Concentration polarisation, membrane

Gas separation Hollow-fibre for high-volume applications with low-flux, low-selectivity membranes in which concentration polarisation is easily controlled (nitrogen from air) Spiral-wound when fluxes are higher, feed gases more contaminated, and concentration polarisation a problem (natural gas separations, vapour permeation). 

S. Yao, A. G. Fane, J. M. Pope 1997, (An investigation of the fluidity of concentration polarisation layers in crossflow membrane filtration of an oil-water emulsion using chemical shift selective flow imaging), Mag. Reson. Imag. 15, 235. 

Permeabilities measured for pure gases can serve as a rough guide for selection of membrane materials. For design, data must be obtained on gas mixtures, where selectivities are often found to be much lower than those calculated from pure-component measurements. This effect is often due to plasticisation of the membrane by sorption of the most soluble component of the gas. This allows easier penetration by the less-permeable components. The problem of concentration polarisation, which is often encountered in small-scale flow tests, may also be responsible. Concentration polarisation results when the retention time of the gas in contact with the membrane is long. This allows substantial depletion of the most permeable component on the feed side of the membrane. The membrane-surface concentration of that component, and therefore its flux through the membrane, decreases. 

The membrane selectively rejects oxygen and nitrogen. The field test showed a selectivity for chlorine over nitrogen of about ten. That this is so much lower than that obtained in the laboratory is attributed to concentration polarisation. Increasing the rate of flow through the module can alleviate this. At the same time, chlorine recovery can be maintained by adding modules in series. This is precisely what would be done in a commercial unit, and so one can reasonably expect better results in full-scale operation. 

The thermodynamic approach does not make explicit the effects of concentration at the membrane. A good deal of the analysis of concentration polarisation given for ultrafiltration also applies to reverse osmosis. The control of the boundary layer is just as important. The main effects of concentration polarisation in this case are, however, a reduced value of solvent permeation rate as a result of an increased osmotic pressure at the membrane surface given in equation 8.37, and a decrease in solute rejection given in equation 8.38. In many applications it is usual to pretreat feeds in order to remove colloidal material before reverse osmosis. The components which must then be retained by reverse osmosis have higher diffusion coefficients than those encountered in ultrafiltration. Hence, the polarisation modulus given in equation 8.14 is lower, and the concentration of solutes at the membrane seldom results in the formation of a gel. For the case of turbulent flow the Dittus-Boelter correlation may be used, as was the case for ultrafiltration giving a polarisation modulus of ... 

Porter, M. C. Ind. Eng. Chem. Prod. Res. Develop, 11 (1972) 234. Concentration polarisation with membrane ultrafiltration. 

In ultrafiltration and reverse osmosis, in which solutions are concentrated by allowing the solvent to permeate a semi-permeable membrane, the permeate flux (i.e. the flow of permeate or solvent per unit time, per unit membrane area) declines continuously during operation, although not at a constant rate. Probably the most important contribution to flux decline is the formation of a concentration polarisation layer. As solvent passes through the membrane, the solute molecules which are unable to pass through become concentrated next to the membrane surface. Consequently, the efficiency of separafion decreases as fhis layer of concentrated solution accumulates. The layer is established within the first few seconds of operation and is an inevitable consequence of the separation of solvent and solute. 

The ultrafiltration of the microemulsion is a very useful operation for separating water and oil in these mixtures [117-120]. Because of the limited availability of solvent stable membranes, most of the work pubHshed so far was performed using ceramic membranes, which show a high adsorption of surfactant at the membrane surface and comparably low rejection rates of reverse micelles. Using electro ultrafiltration, where the concentration polarisation phenomenon of the reverse micelles (using the ionic surfactant AOT) at the membrane surface is depressed by asymmetric high voltage electrical fields, the rejection rates can be increased,but not to economical values [121,122]. 

The change of flux velocity with transmembrane pressure can be explained by the concentration polarisation phenomenon. The physical processes at the membrane surface during the filtration procedure may be described by theo-... 

There have been many models, both simple and sophisticated, that describe the operating patterns of ultrafiltration processes [4]. Most of these models describe how the rate of ultrafiltration is controlled by the properties of a region of very high solute concentration, a filter cake or concentration polarised layer, close to the membrane surface. Relatively few of these models have a genuinely predictive capability. Remarkably, only a very few [5-7] of these models consider the most important feature of the solutes being separated by ultrafiltration—that they fall in the colloidal size range. For colloidal materials, the properties of the filter cake or concentration polarised layer will be controlled by the interparticle interactions in such a region. The important interactions which need to be taken into account are [8] ... 

Boundary layer formulation. Many membrane processes are operated in cross-flow mode, in which the pressurised process feed is circulated at high velocity parallel to the surface of the membrane, thus limiting the accumulation of solutes (or particles) on the membrane surface to a layer which is thin compared to the height of the filtration module [2]. The decline in permeate flux due to the hydraulic resistance of this concentrated layer can thus be limited. A boundary layer formulation of the convective diffusion equation can give predictions for concentration polarisation in cross-flow filtration and, therefore, predict the flux for different operating conditions. Interparticle force calculations are used in two ways in this approach. Firstly, they allow the direct calculation of the osmotic pressure at the membrane. This removes the need for difficult and extensive experi-... 

Ophoff, J. Voss, G.S. Racz, I.G. Reith, T. Flux enhancement by reduction of concentration polarisation due to secondary flow in twisted membrane tubes. Proceedings of Euromembrane University of Twente, The Netherlands, 23-27 June 1997 Kemperman, A.J.B., Koops, G.H., Eds. 400-402. 

The solubility of natural organics is an important issue in membrane processes where concentration polarisation is a common effect. Concentration polarisation may lead to gel layer formation if the solubility is exceeded. Solubilities of mixtures are difficult to determine. Many parameters influence solubility FA is, by definition, more soluble than HA at low pH. The complexation with metal ions (see section 2.8) also influences solubility. Metal complexes of FA are more soluble than those of HA due to the lower MW and the higher charge of FA. The solubility of each compound depends on the saturation of the complex with metal ions (Stevenson (1985)). Tipping et al. (1988) established a direct relationship between solubility and charge of HA. 

This may be of importance in membrane filtration, where significant changes in concentration may take place in the boundary layer, where molecular conformation may influence gel layer permeability. However it may only be relevant for feeds of high NOM, high recovery and significant concentration polarisation. 

Table 3.1 illustrates that the separation between the different processes is not precise, as the processes overlap. Therefore, filtration and separation models are generally applicable to mote than one process. Often several phenomena are operative simultaneously and which one dominates depends on the membrane and the solute or particle in question. Concepts such as the resistance-in-series model, the osmotic pressure model or concentration polarisation are principles which are applicable to any membrane operation. These wiU be described in the MF section. 

The Resistance in Series Model describes the flux of a fouled membrane. This is given in equation (3.4). The resistances Ra>, Ri> and Rc denote the additional resistances which result from the exposure of the membrane to a solution containing particles or solute. Rcp is the resistance due to concentration polarisation, Ri> the internal pore fouling resistance, and Rc the resistance due to external deposition or cake formation. These resistances are usually negligible in RO, where the osmotic pressure effects become more important (Fane (1997)). However, the osmotic pressure can also be incorporated into Rcp. 

Reversible flux decline can be reversed by a change in operation conditions, and is referred to as concentration polarisation. Irreversible fouling can only be removed by cleaning, or not at all. Irreversible fouling is caused by chemical or physical adsorption, pore plu ng, or solute gelation on the membrane. 

Concentration Polarisation is the accumulation of solute due to solvent convection through the membrane and was first documented by Sherwood (1965). It appears in every pressure dri en membrane process, but depending on the rejected species, to a very different extent. It reduces permeate flux, either via an increased osmotic pressure on the feed side, or the formation of a cake or gel layer on the membrane surface. Concentration polarisation creates a high solute concentration at the membrane surface compared to the bulk solution. This creates a back diffusion of solute from the membrane which is assumed to be in equilibrium with the convective transport. At the membrane, a laminar boundary layer exists (Nernst type layer), with mass conservation through this layer described by the Film Theory Model in equation (3.7) (Staude (1992)). cf is the feed concentration, Ds the solute diffusivity, cbj, the solute concentration in the boundary layer and x die distance from the membrane. 

Concentration polarisation can be minimised with turbulence promoters on the feed side of the membrane, such as spacers or introduction of crossflow. 

The Cake Filtration Model describes the filtration of particles which are much larger than the pores and will be retained, without entering the pores. The particles deposit on the membrane surface contributing to the boundary layer resistance. Included in this model is deposition due to concentration polarisation. 

Apart from solute-solute interactions, the deposition of foulants on the membrane can alter rejection. Rejection can increase due to a lower porosity of the fouling layer or pore constriction, or decrease due to a higher concentration in the boundary layer (concentration polarisation effect). 

Concentration polarisation, as described in the previous section, can become irreversible if a gel is formed, which can be the case when solute solubilities are exceeded. Concentration polarisation depends strongly on solute concentration and operational conditions, such as pressure and stirring. Fouling of tight UF and NF membranes tends to occur more on the surface than in pores - similar to MF and loose UF. Cake formation is usually reversible and can, as in MF, form a second membrane. Surprisingly, 0degaard and Thorsen (1989) demonstrated that HS concentration and pressure, which influence precipitation and gel formation, had no influence on flux. The fouling layer thickness was calculated with pressure drop and flow. The film was soft, dark brown, and loosely connected to the surface. 

While gel formation and precipitation are reported frequently as the source of fouling in all membrane processes, only a small amount of work has been done on a quantitative determination of gel layer concentration and the solubility of natural organics. Naturally, flux or transmembrane pressure are important for concentration polarisation, which seems to be the major factor in gel formation. The MTC can also describe this, as was shown above. Organic characteristics such as solubility and hydrophobicity require further investigation as do their interaction with ions. The solubilities of HS and their complexes with salts are relatively unknown, as was discussed in Chapter 2. 

Bian et al (1999) reduced the concentration polarisation and fouling in NF by vibrations. Mallubhotla et al (1998) reduced the concentration polarisation by constructing helical modules from tubular membranes. 

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