A primary, secondary and pseudo-tertiary mathematical model of a chlor-alkali
membrane cell
P. BYRNE
1
*, P. BOSANDER
3
, O. PARHAMMAR
2
and E. FONTES
3
1
Faxe
Â
n Laboratory, Applied Electrochemistry, Royal Institute of Technology (KTH), Teknikringen 42,
SE-100 44 Stockholm, Sweden
2
Permascand-Eka Chemicals R&D, PO Box 13000, SE-850 13 Sundsvall, Sweden
3
Comsol AB, Tegne
Â
rgatan 23, SE-111 40 Stockholm, Sweden
(*author for correspondence, fax: +46 8 10 80 87, e-mail: phil@ket.kth.se)
Received 31 July 1999; accepted in revised form 30 May 2000
Key words: chlor-alkali, current distribution, electrolysis, mass transport, model
Abstract
A theoretical study of current density and potential at the anode, membrane and cathode, of a chlor-alkali
membrane cell where the electrode blades are placed vertically, is presented. A representative unit cell is modelled in
primary, secondary and pseudo-tertiary current distribution models. It is shown that electrolyte and membrane
resistance has the greatest eect on current distribution. Furthermore, it is shown that there is a surprisingly small
in¯uence of mass transport on current distribution, on the assumption that the diusion layer is of constant
thickness. In converse to this, it is shown that mass transport aects the anode overpotential distribution to the
extent that conclusions can be made about the occurrence of side-reactions and where they occur. Finally, it is
shown that it is possible to estimate tertiary behaviour with a secondary current distribution model, by using an
analytic expression at the anode surface.
1. Introduction
Improvements in cell design have seen a remarkable
increase in the amount of current density passing
through chlor-alkali membrane cells [1]. This has led
the process into areas where the feed streams are
signi®cantly close to complete ion depletion, with all
the subsequent problems of concentration overpotential
and side-reactions that this incurs. By creating cell
geometries or conditions that are favourable to a more
uniform current distribution, one is able to utilise larger
portions of electrode area, and still manage localised
increases in current density and therefore production. A
uniform current distribution would diminish localised
corrosion and ensure a more even depletion of the
electrocatalyst, which undergoes wear due to gas evo-
lution [2].
Basically, the general con®guration of the chlor-alkali
membrane cell allows fresh electrolyte to enter to the
front of the electrodes, and bubbles to pass behind the
electrodes through the gap between the electrode blades
[3]. The blades can be layered horizontally, like louvres
in a venetian blind [4], or vertically like slats in a fence
[5]. This paper shall look at the `lantern' cell structure
found in the ICI FM-21 electrolysers, previously pre-
sented in a paper by Martin and Wragg [5]. It is
generally known that the critical region of the mem-
brane cell is the space between the membrane and
electrode. Traini and Meneghini [6] reported that the
membrane cell could be run with the membrane right up
against the anode surface. However, this investigation
will be looking at the case where there is reasonable
room between the membrane and electrodes. The plan
view of a horizontal cross-section of the anode, mem-
brane and cathode is seen in Figure 1.
Primary current distribution models have previously
been written to describe current density and potential
distributions around the membrane cell anode [2, 5, 7, 8].
These models considered only the migratory properties
of the chloride ion, whilst ignoring the convection and
the complete eects of electrode kinetics. This work
aims to present a model that also takes into account the
kinetics of the chlorine reaction, as represented by an
exponential relationship, and thus illustrate the distri-
butions of current and potential in this secondary
current distribution model. Furthermore, the article will
present results from a pseudo-tertiary current distribu-
tion model, which considers the transport of chloride
ions by de®ning a hypothetical diusion layer. The use
of an assumed diusion layer thickness will then be
investigated in the light of the work from Ibl and
Dedicated to the memory of Daniel Simonsson
Journal of Applied Electrochemistry 30: 1361±1367, 2000.
1361
Ó
2000 Kluwer Academic Publishers. Printed in the Netherlands.