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Energy & Fuels 2004, 18, 652-658
Effect of Sodium Aromatic Sulfonate Group in Anionic Polymer Dispersant on the Viscosity of Coal-Water Mixtures Toshio Kakui*,† and Hidehiro Kamiya‡ Chemicals Research Laboratories, Chemicals Division, Lion Corporation, 13-12, Hirai 7-chome, Edogawa-ku, Tokyo 132-0035, Japan, and Graduate School of Bio-Application and Systems Engineering, BASE, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan Received August 21, 2003. Revised Manuscript Received January 20, 2004
This paper focused on the effect of sodium aromatic sulfonate in anionic polymer dispersants on the viscosity of coal-water mixtures (CWMs) with a Tatung coal powder. To determine the optimum molecular structure of a polymer dispersant for the minimum viscosity of a CWM, various anionic co-polymers with different hydrophilic and hydrophobic groups or different molecular weights were prepared, using various types of monomers. Anionic co-polymers with sodium aromatic sulfonate, such as sodium styrene-sulfonate and sodium naphthalene-sulfonate, reduced the viscosity of dense CWMs. In particular, a co-polymer of sodium styrene-sulfonate and sodium acrylate with a molar ratio of 70:30 and a molecular weight of ∼10 000 gave the minimum viscosity of a 70 wt % CWM. To obtain a low viscosity for a CWM, a large electrostatic repulsive force with an absolute value of the zeta potential of the coal particles of >70 mV and >6.5 mg/g of adsorbed polymer on the coal surface were needed. The mixture of sodium polystyrene-sulfonate and sodium polyacrylate with a weight ratio of 50:50 also gave a low viscosity of 70 wt % CWM. On the basis of the results, the adsorption behavior of polymer dispersants on the coal surface is examined by measuring the wettability of coal powder pellets.
1. Introduction A coal-water mixture (CWM) is regarded as an alternate fuel to replace fuel oil for electric power generation. A CWM should have a high solids fraction and sufficient fluidity to replace conventional fuels in existing systems of transportation, storage, and combustion. In regard to decreasing the viscosity of CWMs, the selection of a polymer dispersant is key to their preparation. Many previous papers described various types of polymer dispersants used to obtain CWMs with a low viscosity and a high solids fraction. Homopolymers such as sodium polystyrene-sulfonate (SPS) and sodium naphthalene-sulfonate formaldehyde condensate (NSF) are often studied for the preparation of CWMs. The effect of the molecular weight and degree of sulfonation of SPS on the rheological behavior, the zeta potentials, and the size of agglomerates in a dense CWM were investigated using the colloid vibration potential method.1 The influence of NSF structures with different condensation degrees and substitutes on the fluidity and stability of CWM was examined; however, their molec* Author to whom correspondence should be addressed. E-mail address:
[email protected]. † Lion Corporation. ‡ Tokyo University of Agriculture and Technology. (1) Sugawara, H.; Tobori, N. Electrical Phenomena at Interface; Surfactant Science Series 76; Academic Press: New York, 1998; pp 485-502.
ular structure was not actually clarified.2 Several types of anionic polymer dispersants with various hydrophilic and hydrophobic groups were recently proposed. For example, graft polymers (such as hydrophilic sodium polyacrylate with hydrophobic polystyrene side chains and hydrophobic poly 4-methylstyrene with hydrophilic poly(ethylene oxide) side chains3,4) and poly(vinyl alcohol) with alkyl (C12, C18) end groups and anionic groups such as sodium sulfonate and sodium carboxylate5 were investigated, to determine the optimum molecular structure of the co-polymer for a CWM with high fluidity. Multibranched nonionic polymers with a high molecular weight, such as polynonylphenolic resin with poly(ethylene oxide) side chains and polyethyleneimine with poly(ethylene oxide) side chains, as nonionic dispersants, were reported to obtain a CWM with a high coal solids fraction.6,7 The evaluation of commercial anionic and nonionic dispersants for CWMs with various (2) Sun, C. G.; Xie, Y. X.; Li, B. Q.; Li, Y. X. In Proceedings, ICCS ’97, Vol. 1; DGMK: Hamburg, Germany, 1997. (3) Yoshihara, H. Coal Prep. (Gordon & Breach) 1999; 21 (1), 93103. (4) Bonaccorsi, F.; Lezzi, A.; Prevedello, A.; Lanzini, L.; Roggero, A. Polym. Int. 1993, 30 (1), 93-100. (5) Yamauchi, J.; Terada, K.; Sato, T.; Okaya, T. J. Appl. Polym. Sci. 1995, 55 (11), 1553-61. (6) Nakanishi, T.; Furuya, K.; Hirao, M.; et al. In Proceedings of the 14th International Technical Conference on Coal and Slurry Technologies (Clearwater, FL, 1989): Coal Technology Association: Washington, DC, 1989; pp 321-330. (7) Naka, A.; Nishida, Y.; Sugiyama, H.; Sugiyama, T. J. Chem. Soc. Jpn. 1986, 13, 227-230.
10.1021/ef030154a CCC: $27.50 © 2004 American Chemical Society Published on Web 03/04/2004
Effect of Polymer Dispersants on Viscosity of CWMs
coal powders was performed.8 However, the selection and synthesis of the polymer dispersants was determined by an experimental rule. In particular, the effect of the molecular structure and molecular weight of the anionic polymer dispersant on the behavior of the CWM slurry has not been systematically investigated. During the processing of CWMs, control of the rheological properties is important. Many researchers have discussed the stability, dynamic properties, and structural changes of CWMs from various viewpoints. For example, various additives (such as dispersants,9 flocculants,10,11 and electrolytes12) and coal surface properties (such as acidification8 and hydrophilic functional groups) have been examined. The zeta potential of the coal particles is also one of the factors that affect the flow and stability of CWMs. The factors that control the zeta potential were examined using different coal powders,13 as well as through variation of properties such as the type of coal,14 the pH15 and temperature16 of the slurries, and the electrolytes.17 However, the molecular structure of the polymer dispersants has not been clearly demonstrated in the research studies. The relationship between the molecular structure of the anionic polymers and the zeta potential of the coal particles has not been examined. The effect of coal particle properties on the viscosity of CWMs had been investigated using various types of coal with different surface behavior and components.18 The maximum coal solids fraction in CWMs decreased as the water wettability of each coal, in the presence of polymer dispersants, increased.19 Using various types of coal, the effect of concentration of CWMs with NSF, SPS, and N-(polyalkylene oxide)-polyethyleneimine was also considered.20 Furthermore, the relationship between the viscosity of CWMs with different coal powders, the zeta potential of each coal particle, and the amount of adsorbed polymers was examined.21,22 However, the molecular structure of the polymer dispersants had not been clearly demonstrated in the research. The effect of the molecular structures of the polymer dispersants (such as the hydrophilic-to-hydrophobic group ratio) on the viscosity of CWMs, the zeta (8) Takao, S. Kagaku Kogaku Ronbunshu 1996, 22 (3), 488-495. (9) Naka, A.; Nishida, Y.; Murakami, O.; Sugiyama, H. J. Chem. Soc. Jpn. 1986, 10, 1342-1347. (10) Ushui, H. Nihon Reoroji Gakkaishi 1990, 18 (1), 53-55. (11) Pawlik, M.; Laskowski, J. S. Polymers in Mineral Processing. In Proceedings of the 3rd UBC-McGill Bi-Annual International Symposium on Fundamentals of Mineral Processing; Metallurgical Society of CIM: Quebec City, Canada, 1999; pp 541-555. (12) Kaji, R.; Muranaka, Y.; Miyadera, H.; Hishimura, Y. AIChE J. 1987, 33 (1), 11-18. (13) Higashiani, K.; Shikage, A.; Kurita, N. Kagaku Kogaku Ronbunshu 1986, 12 (5), 557-562. (14) Atesok, G.; Boylu, F.; Sirkeci, A. A.; Dincer, H. Fuel 2000, 81 (14), 1855-1858. (15) Mori, S.; Hara, T.; Aso, K.; Okamoto, H. Powder Technol. 1984, 40, 161-165. (16) Saeki, T.; Usui, H.; Kawamoto, T. J. Chem. Eng. Jpn. 1993, 26 (1), 59-63. (17) Hamieh, T. J. Mater. Sci. 1996, 31 (21), 5665-5669. (18) Higashitani, K.; Umemoto, T.; Kashiwabara, Y.; Ito, H. Proceedings of the 11th International Coal Preparation Congress (Tokyo, 1990), pp 323-326. (19) Naka, A.; Nishida, Y.; Murata, T. Nenryo Kyokaishi 1986, 65 (6), 408-416. (20) Naka, A.; Sugiyama, H.; Honjo, S. J. Chem. Soc. Jpn. 1986, 4, 602-607. (21) Kikkawa, H.; Takezaki, H.; Otani, Y.; Ogawa, J.; Kanamori, S. Powder Technol. 1988, 55, 277-284. (22) Ekaku, K.; Yokouchi, A.; Watanabe, S.; Meguro, K.; Hnda, H. Nenryo Kyokaishi 1985, 64 (5), 345-349.
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potential, and the amount of adsorbed polymer dispersants on the coal particles has not been investigated. On the other hand, in our previous article,23 the effect of the molecular structure of the anionic polymer dispersants on the electrosteric interaction between solid surfaces and the viscosity of dense Al2O3 suspensions was examined and analyzed by atomic force microscopy. The ratio of hydrophilic ammonium acrylate to hydrophobic methyl acrylate (m:n) in an optimum copolymer obtained from the minimum viscosity of the suspension was determined at m:n ) 3:7. This study focused on the effect of sodium aromatic sulfonate in anionic polymer dispersants on the viscosity of CWMs. To obtain the minimum viscosity of the CWM with Tatung coal powders, a series of anionic polymers with various hydrophilic and hydrophobic groups and different molecular weights were prepared, from 11 types of monomers that were used as polymer dispersants. The optimum molecular structure and the essential monomer of the polymer dispersants for a dense CWM were determined. Based on the results, the relationship between the CWM viscosity, the zeta potential of coal particles, and the amount of the adsorbed polymer on the coal surface was discussed. 2. Materials and Methods 2.1. Preparation of Anionic Polymers with Different Compositions. Homopolymers of sodium polystyrene-sulfonate (SPS, molecular weight (Mw) of 15 000; Lion Corp., Japan), sodium naphthalene-sulfonate formaldehyde condensate (NSF, Mw ) 1000; Daiich Kougyo Seiyaku Corp., Japan), and sodium polyacrylate (SPA, Mw ) 10 000; Nippon Syokubai Corp., Japan) were used in this study. Various co-polymers with different hydrophilic and hydrophobic groups or different molecular weights were prepared as dispersants by radical copolymerization using various monomers. Sodium styrenesulfonate (Spinomar NaSS, Tosoh Corp., Japan), acrylic acid (Osaka Organic Chemical Industry, Ltd., Japan), and other monomers and initiators (Junsei Chemical Corp., Japan) were used. Each co-polymer was synthesized via a random co-polymerization of monomers with a polymerization initiator, ammonium persulfate, and hydrogen peroxide in an aqueous system.24 The polymerization was performed under a nitrogen atmosphere for 4 h, whereas a mixture of monomers and the aqueous initiator solution were separately added dropwise into water at a temperature of ∼100 °C. After the dropwise addition ended, the reacted product was maintained at 100 °C for 2 h, to react the monomers completely. The aqueous solution of the co-polymer was neutralized with aqueous sodium hydroxide at pH ∼8. The solids content in the co-polymer solution was adjusted to 25 wt %, based on the calculated value when the monomers completely reacted. The desired solid product was measured based on the dry weight at 150 °C for 1 h, using an infrared moisture balance (model FD-600, KETT Electric Laboratory, Japan). The reaction yield was then determined by the NMR analysis of nonreacted monomer and a comparison between the calculated and measured values. Most of the copolymers reacted completely, and the reaction yields were >99.0 wt %. However, it was difficult to complete the copolymerization of styrene, because the monomers were not very soluble in water. The co-polymerizations were performed in benzene with benzoyl peroxide as an initiator. The reaction (23) Kamiya, H.; Fukuda, Y.; Suzuki, Y.; Tukada, M.; Kakui, T.; Naito, M. J. Am. Chem. Soc. 1999, 82 (12), 3407-3412. (24) Kiyonaga, Y.; Narita, M.; Kakui, T. Japanese Patent No. JP 1,656,710.
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Table 1. Composition of Co-monomers, Molecular Weight, and Reactive Yield of Representative Anionic Co-polymers with Hydrophilic and Hydrophobic Groups Used as Polymer Dispersants Composition of Co-polymers number 1 2 3 4 5 6
acrylic acid acrylic acid acrylic acid acrylic acid acrylic acid acrylic acid
7 8 9
maleic acid methacrylic acid 2-acrylamide 2-methyl-propane sulfonic acid acrylamide 2-vinyl pyridine acrylic acid acrylic acid acrylic acid acrylic acid acrylic acid
10 11 12 13 14 15 16 a
co-monomers of co-polymers maleic acid methyl acrylate 2-vinyl pyridine allyl sulfonic acid allyl sulfonic acid 2-acrylamide 2-methyl-propane sulfonic acid styrene NaSS NaSS NaSS NaSS NaSS NaSS NaSS NaSS NaSS
molar ratio
molecular weight
yield (wt %)
80/20 80/20 80/20 80/20 50/50 50/50
9600 11000 9100 9200 9300 10100
>99.0 >99.0 93.1 >99.0 >99.0 >99.0
50/50 50/50 50/50
10000 10100 10900
95.9 >99.0 >99.0
50/50 50/50 80/20 70/30 50/50 30/70 10/90
12100 9300 11500 10700 10000 10000 9300
>99.0 90.5 >99.0 >99.0 >99.0 >99.0 >99.0
NaSS ) sodium styrene-sulfonate (NaSS).
yields of the co-polymer obtained by removing benzene was ∼90 wt %. The aqueous solution of the sodium salt of the copolymer was obtained via neutralization with aqueous sodium hydroxide. The molecular weight was measured via gel permeation chromatography (GPC-8020 Model II, Tosoh Corp.) with two aqueous gel columns (TSKgel G3000PW and G5000PW, Tosoh Corp.), using a 0.1 mol/L phosphate buffer and a 0.1 mol/L NaCl aqueous solution (pH 6.7) as an eluent. The average molecular weight of each co-polymer was determined from seven types of standard poly(ethylene oxide) materials with molecular weights ranging from 2000 to 100 000 as reference materials, using a refractive index detector (model RI-8012, Tosoh Corp.). All the products had almost a single peak. The molecular weight of each co-polymer was controlled by the amount of initiator. The representative co-polymers used as an anionic polymer dispersant in this work, and their characteristics, are given in Table 1. The co-polymer solutions with nonreacted monomer were used as a dispersant without purification. 2.2. Preparation and Characterization of Coal-Water Mixture (CWM) Slurries. A Tatung coal that had been pulverized by a dry ball mill was used in this study. The particle size distribution was determined using a standard screen method (JIS K0069): >300 µm, 0.5 wt %; 300-150 µm, 12.5 wt %; 150-75 µm, 20.0 wt %; 75-44 µm, 8.8 wt %; and 100 000 mPa s). The NaSS seems to be an essential monomer for the polymer dispersants to achieve fluidity in dense CWMs. 3.2. Effect of the Molecular Ratio of NaSS in Sodium Acrylate and the NaSS Co-polymer. To clarify the effect of NaSS in sodium acrylate and the NaSS co-polymers shown in Figure 2 for CWMs with lower viscosity, the viscosity of a 68 wt % CWM with co-polymers that contained different NaSS ratios was investigated. The molecular weights of the co-polymers were the range of 9300-11 500, as shown in Table 1 (Nos. 12-16). Figure 3 shows the effect of the molecular ratio of NaSS in sodium acrylate and NaSS co-polymers on the apparent viscosity of a 68 wt % CWM. With an increase in the NaSS ratio from 20 mol % to 70 mol %, the viscosity decreased from >100 000 mPa s to 1330 mPa s. The optimum ratio of NaSS needed to obtain the minimum viscosity was determined to be 70 mol % in the sodium acrylate and NaSS co-polymer. An excess of NaSS (>70 mol %) in the co-polymer caused an increase in the CWM viscosity. The CWM viscosity with sodium polystyrene-sulfonate (SPS) was 3200 mPa s. 3.3. Effect of Molecular Weight of Sodium Acrylate and the NaSS Co-polymer. The existence of an optimum molecular weight of the polymer dispersant to obtain the minimum viscosity has been reported by
Figure 5. Molecular structure of sodium styrene-sulfonate (NaSS) co-polymers with various co-monomers at a molar ratio of 50:50 and molecular structure of NaSS with a hydrophobic aromatic group and hydrophilic sodium sulfonate group.
many researchers, with regard to various suspension systems. The optimum molecular weight of the SPS has been reported to be above the range of 15 000-20 000 for the CWM.1 To determine the optimum molecular weight of sodium acrylate and NaSS co-polymers at molar ratios of 50:50 and 30:70, the influence of the molecular weight, in the range of 2000-100 000, on the viscosity of CWMs was investigated. As shown in Figure 4, the apparent viscosity of the 68 wt % CWM decreased as the molecular weight increased up to 10 000. The minimum viscosity appeared at ∼10 000 mol/g, which was almost the same value as that for the SPS homopolymer. When the molecular weight was >10 000, the viscosity of CWM increased as the molecular weight increased. The optimum molecular weight of the copolymer, with respect to the minimum viscosity, was determined to be ∼10 000. 3.4. Effect of the Sodium Aromatic Sulfonate Group in a Polymer Dispersant. Based on the results, the NaSS monomer was determined to be very important, in regard to using the polymer dispersants to obtain CWMs with high fluidity. The NaSS monomer consists of a hydrophilic sodium sulfonate group and a hydrophobic aromatic group, as shown in Figure 5. To identify the effect of the styrene-sulfonate group on the co-polymer, co-polymers with a sodium sulfonate (SO3Na) group or an aromatic group without NaSS were prepared. Each monomer content was fixed at 50 mol %, and the molecular weight of each co-polymer was ∼10 000, because the molecular ratio of the co-polymer
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Table 2. Effect of Various Hydrophilic and Hydrophobic Monomers in Sodium Styrene-Sulfonate (NaSS) Co-polymer on the Apparent Viscosity of 68 wt % CWM co-monomer
apparent viscosity (mPa s)
sodium acrylate sodium methacrylate 2-acrylamide 2-methyl-propane sulfuric acid acrylamide 2-vinyl pyridine
2500 2100 1800 9500 3500
prepared from two types of monomers for the dispersant was generally 50 mol %. The co-polymers with styrene, allyl-sulfonic acid, and 2-acrylamide-2-methyl-propane sulfonic acid (Nos. 5, 6, 7, and 14) were prepared. When styrene without a sodium sulfate group or sodium aliphatic sulfonate without an aromatic group was used as a co-monomer in the carboxylate co-polymers, fluidity of the CWMs was not achieved. However, the co-polymer that contained NaSS (No. 14) reduced the CWM viscosity to 2600 mPa s. The essential monomer in the anionic polymer dispersant for the CWM was NaSS with a sodium sulfonate and an aromatic group. On the other hand, sodium naphthalene-sulfonate formaldehyde condensate (NSF) with hydrophilic sodium sulfonate and hydrophobic naphthalene was generally used as a dispersant. The viscosity of CWM with NSF was 3000 mPa s, which was similar to the viscosity of SPS (3200 mPa s). The sodium aromatic sulfonate group in anionic polymer dispersants seems to be necessary to obtain a low CWM viscosity. 3.5. Effect of Combination of Different Monomers in a NaSS Co-polymer. To define the effect of NaSS in the co-polymers as dispersants for CWMs, the combination of different co-monomers with NaSS shown in Figure 5 was investigated. The co-polymers of various hydrophilic and hydrophobic monomers and NaSS were synthesized and added to CWMs with a solids fraction of 68 wt %. The molecular weight was ∼10 000, and the ratio of NaSS in the co-polymers was fixed at 50 mol %. Table 2 shows the effect of the various monomers in the NaSS co-polymers on the apparent viscosity of the CWM. All co-polymers that contained 50 mol % NaSS were useful in obtaining low CWM viscosities (under 10 000 mPa s). Co-polymers of NaSS with an anionic aliphatic monomer, such as sodium allyl-sulfonate and sodium methacrylate, provided a lower apparent viscosity than the sodium acrylate and the NaSS co-polymer. The optimum molecular structure of the polymer dispersant was that of co-polymers that contained NaSS and aliphatic anionic monomers.
3.6. Effect of Polymer Dispersant Structure on Zeta Potential and Adsorbed Behavior. The viscosity of a CWM with a polymer dispersant is dependent on the repulsive interaction between the coal particles. The repulsive force of adsorbed ionic polymers on a water-particle interface can be classified mainly according to the electrostatic interaction of the electric double layer that surrounds the particles and the steric effect of the adsorbed polymers. The zeta potential of coal particles and the amount of adsorbed polymer on the coal surface in a dense CWM were investigated in the presence of each polymer dispersant. Table 3 shows the effect of the molecular structure of the polymer dispersant on the apparent viscosity of 68 wt % and 70 wt % CWM, the zeta potential, and the amount of adsorbed polymer on the coal particles. A polymer dispersant with a low viscosity of 68 wt % CWM, such as the sodium acrylate and NaSS co-polymer, SPS, and NSF, displayed an absolute value of the zeta potential of >70 mV and an adsorbed polymer of >6.5 mg/g, which was much higher than that of the other polymer dispersants. The increase in the amount of adsorbed anionic polymer dispersant on the coal particles increased the zeta potential. The increase in the electrostatic repulsion force by the zeta potential will promote the dispersion in the CWM viscosity. Sodium aromatic sulfonate with both aromatic and sodium sulfonate groups in the polymer dispersant was a suitable structure to adsorb onto both the hydrophilic and hydrophobic groups on the coal surface. On the other hand, the effect of the coal fraction in CWMs was examined using polymer dispersants with sodium aromatic sulfonate. As shown in Table 3, although NSF with Mw ) 1000 had a large amount of adsorbed polymer on the surface (7.02 mg/g) and a relatively high absolute value for the zeta potential (-71 mV), fluidity of a 70 wt % CWM was not obtained using this polymer. The sodium acrylate and NaSS copolymer, and a SPS homopolymer with Mw > 10 000, displayed a low viscosity of the 70 wt % CWM (3300 and 8500 mPa s, respectively). 3.7. Apparent Viscosity of CWM with Homopolymer Mixture. The effect of mixing sodium polyacrylate and SPS on the apparent viscosity of a CWM was examined. Figure 6 shows the effect of the mixing ratio of the sodium polyacrylate with Mw ) 10 000 and the SPS with Mw ) 15 000 on the apparent viscosity of the 70 wt % CWM. As the mixing ratio of the sodium polyacrylate increased up to 50 wt % with the SPS, the CWM viscosity dramatically decreased, from 8500 MPa s to 1250 mPa s. The optimum mixture ratio of the
Table 3. Effect of Polymer Dispersants with Different Molecular Structures on the Relationship among the Apparent Viscosity of 68 wt % and 70 wt % CWM, the Zeta Potential, and the Amount of Adsorbed Polymer Viscosity (mPa‚s) polymer dispersant(s) sodium polyacrylate sodium maleate and styrene co-polymer sodium acrylate and sodium 2-acrylamide 2-methyl-propane sulfonate co-polymer sodium acrylate and NaSS copolymer SPS NSF a
molecular weight
68 wt % CWM
10000 10000 10000
a
10000 15000 1000
1330 3200 3000
To determine the viscosity, the fluidity of CWM could not be obtained.
70 wt % CWM
a a
3300 8500 a
zeta potential (mV)
adsorption amount (mg/g coal)
-45 -33 -42
2.51 3.52 1.89
-87 -78 -71
8.95 6.54 7.02
Effect of Polymer Dispersants on Viscosity of CWMs
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Figure 7. Contact angles of various powders with water, as a function of the hydrophilic and hydrophobic surface properties.
Figure 6. Synergistic effect of the mixture of sodium polyacrylate with a molecular weight (Mw) of 10 000 and sodium polystyrene-sulfonate (SPS) with Mw ) 15 000 on the apparent viscosity of a 70 wt % CWM.
sodium polyacrylate to the SPS for the 70 wt % CWM to obtain the lowest apparent viscosity was determined to be 50:50 (wt %). Interestingly, even an excess of sodium polyacrylate at 70 wt % maintained a low viscosity (70 mV and >6.5 mg/g of adsorbed polymer on the coal surface. Furthermore, the mixture of the sodium polyacrylate with Mw ) 10 000 and the sodium polystyrene-sulfonate (SPS) with Mw ) 15 000 at a 50: 50 ratio (wt %) reduced the viscosity of the 70 wt % CWM to 1250 mPa s. EF030154A
