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Coal-Oil Mixtures. 2. Surfactant Effectiveness on Coal-Oil Mixture Stability Measured with a Sedimentation Column Device Robert L. Rowell; Stephen R. Vasconcellos, Rlchard J. Sala, and Raymond S. Farlnato Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 0 1003
A sedimentation column device is described and tested for determining the effect of surfactants on the static stability of coal-oil mixtures (COM). The sedimentation ratio (bottomsampled wt % coal in treated slurry/untreated slurry after 24 h settling) was found to be an effective screening parameter for the effectiveness of commercial surfactants on COM stability. From the settling behavior of 25 wt % coal slurries, determined with a Sedimentation balance, a compressive settling (subsidence) model was found to be appropriate. This led to an interpretation of COM stability in terms of network stability rather than classical isotated colloidal particle stability. The effects of water and surfactant structure were assessed and led to identification of the most effective classes of surfactants and interpretation of COM stability at the macroscopic, microscopic, and molecular levels.
Introduction The evaluation of colloidal dispersion stability plays a fundamental role in the currently active development of coal-oil mixtures (COM) for use as boiler fuels (Blake and Sabadell, 1978; Brown, 1977). The eventual utility of a COM, which may include stabilizing additives, depends on several physical and economic properties. These include the rate of coal sedimentation, particle redispersibility, rheological properties of the settled COM, effects of dispersion and additives on combustion and emissions, and the total cost of preparative procedures and additives. The settling of coal in COM’s has been the subject of experimentation by two major interests: liquefaction and combustion. The coal liquefaction process results in unconverted coal and mineral matter which must be separated from the final product. The coal in COM’s for combustion must be kept in either a suspended state, or coaxed to form a fragile, loosely packed network which is easily redispersible. In either case, the stability of a solid dispersed in a nonaqueous medium is the central concern. Destabilization is sought in the coal liquefaction process and stabilization in the production of COM. The effectiveness of stabilizing additives has many facets. We would expect a stabilizer to hinder the settling of coal particles distributed in the oil phase. The mechanism at the microscopic level for this behavior might involve elements of electrostatic and steric stabilization. In addition, practical applications require that the structure of the settled coal bed (and this is usually the case, even with the best stabilizers) be such that it is easily redispersible. In addition to the recognized mechanisms of electrostatic and steric stabilization, we have proposed (Rowell et al., 1978) and emphasized the importance of a third mechanism which may be termed matrix or network stabilization. In network stabilization, the dispersion acting as a whole tends to preserve a uniformity. An effective stabilizer would cause the formation of a loose network with a low yield point to flow. All three mechanisms of stability, electrostatic, steric and network, are significant in nonaqueous systems, although network stability seems to dominate COM. Work with the model system of carbon blacks in nonaqueous solvents have demonstrated the importance of charge stabilization effects (Van der Minne and Hermanie, 1953; Van der Waarden, 1950; Damerell and Mattson, 1944). Results in the work described here show that for COM’s there is also a definite nonelectrostatic component to the stability which we relegate to steric and network effects. 0196-4305/8111120-0283$01.25/0
Differences in the rheological properties of the settled COM’s are explained in terms of network stability. The dynamics of settling particles have been mathematically modeled by Michaels and Bolger (1962) in three particle concentration ranges: (1)dilute, where the settling rate is a constant with time, (2) intermediate, where the settling rate goes through a maximum and the aggregates (roughly spherical flocs of kaolin in their work) settle as a coherent network, and (3) concentrated, where a compressive settling (subsidence) mechanism was indicated, resulting in a monotonically decreasing rate of settling with time. The dilute settling rate equations were derived from the Richardson and Zaki (1954) equation for the group settling of uniform spheres. For the other concentration regions, equations for a compressive settling model were derived. They experimentally verified the relations using kaolin suspensions. More recently, Slagle et al. (1978) have demonstrated the application of these settling rate equations in the dilute and intermediate concentration regions to coal in COM’s. They remarked that the rate law would change from the intermediate to dilute approximations as the temperature is raised at constant particle concentration. In the concentrated range which is of interest in COM, the combination of aggregates and developing network tends to settle en masse or subside. The dynamics of subsidence have been derived by Smellie and La Mer (1956) using mass continuity relations. Such relations were subsequently re-derived by Tory and Shannon (1966) using the more complete continuity equation of Kynch (1952). They also derived concentration profiles of the settled bed which has been shown by X-ray transmission to be nonuniform (Gaudin and Fuerstenau, 1958, 1959, 1960). It is clear that kinetic processes are important in the approach to the steady state. It is also evident that kinetic processes are important in mixing and flow of COM. We have classified (Rowell et al., 1978) all such kinetic processes as dynamic stability as opposed to static stability which is concerned with the structure and homogeneity of the quiescent COM and reactive stability which encompasses all physical and chemical processes that cause rearrangements of physical and chemical bonds. While we tend to think of reactive stability mainly in relation to the COM during actual combustion, it is important to recognize that reactive stability plays a role in dynamic and static stability as well. While the systems studied here are predominantly no@ 1981 American Chemical Society
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naqueous, small amounts of water are inevitably present, either in the coal or in some of the surfactant preparations. Water, even in trace amounts, can upset the stability of a nonaqueous dispersion. This can be due to the coalescing forces of adsorbed water layers on two particles, resulting in irreversible aggregation. In addition, adsorbed water can compete for active sites normally sought by surfactant molecules (Overbeek, 1952). The effect of water was not a major object of this study but did require some consideration. In choosing a test of COM stability we have concentrated mainly on the static and dynamic stabilities. A sedimentation type of test seemed the most direct and embodied the variables of an actual COM application. Consideration of known methods for sedimentation analysis led to a rejection of the Andreasen pipet (side-arm sampling), diver (sinking sphere), hydrometer (surface density), and manometer (side-arm pressure) methods because of sample coarseness and viscosity. Neutron activation analysis was not feasible with available instrumentation, and turbidimetric techniques were not applicable due to the large optical density of COM's. In this work sedimentation columns, applied in a novel way, were chosen to test COM stability. Some corroborative and complementary tests were performed with a sedimentation balance. The sedimentation columns as used in this work allowed a measurement of a static stability parameter (the sedimentation ratio, SR) and a dynamic stability parameter (the drain time, DT).Both of these parameters allow a relative comparison of the effects of additives on COM stability. From the results of stability studies and the effect of small amounts of water on COM properties, interpretations a t the microscopic and molecular levels were begun. In summary, we note from this overview that the problem of stability may be more clearly understood by defining three general classes of stability: static, dynamic, and reactive according to the field acting upon the system. Examples of the fields respectively are: gravity, flow and shear, and chemical potential. A stable system is a system with constant properties, i.e., invariant to the applied field. The present work is directed at the measurement of static stability and reports on methods developed and used in the screening and evaluation of stabilizers for coal-oil mixtures in laboratory research on coal-oil mixtures at the University of Massachusetts for the New England Power Service Co. in relationship to field tests at the Salem Harbor Plant of the New England Power Co. (Rowell et al., 1978). Experimental Section Materials. Coal samples were from a single batch of finely powdered bituminous coal obtained from the General Motors Corp. The coal had been pulverized to 80% -200 mesh and had been reported in a previous study (Brown, 1977) to be a Pittsburgh No. 8 seam coal with low ash (5 to 8%) and medium to high volatiles (35%) and ash fusibility temperature (2350 O F ) . Each sample was ovendried for 6 h a t 100 "C. This was shown to shift the sieve-analysisparticle size distribution histogram to smaller sizes due to the breakup of aggregates (Rowell et al., 1978). Only trace amounb of water were found necessary to cause significant coal aggregation. The size distribution of our standard samples was prepared by sieving: 95% through 200 mesh and 5% between 200 and 170 mesh. The coal density was determined by volume displacement of reagent grade heptane (Fisher) in a Gay-Lussac specific gravity bottle to be 1.22 g/cm3.
-INNER
DIA.2CM
GLASS JACKET FOR WATER HEATING
10 MM BORE
STOPCOCK
Figure 1. Sedimentation column used to sample bottom of slurry as a function of time for determination of density using a pycnometer.
A low sulfur no. 6 fuel oil sample was obtained from the New England Power Service. Its viscosity a t 50 "C was determined using an Eprecht rotating shaft viscosimeter to be 0.78 P. The density, determined with a standard pycnometer, at 50 "C, was 0.915 g/mL. The coal-oil mixture gave a linear plot of specific volume vs. weight percent coal. Surfactants were used as received from commercial sources and are listed in Table I. Sedimentation Column. The sedimentation column (Vasconcellos, 1977) has been described earlier (Rowell et al., 1978), but dimensions were omitted because of patent application. It consisted of a glass column 50 cm high with an inner diameter of 2.0 cm and fitted at the bottom with a stopcock of 1.0 cm bore to withdraw samples. The column was surrounded with a glass jacket allowing temperature control by the circulation of water at 50 "C from a thermostated reservoir. The column geometry is shown in Figure 1. Measurements. Two types of measurements were made on a COM in the sedimentation column: density and flow. After a given settling time a small sample was drawn off the bottom of the column through the stopcock into a 2-mL pycnometer. The sample density was converted to a weight percent of coal by assuming specific volume additivity (experimentallyverified for these samples of coal and oil), or by using a calibration plot of density vs. weight percent coal. The sedimentation ratio (SR)was obtained as the ratio of the weight percent of coal from bottom sampling for a stabilized dispersion to the weight percent of coal under the same conditions for a dispersion with no additive. Values of SR < 1 indicate a stabilizing effect. The use of a ratio minimized the effects of intrinsic coal and oil properties so that the effect of the additive could be measured. The flow measurement was the efflux time for a settled COM with the stopcock wide open. The ratio of drain time for unstabilized to stabilized COM gave an independent measurement of the sedimentation that was used previously (Vasconcellos, 1977). Preparation of COM. The coal-oil slurries were prepared at 25 wt % coal and 0.25 wt % additive based on total COM weight. Above 25 w t % coal the SR begins to increase rapidly and it is difficult to measure differences in the stabilizing effect of the different additives. Below
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Table I. Effect of Stabilizing Agents on COM's: Sedimentation Ratio stabilizer
SR
composition or formula Anionics NH,' salt of sulfite ester of alkylphenoxypolyol polyoxyethylene alkyl aryl sulfonate sorbitan monostearate polyoxyethylene sorbitol oleate-laurate NH4+salt of alkyl aryl sulfonate sorbitan monopalmitate CH3(CH,),,OSO,Na Na' salt of lauryl myristyl-p-aminopropionic acid oleyl N-acylsarcosinate CH3(CHZ)17S03Na sorbitan sesquioleate Na' salt of condensed naphthalenesulfonic acid sorbitan monolaurate
Alepal CO-436 Atlox 3335 Span 6 0 Atlox 1045A Atlox 2081 Span 40 sodium lauryl sulfate Deriphat 170C Hamposyl 0 sodium octadecanesulfonic acid Arlacel 83 Blancol N Arlacel 20
0.70 0.77 0.82 0.82 0.83 0.83 0.88 0.89 0.89 0.95 0.95 0.98 0.98
Cationics CHACHz )19N(CzH4O)mH(CzH4O)nH cetyl trimethylammonium stearate N-soya N-ethyl morpholinium ethosulfate n-alkyl amine derivative CH3(CHz),4N(C*H4O)mH(CzH,O),H benzylalkonium C1 CH3(CHz),,(CH)4N-NC1
Ethomeen C-20 CTAS Atlas G-271 Avitex LN Ethomeen (3-15 Triton X-400 cetyl pyridinium chloride CTAC CTAB
0.58 0.61 0.62 0.69 0.70 0.72 0.75 0.75 0.79
CH3(CHZ)15N(CH3)3C1 CH3(
'HZ
)ISN(
CH 3) 3Br
Nonionics Tetronic 1104 Merpol SH Merpol OE Merpol HC Merpol OJ Brij 96 Rewopon AM-C Brij 78 Surfonic N-95 Cabosil M-5 Brij 58 Tween 40 G-1441 Igepal CO-610 Merpol SE Brij 56 Tween 20 Brij 76
poly01 CH3(CHZ
)l~(°CZH4)80H
polyoxyethylene oxide derivative CH3(CH,),CH=CH( CH,),( OC,H,),,OH polyoxyethylene oxide derivative polyoxyethylene (10) oleyl ether amphoteric nonionic polyoxyethylene (20) oleyl ether CH3(CHZ)8C6H40(C7.H40)9.SH
finely powdered silica polyoxyethylene ( 2 ) cetyl ether polyoxyethylene ( 20) sorbitan monopalmitate polyoxyethylene (40) sorbitol lanolin nonylphenoxypoly( ethy1eneoxy)ethanol CH3(CHZ)14(0C2H4 ),OH polyoxyethylene (10) cetyl ether polysorbate ( 20) polyoxyethylene ( 20) sorbitan monolaurate polyoxyethylene (10) stearyl ether 0
.9
0.66 0.77 0.77 0.79 0.80 0.82 0.82 0.83 0.84 0.85 0.85 0.85 0.86 0.88 0.91 0.92 0.92 0.99
ATLAS 6-271
0 ETHOMEEN C-20
-
V ETHOMEEN C-15
.8 -
p
.7-
t ATLAS 6-271 E ETHOMEEN C.20 A ETHOMEEN C-15 0
0,
.6-
1
g
.5
I
.5
'4
EE
t 0
1
.2
.3
WEIGHT X OF ADDITIVE 10
20
30
40
50
WEIGHT PERCENT COAL
Figure 2. Sedimentation ratio as a function of weight percent coal based on the total weight of the slurry for three of the most effective stabilizers.
25 wt % coal the SR values are well separated for different surfactants (see Figure 2). The effectiveness of several surfactants in lowering the SR as a function of wt %
Figure 3. Sedimentation ratio as a function of weight percent additive based on the total weight of the slurry.
showed a leveling off above 0.2 w t %. This was the basis of choosing 0.25 wt. % of additive as standard (see Figure 3). Slurries were prepared by stirring the surfactant into the heated oil at 50 "C followed by subsequent addition
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- -
4 1
0.5
0.6 Y c1
i P 2 I +-
o
CONTROL
0
ATLAS 6-271
T 7
Y
Y
0.7 SR
ETHOMEEN C-20 ETHOMEEN C.15
0.8
4 1
0.8
2
4
6
8
10
12
14
16
18 20
22
24
TIME (HOURS)
Figure 4. Weight deposited as a function of time for a slurry of coal in no. 6 oil measured with the sedimentation balance for different stabilizers.
of oven-dried coal. The mixing procedure was standardized so that the effects of additives could be measured. Michaels and Bolger (1962) determined for aqueous kaolin suspensions (highly interactive systems) that the method of agitation was an important factor in determining initial settling rates for dilute suspensions. The measurements here, however, were after 24 h had elapsed on systems of low interaction and the settling had nearly equilibrated. Thus, the sedimentation ratio measured the effects of additives and was insensitive to initial mixing procedure. It must also be noted here that small amounts of water have a decided effect on COM stability. Many of the commercial surfactants are themselves aqueous solutions and this water must be considered in any microscopic interpretations. Sedimentation Balance. In order to follow the time course of sedimentation for several additive-treated COMs, a sedimentation balance was constructed along the design of Calbeck and Harner (1927). A thermostated 800-mL beaker was centered under the balance pan of a magnetically damped Chain-0-Matic balance. In this way the time dependence of the mass sedimentation was measured directly a t 50 “C. Subsidence Volume. In a related study on the stability of COMs as determined by the subsidence volume of the settled coal bed, the effect of trace amounts of water on the subsidence volume (SV) was clearly demonstrated (Saia, 1978). The results are given here and a comparison made with the sedimentation ratio. The S V was measured for a 20.7 wt % codf0.45 wt % additivefparaffm oil slurry a t 50 “ C after 48 h using 250-mL graduated cylinders as settling vessels. The height of the subsided coal bed could be read directly from the graduated cylinders because of the clear supernatant paraffin oil. In some of the experiments 250-mL beakers were used to measure the subsidence (Saia, 1978). Results COM’s were allowed to settle in the columns for 24 h at 50 “C before sampling. Results of the sedimentation balance (Figure 4) showed that the settled bed had nearly equilibrated after about 18 h. Stabilizing Additives. A variety of commercially available anionic, cationic, and nonionic surfactants were tested for their effect on COM stabilization based on evaluating their sedimentation ratio relative to an unstabilized COM. All results listed in Table I are for COM’s composed of 25 wt % coal and 0.25 wt % additive. Any differences in the SR less than 3% are within the precision of the experiments. In general, the cationics were the most
1.0
I 0
5
I IO
I 15
I 20
I 25
I
30
SURlAClANI
Figure 5. Correlation of sedimentation ratio SR and subsidence volume SV with surfactant effectiveness. Full-shaded circles for SR of liquid surfactants in no. 6 oil at 50 OC for 24 h. Half-shaded dots for S V of solid Surfactants in paraffin oil at 50 “C for 48 h. Open circles with central dot for S V of liquid surfactants in paraffin oil at 50 OC for 48 h. The terms “solid” and “liquid refer to the ambient temperature state of the surfactants as received from the manufacturer.
effective stabilizers based on the SR. Drain times were measured for several of the samples (Vasconcellos, 1977), and the trend in drain times followed the trend in SR values. The values of SR for coal in no. 6 oil showed a direct correlation with the subsidence volume (SV) of coal in paraffin oil for the case of liquid surfactants. When insoluble solid surfactants were used there was generally little effect on the SV, suggesting a simple solubility problem. The data for SR and S V are compared in Figure 5, which was prepared by first establishing the order of decreasing effectiveness in terms of increasing sedimentation ratio. The subsidence volume of the numbered surfactants was added to the graph and gave a similar trend. The most effective stabilizers gave the lowest sedimentation ratio and the highest subsidence volume. The low effectivenessand lack of correlation for the solid surfactants in paraffin oil (half-shaded circles in Figure 5) was attributed to the fact that these materials were not as soluble as the liquid surfactants and that the paraffin oil was not heated during addition of the stabilizer. Effect of Water. Small amounts of water added to the coalfparaffin oil/surfactant slurry were seen to have pronounced effects on the subsidence volume. Figure 6 summarizes subsidence measurements in 250-mL beakers and shows that the added water initially increases the subsidence bed then decreases it. For this particular experiment 0.30 g of the cationic surfactant Atlas G-271 was used along with 20.0 g of coal and 125 mL of paraffin oil. This stabilizer is actually a 35% aqueous solution, so the mixture without added water contained approximately 0.15 wt % (based on total mixture) water and the addition of 15 mL of water raised the total water to 10.5%. The four points along the maximum in Figure 6 range over a total water content of 1.7 to 3.8 wt % based on the total COM. Addition of 0 to 15 mL of water to the mixture changed the coal concentration from 15.4 wt % to 13.8 wt %. The coal used in this S V experiment was not oven-dried; however, previous work (Saia, 1978) showed that the coal was of such low moisture content that oven drying did not significantly affect values of the SV. Optical microscopic observation of these slurries showed that the addition of
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 287 4.0
30
-
P I
1
I
2.0
1.0 I
I
I
I
0
5
IO
15
VOLUME H,O (MU
Figure 6. Variation of the equilibrium subsidence level H of the settled coal bed in 250-mL beakers for an initial mix of 20.0 g of coal, 125 mL of paraffin oil, and 0.30 g of Atlas G-271 to which various amounts of water were added.
50
-
A=CONTROL B=ATLAS 6-271
u 0
50
100
150
200
250
TIME (HRS)
Figure 7. Variation of the subsidence volume with time for stabilized (0.45 wt % Atlas G-271 based on total COM) and unstabilized slurry using 250-mL graduated cylinders and a mixture of 23.0 g of coal (20.7 wt % ) and 100 mL of heavy white paraffin oil at 22 "C.
water increased the floc size proportionally. In addition to the SV experiments, the effect of water on the SR for an unstabilized COM was determined. In these experiments 30 wt % dried coal in no. 6 fuel oil with different amounts of water (3.5 and 10 wt %) were mixed in a laboratory blender. In both cases, the SR was greater than 1 (1.13 and 1.19, respectively), indicating a destabilization. In addition to the near-equilibrium measurements of subsidence volume, some measurements of subsidence rate were carried out (Saia, 1978). The more effective additives (based on SR and SV) showed a faster initial rate of settling but the final formation of a more-open equilibrium subsidence volume. A typical plot is shown in Figure 7, where a lower temperature, 22 "C, was used to improve observation. Discussion The sedimentation balance showed that near-equilibrium sedimentation was attained at 50 "C in 18 h and that 24 h was certainly a suitable time to look for the effects of different stabilizers. The results of the 24-h screening of various classes of stabilizers given in Table I may be summarized as follows. The average SR (*the average deviation) for the cationic
-
0 01
00
WT
0 02 ADDITIYt/ W T COAL
0 03
Figure 8. Variation of the sedimentation ratio as a function of the weight ratio of additive/coal. The screeninglevel of 0.25 wt % COM corresponds to a weight ratio of 0.01 for a 25 wt % COM.
surfactants was 0.69 f 0.06, which was clearly lower than 0.86 f 0.07 for the anionics or 0.83 f 0.05 for the nonionics. Despite the individual differences in surfactants, the cationic surfactants as a class were the most effective in retarding the accumulation of a dense bottom sediment. The correlation of sedimentation ratio for coal in no. 6 oil with subsidence volume for coal in paraffin oil, shown in Figure 5, shows that the most effective stabilizers give a low sedimentation ratio and a high subsidence volume. Clearly, the more effective stabilizers form a more open or less densely packed subsidence bed. The pronounced effect of water shown in Figure 6 can be understood by the formation of very loosely packed subsidence bed in the range of the maximum. With larger amounts of water the open network is disrupted and a densely packed bed is obtained. The network collapse is explained by a competition between the coal and water for the available surfactant. Optical microscopic examination of some of the slurries at high water content suggested that the surfactant was drawn into phase-separated water droplets. The subsidence curves shown in Figure 7 are of the general type for concentrated suspensions suggested by Michaels and Bolger (1962) in their study of the subsidence of aqueous Kaolin suspensions. Some insight into the nature of the mechanism of stabilization can be obtained by replotting the data of Figure 2 to emphasize the ratio of surfactant to coal as shown in Figure 8. The form of the relationship resembles a titration curve where a pronounced retardation of sedimentation occurs at a critical concentration of additive. The critical concentration occurred at 0.86 wt % additive (based on the coal) identically for the three most effective cationic surfactants tested and at a level of 1% all three additives had reached the plateau of effectiveness. The chemical formula weights of Ethomeen C-15 (CZ2Hd7NO5; 405), Ethomeen (2-20 (C36H75N010; 681) and Atlas G-271 (C,H&O,S; 493) are sufficiently similar that approximately the same number of molecules of each stabilizer are available at the critical concentration. This suggests that the mechanism of stabilization may involve surface coverage by molecular adsorption. Ekmann and Bienstock (1978) have measured sedimentation of COM in large columns (3 X 30 in. and 4 X 36 in.) using the transmission of ultrasound and y radiation. Their results are in terms of the percent change in suspended solids at a given height divided by the time of the test. Clear oil sedimentation or subsidence measure-
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ment has been used by Kugel (1978) to examine the uniformity of the subsidence bed using both white light and black light illumination. The distinctive features of our work are: (1) the use of the sedimentation ratio obtained by direct pycnometric measurement, and (2) utilization of bottom samples which highlights the effect of additives and cancels out the intrinsic properties of a particular combination of coal and oil. This accounts for the effectiveness of the sedimentation column as a device for screening additives. The correlation of sedimentation ratio with subsidence demonstrates an equivalent screening method and visually illustrates the mechanism of network stabilization. Cherry and Stokes (1978) have examined the uniformity of sedimenting COM by obtaining a vertical profile of the dielectric constant in a K-scan instrument. Since their technique is especially sensitive to water (because of the high dielectric constant relative to COM), it should be especially useful in examining the effects of water on COM. Other workers (Ekmann and Bienstock, 1978; Kugel, 1978) have regarded aggregation as an instability where we note here and have noted before (Rowell et al., 1978) that aggregation is an important step in network formation. We introduced the concept of matrix-stabilization (Rowell et al., 1978) and have used the phrase network stabilization as a clearer terminology here. We have stressed the concept of network stabilization as an important mechanism for maintaining uniform properties of COM. It should be mentioned, however, that in studies of aqueous flocculated kaolin suspensions, Michaels and Bolger (1962) pointed out that “aggregates may form networks which extend to the walls of the container and give the suspension its plastic and structural properties”. Postscript Added in Revision. This manuscript was prepared prior to the Second International Symposium on Coal-Oil Mixture Combustion which was held at Danvers, Mass.,on Nov 27-29,1979. We should like to call attention to the fourth paper in our series which was presented at Danvers (Rowell et al., 1979) and to the fact that there are a number of other approaches to the study of the stability
of COM reported in Vol. 2 of the 2-volume proceedings of the Danvers meeting. A discussion of the alternative approaches is beyond the scope of this work and both i n a p propriate and unnecessary.
Literature Cited Blake, J. C.; Sabadell, A. J., Ed. “First International Sympoelum on CoacOli Mixture Combustion Proceedings”, The MITRE Corporation: Mdean, Va., 1978. Brown, A., Jr., Ed.; Final Report of the General Motors corporetkn Powdered Coal-Oil Mixtures (COM) Program, Natbnal Tech. Information Service, U. S. Dept. Commerce, Springfield, Va.. 1977. Calbeck, J. H.; Harm, H. R. Ind. Eng. Chem. 1927, 19, 58. Cherry, N. H.; Stokes, C. S. p 233 In Blake and SabadeH (1978). Darnerel. V. R.; Mattson, R. J. phys. Chem. 1944, 48, 134. Ekmann, J. M.;Bienstock, D. p 273 In Blake and Sabadell (1978). Gaudin, A. M.;Fuerstenau, M. C. Eng. Mln. J. 1958, 159, 110. Gaudln. A. M.; Fuerstenau, M. C.; Mitchell, S. R. Mln. €w. 1959, 11, 613. Gaudin, A. M.; Fuerstenau, M. C. Preprlnt, International Mlnlng Roc. Congress, London, April 1960. Kugel, R. W. p. 300 in Blake and Sabadeli(1978). Kynch, G. J. Trans. Faraday Soc. 1952, 48, 168. Michaels, A. S.; Bolger, J. C. Ind. Eng. Chem. Fundem. 1962, 1 , 24. Overbeek, J. Th. 0. ”Colldd Science”, Vd. 1, H. R. Kruyt, Ed.; Elsevier: Amsterdam, 1952; pp 115, 245. Rlchardson, J. F.; Zakl, W. N. Trans. Inst. Chem. €ng. 1954, 32, 35. Rowell, R. L.; Vesconcellos, S. R.; Ford, J. R.; Lindsey, E. E.; Olennon, C. B.; Tsai, S. Y.; Batra, S. K. In “First International Sympodum on CoaCOll Mixture Combustion Procedlngs”, Blake, J. C.; Sabaddl, A. J., Ed.; The MITRE Corporation: M e a n , Va., 1978; pp 288-299. Rowell, R. L.; Marbw. B. J.; Tsal, T. S.; Batra, S. K.; Pitman. J. R. In “2nd International Symposium on CoaCOll Mixture COmbusUon”, 1979, CONF791160 Vol. 2, National Technical Information Servlce, U.S. Dept. of Commerce, Springfield, Va. 22181. Sala, R. J. M.S. Thesis, Unhrerslty of Massachwetts, Amherst, 1978. Slagle, D. J.; Shah, Y. T.; Kllnzlng, G. E.; Waiters, J. 0. Ind. Eng. Chem. Process Des. Dev. 1978, 17. 500. Smelile, R. H.; La Mer, V. K. J. C d b H Sci. 1956, 11, 704. Tory, E. M.; Shannon, P. T. J. Cob& Interlase Sci. 1966, 21. 107. Van der Minne, J. L.; Herrnanle, P. H. J. J . CohH Scl. 1953, 8, 38. Van der Waarden, M. J. J. CoUoH Sci. 1950, 5, 317. Vanconcellos, S. R. M. S. Thesis, Unlverslty of Massachusetts, Amherst, 1977.
Received for review January 28, 1980 Accepted October 28, 1980 Presented at the ACS/CSJ Chemical Congress, Honolulu, April, 1979, Division of Colloid and Surface Chemistry; supported in part by the New England Power Service Company and in part by the Electric Power Research Institute.
