Pulsating flow, dispersion

For some reactions listed in Table 1-4A, the fixed-bed reactor is operated under cocurrent-upflow conditions. Unlike the trickle-flow condition, this type of operation is normally characterized by bubble-flow (at low liquid and gas rates) and pulsating-flow (at high gas flow rates) conditions. Normally, the bubble-flow conditions are used. In the SYNTHOIL coal-liquefaction process, both pulsating-and spray-flow conditions are used, so that the solid reactant (coal) does not plug the reactor. In bubble flow, the gas is the dispersed phase and the liquid Ls a continuous phase. In pulsating flow, pulses of gas and liquid pass through the reactor. In the spray-flow regime, the gas is a continuous phase and the liquid is a dispersed phase. 

The effects of liquid fuel pulsation without air forcing were visualized at four instances of time (images not included). At time 0, a high concentration of fuel became visible at the nozzle exit. At time 7t/2, the fuel droplets became evenly dispersed through the quarter cycle. Times tt and 37t/2 showed similar droplet distributions, homogeneous throughout the flow. 

Taylor (T4, T6), in two other articles, used the dispersed plug-flow model for turbulent flow, and Aris s treatment also included this case. Taylor and Aris both conclude that an effective axial-dispersion coefficient Dzf can again be used and that this coefficient is now a function of the well known Fanning friction factor. Tichacek et al. (T8) also considered turbulent flow, and found that Dl was quite sensitive to variations in the velocity profile. Aris further used the method for dispersion in a two-phase system with transfer between phases (All), for dispersion in flow through a tube with stagnant pockets (AlO), and for flow with a pulsating velocity (A12). Hawthorn (H7) considered the temperature effect of viscosity on dispersion coefficients he found that they can be altered by a factor of two in laminar flow, but that there is little effect for fully developed turbulent flow. Elder (E4) has considered open-channel flow and diffusion of discrete particles. Bischoff and Levenspiel (B14) extended Aris s theory to include a linear rate process, and used the results to construct comprehensive correlations of dispersion coefficients. 

Solvent Delivery System. Solvent delivery systems provide the flow rate of the carrier liquid through the whole separation system. Highly stable liquid flow rates free of pulsations, a broad range of adjustable flow rates, repeatability, and reproducibility of the adjusted flow rate are their most important parameters. A broad range of adjustable flow rates is important. The pumps must be corrosion resistant and inert to the solvents used. The channels for FFF have very low hydrodynamic resistance and, consequently, the solvent delivery systems should not rely on high pressure operation. Furthermore, it is very important that the flow rate is free of pulsation. The stability of the flow rate and, consequently, of the velocity profile inside the separation channel is the most important requirement for the validity of all the theoretical relationships for retention and dispersion and thus for the choice of the solvent delivery system. 

The dimensioning (diameter) of extraction columns is performed as follows After determining the optimum plate number and the corresponding optimal solvent quantity for a given process, the ratio of the volumetric flow rates of the continuous and disperse phases Vcmal ikp fixed, and one defines an area-specific volumetric throughput (Equation 2.3.4-11) and, in a pilot column, determines the flood point as a function of the pulsation a f, where a is the amphtude (typically 8 mm) and/is the frequency (typically ca. 60 Hz) ... 

Pulsations of less scale possess significantly less energy and are not able to deform particles of disperse phase. Pulsations of big scale carry the elements of disperse phase and do not deform their surface. The fundamental problem under estimation of disperse inclusions of multiphase systems in tubular turbulent apparatus according to (1.23) is calculation of rate of turbulence kinetic energy dissipation e. It requires the development of model describing disperse processes in turbulent flows. 

The study of turbulent mixing in a single-phase reaction mixtnre has shown that the radial inpnt of reactants and conical widening at the input zone of a reactor can decrease diffnsion limitations for fast liqnid-phase reactions. At the same time, the size of dispersed phase particles in a two-phase reaction mixture flow is determined by the hydrodynamic inflnx (shear pressnre valne) in a dispersion medium, which increases with the growth of the intensity of the tnrbulent pulsations. 

Buyevich, Y. A. Internal pulsations in flows of finely dispersed suspensions. Izv. Ross. Akad. Nauk, Mekh. Zhidkosti I Gaza no. 3, 91-100 (1993) (in Russian). 

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