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Calcium sulfate dihydrate (gypsum) scale formation on heat exchanger surfaces: The influence of scale inhibitors
Calcium Sulfate Dihydrate (Gypsum) Scale Formation on Heat Exchanger Surfaces: The Influence of Scale Inhibitors Z. A M J A D BFGoodrich Chemical Company, Specialty Polymers and Chemical Division, Avon Lake Technical Center, Avon Lake, Ohio 44012
Received April 13, 1987; Accepted July 21, 1987 The kinetics of calcium sulfate dihydrate (CaSO4 • 2H20, gypsum) scale formation on heat exchanger surfaces from aqueous solutions has been studied by a highly reproducible technique. It has been found that gypsum scale formation takes place directly on the surface of the heat exchanger without any bulk or spontaneous precipitation in the reaction cell. The kinetic data also indicate that the rate of scale formation is a function of surface area and the metallurgy of the heat exchanger. A variety of polymeric and nonpolymeric scale inhibitors such as polyacrylates (mol wt 900-250,000), acrylate-based copolymers, polyphosphates (pyro-, tripoly-, and hexametaphosphates), and phosphonates [aminotri(methylene phosphonic acid), 1-hydroxyethylidine 1,1-diphosphonic acid, and 2-phosphonobutane 1,2,4-tricarboxylic acid] have been examined for their effects on the rate of scale formation. The results indicate that the amount of gypsum scale formed on heat exchanger surface is strongly affected by changing the functional group, molecular weight, and concentration of the inhibitor. In addition, amount of gypsum scale formed suggests an optimum effectiveness with molecular weight of ~2000 for the polyacrylates studied in the molecular weight range 900-250,000. Scanning electron microscopic investigations of the gypsum crystals grown in the presence of polyacrylates show that structures of these crystals are highly modified. A mechanism based upon surface adsorption of inhibitors on the growing gypsum crystals is discussed. The order, in terms of decreasing effectiveness on the rate of gypsum growth, of various scale inhibitors studied is polyacrylic acid (tool wt ~2,000) ,> hexametaphosphate > phosphonates. © 1988Academic Press, Inc.
INTRODUCTION F o r m a t i o n o f m i n e r a l scales on heat exchangers, reverse o s m o s i s m e m b r a n e surface, a n d e q u i p m e n t surfaces is a persistent a n d an expensive p r o b l e m in cooling water systems, boilers, s e c o n d a r y oil recovery utilizing water flooding techniques, d e s a l i n a t i o n b y evaporation a n d reverse osmosis m e t h o d s , a n d clothing washing m a c h i n e s . P r e c i p i t a t i o n o f m i n e r a l salts such as c a l c i u m c a r b o n a t e has been r e p o r t e d to result in i n c r u s t a t i o n s o n clothes washed with h a r d water. A m o n g the p r o b l e m s caused b y scale deposits are obstruct i o n o f fluid flow, i m p e d a n c e o f heat transfer, wear o f m e t a l parts, localized corrosion attack, a n d u n s c h e d u l e d e q u i p m e n t shutdown. T h e scales consist p r i m a r i l y o f carbonates, sulfates, hydroxides, phosphates, a n d silicates o f alka-
line earth metals, p a r t i c u l a r l y c a l c i u m a n d m a g n e s i u m . T h e p r o b l e m o f scale f o r m a t i o n is intensified at higher t e m p e r a t u r e s because o f the p e c u l i a r inverse t e m p e r a t u r e - s o l u b i l i t y characteristics o f these m i n e r a l s in water. Considerable attention has been given to the various forms o f c a l c i u m sulfate crystallizing f r o m a q u e o u s solution as affected b y t e m p e r ature, p H , solution s t o i c h i o m e t r i c ratio o f lattice ions, a n d i m p u r i t y level (1-4). These a n d o t h e r i m p o r t a n t factors i n v o l v e d in the nuclea t i o n a n d growth o f c a l c i u m sulfate d i h y d r a t e (CaSO4.2H20, gypsum), hemihydrate (CaSO4 • ½H 2 0 , plaster o f Paris), a n d anhydrite (CaSO4) should have direct a p p l i c a t i o n to the c o n t r o l a n d i n h i b i t i o n o f scale f o r m a t i o n in industrial water systems, g e o t h e r m a l energy, oil p r o d u c t i o n , a n d desalination. T o p r e c l u d e an excessive rise o f o p e r a t i n g costs, scale for523 0021-9797/88 $3.00
Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.
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z. AMJAD
mation has to be prevented. One of the most effective methods for controlling scale formation is the use of inhibitors. The deposition of calcium sulfate on heat exchanger surfaces under simulated cooling tower conditions has been the subject of numerous investigations (5-7). According to Hasson and Zahavi, the initial stage of deposition consisted of the nucleation of crystals adhering to the metal surface. The data from this study also indicated that the scale deposit was thickest at a point ahead of the upstream edge of the heat exchanger surface. In other investigations, heated metal billets suspended in supersaturated solutions have been used to study the scale formation on metal surface. The major problem associated with these investigations, although they did simulate heat exchanger surfaces, is that the kinetics of scale formation could not be studied quantitatively. In a recent study Gill and Nancollas (6) reported that during CaSO4.2H20 (gypsum) scale formation on heated metal surfaces, gypsum crystals nucleate preferentially at the gas-liquid interface provided by gas bubbles escaping from the heated surface. Subsequently, crystal growth took place by a surfacecontrolled reaction. More recently it was shown by Nancollas and Klima (7) that by using the constant composition technique, the kinetics of scale formation could be accurately studied at a sustained supersaturation. The effectiveness of a number of inhibitors in preventing or reducing the crystallization of calcium sulfate has been investigated (811). Some of these and notably polyelectrolytes (12, 13) are effective in reducing the deposition of scale. Recently, McCartney and Alexander (14) have examined the effect of a number of polyelectrolytes on the growth rate of calcium sulfate dihydrate. It was found that polymers containing carboxyl groups such as carboxymethyl cellulose, aliginic acid, polymethacrylic acid, and polyacrylic acid were particularly effective as CaSO4.2H20 crystal growth inhibitors. Kuntz (15) arrived at similar conclusions after studying the effect of various protein hydrolysates and their synthetic analogs on the Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
crystallization of calcium sulfate. In another study using the seeded growth method Liu and Nancollas (16) reported that trace amounts of phosphonates can stabilize supersaturated calcium sulfate solutions and lengthen the induction period before the onset of crystallization. The duration of the induction periods of phosphonates was found to be greatly influenced by the concentration, temperature, and amount of seed crystals added. The influence of molecular weight of the polyelectrolytes on the precipitation of CaSO4.2H20 from aqueous solutions has been the subject of a number of investigations. Jones (17), in his study on the effect of molecular weight (50,000-400,000) of carboxymethylcellulose (CMC), reported that highmolecular-weight CMC polymers were the least effective and the effectiveness increases as the molecular weight of the polymer decreases. Flesher et al. (18), in their study using the spontaneous precipitation method for the evaluation of a variety of polyelectrolytes at high temperature as calcium sulfate inhibitors, showed that the efficiency of the polyacrylate decreases with increasing molecular weight in the range 750,000-2000. Libutti et al. (19), in a recent study on the influence of molecular weight of polyacrylates in controlling CaCO3 scale formation on heat exchanger surfaces, reported that the molecular weight of polyacrylate plays an important role in the inhibition of CaCO3 crystal growth from aqueous solution. The rate of scaling was found to be higher in the case of high-molecular-weight polyacrylate (100,000) than that obtained in the presence of 2000 molecular weight polyacrylate. In this paper the kinetics of CaSO4.2H20 scale formation on heat exchanger surfaces has been studied using a highly reproducible technique (6). This study also reports the effects of various inhibitors on the rate of deposition of CaSO4.2H20 from metastable supersaturated solutions. In addition, microscopic observations have been made of CaSO4.2H20 crystal grown on heat exchanger surfaces in the presence and absence of inhibitors. The
525
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analytical characteristics of inhibitors tested in the present investigations are shown in Table I from which it can be seen that the inhibitors vary appreciably in terms of both the functional group and the molecular weight. It was hoped that study of such a variety of inhibitors would not only quantify the effectiveness of these inhibitors, but also throw some light on the mechanism of inhibition of calcium sulfate dihydrate scale formation in aqueous systems. EXPERIMENTAL
Reagent-grade chemicals, distilled water, and grade A glassware were used. Calcium
chloride solutions were standardized by titration with ethylenediaminetetraacetic acid using murexide as an indicator. Sodium sulfate solutions were standardized by passing aliquots through a cation exchange resin (Dowex 50W-×8, 20-50 mesh) in the hydrogen form and titrating the eluted acid with standard sodium hydroxide solution. The inhibitors used in these studies were commercial and experimental polyacrylates, acrylic acid-based copolymers, and polyacrylamide with molecular weights in the range 900 to 250,000 (BFGoodrich Chemical Co.); 2-phosphonobutane 1,2,4tricarboxylic acid, PTCA (Mobay Chemical Co.), phosphonates [aminotri(methylene phosphonic acid) (AMP) and 1-hydroxyethyl-
TABLE I Analytical Characteristics of Inhibitors Inhibitor
Molecular weight
Composition
Experimental-A
Polyacrylic acid (K-XP34)
Commercial-B
Source
900
BFGoodfich
Polyacrylic acid (K-752)
2,100
BFGoodrich
Commercial-C
Polyacrylic acid (K-732)
5,100
BFGoodrich
Experimental-D
Polyacrylic acid
10,000
BFGoodrich
Commercial-E
Polyacrylic acid (K-722)
180,000
BFGoodrich
Commercial-F
Polyacrylic acid (K-702)
250,000
BFGoodrich
Experimental-G
Polyacrylamide
6,000
BFGoodrich
Experimental-H
Poly(acrylic acid:acrylamide, 70:30)
6,000
BFGoodrich
ExperimentaM
Poly(acrylic acid: hydroxypropylacrylate, 70:30)
7,000
BFGoodrich
Aminotri(methylene phosphonic acid)
N(CH2PO3H2)3
299
Monsanto Chemical
1-Hydroxyethylidine 1,1-diphosphonic acid
CH3C(PO3H2)2OH
206
Monsanto Chemical
2-Phosphonobutane 1,2,4-tricarboxylic acid
COOH
COOH
COOH
270
MobayChemical
I
I
I
CH 2 - -
C - - CH2 - - CH2
I POsH2 Sodium pyrophosphate
Na4P207 • 10H20
446
Fisher Scientific
Sodium tripolyphosphate
NasPaOlo
368
Fisher Scientific
Sodium hexametaphosphate
(NaPO3)6
612
Fisher Scientific
Journal of Colloid and Interface Science, Vol. 123,No. 2, June 1988
526
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idine 1,1-diphosphonic acid (HEDP) from Monsanto Chemical Co.], and sodium polyphosphates (Fisher Scientific Co.) were also used. Supersaturated solutions of calcium sulfate were prepared in a double-walled, water-jacketed crystallization cell of about 1000-ml capacity fitted with water-cooled condensers. The heat exchanger U-tubes of about 40 cm in length and 1.0 cm in outer diameter were made of brass, copper, or stainless steel. These tubes were attached to the lid of the crystallization cell for immersion in the supersaturated solution. In most experiments the total surface area of metal in contact with the solution was ~ 7 8 cm 2, although some experiments were conducted where the metal surface area was varied from 40 to 78 cm 2. Most of the experiments reported in this study were performed using brass tubes, although stainless-steel and copper tubings were also used in some experiments. To avoid any surface imperfections and impurities, the tubes were chemically cleaned and rinsed thoroughly with distilled water. The crystal growth experiment was initiated by immersing the metal tube in the calcium sulfate supersaturated solution. A temperature differential was provided by circulating hot water, maintained at 68 +_0.5°C, through the tube, and cold water, 8 ___0.1 °C, through the outside of the crystallization cell. Within ~ 5 rain a steady-state temperature was reached and the bulk solution temperature remained at a constant value depending upon the thermal conductivity of the metal heat exchanger. To minimize corrosion of the heat exchanger tube during a crystallization experiment, 2-5 ppm of a corrosion inhibitor (sodium mercaptobenzotriazole, NaMBT, BFGoodrich Chemical Co., or tolytriazole, T.T. Sherwin Williams Co.) was used. During the crystallization reaction solution samples were withdrawn from time to time and filtered through 0.22-~m filter paper (Millipore Corp.), and the soluble calcium was analyzed by EDTA titration. At the end of the experiment, the amount of CaSO4.2H20 deposited on the brass Journal of Colloid and Interface Science, Vol. 123,No. 2, June 1988
tube was also determined by dissolving CaSO4 • 2H20 in a known amount of distilled water and analyzing for calcium by EDTA. During the crystallization experiments the solution temperature was also recorded. X-ray diffraction studies were also made of the crystals deposited on the metal surface using an X-ray powder diffraction apparatus (General Electric XRD-5 goniometer). In addition, the solid phases were examined by scanning electron microscopy (AMR Model 1400, Amray Inc.). RESULTS AND DISCUSSION
Following the immersion of the heat exchanger surface in the metastable supersaturated solution, the kinetics of CaSO4.2H20 scale formation was followed by withdrawing solution samples at known times and filtering, and the filtrate was analyzed for calcium. The experimental conditions used in this study are summarized in Table II. In Fig. 1 the concentration of calcium ion is plotted against time for experiments 3 and 6 (Table II) in which brass tubes were used. It can be seen in Fig. i that, following an induction period, CaSO4.2H20 scale formation takes place on the heated brass surface. The reproducibility of the scale formation experiments is illusTABLE II Formation of Calcium Sulfate Dihydrate (Gypsum) Scale on Heat Exchanger Surfaces"
Expt
Total Ca (10-2 M)
Heat exchanger
3 6 10 12 11 7 5 9 8
3.434 3.434 3.062 3.062 3.062 2.856 2.402 3.434 3.434
Brass Brass Brass Brass Brass Brass Brass Copper Stainless steel
Surface Induction area time ks (cma) (rain) (M-j rain-~) 78 78 85 52 36 78 78 78 78
13 12 25 35 45 48 125 8 20
2.18 2.07 1.79 0.95 0.69 1.87 1.72 2.81 1.75
Initial conditions: total calcium = total sulfate, reaction volume = 800 ml.
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,
S
2 I °2.8 24 2.0"~
,
8~0
, 240
~
~, 120~
TIME (min)
FIG. 1. Growthcurvesfor calciumsulfatedihydrateon brass surface. Plots of calcium concentration(O, Expt 3; Lx,Expt 6) and solutiontemperature (rq) againsttime. trated by the excellent agreement between the results of experiments of 3 and 6 (Table II). Also shown in Fig. 1 is the plot of the temperature of the calcium sulfate supersaturated solution as a function of time. It is interesting to note that as the amount of calcium sulfate on the heat exchanger increases, the bulk temperature decreases. This reflects the loss in the thermal efficiency of the heat exchanger. The crystallization of calcium sulfate dihydrate from solutions containing equivalent concentrations of lattice ions has been shown (20, 21) to follow the rate law of the form - d C / d t = k s ( C - Cs) 2
[1]
in which C is the total molar calcium ion (or total calcium sulfate) concentration at time t, Cs is the equilibrium solubility value, t is the time in minutes, k is the rate constant, and s is a function of active growth sites on the substrate. Typical plots of the integrated form of Eq. [1] are shown in Fig. 2 and the ks values are given in Table II. It is interesting to note that Eq. [1] satisfactorily represents the rate data even under conditions of metal surfaceinduced nucleation of CaSO4.2H20. In addition, the rates of scale formation were found to be independent of stirring rate, in agreement with the results of seeded growth experiments made using well-characterized CaSO4.2H20 seed crystals (13, 16). The evidence again points to an interfacial rate-controlling process rather than the diffusion of lattice ions on the solid-liquid interface.
It has been reported that gas-solid-liquid interfaces play an important role in the nucleation and subsequent attachment of scale crystals on heat exchanger surfaces. Hasson and Zahavi (5) suggested that the existence of gas bubbles at the heat exchanger surface leads to anomalous results. They proposed that variations in the surface temperature might be partly responsible for the observed large variations in their kinetic data. Nancollas and Klima (7), in their study on the deposition of CaSO4.2H20 on Admiralty brass, proposed that gypsum crystals formed preferentially at the perimeters of the gas bubbles. In the present investigation the nucleation and subsequent growth ofCaSO4 • 2H20 crystals on the brass heat exchanger were examined by scanning electron microscopy. The micrographs of CaSO4.2H20 are shown in Figs. 3(a and b) from which it can be seen that nucleation of crystals (Fig. 3a, 10 min) took place at the perimeter of the gas bubbles. Figures 3b (30 min) and 3c (145 rain) show the subsequent growth of CaSO4 • 2H20 crystals on the brass surface. It may be pointed out that at 145 min growth, not only was the heat exchanger surface completely covered with CaSO4 • 2H20 crystals but also the crystals were about five times larger (Fig. 3c) than those formed during the induction time (10 min, Fig. 3a).
40
CO O I O
"r I
%
i
80
i
240 TIME(rain)
FIG. 2. Kinetic plots ofEq. [ 1] for the growth of calcium sulfate dihydrate. IZ, Expt 3; ZX, Expt 6. Journal ofColloM andlnterface Science, Vol. 123, No. 2, June 1988
528
z. AMJAD
FIG. 3. Scanning electron rnicrographs of C a S O 4 ° 2 H 2 0 grown on brass heat exchanger: (a) 10 min, (b) 30 rain, (c) 145 min.
The influence of the nature of the metal surface (copper, stainless steel, and brass) on the rate of gypsum scale formation was studied by a series of experiments summarized in Table II. Figure 4 shows the calcium-time proJournal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
files for the three different heat exchangers. It can be seen that the excellent linearity of the plots of the integrated form of Eq. [1] illustrated in Fig. 5 confirms the applicability of this rate expression for different metal sub-
529
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FIG. 3 - - C o n t i n u e d .
strates. As shown in Table II, a higher ks ( M -I min -l) value was obtained for copper (2.81) as compared with brass (2.18) and stainless steel (1.75) which is consistent with the thermal conductivity characteristics, i.e., copper > brass > stainless steel. The results of scale formation experiments, in which different surface areas of the heat exchanger were exposed to the supersaturated solution, are summarized in Table II. As
shown in Table II (Expts 10-12) the rate of scale formation is proportional to the area exposed, thus confirming that scale formation occurred only on the metal surface, without any secondary nucleation or spontaneous precipitation. To verify that bulk or spontaneous precipitation did not occur during the crystallization reactions, unfiltered samples were
40 3.6
T % 0
I
0 I I
%
o v el 0
0
i
I,-
O
~10 2.4 i
i
80
i
240 TIME(rain)
FIG. 4. Growth curves of calcium sulfate dihydrate on various heat exchanger surfaces. A, copper; V], brass; O, stainless steel.
80
240 TIME(min)
FIG. 5. Kinetic plots ofEq. [ 1] for the growth of calcium sulfate dihydrate on different heat exchanger surfaces. A, stainless steel; [3, brass; O, copper. Journal of Colloid and Interface Science, Vol. 123,No. 2, June 1988
530
z. AMJAD
also analyzed for calcium ion and found to be within +0.3% of the filtered samples. Results summarized in Table II (Expts 5, 7, 11) indicate that rate of scale formation strongly depends upon solution supersaturation. For example, rate of scale formation obtained for C a S O 4 = 3.63 X 10 -2 M is 2.18 M -1 min -1 compared with 1.72 M -1 min -1 obtained for CaSO4 = 2.40 X 10-2 M. As will be shown later the effects of inhibitors on gypsum scale growth curves reflecting the time-dependent change in calcium ion concentration brought about by immersing a brass heat exchanger of known surface area into metastable supersaturated solutions containing different inhibitors are of two types: those inhibitors that affect the induction period and those that show no significant effect on the induction period preceding the gypsum scale formation. Curves of the first type were observed for the better inhibitors while curves of the second type were obtained for less effective inhibitors. In both cases, the loss of calcium ion concentration from solutions with increasing reaction time was found to follow a second-order rate law of the form shown above. To accommodate both types of behavior, we have chosen to employ as a criterion of inhibitor performance the amount of calcium ion remaining in solution after 4 h of growth. When expressed as a fraction of the total calcium ion present at the beginning of the scale formation experiment, the difference between the initial and 4-h residual calcium ion concentrations becomes a measure of the amount of gypsum scale formed in the presence ofinhibitors over the initial 4 h of growth. The choice of a 4-h growth period is arbitrary and, although the use of a different growth period would lead to a change of absolute weights of gypsum scale formed, it would not affect the findings on the effect of molecular weight, functional groups, and concentration trends on performance that are described below.
Polyacrylates Recently there has been an increasing interest in the application of polyacrylates as Journal of Colloid and Interface Science, Vol. 123,No. 2, June 1988
scale control agents, because they are highly efficient in preventing the nucleation and crystallization of many scale-forming minerals. The main advantage in using polyacrylates lies in their excellent thermal stability compared with polyphosphates and other copolymers. Scale growth experiments made in the presence of varying concentrations (0-2.0 ppm) of polyacrylate (mol wt 2100) are illustrated in Fig. 6. As shown, the crystallization reaction in the presence of 0.05 ppm is preceded by an induction period of ~65 min compared with "-~13 min observed in the absence of polyacrylate. Figure 6 further indicates that an increase in polymer concentration results only in a prolongation of induction period, and at a 2.0-ppm polymer concentration, the rate of crystallization was completely inhibited for at least 24 h. Table III summarizes the weights of gypsum formed at 4 h on brass heat exchanger, in the presence of varying polymer concentration. In Fig. 7, the amount of gypsum scale deposited in 240 min on the heat exchanger tube is plotted against polymer (commercial-C) concentration. It is interesting to note that even at very low polymer concentration (0.2 ppm) the amount of scale is reduced by a factor of ~3. Results of experiments made in the presence of polyacrylates of varying molecular weight are summarized
3.6
'o
°2.8
8'0
'
240 '
'
TIME(mln)
FIG. 6. Growth curves of calcium sulfate dihydrate on brass surfaces. Plots of calcium concentration against time as a function of polyacrylate (Commercial-B) concentration (ppm). V, 0; O, 0.05; O, 0.01; A, 0.50.
SCALE
TABLE
IlI
1.2
Formation of Calcium Sulfate Dihydrate (Gypsum) Scale on Brass Heat Exchangerin the Presence of Polyacrylatesa Expt
Polyacrylate
6 43 18 17 20 19 21 22 23 25 26 28 30 45 29 32
None Experimental-A Commercial-B Commercial-B Commercial-B Commercial-B Commercial-B Commercial-C Commercial-C Commercial-C Commercial-C Commercial-C Commercial-C Experimental-D Commercial-E Commercial-F
53 1
FORMATION
Molecular weight
Dosage (ppm)
CaSO4 • 2H20 b (g)
-900 2,100 2,100 2,100 2,100 2,100 5,100 5,100 5,100 5,100 5,100 5,100 10,000 180,000 250,000
-0.10 0.05 0.10 0.20 0.50 2.00 0.025 0.050 0.10 0.20 0.50 2.00 0.10 0.10 0.10
1.17 0.78 0.74 0.58 0.36 0.10 -0.97 0.88 0.68 0.42 0.15 <0.1 0.81 1.06 1.21
a Total calcium sulfate concentration = 3.43 × 10-2 M, heat exchanger surface area = 78 cm 2. b At 240 min.
in Table III. Figure 8 shows the plot of amounts of gypsum formed as a function of the molecular weight of polyacrylates. It can be seen that at constant polymer concentration the amount of gypsum scale deposited on the heat exchanger surface decreases with decreasing molecular weight (240,000-2100), reaches a minimum at ~2000 molecular
1.2
¢/) D. >" 0 . 4
0.20 0.40 POLYACRYLATE(ppm)
FIG. 7. Plot of the amount of gypsum (CaSO4- 2H20) scale deposited in 240 min on the brass surface with increasing polyacrylate (Commercial-C) concentration.
oD
v :E
co o,.
~ 0.4 i
i 4 MOL.WT.(IO
i
i 8
i
3)
FIG. 8. Plot of the amount of gypsum scale deposited in 240 rain on the brass heat exchanger in the presence of 0.10 ppm of polyacrylate of varying molecular weight.
weight, and thereafter increases with decreasing molecular weight. Under the experimental conditions employed in the present investigation the decrease in the amount of scale formed in the presence of polyacrylates must be attributed to the surface adsorption factor rather than the simple calcium-inhibitor complex formation (the percentage calcium complexed at even higher inhibitor concentrations only amounts to less than 5% of the total calcium) or the concomitant increase in ionic strength of the supersaturated solution in the presence ofinhibitors. It has been shown that the presence of trace amounts of scale inhibitors influences not only the growth rate but also the morphology of the scale-forming minerals. In some cases, such as calcium oxalate (22), calcium sulfate (2), calcium carbonate (23), and calcium phosphate (24), the presence ofinhibitors also affects the nature of the phase that forms. This may be of particular importance in industrial water systems since the presence of certain inhibitors may influence the nature of the scaling mineral. In the present investigation the influence of polyacrylate on the morphology of growing CaSO4.2H20 crystals was also studied by scanning electron microscopy. As shown in Figs. 9b and c, CaSO4.2H20 crystals grown on the brass heat exchanger surface in the presence of polyacrylate (Commercial-C, 0.1 ppm) are highly modified compared with those grown (Fig. 9a) in the absence of polymer. The observed change in morphology may be exJournal of Colloid and Interface Science, VoL 123, No. 2, June 1988
532
z. AMJAD
FIG. 9. Scanning electron micrographs o f C a S O 4 • 2 H 2 0 grown on the brass heat exchanger in the presence of 0 ppm (a, 65 min) and 0.1 ppm K-732 (Commercial-B) polyacrylate (b, c). (d, e) Scanning electron micrographs of CaSO4.2I-I20 grown in the presence of 0 and 1 ppm Commercial-A polyacrylate (CaSO4 = 4.67 × 10~2 M, 67°C, 20 h).
p l a i n e d in t e r m s o f surface a d s o r p t i o n o f p o l y acrylates o n t h e growing C a S O 4 . 2 H 2 0 crystals. Figures 9d a n d e further illustrate the Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
m a r k e d effect o f polyacrylate ( C o m m e r c i a l - A ) o n CaSO4 • 2 H 2 0 crystals grown in the absence (Fig. 9d) a n d the presence (Fig. 9e) o f poly-
SCALE FORMATION
FIG.
533
9--Continued.
acrylate. These experiments were conducted without heat exchanger with C a S O 4 = 4.605 X 10 -2 M, 67°C, and under static conditions. The influence of polymer composition at a constant concentration of 0.10 ppm was stud-
ied using the brass heat exchanger surface. Results presented in Table IV indicate that at constant polymer concentration the amount of gypsum scale formed is a function of polymer composition. The order in terms of effecJournal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
534
z. AMJAD
FIG. 9 - - C o n t i n u e d .
tiveness is polyacrylic acid (mol wt 2000) seen that at 0.1 ppm, AMP exhibits a larger > poly(acrylic acid:hydroxypropyl acrylate) inhibiting effect. For example, the amounts of > poly(acrylic acid:acrylamide) > polyacryl- gypsum formed on the brass heat exchanger amide = control (no polymer). It is interest- surface were 0.82 and 0.98 g for AMP and ing to note that the order of polymer effective- HEDP, respectively, compared with 1.17 g ness observed in the present study in which formed in the absence of phosphonates. Liu brass heat exchangers were used is consistent and Nancollas (20), in their seeded growth with observation made in the kinetic studies study on the evaluation of various phosphoof CaSO4-2H20 using well-characterized nate derivatives as CaSO4.2H20 inhibitors, CaSO4 ° 2H20 seed crystals (13). also made similar observations regarding the poor inhibitory action of HEDP. The effect of PTCA on gypsum scale formation was also Phosphonates and Polyphosphates investigated. Results summarized in Table IV It has been shown that organophosphonates indicate that PTCA exhibits some inhibitory such as aminotri(methylene phosphonic acid), action but not as marked as that observed for AMP, and HEDP markedly inhibit the crys- commercial-B polyacrylate. For example, the tallization of calcium carbonate (25), calcium amounts of gypsum scale formed in the presoxalate (22), calcium phosphates (26), and ence of 0.10 ppm of PTCA and commercialbarium sulfate (27). In studies of spontaneous B polyacrylate were 0.98 and 0.62 g, respecprecipitation, King (28) reported that small tively, compared with 1.17 g obtained in the amounts of phosphonates can stabilize a su- absence of inhibitors. persaturated solution of calcium sulfate and The inhibiting properties of condensed lengthen the induction period before the vis- phosphates of both the meta- and the polyible onset of precipitation. Scale growth ex- phosphate types have been extensively studied periments made in the presence of AMP and in relation to scale formation and biological HEDP are summarized in Table IV. It can be calcification processes. In the former case it Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
SCALE FORMATION
535
TABLE IV
gypsum crystals. For example, the amounts of Formation of Calcium Sulfate Dihydrate (Gypsum) gypsum crystals grown in the presence of SPP, Scale on Heat Exchanger in the Presence of 0.10 ppm STPP, and SHMP are 1.11, 1.06, and 0.84 g, respectively, compared with 1.17 g obtained Inhibitorsa in the absence of polyphosphates. The order Expt lnhibilor CaSO4.2H20 b in terms of effectiveness (based upon weight of gypsum crystals grown on brass heat ex6 None 1.17 changer) is SHMP > AMP ~> STPP ~ SPP 17 Polyacrylate(mol wt 2100) 0.62 control. It is interesting to note that pyrophosphate exhibits a strong inhibitory effect 52 Aminotri(methylene 0.82 on calcium phosphates (31), calcium oxalate phosphoric acid) (22), barium sulfate (30), and calcium carbon33 1-Hydroxyethylidine 0.98 ate (29) systems and only a slight inhibitory 1,1-diphosphonic acid effect for gypsum growth on a heat exchanger 37 Sodiumhexametaphosphate 0.84 surface. 51 Sodiumpyrophosphate 1.06 The results presented in this paper show that the rate of CaSO4.2H20 (gypsum) scale for49 Sodiumtripolyphosphate 0.99 mation on heat exchanger surfaces from the 46 2-Phosphonobutane 0.98 metastable supersaturated solutions follows a 1,2,4-tricarboxylicacid second-order rate law. Furthermore, it has 36 Polyacrylamide 1.12 been found that scale formation takes place directly on the surface of a heat exchanger with 40 Poly(acrylicacid:acrylamide, 0.95 70:30) no evidence of either bulk or spontaneous precipitation. The kinetic data also indicate 39 Poly(acrylicacid:hydroxypropyl 0.82 that the rate of scale formation is a function acrylate, 70:30) of surface area and metallurgy of heat exa Total calcium sulfateconcentration= 3.43 × 10-2 M, changer. The order in terms of decreasing rate heat exchangersurface area = 78 cm2. for different metals is copper > brass > stainless steel. Results of scale growth experiments made has been shown that sodium pyrophosphate in the presence of low levels (0.0-0.5 ppm) of is an effective growth inhibitor when added to polyacrylates (Commercial-B and -C) clearly CaCO3 (29) and BaSO4 (30) systems. In bio- indicate that these polymers exhibit an excellogical calcification processes, pyrophosphate lent inhibitory property. In terms of molecular ion has been found to be an effective crystal weight dependence it has been found that the growth inhibitor for calcium phosphates (31) amount of gypsum scale formed on the heat and calcium oxalate (22) systems. A surface exchanger surface decreases with decreasing adsorption mechanism has been found useful molecular weight of the polymer (250,000in describing the effect of pyrophosphate in 2100), reaches a minimum at ~ 2 0 0 0 molecmany of these systems. ular weight, and thereafter increases with deGypsum growth experiments made in the creasing molecular weight (2100-900). Results presence of 0.10 ppm of sodium pyrophos- also indicate that polymer composition greatly phate (SPP), sodium tripolyphosphate (STPP), affects the rate of gypsum growth. A mechaand sodium hexametaphosphate (SHMP) are nism based upon surface adsorption has been shown in Table IV. In contrast to larger proposed to explain the gypsum growth inamounts of gypsum grown on a brass surface hibition. in the presence of SPP and STPP, SHMP Overall, the order in terms of decreasing efshows a significant decrease in the amount of fectiveness of all the inhibitors studied is polyJournal of Colloid andlnterface Science, Vol. 123, No. 2, June 1988
536
Z . AMJAD
acrylate ( ~ 2 0 0 0 m o l wt) ~> S H M P > phosp h o n a t e ~ P A M ~ control. Scanning electron micrographs of CaSO4 • 2 H 2 0 crystals g r o w n o n brass h e a t exc h a n g e r surface reveal t h a t n u c l e a t i o n takes place preferentially at the p e r i m e t e r o f gas bubbles. A t 165 r a i n o f scale growth, the crystal size increases b y a factor o f a b o u t 5 c o m p a r e d with size o f crystals d u r i n g n u c l e a t i o n ( a b o u t 1 0 - 2 0 rain). F u r t h e r m o r e , crystals o f g y p s u m grown in t h e presence o f p o l y a c r y l a t e s are highly distorted. ACKNOWLEDGMENTS The author thanks BFGoodrich Chemical Company, Specialty Polymers and Chemical Division, for support in publishing this article. The contributions and cooperation of Joe Hooley, Bob Whalen, Dr. W. Masler, and Bud Raike are herewith acknowledged with sincere thanks. REFERENCES 1. Austin, A. E., Miller, J. F., Richard, N. A., and Kircher, J. F., Desalination 16, 331 (1975). 2. Vetter, O. J. G., and Phillips, R. C., J. Petrol. Technol./ 1299 (Oct. 1970). 3. Petrey, E. Q., and Sexsmith, D. R., Desalination 13, 89 (1973). 4. Nancollas, G. H., White, W., Tsai, F., and Maslow, L., Corrosion 35, 304 (1979). 5. Hasson, D., and Zahavi, J., Ind. Eng. Chem. Fundam. 9, 1 (1970). 6. Gill, J. S., and Nancollas, G. H., Z Cryst. Growth 48, 34 (1980). 7. NancoUas, G. H., and Klima, W. F., Paper 81, Corrosion/81, National Association of Corrosion Engineers Conference, Toronto, Ontario, i981. 8. Gritfiths, D. W., and Roberts, S. D., Paper 7862, International Symposium on Oilfield and Geothermal Chemistry, Society of Petroleum Engineers, Houston, 1979.
JournalofColloidand InterfaceScience,Vol.123,No. 2, June 1988
9. Smith, B. R., and Alexander, A. E., J. ColtoM Interface Sci. 34, 81 (1970). 10. Gill, J. S., and Nancollas, G. H., Corrosion 37, 120 (1981). 11. Amjad, Z., Desalination 54, 263 (1985). 12. Weijnen, M. P. C., and van Rosmalen, G. M., Desalination 54, 239 (1985). 13. Amjad, Z., and Hooley, J., J. Colloid Interface Sci. 111, 496 (1986). 14. McCartney, E. R., and Alexander, A. E., J. Colloid Sci. 13, 383 (1958). 15. Kuntz, R. A., Nature (London) 211, 406 (1966). 16. Liu, S. T., and Nancollas, G. H., 3". Colloid Interface Sci. 44, 422 (1973). 17. Jones, L. W., Corrosion 17, 232 (1961). 18. Flesher, P., Streatfield, E. L., Pearse, A. S., and Hyde"s, O. D., 3rd Int. Symp. Fresh Water Sea 1, 493 (1970). 19. Libutti, B. L., Knudsen, J. G., and Mueller, R. W., Paper 179, Corrosion/84, National Association of Corrosion Engineers Conference, New Orleans, 1984. 20. Liu, S. T., and Nancollas, G. H., J. Cryst. Growth 6, 201 (1970). 21. Smith, B. R., and Sweet, F., J. Colloid Interface Sci. 37, 612 (1971). 22. Gardner, G. L., J. Phys. Chem. 82, 864 (1978). 23. Nancollas, G. H., and Sawada, K., Paper 312, Fifth International Symposium on Oilfield, San Diego, 1980. 24. Brecevic, L., Sendijarevic, A., and Furedi-Milhofer, H., Colloids Surf 11, 55 (1984). 25. Reddy, M. M., and Nancollas, G. H., Desalination 12, 61 (1973). 26. Koutsoukos, P. G., Amjad, Z., and Nancollas, G. H., J. Colloid Interface Sci. 83, 599 (1981). 27. Rizkalla, E. N., J. Chem. Soc. Faraday Trans. 79, 1857 (1983). 28. King, T. M., Paper NATR5S, 158th National Meeting of American Chemical Society, New York, 1969. 29. Raistriek, B., Discuss. Faraday Soc. 5, 234 (1949). 30. Nancollas, G. H., and Liu, S. T., Soc. Petrol. Eng. J.~ 509 (1975). 31. Robertson, W. G., Calcif. TissueRes. 11, 311 (1973).
Received April 13, 1987; Accepted July 21, 1987 The kinetics of calcium sulfate dihydrate (CaSO4 • 2H20, gypsum) scale formation on heat exchanger surfaces from aqueous solutions has been studied by a highly reproducible technique. It has been found that gypsum scale formation takes place directly on the surface of the heat exchanger without any bulk or spontaneous precipitation in the reaction cell. The kinetic data also indicate that the rate of scale formation is a function of surface area and the metallurgy of the heat exchanger. A variety of polymeric and nonpolymeric scale inhibitors such as polyacrylates (mol wt 900-250,000), acrylate-based copolymers, polyphosphates (pyro-, tripoly-, and hexametaphosphates), and phosphonates [aminotri(methylene phosphonic acid), 1-hydroxyethylidine 1,1-diphosphonic acid, and 2-phosphonobutane 1,2,4-tricarboxylic acid] have been examined for their effects on the rate of scale formation. The results indicate that the amount of gypsum scale formed on heat exchanger surface is strongly affected by changing the functional group, molecular weight, and concentration of the inhibitor. In addition, amount of gypsum scale formed suggests an optimum effectiveness with molecular weight of ~2000 for the polyacrylates studied in the molecular weight range 900-250,000. Scanning electron microscopic investigations of the gypsum crystals grown in the presence of polyacrylates show that structures of these crystals are highly modified. A mechanism based upon surface adsorption of inhibitors on the growing gypsum crystals is discussed. The order, in terms of decreasing effectiveness on the rate of gypsum growth, of various scale inhibitors studied is polyacrylic acid (tool wt ~2,000) ,> hexametaphosphate > phosphonates. © 1988Academic Press, Inc.
INTRODUCTION F o r m a t i o n o f m i n e r a l scales on heat exchangers, reverse o s m o s i s m e m b r a n e surface, a n d e q u i p m e n t surfaces is a persistent a n d an expensive p r o b l e m in cooling water systems, boilers, s e c o n d a r y oil recovery utilizing water flooding techniques, d e s a l i n a t i o n b y evaporation a n d reverse osmosis m e t h o d s , a n d clothing washing m a c h i n e s . P r e c i p i t a t i o n o f m i n e r a l salts such as c a l c i u m c a r b o n a t e has been r e p o r t e d to result in i n c r u s t a t i o n s o n clothes washed with h a r d water. A m o n g the p r o b l e m s caused b y scale deposits are obstruct i o n o f fluid flow, i m p e d a n c e o f heat transfer, wear o f m e t a l parts, localized corrosion attack, a n d u n s c h e d u l e d e q u i p m e n t shutdown. T h e scales consist p r i m a r i l y o f carbonates, sulfates, hydroxides, phosphates, a n d silicates o f alka-
line earth metals, p a r t i c u l a r l y c a l c i u m a n d m a g n e s i u m . T h e p r o b l e m o f scale f o r m a t i o n is intensified at higher t e m p e r a t u r e s because o f the p e c u l i a r inverse t e m p e r a t u r e - s o l u b i l i t y characteristics o f these m i n e r a l s in water. Considerable attention has been given to the various forms o f c a l c i u m sulfate crystallizing f r o m a q u e o u s solution as affected b y t e m p e r ature, p H , solution s t o i c h i o m e t r i c ratio o f lattice ions, a n d i m p u r i t y level (1-4). These a n d o t h e r i m p o r t a n t factors i n v o l v e d in the nuclea t i o n a n d growth o f c a l c i u m sulfate d i h y d r a t e (CaSO4.2H20, gypsum), hemihydrate (CaSO4 • ½H 2 0 , plaster o f Paris), a n d anhydrite (CaSO4) should have direct a p p l i c a t i o n to the c o n t r o l a n d i n h i b i t i o n o f scale f o r m a t i o n in industrial water systems, g e o t h e r m a l energy, oil p r o d u c t i o n , a n d desalination. T o p r e c l u d e an excessive rise o f o p e r a t i n g costs, scale for523 0021-9797/88 $3.00
Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.
524
z. AMJAD
mation has to be prevented. One of the most effective methods for controlling scale formation is the use of inhibitors. The deposition of calcium sulfate on heat exchanger surfaces under simulated cooling tower conditions has been the subject of numerous investigations (5-7). According to Hasson and Zahavi, the initial stage of deposition consisted of the nucleation of crystals adhering to the metal surface. The data from this study also indicated that the scale deposit was thickest at a point ahead of the upstream edge of the heat exchanger surface. In other investigations, heated metal billets suspended in supersaturated solutions have been used to study the scale formation on metal surface. The major problem associated with these investigations, although they did simulate heat exchanger surfaces, is that the kinetics of scale formation could not be studied quantitatively. In a recent study Gill and Nancollas (6) reported that during CaSO4.2H20 (gypsum) scale formation on heated metal surfaces, gypsum crystals nucleate preferentially at the gas-liquid interface provided by gas bubbles escaping from the heated surface. Subsequently, crystal growth took place by a surfacecontrolled reaction. More recently it was shown by Nancollas and Klima (7) that by using the constant composition technique, the kinetics of scale formation could be accurately studied at a sustained supersaturation. The effectiveness of a number of inhibitors in preventing or reducing the crystallization of calcium sulfate has been investigated (811). Some of these and notably polyelectrolytes (12, 13) are effective in reducing the deposition of scale. Recently, McCartney and Alexander (14) have examined the effect of a number of polyelectrolytes on the growth rate of calcium sulfate dihydrate. It was found that polymers containing carboxyl groups such as carboxymethyl cellulose, aliginic acid, polymethacrylic acid, and polyacrylic acid were particularly effective as CaSO4.2H20 crystal growth inhibitors. Kuntz (15) arrived at similar conclusions after studying the effect of various protein hydrolysates and their synthetic analogs on the Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
crystallization of calcium sulfate. In another study using the seeded growth method Liu and Nancollas (16) reported that trace amounts of phosphonates can stabilize supersaturated calcium sulfate solutions and lengthen the induction period before the onset of crystallization. The duration of the induction periods of phosphonates was found to be greatly influenced by the concentration, temperature, and amount of seed crystals added. The influence of molecular weight of the polyelectrolytes on the precipitation of CaSO4.2H20 from aqueous solutions has been the subject of a number of investigations. Jones (17), in his study on the effect of molecular weight (50,000-400,000) of carboxymethylcellulose (CMC), reported that highmolecular-weight CMC polymers were the least effective and the effectiveness increases as the molecular weight of the polymer decreases. Flesher et al. (18), in their study using the spontaneous precipitation method for the evaluation of a variety of polyelectrolytes at high temperature as calcium sulfate inhibitors, showed that the efficiency of the polyacrylate decreases with increasing molecular weight in the range 750,000-2000. Libutti et al. (19), in a recent study on the influence of molecular weight of polyacrylates in controlling CaCO3 scale formation on heat exchanger surfaces, reported that the molecular weight of polyacrylate plays an important role in the inhibition of CaCO3 crystal growth from aqueous solution. The rate of scaling was found to be higher in the case of high-molecular-weight polyacrylate (100,000) than that obtained in the presence of 2000 molecular weight polyacrylate. In this paper the kinetics of CaSO4.2H20 scale formation on heat exchanger surfaces has been studied using a highly reproducible technique (6). This study also reports the effects of various inhibitors on the rate of deposition of CaSO4.2H20 from metastable supersaturated solutions. In addition, microscopic observations have been made of CaSO4.2H20 crystal grown on heat exchanger surfaces in the presence and absence of inhibitors. The
525
SCALE FORMATION
analytical characteristics of inhibitors tested in the present investigations are shown in Table I from which it can be seen that the inhibitors vary appreciably in terms of both the functional group and the molecular weight. It was hoped that study of such a variety of inhibitors would not only quantify the effectiveness of these inhibitors, but also throw some light on the mechanism of inhibition of calcium sulfate dihydrate scale formation in aqueous systems. EXPERIMENTAL
Reagent-grade chemicals, distilled water, and grade A glassware were used. Calcium
chloride solutions were standardized by titration with ethylenediaminetetraacetic acid using murexide as an indicator. Sodium sulfate solutions were standardized by passing aliquots through a cation exchange resin (Dowex 50W-×8, 20-50 mesh) in the hydrogen form and titrating the eluted acid with standard sodium hydroxide solution. The inhibitors used in these studies were commercial and experimental polyacrylates, acrylic acid-based copolymers, and polyacrylamide with molecular weights in the range 900 to 250,000 (BFGoodrich Chemical Co.); 2-phosphonobutane 1,2,4tricarboxylic acid, PTCA (Mobay Chemical Co.), phosphonates [aminotri(methylene phosphonic acid) (AMP) and 1-hydroxyethyl-
TABLE I Analytical Characteristics of Inhibitors Inhibitor
Molecular weight
Composition
Experimental-A
Polyacrylic acid (K-XP34)
Commercial-B
Source
900
BFGoodfich
Polyacrylic acid (K-752)
2,100
BFGoodrich
Commercial-C
Polyacrylic acid (K-732)
5,100
BFGoodrich
Experimental-D
Polyacrylic acid
10,000
BFGoodrich
Commercial-E
Polyacrylic acid (K-722)
180,000
BFGoodrich
Commercial-F
Polyacrylic acid (K-702)
250,000
BFGoodrich
Experimental-G
Polyacrylamide
6,000
BFGoodrich
Experimental-H
Poly(acrylic acid:acrylamide, 70:30)
6,000
BFGoodrich
ExperimentaM
Poly(acrylic acid: hydroxypropylacrylate, 70:30)
7,000
BFGoodrich
Aminotri(methylene phosphonic acid)
N(CH2PO3H2)3
299
Monsanto Chemical
1-Hydroxyethylidine 1,1-diphosphonic acid
CH3C(PO3H2)2OH
206
Monsanto Chemical
2-Phosphonobutane 1,2,4-tricarboxylic acid
COOH
COOH
COOH
270
MobayChemical
I
I
I
CH 2 - -
C - - CH2 - - CH2
I POsH2 Sodium pyrophosphate
Na4P207 • 10H20
446
Fisher Scientific
Sodium tripolyphosphate
NasPaOlo
368
Fisher Scientific
Sodium hexametaphosphate
(NaPO3)6
612
Fisher Scientific
Journal of Colloid and Interface Science, Vol. 123,No. 2, June 1988
526
z. AMJAD
idine 1,1-diphosphonic acid (HEDP) from Monsanto Chemical Co.], and sodium polyphosphates (Fisher Scientific Co.) were also used. Supersaturated solutions of calcium sulfate were prepared in a double-walled, water-jacketed crystallization cell of about 1000-ml capacity fitted with water-cooled condensers. The heat exchanger U-tubes of about 40 cm in length and 1.0 cm in outer diameter were made of brass, copper, or stainless steel. These tubes were attached to the lid of the crystallization cell for immersion in the supersaturated solution. In most experiments the total surface area of metal in contact with the solution was ~ 7 8 cm 2, although some experiments were conducted where the metal surface area was varied from 40 to 78 cm 2. Most of the experiments reported in this study were performed using brass tubes, although stainless-steel and copper tubings were also used in some experiments. To avoid any surface imperfections and impurities, the tubes were chemically cleaned and rinsed thoroughly with distilled water. The crystal growth experiment was initiated by immersing the metal tube in the calcium sulfate supersaturated solution. A temperature differential was provided by circulating hot water, maintained at 68 +_0.5°C, through the tube, and cold water, 8 ___0.1 °C, through the outside of the crystallization cell. Within ~ 5 rain a steady-state temperature was reached and the bulk solution temperature remained at a constant value depending upon the thermal conductivity of the metal heat exchanger. To minimize corrosion of the heat exchanger tube during a crystallization experiment, 2-5 ppm of a corrosion inhibitor (sodium mercaptobenzotriazole, NaMBT, BFGoodrich Chemical Co., or tolytriazole, T.T. Sherwin Williams Co.) was used. During the crystallization reaction solution samples were withdrawn from time to time and filtered through 0.22-~m filter paper (Millipore Corp.), and the soluble calcium was analyzed by EDTA titration. At the end of the experiment, the amount of CaSO4.2H20 deposited on the brass Journal of Colloid and Interface Science, Vol. 123,No. 2, June 1988
tube was also determined by dissolving CaSO4 • 2H20 in a known amount of distilled water and analyzing for calcium by EDTA. During the crystallization experiments the solution temperature was also recorded. X-ray diffraction studies were also made of the crystals deposited on the metal surface using an X-ray powder diffraction apparatus (General Electric XRD-5 goniometer). In addition, the solid phases were examined by scanning electron microscopy (AMR Model 1400, Amray Inc.). RESULTS AND DISCUSSION
Following the immersion of the heat exchanger surface in the metastable supersaturated solution, the kinetics of CaSO4.2H20 scale formation was followed by withdrawing solution samples at known times and filtering, and the filtrate was analyzed for calcium. The experimental conditions used in this study are summarized in Table II. In Fig. 1 the concentration of calcium ion is plotted against time for experiments 3 and 6 (Table II) in which brass tubes were used. It can be seen in Fig. i that, following an induction period, CaSO4.2H20 scale formation takes place on the heated brass surface. The reproducibility of the scale formation experiments is illusTABLE II Formation of Calcium Sulfate Dihydrate (Gypsum) Scale on Heat Exchanger Surfaces"
Expt
Total Ca (10-2 M)
Heat exchanger
3 6 10 12 11 7 5 9 8
3.434 3.434 3.062 3.062 3.062 2.856 2.402 3.434 3.434
Brass Brass Brass Brass Brass Brass Brass Copper Stainless steel
Surface Induction area time ks (cma) (rain) (M-j rain-~) 78 78 85 52 36 78 78 78 78
13 12 25 35 45 48 125 8 20
2.18 2.07 1.79 0.95 0.69 1.87 1.72 2.81 1.75
Initial conditions: total calcium = total sulfate, reaction volume = 800 ml.
527
SCALE F O R M A T I O N
,
S
2 I °2.8 24 2.0"~
,
8~0
, 240
~
~, 120~
TIME (min)
FIG. 1. Growthcurvesfor calciumsulfatedihydrateon brass surface. Plots of calcium concentration(O, Expt 3; Lx,Expt 6) and solutiontemperature (rq) againsttime. trated by the excellent agreement between the results of experiments of 3 and 6 (Table II). Also shown in Fig. 1 is the plot of the temperature of the calcium sulfate supersaturated solution as a function of time. It is interesting to note that as the amount of calcium sulfate on the heat exchanger increases, the bulk temperature decreases. This reflects the loss in the thermal efficiency of the heat exchanger. The crystallization of calcium sulfate dihydrate from solutions containing equivalent concentrations of lattice ions has been shown (20, 21) to follow the rate law of the form - d C / d t = k s ( C - Cs) 2
[1]
in which C is the total molar calcium ion (or total calcium sulfate) concentration at time t, Cs is the equilibrium solubility value, t is the time in minutes, k is the rate constant, and s is a function of active growth sites on the substrate. Typical plots of the integrated form of Eq. [1] are shown in Fig. 2 and the ks values are given in Table II. It is interesting to note that Eq. [1] satisfactorily represents the rate data even under conditions of metal surfaceinduced nucleation of CaSO4.2H20. In addition, the rates of scale formation were found to be independent of stirring rate, in agreement with the results of seeded growth experiments made using well-characterized CaSO4.2H20 seed crystals (13, 16). The evidence again points to an interfacial rate-controlling process rather than the diffusion of lattice ions on the solid-liquid interface.
It has been reported that gas-solid-liquid interfaces play an important role in the nucleation and subsequent attachment of scale crystals on heat exchanger surfaces. Hasson and Zahavi (5) suggested that the existence of gas bubbles at the heat exchanger surface leads to anomalous results. They proposed that variations in the surface temperature might be partly responsible for the observed large variations in their kinetic data. Nancollas and Klima (7), in their study on the deposition of CaSO4.2H20 on Admiralty brass, proposed that gypsum crystals formed preferentially at the perimeters of the gas bubbles. In the present investigation the nucleation and subsequent growth ofCaSO4 • 2H20 crystals on the brass heat exchanger were examined by scanning electron microscopy. The micrographs of CaSO4.2H20 are shown in Figs. 3(a and b) from which it can be seen that nucleation of crystals (Fig. 3a, 10 min) took place at the perimeter of the gas bubbles. Figures 3b (30 min) and 3c (145 rain) show the subsequent growth of CaSO4 • 2H20 crystals on the brass surface. It may be pointed out that at 145 min growth, not only was the heat exchanger surface completely covered with CaSO4 • 2H20 crystals but also the crystals were about five times larger (Fig. 3c) than those formed during the induction time (10 min, Fig. 3a).
40
CO O I O
"r I
%
i
80
i
240 TIME(rain)
FIG. 2. Kinetic plots ofEq. [ 1] for the growth of calcium sulfate dihydrate. IZ, Expt 3; ZX, Expt 6. Journal ofColloM andlnterface Science, Vol. 123, No. 2, June 1988
528
z. AMJAD
FIG. 3. Scanning electron rnicrographs of C a S O 4 ° 2 H 2 0 grown on brass heat exchanger: (a) 10 min, (b) 30 rain, (c) 145 min.
The influence of the nature of the metal surface (copper, stainless steel, and brass) on the rate of gypsum scale formation was studied by a series of experiments summarized in Table II. Figure 4 shows the calcium-time proJournal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
files for the three different heat exchangers. It can be seen that the excellent linearity of the plots of the integrated form of Eq. [1] illustrated in Fig. 5 confirms the applicability of this rate expression for different metal sub-
529
SCALE FORMATION
FIG. 3 - - C o n t i n u e d .
strates. As shown in Table II, a higher ks ( M -I min -l) value was obtained for copper (2.81) as compared with brass (2.18) and stainless steel (1.75) which is consistent with the thermal conductivity characteristics, i.e., copper > brass > stainless steel. The results of scale formation experiments, in which different surface areas of the heat exchanger were exposed to the supersaturated solution, are summarized in Table II. As
shown in Table II (Expts 10-12) the rate of scale formation is proportional to the area exposed, thus confirming that scale formation occurred only on the metal surface, without any secondary nucleation or spontaneous precipitation. To verify that bulk or spontaneous precipitation did not occur during the crystallization reactions, unfiltered samples were
40 3.6
T % 0
I
0 I I
%
o v el 0
0
i
I,-
O
~10 2.4 i
i
80
i
240 TIME(rain)
FIG. 4. Growth curves of calcium sulfate dihydrate on various heat exchanger surfaces. A, copper; V], brass; O, stainless steel.
80
240 TIME(min)
FIG. 5. Kinetic plots ofEq. [ 1] for the growth of calcium sulfate dihydrate on different heat exchanger surfaces. A, stainless steel; [3, brass; O, copper. Journal of Colloid and Interface Science, Vol. 123,No. 2, June 1988
530
z. AMJAD
also analyzed for calcium ion and found to be within +0.3% of the filtered samples. Results summarized in Table II (Expts 5, 7, 11) indicate that rate of scale formation strongly depends upon solution supersaturation. For example, rate of scale formation obtained for C a S O 4 = 3.63 X 10 -2 M is 2.18 M -1 min -1 compared with 1.72 M -1 min -1 obtained for CaSO4 = 2.40 X 10-2 M. As will be shown later the effects of inhibitors on gypsum scale growth curves reflecting the time-dependent change in calcium ion concentration brought about by immersing a brass heat exchanger of known surface area into metastable supersaturated solutions containing different inhibitors are of two types: those inhibitors that affect the induction period and those that show no significant effect on the induction period preceding the gypsum scale formation. Curves of the first type were observed for the better inhibitors while curves of the second type were obtained for less effective inhibitors. In both cases, the loss of calcium ion concentration from solutions with increasing reaction time was found to follow a second-order rate law of the form shown above. To accommodate both types of behavior, we have chosen to employ as a criterion of inhibitor performance the amount of calcium ion remaining in solution after 4 h of growth. When expressed as a fraction of the total calcium ion present at the beginning of the scale formation experiment, the difference between the initial and 4-h residual calcium ion concentrations becomes a measure of the amount of gypsum scale formed in the presence ofinhibitors over the initial 4 h of growth. The choice of a 4-h growth period is arbitrary and, although the use of a different growth period would lead to a change of absolute weights of gypsum scale formed, it would not affect the findings on the effect of molecular weight, functional groups, and concentration trends on performance that are described below.
Polyacrylates Recently there has been an increasing interest in the application of polyacrylates as Journal of Colloid and Interface Science, Vol. 123,No. 2, June 1988
scale control agents, because they are highly efficient in preventing the nucleation and crystallization of many scale-forming minerals. The main advantage in using polyacrylates lies in their excellent thermal stability compared with polyphosphates and other copolymers. Scale growth experiments made in the presence of varying concentrations (0-2.0 ppm) of polyacrylate (mol wt 2100) are illustrated in Fig. 6. As shown, the crystallization reaction in the presence of 0.05 ppm is preceded by an induction period of ~65 min compared with "-~13 min observed in the absence of polyacrylate. Figure 6 further indicates that an increase in polymer concentration results only in a prolongation of induction period, and at a 2.0-ppm polymer concentration, the rate of crystallization was completely inhibited for at least 24 h. Table III summarizes the weights of gypsum formed at 4 h on brass heat exchanger, in the presence of varying polymer concentration. In Fig. 7, the amount of gypsum scale deposited in 240 min on the heat exchanger tube is plotted against polymer (commercial-C) concentration. It is interesting to note that even at very low polymer concentration (0.2 ppm) the amount of scale is reduced by a factor of ~3. Results of experiments made in the presence of polyacrylates of varying molecular weight are summarized
3.6
'o
°2.8
8'0
'
240 '
'
TIME(mln)
FIG. 6. Growth curves of calcium sulfate dihydrate on brass surfaces. Plots of calcium concentration against time as a function of polyacrylate (Commercial-B) concentration (ppm). V, 0; O, 0.05; O, 0.01; A, 0.50.
SCALE
TABLE
IlI
1.2
Formation of Calcium Sulfate Dihydrate (Gypsum) Scale on Brass Heat Exchangerin the Presence of Polyacrylatesa Expt
Polyacrylate
6 43 18 17 20 19 21 22 23 25 26 28 30 45 29 32
None Experimental-A Commercial-B Commercial-B Commercial-B Commercial-B Commercial-B Commercial-C Commercial-C Commercial-C Commercial-C Commercial-C Commercial-C Experimental-D Commercial-E Commercial-F
53 1
FORMATION
Molecular weight
Dosage (ppm)
CaSO4 • 2H20 b (g)
-900 2,100 2,100 2,100 2,100 2,100 5,100 5,100 5,100 5,100 5,100 5,100 10,000 180,000 250,000
-0.10 0.05 0.10 0.20 0.50 2.00 0.025 0.050 0.10 0.20 0.50 2.00 0.10 0.10 0.10
1.17 0.78 0.74 0.58 0.36 0.10 -0.97 0.88 0.68 0.42 0.15 <0.1 0.81 1.06 1.21
a Total calcium sulfate concentration = 3.43 × 10-2 M, heat exchanger surface area = 78 cm 2. b At 240 min.
in Table III. Figure 8 shows the plot of amounts of gypsum formed as a function of the molecular weight of polyacrylates. It can be seen that at constant polymer concentration the amount of gypsum scale deposited on the heat exchanger surface decreases with decreasing molecular weight (240,000-2100), reaches a minimum at ~2000 molecular
1.2
¢/) D. >" 0 . 4
0.20 0.40 POLYACRYLATE(ppm)
FIG. 7. Plot of the amount of gypsum (CaSO4- 2H20) scale deposited in 240 min on the brass surface with increasing polyacrylate (Commercial-C) concentration.
oD
v :E
co o,.
~ 0.4 i
i 4 MOL.WT.(IO
i
i 8
i
3)
FIG. 8. Plot of the amount of gypsum scale deposited in 240 rain on the brass heat exchanger in the presence of 0.10 ppm of polyacrylate of varying molecular weight.
weight, and thereafter increases with decreasing molecular weight. Under the experimental conditions employed in the present investigation the decrease in the amount of scale formed in the presence of polyacrylates must be attributed to the surface adsorption factor rather than the simple calcium-inhibitor complex formation (the percentage calcium complexed at even higher inhibitor concentrations only amounts to less than 5% of the total calcium) or the concomitant increase in ionic strength of the supersaturated solution in the presence ofinhibitors. It has been shown that the presence of trace amounts of scale inhibitors influences not only the growth rate but also the morphology of the scale-forming minerals. In some cases, such as calcium oxalate (22), calcium sulfate (2), calcium carbonate (23), and calcium phosphate (24), the presence ofinhibitors also affects the nature of the phase that forms. This may be of particular importance in industrial water systems since the presence of certain inhibitors may influence the nature of the scaling mineral. In the present investigation the influence of polyacrylate on the morphology of growing CaSO4.2H20 crystals was also studied by scanning electron microscopy. As shown in Figs. 9b and c, CaSO4.2H20 crystals grown on the brass heat exchanger surface in the presence of polyacrylate (Commercial-C, 0.1 ppm) are highly modified compared with those grown (Fig. 9a) in the absence of polymer. The observed change in morphology may be exJournal of Colloid and Interface Science, VoL 123, No. 2, June 1988
532
z. AMJAD
FIG. 9. Scanning electron micrographs o f C a S O 4 • 2 H 2 0 grown on the brass heat exchanger in the presence of 0 ppm (a, 65 min) and 0.1 ppm K-732 (Commercial-B) polyacrylate (b, c). (d, e) Scanning electron micrographs of CaSO4.2I-I20 grown in the presence of 0 and 1 ppm Commercial-A polyacrylate (CaSO4 = 4.67 × 10~2 M, 67°C, 20 h).
p l a i n e d in t e r m s o f surface a d s o r p t i o n o f p o l y acrylates o n t h e growing C a S O 4 . 2 H 2 0 crystals. Figures 9d a n d e further illustrate the Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
m a r k e d effect o f polyacrylate ( C o m m e r c i a l - A ) o n CaSO4 • 2 H 2 0 crystals grown in the absence (Fig. 9d) a n d the presence (Fig. 9e) o f poly-
SCALE FORMATION
FIG.
533
9--Continued.
acrylate. These experiments were conducted without heat exchanger with C a S O 4 = 4.605 X 10 -2 M, 67°C, and under static conditions. The influence of polymer composition at a constant concentration of 0.10 ppm was stud-
ied using the brass heat exchanger surface. Results presented in Table IV indicate that at constant polymer concentration the amount of gypsum scale formed is a function of polymer composition. The order in terms of effecJournal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
534
z. AMJAD
FIG. 9 - - C o n t i n u e d .
tiveness is polyacrylic acid (mol wt 2000) seen that at 0.1 ppm, AMP exhibits a larger > poly(acrylic acid:hydroxypropyl acrylate) inhibiting effect. For example, the amounts of > poly(acrylic acid:acrylamide) > polyacryl- gypsum formed on the brass heat exchanger amide = control (no polymer). It is interest- surface were 0.82 and 0.98 g for AMP and ing to note that the order of polymer effective- HEDP, respectively, compared with 1.17 g ness observed in the present study in which formed in the absence of phosphonates. Liu brass heat exchangers were used is consistent and Nancollas (20), in their seeded growth with observation made in the kinetic studies study on the evaluation of various phosphoof CaSO4-2H20 using well-characterized nate derivatives as CaSO4.2H20 inhibitors, CaSO4 ° 2H20 seed crystals (13). also made similar observations regarding the poor inhibitory action of HEDP. The effect of PTCA on gypsum scale formation was also Phosphonates and Polyphosphates investigated. Results summarized in Table IV It has been shown that organophosphonates indicate that PTCA exhibits some inhibitory such as aminotri(methylene phosphonic acid), action but not as marked as that observed for AMP, and HEDP markedly inhibit the crys- commercial-B polyacrylate. For example, the tallization of calcium carbonate (25), calcium amounts of gypsum scale formed in the presoxalate (22), calcium phosphates (26), and ence of 0.10 ppm of PTCA and commercialbarium sulfate (27). In studies of spontaneous B polyacrylate were 0.98 and 0.62 g, respecprecipitation, King (28) reported that small tively, compared with 1.17 g obtained in the amounts of phosphonates can stabilize a su- absence of inhibitors. persaturated solution of calcium sulfate and The inhibiting properties of condensed lengthen the induction period before the vis- phosphates of both the meta- and the polyible onset of precipitation. Scale growth ex- phosphate types have been extensively studied periments made in the presence of AMP and in relation to scale formation and biological HEDP are summarized in Table IV. It can be calcification processes. In the former case it Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988
SCALE FORMATION
535
TABLE IV
gypsum crystals. For example, the amounts of Formation of Calcium Sulfate Dihydrate (Gypsum) gypsum crystals grown in the presence of SPP, Scale on Heat Exchanger in the Presence of 0.10 ppm STPP, and SHMP are 1.11, 1.06, and 0.84 g, respectively, compared with 1.17 g obtained Inhibitorsa in the absence of polyphosphates. The order Expt lnhibilor CaSO4.2H20 b in terms of effectiveness (based upon weight of gypsum crystals grown on brass heat ex6 None 1.17 changer) is SHMP > AMP ~> STPP ~ SPP 17 Polyacrylate(mol wt 2100) 0.62 control. It is interesting to note that pyrophosphate exhibits a strong inhibitory effect 52 Aminotri(methylene 0.82 on calcium phosphates (31), calcium oxalate phosphoric acid) (22), barium sulfate (30), and calcium carbon33 1-Hydroxyethylidine 0.98 ate (29) systems and only a slight inhibitory 1,1-diphosphonic acid effect for gypsum growth on a heat exchanger 37 Sodiumhexametaphosphate 0.84 surface. 51 Sodiumpyrophosphate 1.06 The results presented in this paper show that the rate of CaSO4.2H20 (gypsum) scale for49 Sodiumtripolyphosphate 0.99 mation on heat exchanger surfaces from the 46 2-Phosphonobutane 0.98 metastable supersaturated solutions follows a 1,2,4-tricarboxylicacid second-order rate law. Furthermore, it has 36 Polyacrylamide 1.12 been found that scale formation takes place directly on the surface of a heat exchanger with 40 Poly(acrylicacid:acrylamide, 0.95 70:30) no evidence of either bulk or spontaneous precipitation. The kinetic data also indicate 39 Poly(acrylicacid:hydroxypropyl 0.82 that the rate of scale formation is a function acrylate, 70:30) of surface area and metallurgy of heat exa Total calcium sulfateconcentration= 3.43 × 10-2 M, changer. The order in terms of decreasing rate heat exchangersurface area = 78 cm2. for different metals is copper > brass > stainless steel. Results of scale growth experiments made has been shown that sodium pyrophosphate in the presence of low levels (0.0-0.5 ppm) of is an effective growth inhibitor when added to polyacrylates (Commercial-B and -C) clearly CaCO3 (29) and BaSO4 (30) systems. In bio- indicate that these polymers exhibit an excellogical calcification processes, pyrophosphate lent inhibitory property. In terms of molecular ion has been found to be an effective crystal weight dependence it has been found that the growth inhibitor for calcium phosphates (31) amount of gypsum scale formed on the heat and calcium oxalate (22) systems. A surface exchanger surface decreases with decreasing adsorption mechanism has been found useful molecular weight of the polymer (250,000in describing the effect of pyrophosphate in 2100), reaches a minimum at ~ 2 0 0 0 molecmany of these systems. ular weight, and thereafter increases with deGypsum growth experiments made in the creasing molecular weight (2100-900). Results presence of 0.10 ppm of sodium pyrophos- also indicate that polymer composition greatly phate (SPP), sodium tripolyphosphate (STPP), affects the rate of gypsum growth. A mechaand sodium hexametaphosphate (SHMP) are nism based upon surface adsorption has been shown in Table IV. In contrast to larger proposed to explain the gypsum growth inamounts of gypsum grown on a brass surface hibition. in the presence of SPP and STPP, SHMP Overall, the order in terms of decreasing efshows a significant decrease in the amount of fectiveness of all the inhibitors studied is polyJournal of Colloid andlnterface Science, Vol. 123, No. 2, June 1988
536
Z . AMJAD
acrylate ( ~ 2 0 0 0 m o l wt) ~> S H M P > phosp h o n a t e ~ P A M ~ control. Scanning electron micrographs of CaSO4 • 2 H 2 0 crystals g r o w n o n brass h e a t exc h a n g e r surface reveal t h a t n u c l e a t i o n takes place preferentially at the p e r i m e t e r o f gas bubbles. A t 165 r a i n o f scale growth, the crystal size increases b y a factor o f a b o u t 5 c o m p a r e d with size o f crystals d u r i n g n u c l e a t i o n ( a b o u t 1 0 - 2 0 rain). F u r t h e r m o r e , crystals o f g y p s u m grown in t h e presence o f p o l y a c r y l a t e s are highly distorted. ACKNOWLEDGMENTS The author thanks BFGoodrich Chemical Company, Specialty Polymers and Chemical Division, for support in publishing this article. The contributions and cooperation of Joe Hooley, Bob Whalen, Dr. W. Masler, and Bud Raike are herewith acknowledged with sincere thanks. REFERENCES 1. Austin, A. E., Miller, J. F., Richard, N. A., and Kircher, J. F., Desalination 16, 331 (1975). 2. Vetter, O. J. G., and Phillips, R. C., J. Petrol. Technol./ 1299 (Oct. 1970). 3. Petrey, E. Q., and Sexsmith, D. R., Desalination 13, 89 (1973). 4. Nancollas, G. H., White, W., Tsai, F., and Maslow, L., Corrosion 35, 304 (1979). 5. Hasson, D., and Zahavi, J., Ind. Eng. Chem. Fundam. 9, 1 (1970). 6. Gill, J. S., and Nancollas, G. H., Z Cryst. Growth 48, 34 (1980). 7. NancoUas, G. H., and Klima, W. F., Paper 81, Corrosion/81, National Association of Corrosion Engineers Conference, Toronto, Ontario, i981. 8. Gritfiths, D. W., and Roberts, S. D., Paper 7862, International Symposium on Oilfield and Geothermal Chemistry, Society of Petroleum Engineers, Houston, 1979.
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9. Smith, B. R., and Alexander, A. E., J. ColtoM Interface Sci. 34, 81 (1970). 10. Gill, J. S., and Nancollas, G. H., Corrosion 37, 120 (1981). 11. Amjad, Z., Desalination 54, 263 (1985). 12. Weijnen, M. P. C., and van Rosmalen, G. M., Desalination 54, 239 (1985). 13. Amjad, Z., and Hooley, J., J. Colloid Interface Sci. 111, 496 (1986). 14. McCartney, E. R., and Alexander, A. E., J. Colloid Sci. 13, 383 (1958). 15. Kuntz, R. A., Nature (London) 211, 406 (1966). 16. Liu, S. T., and Nancollas, G. H., 3". Colloid Interface Sci. 44, 422 (1973). 17. Jones, L. W., Corrosion 17, 232 (1961). 18. Flesher, P., Streatfield, E. L., Pearse, A. S., and Hyde"s, O. D., 3rd Int. Symp. Fresh Water Sea 1, 493 (1970). 19. Libutti, B. L., Knudsen, J. G., and Mueller, R. W., Paper 179, Corrosion/84, National Association of Corrosion Engineers Conference, New Orleans, 1984. 20. Liu, S. T., and Nancollas, G. H., J. Cryst. Growth 6, 201 (1970). 21. Smith, B. R., and Sweet, F., J. Colloid Interface Sci. 37, 612 (1971). 22. Gardner, G. L., J. Phys. Chem. 82, 864 (1978). 23. Nancollas, G. H., and Sawada, K., Paper 312, Fifth International Symposium on Oilfield, San Diego, 1980. 24. Brecevic, L., Sendijarevic, A., and Furedi-Milhofer, H., Colloids Surf 11, 55 (1984). 25. Reddy, M. M., and Nancollas, G. H., Desalination 12, 61 (1973). 26. Koutsoukos, P. G., Amjad, Z., and Nancollas, G. H., J. Colloid Interface Sci. 83, 599 (1981). 27. Rizkalla, E. N., J. Chem. Soc. Faraday Trans. 79, 1857 (1983). 28. King, T. M., Paper NATR5S, 158th National Meeting of American Chemical Society, New York, 1969. 29. Raistriek, B., Discuss. Faraday Soc. 5, 234 (1949). 30. Nancollas, G. H., and Liu, S. T., Soc. Petrol. Eng. J.~ 509 (1975). 31. Robertson, W. G., Calcif. TissueRes. 11, 311 (1973).