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Effect of Organized Assemblies. Part 4. Formulation of Highly Concentrated Coal-Water Slurry Using a Natural Surfactant Debadutta Das,† Sagarika Panigrahi,† Pramila K. Misra,*,† and Amalendu Nayak‡ Centre of Studies in Surface Science and Technology, Department of Chemistry, Sambalpur UniVersity, Jyoti Vihar 768 019, Orissa, India, and Institute of Minerals and Material Technology, Bhubaneswar 751 013, Orissa, India ReceiVed NoVember 3, 2007. ReVised Manuscript ReceiVed January 27, 2008
Coal-water slurry has received considerable research nowadays due to its ability in substituting energy sources. The present work reports the formulation of highly concentrated coal-water slurry using a natural occurring surface active compound, saponin, extracted from the fruits of plant Sapindous laurifolia. The isolation of saponin from the plant and its surface activity has been discussed. The rhelogical characteristics of coal-water slurry have been investigated as a function of coal loading, ash content of coal, pH, temperature, and amount of saponin. The viscosity of the slurry and zeta potential are substantially decreased with concomitant shift of the isoelectric point of coal on adsorption of saponin to it. In the presence of 0.8% of saponin, coal-water slurry containing 64% weight fraction of coal could be achieved. The slurry is stable for a period of as long as 1 month in contrast to 4-5 h in the case of bare coal-water slurry. The results confirm the use of saponin as a suitable additive for coal-water slurry similar to the commercially available additive such as sodium dodecyl sulfate. Basing on the effect of pH on the zeta potential and viscosity of slurry, a suitable mechanism for saponin-coal interaction and orientation of saponin at the coal-water interface has been proposed.
Introduction The development of substitute of oil is highly essential in the present context due to acute shortage of oil as well as increasing demand for it from different public and private sectors. Initially, a finely ground coal-oil mixture was developed as a promising substitute of oil.1 Since 1980, attention has been focused on coal-water slurry as an alternative fuel for the power generation industry and a suitable substitute for oil in several industrial applications. Consequently, several such plants have been set up around the globe.2,3 For maximum efficiency as fuel, the coal concentration in coal-water slurry should be as high as possible, maintaining its viscosity at the minimum level simultaneously so that it will be suitable for storage and transportation through pipelines. The primary factors responsible for the optimum stability of coal-water slurry depend on physicochemical properties of coal, such as its (i) surface hydrophobicity,4–6 (ii) particle size * To whom correspondence should be addressed. Telephone: +916632430 983. Mobile:09938333244. E-mail:
[email protected]. † Sambalpur University. ‡ Institute of Minerals and Materials Technology (formerly Regional Research Laboratory, Council of Scientific & Industrial Research). (1) Bienstock, D.; Jamgochian, E. M. Coal-oil mixture technology in the US. Fuel 1981, 60, 851–864. (2) Noboru, H.; Harumitsu, Y.; Masao, T. Coal & Manabus Water slurryPilot Plant Scale Preparation by the Carbogel Process. Proc. SeVenth Int. Symp. Coal Slurry Prep. Utilis. (New Orleans, LA) 1985, 21-24, 215– 224. (3) Whaley, H.; Rankin, D. M.; Landry, P. G.; Covill, I. D. Utility Boiler Demonstration of Coal water mixture Combustion at Chatham. Proc. Sixth Int. Conf. Coal Slurry Combust. Technol. (Orlando) 1984, 519–526. (4) Kaushal, K. T.; Basu, S. K.; Kumaresh, C.; Bit, B. S.; Mishra, K. K. High-concentration coal-water slurry from Indian Coals using newly developed additives. Fuel Process. Technol. 2003, 85, 31–42. (5) Leong, Y. K.; Boger, D. V.; Christie, G. B.; Mainwaring, D. E. Rheology of low viscosity, high concentration brown coal suspensions. Rheol. Acta, 32, 277–285.
distribution,6–8 (iii) oxygen content,4 (iv) zeta potential (surface charge),9 (v) pH sensitiveness,10,11 (vi) shear rate-shear stress relation,12 (vii) porosity,5 (viii) temperature sensitiveness of the viscosity of the coal-water slurry,13,14 and (ix) surface chemistry of coal15,16 etc. Most of these factors are controlled by the interfacial characteristics of coal. Coal, being a heterogeneous mixture of carbonaceous and mineral matter,17 has a surface (6) Roh, N.-S.; Shin, D.-H.; Kim, D.-C.; Kim, J.-D. Rheological behaviour of Coal-water mixture 1: Effect of coal type, loading and particle size. Fuel 1995, 74, 1220–1225. (7) Lorenzi, L. De; Bevilacqua, P. The Influence of Particle Size Distribution and Nonionic Surfactant on the Rheology of Coal Water Fuels Produced Using Iranian and Venezuelan Coals. Coal Prep. 2002, 22, 249– 268. (8) Boylu, F.; Dincer, H.; Atesok, G. Effect of coal particle size distribution, volume fraction, and rank on the rheology of coal-water slurries. Fuel Process. Technol. 2004, 85, 241–250. (9) Celik, M. S. Adsorption of ethoxylated sulfonate and non ionic homologous on coal. J. Colloid Interface Sci. 1989, 129, 428–440. (10) Mishra, S. K.; Panda, D. Studies on the adsorption of Brij-35 and CTAB at the coal water interface. J. Colloid Interface Sci. 2005, 283, 294– 299. (11) Mishra, S. K.; Kanungo, S. B.; Rajeev. Adsorption of Sodium dodecyl benzene sulfonate onto coal. J. Colloid Interface Sci. 2003, 267, 42–48. (12) Meikap, B. C.; Purohit, N. K.; Mahadevan, V. Effect of Microwave pre-treatment of coal for improvement of rheological characteristics of coalwater slurries. J. Colloids Interface Sci. 2005, 281, 225–235. (13) Roh, N. S.; Shin, D.-H.; Kim, D.-C.; Kim, J.-D. Rheological behaviour of Coal-water mixtures 2- Effect of surfactant and temperature. Fuel 1995, 74, 1313–1318. (14) Mishra, S. K.; Senapati, P. K.; Panda, D. Rheological behaviour of Coal-water slurry. Energy Sources 2002, 24, 159–167. (15) Leong, Y. K.; Boger, D. V. Surface Chemistry effects on concentrated suspensions rheology. J. Colloid Interface Sci. 1990, 136, 249– 258. (16) Leong, Y. K.; Creasy, D. E.; Boger, D. V.; Nguyen, Q. D. Rheology of brown coal-water suspension. Theol, Acta 1987, 26, 291–300. (17) Mishra, S. K.; Kanungo, S. B. Factors affecting the preparation of Highly concentrated Coal-Water slurry (HCCWS). J. Sci. Ind. Res. 2000, 59, 765–790.
10.1021/ef7006563 CCC: $40.75 2008 American Chemical Society Published on Web 04/12/2008
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which is mostly hydrophobic and, therefore, readily agglomerates to form clusters, reducing the stability of the coal-water dispersion. For stable dispersion, interparticle interaction of the coal particles has to be mutually repulsive. Surfactants and polymers, due to their amphiphilic nature, adsorb strongly on solid/liquid interface, making the surface hydrophilic or hydrophobic.18,19 Depending on the charge of the head groups and size of both hydrophilic group and hydrophobic chain, they provide electrostatic/steric hindrance for the particle-particle association.20,21 Many commercially available surfactants and polymers, on being adsorbed to coal-water interface, have improved the stability of coal-water slurry as well as increased concentration of coal in the coal-water slurry.22–29 Detailed screening of the literature, however, reveals that these additives are mostly of synthetic nature, development of a natural additive for stabilizing coal-water slurry being scanty. The present paper is an attempt to develop saponin, a natural surface-active compound extracted from the fruit of the Sapindous laurifolia plant, as an additive to get relatively concentrated coal-water slurry. Sapindous laurifolia is one type of soapnut tree indigenous to both India and China and is available in abundance mostly in the eastern zone of India. Saponin is a complicated mixture of sachharin derivatives and belongs to a class of naturally occurring nonionic surfactants. The hydrophilic part of the molecule called glycon consists of sachharides such as glucose, galactose, rhamnose, xylose, pentose, etc. and the hydrophobic part called aglycon consists of steroids and triterpene. The hydrophobic part is bonded through oxygen to the hemiketal or hemiacetal carbon of the sachharide residue which in turn is linked through oxygen linkages to other sachharide residues (molecular structure 1, Chart 1). The method of isolation of saponin extracted from the fruits of sapindus laurifolia and its surface activity has been discussed. The physicochemical properties such as particle size distribution, viscosity, zeta potential of coal have been investigated in the (18) Misra, K. P.; Panigrahi, S.; Somasundaran, P. Organization of amphiphiles, Part VIII: Role of polyoxyethylated alkyl phenols in optimizing of the beneficiation process of a hydrophilic mineral. Int. J. Miner. Process. 2006, 80, 229–237. (19) Soto, H.; Iwasaki, I. Selective flotation of phosphates from dolomite using cationic collectors. I. Effect of collector and nonpolar hydrocarbons. Int. J. Miner. Process. 1986, 16, 3–16. (20) Aliferova, S.; Titkov, R.; Noveselov, V.; Panteleeva, N. Application of nonionic surface-active substances in combination with acrylamide flocculants for silicate and carbonate mineral flotation. Miner. Eng. 2005, 18, 1020–1023. (21) Somasundaran, P.; Chander, P.; Chari, K. A study of the interaction between particles and bubbles in surfactants solutions. Colloids Surf. 1983, 8, 121–136. (22) Aktas, Z.; Woodburn, E. T. Effect of addition of surface active agent on the viscosity of a high concentration slurry of a low rank British coal in water. Fuel Process. Technol. 2000, 62, 1–15. (23) Boylu, F.; Atesok, G.; Dincer, H. The effect of carboxymethyl cellulose (CMC) on the stability of coal-water slurries. Fuel 2005, 84, 315– 319. (24) Zhou, M.; Qiu, X.; Yang, D.; Wang, W. Synthesis and evaluation of sulphonated acetone-formaldehyde resin applied as dispersant of coalwater slurry. Energy ConVers. Manage. 2007, 48, 204–209. (25) Guo, Z.; Feng, R.; Zheng, Y.; Fu, X. Improvement in properties of coal water slurry by combined use of new additive and ultrasonic irradiation. Ultrasonic Sonochem. 2007, 14, 583–588. (26) Zhou, M.; Qiu, X.; Yang; Dongjie; Lou, H.; Quang, X. High performance dispersant of coal-water slurry synthesized from wheat straw alkali lignin. Fuel Process. Technol. 2007, 88, 375–382. (27) Majumdar, S. K.; Kama, C.; De, S. D.; Kundu, G. Studies on flow characteristics of coal-water slurry system. Int. J. Miner. Process. 2006, 79, 217–224. (28) Qiu, X.; Zhou, M.; Yang, D.; Lou, H.; Ouyang, X.; Pang, Y. Evaluation of sulphonated acetone-formaldehyde (SAF) used in coal water slurries prepared from different coals. Fuel 2007, 86, 1439–1445. (29) Pawlik, M. Polymeric dispersants for coal-water slurries. Colloids Surf. A: Physicochem., Eng. Aspects 2005, 266, 82–90.
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presence and absence of the additive. The effects of pH, solid loading, and optimum concentration of additive have also been studied and have been compared with a commercial additive, sodium dodecyl sulfate (SDS). The mechanism of adsorption and orientation of additive at coal-water interface have been elucidated. Chart 1. Molecular Structure 1a
a
White ball, hydrogen; blue ball, carbon; red ball, oxygen.
Experimental Section 1. Isolation of Saponin.30 The drupes of Sapindus laurifolia were collected from Paralakhemundi, the forest zone of southern Orissa, India. Pericarp of the drupes of Sapindus laurifolia were cut into pieces and were extracted four times with water at ambient temperature, and finally with hot water (90-95 °C). The solid-liquid ratio was maintained at 1:3 and the sample was soaked with water for 12 h each time. The extract was concentrated on a climbing film through evaporation. On adding ammonium sulfate till saturation, solid mass started floating. The floating mass was separated and squeezed off to remove water. The floating solid compound was extracted and recrystallized from n-butanol phase to give bushy threadlike mass which was identified to be saponin. A single spot was found in the thin layer chromatogram of silica gel taking hexane:chloroform (1:3) as the eluant. The melting point was found to be 145 °C as reported earlier.30 2. Procurement of Coal Sample. Coal samples were collected from Talcher Coal Field, Orissa, eastern part of India. The coals were beneficiated employing controlled crushing and deashing to obtain cleans. Cleans were first crushed in a jaw crusher and subsequently in a double roll crusher to obtain samples with particle size below 100 mm. Three types of coal samples designated as coal A, coal B, and coal C with ash content 8.02% (low-ash coal), 18.14% (medium-ash coal), and 39.06% (high-ash coal), respectively, were selected for preparation of high-concentration slurry in distilled water. The particle size distribution of the coal samples, measured by Malvern particle size analyzer shows that the d50 values of the coal samples are 27.318, 23.949, and 33.131 µm for coals A, B, and C, respectively. A representative log-log plot for the particle size distribution of coal C is given in Figure 1. Physicochemical characteristics of coal particle surfaces vary depending on the source of coal and its treatment. On analyzing proximate and ultimate analyses data (Tables 1 and 2), it is seen that the coal sample C has the highest hydrophilicity (lowest C/O ratio, due to the presence of high percentage of oxygen-containing functional group like OOH and OCOOH) and maximum ash content. The calorific value of coal A is found to be maximum due to high percentage of carbon. 3. Rheological Measurement. The rheological studies of the CWS were carried out using a HAAKE rotational viscometer (Model RV 30), consisting of a measuring drive unit, temperature vessel with circulator, sensor system, and a data logger. A sensor (30) Biswas, H. G. A Note on the saponin from sapindous laurifoliusVahl. J. Ind. Chemical Society 1948, 25, 151–152.
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Figure 1. Particle size distribution of coal samples C (high ash coal).
Figure 2. I3/I1 and excimer intensity of pyrene versus [saponin] in percentage.
Table 1. Proximate Analyses of the Coal Samples
of the fluorescence probe, pyrene, was prepared in pure methanol (AnalR grade, Sisco-chem). Various concentrations of saponin containing 1 × 10-6 M of pyrene were prepared. The emission spectra of pyrene between 370-480 nm were recorded by exciting the saponin solutions at the absorption maximum (λmax) of pyrene in water (335 nm).
moisture (%) ash (%) volatile matter (%) fixed carbon (%) calorific value (kcal/kg)
coal A
coal B
coal C
13.15 8.02 33.44 45.39 6438
13.65 18.14 28.06 39.66 5745
10.02 39.84 29.75 30.39 4937
Results and Discussion Table 2. Ultimate Analyses of the Coal Samples
carbon (%) hydrogen (%) sulfur (%) nitrogen (%) oxygen (%) carbon/oxygen ratio
coal A (low ash)
coal B (medium ash)
coal C (high ash)
78.81 5.83 2.86 1.85 10.06 7.834
78.70 5.91 3.70 1.89 10.62 7.411
77.21 5.63 3.62 1.94 11.27 6.856
system MV I was chosen for the rheological measurements. The sensor and the cup were cleaned and air-dried. About 100 mL of slurry was prepared at weight concentrations ranging from 55 to 64 wt % with distilled water. The additive concentrations for coal-water slurry were varied from 0.4-1.2 wt % in steps of 0.2% for different weight concentrations of coal slurry. The variation in temperature was (0.1 °C, controlled through a constant temperature circulator bath connected to a viscometer. All experiments were conducted at room temperature of 30 °C. The pH of coal-water slurry with natural and commercial additives measured in a pH meter was in the range of 4.5-5.0. The rheological measurement was controlled by a software rotation version 3.0. The best-fit model was fitted to the shear stress-shear rate data to obtain the nature of coal-water slurry. The various parameters such as shear stress, shear rate, true viscosity, apparent viscosity, and temperature along with the curve were displayed on a computer screen. Zeta probe 24V(52-60 Hz) T3A equipped with microprocessor was used for the measurement of the zeta potential of coal particles containing 5% weight fraction of coal in water. 4. Static Stability Tests. Coal water slurries were prepared at weight concentrations of 58%-64% and then poured into glass cylinders of about 100 mL. The tops of the cylinders were sealed and the cylinders were stored at room temperature. The static stability of coal-water slurry was evaluated by applying the rod penetration method.31 5. Fluorescence Measurement. Florescence spectral measurements were made on a Hitachi 650-40 fluorimeter. Stock solution (31) Yuchi, W.; Li, B.; Li, W.; Chen, H. Effects of Coal Characteristics on the properties of coal-water slurry. Coal Prep. 2005, 25, 239–249.
A. Surface Activity of the Aqueous Solution of Saponin. The surface activity of saponin was determined by studying the fluorescence characteristics of saponin solution using pyrene as fluorescent probe. Pyrene has a structured monomeric emission with five peaks out of which the first peak at 370 nm (I1) and third peak at 380 nm (I3) are sensitive to the polarity of the medium32 and hence the ratio I3/I1 is called the polarity parameter. The surface activity of saponin was studied by determining its critical micellar concentration(cmc) through measurement of polarity parameter as a function of surfactant concentration. At very low concentration, the emission spectrum of pyrene is similar to that in pure water with I3/I1 ratio ) 0.63. With increase of saponin concentration, the ratio gradually increases till a value of 1.5 is reached at around 0.8% of saponin. This value does not change with further increase of saponin. Appearance of a plateau in the polarity parameter versus [saponin] curve has been attributed to the consequence of micelle formation and the concentration at onset of plateau of this curve is considered to be cmc33 (Figure 2). The gradual increase in the ratio has been attributed to the gradual decrease in the polarity of pyrene environment due to the formation of micelles in a stepwise manner. The micelles are evolved gradually through association of surfactant monomers to form premicellar aggregates such as dimers, trimers, tetramers,34,35 etc. The stepwise association has been evidenced from the appearance of excimer at low concentrations of saponin. The excimer is a (32) Kalyanasundaram, K.; Thomas, J. K. Environmental Effects on Vibronic Band Intensities in Pyrene Monomer Fluorescence and Their application in Studies of Micellar systems. J. Am. Chem. Soc. 1977, 99, 2039–2044. (33) Chandar, P.; Somasundaran, P.; Turro, N. J. Fluorescence probe studies on the structure of the adsorbed layer of Dodecyl Sulphate at the Alumina-water interface. J. Colloid Interface Sci. 1987, 117, 31–46. (34) Sahoo, L.; Sarangi, J.; Misra, K. P. Organization of amphiphiles, Part I: Evidence in favour of premicellar aggregates through Fluorescence Spectroscopy. Bull. Chem. Jpn. 2002, 75, 859–865. (35) Sahoo, L.; Misra, K. P. Characterization of the structure of polyoxyethylated alkyl phenols micelles at 300 K. SU J. Sci. Technol. 2001, 13B, 18–22.
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Table 3. Effect of Solid Weight Concentration on Apparent Viscosity apparent viscosity, Pa · s concn of coal in CWS (%)
coal A (low ash)
coal B (medium ash)
coal C (high ash)
50.0 55.0 60.0 62.0 64.0
0.382 0.407 0.481 0.627 0.782
0.401 0.473 0.524 0.706 1.081
0.446 0.504 0.543 0.794 1.123
complex formed between a ground-state and excited-state pyrene. Ground-state and excited-state pyrene can be placed at a suitable distance to form an excimer within the domain of the premicellar aggregates due to their small size. The intensity of the excimer increases due to the formation of more premicellar aggregates as the saponin concentration increases. But once the micelles are formed in the solution, the intensity of the excimer goes down due to the distribution of pyrene in a large domain of micelles. The formation of excimer decreases as the probability of the excited singlet P* to leave one micelle and interact with a ground-state pyrene from another micelle to form excimer during its excited lifetime is very low since the exit of pyrene from micelles is a millisecond phenomenon whereas fluorescence is a nanosecond phenomenon.36 The excimers are formed at very low concentrations of saponin due to the large hydrophobic residue of saponin. The large hydrophilic head in the saponin molecule drives the stepwise growth of micelles.34,35 B. Effect of Coal Loading. The apparent viscosity of coal-water slurry for coal A, coal B, and coal C is measured at shear rate 77 S-1 with 0.8% weight (w/v) of saponin (around cmc) by varying coal weight in the concentration range of 55-64%. The values are given in Table 3. The apparent viscosity is found to increase with solid weight concentration due to increase in particle-particle interaction. Beyond 64% weight fraction of coal, the slurry becomes a hard sediment and hence difficult to disperse. The variations of apparent viscosity with shear rate containing 64% and 60% of coals A, B, and C and 0.8% of saponin are shown in the Figure 3a,b, and c. Similar behavior of CWS’s has also been reported by other authors.4,37 The apparent viscosity is found to increase with increase in ash content of the coal for the same weight concentration of coal-water slurry possibly due to strong interaction of the hydrophilic ash with water. Usually, increase in ash content leads to strong aggregation of coal and formation of gel state resulting in increase of the viscosity of the slurry.4 C. Effect of Additive Concentration. The apparent viscosity of coal-water slurry (64%) is found to decrease with increase in additive concentration till a constant value is obtained beyond 0.8 wt % of saponin as shown in Figure 4. Viscosity is a property of liquid which resists the motion of different layers present in it. This opposing force arises largely due to van der Waals forces of the molecules present in the different layers. Saponin adsorption at the coal-water interface increases the surface hydrophobicity as well as hinders the close approach of the different layers due to steric barriers offered by the head groups of saponin. This leads to the decrease in interaction among particles present in the layers. The steady decrease of viscosity of the slurry occurs as long as saponin continues to adsorb on coal particles. But once the critical micellar concen(36) Singer, L. A. In Solution BehaViour of Surfactants; Mittal, K. L. , Fendler, E. J., Eds.; Plenum: New York, 1983; Vol. 1, p 73. (37) Swain, P; Panda, D. Rheology of Coal-Water mixture. Fuel Sci. Technol. Int. 1996, 14 (9), 1237.
Figure 3. (a) Rheological behavior of coal A sample with variation of shear rate having loading 60% and 64% containing 0.8% of saponin. (b) Rheological behaviour of coal B sample with variation of shear rate having loading 60% and 64% containing 0.8% of saponin. (c) Rheological behaviour of coal C sample with variation of shear rate having loading 60% and 64% containing 0.8% of saponin.
tration is achieved in the solution, the saponin monomers prefer to aggregate to form micelles. This limits its monomer activity38 and hence no further adsorption of saponin takes place onto the coal-water interface. It is reported that surfactant adsorbed as monomer at the interface38–41 and hence the effect of saponin on apparent viscosity is labeled up beyond its cmc i.e., 0.8% (w/v). D. Rheological Behavior Studies of Coal-Water Slurry. Figure 5a-c shows the plots of shear stress with variation of (38) Hanna, H. S.; Somasundaran, P. In ImproVed Oil RecoVery by Surfactant and Polymer Flooding; Shah, D. O., Schechter, R. S., Eds.; Academic Press, Inc.: New York, 1977.
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Figure 4. Apparent viscosity of coal-water slurry with variation of saponin concentration.
shear rate of coal-water slurry (64% weight fraction) in the presence of 0.8% of saponin and commercially available surfactant, sodium dodecyl sulfate (SDS). In all cases, a linear shear stress-shear rate relationship with an initial shear stress threshold is found. Thus, these fluids belong to Bingham plastic fluids, obeying eq 1. τ ) τ0 + ηγ
(1)
where τ and γ denote shear stress and applied shear rate, respectively. τ0 is the yield stress and η is defined as the coefficient of rigidity. The yield stress and apparent viscosity values measured at the shear rate of 77 S-1 in the weight concentration range of 55–64% for all coal samples are given in Table 4. On analyzing the rheological measurement data of coal water slurry, it is found that coal samples A, B, and C have non-Newtonian12 characteristics in the weight concentration range 55–64%. E. Effect of pH on Zeta Potential of Coal and Apparent Viscosity of Coal-Water Slurry. The electrophoretic mobility (zeta potential) of the coal particles (5% in water) was measured with variation of pH and is shown in Figure 6a-c. The bare coals exhibit an isoelectric point at around 5.4-6.5, which falls within the same range as reported earlier.42,43 However, in the presence of saponin the zeta potential is decreased in all cases with concomitant shift of isoelectric point of all coals toward alkaline pH (Table 5). Coal surface contains polar groups such as -COOH and -OH attached to the hydrocarbon skeleton connected by cross-links44,45 Depending on the pH of the solution, the polar groups get protonated or ionized when suspended in water (Scheme 1). At acidic pH, protonated hydroxyl and carboxyl (39) Kolbel, H.; Horig, K. Konstitution und Eigenschaften grenzfla¨chenaktiver Stoff. II. Sorption von anionischen grenzfla¨chenaktiven Stoffen an Textilfasern. Angew. Chem. 1959, 71, 691–697. (40) Griffith, J. C.; Alexander, A. E. Equilibrium adsorption isotherms for wool/detergent systems I. The adsorption of sodium dodecyl sulfate by wool. J. Colloid Interface Sci. 1967, 25, 311–316. (41) Misra, K. P.; Mishra, B. K.; Somasundaran, P. Organization of amphiphiles, Part IV: Characterization of Microstructure of the adsorbed layer of decylpolyoxyethylene nonyl phenol. Colloids Surf. 2005, 252, 169– 174. (42) Fuerstenau, D. W.; Pradip. Adsorption of frothers at coal/water interfaces. Colloids Surf. 1982, 4, 213–227. (43) Miller, J. D.; Laskowski, J. S.; Chang, S. S. Dextrin adsorption by oxidized coal. Colloids Surf. 1983, 8, 137–151. (44) Whitehurst, D. D.; Mitchell, T. O.; Farcasiu, M. Coal Liquifaction; Academic Press: New York, 1980. (45) Davidson, R. M. In Coal Science; Gorgaty, M. L., Larsen, J. W, Wender, I., Eds.; Academic Press: New York, 1982; Vol. 1, p 84.
Figure 5. (a) Rheological behavior of coal A sample in the presence of saponin and SDS; (b) rheological behavior of coal B sample in the presence of saponin and SDS; and (c) rheological behaviour of coal C sample in the presence of saponin and SDS.
group will render positive charge and at alkaline pH dissociated hydroxyl and carboxyl group would render negative charge to the coal surface. These charges account for its zeta potential when electric field is applied. The (46) Misra, K. P.; Somasundaran, P. Organization of amphiphiles, Part VI: A comparative study of the orientation of polyoxyethylated alkylphenols at air-water and silica-water interface. J. Surfactants Deterg. 2004, 7, 373– 378.
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Table 4. Apparent Viscosity and Yield Stress for Coal A, Coal B, and Coal C at the Shear Rate of 77 S-1 for Different CWS Weight Concentrations apparent viscosity (Pa · S) wt concn, Cw, %
coal A
coal B
coal C
55 60 62 64
0.42 0.50 0.60 0.82
0.46 0.54 0.72 0.97
0.47 0.58 0.75 1.123
yield stress (Pa) coal A coal B coal C 35 46 57 64
40 51 58 72
40 53 62 74
decrease in zeta potential may be attributed to the mechanical displacement of the shear plane being projected some distance beyond the original shear plane or coverage of a fraction of coal surface due to the adsorption of bulky surfactant molecules at the coal-water interface.9 We have also seen such decrease in zeta potential due to the adsorption of surfactant at the silica-water interface.46 The shift of isoelectric point is due to the large number of -OH and -COOH groups present in adsorbed saponin which may also be dissociated and protonated depending on the pH of the solution. The apparent viscosities of all coal-water slurry (60% w/v) are found to decrease with increase of pH as shown in Figure 7a-c. The electrostatic repulsion between intense negative charges developed on coal surface due to the ionization of the polar groups prevents particle-particle association as the pH of slurry increases. This results in reduction of apparent viscosity. The yield stress is computed graphically from the plot of shear stress versus shear rate at a particular pH. The plot of yield stress with variation of pH (Figure 8), shows a slow increase in yield stress up to certain pH beyond which there is a sharp and steady decrease in yield stress. The maximum yield stress is obtained at a pH that nearly coincides with the isoelectric point (around pH 6-7). Since at the isoelectric point the residual charge on coal particle becomes zero, greater cooperative interaction of the saponin-adsorbed coal particles occurs, resulting in highest yield stress. Hence they settle down relatively easily. To make the slurry flow at this pH, one has to pump the slurry more than any other pH. The slow increase of yield stress before isoelectric point is probably not affecting the apparent viscosity of the slurry too much and hence a continuous decrease in apparent viscosity is observed with increase in pH in all cases. Saponin is by nature nonionic but, because of the presence of a large number of groups containing oxygen having lone pairs of electrons, sometimes behaves like anionic nature. This may be the reason for which the effects of SDS and saponin are more or less equal. In fact, during measurements of pK of a number of aldimines in presence of nonionic and ionic surfactant assemblies,47 we have seen that nonionic surfactant assemblies behave in a similar way to anionic surfactant assemblies. Mishra et al.17 have also observed similarity in behavior between Brij35, a nonionic surfactant, and an anionic surfactant NaDBS during their studies on the adsorption characteristics at the coal-water interface. F. Effect of Temperature on Apparent Viscosity of Coal– Water Slurry. The apparent viscosity of coal-water slurry is found to decrease exponentially with increase of temperature as shown in Figure 9a,b. This is attributed to the decrease in interparticle attraction due to increase in kinetic energy of the coal particles. The relation between viscosity and temperature may be represented (47) Panigrahi, S.; Chakravorty, M.; Misra, K. P. Effect of organized assemblies, Part-II: Environmental effect on the pK of (o/p) hydroxybenzylidene- (4/6) nitro-2-aminobenzothiazoles. J. Colloid Interface Sci. 2007, 306, 137–142.
Figure 6. (a) Variation of zeta potential of coal A with change in pH in the presence and absence of saponin; (b) variation of zeta potential of coal B with change in pH in the presence and absence of saponin; and (c) variation of zeta potential of coal C with change in pH in the presence and absence of saponin.
by a simple Arrhenius expression14 as presented in eq 2 which on rearrangement yields eq 3. η ) A exp(E/RT) (2) ln(η) ) E/RT + ln(A) (3) where η is the viscosity at a particular shear rate, E is the fluid flow activation energy, T is the temperature in kelvin, A is fitting
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Figure 8. Plot of yield stress versus pH of the slurry containing 60% weight fraction of coal and 0.8% of saponin.
Figure 7. (a) Effect of pH with viscosity for coal A (coal concentration ) 60%, shear rate ) 77 1/S and saponin 0.8%); (b) effect of pH with viscosity for coal B (coal concentration ) 60%, shear rate ) 77 1/S and saponin 0.8%); (c) effect of pH with viscosity for coal C (coal concentration ) 60%, shear rate ) 77 1/S and saponin 0.8%). Scheme 1
parameter, and R is the universal gas constant. A linear plot with correlation coefficient near about 0.99 (Figure 10) is
Figure 9. (a) Effect of temperature on apparent viscosity for coals A, B, and C (coal concentration ) 60%, shear rate ) 77 1/S, and saponin 0.8%). (b) Effect of temperature on apparent viscosity for coals A, B, and C (coal concentration ) 64%, shear rate ) 77 1/S, and saponin 0.8%).
obtained for coal-water slurry containing 64% of coal. The fluid flow activation energy is found to depend on the type of coal used being highest for coal A and lowest for coal C. G. Mechanism of Stabilization of Coal-Water Slurry. For a hydrophobic solidlike coal, the most probable mechanism of adsorption of saponin at the coal-water interface is the adsorption of surfactant with its hydrophobic group being anchored on the coal surface while the hydrophilic chain has to remain dangled in the bulk water. This orientation of saponin
1872 Energy & Fuels, Vol. 22, No. 3, 2008
Figure 10. Arrhenius plot of viscosity versus temperature for coals A, B, and C (coal concentration ) 64%, shear rate ) 77 1/S, and saponin 0.8%).
Das et al.
Figure 12. Schematic representation of stabilization of a coal-water slurry. Table 5. Isoelectric Point (IEP) of Different Coals in the Absence and Presence of Saponin and SDS system
isoelectric point
naked coal A coal A with additive coal A with SDS naked coal B coal B with additive coal B with SDS naked Coal C coal C with additive coal C with SDS
5.4 6.8 6.5 6.1 6.5 6.1 6.5 6.8 6.3
coal-water slurry by the additive is mostly due to steric reasons as the zeta potential of coal is decreased with adsorption of the additive Table 5. Figure 11. Schematic representation of a coal particle with adsorbed saponin, the terpenoid part sitting on its surface and the glycoside part being oriented toward bulk water phase.
at the coal-water interface is further supported by adsorbing hederagenin, the hydrolyzed product of saponin (which is essentially nonpolar) on the coal surface. Hederagenin30 was synthesized by hydrolyzing 10 g of saponin with 100 mL of aqueous methanol (1:1) containing 5% hydrochloric acid. The resulting sticky mass was separated and washed with petroleum ether to remove the oils present in it. The insoluble substance on crystallization from methanol gives rhombic crystals of hederagenin (m.p. 328-329 °C). The rheological behavior of coal-water slurry treated with 0.8% of hederagenin was studied, and the viscosity was found to remain unchanged. The decrease in viscosity in the presence of saponin therefore excludes the possibility of orientation of hydrophobic toward bulk aqueous phase. The hydrocarbon part of the molecule lies flat on the hydrophobic surface and the hydrophilic part protrudes to bulk water molecules (Figure 11). The bulky glycosides heads are hydrated and form an effective barrier around each particle. The mechanical barrier increases the resistance of the dispersed particles to maintain a mechanical shock and prevents them for coalescing when they collide, inhibiting the close approach of one particle to another (Figure 12). The stabilization of the
Conclusion Saponin, a surface-active compound extracted from the fruits of plant Sapindous laurifolia, has been developed as a cost effective and environment-friendly additive for the stabilization of coal-water slurry. Since these surfactants contain carbon, hydrogen, and oxygen only, in the combustion of coal-water slurry the contribution of the surfactants to the amount and fusion temperature of ash produced is negligible. The results confirm the use of saponin as a suitable additive for coal-water slurry similar to commercially available additive such as sodium dodecyl sulfate. Acknowledgment. The authors express their sincere thanks to Institute of Minerals and Materials Technology, Bhubaneswar, for providing laboratory facilities. P.K.M. thanks the Department of Science and Technology for financial support through sanction of a project to P.K.M. (Project No. sanction letter no. SR/S1/PC-39/ 2004 dated 14.03.2006). The authors also thank CSIR for award of senior research fellowship to S.P. and the University Grants Commission and Department of Science and Technology, Government of India, for financial support to the Department through sanction of DRS and FIST, respectively. EF7006563
