基于微吸收器的CO2吸收过程研究进展

微化工技术在流体流动、过程强化、传质与反应过程等领域备受关注,本文归纳整理了3种不同类型的微吸收器（微降膜吸收器、微通道吸收器和微网格吸收器）捕集CO₂过程中的水力学性质和传质过程及其机理研究进展,并对3种微吸收器吸收CO₂过程中存在的问题进行分析总结,同时对微吸收器能快速工业化提出展望。其中重点介绍了微通道泰勒流吸收器的水力学流动特性,包括泰勒流气泡的生成机制、气泡和液弹的长度、气泡的输运和运动速度、气泡截面形状及液膜厚度和气液两相流压降;归纳了微通道泰勒流吸收过程的传质过程机理和传质系数的模型以及不同影响因素（通道截面尺寸,通道长度,主通道结构及入口形状,气、液相组成及其流速,吸收剂和系统压力）作用下CO₂吸收效率和传质系数的研究进展。     

加入第二液相强化气液传质

以水吸收二氧化碳为研究体系,考察了第二液相(有机相)的加入对气液传质的增强作用.运用经验关联式,对气泡平均直径进行了预测,应用渗透模型计算并讨论了第二液相的加入对气液接触界面积、体积传质系数的影响.实验结果表明:不同搅拌速度对气泡大小影响不同,适宜的搅拌速度有利于传质;少量的第二液相的加入减少了气泡聚并现象,增大了气液接触界面积.气液传质增强效果用增强因子表示.     

微通道内气-液传质研究

以CO₂-H₂O为模型体系，实验考察了当量直径为667 μm的单通道和16个并行通道内的气-液传质行为.实验发现，液体表观速度增加，单通道内液侧体积传质系数明显提高;同一液体表观速度下，液侧体积传质系数随气体表观速度增加而增加;在实验数据基础上关联了液侧体积传质系数与气-液两相流参数间的关系.微通道内的液侧体积传质系数较常规尺度气-液接触设备至少高1～2个数量级.并讨论了并行微通道内气-液两相流分配特性对整体传质性能的影响，表明合理设计气、液流动分布结构，可保证微通道内优异的传质特性.     

微通道内气-液弹状流动及传质特性研究进展

气-液弹状流,又称Taylor流,是一种以长气泡和液弹交替形式流动的流动形态。微通道内气-液弹状流因其气泡与液弹尺寸分布均一、停留时间分布窄、径向混合强等优点,是一种适于强化气-液反应的理想流型。本文首先介绍了微通道内气泡的生成机理、气泡和液弹长度,以及气泡生成阶段的传质特征。其次系统综述了主通道中弹状流动及传质过程的研究进展,包括气泡形状与液膜厚度、液弹内循环和泄漏流特征、气-液传质系数的测量与预测,以及物理与化学吸收过程中的传质特性等方面内容。最后阐述了当前研究的不足并展望了气-液弹状流的研究方向。     

微通道内MEA/MDEA混合溶液吸收CO2的传质特性

研究了T形微通道内N-甲基二乙醇胺（MDEA）和单乙醇胺（MEA）混合水溶液吸收CO₂的传质过程。考察了弹状流型下气液两相流量、MEA和MDEA浓度对液侧传质系数k_L和体积传质系数k_La的影响。液侧传质系数和体积传质系数均会随着MEA浓度的升高而升高。与MEA相比,MDEA浓度的提高对传质影响较小。传质系数会随着液体流量的增大而增大,但气体流量的变化对其影响较小。体积传质系数随液体流量的增大而增大,但随气体流量的增大先增大,之后趋于稳定。考虑到化学反应对传质的强化作用,引入了Hatta数,提出了一个新的体积传质系数预测式,预测效果良好。     

壁面润湿性对微通道内二氧化碳-水两相流流动及传质性能的影响

在1 mm×1 mm矩形截面下微通道内,以二氧化碳-水为工作流体,研究壁面润湿性和气液表观流速对气-液两相流型和气液传质的影响,并研究了气、液表观流速对弹状流流体力学性质的影响。在亲水微通道中观测到了泡状流、泡状-弹状流、弹状流;在疏水微通道中观测到了非对称弹状流、拉长的非对称弹状流、分层流。实验表明亲水微通道中弹状流区域下气泡长度大体上随气相表观流速的增大而增大,随液相表观流速的增大而减小;液弹长度大体上随气相表观流速的增大而减小,随液相表观流速的增大先增大后减小;液侧体积传质系数k_La均随气、液相表观流速的增大而增大,随通道壁面润湿性的增强而增大。     

小曲率蛇形微通道弯头处弹状流流动及传质特性的数值研究

采用CLSVOF（coupled level set and volume of fluid）方法,以空气和水为工作流体对小曲率矩形截面蛇形微通道内气液两相流动进行模拟研究。验证模型的合理性后,研究了曲率对弯通道内压降的影响,曲率及气相速度对弹状流气泡及液塞长度的综合影响;同时深入分析了弯管内气液两相流动的传质特性,包括不同曲率下气泡长度的变化,弯管内液侧体积传质系数与液膜体积传质系数的比较,曲率及气相速度对液相体积传质系数的影响。同时,对比了回转弯道与直微通道传质系数的差异,发现弯微通道可以强化传质。     

静止非牛顿流体中气泡生成过程的传质

气泡生成过程中气液传质是气液接触设备的设计、优化的重要参考指标。以二氧化碳气泡在羧甲基纤维素钠(carboxymethyl cellulose,CMC)溶液中生成过程中的传质为研究对象,分别考察了气速、CMC溶液浓度、针头直径对气泡生成过程气液传质的影响,采用具有CCD显微相机的动态接触角分析仪测量了气泡形状、表面积和体积的变化,进而获得气泡生成过程气液传质系数k_l。实验结果表明,CMC浓度从0.2%增加到0.8%,黏度逐渐增大,传质系数随CMC浓度的增大而增大;针头直径从2.5 mm增加到4 mm,传质系数也随之增大;气速从1 ml·min^-1增加到9 ml·min^-1,传质系数也逐渐增大。     

基于传质强化的微气泡塔盘流动特性实验的研究

筛板塔塔内件的作用主要是增大气液比表面积,改变气液接触形式,以提高塔内传质、传热。研究者提出了新型微气泡塔盘,通过在筛板上方的泡沫层内增加破泡组件,利用破泡组件使大气泡破裂为小气泡,增加气液接触面积,提高气液传质、传热效果。利用自行设计的筛板塔和微气泡塔盘实验平台及PIV测速系统,测定了塔板上气泡的直径分布和上升速度。实验结果显示:与普通筛板相比较,微气泡塔盘上气泡的上升速度较小,气泡停留时间更长,且径向分布更平坦;气泡直径也仅为筛板的1/16,气液接触的比表面积增大,更利于传质过程的进行。     

超声微反应器内气液传质过程的介尺度强化机制

采用CFD方法对超声微反应器内的Taylor气液两相流的传质过程进行了模拟。针对传质过程中主要的介尺度结构,包括气泡表面波、空化声流、液相内的局部浓度,分析了其空间分布和时间演化规律。模拟结果有效捕捉了实验难以观测的液膜区域,并将液膜厚度与气泡表面波振动进行了关联,阐释了气液界面附近的空化声流对传质过程的强化作用。根据超声微反应器内Taylor流的传质特点,分别研究了不同流动和超声条件对液弹内和液膜处传质过程的影响,比较了各局部传质对整体传质效率的贡献。通过分析整体/局部Sherwood数与Peclet数间的关系,研究了超声效应对气液传质速率的影响。分析结果从介尺度角度验证了文献关于超声微反应器传质系数的计算,完善了超声微反应器内气液传质过程的强化理论。     

CFD simulations of mass transfer from Taylor bubbles rising in circular capillaries

Computational Fluid Dynamics (CFD) is used to investigate mass transfer from Taylor bubbles to the liquid phase in circular capillaries. The liquid phase volumetric mass transfer coefficient k_La was determined from CFD simulations of Taylor bubbles in upflow, using periodic boundary conditions. The separate influences of the bubble rise velocity, unit cell length, film thickness, film length, and liquid diffusivity on k_La were investigated for capillaries of 1.5, 2 and diameter. The mass transfer from the Taylor bubble is the sum of the contributions of the two bubble caps, and the film surrounding the bubble. The Higbie penetration model is used to describe the mass transfer from the two hemispherical caps. The unsteady-state diffusion model of Pigford is used to describe the mass transfer to the downward flowing liquid film. The developed model for k_La is in good agreement with the CFD simulated values, and provides a practical method for estimating mass transfer coefficients in monolith reactors.     

Hydrogenation of 2‐ethylanthraquinone under Taylor flow in single square channel monolith reactors

The hydrogenation of 2‐ethylanthraquinone (EAQ) to 2‐ethylanthrahydroquinone (EAHQ) was carried out under Taylor flow in single square channel monolith reactors. The two opening ends of opaque reaction channel were connected with two circular transparent quartz‐glass capillaries, where Taylor flow hydrodynamics parameters were measured and further used to obtain practical flow state of reactants in square reaction channels. A carefully designed gas‐liquid inlet mixer was used to supply steady gas bubbles and liquid slugs with desired length. The effects of various operating parameters, involving superficial gas velocity, superficial liquid velocity, gas bubble length, liquid slug length, two‐phase velocity and temperature, on EAQ conversion were systematically researched. Based on EAQ conversion, experimental overall volumetric mass transfer coefficients were calculated, and also studied as functions of various parameters as mentioned earlier. The film model, penetration model, and existing semi‐empirical formula were used to predict gas‐solid, gas‐liquid, and liquid‐solid volumetric mass transfer coefficients in Taylor flow, respectively. The predicted overall volumetric mass transfer coefficients agreed well with the experimental ones. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

CFD simulations of wall mass transfer for Taylor flow in circular capillaries

Computational fluid dynamics is used to investigate the mass transfer from the liquid phase to the channel wall for Taylor flow of bubbles rising in circular capillaries. The separate influences of the Taylor bubble rise velocity, unit cell length, gas holdup, and liquid diffusivity on mass transfer were investigated for capillaries of 1.5, 2 and 3 mm diameter. A correlation is proposed for estimation of the wall mass transfer coefficient and this correlation has been tested against published experimental data.     

An experimental study of air-water Taylor flow and mass transfer inside square microchannels

Flow and mass transfer properties under air-water Taylor flow have been investigated in two square microchannels with hydraulic diameters of 400 and 200 μm. Experimental data on Taylor bubble velocity, pressure drop and liquid side volumetric mass transfer coefficient (k_La) have been presented. It was shown that the measured Taylor bubble velocity in square microchannels could be well interpreted based upon an approximate measurement of the liquid film profile therein. Then, the obtained two-phase frictional pressure drop values in both microchannels were found to be significantly higher than the predictions of the correlation proposed by Kreutzer et al. [2005b. Inertial and interfacial effects on pressure drop of Taylor flow in capillaries. A.I.Ch.E. Journal 51, 2428-2440] when the liquid slug was very short, which can be explained by the inadequacy of their correlation to describe the excess pressure drop caused by the strong inner circulation in such short liquid slugs. An appropriate modification has been made to this correlation in order to improve its applicability in microchannels. Finally, the experimental (k_La) values in the microchannel with hydraulic diameter of 400 μm were found to be in poor agreement with those predicted by the existing correlations proposed for capillaries with diameters of several millimeters. The observed deviation was mainly due to the fact that mass transfer experiments in this microchannel actually corresponded to the case of short film contact time and rather poor mixing between the liquid film and the liquid slug, which was not in accordance with mass transfer assumptions associated with these correlations. A new empirical correlation has been proposed to describe mass transfer data in this microchannel.     

Numerical simulation of the mass transfer process of CO2 absorption by different solutions in a microchannel

The flow and mass transfer characteristics of CO₂ absorption in different liquid phases in a microchannel were studied by numerical simulation. The mixture gas phase contained 5 vol% CO₂ and 95 vol% N₂ , and the different liquid phases were water, ethanol solution, 0.2 M monoethanolamine solution, and 0.2 M NaOH solution, respectively. Based on the permeation theory, the distribution of velocity and concentration in the slug flow was obtained by local simulation of flow and mass transfer coupling and was described in depth. The influence of contact time and bubble velocity on the mass transfer of the whole bubble was highlighted. The volumetric mass transfer coefficient on the bubble cap and liquid film, CO₂ absorption rate, and enhancement factor were calculated and analyzed. The results showed that the volumetric mass transfer coefficients of chemical absorption were ~3 to 10 times that of physical absorption and the CO₂ was absorbed more completely in chemical absorption. The new empirical correlations for predicting the mass transfer coefficient of the liquid phase were proposed respectively in physical absorption and chemical absorption, which were compared with the empirical formulas in the literature. The volumetric mass transfer coefficients obtained by predictive correlations are in good agreement with those obtained by simulation in this paper. This work made a basic prediction for CO₂ absorption in microchannel and provides a foundation for later experimental research.     

CFD studies coupling hydrodynamics and solid‐liquid mass transfer in slug flow for matter removal from tube walls

Organic matter deposition on internal surfaces constitutes a drawback that impairs the efficiency of several industrial processes. To overcome this problem, sparging a train of bubbles could be useful since its presence strongly increases the wall shear stress. A detailed numerical mass‐transfer study between a finite soluble wall and the liquid around a rising Taylor bubble was performed, simultaneously solving velocity and concentration fields. The bubble passage throws solute backward and is responsible for radial dispersion. There is also an increase in the transfer rate with enhancements between 10 and 20% (depending on liquid average velocity and bubble length) compared to single‐phase flow. Mass‐transfer coefficients along the different hydrodynamic regions around the bubble nose, liquid film, and wake were characterized and their values compared with those from literature. The results suggest a promising potential of bubble train flow to enhance organic matter removal from walls in biological systems. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2420–2439, 2017     

垂直管内弹状气泡上升中壁面传递的实验研究

垂直管内弹状流壁面传递是诸多工业应用中需研究的重要问题之一。今用极限扩散电流技术,对弹状气泡上升时瞬时壁面剪应力和传质系数进行了测定,结果显示:当基于表观气速的Froude数FrG < 0.74时,壁面剪应力随弹状气泡和液塞的到来呈现方向相反的交替变化,壁面传质系数亦相应变化;而当FrG > 0.74时,剪应力方向一直向下,说明液膜向下流动,且弹状气泡和液塞的到来对壁面传质系数的影响很小。这说明下落液膜射流穿透了液塞段,控制了整个壁面传递过程。研究还对下落液膜区、尾迹区及液塞段的不同传递特征及机理进行了分析, 并结合气泡塔熔融结晶器中弹状气泡上升时的传热,对结晶操作条件的合理选择进行了讨论。     

Predicting inter-phase mass transfer for idealized Taylor flow: A comparison of numerical frameworks

Four numerical frameworks were derived to investigate the impact of underlying assumptions and numerical complexity on the predicted mass transfer between a Taylor bubble and liquid slug in circular capillaries. The separate influences of bubble velocity and film length, slug length, and bubble film thickness on k_La were compared to empirical and CFD-based predictions from existing literature. Reasonable agreement was obtained using a Slug Film model, which accounted for diffusion-limited mass transfer between the slug film and circulating bulk without the need for an iterative numerical solution. Subsequent investigation of the relative contributions of film and cap mass transport for industrially relevant conditions suggests that both mechanisms need to be accounted for during the prediction of k_La.