School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China E-mail:
Water Sci Technol. 2022 Sep;86(5):950-967. doi: 10.2166/wst.2022.241.
The removal of low concentration ammonia-nitrogen in industrial wastewater is necessary before discharged into the environment. In this study, vacuum plate membrane distillation (VPMD) technology was utilized and operating parameters such as pH, feed temperature, vacuum degree, feed flow and time were investigated. Based on the experimental data, the heat and mass transfer mechanism and mathematic model were studied. The experimental results show that low solution pH was significantly beneficial to ammonia-nitrogen removal but permeate flux was nearly changeless. At pH = 4, a removal rate up to 93.33% was achieved. Ammonia-nitrogen mainly exists with NH ions in acidic solution, so only water molecules pass through the membrane to acquire the water product in the permeate side. Increasing the temperature of the solution was disadvantageous to the ammonia-nitrogen removal due to membrane pores expanding and the mass transfer coefficient of NH molecules increasing; therefore a low temperature was chosen if possible. Because vapor pressure of the feed solution increases exponentially with temperature and results in membrane surface pressure difference increases, therefore increasing the temperature enhances the permeate flux. Raising the vacuum degree enhanced ammonia removal rate and permeate flux obviously, a vacuum degree of 0.09 MPa was chosen for the experiment. The effect of feed flow rate on ammonia-nitrogen removal instead of permeate flux is weak, the reason is that the boundary layer wears thin when the feed flow rate is increased, which is conducive to permeate flux increasing. In a two-parameter model of Knudsen diffusion, Poiseuille flow was chosen to demonstrate the heat and mass transfers in the process of VPMD in the study. Based on the experimental values of permeate flux, two parameters C and C in the model were calculated using a nonlinear fitting method software, which indicated that the Knudsen diffusion model more than the Poiseuille flow model was suitable. The maximum values of the relative average deviation (RAD) and root mean square difference (RMSD) of experimental and calculated values with model equations of the permeate flux at the different temperature, vacuum degree and feed flow rate were no more than 8.7% and 3.20 kg · (m · h), respectively.
在工业废水排放到环境中之前，必须去除其中的低浓度氨氮。本研究利用真空平板膜蒸馏（VPMD）技术，考察了 pH 值、进料温度、真空度、进料流量和时间等操作参数。基于实验数据，研究了传热传质机理和数学模型。实验结果表明，低溶液 pH 值有利于氨氮的去除，但渗透通量几乎不变。在 pH = 4 时，去除率高达 93.33%。在酸性溶液中，氨氮主要以 NH 离子形式存在，因此只有水分子通过膜在渗透侧获得水产物。由于膜孔扩张和 NH 分子的传质系数增加，提高溶液温度不利于氨氮的去除；因此，如果可能的话，选择低温。由于进料溶液的蒸气压随温度呈指数增加，导致膜表面压差增加，因此提高温度会增加渗透通量。提高真空度可明显提高氨氮去除率和渗透通量，实验选用真空度为 0.09 MPa。进料流速对氨氮去除率的影响小于对渗透通量的影响，原因是进料流速增加时边界层变薄，有利于渗透通量增加。在 Knudsen 扩散的双参数模型中，选择 Poiseuille 流来表示 VPMD 过程中的传热传质。基于渗透通量的实验值，使用非线性拟合软件计算模型中两个参数 C 和 C，表明 Knudsen 扩散模型比 Poiseuille 流模型更适合。在不同温度、真空度和进料流速下，模型方程计算得到的渗透通量实验值与计算值的最大相对平均偏差（RAD）和均方根差（RMSD）分别不超过 8.7%和 3.20kg·（m·h）。
