(Invited) Investigation on Dynamics of Expansion Forces and Dimensions Changes of the Pilot Scale Electrodialysis Stack
Authors: Kodým Roman , Tichý David , Němeček Michal , Šnita Dalimil , Bouzek Karel 
Published: September 2023

Electrodialysis (ED) represents a matured and well-established desalination and electromembrane separation technology with typical plate-and-frame configuration. In order to satisfy application demands, there is a trend for an increasing dimension and operating temperature of the industrial ED capable of high performance and efficiency in view of process time and system volume. These activities are motivated mainly by process cost reduction and exploiting new application fields benefiting from higher operating temperature and higher performance-to-volume ratio. This is also the case of reverse electrodialysis, currently meeting a growing popularity as a promising green energy source. However, experiences in a development of larger scale electromembrane devices show that scale-up from laboratory or pilot system to industrial ones is not straightforward. The scale-up simply based on increasing membrane active area or number of membranes, without appropriate modifications of entire unit design, often lead to accentuation of negative effects that are normally negligible on a small scale.
One of the critical issues in larger scale represents dimensional changes of the ED stack, mainly due to membranes swelling and changes of mechanical properties of the stack materials as a respond to variation of operating conditions (temperature, electric current, salt concentration). When a robust rigid frame is used for stack assembly, such stack expansion may lead to an increase of the internal expansion forces causing irreversible mechanical damage of the materials used (membranes, spacers). On the contrary, the stack shrinking (e.g. due to membrane drying and deswelling) negatively affects the tightness. Consequently, the performance, reliability and lifetime of the apparatuses can be significantly exacerbated. Therefore, an autonomous (passive or active) system of compression force control has to be employed, which at each moment spontaneously adapts to stack dimensions keeping compression/expansion force at an optimal value. However, prior to a development of such system a deeper fundamental understanding of the stack behaviour is required. We are mainly interested at a) a qualitative and quantitative respond of the stack internal forces upon variation of different operating parameters, b) uniformity of distribution of internal expansion forces, c) dynamics (characteristic times) and reversibility of the stack dimensional changes, d) effect of different compression forces and stack compression-expansion cycles to longer-term operation of the stack and also e) stack tightness.
In the first part of presented contribution the basic principle of electrodialysis process, system design and critical aspects of the ED process operation in industrial practice will be discussed. Next, the results of the research aimed at investigation of the dimensional and expansion forces changes in the pilot electrodialysis stack will be presented. The pilot-scale ED unit consists of 100 membrane pairs equipped by heterogeneous anion- and cation-selective membranes. Working area is of 0.4 × 0.1 m2. The stack is orientated horizontally assembled into the robust frame made of aluminum Item® profiles. A rigid endplate is attached to one side of the stack, whereas the opposite end-plate is segmented and equipped by 6 tensiometers measuring local stack expansion force. This compression force is applied to the stack by six pneumatic cylinders with equal pressure pressurized by a one standard compressor. The aspects of the stack behavior mentioned above are investigated as a dynamic respond of the stack to sudden changes in operating temperature (20 – 65 °C), electric current, salt concentration (0 – 160 g/dm3) and compression force (up to 45 kN). Different regimes are investigated: a) a regime with constant compression force in the pneumatic cylinders, or b) constant stack dimension (rigid compression frame) during the entire experiment. It was found that the effect of temperature was significantly more pronounced than the effect of salt concentration and electrical current. Moreover, up to 3 different characteristic times in the stack dynamic respond to temperature change were found.
The financial support from grant agency of Ministry of Trade and Industry of the Czech Republic within program TRIO (project TOM2022 no. FV40053) is gratefully acknowledged.