矿井水脱盐过程中卷式反渗透膜性能的数值模拟研究

CFD simulations of mass transfer and concentration polarization in a spiral-wound RO element for coal mine water desalination

为了更好地研究矿井水中无机盐组分对于反渗透过程产水、结垢及脱盐效果的影响,以内蒙古某煤矿矿井水水质组分作为进水水质条件,采用计算流体力学(CFD)模拟单支商用标准8寸卷式反渗透膜元件(陶氏BW30-400)内部的传质以及局部浓差极化的分布,预测实际运行情况下微溶盐结垢的风险。进水通道采用以阿基米德螺旋曲线为卷制轨迹的几何模型。无机盐的混盐作用通过混盐渗透压模型模拟。从全尺度卷式反渗透膜元件的模拟结果可以看出,卷式反渗透膜内的水流主要以轴向流速为主,沿切向阿基米德螺旋线的流速较低,对整体盐度分布的影响较小(<1%),可以忽略不计,在后续模拟中采用简化模拟单元或几何模型或网格。在模拟操作条件下,卷式膜元件的浓水网产生的水头损失占整体水头损失约86%,为卷式膜元件中的主要水头损失来源。在没有安装浓水网的进水流道中最高Na_(2)SO_(4)浓度位于元件浓水出口处,高达3594 mg/L,约为有浓水网情况下的1.8倍。而且有浓水网的进水流道内,浓差极化现象主要发生在浓水网背水侧局部区域,影响范围较小。该模型模拟得到的产水量与实测产水量做对比,误差小于5%,同时模拟结果也接近ROSA9.1模拟数据(误差<4.4%),因此可以对卷式反渗透膜的无机盐脱盐过程进行较精确的模拟仿真。与商业设计软件如ROSA(反渗透系统分析)相比,其只提供产水和浓水中的盐浓度信息,本文开发的模型可以提供浓度极化的特征信息,加深了对卷式反渗透膜在不同位置的潜在结垢风险的理解。

The mass transport of mixed solutes in a standard 8-inch spiral wound reverse osmosis(RO)membrane element for coal mine water desalination was simulated by using computational fluid dynamics(CFD).A calibrated Archimedes spiral curve was used to represent the trajectory for the feed flow in the model.Coal mine water was simulated as a liquid with mixed inorganic salts.The interactions between ions were incorporated into the model by using a mixed salts osmotic pressure model.The simulation results showed that the flow velocity along the cross section was less than 1%of the axial flow velocity and,as such,can be neglected.A simplified geometry was used in the subsequent simulations of this work.At the investigated operating conditions,the feed spacer contributed to 86%of the pressure drop,however its presence resulted in a decrease in the concentration polarisation(CP)of Na_(2)SO_(4) from 3594 mg/L to 2036 mg/L where the most significant CP occurred.In addition,with the presence of the feed spacer,CP mainly occurred in the limited areas at the back of the feed spacer.The simulated effluent flowrate was compared with both experimentally measured results and simulated data using ROSA9.1 with less than 5%error obtained for both cases,indicating the high accuracy of this CFD model for simulating the mass transfer of mixed salts in full-scale spiral wound RO elements during filtration processes.Compared to commercial design software such as ROSA(Reverse Osmosis System Analysis)which only provides information on salt concentrations in permeate and brine streamse,the model developed in this work can provide information on concentration polarisation profiles and provide insights into the extent of fouling potential on different locations on a spiral would module.