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Seeds

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Seeds

  1. 1. Chemical & Physical Processes in Glass Melting Quality of glass melting process Buchmayer Ruud Beerkens TNO Glass Group Eindhoven, The Netherlands Glass Service ICG – EFONGA Spring School Montpellier 4-5 May 2009 1
  2. 2. Contents • Overview of processes essential for glass melting • Tools of analysis of industrial glass melting • Melting-in of Raw materials • Kinetics of Sand Dissolution • Removal of Gas bubbles & Dissolved gases • Evaporation processes • Homogenisation ICG – EFONGA Spring School Montpellier 4-5 May 2009 2
  3. 3. ICG – EFONGA Spring School Montpellier 4-5 May 2009 3
  4. 4. refractory corrosion evaporation: NaOH, KOH, NOx and HBO2, heat transfer water infiltration PbO, NaCl, HF, SO2 etc. flue gas chemistry melting kinetics foaming fining and redox refractory corrosion deposition and dust formation 1. Overview Chemistry & Physics of Glass Melting Processes emissions: Na2SO4, Na2B4O7 and PbO dust HCl, HF, SO2, SO3 , SeO2, HBO2, H3BO3 etc. ICG – EFONGA Spring School Montpellier 4-5 May 2009 4
  5. 5. Side-effects • • • • Evaporation from melt Refractory corrosion Combustion & heating Emissions from evaporation & combustion Furnace Raw materials mixed Or Homogeneous molten glass Crucible Melting-in of batch – chemical reactions/endothermic effects Sand* grain dissolution Removal of bubbles/gases Dissolution of seed (fine bubbles) residue Homogenisation - Diffusion (slow) - Velocity gradients – stretching of inhomogeneities ICG – EFONGA Spring School Montpellier 4-5 May 2009 5
  6. 6. Parameters for processes in melting Important parameters: • Heat transfer Temperature viscosity, surface tension, chemical activity, reaction kinetics, gas evolution.. • Flow characteristics – convection in melt, stirring • Residence time: time-temperature history • Exposure of melt to (reactive) atmosphere and refractory lining How to assess: Temperatures and flows in glass melt ? ICG – EFONGA Spring School Montpellier 4-5 May 2009 6
  7. 7. 2. Tools: CFD Simulation Models Glass Furnaces • Discretize total volume of furnace in small volume cells (> 1 million) cells LxBxH: (1-10) x (1-10) x (1-10) cm – GRID / mesh • Input data process: pull, batch composition, fuel distribution, air number • Input furnace: – Design – Wall construction, including insulation • Input glass: viscosity, heat conductivity, density, thermal expansion, electric conductivity, solubility sand, solubility gases,… • For each volume cell in tank & combustion chamber – Energy conservation – Momentum conservation – Mass conservation (continuity) for melt and each chemical element – Respect electro-neutrality ICG – EFONGA Spring School Montpellier 4-5 May 2009 7
  8. 8. CFD Simulation Models example: local conservation of energy ∂ (ρ c pT ∂t ) = − div (ρ c T v ) + div (λ r increase sensible heat p convection of heat grad T ) + q heat conduction local boosting or cooling Energy equation, conservation law for energy in each volume element ICG – EFONGA Spring School Montpellier 4-5 May 2009 8
  9. 9. Results of CFD simulation models • Temperatures at all possible positions – Combustion space – Glass melt – Refractory • Glass melt and Combustion gas velocities • Trajectories (particle tracing) in tank – Thousands of different paths can be identified from charging end to throat or spout • Redox and dissolved gases – Redox state of melt at each position (pO2 or Fe2+/Fe3+) • Residence time distribution – Minimum residence time is of importance for melting process • Glass melt quality indices per trajectory – Trajectory with minimum melting or fining index is decisive for glass ICG – EFONGA Spring School Montpellier 4-5 May 2009 9
  10. 10. Application of CFD models • For furnace design (lowest energy, highest glass quality) – – – – Optimum depth of tank Position bubblers or dam or burners Size and design of throat Design combustion chamber (LowNOx, less evaporation) • For optimum process settings – – – – Optimum fuel-boosting ratio Temperature profile (energy distribution) Bubbling rate Creation of distinct spring zone to avoid short cut • Time-transient (time dependent) for colour or pull change – Optimize colour change process: reduce transition time • Time-transient for process control (rMPC) – Sensors give model continuous new information: model tracking – Model continuously gives recommendation for input parameter changes to follow optimum process path (low energy, high glass quality, constant T) ICG – EFONGA Spring School Montpellier 4-5 May 2009 10
  11. 11. Geometry & Grid for computational fluid dynamics (CFD) analysis of glass furnace Port Necks crown Burner port tank Deep Refiner Batch Boosting electrodes ICG – EFONGA Spring School Montpellier 4-5 May 2009 11
  12. 12. Example result CFD computation Temperature contours ICG – EFONGA Spring School Montpellier 4-5 May 2009 12
  13. 13. NOx End-port fired furnace horizontal cross section at level of burners Base case 4 inch higher crown ICG – EFONGA Spring School Montpellier 4-5 May 2009 13
  14. 14. NOx End-port fired furnace vertical cross section at 25 % from furnace length from port NOx scaling in mole fraction Base case 4 inch higher crown Burner port Exit port (flue gas) Lower NOx-concentration in exit ICG – EFONGA Spring School Montpellier 4-5 May 2009 14
  15. 15. Glass melt path with lowest ‘temperature‘ index canal Temperatur indexpath i e T = ∫ dt η doghouse Temperature course of glass (melt) in typical float glass furnace with minium temperature index 1800 1600 Temperature in oC 1400 1200 1000 800 600 400 200 0 0 2 4 6 8 10 12 14 Time in hours ICG – EFONGA Spring School Montpellier 4-5 May 2009 15
  16. 16. Sensors (short overview) • Combustion gases • gas composition: • laser optics • electrochemical sensors (oxygen) • Glass melt • chemical composition • LIBS = laser induced breakdown spectroscopy emf • redox / colour parameters t/c • Potentiometry • Voltammetry Type B (mV) (mV) Alumina rod Pt / Ni-NiO // ZrO2 // pO2 (glass) / Pt EMF = RT pO 2 (glass) ⋅ ln nF pO 2 (ref.Ni/NiO) Pt measuring electrode ICG – EFONGA Spring School Montpellier 4-5 May 2009 Zirconia cell Ni/NiOreference mix 16
  17. 17. 3. Melting-in of batch • In glass furnaces (industrial) – Kinetics determined by heat transfer through batch blanket • In small crucibles: – Kinetics determined by contact between different batch constituents and temperature ICG – EFONGA Spring School Montpellier 4-5 May 2009 17
  18. 18. Return flow for batch heating Generation blisters from refractory Day hopper Refining Bubble absorption Hot spot & evaporation Conditioning of melt Thermal homogeneity Zone for sand grain dissolution Batch melting • 40-60 minutes • 80-90 % of net heat flux Spring zone & primary fining Return flow from working end ICG – EFONGA Spring School Montpellier 4-5 May 2009 18
  19. 19. Scheme of melting process of batch blanket, charging velocity vg (m/s) combustion space heat transfer gas release thickness temperature profile reaction zone figure 1b glass melt layer normal batch Zipfel glass level reaction zone glassmelt flow heat transferred figure 1c ICG – EFONGA Spring School Montpellier 4-5 May 2009 19
  20. 20. Detailed re-presentation of the batch melting process in glass furnace 1500 o C Layer glassmelt melts Dissolution sand grains loose batch sand grains gas melting reactions batch melting reactions carbonates (soda/lime) gas dissolution sand grains sand grains b. top of batch blanket glass melt 1400 c. o C bottom side of batch blanket ICG – EFONGA Spring School Montpellier 4-5 May 2009 20
  21. 21. Example: Melting reactions of soda lime (dolomite) silica batch • De-hydratation (100 oC physical bonded water & > 100 oC hydrates) – Important for energy consumption: water evaporation is energy intensive • Solid state reactions, formation of silicates, e.g.: Carbonate route < 900 oC at fast heating rate (melts at ±820 oC) High amount of heat required (550-850 oC) Na2CO3 + CaCO3 Na2Ca(CO3)2 Na2Ca(CO3)2 +2SiO2 Na2SiO3/CaSiO3 + 2CO2↑ reaction enhances > 820 oC Na2CO3 + 2SiO2 Na2SiO3 + CO2↑ (790-850 oC) • Formation of primary melt phases (alkali rich carbonates), e.g.: Tm Na2CO3 Tm Na2Ca(CO3)2 Tm K2CO3 = 850 oC = 820 oC = 890 oC ICG – EFONGA Spring School Montpellier 4-5 May 2009 21
  22. 22. Melting reactions of soda lime silica batch limited kinetics may shift some reactions to higher temperatures • Decomposition reactions of (Ca- and Mg-) carbonates: heat required CaCO3 + heat CaO + CO2↑ MgCO3 + heat MgO + CO2↑ MgCO3·CaCO3 + heat MgO + CaCO3 + CO2↑ (910 oC at pressure 1 bar) (540 oC at pressure of 1 bar) (650 oC, 1 bar) MgO still present up to 1150 oC. • Dissolution reactions of SiO2, e.g. (coarse limestone) Na2O·2SiO2 + CO2↑ T > 790 oC forms with SiO2 an eutectic melt Or at further heating fast Na2O·SiO2 formation (850 oC) – limestone decomposes and: 2CaO + (SiO2 + Na2O·2SiO2 )eutectic melt Na2O·2CaO·3SiO2 (> 900 oC) Reactive calcination: Na2CO3 + 2SiO2 Silicate route: Silicate melt + SiO2 silica enriched melt T > 1000-1100 oC Eutectic melt phases are formed above ±800-840 oC ICG – EFONGA Spring School Montpellier 4-5 May 2009 22
  23. 23. Phase diagram for the system Na2O – SiO2 showing eutectic sodium silicate melt phases 100 % SiO2 ICG – EFONGA Spring School Montpellier 4-5 May 2009 23
  24. 24. Scheme of melting reactions of soda lime glass batch 1080 oC: T s Na2SiO 3 910 oC : CaCO 3 CaO + CO 2 (gas) o 850 C : T s Na2CO 3 820 oC : T s Na2Ca(CO 3)2 790 oC : T eut Na 2O.2SiO 2 + SiO 2 740 oC : T eut Na 2Ca(CO 3)2 + Na2CO 3 650 oC : MgCO 3.CaCO 3 MgO+CaCO 3+CO 2 (gas) 540 oC : MgCO 3 -> MgO + CO 2 (gas) Dissolution of SiO 2, CaO, MgO, Al2O 3 e.d. in melt phases primary melts decomposition carbonates solid state reactions volatilisation of water 0 200 400 600 800 1000 temperature in oC 1200 ICG – EFONGA Spring School Montpellier 4-5 May 2009 1400 24
  25. 25. Overall chemical energy demand -1 CaCO3(s) -> CaO(s) + CO2(g) Chemical energy consumption rate [kJ·kgbatch ·K ] MgCO3·CaCO3(s) -> MgO(s) + CO2(g) + CaCO3(s) -1 10 8 Na2CO3(s) + SiO2(q) -> Na2O·SiO2(s) + CO2(g) Na2CO3(s) -> Na2CO3(l) 6 Na2CO3(l) + SiO2(q) -> Na2O·SiO2(s) + CO2(g) Na2O·SiO2(s) + SiO2(q) -> NS(l) CaO(s) + melt 4 2 0 600 650 700 750 800 850 900 950 1000 -2 Temperature [°C] Chemical enthalpy of batch reactions for float glass from soda-sand-dolomite and limestone (positive: endothermic effects) ICG – EFONGA Spring School Montpellier 4-5 May 2009 25
  26. 26. 4. Dissolution of ‘refractory’ type raw material in silicate melt example: sand grains ICG – EFONGA Spring School Montpellier 4-5 May 2009 26
  27. 27. Sand grain Glass melt CSiO2 Ce(T) = saturation level SiO2 in melt 100 Cb Ce (T) Cb = bulk SiO2 level in melt (depends on amount dissolved sand) Moving boundary Diffusion of SiO2 in melt ICG – EFONGA Spring School Montpellier 4-5 May 2009 27
  28. 28. Dissolution front One-dimensional dissolution in multi-component liquid Mass fraction SiO2 in saturated melt: we Multi-component liquid Dissolving material w is mass fraction SiO2 in melt a x dissolution we  ∂w     ∂x  a w ρ SiO2  ∂w  ρe ⋅   da  ∂x  a ⋅ = − D⋅ dt (1 − VA ⋅ ρ e ⋅ w e ) ICG – EFONGA Spring School Montpellier 4-5 May 2009 28
  29. 29. Mathematical description (Ready & Cooper 1966) - Spherical symmetry – diffusion in 3 dimensions Assuming constant diffusion coefficient in melt Ideal solution, partial molar volume of SiO2 in melt is constant Convection (term u) due to change of partial molar volume of SiO2 in sand versus in melt Moving boundary: dissolving sand is partly staying in volume it came from Mass flux (j) of dissolved SiO2 J D r R t ρ C u = mass flux of SiO2 = diffusion coefficient of SiO2 in silicate melt (m2/s) = radial co-ordinate (distance from sand grain centre) (m) = radius sand grain (m) = time (s) = density of melt (kg/m3) = local SiO2 mass concentration (kg/m3) = mass average velocity radial direction due to expansion by dissolution (change in molar volume) (m/s) ICG – EFONGA Spring School Montpellier 4-5 May 2009 29
  30. 30. Solution dissolution sand grain without forced convection Effect of moving boundary a = actual grain size radius (m) VA = partial specific volume of SiO2 in molten glass (m3/kg) Cs = density of sand grain (kg/m3) Ca = mass concentration SiO2 in saturated melt (kg/m3) ICG – EFONGA Spring School Montpellier 4-5 May 2009 30
  31. 31. Sand grain dissolution with convection by glass melt velocity gradients & density differences ρ SiO2 h we ws VA a t ρs ρSiO2 D da ⋅ = − h ⋅ (w e ρ e − w sρ s ) /(1 − VA ⋅ w e ⋅ ρ e ) dt = mass transfer coefficient SiO2 into melt (m/s) = mass fraction SiO2 in saturated melt (depends on T, and glass) (kg/m3) = mass fraction SiO2 in bulk melt (depends on dissolved sand)) (kg/m3) = partial specific volume of SiO2 in molten glass (m3/kg) = actual radius sand grain (m) = time (s) = density of melt (kg/m3) = density sand grain (kg/m3) = diffusion coefficient of SiO2 in silicate melt (m2/s) ICG – EFONGA Spring School Montpellier 4-5 May 2009 31
  32. 32. Mass transfer coefficient     Sh 1 1  h= ⋅D ⋅  + R  2 Sh ⋅D ⋅ t  π⋅  2   h = D = mass transfer coefficient (m/s) diffusion coefficient of SiO2 in the molten glass, based on concentration profiles given in mass fraction (D in m2/s) R = grain radius (m), t time (s), = Sh = Sherwood number for mass transfer from spherical grain, ≈ 2 + 0.89 {Re · Sc + (Gr · Sc)3/4}1/3 * = 2 (no convection) = f (R2/3 , D-1/3, (grad v)1/3) (convection flow of the glass melt) = f (R3/4 , h-1/4 , D-1/4) (free convection of surrounding melt relative to the sand grain: v = flow velocity of the melt relative to the sand grain (m/s), η = viscosity (Pa.s) ICG – EFONGA Spring School Montpellier 4-5 May 2009 32
  33. 33. 30000 no convection Dissoltion time in s 25000 20000 v-gradient 15000 -1 0.001 s 10000 v-gradient 5000 0.025 s 0 1350 -1 1450 1550 1650 1750 1850 T in K Dissolution time required for complete dissolution of sand grains in almost static and stirred soda-lime silica glass melts (forced convection with velocity gradient grad v) at different temperatures. Initial size Ao=100 mm. ICG – EFONGA Spring School Montpellier 4-5 May 2009 33
  34. 34. Dissolution of sand and alumina grains in static and convective sodalime-silica glass melt at 1700 K, moving boundary effect taken into account (not on concentration profiles) 1.20E-04 1.00E-04 sand, grad v=0 sand, grad v= 0 steady state sand, -1 grad v=0.001 s radius in m 8.00E-05 sand, -1 grad v=0.01 s 6.00E-05 alumina, -1 grad v=0.01 s 4.00E-05 alumina, -1 grad v=0.001 s sand, steady state -1 grad v=0.001 s 2.00E-05 0.00E+00 0 5000 10000 15000 20000 alumina, grad v = 0 25000 30000 35000 time [s] ICG – EFONGA Spring School Montpellier 4-5 May 2009 34
  35. 35. 5. Fining Processes ICG – EFONGA Spring School Montpellier 4-5 May 2009 35
  36. 36. Seeds after batch melting Coarse sand Seeds after batch melting Fine sand ICG – EFONGA Spring School Montpellier 4-5 May 2009 36
  37. 37. 10 mm Glass just after batch melting - sample thickness ± 5 mm ICG – EFONGA Spring School Montpellier 4-5 May 2009 37
  38. 38. 0 to ICG – EFONGA Spring School Montpellier 4-5 May 2009 8 mm 38
  39. 39. Bubbles & Seeds just after melting • Many small seeds and bubbles (Blisters) in glass melt combustion space (Mulfinger 1976 GTB) heat transfer gas release – More than 100.000 per kg glass melt thickness reaction zone figure 1b glass melt layer temperature profile glass level normal batch – Most bubble diameters: 0.05 -0.4 mm reaction zone glassmelt flow • heat transferred figure 1c In most glass melts (using carbonates): – bubbles in batch melting area: contain often mainly CO2 • Large concentrations dissolved CO2 in melt • During sand grain dissolution in melt: generation of fine CO2 seeds (Gispen) from Glass Service ICG – EFONGA Spring School Montpellier 4-5 May 2009 39
  40. 40. Fining Objective of Fining: Removal of bubbles and dissolved gases from the glass melt Rising velocity of bubble: v ascension ρ = Density of the glass melt [kg/m3] η = Viscosity of the melt [Pa·s] R = Bubble radius [m] g = Acceleration of gravity [m/s2] c = c ⋅ ρ ⋅ g ⋅R = η 2 Factor (e.g. Stokes c = 2/9) ICG – EFONGA Spring School Montpellier 4-5 May 2009 40
  41. 41. Time to reach glass level at 1 meter [h] Fining Time to reach glass surface (1 meter) 250 1400 OC 200 1450 OC 1350 OC 150 100 1500 OC 50 0 0 100 200 300 400 500 Bubble diameter [µm] ICG – EFONGA Spring School Montpellier 4-5 May 2009 41
  42. 42. III. Fining gases and other dissolved gases diffuse strongly into bubble II. start of fining: gases diffuse into bubble I. static bubble Reaction in melt: release of fining gases Pgases melt > pt (pt is pressure in bubble) ICG – EFONGA Spring School Montpellier 4-5 May 2009 42
  43. 43. Two fining steps • First step: primary fining – High temperatures – Bubble agglomeration and bubble size growth – Dissolved gases diffuse from melt in to bubbles (like bubbles in soda drinks) – Ascension to glass melt surface • Second step: Secondary fining/Refining (secondary fining) – Dissolution of (small) remaining bubbles • Only effective if bubble contains gases (CO2, O2, SO2+O2) that dissolve in cooling melts • Glass melt should be lean in dissolved gases ICG – EFONGA Spring School Montpellier 4-5 May 2009 43
  44. 44. Mechanism sulfate primary fining Fining process in glass melt pSO2 · pO21/2 ·aNa2O K= pSO2· pO21/2 K’ = aNa2SO4 [SO3] Increasing temperatures lead to increasing K-values → extra oxygen gas & SO2 gas release: - oxygen & SO2 molecules diffuse into growing bubbles - bubble ascension increases (vascension~R2) - sulfate retention decreases ICG – EFONGA Spring School Montpellier 4-5 May 2009 44
  45. 45. Fining (primary) – Fining agents added to the batch to enhance the rising velocity of bubbles – Often used fining agent: Sodium sulphate Fining reaction: T > TFining onset Na2SO4 ⇔ Na2O + SO2 (gas) +1/2 O2 (gas) Cm CO2 pSO2 ⋅ pO2 K = [SO3 ] ' CO2 Stripping of CO2 and N2 from melt Cm N2 SO2 O2 N2 Dilution of N2 & CO2 in bubble by fining gases ICG – EFONGA Spring School Montpellier 4-5 May 2009 45
  46. 46. Multicomponent diffusion of gases in bubbles     d Shi Di 1 1  ⋅ ⋅ Rg ⋅ T ⋅ (Csi − Cii) ⋅  + (4πR3 ⋅ pt ) /(3Rg ⋅ T) = 4πR2 ⋅ Σi R  dt 2 pt Shi ⋅ Di ⋅ t  π⋅  2   [ ] Shi = 1+ (1+ 2·v·R/Di )1/3 ICG – EFONGA Spring School Montpellier 4-5 May 2009 46
  47. 47. Fining/Refining: degassing & removal of bubbles • Mostly applied fining agents in glass industry: Na2SO4 & Sb2O5 – Na2SO4 (m) → SO2 (g) + 0.5 O2 (g) + Na2O (m) – Sb2O5 (m) → Sb2O3 (m) + O2 (g) • Na2SO4 added in concentrations 0.1 – 1 wt. % to batches of: – Soda lime glass for container, float and tableware – E-borosilicate glass for fibres • Na2SO4 partly decomposes during batch melting & releasing SO2 in early melting stages • Dissociation temperature of Na2SO4 in melt: – Between 1350 – 1480 ºC, depending on redox state – Between 1100-1350 oC (reduced batches) Na2SO4+Na2S reactions forming SO2 and or S2 gas. ICG – EFONGA Spring School Montpellier 4-5 May 2009 47
  48. 48. Fe2+/Fetotal Sulfur retention (wt.%SO3) 0.6 0.5 80 70 60 Sulfur only in Sulfur in form of S2form of SO42-, S2- 40 25 15 % Sulfur only in form of SO42- (probably also SO32-?) 0.4 0.3 1400 oC 0.2 0.1 1500 oC 0.0 -8 -7 -30 -6 -5 -4 -3 Log pO2 in the melt at 1400°C (bar) -20 -10 0 +10 -2 -1 +20 redox number ICG – EFONGA Spring School Montpellier 4-5 May 2009 48
  49. 49. Mass Spectrometer Synthetic gas 30 mm ICG – EFONGA Spring School Montpellier 4-5 May 2009 49
  50. 50. High temperature test facility ICG – EFONGA Spring School Montpellier 4-5 May 2009 50
  51. 51. Fining/Refining: degassing & removal of bubbles 1. Primary fining – Removal of bubbles by rising of bubbles to melt surface – Bubble growth under influence of fining agents – Stripping of dissolved gases by growing of gas bubbles (dilution) Fining ICG – EFONGA Spring School Montpellier 4-5 May 2009 51
  52. 52. Enhanced Sulfate Fining by Dissolved Water in Melt O2 N2 CO2 O2 H2O H2O Oxy-case SO2 SO2 O2 N2 CO2 O2 SO2 H2O Air case SO2 In oxygen-fired glass furnace: peH2O = 0.25-0.40 bar Fining only if: peSO2 + peO2 > 0.70 - 0.75 bar In air-fired furnace: peH2O = 0.10-0.15 bar Fining only if : peSO2 + peO2 > 0.9 bar ICG – EFONGA Spring School Montpellier 4-5 May 2009 52
  53. 53. Evolution of fining gas: water lean & rich melts 50 water vapor pressure 40 0 bar 30 0.20 bar 20 0.60 bar 3 m /batch Volume of gas in Gas evolution during sulfate fining of soda lime glass melt - effect of water vapor level 60 10 0 1300 1400 1500 1600 o Temperature in C ICG – EFONGA Spring School Montpellier 4-5 May 2009 53
  54. 54. Stripping of dissolved gases from melt ICG – EFONGA Spring School Montpellier 4-5 May 2009 54
  55. 55. Pressure in melt before and during fining & cooling Total internal pressure in melt bar [S]initial= 0.3 mass% SO3 300 mgr water/kg [Fe2+]/ [Fetotal] = 20 % 1.2 1.0 0.8 0.6 0.4 0.2 0.0 1200 1300 1400 1500 1600 o Temperature in C ICG – EFONGA Spring School Montpellier 4-5 May 2009 55
  56. 56. Partial pressure in float glass melt during heating Partial pressure in float glass melt in bar [S]initial = 0.3 mass% SO3 300 mgr water/kg [Fe2+]/ [Fetotal] = 20 % 1.E+00 SO2 1.E-01 H2 O 1.E-02 O2 1.E-03 CO2 1.E-04 N2 1.E-05 1250 1350 1450 1550 o Temperature in C ICG – EFONGA Spring School Montpellier 4-5 May 2009 56
  57. 57. O2 fining gas CO2 mol/m3 mol/m3 Modeling dissolved gas distribution in glass melt tank ICG – EFONGA Spring School Montpellier 4-5 May 2009 57
  58. 58. Fining/Refining: degassing & removal of bubbles 2. Secondary fining (refining) – Re-absorption of residual gases during controlled cooling • Chemical solubility SO2 and O2 increases with decreasing temperature: gases will be re-absorbed during cooling. • Physical solubility of dissolved gases increases slightly with decreasing temperature: these gases will also be reabsorbed during cooling Refining ICG – EFONGA Spring School Montpellier 4-5 May 2009 58
  59. 59. Fining at low pressure - Same amount of gas needs large volume - Low partial pressures in bubble will stimulate gas diffusion from melt into bubble ICG – EFONGA Spring School Montpellier 4-5 May 2009 59
  60. 60. 6. Evaporation processes ICG – EFONGA Spring School Montpellier 4-5 May 2009 60
  61. 61. Multi-component glasses • Different oxides • Oxides in glass with high chemical activity or vapour pressure: – React at glass melt surface with combustion gases – Evaporate from glass melt surface – Show depletion at surface layer INCONGRUENT EVAPORATION ICG – EFONGA Spring School Montpellier 4-5 May 2009 61
  62. 62. Evaporation mechanisms • Direct evaporation of volatile glass components from the surface of the melt, e.g. volatilization of PbO from lead crystal melts; • Evaporation of components by reactions in the melt itself, forming volatile compounds; generally such volatile compounds exhibit high activity coefficients and weak bonding with other glass melt species. – • An example: formation of alkali borates in alkali borosilicate melts, subsequently evaporation of alkali meta-borates/tetra-borates takes place, Na2O(melt) + B2O3 (melt) ⇔ 2NaBO2 (melt) ⇒ 2 NaBO2 (vapor) Evaporation by reactions of certain glass melt components with gas species at the surface of the melt. The evaporation rate & vapor pressure depends on the composition of the gas atmosphere above the melt. B2O3 (glass melt) + H2O ⇒ 2HBO2 (vapor) Na2O (glass melt) + H2O (gas) ⇒ 2NaOH (vapor) Na2O(glass melt) + CO (gas) ⇒ 2Na (vapor) + CO2 ICG – EFONGA Spring School Montpellier 4-5 May 2009 62
  63. 63. Kinetics of evaporation The volatilization rate is often determined by: • mass transport of the relevant species from the melt interior (bulk) to the surface; • the vapor pressures of the volatile components at the surface of the melt, dependent on the glass composition, temperature and gas atmosphere; • the mass transfer of evaporated species from the surface of the melt into the main gas stream above the melt. ICG – EFONGA Spring School Montpellier 4-5 May 2009 63
  64. 64. Reactive evaporation For reactive evaporation of a component j in the melt reacting with a gas k and forming gaseous species i with saturation pressure pi*, according to reaction: n·j (melt) + m·k (gas) Reaction equilibrium: pi*q q·i (gas) = K · ajn · pkm The values of K (equilibrium constant, assuming chemical equilibrium at the glass melt surface) and aj (activity of component j in the molten glass at the surface) can be determined experimentally or by thermodynamic modeling ICG – EFONGA Spring School Montpellier 4-5 May 2009 64
  65. 65. Static melt and static atmosphere above the melt (interface x = 0) Diffusion in melt of reacting glass melt component j: dCj/dt = Dm,j·δCj2/δx2 Time dependency: Cjsurface(t). Dm,j is the inter-diffusion coefficient of the volatile component j in the melt. at t = 0 at t > 0 at t > 0 -∞ < x < 0 x −∞ x =0 Cj = Cjbulk Cj = Cjbulk Cj = Cjsurface(t) For the vapor i in a static gas phase with partial vapor pressure pi, the diffusion process in the gas phase can be described in a similar way: δ(pi/RgT)/δt = Dg,i ·δ2(pi/RgT) /δx2 Dg,i is the diffusion coefficient of the vapor I in the gas phase. at t = 0 at t > 0 at t > 0 0<x<∞ x ∞ x =0 pi = pi,gasbulk pi = pigasbulk pi = pi*(t) ICG – EFONGA Spring School Montpellier 4-5 May 2009 65
  66. 66. -3 Na2 O concentration [mol.m ] 4970 4968 4966 4964 4962 5 sec. 4960 50 sec. 250 sec. 4958 4956 DNa2O=3.3 10-11 m 2.s -1 4954 0 0.01 0.02 0.03 0.04 0.05 0.06 distance from surface [mm] Calculated time dependent- Na2O concentration profiles in static melt Situation: static conditions in semi-infinite gas phase with 0.55 bar vapor pressure and semi-infinite soda-lime-silica melt (13 wt% Na2O, 10 wt% CaO, 5 wt% MgO, 72 wt% SiO2). Dg,NaOH = 2.7 10-4 m2·s-1 , Dm,Na2O = 3.3 10-11 m2·s-1 ICG – EFONGA Spring School Montpellier 4-5 May 2009 66
  67. 67. Evaporation in gas flow above molten glass pb,i main gas stream velocity, vg diffusion of gas i in gas boundary layer, Dgi surface reaction: n·j (melt)+ m·k (gas) ⇒ q·i (gas) p*i glass melt surface C j-profile Example: Na2O (m) + H2O(g) ⇔ 2NaOH (g) Transport of component j in the melt, Dm,j ICG – EFONGA Spring School Montpellier 4-5 May 2009 67
  68. 68. Evaporation in gas flow Average evaporation rate (over length Lg of gas flow above melt ) of component i (formed by reaction of glass compound j) into (turbulent) gas phase: Qm,j =(ni/qi)·A·vg0.8·ρg0.47·µgas-0.47·Dg0.667·Lg-0.2· Rg-1·T-1·B·Cj,x=0(t) The proportionality parameter B depends on the furnace atmosphere composition and the chemical activity of the volatile component in the melt. For NaOH-evaporation, the B value depends on the water vapor pressure in the furnace atmosphere, B ∼ pH2O0.5 ICG – EFONGA Spring School Montpellier 4-5 May 2009 68
  69. 69. Mass transfer equations Average evaporation rate (rate of loss of glass component j) over length Lg from leading edge: Qm,j = -Dm,j·(δCj/δx)x=0 = α·Cj,x=0(t) α = (ni/qi) · A· vg0.8·ρg 0.47·µgas-0.47·Dg0.667·Lg-0.2· Rg-1·T-1· B Turbulent flow of gas v = velocity, g refers to gas phase, Rg is universal gas constant, T in K, B ratio between vapour pressure i and surface concentration component j A = between 0.03 and 0.04 for turbulent gas flow (Re > 300000 or for disturbed flows) ICG – EFONGA Spring School Montpellier 4-5 May 2009 69
  70. 70. Solution in flowing gas and static melt For kd defined as α/DmNa2O the solution for a single component j Evaporating from a static melt in flowing gas phase Assuming complete depletion at surface for t ∞ MQm.j is the total evaporation mass loss per unit surface area between time 0 and τ MQm.j = (Cj,0/kd)·{exp(kd2·DmNa2O·τ)·erfc[kd·(DmNa2O·τ)0.5] -1 + 2kd·(DmNa2O·τ/π)0.5} Cj,x=0(t) = Cj,0· exp(kd2·DmNa2O·τ)·erfc[kd·(DmNa2O·τ)0.5] Cj,0 = bulk concentration compound j at t=0 ICG – EFONGA Spring School Montpellier 4-5 May 2009 70
  71. 71. 5000 -3 melt [mol.m ] Na2O concentration in 5500 4500 stagnant gas 4000 Lg= 2 m, v= 2 m.s-1 Lg= 2 m, v = 5 m.s-1 3500 Lg= 0.5 m, v= 2 m.s-1 3000 0 0.5 1 1.5 2 2.5 distance from surface [mm] Local concentration profile in soda-lime silica melt after 7200 seconds exposure time, calculated for NaOH-evaporation from static melt in static or flowing gas phases, (Lg= downstream distance from leading edge ). Temperature = 1500 oC, pH2O = 0.55 bar. Dm,Na2O = 3.3 10-11 m2·s-1 Glass composition (mass %): SiO2 =72, Na2O =13, MgO = 5, CaO = 10 ICG – EFONGA Spring School Montpellier 4-5 May 2009 71
  72. 72. parameter is temperature: 5000 1723K 1773 K 1823 K 4000 -3 [mol.m ] Na2O-surface concentration 6000 1873 K 3000 2000 1000 0 0 5000 10000 15000 20000 time [s] 25000 30000 Change in Na2O-surface concentration soda-lime-silica melt at different temperatures in flowing gas (5 ms-1), 1 meters downstream. pH2O in gas = 0.55 bar & Na2O in glass = 13 mass%. Dm,Na2O= 8 10-10 exp(-5655/T) in upper graph ICG – EFONGA Spring School Montpellier 4-5 May 2009 72
  73. 73. Experimental – Set up to study (reactive) evaporation from molten glass Thermocouples Platinum funnel Porous plate Gases IN: Platinu m gas samplin g probe Platinum coating (30 cm) Platinum boat N2, H2O, O2 melt ICG – EFONGA Spring School Montpellier 4-5 May 2009 73
  74. 74. Mass transfer in gas phase during transpiration – evaporation test Shtranspiration 0 .5   2   1 .5 =  C1 + C 2 ⋅ Re d ⋅ Sc +   ⋅ C3 ⋅ (Re d ⋅ Sc )     1 + 22 ⋅ Sc    h g ,i = Q g ,i = Vessel with liquid of melt 1 3 Shtranspiration ⋅ D g ,i d hg ,i R ⋅T ( ⋅ pi (t ) * − pi bulk ) 2.5E-03 x=0 pi*(t) (e.g. p*NaOH or p*NaBO2) can be derived from evaporation (transpiration experiments) From measured Qg,i and Sherwood relations derived with model liquids Water evaporation rate QH2O (moles s -1 m-2) 2.0E-03 1.5E-03 Measurerments 1.0E-03 CFD model Empirical equation (2.19) 5.0E-04 0.0E+00 0 100 200 Reynolds number Re (-) ICG – EFONGA Spring School Montpellier 4-5 May 2009 300 400 74
  75. 75. 1.E-06 Modeled aNa2O (-) SiO2:Na2O:CaO = 74:16:10 mol 1.E-07 Na2O.2SiO2 y = 0.9823x R2 = 0.9283 1.E-08 1.E-09 1.E-09 1.E-08 1.E-07 1.E-06 Measured aNa2O (-) Na2O activity at glass melt surface determined by transpiration test measuring p*NaOH: Na2O + H2O 2 NaOH ICG – EFONGA Spring School Montpellier 4-5 May 2009 75
  76. 76. Derivation chemical activity of volatile glass component at surface of melt K = exp(-∆G/RT) = p*NaOH2/aNa2O.pH2O From thermodynamic tables: ∆G = GfNa2O+GfH2O- 2GfNaOH(g) p*NaOH is measured from QNaOH and pH2O is controlled aNa2O (surface) can be determined K is calculated by standard Gibbs free energy values of products & reactants of reaction ICG – EFONGA Spring School Montpellier 4-5 May 2009 76
  77. 77. Non static melt & non static gas phase free convection by density gradients Gas flow Low Na2O Mid Na2O High Na2O Float glass melt with Na2O concentration differences ICG – EFONGA Spring School Montpellier 4-5 May 2009 77
  78. 78. 6. Homogeneity of glass Micro-mixing: transfer from high to low chemical activity by diffusion Cm - π 2 . D.t = A exp Co Lo 2 Macro-mixing: elongation of in-homogeneities exposed to velocity gradient in melt C(x,t) dC d 2C = D⋅ 2 dt dx t=0 Cm (t) Slow diffusion processes t = t1 t = t2 Co Lo ICG – EFONGA Spring School Montpellier 4-5 May 2009 78
  79. 79. Macro-mixing Reduction of diffusion distance, Lo velocity v + dv velocity v y In the case, t dv/dy >> 1: L= L0 dv ⋅ t dy For macro-mixing in combination with diffusion (by approximation): Cm π 2 . D . t 3 .( dv / dy ) 2 = A .exp − Co L2 0 A = proportionality factor dependent on the shape of the cord ICG – EFONGA Spring School Montpellier 4-5 May 2009 79
  80. 80. Macro versus only Micro mixing • Small velocity gradients (> 0.01 m/s per m) enhance homogenisation process with factor 20 to 100 • Velocity gradients by: – Stirring – Bubbling – Temperature gradients free convection ICG – EFONGA Spring School Montpellier 4-5 May 2009 80
  81. 81. Thank you for your attention What does a number tell us without the proper unit? ICG – EFONGA Spring School Montpellier 4-5 May 2009 81

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