Academic literature on the topic 'Packed bed'

Conventional packed bed reactors of low tube to particle diameter ratio suffer from poor heat transfer near the tube wall, and also from increased axial dispersion compared with wide beds. In this dissertation the potential of a new reactor design aimed at overcoming these deficiencies is investigated. The radially-stratified bed provides more particles in the wall region to support improved heat transfer and also to flatten the voidage profile in an attempt to reduce the axial dispersion. An experimental study on the effect of stratification on voidage profiles using an image analysis technique showed that both the voidage and velocity profiles could be flattened. A study into the effect of different packing arrangements on axial dispersion has shown that the best arrangement for flattening the voidage profile can lead to over-compensation, resulting in channelling through the core and an increase in dispersion. However, a packing arrangement consisting of a binary mixture of large and small particles near the wall with the core of the bed packed solely with the larger, was shown to exhibit dispersion characteristics no worse than monosized packing. This same packing arrangement was also found to support improved heat transfer. At Re<SUB>p ></SUB> 1100 the heat transfer coefficient appropriate to the one-dimensional plug flow model was shown to increase by ca. 15%. A novel analysis of the one-dimensional reactor model has shown that use of a stratified bed of the same voidage, heat transfer coefficient, tube diameter, and feed flowrate as a monosized bed, results in a pressure drop which is 57% of that across a conventional bed of the same bed length. A further advantage of the stratified bed is that, on average, the catalyst particles are nearer the wall than in the conventional monosized bed. The magnitude of this and the other advantages was assessed by simulation of a reaction performed at high Reynolds numbers, the partial oxidation of ethylene. For this purpose, a new, plausible but simple two-dimensional model of the packed bed reactor was devised. A stratified bed was predicted to then have a pressure drop only some 34% of that of a conventional bed for the same overall conversion. This further reduction of some 23% in pressure drop stems largely from the reduction in packed length necessary to achieve a stipulated conversion. Alternatively, for the same selectivity, a stratified bed could reduce the pressure drop to about half that of a conventional bed of the same voidage. The potential of this novel design has thus been demonstrated: the next stage is optimisation.

Packed beds find extensive application in a wide variety of industries to cany out a large number of diverse processes. The main objective of the present work is to develop models to predict the arrangement of particles and based on them, to determine and evaluate the structural characteristics of packed beds. These problems have received only a limited attention in the literature. As a first attempt, spheres of uniform size are considered. Beds of aspect ratio up to 2 (referred to as low aspect ratio beds) are analyzed by application of principles of analytical geometry. Expressions are derived for the location of particles and for the structural characteristics of the beds, both of which show periodicity. This leads to the concept of a unit cell which is the repetitive section of the bed whose characteristics are the same as those of the complete bed. The beds fall into three distinct groups — those with aspect ratio between 1 and l√3⁄2, between 1√3⁄2 and 2, and with aspect ratio 2. Equations are distinct for each group. The aspect ratio shows marked influence on the structural characteristics of the beds. Agreement of the predictions on the overall void fraction with the available experimental data is excellent. Radial void fraction profiles are estimated by defining a concentric cylindrical channel (CCC) of an arbitrary thickness and with the cylindrical surface through the radial position of interest located at the middle of the CCC, and by accounting for the solid volumes of all the segments (in this CCC) of spheres with centers lying within a distance of a particle radius on either side of the cylindrical surface. The curved boundaries of the sphere segments are rigorously accounted for. The results show that the entire bed is filled with variations in the void fraction, starting from a value of unity at the wall and zero (or close to zero) towards the axis of the bed. Monte Carlo model for the simulation of high aspect ratio beds has not proved successful even with any of a wide variety of distribution functions for the coordinates of the sphere dropping point. With uniform distribution, the only distribution used in all the reports so far, and with normal distribution, there is not even a qualitative agreement with the reported data on void fraction variations. Distributions with asymmetric density functions such as exponential, Weibull, gamma and beta, show considerable improvement; beta distribution being the best. However even the best results with beta distribution show satisfactory agreement with the experimental data only up to about 2dp from the wall. Simulations with the cluster growth model, modified to account for the confining nature of the wall, lead to more satisfactory results. The proposed algorithm consists of building up the cluster, sphere by sphere, by calculating all possible interior and wall sites for placing an incoming sphere in a stable and non-overlapping position on the current cluster. A preference parameter is defined to place the new sphere at locations along the cross section of the column at which the experimental void fraction profiles show prominent minima, that is, locations around which the bed has relatively high solid volume. Void fraction profiles in beds of various aspect ratios simulated by this model show good agreement with the corresponding experimental data. The structural characteristics of the high aspect ratio beds thus simulated are evaluated. The number of spheres per unit length, Ni is correlated with the aspect ratio. It becomes proportional to the square of the aspect ratio, with the proportionality constant being close to 0.9, for aspect ratios greater than about 10. This follows since in these beds the overall void fraction becomes constant at 0.4. Majority of the spheres have contacts (with neighboring spheres) between 4 and 7, with the lower and upper limits for the coordination number being 2 and 9. The radial profile of the average coordination number (averaged over the height of the bed at the given radial position) shows small oscillations about a mean value of about 6 over almost the entire bed cross section starting from a distance of about ldp from the wall. At a distance of 0.5dp from the wall the predominant number of contacts is four while the mean value is about 4.3. The overall coordination number (averaged over the entire bed) shows inverse dependence on the aspect ratio. For random packings, that is, as the aspect ratio becomes infinity, the overall coordination number tends to six which corresponds to regular cubic arrangement. Cumulative number fraction, CNf is a global measure of the arrangement of spheres in beds of high aspect ratio. Its radial variation shows four distinct regions whose locations are independent of the aspect ratio The CNf values in each region are correlated with aspect ratio The correlations combined with that of NL lead to a very useful and effective model for predicting void fraction profiles in a bed of any specified aspect ratio The validity of the predictive model is demonstrated

Packed beds find extensive application in a wide variety of industries to cany out a large number of diverse processes. The main objective of the present work is to develop models to predict the arrangement of particles and based on them, to determine and evaluate the structural characteristics of packed beds. These problems have received only a limited attention in the literature. As a first attempt, spheres of uniform size are considered. Beds of aspect ratio up to 2 (referred to as low aspect ratio beds) are analyzed by application of principles of analytical geometry. Expressions are derived for the location of particles and for the structural characteristics of the beds, both of which show periodicity. This leads to the concept of a unit cell which is the repetitive section of the bed whose characteristics are the same as those of the complete bed. The beds fall into three distinct groups — those with aspect ratio between 1 and l√3⁄2, between 1√3⁄2 and 2, and with aspect ratio 2. Equations are distinct for each group. The aspect ratio shows marked influence on the structural characteristics of the beds. Agreement of the predictions on the overall void fraction with the available experimental data is excellent. Radial void fraction profiles are estimated by defining a concentric cylindrical channel (CCC) of an arbitrary thickness and with the cylindrical surface through the radial position of interest located at the middle of the CCC, and by accounting for the solid volumes of all the segments (in this CCC) of spheres with centers lying within a distance of a particle radius on either side of the cylindrical surface. The curved boundaries of the sphere segments are rigorously accounted for. The results show that the entire bed is filled with variations in the void fraction, starting from a value of unity at the wall and zero (or close to zero) towards the axis of the bed. Monte Carlo model for the simulation of high aspect ratio beds has not proved successful even with any of a wide variety of distribution functions for the coordinates of the sphere dropping point. With uniform distribution, the only distribution used in all the reports so far, and with normal distribution, there is not even a qualitative agreement with the reported data on void fraction variations. Distributions with asymmetric density functions such as exponential, Weibull, gamma and beta, show considerable improvement; beta distribution being the best. However even the best results with beta distribution show satisfactory agreement with the experimental data only up to about 2dp from the wall. Simulations with the cluster growth model, modified to account for the confining nature of the wall, lead to more satisfactory results. The proposed algorithm consists of building up the cluster, sphere by sphere, by calculating all possible interior and wall sites for placing an incoming sphere in a stable and non-overlapping position on the current cluster. A preference parameter is defined to place the new sphere at locations along the cross section of the column at which the experimental void fraction profiles show prominent minima, that is, locations around which the bed has relatively high solid volume. Void fraction profiles in beds of various aspect ratios simulated by this model show good agreement with the corresponding experimental data. The structural characteristics of the high aspect ratio beds thus simulated are evaluated. The number of spheres per unit length, Ni is correlated with the aspect ratio. It becomes proportional to the square of the aspect ratio, with the proportionality constant being close to 0.9, for aspect ratios greater than about 10. This follows since in these beds the overall void fraction becomes constant at 0.4. Majority of the spheres have contacts (with neighboring spheres) between 4 and 7, with the lower and upper limits for the coordination number being 2 and 9. The radial profile of the average coordination number (averaged over the height of the bed at the given radial position) shows small oscillations about a mean value of about 6 over almost the entire bed cross section starting from a distance of about ldp from the wall. At a distance of 0.5dp from the wall the predominant number of contacts is four while the mean value is about 4.3. The overall coordination number (averaged over the entire bed) shows inverse dependence on the aspect ratio. For random packings, that is, as the aspect ratio becomes infinity, the overall coordination number tends to six which corresponds to regular cubic arrangement. Cumulative number fraction, CNf is a global measure of the arrangement of spheres in beds of high aspect ratio. Its radial variation shows four distinct regions whose locations are independent of the aspect ratio The CNf values in each region are correlated with aspect ratio The correlations combined with that of NL lead to a very useful and effective model for predicting void fraction profiles in a bed of any specified aspect ratio The validity of the predictive model is demonstrated

Tato diplomová práce se zabývá tématem výměny tepla v zásobníku tepla typu ”packed bed”. Cílem je popsat přenos tepla v zásobníku tepla obsahující kamínky malých průměrů, skrz který proudí horký vzduch. Toto je modelováno v prostředí MATLAB. Na začátku je krátký úvod do problematiky zahrnující ukládání tepla a jeho možné využití. Dále je uveden krátký přehled o základech přenosu tepla, typech přenosu tepla a termofyzikální vlastnosti systému vzduch-kámen. Ve třetí kapitole je představen zásobník tepla typu ”packed bed” a rozličné modely a dané podmínky jsou vysvětleny. Další kapitola se zabývá s numerickými metodami, převážně s metodou konečných diferencí použitou v této práci. Pátá kapitola se zaměřuje na obecnou optimalizaci daného problému přenosu tepla. Populačně založený metaheuristický optimalizační algoritmus zvaný Genetický algoritmus je popsán. Sestavení modelu je ukázáno v šesté kapitole, stejně jako prezentace výsledků získaných z programu MATLAB. V poslední kapitole je pak diskutován závěr a doporučení.

This study reports on overfeed packed bed char combustion experiments and modelling. Experimental results are used to verify the assumptions of an existing char combustion simulation. The computer model uses finite volume discretization methods to predict temperature and concentration profiles for packed bed char combustion. Also, the behaviour of ash in a packed bed is explored and incorporated into the model by accounting for the effect of ash on such properties as bed void fraction and heat and mass transfer processes. The effects of air flow rate and bed depth on the burning rate, the bed temperature and gas concentrations are also examined. Experiments were performed in a reactor 23 cm in diameter and 20 cm high. Thermocouples measured bed temperatures and a water-cooled probe sampled bed gases. Gas samples were analysed for CO2, CO and O2 using a gas chromatograph. The fuel was foundry coke with a particle diameter in the 1 cm size range. Experiments were run for bed heights ranging from 5 to 15 cm and air flow rates of 191 to 520 kg/m2hr. Comparison of the experimental bed profiles and burning rates with model predictions shows good agreement, indicating that the model can reliably predict packed bed char combustion. Species concentrations are shown to be only dependent on bed depth; whereas the burning rate is a function of both the bed depth and air flow rate. The data also show that the ash properties are highly dependent on the operating conditions; since the model predictions are very sensitive to the ash properties, a relationship between ash properties and temperature needs to be developed and included in the model. More research is also needed on the CO2 reduction reaction parameters and pore diffusion model and the oxidation kinetics in the ash layer.

Packed bed combustion is the burning of solid fuel particles supported by a grate with the combustion air supplied from below. The combustion process is divided into four main stages: drying, devolatilization, volatiles combustion and char combustion. Biomasses proposed as renewal energy sources, such as wood, have a very high volatile content (&sim;80%). Therefore mechanistic models developed for the prediction of bed characteristics during biomass combustion must include devolatilization and volatile combustion stages in order to correctly predict combustion behaviour for better emissions control and process efficiency. A novel in-situ sampling method for tar, a major pyrolysis product, was developed that allows its concentration to be measured at various heights within the packed bed and appears to work satisfactorily. A series of experiments on packed bed combustion were conducted in a laboratory 'pot' type combustor. Two different equivalent particle size diameters (2.8 cm and 3.2 cm) of untreated spruce wood and two different airflow rates (0.025 kg/m2s and 0.03 kg/m 2s) were tested at a 22 cm bed height. Although the experimental data show scatter, the measurements indicated that pyrolysis occurred primarily within two particle diameters of the top of the bed, with large amounts of tar and CO and somewhat less CO2 being produced. This research also expanded a numerical model for packed bed combustion of solid fuels with the addition of a simple first order pyrolysis reaction, in which fixed proportions of the products were set as light volatiles of CO and CO2 with the balance as tar. The model results compared well with bed temperature, particle size and density measurement throughout the bed and gas concentration (CO, CO2, O2, and CH4) measurements in the reduction and oxidation zone.

Present integrated gasification combined cycle (IGCC) systems demonstrate high system efficiency and impressive environmental performance, giving them an edge over conventional pulverised fuel power stations. A key area in the development of IGCCs is hot fuel gas clean-up (HGCU). Fuel gas cleaning at elevated temperatures reduces thermal efficiency losses associated with gas quenching in conventional cold gas cleaning methods. Current hot gas desulphurisation techniques focus on the use of regenerable metal oxide sorbents, however the long-term sorbent performance issues have yet to be fully addressed. A fresh and radical approach may provide the key to overcoming the inherent limitations associated with metal oxide sorbents. A molten tin irrigated packed bed scrubber adopted in this research project is one such innovative way forward in HGCU. The hot scrubber offers the prospect of a multicomponent clean-up device. High-temperature sulphur removal takes place via absorption of H2S (and COS) into molten tin whilst discrete molten tin droplets and rivulets on the packing surface act as solid particulate collectors. The primary aim of this research project was to investigate the workings of a small-scale room temperature packed bed scrubber operating under non-wetting flow conditions analogous to the molten tin irrigated scrubber. Water irrigation of low surface energy packings simulated the nonwetting flow of liquid metals. The air-water analogue of the liquid metal scrubber provided the platform for hydrodynamics (flow visualisation, flooding and liquid holdup), particulate removal and mass transfer studies under non-wetting flow conditions. The performance of a small air lift for water circulation through the column was also investigated. These cold studies offered insight into the operation and performance of the liquid metal hot scrubber. Prior to the cold gas scrubber studies, preliminary small-scale gasification tests on petroleum coke samples were performed to investigate the effect of molten tin on H2S in the product fuel gas. The tests provided actual experimental evidence of the possibility of sulphur removal by molten tin in a gasification environment. It was shown that the maximum possible size of a liquid droplet hanging from a non-wetting spherical solid surface could be predicted from the liquid surface tension and density based on force balance. The mobility of static holdup in a non-wettable packed bed has been demonstrated, this being due to the tendency for the liquid to form discrete droplets rather than spreading films. Existing flooding and liquid holdup correlations that hold for conventional wettable packed beds were shown to be inadequate where non-wetting systems were concerned. Summary hence alternative methods applicable to the latter were sought. The introduction of a non-wetting tendency factor based on the ratio of the solid critical surface tension to the liquid surface tension, enabled the flooding capacities of non-wetting systems including those of this study to be predicted using Sherwood et al. 's graphical flooding correlation. The total volumetric liquid holdup was well correlated against the bed pressure drop, true gas velocity and gas density, offering the prospects of predicting holdup for systems using the same spherical packing. In general, the water-irrigated packed bed showed good hydrodynamic similarities to liquid metal systems, suggesting a dominating influence of liquid-solid contact angle which overrides striking differences in liquid physical properties. The performance of the small air lift pump was unaffected by varying the number of gas ports on the injector without any change to the hole size. The operating curve of the air lift pump could be predicted with good accuracy using momentum balance and two phase flow theory, provided that all major pressure losses in the system were accounted for, including notably the downcomer friction losses and accelerative effects. The non-wetting packed bed scrubber demonstrated impressive dust removal performance. Total separation efficiencies as high as 99.6% and cut sizes approaching submicron were achieved. Dust particles larger than about 6.5 um can be separated to efficiencies greater than 98%. Complete particle separation was achieved in all cases for dust particles larger than 16 J..lm. Particulate removal in a packed bed of spheres under non-wetting flow conditions has also been modelled using computational fluid dynamics (FLUENT). Simulation results showed that particle separation efficiency increases with particle size and density, but is unaffected by particle concentration. The predicted particle size corresponding to 98% efficiency is about 40 J..lm. In mass transfer, the height of the gas film transfer unit of various non-wetting spherical packed bed systems including those of this study was correlated successfully against the gas phase Reynolds number, the liquid superficial velocity and the packing diameter. Results from the cold gas scrubber studies have offered insight and understanding into the workings and development of the liquid metal packed bed gas scrubber. Findings and correlations derived from the water model studies, occasionally complemented by data from other non-wetting systems, have provided the means to predict the hydrodynamics, particulate removal capability and mass transfer performance of the liquid metal based gas scrubber. The pilot unit of the hot gas scrubber has been designed and fully constructed. The high temperature gas cleaning facility is ready for commissioning.