§ 3 - Flat membrane modules

Membrane fouling and concentration polarization can be greatly mitigated by using the helical membrane modules to enhance the mass transport process. In this study, experiments and computational fluid dynamics were used to investigate the transport phenomena in a helical membrane filter with several helical membrane modules. A model is constructed with a square filter which has three helical membrane modules embedded as not only turbulence promoters but also filtering elements. Direct numerical simulations based on the Navier-Stokes equations are performed over a range of characteristic parameters of membrane and aeration flux. The distributions of local parameters such as velocity, shear stress and turbulent kinetic energy on the membrane surface were obtained by numerical simulations with different helical angle and aeration flux. These parameters are directly related to mass transport enhancement. Results show that both wall shear stress and turbulent kinetic energy obtained from helical membrane modules are larger than those from flat membrane modules, and they increase with an increase of the helical angle. The average shear stress on the membrane surface increases from 0.097 Pa to 0.217 Pa as the helical angle changes from 0° to 360°. In addition, the flow field was analyzed by means of noncontact measuring and visualization device-Particle Image Velocimetry (PIV), and the vorticity as well as the turbulent kinetic energy were obtained from the velocity distribution. The measured data are in agreement with the numerical results. From the research, we can see that the helical membrane modules can enhance the transfer efficiently compared to the flat membrane modules, which means the concentration polarization and membrane fouling can be alleviated efficaciously, it can be concluded that the helical membrane modules can play an important role in government actions membrane separation engineering and its application prospect in industry is very broad.

Modeling the performance of flat and capillary membrane modules in vacuum membrane distillation

The cross-flow shearing action produced from the inferior aeration in the Submerged Membrane Bio-reactor (SMBR) is an effective way to further improve anti-fouling effects of membrane modules. Based on the widely-applied vertical structure of flat membrane modules, improvements are made that ladder-type flat membrane structure is designed with a certain inclined angle θ so that the cross-flow velocity of bubble near the membrane surface can be held, and the intensity and times of elastic collision between bubbles and membrane surface can be increased. This can improve scouring action of membrane surface on aeration and reduce energy consumption of strong aeration in SMBR. By deducing and improving the mathematics model of collision between bubble and vertical flat put forward by Vries, the relatively suitable incline angle θ under certain aeration place and in certain size rang of bubble can be obtained with the computer iterative calculation technology. Finally, for many groups of ladder-type flat membrane in parallel placement in the practical application of SMBR, some suggestions are offered: the interval distance of membrane modules is 8–15 mm, and aeration should be operated at 5–7 mm among membrane modules, and the optimal design angle of trapeziform membrane is 1.7°-2.5°.

The potential of membrane contactors for treating boron containing waters were investigated. In particular, experimental tests at lab scale on flat membrane modules with 40 cm2 of membrane area were carried out, to identify the effect of different parameters, such as temperature, flow rate, boron concentration, etc. on the efficiency of the process. Water was chosen as the extractant in order to avoid the pollution of the feed stream and two symmetric microporous hydrophilic flat membranes with different pore size and porosity were used. From these tests, it results that the boron removal increases with the extractant temperature and with the operating flow rates. However, it is independent on the initial boron concentration in the feed water. Moreover, higher removals are obtained with the membrane with larger pore size and higher porosity. Based on the experimental results, an integrated reverse osmosis-membrane contactor system, where the membrane contactor works on the reverse osmosis permeate, was proposed and designed for a 100 m3/h fresh water production (with a boron content ≤0.4 ppm). In particular, membranes with higher porosity and lower thickness than those used in the experimental tests were considered for the calculations, in order to work at 25°C (so, there is no need of heating the extractant stream) with reasonable membrane areas. The comparison of the proposed plant to that actually used, has shown that the proposed one appears to be more effective in terms of size, energy and chemical consumption, flexibility and modularity.

Hydrolysis of olive oil into fatty acids by lipase in a flat membrane module is reported. Lipase was immobilized by adsorption onto a hydrophilic cellulose acetate membrane. Hydrolysis was carried out by circulating pure olive on the enzyme side of the membrane and water phase on the other side. Conversion reached 85 % after 50 hours reaction time.

Numerical simulation of aeration flow phenomena in bench-scale submerged flat membrane bioreactor

A novel graphene oxide nanocomposite was investigated in a flat membrane module with cross-flow pattern for decontamination of water on optimization of the operating conditions. Response surface optimization methodology was followed in arriving at the best operating conditions. The best module performance in terms of flux and rejection was obtained at the optimum set of operating conditions comprising a pH of 8.0, operating pressure of 14 bars and an hourly cross-flow of 800 L. At the end of a 96-h run, the observed drop in flux was a negligible 3.4%. On rinsing and backwashing at the end of this period, the net drop could be limited to within 2%. The module succeeded in selectively removing 98.5% of arsenic, 96.7% of fluoride, 96–97% of iron and suspended solids from contaminated groundwater while permeating more than 79–81% of the useful calcium and 87–90% of magnesium minerals as desired in potable water. The study shows that if run under properly optimized conditions, a flat membrane module with cross-flow pattern and equipped with the graphene oxide nanocomposite can be a potential technology in producing healthy, tasty and non-toxic drinking water from contaminated groundwater even in the remote areas.

Membrane contactors for the oxygen and pH control in desalination

Computational fluid dynamics simulations for water desalination using forward osmosis were conducted on a flat membrane module. In the simulations, the effect of the porous support layer is assumed negligible. The simulations were performed with two values of flow rate such that the Reynolds number equals 200 and 800 in each channel. The working temperatures of both the feed and the draw solutions were varied from 20°C to 40°C. The feed solution had a concentration of 0.00355 solute mass fraction while the draw concentration was set to 0.0355 solute mass fraction. In all simulations, the laminar model was utilized. The results of the simulations suggest that the osmotic pressure is not the only factor that affects the water flux in forward osmosis when there is a temperature difference between the two sides of the membrane. The solution properties have a significant effect on the separation process. As the solution temperature increases, the viscosity decreases, which in turn increases the water permeation through the membrane. The feed temperature had a more substantial influence on the water flux compared to the draw temperature. Also, the effect of changing the flow rate did not change the results substantially.

Experience with plate-and-frame ultrafiltration and hyperfiltration systems for desalination of water and purification of waste water

Membrane contactor air conditioning system: Experience and prospects

Fouling mitigation using chaotic advection caused by herringbone-shaped grooves in a flat membrane module is numerically investigated. The feed flow is laminar with the Reynolds number () ranging from 50 to 500. In addition, we assume a constant permeate flux on the membrane surface. Typical flow characteristics include two counter-rotating flows and downwelling flows, which are highly influenced by the groove depth at each . Poincaré sections are plotted to represent the dynamical systems of the flows and to analyze mixing. The flow systems become globally chaotic as the groove depth increases above a threshold value. Fouling mitigation via chaotic advection is demonstrated using the dimensionless average concentration () on the membrane and its growth rate. When the flow system is chaotic, the growth rate of drops significantly compared to that predicted from the film theory, demonstrating that chaotic advection is an attractive hydrodynamic technique that mitigates membrane fouling. At each Re, there exists an optimal groove depth minimizing and the growth rate of . Under the optimum groove geometry, foulants near the membrane are transported back to the bulk flow via the downwelling flows, distributed uniformly in the entire channel via chaotic advection.

Effective suppression of concentration polarization by nanofiltration membrane surface pattern manipulation: Numerical modeling based on LIF visualization

Fruit juice concentration by membranes: effect of rheological properties on concentration polarization phenomena

A theoretical analysis of transport phenomena in membrane concentration of liquorice solutions: a FEM approach