Understanding of flow behavior

The entire subject of the viscoelasticity of branched polymers is an active area of research at present. The linear viscoelastic properties of nonsymmetric stars, H-shaped polymers, polymeric combs, and randomly branched species are being investigated both theoretically and experimentally [78-82], and new ideas about their nonlinear responses both during shear and during extension are being considered [83]. With these and other initiatives, the molecular understanding of flow behavior in entangled-polymer liquids will surely expand rapidly in the next few years. 

Dan Luss I believe that there is still a need to study pathological behavior, especially in systems where it is caused by the interaction of the fluid flow and chemical reaction. For example, a major problem in the design and operation of a trickle-bed reactor is the presence of local hot spots, which have caused several major explosions. I ve just been involved in litigation that resulted from a reactor explosion it caused 15 million worth of damage at a plant in Corpus Christi. Most of the existing models in the literature are oversimplified and cannot predict this important feature. A model that will predict this behavior would be an important contribution. Another example is the self-ignition of coal piles, which is the major source of emission of sulfur dioxide into the atmosphere in South Africa. It s definitely desirable to get a better quantitative understanding of this behavior and how to prevent it. 

Viscoelastic properties of starch dispersions are of interest to better understand their structure (Biliaderis, 1992). In addition, studies on synthetic-particle dispersions (Boersma et al., 1992 Chow and Zukoski, 1995b) showed that they may play a role in understanding their flow behavior. As a reminder, one can anticipate that the viscoelastic behavior of starch dispersions also is influenced very strongly by the volume fraction and state of the granules in the dispersion. 

Several sophisticated techniques and data analysis methodologies have been developed to measure the RTD of industrial reactors (see, for example, Shinnar, 1987). Various different types of models have been developed to interpret RTD data and to use it further to predict the influence of non-ideal behavior on reactor performance (Wen and Fan, 1975). Most of these models use ideal reactors as the building blocks (except the axial dispersion model). Combinations of these ideal reactors with or without by-pass and recycle are used to simulate observed RTD data. To select an appropriate model for a reactor, the actual flow pattern and its dependence on reactor hardware and operating protocol must be known. In the absence of detailed quantitative models to predict the flow patterns, selection of a model is often carried out based on a qualitative understanding of flow patterns and an analysis of observed RTD data. It must be remembered that more than one model may fit the observed RTD data. A general philosophy is to select the simplest model which adequately represents the physical phenomena occurring in the actual reactor. 

There is an added interest in developing the understanding of the behavior of molten coal ash systems beyond somewhat empirical correlations of chemical compositions expressed in terms of a variety of ratios. It would be desirable to describe the flow behavior in terms of the Interactions between the individual constituents, and to understand the nature of the acids and bases, such that the reason for the partial success of empirical correlations using these concepts can be understood. 

This book is focused almost entirely on the dynamics of, and transport processes in, Newtonian fluids. Nevertheless, the class of complex or non-Newtonian fluids is extremely common, especially in chemical engineering applications, and it is probably useftd to spend some time discussing what is responsible for the departures from Newtonian behavior. We shall see that the applicability of the Newtonian fluid model is based on a combination of the intrinsic characteristics of the fluid and the characteristics of the flow. Specifically, a fluid may exhibit behavior that can be described accurately by the Newtonian fluid model under one set of flow conditions but appear as non-Newtonian under a different set of conditions. One motivation for discussing the origins of non-Newtonian behavior is that this will help us to understand when we might expect the Newtonian fluid model to apply. A second is that it will give us a very crude and qualitative understanding of the behavior that is characteristic of non-Newtonian materials. 

As downstream pressure Pj is decreased and upstream pressure P is kept fixed, the flow increases until the internal pressure of the tube drops somewhat below the external pressure P. Then the tube partially collapses, decreasing in cross-sectional area according to its pressure-area or tube law relationship A(P), shown in Fig. 4.11i>. As P is further decreased, the tube reduces its cross-sectional area while the average velocity of the flow increases. However, their product, the volumetric flow rate F, does not increase as shown in Fig. 4.11c. A simplified understanding of this behavior may be seen from the conservation of momentum equation for the flow, a Bernoulli equation, where the average fluid velocity U is defined as the ratio of flow to cross-sectional area, U = FIA, all of the terms pressure dependent. 

These implications are important for understanding the flow behavior of immiscible blends. 

Rheology is a part of continuum mechanics that assumes continuity, homogeneity and isotropy. In multiphase systems, there is a discontinuity of material properties across the interface, a concentration gradient, and inter-dependence between the flow field and morphology. The flow behavior of blends is complex, caused by viscoelasticity of the phases, the viscosity ratio, A (that varies over a wide range), as well as diverse and variable morphology. To understand the flow behavior of polymer blends, it is beneficial to refer to simpler models — for miscible blends to solutions and mixtures of fractions, while for immiscible systems to emulsions, block copolymers, and suspensions [1,24]. 

Melt rheology forms an important part of the discussion regarding process-ability of any thermoplastic resin. Here practical information useful for understanding the flow behavior of SPS as a molten polymer is summarized. 

The selective oxidation of CO in the presence of H2 is being carried out in a catalytic, ideal plug-flow reactor at atmospheric pressure and 100 °C. Although the reactions are exothermic, we will assume that the reactor is isothermal in order to develop a preliminary understanding of reaction behavior. We also will neglect pressure drop, assume that transport effects are negligible, and assume that the ideal gas laws are valid. 

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