【作者】 游文婷；

【导师】 许振良；

【作者基本信息】 华东理工大学 ， 环境科学与工程， 2014， 博士

【摘要】 目前,高含铵盐废水是许多化工行业较难处理的废水之一。本文采用间歇式真空膜蒸馏(VMD)—结晶耦合过程对高含铵盐溶液进行了处理研究。系统地讨论了真空膜蒸馏和结晶过程并对其模型分析,获得了良好的效果。论文主要结果如下：首先对铵盐溶液中氨氮形态分布进行了分析,考察了不同进水pH值对溶液中游离氨和VMD渗透出水氮含量的影响；结果表明当料液pH小于6时,溶液中的游离氨含量较低,渗透出水氮含量较低,膜蒸馏过程对铵根离子具有较好的截留作用。利用真空膜蒸馏分别对最低浓度为3wt.%的氯化铵和硫酸铵溶液进行研究；讨论了不同工艺参数下渗透通量以及渗透出水氮含量的变化。在实验条件的范围内,渗透通量最高可达48L.m-2.h1,氨氮截留率均大于99.95%,且当铵盐料液pH为3时,其渗透出水氮含量均在3mg/L以下。其次,在纯水体系下,利用不同进水温度和流量测得的渗透通量对本实验膜组件的传热方程进行拟合,并结合铵盐溶液的各个物性参数和传质经验式建立了铵盐体系下渗透通量预测模型,预测值与实验值吻合。利用预测模型估算了浓度极化系数(CPC)和温度极化系数(TPC),讨论了不同操作条件对其产生的影响。结果表明,进料温度对TPC的影响较小,而CPC则随其升高而升高；提高料液表面流量促进了TPC降低,而CPC则略微上升；大渗透侧的压强将造成TPC升高,CPC则降低；料液的铵盐浓度对TPC和CPC的影响均不明显。随后,通过真空膜蒸馏将起始pH值为3的两种铵盐溶液进行了浓缩处理,随着浓缩实验的进行,渗透通量起初呈缓慢下降趋势,然而当料液浓缩至一定浓度时,渗透通量发生骤降,实验终点时,进水料液过饱和因子均接近1；在氯化铵体系中,渗透出水的氮含量起先均保持在3.2mg/L~4.7mg/L之间,但随着通量骤减,氮含量也发生骤升,最终高达59.8mgL；在硫酸铵体系中,渗透出水氮含量则一直保持在3mgL以下；铵盐的阴离子性质对渗透出水氮含量有着一定的影响；不同操作条件对浓缩过程中的渗透出水回收率也有不同影响,当渗透压强较低时,渗透通量发生衰减较早,单批渗透液回收率也较低；提高进水温度能够增大单批渗透液回收率,但是渗透通量发生衰减也较早；增大膜表面流量既能够促进单批渗透液的回收率还能够减缓渗透通量的衰减点。再次,利用浓缩实验渗透通量变化值估算了浓缩过程中铵盐体系下各个传递阻力分布以及膜表面处过饱和因子。在浓缩实验初期,传递阻力以膜阻力和边界层阻力为主；当膜表面过饱和因子接近1时,污染层阻力迅速增大,成为了膜蒸馏的主要传递阻力；选择膜表面处过饱和因子接近1时,对应的溶液浓度作为临界点；考察临界点前后浓度对渗透通量的影响,当浓度超过临界点时,渗透通量随操作时间大幅下降,污染层阻力迅速上升,产生膜污染现象；氯化铵体系中渗透出水的氮含量也随运行时间增大；通过扫描电镜对被污染的膜进行了观察,结果表明,铵盐的阴离子性质对膜污染的形成有着显著的影响；在氯化铵体系下,受污染的膜表面和膜孔孔壁处均有颗粒沉积,而硫酸铵体系中,仅在膜表面处发现大量颗粒沉积,膜孔内几乎没有颗粒沉积；将两种受污染膜利用纯水清洗,硫酸铵体系下的膜渗透通量恢复率可达98.6%,而氯化铵体系下的渗透通量恢复率仅有53.6%；当料液浓度低于临界点时,渗透通量随运行时间下降缓慢,渗透出水氮含量也均低于5mg/L;当料液浓度小于临界点,并保证其浓度在后续结晶温度下溶液过饱和度大于1,可将此浓度范围作为真空膜蒸馏浓缩料液的最佳终点浓度,能够有效的避免真空膜蒸馏过程中的膜污染。利用结晶器对浓缩后的铵盐溶液进行了结晶实验,结果表明,冷却结晶能够有效地将氯化铵析出,但硫酸氨的浓缩液在333K下的过饱和因子低于0.89时,需要向溶液中加入适量晶种才能够将硫酸铵晶体析出。最后,结合膜蒸馏浓缩实验以及浓缩液结晶实验,提出真空膜蒸馏—结晶耦合工艺处理酸性条件下的铵盐废水；即将铵盐废水浓缩至最佳终点浓度后,通入结晶器,同时回收渗透出水和铵盐晶体,再将低温的上清液回流至真空膜蒸馏过程中。值得注意的是,上清液回流过程需经过热交换器,用于真空膜蒸馏渗透气体的冷凝,以提高膜蒸馏的热效率。更多还原

【Abstract】 High ammonium salt wastewater is one of the most refractory wastewater in industry wastewater. In this paper, the batch mode of vacuum membrane distillation-crystallization process was carried out on the ammonium salt solutions. The results showed that high rejection rate of ammonium salt was achieved in vacuum membrane distillation process. The solutes of ammonium salt were recovered by coupling with crystallizer. The purpose of zero emission was achieved in theory. The detailed conclusions were as follows:Firstly, the effects of feed pH value on the concentrations of both free ammonia and permeate total nitrogen (TN) were investigated. When the feed pH value was below6, the concentrations of both free ammonia and permeate TN were kept at low level. Moreover,3wt.%NH4CI and (NH4)2SO4solutions were respectively treated by vacuum membrane distillation. The effects of operating conditions on permeate flux and permeate TN concentration were investigated. The results showed that the permeate flux reached up to48L·m-2·h-1and the ammonium salt rejection rate was above99.95%. The permeate TN concentration was below3mg/L, while feed pH value was3.Secondly, the empirical equation of heat transfer was fitted by experimental flux datas which were obtained at different feed temperatures and feed flow rates. A predictive model in term of permeate flux was built by combining physical parameters of ammonium salt solutions withthe empirical equation of mass transfer. The predicted values and experimental datas tended to agree. Furthermore, the concentration and temperature polarization coefficients (CPC and TPC) were estimated by the predictive model, and the effects of different operating conditions on the coefficients were investigated. Higher feed temperature leaded to greater CPC. Increasing feed flow rate promoted the decrease of TPC, but CPC was slightly increased. Higher TPC and lower CPC were obtained with the permeate pressure increasing. The influences of feed concentration on TPC and CPC were not obvious.Then, the ammonium salt solutions were concentrated by vacuum membrane distillation when feed pH value was3. The permeate flux decreased slowly at first. However, the permeate flux dropped rapidly at the end of concentration experiment. The feed supersaturation factor was closed to1. Moreover, in NH4C1system, the permeate TN concentration was kept in the range of3.2mg/L~4.7mg/L at first, butrapidly increased to 59.8mg/L at the end of concentration. Whereas, the permeate TN concentration was below3mg/L all the time with (NH4)2SO4system. The negative ions of ammonium salt have great influence on permeate TN concentration. The operating conditions also have effects on batch recovery. Lower permeate pressure resulted in earlier flux decline and lower batch recovery. Increasing feed temperature increased batch recovery, but leaded to flux decline early. Higher feed flow rate promoted the higher batch recovery and delayed the flux decline.The transfer resistances and supersaturation factor at membrane surface (SFm) were estimated by the experimental datas in concentration process. The membrane resistance and boundary layer resistance dominated at first, but the fouling layer resistance increased rapidly while the SFm closed to1. The corresponding feed concentration was taken as the critical point. Chosen the feed concentration below and beyond the critical point respectively, the behavior of permeate flux with operating time was investigated. The sharp decline of permeate flux indicated increase of fouling layer resistance, and the membrane fouling phenomenon with operating time were observed when the feed concentration was beyond the critical point. In addition, the permeate TN concentration also increased with operating time in NH4CI system. The deposition was found on both membrane surface and pore wall in NH4CI system from the SEM images of fouling membrane. However, large amount of (NH4)2SO4crystal deposits were observed on the membrane surface, but little fouling on the pore wall. The permeate flux recoveries were98.6%and53.6%respectively in NH4CI and (NH4)2SO4system after membrane cleaning. When the feed concentration was below the critical point, the permeate flux decreased slowly, and the permeate TN concentrations were below5mg/L.The crystallizer was carried out with the concentrated amminium salt solution. The results showed that the NH4CI crystals were precipitated from concentrated solution by cooling crystallization, whereas the (NH4)SO4crystal was obtained by adding the seed crystal. The optimized cooling method caused the larger NH4CI crystal size.Finally, the treatment of ammonium salt wastewater by coupling vacuum membrane distillation-crystallization was proposed. The membrane distillation was used to obtained the permeate water and desired concentration solutions and then crystals were precipitated by crystallizer. It was worth noting that the effluent with low temperature from crystallizer should be transported through the heat exchanger, in order to condense the permeate gas from membrane distillation process and increase the heat efficiency.更多还原