Recycle of material

The recycling of material is an essential feature of most chemical processes. Therefore, it is necessary to consider the main factors which dictate the recycle structure of a process. We shall start by considering the function of process recycles and restrict consideration to continuous processes. Later the scope will be extended to include batch processes. 

Having considered the separation and recycling of material, the streams entering and leaving the process can now be established. Figure 4.17 illustrates typical input and output streams. Feed... 

Reactors at nonoptimal conditions produce (additional) unwanted byproducts. Not only might this lead to loss of material through additional byproduct formation, but it also might prevent the recycling of material produced during the start-up. 

Several variations of the above process are practiced. In the Sumitomo-Nippon Shokubai process, the effluent from the first-stage reactor containing methacrolein and methacrylic acid is fed directiy to the second-stage oxidation without isolation or purification (125,126). In this process, overall yields are maximized by optimizing selectivity to methacrolein plus methacrylic acid in the first stage. Conversion of isobutjiene or tert-huty alcohol must be high because no recycling of material is possible. In another variation, Asahi Chemical has reported the oxidative esterification of methacrolein directiy to MMA in 80% yield without isolation of the intermediate MAA (127,128). 

Measuring process parameters on full-scale plants is notoriously difficult, but is needea for control. Usually few of the important variables are accessible to measurement. Recycle of material makes it difficult to isolate the effects of changes to individual process units in the circuit. Newer plants have more instrumentation, including on-line viscosimeters [Kawatra and Eisele, International ]. Mineral Processing, 22, 251-259 (1988)], mineral composition by on-line X-ray fluorescence, belt feeder weighers, etc., but the information is always incomplete. Therefore it is helpful to have models to predict quantities that cannot be measured while measuring those that can. 

Comparable fuels. In order to promote the recycling of materials with high fuel values, certain materials that are burned as fuels are excluded from the definition of solid waste, provided that they meet certain specifications (i.e., are of a certain degree of purity). This is to ensure that the material does not exceed certain levels of toxic constituents and physical properties that might impede burning. Materials that meet this specification are considered comparable to pure or virgin fuels. 

Finally, recycling of materials is difficult in batch processes because the connection in time cannot usually be made between the steps involved in the recycling. This is because different steps take place in different time periods. However, time can be bridged through the use of intermediate storage for the recycle. 

The synthesis of reaction-separation systems. The recycling of material is an essential feature of most chemical processes. The use of excess reactants, diluents, solvents or heat carriers in the reactor design has a significant effect on the flowsheet recycle structure. Sometimes, the recycling of unwanted byproduct to the reactor can inhibit its formation at the source. 

Net sedimentation is defined as the flux of material incorporated into the permanent sediment record. 210Pb and 137Cs geochronologies indicate a mass sedimentation rate of 103 g/m2 per year for profundal sediments in Little Rock Lake. By using the mean Hg concentration (118 ng/g) in the top 1-cm slice of our bulk sediment profile, we estimated an annual net sedimentation of 12 xg of HgT/m2 per year. This net accumulation rate is similar to the calculated atmospheric input rate of about 10 xg/m2 per year (18, 19). Additionally, gross deposition rates (from sediment traps) exceeded these estimates by about a factor of 3 this rate suggests substantial internal recycling of material deposited at the sediment-water interface in this lake. 

Recycle. A large number of manufacturing facilities, especially chemical plants, have internal recycle streams that are considered part of the process. In this case, recycle refers to the external recycle of materials, such as polyester film and bottles, Tyvek envelopes, paper, and spent solvents. 

If process units are arranged in a purely series configuration, where the products of each unit feed downstream units and there is no recycle of material or energy, the plantwide control problem is greatly simplified. We do not have to worry about the issues discussed in the previous section and we can simply configure the control scheme on each individual unit operation to handle load disturbances. 

An ideal mixture of n components requires a sequence of n - 1 conventional distillation columns (two product streams) to separate the components completely. The columns can be arranged sequentially without recycle between them. This picture changes when mixtures forming azeotropes must be separated. Nonideal systems sometimes require complex distillation arrangements involving more than n - 1 columns with recycle of material between the columns. For the analysis of such systems, we recommend the use of residue curve maps. We base the following summary on the excellent book by Doherty and Malone (1998), who pioneered the use of these techniques. 

Improve the process diagram by considering recycles of materials and energy, by considering cheaper ingredients, or by reducing the number of steps. 

Fillers play various roles in the recycling of materials. These include the use of waste materials as a substitute for fillers (filler replacement), the effect that fillers have on recycling methods and waste production (filler impact on quantity of wastes), recovery of fillers from scrap (filler recovery), and the application of fillers to make materials recyclable (material improvement). These subjects are discussed below. 

Recycle. Self-regulation must be provided for components and energy in recycle loops. Recycling of material can recycle disturbances if the control is, for example, based on level control. Disturbances in the heat balance can be recycled if extensive heat integration is used. Add trim heaters and coolers to damp out thermal disturbances. 

The above should serve two purposes establish that the reuse of containers is not as simple as first envisaged, and indicate that the recycling of materials may become a... 

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