Study On Fluid Dynamics In CO₂ Absorption Tower By Ionic Liquids

With the rapid increase of CO₂ concentration in environment has resulted the severe consequences on our society such as sharply increasing global warming.Conventionally used aqueous amine based chemical absorption processes have broad industrial applications for CO₂ capture.However,these have few disadvantages,i.e.,volatility,high uneconomical and energy intensive regeneration and corrosive nature.The methanol has also been used for CO₂ capture as physical solvents in industries due to advantages of low surface tension and viscosity,easy availability and good CO₂ capture performance.However,its high volatility results into the huge solvent loss making the separation process uneconomical.Thus,the suitable absorption processes could be desirably adopted.Presently,environmentally benign ionic liquids(ILs)by virtue of their low volatility,great versatility and tunability,are considered as promising candidates for CO₂ capture.Besides pure ILs,using their binary mixture with conventional solvents such as aqueous and pure amine,and methanol could be a suitable choice to achieve a desirable CO₂ capture.Combining the amines with ILs reduces the loss of amine solvent and reduces the cost of solvent regeneration.Similarly,binary mixture of ILs with MeOH integrating the advantages of both,is a preferable candidate over either a pure IL or pure MeOH for CO₂ capture.Bubble columns have been extensively used in industrial separation processes,for an industrial absorption column,it is strongly recommended to develop the better insight into their fluid dynamic behavior and mass transport properties.This study for the first time compared bubble size distribution,mean bubble size(d32),gas holdup and interfacial area in pure IL i.e.,[BMIM][BF₄]with those in its binary mixtures with monoethanol amine(MEA vol.%of 10 and 20)and methanol(MeOH vol.%of 33.33 and 66.66).Experiments were performed in a small-scale cylindrical bubble column with a porous sparger(16?m)and were measured by a high-speed image capture.Further,to develop the insight into the role of design and operating factors,such as sparger pore size(4-9?m and 16?m),column diameter,temperature and gas flow rate,experiments were performed employing gas-pure IL system.It was found that bubble size in binary mixtures was smaller compared with pure IL.With increasing concentration of MEA or MeOH,d32 was decreased,and both the gas holdup and interfacial area were increased.Compared with MEA,the effect of MeOH was more profound.In pure IL,d32 was notably decreased by decreasing the sparger pore size,which increased the overall gas holdup and interfacial area.A similar effect was exhibited by increasing temperature.The effect of column diameter was negligible on d32,however,in small diameter column,gas holdup and interfacial area were relatively higher.Additionally,an empirical correlation was proposed on the basis of experimental measurements for predicting the overall gas holdup in pure IL.The predicted gas holdup was in remarkable agreement with that measured experimentally.The role of different gas sparger configurations and inclusion of horizontal baffles on hydrodynamic characteristics in air-water flat bubble column was studied numerically.Initially,CFD results of time averaged liquid velocity,gas holdup,local and global sauter mean diameter,and overall gas holdup were validated with previous experimental measurements in good agreement.Subsequently,the validated CFD model was extended for investigating effect of different configurations of gas sparger and inclusion of horizontal baffles in columns on the overall bubble size distribution(BSD),gas phase distribution,overall gas holdup and interfacial area at different superficial gas velocities(U_g).The liquid circulation developed due to baffles was also explained.CFD results showed a significant effect of both the sparger and U_g on overall gas holdup and interfacial area.Overall gas holdup and interfacial area were increased by increasing number and decreasing size of individual gas openings in sparger.BSD was significantly influenced by sparger especially at low gas velocity and highest uniformity in BSD was achieved with sparger S72.In the presence of baffles,liquid circulation was greatly improved splitting the global vortexes which resulted into higher gas-liquid interactions.Also,bubble coalescence was significantly promoted resulting into a reasonable increase in fraction of large size bubbles.However,the effect of baffles was not obvious on interfacial area due to their effect of simultaneously increasing gas holdup and bubble size.The flat bubble column used in second part of thesis was simulated and optimized for CO₂IL([BMIM][BF4])absorption system using the combined computational fluid dynamics(CFD)and response surface method(RSM).The Euler-Euler model was modified by using a specific drag force model for ILs and gas and liquid interaction was improved by coupling with population balance model(PBM).Moreover,based on assumption of CO₂ absorption in IL,a scalar transport equation was used to account for CO₂ mass fraction dissolved in IL.CFD results of sauter mean diameter and gas holdup were obtained in well agreement with experimental results.A validated CFD model combined with RSM is capable of predicting the effects of different factors and their interaction on CO₂ dissolution.The operating temperature(T),pressure(P)and gas superficial velocity(U_g)were selected as design factors.While,the CO₂ dissolved in IL and CO₂ mass flux were selected as responses R₁ and R₂.The CFD results suggest that,effect of U_g and P on the dissolved CO₂ was high compared with T.According to ANOVA results,all the individual factors i.e.,A(U_g),B(P)and C(T),AB(interaction of U_g and P)and and second order of A(U_g)were found to be highly significant model terms with respect to R₁.With regards of R₂,A(U_g),B(Pressure),AB(interaction of U_g and P)and second order of A(U_g)were found to be significant model terms.Through the regression analysis,equations were fitted to estimate R₁ and R₂ as a function of model terms and their R² was found to be 0.99 and 0.94 respectively.A satisfactory agreement between the results of CFD and those predicted by regression model was obtained.The optimal conditions were obtained as;U_g of 10mm/s,P of 1MPa and T of 303K.The results received from first part of thesis will give a sufficient data pool to develop and validate CFD models and extend the CFD study for understanding the intricate phenomena of bubble columns over the variety of ILs and their mixtures.In the second part,aim of conducting study on air-water system was to capture the basic fluid dynamic phenomena against various sparger configuration and modified structure of bubble columns which will provide a basic guideline in the extension of same investigations for CO₂-IL and CO₂-IL-mixtures systems,additionally to design new configurations of industrial absorption devices.Presently used combined CFD-RSM has proven to be a useful theoretical approach in the simulation and optimization of CO₂-IL gas absorption system.This approach can be extensively used over a wide range of ILs and its binary mixture with the purpose of developing new applications of CO₂ capture.