A bubble curtain model applied in chlorate electrolysis

Chapter 1.
Introduction
Eka Chemicals is a daughter company of AkzoNobel and one of their main products is sodium chlorate
(NaClO 3 ). It is used in the pulp and paper industry for the production of chlorine dioxide which is an
important bleaching chemical. Chlorate is used as a chemical oxygen generator in airplanes where it
provides emergency oxygen in case of pressure drop. Potassium chlorate in matches is also made from
sodium chlorate. Chlorate is used as a total destructive herbicide in the agricultures sector and as raw
material for the production of perchlorates and perchloric acid.
Chlorate is produced electrochemically. When current is sent through an electrolyte of salt in water,
hydrogen gas is formed at the cathode as a byproduct. This gas drives the convection in the cell due to
the buoyancy caused by the gas bubbles. Gas evolution also has some undesirable effects such as
reducing the conductivity. The effect of a change in conductivity on the mass transfer is still to be
discussed.
Electrolysis is a power consuming process. Assuming 100 % current efficiency, the chlorate process
needs a charge of 1,51 MAh for the production of one ton of chlorate. This is calculated with Faraday´s
Law (Eq.(1.1)). To run an electrolyzer, a superimposed potential around 3 V is needed. The precise cell
voltage depends on the cell design and operating conditions such as current density, flow rate and
temperature. The current efficiency (η eff ) of the sodium chlorate process ranges between 93 and
96%.[27]
It = mzF
M
(1.1)
The total power consumption is given by the equation below: [27]
Power consumption = I.t.U
= 1,51 106 A.1h.3V
= 4,8 MWh (1.2)
η eff 0,95
This makes a power consumption of 4,8 MWh per ton chlorate. When taking in account the price of
one MWh and the annual production of chlorate, the electricity costs at Eka Chemicals is 200-300 million
Euro every year. Even the smallest improvements on current efficiency or on the total voltage would lead
to large savings.
The chlorate process is greatly dependent on mass transfer and transport of reacting species to and from
the electrode surfaces. Since hydrogen bubbles have an effect on the mass transfer and conductivity,
their behaviour has to be examined. Also other information like bubble size, bubble growth and hydrogen
transport will be searched for to create a better understanding of hydrogen bubbles. After all, these
bubbles causes a higher cell voltages and therefore a higher power consumption. The reason why a
better bubble understanding is searched for is to reduce the costs of the cell voltage caused by these
bubbles by bubble coverage on the electrode, gas voidage and bubble curtain.
1