Membranes compaction

This value is taken into account when planning hoUow-fiber dimensions. A partial account of these considerations can be found in References 6 and 7. In practical appUcations, ie, reverse osmosis, membrane compaction with time is experimentally derived as a function of the polymeric material at given temperatures and pressures (8). 

Initial membrane compaction is illustrated by Figure 3. Equation 1 predicts a straight-line response of J to AP, or at P. Owing to the compaction, a lower flux is observed. Once a membrane has been subjected to some pressure (P ), equation 1 is vaHd for predicting flux up to that pressure (Fig. 3, curve B). If the membrane is subsequently subjected to higher pressure (P2), the hydrauHc permeabiUty constant is changed (Fig. 3, curve D). 

Membrane Limitations Chemical attack, fouling, and compaction are prominent problems with RO and NF membranes. Compaction is the most straightforward. It is the result of creep, slow cold flow of the polymer resulting in a loss of water permeability. It is measured by the slope of log flux versus log time in seconds. It is independent of the flux units used and is reported as a slope, sometimes with the minus sign omitted. A slope of—0.001, typical for noncelhilosic membranes, means that for every threefold increase in log(time), 10 seconds, a membrane looses 10 percent of its flux. Since membranes are rated assuming that the dramatic early decline in permeability has already occurred, the further decline after the first few weeks is veiy slow. Compaction is specific to pressure, temperature, and envi-... 

Physical deterioration includes compaction by creeping and surface deteriorations by scratching and vibration. Creeping is accelerated at higher temperatures and pressures, resulting in the membrane compaction. This phenomenon is well analyzed and the membrane characteristics of compaction can be estimated in terms of m-value. Scratching and vibration can develop the microscopic defects in the surface structure of membranes, and give poor performances. We discussed this type of deterioration in Mexico in 1976 ( ). 

The decrease in flux with time under pressure has been attributed to membrane compaction. 

Membrane Structure and Compaction. Membrane compaction has been considered as results of an increase in the effective thickness of the active surface layer and/or an added viscous flow resistance in the porous region underneath. (1, 2 ) It will be more reasonable to consider that water flux is inversely proportional to the effective thickness of the membrane which can be assumed to increase with time. 

Intrinsic Membrane Compaction and Aqueous Solute Studies of Hyperfiltration (Reverse-Osmosis) Membranes Using Interferometry ... 

Effect of Organophilic Bentonites. Membrane compaction reduces the integral product water output. By implication, membrane stabilization is a means to increase the flux. Stabilization, along with some flux improvement, can be achieved by doping the membranes with organophilic bentonites ( ). 

The objective of employing organophilic bentonite is flux stabilization. In terms of the membrane compaction slope the stabilizing effect is exemplified by the following figures (brackish water conditions) reference, -0.10 bentonite-doped, -0.06. In a field test over 1300 hours on well water of 5200 ppm TDS at a pressure of 60 bar, starting with an initial flux of 1780 1/m d and 95 % rejection, a compaction slope of -0.058 was found under the same conditions the reference membrane had a compaction slope of -0.094. 

Finally, it should be mentioned that it is very difficult to accurately measure the values of r when it is close to unity. On the other hand, when the value of r is low, membrane compaction will take place. In spite of these difficulties, experiments are currently being conducted to test the present theory at the Max-Planck-Institut flir Blophyslk in Frankfurt, Germany. 

Concentration polarization may be a possible explanation for this erratic permeability coefficient behavior however, the influence of concentration polarization in gaseous exchange regimes such as the one herein reported is quite doubtful and subject to dispute(7 ). Alternative possibilities include (a) permeator functional loss due to on-going membrane compaction,... 

Membrane fouling and scaling can both lead to a loss in normalized permeate flow. Additionally, membrane compaction will result in decreased permeate flow as well. 

Increases in salt rejection are typically due to membrane compaction (see Chapter 12.1.1.4). As the membrane becomes denser due to compaction, the passage of salts through the membrane is reduced, leading to a loss in salt passage and in increase in salt rejection. 

Membrane compaction and fouling-cleaning studies by ultrasonic time-domain refiectometry... 

Figure 10.6. (A) Experimental set-up used for membrane compaction studies of a high-pressure separation system by ultrasonic time-domain reflectometry (B) Scheme of the separation cell showing the externally mounted transducer and the primary reflections identified as a, b and c, which correspond to the top plate-feed solution interface, feed solution-top membrane surface interface and bottom membrane surface-support plate interface, respectively (C) Change of the arrival time which translates into changes in membrane thickness during compaction. (Reproduced with permission of Elsevier, Ref [63].)...

In chromatographic separations using flat sheet membranes, compact porous disks, fibers, tubes, or rods, the interaction between the ligate molecule and the immobilized ligand takes place in the through-pores of the matrix and not in the dead-end pores of the conventional packed-bed particles. This method resembles affinity membrane separation with a very short affinity chromatography column [2]. 

On examining the effect of membrane compaction on the membrane permeability, Lawson et al. [100] concluded that the transmembrane flux in MD could be increased significantly up to 11 % with relatively small pressure drops <70 kPa. 

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