Article pubs.acs.org/IECR
Functionalized Mesoporous Silica for the Control of Crystallization Fouling Tomer Lapidot and Jerry Y. Y. Heng* Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom ABSTRACT: A seeding study to test the effectiveness of nanoparticles in mitigating crystallization fouling is reported. The effect of functionalized mesoporous silica particles on calcium sulfate crystallization was studied through a series of batch crystallization experiments. Tested surface chemistries include nonfunctionalized silica (hydrophilic), methyl (hydrophobic) propyl amine (basic), propyl sulfonic acid (acidic), and triaminetetraacitic acid (TAAcOH, chelating). Crystallization was tracked online using electrical conductivity to determine concentration as a function of time, which was used to calculate induction time and growth rate. At a loading of 0.1 mg/gsol, amine functionalized particles were found to be the most effective at reducing induction time, while TAAcOH particles were found to significantly increase induction time. The efficacy of TAAcOH particles was further tested at various loadings and was found to increase induction time 6-fold at a loading of 0.5 mg/gsol. Despite having a profound effect on induction time, growth rates remained relatively constant for all surface chemistries and loadings. Here, we show that the seed surface chemistry can play a major role in the control of calcium sulfate crystallization. habit in batch crystallizers.12 Amjad investigated the effect of additives to heat exchangers, finding that various polymers and surfactants can inhibit surface crystallization of gypsum in flow configuration.13 More recently, inorganic particles were investigated as substitutes for molecular and polymeric additives. Such particles can be manufactured at low cost and are environmentally friendly. Bansal et al. found that micrometric calcium sulfate particles can enhance deposition rate by adsorbing to the heat-transfer surface and act as seeds, whereas alumina particles can reduce deposition rate by acting as crystallization surfaces that wash away with the flow.14 Colloidal particles were also found to delay crystallization by interfering with the formation of stable nuclei. Wang et al. reported that colloidal silica particles significantly increased both induction time and rate of crystallization at particle loading of 50 mg/L.15 Particles offer a distinct advantage over traditional additives for mitigating crystallization fouling. They do not corrode the heat exchanger’s metallic surfaces and can be easily separated from the fluid phase. Consequently, their environmental impact can be better controlled when compared to molecular additives. This work explores the potential of using functionalized mesoporous silica particles to mitigate crystallization fouling. Such particles are easy to handle and separate from the fluid phase because of their large size and are known to affect
1. INTRODUCTION Crystallization fouling of heat exchangers is a problem faced by many chemical and power plants that use water as a cooling utility. Crystallization fouling reported in heat exchangers is extensively discussed by Bott1 and reported in other processes such as reverse osmosis membranes.2 Mineralized water rich in inverse solubility salts passes through heat exchanges and causes these salts to deposit on heat exchanger walls (commonly known as “scale”), consequently reducing their heat-transfer efficiency. The result is an increase in operation costs, longer maintenance downtime, and a larger carbon footprint. Traditionally, removal of scale deposits is performed using acids such as inhibited hydrochloric acid (HCl, with protective organic additives) or ethylenediaminetetraacetic acid (EDTA).3,4 Other acids explored in the literature include a range of organic acids and phosphonic acids.5,6 Although acids are extensively used in industry, they have notable drawbacks. Because of their corrosive nature they can damage metallic surfaces of heat exchangers and are difficult to dispose of safely. Calcium sulfate is extensively used as a model compound when investigating scale deposition; a variety of techniques are used in the literature to quantify both its bulk and surface crystallization.7−9 As a result, the effects of additives on calcium sulfate crystallization are also widely investigated in the context of heat exchanger fouling mitigation.10 Acrylic polymers (molecular weight 103−108) were reported to increase induction time several ten-fold at concentrations around 1 ppm.11 Small organic additives were also observed to significantly increase induction time, as well as affect crystal © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
July 29, 2016 October 16, 2016 October 18, 2016 October 18, 2016 DOI: 10.1021/acs.iecr.6b02914 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Here Ci, C*i , and Mi are the concentration, solubility, and molar mass of species i, respectively. The desired concentrations of sodium sulfate and calcium chloride were calculated numerically as the solubility of calcium sulfate depends on both the temperature and concentration of sodium chloride. The conductivity of the solution was measured using a MettlerToledo S47-K SevenMulti conductivity probe (cell constant 0.5 cm, ± 0.005 mS/cm, calibrated to 1413 μS/cm at 25 °C 0.01 M potassium chloride standard).
crystallization because of their large surface area (comparable to that of colloidal particles). Applications of mesoporous silica particles include catalysis,16 production of diffraction grade protein crystals,17 and loading of active pharmaceutical ingredients.18 Mesoporous particles are known to affect nucleation induction time and even shift the thermodynamic stability between polymorphs because of their large surface area and available pore volume.19 Such effects can be further enhanced by modifying the particles’ surface chemistry. This is accomplished by anchoring various functional groups to the particles’ surface through a process known as silanization, which changes surface properties such as hydrophobicity, acidity, and electric charge. Such properties make mesoporous silica particles good candidates for mitigating crystallization fouling.
3. RESULTS AND DISCUSSION 3.1. Functionalization Comparison. Figure 1 compares the crystallization of calcium sulfate in the presence of various
2. EXPERIMENTAL METHOD The effect of various silica particles on calcium sulfate crystallization is investigated through a batch crystallization experiment. Five different surface chemistries were chosen to investigate the effect of functionalized silica on the crystallization of calcium sulfate. Plain irregular silica was used as a reference surface chemistry to observe the effect of functionalization. Five chemical functionalities tested include nonfunctionalized silica, methyl, propyl amine, propyl sulfonic acid, and triaminetetraacitic acid (TAAcOH). The solution temperature in the jacketed crystallizer (500 mL vessel) is controlled using a water bath of type Grant GR150 R1 (±0.01 °C). Stirring is used to overcome thermaland mass-transfer limitations from the crystal surface and is maintained at 200 rpm (stirrer diameter, LI, is 7 cm; four pitched blades) for all experiments, which corresponds to a turbulent Reynolds number of 16 000. Supersaturation of the calcium sulfate solution is attained through an ion exchange reaction. Sodium sulfate anhydrous and calcium chloride dihydrate solutions (both 99% pure, Sigma-Aldrich) were prepared separately and were passed through a 0.22 μm filter upon injection to eliminate particulate contaminants. Functionalized mesoporous silica particles (Silicycle) were dispersed in a 300 mL solution of sodium sulfate. The particles are irregularly shaped amorphous silica with an average diameter of 56 μm and 6 nm pores, as confirmed by gas sorption measurements and BJH analysis. Mesoporous particles were synthesized using surfactant template guided sol−gel polymerization.20,21 Surface modification of the silica was performed using trimethoxysilanes with a desired functional group; for example, propylamine functionalization was performed with (3-aminopropyl)trimethoxysilane.22,23 The reaction takes place in an organic solvent (typically toluene) under reflux after activation of silica surface through acid treatment. The suspension was allowed to thermally equilibrate within the crystallizer, after which 10 mL of calcium chloride solution was injected at a 1:1 molar ratio (total working volume of 310 mL). This is set as t = 0 for all experiments. Only a small volume of calcium chloride was added in order to instantaneously create a supersaturated solution in an isothermal manner. All experiments were performed at a temperature of 30 °C and initial supersaturation of S0 = 2, which was calculated using eq 1 (solubility data was provided by Bock24 and fitted using quartic polynomials). S0 =
CCaSO4 * (C NaCl , T ) CCaSO 4
=
C NaClMCaSO4 * (C NaCl , T ) 2CCaSO 4
Figure 1. Crystallization curves in the absence (black) and presence of various silica particles. Particle types include unmodified (orange) mesoporous silica and functionalized mesoporous silica particles with methyl (purple), propyl amine (green), propyl sulfonic acid (red), and TAAcOH (blue) functional groups. Experiments are conducted at a loading of 0.10 mg/gsol at a supersaturation of S0 = 2.0, temperature of 30 °C, stirring at 200 rpm; experiments are repeated three times.
functionalized particles at a particle loading of 0.10 mg/gsol. Every particle type was tested three times to verify repeatability. Experimentally observed crystallization curves can be divided into three parts. Initially the solution is supersaturated, although no supersaturation is detected; this is known as the crystallization induction time (tD). Once crystallization begins, a noticeable reduction in conductivity is observed. Conductivity continues to drop until the solution achieves equilibrium. Conductivity is linearly related to ion concentration (c ∝ C) as the observed conductivity changes are small. As observed in Figure 1, the induction time conductivity of c = 12.1 mS/cm corresponds to the supersaturation S = 2, and the final conductivity of c = 10.5 mS/cm corresponds to the saturation concentration S = 1. The induction time can be calculated from crystallization curves using eq 2 c − cmax t D = tmax + 0 ′ cmax (2) where c0 is the initial conductivity and c′max is the crystallization curve’s maximum gradient. cmax and tmax are the corresponding conductivity and time where the maximum gradient is observed. The maximum gradient of the curves was determined using central differencing (eq 3) c − ct −Δt ct′ = t +Δt (3) 2Δt
(1) B
DOI: 10.1021/acs.iecr.6b02914 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research where ct refers to the conductivity at time t and Δt is the time increment between samples. Variation in surface chemistry produces crystallization curves with varying induction times; however, the steepness of the curves does not change significantly. Figure 2 compares the average induction time and maximum gradient of the crystallization curves depicted in Figure 1.
surface of the pore by acting as a Brønsted−Lowry acid, donating its proton to form the sulfonate terminal group. This is expected to decrease the local pH of the pore by pushing the equilibrium toward the bisulfate ion. Increased acidity in the pore can increase the solubility of calcium sulfate and can slow the nucleation rate of calcium sulfate. 3.2. TAAcOH Functionalization. The response observed with triaminetetraacitic acid (TAAcOH) particles (blue curves Figure 1) was notably different from all other functionalities. Although the effect of TAAcOH functionalized particles is similar to that of sulfonic acid functionalized particles, the magnitude of the response is considerably higher than that observed with other functionalities. When compared to crystallization without particles, the induction time doubles in the presence of TAAcOH particles. TAAcOH functionalization is capable of chelating multivalent ions, much like EDTA, and binds very strongly to calcium ions, forming an octahedral polydentate coordinate complex (Figure 3). In addition to
Figure 2. Comparison of average induction time and maximum gradient of crystallization curves for the data presented in Figure 1.
Nonfunctionalized silica particles reduced the induction time by 11%. This is attributed to the confined spaces of the particle’s pores which are known to enhance nucleation kinetics. Although nonfunctionlized silica is typically hydrophilic and negatively charged (because of hydroxide terminations at the particle-liquid interface), it is not chemically active in neutral salt solution. Activation of silica is typically performed through reflux in water or acid/peroxide treatment.22,23 Amine functionalization (green curves in Figure 1) reduced induction time by 26% (compared to no particle result, black curves in Figure 1), exhibiting the shortest induction times for all trials. This can be explained by the amine group’s ability to affect local concentration of ions near the walls of the pore. By acting as a Lewis base, amine groups generate positively charged ammonium cations, making the local pH inside the pores higher than that in the bulk solution. This pushes the equilibrium toward the sulfate ion and enhances crystal nucleation kinetics. Particles functionalized with methyl groups are also effective at reducing the crystallization induction time. The hydrophobicity of the methyl functionalization is hypothesized to reduce the wetting angle (and interfacial energy) of calcium sulfate crystal nuclei with the pore wall, thereby enhancing nucleation. This suggests that reduction in nucleation induction time can be achieved through several physical effects. In this case, amine groups affect nucleation by shifting the local pH (and therefore solubility) in the pore, while the effect of methyl groups is driven by variation of interfacial energy. The dominant effect highly depends on the chemistry of the attached functional group. In addition to their effect on nucleation, methyl functionalized particles were difficult to disperse in solution because of their hydrophobic nature. As a result, dispersion of these particles is expected to be problematic at higher loadings. Particles functionalized with sulfonic acid (red curves in Figure 1) were observed to have induction times that were longer than those observed for plain silica. This represents the opposite effect to that observed with amines; sulfonic acid functionalization can produce an acidic negatively charged surface. This may affect the concentration of ions near the
Figure 3. Structure of TAAcOH functionalization chelating a calcium ion, creating an octahedral polydentate ligand.
capturing a calcium ion, the chelation process protonates four sulfate ions (because of the presence of four carboxylic groups per functional group). As a result, crystal formation is inhibited by both passivation of free calcium ions and reduction of local pH. Consequently, this reduces the local supersaturation in the pores and vicinity of a particle, inhibiting the nucleation process. The efficacy of TAAcOH particles is further investigated by varying the particle loading as presented in Figure 4. The experiment was performed at the same conditions used in the experiment presented in Figure 1. As observed in Figure 4, the induction time increases significantly, whereas the gradients of the crystallization curves decrease only slightly with increasing loading. The presence of TAAcOH functionalized particles significantly delays the crystallization of calcium sulfate; at the highest loading (0.50 mg/gsol), the induction time increased nearly 6-fold. At a supersaturation of S = 2.0, the solution is in excess of 4.8 mmol of calcium ions (C* = 2.7 g/L, V = 310 mL), whereas the molecular loading of the TAAcOH functionalized particles is 0.4 mmol/g, determined using titration (reported by Silicycle). At a loading of 0.50 mg/gsol, the particles have a considerable effect on induction time despite having a ratio of free calcium ions to available chelating groups as high as 80:1. Figure 5 C
DOI: 10.1021/acs.iecr.6b02914 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6b depicts calcium sulfate that crystallized in the presence of TAAcOH particles. It can be seen that crystals do not nucleate and grow on the silica particles themselves but primarily nucleate homogeneously. This indicates that although TAAcOH particles can delay nucleation, they do not affect the crystal growth or participate in heterogeneous crystallization. As a result, TAAcOH particles significantly increase induction time but do not affect the gradient of the crystallization curve (which largely depends on crystal growth rate) as observed in Figures 1 and 4. In real-world applications, functionalized silica particles are expected to behave in a manner similar to that observed in Figures 1 and 4. The solution composition used in this study is similar to that of grid water, which is rich in sodium, calcium, sulfate, and chloride ions. Moreover, inverse solubility salts (responsible for scale deposition) are composed of solely divalent cations such as calcium and magnesium, to which TAAcOH groups have a higher affinity toward than monovalent cations. As a result, it is expected that TAAcOH functionalized particles will naturally passivate the fouling cations in a selective manner, like calcium and magnesium, despite the wide variety of ions found in grid water. Because of their significant ability to delay calcium sulfate crystallization, TAAcOH particles can potentially serve as crystallization fouling mitigation additives that are both easily separable and environmentally friendly.
Figure 4. Crystallization curves in the presence of TAAcOH functionalized mesoporous silica particles at 0 mg, 0.10, 0.25, 0.40, and 0.50 mg/gsol loadings, at a supersaturation of S0 = 2.0, temperature of 30 °C, stirring at 200 rpm. Experiments were repeated three times.
shows that the induction time increases nonlinearly with particle loading by a considerable amount, while the maximum gradient varies only slightly.
4. CONCLUSION A series of batch crystallization experiments were conducted to quantify the effect of functionalized mesoporous silica on calcium sulfate crystallization. Five surface chemistries were tested including unmodified silica, propyl amine, propyl sulfonic acid, methyl, and TAAcOH. Amine functionalized mesoporous particles proved to be effective at enhancing calcium sulfate crystallization by reducing the nucleation induction time by 26% at a loading of 0.10 mg/gsol. TAAcOH functionalized mesoporous particles were particularly effective at delaying crystallization by extending the nucleation induction time by as much as 6-fold at a loading of 0.50 mg/gsol. This delay is attributed to the ability of the TAAcOH functionaliza-
Figure 5. Effect of TAAcOH functionalized particle loading on induction time, calculated using inflection point tangent of curves in Figure 4.
Figure 6. (a) TAAcOH functionalized silica particle; (b) calcium sulfate dihydrate that crystallized in the presence of the mesoporous TAAcOH functionalized silica particles. D
DOI: 10.1021/acs.iecr.6b02914 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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tion to chelate calcium ions and provide an acidic pore environment.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +44 (0)20 7594 0784. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Ph.D. studentship, supported by the Alan Howard Scholarships for T.L., is gratefully acknowledged. REFERENCES
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DOI: 10.1021/acs.iecr.6b02914 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
