Carbon materials and catalytic wet air oxidation of organic pollutants in wastewater

The use of carbon materials as catalytic support or direct catalyst in catalytic wet air oxidation (WAO) of organic pollutants is reviewed. The discussion covers important engineering aspects including the characterisation, activity and stability of carbon catalysts, process performance, reaction kinetics and reactor modelling. Recommendations for further research in catalytic WAO are outlined.     

高温抗蚀耐磨涂料在循环流化床锅炉省煤器上的应用

通过对山东恒通化工热电厂的6#锅炉省煤器泄漏进行原因分析,提出了锅炉省煤器抗蚀防磨技术的改造方案,在方案中采用了某高新技术公司研制、生产的KM系列高温抗蚀耐磨涂料,实施后效果良好,此方案在热电厂锅炉省煤器,尤其是在循环流化床锅炉省煤器抗蚀防磨方面,具有推广意义。     

Catalyst pore plugging effects on hydrocracking reactions in an Ebullated bed reactor operation

This paper analyzes the deactivation effects of NiMo/Al₂O₃ catalyst during the operation in an Ebullated bed reactor for Heavy residue hydrocracking. The spent catalysts were characterized by chemical analysis, ¹³C NMR, ESM, DRX, and by using thermal programmed oxidation and diffusion studies in a shallow bed micro-reactor. The deactivations were performed in a 5 l continuously stirred tank reactor, while the spent catalysts were tested in a 0.05 l micro-reactor. The study focused on determining the properties of the external layer of the catalyst and on evaluating the internal coke and metal deposition. The results indicated that initial deactivation is mainly due to coke depositions, while its impact on mass transfer reaction control depends on temperature. In long-term deactivation, the metal deposition plays a more important role in blocking the internal micro- and meso-structures and in building up the external layer of the pellets.     

The influence of geometrical factors and feedstock on gasification in a high temperature fluidised bed

Results are presented for gasification of coal and char by means of air or air-steam mixtures in fluidised bed reactors of three different volumes. Two sizes of coal feedstock particles, 0.5-1.0 mm and 1.0-1.5 mm, and one size of char particles, 0.5-1.5 mm, were used. The calorific value of generated gas and the carbon conversion are presented as a function of particle residence time. For coal gasification higher carbon conversion has been obtained at the same particle residence time than for char gasification. For the steam gasification, a lower gas heating value of about 4 MJ/m³ (S.T.P.) was obtained.     

Modelling of batch fluidised bed drying of pharmaceutical granules

This paper presents a mathematical model based on a three-phase theory, which is used to describe the mass and heat transfer between the gas and solids phases in a batch fluidised bed dryer. In the model, it is assumed that the dilute phase (i.e., bubble) is plug flow while the interstitial gas and the solid particles are considered as being perfectly mixed. The thermal conductivity of wet particles is modelled using a serial and parallel circuit. The moisture diffusion in wet particles was simulated using a numerical finite volume method. Applying a simplified lumped model to a single solid particle, the heat and mass transfer between the interstitial gas and solid phase is taken into account during the whole drying process as three drying rate periods: warming-up, constant rate and falling-rate. The effects of the process parameters, such as particle size, gas velocity, inlet gas temperature and relative humidity, on the moisture content of solids in the bed have been studied by numerical computation using this model. The results are in good agreement with experimental data of heat and mass transfer in fluidised bed dryers. The model will be employed for online simulation of a fluidised bed dryer and for online control.     

Prediction of minimum velocity and minimum bed pressure drop for gas-solid fluidization in conical conduits

Experimental investigations have been carried out for spherical and non-spherical particles using beds comprised of single-sized particles and mixtures in the size and particle density ranges of 439 to 1524 μm and 1303 to 4948 kg/m³, respectively. Five conical fluidizers with varying apex angles of 8.86, 14.77, 19.60, 32.0 and 43.2 degrees were used. Experimental values of minimum velocity and bed pressure drop with air as the fluidizing medium have been compared with their respective values obtained from different models available in the literature. Deviations for each chosen model have been presented.     

Hydrodynamics and steel tube wastage in a fluidized bed at elevated temperature

Hydrodynamic measurements were made in a bubbling fluidized bed operated at 550°C at three different excess gas velocities (0.15, 0.40 and ). The bed has a cross-sectional area of with an immersed tube bank consisting of 59 horizontal stainless steel tubes (AISI 304L), 21 of which are exchangeable, thus allowing erosion studies. Capacitance probe analysis was used to determine the mean bubble rise velocity, the mean bubble frequency, the mean pierced bubble length, the mean bubble volume fraction and the mean visible bubble flow rate. Tube wastage was calculated from roundness profiles obtained by stylus profilometry.A redistribution of the bubble flow towards the center of the bed occurs when the excess gas velocity is increased. Measurements along a target tube, situated next to the capacitance probe, usually show greater material wastage at the central part of the tube, since the mean bubble rise velocity and the mean visible bubble flow rate are higher there. It is suggested that the greater material degradation is also an effect of the through-flow of a particle-transporting gas stream in the bubbles. With increasing height above the distributor plate the circumferential wastage profiles for the lowest excess gas velocity show a gradual change from an erosion pattern with one maximum (Type B behavior) to a pattern with two maxima (Type A behavior). Power spectral density distributions of the fluctuating pressure signals show that this is a result of the formation of larger bubbles, when the fluidization regime is changed in the upper part of the bed. At the highest excess gas velocity the bubble flow becomes more constrained due to a more rapid coalescence of the bubbles and the tubes show Type A wastage profiles throughout the bed.     

Liquid Radial Dispersion in Liquid-solid Circulating Fluidized Beds with Viscous Liquid Medium

Liquid dispersion in the radial direction was investigated in the riser of a viscous liquid-solid fluidized bed 0.102 m in diameter and 3.5 m in height. Pressure fluctuations in the riser were also measured and analyzed to examine the behavior of fluidized particles. Effects of liquid velocity (0.15-0.45 m/s), solid circulation rate (2-8 kg/m²s), particle size (1-3 mm), and liquid viscosity (0.96-38 mPas) on pressure fluctuations and the liquid radial dispersion coefficient were determined. The infinite space model was employed to obtain the radial dispersion coefficient from the radial concentration profiles of the tracer. The pressure fluctuations were analyzed by means of autocorrelation coefficient as well as power spectral density function. The dominant frequency obtained from the autocorrelation coefficient or power spectral density function of pressure fluctuations decreases with increasing liquid viscosity or liquid velocity, but it increases with increasing particle size. The liquid radial dispersion coefficient decreases with increasing liquid velocity or viscosity, but it increases as the solid circulation rate or particle size increases. The liquid radial dispersion coefficient is related closely to the resultant behavior of fluidized particles. The radial dispersion coefficient has been well correlated with operating variables in terms of dimensionless groups.     

Using Electrical Capacitance Tomography to Investigate Gas Solid Contacting

An ECT system has been shown to be useful in observing differences in particle behaviour in a bubbling fluidised bed. An image analysis technique is further described that utilises the construction of solids concentration profiles at three key values, x = 0.2, x = 0.5 and x = 0.8, which have been identified as important solids concentrations in a new Bubble Structure Model. Significant differences in the bubble structure are shown to result when the concentration of fine particles is slightly increased in a bubbling fluidised bed. Changes of this type would seriously alter the gas solid contact efficiency in the fluidised bed, which would significantly influence selectivity of in‐bed catalytic reactions and gas residence time.     

大颗粒尿素流化床造粒机尿液喷射系统故障分析

对严重影响大颗粒尿素流化床造粒机尿液喷射系统正常运行的多项故障进行了剖析,从生产技术改造及操作管理方面提出了改进和防范措施,结果表明,改进后装置运行周期从不足10天延长到40天。