Modeling and analysis of vacuum membrane distillation for the recovery of volatile aroma compounds from black currant juice

A vacuum membrane distillation (VMD) model has been developed and validated with experimental data. The model consists of an extended transport model for the VMD process and is able to predict the effects of concentration and temperature polarization on the overall process performance. To validate the model, first it was tested with few experimental case studies from literature [S. Bandini, G.G. Sarti, Heat and mass transport resistances in vacuum membrane distillation per drop, AIChE J. 45 (7) (1999) 1422–1433; K.W. Lawson, D.R. Lloyd, Membrane distillation. I. Module design and performance evaluation using vacuum membrane distillation, J. Membr. Sci. 120 (1996) 111–121; A.M. Urtiaga, G. Ruiz, I. Ortiz, Kinetic analysis of the vacuum membrane distillation of chloroform from aqueous solutions, J. Membr. Sci. 165 (2000) 99–110]. Then the VMD model has been validated with experimental data collected from the recovery of aroma compounds from black currant [R.B. Jørgensen, A.S. Meyer, C. Varming, G. Jonsson, Recovery of volatile aroma compounds from black currant juice by vacuum membrane distillation, J. Food Eng. 64 (2004) 23–31]. In this work, recovery of 12 characteristic volatile aroma compounds from black currant juice has been studied. The simulated results from the VMD model, in terms of aroma concentration in the permeate have been compared with those obtained from laboratory experiments. The validated model has been used to study the effects of various process and membrane parameters on the concentration factor. The physical properties of various aroma compounds have been predicted using group contribution method as a function of temperature.     

Shell-side mass transfer of hollow fibre modules in osmotic distillation process

The effect of packing density of hollow fibre modules on mass transfer in the shell side of osmotic distillation process was studied. The osmotic distillation experiments were carried out with several modules of the packing densities ranging from 30.6 to 61.2%. It was found that the Reynolds number was a function of packing density and packing density affected mass transfer performance. The shell-side mass transfer coefficient increased with the brine velocity. The membrane permeability can be predicted from the experimental flux at the maximum brine velocity. The mass transfer correlation was proposed in order to determine the shell-side mass transfer coefficient in the randomly packed modules for osmotic distillation process. The empirical correlation proposed was fitted to the experimental results and it was found that the mass transfer coefficients calculated from the proposed correlation were in good agreement with those from the experimental data. Comparison of the results obtained from the proposed correlation with other correlations in the literature was discussed.     

Removal of 2,4-dichlorophenol from wasterwater by vacuum membrane distillation using hydrophobic PPESK hollow hiber membrane

2,4-Dichlorophenol was removed from wasterwater using a new hydrophobic poly(phthalazinone ether sulfone ketone) (PPESK) hollow fiber membrane by vacuum membrane distillation(VMD).     

Influence of polypropylene membrane surface porosity on the performance of membrane distillation process

Two kinds of polypropylene capillary membranes were used in the membrane distillation (MD). These membranes exhibited a similar morphology, but one of them has an additional low porosity layer on the internal surface of capillaries. The changes of membrane performance during MD process of tap water were investigated. The presence of low porosity layer (thickness below 1 μm) caused that the air permeability was reduced from 1.365 to 0.863 dm³/m² s kPa, whereas the MD permeate flux was decreased only by 15%. A significantly larger decline of the flux was caused by CaCO₃ deposit formed during distillation of tap water. This deposit was removed every 30–70 h by rinsing the modules with a 2–5 wt.% HCl. Unfortunately, a repetition of this operation several times resulted in a gradual decline of the maximum permeate flux (distilled water as a feed). However, the module efficiency with the membranes covered by a surface layer of low porosity was found to decreases twice as slowly. The investigations revealed that a low surface porosity does not limit the possibility of surface wetting of polypropylene membranes, but hindered the scale formation inside the pores.     

Influence of temperature on mass transfer in an incomplete trapping single hollow fibre supported liquid membrane extraction of triazole fungicides

The influence of temperature in a single hollow fibre supported liquid membrane extraction of triazole fungicides with a stagnant acceptor phase was investigated. The mass transfer parameters such as diffusion coefficient, flux and apparent viscosity were determined at temperature ranging from 278 K to 313 K. Increase in temperature led to an increase in diffusion coefficient and flux. The apparent viscosity also decreased with an increase in temperature. The degree of trapping in the acceptor phase influenced the mass transfer at higher temperature. At lower temperature, the transport of analytes from the donor solution through the donor-membrane interface and through the membrane mainly affected the transport of triazole fungicides. The effect of temperature in a single hollow fibre SLM extraction technique is therefore more pronounced where transport is donor controlled and/or membrane controlled. The partition coefficient of analytes from the acceptor solution to the membrane, K_A was found to be much higher than that of from the donor solution to the membrane K_D, thus least trapped triazole fungicides preferred to remain in the membrane even with an increased extraction temperature.     

Theoretical modeling and experimental analysis of direct contact membrane distillation

A two-dimensional mathematical model was theoretically developed to predict the temperature polarization profile of direct contact membrane distillation (DCMD) processes. A concurrent flat-plate device was designed to verify the theoretical prediction of pure water productivity on saline water desalination. The numerical results from the temperature polarization profile were obtained using the finite difference technique to reduce the two-dimensional partial differential equations into an ordinary differential equations system. The resultant simultaneous linear equations system was solved with the fourth-order Runge-Kutta method. The results show theoretical prediction agreement with the measured values from the experimental runs. A combination of the Knudsen flow and Poiseuille flow models in the present mathematical formulation for membrane coefficient estimation was used to establish theoretical agreement. The influence of the inlet saline water temperature and volumetric flow rate on the pure water productivity as well as the hydraulic dissipated energy are also delineated.     

Electric mass transfer of sodium chloride through cation-exchange membrane MK-40: A rotating membrane disk study

An experimental device consisting of a rotating membrane disk with horizontally positioned cation-exchange membrane MK-40 is described. The device’s design makes it possible to simultaneously obtain current-voltage curves (CVC) and dependences of effective transport numbers for ions of electrolyte and water dissociation products on the current density. Partial CVC are calculated and limiting current densities and diffusion layer thickness are determined at various disk rotation rates. At current densities below the limiting value, the disk’s CVC obey regularities of electrodiffusion kinetics. Upon raising the current density further, the salt ion fluxes increase due to a decrease in the effective diffusion layer thickness, which is caused by the emergence in the near-membrane region of a space charge and electroconvection. At high current densities there occur oscillations of the potential jump that are caused by hydrodynamic instability of the near-membrane solution layer.     

Experimental and theoretical study on propylene absorption by using PVDF hollow fiber membrane contactors with various membrane structures

Five kinds of asymmetric poly(vinylidene fluoride) (PVDF) hollow fiber membranes with considerable different porosities at the inner and outer surfaces of the membrane were prepared via thermally induced phase separation (TIPS) method and applied for propylene absorption as gas–liquid membrane contactors. A commercial microporous poly(tetrafluoroethylene) (PTFE) hollow fiber membrane was also used as a highly hydrophobic membrane. Experiments on the absorption of pure propylene into silver nitrate solutions were performed and the effects of membrane structure, inner diameter, silver nitrate concentration and absorbent liquid flow rate were investigated at 298 K. PVDF membranes prepared by using nitrogen as bore fluid had lower inner surface porosity than the membranes prepared with solvent as bore fluid. Except the membrane with a skin layer at the outer surface, propylene absorption flux was inversely proportional to the inner diameter of the hollow fiber membrane, and propylene absorption rate per fiber was almost the same. Propylene flux increased with increasing the silver nitrate concentration and also with increasing the absorbent flow rate.A mathematical model for pure propylene absorption in a membrane contactor, which assumes that the membrane resistance is negligibly small and the total membrane area is effective for gas absorption, was proposed to simulate propylene absorption rates. Experimental results were satisfactorily simulated by the model except for the membrane having a skin layer. The model also suggested that propylene is absorbed in silver nitrate solutions accompanied by the instantaneous reversible reaction. This paper may be the first experimental and theoretical study on propylene absorption in membrane contactors.     

On the structure of the single-phase flow field in hollow fiber membrane modules during filtration

Whereas the structure of the flow field generated in the hollow fiber membrane modules operating as liquid–liquid or gas–liquid contactors has been studied extensively theoretically, this is not the case for the much more complex flow field generated during the filtration operation of the same type of modules. The present work is a first approach to analyze and understand the particular flow field. The mathematical problem is formulated and then geometrical and physical simplifications are introduced in order to decompose the full problem in a series of simpler sub-problems. In addition to the presentation of the general structure and the features of the problem, the focus of the present work is to derive all possible analytical solutions of the sub-problems. Several results based on analytical or simple numerical approaches are also presented.     

Performance evaluation of the different nano-enhanced polysulfone membranes via membrane distillation for produced water desalination in Sert Basin-Libya

A Polysulfone-Polyethylene glycol (PS/PEG) flat sheet membrane was prepared by phase inversion technique. Dimethyl Formamide (DMF) was utilized as a solvent and deionized water was utilized as the coagulant. Polyethylene glycol (PEG) of a various dose of PEG 2000 was utilized as the polymeric improvers and as a pore-forming agent in the casting mixture. The single-walled carbon nanotube (SWCNTs), multi-walled carbon nanotube (MWCNTs), aluminum oxide (Al₂O₃) and copper oxide (CuO) nanoparticles (NPs) were utilized to improve the PS/PEG membrane performances. The characterizations of the neat PS, PS/PEG, PS/PEG/Al2O3 (M1) PS-PEG/CuO (M2), PS-PEG/SWCNTs (M3) and PS/PEG/MWCNTs (M14) nanocomposite (NC) modified membranes were acquired via Fourier-transform infrared analysis (FTIR), water contact angle estimation (WCA), scanning electron microscope (SEM), dynamic mechanical analyzer (DMA) and thermogravimetric analysis (TGA). Enhanced Direct contact membrane distillation (EDCMD) unit was used for estimating the efficiency of the performance of the synthesized NC membranes via 60 °C feed synthetic water and/or saline oil field produced water samples containing salinities 123,14 mg/L. Adjusting the operational procedures and water characteristics confirmed a high salt rejection of 99.99% by the synthesized NC membranes. The maximum permeate flux achieved in the order of SWCNTs (20.91) > Al₂O₃ (19.92) > CuO (18.92) > MWCNT (18.20) (L/m².h) with adjusted concentration of 0.5, 0.75, 0.75, 0.1 wt% compared with PS weight, i.e. 16%. The optimum operational circumstances comprised feed and permeate temperatures 60 °C and 20 °C, respectively. The achieved flux was 5.97 L/m².h, using brine oil field produced water, via PS/PEG/SWCNTs membrane with 0.5 wt% of SWCNTs. Moreover, the membrane indicated sustaining performance stability in the 480 min continuous desalination testing, showing that the synthesized PS/PEG/SWCNTs NC modified membrane may be of magnificent potential to be activated in EDCMD procedure for water desalination.     

Experimental and modeling study of oxygen permeation modes for asymmetric mixed-conducting membranes

An asymmetric mixed-conducting membrane consists of a thin dense layer and a porous support, and its application has drawn considerable attention, because it is expected to have a more promising potential in the practical application compared with the symmetric membrane. However, with the introduction of support in the asymmetric membrane, two possible permeation modes are produced. One mode is that oxygen permeates from the support to the thin dense layer (designated as SD mode). The other is in the direction from the thin dense layer to the support (designated as DS mode). Thus, from the viewpoint of choosing an appropriate oxygen permeation mode to make better use of the membrane, it is necessary to study the oxygen flux in these two modes. In this paper, their effects on the oxygen flux of asymmetric membranes were investigated from the experiment and the model. The modeling results showed a good agreement with the experimental data. Our study demonstrates that when the asymmetric membrane adopts the SD mode, it is beneficial for the membrane to obtain higher oxygen permeation flux.     

PVDF membrane formation by diffusion‐induced phase separation‐morphology prediction based on phase behavior and mass transfer modeling

The equilibrium phase behavior of water (nonsolvent)‐DMF (solvent)‐PVDF system at 25°C was investigated via both theoretical and experimental approaches. Using binary interaction parameters, χ_ij, obtained previously, the theoretical phase boundaries were computed and were found to match closely the measured binodal and crystallization‐induced gelation data. Membranes were prepared using the isothermal immersion‐precipitation processes in various dope and bath conditions. The formed membranes demonstrated a broad spectrum of morphologies: At one extreme, asymmetric structure was obtained featuring a continuous tight skin and a sublayer composed of parallel macrovoids and cellular pores; at the other limit, skinless microporous membrane was produced with spherical particles packed into a bi‐continuous structure. The crystalline characters of PVDF gels and membranes were examined by small angle light scattering, scanning electron microscopy, and differential scanning calorimetry techniques. In addition, diffusion trajectories and concentration profiles in the membrane solution before precipitation were calculated for the immersion process. These results predicted reasonably the various morphologies observed in the membranes. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2079–2092, 1999     

Potential of membrane distillation in seawater desalination: Thermal efficiency,sensitivity study and cost estimation

In this work, an extensive analysis on direct contact membrane distillation (DCMD) performance was developed to estimate the mass flux and the heat efficiency, considering transport phenomena, membrane structural properties and most sensitive process parameters, with the aim to provide optimization guidelines for materials and methods. The results showed that an increase of the temperature gradient resulted in the enhancement of both transmembrane flux and thermal efficiency. The investigation of the effects of membrane properties confirmed that better DCMD performance was achieved when using polymeric membranes characterized by low thermal conductivity (flux and thermal efficiency declined by 26% and 50%, respectively, when increasing thermal conductivity from 0.1 to 0.5 W/m K), and high porosity. An optimal thickness value (around 0.7 mm) was identified when operating at low temperature gradient (<5 °C). However, at higher temperature gradient (>10 °C), increasing the membrane thickness from 0.25 to 1.55 mm resulted in a flux decay of about 70% without a significant improvement in thermal efficiency.     

Photocatalytic degradation of ammonia and butyric acid in plug-flow reactor: Degradation kinetic modeling with contribution of mass transfer

Photocatalytic treatment of polluted air by odorous contaminants – ammonia and butyric acid – is investigated in a plug-flow reactor covered by non-woven fiber textile coated with TiO₂. For the first time, the single-component degradation pathway of ammonia by photocatalysis at ambient condition is highlighted. It appears fundamentally different compared to the butyric acid degradation pathway. The ammonia degradation pathway highlights a possible auto-accelerated behavior of the reaction. The chemical degradation kinetics follows the Langmuir–Hinshelwood model, though observed oxidation rates depend upon flow conditions in the reactor. Thus, interpretation of degradation results through a model considering the Langmuir–Hinshelwood approach and mass transfer phenomenon is presented. This model succeeds with a pair of determined kinetic constants and mass transfer coefficients to describe experimental results for different flow rates and for both pollutants, though they present wide dissimilarities in their degradation pathways.     

Development and application of polymer-coated hollow fiber membrane microextraction to the determination of organochlorine pesticides in water

A novel extraction procedure coupled with gas chromatography-mass spectrometric detection for quantification of organochlorine pesticides (OCPs) in water is described. Amphiphilic polyhydroxylated polyparaphenylene (PH-PPP) was synthesized and coated on the surfaces of a porous polypropylene hollow fiber membrane (HFM). Due to the high porosity of the HFM, maximum active surface area to achieve high extraction efficiency is expected. The polymer-coated HFM was used for the extraction of 15 OCPs from water. The extraction efficiency was compared with emerging and established methods such as liquid-phase microextraction (LPME), solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE) techniques. We term the current procedure as polymer-coated hollow fiber microextraction (PC-HFME). PC-HFME showed good selectivity and sensitivity. Detection limits for OCPs were in the range of 0.001-0.008 microg l(-1). The sensitivity and selectivity of the coated HFM could be adjusted by changing the characteristics of the coated PH-PPP film.     

Single strand hollow fiber membrane (SSHFM): an on-line sample preparation for the flow based colorimetric determination of free iron in fruit juices

A single strand hollow fiber membrane (SSHFM) was developed for the on-line sample preparation for the flow based colorimetric determination of free iron levels in fruit juices. The SSHFM, as used, could separate Fe²⁺ from some spectrophotometric interfering agents in the fruit juice, such as pigments, solid suspensions and polysaccharides. The screening process was likely to have been primarily based on dialysis, wherein only ions or molecules that are smaller than the pores of the membrane can diffuse through while relatively larger molecules or particles could not. Two flow modes, a continuous and stopped flow, were studied. Factors that influenced the sensitivity (%dialysis) of the method, such as the flow rate, sample volume, flow direction and stopped flow time, were optimized. The stopped flow mode was found to be relatively more sensitive than the continuous flow mode and displayed a linear range of 1-30 mg L⁻¹ Fe²⁺, a limit of detection of 0.5 mg L⁻¹, and a % relative standard deviation of less than 2% (n = 8) for the analysis of 10 mg L⁻¹ Fe²⁺ spiked grape juice samples. A sample throughput of 24 samples h⁻¹, was attained without any further sample treatment.     

CFD modeling of reaction and mass transfer through a single pellet: Catalytic oxidative coupling of methane

In this study a mathematical model of a small scale single pellet for the oxidative coupling of methane(OCM)over titanite pervoskite is developed.The method is based on a computational fluid dynamics(CFD)code which known as Fluent may be adopted to model the reactions that take place inside the porous catalyst pellet.The steady state single pellet model is coupled with a kinetic model and the intra-pellet concentration profiles of species are provided.Subsequent to achieving this goal,a nonlinear reaction network consisting of nine catalytic reactions and one gas phase reaction as an external program is successfully implemented to CFD-code as a reaction term in solving the equations.This study is based on the experimental design which is conducted in a differential reactor with a Sn/BaTiO3 catalyst(7-8 mesh) at atmospheric pressure,GHSV of 12000 h-1,ratio of methane to oxygen of 2,and three different temperatures of 1023,1048 and 1073 K.The modeling results such as selectivity and conversion at the pellet exit are in good agreement with the experimental data.Therefore,it is suggested that to achieve high yield in OCM process the modeling of the single pellet should be considered as the heart of catalytic fixed bed reactor.     

A comparison of ion and atom behavior in the first stage of an inductively coupled plasma mass spectrometer vacuum interface: Evidence of the effect of an ambipolar electric field

Velocities of argon atoms and calcium ions were measured in the first vacuum stage of an inductively coupled plasma mass spectrometer using high-resolution laser-excited fluorescence spectroscopy. The calcium ions reached terminal velocities in the supersonic expansion that were consistently 5–6% higher than those of argon atoms, despite minimal differences in the masses of the two species. A computational model of the expansion was developed that shows the development of an ambipolar electric field along the expansion axis. With reasonable assumptions about electron temperatures in the expansion, the model accounts for the differences between the terminal velocities of the neutral argon atoms and the singly-charged calcium ions.     

Gel effect in the bulk reversible addition–fragmentation chain transfer polymerization of methyl methacrylate: Modeling and experiments

A model for the evaluation of the kinetics and the chain length distribution in living/controlled radical polymerization mediated by reversible addition–fragmentation chain transfer (RAFT) in bulk is presented. Using the free volume theory, the model accounts for the diffusion limitations over both termination and RAFT exchange reactions. Model predictions are compared to experimental results of methyl methacrylate polymerization with cumyl dithiobenzoate as a RAFT agent. It is shown that the polymerization retardation observed in living systems at large conversions is well predicted. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1071–1085, 2006