基于CO2/H2O(g)再生气膜热回收机制的低能耗沼气CO2化学吸收研究

Chemical absorption technology is worthy of considering for CO2 capture from biogas to generate bio-natural gas in terms of its robust technical superiority. Determining how to reduce the overall energy consumption, especially reduce CO2 regeneration heat requirement herein is the keystone for CO2 chemical absorption from biogas. If part of the cold CO2-rich solvent without being heated by the rich/lean exchanger is bypassed and sent to recover the huge latent heat from the stripping steam, i.e. CO2/H2O(g) in the stripper overhead, CO2 regeneration heat consumption may be reduced greatly. And it is very easy to bypass the cold rich solvent. It should be noted that it is of importance to both enhance the heat exchange performance and suppress CO2 mass transfer from stripping steam to rich solvent in terms of its economy and feasibility. In this study, a new mechanism is put forward to recover heat from the stripping steam by using the porous hollow fibre membrane contactor as a barrier between gas and liquid phases with permitting H2O(g) free to diffuse through the membrane pores. H2O(g) will be condensed into the membrane pores with the appropriate pore size, but the length of condensation will be controlled not to fully fill the pores in order to enhance the coupled heat and mass transfer coefficients greatly. And mass transfer of CO2 will be blocked effectively ascribed to the low solubility of CO2 into water condensed in the pores. Furthermore, the mechanism of H2O(g) mass transfer enhancement in addition to the blocking effect of CO2 mass transfer in the membrane pores by the use of capillary condensation will be analysed and understood. The mechanism of absorbent loss due to the mass transfer of absorbent molecules from solvent to the stripping steam will be investigated as well. Additionally, the overall heat transfer mechanism of membrane heat recovery system will be explored when considering the contribution of H2O(g) mass transfer. When these mechanisms were determined, the operating parameters featuring perfect heat transfer characteristic and negligible CO2 mass transfer coefficient will be ultimately achieved. And the thermodynamic model on heat recovery from the stripping steam using membrane contactor will be built as well. On the basis of these constructive results, a novel and positive concept might be developed for reducing the overall energy consumption of CO2 capture from biogas in the future.

采用化学吸收技术分离沼气中CO2而制备生物天然气的重点在于降低CO2分离能耗，尤其是降低富CO2吸收液的再生热耗。在化学吸收工艺中，将贫富液热交换之前的部分冷富液分流，用于回收再生塔顶高温CO2/H2O(g)再生气携带的巨大潜热，有助于大幅降低再生热耗，且工艺简单，其关键是在经济可行范围内强化气-液换热性能和抑制CO2向富液传质。基于此，本项目采用多孔中空纤维膜接触器作为再生气和分流富液间的换热介质，实现再生气中H2O(g)向富液的热质耦合传递，并通过膜孔径筛选与参数优化保证H2O(g)在膜孔内某一区域冷凝，协同实现H2O(g)热质耦合传递强化与CO2传质阻塞。通过解析H2O(g)膜毛细冷凝传质机理、CO2传质阻塞机制、吸收剂逆向迁移损失机理和H2O(g)传质协同背景下的系统传热机制，建立膜热回收热力学模型，掌握高效传热和低CO2传质操作参数范围，期望为低耗沼气CO2分离提供一种新思路。

在沼气CO2化学吸收工艺中，将进入再生塔前的部分冷富CO2吸收液分流，用于回收再生塔顶热再生气的余热，可降低富液再生热耗。本项目利用具有纳米级膜孔的亲水陶瓷膜管与不锈钢壳体构筑了陶瓷膜换热器（CMHE），并将其作为分流冷富液与再生气间的换热介质，实现余热回收强化。基于CMHE构建了模拟热再生气余热回收试验系统，并在典型MEA体系下进行了约530h的连续试验，发现系统的余热回收性能波动幅度在±10%以内，具有良好的稳定性和可靠性。余热回收中，水蒸气和冷凝水均能透过膜孔传递至冷富液侧，但冷凝水传质占总传质量85%以上。在活跃水蒸气分子数密度梯度的驱动下，水蒸气可在膜孔内传递，但绝大部分水蒸气先在膜表面冷凝，然后在跨膜压差和毛细力梯度共同作用下进入纳米膜孔内，并依靠黏性表面流传质。传质引发的对流换热可强化热传递，但热传递主要以陶瓷膜导热传递为主，其可占总热通量80%以上。当余热回收量为300-800kJ/kg-CO2时，4nm孔径CMHE的换热面积可比不锈钢换热器小11.68-45.82%。由于水蒸气在纳米膜孔内的毛细冷凝机制，吸收剂分子和CO2的传质被阻塞。膜余热回收中，增加富液分流率、降低富液温度和吸收剂浓度，均有助于强化余热回收性能。但过高的富液分流率反而会导致再生能耗大幅上升，因而分流工艺存在最佳富液分流率。研究中，也构建了基于4nm和10nm孔径CMHE的余热回收数学模型，且模型计算值与试验值间的平均绝对相对偏差分别为10.17%和10.67%。最后，搭建了沼气CO2化学吸收-膜余热回收试验系统，系统的质量平衡误差可控制在±3.5%以内。在MEA体系的真实再生中，常规富液分流工艺的最佳富液分流率约为0.1，此时再生能耗可比不分流工艺降低约7.3%。而采用CMHE的富液分流工艺，在0.3分流率下，再生能耗可降低约21.7%，可为低能耗沼气CO2分离提供一种新思路。