Hydrogen Tech World October 2023 | Page 33

[ water treatment ] system , thereby increasing the overall plant recovery .

Electrolyser UPW configuration

A typical UPW system configuration is illustrated in Figure 3 and would include two-stage RO followed by membrane degassing ( MD ) to remove CO 2

, which , if present , represents a significant ionic load . This is followed by either a conventional ion exchange or an EDI system . With the addition of the final ion exchange polishing section , a conductivity of < 0.056 µ S / cm may be achieved .

The UPW configuration may require additional steps depending on the source water quality ( e . g ., surface water , brackish or sea water ). Similarly , fewer steps may be required depending on the exact water specifications required by the electrolyser manufacturer . One relatively constant requirement is the use of RO followed by either EDI or traditional ion exchange resin systems .

The purpose of the EDI or ion exchange process is to further demineralize the RO permeate to levels that typically exceed a conductivity of < 0.1 µ S / cm . It is important to note that the product water specification for the electrolyser should be met at this point .

The point-of-use ion exchange polishing is present as a safeguard and is not sized or designed for bulk demineralization .

Effectively there are three choices to compare , namely : 1 . Ion exchange – single-use resin

• No onsite regeneration , either resin or resin + tanks are exchanged onsite ;

• Very costly for higher-capacity bulk demineralization ;

• Preferable for low capacities such as < 1.0 m 3 / h . 2 . Ion exchange – with onsite chemical regeneration

• Bulk chemical storage and dosing equipment required ;

• Waste neutralization facility often needed to handle chemical waste ;

• Low capital costs , but standby is required during regeneration periods , and infrastructure to handle chemicals increases costs ;

• Hazardous chemical handling

• Operating costs are high .

3 . Electrodeionization

• Continuous operation and regeneration ( no standby required for regeneration );

• Small footprint ;

• No bulk chemicals required for regeneration ;

• Fair capital costs ;

Cathode —

Anode

4H + + 4e → 2H 2 Cathode Reaction

2H 2

O → O 2

+ 4H + 4e – Anode Reaction

Hydrogen Tech World | Issue 12 | October 2023 33