Preparation Of Nanofluids And Their Enhanced CO₂ Absorption By Alcoholamines

The carbon emissions from the burning of fossil fuels continue to strengthen the greenhouse effect and become the main cause of climate warming.Advanced CCUS（carbon capture,utilization and storage）technology will alleviate the greenhouse effect,improve resource utilization,and contribute to the sustainable development of mankind.At present,chemical absorption is still the main way to separate CO₂ from flue gas emitted by power plants,among which alkanolamines are the most widely used CO₂ absorbents.As a new type of gas-liquid mass transfer working medium,nanofluids can effectively expand the mass transfer area and improve the gas diffusion coefficient in the liquid phase.In this paper,nanoparticles were added to traditional alkanolamine absorbents to prepare nanofluids,in order to optimize the gas-liquid mass transfer process between CO₂ and the absorbent,and improve the existing CO₂capture system.In this paper,the“two-step method”combined with ultrasonic treatment and surfactant addition was adopted to prepare stable nanofluids.The optimal ultrasonic time was 40 min,and0.1wt%sodium polyacrylate（ASAP）was added as a dispersant.The stability of the nanofluids could be maintained for more than 18 h.The 30wt%ethanolamine（MEA）,N-methyldiethanolamine（MDEA）and MDEA+MEA blended amine aqueous solutions were selected as base fluids,and the nanoparticles included Ti O₂,Cu O,Si O₂,as well as carbon sphere（CS）,carbon nanotube（CNT）,activated carbon（AC）.A system for CO₂ bubbling absorption by nanofluids was built.Based on the enhanced gas-liquid mass transfer mechanisms of nanofluid and the kinetic analysis of CO₂ absorption by MEA/MDEA base fluids,the processes and impact factors of alkanolamine nanofluids enhanced CO₂ absorption were explored.Starting from the viscosity model of nanofluids,an empirical equation for predicting the enhancement factor of alkanolamine nanofluids was derived.The following conclusions were obtained.The absorption enhancement factor of nanofluids increased first and declined then with the increasing of nanoparticle concentration,so there was an optimal content of nanoparticles in the base fluid.In 30wt%MEA base fluid,the optimal solid contents of Ti O₂,Cu O and Si O₂ nanoparticles with a particle size of 40 nm were 0.6 g/L,0.4 g/L and 0.4 g/L,respectively.The order of enhancing effect on CO₂ absorption by MEA was Ti O₂>Cu O>Si O₂.Among the different shapes of carbon nanoparticles（CS,CNT and AC）,the enhancement of CO₂ absorption by AC nanoparticles was the most significant due to the special properties of the porous structure.Nanoparticles act on diffusion-controlled processes and therefore have a higher enhancement factor in base fluids with fast reaction kinetics.The increase of air pressure,total flow of mixed gas and CO₂ concentration in mixed gas could promote the reaction to a diffusion-controlled process and enhance the absorption of CO₂ by nanofluids.Compared with the one-component MEA/MDEA nanofluids,the absorption enhancement of nanoparticles in MDEA+MEA blended amines was greater.As for MDEA+MEA blended amine nanofluids,the high concentration of MEA in the base fluid is not conducive to the enhancement of gas-liquid mass transfer by nanoparticles.The enhancement factor of Ti O₂ nanoparticles in the base fluid with a ratio of 25%MDEA+5%MEA could reach up to 1.36.