Numerical study on performance and efficiency of batch submerged vacuum membrane distillation for desalination
Graphical abstract
Introduction
Arising from population growth, environmental pollution and uneven potable water distribution in the entire world, freshwater demand is increasing rapidly and intimidating mankind to discover more sustainable and eco-friendly desalination technologies for freshwater production. It is even more challenging for most of space-constrained arid regions as natural freshwater sources are rapidly exploited. The ocean, which is the only nondepletable water source could be a highly potential alternative source. Therefore, simple and space-saving desalination system is a promising alternative to overcome water scarcity problem in the remote areas.
Membrane distillation (MD) is the most promising sustainable and cost-effective thermal-driven separation technology by means of microporous hydrophobic membrane to produce distilled water from seawater (Meng et al., 2015). The volatile vapor molecules are evaporated and diffused through the membrane to the permeate side as a result of the partial vapor pressure gradient. MD requires lower operating temperature than conventional thermal distillation process such as multi-stage flash (MSF) and multi-effect distillation (MED). MD technology is capable of treating high saline water without compromising excellent salt rejection as it is not affected by the osmotic pressure gradient (Safavi and Mohammadi, 2009; Zou et al., 2018; Chang et al., 2020). However, the requirement of thermal energy makes MD less attractive unless renewable energy such as solar energy or waste heat is employed (Mericq et al., 2011; Khayet, 2013; Triki et al., 2017; Lu et al., 2019). The low operating temperature requirement creates an alternative for the integration of MD with renewable energy which could make the system more sustainable and less energy intensive.
There are a few literatures reported that vacuum membrane distillation (VMD) process consumes lower energy and produces higher flux than direct contact membrane distillation (DCMD) (Criscuoli et al., 2008; Cerneaux et al., 2009; Meng et al., 2015). The vacuum created in the permeate side deaerates membrane pores. This helps to reduce the mass transfer resistance of water vapor and improve permeate flux, at the same time reducing the operation time and maintenance costs. However, energy consumption remains high as conventional VMD configuration requires high energy to circulate the huge volume hot feed solution across the membrane module with housing. Moreover, the conventional configuration provides external heat supply and might induce heat loss during pumping the feed solution, consequently limiting the thermal efficiency of the system.
These drawbacks can be eliminated by immersing hollow fiber membranes within the feed tank. The submerged MD is easy to be installed and constructed in a space-saving design, especially in remote area due to the absence of external module housing. The submerged MD does not require continuous feed pumping as the feed solution is stored in the feed tank during operation. Therefore, the operating cost and heat loss as a result of pumping continuous feed are insignificant in submerged MD. In addition, installing heater in the feed tank for direct heat transfer can minimize heat losses in submerged MD system and enhance the feed flow across the membranes boundary layer through the natural convection currents by the heater, consequently ensuring uniform feed temperature and better total heat stored in the tank. These advantages render the future development of submerged MD in larger scale (Gryta, 2020).
Recently, the feasibility of submerged DCMD (S-DCMD) system has been widely reported in several studies yet the submerged VMD (S-VMD) system is still very limited (Francis et al., 2015; Choi et al., 2017; Gryta, 2020). Gryta (2020) analyzed that the process efficiency of the S-DCMD capillary modules without external housing was comparatively high with conventional DCMD modules. Another work also showed that the permeate flux of S-DCMD system that employed commercial polytetrafluoroethylene (PTFE) hollow fiber membrane is comparable with conventional DCMD process when the hot-cold streams are in co-current direction (Francis et al., 2015). Choi et al. (2017) reported that the experimental permeate flux in S-VMD at 50 mbar using ultrapure water as feed solution had achieved 77% higher than S-DCMD due to higher driving force by vacuum enhancement. Therefore, in this paper, S-VMD is chosen to evaluate the feasibility of its performance under different operating conditions.
Furthermore, there have been several studies have focused on the energy efficiency of conventional VMD configuration, but very limited in S-VMD (Summers et al., 2012; Summers and Lienhard, 2013; Shim et al., 2014). Generally, energy efficiency of thermal desalination systems is measured quantitatively by gained output ratio (GOR). Most of the GOR reported in bench scale conventional VMD literatures were below unity. Summers et al. (2012) discovered that single stage conventional VMD that employed large membrane area has poor heat recovery from permeate vapor as it is limited by the low saturation temperature of condensation due to the extremely low pressure. Hence, it is difficult to rise the brine to high temperature in the condenser. GOR obtained was still below unity in the multi-stage conventional VMD with brine recycle ratio of 6.0 due to the absence of latent heat recovery (Shim et al., 2014). Besides, Meng et al. (2015) reported S-VMD system for inland desalination has lower energy consumption than conventional VMD configuration. However, this study is lack of GOR or other efficiency analysis. Therefore, it is notably interesting to investigate and quantify the energetic performance of S-VMD system, which could potentially be alternative to conventional VMD systems.
However, the drawback of submerged MD is the induction of temperature and concentration polarization due to the difficulty of achieving high flow turbulence on the feed side. Several works that employed mechanical agitation have been recently reported in submerged MD process to reduce the negative polarization effects by generating forced convection to further enhance the flow turbulence at feed-membrane interface (Meng et al., 2015; Julian et al., 2016, 2018a, b; Zhong et al., 2018; Zou et al., 2018; Gryta, 2020). In fact, assembling the heater in the submerged MD module also allows a limiting mixing effect on feed flow across the membrane bundle by natural convection. According to Gryta (2020), the efficiency of multiple U-shaped S-DCMD membrane module and thermal efficiency could be improved by over 20%, respectively with the presence of mechanical mixing in the feed tank. However, most of the previous published works only reported the effect of different flow turbulence approach on heat transfer qualitatively. Quantitative analysis of heat transfer on submerged MD is meagre. Therefore, in this work, feed recirculation was employed to quantitatively study the heat transfer coefficient and temperature polarization across the membrane boundary layer in the S-VMD system.
Numerical simulation is important to predict the permeate flux across the membrane based on the integrated heat and mass transfer as well as unmeasurable temperature effect. Nonetheless, only few literatures are available on the numerical study of S-VMD (Julian et al., 2018a, b; Dong et al., 2019; Zou et al., 2019). Zou et al. (2019) developed an iterative simulation to investigate the heat transfer improvement effect by introducing different turbulent modes in submerged vacuum membrane distillation crystallization (S-VMDC) system. It was discovered that the system is feasible to produce pure water and recover salt crystals from high saline water. Julian et al. (2018a, b) incorporated calcium carbonate scaling mechanism model with CFD numerical simulation to study the polarization effects on the scaling rate in S-VMDC process. Simulation results from the work of Dong et al. (2019) found that the low module aspect ratio (tank height/tank diameter) of S-VMD design encourages the industrial scale up due to its minimum axial heat loss compared to conventional DCMD and VMD configurations. The large feed tank diameter (>10 cm) could increase flux stability due to the improved heat stored in the tank. However, qualitative and quantitative numerical study which focused on process efficiency of S-VMD remains scarce.
In this paper, the vapor flux and energy efficiency of S-VMD system were thoroughly investigated. The main objective of this work is to model the S-VMD system and perform the investigation on its energy efficiency under different operating parameters. In order to study the feasibility of the performance of batch type S-VMD in desalination, the S-VMD model had been simulated and verified with bench scale experimental results.
Section snippets
Mass transfer mechanism in S-VMD
The mass transfer mechanism through a microporous membrane in MD application can be generally categorized as Knudsen, molecular and Poiseuille flow diffusion (Zhang et al., 2013). The Knudsen number ():is used to determine the dominant mass transfer mechanism through the membrane pores. is the mean free path of the water vapor molecules () and is the average pore size of the membrane (). Assuming that only water vapour diffuses through the membrane pores, the driving force for
Membrane and materials
The commercial hollow fiber membranes (Accurel PP S6/2) with porosity of 73%, outer diameter of 2.7 mm, inner diameter of 1.8 mm, wall thickness of 450 μm and nominal pore size of 0.2 μm, were supplied by 3 M Deutschland GmbH, Germany. The feed solution used in the S-VMD system is 35,000 mg/L saline water.
S-VMD experiments
The batch operated S-VMD experimental rig is illustrated in Fig. 2. Each membrane bundle contains 4 hollow fibers in 25 cm length with total effective area of 0.00848 m2. The fibers were
Analysis of system performance at different feed temperature and circulation flow rates
The system performance was simulated for different feed temperatures and circulation flow rates. The simulated results were then compared with the experimental results. As seen in Eq. (3), the flux is highly dependent on the feed temperature, which contributed to the partial pressure for vapor diffusion. There is a linear relationship between the flux and temperature in S-VMD as observed in Fig. 3(a). This phenomenon is due to the increased vapour pressure in the feed side as feed temperature
Conclusions
This work aims to simulate the performance of submerged vacuum membrane distillation (S-VMD) under various operating conditions. The approach developed in this paper provides a comprehensive understanding of the interaction of operating parameters (feed temperature, circulation flow and permeate pressure) on S-VMD performance in term of water productivity and energy efficiency. The model was found to be agree well with the experiment data, with the errors less than 5%.
Among the investigated
Conflict or interests
None declared.
Acknowledgement
The authors are thankful to the financial support provided by Transdisciplinary Research Grant Scheme (TRGS) (TRGS/1/2018/USM/01/5/2) (203.PJKIMIA/67612002), Ministry of Higher Education Malaysia.
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