PETROCHEMISTRY: CHLORINE-ALKALI ELECTROLYSIS

PETROCHEMISTRY 

 CHLORINE-ALKALI ELECTROLYSIS

The chlorine-alkali electrolysis is an important procedure in the chemical industry. The products chlorine, hydrogen, hydrochloric acid and caustic soda are generated from sodium chloride. There are three manufacturing methods: the diaphragm, the membrane and the amalgam process.

The LiquiSonic® analyzer provides an advantageous utilization in the varied process steps of the three methods. Foremost, the customer gains the advantages of a reduction in raw material and energy consumption, as well as an increase in the yield.

 

LiquiSonic® measuring points in the processes of the diaphragm method of the chlorine-alkali electrolysis

 

Preparation of end products

1.1  Caustic soda concentration

The market ready caustic soda (NaOH) typically has a concentration between 45 wt% and 50 wt%. Since NaOH gained from electrolyzer cells only yields a concentration range between 12 wt% and 33 wt%, it is concentrated in cascade evaporators.

If together with NaOH the solution contains NaCl (diaphragm method), the excess salt in the caustic soda precipitates in a crystal form during the evaporation. This way, the NaOH-concentration is elevated to 45 wt% and 50 wt%.

The LiquiSonic® analyzer continuously determines the concentration of caustic soda at any time after evaporation. A subsequent dilution of the caustic soda to a customer specific product concentration, can also be monitored.

ADVANTAGES:

  • Continuous concentration monitoring of the caustic soda
  • Reduction in the energy costs during the evaporation process

1.2 Chlorine gas drying

Water content needs to be removed from the chlorine gas before it is further processed as its corrosives elevate at a moisture content of over 30 ppm. For the drying, the chlorine gas is routed into the absorption towers, where the water content in the chlorine gas is absorbed with highly concentrated sulfuric acid (80 – 99 wt% H2SO4).

The effectiveness of this drying process significantly influences the productivity and quality of the gas. Therefore, a reliable measurement of the H2SO4 concentration is vital. The LiquiSonic® analyzer offers a continuous and safe monitoring of the H2SO4-concentration, in opposition to conductivity and density measurement.

ADVANTAGES:

  • Elimination of labour intensive sampling
  • Continuous monitoring of H2SO4-concentration
  • Clear concentration determination signal between 80 wt% and 100 wt% H2SO4
  • Ensuring the desired Cl2 dryness to avoid corrosion in the system

1.3  Hydrochloric acid production

The chlorine gas that is generated on the anode of the electrolyzer with the added hydrogen form the base materials for the synthesis of hydrochloric acid. Both gases are fed into a burner and react to form hydrogen chloride. Subsequently, the formed HCI-gas streams from the burning chamber into the integrated isothermic falling-film-absorber. Here, the gas is absorbed with the help of water or diluted acid, whereby concentrated hydrochloric acid (37 wt% HCI) is formed.

Using the LiquiSonic® analyzer, a continuous monitoring of the hydrochloric acid concentration is possible. This allows for deviations to be recognized from the target concentration and to react accordingly.

ADVANTAGES:

  • Continuous concentration monitoring of hydrochloric acid (20-40 wt% HCl)
  • Ensuring an extremely precise target concentration

Advantage of sonic velocity as compared to conductivity and density

 

 

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