Study On Carbon Dioxide Capture Using A Novel Biphasic Solvent

Chemical absorption with aqueous alkanolamine solutions as absorbents is a widely used carbon dioxide（CO₂）capture method during industrial processes.However,a large amount of energy is required for the regeneration of alkanolamine absorbents.Biphasic solvents,which show great potential to reduce the energy consumption of CO₂ capture,have been regarded as ideal alternatives to alkanolamine absorption.This new class of solvents can form two immiscible phases after CO₂absorption,and only the CO₂-rich phase needs to be sent to the stripper.Thus,the energy requirement is excepted to be reduced due to the decreased amount of solution for regeneration.The present work aims at developing novel biphasic solvents for energy-efficient CO₂ capture.The main contents of this thesis are as follows:（1）A novel biphasic solvent consisted of a polyamine（diethylenetriamine,DETA）and a tertiary amine（pentamethyldiethylenetriamine,PMDETA）was developed.It was found that the CO₂ loading of the DETA-PMDETA biphasic solvent（5 mol/L（M）,1:4）could reach 0.613 mol CO₂/mol of total amine,in which 99.7%of the absorbed CO₂ was concentrated in the lower phase with 57%of total volume.The reaction mechanism indicated that DETA not only played as an absorption accelerator but also contributed the CO₂ absorption capacity in the biphasic system,while PMDETA functioned as a proton acceptor and was involved in the deprotonation process.The phase change mechanism revealed that CO₂ products had limited solubility in PMDETA,and migrated to the lower phase due to their high density,while PMDETA migrated to the upper phase.（2）2-Amino-2-methyl-1-propanol（AMP）was used as a regulator to overcome the high viscosity and inferior regenerability of the CO₂-rich phase of DETA-PMDETA biphasic solvent.By optimizing the composition,the optimal biphasic solvent was considered 0.5 M DETA+1.5 AMP+3 M PMDETA（0.5D1.5A3P）.Based on various characterizations,the regulatory mechanism of AMP in the biphasic system was elucidated.In the DETA-PMDETA system,DETA-carbamate was the dominant CO₂ product.But a large proportion of absorbed CO₂ was in form of HCO₃^-/CO₃^2-as AMP was introduced into the biphasic system.Since HCO₃^-/CO₃^2-did not tend to form hydrogen bonds,and decomposed more easily than DETA-carbamate,the saturated DETA-AMP-PMDETA system presented a lower viscosity and better regenerability.Moreover,in the presence of AMP,free protons could be easily produced by multiple ways to facilitate the dissociation of DETA-carbamate.（3）Based on the"zwitterion mechanism"and"two-film theory",kinetics models for CO₂ absorption into DETA,AMP,PMDETA solutions and DETA-AMP-PMDETA biphasic solvents were established.Compared with the kinetics of CO₂ absorption into the individual amine solution,it revealed that the CO₂ reaction rate was mainly determined by DETA and AMP in the biphasic system.In addition,PMDETA molecules would aggregate to form PMDETA clusters in the biphasic system,limiting the absorption of CO₂ to some extent.The kinetics of CO₂ absorption into the DETA-AMP-PMDETA biphasic system could be described by a simple pseudo-first-order kinetics model.The values of the total reaction rate constant k_ov,mixv,mix predicted by the model were in agreement with the experimental data.（4）An evaluation of regeneration energy consumption of CO₂ capture by DETA-AMP-PMDETA biphasic solvents was conducted.The result showed that the regeneration heat of the 0.5D1.5A3P system was 1.83 GJ/ton CO₂,which was approximately 52%lower than that of the MEA solution.The energy-saving advantages of biphasic solvents were mainly attributed to the low sensible heat and latent heat during regeneration.In summary,the biphasic solvent studied in the present work has good CO₂absorption-desorption performances and low regeneration energy consumption,which can be a promising candidate for efficient and energy-saving CO₂ capture.