1) General

The scrubbing system design must follow good engineering practice and adhere to state and local regulations and company policy. Permitting is generally required. Specific performance criteria are beyond the scope of this pamphlet. Sections 2) through 5) contain process design considerations specific to chlorine that 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 be scrubbed is defined. The composition must be predicted fairly accurately. A stream containing a high concentration of chlorine can be neutralized readily using a large volume of caustic in a relatively small contact area system. When the stream to be scrubbed contains more than 20 to 30% by volume inert, care must be taken to assure adequate contact in order to remove the last traces of chlorine from the gas stream.

Generally, caustic at 20% or less concentration is used for scrubbing purposes. The freezing temperature of a 20% solution is -16.6^oF (-27^oC). 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 peak reaction temperatures and crystal salt precipitation with attendant pluggage potential. It must be realized that if a batch scrubber is designed properly, the starting solution can exceed 20% by weight, if the solution is not chlorinated to the end point. Each application will need careful review to ensure salt precipitation can not occur.

Potentially reactive or hazardous components must be defined and considered in the scrubbing system design. For example, in chlorine producing plants, it is sometimes necessary to neutralize a stream made up of chlorine, air, and hydrogen. When the chlorine in this stream is neutralized, the hydrogen concentration may increase through the reactive/explosive ranges. (See Pamphlet MIR-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 design process. If liquid chlorine is fed to a system designed to process gas, a violent and uncontrolled reaction will result. This can lead to a chlorine release.

In a batch reaction system, the duration and concentration of the vent stream flow 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 reacting chlorine at an instantaneous rate equivalent to 100 tons (90.7 metric tons) per day. On a hourly basis this is equivalent to 8,333 lbs/hr (3,720 kg/hr). The "no decomposition " line assumes that all chlorine reacts to sodium hypochlorite. The "decomposition" line assumes that 25% of the sodium hypochlorite produced decomposes to oxygen and salt.

Caution!

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

When neutralizing water saturated chlorine with stoichiometric quantities of 15 to 20% caustic, the heat generated can bring the solution to the boiling temperature. The water vapor generated by the boiling solution dilutes the chlorine and reduces the mass transfer efficiency of the scrubber. Thus, it is desirable to maintain the solution temperature well below the boiling temperature. The transfer of heat from the solution to an external cooling system can be the obvious choice if capacity is available. If external cooling is not available, temperature control can also be accomplished by reducing the initial caustic concentration or scrubbing with excesses of caustic. Graphs 6.3A and 6.3B show the effect on scrubber liquor temperature rise when initial caustic strength varies from 5 to 20% and when one to four times the stoichiometric quantity of caustic is used for neutralization. Note that the following graphs illustrate chlorine saturated 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. Temperature increases are approximates. A rigorous thermal analysis is required for each scrubber 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 be considered.

• In order to maintain scrubber capacity to react chlorine, there must always be some excess of caustic. In emergency scrubber applications where flows and concentrations cannot be guaranteed, sufficient excess caustic should be made available. For in-process scrubbers where flows are known only minimal excess caustic is necessary.
• In many applications, it is desirable and technically feasible to deplete the scrubbing liquor to as low as 10 grams per liter of NaOH. When low concentrations of caustic are used, several items should be considered. As pH drop below 10, conditions become favorable for the formation of sodium chlorate. Under basic conditions sodium chlorate is quite stable and will contaminate the effluent stream.
• Total depletion of caustic is to be avoided. Accidental depletion will negate the reaction 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 columns shall have sufficient caustic soda solution flowing to always exceed the 1.128 caustic to chlorine ratio. At the end of the batch scrubbing cycle when the caustic concentration has been reduced to low levels, care must be taken to assure adequate mass transfer.
• When strong caustic solutions are chlorinated to the end point, the salt concentration can be high enough to become saturated in the resulting solution 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, care must be taken of the temperature at which the solutions separate solid hydrates. These "freezing curves" are available in literature published by producers 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 of caustic. The freezing point of this solution is -18.4^oF (-28^oC). 50% caustic soda, the common commercially available strength, freezes at 54^oF (12.2^oC)

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 design criteria. 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 to prevent same, e.g. provisions for dilution with air.
• Installation of a system to prevent hypochlorite of caustic solutions from flowing back into the chlorine lines and corroding piping and valves, such as a barometric loop.
• Caustic and water when mixed will have less volume than the sum of the two streams. However, the resultant solution when chlorinated to low excess caustic levels will expand. This expansion at high solution strengths of beginning caustic can result in an approximate 10% increase in volume more than the total sum of caustic and water volume. Always design the scrubbing system for the theoretical maximum volume.
• Batch scrubber system, when used for room scrubbing or many types do process scrubbing, will cause the caustic to react with any CO₂, present to produce sodium carbonate. Each application must be reviewed to ensure the caustic depletion and carbonate/bicarbonate precipitation during operation are not problems.
• Materials of construction should be consistent with the process under both design and upset conditions, e.g. if titanium, which is excellent in wet chlorine, is allowed to contact dry chlorine, spontaneous combustion will result.