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.
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.
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
- 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
- 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 :
- Reduced energy consumption in the membrane Chlor-alkali process through the utilization of perfluoro membranes suitable for the production of 30-33 % NaOH.
- Lower investment cost due to simplicity of electrolysis cell and fewer space requirements for
- Ease of operation, high operational
- 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.
- 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 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 that 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
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 in two separated cell electrode reactions: the anode and the cathode reaction.
At the anode the reaction is
Cl– – – > ½ Cl2 + e–
At the cathode the reaction is the discharge of H+ ions according to
H2O + e– = ½ H2 + 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)
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