Yongjun Choi

Application of response surface methodology (RSM) in the optimization of dewetting conditions for flat sheet membrane distillation (MD) membranes

AbstractThis study focuses on the exploration of the optimum dewetting conditions to remove water in pores in membrane distillation (MD) process. Response surface methodology (RSM) was applied to build statistical models for the analysis of flux and liquid entry pressure (LEP) as a function of dewetting temperature and time. A set of MD experiments based on central composite design of experiments method are carried out. Using these experimental results, two response surface (RS) models were developed to predict flux and LEP. The RS models were further used to optimize the dewetting conditions for desired membrane properties. Moreover, the optimum dewetting condition derived from RSM was experimentally verified.

Comparison of fouling propensity and physical cleaning effect in forward osmosis, reverse osmosis, and membrane distillation

AbstractMembrane-based desalination technologies including reverse osmosis (RO), forward osmosis (FO), and membrane distillation (MD) hold promise as efficient methods to produce fresh water from saline water sources. However, the fouling properties of these membranes are quite different due to the difference in the driving forces among them. Accordingly, the objective of this study is to compare fouling behavior and physical cleaning efficiency for these three membranes under similar operating conditions. Colloidal silica and alginate were used as model foulants and NaCl was added to feed solutions. Laboratory-scale experiments were carried out to compare fouling rates and recovery of flux by physical cleaning. Results showed that fouling propensity was the highest in FO membrane and the lowest in MD membrane, which may be attributed to the effect of cake-enhanced concentration polarization. On the other hand, physical cleaning was more efficient to recover flux in FO and RO than in MD, suggesting that t...

Evaluation of economic feasibility of reverse osmosis and membrane distillation hybrid system for desalination

AbstractThis study seeks to evaluate the economic feasibility of membrane distillation (MD) and reverse osmosis (RO)-MD hybrid system for seawater desalination. A theoretical cost model was applied to analyze the effects of flux, recovery, membrane properties, and energy price on RO, MD, and RO-MD hybrid system. The simulation results showed that MD stand-alone system and RO-MD hybrid system can be cost-competitive compared with RO systems when the recovery and flux of MD system are higher than those of RO system and the steam cost is relatively cheap. It is also revealed that the water costs of RO-MD hybrid system and RO system are same under similar operating conditions, but the water cost of MD stand-alone system is higher. The effect of thermal energy cost on water cost for MD and RO-MD systems was also analyzed. Based on these results, guidelines for an analysis of economic feasibility of MD and RO-MD were suggested.

Hydrophobic surface modification of membrane distillation (MD) membranes using water-repelling polymer based on urethane rubber

AbstractMembrane distillation (MD) is a unit process that uses hydrophobic membranes to separate vapor from saline water. The performance of the MD process is largely affected by the properties of the membranes, which should be porous, hydrophobic, and stable under high temperature conditions. Accordingly, it is essential to develop highly efficient membranes for practical implementation of MD technology. In this study, we applied a water repellent chemical (WRC) made of urethane rubber onto hydrophilic membranes to develop a novel approach for MD membrane preparation. A spin coating method was adopted to introduce hydrophobic coating layers on polyamide membranes. Experiments were carried out in the direct contact membrane distillation mode. Contact angle and liquid entry pressure (LEP) were measured before and after the surface coating. In addition, scanning electronic microscope, FT-IR, and atomic force microscope analysis were conducted to confirm a coating layer of the membrane. The optimum condition...

Theoretical and experimental investigation of boron rejection by forward osmosis membrane

AbstractThis study focused on the investigation of the boron rejection from model solutions using a forward osmosis (FO) membrane. A laboratory-scale plate-and-frame membrane module and stirred cell device were used for the experiments. The boron rejection were examined in both reverse osmosis and FO operating modes. A theoretical model based on the film theory was applied for the analysis of boron rejection. Experimental results indicated that the boron rejection by FO membranes was improved by raising pH, suggesting that the removal of boron is done either by charge repulsion or by size exclusion. FO treatment of feedwater containing a high proportion of boric acid led to relatively high concentration of boron in the permeate water. This implies that FO membrane may require another process to attain sufficient rejection to boron. The model calculation was also compared with the experimental data for better understanding of boron rejection in FO membrane systems.

Economic evaluation of the reverse osmosis and pressure retarded osmosis hybrid desalination process

AbstractThis paper presents a performance evaluation and economic analysis of a reverse osmosis (RO) and pressure retarded osmosis (PRO) hybrid process to propose guidelines for its economic competitiveness use in the field. A model to predict the performance of a hybrid process using RO and PRO was developed based on a solution-diffusion model modified with film theory. The effects of external and internal concentration polarization (ICP) on PRO efficiency were considered in the model. Moreover, a simple cost model was applied to analyze the effects of seawater TDS and feedwater for the PRO process, water and salt permeability of PRO membrane, and membrane and energy cost on the RO-PRO hybrid process. The results show that the water transport coefficient, ICP, and seawater and feedwater TDS are important factors affecting the performance of the PRO process. On the other hand, the effect of the salt transport coefficient is not substantial. The RO-PRO hybrid process can be economically competitive with th...

Comparison of hollow fiber membranes in direct contact and air gap membrane distillation (MD)

AbstractHollow fiber microporous membranes was used for seawater desalination in direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD) configurations. The efficiencies of hollow fiber membranes with different membrane materials and pore size were compared in a laboratory-scale DCMD system. The water flux and salt rejection were shown to be different depending on the membrane properties. The apparent permeabilities for water vapor and NaCl ion were estimated to determine the optimum membrane distillation (MD) membranes. A novel AGMD membrane module utilizing hollow fiber membranes was designed, and its prototype was fabricated using a 3D printer. Using this AGMD module, the performance of the hollow fiber membrane was examined and compared with the results from DCMD experiments. The water flux in AGMD configuration was less than 40% of that in DCMD configuration due to high mass transfer resistance and insufficient cooling plate area. However, it holds promise for practical a...

The use of ultrasound to reduce internal concentration polarization in forward osmosis

Unlike reverse osmosis (RO) that is dominated by the hydraulic pressure differential, forward osmosis (FO) uses the osmotic pressure gradient as the driving force between a dilute feed solution and a concentrated draw solution across a membrane. High pressure is not required in FO, which means that FO can be used as an alternative to RO as an energy-saving separation process in desalination technology. However, a major limiting factor of the FO process is the internal concentration polarization (ICP). Because of the stagnant environment inside the porous supporting layer of a FO membrane, it is difficult to mitigate the ICP by simply increasing the shear stress or promoting turbulence. In this study, the ICP is reduced by ultrasound. The effect of the ultrasound frequency and output power on the ICP coefficient is investigated in a flat-sheet FO membrane module with counter-current flow. The ultrasound frequency and output power are varied between 25, 45, and 72 kHz and over the range of 10-70 W, respectively. NaCl solution is used as both the feed and draw solution. The results illustrate that moderate ultrasonic irradiation is effective for reducing the ICP in a FO process. A modified solution-diffusion model based on film theory is used to assess the effect of ultrasound on the ICP in a FO process. The ICP coefficient is estimated using this model.

Influence of microbial floatation on membrane fouling due to particles and organic matters in submerged microfiltration

AbstractThis study investigated the effect of microbubble floatation on the fouling of microfiltration membranes. Synthetic feed solutions containing either kaolin or alginic acid were used for the membrane filtration tests. A dissolve air flotation system using a pump-type microbubble generator was adopted as a pretreatment for microfiltration system. A multi-array submerged membrane filtration system was used to monitor the changes in transmembrane pressure with time under various operating conditions. Turbidity removal efficiency, particle counts, and fouling rate for the microfiltration membrane were measured after the microfiltration floatation. The effect of coagulant dose on the treated water quality and membrane performance was also investigated. Results showed that microbubble floatation without coagulant was effective to reduce membrane fouling by kaolin but it was not very effective to control fouling by alginate. With the aid of coagulant, microbubble flotation could control fouling by alginat...