低温多效蒸发海水淡化系统热力性能分析与优化研究

发布时间：2018-09-10 06:02

【摘要】：随着人口的急剧增长、工业化和城市化程度的不断加深、生活水平的日益提高以及环境污染的加剧,淡水资源短缺问题已成为制约经济发展的关键因素。海水淡化是从源头解决淡水资源匮乏的有效手段之一。低温多效蒸发(LT-MEE)海水淡化技术由于具有能耗较低、产品水纯度高、化学药剂用量少以及腐蚀和结垢风险小等诸多优点,受到了学术界和工程界的广泛关注。对热力过程进行细致分析和准确把握,是发展多效蒸发海水淡化装置优化设计技术的基础。本文基于针对性实验所建立的数据库,建立了全面的LT-MEE海水淡化系统热力过程数学模型,对装置的热力性能分析与优化方法进行了深入研究,旨在推动具有完整科学内涵的LT-MEE海水淡化装置热力性能分析理论的形成和先进技术的发展。基于热力学第一定律和第二定律,建立了全面的LT-MEE海水淡化系统热力过程数学模型。模型中包含了海水沸点升高和蒸汽流过传热管束、除沫器、蒸汽通道以及管内冷凝过程的流动阻力引起的热力损失,流动阻力和传热系数由针对性实验所建立的数据库得到,使得模型更为准确全面。通过与实际装置和文献模型数据对比,验证了本文所建立数学模型的有效性和先进性。通过模拟计算,得到了二次蒸汽量、浓盐水盐度、温度、传热系数和热力损失等参数在各效蒸发/冷凝器中的分布及其随设计变量的变化规律,对LT-MEE海水淡化系统的热力过程进行了深入分析,探讨了进料方式、预热方式以及与热力蒸汽压缩器(TVC)相结合等因素对系统热力性能的影响,对比分析了MEE和MEE-TVC两种系统的热力性能及炯损分布情况。研究结果表明,流动阻力和沸点升高引起的热力损失对系统热力性能具有重要影响,通过对热力过程的分析,归纳出LT-MEE海水淡化装置“小温差、低流阻、饱和态、高敏感”的热力过程特征：当以二次蒸汽为预热热源时,预热器位置后移、预热器温升增大,均有利于提高装置的热力性能；与MEE系统相比,MEE-TVC系统的造水比显著提高,比冷却水量和比传热面积均有不同程度下降。将准确热力过程模型-响应面近似模型-多目标遗传算法相结合的方法应用于LT-MEE海水淡化系统的优化设计中。首先基于总收益需求法,建立了LT-MEE海水淡化系统的经济学模型,即经济性能评价指标-单位质量造水成本的详细数学模型。然后基于准确的热力过程模型建立LT-MEE海水淡化系统性能参数的响应面近似模型,针对系统的多目标优化问题(造水比最大、比炯耗最大和单位质量造水成本最小),采用改进的多目标遗传算法(NSGA-II)对其进行求解,最终得到Pareto最优解集。从目标函数、设计变量、(火用)损分布和成本构成方面,对比分析了基于不同偏好的最优方案的设计特点。优化结果显示：对于MEE-TVC海水淡化系统,与算例的初始方案相比,从Pareto最优解集中筛选出的偏好热力学性能最优的方案,其造水比提高44.6%,比(火用)耗降低26.7%,偏好经济学性能最优的方案单位质量造水成本减小13.7%,从而表明本文所采用的优化方法能够取得显著的优化效果,同时也显示出对低温多效蒸发海水淡化系统进行优化设计的必要性。
[Abstract]:With the rapid growth of population, the deepening of industrialization and urbanization, the improvement of living standards and the aggravation of environmental pollution, the shortage of fresh water resources has become a key factor restricting economic development. Chemical technology has attracted much attention from academia and engineering circles because of its advantages such as low energy consumption, high purity of product water, low dosage of chemical reagents and low risk of corrosion and scaling. Detailed analysis and accurate grasp of the thermal process are the basis for the development of optimal design technology of multi-effect evaporative seawater desalination unit. A comprehensive mathematical model of thermal process of LT-MEE seawater desalination system is established based on the database established by the sex experiment. The thermal performance analysis and optimization methods of the device are studied in depth. The purpose is to promote the formation of the theory of thermal performance analysis of LT-MEE seawater desalination device with complete scientific connotation and the development of advanced technology. Based on the first and second laws of mechanics, a comprehensive mathematical model for the thermal process of LT-MEE seawater desalination system is established. The model includes the thermal losses caused by the elevation of seawater boiling point and the flow resistance of steam through heat transfer tube bundles, demisters, steam passages and condensation process in tubes. The flow resistance and heat transfer coefficient are established by pertinent experiments. The validity and advancement of the mathematical model established in this paper are verified by comparing with the actual equipment and the data of the literature model. The parameters of secondary steam, salinity of concentrated brine, temperature, heat transfer coefficient and thermal loss are obtained by simulation calculation. The thermal process of LT-MEE seawater desalination system is analyzed in detail. The effects of feed mode, preheating mode and combination with thermodynamic vapor compressor (TVC) on the thermal performance of the system are discussed. The thermal performance and the distribution of the enthalpy loss of MEE and MEE-TVC systems are compared and analyzed. The results show that the thermal loss caused by flow resistance and boiling point increase has an important influence on the thermal performance of the system. Through the analysis of the thermal process, the thermal process characteristics of LT-MEE seawater desalination unit with "small temperature difference, low flow resistance, saturated state, high sensitivity" are summarized. Compared with MEE system, the water-making ratio of MEE-TVC system is significantly increased, and the specific cooling water volume and specific heat transfer area are reduced to varying degrees. In the optimization design of the system, the economic model of LT-MEE seawater desalination system, i.e. the detailed mathematical model of economic performance evaluation index-unit quality water production cost, is established based on the total revenue demand method. Then the response surface approximation model of the performance parameters of LT-MEE seawater desalination system is established based on the accurate thermodynamic process model. An improved multi-objective genetic algorithm (NSGA-II) is used to solve the traditional multi-objective optimization problem (maximum water-making ratio, maximum specific energy consumption and minimum unit quality water-making cost). The Pareto optimal solution set is obtained. The optimization based on different preferences is compared and analyzed in terms of objective function, design variables, (exergy) loss distribution and cost composition. The optimization results show that, for MEE-TVC seawater desalination system, compared with the initial scheme of the example, the scheme with the optimum thermodynamic performance selected from the Pareto optimal solution set has 44.6% higher water-making ratio, 26.7% lower exergy consumption and 13.7% lower water-making cost per unit quality of the scheme with the optimum economic performance. This shows that the optimization method adopted in this paper can achieve remarkable optimization results, and also shows the necessity of optimizing the design of low-temperature multi-effect evaporative seawater desalination system.
【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P747

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