CFD IMPELLER SPEED EVALUATION OF AN INDUSTRIAL SCALE TWO-PHASE FLOW STIRRED TANK

Transcription

1 Eleventh International Conferene on CFD in the Minerals an Proess Inustries CSIRO, Melbourne, Australia 7-9 Deember 015 CFD IMPELLER SPEED EVALUATION OF AN INDUSTRIAL SCALE TWO-PHASE FLOW STIRRED TANK Tao SONG 1,, Kaixi JIANG 1, Junwu ZHOU 1,, Zhenghang SHEN 1, Yuqing FENG 3* 1 Beijing General Researh Institute of Mining an Metallurgy, Beijing , CHINA Beijing Key Laboratory of Automation of Mining an Metallurgy Proess, Beijing 1068, CHINA 3 CSIRO Mineral Resoures Flagship, Clayton VIC 3168, AUSTRALIA *Corresponing author, aress: Yuqing.Feng@siro.au ABSTRACT The inustrial appliation of bio-oxiation in the gol extration has been realize for years. The gol pareling in sulfies like pyrite an arsenopyrite oul be expose through baterial oxiation pretreatment, an extrate with high reovery in the yaniation. Large sale stirre tank bioreators with high performane are essential to save operation osts for low grae refratory gol onentrates. In this paper, a gas-liqui two phase CFD moel has been applie to investigate the air-water flow in an inustrial sale bioreator. The effets of the impeller spees on tank performane are assesse in terms of the overall flow patterns, gas holup, bubble resiene time istribution, as well as water mixing. It was foun that the water flow in the tank is ominate by stirre impellers with axial flow in the impeller zones. Small air bubbles generate by six gas nozzles istribute to the whole tank exept the bottom area. The impeller spees are essential to ahieve a goo gas-liqui flow pattern in the reator. Simulation results obtaine emonstrate the feasibility of the present moelling approah as a useful numerial tool to help potential improvement of inustrial sale stirre tank bioreator esign an/or operation. NOMENCLATURE Cμ bubble inue turbulent ey visosity oeffiient CD rag fore oeffiient bubble iameter g gravity aeleration k turbulent kineti energy M interfaial momentum transfer between phases P pressure SMα momentum soures ue to external boy fores U veloity US speifie bubble terminal veloity UT bubble terminal veloity γ volume fration turbulent energy issipation rate ensity graient operator ynami visosity t bubble inue turbulent ey visosity t gas phase turbulent ey visosity σ turbulent Prantl number ν ontinuous-phase kinemati visosity t α ontinuous liqui phase isperse gas phase turbulene phase, either gas () or water () INTRODUCTION Many gol eposits in China ontain so-alle refratory ore, where the gol is loke insie sulphies suh as pyrite an arsenopyrite. These ores have to be speially treate to break own the sulphies before the gol an be extrate. This is onventionally one by roasting, neessitating substantial energy input an resulting in environmentally unesirable emissions, espeially when ealing with arseni minerals (Pinhes, 005). Pressure oxiation metho is a more avane refratory gol onentrates pretreatment. The hyro-proess oes not proue harmful gases, an an ahieve a high gol exation. However, the pressure oxiation proess also nee a high investment in failities, maintains an trainings of the workers. Bio-oxiation tehnology uses the ability of ertain miroorganisms to oxiise sulphie minerals, an exothermal proess from whih these organisms gain energy. At the same time metals may be release into an aqueous solution. From 1980s, the gol onentrates tank bioleah has evelope rapily in the whole worl. In last ten years, some biogol plants in China, suh as Shanong Tarzan Biogol Company in Laizhou, Tianli Gol Plant in Liaoning, Jinfeng Biogol Plant in Guizhou, have been built, an high gol exations an goo environmental performanes have been ahieve (Yang et al., 00). This kin of moern, simple, heap an environmentally soun tehnology is suitable for a large number of small gol mines in China, in whih mostly ontain refratory ores. As the key failities in gol bioleah plants, stirre tank bioreators an airlift bioreators are ommonly use. In the main biogol ompany, suh as Ashanti (Ghana), Fairview (Zambia), Sao Bento (Brazil), Wiluna (Australia), the stirre tank bioreators are mainly use, an lea a goo performane (Morin et al., 006). Bioreators with high effiieny require proper flow fiels for mineral partile suspension, low shear an nutrient environment for growth of bateria, goo bubble istribution for oxygen mass transfer. Sale-up of the high performane bioreator is still a hallenging task, espeially for large- Copyright 015 CSIRO Australia 1

2 sale reators. Using CFD moelling, a wie range of variations in physial esign an operational parameters an be teste an refine until a esign that gives optimum performane. Nowaays, CFD moels have been generally aepte as a powerful tool to assist esigning new equipments an proesses or proess optimisation while minimising risk (Shwarz, 1994; Shwarz an Feng, 014). In this paper, a large sale stirre tank bioreator with high performane esigne by BGRIMM has been moele. The CFD moel was use to evaluate the performane of the bioreator uner several impeller spee onitions. The overall flow ynamis is investigate in terms of liqui veloity vetors. The relative performanes have been evaluate in terms of gas holup, bubble resiene time istribution an liqui mixing. MODEL DESCRIPTION There are vast literatures on moelling tehniques for gas liqui flows in stirre tanks (Bakker an van en Akker, 1994; Bruato et al., 1998; Aubin et al. 004). Usually, the Eulerian-Eulerian moel was foun as a proper metho in simulation of multiphase flow. The steay multiple referene frame (MRF) metho an sliing mesh metho were use to simulate impeller rotation. The Eulerian-Eulerian moel requires less omputing power than iret interfae resolution, but the etaile bubbling hyroynamis annot be obtaine. This moel is still suitable for proess simulation, an has been wiely use in various multiphase flow systems (Shwarz an Turner, 1988; Lane et al., 005). The time average two flui (Euler-Euler) moelling approah has been aopte for this stuy. For the air-water system being stuie in the stirre tank bioreator, the equations are average over the phase struture so as to give time-average equations for eah phase. The onservation of mass an momentum governing equations are summarize below: ( U ) 0 (1) ( ( U ( ( U U )) P ( U T ) )) S M M Phase epenent turbulene moels have been use, e.g. the isperse phase zero equation moel for the gas phase an the k-ε two-equation moel for the liqui phase. In the liqui phase, an extra ontribution to the turbulent visosity ue to bubble slip is inlue, following Sato an Sekoguhi (1975). The effetive visosity is a sum of the moleular an turbulent visosities, with the turbulent ey part alulate in: t (3) t k t C 0. 6 U U Coupling between the isperse gas phase an ontinuum liqui phase is moelle using the rag law propose by Fajner et al. (008): 4 g CD (5) 3 U S () (4) UT g (6) ρ ρ U S UT 0.3 tanh (7) ρ Here λ=(v 3 /ε) 0.5 is the Kolmogorov length. This moel onfirme the signifiant role of turbulene in reuing the rising veloity of the buoyant partiles an exhibite the same behaviour as the settling partiles. The lift fore esribes the interation of the bubble with the shear fiel of the liqui. This results in net fore ating perpeniular to the motion of the bubble relative to the liqui on the bubble. Tomiyama (1998) lift fore moel was use here. A turbulene ispersion fore is propose to aount for the iffusion of bubbles ue to the ranom influene of turbulent eies in the liqui using the Lopez turbulene ispersion fore moel (Lopez et al., 1994). In simulation, a steay-state moel for the slurry an bubble flow within the tank was solve first. Then, by holing the gas-liqui flow fiel fixe, a transient moel of two aitional variables were use to preit the time varying of bubbles an slurries in the tank. The transport equation for the two variables followe: t U D S t (8) t whih were set the respetive properties of gas an liqui to trak the flow of bubbles an slurries separately. Figure 1: The geometry of this inustrial stirre tank bioreator. The stirre tank bioreator esigne for refratory gol ores by BGRIMM is a kin of baffle tank with multiple impellers. The impeller is esigne to be an axial flow impeller an is similar to hyrofoil impeller. The iameter of the impeller is 1 m, an eah impeller has 3 blaes. At the bottom of tank, there are 6 high spee gas nozzles, whih an generate small bubbles, installe evenly at the sie wall 0.3 m above the flat tank bottom. The height of the tank is 10 m with an internal iameter of m. The total volume of the reator is 31.4 m 3. The efault esign has 4 impellers installe, an bottom learane is 0.9 m. The interval between impellers are.5 m. The istane between the top impeller the free liqui surfae is.05 m. Figure 1 shows the geometry struture of the impeller an this stirre tank bioreator. Copyright 015 CSIRO Australia

3 The CFD moel has been setup using the geometry of the full-sale bioreator. In this stuy, only water an air were onsiere for inustrial experiments in water environment. The simulations were teste uner two ifferent mesh sizes. The fine gri inlue 69,78 ells. The oarse strutural gri was 373,19 ells. The ifferenes between simulation results of gas volume fration in the whole tank uner two kins of gris were less than 4%. Consiering the omputing time, the oarse mesh was use in this paper. A ommerial CFD oe ANSYS CFX14.5 has been use to obtain a solution of the above equations. A gas outlet bounary onition has been use on the top surfae of the stirre tank bioreator through whih gas leaves the tank at the rate it arrives from below (an option alle egassing onition in CFX). Wall soli bounaries were set as no slip for water an free slip for air. RESULTS In the bioreator, hyrofoil impellers were evelope for appliations where axial flow is important an low shear is esire. For the pratie in bateria bio-oxiation proess this type of impeller is proper to generate irulation flow an bubble istribution. A low shear environment is also goo for growth of bateria. In the reator, bubble size was assume to be in a uniform size of mm, whih was base on observation of the gas nozzle experiments in laboratory. Several simulations were onute to test the effet of impeller spee of 0, 16, 3 an 48 rpm, an the gas flow rates were all 0.3 Figure an Figure 3 show the water an air veloity istribution along the mile vertial plane uner ifferent impeller spees. When the impeller spee was 0 rpm, the injete bubbles rose up iretly to the free surfae ue to the buoyany fore, an the water move to the vaany where bubble left. So no regular irulation was generate in the bioreator. When the impeller spee inrease, axial flow irulations were generate near the impeller areas. In the same time, air bubbles near impeller zones were also ragge into the swirls an move along the irulation routes. Figure 4 preits gas volume fration uner 4 ifferent impeller spee. When the impeller i not spin, the gas volume fration was apparently smaller. As the inrease of the impeller spee, air bubble move to more area in the tank. But there is nearly no gas in the bottom of the reator. Figure 3: Gas veloity vetors in the mile slie plane for ifferent impeller spees uner the gas flow rate of 0.3 Figure 4: Gas volume fration in the mile slie plane for ifferent impeller spees uner the gas flow rate of 0.3 Figure : Water veloity vetors in the mile slie plane for ifferent impeller spees uner the gas flow rate of 0.3 The shear strain rate at some point within the material measures the rate at whih the istanes of ajaent parels of the material hange with time in the neighborhoo of that point. It reflets the rate at whih it is being eforme by progressive shearing without hanging its volume. It is zero if these istanes o not hange, as happens when all partiles in some region are moving with the same veloity (same spee an iretion) an/or rotating with the same angular veloity, as if that part of the meium were a rigi boy. When the impeller spee inreases, veloity ifferene turns to be bigger in the rotational area, an brings higher shear whih is harmful for the bateria in bio-oxiation proess. Figure 5 Copyright 015 CSIRO Australia 3

4 shows the simulate shear strain rates uner 4 ifferent impeller spees. Higher impeller spee brought higher shear strain rate in the interation zones. Although mehanial estrution of bateria has been notie uner various operating onitions, the amount of shear at the onset of ell amage has not been quantifie for biooxiation (Bailey an Hansfor, 199). Hak et al. (1989) foun low leah rates were ause by exessive shear stress uring the pilot plant test work. Leahing was improve when impeller tip spee was reue from 5.3 m/s to 3.3 m/s. In this work, when the impeller spee was 48 rpm, the tip spee was nearly.5 m/s. Taking the researh results propose by Hak et al. (1989) as referene, the impeller spee of 3 an 48 rpm are all aeptable for the leahing proess. gas volume fration an mean bubble resiene time, the mean bubble resiene time is linearly proportional to the gas holup. Bubble RTD (s -1 ) Time (s) 0 rpm 16 rpm 3 rpm 48 rpm Figure 6: RTD urves for the step input bubble traer for ifferent impeller spees uner the gas flow rate of 0.3 (the urve of 3 rpm oinies with the urve of 48 rpm) Gas Volume (m 3 ) a Impeller Spee (rpm) Mean Bubble Resiene Time (s) b Impeller Spee (rpm) 95% Mixing Time (x10 s) Impeller Spee (rpm) Figure 5: Water shear strain rate in the mile slie plane for ifferent impeller spees uner the gas flow rate of 0.3 In the present bateria bio-oxiation operation, a ertain amount of gas holup is require to ensure the oxygen an arbon ioxie mass transfer between air bubble an liqui phase, an the bubble resiene time shoul be as long as possible to inrease the reation spee an fee the miroorganism. Cheking the bubble resiene time istribution (RTD) oul present the bubble resiene time in the bioreator. The bubble RTD is alulate by step traer methos, e.g. ontinuously injeting bubble traers from the gas inlet an monitoring gas esaping rate from the tank top. One the bubble traers reah a ynamially stable state, e.g. the bubble esaping rate is equivalent to the gas injetion rate, the bubble RTDs an be obtaine. Figure 6 ompares the alulate bubble RTD ensity urves uner ifferent impeller spees. The bubble RTD ensity urves in the bioreator use here were similar to log normal istribution ensity urves. When there was no impeller spin, the mean of bubble RTD ensity urve was smaller than the onitions when impeller spun. When the impeller was 16 rpm, the peak value was a little bigger than the onitions when the impeller spees were 3 an 48 rpm. Uner the impeller spee of 3 an 48 rpm, the mean an the stanar eviation of the istributions were similar to eah other, but the peak value was smaller than the onition when the impeller spee was 16 rpm. Taken the onsieration of Figure 7a an Figure 7b, whih quantitatively present the Figure 7: Simulation results for ifferent impeller spees uner the gas flow rate of 0.3 m 3 m - min -1 : (a) gas volume in tank; (b) mean bubble resiene time; () 95% mixing time. Figure 8: Water traer istributions in the leahing tank uner impeller spee of 3 rpm an gas flow rate of 0.3 m 3 m - min -1 after 0s, 100s, 300s, 600s, 900s an 1660s. The level of liqui mixing is an important parameter to assess the tank performane. Pulse water traers are put into the leahing tank from the slurry inlet loate at the bottom area to evaluate the mixing behavior of the bioreator. Figure 8 shows the water traer istributions over a vertial plane in the bioreator uner impeller spee of 3 rpm an gas flow rate of 0.3 After the injetion, the water traers flow upwar, an isperse in a Copyright 015 CSIRO Australia 4

5 wier region. Before evenly istributing in the tank, the traer experienes a ispersion from the bottom to the up area in the reator. After about nearly 1660 seons, an almost uniform istribution of slurry is ahieve. For the other three impeller spees, the traer mixing pattern are very similar, thus, their temporal-spatial istribution are not plotte. The 95% mixing time are presente in Figure 7, ereasing with the inrease impeller spee. CONCLUSION Air-water flow in a bio-oxiation reator for refratory gol ore has been investigate using a gas-liqui two phase CFD moel with ANSYS CFX14.5 being the numerial platform. The likely effets of the impeller spee, the impeller installation onition an gas flow rates on tank performane are assesse in terms of the overall flow patterns, gas holup, bubble resiene time istribution, as well as the water mixing. The impeller rotation is the main mixing power input in this bioreator. The impeller spee is the eisive fator for the flow pattern of air an liqui, an also for the mass transfer an the partile suspension. The impeller spee of 3 an 48 rpm showe better performane than the spee of 0 an 16 rpm, an presente similar gas volume fration an bubble RTD. So these two spee settings are reommene in this stuy. Simulation results obtaine emonstrate the feasibility of the present moelling approah as a useful numerial tool to help unerstaning the influene of impeller settings an gas flow rates for the bioreator. Further work with the onsieration of oxygen/ioxie mass transfer between the air an water/slurry will be performe uner a wier esign/operation onitions to fully larify this issue in etail. 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