Study On Intensification On CO₂ Capture And Its Mass Transfer Process Using Rotating Packed Bed

Combustion process of fossil fuels leads to large emission of carbon dioxide(CO2),which is considered as a vital contributor to global waring.CO2 capture by chemical method using amine-based absorbents is the most matured technology,and monoethanolamine(MEA)is the most commonly-used absorbent.However,MEA faces many drawbacks,like low CO2 capacity,strong corrosive to equipments,high oxidative degradation and thermal degradation loss,and large regeneration energy consumption,and moreover,it needs high capital cost for absorber and desorber.Therefore,developing high-efficiency absorbent and absorber is the key to the development of CO2 capture technology which can be applied in an industrial scale.Rotating Packed Bed(RPB),a classic process intensification device,can efficiently intensify the mass transfer process and lower equipment's size and room space,thereby reducing the investment cost.This work aims to explore a promising CO2 capture technology which can be applied in an industrial scale,which adopted high-gravity technology to intensify CO2 capture into amine absorbents in order to lower equipment's size and reduce investment cost.In this work,the applicability of steady-state and unsteady-state mass transfer process under high-gravity environment was firstly discussed and the mechanism of intensification on mass transfer process by RPB was simultaneously analyzed.Then,CO2-NaOH system was adopted to measure the effective mass transfer area of RPB,and pipeline-type liquid distributors was used to explore the effect of initial liquid distribution on effective mass transfer area.Further,the intensified CO2 capture process into amine absorbents by RPB was studied and a gas-liquid mass transfer model accompanied by reversible chemical reaction was established using penetration theory,which can provide a valuable assistance in reactor design and process development.Main conclusions are list below:1.Steady-state and unsteady-state liquid mass transfer coefficients accompanied by chemical reaction were obtained through solving reaction-diffusion equation to discuss the applicability of steady-state and unsteady-state mass transfer assumptions for establishing gas-liquid mass transfer model and analyze the mechanism of intensification on mass transfer process.Results indicate that,chemical reaction rate plays a key role in the assumption of steady-state mass transfer,and whether the liquid-phase mass transfer process can be considered to be steady-state may be pre-judged by chemical reaction rate and the lifetime of liquid elements;the steady distribution of soluble gas components in the liquid phase cannot be established due to the very short lifetime of liquid elements in the RPB and the sharp concentration profile near gas-liquid interface can greatly intensify the liquid-phase mass transfer process during the movement of liquid along with the radial direction,which is the essential reason that RPB can intensify the liquid-phase mass transfer process.Through the further analysis and comparison,it is found that high effective mass transfer area of RPB is a vital factor for improving the mass transfer efficiency;2.CO2-NaOH system was adopted to measure the effective mass transfer area of RPB,and it is found that the source of parameters shows a vital effect,which becomes more significant with increasing ionic strength,on the measurement results.Results indicate that,the commonly-used test conditions used in traditional packed beds may not be suitable for RPB and the method through changing the liquid composition(with CNaOH in the range of 0.5-4 mol/L)is proposed to search the suitable range with lowest variation in the measurement results and the result measured within this range is close to the true value.This method can efficiently avoid the effect of gas-phase mass transfer resistance.At the same time,special attention needs be paid to avoid the inconsistency between the selected sources of parameters,thereby avoiding the potential calculating error;3.Pipeline-type liquid distributor was used to explore the effect of initial liquid distribution on effective mass transfer area,and the liquid distribution in the packing was also analyzed.It is found that the initial liquid distribution on the axial direction shows an important effect on effective mass transfer area.On the basis of the analysis of experimental data and literature results,the effect of initial liquid distribution mainly exists in end-effect zone and the nearby zone with thin packing thickness,and improving initial liquid distribution on the axial direction is helpful to the uniform distribution of liquid in packing,thereby increasing effective mass transfer area.The effect of initial liquid distribution on the circumference also mainly exists in the packing zone close to inner edge of packing,but the effect is limited;4.CO2 capture performance largely depends on the reaction rate and CO2 capacity,and the dependences of absorbent type and operation conditions were explored using amine absorbents.Results indicate that,the order of CO2 capture performance and the promoted effect on MEA obtained by polyenamines is followed as:DETA>TETA>TEPA.Among the blended amine absorbents,PZ+DETA shows better CO2 capture performance than DETA+MEA and TETA+MEA,and it can be regarded as a promising absorbent for CO2 capture.The suitable operation conditions for amine absorbents are:mean gas retention time of 0.7-1 s(lowest of 0.6 s),rotation speed of 1000-1200rpm,gas-liquid ratio of 120-190;5.A gas-liquid mass transfer model accompanied by reversible chemical reaction was established for CO2-MEA system using penetration theory,and the calculated results agreed well with experimental data with a deviation within±15%.Modelling results indicate that,CO2 is mainly captured in the packing zone,and the reaction between CO2 and MEA is the main reaction that occurs during the absorption process.Through using this model,CO2 capture performance and the optimal packing size under different operation conditions can be well predicted,and the chemical reaction occurred and the distribution of reacting components during the absorption process can also be well described,which can provide an assistance of high value in reactor design and process optimization.