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 this paper, we discuss the dilution of a gaseous release by deliberate and cautious mixing with air to reduce its concentration to a

harmless level. The idea bears its origin to the Bhopal Gas Tragedy where some families saved themselves by turning the ceiling fans on

when MIC reached their bedrooms at the dead of very cold night on December 2–3, 1984. The air pushed in by the fans diluted the MIC to

below the harm level.

Some of the advantages of using air dilution are: no cost of air, no air storage needed, no need to treat the air after use as in case of water

curtains; required equipment, its maintenance and staff training in its use are very likely to cost less than in other ways of handling a release.

Air dilution may not be feasible in all cases, such as gaseous release within a congested equipment layout, release that forms a liquid pool,

etc. The method needs to be evaluated for each case.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Toxic gas release; Flammable gas release; Air dilution; Gas dispersion

1. 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 surroundings. The released material can be in

gaseous form, or 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),

0950-4230/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jlp.2005.07.005

* Tel.: C91 512 2597175; fax: C91 512 259 0104.

E-mail address: jpg@iitk.ac.in.

released as 2-phase and the pool formed evaporates

gradually (LNG, liquefied ammonia), etc.

The released material can be toxic or flammable of both.

Depending upon the amount released and confinement by

the surroundings, a flammable material may explode. Need

is to avoid such releases and to minimise consequences of

such releases. This has engaged many researchers and

regulators over the last several decades. Some of the

techniques that have been developed to reduce the

probability/amounts of releases are:

† High-pressure container to hold the maximum possible

pressure of the stored material under the worst situation,

such as runaway reaction, fire underneath the container,

etc. This does not allow any material to get out of the

container. However, the design of such equipment is a

specialized art, and costly.

† Store under refrigeration at atmospheric pressure. There

is no flashing or significant vapour formation. Backup

power source is required for refrigeration system which

must be maintained under all circumstances.

† Reduce amount of storage at one site. Frequent

deliveries may be required with their related

Journal of Loss Prevention in the Process Industries 18 (2005) 502–505

www.elsevier.com/locate/jlp

J.P. Gupta / Journal of Loss Prevention in the Process Industries 18 (2005) 502–505 503

transportation hazards.

† Reduce amount of storage in one container. Connecting

several small containers to the piping system has its

increased leakage probability.

† Double-walled containers. These have a high cost of

construction and monitoring.

† Mounded/buried containers. Precautions are needed to

avoid corrosion, etc.

† 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/com-

bustible nature of the gas. Cost of construction is high.

Some techniques used to control the releases/minimise

their effects are given below. Some of these are speculative.

The main aim always is to dilute the material released, or

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, Dimbour, Gilbert, Dandrieux,

& Dusserre, 2003), vertically up with peacock tail type

(Dandrieux, Dusserre, & Ollivier, 2002), 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. Heats up the gas release and dilutes it

significantly. Requires steam generation system as well

as nozzles to spray the steam.

† Diverting the gas to pass through a container of

neutralizing liquid or spraying a neutralising liquid

into the gas stream (scrubbing action). This requires

storage of the neutralising chemical, its recirculation,

collection and disposal of the neutralised material.

Generally, acid or base is used. Stainless steel or

other expensive material of construction is needed in

such cases.

† Greenbelts to reduce and delay the gas from crossing

the company boundary, thus giving more time to

arrange for emergency handling.

† 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. This can be done for small releases.

† Emergency repairs of the release/leaking point.

† People advised to stay indoors until the gas disperses

to a harmless level. This requires coordinating with

the civic authorities.

† People evacuated to safer areas. Coordination with

civic authorities is required.

† Stopping traffic on affected roads so no ignition is

possible from autos although a flammable gas cloud

may get ignited from any hot spot in the company:

canteen, boiler house, etc.

† Diluting with extensive and instantaneous flow of air

using large fans and ducts similar to those used in

ventilation systems, or 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. Air is already used in dilution of releases in many

ways:

† Regular emissions are directed from a tall chimney,

since the wind velocity is higher there and the gas gets

diluted before it hits the ground.

† It is known that in Stability Class—D 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.

† In research laboratories using chemical or biological

substances, a hood is provided to dilute the

vapours/fumes which are then exhausted out of the

laboratories.

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.

The motive for use of air dilution in case of

toxic/flammable releases comes from several cases during

the Bhopal Gas Tragedy of December 1984. Many families

were saved when they switched on the ceiling 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

generally 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 and elegant for

the following reasons:

† The primary equipment is a set of fans/blowers. Their

installation, maintenance and operator training, etc. are

expected to cost less than most other methods.

J.P. Gupta / Journal of Loss Prevention in the Process Industries 18 (2005) 502–505504

† There is no storage tank needed since air is everywhere,

while most other methods require water or neutralising

chemical to be stored.

† There is no post-use collection and treatment system

needed as is required for water or neutralising

chemical.

† 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.

There are several precautions that need to be taken at the

design stage while considering the use of air dilution:

† Account for the wind pressure generated on the side

walls, the trees and other structures by the sudden gush

of air.

† Account for the vibrations of the fan affecting any

instrumentation.

† Consider the fan noise effect on the surrounding

although during emergencies, noise is not a major

consideration.

† Ensure that the sparks from the fan motor do not

cause explosion Also there should be no source of

ignition present in the vicinity 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.

We need to calculate the extent of dilution required for

a release scenario. It would be based upon the

concentration and rate of release of the hazardous gas

and the extent of dilution required to reduce it to a

harmless level. The fans for the purpose can be designed

as per the procedures used for air coolers in the chemical

process industry. The mounting of fans is a mechanical

exercise and not difficult to handle. Where flammable/ex-

plosive gaseous mixtures are likely to be encountered, the

fan motor and all the electric fittings would need to be

explosion proof.

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.

In a company where hazardous gas release is possible

at several places, mounting of permanent fans at all

places would become costly. In such cases, fans/blowers

can be mounted on trucks and driven to the place where

needed, just like fire trucks with water are driven to the

place on fire. A compressed air supply system can also

be provided around the plant similar to the fire water

headers.

For possible release in congested space one can have air

ducts installed like it is done by HVAC people (heating,

ventilating and air conditioning). It will provide a sweeping

flow of air, yet not put undue pressure on the structure,

equipment or instruments.

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 most 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/flam-

mable 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 already 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 from

the pool.

In case of formation of aerosol droplets, two matters of

concern show up which require careful evaluation before

using air dilution:

† 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.

† 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 through the

use of air dilution. 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 earlier direction with reduced or no benefit of

dilution.

J.P. Gupta / Journal of Loss Prevention in the Process Industries 18 (2005) 502–505 505

2. Conclusions

The motivation for air dilution has come through actual

cases in Bhopal where those who turned on the ceiling

fans during the MIC release were saved since the gas was

diluted to harmless levels. Time is therefore ripe to have a

fresh look at air dilution of release of toxic/flammable

gases. Several advantages (no cost of air, no post-use

treatment, universal and abundant availability, etc.) and

cautions (worsening situation in case of pool formation,

evaporating aerosol droplets quickly thus increasing

concentration of hazardous gas, putting pressure on

structure, equipment and instruments, etc.) listed above

will need to be kept in mind. Experimental validation

would be required.

Acknowledgements

Comments by Prof. Trevor Kletz, Mr Dennis Hender-

shot, Prof. Paul Amyotte and Dr Faizal Khan are greatly

appreciated.

References

Dandrieux, A., Dusserre, D., & Ollivier, J. (2002). Small scale field

experiments of chlorine dispersion. Journal of Loss Prevention in the

Process Industries, 15, 5–10.

Dimbour, J. P., Gilbert, D., Dandrieux, A., & Dusserre, G. (2003).

Assessment of the effectiveness of downward water sprays for

mitigating gaseous chlorine releases in partially confined spaces.

Journal of Hazardous Materials, A96, 127–141.