Membrane permeability rejection measurements

Most membrane processes - which is another TA ay of saying most membranes - are characterised by two key process parameters flux (i.e. the rate of flow of fluid through the ntembrane) and selectivity (i.e. the ability of the membrane to reject" - prevent the passage of - one or more species in the feed suspension or solution). The selectivity is governed by the intrinsic nature of the membrane material, built into it by its method of manufacture, and measured by its permeability to the species in question. The... 

However, in actual test measurements, at what point, if any, can it be said that all the feed passes through the membrane That is to say, does not holdup occur on the upstream pressure side For in any kind of short-term or transient test (say, in what might be called a batch or semi-continuous laboratory or bench-scale test), does a reject phase not exist at any point At any point in time, is there no situation in which the feed that has not yet passed through the membrane constitute a reject phase Only for a long-term, steady-state test—with no reject sidestream—can it truly be said that all the feed passes through the membrane. This sort of long-term test, properly speaking, then provides the true measure of membrane permeability for the components within a mixture. Whether or... 

The transition between reverse osmosis membranes with a salt rejection of more than 95 % and molecular weight cutoffs below 50 and ultrafiltration membranes with a salt rejection of less than 10% and a molecular weight cutoff of more than 1000 is shown in Figure 2.42 [74], The very large change in the pressure-normalized flux of water that occurs as the membranes become more retentive is noteworthy. Because these are anisotropic membranes, the thickness of the separating layer is difficult to measure, but clearly the permeability of... 

The permeability Ps is a measure of the transport of a molecule by diffusion. The reflection coefficient a of a given component is the maximal possible rejection for that component (at infinite solvent flux). Various models have been proposed for the reflection coefficient [75-77]. In the lognormal model [78], a lognormal distribution is assumed for the pore size. No steric hindrance in the pores or hydrodynamic lag is taken into account, but it is assumed that a molecule permeates through every pore that is larger than the diameter of the molecule. Moreover, the diffusion contribution to the transport through the membrane is considered to be negligible. Therefore, the reflection curve can be expressed as ... 

The aspect of hole filling by plasma deposition can be demonstrated by the transport characteristics of LCVD-prepared membranes. First, the porosity as porous media calculated from the gas permeability dependence on the applied pressure can be correlated to the salt rejection of the composite membrane as shown in Figure 34.13. The effective porosity s/, where s is the porosity and q is the tortuosity factor, is measured in dry state and may not directly correlate to the porosity of the membranes in wet state. The effective porosity of LCVD-prepared membranes was measured before the reverse osmosis experiment. The decrease of porosity (as porous media) is clearly reflected in the increase in salt rejection in reverse osmosis. 

Transient vs. Steady-State Behavior in Permeability Determinations. The foregoing derivations raise some intriguing speculations about the measurement and determination of permeabilities for the respective components in a mixture. Thus, if a true or complete steady-state condition exists during the experiment, whereby all of the feed stream passes through the membrane, then the ratio V/F = and the ratio L/F = 0. That is, it can be said that no reject phase whatsoever is produced. 

Roth et al. [80] proposed a method to determine the state of membrane wear by analyzing sodium chloride stimulus-response experiments. The shape of the distribution of sodium chloride in the permeate flow of the membrane revealed the solute permeation mechanisms for used membranes. For new membranes the distribution of sodium chloride collected in the permeate side as well in the rejection side was unimodal. For fouled membranes they noted the presence of several modes. The existence of a salt leakage peak, as well as an earlier detection of salt for all the fouled membranes, gave evidence of membrane stmcture modification. The intensive use of the membranes might have created an enlargement of the pore sizes. Salt and solvent permeabilities increased as well. While this is a difficult paper to follow, it may be of use to those who want to develop new methods for measuring membrane degradation. 

The transport of co-ions is a function of the leakage flow of ions through the fiber wall. No flow occurs through a perfectly semi-permeable fiber wall and the electromotive potential observed corresponds to the Nernst potential. However, when leakage flow occurs, as it does with real membranes, one can assume that the flow occurs through water filled pores. The cross-sectional area of pores allowing such flow, divided by the total available cross-sectional area is then a measure of the semi-permeability of the membrane. On this basis the rejection of ionic species is estimated by ... 

Rejection for BSA at 90%, Water permeability measured at 20°C. Salt concentration =1000 mg trans-membrane pressure = 0.5 MPa, MWCO molecular weight cutoff... 

Kim and co-workers demonstrated a novel RO Alter, made of a sulfonated poly(arylene ether) top barrier layer and a middle layer of end-group cross-linked poly(arylene ether) electrospun nanofibers [93]. The salt rejection was measured at 1,000 psi in a dead-end filtration system. The order of salt rejection was as follows Na2S04 (88.4 %), NaCl (79.3 %), MgS04 (70.2 %), and MgCh (62.3 %). Also, the NaCl rejection and permeability of the composite membranes were controlled as the different sulfonation group ratio of poly(arylene ether). 

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