Elsevier

Energy

Volume 194, 1 March 2020, 116891
Energy

An experimental study on liquid regeneration process of a liquid desiccant air conditioning system (LDACs) based on vacuum membrane distillation

https://doi.org/10.1016/j.energy.2019.116891Get rights and content

Highlights

  • A liquid regeneration method by vacuum membrane distillation is proposed for LDACs.

  • The effects of various influence parameters on regeneration ability were studied.

  • The mathematical model of membrane flux was optimized, and its accuracy was verified.

Abstract

In this paper, a liquid regeneration method by vacuum membrane distillation (VMD) is proposed for the liquid desiccant air conditioning system (LDACs), and the experimental study on this method is carried out. VMD regeneration experiments were carried out with LiCl solution. The effects of temperature, concentration of feed solution, length, number of fiber membranes and vacuum pressure on the membrane flux, mass transfer coefficient, rejection rate and regeneration ability were studied. The results show that the error between experimental and calculation results is reduced from less than 15% to less than 5% by the optimized calculation model. The temperature of feed solution has a great influence on the regeneration performance of VMD, and the regeneration ability of VMD process increases approximately exponentially from about 0.1% to 0.8–1.2% with the increase of regeneration temperature. The VMD regeneration process of LiCl solution is the result of Poiseuille flow and Knudsen diffusion, and Poiseuille flow dominates. Under the same regeneration capacity, the regeneration temperature of 30 wt% LiCl solution is about 7 °C higher than that of 20 wt% LiCl solution, and this temperature difference also increases as the target regeneration amount increases. In order to improve the regeneration effect of high concentration solution, the regeneration temperature can be increased appropriately. The water flux of the membrane decreases with the increase of the length of the membrane. The membrane length of Type1 is 2.1 times longer than Type 2, but regeneration capacity of Type 1 is only 1–1.7 times higher than Type 2. Further, both the water flux and regeneration ability of the solution decrease first and then increase with the increasing number of membranes. Therefore, the reasonable selection of number of fiber membranes can significantly save materials and also improve the regeneration ability.

Introduction

Rising comfort requirements and increasing population have led to a significant increase in the energy consumption for air-conditioning systems in the last few decades. Heating, Ventilating & Air Conditioning (HVAC) system consumes over 50% of building energy in hot and humid climates [1]. Liquid desiccant air conditioning systems (LDACs) have shown promising potential and application in refrigeration and air conditioning by lowering energy consumption and improving indoor humidity comfort level [2]. Compared with conventional air conditioners, LDACs has a higher air dehumidification capacity, which can effectively purify the treated air and has almost no pollution to the environment [3]. LDACs are usually composed of collectors, regenerators, dehumidifiers, liquid storage tanks, solution pumps, etc. [4]. The regenerator is one of the most important heat and mass transfer devices in the LDACs [5], and the efficient solution regeneration method can effectively improve the efficiency of the solution dehumidification air conditioning system. Existing thermal regeneration methods can be roughly divided into the air type and the boiling type. The principle of the air type regeneration is usually to contact the air and hot dilute solution in the packed tower, and the air takes away the water in the solution to realize the concentrated regeneration of the solution [6]. The temperature of heat source is relatively low. The concentration and efficiency of solution regeneration in this regeneration method are low, and the regeneration process is susceptible to environmental impact. Yin et al. [4,7] carried out an experimental study on the solution desiccant evaporative cooling air conditioning system of a packed tower regenerator. It was found that the heat source temperature of the regenerator had an important influence on the regeneration performance. The results showed that the internal heat regenerator had a higher regeneration rate and energy utilization efficiency than the traditional adiabatic regenerator. Air-type solution regeneration depends heavily on the surrounding environment. Under high temperature or humid climate conditions, the regeneration effect often fails to meet the needs of dehumidification [8]. In contrast, the boiling type solution regeneration can reduce the dependence of the air conditioning system on the outdoor environment. Boiling type regeneration equipment usually distills its water by boiling the dilute solution. This regeneration method is little affected by environmental factors, but requires a high regeneration temperature.

Membrane distillation (MD) is a membrane separation technology combination of thermal evaporation and membrane separation [9]. The hydrophobicity of the membrane prevents mass transfer of the liquid, where by a gas-liquid interface is created. Vapor pressure difference caused by liquid temperatures on both sides of the membrane or vacuum pump on one side, where by volatile components in the supply mix evaporate through the pores (10 nm –1μm). For feed solutions that only contain non-volatile substances, such as salts, water vapor will be transported through the membrane whereby the demineralised water is obtained on the distillation-side and a further concentrated salt flow on the feed side [10]. In addition, in comparison with Reverse Osmosis (RO), MD is less susceptible to flux limitations caused by concentration polarization, whereby a higher concentration of matter is obtained on the feed side. Theoretically, MD offers about 99% retention for non-volatile dissolved substances, whereby there is no limit of the supply concentration [11]. At present, MD technology has four basic operating modes, namely direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), scavenging membrane distillation (SGMD), and vacuum membrane distillation (VMD) [12]. Among them, the water flux of VMD is usually higher than that of other MDs [13]. In addition, VMD has the advantage of low heat loss, because heat does not pass through the membrane through vacuum. Therefore, the heat transfer through the membrane can be neglected [14,15]. VMD is a process driven by both heat and pressure. The membrane separation of the solute and the solvent is achieved by using the steam pressure difference on both sides of the hydrophobic membrane as the driving force [16]. Compared with the traditional thermal regeneration process, MD not only needs a lower heat source temperature (40–80 °C), but also can be operated stably in high temperature and humidity environments [17].

Membrane distillation can evaporate at a lower temperature (than the boiling temperature), so it can utilize industrial waste heat, geothermal, solar energy and other cheap types of energy [10]. At present, research of membrane distillation technology is basically in the process of seawater desalination and wastewater treatment, but there is a lack of research in LDACs. Membrane distillation regeneration in LDACs is different from seawater desalination in drinking water production, which involves removing only a small amount of water in a high concentration solution. Bodell et al. first introduced membrane distillation in 1963, using microporous hydrophobic membrane evaporation to provide clean water [14]. Duong et al. [18] examined the application of membrane distillation (MD) in the regeneration of the air-conditioned lithium chloride liquid desiccant. The process can increase the concentration of lithium chloride to 29 wt% at the feed temperature of 65 °C without any significant loss of lithium chloride. Zhou et al. [19] studied the regeneration performance under various operating parameters and climatic conditions through model simulation, and compared the solar energy consumption of regenerating 1 kg desiccant between VMD regeneration and TH regeneration. The results showed that VMD regeneration is more suitable for solution regeneration in wet areas than the TH regeneration. Lefers et al. [20]. Described the testing of a vacuum membrane distillation system for liquid dehumidification. It was found that 30 wt% magnesium chloride solutions obtained an average flux of 8 kgm−2h−1 at a temperature of 50 °C. Previous studies have mainly discussed the feasibility of solution regeneration by the membrane distillation technology. However, few studies have been conducted on the influence of different factors on the performance of VMD liquid regeneration. Therefore, in this work, we performed an experimental study to examine the various factors on the VMD liquid regeneration performance and it is believed that our work will certainly promote the application of VMD liquid regeneration in LDACs.

Section snippets

Materials

Membrane material: Fiber membrane is PTFE membrane (manufacturer: Nanjing Bidun New Membrane Co., Ltd., Nanjing, China). The specific parameters of the membrane are shown in Table 1.

Experimental device

The experimental device consists of a VMD module, constant temperature water tank, peristaltic pump, condenser, collection bottle and a water ring vacuum pump. The VMD regeneration system is shown in Fig. 1. The PTFE film assembly is sealed by high temperature resistant epoxy resin. Major equipment and instruments

Optimization of membrane flux calculation

Different mechanism models have been proposed from different perspectives, such as the Knudsen flow model and the viscous flow model, but their application is limited to a certain extent. According to the characteristics of vacuum membrane distillation, some researchers combined the above two models to obtain membrane flux formulas in different ranges.

The diffusion mechanism of the gas in the membrane during VMD regeneration is determined by the mean free path λ of the molecular motion and the

Conclusions

In this paper, a liquid regeneration method by VMD is proposed for the LDACs, and the experimental study on this method is carried out. VMD regeneration experiments were carried out with LiCl solution. The effects of temperature, concentration of feed solution, length, number of fiber membranes and vacuum pressure on membrane flux, mass transfer coefficient, rejection rate and regeneration ability were studied. The specific conclusions are as follows:

  • 1.

    Through analysis, the mathematical model of

Declaration of competing interest

We have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work of this paper is financially supported by the National Natural Science Foundation of China (No. 51520105009), the Scientific Research Foundation of Graduate School of Southeast University (No. YBPY1912), Postgraduate Research & Practice Innovation Program of Jiangsu Province(No. KYCX19_0100). The supports are gratefully acknowledged.

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