G – 127 Dilution with Air to Minimise Consequences of Toxic/Flammable Gas Releases

J.P.Gupta

Department of Chemical Engineering

Indian Institute of Technology

Kanpur- 208016, India

Abstract:

Dilution has long been considered a solution to many problems of toxic/ flammable material releases. It implies diluting to a concentration that is below physiologically dangerous levels for a toxic substance (generally below TLV), or to a level below LFL for a flammable material release, ensuring that the process adopted for dilution does not itself enhance the risks.

In practice, dilution is achieved by dissolving the released material in a solvent (ammonia in water), reacting it with a chemical (MIC reacting with NaOH), absorption in solid material, water or steam sprays to trap the gas and thus slow its movement.

Air is also used in many ways: directing the release from a tall chimney since the wind velocity is higher there than at the ground level and hence, should the gas hit the ground, it would have been significantly diluted by then. It is also known that in Stability Class - A the wind velocity is very high and the release gets diluted early. Plants located near the sea coast have natural dilution by air since the wind velocity is usually high. Compressors for flammable gases are housed in the open, or just a cover on top with sides open (if the noise is tolerable), so that the natural air flow can dilute any leakage of the flammable material. Ofcourse, in research laboratories of chemical, biological and drug industries, and in universities, the hood is used to dilute (and direct away) the vapours.

This all leads to the conjecture that dilution by deliberately, and cautiously, introducing large volumes of air into a gaseous release, may lead to the desired effect of minimising the consequences. This is most easily done when the release is at the outer periphery of the plant equipment since the air will drive the released gas further away from the plant. However, the utility of direct air dilution approach should be evaluated in all cases. For releases inside a congested equipment set-up, air-dilution may increase the wind load on structure and equipment. In case of a liquid or two phase release, the pool formed on the ground will evaporate faster and contribute to the risk if air is blown directly over the pool. The pool needs to be covered by foam or diluted by water, or otherwise taken care of to ensure that air flow does not cause higher evaporation rate.

The advantages, in situations where analysis shows that air dilution will work, are obvious: No cost of air, no storage needed, no need to treat the material after use as in case of water curtains; testing and maintenance of equipment and training of workers are cheaper than in other methods; the equipment costing has to be looked into, though the initial thoughts are that it would be cheaper compared to other modes of handling such releases. Air can be provided through headers like fire water is, or by blowers and compressors trucked to the location of the leak, or by industrial sized fans permanently mounted or trucked to the desired place. Explosion proof motors and other safety measures will need to be taken.

In this paper, we will develop this concept further.

Key words: Toxic gas release, air dilution, flammable gas release, mitigating consequences

Introduction:

During manufacture, storage, transport and usage of chemicals, accidents sometimes do happen that breach the containment of the material. Except where the total released material is successfully impounded, neutralised or burnt, it gets into the surrounding. The material can be in gaseous form, liquid, solid, two-phase or three-phase.

In this paper, we are concerned with gaseous releases and two-phase (gas-liquid) where liquid evaporates as its enthalpy increases. In this restricted regime, there are several types of release situations possible: Heavier than air (e.g.: chlorine, propane), lighter than air (hydrogen, methane), combustible or explosible in air (hydrogen), non-combustible (chlorine), supporter of combustion (oxidizer: chlorine, oxygen), high solubility in water (ammonia), negligible solubility in water (chlorine), released as 2-phase and the pool formed evaporates gradually (LNG, liquefied ammonia), etc.

Details:

Techniques used to reduce the probability/ amounts of releases:

  • High-pressure container to hold the maximum possible pressure of the stored material under the worst situation.
  • Store under refrigeration at atmospheric pressure
  • Reduce amount of storage at one site
  • Reduce amount of storage in one container
  • Double-walled containers
  • Mounded / buried container
  • Keep gas cylinders in strong rooms that close airtight in case of a leak at the cylinder. The room should be able to withstand increase in pressure and the corrosive/ combustible nature of the gas.

Techniques used to control the releases/ minimise their effects (some are speculative):

The main aim always is to dilute the material released, reduce the pressure or the temperature of the release, as the case may be.

  • Water curtains: Vertically down (flat spray or circular nozzles that entrap air), vertically up with peacock tail type, etc. More than one line of spray set-up may be required. Requires water storage, high pressure pumps, bank of nozzles, collection of used water and its treatment. (Entraps air and released gas by jet effect. Pushes it down thus reducing rate of release. Also reduces density differences between air and dense gas release, enhances absorption in water, provides some heating due to ‘hot’ air entrapment [Gilles Dusserre, personal communication].)
  • Steam curtains
  • Spray of a neutralising liquid (scrubbing action)
  • People advised to stay indoors and let the gas disperse to a harmless level
  • People evacuated
  • Stopping traffic on affected roads so no ignition possible from autos.
  • Greenbelts to reduce and delay the release of gas thus giving more time to arrange for emergency handling.
  • Putting up a wall like structure that will not corrode or burn. It will delay the movement briefly. It will increase the source size and thus dilute the gas. Such a structure can either be fixed in advance at the vulnerable locations or be erected from mobile / folding units.
  • Deliberate setting fire to a combustible gas release. Ensure that fire or explosion will not cause serious damage to the equipment, which may cause more release.
  • Expandable non-reacting balloon put on the source of release
  • Using released gas as an oxidiser to burn wood/ other material
  • Diverting the gas to pass through a neutralizing solution
  • Sucking the gas into a neutralising solution: If gas stream has a lot of momentum, sucking may be difficult; diversion as stated above might work.
  • Emergency repairs of the release/ leaking point
  • Diluting with extensive and instantaneous flow of air using large fans ducts similar to those used in ventilation systems, pipes and hoses, fixed nozzles that release high pressure air when triggered, etc.
  • A possible combination of some of the above methods can also be used.

This write-up is concerned with the air-dilution mode of reducing the consequences of release of combustible/ toxic material. In effect, one can possibly use the same for air delivery, all the systems currently used for water/ foam delivery to counter toxic/ combustible material releases with some modifications. Naturally, air dilution will not work for all the releases. It may worsen the situation in some cases. However, it is worth considering and only then a decision should be taken as to whether or not it would work. It should not be rejected out of hand, without due thought. The advantages are too many to do that. These are listed later in the paper.

Ceiling fans are used extensively in India (and other tropical countries) to blow the hot air away and cool down the surroundings during the summer months. In some wedding parties, a perfume is spread in the marriage hall by creating a fine spray in front of a fan which evaporates the perfume and spreads it far and wide in the marriage hall in an acceptably low concentration. In Bhopal many families were saved when they switched on the fan when the MIC gas reached their bed rooms in the dead of the night, even though it was a cold winter night when the fans are not used. The extensive airflow generated by the fans diluted the MIC concentration to below the serious damage level.

Dilution with air seems rather promising since its installation, testing, maintenance and operator training, etc., cost less than in most other methods. There is no water to store and hence no tank is needed. There will be no water to collect and clean since there is no water spray used. The system can be made portable for a large site having several locations where hazardous gas release can possibly occur. It can be mounted on a trailer and driven to the place like the company fire brigade comes with its water tanks. Or a large complex of chemical companies can share its cost and use. It is an elegant method.

Amongst the precautions that need to be taken while considering the use of air dilution:

Ensure that the sparks from the fan motor do not cause explosion; Also there should be no source of ignition present since dilution with air may bring a combustible gas release in to the flammable range. The fan assembly should be able to withstand the corrosive nature of the released gas. Else, forced draft fans can be used. With these precautions, the dilution with air can be used both with toxic and combustible gases.

The design will have to account for the wind pressure generated on the side walls, the trees and other structures; the vibrations of the fan affecting any instrumentation, the noise effect on the surrounding (during emergencies, noise is not a major consideration).

All the fans need not be erected at one place; there may not be enough space available. These can be put in a cascading manner, the outflow from one feeding the inflow to two or more in an induced draft mode. If the wind direction is away from the habitation, less fan power may be needed and the concentration may even be allowed to fall by natural dispersion without the use of any fans.

We need to calculate the extent of dilution required for a release scenario. The fans needed to supply the required airflow would be known from the design of air coolers in the chemical process industry. The mounting of fans is a mechanical exercise and not difficult to handle. Where flammable / explosive gaseous mixtures are likely to be encountered, the fan motor and all the electric fittings should be explosion proof.

Great care has to be exercised in the use of air dilution by fans since they should not aggravate the problem. If the release is liquid, or two phase, with all or some of the released material falling on to the ground, the fans can significantly increase the rate of evaporation from the pool thus formed. This will increase the toxic/ flammable vapour concentration and may lead to adverse consequences. In such a case, the use of the suggested air dilution using fans should be carefully evaluated. One way out could be to first use foam/ other inert material to cover the pool and then the fans to dilute the material in the vapour form. The induced airflow created by the fans should not disturb the layer of the foam/ inert material covering the pool. For this the fans may be placed at a sufficient distance away from the pool.

In case of aerosol droplets, two matters of concern show up:

  1. The draft of air caused by the fan will result in more of the aerosol evaporating into vapour instead of falling to the ground since heat of evaporation will be readily supplied by the increased movement of air.
  1. The aerosol droplets do not just settle on the ground close to the equipment from which the material is released but they get carried away to certain distances and rain down on the ground along the way. This requires a more careful planning about the location of fans so as not to increase the vapour concentration of toxic/ flammable material by increasing evaporation from the rained down material. It may not be feasible in such a case to spread foam or other inert material to cover the long, though shallow, pool formed on the ground.

Wind direction should be monitored all along. This will greatly aid in the effective use of the fans. Should the change in wind direction not be monitored, it might be that the gas release moves in a different direction while the fans keep pushing the air in the old direction with reduced or no benefit of dilution.

Conclusions:

Time is ripe to consider air dilution of release of toxic/flammable gases. Several cautions and advantages listed above needed to be kept in mind. Experimental validation would be required.

Acknowledgement:

Comments by Prof. Trevor Kletz, Mr. Dennis Hendershot, Prof. Paul Amyotte and Dr. Faizal Khan are greatly appreciated.

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