Heat and mass transfer in a polymeric electrolyte membrane-based electrochemical air dehumidification system: Model development and performance analysis

https://doi.org/10.1016/j.ijheatmasstransfer.2018.06.010Get rights and content

Highlights

  • A 2D theoretical model for PEM-based electrolytic dehumidifier was developed.

  • The inside heat/mass transfer mechanisms of five-layer elements can be disclosed.

  • The model predictive results showed good agreement with experimental ones.

  • The largest moisture gradient occurred inside the PEM during dehumidification.

  • Increasing PEM water content can significantly improve the system performance.

Abstract

In this paper, a two-dimensional steady-state theoretical model was established, to model the internal transport phenomena in a polymeric electrolyte membrane-based (PEM-based) air dehumidification element. The influences of electrochemical reactions, activation and concentration over-potentials, and Ohmic and electro-osmotic effects were considered. The model was solved by the finite difference method with conjugate boundary conditions. So, with this model, the heat, mass and current transfer through the five layers of the element (diffusion layers, catalyst layers and a PEM) could be described theoretically, as well as the convective heat and mass exchange with adjacent airflows. Compared with the results from previous models, this model showed a much closer trend to the experimental data. The overall error was less than 15%, with an acceptable average error of 8.6%. However, greater deviations were observed under larger airflow conditions, probably due to the assumption of laminar airflow and steady-state heat conduction. Furthermore, by the performance analysis, the maximum moisture gradient was found inside the PEM, so the PEM’s parameters could largely affect the system performance. With the increase in PEM water content, the dehumidification was significantly enhanced, especially when the air humidity was high. If the PEM water content was doubled, the dehumidification rate was increased by 42%. Then, decreasing the PEM thickness also improved the performance. However, the effect became minor if the thickness was less than 100 μm. It was also helpful by increasing the PEM conductivity, although the effect of this variable was relatively small. This study provided theoretical guidance for further system improvement and material preparation for PEM-based dehumidification systems.

Graphical abstract

Temperature and moisture profiles inside the PEM-based element during the dehumidification.

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Introduction

Air humidity control is a key factor for ensuring the product quality in industrial environments, especially for electronics and precision manufacturing [1], [2]. Excessive air humidity can cause many problems such as mould growth, surface corrosion and electrical breakdown [3], which are more serious in humid areas. Over 70% of China’s manufacturing industries are located in moist south-eastern areas, leading to the fact that about 25% of product defects are related to humidity issues. However, current dehumidification methods have some limitations in industrial applications [4]. The widely-used cooling-condensation dehumidification process requires 7–12 °C cooling water, causing many issues such as insufficient humidity control, low system efficiency and condensation-induced water droplets [5]. Recently-developed desiccant absorption systems, including the liquid desiccant absorption and desiccant wheel adsorption, could improve the dehumidification efficiency by dealing with the water vapour independently. However, it still has practical limitations, as follows. (a) The process air may carry the corrosive desiccant droplets and damage the devices [6]. (b) Regeneration systems are often complex and large [7]. (c) Rotation of the desiccant wheels or the flow of liquid desiccants may cause noise and even lead to safety risks [8].

As a kind of humidity independent control methods, electrochemical dehumidification processes have drawn many attentions in these years. This kind of systems can drive the moisture transfer directly by an electric field, without the use of cooling water or desiccants. So the system can be simple and compact, requiring only power supply instead of cooling and/or regeneration components. The researchers studied such methods as electro-osmotic [9], [10], thermoelectric [11], [12], [13], electrodialysis [14], [15] and electrolytic [16] variants. However, the thermoelectric dehumidification still has condensation problems, and the electro-osmotic and electrodialysis ones are not suitable for practical uses. Recently, the dehumidification with electrolytic materials is a creative and innovative technology. So, this study focused on the electrolytic air dehumidification system.

The electrolytic dehumidification with solid electrolytic materials was firstly proposed by Iwahara et al. in 2000 [17]. Then, Greenway et al. [18] and Sawada et al. [19] tested the water vapour removal possibility within electrolytic cells. However, the above studies were conducted at high temperatures (>600 °C). The possibility of electrolytic dehumidification at normal temperatures (20–40 °C) was experimentally validated by Sakuma et al. in 2009 [20]. In 2010, they also measured the effect of air temperature in an enclosed space, and built an empirical formula [21]. Then in 2011, Lewis et al. conducted an experiment [22], and developed an electrochemical model to simulate the water transfer [23]. In their study, only pure water vapour was used, rather than the actual airflow through the element. In 2017, Qi et al. validated the possibility of polymeric electrolyte membrane-based (PEM-based) electrolytic dehumidification with airflows [16]. The elements developed were compact, as small as 10−2–10−3 m3/kW. Additionally, the dehumidification performance was competitive to other electrochemical methods. A semi-empirical model was also developed for evaluating the performance. The comparison of different dehumidification methods was summarized in Table 1.

From the literature it could be found that the key advantage of the PEM-based dehumidification is its compact size and the ability in effective humidity control, which is suitable for industrial environments. However, most previous studies only focused on experiments. Until now, the system performance could only be predicted by empirical models. These models were seriously limited to the developers’ experimental conditions, leading to discrepancies in different studies. Furthermore, different layers of the PEM-based element cannot be discerned in previous models, and the influence of airflows was not considered. So, the internal heat and mass transfer of the PEM-based dehumidification element was not yet clear. Without accurately predicting the effects of operating parameters on dehumidification performance, the system optimization became very difficult. Therefore, the development of a theoretical model for the electrolytic dehumidification element is of great significance to the practical applications of this novel system.

In this paper, a two-dimensional steady-state theoretical model of the PEM-based electrolytic dehumidification element was developed, focusing on the heat, mass and current transfer through the multi-layer element with air flows. The element was composed of a proton PEM in the middle, catalytic layers with porous electrodes on both sides and diffusion layers on both outer sides. Two air channels were assembled on both outermost sides. Experiments were conducted to validate the model, with an element using Nafion 117 as the PEM. Then, the influences of operating parameters were analysed with the model.

In short, the contributions made by this study are as follows. (a) A complete theoretical model of the PEM-based electrolytic dehumidification system was developed to describe the temperature, moisture and current density fields inside the multi-layer element and the airflows. (b) The effects of reactions, over-potentials, and Ohmic and electro-osmotic effects on both anode and cathode sides were considered. (c) The effects of operating parameters on the dehumidification performance were investigated to provide guidance for system optimization.

Section snippets

Description of the electrolytic dehumidification

The PEM-based element for an electrolytic dehumidification system has a multi-layer sandwiched structure, as shown in Fig. 1: a proton-conductive PEM in the middle, porous electrodes with catalytic layers on both sides, followed by diffusion layers on both outer sides. The process air and sweep air flowed through the air channels facing the element anode and cathode, respectively. Under an external electric field, the following reactions occurred.Anode:H2O2H++2e-+0.5O2Cathode:2H++2e-+0.5O2H2O

Data validation

The modelling process should be validated. Two performance indices were defined.Moisture removal rate(kg/s:)ṁremoval=ṁpMH2O(ζp,out-ζp,in)where ṁ means the mass flow rate. Subscripts ‘in’ and ‘out’ mean the inlet and outlet, respectively. ṁremoval represents the rate of moisture removal from the process air.Dehumidification rate(kg/(s·V·m2:))αde=ṁremovalMH2O×ζp,in×V×Aareawhere V means the electric field supply. αde represents that the moisture removal rate per area at a certain air inlet

Conclusions

The PEM-based electrolytic dehumidification system is promising, as it can achieve an independent, portable and energy-efficient moisture removal of air. However, most previous studies focused on limited experiments. The predictions from past empirical models varied seriously in different studies. In this study, a two-dimensional steady-state theoretical model was developed, focusing on the heat, mass and current transfer through the multi-layer PEM-based element, as well as the air flows. The

Conflict of interest

The authors declared that there is no conflict of interest.

Acknowledgment

The project is supported by the National Key Research and Development Program (No. 2016YFB0901404), National Natural Science Foundation of Guangdong Province (No. 2017A030313264). It is also supported by the Science and Technology Planning Project of Guangdong Province: Guangdong-Hong Kong Technology Cooperation Funding Scheme (TCFS), No. 2017B050506005 and the National Science Fund for Distinguished Young Scholars (No. 51425601).

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