Observations and Mechanism of CaSO4 Fouling on

Article pubs.acs.org/IECR

Observations and Mechanism of CaSO4 Fouling on Hydrophobic Surfaces Yongwei Cai,† Mingyan Liu,*,†,‡ and Longfei Hui† †

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ‡ State Key Laboratory of Chemical Engineering, Tianjin 300072, China ABSTRACT: Pool boiling of CaSO4 solution on prepared microscale and nanoscale hydrophobic titania−fluoroalkylsilane (TiO2−FPS) composite coatings on polished AISI304 stainless steel (SS) substrates was carried out to evaluate the antifouling behavior of these surfaces. Lower fouling resistance and looser, slender, and larger CaSO4 crystals on hydrophobic TiO2−FPS coatings were observed compared to those on the TiO2 coatings and SS surfaces. The colloidal interaction energies between crystalline particles and coated surfaces were analyzed by using the extended Dejaguin− Landau−Verwey−Overbeek (XDLVO) theories to explore the possible mechanism of inhibition of fouling. The results of the XDLVO analyses generally agree to the experimental observations. The Lewis acid−base component contributes most of the total XDLVO interaction energy. Low surface free energy and electron donor component of heat transfer surface lead to a low fouling resistance and a small initial deposition rate of CaSO4 fouling. On the basis of the XDLVO evaluations, a key strategy to reduce the CaSO4 deposition rate on heat transfer surface is suggested.

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INTRODUCTION Fouling on heat transfer surface is a troublesome and not wellsolved problem in the process industries, and fouling on boiling surfaces is more serious. Surface modification technologies are effective measures of fouling mitigation by reducing the surface free energy.1,2 Several technologies were applied to prepare the low-energy antifouling coatings, such as ion implantation with SiF3+, MoS2, H+, and F−,1,3 plasma chemical vapor deposition for SiOx coating,4 electroless technology for Ni−P−PTFE,3,5 Ni− Cu−P−PTFE,6,7 and Ni−P−BN8 coatings, magnetron sputtering technology for TiN,9 TaC,3 and diamond-like carbon (DLC)10,11 coatings, liquid phase deposition (LPD) method12,13 or the vacuum vapor plating technique14 for TiO2 coatings, etc. Fluoroalkylsilane (FPS) is often used to prepare hydrophobic and superhydrophobic surfaces with low surface free energy.15−17 Hence, it is feasible to reduce the surface free energy by modification of the coatings with FPS. Liu18 investigated the performance of CaCO3 deposition on the hydrophobic FPS coatings on the copper substrate. The results showed that the nucleation rate on the hydrophobic coating was lower than that on the hydrophilic substrate surface and the fouling formed on the FPS coating was very thin and fell off easily. Ning19 prepared the hydrophobic coating with a dipping method on the copper substrate in the hybrid fluorosilicone oligomer colloidal solution and experimentally found that the adhesion between CaCO3 and hydrophobic coating was relatively poor. However, there are some disadvantages for direct modification of organic composite coating on the copper. For example, the organic coating is not resistant to wear, and the surface morphology of the substrate cannot be modified with a thin organic coating. One can prepare TiO2 coatings first on the substrates and then further modify the TiO2 coatings with FPS to obtain hydrophobic coatings. These TiO2−FPS coatings have been done recently by the authors.20 The characteristics of CaCO4 fouling on these surfaces © 2014 American Chemical Society

during pool boiling have been evaluated and good antifouling and heat transfer results were obtained. However, fouling mechanisms have not been investigated. Generally, fouling results from the deposition of particles onto the heat transfer surfaces through a combination of physical and chemical interactions. While the drag forces bring the particles close to the surface, it is the colloidal interactions that cause the particles adhering to the surface.21−24 The particle gravity is often negligible if its diameter is less than 1 μm.21 The fouling deposition on heat transfer surface is a combination of fluid drag force and colloidal force interaction energies.22 It is necessary to analyze the colloidal interaction energies between particles and surface to predict the fouling tendency. Meanwhile, it is not enough to consider only the surface wetability (hydrophobicity or hydrophilicity), and the wettability of colloidal particles should also be considered.25 Previous studies often focused on the effects of surface contact angles or surface free energy on fouling deposition behavior. It is necessary to assess the adhesive energy between fouling particles and the surfaces to predict the antifouling properties of various surfaces. Many methods have been applied to calculate the surface free energies with the measurement of contact angles on a solid surface, i.e. the Zisman approach,26 two liquid method,27 one liquid method,28 Owens−Wendt equation,29,30 and Young− Good−Girifalco−Fowkes equation.28 Among them, the Young− Good−Girifalco−Fowkes equation has often been used to obtain the nonpolar Lifshitz−van der Waals (LW) component, γLW i and the polar Lewis acid−base (AB) component, γAB i , as well as the electron-acceptor component, γ+i , and that of the electrondonor, γ−i . Received: Revised: Accepted: Published: 3509

July 19, 2013 January 22, 2014 January 29, 2014 January 29, 2014 dx.doi.org/10.1021/ie402308m | Ind. Eng. Chem. Res. 2014, 53, 3509−3527

Industrial & Engineering Chemistry Research

Article

The fluoroalkylsilane used for preparation of the hydrophobic coating is 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyltri-isopropoxysilane (CAS registry number 24623480-0), and the molecular formula is C19H25F17O3Si. The mass percentage of C in FPS-17 is 32.57%. This chemical was produced by Tianjin Kilo Pharmaceutical Sci-Tech Co., Ltd. The company’s internal product serial number is FPS-17. The structure formula and three-dimensional sphere model diagrams are shown in Figures 1 and 2.

The stability of the foulant system is often interpreted with classical Dejaguin−Landau−Verwey−Overbeek (DLVO) theory.31,32 The theory consists of two forces: van der Waals force (LW) and electrostatic double-layer force (EL). Recent investigations suggested that certain special interaction forces exist between hydrophilic or hydrophobic colloid particles and play a determinative role for the stability of colloidal dispersion systems. Colloidal stability behaviors cannot be explained fully by the classical DLVO theory, and the extended DLVO approach (XDLVO) is based on the classical DLVO theory.28 These additional interaction energies in the XDLVO theory are often attributed to a short-ranged acid−base (AB, electron donor/ electron acceptor) interaction energy and Brownian motion (Br) force,33 etc. Ohki and Ohshima34 studied the interaction energy and aggregation of acidic phospholipid vesicles in vesicle suspensions and found that the XDLVO approach is more accurate in describing interaction energies occurring at a separation distance less than 2 nm. Brant et al.25 investigated the contribution of the AB interaction energy to the total interaction energies between the membranes and the colloids. The interaction energy based on the XDLVO approach to evaluate the membrane-colloid interaction energies was compared to those from the classical DLVO theory. The results showed that DLVO potentials were almost equal in all the membrane−colloid systems. However, inclusion of AB interactions resulted in a substantially different (qualitative and quantitative) prediction of short-range (separation distances < 5 nm) interaction energies for several of the membrane−colloid combinations investigated. Our preliminary studies indicate that the results of CaCO3 fouling experiments reported in ref 20 could not be explained by the XDLVO theory well. As insoluble salts, CaCO3 solution was prepared by CaCl2 and NaHCO3 to obtain fouling solution of high concentration. During the process in which the solution was heated to saturation temperature, much CO2 gas escaped from the fouling solution. Therefore the concentration of CaCO3 foulant was not constant during the fouling test. The inconstant concentration of the CaCO3 foulant would affect the calculation accuracy of the parameters of the XDLVO. Hence here we have replaced the fouling solution to CaSO4 solute. The concentration of the CaSO4 foulant stayed almost constant because of the slightly soluble properties of CaSO4 in pool boiling. On the basis of the pool boiling experiments of CaSO4 solution, fouling mechanism and tendency on the microscale and nanoscale hydrophobic TiO2−FPS composite coatings on polished AISI304 stainless steel (SS) substrates were explored in this work with XDLVO theory. TiO2−FPS composite coatings were first prepared based on the LPD and characterized according to the surface morphologies, depth profiles, surface free energy, and chemical elements by corresponding analysis instruments. Experiments of pool boiling of CaSO4 solution on these coatings were then carried out and fouling and heat transfer performances were evaluated. Finally colloidal interaction energies between coated surfaces and CaSO4 particles were estimated with XDLVO and DLVO theories and deposition mechanism and tendency on these coated surfaces were discussed.

Figure 1. Structural formula of FPS-17.35

Figure 2. Three-dimensional spherical model of FPS-17.

TiO2 coatings were obtained by LPD and heat treatment methods (heating rate 2−5 K·min−1, sintering temperature 773− 873 K) on polished AISI304 type SS substrates with diameter of 180 mm and thickness of 5.07 ± 0.02 mm. TiO2 coatings were immersed into the hydrophobic solutions with FPS mass concentration of 0.1−0.3% and, then, were transferred into an stainless steel oven (101−3BS, Shandong Longkou furnace factory, China) to be dried at 100−200 °C. Different degrees of the hydrophobic properties of TiO2−FPS composite coatings could be available with different TiO2 coating surface roughness, FPS solution concentration, and immersion time. TiO2 coating samples named TA, TB, TC and TD and hydrophobic TiO2− FPS composite coatings samples named TFA, TFB, TFC, and TFD were prepared for experimental investigations. Film thickness was measured by the optical measuring instrument of thin film thickness (SGC-10, Tianjin Gangdong Scientific and Technical Development, Co., Ltd., Tianjin, China). The thickness of TiO2 coatings TA, TB, TC, and TD are 157.2 ± 2.3, 130.4 ± 1.6, 189.9 ± 1.9, and 335.9 ± 0.8 nm, respectively. The TFA, TFB, TFC, and TFD were obtained by treating the TA, TB, TC, and TD in different hydrophobic solution of the FPS. The thin layer of the FPS formed on TiO2 coatings is less than 2 nm in thickness.36 Hence, the thickness change of TiO2−FPS composite coating caused by FPS is negligible compared with TiO2 coating. Surface morphology of the coating was analyzed by the field emission scanning electron microscopy (FE-SEM, Supra 55VP, Zeiss Co., Germany). Chemical elements in the coating were determined by the X-ray photoelectron spectroscopy (XPS) technology with a PHI 1600 ESCA (Perkin-Elmer Co., USA) and energy dispersive X-ray spectroscopy (EDS). The distributions of various elements along the coating surface depth were measured with the instrument of Auger depth profile analysis (PHI-4300/SAM, Perkin-Elmer Co., USA). The technique combines the Auger

1. EXPERIMENTAL SECTION Detailed contents on experimental aspects can be found in another published paper,20 and here, only a brief description is given. 3510

dx.doi.org/10.1021/ie402308m | Ind. Eng. Chem. Res. 2014, 53, 3509−3527


Article

electron spectroscopy (AES) and ion sputtering technologies. The measuring parameters are as follows: Ar+ ion beam energy of 10 keV; ion beam current density of 23 μA·cm−2. The contact angles of the coating surfaces were measured using the sessile drop method31 with the video optical contact angle measuring instrument (OCA20, DataPhysics Instruments GmbH, Filderstadt, Germany) at 293 ± 1 K. The standard liquids used were water (doubly distilled), formamide (Tianjin Jiangtian Chemical Co., Ltd., Tianjin, China), and diiodomethane (≥98.0%, Tianjin Chemical Reagent Research Institute, Tianjin, China). Each injection volume of standard liquid was 2 × 10−9 m3 (i.e., 2 μL). The solid surface free energy was calculated with the system of equations method.29 To measure the contact angles of CaSO4, CaSO4·2H2O crystals were compacted into a circular disc with the diameter of 13 mm and thickness of 1.5 mm using a handpunch tablet machine (TDP-5, Guangzhou Hengxing Machinery Co., Guangzhou, China). The surface force topography as well as surface roughness of the coating was measured by atomic force microscope (AFM) (D3100 M MMAFM/STM, USA).The images were taken in a tapping mode with a Si3N4 tip. The scan areas of 1.5 × 1.5 μm2 for SS substrate and 2.0 × 2.0 μm2 for the coatings were chosen for AFM topographic images with a resolution of 256 × 256 data points, and the scan rate is 0.9766 Hz. The ζ potential of CaSO4 particles in the prepared solution was measured with zeta potential analyzer (ZetaPlus, Brookhaven instruments Corp., USA). The measuring parameters are as follows: conductance 36501 μS, current 150 mA, electric field strength 12.18 V·cm−1, and pH value of the solution 7.57. Each measurement was repeated three times. The size distribution of the fouling particles was measured with the ZetaPlus particle size software (Brookhaven Instruments Co., USA). The test parameters are as follows: temperature 298 K, solution viscosity 0.890 cP, referenced fluid 1.330, angle 90°, and wavelength 659.0 nm. Each measurement was repeated three times. Calcium sulfate crystallizing from an aqueous solution appears in three formsgypsum (CaSO4·2H2O), calcium sulfate hemihydrate (CaSO4·1/2H2O), and anhydrite (CaSO4).37 All three forms are the inverse solubility salt and only gypsum is expected to crystallize and form deposits on the heated surface. Calcium sulfate solution was prepared by dissolving two kinds of analytical grade chemicals of calcium nitrate tetrahydrate and anhydrous sodium sulfate38 in deionized water with temperature of 293 K, as shown in eq 1.

Figure 3. Schematic diagram of pool boiling apparatus: (1) online acquisition and control system, (2) control cabinet and power supply, (3) thermocouples of solution bulk temperature measurement, (4) straight tube auxiliary heater of solution, (5) spiral tube auxiliary heater of solution, (6) solution outlet, (7) boiling pool holder, (8) ground lead, (9) high-speed video camera, (10) monitor, (11) lighting source, (12) metering tank of condensate, (13) condensate storage tank, (14 and 23) liquidometers, (15) outlet of condensate, (16) vapor condenser, (17) inlet of cooling water, (18) outlet of cooling water, (19 and 32) heat insulation layer, (20) inlet of vapor stream, (21 and 31) vapor vent valves, (22) pressure gauge, (24) solution sample outlet, (25) glass view port, (26) reference line for bubble size measurement, (27) bolts, (28) heat-resistant silicone gasket, (29) coated disk, (30) vapor bubbles, (33) copper heating unit, (34) thermocouples of copper and disk temperature measurement.

pressure. Some images were taken by Sony digital camera (SONY DSC-F717, Shanghai Suoguang Electronics Co., Ltd., Shanghai, China). Heat flux through polished or coated surface was determined by a two-dimensional stationary heat conduction measurement and calculation. Heat flux distribution in radial and axial directions through the substrate disk or sample was numerically simulated with ANSYS V14.0 commercial software (ANSYS, Inc. USA). The heat flux was validated with the volumes of the steam condensate per unit pool boiling time. On these experimental data, fouling resistance can be calculated.20,39 The maximum uncertainty for measuring the heat flux is 5.15%. The maximum error for the heat transfer coefficients evaluation is about 6.69%. The average heat loss of the apparatus in the course of the experiments is 6.42%.

Ca(NO3)2 ·4H 2O + Na 2SO4 → CaSO4 ·2H 2O + 2NaNO3 + 2H 2O

(1)

2. RESULTS AND DISCUSSION 2.1. Coating Characterization before and after Fouling Experiments. 2.1.1. Contact Angles and Surface Free Energy of Coatings and CaSO4·2H2O Crystals. Table 1 shows the contact angles of polished SS and different coatings. The symbols of θw, θf, and θd, in Table 1 represent the contact angles of water, formamide, and diiodomethane, respectively. We use DAs method29 with one nonpolar standard liquid, diiodomethane, and two polar standard liquids, water and + − formamide, to calculate γLW S , γS , and γS . Then the solid surface free energy is equal to

Figure 3 shows the schematic diagram of SS pool boiling apparatus. The apparatus consists of a cylinder pool with inner diameter of 260 mm and depth of 550 mm, a vapor condensation unit, a cylinder cooper heater with diameter of 70 mm, and length of 200 mm wound electric heating coil as power source mounted horizontally with one end contacting a polished or coated cylinder plate placed vertically, a power control unit, a solution injection unit, an auxiliary heating system, an online data acquisition system, and a CMOS high-speed camera system (BASLER A504k, 500−1000 fps, Germany). Pool boiling and fouling deposition phenomena on the polished or coated surfaces can be observed through the front or side glass windows. Solution bulk temperature, central temperature of copper heater, and wall temperature of the polished or coated substrates were measured by eight E-type thermocouples. Nucleate pool boiling and fouling experiments were carried out at atmospheric

γS = γSLW + γSAB = γSLW + 2 γS+γS−

(2)

+ − The calculated three surface tension components (γLW S , γS , and γS ) AB as well as the polar energy component (γS ) and the total free energy

component of each heat transfer surface are shown in Figure 4. 3511



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is 33.85 nm. So they have different surface free energy. However, the reason for the correlation between the surface properties and the coating thickness is not clear. Moreover, the quantitative relationship among the surface materials, surface roughness, and the coating thickness needs in-depth research. Table 2 lists the results of contact angles and surface free energy of CaSO4·2H2O crystals. 2.1.2. Surface Morphology Observations with FE-SEM. Figure 5 shows the surface morphology amplified by 5 000 and 100 000 times for SS, TB, TFA, TFC, and TFD, respectively. It can be found from Figures 5a, c, e, g, and i that there are many stripes on the surfaces, and these were formed when the substrates were polished. There are many TiO2 cracking stripes on TFD surface, as shown in Figure 5i. The crystal structure is not observed on polished SS surface, as shown in Figure 5b; while many TiO2 nanoscale particles can be seen on TiO2 coating surfaces, as shown in Figures 5d, f, h, and j. On TFA, TFC, and TFD surfaces, FPS material had been modified on these particles and thus these coatings have good hydrophobic properties. It should be noted that all FE-SEM images were chosen at the representative location on the surfaces. The EDS analyses were carried out on the TB and TFA coatings, as shown in Figure 6. The main peaks in the EDS images are Fe, Cr, and Ni elements which come from the SS substrates because the depth of EDS beam irradiation is several micrometers, and it is far greater than the thickness of the coatings. The intensity of Ti element is also strong, and the atom content (at %) is 3.18% in TB coating. Besides, the atom content of O element is 15.84% which is significantly higher than the stoichiometric ratio of TiO2 compound, which indicates that the O atoms in the coating also come from other substances, such as organic pollutants or dust. Trace Si element was also detected in the TB coating, which may come from the polishing paste because the polished SS substrate used for coating preparation may not be completely cleaned. The elements of Ti, F, and Si in TFA coating were detected, and the atom content is 3.74%, 3.35%, and 1.02%, sequentially. The results indicate that FPS film has been modified on the TiO2 coating. 2.1.3. Surface Force Topography and Roughness of Coatings with AFM. Figure 7 presents two- and threedimensional AFM images of polished SS, TB, TFA, and TFD surfaces and corresponding values of the surface roughness are listed in Table 3. Ra is the arithmetical mean deviation of assessed profile,41 l is sampling length, and z(x) is the height of the assessed profile at any position x.

Table 1. Contact Angles of Different Surfaces contact angle/deg

a

specimen

θw

SS TA TB TFA TFB TFC TFD

43.2 ± 3.6 74.7 ± 5.0 91.4 ± 7.9 120.1 ± 1.2 114.0 ± 6.6 109.9 ± 4.7 105.0 ± 2.4

a

θf

θd

36.0 ± 2.5 45.8 ± 7.5 70.1 ± 3.2 112.1 ± 0.8 102.0 ± 1.3 107.2 ± 6.1 100.6 ± 1.5

36.9 ± 1.7 34.8 ± 5.6 69.1 ± 1.4 104.6 ± 1.9 89.0 ± 0.9 91.6 ± 2.7 88.0 ± 2.9

Symbol “±” represents the standard deviation.

Figure 4. Surface free energy and its components of different surfaces.

From Table 1 and Figure 4, we can see that the polished SS substrate has the largest surface free energy, 47.4 mJ·m−2, while the value of γsAB is also very large, which indicates that the polarity of the polished SS substrate is very strong. The surface free energy of TA coating is very close to that of the polished SS; however, the value of γsAB has dropped a lot. Compared with SS, TA, and TB, the surface free energy of the composite coatings reduces greatly, which means that the hydrophobicity of the composite coating has increased greatly. The γLW component s makes the most contribution to the total surface free energy of γs. Although TFB, TFC, and TFD coatings have almost the same values of γs, their components of are quite different, which indicates that the mechanism of the effect of each energy component on its total surface free energy is very complex and the reasons should be further investigated. In addition, the electron donor component γ− is significantly higher than the electron acceptor component γ+ of each surface. The smaller the total surface free energy is, the smaller the electron donor component.5,40 Generally, the surface free energy is affected by the surface material properties and the surface roughness.29 TiO2 coatings of TA and TB have the same material, but they have a different surface roughness. The surface roughness of TFA is 27.11 nm, as shown in the following section 2.1.3. The FPS hydrophobic film is of a self-assembled monolayer, which has little effect on the surface morphology.36 So the surface roughness of TA is about 27 nm. The surface roughness of TB

Ra =

1 l

∫0

l

| Z ( x )| d x

(3)

Rq is the mean square deviation of the assessed profile. Rq =

1 l

∫0

l

41

z 2(x) dx

(4)

Figure 7a and b show that the polished SS substrate has the smallest surface roughness of 4.75 nm compared with other coating surfaces. Many parallel stripes are found on the polished

Table 2. Contact Angles and Surface Free Energy of CaSO4·2H2O Crystals surface free energy/mJ·m−2

contact angle/deg specimen

θw

θf

θd

γs

γLW s

CaSO4·2H2O

16.1 ± 1.8

19.4 ± 0.2

35.8 ± 5.7

55.4

31.2

3512

γAB s

γ+

γ−

24.3

2.8

53.4



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Figure 5. FE-SEM of different surfaces: (a, b) SS; (c, d) TB; (e, f) TFA; (g, h) TFC; (i, j) TFD.

TiO2 coating was coated with a layer of the FPS. The FPS coating is very thin and has very little contribution to the surface morphology of the coating. Figure 7g and h show that the TFD coating is cracked in certain degree due to the thermal stress when the coating is sintered with high temperature.

SS surface which is the scratch after the substrate was polished. From Figure 7c and d, we can see that the coating TB has many white points, which results from the accumulation of TiO2 particles. These particles are also observed on the composite hydrophobic coatings, as shown in Figure 7e and f; although the 3513



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about 70 nm, which demonstrates that the interface between the coating and the substrate is not obvious. This phenomenon could be explained in two aspects. On the one hand, the thermal diffusion happened during the sintering of the coating, and on the other hand, the SS substrate is not an ideal smooth surface. The total sputtering time is about 6 min, so the depth of the groove in TD can be estimated as 600 nm, as shown in Figure 8c. Figure 9c shows that the cross-section curve of coating elements can be divided into three regions, FPS coating at left, TiO2 coating in the middle, and the interface between the TiO2 coating and the SS. When the sputtering time is 0.4 min (about 8 nm in depth), the elements of Si and F both had the maximum content; when the sputtering time was longer than 20 min, the SS substrate was detected. The content of the element C on the coating surface is very high, which mainly comes from the FPS compound. The atomic percentage contents of Ti and O elements are approximately 20% and 70% in the TiO2 coating, respectively. In addition, the trace content of Fe element was observed in the coating as well as Ti element also existed in the SS substrate. These results indicate that both the Ti and Fe elements had undergone the atomic diffusion during the sintering process of the TiO2 coating. The content of element O increases slightly near the interface of the coating and substrate, which can be seen in Figure 9c. The reason might be that the SS substrate surface had been partially oxidized when polishing and cleaning. Furthermore, the changing trends of the elements of Ti, O, and Fe at the interface are not very steep, which indicates that there is no apparent interface between TiO2 coating and SS substrate. These results can be explained by two aspects. One is that the thermal diffusion of the various elements at the interface has been happened; the other is that the substrate is not an ideal smooth surface and has a certain surface roughness. On the coating surface, the contents of F and Si elements are about 2.3% and 10.2%, respectively. In addition, the trace amounts of F and Si elements have been found in the internal of the TiO2 coating, which may be a small amount of the FPS compound had penetrated into the interior of the TiO2 coating. The total sputtering time is about 25 min, so the depth of the groove in TFD can be estimated as 500 nm, as shown in Figure 9c. 2.1.5. Chemical Elements on Coating Surface with XPS. Figure 10 shows the XPS of representative coatings of TB and TFC. The binding energies of 529.4 and 458.7 eV present the characteristic peaks of the elements of O and Ti in Ti−O chemical bonding,36 respectively, as shown in Figure 10a. The atomic content ratio of Ti to O element is 0.445, which is slightly less than the chemical stoichiometric ratio of TiO2 crystals indicating that some O element in the TiO2 coating may come from other compounds, such as organic pollutants. It can be seen from Figure 10b that, the binding energy of 102.6 eV for Si2p orbit electrons had been excited, which is the characteristic peak of the Si−O bond.42 The XPS peaks at the binding energies of 529.4 and 532.5 eV correspond to the O element in the O−Ti and O−Si bonds, respectively, and the peaks at the binding energies of 458.7 and 464.9 eV correspond to the Ti element in the T−O bonds. The binding energy of 688.9 eV belongs to −CF2 bond group. In other words, the Ti−O bond comes from the TiO2 coating, and the Si−O bond is the chemical bond connected between the FPS and the TiO2 coating, while the −CF2 group comes from the FPS coating. 2.2. CaSO4 Fouling Investigations. Figure 11 shows the fouling experiments on surfaces of SS, TA, TFA, and TFB at the heat flux of 50.69 kW·m−2 and the CaSO4 fouling concentration

Figure 6. EDS analyses of TB and TFA surfaces.

2.1.4. Auger Depth Profile of Coating Elements. TD and TFD were selected to measure the depth profile of the chemical elements. The ion sputtering rate for TD was about 100 nm·min−1. A smaller sputtering rate of 20 nm·min−1 was chosen to measure TFD composite coating because the FPS materials on TFD surface is very thin. The morphology before sputtering, the groove morphology, and the cross-section curve of the chemical elements of the coatings TD and TFD are shown in Figures 8 and 9, respectively. Figures 8a and 9a show that both coatings of TD and TFD are obviously cracked. The sizes of the groove for the coatings of TD and TFD are 1375 × 375 and 1633 × 519 μm, respectively, as shown in Figures 8b and 9b. Figure 8c shows that TiO2 coating is detected when the sputtering time is less than 2.5 min (about 230 nm) and the chemical elements from the polished SS substrate can be found if the sputtering time is larger than 4.0 min (about 360 nm). The content of C element is about 80% on the TiO2 coating surface, and these C atoms might come from the organic pollutants. The coating is mainly composed of Ti and O elements, and the atomic percentage is about 30% and 50%, respectively. In addition, trace Fe element in the TiO2 coating and Ti in the SS substrate are both found, which indicates that the diffusion of the elements took place through the interface of the coating and the substrate during the sintering process. The content of O element near the interface is slightly higher than that in the TiO2 coating, which indicates that part of Fe atoms on the SS substrate surface had been oxidized when the SS was polished and cleaned. Moreover, it can be seen from Figure 8c that the width of the interface is 3514



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Figure 7. 2D and 3D AFM topography of different surfaces: (a, c, e, and g) 2D AFM images of polished SS, TB, TFA, and TFD, respectively; (b, d, f, and h) 3D AFM image of polished SS, TB, TFA, and TFD, respectively.

of 2.1 g·L−1. The results indicate that the fouling resistance on the SS surface is higher than that of the TiO2 and TiO2−FPS composite coatings at the same heat flux and CaSO 4 concentration and is about two times of TiO2−FPS composite

coatings. TiO2−FPS composite coatings can significantly reduce the fouling resistance on heat transfer surfaces and improve the antifouling properties. In addition, the fouling deposition rates at initial time on the TFA and TFB coatings are less than those on 3515



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coating surface were found, as shown in Figure 14a and b. On the TFA surface, the CaSO4 crystals are looser, slender, and larger than those on the TB surface, as shown in Figure 14c and d. Compared with CaSO4 crystals on the DLC sputtered surface,1 CaSO4 crystals on TFA surface are more slender and fluffy. The previous studies were often focused on the fouling morphology after tests, and little attention was paid on the formation process of the fouling during test. This study attempts to photograph the morphology of the fouling deposited on the heat transfer surface online and to analyze the formation process of fouling. Figure 15a shows the pictures of fouling formation versus time with high speed CMOS camera. Figure 15b shows the fouling resistance curve on coating TFD at heat flux of 33.47 kW·m−2 and CaSO4 concentration of 2.3 g·L−1. The copper heater was removed from the back of the coating disk at certain time intervals. After some time, there were no bubbles were found on the coating disk, and then, the fouling pictures were photographed quickly. Figure 15a shows that small and sparse fouling points were observed on the heat transfer surface after fouling test was continued for 60 s (time point ①, as shown in Figure 15b). The scale points were the locations for the bubbles formation and growth. The heat transfer surface between the fouling points remained clean and almost no foulant was observed. Once a small number of fouling points were formed, the fouling points would quickly grow up and connect with the adjacent fouling points. This phenomenon could be demonstrated by the rapid increase of fouling resistance at time of 3600 s (②), as shown in Figure 15b. The relatively denser layer of CasO4 fouling had been formed after

Table 3. Surface Roughness of Different Surfaces specimen

SS

TB

TFA

TFD

Ra/nm Rq/nm

4.75 6.64

33.85 74.16

27.11 55.40

53.39 80.29

the SS and TA surfaces. It should be noted that the fouling curve has not reached the asymptotic fouling resistance after fouling time of 11 h on the TFA coating. The explanation and discussion will be given in the following section. Figure 12 shows the morphology of CaSO4 crystallization fouling on SS surface at heat flux of 50.69 kW·m−2 and CaSO4 concentration of 2.1 g·L−1. Two types of crystallization scale are found: relatively loose fouling with snowy white color at position (I) and more hard and compact fouling with pale yellow color at position (II), as shown in Figure 12a. The fouling at location (I) is of gypsum crystals with elongated hexagonal form (Figure 12b and c), and the fouling at position (II) is of amorphous of solid blocks (Figure 12d and e). The result indicates that the surface temperature at position (II) was higher than that at (I), and gypsum crystals might lose water and became denser. The morphology of fouling at position (II) is similar with that of the CaSO4 fouling deposited on the SS tube observed by Zhao et al.43 The EDS analysis of fouling at position (I) is shown in Figure 13. The results show that the elements of Ca, S, and O are detected, and the atom percentage (at %) is 27.03%, 26.83%, and 46.14%. Polymorphs of CaSO4 fouling on coated TB and TFA surfaces at heat flux of 50.69 kW·m−2 and CaSO4 concentration of 2.1 g·L−1 are given in Figure 14. Hexagonal columnar crystals of CaSO4 on TB

Figure 8. Auger depth profiles of TD coating. 3516



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Figure 9. Depth profiles of TFD coating.

2.3.1. XDLVO Theory. We assume the fouling particle as a sphere, the subscripts of “1”, “2”, and “3” in the symbols represent the spherical fouling particle, heat transfer surface (plane), and water medium (similarly hereinafter), respectively, in the following equations. It was suggested by van Oss45 that the energy balance performed for aqueous systems accounts for the LW, EL, AB, and Br interaction energies, which results in the extended DLVO theory and may be written as46

the fouling test continued to 18 000 s, and fouling resistance increased obviously. The fouling layer became very dense after the time reached 21 600 s (⑤). Some fouling peeled off from the heat transfer surface after the fouling time continued to 25 860 s. At the same time, fouling resistance suddenly decreased, and the fouling curves became discontinuous, as shown in Figure 15b. The fouling resistance increased slowly again after that because the new fouling was deposited at both the previous peeling off area and other location of the heat transfer surface. The peeling process of fouling depends on many factors, such as the thermal stress in the heat transfer process, and the thermal effects caused by changes in the structure of the fouling, as well as bad combination between the fouling and the wall.44 It is noted that more discrete fouling points are found in the pool boiling of CaCO3 solution and that not much of the foulant remains on the coating surface.20 The reason is that most CaCO3 has been crystallized from the fouling solution with the CO2 gas escapes continuously in the early stage of the fouling test.20 However, a relatively denser layer of CasO4 foulant is found on the heat transfer surface in the CaSO4 fouling test. 2.3. Interfacial Interaction Energy Analyses between Coating and Crystal Particle in CaSO4 Solution with XDLVO Theory. The XDLVO theory can only apply to particulate deposition. However, even for the fouling type of scaling, the formation of the first fouling layer on the heat transfer surface is the most important thing in the entire scaling process. In this layer, the particulate deposition of CaSO4 crystal particles dominates, and the formation mechanism of fouling can be analyzed be the XDLVO theory.

ΔGTOT = ΔGLW + ΔGEL + ΔG AB + ΔGBr

(5)

According to eq 5, the colloid particles will adhere to the surface if ΔGTOT < 0. 2.3.1.1. Lifshitz−van der Waals Interaction Energy. The LW interaction energy component between the fouling particle 1 and the heat transfer surface 2 in aqueous solution 3 changing with the separation distance, H, can be expressed as46 LW ΔG132 (H ) = −

A132 R 6H

(6)

where R is the radius of the fouling particles, H is the distance between spherical particle and plane of heat transfer surface, and A is the Hamaker constant. For two spherical particles interacting in water media, the LW interaction energy changing with the distance H can be written as47 LW ΔG131 (H ) = −

3517

A131R 12H

(7)



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For a three-component system, the Hamaker constant A132 can be gained by calculating Aii of the three kinds of pure substances46 A132 = ( A11 −

A33 )( A 22 −

A33 )

(8)

For common water system, the right side of eq 8 is always positive, namely, ΔGLW 132 < 0, which means that the fouling adhesion would exist when van der Waals force is the only interaction in the system. The Hamaker constant between two identical fouling particles in water can be written as28 A131 = ( A11 −

A33 )2

(9)

where, the Hamaker constant of the pure substance Aii can be calculated with LW force at the minimum distance between two plane surfaces28 ΔGLW (H0) = −2γi LW = −

Aii 12πl0 2

(10)

Aii = 24πH0 2γi LW = −12πH0 2ΔGLW (H0)

(11)

where H0 is the minimum distance between two surface, 0.158 ± 0.009 nm.28 The LW force between fouling particle 1 and heat transfer surface 2 in water 3 at the minimum distance of H0 can be expressed as28 LW ΔG132 (H0) = −2( γ1LW −

γ3LW )( γ2LW −

γ3LW ) (12)

LW LW LW LW LW LW When γLW 1 < γ3 < γ2 or γ1 > γ3 > γ2 , ΔG132 (H0) > 0, the particle 1 and heat transfer surface 2 would be exclusive to each other. The LW interaction energy between two identical fouling particles in water can be written as28

LW (H0) = −2( γ1LW − ΔG131

γ3LW )2 < 0

(13)

The value of ΔGLW 131(H0) is always negative, i.e., two identical particles are always attractive mutually. The average diameter of CaSO4 particles is 2618.5 ± 74.4 nm in the prepared fouling solution before pool boiling experiments. The size distribution of CaSO4 particles is close to the normal distribution, as shown in Figure 16. Many large CaSO4 particles have precipitated to the bottom of the pool during the heating of the solution. Hence, the size of the crystal particles in the pool during boiling may be less than that of the prepared foulant solution before boiling experiments. 2.3.1.2. Electrostatic Double-Layer Interaction Energy. If the surface potential of fouling particles and heat transfer surface is less than 50 mV and remains constant, the electrostatic doublelayer interaction energy can be expressed as48,49

Figure 10. XPS of specimens TB and TFC.

⎡ 1 + e−κH EL ΔG132 (H ) = πεrε0R ⎢2ζ1ζ2 ln ⎣ 1 − e−κH ⎤ + (ζ12 + ζ2 2)ln(1 − e−2κH )⎥ ⎦

(14)

The EL interaction energy between two identical particles can be written as50 EL ΔG131 (H ) = 2πεRζ 2 ln(1 + e−κH )

(15)

where R is the radius of the spherical particles, m; ζ is the zeta potential, V; ε is the dielectric constant of the medium, ε = ε0εr = 4.93 × 10−10 C2·J−1·m−2, εr is the relative dielectric constant of

Figure 11. Fouling curves of different surfaces at CaSO4 concentration of 2.1 g·L−1 and heat flux of 50.69 kW·m−2. 3518



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Figure 12. SEM morphology and CaSO4 crystal type on SS surface at heat flux of 50.69 kW·m−2 and CaSO4 concentration of 2.1 g·L−1.

The ζ potential of CaSO4 particles was measured with zeta potential analyzer, which is based on the principle of the capillary electrophoretic mobility and the Smoluchowski formulation as well as the value of electrophoretic mobility of the particles to calculate the ζ potential.54,55

water, 55.65 at 273 K,51 ε0 is the vacuum dielectric constant, 8.85 × 10−12 C2·J−1·m−2; κ the reciprocal of the Debye length, which can be calculated with eq 1633,52,53 κ=

e 2NA ∑ (zi 2ci) εkBT

(16)

ζ=

where kB is Boltzmann constant, 1.38 × 10−23 J·K−1; T is the Kelvin temperature, K; zi is the valent state of the ion i in the solution, ci the molar concentration of the ion i in the solution, e is the electronic charge, 1.602 × 10−19 C; and NA is Avogadro’s constant, 6.02 × 1023 mol−1. The value of κ is 1.06 × 109 m−1 in the CaSO4 solution (mainly consisting of Ca2+, SO42−, Na+, and NO3− ions) with the fouling concentration of 2.1 g·L−1.

Uη εrε0

(17)

where ζ is the zeta potential, η is the viscosity of the solution, εr is the dielectric constant of water, ε0 is the vacuum dielectric constant, U is the electrophoretic mobility of particles. The ζ potential of the SS and anatase type TiO2 coating surface at pH of 6.8 (293 K) is −0.03547 and −0.0475 V,53 respectively. The ζ potential of the FPS coating is −0.0305 V.56 The average value of the ζ potential of CaSO4 particles in the prepared 3519



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The AB interaction energy between two identical fouling particles at the distance of H0 is expressed as28 AB ΔG131 (H0) = −4( γ1+ −

γ3+ )( γ1− −

γ3− )

(19)

The AB interaction energy between the fouling particle 1 and heat transfer surface 2 in aqueous solution 3 at the distance of H is written as28 AB ΔG132 (H ) = 2πRλ

∞

∫H

AB ΔG132 (H0)e(l0− l)/ λ dH

AB = 2πRλΔG132 (H0)e(H0− H )/ λ

(20)

The AB interaction energy between two identical fouling particles at the distance of H is expressed as28 AB ΔG131 (H ) = πRλ

Figure 13. EDS analysis of CaSO4 fouling at position (I) with heat flux of 50.69 kW·m−2 and CaSO4 concentration of 2.1 g·L−1.

γ2+ )( γ1− −

γ2− )

− ( γ1+ −

γ3+ )( γ1− −

γ3− )

− ( γ2+ −

γ3+ )( γ2− −

γ3− )⎤⎦

AB ΔG131 (H0)e(l0− l)/ λ dH

AB = πRλΔG131 (H0)e(H0− H )/ λ

(21)

where λ is the characteristic decay length, and its value is 1.0 nm at 293 K. 2.3.1.4. Brownian Motion. The free energy of the Brownian motion adhering to the surface is expressed as28 47

solution is −21.82 ± 1.56 mV, which agrees with the reported value in the literature.57 Hence, prepared CaSO4 solution is stable, and the particles have no trend of aggregation at 293 K. 2.3.1.3. Lewis Acid−Base Interaction Energy. The AB interaction energy between the fouling particle 1 and heat transfer surface 2 in aqueous solution 3 at the minimum distance of H0 is written as28 AB ΔG132 (H0) = 2⎡⎣( γ1+ −

∞

∫H

ΔGBr = 1kT = 5.147 × 10−21J

(373 K)

(22)

According to the calculation result, the values of ΔGBr is 2 orders of magnitude smaller than other components of the XDLVO approach. So, it has been ignored when calculating the XDLVO values. The decay of ΔGTOT 132 (H) and each interaction energy component of LW, EL, and AB with the distance of H in the fouling solution can be calculated with eqs 5, 6, 14, and 20, respectively. The decay of ΔGTOT 131 (H) and each interaction

(18)

Figure 14. FE-SEM images of CaSO4 crystals on coated surfaces at heat flux of 50.69 kW·m−2 and CaSO4 concentration of 2.1 g·L−1: (a and b) TB surface; (c and d) TFA surface. 3520



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Figure 15. Photograph sequence of fouling formation and deposit resistance on coating TFD at heat flux of 33.47 kW·m−2 and CaSO4 concentration of 2.3 g·L−1.

energy component of LW, EL, and AB with the distance of H in the fouling solution can be calculated with eqs 5, 7, 15, and 21. 2.3.1.5. Parameter Sensitivity Analyses. It should be noted that all of the fouling tests were carried out in saturated boiling solution (about 373 K), and the temperature of the heat transfer surface was higher than 373 K. However, the data of several parameters needed for the calculation of the XDLVO theory at

these temperatures were difficult to obtain from the theory calculations or the measurements at present. The parameters values provided in the literature were often obtained at 293 K. Furthermore, the surface free energy and its components were measured at 293 K in this paper. Hence, the calculated results of XDLVO and DLVO interaction energies may have some deviations compared with the actual fouling system. 3521



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and valence zi of the fouling particle affect the reciprocal of Debye length, κ, as shown in eq 16. Debye length is the characteristic length of the electric double layer interaction which decays with the distance H.58 If the valence zi remains unchanged and the ion concentration ci becomes higher, the value κ increases. While the static double layer becomes thinner, the potential gradient in the diffusion layer decreases faster, the ζ potential decreases more seriously, and finally the repulsion interaction of ΔGEL 132 decays faster with the distance H. Hence, the fouling particles are more easily approaching to the heat transfer surface. Therefore, the ion concentration may also affect the calculation results of the XDLVO theory. 2.3.2. Interaction Energy Profiles. Different interaction energy profiles were calculated with both XDLVO and DLVO theories. The relationship between interaction energy and separation distance H of TFB surface are shown in Figures 17. The interaction energy curves of other surfaces are similar to that of TFB surface. The comparisons of XDLVO curves of surfaces SS, TA, TFA, and TFB are shown in Figure 18. The interaction energy with separation distance H between two CaSO4 particles is shown in Figure 19. The horizontal axis (separation distance of H) is plotted using a logarithmic scale in order to emphasize the short-range differences between different prediction curves. It can be seen from Figure 17 that there is almost no difference among the values of all interaction energies when the separation distance is larger than 10 nm. Hence, the colloidal forces between the CaSO4 fouling particles and heat transfer surface are only effective at a short separation distance. For all heat transfer

Figure 16. Size distribution of CaSO4 particles at concentration of 2.1 g·L−1 and 293 K.

With a given distance of H, ΔGLW 132(H) is proportional to A132 AB and R. ΔGEL 132(H) is proportional to R, ΔG132(H) is proportional (H ). If the condition of the solution remains to R and ΔGAB 132 0 constant, two parameters of A132 and ΔGAB 132(H0) are related to the surface free energies and their components of CaSO4 particles and the heat transfer surface. Another key parameter is the fouling particle radius of R, which has a certain distribution in the solution; therefore, the average value was used for calculation in this paper. Surface free energies of the CaSO4 particles and the heat transfer surfaces, as well as the radius of the CaSO4 particles should be accurately measured to improve the accuracy of the calculation results. Both the ion concentration ci

Figure 17. XDLVO and DLVO interaction energy profiles for TFB surface. 3522



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We can take the interaction energy value of XDLVO theory at separation distance of 0.498 nm as an example, as shown in Figure 18. The total interaction energy on the TA surface is −1.30 × 10−16 (−2.6 × 104kBT) J·m−2. However, the XDLVO interaction energy on TFA or TFB surface is about −2.0 × 10−17 (−4.0 × 103kBT) J·m−2, which is 1 order of magnitude higher than that of TA surfaces. These results can explain why the antifouling property of FPS surface is better than that of TiO2 surface. By comparing the various components of the XDLVO theory, we can predict the antifouling trends of various surfaces by only calculating the AB component. The key strategy to decrease the tendency of fouling deposition is trying to reduce the AB component of the XDLVO theory. As mentioned above, TiO2−FPS coating has a relatively significant antifouling effect. This is mainly due to the coating surface has many nonpolar F−C bonds with low free energy, which results in a smaller AB component of the coating. This result consists with the conclusion in the literature59 that the nonpolar surface has a smaller AB component. Zhao and Müller-Steinhagen21 pointed out that F element could be added into the coating to adjust the surface free energy and to obtain a low free energy surface. Therefore, the AB component can be decreased by adding nonpolar F−C chemical bonds into the heat transfer coatings if the physical and chemical properties of the fouling solution cannot be changed. However, the abnormal trend of the XDLVO curve on SS surface is also found, as shown in Figure 18. The surface forces on SS surface are positive, which means that there would not be

Figure 18. Comparisons of XDLVO curves on the surfaces of SS, TA, TFA, and TFB.

surfaces, the AB interaction energy component of the XDLVO theory is 1 order of magnitude higher than the LW interaction energy component and two orders higher than the EL component. In other words, the AB interaction energy component contributes the most part of the total XDLVO energy. This result consists with the conclusion obtained by Al-Janabi et al.8 that the AB component is the main contribution for the total interaction energies at the distance of H0. Hence, the fouling tendency on the heat transfer surface can also be predicated by comparing the component values of the XDLVO theory.

Figure 19. XDLVO and DLVO interaction energy profiles for CaSO4 particles. 3523



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hydrophobic surface and the adhesion strength of bacteria on the surface could be increased if the hydrophilic surface was coated with a layer of oil or hydrocarbons. Hence, in the research area of organic fouling, the surface should be made more hydrophilic to prevent fouling deposition. In the research region of inorganic fouling, Al-Janabi et al.8 concluded that the heat transfer surface should have high γ− value to reduce the deposition rate of CaSO4 fouling. They attributed this result to the strong mutual repulsion interaction between the fouling and the heat transfer surface with high γ− value. However, Rosmaninho and Melo5 found that the heat transfer surface with high γ− value was easy to absorb calcium phosphate particles, while the fouling particles were difficult to nucleate on the surface with low γ− value. Since the initial deposition rate of calcium phosphate was high on the low γ− surface, the fouling was looser and easy to remove with shear stress of the fluid and, finally, the quantity of the fouling decreased. The results shown in Figures 4 and 11 indicate that the lower fouling resistance and smaller initial deposition rate of CaSO4 fouling can be obtained on the heat transfer surface both with lower surface free energy and lower γ− value. However, this conclusion is inconsistent with that obtained by Al-Janabi et al.8 This discrepancy could be attributed to the different experimental conditions because the fouling test was conducted in a convective heat transfer with bulk solution temperature of 313 K in the literature.8 The conclusion in the literature5,40,61 is consistent with the results obtained in this paper that the surface free energy is usually larger for a surface with a higher γ− value. Although the conclusion that the heat transfer surface with low γ− is favorable to the antifouling effect is consistent with that obtained by Rosmaninho and Melo,5 the results in this paper were obtained according to the fouling resistance rather than the deposition mass on the heat transfer surface.

fouled on SS surface. This conclusion contradicts the fouling curves results of Figure 11. This result shows that there are some defects to understand the fouling deposition mechanism in the pool boiling only according to the calculation results of XDLVO theory. When the AB interaction energy is considered in the XDLVO theory, there is a marked difference in the total interaction energies. The total interaction energies between fouling particles and heat transfer surfaces predicted by the DLVO theory are always positive within all of the separation distances on the surfaces of SS, TA, and TFB (Figure 17b and c), which indicates that these heat transfer surfaces will not be fouled, which is inconsistent with the experimental curves, as shown in Figure 11. Therefore, compared to DLVO theory, XDLVO theory can accurately predict fouling tendency on heat transfer surfaces. Zhao and Müller-Steinhagen21 proposed a criterion for judging the heat transfer surface whether fouled or not (immediately or TOT later) in the fouling solution. If |ΔGTOT 132 − ΔG131 | > 0, the surface TOT TOT will be fouled; otherwise, if |ΔG132 − ΔG131 | = 0, it will not be fouled. From the XDLVO curves shown in Figures 17c, it can be TOT seen that ΔGTOT 132 < 0 and ΔG131 > 0 at a shorter separation distance of H. Therefore, all the heat transfer surfaces we prepared can be fouled, and this conclusion consists with Zhao’s criterion. Figure 19 shows that the interaction energy between CaSO4 particles is mutually exclusive throughout the separation distance, which is predicted by either XDLVO or DLVO theory. The result indicates that the fouling solution is stable, and the CaSO4 particles have no trends of aggregation. The AB component is the maximum one in all the components of the XDLVO theory and is always repulsive within the separation distance. In addition, the total repulsive interaction of XDLVO theory is two orders higher than that of the DLVO theory, as shown in Figure 19. It should be noted that several other interaction energy components should be included in the XDLVO theory, such as the spatial force, entropy contribution, osmotic pressure, and dissipative force,28 which usually exit in the organic fouling. However, we considered only four interaction energies, including LW, EL, AB, and Br components, which widely exist in the crystallization and particulate fouling24 processes. The conclusions obtained from the XDLVO theory can be merely applied to the crystallization or particulate fouling systems. In addition, the surface topography of fouling particles as well as the surface roughness, three-dimensional nanoscale morphology, and the chemical composition of the heat transfer surfaces have not been included in the calculation of the interaction energies with XDLVO and DLVO theories. Hence, the results have some limitations to predict the interaction energies in the actual fouling system. Moreover, there is a controversy on the relationship between fouling deposition and the surface free energy or the Lewis electron donor component, γ−, in the literature. For instance, Visser46 indicated that the hydrophilicity and the γ− value of the SS should be improved to prohibit the deposition of calcium phosphate and whey. Brant and Childress25 mentioned that the hydrophilicity of the organic separation membranes should be enhanced in order to reduce the fouling deposition. This conclusion showed that the organic compounds contain lipophilic groups, and the combination strength between the lipophilic groups of the fouling and the surface might became strong if the hydrophobicity of the surface was increased. Besides, Oliveira60 indicated that bacteria were easy to adhere onto the

3. CONCLUDING REMARKS Compared to TiO2 coating and SS surfaces, hydrophobic microscale and nanoscale TiO2−FPS composite coatings were characterized with lower initial CaSO4 fouling deposition rate and smaller fouling resistance at the same fouling conditions. CaSO4 crystals on TiO2−FPS coatings were looser, slender, and larger in size than those on TiO2 and SS surfaces and easy to remove. The mechanism analyses of colloidal forces between fouling particles and heat transfer surfaces according to theories of XDLVO and DLVO interaction energies confirmed the above results and XDLVO analyses agreed more accurately to the CaSO4 fouling observations on various heat transfer surfaces compared to the DLVO calculations. Colloidal forces between fouling particles and heat transfer surfaces were only effective when the separation distance was less than 10 nm. For all heat transfer surfaces, the Lewis acid−base component contributed most part of the total XDLVO interaction energy and thus the antifouling tendencies of different heat transfer surfaces could be predicted only by estimating the Lewis acid−base component of the XDLVO interaction energy between fouling particles and heat transfer surfaces. The key approach to reduce the tendency of CaSO4 fouling deposition was to try to reduce the AB interaction energy component of the XDLVO theory. If the physical and chemical properties of the fouling solution could not be changed, the AB component of the surface free energy could be decreased by adding the low free energy materials with nonpolar F−C chemical bonds into the heat transfer coatings. 3524



Article

ε = dielectric constant of the medium, C2·J−1·m−2 ε0 = vacuum dielectric constant, 8.85 × 10−12 C2·J−1·m−2 εr = relative dielectric constant of water ζ = zeta potential, V η = viscosity of the solution, Pa·s θd = contact angle of diiodomethane, ° θf = contact angle of formamide, ° θw = contact angle of water, ° κ = reciprocal of the Debye length, m−1 λ = characteristic decay length, nm γ = interfacial tension, mJ·m−2 μ = chemical potential of the fouling solution, V ν = volume of the crystal nucleus, m3

Moreover, low surface free energy and low electron donor component of the heat transfer surface leads to low fouling resistance and small initial deposition rate of CaSO4 fouling. The conclusions obtained from the XDLVO theory analyses could be merely applied to the crystallization or particulate fouling systems. The surface topography of the fouling particles and coating surfaces, such as the surface roughness and threedimensional morphology, were not included in the calculation of the interaction energies with XDLVO and DLVO theories. There are some limitations to predict the interaction energies in the actual fouling system using XDLVO and DLVO theories.

■

AUTHOR INFORMATION

Corresponding Author

Abbreviation

*School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China No.92 Weijin Road, Nankai District, Tianjin 300072 Tel:+86-22-27404614(O) +86-2227892067(L) E-mail: [email protected].

AB = acid−base (polar) interaction energy component AES = auger electron spectroscopy AFM = atomic force microscope DAs = donor acceptor system DLVO = the classical Dejaguin−Landau−Verwey−Overbeek theory EL = electrostatic double-layer force F-DLC = fluorine-diamond-like carbon FE-SEM = field emission scanning electron microscopy FPS = fluoroalkylsilane LPD = liquid phase deposition method LW = van der Waals interaction energy component SS = AISI304 stainless steel TA, TB, TC, TD = TiO2 coating TFA, TFB, TFC, TFD = TiO2−FPS composite hydrophobic coating XDLVO = extended DLVO theory XPS = X-ray photoelectron spectroscopy

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (grant number: 20876106), National High Technology Research and Development Program 863 (No.2012AA052804), and Tianjin Research Program of Application Foundation and Advanced Technology (No. 09JCZDJC24100) for the financial support.

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NOTATION A = Hamaker constant Br = Brownian motion ci = molar concentration of the in the solution, mol·L−1 D = diameter of the fouling particles, m e = electronic charge, 1.602 × 10−19 C Eb = binding energy, eV H = separation distance, nm H0 = the minimum distance between two surface, 0.158 ± 0.009 nm kB = Boltzmann’s constant, 1.38 × 10−23 J·K−1 NA = Avogadro constant, 6.02 × 1023 mol−1 rc = critical radius of the embryos, nm Ra = arithmetical mean deviation of profile, nm Rq = arithmetic mean square root deviation of profile, nm S = supersaturation of the bulk solution R = gas constant, 8.314 J·mol−1·K−1 R = radius of the fouling particles, nm Rf = fouling resistance, m2·W1−·K−1 T = temperature, K Tb = temperature of the bulk solution, K U = electrophoretic mobility of particles zi = valent state of the ion i in the solution

Subscript

■

1 = spherical fouling particle 2 = heat transfer surface (plane) 3 = water b = bulk crit = critical i = ion TOT = total

REFERENCES

(1) Bornhorst, A.; Müller-Steinhagen, H.; Zhao, Q. Reduction of scale formation under pool boiling conditions by ion implantation and magnetron sputtering on heat transfer surfaces. Heat Transfer Eng. 1999, 20, 6−14. (2) Müller-Steinhagen, H.; Zhao, Q.; Helalizadeh, A.; Ren, X. G. The effect of surface properties on CaSO4 scale formation during convective heat transfer and subcooled flow boiling. Can. J. Chem. Eng. 2000, 78, 12−20. (3) Zettler, H. U.; Wei, M.; Zhao, Q.; Müller-Steinhagen, H. Influence of surface properties and characteristics on fouling in plate heat exchangers. Heat Transfer. Eng. 2005, 26, 3−17. (4) Zhao, Q.; Liu, Y. Investigation of graded Ni-Cu-P-PTFE composite coatings with anti-scaling properties. Appl. Surf. Sci. 2004, 229, 56−62. (5) Rosmaninho, R.; Melo, L. F. Calcium phosphate deposition from simulated milk ultrafiltrate on different stainless steel-based surfaces. Int. Dairy J. 2006, 16, 81−87. (6) Zhao, Q.; Liu, Y.; Wang, C.; Wang, S.; Müller-Steinhagen, H. Effect of surface free energy on the adhesion of biofouling and crystalline fouling. Chem. Eng. Sci. 2005, 60, 4858−4865. (7) Zhao, Q.; Liu, Y.; Wang, S. Surface modification of water treatment equipment for reducing CaSO4 scale formation. Desalination 2005, 180, 133−138.

Greek Symbols

at = atomic percentage γ+ = electron acceptor component of acid−base interaction energy, mJ·m−2 γ− = electron donor component of acid−base interaction energy, mJ·m−2 γs = surface free energy, mJ·m−2 −2 γAB s = acid−base component of surface free energy, mJ·m LW γs = van der Waals component of surface free energy, mJ·m−2 ΔG = Gibbs free energy, mJ·m−2 3525



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Observations and Mechanism of CaSO4 Fouling on

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