Performance of Membrane Separators

Membrane separators are built with a variety of membrane types assembled in modules that can take any of several forms such as spiral-wound, tubular, plate-and-frame, and so on (Hsieh et al., 1988). 

The performance of a membrane separator may be predicted on the basis of permeation fluxes and material balances. The formulation of these relationships for a particular module depends on the flow patterns of the fluid on both sides of the membrane. 

---

Ab Initio Prediction of the Performance of Membrane Separation Processes... 

Ab Initio prediction of the performance of membrane separation processes Ch. 14... 

The development of ab initio methods for the prediction of the performance of membrane separation processes has made substantial developments. Sophisticated methods now exist for such prediction, and these have been experimentally verified in the laboratory. The present challenges are two-fold. Firstly, to continue the fundamental development to more complex separations. Secondly, to apply the verified methods in the design of full-scale industrial processes. The existence of good predictive methods is expected to further expand the application of membrane processes. 

Parameters Affecting the Performance of Membrane Separation in the Pulp and Paper Industry.983... 

PARAMETERS AFFECTING THE PERFORMANCE OF MEMBRANE SEPARATION IN THE PULP AND PAPER INDUSTRY... 

Another approach to enhance separation performance of membrane for dehydration of isopropanol is the modification of PVA membranes in gaseous plasma [30], The modification of membrane properties in nitrogen plasma environment lead to increase in selectivity by about 1477 at 25 °C such increase in the selectivity is justified by an increase of cross-linking on membrane surface provoked by plasma treatment. 

The structure of this interface determines fhe sfabilify of PEMs, the state of water, the strength of interactions in the polymer/water/ion system, the vibration modes of side chains, and the mobilities of wafer molecules and protons. The charged polymer side chains contribute elastic ("entropic") and electrostatic terms to the free energy. This complicated inferfacial region thereby largely contributes to differences in performance of membranes wifh different chemical architectures. Indeed, the picture of a "polyelectro-lyfe brush" could be more insighttul than the picture of a well-separated hydrophobic or hydrophilic domain structure in order to rationalize such differences. ... 

Very little work (relative to research of electrode materials and electrolytes) is directed toward characterizing and developing new separators. Similarly, not much attention has been given to separators in publications reviewing batteries.A number of reviews on the on cell fabrication, their performance, and application in real life have appeared in recent years, but none have discussed separators in detail. Recently a few reviews have been published in both English and Japanese which discuss different types of separators for various batteries. A detailed review of lead-acid and lithium-ion (li-ion) battery separators was published by Boehnstedt and Spot-nitz, respectively, in the Handbook of Battery Materials. Earlier Kinoshita et al. had done a survey of different types of membranes/separators used in different electrochemical systems, including batteries."... 

The technology to fabricate ultrathin high-performance membranes into high-surface-area membrane modules has steadily improved during the modem membrane era. As a result the inflation-adjusted cost of membrane separation processes has decreased dramatically over the years. The first anisotropic membranes made by Loeb-Sourirajan processes had an effective thickness of 0.2-0.4 xm. Currently, various techniques are used to produce commercial membranes with a thickness of 0.1 i m or less. The permeability and selectivity of membrane materials have also increased two to three fold during the same period. As a result, today s membranes have 5 to 10 times the flux and better selectivity than membranes available 30 years ago. These trends are continuing. Membranes with an effective thickness of less than 0.05 xm have been made in the laboratory using advanced composite membrane preparation techniques or surface treatment methods. 

Figure 8.12 Parameters affecting the performance of membrane gas separation systems...

The separation performance of membranes with nonporous barriers is - because of the transport via solution-diffusion (cf. Section 2.2) - predominantly influenced by the polymer material itself. Therefore, the material selection is directly related to the intrinsic (bulk) properties of the polymer, but - as for porous membranes - filmforming properties, mechanical and thermal stability form the basis of applicability (cf. Section 2.3.2.1). The following characteristics should be considered ... 

The mass flow is effected by keeping the downstream side of the membrane at reduced pressure. The performance of membranes for the pervaporation of ethanol-water mixtures is evaluated by the separation factor a H and the specific permeation rate R. is defined as follows ... 

The overall performance of membranes is related to two main characteristics their permeability and their permselectivity (separation ability). For porous membranes, the selectivity and the membrane cutoff depend on the pore size and the pore size distribution of the separative layer. The membrane permeability and the membrane thickness fix the viscous flux for a given transmembrane pressure. The viscous flux of a liquid, J, across a porous medium is given by Darcy s law ... 

Since chromatographic membranes consist of a substrate to which the interactive ligand is coupled, three main steps are usually involved in their preparation (i) basic membrane preparation (ii) functionalization (activation) of the basic membranes and (iii) spacer arms and ligand molecules coupling on the activated porous membrane surface [9]. The preparation of basic materials is essential for the performances of the separation process. 

This section aims to explain the unique features of membrane separation methods, their superior performance in contaminant removal, and their operational sensitivities and limitations. We focus particularly on the factors that need to be carefully assessed when the membrane technology to be used in the treatment of liquid radioactive waste is being considered. These include membrane configuration and arrangement, process application, operational experience, data related to key performance parameters, and plant and organizational impacts. 

As mentioned above, the top skin layer governs the performance of a separation membrane. The surface deposition of contaminants from solutions or from gas mixtures is also affected by the surface properties of the membrane. This is particularly important when decline in the membrane flux with a prolonged operating period is observed, because it is often caused by the contaminant deposition. Hence, many attempts have been made to modify the membrane surface, aiming at prevention of contaminant deposition and maintenance of high flux. Several methods of surface modification are described below. 

For an existing separator where the membrane area is known, the performance of the separator is determined by an iterative computational procedure as follows ... 

The performance of membranes often decreases over time due to effects such as fouling and concentration polarization. This is seen as a decrease in flux (Figure 9.15). This performance decline is a major concern for filtration processes, but less so for gas separation processes. 

---