Membrane heat exchanger for novel heat recovery in carbon capture
Graphical abstract
Introduction
Because of its technical superiority and successful commercial implementation, post-combustion carbon capture (PCC) using aqueous alkanolamine solvents is considered as one of the most mature methods to reduce CO2 emissions [1], [2], [3]. However, CO2 chemical absorption still requires huge energy inputs for absorbent regeneration. Many strategies, such as screening new CO2 absorbents with low regeneration energy [4], inter-cooling [5], absorbent splitting [6], absorption-mineralization integration [3], [7], and latent heat recovery from water vapor [8], [9], [10], have been studied to reduce the CO2 capture energy penalty.
The rich-split process modification has been used to recover the waste heat by splitting a partial cold CO2 rich solvent before the lean-rich heat exchanger to contact with the stripped gas mixture (i.e., H2O(g)/CO2) on top of the stripper in carbon capture pilot plants [6], [11]. Metal heat exchangers have been demonstrated for such heat recovery from the H2O(g)/CO2 during regeneration [12]. However, metal heat exchangers may not be efficient for a low-temperature gas mixture and face the issue of equipment corrosion caused by the gas mixture of CO2 and water vapor. Membrane heat exchangers can offer better performance in heat recovery since both mass and heat transfer occurs, whereas only heat transfer occurs in conventional heat exchangers.
In our previous study, thermodynamic analysis of mass and heat transfer in a membrane heat exchanger with rich split was performed to estimate the heat recovery potential in post-combustion carbon capture [10]. In the concept (Fig. 1), water vapor from the H2O(g)/CO2 transfers through a membrane contactor into the bypassed cold rich solvent to enhance the heat recovery performance, and the gas-liquid membrane contactor acts as a novel membrane heat exchanger. Similar gas-liquid membrane contactors have been used as membrane condensers for water and heat recovery from waste gas streams, such as power station flue gas [13], [14], [15], [16], [17], [18]. However, membrane heat exchangers have not been experimentally demonstrated for heat recovery in carbon capture yet.
In this study, we for the first time experimentally investigated the heat recovery performance by a hydrophilic nanoporous ceramic membrane heat exchanger (CMHE) with rich split modification in post-combustion carbon capture (Fig. 1). CO2 loaded monoethanolamine (MEA) was selected as the rich split. A CMHE and a conventional stainless-steel heat exchanger (SSHE) were used and compared in terms of heat recovery performance and overall heat transfer coefficients. The superiority of the permeable CMHE to the impermeable SSHE was investigated. Furthermore, the heat and H2O transfer mechanisms were explored to provide perspectives on further tailoring membrane characteristics.
Section snippets
Heat exchangers
A monochannel nanoporous tubular ceramic membrane (Nanjing Aiyuqi Membrane Technology Co., Ltd., China) was used as the ceramic membrane heat exchanger (CMHE) and a custom-built 304 stainless steel tube was used as the conventional stainless-steel heat exchanger (SSHE) for heat recovery. Both heat exchangers had the same dimensions: inner diameter (ID) 8 mm, outer diameter (OD) 12 mm and total length 300 mm. The SSHE had an effective length of 300 mm, while the CMHE had an effective length of
Results and discussion
For heat recovery with a membrane heat exchanger, parameters of the gas phase and the liquid phase will affect the heat recovery performance. These parameters include the liquid phase (i.e., rich MEA solvent) inlet temperature and flow rate, and the gas phase (i.e., CO2 and water vapor) inlet pressure, flow rate, temperature and ratio of water vapor to CO2. Therefore, these parameters were systematically investigated.
Conclusion
This work, for the first time, experimentally demonstrates the heat recovery performance of a commercial hydrophilic ceramic membrane heat exchanger (CMHE) in post-combustion carbon capture through a rich-split process modification. In the CMHE, there is a strong linear relationship between heat flux and water flux. For the membrane heat exchanger, liquid water transfer dominates the mass transfer mechanism. Thermal conduction is the key heat transfer mechanism through the membrane heat
Acknowledgements
The project was supported by the National Key R & D Program of China (No. 2017YFB0603300), National Natural Science Foundation of China (No. 51676080), Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control (No. 2017B030301012), and State Environmental Protection Key Laboratory of Integrated Surface Water-Groundwater Pollution Control.
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