Vladimír Staněk

A study of the area porosity profiles in a bed of equal-diameter spheres confined by a plane

Abstract Profiles have been measured of area porosity in random beds of equal-diameter spheres confined by a plane surface. The results have been interpreted in terms of a proposed stratified model of the random bed. The assumptions made in the formulation of the model seem to be justified in view of the generality of results obtained with spheres of different diameter and the good agreement of the experimental and predicted porosity profiles. The model has been tested also for describing porosity oscillations near the top of beds compressed by a plane surface and beds prepared by charging the spheres by portions while compressing simultaneously each portion added.

HYDROGENATION OF 1,5-CYCLOOCTADIENE IN A TRICKLE BED REACTOR ACCOMPANIED BY PHASE TRANSITION

This paper examined the evaporation and condensation of the reaction mixture within the trickle bed reactor during 1,5-cyclooctadiene hydrogenation. The aim of study has been to formulate a mathematical model of heat and mass transfer influenced the exothermic reaction of a volatile reaction mixture apt to evaporation by the reaction heat in the system which is often accompanied by a hot spot temperature formation in the reactor followed by the enhancement of undesirable side reactions and/or catalyst deactivation. The numerical solution of the proposed model agreed quite well with the experimental temperature profiles in the trickle bed reactor.

The effect of liquid flow distribution on the behaviour of a trickle bed reactor

Abstract A mathematical model has been formulated of the effect of flow distribution of the liquid phase carrying a dissolved reactant on the progress of an n th order, irreversible, catalytic reaction with heat effects in an adiabatic trickle bed reactor. The model has been stated in terms of the density of irrigation, temperature and concentration of the reactant in the liquid, all treated as spatially distributed variables. Provisions have been made to account for the existence of the flow down the surface of the wall, which has no catalytic effect. Local concentration and temperature have been proven to be coupled by the invariant T + U γ c = γ U . The same invariant governs also local concentration and temperature of the wall flow. Mathematically, the model is represented by a coupled set of nonlinear parabolic partial differential equations enabling concentration and temperature fields to be obtained for an arbitrary type of liquid distribution and intensity of the wall flow. Numerical solutions have been obtained by the finite-difference method simulating reactors irrigated by liquid distributors as central discs of different radii, or a central annulus, and strongly exothermic reactions with the reaction order ranging between 0.1 and 2. Numerical results have shown the effect of liquid distribution on the overall reaction conversion to be very complex. Optimum initial distribution varies depending on the reaction order as well as the required degree of conversion. In general, however, the entrance region flow pattern may play a significant role in affecting especially reactions exhibiting kinetics close to zero order (hydrogenations). The effect of the wall flow has been found unambigously adverse to reaching high conversions and of increasing importance for low order reactions.

Transients of the hydrodynamics of counter-current packed-bed columns

Abstract Transients of the hydrodynamics of the counter-current flow of air and water have been investigated experimentally in a 190-mm column packed with 25 × 25 × 2 mm Pall rings. The transients induced by a step change in liquid or gas velocity to higher or lower values have been processed by a simple first-order model. Transitions to higher phase velocities have been found in all cases to be slower than transitions to lower velocities. Transients induced by changes in gas velocity have been found in all cases to be slower than transients induced by changes in liquid velocity. Two parallel transition mechanisms have been proposed explaining qualitatively the observed behaviour which refutes the validity of the linear first-order model.

ON THE LENGTH EFFECT OF THE HEAT DISPERSION PARAMETERS

Abstract Considerable attention has been paid to the dispersion of heat in fixed beds passed by a flowing gas. This is best documented by ample experimental evidence and correlations available in the literature. However, the results of individual studies differ often significantly. As a possible cause of the discrepancies have been put forth various experimental techniques and data processing methods (Gunn, 1970; Froment, 1972; Li and Finlayson, 1977), particularly the use of different mathematical models. As a fact of considerable significance, however, remains the dependence of results on the geometry of the experiment. All parameters of the pseudohomogeneous one- and two-dimensional models have been found to depend on the length of the bed (Li and Finlayson, 1977; De Wasch and Froment, 1972; Bunell el al., 1949; Calderbank and Pogorski, 1957), while their values invariably decrease with the increasing length. In contrast, neither of the existing models predicts such a dependence, nor is there a satisfa...

An experimental study of the flow of liquid in counter-current trickle beds by the frequency response technique

Abstract An experimental study has been carried out of the trickle flow of water in a 106 mm in diameter column packed by 10 mm spheres. Experimental frequency characteristics have been used to optimize, directly in the complex domain, parameters of the plug flow, axially dispersed, stagnant zone and the axially dispersed stagnant zone model. The plug flow has been found clearly unsatisfactory. At high irrigation rates the response of the column is dominated by the stagnant zones. At low irrigation rates the real flow is most complex and both axial dispersion and stagnant zones should be considered. A comparison of the results with literature has shown that present understanding of the trickle flow is only at the level of identification of proper trends of individual parameters in dependence on process and system variables.

The effect of liquid flow distribution on catalytic hydrogenation of cyclohexene in an adiabatic trickle-bed reactor

Abstract The earlier formulated mathematical model of the distributed-flow adiabatic trickle-bed reactor has been used to predict the course of liquid-phase hydrogenation of cyclohexene. Computed solutions for independently obtained parameters and reaction kinetics have been compared with experiments on a laboratory trickle-bed reactor 3 cm in diameter packed with 3% Pd on activated carbon pellets. The model has been able to predict the experimentally found tailings on the cup-mixing mean outlet concentration versus bed depth curves. The existence of these tailings at the high conversion end cannot be accounted for by other mechanisms than the flow distribution effects. The model has been found useful for predicting the required depth of the catalyst under the real flow maldistribution conditions.

A study of the dependence of radial spread of liquid in random beds on local conditions of irrigation

Abstract Local values have been determined experimentally of the coefficient of radial spread in trickle beds randomly packed by 2O mm spheres and 15 and 25 mm Raschig rings in dependence on local conditions of irrigation. Three regions, A, B, C, have been found of different behaviour of trickling liquid. In regions A and B the coefficient remains constant but mutually different. This difference has been attributed to different regime of flow. In region C the coefficient depends on local value of the density of irrigation and the gradient of the density of irrigation. In spite of these findings the distribution of liquid in random beds may be described with the aid of a single suitably selected effective value of the coefficient of radial spread.

Experimental observation of pressure drop overshoot following an onset of gas flow in counter-current beds

Abstract The paper presents results of an experimental study of the hydrodynamics of a counter-current packed bed column undergoing a step change in the inlet velocity of the gas phase. The experimental transient curves are presented for the total pressure drop across the column and the pressure profile along the column height. Characteristic overshoots have been observed on the transient curves of the pressure drop and pressure profiles within the bed when the irrigated bed is suddenly exposed to the flow of gas. These rather unexpected overshoots appear temporarily after the onset of gas flow and cannot be explained by existing two-phase flow models.

Chapter 16 Application of Fixed-Bed Reactors to Liquid-Phase Hydrogenation

Publisher Summary This chapter discusses the application of fixed-bed reactors to liquid-phase hydrogenation. Multi-phase reactions may be performed either in slurry reactors in a batch-wise manner, or continuously in fixed-bed reactors. Depending upon the method of addition of the liquid reaction mixture, the bubble columns or trickle-bed reactors (TBRs) are discussed, where the reaction mixture trickles down the elements of the catalytic bed, concurrently with the gaseous reactant. Three-phase catalytic hydrogenations occur when the volatilities of the two reactants—that is, hydrogen and substrate—and thus two-fluid-phase reactors result from economic and technical considerations. The role of the hydrodynamics and heat- and mass-transfer phenomena are better understood, but there is a lack of pertinent physicochemical and transport data, necessary for adequate modelling, design and scale-up of these systems. The simple pseudohomogeneous reactor model can be recommended for the description of most hydro-treatments of petroleum fractions or organic hydrogenations. Experimental work has improved the understanding of the complex phenomena such as phase transitions and the associated pathological effects occurring in these systems.