High Feed Temperature Research Articles

Dynamics and control of loop reactors: a review

In the loop reactor (LR) the system is composed of several reactor units that are organized in a loop and the feeding takes place at one of several ports with switching of the feed port in a periodic way. In its simplest operation a pulse is formed and rotates around it, producing high temperatures which enable combustion of dilute streams. A limiting model with infinite number of units was derived. Rotating pulses, that are steady in a coordinate moving with the switch velocity, emerge in both asymptotic and discrete models when the ratio of switching to front propagation velocities is around unity. But this behavior exists over a narrow domain of this ratio. Simulations were conducted with generic first order Arrhenius kinetics. Experimental observations of simple frozen rotating pulses are reviewed. Outside the narrow frozen rotating patterns domain the system may exhibit multi- or quasi-periodic operation separated by domains of inactive reaction. The bifurcation set incorporates many ’finger’-like domains of complex frequency-locked solutions that allow to significantly extend the operation domain with higher feed temperature or concentration. Control is necessary to attain stable simple rotating frozen patterns within the narrow domains of active operation. Various control approaches that were suggested, or experimentally applied for this purpose, are reviewed. Actual implementation of combustion in LR will involve several reactants of different ignition temperatures and varying concentration. Design and control should be aimed at producing locked fronts and avoid extinction of the slower reaction.

Evaluation of direct contact membrane distillation coupled with fractionation and ozonation for the treatment of textile effluent

Direct contact membrane distillation (DCMD) coupled with fractionation or ozonation as a pretreatment method was investigated for the treatment of combined textile effluent. The stand-alone DCMD process has difficulty in directly treating the effluent due to the wetting issue, in particular at higher feed temperatures. Fractionation was conducted to 21 L combined effluent for 24 h. After the pretreatment, total organic carbon (TOC) and conductivity decreased by 47 % and 28 %, respectively, but an additional 5% concentrate was also generated. A long-term DCMD experiment was conducted to treat the fractionated effluent for 50 h and a high water recovery of 98 % was achieved. The optimum recovery of 95 % was identified to maintain stable performance of membrane. Ozonation was used to treat 550 mL combined eluent in batch mode. The TOC removal increased with time and reached the plateau after 1 min. The TOC was reduced by 38 %, while the colour of the combined effluent became lighter after 1 min treatment. As the temperature increased, longer ozonation time was required to inhibit the membrane wetting. For the feed inlet temperatures of 50 °C and 70 °C, the ozonation times of 40 s and 60 s were able to suppress the membrane wetting in the short-term tests. For the fractionated effluent, near zero discharge (water recovery of 98 %) could be achieved without serious wetting issue. To avoid membrane fouling issues, 95 % water recovery is recommended. For the 40 s ozonation pretreated effluent, DCMD could only achieve 38 % of the feed recovery before occurrence of wetting.

Modeling and simulation of non-isothermal packed-bed membrane reactor for decomposition of hydrogen iodide

HI decomposition using a membrane reactor is one of the important steps in hydrogen production by the thermal splitting of water using the Sulfur–Iodine cycle. Though, this reaction is endothermic. The mathematical model available to study the performance of such a reactor is an isothermal model. In this study, a non-isothermal mathematical model is developed by microscopic material and energy balance across the length of the membrane reactor. Experimental results from the literature are used for validation of the developed model. Comparing the simulations from the developed model and already reported isothermal model showed significant differences, which endorses the importance of the developed model. Later, the effects of different operating and design parameters were analyzed. Higher feed temperature (950–1000 K), lower feed pressure (100,000–150,000 Pa), lower feed flow rate (<200 ml/min), lower permeate pressure (<2500 Pa) and low to moderate N 2 /HI ratio (0.3–0.4) were found to be the optimum operating conditions. Similarly, the conversion also increased with increasing reactor length, membrane area, and membrane permeance but eventually became constant. Reactor length around 0.4 m, membrane area around 0.008 m 2 and membrane permeance around 2 × 10 −7 mol m 2 Pa −1 s −1 were estimated to be the optimum design conditions for the maximum HI decomposition. • A non-isothermal model for HI decomposition using membrane reactor is developed. • Developed model is validated using experimental results. • Performances of developed model are significantly different from isothermal model. • Operating and design parameters are optimize using developed model.

Multi-effect distillation brine treatment by membrane distillation: Effect of antiscalant and antifoaming agents on membrane performance and scaling control

One of the main advantages of the membrane distillation (MD) process is its ability to treat highly saline feed waters such as thermal desalination brines at moderate temperatures. However, scaling remains one of the major obstacles, causing a significant flux decline and membrane pore wetting. Furthermore, antiscalant and antifoaming agents are commonly utilized in conventional thermal desalination to prevent scale formation, and their effects on MD operation are not yet well understood. This study explores a multi-effect distillation (MED) brine as a potential feed source for MD system with respect to process performance and membrane scaling. The influence of chemicals present in MED brine as well as feed temperature on the scaling process is addressed in terms of vapor flux and salt crystals formation. The scale formation was monitored with the non-invasive optical coherence tomography (OCT) imaging, and results were validated by scanning electron microscopy (SEM). Additionally, the elemental composition of the scale was determined and its effect on membrane contact angle was evaluated. We found that depending on its concentration, the antiscalant prolonged the induction time of salt crystallization whereas antifoaming showed the opposite effect. Scaling mostly occurred due to calcium sulfate crystals formation with the large size needle-shaped crystals favored at higher feed temperature. However, no pore wetting was observed including locations where crystal deposition occurred. Results show that thermal desalination brine, which is already preheated and chemically pretreated, could be an appropriate feed source for MD to further increase the overall water recovery and reduce the marine environmental impact by reducing the brine discharge volume and its temperature.

Experimental and numerical evaluation of the energy requirement of multi-stage vacuum membrane distillation designs

Although vacuum membrane distillation (VMD) has a high distillate flux, its potential is limited by its high specific heat consumption (SHC). Flat sheet VMD systems recover the brine energy by employing a cascade system to improve the SHC. Hollow fiber VMD systems, however, have not yet been optimized for energy recovery. To overcome this knowledge gap, four multi-stage configurations (with and without brine energy recovery) were investigated via experimentation and through numerical simulations. The results show that having permeate flux promoting conditions, such as high feed temperature or lower vacuum pressure, improves the SHC for VMD configurations without brine recirculation, but have little impact for configurations with brine energy recovery. A Pareto multi-objective optimization showed that the optimized first-stage heating MS-VMD without brine recirculation has the highest SHC (908 kWh/m3) but one of the lowest LCOW (2.37 USD/m3). In contrast, the configuration with the first-stage heating with brine recirculation provided the lowest SHC (585–629 kWh/m3) and an acceptable LCOW (2.27–8.30 USD/m3). This study reveals that the development of MS-VMD represents a promising direction for thermal desalination technologies, particularly for high saline water applications.

Variance based sensitivity analysis and statistical optimization of design and operating parameters of spiral wound pervaporation modules for thiophene removal from FCC gasoline

Global sensitivity analysis and optimization is an essential step to understand the overall influence of input parameters. In this study, the influence of different operating and design parameters of the spiral wound pervaporation module is examined for the desulfurization of model gasoline. Feed temperature is found most influential parameter among all parameters for flux and enrichment factor. The sensitivity of other parameters is found relatively lesser. For temperature drop in the feed section and pressure drop in the permeate section, influences of design parameters become significant. The influence of interactions among input parameters on the decision variables is also found substantial except for total flux.Later, different operating and design parameters were also optimized. Lower permeate pressure, higher feed temperature, and Reynolds number are found better-operating conditions, and lower L/W (length to width ratio of a membrane leaf), smaller membrane area and the higher number of leaves are found better design conditions.

Simulation and multi-objective optimization of heat and mass transfer in direct contact membrane distillation by response surface methodology integrated modeling

Membrane distillation (MD) traditionally utilizes low-grade thermal energy to drive vapor transport through hydrophobic membrane pores for water treatment. The concurrent enhancement of water flux, thermal efficiency and water productivity to achieve the global optimization of MD is still a challenge. In this work, a new simulation and optimization approach is proposed by integrating theoretical heat and mass transfer models with response surface methodology to investigate the complicate interaction effects of various influencing factors on heat and mass transfer parameters, including overall mass transfer coefficient, feed/permeate side heat transfer coefficients, membrane surface temperature and temperature polarization coefficient, in direct contact membrane distillation (DCMD). The operating and module configuration variables show important interaction effects on enhancing the heat and mass transfer in the membrane module. The significantly elevated heat transfer coefficient on account of the combined high feed temperature, short module length and high feed velocity leads to a great improvement of water flux and thermal efficiency. The ultimate global optimum conditions for the high water productivity were determined where the high levels of heat transfer coefficient (5905W/(m2·K)), temperature polarization coefficient (0.66), water flux (63.0kg/(m2·h)) and thermal efficiency (0.83) all synchronously approach to their own pinnacles. The results indicate that it is an effective way to take advantages of the interaction effects of operating conditions with module configuration parameters to achieve the overall enhancement of MD performance.

The Performance of Membranes Interlayer-Free Silica-Pectin Templated for Seawater Desalination via Pervaporation Operated at High Temperature of Feed Solution

Recently, water scarcity is the big issues around the world. Especially in coastal area where the water distribution could not entranced and able to supply clean water for the citizen. The one and only solution is processing seawater to produce fresh and potable water. The desalination process using membrane was recommended to solve this issue. Due to that, the membrane with good structure and high hydro-stability was necessary to fabricate. The aim of this work is to investigate the performance of silica-pectin membranes for treating seawater by pervaporation employing silica based membranes. In this work, the silica-pectin membranes were successfully fabricated using Tetraethyl orthosilicate (TEOS) as silica precursor. Then, pectin from apple was also using in various concentrations (0; 0.1 to 0.5%). This organic material was implemented as a templating agent to produce in silica-pectin thin film. This thin films were dipcoated onto membranes support during membranes fabrication. These membranes were calcined in air at 300 and 400°C using rapid thermal processing (RTP) technique. All membranes were tested for water desalination via pervaporation set-up in various feed temperatures (25, 40 and 60°C). Results show that the membranes produced were crack-free and no pore dense. The FTIR-spectra and Fityk analysis refer to membrane of 2.5% at 300°C and 0.5% at 400°C are the optimum condition due to silanol and siloxane concentrations. An excellent performance was obtained at 0.5% at 400°C with water flux of 8.3 kg.m-2.h-1 and high salt rejection of 99.4% at 60 °C of feed temperature. It clearly demonstrates that the silica-pectin membrane has a robust structures due to the templating of carbon chains into silica matrices. The presence of carbon chains in silica matrices may form the smaller and robust pores as expected, that makes the excellent salt rejection in high feed temperature.

Simultaneous recovery of phosphorus and nitrogen from liquid digestate by vacuum membrane distillation with permeate fractional condensation

Abstract A vacuum membrane distillation (VMD) process with permeate fractional condensation on membrane downstream has been developed for simultaneous recovery of phosphorus and nitrogen from liquid digestate. The polytetrafluoroethylene (PTFE) membrane flux could reach 6000 g·m−2·h−1 with the rejection efficiency of total phosphorus (TP) over 0.99, under the condition of flowrate being 120 L·h−1 and temperature being 40 °C. Membrane fouling occurred with a film of organics and microorganism deposited on the surface of the membrane. Membrane flux could be reversed after the membrane was rinsed by water. Higher feed temperature and flowrate could improve the membrane flux, while hardly affect the rejection efficiency of total phosphorus. The concentration of TP could reach 1600 mg·L−1 after membrane distillation, which is about 5 times of that in initial liquid digestate. On the downstream of the membrane, some of the permeate vapor was condensed under the vacuum condition and most of water was collected here. The remaining vapor enriched with total nitrogen (TN) was compressed and pumped to the atmospheric condition to condense. The TN concentration in atmospheric condensate was as high as 7000 mg·L−1 with the process separation factor for ammonia being enhanced to 114.

Analysis of water gap membrane distillation process for water desalination

A detailed theoretical model of water gap membrane distillation process is presented for water desalination. The model predicts the permeate flux through the analysis of the coupled heat and mass transfer and steady-state energy balance through membrane and module layers. Natural convection inside the water gap is studied. Effects of membrane support plate and its geometry are included to account for different areas of heat and mass transfers. Model validation shows that considering the natural convection in the water gap is critical, except for gap widths less than 1 mm, where pure conduction prevails inside the gap. A modified correlation of the gap Nusselt number is adopted and validated. The new correlation provides very good match to experimental results for the water gap module under study. System performance indicators like evaporative efficiency, polarization coefficients, and gain output ratio (GOR) are studied. The efficiency reached a maximum of 93% at high feed temperature of 90 °C and high feed flow rate of 6 L/min. GOR reached a maximum value of about 1.3 at 90 °C and low feed rate of 1.5 L/min. The width of water gap has marginal effects on EE and GOR.

Efficient Production of Cyclopropylamine by a Continuous-Flow Microreaction System

Cyclopropylamine (CPA) is an essential intermediate in the preparation of many biologically active substances. The traditional batch method to prepare CPA is facing many problems in the Hofmann rearrangement step, such as low production efficiency, poor stability, and complex process. In this paper, a novel continuous-flow method was developed to prepare CPA with high efficiency. Taking advantages of characteristics of the continuous-flow microreaction system, the simple process with a one-stage reaction was successfully realized instead of the original two-stage reaction in batch reactors. The yield of CPA reached up to 96% in only 4 min residence time at 90 °C. Interestingly, with the higher feed temperature, the reaction concentration in continuous-flow approach can be even higher than that in the batch method.

Rejection of rare earth elements from a simulated acid mine drainage using forward osmosis: The role of membrane orientation, solution pH, and temperature variation

The high demand for the rare earth elements (REEs) in the clean technological industries implies the increased demand to produce these elements from ores and REEs containing impaired water sources such as acid mine drainage (AMD). However, the presence of these elements in AMD has increased their toxic exposure to the water environment. Herein, we investigated the role of the orientation of the membrane (such as active layer facing feed solution (AL-FS) or active layer facing draw solution (AL-DS) mode), solution pH, and temperature variation on the flux performance and rejection of three REEs (e.g., lanthanum, cerium, and dysprosium) from a simulated AMD solution using the forward osmosis (FO) process. It was found that there was a greater water flux at high pH, AL-DS mode of the FO system and high feed and draw solution temperature. FO could effectively reject all three REEs in both AL-FS and AL-DS membrane orientations mode due to steric hindrance. The rejection of these REEs in the AL-DS mode was slightly lower (82–88%) than that in the AL-FS mode (86–96%). The rejection of all REEs was noticeably influenced by solution pH in both mode. This was due to the changes in membrane surface with pH variation. The rejection of REEs increased when the temperature of the feed and draw solution was 20 and 40 °C, respectively. However, the rejection of all REEs decreased markedly when the temperature of the feed and draw solution was 40 and 20 °C, respectively. The lower rejection of these REEs was due to the enhancement of their diffusivity at a higher temperature in the feed side.

Co-axial hollow fiber module for air gap membrane distillation

A novel air gap membrane distillation (AGMD) module in which non-porous polymeric hollow fiber condensers (i.e., heat exchangers) were inserted inside the porous hollow fiber membranes was developed. In this module the hot feed was circulated on the outer side of the membrane's lumen and the coolant was circulated counter-currently inside the condenser fibers. The condensation of water vapor occurred in the air gap between the inner surface of the membrane fibers and the outer surface of the condenser fibers. By varying the number of condenser fibers inside the lumen, a different ratio of membrane fiber active surface area to the total surface area of condenser fibers and corresponding packing densities were achieved and examined in desalination of Red Sea water. The effect of membrane type on process performance was investigated with three different hollow fiber membranes with varied wall thickness (two capillary and one tubular). At a feed temperature of 85 °C, the water vapor flux increased from 12 kg/m2h to 18 kg/m2h with the increase in condenser fibers packing density from 9% to 28%, and then decreased to 14 kg/m2h when packing density was increased to 36% due to condensing surface constrain inside the lumen. A higher efficiency of the AGMD process was observed in the case of capillary membranes as compared to tubular membranes due to reduction in wall thickness which facilitated lower mass transfer resistance. The effect of operating conditions including feed and coolant flow rates and temperature difference between the feed and coolant solutions was also investigated. The increase in the feed flow rate had significant effect on vapor flux comparing to that of coolant for all tested AGMD configurations. This effect was more pronounced at high feed temperatures. Based on observed results, an optimal module design was suggested.

Poly(vinyl alcohol)/ZSM-5 zeolite mixed matrix membranes for pervaporation dehydration of isopropanol/water solution through response surface methodology

Mixed matrix membranes (MMMs) of poly(vinyl alcohol) (PVA) containing certain amounts of ZSM-5 zeolite were evaluated for pervaporation dehydration of highly concentrated isopropanol aqueous solution. The effects of zeolite content, feed composition and feed temperature on the membrane separation performance are in detail examined by using a preliminarily one-factor-each time method and a systematically response surface methodology (RSM). Preliminarily results show the dehydration separation factor/selectivity have been greatly boosted but without the cost of pervaporation flux/permeance after adding zeolite ZSM-5, and it is consistent very well with the Arrhenius activation energy estimations where water molecules require much less energy whereas ethanol molecules need much more energy to transport through the membrane, probably because of the favorable hydrophilic and porous features of zeolite ZSM-5 as revealed by swelling and water contact angle results. High feed isopropanol concentration and low feed temperature are both observed to lead to very low pervaporation flux/permeance but very relatively high separation factor/selectivity. The RSM results suggest that zeolite content, feed composition and feed temperature all have highly significant impacts on total pervaporation flux and separation factor. The interaction effect of zeolite content and feed temperature on separation factor is for the first time found to be also significant. The polynomial models established according to the RSM analysis can fit very well against the experimental data with a very high coefficient of determination and the predictions given for optimized conditions have been experimentally confirmed by the validation results with a deviation of less than 2.0%.

The effect of feed temperature on biofouling development on the MD membrane and its relationship with membrane performance: An especial attention to the microbial community succession

The influence of feed temperature on biofouling of membrane distillation (MD) with respect to the microbial community composition developed on the membrane surface was detailedly investigated in this study. The microbial community profile of MD-membrane biofilm was elucidated during the treatment of Xuanwu Lake water in a flat-sheet bench-scale direct contact membrane distillation (DCMD) system operated under higher feed temperature (60 °C) and lower feed temperature (40 °C), respectively. The fouling data showed that a more severe flux decline was observed under 60 °C, which exhibited a three-phase decline pattern. Comparatively, the normalized flux (NF) of 40 °C continuously demonstrated a higher value than that of 60 °C, and rather less variations were observed for its decline rate. The combination of SEM-EDS and EEM analysis revealed that more obvious deposition of salt crystals and protein-like substances was observed for the MD membrane operated under 60 °C. Phylogenetic analysis using next-generation sequencing of 16S rDNA showed that rather significant shifts of microbial community were observed for the 60 °C developed biofilm, while the 40 °C formed biofilm bacteria were relatively stable during the MD operation process. Finally, a three-phase model was suggested for the biofilm formation on the MD membrane.

Dynamic simulation of the reverse osmosis process for seawater using LabVIEW and an analysis of the process performance

The reverse osmosis (RO) process has become one of the prominent membrane-based technologies for water desalination to meet the demand for fresh water. This paper presents the results of simulations of the RO desalination process based on simplified functional-decomposition approach-based modelling to understand the process dynamics. The simulation model was validated by comparing the transient behaviour predicted by the model with experimental data from an industrial seawater desalination process. The proposed work was carried out using the Control Design and Simulation (CDSim) toolkit in the LabVIEW 2011 environment. The simulation analysis showed that the specific energy consumption of the RO system could be reduced at different fractional recovery rate by maintaining a reasonably high feed temperature.

Experimentally-validated computational simulation of direct contact membrane distillation performance

A Direct Contact Membrane Distillation system using a flat-sheet membrane module was fabricated to experimentally study the effects of operating conditions (inlet temperatures and flow rates of feed and permeate streams and salt concentration) on its distilled water production. A two-dimensional Computational Fluid Dynamics (CFD) model considering transmembrane mass flux and heat conduction and associated phase change heat transfer (evaporation and condensation) was developed. It was found that the computational predictions of distilled water production agreed well with the experimental measurements. Furthermore, the CFD model analyzed the effects of filament spacing of a screen spacer in the flow channels and flow configuration (counterflow and parallel) on the water distillation performance. In this study, it is concluded that high water distillation is achieved by using low permeate temperature, high feed temperature, high feed and permeate flow rates, and less salt concentration in a counterflow configuration. The water distillation is more influenced by the feed temperature than the permeate temperature due to the water saturation pressure-temperature relation. The simulation results also showed that the convection thermal resistances in the feed and permeate channels are decreased by using screen spacers and therefore, the temperature and concentration polarization effects are decreased in the channels.

Sustainable operation of membrane distillation for hypersaline applications: Roles of brine salinity, membrane permeability and hydrodynamics

This study aims to explore the role of brine salinity in achieving sustainable operation of membrane distillation (MD), particularly in hypersaline applications where highly concentrated or saturated solutions are treated. Given the state-of-the-art MD modeling work mainly focused on the mass and heat transfer phenomena for dilute systems, our simulation work predicts the trends of permeation flux in direct contact MD (DCMD) with elevated feed concentrations up to saturation, by a newly-developed exponential decay function. Also, a semi-empirical equation of the solute transport coefficient Sherwood number (Sh) is derived as Sh = (α1ωF + α2) ReβSc0.33, which for the first time incorporates the influence of feed concentration into the concentration polarization calculation in MD. Numerical analysis on the supersaturation ratio, concentration factor and concentration polarization effect showed that low to modest membrane permeability, reasonably high feed temperature and modest hydrodynamics (500 < Re < 2000) may help to prevent supersaturation and potentially reduce membrane scaling in hypersaline applications.

Bio-inspired deposition of polydopamine on PVDF followed by interfacial cross-linking with trimesoyl chloride as means of preparing composite membranes for isopropanol dehydration

Polydopamine (PDA) simulates the adhesive properties of mussels and modifies the surface of hydrophobic materials. In this study, the mussel-inspired deposition of PDA on a polyvinylidene fluoride support was adopted. Afterward, the catecholamine in PDA was cross-linked with trimesoyl chloride (TMC). Attenuated total reflectance-Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy validated the success of the cross-linking process. Scanning electron microscopy illustrated that the cross-linking induced the reduction of nodules. Water contact angle measurements indicated that the cross-linked membrane exhibited a higher contact angle compared with the pristine membrane. Atomic force microscopy showed that the cross-linked membrane surface was smoother than that of the membrane without cross-linking. Pervaporation tests demonstrated that the cross-linked membrane could separate water from isopropanol. Several parameters were varied: dopamine concentration, buffer solution pH, self-polymerization time, TMC concentration, cross-linking time, and annealing time. At optimum conditions, the permeate flux was 2411 ± 33 g·m−2·h−1 and the water concentration in permeate was 95.72 ± 0.44 wt% (feed = 70 wt% isopropanol at 25 °C). At a feed temperature of 70 °C, the permeate flux was 11001 ± 989 g·m−2·h−1 and the water concentration in permeate was 93.61 ± 0.45 wt%. This indicates that a composite membrane containing cross-linked PDA is stable at a high feed temperature.

Performances of air gap and water gap MD desalination modules

Abstract Membrane distillation (MD) is a promising thermally-driven membrane separation technology for water desalination. In MD, water vapor is being separated from the hot feed water solution using a micro-porous hydrophobic membrane, due to the difference in vapor pressures across the membrane. In the present work, experiments are conducted to compare the performance of water gap membrane distillation (WGMD) and air gap membrane distillation (AGMD) modules under the main operating and design conditions including the feed and coolant temperatures, membrane material and pore sizes, and the gap width. Results showed that the WGMD module produced higher fluxes as compared to the AGMD module, for all test conditions. The feed temperature is the dominant factor affecting the system flux. The permeate flux increases with reducing the gap width for both water and air gap modules. However, WGMD module was found to be less sensitive to the change in the gap width compared to the AGMD module. The PTFE membrane produced higher permeate flux as compared to the PVDF membrane. Bigger mean pore diameter enhanced the permeate flux, however, this enhancement is marginal at high feed temperatures. With increasing the feed temperature, the GOR values increase and the specific energy consumption decreases.