Waste Heat Recovery From Stripped Gas Overhead CO₂ Stripper Using Hydrophobic Membrane Contactor

Purifying biogas can obtain biological natural gas and alleviate the contradiction between supply and demand of natural gas.Among many kinds of CO₂ separation technologies,CO₂ chemical absorption process has the advantages of mature technology and high purity of product gas CH₄,which are very competitive.However,in chemical absorption process,the problem of high CO₂ separation cost needs to be solved sharply,and especially the regeneration heat consumption of CO₂-rich absorbent solution（generally called as rich liquid）is high,which account for more than 60%of the total energy consumption.In the thermal regeneration part of the CO₂ chemical absorption process,the stripping gas（usually the mixture gas of CO₂ and H₂O（g））discharged from the top of the stripper carries a large amount of saturated water vapor,and the temperature is higher,carrying a large amount of latent heat,which is usually taken away by the external circulating cooling water outside the stripper,causing heat waste.Therefore,recovering this part of heat will help to reduce CO₂ regeneration heat requirement.This paper takes the system heat transfer flux as the main index,and discusses the waste heat recovery performance of hydrophobic polytetrafluoroethylene（PTFE）membrane heat exchanger and hydrophilic polyetheretherketone（PEEK）membrane heat exchanger.The influence of parameters and the membrane arrangement and combination mode were studied,and the main conclusions are as follows:In the research of hydrophobic membrane heat exchanger and hydrophilic membrane heat exchanger,cold rich solvent was used as recovery medium to recover waste heat from stripping gas.Heat transfer flux and mass transfer flux were taken as the main indexes,the influences of the parameters such as rich liquid flow rate,temperature,MEA mass fraction,stripping gas flow rate and the H₂O（g）molor fraction of stripping gas were discussed,and the recovery performance of PTFE hydrophobic membrane heat exchanger with pore diameters of 100 nm,200 nm,450 nm and PEEK hydrophilic membrane heat exchanger with pore diameters of 5 nm,20 nm and 100 nm on waste heat of stripping gas was evaluated.The results show that both PTFE membrane and PEEK membrane,increasing the flow rate of rich liquid and decreasing the inlet temperature of rich liquid will greatly increase the heat and mass transfer performance of the system,while changing MEA mass fraction can make little affection on heat transfer performance of the system.However,it is worth noticing that the rich liquid split ratio should not exceed30%.Increasing stripping gas flow rate is helpful to improve the heat and mass transfer flux of the system.The higher the content of H₂O（g）in the stripping gas,the higher the condensation rate of vapor on the gas phase side in waste heat recovery,and the better the heat and mass transfer performance of the system.In addition,the larger the membrane pore size,the higher the recovery rate of waste heat of PTFE and PEEK membranes.On the basis of experimental data,the empirical models of water recovery and heat recovery by PTFE hydrophobic membrane heat exchanger and PEEK hydrophilic membrane heat exchanger with pore diameter of 100 nm was built.It is found that the simulated water recovery flux and total heat transfer flux were basically consistent with the experimental results,and the average absolute percentage error（MAPE）was within10%,indicating that the model has good accuracy.In the experiments of flat membrane waste heat recovery performance under different combination modes,four different membrane series combination modes were studied.Namely,front hydrophobic membrane heat exchanger and rear hydrophilic membrane heat exchanger were connected in series（mode A）.Hydrophobic membrane heat exchanger and hydrophobic membrane heat exchanger were connected in series（mode B）.Front hydrophilic membrane heat exchanger and rear hydrophobic membrane heat exchanger were connected in series（mode C）.Hydrophilic membrane heat exchanger and hydrophilic membrane heat exchanger were connected in series（mode D）.The influences of liquid phase flow rate,temperature,MEA mass fraction,stripping gas flow rate,H₂O mole fraction were deeply studied.The results show that increasing the gas-liquid two-phase flow rate is conducive to improving the mass transfer and heat transfer performance of the system.Increasing the H₂O（g）mole fraction also promotes the water and heat recovery.However,increasing the MEA mass fraction and the rich liquid temperature will lead to the decrease of the heat and mass transfer performance of the system.And under all parameters,mode A shows the best heat transfer performance.For example,when the rich liquid flow rate is 0.72L/h,the heat transfer flux of mode A is18.81 MJ/（m²·h）,while mode B,mode C and mode D are 16.53 MJ/（m²·h）,15.17MJ/（m²·h）and 15.01 MJ/（m²·h）,respectively.In addition,there are three diversion methods that are adopted to investigate the effect of diversion methods,which are gas&liquid splitless,gas splitless&liquid split,and gas split&liquid splitless.The results show that the heat transfer performance of gas&liquid splitless is the best,with a maximum of 42.01MJ/（m²·h）,and the latent heat recovery in water vapor is maximized.