Reuse approach

For most applications, enzymes are purified after isolation from various types of organisms and microorganisms. Unfortunately, for process application, they are then usually quite unstable and highly sensitive to reaction conditions, which results in their short operational hfetimes. Moreover, while used in chemical transformations performed in water, most enzymes operate under homogeneous catalysis conditions and, as a rule, cannot be recovered in the active form from reaction mixtures for reuse. A common approach to overcome these limitations is based on immobilization of enzymes on solid supports. As a result of such an operation, heterogeneous biocatalysts, both for the aqueous and nonaqueous procedures, are obtained. 

A very important part of such an undertaking is to be clear about what stages of a chemical process generate the most waste. Often this is found to be the separation stage, after the transformation of reactants to products, where all the various components of the final mixture are separated and purified. Approaches to chemical reactions which help to simplify this step are particularly powerful. Such an approach is exemplified by heterogeneous catalysis. This is an area of chemistry where the catalysts used are typically solids, and the reactants are all in the hquid or gas phase. The catalyst can speed up the reaction, increase the selectivity of the reaction, and then be easily recovered by filtration from the liquid, and reused. 

Finally, these aqueous suspensions of rhodium(O) and iridium(O) are the most efficient systems for the hydrogenation of a large variety of mono-, di-substituted and/or functionalized arene derivatives. Moreover, in our approach, the reaction mixture forms a typical two-phase system with an aqueous phase containing the nanoparticle catalyst able to be easily reused in a recycling process. 

One approach to waste reduction is to recover process materials for reuse. Materials used in metal finishing processes can be effectively recovered using available technologies such as dragout, evaporation, reverse osmosis, ion exchange, electrodialysis, and electrolytic recovery.22-26... 

To include all of these complexities requires a different approach from the one described so far. The design approach based on the optimization of a superstructure can be used to solve such problems14. Figure 26.36 shows the superstructure for a problem involving two operations and a single source of fresh water14. The superstructure allows for reuse from Operation 1 into Operation 2, reuse from Operation 2 to Operation 1, local recycles around both operations, fresh water supply to both operations and... 

Maximum water reuse can be identified from limiting water profiles. These identify the most contaminated water that is acceptable in an operation. A composite curve of the limiting water profiles can be used to target the minimum water flowrate. While this approach is adequate for simple problems, it has some severe limitations. A more mathematical approach using the optimization of a superstructure allows all of the complexities of multiple contaminants, constraints, enforced matches, capital and operating costs to be included. A review of this area has been given by Mann and Liu21. 

Instead of splitting the problem into concentration intervals and time subintervals, the problem is split into time intervals and concentration subintervals, with water demand plotted on the horizontal axis. The boundaries for time intervals and concentration subintervals are set by the process end-points. However, unlike in a case where time is taken as a primary constraints, the streams that are required or available for reuse in each concentration subinterval are plotted separately. This approach has proven to ease the analysis as will be shown later in this section. 

Jodicke, G., Fischer, U., Hungerbtihler, K., 2001. Wastewater reuse a new approach to screen for designs with minimal total costs. Comp. Chem. Eng., 25 203-215... 

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