SODIUM HYPOCHLORITE
Process Description:
The absorption of chlorine takes place in a two stage jet scrubbing system and a final absorber arranged in series. Chlorine and caustic soda are fed in counter-current. The suction necessary to absorb chlorine bearing waste air from every part of the plant is generated by the two stage jet system. As driving medium for the ejectors serves a 18% NaOH hypochlorite solution.
Heat generated during absorption and recirculation process is removed with chilled water in a cooler.
The first stage jet system consists of the Jet Scrubber 1, together with Suction Tank 1 and Bleaching Lye Circulation Pumps A/B, and Bleaching Lye Cooler 1.
Bleaching Lye Circulation Pump B serves as stand-by pump for both stages 1st and 2nd.
The second suction stage consists mainly of Jet Scrubber 2, Suction Tank 2 and Bleaching Lye Circulation Pump C.
The final absorber A is a packed column, where final air scrubbing takes place with 18% NaOH solution fed on the top of the column.
The 18% caustic soda necessary for absorption is produced discontinuously mixing 33% NaOH coming from Catholyte Tank with process water in the corresponding proportions. Dilution of the caustic soda takes place in the 18% Caustic Soda Mixing Jet. The diluted 18% caustic soda is passed through the 18% Caustic Soda Cooler and cooled by means of chilled water to less than 30ºC and stored in 18% Caustic Soda Tank.
18% Caustic Soda is continuously fed to the circulation loop of the Suction Tank 2 through flow meters 1/2. Flow rate for low rates is monitored through 1st and high flow rates through 2nd flow meter. The necessary amount of caustic is computed from the amount of free caustic in hypo from 1st stage which should be maintained in a range of 30-50 g NaOH/l. In emergency cases higher quantities of chlorine can thus be absorbed in the 1st stage. Caustic can be added to the 1st stage hypo loop on the suction side of Bleaching Lye Circulation Pump A/B by opening control valve. This measure may be necessary in the event higher quantities of chlorine are to be absorbed.
Hypochlorite solution overflows from the 2nd stage to the 1st stage and from there to the Bleaching Lye Storage Tank A.
The hypo from the 2nd stage is circulated through Jet Scrubber 2 maintaining the necessary suction (about 10 mbar) to absorb the waste gas from the first stage. The waste gas is scrubbed in the jet and chlorine absorbed to high extent. The waste air from this stage is sent through the final absorber A to absorb the residual chlorine with caustic to a final content of max. 5 ppm. The pressure drop through the absorber should be approx. +5 mbar.
The caustic necessary for absorption is taken from the hypo loop of the 2nd stage. The flow rate is monitored through flow meter. The process flow rate value shall be 6-7 m3/hr.
In the 1st stage, the hypochlorite solution is circulated through the Bleaching Lye Cooler 1, to remove the reaction heat. The hypo is cooled to less than 30ºC by means of chilled water; the temperature is monitored and adjusted manually. When cooling water at low temperature is available, chilled water supply and return valves can be closed and cooling water supply be opened.
The hypo of 2nd stage loop can be cooled by the same cooler. For this the 1st stage loop can be by-passed from the cooler.
Waste gas is absorbed by Jet Scrubber 1, at a suction of - 25 mbarg. The suction pressure is controlled by pressure controller which controls the flap valve on the gas by-pass line between 2nd stage and 1st stage. Thus constant suction at different absorption loads can be maintained in the waste gas absorption system.
The system is secured against low vacuum by the Low Pressure Safety Vessel A. At pressure less than – 30 mbarg, air will be sucked into the waste gas system passing through liquid seal of the vessel. Hypo liquor is continuously fed to the vessel from the 2nd stage loop and returns to the same by gravity overflow. Overflow can be monitored on the sight glass installed on the overflow line.
In emergency cases, the 1st stage absorption system can be by-passed and taken out of line in the event maintenance is necessary. For this all necessary piping connections to by-pass the 1st stage are provided.
A small part stream from the hypo solution sent through the Jet Scrubber 1 is diverted to the redox analyser and returned to the Suction Tank 1 by means of Bleaching Lye Sample Ejector A.
The hypo solution overflows from the 1st Stage Tank into the Bleaching Lye Storage Tank A. To avoid decomposition of hypo due to high ambient temperatures, the liquor is cooled by circulation through the Bleaching Lye Cooler 2. Cooling is effectuated by means of chilled water, or with cooling water in case low cooling water temperature is available. Bleaching lye should be maintained at temperature below 30ºC and can be monitored.
