1) General

Chlorine can be reacted with alkaline solutions to produce hypochlorites, salts and other byproducts. Although many chemicals can be used, the most common is sodium hydroxide (caustic soda). Caustic soda solutions are the most commonly used for typical scrubbing applications. The technical information is written primarily for the caustic soda solutions and its reactions with chlorine. All of the scrubbing application are chemical processes requiring detailed knowledge of the reactions 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 special handling. Careful attention must be given to the nature of the chemicals involved in the scrubbing process.

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

Reducing agents have specific safety hazards which must be considered as part of the design for the sodium hypochlorite neutralization system. In some cases, attention must also be given to the potential explosive gas mixtures when hydrogen (or other reactive) gas is present. A review and analysis of the system should be performed to identify and provide plans and procedures to avoid or minimize the potential hazards found in the operation and maintenance activities. Written guide lines and operation standards should be prepared, maintained, and used. Material Safety Data sheets are available from manufacturers and should be included in guidelines, standards and training.

3) Chemistry

(1) Chlorine and Sodium Hydroxide

The addition of chlorine to a solution of sodium hydroxide (NaOH) produces sodium 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 sodium hydroxide will produce 1.05 pounds (kg) of sodium hypochlorite.

The reaction of chlorine and caustic is exothermic, liberating 626 BTU/lb (348 cal/g) of chlorine gas absorbed. If liquid chlorine is directly reacted with the sodium hydroxide, 526 BTU/lb (292 cal/g) of liquid chlorine is produced since the 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) when carried to completion produces sodium hypochlorite (NaOCl), salt (NaCl), and sodium bicarbonate (NaHCO₃):

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

The theoretical quantities if carried to completion are one pound (kg) of chlorine reacts with 2.99 pounds (kg) of sodium carbonate to produce 1.05 pounds (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 calcium hypochlo-rite, calcium chloride and water. The calcium hydroxide may be prepared by mixing hydrated lime (Ca(OH)2) with water or by slaking calcium oxide (quicklime) with water [one pound (kg) of CaO = 1.32 pounds (kg) of Ca(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 is temperature. Significant quantities of heat are released by the caustic-chlorine reaction. The most significant reactions involved in the chlorine scrubbing and the corresponding heats of reaction are as follows:

• Scrubbing Reaction
  2NaOH + Cl₂ = NaOCl + NaCl + H₂O
  DH25 = - 626 BTU/lb (- 348 cal/g) Chlorine Gas
  
• Decomposition Reactions of Sodium Hypochlorite
  NaOCl = NaCl + 1/2 O₂
  DH25 = - 336 BTU/lb (- 187 cal/g) Hypochlorite decomposed
  3NaOCl = NaClO₃ + 2NaCl
  DH25 = - 188 BTU/lb (- 104 cal/g) Hypochlorite decomposed

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

DH25 NaOH = - 112.93 KCal/ g-mole
DH25 NaCL = - 97.234 KCal/g-mole
DH25 H2O = - 68.3174 KCal/g-mole
DH25 NaOCl = - 83.39 KCal/g-mole
DH25 NaOCl3 = -78.92 KCal/g-mole

4) Scrubbing Solutions

(1) Sodium Hydroxide

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

(2) Sodium Carbonate (Soda Ash) Solutions

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

6.5 wt % at 32^oF (0^oC)
10.8 wt % at 50^oF (10^oC)
18.1 wt % at 68^oF (18.9^oC)

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 to use 3-1/3 pounds of sodium carbonate per pound of chlorine. Due to the solubility of sodium carbonate and the above ratios, only low strengths of the resulting sodium hypochlorite are produced. Generally the reaction temperatures in these applications are not a design consideration.

Important!!

Sodium carbonate for chlorine absorption has limited application for scrubbing and 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 to provide further details specific to the use of sodium carbonate for chlorine scrubbing. Many of the guidelines used for caustic scrubbing can be applied to the 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 and quicklime contains 95% CaO. Not including the excess lime required, the following ratios are required:

1.10 pounds (kg) of 95% Ca (OH)₂ per pounds (kg) of Cl₂
0.833 pounds (kg) of 95% CaO per pound (kg) of Cl₂

The amount of either quicklime or hydrated lime is determined by the solubility in the resulting chlorinated solution. Due to the low solubility of the calcium hydroxide solutions, temperatures due to chlorine absorption are low and are not normally a design consideration.

Important!!

Calcium hydroxide for chlorine absorption has limited application for scrubbing and is generally used for specific reasons, i.e. desired final product or availability of the calcium hydroxide. It is not the intent of this pamphlet to provide further details specific to the use of calcium hydroxide for chlorine scrubbing. Many of the guidelines used for caustic scrubbing can be applied to the calcium hydroxide systems but must be analyzed item by item.

5) Additional Reactions

Due to the higher strengths of sodium hypochlorite produced with caustic soda scrubbing, additional reactions occurring during the absorption of chlorine may become 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 pH near the point of chlorine addition to the caustic. When sodium hypochlorite is over chlorinated, the rate of chlorate formation is greatly accelerated. HOCl is formed 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 of reaction [5.1b]. The HCl formed in reaction [5.1e] combines with the hypochlorite ion to form more HOCl so the excess chlorine gas a catalytic effect on chlorate formation. The increased acidity causes reaction [5.1c] to proceed. This is an exothermic reaction which can become violent in scrubbers under these conditions.

CAUTION!

IF EXCESS CHLORINE IS ADDED TO A SCRUBBER, DECOMPOSITION OF SODIUM HYPOCHLORITE TAKES PLACE. REACTION [5.1c] WILL BECOME APPRECIABLE. THIS CONDITION 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 EXCESS CAUSTIC.