Holding hydrodynamic

The results of research into the fluidised bed pyrolysis of plastic wastes are reported, with reference to determining the optimum process conditions for the process with respect to the reactor behaviour. The study investigates the effects of process variables such as bed temperature, polymer feed rate, bed hold-up, fluidising velocity, and size of inert material. Findings illustrate the importance of the knowledge of the hydrodynamics of the fluidised bed and of the interactions between bed and polymer particles in the design and operation of the reactor. 15 refs. 

Eulerian two-fluid model coupled with dispersed itequations was applied to predict gas-liquid two-phase flow in cyclohexane oxidation airlift loop reactor. Simulation results have presented typical hydrodynamic characteristics, distribution of liquid velocity and gas hold-up in the riser and downcomer were presented. The draft-tube geometry not only affects the magnitude of liquid superficial velocity and gas hold-up, but also the detailed liquid velocity and gas hold-up distribution in the reactor, the final construction of the reactor lies on the industrial technical requirement. The investigation indicates that CFD of airlift reactors can be used to model, design and scale up airlift loop reactors efficiently. 

The presence of a gas in the suspension results in an increase of the stirrer speed required to establish the state of complete suspension. The propeller usually requires a higher speed than the turbine. Furthermore, a critical volume gas flow exists above which drastic sedimentation of particles occurs. Hence, homogenisation of the suspension requires an increase of the rotational speed and/or a decrease of the gas flow rate. The hydrodynamics of suspensions with a solid fraction exceeding 0.25-0.3 becomes very complex because such suspensions behave like non-Newtonian liquids. This produces problems in the scale-up of operations. Hydrodynamics, gas hold-up, mass-transfer coefficients, etc. have been widely studied and many correlations can be found in literature (see e.g. Shah, 1991). 

Table 7.4 gives the hydrodynamic data used for the calculations. Gas hold-ups were taken from Table 7.3. 

While there is no reason to assume that basic features of the hydrodynamic vapor-removal mechanism, q"vap, do not hold for liquid metals, the term q" c for liquid metals is much larger than that of ordinary liquids, and even greater than q vap. As... 

The hydrodynamics were modelled simply, using an average hold-up and slip velocity approach... 

Additional experiments in a loop reactor where a significant mass transport limitation was observed allowed us to investigate the interplay between hydrodynamics and mass transport rates as a function of mixer geometry, the ratio of the volume hold-up of the phases and the flow rate of the catalyst phase. From further kinetic studies on the influence of substrate and catalyst concentrations on the overall reaction rate, the Hatta number was estimated to be 0.3-3, based on film theory. 

Figure 3.29.A shows a flow-cell of 20 iL inner volume used to hold immobilized anti-mouse IgG bound to a rigid beaded support (activated Pierce trisacryl GF-2000). The cell was used to develop a two-site immunoassay for mouse IgG by consecutive injection of the sample, acridinium ester-labelled antibody and alkaline hydrogen peroxide to initiate the chemiluminescence, which started the reaction sequence shown in Fig. 3.29.B. Regenerating the sensor entailed subsequent injection of an acid solution, which resulted in a determination time of ca. 12 min (this varied as a fimction of the flow-rate used, which also determined the detection limit achieved, viz. 50 amol for an overall analysis time of 18 min) [218]. The sensor was used for at least one week with an inter-assay RSD of 5.9%. Attempts at automating the hydrodynamic system for use in routine analyses are currently under way.

Figure 7.7 CCC chromatogram of the 4-nitrobenzoic elution. Hydrodynamic CCC column 53 mL rotor speed 830 rpm liquid system [C4CiIm][PF5]-acetonitrile-water 40/20/40 %w/w stationary phase IL-rich lower phase, Vg = 38 mL mobile phase acetonitrile-rich upper phase, f = 1 mL/min in the tail-to-head direction injection volume 500 pL of a mixture of 6 ppm of 4-nitrobenzoic acid (retention time 18.595 min) + 2.5 ppm phenylalanine (retention time 15.02 min) as a hold-up volume tracer UV detection 254 nm.

The following analysis holds for Type B fluidization and for Type A bubbling fluidization, when the region of particulate fluidization is so small that it can be ignored. In the framework of the two-phase model (see the subsection Hydrodynamic modeling of bubbling fluidization), the bed expansion in terms of the fraction of the bed occupied by bubbles is... 

The second section presents a review of studies concerning counter-currently and co-currently down-flow conditions in fixed bed gas-liquid-solid reactors operating at elevated pressures. The various consequences induced by the presence of elevated pressures are detailed for Trickle Bed Reactors (TBR). Hydrodynamic parameters including flow regimes, two-phase pressure drop and liquid hold-up are examined. The scarce mass transfer data such gas-liquid interfacial area, liquid-side and gas-side mass transfer coefficients are reported. 

Generally speaking, the design of TBR requires knowledge of hydrodynamics and flow regimes, pressure-drop, hold-ups of the phases, interfacial areas and mass-transfer resistances, heat transfer, dispersion and back-mixing, residence time distribution, and segregation of the phases. 

For the TBR design the dynamic liquid hold-up is a basic parameter because it is related to other important hydrodynamic parameters (including the pressure drop, wetting, and mean-residence-time of liquid). 

Then, in contrast to the operation at atmospheric pressure a small increase in the superficial gas velocity reduces considerably the dynamic liquid hold-up. This effect is more pronounced at higher liquid flow-rate values and total reactor pressures. Because of this high influence of the gas flow on the hydrodynamics, Wammes et al. [34] recommend avoidance of the use of the term, "low interaction regime", for the trickle-flow regime at high pressure. 

W.J.A. Wammes, J. Middekamp, W.J. Huisman, C.M. Debaas and K.R. Westerterp, Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures, Part 2 liquid hold-up, pressure drop, flow regimes, AIChE Journal, 37, 12 (1991) 1855-1862. 

CottrelUPoterson Equation of State, An equation of state, applicable to gases at densities near that of the solids and to temps far above the critical, is derived by Cottrell Paterson (Ref 1). It is shown that this equation is likely to hold in the range of density temperature characteristics of the detonation wave in condensed expls. The hydrodynamic equations of deton are developed on the basis of the equation of state. They were applied to PETN and the theory predictions were shown to agree with observations. Murgai (Ref 2) extended the application of the equations to oxygen-deficient expls, specifically TNT... 

While the hydrodynamic theory always predicts this near equivalence of the friction and the viscosity, microscopic theories seem to provide a rather different picture. In the mode coupling theory (MCT), the friction on a tagged molecule is expressed in terms of contributions from the binary, density, and transverse current modes. The latter can of course be expressed in terms of viscosity. However, in a neat liquid the friction coefficient is primarily determined not by the transverse current mode but rather by the binary collision and the density fluctuation terms [59]. Thus for neat liquids there is no a priori reason for such an intimate relation between the friction and viscosity to hold. 

Equation (4.23) holds true when H/q0 > e1/s. The existance of 5+, which minimizes the filling time for radial forming cavity, is conditioned by the same physical (hydrodynamical) reasons as in the case of a flat moulding. 

Gas hold-up is one of the most important parameters characterizing the hydrodynamics in a fermenter. Gas hold-up depends mainly on the superficial gas velocity and the power consumption, and often is very sensitive to the physical properties of the liquid. Gas hold-up can be determined easily by measuring level of the aerated liquid during operation ZF and that of clear liquid ZL. Thus, the average fractional gas hold-up H is given as... 

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