USE OF PERFLUOROPOLYETHER COATINGS TO MITIGATE FOULING ON HEAT EXCHANGER METAL SURFACES

V. Oldani1, C.L. Bianchi1, S. Biella2, C. Pirola1 and G. Cattaneo3

1 Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy & Consorzio INSTM, Via Giusti 9, 50121 Firenze, Italy

valeria.oldani@unimi.it 2 Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Piazza Leonardo da Vinci

32, 20133 Milano, Italy 3 ST special tanks S.r.L., Via Ai Pascoli snc, 23841 Annone di Brianza (LC), Italy

ABSTRACT

Scope of this research is to have a film coating on steel surfaces in order to reduce interaction between the metal surface and the precipitates, so to reduce the effect of the fouling in heat exchangers.

Perfuoropolyethers are used to obtain nano-range fluorinated layer in order to make steel surfaces hydrophobic. A pilot plant with two identical heat exchangers AEW TEMA TYPE was built to investigate the ability of the hydrophobic coating to prevent fouling. The heat exchangers, installed in parallel, were operating at the same temperature and pressure conditions. The thermodynamic performance of the two heat exchangers was compared. After a five months operation the results obtained demonstrate a significant increase of heat transfer resistance due to scaling on the uncoated heat exchanger respect to the coated one. INTRODUCTION The term fouling is usually referred to the accumulation of unwanted deposit on solid surfaces. Related to heat exchangers, fouling phenomenon occurs specially on heat transfer surfaces and the consequences are not negligible. The presence of the deposit, in fact, represents another resistance to heat transfer and it is also responsible of the increasing of the pressure drop. The results is the loss of efficiency thus and a more consumption of energy and an higher operating cost (Bott, 1995).

Several fouling mitigation techniques have been adopted and a great attention has been paid in the past for the on-line physical mitigation of the fouling in the heat exchangers. Physical mitigation techniques try to reduce fouling phenomenon modifying the interaction of the fouling precursors and the heat transfer surfaces. (Müller-Steinhagen et al., 2011).

According to Malayeri et al. (2009), if we consider the physical and geometrical properties of the surfaces, the foulant deposition process can be altered in two ways, or by reducing the work of adhesion between the surface and the fouling precursor or by increasing the shear stress. Key

elements for this is the surface roughness, surface free energy and surface geometry both at micro- and macroscopic scale.

In this research the use of hydrophobic coatings was made in order to modify the surface energy of steel so to reduce the interaction between the fouling precursor and the heat transfer surface. Several researches show that lower surface energies can reduce the propensity of the surface to foul; Förster et al. (1999) demonstrated that the nucleation rate and the adhesion of crystals on the heat transfer surfaces decreases for low energy surfaces. Zhang et al. confirmed the capacity of superhydophobic surfaces (i.e. very low surface energy) to delay bio-fouling phenomena, on the basis that the air bubbles entrapped in micro and nano-pores on the superhydrophobic surface can create a barrier preventing the adsorption of micro-organism. Modified polytetrafluoroethylene (PTFE) composite coatings, characterized by non-stick properties and low surface energy, have been developed with good results in fouling protection. Zhao et al. (2005) developed graded Ni-Cu-P-PTFE coatings for minimize adhesion of both micro-organism and scale, showing minimal adhesion of E. Coli bacteria and CaSO4 at lower surface free energy values. Rosmaninho et al. (2007) attested easy removal of non-microbiological milk deposits on stainless steel surfaces covered with Ni-P-PTFE coatings. Other materials have been used in order to obtain anti-fouling coatings. Wang et al. (2009) used nanometer TiO2 layer on copper surfaces, obtaining hydrophobic surfaces, able to enlarge the fouling induction period for scale formation. Martinelli et al. (2008) used amphiphilic polymer coatings in order to reduce both the hydrophobic and hydrophilic interactions between organisms responsible of bio-fouling and the surfaces. Results show weaker adhesion of some micro-organism on films reach in the amphiphilic polymer.

In this work functionalized perfluoropolyethers (PFPE) are used to obtain hydrophobic coatings. Typical properties of PFPE are high thermal stability, chemical inertness, hydrophobicity, oleophobicity and very low surface energy; moreover the presence of functional groups strongly changes chemical reactiveness of the polymer, allowing for example the formation of chemical bonds with chemical

Proceedings of International Conference on Heat Exchanger Fouling and Cleaning - 2013 (Peer-reviewed) June 09 - 14, 2013, Budapest, Hungary Editors: M.R. Malayeri, H. Müller-Steinhagen and A.P. Watkinson

Published online www.heatexchanger-fouling.com

316

sites present on metal surfaces, such as –OH sites (Tonelli et al.). Moreover Fabbri et al. attested high hydrophobicity and low surface energy for coatings obtained from different functionalized PFPE, specifically trietohoxysilane and hydroxy α-ω terminated poly(caprolactone-b-PFPE-b-caprolactone).

Coatings features and surface energy have been studied by XPS and contact angle analysis. Coatings resistance to different liquid environment was tested.

In order to demonstrate the anti-fouling efficacy of the fluorinated coatings developed, a pilot plant with two identical shell and tube heat exchangers has been built. Shell and tubes heat exchangers are the most common heat transfer equipment used in chemical industries, due to their versatility, robustness and reliability. However this type of heat exchangers are greatly involved in fouling phenomena if the design practice is not proper (Costa et al. 2008). Internal and external tubes surfaces of one of the pilot heat exchangers have been treated with a PFPE solution in order to obtain a protective coating. Heat exchangers performances were monitored and fouling resistance was determined along a period of five months. 1 EXPERIMENTAL 1.1 Preparation of fluorinated coated stainless steel

surfaces by liquid phase deposition

The fluorinated coatings were first applied on stainless steel (AISI 316) samples, used for coating characterization. The size of that samples was 2 cm x 1 cm and 0,5 cm in thickness.

Samples used for XPS analysis have been submit to a cleaning treatment before depositing the fluorinated cover. Such treatment consists in a cleaning with water and acetone, in order to remove the rough impurities present on the samples.

For contact angle analysis fluorinated coatings were applied on both previously cleaned or un-cleaned surfaces.

PFPE’s used for obtaining the coatings have a general formula X− [OCF2]n[OCF2CF2]p−X, where X is a functional polar group and the sum x+p is between 9 and 15, while the ratio x/p is between 1 and 2.

The coatings have been obtained dissolving in water or in a mixture of water and 2-propanol the functional PFPE derivatives. The weight percentage of the fluoropolymer in each solution ranges from 0,5% wt to 10% wt; the solution may include also a catalytic amount of acetic acid in the same weight percentage of the fluoropolymer used. The coating deposition on stainless steel surfaces has been obtained by dipping the samples in the PFPE solution, immersion time can vary from 10 minutes to several hours. Finally a thermic treatment has been performed in a stove at temperatures varying form 80°C to 150°C for 3 to 24 hours. The tubes of the heat exchanger pilot plant were coated by using a solution obtained from a completely water formulation of a PFPE derivative, using 1% wt of the polymer. Tubes were consequently dipped into the solution for 20 minutes and dried in an industrial stove for 19 hours

at a temperature of 80°C. Tubes surfaces were not specifically treated (for example with some chemicals or mechanical equipment), before the deposition procedure, but only a carefully washing was made. 1.2 Characterization of fluorinated coated surfaces

X-ray photoelectron spectroscopy spectra were obtained by using M-probe apparatus (Surface Science Instruments). Monochromatic Al K α radiation (1486.6 eV) was used as source. Analysis were performed on spot of size 200 μm 750 μm and pass energy was 25 eV. Survey analyses in the whole range of X-ray spectra and high-resolution analyses in the typical zone of C-bonds, F-bonds and O-bonds were performed. Fittings were performed using pure Gaussian peaks, Shirley’s baseline and no constraints.

Contact angle (CA) measurements were performed by a Krüss Easy Drop instrument. Free surface energy of coated samples has been determined by using Lewis acid-base approach. Three pairs of SFE-Theta values were obtained for each analysis by using di-iodomethane, water and formamide as solvent. CA values here reported are the average values obtained from at least five different determinations, obtained by deposition of liquid drops at different sample location.

1.3 Heat exchanger pilot plant A heat exchanger pilot plant was built in order to verify the antifouling properties of the fluorinated coating developed for stainless steel. The experimental equipment consisted of two tube and shell heat exchangers AEW TEMA type. Shells had horizontal orientation, with an internal diameter of 90 mm. Tubes were plain type, the outside diameter was 10 mm and the thickness was 2 mm. The length of tubes was 500 mm, the layout angle was 30° and the pitch was 12,5 mm. Tubes material was stainless steel AISI 316. Both heat exchangers had 18 tubes, one shell-pass and six tube-passes. Flows were countercurrent.

The only difference between the two heat exchangers was the presence of a tailor made PFPE coating on the tubes surfaces of only one of them.

The schematic diagram of the pilot plant set up is shown in Figure 1. In the flow sheet coated heat exchanger is named with the letter A (HX A), while uncoated heat exchanger is named B (HX B).

Oldani et al. / Use of perfluoropolyether coatings to mitigate fouling on …

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Heat Exchanger Fouling and Cleaning – 2013

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RESULTS AN

1 Characterizmple

ontact angle apical images oated stainless sults of contatermination, pated samples,

g. 2 Liquid coated stainlerface (b).

ble 2. Surfacmple by watee energy deter

Coating rmulation typncoated sampl

A B C D E F

om Table 2 is e surface ener

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Samples E aater formulatiours at a temtween the twoeviously cleanrface of sampl

ND DISCUSS

zation of fluo

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act angle anaperformed on are reported.

droplet of ss steel surfac

e characterizaer contact angrmination.

pe Water c

angle (θle 76 ±

132 ±117 ±128 ±114 ±135 ±138 ±

possible to obrgy for fluorinmple. Coating

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minutes imme

and F were coon, immergingmperature of o samples is ned before tle F was not c

SION

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contact degree)

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bserve a consnated coated sg formulationshich 2-propanwater was usedformulation an

different α-dure for this srsion and 20

oated by using for 5 hours a130°C. The that surface o

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In Figure on uncoated d. In Table 2e surface ene

pe of fluorina

er deposited ted stainless s

ed stainless sments and surf

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istent decreassamples relates named A annol was usedd as solvent. nd B and D is-ω terminatiosamples were

hours drying

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oropolymers ted that in

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ating nature arformed in ord fluoropolymults of these

gh content of frcentage 41,6alysis, confireater than 50 Åe results obt

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no significant ce free energible to suppoemicals or m

e obtainment obtained by PF

lower surfacemulations, duin the organicreasing ims possible to osurface energiE formulations

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F = 41,6 % at.

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whole range oSi = 3,6 % at;

in contact anhis two sampcific step of pquipment, is ic surfaces. ion A and C coating obtaisolubility of

vertheless it wme and dryater contact ancoatings obtaiand F).

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s is possibleon stainless s

each other maiable and resis

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are high The C-C tion and

e to steel inly tant

at;

Oldani et al. / Use of perfluoropolyether coatings to mitigate fouling on …

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Fig= 2287(CF(CF

FigA =(C- 2.2 Tochethediproovalforsta coasamNasoltemtemweTab

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2 Coating che

assess the stemical interace stainless stpped into THFom temperatulues have beermation of cheinless steel su

Preliminaryatings have bmples in MiaOH solution lution (300 mperature or mperatures; saeek, maintainble 3 some res

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tability of thections betweenteel surfaces,F and aceton

ure no significen detected foemical bondinurface. y investigatiobeen performlan municipa(pH 9), HClppm). Testsby using

amples were kning continuosults are prese

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81 eV (C=O)(CF2/CF3); G

spectrum in th1,21 eV (C=O

ance

e coating and n the function, coated sam

ne. After 1 hocant changes

or both solvenng between th

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kept immersedously stirred ented.

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; E = 290,30G = 293,52

he O bonds zoO); C = 534,07

the formationalized PFPEs

mples have bour immersionin contact an

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bath at higd for a period

the solution.

e: A C =

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one: 7 eV

n of and

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Conditions

THF Acetone

Municipal wate50°C

Municipal wate80°C

eawater - 50°HCl solution -

50°C HCl solution -

80°C NHCl2 - 40°C

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NaOH solution80°C

e results showution and aci

mperatures. Smersion in synunder 50°C. A

the PFPE cem to deteriora

Heat trantermination

at exchanger ti-fouling properative condit

minar flows anes at first the

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at exchangersculated. Suppating, at nanoriation to floults obtained ta presented erating day un

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ntil the 172nd. the overall

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6 ± 3.7 1 ± 8.8

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at transfer efficheat transfer

he presence othickness, doet is possible and uncoated k are collecte

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to water, chloreratures and hhobic also as the temperatmore aggress

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t for determinoating developre kept mild, ences, in ordec layer to redes. ciency of the r coefficient wof the fluorinaes not cause to compare

heat exchanged from the

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Heat Exchanger Fouling and Cleaning – 2013

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Fighea Thincrescoe

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Figtran

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g. 6 Overall hat exchanger;

e decrease of crease of thsistance depenefficient and o

In Figure efficient valuat these valueschangers. Onsequently driation with riation.

g. 7 Comparinsfer coeffic

changers: coated heat exfor uncoated h

heat transfer c Uncoated h

f U values withe thermal nds on shell son fouling coe7 shell side es are reports are almost tverall therm

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able to influeers are the fougeometry and ratures and veing resistancehis parametervariable is thesurface of on

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changer after reater quantityby the coated

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me: Coated

ence fouling rulant concentrad the operatingelocities. For te values betwrs were kept sie presence of ne of them. tion with timeobserve differat exchangers.ime is compaicient. Foulingculated for ththan the ones eover, Rf valuring the 60th anger at day 1he hydrophobnsfer surfacesthe effect of

he quantity ofwo heat excha

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rate and behavation in operatg properties, sthe obtainmen

ween the two himilar for both

f the hydropho

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FigCo

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Fig

inc

exc Afttubexcwecom depcoa

g. 9 Absolute voated heat exch

A comparisfouling resistaFigure 10.

ported the foucoated heat elculated for tcrease in time r the uncoated0 days.

g. 10 Average

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changer; U

fter 5 months bes samples, cchangers, havere performedmposition of t

Images in posits on bothated and unco

value of the hhanger; Un

on between thance of the twFor each peuling resistan

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operation, pilcovered by a ve been collecd on this sampthe deposits. Fig. 11 show

h the internal toated heat exc

heat transferredcoated heat ex

he progressivewo heat exchaneriod of worknce average vabout the dou

heat exchangeg resistance seger than the c

istance value

the pilot plant

exchanger.

lot plant was sfoulant layer,cted. SEM anples in order

w the presentube surfaces,changer. EDX

d vs time: xchanger.

e increase in tngers is presenk consideredvalue. Rf for uble of the vaer, moreover eems to be higcoated one wi

s determined

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Heat Exchanger Fouling and Cleaning – 2013

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1. Low surface energy was evaluated for stainless steel samples coated with the PFPE derivatives (3.3-11.3 mN/m). Hydrophobicity was attested by water contact angle analysis; depending on the type of PFPE formulation used and coating deposition procedure, it possible to obtain high contact angle values, from 100° up to 140°;

2. Immersion in THF or acetone didn’t affect the stability and the hydrophobicity of the coatings, consequently, the formation of chemical bonds between the PFPE functional group and the chemical sites present on the stainless steel surfaces was assumed. It is supposed the formation of a nanometer level thickness coating.

3. Coating chemical resistance has been studied. Tests performed for 1 week immersion in different chemical and aggressive environments highlighted good resistance of the coatings.

4. The effective anti-fouling efficiency of the hydrophobic coating has been tested by a pilot plant constituted of two shell and tube heat exchangers, tubes of one of them were coated: during 5 months experimentation it was possible to observe lower heat transfer resistance due to fouling for the coated heat exchanger compared to the uncoated one (0.0024 vs 0.0066 h m2 °C / kcal). The fluorinated coating actually limited the scale formation on the heat transfer surfaces.

5. Coating influence on flow regime and on heat transfer is under investigation; resistance to abrasion and turbulent flow effects will be considered in future work. .

NOMENCLATURE A Heat transfer surface area, 2DL, m2

cp Specific heat of fluid, J/kg K D Diameter, m Gz Graetz number, Re Pr D/L, dimensionless h Heat transfer coefficient, W/m2 K k Liquid conductivity, W/m K L Length, m lb Baffle spacing, m Nu Nusselt number, hD/k, dimensionless Pr Prandlt number, cp μb /k, dimensionless q Heat quantity, J/h ReG Reynolds number defined by Gnielinsky, W L/ εF D lb μb, dimensionless T Temperature, K U Overall heat transfer coefficient, W/m2 K W Total mass flowrate, kg/h εF Shell void fraction, m θ Contact angle ° μ Fluid viscosity, Pa·s

Subscript b Bulk C Cold f Fouling H Hot in Inner mlg Logarithmic mean out Outer w Wall REFERENCES Bott T.R., 1995, Fouling of heat exchangers, ISBN 0444821864, Elsevier Science & Technology Book. Costa A.L.H., Queiroz E.M., 2008, Design optimization of shell ant tubes heat exchangers, Applied Thermal Engineering,Vol. 28, pp. 1798-1805. Fabbri P., Messori M., Pilati F., Taurino R., 2007, Hydrophobic and oleophobic coatings based on perfluoropolyethers/silica hybrids by the sol-gel method, Advances in Polymer Technology, Vol. 25, No. 3, pp. 182-190. Förster M. Bohnet M., 1999, Influence of the interfacial free energy crystal/heat transfer surface on the induction period during fouling, International Journal of Thermal Sciences, Vol. 38, pp. 944-954 Malayeri M.R., Al-Janabi A., Müller-Steinhagen H., 2009, Application of nano-modified surfaces for fouling mitigation, International Journal of Energy Research, Vol. 33, pp.1101-1113. Martinelli E., Agostini S., Galli G., Chiellini E., Glisenti A., Pettit M.E., Callow M.E., Callow J.A., Graf K. And Bartels F.W., 2008, Nanostructured films of amphiphilic fluorinated block copolymers for fouling release application, Langmuir, Vol. 24, pp. 13138-13147. Müller-Steinhagen H., Malayeri M.R. and Watkinson P., 2011, Heat exchanger fouling: mitigation and cleaning strategies, Heat Transfer Engineering, Vol. 32(3-4), pp. 189-196. Rosmaninho R., Santos O., Nylander T., Paulsson M., Beuf M., Benezech T., Yantsios S., Andristos N., Karabelas A., Rizzo G., Müller-Steinhagen H. and Melo L.F., 2007, Modified stainless steel surfaces targeted to reduce fouling – Evaluation of fouling by milk components, J. of Food Engineering, Vol. 80, pp. 1176-1187. Tonelli C., Gavezotti P., Strepparola E., 1999, Linear perfluoropolyethers difunctional oligomers: chemistry, properties and applications, Journal of fluorine chemistry, Vol. 95, pp.51-70. Zhang H., Lamb R., Lewis J., 2005, Engineering nanoscale roughness on hydrophobic surface-preliminary assessment of fouling behavior, Science and Technology of advanced materials, Vol. 6, pp. 236-239. Zhao Q., Liu Y., Wang C., Wang S. and Müller-Steinhagen H., 2005, Effect of surface free energy on the adhesion of biofouling and crystalline fouling, Chemical Engineering Science, Vol. 60, pp. 4858-4865.

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