Performance Enhancement Of Waste Heat Recovery From Desorbed Gas Discharged From The Top Of CO₂ Desorber Using Transport Membrane Condenser

Among numerous CO₂ emission reduction technologies,Carbon Capture,Utilization,and Storage（CCUS）is one of the technologies that can significantly reduce CO₂ emissions from fossil fuels use.In CCUS,CO₂ capture technology is the core.As one of the most promising CO₂ capture technologies,organic amine absorbent-based CO₂ chemical absorption technology is one of the most promising CO₂ capture processes for large-scale application due to the advantages of technological maturity and simple operation.However,the huge desorption heat consumption still restricts the commercial application of CO₂chemical absorption process.In the desorption process,the desorbed gas（mixture gas of CO₂and H₂O（g）,CO₂/H₂O（g））discharged from the top of the CO₂ stripper carriers amounts of heat,which is often taken away by external circulating cooling water in traditional CO₂chemical absorption process,resulting in heat loss.Therefore,recovering the waste heat from desorbed gas will help to reduce the desorption heat consumption.Based on this,this study proposes to use a single-channel nano-porous Al₂O₃ ceramic membrane in modified rich-split CO₂ capture process for recovering the waste heat and water vapor from desorbed gas and then returning that heat into desorber to reduce the heat consumption of CO₂desorption process using the rich-split solvent as the heat recovery receptor.However,since ceramic membranes are expensive,it is necessary to boost the waste heat recovery performance of ceramic membrane heat exchangers（CMHE）.This study mainly investigates the following aspects,including building the steady transmembrane condensation waste heat recovery system,the adaptability of the waste heat recovery system from desorbed gas,the heat and mass transfer mechanism of CMHE,the screening of the characteristics of CMHE,and the enhancement of waste heat recovery performance of CMHE.The main research conclusions are as follows:（1）A stable operation of absorption gas waste heat trans-membrane condensation recovery device was constructed,and the feasibility of the waste heat recovery system were studied in different absorbent systems.The results showed that,when CMHE consisting of mono-channel nano-porous ceramic membrane tube and stainless-steel shell was adopted to recover the waste heat from simulated desorbed gas formed by H₂O（g）and CO₂ during a cumulative operation time of 1527 hours,the inlets and outlets of the desorbed gas and the rich solvent were stable and the water and heat recovery fluxes fluctuated within 15%,indicating that the membrane waste heat recovery system had good long-term stability.The feasibility of the transmembrane condensation heat recovery system was explored in the systems of monoethanolamine（MEA）,diethanolamine（DEA）,potassium glycinate（PG）,and piperazine（PZ）.The study found that regardless of which solvent adopted,the transmembrane condensation waste heat recovery system could recover the waste heat from desorbed gas.Under the four absorbent systems,the waste heat recovery flux was linearly related to the water recovery flux,suggesting that the membrane waste heat recovery performance could be improved by increasing the water recovery performance of the membrane.The economic analysis of the waste heat recovery system based on the four absorbents was built and the results showed that recovering the heat from desorbed gas could help to reduce the cost of carbon capture process.The PG and the PZ absorbent systems had the largest cost reduction,which accounted for 35.56 RMB/t-CO₂ and 45.70RMB/t-CO₂at most,respectively.（2）The coupled heat and water transfer mechanism in the membrane was analyzed,and an empirical formula of the waste heat recovery flux of the membrane and the operating parameters was established.During the mass transfer process,due to the initial hydrophilicity of the ceramic membrane,most of the water vapor first condensed through film condensation on the membrane surface in the gas side and also through capillary condensation inside the membrane pores,and then the condensate was transferred to the solvent side in a viscous flow manner driven by the capillary force and transmembrane pressure difference.At the same time,H₂O（g）would also transfer to the solvent side in the form of multi-layer diffusion driven by the transmembrane pressure difference of H₂O（g）.The transition of H₂O（l）dominated the whole mass transfer,which accounted for over 85%of the total mass transfer.During the heat transfer process,the heat was transferred through either membrane conduction or heat convective related to mass transfer,and the heat conduction dominated the whole heat transfer process which accounted for more than 80%of the total heat transfer flux.In the heat convective transfer process,the heat convective relevant to H₂O（g）transition accounted for more than 70%of the entire heat convection flux.The transition of MEA and CO₂ were blocked by capillary condensate formed in the membrane pores by H₂O（g）,which could be ignored.The empirical formula of the heat recovery flux and the operation parameters showed that,the heat recovery flux errors between the calculated values and the experimental values were within±15%,proving that the empirical formula was reliable.According to the empirical formula,the influence degree of the operating parameters on the membrane waste heat recovery performance could be ranked as follows:the mole fraction of H₂O（l）in desorbed gas,solvent inlet flow rate,solvent inlet temperature,desorbed gas inlet flow rate,absorbent concentration/desorbed gas inlet temperature.（3）The waste heat recovery performance was optimized by varying the arrangement of ceramic membrane tube or CMHE under the condition of same membrane area.The CMHE,composed of 4 nm pore-size ceramic membrane tubes and a stainless-steel shell,was selected in this study.Five different arrangements of CMHEs were considered:Mo CMHE-A（two 200 mm long CMHEs were connected in series and both gas and liquid were not distributed）,Mo CMHE-B（two 200 mm long CMHEs were connected in parallel,and both gas and liquid were parallel distributed）,Mo CMHE-C（two 200 mm long CMHEs were connected in series and the liquid is parallel distributed but gas was not distributed）,Mu CMHE-D（two cores,consisted of two 200 mm long membrane tubes and a 200 mm long shell）,and Mu CMHE-E（four cores,consisted of four 100 mm long membrane tubes and a 100 mm long shell）.The results showed that Mo CMHE-A had the best heat recovery performance,followed by Mu CMHE-E,Mu CMHE-D,Mo CMHE-B,and Mo CMHE-C had the worst heat recovery performance.In addition,the waste heat recovery performances of Mo CMHE-A and Mu CMHE-E were 8.71%-20.09%and 4.74%-32.75%higher than that of monotube-monochannel CMHE with the same membrane area,respectively,demonstrating a good potential for industrial applications.（4）The Janus membrane was prepared and its heat recovery performance from desorber gas was explored and strengthened.The hydrophobic membrane surface of the Janus membrane was successfully prepared by a simple and efficient immersion method,and the modified membrane was stable at temperatures below 170°C.When using the Janus membrane as the heat exchanger medium,the hydrophobic membrane surface of the Janus membrane should face to the desorbed gas to achieve a higher waste heat recovery performance.Meanwhile,the waste heat recovery performance of the Janus membrane increased first and then decreased with the increase in the hydrophobicity of the hydrophobic membrane surface of the Janus membrane.In this study,the optimal water contact angle of the membrane surface was approximately 106±1.3°,and the waste heat recovery performance of the Janus membrane was 9.04%-42.92%higher than that of hydrophilic ceramic membranes of the same size.Moreover,once the hydrophobicity of the hydrophobic membrane surface of the Janus membrane was fixed,the heat recovery performance of the Janus membrane was almost constant regardless of the grafting conditions used.（5）A hydrophilic-hydrophobic composite ceramic membrane with both hydrophobic and hydrophilic surfaces on the same side of the membrane surface was constructed,achieving efficient heat recovery from hot desorbed gas.The results showed that when the desorbed gas entered the shell of CMHE from the hydrophobic segment of the composite membrane,the CMHE could achieve a higher heat recovery performance.The heat recovery performance showed a trend of first increasing and then decreasing with the increase of the length of the hydrophobic segment.At the same time,increasing the water contact angle of the membrane surface of the hydrophobic segment was conducive to increase the heat recovery performance of the composite membrane.When the length proportion of the hydrophobic segment is 25%and the membrane surface water contact angle of the hydrophobic segment is 137.35°,the heat recovery performance of the integrated membrane is 7.44%-25.15%higher than that of the hydrophilic membrane.In conclusion,the constructed transmembrane condensation waste heat recovery system of hot desorbed gas in this study had good stability.At the same time,the Janus ceramic membrane and the hydrophilic-hydrophobic composite ceramic membrane proposed in this paper had higher heat recovery performances than the hydrophilic ceramic membrane with the same scale and adopting that CMHEs helped to reduce desorption heat consumption of carbon capture process.The present study provides some new tack for efficiently recovering the waste heat from the mixture gas with H₂O（l）using ceramic membrane heat exchangers.