Membranes Problem-solving

Newer fuel cells, such as the cell in the problem-solving LAB and the cell shown in Figure 21-12, use a plastic sheet called a proton-exchange membrane,... 

Many refineries install membranes to recover hydrogen from fuel gas and purge gas. Membrane reliability problems solved. 

The use of purified proteins facilitates SPR determinations. Experiments with membrane proteins offer additional complexity, as their expression and purification can alter conformation, stability and/or function outside the cell environment. Solubilization of membrane proteins solves the problem in some cases, although detergents must be selected carefully (le Maire,... 

Combining wrinkling and diffusion provides thus an interesting method to produce complex patterns with tunable dimensions. However this physical method by itself is not suitable as a patterning technique due to the randomness of the wrinkle nucleation events. Indeed, the random distribution of the wrinkled domains is related to the uncontrolled localization of defects in the metal membrane. To solve this problem, we use thicker titanium layers and an AFM tip (Fig. 8.14) to make small holes in the metal layer with a specific geometry. As shown in... 

For example, more and more manufacturers, contractors, consultants, and building owners are turning to these techniques to solve in-service roofing membrane problems as well as to evaluate new roofing membrane materials. A typical example is the paper on the problem of EPDM membrane shrinkage The study was conducted in 1995 to investigate the causes of... 

Section 6.3.3.3 studies RO in bulk flow parallel to the force configuration and describes various membrane transport considerations and flux expressions. Practical RO membranes are employed in devices with bulk feed flow perpendicular to the force configuration, as illustrated in Section 7.2.I.2. A simplified solution for a spiral-wound RO membrane is developed analytical expressions for the water flux as well as for salt rejection are obtained and illustrated through example problem solving. A total of sbt worked example problems have been provided up to Chapter 7. Chapter 9 (Figure 9.1.5) shows a RO cascade in a tapered configuration. Section 10.1.2 calculates the minimum energy required in reverse osmosis based desalination and compares it with that in evaporation. Section 11.2 covers the sequence of separation steps in a water treatment process for both desalination and ultrapure water production. The very important role played by RO in such plants is clearly illustrated. 

The first successful chiral resolutions through enantioselective membranes have been published recently, but few cases are applicable to the preparative scale, mainly due to mechanical and technical limitations. Low flow rates, saturation of the chiral selectors and loss of enantioselectivity with time are some of the common problems encountered and that should be solved in the near future. 

Many procedures have been suggested to achieve efficient cofactor recycling, including enzymatic and non-enzymatic methods. However, the practical problems associated with the commercial application of coenzyme dependent biocatalysts have not yet been generally solved. Figure A8.18 illustrates the continuous production of L-amino adds in a multi-enzyme-membrane-reactor, where the enzymes together with NAD covalently bound to water soluble polyethylene glycol 20,000 (PEG-20,000-NAD) are retained by means of an ultrafiltration membrane. 

A fuel cell is an electrochemical reactor with an anodic compartment for the fuel oxidation giving a proton and a cathodic compartment for the reaction of the proton with oxygen. Two scientific problems must be solved finding a low-cost efficient catalyst and finding a membrane for the separation of anodic and cathodic compartments. The membrane is a poly electrolyte allowing the transfer of hydrated proton but being barrier for the gases. 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

Smart chemical reaction engineering can also solve the separation problem. Multifunctional reactors based on distillation or membrane separation wil gain importance in the future (see also Chapter 6). 

An important problem in emulsified organic-aqueous systems is that of scale-up, which is concerned with the realization of stable emulsions and the separation of phases after the reaction. The use of biphasic membrane systems that contain the enzyme and keep the two phases separated is likely to solve this problem. In the case of 5-naproxen an ee of 92% has been demonstrated without any decay in activity over a period of two weeks of continuous operation. A number of examples of biocatalytic membrane reactors have been provided by Giorno and Drioli (2000) and include the conversion of fumaric acid to L-aspartic acid, L-aspartic acid to L-alanine, and cortexolone to hydrocortisone and prednisolone. 

---