ELECTROCHEMISTRY

Process & plant

The Process

The flow of and electric current between two electrodes in a strong alkaline water electrolyte produces the decomposition of water into its elements, hydrogen and oxygen. The electrode reactions can be expressed by the following equations: 

At the cathode 
2H2O + 2e ---> H2 + 2OH- 

At the anode 
2OH- ---> ½O2 + H2O + 2e 
------------------------------------------ 
Process reaction H2O ---> H2 + ½O2 

A certain electrical potential difference has to be applied to the electrodes in order to make the current flow. The thermodynamics of the reaction process shows that the minimum voltage (reversible) necessary to start the current flowing is equal to 1.23 V, corresponding to the free energy (deltaG) of the processing reaction. At this voltage the system produces hydrogen, but requires and additional amount of energy corresponding to the process reaction entropy to operate isothermally. Therefore, the minimum voltage at which hydrogen can be produced without additional heat is 1,49V. 
At this voltage the irreversible energy losses compensate the entropy of the process reaction. In practice, the cell voltage has to be kept well over the reversible value to account for the other losses such as the over voltage of electrodes, the bubble effect, and the electrical resistance of metals, electrolyte, an diaphragms. 

SESPI system runs at a voltage as low as 1,8 – 1,9 V over long periods of time, due to: 

 Special activation of the electrodes for longer operational life 
 Separators with low gas permeability and high ion conductivity 
 Filter press assembly with well designed bipolar metal conductors. 

The Plant

The plant includes the electrical rectifier, the bipolar electrolyser, the hydrogen and oxygen purification units and the drying and compression systems. 
The electrical rectifier, providing direct current to the electrolysers, can operate in a range between 20 and 100% of the rated capacity with very high efficiency. 
Hydrogen and oxygen leaving the electrolyser are stored in separate gasholders, which provide a constant feed to the purification and compression systems. 
Purification is performed by a catalytic system from which hydrogen with a purity of up to 99,000% can be obtained. 
Oil free piston or membrane compressor ensure high purity gases. 
Drying up to – 60°C dew point is automatically controlled. 
Trouble free operation of the plant is guaranteed through a complete instrumentation and well proved safety measures. 

Plant Operating Data

Requirements given here are for a general case. More specific and detailed figures can be supplied to meet the demand of particular plant conditions or locations, or for integration with existing facilities. 

For the production of 1 Nm3 pure hydrogen : 

Electric power 5,8 kWh 
Process water 0,85 l 
Caustic potash (100% basis) 0,1 g 
Cooling water 0,35 m³