摘要
膜蒸馏是一种新型的膜分离技术,可用于海水和苦咸水淡化等领域。目前膜蒸馏海水淡化存在水通量低、产水成本高等问题,制约了其大规模应用。因此,深入研究膜蒸馏系统,提高膜蒸馏海水淡化的产率、降低产水成本,具有重要的现实意义。本文将真空膜蒸馏与太阳能集热系统结合,从真空膜蒸馏过程设计出发,同时对中空纤维膜组件、操作条件等因素对膜蒸馏过程通量的影响进行了模拟优化,并在此基础上进一步研究太阳能真空膜蒸馏集成工艺。
论文首先考察了中空纤维膜组件装填系数、组件长径比、操作条件、膜结构等对真空膜蒸馏过程的影响。结果表明:膜通量随组件装填系数增加而增加,在装填系数约为11%时达到最大值,随着装填系数继续增加,膜通量减少;膜通量随组件长径比增加而减少,产水量随组件长径比增加而增加,综合以上两方面的因素,组件长径比在4.3-5左右时比较合适;进料温度增加,膜通量显著增加;料液在层流状态下,膜通量随料液流速增加而显著增加,在湍流状态下,膜通量随料液流速增加不明显;增加真空度可以显著提高膜通量;平均孔径适中、空隙率大、厚径比小的膜具有较好的真空膜蒸馏性能。
其次,基于能量衡算建立了膜管内热量传递方程,利用数值逼近法对中空纤维膜内温度分布进行了模拟计算;基于气相传质关联式,将装填系数等膜组件参数以壳程传质系数形式进行关联;在此基础上利用总传质系数计算真空膜蒸馏的膜通量。通过实验结果与模拟计算结果的比较表明,该模型具有较高的准确性。模型模拟了膜组件参数及膜蒸馏操作条件对真空膜蒸馏过程中温度和通量的影响规律,并为预测系统通量、优化系统操作条件提供了理论依据和指导。
最后,以杭州地区太阳辐射的理论数据为基础,计算了太阳能真空膜蒸馏集成工艺的生产能力,并研究太阳辐射强度、组件连接方式、组件数目等因素对该工艺的影响。结果表明:我国沿海地区太阳能资源丰富,年产水量约为954kg·m~(-2)(太阳能集热板)。较合适的太阳能集热板面积与膜面积的比例约为11:1。太阳能真空膜蒸馏集成工艺的日产水量随太阳辐射强度增加而显著增加,阴雨天气时工艺无法正常运行。集成工艺的最大膜通量达14L·m~(-2)·h~(-1)。工艺中组件连接方式对通量和产水量影响不大。集成工艺的通量随组件数目的增加而降低,产水量随组件数目的增加而增加。
Membrane distillation(MD)is an emerging separation technology,which can be applied for seawater desalination.At present,low water flux and high cost are still the main barriers for the application of MD in industry.As a result,it is important to study the MD system to increase the water flux and reduce the cost.The vacuum membrane distillation(VMD)system was integrated with the solar still in this paper.And,in the work a VMD process had been designed,and a mathematic model had been built to describe the effect of the membrane module and the operation conditions on VMD performance.Based on these results,the integrated process of vacuum membrane distillation and solar still was studied in detail.
The effects of some facts on the performance of VMD process were studied,including the packing fraction of the hollow fibers,the ratio of length to diameter of the module,the operation conditions and the membrane.The experiment showed that,with the packing fraction increasing,the flux of the VMD system increased to a maximum at the packing fraction of 11%,and then decreased.The flux of the VMD system decreased with the increasing of length to diameter ratio of the module while the yield of the pure water increased with the ratio of the module.Considering the yield and the flux,the optimum ratio was about4.3-5.As the inlet temperature of the feed increased,the flux of the VMD increased. It also increased with the velocity of the feed water and vacuum degree.An excellent membrane for VMD should exhibit large pore size,high porosity and small ratio of thickness to diameter.
A model of heat transfer was acquired based on the conservation equation of energy,and solved by the numerical approximation method.The parameters of the membrane module were correlated in the shell-side mass transfer coefficient based on the correlations of the gas phase mass transfer coefficient.And then the model of flux was built based on the overall mass transfer coefficient.It was found that the model values were in qualitative agreement with experiment results.The model could be used for forecast flux and optimize operation of the process.
The portable water yield of the integrated system in Hangzhou city was estimated theoretically,and the effects of some facts on the performance of the integrated system were studied,including the intensity of the solar radiation,connecting styles of the modules and the number of membrane modules used.It was found that the solar energy was rich in the costal area of China and the yield of the pure water in a year could reach to 954kg·m~2 of the solar collector area.The preferred ratio of the area of the solar collector to the area of the membrane was about 11.The flux of the integrated system varied with the solar intensity in a day,and the maximum reached 14L·m~(-2)·h~(-1).At the same time,the performance of the integrated system was influenced little by the connecting styles of the modules.The flux of the system decreased as the number of membrane modules used increased,while the yield of the pure water increased slowly as the number of membrane modules increased.
引文
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