The influence of transport phenomena on the fluidized bed combustion of a single carbon particle

The burning rate and temperature of the carbon particles are known to affect the efficiency of a fluidized bed combustor, and also the emission levels of undesired noxious components. The main results of an extensive study on the fluidized bed combustion behaviour of a single carbon particle [1] are summarized. Calculations have been carried out with a newly developed transient model, the ASPC model, and also with the much simpler progressive conversion model. Besides, many experiments have been performed in a lab-scale fluid bed construction to measure the burning rate and temperature of individual carbon particles for various conditions. From the comparison between experimental results and model predictions it has been overall concluded that the ASPC model is especially useful in i) describing the complex behaviour of progressive carbon conversion for the regime of combustion controlled by carbon reactivity plus intraparticle oxygen diffusion, and ii) estimating the conditions for which transition to the regime of external mass and heat transfer control occurs. Accurate prediction of the carbon particle burning rate and temperature is only possible for the latter combustion regime

Tanzanian manufacturing performance in comparative perspective

This paper presents a summary of new estimates of employment, nominal and real GDP in Tanzanian Manufacturing, 1961-1995. Time series of GDP and employment are placed in comparative perspective by linking them to benchmark level comparisons of GDP and employment for 1989. The first part of the paper (sections 2-4) deals with adjustments to nominal GDP, based on in depth analysis of the data of the 1989 industrial census, earlier census data and industrial surveys. Adjustments are made for undercoverage, omitted establishments, non-response and conceptual adjustments in the concept of value added. After adjustment, nominal manufacturing value added in establishments with 10 is substantially higher. The adjustments vary from 3% in 1978 to 127% in 1988. On average the upward adjustment is 52 per cent. New consistent time series of nominal GDP are presented both for aggregate 10+ manufacturing (1961-1995) and for six branches of manufacturing (1965-1995) The second part of the paper (section 5) focuses on the construction of a consistent long term index of industrial production, using weighted quantity relatives. The index is constructed for aggregate 10+ manufacturing and six branches. Corresponding indices of employment are derived, using the same adjustments for undercoverage, omitted establishments and non-response as in the case of GDP. The third section of the paper (section 6) presents a benchmark comparison of real value added relative to world manufacturing productivity leader, the USA. The benchmark uses average unit value ratio’s to convert value added for purposes of real comparisons. These unit value ratio’s are derived from the industrial census product listings in Tanzania and the USA, according to the industry of origin methodology of the international comparisons of output and productivity project (ICOP). Census listings contain quantity and value information, which are used to make product matches. In 1965 comparative labour productivity in aggregate manufacturing was around 9 per cent of the US level. It increased until 1973 to 11 per cent, followed by a long period of decline. By 1989 labour productivity stood at 3.7 of the US level.

A grain size distribution model for non-catalytic gas-solid reactions

A new model to describe the non-catalytic conversion of a solid by a reactant gas is proposed. This so-called grain size distribution (GSD) model presumes the porous particle to be a collection of grains of various sizes. The size distribution of the grains is derived from mercury porosimetry measurements. The measured pore size distribution is converted into a grain size distribution through a so-called pore-tosphere factor whose value is also derived from the porosimetry measurements. The grains are divided into a number of size classes. For each class the conversion rate is calculated either according to the shrinking core model, involving core reaction and product layer diffusion as rate-determining steps or according to a new model in which some reaction at the grain surface is assumed to be limiting. The GSD model accounts for the phenomenon of pore blocking by calculating the maximum attainable conversion degree for each size class. In order to verify the model, two types of precalcined limestone particles with quite different microstructures were sulphided as well as sulphated. Furthermore, a single sample of sulphided dolomite was regenerated with a mixture of carbon dioxide and steam. For each reaction good agreement was attained between measured and simulated conversion vs. time behaviour

Theoretical and experimental bubble formation at a single orifice in a two-dimensional gas-fluidized bed

An earlier developed, first principles hydrodynamic model of gas-fluidized beds has been employed to study theoretically bubble formation at a single orifice in a two-dimensional bed. For several orifice discharge rates, theoretically predicted bubble sizes, formation times and shapes have been compared with experimental data obtained from triggered photographs. Besides, the present experimental and theoretical results were compared with predictions from two approximate models reported in the literature which are based on an idealized picture of the process of bubble formation. The advanced hydrodynamic model appears to predict the experimentally observed diameters, formation times and shapes of bubbles quite satisfactorily. The observed and calculated bubble diameters fall between the predictions from the two approximate models. Both the experimental and theoretical results clearly indicate that a strong leakage of bubble gas into the surrounding porous emulsion phase occurs, especially during the initial stage of bubble formatio

Reversible electrowetting and trapping of charge: model and experiments

We derive a model for voltage-induced wetting, so-called electrowetting, from the principle of virtual displacement. Our model includes the possibility that charge is trapped in or on the wetted surface. Experimentally, we show reversible electrowetting for an aqueous droplet on an insulating layer of 10 micrometer thickness. The insulator is coated with a highly fluorinated layer impregnated with oil, providing a contact-angle hysteresis lower than 2 degrees. Analyzing the data with our model, we find that until a threshold voltage of 240 V, the induced charge remains in the liquid and is not trapped. For potentials beyond the threshold, the wetting force and the contact angle saturate, in line with the occurrence of trapping of charge in or on the insulating layer. The data are independent of the polarity of the applied electric field, and of the ion type and molarity. We suggest possible microscopic origins for charge trapping.Comment: 13 pages & 5 figures; the paper has been accepted for publication in Langmui

A small scale regularly packed circulating fluidized bed. Part II: Mass transfer

The underlying objective of the present study is to increase gas¿solids contact in a circulating fluidized bed by the introduction of obstacles in the riser portion. The presence of such obstacles leads to suppression of radial inhomogeneities in the solids mass flux and concentration, and break-up of solids clusters. At ambient conditions, gas¿solids mass transfer was investigated for cocurrent upward flow of air and microsize solid particles (FCC, 70 ¿m diameter) over a regularly structured inert packing introduced into the riser part of a circulating fluidized bed unit. The packed section has a height of 0.48 m, a cross-sectional area of 0.06 × O.06 m2, and contains regularly stacked 0.01 m diameter Perspex bars as the obstacles meant to enhance the gas¿solids contact. Gas mass fluxes used were 1.4 and 2.7 kg m¿2 s¿1. Solids mass fluxes were varied in the range 0Gs 12 kg m¿2 s¿1. Experimental mass transfer data were obtained by applying the method of adsorption of naphthalene vapor on FCC particles. A conservative estimate of the apparent gas¿solids mass transfer coefficient kg* could be derived from the naphthalene vapor concentration profile along the packed section on the basis of a plug-flow-model interpretation, while assuming single-particle behaviour and neglecting intraparticle diffusion effects. Such kg* values appear to increase with increasing gas mass flux, but decrease with increasing solids mass flux (and consequently increasing solids volume fraction) probably due to the corresponding increase in particle shielding. Comparison of the present results with available literature data for similar solid materials suggests that the effect of the packing inserted into the CFB is significant: the Sherwood numbers derived from the present study are relatively high