Pressure drop and dispersion

Pressure drop and dispersion were the focus of work by Magnico (2003) who simulated flow at lower Re by direct numerical simulation (DNS) in beds of spheres with an in-house code. Tobis (2000) simulated a small cluster of four spheres with inserts between them to compare to his experimental measurements of pressure drop. Gunjal et al. (2005) also focused on flow and pressure drop through a small cell of spheres, in order to validate the CFD approach by comparison to the MRI measurements in the same geometry made by Suekane... 

When the model parameters of the plant are known, the packing parameters i.e. void fraction, pressure drop and dispersion are determined (step 3 in Fig. 6.9). 

Thus, the barrier flight width and the barrier clearance 6 have to be designed for both pressure drop and dispersive mixing capacity. 

A pipe that releases gas to disperse into the atmosphere is called a vent. If the gas is burned at the tip, it is called a flare. In its simplest form, a vent or flare tip is a pipe. Sometimes the pipe diameter is reduced for the last 5 ft or so to increase exit velocity for better mixing with the air. The operating pressure of the vent scrubber can be adjusted by reducing the tip diameter to increase pressure drop across the tip. Fluidic seal., also give increased pressure drop and can be used to reduce the infusion... 

This section has based scaleups on pressure drops and temperature driving forces. Any consideration of mixing, and particularly the closeness of approach to piston flow, has been ignored. Scaleup factors for the extent of mixing in a tubular reactor are discussed in Chapters 8 and 9. If the flow is turbulent and if the Reynolds number increases upon scaleup (as is normal), and if the length-to-diameter ratio does not decrease upon scaleup, then the reactor will approach piston flow more closely upon scaleup. Substantiation for this statement can be found by applying the axial dispersion model discussed in Section 9.3. All the scaleups discussed in Examples 5.10-5.13 should be reasonable from a mixing viewpoint since the scaled-up reactors will approach piston flow more closely. 

A number of studies have used CFD results to obtain information on dispersion or mass transfer in packed tubes, in addition to the usual hydrodynamic results. Some relatively recent work has also included reaction. This area is so far not as well developed as the pressure drop and flow fields results of the previous section however, some promising first steps have been taken. 

In bubble columns the static head of the fluid is the dominant component of the pressure drop and consequendy it is important to determine the void fraction of the dispersion. All quanuties will be measured as posidve in the upward direction, this being the direction of flow of the dispersed phase. Assuming that the gas bubbles are of uniform size and are uniformly distributed over any cross section of the column, the gas and liquid velocities relative to the column are... 

The pressure drop and pumping requirements are functions of the type of flow and of the rheological properties of the dispersion. If the flow rate in a pipeline falls below the critical deposit velocity then particles or emulsion droplets will either sediment or cream to form a layer on the bottom or top wall, respectively, of the pipe. Some correlations that have been developed for the prediction of critical deposit velocity are discussed by Nasr-El-Din [86] and Shook et al. [90]. 

All the results presented so far give reason to conclude that the avalanche-like destruction of a foam column at definite temperature, pressure drop and foam dispersity, depends mainly on the equilibrium pressure reached. However, in order to establish the mechanism of action of the critical pressure drop, further studies of single foam films and foams are required. They should be performed under conditions that reveal the role of all elements of the foam (films, borders and vertexes) in the process of foam destruction. 

Even when the laboratory test reactor is intended to be representative in a reaction kinetic sense only (thus waiving the demand for correspondence in terms of pressure drop and hold-ups), the process performance data can be affected by differences in mass transfer and dispersion caused by scale reduction. When interphase mass transfer and chemical kinetics are both important for the overall conversions, the above test reactor, which is a relatively large pilot plant reactor, cannot be further reduced in size unless one accepts deviations in test results. 

In their second method, as in the case of a vertical riser, the total pressure drop along the spout height is composed of (i) a solids static head equivalent to the dispersed-solids bulk density, (ii) an acceleration pressure drop, and (iii) a solids friction loss due to relative motion of the particles with respect to the gas and to the spout wall. Thus,... 

A comparison of the measured pressure drops and those calculated using the intermittent and annular/disperse-wave/mist flow models is shown in Figure 11 for each tube considered. In the overlap zone (Figure 7), the flow exhibits both the adjoining mechanisms (intermittent and annular/disperse-wave/mist flow). Therefore, for calculating the pressure drops in the overlap zones in Figure 11, the four-point interpolation scheme described above in connection with the transition between laminar and turbulent data was applied to the pressure drops calculated using the intermittent and annular/disperse-wave/mist flow models. This combined model for the... 

Rigorous. This involves solutions to numerous governing equations they may include mass transfer resistances, heat effects, complex equilibria, pressure drop, axial dispersion, and others. The last can be time consuming it lacks intuitive connections between performance and variables. Many applications require rigorous methods. An example is multicomponent adsorption of the type shown in Eigure 14.13. 

In the case of liquid/liquid and gas/liquid dispersion, the above mixing mechanism does not hold and the operation is usually carried out in the turbulent region. The break-up of gas bubbles or liquid droplets to give a system of high interfacial area for mass transfer is brought about by the shear stresses in the system. These stresses are related to the pressure drop and hence flow rate through the mixer. Thus to get a smaller droplet size the fluid flow rate must be increased. It will not be effective to merely increase the number of elements (as was the case in laminar blending). 

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