Calcium Phosphate Scale Formation From Simulated Milk Ultrafiltrate Solutions

8/10/2019 Calcium Phosphate Scale Formation From Simulated Milk Ultrafiltrate Solutions

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09603085/02/$23.50+0.00# Institution of Chemical Engineers

www.ingentaselect.com=titles=09603085.htm Trans IChemE, Vol 80, Part C, December 2002

CALCIUM PHOSPHATE SCALE FORMATION FROMSIMULATED MILK ULTRAFILTRATE SOLUTIONS

N. ANDRITSOS, S. G. YIANTSIOS and A. J. KARABELASChemical Process Engineering Research Institute, and Department of Chemical Engineering,

Aristotle University of Thessaloniki, Thermi-Thessaloniki, Greece

C alcium phosphate deposit formation caused by the ow of SMUF (simulated milkultra ltrate) solutions was studied under well-de ned hydrodynamic conditions at 60and 70 C in the pH range 5.87.0. At 60C maximum deposit formation of mostlyamorphous calcium phosphate occurs in the pH range of 6.26.4, and at a somewhat lower pHat 70 C. Above pH 6.3 bulk precipitation occurs, depleting the solution of scale-forming ionsand resulting in reduced deposition rates. At the maximum deposition rate, an initially thin andwell-adhering layer is formed, entirely covering the substrate within minutes from the onset of

ow. Further growth occurs mostly in the form of overgrowths consisting of clusters of submicron particles. By employing two different pipe diameters, a range of Reynolds numbersfrom 7000 to 17,000 was covered. With increasing ow velocity the deposition rate tends toincrease, while more compact deposit layers are obtained. Preliminary experiments provide noclear evidence that the deposited mass is affected by the surface treatment or the type of substrate material; however, reduced adhesion to the modi ed surfaces is likely.

Keywords: milk; model uid; fouling; calcium phosphate; pipe ow.

INTRODUCTION

Fouling of heat exchangers (HE) during milk processing is amajor problem in the dairy industry, with a negative impacton operating costs and product quality14 . Fouling leads to arise in pressure drop across the HE (with reduction of the

ow rate) and to possible deterioration of product qualitydue to failure of the process uid to reach the requiredtemperature. Another serious problem associated with foul-ing is cleaning the fouled surfaces by means of costly andtime-consuming techniques where environmentally offen-sive chemicals are employed2 . Despite the large number of investigations aiming at elucidating the fouling process andthe success in understanding several aspects of the process,a real breakthrough in controlling fouling has not beenreached, mainly due to the great complexity of the dairysystem3,5 .

Burton1 and other investigators recognize two distincttypes, A and B, of milk deposits. Type A is soft, voluminousand protein-rich material starting to form at temperaturesexceeding 70C, reaching a maximum rate at about 100C.Type B, the high-temperature deposits, have a hard, compactand cracked structure, consisting mainly of minerals.Calcium phosphates constitute a signi cant part of themilk deposits (up to 80% in type B) and their percentage

tends to increase with temperature. The Ca=P systemequilibrium is quite complex and as a result various calcium

phosphate phases are encountered in the milk deposits,depending upon the bulk temperature, level of supersaturation, pH, ionic environment etc. From analysisof milk deposits calcium phosphate appears in the following

phases: amorphous calcium phosphate [ACP, stoichiometrycorresponding to Ca3(PO4)2 xH2O], dicalcium phosphatedihydrate (DCPD, CaHPO42H2O), octacalcium phosphate[OCP, Ca8H2(PO4)65H2O], and hydroxyapatite [HAP,Ca5(PO4)3OH], the least soluble phase27 . Whitlokite orb-TCP [b-Ca3(PO4)2] is also a probable calcium phosphatephase reported in the literature6. However, this is a hightemperature calcium phosphate phase, which reportedlydoes not form at temperatures below 100C. It is generallyagreed that the formation of HAP from a highly super-saturated solution at neutral pH is usually preceded by ACPor other precursor phases.

As has been pointed out, fouling in milk processing is acomplicated process because of the phenomena associatedwith protein aggregation and deposition as well as mineraldeposition. In particular, calcium phosphate may precipitatedirectly on the metallic wall or can be deposited in the formof particles precipitated in the bulk or associated with milkproteins5 . Nucleation of the calcium phosphate system canoccur both at the wall surface of the heat exchanger and oncasein centres in the bulk7. Barton et al. 8 suggested thatcalcium phosphate deposition occurred by precipitationfouling. Several investigators have used model uids tobetter understand the effect of solids deposition on milkfouling. Simulated Milk UltraFiltrate (SMUF), prepared

according to Jenness and Koops9

, has been probably themost commonly used model uid5,10 .The scope of this work, which is part of the European

research project MODSTEEL, is to contribute to furtherunderstanding of the fouling mechanisms upon milk heatingby conducting experiments of calcium phosphate deposition

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from model uids excluding the presence of proteins.Calcium phosphate deposition is studied under controlledhydrodynamicconditions at temperatures of 60 and 70C bythe once-through ow of SMUF solutions. A summary of the MODSTEEL project and of milk fouling in generalcan be found in Visser11 . Stainless steel AISI 316L is thestandard material of construction of piping and equipment inthe dairy industry. There are numerous reports in the recentliterature that coated or modi ed steel surfaces can helpmitigate the fouling problem in the process industries12,13 .Thus, another objective of this work is to examine theeffectiveness of surface modi cation or coatings in eliminat-ing or reducing scale formation by using SMUF solutionsand the present experimental set-up.

EXPERIMENTAL

Detailsconcerningthe experimental set-up, the tubular testsection and the specimen (semi-annular coupons) on which

depositsdevelopare given in Andritsosand Karabelas14

. Thesemi-annular coupons are mounted in pairs to give a tubulartestsurface arrangement:the tubular testsectionallows one tostudy the fouling process in a well-characterized hydrody-namic system. A schematic of the set-up is presented inFigure 1. In order to assess the effect of ow velocity, a newtubular test section was installed (in addition to that alreadyavailable,of i.d.13 mm)comprisingof semi-annularcouponsforming an i.d. of 7 mm. These coupons were machined outof AISI 316L. Some PTFE semi-annular coupons were alsoused. Evaluation of the modi ed surfaces was carried out inanother test section placed at the downstream end, having a10 10mm2 free cross-sectional area. Modi ed and unmodi-

ed surfaces of dimension 10 40mm2 were glued onspecial trapezium-shapedstainlesssteel couponsand insertedinto the test section forming a channel of square cross-section. The unmodi ed at coupons were of stainless steelAISI 316L with 2R surface nish, provided by UGINE,France. The following modi ed surfaces were tested:

diamond-like carbon (DLC) coated, Ni-P-PTFE coated,SiF3

ion implanted and silica coated. The rst threecoupon types were provided by University of Stuttgart,while the last one was coated in this laboratory. Allcoupons were cleaned before use following a standardcleaning procedure. All at surfaces were characterizedby measuring contact angle (in a KRU SS G10 contactangle measuring system) and roughness characteristics( Ra parameter) with a DEKTAK3 ST pro ler.

The SMUF solutions were prepared with deionized waterand appropriate amounts of the reagents as suggested byJenness and Koops9 . The nal reagent concentrations aresummarized in Table 1. All reagents were of analyticalgrade. The resulting SMUF solution had a pH close to6.1. For runs at different pH, adjustment was made byaddition of KOH or HCl in reagent solutions 2 and 3,respectively. The water was rst passed through a lter forremoval of any particulates and then through an electricheater. The rating of the heater determined essentially themaximum operating ow rate at 70C, i.e. 25ml s 1.

The bulk temperature was measured with a K-type thermo-couple located upstream of the test section and control-led within 0.5C of the desired value. The pH wascontrolled within 0.05 units of the desired value. ThepH electrode was standardized before each experiment (andoccasionally during the experiment) with standard buffersolutions at pH 7.0 and 4.0 at 25 or 60C. The three reagentsolutions were kept under continuous stirring and wereintroduced into the test section via FMI metering pumps.Cleaning of the test section was carried out by circulating anacid solution of HNO3 and rinsing with deionized water.

The deposits, after drying at room temperature, werecharacterized by powder X-ray diffraction (XRD, SiemensD500), scanning electron microscopy (SEM, JEOL 6300),energy dispersion X-ray system (EDXS, OXFORD ISIS300) and inductively coupled plasma spectroscopy (ICP,Perkin-Elmer Plasma 40). SEM observations were madeon the modi ed surfaces and on small 10 10mm2 couponsafter gold sputtering. The speci c surface area of thedeposits was determined by the BET method (Quanta-chrome). To check the onset and the extent of bulk preci-pitation, numerous samples were withdrawn during each runand the solution absorbance was measured at 432nm in aspectrophotometer (UNICAM Helios a).

Figure 1 . Schematic diagram of the experimental set-up.

Table 1. Composition of SMUF solutions.

ReagentSolution

concentration (mM)

Solution 1KH2PO4 11.61K3 citrateH2Oa 3.70Na3 citrate2H2O 6.09K2SO4 1.03Solution 2K2CO3 2.17KCl 8.05Solution 3CaCl22H2O 8.98MgCl26H2O 3.21

aCitrate C6H8O73 .

224 ANDRITSOS et al.

Trans IChemE, Vol 80, Part C, December 2002


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RESULTS AND DISCUSSION

Experiments were conducted at two liquid temperatures,60 and 70 C; two ow velocities were examined simulta-neously at each temperature. The pH was one of the mainparameters investigated in this work and was varied between5.8 and 7.0. In this pH range the solution was supersaturatedwith respect to almost all possible calcium phosphate

crystalline phases. The supersaturation ratio of each phos-phate phase was determined by

S IAPK sp

1

where IAP is the ionic activity product of the phaseconsidered and K sp the respective thermodynamic solubilityproduct. The solution speciation and the supersaturationratios with respect to several calcium phosphate phaseswere computed by the MINEQL computer code15 . Figure 2illustrates the supersaturation ratio of three calcium phos-phate phases and of a magnesium one in the pH range of interest. Although the SMUF solution was supersaturatedwith respect to magnesium phosphate, this mineral was notidenti ed in the deposits by XRD and the magnesiumcontent of the deposits was always less than 1% w=w.

Formation of Deposits

The deposited mass tends to increase with time, asillustrated in Figure 3 for two typical runs at 60 and 70C.The same gure also provides the effect of ow velocity onthe deposited mass. In the great majority of runs an apparentinduction period was identi ed by the delayed appearance of measurable deposited mass, followed by an increasingdeposition rate, which seemed to become constant at longertimes. The macroscopicallyobserved inductionperiod was inthe range of 520min. Because of the moderately variabledeposition rate, for the purpose of this work comparison of deposited mass at different conditionswas made at a runningtime of 60 min.

Effect of pH and Temperature

At 60 C and at pH lower than 6.0 no deposits wereobserved for a running time less than 2 h. Above this pH

value, deposits started to form and the deposition rateincreased sharply with a slight increase in pH. Maximumdeposition rate occurred in the pH range of 6.256.35,where the solution started to become turbid. Above thispH range the deposited mass decreased, since bulk precipi-tation (evidenced from increased solution turbidity) relievedsupersaturation and tended to reduce the driving force forwall precipitation. Figure 4 illustrates this deposition behav-iour at 60C, depicting the mass deposited at 60 min on the13 mm i.d. semi-annular coupons as a function of pH; thecorresponding initial values of light absorbance were alsoplotted. The size and shape of bulk precipitated particlesalso depended on pH. At the pH values corresponding tolow absorbance, the particles were relatively large with agrain-like appearance, but their characteristic size tended tobe reduced to less than 50 nm at higher pH, as shown inFigure 5. EDXS showed that these particles consisted of Ca,

Figure 2 . Supersaturation ratios of phosphate minerals in the pH range of interest.

Figure 3 . Temporal evolution of the deposited mass at two ow velocitiesand two temperatures. Circles: pH 6.55, ow rate 42 cm3 s 1. Squares:pH 6.15. Flow rate 28cm3 s 1.

Figure 4 . The deposited mass at 60min run time vs pH, and thecorresponding change in initial light absorbance. T 60C, V 0.32ms 1, i.d. 13mm ( Re 8800).


CALCIUM PHOSPHATE SCALE FORMATION FROM SIMULATED MILK ULTRAFILTRATE SOLUTIONS 225


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P and O, while XRD analysis indicated an amorphous phaseand poorly crystallized HAP.

At 70

C the deposition behaviour followed that obtainedat 60 C, with the maximum being shifted to lower pH by0.10.2 pH units. As expected, the deposited mass increasedwith increasing temperature due to enhancement of drivingforce and mass transfer. The maximum deposition rate at70C was about 1.3 times higher than that at 60C. It isfurther noted that, working at only two temperatures, theactivation energy of the process could not be estimated.

Morphology and Characterization of Deposits

At the maximum deposition rate, the sequence of depositformation shown in the micrographs of Figure 6 proceededas follows: initially, a thin (less than 1mm) and well-adhering layer was formed very quickly (i.e. in less than5 min) covering almost entirely the substrate [Figure 6(a)].Further growth occurred mostly in the form of overgrowths[Figure 6(b)], developing rather randomly on the coveredsubstrate. It is noted that these two deposit formation stagesresembled those reported on the deposition of calciumphosphate colloids on a rotating disk11 . The overgrowthsdid not appear to be compact and consisted of submicronspheroids of sizes less than 100 nm, as shown in Figure 6(c),forming tightly packed clusters. The small size of theparticles comprising the deposits was also re ected in the

large values of speci c area of the deposits (> 80 m2

g 1

).This type of deposit on the stainless steel coupons was notremoved by simple water rinsing, but they could be removedrather easily by mechanical means when the deposits weredry, leaving the adhering initial layer on the substrate. Whenthese overgrowths exceeded a certain surface density andmass, i.e. at relatively long times, they tended to form largedeposit ridges, which appeared to be normal to the owdirection [Figures 6(e) and (f)] with heights up to 200mm.These ridges, appearing mainly in the lower part of the pipe,did not usually cover the whole circumference of the 13 mmtube for runs up to 2 h. However, they did so in the smaller

diameter section. Very often, the initial layer exhibitedmicroscopic cracks, which seemed to increase at higherpH and upon electron beam irradiation. It is likely thatthese cracks developed during the drying process.

EDXS and ICP analysis show that the major constituentsof the depositswere P, O and Ca, with small quantities of Mg,

Cl and K. ICP analysis of deposits at 60C gave a molecularratio Ca=P about 1.5, typical of ACP. This stoichiometric

molar ratio may also be attributed to the formation of precursor phases (DCPD and=or OCP), which rapidly hydro-lyse to the thermodynamically more stable HAP. The XRDpatterns of deposits formed at 60 and 70C are illustratedin Figure 7. The main HAP peaks are also shown in the

gure. At 60C an apparent lack of crystalline order isevident, although a broad peak at about 32 can be observed.This peak is intensi ed at 70C, where poorly crystallizedHAP can be identi ed. Visser and Jeurnink4 also repor-ted that, upon heating of SMUF solution at 60C in the pHrange 67, HAP is formed, preceded by a precursor phase,which at pH 6.7 is OCP.

Figure 5 . Size and shape of bulk precipitated calcium phosphate particles at 60C collected on 0.2mm polycarbonate membrane lters. Left: pH 6.25;right: pH 6.4.

Figure 6 . SEM micrographs of the mostly amorphous-like deposits formedat pH 6.3 and 60C ( ow velocity 0.32ms 1) at various run timesindicated on the pictures. Micrographs (b), (d) and (f) are viewed at an angleof 60 . Micrograph (c) presents a detail of the deposit overgrowths.




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At pH values lower than that corresponding to themaximum deposition rate, only parts of the substrate werecovered by an amorphous-like adhering deposit, as illu-strated in the micrographs (a) and (b) of Figure 8. On theother hand, at higher pH values the initial layer had apowdery appearance, did not cover the substrate entirelyand did not seem to adhere rmly to the substrate[Figure 8(c) and (d)].

On the Mechanisms of Scale Formation

The above microscopic observations, coupled withdeposition and turbidity measurements, suggest that calciumphosphate scale formation from SMUF solutions essentiallyoccurs in a relatively narrow pH range and that at low pHvalues the deposits result from direct wall crystallization. AspH and supersaturation ratio increase, bulk precipitation

starts to occur almost instantly upon mixing of the reagentsolutions. Bulk precipitation consumes a portion of scale-forming ions, thus reducing the driving force for crystal-lization on the tube wall. The calcium phosphate particlesand aggregates can be transported and deposited onto thesubstrate or on the deposit layer, but at rates considerablylower than those of individual ions (on the basis of the samemass concentration). This is due to much lower diffusivities(and mass transfer coef cients) of the colloidal particlescompared with those of ions. The particulatedeposits adhereless strongly on the pipe surface that the deposits crystal-lizing directly on the wall, as observed experimentally.Similar deposition behaviour has been also observed withother scaling systems14 .

Apart from the reduced rate of particulate deposition,another aspect that may be of interest is whether particlesformed by bulk precipitation experience any signi cantelectrostatic forces that could prevent them from depositingon the developing deposit layer. To address this question,z-potential measurements were performed by employing the

cylindrical cell of a Rank Brothers Mark II electrophoresisapparatus. Particles taken from deposits formed at pH 6.7were dispersed in a 0.1 N KCl solution (close to the ionicstrength of the SMUF solution), with the pH adjusted to 6.7.At these conditions the particle z-potential was found to beapproximately 15 mV. A simple estimate of the potentialof interactionF between two particles may be obtained fromthe relation

F AHa12h

2pee0c2a ln 1 e kh 2

where AH is the Hamaker constant, a the particle radius, h

the particle separation distance, e the water dielectricconstant,e0 the vacuum dielectric permittivity,c the particlesurface potential and k the inverse Debye length. At theparticular ionic strength employed, the Debye length is1 nm. Assuming a typical value of 10 20 J for AH andusing the determined z-potential for c , the potential of interaction is found to be purely attractive over all separationdistances, as shown in Figure 9. Hence, electrostatic forcesare not expected to play any signi cant role in particledeposition. This is mainly due to the relatively high ionicstrength, which effectively screens such forces between

owing and deposited particles.

Effect of Flow Velocity

Flow velocity is one of the most important parameters infouling. It was recognized very early that velocity affects

Figure 7 . XRD patterns of deposits formed from SMUF solutions with andwithout citrates.

Figure 8 . SEM micrographs of deposits formed at 60C at relatively lowand high pH. (a) and (b) pH 6.05, run time 60min; (c) pH 6.7, run time20min; (c) pH 6.7, run time 83min. Figure 9 . Normalized interaction potential between two particles of size a .




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the fouling process with respect to both deposition andremoval stages and that, in general, high velocities mayreduce deposit formation (due to increased detachment),something that is true especially for particulate fouling.However, in the cases where precipitation fouling occurs,leading to compact and adherent deposit layers (thus mini-mizing the removal process), high ow velocities result inhigher deposition rates16,17 .

In the present experimental system, an increase in the owvelocity tends to signi cantly increase the amount of depos-ited mass, at least for the duration of these experiments,which is less than 2 h, as shown in Figure 3. Although testingof ow velocity effects is limited to only two values, it maybe deduced that the depositedmass depends on ow velocity,as V n . The exponent n is approximately 0.7 0.05 (aftercorrecting for the different pipe diameter), close to thattheoretically expected for a mass transfer-controlled process,i.e. 0.87516 . Certainly, more data are needed to elucidate theeffect of ow velocity Furthermore, it is pointed out thatthe in uence of velocity is also pronounced regarding the

morphology of deposits. At the higher velocity, the depositsare more compact (i.e. they can be removed with muchgreater dif culty), the deposit ridges are shorter (or thesurface becomes smoother) covering almost the whole pipecircumference and leaning in the downstream ow direc-tion. A video image of such deposits appears in Figure 10.The smoother appearance of the deposits with increasingvelocity has also been observed in other scaling systems,such as CaCO317,18 .

Assessment of Modi ed Surfaces

Preliminary experimental results on the in uence of thesubstrate (shown in Figure 11) indicate that the substratematerial and the modi ed surfaces do not exhibit anydiscernible difference with respect to deposited mass anddeposit morphology, suggestingthat the interfacial properties

do not control the scale formation process. This behaviour

may be explained by considering the aforementioned forma-tion of the initial compact and adhering layer due to wallcrystallization. Once this layer forms, the nature of thesubstrate is unlikely to affect the subsequent depositformation. The measured roughness average, Ra, of allsurfaces tested is relatively low and is also given in Figure 11.

In the present experiments, smaller amounts of depositswere systematically obtained on the top coupon, regardlessof the substrate material or surface modi cation. The onlydifference that can be observed among the different couponsis that the probability of having cracks (after drying) in theinitial coherent layer was greater in most of the modi edsurfaces than in unmodi ed ones (see micrographs inFigure 12). It is also of interest, that, although the mass of deposits on PTFE semi-annular coupons was similar to thaton the stainless steel ones, the initial layer on the formercoupons could be easily removed by mechanical means.

Figure 10 . Video image of the morphologyof deposits in the 7 mm i.d. pipesection. Conditions: T 60C; running time 100min; pH 6.3; Re 16,000 (V 1.10ms 1).

Figure 11 . Mass of deposits on modi ed and unmodi ed surfaces at various conditions. Each number corresponds to the position of the coupon in the tsection ( ow velocity 0.32m s 1).




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This might be interpreted as an indication of reducedstrength of adhesion of scale onto the non-metallic substrate.Several investigations have also found that the substratematerial becomes unimportant after the formation of the initial layer1921 . Britten et al. 19 found that non-metallicsurfaces did not affect the amount of deposits upon milkheating at 100C, but the material affected the strength of adhesion. Yoon and Lund20 reported no effect of surfacematerials (electropolished stainless steel, 304 stainless steel,titanium, polysiloxane and Te on) on calcium phosphatefouling. More work is certainly needed before a nalassessment on the ability of modi ed surfaces to reducefouling is made.

Contact angle measurements with three different liquids(water, a-bromonaphtaleneand glycerol) were used to obtainthe surface energy parameters of the modi ed surfaces22 ,namely the apolar Lifshitzvan der Waals component (gLW )and the acidbase polar component (gAB ). The latter consists

of the electron donor (g

) and the electron acceptor compo-nent (g ). Comparison of the different surfaces suggests thatthe apolar component is approximately the same for allsurfaces and close to 40 mJm 2 and the electron acceptorcomponent is close to zero for all surfaces. The electrondonor component is the one which differentiates the surfacesinto low-energy ( 0mJm 2) for the NiPPTFE coated,medium-energy (1530 mJ m 2) for the DLC coated andhigh-energy (4050 mJ m 2) for the ion-implanted andsilica-coated surfaces.

In uence of Citrates

It is well known that citrates present in the milk act ascrystal growth inhibitors of the phosphate minerals, throughtheir complexation to calcium ions5,23 and, possibly, throughother mechanisms (threshold and=or dispersing effects). It isalso likely that other organic substances in small orlarge quantities (such as casein) play an inhibitory roleto stabilize various colloidal and dissolved species. Thisis probably the reason why SMUF solutions are less stablethan milk and whey upon heating5,23 .

Runs with SMUF solutions in the absence of citrate ionslead to some interesting ndings. First, the maximum

deposition rate is shifted towards a signi cantly lower pHvalue. Second, the deposited mass is signi cantly higherthan that obtained in the presence of citrates, as shown inFigure 13. Third, the deposits after the coupon removal hada gel-like appearance. XRD and SEM results show that thephase composition and the morphology of these deposits

are different, as illustrated in Figures 7 and 14, where plate-like DCPD and OCP can be identi ed along with HAP.Citrates, at much lower concentrations, have been reportedto inhibit crystal growth and induce habit modi cations of calcium phosphate precipitating in supersaturated solu-tions24 . It is stated that these phenomena were caused bysurface absorption of negatively charged ions.

CONCLUDING REMARKS

Experiments have been carried out to investigate themechanism of calcium phosphate deposition from SMUFsolutions at 60 and 70C, by using tubular test sections.At both temperatures the deposition rate vs pH exhibited abell shape, with the maximum corresponding to the onsetof system bulk precipitation. Under the present experimentalconditions it seems that mass transport plays an importantrole in the scale formation and may control the wholeprocess. An increase in ow velocity leads to more compactand smoother deposits. Our preliminary experiments

suggests that surface modi cation does not result in notice-able deposit reduction, but some indications exist thatadhesion strength on modi ed surfaces might be smaller,possibly facilitating clean-up. However, more experimentsare required to investigate scaling characteristics of modi edsurfaces and obtain more de nitive results.

Figure 12 . SEM micrographs of the deposits on different substrates from the same run (T 60C, pH 6.25, run time 1 h, Re 8800).

Figure 13 . Comparison of the maximum deposited mass from SMUFsolutions in the presence and absence of citrates (T 60C, V 0.32ms 1, Re 8800).




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Finally, it is suggested that, since fouling and cleaning

processes are interrelated and can be tackled by employingthe same physicochemicalconsiderations, the present experi-mental set-up and procedures can also be used to assess themechanisms and ef ciency of common cleaning techniques.

REFERENCES

1. Burton, H., 1988, Ultra-high Temperature Processing of Milk and Milk Products (Elsevier, London, UK).

2. Rene, G. and Lalande, H., 1990, Int Chem Engng , 30: 643653.3. Visser, J. and Jeurnink, Th. J. M., 1997, Exp Thermal Fluid Sci , 14:

407424.4. Changani, S. D., Belmar-Beiny, M. T. and Fryer, P. J., 1997, Exp

Thermal Fluid Sci , 14: 392406.

5. Jeurnink, Th. J. M., Walstra, P. and de Gruif, C. G., 1996, Netherlands Milk Dairy J , 50: 407426.6. Burton, H., 1968, J Dairy Res , 35: 317330.7. Dau n, G. and Labbe, J-P., 1998, Equipment fouling in the dairy

application: problem and treatment, in Amjad, Z. (ed). Calcium Phos- phates in Biological and Industrial Systems , 437463 (KluwerAcademic Publishers, Boston, MA, USA).

8. Barton, K. P., Chapman, T. W. and Lund,D., 1985, Biotechnol Progr , 1:3945.

9. Jenness, R. and Koops, J., 1962, Netherlands Milk Dairy J , 16: 153164.

10. Rosmaninho, R., Visser, H. and Melo, L., 2001, In uence of thesurface tension components on fouling caused by calcium phosphate,in Proceedings of Conference on Heat Exchanger Fouling Fundamental Approachesand Technical Solutions , Davos, Switzerland,

813 July.11. Visser, H., 2001, Improvement of construction materials used in thefood industry to lengthen processing timea new European project(MODSTEEL), in Proceedings of Conference on Heat Exchanger FoulingFundamental Approaches and Technical Solutions , Davos,Switzerland, 813 July 2001.

12. Muller-Steinhagen, H. and Zhao, Q., 1997, Chem Engng Sci , 52:33213332.

13. Forster, M. and Bohnet, M., 2000, Int J Thermal Sci , 39: 697708.14. Andritsos, N. and Karabelas, A. J., 1991, J Colloid Interface Sci , 145:

158169.15. Schecher, W. D. and McAvoy, D. C., 1998, MINEQL : A Chemical

Equilibrium Modeling System: Version 4 .0 for Windows Users Manual(Environmental Research Software, Hallowell, ME, USA).

16. Andritsos, N. and Karabelas, A. J., 1993, Crystallization fouling: theeffect of ow velocity on the deposition rate, in Bohnet, M. et al. (eds).Proceedings of EUROTHERM Seminar No. 23 : 2938, (Edit. Eur.Therm. et Industrie, Grenoble).

17. Hasson, D., 1981, Precipitation fouling, in Somerscales. E. F. C. andKnudsen, J. G., (eds). Fouling of Heat Transfer Equipment , 527568(Hemisphere, Washington, DC, USA).

18. Andritsos, N., Kontopoulou, M., Karabelas, A. J. and Koutsoukos, P. G.,1996, Can J Chem Engng , 74: 911919.

19. Britten, M., Green, M. L., Boulet, M. and Paquin,P., 1988, J Dairy Res ,55: 551562.

20. Yoon, J. and Lund, D. B., 1994, J. Food Sci , 59: 964969.21. Dupeyrat, M., Labbe, J-P., Michel,F., Billoudet, F. and Dau n, G., 1987,

Lait , 67: 465486.22. Van Oss, C. J., 1994, Interfacial Forces in Aqueous Media

(Marcel Dekker, New York, USA).23. Schmidt, D. G. and Both, P., 1987, Netherlands Milk Dairy J , 41:

105120.24. Brecevic, L. and Furedi-Milhofer, H., 1979, Calcif Tissue Int , 28:131136.

ACKNOWLEDGMENTS

The paper is dedicated to the memory of Dr H. Visser, the coordinatorand the driving force behind the MODSTEEL project, who will beremembered by the present authors for his contributions in the area of fouling, his enthusiasm for research and above all his kindness. This workhas been nancially supported by the Commission of European Commu-nities under Contract G5RD-CT-1999-00066.The authors wish to thank theAnalytical Laboratory of CPERI for the XRD spectra and the SEM picturesand Professor P.G. Koutsoukos of the University of Patras, Greece, for

helpful suggestions and discussions.

ADDRESS

Correspondence concerning this paper should be addressed toDr N. Andritsos, CPERI=CERTH, PO Box 361, GR 570 01, Thermi-Thessaloniki, Greece.E-mail: [email protected]

The paper was presented at the Fouling, Cleaning and Disinfection inFood Processing conference held at the University of Cambridge, UK, 35 April 2002. The manuscript was received 31 May 2002 and accepted for publication after revision 29 October 2002.

Figure 14 . Left, picture from a stereomicroscope (magni cation 40) and, right, SEM micrograph of deposits formed from SMUF solution without citrates(T 60C, pH 5.7, Re 8800).



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Calcium Phosphate Scale Formation From Simulated Milk Ultrafiltrate Solutions