流化床与沸腾床内部微生物群落演变与分析

用流化床反应器和沸腾床反应器处理焦化废水,对两种反应器运行效能及微生物群落变化进行对比研究。结果表明,提高污泥负荷后流化床COD去除率优于沸腾床反应器。提高进水污泥负荷对沸腾床内微生物的冲击更大,致使其种群丰富程度下降明显。研究结果证明,流化床在宏观去除率、维持菌种丰富度方面具有较大优势。     

Hydrodynamic characteristics of a trickling bed of peat moss used for biofiltration of wastewater

The retention time distribution of liquid in a trickling bed of peat moss was studied. Two different flow regimes have been detected. This is due to the fractal-like structure of peat moss. At low velocities, the liquid flows through the aggregates of peat moss particles. In this case, the tracer is distributed evenly in all the volume of the liquid because of the small characteristic time of diffusion. When the superficial liquid velocity is high, the liquid mainly flows between the solid aggregates and only a small amount of tracer penetrates the liquid inside and between the particles forming the aggregates. The results obtained are important for modelling the process of biofiltration with peat moss, a new system for environmental protection.     

Particle growth kinetics of calcium fluoride in a fluidized bed reactor

Crystallization process in a fluidized bed reactor to remove fluoride from industrial wastewaters has been studied as a suitable alternative to the chemical precipitation in order to decrease the sludge formation as well as to recover fluoride as synthetic calcium fluoride.In the modeling, design and control of a fluidized bed reactor for water treatment it is necessary to study the particle growth kinetics. Removal of fluoride by crystallization process in a fluidized bed reactor using granular calcite as seed material has been carried out in a laboratory-scale fluidized bed reactor in order to study the particle growth kinetics for modeling, design, control and operation purposes.The main variables have been studied, including superficial velocity (SV, ), particle size of the seed material (L₀, m) and supersaturation (S). It has been developed a growth model based on the aggregation and molecular growth mechanisms. The kinetic model and parameters given by the equation fits well the experimental data for the studied range of variables.     

Effect of longitudinal mixing on microbial growth in a multi-stage column bioreactor

The performance of a multi-stage column bioreactor for nonlimited microbial growth was studied. The back-flow model was employed to describe the flow pattern of the microbial suspension. Analytical solutions for calculating the microbial cell concentrations under steady-state and unsteady-state operations, with and without cells in feed, were obtained and the cell growth behavior was examined. Only when microbial cells were present in the feed and the reaction number was less than a critical number was a stable steady state achieved: the microbial cell concentration was then significantly affected by the extent of longitudinal mixing. In the absence of microorganisms in the feed, the steady state was not stable with either washout or nonlimited growth resulting.     

Parametric study for the growth of carbon nanotubes by catalytic chemical vapor deposition in a fluidized bed reactor

Multiwalled carbon nanotubes have been produced from H₂-C₂H₄ mixtures on Fe-SiO₂ catalysts by a fluidized bed catalytic chemical vapor deposition process. Various parameters such as the catalyst preparation, the residence time, the run duration, the temperature, the H₂:C₂H₄ ratio, the amount of metal deposited on the support have been examined. The influence of these parameters on the deposited carbon yield is reported, together with observations of the produced material. This process allows an homogeneously distributed deposition of nanotubes (10-20 nm diameter), that remain anchored to the support.     

Effect of external mass transfer in a packed bed reactor system for a reversible enzyme reaction

A mathematical model was developed to describe the effect of external mass transfer for a packed-bed enzyme reactor in which a reversible, one-substrate, two-intermediate enzyme reaction took place. The model equation was applied to the analysis of an immobilized glucose isomerase reactor system. A Colburn-type mass transfer correlation was obtained from the Colburn j-factor versus Reynolds number plot: i.e., j_D = 0.045N_Re^−0.48. The values of mass transfer coefficient for the system under study ranged from 0.01 to 0.1 cm h⁻¹ depending on the substrate flow rate. Very good agreements were observed between the computer simulation using a plug flow reactor model with the derived mass transfer correlation and the experimental results obtained from the packed-bed reactor operation.     

A mathematical model for the dehydrogenation of methylcyclohexane in a packed bed reactor

A model for the dehydrogenation of methylcyclohexane in a tubular reactor over an industrial catalyst Pt-Sn/Al₂O₃ has been established. This model takes into account the axial dispersion at the inlet of the catalytic bed reactor as well as the heat transfer at the wall of the reactor. The heat transfer at the wall is satisfactorily represented by using a heat transfer coefficient correlation for which the parameters are obtained by fitting to the experimental data. The model provides a good representation of the radial and axial temperature profiles in the packed bed and can be also used to calculate the conversion.     

A method for isolation of soil microbial DNA that is suitable for analysis of microbial cellulase genes

We developed a method for isolation of DNA of cellulolytic microbies from soil, so that DNA sample was free from soil organic matter (SOM). The method consists of three procedures: propagation of cellulolytic microbes, separation of microbial cells from SOM, and DNA extraction using cetyl trimethyl-ammonium bromide (CTAB). Differential centrifugation with Ficoll-PM400 successfully separated the cells from soil particles and SOM. From 1 g of soil, 0.12-0.51 μg of microbial DNA with an average size of 15-20 kbp was isolated. Denaturing Gradient Gel Electrophoresis showed that DNA from a variety of cellulolytic microbes was recovered.     

A mathematical model of a fluidized bed reactor for the continuous production of solvents by immobilized Clostridium acetobutylicum

A fluidized bed reactor has been employed for the continuous production of solvents from whey permeate using cells of Clostridium acetobutylicum immobilized by adsorption onto bonechar. Substrate diffusion equations have been developed for the bioparticles, and a mathematical model has been advanced to describe the operation of the reactor. The model was fitted to the experimental data using the concept that not all of the biomass within the reactor was active in solvent production. On this basis, less than 5% was ‘active’ biomass.     

A model for the rate of catalytic oxidation of methanol in a fixed bed reactor

Oxidation of methanol to formaldehyde, using a mixture of ferric and molybdenum oxides as catalyst, has been studied in a fixed bed integral reactor, 150 mm long, 25 mm o.d. and 10 mm i.d. The catalyst was prepared by reaction between aqueous solutions of ammonium heptamolybdate and ferric chloride. It has been shown that, under experimental conditions similar to those employed in industry, neither external nor internal diffusion had been effective in the process and isothermal conditions prevailed in both gas and solid phases. A two-step mechanism has been put forward for the oxidation of methanol. According to such a mechanism, methanol is first oxidized to formaldehyde, accepting an oxygen molecule from the catalyst and changing the latter into a reduced form. In the second step, the reduced catalyst is transformed into the original form on obtaining an oxygen molecule from the gas phase. Based on this scheme, a rate model has been derived and verified by experimental results.     

Analysis of nitrogenous compounds in the effluent streams from a fluidized bed coal gasification reactor

A Texas lignite and a New Mexico subbituminous coal were gasified with steam and oxygen in a pilot-scale fluidized bed reactor at pressures from 770 kPa to 830 kPa, and temperatures form 795°C to 980°C. The make gas passed through a cyclone separator, and then a venturi scrubber in which condensable and water-soluble compounds were removed. The gasifier effluents (spent char, cyclone fines, tar, wastewater, and dry make gas) were analyzed for nitrogenous compounds.

For both coals, 6–12% of the nitrogen in the feed was retained in the spent char, with greater quantities being retained in the subbituminous coal char. Of the nitrogen volatilized from both coals, roughly 5% appeared in the tar, less than 0.2% appeared in the dry make gas as ammonia and NO_x, and the balance appeared in the wastewater as ammonia (60%), hydrolyzable nitrogenous compounds and possibly cyanate (10–15%), thiocyanate (1%), cyanide (0.5%), and other compounds (3–10%). The average concentration of NO_x in the dry gas was 7 ppm for lignite. No NO_x data for subbituminous coal were obtained. Reactor conditions (temperature, pressure, steam-to-carbon feed ratio) had no measurable effect on the production rates of nitrogenous compounds over the range of conditions investigated.     

A fluidized bed membrane reactor for the steam reforming of methane

In the present investigation a realistic two-phase model accounting for the change in the total number of moles accompanying the reaction is utilized to explore a novel reactor configuration suggested for the methane steam reforming process. The suggested design is basically a fluidized bed reactor equipped with a bundle of membrane tubes. These tubes remove the main product, hydrogen, from the reacting gas mixture and drive the reaction beyond its thermodynamic equilibrium. The proposed novel design is also equipped with sodium heat pipes which act as a thermal flux transformer to provide the large amount of heat needed by the endothermic reaction through a relatively small heat transfer surface, assuring better reactor compactness. Two options for fluid routing through the membrane tubes are proposed; each is suitable for a certain industrial application. The performance of this novel configuration is compared with that of an industrial fixed bed steam reformer and the comparison shows the potential advantages of the suggested configuration.     

A model for the rate of hydrodesulfurization of thiophene in a continuous catalytic fixed bed reactor

Hydrodesulfurization of thiophene in n-heptane, using a mixture of cobalt and molybdenum oxides as catalyst, has been studied in a fixed bed integral reactor, 200 mm long, 30 mm outer diameter and 20 mm inner diameter. It has been shown that, under experimental conditions employed in this study, neither external nor internal diffusion had been effective in the process and isothermal conditions prevailed in both gas and solid phases. A three step mechanism has been proposed for the hydrodesulfurization of thiophene. According to such a mechanism the diluent (n-heptane) acts as an inhibitor by occupying free active sites. In further steps, thiophene is adsorbed on the catalyst surface and then undergoes reaction with hydrogen gas. Based on this scheme a rate model has been derived and verified by experimental results.     

Effectiveness factor of a catalyst pellet in a trickle bed reactor: Limiting reactant in the gas phase

The influence of both internal and external wettings on the effectiveness factor of a partially wetted catalyst pellet in a trickle-bed reactor when the limiting reactant is in the gas phase is analyzed.A new parameter, not considered up to now, measuring the gas-liquid interfacial area inside and on the pellet surface, is introduced to study different forms of wetting.The reaction in the dry zone is also taken into account and its influence on the effectiveness factor for both the pores mouth and pores end dry is studied.An approximate analytical expression for the effectiveness factor, giving close results to the numerical solution, is proposed.     

Multiple reactor testing of a synthetic sorbent for regenerative sulfur capture in fluidized bed combustion of coal

A new synthetic regenerable sorbent was developed as an alternative to limestone and dolomite for regenerative sulfur capture in fluidized bed combustion (FBC of coal. The sorbent consists of CaO (8-9 wt%) on a spherical 3 mm diameter γ-Al₂O₃ support. A shrinking unreacted core model (SURE2) is proposed to describe the sulfation process. Experiments have been carried out in a 12 mm internal diameter fixed bed reactor, a 0.05 m ID fluidized bed reactor and a thermobalance. The results obtained for all three reactors are in good agreement with the predictions of the SURE2 model.     

A comparative study of a fluidised bed reactor and a gas lift loop reactor for the IBE process. Part II: Hydrodynamics and reactor modelling

A fluidised bed reactor with liquid recycle (FBR) and an external loop gas lift reactor (GLR) were designed for the production of isopropanol—butanol mixtures by immobilised Clostridium spp. and scaled down to laboratory scale (part I). Hydrodynamic models were set up for the two laboratory scale reactors. Liquid mixing in the 10 dm³ FBR was described by 10 tanks in series. Fluidisation velocities, bed expansions and axial dispersion coefficients agreed well with literature data. Liquid mixing in the 15 dm³ GLR was described by 100 tanks in series. The gas hold-up and circulation velocity were found to decrease with increasing hold-up of solids, in accordance with literature indications. No influence of the hold-up of solids on the axial dispersion coefficient was determined. An integrated reactor model was set up for both reactors, using the hydrodynamic and kinetic model. Actual fermentation data are presented and compared with model predictions in part III of this study; this part will also include a comparison of reactor performances and scale up aspects.     

Layered double hydroxides as catalysts for the efficient growth of high quality single-walled carbon nanotubes in a fluidized bed reactor

A family of layered double hydroxides (LDHs), such as Fe/Mg/Al, Co/Mg/Al, and Ni/Mg/Al LDHs, were used as catalysts for the efficient growth of single-walled carbon nanotubes (SWCNTs) in a fluidized bed reactor. The LDH flakes were agglomerated into clusters with sizes ranging from 50 to 200 μm, and they can be easily fluidized with a gas velocity ranging from 2.3 to 24 cm/s. After calcination and reduction, small metal catalyst particles formed and distributed uniformly on the flakes. At the reaction temperature, the introduction of methane realized the growth of SWCNTs with the diameter of 1-4 nm. The loose structure of LDH agglomerates afforded a yield as high as 0.95 g_CNT/(g_cat h) of SWCNTs with a surface area of 930 m²/g. Compared with Fe/Mg/Al LDH, Ni/Mg/Al and Co/Mg/Al LDHs showed a better selectivity to SWCNTs. The highest selectivity for metallic SWCNTs was obtained using Co/Mg/AI LDHs as the catalyst.     

A comparative study of residence time distribution and selectivity in a monolith CDC reactor and a trickle bed reactor

A comparative study of the performance of a trickle bed reactor (TBR) and a monolith cocurrent downflow contactor (CDC) reactor in terms of selectivity and residence time distribution was conducted for the hydrogenation of 2-butyne-1,4-diol (B). Selectivity (S) towards 2-butene-1,4-diol was investigated with the solvent 2-propanol and a 30% (v/v) 2-propanol/water mixture (M) in batch recycle mode. Liquid residence time distribution (RTD) curves were obtained for both reactors. Although both reactors presented almost identical hydrodynamic behaviour, i.e. RTD, significant differences regarding selectivity towards the alkene were observed in both solvents. The use of 2-propanol gave lower selectivities in both reactors, but even then the monolith reactor was superior. In the monolith CDC, the liquid RTD curve was also obtained at different radial positions. RTD profiles across the monolith showed that from the centre to the column wall there is possibly an increased retention of material and despite this, overall selectivity does not appear to be considerably depressed by the backmixing that the above result implies in 2-propanol/water where the selectivity was found to be 100% towards the intermediate (C).

Modelling of the monolith CDC reactor was also conducted to predict RTD. The models tested were tanks-in-series, piston exchange and piston dispersion exchange; from which, piston exchange model was found to best predict and fit the experimental data.     

活性污泥的主要微生物菌群及研究方法

活性污泥是活性污泥法生物处理污水系统的功能主体.人类对活性污泥微生物菌群的认识随着其研究方法的发展而逐步深入.对活性污泥处理污水过程中的主要微生物菌群及其特性进行了全面描述,并且简要介绍了菌群的研究方法,对研究趋势作了展望.新的研究方法有待进一步开发和利用.