Answer to Question #140786 in General Chemistry for Sam

Question

Discuss the most suitable method for manufacturing NaOH.
Include a reference to each of the following:
The relative cost of making each cell.
Any environmental impact that each process may have.
Any maintenance costs involved.
The purity of the NaOH produced.
The relative cost of electricity for each process.

Answer 1

The Choroalkali process is the most suitable method used in the manufacture of NaOH.

The Choroalkali process is an industrial process usually conducted on brine (an aqueous solution of NaCl), in which case NaOH, hydrogen, and chlorine result.

This process has a high energy consumption, for example around 2500 kWh of electricity per tonne of sodium hydroxide produced. Because the process yields equivalent amounts of chlorine and sodium hydroxide (two moles of sodium hydroxide per mole of chlorine), it is necessary to find a use for these products in the same proportion. For every mole of chlorine produced, one mole of hydrogen is produced. Much of this hydrogen is used to produce HCl, Ammonia, Hydrogen peroxide or is burned for power and/or steam production.

There are three production systems used in this process.

1. The Mercury cell method; (also known as the also known as the Castner Kellner process). In this method chlorine-free sodium hydroxide is produced, but the use of several tonnes of mercury leads to serious environmental problems.
2. The Membrane cell method; This is the most common chloralkali process and it involves the electrolysis of brine in a membrane cell such as such as one made from Flemion or Aciplex, which prevents the reaction between the chlorine and hydroxide ions.
3. The Diaphragm cell process; In this cell process, there are two compartments separated by a permeable diaphragm, often made of asbestos fibres.

References:

-  R. Norris Shreve; Joseph Brink(1977). Chemical Process Industries (4th ed.). p. 219. ASIN B000OFVCCG

https://www.tandfonline.com/doi/abs/10.1080/15275922.2016.1177755?scroll=top&needAccess=true&journalCode=uenf20

https://en.m.wikipedia.org/wiki/Castner%E2%80%93Kellner_process