【作者】 伍灵；

【导师】 王晓琳；

【作者基本信息】 清华大学 ， 化学工程与技术， 2009， 博士

【摘要】 纳滤是介于超滤与反渗透之间的膜分离技术,具有操作压力低、通量高等优点。纳滤分离原理为筛分效应和电荷效应,分离性能具两个特征,一是其截留物质的相对分子量介于反渗透膜和超滤膜之间;二是膜面带电荷,对不同价态和扩散性能的带电离子具有选择透过性。纳滤分离技术可广泛应用于处理地表水及地下水制备饮用水过程。但与反渗透技术相比,纳滤技术缺少具适用性过程模拟软件,在一定程度上限制了纳滤技术在大规模水处理工业中的推广速度。本实验室针对混合无机盐溶液的纳滤分离过程提出一评价模型策略,通过单盐分离性能模拟混合盐分离性能。该模型的不足之处在于涉及体系中不含有弱电解质,且未考虑pH值变化对分离性能的影响。本论文中研究了三种纳滤膜(ESNA 1-LF、ESNA 1-K和LES 90)对单组分及混合铵盐和碳酸氢盐分离性能。实验结果表明,在不对原料液pH值进行调节的条件下可认为铵盐和碳酸氢盐溶液中的氨和碳酸完全解离,此时溶液中主要成分为铵离子与碳酸氢根,在透过纳滤膜时受筛分与电荷效应作用,其表观透过率随透过通量增加而降低,随原料液浓度增加而上升,变化规律与其他强电解质相同。当原料液pH值发生改变时,不同pH值下纳滤膜对铵盐与碳酸氢盐的分离性能与其解离平衡相关,纳滤膜对总氨氮和总无机碳的表观透过率随中性分子所占分配系数的增加而显著上升。中性小分子透过纳滤膜时仅受筛分作用,其表观透过率不随原料液通量及浓度变化,仅受膜孔径影响,且表观透过率数值远大于带电离子。针对纳滤膜分离铵盐和碳酸氢盐,在原有评价模型上进一步演绎,引入弱电解质解离平衡,将其适用范围扩展到含氨和碳酸氢盐混合体系。扩展的评价模型中由原料液pH值分别计算铵盐和碳酸氢盐中带电离子与不带电荷的中性分子所占分布系数,其中带电离子与强电解质离子一同代入加和规则计算,其透过率与中性小分子透过率通过分布系数加和可得总氨氮和总无机碳透过率。将扩展的评价模型用于预测三种纳滤膜对含铵盐和碳酸氢盐的复杂混合溶液中各种溶质的表观透过率,实验结果与预测结果符合良好。更多还原

【Abstract】 Nanofiltration (NF) is defined as the separation progress between Ultrafiltation (UF) and Reverse Osmasis (RO). The separation principle of NF contains sieve effect and charge effect, presenting two typical separation characteristics: one is the intermediate molecular weight cut-offs (MWCO) between RO and UF membranes; the other is the selective permeate of ions of different charge and diffusion property. As a consequence, NF is a promising separation technology for surface and ground water treatment. However, compared with RO, the absence of mature modeling software of NF is to some extent limiting the generalization of large scale application of NF in drinking water production. In the preceding works, a noval model was proposed to evaluate the separation performance of NF for the mixed electrolytes solution based on the observed transmissions of single salts. Further considered, this model still has inadequacies, including the inconsideration of weak electrolyte and pH values.In this work, the separation experiments of three NF membranes (ESNA 1-LF, ESNA 1-K and LES 90) for single and mixed ammonium and bicarbonate salts solutions were carried out. The experimental results indicated that under the original pH value of ammonium and bicarbonate salts solutions, ammonia and carbonic acid dissociated into the form of ammonium and bicarbonate ions almost totally. The rejection progress of ammonium and bicarbonate ions was a combination of sieve and charge effect. As a result the observed transmission of these single salts decreased with flux increasing, while it increased with feed concentration increasing. In conclusion, the rejection properties of ammonium and bicarbonate salts present the same regulation with strong electrolytes. Furthermore, with pH variation, the rejection of ammonium and bicarbonate salts is associated with dynamical equilibrium. The observed transmissions of total ammonia and inorganic carbon were increased obviously with the growth of proportion of uncharged molecular in solutions. The rejection of uncharged molecular by NF membrane was influenced by sieve effect only, and the observed transmission of uncharged molecular through NF membrane is neither influence with flux nor feed concentration, the only influence is membrane pore size. Another important conclusion is that the transbility of uncharged molecular is much higher than ions.Aiming on modeling the rejection of ammonium and bicarbonate salts solutions, the model proposed before is extended by introducing dynamical equilibrium of weak electrolyte to expanding the application of this model. In the extended model, the proportions of charged ions and uncharged moleculars disassociated from ammonium and bicarbonate salts are calculated separately with pH value of feed solution. The permeate performance of charged ions of ammonium and bicarbonate salts is taken into summation model together with strong elelctrolyte ions, and the calculated transmissions of ions are summated to the transmission of uncharged moleculars with their proportions, result in the transmissions of total ammonia and inorganic carbon. In conclusion the transbility of weak electrolyte is directly related with proportions of charged ions and uncharged moleculars, influenced by pH values through dynamic equilibrium. At last the extended model was applied on prediction the observed transmissions of components of complex mixed salts solution containing weak electrolytes such as ammonium and bicarbonate salts. The deviation between model evaluation and experiment results was limited.更多还原