Hollow Fiber Reverse Osmosis Research Articles

Improvements of a hollow fiber reverse osmosis desalination model: Analysis of numerical results

In this work, a rigorous model describing the processes taking place in hollow fiber modules for reverse osmosis desalination is analyzed. The Kimura–Sourirajan model is used for describing transport phenomena through the membrane. The concentration polarization phenomenon is mathematically described using the film theory, while the Hagen–Poiseuille and Ergun equations describe the pressure drop in the fiber bore and on the shell side of the fiber bundle, respectively. Improving the previous model, in this work the salt concentration of the permeate accumulated along the fiber is calculated from appropriate mass balances. Hence, the osmotic pressure and the water and salt fluxes through the membrane that depend on this concentration change through the module; and it also influences indirectly the calculation of other process parameters. The solutions of all the differential equations involved in the model are accurately approximated by the finite differences method applied over an appropriate discretization. The value of the output variables changes less than 1% when the finite difference mesh is increased from 6 to 7 grid points in the range of each domain, axial and radial. The flow rates and salt concentrations profiles obtained by the proposed model are analyzed. The influences of the transmembrane and osmotic pressures over the permeate flow rates and salt concentrations are studied. The effect of incorporating the accumulated permeate salinity is showed. It is proved that errors committed by ignoring the permeate accumulated salinity can be significant. Sensitivity analysis for the permeate flow rate and permeate salt concentration is performed by studying the influence of different kind of data: input variables, physical coefficients and design variables.

Study of mass transfer characteristics for a hollow fiber reverse osmosis module

The reverse osmosis separation process has become important as an energy saving (no phase change) process for brackish water and seawater desalination. Several analyses on hollow fiber reverse osmosis modules in which permeate water flows from outer side to inner side of hollow fiber bore have been attempted over the past twenty years. An FCP Model (Friction Concentration Polarization Model) was previously presented that showed good fit with a wide range of actual performance data on a hollow fiber reverse osmosis module. However, this analytical model contained the anomaly that the simulated boundary layer thickness is larger than the apparent length of the clearance among hollow fibers. Here, we propose a hollow Fiber Group Model that is consistent with the features of the hollow fiber arrangement of the practical reverse osmosis module. As a result of verifying this model, it was confirmed that the boundary layer thickness on the surface of the fiber group is smaller than the length of clearance between the fiber groups. Consequently the anomaly was broken off by this Fiber Group Model.

A speculative hybrid reverse osmosis/electrodialysis unit operation

We have considered a speculative water desalination concept that combines high-pressure reverse osmosis (RO) and electrodialysis (ED) in intimate proximity to each other. In the concept model’s configuration, the RO membranes are hollow-fibers that form the spacer channels between the alternating anion and cation exchange membranes. The entire system is under high-pressure (as needed for the pressure-driven RO process) and produces two main streams, an RO-quality permeate and a concentrated brine. The schematic utilizes an internal recycle between the alternate ED flow channels. The anticipated process advantages are that some salt is removed from the RO concentrate—by ED—during the pressure-driven filtration and that electrophoretic flow also helps reduce concentration polarization. Both of these serve the purpose of lowering the osmotic pressure at the RO membrane interface and saving energy (and/or increasing the recovery of pure water per unit energy input). Our hybrid approach is conceptually superior to operating an ED step followed by RO because the energy expenditure for the ED (in the hybrid case) is lower—there are higher salt concentrations in all the channels, which lowers the overall cell pair resistance. The figure-of-merit is the W·h·L −1 expended to produce RO-quality permeate. This initial process analysis focuses on a single electrolyte salt, NaCl, and calculated the energy savings for several operating conditions. The process parameters include the overall transport properties of the ion-exchange (IEX) and RO membranes; the recovery of feed as permeate; the concentration of various streams; and the RO membrane area per unit area of IEX membrane. Many conditions have been identified for which energy savings from 10 to 20+% can be realized. We also include initial measurements of NaCl and water transport figures-of-merit through a number of IEX membranes and a polyaramid-based RO hollow fiber, in order to consider the viability of current membrane materials for enabling the proposed, novel process concept. Our overall analysis indicates that the conductivity of the ion exchange membranes becomes the limiting factor on the possible energy savings.

Modeling of a radial flow hollow fiber module and estimation of model parameters for aqueous multi-component mixture using numerical techniques

A mathematical model is developed for the separation of two solutes from aqueous solutions in a hollow fibre module. The model combines the extended Spiegler–Kedem model with the film theory model and takes into account the pressure drops in both feed side and permeate side streams. This model is then used for the simulation of the separation of phenol and NaCl in an aqueous solution at different operating conditions and the role of the solute–solute interaction parameter appearing in the extended Spiegler–Kedem model is further investigated. To validate this model, the separation data for a ternary NaCl–KBr aqueous solution are obtained in a hollow fiber reverse osmosis (HFRO) module. Using part of the experimental results for the binary and ternary systems separation, the six unknown membrane parameters of the model, namely the pure water hydrodynamic permeability, the two solute permeabilties, two reflection coefficients and the solute–solute interaction parameter were determined by using an optimization technique—the simplex search. These values are then used to predict and compare the performances at other operating conditions. The theoretically predicted and the experimental values are found to be in excellent agreement.

Global optimal design of reverse osmosis networks for seawater desalination: modeling and algorithm

A novel global optimization algorithm to solve nonconvex problem is used to find the global optimal design of reverse osmosis networks for seawater desalination. The objective is to determine the optimal process design and operating conditions for a given water production. The networks were designed by using hollow fiber reverse osmosis modules. The Kimura–Sourirajan model was used for describing transport phenomena of solute and water transport through the membrane. The concentration polarization phenomenon has been taken into account. It was mathematically described using the film theory. The objective function to be minimized is the cost, which includes capital investment (membrane cost, pumping and energy recovery system, intake and pre-treatment systems, etc.) and operation and maintenance costs (membrane replacement, chemical treatment, spares, required and recovered energy, etc.). The proposed algorithm is deterministic and attains finite convergence to the global optimum. It is iterative and a main problem is solved each iteration. The main problem has convex constraints and a nonconvex objective function. The main problem solution indicates either a better solution for the original problem, or a region which can be discarded. Therefore, the feasible region to improve the objective function is reduced each iteration. The algorithm finishes when the whole region has been analysed and discarded. A bound reduction technique is performed in order to accelerate the convergence speed. The algorithm shows a good performance and efficient execution time. Different cases are solved in order to show the methodology and computational performance.

Modeling of a radial flow hollow fiber module and estimation of model parameters using numerical techniques

A numerical solution is developed for modeling as well as predicting the performance of a radial flow hollow fiber reverse osmosis (HFRO) module. The three-parameter Spiegler–Kedem (S–K) model is used for describing the mass transport across the membrane. The solution proposed considers pressure drops in both permeate and bulk streams and includes concentration polarization. The numerical solution uses finite difference method. Two limiting cases of the system of equations are used to arrive at analytical solutions and check the validity of the numerical solution. The simulation of the above three-parameter membrane model is compared with a two-parameter membrane model and an analytical solution available in literature and the significance of the third membrane parameter, the reflection coefficient, is investigated in detail. It is found that when the reflection coefficient is decreased from its maximum value of one, the permeate flow rates are higher and the permeate concentration differs substantially from that reported by the two parameter membrane model of Sekino [J. Membr. Sci. 85 (1993) 241]. Under the simulation conditions investigated (1> σ>0.9), the maximum difference in the permeate flow rates between the two can be as large as 14% while the differences in the permeate concentrations can be as large as 4431%. The mass transfer coefficient and the membrane model parameters are determined by using an optimization technique—the Simplex Search. The data for phenol separation obtained on a laboratory scale B9 HFRO experimental setup and the experimental data available in literature are used for the estimation of the model parameters as well as the mass transfer coefficient in the B9 module. These estimated parameters are then used to predict and compare performance at some other operating conditions. The theoretically predicted and the experimental values for both present and previous studies are found to be in good agreement. The analysis of the phenol separation data clearly shows the two parameter membrane transport model used in previous studies may not be sufficient for accurate design and analysis of a radial flow hollow fiber system for many solute-membrane systems. Finally, the mass transfer correlation for the shell side of the hollow fiber module is also developed and compared with the correlation available in literature.

Solvent treatment of CTA hollow fiber membrane and its pervaporation performance for organic/organic mixture

Cellulose triacetate (CTA) hollow fiber membrane used to separate methanol/methyl tert-butyl ether (MTBE) by pervaporation (PV) has been prepared from CTA hollow fiber reverse osmosis (RO) membrane for desalination of brackish water with high salinity. Acetone was selected as a modification agent of CTA membrane. PV performance depended on the solvent concentration, the treatment time and modification temperature of CTA RO hollow fiber membrane soaked in the aqueous acetone. The results show that CTA hollow fiber membrane modified with the solvent has a superior performance both the separation factor and the permeate flux in the PV experiment conditions.

Measurement of rheologically induced molecular orientation using attenuated total reflection infrared dichroism in reverse osmosis hollow fiber cellulose acetate membranes and influence on separation performance

The objective of this study is to investigate the pure influence of shear and its capacity to increase the separation performance of cellulose acetate reverse osmosis hollow fiber membranes, produced using a dry/wet spinning process with forced convection in the dry gap. In order to achieve this, the experiments are designed to decouple the effect of extrusion shear from forced convection residence time by fixing the residence time at 0.615 s. RO hollow fibers were spun at various dope extrusion shear rates ranging from 3.0 to 4.5 ml/min corresponding to shear rates of around 13,479–20,219 s −1, respectively, at the outer spinneret wall. Fourier transform attenuated total reflection (ATR) also known as internal reflection spectroscopy was used to probe the membrane active layer so as to examine if high shear rates may induce anisotropy at the molecular level in the active layer. The rejection rates of the membranes were evaluated using salt water. Results revealed that: (i) ATR can be used to determine qualitatively and quantitatively—the degree of molecular orientation in the sheared membranes; (ii) molecular orientation is enhanced as the dope extrusion rate increases; and (iii) there exist an optimum shear rate that can induce a certain degree of molecular orientation to yield membrane morphology with optimum separation performance.

Rheology assessment of cellulose acetate spinning solution and its influence on reverse osmosis hollow fiber membrane performance

Cellulose acetate spinning solution used to produce reverse osmosis (RO) hollow fiber membranes was rheologically assessed using a rotational rheometer and an optical shear cell. Rheology measurements which involved flow curves were carried out so as to obtain the values of power law coefficients, n and k. The power law behaviour, normal force and flow profiles generated provided clues regarding phase inversion and molecular orientation. These rheological results are then correlated to the performance of cellulose acetate RO hollow fibers spun at different extrusion shear rates. The results suggest that extrusion shear is linked indirectly to phase inversion through induced molecular orientation, which in turn, affects the subsequent dry/wet precipitation stages in spinning. As the extrusion shear rate increases, the level of shear experienced at the walls of the spinneret also increases, thus leading to greater molecular orientation, resulting in membranes with higher rejection and flux rates.

The deduction of fine structural details of reverse osmosis hollow fiber membranes using surface force–pore flow model

In order to elucidate the relationship between the dope extrusion shear rate and membrane performance, sodium chloride transfer through the asymmetric cellulose acetate reverse osmosis hollow fiber membranes is modeled, allowing fine details of the fiber structure to be deduced from the NaCl-H2O rejection characteristics. The structural information such as the pore size radius and skin thickness of the active layer deduced from the sodium chloride separation experimental data and surface force-pore flow model (SF-PF) is then used to interpret the relationship between the rheological conditions during spinning and membrane performance. The modeling results revealed that increased extrusion shear rate would decrease both pore size and thickness of the active layer, thus increasing the separation performance of the RO hollow fiber membranes.

Optimization of cellulose acetate hollow fiber reverse osmosis membrane production using Taguchi method

Cellulose acetate hollow fiber membranes for reverse osmosis (RO) were spun using a forced convection technique. In this study, a systematic experimental design based on Taguchi’s method (which is a fractional factorial method) has been employed for discussing the relationship between the rejection rate coefficient, permeation rate and the dry–wet spinning conditions for making cellulose acetate hollow fibers for RO. The factors considered in the experimental design included the polymer contents (PCs), the ratio of the solvent (acetone) to swelling agents (formamide) in the dope solution, the dope extrusion rate (DER), the type of bore fluid (BF), the residence time (RT) and the nitrogen gas flushing rate (GR). The results indicate that the BF and the DER are the two most important factors in determining the performance of the RO membranes.

Study of shear rate influence on the performance of cellulose acetate reverse osmosis hollow fiber membranes

The effect of shear rate on the separation performance of reverse osmosis hollow fiber membrane is discussed. Experiments involving six different dope extrusion rates (DERs) (ranging 2.5–5 ml/min) are performed with the other process factors set at the optimum conditions determined by the Taguchi analysis. This will enable an assessment to be made on the relationship between the DER and the rejection rate. The regression method is used to analyse the experimental results and an empirical model has been developed. Simultaneously, it is found that there is a fairly strong correlation between extrusion shear rate and the rejection rate of the membranes, whereby as the shear rate increases, the rejection rate increases until a critical level of shear is achieved, beyond which reverse osmosis membrane performance deteriorates, suggesting that there exists an optimum shear rate which yields optimal membrane morphology for reverse osmosis hollow fiber membranes.

Seawater Desalination Technology by Hollow Fiber Reverse Osmosis Module.

Seawater Desalination Technology by Hollow Fiber Reverse Osmosis Module.

New chlorine-resistant polyamide reverse osmosis membrane with hollow fiber configuration

New asymmetric hollow fiber reverse osmosis (RO) membrane was developed from a new chlorine-resistant copolyamide [4T-PIP(30)] with a piperazine moiety by a conventional phase-separation method. The new 4T-PIP(30) hollow fiber membrane has the same low-pressure RO performance as cellulose triacetate hollow fiber membrane (FR = 205 L/m2 day, Rj = 99.6%) and superior chlorine resistance as well as pH resistance to conventional aramid RO membranes. Structural analysis and viscoelastic study revealed that the new hollow fiber consisted of a top skin, dense layer, and microporous layer, and that it began to decrease its elasticity at 80°C in water, which is possibly related to its good and stable RO performance around room temperature. Several kinds of RO modules were made from the new hollow fiber membranes, for which RO performances were stable for 2 years in chlorinated feed water desalination (the free residual chlorine ranged from 0.l to 1.1 mg/L). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 517–527, 2001

Newly Developed Economical Sea Water Desalination System by Hollow Fiber Reverse Osmosis Module

Newly Developed Economical Sea Water Desalination System by Hollow Fiber Reverse Osmosis Module

Analysis and Simulation of Hollow-Fiber Reverse-Osmosis Modules

Abstract The purpose of this work is to analyze the performance of hollow-fiber reverse-osmosis (HFRO) systems. First, the mass and momentum balances on the fluids inside and outside of the hollow fibers are derived. Consequently, these equations are coupled with the membrane transport expressions to provide a two-phase model for completely describing the hydrodynamic behavior of HFRO units. The model equations are solved numerically via discretization techniques. Experimental data are used for two objectives; transport-parameters estimation and model validation. Based upon optimization principles, a parameter-estimation technique is proposed for correlating the membrane-transport coefficients with the operating pressures and concentrations. The model validity is elucidated by comparing the theoretical predictions with the experimental data for different HFRO modules and over a wide range of operating parameters.

Performance optimization of hollow fiber reverse osmosis membranes. Part II. Comparative study of flow configurations

Sensitivity evaluation of overall performance of hollow fiber membranes was performed to study the effects of such operating parameters as pressure, packing density, and fiber diameter. It is shown that in a wide range of operating conditions, fiber productivity and selectivity as dependent upon hollow fiber length exhibit a similarity property. This is demonstrated in all three flow configurations of concurrent, countercurrent, and flow inside hollow fibers.

Performance optimization of hollow fiber reverse osmosis membranes, part I. development of theory

The overall performance of hollow fiber membranes can be engineered based upon the interplay of fiber productivity and fiber selectivity. Fiber length and diameter can be optimized to render the desired balance of fiber performance. Fundamental analytical models are developed for feed outside (concurrent and countercurrent flow configurations) and feed inside. Part II of this series of papers will evaluate the sensitivity of fiber performance to such operating conditions as pressure, packing density, and fiber diameter.

Study of an analytical model for hollow fiber reverse osmosis module systems

A number of investigators have made efforts to develop various analytical models for hollow fiber type reverse osmosis (RO) module systems since the 1970s. However, a perfect analytical model, which can precisely explain the observed RO performances under a wide range of operating conditions has not been developed yet. The author previously proposed a precise analytical model called a friction-concentration-polarization model (FCP model) [1], which used the Kimura-Sourirajan model for transport phenomena of solute and water transport through a membrane, taking a mass transfer coefficient as local variables and taking a fiber-bore side pressure drop into account. In the application of this model, fundamental transport parameter of hollow fiber membranes were needed, and they were initially determined by a U-tube membrane test where the effect of concentration polarization could be neglected. Then a local mass transfer coefficient was estimated from experimental data using actual modules as a function of Reynolds and Schimidt numbers by a trial-and-error method for both brackfish water and seawater desalination cases. Using all of the above results, behaviors of hollow fiber modules under various operating conditions were estimated and compared with the results obtained from commercial size module experiments. Compared to other previous models, the FCP model is verified to be the best one to predict actual module performances. This model will be further extended to anlyze a change of transport parameters during long-term runs.

Precise analytical model of hollow fiber reverse osmosis modules

A highly reliable and detailed analytical model for the hollow fiber reverse osmosis module is reported. The model introduced concentration polarization into the transfer equations, which are usually neglected, combined with the pressure drop in the fiber bore. In this model, friction-concentration-polarization model (FCP model), the membrane transport equations were derived from the solution-diffusion model and the concentration polarization model while the pressure drop in the fiber bore was introduced with the Hagen-Poiseuille equation. The membrane permeability constants in the solution-diffusion model were experimentally determined. The mass transfer coefficients in the concentration polarization model were calculated by previously derived equations. The model was verified by reverse osmosis using Toyobo's hollow fiber modules for both brackish water and seawater desalination, and showed good agreement.