OPTICAL AND THERMAL INVESTIGATIONS OF CRYSTALLIZATION FOULING IN A MICRO HEAT EXCHANGER

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1 Proceedings of International Conference on Heat Excanger Fouling and Cleaning - 2 (Peer-reviewed) June 5 -, 2, Crete Island, Greece Editors: M.R. Malayeri, A.P. H. Müller-Steinagen Watkinson and H. and Müller-Steinagen A.P. Watkinson Publised online OPTICAL AND THERMAL INVESTIGATIONS OF CRYSTALLIZATION FOULING IN A MICRO HEAT EXCHANGER M. Mayer, J. Bucko 2, W. Benzinger 3, R. Dittmeyer 4, W. Augustin 5 and S. Scoll Tecnisce Universität Braunscweig, Institute for Cemical and Termal Process Engineering (ICTV), Langer Kamp 7, 386 Braunscweig, Germany moriz.mayer@tu-braunscweig.de Karlsrue Institute of Tecnology (KIT), Institute for Micro Process Engineering (IMVT) Hermann-von-Helmoltz-Platz, Eggenstein-Leopoldsafen, Germany ABSTRACT Micro eat excangers ave particular advantages regarding eat and mass transfer suc as intensive eat excange, low material costs, ig mass transfer rates and an excellent termal efficiency. Te sensitivity of micro structures to undesirable deposition is a fundamental knockout criterion for many industrial applications. Crystallization fouling of inverse soluble salts on eat excanger surfaces leads to an increase of te pressure drop, as far as total blocking of te micro cannels. Furtermore fouling results in a reduction of te eat excanger performance, in a maldistribution of flow in te micro structures and in a sorter residence time of te fluid. Te intension of tis work is te basic investigations of te fouling beavior in micro eat excangers. Terefore fouling experiments wit calcium carbonate (CaCO 3 ) in an experimental micro eat excanger were carried out and observed wit a digital microscope. Te investigations included local temperature measurements confirmed by CFD simulation as well as te optical visualization of te fouling process inside te micro cannels. Te detected fouling resistances R f were in te range of m 2 K W -. Te fouling developed eterogeneously in te micro cannels and in a number of runs te deposit built up a parabolic profile wit less fouling at te inlet of te micro structure. Cleaning in place (CIP) was possible and also optically observed. INTRODUCTION Crystallization fouling on eat transfer surfaces can cause ig damage to te eat excanger as well as to te connected equipment (Epstein, 983) (Müller-Steinagen, 999). It often raises considerable costs in construction and operation. Te crystal deposition is usually formed by inverse soluble salts suc as CaCO 3 and CaSO 4 2H 2 O wic crystallize eterogeneously on te ot eat excanger surface (Bonet, 987). Tis process can be divided into eterogeneous nucleation, crystal growt, adesion, removal and aging of te fouling (Bott, 995). During nucleation and to some extend during crystal growt an interface between te crystal and te eat excanger surface comes to existence (Mayer et al., 2). Consequently te eat flux is reduced and te pressure drop increases. Tis results in a decline of te eat excanger performance and an extensive cleaning task. Hence it is reasonable to formulate a fouling resistance (Eq. ) from te eat balance. Figure sows te fouling on a eat transfer surface scematically. Fig. Scematic diagram of te eat flux troug a clean and a fouled eat excanger wall. R f Q U f x U,x,2 f,f, f, 2 f f, x λ f, f,, f,2 x λ f,2 f,2 () Typical distributions of te fouling resistance are sown in Figure 2. At least two periods can be distinguised: Te induction period, were te eat transfer is not reduced significantly, and te crystal growt period. Many industrial eat excangers are operated wit a ph value above 7 in order to avoid corrosion. Unfortunately fouling rates rise wit increasing ph value (Augustin et al., 995). Te driving power for crystallization fouling is te level of supersaturation at te eat excanger surface wic can be expressed by te saturation index SI (Eq. 2). Q xf, x f, 2,2,2 68

2 Mayer et al. / Optical and Termal Investigations R f R f * (a) (d) (b) (c) MICRO HEAT EXCHANGER DESIGN Te experimental eat excanger was constructed at te IMVT, KIT Karlsrue. Figure 4 sows te experimental counter current micro eat excanger for te fouling experiments (Mayer et al., 2). Te micro structured devices of te solution and te eating side as well as of te local temperature measuring points are sown in Figure 5 (Bucko et al., 2). I II t ind t Fig. 2. Fouling resistance over time: I induction period, II layer growt period; (a) no removal, (b) decreasing deposition rate due to removal, (c) asymptotic beavior: deposition and removal rate become equal, (d) saw toot sape: removal of bigger deposit fractions. Te saturation index is te common logaritm of te ratio of te actual ionic activity product to te solubility product. Te solution at te wall must be supersaturated, i.e. te saturation index must be greater tan zero for crystallization fouling (Mullin, 25). i a i IAP SI log log logs(c,t m ) i K (2) SP eq ai Micro process engineering tries to adopt unit operations in a volume at micro scale. Typical micro eat excangers are endowed wit micro cannels tat ave diameters of less tan mm. Due to te small dimensions micro eat excangers are appropriate for processes wit ig eat and mass transfer rates. Hence tey ave advantages over conventional eat excangers concerning te rate of product conversion, directed eating and sort residence times (Scubert et al., 2). However te usage in cemical industries is rare today since te micro structures are very vulnerable to corrosion and fouling. Terefore te contamination of micro eat excangers and teir cleaning needs to be studied. Figure 3 sows an example of a cross flow micro eat excanger (Bucko et al., 2). Fig. 4 Scematic exploded draft of te experimental micro eat excanger (left) and assembled experimental micro eat excanger for fouling investigation (rigt). (a) (b) (c) Fig. 3 Crossflow micro eat excanger wit a volume of 8 cm 3 (left) and micro structured cores for crossflow micro eat excanger of different sizes (rigt). Fig. 5 Micro structured devices: (a) polycarbonate element wit one micro cannel, (b) eat transfer foil wit 2 micro slots for termocouples, (c) titan element for eating wit 693 cannels. 69

3 Heat Excanger Fouling and Cleaning 2 FOULING EXPERIMENTS Te upper test section is made from polycarbonate and tus transparent for te optical investigations. It is furter removable for cleaning and for te insertion of different geometries. Te investigated cannels were rectangular wit 4 µm widt, 2 µm dept and 24 mm lengt. Te corresponding structures ad one, 2 respectively 8 cannels. Te eating device ad 693 cannels (2 x 2 x 6 µm 3 ) and provides nearly constant wall temperature on te eating side. Six respectively 2 termocouples wit a diameter of 5 µm were placed in micro slots wit a cross section of 6 x 55 µm 2 between te two eat excanger foils (eac 2 µm tickness) centered in flow direction. All parts were clamped togeter wit screws and sealed wit Viton. Figure 6 sows te experimental setup. For te fouling experiments a supersaturated solution (25 C) was pumped troug te micro eat excanger wit a HPLC pump (Knaur, Smartline ). Te HPLC pump provided constant flow rate over te wole fouling experiment. In order to avoid fouling at te eating side distilled water was pumped counter current wit 8 ml s -. Temperatures were measured at te inlet and outlet on bot sides. Te pressure was measured at te inlet and outlet of te solution side. In order to obtain te mean initial surface temperature T m te temperatures from te six local measurement points were used. and between 44 and 76 for 8 cannels. To avoid damage to te micro structure te maximum pressure inside te HPLC pump was set to 3 MPa. After te fouling experiments te micro structures were cleaned by rinsing wit 6% HCl solution for about 2 minutes and rinsing again for about 3 minutes wit distilled water in order to remove te acid. Te formation of te crystallization fouling was optically captured by a digital microscope (see Figure 7) (Keyence, VHX 5F) at different magnifications. Fig. 7 Setup of micro eat excanger wit digital microscope at rigt (view from top). RESULTS In order to validate te temperature measurements CFD simulations (Ansys, Fluent 2.) were carried out using te Navier-Stokes equations for incompressible fluids. Te geometric model corresponding to te single cannel micro structure (see Figure 5a) is sown in Figure 8. outlet polycarbonate inlet cannel metallic foil Fig. 6 Experimental setup. Te solution was aerated wit CO 2 for 2 in order to keep a ig solubility of CaCO 3. Afterwards te solution was brougt from about ph 5.3 to 7 wit NaOH. Te concentration of CaCO 3 was 5 mmol l - and te corresponding SI at te eat excanger surface varied from Flow rates were varied from to ml min - for one cannel and from to 4 ml min - for 2 cannels respectively 8 cannels. Under consideration of a uniform flow distribution at te inlet of te micro structures te corresponding Reynolds numbers Re were between 79 and 792 for one cannel, between 66 and 264 for 2 cannels Fig. 8. Geometric model of te flow system: micro cannel in polycarbonate wit metallic bottom side. Te simulation was carried out wit constant wall temperature and data for pure water at te same conditions as te experiments wit different mass flows. Figure 9 sows a profile of te micro cannel and te foil wit micro slots for te termocouples. 7

4 Mayer et al. / Optical and Termal Investigations 8 Fig. 9 Scematic profile for te micro cannel and te metal foil wit a micro slot. Arrows mark te possible measuring points of te termocouple in te micro slots of te eat excanger foil. Figure sows te experimental and simulation results of te local temperature measurements corresponding to Figure 9 for kg s - ( ml min - ) and for.4-4 kg s - (8 ml min - ). Due to te tetraedral mes used in te simulation small fluctuations of te simulated curves occurred. At iger flow rates te difference between te experimental and te calculated values increases. Te experimental values were sligtly iger tan tose obtained from te simulation. Figure sows te experimental detected temperatures of te fluid and te temperatures from te simulation at te outlet of te micro structure. As a result from te eat balance te outlet temperature at te solution side sinks wit increasing flow rate. Te results are in good accordance for iger mass flow rates. An average standard deviation of.5 K was determined for te experimental results. Fluid outlet temperature T F,out [ C] ,,2,4,6,8 Mass flow rate M [ -4 kg s - ] T F,out (exp) T F,out (sim) Fig. Fluid temperature at te outlet versus te mass flow rate. Typical examples for te optical detection of te fouling progress are sown in Figure 2. Te crystal deposition appears dark in te pictures. In some cases a parabolic fouling layer was observed at te end of te experiment. Tis can be attributed to an uneven eat flux distribution inside te eating structure (Figure 4c). Te corresponding integral fouling resistances from te eat balance are sown in Figure 3. Te integral fouling resistance was calculated wit Eq. (3) and te eat flux wit Eq. (4). 9 (a) Local wall temperature T w [ C] µm 2 µm µm experimental (t = min) Lengt L [mm] 9 (b) (t = 48 min) Local wall temperature T w [ C] µm 2 µm µm experimental Lengt L [mm] Fig. Experimental vs. simulation results of te local temperature measurements: (a) kg s -, (b).4-4 kg s -. (t = 96 min) Fig. 2 CaCO 3 in micro structures over time (flow from rigt to left, crystals appear dark). Left column: 2 cannels (4 ml min - ; T m = 6 C; 3x magnified, parabolic fouling 7

5 Heat Excanger Fouling and Cleaning 2 profile at te end. Rigt column: single cannel ( ml min - ; T m = 8 C; 2x magnified). Fouling resistance R f [ -5 m 2 K W - ] Single cannel: ml min -, Re=792, T m =8 C 2 cannels: 4 ml min -, Re=66, T m =63 C Time t [min] Fig. 3 Fouling resistances corresponding to Figure 2. A ΔTlog,f (t) A ΔTlog, R f (t) (3) Q f (t) Q Q (t) M(t) c (t) T (t) T (4) p out In Eq. (3) te eat transfer area of te micro cannels A was assumed to be constant since te crystal deposition acts as an insulator for te conductive eat transfer. In Eq. (4) only te outlet temperature canged significantly over time; te mean mass flow rate and te mean eat capacity of te solution canged ardly wile te volume flow rate and te inlet temperature were kept constant. Te cleaning in place (CIP) was also detected wit te digital microscope. Figure 4 sows te cleaning of te CaCO 3 fouled 8 micro structure cannels wit a 6% HCl solution at different times. Fig. 4 CIP (HCl 6 wt-%) of a 8 micro structure canels fouled wit CaCO 3 ; from top left to bottom rigt ( min;.5 min; 6.5 min; 8.5 min) 2x magnified, flow from rigt to left. in DISCUSSION It was sown tat it is possible to measure te local temperature inside te eat excanger foil of a micro eat excanger troug micro slots. Te obtained values are in good accordance wit te results of te CFD simulation. Te initial mean surface temperature T m and te corresponding saturation index SI could be determined wit te local temperature measurements. During te fouling progress te local temperature sowed an inomogeneous distribution due to te cross conduction inside te eat excanger foil. Tis made it difficult to summarize te local fouling resistances. In te future te fouling process itself needs to be modeled and simulated troug a mass and a eat balance. Te pictures taken wit te digital microscope sowed te fouling inside te micro structure. It was observed tat te fouling occurred also below te ligaments between te cannels. Tis can be attributed to te fact tat te micro cannels were not sealed. Furtermore te optical investigations sowed a maldistribution for te 2 cannel micro structure caused by fouling wit a parabolic fouling layer for longer periods tat can be attributed to an uneven eat flux distribution in te eating structure (see Figure 2). Te optically detected fouling density (see Figure 2) is not directly comparable to te fouling resistance from te integral eat balance (see Figure 3) because tere was a ig eat transfer in te inlet and outlet distributors of te micro structures (see Figure 5 (a)) wic cannot be neglected. Te integral fouling resistance sowed an asymptotic beavior wit very sort induction periods (see Figure 3). Te asymptotic beavior can be explained troug a decreasing saturation index (Eq. ) due to a decreasing surface temperature. Wen fouling occurs also te influence of an increasing pressure drop needs to be taken into consideration. Tere was no removal detected neiter troug temperature measurements nor troug optical capture (Mayer et al., 2). In comparison to an industrial sized eat excanger te flow velocities and te according wall sear stresses in te investigated micro eat excangers are ig but te flow regime is laminar (Re < 23). It is possible tat a flow induced removal on microscale depends mainly on te degree of turbulence rater tan on wall sear stress. Since te observed crystallization fouling is reaction controlled te effect of te mass transfer as been not considered yet. Also te deposit tickness could not be measured. Te CIP wit acid was enanced by possible bypass flows below te ligaments of te micro cannels due to te clamped construction of te micro eat excanger. Once a number of te cannels were free te cleaning rate increased (see Figure 9). A measure to reduce te adesion could be a surface treatment. Investigations of te adesion of crystals on eat excanger surfaces sowed a significant reduction of CaCO 3 fouling on surfaces modified wit diamond-like-carbon based coatings (Mayer et al., 2). An additional surface layer or coating on te eat excanger foil would not decrease te cannel diameter nor increase te pressure drop due to te clamped construction. Only te additional termal resistance of te coating must be considered. For a standard DLC coating wit tickness of 3 µm tis is negligible. It 72

6 Mayer et al. / Optical and Termal Investigations could be also elpful to use electro polised eat excanger surfaces in micro process engineering in order to enance te CIP. CONCLUSIONS. It could be pointed out tat crystallization fouling of CaCO 3 at a relatively ig supersaturation level as a significant and fast impact on te degradation of te performance of te micro eat excanger. Te measurement of local wall temperatures was possible and was in good agreement wit CFD simulations. Troug te experimental eat excanger wit optical accessible micro cannels te fouling progress and its cleaning was observed. Te fouling resistance was determined from an integral eat balance. Te cross conduction inside te eat transfer foil as not been considered yet. 2. Te obtained results could be applied at te construction of micro eat excangers. In general a similar fouling beavior like in macroscale eat excangers was observed. Since crystallization fouling causes ig damage to te micro structured system it is eiter crucial to avoid fouling troug closed systems or to construct a micro eat excanger tat bears a certain amount of unwanted deposit and can be cleaned easily. Te influence of te pressure drop in fouling in micro structures needs to be investigated. Furter studies sould concentrate on te usage of modified surfaces in micro eat excangers and cleaning possibilities. It was demonstrated tat in principle crystallization fouling in micro structures can be removed by applying HCl and acid-resistant surfaces. Te design of micro structures must avoid a maldistribution of te flow if fouling occurs. Since te impact of crystallization fouling depends strongly on te flow velocity and surface temperature flow dead zones particularly in te distributors need to be avoided. Flow dead zones can be prevented troug small inlet and outlet distributors wic sould be also at microscale. Also gauging accesses to te micro structures for example for pressure sensors can ave a negative effect. Te optical results could be applied to time scedules for CIP of micro structures tat do not ave optical accessibility. NOMENCLATURE A eat transfer area, m 2 a activity, dimensionless C concentration, mol m -3 c p eat capacity, kj kg - K - eat transfer coefficient, W m -2 K - IAP ionic activity product, dimensionless K sp equilibrium solubility product, dimensionless L lengt, m M mass flow rate, kg s - Q eat duty, W R f fouling resistance, m 2 K W - Re Reynolds number, dimensionless S supersaturation, dimensionless SI saturation index, dimensionless T F,out T m T w fluid outlet temperature, K initial mean surface temperature, K wall temperature, K t Time, s U overall eat transfer coefficient, W m -2 K - x tickness, m λ termal conductivity, W m - K - T log logaritmic mean temperature difference, K Subscript eq equilibrium f fouled i encounter ind induction clean,2 sides REFERENCES Augustin, W. and Bonet, M., 995, Influence of te ratio of free ydrogen ions on crystallization fouling, Cem. Eng. Process., Vol. 34, pp Bonet, M., (987), Fouling of Heat Transfer Surfaces, Cem. Eng. Tecnol., Vol., pp Bott, T., 995, Fouling of eat excangers, Elsevier. Bucko, J., Benzinger, W., Mayer, M., Augustin, W., Scoll, S., Dittmeyer, R., 2, Experimental and CFD Results for Local Temperature Measurement in Microstructure Devices, Proceedings 2 nd µfluidic Conference - 2, Toulouse, France. Epstein, N., 983, Tinking about eat transfer fouling: A 5 x 5 matrix, Heat Trans. Eng., Vol. 4, pp Mayer, M., Augustin, W. and Scoll, S., 2, Experimental study on te adesion of single crystals on modified surfaces in crystallization fouling, Proceedings of te International Conference on Heat Excanger Fouling and Cleaning IX 2, Crete, Greece. Mayer, M., Bucko, J., Benzinger, W., Dittmeyer, R., Augustin, W. and Scoll S., 2, Investigation and Visualization of crystallization fouling in a micro eat excanger, Proceedings of te 2 nd European Conference on Microfluidics - 2, Toulouse, France. Mullin, J., 25, Crystallization and Precipitation, Wiley-VCH, Weineim, Germany. Müller-Steinagen, H., 999, Cooling-water fouling in eat excangers, Adv. Heat Trans., Vol. 33, pp Scubert, K., Brandner, J.J., Fictner, M., Lindner, G., Scygulla, U., Wenka, A., 2, Microstructure Devices for Applications in Termal and Cemical process Engineering, Microscale Termopys. Eng, Vol. 5, pp