Flux decline

Flux Decline Plugging, Fouling, Polarization Membranes operated in NFF mode tend to show a steady flux decline while those operated in TFF mode tend to show a more stable flux after a short initial decline. Irreversible flux decline can occur by membrane compression or retentate channel spacers blinding off the membrane. Flux decline by fouling mechanisms (molecular adsorption, precipitation on the membrane surface, entrapment within the membrane structure) are amenable to chemical cleaning between batches. Flux decline amenable to mechanical disturbance (such as TFF operation) includes the formation of a secondary structure on the membrane surface such as a static cake or a fluid region of high component concentration called a polarization layer. 

In fact, the main problem with UF, however, is the flux decline caused by the irreversible adsorption of foulants onto the surface or even inside the pores of the membrane. 

Also included are sections on how to analyze mechanisms that affect flux feature models for prediction of micro- and ultrafiltration flux that help you minimize flux decline. Descriptions of cross-flow membrane filtration and common operating configurations clarify tf e influence of important operating parameters on system performance. Parameters irdlucnc irxj solute retention properties during ultrafiltration arc identified and discussed or treated in detail. 

Performance. Figure 2 shows a rejection-flux pattern (r-f pattern). Compaction, as it is well known, results in the flux decline with salt rejection Increase. Contrary to this, other types of membrane deterioration give the flux increase with salt rejection decline. In case of scratching, vibration, or microbiological deterioration, small cracks or pinholes develop over membrane surfaces. If the flux Increase is solely attributed to the crack or pin-holes, and these sites do not reject salt at all, the relation between salt rejection and flux can be calculated. 

Toray s data are plotted as the reciprocal of membrane constants against time elasped like Fig. U. The first generation of Toray module tested at the laboratory might have some defect In the membrane backing material. After 6000 hours in operation, they exchanged half of the modules for the second generation chlorine resistant. The second had been improved about twice in its resistance against flux decline. 

Still we have to obtain relationship between half time or the slope of the lines and the operating conditions such as operational pressure, temperature and so on. But taking pilot plant data only for 1000 to 2000 hours at some designated conditions, this type of plot proved to be able to predict the flux decline over 10 000 hours quite accurately. 

As for the flux decline over long term operation 10 000 hours at high pressure, the reciprocals of flux through the membranes may be expressed as a linear function of time elasped. 

The basis for the experiments as carried out was that the flux decline with time, which is an easily measured quantity, could be correlated with the foulant film growth by the model of Eq. (2), or some other appropriate semi-empirical model. In this way the fouling film thickness could be deduced indirectly. To show this we write from Eqs. (5)-(7)... 

This shows the film thickness is linear in the relative flux decline. Although this result has been developed using the semi-empirical Eq. (5), it can be derived on more rigorous grounds. From Eq. (3) it follows that... 

Figures 2 and 3 show typical test results for flux decline in laminar flow where the pressure and temperature are varied and the Reynolds number is held fixed. Similar behaviors are found with variations in Reynolds number and for turbulent flow. The important feature of the data is that the flux decline is exponential with time and an asymptotic equilibrium value is reached. Each solid curve drawn through the experimental points is a least-square fit exponential curve defined by Eq. (19). It is interesting to note that Merten et al ( ) in 1966 had observed an exponential flux decay in their reverse osmosis experiments. However, Thomas and his co-workers in their later experiments reported an algebraic flux decay with time (4,5).

Figure 2. Flux decline vj. time at different pressures in laminar flow...

In Figures 8 and 9 are shown the data for the dependence of the characteristic film buildup time t on Apg and U. In accord with the model, t is found to be independent of U, with only a very weak dependence on Apg indicated. This latter result could in part be a function of experimental inaccuracy. The data reduction for t introduces no assumptions beyond that needed to draw the exponential flux decline curves such as those shown in Figures 2 and 3. However, an error analysis shows that the maximum errors relative to the exponential curve fits occur at the earlier times of the experiment. This is seen in the typical error curve plotted in Figure 10. The error analysis indicates that during the early fouling stage the relatively crude experimental procedure used is not sufficiently accurate or possibly that the assumed flux decline behavior is not exponential at the early times. In any case, it follows that the accuracy of the determination of 6f is greater than that for t. 

Thomas, D.G., and Mixon, W.R., "Effect of Axial Velocity and Initial Flux on Flux Decline of Cellulose Acetate Membrane in Hyperfiltration of Primary Sewage Effluents," I EC Process Design and Development 11, 339-343 (1972). 

Tubular membranes of 8 long were prepared from blend composition consisting of CA and PMMA and performance data for one month operation was collected. These datas show high product water flux (18-20 gfd.) with low flux decline slope. However, it was observed that these membranes initially showed fountains" which disappeared in about 30 minutes time. This was attributed to the peculiar membrane rheology and orientation of PMMA molecule with respect to CA molecule. This needed further study for confirmation. 

Tests were also run with simulated brackish agricultural drainage water, as illustrated in Table 4. A feedwater composition containing sodium, calcium, chloride, sulfate, and bicarbonate ions was prepared in such a way as to duplicate the water in the Mohawk-Wellton drainage canal at Yuma, Arizona. Salt rejections were relatively poor toward this synthetic feedwater, but when this water was line-softened and acidified to pH 5.5 with sulfuric acid, salt rejection of the 90 10 copolyamide improved markedly. However, the membrane s water flux declined by nearly 50 percent. Salt rejection and flux were found in this and other examples to be markedly dependent on pH. As the pH approached the pKa of... 

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 poly(ether/amide) thin film composite membrane (PA-100) was developed by Riley et al., and is similar to the NS-101 membranes in structure and fabrication method 101 102). The membrane was prepared by depositing a thin layer of an aqueous solution of the adduct of polyepichlorohydrin with ethylenediamine, in place of an aqueous polyethyleneimine solution on the finely porous surface of a polysulfone support membrane and subsequently contacting the poly(ether/amide) layer with a water immiscible solution of isophthaloyl chloride. Water fluxes of 1400 16001/m2 xday and salt rejection greater than 98% have been attained with a 0.5% sodium chloride feed at an applied pressure of 28 kg/cm2. Limitations of this membrane include its poor chemical stability, temperature limitations, and associated flux decline due to compaction. 

Time-dependent flux decline is plotted in Figure 8. The C02 permeation rate, in units of SCFH/ft2.100 psi, was calculated from performance data for the fifteen elements in series. This rate corresponds to that which would be obtained for pure CO2 under similar partial pressure differences. After an initial (40 hrs) nonlinear flux decline, the C02 rate experienced a 10% loss over the next 1,000 hours (40 days). As this flux decline is a log/log relationship, only 6% more would be lost over the next three years, the minimum expected element lifetime. Membrane systems can be easily designed to adapt to a changing permeation rate by adjusting the feed flow rates and/or pressure, by allowing for addition of more elements in series after a period of time, and by using a progressive element replacement schedule. 

Membrane fouling is the main cause of permeant flux decline and loss of product quality in reverse osmosis systems, so fouling control dominates reverse osmosis system design and operation. The cause and prevention of fouling depend greatly on the feed water being treated, and appropriate control procedures must be... 

A typical plot illustrating the slow decrease in flux that can result from consolidation of the secondary layer is shown in Figure 6.5 [14], The pure water flux of these membranes is approximately 50 gal/min but, on contact with an electrocoat paint solution containing 10-20% latex, the flux immediately falls to about 10-12 gal/min. This first drop in flux is due to the formation of the gel layer of latex particles on the membrane surface, as shown in Figure 6.4. Thereafter, the flux declines steadily over a 2-week period. This second drop in flux is caused by slow densification of the gel layer under the pressure of the... 

Figure 6.5 Ultrafiltration flux as a function of time of an electrocoat paint latex solution. Because of fouling, the flux declines over a period of days. Periodic cleaning is required to maintain high fluxes [14]. Reprinted from R. Walker, Recent Developments in Ultrafiltration of Electrocoat Paint, Electrocoat 82, 16 (1982) with permission from Gardner Publications, Inc., Cincinnati, OH...

Figure 6.15 Effect of membrane surface charge on ultrafiltration flux decline. These membranes were used to ultrafilter cathodic electrocoat paint, which has a net negative charge. Electrostatic repulsion made the negatively charged membrane significantly more resistant to fouling than the similar positively charged membrane [13]...

The importance of membrane surface characteristics on performance is illustrated by Figure 6.15. The feed solution in this example was a cathodic electrocoat paint solution in which the paint particulates had a net positive charge. As a result, membrane flux declined rapidly with the positively charged membranes but much more slowly with essentially identical membranes that had been treated to give the surface a net negative charge [13]. 

D.J. Edlund and J. McCarthy, The Relationship Between Intermetallic Diffusion and Flux Decline in Composite-metal Membranes Implications for Achieving Long Membrane Lifetime, 7. Membr. Sci. 107, 147 (1995). 

The pressure-driven membrane processes can be operated at fixed pressure (FP) or fixed flux (FF), and FP tends to be lab and small scale and FF is large-scale commercial. Fouling for FP shows as a flux decline and for FF as TMP rise (Figure 6.1(b)). The fouling kinetics differ since FP becomes self-limiting as flux-driven fouling slows down, whereas for FF it is self-accelerating as foulants steadily accumulate and concentration polarization accelerates. These differences mean that extrapolation of FP trends to FF requires caution. 

To compensate for fouling (RF and CEOP) it is necessary to increase transmembrane pressure (Figure 6.1(b)). The constant flux strategy has important implications. Firstly, if J > Jcrit of foulant species, that species will continue to deposit. Secondly, as CP is (exponentially) flux-driven (see Equation 6.6 in Table 6.1) it will rise due to CEOP in a self-accelerating fashion. This is in contrast to a fixed-pressure strategy where the flux declines, net convection of foulant drops and CEOP become self-limiting. 

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