Tadatoshi Chiba

The minimum fluidisation velocity, bed expansion and pressure-drop profile of binary particle mixtures

The total bed pressure drop, the pressure-drop profile, bed expansion and bed voidage have been measured for a variety of binary particle mixtures over a wide range of gas velocities. Apparent minimum fluidisation velosities have been defined for segregating systems, and the addition of dense particles of lower minimum fluidisation velocity can cause a decrease in apparent minimum fluidisation velocity of the mixture in a very similar fashion to the addition of finer particles to larger ones of the same density. The measured umf s are compared with presently derived simplified theoretical equations and with equations from the literature. It is clearly shown that because of the sensitivity of umf determination to voidage, such relationships cannot be used with confidence. However, the empirical equation of Cheung on average follows the shape of the experimental curves well, includig those for binary systems of different density, provided the bed is in a well-mixed condition. Bed pressure-drop profiles are related to the mixing/segregation state and to the amount of fluidisation of the bed and may offer a simple indirect method of determining these conditions in practice.

A comprehensive interpretation of solid layer inversion in liquid fluidised beds

Abstract 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.

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part V. Combined effects of Na concentration and char structure on char reactivity

Abstract A set of NaCl-loaded Loy Yang brown coal was pyrolysed in a thermogravimetric analyser between 600 and 900 °C. The char sample after pyrolysis was cooled down directly for in situ reactivity measurement with air. The results indicated that the volatilisation of Na during pyrolysis is an important reason for the existence of catalyst loading saturation level with Na as a catalyst in char because the char prepared at high temperature had a limited holding capacity for Na. Under the experimental conditions in this study, the char reactivity showed good linear correlation with the Na concentration in the reacting char. Peak pyrolysis temperature, affecting the release of Cl and distribution of Na in char, is an important factor governing the correlation between the char reactivity and Na concentration in char. The catalytic activity of Na is a result of the interaction between Na and char and thus is greatly dependent on the char/carbon structure. At high char conversion levels where the char structure is more inert and highly condensed, the catalytic activity of Na is reduced compared with its activity at low char conversion levels. The catalytic activity of Na depends on the structure of char.

Prediction of complete mixing of liquid-fluidized binary solid particles

Abstract In a liquid-fluidized bed consisting of density- and size-different binary solid particles with a given bulk composition, complete mixing may be observed at a liquid velocity less than the terminal velocities of the constituent particles. In such cases a change either in liquid velocity or in bulk composition results in stratification of the bed. A theoretical model to predict relationships between the bulk composition and the critical velocity for complete mixing is developed on the basis of momentum equations for each particle component. The voidage function involved in the equations is independently given from a unit cell model with assumptions of creeping flow around the particle within the fictitious spherical cell and of a potential flow outside the cell. Although the voidage function estimates monocomponent bed expansions to an accuracy comparable to that of the empirical Richardson-Zaki equation, it is not accurate enough when used for predicting the volume fractions of binary particles at complete mixing. The prediction is highly sensitive to monocomponent bed-expansion characteristics, and closer agreements between the predicted and observed results are obtained when experimental expansion data are used in the prediction. Nonetheless, the present model gives correct predictions for totally segregating particle systems. Thus, a segregation map is then illustrated which predicts the mixing state of any given binary particle system and is well comparable with the observed results not only from the present work but also from other existing investigations.

Fluidised binary mixtures in which the denser component may be flotsam

Abstract From previous work, some simple, general rules can be written for determining with binary mixtures which of the two components is jetsam. In general, if there is a density difference, it is the denser. This paper reports some experiments which show that in certain special cases this density rule is broken. If the dense particles are sufficiently small (size ratio The concept of segregation maps is introduced as a convenient way of showing for a particular set of conditions whether small dense particles are jetsam or flotsam, or whether the whole system is well mixed. Finally, some simple theoretical arguments are put forward to explain the observations.

A quantitative definition and flow regime diagram for fast fluidization

Abstract The name ‘fast fluidization’ was proposed by Yerushalmi et al. to describe a class of gas-solid fluidized beds with high gas velocity and high solid circulation rates. Although much work has been published on fast fluidization and its applications, a generally accepted definition of fast fluidization is still lacking. In this paper, we propose a definition of fast fluidization. A quantitative flow regime diagram for catalytic cracking catalyst in a column of 0.1 m diameter and 5.5 m high, showing the limiting gas and solid velocities for fast fluidization, is presented. The diagram is obtained from experimental studies in which both solid flowrate and gas flowrate were manipulated independently. Previous work on fast fluidization suffers from the disadvantage that the solid circulation rate cannot be independently manipulated.

Regime classification of macroscopic gas-solid flow in a circulating fluidized bed riser

Abstract A conceptual flow regime diagram for a circulating fluidized bed riser is proposed, combining existing investigations with experimental data obtained under idealized conditions in which a fully independent control of gas velocity and solid circulation rate was conducted by use of a screw feeder for solid feed into the riser. The diagram classifies the flow state into five regimes by qualitative transition lines which describe the relationship between gas velocity and solid circulation rate. These regimes are particulate fluidization, bubbling fluidization, turbulent fluidization, dense-phase transport and dilute-phase transport. The diagram suggests that S-shaped bed-density distribution or dense/dilute region interface appears only at limited conditions in the bubbling and turbulent fluidization regimes. These experimental findings were generalized by further experiments in a conventional circulation system with a ball valve between the riser and the downcomer which permits changes in the solid circulation rate and the bed height in the downcomer. The experimental results showed that the bed height in the downcomer has no particular effect on the bed density distribution or the height of the dense/dilute region interface, but an appreciable effect on the lowest gas velocity to maintain steady solid circulation at a given rate. These results are consistent with the above diagram.

Gas exchange between the bubble and emulsion phases in gas-solid fluidized beds

Abstract Experimental studies for measuring the extent of gas exchange between the bubble and emulsion phases in gas-solid fluidized beds were conducted using ozone as a tracer. Measurements were made on tha changes of ozone concentration immediately inside the bubbles which were injected successively into the incipiently air fluidized bed. Exchange coefficients thus obtained were compared with those predicted theoretically and were found to be in good agreement. It was concluded from these results that the model used in this study based on the diffusional gas exchange between clouds and the emulsion phase was reasonable. The exchange coefficient values measured for single bubbles, as seen in this study, were found to be roughly one third of that obtained from freely bubbling fluidized beds, which fact was attributed to the coalescence of bubbles in the freely bubbling beds. The effect of gas adsorption on the surface of the particles was estimated and discussed.

Evidence of poly-condensed aromatic rings in a Victorian brown coal

An attempt was made for obtaining UV/VIS absorption and fluorescence spectra of a whole Victorian brown coal and then examining the presence of aromatic ring systems (ARSs) with poly-condensed rings in the coal. Loy Yang brown coal was subjected to an alkali-promoted depolymerization in an aqueous solution of sodium hydroxide at 473 K and then dissolved nearly completely into the solution. The solution of the solubilized coal was analyzed by UV/VIS absorption and fluorescence spectroscopies. The spectra of the solubilized coal were compared with those of a tar that was produced from the rapid pyrolysis of the coal in a wire-mesh reactor. Absorbance of the solubilized coal per unit molar concentration of aromatic carbon and unit light path length was averaged over 50 nm wavelength intervals in a range from 250 to 600 nm. The averaged absorbance of the solubilized coal was much greater than those of mono-aromatic compounds and lignin at intervals from 400 to 600 nm while comparable to those of tetra- to hexa-aromatic compounds, suggesting that the coal is abundant of ARSs with poly-condensed rings. The fluorescence spectra of the solubilized coal also give indication of the presence of ARSs with three to six condensed rings. Furthermore, the presence of even larger ARSs is evidenced from appreciable difference in apparent quantum yield of the solubilized coal and that of the tar. By comparing the absorption and fluorescence spectra of the solubilized coal with those of the tar, it was revealed that the tar is richer in relatively small poly-ARSs than the solubilized coal while the larger poly-ARSs are abundant in the solubilized coal. Smaller poly-ARSs were thus released from the coal preferentially to larger poly-ARSs during the pyrolysis.

Behaviour of bubbles in gas—solids fluidized beds: initial formation of bubbles

Abstract The successive formation of gas bubbles on the gas distributor of a multi-orifice plate in two-dimensional air-fluidized beds was studied both theoretically and experimentally. A brief theory for the initial bubble formation, which reflects the effects of fluidized particles, was developed based on the assumption that the forming bubbles do not coalesce with the foregoing bubbles until they grow up to a suitable enough size to detach themselves from the spout formed on the orifice. Measurements were made on the change in the size and the position of forming bubbles and bubbles immediately after formation in the two-dimensional fluidized beds consisting of particles of different shapes and dia. A good agreement was found between the theoretical and the experimental results.