Concentration polarization and membrane fouling

As a result of these improvements in membrane performance, the major factors determining system performance have become concentration polarization and membrane fouling. All membrane processes are affected by these problems, so... 

The need to control concentration polarization and membrane fouling dominates the design of ultrafiltration modules. The first commercially successful... 

Advantages Minimal filtration pretreatment Easier membrane cleaning Higher surface area means more filtration area per fiber True cross flow velocity minimizes concentration polarization and membrane fouling When no air is utilized for backwashing the less fiber movement leads to breakage of fewer fibers Can be created with a variety of inside fiber diamters... 

Methods to Control Concentration Polarization and Membrane Fouling. 197... 

The efficiency of UF in whey processing is limited by a few factors, the most significant of which are concentration polarization and membrane fouling [6,39 1]. While both factors, which adversely affect permeate flux, may be aggravated by protein-protein and membrane-protein interactions [23,40,42-44], they may also be minimized by choosing suitable membrane material and configuration as well as the appropriate process conditions such as TMP, feed velocity or recirculation rate, temperature, and the chemical environment of whey [42,45,46]. 

When discussing membrane preparation, not only must the physical structure be considered, but one must also consider the membrane form or shape. In an effort to combat concentration polarization and membrane fouling and to maximize the membrane surface area per unit module volume, membranes are produced in the form of flat sheets (used either In plate-and-frame or spiral wound modules), supported and unsupported tubes, and hollou fibers. Although much of the technology associated with membrane development and membrane production Is closely guarded as proprietary Information, some of the details are beginning to appear in the literature (6,9-13,16-20). 

As any membrane separation process elcctrodialysis is effected by concentration polarization and membrane fouling. The magnitude of concentration polarization is largely determined by the electrical current density, by the cell and particularly spacer design, and by the flow velocities of the diluate and brine solutions [39]. Concentration polarization effects electrodialysis lead to a depletion in the laminar boundary layer at the membrane... 

The concept of the VSFP (New Logic International, Fmeryville, CA), proposed by Armando et al. [70], came with the idea to combat concentration polarization and membrane fouling by directly moving the membranes rather than by moving the liquid. The VSFP system consists of a stack of parallel circular membranes mounted in a cylindrical housing that is spun in torsional oscillation at a resonant frequency of about 60 Hz as shown in Figure 10.33. 

In tubular membrane modules membranes are cast on a relatively thick and mechanically strong porous support material. Tube diameters are typically in the range of 10-25 mm. The feed solution is fed through the tubular bundle while the permeate is collected on the shell side of the module (Fig. 2.3c). These systems allow an efficient control of the concentration polarization and membrane fouling phenomena and are easy to clean. However, as the tube diameter increases, they occupy a larger space and require high pumping costs. 

Hollow fiber membrane modules (Fig. 2.3d), consisting of fibers with diameters of 0.001-1.2 mm, offer the highest packing density of all modules available on the market and can withstand relatively high pressures. However, the control of concentration polarization and membrane fouling is difficult and an extensive pretreatment of the feed solution is required in order to remove particles, macromolecules or other materials which can precipitate at the membrane surface. 

Although the flux decline is a typical aspect involved in concentration polarization and membrane fouling, there are some substantial differences between these phenomena. Concentration polarization is a reversible process which takes place over a few seconds. It can be easily controlled by decreasing the TMP, lowering the feed concentration or increasing the cross-flow velocity. In membrane fouling, the flux decline is irreversible and takes place over many minutes, hours or days. It is more difficult to describe and to control experimentally. In addition, a continuous flux decline can often be observed. 

An ultrafilter works like a sieve on a macromolecular scale. Theoretically, ultrafiltration would be an ideal technique for studying macromolecular binding. Unfortunately, two main drawbacks generally limit its appication (1) the lack of membranes of suitable cut-off, and (2) concentration polarization and membrane fouling during the experiments. The latter phenomenon modifies the separative properties of the membrane, i.e. the sieving characteristic is not constant during the experiments. 

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