Effect of gas properties on the characteristics of a bubble column equipped with fine porous sparger.

Transcription

1 ICFM-16-9 th Internatonal Conference on Multphase Flow May nd -7 th, 16, Frenze, Italy Effect of gas propertes on the characterstcs of a bubble column equpped wth fne porous sparger. Aradn P. Chatzdafn, Agathokls D. Passos, Spros V. Paras and Akatern A. Mouza* Laboratory of Chemcal Process and Plant Desgn Department of Chemcal Engneerng Arstotle Unversty of Thessalonk Thessalonk, reece *Correspondng author: Tel.: ; Emal: mouza@auth.gr Abstract The purpose of ths work s to check the valdty of prevously proposed correlatons concernng the parameters that affect the operaton of bubble columns by conductng experments wth varous gases. The effect of gas propertes on the performance of a bubble column reactor wth fne pore sparger s nvestgated by employng varous gases (.e., ar, CO, He) that cover a wde range of physcal property values, whle the lqud phase s de-onzed water. A fast vdeo technque s employed for vsual observatons and, combned wth mage processng, s used for collectng data regardng gas holdup and bubble sze dstrbuton. Prevously proposed correlatons for predctng the transton pont from the homogeneous to the heterogeneous regme, the gas holdup and the Sauter mean bubble dameter are slghtly modfed to nclude the effect of gas phase propertes. It s found that the new correlatons can predct the aforementoned quanttes wth reasonable accuracy. Keywords: Bubble column, Porous sparger, as propertes, Transton pont, as holdup, Sauter mean dameter 1 Introducton Bubble columns offer many advantages when used as gaslqud contactors, due to ther smple constructon, low operatng cost, hgh-energy effcency and good mass transfer capabltes. Consequently, they are wdely used n many ndustral gas-lqud operatons (e.g. gas/lqud reactons, agtaton by gas njecton, fermentatons, waste water treatment, etc.) n chemcal and bochemcal process ndustres. In all these processes gas holdup and bubble sze are mportant desgn parameters, snce they defne the gas-lqud nterfacal area avalable for mass transfer. In turn, bubble sze dstrbuton and gas holdup n gas-lqud dspersons greatly depend on column geometry, operatng condtons, physcochemcal propertes of the two phases and the type of gas sparger [1]. It has been proved that a fne porous sparger holds advantages over other types of gas dstrbutors, snce t produces more numerous and smaller bubbles and, thus, offers a greater gas-lqud contact area []. Dependng on the gas flow rate, two man flow regmes can be readly observed n bubble columns [3],.e. the homogeneous bubbly flow regme encountered at low gas veloctes and characterzed by a narrow bubble sze dstrbuton and radally unform gas holdup; and the heterogeneous regme observed at hgher gas veloctes and characterzed by the appearance of large bubbles, formed by coalescence of the small bubbles and bearng a hgher rse velocty, hence leadng to relatvely lower gas holdup values. The mechansm of bubble formaton s of crucal mportance to bubble column hydrodynamcs. Fgure 1 and Table 1 gve the forces that act on an under formaton bubble (Eqns 1-6). A bubble s detached, when the sum of the upward forces (.e. buoyancy, gas momentum, pressure) outwegh the sum of the downward ones (.e. drag, nertal, surface tenson). In prevous works conducted n ths laboratory [,3] the effect of the sparger characterstcs (.e. dameter, pore sze) and the lqud physcal propertes on the performance of a bubble column equpped wth fne pore sparger, have been expermentally studed. Desgn correlatons applcable n bubble columns equpped wth fne porous sparger have been proposed, takng nto account the effect of column dameter and pore sze [3], as well as the lqud propertes (.e. effect of surfactant addtves [4,5]). The aforementoned correlatons are based on data where the gas phase s ar, although several bubble column applcatons use other gases, lke CO. In ths case, the dfference n gas densty affects the gas momentum force (Eqn. ). Fgure 1: Forces actng on an under-formaton bubble Buoyancy Table 1: Forces actng to bubble formaton Upward Forces Downward Forces F L gv (1) b b as momentum F d W () 4 p Pressure F L p 4 d p P P (3) Drag 1 d F b d W C (4) L 4 D Inertal F a V L b b L Surface tenson (5) F d (6) p

2 ICFM-16-9 th Internatonal Conference on Multphase Flow May nd -7 th, 16, Frenze, Italy Thus, the purpose of ths work s to check the valdty of prevously proposed correlatons, by conductng experments wth varous gases. Expermental set-up and procedure The expermental set-up (Fg. ) conssts of a cylndrcal bubble column, equpped wth a fne pore sparger for the njecton and the unform dstrbuton of the gas phase, an approprate flowmeter for gas flow control, a hgh speed dgtal vdeo camera (Redlake MotoScope PCI 1S) for bubble sze and gas holdup measurements and a computer for acqurng and processng the data. A Plexglas rectangular box, flled wth the same flud wth the one used at the correspondng experment was placed out of the bubble column to elmnate mage dstorton caused by lght refracton. A recordng rate of 15 frames per second (fps) was used for the measurement of gas holdup, whle a speed of 5 fps was selected for bubble sze measurements. where Ηο and Η are the lqud level before and after gas njecton respectvely, ΔΗ s the lqud level dfference and n s the number of recurrent measurements for each gas flow rate (n ths case n=5). The maxmum uncertanty of the measurements s estmated to be less than 15%. Table 3: Lqud phase propertes at 5 C ρ Lqud L μ L (Kg/m 3 ) (1-3 Pa s) σ L (mn/m) Water Table 4: as phase propertes at 5 C ρ μ as (Kg/m 3 ) (1-5 Pa s ) Ar CO He.19. From bubble mages taken by the vdeo camera, the Sauter mean dameter (d3), defned as: N 3 nd b d (8) 3 N nd b was calculated, where db and n are the dameter and the number of the bubbles of sze class respectvely and N s the number of classes used for the dstrbuton. The mnmum number of classes requred for the constructon of the sze dstrbutons,k was estmated usng the Sturges rule gven by: k 1log S (9) Fgure : Expermental set-up For the purpose of the present work, two vertcal cylndrcal Plexglas columns of 5 and 9 cm.d., and 1 cm n heght were used. The bubble column dmensons are gven n Table. The lqud phase was de-onzed water (Table 3), whle three gases (.e. ar, CO and He), coverng a suffcently wde range of densty values were used (Table 4). All the experments were performed wth no lqud throughput at atmospherc pressure and ambent temperature condtons. Table : Bubble column dmensons d C (cm) H C (cm) d S (cm) Nomnal pore sze dp (μm) Mnmun pore dameter (μm) Maxmumn pore dameter (μm) The average gas holdup (ε ) s estmated by calculatng the bed expanson as follows: n n HHo, n H, 1 1 H 1 H (7) n n n where S s the sample sze (~1 bubbles). The number of classes used for the constructon of the dstrbutons n the present work s 1 equal nterval. 3 Results and dscusson 3.1 Regme transton The transton pont from homogeneous to heterogeneous regme s estmated by applyng the drft flux analyss, whch takes nto account the relatve moton of the two phases [6]. The basc quantty s the drft flux, j, gven by: j U 1 (1) S where US s the superfcal gas velocty and ε s the gas holdup. The gas superfcal velocty s defned as: Q U (11) S A where Q s the gas flow rate and A the column cross secton area. When the drft flux s plotted versus the gas holdup, the change n the slope of the curve ndcates the transton from homogeneous to heterogeneous regme [7]. Fgure 3 presents the effect of column dameter on transton pont. It s clear that the transton velocty depends on the column dameter,.e. for the smaller dameter used, transton occurs at

3 ICFM-16-9 th Internatonal Conference on Multphase Flow May nd -7 th, 16, Frenze, Italy lower gas flow rates. Ths observaton s n agreement wth Sarraf et al. [8], who also notced that as the column dameter ncreases, the transtonal superfcal gas velocty ncreases sharply. j, m/s d C, cm 5 cm 9 cm Water-Ar where Frtrans, the Froude number at the transton pont and Eo, the Eotvos number, gven by: U S, trans Fr (14) trans dpg dclg Eo L (15) In Fg.5 the predcted Frtrans s compared wth the expermental data, provng that the correlaton accuracy s better than % Equaton 13 Expermental data % ε, % Fgure 3: Effect of column dameter on regme transton The effect of type of gas on regme transton s shown n Fg. 4. It s obvous that as gas densty decreases (.e. He), the homogeneous regme s extended to hgher US values. Ths behavor s attrbuted to the lower value of the gas momentum force actng to bubbles (Eqn ), due to dfference n gas densty. j, m/s Lqud phase as phase Water Ar CO He ε, % d C =9 cm Fgure 4: Effect of type of gas on regme transton for water Predcton of transton pont A general correlaton for predctng the transton pont based on dmensonless numbers, has been prevously proposed [4,9], takng nto account the physcal propertes of the lqud phase, the column dameter, as well as the dameter and the mean pore sze of the porous sparger. To ncorporate the effect of type of gas on the transton pont, t was found that the rato of gas to ar densty (ρ /ρ ar) and the gas Reynolds number, defned as follows: USdCρ Re = (1) μ should be ncluded. The proposed correlaton s sutable for predctng the transton pont from homogeneous to heterogeneous regme, takng nto account the gas propertes: Re.5 d Fr S Eo trans (13) d C ar Fr trans as holdup - % Eo.58 Re.5 (ρ /ρ ar ) -.6 Fgure 5: Transton pont predcton for all data In ths secton the measured gas holdup values are gven. The flow regmes can be dstngushed by plottng the average gas holdup (ε ) versus the gas flow rate (Q ). Fgure 6 shows the dependence of gas holdup on correspondng gas superfcal velocty for the two bubble columns used. As t s expected, gas holdup ncreases wth the gas velocty. The frst part of the curve corresponds to the homogeneous regme. A transton regme follows where a slght decrease n gas holdup s observed. Fnally, at the heterogeneous regme the gas holdup contnues to ncrease, but wth a lower slope than the homogeneous regme [3]. ε, % Water-Ar U S, m/s d C, cm Fgure 6: Effect of column dameter on gas holdup for the water-ar system 5 9

4 ICFM-16-9 th Internatonal Conference on Multphase Flow May nd -7 th, 16, Frenze, Italy It s obvous from Fg. 6 that by ncreasng the column dameter the gas holdup ncreases, especally for hgher gas flow rates. However, the lterature results concernng the effect of column dameter on gas holdup are contradctory. Some researchers report that the column dameter has no effect on gas holdup [1-13]. The above works concern bubble columns wth dameter larger than 1 cm, where the gas dstrbutor s a perforated plate. Ruzcka et al. [14] also state that the gas holdup s ndependent of column dmensons provded that the column dameter s larger than 1 cm, the column heght s larger than 15 cm and the column heght to dameter rato s n excess of 5. On the other hand, some works report that the column dameter affects the gas holdup. Botton et al. [15] report that gas holdup ncreases when the column dameter decreases, whereas Kumar et al. [16] who conducted experments n bubble columns wth dameter larger than 1 cm, state that there s a contnuous ncrease n the gas holdup wth ncreasng column dameter. To the author s best knowledge, there are no expermental results concernng bubble columns wth dameter less than 1 cm, equpped wth fne porous sparger. Dhotre et al. [17] have numercally studed the effect of sparger type and heght to dameter rato on radal gas holdup profles. The above nvestgators report that for multpont spargers, an ncrease to the H/d C rato results nto margnal decrease n gas holdup. It s obvous that, by decreasng the column dameter and, thus ncreasng the H/d C rato, the wall effects become more ntense. Fgure 7 presents the effect of the type of gas on gas holdup when the bubble column s flled wth water. Wth ncreasng gas densty gas holdup ncreases, e.g. helum that has lower densty exhbts lower values of gas holdup than ar and CO. Ths s expected, f we take nto account the forces that nfluence an under formaton bubble (Table 1). The only force that s affected by gas densty s the gas momentum force (Eqn ). The gas momentum force s greater n gases of hgher densty (e.g. CO) and therefore, the formaton of smaller bubbles s more pronounced. Ths observaton s n agreement wth other researchers [18,19] who also reported that gases of hgher densty produce hgher gas holdup values, attrbutng ths behavor on phenomena occurrng durng bubbles formaton on the sparger. dmensonless numbers. In the case that the gas phase s other than ar, t s necessary to ntroduce a term that ncorporates the propertes of the gas phase. For that purpose, the rato of gas to lqud vscosty (μ /μ L) and the gas Reynolds number (Eqn 1), were ntroduced. The proposed correlaton s approprate for predctng the gas holdup, ncludng the gas phase propertes: d p ds. FrAr Eo Re (16) ds dc L where ε (%) s the gas holdup and Froude (Fr), Archmedes (Ar) and Eotvos (Eo) are defned as: US Fr dc g 3 dcl g Ar L dclg Eo L (17) (18) (19) Ths correlaton s plotted n Fg. 8 and s n farly good agreement (.e.±%) wth the expermental data. ε, % Equaton 16 Expermental data + % - % 1 8 d C =9 cm FrAr.1 Eo 1.6 Re.18 (d p /d S ).5 (μ /μ L ) -.6 ε, % 6 4 Lqud phase as phase Water Ar CO U S, m/s Fgure 7: Effect of type of gas on gas holdup Predcton of gas holdup In prevous studes conducted n ths lab [3,4,9] a correlaton for predctng the average gas holdup, ε, was proposed based on He 3.3 Bubble sze dstrbuton Fgure 8: Predcton of gas holdup Fgure 9 llustrates typcal bubble sze dstrbutons when the sparger of 4 μm was used (dc=9 cm), for all gases studed n the present work and for a constant US value. It s obvous that the dstrbutons are log-normal, whle t seems that as the gas densty decreases (.e. He), the sze dstrbuton curve shfts to hgher values. Fgure 1 presents bubble sze dstrbutons for the water-ar system, for a constant US value when the two spargers of dfferent mean pore dameter were used. It s obvous that as the sparger mean pore dameter ncreases, the sze dstrbuton curve s bmodal and shfts to hgher values. Ths s n accordance wth Kazaks et al. [], who ascrbed ths dfference to the pore sze range of the two spargers. The two peaks shown n the dstrbuton of 1 μm sparger, can be attrbuted to the broad sze range of ths sparger, whereas for the sparger of 4 μm the pore sze

5 ICFM-16-9 th Internatonal Conference on Multphase Flow May nd -7 th, 16, Frenze, Italy range s much narrower and as a result there are not large devatons n bubble sze. Frequency, % d p =4 μm U S =.1 m/s Lqud phase Water Ar CO He d b, mm as phase Fgure 9: Effect of type of gas on bubble sze dstrbuton 5 4 Water-Ar U S =.1 m/s d p, μm 4 1 Predcton of mean bubble sze In prevous works n our lab [,4] a correlaton for predctng the Sauter mean dameter (d3) based on dmensonless numbers, was proposed. In the present work, a term was ntroduced to account for the dfferent gas type (.e. gas Reynolds number, Eqn 1). The new equaton s as follows:. d FrRe.6 Re.1 d p We () d d S S where We s the Weber, Re the Reynolds and Fr the Froude number respectvely, defned as: LUS dc We (1) L USdC L Re () L US Fr dc g (3) Ths correlaton s plotted n Fg. 11 and s n good agreement wth the expermental data (± %)..7.6 Equaton Expermental data Frequency, % 3 d 3 /d S % % d b, mm Fgure 1: Effect of sparger mean pore dameter on bubble sze dstrbuton Table 5 contans the calculated Sauter mean dameter d3 values for all gases and for both column dameters used. It s clear that d3 s unaffected by the gas flow rate and t ncreases as the column dameter decreases. Regardng the effect of gas densty on Sauter mean dameter, t s obvous that as the gas densty decreases (.e. He), d3 obtans hgher values for water regardless of the bubble column Table 5: Sauter mean dameter, d3 US=.5 m/s US=.1 m/s Lqud as d C=9 cm d C=5 cm d C=9 cm d C=5 cm Water Ar CO He We - FrRe.66 Re -.1 (d p /d S ). Fgure 11: Comparson of the Sauter mean dameter predcton wth expermental data 4 Concludng remarks In ths work we have expermentally nvestgated n what extent the column dameter and the type of gas phase nfluence the performance of a bubble column reactor. The experments revealed that, as gas densty ncreases, the transton pont shfts to lower gas flow rates and the gas holdup ncreases. Ths can be attrbuted to the ncreased gas momentum force when gas of hgher densty s employed. Thus, the prevously proposed correlatons for predctng the transton pont from the homogeneous to the heterogeneous regme, the gas holdup and the Sauter mean dameter are slghtly modfed to nclude the gas phase propertes. The new correlatons were tested wth date from the current as well as from prevous works and are found that can predct the aforementoned quanttes wth reasonable accuracy. More work s currently n progress n an effort to nvestgate the wall effects on the operaton of small bubble columns (e.g.

6 ICFM-16-9 th Internatonal Conference on Multphase Flow May nd -7 th, 16, Frenze, Italy d C<1 cm) when varous fluds are employed as lqud and gas phases. Nomenclature A column cross secton, (m ) d b bubble dameter, (m) d 3 Sauter mean dameter, (m) d C column dameter, (m) d p pore dameter, (m) d S sparger dameter, (m) F b buoyancy force, (N) F d drag force, (N) F g gas momentum force, (N) F nertal force, (N) F p pressure force, (N) F σ surface tenson force g acceleraton of gravty, (m/s ) H C column heght, (m) j drft flux, (m/s) Q gas flow rate, (m 3 /s) U S superfcal gas velocty, (m/s) bubble formaton velocty, (m/s) W g reek letters ε average gas holdup, (dmensonless) μ gas phase vscosty (Pa s) μ L lqud phase vscosty, (Pa s) ρ gas densty, (Kg/m 3 ) ρ L lqud densty, (Kg/m 3 ) σ gas nterfacal tenson, (mn/m) lqud surface tenson, (mn/m) σ L 5 References Dmensonless Ar Archmedes number Eo Eotvos number Fr Froude number Fr tans Froude number at transton pont k mnmum number of classes N number of classes used for the dstrbutons n number of bubbles of sze class Re Reynolds number Re gas Reynolds number S sample sze We Weber number [1] Camarasa, E., Val, C., Poncn, S., Wld,., Mdoux, N., Boullard, J., Influence of coalescence behavour of the lqud and of gas spargng on hydrodynamcs and bubble characterstcs n a bubble column, Chem. Eng. Proc., 38, pp , [] Kazaks, N.A., Mouza, A.A., Paras, S.V., Expermental study of bubble formaton at metal porous spargers: Effect of lqud propertes and sparger characterstcs on the ntal bubble sze dstrbuton, Chem. Eng. J., 137, pp , 8. [3] Mouza, A.A., Dalakoglou,.K., Paras, S.V., Effect of lqud propertes on the performance of bubble column reactors wth fne pore spargers, Chem. Eng Sc., 6, pp , 5. [4] Passos, A.D., Voulgaropoulos, V.P., Paras, S.V., Mouza, A.A., The effect of surfactant addton on the performance of a bubble column contanng a non-newtonan lqud, Chem. Eng. Res. Des., 95, pp , 15. [5] Anastasou, A.D., Kazaks, N.A., Mouza, A.A., Paras, S.V., Effect of organc surfactant addtves on gas holdup n the pseudo-homogeneous regme n bubble columns equpped wth fne pore sparger, Chem. Eng. Sc., 65, pp , 1. [6] Walls,.B., One-Dmensonal Two-Phase Flow, Mcraw-Hll, New York [7] Shah, Y.T., Kelkar, B.., odbole, S.P., Deckwer, W.D., Desgn parameters estmatons for bubble column reactors. Alche. J., 8, pp , 198. [8] Sarraf, A., Jamalahmad, M., Muller-Stenhagen, H., Smth, J.M., as Holdup n Homogeneous and Heterogeneous as-lqud Bubble Column Reactors, Can. J. Chem. Eng., 77, pp. 11-1, [9] Kazaks, N.A., Papadopoulos, I.D., Mouza, A.A., Bubble columns wth fne pore sparger operatng n the pseudo-homogeneous regme: as hold up predcton and a crteron for the transton to the heterogeneous regme, Chem. Eng. Sc., 6, pp , 7. [1] Su, X., Hol, P.D, Talcott, S.M., Staudt, A.K., Hendel, T.J., The effect of bubble column dameter on gas holdup n fber suspensons, Chem. Eng. Sc., 61, pp , 6. [11] Forret, A., Schwetzer J-M., auther T., Krshna R., Schwech, D., Influence of scale on the hydrodynamcs of bubble column reactors: an expermental study n columns of.1,.4 and 1m dameters, Chem. Eng. Sc., 58, pp , 3. [1] Wlknson, P.M., Spek, A.P., Van Derendonck, L.L., Desgn Parameters Estmaton for Scale-Up of Hgh-pressure Bubble Columns, AlChE J., 38, pp , 199. [13] Vata,., Tekc, M.N., as hold-up and mass transfer n bubble columns wth pseudoplastc lquds, Chem. Eng. Sc., 44, pp. 4-47, [14] Ruzcka, M.C., Drahos, J., Falova, M., Thomas, N.H., Effect of bubble column dmensons on flow regme transton, Chem. Eng. Sc., 56, pp , 1. [15] Botton, R., Cosserat, D., Influence of Column Dameter and Hgh as Throughputs on the Operaton of a Bubble Column, Chem. Eng. J., 16, pp , [16] Kumar, S.B., Mosleman, D., Dudukovc, M.P., as- Holdup Measurements n Bubble Columns Usng Computed Tomography, AlChE J., 43, pp , [17] Dhotre, M.T., Ekambara, K., Josh, J.B., CFD smulaton of sparger desgn and heght to dameter rato on gas holdup profles n bubble column reactors, Exp. Therm. Flud Sc., 8, pp , 4. [18] Krshna, R., Wlknson, P.M., Van Derendonck, L.L., A model for gas holdup n bubble columns ncorporatng the nfluence of gas densty on flow regme transtons, Chem. Eng. Sc., 46, pp , [19] Hecht, K., Bey, O., Ettmuller, J., raefen, P., Frehmelt, R., Nlles, M., Effect of as Densty on as Holdup n Bubble Columns, Chem. Ing. Tech., 87, pp , 15.