Design Of Hollow Fiber Modules Research Articles

Osmotically enhanced reverse osmosis using hollow fiber membranes

Osmotically enhanced reverse osmosis (OERO) effectively increases the water recovery in desalination as it can reduce the transmembrane osmotic pressure by a sweep solution. Utilizing a model based on mass-transfer principles, we report the performance of a hollow-fiber RO membrane module in OERO as a function of the operating conditions, the fiber geometry, and the membrane properties. The hollow fiber system allows the feed and sweep solutions to flow on both sides of the membrane. To avoid energy-intensive solute/water separation, fertilizer can be employed as a “green” sweep solution as it can be directly used for fertigation (fertilized irrigation). Simulations indicated that the water recovery is significantly enhanced by increasing the water permeance and decreasing the structure parameter of the hollow fiber membranes. Further, an investigation into the role of feed salinity suggests that longer fibers can provide a higher water recovery in the case of low-salinity water reuse, while larger-diameter fibers achieve a more efficient seawater desalination. A single-stage OERO process facilitates the enhancement of water recovery from 35.5 % and 14 % to 36.5 % and 34 % in the case of low-salinity and high-salinity desalinations, respectively. This study provides theoretical perspectives into the design of hollow fiber modules for OERO processes.

Fundamental understanding on the preparation conditions of high-performance polyimide-based hollow fiber membranes for organic solvent nanofiltration (OSN)

Integrally skinned asymmetric (ISA) polyimide-based hollow fiber (HF) membranes were prepared via phase inversion followed by chemical crosslinking for organic solvent nanofiltration (OSN). The effect of preparation conditions including air gap length, bore composition, and dope flow rate (expressed as shear rate in the spinneret) on structure, separation performance and solvent stability of the HF-OSN membranes were extensively investigated. Among these conditions, the air gap length has a profound effect on the compactness of the skin layer. The separation performance of the OSN membrane can be effectively tuned by changing the air gap length. The composition of the bore fluid influenced the membrane structure from the synergetic effect of both thermodynamic and kinetic aspects. The variation in elongation ratio due to the swelling of the HF-OSN membrane in different organic solvents is a crucial factor that should be considered during the design of hollow fiber modules for real OSN applications. The shear rate of the spinning solution in the spinneret remarkably impacted the swelling properties and solvent stability of the HF- OSN membrane. Under the optimized shear rate, the prepared HF-OSN membranes had a relatively low swelling tendency in both NMP and DMF and maintained a 99.5% rejection for RDB after 7 days’ immersion in DMF. The above investigation might provide both fundamental understanding for the preparation mechanism of high-performance HF-OSN membranes and insightful guidance for the scaling-up of them.

Evaluation of mass transfer in a novel hollow fiber module design using an electrochemical technique

Abstract The mass transfer coefficient (K L ) determined using an electrochemical technique was used in this work as a parameter to evaluate the hydrodynamics of hollow fiber membrane modules. A new microfiltration module configuration was investigated, taking advantage of the hydrocyclone concept aimed at reducing the concentration of the polarization layer near the membrane surface promoted by the centrifugal field. The mass transfer coefficient for the new configuration was compared with that of a conventional longitudinal module. The experimental determination of K L was obtained by monitoring the electrochemical reactions that occur at the electrode surface under mass transfer-limiting conditions. The performance of the microfiltration modules, both hybrid and longitudinal, was evaluated based on parameters such as packing density and fluid flow regimes. The results achieved for the mass transfer coefficient with the electrochemical technique allowed for performance evaluations of the proposed new module configuration and a comparison with the longitudinal module.

Energy recovery by pressure retarded osmosis (PRO) in SWRO–PRO integrated processes

Pressure retarded osmosis (PRO) is a promising technology to reduce the specific energy consumption of a seawater reverse osmosis (SWRO) plant. In this study, it is projected that 25.6–40.7millionkWh/day of energy can be recovered globally, if the brines from SWRO are used as the draw solution and diluted to the seawater level in a PRO system. Detailed integrated SWRO–PRO processes are developed in this study with the option to form a closed-loop SWRO–PRO process that can substantially reduce the pretreatment cost of desalination. The governing mathematical models that describe both the transport phenomena on a module level and the energy flow on a system level are developed to evaluate the performances of the SWRO–PRO processes. The model aims to investigate the performance of the hollow fibers as dilution occurs and provides guidelines on hollow fiber module design and process operation. Determining the dilution factor and the corresponding operating pressure of PRO is the key to optimize the integrated process. The specific energy consumptions of three SWRO-involved processes; namely, (1) SWRO without a pressure exchanger, (2) SWRO with a pressure exchanger, and (3) SWRO with pressure exchangers and PRO are compared. The results show that the specific energy consumptions for the above three processes are 5.51, 1.79 and 1.08kWh/(m3 of desalinated water) for a 25% recovery SWRO plant; and 4.13, 2.27 and 1.14kWh/(m3 of desalinated water) for a 50% recovery SWRO plant, using either freshwater or wastewater as the feed solution in PRO.

A New Short–Cut Design Method for Ultrafiltration and Hyperfiltration Cross–Flow Hollow Fiber Membrane Modules

Although ultrafiltration and hyperfiltration have replaced many liquid phase separation equipment, both are still considered as “non–unit operation” processes because the sizing of both equipments could not be calculated using either the equilibrium stage, or the rate–based methods. Previous design methods using the dead–end and complete–mixing models are unsatisfactory because the dead–end model tends to underestimate the membrane area, due to the use of the feed concentration in the driving force, while the complete–mixing model tends to overestimate the membrane area, due to the use of a more concentrated rejection concentration in the driving force. In this paper, cross–flow models for both ultrafiltration and hyperfiltration are developed by considering mass balance at a differential element of the cross–flow module, and then integrating the expression over the whole module to get the module length. Since the modeling is rated–based, the length of both modules could be expressed as the product of the height of a transfer unit (HTU), and the number of transfer unit (NTU). The solution of the integral representing the NTU of ultrafiltration is found to be the difference between two exponential integrals (Ei(x)) while that representing the NTU of hyperfiltration is found to be the difference between two hypergeometric functions. The poles of both solutions represent the flux extinction curves of ultrafiltration and hyperfiltration. The NTU for ultrafiltration is found to depend on three parameters: the rejection R, the recovery S, and the dimensionless gel concentration Cg. For any given Cg and R, the recovery, S, is limited by the corresponding flux extinction curve. The NTU for hyperfiltration is found to depend on four parameters: the rejection R, the recovery S, the polarization β, and the dimensionless applied pressure difference ψ. For any given ψ and R, the recovery, S, is limited by the corresponding flux extinction curve. The NTU for both ultrafiltration and hyperfiltration is found to be generally small and less than unity but increases rapidly to infinity near the poles due to flux extinction. Polarization is found to increase the NTU and hence the length and membrane area of the hollow fiber module for hyperfiltration. Key words: Ultrafiltration; hyperfiltration; reverse osmosis; hollow fiber module design; crossflow model; number of transfer unit; height of a transfer unit

Numerical simulation of heat and mass transfer in direct membrane distillation in a hollow fiber module with laminar flow

The heat and mass transfer processes in direct contact membrane distillation (MD) under laminar flow conditions have been analyzed by computational fluid dynamics (CFD). A two-dimensional heat transfer model was developed by coupling the latent heat, which is generated during the MD process, into the energy conservation equation. In combination with the Navies–Stokes equations, the thermal boundary layer build-up, membrane wall temperatures, temperature polarization coefficient (TPC), local heat transfer coefficients, local mass fluxes as well as the thermal efficiency, etc. were predicted under counter-current flow conditions. The overall performance predicted by the model, in terms of fluxes and temperatures, was verified by single hollow fiber experiments with feed in the shell and permeate in the lumen. Simulations using the model provide insights into counter-current direct contact MD. Based on the predicted temperature profiles, the local heat fluxes are found to increase and then decrease along the fiber length. The deviation of the membrane wall temperature from the fluid bulk phase on the feed and the permeate sides predicts the temperature polarization (TP) effect. The TP coefficient decreases initially and then increase along the fiber length. It is also found that the local Nusselt numbers ( Nu) present the highest values at the entrances of the feed/permeate sides. Under the assumed operating conditions, the feed side heat transfer coefficients h f are typically half the h p in the permeate side, suggesting that the shell-side hydrodynamics play an important role in improving the heat transfer in this MD configuration. The model also shows how the mass transfer rate and the thermal efficiency are affected by the operating conditions. Operating the module at higher feed/permeate circulation velocities enhances transmembrane flux; however, the thermal efficiency decreases due to the greater heat loss at a higher permeate velocity. The current study suggests that the CFD simulations can provide qualitative predictions on the influences of various factors on MD performance, which can guide future work on the hollow fiber module design, module scale-up and process optimization to facilitate MD commercialization.

A numerical approach to study the impact of packing density on fluid flow distribution in hollow fiber module

The aim of this study was to analyze the influence of hollow fiber module design, specially packing density, and filtration operating mode on the filtration performance. In order to perform this analysis, a model based on the finite element method was used to simulate numerically the flow and filtration velocity along the fiber. An annular region of fluid surrounding the fiber was considered in order to account for the packing density Φ of the module. The originality of this approach lies in the study of fiber density effect on the hydrodynamic conditions, both for inside/out (IO) and outside/in (OI) filtration modes. The numerical simulations of fluid flow have shown a modification of the axial filtration velocity profile with packing density. When the density of fibers was high, filtration took place preferentially in the bottom of the fiber. In contrast, when the packing density was low, permeate flow was higher at the top of the fiber, i.e. the filtration module. Two experimental hollow fiber modules with two packing densities were tested and showed good agreement with the numerical data. These results underline the variations of filtration velocity along the fiber that will allow some predictions on fouling deposit to be done.

Theoretical analysis on the design of hollow fiber modules and modules cascades for the separation of diluted species

The transport of dilute species through a hollow fiber membrane has occurred in several separation processes referred to in the literature as membrane extraction, supported liquid membrane, supported gas membranes, pervaporation, gas (vapor) permeation, pertraction and permabsorption. In these processes, the mass transfer often occurs with a constant external resistance and a constant shell concentration, or with a constant external resistance and a varied shell concentration resulting from an operational mode of co-current or counter-current. A set of algebraic expressions are derived to design a single module or modules-in-series and predict the operating performance, using the correlations of lumen mass transfer coefficient as described in our previous study [1]. The theoretical analysis demonstrates that modules-in-series are superior to a long single module or modules-in-parallel for a given separation, since it allows an improved lumen mass transfer rate. The simulations show that the equations suggested in this paper can provide a more accurate design than those reported in the literature.

Design of hollow fiber modules for uniform shear elution affinity cell separation.

Large-scale monoclonal antibody based systems for the selection of cell subsets will play a prominent role in the development of hematotherapy and graft engineering. Hollow fiber systems for affinity cell separation rely on the generation of uniform fluid shear stress at the lumenal attachment interface. Potential mechanisms for nonuniformity of lumenal wall shear stress are fiber wall permeation fluxes driven by the pressure gradient along individual fibers and the influence of inlet header dynamic pressure on the radial distribution of axial flow within the fiber module. Dimensional analysis and numerical solution of the flow field within the lumen of a hollow fiber module illustrate the main physical criteria for design of hollow fiber modules. There will be a nearly uniform distribution of flow within the fiber bundle provided that the dynamic inlet pressure is small in comparison with the pressure drop along fibers. Fiber wall permeation fluxes will have a negligible effect on axial flow rate for nonporous membranes such as Cuprophan.

Optimal dimensions of capillary membranes for plasma separation.

The work presented in this article examines the relationship between the efficacy of the design (membrane consumption) and the design parameters for plasma separation modules. A computer simulation program for the design of hollow fiber modules was developed. It is based on a formula for filtrate flux prediction by Jaffrin. The limiting conditions set by red and white blood cell lysis are also taken into account. The results show that membrane area consumption is strictly related to the internal hollow fiber diameter. Low-efficiency devices (low infiltrate flux) can be designed nearly optimally, using 330-micron fibers. On the other hand, for high-efficiency devices, our model predicts lowest membrane consumption and, therefore, lowest costs using 220-micron fiber diameter. Furthermore, the results demonstrate that shear rates in commercially available plasma filters are too low.

Physical aspects governing the design of hollow fiber modules

For the design and use of hollow fiber membrane separation units, the engineer is in need of a quantitative or at least semi-quantitative description of the process, which will enable him to predict the effect of fiber length, fiber diameter, packing density, flow rates, etc. To this end, models are considered for which an approximate mathematical description can be developed. By way of illustration, two processes are discussed in particular: reverse osmosis and dialysis. In reverse osmosis, the relevant physical quantities for a given permeability are flow rates and pressure gradients. The model assumes a rate of permeation which is proportional to the pressure difference between the two sides of the membrane. Special attention is paid to the serious problem of polarisation. The dialysis process considered is one in which the shell-side flow is radial, volume changes due to permeation of solute are small, and the rate of permeation of solute is proportional to the concentration difference. The conditions to be fulfilled to neglect the role of diffusion are examined and, by way of illustration, corrections for diffusion are developed for the special case of axial flow of a solution through a fiber when the solute is removed by dialysis through the fiber wall.