Hydrodynamics and Mass Transfer Performance of Annulus-Rising Airlift Reactor The Effect of Reactor Scale

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1 Internationa Journa of Chemica Enneering and Appications, Vo. 8, No. 1, February 017 Hydrodynamics and Mass Transfer Performance of Annuus-Rising Airift Reactor The Effect of Reactor Scae Mei Han, Arto aari, and Tuomas Koiranen AR_ARs have in fact higher gas hodups and mass transfer rates than CR_ARs, especiay for iquid phase systems which have frothing properties. The anti-frothing abiity of the AR_ARs is higher than CR_ARs due to much more bubbes being entrained into the downcomer. Moreover, most of the studies, typicay in bioprocesses, have been performed in bench-scae (1 10 ) ARs [15] and are imited to the specified aeration rates []. The yied of desired products may be ower than expected even in piot-scae appications [19] due to the effects of the reactor scae, aeration and atation. It is therefore necessary to investigate the effect of aeration and scae on hydrodynamics and mass transfer in AR_AR so that the performance of the reactor woud satisfy the process needs as we as possibe. The aim of this work was to investigate the effects of aeration and reactor scae on hydrodynamics and mass transfer performance of AR_ARs in order to promote their industria appications. This study on a aboratory AR_AR was, therefore, conducted with experiments and CFD simuations for different gas superficia veocities. An Euerian mode with two bubbe phases was deveoped to simuate the hydrodynamics and mass transfer performance of the AR_AR. Furthermore, the effect of reactor scae was predicted and compared with experimenta studies from the iterature. Abstract Hydrodynamic and mass transfer characteristics of an annuus-rising airift reactor (AR_AR) were investigated with experimenta and CFD simuation methods. An Euerian mode with two bubbe phases was deveoped to simuate the AR_AR in three circuation fow remes. 3D steady state CFD simuations were performed under different gas superficia veocities (Ug). Good agreements on gas hodup and voumetric mass transfer coefficient were obtained over the range of the studied Ug. The simuated averaged iquid veocities in AR_AR with different scaes were compared and accounted for the infuence of reactor scae on gas hodup. The three fow remes in AR_AR were captured we and are simiar to those observed in experiments. The deveoped CFD mode can be used to predict the hydrodynamics and mass transfer in AR_ARs with different scaes. Index Terms Annuus-rising airift reactor, CFD, mass transfer, scae-up effect. I. INTRODUCTION Airift oop reactors (ARs) have received more and more attention in chemica, petrochemica and biochemica industries. They are used in fermentation, waste water purification, hydrogenation and exhaust-gas treatment [1], []. ARs have some advantages over bubbe coumns such as enhanced mixing, mass and heat transfer, and suspension of partices with ow energy consumption. The mid and constant shear environment in ARs, contrary to mixing tanks, is preferabe for bioprocesses with frae partices [3]. ARs usuay are composed of a riser with a gas feed, and a downcomer where the iquid phase fows downwards. An enarged expansion, caed gas separator, is often paced on the top of the coumn in order to achieve good separation of gas and iquid phases. Generay, two types of ARs, an interna-oop airift reactor (I-AR) and an externa-oop airift reactor (E-AR), are cassified according to the arrangement of the riser and the downcomer. For I-AR, two operating modes can be used which are the gas-feeding in the draft tube (center-rising airift reactor, CR_AR) and the gas feeding in the annuus (annuus-rising airift reactor, AR_AR) modes. A arge number of studies on CR_ARs have been reported and great achievements have been obtained with experimenta [4]-[8] or simuation [9]-[14] methods. However, studies on AR_ARs are quite scarce [15]-[18] athough Koide et a. [17],[18] have shown in their experimenta studies that II. EXPERIMENTS A schematic diagram of the experimenta setup used in the studies is shown in Fig. 1. Manuscript received October 15, 015; revised August 5, 016. The authors are with appeenranta University of Technoogy (UT), Finand (e-mai: mei.han@ut.fi, arto.aari@ut.fi, tuomas.koiranen@ut.fi). doi: /ijcea Fig. 1. Schematic diagram of the experimenta setup. 47

2 Internationa Journa of Chemica Enneering and Appications, Vo. 8, No. 1, February 017 The aboratory-scae AR_AR consists of two cyinders made of acryic resin with a wa thickness of 05 m. A pastic ring gas distributor with a diameter of 0.1 m was instaed in the annuus between the outer and the inner cyinders at 7 m above the bottom. There are 36 hoes of 0.5 mm diameter distributed equidistanty at the top of the ring gas distributor. Tabe I ves the dimensions of the AR_AR in detais. TABE I: THE DIMENSIONS OF THE AR_AR Dimension, m Riser Downcomer Diameter Height Compressed air was used as the dispersed phase and it was operated in once-through mode. Tap water was the continuous phase and it was used as a batch. The unaerated iquid height was 0.8 m. A the experiments were carried out at room temperature and atmospheric pressure. The operating gas superficia veocity, U g, was cacuated based on the cross-sectiona area of the riser and was in the range 0.41~.55 cm/s, as isted in Tabe II. The overa gas hodup, <a g,overa >, in the coumn was measured by the height expansion method and cacuated by Equation (1). H m H H m ag, overa (1) where H m and H are the height of the mixture after aeration and the height of the iquid before aeration, respectivey. Dita imang method was used to evauate the bubbe size at the height of 0.5 m above the base of the AR_AR images were acquired and anayzed with image anaysis software PORA [0]. More than 10 5 bubbes were detected for each U g, which was found to be enough for the measured bubbe size vaue to converge to a constant vaue. The voumetric mass transfer coefficient, k a, was determined by using a dynamic oxygen absorption method. The dissoved oxygen concentration was monitored and recorded using a MARVET BASIC 000 Dissoved Oxygen Meter (Eke Sensor OÜ) with the oxygen sensor ocated at 0.6 m above the gas distributor. Nitrogen was first sparged into the AR_AR unti the dissoved oxygen concentration was cose to zero and steady-state hydrodynamic conditions were reached. Then, nitrogen fow was shut down and compressed air feed was started. A pressure vave and a dita mass-fow controer were used to keep the nitrogen and the compressed air feeds at constant eve in order not to interrupt the fow fieds. Sufficient time was ven in each experimenta run to strip out the dissoved nitrogen and to saturate the iquid phase with oxygen to the maximum eve. To cacuate the voumetric oxygen mass transfer coefficient, it is assumed that the iquid phase is perfecty mixed and that oxygen depetion from the gas bubbes is negibe. The rate of oxygen dissoution in the iquid phase can be described by Equation (), dc * dt k C C () where k a is the voumetric mass transfer coefficient per unit voume of iquid in the AR_AR. C is the dissoved-oxygen concentration in the iquid and C* is the oxygen saturation concentration. The sensor of the dissoved oxygen meter has a time constant, k sensor, corresponding to the deay in recording due to the inherenty finite response time. The oxygen concentration vaue indicated by the sensor, C sensor satisfies [1], dc sensor sensor dt k C C (3) sensor where C is the concentration of dissoved oxygen. In the experiments to determine the vaue of k sensor, two beakers with water saturated with nitrogen and oxygen were used, respectivey. A vaue of 65 s -1 was obtained for the k sensor by making a regression of the C sensor /C vaues versus time. Anaytica soution of () aong with the sensor dynamics ven by (3), yieds [], t C C 1 k e k k a * kat sensor sensor sensor ksensor + k a e k k a sensor The voumetric mass transfer coefficient per voume of the reactor can then be determined by using (5): A. Governing Equations k a ka 1 ag, overa (4) (5) III. SIMUATION An Euerian mode with two bubbe phases was deveoped to simuate the hydrodynamics and mass transfer of AR_ARs. This mode incudes two bubbe phases as the dispersed phase and one iquid continuous phase. Each bubbe phase has a uniform size but a different bubbe diameter vaue. The arge bubbe phase (g1) and the sma bubbe phase (g) were defined according to measured bubbe sizes. The diameter varies from 05 to 08 m for the arge bubbe phase and from 01 to 03 m for the sma one corresponding to U g. The exact diameter vaues for both bubbe phases are isted in Tabe II according to U g. TABE II: BUBBE SIZES CORRESPONDING TO GAS SUPERFICIA VEOCITIES Gas superficia veocity, cm/s Bubbe size of g1, mm Bubbe size of g, mm Interaction between g1 and g was not considered and no bubbe coaescence or breakage was invoved. The momentum transfer (M k ) between the gas phases and the iquid phase was formuated by taking the drag (F D, k ), ift (F, k), wa ubrication (F W, k ) and turbuent dispersion forces (F T, k) into account [13]. The turbuence of the iquid phase was resoved by the dispersed standard k - ɛ turbuent mode. The 48

3 Axia iquid veocity, m/s Internationa Journa of Chemica Enneering and Appications, Vo. 8, No. 1, February 017 governing equations are isted in Tabe III in detai. TABE III: THE DEVEOPED THREE-PHASE EUERIAN MODE Item Mode u 0, k, g1, g Continuity k k k ' T uu P eff, u u Momentum F D, gm ' T uu P eff, u u gm i g1, g M ( M g1 M g ) F C i1 F D, D, 3 4 CD, d i1 F u, F i1 u W, u FT, i1 u max min 4 Re 10.15Re,7 Re, D, g1 r, g1 r, g1 r, g1 8 3 Eog1 Eog Rer, g Re r, g, Rer, g 1000 CDg, 0.44, Rerg, 1000 F, C, u u u F, min(0.88tanh n(re r, ), f ( Eo )), Eo 4 C, f Eo, 4< Eo , 10< Eo f Eo Eo Eo Eo ' 3 ' ' ( ) F T, F, C, C 0.1 T T T F W, FW, ( usip, ( usip, nw ) nw) d k - ε max( C C d y,0), C 1, C 5 W1 W W W1 W ( ku ) ( ) / k am, t tb k ( G ) S k, k, ( u ) ( ) / am, t tb ( C G C ) S 1 k,, k eff am,,, t eff, g eff, C µ = 9; σ k =1.00; σ ɛ =1.3; C ɛ1 = 1.44, C ɛ = 1.9 B. Mass Transfer Mode Mass transfer rate between gas and iquid phase is an important factor in mutiphase reactor performance. Interfacia mass transfer is usuay expressed by voumetric mass transfer coefficient k a. Four different categories of mass transfer modes have been reported in [13]. These are (1) phenomenooca correations and/or modes; () spatia modes; (3) time modes and (4) combined fim-penetration modes. Huang et a. [13] compared severa typica mass transfer modes for one AR and recommended the one time mode proposed earier by Cocke et a. [3] based on the penetration theory, as shown in (6). k a 1 d D U d (6) g b Equations (7) and (8) in the current study were empoyed to cacuate the k a between the iquid and each bubbe phase, respectivey. The overa k a, the sum of (k a) g1 ( arge bubbe size cass) and (k a) g ( sma bubbe size cass), was vaidated against the experimenta resuts. sip ka g1 dg1 DU sip, g1 d 1 (7) 1 g1 g b ka g dg DU sip, g d C. Numerica Detais (8) 1 g g To reduce computationa expense, one quarter of the 3-dimensiona AR_AR was taken as the computationa domain. The configuration of the AR_AR was generated according to the experimenta equipment. The veocity inet boundary was appied to the upper face of the gas distributor, where the gas inet veocity and its voume fraction were specified according to the experiments. On the top domain surface, pressure outet boundary condition was set. Symmetry boundaries were used for the two symmetrica surfaces of the quarter of the AR_AR due to computationa economics. The other boundary conditions were a standard non-sip condition for the iquid and gas phases on a the reactor was. 3D steady state simuations were carried out with the commercia software ANSYS Fuent For a the simuations, the phase-couped SIMPE agorithm was used for the soution of the pressure-veocity couping. The second order upwind scheme was used for the momentum, turbuence kinetic energy and dissipation rate equations and the first-order scheme for the voume fraction equation. The residua convergence criterion for a the variabes was set to The mass baance of bubbe phases and the voume-weighted average gas hodup for each bubbe phase were additionay used to examine the convergence for a simuations. A. Mesh Independence IV. RESUT AND DISCUSSION The infuence of the mesh size on the numerica soutions was examined with one coarse mesh case and another finer mesh. The coarse mesh had 43,84 hexahedron eements with the maximum edge size of 5 mm. The number of the ces of the fined one was 400,896 with the maximum edge size of.5 mm. The simuations were performed for both cases at gas superficia veocity U g 1.18 cm/s. The reative discrepancy between the coarse and the finer mesh cases is 1.58% for the overa gas hodup and 0.81% for the k a. Fig. shows the radia profie of the axia iquid veocity for both cases, where the difference is around 10%. It can be seen that the coarse mesh is sufficient to obtain mesh independent soution. Therefore, it was used to conduct a the subsequent simuations Coarse mesh Fine mesh Fig.. Infuence of mesh size on radia profie of axia iquid veocity. r/r 49

4 Overa gas hodup, % Voumetric mass trasnfer coefficient, 1/s Internationa Journa of Chemica Enneering and Appications, Vo. 8, No. 1, February 017 B. Simuation and Vaidation of the ab-scae AR_AR The simuated overa gas hodup agrees we with the experimenta resuts over the range of the studied U g, as shown in Fig. 3. The overa gas hodup increases with the increase of U g CFD of k a is core-peaking in the riser and wa-peaking in the downcomer (except for the case (a) of no bubbe entrained into the downcomer fow reme ), which is in accordance with the gas hodup profie aso. Ony sma bubbes can be entrained further into the downcomer. These sma bubbes are ocated cose to the wa of the downcomer. These resuts suggest that the deveoped three-phase Euerian mode can be used to accuratey predict the hydrodynamics and mass transfer of the studied AR_AR CFD Gas Superficia veocity, cm/s Fig. 3. Overa gas hodup at different U g: square-experiments; star-cfd Three fow remes can be distinguished in ARs depending how far the gas bubbes reach in the reactor. These are the no gas entrainment reme (reme I), gas entrainment, but no recircuation reme (reme II) and compete gas recircuation reme (reme III). As shown in Fig. 4 (a), (b) and (c), a the three fow remes, which were aso observed in the experiments, were captured with the deveoped mode at different gas superficia veocities Gas superficia veocity, cm/s Fig. 5. Mass transfer at different U g: square-experiments; star-cfd. (a) (b) (c) Fig. 6. Mass transfer profie for three remes: (a) U g =0.65 cm/s (b) U g =1.18 cm/s (c) U g =.55 cm/s. (a) (b) (c) Fig. 4. Gas hodup profies for three remes: (a) U g =0.65 cm/s (b) U g =1.18 cm/s (c) U g =.55 cm/s. The voumetric mass transfer coefficient was vaidated against the experimenta resuts under different U g, as shown in Fig. 5. In overa, good agreement was obtained between the simuations and experiments athough some discrepancy can be seen at U g = 1.18 cm/s. This is probaby caused by the more compex hydrodynamic characteristics of the fow at the transition from homogeneous to heterogeneous fow. The oca voumetric mass transfer coefficient from simuations is shown in Fig. 6 for the three different fow remes. The maximum voumetric mass transfer coefficient appears at the inet of the downcomer due to substantia accumuation of bubbes. It can be seen that the radia profie C. The Scae Effect Simuated by CFD The effect of reactor scae on the hydrodynamics and mass transfer of AR_AR was investigated with CFD simuations. 3-D steady-state simuation was performed for an AR_AR with a height of 4.5 m. The ratio of the cross-sectiona area of the riser to that of the downcomer was maintained the same as in the aboratory AR_AR. The dimension of the computationa domain for the arger AR_AR is shown in Fig. 7. The number of the eements was 773,65 with the maximum edge of 5 mm. Fig. 8 shows the simuated overa gas hodup in the AR_ARs with different scaes and the comparison with the experiments. It can be seen that the overa gas hodup simuated with the arger scae AR_AR is sighty higher than that with the aboratory scae. This is probaby because much more bubbes can be entrained into the downcomer due 50

5 Overa gas hodup, % Voumetric mass transfer coefficient, % H1 H H3 iquid veocity in riser, m/s H4 iquid veocity in downcomer, m/s Internationa Journa of Chemica Enneering and Appications, Vo. 8, No. 1, February 017 to increasing iquid circuation. As pointed out by van Baten et a. [4], the friction osses of iquid phase encountered in arger scae reactor are reduced and therefore iquid circuation veocity increases. In order to vaidate the deveoped CFD mode, further simuations with the arger scae reactor were performed on two more high gas superficia veocities and compared with the experimenta data from [5]. As shown in Fig. 8, somewhat higher simuated gas hodup is, however, obtained probaby due to absence of the enarged gas separator part in the simuated AR_AR (b) CFD_ab-scae CFD_Scae-up Gas superficia veocity, cm/s Rd_i Rd_o Unit: m 0.3 CFD_ab-scae CFD_Scae-up H1= 0.5 H= H3= 3.50 H4= 0.75 R = Rd_i= 9 Rd_o= (a) R Fig. 7. Dimension of the scae-up AR_AR Gas superficia veocity, cm/s Fig. 9. Cross-sectiona area averaged iquid veocity at the haf of reactor height: (a) in riser, (b) in downcomer Data from Deng et a. CFD_ab-scae CFD_Scae-up Data from Deng et a. CFD_ab-scae CFD_Scae-up Gas superficia veocity, cm/s Fig. 8. Effect of reactor scae on overa gas hodup: square-experiments; diamond-from [5]; star-cfd with aboratory scae AR_AR; cross-scae-up AR_AR. The cross-sectiona area averaged iquid veocity in riser and downcomer obtained from simuation, are presented in Fig. 9 for the AR_ARs with different scaes. The average iquid veocities in the riser and downcomer increase with the increasing of gas superficia veocity. The increasing trend is significant at ow U g but negibe at high U g. The average iquid veocities are much higher in the arge scae AR_AR than in the ab-scae reactor. The average iquid veocity is high in the arge-scae AR_AR mosty because of the reduction in frictiona resistance compared to the ab-scae. This aso accounts for the effect of the reactor scae on the gas hodup, as shown before. In Fig. 10, the voumetric mass transfer coefficients obtained in simuations and experiments are compared in the two reactor scaes. The simuated resuts with the arger scae reactor are higher than that of the aboratory scae reactor, which is in accordance with the predicted gas hodup Gas superficia veocity, cm/s Fig. 10. Effect of reactor scae on voumetric mass transfer coefficient: square-experiments; diamond-from [3]; star-cfd with aboratory scae AR_AR; cross-scae-up AR_AR. V. CONCUSION Hydrodynamic characteristics and mass transfer performance of the annuus-rising airift reactor were investigated with experimenta and CFD simuation methods. The effects of superficia gas veocity and reactor scae are presented and compared with our experimenta resuts and those from iterature. An Euerian mode with two bubbe phases was deveoped for simuating the AR_AR. Good agreements on gas hodup and voumetric mass coefficient were obtained over the range of the studied gas superficia veocities. The three fow remes observed in experiments in AR_AR are aso captured by the deveoped mode. The effect of the reactor scae on average iquid veocities in AR_AR is presented 51

6 Internationa Journa of Chemica Enneering and Appications, Vo. 8, No. 1, February 017 and it agrees we with resuts from van Baten et a. According to the comparison of experiments and simuations, the gas hodup and the voumetric mass transfer coefficients can be predicted we at different scaes and gas superficia veocities using the deveoped Euerian mode with the proposed momentum terms. REFERENCES [1] M. Y. Chisti and M. Moo-Young, Airift reactors: Characteristics, appications and design considerations, Chem. Eng. Comm., vo. 60, pp , [] R. Bannari, A. Bannari, P. Vermette, and P. Proux, A mode for ceuase production from Trichoderma reesei in an airift reactor, Biotech. Bioeng., vo. 109, pp , 01. [3] M. Y. Chisti, Airift Bioreactors, ondon: Esevier, 1989, pp [4] C. C. Fu,. S. Fan, and W. T. Wu, Fow reme transitions in an inter-oop airift reactor, Chem. Eng. Tech., vo. 30, pp , 007. [5]. C. Shing and H. S. 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Krishna, Voumetric mass transfer coefficient in surry bubbe coumns operating in the churn-turbuent fow reme, Chem. Eng. Process., vo. 43, pp , 004. [] H. M. etze, J. C. Schouten, R. Krishna, and C. M. van den Beek, Gas hodup and mass transfer in bubbe coumn reactors operated at eevated pressure, Chem. Eng. Sci., vo. 54, pp , [3] A. Cockx., Z. D. Quang, J. M. Audic, A. iné, and M. Roustan, Goba and oca mass transfer coefficients in the waste water treatment process by computationa fuid dynamics, Chem. Eng. Process., vo. 40, pp , 001. [4] J.M. van Baten, J. Eenberger, and R. Krishna, Hydrodynamics of interna air-ift reactors: experiments versus CFD simuations, Chem. Eng. Process., vo. 4, pp , 003. [5] Z. H. Deng, T. F. Wang, N. Zhang, and Z. W. Wang, Gas hodup, bubbe behavior and mass transfer in a 5 m high interna-oop airift reactor with non-newtonian fuid, Chem. Eng. J., vo. 160, pp , 010. Mei Han is a PhD student at appeenranta University of Technoogy (UT) in Finand. Her current research topic is fuid dynamic in chemica enneering and mutiphase reactor design and deveopment in the aboratory of Product and Process Deveopment at UT. Arto aari is a research scientist at appeenranta University of Technoogy (UT). He received his PhD (005) and the tite of Docent (010) from UT in the fied of product and process deveopment. He has been working as an associate professor in the same department in His current scientific interests are in the deveopment of sustainabe meta eaching processes and in the storage and transformation of renewabe energy. Tuomas Koiranen is a professor at appeenranta University of Technoogy (UT). He received his PhD (1998) in Hesinki University of Technoogy, and the tite of Docent (004) from appeenranta University of Technoogy. After having 1 years industria experience in pharmaceutica R&D as a research scientist he started the professorship at appeenranta University of Technoogy in 013. His current scientific and teaching interests are fuid dynamics in chemica enneering appications, iquid dropet behavior in reactive fows, process deveopment and intensified processes. 5