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Papers by Mohamed Essalhi

Response surface methodology (RSM) and artificial neural network (ANN) have been used to develop ... more Response surface methodology (RSM) and artificial neural network (ANN) have been used to develop predictive models for simulation and optimization of reverse osmosis (RO) desalination process. Sodium chloride aqueous solutions were employed as model solutions for a RO pilot plant applying polyamide thin film composite membrane, in spiral wound configuration. The input variables were sodium chloride concentration in feed solution, C, feed temperature, T, feed flow-rate, Q, and operating hydrostatic pressure, P. The RO performance index, which is defined as the salt rejection factor times the permeate flux, has been considered as response. Both RSM and ANN models have been developed based on experimental designs. Two empirical polynomial RSM models valid for different ranges of feed salt concentrations were performed. In contrast, the developed ANN model was valid over the whole range of feed salt concentration demonstrating its ability to overcome the limitation of the quadratic polynomial model obtained by RSM and to solve non-linear problems. Analysis of variance (ANOVA) has been employed to test the significance of response surface polynomials and ANN model. To test the significance of ANN model, the estimation of the degree of freedom due to residuals has been detailed. Finally, both modeling methodologies RSM and ANN were compared in terms of predictive abilities by plotting the generalization graphs. The optimum operating conditions were determined by Monte Carlo simulations considering: (i) the four input variables, (ii) for typical brackish water with a fixed concentration of 6 g/L and (iii) for typical seawater with a fixed concentration of 30 g/L. Under the obtained optimal conditions maximum RO performance indexes have been achieved experimentally.▶ The highest factors interaction effects exists between the pressure and salt concentration. ▶ The feed flow rate has the smallest non-linear effect on the RO performance index. ▶ The effect of pressure is more significant at higher feed temperatures. ▶ The effect of the feed temperature is higher at higher operating pressures. ▶ The optimal conditions offered by ANN are better than those given by RSM.

A fluorinated surface modifying macromolecule (SMM) was synthesized and blended into the casting ... more A fluorinated surface modifying macromolecule (SMM) was synthesized and blended into the casting solution of polyetherimide used as host polymer. A composite porous hydrophobic/hydrophilic membrane was prepared by the phase inversion technique in a single casting step. The membrane was characterized by different techniques. During membrane formation, SMM migrates to the top membrane surface increasing its hydrophobicity and decreasing its pore size, nodule size and roughness parameters. The thickness of the porous hydrophobic top layer was found to be around 4 μm. The membrane was used for desalination by air gap membrane distillation and direct contact membrane distillation. The experiments were performed for different sodium chloride aqueous solutions and various operating conditions. The water production rate was found to be high for direct contact membrane distillation because of the low resistance to mass transport achieved by the diminution of the water vapour transport path length through the hydrophobic thin top-layer of the membrane.Migration of fluorinated surface modifying macromolecule (SMM) to the air/polymer interface and formation of porous composite hydrophobic/hydrophilic membrane.► Synthesis of a fluorinated surface modifying macromolecule (SMM) and blend with polyetherimide. ► SMM migrates to the top membrane surface during membrane formation by phase inversion method. ► Composite porous hydrophobic/hydrophilic membrane was prepared using SMM and polyethermide. ► The hydrophobic top layer has a thickness of about 4 μm with a smooth surface and nano-metric pore size. ► The hydrophobic/hydrophilic membrane was applied for desalination by air gap and direct contact MD.

Desalination

A fractional factorial design and a steepest ascent method were applied for possible fabrication ... more A fractional factorial design and a steepest ascent method were applied for possible fabrication of hollow fibers by the dry/wet spinning technique. Seven spinning factors were taken into account. Different concentrations of the copolymer poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP with 400,000 g/mol molecular weight and the additive polyethylene glycol, PEG with 10,000 g/mol molecular weight were dissolved in N,N-dimethyl acetamide, DMAC. The developed approach permits localization of the region of experimentation, defect-free spinning conditions, to produce hollow fibers. The obtained hollow fiber membranes were characterized by scanning electron microscopy and atomic force microscopy. Penetration liquid in membrane pores and porosity were also determined. Finally the membranes were tested for desalination by direct contact membrane distillation. An optimal hollow fiber membrane was finally fabricated using the determined optimum spinning conditions: a copolymer concentration of 20% w/w, a PEG concentration of 6% w/w, an air gap length of 25 cm, an internal/external coagulation temperature of 37.5 °C, an internal coagulant flow rate of 19 ml/min, a pressure of 0.3 bar and free falling. This membrane exhibits the highest performance index and the greatest global desirability (i.e. high permeate flux and salt rejection factor).► Application of fractional factorial design and Box–Wilson steepest ascent method for hollow fiber spinning with seven factors. ► Fabrication of porous hydrophobic copolymer hollow fiber membranes by dry/jet wet spinning method for membrane distillation. ► Optimization of spinning process for fabrication of hollow fiber membranes. ► Characterization of spun hollow fibers by different techniques, SEM, AFM, liquid entry pressure and desalination by DCMD. ► The optimal spinning conditions were confirmed experimentally.

Journal of Membrane Science, 2011

Response surface methodology (RSM) and artificial neural network (ANN) have been used to develop ... more Response surface methodology (RSM) and artificial neural network (ANN) have been used to develop predictive models for simulation and optimization of reverse osmosis (RO) desalination process. Sodium chloride aqueous solutions were employed as model solutions for a RO pilot plant applying polyamide thin film composite membrane, in spiral wound configuration. The input variables were sodium chloride concentration in feed solution, C, feed temperature, T, feed flow-rate, Q, and operating hydrostatic pressure, P. The RO performance index, which is defined as the salt rejection factor times the permeate flux, has been considered as response. Both RSM and ANN models have been developed based on experimental designs. Two empirical polynomial RSM models valid for different ranges of feed salt concentrations were performed. In contrast, the developed ANN model was valid over the whole range of feed salt concentration demonstrating its ability to overcome the limitation of the quadratic polynomial model obtained by RSM and to solve non-linear problems. Analysis of variance (ANOVA) has been employed to test the significance of response surface polynomials and ANN model. To test the significance of ANN model, the estimation of the degree of freedom due to residuals has been detailed. Finally, both modeling methodologies RSM and ANN were compared in terms of predictive abilities by plotting the generalization graphs. The optimum operating conditions were determined by Monte Carlo simulations considering: (i) the four input variables, (ii) for typical brackish water with a fixed concentration of 6 g/L and (iii) for typical seawater with a fixed concentration of 30 g/L. Under the obtained optimal conditions maximum RO performance indexes have been achieved experimentally.▶ The highest factors interaction effects exists between the pressure and salt concentration. ▶ The feed flow rate has the smallest non-linear effect on the RO performance index. ▶ The effect of pressure is more significant at higher feed temperatures. ▶ The effect of the feed temperature is higher at higher operating pressures. ▶ The optimal conditions offered by ANN are better than those given by RSM.

Desalination, 2009

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP) hollow fiber membranes were prepared ... more Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP) hollow fiber membranes were prepared by the dry/wet spinning technique at different copolymer concentrations from 17 to 24 wt%. All the spinning parameters were kept constant except the copolymer concentration. The temperature of both the internal and external coagulants was maintained at 40ºC. The effects of the copolymer concentration on the morphological properties of the hollow fibers were studied in terms of external and internal diameter and scanning electron microscopy (SEM). It was found that the thickness of all tested hollow fibers did not change significantly. An evolution of the cross-section structure with the increase of the copolymer concentration was detected. The cross-section of the hollow fiber prepared with the lowest copolymer concentration exhibited a finger-like structure in both the external and internal layers disappearing in the internal layer as the copolymer concentration increases. Finally, a sponge-like structure is formed through all cross-section of the hollow fiber prepared with the highest concentration. This may be explained based on the decrease of the coagulation rate with the increase of the copolymer concentration in the dope solution.

Journal of Membrane Science, 2010

Poly(vinylidene fluoride-hexafluoropropylene), PVDF-HFP, hollow fiber membranes were prepared by ... more Poly(vinylidene fluoride-hexafluoropropylene), PVDF-HFP, hollow fiber membranes were prepared by the dry/wet spinning technique using different copolymer concentrations in the dope solutions ranging from 17 to 24 wt.%. All the spinning parameters were maintained constant except the copolymer concentration. The morphological properties of the hollow fiber membranes were studied in terms of scanning electron microscopy (SEM), atomic force microscopy (AFM) and void volume fraction. The effects of PVDF-HFP content in the spinning solutions were also studied by measuring the water entry pressure and direct contact membrane distillation (DCMD) permeate flux of the hollow fiber membranes. An increase in the copolymer concentration of the spinning solution resulted in a decrease in the precipitation rate and a transition of the cross-section structure from a finger-type structure to a sponge-type structure. Pore size, nodule size and roughness parameters of both the internal and external hollow fiber surfaces were determined by AFM. It was observed that the pore size decreased in both the internal and external surfaces of the hollow fiber membranes with increasing the copolymer concentration and reached a minimum value at the outer surface for PVDF-HFP concentrations greater than 20 wt.%. Water entry pressure values were decreased whereas both the void volume fraction and the DCMD permeate flux increased with decreasing the copolymer concentration.

Nanoscale research …, 2011

We employed an easy and direct method to measure the thermal conductivity of epoxy in the liquid ... more We employed an easy and direct method to measure the thermal conductivity of epoxy in the liquid (nanofluid) and solid (nanocomposite) states using both rodlike and platelet-like carbon-based nanostructures. Comparing the experimental results with the theoretical model, an anomalous enhancement was obtained with multiwall carbon nanotubes, probably due to their layered structure and lowest surface resistance. Puzzling results for functionalized graphene sheet nanocomposites suggest that phonon coupling of the vibrational modes of the graphene and of the polymeric matrix plays a dominant role on the thermal conductivities of the liquid and solid states.PACS: 74.25.fc; 81.05.Qk; 81.07.Pr.

Desalination, 2010

A solar thermal and photovoltaic-powered reverse osmosis (RO) desalination plant has been constru... more A solar thermal and photovoltaic-powered reverse osmosis (RO) desalination plant has been constructed and optimized for brackish water desalination. The central composite experimental design of orthogonal type and response surface methodology (RSM) have been used to develop predictive models for simulation and optimization of different responses such as the salt rejection coefficient, the specific permeate flux and the RO specific performance index that takes into consideration the salt rejection coefficient, the permeate flux, the energy consumption and the conversion factor. The considered input variables were the feed temperature, the feed flow-rate and the feed pressure. Analysis of variance (ANOVA) has been employed to test the significance of the RSM polynomial models. The optimum operating conditions have been determined using the step adjusting gradient method. An optimum RO specific performance index has been achieved experimentally under the obtained optimal conditions. The RO optimized plant guarantees a potable water production of 0.2 m3/day with energy consumption lower than 1.3 kWh/m3.

Response surface methodology (RSM) and artificial neural network (ANN) have been used to develop ... more Response surface methodology (RSM) and artificial neural network (ANN) have been used to develop predictive models for simulation and optimization of reverse osmosis (RO) desalination process. Sodium chloride aqueous solutions were employed as model solutions for a RO pilot plant applying polyamide thin film composite membrane, in spiral wound configuration. The input variables were sodium chloride concentration in feed solution, C, feed temperature, T, feed flow-rate, Q, and operating hydrostatic pressure, P. The RO performance index, which is defined as the salt rejection factor times the permeate flux, has been considered as response. Both RSM and ANN models have been developed based on experimental designs. Two empirical polynomial RSM models valid for different ranges of feed salt concentrations were performed. In contrast, the developed ANN model was valid over the whole range of feed salt concentration demonstrating its ability to overcome the limitation of the quadratic polynomial model obtained by RSM and to solve non-linear problems. Analysis of variance (ANOVA) has been employed to test the significance of response surface polynomials and ANN model. To test the significance of ANN model, the estimation of the degree of freedom due to residuals has been detailed. Finally, both modeling methodologies RSM and ANN were compared in terms of predictive abilities by plotting the generalization graphs. The optimum operating conditions were determined by Monte Carlo simulations considering: (i) the four input variables, (ii) for typical brackish water with a fixed concentration of 6 g/L and (iii) for typical seawater with a fixed concentration of 30 g/L. Under the obtained optimal conditions maximum RO performance indexes have been achieved experimentally.▶ The highest factors interaction effects exists between the pressure and salt concentration. ▶ The feed flow rate has the smallest non-linear effect on the RO performance index. ▶ The effect of pressure is more significant at higher feed temperatures. ▶ The effect of the feed temperature is higher at higher operating pressures. ▶ The optimal conditions offered by ANN are better than those given by RSM.

A fluorinated surface modifying macromolecule (SMM) was synthesized and blended into the casting ... more A fluorinated surface modifying macromolecule (SMM) was synthesized and blended into the casting solution of polyetherimide used as host polymer. A composite porous hydrophobic/hydrophilic membrane was prepared by the phase inversion technique in a single casting step. The membrane was characterized by different techniques. During membrane formation, SMM migrates to the top membrane surface increasing its hydrophobicity and decreasing its pore size, nodule size and roughness parameters. The thickness of the porous hydrophobic top layer was found to be around 4 μm. The membrane was used for desalination by air gap membrane distillation and direct contact membrane distillation. The experiments were performed for different sodium chloride aqueous solutions and various operating conditions. The water production rate was found to be high for direct contact membrane distillation because of the low resistance to mass transport achieved by the diminution of the water vapour transport path length through the hydrophobic thin top-layer of the membrane.Migration of fluorinated surface modifying macromolecule (SMM) to the air/polymer interface and formation of porous composite hydrophobic/hydrophilic membrane.► Synthesis of a fluorinated surface modifying macromolecule (SMM) and blend with polyetherimide. ► SMM migrates to the top membrane surface during membrane formation by phase inversion method. ► Composite porous hydrophobic/hydrophilic membrane was prepared using SMM and polyethermide. ► The hydrophobic top layer has a thickness of about 4 μm with a smooth surface and nano-metric pore size. ► The hydrophobic/hydrophilic membrane was applied for desalination by air gap and direct contact MD.

Desalination

A fractional factorial design and a steepest ascent method were applied for possible fabrication ... more A fractional factorial design and a steepest ascent method were applied for possible fabrication of hollow fibers by the dry/wet spinning technique. Seven spinning factors were taken into account. Different concentrations of the copolymer poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP with 400,000 g/mol molecular weight and the additive polyethylene glycol, PEG with 10,000 g/mol molecular weight were dissolved in N,N-dimethyl acetamide, DMAC. The developed approach permits localization of the region of experimentation, defect-free spinning conditions, to produce hollow fibers. The obtained hollow fiber membranes were characterized by scanning electron microscopy and atomic force microscopy. Penetration liquid in membrane pores and porosity were also determined. Finally the membranes were tested for desalination by direct contact membrane distillation. An optimal hollow fiber membrane was finally fabricated using the determined optimum spinning conditions: a copolymer concentration of 20% w/w, a PEG concentration of 6% w/w, an air gap length of 25 cm, an internal/external coagulation temperature of 37.5 °C, an internal coagulant flow rate of 19 ml/min, a pressure of 0.3 bar and free falling. This membrane exhibits the highest performance index and the greatest global desirability (i.e. high permeate flux and salt rejection factor).► Application of fractional factorial design and Box–Wilson steepest ascent method for hollow fiber spinning with seven factors. ► Fabrication of porous hydrophobic copolymer hollow fiber membranes by dry/jet wet spinning method for membrane distillation. ► Optimization of spinning process for fabrication of hollow fiber membranes. ► Characterization of spun hollow fibers by different techniques, SEM, AFM, liquid entry pressure and desalination by DCMD. ► The optimal spinning conditions were confirmed experimentally.

Journal of Membrane Science, 2011

Response surface methodology (RSM) and artificial neural network (ANN) have been used to develop ... more Response surface methodology (RSM) and artificial neural network (ANN) have been used to develop predictive models for simulation and optimization of reverse osmosis (RO) desalination process. Sodium chloride aqueous solutions were employed as model solutions for a RO pilot plant applying polyamide thin film composite membrane, in spiral wound configuration. The input variables were sodium chloride concentration in feed solution, C, feed temperature, T, feed flow-rate, Q, and operating hydrostatic pressure, P. The RO performance index, which is defined as the salt rejection factor times the permeate flux, has been considered as response. Both RSM and ANN models have been developed based on experimental designs. Two empirical polynomial RSM models valid for different ranges of feed salt concentrations were performed. In contrast, the developed ANN model was valid over the whole range of feed salt concentration demonstrating its ability to overcome the limitation of the quadratic polynomial model obtained by RSM and to solve non-linear problems. Analysis of variance (ANOVA) has been employed to test the significance of response surface polynomials and ANN model. To test the significance of ANN model, the estimation of the degree of freedom due to residuals has been detailed. Finally, both modeling methodologies RSM and ANN were compared in terms of predictive abilities by plotting the generalization graphs. The optimum operating conditions were determined by Monte Carlo simulations considering: (i) the four input variables, (ii) for typical brackish water with a fixed concentration of 6 g/L and (iii) for typical seawater with a fixed concentration of 30 g/L. Under the obtained optimal conditions maximum RO performance indexes have been achieved experimentally.▶ The highest factors interaction effects exists between the pressure and salt concentration. ▶ The feed flow rate has the smallest non-linear effect on the RO performance index. ▶ The effect of pressure is more significant at higher feed temperatures. ▶ The effect of the feed temperature is higher at higher operating pressures. ▶ The optimal conditions offered by ANN are better than those given by RSM.

Desalination, 2009

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP) hollow fiber membranes were prepared ... more Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP) hollow fiber membranes were prepared by the dry/wet spinning technique at different copolymer concentrations from 17 to 24 wt%. All the spinning parameters were kept constant except the copolymer concentration. The temperature of both the internal and external coagulants was maintained at 40ºC. The effects of the copolymer concentration on the morphological properties of the hollow fibers were studied in terms of external and internal diameter and scanning electron microscopy (SEM). It was found that the thickness of all tested hollow fibers did not change significantly. An evolution of the cross-section structure with the increase of the copolymer concentration was detected. The cross-section of the hollow fiber prepared with the lowest copolymer concentration exhibited a finger-like structure in both the external and internal layers disappearing in the internal layer as the copolymer concentration increases. Finally, a sponge-like structure is formed through all cross-section of the hollow fiber prepared with the highest concentration. This may be explained based on the decrease of the coagulation rate with the increase of the copolymer concentration in the dope solution.

Journal of Membrane Science, 2010

Poly(vinylidene fluoride-hexafluoropropylene), PVDF-HFP, hollow fiber membranes were prepared by ... more Poly(vinylidene fluoride-hexafluoropropylene), PVDF-HFP, hollow fiber membranes were prepared by the dry/wet spinning technique using different copolymer concentrations in the dope solutions ranging from 17 to 24 wt.%. All the spinning parameters were maintained constant except the copolymer concentration. The morphological properties of the hollow fiber membranes were studied in terms of scanning electron microscopy (SEM), atomic force microscopy (AFM) and void volume fraction. The effects of PVDF-HFP content in the spinning solutions were also studied by measuring the water entry pressure and direct contact membrane distillation (DCMD) permeate flux of the hollow fiber membranes. An increase in the copolymer concentration of the spinning solution resulted in a decrease in the precipitation rate and a transition of the cross-section structure from a finger-type structure to a sponge-type structure. Pore size, nodule size and roughness parameters of both the internal and external hollow fiber surfaces were determined by AFM. It was observed that the pore size decreased in both the internal and external surfaces of the hollow fiber membranes with increasing the copolymer concentration and reached a minimum value at the outer surface for PVDF-HFP concentrations greater than 20 wt.%. Water entry pressure values were decreased whereas both the void volume fraction and the DCMD permeate flux increased with decreasing the copolymer concentration.

Nanoscale research …, 2011

We employed an easy and direct method to measure the thermal conductivity of epoxy in the liquid ... more We employed an easy and direct method to measure the thermal conductivity of epoxy in the liquid (nanofluid) and solid (nanocomposite) states using both rodlike and platelet-like carbon-based nanostructures. Comparing the experimental results with the theoretical model, an anomalous enhancement was obtained with multiwall carbon nanotubes, probably due to their layered structure and lowest surface resistance. Puzzling results for functionalized graphene sheet nanocomposites suggest that phonon coupling of the vibrational modes of the graphene and of the polymeric matrix plays a dominant role on the thermal conductivities of the liquid and solid states.PACS: 74.25.fc; 81.05.Qk; 81.07.Pr.

Desalination, 2010

A solar thermal and photovoltaic-powered reverse osmosis (RO) desalination plant has been constru... more A solar thermal and photovoltaic-powered reverse osmosis (RO) desalination plant has been constructed and optimized for brackish water desalination. The central composite experimental design of orthogonal type and response surface methodology (RSM) have been used to develop predictive models for simulation and optimization of different responses such as the salt rejection coefficient, the specific permeate flux and the RO specific performance index that takes into consideration the salt rejection coefficient, the permeate flux, the energy consumption and the conversion factor. The considered input variables were the feed temperature, the feed flow-rate and the feed pressure. Analysis of variance (ANOVA) has been employed to test the significance of the RSM polynomial models. The optimum operating conditions have been determined using the step adjusting gradient method. An optimum RO specific performance index has been achieved experimentally under the obtained optimal conditions. The RO optimized plant guarantees a potable water production of 0.2 m3/day with energy consumption lower than 1.3 kWh/m3.