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含有亚微米/微米级气泡的气-液系统具有传质效率高、吸附性能强和自持时间长的特点[1-2],在化学反应、水体增氧、污水净化、生物医药、矿物浮选和流动减阻等领域具有广泛的用途[3-6]。在多相传质与反应的动态体系中,若气泡被破碎成微米尺度时,其尺寸与形状通过肉眼已无法辨别,其数量也会呈几十、几百甚至上千倍增加,单位体积气泡数量高达109~1011 m−3,气液相界面积可达103~105 m2·m−3(通常的鼓泡塔反应器气液相界面积为102 m2·m−3 量级)[7]。气-液或气-液-固反应一般受界面传质控制,而颗粒与界面将直接决定反应过程的传质速率与表观反应速率。准确测试和掌握微界面体系的颗粒动态特征与变化(微气泡或催化剂颗粒直径、个数、粒径分布、形状和流动状态等)对研究微界面状态下气-液或气-液-固体系的界面传质与反应特性有重要作用。尺度减小的效应导致微气泡臭氧传质速率是普通气泡的1.3~1.5 倍,故可以采用微气泡催化臭氧化体系深度处理实际制药废水:一方面臭氧微气泡通过臭氧分解以及微气泡破裂可提高制药废水的降解;另一方面臭氧微气泡类催化效应与催化剂催化效应协同,可显著提高臭氧利用率和有机物矿化率[3]。同时,微气泡催化臭氧化工艺可通过将臭氧转化为强氧化性自由基或通过催化剂对污染物的吸附来提高污染物去除率[5]。微气泡不仅在水体污染治理方面有重要应用,而且在挥发性有机物(VOCs)的治理技术方面也具有潜在应用价值。张静等[4]以易溶性乙酸乙酯气体模拟VOCs 气体,采用微气泡臭氧化技术对模拟高浓度乙酸乙酯气体进行处理,发现微气泡臭氧化提高了臭氧利用效率,同时可强化·OH 氧化反应,提升了臭氧化反应效率。因此,在多相微气泡群中获得气泡粒径的分布及变化规律对微气泡技术的开发具有重要意义。
随着国内外多相流体系的深入研究,针对不同微界面多相体系参数(气泡的尺寸[8-10]、运动速度[11-14]、停留时间[15-18]等)的测量技术的开发应运而生。目前,测量和表征气泡群和液滴群颗粒的粒径及运动情况的技术及方法有许多,包括相位多普勒测量技术(PDA)、图像分析技术、探针法等[19-29]。PDA技术的测量精度取决于气泡和液滴的圆形度,仅适用于粒径较小、圆形度较高的单个颗粒或小气泡,对于大小尺寸不一的复杂气泡群不适用[19-21]。图像分析技术是非接触式方法中的一类,如高速摄像法[8-10]和粒子成像速度仪 (PIV) [22-23],拍摄透明容器中运动的气泡和液滴群,可以获得气泡的瞬时运动轨迹,同时这类方法对气泡运动没有干扰,但容易受液体清晰度和气泡的影响而产生实验误差,仅适用于低气含率和无固体粒子的条件。探针法[24-29]是最常用的方法,优点是简单易行,信号后处理不需要复杂模型支持,时间(秒级)和空间(毫米级)分辨率较好;其缺点是属于接触式测量,探针对流场有干扰,测量结果又容易受气泡速度、方向、接触位置的影响,在测量气泡的大小和速度时需要使用双探针或多探针探头(电导探针和光纤探针),在透明设备中进行。探针法包括电导探针法和光纤探针法。电导探针法[24-26]适用于测试直径为2 ~ 5 mm 的气泡,并且气液两相流中气泡密度比较小的液相导电体系;光纤探针法[27-29]要求液相透明,固体颗粒少,故非透明设备不适用。此外,近年来出现的基于射线辐射强度变化来进行二维成像的X 射线和γ射线层析成像法,所需测试时间较长,空间分辨率为毫米级或更低[18, 30]。而基于CT 技术[14]的多相检测法的时空分辨率更高,但是其传感器置于反应器外部,仪器极其昂贵且只能用于小型实验装置的测量。
基于以上研究,本研究构建一种在线测试系统,将含有亚微米级和微米级气泡与水的气液相流动体系引入到粒度仪测试系统中,在样品测试系统和微气泡发生装置中间设置分流阀和分散剂阀,将测试系统和含微气泡水的流动系统有效串联;考察遮光率、进样距离和测试时间对测试过程的影响;探究不同进气量和水样对气泡粒径分布的影响;实时监测并分析污水处理、水体增氧和气液相化反应的过程中气泡尺寸的变化情况,旨在为微气泡在污水处理、水体增氧和气液相反应等场合的应用提供参考。
在线实时监测微气泡尺寸系统的构建及效果分析
Construction and effect analysis of a test system for online real-time monitoring the microbubble size
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摘要: 为在线监测污水处理、水体增氧和气液相化学反应过程中的气泡粒径的实时变化情况,构建了一种在线测试微气泡群的测试系统,并进行气泡粒径的实时监测;将微细气泡发生装置产生的气泡水引入到粒度分析仪的测试区样品池,形成一个稳定的动态循环流动的系统;通过调节分流阀和分散剂阀同时实现样品的均匀混合和在线取样,再将样品输送至样品池进行快速检测,实现气泡尺寸的在线实时监测。结果表明:当遮光率为10%,从样品槽混合好的样品到样品池的距离为6 cm,测试时间为10 s时,可获得测试微细气泡粒径的最佳条件,精度为±6%;溶气-释气法产生的微细气泡粒径为10~150 μm;随着进气量的增加,气泡的尺寸逐渐增加,其对应的体积比逐渐较小;气泡的粒径还会受到液相水样的影响,当液相为循环水时,气泡粒径呈双峰分布;当液相为新鲜自来水时,气泡粒径呈单峰分布。微气泡尺寸在线测试平台具有具有重复性好、精度高、可控性强、操作简单等特点。本研究结果可为微气泡在污水处理、水体增氧和气液相反应等场合的应用提供参考。Abstract: In order to online monitor the real-time variation of bubble size in sewage treatment, water oxygenation and gas-liquid chemical reaction, a test system for online testing of micro-bubble groups was constructed to realize the real-time monitoring of bubble size. The bubble water produced by the micro-bubble generator was introduced into the sample cell of the particle size analyzer in the test area and a stable dynamic circulating flow system was formed. By adjusting the diverter valve and the dispersant valve, the uniform mixing and online sampling could be realized at the same time. Then the sample was transported to the sample cell for rapid detection which could realize online real-time monitoring of bubble size. The tested results with an accuracy of ±6% showed that the optimal conditions for testing the particle size of micro-bubble were following: the shading rate was 10%, and the distance from the mixed sample in the sample tank to the sample cell was 6 cm, and the test time was 10 s. The particle size of the micro-bubble produced by the dissolved gas-release method was 10~150 μm, which was in good agreement with the literature results. Simultaneously, with the increase of air inlet, the size of the bubble gradually increased, and its corresponding volume ratio gradually decreased. The bubble size was also affected by the liquid water inlet. The bubble size displayed a bimodal distribution with the liquid phase of circulating water, while it displayed a unimodal distribution with the liquid phase of fresh tap water. The microbubble size online test platform presented the characteristics of good repeatability, high precision, strong controllability and simple operation, the results of this study can provide a reference for the application of microbubbles in sewage treatment, water oxygenation and gas-liquid phase reactions.
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表 1 微气泡表征实验的实验条件及估算的气含率
Table 1. Experimental conditions and estimated gas holdup in this work
样品序号 进气量/
(mL·min−1)气含率 /% 微气泡发生器
出口压力/MPa1 40 4.10 0.41 2 60 3.80 0.40 3 80 3.74 0.38 4 100 3.70 0.36 5 120 2.99 0.30 6 140 2.31 0.20 表 2 循环水引入微气泡发生器时粒径的体积占比
Table 2. Volume proportion of bubble size with the liquid phase inlet of circulating water
粒径/μm 微气泡体积占比/% 进气量
40 mL·min−1进气量
60 mL·min−1进气量
80 mL·min−1进气量
100 mL·min−1进气量
120 mL·min−1进气量
140 mL·min−1<10 0 0 0 0 0 0 10 ~ 30 17.01 26.39 15.43 20.03 12.48 8.55 30 ~ 60 40.99 38.82 38.58 41.72 43.24 39.04 60 ~100 6.11 4.53 5.52 5.35 6.72 7.78 100 ~ 120 0.003 0.74 0.003 0.04 0.002 0.005 120 ~ 150 0.59 3.48 0.53 0.97 0.36 0.66 >150 35.30 26.04 39.93 31.90 37.20 43.96 表 3 新鲜自来水引入微气泡发生器时微气泡的体积分布
Table 3. Surface area/volume diameter of microbubble with the liquid phase inlet of fresh tap water
粒径/μm 微气泡体积占比 /% 进气量
40 mL·min−1进气量
60 mL·min−1进气量
80 mL·min−1进气量
100 mL·min−1进气量
120 mL·min−1进气量
140 mL·min−1<10 0.02 0.01 0.001 0.000 06 0 0 10 ~ 30 25.93 41.15 39.72 32.56 16.29 5.05 30 ~ 60 24.40 37.24 41.35 52.50 48.61 36.74 60 ~100 4.18 2.98 3.06 10.13 19.63 23.41 100 ~ 120 0.48 0.002 0.000 05 0.22 2.39 3.93 120 ~ 150 0.62 0.25 0.05 0.03 1.55 2.75 >150 44.37 18.37 15.81 4.47 11.53 28.12 -
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