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Efeitos Da Altura Do Sparger e a Direcao Dos Furos

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Fala sobre a questão do uso de sparger nos fermentadores. Pode ser utilizado na industria quimica para dúvidas relacionadas e fermentação
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  Effects of Sparger Height and Orifice Orientation on Solids Dispersion in a Slurry Bubble Column BIMAL GANDHI, ANAND PRAKQSH* and MAURICE A. BERGOUGNOU Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, ON N6A 5B9, Canada The effects of gas distributor height and the orientation of its orifices are investigated on solids dispersion and gas holdup profiles in a three-phase slurry bubble column. The height of the distributor was varied to cover locations from near column bottom to above the settled solids bed height. The orifice orientations were changed from upward facing to downwards facing directions. The measurements were conducted in a Plexiglas column of 0.15 m D and 2.5 m height. The gas phase was oil-free compressed air while tap water was used as liquid phase. Glass beads with an average particle diameter of 35 pm and density of 2450 kg/m3 constituted the solid phase. The settled bed height was about 0.4 m which provided an average slurry concentration of about 15 (v/v) when all solids were dispersed. Both axial and column average phase holdups were measured. Effects of sparger location, gas jets formation and liquid circulation patterns on gas holdups and solids dispersion are analyzed. Empirical correlations are developed to relate sparger location to solids dis- persion as a hnction of gas velocity. Optimum sparger height and orifice orientation is proposed based on the measure- ment of this study. On a etudie les effets de la hauteur du distributeur de gaz et de I’orientation de ses orifices sur les profils de disper- sion des solides et de retention de gaz dans une colonne a bulles en suspension triphasique. On a fait varier la hauteur du distributeur afin de couvrir les differents emplacements, en partant du fond de la colonne jusqu’a la hauteur de lit de solides sedimentes. Les orientations d’orifices ont ete modifiees de la direction “vers le haut” a la position “vers le bas”. Les mesures ont ete menees dans une colonne en plexiglass de 0,15 m de diametre interieur et de 2,5 m de hauteur. La phase gazeuse etait de I’air comprime sans huile tandis que de I’eau du robinet etait utilisee comme phase liquide. Des billes de verre ayant un diametre de particule moyen de 5 pm et de masse volumique egale a 2450 kg/m3 constituaient la phase solide. La hauteur de lit fixee etait d’environ 0,4 m e qui donnait une concentration moyenne de boues d’environ 15 en volume lorsque tous les solides etaient disperses. Les retentions de phase axiales et moyenne pour la colonne ont toutes deux etaient mesurees. Les effets de la position de I’aerateur, de la formation de jets de gaz et des profils de circulation des liquides sur les retentions de gaz et la dispersion de solides sont analyses. Des correlations empiriques sont etablies pour relier la position de I’aerateur a la dispersion de solides en fonction de la vitesse de gaz. On propose une hauteur d’aerateur et une orientation des orifices optimales a partir des mesures de cette etude. Keywords: sluny bubble column, sparger height, orifice orientation, solids dispersion. he slurry bubble column reactor is an important multi- T hase particulate system used for a number of processes in chemical, petrochemical and biochemical industries (Deckwer, 1985; Fan, 1989; Dudukovic and Devanathan, 1992). The advantages offered by slurry bubble columns include: high liquid (slurry) phase content for reactions to take place, reasonable interphase mass transfer rates at low energy input, high selectivity and conversion per pass, excellent heat transfer properties and easy temperature con- trol (isothermal operation), and online catalyst addition and withdrawal. Also, there is a low maintenance requirement due to simplicity in construction and absence of any moving parts. Some of the drawbacks of slurry bubble columns include: considerable backmixing in both the continuous liquid (sluny) phase and the dispersed gas phase, low volu- metric catalyst loading, bubble coalescence, and difficulties in scaling up. The productivity of catalytic slurry bubble column reactors could be improved by increasing catalyst loading. This can, however, lead to regions of poor mixing and mass transfer, especially in the distributor region. The distributor design therefore, is expected to play an important role in proper design and operation of slurry bubble columns. The placement of the gas distributor system is also critical for an efficient and trouble Free operation of the slurry ‘Author to whom correspondence may be addressed. E-mail address: aprakas2@julian.uwo.ca bubble column reactor. The placement of a sparger with downward facing orifices too close to the column base can lead to eventual erosion of the base place. Moreover, for higher solids corrcentration systems, there may be startup problems due to solids plug formation (Gandhi, 1997). There is currently a lack of information on the hydro- dynamic behavior of slurry bubble columns with varying sparger heights which can lead to optimum sparger location. This study investigates the effects of sparger height, orifice orientations and superficial gas velocities on hydrodynamics of a slurry bubble column. The hydrodynamic parameters investigated are solids dispersion and gas holdup. Experimental Experiments were conducted in a Plexiglas column which had an inner diameter of 0.15 m and a total height of 2.5 m. The column was designed with four sections for easy con- struction and flexibility (Figure 1). The gas phase was oil-free compressed air. Filtered air passed through a sonic nozzle and entered the column through a gas distributor at the bottom of the column. The sonic nozzle provided the advantage of a controlled air flow which is independent of downstream pressure (which would fluctuate during experimental runs). The air flow rate was varied by adjusting the pressure upstream of the sonic nozzle with a pressure regulator. The superficial gas velocity was varied between 0.05 ds nd 0.28 m/s based on ambient conditions. Air exited the column THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 77, APRIL, 1999 383  Section 4 (0.30m) Section t 3 (0.50m) Section # 2 (1.20m) Section W 1 (0.50m) / rl .D. = 0.15m Cyclonic Separator i I I E Sluny Sampling Taps (9.53mm) .. 1 IAir Pressure Taps (6.35mm) Drain Sonic Nozzle Figure 1 etails of experimental setup. top via a fume hood. Prior to exiting in the fume hood, the air passed through a cyclonic separator and bag filter to remove any fine particulates which may have been entrained. Tap water was used as the liquid phase for both two phase (G-L) and three phase (G-L-S) systems. Since the system was operated in batch mode, the static slurry height was maintained at 1.5 m above the bottom of the column. Glass beads with an average diameter of 35 pm and particle den- sity of 2452 kg/m3 constituted the solid phase. The settled solids bed height was about 0.395 m giving an average slurry concentration of about 15% (vh) solids. The gas was distributed at the column bottom through a four arm sparger with orifices facing downwards or upwards. The distance between the orifice and the column bottom (base plate) could be adjusted with a special arrangement shown in Figure 2a. It consisted of a Plexiglas ring, with its inner diameter flush with the gas inlet pipe. The sparger could be moved up or down with respect to the ring and an O-ring prevented any leakage of slurry. The following pro- cedure was followed for adjusting sparger height: first the screws on the Plexiglas ring were loosened, then the sparger height adjusted by either raising or lowering the sparger and finally the screws on the Plexiglas ring were tightened. The height of the orifices from the base plate was varied between 0.015 to 0.45 m. Figure 2a shows the sparger with down- ward facing holes. For upward facing holes, the sparger arms were turned up by 180 . Each arm of the sparger had five orifices of 1.5 mm diameter. The orifices were spaced as shown in Figure 2b, based on the criterion of uniform dis- tribution of gas across column cross-section. Average and axial gas holdups were measured by the pressure profile technique using manometers located at Spaier Adjustable) eight 550 Column Wall Plexiglass Ring ___-________-------------- crew 0-nng Air from sonic nozzle Figure 2a djustable height sparger design details. Sparger Arm Column Wall / 0 OI j I n Orifice (diameter = 1 5 mm) Figure 2b - op view of sparger with orifice spacing. approximately 0.05, 0.25, 0.45, 0.75 and 1.15 m, above the base plate. To avoid plugging of pressure ports by fine solid particles, a U-tube manometer system with air backflushing was used. Each rotameter allowed a small amount of air to enter the column, thereby preventing liquid and/or solids from entering the lines. To minimize any errors due to fric- tional pressure drop in the back flushing lines, the length of tubing from the tee splitter to the column wall was minimized 384 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 77, APRIL, 1999  and kept constant for all pressure taps. The pressure gradient can be related to the manometer pressure differential by: Column Wall w I 165 mm ; P = p,p( ) ........ A 2 .................... (1) Where p,is the density of the manometer fluid, Ay is the manometer pressure differential and Az is the height differ- ence of the pressure taps. However, for the three-phase flu- idized system, pressure gradient is also defined as: ..... ... 2) where pd is the dispersion density. Since water was used as the manometer fluid, a relationship between dispersion den- sity and pressure profile can be obtained: Pd = -Ow *y ) ......................... (3) For bubble column systems (G-L), the gas holdup between two adjacent pressure taps can be directly correlated from the ratio of pressure differential to the height differ- ence of the pressure taps as: ............................ g = +(Ay ) 4) For slurry systems, Equation (2) can be rearranged to give: or. Pd - PSI & =- .......................... Pg PSI Finally, for low pressure operations the gas density (p,) is small compared to slurry density (p,,), therefore: Thus, the gas holdup in a differential section of the col- umn Az), ould be calculated from Equations 3) and (6) if the slurry density in this section was also known. Therefore, slurry samples were taken along the column height and slurry density was determined by the pycnometric technique. The set of slurry samples withdrawn along the column height gave an axial profile of the slurry density. From this profile and Equations 3) and (6), he axial gas holdup profile in the column was determined. The average gas holdup in the col- umn was calculated from the pressure difference between the top and the bottom pressure taps and the average slurry density obtained from total solids dispersion. Since back- flushing introduced a small amount of air into the system, tests were performed to measure the effect of back-flushing .-t -.J 9 53 mm Slurry Sample Figure 3 -Details of slurry sampling probe design and operation. on average gas holdups. Measurements were made for a gas-liquid system with air back-flushing and with back- flushing turned off. The effects of back-flushing were found to be less than 1%. Local slurry concentrations were obtained from slurry samples withdrawn with a specially designed sampling probe shown in Figure 3. The sampling probe was designed so as to avoid entrainment of gas bubbles and prevent solids from settling within its shaft. Five sampling ports were available along the axial column height to collect slurry samples. They were located at 0.05, 0.25, 0.65, 1.05 and 1.45 m respectively, above the base of the column. Each sampling probe consisted of an outer sleeve and a piston rod. Figure 3 shows the probe assembly and direction of flow of slurry sample. As can be seen in Figure 3, when the piston rod was pulled outward the slurry flowed out of the nozzle pointing downwards. After collection of the slurry sample the piston was pushed back into the sleeve to stop the flow of slurry. A brush attached at the tip of the piston cleaned the sleeve when the piston was pushed back. Samples sizes of 75 mL to 100 mL were withdrawn and the solid fraction in slurry sample y,) was obtained by the pycno- metric technique. Average solids dispersion density was determined by measuring static solid heights prior to and during experimental runs. Results and discussion Measurements were made for solids dispersion and gas holdups for varying sparger heights. The sparger was above the initial bed of solids when positioned at 0.45 m and 0.40 m from the bottom, and within the bed for other sparger positions. When air was sparged into the column some of the settled solid got dispersed, causing a reduction in the set- tled bed height. Figure 4 ives the settled solid bed height as a function of superficial gas velocity for varying sparger positions for the downward facing orifices. A relatively even interface between the solid bed and slurry could be observed for different conditions, allowing visual measure- ments of the settled bed heights. For a given sparger posi- tion, the settled bed height decreased with increasing gas velocity due to the dispersion of the solids. The total amount of solids dispersed had two components; the solids dis- persed from above the sparger and those dispersed from below it. Dispersion of solids from above the sparger was always complete and dependent only on the sparger posi- tion. This is illustrated in Figure 4, where the height of the undisturbed solid bed is always below the sparger position at any given velocity. When the sparger was positioned at 0.45 m, there was practically no dispersion of solids at the superficial gas velocity of 0.05 m/s. The distance between THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 77, APRIL, 1999 85    Spargar I 0.40m A Smrasr at 0.35m 1 o 0.8 P H i5 UI 0.6 P 5 u -6 C . 0.4 P LL 0.2 0 0 0.15 -/ ------v , 0.05 0 00 0 00 0.05 0.10 0 15 0.20 0.25 0.30 Superficial Gas Velocity (m/s) Figure 4 ettled solid bed height for various sparger positions and superficial gas velocities (downward facing orifices). the sparger and the solid bed was about 0.065 m, indicating that any effect of bubbles generated turbulence and liquid recirculation extended to a lower distance. Some reduction in the settled solid bed height was, however, observed at the same gas velocity when the sparger was positioned at 0.40 m. The distance between the sparger and the defluidized bed was about 0.035 m, indicating that the effect of the gas bubbles and the turbulence induced by it extended to about 0.035 m. The length to which the solid bed height was reduced for the superficial gas velocity of 0.05 m/s remained substantially constant (at about 0.035 m) for all other sparger positions. As the gas velocity was increased further, the dispersion of solids increased and the solids bed height decreased. It can also be seen in Figure 4 that the settled solid bed height lines are almost parallel to each other for different sparger heights. This implies that the amount by which the settled solid bed height is reduced from below the sparger is mainly a function of the gas velocity and relatively independent of the sparger position. However, when the sparger height is decreased, there is higher dispersion of solids from the bed leading to a maximum slurry concentration of 15% (v/v) when all solids were dispersed. Therefore, it may be con- cluded that for the downward facing orifices there is no sig- nificant effect of slurry concentration on solids dispersion up to the highest slurry concentration (1 5% v/v) used in this study. With increasing gas velocity through sparger orifice, there is a transition from the bubbling regime to gas jetting regime. The transition from uniform bubbling to bubble coa- lescence and gas jetting has been reported in the literature (Leibsan et al., 1956; Ozawa and Mori, 1983; Rabiger and Vogelpohl, 1983). For a superficial gas velocity of 0.05 m/s the orifice Reynolds number was about 2500, indicating operation in the bubble coalescence regime (Leibsan et al., 1956) between uniform bubbling and gas jetting. For the superficial gas velocity of 0.1 m/s, the orifice Reynolds number was about 6000, indicating transition to gas jetting regime (Leibsan et al., 1956). For the downward pointing orifices, the solids dispersion below the sparger is aided by the momentum of gas jets. The dispersion of solids would h I I I I 0.0 0 1 0.2 0.3 0.4 0.5 Sparger Height from Base Plate (m) Figure 5 -Fractional solid dispersion with varying sparger height for downward facing orifices. increase with increasing gas velocity, due to increasing gas jet penetration and upward solids entrainment by the rising gas bubbles. The kinetic energy of the gas jets (1/2p,V 2 would increase with increasing gas velocity. The gas bubgles formed from the penetrating gas jets, create an upward momentum of the suspension. In turn he suspension travels up the column center and back down at the column walls, creating circulation patterns. If there is enough kinetic energy in this recirculation, solids may also be entrained and dis- persed: The length of the gas jets and the recirculation pattern in the liquid both increase with an increase in gas superficial velocity. This can account for the increased dispersion (smaller undispersed bed) with the gas velocity seen in Figure 4. Local slurry samples were taken along the column with sampling probes #2 to 5 (wherever possible). The average concentration of the dispersed solids could be calculated from these local slurry concentrations. Figure 5, shows the amount of dispersed solids in the slurry, (represented as a fraction of the maximum solid concentration), as a function of the gas superficial velocity, at different sparger locations. Nearly complete solids dispersion could be achieved up to a sparger height of about 0.1 m at the highest gas velocities (>0.2 ds). For sparger heights above 0.1 m, complete dis- persion of solids could not be achieved. Figure 5 can be used for a quick estimation of sparger height for uniform disper- sion of solids at a given velocity and vice versa. The distance between the sparger and the undisturbed solid bed (Ldis), could be related to the modified Froude number based on the orifice velocity and orifice diameter. Figure 6 presents the distance of the settled solid bed from the sparger as a function of the modified Froude number for various sparger positions. Data for the runs with gas super- ficial velocity varying from 0.10 m/s to about 0.25 m/s, and with sparger positions ranging from 0.45 m to 0.10 m above the bottom, have been included in this figure. It is seen from Figure 6, that LdiS is relatively independent of the sparger location (as indicated by the 95% confidence interval), except for the sparger position of 0.45 m from the bottom. However, L, for the sparger location at the 0.45 m con- verges with those for the other sparger positions at high 3 86 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 77, APRIL, 1999
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