Explosion characteristics and flammability limits of various aqueous ammonia vapours in air

Abstract

 On 22 August 2016, an enormous quantity of toxic ammonia gas was released because of overpressure explosion of an ammonia tank in the Di-ammonium Phosphate Factory Limited known as DAP-1. The factory was established in 2006 on the premises of the Chittagong Urea Fertilizer Limited (CUFL) on the south bank of the Karnaphuli River, Bangladesh. The Di-ammonium phosphate factory has a production capacity of 1600 ton di-ammonium phosphate per day. It has two 500 ton ammonia tanks which supply ammonia to the process through pipeline during the operation. One of the tanks which exploded was partially full containing 325 ton of anhydrous ammonia at the time of the incident, completely rift from its base and landed about 35 feet away from its base. Hence, a huge gas cloud was formed and dispersed into air. The toxic ammonia gas spread over several kilometres and wind carried away the gas to the corresponding side of the Karnaphuli River leaving nearly 250 people falling sick due to inhaling the toxic ammonia gas. About fifty individuals were required hospitalization the same night. Locals in the affected area were advised to stay home but they were feeling uncomfortable even staying inside homes with windows and doors closed. Many of them developed sickness, respiration problems and severe eye irritation.

Introduction

It is well known today that anhydrous gaseous ammonia can form an explosive atmosphere when mixed with air. However, the flame speed and maximum rate of pressure rise are rather low. Besides, explosion risks mainly exist due to the handling of ammonia in confined spaces. Indeed, there is no example of anhydrous gaseous ammonia explosion in the open air. At the same time, ammonia solution vapours are often not considered as flammable in air, although they have been involved in some explosions as reported by the researchers. In fact, the available data are limited for proper explosion risk assessment [1].

Ammonia gas is corrosive, toxic and can rapidly penetrate the eye even in low concentration (below 20 PPM-parts per million). The degree of damage for a human depends on the length of exposure and toxicant concentration. At high enough concentrations, it can react with moisture in the skin, eyes and respiratory systems to cause tissue burns, blindness or potentially fatal pulmonary diseases. It can be fatal if the concentration exceeds 2500 PPM for 2 hours of exposure. The tolerable limit for most individual is as high as 250 PPM for 30 minutes exposure. The maximum exposure limit of ammonia gas as recommended by ACGIH is 25 PPM (17 mg/m³) [2] for 8 hours and short term exposure limit is 35 PPM (24 mg/m³) for 15 minutes.

The concentration was quite high at the time of the incident. It is also worth noting that the toxic gas released from the explosion is dispersed to the atmosphere and move with wind direction. Therefore, it becomes rather important to estimate the downwind concentration of toxic gas and the total affected area. The area should be evacuated where the concentration exceeds maximum exposure limit. However, the concentration of ammonia gas was unknown in the affected area during the incident. No prior study has been conducted to estimate the concentration and area that can be affected if any accident occurred. Thus, this is alarming that workers and other people are overexposed to a high concentration which can cause irreversible health effect even if treatment is performed [1].

The release of toxic anhydrous ammonia is harmful for workers and public as well as the environment. Ammonia that diffused into water bodies would increase pH and have an adverse impact on overall aquatic ecosystem. Fishermen already found plenty of dead fish from nearby contaminated pond. The industries which are dealing with hazardous materials or toxic chemicals should perform risk evaluation and consequences analysis for the worst possible scenario. Based on atmospheric conditions, wind direction and wind velocity, it is possible to estimate the affected area should maximum exposure limit be reached for a certain amount of chemical release. This helps to identify potential emergency scenarios and develop a proper emergency response plan. Public should be aware of the potential hazard that exits in their proximity.

Deficiencies at many levels are often hold responsible for any catastrophic incident occurring. Failure of safety instrumentation, lack of protection layers, and lack of risk understanding, mechanical integrity failures and absence of safety culture often lead to a safety incident. Therefore, it is recommended to conduct a thorough process safety analysis, risk evaluation and consequence analysis for the industries dealing with hazardous materials. Process safety analysis and proactive actions are necessary to avoid any potential incident occurring.

There are many possible causes that can lead to an explosion of a pressurized tank, i.e. faulty tank, internal corrosion, external corrosion, flow interruption, failure of control valve, failure of relief valve, increase of temperature / pressure and / or human error. However, the way tank explodes indicates that the tank was over pressurized due to operational error or mechanical integrity failures. In addition, there was no additional layer of protection that can contain the released ammonia and minimize the consequences. Process hazard analysis should be conducted to identify all causes, estimate the risk and develop strategies to address the risk and install additional layer of protection to minimize the consequences if accident occurs. Continuous inspection, risk monitoring, risk communication among employees and proactive actions can prevent such an accident from occurring in the future.

Ammonia Explosivity

Explosivity of ammonia has been studied for a long time: the first experimental flammability limits for an ammonia and oxygen mixture were published in 1809. Despite these first results, the ability of ammonia to create explosive mixtures was denied for a long time until German experts put forward an ammonia decomposition to explain explosions in refrigeration equipment at the beginning of the 20^th century [3]. Nowadays, it is well recognized that ammonia can ignite and explode under certain conditions of containment, within enclosures for instance, when its concentration in air is between 15 and 28% v/v. These are the most accepted concentrations limits. The main combustion products of ammonia in air are nitrogen and water. Ammonia explosiveness played a key role in various industrial accidents, in particular in refrigeration processes [4]. None of the reported accidents was related to ammonia explosion in open air.

The main characteristics of ammonia combustion are [5]:

• Ammonia combustion flame temperature is about 1,470 K. It can therefore cause severe damages to an exposed person without personal protective equipment. High temperatures can persist for up to 30 seconds after combustion takes place;
• the fundamental ammonia / air flame velocity (ammonia 23%) is about 0.07 m/s compared with fundamental flame velocities of about 0.4 – 0.5 m/s for the majority of hydrocarbons;
• The minimum ignition energy (MIE) of ammonia and air mixtures ranges between 380 and 680 mJ. Ammonia MIE is relatively high in comparison to other usual values for gas and vapours;
• This combustion energy does not generate a significant pressure wave. It was observed that a discharge surface of 1 m² in a total volume of 40 m³, would experimentally lead to an overpressure that is slightly less than 1 kPa (10 mbar).

Ammonia Solutions

Solubility

Anhydrous ammonia is very soluble in water: at 293 K, 688 L of gaseous anhydrous ammonia can be dissolved in one litre of water. The amount of anhydrous ammonia released at the water / ammonia solution surface can be very important. The solubility rises when the temperature drops (Table 1). Only anhydrous ammonia is present in the vapour phase.

This high solubility is due to chemical reactions between ammonia and water both under ionized forms:

Table1. Ammonia Solubility in water

Ammonia Combustion Experiments

These experiments are conducted in a standardized 20 litre sphere (Kuehner) previously described in Dupont and Accorsi in 2006 [6]. A typical diagram is shown in Figure 1. The pressure variation during explosions is recorded with two independent piezo-mechanical pressure transducers (Kistler). The explosion is triggered by an electrical arc generated by a fusing wire located at the centre of the sphere. It delivers ignition energy of 20 J. This high energy level is sufficient to ignite any potentially explosive atmospheres but at the same time does not contribute directly to pressure rise inside the sphere.

Figure 1. Illustration of the 20-litre sphere apparatus

The 20-litre sphere is heated up at the required temperature with a water jacket and the entire sphere is thermally insulated to avoid water condensation inside the sphere on cooler parts such as on the sphere lid. For temperatures below 288 K, the thermo fluid used is glycol ether.

The fuel / air desired mixtures were obtained by the partial pressure method at a total pressure of 1 bar. To perform a test, the desired quantity of the liquid solution, calculated to be in excess for the sphere atmosphere volume to be saturated with water vapour, was injected in the empty sphere with a syringe.

Then, after the time necessary to reach liquid / vapour equilibrium, air was introduced.

Lower Explosion Point

The lower explosion point (LEP) of a flammable liquid [5] is the temperature at which the concentration of the saturated vapour / air mixture equals the lower explosive limit (LEL) of the vapour phase composition.

Flash Point

The flash point is the lowest temperature of a flammable liquid at which an ignition in the vapour phase initiated by a test flame will propagate.

Figure 2. Typical explosivity diagram for vapours

Liquid Compounds Explosivity

In an enclosed vessel containing ammonia solution and atmosphere made of vapours in equilibrium with the liquid phase and air, the vapour concentration is set by the saturated vapour pressure at this temperature. The vapour concentration cannot exceed this value.

As the temperature increases, the concentration of flammable vapours in air increases. At low temperatures, the molar fraction of combustible vapours is under the LEL. The temperature at which the concentration reaches the LEL is the LEP. Above a given temperature, the concentration becomes higher than the upper explosive limit (UEL) and the mixture is not combustible. The correspondent temperature is known as the upper explosion point (UEP).

Besides that, ammonia solutions do not exhibit a flash point; however, their vapours can explode when mixed with air: they have an LEP. This fact results from definitions and from experimental determination techniques of these characteristics. Similar conclusions are often drawn with aqueous solutions in which water vapours can inert the atmosphere. Therefore, explosion hazards of ammonia vapours should not be neglected in safety studies [6].

Partial Pressure

Vapours of ammonia solutions contain two components: water and ammonia. The partial pressure of each component depends upon its interaction with the other component present in the solution. Henry’s Law gives the relation between partial pressure of a gas above the liquid phase in a solvent and its concentration in the liquid phase. It does not apply because the ammonia water solution is not an ideal mixture. Indeed, in this case, the dissolved gas is chemically bounded with the solvent (water). The ideal mixture assumption that does not consider any chemical interaction between mixed liquids would introduce a significant error in the estimation of the partial pressure. Figure 3 shows how Henry’s approximation can overestimate the partial pressure of ammonia in the case of 20.5% ammonia in water solution: it overestimates partial pressure values with an error of almost 200%.

Figure 3. Comparison of non-ideality with Henry’s law case

Explosion Points and Explosivity Limits

The lower explosion point range for each ammonia solution is shown in Figure 4. It should be noticed that temperatures for which vapours are explosives increase with the decrease of ammonia concentrations. Hence, vapours coming out of 28% ammonia solution form an explosive atmosphere in air for temperatures ranging from 269.7 to 286.1 K. This is more severe than the lower concentrations of ammonia solutions.

Figure 4. Explosion temperature ranges for different concentrations.

On the other hand, Figure 5 shows ammonia concentrations corresponding to the explosion limits above the surface of the studied solutions (water vapour partial pressure is deduced from the total pressure). It can be seen that the lower explosive limit remains roughly constant with increased ammonia concentrations in the solution while the upper explosive limit rises considerably; the explosion range widens while increasing ammonia concentration in aqueous solutions.

Figure 5. Ammonia concentrations corresponding to the explosion limits

Conclusions

In this article, it was shown that the explosion characteristics and flammability limits of various aqueous ammonia vapours in air are measurable at atmospheric pressure and at various temperatures using a 20-L explosion sphere.

Atmospheres above ammonia solutions consist of gaseous ammonia, air and water vapour, in concentrations depending on working temperatures. Both water vapour and ammonia concentrations increase with temperature but as ammonia and water react together in solution to form ammonium hydroxide ions, partial pressures do not fit with saturated vapour curves of each component.

As for the working temperature ranges, LEL and UEL, we can conclude that:

1. The lower oxygen concentration of ammonia in water vapours is 15%.
2. Temperature ranges for which ammonia vapours are explosive in air increase with decreasing ammonia concentration in solutions. For example, ammonia vapours from the 28% solution in air form an explosive atmosphere between 270 K and 282 K, whereas it is between 309 K and 320 K for a 10% ammonia solution.
3. At low temperatures, the explosion range corresponds to the one of gaseous ammonia in air (between 15 and 28% v/v) because of the low and insignificant water vapour content. The explosion range shortens while temperature increases by decreasing the UEL, whereas the LEL remains nearly constant.
4. The limiting concentration of ammonia in aqueous ammonia solutions below which vapours are not flammable is 5% regardless of the working temperature.
5. Explosion violence (pmax and (dp/dt) max) values are low for pure gaseous ammonia in air. However, the maximum explosion pressure is still above 3 bars with the 10% solution. Hence, these aqueous solutions can cause severe damages, if ignited, in case they are handled in closed spaces.

References

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