Flux decay

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 25. Tap water flux decay and regeneration with flush on BST-1...

In order to determine profound subcriticality (Keir 0.9) on the basis of the measured value of asymptotic prompt a-flux decay constant, the following formula is used [15] ... 

Flux decay in the reverse osmosis test has been determined and minimized using appropriate fluid dynamic regimes. Typical flux decay with DDS 800 and PA 300 are shown in Figure 5 and 6. The feed was must ultrafiltered through BMR capillary membranes. 

Figure 5. Typical flux decay with CA DDS-800 membranes. Feed white must previously ultrafiltered through BMR-100515 modulus T = 10°C, P — 35 atm.

Flux decays and rejection changes in the UF steps, depending upon the feed concentration and experiment history, must be mainly attributed to concentration polarization phenomena with consequent gel layer formation of the pressurized membrane surface, for the presence of proteins, colloids and in general high molecular species in the feed (3). 

The theory of t angential flow filtration as it applies to cell separations is discussed. Major emphasis, however, is placed on presenting the relationship of experimental results to theoretical performance. Topics highlighted are flux decay with time, effects of operating pressures and flow, membrane fouling, prefiltration requirements and filter geometries. ... 

Let the solar flux decay as q = q e YX in a solar pond (or a liquid layer) of thickness l [Fig. 2.17(a)]. The lower surface of the pond is conductively insulated but radiatively transparent, while its upper surface loses heat to the ambient with a heat transfer coefficient h. The entire system is at temperature Tx in the absence of solar flux, (a) We wish to determine the steady temperature of the pond, (b) What would happen to this temperature if the lower surface were radiatively opaque (c) Find the difference between the bottom surface temperature corresponding to parts (a) and (b). 

For suspensions of particles with sizes nearer to the pore size, some internal pore fouling will occur but at a greatly reduced rate. Figure 2.4122 shows cross-flow filtration of a single cell protein suspension on a "tortuous-pore" membrane. The flux declines rapidly at first, as boundary layer conditions are established, and then levels off with a diminishing rate of flux decay. 

With some process streams the flux can be stable for months or even years without cleaning or membrane replacement. For most applications, however, there is a gradual flux decay with time as in Figure 3.59. This is not due to internal pore fouling (as in symmetrical MF membranes). Rather it is the result of the accumulation of materials on the membrane which no longer participate in the mass-transport to or away from the membrane. In effect, they "blind" small sections of the membrane, thereby reducing the effective area and the flux through the membrane. 

Figure 3.59 Long term flux decay and restoration by cleaning.

Figure 3.60 Effect of cross-flow velocity on long term flux decay.

Figure 3.63 Effect of "charged" membrane on long term flux decay with cathodic paint.

Often a ratio controller is used to keep the feed to bleed ratio constant-equal to the volumetric concentration ratio required. This means that even with flux decay, the concentration ratio and recovery will remain constant. 

If the feed is preconditioned properly, the UF flux is often quite stable. One thousand hours of continuous operation between cleanings is common. When flux decay does occur, detergent washing is usually sufficient to restore flux. In some cases, polymer solvents may be required. Proper selection of a solvent resistant membrane and/or solvents which will dissolve the latex but not affect the membrane is crucial. For PVC latex, the solvents of choice are methyl isobutyl ketone (MIBK) and methyl ethyl ketone (MEK). Styrene butadiene rubber will swell in MIBK, MEK or toluene. Polyvinyl acetate will dissolve in the low MW alcohols such as propyl alcohol. Generally, the membranes are first washed with water, then detergent, followed by another water flush. The system is then drained of all water since it will affect polymer solubility in MEK. Finally, a solvent rinse is employed. If the module is tubular, sponge balls will enhance cleaning. 

UF can remove metal values from metal plating wastes provided the metals are first precipitated to form a colloidal suspension. The process is described in Chapter 2 in the section on cross-flow applications. Even though MF membranes provide a higher flux, they are more prone to flux decay than UF due to internal pore fouling. The long term UF flux may be higher than that from MF. 

Yet to be solved are the fouling and flux decay problems encountered in the processing of milk and cheese whey. Permanent surface treatments of mem-... 

Even though enzymatic conversion is not too effective, it is possible to prepare semipermeable membranes whose ultrafiltration yields are higher than those of passive membranes.74 75 Ultrafiltration experiments of cheese whey through cellulosic membranes to which papain was covalently bound, show that flux decay curves of enzymatic membranes are even less sensitive to pH changes.74... 

Akay G., Wakeman R.J. 1994. Mechanism of permeate flux decay, solute rejection and concentration polarisation in crossflow filtration of a double chain ionic surfactant dispersion, J. Membr. Sci., 88, 177-195. 

Gain bandwidth Type, gain saturation Homogeneous saturation flux Decay lifetime (lower level) Inversion density Small signal gain coefficient Pump power density Output power density Laser size (diameter length) Excitation current/voltage Excitation current density Excitation power Output power Efficiency... 

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