Membrane materials evaluation

In this paper an approach has been presented that will facilitate selection and evaluation of possible membrane materials. Having selected potential membrane materials for desired separations, steric considerations are taken Into account In membrane preparation. This approach avoids starting with and subsequently modifying aqueous-separation membranes, allows a vast spectrum of polymers to be considered as potential membrane materials and focuses the selection process. A brief review of membrane material evaluation procedures have been discussed with emphasis on those techniques which do not require the fabrication of membranes. Finally, the survey of materials evaluated for possible membrane use Indicates both the Interest In this field and the need for appropriate material selection and evaluation procedures. The ideas presented here will continue to grow In value In the future as membranes are called upon to achieve more difficult separations in an energy efficient fashion. 

Lloyd, D.R. and Meluch, T.B. 1985, Selection and evaluation of membrane materials for liquid separations. In Materials Science of Synthetic Membranes, Lloyd, D.R., Ed. American Chemical Society Washington, DC. 

In evaluating this exponential term (the Poynting correction), it is important to recognize that u is not the molar volume of i in the gas phase, but the molar volume of i dissolved in the membrane material, which is approximately equal to the molar volume of liquid i. 

Typically, the integrity of the cell membrane is evaluated by identifying the percentage of cells taking up a vital dye termed trypan blue. Cells that take up trypan blue have defective cell membranes and therefore external loading or contact with materials that cause cells to have faulty membranes are likely to cause cell injury and death. Cell cytotoxicity is evaluated by contacting a surface of a material or the extract of a material or implant with cells in culture. If more than five percent of the cell population stains with dye after contact with the material or an extract then it is considered cytotoxic. 

Pereira CC, Rufino JRM, Habert AC, Noberga R, Cabral LMC, and Borges CP. Aroma compounds recovery of tropical fruit juice by pervaporation Membrane material selection and process evaluation. J. Food Eng. 2005 66(l) 77-87. 

Another factor that has been shown to significantly affect flux and fouling is the hydrophilic character of the membranes [4,8-12]. Water spreads over the surface of a hydrophilic membrane but not on a hydrophobic membrane. The membrane material determines its hydrophilicity, which is typically evaluated by measuring the contact angle at a membrane-water-air interface. Cellulose and hemicelluloses, which form the main part of wood and paper, are extremely hydrophilic. Wood extractives such as fatty and resin acids, on the other hand, have hydrophobic characteristics and, therefore, they are found more on fouled and used hydrophobic membranes than on hydrophilic ones (Figure 35.1) [9,13]. 

A recent study has shown that membranes made of a modified polyetheretherketone (PEEK-WC) are interesting materials for biomedical applications [23,24]. The cytocompatibility of PEEK-WC membranes was evaluated by culturing hepatocytes isolated from rat liver (Figure 43.6). The properties of PEEK-WC membranes were compared to polyurethane membranes prepared using the same technique, and commercial membranes (made of Nylon, polyethersulphone, and polyester). The results have shown that PEEK-WC membranes promoted hepatocyte adhesion most effectively and metabolic activities of cells cultured on these membranes improved significantly. 

Filters are available in several constructions, effective filtration areas, and configurations. Depending on the individual process, the filter construction and setup will be chosen to fit its purpose best. Most commonly used for RO filters are tubular devices, so-called spiral wound modules due to the spiral configuration of the membrane within the support construction of such device. UF systems can be found as a spiral wound module, a hollow fiber, or a cassette device. The choice of the individual construction depends on the requirements and purposes towards the UF device. Similar to the different membrane materials, UF device construction has to be evaluated in the specific applications to reach an optimal functioning of the unit. Microfilters and depth filters can be lenticular modules or sheets but are mainly cylindrical filter elements of various sizes and filtration areas, from very small scale of 300 cm to large scale devices of 36 m. A 10-inch high cylindrical filter element can be seen in Fig. 6. 

For an ideal system, all contaminants to be removed are separated by the membrane and exit in the concentrate stream. In reality, no membrane is perfect. The actual quantity of solute that passes through the membrane depends on the chemistry of solute on the feed, the nature of membrane material, and the operational conditions. Cell test, applications test, and pilot test can be used to evaluate membrane technology with a particular stream. 

Material science aspects of synthetic polymeric membranes are presented In this survey. The objective Is to place each of the subsequent chapters of this volume Into proper perspective. Therefore, frequent reference Is made to the accompanying chapters and, where necessary, to alternative Information sources. By way of Introduction, this chapter considers In turn material selection, material characterization and evaluation, membrane preparation, membrane characterization and membrane evaluation. Membrane module design and manufacture, transport phenomena and process performance are Introduced In the discussion only as they pertain to membrane materials science. Following this Introduction, the various chapters of this volume are previewed. 

Membrane science can arbitrarily be divided Into seven Intimately related categories material selection, material characterization and evaluation, membrane preparation, membrane characterization and evaluation, transport phenomena, membrane module design and process performance. This chapter and those to follow emphasize the materials science aspects of synthetic polymeric membranes that Is, the selection, characterization and evaluation of membrane materials as well as the preparation, characterization and evaluation of membranes. Transport phenomena, membrane module design and process performance enter the discussion only as these topics pertain to materials science. 

Material Characterization and Evaluation. PhysIcochemlcal considerations can be useful in membrane material selection. However, it would be beneficial if one could experimentally verify that the proper choice has been made prior to undertaking the often difficult tasks of membrane preparation and characterization. In addition, it is frequently beneficial to have fully characterized the polymer prior to forming the membrane. 

Lloyd and Meluch ( ) summarize several methods of evaluating potential membrane materials for liquid separations without actually preparing membranes. They point out that once a membrane or film has been formed, it is often difficult to distinguish the intrinsic properties of the polymer from the structural characteristics of the membrane, which are themselves dependent... 

Figure 8. Simple apparatus for evaluating stress cracking potential of candidate membrane materials.

Selection and Evaluation of Membrane Materials for Liquid Separations... 

Physicochemical Interactions between permeating molecules and the macromolecules which comprise the membrane structure are considered. Dispersive, polar and hydrogen-bonding Interactions are used to establish an Index which can be useful In membrane material selection. A number of material characterization and evaluation procedures are outlined. 

Whether the approach outlined above Is used to select a pl oz i the membrane material or an approach based simply on polymer availability Is used. It would be convenient to evaluate the material for Its potential to accomplish the desired separation prior to forming a membrane. Once the membrane has been formed. It Is difficult to distinguish the Intrinsic properties of the polymer from the physical characteristics of the membrane such as porosity and asymmetry (that Is, properties which are governed In large part by membrane formation procedures). In this section a number of techniques for evaluating the material In particulate form as well as In the form of a dense film are discussed. Recall that the membrane Is to be used to separate a mixture of components A and B where B Is the bulk or major solution component and that Interaction between either solution component and the membrane material, M, may be Influenced by the presence of the other solution component. Thus, It would be difficult to evaluate a... 

Lloyd DR, Meluch TB, Selection and evaluation of membrane materials for hquid separations. In Lloyd DR, Ed., Material Science of Synthetic Membranes, ACS Symp. Ser. No 269, Washington DC, ACS, 1985. 

The zeolite membranes quality and the development of methods able to rapidly identify the defects are key-factors for large scale applications. A large number of static and dynamic methods can be used to evaluate the quality of membranes [125J. Static methods allow to study the physico-chemical characteristics of the membrane material. Dynamic methods allow the detection of defects affecting the transport properties and consequently the membrane performance. For zeolite membranes the quality of the adhesion between the zeolite layer and the support, the orientation of zeolite crystals, the layer thickness, and the number and quality of... 

Struis and Stuck [6.12] have evaluated the application of membrane reactors for methanol synthesis using methanol permselective Nafion membranes. In their design calculations they utilize kinetic and membrane permeation data measured in their laboratory. They estimate that with 10 im thin membrane under methanol synthesis plant technically relevant conditions (T = 200 C, P = 40 bar, GHSV = 5000 h ), the single pass reactor yield improves by 40 %, and that the additional costs for the membrane materials correspond only to two production months. The ability of the Nafion membranes to withstand such conditions for prolonged periods still remains, however, questionable. 

In Task 2.1, "Commercial Plant Economic Evaluation," advanced ITM Syngas/ITM H2 processes will be developed, and the economics of operation at the commercial plant scale will be evaluated based on the results of the Phase 2 program. In Task 2.2, "Materials and Seals Development and Evaluation," membrane materials and seals will be tested at the laboratory scale under ITM S mgas/ITM H2 process conditions to obtain statistical performance and lifetime data. In Task... 

Physical sieving applies to colloids and large molecules. Apart from that, rejection is a function of the relative chemical affinity of the solute to the membrane material. Ion rejection follows the tyotropic series, which means that rejection is increased with the increased hydrated radius of the ion. The order of the ions, however, may change due to ion pairing, complexadon, or other solute-solute interactions, and it is, therefore, difficult to predict rejection for mixtures of ions. The rejection behaviour in the presence of organics, or even of organics themselves is poorly understood and only trends can so far be noted. Rejection is usually evaluated with NaCl or MgS04 solutions. 

Section 3 presents a look at the current state of membrane reactor evaluation. Despite the advances of recent years, many challenges and difficulties still face catalytic membrane reactors. Some are technical the materials available, although much improved, still suffer from problems of stability. The difficulty of obtaining high permeation rate and permselectivity in the same material still confronts us. Some problems are commercial or competitive in nature. For example, membrane reactors may be too expensive. In certain applications, reactors that are more conventional can outperform them. An active area of research answers the question of how to make such comparisons. Simple measures, such as conversion or yield, do not always give the whole story. 

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