Energy & Fuels 1998, 12, 1313-1321
1313
Measurements of the Vapor Pressures of Coal Tars Using the Nonisothermal Knudsen Effusion Method Vahur Oja† and Eric M. Suuberg* Division of Engineering, Brown University, Providence, Rhode Island 02912 Received April 23, 1998. Revised Manuscript Received August 19, 1998
The vapor pressures of primary coal tars have been determined using a modified Knudsen effusion method. Vapor pressure data are correlated with the molecular weight and temperature for tars from three Argonne coal samples (Pittsburgh No. 8, Upper Freeport, and Wyodak). These results show significant differences in the vapor pressures of similar molecular-weight tars from different coals. The results likewise warn that the commonly used estimation techniques for vapor pressures of the coal tars oversimplify the actual behavior but at the same time provide reasonable estimates for some modeling applications.
Introduction Vapor pressures of high molecular weight thermal decomposition products of coals (i.e., tars) are an important parameter in modeling the pyrolysis behavior of the coals. The extent to which the tars can vaporize, before retrograde reactions reincorporate them into a char, plays a key role in determining the volatiles yield. This is reflected in pyrolysis models.1-8 There has been some debate in the literature regarding what values to use for the vapor pressures of coal tars, since there have been no actual measurements of this property. What data have been available have come from hydrogenated coal liquids9,10 or from pure model compounds, and uncertainties of an order of magnitude have been reported.5,11 In this study, the vapor pressures of primary coal tars have been studied using a modification of the Knudsen effusion method, which has been widely used for measuring vapor pressures of low-volatility materials.12 In addition to the requirement that the technique be suitable for low vapor pressures, it is necessary to use † Present address: Institute of Chemical Reaction Science, Tohoku University, Sendai, Japan. (1) Suuberg, E. M. In Chemistry of Coal Conversion; Schlosberg, R., Ed.; Plenum: New York, 1985. (2) Unger P. E.; Suuberg, E. M. 18th Symposium (International) on Combustion, Combustion Institute: Pittsburgh, PA, 1981; p 1203. (3) Niksa, S. AIChe J. 1988, 34, 790. (4) Niksa, S.; Kerstein, A. Energy Fuels 1991, 5, 647. (5) Fletcher, T.; Kerstein, A.; Pugmire, R.; Solum, M.; Grant, D. Energy Fuels 1992, 6, 414. (6) Solomon, P. R.; Serio, M. A.; Suuberg, E. M. Prog. Energy Comb. Sci. 1992, 18, 133. (7) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. Energy Fuels 1988, 2, 405. (8) Oh, M. S.; Peters, W. A.; Howard, J. B. AIChE J. 1989, 35, 776. (9) Tsonopoulos, C.; Heidman, J.; Hwang, S. C. Thermodynamic and Transport Properties of Coal Liquids; Wiley: New York, 1986. (10) Gray, J. A.; Brady, C. J.; Cunningham, J. R.; Freeman, J. R.; Wilson, G. M. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 410. Gray, J. A.; Holder, G. D.; Brady, C. J.; Cunningham, J. R.; Freeman, J. R.; Wilson, G. M. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 97. (11) Oja, V.; Suuberg, E. M. Prepr. Pap.sAm. Chem. Soc., 1996, 41, 82. (12) Dushman, S. Scientific Foundations of Vacuum Technique 2nd ed.; Wiley: New York, 1962.
a continuous nonisothermal method since our interest centers on the vapor pressures of complicated mixtures, such as coal tar, for which there is no unique vapor pressure for any given temperature. The vapor pressure is a function of composition as well as temperature. This latter problem is well-known and has led to the adoption of empirical nonisothermal batch distillation methods such as ASTM D86-96, D447-93, D1078-95, and D85093, which provide boiling-point curves as a function of mass distilled. These have been supplemented by the now widely practiced “simulated distillation” alternatives such as D2887-93e1 or D5399-95 and other techniques.13 All of the traditional distillation techniques suffer the drawback of being atmospheric or mild-vacuum techniques, therefore, requiring quite high temperatures to be reached when dealing with fairly nonvolatile materials such as high molecular weight tars. It is quite likely that the high temperatures involved when applying these distillation techniques to low-volatility materials lead to thermal degradation of the materials of interest. For this reason, it is typically recommended in using the “true boiling point” (TBP) technique that temperatures in excess of about 260 °C be avoided. If degradation does occur, the boiling-point distribution curve which is obtained does not accurately reflect vaporliquid equilibrium, particularly when significant gas evolution occurs during reaction. The simulated distillation techniques can generally be used at lower temperatures but suffer from questions regarding calibration materials. In addition, even simulated distillations may be carried to temperatures at which degradation may be of concern. The vapor pressure results obtained by one of the traditional distillation techniques are difficult to apply in coal pyrolysis models. Unlike applications such as crude-oil refining, in which there is a particular mixture (13) (a) Huang, H.; Wang, K.; Wang, S.; Klein, M.; Calkins, W. H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41, 87. (b) Padlo, D. M.; Kugler, E. L. Energy Fuels 1996, 10, 1031. (c) Mondragon, F.; Ouchi, K. Fuel 1984, 63, 61.
10.1021/ef980093u CCC: $15.00 © 1998 American Chemical Society Published on Web 09/25/1998
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which must be distilled, in coal pyrolysis the mixture to be “distilled” is dynamically defined. A “boiling point curve”, if it could be obtained for coal tars, would be time varying. Evaporable components are formed, vaporized, or further reacted all at the same time during pyrolysis. There is, thus, a need to link the statistical picture of how the coal macromolecular structure breaks down into fragments to the vaporization behavior of those fragments. Generally, little information beyond the molecular-weight distribution of the fragments is readily available in the pyrolysis models, which is why these models have been developed using the molecular weight as the key correlative parameter for determining tar vapor pressure.1,2 Few reliable data have been available to guide the development of such correlations, and early attempts involved use of sparse data on vapor pressures of aromatic compounds, with or without alkyl side chains.2 Even thought some more recent correlations were developed based upon data on coal liquefaction products,5 the nature of the basic correlations did not change. They are still all based upon the molecular weight of the decomposition fragments. In recognition of the need to develop greater confidence in the vapor pressure correlations which are now part of all advanced coal pyrolysis models, the objective of this study was to examine the true vapor pressures of coal pyrolysis tars while at the same time characterizing them with respect to molecular weight. Again, the study also needed to be performed under conditions at which the tars would not thermally degrade and change the molecular weight. This led to adoption of the effusion technique. In this first paper on the topic, attention will be focused on the behavior of whole coal tars. Later papers will describe the results obtained on fractionated tars.
Oja and Suuberg
Coal Samples. The coals from which the tars for this study were derived were provided by the Argonne Premium Coal Sample Program. The three coals of interest here are the Wyodak, Pittsburgh No. 8, and Upper Freeport coals. These coals will not be described further here, since detailed characterizations have been published elsewhere.14 Tar Preparation. The coal tars were produced using rapid pyrolysis techniques described below and collected by washing the reactor with inhibitor-free THF. To prepare dry tars for testing, solvent was evaporated from the collected tar solutions in a vacuum oven at 45 °C. It is known that vacuum drying will result in the loss of some light material, especially that with a volatility greater than that of anthracene. Thus, the starting tars can be defined, operationally, as a collection of room-temperature condensables with a molecular weight generally greater than 150 daltons (corresponding to anthracene). As the project emphasized the study of primary tars (i.e., tars that have not been subject to extensive secondary reactions), the tar preparation systems have been selected to minimize the residence times of evolved volatiles in hot regions of the reactors and to avoid contact with oxidizing gases. Two reactors were used to achieve such conditions. The wire mesh reactor (or heated grid reactor) is a rapid pyrolysis apparatus which has been described in detail elsewhere.15 In this device, a small sample of coal is enclosed in the folds of a wire mesh,
which is electrically heated as the resistance element of a highcurrent circuit. Using this device, a pulverized coal sample was typically heated at a rate of roughly 3000 °C/min to a temperature of 700 °C, at which it was held for 3 s before cooling. Because production rates of tar were limited when using this system, a higher production-rate system was also employed. Using a tubular reactor, it was possible to pyrolyze larger masses of coal than in the heated wire mesh reactorsseveral grams as compared to less than 100 mg per experiment. In the case of the tubular reactor preparation experiments, the coal was first placed into a wire mesh holder, which was inserted into the cold end of an inert-gas-purged tube. The sample was then quickly pushed into the opposite end of the tube, which had been preheated to the desired temperature in a tube furnace. This procedure was used to provide a moderately high heating rate for the coal and to minimize heat soaking at low temperatures. The desire was to maximize the tar yield and avoid secondary reactions. Using this procedure, the tar evolved in a quick burst, which could be rapidly purged from the tube. The volatiles were swept away by the inert purge gas, and tar was collected on the cooled walls of a small tube, downstream of the hot zone. The average tar residence time in the hot zone of the reactor was estimated to be around 5 s. Particle temperatures were measured directly by a chromelalumel thermocouple connected to the wire mesh holder. The tubular reactor wall temperature was kept at approximately 700 °C, leading to an ambient inert-gas temperature of approximately 670-680 °C and a comparable final particle temperature. The average particle heating rate from room temperature to 500 °C was 900 °C/min, and from there the particle temperature reached 670 °C in 2 min. The total reaction time was 3 min, after which the char was pulled out from heated zone. The handling procedures for the tars produced in this system were identical to those used in the case of the heated wire mesh reactor. Tar Characterization. General. In the work reported here, only the behavior of whole tars will be considered. In a future publication, we will discuss the vapor-pressure behavior of distinct fractions derived from these tars. Analyses of the different tars were performed using vaporphase osmometry (VPO) and analytical-mode gel-permeation chromatography (GPC). The latter provides the number average molecular weight of a sample, and the former allows a crude characterization of molecular weight and chemical nature, as will be discussed more fully in a future publication. An elemental analyzer was used to determine the elemental composition of the tars. A Knauer model 11 vapor-phase osmometer was used for the measurement of the number average molecular weights. All measurements were made at 90 °C in pyridine. Calibration was accomplished using pyrene (MW ) 202) as the standard, and the accuracy of the VPO measurement was checked with 3-hydroxy-1H-phenalen-1-one (MW ) 196) and phenanthridine (MW ) 179) to ascertain the minimal influence of heteroatoms on the calibration. General procedures were along the lines discussed by earlier workers.16 Vapor Pressure Measurements. The vapor pressures of coal tars have been measured using a molecular effusion/TGA technique. The various so-called “effusion” methods are based on the molecular effusion of a vapor from a surface or through an orifice.12 The method selected for use here is based on the Knudsen method,17,18 in which the vapor of a substance of interest effuses through a small pinhole of known area in an otherwise sealed container or cell. Our implementation of the technique has been described in detail elsewhere.19,20 Because
(14) Vorres, K. Energy Fuels 1990, 4, 420. (15) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 37.
(16) Chung, K. E.; Anderson, L. L.; Wiser, W. H. Fuel 1979, 58, 847. (17) Knudsen, M. Ann. Phys. 1909, 28, 999. (18) Knudsen, M. Ann. Phys. 1909, 29, 179.
Experimental Section
Vapor Pressures of Coal Tars coal tars are complex mixtures, they cannot be conveniently characterized by the traditional Knudsen effusion method involving a series of measurements in isothermal steps. As noted above, mixtures of components exhibiting a wide range of volatility are typically examined by a continuous distillation procedure. In this case, the Knudsen effusion method was modified to allow for a continuous temperature ramp, as opposed to isothermal operation. In this way, mixtures showing a continuous range of vapor pressures may be characterized much more conveniently than by using a great number of isothermal steps. In any event, an isothermal-step approach could not be implemented because in the time that it would take to reach equilibrium at any temperature, the composition of the mixture would change. Since the details of our technique are availabe elsewhere, only key features of the method are provided. The cells for holding the tar samples were made in-house out of either brass or stainless steel foil of 25 µm thickness, using an appropriate die. They were sealed by crimping a circular top onto a stamped base. The cell was cylindrical and had approximate dimensions of 0.5 cm height by 0.5 cm diameter and a volume of 0.4 cm3. The foil cell had a final assembled mass of about 0.1 g. After fabrication, the cells were outgassed under vacuum for several hours at a temperature of at least 300 °C or cleaned by heating in a propane flame. The effusion holes were made in the top plate of the sample cell using a fine drill. The effusion rate was itself measured in a thermogravimentric analyzer (TGA). The effusion cell was suspended on the arm of a Cahn 2000 recording electrobalance, which has a sensitivity of 0.5 µg, capacity of 1.5 g, and zero-stability better than 10 µg. The backpressure in the TGA system was maintained at 10-7 Torr, which has been determined to be sufficient to provide accuracy in the 10-6 Torr range of vapor pressures.21 We have relied upon high pumping rates and a condenser slightly downstream of the cell to give us the necessary low pressures outside of the cell. A mechanical vacuum pump is used both for pre-evacuating the system and for backing a 4 in. oil-diffusion pump. The pumping system is isolated from the main part of the apparatus by a liquidnitrogen trap. The sample cell itself hangs inside of an aluminum capsule which surrounds, but does not touch, the cell. The capsule is cylindrical and has a diameter of about 1.5 cm, a length of 4 cm, and a mass of 5 g. The capsule is painted black, with hightemperature paint, to increase the effectiveness of radiative transfer. The temperature is measured to 0.1 K accuracy using a calibrated thermocouple in direct contact with the capsule. The cell-within-a-capsule arrangement was found to be necessary in order to improve heat transfer to the cell and to allow accurate measurement of the temperature in the immediate vicinity of the cell. The heating system is entirely outside of the vacuum enclosure and consists of an aluminum block oven surrounded by insulating material. Heat is provided by cartridge heaters embedded in the aluminum. The nonisothermal Knudsen effusion method was developed in response to the need to scan a wide range of temperatures in a modest time. A temperature ramp is imposed by raising the aluminum block temperature at a constant rate, and the capsule responds by rising in temperature at a rate which is in some way related to the rise in block temperature. The important thing is that the capsule need not track the block temperature perfectly, so long as it rises at a nearly constant rate, comparable to the rate for the block. Since heat transfer through the vacuum enclosure occurs mainly by radiation, the decoupling of the furnace and capsule temperature is useful, as it avoids the need for long (19) Oja, V.; Suuberg, E. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 381. (20) Oja, V.; Suuberg, E. M. Anal. Chem. 1997, 69, 4619. (21) Ribero Da Silva, M.; Monte, M. Thermochim. Acta 1990, 171, 169.
Energy & Fuels, Vol. 12, No. 6, 1998 1315
Figure 1. Simulated nonisothermal Knudsen effusion experiment. The “experiment” was numerically simulated at a heating rate of 1 °C/min, assuming a mixture consisting of five pseudocomponents. The molecular weights M of the components and the percentages of each (in parentheses) in the starting mixture are as follows: 220 (10%), 240 (20%), 260 (35%), 280 (25%), and 300 (10%). The mixture was assumed to be ideal and follow Raoult’s law. The individual component vapor pressures were calculated from an expression of the form ln P ) (-∆H/RT + ∆S/R), where ∆H and ∆S were functions of the molecular weight as follows ∆H ) a1 + a2M and ∆S ) a3 + a4M. The constants ai were fit from actual vapor pressure data for anthracene, naphthacene, and pentacene. periods of thermal equilibration. On the other hand, the lowmass sample cell which sits directly inside of the capsule tracks the capsule temperature well. Vapor pressure is calculated from the measured mass-loss rate from the cell. The information on cell mass as a function of time is converted to a time derivative, allowing each recorded sample temperature to be related to an instantaneous value of mass-loss rate, and therefore, vapor pressure.20
Results Interpreting the Results of Nonisothermal Knudsen Measurements. In order to better understand the actual nonisothermal Knudsen effusion data, the results of a simple model calculation are presented first. The behavior of a hypothetical model mixture, consisting of five components with vapor pressures similar to those found in tars, is shown in Figure 1. What is represented is the vapor pressure behavior of an open system (simulating an effusion cell), starting from the lower right-hand corner of the figure. The “experiment” starts as the sample is “heated”, tracing the curve labeled heating on the far right of the figure. The usual sort of dependence of vapor pressure on temperature is observed, consistent with the Clausius-Clapeyron equation
d[ln P]/d[1/T] ) -∆HV/R
(1)
in which P is the vapor pressure, T is temperature, R is the universal gas constant, and ∆HV is the enthalpy of vaporization of the vaporizing material. A simple linear behavior is observed because at the lowest temperatures of the experiment, the vapor-pressure behavior is dominated by that of the most volatile component of the mixture. It is, however, observed that near the end of the first heating cycle, the straight line begins to curve slightly toward the horizontal direction, due to a change in mixture composition associated with
1316 Energy & Fuels, Vol. 12, No. 6, 1998
loss of the most volatile component. This emphasizes that the “measured” vapor pressure at any point is, of course, not a pure-component vapor pressure but is, instead, governed by Raoult’s law for this ideal mixture (i.e., is given by the product of the mole fraction of each component times its respective pure-component vapor pressure). At the end of the first hypothetical heating cycle, a “cooling” cycle is initiated. The temperature of the sample drops with time, following the trajectory toward point 1. A different (lower) vapor-pressure curve is traced during cooling than during the first heating, because of the loss of volatile components from the cell during the initial heating. Upon reaching point 1, the cooling is halted and heating reinitiated. During the second heating cycle, the curvature of the vapor-pressure curve toward horizontal is even more marked. Again, the decrease in vapor pressure, which this implies, is due to loss of volatile components. It is clear that when one cooling cycle is ended and a heating cycle begun, there is a fairly close retracing of the same vapor-pressure curve. This is expected, since at the end of any cooling cycle the sample temperature has dropped so low that the mass-loss rate becomes very low (note the very low vapor pressures at points 1 and 2). The composition cannot change very much at this time because the mass-loss rate is proportional to pressure in the effusion technique.20 Thus, when the next heating cycle begins, the sample composition is almost the same as at the end of the cooling cycle. Therefore, the points 1 and 2 are convenient places at which to obtain the vapor-pressure dependence at a particular composition or molecular weight; the lines bisecting the cooling and heating curves are good approximations of the vapor pressure curves at those compositions. This general approach will be used to establish the vapor pressures of the tar samples. Vapor Pressures of Pittsburgh No. 8 Coal Tar. The Pittsburgh No. 8 coal tars were all found to be very similar in properties, regardless of whether they had been prepared in the wire mesh reactor or in the tubular reactor. The H/C ratio was measured to be 1.09 for the former sample and 1.03 for the latter, which is within experimental uncertainty. The former had an initial number average molecular weight, by VPO, of 311 daltons and the latter 320 daltons, again the same within experimental error. These values are a bit lower but still in reasonable agreement with the reported number average molecular weight of Pittsburgh No. 8 tar produced at 550 °C at near ambient pressure and as measured by field ionization mass spectrometry (FIMS).22 In addition, the vapor-pressure measurements themselves confirmed that there was no significant effect of how the tar was produced. Thus, no distinction will be drawn hereafter between the samples prepared in the wire mesh reactor and the sample produced in the tubular reactor. The results of the vapor-pressure determinations on the whole Pittsburgh coal tars are shown in Figure 2. Note the similarity of the results to the model mixture results of Figure 1. The results of the vapor pressure (22) Solomon, P. R.; Hamblen, D. G. In Chemistry of Coal Conversion; Schlosberg, R. H., Ed.; Plenum: New York, Chapter 5.
Oja and Suuberg
Figure 2. Nonisothermal Knudsen effusion results obtained for a sample of Pittsburgh No. 8 coal tar, showing seven heating and cooling cycles. The data may be interpreted as discussed in connection with Figure 1, and the results are tabulated in Table 1. The curves, corresponding to the different number average molecular weights of Table 1 are as follows: 230, thin solid line, 245, thin dashed line, 265, thin dotted line, 285, heavy solid line, 315, heavy dashed line, 340, heavy dotted line, 360, heavy dashed-dotted line. Table 1. Vapor Pressures of Pittsburgh No. 8 Coal Tar at Different Extents of Evaporationa mass loss (%)
MN
a
b
6 16.5 30 48 60 69 75
230 245 265 285 315 340 360
39.42 40.03 39.75 42.04 41.17 42.54 43.39
17285 18799 19649 22431 23557 25992 27729
a The quantity M is number average molecular weight for the N tar which evaporates at the indicated extent of mass loss. The values of a and b refer to eq 2.
measurements are given in Table 1 for seven different extents of mass loss, assuming a simple ClausiusClapeyron correlation equation of the form
ln P [Torr] ) a - b/T [K]
(2)
where a constant enthalpy of vaporization ∆HV ) bR has been assumed in integrating eq 1. The molecular weights of the tars collected at differing extents of mass loss were determined by VPO in separate experiments. These are the molecular weights reported in Table 1. It should be recalled that the molecular weights provided by VPO are number average molecular weights. The number average molecular weight of a mixture is defined by
∑(mi/Mi)]-1
MN,m ) [
(3)
where mi is the mass fraction of species i in the mixture and Mi is its molecular weight. The molecular weights of individual tar species can go up to 103 daltons and the molecular weight of unevaporated coal extracts considerably higher than that.6 For example, a Pittsburgh No. 8 tar with a number average molecular weight of around 340 daltons has been seen in FIMS to include species with actual molecular weights over 600 daltons.22 The variation of ∆HV with the number average molecular weight of the evaporated fraction is shown in Figure 3, together with a comparison of the values
Vapor Pressures of Coal Tars
Energy & Fuels, Vol. 12, No. 6, 1998 1317
Figure 3. Enthalpy of vaporization of coal tars as a function of the molecular weight of the evaporating fraction. Results are shown for Pittsburgh No. 8 tars (circles and dashed line), Upper Freeport tars (triangles and dotted line), Wyodak tars (squares and solid line). Shown for comparison are the results for the unsubstituted aromatics24 (open diamonds) including anthracene (MW ) 178), naphthacene (MW ) 228), pentacene (MW ) 278), pyrene (MW ) 202), perylene (MW ) 252), and coronene (MW ) 300); alkyl compounds (open triangles), including n-eicosane (MW ) 282),27 n-heptacosane (MW ) 380),28 and 9-butylanthracene (MW ) 228),29 heteroatomic aromatics24 (open squares), including phenanthridine (MW ) 179), 1,2-benzodiphenylene sulfide (MW ) 234), and benz[g]isoquinoline-5,10-dione (MW ) 209); hydroxyl-containing aromatics (open circles), including 1,8-dihydroxyquinone (MW ) 240),30 bisphenol A (MW ) 228),31 hydroxypyrene (MW ) 218),24 6,11-dihydroxy-5,12-naphthacenedione (MW ) 290),24 3-hydroxy-1H-phenalen-1-one (MW ) 196).24
for some pure model compounds (the value for bisphenol A has been calculated from the published correlation). The results for several different experiments with Pittsburgh seam coal tars have been combined in this figure. It is apparent that there is a good, fairly linear correlation of ∆HV with molecular weight. An increase of ∆HV with molecular weight is naturally to be expected. This would imply that the constant b in eq 2 is a function of molecular weight and supports the type of correlation which has often been used for characterizing tars
ln Pi° ) A - BMic/T
(4)
where the constant c ) 1 for a linear increase in ∆HV with Mi. The observed unity value for c was surprising since most of the currently use literature vapor-pressure correlations for coal tars do not assume c ) 1. Typically employed values of c are 0.6,4 0.586,23 and 0.59.5 This point will be explored further below. It is apparent when comparing the results for the model compounds with those for the coal tars that the pure alkyl compounds do not fit the general trend. This is not surprising since coal tars are highly aromatic in character. The remaining classes of compounds generally fit the pattern defined by the coal tars reasonably well on the basis of the enthalpy of vaporization. The parameter A will further help discriminate between good and poor model compounds, as noted below. It needs to be recognized that the vapor-pressure curves defined by the constants in Table 1 are for mixtures of a particular composition. There is a finite amount of any material with any given volatility. In order to begin to cast the results of the present study into a form more like that of eq 3, which applies to a (23) Suuberg, E. M.; Unger, P. E.; Lilly, W. D. Fuel 1985, 64, 956.
Figure 4. Constant A for vapor pressures of different fractions of coal tars, vapor pressures, in Torr. Results are shown for Pittsburgh No. 8 tars (circles and dashed line), Upper Freeport tars (triangles and dotted line), Wyodak tars (squares and solid line). The values of A are also shown for pure compounds as described in the caption for Figure 3.
particular molecular weight, it is necessary to assume a mixing model and a composition. In work which will be published separately, we demonstrate that a behavior approaching Raoult’s-law behavior is appropriate for many, though not all, components of the mixture. Thus, Raoult’s law will be assumed to apply in a version commonly applied in macromolecular systems, in which mole fractions are replaced by volume fractions. If all species are further assumed to have roughly the same density, consistent with their very similar liquid chromatograms and, therefore, structures, then the volume fractions are equivalent to mass fractions. Thus
Ptot )
∑Pi ) ∑miPi°
(5)
where Pi° is the pure-component vapor pressure for i. The pressure Ptot is the total vapor pressure which is experimentally measured for the mixture. Consistent with the discussion of Figure 1, it is assumed that at any particular measurement point, the vapor pressure is dominated by one particular fraction i of the mixture and that the inherent vapor pressure of that fraction Pi° ) P/mi. The mass fractions mi could be determined from the actual experimental curves of mass loss. For simplicity, the assumption was made that the mixture was made up of a finite number of components corresponding to the measurement points at which the vaporpressure curves were fit. Then it made sense to measure mi in terms of one-half of the mass loss to the next measurement point. The uncertainty in mi that this procedure introduces is small. In this way, a fit of the data to the form implied by eq 4 was possible. The value of the constant A could then be determined. Figure 4 shows the variation of the constant A with the number average molecular weight. The constant A is related to the entropy of vaporization, and the entropy of vaporization of pure substances is often calculated from a second-law analysis of results such as these. Generally, ∆S ) RA - R ln P, where the vaporization is taking place at a pressure P, with proper regard given to consideration of standard states. The results for A in Figure 4 are observed to be somewhat more scattered than the results for ∆HV. Still, a distinct variation with moelcular weight may be discerned. This variation is again inconsistent with the generally accepted form of eq 4, in which A is assumed independent of molecular weight.4,5,23 This might help
1318 Energy & Fuels, Vol. 12, No. 6, 1998
Figure 5. Vapor pressures of different molecular-weight fractions of Pittsburgh No. 8 tar. The indicated values are number average molecular weights of the fractions.
Figure 6. Mass percentage of Pittsburgh No. 8 tar evaporated as a function of the number average molecular weight of the evaporated fraction. The results of several experiments have been combined.
explain why the exponent c in eq 4 is empirically observed to not have a value of unity; the molecular weight variation in A is, in a sense, also being represented in the value of c. It is striking how different model compounds perform with regard to capturing the trends exhibited by the coal tars. The tars from Pittsburgh No. 8 coal are not wellrepresented by the behavior of alkyl or unsubstituted aromatic compounds. Only hydroxyl-containing aromatics begin to approach the behavior of these compounds. This point will be further discussed in connection with the other tars. Figure 5 shows the different correlations which apply to the different molecular weight fractions of the Pittsburgh No. 8 coal tar. It should be recalled that the actual temperature range over which the data were obtained is from about 1000/T ) 1.6 to 1000/T ) 3.2. This is in contrast to the temperature range for coal pyrolysis, in which tar evolution typically occurs in the range of 1000/T from about 1 to 1.5. Thus, a significant extrapolation of the experimetal results will be required and issues related to correct prediction of ∆HV are quite important. Figure 6 shows the variation in the number average molecular weight of the vaporized species with the extent of mass loss from the sample. The data show that this number average molecular weight increases in what appears to be a linear fashion with the mass loss. The increase with mass loss is of course expected,
Oja and Suuberg
since the higher molecular weight species will evaporate later (at higher temperatures) than the lower molecular weight species. The behavior seen in Figure 6 implies that the molecular weight distribution in the sample, as a function of the number average molecular weight MN, is essentially flat. It proved impossible to reliably characterize the molecular weight of the remaining nonvolatile fractions of the tar after an experiment. Despite our efforts to work at temperatures low enough to avoid secondary reactions in the tars, we have observed that when temperatures reach the values needed to evaporate the last tar fractions, some reactions do occur. Note from Figure 2 that the evpaoration of the least-volatile fractions of tar involved heating to almost 300 °C, which is certainly too high to assure thermal stability. These reactions involved condensation processes within the tars and shift the molecular weight of the residue to higher values. The nature of the processes has not been precisely determined yet, but it was observed in separate experiments that they do not involve reactions which liberate significant quantities of gas. The occurrence of the condensation reaction does not influence the reported vapor pressures very much for several reasons. First, the reactions are believed to be of importance mainly at the higher temperature range of interest in this study, after much of the tar has already evaporated; we have observed some reaction after several hours of exposure to 200 °C. Second, GPC results suggest that the amount of such reaction which occurs is not large. When the chromatograms of the different evaporated and residue fractions are co-added, they give essentially the original tar chromatogram, except for a small shoulder at an elution time which suggests formation of higher molecular weight material. When viewed from the perspective of the whole tar, the change in composition is small. The GPC results further suggest that reactions are most important mainly in the larger, less-volatile species, as would be expected from arguments based upon numbers of reactive sites per molecule. This means that reactions have no influence on fitting the data on volatile fractions with eq 5, insofar as the shift in the molecular weights of the nonvolatile fractions do not affect calculations relating to volatile species (this is true because eq 5 is written in terms of mass fractions). One final piece of evidence can be offered in support of the view that the process of evaporation in these experiments is not influenced much by reactions. This evidence comes from Figure 6 itself. We do not know the true shape of the molecular-weight distribution at high extents of mass loss but have chosen to represent it by a linear extrapolation of the curve in Figure 6 to 100% mass loss. It is then possible to determine the number average molecular weight of the original sample from
[MN,M]-1 )
∫(f/MN) dMN
(6)
where f is the slope of the line in Figure 6 and MN represents the number average molecular weight of the fraction which is lost at any instant of time. The limits of the integral are from MN ) 220 daltons at the beginning of mass loss to an extrapolated value of 400
Vapor Pressures of Coal Tars
Energy & Fuels, Vol. 12, No. 6, 1998 1319
Table 2. Vapor Pressures of Upper Freeport Coal Tar at Different Extents of Evaporationa
Table 3. Vapor Pressures of Wyodak Coal Tar at Different Extents of Evaporationa
mass loss (%)
MN
a
b
mass loss (%)
MN
a
b
6 14 21 27 34 40 51 62
215 230 234 240 250 255 260 270
28.12 28.50 28.79 28.78 29.37 30.04 30.46 31.46
12805 13063 14130 14510 15103 15703 16724 17521
10 22 35 47 59
240 255 280 310 345
38.45 37.89 40.00 38.89 41.35
17292 17636 19442 19752 21915
a The quantity M is number average molecular weight for the N tar which evaporates at the indicated extent of mass loss.
daltons at 100% mass loss. The resulting value of MN,m is 302 daltons, which is in reasonable agreement with the measured value for the original unheated sample (311-320 daltons). The agreement is especially impressive considering that we used a linear extrapolation for the higher end of molecular weight. Thus, these results confirm that for the species whose vapor pressures are measured in this work, there is an accurate picture of number average molecular weights throughout the process. Vapor Pressures of Upper Freeport Coal Tars. Table 2 provides the constants obtained for Upper Freeport coal tar, fit to eq 2. The H/C ratio of the tars averaged 1.14, which was a bit higher than the value for the Pittsburgh No. 8 coal tar. The molecular weight of the Upper Freeport tars obtained from the wire mesh reactor was 290 and that from the tube reactor was 276 daltons. Again, this difference was not considered significant. Thus, these tars were of a slightly lower number average molecular weight than the Pittsburgh No. 8 coal tars. The variation of ∆HV with molecular weight is also shown in Figure 3 for this tar. It is seen that for any given molecular weight, the enthalpy of vaporization of Upper Freeport tar is less than that for Pittsburgh No. 8 tar. It is believed that this is attributable to the higher rank of the Upper Freeport coal. Because of its higher rank, the tars from this coal contain considerably fewer heteroatoms than those from Pittsburgh No. 8 coal. All of the Upper Freeport coal tars contain approximately 4% by mass of sulfur plus oxygen, whereas the Pittsburgh No. 8 tars contain about 10%. Heteroatoms can significantly affect the vapor pressures of tars, because the functional groups that they are involved in can be responsible for forming hydrogen bonding or other electron donor-acceptor interactions. This has been observed in the strong influence the heteroatoms have on the vapor pressures of otherwise very similar molecular-weight aromatics.24 Such hydrogen bonding or electron donor-acceptor interactions will generally raise the enthalpy of vaporization per mole of compound. The value of the preexponential A in eq 4 is shown in Figure 4. It is seen that the values of this parameter are, for Upper Freeport tar, much below the values for Pittsburgh No. 8 tars. A lower value of ∆SV, as would be implied by a lower value of A, is also to be expected for the Upper Freeport tar. Hydrogen-bonded compounds, more abundant in the Pittsburgh No. 8 tar, will generally have a larger ∆SV because their energetically (24) Oja, V.; Suuberg, E. M. J. Chem. Eng. Data 1998, 48, 486.
a The quantity M is number average molecular weight for the N tar which evaporates at the indicated extent of mass loss.
most favorable configurations in the condensed phase will be more constrained. It is notable that the unsubstituted aromatics are a fairly good model for the Upper Freeport tars, whereas they are quite poor for the lower rank tars. The rise of the number average molecular weight in the evaporated fractions was again approximately linear in mass loss. The value of the sample mean molecular weight calculated using eq 6 was 267 daltons, again in reasonable agreement with the original sample values. Vapor Pressures of Wyodak Coal Tars. Table 3 gives the constants for the fit of eq 2 to the data obtained on Wyodak coal tar. This tar had an H/C ratio of 1.2, the highest of the three tars examined in this study. The molecular weights of the tars from the wire mesh and tube reactor were 325 and 340 daltons, respectively. Figure 3 gives the variation ∆HV for this tar as well. It is quite clear that this property of the Wyodak coal tars is somewhat more similar to that observed for Pittsburgh No. 8 tar than for Upper Freeport tar, at low molecular weights. Since the sulfur-plus-oxygen content of this tar is about 12%, the heteroatomic content of Wyodak coal is much more similar to that of the Pittsburgh No. 8 tar. Still, as Figure 3 clearly illustrates, the Wyodak tar is distinctly different from the other two tars. Figure 4 gives the value of the preexponential factor A for this tar. Once again, the properties of the Wyodak tar are more similar to the properties of the Pittsburgh No. 8 tar than to the properties of the Upper Freeport tar. The unsubstituted aromatics and the aromatics which merely contain heteroatoms in the ring (or nonhydrogen-bonding heteroatoms) are not good models for the Wyodak or Pittsburgh tars. The hydroxyl-containing aromatics are the only suitable models for these tars. In fact, the very high values of A for these tars imply quite a strong hydrogen-bonding character. We have observed such high values of A (and even higher) in our recent work on the vapor pressures of saccharides. The value of the sample mean molecular weight for Wyodak tar, calculated from eq 6 using the molecular weights of the evaporated fractions, was 327 daltons, again in good agreement with the initial value of the sample molecular weight. Discussion It is useful to compare the results obtained in this study with results on coal liquids, in order to establish how well the latter materials can serve as models for coal tar. The comparison is shown in Figure 7. The data of Gray et al.10 on certain narrow distillation cuts are used for the comparison. These latter samples were obtained from the SRC-II direct liquefaction processing
1320 Energy & Fuels, Vol. 12, No. 6, 1998
Figure 7. Comparison of some measured vapor pressures from this study with those from Gray et al.10 Heavy solid line, Pittsburgh No. 8 tar of 315 molecular weight; heavy dotted line, Upper Freeport tar of 215 molecular weight; thin dasheddotted curve, Pittsburgh tar of 230 molecular weight; results of Gray et al.; thin solid line, cut 18; thin dashed line, cut 11.
of Pittsburgh seam high-volatile bituminous coal. Fraction 11 had an H/C atomic ratio of 1.09 and a number average molecular weight of between 214 and 230 daltons. Its oxygen-plus-sulfur content was 1.93%. Consequently, this material was most similar to the Upper Freeport tars in several respects. Fraction 18 had an H/C atomic ratio of 0.94 and a number average molecular weight of between 304 and 310 daltons. Its oxygen-plus-sulfur content was 2.6%. This fraction was, therefore, not directly comparable to any of our samples, except for its molecular weight. The comparison in Figure 7 shows that a Pittsburgh seam tar of a molecular weight comparable to fraction 18 exhibits a much stronger temperature dependence of the vapor pressure than does fraction 18. This might have been anticipated from the earlier discussion on the influence of heteroatomic species on the enthalpy of vaporization. The higher heteroatomic content of the Pittsburgh No. 8 tar will lead to a higher enthalpy of vaporization and result in a greater temperature sensitivity of the vapor pressure. Comparison is also made between fraction 11 of Gray et al. and the present results for Upper Freeport tar of 215 molecular weight and the results for Pittsburgh No. 8 tar of 230 molecular weight. The best agreement is seen between the Upper Freeport sample and fraction 11. This was also to be anticipated since these two materials were most similar in nature. The higher vapor pressure exhibited by comparable molecularweight Pittsburgh No. 8 tar came as a bit of a surprise. The reasons for this are not yet clear. Thus, it is apparent that while agreement is seen over certain ranges, generally speaking, coal tars exhibit a wide range of behavior for which these coal liquids are not necessarily good models. The variability is ascribed to chemical characteristics of the tars, which cannot be captured in correlations based purely upon molecular weight. When different volatility fractions are examined for vapor pressure, as in the present study or in the study of Gray et al.,10 it must be recognized that the molecular weight which is given for a particular sample can represent an average over a broad distribution of species. For example, under a particular set of conditions, a hydrocarbon species of high molecular weight
Oja and Suuberg
Figure 8. Comparison of predictions of various models of coal tar vapor pressure with the results of this study, all evaluated at 500 °C.
Figure 9. Comparison of predictions of various models of coal tar vapor pressure with the results of this study, all evaluated at 600 °C.
may exhibit the same vapor pressure as a heteratomic species of somewhat lower molecular weight. Since both will evaporate, both will contribute to the observed number average molecular weight. Thus, it should not be assumed that the number average molecular weight of a fraction defines a “typical” molecule in the sample. The above conclusion leads to difficulties in application of the results of Gray et al. or this study to pyrolysis models in which a statistical approach is used to define fragment molecular weight distributions. Depending upon what functional groups a fragment of a particular size (molecular weight) contains, it may or may not be volatile under any given conditions. There is a statistical way in which to deal with this issue, involving simultaneous examination of not only fragment molecular weights, but also distributions of functional groups. This will be reported on separately. Ultimately, the above concerns must be placed into perspective by comparing the performance of the existing models of coal vapor pressure with the values of vapor pressure based upon the present measuresments. Again, existing literature models presume a single species of a particular molecular weight, whereas the data presented here involve correlation with a number average molecular weight of a fraction. A simple comparison is shown at 500 °C in Figure 8, 600 °C in Figure 9, and 700 °C in Figure 10. The nonshaded areas indicate the regions in which we believe that the results of this study, and the companion study on fractionated tars, are of greatest reliability. Despite the concerns raised above, it may be seen that the earlier published models provide reasonable esti-
Vapor Pressures of Coal Tars
Energy & Fuels, Vol. 12, No. 6, 1998 1321
these methods at the time these measurements were performed (the tars are also quite unstable to prolonged storage and extensive handling). This is why the vaporpressure results were correlated with the only conveniently measurable parameter, the number average molecular weights of the evaporated species. The problem of modeling the vapor pressures of tars would no doubt be better handled by a modification of eq 5 to
Ptot )
Figure 10. Comparison of predictions of various models of coal tar vapor pressure with the results of this study, all evaluated at 700 °C.
mates of vapor pressure over some range of molecular weight. The range of molecular weight over which the different model predictions intersect with the extrapolation of the present data is known to be a key range for pyrolysis modeling. Thus, it is not surprising that all of the current pyrolysis models give “reasonable” vaporpressure behavior, in one range or another. Still it is clear that the earlier published models cannot be judged to be quantitatively reliable over a wide range of molecular weights or temperatures. Note that the use of the logarithmic pressure scale minimizes differences. The agreement between the published models and the Upper Freeport tar data is generally better than it was in the cases of the Pittsburgh No. 8 and Wyodak tars. Again, this is probably because the published correlations have been developed for model systems which are much more like Upper Freeport coal tar than like Pittsburgh No. 8 or Wyodak coal tars, because of the higher heteroatomic content of the latter two. Figures 8-10 essentially compare models of pure component vapor pressures Pi° with measurements of Ptot for real mixtures. While theoretically possible, the presently available data cannot be used to backcalculate the values of Pi° for a more direct comparison. This is because the true molecular weight distributions of the mixtures are unknown and extremely difficult to determine reliably. The use of GPC proved unacceptable for this purpose. It has been shown that when applied to complex mixtures, as opposed to oligomers or polymers of a particular chemical structure, adsorption effects will preclude uniquely relating elution time to molecular weight.25,26 Thus GPC was only used for qualitative characterization of the tars. Mass spectrometric characterization is also problematic for several reasons, including sample evaporation difficulties and fragmentation in the inlet. While techniques exist to deal with some of the issues, we did not have access to (25) Philip, C. V.; Anthony, R. G. Fuel 1982, 61, 375. (26) Lafleur, A. L.; Sarofim, A. F.; Wornat, M. J. Energy Fuels 1993, 7, 357. (27) Macknic, A.; Prausnitz, J. M. J. Chem. Eng. Data 1979, 24, 175. (28) Piacente, V.; Pompili, T.; Scardala, P. Ferro, D. J. Chem. Thermodyn. 1991, 23, 379. (29) Morecroft, D. J. Chem. Eng. Data 1964, 9, 488. (30) Bardi, G.; Gigli, R.; Malaspina, L.; Piacente, V. J. Chem. Eng. Data 1973, 18, 126. (31) Yaws, C. L. Handbook of Vapor Pressure; Gulf Publishing Co.: Houston, 1994; Vol. 3.
∑∑mijPij°(Mi,T)
(7)
where the second summation over the index j represents the use of the appropriate model of vapor pressure, represented by eq 4, for a particular class of compounds j, e.g., polar with hydroxyl, nonpolar hydrocarbons. The data needed to obtain such detailed correlations were not obtained in the first phase of our study. In the absence of the data to allow models of the form of eq 7 to be developed, we must continue to work with models involving use of eq 4 together with eq 5. In this context, correlation with an average molecular weight more explicitly recognizes that there exists a variety of species with distinctly different chemical structures and molecular weights which can all have comparable vapor pressures at the same temperature. Conclusions The present study has presented the first vaporpressure data on actual primary coal pyrolysis tars. In order to obtain reliable data under conditions for which the tars exhibit reasonable thermal stability, a lowtemperature/low-vapor pressure technique (Knudsen effusion) had to be adopted. The classical Knudsen effusion technique had to be modified to allow examination suitable for a mixture which shows a classical batch distillation behavior. The resulting nonisothermal Knudsen effusion method provided the desired data, which were correlated with the number average molecular weights of the different evaporating fractions. The vapor-pressure correlations which were obtained for the Pittsburgh No. 8, Upper Freeport, and Wyodak samples of the Argonne sample bank showed that there is no one vapor-pressure correlation which is suitable for all ranks. Comparison of the results for these three coals implied that heteroatoms may play an important role in influencing the vaporization behavior. Published data on coal liquids showed some similarity to the results for Upper Freeport tars but were considerably different from results on the other coal tars. Commonly used models of coal tar vapor pressure are seen to provide reasonable estimates of vapor pressures under some conditions but not others. The deficiency in the existing models is at least partly attributable to the fact that they are based entirely upon the molecular weight of the tar species. Thus, they do not take into account important differences in vapor pressures which are known to be caused by functional groups, particularly those containing heteroatoms.24 Acknowledgment. The financial support of the U.S. Department of Energy, under grant DE-FG22-92PC92544, is gratefully acknowledged. EF980093U
