Numerical simulation studies of mass transfer under steady and unsteady fluid flow in two- and three-dimensional spacer-filled channels

Hollow fibre and spiral wound membrane (SWM) modules are the most common commercially available membrane modules. The latter dominate especially for RO, NF and UF and are the focus of this study. The main difficulty these types of modules face is concentration polarisation. In SWM modules, the spacer meshes that keep the membrane leaves apart also help reduce the effects of concentration polarisation. The spacer filaments act as flow obstructions, and thus encourage flow destabilisation and increase mass transfer enhancement. One of the detrimental aspects of the use of spacers is an increase of pressure losses in SWM modules. This study analyses the mechanisms that give rise to mass transfer enhancement in narrow spacer-filled channels, and investigates the relationship between flow destabilisation, energy losses and mass transfer. It shows that the regions of high mass transfer on the membrane surface correlate mainly with those regions where the fluid flow is towards the membrane. Based on the insights gained from this analysis, a series of multi-layer spacer designs are proposed and evaluated. In this thesis, a Computational Fluid Dynamics (CFD) model was used to simulate steady and unsteady flows with mass transfer in two- and three-dimensional narrow channels containing spacers. A solute with a Schmidt number of 600 dissolving from the wall and channel Reynolds numbers up to 1683 were considered. A fully-developed concentration profile boundary condition was utilised in order to reduce the computational costs of the simulations. Time averaging and Fourier analysis were performed to gain insight into the dynamics of the different flow regimes encountered, ranging from steady flow to vortex shedding behind the spacer filaments. The relationships between 3D flow effects, vortical flow, pressure drop and mass transfer enhancement were explored. Greater mass transfer enhancement was found for the 3D geometries modelled, when compared with 2D geometries, due to wall shear perpendicular to the bulk flow and streamwise vortices. Form drag was identified as the main component of energy loss for the flow conditions analysed. Implications for the design of improved spacer meshes, such as extra layers of spacer filaments to direct the bulk flow towards the membrane walls, and filament profiles to reduce form drag are discussed

Optimal Pipeline Design with Increasing CO2 Flow Rates

AbstractWide deployment of carbon capture and storage (CCS) will require extensive transportation infrastructure, quite often in the form of pipelines. The rollout of such large-scale infrastructure would undoubtedly require very large investments. In regions with several CO2 emission sources, it is possible that not all of the major CO2 sources will implement CCS at the same time. Shared oversized pipeline designs are often proposed in order to form a “cluster” of CO2 sources and serve as the backbone for an expanding CO2 transportation infrastructure, to which emission sources will be connected. This paper analyses the economics of using oversized and parallel pipelines for different typical pipeline length and CO2 flow rate combinations. For new CCS projects, the expansion methodology presented in this paper can identify the optimal pipeline design that minimises the cost per tonne of CO2 avoided over the life of the project. For existing projects, the expansion methodology identifies the optimal pipeline design change, which may include either using an existing pipeline as CO2 supply increases or duplicating pipelines

Quantifying the potential of pressure retarded osmosis advanced spacers for reducing specific energy consumption in hybrid desalination

A hypothetical PRO advanced spacer that delivers a 50 % mass transfer enhancement (i.e., 50 % higher Sherwood number) is simulated for a range of feed conditions and membrane properties, to shed insights into the effect of improved PRO spacer on the overall specific energy consumption (SEC) of RO-PRO hybrid desalination. Results show that a large increase in pressure drop in the PRO module has negligible impact on power density (PD) and SEC for RO-PRO. The analysis revealed that the PRO advanced spacer has marginal impact on SEC for a typical current PRO membrane. Even so, the PRO advanced spacer has an important impact in terms of PD, which can increase by 10 %, especially under severe external concentration polarization. The sensitivity analysis demonstrates that the extent of SEC reduction or power density enhancement related to the advanced spacer is most sensitive to the structural parameter. This is because internal concentration polarization is the major cause for osmotic pressure loss in PRO, which limits the potential PRO performance improvements from advanced spacers. Nevertheless, the benefits of PRO advanced spacers can be further exploited through the continuous development of new materials for novel membranes with a reduced structural parameter

Analysis of the effect of advanced FO spacer on the specific energy consumption of hybrid RO desalination system

Research involving hybrid forward osmosis-reverse osmosis (FO-RO) desalination has gained attention recently due to its potential to reduce energy consumption compared to traditional RO. This paper aims to understand the degree of the impact of advanced spacers in the FO process on the overall specific energy consumption (SEC) of FO-RO systems. The SEC for a representative advanced spacer is simulated and analysed under standard recovery and operating conditions. The results show that advanced spacers can significantly reduce SEC by up to 9.27% under the operating conditions considered. The results also show that placing an advanced spacer on the FO membrane draw side has a greater effect in reducing SEC compared to placing it on the feed side, due to the larger extent of ECP. It was found that FO channel pressure drop has insignificant impact on the SEC. The performance of advanced spacers in SEC reduction is most effective if the contribution of external concentration polarisation (ECP) to transmembrane osmotic pressure is high, if the contribution of internal concentration polarisation (ICP) is low, and if the effective transmembrane osmotic pressure is low. Optimal mixing in FO systems is therefore crucial to reduce SEC, especially for systems with severe ECP

Using Desalination to Improve Agricultural Yields: Success Cases in Mexico

Water scarcity is a global problem, motivating growth and development of new technologies for water treatment, reuse and desalination. For many arid regions in Mexico, especially in the northwest, agriculture is an important economic activity. The Yaqui Valley in Sonora, Mexico, faces problems related to aquifer overexploitation and saline intrusion, which have increased salt concentration in well water to 2000–9000 mg/L total dissolved solids (TDS) and led to soil salinization and low crop yields. This work evaluates the effect of TDS in irrigation water on crop yield. A 150 m3/d desalination plant was used, consisting of 12 SWC4B-MAX membrane modules, with 98% rejection and 75% recovery. Two crops were irrigated with control (4000 mg/L) and desalinated water (200 mg/L). Sorghum (Sorghum) had yields of 7.9 and 8.8 ton/ha, whereas tomatillo (Physalis philadelphica) had yields of 30.82 and 35.88 ton/ha, respectively. Evidently, the desalination process influences agricultural yields

3D CFD study of hydrodynamics and mass transfer phenomena for spiral wound membrane submerged-type feed spacer with different node geometries and sizes

Modification of the spacer geometry is a promising approach to increase the efficiency of reverse osmosis (RO) spiral wound membrane modules. Column nodes and spherical nodes are considered in this three-dimensional computational fluid dynamic (CFD) study to evaluate the hydrodynamic and mass transfer performance of submerged spacers with different node geometries and sizes. Small-scale CFD analysis results reveal that the column node has better mass transfer performance than the spherical node geometry because column nodes divert more flow to the filaments, leading to higher local velocity at the region between the filament and wall. Furthermore, when the dimensionless node diameter ratio of the column nodes increases from 0.3 to 1.2, Sherwood number and wall shear increase by 25% and 8% respectively at the expense of higher global friction factor (44%). A sea water RO full-scale analysis revealed that column node spacers yield higher average flux than spherical nodes and conventional spacers at high feed inlet velocity (> 0.1 m/s), because the mixing effects by the spacer that improve mass transfer are more prominent

Development, Characterization, and Applications of Capsaicin Composite Nanofiltration Membranes

Biofouling in reverse osmosis (RO) membranes is a severe problem, causing a decrease in both permeate flux and salt rejection and increasing transmembrane pressure. Capsaicin extract inhibits bacterial growth and is therefore used in this study to prepare a thin-film composite membrane and membrane support. Four types of nanofiltration (NF) membranes were prepared by interfacial polymerization onto a porous support prepared by the phase inversion method. Membrane A was the control membrane with no capsaicin extract, membrane B contains capsaicin in the polyamide thin film, membrane C contains capsaicin in the porous support, and membrane D contains capsaicin in both the thin film and support layers. Three different salts (Na2SO4, MgSO4, and NaCl) were used at different concentrations (1000, 3000, and 5000 ppm) to test the performance of the membranes in terms of salt rejection and permeate flux. Membrane B showed the highest rejection for all the salts and concentrations tested. For 5000 ppm NaCl, the permeate flux for membrane B was 14.81% higher, and salt rejection was 19.6% higher than membrane A. Future work will evaluate the anti-biofouling properties of the membranes prepared with capsaicin, when exposed to seawater microorganisms

Desalación por ósmosis inversa y su aprovechamiento en agricultura en el valle del Yaqui, Sonora, México

Sonora, situado al noroeste de México, ocupa el segundo lugar en cultivos de riego en el país. Los problemas de disponibilidad de los recursos hídricos, principalmente para agricultura en ese estado, se deben a la alta concentración de sales en los pozos, que van desde 2 000 hasta 5000 mg/l de sólidos disueltos totales (SDT). Estos altos valores de SDT son atribuidos a efectos de intrusión salina (Dévora, Gonzalez, & Saldivar, 2009), al tener una excesiva explotación del manto acuífero (Conagua, 2011). Sin embargo, el uso de tecnologías de desalación y modelos de predicción del proceso permiten aprovechar el recurso de manera óptima tanto en agua producto como en el rechazo. Es por esto que el objetivo es definir un modelo matemático de simulación para la predicción de la operación de una planta desaladora de ósmosis inversa alimentada por agua salobre, validando que el agua producto cumpla con los límites permisibles para su uso en agricultura. Con motivo de validar el modelo, el experimento consistió en habilitar una planta desaladora por ósmosis inversa (OI), con capacidad nominal de 100 m3/d, compuesta por ocho membranas SWC4 de 8”X40”, para ser utilizada en el riego del cultivo de sorgo (Sorghum), el cual presenta un rendimiento de 100%, a una concentración máxima de 2 000 mg/l de SDT en el agua de riego (4 mS/cm), en un área de 0.25 ha, con la finalidad de evaluar rendimiento en kg/m2 (ton/ha). Se regó la mitad del terreno con agua desalinizada (promedio de 64.8 mg/l de SDT) y la otra mitad con agua salobre de pozos subterráneos (promedio de 6 610 mg/l de SDT). Con el apoyo de un equipo de medición multiparamétrico modelo YSI 556 se determinó en el agua de alimentación, rechazo y permeado, la conductividad eléctrica (ìS/cm), sólidos disueltos totales (mg/l), pH y temperatura (°C). Con los datos obtenidos de la planta piloto se diseñó un modelo de simulación en la plataforma de MATLAB R2009a, usando Simulink, que cumple con la función de predecir el comportamiento de la planta desaladora, obteniendo concentraciones de agua producto y rechazo en diferentes arreglos con iteraciones de control, que incluye recirculación de salmuera en diferentes volúmenes. De forma subsecuente, este modelo fue utilizado para simular el aprovechamiento de caudal en el agua de rechazo, con la finalidad de incrementar la sustentabilidad del proceso. Los resultados del estudio muestran que al alimentar un agua de 6 610 mg/l de SDT, las corrientes de producto y rechazo son de 64.8 y 21 300 mg/l de SDT, respectivamente. La evaluación del proceso muestra que el costo de producción de agua es de 6.05 MX$/m3, muy similar a lo reportado por la International Desalination Association, que es de 6.70 MX$/m3. Con el uso de esta agua desalinizada se logró un incremento de producción de 1 ton/ha de sorgo, comparado al riego con agua salada. Estos datos se usaron para validar y calibrar el modelo. Los resultados de la modelación de recirculación muestran que conforme sea menor el porcentaje de recirculación de salmuera se pueden realizar hasta cinco iteraciones, sin incrementar la concentración del agua de alimentación por arriba de 42 000 mg/l, que es lo máximo permitido por la membrana utilizada en la planta. Se incrementó la productividad agrícola, por lo cual las inversiones públicas y privadas en el sector rural se consideran viables en el corto plazo. Se encontró que es posible, para las condiciones de la planta de 100 m3/d, recircular total o parcialmente la corriente de salmuera. Se pueden hacer varias iteraciones con esta corriente sin afectar de modo significativo el agua producto ni llevar al máximo la capacidad de la planta. Entre mayor sea el porcentaje de salmuera recirculada, menos iteraciones podrán realizarse, pero serán reducidas en mayor medida las descargas al medio ambiente

Investigation into the effectiveness of feed spacer configurations for reverse osmosis membrane modules using Computational Fluid Dynamics

© 2016 Elsevier B.V.Reverse osmosis operations for water treatment are usually energy intensive and responsible for most of the product price. Several studies used flow characteristics to compare different geometries of feed spacers, but these cannot completely explain the effectiveness of feed spacers for promoting mass transfer near membranes. A few recent studies introduced a concept (Spacer Configuration Efficacy, SCE) combining mass transfer and energy consumption, but SCE has been applied only to a limited extent. The present study uses 3-dimensional steady state Computational Fluid Dynamics with mass transfer to compare four channels with feed spacer configurations (Ladder-type, Triple, Wavy and Submerged) and an empty plain channel using SCE and other performance measures. In contrast to previous studies, a saturated concentration boundary condition is employed at the membrane surface and optimised meshing of the domain is discussed. Power law correlations for SCE and other performance measures developed from the simulation results enable quick evaluation of the spacers. Results indicated that the assumed saturated solute concentration at the membrane strongly affects the mass transfer coefficient. Based on SCE, the Wavy spacer configuration showed the highest performance for Re>120 among the obstructed geometries considered, while Ladder-type was better for Re<120

An evaluation of membrane properties and process characteristics of a scaled-up pressure retarded osmosis (PRO) process

YesThis work presents a systematic evaluation of the membrane and process characteristics of a scaled-up pressure retarded osmosis (PRO). In order to meet pre-defined membrane economic viability ( ≥ 5 W/m2), different operating conditions and design parameters are studied with respect to the increase of the process scale, including the initial flow rates of the draw and feed solution, operating pressure, membrane permeability-selectivity, structural parameter, and the efficiency of the high-pressure pump (HP), energy recovery device (ERD) and hydro-turbine (HT). The numerical results indicate that the performance of the scaled-up PRO process is significantly dependent on the dimensionless flow rate. Furthermore, with the increase of the specific membrane scale, the accumulated solute leakage becomes important. The membrane to achieve the optimal performance moves to the low permeability in order to mitigate the reverse solute permeation. Additionally, the counter-current flow scheme is capable to increase the process performance with a higher permeable and less selectable membrane compared to the co-current flow scheme. Finally, the inefficiencies of the process components move the optimal APD occurring at a higher dimensionless flow rate to reduce the energy losses in the pressurization and at a higher specific membrane scale to increase energy generation