Tiefeng Wang

Application of Doppler ultrasound velocimetry in multiphase flow

Abstract This work aims to extend the application of a commercial Doppler ultrasound velocimetry DOP2000 (Model 2030, signal processing S.A.) to multiphase systems. We firstly make a correction to the determination of Doppler angle and measuring location taking into account the ultrasound refraction and the ultrasound velocity difference in different medias, and then investigate the measurement of the solid concentration in a liquid–solid system and the bubble behavior in a gas–liquid system with low gas holdup. The experimental results show that the attenuation coefficient increases monotonously. A model based on the ultrasound reflection and refraction law is proposed to predict the received echo energy in homogeneous liquid–solid system. Furthermore, the signal response of bubbles is also discussed. By placing the probe in the direction of the flow, the difficulty of determining Doppler angle is avoided and the liquid and bubble rise velocities are simultaneously obtained.

Bubble behavior in gas–liquid–solid three-phase circulating fluidized beds

Abstract A novel fiber optic probe system has been developed for studying the bubble behavior in gas–liquid–solid three-phase circulating fluidized beds (TPCFBs). Mathematical method to analyze data by probe technique has also been discussed in this paper. The bubble size and its distribution, bubble Sauter diameter, gas–liquid interfacial area and bubble rise velocity, have been experimentally studied using the fiber optic probe. The experimental results show that the bubble size distribution in TPCFBs follows lognormal function. The bubble Sauter diameter has the radial profile with smaller value in the central region than in the near-wall region, different from the conventional three-phase fluidized beds (CTPFBs) without outer particle circulation. The distribution of bubble rise velocity, gas–liquid interfacial area and effect of the operation conditions have been experimentally studied.

Optimum design of radial flow moving-bed reactors based on a mathematical hydrodynamic model

An integrated hydrodynamic mathematical model is established from experimental results and a more rigorous definition is proposed for the uniformity of the axial gas distribution in a radial flow moving-bed reactor (RFMBR). A new iteration strategy is provided for the solution of the model. Simulations show the influence of the flow structure, the reactor dimension, the design of the axial gas distributor and the operation conditions on the axial uniformity of the gas distribution. This work provides a theoretical basis and useful method for the optimum design and operation of RFMBRs.

Highly selective catalytic conversion of furfural to γ-butyrolactone

A two-step process was developed for the production of γ-butyrolactone (GBL) from furfural. Furfural was first oxidized using homogeneous acid catalysts under mild conditions in an aqueous/organic bi-phasic system to obtain 2(5H)-furanone, which was then selectively hydrogenated to GBL with a high yield over supported metal catalysts.

EXPERIMENTAL STUDY ON GAS-HOLDUP AND GAS-LIQUID INTERFACIAL AREA IN TPCFBs∗

Abstract A new fiber optic probe system has been developed for measuring the gas-holdup εg and bubble size distribution Ps(db) in multiphase reactors. Experiments have been carried out in a gas-liquid-solid three-phase circulating fluidized bed (TPCFB) with 140mm inner diameter and 3 m height, in which air, tap water and 0.4 mm glass beads are used as the gas, liquid and solid phase, respectively. Based on the measured data, the gas-liquid interracial area a, one or the most important parameters concerning the mass transfer, has been derived. The radial profile of the gas holdup and the gas-liquid interracial area, and the influence or main operating conditions have been studied experimentally in TPCFBs.

Analysis and Measurement of Mass Transfer in Airlift Loop Reactors

Abstract Inter-phase mass transfer is important to the design and performance of airlift loop reactors for either chemical or biochemical applications, and a good measurement technique is crucial for studying mass transfer in multiphase systems. According to the model of macro-scale mass transfer in airlift loop reactors, it was proved that the airlift loop reactor can be regarded as a continuous stirred tank reactor for measuring mass transfer coefficient. The calculated mass transfer coefficient on such a basis is different from the volumetric mass transfer coefficient in the macro-scale model and the difference is discussed. To describe the time delay of the probe response to the change of oxygen concentration in the liquid phase, a model taking into account the time constant of response is established. Sensitivity analysis shows that this model can be used to measure the volumetric mass transfer coefficient. Applying this model to the measurement of volumetric mass transfer coefficient in the loop reactor, results that coincide with the turbulence theory in the literate were obtained.

A Study of Acetylene Production by Methane Flaming in a Partial Oxidation Reactor

Abstract The partial oxidation of hydrocarbons is an important technical route to produce acetylene for chemical industry. The partial oxidation reactor is the key to high acetylene yields. This work is an experimental and numerical study on the use of a methane flame to produce acetylene. A lab scale partial oxidation reactor was used to produce ultra fuel-rich premixed jet flames. The axial temperature and species concentration profiles were measured for different equivalence ratios and preheating temperatures, and these were compared to numerical results from Computational Fluid Dynamics (CFD) simulations that used the Reynolds Averaged Navier-Stokes Probability Density Function (RANS-PDF) approach coupled with detailed chemical mechanisms. The Leeds 1.5, GRI 3.0 and San Diego mechanisms were used to investigate the effect of the detailed chemical mechanisms. The effects of equivalence ratio and preheating temperature on acetylene production were experimentally and numerically studied. The experimental validations indicated that the present numerical simulation provided reliable prediction on the partial oxidation of methane. Using this simulation method the optimal equivalence ratio for acetylene production was determined to be 3.6. Increasing preheating temperature improved acetylene production and shortened greatly the ignition delay time. So the increase of preheating temperature had to be limited to avoid uncontrolled ignition in the mixing chamber and the pyrolysis of methane in the preheater.

LOCAL HYDRODYNAMICS IN AN EXTERNAL LOOP AIRLIFT SLURRY REACTOR WITH AND WITHOUT A RESISTANCE-REGULATING ELEMENT

The hydrodynamic behavior of an external loop airlift slurry reactor (ALSR) with and without a resistance-regulating element was studied with a fiber optic probe and ultrasound Doppler velocimetry. The influences of the superficial gas velocity and solid holdup on the global gas holdup and radial profiles of the suspension circulation velocity in the downer and of gas holdup, bubble size, and bubble rise velocity in the riser were studied. Local measurements allow a better understanding of the flow behavior in the reactor and can be used for CFD modeling and validation. Experimental results show that the resistance-regulating element increases the gas holdup and decreases the suspension circulation velocity, indicating that an optimum design of the flow resistance is needed to obtain the maximum gas-liquid volumetric mass transfer coefficient for a specific superficial gas velocity. A high superficial gas velocity and low solid holdup are favorable for increased uniformity of the radial profile of the gas holdup and bubble rise velocity. Hydrodynamic models that predict the gas holdup and suspension circulation velocity were developed for an ALSR with and without a resistance-regulating element. Good agreement was obtained between measured and predicted values.

An Adsorption Kinetic Model for Sulfur Dioxide Adsorption by ZL50 Activated Carbon

Abstract A semi-empirical adsorption kinetic model was proposed with the time compensation method to describe the chemisorption of SO 2 in flue gas by carbon adsorbents for flue gas purification. The change in adsorption capacity and adsorption rate with time at different water vapor concentrations and different SO 2 concentrations was studied. The model was in good agreement with experimental data. The surface reaction was probably the rate controlling step in the early stage for SO 2 adsorption by ZL50 activated carbon. The parameters m and n in the n th order adsorption kinetic model were related to the magnitude of the time compensation and adsorption driving force, respectively. The change of parameter n with water vapor concentrations and sulfur dioxide concentrations was studied and some physical implications were given. The sum of square errors was less than 1.0 and the average absolute percentage deviations ranged from 0.5 to 3.2. The kinetic model was compared with other models in the literature.

Efficient production of acrylic acid by dehydration of lactic acid over BaSO4 with crystal defects

BaSO4 catalysts with different micromorphologies and crystal texture were prepared and used to investigate the structure–activity relationship in the dehydration reaction of lactic acid (LA) to acrylic acid (AA). SEM and N2 physisorption were used to study the micromorphology. XRD and photoluminescence spectra were employed to analyze the crystal texture of samples prepared with different methods and treatments. The results revealed that BaSO4 with smaller crystals and more defects had higher activity and selectivity to AA. It was likely that the crystal defects provided the active acid sites for dehydration of LA to AA, as evidenced by XPS and NH3-TPD measurements. Using ethanol as the solvent and ultrasound treatment during the preparation of BaSO4, imperfect small crystals with more defects were formed, which increased the AA selectivity to 78.8%.