往复振动筛板萃取塔萃取机理研究进展

介绍了往复振动筛板萃取塔（RPEC）流体力学性能和传质性能研究现状。     

往复振动筛板塔对低界面张力体系萃取过程的强化

对低界面张力体系正丁醇-丁二酸-水在往复振动筛板塔中的萃取性能进行研究,体系中水为萃取剂,萃取正丁醇中的丁二酸。实验考察了两相流速、相比、传质方向和筛板振动速率对流体力学性能和传质性能的影响,并且与相同操作条件下固定筛板萃取塔的性能作对比。结果表明,筛板振动速率不高于3.5 cm/s的情况下体系没有发生乳化现象,相比增大到2.8时接近液泛点,实验稳定性较差。流速和相比增大能够获得更好的液滴分布和更大的体积传质系数,但增大的幅度要综合考虑设备的最大通量和两相在塔内的停留时间。分散相到连续相的传质方向传质相界面积大,更有利于提高传质效率。相同操作条件下,连续相中的轴向混合远大于分散相的轴向混合。与固定筛板塔的萃取性能相比,振动筛板改善液滴分布、增大处理能力和强化传质的作用都很明显。     

Hydrodynamics of a vibrating perforated plate extraction column

Hydrodynamic behaviour of several liquid-liquid systems has been measured in a 5 cm diameter extraction column agitated by finely perforated plates vibrating at frequencies (f) 10–100 Hz and amplitudes (a) 0.024–1.5 mm. The Sauter mean droplet diameter is generally in the range 0.1 to 1 mm. It has been correlated empirically with af², system properties and operating conditions. Dispersed phase holdup, when plotted against the product af, showed minimum values corresponding to the transition from mixer-settler to emulsion regime. Holdup was also found to be affected by mass transfer and its direction. Entrainment of fine droplets in the continuous phase occurs at high agitation levels, but can be reduced by the addition of coalescence plates at the base of the column. Energy dissipation rates greatly exceed predictions from the quasi-steady-state model.     

往复式振动筛板塔净化湿法磷酸的研究

为了开发湿法磷酸(WPA)净化的新工艺及设备,根据溶剂萃取净化湿法磷酸的物性与振动筛板塔的特性,设计研制了振动筛板塔,并进行溶剂法净化湿法磷酸的设备结构参数与操作条件的实验,找出了萃取、洗涤、反萃3个主要过程的设备结构参数与适宜操作条件,萃取:筛板间距为7 cm,振动频率为200 r/min;洗涤:振动频率为100 r/min,相比(体积)为20∶1;反萃:筛板间距为6 cm,振动频率为250 r/min。经3塔串联连续实验,最终制得符合工业品级质量的磷酸,效果良好。成功开发了WPA净化的新工艺、新设备,为中试及工业化放大创造了有利条件。     

The use of an ionic liquid in a Karr reciprocating plate extraction column

Ionic liquids offer an alternative to volatile organic solvents for extraction of a range of materials. This work examines the mass transfer performance of a Karr reciprocating plate column with an ionic liquid as an alternative solvent for the extraction of phenol from water. The results indicate that ionic liquids can be used in existing solvent extraction equipment and that existing correlations and models can be used to predict mass transfer performance.     

Axial dispersion in a reciprocating plate column

Axial mixing in a reciprocating plate column with different amplitudes has been investigated for single-phase and two-phase (air-water) systems. Experiments were performed in water in a semi-batch scheme (no water flow) in a 1.26 m high and 101.6 mm internal diameter column. It was found that the axial dispersion coefficient is a strong function of reciprocation speed and amplitude. The presence of gas considerably affects the axial dispersion coefficient but its contribution appears to saturate at low gas flow rates and then, over a large range of gas flow rates, the axial dispersion coefficient was almost constant. Models are proposed based on experimental data to account for the effects of gas superficial velocity, amplitude and reciprocating frequency. The models predictions are compared with experimental data, obtained over a wide range of operating conditions, and the agreement between them was found to be good. A new class of nonlinear models, for which it is not necessary to have a constant dependency on each operating variable, was also used to correlate the axial dispersion coefficient. This new model allows to carry out sensitivity analysis on each operating variable.     

Steady state hydrodynamics and mass transfer characteristics of a karr extraction column

The steady state hydrodynamics of the holdup in a 15 cm column agree quite well with mechanistic predictions at conditions approaching flooding. In the presence of mass transfer (acetic acid from the dispersed kerosene phase to the continuous water phase), the holdup data below flooding are not well predicted by the model due to enhanced droplet coalescence. The prediction of holdup at flooding is a function of the ratio of the flow rates but is independent of the physicochemical properties of the extraction system, and therefore can be used to estimate the flooding holdup regardless of the occurrence of mass transfer. In the second part of the paper, steady state mass transfer characteristics of the column were also simulated using a mechanistic model. Despite the difficulty of estimating parameters such as the mass transfer coefficient, the model curves could be arbitrarily adjusted to fit observed exit concentrations. These steady state results provide useful insights on the non-linearity of the system from a control viewpoint.     

Axial mixing and mass transfer in a vibrating perforated plate extraction column

The axial mixing and countercurrent mass transfer characteristics of a 5 cm diameter extraction column agitated by vibrating perforated Teflon plates have been investigated. The dispersed phase was an organic liquid (usually kerosene) and the continuous phase was water. Axial mixing was measured in both phases using pulse tracer techniques; in the continuous phase the axial mixing was estimated to have a significant effect on mass transfer, but axial mixing in the dispersed phase had a negligible effect. Mass transfer was measured for several different solutes; n-butyric acid, benzoic acid and phenol. The overall heights of a transfer unit (cont. phase) were in the order of 10-20 cm for the organic-acids but higher for transfer of phenol from very dilute solutions. The characteristics of the vibrating plate column have been compared with those of other types of extractor and suggestions are made for further development.     

Gas holdup in a gas-liquid-solid reciprocating plate column

The presence of solid particles at lower contents (up to 6% by volume) caused an increase in the gas holdup in the three-phase reciprocating plate column, compared to that in the two phase (gas-liquid) column of the same type, under the same other operating conditions. The decrease in the gas holdup, which was normally observed in the two-phase column, was not found in the three-phase column as the vibration speed was increased at lower levels.     

Gas liquid contacting in reciprocating plate columns: I. hydrodynamics

The hydrodynamic characteristics of a 10-cm reciprocating plate column used for counter-current gas-liquid contacting was investigated. The values of gas holdup, interfacial area and maximum throughputs achievable in this unit put it in a favourable position in comparison with other types of contactors.     

Mass transfer from discrete gas bubbles in a reciprocating plate column

Measurements have been made of the uptake of oxygen in water from a series of discrete bubbles released into a 5 cm diameter reciprocating plate column. The rate of increase of dissolved oxygen in the water has provided data on the average mass transfer coefficients under the following conditions; (a) free rise in the absence of plates, (b) bubbles rising through a static plate assembly and (c) bubbles rising through moderately agitated plates (frequency up to 1.28 Hz). It was found that the mass transfer coefficients were higher under condition (b) than (a), and were further increased under condition (c). The results were compared with earlier data on mass transfer to agitated bubble dispersions.     

Mass transfer characteristics in a gas–liquid reciprocating plate column. II. Interfacial area

This paper is the second part of a continuing study on mass transfer in a reciprocating plate column. The first part dealt with k_La. The bubble size distribution, the Sauter mean diameter and the interfacial area are the subject of this paper. The bubble size increases slightly with gas flow rate and decreases with agitation intensity above a “critical” level. The interfacial area increases with increasing agitation and aeration intensities, while the liquid flow rate and coalescing properties of the liquid have no significant effect. The specific interfacial area is correlated in terms of the superficial gas velocity and the maximum power consumption. The correlations obtained for k_La and a were used to calculate k_L. It was found that k_L depends on the agitation intensity and the bubble size.     

Dispersed phase holdup and bubble size distributions in gas–liquid cocurrent upflow and countercurrent flow in reciprocating plate column

Dispersed phase holdup and the bubble size distribution were measured in a reciprocating plate column under cocurrent upflow and countercurrent flow of gas and liquid phases. The response of the system to a variation in design and operating conditions was found similar to that for liquid–liquid contacting; the magnitude of response, however, differed significantly between them. Taking into consideration the dominant forces encountered in gas–liquid dispersions, the experimental data are satis–factorily correlated in terms of Froude, Weber and Gallileo numbers.     

Axial mixing and scaleup of reciprocating plate columns

Axial mixing measurements in single phase (water) flow have been taken in open-type reciprocating plate columns of diameters 25.4 and 508 mm. In the case of the smaller column, two-phase axial mixing was measured, both in the dispersed phase (water dispersed in n-heptane) and the continuous phase (with n-heptane dispersed in water). Pulse injection of a tracer solution of ammonium chloride and methanol in water was used. Under single phase conditions, the axial dispersion coefficients were found to go through a minimum as the agitation level was increased from zero. The coefficients were nearly an order of magnitude higher in the 508 mm column than in the 25,4 mm column. In two phase flow in the 25,4 mm column, the continuous phase axial dispersion coefficients also went through a minimum as agitation was increased. The dispersed phase axial dispersion coefficients decreased monotonically as agitation was increased from zero. The results of this work and previous data are used in modelling the scale-up of reciprocating plate columns by means of Pratt's simplified technique. The existing empirical scale-up equation is consistent with an assumption that continuous phase mixing increased with column diameter but dispersed phase mixing remains unchanged.     

On the possibility of general prediction of gas hold-up in reciprocating plate columns

Gas hold-up was measured in a 2.54 cm diameter reciprocating plate column for countercurrent flow of air and tap water. The gas hold-up data obtained could not be correlated by the relationship based on slip velocity, modified for circulation effects, as proposed by Rama Rao and coworkers. An analysis using other gas hold-up data available in the literature, obtained in reciprocating plate columns of different sizes and geometries, confirmed that this relationship could not be used for the general prediction of gas hold-up in this type of gas-liquid contactor.     

Hydrodynamics and power consumption of a reciprocating plate gas–liquid column

An experimental program has been initiated to use a reciprocating plate column for aerobic fermentations. This paper reports the results of the initial phase consisting of the hydrodynamic studies and the evaluation of the power consumption as a function of the level of agitation and gas superficial velocity. The variation of the gas holdup is presented and compared to a correlation. It is shown that a simple correlation can predict adequately the gas holdup when water is used. The power consumption, necessary for scale-up and design considerations, was evaluated using the pressure variation at the base of the column, the pressure difference across the column and the force exerted on the central shaft to reciprocate the plate stack. Results for the power consumed by the fluid mixture obtained with the three methods are presented and analyzed. The three methods give approximately the same value of the power consumption.     

Mass transfer characteristics in a gas-liquid reciprocating plate column. I. Liquid phase volumetric mass transfer coefficient

In a series of two papers the results of investigating the mass transfer characteristics of two gas-liquid reciprocating plate columns of the Karr type by different methods are presented. The subject of the first part is a study of the liquid phase volumetric mass transfer coefficient, k_La, while the second part deals with the interfacial area. The volumetric coefficient k_La was investigated using the sulphite method, the pure physical absorption of oxygen, and a dynamic method under culture conditions, the second of these three methods being the most favorable. Very good agreement among these methods was found. Generally, k_La increased with increasing vibration intensity, superficial gas velocity, and the number of perforated plates. Liquid-phase properties appeared to affect k_La only slightly. The coefficient k_La was correlated in terms of the maximum power consumption and the superficial gas velocity:      

Interfacial area and mass transfer in gas-liquid cocurrent upflow and countercurrent flow in reciprocating plate column

The volumetric mass transfer coefficient and the interfacial area were measured for carbon dioxide absorption into water using a reciprocating plate column of plate geometry different from a Karr column. The specific interfacial area was governed by a change in bubble size at low agitation rates and by a variation in gas holdup at high agitation rates. The liquid phase mass transfer coefficient was strongly influenced by the agitation rate, the phase velocities and the plate geometry.     

Characteristics of a countercurrent reciprocating plate bubble column. I. Holdup,pressure drop and bubble diameter

The hydrodynamics of countercurrent air/water flow in a 5 cm diameter reciprocating plate bubble column have been studied; the plates contained 14 mm diameter perforations and had a fractional open area of 0.57. The ranges of superficial velocities of air and water were respectively 0-0.99 cm/s and 0-3.95 cm/s. The stroke was in most cases 4.5 cm and the reciprocation frequency was in the range 0–5 Hz. The pressure drops were measured in the absence of reciprocation for single phase and two phase flow conditions. Pressure fluctuations and time-averaged pressure drops were measured with plate reciprocation under single and two-phase conditions. The results were described in terms of the simple quasi-steady state model; the effective orifice coefficients of the perforations were within the range 0.4 to 0.97 depending on the reciprocation conditions. The Sauter mean diameters of the bubbles decreased with agitation; they were about twice the values predicted from an earlier correlation developed for liquid-liquid systems. The gas holdups were also substantially greater than predicted from correlations based on liquid-liquid systems. Both these effects were explained as due to the tendency for bubbles to cluster in the plate region.