Safer water treatment

The generation of waste gases in electrolytic processes is a problem. This article presents an integrated system that is capable of separating the explosive mixture of hydrogen and oxygen from a fluid and converting it in a controlled manner into water vapour. The concept is suitable for use in many different water-treatment processes.

In industrial water-treatment systems, electrolytic processes are used, among other things, for the generation of ozone and for electro-deionisation (EDI). In electrolytic ozone generators, oxygen is converted into ozone at the anode of a PEM electrolysis cell, which acts as a catalyser. At the same time, hydrogen is produced at the cathode of the cell and must be removed and disposed of in order to prevent explosions.

In EDI systems, ions for the continuous regeneration of the ion exchanger are produced by decomposition of water. In this process, hydrogen and oxygen are produced at the electrodes and carried out of the system with the rinsing water. The water leaving the system thus carries a dispersed gas mixture whose composition is equivalent to an explosive gas mixture with a stochiometric volume ratio of H2:O2=2:1. In a homogeneous dispersion, this gas mixture is not explosive.

However, as soon as a large volume of this gas mixture accumulates, there is a risk of gas explosions. This can happen, for example, when the water leaving the system runs into a drain.

The user manuals of EDI systems contain warnings about the possible danger, but the manufacturers of such systems have, until now, done little to provide convincing solutions to the problem. The suggested precautions, such as discarding the water into well-ventilated drains, are not very practical. One solution that complies with the safety requirements is to use a gas separator and to direct the collected gases via an explosion-protected exhaust line into the open air, but this is very costly. An integrated solution that is fully independent of the installation in the building would be far better.

Catalytic converter

With the Hydrokat, treatment of waste gases from electrolytic processes can be carried out far more easily. The unit consists of two modules: a gas separator and a catalytic burner. The gas separator is needed in cases, such as EDI systems, where it is necessary to extract the gases from a flow of liquid. The outlet line of the gas separator is designed as a riser to ensure that the gas separator is always filled with water, preventing the formation of explosive gas bubbles.

On top of the gas separator, there is a droplet separator that catches any water carried in the gas flow. The droplet separator is packed with wire-mesh elements and, due to their high specific surface area, also acts as a flame trap. The gas then enters the catalytic burner, where the hydrogen is burnt. The noble-metal catalyst used for this is applied as a coating to a stainless-steel foil. This foil is in direct contact with the inner side of the combustion tube, which means that the heat resulting from combustion passes directly to the tube, preventing local overheating.

The combustion tube also has an additional electric heater on the outside, whose power is regulated by means of a temperature sensor. The additional heater is used primarily to ensure that the combustion reaction starts reliably and to prevent condensation of the water vapour that results from the combustion process.

In addition, it has an important function in supplying air for the combustion process: in many applications, the waste gas does not contain the necessary excess oxygen for complete conversion of the hydrogen. The additional heater draws the necessary combustion air from the surroundings since it, together with the cover of the burner, generates a chimney effect. This means that even waste gas consisting of pure hydrogen can be converted safely.

The concept of catalytic combustion implemented in the Hydrokat from Christ is a safe solution for the treatment of the waste gases generated by electrolytic processes. At the heart of this solution is a catalytic burner with an automatic supply of oxygen and integrated temperature regulation. It is capable of completely converting mixtures of hydrogen and oxygen in any ratio.

Compared with the ventilation systems commonly used today, the Hydrokat concept offers several advantages: the water treatment system can be installed anywhere, without having the need for special ventilation systems. Since it is designed as an independent component, the Hydrokat can be installed without having to modify the piping or the control system of the water-treatment system. This means that it can be retrofitted easily in existing systems.