Results of Dynamic FBRM Measurements in a Confined Impelled Stirrer Tank (CIST) for O/W, W/O, Phase Inversion, and Bitumen Dewatering Studies

Transcription

1 Results of Dynamic FBRM Measurements in a Confined Impelled Stirrer Tank (CIST) for O/W, W/O, Phase Inversion, and Bitumen Dewatering Studies Márcio B. Machado and Suzanne M. Kresta marcio@ualberta.ca Chemical and Materials Engineering, University of Alberta Process Development & Scale-up June 28, 2017 Houston, Texas

2 Introduction Mixing Multi-phase liquid/liquid Scale-up/down Mixing sensitive problems What needs to be matched between scales? Scaling approaches Mixing devices Confined impeller stirred tank (CIST) 2

3 What do you mean by Well Mixed? This is a well mixed CSTR. Coffee mug Well mixed is not a clear definition, and perfectly mixed in a very short time is only a safe assumption for perfectly miscible low viscosity fluids. References: Paul et al. (2004); Kresta et al., (2015) 3

4 Technical Definition of Mixing Mixing has three physical dimensions (concentration, scale, and rate) It requires three design specifications: Uniformity of concentration A specified scale of segregation A required rate of mixing, or mixing time, which is often in competition with other rates. Reference: Kukukova et al. (2009) 4

5 Identifying Mixing Problems Multiphase mixing involves many scales of mixing, frequently mass transfer, and complex physics. Oversimplifying is dangerous. Mixing frequently gets worse on scale-up. Competing rate processes are frequently sensitive to mixing. Design for local conditions. 5

6 Liquid/liquid dispersion Two (or more) immiscible liquids (or partially soluble) Function of turbulence Smallest droplets generated by turbulence are around the same size of the Kolmogorov scale Satellites drops smaller than Kolmogorov scale may occur Droplets are not necessarily spherical How to scale-up? See papers by Calabrese or Tavlarides 6

7 Principles of robust scale-up CIJR 4 mm, 1 ml CIST 120 mm, 1 L 40 ft = 12 m, 10 ML 7

8 Requirements for robust scale-up Active flow in the entire vessel (no inactive regions) Entire vessel must be fully turbulent (transitional flow is not scalable) Regions of high local concentration are minimized (mesomixing) Single-impeller stirred tanks require Re > to sustain fully turbulent flow at the top third part (Machado et al., 2013) Control the Kolmogorov scale 8

9 Classical scale-up - complications Simple scale-up cases 1. Geometric similarity 2. Constant property (power per volume, tip speed ) 3. Empirical design equations (Kawase and Moo-Young, Zwietering, Grenville) See Paul et al. (2004) This approach works well for many cases Some cases require more robust techniques Geometric similarity is not feasible Multiple process objectives Large scale-up factor Local mixing effects 9

10 Sources of scale-up problems Reasons why bench scale tests sometimes do not scale Non-homogenous energy dissipation Different flow regimes at different scales Feed plume effects (mesomixing) Change of geometry Are bench scale test vessels well designed? Magnetic stir bars Shaker table Jar test Stirred tank 10

11 Design objectives for new mixers Increase uniformity of turbulence over the volume of the vessel Allow a wider range of settings for energy dissipation (change impeller, N) Increase volume fraction of the mixer in fully turbulent flow Reduce mixer volume and eliminate stagnant or inactive zones Often achieved by miniaturizing the vessel and/or confining the flow (Kresta et al., 2015) 11

12 CIST Confined impeller stirred tank Fully turbulent to a much smaller Re Allows a 1L batch (or less) for additive testing Dramatic reduction in inactive volume (5% versus 30% in a stirred tank) Allows gravity separation test without material transfer References: Machado and Kresta (2013), Komrakova et al. (2017) 12

13 CIST Confined impeller stirred tank H = 3T T = 7.6 cm Source: Machado and Kresta (2013) Baffled tank filled with 5 or 6 impellers High H/T Large impeller diameter (D=T/2 or 2T/3) Uniform energy dissipation Re t as low as at the impeller region ( for ST) V = 1 L (small footprint and less sample and less waste) 13

14 Mixing energy The total mixing energy (J, J/kg) in a mixing vessel is obtained from the total energy dissipated times the mixing time (Machado and Kresta, 2015) J = ε*t mix It is an alternate scaling variable when geometric similarity is not feasible If a process is shear sensitive, ε can be decreased and J kept constant by increasing t mix Successful scale down from a pipeline to a CIST using J: Demulsifier addition tested in CIST and results scaled up to an industrial pipeline 50% reduction in chemical usage (Laplante et al., 2015; Chong et al., 2016a; Chong et al. 2016b) 14

15 Experimental Focused beam reflectance method (FBRM) Design of the confined impeller stirred tank (CIST) 15

16 FBRM Laser beam directed fluid from in situ probe. Backscatter reading. Measures chord length distribution (not diameter!). Image from: Mettler Toledo 16

17 Chord Length: 0< cl<d but not all drops are spherical Image from: Mettler Toledo Image from: Kresta, HIM (2004) Instead of considering the difficulty of measuring spherical drops, perhaps we should consider the advantages of collecting a distribution of length scales, just as we do for turbulent eddies. 17

18 Confined Impeller Stirred Tank CIST with a side port for the FBRM and top port for the PV19 Extra port for feeding or sample withdraw Lid allows for control of air entrainment Extra layer of liquid may be added up to the top of the lid Several set of impellers can be used different flow patterns and energy dissipation ranges 18

19 Confined Impeller Stirred Tank CIST operating with a FBRM and a PV19 19

20 Results Oil-in-water dispersion Water-in-oil dispersion Phase inversion Bitumen de-watering 20

21 Oil-in-water dispersion Canola oil (dispersed phase) in water CIST vs. stirred tank Different hold ups (10 % or 30 %) Liquid draw-down vs. impeller feeding 21

22 Experimental overview One set of fluids no change in interfacial tension (Canola oil with red Leak-Detecting Dye (Kingscote Chemicals) dispersed into tap water) 2 Vessels (ST and CIST) 3 impeller geometries (RT, PBT and A310) In the CIST, H=3T and there are 5 impellers. For the PBT, three configurations tested (DDDDD, DUDUD, UDUDU) Feed is done from a surface layer, and at the impeller Visual observations (simple video), Chord length measurements (FBRM) 22

23 CIST vs. stirred tank 23

24 Stirred Tank (ST) and Confined Impeller Stirred Tank (CIST) with 10% and 30% hold-up 24

25 Fluid Specifications Water continuous phase (c) Canola oil dispersed phase (d) Φ=10% Φ=30% Red Leak-Detecting Dye (Kingscote Chemicals Inc.) No stabilizer Fluid Properties Water Canola Oil Interfacial tension ± 0.58 (, mn/m) Density (, kg/m 3 ) Viscosity (, Pa s) 1E E-03 25

26 Experimental Procedure 1 Tank setup per conditions Liquids agitated by changing N: Tq at each N recorded N id determined and P and Re calculated First drop of oil separated 26

27 Experimental Procedure 2 N cd measured, P cd and Re calculated Clean and refill the tank. Set N = N cd Determine t cd at complete dispersion 27

28 Part 1: Energy for Complete Dispersion (J/kg) Normalized to base case RT in ST at 10% (17.5 J/kg) 8 geometries tested In ST, increasing hold-up increases J cd, but in the CIST, increasing holdup decreases J cd At the end of Part 1: Hypothesis: configuration with higher J should result in lower drop size Need to measure transient CLD using FBRM from Bhalerao et al. (2015) 28

29 A new variable in Part 2: feed at the impeller Wanted to compare surface draw-down with impeller feed to determine if there is a difference in performance. Limited to 10% feed in the CIST so that the top impeller is always submerged. At this point, we could not do impeller feed in the ST because the required feed times are too short (flowrates are too high) for the pumps we have in the lab. 29

30 FBRM Probe Position 30

31 Evolution of chord length distribution with time for t<t eq (a) Cumulative, (b) Number and (c) Square Weighted chord length distribution for surface layer CIST-PBT DU-30 at the beginning of mixing process (from t=0) 31

32 Stable CLD for t>t eq (a) Cumulative, (b) Number and (c) Square Weighted chord length distribution for surface layer CIST-DU-30 after equilibrium 32

33 Results 1. Evolution of mean cl : from completely dispersed (cl cd ) to equilibrium (cl eq ) Compare vessels, feed conditions, and hold-up 2. Compare the mean chord length to the Kolmogorov scale: where are we on the spectrum of eddies? 3. Scaling condition 1: Power (W/kg) power does not consider mixing time, so choose cl eq 4. Scaling condition 2: Mixing energy (J/kg) which is (J=P*t/m) Better to use equilibrium time or completely dispersed time? Compare impeller geometries and vessels to group data Compare feed conditions 33

34 Evolution of mean chord length Mean area weighted (cl 32 ) chord length (ST-SL) The first and second dots are t cd and t eq Most of the change in mean drop size happens during the initial dispersion (t<black dot). t cd t eq In fact, the change in square weighted cl from cd to eq is less than 10% for most cases tested. There is a lot of variation around the mean, even after t eq What about other configurations? 34

35 counts (%) counts (%) cl32 ( micron) cl32 ( micron) Evolution of mean chord length Compare two cases more carefully: PBT-UD-10 and RT SL-UD-10 IF-UD-10 PBT mean Time (Sec) IF-DU-10-N SL-DU-10- N PBT distribution SL-RT-10 IF-RT IF-RT-10-N SL-RT-10-N Time (Sec) RT mean changes with IF RT distribution Looking in more detail at these two cases, we see that when there is surface feed with the RT, some of the large drops are never broken up. Both of these cases are in the CIST Chord length (µm) Chord length (µm) 35

36 cl32 ( micron) cl32 ( micron) Evolution of mean chord length Mean cl: The effect of increasing hold-up in the CIST CIST-10-SL CIST-30-SL UD-10 DD-10 DU-10 RT-10 t cd t eq RT-30 UD-30 DD-30 A t cd t eq Time (Sec) Time (Sec) Once the upper impeller is submerged in the oil, there is a dramatic change in variability in the final mean chord length. 36

37 counts (%) Chord length vs. Kolmogorov scale: Where do we fall on the spectrum? The range of cl 32 (110 m<cl 32 <500 m) is much larger than the Kolmogorov scales (9.9 m< <26 m) and much smaller than the height of the trailing vortices (L I 3.5mm) so the drops which dominate the square weighted distribution and the mean chord length fall in the inertial range sample CLD number square The number weighted distribution shows roughly half of the drops in the 1-10 micron range, suggesting that mechanisms of break-up are present which act at scales smaller than eddy break-up Chord length (µm) By selecting cl 32 for the scaling comparisons, we are focussing on the turbulent eddy break-up. 37

38 cl32-eq (micron) Scaling with P/m at t eq Power per mass seems to have little effect on the drop size over the range tested, from 0.1W/kg to 2W/kg. Note that this is the power needed to just disperse the liquid, so the range is smaller than we would select for a study of drop break-up. Fitting a line to the data on a log-log plot gives an exponent of %-CIST 10%-IF 10%-ST 30%-ST LINE slope=-0.01 RT-IF Power/m (W/kg) 38

39 cl32-cd (micron) Scaling with J cd at t cd and cl cd The range of J cd is wider, from J/kg The slope of most of the data falls on one line, giving an exponent of (eight times larger than the power result) %-CIST 10%-IF-CIST 30%-CIST 10%-ST 30%-ST Series5 Using t cd rather than t eq brings the RT-IF point into line with the other configurations. For the one case where an impeller is submerged in the surface layer, the exponent is dramatically different, at slope=-1.4 slope= %-CIST Jcd / m- (J/kg) 39

40 cl32-cd (micron) J cd scaling grouped by impeller Grouping the data by impeller leads to the same conclusion: for all impeller configurations, the results change dramatically if one impeller is submerged in the surface layer. This result is in close alignment with the literature on the effect of geometry on phase inversion during emulsion formation RT PBT-UD PBT-DU PBT-DD slope= %-CIST slope= Jcd / m- (J/kg) 40

41 Conclusions oil-in-water dispersion The CLD shows that almost half of the drops are smaller than the Kolmogorov scales, while the weighted mean falls conveniently inside the inertial range. More work is needed on the mechanisms which lead to formation of these very tiny drops. At the just-dispersed condition, cl 32 scales well with J cd, in contrast with N cd and t cd which showed no trends energy required to completely disperse the second phase, which showed conflicting trends depending on geometry P/m at equilibrium, which showed no effect on cl 32 The hypothesis that mixing energy is a useful scaling variable is upheld for the moment. 41

42 Water-in-oil dispersion Number averaged droplet counts Effects of the position of the liquid interface relative to the impeller blade 42

43 Experimental Conditions Interface at 2mm below lowest impeller Interface at 2mm above lowest impeller Continuous Phase Canola Oil Canola Oil Dispersed Phase Water Water Water Hold-up 4.77 wt% 9.91 wt% Average Energy Dissipation, ɛ (W/kg) 0.25 to to

44 Number averaged droplet counts experiments with 4.77wt% of water - Maximum is at the limit detection of the equipment - Kolmogorov scale is larger than 15 µm for all cases - What is generating these small droplets? 44

45 Dispersion of 4.77 wt% of water in canola oil Interface: 2mm below the lowest impeller Lower energies do not have peaks, but it takes longer for them 45 to get to equilibrium 10

46 Comparing the position of the interface Energy dissipation of 2 W/kg Peak is higher when the interface is located above the impeller 46

47 Dispersion of water (10 wt%) in canola oil 47

48 Conclusions water-in-oil dispersion Most part of the readings are on the order of 2-3 µm (smaller than Kolmogorov scale) limit detection of the equipment The initial dynamics of the dispersion are effected not only by the hold up, but also by the placement of the interface relative to the Rushton impellers. At the initial stages of the dispersion, the continuous phase (canola oil) may be pumped towards the dispersed phase (water). So canola oil may be initially dispersed into water

49 Bitumen de-watering Bitumen contains water and solids that need to be removed Chemical demulsifier is used for that Demulsifier concentration Mixing (energy dissipation and mixing time) Injection concentration (pre-mixing, demulsifier dilution) 49

50 Experimental Setup 50

51 Sampling Sampling Point Height Below Interface Relative Height FBRM 30 mm 0.13 Z1 52 mm 0.23 Z2 96 mm 0.43 Z3 140 mm 0.62 Z4 184 mm 0.82 liquid height = 225 mm rendering by Jairamdas,

52 Main results - Demulsifier concentration is the most important variable as expected - Correct levels of mixing energy (energy dissipation x mixing time) can cut the amount of demulsifier by half. - Right mixing conditions also improve settling - Pre-dilution of the demulsifier (lowering injection concentration) significantly improves performance avoid high local concentration 52

53 FBRM Fouling Conclusion: Fouling must be accounted for in LQ froth Sapphire repellency treatment (Aculon) is very effective 53

54 FBRM Concluding remarks - The FBRM measures chord length instead of drop size. This seems physically meaningful when placed in the same frame as the spectrum of length scales in turbulent flow - Air entrainment is a major factor that has to be carefully addressed - We tested several fat soluble fluids as continuous phase Conosol, Silicon and canola oil - Canola oil provides the most stable results - We are trying n-heptane and mineral oil - Saphire repellency treatment was very effective for bitumen experiments 54

55 Acknowledgments Several people worked on the results: Dr. John Nychka, Runzhi (Anna) Xu, Colin Saraka, Akshay Bhalerao, Fatemeh Safari Alamuti, Tanya Varma, Khilesh Jairamdas, Alexandra Komrakova 55

56 Handbook of Industrial Mixing & Advances in Industrial Mixing 56

57 References Bhalerao, A., Alamuti, F.S., Kresta, S.M., Machado, M.B., Komrakova, A.E., Liquid Drawdown in a Stirred Tank and Confined Impeller Stirred Tank (CIST), AIChE annual meeting Salt Lake City, Utah, USA. Calabrese, R.V., Chang, T.P.K., Dang, P.T., 1986a. Drop breakup in turbulent stirred-tank contactors. Part I: Effect of dispersed-phase viscosity. AIChE Journal 32, Calabrese, R.V., Wang, C.Y., Bryner, N.P., 1986b. Drop breakup in turbulent stirred-tank contactors. Part III: Correlations for mean size and drop size distribution. AIChE Journal 32, Chong, J.Y., Machado, M.B., Arora, N., Bhattacharya, S., Ng, S., Kresta, S.M., 2016a. Demulsifier Performance in Diluted Bitumen Dewatering: Effects of Mixing and Demulsifier Dosage. Energy & Fuels 30, Chong, J.Y., Machado, M.B., Bhattacharya, S., Ng, S., Kresta, S.M., 2016b. Reduce Overdosing Effects in Chemical Demulsifier Applications by Increasing Mixing Energy and Decreasing Injection Concentration. Energy & Fuels 30, Coulaloglou, C.A., Tavlarides, L.L., Drop Size Distributions and Coalescence Frequencies of Liquid-Liquid Dispersions in Flow Vessels. AIChE Journal 22, Coulaloglou, C.A., Tavlarides, L.L., Description of Interaction Processes in Agitated Liquid-Liquid Dispersions. Chemical Engineering Science 32, Komrakova, A.E., Liu, Z., Machado, M.B., Kresta, S.M., Development of a zone flow model for the confined impeller stirred tank (CIST) based on mean velocity and turbulence measurements. Chem Eng Res Des submitted. Kresta, S.M., Etchells, A.W., Dickey, D., Atiemo-Obeng, V.A., Advances in Industrial Mixing: A Companion to the Handbook of Industrial Mixing. Wiley. Kukukova, A., Aubin, J., Kresta, S.M., A new definition of mixing and segregation: Three dimensions of a key process variable. Chemical Engineering Research and Design 87, Laplante, P., Machado, M.B., Bhattacharya, S., Ng, S., Kresta, S.M., Demulsifier Performance in Froth Treatment: Untangling the Effects of Mixing, Bulk Concentration and Injection Concentration using a Standardized Mixing Test Cell (CIST). Fuel Processing Technology 138, Machado, M.B., Bittorf, K.J., Roussinova, V.T., Kresta, S.M., Transition from turbulent to transitional flow in the top half of a stirred tank. Chemical Engineering Science 98, Machado, M.B., Kresta, S.M., The confined impeller stirred tank (CIST): A bench scale testing device for specification of local mixing conditions required in large scale vessels. Chem Eng Res Des 91, Machado, M.B., Kresta, S.M., When Mixing Matters: Choose Impellers Based on Process Requirements. Chemical Engineering Progress 111, Paul, E., Atiemo-Obeng, V.A., Kresta, S.M., Handbook of Industrial Mixing: Science and Practice. John Wiley and Sons, Inc., Hoboken, New Jersey. Tsouris, C., Tavlarides, L.L., Breakage and coalescence models for drops in turbulent dispersions. AIChE Journal 40, Wang, C.Y., Calabrese, R.V., Drop breakup in turbulent stirred-tank contactors. Part II: Relative influence of viscosity and interfacial tension. AIChE Journal 32, Zhou, G.W., Kresta, S.M., Impact of tank geometry on the maximum turbulence energy dissipation rate for impellers. AIChE Journal 42,

58 Extra slides 58

59 Video DUDUD PBT in CIST at φ = 10% first 27s of 270s 59

60 PV19 images oil-in-water dispersion Small oil droplets and large droplet in background 60

61 PV19 images oil-in-water dispersion Oil droplets in background and foreground 61

62 PV19 images oil-in-water dispersion Small droplets on surface of larger droplet 62

63 PV19 images oil-in-water dispersion Complex droplets 63

64 Flow field CIST operating with 5 Rushton impellers Same flow behaviour in all jets leaving the impellers 5 recirculation zones between impellers 64

65 Power per volume comparison Impeller * ε max /ε average for the CIST ε max /ε average for stirred tank Ϯ Rushton A Intermig 4.8 N/A When Rushtons are used, CIST is 13 times more homogeneous When A310s are used, CIST is 6 times more homogenous # of impellers forces the energy dissipation to be well distributed * Impeller sketches: Hemrajani and Tatterson in Paul et al., Ϯ Zhou and Kresta (1996) 65