Study On Processes And Mechanism Of CO₂ Absorption Intensified By A Multistage Rotating Zigzag Bed

The massive emission of CO₂ breaks the equilibrium and stability of the climate and ecological environment,leading to many serious problems,including extreme weather and global warming.The conventional CO₂capture process has the disadvantages of slow absorption rate,high regeneration energy consumption,low absorption capacity,large device size and low mass transfer efficiency.Therefore,the development of efficient devices and absorbents are key to improving CO₂ capture efficiency.As a novel high-gravity device,rotating zigzag bed（RZB）with a zigzag channel elevates the gas-liquid contact time and liquid holdup markedly compared to rotating packed bed（RPB）,thereby being favorable for liquid phase mass transfer.Moreover,RZB simplifies the device structure and expediently achieves multistage operation relative to RPB,leading to an enhanced device stability and mass transfer.These features of RZB suggest that it has great potential for the intensifcation of CO₂ absorption process.In this study,an RZB was employed to enhance absorption and mass transfer performance in CO₂ absorption process.Gas-liquid mass transfer performance and mechanism of the RZB were studied,and the CO₂ absorption performance of new types of absorbents in the RZB was investigated.The main results are as follows:（1）The gas-liquid effective interface area（a）in the RZB was studied by chemical absorption method with the CO₂-Na OH system.The effects of different experimental conditions on a in the RZB were examined,and a in the top and bottom stages of the RZB were compared.An increasing Na OH concentration led to a rising a.When the Na OH concentration in the liquid phase reached 3 mol/L,it was deduced that Ha>3 and the premise of pseudo-first-order rapid reaction was established.Thus 3 mol/L Na OH solution was selected as the experimental system to measure a in the RZB.The increase of rotational speed,liquid flow rate and gas flow rate caused an elevated a,and the a in the top stage of the RZB was higher than that in the bottom stage of the RZB.The empirical correlation model of a in the RZB was established.The calculated values were in good agreement with the experimental values,with the deviations less than 15%.（2）A mathematical model was established to quantitively reveal the mechanism of the gas-liquid mass transfer process in the CO₂-Na OH system in the RZB.The calculated overall gas-phase volumetric mass-transfer coefficients（K_Ga）were in good agreement with the experimental data,with deviations generally within 10%,demonstrating that this model possesses a good predictability for CO₂ absorption in RZB and can be employed to predict the effects of various operating conditions on K_Ga.（3）Piperazine（PZ）and 1-methylpiperazine（1-MPZ）were used to enhance the CO₂ absorption performance in the diethylenetriamine（DETA）solution,and the CO₂ absorption and mass transfer performance with blended amine absorbents in the RZB was studied.The effects of different experimental conditions on the CO₂ absorption efficiency,K_Ga and height of mass transfer unit（HTU）in the RZB were investigated.Experimental results show that both PZ and 1-MPZ were efficient promoters,in which PZ exhibited better performance.An elevated promoter concentration,lean solution flow rate and rotational speed were beneficial to CO₂ absorption.Relative to one-stage operation,a two-stage operation of the RZB presented greater absorption performance of CO₂.A comparison with RPB reveals that the RZB can reach better CO₂ absorption performance and higher mass-transfer efficiency with lower absorbent consumption and smaller rotor volume.The maximum CO₂ absorption efficiency and K_Ga reached 99.3%and7.43 kmol/k Pa m³ h,respectively,in the RZB with DETA+PZ absorbent.（4）An enhanced CO₂ absorption process with aqueous diethylaminoethanol（DEEA）+DETA biphasic absorbents in the RZB was investigated.The influences of the absorbent composition on CO₂ absorption performance and phase separation behavior were studied.The variation of K_Ga and CO₂ absorption efficiency in different operating conditions was explored.Results reveal that over 97%absorbed CO₂ was concentrated on the bottom layer in the rich solution.Compared to absorbents with other compositions,the 4 mol/L DEEA+1 mol/L DETA and 3.5 mol/L DEEA+1.5mol/L DETA solutions had much lower volume of the bottom layer.The K_Ga and CO₂ absorption efficiency with the 3.5 mol/L DEEA+1.5 mol/L DETA solution in the RZB reached 4.82 kmol/k Pa m³ h and 98.7%,respectively,suggesting the obvious advantage in absorption performance of CO₂compared to the 5 mol/L monoethanolamine.An artificial neural network model was established to predict K_Ga and CO₂ absorption efficiency.The predicted data and experimental results had a good agreement with the deviations generally less than 10%and the average absolute relative deviations were 3.40 and 1.71%,respectively.It was found that RZB can achieve higher mass transfer efficiency compared to wetted-wall column.In addition,the mechanism of phase separation in the DEEA+DETA solution was analyzed based on the experiments of CO₂ absorption and the detection of¹³C nuclear magnetic resonance.（5）A blended absorbents composed of aqueous phase（K₂CO₃+PZ）and organic phase（cyclohexane or n-heptane）were adopted to absorb CO₂ in the RZB.The dependences of CO₂ absorption efficiency on experimental conditions in the RZB were examined.Experimental results demonstrate that an elevated PZ concentration could improve the CO₂ absorption efficiency.These two organic phases can promote the CO₂ absorption efficiency in the K₂CO₃+PZ solution,while cyclohexane showed better performance.CO₂absorption efficiency rose with an elevated rotational speed and liquid flow rate,and the CO₂ absorption efficiency in the K₂CO₃+PZ+cyclohexane solution reached 93.2%.An empirical correlation of CO₂ absorption efficiency in the gas-liquid-liquid system in the RZB was obtained,and the calculated data derived from the correlation and the experimental values were in good agreement,with the deviation within 15%.