| Global warming is a major concern currently in the world,which is mainly caused by C02 emission in industry,e.g.,40%from petroleum refinery,33%from coal power plant and 26%from natural gas processing.The commercial method for capturing C02 with alcoholamine solution is facing a challenge for the disadvantages such as solvent loss,large energy consumption and equipment corrosion.Recognized as green solvents,ionic liquids(ILs)have been attracting more and more interests in gas processing including C02 capture in the last twenty years.To our best knowledge,the understanding of hydrodynamics of C02 in ILs,however,is rarely touched,which is necessitated knowledge for the final application of ILs in C02 capture in industry.In this thesis,we performed a systematical CFD study on the hydrodynamics of C02-ILs system,including CO2 bubble formation through submerged orifice in ILs,comparative modeling of ionic liquids and conventional amine solvents in bubble column,and modeling of CO2-ILs in industrial tray column and packed column.Some results are summarized as follows.Firstly,fundamental mechanism of C02 bubble formation was investigated in the pool of ILs by developing a Computational Fluid Dynamics(CFD)model.In new model,conventional VOF(Volume of Fluid)method was improved by adding suitable drag force for C02-ILs interactions and ILs viscosity was considered as a variable with dissolved C02 mass fraction.The mass transfer phenomenon was captured by adding C02 diffusion term.The developed VOF model was further improved to get more information like bubble detachment time through pressure signals.Therefore,current VOF model was coupled with FW-H(Ffowcs Williams and Hawkings)acoustic formulation to discretize acoustic pressure equations.The predicted results were in good agreement with experimental measurements.Based on developed CFD model,distinct acoustics pressure signals were examined to characterize CO2 bubble formation stages as bubble growth,necking,and detachment.The CO2 bubble was formed at 0.8 mm orifice size in[Bmim]BF4 and bubble growth,necking and detachment period was observed 0.001 s to 0.040 s,0.050 s to 0.080 s and 0.082 s to 0.088 s.Along with orifice diameter 0.5 mm,0.6 mm,0.7 mm and 0.8 mm,the C02 bubble diameter in[Bmim]BF4 was observed to increase as 2.43 mm,2.58 mm,2.82 mm,3.07 mm respectively.The detachment time of C02 in[Bmim]BF4 were also increased with same four orifice sizes as 0.063 s,0.074 s,0.081 s,0.0884 s respectively.It was shown that as increase of C02 inlet velocity from 0.1 m/s to 0.3 m/s at 1 mm orifice size in[Bmim]BF4,the bubble formation was completed at early time from 0.0947 s to 0.0822 s.The more effects of gas inlet velocities were explained as the Reynolds number was exponentially increased during bubble growth and it was seen to fluctuate abruptly during unstable necking stage.The effects of ILs inlet velocity were investigated under the range from 0.01 m/s-0.1 m/s,while CO2 gas inlet velocity 0.21 m/s was kept constant at 0.8 mm orifice size.By the help of acoustic pressure signals time series,it was demonstrated that as increase of ILs inlet velocity from 0.01 m/s to 0.1 m/s,the CO2 bubble growth stage was observed to shrink from 0.036 s to 0.023 s,bubble necking period was also reduced and pinch-off time was achieved at early time from 0.079 s to 0.039 s.In this ILs velocity range,the C02 bubble diameter was also decreased sharply from 2.84 mm to 1.97 mm.Because of decreasing bubble diameter and continuously increasing in ILs velocity,the C02 local bubble velocity was improved from 0.058 m/s to 0.178 m/s.Mass transfer aspects are explained by means of C02 mass fraction distribution in two imidazolium based pure ILs having different cations[umim]+and[Bmim]+and in two aqueous solutions(90%and 95%)of[Bmim]BF4.In both pure ILs,as C02 bubble moved upward,the C02 mass fraction was seen under the bubble wake region which was more concentrated in straight line.Unlike pure ILs,C02 bubble was slightly declined in radial direction in ILs aqueous solutions.In case of aqueous ILs solution,it was also noted that aqueous solution were tended to dissolve more CO2 than pure ILs.Furthermore,the C02 bubble was detached from 0.8 mm orifice after time 0.088 s and 0.092 s in pure[Bmim]BF4 and[Omim]BF4 respectively.Whereas,the bubble detachment time in 95%aqueous[Bmim]BF4 and 90%aqueous[Bmim]BF4 was achieved as early as 0.047 s and 0.045 s respectively because of attaining less viscosity.Secondly,hydrodynamics of ILs were compared with amine based solutions.A flat rectangular bubble column has quiescent liquid height of 45 cm,width of 20 cm and depth of 5 cm was adopted.The ILs were chosen as cations([Bmim]+and[Hmim]+)were different with each other,in a case,if cations of ILs were same then anions(BF4-,PF6-,and SCN-)should be alike.For a hydrodynamic comparison,three conventional amine solvents(30%-APM,30%-MDEA and 30%-MEA)were used.As a primary target of this research,another CFD model was developed for ILs and amine solvents.Eulerian model was coupled with Population Balance Model(PBM)to increase the gas-liquid interactions by means of bubbles breakup and coalescence phenomena.Conventional Eulerian model was improved by means of adding special drag correlations because of facing higher viscous ILs.The CFD predicted results were compared with experimental measurements and showed good agreements.By using developed Eularian-PBM model,the flow pattern,liquid velocity magnitude,C02 holdup and BSD(bubble size distribution)were investigated,compared and explained in a glance.In the comparison with amine solvents,it was observed that C02 plume oscillation behavior was not seen in ILs because of inherent higher viscosity.Alike with ILs,C02 plume were continuously oscillated as fully developed plume was started to oscillate after 20 s and completed its one cycle in average of 12-13 s.It was reported that the liquid velocity were decreased as increasing the ILs viscosity as[Bmim]SCN>[Hmim]BF4>[Hmim]PF6.Furthermore,the liquid velocity magnitude of ILs was increased with the height in bubble column,but a decreasing trend with column height was followed in the case of amine solvents.It was noted that at higher viscosity(>0.01 Pa.s),gas holdup of C02 was apparently independent of viscosity and surface tension were played significant part in that higher viscosity conditions.It was also observed that C02 exhibited non-coalescence behavior in ILs,but coalescence and breakup phenomena were seen in amine solvents.Finally,the developed and validated CFD model was extended over baffled type industrial tray column and packed column.Two horizontal trays were inserted in bubble column at vertical distance of±10 cm from the mid length(22.5 cm)of bubble column to replicate industrial tray column.The inlet dimension and position were set as each inlet size was 2.4×1.2 cm that were±5 cm apart horizontally from central inlet.The hydrodynamic parameters,gas holdup,liquid velocity and BSD were investigated in tray column for pure[Bmim]BF4,90%-[Bmim]BF4 and 30%-MEA.It was observed that horizontal baffles significantly affect the CO2 plume and more gas-liquid circulation was achieved.It was further observed that,non-coalescence behavior in ILs was notably improved which was not seen in bubble column without baffles.In the case of packed column,porous resistance force was implemented to resemble the structured real packings MellapakTM.Both liquid distributer and gas sparger consist of nine small holes(3×3mm2)arranged in diamond configuration.Both phases(solvent and C02 gas)were applied to move in counter current direction.With help of developed CFD model,it was reported that more gas-liquid mixing was achieved in packed column than tray column.Particularly,in terms of intimate contact,mixing and vortices formation,M-500-X structured packing is more suitable for CO2-ILs system than M-250-X.The experimentally validated CFD model shows the ability to resolve the complex interphase between ILs&CO2.The reported results may be helpful to design industrial units for C02-ILs.The experimentally validated CFD model can be applied with new ILs in future by adopting more real industrial units like packed column with real packings and tray column with bubble cap and sieve tray.Indeed,the current experimentally validated CFD model may further apply for the investigation of the series of different ILs.For future studies,combined effects of heat and mass transfer can be applied in CFD studies. |
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