z. amjad gypsumscaleformationonheat exchangersurfaces

❙ Z. Amjad

Gypsum Scale Formation on HeatExchanger Surfaces: The Influenceof Natural and Synthetic Polyelectrolytes

The precipitation and deposition of calcium sulfate dihydrate(gypsum) on brass heat exchanger surfaces from aqueous solu-tions has been studied using a highly reproducible technique. Ithas been found that gypsum scale formation takes place directlyon the surface of the heat exchanger without any bulk or spon-taneous precipitation in the reaction cell. A variety of natural andsynthetic polyelctrolytes such as fulvic acid, tannic acid, poly-(acrylic acid) and acrylic acid – based copolymers containingsulfonic group(s) have been examined for their effects on therate of scale formation. The results indicate that the amount ofgypsum scale formed on heat exchanger surface is strongly af-fected by polyelectrolyte architecture and concentration of poly-electrolyte. Scanning electron microscopic investigations of gyp-sum crystals grown in the presence of polyelectrolytes (naturaland synthetic) show that structures of these crystals are highlymodified.

Key words: gypsum, polyelectrolytes, crystal morphology, heatexchanger

Gips-Scaling Ablagerung auf Wärmetauscherflächen: DerEinfluss natürlicher und synthetischer Polyelektrolyte. DieAusfällung und Ablagerung von Kalziumsulfatdihydrat (Gips)auf Messingwärmetauscherflächen aus wässerigen Lösungen istmit einer hochreproduzierbaren Technik studiert worden. Es hatsich herausgestellt, dass Gips-Scaling direkt auf der Oberflächedes Wärmeaustauschers stattfindet, ohne dabei größere oderspontane Ausfällungen in der Reaktionszelle zu bilden. EineVielzahl natürlicher und synthetischer Polyelektrolyte wie Fulvin-säure, Gerbsäure, Polyacrylsäure und Acrylsäure, die auf Sulfo-gruppen enthaltende Copolymere basieren, sind für ihre Effekteauf die Scaling-Rate überprüft worden. Die Resultate zeigen,dass die Menge an abgelagertem Gips-Scaling auf Wärme-tauscherflächen stark vom Aufbau und Konzentration des Poly-elektrolyts beeinflusst wird. Untersuchungen mit einem Rastere-lektronenmikroskop von Gipskristallen, die in Gegenwart vonPolyelektrolyten (natürliche und synthetische) gewachsen sindzeigen, dass die Strukturen dieser Kristalle stark modifiziert wer-den.

Stichwörter: Gips, Polyelektrolyte, Kristallmorphologie, Wär-meaustauscher

1 Introduction

The deposition of sparingly soluble salts (i. e., calcium sul-fate, calcium carbonate, calcium phosphate, magnesium hy-droxide, etc.) on heat exchanger surfaces continues to posechallenges from both technical and economical points ofview. In cooling water and reverse osmosis based systems,gypsum (calcium sulfate dihydrate) is the most commonlyencountered calcium sulfate scale whereas calcium sulfate

hemi-hydrate and calcium sulfate anhydride are the mostfrequently formed salts in high temperature processes(boiler, multi-stage distillation) [1–3]. The scale formationmainly depends upon super-saturation of scale forming salt,pH, temperature, and flow velocity. The scale gets depositedon heat exchanger and equipment surfaces and affects heattransfer and material damage especially when coupled withcorrosion.

A number of methods have been proposed to reduce orprevent the precipitation and deposition of scale formingsalts on heat transfer surfaces. These common approachesinclude (a) operating a system where solubility of scaleforming salt is not exceeded, (b) reducing the cation ionconcentrations of scale forming salt by ion exchange, and(c) using scale inhibitors to control the precipitation and de-position of scale forming minerals. However, one of thecommon methods for controlling scale formation involvesadding scale inhibitors to the water.

Over the years, different polyelectrolytes have been devel-oped and are extensively used as precipitation inhibitors forvarious sparingly soluble salts i. e., calcium phosphate, cal-cium carbonate, calcium sulfate, barium sulfate, and cal-cium fluoride. Studies have shown that inhibitor perfor-mance strongly depends on polyelectrolyte architecture,molecular weight, and ionic charge [4–6]. Typically, an effec-tive scale inhibitor (or polyelectrolyte) contains at least oneof the following functional groups: carboxylic acid, –COOH;acrylamide, –CONH2, sulfonic acid, –SO3H; ester, –COOR;phosphonic acid, PO3H2, etc.

The influence of molecular weight of the polyelectrolyteson the precipitation of gypsum from aqueous solutions hasbeen the subject of number of investigations. Flesher et al.[7] in their study using the spontaneous precipitation meth-od for the evaluation of a variety of polyelectrolytes at hightemperature as calcium sulfate inhibitors, showed that theefficacy of polyacrylate decreases with increasing molecularweight in the range 2000 to 750 000. Amjad [8] and Amjadet al. [9] in their studies on the molecular weight of polyacry-lates as precipitation inhibitors for calcium carbonate andcalcium phosphonates, reported that the inhibitors perfor-mance depends upon polymer composition and molecularweight. The rate of precipitation was found to be higher inthe case of high molecular weight polyacrylate (10 000) thanthat obtained in the presence of 2000 molecular weight poly-acrylate.

Natural organic compounds (i. e., humic substances) arecommonly encountered in surface water and ground waterused for water supply. Humic substances are generally con-sidered to be composed of three operationally distinct frac-tions: 1) fulvic acid, which is soluble in both acidic and basicsolutions, 2) humic acid, which is soluble in basic solutions,but insoluble in acidic solutions, and 3) humin, which is in-soluble in both acidic and basic solutions. The bulk of riverwater humic substances generally resemble the more readily

APPLICATION

214 Carl Hanser Publisher, Munich Tenside Surf. Det. 41 (2004) 5

solubilized fulvic acid, with the relative amount of the lesssoluble humic acid probably being dependent on the pH ofthe natural water [10]. In addition, humic substances mostlycontain carboxyl and phenolic functionalities, and can be-have as negatively charged colloids or anionic polyelectro-lytes in natural waters.

Several studies have been reported pertaining to the in-fluence of natural organic compounds on the precipitationof various scale forming salts. Results of these studies revealthat low levels of fulvic acid markedly inhibit the crystalgrowth of calcium carbonate and calcium phosphate salts[11, 12]. To understand humic substance interactions withvarious scale-forming salts, we examined fulvic acid and tan-nic acid growth inhibition of gypsum scale. This study alsopresents results on the effect of synthetic polyelectrolytescontaining different functional groups i. e., carboxyl, sulfo-nic, and sulfonated styrene. In addition, scanning electronmicroscopy and x-ray diffraction methods were used tostudy the morphology of gypsum crystals grown on the heatexchanger surface in the presence and absence of polyelec-trolytes.

2 Experimental

Reagent grade chemicals, grade A glassware, and de-io-nized, CO2-free water were used. Calcium chloride solutionswere standardized using ethylenediamine tetraacetic acid(EDTA) method. Sodium sulfate solutions were standar-dized by ion exchange method. Swannee River fulvic acidused in this investigation was donated by Dr. M. M. Reddy,US Geological Survey, Boulder, CO. Tannic acid used in thepresent study was obtained from Sigma-Aldrich and wasused without any further purification. Synthetic polyelectro-lytes i. e., poly(acrylic acid), copolymers of acrylic acid, 2-ac-rylamido 2-methylpropane sulfonic acid, and sulfonatedstyrene were commercial materials from Noveon, Inc., Cle-veland, Ohio. The stock solutions of all polyelectrolytestested were prepared on as – active solids basis.

Supersaturated solutions of calcium sulfate (34.3 mM)were prepared in a double-walled, water jacketed glass cellof about 950 mL capacity by adding known volume of stocksolution of calcium chloride to sodium sulfate solution.The total volume of supersaturated solution was 800 mL.The brass heat exchanger tubes (40 cm long, 1.0 cm outerdiameter) were used. These tubes were suspended fromthe lid of the glass cell and immersed in the supersaturatedsolution as illustrated in Figure 1. The total surface area incontact with calcium sulfate solution was typically about80 cm2. The new tube was used for each experiment andwas chemically cleaned and rinsed thoroughly with dis-tilled water to avoid any surface imperfections and impuri-ties.

Scale deposition experiments were initiated by immer-sing the metal tube in the calcium sulfate solution. A tem-

Z. Amjad: Gypsum scale formation on heat exchanger surfaces: The influence of natural and synthetic polyelectrolytes

Tenside Surf. Det . 41 (2004) 5 215

Figure 1 Experimental set-up used to study gypsum scale formation on heatexchanger surfaces

Additive Structure Acronym

Poly (acrylic acid) PAA

Poly (acrylic acid : 2-acrylamido-2-methylpropane sulfonic acid)

PSA

Poly (acrylic acid : 2 acrylamido-2-methylpropane sulfonic acid : sulfonated styrene)

PSS

Fulvic Acid phenolic, carboxyl groups FA

Tannic Acid phenolic, carboxyl groups TA

Table 1 Polyelectrolytes tested as gypsum growth inhibitors

perature differential was provided by circulating hot water,maintained at 67 ± 0.5 °C, through the tube, and coldwater, 6.0 ± 0.4 °C through the outside of the glass cell.The experimental set-up is illustrated in Figure 1. Within∼ 5 minutes a steady state temperature was reached andthe bulk solution temperature remained at a constantvalue. To minimize corrosion of the brass heat exchangerduring scale deposition experiment, an azole-based corro-sion inhibitor was used. During the scale deposition expe-riment solution samples were taken from time to time, fil-tered through 0.22 micron filter paper, and soluble calciumwas analyzed by EDTA titration. After the end of experi-ments, the solids formed on the outer surface of the metaltube were collected, dried overnights and were further ana-lyzed for the identification of the mineral phase by powderx-ray diffraction (xrd) and scanning electron microscopy.The names of the polyelectrolytes and their chemical struc-tures are presented in Table 1.

3 Results and Discussion

Solution species concentrations were calculated by usingmass balance, proton dissociation, electroneutrality, andequilibrium constants involving calcium ions with polyele-trolytes, by an iterative procedure described previously [1].The driving force for gypsum scale formation can be ex-pressed in terms of a Gibbs free energy of transfer, given inTable 2, from a supersaturated to saturated solution at themetal surface by equation 1

DG = –RT/2 ln (IP/Ke) (1)

In equation 1, IP is the concentration product of free cal-cium and sulfate ions at time t and Ke is the correspondingsolubility product. The DG values in Table 2 refer to the in-itial values, calculated for temperature of 35 °C.

3.1 Influence of Natural Polyelectrolytes:

Table 2 summarizes the experimental conditions and thegypsum growth data. Typical calcium concentration versustime for gypsum deposition experiments on brass tube inthe absence of polyelectrolyte are shown in Figure 2. It isevident from Figure 2 that following an induction period(∼ 15 min), decrease in calcium concentration with time re-flects gypsum growth on the heated brass surface. The re-producibility of the scale growth experiments is illustratedby the excellent agreement between the results of experi-ments 1 and 2 (Table 2).

Figure 2 also shows the temperature-time profile for gyp-sum growth experiment in the absence of polyelectrolyte. Itis worth noting that as the concentration of calcium de-creases (or the amount of gypsum deposited on heat exchan-ger increases), the calcium sulfate solution temperature inglass cell decreases. This observed decrease in solution tem-perature reflects the thermal loss of heat exchanger. To veri-fy that spontaneous precipitation did not occur during thescale deposition experiment, unfiltered samples were alsoanalyzed for calcium ion and were found to be within± 0.5% of the filtered sample.

It has been previously reported that the influence ofpolymeric and non-polymeric additives as gypsum growth


216 Tenside Surf. Det. 41 (2004) 5

Expt. Polyelectrolyte Acronym Polyelectrolyteconcentration

(ppm)

Mass of gypsumdeposited (g)

1. None – 0.0 1.57

2. None – 0.0 1.62

3. Tannic acid TA 1.0 1.31




7. Fulvic acid FA 1.0 1.20



10 Poly-AA PAA 0.075 1.02

11. P-AA PAA 0.10 0.86

12. P-AA PAA 0.25 0.48

13. P-AA PAA 0.50 0.20

14. P-AA PAA 1.0 0.05

15. P-AA:SA PSA 0.20 0.79

16. P-AA:SA:SS PSS 0.20 0.88

Total calcium = sulfate = 3.45 ×10–2 M, brass heat exchanger surfacearea = 78 cm2

DG = –.10.7 KJ mol–1, PAA: poly(acrylic acid), PSA, copolymer of acrylic acidand 2-acrylamido 2-methylpropane sulfonic acid, PSS, terpolymer of acrylicacid, 2-acrylamido 2-methylpropane sulfonic acid, and sulfonated styrene

Table 2 Results of gypsum growth experiments

Figure 2 Gypsum growth on brass heat exchanger in the absenceof polyelectrolyte

Figure 3 Inhibition of gypsum by tannic acid

inhibitors falls into two categories: those additives that affectthe induction period and those that show no significant ef-fect on the induction period preceding the gypsum crystalgrowth. The calcium-time profiles for the first type were ob-served for the better additive while profiles for the secondtype were obtained for less effective additives. In both casesthe decrease in calcium ion concentration from solutionswith increasing reaction time was found to follow second-order rate law [13]. To accommodate both types of behavior,in the present study we have selected, for comparing poly-electrolyte performance, the amount of calcium remainingin solution after 6 hours of gypsum scale growth on the heatexchanger. When expressed as a fraction of the total calciumion present at the beginning of the scale formation experi-ment, the difference between the initial and 6-hr residualcalcium ion concentrations becomes a measure of theamount of the gypsum scale deposited on the heat exchan-ger. The choice of 6-hr growth time is arbitrary and,although the selection of different growth time would leadto a change of absolute amount of gypsum scale deposited,it would not affect the relative ranking of the polyelectrolyteeffectiveness.

Gypsum growth experiments carried out in the presenceof varying concentrations of tannic acid, TA, are presentedin Figure 3. It is evident from the data presented in Figure 3and Table 2 that TA concentration as low as 1.0 ppm (partsper million) reduces the amount of gypsum grown on heatexchanger by ∼ 25% and at 2.5 ppm, a growth reduction of∼ 50% is observed. In addition, increasing the TA concentra-tion from 5.0 to 10 ppm resulted in > 75% reduction of gyp-sum growth and, at 15 ppm, the gypsum growth is almostcompletely inhibited for at least 6 hr.

Gypsum growth reduction by fulvic acid, FA (Table 2) in-dicates that FA is an effective gypsum growth inhibitor atrelatively low concentrations. For example, mass of gypsumdeposited in the presence of 1.0, 2.5, and 5.0 ppm are 1.2,0.85, and 0.67 g respectively, compared to 1.6 g obtained inthe absence of FA (Table 2). It is interesting to note that theorder of performance (i. e., FA > TA) observed in the presentstudy has also been noted in the precipitation of calcium car-bonate and calcium phosphate [14, 15].

3.2 Influence of Synthetic Polyelctrolytes

The results of gypsum growth experiments in which poly-(acrylic acid), PAA, was used are shown in Table 2. Calciumconcentration–time profiles for experiments conducted inthe presence of varying concentration of PAA are illustratedin Figure 4. In Figure 5, the amount of gypsum deposited in6 hr on the heat exchanger tubes, is plotted against PAAconcentration. As can be seen in Figure 5, the amount ofgypsum deposited strongly depends on PAA concentration.For example, mass of gypsum deposited in the presence of0.10 ppm is 0.8 g compared to 0.2 g obtained in the pre-sence of 0.5 ppm of P-AA concentration. Thus, a five fold in-crease in PAA concentration results in ∼ four fold decreasein gypsum growth.

The influence of polymer composition at a constant con-centration of 0.25 ppm was studied using the brass heat ex-changer surface. Results presented in Table 1 and illustratedin Figure 6 indicate that at constant polymer concentrationthe amount of gypsum deposited is a function of polymercomposition. For example, the mass of gypsum depositedin the presence of 0.25 ppm of PAA is 0.48 g compared to0.75 and 0.88 g obtained in the presence of PSA and PSS,thus suggesting that carboxyl group present in the polymer

plays an important role in preventing the deposition of gyp-sum on heat exchanger. It is interesting to note that in thecase of calcium phosphate and calcium phosphonates preci-pitation, the copolymers i. e., PSA and PSS were found to bemore effective inhibitor than PAA [16, 17].

3.3 Influence of Polyelectrolytes on Gypsum Crystal Morphology

It has been reported that the presence of trace amounts ofscale inhibitors influences not only the growth rate but also



Figure 4 Inhibition of gypsum by poly(acrylic acid)

Figure 5 Gypsum growth in the presence of varying concentrationof poly(acrylic acid)

Figure 6 Gyspum growth in the presence of tannic acid, (2.5 ppm), fulvic acid,(2.5 ppm), PAA (0.2 ppm), and PSA (0.2 ppm)

the morphology of the scale-forming minerals. In somecases, such as calcium oxalate, calcium sulfate, calcium car-bonate, and calcium phosphate, the presence of inhibitorsalso affects the nature of phase that forms. In the presentinvestigation the influence of polyelectrolytes on the mor-phology of growing gypsum crystals was also studied byscanning electron microscopy. The electron micrographs ofgypsum grown at 1 hr and 6 hr are shown in Figure 7 (a)and Figure 7 (b), respectively. It can be seen that at 6 hrgrowth (Figure 7 (b)), not only was the heat exchanger sur-face completely covered but also the crystals were about 3to 4 times larger than those formed at 1 hr (Figure 7 (a)).

The electron micrographs of gypsum crystals grown at6 hr in the presence of TA (2.5 ppm) and PAA (0.15 ppm)are shown in Figure 7 (c) and Figure 7 (d), respectively. Asshown in Figure 7 (c, d), gypsum crystals grown in the pre-sence of both natural and synthetic polyelectrolytes arehighly modified compared with those grown in the absenceof polyelectrolytes.

The XRD spectra of gypsum deposited on the heat ex-changer from the calcium sulfate supersaturated solutionsin the absence and presence of tannic acid, and poly(acrylic


218 Tenside Surf. Det. 41 (2004) 5

a)

b)

c)

d)

Figure 7 Scanning electron micrographs of gypsum grown in the absence ofpolyelectrolyte at 1 hr (a); 6 hr (b); in the presence of 2.5 ppm TA at 6 hr (c);and 0.25 ppm PAA at 6 hr (d)

Figure 8 X-ray diffraction spectra of gypsum grown in the absence of poly-electrolyte (a), in the presence of 2.5 ppm of TA, and in the presence of0.25 ppm of PA

acid) are shown in Figure 8 (a), Figure 8 (b), and Figure 8(c), respectively. For gypsum deposit (control, Figure 8 (a)),the structure is proved to be CaSO4 · 2 H2O. The ‘d’ and ‘H’values of gypsum are in conformity with the reported values.In both the cases with the polyelectrolyte addition the crystalstructure has not been altered (Figure 8 b, c), only the mor-phology is changed and this is confirmed by the variation inthe intensity values and no change in the ‘d’ and ‘H’ valuescompared to that of the control.

References

1. Amjad, Z. and Hooley, J.: J. Colloid Interface Sci. 111 (1986) 496.2. Gill, J. S. and Nancollas, G. H.: Corrosion 37 (1981) 81.3. Amjad, Z.: Desalination 54 (1985) 263.4. Oner, M., Dogan, O. and Oner, G.: J. Crystal Growth 186 (1998) 427.5. Amjad, Z.: Langmuir 7 (1991) 2401.6. Amjad, Z.: Water Treatment 9 (1994) 47.7. Flesher, P., Streatfield, E. L., Pearse, A. S. and Hydes, O . D.: 3RD International

Symp. Fresh Water Sea 1 (1970) 493.8. Amjad, Z. and Hooley, J.: Tenside 31 (1994) 12.9. Masler, W. F. and Amjad, Z.: Corrosion/88 Paper No.11, National Association of

Corrosion Engineers, Houston, TX (1988).10. Reuter, J. H. and Perdue, E. M.: Geochim. Cosmochim. Acta 41 (1977)

325.11. Klepetsanis, P. G., Kladi, A., Ostvold, T., Konotoyiannis, C. G., Koutsoukos, P. G.,

Amjad, Z. and Reddy, M. M.: Chapter No. 9 in Advances in Crystal GrowthInhibition Technologies (Z. Amjad, Ed.), Kluwer Academic Publishers, NY,(2000).

12. Amjad, Z.: Phos. Res. Bull. 8 (1998) 71.13. Liu, S. T. and Nancollas, G. H.: J. Colloid Interface Sci. 52 (1975) 593.14. Amjad, Z. and Reddy, M. M.: Chapter No. 7 in Water Soluble Polymers:

Solution Properties and Applications, Kluwer Academic Publishers, NY,(1998).

15. Amjad, Z.: Phosphorus Research Bulletin 9 (1999) 41.16. Amjad, Z., Pugh, J., Zibrida, J. and Zuhl, R. W.: Materials Performance 36 (1997)

32.17. Amjad, Z.: Tenside 34 (1997) 102.

Received: 20. 07. 2004Revised: 27. 09. 2004

y Correspondence to

Amjad, Z.Performance Coatings GroupNoveon, Inc., 9911 Brecksville RoadBrecksville, Ohio 44141, USA

The author of this paper

Zahid Amjad, received his M.Sc. in Chemistry from Punjab University, Lahore,Pakistan, and his Ph.D. in Chemistry from Glasgow University, Scotland. He is cur-rently a Research Fellow in the Performance Coatings Group of the Noveon, Inc.His areas of research include water soluble/swellable polymers, adsorption ofpolymers at solid-liquid interface, and prevention of scaling in industrial water sys-tems.

Z . Amjad: Gypsum scale format ion on heat exchanger surfaces: The influence of natural and synthet ic polyelectrolytes


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z. amjad gypsumscaleformationonheat exchangersurfaces

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