Mechanical recycling flow scheme

After ammonolysis, ammonia is evaporated and can be reused after liquefaction, while degradation products of polyurethane hard segments (e.g., amines and chain extenders) and urea are removed by extraction. The pure polyol is left in the reactor. It can be removed mechanically or by extraction with liquid ammonia in which it is soluble. The recovered amines can be converted to the corresponding isocyanates and can be reused, along with polyols, in the same applications as before. A flow scheme of the recycling process is shown in Figure 6.9. 

As for the major PVC flows, several recycling schemes mainly based on mechanical recycling have been and will be set up, particularly in the northern EU countries (notably Scandinavia, Germany, Austria and the Netherlands). These schemes rely on covenants between the authorities and the PVC industry, or voluntary incentives of the PVC industry. The potential of PVC waste (pure resin) for chemical recycling in ktons in the period 2000-2010 is given in Table 7.8 [258]. 

Finally, some remarks on the operation of mechanically agitated gas-liquid reactors are worth mentioning. The mode of operation (i.e., batch, semibatch, continuous, periodic, etc.) depends on the specific need of the system. For example, the level of liquid-phase backmixing can be controlled to any desired level by operating the gas-liquid reactor in a periodic or semibatch manner. This provides an alternative to the tanks in series or plug flow with recycle system and provides a potential method of increasing the yield of the desired intermediate in complex reaction schemes. In some cases of industrial importance, the mode of operation needs to be such that the concentration of the solute gas (such as Cl2, H2S, etc.) as the reactor outlet is kept at a specific value. As shown by Joshi et al. (1982), this can be achieved by a number of different operational and control strategies. 

At still higher temperatures, 3,4-dimethylenecyclobutene gave both fulvene and benzene in a 2 1 ratio, respectively, in a flow system. Further, when the 1,2-dideuterio material was pyrolyzed, the fulvene appears to have deuterium on the 2 and 3 positions while the benzene is 93% ortho labeled with only 5% meta labeled and 2% para labeled. The proposed mechanism for these conversions invoked reversion to the bisallene follwed by recyclization in two different modes to produce biradicals whose resonance structures are carbenes which undergo insertion(s) into adjacent CH bonds (Scheme 7.17). 

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