefins do not result from isomerization, but from a previous catalytic cracking unit. It may be concluded that the purposes of alkylation and catalytic cracking are opposite: while catalytic cracking aims at reducing the size of long hydrocarbons, some olefins produced in this process, such as propylene and butadiene, are used in alkylation and added to isobutane (a paraffin or alkane) to form an alkylate, a larger branched hydrocarbon.

Alkylates have a high research octane number (RON) ranging between 92 and 96, which gives them value as antiknock additives for gasoline. They also have low steam pressure and they do not generate byproducts such as olefins or alkenes or aromatic substances (such as benzene). Due to these features and the fact that light olefins and isobutene come from light cuts of hydrocarbons and do not have a significant commercial value, the alkylation process is necessary to increase the size of hydrocarbons in refineries that have a catalytic cracking unit to reduce it.

Only Flammability Hazard?

Certainly, upon evaluating the hazads inherent in alkylate production, the flammability hazards will be taken into account due to the great amount of hydrocarbons involved in the process. A typical situation would be a fire in a fuel deposit or an alkylate deposit. However, it is important to consider hazards not related to flammability.

We have already highlighted that the hydrocarbon alkylation reaction implies the use of a strong acid able to give away protons to break the alkene or olefin double bond so as to then allow an alkane enter in its structure.  Sulfuric acid and hydrofluoric acid are good for this purpose.

The first hazard to be taken into account with many acids is corrosivity, which entails an action that may destroy not only human tissue, but also materials, for instance pipes or tanks. Corrosive action on the facilities may cause the release of those substances.

This aspect may be controlled using tools such as corrosion rate prediction (such rates closely depend on the process temperatures) and periodic audits (¹⁰).

Any corrosive substance may kill an individual when in contact with him/her if such contact is significant. However, a corrosive substance is not the same as a toxic substance.

Corrosivity refers to the capacity of a substance to cause an irreversible damage to skin, such as a visible necrosis, from the epidermis to the dermis, after an application of up to 4 hours. A skin corrosion reaction shows injuries to the skin, bleeding, bleeding sores and after a 14-day observation period, complete areas of alopecia and scars (⁴).

On the other hand, the acute toxicity of a product refers to the adverse effects experienced after the oral or skin administration of a single dose of the substance, or as a result of inhalation throughout 4 hours (⁴). In the case of acute toxicity, the adverse effect to be observed is the death of the affected individual. Therefore, the acute toxicity values are usually stated as Lethal Dose 50 (LD₅₀) when the toxic substance enters the body orally or through the skin, or Lethal Concentration 50; (LC₅₀) when the toxic substance enters by inhalation.

Hydrofluoric acid involves both types of hazards: corrosivity, thus, being able to destroy the skin and some materials, and toxicity, being able to cause the death of an individual with a very low dose.

Burns with hydrofluoric acid are more serious than those caused by sulfuric acid, and they may not be immediately visible or painful, as the first symptoms may appear 8 hours after the exposure. Hydrofluoric acid penetrates the skin quickly destroying all deep tissues, including bones.

Hydrofluoric acid may also cause serious burns in the eyes and the respiratory tract, given that it is extremely volatile and, in the event of a leakage, as gas is denser than air, it remains at a low- height level, and it is incompatible with many compounds, such as glass, rubber, leather, ammonia, ethylenediamine, calcium oxychloride, etc. In contact with metals, it may release hydrogen, an extremely flammable gas, and in contact with water, it may generate a strong exothermic reaction (⁵), though a water supply system may be used to respond to a leakage as both substances are miscible.

Accidents involving hydrofluoric acid (HF) leakages at a large scale are not very common.  In the accident occurred on July 19, 2009 at CITGO's refinery in Texas, USA, there was a HF release due to a previous loss of flammable gaseous hydrocarbon, which accumulated in areas where there was hydrofluoric acid and caused a fire which affected the pipes that contained the acid and released it.  In total, 21 tons of HF were released, 2 of which evaporated in the atmosphere, and could not be recovered, though the facilities had a water mitigation system (¹¹).

Some Comparisons

It is reasonable to think that, compared with an alkylation process with sulfuric acid, the process that uses hydrofluoric acid has more safety and environmental disadvantages.

We have already discussed that hydrofluoric acid may cause burns that are more serious than those caused by sulfuric acid. However, it is also important to consider that the former is much more volatile and that its vapors are corrosive to the respiratory tract (⁵). More precautions should be taken in the case of hydrofluoric acid leakages, considering the volatility of such substance. This factor is very important when evaluating the impact of a possible accident with leakage in a populated area. 

One of the main reasons why this process has been so successful during the last 50 years has been the economic reason. Processes involving hydrofluoric acid (HF) have the following advantages(⁵)(⁹):

• Units with HF do not require such a strict temperature control as sulfuric acid, which requires a strict control as it involves an exothermic reaction.
• The capacity of the HF to catalyze the alkylation reaction is larger than that of the sulfuric acid.  
• Hydrofluoric acid is more expensive than sulfuric acid, but it may be used in much smaller amounts. In fact, HF consumption may be about 100 times less.
• HF may be regenerated at the user's facilities without any need to transport it to third parties' facilities, which is not the case in processes involving sulfuric acid.

In Argentina, for instance, alkylation units with hydrofluoric acid are used at the refineries of Shell CAPSA (Dock Sud) and Repsol-YPF (La Plata).

On the other hand, two factors considerably favored the use and production of alkylates during the last ten years:

• The recent prohibition to use the most popular antiknock additive, methyl t-butyl ether (MTBE) in most of the States of the USA in the last 20 years, as a result of the 1990 Clean Air Act Amendments (CAAA90) in that country (⁶).
• The most used substitute for the MTBE has been ethanol, but when ethanol is mixed with gasoline, the mixture is more volatile (i.e., it has more Reid Vapor Pressure, an indirect measure of the actual vapor pressure of the mixture) which makes it increasingly difficult to comply with the emission standards issued by the authorities in the different countries, where there is a significant trend to reduce the RVP value in commercial gasoline (⁷).

 Alkylates do not have such complications and have had no obstacle to gain their market share. Meanwhile, the alkylation technology is still searching for an increasingly safer production.

(1)    Química Orgánica (Organic Chemistry) - Chapter 3.18. Robert Morrison and Robert Boyd.  Addison – Wesley Iberoamericana. 5th ed. 1990.

(2)    Encyclopedia of Occupational Health and Safety. Chapter 78 – Chemical Industries/Oil and Natural Gas – Oil Refinery Process. International Labour Organization, 4th ed. 1998. Editor-in-Chief: Jeanne Mager Stellman, PhD. Chapter Director: Richard Graus, PE, CSP.

(3)    Energy and Environmental Profile of the US Petroleum Refining Industry – US Department of Energy – Office of Industrial Technologies. 1998.

(4)    Globally Harmonized System of Classification and Labeling of Chemicals (GHS). Third Revised Edition – Part 3: Health Hazards. United Nations, 2009.
(5)    Material Safety Sheet of Hydrofluoric Acid, prepared by Mallinkrodt Baker Inc, USA. http://www.jtbaker.com/msds/englishhtml/h3994.htm

(6)    Status and Impact of State MTBE Bans. US Energy Information Administration. http://tonto.eia.doe.gov/ftproot/service/mtbe.pdf

(7)    Determination of Environmental Contamination due to the Evaporation Percentage of Colombian Gasoline – Final Report.  Corporación para el Desarrollo Industrial de la Biotecnología y Producción Limpia – CORPODIB. March 2004.

(8)  http://www2.dupont.com/Clean_Technologies/en_US/assets/downloads/H2SO4_vs._HF.pdf

(9)    Advances in Hydrofluoric (HF) Acid Catalyzed Alkylation. J. Frank Himes, Robert L. Mehlberg PhD-ChE, Franz-Marcus Nowak. UOP, LLC. Document presented in 2003 at the annual meeting of the National Petrochemical & Refiners Association, USA.
(10)The American Petroleum Institute (API), in its Recommended Practice 751, Safe Operation of Hydrofluoric Acid Alkylation Units, recommends that these safety audits be performed on a quarterly basis.

(11)“Urgent Recommendation" of the US Chemical Safety Board (CSB) to CITGO, issued on December 9, 2009. http://www.csb.gov/newsroom/detail.aspx?nid=298. The investigation of this accident is still open, although, for the time being, the investigation agency has recommended the performance of an audit plan according to API Rule RP 751, and the enhancement of the spill response system with water, given that the estimated capacity to absorb the spilled product was of about 90 %.




Article translated by Camila Rufino, accredited translator.

The Andrews’s Critical Point

Thomas Andrews was born in Belfast, Northern Ireland, on December 19th, 1813. He was a chemist; physicist and physician who studied phase transitions in the 1860's.

Andrews studied chemistry and physics in Great Britain and Paris. In 1835 he received his medical degree at the University of Edinburgh. After developing his career in medicine in Belfast and teaching in chemistry at Belfast Royal Academy for ten years, in 1845 he was Vice President of Northern College in that city, contributing to its reorganization and creating the Queens College in 1849, where he was professor of chemistry until the end of his career in 1879 at age 66 (¹).

In 1869, he discovered the necessary conditions for the liquefaction of gases by studying the relationship between the pressure, temperature and volume of carbon dioxide, by measuring the pressures at different volumes at constant temperature. When repeating these measurements at different temperatures he plotted the isotherms for carbon dioxide (²) and established the critical constants, which then enabled the development of liquefaction techniques for gasses which until that moment could not be taken to the liquid state. Oxygen, hydrogen, nitrogen and helium were in this group, the so called “stable gases”, during mid-nineteenth century.

The critical constants are the critical temperature, the critical pressure and the critical volume (some authors as D. Fernandez and Fernandez Prini (³) prefer to refer to the critical density, which is inversely proportional to the critical volume). Considering the isotherm corresponding to the critical temperature, critical pressure and critical volume converge at the critical point.

By increasing the pressure (and decreasing the volume) in a system containing carbon dioxide at a constant temperature which is lower than the critical temperature, Andrews noted that to a certain volume, the gas phase abruptly starts to coexist with a liquid phase. This occurs during a range of volumes, with constant pressure throughout the transition phase. This pressure is the vapor pressure of the liquid.

At the critical point, the properties of liquid and gas phases are indistinguishable. It cannot be stated that there is a liquid phase and a gaseous phase.

At temperatures above the critical temperature, Andrews observed a behavior consistent with Boyle's law, which is valid for ideal gases: P1V1 = P2V2, temperature and number of moles of gas constant. He found that at temperatures higher than the critical temperature it is impossible to liquefy a gas.

Why at lower temperatures the behavior of the gas deviates from Boyle's law? Because when the temperature drops, interactions between gas molecules, especially the attraction, starts to be more important, depending on the distance between them, and showing a real gas behavior.

By increasing the system pressure (T always constant and less than the critical T), the molecules come to be at a distance in which the intermolecular attraction force is maximum, joining each otherand forming the liquid phase. If the pressure increases further, then the repulsive intermolecular forces start to prevail.

These studies have been the background to Johannes van der Waals in 1873, who proposed the equation of state for real gases in his doctoral thesis ("Over the Continuïteit van den Gas - in Vloeistoftoestand" or "On the Continuity of liquid-gas state")

Considering the isotherms obtained empirically by Andrews, van der Waals tried to find an explanation for the experiments that revealed the existence of "critical temperatures" of the gases. He could finally establish a relationship between pressure, volume and temperature of gases and liquids taking into account the molecular volume and intermolecular attractive forces, the "Van der Waals forces" (⁴).

Indeed, the "new" equation of state, which took into account the repulsive and attractive forces between molecules, were consistent with the isotherms observed by Andrews.

P=RT/(Vm-b) – aVm2

Where P is pressure, R is the constant of gases (8.3144 J/Kmol), T is the temperature in Kelvin, Vm is the molar volume of gas. a and b are the Van der Waals constants: a, related to the attractive forces, and b related to the molecular volume, and hence the repulsive forces imposed at high pressures. At very high temperatures, the components of the equation for these constants (or rather the corresponding intermolecular forces) become negligible compared to the component dominated by T, leaving as a result the old equation of state for ideal gases, and assuming that also are negligible interactions between molecules.

PVm=RT

Without going into detail on the limitations of the equation of state for real gases formulated by van der Waals, we can say that his studies made it possible to calculate the conditions for the liquefaction of gases, which opened the door to modern techniques of cooling (⁵).

The critical point and the hazardous substances

Almost one hundred and forty years after the experiments of Andrews, the critical point remains.

At a first sight, the critical constants don’t seem to have much importance in the world of hazardous substances. Is it important for safety to know at what temperature a gas can be liquefied if it is compressed?

I have never seen a product safety sheet (MSDS) containing the critical constants of a liquid or gas. These parameters are not even mentioned in any regulation relating to MSDS authoring. Taking this into account we can assume that in fact the critical point is not so important after all.

Wrong. The Globally Harmonized System of Classification and Labelling of Chemicals (GHS) establishes the criteria for classification of gases under pressure according to the critical temperature. It identifies four groups of gases:

• Compressed gases: those with critical temperature less than or equal to -50 °C.
• Liquefied gases: at high pressure, those with critical temperature between -50 °C to 65 °C, and low pressure, those having a critical temperature higher than 65 °C.
• Refrigerated liquefied gases: partially liquefied gases when they are at low temperatures.

·      Dissolved gases in a liquid phase.

The critical temperature defines the type of gas, if it is compressed or liquefied, at high or low pressure. In the definition of this temperature, the GHS only refers to pure gases (⁶).

However, it is necessary to note that GHS states these groups are only valid for gases under pressure, with risks of explosion by heating or cryogenic burns or injuries. GHS consider these gases in Chapter 2.5, separately from gases presenting other hazards, such as oxidizing gases (GHS Chapter 2.4), gases which are flammable and chemically unstable (GHS Chapter 2.2), and toxic gases (GHS Chapter 3.1) thus the concept of critical temperature is limited only to the pressurized gases.

At the end of Chapter 2.5, the GHS contains an informative paragraph regarding the classification of gases under pressure (GHS Chapter 2.5, paragraph 2.5.4.2). According to this paragraph, It is necessary to know three characteristics of the substance in order to classify it as a gas:

• The vapor pressure at 50 °C.

• The physical state at 20 ° C at standard pressure.

• The critical temperature.

The GHS does not indicate further details on these parameters, or why they should be considered, or what vapor pressure is at 50 °C, or what does it mean with “physical state at 20 ° C and standard pressure”. Regarding the latter, I suppose I would have to be completely gaseous. But perhaps it could include both gaseous and liquid phases present in the system in equilibrium, so as to consider the substance as a gas for the purposes of classification risk according to the GHS.

GHS only indicates that the information can be found in the literature, calculated or determined by tests. For pure gases, it indicates that the majority are classified on the Recommendations on the Transport of Dangerous Goods of the United Nations. This is true. Almost all pure gases are identified in the list of dangerous goods of Chapter 3 of the Recommendations, with a UN number assigned along with a proper shipping name and the risk class.
What happens with gas mixtures? The GHS is not conclusive on this issue, and it indicates that "most of mixtures require additional calculations that can be very complex." This statement is special for the determination of the critical temperature.

To take one example, which may not have much to do with "gases under pressure" referred to the GHS, but can illustrate the complexity of the issue, F. Escobar suggests a procedure for determining the critical properties of hydrocarbon mixtures (⁷). The critical temperature of each component is taken and then they are multiplied by the corresponding volume fraction. The sum of all is the critical temperature of the mixture. This procedure would only be valid when the components are hydrocarbons lighter than heptanes.

Trying to find an answer for the other two aspects mentioned in the GHS (vapor pressure at 50 °C, and physical state at 20 °C and standard pressure), this can be found in the UN Recommendations, or Orange Book.
The Orange Book of the United Nations provides the same four categories of gases according to their physical state, but unlike the GHS, this classification is valid for all gases, regardless of the risk involved. On the other hand, although there is no definition of the critical temperature, there is a definition of "gas".

For the Orange Book, gases are substances with a vapor pressure exceeding 300 kPa at 50 °C, or gases which are completely gaseous at standard pressure of 101.3 kPa. Even though these concepts are applicable to all gases according the UN Recommendations for the Transport of Dangerous Goods, they are consistent with the GHS indications valid only for the "gases under pressure".

Once we know whether a substance can be defined as a gas or liquid (a typical case where the situation is not so obvious could be a mixture of light hydrocarbons), if it is a gas, then the critical temperature may help making the classification according to their physical state.

If we consider the Orange Book, the categories will define the conditions of transport, for instance, Packing Instruction P200.

The first impression when I studied the critical parameters when I began my studies was that these concepts are of little practical application. Then I realized that the legacy of Andrews was much more important than I imagined. His contribution to the history of science, which enabled the subsequent liquefaction of stable gases, which led to the studies of Van der Waals and his equation of state and deserved Nobel Prize, and which enabled the development of applications such as refrigeration or exploitation of hydrocarbons, also left consequences in the world of chemical safety.


(1) "Thomas Andrews." Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2012. Web. 18 Feb. 2012.

http://www.britannica.com/EBchecked/topic/24010/Thomas-Andrews.

(2) Elements of Physical Chemistry, Samuel Glasstone, Surgical Medical Publishing, New York, 1946.

(3) "Fluídos supercríticos", Diego Fernandez and Roberto Fernandez Prini, Science Today, Volume 8, No. 43, November-December 1997.

(4) "J. D. van der Waals - Biography". Nobelprize.org.17 February 2012

http://www.nobelprize.org/nobel_prizes/physics/laureates/1910/waals.html.

(5) "Nobel Prize in Physics 1910 - Presentation Speech". Nobelprize.org. 20 Jan 2012

http://www.nobelprize.org/nobel_prizes/physics/laureates/1910/press.html

(6) "Critical temperature is the temperature above which a pure gas can be liquefied, regardless of the degree of compression," according Chapter 2.5 "Gas Pressure", GHS United Nations, Edition 4th, 2011.

(7) " Fundamentos de Ingeniería de Yacimientos ", Freddy Humberto Escobar Macualo, PhD, Surcolombiana University Editorial, First Edition.

(8) Recommendations on the Transport of Dangerous Goods, United Nations, Edition 17th.

El Punto Crítico de Andrews

Thomas Andrews nació en Belfast, Irlanda del Norte, el 19 de diciembre de 1813. Fue un químico, físico y médico que estudió las transiciones de fase en la década de 1860.

Andrews llevó a cabo sus estudios de química y física en Gran Bretania y en París. En 1835 recibió su título de Médico en la Universidad de Edimburgo. Luego de comenzar su carrera profesional en medicina en Belfast y de ejercer la docencia en química en la Academia Real de Belfast durante diez años, en 1845 fue Vicepresidente del Northern College de esa ciudad, contribuyendo a su reorganización y dando forma al Queens College en 1849, donde fue profesor de química hasta el final de su carrera en 1879, a los 66 años (1).

En 1869 descubrió las condiciones indispensables para la licuefacción de los gases estudiando la relación entre la presión, la temperatura y el volumen del dióxido de carbono, midiendo los las presiones a distintos volúmenes a temperatura constante. Al repetir estas mediciones a distintas temperaturas logró graficar las isotermas para el dióxido de carbono (2) y estableció las constantes críticas, que posibilitaron el desarrollo de técnicas de licuefacción de gases que hasta entonces era imposible llevar al estado líquido (a mediados del siglo XIX estos gases eran llamados “gases estables”). El oxigeno, el hidrógeno, el nitrógeno y el hélio se encontraban en este grupo.

Las constantes críticas son la temperatura crítica, la presión crítica y el volumen crítico (algunos autores como Fernandez Prini y D. Fernandez (3) prefieren referirse a la densidad crítica, siendo esta inversamente proporcional al volumen crítico). Teniendo en cuenta la isoterma correspondiente a la temperatura crítica, la presión crítica y el volumen crítico confluyen en el punto crítico.

Al aumentar la presión (y disminuir el volumen) en un sistema conteniendo dióxido de carbono a temperatura constante, siendo esta menor que la temperatura crítica, Andrews observó que a un dado volumen la fase gaseosa comienza abruptamente a coexistir con una fase líquida. Esto ocurre durante un rango de volúmenes permaneciendo la presión constante durante toda la transición de fases. Esta presión no es otra que la presión de vapor del líquido.

En el punto crítico, las propiedades de las fases líquida y gaseosa se hacen indistinguibles. No se puede decir que hay una fase líquida y una fase gaseosa.

A temperaturas superiores a la temperatura crítica, Andrews observó un comportamiento acorde con la ley de Boyle, que es válida para gases ideales:

P1V1= P2V2 a temperatura y cantidad de moles de gas constantes. Comprobó que a temperaturas mayores que la temperatura crítica es imposible licuar un gas.

¿Por qué a menores temperaturas el comportamiento del gas se aparta de la ley de Boyle? Porque cuando baja la temperatura, comienzan a ser más importantes las interacciones entre las moléculas de gas, especialmente la atracción, dependiendo de la distancia entre ellas, y mostrando un comportamiento de gases reales.

Al aumentar la presión del sistema (siempre a T constante y menor que la T crítica), las moléculas llegan a estar a una distancia en la que la fuerza de atracción intermolecular es máxima, cohesionando entre sí y formando la fase líquida. Si la presión aumenta un poco más, las fuerzas de interacción que comienzan a prevalecer entre las moléculas son las fuerzas repulsivas.

Estos estudios sirvieron como antecedentes para que Johannes van der Waals planteara en 1873 la ecuación de estado para gases reales en su tesis de doctorado (“Over de Continuïteit van den Gas - en Vloeistoftoestand”, o “Sobre la Continuidad del estado líquido-gaseoso”).

Teniendo en cuenta las isotermas obtenidas empíricamente por Andrews, van der Waals trató de encontrar una explicación a los experimentos que revelaban la existencia de “temperaturas críticas” de los gases. Finalmente pudo establecer una relación entre la presión, el volumen y la temperatura de los gases y de los líquidos teniendo en cuenta los volúmenes moleculares y las fuerzas de atracción intermoleculares, luego llamadas “fuerzas de van der Waals” (4).

Efectivamente, la “nueva” ecuación de estado, que tenía en cuenta a las fuerzas repulsivas y atractivas entre las moléculas, pudo ser coherente con las isotermas observadas por Andrews.

P=RT/(Vm-b) – aVm2

Donde P es la presión, R es la constante de los gases (8,3144 J/Kmol), T es la Temperatura en grados Kelvin, Vm es el volumen molar del gas. a y b son las constantes de van der Waals: a, relacionada con las fuerzas atractivas; y b relacionada con el volumen molecular, y por lo tanto con las fuerzas de repulsión que se imponen a altas presiones. Notar que a muy altas temperaturas, los componentes de la ecuación correspondientes a estas constantes (o mejor dicho los correspondientes a las fuerzas intermoleculares) se vuelven despreciables frente al componente dominado por T, quedando como resultado la antigua ecuación de estado para gases ideales, y asumiendo que también son despreciables las interacciones entre las moléculas.

PVm  =  RT

Sin entrar en detalle en las limitaciones de la ecuación de estado para los gases reales formulada por van der Waals, se puede afirmar que sus estudios posibilitaron calcular las condiciones para la licuefacción de gases, lo que abrió la puerta a las técnicas modernas de refrigeración (5).

El punto crítico y las sustancias peligrosas

Casi ciento cuarenta años después de los experimentos de Andrews, el punto crítico sigue vigente.

En principio las constantes críticas no parecen tener mucha importancia en el mundo de las sustancias peligrosas. ¿Es importante para la seguridad saber a qué temperatura un gas puede ser licuado si se lo comprime?

No he visto nunca una hoja de seguridad de producto (MSDS) conteniendo las constantes críticas de una sustancia líquida o gaseosa. Estos parámetros ni siquiera son mencionados en las normas relacionadas con la elaboración de las MSDS. Teniendo en cuenta este detalle podemos suponer que en realidad el punto crítico no es tan importante después de todo.

Error. El Sistema Globalmente Armonizado para la Clasificación y el Etiquetado de Productos Químicos (SGA, o GHS en inglés) establece los criterios de clasificación de los gases a presión en función de la temperatura crítica. Indica cuatro grupos de gases:

• Los gases comprimidos, aquellos con temperatura crítica menor o igual a -50ºC.

• Los gases licuados: a alta presión, los que tienen temperatura crítica entre -50ºC y 65ºC; y a baja presión, aquellos cuya temperatura crítica es mayor que 65ºC.

• Gases licuados refrigerados: son gases que se encuentran parcialmente licuados cuando se encuentran a bajas temperaturas

• Gases disueltos en una fase lìquida.

Claramente, la temperatura crítica define qué tipo de gas tenemos, si es comprimido o si es licuado, a alta o baja presión. En la definición de esta temperatura, el GHS solamente se refiere a los gases puros (6).

Sin embargo, es necesario tener en cuenta que para el GHS estos grupos son solamente válidos para los gases a presión, que implican riesgos de explosiones por calentamiento o por quemaduras o lesiones criogénicas. El GHS los considera en el Capítulo 2.5, por separado de los gases que presentan otros riesgos, como los comburentes (GHS Capítulo 2.4), los inflamables y los que son químicamente inestables (GHS Capítulo 2.2), y los tóxicos (GHS Capítulo 3.1), de este modo el concepto de temperatura crítica queda encerrado solamente en los gases a presión.

Al final del Capítulo 2.5, el GHS contiene un párrafo informativo referente a la clasificación de los gases (GHS, Capítulo 2.5, Párrafo 2.5.4.2). De acuerdo a este párrafo, es necesario conocer tres características de la sustancia para clasificarla como gas:

• La presión de vapor a 50ºC.

• El estado físico a 20ºC a presión estándar.

• La temperatura crítica.

El GHS no indica mayores precisiones respecto a estos parámetros, ni por qué deberían ser considerados, ni qué presión de vapor tomar a 50ºC, ni cómo tiene que ser el estado físico a 20ºC y presión estándar. Respecto a este último aspecto, supongo que tendría que ser completamente gaseoso. Pero  tal vez podría referirse a que esté presente la fase gaseosa junto con el líquido, o sea, que a esa presión y temperatura las dos fases se encuentren en equilibrio, como para considerar a la sustancia como un gas a los efectos de su clasificación de riesgos según el GHS.

Solamente se indica que la información puede ser encontrada en la literatura, calculada o determinada por ensayos. Para los gases puros, indica que la mayoría se encuentra clasificada en las Recomendaciones para el Transporte de Mercancías Peligrosas de las Naciones Unidas. Esto es cierto. Casi todos los gases puros se encuentran identificados en el listado de mercancías peligrosas del Capítulo 3 de las Recomendaciones, con un Número de Naciones Unidas asignado junto con un nombre apropiado de expedición y una clase de riesgo.

Qué pasa en tanto con las mezclas de gases? El GHS no es concluyente en este tema, e indica que “la mayoría de estas requieren cálculos adicionales que pueden resultar muy complejos”. Esta afirmación es especial para la determinación de la temperatura crítica.

Para mencionar un ejemplo, que tal vez no tenga que ver con los “gases a presión” a los que se refiere el GHS, pero que puede ilustrar lo complejo del tema, F. Escobar indica un procedimiento para la determinación de las propiedades críticas de mezclas de hidrocarburos (7). Se toma la temperatura crítica de cada componente de la mezcla y se multiplica cada una por su correspondiente fracción volumétrica; la sumatoria de cada una constituye la temperatura crítica de la mezcla. Este procedimiento solamente sería válido cuando los componentes son hidrocarburos más livianos que los heptanos.

Tratando de buscar una respuesta respecto a los otros dos aspectos mencionados en el GHS (presión de vapor a 50ºC y estado físico a 20ºC y presión estándar), esta puede ser encontrada en las Recomendaciones de Naciones Unidas, o Libro Naranja.

El Libro Naranja de las Naciones Unidas establece las mismas cuatro categorías de gases de acuerdo a su estado físico, pero a diferencia del GHS, esta clasificación es válida para todos los gases, independientemente del riesgo que involucren. Por otro lado, si bien no hay una definición de la temperatura crítica, sí hay una definición de lo que son “gases”.

Para el Libro Naranja, gases son sustancias que presentan una presión de vapor superior a 300 kPa a 50ºC, o que son completamente gaseosos a una presión estándar de 101,3 kPa. Si bien aplica para todos los gases, estos conceptos para discriminar a un gas de un líquido son coherentes con las indicaciones que el GHS realiza solamente para los “gases a presión”.

Una vez que sabemos si una sustancia puede ser definida como gas o como líquido (un caso típico donde la situación no es tan obvia podría ser un recipiente conteniendo una mezcla de hidrocarburos livianos), en caso de que sea un gas, la temperatura crítica nos podrá ayudar a realizar la clasificación según su estado físico.

Si nos guiamos por el Libro Naranja, las distintas categorías van a definir las condiciones de transporte, por ejemplo, en las tablas de la Instrucción de Embalaje P200 de dicho Libro.

La primera impresión cuando estudié los parámetros críticos en los inicios de la carrera universitaria fue que estos son conceptos de poca aplicación práctica. Luego me fui dando cuenta de que el legado de Andrews fue mucho más importante que lo que imaginaba. Su contribución a la historia de la ciencia, que posibilitó la posterior licuefacción de gases estables, que derivó en los estudios de van der Waals con su ecuación de Estado y el merecido premio Nobel, y que posibilitó el desarrollo de aplicaciones tales como la frigorífica o la explotación de hidrocarburos, también dejo secuelas en el mundo de la seguridad química.

(1) "Thomas Andrews." Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2012. Web. 18 Feb. 2012. .
(2) Elementos de Fisicoquímica, Samuel Glasstone, Editorial Médico Quirúrgica, Buenos Aires, 1946.
(3) “Fluidos Supercríticos”, Diego Fernandez y Roberto Fernandez Prini, Ciencia Hoy, Volumen 8, nº 43, noviembre-diciembre de 1997.
(4) "J. D. van der Waals - Biography". Nobelprize.org.17 Feb 2012
http://www.nobelprize.org/nobel_prizes/physics/laureates/1910/waals.html. (5) "Nobel Prize in Physics 1910 - Presentation Speech". Nobelprize.org. 20 Jan 2012 http://www.nobelprize.org/nobel_prizes/physics/laureates/1910/press.html (6) “Temperatura crítica es aquella por encima de la cual un gas puro no puede ser licuado, independientemente de su grado de compresión”, definición dada en GHS, Naciones Unidas , Edición 4, 2011. Capítulo 2.5 “Gases a Presión”
(7) “Fundamentos de Ingeniería de Yacimientos”, Dr. Freddy Humberto Escobar Macualo, Editorial Universidad Surcolombiana, Primera Edición.
(8) Recomendaciones para el Transporte de Mercancías Peligrosas, Naciones Unidas, Edición 17.