Coal-Oil Mixtures. 3. Stabilization of Powders vs. Colloids - American

Ind. Eng. Chem. Process Des. Dev. 1881, 20, 289-294

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Coal-Oil Mixtures. 3. Stabilization of Powders vs. Colloids: CoaVOil vs. Carbon Black/Oil Systems Robert L. Rowell,’ Stephen R. Vasconcellos, and Rlchard J. Sala Depatfment of Chemistry, Universtty of Massachusetts, Amherst, Massachusetts 0 1003

Avrom I. Yedalla and Bruce S. Yarmoska Cabot Corporation, Billerica, Massachusetts 0 1821

Robert E. Cohen

Downloaded by UNIV OF CAMBRIDGE on September 2, 2015 | http://pubs.acs.org Publication Date: April 1, 1981 | doi: 10.1021/i200013a017

Depatfment of Chemical Engineering, Massachusetts Institute of Technology, CambrMge, Massachusetts 02 139

The concept of colloid stability is explored in a practical study of the effectiveness of additives as stabilizers for mixtures of powdered coal (80% -200 mesh) and no. 6 fuel oil. Further insight is added by the results of a parallel study of the stability of colloidal suspensions of carbon black at high concentratin In mineral oil. Sedimentation, subsidence, consistency, and viscosity are measured and interpreted in terms of practical stability and colloid stabiiity. It is shown that flocculation and network formation are important in powders as well as in colloids. The dependence

of the effectiveness of stabilizers on molecular weight of the additive suggests a mechanism of bridging flocculation and network formation. Certain anionic surfactants stabilize carbon black in mineral oil as shown by the slow sedimentation of dilute suspensions and the low viscosity of pastes, but the same surfactants destabilize coal-oil mixtures (COM) since the particles readily settle to a closely packed bed. Surfactants that confer practical stability on COM by promoting rapid flocculation and network formation increase the viscosity of carbon black pastes.

Introduction A suspension is ideally stable if no volume element which contains a statistically large number of particles undergoes a measurable change in particle concentration and properties within a reasonable time. Unless otherwise stated, a normal gravitational field is generally assumed to be present, and agitation (including thermal convention) is assumed to be absent. The factors influencing stabilization depend greatly on the size, relative density, and concentration of the particles. In a dilute suspension of colloidal particles, settling is prevented by Brownian motion unless the particles first flocculate (or coagulate). Thus, while stabilization may be measured by sedimentation, aggregation (by flocculation or coagulation) is the primary process. Dilute suspensions of powders (>1pm) settle out spontaneously and thus are never “stable”. However, if the medium is of high viscosity or otherwise mechanically supportive, the rate of settling may be negligible giving a practical stability. The IUPAC (Everett, 1971) defines stability in terms of aggregation, but recognizes the absence of settling as a practical form of stability. Ideal stability, as defined above, implies the absence of both aggregation and settling. If the concentration of particles (of whatever size) is raised, a packing limit is reached at the critical pigment volume concentration (CPVC) beyond which an additional phase (gas or vapor-filled voids) is present. At concentrations somewhat below the CPVC, powders can settle out, or they can set up a network or other form of stable packing. This can be promoted by additives, such as polymeric flocculants, which bring about bridging between the particles. Colloidal particles at high concentrations can be prevented from flocculating by the same mechanisms which are effective in dilute suspensions (electrostatic, entropic). 0196-4305/81/1120-0289$01.25/0

However, if flocculation is not prevented, colloidal particles can set up a supporting network a stable gel. Thus, at high concentrations flocculation (whether spontaneous or due to bridging additives) can promote stability of colloidal suspensions and may be the only way of conferring practical stability on a suspension of a powder in a medium of ordinary viscosity. Stabilization of concentrated suspensions of powders, or slurries, is of considerable industrial importance. It might not be practical to raise the concentration to the CPVC since the slurry might be too viscous (and possibly dilatant) to be pumped. In many processes slurries are kept from settling by constant agitation, but this is not always feasible or desirable. Slurries of powdered coal in heavy fuel oil are of current interest because they would provide an alternative way of utilizing an abundant domestic energy source (NAS, 1977; Blake and Sabadell, 1978). These slurries are typically fairly concentrated (volume fraction 0.20) and the oil is so viscous that it must be heated to be pumpable; thus the slurries are normally stored at about 50 “C. Part of the work described in this paper consists of a search for additives which would stabilize coal-oil slurries, Le., prevent settling. After the discovery of certain effective additives, the work was extended to slurries of coal in refined mineral oil, in order to gain more insight into the mechanism. Further insight was provided by the results of a parallel study of the stability of colloidal suspensions of carbon black in mineral oil. Experimental Section Materials. The finely powdered (95% -200, 100% -170) bituminous coal was the same as that used in previous work (Rowell et al., 1981). Samples of no. 6 fuel oil were obtained from the New England Power Service Co., Salem Harbor, Mass., as in previous work (Rowell et al., 0 1981 American Chemical Society


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Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981

1981). Densities of the oil at 50 "C were measured with a standard pycnometer and ranged from 0.91 to 0.94 g/mL. A transparent heavy mineral oil (HMO), USP grade, was used as obtained from the Fisher Scientific Co. The carbon blacks were products of Cabot Corporation. Most work was done with Vulcan J, a rubber-reinforcing furnace black (ASTM grade N375). Some experiments were also carried out with Black Pearls L, an oxidized black used in inks. The important properties of these blacks are as follows: surface area (NJ, 90.9 and 138.1 m2/g; dibutyl phthalate absorption (DBPA), 118 and 58 cm3/100 g; volatile content (weight loss on heating to 950 "C), 2% and 5 % , respectively. A large number of surfactants were tested including anionic, cationic and nonionic types. Procedure. The static stability of the coal-fuel oil slurries was measured in a sedimentation column (Rowell et al., 1981) giving the sedimentation ratio from a bottom sample after 24 h sedimentation, i.e., the ratio of the per cent coal in a stabilized slurry to that of an unstabilized slurry. After sedimentation for 24 h, the time required to drain the column was measured, and divided by the time required for an unstabilized mixture, to give the drain time ratio. With slurries of coal in mineral oil, sedimentation was carried out in 250-mL graduated cylinders in a thermostated water bath. The coal-oil boundary was sharp and was easily observed due to the lack of color of the mineral oil. This is the technique ordinarily used for measurement of subsidence (Smellie and LaMer, 1956) as discussed below. After sedimentation in the graduated cylinders, viscosity measurements were made with a "consistometer". The device included a small metal cone which was pulled through the slurry bed by a wire and pulley arrangement and a fixed weight so that the viscosity was expressed in terms of the time required for a constant force to pull a fixed weight a short distance through the slurry bed. In practice, after the equilibrium subsidence height was measured, the top layer of oil was removed with a pipet. The consistometer cone was then allowed to settle slowly into the settled bed of coal until it just touched the bottom of the graduated cylinder. The counterweight was then added to the other end and the time required for the weight to pull the cone up 2 in. through the packed coal was measured. Carbon black pastes were prepared on a 3-roll mill at 43 pho (parts by weight of carbon black per 100 parts of oil) and then let down on the mill to the desired concentration (Graziano et al., 1979). A relatively large quantity (1200 g) of paste was prepared in this way; 4-g aliquots were then taken and mixed with the desired amount of stabilizer by means of a spatula on a glass plate, immediately prior to viscosity measurement. (In separate experiments with one additive (Neutral Ca Petrosul) it was established that mixing with a spatula gave the same reduction in viscosity as incorporation of the stabilizer on the 3-roll mill.) A control paste was prepared by spatula-mixing with the hydrocarbon instead of a stabilizer (thus, in effect, reducing the concentration of carbon black). Viscosity measurements were carried out with a Laray viscometer (Lhomargy, Draveil, S. & O., France; available from Testing Machines Inc., Mineola, Long Island, N.Y.). This is a Pochettino type of viscometer in which a steel rod of 1.200 cm diameter falls through an orifice of 1.205 cm diameter and 2.1 cm length. The paste is placed at the entrance to the orifice so that the rod drags fresh paste

0.5

7

300

400

500

600

700

MOLECULAR WEIGHT

Figure 1. Sedimentation ratio as a function of molecular weight of cationic surfactant. The numbers on the points refer to the surfactants reported in Figure 5 of the preceding paper in this series.

into the annulus as it falls. In general, the rod acquired a smooth coating of paste, indicating good wetting; occasional measurements (e.g., at too high shear rate) where this was not found were discarded. The time of fall was measured between two photoelectric cells, 10 cm apart; the average of three or four runs was used in the calculations. Generally the runs agreed to within i5%, except at very short times of fall, where the agreement was poorer. The Laray viscometer may be regarded as nearly equivalent to a classical parallel plate viscometer. The exact theory has been given (Van Wazer et al., 1963) and was used to calculate the viscosity; the values agreed closely with calibration curves supplied by the manufacturer. With the indicated dimensions the theory leads to the following equations: 7 = 31.31 W shear stress at wall (Pa): where W is the total weight of the rod plus applied weights (grams); shear rate at wall (s-l): i/ = 7979/t where t is the time of fall (9); apparent viscosity (Pa-s):

17 =

7/+

Results A. Coal in Fuel Oil. The general screening of surfactants to select effective stabilizers using the sedimentation ratio has been reported in the previous paper (Rowell et d.,1981). It was shown that the most effective class of stabilizers was the cationic surfactants (Vasconcellos, 1977). Among the cationic surfactants, there is a general trend toward increasing effectiveness (as measured by decreasing sedimentation ratio) with increasing molecular weight as shown in Figure 1. B. Coal in Mineral Oil. Subsidence Behavior. Subsidence studies of the near-equilibrium sedimentation of coal-mineral oil slurries at 50 "C have been reported in the previous paper (Rowell et al., 1981)and a correlation of low sedimentation ratio with high subsidence volume has been demonstrated (Saia, 1978). The earlier work (Rowell et al., 1981) also showed that effective stabilizers gave rapid initial subsidence and led to a larger final settled volume. The rapid subsidence is explained by rapid flocculation giving coal particles with a larger effective size (equivalent Stokes diameter) resulting in faster settling. However, with flocculation, network formation also occurs and this results in a higher final settled volume or macroscopic stability characteristic of an effective surfactant.

Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 291

Table I. Fluidity of S h i e s of Coal in Heavy Mineral Oil and in Fuel Oil, after Sedimentation at 50 "C

surfactant a Atlas G-271 Ethomeen c/20 Ethomeen C/15 Merpol SH Merpol HC Triton X-400 CTAB

A = MERPOL SH B = ETHOMEEN C-20

I

I

I

I

I

1

0

50

100

150

200

250

TIME

1

(HRS)


Figure 2. Smellie-LaMer p1ot:dependence of t / ( h - h,) on t for representative surfactants.

The type of settling observed in these concentrated slurries, with or without surfactant, has been described as subsidence (Smellie and LaMer, 1956). In this process there is a uniform compression and rearrangement of the network of particles, under the influence of gravity. Smellie and LaMer (1956) have shown, both empirically and theoretically, that if the network is always of uniform density from top to bottom, subsidence follows an equation of the form 1 t --t (1) ho - h U O (Ho- hf + C)

--+

where hois the initial height of the column of a slurry, h is the height of the packed bed after time t , uo is the initial velocity of subsidence, and c represents the fluid remaining in the network when h has reached its final value hf. We have adopted the Smellie-LaMer model as a first approximation and show the dependence of t / ( h- ho)on t for three representative surfactants in Figure 2. In the case of Atlas G-271, a straight line is obtained, indicating that the network of flocs is complete at the earliest time of observation, and further settling is by subsidence. With the control, and also the Merpol SH slurry, the curve has a negative slope at first. This initial failure to obey the Smellin-LaMer relation indicates that another mechanism of settling is operating instead of, or in addition to, subsidence. Smellie and LaMer interpreted such negative slopes as due to slow initial flocculation. Presumably the particles are undergoing hindered settling while also flocculating and forming a network, but the particles or flocs have not formed a completely space-filling network until the straight line portion of Figure 2 is reached. With Ethomeen C-20, intermediate behavior is found. The nonionic surfactant, Merpol SH, showed little effectiveness and a similar curve is obtained for coal-mineral oil systems with no added stabilizer. Consistometer Measurements. Further insight comes from measurement of the consistency of the packed bed after sedimentation at 50 "C in the screening experiments as shown in Table I. The surfactants which gave the shortest drain time and lowest sedimentation ratio (in fuel oil) also gave the softest packed beds (in mineral oil), as shown by the shortest consistometer times. This confirms that with the most effective surfactants the packed bed is a loose network of flocs, whereas with the least effective surfactants a compact bed of particles if formed. Thus, in the settling experiments, the Atlas G-271 slurry must

sedimentation column drain time ratiob

consistometer time, s c

0.44

3.8

0.48

6.0

0.50

6.5

0.60 0.64 0.69

11.2 12.9 14.2

0.70

22.7

a Chemical nature of surfactants is summarized elsewhere (Rowell, 1981). Fuel oil. Heavy mineral oil.

Table 11. Sedimentation of Coal-Mineral with Carbon Black Stabilizers concn of bottom sedimenlayer after tation surfactant 10 h, wt % ratio Neutral Ca 74.8% 1.41 Petrosul Ca naphthenate 74.0% 1.40 Atlas G-271 48.0% 0.906 Control (no 53.0% 1 stabilizer)

Oil Slurries

flow time, min

drain time ratio

10.5

1.4

10.5 0.5 7.5

1.4 0.067 1

flocculate rapidly, and the network must be built from the flocs, while with the control or Merpol SH, sedimentation is relatively more rapid than flocculation. Destabilization by Anionic Surfactants. Experiments with carbon black showed that two anionic surfactants acted as stabilizers for carbon black-oil suspensions. These surfactants, Neutral Calcium Petrosul and calcium naphthenate, were evaluated at 0.25% concentration with 20% slurries of C-8oven-dried coal in heavy mineral oil. After 24 h at 50 "C, difficulty was experienced in taking reproducible samples. Therefore, tests were run using the sedimentation columns at 50 "C over a period of 10 h, which previous work had shown to be sufficient for measurable changes to occur. The results (Table 11) indicate that both anionic additives appear to destabilize coal-mineral oil slurries, since weight percent values were obtained that are greater than the unstabilized slurry. Visually, with both anionic surfactants, no subsidence bed was observed, even when a similar test was run for a period of 48 h, whereas such beds were observed for both the Atlas G-271 stabilized and unstabilized slurries. The lack of a clear supernatant fluid and development of a subsidence interface indicated the presence of suspended coal, material that would have been incorporated in the flocs and network developed by effective stabilizers. These results indicate that practical stabilization of coal-oil slurries is hindered by surfactants which retard flocculation, just as the previous results showed that stabilization of the slurries is enhanced by surfactants which promote flocculation. C. Carbon Black i n Mineral oil. Viscosity of Pastes. Carbon blacks are of colloidal dimensions. In the grades used in this work, the independent units are principally of about 0.02-0.2 pm in size, each typically composed of from 10 to several hunderd particles about 10-40 nm in diameter fused together. When dispersed under high shear in a noninteracting liquid such as water (Me-

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-

control Paste+Mineral 011I5 5 % of Carbon Black) PasteiNeutral Ca Petrosul 15 5'" o f Carbon Black1 -179g,

;:

$. 2 2 9 8

P : $ 100 20 50 I

4


l01O

I 20

T1T

5604 Pal

- 7170 Pal

;:;::\ I

I

I

50

100

200

Table 111. Apparent Viscosity of Pastes with Various Additives. Stock Paste: 38 pho of Vulcan J (N375) in Heavy Mineral Oil (HMO). All Viscosity Measurements at 25 * 1"C, with 100 g Weight Added to Rod (229 g Total Weight, or 7 = 7170 Pa) concn of additive, type0

carbon black

apparent viscosity, Pa+s

A A A A A A A A A A C C C C P

5.70 6.09 6.26 5.65 6.14 2.24 5.60 2.49 5.94 2.28 6.10 5.13 6.15 5.64 1.56 5.91 5.57 5.95 6.10 6.26 6.06

149.2 92.7 94.5 99.0 61.0 7.9 45.9 16.7 100.6 7.1 100.1 117.6 157.4 175.1 175.5 155.3 181.8 96.1 96.6 401.7 23.4 112.5

%of

additive none HMO HMO mineral spirits Zn naphthenate 8% Ca naphthenate 5% Ca naphthenate 5% Neutral Ba Petrosul Neutral Ba Petrosul Neutral Ca Petrosul Neutral Ca Petrosul Witconate 605A Wayfos M-60 Emphos PS-21A Ethomeen C/15 Ethomeen C/15 Ethomeen C/20 Witcamine PA-78B FOA-2 Gilsonite Pioneer 24 Zn stearate

IX

500

a

A

A = anionic; C = cationic; P = polymeric,

the former is 5 % calcium by weight and contains approximately 50% of calcium naphthenate in mineral spirits, an the latter, 40% in light mineral oil. The amounts referred to in Table I11 are based on the material as supplied; i.e., a nominal concentration of 6.14% of calcium naphthenate only contains 3.07% of the active ingredient and 3.07% of mineral spirits. Calcium naphthenate is used as a dispersing agent for carbon black and other pigments in coating formulations. The Petrosuls are salts of a narrow molecular weight range petroleum sulfonate, average molecular weight 1000-1040. Both sulfonates are used commercially as additives in fuel oil and greases. Pioneer 24 is a "bituminous compound of petroleum origin" and is used for wetting and viscosity control in carbon black inks. Other surfactants actually increased the viscosity considerably, especially the Ethomeens. These are the same surfactants which, as shown above, act as stabilizers for concentrated suspensions of powdered coal is fuel oil and mineral oil. Emphos PS-21A also gave considerable thickening. Atlas G-271 could not be used because it was not soluble in oil at these concentrations. The apparent thickening effect of Gilsonite and zinc stearate is an artifact attributable to incomplete dissolution of these materials in the paste. Viscosity Results with Oxidized Carbon Black. The effect of carbon black surface chemistry was explored briefly by using an HMO paste prepared with a surfaceoxidized black (Black Pearls L). As shown in Table IV, Neutral Ca Petrosul caused tremendous viscosity reduction, just as with Vulcan J. Ethomeen C/20 thickened the paste and so a heavier weight was used on the Laray rod, resulting in a higher shear rate than the control; thus, the apparent s m d increase in viscosity is actually comparable with the larger increase in viscosity reported for Vulcan J (Table 111). Duomeen TDO, which is used as a dispersant in paint, reduced the viscosity of the Black Pearls L paste.

Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 203

Table IV. Effect of Dispersant on Viscosity of Pastes with Black Pearls L (43pho in HMO)

additive control (5.13%

shear stress, kPa

shear rate, s-I

7.17

63.8

apparent viscosity, Pes

112.4

HMO)


Neutral Ca Petrosul (5.06%) Ethomeen C/20 (5.46%) Duomeen TDO (5.18%)

7.17

3325

13.4

2.2

112.1

7.17

120

499

14.4

\

i Distance of Bound: from Menircur m n

\

Ethomeen C/20’

I

0

I

2

I

I

6 Time (minutesl

4

I 8

I

10

Figure 4. Sedimentation of carbon black in hexane. Pastes diluted to 1.68g/lW cm3. Pastes contained 5.5% additive based on content of carbon black e, Ca naphthenate; A, Neutral Ca Petrosul; 0,ut control (mineral oil); Sr, Ethomeen C/20.

Sedimentation of Diluted Pastes. In order to confirm that the effects of surfactants on paste viscosity were related to stabilization or flocculation, a simple test-tube experiment was carried out on diluted suspensions of paste (37.5 pho of Vulcan J. in HMO). A l-g aliquot of paste was diluted with 15 cm3 of reagent grade hexane, giving a carbon black concentration of 1.68 g/cm3. The suspensions were shaken 100 times in capped test tubes and then allowed to sediment. The height of the sharp upper boundary was noted after time intervals (Figure 4). Surfactants which thinned the paste gave slow sedimentation, indicating colloidal stabilization (Le., reduction of rate of aggregation or flocculation), while the thickening surfactant promoted sedimentation, indicating increased rate of flocculation. Mechanism of Stabilization by Metal Soaps. The mechanism by which metal soaps stabilize carbon particles in hydrocarbons (such as lubricating oils) is not well established. Careful microelectrophoresis has shown (van der Minne and Hermanie, 1952, 1953) that carbon black particles suspended in benzene acquire a positive charge on addition of calcium diisopropyl salicylate. It has been suggested (Fowkes et al., 1965) that this is due to the formation of invert micelles in the liquid which abstract an electron from the carbon particles. Proton transfer may also play a role (Tamaribuchi and Smith, 1966). To test this hypothesis, some electrodeposition experiments were carried out, using carbon black (Vulcan J) pastes diluted in reagent grade hexane to give a concentration of 0.1 % carbon black. Electrodes of about 4 cm2 area were dipped into 100 mL of sol in open beakers; the electrodes were kept 1cm apart and a potential of 100 V was applied. In the absence of surfactant, and also in the presence of either Ethomeen C/15 or even calcium na-

phthenate at various concentrations, flocculation was sufficiently rapid to prevent appreciable deposition of carbon black on either electrode. With Neutral Calcium Petrosul (added to the paste, 5 % based on the carbon black), there was a significant accumulation of carbon black on the positive electrode in 30 min. This was confirmed in several experiments. The origin of the negative charge is not clear but may be due to simple adsorption of the metal soap on the carbon black, just as would be expected in aqueous systems. Note that no precatutions were taken to exclude moisture in these experiments. Discussion This work has brought out the inherent difference between practical stability (absence of settling) and colloidal stability (absence of aggregation or flocculation). In concentrated slurries and pastes, the one is the converse of the other since the mechanism of practical stabiliztion actually required flocculation as an initial stage of network formation. Evidently the same interaction between particles which leads to flocculation also leads to interaction between flocs to form a network. In the absence of such interaction, powders settle to a dense sludge which is nearly close-packed and is thus difficult to pump. It is interesting that even with particles of much larger than colloidal dimensions, such as powdered coal, flocculation can play a key role. The mechanism by which certain Surfactants enhance flocculation and thus improve practical stability is not established in molecular detail. It is clear that effective stabilizers gave a network that is open and can flow readily. The fact that surfactants of some degree of effectiveness are found among anionic, cationic, and even nonionic classes appears to rule out simple electrical (charge neutralization) mechanisms as essential for stabilization. The dependence of practical stability on the molecular weight of the surfactant (Figure 1)suggests that bridging is involved. In the past, bridging has generally been associated with high-polymer flocculants rather than with surfactants. It is conceivable that surfactant micelles or similar structural elements might act as bridges rather than individual molecules. In the presence of water, a surfactant-water mixture may link the particles by capillary action. Concentrated suspensions (pastes) of colloidal particles, such as carbon black, do not settle to a sludge, even in the absence of bridging. This is because the high surface area and small Stokes diameter promote extremely rapid flocculation and network formation. This is undoubtedly assisted by the fact that the carbon black exists ab initio as fused aggregates which resemble floca in their shape and bulkiness. Addition of a surfactant which confers colloidal stability does not lead to sludging, but simply to a reduction in viscosity, as the inter-aggregate bonds are weakened or reduced in number but are not entirely destroyed. Acknowledgment Two of the authors (S.R.V. and R.J.S.) acknowledge the financial support of the New England Power Service Company and the Electric Power Research Institute. The authors are grateful to their respective institutions for support and permission to publish. Literature Cited Blake, J. C.; Sabadell, A. J., Ed. “Proceedings of the First Internatlonal S y m posium on Coal-Oil Mlxture Combustion". The MITRE Corp.: Mdean. Va. 22101, 1978. Everett, D. H., Preparer “IUPAC Division of Physical Chemistry Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix, Definitions, Terminology and Symbols In ColloM and Surface Chemlstry”, Part I , Butterworths: London, 1971.

Ind. Eng. Chem. Process Des. Dev. 1981, 20, 294-298

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Fowkes, F. M.; Anderson. F. W.; Moore, R. J. Preprlnts, 150th National Meeting of the American Chemical Society. Atlantic City, N.J., Sept 1965. Grazlano. F. R.; Cohen, R. E.; Medalla. A. I. Rheol. Acta. 1979, 18, 640. Medalia, A. I.; Hagopian, E. Rheol. Acta. 1983, 3 , 100. Medalla, A. I. J. Powder Bulk Sollds Technol. 1978, 2 , 19. National Academy of Science “Coal as an Energy Resource”, Washington, D.C.. 1977. Rowell, R. L.; Vasconcellos, S. R.; Saia, R. J.; Farinato, R. S. Id. Eng. Chem. Process Des. Dev. 1981, 20, precedlng article in this issue. Sala. R. J. M.S. Thesis. University of Massachusetts, Amherst. Mass., 1978. Smellie, R. H., Jr.; LaMer, V. K. J. Collold Scl. 1958, 1 7 , 720. Tamerlbuchi, K.; Smith, M. L. J . Colbld Interface Sci. 1988. 22, 404. van der Minne, J. L.; Hermanie, P. H. J. J . Colloid Sci. 1952, 7, 600. van der Minne, J. L.; Hermanie, P. H. J. J . Colloid Sci. 1953, 8, 38.

Van Wazer, J. R.; Lyons. J. W.; Kkn, K. Y.; CdweH, R. E. “Vlscoaity and Flow Measurement”. interscience: New York, 1963; p 287. Vasconcellos, S. R. M.S. Thesis, Unhwsity of Massachusetts, Amherst, Mass.. 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 Electric Power Research Institute and the New England Power Service Company.

Stoichiometric Analysis of Synthetic Fuels and Chemicals from Coal James Wei


Depaflment of Chemical Engineerhg, Massachusetts Institute of Technobgy, CambMge, Massachusetts 02 139

The production of synthetic fuels and chemicals from coal involves many chemical species and numerous reactions. The stoichiometry of mass and energy balance places many constraints on what is possible. The basic principles of analysis by vectorial representation and geometric interpretation are given here. An illustration is given on a comparison of the overall efficiencies of a number of processes to produce 1-octene.

Introduction Coal is being proposed as the feedstock for a wide variety of synthetic fuels and chemicals-all of them with higher H/C ratios than the fixed carbon or the volatile matter component of coal. To produce these clean and useful hydrogen-rich products, it is necessary either to add hydrogen or to reject carbon. Since many of the product formation reactions are endothermic, it is necessary to burn some of the carbon to produce needed heat. The processes proposed include mild solvent refining, direct liquefaction, gasification followed by further gas processing, or indirect liquefaction (Hottel and Howard 1971, Fumich 1979). The starting ingredients are mostly carbon, oxygen, and water; the reactions mainly involve only the three elements C, 0, and H. The number of possible chemical species and reactions are numerous. The stoichiometric constraints of mass and energy balances can be analyzed in a triangular diagram of the three elements, each chemical species represented by a point and each reaction represented as straight lines connecting the points as reacting species. Vectorial Representation of Compounds and Reactions Each compound of C and 0 can be represented in a formula C,O,, or as a vector in two-dimensional space (m,n). Figure 1 gives the geometric representations of compounds C, CO, COz, and O2 in the coordinate axes of C and 0. One mole of COzis the point (1,2) and two moles of COz is the point ( 2 , 4 ) . The combustion reaction C + O2 = C 0 2is represented by the vector sum (1,O) + (0, 2) = (1, 2). The geometric interpretation is more convenient with normalized vectors, where the sum of the components is one. The species C02is represented thus as c&/3, which is represented in Figure 1 by the point P. The normallzed vector space (NVS) is the set of vectors where each component is nonnegative (zero or positive) and the sum of the components is 1. In Figure 1, the line between the points (1,O) and (0, 1) is the W S . The point P i s obtained 0196-4305/81/1120-0294$01.25/0

by central projection of the point (1, 2) to intersect with the NVS. Similarly, the projection of O2 is point Q, and the projection of point CO is point S ; since C is already on the NVS, the projection point R is identical with C. When all chemical species and reactions are projected onto the normalized vector space, the combustion reaction C + O2 = C02 becomes R 2Q = 3P. Since P consists of R and Q it must lie between them but closer to Q due to its larger contribution. The reaction is represented in Figure 1by a straight line connecting the points Q, P, and R; the distance between Q and P is ’Iz the distance between P and R. In the same manner, the gasification reaction C 0 2 + C = 2CO becomes 3P + R = 4s; the distance between P and S is of the distance between S and

+

R. For a three-component system of C-O-H, each chemical species can be represented by a vedor in three-dimensional space, or more conveniently projected on the normalized vector space of the 111 plane. The triangular diagram in Figure 2 is this normalized vector space, where each point represents three numbers: (fraction of C, fraction of 0, fraction of H). The compound CO is represented by the point (0.50,0.50,0), COPby the point (0.333,0.667,0),CzHz or C6H6by the point (0.50, 0, 0.50), and CH30H by the point (0.167, 0.167, 0.667). The methanol synthesis reaction 2Hz + CO = CH30H is represented by 4(0,0, 1) + 2(0.5,0.5,0) = 6(0.167,0.167, 0.667). To represent this reaction geometrically in Figure 3, we connect the points CO, CH30H,and Hz by a straight line; the distance between CH30H and Hz is 2 / 4 the distance between CO and CH30H. The oxo reaction C2H4 + CO + Hz = CzHsCHOis represented by 6(0.333,0,0.667) + 2(0.5, 0.5,O) + 2(0,0,1) = 10(0.3,0.1,0.6). Geometrically, since CzH5CH0is the positive sum of CzH4,CO, and Hz, it must lie within the triangle bounded by these three points in Figure 3. The proprionic aldehyde point is constructed by first joining CO and Hz to obtain the mid-way point P, and then joining P and CHz-the distance between CzH5CH0 and P being the distance 0 1981 American

Chemical Society

Coal-Oil Mixtures. 3. Stabilization of Powders vs. Colloids - American

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