Mathematical modelling of coal combustion in fluidized beds with sulphur emission control by limestone or dolomite

A model is developed for the combustion of coal in fluidized beds with sulphur emission control by limestone or dolomite. The gas and solid flow analysis is based on multiple gas bubbles of varying sizes which, accompanied by the cloud and wake, rise through the particulate emulsion phase. Solids population balance relating the feed, overflow, and elutriation with the physiocochemical changes of particles in the bed is carried out for coal and sulphur-absorbent respectively. The reactions in the bed are then formulated in terms of the above determined gas velocities and distribution functions for solids. Experimental data from various pilot-plant operations are used to assess the validity of the proposed model. The observed coal conversion and sulphur retention under various operating conditions are in good agreement with the predicted values.     

Consideration of primary creep at a stationary crack tip: Implications for the C t parameter

A finite element creep analysis of a center crack specimen has been carried out under small scale to extensive creep conditions. The crack was assumed to be stationary. Several constitutive models were used; these consisted of elastic, power-law creep with and without rate-independent plasticity, as well as one which also included primary creep. The mechanics basis of the C _t parameter, which has been proposed for correlating creep crack growth behavior under conditions ranging from small scale to extensive creep, is explored.For the aforementioned specimen geometry, consideration of primary creep seems to explain the differences between the measured and previously calculated load line deflection rates based on power-law creep only. It is also concluded that in small scale creep, C _t does not characterize the instantaneous crack tip singular stress field, but it accurately characterizes the rate of expansion of the crack tip creep zone regardless of whether primary or secondary creep is occurring. This result provides a rationale for using C _t to correlate creep crack growth rates even in the presence of significant primary creep deformation.

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Résumé On a procédé à une analyse par éléments finis du fluage d'une éprouvette comportant une fissure centrale, sous des conditions allant du fluage à petite échelle jusqu'à un fluage important. On suppose que la fissue est stationaire. Divers modèles ont été utilisés, à savoir la loi de fluage élastique ou parabolique, avec ou sans plasticité indépendante de la vitesse, ainsi qu'une loi incluant également le fluage primaire. On a étudié la base mécanistique du paramètre C _t, proposé pour relier le comportement de la croissance de la fissure de fluage sous des conditions entraînant un fluage depuis une petite échelle jusqu'à une grande échelle.Pour la géométrie citée, les différences entre les vitesses de variation de la courbe de la charge, mesurées et précédemment calculées en se basant sur la seule loi parabolique de fluage, semblent applicables par la prise en considération du fluage primaire. On conclut également que lors de fluage à petite échelle, C _t ne caractérise pas le champ singulier de contraintes instantanées à l'extrémité de la fissure, mais plutôt, et de maniére sûre, la vitesse d'extension de la zone de fluage à l'extrémité de la fissure, qu'il y ait fluage primaire ou fluage secondaire. Ce tésultat fournit une base d'utilisation de C _t pour connaître les vitesses de croissance d'une fissure de fluage, même en présence d'une déformation de fluage primaire significative.

     

Residues of DDT and HCH in wheat samples collected from different states of India and their dietary exposure: A multicentre study

Under a multicentre study conducted by the Indian Council of Medical Research, 1712 samples of wheat grain/flour were collected from urban and rural areas in 11 states representing different geographical regions of India. These samples were analysed for residues of DDT (2,2-bis (p-chlorophenyl)-1,1,1-trichloro ethane) and different isomers of HCH (1,2,3,4,5,6-hexachloro cyclohexane, a mixture of isomers) by gas-liquid chromatography. Residues of DDT were detected in 59.4% of 1080 samples of wheat grain and in 78.2% of 632 samples of wheat flour. Different isomers of HCH were present in about 45-80% of the samples of wheat grain/flour. Medians of DDT and total HCH, respectively, for pooled samples of wheat grain were 0.013 and 0.035 mg kg^-1, while those for wheat flour were 0.01 and 0.02 mg kg^-1. Estimated daily intakes of DDT and different isomers of HCH through the consumption of wheat contaminated at their median and 90th percentiles constituted a small proportion of their acceptable daily intakes. Amongst the pesticide residues analysed, statutory maximum residue limits have been fixed only for γ-HCH in wheat in India, as 0.1 mg kg^-1 in wheat grain and zero in wheat flour. Residue levels of γ-HCH exceeded these maximum residue limits in five of 1080 samples of wheat grain and in 340 of 632 samples of wheat flour. The failure to meet the requirement of the γ-HCH maximum residue limit in large number of wheat flour samples was attributed to the fixation of practically unachievable zero limit. Comparing the previous studies and the present one, the levels of residues of DDT and HCH in wheat were significantly decreased.     

GAS HOLDUP IN BAFFLED BUBBLE COLUMNS OF DILUTE SLURRIES OF FINE POWDERS AND VISCOUS LIQUIDS

Experimental gas phase holdup data reported by the authors as taken on two baffled bubble columns and involving slurries of fine powders (average particle diameter ≤90 μm) and dilute suspensions (mass fraction ≤ 20 percent) in viscous fluids are re-examined after correction for a small calculation error in superficial gas velocity. The two bubble columns are: a Plexiglas bubble column, 0.108 m in diameter and 2.25 m tall, equipped with seven 19 mm tubes arranged in equilateral triangular configuration with a pitch of 36.5 mm and a Pyrex glass column, 0.305 m in diameter and 3.24 m tall, equipped with thirty-seven 19 mm tubes arranged in the same configuration. Air and nitrogen are used as gas phase, water and Therminol as liquid phase, and iron oxide powders, glass beads and sand as solid phase. The small column data are at ambient temperature while those belonging to the large column extend from ambient to 473K. These data are examined to assess the influence of column diameter (scale-up), temperature, slurry concentration and viscosity, and superficial gas velocity on gas holdup for baffled columns.

It is found that in the 0.108 m diameter column the holdup is about the same for axial probes of different diameters, 19 mm, 31.8 mm and 50.8 mm, for two-phase systems involving liquids of small (water) and large (Therminol) viscosities. However, when a seven-tube bundle is installed, the holdup increases. This is consistent with the bubble dynamics and observed bubble sizes. This qualitative trend is also upheld by three-phase systems involving dilute slurries of fine powders. In the larger column when fully packed with a thirty-seven tube bundle, the holdup is found to be the same as for the small column. A loosely packed bundle gives smaller holdup at temperatures greater than the ambient due to the larger size of bubbles. The gas holdup is greater for a less viscous system and this is again due to the larger size of bubbles in a more viscous system. The influence of temperature is pronounced and is very characteristic of the nature of liquid involved. For example for water and Therminol the variations are in opposite directions with change in temperature. Bubble splitting and foaming control the nature of these dependencies. With the addition of solids the holdup is almost insignificantly altered at all temperatures.     

Detection of M. leprae by gene amplification; combined ethidium-bromide staining and probe hybridization

Human and animal studies have clearly demonstrated the advantageous effects of sensorially enriched rearing environments. Nevertheless, little work has been done concerning the long-lasting persistence of all these behavioral modifications. To undertake this question, a very early enrichment animal model was used. From days 10 to 24 after birth, 28 male albino rats were exposed to a multisensory stimulated environment, while other 28 littermates constituted the control group. At 3 and 6 months old two cognitive abilities were analyzed; the spatial working memory (short term memory) and the latent learning capacity (long term memory). The results evidenced an improved working memory in both 3 and 6 months old rats exposed to the early enriched environment. Moreover, the adult early stimulated group performed as well as younger subjects both on error scores and speed to solve this test. Only in the adult group of rats a superior latent learning capacity of stimulated subjects was evidenced. To conclude, the early enriched environment induced: a) persistent cognitive benefits in the adult rat and b) a more relevant influence on the subsequent behavior of older rather than younger subjects.     

Annular crack surrounding an elastic fibre in transversely isotropic elasticity

The axisymmetric problem of an infinitely long transversely isotropic elastic fibre perfectly bonded to a dissimilar transversely isotropic elastic matrix containing an annular crack is considered. The annular crack, surrounding the fibre, is subjected to prescribed longitudinal tension. A potential function approach is used to find the solution of the basic equations. The mixed boundary value problem is reduced to the solution of a singular integral equation, which is further reduced, by using Chebyshev polynomials, to a system of algebraic equations.     

Volume of reaction by the Archibald ultracentrifuge method (lobster hemocyanin)

Samples of lobster hemocyanin (Homarus americanus) under conditions of reversible reaction between whole (25 S) and half (17 S) molecules have been subjected to accurately known nitrogen pressures in analytical ultracentrifuge cells. A modified pressurization chamber of the type developed by Schumaker and colleagues has been constructed for this purpose. The molecular weight was then determined at the top (liquid-gas) meniscus, by means of the Archibald method. The logarithmic dependence upon pressure of the derived equilibrium constant then gave directly the volume of reaction. Experiments were performed in veronal-citrate buffers at pH 8, where the molar volume of formation of whole (dodecameric) molecules from half molecules appears to be negative, and at pH 8.46 in veronal-citrate buffer in the presence of 0.003 molar free calcium ion, where the molar volume of formation was estimated to be + 390 cm3/mole. In glycine-sodium hydroxide buffer at pH 9.6 containing 0.0047 molar free calcium, the molar volume of formation of whole molecules was estimated to be +120 +/- 70 cm3, corresponding to an estimated difference in partial specific volume between whole molecules and half molecules of only 1.3 (10)-4cm3/gram. The correctness of the sign of this value in glycine buffer has been verified by pressure-jump light-scattering experiments.     

Optical technique for the study of convective diffusion from falling drops

A recently developed deflection mapping technique has been exploited for measurement of rate of mass transfer from a liquid drop falling freely in another liquid medium. Results have been found to be in good agreement with theory. Simultaneously, measurement of the diffusion coefficient is also reported.