Feasibility of membrane processes for regeneration of liquid desiccant solutions used in air-conditioning systems

Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” DOI: 10.15625/vap.2019.000229 661 FEASIBILITY OF MEMBRANE PROCESSES FOR REGENERATION OF LIQUID DESICCANT SOLUTIONS USED IN AIR-CONDITIONING SYSTEMS Hung Cong Duong 1 , Anh Van Nguyen 1 , Nguyen Cong Nguyen 2 , Khac Uan Do 3 1 Le Quy Don Technical University, Email: hungcongduong@hotmail.com 2 Da Lat University, Email: nguyennc@dlu.edu.vn 3 Hanoi University of Science and Technology, Email: uan.dokhac@hust.edu.vn ABSTRACT Liquid desiccant air conditioning (LDAC) has emerged as an energy-saving process to provide thermal comfort to buildings. Unlike traditional vapor compression air conditioners, the LDAC process relies on concentrated liquid desiccant solution to regulate the air humidity and temperature. Thus, the regeneration of liquid desiccant solution is crucial to sustain the efficiency of the LDAC process. In most LDAC processes, liquid desiccant solution regeneration is carried out using the thermal evaporation method which is deemed energy-intensive and prone to desiccant carry-over. Recently, membrane processes including reverse osmosis (RO), membrane distillation (MD), and electro-dialysis (ED), have been proposed and demonstrated for liquid desiccant solution regeneration. Using membrane to facilitate the selective transport of water, the membrane processes can regenerate liquid desiccant solution without any issue of desiccant carry-over. In this paper, fundamental knowledge, working principles, and applications of the three membrane processes are thoroughly analyzed and discussed. The ultimate purpose of this review paper is to shed light on the feasibility of and challenges to the membrane processes for the liquid desiccant solution regeneration application. Keywords: Liquid desiccant air conditioning (LDAC); liquid desiccant solution regeneration; liquid desiccant solution recovery; membrane process. 1. INTRODUCTION Liquid desiccant air conditioning (LDAC) has emerged as an energy-saving alternative to the conventional vapor compression air conditioners [1]. The LDAC process regulates the air humidity and temperature by using liquid desiccant solution to absorb the air moisture, thus obviating the need for over cooling and subsequent reheating the air like in the conventional air conditioning systems. In the LDAC process, liquid desiccant solution regeneration is a critical step because it directly affects the process dehumidification efficiency and energy consumption [2]. As a result, methods to regenerate liquid desiccant solutions used for LDAC have been the focus of many recent studies. This paper aims to provide a comprehensive review of liquid desiccant solution regeneration using membrane processes. The review begins with a brief description of the LDAC process, and then fundamental knowledge of the mature as well as emerging membrane processes are provided. Based on this knowledge, the feasibility of and challenges to each membrane process for the liquid desiccant solution regeneration application are critically discussed. 2. WORKING PRICIPLES OF THE LDAC PROCESS A basic LDAC process entails air dehumidification and liquid desiccant solution regeneration (Fig. 1). During air dehumidification, the air moisture is absorbed into the cool and concentrated liquid desiccant solution to condition the air. The moisture absorption leads to an increase in the temperature but a decrease in the concentration of the solution, hence reducing its dehumidification efficiency. To be recycled for air dehumidification, the weak (i.e. warm and diluted) liquid desiccant solution needs to be regenerated. Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” 662 Most current LDAC systems utilize traditional thermal evaporation methods to regenerate the weak liquid desiccant solution [2]. Using these methods, the weak liquid desiccant solution is heated to about 90 C and then blown counter-currently with an air stream through the packed bed in regenerators. Desiccant carry-over is a vexing technical issue associated with the traditional thermal methods due to the direct contact between the desiccant solution and the air streams [3]. The desiccant carry-over inevitably leads to desiccant solution replenishment and detriment to building occupants and equipment. Fig. 1. A schematic diagram of a LDAC process. 3. MEMBRANE PROCESSES FOR REGENERATION OF LIQUID DESICCANT SOLUTION 3.1. Reverse osmosis Reverse osmosis (RO) is a benchmarking process for seawater desalination applications. In the RO process, a dense and semi-permeable membrane is used to achieve the salt-water separation. Given its high water-salt selectivity, the RO membrane allows water to permeate through it but rejects mostly all dissolved salts. Thus, the RO regeneration process of liquid desiccant solution might deftly avoid the desiccant carry-over issue. Several studies have evaluated the feasibility of the RO process and highlighted the incompatibility of current RO membranes for regeneration of liquid desiccant solution due to the high osmotic pressure of liquid desiccant solutions. The water flux (J) of the RO process is expressed as: ( )mJ A P (1) where A is the membrane water permeability coefficient, P is the applied hydraulic pressure, and m is the osmotic pressure difference between the feed and permeate sides of the membrane. The applied P must be higher than m to achieve a practical water flux. For the RO process of a 25 wt.% LiCl solution, the m is around 180 bar [4], hence a hydraulic pressure higher than 180 bar is required on the feed side of the RO membrane. It is noteworthy that 180 bar is far above the workable pressure of all current RO membranes. Thus, RO is currently not technically feasible for regeneration of liquid desiccant solutions used in the LDAC systems. Hồ Chí Minh, tháng 11 năm 2019 663 3.2. Membrane distillation Membrane distillation (MD), which is a hybrid thermally driven membrane separation process, has emerged as a promising candidate for regeneration of liquid desiccant solutions used in LDAC systems. Given the hydrophobic nature of its membrane, the MD regeneration process of liquid desiccant solution can theoretically achieve a 100% salt rejection, hence completely preventing the desiccant carry-over issue. The MD process is driven by a temperature difference between two sides of the membrane but not by the hydraulic and osmotic pressure as in RO. The MD regeneration process of liquid desiccant solution is not heavily affected by the osmotic pressure of liquid desiccant solutions compared to the RO process. Thus, the MD process is compatible with most liquid desiccant solutions used for LDAC systems. The feasibility of MD for regeneration of liquid desiccant solutions used for LDAC systems has been demonstrated at both bench-scale and pilot-scale levels. Duong et al. [3] employed a bench-scale MD system to regenerate a LiCl liquid desiccant solution. The MD process at feed temperature of 65 C could increase the LiCl concentration up to 29% without any observable LiCl loss. Lefers et al. [5] utilized a bench-scale MD system to manifest the capability of MD for regeneration of liquid desiccant solutions (e.g. CaCl2 and MgCl2 solutions). A pilot MD process using solar thermal energy for regeneration of liquid desiccant solution (i.e. LiBr) was examined by Choo et al. [6]. The experimental results showed the heavy dependence of the MD process performance indicators on the process operating conditions. The most considerable technical challenges to MD regeneration of liquid desiccant solutions are the polarization phenomena, particularly the concentration polarization effect. Temperature and concentration polarization effects are intrinsic problems for the MD process. The temperature polarization reduces the temperature while the concentration polarization increases the salt concentration of the feed solution at the membrane surface compared the those in the bulk feed solution; therefore, they reduce the MD process water flux. Given their hyper salinity, the MD process of the liquid desiccant solutions is envisaged to suffer severe polarization effects [7]. 3.3. Electro-dialysis Electro-dialysis (ED) is an electrically driven process relying on cation and anion exchange membranes to achieve the salt-water separation. The ED process can regenerate the weak liquid desiccant solution without the need for high hydraulic pressure as in RO and heating the solution as in MD. Therefore, ED is considered an energy-efficient process for regeneration of liquid desiccant solutions for LDAC systems. The ED process for regeneration of liquid desiccant solutions has been investigated. As a notable example, Guo et al. [8] examined the influences of the ED operating conditions on the regeneration capacity of a bench-scale ED system with LiCl solution. The authors reported that ED regeneration capacity increased with decreased circulation flow rates, increased current density, and lowered regenerated solution initial concentration. Particularly, the difference in concentration between the regenerated and spent solutions exerted a profound impact on the system regeneration capacity due to the influences of osmosis and salt diffusion. Thus, the ED system could only regenerate the LiCl solution with a concentration difference between the regenerated and the spent solutions below 5.9 wt.%. Cheng et al. [9] and Guo et al. [10] demonstrated that the ED regeneration of liquid desiccant solutions exhibits limited energy efficiency, and the process energy efficiency decreases when regenerating liquid desiccant solutions with higher initial concentrations. More studies are required to the realize the ED process for regeneration of liquid desiccant solutions. Novel approaches are needed to improve the energy efficiency of the ED process, especially for the regeneration of liquid desiccant solutions in LDAC systems given their hyper saline nature. In addition, the recycle of the spent solution used in the ED process is worth considering. Because the concentration difference between the regenerated and the spent solutions Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” 664 critically affects the ED process regeneration capacity and energy efficiency, the spent solution eventually needs regenerated. 4. CONCLUSIONS The three key membrane processes for regeneration of liquid desiccant solutions are reviewed in this study. The RO process can avoid the desiccant carry-over issue; however, the current RO process is not viable for regenerating the liquid desiccant solutions due to their extreme osmotic pressure. The MD process has proven to be promising for regeneration of liquid desiccant solution. Nevertheless, further studies are required to address the issue the polarization effects associated with the hyper salinity of liquid desiccant solutions. Finally, the ED process demonstrates its technical feasibility for liquid desiccant solution regeneration because it can regenerate liquid desiccant solutions without the need for heating the feed solution and extreme applied hydraulic pressure. Research into the process energy efficiency and the recycle of spent solution is necessary for the commercialization of the ED process for regeneration of liquid desiccant solutions. Acknowledgement This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 105.08-2019.08. REFERENCE [1]. Yon HR, Cai W., Wang Y., and Shen S. (2018). Performance investigation on a novel liquid desiccant regeneration system operating in vacuum condition, Applied Energy, 211, 249-258. [2]. Gurubalan A., Maiya MP., and Geoghegan PJ. (2019). A comprehensive review of liquid desiccant air conditioning system, Applied Energy, 254, 113673. [3]. Duong HC, Hai FI, Al-Jubainawi A, Ma Z, He T, and Nghiem LD. 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