Volume 11, Issue 1 - Special Issue: Selected Extended Papers from the 12th International Conference on Membrane Science and Technology (MST2015) Symposium on Modeling and Simulation, Guest Editors: Reza Zarghamia and Ahmad Fauzi Ismail

Graphene-derived membranes has gained much interest recently due to its promising potential in filtration and separation applications. Molecular Sieving phenomena of gas molecules in the interlayer of graphene oxide nanopore have been investigated using molecular dynamic (MD) simulation. We explore I-129 gas radionuclides sequestration from natural air in nanoporous graphene oxide membranes in which different sizes and geometries of pores were modeled on the graphene oxide sheet. In the present work, mean-squared displacement (MSD), diffusion, flow of gas and the number of crossed gas molecules through graphene oxide nanopore membrane have been calculated and the results showed, selective proper nanopores in graphene oxide membrane could dramatically improve separation. The aim of this paper is to show that for the well-defined pore size called P-12, it is possible to separate I-129 from a gas mixture containing I-129, O 2 and N 2 . The results would be benefited by the oil industry and others.

Due to special features, modules comprising asymmetric hollow fiber membranes are widely used in various industrial gas separation processes. Accordingly, numerous mathematical models have been proposed for predicting and analyzing the performance. However, majority of the proposed models for this purpose assume that membrane permeance remains constant upon changes in temperature and pressure. In this study, a mathematical model is proposed by taking into account non-ideal effects including changes in pressure and temperature in both sides of hollow fibers, concentration polarization and Joule-Thomson effects. Finite element method is employed to solve the governing equations and model is validated using experimental data. The effect of temperature and pressure dependency of permeance and separation performance of hollow fiber membrane modules is investigated in the case of CO 2 /CH 4 . The effect of temperature and pressure dependence of membrane permeance is studied by using type Arrhenius type and partial immobilization equations to understand which form of the equations fits experimental data best. Findings reveal that the prediction of membrane performance for CO 2 /CH 4 separation is highly related to pressure and temperature; the models considering temperature and pressure dependence of membrane permeance match experimental data with higher accuracy. Also, results suggest that partial immobilization model represents a better prediction to the experimental data than Arrhenius type equation.

Hollow fiber membrane permeators used in the separation industry are proven as preferred modules representing various benefits and advantages to gas separation processes. In the present study, a mathematical model is proposed to predict the separation performance of natural gas using hollow fiber membrane modules. The model is used to perform sensitivity analysis to distinguish which process parameters influence the most and are necessary to be assessed appropriately. In this model, SRK equation was used to justify the nonideal behavior of gas mixtures and Joule-Thomson equation was employed to take into account the changes in the temperature due to permeation. Also, the changes in temperature along shell side was calculated via thermodynamic principles. In the proposed mathematical model, the temperature dependence of membrane permeance is justified by the Arrhenius-type equation. Furthermore, a surface mole fraction parameter is introduced to consider the effect of accumulation of less permeable component adjacent to the membrane surface in the feed side. The model is validated using experimental data. Central Composite Designs are used to gain response surface model. For this, fiber inner diameter, active fiber length, module diameter and number of fibers in the module are taken as the input variables related to the physical geometries. Results show that the number as well as the length of the fibers have the most influence on the membrane performance. The maximum mole fraction of CO 2 in the permeate stream is observed for low number of fibers and fibers having smaller active lengths. Also results indicate that at constant active fiber length, increasing the number of fibers decreases the permeate mole fraction of CO 2 . The findings demonstrate the importance of considering appropriate physical geometries for designing hollow fiber membrane permeators for practical gas separation applications.

The main aim of this study is the investigation of dense palladium membrane reactor (MR) performance during methane steam reforming (MSR) reaction using computational fluid dynamic (CFD). To this purpose, a two-dimensional isothermal CFD model was developed and its validation was realized by comparing the theoretical results with our experimental data achieved in ITM of Italy. In this work, the CFD model was presented by COMSOL- Multiphysics software version 5. The reaction rate expressions and kinetics parameters were used from literatures. According to validation results, a good agreement between modeling results and experimental data was found. After model validation, the effect of the some important operating parameters (temperature and pressure) on the performance of palladium MR was studied in terms of methane conversion and hydrogen recovery. The CFD model presented velocity and pressure profiles in both side of MR and also molar fraction of different species in permeate and retentate streams. The modeling results showed that the palladium MR presents comparable performance with respect to traditional reactor (TR) in terms of the methane conversion, especially, at lower temperatures and higher pressures. In fact, CFD results indicated that palladium MR performance was improved by increasing the reaction pressure, while this parameter had negative effect on the TR performance. This result related to increasing the hydrogen permeance through the palladium membrane by enhancement of pressure gradient. Indeed, this shift effect can provide a higher methane conversion in lower temperatures in the palladium MR. In particular, 99% methane conversion and 43% hydrogen recovery was achieved at 500°C and 1.5 atm.

The main purpose of this work is the numerical investigation of PEBX membrane performance for natural gas sweeting. Hence, a single-stage process for PEBX membrane was considered in various flow patterns, namely, cocurrent, cross flow and countercurrent to separate a typical natural gas mixture. To this target, a black box numerical model was extended for the ASPEN HYSYS commercial package and also its validation was realized by litterature experimental data. The validation results indicated a good agreement between thoritical results and experimental data. After model validation, the effect of the some significant operating parameters (pressure gradient, stage cut and membrane area) on the performance of PEBX membrane was analysed in terms of acid gases removal percentage. The simulation results presented a noticeable performance of PEBX membrane to produce high purity CH 4 . In particular, concerning the stage cut effect, it was found that the CO 2 and H 2 S compositions in the permeate side were decreased through the enhancment of stage cut from 0.005 to 0.03, whereas the CH 4 composition increased for whole the flow patterns. Moreover, a similar effect was achived for membrane surface area. On the other hand, the transmembrane pressure effect was positive on the PEBX membrane performance during natural gas sweeting.

Membrane separation processes are energetically efficient compared to the other techniques such as cryogenic distillation and gas adsorption techniques. It is well known that a membrane's permeance is inversely proportional to its thickness. Regard to its single atom thickness and its mechanical strength, nanoporous graphene has been proposed as a very promising candidate for highly efficient gas separation applications. In this work, using classical molecular dynamics, we report the separation performance of such membrane in a molecular-sieving process as a function of pore size and chemical functionalization of pore rim. To investigate the membrane separation capability, we have calculated the permeance of each gas molecule of the considered binary mixtures through the membranes and therefore the separation selectivity. We investigated the separation performance of nanoporous graphene for CO 2 /N 2 , H 2 /CH 4 and He/CH 4 with 50:50 proportions of each component and the separation selectivity has been calculated. We also calculated the potential of the mean force to characterize the energy profile for gas transmission. The separation selectivity reduced by increasing the pore size. However, presence of chemical functionally pores in the membrane increased the separation selectivity. Furthermore, the gas permeance through nanoporous graphene membranes is related not only to transport rate to the graphene surface as well as kinetic diameters but also to molecular adsorbed layer which is formed on the surface. The flux of molecules through the nanopores is also dependent on pore chemistry which is considered as gas-pore interactions in the molecular simulations and can be a sizable factor in simulation in contrast to experimental observations. This study suggests that nanoporous graphene could represent a suitable membrane for gas separation.

Some features of Direct Contact Membrane Distillation (DCMD), as one of the interesting membrane processes, has been studied in this effort. 3D computational fluid dynamic simulations were carried out to investigate some geometric parameter effects on flat sheet membrane module performance. It is obvious that using of baffles could noticeably improve the performance of the system. Hence, in present work, some baffle configurations were simulated and some parameters like temperature polarization, vapor flux and pressure drop through module length were investigated. The Simulation was performed based on neglecting viscous flow in membrane pores and dusty gas model was applied to predict vapor flux through membrane. Simulation results predicted that by using the new configuration we could have 40–60% vapor flux improvement (depend on inflow velocity) compared to a module without baffle. It was found that the average temperature polarization (TP), as a proper criteria, was higher for baffled one in all situations.

Some challenges, including inappropriate distribution of currents on the membrane surface, poor hydrodynamics and existing severe temperature polarization (TP) phenomenon in MD modules, impede industrialization of MD process. Computational fluid dynamics (CFD) method was used for numerical simulation of hydrodynamics in air gap membrane distillation modules. One of two simulated modules in this work is a novel developed one in which heat and mass transfer data was compared with available literature data. Moreover, the effect of using baffles in module was investigated. Comparison between the novel module and conventional module indicates higher trans-membrane mass flux and gained output ratio (GOR) coefficient by 7% and 15%, respectively. Moreover, the effects of different operating conditions including feed temperatures and feed flow rates on permeate flux were investigated.

Multi-objective optimization is used in many chemical engineering fields that have conflict objective functions. Prevaporation is an effective process for removing trace or minor amount of the component of diluting solutions. This process is used for dehydration of alcohols containing small amounts of water. In this process membrane flux and separation factor have conflict with each other. So a multi-objective optimization approach can be used for optimization of the process. In this paper, in first stage a neural network based model was developed for preparation conditions for polyetherimide membrane in isopropanol prevaporation. Four major variables involved in the membrane preparation procedure, including polymer concentration, additive content, solvent evaporation temperature and time was considered. Membrane flux and separation factor were considered as objective functions. Elitist Non-dominated sorting genetic algorithm with jumping gene and altruistic adaptation (Alt-NSGA-aJG) was applied for simultaneous maximization of flux and separation factor. Pareto optimal solutions for membrane preparation conditions and effect of decision variables (four preparation variables) on Pareto front were investigated.

The main purpose of present study is the analysis of dense palladium membrane reactor (MR) performance during ethanol steam reforming (ESR) reaction using computational fluid dynamic (CFD). To this aim, a two-dimensional and isothermal model based on CFD method was developed and results validation was tested by our experimental data achieved in ITM-CNR of Italy. In this work, Pd-based MR modeling was performed by using COMSOL-MULTIPHYSICS software. Regarding to model validation results, a good agreement was found between CFD model results and experimental data. Moreover, in this study, the effects of the some important operating parameters (reaction temperature and pressure) on the performance of Pd-based MR was studied in terms of ethanol conversion and hydrogen recovery. Concerning to simulation results, the CFD model presented velocity and pressure profiles in both side of MR and also compositions of various species in permeate and retentate streams. The simulation results indicated that the Pd-based MR has better performance with respect to traditional reactor (TR) in terms of the ethanol conversion, especially, at lower reaction temperatures and higher reactions pressures. As a consequence, CFD model results illustrated that Pd-based MR performance was improved by increasing the reaction pressure, while this parameter had negative effect on the TR performance. This result related to enhancement of hydrogen permeance through the palladium membrane by increasing the pressure gradient. Indeed, this shift effect can provide a higher ethanol conversion in lower temperatures in the Pd-based MR. In particular, 98% ethanol conversion and 37% hydrogen recovery was achieved at 350°C and 2 atm.

In the last decades, a large number of studies have been carried out on the utilization of membranes in separation processes. However, most of these studies deal with material properties, experimental investigations and process modeling. Only quite a few authors utilized computational fluid dynamics (CFD) to analyze the flow and mass transfer in membrane modules. Using CFD it is possible to obtain spatially resolved information on the behavior of membrane modules, allowing for the investigation of geometric effects on the performance of the module. This includes e. g. the positioning of the permeate outlets, the flow alignment (co- and/or counter-current), the use of spacers and other mixing promoters and also the subject of concentration polarization close to the membrane surface. In our present study we made use of OpenFOAM®, which is a free open sourced CFD toolbox. The toolbox enables for introducing new solver code, membraneFoam, based on the standard multicomponent solver reactingFoam. In membraneFoam suitable source and sink terms have been added to account for trans-membrane flux – in this case based on the solution-diffusion model for glassy polymer gas permeation membranes. The solver has been preliminary validated using literature data obtained from a process simulation code. In a first stage of the research work the positioning of the permeate outlet and the flow alignment have been investigated for a hollow fiber gas permeation module. By adjusting the position of the permeate outlet the shell side flow can be co-current, counter-current or mixed type relative to the retentate flow inside the fibers. Since this influences the driving force for the trans-membrane flux, effects on the module performance are expected which have been analyzed using the described membraneFoam CFD approach.

Implementing membrane gas separation systems have led to remarkable profits in both processes and products. This study presents the modeling and simulation of membrane gas separation systems using Aspen Plus ® V8.6. A FORTRAN user model and a numerical solution procedure have been developed to characterize asymmetric hollow fiber membrane modules. The main benefit of this model is that it can be easily incorporated into a commercial simulator and used as a unit operation model in complex systems. A comparison between the model and the experimental cases at different operation conditions shows that calculated values are in good agreement with measured values. This model is suitable for future developments as well as design and performance analysis of multicomponent gas permeation systems prior to experimental realization.

In recent years carbon nanotubes and other carbon nanostructures such as graphene sheets have attracted a lot of attention due to their unique mechanical, thermal and electrical properties. These structures can be used in desalination of sea water, removal of hazardous substances from water tanks, gases separation, and so on. The nanoporous single layer graphene membranes are very efficient for desalinating water due to their very low thickness. In this method, water-flow thorough the membrane and salt rejection strongly depend on the applied pressure and size of nanopores that are created in graphene membrane. In this study, the mechanism of passing water and salt ions through nanoporous single-layer graphene membrane are simulated using classical molecular dynamics. We examined the effects of applied pressure and size of nanopores on desalination performance of NPG membrane. Unlike previous researches, we considered the flexibility of the membrane. The results show that by increasing the applied pressure and diameter of the nanopores, water-flow through membrane increases, meanwhile salt rejection decreases.

Separation of light gaseous olefins from paraffin’s of the refinery process off-gasses has been traditionally performed by cryogenic distillation, which is a highly capital and energy intensive operation. This handicap creates an incentive for the investigation of alternative olefin/paraffin separation technologies. In this regard, membrane technology supposes a potential solution for process intensification. Previous works of our research group reported the use of facilitated transport composite membranes integrating the use of PVDF-HFP polymer, BMImBF 4 ionic liquid and AgBF 4 silver salt. In this type of membranes, the silver cations react selectively and reversibly with the olefin, allowing the separation via mobile and fixed carrier mechanisms. Ionic liquids were selected as membrane additives because in addition to their negligible vapor pressure that avoids solvent losses by evaporation, they provide stability to the metallic cation dissolved inside, and modify the structure improving the facilitated transport. This technology offers a commercial attractive separation alternative thanks to their modular form of operation, high values of selectivity and permeability and low operational costs. In the present work, propane/propylene permeation experiments involving the use ionic liquids and different membrane compositions were performed. Moreover, basing on the transport and equilibrium parameters previously obtained, a mathematical model description of the system will be proposed fitting the remaining parameters and allowing the design and optimization of the propane/propylene separation process at industrial levels.

In this article, hydrogen adsorption from gas mixture involve nitrogen and hydrogen by palladium hollow fiber membrane investigated and two dimensional model proposed for hydrogen adsorption in this model. This model has been evaluated based on equations (momentum and mass transfer) in all three parts (shell, membrane and tube) for existing gaseous compounds with using finite element method. The results of simulation validated by experimental data of hydrogen adsorption by palladium hollow fiber membrane. Modeling predictions shows good agreement with experimental data at different operating conditions such as different gas flows, temperature, pressure and etc. the result of simulation shows hydrogen separation efficiency increases with increasing temperature and pressure and decreases with increasing inlet rate. Also with using this model better performance of hollow fiber membranes can be obtained. In fact, hollow fiber membrane can be designed at different conditions and for different rates that it can save cost of various tests.

In this paper, a 3D modeling of Hydrogen separation from H 2 /N 2 mixture by pd/α-Al 2 O 3 hollow fiber membrane in steady and unsteady state using computational fluid dynamic was considered. The effect of operating condition such as temperature, pressure and feed flow rate on concentration polarization was examined. Using concept of concentration polarization, controlling mass transfer in membrane module was determined. Also by applying sensitivity factor of flux that is used for analysis of concentration polarization, the best performance of membrane was found. The CFD results show good agreement with experimental data.