Liquid/Solid Mass Transfer in Fixed Beds

The liquid/solid mass transfer behavior of a fixed bed of cylinders was studied using the diffusion‐controlled dissolution of copper in acidified potassium dichromate. Variables studied were solution velocity and cylinder diameter. For a steady flow, the data were correlated for the conditions 25 < Re < 600 and Sc = 990 by the equation Sh = 2.65 Sc^0.33 Re^0.5. A comparison between the present data and previously obtained data for a fixed bed of Raschig rings has revealed that cylinder packing produces higher rates of mass transfer than Raschig rings.     

Investigating Gasification Processes of Solid Fuels by in situ Gaspotentiometric Oxygen Probes

This paper describes a novel analytical method that applies a gaspotentiometric oxygen probe (GOP) for characterizing gasification behavior of solid fuels. On the basis of GOP signals a developed gasification model enables the determination of fuel‐specific properties, like effective reaction rate constant and overall activation energy. For its experimental validation two coal cokes were converted with four different gasification agents in a lab‐scale fluidized‐bed reactor. The results obtained will be discussed and compared with literature data. As this advantageous in situ measurement technique is fast, inexpensive, and easy to handle, it makes the GOP a predestined tool for monitoring and controlling gasification processes.     

Experimental investigation of a 125 kW twin-fire fixed bed gasification pilot plant and comparison to the results of a 2 MW combined heat and power plant (CHP)

Fixed bed biomass gasification is a promising technology to produce heat and power from a renewable energy source. A twin-fire fixed bed gasifier based CHP plant was realized in the year 2003 in Wr. Neustadt, Austria. Wood chips are used as fuel, which are dried and sieved before being gasified to a low calorific gas of about 5.8 MJ/Nm³_dry. Before the clean gas is fed into a gas engine a cyclone and a RME (rapemethylester)/H₂O quench system followed by a wet electrostatic precipitator (ESP) is used for gas cleaning. The CHP plant has a fuel power of 2 MW_th and an electric output of 550 kW_el. As scale up and optimization tool a hot test rig with a capacity of 125 kW_th was built. Basic parameters like the type of wood chips, power and air distribution were varied to investigate the effect on gas composition, tar content in the producer gas and carbon content in the ash. Additionally a temperature profile over the height of the 125 kW hot test rig was measured. Furthermore, the results from the hot test rig are discussed and compared with the results from the 2 MW_th demonstration plant.     

Adiabatic fixed bed gasification of dairy biomass with air and steam

Including dairy biomass (DB) as feedstock in gasification processes for locally based power generation could mitigate the environmental impact from DB produced in large US farms (56 million dry tons per year) and fossil-fuels emissions, since biomass is a CO₂ neutral fuel. The current paper presents experimental results obtained from adiabatic, fixed bed gasification of DB using air and steam as oxidizers. The effect of equivalence ratio (ER) and steam to fuel ratio (S:F) ratio on temperature profile, gas composition (CO, CO₂, H₂, N₂, CH₄, and C₂H₆), gross heating value (HHV) and energy conversion efficiency (ECE) are discussed. The results show that the peak temperature (T_peak), ECE, and CO decrease and H₂ and CO₂ increase with increase in ER; the increase in S:F at same ER increases H₂, CO₂, CH₄, HHV, and ECE, and decreases CO.     

A new experimental Continuous Fixed Bed Reactor to characterise wood char gasification

Two-stage fixed bed gasification is one of the most promising technologies for low and medium energy production from biomass. In industrial processes, control and optimisation is often based on constructor know-how rather than on an understanding of the mechanisms involved. We present a new original tool, the Continuous Fixed Bed Reactor (CFiBR), which was specifically designed and built to enable a fine understanding of the limiting stage of a gasifier: the char bed gasification zone. The reactor, the instrumentation, the operating procedure and set-up tests are described in detail. The potential of the reactor is demonstrated through the characterisation of the gasification of a continuous wood char bed. Temperature profiles and gas concentrations along the 65 cm bed were established and showed that the most reactive zone was the first 10 cm of the char bed. Accurate energy and mass balances provided relevant information regarding the contributions of the main reactions involved in the fixed char bed gasification process.     

‘High Conductivity’ Monolith Catalysts for Gas/Solid Exothermic Reactions

The replacement of conventional packed beds of catalyst pellets with novel ‘high conductivity’ honeycomb catalysts in industrial externally cooled multitubular fixed‐bed reactors for exothermic gas/solid processes offers potential advantages besides reduced pressure drops. Near‐isothermal operation could result in safer reaction operation, better catalyst thermal stability, improved selectivities, higher throughputs and more economical reactors with larger tubes. We report herein an experimental and theoretical investigation of this concept.     

Removal of Cyanides from Water by Air Oxidation in a Cocurrent Downflow Fixed Bed Reactor

A method for removal of cyanides from water is described. The method involves the air oxidation of cyanides in a fixed bed reactor with cocurrent downflow. Effects of parameters such as temperature, concentration, gas and liquid flow rates on the oxidation yield are studied. It was observed that the yield increases by increasing temperature and decreasing gas and liquid flow rates. Altering the concentration had no clear effect on the yield. A yield of 86 % was achieved at high temperature (60°C) while the maximum yield was 68 % at room temperature.     

Axial Dispersion and Wall Effects in Narrow Fixed Bed Reactors: A Comparative Study Based on RTD and NMR Measurements

Axial dispersion and wall effects in narrow fixed beds with aspect ratios < 10 were investigated, both by classical methods and by NMR imaging. The residence time distribution (RTD) in the center and at the wall was measured, system water/NaCl‐solution as tracer, and subsequently compared with radial velocity profiles based on NMR imaging. The influence of the aspect ratio and Re_p on dispersion and on the degree of non‐uniformity of the velocity profile was studied. The NMR results are consistent with the RTD and also with literature data of numerical simulations. For low aspect ratios, dispersion/wall effects have a strong influence on the reactor behavior, above all, in cases where a low effluent concentration is essential, as proven by breakthrough experiments with the reaction of H₂S with ZnO.     

Gasification of high moisture rubber woodchip with rubber waste in a bubbling fluidized bed

This study investigated co-gasification of high moisture rubber woodchip (RW) with shredded rubber waste (SR) as an alternative to pre-drying of woodchip and effective utilisation of wastes. The shredded rubber waste was added into the rubber woodchip having originally 27% moisture content at 10 and 20 wt.% based on the dry weight of rubber woodchip. The mixtures were gasified with air in the 100 kWth bubbling fluidized bed gasifier at various equivalence ratios (ER). Operated at the same ER, the SR addition was found to raise the gasification temperature, e.g. an increase from 700 °C without SR addition by ~ 70°C with 20% SR addition. The heating value of synthesis gas generally increased with increasing % SR addition and with decreasing ER. Due to the resulting higher gasification temperature obtained from gasification with SR addition, it is possible to operate gasification at lower ER compared to the case without SR addition leading to the higher CO/CO₂ production ratio. Both ER and SR addition gave a positive effect on the carbon conversion efficiency; while the gasification efficiency was more influenced by the fuel chemistry. The amount and heating value of fly ash were slightly decreased in all mixtures with increasing ER, while increasing the SR addition ratio resulted in the higher fly ash due to the high ash originally present in the rubber waste. Overall, the performance and products from co-gasification were significantly improved compared to those from gasification using the high moisture woodchip alone and almost as good as that obtained when using low moisture woodchip. This implies the direct usability of high moisture rubber woodchip by adding shredded rubber waste without the need of pre-drying and moreover an improvement of the gasification performance and products.     

Dynamics of Capillary Electrochromatography: Experimental Study of Flow and Transport in Particulate Beds

The chromatographic performance with respect to the flow behavior and dispersion in fixed beds of nonporous and macroporous particles (having mean intraparticle pore diameters of 41, 105, and 232 nm) has been studied in capillary HPLC and electrochromatography. The existence of substantial electroosmotic intraparticle pore flow (perfusive electroosmosis) in columns packed with the macroporous particles was found to reduce stagnant mobile mass transfer resistance and decrease the global flow inhomogeneity over the column cross‐section, leading to a significant improvement in column efficiency compared to capillary HPLC. The effect of electroosmotic perfusion on axial dispersion was shown to be sensitive to the mobile phase ionic strength and mean intraparticle pore diameter, thus, on an electrical double layer interaction within the particles. Complementary and consistent results were observed for the average electroosmotic flow through packed capillaries. It was found to depend on particle porosity and distinct contributions to the electrical double layer behavior within and between particles. Based on these data an optimum chromatographic performance in view of speed and efficiency can be achieved by straightforward adjustment of the electrolyte concentration and characteristic intraparticle pore size.     

催化剂在生物质超临界水汽化制氢中的应用

氢是洁净而高效的能源,并有望在未来能源供求中占有重要位置。用价格低廉的生物质超临界水汽化催化制氢是一种很好的方法。集中讨论了生物质超临界水汽化制氢中催化剂的应用,及各类催化剂的比较。     

生物质气化焦油采样系统的研究进展

介绍了国内外主要生物质气化焦油的采样方法和采样系统,总结了各自的特点,为研究焦油成分提供了参考。     

Gasification of food waste with steam in fluidized bed

Biomass has became an important renewable alternative energy resource. Million tons of food sludge, which is difficult to handle because of its rank smell and water content, is generated in Korea. Thermochemical conversion is one way to convert biomass to energy; it can be divided into carbonization, liquefaction, and gasification. Carbonization of food waste was carried out in a conventional stainless steel autoclave of 2 L capacity at different temperatures. Since gasification produces hydrogen-rich synthesis gas, which can be used for methanol synthesis, gasification of carbonized solid was studied in the fluidized-bed gasifier. The reaction parameters in the gasification of carbonized solids were investigated. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Experimental and Theoretical Studies on Hydrogenation in Multiphase Fixed‐Bed Reactors

Multiphase fixed‐bed reactors have complex hydrodynamic and mass transfer characteristics. The modeling and scale‐up are therefore difficult. The present work focuses on the role of mass transfer on the effective reaction rate. The catalytic 1‐octene hydrogenation was taken as a model reaction. The reaction rate in the trickle‐bed reactor is by a factor of 20 smaller than (theoretically) in the absence of any mass transfer limitations. For high octene concentrations (> 10 %), the effective reaction rate is limited by the H₂ consumption, above all by the gas/liquid and liquid/solid mass transfer. For lower octene concentrations the reaction is zero order with respect to H₂ and only depends on the octene consumption, i.e., on the interplay of chemical reaction, L/S and intraparticle mass transfer of octene.     

Gasification of rice husk in a cyclone gasifier

The experimental results of air gasification of rice husk in the cyclone gasifier were presented at the fuel rate of 20.1 kg/h. With the equivalence ratios varied in the range of 0.21–0.32, the heating value of the producer gas decreases from 6.98 MJ/Nm³ to 3.11 MJ/Nm³ and the cold gas efficiency decreases from 64% to 31%. However, the tar content in the prouder gas decreases with the increase of the equivalence ratio. The rice husk and ash were examined under a scanning electron microscope (SEM) and energy dispersive X-ray (EDX) elemental analysis. The outer surface of the fuel particle which is of scale structure does not change basically during the gasification. The pyrolyzed gas is mainly released from the inner surface of the fuel particle. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Comparison of Mechanistic Models for the Catalytic Oxidation of Trichloroethylene over Cr/Al2O3 and Al‐Cr/Porous Glass Catalysts

In this study the catalytic oxidation of trichloroethylene was investigated in a fixed bed tubular reactor system that consisted of a 50 cm long and 3 cm inner diameter quartz glass tube packed with the catalyst. The Cr/Al₂O₃ catalyst (Cat. I) contains 3.8 w/o Cr and the Al‐Cr/porous glass catalysts (Cat. II) contain in one set 5.5 w/o Al and 9.6 w/o Cr and in the other set 6.2 w/o Al and 11.7 w/o Cr. The two types of catalysts were prepared by impregnation procedure. A number of kinetic rate expressions were evaluated for their ability to fit the experimental data to the integral reactor equation using SimuSolv packet program. The temperature influence on the reaction rate constants and the adsorption equilibrium constants were correlated simultaneously using Arrhenius and van't Hoff equations, respectively. The kinetic rate expression, based on Rideal‐Eley type model, describes well the integral conversion data for Cat. I while Langmuir‐Hinshelwood/Hougen‐Watson type model describes well the integral conversion data for Cat. II over the range of conditions investigated.     

Modeling of a Fixed Bed Reactor and Kinetic Investigations for the Catalytic Reduction of Nitrogen Oxides with Methane

For the catalytic reduction of nitrogen oxides with methane in oxygen rich exhaust gases zeolite catalysts are used. During a catalyst screening the beta-zeolite (BEA) which was modified with Indium has been chosen for kinetic measurements in a fixed bed reactor. Therefore zeolite pellets were prepared. The influence of the residence time, temperature and gas composition at the inlet of the reactor were investigated by experiments and simulations. An isothermic process model was developed which is based on a balance scheme.     

Gasification characteristics of combustible wastes in a 5 ton/day fixed bed gasifier

The gasification characteristics of combustible wastes were determined in a 5 ton/day fixed bed gasifier (1.2 m I.D. and 2.8m high). The fixed bed gasifier consisted of air compressor, oxygen tank, MFC, fixed bed gasifier, cyclone, heat exchanger, solid/gas separator, water fluidized bed reactor and blower. To capture soot or unburned carbon from the gasification reaction, solid/gas separator and water fluidized bed were used. The experiments with 10–50 hours of operation were carried out to determine the effects of bed temperature, solid/oxygen ratio and oxidant on the gas composition, calorific value and carbon conversion. The calorific values of the produced gas decreased with an increase of bed temperature because combustion reaction happened more actively. The gas composition of partial oxidation of woodchip is CO: 34.4%, H₂: 10.7%, CH₄: 6.0%, CO₂: 48.9% and that of RPF is CO: 33.9%, H₂: 26.1%, CH₄: 10.7%, CO₂: 29.2%. The average calorific values of produced gas were about 1,933 kcal/Nm³, 2,863 kcal/Nm³, respectively. The maximum calorific values were 3,100 kcal/Nm³ at RPF/oxygen ratio: 7