Outline of Recycling Methods

Another approach towards asymmetric heterogeneous catalysts is the immobilization of chiral homogeneous complexes via different methods. In this way the advantages of homogeneous catalysts (high activity and selectivity) and heterogeneous catalysts (easy recovery) can be combined. For a complete overview of this active research field the reader is referred to several reviews on this topic [50, 51]. The practical applicability of these catalysts is hampered by the fact that severe demands of recyclability and stability need to be obeyed. In certain cases promising results have been obtained as outlined here. 

As we noted at the beginning of this chapter, there are two broad approaches to the automated solution of the balance equations for a process system the sequential modular approach and the equation-based approach. This section outlines the first of these methods. The balance equations (and any other equations that may arise from physical considerations or process specifications) for each unit are written and solved. If there are no recycle streams, the calculation moves from one unit to another, until all units have been covered. If there is a cycle (the conventional term for a recycle loop in a process flowchart), a trial-and-error procedure is required values of one or more stream variables in the cycle are assumed the balance equations for units in the cycle are solved, one unit at a time, until the values of the assumed variables are recalculated new variable values are assumed and the procedure is repeated until the assumed and calculated values agree. 

A computer program may be written to solve this problem for residence times as a function of the recycle ratio, 7 . The above outline of the solution procedure may be programmed using MATHCAD, TK SOLVER, FORTRAN, or any other method. The printouts below show the saturation temperature, Ts, and reciprocal rates, 1/Rate, for recycle ratios of 7 = 0, 1, and 5, and for 7 = 1 with only the fresh air rate increased, using Eq. 8. When there is no recycle, use Eq. (la) instead of Eq. (1). The residence times for the four cases are ... 

Cold recycling is a pavement recycling method in which no heating is required at any stage of work. The advantages and disadvantages of cold recycling are outlined in Table 18.4. 

AECL has evaluated some of the basic information and development requirements in some detail (24, 25) and has outlined the type of fuel recycle development program which would be required. It would involve research and development of thorium fuels and fuel fabrication methods, reprocessing, demonstration of fuel management techniques and physics characteristics in existing CANDU reactors and demonstration of technology in health, safety, environmental, security and economics aspects of fuel recycle. 

Outline an overall process flow sheet and material balance including solvent recovery and recycle. This should be done with the aid of process simulation software. [See Seider, Seader, and Lewin, Product and Process Design Principles Synthesis, Analysis, and Evaluation, 2d ed. (Wiley, 2004) and Turton et ah. Analysis, Synthesis, and Design of Chemical Processes, 2d ed. (Prentice-Hall, 2002)]. In the flow sheet include methods needed for controlling emissions and managing wastes. Carefully consider the possibility that impurities may accumulate in the recycled solvent, and devise methods for purging these impurities, if needed. 

The difficulty for secondary plants is that, whilst the de-silverizing process (outlined in Section 15.3) can be used for the removal of low levels of silver in secondary lead bullion (i.e., up to 0.01 wt.%), the same amount of zinc must be added as for primary bullion that contains up to 0.5 wt.% silver. With this method, silver cannot be removed economically from recycled lead, as the cost of zinc alone... 

An unavoidable by-product of the Swem reaction is the volatile dimethylsulphide which, on account of its unpleasant smell, is a reagent regulated by offensive odour control laws. This makes large scale chemistry problematic, especially in industry. To overcome this, several methods exist to perform the Swem oxidation under odourless conditions. For example, Node et al. outline a protocol for the Swem oxidation which uses dodecyl methyl sulfoxide in place of methyl sulfoxide,12 while Crich and co-workers have developed a fluorous Swem oxidation reaction that uses tridecafluorooctylmethyl sulfoxide 17,l3a,b This reagent can be recovered via a continuous fluorous extraction procedure and recycled by reoxidation with hydrogen peroxide. Additionally, the fluorous DMSO is crystalline, odourless and soluble in CH2CI2 to —45 °C. 

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