具有双层桨结构的自吸式搅拌反应器的流体力学性能

在直径0.48 m的搅拌槽中以水?空气为介质,对具有双层桨结构的自吸式反应器的流体力学性能进行了实验研究,考察了自吸式桨浸没深度、底层桨结构和搅拌桨层间距对自吸式桨的临界吸气转速、吸气速率和气含率的影响。结果表明,临界吸气转速随自吸式桨浸没深度增加而增加,临界吸气转速几乎与下层桨的结构无关;吸气速率与气含率随浸没深度增加而减小,吸气速率与气含率受下层桨影响较大,层间距为自吸式桨直径(D)且采用上推式的四叶宽叶翼形轴流式桨作下层桨时,自吸式桨的吸气性能最佳。     

A novel design for a gas-inducing impeller at the lowest critical speed

To disperse the unreacted overhead gas phase into the liquid in an agitated reactor without gas outlet, a gas-inducing impeller is usually employed. To determine the lowest critical rotating speed, the gas-induction mechanism was reconsidered by constituting a mechanical energy conservation equation between the gas inlet orifice in the gas phase and the gas outlet orifice in the liquid phase under a certain rotational speed. According to this model, the critical speed of the gas-inducing impeller could be basically determined by the submersion depth and the radial position of the gas outlet, and a novel design was proposed by introduction of six short pipes stretched radially from the axis of the impeller. The final design of the gas-inducing impeller was obtained by an optimal combination of the gas-inducing pipes, the blades of the impeller and the baffle dimension.     

Performance characteristics of an axial-flow gas induction impeller

Power number, pressure coefficient, and induced gas flow rate of a novel axial-flow gas induction impeller with large openings for gas flow are reported. The effects of rotational speed, impeller diameter, submergence, and pumping mode are considered in turbulent operation. The pressure coefficient is measured using the reduction in pressure at speeds below the minimum induction speed as well as from the minimum induction speed. The minimum induction speed approach consistently yields lower pressure coefficient values, indicating the existence of impeller exit losses that are required to force gas from the centre of the impeller into the liquid. The induced gas flow rate is modelled using two approaches. The first approach relates the gas flow rate to the available pressure difference, and in this case, the relation is found to be strongly affected by impeller diameter. The second approach is a literature model that likens the induction process to water jet injection. While this approach appears promising, flow measurement device pressure losses make it difficult to definitively evaluate this method.     

Critical Rotational Speed for a Floating Particle Suspension in an Aerated Vessel

The critical suspension speeds of floating particles in a gas‐liquid‐floating particle three‐phase system were measured in a multiple‐impeller agitated vessel. Three types of impellers, i.e., simple axial‐flow impeller upflow (SPU) and downflow (SPD), disk turbine (DT) and wing turbine (WT), twelve types of baffles and three kinds of gas spargers were used. The influences of impeller types, baffle configurations, gas spargers, gas superfacial velocity and particle loading on the critical suspension speeds of floating particles were systematically investigated. The optimum regressions of critical suspension speeds were respectively obtained for some better combinations of impellers, bafffles and spargers, such as (a) the 45SPU+45SPD+DT triple impellers, two high‐level baffles and two low‐level baffles (symmetric allocation), gas spargers, (b) the 45SPU+45SPD+DT three‐impeller, standard baffle and small gas sparger. Their errors were smaller than 11 %.     

Interrelation among Liquid Velocity,Impeller Speed and Gas Flow Rate in the Stirred Airlift Reactor

We investigate the liquid circulation velocity in the draft‐tube airlift reactor, with mechanical agitation in the internal column as a riser, at the impeller speed of up to 40 s^–1. An influence of the gas flow rate and the stirrer speed on the riser and downcomer gas hold‐up difference and on the liquid circulation velocity is also investigated. The character of the liquid circulation velocity changes depends on the relation between the gas flow rate and the impeller speed. A monotonic increase of the liquid circulation velocity with an increase of the gas velocity is observed at the impeller speed of lower than 15 s^–1. A distinct decrease of the liquid velocity is found at higher impeller speeds and at low gas velocities. The decrease is larger for higher impeller speeds. We observe the minimum on the curve of the liquid velocity dependence on the gas velocity followed by the monotonic increase of the liquid velocity with an increase of the gas flow velocity. The minimum of the liquid circulation velocity appears if the ratio of the gas flow number to the impeller speed is about 0.0006. The minima are shifted towards the higher gas velocities at higher impeller speeds. An experimental equation for the prediction of the liquid circulation velocity in the stirred airlift reactor is presented.     

Critical impeller speeds for a gas-inducing stirring tank loaded with solid particles

The influence of solid particles size,density and loading on the critical gas-inducing impeller speed was investigated in a gas–liquid–solid stirring tank equipped with a hollow Rushton impeller.Three types of solid particles,hollow glass beads with diameters of 300 μm,200 μm,100 μm,and 60 μm,silica gel and desalting resin,were used.It was found that the adding solid particles would change the critical impeller speed.For hollow glass beads and silica gel,whose relative densities were less than or equal to 1.5,the critical impeller speeds increased with the solid loading before reaching the maximum values,and then decreased to a value even lower than that without added solids.The size of the solids also had apparent influence on the critical impeller speed,and larger solid particles correspond to a smaller critical impeller speed.The experimental data also showed that the gasinducing was beneficial to the suspension of the solid particles.     

γ-CT measurement and CFD simulation of cross section gas holdup distribution in a gas–liquid stirred standard Rushton tank

Cross section gas holdup distributions at 3/4 dimensionless static liquid height in a gas–liquid stirred standard Rushton tank were measured using ¹³⁷Cs γ-CT scan measuring technology at larger gas flow rates and higher impeller rotating speeds. The obtained CT scan images and digital distribution curves of gas holdup with dimensionless radius based on the CT images could explain the fluctuation changes of gas holdup distribution. The dense area of gas holdup distribution appeared in the upper space of impeller blades. Gas holdup increased both with gas flow rate and impeller rotating speed, but gas flow rate had more influence on gas holdup than impeller rotating speed. The Eulerian–Eulerian two-fluid model coupling with the bubbles' coalescence and break-up models, and the drag coefficient model were established to make CFD simulation of gas holdup distributions for the gas–liquid stirred Rushton tank under different gas flow rates and impeller rotating speeds.     

Gassed power dynamics of disc turbine impeller

Measurements have been made of the dynamic power response of a disc turbine impeller, following sudden changes in the input gas flowrate. Transient and frequency response results are presented for a number of impeller speeds. A very satisfactory representation of the smoothed-transient response is obtained using a simple linear first order model. The power change that occurs over the transient period is explained by the process of gas cavity formation described by Warmoeskerken, Feijen and Smith. It is found that the speed of formation and stripping of gas cavities depends on the impeller speed and gassing rate.     

LDA Measurements on a Turbulent Gas/Liquid/Fibre Suspension

Detailed continuous phase flow measurements (mean and RMS velocities) by Laser Doppler Anemometry (LDA) of a turbulent gas/fibre/liquid suspension in a rotary shear tester have been performed with simultaneous torque measurements. The model system comprises a refractive‐index matched suspension in the 4‐12% wt. range with gas contents up to 15% vol. The two transition points, found by Andersson and Rasmuson (2000), were detected also with gas present and appear at higher impeller speeds with increasing gas and fibre concentrations. Plotting RMS and mean velocities versus impeller speed and power input, it is found that both decrease with increasing gas and fibre contents.     

翼形桨在气液固三相搅拌反应釜中的混合性能

<正>机械搅拌槽中高速旋转的叶轮产生的排出流中,径向速度主要用于气体的剪切分散,而其轴向速度是固体悬浮和液相轴向混合的主要动力。三相体系的混合不仅要求叶轮具有较强的径向剪切分散能力,而且要有一定的轴向混合能力,以此达到对气体和固体的同时分散。理想的混合水平不仅要求达到二者在宏观上的均匀,而且在微观混合上也要达到一定的均匀度,为此对搅拌桨的设计提出了较高的要求。在气液二相的混合操作中比较多的采用了盘式涡轮桨,而在液固二相的混合操作中比较多的使用螺旋桨,这都是为了利用各自不同的混合优势。涉及到气液固三相混合,由于气体和固体的分散是一个相互制约的问题,完全的径向     

气液固三相体系搅拌混合研究进展

综述了桨型、挡板及气体分布器等结构因素；转速、气量及颗粒分率等操作因素；颗粒直径及密度等物性因素对气液固三相体系的临界搅拌转速、功耗、气含率的影响。     

MIXING CHARACTERISTICS IN SLURRY STIRRED TANK REACTORS WITH MULTIPLE IMPELLERS

Hydrodynamic parameters such as power consumption, gas holdup, critical impeller speed for solid suspension and mixing time were measured in slurry stirred tank reactors with multiple impellers. The experiments were mainly conducted in a stirred tank of 0.2mi.d. with baffles. It contained two four-pitched blade downflow turbines for gas dispersion and one Pfaudler type impeller for solid suspension. As a part of scaling studies, additional experiments were also carried out in a larger stirred tank reactor (0,8m i.d.) geometrically similar to the smaller one. Glass beads and polymeric particles were used as a solid phase. Solid concentration was in the range of 0-20% (K/K). Tap water and methanol were used as a liquid phase

The power consumption decreased due to an introduction of gas and the presence of solids caused a decrease in the extent of reduction in power consumption. A correlation for power consumption in aerated slurry systems was proposed, It was found that the presence of solids is responsible for a decrease in gas holdup. A new correlation for gas holdup in gas-liquid-solid three-phase stirred tank reactors was developed. It fit the present experimental data reasonably. The critical impeller speed for solid suspensions increased with increasing gas flow rate. However, its increase was rather smaller as compared with the predictions of the correlations available in the literature. We proposed a correlation of the critical impeller speed for solid suspension in the presence of gas. The mixing time complicatedly increased or decreased depending on gas flow rate, impeller speed, solids type and concentration.     

Critical impeller speed for solid suspension in multi-impeller three phase agitated contactors

The critical impeller speed for solid suspension in gas-liquid-solid systems has been measured in multi-impeller agitated contractors of 0.15 and 0.30 m and ID and 1.0 m height. Three types of impellers, i.e. disk turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were used. Air, deionised water and sand particles were used as the gas, liquid and solid phases, respectively. The superficial gas velocity and solid loading were varied in the ranges 0–15 mm/s and 0.5 to 10% w/w, respectively. The effects of impeller type and its diameter, particle size and loading and gas flow rate were studied. Some measurements of gas hold-up and mixing time were also made in order to get some insight of the hydrodynamic behaviour of the reactor. The critical impeller speed for solid suspension in the presence of gas (n_isg) was found to be more than that in the absence of the gas and the increase of critical speed correlated well with the gas flow rate. The influence of particle—liquid parameters on solid suspension speed in the gassed system was similar to but relatively weaker than that in the ungassed condition.     

GAS DISPERSION AND SOLID SUSPENSION IN A THREE-PHASE STIRRED TANK WITH MULTIPLE IMPELLERS

Despite much research on gas-liquid-solid systems and their widespread application in industry, gas dispersion with solid suspension in multistage stirred reactors equipped with multiple impellers has received little attention. We report here the critical just-suspension impeller speed for different concentrations of solid particles, gas holdup, and shaft power in a vessel of 0.48 m diameter with four baffles and dished base. Five agitator configurations, each with three impellers mounted on a single shaft, have been used in the experiments. Two novel impeller designs were used, a deep hollow blade (semi-ellipse) disc turbine (HEDT) and four-wide-blade hydrofoil impellers. The hydrofoils were used in both up-pumping (WH_U) and down-pumping (WH_D) modes. Glass beads of 50 ∼ 150 μm diameter and density 2500 kg · m^-3 were suspended at solid volumetric concentrations of 1.5, 3, 6, 9, and 15%. Results show that these suspended solids have little effect on the relative power demand. Agitators using the HEDT radial dispersing impeller at the bottom have a higher relative power demand (RPD = P_G/P_U) than those with WH_D or WH_U as the lowest one. For all impeller combinations there is little or no effect on gas holdup with increasing solid concentrations. Of the five different impeller combinations, those with an axial flow bottom impeller have significantly higher just-suspension agitation speeds and power consumption, so mounting the hydrofoil impeller at the bottom is not the optimal configuration for particle suspension. Of these impeller combinations, at a given gas flow rate the arrangement of HEDT + 2WH_U has the highest relative power demand, gas holdup, and power input for both the suspension of settling particles and gas dispersion.     

Gas—liquid dispersion and mixing characteristics and heat transfer in a stirred vessel

Properties of gas—liquid dispersion and mixing of seven types of impellers were studied and compared in a stirred vessel with aeration. New correlations for the properties including critical dispersion impeller speed, dispersion regime, power consumption, gas hold-up, discharge flow number and discharge efficiency have been developed. The fluid/wall heat transfer was also studied with several types of dual impeller combinations. There is a critical impeller speed which determines how aeration changes the heat transfer coefficient. Operating conditions influence heat transfer interactively by three factors, which can be expressed by proper dimensionless variables.     

GAS DISPERSION AND SOLID SUSPENSION IN A THREE-PHASE STIRRED TANK WITH MULTIPLE IMPELLERS

Despite much research on gas-liquid-solid systems and their widespread application in industry, gas dispersion with solid suspension in multistage stirred reactors equipped with multiple impellers has received little attention. We report here the critical just-suspension impeller speed for different concentrations of solid particles, gas holdup, and shaft power in a vessel of 0.48 m diameter with four baffles and dished base. Five agitator configurations, each with three impellers mounted on a single shaft, have been used in the experiments. Two novel impeller designs were used, a deep hollow blade (semi-ellipse) disc turbine (HEDT) and four-wide-blade hydrofoil impellers. The hydrofoils were used in both up-pumping (WH_U) and down-pumping (WH_D) modes. Glass beads of 50 ～ 150 μm diameter and density 2500 kg · m^?3 were suspended at solid volumetric concentrations of 1.5, 3, 6, 9, and 15%. Results show that these suspended solids have little effect on the relative power demand. Agitators using the HEDT radial dispersing impeller at the bottom have a higher relative power demand (RPD = P_G/P_U) than those with WH_D or WH_U as the lowest one. For all impeller combinations there is little or no effect on gas holdup with increasing solid concentrations. Of the five different impeller combinations, those with an axial flow bottom impeller have significantly higher just-suspension agitation speeds and power consumption, so mounting the hydrofoil impeller at the bottom is not the optimal configuration for particle suspension. Of these impeller combinations, at a given gas flow rate the arrangement of HEDT + 2WH_U has the highest relative power demand, gas holdup, and power input for both the suspension of settling particles and gas dispersion.     

充气条件下多层桨悬浮颗粒的临界转速

在直径为386 mm 的通气多层桨搅拌釜中,实验考察了下层搅拌桨型、挡板和气体分布器等对颗粒悬浮临界搅拌转速的影响,这对工业过程的设计和放大具有一定的指导意义.     

Suspension of solid particles in multi‐impeller three‐phase stirred tank reactors

Solids suspension characteristics in gas—liquid–solid three‐phase stirred tanks with multi‐impellers were experimentally examined. Minimum impeller speeds for ultimately homogeneous solid suspension have been measured stirred tank reactors. Three impellers were installed: two four‐pitched blade downflow disk turbines and one Pfaudler type impeller chosen to provide good gas dispersion and to accomplish off‐bottom suspension of solid particles, respectively. Gas dispersion causes an increase in particle sedimentation associated with a decrease in power consumption and as a result, minimum impeller speeds for ultimately homogeneous solid suspension increase with increasing gas flow rates. A correlation was developed to predict minimum impeller speeds for ultimately homogeneous solid suspension. The proposed correlation, which agrees satisfactorily with the experimental results, is expected to be useful in design and scale‐up.     

Mixing in multiple-impeller stirred tank reactors with boiling liquids

Mixing in a boil-off mechanically stirred tank reactor with multiple impellers was examined. Power consumption and gas hold-up were measured in boiling water in a 0.2 m i.d. stirred tank reactor with three four-pitched blade downflow disk turbines. Vapour was generated from both the immersed ring heater and the impellers. At low vapour generation rates, vapour was mainly generated from the impellers rather than from the heater, whereas nucleation occurred at the heater instead of the impeller at higher vapour generation rates. The mechanical power consumption decreased due to vapour generation. The change in boiling-to-non-boiling mechanical power ratio with varying impeller rotational speed and boiling rate was complicated and not monotonous except at higher impeller speeds and boiling rates. The gas hold-ups increased with increasing vapour generation rate but were rather small as compared to those in cold gas dispersing systems. Empirical correlations for power consumption and gas hold-up in boiling liquids were developed using the present experimental data.     

Hydrodynamics and volumetric gas-liquid mass transfer coefficient of a stirred vessel equipped with a gas-inducing impeller

Hydrodynamic and mass transfer characteristics of a gas-liquid stirred tank provided with a radial gas-inducing turbine were studied. The effect of the rotation speed and the liquid submergence on global hydrodynamic and mass transfer parameters such as the critical impeller speed, the induced gas flow rate, the gas holdup, the power consumption and the volumetric gas-liquid mass transfer coefficient were investigated. The experiments are mainly conducted with air-water system. In the case of critical impeller speed determination, two liquid viscosities have been used. The volumetric gas-liquid mass transfer coefficient k_La has been obtained by two different techniques. The gas holdup, the induced gas rate and the volumetric gas-liquid mass transfer coefficient are increasing functions with the rotation speed and decreasing ones with the liquid submergence. The effects of these operating parameters on the measured global parameters have been taken into account by introducing the dimensionless modified Froude number and correlations have been proposed for this type of impeller.