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Contenu archivé le 2019-03-15

ENERGY CONSERVATION IN THE CHLOR-ALKALI INDUSTRY

Objectif



THE THEORETICAL CELL VOLTAGE FOR ELECTROLYSIS OF BRINE (NACL IN WATER) IS 2.1 V. HOWEVER, DUE TO LOSSES AND POLARIZATION AT THE ELECTRODE, THE PRACTICAL OPERATING POTENTIAL LIES AROUND 3 V. THE VERY LARGE CHLOR-ALKALI ELECTROLYSIS INDUSTRY IS BY ITS NATURE HIGHLY ENERGY-INTENSIVE. A SMALL REDUCTION IN CELL VOLTAGE OF E.G. 150 MV ON 3 V AT A GIVEN CURRENT DENSITY REPRESENTS A SAVING IN ELECTRIC ENERGY OF 5 %.

THE VOLTAGE OF THE CELL IS DETERMINED BY THE OVERVOLTAGES AT BOTH ELECTRODES WITH THE MAIN CONTRIBUTION COMING FROM THE CATHODIC OVERVOLTAGE REQUIRED FOR THE HYDROGEN EVOLUTION (PRESENCE OF HYDROGEN BUBBLES AT THE CATHODE WHICH PREVENT EFFICIENT CONTACT BETWEEN THE IONS IN THE SOLUTION AND THE CATHODE) AND BY THE OHMIC RESISTANCE OF THE ELECTROLYTE (AND DIAPHRAGM OR MEMBRANE). APPLYING A CATALYST TO THE SURFACE OF THE CATHODE REDUCES THE OVERVOLTAGE. HOWEVER, IT IS NECESSARY THAT THE CATALYTIC ACTIVITY IS STABLE FOR A LONG PERIOD UNDER THE PREVAILING CELL CONDITIONS.

TWO TYPES OF LOW OVERVOLTAGE COATING BASED ON NI-CO-S FOR THE H2 CATHODE HAVE BEEN DEVELOPED AND TESTED IN A CHLOR-ALKALI ENVIRONMENT. THE ELECTRODEPOSITED AND TEFLON BONDED COATINGS GAVE THE RESULTS :
- 126 MV AND - 68 MV VS THE DYNAMIC HYDROGEN ELECTRODE (D.H.E.) AT 1 A/CMEXP2, ELECTROLYTE 5 M NAOH, 80 DEGREES C, COMPARED TO OVER - 350 MV FOR STANDARD STEEL CATHODES.

THEREFORE, NI-CO-S ELECTRODES, BOTH ELECTROPLATED AND BONDED, CAN PRODUCE VERY ACTIVE HYDROGEN EVOLUTION CATHODES. THE TESTS HAVE, HOWEVER, SHOWN THAT THE ELECTRODES SHOULD NOT BE LEFT AT OPEN CIRCUIT WHEN THEY ARE USED IN ALKALI SOLUTION. CO IS CORRODED ON OPEN CIRCUIT SINCE IT BECOMES ANODIC RELATIVE TO NICKEL OR IRON. THE BEST WAY TO PREVENT THIS IS TO ENSURE THAT THE CATHODES ARE ALWAYS PRACTICAL IN AN INDUSTRIAL ENVIRONMENT.

ANOTHER PROMISING CATALYST, NI-MO-S, WHICH HAD BEEN SHOWN TO BE VERY ACTIVE (- 100 MV VS DHE AT 1 A/CM2 IN 5 M KOH AT 70 DEGREES C) AND CLAIMED TO BE IMMUNE TO LOSS OF PERFORMANCE AFTER OPEN CIRCUIT, WAS INVESTIGATED. HOWEVER, IN THE EXPERIMENTS CARRIED OUT THIS ELECTRODE WAS ALSO OBSERVED TO LOSE ACTIVITY.

THE PRESENT STUDY INDICATED THAT THE ADDITION OF LESS THAN 0.1 M NA2S TO THE NAOH ELECTROLYTE DURING OPEN CIRCUIT REGAINED THE ORIGINAL PERFORMANCE OF THE OXIDISED NIS ELECTRODE. THE PERFORMANCE WAS IMPROVED BECAUSE IT AIDED THE REDUCTION OF THE OXIDISED NIS ELECTRODE.

NI-S CATHODES ARE BEING USED BY NORSK HYDRO IN THEIR WATER ELECTROLYSERS.

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Coordinateur

City University
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Adresse
Northampton Square
EC1V 0HB London
Royaume-Uni

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