Fundamentals of Electrolysis

Fundamentals of Electrolysis

Different types of reactions take place in chemical industries and these reactions are helpful in manufacturing the final product. Previously we had talked about different types of chemical reactions among which decomposition reaction was one of them. A decomposition reaction is also of various types and among those electrolysis reaction is one of them. Today, we are going to see what is electrolysis reactions, the Fundamentals of Electrolysis, and the practical application of the electrolysis process.

What is Electrolysis – Introduction to Electrolysis process

Production of the caustic solution, Chlorine, and Hydrogen from an aqueous solution of alkaline chlorides by application of direct current known as the “Electrolysis of Alkaline Chlorides” is being practiced in the Chloralkali industry for several years. Continuous development is being conducted by engineers on membrane design to the power-efficient membrane which can produce sodium hydro-oxide while consuming less power.

Different methods of electrolysis process for chloralkali process

  1. Amalgam  process – The electrolysis  using  graphite  anodes  or  metal  anodes  and  mercury  cathodes
  2. Diaphragm process – The electrolysis using  graphite anodes or metal  anodes and iron cathodes  partitioned by diaphragms
  3. Membrane process – The electrolysis  using  metal  anodes  and  cathodes  partitioned  by  Cation  Exchange  Membranes

The membrane process has,  indeed,  passed through various stages of design and development.  It has become the consensus of expert opinion  that membrane electrolysis will be the predominant process for Chlor alkali production in the future.   This  is  based  on  the  following  advantages :

  1. Reduced energy consumption in the membrane  Chlor-alkali process through the utilization of perfluoro membranes suitable for the production of  30-33 % NaOH.
  2. Lower investment  cost  due  to  simplicity  of  electrolysis  cell  and fewer space  requirements  for
  3. Ease of  operation,  high  operational
  4. Lower operating cost due to high life expectancy for electrolyzers and  3  years service life of the membranes and also due to less personnel requirement for  cell operation and maintenance.
  5. High product purity  (less  than  35  ppm  NaCl  in  33  %  NaOH  and  practically  no  Hydrogen  in Chlorine gas)
  6. No Environmental  pollution  due  to  mercury  or  asbestos or  any  other

Electrolysis Cells &  Definitions :

Chemical reactions that can be made to occur, via ionic mechanism,  by application of electric energy to a  suitably conceived reactor system is called electrochemical cell.

Central to the operation of any cell is the occurrence of ionic reactions which produce or consume electrons at isolated phases on the cell.  These phases are called electrodes and must be good electricity conductors.  In operation,  a  cell (or a  series of cells)  is connected to an external voltage source  (rectifier unit),  and the charge is transferred by electrons between electrodes through the external circuit.

The electric circuit through the cell is completed by electrolytes which support charge transfer by ionic conduction.  The  electrode  at  which  an  electron-producing  ionic  reaction occurs  (e.g. ,  Cl- –>  ½  Cl2  + e-  ) is  the  anode;  the  electrode  at  which  an  electron  consuming  reaction  occurs  (e.g., H2O  + e-  –>  ½  H2  + OH-  )   is called  the  cathode. The direction of electron flow in the external circuit is always from the anode (+)  to  Cathode  (-).

Definitions related to chloralkali Plant

Chloralkali  Plant is a Chemical Plant  comprising  all equipment and Chloralkali  process for  the production  and  Treatment  of  Chlorine, hydrogen  and  Caustic  Soda ( Sodium hydroxide)

Cell Room:- Unit comprising electrolyzes.

Electrolysers (cell) :- Membrane Cell Package.

Rack :- CS  Structure  housing  34  elements.

Element :- Assembly of one each of Cathode, membrane and anode.

Principles of  the  Membrane  Process for chlor alkali process

The ion exchange membrane is the heart of the membrane cell system. It acts as a partition between the anode and the cathode compartment of the cell. The effectiveness of the membrane as a  separation device between the anode and the cathode compartments defines the current efficiency of the cell.  The chemical composition of the membrane,  based on fluorocarbon matrices,  defines the range of caustic concentration at which optimum operating performance is given.

Membranes are currently available to produce caustic concentrations of up to 33%  NaOH with optimum performance. The physical and electrochemical properties and ion-exchange capability can vary widely.

The ion exchange membrane is impermeable for liquid and gas.   It is selectively permeable to Na+-Cations  and the passage of   OH- ions back into the anode compartment is blocked,  thus allowing a  high current efficiency.

The membrane effectively allows the passage of Na+-ions only and prevents the diffusion of anions from the anode compartment to the cathode compartment,  thereby making it possible to obtain caustic soda of very high purity.

1 Faraday of electricity produces  1  equivalent of  Cl2  (theoretically,  when no  Oxygen is discharged on the anode,  and when feed brine is acidified)  in the anode compartment and 1  equivalent of  H2  and  Ce equivalent of  NaOH in the cathode compartment.  The value of Ce,  which is referred to as the current efficiency,  thus becomes a  determining factor in the economics of the ion exchange membrane process.

In practice, alkaline feed brine,  i.e. without the addition of  HCl, is supplied to the cell.  In that case, one  Faraday of electricity will produce less than one equivalent of  Cl2  in the anode compartment.   This is because a  small amount of electricity is consumed by the discharge of  OH- anions at the anode to form oxygen.   Chlorine gas will thus contain about  1.5  vol.%  O2.    However,  it is not possible in practice to avoid  O2  generation at the anode completely.  Even adding the equivalent amount of  HCl necessary to neutralize the total amount of OH- which migrates from the cathode to the anode compartment,  the oxygen content in chlorine cell gas may not be less than about 0.2  Vol. %. This depends on the electrocatalytic properties of the anode activation layer too.

In addition,  a  small part of the chlorine generated in the anode compartment is transformed to hypochlorite   (OCl-)  and chlorate  (ClO3) anions in case of sufficient amount of  OH   anions are present in the anolyte.  When the anolyte from the cells is acidified outside the cell with the equivalent amount of  HCl,  the transformed amount of chlorine is recovered back.

Electrode Potentials, Chemical  and  Electrochemical  Reactions

Gross Reaction

The  gross reaction  for the formation of chlorine, caustic  soda, and hydrogen from a sodium chloride  solution  can  be  expressed  as  follows :

NaCl  +  H2O   –>  NaOH  +  ½ Cl2   +  ½ H2           (  1 )

This reaction is taking part as two  separated  cell  electrode  reactions  : the anode and  the  cathode reaction.

Anode Reaction 

At  the  anode  the  reaction  is

Cl–   – – >   ½  Cl2   +   e

Cathode Reaction 

At  the  cathode  the  reaction is  the  discharge  of  H+   ions  according to

H2O  +  e-   =   ½  H  +  OH

For  each  hydrogen  equivalent  set free  one  equivalent  hydroxyl remains  in  solution  forming  NaOH  with  the   Na+  – ions  migrating  into  the  cathode  compartment :

H2O  —>   H+

H2O  +  e–    +   Na+    —>  NaOH   +  ½    H2

Cell Decomposition Voltage

The  cell  decomposition voltage  is defined  as

Uo   =   E  = ECl2/Cl–      –    EH2/H+,  volts

It is the minimum voltage required  to start  the  reaction

NaCl  +   H2O  =  ½    Cl2   +  ½  H2   +  NaOH

At minimum (zero) current load.

Different  values as function of electrolyte  temperature and  concentration  are  given  in  following  table  for  the  gross  reaction

Cl–     +   H+    =    ½   Cl2  +  ½  H2   at   1  atm  cell  gas  paressure.

Side Reactions and Inefficiencies

The main reaction at the  anode is the electrolytic  oxidation of chloride ions to chlorine.

2 Cl —>  Cl2    +  2e 

The  chlorine  evolved is the desired product,  however  some  chlorine  is  partially  dissolved in  the  water  and  reacts  accordingly.

Cl2  +  H2O  —>  HOCl  +  H+  +  Cl–                  ( 1 )                  

 The hypochlorous acid and the hypochlorite ion   (OCl- )  originating from  ( 1) and  (2)   can give rise to a  third reaction,  which is also purely chemical in nature and produces chlorates.

The  hypochlorous  acid   formed  in  reaction  (1)  is  a  weak  acid  that  readily  dissociates :

HOCl  — >  OCl   +  H+ ( 2 )         

2HOCl + OCl- —> ClO3 + 2 H+ 2 Cl ( 3 )

By  combining  reactions ( 1 ), (2)  and (3)   it  is  possible  to write  down the  following  equation

3 Cl2  + 3  H2 O  —->  ClO3  +  6  H+  + 5  Cl    ( 4 )

The  formation  rate  of  chlorate  which  reduces  the  cell  efficiency,  is  clearly  a  function  of  several  parameters.

  • The partial pressure of chlorine
  • The concentration of  Cl ions;  i.e. the concentration of salt in anolyte.
  • The pH  of  the

The  amount  of  chlorate  formed can be  minimized  by increasing the concentration of Cl- ions  and reducing  the anolyte  pH.

Chlorates may also be  formed in lesser extent electrochemically at  the anode, i.e.  according to a primary reaction involving hypochlorite  ions.

60Cl +3H2  O  —>  2 ClO3  + 6H +    +  4Cl  +  1.5  O2  +  6e–      ( 5)

This reaction is favored  by decreasing the  acidity  of  the  anolyte.

Another  important  side reaction  on  the anode,  which  is  responsible for a  further  loss  of current efficiency,  is  the  generation of  oxygen at  the  anode :

2 H2 O  —>  O2    +  4 H+   +   4e–                   ( 6 )

Despite the fact that the standard electrode potential for this reaction is  +1.229  V  at  25°C,  i.e. less than  EoCl2/Cl- =  1.358,  only minor amounts of  O2  are formed. This is due to the high oxygen over-potential at the anode coating by which the discharge potential of  O2   is higher than that of the  Cl2.

The rate of   O2   generation depends on the available concentration of OH-  ions in the anolyte,  i.e. on the pH-value. At low cell current efficiencies,  i.e. when higher amounts of  OH-  migrate into the anode compartment,  the pH value increases and so also oxygen and chlorate formation.

The Cell Voltage

The operating cell voltage is substantially higher than the equilibrium or decomposition voltage obtained from theoretical considerations. The  cell  voltage  may  be   expressed  as  several  components  :

  • Decomposition voltage, Uo
  • Anode over-potential.
  • Cathode over-potential.
  • Structural voltage  drop  (ohmic  loss)
  • Electrolyte and  gas voltage  drop (ohmic  loss)
  • Membrane voltage  drop  (ohmic  loss + membrane  polarization)

Wrapping Up

This was the tutorial on the fundamentals of electrolysis and I am sure that this article has provided you the Chloralkali process overview in the simplest words. If you had any problem understanding the fundamentals of electrolysis process then feel free to comment below. If you are interested in learning chemical engineering-related articles the do check our other articles. We also write on workplace safety topic i.e. What is PTW in Industries and Types of Safety Work Permit so don’t forget to check those.

Reference: Handbook of Chlor-Alkali Technology

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