Academic literature on the topic 'Transmembrane distillation'

Finned tubular air gap membrane distillation is a new membrane distillation method, and its functional performance, characterization parameters, finned tube structures, and other studies have clear academic and practical application value. Therefore, the tubular air gap membrane distillation experiment modules composed of PTFE membrane and finned tubes were constructed in this work, and three representative air gap structures, including tapered finned tube, flat finned tube, and expanded finned tube, were designed. Membrane distillation experiments were carried out in the form of water cooling and air cooling, and the influences of air gap structures, temperature, concentration, and flow rate on the transmembrane flux were analyzed. The good water-treatment ability of the finned tubular air gap membrane distillation model and the applicability of air cooling for the finned tubular air gap membrane distillation structure were verified. The membrane distillation test results show that with the tapered finned tubular air gap structure, the finned tubular air gap membrane distillation has the best performance. The maximum transmembrane flux of the finned tubular air gap membrane distillation could reach 16.3 kg/m2/h. Strengthening the convection heat transfer between air and fin tube could increase the transmembrane flux and improve the efficiency coefficient. The efficiency coefficient (σ) could reach 0.19 under the condition of air cooling. Compared with the conventional air gap membrane distillation configuration, air cooling configuration for air gap membrane distillation is an effective way to simplify the system design and offers a potential way for the practical applications of membrane distillation on an industrial scale.

The main idea of membrane distillation is to use a porous hydrophobic membrane as a barrier that isolates vapor from aqueous solutions. It is similar to the evaporation process from a free water surface but introduces solid–liquid interfaces and solid–vapor interfaces to a liquid–vapor interface. The transmembrane mass flux of a membrane-distillation process is affected by the membrane’s intrinsic properties and the temperature gradient across the membrane. It is interesting and important to know whether the evaporation process of membrane distillation is faster or slower than that of a free-surface evaporation under the same conditions and know the capacity of the transmembrane mass flux of a membrane-distillation process. In this work, a set of proof-of-principle experiments with various water surface/membrane interfacial conditions is performed. The effect and mechanism of membrane-induced evaporation are investigated. Moreover, a practical engineering model is proposed based on mathematical fitting and audacious simplification, which reflects the capacity of transmembrane flux.

It has been identified that temperature polarization and concentration polarization are typical near-surface phenomena limiting the performance of membrane distillation. The module design should allow for effective flow, reducing the polarization effects near the membrane surfaces and avoiding high hydrostatic pressure drops across and along the membrane surfaces. A potential route to enhancing the membrane distillation performance is geometry modification on the flow channel by employing baffles as vortex generators, reducing the polarization effects. In this work, various baffles with different structures were fabricated by 3D printing and attached to the feed flow channel shell in an air gap membrane distillation module. The hydrodynamic characteristics of the modified flow channels were systematically investigated via computational fluid dynamics simulations with various conditions. The membrane distillation tests show that adding the baffles to the feed channel can effectively increase the transmembrane flux. The transmembrane flux with rectangular baffles and shield-shaped baffles increases by 21.8% and 28.1% at the feed temperature of 70 °C. Moreover, the shield-shaped baffles in the flow channel not only enhance the transmembrane flux but also maintain a low-pressure drop, making it even more significant.

In order to enhance the separation performance and reduce the heat loss of transmembrane for membrane distillation, the thermal efficiency and hydrophobicity of the membrane distillation need to be simultaneously enhanced. In this work, a polyvinylidene difluoride/polyethylene glycol terephthalate (PVDF/PET) hydrophobic/hydrophilic membrane has been prepared by non-solvent phase induction method. Nanosized silica aerogel (SiAG) with high porosity has been added to the composite membranes. The modifying effects and operating conditions on permeate flux and thermal efficiency in direct contact membrane distillation (DCMD) are investigated. Furthermore, the latent heat of vaporization and the heat transfer across the membranes have been compared for SiAG addition, which indicates that the composite PVDF@SiAG/PET membranes demonstrate a great potential for distillation-separation application due to their high heat efficiency.

Vacuum-enhanced direct contact membrane distillation (VEDCMD) has been proven experimentally to improve the permeate flux, compared with direct contact membrane distillation (DCMD). However, the theoretical mechanism for its transmembrane transfer process has not been revealed sufficiently. In this paper, with full consideration of the different driving forces of diffusion and Poiseuille flow under the vacuum enhancing condition, a theoretical transmembrane model for mass and heat transfer in VEDCMD is proposed. The CFD model and experimental platform are established to verify the theoretical model. The simulated results agree with the experimental data well, and nearly 200% improvement of the permeate flux is obtained when the permeate pressure drops to 30 kPa. The flow fields of the flow along the membrane surface are obtained and analyzed, with good consistency in the variation of the permeate flux. Since all the parameters of the proposed model are independent of the operating condition, the model is much easier for use and has better adaptability to fluctuating operating conditions.

In the design of membrane distillation systems, the effect of different heat transfer coefficient models on the transmembrane flux seems to have been overlooked thus far. Interestingly, the range of discrepancy in the results of the transmembrane flux is wide, especially in the laminar flow region, where MD is often operated. This can be inferred by studying the design and parameters of the direct contact membrane distillation system. In this study, the physical and physiochemical properties that affect the design of MD are comprehensively reviewed, and based on the reviewed parameters, an MD design algorithm is developed. In addition, a cost analysis of the designed MD process for low-grade-energy fluids is conducted. As a result, a total unit product cost of USD 1.59/m3, 2.69/m3, and 15.36/m3 are obtained for the feed velocities of 0.25, 1 and 2.5 m/s, respectively. Among the design parameters, the membrane thickness and velocity are found to be the most influential.

Water scarcity raises important concerns with respect to human sustainability and the preservation of important ecosystem functions. To satisfy water requirements, seawater desalination represents one of the most sustainable solutions. In recent decades, membrane distillation has emerged as a promising thermal desalination process that may help to overcome the drawbacks of traditional desalination processes. Nevertheless, in membrane distillation, the temperature at the feed membrane interface is significantly lower than that of the bulk feed water, due to the latent heat flux associated with water evaporation. This phenomenon, known as temperature polarization, in membrane distillation is a crucial issue that could be responsible for a decay of about 50% in the initial transmembrane water flux. The use of plasmonic nanostructures, acting as thermal hotspots in the conventional membranes, may improve the performance of membrane distillation units by reducing or eliminating the temperature polarization problem. Furthermore, an efficient conversion of light into heat offers new opportunities for the use of solar energy in membrane distillation. This work summarizes recent developments in the field of plasmonic-enhanced solar evaporation with a particular focus on solar-driven membrane distillation applications and its potential prospects.

A new crystallization process for sodium bicarbonate (NaHCO3) was studied, proposing the use of osmotic membrane distillation crystallization. Crystallization takes place due to the saturation of the feed solution after water evaporation on the feed side, permeating through the membrane pores to the osmotic side. The process operational parameters, i.e., feed and osmotic velocities, feed concentration, and temperature were studied to determine the optimal operating conditions. Regarding the feed and osmotic velocities, values of 0.038 and 0.0101 m/s, respectively, showed the highest transmembrane flux, i.e., 4.4 × 10−8 m3/m2·s. Moreover, study of the temperature variation illustrated that higher temperatures have a positive effect on the size and purity of the obtained crystals. The purity of the crystals obtained varied from 96.4 to 100% In addition, the flux changed from 2 × 10−8 to 7 × 10−8 m3/m2·s with an increase in temperature from 15 to 40 °C. However, due to heat exchange between the feed and the osmotic solutions, the energy loss in osmotic membrane distillation crystallization is higher at higher temperatures.

Le sujet majeur de cette thèse porte sur l'étude conceptuelle d'un procédé de distillation transmembranaire pour la valorisation d'effluents industriels. Les enjeux scientifiques étudiés sont propres aux domaines couverts par les procédés de séparations avancés : Matériaux : polymères denses ou poreux, validations expérimentales à l'échelle laboratoire des modèles de transfert, modélisation du transfert de matière multiconstituants, transferts couplés matière / chaleur, simulation et optimisation de procédés. Dans cette thèse, les enjeux sont traités par simulation des procédés. Une brique d'opération unitaire dédiée aux calculs des technologies membranaires (telle que la distillation transmembranaire) est proposé. Il tient en compte des phénomènes physiques et chimiques à trois niveaux : à l'échelle du matériel, du module (géométrie) et du procédé (conditions opératoires). Le modèle mathématique générique présenté est adapté aux trois systèmes d'intérêt industriel : le dessalement d'eau de mer (composant non volatil), le traitement des effluents contenant du bore (composant moins volatil que l'eau) et la récupération de l'ammoniaque (composant plus volatil que l'eau). L'adaptation des modèles mathématiques est appuyée sur des données expérimentales de la littérature et complétée par des expériences réalisées sur des dispositifs à l'échelle du laboratoire. Ces expériences ciblent la caractérisation des deux phénomènes principaux nécessaires pour le calcul de la sélectivité du procédé : la modélisation des équilibres liquide-vapeur et l'évaluation des coefficients de transfert pour chaque composant au niveau du matériel. Ce code dédié, modifié avec des résultats expérimentaux, est ensuite intégré aux logiciels de simulation de procédés, permettant l'optimisation d'architecture, la synthèse et la simulation de procédés. Les enjeux industriels sont évalués par comparaison entre les technologies déployées traditionnellement et les technologies membranaires à partir de l'évaluation des performances énergétiques et de production. Finalement, les résultats permettent d'identifier le potentiel de la distillation transmembranaire pour la séparation sélective des composants d'intérêt industriel comme des procédés alternatifs aux technologies existantes
The major subject of this research is the conceptual study of a transmembrane distillation process for the recovery of industrial effluents. The scientific issues studied in this work are covered by advanced separation processes: Materials: dense or porous polymers, experimental quantification at laboratory scale of the mass transfer mechanism, modelling of mass and heat coupled transfers, process simulation and optimization. In this thesis, the technological issues are addressed by a process simulation approach. A numerical unit operation brick for membrane technology simuation is proposed. It considers the physical and chemical phenomena at three levels: material, module (geometry, flow patterns) and process (operating conditions). The generic mathematical model presented in this work is adapted to three systems of industrial interest: water desalination (non-volatile component), boron-containing effluent treatment (component less volatile than water) and ammonia recovery (component more volatile than water). The adaptation of the mathematical model is based on experimental data found in the literature and complemented by experiments performed on laboratory-scale equipment. These experiments target the comprehension of the two main phenomena necessary for the calculation of the process selectivity: the modelling of the liquid-vapour equilibrium and the evaluation of the transfer coefficients for each component at the material level. This dedicated code, modified with the experimental results, is then integrated into a process simulation software, allowing architecture optimization, process synthesis and optimization. Industrial issues are evaluated by comparing traditionally deployed technologies (thermal-based technologies) with membrane technologies based on the evaluation of energy and production performances. Finally, the results allow the identification of the transmembrane distillation potential for the selective separation of components as alternative processes to existing technologies