Membrane transmembrane flux enhancement

In spite of their advantage and potential over conventional processes, membrane distillation suffers from the drawback of low transmembrane flux when compared to RO. In the past few years, additional innovations in module design and new strategies have been explored to reduce concentration polarization and to enhance mass transfer. Some of these attempts are discussed here. 

Enhancement of transmembrane flux has been shown in OMD of grape juice pretreated by UF [131]. The increase in flux has been attributed to a reduction in the viscosity of the concentrated juice-membrane boundary layer as the result of removal of high-molecular weight biopolymers present in juice. UF is a powerful method for removing natural polymers (polysaccharides, proteins) from fruit and vegetable juices. Lukanin et al. [12] have improved the concept of (UF + OMD), by enzymatic pretreatment of the apple juice prior to the UF step. Introduction of an additional enzymatic deproteinization step with the pectinase/amylase treatment of apple juice followed by UF has yielded minimal biopolymer content. Such a treatment is found to enhance transmembrane flux during concentration of clarified juice by MD. As in the case of grape juice this has been... 

An ultrasonic technique was applied to AGMD to enhance the permeability for the concentration of various aqueous solutions. In this study, an ultrasonic stimulation of resonance frequency of 20 kHz and power up to 90 W was applied to a flat-plate AGMD system of 1 p,m PTFE membrane with a temperamre difference of 55°C. A 200% improvement in the transmembrane flux was observed with an ultrasonic intensity of 5 W cm . Experimental investigation of the basic mechanism of ultrasonic enhancement of AGMD suggests that, for a continuous ultrasonic stimulation the dominating mechanisms are microstreaming and cavitation [70,130]. 

Narayan et al. [Ill] have tried to enhance the transmembrane flux by using an acoustic field (1.2 MHz), by placing the membrane cell over an ultrasonic transducer (Figure 19.16). Their results show an increase in flux by approximately 35% to 98% when compared with control runs (without acoustic field). The increase in flux is due to the fact that acoustic field induces mild circulation currents, which disturbs the hydrodynamic boundary layers of feed and OA solutions, thereby reducing the effect of concentration polarization. 

The MD process has several technical advantages and scope for promoting its industrial application. However, implementation of MD on process scale would require long-term validation on a pilot scale. For achieving these goals more intensive and focused research effort is needed in this field. Research on new membrane materials and composite membranes that enhance transmembrane flux and selectivity is required. Modeling and scale-up studies will form an integral part of this activity. 

Belleville et al. [54] give a full description of the chemical nature of the fouling species. The use of enzymes to enhance the filter ability might also be effective for raising the economy in the application of ceramic membranes [55]. Another means of achieving economical operation could be flux enhancement by the application of pulsating flow as outlined by Jaffrin et al. [56,57]. The simultaneous introduction of pulses, and a rise in circulation velocity from 3 to 4.37 m/s and of the transmembrane pressure from 1 to 4.5 bar, increases the flux from 351/m h to 501/m h. 

The ability of limiting aging phenomena in high free volume glassy polymers is a recent discovery for hybrid membranes and can open up a new branch of study for this polymers family. If selectivity enhancement via the sieving mechanisms and antiaging properties of the fillers can be coupled with the high transmembrane flux offered by the polymer phase, superior separation features can be achieved. 

Both secondary active transport and positive cooperativity effects enhance carrier-mediated solute flux, in contrast to negative cooperativity and inhibition phenomena, which depress this flux. Most secondary active transport in intestinal epithelia is driven by transmembrane ion gradients in which an inorganic cation is cotransported with the solute (usually a nutrient or inorganic anion). Carriers which translocate more than one solute species in the same direction across the membrane are referred to as cotransporters. Carriers which translocate different solutes in opposite directions across the membrane are called countertransporters or exchangers (Figs. 10 and 11). 

Membrane structures for MF include screen filters, which collect retained matter on the surface, and depth filters, which trap particles at constrictions within the membrane. Depth filters have a much sharper cutoff, resulting in enhanced separation factors. For example, a Nuclepore membrane of type 2 can separate a male-determining sperm from a female-determining sperm (Seader and Henley, 2006). Nuclepore MF membranes come in pore sizes from 0.03 to 8.0 microns with water permeate flux rates, at 294 K and a transmembrane pressure difference of 70 kPa, ranging from 15 to 350,000 L/m2-h. 

It is well documented that the surface chemistry and morphology of the membranes play an important role in the transmembrane transport of permeates (Khayet and Matsuura 2003a,b). To enhance the overall performance of a membrane, it is often necessary to modify the membrane material or its structure. Generally, the objective of modification is not only to increase the flux and/or selectivity, but also to control the pore size, eliminate defects, and improve the chemical resistance, for example, the solvent resistance, swelling, or fouling resistance. 

Dead end filtration operation mode was used, and the installation was continuously running 24 h daily. The membrane flux was 60 L/(m h) and filtration cycle was 30 min, and the backwash time was set up at 1 min. To inhibit the growth of bacteria on the surface, membrane was immersed with 10 mg/L of NaClO solution for 5 min every 8 h. When the membrane flux reduced or transmembrane pressure (TMP) increased significantly, the backwash flow was increased to enhance washing. The quality of membrane inflow and effluent were examined and monitored during the experiment. 

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