【作者】 刘大伟；

【导师】 刘晓华；

【作者基本信息】 大连理工大学 ， 热能工程， 2011， 硕士

【摘要】 目前,全球大约有25%的人口缺乏保质保量的淡水供应,超过80%的国家存在淡水资源不足的问题。海水淡化技术已经成为解决淡水短缺问题的一条重要途径。低温多效蒸发海水淡化技术是主要的海水淡化方法之一。低温多效蒸发海水淡化系统通常被分为顺流、逆流、并流和混流流程。低温多效蒸发海水淡化系统中各效蒸发器的蒸发管的喷淋密度的大小对装置正常运行非常重要,本文的计算分析考虑到工程实际,所有的计算结果都保证各效喷淋密度都满足装置正常运行的要求,即喷淋密度在0.03-0.08kg/(m·s)范围内。在质量和能量平衡方程的基础上,建立了低温多效蒸发海水淡化系统的数学模型,其中包括蒸发器、冷凝器、预热器、海水冷却器和热力压缩器等；考虑了盐水沸点升高及蒸汽流经除沫器以及在效间通道和管束中流动时因阻力而造成的温度损失；海水的物性参数为浓度和温度的函数,饱和蒸汽和淡水的物性参数为温度的函数；针对系统的数学模型编制了计算求解程序。以额定产水量为10000 t/d的系统为例,研究了不同的因素对无热压缩的低温多效蒸发海水淡化混流系统的影响。在其他条件不变的情况下,首效加热蒸汽温度对造水比的影响不大,对蒸发器总面积的影响较大,提高首效加热蒸汽温度有利于降低蒸发器总面积。增加系统效数可以显著提高系统的造水比。建议尽量将预热器放在混流点前,而且越靠近首效越好,尤其是当预热量比较大时,有利于避免蒸发器总面积的急剧增加。对于带热压缩的低温多效蒸发海水淡化混流系统,以额定产水量3500 t/d为例,研究了不同的因素对系统的影响。结果表明：TVC引射蒸汽位置位于混流点位置后一效蒸发器之后时,系统的造水比达到最大值,蒸发器总面积达到最小值。为了减少系统蒸发器的换热面积可以适当的升高首效加热蒸汽的温度。TVC动力蒸汽的状态对系统的造水比的影响较大,对蒸发器总面积的影响不大,提高TVC动力蒸汽的温度和压力将有利于系统造水比提高。当系统可设置的预热器个数有限时,要尽量将预热器设置在TVC引射蒸汽位置以前的位置,而且越靠近首效越好,这样既有利于提高系统造水比,也有利于降低蒸发器总面积,有助于降低海水淡化成本。对无热压缩的低温多效蒸发海水淡化混流和并流系统进行了对比研究。在其他条件相同的情况下,混流系统的造水比始终高于并流系统,混流系统和并流系统的造水比都随系统效数的增加而增加。混流系统的蒸发器总面积随效数的变化幅度不大,而并流系统的变化幅度较大。两系统的造水比和蒸发器总面积都随首效加热蒸汽温度的升高而减小,但变化幅度不大。混流系统和并流系统的造水比和蒸发器总面积都随额定产水量的增加而增加,但是,增加系统的额定产水量将导致混流系统的造水比和蒸发器总面积相对于并流系统的优势降低。对带热压缩的低温多效蒸发海水淡化混流和并流系统进行了对比研究。结果表明：在相同的条件下,混流流程的系统造水比具有相当大的优势,但需要的蒸发器换热面积也较大。带TVC的混流流程受TVC引射蒸汽位置的影响较大。首效加热蒸汽温度对混流和并流系统的造水比和蒸发器总面积的影响都不大,对并流系统的影响要略强于混流系统。混流和并流系统的造水比受TVC动力蒸汽参数的影响都很大,蒸发器总面积受TVC动力蒸汽参数的影响都很小。混流系统受TVC动力蒸汽参数的影响更大一些。预热位置越靠前混流和并流系统的性能会越好,混流系统受预热位置的影响更大一些。更多还原

【Abstract】 At present, about 25% of the world’s people lack the quality and quantity of freshwater supplies. More than 80% of the countries have serious water problems. Seawater desalination has emerged as an important source of fresh water. Low-temperature multi-effect distillation (LT-MED) is one of main methods of seawater desalination. LT-MED desalination system is commonly divided into forward feed configuration, backward feed configuration, parallel feed configuration and mixed feed configuration. The spray density is very important for operation. Taking the actual conditions into account, the processes with spray density of evaporation tube in the range of 0.03-0.08 kg/(m·s) were selected as feasibility ones.Based on the mass and energy conservation, the mathematical models of a mixed feed LT-MED desalination system and a mixed feed LT-MED desalination system with thermal vapor compressor (TVC) were established, in which included evaporator, condenser, preheater, brine cooler etc. The temperature losses were considered caused by the boiling point elevation (BPE) of seawater and the flow pressure drop when steam flow through tube bundle, demisters and pipelines. The property parameters of seawater were considered as the functions of temperature and concentration. And the property parameters of fresh water and saturated steam were considered as the functions of temperature. A computer program was carried out to solve the model.Different factors impacting on the mixed feed LT-MED desalination system without thermal vapor compressor (TVC) were analyzed. The calculation was based on the system with a rated water production 10000 t/d. When other relative parameters are unchanged, the total area of evaporators is more sensitive to the change of the heating steam temperature of the first effect than gained output ratio (GOR). As the number of effects increase, GOR rises significantly while within some range, the total area of evaporators decreases. It is benefit for the performance of the system that the position of preheater moves toward the first effect.For the mixed feed LT-MED desalination system with TVC, The calculation was based on the system with a rated water production of 3500 t/d. Many factors impacting on the system performance were analyzed. The results show that when the entrained steam position of TVC is at the effect just behind the mixed feed position, GOR reaches the maximum value and the total area of evaporators reaches the minimum value which is benefit to reduce the water cost. The impact of motive steam parameters of TVC on GOR is greater than on total area of evaporators and it is better to increase the parameters of motive steam for reducing the cost of water. For reducing the water cost it is also better to put preheater at the position before the entrained steam position of TVC.Comparative study on parallel feed and mixed feed LT-MED desalination systems without TVC was done under different factors impacting on the systems. When the other factors are unchanged, the more the number of evaporators, the better the performance of mixed feed system compared to the parallel feed system. The impact of the heating steam temperature of the first effect on the performance difference of the two systems is very little. The less the rated water production, the better the performance of mixed feed system compared to the parallel feed system.Comparative study on parallel feed and mixed feed LT-MED desalination systems with TVC was done under different factors impacting on the systems. Under the same conditions, GOR of the mixed feed LT-MED desalination system has a considerable advantage, but the total area of evaporators is also larger. The impact of the entrained steam position of TVC on the mixed feed LT-MED desalination system is greater. The influence of motive steam parameters of TVC on GOR is greater than on total area of evaporators and it is better to increase the parameters of motive steam for reducing the cost of water. It is benefit for the performance of the two systems that the position of preheater moves toward the first effect. And the impact of the position of preheater on the mixed feed LT-MED desalination system is more significant.更多还原