Effects of surface-active agents on mass transfer inside drops

The transfer of iodine from a drop of carbon tetrachloride falling in water has been studied in the presence of two surface-active agents, Teepol and sodium dodecylsulfate.The presence of Teepol is found to decrease the drop's terminal velocity more than that of sodium dodecylsulfate, this effect being more pronounced for larger drops.Small amounts of these surfactants reduce the internal circulation and consequently the mass transfer rates. Efficiencies can even fall below those for rigid spheres when the surfactant concentration is high. The maximum reduction for the individual mass transfer coefficient is about 77% with both surface-active agents.Results are compared with the model of Vermeulen and the model of Calderbank and Korchinski. These results are also expressed using the Galileo number and diffusivity effective factor $\text{R}$.     

Effects of surfactants on mass transfer outside drops

A study was made of the effect of a surface-active agent, on mass transfer of Iodine from an aqueous phase to a falling drop of carbon tetrachloride. The size of drops was varied from 0.2 to 0.4 cm equivalent spherical diameters and the concentrations of Teepol ranged from 0 to 0.5 cm³/l. The experimental falling velocities have been compared with various correlations proposed by several workers. It has been found that the experimental results for the pure or slightly contaminated system (0.05 cm³/l) agree well with Vignes' correlation. In case of mass transfer, a maximum reduction in the mass transfer coefficient of 58% due to the presence of Teepol has been observed. Results have been expressed in dimensionless form using the Galileo and Sherwood numbers.     

Déformation et oscillation des gouttes en présence d'agents tensio-actifs

A study was made of the fall of a single drop of carbon tetrachloride through pure or contaminated water. Results are presented for the variations of the shape of the drop and of the frequency of oscillation with diameter and Teepol concentration. It has been found that presence of surface active agents, increases the deformation of drops but reduces the frequency of oscillations. The comparison between these experimental data and several relations has shown that no previous correlations of either the eccentricity or the frequency of oscillation are satisfactory when the system is contaminated by some surfactants.     

Catalytic steam reforming of biomass-derived tar for hydrogen production with K2CO3/NiO/γ-Al2O3 catalyst

A major problem of using Ni-based catalysts is deactivation during catalytic cracking and reforming, lowering catalytic performance of the catalysts. Modification of catalyst with alkali-loading was expected to help reduce coke formation, which is a cause of the deactivation. This paper investigated the effects of alkali-loading to aluminasupported Ni catalyst on catalytic performance in steam reforming of biomass-derived tar. Rice husk and K₂CO₃ were employed as the biomass feedstock and the alkali, respectively. The catalysts were prepared by a wet impregnation method with γ-Al₂O₃ as a support. A drop-tube fixed bed reactor was used to produce tar from biomass in a pyrolysis zone incorporated with a steam reforming zone. The result indicated that K₂CO₃/NiO/γ-Al₂O₃ is more efficient for steam reforming of tar released from rice husk than NiO/γ-Al₂O₃ in terms of carbon conversion and particularly hydrogen production. Effects of reaction temperature and steam concentration were examined. The optimum temperature was found to be approximately 1,073 K. An increase in steam concentration contributed to more tar reduction. In addition, the K₂CO₃-promoted NiO/γ-Al₂O₃ was found to have superior stability due to lower catalyst deactivation.     

Parametric studies on catalytic pyrolysis of coal-biomass mixture in a circulating fluidized bed

Pyrolysis is an efficient way of thermally converting biomass into fuel gas, liquid product and char. In this research, pyrolysis experiments were carried out in a circulating fluidized bed reactor with a riser diameter of 25 mm and height 1.65 m. The biomass used was corn cobs. The experiments were conducted systematically using two level factorial design with temperature ranging from 650 to 850 degree Celsius, corn cobs and catalyst contents in feed ranging from 0 to 100%, and from 1 to 5 wt%, respectively, and Ni loaded on catalyst ranging from 5 to 9 wt%. The results showed that when temperature and catalyst contents in feed and Ni loaded on catalyst increased, the percent of hydrogen and carbon monoxide increased. The amount of corn cobs was found to have an effect only on the composition of hydrogen. Carbon dioxide was also observed to increase slightly. On the other hand, the percent of methane was considerably decreased. The optimum conditions were 850 degree Celsius, corn cob content in feed of 100%, catalyst content in feed of 5% and Ni loaded on catalyst of 9%. At this condition the percentages of hydrogen and carbon monoxide were 52.0 and 18.0, respectively.     

Experiment and 3D simulation of slugging regime in a circulating fluidized bed

A circulating fluidized bed (CFB) is widely applied in many industries because it has high efficiency. To develop and improve the process, an understanding of the hydrodynamics inside the CFB is very important. Computational fluid dynamics (CFD) represents a powerful tool for helping to understand the phenomena involved in the process. In this study, a CFD model was developed to represent a cold model of the laboratory scale CFB which was designed to study the hydrodynamics of a CFB using commercial CFD software. The Eulerian approach with kinetic theory of granular flow was used for simulating the hydrodynamics inside the system. After proper tuning of relevant parameters, the pressure profile along the equipment from the simulation was well agreed with that from the experiment. The simulation result expresses the hydrodynamic parameters of the slug flow such as solid volume fraction, gas and solid velocities and granular temperature in the riser.     

Co-combustion of coal and biomass in a circulating fluidized bed combustor

In this research, co-combustion of coal and rice husk was studied in a circulating fluidized bed combustor (CFBC). The effects of mixed fuel ratios, primary air and secondary air flow rates on temperature and gas concentration profiles along riser (0.1 m inside diameter and 3.0 m height) were studied. The average particle size of coal from Maetah used in this work was 1,128 mm and bed material was sand. The range of primary air flow rates was 480–920 l/min corresponding to U _g of 1.0–2.0 m/s for coal feed rate at 5.8 kg/h. The recirculation rate through L-valve was 100 kg/hr. It was found that the temperatures along the riser were rather steady at about 800–1,000 degrees Celsius. The introduction of secondary air improved combustion and temperature gradient at the bottom of the riser, particularly at a primary air flow rate below 1.5 m/s. Blending of coal with biomass, rice husk, did improve the combustion efficiency of coal itself even at low concentration of rice husk of 3.5 wt%. In addition, the presence of rice husk in the feed stocks reduced the emission of both NO _x and SO₂.     

Effect of Regeneration Temperature on the Composition and Carbon Dioxide Sorption Ability of a K2CO3/Al2O3 Solid Sorbent in a Bubbling Fluidized Bed Reactor

The effect of the regeneration temperature (150°, 250°, and 350°C) during multiple CO₂ cyclic sorption-regeneration cycles of a K₂CO₃/Al₂O₃ solid sorbent in a bubbling fluidized bed reactor was evaluated in terms of the CO₂ capture capacity and chemical composition of the solid sorbent. The CO₂ capture capacity after regeneration at 150° and 250°C decreased with increasing cycle numbers, reaching approximately 57 and 78%, respectively, and 19.0 and 39.3%, respectively, of the original capacity after one and five regeneration cycles. This decline in the CO₂ capture capacity was due to the accumulation of KHCO₃ (at 150°C) and KAl(CO₃)₂(OH)₂ (150° and 250°C) from their incomplete degradation back to the K₂CO₃/Al₂O₃ solid sorbent. When regenerated at 350°C, the CO₂ capture capacity remained essentially constant in each cycle number because of complete desorption (no residual KHCO₃ and KAl(CO₃)₂(OH)₂). The formation mechanism of complex structure occurred similar to the one in a fixed bed reactor/thermogravimetric analyzer with lower regeneration temperature. The general operation conditions for K₂CO₃/Al₂O₃ solid sorbents are summarized.