Fluid mean resistance times

Slurry Modes of Transportation, 237 Slurry Hold-up and Solid Concentration, 239 Solids and Fluid Mean Resistance Times, 243 Prediction of Hold-up Solids Concentration, 243... 

Solids mean resistance time, s Xf Fluid mean residence time, s... 

Liquid holdup is defined as the volume of liquid contained in the bed per unit bed volume. It is a function of the physical properties of the fluid phases and the bed characteristics. It is a basic parameter for reactor design, because it is related to other important parameters, namely, pressure gradient, gas-liquid interfacial area, the mean residence time of the liquid phase, catalyst loading per unit volume, axial dispersion coefficient, mass transfer characteristics, and heat transfer coefficient at the wall, etc. The optimal value of liquid holdup is desirable for better performance of TBR as a high value of liquid holdup will increase mass transfer resistance while too low a value of liquid holdup will decrease the proper utilization of the catalyst bed. Sometimes, the term total liquid saturation (j t) is used to describe the amount of liquid in the bed. It is defined as the volume of liquid present in a unit void volume of the reactor. Thus, the liquid holdup and total liquid saturation are related as ... 

As flow rate increases, the liquid film thickness increases up to the maximum value given by the height of the channel walls. An increase in film thickness will increase the diffusion time for solutes across the liquid film to the catalyst surface, thus increasing the mass transfer resistance and decreasing the effective reaction rate and diminish the conversion. Higher flow rates will decrease the mean residence time of the fluid, thus reducing the conversion as well. 

Because of their properties as chemically stable, viscous liquids with low electrical resistance, PCBs have been used widely in electrical equipment such as transformers and capacitors, as plasticizers, and as fluids in pumps. They have trade names such as Aroclor 1242 (which means it is a PCB which contains 42 per cent chlorine). PBBs have been used as fire retardants (see p. 258). The wide use of PCBs has led to human exposure as a result of disposal of old transformers, and so on. Their persistence means that they are detectable in the environment and in the fat and tissues of animals and humans. Their manufacture and use is now restricted, but because of their persistence they will remain in the environment for a long time. 

In turbulent flow, momentum is constantly fed into the layer adjacent to the wall because of the momentum transfer between layers at different velocities. The kinetic energy of the fluid elements close to the wall does not decrease as rapidly as in laminar flow. This means that turbulent boundary layers do not become detached as quickly as laminar boundary layers. Heat and mass transfer close to the wall is not only promoted by turbulence, the fluid also flows over a larger surface area without detachment. At the same time the pressure resistance is lower because the fluid flow does not separate from the surface for a longer flow path. 

Boundary layer diffusion resistance can be reduced by increasing the linear velocity of the flow passing over the surface. In practice this means increasing the turbulence of the fluid in die presence of the solid particle. To see the effect of turbulence (or fluid linear velocity) at a given space time and the other reaction conditions (temperature, pressure and composition), one has to change the relative velocity between the solid and the fluid. 

To determine the mean volumetric heat transfer coefficient, h, in a deep bed of gravel it was assumed that (1) any particle is at a uniform temperature at any given time, (2) resistance to heat transfer by conduction in the fluid or solid is negligible, and (3) the rate of heat transfer is described by Newton s equation [61] ... 

In obtaining the heat balance equation, we have assumed that the heat resistance by conduction inside the particle is insignificant compared to the heat resistance of the fluid film surrounding the particle, which is described by the second term in the RHS of eq.(9.3-12a). Because of such assumption, the temperature inside the particle is uniform. The first term in the RHS of eq.(9.3-12a) is the amount of heat released per unit volume per unit time as a result of the adsorption rate per unit volume d/dt. The parameter hf is the heat transfer coefficient per unit surface area, an is the heat transfer surface area per unit volume, Q is the molar heat of adsorption, T is the surrounding temperature, is the mean heat capacity per unit particle volume and is the volumetric average concentration of the adsorbed species, defined as... 

While it is relatively easy to select alternatives to DEHP that have good resistance to extraction in oils and fatty fluids, it is much more difficult to find materials which at the same time have as good resistance to extraction by water as DEHP. This can be seen in Table 1 which summarises some of the typical aqueous extraction tests that are required by most national standards or pharmacopoeia as a means of demonstrating freedom from chemical toxicity. 

ABSTRACT The characteristic of turbulent flow in jackets with triangular helical ducts was simulated and the velocity fields of fully developed turbulent fluid flow in the jackets were obtained. The features of the local coefficient of resistance C/Reiocai) on outer walls and inner wall were summed up and the effects of dimensionless curvature ratio and Reynolds number on the flow field and the flow resistance were analyzed. The results indicate that the structure of secondary flow is with two steady vortices at turbulent flow conditions. The distribution of/ eiocai on the outer walls differs from that of/ eiocai on the inner wall. The mean coefficient of resistance (/Rem) on the outer walls is about 1.41 1.5 7 times as much as that on the iimer wall. With the increase of dimensionless curvature ratio or Reynolds number,/Rem on the boundary walls increases. 

Turbulent flow in the jackets with triangular helical ducts is simulated applied CFD software. The fully developed flow field and the distribution of the turbulence kinetic energy are obtained. When turbulent fluid flows in the jackets, the structure of secondary flow in the cross section is steady two vortices. The turbulent kinetic energy near the outer walls is larger than that near the inner wall. The fRe oc on the inner wall is almost symmetric about / = 0 and the variation of/ iocai on the outer walls with y is vastly different from that on the inner wall. In the study range, the mean coefficient of resistance on the outer walls is about 1.41 1.57 times of that on the inner wall. The effects of Re and k on the flow field, the local coefficient of resistance at the boundary walls and the mean coefficient of resistance are analyzed. With the increase o Re, the intensity of secondary flow and the turbulent kinetic energy are all enhanced and the /K iocai on the boundary walls is increased as well. However, the locai near the center of the inner wall decreases with increasing k. The mean coefficient of resistance can increase as Re or k increases. 

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