What is Electrolysis – Introduction to Electrolysis
Production of caustic solution, Chlorine, and Hydrogen from aqueous solution of alkaline chlorides by application of direct current known as the “Electrolysis of Alkaline Chlorides” is being practice the Chloralkali industry from several years. Continuous development is being conducted by engineers on membrane design to
Different methods of electrolysis process for chloralkali process
- Amalgam process – The electrolysis using graphite anodes or metal anodes and mercury cathodes
- Diaphragm process – The electrolysis using graphite anodes or metal anodes and iron cathodes partitioned by diaphragms – the .
- 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 the membrane electrolysis will be the predominant process for chlor alkali production in the future. This is based on the following advantages :
- Reduced energy consumption in the membrane chlor-alkali process through the utilization of perfluoro membranes suitable for production of 30-33 % NaOH.
- Lower investment cost due to simplicity of electrolysis cell and less space requirements for
- Ease of operation, high operational
- Lower operating cost due to high life expectancy for electrolysers and 3 years service life of the membranes and also due to less personnel requirement for cell operation and maintenance.
- High product purity (less than 35 ppm NaCl in 33 % NaOH and practically no Hydrogen in Chlorine gas)
- 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 electric conductors. In operation, a cell (or a series of cells) is connected to an external voltage source (rectifier unit), and 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 election flow in the external circuit is always from anode (+) to Cathode (-)
Chloralkali Plant is a Chemical Plant comprising all equipments and Chloralkali i process for the production and Treatment of Chlorine, hydrogen and Caustic Soda ( Sodium hydroxide)
Cell Room:- Unit comprising electrolysers.
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 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 an 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 a 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 addition of HCl, is supplied to the cell. In that case one Farady 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 2 + 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 )
2 HOCl + 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; 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.
An other 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, 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 substantial 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)
This was the tutorial on fundamental of electrolysis and i am sure that this article have provided you the Chloralkali process overview in simplest words. If you had any problem in understanding the fundamental of electrolysis then feel free to comment below. If you want to gain more knowledge on Chloralkali process or Chloralkali manufacturing indept then don’t forgot to bookmark us.