I've been reading "Ion Exchange Membrane Separation Processes" closely this week, and I came across a statement in chapter 4 that confused me:

"The total energy used for the actual ion transfer from a feed to a concentrate is the same for a system in which the required membrane area is installed in one cell pair between two electrodes as for a system in which the membrane area is installed in a multitude of cell pairs since the energy consumption is the product of current and voltage drop."

I'm confused by this because it is my understanding that if there is only 1 cell pair, then for every mole of salt separated you would need to split 1 mol of water at the electrodes. But for n cell pairs, to separate 1 mol of salt you would need to split 1/n mole of water at the electrodes to drive the current.

In the first case, the energy input for electrode reactions is about 237 kJ

In the second case, the energy input for electrode reactions is 237 kJ / n.

I understand that the more cell pairs you have, the larger the voltage drop needed, but does this voltage increase with increasing cell pair truly exactly compensate the lowering of the energy input at the electrodes?

It seems to me that there would be an optimum number of cells in a stack that would provide lower energy, but the statement suggests that the energy consumption is sort of unrelated to the number of cells.