Recycling of byproducts

Full recycling of byproducts and waste in the manufacturing process itself. 

Understand the conditions under which recycle of byproducts to extinction can be employed to reduce waste and increase yield. 

S.2 Recycle of byproducts. As already discussed, one mechanism to reduce waste for disposal is to explore whether any byproducts formed in the reaction can be recovered and processed for recycle. Another mechanism is to alter the nature of the byproducts formed such that they become saleable in their own right or easily recovered for reuse. 

When more than one reactant is used, it is often desirable to use an excess of one of the reactants. It is sometimes desirable to feed an inert material to the reactor or to separate the product partway through the reaction before carrying out further reaction. Sometimes it is desirable to recycle unwanted byproducts to the reactor. Let us now examine these cases. 

Where possible, introducing extraneous materials into the process should be avoided, and a material already present in the process should be used. Figure 4.6h illustrates use of the product as the heat carrier. This simplifies the recycle structure of the flowsheet and removes the need for one of the separators (see Fig. 4.66). Use of the product as a heat carrier is obviously restricted to situations where the product does not undergo secondary reactions to unwanted byproducts. Note that the unconverted feed which is recycled also acts as a heat carrier itself. Thus, rather than relying on recycled product to limit the temperature rise (or fall), simply opt for a low conversion, a high recycle of feed, and a resulting small temperature change. 

The use of excess reactants, diluents, 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. If this can be achieved, it improves the overall use of raw materials and eliminates effluent disposal problems. Of course, the recycling does in itself reuse some of the other costs. The general tradeoffs are discussed in Chap. 8. 

Some small amount of byproduct formation occurs. The principal byproduct is di-isopropyl ether. The reactor product is cooled, and a phase separation of the resulting vapor-liquid mixture produces a vapor containing predominantly propylene and propane and a liquid containing predominantly the other components. Unreacted propylene is recycled to the reactor, and a purge prevents the buildup of propane. The first distillation in Fig. 10.3a (column Cl) removes... 

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. 

Waste HBr is a common byproduct of organic brominations. Frequently, this waste is neutralized with caustic, the resulting sodium bromide salt is discharged, and valuable bromine is lost. The economic advantages of recovery and recycle of this HBr have long been recognized (refs. 1, 3). In practice, recovery typically takes the form of conversion of the HBr to clear drilling fluids or alkylbromides (ref. 4) as shown in equations 1 and 2. 

Coke byproduct wastes. Coke, used in the production of iron, is made by heating coal in high-temperature ovens. Throughout the production process many byproducts are created. The refinement of these coke byproducts generates several listed and characteristic wastestreams. However, to promote recycling of these wastes, U.S. EPA provided an exclusion from the definition of solid waste for certain coke byproduct wastes that are recycled into new products. 

Having considered the feed, reaction, separation and recycling of material, the streams entering and leaving the process can be established. Figure 13.16 illustrates typical input and output streams. Feed streams enter the process and product, byproduct and purge streams leave after the separation and recycle system has been established. 

In 2000, Benaglia and coworkers reported preparation of MeO-PEG supported quaternary ammonium salt (10) and examined the catalytic efficiency in a series of phase-transfer reactions (Fig. 5.3) [69]. The reactions occurred at lower temperatures and with shorter reaction times than with comparable insoluble 2% cross-linked polystyrene-supported quaternary ammonium salts, although yields varied with respect to classical solution phase quaternary ammonium salt catalyzed reactions. It was observed that yields dropped with a shorter linker, and that PEG alone was not responsible for the extent of phase-transfer catalysis. While the catalyst was recovered in good yield by precipitation, it contained an undetermined amount of sodium hydroxide, although the presence of this byproduct was found to have no effect on the recyclability of the catalyst... 

The choice of an ionic liquid was shown to be critical in experiments with [NBuJBr (TBAB, m.p. 110°C) as a catalyst carrier to isolate a cyclometallated complex homogeneous catalyst, tra .s-di(ri-acetato)-bis[o-(di-o-tolylphosphino) benzyl] dipalladium (II) (Scheme 26), which was used for the Heck reaction of styrene with aryl bromides and electron-deficient aryl chlorides. The [NBu4]Br displayed excellent stability for the reaction. The recycling of 1 mol% of palladium in [NBu4]Br after the reaction of bromobenzene with styrene was achieved by distillation of the reactants and products from the solvent and catalyst in vacuo. Sodium bromide, a stoichiometric salt byproduct, was left in the solvent-catalyst system. High catalytic activity was maintained even after the formation of visible palladium black after a fourth run and after the catalyst phase had turned more viscous after the sixth run. The decomposition of the catalyst and the formation of palladium... 

Assumptions Synthesis of phenylpyruvic acid Batch synthesis process for precursors overal yield of 95+% of theoretical to pheny Ihydantoin overall yield of 90+% of theoretical from phenylhydantoin to phenylpyruvic acid recovery and recycle of acetic acid no byproduct crec taken for acetic acid formed from acetic anhydride addition. Conversion of phenylpyruvic acid and aspartic acid. Bioreactor productivity of-18 g PHE/L/h (four columns in parallel) 98% overall conversion no byproduct credit taken for pymvic acid (recovery cost assumed to be of by revenue from sale) 80% recovery of L-PHE downstream of bioreactor. 

Polymer-supported reactants are most useful for cases where reactant byproducts would otherwise be difficult to sequester. Recycling of the... 

Note There is a tendency to byproduct diphenyl ether formation in reactor R-101. However, a recycle of 100 pph of DPE in the feed to the reactor prevents any further formation of this substance. 

Oxidative reactions of pyridines are commercially more interesting than reductive ones because catalytic hydrogenation of pyridines is a generally useful method, whereas catalytic oxidation is not. In contrast, anodic oxidation of pyridines is widely applicable and can replace methods that use expensive oxidants such as permanganate salts or chromic oxide. Consider, as an example, oxidation of a methylpyridine to produce 1 kg of the pyr-idinecarboxylic acid this process would consume about 3 worth of potassium permanganate at 100% efficiency and would produce 0.7 kg of byproduct Mn02 for disposal or recycle. The same anodic reaction would consume only 0.30 of electrical power (for oxidation) and would not produce a significant amount of material for disposal. 

In a new process proposed by Kellogg the oxidation of HCI makes use of nitrosyl-sulfuric acid (HNSOs) at 4 bar and 260-320 °C [9], The amount of byproducts diminishes drastically raising the overall yield over 98%. The gaseous emissions are reduced practically to zero. Chlorinated waste from other processes may be incinerated to HCI, and in this way recycled to the manufacturing of VCM. The process is safer because the contact of hydrocarbon mixtures with oxygen is eliminated. A capital cost reduction of 15% may be estimated compared with the oxy-chlorination process. 

An economic analysis can show that the most effective factor in reducing the manufacturing costs could be replacing the propene by propane, which is 30% cheaper. An industrial process is attributed to Asahi [18]. The key competitive element is the availability of a suitable catalyst. Despite intensive research the performance of today s catalysts remains rather modest. The conversion should be kept low, below 50%, while the selectivity cannot be pushed beyond 60%. Because the recycle of unconverted propane and larger spectrum of byproducts the advantage of lower price seems to be not sufficient for a technological breakthrough. 

A nucleophilic attack by 4.7 on CH3I produces 4.8 and I. Conversion of 4.8 to 4.9 is an example of a carbonyl insertion into a metal alkyl bond. Another CO group adds onto the 16-electron species 4.9 to give 4.10, which in turn reacts with I to eliminate acetyl iodide. Formation of acetic acid and recycling of water occur by reactions already discussed for the rhodium cycle. Apart from these basic reactions there are a few other reactions that lead to product and by-product formations. As shown in Fig. 4.4, both 4.9 and 4.10 react with water to give acetic acid. The hydrido cobalt carbonyl 4.11 produced in these reactions catalyzes Fischer-Tropsch-type reactions and the formation of byproducts. Reactions 4.6 and 4.7 ensure that there is equilibrium between 4.7 and 4.11. 

The synthesis as well as the recycling and disposal of halogenated compounds such as Polyvinylchloride (PVC), flame retardants (for example Tetrabromobisphenol A), y-Hexachlorocydohexane (y-HCH), Hexachlorobenzene (HCB) leads to the formation of byproducts and residues. Polymers, for example, as well as printed circuit boards or shredder residues include problematic substances which, for ecological reasons, cannot be passed on to the environment but have to be supplied to a specific and proper recycling or disposal process. Prior to this background so called "Supercritical Fluids" are of special interest. Supercritical fluids can be used for the synthesis of polymers in an enviromentally friendly way as well as for recycling and disposal processes. 

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