Fouling Control in a Submerged Flat Sheet Membrane System: Part II—Two‐Phase Flow Characterization and CFD Simulations

Abstract

Gas‐liquid two‐phase flow has been shown to be very effective in reducing fouling for different membrane modules with different feeds, including submerged flat sheet membranes used in membrane bioreactors for treatment of wastewater. Although gas‐liquid two‐phase flow occurring on the lumen side of tubular or hollow‐fiber membranes has been very well characterized the two‐phase flow regime in submerged membrane processes is different to that inside external membranes. Characterization of two‐phase flow in submerged flat sheet membrane modules has not been previously reported and hence the use of two‐phase flow in these modules has not yet been optimized. This paper reports on characterization of two‐phase flow for a submerged flat sheet membrane module with the aim of identifying the most effective flow profiles for fouling minimization. In order to better understand the fouling control process by two‐phase flow, CFD simulations were also conducted. It was found experimentally that an increase in the bubble size leads to an increase in the cleaning effect, however, for bubbles larger than the channel gap between the submerged flat sheet membranes, any further increase in the bubble diameter had only a minor effect on the cleaning process. CFD simulations revealed that flux enhancement was primarily due to an increase in the overall shear stress on the membrane and to more turbulence generated by introduction of the gas phase.     

Synthesis and optimization of membrane cascade for gas separation via mixed‐integer nonlinear programming

Currently, membrane gas separation systems enjoy widespread acceptance in industry as multistage systems are needed to achieve high recovery and high product purity simultaneously, many such configurations are possible. These designs rely on the process engineer's experience and therefore suboptimal configurations are often the result. This article proposes a systematic methodology for obtaining the optimal multistage membrane flow sheet and corresponding operating conditions. The new approach is applied to cross‐flow membrane modules that separate CO₂ from CH₄, for which the optimization of the proposed superstructure has been achieved via a mixed‐integer nonlinear programming model, with the gas processing cost as objective function. The novelty of this work resides in the large number of possible interconnections between each membrane module, the energy recovery from the high pressure outlet stream and allowing for nonisothermal conditions. The results presented in this work comprise the optimal flow sheet and operating conditions of two case studies. © 2017 American Institute of Chemical Engineers AIChE J, 63: 1989–2006, 2017     

Separation of greenhouse gases from gas mixtures using nanoporous polymeric membranes

Removal of greenhouse gases from gas streams using porous membranes was carried out in this work. Theoretical studies were performed in terms of mathematical modeling and numerical simulation of CO₂ capture in a flat‐sheet membrane contactor. Numerical simulation was performed using computational fluid dynamics (CFD) of mass and momentum transfer in the membrane module for laminar flow conditions. Physical absorption was considered in the simulations for absorption of CO₂ in pure water. CO₂ concentration distribution in the membrane module was determined through numerical solution of continuity equation coupled with the Navier‐Stokes equations. The modeling predictions indicated that the CO₂ concentration difference is not appreciable in the membrane direction. Moreover, velocity distribution was determined in the liquid side of membrane contactor. CFD also represents a design and optimization tool for membrane gas separation processes. POLYM. ENG. SCI., 55:975–980, 2015. © 2014 Society of Plastics Engineers     

Experimental Study and Design of a Submerged Membrane Distillation Bioreactor

A hybrid process incorporating membrane distillation in a submerged membrane bioreactor operated at elevated temperature is developed and experimentally demonstrated in this article. Since organic particles are rejected by an ‘evaporation’ mechanism, the retention time of non‐volatile soluble and small organics in the submerged membrane distillation bioreactor (MDBR) is independent of the hydraulic retention time (mainly water and volatiles). A high permeate quality can be obtained in the one‐step compact process. The submerged MD modules were designed for both flat‐sheet membranes and tubular membrane configurations. The process performance was preliminarily evaluated by the permeate flux stabilities. The module configuration design and air sparging used in the MDBR process were tested. Flux declines were observed for the thin flat‐sheet hydrophobic membranes. Tubular membrane modules provided more stable permeate fluxes probably due to the turbulent condition generated from air sparging injected inside the tubular membrane bundles. The experiments with the submerged tubular MD module gave stable fluxes of approximately 5 L/m² h over 2 weeks at a bioreactor temperature of 56 °C. The total organic carbon in the permeate was consistently lower than 0.7 mg/L for all experiments.     

Modelling of multicomponent gas separation with asymmetric hollow fibre membranes—methane enrichment from biogas

A mathematical model for the dynamic performance of gas separation with high flux, asymmetric hollow fibre membranes was developed considering the permeate pressure build‐up inside the fibre bore and cross flow pattern with respect to the membrane skin. The solution technique provides reliable examination of pressure and concentration profiles along the permeator length (both residue/permeate streams) with minimal effort. The proposed simulation model and scheme were validated with experimental data of gas separation from literature. The model and solution technique were applied to investigate dynamic performance of several membrane module configurations for methane recovery from biogas (landfill gas or digester gas), considering biogas as a mixture of CO₂, N₂ and CH₄. Recycle ratio plays a crucial role, and optimum recycle ratio vital for the retentate recycle to permeate and permeate recycle to feed operation was found. From the concept of two recycle operations, complexities involved in the design and operation of continuous membrane column were simplified. Membrane permselectivity required for a targeted separation to produce pipeline quality natural gas by methane‐selective or nitrogen‐selective membranes was calculated. © 2012 Canadian Society for Chemical Engineering     

Fouling Control in a Submerged Flat Sheet Membrane System: Part I – Bubbling and Hydrodynamic Effects

Abstract

Submerged flat sheet membranes are mostly used in membrane bioreactors for wastewater treatment. The major problems for these modules are concentration polarization and subsequent fouling. By using gas‐liquid two‐phase flow, these problems can be ameliorated. This paper describes a study of the use of gas‐liquid two‐phase flow as a fouling control mechanism for submerged flat sheet membrane bioreactors. The effect of various hydrodynamic factors such as airflow rate, nozzle size, intermittent filtration, channel gap width, feed concentration, imposed flux, and the use of membrane baffles were investigated. Experiments conducted on model feeds showed that fouling reduction increased with air flow rate up to a given value and beyond this flowrate no further enhancement was achieved. The effect of bubbling was also found to increase with nozzle size at constant airflow. Using intermittent filtration as an operating strategy was found to be more effective than continuous filtration and it also reduced energy requirements. The study showed the importance of the size of the gap between the submerged flat sheet membranes. As the gap was increased from 7 mm to 14 mm, the fouling became worse and the degree of fouling reduction by two‐phase flow decreased by at least 40% based on suction pressure rise (dTMP/dt). This is the first study which has reported the effects of baffles in improving air distribution across a flat sheet submerged membrane. It was found that baffles could decrease the rate of fouling by at least a factor of 2.0 based on the dTMP/dt data, and significantly increase critical flux.     

Model Based Quantification of Internal Flow Distributions from Breakthrough Curves of Flat Sheet Membrane Chromatography Modules

A novel model for quantifying radial flow distributions in flat sheet membrane chromatography modules under non‐binding conditions is presented and applied for the practical analysis of two modules. The proposed model partitions the total void volume of the chromatography module into zones that are considered homogeneous with respect to flow velocity. The corresponding solute concentrations are time variant, but also spatially homogeneous within each zone. The model is mathematically represented and analytically solved as a network of continuously stirred tank reactors (CSTR). An additional plug flow reactor (PFR) is connected in series with the CSTR network in order to account for a time‐lag that is not associated with the system dispersion. The capability of the model to describe experimental breakthrough data is compared to the frequently applied standard model for extra‐membrane system dispersion, which consists of a single CSTR in series with a PFR. Non‐binding conditions are deliberately chosen for studying the impact of module geometries on breakthrough curves separately from chromatographic membrane performance. The commercial CIM module and a custom designed cell (Scell) are studied with acetone and lysozyme as test tracers at varied flow rates and for various membrane pore sizes under non‐binding conditions. In all studied cases, the proposed model fits the measured breakthrough curves better than the standard model. Moreover, the minimal number of radial flow zones that are required to accurately describe observed breakthrough curves and the estimated flow fractions through these zones provide valuable information for the analysis and optimization of internal module designs.     

Double‐deck aerated biofilm membrane bioreactor with sludge control for municipal wastewater treatment

Alternative designs of an aerated moving‐bed biofilm reactor and a flat‐sheet membrane module for a biofilm membrane bioreactor process have been investigated to overcome a membrane clogging problem and to determine the performance of a new membrane module. Double‐deck aerated biofilm reactor with integrated designs of sludge hopper, thickener, and velocity‐zone concept for particle settlement was evaluated for the suspended solid control and removal. Hydrodynamics of bubbling, liquid, and solid particles were arranged in the bioreactor to obtain a particle settlement. New membrane modules used under low suspended solid environment having smaller membrane gaps were evaluated for filtration performance and clogging problems for long‐term operation. The average suspended solids concentration in the bioreactor effluent was 44.6 mg/L. Relaxation applied with the membrane module provided the most optimum result for fouling control, and no clogging problems in the modules were observed in the system after continuous operation of 3 weeks. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Optimization of CO2–CH4 separation performance of integrally skinned asymmetric membranes prepared from poly(2,6‐dimethyl‐1,4‐phenylene oxide) by factorial design

Integrally skinned asymmetric flat sheet membranes were prepared from poly(2,6‐dimethyl 1,4‐phenylene oxide)(PPO) for CO₂–CH₄ separation. Various experiments were carried out to identify PPO membranes, which have good mechanical strength and gas separation abilities. Membrane strength and selectivity depend on the interplay of the rate of precipitation and the rate of crystallization of the PPO. The effects of major variables involved in the membrane formation and performance, including the concentration of the polymer, solvent, and additive, the casting thickness, the evaporation time before gelation, and the temperature of the polymer solution, were investigated. Factorial design experiments were carried out to identify the factor effects. The membrane performance was modelled and optimized to approach preset values for high CO₂ permeance and a high CO₂ : CH₄ permeance ratio. Membranes were prepared based on the optimum conditions identified by the model. Essentially, defect‐free membranes were prepared at these conditions, which resulted in a pure gas permeance of 9.2 × 10⁻⁹ mol/m² s Pa for CO₂ and a permeance ratio of 19.2 for CO₂ : CH₄. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1601–1610, 1999     

Impact of module design on the performance of membrane bioreactors treating municipal wastewater

Membranes are located in a membrane module that physically seals and isolates the feed stream from the permeate flux in membrane bioreactors (MBRs). Therefore, module type, structure, and geometrical configuration are critical design considerations affecting membrane performance in MBRs. In this study, impact of membrane module design on treatment and filtration performance of MBRs was investigated. For this purpose, two flat sheet membrane modules with different outlet structures and module geometries, including rectangular- and D-shaped, were tested. In addition to the differences in outlet structure and module geometry, size of circular structures which supported membranes in rectangular- and D-shaped modules differed from each other. Considering the results, permeate quality was not affected from the change in the module design. However, the most remarkable impact of the module design was observed on the transmembrane pressure (TMP) evolution and fouling potential. D-shaped membrane module including smaller circular structures resulted in a decrease in fouling potential and thus, this module could be operated longer time in comparison to rectangular-shaped membrane module without a severe TMP increase. The observed differences in TMP increase and fouling potential lead to the hypothesis that module design is a critical factor affecting filtration performance in MBRs.     

Flux Enhancement in a Helical Microfiltration Module with Gas Injection

Abstract

A helical flow module with an inner rod mounted membrane was designed and built to reduce gel layer deposit and membrane fouling during microfiltration. Controlled centrifugal instabilities resulting from flow in a helically grooved channel, as well as the leakage flow between adjacent grooves, generated secondary vortex flows. The permeation fluxes for helical modules with Dean vortex flow were compared with flat crossflow modules at different operating pressures, concentrations, and feed flow rates. The permeation flux of the helical module for a feed solution containing 0.3 wt% kaolin solution at 1.2 bar was 57% higher than that of the flat module. Moreover, in addition to secondary vortex flow, compressed air was introduced to the membrane module. The increase in flux for the helical module with air injection was significant: the flux enhancements at 1.3 bar, 2 L‐solution/min and 1.3 L‐air/min for 0.1 wt% solutions of kaolin and bentonite were 47 and 73%, respectively.     

Modeling and simulation of hollow fiber CO<Subscript>2</Subscript> separation modules

We developed two models for the CO₂ separation process by hollow-fiber membrane modules. The explicit model, which is based on mass balances for the separation modules, is compared with the multilayer perceptrons (MLP) back-propagation neural networks model. Experimental data obtained from single-stage module with recycle are used to validate the explicit model as well as to train the MLP neural model. The effectiveness of the model is demonstrated by little discrepancy between experimental data and computational results. The explicit model for the single-stage module can easily be extended to the multi(three)-stage module. Because of the lack of experimental data for multi-stage modules, computational data from the explicit model with and without recycle are used as training data set for the MLP neural model. We examined the effects of recycle on the recovery based on the results of numerical simulations, and could see that the predicting performance is improved by recycle for multi-stage module. From the results of numerical simulations, the proposed models can be effectively used in the analysis and operation of gas separation processes by hollow-fiber membrane modules.     

A theoretical comparison of facilitated transport and solution-diffusion membrane modules for gas separation

Computer models were developed to compare the performances of facilitated transport (FT) and conventional solution-diffusion (SD) membrane modules, and sample calculations given for the separation of CO₂/CH₄ mixtures. Previous studies have indicated that high CO₂ fluxes can be obtained with laboratory-sized sheets of FT membranes; but these studies have not examined the performance of these membranes as modules. In the present study, the FT membrane properties are those of an ion exchange membrane with ethylene diamine as the CO₂ carrier, and the SD membrane properties are those of a typical, commercial cellulosic membrane. For the conditions examined, the facilitation effect in the FT membrane module is significant only when the partial pressure of CO₂ is relatively low (< 10 psia). For a low CO₂ partial pressure (7.5 psia) in the feed, the FT membrane with an ethylene diamine concentration of 8 M has a factor of 2 lower area requirement and 2% greater methane recovery than for an SD membrane with a selectivity of 30. Above 50 psia CO₂ partial pressure, the SD and FT membrane modules function identically. We have also shown that the fraction of the total CO₂ flux contributed by the ethylene diamine carrier increases along the flow path of the membrane module, thereby making the choice of an optimum equilibrium constant for the CO₂/diamine reaction more difficult than in a membrane sheet with constant boundary conditions. However, even for the low CO₂ feed partial pressure the required membrane area increases by less than 35% when the actual equilibrium constant is a factor of 10 greater than the optimum value.     

The effect of operating conditions on the performance of hollow fiber membrane modules for CO2/N2 separation

A commercialized polysulfone (PSf) hollow-fiber membrane module was tested for CO₂/N₂ separation performance for application in post-combustion capture. Cost efficiency, easy module manufacturing, and efficiency in gas separation are the main advantages of using PSf hollow-fiber modules for CO₂ separation. The effects of operating conditions such as temperature, pressure, and feed composition on separation performance were examined at various stage cuts. A 2-stage system including concentration of feed composition at stage 1 and production of high-purity CO₂ at stage 2 was constructed to improve separation efficiency. Higher operating temperature and pressure increased CO₂ permeance, but the loss of selectivity and higher energy consumption are a concern. Modules with various membrane areas were also used to test the effect of area on CO₂ separation.     

Evaluation of hollow fiber T-type zeolite membrane modules for ethanol dehydration

This work presents the design of hollow fiber T-type zeolite membrane modules with different geometric configurations. The module performances were evaluated by pervaporation dehydration of ethanol/water mixtures. Strong concentration polarization was found for the modules with big membrane bundles. The concen-tration polarization was enhanced at high temperature due to the higher water permeation flux. The increase of feed flow could improve water permeation flux for the membrane modules with small membrane bundle. Computational fluid dynamics was used to visualize the flow field distribution inside of the modules with different configurations. The membrane module with seven bundles exhibited highest separation efficiency due to the uniform distribution of flow rate. The packing density could be 10 times higher than that of the tubular membrane module. The hollow fiber membrane module exhibited good stability for ethanol dehydration.     

Membrane separation of low volatile organic compounds by pervaporation and vapor permeation

Pervaporation of dilute benzyl alcohol solutions was studied using polydimethylsiloxane (PDMS) membranes. The membrane performance was investigated under various temperatures, downstream pressures and feed concentrations. The benzyl alcohol concentration in the permeate decreased dramatically by increasing the downstream pressure, but increased by increasing the operating temperature. The separation and recovery of benzaldehyde from nitrogen was also investigated. Polyetherimide flat sheet membranes were prepared and then coated with silicone material. The performance of membranes in both coupon form and spiral-wound configuration was studied. When the experimental results from both configurations were compared, it was found that for the same material the membrane coupons had higher separation efficiency than the spiral-wound prototype modules. The ratio of benzaldehyde mole fraction in the permeate to the feed reached 500 for a small membrane coupon of effective surface area 10.2 cm². This ratio was only 26 for the prototype modules, which can be attributed to some limitations of the membrane system design. These limitations are discussed and some recommendations to improve the performance are given.     

Specification of commercial reverse osmosis modules and predictability of their performance for water treatment applications

Techniques for specifying membranes in commercial modules and reverse osmosis systems involving such modules, and for predicting data on performance of the modules so specified are illustrated with particular reference to water treatment applications. Four commercial modules (Roga-4000, Westinghouse, Raypak and Du Pont-B9) were studied in. this work for such specification and prediction. Experimental reverse osmosis data using NaCl-H₂O feed solutions were used for obtaining data on membrane specifications. Equations of system analysis were used for prediction of data on module performance. Three sets of calculated data are reported for the operating pressure of 2758 kPa gauge (400 psig). The first set of data shows good agreement between calculated and experimental results on the performance of the modules with aqueous sucrose feed solutions. The second set of data shows the variations in solute separation and membrane productivity for each of the four- modules studied as functions of volumetric fraction product water recovery and membrane compaction for a 3000 ppm NaCl-H₂O feed solution. The third set of data shows the variations in solute separation as a function of solute transport parameter at different levels of mass transfer coefficient on the high pressure side of the membrane for very dilute aqueous feed solutions.     

CONCENTRATION POLARIZATION ANALYSIS AT THE ENTRANCE REGION OF A FLAT SHEET PERVAPORATION MODULE FOR VOC REMOVAL

Concentration polarization is a phenomenon that is inherent in all membrane separation processes, which is difficult if not impossible to measure experimentally. Concentration polarization in a pervaporation module causes flux decline and is therefore an important issue in predicting the performance of the membrane unit for evaluation and optimization. Short-form (small L/D ratio) membrane configurations, commonly used for membrane evaluations or certain material separations, compound the complexity of process modeling that addresses concentration polarization since a substantial portion of the membrane flow channel would be considered as an “entrance region” based on the flow profile that is not fully developed. This article employed the classic boundary layer theory, combined with mass transfer phenomena in a pervaporation process that is used in volatile organic compound (VOC) removal from contaminated water sources, to theoretically analyze the concentration polarization severity in the entrance region of a flat sheet membrane module.     

Single and Binary CO2/CH4 Separation of a Zeolitic Imidazolate Framework‐8 Membrane

The performance of a zeolitic imidazolate framework‐8 (ZIF‐8) membrane in single and binary CO₂/CH₄ gas separation was investigated by means of a gas transport model that included generalized Maxwell‐Stefan and binary friction models. The model concerns gas diffusion through the membrane layer, gas flow through membrane intercrystalline pores, and resistance of the support layer. The effective membrane area considering the actual area for the gas permeated through the membrane was also introduced in this model. The selective ZIF‐8 membrane was successfully synthesized using a microwave‐assisted solvothermal method on an α‐alumina support pre‐attached with ZIF‐8 seeds by solvent evaporation. The simulated data agreed well with the experimental data. The model revealed that the membrane intercrystalline pores and its effective area significantly affected the CO₂/CH₄ gas permeation and separation performance.     

CONCENTRATION POLARIZATION ANALYSIS AT THE ENTRANCE REGION OF A FLAT SHEET PERVAPORATION MODULE FOR VOC REMOVAL

Concentration polarization is a phenomenon that is inherent in all membrane separation processes, which is difficult if not impossible to measure experimentally. Concentration polarization in a pervaporation module causes flux decline and is therefore an important issue in predicting the performance of the membrane unit for evaluation and optimization. Short-form (small L/D ratio) membrane configurations, commonly used for membrane evaluations or certain material separations, compound the complexity of process modeling that addresses concentration polarization since a substantial portion of the membrane flow channel would be considered as an “entrance region” based on the flow profile that is not fully developed. This article employed the classic boundary layer theory, combined with mass transfer phenomena in a pervaporation process that is used in volatile organic compound (VOC) removal from contaminated water sources, to theoretically analyze the concentration polarization severity in the entrance region of a flat sheet membrane module.