Direct Contact Membrane Distillation Module Research Articles

PVDF hollow fiber and nanofiber membranes for fresh water reclamation using membrane distillation

Polyvinylidene fluoride hollow fiber and nanofibrous membranes are engineered and successfully fabricated using dry-jet wet spinning and electrospinning techniques, respectively. Fabricated membranes are characterized for their morphology, average pore size, pore size distribution, nanofiber diameter distribution, thickness, and water contact angle. Direct contact membrane distillation (DCMD) performances of the fabricated membranes have been investigated using a locally designed and fabricated, fully automated MD bench scale unit and DCMD module. Electrospun nanofibrous membranes showed a water flux as high as 36 L m−2 h−1 whereas hollow fiber membranes showed a water flux of 31.6 L m−2 h−1, at a feed inlet temperature of 80 °C and at a permeate inlet temperature of 20 °C.

Performance improvement of countercurrent-flow direct contact membrane distillation in seawater desalination systems

The modeling equations for the temperature distribution and pure water productivity in a parallel-plate direct contact membrane distillation module under countercurrent-flow were derived theoretically with the use of the mass balance on each subchannel. The analytical solution is obtained by using the separated variables method with an orthogonal expansion technique extended in power series. The pure water productivity, average Nusselt number, and temperature distributions of both hot and cold feed streams are represented graphically with the feed velocity and the inlet saline temperature as parameters. The improvements of device performance were achieved under the countercurrent-flow operation. The influences of operation and design parameters on the pure water productivity are also discussed.

Performance investigation of a solar-assisted direct contact membrane distillation system

This paper presents a solar-assisted direct contact membrane distillation (DCMD) system with novel energy recovery concepts for a continuous 24-h-a-day operation. A temperature modulating scheme is introduced to the solar–thermal system that supplies feed seawater to the DCMD modules. This scheme attenuates extreme temperature fluctuations of the feed water by storing the collected energy during solar-peak hours and reutilizing it throughout the day. Thus, the energy savings is realized yet the feed seawater temperature is maintained within the desired range. Additionally, the system employs heat recovery from the permeate and brine streams to the feed seawater. The simulations for such a system with a shell-and-tube type DCMD modules are carried out to examine the spatial property variations and the sensitivity of system performance (i.e., transmembrane pressure, permeate flux and performance ratio) to the operating conditions (inlet temperature and flow rate) and the fiber dimensions (fiber length and packing density). It is found that there are trade-offs between mean permeate flux and performance ratio with respect to permeate inlet temperature and flow rate and between total distillate production and performance ratio with respect to packing density. For the solar-assisted DCMD system having evacuated-tube collectors of 3360m2 with 160m3 seawater storage tanks and 50 DCMD modules, the annual solar fraction and the collector efficiency are found to be 77% and 53%, respectively, whilst the overall permeate production capacity is 31m3/day. The overall specific thermal energy consumption of the DCMD system with heat recovery is found to be 436kWh/m3 and it is about 43% lower as compared to the system without heat recovery. It is observed that the specific thermal energy consumption decreases significantly by 55% with increased collector area from 1983m2 to 3360m2 whereas the specific electrical energy consumption increases slightly by 16%.

High water recovery in direct contact membrane distillation using a series of cascades

Like all thermal processes, water recovery in membrane distillation (MD) is not limited by osmotic pressure, so that high recoveries are in principle achievable. The process can be carried out in batch recirculation mode or in continuous mode. In a number of earlier studies we have experimentally demonstrated very high water recovery in direct contact membrane distillation (DCMD) using cross flow hollow fiber membrane modules by operating in a batch recirculation mode without scaling seriously impairing membrane operation. However continuous operation is desirable for larger scale plants. Such operation may be carried out in a series of MD units each of which may consist of a countercurrent cascade of cross flow DCMD modules. A theoretical analysis of such a series of cascades has been carried out and expressions have been generated to show the relationship between recovery, temperature differences across and between the cascades, and the possible theoretical gained output ratio (GOR) and recoveries that are attainable. Depending on configuration (interstage heating or not) and temperature differences, recoveries of 60% and GOR values above 20 may be attainable. This could have important implications for the use of MD in the treating of desalination and other industrial brines.

Desalination with a cascade of cross-flow hollow fiber membrane distillation devices integrated with a heat exchanger

Cost-efficient desalination technology was developed successfully by integrating a countercurrent cascade of the novel cross-flow direct contact membrane distillation (DCMD) devices and solid polymeric hollow fiber-based heat exchange devices. Simulations have been carried out for the whole DCMD cascade to project values of gained output ratio (GOR) as a function of the number of DCMD stages as well as other important factors in the cascade vis-a-vis the temperatures and flow rates of the incoming hot brine and cold distillate streams. The simulation results were verified with experimental results from cascades consisting of two to eight stages. The numerical simulator predicts a GOR of 12 when unequal flow rates of the incoming brine and distillate streams are used. An artificial sea water was concentrated eight times successfully when a countercurrent cascade composed of four stages of the DCMD modules and a heat exchanger was used during the DCMD process. © 2010 American Institute of Chemical Engineers AIChE J, 2011

A theoretical study of a direct contact membrane distillation system coupled to a salt-gradient solar pond for terminal lakes reclamation

Terminal lakes are water bodies that are located in closed watersheds with the only output of water occurring through evaporation or infiltration. The majority of these lakes, which are commonly located in the desert and influenced by human activities, are increasing in salinity. Treatment options are limited, due to energy costs, and many of these lakes provide an excellent opportunity to test solar-powered desalination systems. This paper theoretically investigates utilization of direct contact membrane distillation (DCMD) coupled to a salt-gradient solar pond (SGSP) for sustainable freshwater production at terminal lakes. A model for heat and mass transport in the DCMD module and a thermal model for an SGSP were developed and coupled to evaluate the feasibility of freshwater production. The construction of an SGSP outside and inside of a terminal lake was studied. As results showed that freshwater flows are on the same order of magnitude as evaporation, these systems will only be successful if the SGSP is constructed inside the terminal lake so that there is little or no net increase in surface area. For the study site of this investigation, water production on the order of 2.7 × 10 −3 m 3 d −1 per m 2 of SGSP is possible. The major advantages of this system are that renewable thermal energy is used so that little electrical energy is required, the coupled system requires low maintenance, and the terminal lake provides a source of salts to create the stratification in the SGSP.

Integration of accelerated precipitation softening with membrane distillation for high-recovery desalination of primary reverse osmosis concentrate

Accelerated precipitation softening (APS) was integrated with direct contact membrane distillation (DCMD) to establish a desalination process for high-recovery desalting of primary reverse osmosis (PRO) concentrate. The APS process, between the PRO concentrate and the DCMD process, involved pH adjustment with sodium hydroxide along with calcite seeding, followed by microfiltration to avoid seeds clogging of the DCMD module. The PRO concentrate was obtained from the RO process of the direct drinking water preparation system designed for the 29th Olympic Games with 50% recovery. pH adjustment to 10.10, 5 g/L calcite dosage and 200 r/min agitation rate was determined for APS treatment via initial small-scale calcium removal tests. Elemental analysis revealed that APS treatment enabled 92% removal of calcium, 58.4% removal of total hardness, 4.4% removal of magnesium, 1.1% removal of sulfate and 1.6% removal of silica. Compared with the sharp decline found in the DCMD process of the PRO concentrate, the permeate flux declined only 20% within 300 h running after APS treatment. Then the PRO concentrate was concentrated 40 times and the whole recovery was enhanced to 98.8%. The permeate conductivity varied between 2.0 and 4.0 μS/cm during the DCMD process, and slightly increased to 6.0 μS/cm at the end due to partial wetting phenomenon.

Spatial variations of DCMD performance for desalination through countercurrent hollow fiber modules

To understand the spatial variations of the desalination behavior, a mathematical model is systematically formulated to simulate a direct contact membrane distillation (DCMD) with the hollow fiber module for desalination. The model equation for the entire module is derived by integrating the mass, momentum and energy balances on both feed and permeate sides with the permeate flux across the membrane. The property variations of feed and permeate sides along the length of the membrane module are thoroughly simulated. Sensitivity of the permeate flux and the thermal efficiency along the fiber length to operating conditions is further investigated over a range of temperature and flow rate. It is shown that both permeate flux and thermal efficiency decrease along the fiber length as the feed gets cooled down. Although the velocity variations of both streams are not big, the flow rate ratio of the hot feed to the cold solution has a dramatic impact on the distribution of the permeate flux and the thermal efficiency along the fiber length. The effect analysis could potentially be applied to optimal design and scale-up of hollow fiber DCMD modules.

Modeling and optimization of hollow fiber DCMD module for desalination

To maximize the desalination performance of both productivity and thermal efficiency, model-based optimal design of a hollow fiber direct contact membrane distillation (DCMD) module is proposed. The mathematical model equations for the entire module are derived by integrating the permeate flux across the membrane with the mass, momentum and energy balances on both feed and permeate sides. The property variations of feed and permeate sides along the length of the membrane module are simulated. It is found that there is a trade-off between the permeate flux (productivity) and thermal efficiency. The trade-off depends on not only the operating variables, such as the temperature and the flow rate of the cold solution, but also the fiber dimensions, such as the fiber length and the packing density. Based on the above decision variables, optimization of the hollow fiber DCMD module is carried out using the Pareto genetic algorithm. It is demonstrated that the optimal values of the inlet cold flow rate and the fiber length are more sensitive than the module packing density. With the synchronous increase in the inlet cold flow rate and the fiber length, the productivity increases at the cost of higher thermal efficiency.

Characterization of membrane distillation modules and analysis of mass flux enhancement by channel spacers

The first aim of this work is to present a method in order to characterize a direct contact membrane distillation (DCMD) module, namely a method to determine the value of the parameters included in a commonly accepted transport model. The model includes membrane and module design characteristic parameters which have the same values regardless of feed used. The characterization method proposed here, includes DCMD measurements (both, water flux and evaporation efficiency measurements) using pure water as feed, and air–liquid displacement measurements. The latter are used to extract previous structural information about the membrane used. The method is applied to characterize two DCMD modules with different designs. The first module has open flow channels while the second one includes a spacer within the flow channels. The transport model together along with the obtained characteristic parameter values adequately predicts the water flux measured with both modules when different operating conditions were imposed and aqueous solutions of different solutes were used as feed. The results given here are for NaCl and sucrose feed solutions. They show that the membrane distillation performance is highly dependent on the design of membrane module, the specific separation being performed and the operating conditions (besides membrane characteristics). The second aim of this work is to study the water flux enhancement induced by the improvement of the fluid-dynamic conditions within the module with spacer in relation to the module with open channels. The dependence of flux enhancement on operating conditions (average temperature, temperature difference between feed and permeate solutions, type of solute, and feed concentration) is analysed. In the range of operating variables studied, the systems with high vapour pressure polarization have water fluxes especially sensitive to the fluid-dynamic changes induced by the spacer.