•Air gap and water gap membrane distillation systems are experimentally compared.•Effects of operating variables and design parameters are studied.•Water gap enhances the permeate flux significantly ...(80–140% increase) as compared to the air gap.•Temperature inside the water gap is always lower than that of air gap under the same conditions.•The water gap is less sensitive to the increase in gap width as compared to air gap.
Experiments are conducted to compare and contrast the performance of Water Gap Membrane Distillation (WGMD) and Air Gap Membrane Distillation (AGMD) designs under different operating and design variables. The membrane module is designed to work on both designs exchangeably. The effects of feed flow rate, feed temperature, gap width, coolant flow rate, feed concentration, and the material of membrane supporting plate on the permeate flux are investigated experimentally. The temperatures inside the gap are measured to evaluate the changes in performance in relation to the gap temperature. Results showed that the water gap design enhances the permeate flux significantly. The increase is flux ranges between 90% and 140%, mainly depending on the feed temperature, when using the water gap as compared to the air gap. The temperature inside the water gap is lower than that of the air gap under the same operating conditions. Having a liquid interface on the cold side of the membrane increases the evaporation in feed side, maintains lower gap temperature, promotes the condensation process, and thus enhances the flux. Increasing the gap width reduces the flux, particularly at higher temperature. However, the water gap is found to be less sensitive to gap increases compared to air gap. It is recommended to use brass plate for supporting the membrane with the water gap, regardless of the gap width. On the other hand, with the air gap, the material thermal properties become less effective as the gap increases. A clear decrease in the flux is recorded with increases of the feed concentration due to the effect of concentration polarization on the feed side of the membrane. A salt rejection factor up to 99.98% is achieved with both air and water gaps.
•Water desalination using an innovative water gap membrane distillation (MD) process.•Circulation of distilled water inside the gap in a separate closed loop.•Experimental investigations of operating ...conditions and design parameters.•Comparison with the conventional water gap module without circulation.•Energy analysis and efficiency; comparative study.
A New process and design of flux-enhanced and energy-efficient water gap membrane distillation (WGMD) module is experimentally tested and evaluated. The gap water is circulated in a separate closed loop to enhance the heat and mass transfer characteristics inside the MD module. A fundamental investigation is conducted to compare the performance of WGMD system with and without gap circulation at different operating conditions. Results showed that circulation of the gap water increases the system flux by 80 to 96% compared to without gap circulation, at the expense of some increase in energy consumed as indicated by the changes in feed and coolant water temperatures through the module. With gap circulation, a reduction in the electric specific energy consumption (SEC) between 15% and 25% was measured, associated with an increase in the gain output ratio (GOR) between 5 and 22%, compared to the case of no gap circulation. The energy utilization efficiency increase with increasing feed temperature. Higher circulation flow rates enhanced the flux while gap width showed marginal effects with circulation due to dominating forced convection inside the gap.
The performances of multistage air gap membrane distillation (MS-AGMD) and water gap membrane distillation (MS-WGMD) systems are experimentally investigated and compared using three-stages systems ...for different operating conditions. Parallel, series, and mixed stage-connections are considered. The flux of MS-WGMD is more than double the flux of MS-AGMD. The parallel stages-flow connections produced higher output flux than series connections (15% on average for MS-WGMD and 10% on average for MS-AGMD). The MS-AGMD system is found to be more sensitive to changes in the feed temperature and gap width than the MS-WGMD system. The effect of feed flow rate on permeate flux is higher than the coolant flow rate. The series stage connections are more sensitive to the change in hot and cold flow rates than the parallel connection; particularly the feed flow rate. The productivity of the three-stages system is almost three times the single stage system. The calculated specific electric energy consumption ranged from 5 to 10 kWh/m3 and it values for MS-WGMD system are lower than that of MS-AGMD system, and decreases with increasing the feed temperature.
•Air gap and water gap membrane distillation (MD) for water desalination•Multistage systems•Parallel, series, and mixed stages connections•Experimental investigations of operating conditions•Energy analysis and comparisons
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.
•Modeling of water-gap membrane distillation (WGMD) process•Effect of natural convection inside the water gap, with new correlation for Nu•Effect of membrane supporting plate and other internal details of module design•Model validation with experimental data of the same module design•Comprehensive investigations of operating and design parameters•Energy analysis and performance indicators
A hybrid Multi-Stage Flash–Humidification Dehumidification (MSF-HDH) desalination system is investigated for energy recovery from an MSF system. The hybrid MSF-HDH system increases total productivity ...and performance ratio and reduces brine rejection. Hot condensed steam that leaves the MSF brine heater is used to warm the rejected pretreated brine from MSF to a higher temperature suitable for HDH system operation (about 60 °C). This allows us to increase the product (desalinated water) without additional “external” energy input to the hybrid system. Four different layouts of the integrated MSF-HDH system are presented and compared. The results show that an HDH system can utilize over 66% of an existing MSF brine blowdown, while the hybrid system can achieve a gained output ratio—GOR, water recovery ratio—RR, productivity and freshwater cost of 8.73, 44.86%, 30,549 m3/day and 1.068 $/m3 of freshwater, respectively. Utilizing 66.96% of MSF brine blowdown by the HDH system leads to a daily HDH productivity of about 670 m3 of drinking water, which is enough to support 134,000 persons considering a daily consumption of 5 L of drinking water per person.
•Modeling of multistage water-gap membrane distillation (MSWGMD) system based on heat and mass transfer analysis.•Consideration of water gap natural convection and module internal design ...details.•Model validation with experimental results of the same module design.•Comprehensive study of operating and design variables, energy analysis with focus on effects of number of stages.•Freshwater production cost estimation and sensitivity analysis.
A theoretical modeling of multistage water gap membrane distillation (MSWGMD) process is presented to predict its performance with economic evaluation. The theoretical model is based on coupled heat and mass transfer analysis inside the water gap MD module and considering the effect of gap natural convection. The model is validated and used to study the variation of feed temperature, coolant temperature, and system productivity with the number of stages for the MD system with and without insulation. It is also used to predict the maximum and optimal number of stages that can be employed under specific operating conditions. In addition, the cost analysis of MSWGMD system is carried out. Results show that under the conditions of 20 °C coolant temperature, 50–90 °C feed temperature, 2.3 L/min feed and coolant flow rates, 4 mm gap thickness; the maximum number of stages that can be used is found to be 35 stages. In this case, the system productivity is about 5.2 L/h. It is also found that, by insulating pipe connections and the MD module, the total productivity increases by 36%. The optimum number of stages was found to be 15 stages, and the corresponding cost of freshwater production of the insulated multistage water gap MD process is about 3 $/m3.
The performance of multistage air gap membrane distillation (MS-AGMD) system is experimentally investigated. The effects of feed temperature and flow rate, coolant temperature and flow rate, air gap ...width, and feed concentration on the system performance are studied. Parallel and series flow stage connections for the feed stream and coolant stream are tested. Energy analysis is performed for the MS-AGMD and compared with the single stage system. Parallel stage connection shows better performance than series connections in terms of permeate flux and energy consumption. The distillate volume of MS-AGMD module is measured to be 2.6 and 3 times the distillate volume of single stage AGMD module, for series and parallel stage connections; respectively. However, the specific energy consumption of multistage system is only 1.5 times the single stage system. The gain output ratio of the parallel stage MS-AGMD system is higher than the series stage MS-AGMD system, and it reached 0.6 for parallel MS-AGMD system and 0.45 for the series MS-AGMD system, at 90°C.
•Water desalination using membrane distillation (MD) technique•Design and construction of Multistage air gap MD system•Parallel and series stage connections•Comprehensive experimental investigations of operating conditions and design parameters•Energy analysis and performance indicators.
This article introduces a dynamic theoretical analysis, which is not available in the literature, for the average hourly and monthly performances of a solar-driven multistage water gap membrane ...distillation system. Based on simultaneous solutions of the combined heat and mass transfer equations using EES software, the dynamic model successfully predicted the average hourly and monthly output from the system. The mid-latitude meteorological data from Dhahran city, Saudi Arabia, are employed in this analysis. This analysis is carried out to examine the variations of the system productivity and energy efficiency with the operating and design parameters of the integrated system, which include the average monthly and hourly solar radiation, the number of the MD units and the number of solar collectors that are used for water heating. Results reveal that an eight-stage MD system with three solar collectors can produce up to 70 L/day of freshwater. The integrated solar MD system is able to effectively improve energy efficiency, with a maximum GOR of 0.95.
Summary
A novel configuration of humidification dehumidification (HDH) desalination system is presented using direct contact bubble column (BC) humidifier and dehumidifier. Experiments are carried ...out for performance analysis and comparison between a single‐stage, HDH air heated system (one humidifier and a dehumidifier) and a double‐stage HDH system (two BC humidifiers with air reheating). Effect of various operating parameters such as airflow rate, air temperature, and humidifier water levels is studied on performance metrics such as productivity, gain output ratio (GOR), input energy, specific energy consumption, and effectiveness. Results show that higher airflow rate, air temperature, and humidifier water level favor the system performance. The single‐stage system has a maximum productivity rate of 0.19 L/h and GOR of 0.21 obtained at 140°C and 1.5 standard cubic feet per minute (SCFM). However, the productivity of the single‐stage HDH system is limited due to low air outlet temperature at the exit of the BC humidifier. The double‐stage humidification system performs better in terms of productivity and GOR. The two‐stage humidification HDH system produced a distillate rate of 0.46 L/h and GOR of 0.4 with thermal energy increment of only 0.27 kW compared to the single‐stage system. The obtained results are promising in comparison with previous HDH studies, especially of air heated cycle. The estimated cost of production is 0.090 USD/L for double‐stage HDH BC.
An experimental investigation of a small-scale air-heated humidification-dehumidification (HDH) desalination system with bubble-column humidification and dehumidification units was conducted. The ...study addressed the performance of the multistage air-heated bubble-column HDH system, which has limited coverage in the literature, by operating two bubble-column humidifiers in series for the air humidification process with air reheating. The effect of operating parameters such as airflow rate, air temperature, and saline water levels in both humidifiers on the performance metrics of the system were investigated. The product distillate rate, energy consumption, gain output ratio (GOR), and specific energy consumption (SEC) are the main indicators of performance for the proposed desalination system. Response surface methodology (RSM) was applied to the current system using the design of experiment (DoE) for the prediction of variables that greatly affect productivity and energy input. The airflow rate, air temperature, and water level of the second humidifier have a favourable effect on the distillate rate and GOR of the system. In contrast, the effect of the water level inside the first humidifier is insignificant. Furthermore, the RSM optimization approach was used to obtain the optimum distillate productivity. An optimized distillate rate of 0.45 L/h and a GOR of 0.4 are achieved at 1.5 SCFM (standard cubic feet per minute) of airflow rate, and 6.5 cm of water level in the second humidifier with 140 degreesC air inlet temperature. The numerical optimization reveals the optimal operating parameters, that correspond to maximum distillate production of 0.3 L/h with minimum input energy of 0.71 kW, to be 139 degreesC air temperature, 1.13 SCFM of airflow rate, 6.5 cm and 3 cm water levels of second and first humidifier, respectively.
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