CFD, A DESIGN TOOL FOR A NEW HOT METAL DESULFURIZATION TECHNOLOGY

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1 Seond International Conferene on CFD in the Minerals and Proess Industries CSIRO, Melbourne, Australia 6-8 Deember 1999 CFD, A DESIGN TOOL FOR A NEW HOT METAL DESULFURIZATION TECHNOLOGY Stefan PIRKER 1, Philipp GITTLER 1, Hermann PIRKER and Joahim LEHNER 1 Johannes Kepler University, 4040 Linz, AUSTRIA, pirker@mehatronik.uni-linz.a.at VOEST ALPINE Industrieanlagenbau GmbH, Postfah 3, 4031 Linz, AUSTRIA ABSTRACT A big vessel ontains hot metal overed by a thik syntheti slag layer. During nitrogen stirring sulfur transits to the slag. Sulfur ions in the slag have to be removed by oxidation (O lane) to restore slag desulfurization apability. The marosopi metallurgial reations ourring during hot metal desulfurization and slag regeneration are studied by kineti laws for permanent and transient phase ontat. Mehanial metal-slag interations are alulated by shear stress onsiderations for stratified flows. The position of the phase interfae and the slag surfae is obtained by an adaptive grid algorithm. Flow initiation by rising gas bubbles is simulated by means of an algebrai drift flux model. As a result of these simulations the fully three-dimensional flow- and temperature fields in the slag and metal phase are obtained. Furthermore the variation of the timedependent onentration fields of the main reatants an be observed. NOMENCLATURE A area g gravitational aeleration h bath height H molar reation enthalpy I 0 initial nozzle momentum K S equilibrium onstant for slag regeneration L S equilibrium onstant for sulfur removal M S molar weight of sulfur Q heat ontent p stati pressure of fluids, partial pressure of gases t time T temperature u veloity V volume z deformation of free surfaes α gas volume fration, heat transfer oeffiient ε emission oeffiient κ mass transfer oeffiient µ visosity ρ mixture density σ Stefan-Boltzmann onstant τ shear fores subsripts: b drift e f-str g hm inter reat sl surf tr wall lam turb bubble ell drift (-veloity) environment free-stream gas hot metal metal slag interfae reation slag bath surfae terminal rising (-veloity) wall and bottom lining of the vessel laminar turbulent hemial nomenlature: [ ] mass fration of speies dissolved in hot metal ( ) mass fration of speies dissolved in slag { } mass fration of gaseous speies INTRODUCTION This new desulfurization tehnology, whih is investigated by means of CFD-simulations, an be divided into two proess steps. First the dissolved sulfur transfers from the hot metal into the syntheti slag layer during the hot metal desulfurization. This proess is onsidered in the next two hapters. Seond the syntheti slag has to be regenerated by oxygen injetion after reahing its sulfur apaity. This seond step an be studied independently from the first one. After the slag regeneration new hot metal an be desulfurized and the proess starts from its beginning. During the development of the simulation models are is taken to reate results that are omparable with ongoing high temperature experiments. SIMULATION OF THE DESULFURIZATION PROCESS - MODELLING Hot metal and slag are both onsidered as Newtonian fluids. As a onsequene the well known Reynolds averaged Navier-Stokes equations have to be fulfilled in both phases. The turbulent harateristis of the flow pattern are modelled by the standard k-ε model (Launder and Spalding, 197). Speies transport, i.e. sulfur transport in the hot metal or sulfur ions transport in the slag phase, is alulated by means of a salar transport equation onsidering onvetive and diffusive transport mehanism (Patanker, 1980). Turbulent diffusion is alulated by applying a onstant Shmi number 53

2 (Pranl et al, 1993). For the simulation of the temperature field an additional energy transport equation is introdued aounting for heat transport based on onvetion and ondution. In priniple alulations are performed in hot metal and slag independently. Reiproal influene, i.e. mehanial and hemial, is assumed to happen only at the metal-slag interfae. Shear Stress Considerations at the Metal-Slag Interfae If the bath agitation is not too strong and emulsifiation does not our, the two fluids, hot metal and slag, represent stratified flow onditions. To explain the modelling of the reiproal mehanial influene at the interfae a Couette flow is onsidered. A hot metal phase and an upper slag layer is situated between two parallel horizontal plates. The upper plate moves with onstant veloity u to the right. Beause of the no slip ondition the uppermost slag layer has to move with the same veloity. On the phase-interfae in the middle between the plates shear fores at from both the hot metal as well as from the slag. In the equilibrium state they have to nullify eah other: sl inter = τ τ (1) hm inter In the omputer ode this priniple is implemented by a moving interfae wall. The veloity of that fititious wall has to be in between the veloities in the neighboring hot metal and slag ells. The exat value of the wall veloity at a given position an be alulated by omparing the shearfores ating from the hot metal flow with those ating from the slag layer. Figure 1 shows the alulated results in the ase of turbulent Couette flow. In this simulation slag visosity is supposed to be ten times higher than hot metal visosity. u inter µ lam Figure 1: Turbulent Couette Flow. The omputed veloity profile is displayed on the left side. In the laminar sublayers of the hot metal phase higher veloity gradients exist due to its lower laminar visosity. On the right side the effetive visosity formed by the sum of the laminar and turbulent visosity is U µ lam µ turb µ turb slag hot metal shown. Although hot metal phase laminar visosity is lower than that of the slag, its effetive visosity is higher than the effetive slag visosity. This surprising result an be explained by the different thikness of the boundary layers. In the slag phase turbulent flutuations are suppressed by the thiker boundary layers. As a onsequene of this effet the veloity gradients in the middle of the hot metal layer are smaller than in the middle of the slag. Position of Phase Interfae and Bath Surfae A hydrostati deformation of the free surfaes an be dedued from the differenes in the pressure distribution at the phase interfae and the bath surfae: ( ρ ρ ) g = p ( h ρ g + p ), zhm hm sl sl sl e () zsl ρ sl g = p pe. After deformation both fluid volumes have to be resaled in vertial diretion in order to fulfil the mass balanes in eah phase. The deformation of the free surfaes influenes the flow field in the liquid phases and as a onsequene the sulfur transport to and from the surfae. Therfore this alulation is inluded into the simulation although it is not of major importane for the desulfurization rate at first glane. Bath Agitation by Nitrogen Injetion In order to stir the bath, nitrogen is injeted through a porous plug at the bottom of the vessel. The uprising gas bubbles indue a flow in the ontinuous phases of hot metal and slag. The effet of the gas bubbles is aounted for by an algebrai slip model (Manninen et al, 1996). This mixture model is supplemented by a more general alulation of the drift veloity between the gas bubbles and the ontinuous phases. In this simulation the drift veloity is set to (Pranl et al, 1993) ( 1 α g ) tr u drift = u. (3) The gas bubbles are assumed to pass the metal-slag interfae without rest and to leave the slag phase at the bath surfae immediately. Sulfur Transfer At the phase interfae dissolved sulfur migrates from the hot metal to the slag phase: [ S ] + ( O ) ( S ) + [ O] The rate of sulfur loss in a ell adjoining the metal-slag interphase is assumed to be proportional to the differene between a free stream sulfur onentration and the equilibrium sulfur onentration at the phase interfae: κ hm sl [ S] d M A [ S] f, hm -str = ( S ) L inter The same sulfur mass flow has to be added in the very slag side ell: S (4) (5) 54

3 d [ S] d ( S ) M, hm = M, sl The mass transfer oeffiient κ is influened by veloity and turbulene fields and an be predited by empirial orrelations (Deo and Boom, 1993). However, in this simulation it is kept onstant. It is only varied in two simulations from 0.1 to 1 kg/m s to show the prinipal dependenies on this parameter. Temperature Field At the outer walls and the bottom of the vessel a heat flux ondition is set: Q wall = α wall A ( T Te ) (7) At the bath surfae heat is lost due to onvetion and radiation: Q surf surf 4 4 ( T T ) + ε σ ( T T ) e e (6) = α (8) A The heat transfer oeffiients as well as the emissivity oeffiient an be found in literature (Oeters, 1989). The sulfur transfer is linked to the temperature field by the reation enthalpy d S M Q reat = [ ] M S H reat, (9) while the ooling effet of the inoming nitrogen is negleted in these simulations. SIMULATION OF THE DESULFURIZATION PROCESS RESULTS Numerial simulations are performed by a modified and enhaned FLUENT ode on a SGI Indigo workstation. Slag Hot Metal = 0, m/s Figure : Veloity Field in Slag and Hot Metal (P=Porous Plug). As hot temperature experiments are not available at the moment the feasibility of the simulation tehnique is proven of parametri studies. P (S - ) 0,0 % 0,04 % 0,06 % 0,08 % [S] 0,05 % 0,051 % 0,05 % 0,053 % Figure 3: Sulfur Content in Hot Metal and Slag after 1 min. of Desulfurization ([S] 0 = 0,06 %). Figure shows the veloity field in both phases during eentri nitrogen injetion. In the region of the uprising bubble plume the deformation of the free surfaes an be seen. In Figure 3 the sulfur onentration in both phases one minute after the beginning of desulfurization is displayed. In Figure 4 the transient sulfur ontent in both phases is given. Several parameter studies are performed to show the funtionality of the simulation model. The variation of both the thermodynami equilibrium L s as well as the sulfur transfer oeffiient κ influene the desulfurization urves in a reasonable way. A higher gas throughput auses higher fluid veloities and therefore inreases the desulfurization rate due to improved sulfur transport to and from the phase interfae. This learly shows the purpose of CFD in the predition of mass transport ontrolled interfaial reations. The flow pattern in the liquid phases influenes the reation rate distribution at the phase interfae diretly. With the help of CFD it an be shown that the desulfurization rate varies with the gas flow rate as well as with the position of the porous plug on the bottom of the vessel or the vessel geometrie. These results annot be obtained by onventional mixed reator models. SIMULATION OF THE SLAG REGENERATION PROCESS MODELLING During the slag regeneration proess oxygen is blown through a submerged lane into the slag phase, whih is enrihed with sulfur-ions by that time. One injeted the oxygen reats with the sulfur to gaseous sulfur dioxid whih forms the offgas of the proess. While developing the simulation tools speial attention was given to the omparability of simulation results with first high temperature experiments. 55

4 a) b) ) Sulfur Content [kg] Sulfur Content [kg] Sulfur Content [kg] 0,6 0,5 0,4 0,3 0, 0,1 0,0 0,6 0,5 0,4 0,3 0, 0,1 0,0 0,7 0,6 0,5 0,4 0,3 0, 0,1 0,0 L S inreases κ inreases Q N inreases Figure 4: Parametri studies of the desulfurization proess: Sulfur ontent in hot metal ( ) and slag ( ). Bath Agitation by Oxygen Injetion Basially the same mixture approah as desribed in the previous hapter is applied. Nevertheless, two new aspets have to be onsidered in this onfiguration. First, the initial momentum of the inoming gas is no longer negligible and has to be added to the Navier-Stokes equations as a soure term: I Time [s] Time [s] Time [s] 0 V O u O 0 = ρ (10) Bath Swelling due to Gas Injetion In this simulation approah the upper bound of the alulation domain is onsidered to oinide with the bath surfae. At the beginning of gas injetion the surfae will rise due to the additional gas volume inside the slag phase. Therefore the omputational grid has to be adapted eah time step in order to fulfil the global mass balanes: d ρ dv = V O ρo V Offgas ρoffgas (11) V The influene of grid movement is onsidered in all field equations (Fluent, 1996). Although the alulation of the bath height is not believed to be of major importane for the predition of desulfurization it is inluded into this simulation to get just another variable whih an be ompared with experiments. Chemial Reation The main reation ourring is given by (Glovaskii, 198) ( CaS ),5 { O } ( CaO) + { } 1 SO +, (1) with the thermodynamial equilibrium onstant K reg ( CaO) pso 1, 5 ( CaS) p =. (13) The reation takes plae at the interfae between the uprising gas bubbles and the liquid slag. In this ase the reation area in a omputational ell is given by O ( α +α ) O SO Areat 3V rb =. (14) It diretly depends on the mean radius of the gas bubbles. Obviously smaller bubbles are hemially more effiient than big ones. The rate of sulfur loss in a ell is given by a mass ation law and follows to: κ sl b A reat - ( ) d S ( ) CaS p M 1,5 O = ( ) CaO p K reg SO (15) The mass transfer oeffiient κ is set to 0.1 kg/m s in this simulation. In futur simulations mass transfer orrelations (Ray, 1993) should be used to aount for the influene of veloity and turbulene fields on the mass transfer oeffiient. Temperature Field Basially the modelling is the same as in the ase of the desulfurization proess. Seond, the prodution of {SO } and (CaO) has to be aounted for. Due to this reation the total volume of the gas bubbles in the vessel is less than in the ase of inert gas purging with idential inoming gas flow rate. 56

5 O O O O {O } (CaS) {SO } (CaO) 3,5 ppm 15 ppm 90 ppm 0,997 % 0,99 % 0,988 % 8,9 ppm 5 ppm 1,5 ppm 89 ppm 55 ppm 3 ppm Figure 5: Spaial distribution of the main reatants during slag regeneration shortly after the beginning of the blow, Initial onditions: 1 % (CaS) and all others 0 %. SIMULATION OF THE SLAG REGENERATION PROCESS RESULTS Parametri studies demonstrate the expeted tendenies in the reation rate. The following parameter variations result in an improved reation rate: - Redution of mean bubble diameter - Redution of atmospheri pressure (vauum proess) - Redution of partial gas pressures by additional inert gas purging - Inreased O flow rate While simulating metallurgial proesses the question arises how alulation results an be ompared with real measurements. In the planned hot temperature experiments the following variables seem to be measurable: - slag surfae veloity (by video methods) - surfae temperature (by infrared thermometer) - bath height (by video methods) - offgas omposition (by in situ gas analysis) - sulfur ontent (by onventional probe) Figure 6 shows the veloity in the slag phase one minute after the beginning of oxygen injetion. As a further result the transient rising of the bath surfae is alulated. Both results an be ompared with hot temperature measurements. O 0,35 h O 0,30 0, t time-dependent bath swelling at the beginning of the blow Temperature (K) ,1 m/s Figure 6: Veloity field in the slag vessel during slag regeneration inluding alulation of initial bath swelling Figure 7: Temperature distribution during desulfurization proess. 57

6 The onentration fields of dissolved (CaS) and (CaO) as well as that of {O } and {SO } bubbles are given in Figure 5. In this simulation a homogeneous (CaS) onentration of 1 % is assumed at the beginning. The amount of oxygen injetion is set onstant during the proess. A signifiant disadvantage of these properties is that the spaial onentration distributions are hardly measurable in hot temperature experiments. The temperature distribution in several horizontal planes is given in Figure 7. Heat is produed by the exothermi desulfurization reation at the submerged lane tip while it is lost at the bath surfae and at walls. The hottest spot at the bath surfae indiates the stagnation point of the uprising hot plume. As the surfae temperature distribution is measurable in hot temperature experiments this simulation result an be validated. gas flowrate [Nl/min] Figure 8: Offgas omposition during stepwise oxygen injetion CaS ontent [kg] 0,16 0,15 0,14 0,13 0, O,in [Nl/min ] time [s] time [s] CaO CaS O,in SO,out O,out Figure 9: (CaS) derease and (CaO) aumulation during stepwise oxygen injetion. In order to study the time dependent variation of the offgas omposition the lane is supplied by stepwise varied oxygen throughput. Figure 8 shows the {O } and {SO } ontent of the offgas during this simulation. The solid line indiates the oxygen flow rate variation. It should be possible to measure the offgas omposition online in real experiments. The same simulation is performed to examine the timedependent desulfurization rate of the slag. Figure 9 depits the variation of the (CaS) and (CaO) ontent in the slag. Due to the desulfurization reation alium exhanges a 0,030 0,05 0,00 0,015 0,010 0,005 0,000 CaO ontent [kg] sulfur atom with an oxygen atom. Therefore the two urves differ from eah other only by a negative sign. The omposition of the slag an be measured only at a ouple of distint time steps. Therefore omparison with these simulation results is rather limited. CONCLUSION A simulation tool for the alulation of a new desulfurization proess is presenteded. This tehnology an be divided into two main steps, whih an be treated separately. During the development of the simulation tools speial are is taken to generate simulation results that an be ompared with high temperature results. The main onlusions of this study are - The oupled simulation of the hot metal desulfurization proess inluding hemial interfae reations and thermal onsiderations in hot metal and slag phases, turns out to be feasible. Parameter variations prove that the implemented models work reasonably. - As hot metal desulfurization is ontrolled by transport mehanisms the flow pattern in the liquid phases determines the reation rate. Therefore porous plug position as well as gas flow rate influenes the desulfurization rate as an be shown with the help of CFD. - The simulation of the slag regeneration proess also seems to be feasible and parameter studies work well. - CFD turns out to be a powerful investigation tool for metallurgists as soon as in the design phase of new tehnologies. REFERENCES DEO B. and BOOM R. (1993), Fundamentals of Steelmaking Metallurgy, Pretie Hall Inernational., New York, USA. FLUENT INC. (1996), FLUENT User s Guide - Release 4.4, Fluent Inorporated, Lebanon, USA. GLOVATSKII V.A. et al. (198), Regeneration of blast furnae slags used for desulphurizing pig iron, Steel in the USSR, pp LAUNDER B.E. and SPALDING D.B., (197), Letures in Mathematial Models of Turbulene, Aademi Press, London, England. MANNINEN M.; TAIVASSALO V. and KALLIO S. (1996), On the mixture model for multiphase flows, VTT Publiations 88, Espoo, Finnland. OETERS F. (1989), Metallurgie der Stahlherstellung, Verlag Stahleisen,Düsseldorf, Germany. PATANKAR S.V. (1980), Numerial Heat transfer and Fluid Flow, Hemisphere Publishing Corporation, New York, USA. PRANDTL L.; OSWATITSCH K. and WIEGHARDT K. (9. Edition, 1993), Führer durh die Strömungslehre, Vieweg, Braunshweig, Germany. RAY H.S. (1993), Kinetis of Metallurgial Reations, Siene Publishers, In., Lebanon, USA. 58