Electrolyser design

As indicated in Fig. 17.2, the membrane process has long been characterised by substantial reductions in electric power consumption, through constant advances in membrane, electrolyser and electrode technologies. In the early years of its commercial establishment, some 25 years ago, it yielded a caustic soda concentration of 20% or lower, with less than 90% current efficiency. Today, the caustic soda concentration is 33%, the current efficiency is 97%, and the ohmic drop of the membrane has been lowered by approximately 1.0 V. During the same period, advances in electrolyser design have improved the uniformity of intracell electrolyte concentra-... 

A proper estimation of the electrolysis efficiency provides guidance to the improvement of the processes and the electrolyser design. In this study, the current efficiency, r c, of the electrolysis system was determined from the experimentally produced amount of hydrogen, H2(exp), divided by the theoretical value, H2(theor) calculated from the Faraday s law ... 

Figure 6.4 The FM01-LC electrolyser designed to ape the characteristics of the FM21 electrolyser.

In the laboratory, preparative electrolyses on the one gram scale can readily be carried out in simple three-electrode cells. The connection of such a cell to a typical potentiostat (feedback system) is illustrated in Fig. 15. It is normally desirable that the electrolysis should be carried out at constant temperature and potential and at a high rate. Hence when designing such cells it is necessary to consider a number of factors. These include the following. 

Following successful testing of the bubble jet system [3] at pilot scale, the plant was scaled to full technical size (2.5 m2 elements) and successfully tested. The anolyte flow-out of the elements showed a completely pulsation-free operation with all benefits for the membrane lifetime. Despite the rather good results of this first run a design review was started to improve the electrolyser element design. 

DuPont research into high current density and the associated effect on membrane and electrolyser performance has been underway for a decade. It has been the area of greatest concentration for the company during the last 5 years. Studies at the DuPont Experimental Station and Fayetteville Nafion Customer Service Laboratories resulted in polymer innovation and new membrane designs. This work has also identified interactions between membranes and electrolysers... 

EMOS has to date been mostly used in chlorate manufacture, but R2 in Montreal, Canada has recently installed its system on an FM-21 1500-type cell chlor-alkali production facility. This is presently a pilot installation, with only six cells currently being monitored. This installation has led to the monitoring of cell currents rather than cell voltages owing to the monopolar design of these electrolysers. It is too soon to make detailed conclusions about this installation as it has only been fully operational since January 2000. 

The three tenders submitted were all of a very high standard and achieved Orica s requirements and specifications. The tender selected was therefore based on lowest net operating cost and was awarded to Kvaerner Chemetics in conjunction with Chlorine Engineers Corporation for electrolyser supply. Kvaerner Chemetics also separately contracted Kvaerner Process Australia to provide local construction management and some local design expertise. 

Kvaerner Chemetics have developed a novel, patented process [1] for the removal of multivalent anions from concentrated brine solutions. The prime market for this process is the removal of sodium sulphate from chlor-alkali and sodium chlorate brine systems. The sulphate ion in a brine solution can have a detrimental effect on ion-exchange membranes used in the production of chlorine and sodium hydroxide consequently tight limits are imposed on the concentration of sulphate ions in brine. As brine is continuously recycled from the electrolysers back to the saturation area, progressively more and more sulphate ions are dissolved and build up quickly in concentration to exceed the allowable process limits. A number of processes have been designed to remove sulphate ions from brine. Most of these methods are either high in capital or operating cost [2] or have large effluent flows. 

All these modifications lead to the fulfilment of all aspects for an improvement of the Rol by KU single element technology and high efficiency of the latest cell design, such as low energy consumption with a high on-stream factor for the plant, simple and rapid maintenance of the electrolysers, plant load flexibility and high current densities. 

The ML32NCH high current density electrolyser was specifically designed and developed to meet all of these requirements. 

Given the analysis described in Section 18.2.2, it was decided to develop an electrolyser that was capable of running comfortably at 6kA m-2. To ensure this requirement, BiChlor is designed for 8 kA m-2. This provides the flexibility to use BiChlor at very high current densities to suit the needs of particular clients and allows for advances in membranes which might move the optimum current density upwards. 

Finally, an electrolyser which has low voltage at start-up and very stable operation during its lifetime was designed. Advantages of these features are longer component lifetimes and reliable operation. 

A modular electrolyser contains a number of discreet modules consisting of anode, membrane and cathode. These sealed modules can be individually removed from the electrolyser without the other modules being affected. The key to this type of design is to ensure a good electrical contact between adjacent modules. 

Of the factors just listed, those that critically influence the design of the electrolyser are described below. 

To remove gas from an electrolyser compartment the obvious choice is to take it out at the top. In addition, it is important that the gas can leave the compartment along its entire length in order to avoid any back pressure. This is exactly the way BiChlor is designed, which is illustrated in Fig. 18.4. 

BiChlor contains a number of features to lower the electrolyser voltage by reducing the resistance. One of these features is the design for zero gap operation. This lowers the electrolyser resistance by keeping the distance from the electrodes to the membrane at a minimum. 

Furthermore, a cell header with its 70-100 connections is not easy to replace, and considerable numbers of safety features have to be incorporated into its design. Composite materials do not lend themselves to effective non-destructive testing and a leaking joint may lead to delamination of the chemical-resistant liner from its mechanical support. Since any leaks cannot be detected easily in such circumstances, the entire header must be replaced with a corresponding electrolyser downtime. 

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