Fluid dynamic conditions

Schreiner etal. (2001) modelled the precipitation process of CaC03 in the SFTR via direct solution of the coupled mass and population balances and CFD in order to predict flow regimes, induction times and powder quality. The fluid dynamic conditions in the mixer-segmenter were predicted using CFX 4.3 (Flarwell, UK). 

Although the absorption of a gas in a gas-liquid disperser is governed by basic mass-transfer phenomena, our knowledge of bubble dynamics and of the fluid dynamic conditions in the vessel are insufficient to permit the calculation of mass-transfer rates from first principles. One approach that is sometimes fruitful under conditions where our knowledge is insufficient to completely define the system is that of dimensional analysis. 

If the fluid dynamic conditions of a droplet are within the spreading regime, i.e., below the threshold curve in the We-Oh map, liquid splashing and ejection from the impact surface can be eliminated. The corresponding threshold velocity can be written as ... 

The specific platelet surface receptors that support these initial adhesive interactions are determined by the local fluid dynamic conditions of the vasculature and the extracellular matrix constituents exposed at the sites of vascular injury. Konstantopolous et al. (1998) and Alveriadou et al. (1993) demonstrated that under high shear conditions, the adhesion of platelets to exposed subendothelial surfaces of atherosclerotic or injured vessels presenting collagen and von Willebrand factor (vWF) is primarily mediated by the platelet glycoprotein (GP)Ib/IX/V complex. This primary adhesion to the matrix activates platelets, leading ultimately to platelet aggregation mediated... 

Roughly, an increase of residence time x by a factor of ten, results in a decrease of fuel content of 1 % 2% provided that all fluid dynamic conditions are kept constant. 

Physical degradation improper fluid dynamics conditions can cause a loss of the catalyst integrity with sphere breakage, fine powder formation and increase of pressure drop. 

The maximum temperature of the specimens was observed at the top of the nose due to the fluid dynamic conditions. The target temperature was set at 1,800 K to maintain the margin at the melting point of Si02, the oxidation resistant layer produced from SiC. 

Enzymatic systems in which membranes are simply used as separation media and not as catalyst carriers are traditionally called "enzyme membrane reactors" (EMR). Concentration polarization phenomena severely affect the performance of such reactors so that it is necessary to control the polarization layer onto membrane pressurized side by means of reactor fluid dynamics or design tricks. Fluid dynamic conditions in some of these reactors make them especially suitable for enzymatic systems for which a homogeneous catalyst distribution is particularly important, such as cofactor-requiring mono- and multi-enzyme systems. 

Enzyme gel layers can be built up under a number of fluid dynamic conditions. Unstirred and stirred batch reactors have been used as well as systems where the enzymatic solution is kept continuously flowing along semipermeable membranes until gel formation sets in. 

Similar equations under different fluid dynamic conditions can be derived from Michaels gel formation theory100 or from similar theories modeling concentration polarization phenomena.101 102 107 108... 

Except for adiabatic reactor operation, precise control of heat exchange is difficult. The local rate of heat exchange between the reactor and the surrounding and the local temperature are interrelated and depend on the fluid-dynamic conditions inside the reactor. 

In order to check the validity of the model described in Section 2, integration of Eqs. (6, 7) was carried out, considering the experimental conditions of the three dynamic runs in the boundary conditions (10). As already indicated, once the boundary conditions and the equilibrium isotherms are assigned, the only parameters of the model are the mass transfer coefficients kg and A simple best fit procedure led to a very satisfactory agreement between model and experimental results. It is important to observe that all three breakthrough curves were obtained using the same value for the external mass transfer coefficient (namely, e=5.M0 m/s) this is in agreement with the physical nature of kg, which only depends on fluid dynamic conditions. On the other hand, the internal mass transfer coefficient ki varied with the total normality of the solution used. In particular, for A =0.89 eqW it was, =1.4-10 eq/m for A7=22.6eq/m it was eq/m for A =44.4eq/m it was... 

Feed-side concentration polarization When feed components sorb in the membrane, a local concentration gradient develops in the feed phase adjacent to the membrane upstream face. Owing to this gradient, transport of components from the bulk into the bulk-membrane interface occurs, thus replenishing the components that were absorbed by the membrane. The transport of a solute across the phase adjacent to the membrane can be either convective or diffusive, depending on the solute concentration as well as the fluid dynamic conditions over the membrane surface. 

Industrial-size plate-and-frame modules, for example, consist of a stack of tightly packed membranes over which the feed solution is recirculated (Mulder, 1997). The membranes are separated by spacers and the permeate is withdrawn by a central permeate pipe (Stiirken, 1994). Pressure losses occur on both the feed and the permeate side of the packed membranes and need to be accounted for in the module design. On the feed side, the fluid dynamic conditions over the membrane may be less uniform than on the laboratory scale, resulting in more pronounced concentration polarization. On the permeate side, the packed configuration of the membranes may lead to considerable pressure losses, rendering the instantaneous removal of solutes from the membrane downstream surface more difficult. Both aspects may cause solute fluxes lower than expected (Chapter 3.2) and a possible... 

Initially the kinetics were investigated in a stirred autoclave in order to develop a microkinetic model. Also of interest were the fluid-dynamic conditions and the axial dispersion (residence-time behavior). Here we shall only deal with the measured residence-time distributions. 

Again in the case of flowing liquids, the supply of the cathodic reagents or of the inhibitors to the surface depends on the fluid dynamic conditions, and it may have contrasting effects. The general increase in the thickness of the boundary layer can reduce the supply of the cathodic reactant and that of the inhibitor, and, within the dead spaces, a cathodic process can be substituted for another, such as in the case of aerated and sulfatic waters, where anaerobic conditions that are established in the dead spaces permit the development of sulfate-reducing bacteria. 

FIGURE 12.41 (a and c) Examples of the perturbation of the fluid dynamic conditions determined by an inappropriate design that leads to corrosion phenomena (b and d) preferred situations. 

According to the CPC definition reported in eqn (14.8), CPC was evaluated as a function of several operating conditions, like temperature, feed and permeate pressure, feed composition and fluid-dynamic conditions by means of an elementary steps permeation model (eqn (14.11)) ... 

Rigorously speaking, the CO partial pressure present in eqn (14.16) is that immediately adjacent to the membrane surface. Therefore, in order to estimate correctly the parameters that appear in this expression, experimental systems that minimize the concentration polarization are necessary. This can be done by using a thick membrane or choosing the fluid-dynamic conditions in such a way for the external mass transfer resistance to be negligible. However, if the first method is quite simple to perform, the second one requires much more effort, because turbulence promoters or very high flow rate are necessary. 

Mixture 1 Mixture 2 Total Fluid dynamic conditions... 

Whether a stable two-phase flow can be obtained in a fluidic system depends on both fluid dynamic conditions and surface chemistries. The aim is to open the way to control the flow and to achieve versatility and reproducibility in on-chip flow management. The design will require smart network topologies and interplay of multiple physical and chemical effects. 

Fluid to particle mass transfer coefficient ki is determined from fluid dynamic conditions as well as diffusion property of the fluid. 

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