With the increase of population every year, fresh water scarcity has rapidly increased and it is reaching a risky level, particularly in Africa and the Middle East. Desalination of seawater is an essential process for fresh water generation. One of the methods for desalination is membrane distillation (MD). MD process separates an aqueous liquid feed across a porous hydrophobic membrane to produce pure water via evaporation. Polyvinlidene fluoride (PVDF) membranes reinforced with a polyester fabric were fabricated as potential candidates for MD. Non-solvent induced phase separation coupled with steam treatment was used to prepare the PVDF membranes. A portion of the prepared membrane was coated with Teflon (AF2400) to increase its hydrophobicity. In the first study, the fabricated membranes were characterized using scanning electron microscopy and other techniques, and they were evaluated using direct contact MD (DCMD). The fabricated membranes showed a porous sponge-like structure with some macrovoids. The macrovoid formation and the spongy structure in the membrane cross-sections contributed significantly to a high permeate flux as they provide a large space for vapor water transport. The modified PVDF membranes with steaming and coating exhibited a permeate flux of around 40 L/h m2 (i.e. 27-30% increase to the control PVDF membrane) at temperatures of 60 °C (feed) and 20 °C (permeate). This increase in the permeate flux for the modified membranes was mainly attributed to its larger pore size on the bottom surface. In the second study, the control PVDF membrane was tested in two different module designs (i.e. semi-circular pipe and rectangular duct module designs). The semi-circular module design (turbulent regime) exhibited a higher permeate flux, 3-fold higher than that of the rectangular duct module design (laminar regime) at feed temperature of 60 °C. Furthermore, a heat energy balance was performed for each module design to determine the temperature polarization coefficients (TPC). The turbulent module design showed higher TPC (0.5-0.58) than the laminar module (0.1-0.14) (i.e. a poor module design). This indicates that the effect of temperature polarization on the laminar flow was significant, which is below the reported TPC range of 0.4-0.70.

The goal of this case study was to investigate the behavior of real textile wastewater in DCMD (Direct Contact Membrane Distillation) treatment and subsequently to develop a simple and effective pretreatment for it. To this moment, this work is one of the only studies to make an in-depth analysis of the treatment while considering the complexity of this effluent, which is inherently composed of Volatile Organic Compounds (VOCs) and surfactants. After the application of pretreatment, it became clear that the main concern with textile wastewater treatment using MD is wetting, not fouling. Sedimentation and filtration alone were effective in removing suspended solids, but insufficient in stopping wetting. However, neutralization before sedimentation and filtration was proven to be a fundamental step in reducing wetting rates. This improved performance happens due to the change in pH of the wastewater sample, which increases the rejection rates by the membrane. The best experiments, neutralized to pHs 7.40 and 9.06, achieved up to 99.89% rejection by the membrane, with up to 97% conductivity decrease when compared to an experiment without neutralization, 97% removal of COD, and 98% TOC. Overall, the permeate obtained in this work after pretreatment demonstrated excellent quality, and the recovered effluent can possibly be reused in the textile industry, aiming for Zero Liquid Discharge (ZLD) processes. Thus, scaling up this technology for real industrial use is still necessary, tailoring the treatment to the effluent's characteristics to obtain the best results.

Yes / In this work, an artificial neural network (ANN) model based on the experimental data was developed to study the performance of vacuum membrane distillation (VMD) desalination process under different operating parameters such as the feed inlet temperature, the vacuum pressure, the feed flow rate and the feed salt concentration. The proposed model was found to be capable of predicting accurately the unseen data of the VMD desalination process. The correlation coefficient of the overall agreement between the ANN predictions and experimental data was found to be more than 0.994. The calculation value of the coefficient of variation (CV) was 0.02622, and there was coincident overlap between the target and the output data from the 3D generalization diagrams. The optimal operating conditions of the VMD process can be obtained from the performance analysis of the ANN model with a maximum permeate flux and an acceptable CV value based on the experiment.

Yes / This paper presents a simulation study of vacuum membrane distillation (VMD) for desalination. A simulation model was built on Aspen Plus® platform as user defined unit operation for VMD module. A simplified mathematical model was verified and the analysis of process performance based on simulation was also carried out. Temperature and concentration polarization effects are significant in the conditions of higher feed temperature and/or vacuum pressure. The sign of difference of the vapour pressures between at the membrane interfaces, is a pointer of the vacuum pressure threshold. Increasing the vacuum pressure at lower feed temperature is an effective way to increase the permeate flux and reduce the energy consumption simultaneously.

Poly(vinylidene) fluoride (PVDF) nano-composite membranes were prepared. The dope solution contained varied concentrations of superhydrophobic SiO2 nano-particles. The fabricated flat sheet membranes were characterized extensively by SEM, FTIR, water contact angle, LEPw, surface roughness, pore size diameter and pore size distribution. The effect of the nano-particles on the membrane performance was then analysed. The nano-composite membranes showed increased surface pore diameter, elevated water contact angle measurements with lower LEPw when compared to the neat membrane. The 7 wt. % nano-composite membrane showed the greatest flux in a VMD process with 2.9 kg/m2.h flux achieved accounting to a 4 fold increase when compared to the neat membrane. Desalination test were carried out using a 35 g/L synthetic salt water and rejection >99.98% was obtained. The best performing nano-composite dope solution (7 wt. %) was then further treated for performance enhancement by increasing the water content to increase pore size and pore size distribution followed by coating with nano-fibres. The uncoated and coated flat sheets, were characterized by SEM, surface roughness, LEPw and CAw. Flux analysis showed that the increase in water content had little effects on the VMD flux. It also suggests that; the nano-fibre layer posed very little resistance to mass transfer. A comparison of VMD and DCMD was also done experimentally.

Desalination is one of the proposed methods to meet the ever increasing water demands. It can be subdivided into two broad categories, thermal based desalination and electricity based desalination. Multi-effect Distillation (MED), Multi-Stage Flashing (MSF), Membrane Distillation (MD) fall under former and Reverse Osmosis (RO), Electro-Dialysis (ED) fall under later. MD offers an attractive solution for seawater as well as brackish water distillation. It shows highly pure yields, theoretically 100% pure. The overall construction of a MD unit is way simpler than any other desalination systems. MD is a thermally driven diffusion process where desalination takes places in the form of water vapor transport across the membrane. It has low second law efficiency due to parasitic heat losses. The objective of the first part of the investigation is to thoroughly analyze a Direct Contact Membrane Distillation (DCMD) system from the view point of yield and exergy. The insights from exergy analysis are used in a design study, which is used for performance optimization. The first part concludes with a design procedure and design windows for large scale DCMD construction. In the second part of the investigation, focus is moved to waveguide solar energy collector. The idea behind an ideal waveguide is to reduce the complexity of modeling solar energy collection. The mathematical model provided in this analysis can be extended to a family of non-imaging optics in solar energy and serves as a benchmarking analysis tool. A waveguide is suitable for low temperature operations due to limitations on maximum continuous temperature of operation. Thus, it becomes an ideal solution for DCMD applications. A levelized cost analysis is presented for a waveguide powered DCMD plant of a 30,000 capacity. A combination of waveguide and DCMD shows levelized cost of water at $1.80/m³, which is found to be lower than previously reported solar desalination water costs. / Master of Science

Dans cette thèse ont été développés des modèles mathématiques pour les procédés de distillation membranaire à contact direct (DCMD) et avec balayage gazeux (SGMD) ainsi qu’un modèle sur l’hydrodynamique des bioréacteurs membranaires anaérobiques (AnMBRs) équipés d’un système de vibration membranaire induite (MMV). Les modèles pour la DCMD et la SGMD permettent de simuler le comportement des modules plats ou à fibres creuses sous différentes conditions opératoires, sans avoir recours aux données expérimentales ou à des équations empiriques pour les transferts de masse et de chaleur. Les modèles ont été validés avec des résultats expérimentaux et de la littérature et ont permis de déterminer l'influence de différents paramètres opérationnels et de la géométrie des modules sur les performances des procédés. Le modèle développé pour les AnMBRs équipés du système MMV permet d’étudier l’effet de la vibration membranaire sur l’hydrodynamique du réservoir. L’analyse paramétrique a permis d’étudier l’effet de la fréquence et de l’amplitude des vibrations sur la vitesse du fluide et la fraction volumique des solides dans le réservoir. Dans ce travail il a été démontré que les modèles proposés pourront être potentiellement appliqués à des études expérimentales préliminaires, l’optimisation des conditions opératoires, la conception des modules membranaires ainsi que pour l’estimation des coûts des procédés. / In this work have been developed general predictive models for direct contact membrane distillation (DCMD) and sweeping gas membrane distillation (SGMD) as well as a hydrodynamic model for anaerobic membrane bioreactors (AnMBRs) equipped with the induced membrane vibration (MMV) system. The DCMD and SGMD models allow simulating hollow fibre and flat sheet configurations under wide range of process conditions without empirical mass and heat transfer coefficients or laboratory experiments. The models have been validated with experimental and literature data. Indeed, the influence of operating conditions and membrane geometric characteristics on the process performance has been investigated. The model for AnMBRs with MMV studies the effect of the membrane vibration on the hydrodynamics of the AnMBR tank. The parametric study allows knowing, the effects of the vibration frequency and amplitude on the fluid velocity and volume fraction of solids. The conducted studies prove that all the proposed models would be potentially applied for the pre-experimental study, optimization of process conditions, design of membrane modules as well as for the further cost estimation of the processes.

A bench-scale, sweeping gas, flat-sheet Membrane Distillation (MD) unit was used to assess the importance of membrane architecture and operational variables to distillate production rate. Sweeping gas membrane distillation (SGMD) was simulated for various membrane characteristics (material, pore size, porosity and thickness), spacer dimensions and operating conditions (influent brine temperature, sweep gas flow rate and brine flow rate) based on coupled mass and energy balances. Model calibration was carried out using four membranes that differed in terms of material selection, effective pore size, thickness and porosity. Membrane tortuosity was the lone fitting parameter. Distillate fluxes and temperature profiles from experiments matched simulations over a wide range of operating conditions. Limitations to distillate production were then investigated via simulations, noting implications for MD design and operation. Under the majority of conditions investigated, membrane resistance to mass transport provided the primary limitation to water purification rate. The nominal or effective membrane pore size and the lumped parameter epsilon/delta tau (porosity divided by the product of membrane tortuosity and thickness) were primary determinants of membrane resistance to mass transport. Resistance to Knudsen diffusion dominated membrane resistance at pore diameters <0.3 mu m. At larger pore sizes, a combination of resistances to intra-pore molecular diffusion and convection across the gas-phase boundary layer determined mass transport resistance. Findings are restricted to the module design flow regimes considered in the modeling effort. Nevertheless, the value of performance simulation to membrane distillation design and operation is well illustrated.

No que diz respeito às águas residuais industriais, além da elevada frequência direta de efluentes salinos, o tratamento convencional de efluentes aquosos de diferentes tipos de indústria quase sempre resulta em uma solução contendo sais solúveis e insolúveis que podem ter qualidade suficiente para serem descartados em corpos aquosos, mas qualidade insuficiente para permitir a reutilização da água em processos industriais. Para reutilizar a água, visando o descarte zero de líquido, a cristalização se mostrou tecnicamente viável e tem sido utilizada nesta separação. Simultaneamente com a evaporação da água, os sais dissolvidos tornam-se materiais particulados que são subsequentemente separados por filtração ou centrifugação. Contudo, para atender às necessidades modernas da indústria em relação aos tratamentos com efluentes salinos, o processo de cristalização deve mostrar melhorias, especialmente devido à sua importância para atingir as metas de descarga de líquidos. Neste sentido, a cristalização simultânea representa a possibilidade de remover vários compostos em uma única operação de cristalização a partir de uma solução multicomponente. Isso significa que além da purificação da água, essa operação também pode permitir a recuperação dos compostos nos efluentes, que podem ser considerados como matéria-prima, eliminando resíduos - ou dando-lhes a destinação mais adequada. Para isso, é necessário projetar processos para separar os diferentes componentes que compõem as correntes de efluentes salinos e produzir produtos cristalinos com características morfológicas conhecidas. Dessa forma, o presente trabalho visou investigar o comportamento de sistemas ternários em cristalização, utilizando como modelo o sistema NaCl-KCl-H2O no processo de cristalização evaporativa em batelada, no intuito de avaliar a influência dos parâmetros de processo nos cristais formados e de traçar estratégias que possibilitassem o aproveitamento dos sais contidos nos efluentes. Três diferentes rotas de operação foram testadas para avaliar o comportamento dos cristais: uma inicialmente saturada apenas em NaCl, uma inicialmente saturada apenas em KCl e uma já de início saturada em ambos sais. Diferentes taxas de evaporação, tamanhos e teores de sementes foram testados. Verificou-se, de maneira geral, que o controle de dois fenômenos elementares é a chave para contornar os principais obstáculos desse processo: crescimento epitaxial e nucleação secundária. Em determinadas condições foi possível obter o produto das bateladas em distribuições bimodais, com cada pico rico em um dos sais, com purezas superiores a 90 %. Estes produtos, passíveis de separação simultânea dos cristais por composição e tamanho, foram obtidos em condições de supersaturações mais baixas e semeadura de cristais grandes de KCl e pequenos de NaCl. A partir destes resultados, considerando-se a necessidade de desenvolver técnicas de purificação de água menos intensivas em energia e mais favoráveis ao meio ambiente, um sistema de cristalização por membranas (MC) foi testado, utilizando-se o mesmo sistema modelo. O design proposto para MC possibilitou operar com este sistema por tempos superiores a 6 horas, com decaimento do fluxo em torno de 20 %. Contudo, o produto da cristalização simultânea por membranas não apresentou melhorias quanto à segregação. / In the industrial wastewater treatment, besides high direct frequency of saline effluents, conventional treatment of aqueous effluents of different types of industry usually results in a solution containing soluble and insoluble salts that may be of sufficient quality to be disposed in aqueous bodies, but insufficient quality for water reuse in industrial processes. In order to reuse the water, aiming at zero liquid discharge, crystallization has proved to be technically feasible and has been used in this separation. At the same time, with the evaporation of the water, the dissolved salts become particulate materials which are subsequently separated by filtration or centrifugation. However, to meet the industry\'s modern needs for saline effluent treatments, the crystallization process should show improvements, especially because of its importance in achieving liquid discharge goals. In this sense, simultaneous crystallization represents the possibility of removing several compounds in a single crystallization step from a multicomponent solution. This means that in addition to the purification of the water, this operation can also allow the recovery of the compounds in the effluents. In addition, particulate materials can be considered as raw material, eliminating waste - or disposing the residues appropriately. To this end, research is still needed to design processes to separate the different components that compose saline effluent streams and produce crystalline products with known morphological characteristics. Thus, the present work aimed to investigate the behaviour of ternary systems in crystallization, using as model the NaCl-KCl-H2O system in the batch evaporative crystallization process, in order to evaluate the influence of the process parameters on the crystals formed and to trace strategies to enable the use of the salts contained in the effluents. Three different routes of operation were tested to evaluate the behaviour of the crystals: one initially saturated only in NaCl, one initially saturated only in KCl and one initially saturated in both salts. Different evaporation rates, seed sizes and contents were tested. It was generally verified that control of two elementary phenomena is the key to overcome the main obstacles of this process: epitaxial growth and secondary nucleation. Some experimental conditions enablesd the obtaining of the batch product in bimodal distributions, where each peak was rich in one salt, with purities higher than 90 %. These fractions of the yield, that can be simultaneously separated by composition and size, were achieve mainly at lower supersaturations and using big KCl and small NaCl seeds. From these results, considering the need to develop energy-less and more environmentally friendly water purification techniques, a membrane crystallization (MC) system was tested using the same model system. The proposed design for the MC enable the operation for over 6 hours with a flux decay around 20 %. Yet, the proposed the yield of simultaneous membrane crystallization has not shown improvements in composition segregation.

Thesis (MTech (Mechanical Engineering))--Cape Peninsula University of Technology, 2018. / A membrane distillation crystallization (MDC) experimental setup was designed, constructed and commissioned with rectangular mixing crystallizer tanks. The advantages and disadvantages of a rectangular mixing tank are compared to the traditional cylindrical mixing tank with baffling by means of a computational fluid dynamic (CFD) analysis in Ansys Fluent. The effect of tank configuration and geometry on the hydrodynamic and mixing characteristics for efficient momentum, solid suspension, heat and mass transfer were investigated. The hydrodynamic conditions in a crystallizer-mixing tank determine the quality of fluid mixing essential for optimal crystallization. Forty-five degree pitched blade turbines (PBT) were used to provide the agitation in the stainless steel rectangular jacketed tanks. Clear polycarbonate replicas of the rectangular tanks were manufactured to visually observe the mixing process in the tanks. Silica particles were used to represent the calcium carbonate crystals in the experiment. The data gathered from these experiments showed that the tanks should be operated between 600 to 750 rpm in the CFD simulations to simulate partial to complete suspension. In the numerical simulations a rectangular tank was compared to a cylindrical tank with baffling of the same volume. The partial differential equations solved in the numerical simulation were the conservation of mass (continuity), conservation of momentum and additional turbulence equations. In order to solve the turbulent fluid flow characteristics, the industry standard two-equation model, namely the K-epsilon model was used. This model was refined by the addition of the Wen-Yu drag model, the Simonin turbulent dissipation and the Simonin et al. turbulence interaction models. The RANS based RNG (k-ε), derived from the instantaneous Navier-Stokes equation was selected as the preferred model to analyse the hydrodynamic flow fields in the tanks. The 3D sliding mesh method was used to compute a time accurate solution. The Eulerian-granular multiphase model was used to predict the degree of solids suspension in the tanks. The efficiency of mixing within the tank was measured by the tank’s ability to keep the crystals in suspension and preventing any particle from settling at the bottom for more than 1-2 second(s). The mixing tanks were initially loaded with 5% v/v, which equates to a loaded height of approximately 10 mm. The simulations were done with the use of the volume fraction function to visually observe the cloud height and gauge the homogeneity and distribution of the particulates within the fluid flow fields. The results from the experimental setup were compared to the CFD simulations to qualify the use of CFD simulations for the comparison of the geometrically different tanks. Lastly, the findings from the CFD simulations were used to compare the tanks and determine if the rectangular tank built for the MDC experiment perform satisfactorily to replace a standard cylindrical tank with baffling for this application.