CAS OpenIR
挥发性有机物的多层流化床吸附浓缩
郑家乐
Thesis Advisor宋文立
2020-07-01
Degree Grantor中国科学院大学
Degree Name博士
Degree Discipline化学工程
Keyword多层流化床,挥发性有机物,吸附,脱附,浓缩
Abstract

挥发性有机物(Volatile Organic Compounds,VOCs)是PM2.5和O3形成的关键前驱体,其严重破坏生态环境,影响人类身体健康。我国VOCs污染的防治工作相对滞后,开发高效的VOCs处理技术已刻不容缓。针对低浓度、大风量的VOCs废气,目前最佳的处理工艺为先进行吸附浓缩将其转变为高浓度、小风量的废气,再进行催化燃烧或冷凝回收。流化床具有床层温度均匀、传热传质快、处理量大、易于放大设计等优点,且多层浅流化床可以有效抑制固相返混和气泡形成,保证较高的传质效率和良好的气固接触。因此,本论文提出了多层浅流化床VOCs吸附浓缩技术方案,该方案采用双流化床进行吸附和脱附,实现了VOCs的富集浓缩。基于该技术方案,本论文通过实验和数值模拟相结合的方法,系统的研究了多层浅流化床VOCs吸附和脱附规律。论文以HCP-5超高交联吸附树脂为吸附剂,测定了吸附剂的物理性质和流化性质,测量了30~160 °C下HCP-5吸附剂和邻二甲苯、乙酸乙酯、丙酮体系的吸附等温线,并用Langmuir吸附等温方程进行拟合。建立了固定床VOCs脱附穿透数学模型,结合固定床脱附穿透实验,采用最小二乘法拟合得到了邻二甲苯、乙酸乙酯、丙酮在不同温度下的脱附内传质系数。对比流化床吸附和脱附内外传质系数,研究发现,流化床吸附是以外传质为主的内外传质共同控制过程,而流化床脱附是以内传质为控制步骤的气固传质过程。与吸附过程相比,脱附过程吸附剂孔内的表面扩散对内传质的贡献减少,导致内传质系数显著下降。在多层浅流化床吸附塔和脱附塔中,开展了VOCs吸附和脱附实验,研究了床层高度、气固流量比、床层总数对气体出口VOCs浓度的影响。建立了基于内外传质系数的多层浅流化床VOCs吸附和脱附过程的数学模型,该数学模型可以较好的描述多层浅流化床内的VOCs吸附和脱附过程。通过数学模型计算,研究了废气VOCs浓度、床层总数、气固流量比、脱附气体温度等对多层流化床吸附塔和脱附塔气体出口VOCs浓度的影响,并考察了不同废气VOCs浓度和VOCs种类对浓缩比的影响。研究发现,在多层流化床脱附过程中,当最上层的气相VOCs浓度大于该层吸附剂对应的平衡气相VOCs浓度时,吸附剂在最上层会发生吸附,VOCs浓度下降;对吸附剂进行预热可以消除此现象,并提高脱附塔的气体出口VOCs浓度。随着脱附塔气固比的变化,多层流化床脱附塔的气体出口VOCs浓度存在最大值,该最大值随床层总数、脱附气体温度、吸附剂预热温度的增加而增加。针对300 mg/Nm3、20,000 Nm3/h的VOCs废气,设计了一套多层浅流化床吸附浓缩示范装置,采用建立的多层浅流化床吸附和脱附数学模型,设计了吸附塔和脱附塔的床层总数、吸附剂循环量、脱附气体流量和温度等工艺参数。该装置运行稳定,VOCs去除率达98%,浓缩比达17,满足VOCs排放和催化燃烧要求。多层浅流化床VOCs吸附浓缩工艺为低浓度、大风量的VOCs净化治理提供了一条切实可行的技术途径。;Volatile Organic Compounds (VOCs) are crucial precursors for the formation of PM2.5 and O3, which seriously damage the ecological environment and affect human health. The control of VOCs emissions in China relatively lags behind. Hence, it is very urgent and crucial to develop efficient technologies to control VOCs emissions.To control the VOCs emissions with dilute concentration and large flow rate, the efficient technology is that the VOCs are first pre-concentrated by adsorption and desorption cycle and then oxidized to H2O and CO2 by catalytic oxidation process or recovered by condensation process. Fluidized bed stands out for the advantages of uniform temperature distribution, fast heat and mass transfer, high handling capacity, and easy scale-up, etc. The shallow multistage fluidized bed with has the additional advantages of inhibiting the solid particles back mixing and the formation of bubbles, which ensures high mass transfer efficiency and good gas solid contact. A concentration technology with double multistage shallow fluidized beds that realizes the enrichment of dilute VOCs by the adsorption and desorption cycle is proposed. The adsorption and desorption of VOCs in multistage fluidized beds are studied by experiments and numerical simulations.In the thesis, the HCP-5 hypercrosslinked polymer was used as adsorbent. The physical and fluidization properties of the adsorbent were measured. The adsorption isotherms of HCP-5 adsorbent and o-xylene, ethyl acetate, acetone system at different temperatures were measured. And, the Langmuir isotherm was used to describe the experimental results in the experimental concentration and temperature ranges.A mathematical model of desorption breakthrough curves in fixed bed was established. By comparison of the experimental and calculated breakthrough curve, the internal mass transfer coefficients of o-xylene, ethyl acetate and acetone at different temperatures during desorption process were obtained by using the least square method. By comparing the internal and external mass transfer coefficients of the fluidized bed in adsorption and desorption process, it is found that in the adsorption process, the mass transfer process is mainly controlled by the external one. But in the desorption process, the limiting step of mass transfer process is the internal one. Compared with the adsorption process, the contribution of surface diffusion to the internal mass transfer is reduced in the desorption process, resulting in a significant decrease of the internal mass transfer coefficient.The VOCs adsorption and desorption experiments in the multistage fluidized bed were carried out, and the effects of bed height, gas solid flow ratio and total stage number on gas outlet VOCs concentration were studied. A mathematical model based on internal and external mass transfer dynamics was proposed, which was used to describe the adsorption and desorption process in multistage fluidized bed. Through the mathematical model, the effects of effluent gas VOCs concentration, total stage number, gas solid flow ratio, desorption gas temperature on gas outlet VOCs concentration of adsorber and desorber were studied, and the effects of effluent gas VOCs concentration and types of VOCs on concentrating ratio were also investigated. It is found that in the desorption process, when the VOCs concentration of gas phase in the upper stage is greater than the equilibrium VOCs concentration corresponding to the adsorbent in this stage, adsorption phenomenon will occur in the upper stage and the VOCs concentration will decrease. Preheating the adsorbent can eliminate this phenomenon and increase the gas outlet VOCs concentration of the desorber. With the change of gas solid flow ratio of the multistage fluidized bed desorber, there is a maximum value of the gas outlet VOCs concentration of the desorber, which increases with the increase of total stage number, desorption gas temperature, adsorbent inlet temperature. A demonstration device of concentration of VOCs by adsorption and desorption in multistage fluidized beds was designed for the effluent gas VOCs concentration and flow rate of 300 mg/Nm3 and 20,000 Nm3/h respectively. According to the established mathematical model, the parameters such as total stage number, adsorbent flow rate, desorption gas flow rate and temperature were determined. The device has already been constructed and put into use,with the total VOCs removal efficiency up to 98% and concentrating ratio up to 17, meeting the requirements of VOCs emission standards and catalytic oxidation. The technology of concentration of VOCs by adsorption and desorption in multistage fluidized beds provides a feasible technical approach for the purification of VOCs emissions with dilute concentration and large flow rate.

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/49672
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
郑家乐. 挥发性有机物的多层流化床吸附浓缩[D]. 中国科学院大学,2020.
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