Handbook on Chlorine Handling

7/28/2019 Handbook on Chlorine Handling

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HAND BOOK

ON

CHLORINEHANDLING

Chapter 1. Chlorine Leaks


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1) Definitions

These leaks are usually referred to as emissions or releases. They are simplyinadvertent discharges of either chlorine liquid or vapor into the surroundingatmosphere. There are both minor and major leaks. When liquid chlorine isinvolved, the leak is almost always major.

Minor leaks usually occur at the start-up of a new installation or right aftermaintenance or inspection procedures. They usually are the result of a gasket

failure, valve packing adjustment, or equipment malfunction.Major leaks are generally referred to as catastrophic leaks. They include suchthings as container rupture, guillotine break in a pipe line under pressure, brokenflexible connection, fusible plug failure, and/or repair work while a system isunder chlorine supply pressure. Not to be overlooked is the real possibility ofspontaneous combustion of steel and chlorine if the temperature reaches 483oFfor some reason.

2) Frequency and Magnitude of Chlorine Leaks

A list of the chlorine accidents reported to the Chlorine Institute over the past 50years indicates by frequency and magnitude of chlorine emission. The followingcategory of causes is in order of potential hazard:

Fire

Flexible connection failure

Fusible plug failure

Freak accidents caused by carelessness and ignorance

Valve packing failure

Piping failure

Equipment failure

Container failure

Chlorine pressure gages

(1) Due to Fire

Fire was listed by the Chlorine Institute as the most serious hazard primarilybecause companies that stored swimming pool chemicals were often wiped outwhen stored granular hypochlorite suffered a spontaneous explosion. If 150-1bcylinders of chlorine were in the area, these too would explode because thefusible plug melts at a few degrees after the cylinder becomes skin-full. Toncontainers are different. When they become skin-full, the dished heads blowoutward without rupturing the container. This maneuver increases the volumeof the container which is sufficient to allow the necessary expansion of thechlorine during the next 4oF rise required for the fusible plug to melt.Fortunately, such fires as described above do not happen in water orwastewater treatment plants. Moreover, the plants that use modernchlorination or sulfonation equipment handle these vapors under a vacuum sothat excessive pressures, for whatever reason, rarely if ever occur.

In the early years of ton container usage, the industry suffered containerexplosions due to the spontaneous combustion of nitrogen trichloride (NCl3).When chlorine is made from electrolytic cell water containing ammonia N in

sufficient quantities, the chlorine produced will contain NCl3, which is soluble inliquid chlorine. When the liquid is withdrawn, the NCl3 evolves as a vapor andwill explode. This first occurred at a New York City water treatment plant in the1920s. It became a simple matter to remove the ammonia N from the cell


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water, but Americans did not do a very good job of exporting this information.In 1981, there were such explosions at the plant of local chlor-alkali locatedBogota, Colombia, and S.A. It was reported that more than a dozen U.S.-madeton containers with their dished heads still intact although they had been blownfrom a concave to a convex position.

There never has been any report of a container failure of 150-1b cylinders &one-ton containers at any potable water, wastewater, or industrial coolingwater chlorination or sulfonation installation in the United States or Canada.

Fires in these locations are practically unheard of. If a fire were to occur thatcaused a major liquid chlorine release it would be vaporized immediately andwould disappear in the chimney effect of the fire.

(2) Due to Flexible Connection Failure

Probably the most frequent cause of chlorine emissions resulting inoverexposure to personnel is the failure of connecting lines between thecontainer and the metering and control equipment. These connecting lines aretraditionally made of annealed copper, 2000 psi strength, and cadmium plated.Copper is used because it is flexible and has the proper structural strength;however, at each container change, the chlorine remaining inside the tubingwill absorb moisture from the atmosphere, and a cycle of corrosion will begin.

Therefore, any flexible connection has a life dependent upon how many times itis disconnected and left open to the atmosphere. To check the worthiness of aflexible connection, bend it carefully; if it screeches slightly, it is due forreplacement. The noise produced by the bending is indicative of the magnitudeof corrosion products on the inside of the tube.

(3) Due to Fusible Plugs Failure

Fusible plug failure without any evidence of elevated temperature caused byfire or direct sunlight is next in order of hazard magnitude. A fusible plug whichis supposed to melt at 158? may leak from corrosion or a poor bond betweenthe lead alloy and the plug retainer. There is only one fusible plug on a 150-1bcylinder, at the base of the outlet valve . There are three in each of the dished

heads of a ton container. Gas emissions through faulty fusible plugs have beennumerous enough to question their safety value.

(4) Due to Freak Accidents caused by Carelessness and Ignorance

Carelessness is high on the list of causes of chlorine accidents, sometimesinvolving unusual situations. One case involved a 6-inch buried chlorine gas lineoriginally and entirely within the property of two chemical plants. Adjoiningproperty became subdivided 20 years later, and some of the undergroundpiping, including that for chlorine gas, became part of the pavement area ofnew streets. This chlorine gas line was first cut into by mistake by a welder whowas supposedly inactivating obsolete natural gas lines. The heat from the torchburned a small hole in the pipe, so that the chlorine could support combustion

of the carbon steel pipe. Within seconds, the hole was approximately 8 inchesin diameter, resulting in an almost immediate discharge of the contents of 7000feet of 6-inch pipe. A few months later, on successive days, this same line wasbroken twice again by a backhol excavating for additional underground lines.After these experiences, the decision was made to lower the pipe from itsformer depth of approximately 2 ft to approximately 5 ft.

(5) Due to Valve Packing Failure

Valve packing failures have never caused any serious problems. If the leak isminor, it can often be corrected by tightening the packing nut. Serious leaks aretaken care of by the application of a proper container safety kit.

(6) Due to Piping Failure

Piping failures have been rare, and are sometimes the results of using impropermaterials. Lines carrying liquid chlorine can be a potential hazard. One of the


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few fatalities attributed to chlorine in recent years was caused by the failure ofa liquid chlorine line. The worker's death was the result of his constrictedbreathing caused by the amount of chlorine fumes, which made it impossiblefor him to climb out of the confined location of the leak.

(7) Due to Equipment Failure

Equipment failure usually refers to vaporizers used between the containers andthe chlorine metering and control equipment. These failures are due primarily

to corrosion by the chlorine, and are a function of the amount of wall thicknessversus the amount of chlorine passing through the vaporizer. The frequency ofthis type of accident is low.

(8) Due to Container Failure

Container failures, except those caused by fire, are rare. The chlorinepackagers through-out the United States and Canada are keenly aware of thepotential hazards of handling chlorine containers. This has resulted in a strictmonitoring of container condition. Perusal of the Chlorine Institute accidentreports indicates two container failures over a period of 15 years. Consideringthat shipments in 150-1b and ton containers amount to nearly 100,000 peryear, this sort of accident is a rarity and is usually a result of using an over-age

cylinder.One of these accidents occurred at a military base. A ton container was beingloaded at a dock. It slipped out of the sling and fell on its end. The tank split atthe "chime," which is the joint between the container and the dished head.Investigation of this accident revealed that this container had been in serviceby the military for 45 years.

(9) Due to Chlorine Pressure Gauge

This type of gage is vulnerable to leaks because of the silver or hastalloydiaphragm that protects the brass Borden tube. The Borden tube flexes toindicate pressure changes. Constant flexing of the diaphragm causes metalfatigue that results in diaphragm failure. When this occurs, the brass Bordentube corrodes quickly from the inherent moisture in the chlorine. This results ina severe leak. These gages should be replaced about once every five years.

3) Calculating Chlorine Leak Rates

All of the following calculations are based upon the Chlorine Institute formula ,"Release rate Formulas."

(1) Liquid Release

Where:

Where:A = area of opening to atmosphere, ft2

P1 = upstream pressure, psiP2 = downstream pressure, psir = density of liquid chlorine upstream from the opening to atmosphere, lb/ft3

(2) Vapor Release


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Where:A = area of opening to the atmosphere, ft2

P = upstream pressure, psiV = upstream specific volume (1/r), ft /lb

(3) Guillotine Break in a Pipeline

Ton Container Supply:

The maximum size of a chlorine header system under pressure in anyapplications should never be larger than one inch. If the installation involvesliquid withdrawal from ton containers, then evaporators will be an integral partof the chlorine supply system. Therefore, the worst-case scenario would be arupture in the liquid header between the containers and the evaporators. Tosimplify the concept let the calculations be confined to one container, oneevaporator, and 100 ft length of one-inch header pipe in the length.

The liquid exiting the container must pass through a 3/8 inch tubing in thedished head -then through the container shutoff valve, then through theauxiliary container shutoff valve, and finally through the header valve. All ofthese components are flow restrictors compared to a one-inch pipe. So how canthese restrictions be accounted for in calculating the chlorine leak rate?

Circa 1950, the operating personnel needed to know the maximum possibleliquid withdrawal rate from a single ton container, at the East Bay MunicipalUtility District wastewater treatment plant, Oakland, California. TheirChlorinator capacity was 18,000 lb/day. Their test, which they performedseveral times, indicated that the maximum rate was only about 10,200 lb/day.The pressure drop between the container and the entrance to the chlorinatorwas on the average about 85-40 psi = 45 psi because there was a pressure-reducing valve between the evaporator and the chlorinator. The flow at thispressure drop has to be recalculated to reflect a zero pressure at the leak. Toapply a worst-case situation, let us assume a container pressure of 120 psi.

Using the Chlorine Institute formula:

Where:Q = 10,200 lb/day = 0.1181 lb/secr = 88 lb/ft3

Substituting this in the above formula, the value of the unknown, 77A, can befound:

Q = 0.1181 lb/sec = 77A x 62.9377A = 0.00188

Assuming a cylinder pressure of 120 psi, chlorine density at 88 lb/ft andsubstituting 77A = 0.00188 in above equation, the leak rate Q will be:

Q = 0.1899 lb/sec x 60 = 11.4 lb/min

This then is the worst-case leak rate from a single ton container "on-line", when

there is a guillotine break in the liquid chlorine header piping. It is obvious thatif ton containers are being used for liquid withdrawal, an evaporator is part of


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the system. Therefore, when there is a guillotine break in the liquid header, thecontents of the evaporator becomes part of the leak.

Powell found by actual test that the maximum liquid withdrawal from aninverted 150-lb cylinder was 20 lb/min at 90 psi cylinder pressure. Therestrictions in this instance were the cylinder valve and 3-4ft of 3/8 inch tubing.

All chlorine evaporators are designed to vaporize at a temperature variationbetween 160 and 180oF, regardless of the chlorine feed rate. This means that

the level of liquid chlorine in the evaporator remains fairly constant. It is safe toconclude that the evaporator content is never more than 100 lb. At 20 lb/min.,the evaporator will empty in about 5 minutes because of the chlorine headerrupture. Therefore, the probable maximum chlorine release rate will be 11.4 +20 lb/min. for the first 5 minutes and then 11.4 lb/min after that interval. This isfor each ton container "on line" and each evaporator.

Such a leak will cool the room so quickly that vaporization is temporarilystopped. During this time, if the container floor area has been designedproperly, the liquid chlorine will flow through the collecting slots in the floor andbe hustled off to the scrubber system. This reduces enormously the amount oftime required to clean up a major spill. This maneuver capitalizes on one of thelittle-known properties of liquid chlorine-its solubility in water. Under a slight

pressure, such as the discharge from an eductor or pump, its solubility is 3 to 4times that of chlorine vapor.

(4) Ton Container Flexible Connection Failure

Assuming a worst-case situation, the flexible connection breaks at the auxiliarycontainer valve. Logic dictates that for a 120-psi cylinder pressure without therestriction of a header valve plus a 4 ft flexible connection, the release rate willexceed the rate from a header pipe rupture. A reasonable estimate would be a20 percent increase: 11.4 x 0.2 = 13.68 1b/min.

(5) Fusible Plug Failure from Corrosion

A. Description

This is the most common problem of fusible plug failure. A 3/4-inch plugcontains a lead core about 3/16 inch in diameter in a brass body. Theinherent moisture in "dry" chlorine begins an immediate attack on thevulnerable brass body. Therefore, the hole generated by the corrosion isshaped like a cone with the base of the cone on the inside of the ton cylinder.The end result of this corrosive attack leads to a pinpoint hole between thebrass body and the threaded steel of the dished cylinder head. Fieldobservations indicate that this hole is never larger than 0.1 inch in diameter.

B. Liquid Release

For a worst-case situation the following calculations will be based upon a hole

diameter of 0.15 inch, with the fusible plug located below the chlorine liquidlevel in the ton container. Here again the container pressure is assumed tobe 120 psi.

Therefore:

A = PI x D2 / 4 = 3.1416 x (0.15)2 / 4 =0.018 in2

A = 0.000125 ft2

Q = 77 x 0.00125 x (120 x 81)1/2 lb/secQ = 77 x 0.00125 x 98.59 = 0.949 lb/sec

Q = 56.94 lb/min

C. Vapor Release


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This is an important comparison because knowing the huge difference in therelease rate, the safety crew should attempt to rotate the leaking plug to thevapor area. If this is done, the escaping vapor will cool the liquid to 40oF in 3-4 minutes. This has to be taken into account when using the Chorine Instituteformula:

The container pressure will have been reduced enormously because theescaping vapor is at zero gage pressure. There is little doubt that thecontainer pressure will be as low as 40 psi. Then V will be chosen for thedensity of chlorine vapor at 40 oF, which is 0.77 lb/ft.

Then:

Therefore:

Q = 1.52 lb/min

D. Fusible Plug Blow-out

There is no such occurrence on record but it always remains a possibility.This is almost equivalent to a container rupture. It is assumed that the totaldischarge will be liquid chlorine. The container pressure will drop dramatically

in the first few seconds, similar to a flash-off. For the sake of a rationalcalculation it will be assumed that the container pressure drops to 30 psi.This is equivalent to a liquid temperature of 20oF; therefore, the density ofthe liquid chlorine is 93 1b/ft. So the leak rate is calculated as follows;

r = density at 20oF = 93 lb/ft3

A = PI x D2 / 4 = 3.1416 x (0.75)2 / 4 = 0.44 in = 0.003 ft2

Q = 732.09 lb/min

It is quite obvious that the calculations indicate an impossibility. The contentsof the container could never be discharged at that rate; otherwise thecontainer would be empty in less than 3 minutes! The scenario that is closerto what will happen is a sudden cooling of the liquid chlorine that brings thecontainer pressure to atmospheric, whereby the liquid chlorine will go into afreezing and thawing cycle that may take hours for the chlorine to escape.

4) Summary of Catastrophic Leak Events

Whenever there is a major leak, the flash-off phenomenon will always preventa positive pressure condition in a containment structure. The suddenvaporization due to flash-off cools the closed area so fast and so much that anegative pressure in the room is the result.


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There will always be a significant amount of liquid chlorine that must be dealtwith as soon as possible. Because it much more soluble in water than chlorinevapor, it can be disposed of quite easily by a water-operated eductor or a liquidchlorine pump.

The only way that liquid chlorine can be cooled by a leak is to withdraw vaporfrom it. Liquid flowing out of container due to a major leak will not cool thecontainer or reduce the vapor pressure unless the leak is a large hole in thecontainer such as a fusible plug blow-out. When this type of leak occurs, the

flash-off phenomenon goes into action as soon as the liquid chlorine is exposedto the open room. This will always cool the room so quickly that it will produce anegative head in the room.

5) Calculating the Area Affected by Chlorine Releases

Whenever there is a major liquid chlorine spill, a vapor cloud is certain to form ifthe vapor is released to the atmosphere. This may not occur if the leak is from thegas phase of the system, because of the initial dilution of the vapor with air at itssource. Chlorine vapor is readily amenable to following air currents, whether theybe ventilation air or atmospheric air, largely owing to the available moisture in theatmosphere. The higher the humidity, the greater is the attraction of chlorine. Thebehavior is synonymous with the suck-back phenomenon.

A great many researchers have investigated the phenomenon of major releasesof hazardous chemicals. The equation used by most investigators is the GaussianPlume Model equation. This predicts the length and shape of the cloud formedfrom the initial release provided weather conditions are known. The cloud thatemerges from this model is shaped like a cone sliced in half with the flat part atground level and the apex at the source of the release. The value of themathematical model is to assist authorities to set reasonable boundaries forevacuation after a release has occurred. The Gaussian equation takes intoaccount release rate, the standard deviation of the crosswind plume, width andheight of plume, height of initial source, and downwind, crosswind and verticaldistances, and chlorine concentration as follows:

where:C = concentration units/mQ = release rate, units/secy, z = the standard deviation of the crosswind plume (width and height inmeter)U = mean wind speed velocity (m/sec) at h.h = release source height (meters)x, y, z = downwind, crosswind, and vertical distances (meters)

There are three factors not accounted for in the above plume model. These are:ambient temperature, relative humidity, and local terrain. These factorscontributed significantly to the fatalities in the Youngstown accident. A release ina fog- shrouded area is probably the worst case. Air movement in a low-lying fog-shrouded area is usually nil. The relative humidity is at the saturation point, whichallows the moisture-seeking chlorine gas to saturate the fog shroud. Clothing onthe people in the release area will absorb the chlorine-laden moist air, thusmultiplying the inhalation of chlorine. In such cases it is not sufficient to merelyevacuate the area quickly. Exposed persons must remove their clothing as soonas possible. This adds another dimension of risk because a fog usually occurs inan area where the ambient temperature is quite low.

While fire is to be avoided at all costs where chlorine containers are stored, abrisk fire adjacent to a chlorine release can be a big help. This was demonstratedin a recent derailment when a tank car was ruptured by a following propane car,which exploded and caught on fire. The heat from the burning propane produced


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a chimney effect and the entire contents of the 90-ton tank car escaped withoutanyone suffering from chlorine inhalation.

2. The Uniform Fire Code

The new edition of this national code was published in 1988. It touched off waves ofdeep concern on how to handle a major leak and at the same time be within therequirements of the UFC. The basic objective of this code as it pertains to toxicgases is that the neutralizing system must be able to handle any "worst-case" type

of leak so that the fire-fighters will never have to be required to enter the facility.

In Section 80.303, page 332, it states: "Such exhaust systems shall be capable ofdiluting, absorbing, neutralizing, burning or otherwise processing the entirecontents of the largest single tank or cylinder of gas stored."

On p.333, it says that: "For portable tanks and cylinders, the maximum flow rate ofrelease shall be calculated based on assuming the total release from cylinder ortank within the time specified in Table No. 80.303-B." For liquid chlorine release thetable specifies 30 minutes for chlorine cylinders and 240 minutes for portable tank.Then on page 333 it says further that: "When portable tanks or cylinders areequipped with approved excess flow or reduced valves, the worst case release willbe determined by the maximum achievable flow from the valve as determined by

the valve manufacturer or the gas supplier."

These statements mean that the scrubber system would have to be able to handlethe entire contents of a ton cylinder in 30 minutes and a 25- or 35-ton tank in 240minutes. This will require a scrubber capacity of 67 lb/min for a ton cylinder and 208lb/min for a 25- or 35-ton tank. These tanks are usually equipped with 14,000 lb/hrexcess flow valves. In spite of the fact that 4000 lb/hr flow valves are not currentlyavailable, they should be put on 25-and 35-ton tanks. This calculates to 96,000lb/day chlorine capacity, which is more than ample for this tank size. For largertanks, 7000 lb/hr flow valves should be used instead of the 14,000 lb/day valves.The 7000 lb/day valves are currently available.

If the 25- or 35- ton tanks were equipped with 4000 lb/hr excess flow valves, then

the scrubber system would only have to handle 67 lb/min -the same as that of a toncontainer.

3. A Plan for Major Chlorine Leak

1) Definition of Major Leak

Trying to deal successfully with a major leak is formidable task. The two mostdiscussed possibilities are a direct hit by an aircraft and a planned act ofsabotage. The latter is usually dismissed on the basis that proper securitymeasures can provide a necessary deterrence. The air crash scenario is usuallydismissed as an improbability. However, an air crash accompanied by explodingfuel would result in the following: The aircraft impact or subsequent explosion

would probably rupture the chlorine container(s). The ensuring fire wouldinstantly vaporize the liquid chlorine, and the chlorine vapor would rise quicklywith the heat of the fire.

This sequence of events would serve to greatly diminish or eliminate chlorineexposure in the surrounding area. This was clearly demonstrated when a freighttrain derailment severely damaged a 90-ton chlorine tank car. The car wasruptured by the couplers from an adjoining butane tanker. All 90 tons of chlorinewere released. The butane tank car exploded, and all of its contents wereconsumed by fire. The heat from this fire vaporized the liquid chlorine whichdisappeared into the upper atmosphere because of the rising hot air from thebutane fire. A subsequent investigation revealed that no one in the surroundingarea or at the scene of the accident was found who had experienced any

exposure to chlorine. The consensus definition of the most probable major leak isa guillotine break in the liquid chlorine header between the chlorine supplysystem and the chlorine evaporators.


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2) Important Aspects of a Major Leak

If a leak is to be considered a major one, there has to be a liquid spill. Any majorgas leak can be dealt with quickly by the use of container kits and the proper useof chlorinator injectors to evacuate the vapor that is leaking. When a liquid spill isinvolved, the designer must make provisions for collecting the liquid in a confinedsump and be able to hustle it off to either a scrubber or an absorption tank.

A major leak will never create a high atmospheric pressure condition in the room

where the leak has occurred. Because of the enormous cooling effect in the leakarea due to the liquid chlorine attempting to vaporize, the room pressure will benegative. This situation assures the flow of fresh outside air into the leak area.Therefore, a containment room for chlorine storage should always be designed forproper continuous outside ventilation. The fresh air from the outside should enterthe storage room at ground level and exit at rooftop level. However, during amajor leak event, the fresh air should enter at ceiling level and discharge to thescrubber at floor level. The scrubber system should be a one-pass system; neveruse a recirculating system, as it would cause untold corrosion damage in theroom where the leak occurred.

3) Liquid Chlorine Collection System

This part of the design focuses on the storage room floor configuration. The floorshould have a dramatic slope(2-1/2 in./10 ft) to a common point terminating inthe liquid collection sump. The collecting slots should be narrow (2 inches max.)and deep (5-6 inches) to shield the liquid from room temperature. This willsignificantly diminish the liquid evaporation rate. When liquid chlorine spills on aflat surface, about 20 percent will "flash off" as vapor. This causes a thin sheet ofchlorine hydrate ice to form on the remaining liquid, which prevents furtherevaporation until the ambient temperature melts the icy film. During the freezingand thawing cycle the vaporization rate of the remaining liquid is typically 8 lb ofchlorine per square foot per hour.

The collection slots in the floor should terminate in the lowest part of the slopingfloor (see fig. 3-1). At this point the floor should be constructed to accept either a

pump or an eductor. (Pumps are available from both the Duriron Co. and Powell.These pumps routinely handle liquid chlorine.) Dealing with the liquid chlorinespill in the design of a neutralizing system is a number one priority because itreduces by a factor of ten the time required for a scrubber system to complete itsobjective.

4) Fundamentals of Estimating Leak Rates

One of the major errors usually committed when calculating leak rates from one-ton container, railcars, and/or bulk storage tanks is trying to estimate therelevance of the physical dimensions of a given leak. When a guillotine break in aone-inch chlorine header is mentioned, this dimension is used as the area of thechlorine leak. In reality the chlorine liquid has to pass through a long series of

restrictions to the flow. These are: the 22 inches of 1/2-inch tubing inside the toncontainer, the ton container shutoff valve, the auxiliary container valve, 4 ft of9/32-inch inside diameter flexible copper tubing (maybe an auxiliary headervalve), and a header valve.

There is no method of quantifying the restrictions in terms of making it possible tocalculate the chlorine leak rate from a broken or ruptured one inch-diameter pipe.The only possible way that this problem can be solved is by simulating such aleak. Such a simulation was performed at the EBMUD, Oakland, California WWTPin the early 1950s - but for an entirely different reason. Operators needed to knowthe max. liquid chlorine withdrawal rate from a single one-ton container in orderto verify the necessity to go to bulk storage.

The amount of liquid chlorine that the one-ton container could deliver to three6000 lb/day chlorinator was only 10,200 lb/day with a 45-lb pressure dropbetween the container and the chlorinator. Converting this flow rate to thepressure drop due to a header rupture, assuming a worst case of 120 psi pressure


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drop, was only 11.4 lb/min. or 16,416 lb/day as described previously. The sameapproach has to be made for the case of non insulated bulk storage tanks andinsulated railcars. These restrictions limit the ability to calculate major leakpatterns. The only way to arrive at reasonable leak flow rates is to simulate a leakon site and use scales for actual weight loss.

4. Neutralizing Major Chlorine Leak

1) Historical Background

The oldest and most popular method of dealing with a major chlorine leak is theuse of the absorption tank. This was developed by the pulp and paper industry,where chlorine is used in enormous quantities for bleaching and control ofbiological growth during pulp preparation. The absorption tank was filled withenough caustic to neutralize all the chlorine contained in the piping system andits components downstream from the chlorine supply shutoff valve. This wasdeemed the only logical way to deal with a leak where all of the chlorine beingused was under supply tank vapor pressure. This is not necessarily the case forthe 5-7 percent of all the chlorine manufactured in the United States that is usedin 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 aninjector, and the power of the venturi is obtained from a supply of water usuallyat 50-60 psi pressure. Each chlorinator is fitted with an injector capable of feedingits maximum capacity. This makes each chlorinator a primary safety devicebecause the injector can dispose of the chlorine in the piping system in a fewminutes. In addition to this feat, the chlorinators (if piped properly) can reduceand control pressure in the supply system; i.e., the chlorine cylinders or storagetanks.

The chlor-alkali plants have made significant advancements in developing saferways to store their chlorine production. The latest innovation involves therecirculation of liquid chlorine in an enormous spherical vessel through arefrigeration system that keeps the liquid at atmospheric pressure. All of this is

supplemented by a containment structure with a sloping floor to confine a liquidleak in as small a space as possible. During a leak, an insulating foam is sprayedon top of the spill to prevent vaporization while the liquid is pumped to aneutralizing tank.

The Uniform Fire Code of 1988 changed the industry approach to safetyprecautions associated with the handling of major leak. The major obstacle of thiscode is requirement that the neutralizing system should be able to handle the fullcontents of the largest single storage container. The code has led to a lot ofconfusion because the people who generated the code do not understand thebasic 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 majorchlorine leaks. Typically this scrubber is designed to recirculate the containmentroom air until all of the chlorine spill has been neutralized. The system usually isdesigned to provide one complete room air turnover every ten minutes. Thescrubber system depends upon a chlorine detector to close the normal ventilationsystem and to activate the scrubber recirculating pump. This delivers caustic tothe inlet of the venturi. Simultaneously, room air is drawn into the suction throatof the venturi where it mixes with the caustic. This is similar to a chlorinatorinjector operation.

The standard venturis on this type of system are only 85-90 percent efficient inthe 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 thetank from the venturi outlet. These gases then are forced up the vent stack andout the mist eliminator. The scrubbed air is returned to the chlorine-conta-


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minated containment room. The caustic tank is designed for any given expectedmajor chlorine spill. The stoichiometric ratio of caustic (NaOH) and chlorine is 1.13lb caustic per pound of chlorine. The scrubber operates until the chlorineconcentration in the room air is reduced to 1 ppm.

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

3) Spent Caustic DisposalThe 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 bemetered in small quantities over given period of time. The hypochlorite can beeasily destroyed by catalytic decomposition using nickel and iron as catalysts (Ifavailable, seawater will destroy the hypochlorite solution in a few hours owing tothe presence of heavy metal ions). The use of sulfites to dechlorinate thehypochlorite solution is not recommended. The heat of reaction between thesulfite 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. Thissystem 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 chlorinevapor will be increasing in volume during the initial phase of the leak. This causedpositive pressure in the contaminant room. This does not comply with the 1988edition of the UFC guidelines. In addition to the room pressure increase, therecycled chlorine vapor will contain both NaOH and NaOCl mist particles, whichwill not be removed by the mist eliminator. Furthermore, this mist will be createdwhenever the scrubber system is activated for testing or neutralizing a leak. Thisnot only is a health and safety issue but also caused severe corrosion to electricalequipment and other metal components in the room.

Another serious flaw in the recycle system is the lack of refresh air dilution. Thiscan affect the emergency response team's decision to enter the room to proceedwith their efforts to stop the leak. These Chemtrec teams are scattered all overthe United States, and their entrance limitations for repair of a leak (only acontainer 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, toproved to a prospective client that the system could neutralize a one-ton spill ofliquid chlorine. This field demonstration represented a continuing effort by a largechemical manufacturing company to reduce, at all costs, the danger of a majorchlorine leak. This was a follow-up to their production of a fail-safe automaticrailcar shutoff valve. Powell built a 10 x 12, 8-ft-high room and executed a 600-lbliquid chlorine leak using four 150-lb inverted chlorine cylinders. This test wasthoroughly documented by a 45-minute video tape. The following description ofthe Powell single-pass neutralizing system is based upon the results of the abovetests and substantiated by the known characteristics of liquid chlorine.

The schematic shown in Fig. 3-1 is based upon current recommendations for a100 lb/min leak. This is an enormous leak, equal to 6000 lb/hr. This rate producesa theoretical 500 cu ft of chlorine. Therefore, this size leak will require two 250cfm units in tandem using a single pump for motive power. It is important torealize that actually the vaporization rate of this liquid leak is going to be a lotslower than a continuous 500 cfm owing to the freezing and thawing cycle ofliquid chlorine.

Regardless of room size or leak rate, the room pressure will always be slightlynegative during the entire leak episode. This is one of the most importantfeatures of the single-pass system. This means that fresh air is entering anddiluting the chlorine-contaminated air space. It has been found that during even a


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small leak of 4-5 lb/min, the cubic feet of chlorine required to maintain negativeroom pressure is very small. This results in better system absorption efficiency.

In the Powell scrubber system, a proprietary reactor downstream from theventuris is used to reduce the vent stack emissions to meet the UFC requirementof 15 ppm detectable chlorine. This eliminates the need for a packed tower on thedischarge vent stack. Any system requiring the addition of a packed towerpresents a leak hazard situation. Any system could increase the safety factor forthe 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 evaporatorrelief system.

Any packed tower requires a finite time to become wetted, which is imperative tothe 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 ventemissions.

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

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

The length of time required to evacuate the room is not critical. Keeping the roomat negative pressure means that the leak has been contained, and theenvironment has been protected during the entire leak episode. The lower theroom temperature at the onset of a leak, the slower will be the vaporization ofchlorine. This increases the efficiency and capacity of the system. The useralways has the option of air-conditioning the chlorine storage area to atemperature of 60-65 Fo. It is a basic characteristic of chlorine that thevaporization 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 rateand not the room size. This allows a standard packaged system and does notrequire each installation to be a unique design.

In summary, these scrubber systems need only to be sized for 78 lb/min for atotal amount of approximately 2400 lb of liquid chlorine. This is far more chlorinethan could ever be vaporized in 30 minutes. This results in much smaller andeasily 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 atmosphereon the storage room floor. The floor should be sloped 2 in./10 ft to a sump that isdesigned to act as a suction well for a liquid chlorine educator. The liquid can thenbe hustled off quickly - before waiting for it to vaporize - to the caustic tank forquick neutralization, as is done with an absorption tank. If this part of the systemis designed properly, the time required for neutralizing a liquid leak will probablybe 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 asump or slot in the floor, atmospheric pressure is the only motive force availableuntil the water or caustic-starts flowing through the eductor. Then the eductorprovides the vacuum needed to create a pressure differential in the flow system.This differential will cause the liquid chlorine to flow under a small negativepressure (10-15 in. Hg) to the eductor throat without any off-gassing that might

cause a gas binding situation.7) Evaporator Pressure Relief Lines


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These lines can be manifold together into a common header pipe that terminatesin a chlorine sparger in the caustic tank, as shown on Fig. 3-1. The piping to thecaustic tanks must contain a barometric loop; this prevents "suck back" of thecaustic 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 corrosionin 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 couldprevent 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 reverseposition 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 goodchance that the recycled caustic will develop foam on the surface of the caustic inthe tank if the scrubber is not properly designed. This foam may eventually bereleased in the discharge stack. Powell advises that 25 square feet of tank surfaceper venturi reactor is sufficient to eliminate any possibility of foaming. Whensizing the caustic tank, always include a 10 percent excess of caustic for anyproposed leak episode.

9) Materials of ConstructionThe preferred materials for a long-life project with very few major leak episodeswill always be rubber-lined steel tanks, halar-lined pipe and fittings includingventuri/reactor bodies, Teflon-lined plug valves, and titanium pumps. The onlyreason for considering plastics would be the cost differential. Carefulconsideration must be given to the possibility of damage to the plasticcomponents from seismic forces, reaction temperatures, and sunlight. Otherpumps such as Durco steel, Durco high silicon iron, or TFE lined FRP should beinvestigated.

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

5. Neutralization Chemical Reaction

1) General

Chlorine can be reacted with alkaline solutions to produce hypochlorites, salts andother byproducts. Although many chemicals can be used, the most common issodium hydroxide (caustic soda). Caustic soda solutions are the most commonlyused for typical scrubbing applications. The technical information is writtenprimarily for the caustic soda solutions and its reactions with chlorine. All of thescrubbing application are chemical processes requiring detailed knowledge of thereactions including heat generated, end products, disposal of the finished

products and safe handling of all the chemicals involved in the reaction.2) Hazards

Chlorine, caustic soda, and sodium hypochlorite are materials that require specialhandling. Careful attention must be given to the nature of the chemicals involvedin the scrubbing process.

All personnel must use appropriate protective safety equipment. Regulationsspecific to chlorine can be found in OSHA publications. Chemical burns are ahazard when handling caustic or scrubber effluent. Thermal burns can result fromelevated temperatures associated with caustic dilution and chlorineneutralization. Chorine leaks can pose a severe respiratory hazard.

Reducing agents have specific safety hazards which must be considered as partof the design for the sodium hypochlorite neutralization system. In some cases,attention must also be given to the potential explosive gas mixtures when


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hydrogen (or other reactive) gas is present. A review and analysis of the systemshould be performed to identify and provide plans and procedures to avoid orminimize the potential hazards found in the operation and maintenance activities.Written guide lines and operation standards should be prepared, maintained, andused. Material Safety Data sheets are available from manufacturers and shouldbe included in guidelines, standards and training.

3) Chemistry

(1) Chlorine and Sodium HydroxideThe addition of chlorine to a solution of sodium hydroxide (NaOH) producessodium hypochlorite (NaOCl) and salt (NaCl):

2NaOH + Cl2 = NaOCl + NaCl + H2O

On a weight basis, one pound (kg) of chlorine plus 1.128 pounds (kg) of sodiumhydroxide will produce 1.05 pounds (kg) of sodium hypochlorite.

The reaction of chlorine and caustic is exothermic, liberating 626 BTU/lb (348cal/g) of chlorine gas absorbed. If liquid chlorine is directly reacted with thesodium hydroxide, 526 BTU/lb (292 cal/g) of liquid chlorine is produced sincethe latent heat of vaporization of liquid chlorine is approximately 100 BTU/lb

(55 cal/g) at room temperature.

(2) Chlorine and Sodium Carbonate

The addition of chlorine to a solution of sodium carbonate (soda ash) whencarried to completion produces sodium hypochlorite (NaOCl), salt (NaCl), andsodium bicarbonate (NaHCO3):

2Na2CO3 + Cl2 + 2H2O = NaOCl + 2NaHCO3 + NaCl + H2O

The theoretical quantities if carried to completion are one pound (kg) ofchlorine reacts with 2.99 pounds (kg) of sodium carbonate to produce 1.05pounds (kg) of sodium hypo-chlorite plus 2.37 pounds (kg) of sodium

bicarbonate, plus 0.82 pounds (kg) of sodium chloride.

(3) Chlorine and Calcium Hydroxide (Milk of Lime)

The addition of chlorine to a solution of calcium hydroxide produces calciumhypochlo-rite, calcium chloride and water. The calcium hydroxide may beprepared by mixing hydrated lime (Ca(OH)2) with water or by slaking calciumoxide (quicklime) with water [one pound (kg) of CaO = 1.32 pounds (kg) ofCa(OH)2 ]. In either case the reaction occurs as follows:

2Ca(OH)2 + 2Cl2 = Ca(OCl)2 + CaCl2 + 2H2O

On a weight basis one pound (kg) of chlorine plus 1.045 pounds (kg) of calcium

hydroxide (or 0.791 pound (kg) of calcium oxide) will produce 1.008 pounds(kg) of calcium hypochlorite and 0.783 pound (kg) of calcium chloride.

(4) Heats of Reaction

A very important factor in operation and/or design of a chlorine scrubber istemperature. Significant quantities of heat are released by the caustic-chlorinereaction. The most significant reactions involved in the chlorine scrubbing andthe corresponding heats of reaction are as follows:

Scrubbing Reaction2NaOH + Cl2 = NaOCl + NaCl + H2O

H25 = - 626 BTU/lb (- 348 cal/g) Chlorine Gas

Decomposition Reactions of Sodium HypochloriteNaOCl = NaCl + 1/2 O2

H25 = - 336 BTU/lb (- 187 cal/g) Hypochlorite decomposed


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3NaOCl = NaClO3 + 2NaClH25 = - 188 BTU/lb (- 104 cal/g) Hypochlorite decomposed

The above heats of reaction were calculated at 77 oF (25 o) using these valuesof heats of formation.

H25 NaOH = - 112.93 KCal/ g-moleH25 NaCL = - 97.234 KCal/g-moleH25 H2O = - 68.3174 KCal/g-mole

H25 NaOCl = - 83.39 KCal/g-mole

H25 NaOCl3 = -78.92 KCal/g-mole

4) Scrubbing Solutions

(1) Sodium Hydroxide

Large quantities of sodium hydroxide are typically purchased in 50% by weightsolution. Dilute strengths of caustic can be purchased in small quantities, andanhydrous caustic soda can be dissolved in water to produce solution strengthsas required. Large amounts of heat will be generated when diluting caustic withwater to prepare scrubber solutions. The amount of heat generated depends onthe strength and the temperature of the starting caustic solution, watertemperature and the final desired dilution strength. This heat should becalculated using standard heat content data to determine final dilutiontemperature. Final dilution temperatures may affect the design of the scrubber.Example: When a solution of 50% caustic at 80 oF (26.6oC) is diluted to a 20%concentration, the resultant temperature is approximately 120oF (49.9oC)

(2) Sodium Carbonate (Soda Ash) Solutions

Sodium carbonate solution strengths are limited to the solubility of sodiumcarbonate in water. The solubility of sodium carbonate increase withtemperature. Typical solubility limitations are as follows:

6.5 wt % at 32oF (0oC)10.8 wt % at 50oF (10oC)

18.1 wt % at 68oF (18.9oC)

Since the sodium carbonate must always exceed the chlorine by a ratio of 2.99,care must be practiced to avoid over chlorination. Industry practice has been touse 3-1/3 pounds of sodium carbonate per pound of chlorine. Due to thesolubility of sodium carbonate and the above ratios, only low strengths of theresulting sodium hypochlorite are produced. Generally the reactiontemperatures in these applications are not a design consideration.

Important!!

Sodium carbonate for chlorine absorption has limited application for scrubbingand is generally used for specific reasons, i.e., desired final product or

availability of the sodium carbonate. It is not the intent of this pamphlet toprovide further details specific to the use of sodium carbonate for chlorinescrubbing. Many of the guidelines used for caustic scrubbing can be applied tothe sodium carbonate systems but must be analyzed item by item.

(3) Calcium Hydroxide (Milk of Lime) Solutions

It is assumed in actual practice hydrated lime contains 95% Ca (OH)2 andquicklime contains 95% CaO. Not including the excess lime required, thefollowing ratios are required:

1.10 pounds (kg) of 95% Ca (OH)2 per pounds (kg) of Cl20.833 pounds (kg) of 95% CaO per pound (kg) of Cl2

The amount of either quicklime or hydrated lime is determined by the solubilityin the resulting chlorinated solution. Due to the low solubility of the calcium


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hydroxide solutions, temperatures due to chlorine absorption are low and arenot normally a design consideration.

Important!!

Calcium hydroxide for chlorine absorption has limited application for scrubbingand is generally used for specific reasons, i.e. desired final product oravailability of the calcium hydroxide. It is not the intent of this pamphlet toprovide further details specific to the use of calcium hydroxide for chlorine

scrubbing. Many of the guidelines used for caustic scrubbing can be applied tothe calcium hydroxide systems but must be analyzed item by item.

5) Additional Reactions

Due to the higher strengths of sodium hypochlorite produced with caustic sodascrubbing, additional reactions occurring during the absorption of chlorine maybecome a design consideration. As discussed, the desired reaction is as follows:

2NaOH + Cl2 = NaOCl + NaCl + H2O ------ [5.1a]

The sodium hypochlorite formed can decompose as follows:

3NaOCl = NaClO3 + 2NaCl ------ [5.1b]

2NaOCl = 2NaCl + O2 ------ [5.1c]

The rate of the reaction [5.1b] is strongly affected by the temperature and the pHnear the point of chlorine addition to the caustic. When sodium hypochlorite isover chlorinated, the rate of chlorate formation is greatly accelerated. HOCl isformed in the event of over chlorination by the reaction:

NaOCl + Cl2 + H2O = 2H2O + NaCl ------ [5.1d]

Chlorate formation then occurs by the following reaction:

2HOCl + NaOCl = NaClO3 + 2HCl ------ [5.1e]

The rate of reaction [5.1e] is several orders of magnitude greater than the rate ofreaction [5.1b]. The HCl formed in reaction [5.1e] combines with the hypochloriteion to form more HOCl so the excess chlorine gas a catalytic effect on chlorateformation. The increased acidity causes reaction [5.1c] to proceed. This is anexothermic reaction which can become violent in scrubbers under theseconditions.

CAUTION!

IF EXCESS CHLORINE IS ADDED TO A SCRUBBER, DECOMPOSITION OF SODIUMHYPOCHLORITE TAKES PLACE. REACTION [5.1c] WILL BECOME APPRECIABLE. THISCONDITION RESULTS IN FOAMING CAUSED BY THE STEAM AND OXYGEN.CHLORINE WILL NO LONGER BE ABSORBED IN THE SCRUBBER AND WILL BE

EVOLVED. THIS SITUATION MUST BE AVOIDED BY GUARANTEEING EXCESSCAUSTIC.

6. Process Design

1) General

The scrubbing system design must follow good engineering practice and adhereto state and local regulations and company policy. Permitting is generallyrequired. Specific performance criteria are beyond the scope of this pamphlet.Sections 2) through 5) contain process design considerations specific to chlorinethat can aid design engineers in developing a chlorine scrubbing system.

2) Capacity Decision

The scrubbing system capacity decision cannot be made until the stream to bescrubbed is defined. The composition must be predicted fairly accurately. A


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stream containing a high concentration of chlorine can be neutralized readilyusing a large volume of caustic in a relatively small contact area system. Whenthe stream to be scrubbed contains more than 20 to 30% by volume inert, caremust be taken to assure adequate contact in order to remove the last traces ofchlorine from the gas stream.

Generally, caustic at 20% or less concentration is used for scrubbing purposes.The freezing temperature of a 20% solution is -16.6oF (-27oC). Also, when a 20%solution of caustic is reacted with chlorine a nearly saturated salt solution is

formed. Scrubbing with higher caustic concentrations will result in higher peakreaction temperatures and crystal salt precipitation with attendant pluggagepotential. It must be realized that if a batch scrubber is designed properly, thestarting solution can exceed 20% by weight, if the solution is not chlorinated tothe end point. Each application will need careful review to ensure saltprecipitation can not occur.

Potentially reactive or hazardous components must be defined and considered inthe scrubbing system design. For example, in chlorine producing plants, it issometimes necessary to neutralize a stream made up of chlorine, air, andhydrogen. When the chlorine in this stream is neutralized, the hydrogenconcentration may increase through the reactive/explosive ranges. (See PamphletMIR-121, ref. 6.1). Special attention must be paid to potentially reactive or

explosive components during the process design.

The fluid state of chlorine, gas or liquid must be considered during the designprocess. If liquid chlorine is fed to a system designed to process gas, a violent anduncontrolled reaction will result. This can lead to a chlorine release.

In a batch reaction system, the duration and concentration of the vent streamflow must be known to size the process equipment appropriately.

3) Reaction Temperatures

Table 6.1 shows the overall heat load on a chlorine scrubber that is reactingchlorine at an instantaneous rate equivalent to 100 tons (90.7 metric tons) perday. On a hourly basis this is equivalent to 8,333 lbs/hr (3,720 kg/hr). The "nodecomposition " line assumes that all chlorine reacts to sodium hypochlorite. The"decomposition" line assumes that 25% of the sodium hypochlorite produceddecomposes to oxygen and salt.

Caution!

The assumed 25% decomposition is noted for illustrative purposes only. Theamount of decomposition will be in influenced by the reaction temperature andthe presence of impurities which can catalyze the decomposition reaction. Theexpected decomposition must be developed for each individual system. In theabsence of external cooling and in the absence of information on the specificcatalysts present, then a conservative estimate of the temperature rise isdetermined by assuming 75% decomposition.

Table 6.1

DRY CHLORINE FEED

No Decomposition 5.2 x 106 BTU/hr (1.31 x 10 Kcal/hr)

Decomposition 6.9 x 10 6 BTU/hr (2.02 x 10 Kcal/hr)

(saturated at 190oF)

No Decomposition 7.8 x 106 BTU/hr (1.97 x 10 K-cal/hr)

Decomposition 8.5 x 106 BTU/hr (2.15 x 10 K-cal/hr)

When neutralizing water saturated chlorine with stoichiometric quantities of 15 to20% caustic, the heat generated can bring the solution to the boilingtemperature. The water vapor generated by the boiling solution dilutes thechlorine and reduces the mass transfer efficiency of the scrubber. Thus, it is


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desirable to maintain the solution temperature well below the boilingtemperature. The transfer of heat from the solution to an external cooling systemcan be the obvious choice if capacity is available. If external cooling is notavailable, temperature control can also be accomplished by reducing the initialcaustic concentration or scrubbing with excesses of caustic. Graphs 6.3A and 6.3Bshow the effect on scrubber liquor temperature rise when initial caustic strengthvaries from 5 to 20% and when one to four times the stoichiometric quantity ofcaustic is used for neutralization. Note that the following graphs illustrate chlorinesaturated with water vapor. Dry chlorine scrubbers have lower heat loads which

can be derived from the data in Table6.1.

Graphs 6.3A and 6.3B illustrate the heat effects of the reaction. Temperatureincreases are approximates. A rigorous thermal analysis is required for eachscrubber design to ensure proper materials of construction are employed.

4) Caustic Soda Scrubbing Solution

When caustic soda is used as a scrubbing solution, these guidelines should beconsidered.

In order to maintain scrubber capacity to react chlorine, there must always besome excess of caustic. In emergency scrubber applications where flows and

concentrations cannot be guaranteed, sufficient excess caustic should bemade available. For in-process scrubbers where flows are known only minimalexcess caustic is necessary.

In many applications, it is desirable and technically feasible to deplete thescrubbing liquor to as low as 10 grams per liter of NaOH. When lowconcentrations of caustic are used, several items should be considered. As pHdrop below 10, conditions become favorable for the formation of sodiumchlorate. Under basic conditions sodium chlorate is quite stable and willcontaminate the effluent stream.

Total depletion of caustic is to be avoided. Accidental depletion will negate thereaction process and chlorine gas will be evolved. The resulting acidic

conditions will cause sodium hypochlorite to decompose to salt and oxygen.The oxygen evolution can be violent.

Batch scrubbing operations using ejector venturi devices or packed columnsshall have sufficient caustic soda solution flowing to always exceed the 1.128caustic to chlorine ratio. At the end of the batch scrubbing cycle when thecaustic concentration has been reduced to low levels, care must be taken toassure adequate mass transfer.

When strong caustic solutions are chlorinated to the end point, the saltconcentration can be high enough to become saturated in the resultingsolution and it can precipitate from the solution. System pluggage is a hazard.Precipitation will occur if the beginning solution is greater than approximately

22% by weight. The salt precipitation is also temperature dependent.

Although caustic soda dissolves in water to form various concentrations, caremust be taken of the temperature at which the solutions separate solidhydrates. These "freezing curves" are available in literature published byproducers and typical values are as follows:

5% @ 25oF (-3.9oC) 10% @ 18oF (-7.8oC) 15% @ 4oF (-15.6oC)

18% @ -11oF (-23.8oC)

20% @ -16.6oF (-27oC)

25% @ 0oF(-17.8oC)

19.09% caustic has the lowest freezing temperature of any concentration ofcaustic. The freezing point of this solution is -18.4oF (-28oC). 50% caustic soda,the common commercially available strength, freezes at 54oF (12.2oC)


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5) Specific Safety Considerations

The following areas are critical when the initial design decisions are made:

Adequate instrumentation should be provided for monitoring, analyzing,recording, and controlling the critical operating parameters.

The fluid state of the chlorine should be consistent with the process designcriteria. The process should be designed to prevent liquid chlorine from

entering a scrubber designed for gas.

If the possibility of explosive gas mixtures exists, steps should be taken toprevent same, e.g. provisions for dilution with air.

Installation of a system to prevent hypochlorite of caustic solutions fromflowing back into the chlorine lines and corroding piping and valves, such as abarometric loop.

Caustic and water when mixed will have less volume than the sum of the twostreams. However, the resultant solution when chlorinated to low excesscaustic levels will expand. This expansion at high solution strengths ofbeginning caustic can result in an approximate 10% increase in volume more

than the total sum of caustic and water volume. Always design the scrubbingsystem for the theoretical maximum volume.

Batch scrubber system, when used for room scrubbing or many types doprocess scrubbing, will cause the caustic to react with any CO2, present toproduce sodium carbonate. Each application must be reviewed to ensure thecaustic depletion and carbonate/bicarbonate precipitation during operationare not problems.

Materials of construction should be consistent with the process under bothdesign and upset conditions, e.g. if titanium, which is excellent in wetchlorine, is allowed to contact dry chlorine, spontaneous combustion willresult.

7. System Design

1) General

The primary function of a scrubber is to contact chlorine with a scrubbing fluid.Chlorine scrubbing systems have been installed to abate a wide range of processstreams. Depending on the size and on the main objective of the scrubber, thereare various options to select from when basic design choices are made. Examplesof streams being scrubbed are continuous process vents, system emergencyrelief, and vents from rooms storing chlorine containing vessels. The selection ofa continuous system over a batch or emergency unit will impact the method usedfor transporting the fluids and the method for bringing them in contact. Each type

of vent poses different demands on equipment. For emergency chlorine spillcontrol the conservative design system would assume the chlorine to be wet. Areview of the location for the scrubbing system should consider the safety ofadjacent areas.

The use of redundant instrumentation, increased safety factors in the design, andstand-by electrical power supply should be considered. Reliability takesprecedence over economics. Below sections 2) through 8) give guidance to thedesigner relative to the selection of system components.

2) Materials of Construction

In the selection process for the materials to be used, the engineer must consider

the operating parameters in which the scrubbing system will be expected toperform not only under normal process conditions but also during process upsets.Different concentrations and temperatures of hypochlorite and caustic can affect


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the corrosiveness of these solutions. Therefore, the particular conditions willimpart the selection of the appropriate materials of construction. Each installationmay have a different application which must be studied to obtain a safe,economic, and efficient system. The use of improper materials of construction canlead to premature failure of piping and/or the scrubber.

As reference and without the intention to exclude others, a common set ofconditions found in the operation of a scrubber system show the demandingenvironment in which the materials must perform. These conditions are:

Component Concentration Temperature

Wet chlorine gas 50 to 99 % 180 to 200oF (82 to 93oC)

Caustic solution 10 to 50 % Ambient

Hypochlorite 5 to 15 % 70 to 200oF (21 to 93oC)

3) Contactors

Table 7.2 represents various types of contacting apparatus presently being usedin scrubbing systems throughout the industry. Comments are given to providesome of the positive and negative points of each system. Selection of thecontactor must be dependent on specific need or application.

CHLORINE CONTACTORS (TABLE 7.2)

Type Descriptions Properties Conditions

Sparger This device is made bysubmerging adistribution pipe belowthe surface of theneutralizing liquid andallowing the stream tobe scrubbed to bubblethrough.

1. Very simple,passive andcommonly used.2. Usually batchoperation3. Very economicalto build4. Uses system

pressure forcontacting

1. High P2. Misting may be aproblem3. Performance istypically predictedby experience ortest

Spray Tower This device is usually ofcounter flow design withmultiple layers ofnozzles with overlappingspray patterns ofneutralizing solution.

1. Low P2. Simple design3. Most economicaltower design

1. Limitedcontacting stages2. Nozzle pluggagepossible3. Performance istypically predictedby experience ortest

Packed Tower This device is usually of

counter flow design withbottom gas entry intoopen area under packedsection. Liquid enters attop of packed section inliquid distribution traysor pipes, liquid flowsdown through thepacked section creatingmulti stage contacting.

1. High contacting

surface - multi-stagecontacting2. Moderate P3. Performancereadily predicted4. operates over awide range of gasflow

1. Pluggage of

packing possible2. Complete wettingof packing isnecessary for propertower performance

Tray Tower This device is usually ofcounter flow design with

gas entering at thebottom of the columnwith scrubbing liquid

1. Moderate to highP

2. Multi-stagecontacting3. Performance

1. Tray pluggagepossible

2. Most expensivetower to build3. Trays must be


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entering on the top tray.Gas bubbles through theliquid on each traycreating a multistageneutralization column

readily predicted filled for properperformance

Venturiscrubber

This device uses a highpressure liquid stream tocreate as vacuum using

an ejector. As the highpressure liquid passesthrough the throat of theventuri intimatecontacting occurs.

1. Eliminates theneed for gas mover2. Pre-engineered

units are available

1. Number oftransfer units limitedin a single stage

2. Efficiencydetermined only bytesting3. Less effective fordilute chlorinestream

4) Fluid Movers

(1) Chlorine Movers

The selection of the proper mover for chlorine is completely dependent on theexact specifications of the situation. In general, the proper material of

construction depends on whether or not chlorine is either "wet" or "dry". For dryservice carbon steel is suitable. For wet chlorine service the choice materials ofconstruction are selected FRP and CPVC. Titanium is very useful where waterconcentration will always be above 2000 ppm. Under no conditions shouldtitanium be used for dry chlorine service as titanium "fires" can result.

For emergency chlorine spill control the conservative design system wouldassume the chlorine to be wet.

There are a number of methods to move chlorine from one point in a system tothe next. The following four sections identify the most commonly used chlorinemovers. The main variables to be considered are required volumes to be movedand the required pressures. Each method listed has limitations for either

volume or pressure.

(2) Process Pressure

Chlorine can be transferred from one vessel to another by the differentialpressure between the two vessels. In warm weather chlorine may betransferred by its own vapor pressure. Because vapor pressure is a function oftemperature, cold weather usually reduces transfer rates unless the pressure isincreased by padding with "dry" air or inert gas. Reference Chlorine InstitutePamphlet #66, see 6.1.4.

(3) Ejectors

An ejector is a simple mechanical device with no moving parts. All types ofejectors operate on the principle of one fluid entraining a second fluid. Theejector is constructed in three sections: motive fluid inlet, suction chamber anddischarge. An ejector consists of a relatively high pressure motive fluid nozzledischarging a jet across a suction chamber into a ventrui shaped diffuser. Thejet stream across the suction chamber entrains the secondary fluid into themotive fluid. The mixture enters the discharge section where the velocityenergy is converted to pressure by the diffuser shape geometry.

For chlorine service an ejector must be constructed of materials suitable for wetchlorine service. True performance of an ejector is difficult to predict andusually requires close testing.

(4) Compressors

Due to the economics of the required materials of construction, conventionalcompressor designs (reciprocating and rotary) are not recommended for wet


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chlorine service. Typically, the liquid ring compressor is used in dry chlorineservice but in limited situations this design can be used in wet chlorine service.The liquid ring compressor is a positive displacement machine is which a barrierfluid is used as the piston to compress and displace the gas stream. For drychlorine service the barrier fluid will be high strength (98%) sulfuric acid. Theacid will tend to dry the chlorine gas during compression. The danger in wetservice is the dilution of the acid and destruction of the equipment. Wherewater content of the chlorine is always above 2000 ppm, titanium liquid ringcompressors, using water as a seal fluid, have been used. Special design

expertise is required.

(5) Blowers

The term "blowers" is a common industrial term to refer to a single stagecentrifugal compressor. Typically a blower will deliver high volumes at lowdischarge pressures. FRP or titanium blowers are used in wet chlorine service.Steel or Alloy 20 blowers with Hastelloy C shafts are used in dry chlorineservice.

(6) Scrubbing Solution Movers

Scrubbing solutions are generally transferred using a centrifugal pump, but

gravity or feed from pressurized tanks can not be excluded. Specifics of pump,vessel design and pump seal needs must be engineered for each site. Followingare comments on pumping caustic and hypochlorites.

Centrifugal Pumps: For pumping caustic liquor to scrubbing systems, all-ironconstruction is generally suitable, although nickel and nickel-cast-ironpumps give longer service life. Above 140oF (60oC), a nickel or nickel alloypump must be used. Brass fittings and bearings should be avoided onwetted parts, especially at high temperatures or concentrations.

For pumping various hypochlorite solutions the materials of construction ofthe pump are dependent on the temperature and concentration of thesolution. At lower temperatures, [below 70oF (21oC)] high silicon iron and

other non-metallic corrosion resistant materials work well. Titanium may beused at any temperature but as the temperature rises above 120oF (49oC),it is the only economical material. It is important when using titanium thatcomplete wetting is maintained.

Differential Pressure: Caustic can be fed to scrubbers from head tanks orpressurized feed tanks. These systems may be particularly useful to assurefeed during transient conditions such as during the time required to put abackup power source into service.

5) Heat Removal

It may be desirable to remove the heat of reaction and the heat of solution of the

caustic soda or process reasons, (i.e., reduced chlorate formation, equipmenttemperature design limits, etc.).

Regardless of the scrubber design, heat removal is accomplished by use ofcirculating pumps and heat exchangers. The process fluid is circulated in a closedloop and the cooling liquid or gas does not come in direct contact with thescrubbing liquid. The typical and most commonly used material for construction istitanium.

Sizing of the system is accomplished by review of the flow rate of the chlorine gasor liquid and using 526 BTU/lb (292 cal/g) for liquid and 626 BTU/lb (348 cal/g) forvapor, plus the calculated heats of solution for the caustic if applicable. The heatof the NaOCl decomposition reaction must be added if conditions are appropriate.

Heat exchangers, pumps, and process flow rates can only be determined byindividual scrubber design and is left to the engineer. Plate and frame heatexchangers can more easily "salt out" and pluggage may occur.


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6) Storage and Decomposition

Tanks for storage of scrubbing solution and/or decomposition of spent scrubbingsolutions may vary according to the end use; i.e. sale of the hypochlorite by-product or decomposition for environmental reasons. If the solution is to be sold,the fluid is usually cooled during scrubbing and may be stored as a cool,hypochlorite solution in lined steel tanks or polyester and vinyl ester fiber-glass-reinforced plastic tanks. Linings that may be economically used are chlorbutyland ethylene propylene rubbers, polyvinyl chloride, and polypropylene.

Decomposition tanks usually handle solutions at elevated temperatures, andtherefore are in extremely aggressive service. At the higher temperatures,titanium, brick lined steel and in some cases rubber lined steel are the materialsof choice. It should be noted that titanium is attacked by dry chlorine, thereforecare should be taken to keep all parts of the decomposition system wetted.

7) Controls

The use of any necessary controls with the attendant instrumentation is dictatedby the type of scrubbing system chosen, the conditions of the installation and thedegree of automation desired. Controls are necessary to start up or shut downsystems, alert operating personnel to any problems - real or potential, and

maintain operation within design parameters.In general, the keys to successful control are the availability of operatingcondition information. the designer should evaluate the need for instrumentationin the following areas:

Chlorine Gas Detector - scrubber gas vents; chlorine gas inside the isolatedarea and external around the area and some process relief headers

Pressures - Pumping solutions

Temperature - Scrubbing Liquids

Scrubbing Liquid End Point Indicator - oxidation reduction potential or pH

Level Control - Storage and Reaction Tanks

Pressure - Chlorine process

Flow of scrubbing liquids

Caution: Although pH and oxidation reduction potential are useful indicators ofreaction end points, they should not be relied upon for insurance against chlorinerelease mitigation. Periodic analysis of the scrubbing solutions must be performedto ensure that adequate excess caustic is maintained.

Alarms should be considered to alert personnel to potential or actual abnormalperformance. Stand-by electrical power supply should be considered.

The process and the desires at the installation should dictate whether a sensor isindicating only or transmitting a signal for indication or control.

Start-up or shut down can be automatic or manual, local or remote, as the needsand design of the situation dictate.

Housing for instrumentation should conform to NEMA 4X standards as a minimumto ensure proper corrosion protection. Review all local regulations and follow goodinstrumentation practice and principles in the design and choice of components.

8) Analysis


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From time to time, the analysis of the scrubbing solution is recommended toensure that the solution is at sufficient strength and can handle the amount ofchlorine release for which the system is designed.

Analysis of solution strengths are determined normally by titration. Care shouldbe taken to differentiate carbonate alkalinity from caustic alkalinity.

8. By-Product Handling

1) Stream Characteristics

The by-product streams for chlorine scrubbing systems vary with the intent fortheir use. The strength and content depends on the solution chosen for scrubbing.In addition, the final use final use of the by-product will be a factor in the streamcharacteristics, especially strength and purity.

2) Product or waste

Will the by-product stream be a product or waste?

As a product, usually a hypochlorite, the material is best handled in lined pipingand tanks to keep the purity as high as possible. The product is kept cool,minimum exposure to light, and with an alkaline pH to minimize decomposition.The most common by-product of a chlorine scrubbing system is sodiumhypochlorite which is used as chlorine bleach. If this product is to be sold, carefulanalysis of potential contaminants must be made.

As a waste, the by-product stream must be decomposed to get rid of the chlorinevalue prior to disposal for environmental compliance. In some cases the by-product streams are acidified for the recovery of the chlorine gas and theresulting stream becomes a waste.

3) Environmental

Before disposal of the chlorine scrubber streams the environmental permits withthe respective agencies must be checked. The choice of decomposition methods

will be dependent upon the regulations. Free chlorine is almost always limited,but in addition certain metals (usually from metal catalysts) cannot bedischarged. The total dissolved solids (which are the salts of decomposition)usually have a limit in discharge permits. It may be necessary to adjust pH priorto discharge.

4) Decomposition

There are several factors that accelerate decomposition of scrubber by-productstreams:

Increasing concentration of hypochlorite

Increasing temperature

Decreasing alkalinity (or pH)

Presence of catalysts

Exposure to light

The effect of variations and combinations of these factors have been used to builddecomposition processes.

(1) Thermal

Thermal decomposition is based upon the fact that hypochlorine solutiondecomposition rate increases with higher temperature. Also higherconcentration and low pH accelerate the decomposition. A common method of


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decomposition is to heat a tank containing a maximum strength hypochloritesolution with live steam. This heating combined with the heat of decompositionresults in rapid decomposition of sodium hypochlorite to oxygen and salt.

(2) Catalytic

The presence and/or addition of dissolved cobalt, nickel, copper, and iron inhypochlorite solutions catalytically accelerates decomposition. The presence ofsalts of these elements combined with the thermal process results in almost

complete decomposition of sodium hypochlorite.(3) Time and Light

All hypochlorite solutions will decompose with time. The presence of sunlightaccelerates the process. In some cases shallow ponds with exposure to lighthave been used to decompose weak solutions of hypochlorite.

(4) Chemical

Certain chemicals react with hypochlorite solutions. Some of these are SO 2,sulfites (Na2SO3, NaHSO3), thiosulfate, and hydrogen peroxide. In most casesthe use of these chemicals are too expensive to use as the method for stronghypochlorite decomposition. They are used most of the time in polishing and

small batch reactions to remove traces of chlorine. Certain chemicals result inhigh chemical oxygen demand and therefore discharge permits must bechecked.

The combination of waste acids are sometimes used to recover chlorine andreturn chlorine back to the process. The reaction of acids such as HCl and H2SO4liberate chlorine gas in their chemical decomposition and thus must be usedwith care.

(5) Effluent pH Adjustment

After decomposition of hypochlorite solutions by any method, acids may beused for pH adjustment prior to effluent discharge.

CAUTION - Acids should not be added before complete decomposition becausechlorine will be evolved.

9. RJ Environmental Chlorine Scrubber

1) Reaction Chemistry

A chlorine leak in the room housing the chlorine cylinders / containers / tanks or inthe chlorinator room should be neutralized by means of neutralizing solution(Soda, Sodium Hyposulphite). Mainly caustic soda solution (NaOH) is used forneutralization of chlorinated air.

The chemical reaction of chlorinated air and caustic soda (Sodium Hydroxide) is:

Cl2 + 2NaOH + H2O = NaCl + NaOCl + 2H2O + 44,600 Btu/mole Cl2

Through the contact of chlorinated air and neutralization solution, theconcentration of exhausted air could be reduced to 1ppm. Two moles of sodiumhydroxide (80 pounds) is required to neutralize each mole of chlorine (70.9pounds). The amount required to neutralize the 1 pound of leaked Cl 2 is obtainedby the below formula;

NaOH = 80.0 / 70.9 = 1.13 times to Cl2

The 20% by weight of sodium hydroxide is usually used to neutralize the chlorinegas considering the freezing point in the winter. Fig. 9-1 is showing therelationship between the temperature and percent of NaOH. Each gallon of 20%solution contains 2.04 lbs of NaOH.


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2) Maximum Room Initial Concentration

This is obtained from

Maximum Room Initial Concentration = (Rate of Gas Vaporization / Rate ofVentilation of Blower) x 106 (ppm).

3) Chlorine Concentration vs. time

By performing a material balance, the following differential equation is obtained

for the decrease of chlorine vapor in the room as a function of time.

Where,V = room volumec = Cl2 vapor concentration in room, ppmQG = room ventilation rate, ft /mint = time, min

Upon integrating, the following equation is obtained:

Where:Ci = initial chlorine vapor concentration, ppmCo = chlorine vapor concentration at any instant, ppm

4) Sump Temperature Rise

Neutralization of chlorine with sodium hydroxide is an exothermic reaction. The

heat of reaction is 44,600 Btu/mole of chlorine. Reaction is in liquid phase. If thereis no heat loss to the air and other components of neutralization system, thetemperature rise of sodium hydroxide solution can be calculated from thefollowing equation:

H = mCp T

Where,H = total heat released by reaction, BTUm = total weight of caustic solution, lbCp =heat capacity of causitic solution, BTU/lb

T = temperature rise, oF

Heat capacity for 20% by weight caustic solution is 0.9 Btu/lb. One (1) gallon of20% NaOH is equal to 10.21 lbs.

5) Chemical Utilization during Periodic Equipment Checkouts

The neutralization system is a safety device to be run when the chlorine gas isleaked. Since this system is not continuously being operated, it is essential tooperate the system periodically for preventing a possible malfunction of rotatingparts. In general, it is highly recommended to checkout the system weekly.

Carbon dioxide (CO2) will be absorbed by the caustic solution during bi-weeklytesting of the system. The reaction between sodium hydroxide and carbon dioxideis:

CO2 + 2NaOH = NaCO3 + H2O


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Two (2) moles of sodium hydroxide (80 pounds) is required to react with eachmole of carbon dioxide (44 pounds). Ambient air contains 0.033% by volume (ormole) of carbon dioxide. Each mole of air at ambient conditions occupies avolume of 386 cubic feet. Therefore, 3,000 cfm of air blower is equal to 7.77moles/min. The carbon dioxide flow rate will be 0.11 lbs/min. Caustic consumptionfor each minute of neutralization system testing is 0.21 pounds/min. If the systemtesting lasts 15 minutes, then the weekly caustic consumption is 3.0 pounds.After one (1) year of regular weekly check-out, the total caustic used is: 3.0 x 52= 156 lbs/year. About 30 gal. of 50% of caustic solution has to be added into the

NaOH solution tank every year. (50% caustic =6.643 lbs/gal of NaOH)

6) RJ Scrubber System

This scrubber system was designed to meet the Uniform Fire Code (revised 1990),Section 80.303 of Article 80 as it pertains to indoor storage of compressed gases.It was specially designed to meet the UFC maximum allowable dischargeconcentration of the Cl2 vapor, to one-half of IDLH (Immediate Danger to Life andHealth) at the point of discharge to the outside atmosphere. For chlorine, theIDLH is 30 ppm. Therefore the maximum allowable discharge concentration in thescrubber vent stack is 15 ppm as stated in the UFC. The RJ scrubber, though, isdesigned to treat a release rate much higher than the UFC requirement. A fullscale test with a chlorine rate at about 100 lb/min. resulted in vent stack chlorine

concentrations of less than 4 ppm. The entire unit is a skid-mounted packagemeasuring 16 feet long 8 feet wide and 8 feet high.

(1) Scrubber

This is a single-pass three stage absorption system that operates entirely undera vacuum (negative pressure), including all the ducting. This eliminates thepossibility of any release of chlorine contaminated air. This system is shown inFig. 9-2. The three stages of absorption consists of one horizontal sprayscrubbing stage, followed by two horizontal cross-flow packed bed sections. Thedesign of each stage provides an overall performance of 99.998% removal ofthe chlorine vapor in the vent discharge. This automatically guarantees that theremoval efficiency on a once through basis will easily neutralize the worst-case

chlorine leak occurrence. The movement of air through the scrubber is providedby a 5 HP 3000 cfm exhaust fan.

(2) Caustic Storage

This amounts to about 2400 gallons of 20 percent NaOH solution which is about85 percent excess caustic over the theoretical requirement. This means thatthe maximum possible concentration of hypochlorite after the neutralization ofa capacity leak would be less than 12.5 percent.

(3) Activation System

A chlorine leak detector activates the scrubber system in two steps:

The caustic recirculating pump is started to provide proper atomization forthe first stage plus the proper wetting of the packing material in the othertwo stages before the exhaust fan is activated.

After a 5 seconds interval of step one, the 3000 cfm exhaust fan is thenautomatically started and this begins the scrubbing of the contaminated air.This interval before the fan is actuated is not nearly long enough to changethe air pressure in the room. This automatic sequence during the initialstart-up prevents the discharge of any partially treated contaminated airbefore the scrubber is operating at design conditions.

(4) Absorption Details

The absorber is located on top of caustic tank which is an integral part of thesystem. The caustic solution is recirculated continuously through the scrubberat the rate of 550 gpm at 25 psi which means that this is classified as a "low


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pressure syste

Handbook on Chlorine Handling

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