Electrodeionisation, and why the resin is there

Ions dragged out under a field while the water stays put — and the limiting current that stops plain electrodialysis reaching ultrapure.

Salt transport straight from Faraday's law•The limiting current that stops plain ED reaching ultrapure•Resin-filled stacks run overlimiting on purpose

Site effluent

Reverse osmosis

EDI polisher

Ultrapure water

Brine header

Waste heat

Membrane distillation

Distillate

Crystallizer

Salt

Purge

The actual dark-mode canvas: reverse osmosis does the bulk desalting, the EDI stack polishes its permeate to boiler-feed quality, and both concentrates go to the brine train.

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Why the polisher is resin-filled

Electrodialysis drags ions out under a field while the water stays put, so its energy scales with the salt removed rather than the volume produced — hopeless on seawater, excellent on a permeate that is already 99% desalted. But the limiting current density is proportional to the diluate concentration, so as the water approaches pure the boundary layer runs out of ions to deliver and the salt flux plateaus: plain ED cannot reach ultrapure at any voltage. Packing the compartment with ion-exchange resin fixes both halves. The resin carries the current, and the water splitting that occurs above the classical limiting current — a fault in a solution-filled stack, causing a pH excursion and scale — is exactly what regenerates the resin in place. Same operating point, opposite meaning, which is why an EDI polisher needs no chemical regeneration.

\dot{n}_{salt} = \frac{\xi\, i\, A_{cell}\, N_{cp}}{z F} \quad\text{(you pay per ion, not per litre)}

i_{lim} = \frac{z F k\, c_d}{t_m - t_s} \quad\text{(falls with the diluate — why plain ED cannot reach ultrapure)}

i_{eff} = \min(i,\, i_{lim})\quad\text{(solution-filled: the flux plateaus)}

i_{eff} = i\quad\text{(resin-filled: EDI runs overlimiting by design)}

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Membrane (RO)

Solution-diffusion rating model, bounded by van't Hoff osmotic pressure.

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Electrodialysis / EDI

Faraday-law ion transport with the limiting current that caps a solution-filled stack and the resin filling that lifts it.

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Mixer

Combines multiple streams into one, closing the mass and energy balance.

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Heater / cooler

Duty- or outlet-condition-specified energy-balance stage.

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Membrane distillation

Vapour-pressure driven, so no osmotic ceiling — with the temperature polarisation a bulk-temperature model gets wrong.

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Crystallizer

MSMPR population balance — nucleation and growth set the mean crystal size.

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Live plant integration

Stream stack current, voltage and product resistivity from the EDI skid into this flowsheet's twin comparison — a stack drawing its set current at a rising voltage is losing cell-pair conductivity, and the product resistivity follows it down.

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