melting sodium chloride vs electrolysis of brine
melting sodium chloride vs electrolysis of brine
(OP)
Hi guys i really want to know why melting sodium chloride (downs cells process) is not selected over brine electrolysis for chlor-alkin production.
to make my confusion more clear let me make qualitative energy consumption for both operation
1. downs cell (melting salt):- in this operation slat is heated and melted (the first energy consumption) then current pass through it (the second huge electric energy consumption) then we will have liquid sodium and chlorine gas
2. brine electrolysis:- first salt preparation (i think difficult step) then brine electrolyzed (the first energy consumption) then caustic soda , chlorine and hydrogen produced. finally caustic soda is evaporated (another huge energy consumption).
in the first case we have sodium metal so we can react it with water with the correct proportion to get sodium hydroxide (solid perhaps) and we have the hydrogen
in my view brine electrolysis seems more challenging so what do you guys think?
to make my confusion more clear let me make qualitative energy consumption for both operation
1. downs cell (melting salt):- in this operation slat is heated and melted (the first energy consumption) then current pass through it (the second huge electric energy consumption) then we will have liquid sodium and chlorine gas
2. brine electrolysis:- first salt preparation (i think difficult step) then brine electrolyzed (the first energy consumption) then caustic soda , chlorine and hydrogen produced. finally caustic soda is evaporated (another huge energy consumption).
in the first case we have sodium metal so we can react it with water with the correct proportion to get sodium hydroxide (solid perhaps) and we have the hydrogen
in my view brine electrolysis seems more challenging so what do you guys think?





RE: melting sodium chloride vs electrolysis of brine
I suggest you look at the half cell potentials:
Compare sodium reduction:
Na+ + e -> Na and Cl- -> 1/2 Cl2 + e
...versus what is happening in a chlor-alkali cell:
H2O + 2 e - > H2 + 2 OH- and Cl2-> 2Cl- + 2 e
The same number of electrons must flow per mole of Cl2 and NaOH produced- but these are NOT at the same VOLTAGE by any stretch of the imagination! Power = voltage x current...
In one case you're fusing NaCl with a flux (at high temperature), electrochemically reducing sodium, then re-oxidizing it to sodium ion by reaction with water to produce hydrogen and a tremendous amount of heat that the process no doubt just wastes. In the other, you're leaving the sodium ions alone- you are not reducing them at all.
In the old days, electrolysis was done in mercury cells where a sodium-mercury amalgam was produced, then reacted with a smaller quantity of water to generate the hydroxide and hydrogen. I suspect that process was superceded by the current membrane electrolysis process not just because of mercury toxicity- I suspect it was also a matter of efficiency improvement. Heat, especially low-grade heat to run a water evaporator, is usually greatly cheaper than electricity for simple thermodynamic reasons.
RE: melting sodium chloride vs electrolysis of brine
A couple comments regarding brine electrolysis: brine preparation is actually quite simple, and doesn't require that much energy. It's simply precipitation with soda ash and caustic followed by filtration, ion exchange, reverse osmosis, and sometimes evaporation. All evaporation done in a chlor-alkali plant (NaCl and NaOH) is done by utilizing recycled heat from the exothermic electrolysis process.
RE: melting sodium chloride vs electrolysis of brine
RE: melting sodium chloride vs electrolysis of brine
Unless you want the hydrogen or the sodium for another process there would be no reason to go down that path. You could use the hydrogen to generate power but all your doing is inefficiently recovering the electrical energy you put in during the molten salt electrolysis phase.
Also operating at 500degC (Downs cell) vs <100degC (brine electrolysis) makes for cheaper construction, easier on materials and maintenance etc.
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
RE: melting sodium chloride vs electrolysis of brine
as you told me i understand operating at lower temperature is easier in many aspects. but overall simplicity of technology downs cell seems simple do not you agree?
RE: melting sodium chloride vs electrolysis of brine
There's nothing more complex about the membrane that isn't made trivial in comparison to handling liquid sodium.
RE: melting sodium chloride vs electrolysis of brine
RE: melting sodium chloride vs electrolysis of brine
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
RE: melting sodium chloride vs electrolysis of brine
RE: melting sodium chloride vs electrolysis of brine
RE: melting sodium chloride vs electrolysis of brine
RE: melting sodium chloride vs electrolysis of brine
Strong caustic, excess salt, near boiling temp, there are only two materials that work in this environment.
commercially pure Ni (Ni200), but it wears out due to abrasion from the salt, and E-Brite (26-1).
The E-Brite is a very special grade of stainless (this is one of its few uses) and it lasts for decades in this service.
There is no way to do this on a small scale that is safe even by developing country standards.
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P.E. Metallurgy, Plymouth Tube
RE: melting sodium chloride vs electrolysis of brine
RE: melting sodium chloride vs electrolysis of brine