Concentration polarization in membrane distillation: I. Development of a laser-based spectrophotometric method for in-situ characterization

Challenges and advancements in membrane distillation crystallization for industrial applications

Membrane distillation crystallization (MDC) is an emerging hybrid thermal membrane technology that synergizes membrane distillation (MD) and crystallization, which can achieve both freshwater and minerals recovery from high concentrated solutions. Due to the outstanding hydrophobic nature of the membranes, MDC has been widely used in numerous fields such as seawater desalination, valuable minerals recovery, industrial wastewater treatment and pharmaceutical applications, where the separation of dissolved solids is required. Despite the fact that MDC has shown great promise in producing both high-purity crystals and freshwater, most studies on MDC remain limited to laboratory scale, and industrializing MDC processes is currently impractical. This paper summarizes the current state of MDC research, focusing on the mechanisms of MDC, the controls for membrane distillation (MD), and the controls for crystallization. Additionally, this paper categorizes the obstacles hindering the industrialization of MDC into various aspects, including energy consumption, membrane wetting, flux reduction, crystal yield and purity, and crystallizer design. Furthermore, this study also indicates the direction for future development of the industrialization of MDC.

Assessment of pilot direct contact membrane distillation regeneration of lithium chloride solution in liquid desiccant air-conditioning systems using computer simulation

Membrane distillation (MD) has been increasingly explored for treatment of various hyper saline waters, including lithium chloride (LiCl) solutions used in liquid desiccant air-conditioning (LDAC) systems. In this study, the regeneration of liquid desiccant LiCl solution by a pilot direct contact membrane distillation (DCMD) process is assessed using computer simulation. Unlike previous experimental investigations, the simulation allows to incorporate both temperature and concentration polarisation effects in the analysis of heat and mass transfer through the membrane, thus enabling the systematic assessment of the pilot DCMD regeneration of the LiCl solution. The simulation results demonstrate distinctive profiles of water flux, thermal efficiency, and LiCl concentration along the membrane under cocurrent and counter-current flow modes, and the pilot DCMD process under counter-current flow is superior to that under cocurrent flow regarding the process thermal efficiency and LiCl concentration enrichment. Moreover, for the pilot DCMD regeneration of LiCl solution under the counter-current flow, the feed inlet temperature, LiCl concentration, and especially the membrane leaf length exert profound impacts on the process performance: the process water flux halves from 12 to 6 L/(m²·h) whilst thermal efficiency decreases by 20% from 0.46 to 0.37 when the membrane leaf length increases from 0.5 to 1.5 m.

Rejection of trace organic compounds by membrane processes: mechanisms, challenges, and opportunities

This work critically reviews the application of various membrane separation processes (MSPs) in treating water polluted with trace organic compounds (TOrCs) paying attention to nanofiltration (NF), reverse osmosis (RO), membrane bioreactor (MBR), forward osmosis (FO), and membrane distillation (MD). Furthermore, the focus is on loopholes that exist when investigating mechanisms through which membranes reject/retain TOrCs, with the emphasis on the characteristics of the model TOrCs which would facilitate the identification of all the potential mechanisms of rejection. An explanation is also given as to why it is important to investigate rejection using real water samples, especially when aiming for industrial application of membranes with novel materials. MSPs such as NF and RO are prone to fouling which often leads to lower permeate flux and solute rejection, presumably due to cake-enhanced concentration polarisation (CECP) effects. This review demonstrates why CECP effects are not always the reason behind the observed decline in the rejection of TOrCs by fouled membranes. To mitigate for fouling, researchers have often modified the membrane surfaces by incorporating nanoparticles. This review also attempts to explain why nano-engineered membranes have not seen a breakthrough at industrial scale. Finally, insight is provided into the possibility of harnessing solar and wind energy to drive energy intensive MSPs. Focus is also paid into how low-grade energy could be stored and applied to recover diluted draw solutions in FO mode.

Impact of Operating Conditions on Measured and Predicted Concentration Polarization in Membrane Distillation

Concentration polarization (CP) occurs in almost all membrane-based separation processes. In this study, the concentration profile of the dissolved salt has been accurately characterized using a previously developed laser-based spectrophotometric method which had a spatial resolution of 4.5 μm. The objective of the current work was to probe the concentration profile of the solute and analyze the impact of operating parameters, such as feed concentration, hydrodynamic conditions, and feed temperature, on the solute concentration profile in the boundary layer. This study also examined the validity of the conventional approach, where semi-empirical models are used to estimate the boundary layer thickness (BLT) and concentration polarization coefficient (CPC)-based on experimental results. Nusselt correlations were developed specifically for the membrane cell and validated through experimental observations at the operating conditions used in this study. A key finding of this study is that the conventional approach of estimating the effect of CP severely underpredicts the BLT and CPC. The results of this study highlight the need to develop new methods to estimate the BLT and CPC as the conventional approach of using semi-empirical Nusselt and Sherwood correlations does not agree with experimental observations obtained for a membrane distillation system employed in this study.