Article pubs.acs.org/IECR
Preparation of Calcium Benzene Sulfonate Detergents by a Microdispersion Process Le Du, Yujun Wang, Kai Wang, and Guangsheng Luo* State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: This study presents a novel technique for the controllable preparation of calcium benzene sulfonate [Ca(Ar− SO3)2] detergents using a microdispersion process. Neutralization and carbonation were conducted in an oil-in-water system instead of the typical water-in-oil system. n-Butanol was employed as the oil phase for the in situ formation of the hydrophobic cores of the detergents. The mechanism of synthesis in different bulk phases was investigated. Microreactors were used to enhance the mixing performance and mass transfer. Low-cost Ca(OH)2 and dodecyl benzene sulfonic acid (Ar−SO3H) were supplied as the reactants. An efficient synthesis with a high utilization ratio of CO2 and a high product yield was achieved. CaCO3 nanoparticles with average sizes from 8 to 20 nm were used as detergent cores, and Ca(Ar−SO3)2 detergents with total base numbers (TBNs) ranging from 300 to 420 mg of KOH/g were successfully prepared.
1. INTRODUCTION Lubricating oil is used in various types of machinery to reduce friction and protect mechanical equipment.1,2 Several types of additives are used in small percentages in lubricating oil formulations.3,4 Detergents are additives that are very important components of engine oils. Since the 1950s, nanosized colloidal inorganic particles have been widely used as detergents.5 The detergents essentially consist of an inorganic core (10−20 wt %) stabilized by oil-soluble surfactants (15−30 wt %) and are incorporated into lubricating base oil (Figure 1a). Generally, the detergents contain CaCO3 as the core, and the surfactants are capped by the alkaline salts of alkylated sulfonates, phenols, salicylates, and so on.6,7 The detergents exhibit special
properties such as detergency (i.e., ability to cleanse by combining with impurities and dirt to make them more soluble), solubilization, neutralization, and wear resistance.8−10 Detergents prevent damage to engine parts by homogeneously distributing the sediments produced during the running of an engine; protect engine parts from corrosion; and neutralize H2SO4/SO2 and HNO3/NOx, which are produced during the combustion of fuels by reacting with H2O.11,12 CaCO3 plays an important role in preventing mechanical corrosion, especially in the neutralization of acids.13 The total base number (TBN) is one of the most important parameters for identifying detergents.14 The base number is defined as the amount of KOH equivalent to 1 g of the substance and is expressed in milligrams of KOH per gram. The TBN depends on the amount of CaCO3 contained in the detergent. Detergents are classified according to the TBN as follows: 100−280 mg of KOH/g, medium-alkali; 280−400 mg of KOH/g, high-alkali; and ≥400 mg of KOH/g, excessively overbased detergent. The core size is another important parameter that strongly influences the stability and turbidity of detergents. Generally, the core size should be controlled to be below 20 nm for the final products. Among these surfactants, low-cost calcium sulfonates have attracted significant attention and have been reported to exhibit enhanced stability even at relatively large core sizes.8 The industrial synthesis of calcium sulfonate detergents consists of neutralization and carbonation (Figure 1b). Both steps are performed in lubricating base oil, which is the main phase of the system.15,16 The neutralization process includes several reactions. Taking methanol accelerant and dodecyl benzenesulfonic acid (Ar−SO3H) surfactant as an example, the process can be described as follows 2Ar−SO3 H + Ca(OH)2 → Ca(Ar−SO3 )2 + 2H 2O Received: Revised: Accepted: Published:
Figure 1. (a) Structure of lubricating detergents. (b) Typical industrial technique for producing lubricating detergents. © 2013 American Chemical Society
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April 27, 2013 July 11, 2013 July 12, 2013 July 12, 2013 dx.doi.org/10.1021/ie4013425 | Ind. Eng. Chem. Res. 2013, 52, 10699−10706
Industrial & Engineering Chemistry Research 2CH3OH + Ca(OH)2 → Ca(CH3O)2 + 2H 2O
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is achieved because the concentration distribution of the surfactant in the system can be controlled. Ca(Ar−SO3)2 is soluble in n-butanol, and the transferred CaCO3 can be modified further. Third, CaCO3 nanoparticles of small sizes can be prepared through the microreaction system. Given the efficient mass transfer and uniform reaction environment that they provide, microreactors have been employed to synthesize nanoparticles with novel morphologies and special properties.22−24 In previous studies, we also successfully used microreactors to synthesize nanoparticles in homogeneous and heterogeneous reaction systems.25−27 In this study, a novel preparation technique of Ca(Ar−SO3)2 detergents using membrane dispersion microreactors has been developed. The neutralization and carbonation processes in an oil-in-water system were realized with n-butanol as the oil phase for the in situ formation of hydrophobic CaCO3 nanoparticles. High-alkali and overbased Ca(Ar−SO3)2 detergents were prepared using the new technique, and the effects of the operating conditions were investigated.
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2Ar−SO3 H + Ca(CH3O)2 → Ca(Ar−SO3 )2 + 2CH3OH (3)
The calcium sulfonate is made in situ through the reaction of the corresponding organic acid with Ca(OH)2. After the neutralization process, CO2 gas is added to the system to synthesize CaCO3 in reverse micellar cores Ca(OH)2 + CO2 → CaCO3 + H 2O
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Ca(CH3O)2 + 2CO2 → Ca(CH3O ·CO ·O)2
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Ca(Ar−SO3 )2 + xCa(CH3O ·CO·O)2 + H 2O → Ca(Ar−SO3 )2 ·xCaCO3 + CH3OH
(6)
However, several challenges remain in the microemulsionmediated process (Figure 2a). First, both the neutralization and
2. EXPERIMENTAL SECTION 2.1. Materials. Ar−SO3H with ≥90 wt % purity (purchased from Aladdin Industrial Co.) and lubricating base oil (purchased from Shandong Jiangshan Polymeric Material Co., Ltd.) with a high viscosity index of ≥90 (analyzed by standard method ASTM D2270) were used. n-Butanol (99.5% analytical purity, purchased from Beijing Modern Eastern Fine Chemical Co., Ltd.) was used as the oil phase. The other main raw materials, namely, Ca(OH)2 powder (analytically pure) and CO2 mixed gas (29.8 vol %, mixed with N2), were purchased from Beijing Chemical Works and Beijing Hua Yuan Gas Chemical Industrial Co., Ltd., respectively. 2.2. Synthesis Process. Ca(Ar−SO3)2 detergents are synthesized using neutralization, carbonation, emulsification, and residue deprivation processes, as shown in Figure 3a. In
Figure 2. Schematic representation of the preparation processes of detergents in different systems: (a) traditional technique in the oil phase and (b) microreaction route with n-butanol.
carbonation reactions are conducted in multiphase systems, which result in low controllability and efficiency of mass transfer.3,17 Thus, high consumption of energy and low utilization of CO2 occur.19,21 With the development of cosolvents, mass transfer has apparently been improved. Second, traditional nonuniform stirring leads to a wide size distribution of the droplets (10−100 nm), generating several relatively large-sized (30−50-nm) CaCO3 particles.18 A more serious problem is the presence of excess Ca(OH)2, which tends to form a gel with H2O in the lubricating oil.17 Third, a low degree of CaCO3 modification occurs because of the uncontrollable complicated microemulsion process. CaCO3 particles with less surfactant on the surface are unstable in the detergents, resulting in low final conversion rates. Therefore, we developed a microreaction process with an oilin-water system as the working system (Figure 2b). First, mass transfer can be enhanced in the aqueous phase. The hydrophobic CaCO3 and calcium benzene sulfonate [Ca(Ar− SO3)2] tend to be transferred to n-butanol. Therefore, the diffusion of Ca2+ ions from the Ca(OH)2 solid particles can be easily completed. Second, a high degree of CaCO3 modification
Figure 3. (a) Microreaction route for producing lubricating detergents. (b) Experimental setup.
this work, neutralization and carbonation were performed in two membrane dispersion microreactors. Figure 3b shows the experimental setup. A 1-μm microfiltration membrane with a working area of 12.5 mm2 was used as the dispersion medium in the microreactors. The main channel size of the microreactors was 20 mm × 4 mm × 0.5 mm (length × width × height). Ca(OH)2 suspension (0.135−1.35 mol/L) as the continuous feed was mixed with Ar−SO3H in n-butanol (0.06−0.6 mol/L) 10700
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Figure 4. Effects of accelerants and raw material ratio on the synthesis and products: (a) molar ratio of Ar−SO3H to Ca(OH)2, (b) volume ratio of n-butanol to H2O, and (c) mass ratio of lubricating base oil to CaCO3.
of the surface-modified CaCO3 was recorded by transmission electron microscopy (TEM; JEOL-2010, 120 kV). TEM samples were prepared by freezing the detergents on a carbon-supported membrane with liquid nitrogen. Dynamic light scattering (DLS) of Ca(Ar−SO3)2·CaCO3 micelles in detergents was measured using a Brookhaven BI-200SM laser light scattering spectrometer equipped with a semiconductor laser light source operating at 532 nm.17 Fourier transform infrared (FTIR) spectroscopy (Bruker Corporation, TENSOR 27) was performed to identify the molecular structures.5 The weight loss was measured by thermogravimetric analysis (TGA; STA 409 PC).20 Photographs of the contact angle were recorded using a high-speed CCD camera attached to an Olympus U-TV0-5xC-3 microscope.30 Additionally, the contact angles were measured using DataPhysics SCA20 contact angle measurement software (version 3.12.11).
as the dispersed feed to allow the neutralization reaction to proceed in microreactor 1 to generate Ca(Ar−SO3)2. The oilin-water emulsion was then pumped into microreactor 2 to mix with CO2 gas to complete the carbonation reaction. To complete the Ca(OH)2 reaction, the suspension in the continuous feed was circulated at a certain feed rate. The initial pH of the reaction mixture was 12.3. The reaction required 0.2−1 h for different conditions and was stopped when the pH reached a value of 8. After neutralization and carbonation, the suspension was mixed with lubricating base oil to transfer the Ca(Ar− SO3)2·CaCO3 micelles into the oil phase. The suspension was slowly added dropwise (5−10 mL/min) into the lubricating base oil at different temperatures in a 1000 mL beaker equipped with a propeller agitator (1800 rpm). After mixing for 1.5−2 h, the emulsion was standing stratified, and the oil phase was separated. The final Ca(Ar−SO3)2 detergents were separated from the oil mixture using a centrifugal separator (LD5-2A, Beijing Medical Centrifugal Separator Factory). For comparison, the detergents were also prepared in a stirred tank reactor. Ar−SO3H in n-butanol (0.06−0.6 mol/L) was slowly added dropwise (1−3 mL/min) to a Ca(OH)2 suspension (0.135−1.35 mol/L) at 20−60 °C in a 1000 mL beaker equipped with a propeller agitator (3000 rpm). CO2 gas was then bubbled into the oil-in-water emulsion to allow carbonation. The subsequent emulsification and separation followed the same procedures as used for the microreaction. 2.3. Characterization Methods. ASTM testing methods were employed. The TBN was determined according to standard method ASTM D664 by measuring the amount of KOH (in mg) required to neutralize the base reserve with perchloric acid in 1 g of oil (mg of KOH/g). The morphology
3. RESULTS AND DISCUSSION 3.1. Influence of Operating Conditions on Ca(Ar− SO3)2 Detergents. A series of experiments was performed to optimize the operating conditions. The ratio of the reactants, concentrations, feed flow rates, reaction temperature, and reaction time were optimized. The experiments confirmed the controllability and reproducibility of the microreaction. The preparation can be achieved at relatively low temperature and high reactant concentrations. Figure 4a shows the optimization results for the molar ratio of Ar−SO3H to Ca(OH)2. An increase in molar ratio resulted in increases in the TBN and yield to their maximum values at a molar ratio of 0.15. Figure 4b shows the influence of the volume ratio of n-butanol to H2O. The concentration distributions of the surfactant and transferring hydrophobic 10701
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Figure 5. Effects of reactant concentrations on the synthesis and products: (a) [Ca(OH)2] = 0.675 mol/L and (b) [Ar−SO3H] = 0.36 mol/L. Other conditions: T = 25 °C, FC = 160 mL/min, FD = 5 mL/min, FG = 120 mL/min.
Figure 6. Effects of feed flow rates on the synthesis and products: (a) FC varying, FD = 5 mL/min, FG = 120 mL/min; (b) FD varying, FC = 160 mL/ min, FG = 120 mL/min; (c) FG varying, FC = 160 mL/min, FD = 5 mL/min. Other conditions: T = 25 °C, [Ca(OH)2] = 1.01 mol/L, [Ar−SO3H] = 0.36 mol/L.
particles were controlled by the addition of more n-butanol. The enhancement became stable when the volume ratio was increased to 0.3. The effects of the mass ratio of lubricating base oil to CaCO3 are shown in Figure 4c. Although an increase in the mass of oil allowed more CaCO3 to be transferred into the oil phase, the TBN decreased considerably per unit mass of detergent products. Therefore, a mass ratio of 5 was chosen for the emulsification process. Figure 5a shows the influence of the Ar−SO3H concentration on the microreaction. Minimal enhancement occurred at concentrations greater than 0.35 mol/L, which resulted in the efficient modification and control of the concentration distribution of the surfactant. Figure 5b shows that the concentration of Ca(OH)2 did not influence the particle size
for efficient mixing (d32). The average particle size varied from 10 to 20 nm in the concentration range of 0.1−1.01 mol/L. However, increasing the Ca(OH)2 concentration caused severe agglomeration of CaCO3 and Ca(Ar−SO3)2 micelles.30 As a result, both the TBN value and the Ca(Ar−SO3)2 yield decreased. The flow rates of the feeds greatly affected the mixing performance. The average particle sizes under different continuous-phase [Ca(OH)2 suspension] flow rates (FC) are presented in Figure 6a. Increased continuous-phase flow rate provided a strong cross-flow drag force, which caused the nucleation to lose a large amount of reactants and the particle size to decrease. Micelles with small core sizes were relatively stable in the detergents, resulting in high TBN and yield values 10702
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Figure 7. Effects of (a) microreaction temperature and emulsification (b) temperature and (c) time on the synthesis and products: (a) [Ca(OH)2] = 1.01 mol/L, [Ar−SO3H] = 0.36 mol/L, FC = 160 mL/min, FD = 5 mL/min, FG = 120 mL/min; (b) CaCO3 core = 18 nm; (c) CaCO3 core = 18 nm, T2 = 50 °C.
Figure 8. (a−d) TEM images of CaCO3 nanoparticles. (e−h) Micelle size distributions of the detergents measured by DLS according to different CaCO3 core sizes: (e) 12, (f) 18, (g) 25, and (h) 36 nm.
at high continuous-phase flow rates. The influence of the dispersed-phase (Ar−SO3H solution) flow rate (FD) is shown in Figure 6b. When the surfactants were added too fast, the excess hydrophobic organic chains tended to coat the solid particles and result in the subsequent production of CO2 bubbles. Figure 6c shows the effects of the CO2 gas flow rate (FG) on the TBN, yield (X), and particle size. Increasing the gas-phase flow rate resulted in a stronger disturbance of the mass transfer, which caused decreases in the CaCO3 particle size and the mass-transfer efficiency of the micelles. Reaction temperature is one of the most important factors affecting the reaction system. The reaction temperature influences the solubilities, diffusion coefficients, and supersaturations of the reactants. Figure 7a shows the effects of
temperature on the microreaction. A decrease in temperature was accompanied by an increase in the solubility of Ca(OH)2 and a decrease in the CO2 bubble sizes, conditions that generated the preferred smaller particle sizes.21 The particle size decreased to 12 nm, and a TBN of 385 mg of KOH/g was achieved, with a 92% yield at 0 °C. The resulting detergents were still satisfactory, with small core sizes and high TBN values, even at temperatures ranging from 20 to 30 °C. The experimental results of subsequent emulsification demonstrated that temperature control was a very important factor, as shown in Figure 7b. The molecular thermal motion and diffusion rates of the micelles were significantly enhanced with increasing temperature, which increased the CaCO3 and Ca(Ar−SO3)2 contents in the detergents. Therefore, a relatively high TBN can 10703
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Figure 9. (a) Thermogravimetric curves and (b) FTIR spectra of detergents with different core sizes. Δm1, light component in base oil; Δm2, dodecyl benzenesulfonic groups.
be achieved at emulsification temperatures greater than 50 °C. Figure 7c shows the influence of emulsification time on the microreaction. The yield was greater than 98%, and the TBN was 403 mg of KOH/g after the reaction mixture had been stirred for 2.5 h, which could be classified as an overbased detergent. 3.2. Characterization of Ca(Ar−SO3)2 Detergents with Different Micelle and Core Sizes. The Ca(Ar−SO3)2 detergents were characterized by various techniques. The core and micelle sizes of the prepared detergents were comparable to those of overbased industrial products. Characterization of the organic group types and component contents also confirmed the high standard of the product ingredients. CaCO3 cores of different sizes were observed by TEM, as shown in Figure 8a−d. The images illustrate that the produced particles were monodisperse and uniform in size. With efficient micromixing, the particle sizes could be reduced to the range of 8−12 nm, an enlarged view of which is shown in Figure 8b. ImageTool v3.0 software was used to measure particle sizes in the TEM images (each experimental data point was determined by measuring over 200 particles). Even under unfavorable conditions, the core size was still approximately 35 nm with a narrow size distribution. In addition, the detergents were investigated by DLS to determine the micelle sizes. The micelle size distributions with different sizes of CaCO3 cores are shown in Figure 8e−h, where num refers to the number fraction. The results show that the micelle size ranged from 19 to 58 nm, with a narrow size distribution. The TEM images mainly reflect the CaCO3 core size, which is, on average, 30−40% smaller than the hydrodynamic diameter measured by DLS. The results on the micelle sizes in the detergents are consistent with other measurements, which confirm the stability of micelles under 60 nm in base oil.28,29 The TGA curves in Figure 9a show the masses of Ca(Ar− SO3)2·CaCO3 with different core sizes in base oil. For the modified hydrophobic particles, significant decreases in weight occurred at temperatures ranging from 450 to 530 °C. The value of the weight loss was exactly the same as the Ar−SO3− dosage. The products remained stable until the temperature was raised beyond 700 °C, at which point CaCO 3 decomposition occurred. The mass of CaCO3 in the detergents indicates the capacity of neutralization to prevent mechanical corrosion. The cores with smaller sizes tended to absorb more surfactants and could easily be transferred into the oil phase, which resulted in a high TBN.
FTIR spectra of Ca(Ar−SO3)2·CaCO3 micelles are shown in Figure 9b. Three strong peaks at 1454.9, 873.3, and 706 cm−1, corresponding to the ν2, ν3, and ν4 vibrations, respectively, are considered to arise from −CO−.5 The sharp peak at 873.3 cm−1 is especially attributed to amorphous CaCO3.13,14,18,20 FTIR spectra of Ar−SO3− shows strong peaks at 2920.2 and 2843.5 cm−1 corresponding to νC−H and νC−C vibrations, respectively, which are inconspicuous in the detergents with larger core sizes.20 In addition, the intensities of νC−H and νC−C increased with decreasing core size, indicating the capacity to absorb surfactants. Figure 10 exhibits the relation between the Ca(Ar−SO3)2 absorbance and the contact angles of the hydrophobic particles
Figure 10. Contact angles of surface-modified CaCO3 nanoparticles with different sizes: (a) 12, (b) 18, (c) 25, and (d) 36 nm.
before the emulsification process. The powders were compressed into flakes and held a drop of deionized water on the surface. The contact angle increased from 85.6° to 116.1° with decreasing core size, which also represents the capacity of the resulting powders to absorb surfactants. The results coincide with the DLS and FTIR analyses. The preparation of Ca(Ar−SO3)2 detergents by microreaction was compared with commonly employed methods, which are listed in Table 1.14,20 Both processes with low energy 10704
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Table 1. Comparison of Different Methods for Synthesizing Lubricating Detergents microreactor a
synthetic time (h) reaction temperature (°C) utilization ratio of CO2 (%) yield (%) residue of Ca(OH)2 (wt%) TBN (mg of KOH/g) a
b
0.2−1, 2−2.5 20−30, 20−50 80−90 >98 undetected 360−415
batch reactor 0.2−1.2, 2.5−4 20−60, 20−50 40−60 80−85 undetected 235−320
bubble column14
batch reactor20
−, 1.3−1.5 −, 85 30−45 >96 11.87 274−436
0.75, 2.5 45−54 − 90
