30-06-2012, 02:51 PM
Dispersion of Chlorine and Its Containment
Introduction
The growth of chemical industries has led to an increase in the risk of occurrence of incidences associated with hazardous chemicals which may result in fire, explosion and/or toxic release. Meteorological conditions such as wind speed, wind direction, height of inversion layer, stability class affects the dispersion pattern of toxic gas clouds. Chemical Disaster Management plan is possible by the adoption of preventive and mitigation strategies. National guidelines have been worked out which calls for Proactive, Participatory, Well Structured, Fail Safe, Multi-disciplinary and Multi–Sectoral approach at various levels. Organizations world over have learnt in hard ways the importance of adopting the principles of prevention and protection in chemical industries. Each chemical industries including chlor-alkali industry aims to achieve “zero” accident in their plant. Prevention, protection and suppression techniques have been applied to reduce the probability of disaster in case of chlorine leakage in chlor-alkali plant. To reduce the effect of disaster, it is very much important to understand the dispersion characteristics and containment system of chlorine.
Effect of release is mitigated even though actual release is not preventable.
Characteristics of Liquid Chlorine Spill
In case of liquid chlorine spill, 20-25% of liquid chlorine shall flash into vapor with in 5-6 seconds and bring down the temperature of containment considerably low. Balance quantity of liquid chlorine shall spill on the floor within 20-22 seconds. Sudden vaporization of liquid chlorine shall develop negative pressure in the containment structure. Vaporization of liquid chlorine lying on the floor shall be intermittent because of cyclic stages of freezing to thawing. Flash Phenomenon occurs in case of leakage/spillage from large hole. Liquid chlorine flowing out of a container will not cool the container (Tonner / Cylinder) or reduce the vapor pressure. Cooling can occur by the withdrawal of vapor from the container. If the temperature of chlorine laden air trapped inside the room is at much higher temperature than the temperature of outside air, the plume of chlorine will move upwards in the atmosphere and cause little ground level damage.
Behavior of Gas Dispersion
After the release of a toxic gas, the gas mixes with ambient air. Depending upon various parameters like Temperature, Pressure of the gas stored, atmospheric conditions, the condition of the neighborhood etc, the gas disperses in the air and air/gas clouds spreads. As the dispersion starts and the clouds starts going away from the source of the leakage, the concentration of the toxic gas goes on decreasing. In case of catastrophic failure of the storage tanks and steady leakage from the largest valve, the ill effects could be felt to distance of more than 10 km. ALOHA (Aerial location of hazarding atmosphere is on atmosphere) dispersion model used for evaluating release of hazards chemical vapors.
Factors Affecting Dispersion of Gas Clouds and Plumes
Turbulence disperses a gaseous containment in the atmosphere. The following factor influences formation of turbulence.
Insolation and Air stability
Incoming solar radiation warms the air layer next to the ground and this rise cause turbulence. Greater the insolation, greater is the turbulence and atmospheric instability. By night, heat is lost front the ground, the closest air layer cool, becomes denser and stratify, with loss of turbulence. These conditions lead to greatest impact on people.
Wind Flow
Wind velocities affect the time of arrival of toxic cloud and duration of passage and thus influencing dispersion. The pressure pattern in the atmosphere gives rise to the flow in the atmosphere. There is a frictional resistance close to the earth’s surface and velocities are lower at ground level than at heights.
Ground Roughness
Surface of the earth provides friction and slows the lowest strata of air movement. Degree of friction is controlled by the roughness. The dispersion process is accelerated by the higher value of ground roughness
Obstacles
Localized air pattern changes in the presence of obstacles like Building and trees. Where dense chlorine clouds are formed at depths of 1-3 meters, they will be forced to go around the building or pass through the gaps between buildings. The Observed effect in built up area is for dense chlorine clouds to be spread very much wider as they advance downwind. Belts of trees around chemical plant may improve dispersion of gas release as well as aesthetic aspects.
Terrain
Natural/man made obstacles (canals and embankments) terrain of the ground influence the directional flow of air and dense gas clouds. People living at lower elevation are at higher risk than the people living at higher elevations.
Humidity
Presence of water vapors in the atmosphere is a source of heat for Soluble gases which can increase the dispersion rate Removal of containment takes place if there is precipitation. Chlorine removal does not takes places in this process.
Hydrate Formation
Chlorine and water forms solid hydrate below 9 ̊c and is seen as floating slush in pool of liquid chlorine. This is due to atmospheric moisture condensation on pool surface. Reaction to form hydrate is exothermic and provides heat for further cl2 vaporization. Solid hydrate does not remove the problem. It decomposes to release of chlorine on warm up.
Vegetation
Chlorine reacts with chlorophyll of the vegetation. The efficiency of the process is not known. It is doubtful that much chlorine can be removed by this process and should be ignored when planning emergencies.
Rain Out
Rain out from a release can occur with particles and liquid droplets. They advent with the vapor and cloud until evaporated by the heat in the cloud or by their own vapor pressure. Pressurized release of liquefied gases are the most vulnerable one for aerosol formation. Storage at atmospheric pressure can reduce aerosol formation very significantly and is a positive way of reducing hazards.
Alkali
Addition of any alkali to liquid chlorine releases more heat and vaporizes more chlorine during the process of neutralizing. Vaporization increases even though its duration is reduced.
Effective Mitigation System (Prevention Spillage of Liquid Chlorine)
Effective mitigation method of liquid chlorine can be developed by Hazard study at project stage while implementing the projects and by adopting process safety management including chlorine dispersion model study, regular risk assessment and mock drill practices. Method of storage of liquid chlorine plays an important role minimization of liquid chlorine leakage and dispersion of liquid chlorine in case of leakage. There are various method of liquid chlorine storage such as:
Atmospheric Storage of liquid chlorine
Storage of liquid chlorine at atmospheric pressure (2.5 Bar absolute) with jacketed cooling releases smaller quantity in a spill. The storage tank is mounted above a submerged concrete tank and is partly covered compartment to receive the quantity of liquid chlorine stored in the storage tank. The compartment is equipped with a pump to remove rain water and blower to send chlorine vapor to neutralizing system. The momentum of the release shall be much less and the released liquid chlorine can be easily stored in a diked area and can be pumped back. Storage at atmospheric pressure can reduce very significantly aerosol formation and thus reduces hazards. Much larger vessel up to 5ooo MT can be designed which can replace a number of pressurized storage tanks. Inherently safer and economical size up to 400 MT storage capacities has been designed. This aspect need very thorough study and safety review at various stages of design. The main drawback of such a system is the increased severity of chlorine release whenever it occurs.
Heavy bunker – Type and Fully Impervious
It is made of metal sheet or concrete suitable to stand excess pressure and sudden vaporization of the product or an external impact such as falling of aircraft. The building is connected to absorption unit. Underground construction offers a similar alternative. This design is restricted to small buildings. This type of construction is not recommended for conventional chlorine unit.
Light Containment
The chamber consists of conventional building e.g. cladding panels. This building is not totally impervious. It can stand slight low or excess pressure. In the event of leak a collection system makes it possible to extract gas to absorption unit.
Double Wall Containment System Room
This method of confinement involves that each piece of equipment is designed and built with a double wall such as Vessels, Pipes and fittings, Pumps and transfer equipments, Measuring sensors and Storage inside a building.
Monitoring, maintenance and operation are difficult in such units. It is easy to detect a leak but difficult to locate. This can be considered in some type of equipments. The problem of delay in starting up exhaust fan can result highest vapor rate. In case of high release rate, the gas evolution may be higher than the exhaust system. The internal building pressure suddenly rise and most probably it will fail. Such failures have taken place in European countries at Zarnesti, Ranma and Walsum.
Effective Containment
Physical Barriers And Basins
Specific types of barriers and basins are generally used for secondary containment system to prevent the accidental discharge of toxic material to uncontrolled areas. Type of containment may be Concrete dikes and High impounding walls.
Maximum vapor generation will be lower for a high wall impoundment than for a low wall dikes or impoundments due to reduced surface area contact. These rates can be further reduced by using insulation on the wall and floor in the annular space. A weather shield may be installed between the tank and wall with the annular space open to atmosphere. For high wall impoundments, the wall may be designed with a volumetric capacity greater than the tank. High wall impoundments hinder routine external observations and maintenance jobs.
Suppression of Pool Evaporation
To avoid possible dispersion of Liquid chlorine, provide a bund by which it is possible to retain the volume of liquid corresponding to the largest capacity storage tank taking into account vaporization and also make the floor with concrete having slope which will facilitate the liquid chlorine to flow into a sump which will minimize the evaporation rate of liquid chlorine. Impervious substrate to be used to avoid pool evaporation Buried tanks, lined with insulation material is being used. These tanks connected to ventilation system. Ventilation can be kept in operation preferably connection into cl2 sensor to avoid delay. As per experiment it is observed that sloped dykes are not successful for containment. Vertical walls or wave breakers may be advisable with in the bund. Take care not to allow rain water to accumulate and liquefied chlorine to flow into drains.
Water Sprays
Water sprays can be effective in aiding the dispersion and dilution of gas or clouds resulting from an accident. Water spray is the dispersion of liquid droplets in a gas. Spray medium is typically applied to the vapor by hand-held hoses and/or by stationary water-spray barriers. The effectiveness of spray system depend on following factors such as Distance, The fog pattern, Nozzle gallon age, Pressure and Nozzle rotation. Do not direct water on the liquid chlorine in dike wall.
Water sprays are more efficient air mover than they are gas absorber and can improve local dilution by increasing the mass flux. It has little effect on the ultimate concentration as long range of dilution TLV or STEL is concerned. Water used should not drain back to pool of liquid chlorine Reactive spray system may be injected to neutralize the release. Evolution . of gas can be reduced from the spilled liquid chlorine by spraying chilled water to form chlorine hydrate at 9.6 degree. These crystal contain 33% cl2 by weight and evolve at 0 degree . It is not advantageous in summer season.
Steam Curtains
Steam curtains act similar to water curtains. Steam curtains provide enhanced buoyancy to the toxic gas. In practice steam curtains operate manually. Steam curtains are useful for toxic and flammable vapors which are heavier than air. 0.15 ton/hr per foot of curtain steam is required which is quite large. Steam curtains are limited to small scale uses. Water spray system is more reliable and applicable where steam is not available.
Coverings
A cheap but very effective cover for toxic liquefied gas pool is to cover with large sheet or sheets of polythene. The advantages of such systems are No heat gain from the material, Insulation suppressed, impervious cover, Vapor can be withdrawn by large bore hose to the destruction system and no problem due to wind which can easily be over come by weighting the sheet at the ends. All installations should carry these sheets.
Floatation Devices
Floatation devices can be used to reduce the surface area of a toxic liquid to minimize the release of toxic vapor in the environment. Impermeable floatation devices can be used over a spilled liquid. It is highly efficient method for reducing evaporation and containing vapors. Deployment may be difficult but useful for small spills. Such a system require 280 particles per square foot of a spill. Cost of a such system depending on the material and dispersal equipment used.
Suitability Of Foam
Chemical foam, high expansion foam or mechanical foam or alcohol resistant foam compounds MUST NOT be used as they are not stable. Use of medium density, chlorine compatible foam may be used. The first effect of the high water content greatly increases the vaporization rate of liquid chlorine. The best practice is to use protein based foam produced and applied on the pool with a appropriate system of foam generator. TUTOGEN L made by CHMISCHE FABRIK PIRNACOPITZ. STHAMEX with a 1/75 dilution made by STHAMER. POLPETROFILM made by EAU et FEU. Kerr’s Proform Angus’Micerol or Pyrene Standard Protein compounds are suitable for liquid chlorine spillage. A crust of ice forms under the foam which is protecting the protein compound. Crust of ice may give some time to achieve emergency activities such as evacuation of personnel, preparation for permanent removal or destruction of chlorine. It is difficult to quantify the benefits of foam system. Slow draining foam, though higher in cost, is superior in performance.
Other Ways of Preventing Spill
Remote shut of valves at all necessary locations in all chlorine transfer lines to be provided. One emergency tank to be kept spare for transfer of material in case of emergency. Provision of Excess flow valve in liquid chlorine outlet line. Rotation of tonner by 180 degree in case of liquid chlorine leakage to reduce gas by 15 times. Spraying of carbon- di -oxide can reduce the vaporization of liquid chlorine. Liquid nitrogen is not recommended. Its rapid vaporization carries too much chlorine and cause brittleness to metal. Resources are available in case of major spill through mutual aid system.
Stopping a Leak from Tonners
Leak from a cylindrical or convex wall. Leakage can be stopped by applying thick sheet of Teflon and inflating with clamp fitting. Leak from small diameter hole It can be stopped with a wooden, copper, lead or brass pin. The metal surrounding the hole is sufficiently sound so that size is not aggravated. Leaky valves of Tonner/cylinders. These can be handled with respective emergency kits. Using a sealed container. Several companies have developed sealed containers. These can be used to transport leaky cylinders. These have built in facility to drill a hole in the cylinder, open/unscrew the damaged valve and connect it to scrubber system to release the gas
Conclusion :
Preventing and minimizing accidental releases of hazardous gases through the choice of inherently safer technologies, process and safety procedures are the utmost importance. Techniques must be available to mitigate any chlorine which may release occur in spite of all precautions. Such techniques are not only limited to secondary confinement such as de-inventory, vapor barriers, foam spraying and water sprays. Dikes are necessary in any situation involving liquid spill. Vapor barriers should be considered only as a field option to assist a partial and local dilution of gaseous release. De-inventory system can reduce the magnitude and the duration of a release. Water sprays are probably the most tested unconfined/vapor mitigation systems. Their effectiveness in dispersing gaseous release is low.