Dynamic material recycle

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. 

Example 3.1 highlights the fact that the presence of (and need for) material recycle streams with significant flow rates is entirely a steady-state design feature of a process. In what follows, we will focus on investigating the profound impact of this feature on the dynamics and control of the processes under consideration. 

According to the developments in Section 2.3, the model of Equation (3.10) is in a nonstandard singularly perturbed form. We thus expect its dynamics (and, consequently, the dynamics of integrated process systems with large material recycle) to feature two distinct time scales. However, the analysis of the system dynamics is complicated by the presence of the term u1, which, as we will see below, precludes the direct application of the methods presented in Chapter 2 for deriving representations of the slow and fast components of the system dynamics. 

Note that the model in Equation (3.27) does not include the secondary column required to separate Pi and P2. This unit is not part of the material recycle loop and has no dynamic interaction with the reactor or the first column. Consequently, the control problem for this column can be formulated and addressed independently and will not be considered in the remainder of the present study. 

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. 

The purpose of this chapter is to create a general framework that captures the dynamic effect of the simultaneous presence of both significant material recycle streams and purge streams. We will use this result presented in (Baldea and Daoutidis 2007) to rationalize at the theoretical level the development and use of a hierarchical process control structure, consisting of several interconnected control and optimization layers. 

Clearly, the general model (5.10) contains terms of very different magnitudes, corresponding, respectively, to the process input and output flows and to the chemical reactions, to the presence of the large material recycle stream, and to the presence of the impurity and the purge stream used for its removal. While (as we have argued in the previous chapters of the book) the presence of these terms is purely a steady-state, design feature of the process, it is intuitive that their impact on the process dynamics will also be very different. 

Gilliland, E. R., Gould, L. A., and Boyle, T. J. (1964). Dynamic effects of material recycle. In Preprints of the Joint American Control Conference, pp. 140-146. 

Suppose that the reactant A is totally consumed. If the reaction rate is not infinite, the reactant B must be recycled at a convenient rate, in such a way that the resulting reaction rate leads to the total consumption of A. Therefore, we may speak about total conversion of the one-pass reactant, and partial conversion of the recycled reactant. Because the feed of fresh reactants must respect the stoichiometry, the feed policy of B must be adapted to fulfil the dynamic material balance. The above situation can be found often in industry, as for instance the synthesis of ethers from alchene-oxides and alcohols, the alkylation of benzene with ethylene or propylene to ethylbenzene or cumene, the addition of HCN to ketones, etc. 

Summing up, the above example demonstrates that the feed policy is a plantwide control problem. When several reactants are involved, only one can be set on flow control. The make-up of the other reactants must be implemented in a way that prevents a permanent accumulation (positive or negative) in the dynamic material balance. Thus, fixing the recycle flow of the second partner in a bimolecular reaction has proved to be an efficient method to stabilise the variability of recycle streams. However, this is not the panacea for feeding the reactants. The next section will bring supplementary insights. 

More elaborated make-up schemes are necessary for multi-reactant processes. If one reference reactant may be set on flow control, the make-up of others should be done in a way that satisfies the dynamic material balance. In this respect is important to note the Luyben s mle that a stream somewhere in a liquid recycle loop should be flow controlled . 

E. R. Gilliland, L. A. Gould, and T. J. Boule. Dynamic Effects of Material Recycle. In Preprints JACC, Stanford, 1964pp 140-146. 

Phenolics are consumed at roughly half the volume of PVC, and all other plastics are consumed in low volume quantities, mosdy in single apphcation niches, unlike workhorse resins such as PVC, phenoHc, urea—melamine, and polyurethane. More expensive engineering resins have a very limited role in the building materials sector except where specific value-added properties for a premium are justified. Except for the potential role of recycled engineering plastics in certain appHcations, the competitive nature of this market and the emphasis placed on end use economics indicates that commodity plastics will continue to dominate in consumption. The apphcation content of each resin type is noted in Table 2. Comparative prices can be seen in Table 5. The most dynamic growth among important sector resins has been seen with phenoHc, acryUc, polyurethane, LLDPE/LDPE, PVC, and polystyrene. 

Addition of rubber particles of 30% to 100% by weight to cement with a grain size of approximately 40 to 60 mesh (0.4 to 0.25 mm) will produce a lightweight cement. The addition of rubber particles also creates a low permeability. The compositions are advantageous for cementing zones subjected to extreme dynamic stresses such as perforation zones and the junctions of branches in a multi-sidetrack well. Recycled, expanded polystyrene lowers the density of a hydraulic cement formulation and is an environmentally friendly solution for downcycling waste materials. 

The structure and properties of water soluble dendrimers, such as 46, is, in itself, a very promising area of research due to their similarity with natural micellar systems. As can be seen from the two-dimensional representation of 46 the structure contains a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups (Fig. 12). However these dendritic micelles differ from traditional micelles in that they are static, covalently bound structures instead of dynamic associations of individual molecules. A number of studies have exploited this unique feature of dendritic micelles in the design of novel recyclable solubilization and extraction systems that may find great application in the recovery of organic materials from aqueous solutions [84,86-88]. These studies have also shown that dendritic micelles can solubilize hydrophobic molecules in aqueous solution to the same, if not greater, extent than traditional SDS micelles. The advantages of these dendritic micelles are that they do not suffer from a critical micelle concentration and therefore display solvation ability at nanomolar... 

The use of cross-functional teamwork can enhance the benefits that can be realized during each component of the LCA process. This underscores the fact that the life-cycle assessment is a dynamic and iterative process of evaluation. In order for this dynamism to be fostered, the LCA must encompass as much of the life cycle of the product, process, or activity, as possible. This will necessarily include total cost of raw materials, manufacturing, transportation, and distribution use/reuse/maintenance recycling, and final disposal (Fava and Page, 1992). 

Such conventional kinetic resolution reported above often provide an effective route to access to the enantiomerically pure/enriched compounds. However, the limitation of such process is that the resolution of two enantiomers will provide a maximum 50% yield of the enantiomerically pure materials. Such limitation can be overcome in several ways. Among these ways are the use of meso compounds or prochiral substrates,33 inversion of the stereochemistry (stereoinversion) of the unwanted enantiomer (the remaining unreacted substrate),34 racemization and recycling of the unwanted enantiomer and dynamic kinetic resolution (DKR).21... 

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