Caustic Soda (NaOH)

KALF development the process of using electrolysis to develop sodium hydroxide, has been adapted in all chlor-alkali industry.

Chlor-Alkali Manufacturing Processes By KALF

 

 

 

 

 

 

 

 

 

 

 

 

Electrochemical & chemical reactions occurring in diaphragm & membrane cells

 

      • 2Cl ==> Cl2 + 2e(Anodic reaction)
      • 2H2O + 2e ==> 2OH + H2 (Cathodic reaction)
      • 2Cl + 2H2O ==> Cl2 + H2 + 2OH (Overall ionic reaction)
      • 2NaCl + 2H2O ==> Cl2 + 2NaOH + H2 (Overall reaction)
      • Cl2 + 2NaOH ==> NaOCl + NaCl + H2O (Side reaction)
      • 3NaOCl ==> NaClO3 + 2NaCl (Side reaction)
        1. Reaction [9] will contaminate the caustic product with chlorate.

 Chemical reactions occurring in brine processing

      • CaSO4 + Na2CO3 ==> CaCO3 + NaSO4 (CaCO3 precipitates)
      • MgCl2 + 2NaOH ==> Mg(OH)2 + 2NaCl (Mg(OH)2 precipitates)

Sodium Hypochlorite/Chlorate Manufacturing Process

Electrochemical and chemical reactions occurring in cells

      • 2Cl ==> Cl2 + 2e(Anodic reaction)
      • 2H2O + 2e ==> 2OH + H2 (Cathodic reaction)
      • Cl2 + 2OH ==> OCl + Cl + H2O (Hypochlorite formation)
      • 3OCl ==> ClO3 + 2Cl (Chlorate formation)
      • NaCl + H2O ==> NaOCl + H2 (Overall hypochlorite reaction)
      • NaCl + 3H2O ==> NaClO3 + 3H2 (Overall chlorate reaction)
      • 3Cl2 + 6NaOH ==> NaClO3 + 5NaCl + 3H2O (Chemical chlorate formation)

    Hypochlorite formation is promoted by the use of weak brine, basic solution, and low cell temperatures. Chlorate formation is promoted by the use of saturated brine, acidic solution, and temperatures close to the boiling point of the solution.

 

 

 

 

Kalf is able to provide the engineering solution to the client with a highly efficient design which had the optimized process and maximum throughput. With this optimized solution, it provides a huge amount of saving in the energy consumptions which in-term having much lower operating cost.

The energy consumption in chlor-alkali production originates from four main processes:

  • Energy to  prepare  and  purify  the  raw  materials,  mainly  the  salt  (sodium  chloride or potassium chloride),
  • Electrical energy used for the electrolysis process itself,
  • Energy (steam) to obtain the caustic soda (or potash) at its commercial concentration,
  • Energy for auxiliary equipment such as heating devices, pumps, compressors,transformers, rectifiers and lighting.

Energy consumption depends on a number of factors such as:

  • The cell technique used,
  • The purity of the salt used as raw material,
  • The specific cell parameters, such as nominal current density, anode/cathode gap, adherence of developed gas bubbles on electrode structures, diaphragm/membrane type and thickness, catalytic electrode coatings,
  • The age of the diaphragm, the membrane and the catalytic electrode coatings,
  • Other technical characteristics of the installation such as the configuration of the electrolysers (monopolar or bipolar), the number of evaporative stages in the caustic concentration unit and the chlorine liquefaction conditions,
  • The actual current density.

Energy consumption for the electrolysis:

The operation of a chlor-alkali plant is dependent on the availability of huge quantities of direct current (DC) electric power, which is usually obtained from a high voltage source of alternating current (AC). The lower voltage required for an electrolyser circuit is produced by a series of step down transformers. Silicon diode or thyristor rectifiers convert the alternating current electricity to direct current for electrolysis. Direct current is distributed to the individual cells of the electrolysers via busbars. There are energy losses across the transformer, the rectification equipment and the busbars. In 2010, the efficiency of rectifier and transformer units varied from  approximately  94 %  (older units) to 98%. To remove the dissipated heat, the units are cooled by circulated air or by special water Circuits.Connections between cells/electrolysers, along with the corresponding energy losses, have to be considered for the measurement of the total energy requirement per tonne of chlorine produced. The  definition  of  the  exact  measurement  point is  necessary  for an  appropriate comparison  of energy consumption figures of different plants.For the usual operating conditions, the specific electricity consumption w (in kWh/t Cl2 produced), which is the electricity consumed divided by the production rate, is proportional to the cell current density j (in kA/m2).

Energy consumption of membrane cells:

The electrical energy consumption of membrane cells ranges from approximately 2300 to3000 AC kWh/t Cl2 produced, the median being approximately 2600 AC kWh/t Cl2 roduced,with current densities ranging from 1.0 to 6.5 kA/m2. All electrolysers are equipped with titanium anodes coated with a catalyst, and the nickel cathodes are usually activated with a catalyst to improve the terms U0 and K, and so consequently reduce the energy consumption. For non-activated cathodes, U0 is approximately the same for all units  and is similar to the diaphragm electrolysers; it has a lower value if the cathode is activated, depending also on the type of catalyst used. Compared to the diaphragm cells, the factor K of membrane cells has a lower value due to a thinner separator (the membrane), a shorter distance between anode and cathode, and due to a lower electric resistance in the electrolyser structure (0.1–0.3 V·m2/kA).

The operating conditions and electrical energy consumptions of monopolar and bipolar electrolysers are different. For monopolar membrane cells, the electrical energ consumption ranges from approximately 2700 to 3000 AC kWh/t Cl2 produced, the median being approximately 2800 AC kWh/t Cl2 produced, with current densities ranging from1.0 to 4.0 kA/m2. For bipolar membrane cells, the electrical energy consumption ranges from approximately 2300 to 2900 AC kWh/t Cl2 produced, the median being approximately 2500 AC kWh/t Cl2 produced, with current densities ranging from 1.4 to 6.5 kA/m2. Within both techniques, there are differences in the design distance of the cathode to the membrane. These differences vary from 0 to almost 2 mm. This distance significantly affects the energy consumption (the shorter the distance the lower the energy requirement), as well as the operational requirements, such as brine purity, and the risk of membrane damage The electrical energy consumption of a membrane cell rises with the lifetime of the membranes and the electrodes (coating ageing), by approximately 3–4% during a period of three years.

Comparison of cell technologies

Mercury Diaphragm Membrane
Operating current density (kA/m2) 8 – 13 0.9 – 2.6 3 – 6
Cell voltage (V) 3.9 – 4.2 2.9 – 3.5 3.0 – 3.6
NaOH strength (wt%) 50 12 33 – 35
Energy consumption (kWh/MT Cl2) at a current density of (kA/m2) 3360 (10) 2720 (1.7) 2650 (5)
Steam consumption (kWh/MT Cl2) for concentration to 50% NaOH 0 610 180