Gas backmixing in the dense region of a circulating fluidized bed

The gas backmixing characteristics in a circulating fluidized bed (0.1 m-IDx5.3-m high) have been determined. The gas backmixing coefficient (D_ba) from the axial dispersion model in a low velocity fluidization region increases with increasing gas velocity. The effect of gas velocity onD _ba in the bubbling bed is more pronounced compared to that in the Circulating Fluidized Bed (CFB). In the dense region of a CFB, the two-phase model is proposed to calculate D_bc from the two-phase model and mass transfer coefficient (k) between the crowd phase and dispersed phase. The gas backmixing coefficient and the mass transfer coefficient between the two phases increase with increasing the ratio of average particle to gas velocities (U_p/U_g).     

填料塔内流体轴向返混系数的确定

阐述了以示踪响应技术为基础确定填料塔内流体轴向返混系数的一种方法,首先对实验得到的示踪剂停留时间分布曲线采用等距法进行异常数据的剔除,再采用五点三次平滑公式进行滤波处理。然后用一维轴向扩散模型对曲线进行拟合,根据扩散模型在一定初始条件和边界条件下的解析解,采用时间域最小二乘法确定了轴向返混系数。结果表明,阐述的方法具有较高的精度。示踪停留时间曲线的实验测定值与其拟合值的均方差在0．034～0．096之间。     

Liquid backmixing in oscillatory flow through a periodically constricted meso-tube

This paper deals with the measurement and modelling of axial liquid dispersion in a 4.5 mm internal diameter tube provided with smooth-periodic constrictions (meso-tube) in steady and oscillatory flow conditions. The residence time distribution (RTD) in the meso-tube was monitored for a range of fluid oscillation frequency (f) and amplitude (x₀) at laminar flow. The RTD response was modelled with three hydrodynamic models: (i) tanks-in-series, (ii) tanks-in-series with backflow and (iii) plug flow with axial dispersion. The steady flow through the meso-tube at flow rates up to 21.30 ml/min resulted in broad RTDs, mainly due to the parabolic velocity profile. The use of fluid oscillations allowed a fine-control of the axial liquid dispersion in the meso-tube due to generation of secondary flow in the regions between the constrictions. The axial dispersion coefficient D was reduced by up to 13-fold in comparison with the steady flow situation. Values of x₀ ≤ 1 mm and f = 10 Hz generally resulted in a maximum reduction in axial dispersion through, therefore maximum improvements in RTD. The tanks-in-series model was generally not capable of predicting RTDs in the meso-tube. The potential of this platform for the continuous, sustainable production of added-value products is herein demonstrated.     

Pulse testing of an agitated vessel

The dynamic study of an agitated insulated vessel was performed using the pulse test method. Adequate pulse characteristics which will disturb a first- and a second-order system in a single pulse test were found by analysing the effects of pulse height and duration, integration step size in computation and the type of approximation used on the accuracy of the results. The theoretical multivariable matrix for the system was found and the best manipulated-controlled variable couples were determined by Bristol's technique.     

Liquid-phase backmixing in bubble columns, structured by introduction of partition plates

Installation of perforated sieve plates into a bubble column has the effect of introducing structure into an otherwise chaotic hydrodynamic behaviour. In this study, we focus on the reduction of backmixing of the liquid phase in compartmentalised bubble columns. Liquid-phase residence time distribution (RTD) measurements were carried out in bubble columns with diameters D_T=0.10, 0.15 and 0.38 m with air–water system operating at superficial gas velocities of U_G=0.05–0.4 m/s. Partition sieve plates with open areas of 18.6 and 30.7% were used in the studies. The measured data on RTD were interpreted in terms of an axial dispersion model extended to allow for liquid interchange between compartments. The interchange velocity was found to be strongly dependent on the free area of the plates but practically independent of the column diameter.     

Local drop size variation in an agitated vessel

The local droplet diameters were measured at two positions inside the circulation region of a stirred tank. The Sauter mean diameter near the top of the vessel was always 1.4 times greater than the one just outside the impeller region for rotational speeds of 250, 300, 350 and 400 rpm and for hold-up fractions of 0.01, 0.025 and 0.05. The drop size distributions at a hold-up fraction 0.01 had one mode for all impeller speeds studied and at both positions. At higher hold-up fractions the distributions usually were bimodal.     

Effects of internals on phase holdup and backmixing in a slightlyexpanded-bed reactor with gas-liquid concurrent upflow

Five different internals were designed, and their effects on phase holdup and backmixing were investigated in a gas-liquid concurrent upflow reactor where the spherical alumina packing particles of three diameters (3.0, 4.5 and 6.0 mm) were slightly expanded under the conditions of varied superficial gas velocities (6.77×10^-2-3.61×10^-1 m·s^-1) and superficial liquid velocities (9.47×10^-4-2.17×10^-3 m·s^-1). The experimental results show that the gas holdup increases with the superficial gas velocity and particle size, opposite to the variational trend of liquid holdup. When an internal component is installed amid the upflow reactor, a higher gas holdup, a less liquid holdup and a larger Peclet number characterizing the weaker backmixing are obtained compared to those in the bed without internals under the same operating conditions. Additionally, the minimal backmixing is observed in the reactor equipped with the internals with a novel multi-step design. Finally, empirical correlations were proposed for estimating gas holdup, liquid holdup and Peclet number with the relative deviations within 11%, 12% and 25%, respectively.     

Transport phenomena under unsteady conditions in an agitated vessel

An experimental study of transport phenomena at the wall of an agitated vessel with a propeller impeller under unsteady conditions is reported. The transient in heat transfer at the wall is caused by a step change in the speed of agitation from an initial static condition. The results are expressed in terms of a rise in modified “j-factor” above the initial free convection value and the number of revolutions taken by the impeller from the start of agitation. Also, an attempt has been made to find an analogy beween heat and momentum transfer. Results are given for the power in terms of Euler number plotted against the impeller Reynolds number. An empirical expression is also given for the transient heat transfer considering the system as a pure first order one with time delay. A “time-factor” constant which gives the number of impeller revolutions for 63.2% of the change from the initial to a final steady state is expressed as a function of impeller Reynolds number (10² to 10⁶) and fluid Prandtl number (6 to 10⁶).     

Mixing studies of non-Newtonian fluids in an anchor agitated vessel

The success of any mixing operation involving liquid–liquid, gas–liquid and gas–liquid–solid systems depends mainly on the geometry of the vessel and impeller, operating conditions and properties of the system. Transformation of laboratory results to commercial scale unit is very difficult due to the complexity of flow phenomena and the scale up is being done by adopting a conservative approach which is based on the geometric, kinematic and dynamic similarities. This approach does not take into account the non-ideal flow behavior of the fluid and the design of commercial unit will be more rational if this information is included in the design of the unit.     

Desorption of carbon dioxide from supersaturated water in an agitated vessel

The rates of desorption of carbon dioxide from supersaturated water solutions into pure carbon dioxide or nitrogen gas stream were measured at 15,25, and 35°C in a baffled agitated vessel with a flat gas-liquid interface operated in a continuous manner. The volumetric liquid-phase mass transfer coefficients for the bubbles generated in the agitated liquid and the enhancement factors of the volumetric liquid-phase mass transfer coefficient for the free liquid surface due to the bubbling were calculated from the measured desorption rates and correlated as functions of the relative supersaturation of the solution and the liquid-phase Reynolds number.     

A coalescence redispersion model for drop-size distributions in an agitated vessel

A population balance model has been developed accounting for coalescence and redispersion of drops in an agitated liquid—liquid dispersion. It is assumed that drop coalescence is followed by immediate redispersion into two drops whose sizes are distributed according to a uniform distribution in the ratio of the daughter drop-volume to the parent drop-volume. If it is assumed that the coalescence frequency is independent of drop-size, the resulting non-linear integral equation admits an exponential solution for the equilibrium drop-volume distribution in a batch vessel. On comparing experimental data obtained from several sources with the prediction of this model, it is found that for flat-blade turbine agitated systems, the agreement is very good; in fact, the sign test ranks the model as better fitting than empirical expressions for drop-sizes proposed by other workers. For circulation stirring systems, e.g. those having Waldhof agitator, the model predictions are substantially improved by assuming that the coalescence frequency is proportional to the diameters of the interacting drops.     

Design of a laboratory experiment on heat transfer in an agitated vessel

A novel teaching laboratory experiment is described, which demonstrates heat transfer under agitation. The experiment involves a simple and inexpensive apparatus with necessary basic components. The laboratory deals with the experimental determination of the heat transfer rates and the overall heat transfer coefficient between steam and water in an agitated vessel. The apparatus can be operated under both static and flow conditions, which affords the student an improved understanding of heat transfer during transient and steady-state modes. Further, student is trained to study the effect of the impeller speed and water flow rate on the rate of heat transfer. By performing the experiment, the student is able to determine the overall heat transfer coefficients experimentally, and compare the results with those obtained from theoretical calculations using correlations available in the literature. In addition, while working in groups, students develop team work and technical writing skills in preparing a comprehensive laboratory report.     

PTA打浆釜搅拌传动装置扩能的计算

对PTA打浆釜的搅拌能力进行分析,校核各传动部件的使用余量,确认其传动部件工作能力可以在目前工作状态下,满足投料量提升1倍的使用要求,从而节约提产改造的费用。     

Liquid flow in impeller region of an unbaffled agitated vessel with an angularly oscillating impeller

The characteristics of a liquid flow were studied in the impeller region of an unbaffled agitated vessel with an angularly oscillating impeller whose unsteady rotation proceeds while periodically reversing its direction at a set angle. The measurement of the velocity of the liquid flow was performed by particle tracking velocimetry (PTV), abreast of that of the torque of the shaft to which the impeller was attached. When a disk turbine impeller with six flat blades was used with variations in operating conditions, such as the frequency and amplitude of impeller angular oscillation, a series of images obtained during one oscillation cycle were analyzed to characterize the internal and discharge streams inside and outside the impeller rotational region. Energy data were inferred on the basis of the circumferential and radial velocities of an internal flow. Results showed that although the total head provided to the liquid by the impeller blades is almost similar, independent of the amplitude of impeller angular oscillation, namely, the acceleration of its movement, the transformation of energy from the pressure head to the velocity head is more efficient at a larger amplitude. In addition, the discharge flow was characterized in terms of volumetric flow rates calculated from the radial and axial velocities. The operation at a smaller amplitude was shown to transform the flow more successfully from the radial direction to the upward and downward axial directions near the vessel wall.     

筒式搅拌器性能试验研究

介绍了获得实用新型专利的筒式搅拌器的结构和工作原理,对筒式搅拌器的搅拌性能进行了试验研究和分析,并与推进式搅拌器和涡轮式搅拌器进行了比较。试验结果表明,3种搅拌器在同等条件下,筒式搅拌器的能耗最小,且其排液量和循环流量最大。     

Macro- and micromixing studies in an unbaffled vessel agitated by a Rushton turbine

Macromixing characteristics, power number and visual observation of the vortex behaviour and micromixing in an unbaffled tank agitated with a Rushton turbine are reported. The latter has also been compared in detail with earlier results from an identical tank containing baffles. The maximum mean specific energy dissipation rate, , in the unbaffled tank that can be utilised without severe air incorporation is compared to with baffles. However, at this lower , the micromixing efficiency is always significantly greater without baffles except when addition is made onto the top of the liquid or into the trailing vortex very close to the impeller. In these latter cases, they are approximately the same but even a small submergence of the feed tube below the liquid surface greatly enhances micromixing in the unbaffled case whilst it is still very poor with baffles. This good micromixing performance of the unbaffled vessel was very unexpected. Furthermore, an established method of estimating the local ε_T gave values of at every feed position where measurement was undertaken. Since the spatially averaged value of φ=1, this result suggests the possibility that the accepted concept of micromixing being totally controlled by the local ε_T at the feed point may not be valid for such swirling flows.     

Relation of mechanical power to gas holdup and mass transfer in an agitated vessel

Two devices were developed, mechanical and electrical, to measure the mechanical power relative to superficial gas velocity and stirring speed in a mechanically agitated reactor. Two bubble regimes were demonstrated. The study of gas holdup, obtained from the residence time distribution, and the bubble size, determined by interfacial area measurement, confirm these two regimes of flow. Our results show that the gas holdup, the interfacial area, and the liquid-side mass transfer coefficient are increasing functions of the energy dissipated in the solution.