Parallel processing terminology

As a result of these heavy-atom KIE experiments the principal features of the benzidine rearrangements have now been firmly established. The two main products arise from two parallel reactions one of which is concerted and the other is not. Other concerted processes have been identified and all of the concerted processes can be readily classified in the terminology of sigmatropic rearrangements within the general class of percyclic reactions. 

The recommendations embodied in this document are concerned with the terminology relating to the structure of crystalline polymers and the process of macromolecular crystallization. The document is limited to systems exhibiting crystallinity in the classical sense of three-dimensionally periodic regularity. The recommendations deal primarily with crystal structures that are comprised of essentially rectilinear, parallel-packed polymer chains, and secondarily, with those composed of so-called globular macromolecules. Since the latter are biological in nature, they are not covered in detail here. In general, macromolecular systems with mesophases are also omitted, but crystalline polymers with conformational disorder are included. 

Planning in both processes is similar. In this manner, one can think of the steps in the CQI cycle as parallel to the sections of a scientific article background, methods, results, conclusions, and recommendations. Considering CQI in this manner diminishes the need to memorize additional terminology. See Fig. 7-1 for a flowchart representation of the process. 

In these circumstances, where routine kinetic measurements are uninformative and direct measurements of the product-forming steps difficult, comparative methods, involving competition between a calibrated and a non-calibrated reaction, come into their own. Experimentally, ratios of products from reaction cascades involving a key competition between a first-order and a second-order processes are measured as a function of trapping agent concentration. Relative rates are converted to absolute rates from the rate of the known reaction. The principle is much the same as the Jencks clock for carbenium ion lifetimes (see Section 3.2.1). However, in radical chemistry Newcomb prefers to restrict the term clock to a calibrated unimolecular reaction of a radical, but such restriction obscures the parallel with the Jencks clock, where the calibrated reaction is a bimolecular diffusional combination with and the unknown reaction a pseudounimolecular reaction of carbenium ion with solvent. Whatever the terminology, the practical usefulness of the method stems from the possibility of applying the same absolute rate data to all reactions of the same chemical type, as discussed in Section 7.1. 

Time-resolved measurements were initiated both by physicists, who were principally interested in photophysical processes that left the chemical structures of the molecules intact, and by chemists, who were mainly interested in the chemical alterations of the irradiated molecules, but also in the associated photophysical steps. The parallel development of these two lines of research is reflected in the terminology. For example, the term flash photolysis, as used by chemists, applies to time-resolved measurements of physical property changes caused by chemical processes induced by the absorption of a light flash (pulse). Flash photolysis serves to identify short-lived intermediates generated by bond breakage, such as free radicals and radical ions. Moreover, it allows the determination of rate constants of reactions of intermediates. Therefore, this method is appropriate for elucidating reaction mechanisms. 

An important factor that influences the performance of membrane filtration is its mode of operation. According to Gekas et al. [48], the filtration mode for most unit operations with manbranes, among which processes of NF and UF are in cross-flow mode, in which the feed stream is parallel to the manbrane surface, while the permeate flux is transverse to it. Thus, the flows of feed and permeate are intertwined, justifying the terminology cross-flow. Moreover, in the conventional mode of filtration, the feed stream is perpendicular to the m brane or filter media and is known as dead end or flow through. 

The cycloaddition of alkenes and dienes is a very useful method for forming substituted cyclohexenes. This reaction is known as the Diels-Alder reaction The concerted nature of the mechanism was generally accepted and the stereospecificity of the reaction was firmly established before the importance of orbtial symmetry was recognized. In the terminology of orbital symmetry classification, the Diels-Alder reaction is a [AUg + lUg] cycloaddition, an allowed process. The transition state for a concerted reaction requires that the diene adopt the s-cis conformation. The diene and substituted alkene (which is called the dienophile) approach each other in approximately parallel planes. The symmetry properties of the n orbitals permit stabilizing interations between C-1 and C-4 of the diene and the dienophile. Usually, the strongest interaction is between the highest occupied molecular orbital (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the dienophile. The interaction between the frontier orbitals is depicted in Fig. 6.1. 

Although some of the terminology is related to chemical plant, the essential steps in the process are also apphcahle to other processes. In general, however, chemicals and petrochemicals have received most of the attention to date in this area. There are parallels with process integration (discnssed earher in this chapter), where it took some time before the techniqne developed for chemical plant was nsed ontside the sector. 

The analysis of nearly parallel flows originated in the study of problems of lubrication, and the approach is often called the lubrication approximation. The terminology is unfortunate from our perspective, given that this approach is at the heart of all analytical treatments of polymer processing operations - we would prefer that it be called the polymer processing approximation - but the historical name is well established. The major figure in the analysis of lubrication flows was Osborne Reynolds, and one widely used form of the resulting equations is often called the Reynolds lubrication equation. 

---