Preparation And Modification Of Polyvinylidene Fluoride-based Composite Membrane For Waste Heat Recovery

Carbon capture,utilization and storage（CCUS）is one of the key technologies to reduce CO₂ emissions and achieve the goal of carbon neutrality.Among the many carbon capture technologies,CO₂ chemical absorption is the most mature and widely used,which is worthy of attention.However,it also has the bottleneck of high energy consumption of CO₂-rich solvent（Short for rich solvent）.The heat exchanger is added to the top of heat CO₂ stripper of the CO₂ chemical absorption system,and the waste heat in the heat stripped gas（CO₂ and H₂O（g）mixture）discharged from the top of CO₂ stripper is recovered by split CO₂-rich solvent（Namely the rich-split carbon capture process）,which can reduce the energy consumption of the rich solvent regeneration.Under the same rich-split condition,the better the waste heat recovery performance is,the more obvious the consumption reduction effect of the renewable energy consumption is.Using membrane heat exchanger instead of traditional steel heat exchanger,the waste heat recovery performance of heat exchanger can be enhanced through heat-mass coupling transfer.Compared with the ceramic membrane which has been studied more,the organic membrane has the advantages of lower cost and better toughness.Theoretically,it can also be applied to the recovery of waste heat from stripped gas.However,organic membrane also has some defects such as poor thermal conductivity,which leads to the limitation of waste heat recovery.Based on this,this paper takes polyvinylidene fluoride（PVDF）organic membrane as the main body to prepare composite organic membrane.At the same time,the basic characteristics of the composite membrane are changed through modification,so as to strengthen the waste heat recovery performance,and realize the efficient recovery of waste heat from the top of the CO₂ stripper.Using heat recovery flux as the main index,the waste heat recovery performance of PVDF/BN-OH composite membrane with enhanced thermal conductivity and Janus membrane with one side hydrophilic and one side hydrophobic was investigated,and the empirical relationship between heat recovery rates of different composite membranes and key experiment parameters was fitted.Relevant research conclusions are as follows:（1）The composite membrane was prepared under different PVDF concentrations and PVP addition amount,and the basic characteristic parameters and waste heat recovery performance of the membrane were compared and analyzed to determine the optimal PVDF concentration and PVP addition amount during the preparation of the composite membrane.The results showed that when the PVDF concentration increased from 10 wt.%to 18 wt.%,the water contact angle on the PVDF membrane increased by about 18.6%.The porosity of composite membrane decreased by about 25.8%.The tensile strength of the composite membrane increased by about 33.8%.At the same time,the pore size of the composite membrane decreases gradually and the overall density of the composite membrane increases.When the amount of PVP increased from 1 wt.%to 5 wt.%,the average pore diameter of the composite membrane increased by about 17.4%.The tensile strength decreased by 16%.Water contact Angle decreased by about 8.3%.The porosity increased by 13.8%.Considering the basic characteristics and waste heat recovery performance of the composite membrane,the PVDF concentration and PVP addition amount during the preparation of the composite membrane were determined to be 14 wt.%and 5 wt.%,respectively.（2）The PVDF/BN-OH plate composite membrane was prepared by using the PVDF composite membrane as the main body,adding hydroxylated boron nitride（BN-OH）blending modification,and using PET non-woven fabric as the support layer.The waste heat recovery performance of the composite membrane was discussed in the ethanolamine（MEA）based rich-split chemical absorption process.The results were compared with commercial PVDF membrane.The results showed that the average pore size of the composite membrane（M3 composite membrane）prepared by adding 1 wt.%BN-OH increased by 11.32%,and the porosity decreased by 7.14%,compared with that of the composite membrane（M1 composite membrane）without BN-OH.Meanwhile,the thermal conductivity also increased by about 52.25%,which has the potential to strengthen the heat-mass coupling transfer characteristics.The research also shows that both M1 and M3membranes can be successfully applied in the split-rich carbon capture process to recover waste heat from heat stripped gas,and the waste heat recovery flux increases with the increase of the rich solvent inlet flow,stripped gas flow and water vapor content in the stripped gas,but decreases sharply with the increase of the rich solvent inlet temperature.Meanwhile,compared with M1 composite membrane,under the same operating parameters,the waste heat recovery flux and heat recovery of M3 membrane can increase by 95.5%and 31.6%,respectively.Compared with commercial PVDF membrane with smaller thickness,the waste heat recovery flux and heat recovery of M3 composite membrane can be increased by 54.8%and 9.6%at the maximum,showing better waste heat recovery characteristics.（3）PTFE/PET composite membrane was used as the base,PVDF membrane was coated on PET side,and PTFE/PET/PVDF composite membrane（Janus membrane M4）with one side hydrophilic and one side hydrophobic was prepared through phase separation,and used for waste heat recovery of heat stripped gas.The results showed that compared with PTFE/PET composite membrane,the pore size of PTFE/PET/PVDF composite membrane increased by 7.4%,and the porosity decreased by 9.3%.The tensile strength increased by 13.7%,while the water contact angle of the hydrophilic side decreased by21.8%（enhanced hydrophilicity）.Among the two modes of M4 membrane,M4-A mode（hydrophilic PVDF membrane surface side to stripped gas,hydrophobic PTFE membrane surface side to rich solvent）has better waste heat recovery performance than M4-B mode（hydrophilic PVDF membrane surface side to rich solvent,hydrophobic PTFE membrane surface side to stripped gas）.Compared with M1 membrane with double-sided hydrophilic membrane with similar thickness,M4 membrane has better waste heat recovery performance in M4-A mode,and its waste heat recovery flux and heat recovery are improved by 26.9%and 27.7%compared with M1 membrane at the highest.（4）The empirical relationship between heat recovery and key operating parameters of M1 composite membrane,enhanced thermal conductivity composite membrane（M2,M3）,commercial PVDF membrane and Janus membrane with one side hydrophilic and one side hydrophobic（M4 membrane includes M4-A and M4-B）was constructed.The average relative errors of the calculated thermal recovery（HRR）values of M1 membrane,M2membrane,M3 membrane,commercial PVDF membrane and M4 membrane（M4-A and M4-B）and their actual experiment values are 7.37%,7.95%,8.19%,6.06%,9.74%and9.49%,respectively,indicating that the empirical model constructed has good accuracy.In addition,based on the empirical model,the significant influence of key operating parameters on the waste heat recovery performance of different types of composite membranes was determined.For M1 composite membrane,thermal conductivity enhanced composite membrane（M2,M3）and commercial PVDF membrane,the inlet temperature of rich solvent has the most significant influence,while the influence of stripped gas temperature is not obvious.For Janus membranes（M4-A,M4-B）with one side hydrophilic and one side hydrophobic,the effect of split rich solvent temperature is the most significant among all parameters,but the effect of stripped gas temperature is also very significant.Therefore,in waste heat recovery,the inlet temperature and stripped gas temperature of split rich solvent should be kept stable to ensure the stability of waste heat recovery performance.In general,enhancing the thermal conductivity of organic membranes and changing the hydrophilic and hydrophobic properties of organic membranes by adding thermal conductive fillers can help strengthen the waste heat recovery performance of organic membranes,which can provide a new idea for reducing the consumption of rich solvent regeneration in CO₂ chemical absorption process.