1) Historical Background

The oldest and most popular method of dealing with a major chlorine leak is the use of the absorption tank. This was developed by the pulp and paper industry, where chlorine is used in enormous quantities for bleaching and control of biological growth during pulp preparation. The absorption tank was filled with enough caustic to neutralize all the chlorine contained in the piping system and its components downstream from the chlorine supply shutoff valve. This was deemed the only logical way to deal with a leak where all of the chlorine being used was under supply tank vapor pressure. This is not necessarily the case for the 5-7 percent of all the chlorine manufactured in the United States that is used in the treatment of drinking water and wastewater.

In these applications, chlorine is metered and controlled under 12-18 in. Hg vacuum. This vacuum is created by the power of a venturi device called an injector, and the power of the venturi is obtained from a supply of water usually at 50-60 psi pressure. Each chlorinator is fitted with an injector capable of feeding its maximum capacity. This makes each chlorinator a primary safety device because the injector can dispose of the chlorine in the piping system in a few minutes. In addition to this feat, the chlorinators (if piped properly) can reduce and control pressure in the supply system; i.e., the chlorine cylinders or storage tanks.

The chlor-alkali plants have made significant advancements in developing safer ways to store their chlorine production. The latest innovation involves the recirculation of liquid chlorine in an enormous spherical vessel through a refrigeration system that keeps the liquid at atmospheric pressure. All of this is supplemented by a containment structure with a sloping floor to confine a liquid leak in as small a space as possible. During a leak, an insulating foam is sprayed on top of the spill to prevent vaporization while the liquid is pumped to a neutralizing tank.

The Uniform Fire Code of 1988 changed the industry approach to safety precautions associated with the handling of major leak. The major obstacle of this code is requirement that the neutralizing system should be able to handle the full contents of the largest single storage container. The code has led to a lot of confusion because the people who generated the code do not understand the basic characteristics of either liquid or gaseous chlorine.

2) Fume Scrubber

The system illustrated by Fig. 3-2 was the first system to be considered for major chlorine leaks. Typically this scrubber is designed to recirculate the containment room air until all of the chlorine spill has been neutralized. The system usually is designed to provide one complete room air turnover every ten minutes. The scrubber system depends upon a chlorine detector to close the normal ventilation system and to activate the scrubber recirculating pump. This delivers caustic to the inlet of the venturi. Simultaneously, room air is drawn into the suction throat of the venturi where it mixes with the caustic. This is similar to a chlorinator injector operation.

The standard venturis on this type of system are only 85-90 percent efficient in the chlorine reaction with the caustic. The remaining 10-15 percent of the chlorine and all the inert gases along with the caustic descend into the top of the tank from the venturi outlet. These gases then are forced up the vent stack and out the mist eliminator. The scrubbed air is returned to the chlorine-conta-minated containment room. The caustic tank is designed for any given expected major chlorine spill. The stoichiometric ratio of caustic (NaOH) and chlorine is 1.13 lb caustic per pound of chlorine. The scrubber operates until the chlorine concentration in the room air is reduced to 1 ppm.

During a leak episode it is necessary to monitor the room air for chlorine concentration and capacity of the caustic to absorb the remaining chlorine. This is accomplished by titration procedures.

3) Spent Caustic Disposal

The spent caustic will be a sodium hypochlorite (NaOCl) solution about 7000 mg/l. This is easily disposed of at a water or wastewater plant provided it can be metered in small quantities over given period of time. The hypochlorite can be easily destroyed by catalytic decomposition using nickel and iron as catalysts (If available, seawater will destroy the hypochlorite solution in a few hours owing to the presence of heavy metal ions). The use of sulfites to dechlorinate the hypochlorite solution is not recommended. The heat of reaction between the sulfite ion and hypochlorite is far too great at these concentrations.

4) Conclusions

This type of design bas been all but abandoned, for a variety of reasons. This system has to shut off all ventilation, so no fresh air can be used for dilution. Because the recirculating scrubber is only 85-90 percent effective, the chlorine vapor will be increasing in volume during the initial phase of the leak. This caused positive pressure in the contaminant room. This does not comply with the 1988 edition of the UFC guidelines. In addition to the room pressure increase, the recycled chlorine vapor will contain both NaOH and NaOCl mist particles, which will not be removed by the mist eliminator. Furthermore, this mist will be created whenever the scrubber system is activated for testing or neutralizing a leak. This not only is a health and safety issue but also caused severe corrosion to electrical equipment and other metal components in the room.

Another serious flaw in the recycle system is the lack of refresh air dilution. This can affect the emergency response team's decision to enter the room to proceed with their efforts to stop the leak. These Chemtrec teams are scattered all over the United States, and their entrance limitations for repair of a leak (only a container leak) vary with the local authority.

5) The Single-Pass Absorption system

This system was thoroughly tested by a field test, done by Powell in July 1985, to proved to a prospective client that the system could neutralize a one-ton spill of liquid chlorine. This field demonstration represented a continuing effort by a large chemical manufacturing company to reduce, at all costs, the danger of a major chlorine leak. This was a follow-up to their production of a fail-safe automatic railcar shutoff valve. Powell built a 10 x 12, 8-ft-high room and executed a 600-lb liquid chlorine leak using four 150-lb inverted chlorine cylinders. This test was thoroughly documented by a 45-minute video tape. The following description of the Powell single-pass neutralizing system is based upon the results of the above tests and substantiated by the known characteristics of liquid chlorine.

The schematic shown in Fig. 3-1 is based upon current recommendations for a 100 lb/min leak. This is an enormous leak, equal to 6000 lb/hr. This rate produces a theoretical 500 cu ft of chlorine. Therefore, this size leak will require two 250 cfm units in tandem using a single pump for motive power. It is important to realize that actually the vaporization rate of this liquid leak is going to be a lot slower than a continuous 500 cfm owing to the freezing and thawing cycle of liquid chlorine.

Regardless of room size or leak rate, the room pressure will always be slightly negative during the entire leak episode. This is one of the most important features of the single-pass system. This means that fresh air is entering and diluting the chlorine-contaminated air space. It has been found that during even a small leak of 4-5 lb/min, the cubic feet of chlorine required to maintain negative room pressure is very small. This results in better system absorption efficiency.

In the Powell scrubber system, a proprietary reactor downstream from the venturis is used to reduce the vent stack emissions to meet the UFC requirement of 15 ppm detectable chlorine. This eliminates the need for a packed tower on the discharge vent stack. Any system requiring the addition of a packed tower presents a leak hazard situation. Any system could increase the safety factor for the emission content of residual chlorine by allowing the stack to terminate at the same height as the barometric loop provided for the discharge of the evaporator relief system.

Any packed tower requires a finite time to become wetted, which is imperative to the success of the tower to absorb any remaining chlorine in the stack emissions. During the wetting period, chlorine would be escaping along with the other vent emissions.

The Powell system depends entirely upon the venturi units to supply the power to evacuate the contaminated room air. Absorption is not the function of the venturi. The absorption occurs by the hydraulic and chemical kinetics in the proprietary reactors mounted adjacent to but downstream from the venturis.

These reactors provide the hydraulic kinetics, and the caustic provides the chemical kinetics. This feature assures the user of system reliability, with every leak episode achieving a 100 percent chlorine-caustic reaction and meeting the UFC regulations.

The length of time required to evacuate the room is not critical. Keeping the room at negative pressure means that the leak has been contained, and the environment has been protected during the entire leak episode. The lower the room temperature at the onset of a leak, the slower will be the vaporization of chlorine. This increases the efficiency and capacity of the system. The user always has the option of air-conditioning the chlorine storage area to a temperature of 60-65 F^o. It is a basic characteristic of chlorine that the vaporization after a major liquid spill will be much slower than 78 lb/min. It is important to emphasize that the Powell system is based solely upon the leak rate and not the room size. This allows a standard packaged system and does not require each installation to be a unique design.

In summary, these scrubber systems need only to be sized for 78 lb/min for a total amount of approximately 2400 lb of liquid chlorine. This is far more chlorine than could ever be vaporized in 30 minutes. This results in much smaller and easily packaged systems such as those provided by Powell.

6) Removal of Liquid Chlorine

A major chlorine leak means that liquid chlorine has been released to atmosphere on the storage room floor. The floor should be sloped 2 in./10 ft to a sump that is designed to act as a suction well for a liquid chlorine educator. The liquid can then be hustled off quickly - before waiting for it to vaporize - to the caustic tank for quick neutralization, as is done with an absorption tank. If this part of the system is designed properly, the time required for neutralizing a liquid leak will probably be reduced by a factor of 10-20-fold.

It is important to understand the hydraulic kinetics when an eductor (or injector) is used for transporting liquid chlorine. When the liquid chlorine is collected in a sump or slot in the floor, atmospheric pressure is the only motive force available until the water or caustic-starts flowing through the eductor. Then the eductor provides the vacuum needed to create a pressure differential in the flow system. This differential will cause the liquid chlorine to flow under a small negative pressure (10-15 in. Hg) to the eductor throat without any off-gassing that might cause a gas binding situation.

7) Evaporator Pressure Relief Lines

These lines can be manifold together into a common header pipe that terminates in a chlorine sparger in the caustic tank, as shown on Fig. 3-1. The piping to the caustic tanks must contain a barometric loop; this prevents "suck back" of the caustic in the tanks. Each of the evaporator relief valves must be fitted with a 25-30 psi rupture disc on the discharge side of the valve to protect it from corrosion in the event of a leak from one of the other valves.

Another method would be to use an air purge system of 1 SCFM. This could prevent the migration of caustic to the relief valve seats. The use of a 400 psi rupture disc in the reverse position is standard practice because in the reverse position the rupture disc will burst at less than 25 psi.

8) Foaming Prevention

After the system has been in operation during a leak episode, there is a good chance that the recycled caustic will develop foam on the surface of the caustic in the tank if the scrubber is not properly designed. This foam may eventually be released in the discharge stack. Powell advises that 25 square feet of tank surface per venturi reactor is sufficient to eliminate any possibility of foaming. When sizing the caustic tank, always include a 10 percent excess of caustic for any proposed leak episode.

9) Materials of Construction

The preferred materials for a long-life project with very few major leak episodes will always be rubber-lined steel tanks, halar-lined pipe and fittings including venturi/reactor bodies, Teflon-lined plug valves, and titanium pumps. The only reason for considering plastics would be the cost differential. Careful consideration must be given to the possibility of damage to the plastic components from seismic forces, reaction temperatures, and sunlight. Other pumps such as Durco steel, Durco high silicon iron, or TFE lined FRP should be investigated.

The caustic recirculating pump and the liquid chlorine pump should be Durco's titanium pumps. Their high- silicon iron pump line should be investigated for this kind of service.