PROCESSES FOR THE RECOVERY OF CHLORINE FROM HYDROCHLORIC ACID
1. Catalytic oxidation processes
The catalytic oxidation processes are based on the Deacon reaction:
4 HCl + O2 catalyst -> 2 Cl2 + 2 H2O
Some examples of such processes are Shell-Chlor, Kel-Chlor, MT-Chlor, Carrier Catalyst Process.
Up to now the above processes have not encountered the favour of the market and they do not seem to represent a viable solution.
2. Electrochemical processes
2.1 Diaphragm based electrolysis
Some industrial plants exist in the world (Germany, ex Soviet Union, China) that recover chlorine by means of the electrolysis of hydrochloric acid with diaphragm electrolysers.
Each one of the elementary cells which compose an electrolyser is constituted by an anodic compartment made of impregnated graphite, a separation diaphragm made of PVC or PVDF and a cathodic compartment which is made of impregnated graphite too. With this technology also the electrodes – anode and cathode - are made of graphite. Aqueous HCl is fed to the anodic chamber at approx. 21% concentration. At the anode the oxidation of the chloride ions takes place and chlorine is formed. At the outlet of the chamber we have the depleted acid (at approx. 17% concentration) and the produced chlorine. H+ ions migrate through the diaphragm towards the cathodic chamber. At the cathode the reduction of the hydrogen ions occurs and hydrogen is formed. The hydrochloric acid solution is fed to the cathodic chamber too. This process is known as Uhde-Bayer process and in the past it was commercialised by Uhde and by De Nora.
The average power consumption that it is possible to achieve with the diaphragm technology is about 1700 kWh/t Cl2.
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Figure 1
CONVENTIONAL HYDROCHLORIC ACID ELECTROLYSIS
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Figure 2
ELECTROLYSIS WITH OXYGEN DEPOLARIZED CATHODE
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2.2 GDE based electrolysis
Now, let us analyse the operation of an elementary cell where the GDE technology is applied and that adopts an Oxygen Depolarised Cathode (ODC).
The anodic reaction in a cell equipped with ODC is the same as the one that takes place in a diaphragm cell: hydrochloric acid is fed to the anodic compartment, the chloride ions are oxidised at the anode while the hydrogen ions pass through the ion exchange membrane and migrate to the cathodic chamber.
On the other side, the cathodic reactions that occur in a diaphragm cell and in an ODC cell are very different. With ODC, oxygen is fed to the cathode (instead of hydrochloric acid) and it is made to react with the H+ ions that migrate through the membrane, thus producing water. At the outlet we have the excess oxygen and the so-called acidic drain constituted by the reaction water, the water that migrates trough the membrane together with the H+ ions and a small amount of acid.
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2.3 Thermodynamic comparison between diaphragm and ODC technologies
The different operating principles of electrolysers equipped with ODC and with diaphragm have been described in the previous two paragraphs.
Now we would like to analyse the thermodynamic consequences of the above principles and how the operating voltage of a diaphragm and of an ODC cell is affected.
As already explained, the anodic reaction is the same for both the technologies, i.e. the oxidation of chloride ions to form chlorine. The potential of such a reaction is around 1,36 V.
The hydrogen ions reduction is the cathodic reaction for the diaphragm technology. The potential of this reaction is around 0 V.
The cathodic reaction for the ODC technology is the formation of water by reaction of hydrogen ions with oxygen. The potential of such a reaction is 1.23 V.
In Fig. 3 it is possible to see that the difference between the potentials of the two cathodic reactions is around 1 Volt in favour of the ODC and this is what makes the ODC technology especially attractive.
Figure 3
HCl ELECTRODE POTENTIALS:
OXYGEN REDUCTION vs HYDROGEN PRODUCTION
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Figure 4
FUNCTIONALITY OF ODC IS SHOWN
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