Process for the on-site production of chlorine and high strength sodium hypochlorite

A process for electrolytically producing chlorine gas, sodium hydroxide, and sodium hypochlorite, comprising:

• a. contacting solid sodium chloride and softened water in a brine module to form a saturated aqueous solution of sodium chloride (saturated brine);
• said brine module comprising:
• at least one briner tank, in which solid sodium chloride having a low total hardness content and softened water can be contacted to form a saturated aqueous solution of sodium chloride (saturated brine);
• and an outlet for saturated brine which is in fluid communication with the briner tank;
• b. feeding the saturated brine to a brine softener module to form softened brine, wherein the brine is continuously circulated through at least one chelating ion exchange resin bed located in the brine softener module;
• said brine softener module comprising:
• a brine recirculation tank, an inlet for saturated brine, an outlet for softened brine, a sodium hydroxide (NaOH) source, a pH sensor/controller and at least one chelating ion exchange resin bed;
• said brine module being in fluid communication with said brine softener module;
• and said brine recirculation tank, said inlet for saturated brine, said outlet for softened brine, said NaOH source, said pH sensor/controller and said at least one chelating ion exchange resin bed being in fluid communication with each other;
• wherein the saturated brine is circulated through at least one chelating ion exchange resin bed and the brine recirculation tank to form softened brine;
• c. feeding the softened brine to a plurality of anolyte compartments in an electrolyzer module, each electrolyzer module comprising:
• an anolyte tank, a conductivity sensor, a chlorine head tank, an NaOH head tank, an NaOH receiver tank, and a plurality of electrolyzers, wherein:
• (i.) each electrolyzer is a membrane cell electrolyzer, comprising:
• an anolyte compartment, said anolyte compartment capable of oxidizing a softened brine mixture comprising softened brine and depleted brine, and having an inlet for the softened brine mixture and an outlet for chlorine gas/depleted brine;
• a catholyte compartment having an inlet for aqueous NaOH and an outlet for hydrogen gas/NaOH solution;
• and a cation ion exchange membrane which is interposed between said anolyte compartment and said catholyte compartment and which is in fluid communication therewith;
• wherein sodium ions can pass through the cation ion exchange membrane from the anolyte compartment to the catholyte compartment;
• and
• wherein said plurality of electrolyzers, conductivity sensor, and anolyte tank are in fluid communication with each other;
• wherein softened brine is capable of being added to depleted brine to provide the softened brine mixture at a selected concentration, and wherein said selected concentration of the softened brine mixture is maintained by controlling an addition of softened brine to the depleted brine with the conductivity sensor;
• (ii.) the chlorine head tank comprises:
• a chlorine tank header having an inlet for chlorine gas/depleted brine, an outlet for depleted brine, a chlorine gas takeoff and, optionally, a second chlorine gas takeoff;
• said header being in chlorine/depleted brine fluid communication with the plurality of electrolyzers, in depleted brine fluid communication with the anolyte tank and, optionally, in chlorine fluid communication with a second chlorine takeoff;
• in which the chlorine gas formed in the anolyte compartments is separated from the chlorine gas/depleted brine that exits the plurality of electrolyzers to form a chlorine gas stream and a depleted brine stream;
• and
• (iii.) the NaOH head tank for separating hydrogen gas from a hydrogen gas/NaOH solution comprises:
• a NaOH tank header having an inlet for the hydrogen gas/NaOH solution, an air diluted hydrogen gas outlet, a concentrated NaOH solution outlet, an overflow concentrated NaOH solution outlet, and an air inlet comprising a blower,
• said NaOH tank header being in fluid communication with:
• the electrolyzers through the hydrogen gas/NaOH solution inlet and through the first concentrated NaOH solution outlet in which the concentrated NaOH solution outlet fluid communication also contains an inlet for water for diluting said first concentrated NaOH solution,
• the NaOH receiver tank through the overflow concentrated NaOH solution outlet, and
• the air inlet;
• in which air disengages hydrogen gas from the hydrogen gas/NaOH solution to form a hydrogen/air stream, a concentrated NaOH solution stream and an overflow concentrated NaOH solution stream;
• said hydrogen gas in said hydrogen/air stream being diluted to below its lower explosive limit (LEL) in air;
• d. feeding an aqueous solution of NaOH to the catholyte compartments of the electrolyzers;
• e. applying a current to the electrolyzers sufficient to produce a chlorine gas/brine mixture in the anolyte compartments and a hydrogen gas/NaOH solution mixture in the catholyte compartments;
• f. drawing off the chlorine gas from the chlorine gas/brine mixture to form a chlorine gas stream and a depleted brine stream;
• g. drawing off the hydrogen gas from the hydrogen gas/NaOH solution with air to form a hydrogen/air stream and a concentrated NaOH solution stream;
• and,
• h. contacting the chlorine gas stream or portion thereof from step f with an NaOH solution in a sodium hypochlorite conversion module to directly form a 2-15% trade NaOCl solution;
• wherein from 1-100% of the chlorine gas generated in step f is contacted with the NaOH solution in step h to form the trade NaOCl solution;
• said sodium hypochlorite conversion module comprising:
• a conversion tank, an ORP sensor, an NaOH solution inlet, a chlorine gas inlet, a trade NaOCl solution outlet, and a water inlet,
• said conversion tank comprising:
• a tank for generating the NaOCl solution and containing therein a solution comprising NaOH, and
• a circulation loop for circulating at least a portion of the conversion tank mixture, said circulation loop comprising first, second, and third sub-loops, wherein:
• said first sub-loop is in fluid communication with an eductor for drawing chlorine from the chlorine head tank into the conversion tank mixture circulating through the eductor, wherein said eductor is in fluid communication with the chlorine head tank;
• said second sub-loop is in fluid communication with a chlorine absorption tower;
• and
• said third sub-loop is in fluid communication with a heat exchanger for cooling the conversion tank mixture passing through said heat exchanger;
• said NaOCl conversion module being in chlorine fluid communication with the chlorine tank header;
• said conversion tank being in fluid communication with the NaOH receiver tank, the chlorine head tank, and a water source;
• said conversion tank, said ORP sensor, said NaOH solution inlet, said trade NaOCl solution outlet, and said water inlet being in fluid communication with each other, and
• said chlorine gas inlet being in fluid communication with said NaOCl conversion module,
• in which the chlorine gas can be contacted with a NaOH solution in the conversion tank to form a 2-15% trade NaOCl solution, the ORP sensor is capable of maintaining a residual NaOH concentration in the hypochlorite solution, and the water inlet allows for an adjustment of the concentration of NaOCl solution formed;
• and
• wherein from 1-100% of the chlorine gas generated in the electrochemical generating system can be converted to trade NaOCl solution.