melting sodium chloride vs electrolysis of brine 1

[tinsnano]

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?

 

[moltenmetal]

This is not my area of expertise, so someone from the chlor-alkali industry can of course jump in here and correct my misapprehension.

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 I suspect that process was superceded by the current membrane electrolysis process not just because of mercury toxicity- I suspect it was also a matter of efficiency improvement.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.

 

[chemEcaleb]

I work at a sodium chlorate manufacturing facility, which is similar to the chlor-alkali process. I would guess manufacturers prefer the brine electrolysis because the downs cell requires more energy due to the melting of the salt, and it's also inherently more dangerous because of the higher temperatures involved.

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.

 

[tinsnano]

Thanks guys. but do you think that downs cell process seems simpler except its high heat area and large energy consumption. when i say simpler. i mean it does not require membrane (which is expensive and prone to be clogged), does not need evaporator and perhaps no salt purification required (if i am right)

 

[itdepends]

Simpler does not = cheaper
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"

 

[tinsnano]

i am thinking as building small scale project in poor country of mine where sophisticated (in case of brine electrolysis) technology might be expensive than fuel(like wood) energy. the cottage project could be melting down salt (with cheap wood fuel) in downs cell and extracting sodium and chlorine, the sodium will be reacted with appropriate quantity of water to give soda and hydrogen then the hydrogen will be reacted with chlorine to form HCl (dissolving this hydrochloric acid).

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?

 

[moltenmetal]

Availability of electricity, not heat, is your biggest worry in a poorly developed location. One cell uasea a lot mire electrical energy than the other.

There's nothing more complex about the membrane that isn't made trivial in comparison to handling liquid sodium.

 

[tinsnano]

Do you think the purification of salt for the membrane to operate perfectly is easy as cottage industry in poor country also what do you think the price of membrane will it be fair to begin as cottage industry

 

[itdepends]

Handling Liquid sodium, hydrogen and chlorine gas in a cottage industry- what could go wrong?

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[tinsnano]

Guys thank you very much! for your great cooperation discussion and solution

 

[tinsnano]

but i wonder if you guys tell me how in very early time chlor-alkin cottage/small scale industries work

 

[moltenmetal]

With respect tinsnano, you were already told. In the old days, they used the mercury cell instead of the membrane cell. This cell is easy to use but the use of mercury makes its continued use no longer acceptable.

 

[EdStainless]

And then the evaporators on the back end of the process (which you didn't have in the old days with Hg).
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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[chemEcaleb]

Just saying, your idea is the worst. Don't do it... I work in the industry, and even in a modern plant with great, explosions happen, people get exposed to chlorine gas, and people get chemical burns. What you're planning on doing sounds like cooking "batches" in the basement of your "cottage"- you're going to get yourself of someone else killed.

 

[tinsnano]

chemEcaleb what do think the arrangement could be for cottage