Prediction of liquid phase hold-up in random packed beds

Experimental data on liquid phase hold-up in packed beds consisting of Raschig rings, Lessing rings and spheres are presented. A generalized correlation has been proposed for predicting the liquid phase hold-up in terms of packing density equivalent spherical diameter, liquid viscosity, density and flow rate.     

A three-dimensional model for simulating the maldistribution of liquid flow in random packed beds

The hydrodynamic characteristics of liquid flow in a packed column are studied using model equations that are based on the volume average concept. The objective is to track radial variations in flow rates (or maldistributions) on a scale smaller than the equipment scale, but larger than the scale of the packing. Such models remain valid on a wide range of scales. Once a number of parameters such as voidage and permeability are determined, the model could be used for scale up purpose. We present the results of two and three dimensional simulation using a computational fluid dynamics solver. For several different inlet distributions, the development of flow patterns and the formation of the natural flow patterns near the wall are simulated. The simulation results on wall flow patterns are in reasonably good agreement with published experimental data.     

A study of local liquid/solid mass transfer in packed beds under trickling and induced pulsing flow

Induced pulsing flow (by cyclic liquid feeding) in packed beds, operated in the trickling flow regime, is studied as a method of overall improvement of catalytic reactor operation. In this paper results are reported of experiments aimed at determining local and global liquid/solid mass transfer rates, mainly for the so-called fast mode of ON-OFF periodic liquid feeding, with frequencies of order 0.1 Hz. Such mass transfer data for the fast mode of induced pulsing are not available in the literature. Uniform 6 mm glass spheres and alumina cylindrical extrudates, of 1.5 mm diameter and a narrow distribution of lengths, are employed in the tests. For completeness, results are also reported for single-phase (liquid) and trickling flow through the same packed beds. A well-known electrochemical technique is employed to measure instantaneous local mass transfer coefficients by means of quite a few probes distributed throughout the bed. The hydrodynamic characteristics under the above conditions, reported in companion papers, are helpful in interpreting the new mass transfer data.There is a wide spread of the time-averaged local mass transfer rates, in all cases tested, apparently due to packing and flow non-uniformities. This spread is much smaller in the case of packed uniform spheres. In general, the benefits of cyclic liquid feeding are more evident in the packed bed of spheres than in that of cylindrical extrudates; for instance, with increasing mean liquid rate, induced pulsing tends to reduce the spread of local mass transfer coefficients, which suggests that more uniform fluids distribution is promoted. The imposed liquid pulses are reflected in the observed periodic variation of local mass transfer coefficients; the latter appear to decay along the bed in the same manner as the liquid pulses. Other trends of local mass transfer rates are identified and discussed in relation to measured variation of liquid holdup, under the same conditions. For packed spheres, the measured global mass transfer rates are in fair agreement with literature correlations obtained for the trickling flow regime, unlike the case of packed extrudates where significant deviation is observed.     

Numerical simulations of the spread of point-source liquid spills in inclined and rolling rectangular packed beds

Liquid spreading in thin rectangular vertical/inclined and oscillating porous media was simulated using a two-fluid dynamic model as a preliminary step in the design of fixed-bed reactors dedicated to marine applications. The model assessed the influence of capillary pressure and mechanical dispersion forces arising from the spatial inhomogeneity of point-source liquid injections in packed beds with different particle sizes, liquid and gas flow rates, liquid viscosity, static bed tilts, and rolling amplitudes and periods. In mildly static inclined beds, the lateral gravity force component and the capillary pressure force were the main factors affecting the liquid spreading. However, at considerable bed inclinations, the liquid phase accumulation in the lowermost regions of the packed bed tended to shrink the liquid spreading. Dynamic oscillatory evolutions of the liquid spills in the rolling bed enlarged the liquid spreading length as compared to the static vertical bed due to combined lateral liquid flow and increased liquid residence time.     

气液固三相流化床局部相含率轴径向分布

应用开发的微电导探针测试技术,对小尺寸颗粒玻璃珠（粒径分别为0.48、0.80、1.25 mm）和1.45 mm苯乙烯树脂为固相,空气和水分别为气相和液相的三相流化床的各相局部含率进行了同时测定.在较宽操作条件下,获得了一系列局部相含率实验数据.通过对实验数据研究发现：在充分发展段,局部固含率在r/R=0.75～0.85范围内有一个极大值,床层截面平均气含率随表观液速的变化存在一个极小值.在气体分布器区,在距气体分布器轴向一定距离范围内,三相局部含率的径向分布存在明显的不对称分布.随着轴向距离的增加,局部气含率径向不对称分布逐渐消失,最终过渡到对称分布,而局部固含率的径向不对称分布消失得不明显.     

Radial liquid spread and maldistribution in packed columns under different wetting conditions

Liquid spreading and distribution patterns were measured under variation of the wettability of the packing, ringsize, bed height and flow rate. A point source of liquid was fed into a column of 0.2 m diameter in the absence of a gas stream. The liquid was collected at the bottom of the column in a grid plate consisting of 177 sampling sections. For the purpose of obtaining a wide range of wetting conditions glass and teflon coated Raschig rings were used in combination with water and water-butanol mixtures of various composition. The radial spreading was found to be dependent only on the ringsize. Further it was shown that the local flow rate deviates strongly from the flow rate predicted by diffusion theory. A maldistribution factor was therefore defined and the ringsize was again the dominant parameter. From this it was concluded that the bulk of the liquid is transported by means of stable channels through the bed even at good wetting conditions. The residence time distribution of this highly irregular liquid flow agrees more or less with residence time distribution data reported for tracer experiments. This indicates that a large proportion of the axial mixing in the liquid phase has to be attributed to the irregular flow distribution.     

A correlation for heat transfer in liquid fluidized beds

A relationship, of the Colburn j-factor type⁽⁹⁾, is derived for heat transfer in fluidized beds. This correlation (10) successfully accounts for the existence of the characteristic maximum heat transfer coefficient at a particular porosity. The derived relationship also predicts the experimentally observed variation of h with change of particle size and density. These variations are mainly a function of the hydrodynamics of the system.     

A study on mixing mechanism in air spouted beds

The mixing mechanism of solid particles in the air spouted bed was studied by employing an impulse response technique. The particles, german millet and barley, were spouted by air in columns of diameter of 8.4 cm and 12.6 cm. In the proposed theory, it was assumed that the mixing of the particles in the spouted bed occurs when they circulate through spout, fountain and annulus. Also a theoretical model was derived by assuming that the particle flow in the annulus is a combination of many annular plug flows while the flow in the spout as well as in the fountain is a mixed flow The residence time distribution of the particles in the bed was measured by injecting a portion of colored particles into the feed line and analyzing the concentration of the colored particles in the discharge line. The experimental results and the proposed theory were most satisfactorily agreed when the null residence time in the spout and in the fountain was assumed in the theoretical model.     

A comprehensive interpretation of solid layer inversion in liquid fluidised beds

Solid mixing and segregation in liquid fluidised beds containing binary mixtures of spherical particles of different density and size has been studied for a range of liquid velocities, bulk bed compositions and particle properties. It was shown that a bed of denser particles expands with liquid velocity independently of the presence of the lighter particles. When the bulk volume fraction of the lighter particles is high and the liquid velocity is relatively low, the bed forms two layers, i.e. the upper layer consisting almost entirely of the lighter and the lower mixed layer consisting of both components in which the volume of the lighter increases with liquid velocity. A completely mixed bed is obtained at a certain velocity and then a further increase of the velocity causes “layer inversion”. The liquid velocity at which complete mixing occurs depends on the bulk bed composition, and at that velocity the volume fraction of the lighter in the lower mixed layer is constant regardless of the bulk bed composition. It is shown that layer inversion occurs for a given particle mixture when the liquid velocity passes through a value at which the volume fraction of the lighter in the lower layer becomes equal to the bulk bed composition; or for a given velocity, when the bulk bed composition becomes equal to the fraction of the lighter component which exists in the lower layer. The dependency of the fraction on the liquid velocity and the particle properties is examined to some extent.     

Local study of wall to liquid mass transfer in fluidized and packed beds. I. Mass transfer in fluidized beds

This paper deals with the local influence of particles on mass and momentum transfer at a surface immersed in a liquid fluidized bed. The experimental distributions of mass-transfer coefficients and shear stresses along the surface appear to be similar and suggest large hydrodynamic perturbations near the leading edge of the diffusional boundary layer. It is shown that the fluidized bed behaves as a turbulent pseudo-fluid and that the Chilton-Colburn analogy, expressing the equivalence between mass and momentum transfer applies locally. The results of the study lead to a qualitative explanation of the influence of the fluidization parameters on the overall surface to liquid mass transfer coefficient in fluidized beds.Nomenclature a coefficient in Equation 6 - D molecular diffusion coefficient - d _p particle diameter - d microelectrode diameter - J _M k/(u/)(Sc) ^2/3 Colburnj-factor - k local mass transfer coefficient - k local mass transfer coefficient defined by Equation 1 - k _c k mass transfer coefficient given by the Chilton-Colburn analogy - ¯k overall mass transfer coefficient - L length of the transfer surface - m, n, p exponents in Equations 6 and 7 - s V _x /y) _y=0 velocity gradient at the transfer surface - (Sc) /D Schmidt number - u liquid superficial velocity - u _max maximum fluidization velocity - V _x length velocity - x length coordinate - x ₀ length of the inactive part of electrode - y normal coordinate - bed porosity - _max porosity corresponding to the maximum ofk in a fluidized bed - _F fluid density - _S particle density - dynamic viscosity - kinematic viscosity - s shear stress at the wall     

Local study of wall to liquid mass transfer in fluidized and packed beds. II. Mass transfer in packed beds

Mass and momentum transfer at a wall in liquid-particle systems are studied with a two-dimensional model which consists of fixed spherical turbulence promoters arranged in a simple cubic lattice in a rectangular channel. Local values of the mass transfer coefficient and shear stress at a wall of the channel have been measured at identical locations. The results show that there are large differences between the local values but their distribution along the transfer surface is reproduced identically. The dependence of these local values on each other allows one to obtain a general relationship between overall mass and momentum transfer as well as a correlation of mass transfer results for exchange between a wall and a flowing liquid in a fixed bed of particles.Nomenclature a _g particle specific area - a coefficient in expression s=a ^q (q>0) - a, b coefficients in expressionJ _M=a(Re) ^–b - d _p particle diameter - d microelectrode diameter - D molecular diffusion coefficient - h _K,h _B constants in Ergun equation - J _M=(¯k/u/)(Sc) ^2/3 Colburnj-factor - k local mass transfer coefficient - k local mass transfer coefficient in inert wall - ¯k overall mass transfer coefficient - L length of the transfer surface - q exponent in expressions=a ^q - (Re)=(ud _p)/[v(1-)] modified Reynolds particle number - (Sc)=v/D Schmidt number - s, ¯s velocity gradients at the wall - u superficial liquid velocity - coefficient in Equation 1 - characteristic length - bed porosity - _F fluid density - dynamic viscosity - kinematic viscosity - shear stress at the wall - P/L fluid pressure gradient     

A local transient approach to monitoring fluidization quality in freely bubbling beds

In fluidization processes, the optimal contacting regime is dictated by the physical or chemical process in question, and can vary significantly from application to application. Even within the specified regime, it is often necessary to place constraints on the hydrodynamic behavior in order to meet operational requirements. While the ability to do this is very important, there is a lack in current technologies' ability to identify the operational state via direct measurement. This work adopts a local transient approach towards addressing this problem. In order to study the local fluid bed dynamics, needle-type capacitance probes were employed to gather local transient solids fraction data in a bench-scale bubbling bed. The time-series were then subject to analysis in order to extract various signal invariants. The parametric effects of axial measurement location, superficial gas velocity, and mean particle size were examined in order to identify favorable heuristic trends in the signal invariants with respect to global parameters governing fluidization quality (i.e., superficial gas velocity and mean particle size). This work demonstrates how these invariants then constitute the basis for a novel regime map based monitoring scheme for quality of fluidization with potential application to feedback control.     

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

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.     

Distribution of solid particles in liquid fluidized beds

A method is proposed for calculating the steady-state distribution of mixtures of solid particles and of the axial bed porosity in liquid fluidized beds. The extent of bed stratification is assumed to be determined by the differences among the settling velocities of the fluidized particles and the random motion of these particles that is a result of fluidization. Predictions of particle distribution based on the present method were found to agree reasonably well with experimental data.     

A study of agglomeration of coal-ash in fluidized beds

Experimental data on agglomeration of coal-ash particles in a fluidized bed have been presented. It has been observed that above the “initial sintering temperature”, the ash particles are defluidized at velocities above their minimum fluidization velocity. The expression for minimum fluidization velocity has been modified by including a force-term due to the phenomenon of sintering in addition to those due to drag, gravity and kinetic energy.     

Flame spread over a liquid surface in a channel of finite section under oncoming air-flow conditions

The propagation of a combustion wave over a shallow hot liquid (n-butanol) blown over by an air flow was studied experimentally. The flame spread was accompanied by pulsations, whose amplitude depended on the oncoming gas velocity. Dependences of the average flame speed on temperature and oncoming gas velocity were obtained. The average speed was found to be independent of the liquid depth ahead of the flame within the experimental error. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 29–34, January–February, 2008.     

An investigation of liquid maldistribution in trickle beds

The influence of liquid maldistribution at the top of the packing on flow characteristics in packed beds of gas and liquid cocurrent downflow (trickle beds) is experimentally investigated. Particular attention is paid to the effect of gas and liquid flow rates on flow development. Tests are made in the trickling and pulsing flow regimes. A uniform, a half-blocked and a quarter-blocked liquid distributor is tested. Packings of various sizes and shapes are employed. Data are presented on pressure drop and liquid holdup as well as trickling to pulsing flow transition. Diagnosis of radial and axial liquid distribution is made by means of conductance probes. The effects of liquid foaming, bed pre-wetting, top-bed material, and blockage midway the bed on liquid distribution are also examined. Overall, liquid waves in the pulsing flow regime have a beneficial effect, promoting uniform liquid distribution in the bed cross section.