Flow properties instabilities

After the process of granulation, the product exists as a wet mass from which the liquid must be removed, since the presence of water leads to the impairment of flow properties, and perhaps to chemical instability. Water is usually removed by evaporation for which energy is needed. This is normally provided as heat, though microwave energy is being increasingly used for drying in tablet manufacture. 

The flow of the continuous phase is considered to be initiated by a balance between the interfacial particle-fluid coupling - and wall friction forces, whereas the fluid phase turbulence damps the macroscale dynamics of the bubble swarms smoothing the velocity - and volume fraction fields. Temporal instabilities induced by the fluid inertia terms create non-homogeneities in the force balances. Unfortunately, proper modeling of turbulence is still one of the main open questions in gas-liquid bubbly flows, and this flow property may significantly affect both the stresses and the bubble dispersion [141]. 

The best designed die or feedblock does not necessarily ensure a commercially acceptable product. Layered melt streams flowing through a coextrusion die can become unstable leading to layer nonuniformities and even intermixing of layers under certain conditions. The causes of these instabilities are related to non-Newtonian flow properties of pol5uners and viscoelastic interactions. 

The polymer melt flows through a die to the next process step flat, circular, or slot dies are preferred. The flow through the die must be uniform across the exit plane. However, this is complicated by the nonlinear dependence of melt viscosity on both temperature and shear rate in the die (23,24). The suitability of a material is determined by measuring the flow properties with a capillary rheometer in the temperature and shear-rate range expected. Melt elasticity can cause flow instabilities, which affect haze and thickness (27,28) or the operation of downstream equipment. Exit melt velocity, flow characteristics, and quenching rate may impart significant orientation to the polymer. In some instances, melt orientation is reduced in others, it is maintained by quenching. 

Due to their strongly anisometric particle shape, triglyceride SLN tend to self-aggregate and build-up stacked lamellae at higher concentrations. The formation of such stacked lamellae was found to be reversible upon dilution and did not lead to an accelerated physical instability of the dispersions. The anisometric particle shape also determines the flow properties of SLN dispersions and high lipid contents (up to 40%) led to the formation of an elastic gel with viscoelastic properties comparable to standard dermal preparations. ... 

The onset of flow instability in a heated capillary with vaporizing meniscus is considered in Chap 11. The behavior of a vapor/liquid system undergoing small perturbations is analyzed by linear approximation, in the frame work of a onedimensional model of capillary flow with a distinct interface. The effect of the physical properties of both phases, the wall heat flux and the capillary sizes on the flow stability is studied. A scenario of a possible process at small and moderate Peclet number is considered. The boundaries of stability separating the domains of stable and unstable flow are outlined and the values of the geometrical and operating parameters corresponding to the transition are estimated. 

The properties of liquid metals can cause flow instability (oscillation) because of vapor pressure—temperature relationship. Most liquid metals, especially alkali metals, show a greater change in saturation temperature, corresponding to a given change of pressure, than does water. In a vertical system under gravitational force, the change of static pressure could appreciably alter the saturation temperature such that explosion -type flow oscillation would occur that would result in liquid... 

Process R D literature review patent review research reports bench scale pilot reports sketch of flow sheet chemicals and their characteristics chemical reactions and interactions thermodynamics physical properties preliminary process concept Laboratory screening and testing for chemicals (toxicity, instability, explosibility) for reactions (explosibility) for impurities Pilot plant tests... 

Most examples of flow in nature and many in industry are turbulent. Turbulence is an instability phenomenon caused, in most cases, by the shearing of the fluid. Turbulent flow is characterized by rapid, chaotic fluctuations of all properties including the velocity and pressure. This chaotic motion is often described as being made up of eddies but it is important to appreciate that eddies do not have a purely circular motion. 

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