摘要
膜气提(Membrane Air Stripping,MAS)法是一种近年发展起来的进行水污染治理的膜分离技术,对治理与修复水体挥发性有机物(Volatile Organic Compounds,VOCs)污染具有较大的优势和广阔的前景。本文以VOCs中典型且难处理的甲基叔丁基醚(Methyl Tert-Butyl Ether,MTBE)污水体系为主要处理对象,从MAS去除MTBE方面进行了系统研究。
为研究各种操作条件对MTBE去除效果的影响,建立了用于MAS实验的装置,进行了一系列的MAS实验。MAS去除MTBE的实验结果表明:料液初始浓度对MTBE的去除率几乎没有影响;系统温度升高、液速和气速增大对去除率具有一定的促进作用;气相压力对去除率的影响很小;溶液中有些共存有机物的存在也会对MAS去除MTBE产生一定的影响。正交实验结果表明气体流速对去除的影响最为显著;用MAS技术去除MTBE,去除率可达到97%左右。因此,将MAS技术用于去除水体中的VOCs是可行且有效的。
文中还对膜气提过程去除水溶液中的VOCs的传质机理进行了理论分析和实验研究。以双膜理论为出发点,建立了包括膜阻在内的MAS传质模型。由理论推导得到基于液相的总传质系数计算公式,对料液初始浓度、气体流速对传质系数的影响进行了实验研究。结果表明,料液初始浓度对传质系数几乎没有影响,MAS技术可用于浓度范围较大的含VOCs的废水处理;随着气体流速的增大,总的液相传质系数是增大的,当气体流速增大到一定程度时,增幅很小。在应用上,存在一个比较经济的气体流速。
在前人对中空纤维膜组件内流体流动情况研究的基础上,探讨了MAS去除VOCs的传质机理,通过对比预测与实验得到的总传质系数值,尝试对Lévésque公式进行了修正,修正后的公式与实验数据能较好得吻合;建立了MAS全过程的传质动力学模型。
Membrane air stripping (MAS) is an innovative membrane sepration technology developed in recent years for removing volatile organic compounds (VOCs) from wastewater. This study mainly focused on the removal of methyl tert-butyl ether (MTBE), which is one of the most typical VOCs, from simulated wastewater by MAS.
The installations for MAS were set up and a series of experiments were carried out. The mainly factors affecting the removal rate such as initial MTBE concentration, temperature, gas velocity, liquid velocity and gas pressure were mostly considered. The results showed that MTBE removal was enhanced by increasing the temperature, gas velocity, liquid velocity and an increase in initial MTBE concentration of the aqueous solution, gas pressure had little effect on the removal efficiency. The MTBE removal efficiency was certainly affected when there is some other organic in the simulated wastewater. The results of the orthogonal experiment indicated that the gas flowrate was the most significant influential factor, and the membrane air stripping was an effective technology to remove MTBE from water as the removal rate can achieve 97%.
The process of mass transfer for VOCs was also investigated and analyzed during MAS. A mass transfer model describing the mass transfer during the MAS between the liquid, the gas and the membrane in the hollow fiber membranes was then established based on the double-film theory. The equations to calculate the overall mass transfer coefficient were derived. The mass transfer coefficients in different operating conditions were measured. The calculating results show that the overall liquid-phase mass transfer coefficient did not change when the initial VOCs concentration had changed; hence MAS had potential for industrial wastewater application for removal/recovery of organics in a wide range of concentrations. The overall liquid-phase mass transfer coefficient increased with gas velocity increased, however when the gas velocity became a certain extent, the increment became gradual. There existed a certain optimum air velocity for applacation.
The mechanism of mass transfer for VOCs was also investigated. Comparing the experimental data with the theoretic data, Lévésque correlation was found to overestimate the local mass transfer coefficient in a cylindrical tube at low velocities. A modification of this correlation had been proposed to predict the local air film mass transfer coefficient. The proposed correlation predictions matched well with the experimental data. A complete mathematical model was proposed for the removal of VOCs from aqueous solutions by MAS.
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