基于PTFE平板膜的膜蒸馏脱盐过程性能模拟和响应面优化研究

发布时间：2018-01-24 04:39

本文关键词： 直接接触式膜蒸馏(DCMD) 真空膜蒸馏(VMD) 响应面法(RSM) 数值模拟 PTFE平板膜　出处：《江苏大学》2017年硕士论文　论文类型：学位论文

【摘要】：膜蒸馏是将热法和膜法结合起来的分离技术,最早用以海水淡化,同其他海水淡化技术相比(比如RO)膜蒸馏具有可在低压下操作,可利用其他过程废热的特点,使得膜蒸馏成为一种节约能源、经济并对环境友好的膜分离技术。但是由于膜蒸馏热效率低、膜通量小、膜蒸馏用膜研究进展缓慢、膜蒸馏传热传质机理不完善等导致膜蒸馏一直未实现产业化应用,因此有必要针对膜蒸馏展开优化设计,包括过程优化及膜蒸馏机理优化,进而提高膜蒸馏效率和膜通量,加快其产业化进程。膜蒸馏用膜的材料主要有三种:聚四氟乙烯(PTFE)、聚偏氟乙烯(PVDF)和聚丙烯(PP),其中PTFE由于其优越的物理和化学性能是市面上应用最广的膜材料。PTFE膜可被制成平板膜和中空纤维膜,为了方便拆卸和清洗在实验阶段多选择平板膜进行研究。目前膜蒸馏的四种形式中直接接触式膜蒸馏(DCMD)和真空膜蒸馏(VMD)是研究最多、应用最广泛的技术。因此本文针对基于PTFE平板膜的DCMD和VMD过程处理Nacl溶液展开实验和模拟研究。本文首先基于实验设计DoE中的响应面法(RSM)对利用PTFE平板膜处理Nacl溶液的DCMD和VMD过程展开模拟优化设计。为了确保RSM有效,针对DCMD和VMD处理去离子水的过程开展了单因素实验,确定对DCMD和VMD过程影响显著的因子及其最佳取值范围。其次,利用软件Design-Expert8.6对DCMD及VMD设计实验,每个因素设计五个水平,结合实验数据拟合出膜通量关于各因子的二阶多项式模型。通过方差分析验证模型的准确性。然后通过响应面分析确定各因子之间影响的水平,再对DCMD和VMD过程进行优化设计。结果表明使用拟合出的模型对DCMD和VMD进行优化是合理可行的,特别是VMD过程。同时也进一步说明,利用响应面法优化工艺参数可以使得膜蒸馏过程获得最大的膜通量,为后续大规模技术集成提供理论依据。最后,通过分析已建立的膜蒸馏传热传质机理,结合实验数据对VMD模型进行修正,利用EXCEL对DCMD和VMD进行迭代模拟计算。DCMD模拟计算过程中蒸发焓不随温度变化,传质模型选择努森扩散模型;VMD过程中分别考虑了蒸发焓随温度变化和不随温度变化时膜蒸馏的性能,传质模型选择努森-分子结合模型。模拟计算考察了热侧料液温度、热侧料液流量、冷侧液体流量、冷侧液体温度、Nacl溶液浓度和真空度对膜蒸馏性能的影响。把单因素实验和RSM设计的实验测得的实验结果与模拟数据对比验证模型的准确性。研究发现对于DCMD和VMD过程,模拟所得的膜通量大部分都要大于实验测得的膜通量,在流体流速很低和Nacl溶液浓度很高的条件下部分实验点的实验值会超过模拟值,但是两者总体变化趋势一致,本文建立的膜蒸馏传热传质模型可以较准确的预测膜蒸馏过程。
[Abstract]:Membrane distillation is a separation technology which combines thermal and membrane processes. It is first used for desalination of seawater. Compared with other desalination technologies (such as ROM) membrane distillation can be operated at low pressure. Because of the waste heat of other processes, membrane distillation has become a kind of energy saving, economical and environmentally friendly membrane separation technology, but because of the low heat efficiency of membrane distillation, the membrane flux is small. The research progress of membrane for membrane distillation is slow, and the heat and mass transfer mechanism of membrane distillation is not perfect. Therefore, it is necessary to optimize the design of membrane distillation. It includes process optimization and membrane distillation mechanism optimization, which can improve the efficiency and flux of membrane distillation and accelerate the industrialization process. There are three kinds of membrane materials for membrane distillation: PTFEs. Polyvinylidene fluoride (PVDF) and polypropylene (PP). PTFE membrane can be made into flat membrane and hollow fiber membrane because of its excellent physical and chemical properties. In order to facilitate the disassembly and cleaning of the plate membrane in the experimental stage, the direct contact membrane distillation (DCMD) and the vacuum membrane distillation (VMD) are the most studied in the four forms of membrane distillation. The most widely used technology. Therefore, this paper focuses on the treatment of Nacl solution by DCMD and VMD process based on PTFE flat membrane. Firstly, the response of DoE is designed based on the experiment. Surface method (. In order to ensure the effectiveness of RSM, the DCMD and VMD processes of Nacl solution treated with PTFE plate membrane were simulated and optimized by RSM. Single factor experiments were carried out on the treatment of deionized water with DCMD and VMD to determine the factors and the optimum value range of the process of DCMD and VMD. The software Design-Expert8.6 is used to design DCMD and VMD. Each factor is designed at five levels. The second order polynomial model of membrane flux on each factor was fitted with the experimental data. The accuracy of the model was verified by ANOVA. Then the level of influence between the factors was determined by response surface analysis. The results show that it is reasonable and feasible to optimize DCMD and VMD by using the fitting model, especially the VMD process. Using response surface method to optimize the process parameters can make the membrane distillation process obtain the maximum membrane flux, and provide the theoretical basis for the follow-up large-scale technology integration. Finally, the established heat and mass transfer mechanism of membrane distillation is analyzed. Based on the experimental data, the VMD model is modified, and the enthalpy of evaporation does not change with the temperature during the simulation of DCMD and VMD by EXCEL. Kucson diffusion model is selected for mass transfer model. In the VMD process, the performance of membrane distillation with and without the change of enthalpy of evaporation was considered, and the mass transfer model was selected as the Knuden-molecular binding model. The temperature of the hot side feed was investigated by simulation and calculation. Hot side liquid flow, cold side liquid flow, cold side liquid temperature. The effects of concentration and vacuum of Nacl solution on the performance of membrane distillation. The accuracy of the model was verified by comparing the experimental results obtained from the single factor experiment and the experiment designed by RSM with the simulated data. And the VMD process. Most of the simulated membrane fluxes are larger than the measured membrane fluxes. Under the condition of low fluid flow rate and high concentration of Nacl solution, the experimental values of some experimental points will exceed the simulated values. The heat and mass transfer model established in this paper can accurately predict the membrane distillation process.
【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P747

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