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磷是生命体不可或缺的营养元素,在生物的生长、发育和繁殖过程中起着至关重要的作用[1]1。它是一种不可再生资源,主要源自磷矿石的开采。有研究表明,按目前的开采速率,现存的磷矿储备最多仅够维持372a[2]。另一方面,水体中过量的磷容易引发水体富营养化,进而破坏生态环境[3]。为了保护资源和环境,磷回收技术应运而生,并逐渐受到人们的关注。其中,鸟粪石(MgNH4PO4·6H2O)结晶法由于具备氮磷去除效果好、反应速率快、产品为优质缓释肥料等特点而备受青睐[4-5]。
鸟粪石产品的商品化是鸟粪石法能否实际应用的关键,而鸟粪石的商品化价值取决于其产品质量。鸟粪石的产品质量与所使用的结晶反应器密切相关,目前主流的鸟粪石结晶反应器为搅拌釜和流化床[6]。有研究表明,完全混合式的搅拌釜无法将杂质与鸟粪石产品进行有效分离,所得的鸟粪石污染物含量较高;流化床采用上升水流作为物料混合和颗粒流化的推动力,可实现轴向上的水力分级,大幅降低产品中杂质含量,所得产品纯度高,安全性好[7-8]。
截至目前,鸟粪石结晶流化床尚无设计规范。研究人员多根据经验或半经验公式计算流化速度,并设计不同的管径以实现鸟粪石在反应器中的分级[9-11]。管径的变化除了具备分级效果,还可创造一定的湍流以促进物料的混合及晶体的聚并[12]。目前较为常见的流化床构型主要有多段式和锥体式。FATTAH等[11]使用多段式流化床对污水处理厂的污泥压滤浓缩液开展磷回收,磷酸盐去除率超过90%,磷回收率高于85%,所得产品中鸟粪石纯度高达96%。李咏梅等[7]采用锥体式流化床反应器对污泥脱水上清液进行处理,
${\rm{PO}}_4^{3-} $ -P的去除率最高可达90.5%,产生颗粒的最大粒径在2.0~3.2 mm之间,纯度在80%以上。2种流化床反应器构型均具备理想的磷去除效果及产品特性,但结构较复杂,加工难度大。鸟粪石微晶的流失会导致总磷(TP)去除率下降,是鸟粪石流化床面临的首要问题,目前主要通过设置沉淀池、安装筛网或投加混凝剂进行截留[13-15]。其中,外置沉淀池的目的是为了保证沉淀效果,须设计较大容积的沉淀池,从而增加了基建成本;安装筛网虽经济有效,但须频繁清理,人工维护成本较高;投加混凝剂则需额外的药剂费用及污泥处置费用。综上所述,无论是复杂的反应器外部构型或是额外的微晶截留措施,均不可避免地增加了加工难度及运行维护成本。鉴于结晶反应器内流体运动的复杂性,完全从实验角度开展反应器优化研究将会非常费时、费力。随着计算机性能的不断提高,计算流体力学(computational fluid dynamics,CFD)已被广泛应用于反应器结构的优化,其避免了传统经验方法中繁复的实验过程,对结晶反应器的设计、优化及放大提供更加可靠的依据和详尽的信息。针对鸟粪石结晶流化床构型设计的不确定性及复杂性,本研究首先采用数值模拟的方法,探明多粒径体系下不同构型流化床的湍流强度、分级特性和微晶截留效率;然后通过实验研究新构型鸟粪石结晶流化床的磷去除效果与产品特性,以验证数值模拟优化方法的可靠性和合理性。
鸟粪石结晶流化床结构优化
Geometry optimization of struvite crystallization fluidized bed
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摘要: 针对鸟粪石结晶流化床结构设计的不确定性及复杂性,采用数值模拟的方法模拟多粒径体系下不同构型流化床的湍流强度、分级特性和微晶截留效率。模拟结果表明,一段式流化床对鸟粪石颗粒的分级效果及湍流条件均优于多段式和锥体式流化床;沉淀区内增设沉淀组件未能显著提升微晶截留效率。验证实验结果表明,在不同的进水磷浓度(240,480和1 000 mg·L−1)下,一段式流化床对磷的去除效果与传统的多段式流化床无显著差异,且具有良好的造粒效果,大于1.25 mm的产品占比分别为88.1%、96.4%和70.1%。实验结果验证了数值模拟优化方法的可靠性和合理性,简化后的一段式鸟粪石结晶流化床具备良好的磷处理效果及产品特性,是理想的鸟粪石流化结晶反应器。Abstract: In view of the uncertainty and complexity in the geometry design of struvite-crystallization fluidized bed reactors, numerical simulation was first adopted to explore the turbulence intensity, classification characteristics and fines entrapment behaviors of fluidized bed reactors with different geometries under multi-particle systems. The simulation results showed that the one-sectional fluidized bed reactor was superior to the multi-sectional and cone-shaped fluidized bed reactor in classifying pellets and turbulent conditions. The fines retention efficiency cannot be significantly improved by installing sedimentation components in the precipitation zone. As for the consequences of experiments, no significant difference of phosphorus removal efficiencies was observed between the one-sectional fluidized bed reactor and the traditional multi-sectional fluidized bed reactor at different influent phosphorus concentrations (240, 480 and 1 000 mg·L−1). It was also proved that the one-section fluidized bed reactor had good capability in granulation, where particles with sizes larger than 1.25 mm took up the percentage of 88.1%, 96.4% and 70.1%, respectively. The experimental results validated that the reliability and rationality of the numerical optimization method. The simplified one-section fluidized bed reactor, which displayed good performance in phosphorus removal and product characteristics, was a promising struvite-crystallization fluidized reactor.
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表 1 模拟条件设定
Table 1. Modeling conditions set-up
工况 粒径组成特征 粒径/mm 外部构型 内部构件 1 宽粒径组合 0.5/1.0/4.0 锥体/多段/一段 无 2 小粒径组合 0.2/0.5/1.0 锥体/多段/一段 无 3 大粒径组合 2.0/3.0/4.0 锥体/多段/一段 无 4 宽粒径组合 0.2/0.5/1.0 一段 三相分离器 5 宽粒径组合 0.2/0.5/1.0 一段 斜板 -
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