Recycling efficiency

Our case studies prove that optimization objectives generally followed in synthesis design and during scale up show a high potential for increasing resource efficiency. These objectives are, for example, increases of yield and the recycling efficiency of solvents and auxiliary materials. 

Additional separation and recycling. Once the possibilities for recycling streams directly have been exhausted, feed purification and extraneous materials for separation eliminated that cannot be recycled efficiently, attention is turned to the fourth option in Figure 28.2, the degree of material recovery from the waste streams that are left. It should be emphasized that once the waste stream is rejected, any valuable material turns into a liability as an effluent material. The level of recovery for such situations needs careful consideration. It may be economic to carry out additional separation of valuable material with a view to recycling that additional recovered material, particularly when the cost of downstream effluent treatment is taken into consideration. 

Vorlop et al. described a novel cross-linked and subsequently poly(vinyl alcohol-entrapped PaHNL for synthesis of (//(-cyanohydrins. These immobilized lens-shaped biocatalysts have a well-defined macroscopic size in the millimeter range, show no catalyst leaching, and can be recycled efficiently. Furthermore, this immobilization method is cheap and the entrapped (/ )-oxynitrilases gave similar good results compared with those of free enzymes. The (//(-cyanohydrin was obtained in good yields and with high enantioselectivity of up to >99% ee [55],... 

To be fair, it should be realized that if a catalyst must be recycled for economic reasons, the recycling efficiency compared to the nonfunctionalized catalyst must be higher in order to compensate for the increased price of the fluorous catalyst itself. However, every recycling technique has its own cost that must be evaluated for each specific case. 

Figure 1.8 The loss of material into the environment as a result of recycling processes, assuming a recycling efficiency of 90 % and product lifetimes of 1 year, 10 years and 50 years.

The surface-ocean recycling efficiency is calculated as the fraction of the element in the surface box that is removed in particulate form. This fraction, g, is given by Amount of particles leaving the surface ocean... 

Thus, to compute the surface-water recycling efficiency of an element, all that must be known are its average surfece-, deep-, and river-water concentrations. 

The overall oceanic recycling efficiency of a biolimiting element is given by the fraction of the river input that is buried in the sediments during one complete mixing cycle. This is calculated as... 

Table 9.1 contains a summary of recycling efficiencies for a variety of representative elements. The results fall into three groups, which have been termed biolimiting, biointermediate, and biounlimited. The biolimiting elements have 1 and, thus, are almost... 

Table 9.1 Broecker Box Model unlimited Elements. Ratios and Recycling Efficiencies for Representative Biolimiting, Biointermediate, and Bio-... 

Nutrients are carried back to the sea surface by the return flow of deep-water circulation. The degree of horizontal segregation exhibited by a biolimiting element is thus determined by the rates of water motion to and from the deep sea, the flux of biogenic particles, and the element s recycling efficiency (/and from the Broecker Box model). If a steady state exists, the deep-water concentration gradient must be the result of a balance between the rates of nutrient supply and removal via the physical return of water to the sea surface. 

Recycling efficiency The degree to which a biolimiting element is remineralized prior to its removal from the ocean via sedimentation. 

For the production of chemicals, food additives and pharmaceutical products, homogeneous catalysis offers some attractive features such as a high selectivity and activity, e.g. in asymmetric synthesis. However, since most homogeneous catalysts are relatively expensive, their current industrial application is limited [3]. On the other hand, heterogeneous catalysts can easily be separated from the products and can be recycled efficiently. Membrane separations with emphasis on nanofil-tration and ultrafiltration will allow for a similar recyclability of homogeneous catalysts, which is important both from an environmental as well as a commercial... 

Good reactor productivities and cofactor recycling efficiencies with reuse of the... 

Reactor productivities of 640 g.l day and a cofactor recycling efficiency of 130,000 (mol product formed/mol cofactor used) are achieved making cofactor costs very low. 

Several organocatalysts have been recycled efficiently (selected examples are shown in Scheme 14.2). For example, the Jacobsen group has reported results from an impressive study of the recycling of the immobilized urea derivative 6, a highly efficient organocatalyst for asymmetric hydrocyanation of imines (Scheme 14.2) [11]. It was discovered that the catalyst can be recycled and re-used very efficiently - over ten reaction cycles the product was obtained with similar yield and enantioselectivity (96-98% yield, 92-93% ee). 

Immobilized proline has also been tested but, in contrast, gave less satisfactory results than L-proline itself [12b], L-Proline is, however, an inexpensive catalyst which can be recycled efficiently without immobilization [10] and was also found to be suitable in catalytic amounts below 10 mol% [13b]. There might, therefore, be less need for immobilization of L-proline than for other, more expensive, catalysts. 

Although a large number of chiral dienophiles have been developed (Table 26.2), their ability to provide high asymmetric induction appears to be limited to specific dienes. However, there are some dienophiles that tolerate a wider variety of dienes including menthol derivatives,117 118 camphor derivatives,6 39 40 105 107-113 181 182 and oxazolidinones.120 165 183 184 It should be noted that even these auxiliaries would require an efficient recycle protocol for economic scale up. One exception is the use of sacrificial chiral oxazolidinones, which are relatively inexpensive. This approach has been used in the large-scale preparation of the base cyclohexane unit of Ceralure Bj.168 A procedure has been developed for the preparation of (75,25)-5-norbomene-2-carboxylic acid where the D-panta-lactone auxiliary can be recycled efficiently.185186... 

Figure 18.12 Bar graph of N-recycling efficiency as a function of bottom water dissolved oxygen concentration. Bars were based on Chesapeake Bay data reported in Kemp et al. (1990) and Cornwell (unpublished data). Solid dots were based on data from Rysgaard-Petersen et al. (1994) collected from Vilhelmsborg So, Denmark. Terms in the recycling efficiency calculation (y-axis) are Ffjj — flux of N2 from sediments = flux of ammonium from sediments FjsjOj flux of nitrate from sediments.

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