【作者】 刘飞；

【导师】 王涛； 骆仲泱； 方梦祥；

【作者基本信息】 浙江大学 ， 工程热物理， 2020， 博士

【摘要】 化学吸收法是现阶段烟气二氧化碳（CO₂）捕集较为成熟的技术路线之一,传统有机胺（如一乙醇胺,30%MEA）吸收剂再生能耗较高,是限制该技术工业应用的主要瓶颈。两相吸收剂具有大幅度降低再生能耗的潜力,有希望成为新一代CO₂捕集溶剂。现有两相吸收剂大多是基于亲脂胺混合、单一胺中添加物理溶剂,存在亲脂胺挥发性大、粘度高、CO₂吸收容量小、分相条件不明等问题。本文对基于烷醇胺/烷醇胺、混合胺与物理溶剂混合的新型两相吸收剂进行筛选和优化,探索其CO₂反应机理、分相机理及分相动力学,研究两相吸收剂的CO₂捕集性能、反应动力学、挥发性、吸收系统水平衡与胺排放特性等。本文对基于烷醇胺混合的两相吸收剂50%N,N-二乙基乙醇胺（DEEA）/25%羟乙基乙二胺（AEEA）进行研究,结果表明该两相吸收剂吸收CO₂后会形成有机相和水相;有机相的CO₂吸收速率比30%MEA快1-3倍,水相的吸收容量比30%MEA高1.5倍。基于瞬时反应传质模型,建立有机相吸收CO₂的反应动力学-传质模型,DEEA/AEEA的CO₂解吸速率和循环容量比30%MEA分别提高34%、58%。通过胺浓度优化,再生能耗可降低至2.2 GJ/t CO₂,比相同条件下30%MEA低45%。10次循环吸收-解吸实验中,CO₂捕集性能和分相特性稳定。研究发现DEEA/AEEA分相与胺浓度、CO₂负荷有关。在少水（10-40%）、特定胺浓度和一定CO₂吸收负荷范围时,DEEA/AEEA发生液液分相。分相机理表明,初始CO₂吸收阶段,AEEA与CO₂反应占主导,生成AEEA氨基甲酸盐;当CO₂负荷较高时,DEEA开始参与反应,生成HCO₃^-/CO₃^2-。氨基甲酸根、质子化胺以及碳酸氢根离子因与与水之间较强的离子-偶极键作用力而相互溶解,并富集在水相;而有机相主要成分为DEEA和少量的AEEA。DEEA与AEEA在水相和有机相中分配系数的差异性是造成分相比例不同的主要原因。根据线性规划求解出10-50%水含量范围内,DEEA/AEEA混合胺能发生分相的浓度区间及分相比例,计算结果与实验吻合较好。DEEA/AEEA分相过程的动力学受分子间作用力、重力和粘性力控制,其中分子间作用力和重力是分相驱动力,粘性力是分相阻力。提高温度、降低粘度,有利于降低分相时间;气体流量主要影响初始扰动,气体流量越小,分相时间越短;低高径比的分相器有利于减少分相时间。向2-氨基-2-甲基-1-丙醇（AMP）/MEA中添加物理溶剂,开发出AMP/MEA/二乙二醇二甲醚（DEGDME,30-50%）两相吸收剂。与烷醇胺混合两相吸收剂分相机理不同的是,物理溶剂型两相吸收剂的分相机理主要是盐析效应。物理溶剂不与CO₂发生化学反应,分相时,CO₂反应产物（氨基甲酸根、碳酸氢根、质子化胺等）因与水有着较强的离子偶极力作用而溶于水相,物理溶剂由于盐析效应被排挤出水相,形成有机相。AMP/MEA两相吸收剂的CO₂循环容量比AMP/MEA水溶液高69%,再生能耗低至2.5 GJ/t CO₂。本文首次提出了基于分相比例和粘度的χ修正方法,用于修正两相吸收剂的再生能耗。修正后DEEA/AEEA和AMP/MEA/DEGDME的再生能耗分别为2.4、2.7 GJ/t CO₂。该χ修正方法同样适用于其他高粘度吸收剂,如少水混合胺、离子液体等,与离子液体等高粘度吸收剂相比,两相吸收剂的再生能耗优势较明显。结合两相吸收剂的挥发性研究,分析了两相吸收系统的水平衡和胺排放问题。吸收剂挥发性受胺分子结构、温度、胺浓度及CO₂负荷影响较大,NRTL模型对有机胺气液平衡的热力学参数拟合效果较好。CO₂吸收系统的水平衡主要是由水洗塔温度决定。基于Aspen Plus仿真进行化学吸收工艺流程计算,烟气温度为40℃时,水洗塔温度控制在41℃,进、出吸收系统达到水平衡。胺排放受吸收塔温度、水洗塔温度以及吸收剂挥发性的影响较大。吸收系统在水平衡状态下,30%MEA、混合胺（ZJU-1）、AMP/MEA/DEGDME和DEEA/AEEA在水洗塔出口的胺排放分别为0.7、18、7、50 ppm。降低水洗塔温度能够进一步降低胺排放。水洗塔温度为25℃时,混合胺（ZJU-1）、AMP/MEA/DEGDME和AEEA/DEEA的胺排放降低至3.5、1.5、10 ppm。更多还原

【Abstract】 Chemical absorption using aqueous amines（such as monoethanolamine,30%MEA）is currently one of the most mature technologies for carbon dioxide（CO₂）capture from flue gas.The high energy consumption of amine regeneration process is the bottleneck of this technology and is hindering its commercial application in industry.The biphasic solvent is a promising solution for next generation CO₂ capture technology as it can achieve a significant reduction in the regeneration energy.There are two types of biphasic solvents,lipophilic amines blends and physical-chemical biphasic solvents.However,researches on phase separation behaviors,CO₂ absorption rate,CO₂ abasorption capacity and regeneration energy are insufficient.Phase separation mechanism upon CO₂ loading and amine concentration are unclear.In this paper,alkanolamine/alkanolamine blended and amine blends based physical-chemical biphasic solvents are screened and optimized to explore the CO₂ reaction mechanism,separation mechanism and phase separation dynamics.CO₂ capture performance,reaction kinetics,amss transfer,and volatility of the biphasic solvents are studied.Water balance and amine emission in the biphasic system are analyzed.Aqueous alkanolamine blends of 50%N,N-diethylethanolamine（DEEA）/25%hydroxyethylethylenediamine（AEEA）is first proposed as the biphasic solvent.More than 95%of absorbed CO₂ is concentrated in the lower phase.Only send the CO₂-rich phase to the lean/rich heat exchanger and stripper,thus saving heat duty of the reboiler.DEEA/AEEA forms aqueous and organic phase.The organic phase absorbs CO₂ 1-3times faster than 30%MEA,while the aqueous phase has a 1.5 greater CO₂ capacity than 30%MEA.The reaction kinetics-mass transfer of CO₂ absorption into the organic phase is modeled based on instantaneous reaction mechanism.The CO₂ desorption rate and capacity of DEEA/AEEA are 34%and 58%higher than that of 30%MEA without process optimization.The regeneration energy of DEEA/AEEA is 2.2 GJ/t CO₂,which is 45%lower than that of 30%MEA under the same conditions.CO₂ capture and phase separation performance were stable in 10-cycle absorption-desorption experiment.Amine concentration and CO₂ loading play important roles in phase separation behaviors.DEEA/AEEA blends formed two phases under lean water condition（10-40%）with specific amine concentrations and CO₂ absorption loading.The phase separation mechanism indicates that CO₂ reaction with AEEA dominates the initial CO₂absorption process with the products of AEEA carbamate.DEEA involves CO₂ reaction at high CO₂ loading and produces bicarbonate.The carbamate,protonated amine and bicarbonate are ionic and dissolve in aqueous phase due to the strong ion-dipole interaction with water.While the organic phase are DEEA and little unreacted AEEA.The difference between distribution coefficients of DEEA and AEEA in aqueous and organic phases results in the species distribution.A linear programming model based on distribution coefficients is solved for critical amine concentration which enables biphasic formation upon CO₂ absorption.The calculated results are in good agreement with experimental results.The dynamics of DEEA/AEEA separation process is controlled by intermolecular forces,gravity and viscous forces,in which intermolecular forces and gravity are the driving force of phase separation,and viscous force is phase separation resistance.Increasing temperature and reducing viscosity help to reduce phase separation time.The gas flow rate affects initial disturbance.The smaller gas flow rate,the shorter phase separation time.Phase separator with a lower height-diameter ratio is beneficial to reduce phase separation time.Diethylene glycol dimethyl ether（DEGDME,30-50%）was added into aqueous blends of 2-amino-2-methyl-1-propanol（AMP）/MEA to develop physical-chemical biphasic solvent.Unlike amine blended biphasic solvents,phase separation of physical-chemical biphasic solvent is triggered by the salting-out effect.Physical solvent is unreactive with CO₂.Products including carbamate,bicarbonate,protonated amine dissolve in aqueous phase due to a strong ionic dipole interaction with water.Physical solvent is squeezed out from aqueous phase due to the salting-out effect and forms organic phase.CO₂ capacity of AMP/MEA/DEGDME is 69%higher than that of aqueous AMP/MEA,and the regeneration energy is 2.5 GJ/t CO₂.This paper first proposed theχmethod to normalize the sensible heat based on the effect of phase separation on reduced heat duty and the effect of increased viscosity on the reduced heat transfer coefficient.The normalized regeneration energy of DEEA/AEEA and AMP/MEA/DEGDME is 2.4 and 2.7 GJ/t CO₂,respectively.Theχmethod is also applicable to other solvents with high viscosity,such as water lean amines,ionic liquids.Compared with ionic liquids and other high-viscosity absorbents,biphasic solvents are more advantage in the regeneration energy.Water balance and amine emissions of the biphasic process for CO₂ capture were analyzed based on amine volatility measurements.Amine volatility is greatly affected by molecular structure,temperature,amine concentration,and CO₂ loading.The NRTL model is effective in fitting the thermodynamic parameters of vapor-liquid equilibrium.The results indicate that water balance of CO₂ absorption system is mainly determined by temperature of water wash tower.At the inlet flue gas temperature of40℃,water balance between in and out of CO₂ absorption system is maintained when the temperature of water wash tower is controlled at 41℃.Amine emission is greatly affected by absorber temperature,water wash temperature,and amine volatility.When water balance is achieved for a 150000 t/y CO₂ absorption system,amine emissions of30%MEA,amine blends（ZJU-1）,AMP/MEA/DEGDME and DEEA/AEEA at the outlet of water wash tower are 0.7,18,7,and 50 ppm,respectively.Lowering the water wash tower temperature can further reduce amine emissions.At water wash tower temperature of 25℃,amine emissions of mixed amine（ZJU-1）,AMP/MEA/DEGDME and AEEA/DEEA are reduced to 3.5,1.5,10 ppm,respectively.更多还原