反渗透条件下粘性土中溶质运移模型及粘土膜效率系数的试验研究

发布时间：2019-06-17 07:49

【摘要】：传统水文地质学理论是以地下水的水头压力作为地下水流的唯一驱动力来建立地下水水流模型,并将地下水水流模型应用到描述溶质迁移规律的对流-弥散方程中,得到溶质迁移的规律。而对于具有半透膜特性的粘性土弱透水层中溶质迁移问题研究中,需要考虑粘性土半透膜效应对溶质迁移影响,由于弱透水层两侧存有浓度差,弱透水层中水流驱动力不仅只有水头压力,还应包含渗透压,因此,传统水文地质理论是难以适用此情况。本文主要研究内容是:在前人研究基础上,推导了粘性土中溶质迁移的简化模型,并利用简化模型对反渗透系统在给定流速和渗入端定浓度条件的室内试验进行模拟和计算。本文先从非平衡热力学角度,确定溶质的质量通量表达式,在粘性土中选取典型单元体,根据质量守恒原理,推导出溶质迁移模型。对溶质迁移模型的简化分别从化学渗透与反渗透方面进行讨论:化学渗透的室内试验可控制渗透条件,使得渗透系统近似无流状态,可得到粘土中溶质的浓度随时间变化率主要取决于分子扩散项,忽略由流体流速所引起的浓度的变化情况;反渗透的简化模型是基于粘土层对溶剂主要起通道作用所建立,将粘土中流体流速视为与时间有关而与位置无关的函数,使用时要求反渗透系统中越流量远远大于粘土层同时段所"储存"的水量或者粘土层厚度能符合要求。随后,设计了给定流速和渗入端定浓度的反渗透室内试验,利用反渗透的简化模型推导出试验条件下的解析解去计算粘土膜效率系数,并对这种计算方法的可靠性进行分析。结果发现,试验条件下重塑粘土膜效率系数为0.9010～0.9064,表明粘土的膜性能好,但120 d后计算的渗出浓度达到0.0998 mol·L-1,接近于渗入浓度0.1 mol·L-1,说明粘土膜拦截盐的能力随时间下降。此外,在拟稳定状态下的敏感性分析表明,流速假定值在偏离原有值的±10%范围内,对膜效率系数的影响不超过±1.25%,拟稳定时长变化对膜效应系数的影响不超过0.71%,说明计算方法可靠性高。最后,讨论渗入浓度对粘性土的膜效率系数影响。调试膜效率系数值,同时考虑到有效分子扩散系数与膜效率系数的关系,对应调整有效分子扩散系数值,利用有限元数值模拟软件comsol对第二次反渗透试验的渗出浓度数据进行模拟计算,结果发现,当渗入浓度增大时,第二次反渗透试验的膜效率系数值取值比第一次反渗透试验值要小时,计算的结果更吻合试验结果,表明粘性土的膜效率系数是随渗入溶液的浓度增大而减少。
[Abstract]:The traditional hydrogeological theory takes the head pressure of groundwater as the only driving force of groundwater flow to establish the groundwater flow model, and applies the groundwater flow model to the convective dispersion equation describing the solute migration law, and obtains the solute transport law. In the study of solute transport in weak permeable layer of clayey soil with semi-permeable film characteristics, it is necessary to consider the effect of semi-permeable film effect on solute transport. Because of the concentration difference between the two sides of weak permeable layer, the driving force of flow in weak permeable layer is not only water head pressure, but also osmotic pressure. Therefore, the traditional hydrogeological theory is difficult to apply to this situation. The main contents of this paper are as follows: on the basis of previous studies, a simplified model of solute migration in clay is derived, and the laboratory tests of reverse osmosis system at a given velocity and at the end of infiltration are simulated and calculated by using the simplified model. In this paper, the mass flux expression of solute is determined from the point of view of non-equilibrium thermodynamics, and the typical unit body is selected in clay. According to the principle of mass conservation, the solute transport model is derived. The simplification of solute transport model is discussed from the aspects of chemical infiltration and reverse osmosis respectively: the laboratory test of chemical osmosis can control the infiltration conditions, so that the infiltration system can approximate the flow-free state, and it can be obtained that the change rate of solute concentration in clay with time mainly depends on the molecular diffusion term, neglecting the change of concentration caused by fluid velocity; The simplified model of reverse osmosis is based on the channel effect of clay layer on solvent. The fluid velocity in clay is regarded as a time-dependent and position-independent function. When used, the flow rate in reverse osmosis system is much larger than the amount of water stored in clay layer or the thickness of clay layer in the same period. Then, the laboratory test of reverse osmosis with given flow rate and fixed concentration at the end of infiltration is designed, and the analytical solution under the test conditions is derived by using the simplified model of reverse osmosis to calculate the efficiency coefficient of clay membrane, and the reliability of this calculation method is analyzed. The results show that the efficiency coefficient of remolded clay film is 0.9010 鈮，

本文编号：2500818