外压浸没式中空纤维膜清洗过程数值模拟分析

发布时间：2018-06-15 08:54

本文选题：膜污染 + CFD模拟　；参考：《天津工业大学》2017年硕士论文

【摘要】：膜污染限制了中空纤维膜在水处理中的进一步应用,而水力反洗和气体冲洗能够有效地缓解外压浸没式中空纤维膜系统中的膜污染问题。本文通过计算流体力学(CFD)模拟手段来研究反洗过程中膜面污染层的变化情况,以及气体冲洗过程中剪切力对膜污染去除的影响。在模拟的过程中,放置在几何体内的膜组件反映了中空纤维膜的真实尺寸,并且在反应器的底部有一个简化的曝气装置用来模拟气体冲洗过程。此外,膜丝被设置成多孔介质区域而非无滑移壁面。首先,利用CFD数值模拟,研究了水力反洗过程中膜面污染层的变化情况。假设过滤过程结束之后颗粒在膜面形成极薄的垢层,随着反洗过程的进行,以膜面污染颗粒的体积分数变化来揭示膜面污染层的剥落过程。根据模拟运算的结果,用三维图形展示出反洗过程中膜反应器内的速度分布与变化,同时通过颗粒运动的分布特征对膜面污染层的动态剥落过程进行分析。CFD模拟结果表明:水力反洗能够有效去除膜面的污染颗粒,并且随着反洗过程的继续,膜面污染会被进一步清除。在反洗过程中,出现了清洗的不均匀现象,研究结果表明膜丝上端具有最佳的清洗效果,其清洗程度优于膜丝的中段,下端区域清洗效果最差。其次,通过CFD技术对气体冲洗过程中膜反应器内的气-液两相流运动进行了数值模拟,研究选取0.167m/s,0.387m/s,0.512m/s三种不同的气体冲洗速度,对比了反应器内气泡流的运动变化情况。分析膜面经受的剪切力大小,同时结合反洗对膜丝不同区域污染去除效果的优劣,从而确定最优的气体冲洗速度。通过分析最优气体冲洗速度下反应器内的速度和矢量等流体力学性质,得出膜丝的下端和中段部位具有较高的剪切力,能够进一步提高中空纤维中部以及下端的污染去除效果,并可有效阻止膜面滤饼的形成。此外,气体冲洗能够有效地作用于膜面反洗效果较差的区域,如膜丝位置0.25,0.275和0.425m处,进一步去除这些区域的污染。这同样也表明水力反洗后利用气体冲洗能够进一步清洗中空纤维膜,并且减轻膜污染的不均匀分布。最后,通过混凝-膜过滤小试装置的过滤及清洗实验,对CFD模拟结果进行验证。实验对比两种不同的清洗方式(水力反洗、气体冲洗)下的膜比通量(J/J0)及跨膜压差,研究被污染中空纤维膜过滤性能的恢复状况。研究发现相比单纯的水力反洗,反洗结合气体冲洗可更好抑制TMP的增长,且膜比通量的下降也相对缓慢。实验结果表明,反洗结合气体冲洗的清洗方式能更有效的去除膜面污染,在更大程度上恢复膜性能,实验结论与模拟研究中气体冲洗能够进一步去除膜面污染的结论相吻合。
[Abstract]:Membrane fouling limits the further application of hollow fiber membrane in water treatment, while hydraulic backwashing and gas washing can effectively alleviate the membrane fouling in the external pressure immersed hollow fiber membrane system. In this paper, the change of membrane fouling layer during backwash and the effect of shear force on membrane fouling during gas flushing are studied by means of computational fluid dynamics (CFD) simulation. In the simulation process, the membrane assembly placed in the geometry reflects the true size of the hollow fiber membrane, and there is a simplified aeration device at the bottom of the reactor to simulate the gas flushing process. In addition, the filaments are arranged in the porous media area instead of the sliding wall surface. Firstly, CFD numerical simulation was used to study the change of membrane fouling layer during hydraulic backwashing. Assuming that the particles form an extremely thin scale layer on the membrane surface after the filtration process, the spalling process of the fouling layer on the membrane surface is revealed by the change of the volume fraction of the fouled particles on the membrane surface with the process of backwashing. According to the result of simulation operation, the velocity distribution and variation in membrane reactor during backwash process are shown by three-dimensional graphics. At the same time, the dynamic spalling process of fouling layer on membrane surface is analyzed by the distribution characteristics of particle movement. The results of CFD simulation show that hydraulic backwash can effectively remove contaminated particles from membrane surface, and with the continuous backwash process, Membrane fouling will be further removed. The results show that the upper end of the membrane wire has the best cleaning effect, and the cleaning degree is better than the middle part of the membrane wire, and the cleaning effect of the lower end area is the worst. Secondly, the gas-liquid two-phase flow in the membrane reactor was numerically simulated by CFD technique. Three different gas flushing velocities, 0.167 m / s 0.387 m / s and 0.512 m / s, were studied and compared. The shearing force on the membrane surface was analyzed, and the effect of backwash on the removal of fouling in different areas of the membrane was analyzed, so as to determine the optimal gas washing speed. By analyzing the hydrodynamic properties such as velocity and vector in the reactor at optimal gas washing rate, it is concluded that the lower end and middle part of the membrane filament have high shear force, which can further improve the pollution removal effect of the hollow fiber in the middle and lower end. And can effectively prevent the formation of membrane filter cake. In addition, gas flushing can effectively remove the pollution in the areas where the backwash efficiency of the membrane surface is poor, such as the position of the filaments at 0.25m 0.275 and 0.425m. It also shows that the hollow fiber membrane can be further cleaned by gas flushing after hydraulic backwash and the uneven distribution of membrane fouling can be reduced. Finally, the CFD simulation results were verified by the filtration and cleaning experiments of the coagulation-membrane filtration plant. The membrane specific flux of J- / J0 and the pressure difference between the two different cleaning methods (hydraulic backwash and gas flushing) were compared and the recovery of the filtration performance of the fouled hollow fiber membrane was studied. Compared with pure hydraulic backwash, backwash combined with gas flushing can inhibit the growth of TMP and decrease the specific flux of TMP more slowly. The experimental results show that the backwash combined with gas washing can remove membrane surface pollution more effectively and recover membrane performance to a greater extent. The experimental results agree with the conclusion that gas flushing can further remove membrane surface fouling in the simulation study.
【学位授予单位】：天津工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ028.8

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