Process systems with significant material recycling

3 Process systems with significant material recycling 

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This chapter addressed the dynamics and control of process systems with material recycling. We established that whenever the flow rate of the recycle stream is significantly larger than the flow rates of the feed/product streams, the overall process exhibits a time-scale separation in its dynamics. 

N-McLhylmorpholine-N-oxidc monohydrate, a tertiary, aliphatic amine N-oxide, is able to dissolve cellulose directly, i.e. without chemical derivatization, which is used on an industrial scale as the basis of the Lyocell process [ 1, 2], This technology only requires a comparatively low number of process steps compared for instance to traditional viscose production. Cellulose material - mainly fibers - are directly obtained from the cellulose solution in NMMO no chemical derivatization, such as alkalization and xanthation for rayon fibers, is required [3]. The main advantage of the Lyocell process lies in its environmental compatibility very few process chemicals are applied, and in the idealized case NMMO and water are completely recycled, which is also an important economic factor. Even in industrial production systems NMMO recovery is greater than 99%. Thus, compared with cotton and viscose the Lyocell process pertains a significantly lower specific environmental challenge [4]. Today, Lyocell fibers are produced on an industrial scale, and other cellulosic products, such as films, beads, membranes and filaments, are also currently being developed or are already produced commercially. 

In this case, the model of the process with recycle is a regular perturbation of the nominal (no recycle) series system. In light of the concepts introduced in Chapter 2, we can expect that the presence of the (small) material recycle stream will not have a significant impact on the dynamics of the process. 

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