1) Reaction Chemistry

A chlorine leak in the room housing the chlorine cylinders / containers / tanks or in the chlorinator room should be neutralized by means of neutralizing solution (Soda, Sodium Hyposulphite). Mainly caustic soda solution (NaOH) is used for neutralization 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, the concentration of exhausted air could be reduced to 1ppm. Two moles of sodium hydroxide (80 pounds) is required to neutralize each mole of chlorine (70.9 pounds). The amount required to neutralize the 1 pound of leaked Cl₂ is obtained by 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 chlorine gas considering the freezing point in the winter. Fig. 9-1 is showing the relationship between the temperature and percent of NaOH. Each gallon of 20% solution contains 2.04 lbs of NaOH.

2) Maximum Room Initial Concentration

This is obtained from

Maximum Room Initial Concentration = (Rate of Gas Vaporization / Rate of Ventilation 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 volume
c = Cl2 vapor concentration in room, ppm
QG = room ventilation rate, ft /min
t = time, min

Upon integrating, the following equation is obtained:

Where:
C_i = initial chlorine vapor concentration, ppm
C_o = 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 there is no heat loss to the air and other components of neutralization system, the temperature rise of sodium hydroxide solution can be calculated from the following equation:

H = mC_pDT

Where,
H = total heat released by reaction, BTU
m = total weight of caustic solution, lb
C_p =heat capacity of causitic solution, BTU/lb
DT = temperature rise, ^oF

Heat capacity for 20% by weight caustic solution is 0.9 Btu/lb. One (1) gallon of 20% 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 is leaked. Since this system is not continuously being operated, it is essential to operate the system periodically for preventing a possible malfunction of rotating parts. In general, it is highly recommended to checkout the system weekly.

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

CO₂ + 2NaOH = NaCO₃ + H₂O

Two (2) moles of sodium hydroxide (80 pounds) is required to react with each mole of carbon dioxide (44 pounds). Ambient air contains 0.033% by volume (or mole) of carbon dioxide. Each mole of air at ambient conditions occupies a volume of 386 cubic feet. Therefore, 3,000 cfm of air blower is equal to 7.77 moles/min. The carbon dioxide flow rate will be 0.11 lbs/min. Caustic consumption for each minute of neutralization system testing is 0.21 pounds/min. If the system testing 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 discharge concentration of the Cl2 vapor, to one-half of IDLH (Immediate Danger to Life and Health) at the point of discharge to the outside atmosphere. For chlorine, the IDLH is 30 ppm. Therefore the maximum allowable discharge concentration in the scrubber vent stack is 15 ppm as stated in the UFC. The RJ scrubber, though, is designed to treat a release rate much higher than the UFC requirement. A full scale 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 package measuring 16 feet long 8 feet wide and 8 feet high.

(1) Scrubber

This is a single-pass three stage absorption system that operates entirely under a vacuum (negative pressure), including all the ducting. This eliminates the possibility of any release of chlorine contaminated air. This system is shown in Fig. 9-2. The three stages of absorption consists of one horizontal spray scrubbing stage, followed by two horizontal cross-flow packed bed sections. The design of each stage provides an overall performance of 99.998% removal of the chlorine vapor in the vent discharge. This automatically guarantees that the removal efficiency on a once through basis will easily neutralize the worst-case chlorine leak occurrence. The movement of air through the scrubber is provided by a 5 HP 3000 cfm exhaust fan.

(2) Caustic Storage

This amounts to about 2400 gallons of 20 percent NaOH solution which is about 85 percent excess caustic over the theoretical requirement. This means that the maximum possible concentration of hypochlorite after the neutralization of a 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 for the first stage plus the proper wetting of the packing material in the other two stages before the exhaust fan is activated.
• After a 5 seconds interval of step one, the 3000 cfm exhaust fan is then automatically started and this begins the scrubbing of the contaminated air. This interval before the fan is actuated is not nearly long enough to change the air pressure in the room. This automatic sequence during the initial start-up prevents the discharge of any partially treated contaminated air before 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 the system. The caustic solution is recirculated continuously through the scrubber at the rate of 550 gpm at 25 psi which means that this is classified as a "low pressure system". The recirculating pump is 20 HP. The scrubbed air passes through a mist eliminator before it is discharged to the outside atmosphere. The discharge is monitored by an EIT series 4000, vent stack monitor. The scrubber provides a vapor residence time of 5 seconds which is much longer than in a venturi scrubber.

(5) Major System Features/Advantages

J has conducted full scale tests of two RJE Vapor Scrubbing Systems during April 1992 at a nationally recognized testing laboratory accredited by the International Conference of Building Officials (ICBO).

Testing was performed under rigidly controlled procedures with continuous on-line data recording and video taping of each of the tests. Liquid chlorine was released directly from the cylinders in a specially designed flash room (13' x 12' x 12'). The systems were evaluated with chlorine release rates from 30 lb/minute to 100 lb/minute.

The Uniform Fire Code requires a maximum concentration of chlorine in the scrubber exhaust of 15 ppm. The chlorine concentration in the RJE scrubber exhausts were 2 ppm or less during all tests.

In addition to proven system designs, RJE scrubber system offers many advantages that are not available with conventional systems.

• Conservative Design: Although design requirements are based on the UFC release rate of 78 lb/min, the scrubber system designed for 3,000 cfm is capable of neutralizing completely more than 500 lb/min of chlorine vapor per minute on a once through basis. This could be equivalent to a complete cylinder failure with an average 20% flash-off rate.
• Low Profile: With a special design concept, our system provides the highest scrubber performance (three stages) with very low profile. The overall system size is about 15' L x 8' W x 9' H.
• Low Horsepower: RJE full scale system only requires about 1/2 to 2/3rd the horsepower, because of low pressure recirculation of chemicals. The table below shows the horsepower required for RJE system compared to an eductor type system.
  

  Air Flow Rate, cfm	RJE Type	Eductor Type
  3000	25	40
  4000	27.5	60
  5000	27.5	60 +

  
  

• Induced Draft Fan: This fan provides "negative pressure" throughout the system, including the room, ducting, and scrubber.
• Low Pressure Recirculation: 30 psig (vs. 70 psig) further enhances the safety of the system.
• Economics: Because of the low profile and special design, this system is delivered as a completely skid mounted, piped, wired, and factory tested unit, thus reducing installation costs tremendously.

References
1. Chlorine Scrubbing System, Chlorine Institute, Inc.
2. Chlorine Handbook, George White
3. RJ Technical Manual