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.