Bohnet, M. W.

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Engineering Conferences International

 CRYSTALLIZATION FOULING ON HEAT TRANSFER SURFACES – 25 YEARS RESEARCH IN BRAUNSCHWEIG  M.W. Bohnet  Institute of Chemical and Thermal Process Engineering Technical University of Braunschweig Langer Kamp 7, D – 38106 Braunschweig, Germany E-mail: m.bohnet@tu-braunschweig.de  ABSTRACT Mass transfer by diffusion:   Fouling research at the Technical University of Braun-schweig during the last 25 years is mainly focused on the formation of crystal layers. A model for the description of the fouling process was developed and contentiously ex-panded. Besides the process parameters such as fluid veloc-ity, pulsating flow, heat flux, pH etc., the properties of the fouling layer itself was examined. Shear strength and poros-ity were determined and correlated with the fouling resis-tance. Another focus was put on the interactions between heat transfer surface and the fouling layer. The build-up of the layer was optically observed with the holographic inter-ferometry. Hence concentration and temperature profiles in the immediate vicinity of the wall could be calculated. Structured and modified surfaces were applied to lengthen the induction period. )( fFddf ccmdtdm−==β&  (1)  Surface reaction:  nsfRddf cckmdtdm)( −==&  (2)  For a reaction of 2. order it follows:  ( )( )−+−−+=2/124121sFRRsFRdcckkcckmββββ& (3)  INTRODUCTION  Forced by industry fouling research began in the late seventies. At this time nobody at German Universities was working in this field. The reason was simple. Doing re-search means: working for a PhD-thesis. But fouling was at this time not a scientific topic. Besides experimental inves-tigations we focused therefore on the physical/chemical understanding of the fouling process.  Unsatisfactory is the fact that due to the unknown con-centration cf at the fouling layer surface a detailed look into the deposition procedure is impossible.  FOULING BY CRYSTALLIZATION  Modeling  More difficult was the lack of knowledge with respect to the solids removal. It depends on the shear stress at the layer surfaces due to the liquid flow, τf ~ ρw2. For the shear strength of the layer we use an assumption of Bohnet (1987):  The deposition of crystals can be described taking dif-fusion and reaction into account as shown in Fig. 1.   pff dTNxPk)1(1 ∆+=δσ  (4)  with P for the intercrystalline force, N number of defects in the fouling layer, xf layer thickness, σ linear coefficient of thermal expansion, ∆T temperature difference within the fouling layer, dp crystal size.   Fig. 1: Concentration and temperature profile in the             vicinity of the fouling layer  ECI Symposium Series,  Volume RP2:  Proceedings of 6th International Conference on Heat Exchanger Fouling and Cleaning - Challenges and Opportunities,Editors Hans Müller-Steinhagen,  M. Reza Malayeri,  and  A. Paul Watkinson,  Engineering Conferences International,  Kloster Irsee,  Germany,  June 5 - 10, 2005Produced by The Berkeley Electronic Press, 2016Shear strength of the fouling layer  For the removal rate can be written:  3/12 ==ρηρστ gkmdtdmfffrrf &  (5)  and it follows:  ( ) ( ) 23/123 1 wxgdpTPNkm ffr ηρδρ ∆+=&  (6)  To receive information on the shear strength as defined in Eq.(4), Hirsch (1996, 1999) developed a device for the measurement of the shear strength of crystal cylinders. Due to the fact, that the real fouling layers are rather thin he combined the shear strength measurements with a wearing procedure (Fig. 3) and was able to describe the shear strength with the abraded thickness of the fouling layer (Fig. 4).    The rate of growth of the layer is:  rdf mmdtdm&& −=  (7)    Introducing the fouling resistance Fig. 3: Shear strength device and wearing disc  Rf = ffxλ with xf = ffmρ (8)    With Eq. (3), (6) and (7) the fouling resistance can be calcu-lated. The only unknown properties are k3, N and P. Krause (1986) determined from his experiments with CaSO4 solu-tions:  NkP3= 83,2 w0,54 (9)   As example Fig. 2 gives a comparison of measured and cal-culated fouling resistances. The empirically received  Eq. (9) was needed to calculate the shear strength in Eq. (4). It was therefore necessary to have a closer look at the shear strength.    Fig. 4: Abraded fouling layer thickness versus shear             strength  Fig. 2: Fouling curves for a CaSO4 solution  296 Heat Exchanger Fouling and Cleaning - Challenges and Opportunities [2005], Vol. RP2, Article 43http://dc.engconfintl.org/heatexchanger2005/43concentration profiles in the vicinity of the surface using two wave length holographic interferometry (Fig. 7). As salt he used Na2SO4. As examples Fig. 8 shows temperature profiles for a clean heat transfer surface and Fig. 9 concen-tration profiles for progressing crystallization. Drawing the concentration against the temperature an unexpected result was received. In the vicinity of the layer surface a strong supersaturation is observed with a very sharp decline of the concentration directly at the surface as can be seen from Fig. 10.      Fig. 5: Shear strength and density versus fouling layer               thickness     The shear strength and the density of the fouling layer strongly depend on the layer thickness as shown in Fig. 5. The results were obtained with CaSO4 layers of a total thickness of about 6 mm. Introducing the shear strength into the model equation gave very good results as shown in  Fig. 6. Fig 7: Holographic interferogram with crystallization         Fig. 6: Measured and calculated fouling resistances for            different concentrations   Fig. 8: Temperature profiles without crystallization Two wave length holographic interferometry   To receive information on the concentration at the foul-ing layer surface, Seyfried (1993) measured temperature and  297Bohnet:Produced by The Berkeley Electronic Press, 2016    Fig. 11: Fouling curves without and with pulsation of the                 liquid flow  Fig. 9: Concentration profiles for progressing crystallization        Fig. 12: Fouling curves with pulsation of the liquid flow Fig. 10: Concentration versus temperature for a fouling layer of 0,64 mm thickness  Based on these results further investigations were carried out varying the oscillation frequency. Single strokes of higher velocity superimposed on the stationary flow and the time interval were varied. The fouling behaviour of the test runs with short intervals was simular to the experiments with continuous pulsation. Going to longer intervals leads to a limiting value where no difference could be observed be-tween experiments with and without pulsation.  Pulsating flow  The question whether a pulsating liquid flow influences fouling was answered by Augustin (2001). Fig. 11 shows the fouling resistance without and with pulsation, Fig. 12 the results in detail. The test runs without pulsation showed the expected decrease of the fouling resistance with increas-ing flow velocity. Test runs with pulsation led to a little improvement of heat transfer which displays a negative value of the fouling resistance. This is caused by small crys-tals growing on the heat transfer surface which increase turbulence. Optical observations with a CCD-camera showed that single crystals or small crystal clusters that grew at the surface were removed continuously and started growing at another part of the heat transfer surface. The adhesion force between crystals and surface is lower than the hydrodynamic forces acting on the crystals which means that the crystals can be removed. A quasi-stationary state ensues.  Surface structure  The behavior of structured surfaces is completely dif-ferent. Augustin (1992, 1993) investigated different struc-tures and received results as shown in Fig. 13. The fouling curves for roughened pipes show a different shape com-pared to smooth pipes. In the initial region the deposition rate increases, comes to a turning point, decreases and reaches the final value of zero. This initial region can be explained by the fact that during the fouling process the first deposits occur within the roughness of the structure. Once the deposit is sufficient to fill the roughness structure and creates a smooth pipe the fouling curve becomes similar to to the curves obtained for smooth pipes. Theoretical calcu-lations of the volume of the deposit required to fill the   298 Heat Exchanger Fouling and Cleaning - Challenges and Opportunities [2005], Vol. RP2, Article 43http://dc.engconfintl.org/heatexchanger2005/43roughness structure (dashed lines) show that it is possible to explain the experimental results on this basis. saturation index is proportional to the deposited mass flow rate of the solids. Experiments have been carried out for the pH-value range of technical interest between pH 6 and pH 10. The results are shown in Fig. 14. The asymptotic fouling resistance increases with the pH-values due to a higher strength of the fouling layer caused by an increasing num-ber of nucleation sides for higher supersaturation. Taking into consideration the complexity of the fouling process the comparison of results of fouling experiments carried out at different pH-values is in good agreement with the calculated data.        Fig. 14: Asymptotic fouling resistance depending on              pH-value and liquid velocity for aqueous solutions              of CaCO3 (Temperature 70 – 90 °C)  Modification of the Interface Crystal/Heat Transfer Surface   An approach for fouling mitigation is the reduction of the adhesive strength between the crystals and the heat transfer surface as shown in Fig. 15.    Fig. 13: Fouling resistance depending on time for pipes with              knurling and different threads (aqueous solution of              CaSO4) pH-value A very strong influence shows the pH-value. For  CaCO3 solutions Augustin (1992, 1995) determined the asymptotic fouling resistance. The saturation index has been calculated using the reaction equations for the dissociation of CaCO3. From these calculations it can be seen, that the  Fig. 15: Approach for fouling mitigation during the                   induction period 299Bohnet:Produced by The Berkeley Electronic Press, 2016In a first step Förster (1999, 2001) determined the surface free energy by wetting experiments.          harmonic mean approach                 dispersive     polar  geometric mean approach                dispersive     polar       Lewis acid-base approach                LW -comp.  AB-comp.Bz CuAlStDLCMs(II)Ms(I)FEPPFAPTFEMaterial010203040506070Surface energy γ 23 / (10-3Nm-1)   tind 0100200300400500600Induction time t ind / h Fig. 16: Surface energy and induction time Fig. 18: Interfacial defect model   A comparison of surface energy and induction time showed, that there is no correlation (Fig. 16). What was really needed was the interfacial energy between crystals and sur-face, which was determined also by wetting experiments  (Fig. 17). The reason is, that the roughness of the heated surface has to be taken into account. With the roughness Rz and the width of the elements Rs a surface texture parameter K = Rz ⋅ Rs was introduced. Comparing the experimentally received data for a reference topography gave very good results. (Fig. 19)     Fig. 17: Interfacial energy crystal/surface  Fig. 19: Induction time versus spreading coefficient Plotting the induction time against the spreading coefficient (Fig. 18) gave good results for metallic heat transfer sur-faces, but could not describe polymeric materials.    300 Heat Exchanger Fouling and Cleaning - Challenges and Opportunities [2005], Vol. RP2, Article 43http://dc.engconfintl.org/heatexchanger2005/43  Numerical Simulation   Brahim (2003) simulated the fouling behavior on the basis of a realistic heat flux distribution during the fouling proc-ess. His model is shown in Fig. 20, his results in Fig. 21. 0 50 100 150 200 250 300 350 40001020304050TF=42°C.q=18,5 kW /m² 0,0275 mol/l CaSO 4 0,005  mol/l CaCO 3   pH 4,0 pH 5,5 pH 6,5 pH 7,0Fouling resistance Rf [10-4 m²K/W]time t [h] . . .q1 = qmaxq3q2qn. . .q1 q3q2 qnq1 = q2 = q3 = ... = qn = q0.. ...... .....t = 0t > 0xf,nxf,3.qi = f (xf,i).Heat Transfer SurfaceFouling Layer Fig. 20: Realistic distribution of the heat flux along the heat              transfer surface. Fig. 22: Fouling resistance for different pH-values       Fig. 23: Heating element   Fig: 21: Comparison of simulation and measurement results CONCLUSIONS   Multi-component systems This paper summarizes the results of fouling research at the Technical University of Braunschweig. The author thanks with great respect his PhD.-students working in this field. They performed their research with great enthusiasm and ability. It was a real pleasure having them at the institute.  The fouling behavior of CaSO4/CaCO3 solutions can not be described with the behavior of the single salts. Höfling (2004) determined a strong influence of the pH-value not only on the fouling resistance as shown in Fig. 22 but espe-cially on the structure of the fouling layer which is deter-mined by different modifications of the crystals  (Fig. 23). 301Bohnet:Produced by The Berkeley Electronic Press, 2016NOMENCLATURE  W. Augustin and M. Bohnet, 2001, Influence of pul-sating flow on fouling behaviour, in: Mitigation of heat ex-changer fouling and its economic and environmental impli-cations, eds. T.R. Bott et al., Begell House, New York, pp. 161-168  Fc  concentration of salt solution [kg/m3] Sc  saturation concentration  [kg/m3] fc  concentration in the vicinity of the fouling layer  [kg/m3] D diffusion coefficient  [m2/s] g  gravitational acceleration  [m/s2] P/k3 cohesion coefficient [kg m/s2] Rk  rate of reaction  [m4 kg/s] m&  total mass rate  [kg/m2 s] dm&  deposition mass rate  [kg/m2 s] rm&  removal mass rate  [kg/m2 s] n  order of the surface reaction [-] P  intercrystalline adhesion force  [N] q&  heat flux  [W/m2 K] fR  fouling resistance  [W/m2 K] fT  temperature of fouling layer surface  [K] FT  temperature of salt solution  [K] fx  thickness of crystal layer  [m] w  fluid velocity  [m/s] t time [s] T temperature [K] β  mass transfer coefficient  [m/s] γ  iηsurface energy [N/m]  viscosity of salt solution  [kg/m s] fλ  thermal conductivity of the fouling layer  [W/m K] λ1,2 spreading coefficient [N/m] θ contact angle [°] ρ  density of salt solution  [kg/m3] fρ  density of fouling layer  [kg/m3] σf shear strength [N/m2] τf shear stress [N/m2] ζ adhesive strength [N/m2]  M. Bohnet, 1987, Fouling of heat transfer surfaces Chem. Eng. Techn. 10, 2, p. 113-125  M. Bohnet, W. Augustin, 1993, Effect of surface struc-ture and pH-value on fouling behaviour of heat exchangers in: Transport Phenomena in Thermal Engineering, eds. J.S. Lee et al., Begell House, New York, p. 884-889  F. Brahim, 2003, Numerische Simulation des Kristall-wachstums auf wärmeübertragenden Flächen (Fouling), Diss. TU Braunschweig  F. Brahim, W. Augustin, M. Bohnet, 2003, Numerical simulation of the fouling process, Int. J. of Therm. Sci. 42, 3, p. 323-334  M. Förster, M. Bohnet, 1999, Influence of the Interfa-cial Free Energy Crystal/Heat Transfer Surface on the In-duction period during Fouling, Int. J. Th. Sci. 38, p. 944-954  M. Förster, 2001, Verminderung des Kristallisations-foulings durch gezielte Beeinflussung der Grenzfläche zwi-schen Kristallen und Wärmeübertragungsfläche, Diss. TU Braunschweig  H. Hirsch, 1996, Scher- und Haftfestigkeit kristalliner Foulingschichten auf wärmeübertragenden Flächen, Diss. TU Braunschweig  H. Hirsch, W. Augustin, M. Bohnet, 1999, Influence of fouling layer shear strength on removal behaviour, in: Un-derstanding Heat Exchanger Fouling and its Mitigation, eds. T.R. Bott et al., Begell House, New York, p. 201-208  V. Höfling, 2004, Kristallisationsfouling auf wärme-übertragenden Flächen durch Mehrkomponentensysteme, Diss. TU Braunschweig  V. Höfling, W. Augustin, M. Bohnet, 2004, Crystalliza-tion fouling of the aqueous two-component system CaSO4/CaCO3 – Fouling of salt mixtures, Int. J. Trans. Phe-nomena 6, 2, p. 99-109   S. Krause, 1986 and Translation 1993, Fouling an Wärmeübertragungsflächen durch Kristallisation und Sedi-mentbildung, Diss. TU Braunschweig, Fouling of heat-transfer surfaces by crystallization and sedimentation, in  Int. Chem. Eng. 33, 3, p. 355-401  REFERENCES   W. Augustin, 1992, Verkrustung (Fouling von Wärme-übertragungsflächen, Diss. TU Braunschweig  F. Seyfried, 1993, Untersuchung der Kristallisation von Salzen an wärmeübertragenden Wänden mit Hilfe der ho-lographischen Interferometrie, Diss. TU Braunschweig W. Augustin, M. Bohnet, 1995, The influence of the ra-tio of free hydrogen ions on crystallisation fouling, in Chem. Eng. Process. 34, 2, p. 79-85 302 Heat Exchanger Fouling and Cleaning - Challenges and Opportunities [2005], Vol. RP2, Article 43http://dc.engconfintl.org/heatexchanger2005/43

CRYSTALLIZATION FOULING ON HEAT TRANSFER SURFACES – 25 YEARS RESEARCH IN BRAUNSCHWEIG

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CRYSTALLIZATION FOULING ON HEAT TRANSFER SURFACES – 25 YEARS RESEARCH IN BRAUNSCHWEIG

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