During the electrolysis of seawater the hydroxide by-products naturally adhere to the cathodes unless prevented by the employment of specific design parameters. It should be unnecessary for concentric tube technology electrolysers but will be a periodic requirement for parallel plate technology electrolysers.
Hydroxide contamination of the cathodes will, over a period of time, be detrimental to the efficiency of the electrolysis eventually causing high voltage requirements and in the final stages reduce electrolyte flow.
The process of acid washing includes recirculation of a 5-7% HCl solution through an electrolyser stream to dissolve the hydroxides therein.
It is difficult to predict the frequency at which acid washing will be required depending much on seawater quality and mode of operation. Typically acid washing would be required once every 2-4 weeks for a period of approximately 2-4 hours.
Given the frequency of operation it is usually provided as a manual system although a degree of automation can be provided.
The sizing of the acid wash system will vary according to the size of the ECP installation due to the differing volumetric requirements. The acid storage tank at a minimum has to be capable of filling both the electrolyser stream and the associated acid delivery pipe work. It should also be considered that the larger the acid wash tank volume the less frequent will be the requirement for strengthening and / or neutralising and disposing of the spent acid.
It is preferable within the system design to provide for evacuation of the acid from the electrolyser stream back to the acid wash tank on completion of a wash cycle. This can be achieved by the provision of an eductor system driven by means of the acid wash pump or through gravity if respective elevations of the equipment permit.
In areas where there is a high Manganese content in the seawater there could be a requirement to introduce a periodic acid wash of a hydrogen peroxide mixture to remove Manganese Dioxide electrolysis by-product which can coat the electrodes and similarly adversely impact their efficiency. Alternatively it has been reported that Electrolysers with Hastelloy C cathodes can be cleaned by brief periods of reversed polarity operation, this is not possible with Titanium cathodes.
