【作者】 沈春枝；

【导师】 于建国； 李平；

【作者基本信息】 华东理工大学 ， 化学工程， 2011， 博士

【摘要】 CO2是导致全球气候变化的主要温室气体之一,其减排已经成为国际社会重大的政治和经济议题。捕集回收燃煤电厂、钢铁厂等固定源排放的CO2不仅是缓解C02排放危机最直接有效的手段,还能通过副产CO2降低减排成本。CO2的捕集技术主要有吸收、吸附和深冷分离等方法,其中真空变压吸附法(VPSA)以其设备简单、能耗低、易于实现自动化操作和腐蚀问题容易处理等优点受到越来越多的关注。然而,目前吸附法的捕集成本仍然偏高,限制了其大规模地工业应用；而降低成本的主要手段在于新型吸附材料的开发和吸附工艺的系统优化。本文采用新型碳材料(沥青基活性炭小球)为吸附剂,系统研究了与之匹配的低能耗真空变压吸附工艺,以满足工业化的需求。首先,采用磁悬浮天平分别测定了0-100 kPa和0-4000 kPa下CO2和N2在沥青基活性炭小球上不同温度下(303、333、363、393和423 K)的吸附等温线,并采用Virial和Multi-site Langmuir等温线模型对吸附平衡进行拟合比较。结果表明,活性炭小球对C02具有良好的吸附量(100 kPa,303 K时吸附量为2.2mmol/g),具有适宜的C02/N2选择性。无论是在低压还是高压下,Virial等温线模型均可以很好地对实验结果进行拟合；Multi-site Langmuir等温线模型能够很好地描述低压下的吸附平衡,但在高压下其拟合结果较差。根据纯组分气体的Virial模型参数可以进一步预测C02/N2双组分的吸附平衡,从而为变温／变压吸附过程的系统优化提供基础数据。其次,采用稀释穿透曲线法测定了303、333、363、393和423K下CO2和N2在沥青基活性炭小球上的扩散系数,建立了双分散二级孔结构扩散模型。结果表明,该模型能够很好地对实验结果进行拟合,N2在活性炭小球上的扩散远比CO2快的多；无论是对于C02还是N2,微孔扩散均为扩散控制步骤。CO2和N2在活性炭小球上的扩散模型的建立以及相关扩散参数的确定,为变温／变压吸附过程的设计和模拟提供另一重要的基础数据。进一步,建立了VPSA过程的数学模型和能耗计算模型,采用单塔实验对模型进行验证,并通过实验和理论模拟,研究了不同进料流速、不同CO2进料浓度、不同操作压力、不同操作温度和不同真空压力对VPSA过程分离性能(包括：产品气纯度、回收率、生产能力和单位能耗)的影响。在此基础上,模拟研究了三塔均压、产品气吹扫等步骤以及不同P/F值、产品气回流比对VPSA分离性能的影响。结果表明：数值模拟结果与VPSA实验过程中的压力、温度和CO2出口流率变化曲线均拟合地较好；在不同操作条件下,采用简单的四步法Skarstrom循环,以沥青基活性炭小球为吸附剂的单级VPSA过程可以将烟道气中的CO2(15%左右)提纯至40%-60%、回收率为40%-96%。最后,模拟设计了两级VPSA流程以得到高纯度的C02,其中：第1级VPSA采用四步法Skarstrom循环,在保证高回收率的情况下,将烟道气中的CO2在近常压下从15%提纯至40%-60%；第2级VPSA采用包含均衡压力的五步循环操作,将第1级VPSA的产品气加压至(2-5)bar后,进一步分离提纯至浓度大于95%(满足CO2储存要求)。结果表明,采用活性炭小球为吸附剂,经过两级VPSA分离后产品气的纯度可达95.29%、回收率为74.36%、吸附剂的生产能力为0.85 mol-CO2/kg/h、总能耗为723.56 kJ/kg-CO2,显示了碳基材料在CO2捕集中良好的工业应用前景。更多还原

【Abstract】 It is widely acknowledge that CO2 is one of the most important greenhouse gases and its mitigation is urgently needed. Capture of CO2 from major industries such as power plants and steel mills has been considered as the most effective solution. Moreover, the capture cost may be reduced by the industrial use of CO2. Among the various capture approaches, absorption, membranes, cryogenic, and adsorption, CO2 capture by vacuum pressure swing adsorption (VPSA) process is a promising option for separating CO2 from flue gas, since it has a number of advantages, such as possible low energy requirement, low capital investment cost and easy to achieve automated operation. The major problem of VPSA process is the cost is relative high for CO2 capture, which restricts its industrial applications. The crucial point to reduce the CO2 capture cost is the use of new adsorbents and the optimization of adsorption process. Thus, this paper was focused on the design and optimization of VPSA process with the use of novel CO2 adsorbent material (pitch-based activated carbon beads), which was aimed to reduce the capture cost.Firstly, adsorption isotherms of CO2 and N2 on AC beads were gravimetrically measured at 303,333,363,393 and 423 K in the range of 0-100 kPa and isotherms at 303 and 333 K were also measured up to 4000 kPa in a magnetic suspension microbalance. A good CO2 capacity (at 100 kPa and 303K,qCO2=2.2 mmol/g) and good selectivity of CO2/N2 can be observed on AC beads. Virial equation can fit the isotherms quite well both at low and high pressure ranges, while Multi-site Langmuir model only showed good fitting at low pressures. The fitting parameters were reported and can be employed in the prediction of multi-component adsorption equilibrium, which provided basic data to optimize a VPSA unitSecondly, adsorption kinetics of CO2/N2 on AC beads was measured by diluted breakthrough curves at 303,333,363,393 and 423 K, respectively. An isothermal mathematical model was established for components diffusion in bi-disperse adsorbent. It was showed that the model can fit the breakthrough curves very well, and micropore diffusions were obtained by the numerical fitting. At the above mentioned temperatures, the diffusion of N2 on AC beads was much faster than CO2, and micropore resistances controlled the diffusion mechanism for both CO2 and N2. With the parameters of the mathematical model for diffusion, diffusivity at various temperature can be calculated which provided another important basic data for the design of VPSA process.Thirdly, a mathematical model taking into account mass balance, energy balance, and Ergun relation for pressure drop, was derived to describe the VPSA process. Single-column experiments were performed to verify the model and to evaluate the effect of different operating parameters on the VPSA performance (purity, recovery, productivity and specific energy consumption). Moreover, effects of operating parameters and process configurations on the multi-bed VPSA performance were investigated theoretically. It was showed that the mathematical model described the pressure histories, temperature change and CO2 flowrate very well. Using the AC beads, CO2 purity of (40-60)% with recovery of (40-96)% was obtained using a four-step Skarstrom cycle VPSA.Finally, a multi-bed two-stage VPSA process was designed for CO2 capture. At the lst-stage, CO2 was concentrated to about (40-60)% when the flue gas feeding at almost atmospheric pressure and employing four-step Skarstrom cycle. Then the product gas of the first stage was compressed to (2-5) bar and feed to the second stage VPSA process, where CO2 was further concentrated to above 95%. The 2nd-stage VPSA unit operated with a cycle with feed pressurization, adsorption, pressure equalization, blowdown and pressure equalization. With the designed two-stage VPSA process, a CO2 purity of 95.29% was obtained with 74.36% recovery. The total specific power consumption of the two-stage VPSA process is 723.56 kJ/kg-CO2, while the unit productivity is 0.85 mol-CO2/kg/h.更多还原