228
Energy & Fuels 1991,5, 228-229
of Figure 1, the values for x , (from the intercept) and K (from the slope) were determined as 0.310 f 0.008 and 0.378 f 0.012 atm-l, respectively. In this case the particle weight was about 1 pg. Figure 2 shows the results of Figure 1 in a DubininPolanyi representation plotted as log 3c vs log2 (PO/Pco,). The value of Powas taken from Weast9as Po= 48 250 Torr a t 25 "C. The value of the intercept was found -0.492 f 0.003 equivalent to x , = 0.321 f 0.003. The discrepancy between the two methods for extrapolating the value of x , is about 3 % . It should be emphasized that the error bars reported above are one standard deviations obtained directly from the linear regression of the data. To calculate the total surface area, the cross-sectional area of C02was taken as 24.2 A2.4 The total surface area of this particle was found 1070 f 20 m2/g (the particle was 155 pm in diameter with apparent density of 0.6 g/cm3). Measurements of COPdesorption were also carried out for Spherocarb particles to yield a point to point difference from the adsorption measurements of about 1 % . Adsorption-desorption cycles were carried out for 10 particles to yield a scatter of less than 5% in the measured value for saturation adsorption. The major advantage of using high-pressure measurements for evaluating the saturation value for C02 adsorption is the improved accuracy of the extrapolation procedure. Previous measurements with the EDC a t atmospheric pressure' yielded values for the surface area for similar particles of comparable value with a relative error of about 15%. The results of the high-pressure measurements are, however, bounded with an error of about 3 '3°C. Also, the equilibrium adsorption-desorption coefficient was found with a high accuracy, whereas from atmospheric measurements it was not practical to obtain an accurate value. Acknowledgment. We are indebted to A. Modestino for his assistance and support. Registry No. CO,, 124-38-9. Ezra Bar-Ziv* Nuclear Research Center-Negev
P.O. Box 9001, Beer-Sheua, Israel John P.Longwell, Adel F. Sarofim Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Received June 25, 1990 Revised Manuscript Received September 21, 1990
Evidence for Low-Temperature Second-Order Phase Transitions in Coal/Solvent Systems Using Differential Scanning Calorimetry Sir: The analysis of coal structure as a cross-linked macromolecular system is well established.' A wide variety of techniques have been used to confirm this and among these is differential scanning calorimetry (DSC). Lucht et a1.2have used DSC to demonstrate the existence of a second-order phase transition for a variety of coals at about 630 K, below the onset of significant thermal deg( 1 ) Green, T. K.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Meyer, R. A., Ed.; Academic Press: New York, 1982. (2) Lucht, L. M.; Larson, J. M.; Peppas, N. A. Energy Fuels 1987,1, 56-58.
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Figure 1. Differential scanning calorimetry at 10 K/min for a Illinois NO.6 coal mixture of 1-methyl-2-pyrrolidinone-extracted
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Temperature (K) Figure 2. Differential scanning calorimetry at 10 K/min for a mixture of pyridine-extractedIllinois No.6 and pyridine for a weight fraction of 0.718 of pyridine in the mixture.
radation. We have confirmed this observation, They attributed this to a glass transition (TJ for the system. Lucht et ala2went on to demonstrate that the thermal Tg reduced to a limiting value of about 400 K following sorption of pyridine vapor to 0.4 pyridine weight fraction. I t has been reported by Brenner3 that thin sections of Illinois No. 6 coal display many of the physical properties of rubbers (e.g., not fracturing under considerable strain) when they are in equilibrium with basic solvents such as pyridine. Brenner3 suggested that T for pyridine-swollen coals lay below room temperature. f i e existence of a high temperature T in these systems does not logically exclude the existence of another lower temperature TB' Using coals more swollen than the samples of Lucht et aL2, we have observed very low temperature second-order phase transitions. A Mettler DSC 30 was used with standard aluminum sample holders. The lids were pierced with a small hole to allow evaporation of solvent into the nitrogen carrier gas when the samples are heated. The typical sample size was 20 mg, and a DSC heating rate of 10 K/min was chosen. The coals were selected from the Argonne Premium Coal Bank. Prior to DSC the coals were exhaustively Soxhlet extracted under a dry N2 atmosphere with the solvent under investigation. Figure 1 shows the DSC from NMP-swollen NMP-extracted Illinois No. 6 coal. There is a second-order phase transition at 180 K. There is one endotherm that corresponds to solvent melting and the temperature of the maximum suggests that this is free NMP. Figure 2 shows the DSC result for a mixture of pyridine-extracted Illinois (3) Brenner, D.Fuel 1984, 63,1324-1328.
0 1991 American Chemical Society
Book Reviews
No. 6 and pyridine. There is a second-order phase transition evident centered a t 130 K and two endotherms. The higher endotherm corresponds to free pyridine melting. The lower may be the result of bound pyridine. This is a much less well defined transition than for the pyridine-swollen coal which may indicate that there is a wider distribution of activation energies for the relaxation of the pyridine-swollen coal. It was found that the existence of all of the phase transitions was reproducible on different samples of the same coal/solvent system and that cycling the same sample between 100 and 200 K also reproduced the second-order phase transitions (we are grateful to a reviewer for this suggestion). Similar phase transitions have been noted for a number of coal/solvent systems. Figure 1 also shows an exotherm centered at 220 K during which enough energy is produced to make the measured C, values become negative. A similar phenomenon has been observed for the DSC of rubbery polymers and polymer melts quenched to below their TC4 For conventional polymers it has been attributed to cold recrystallization and an analogous effect may be operating here. When rubbery polymers are quenched, the hightemperature disorder is "frozen" into the structure. Upon reheating to above the Tgthe molecular chains eventually have enough energy to realign to form a more stable structure. In other experiments a slight exotherm has been noted for pyridine-Illinois No. 6 systems and the apparent absence of a corresponding exotherm in Figure 2 may be due to its occurrence a t a similar temperature to that of the solvent melting. Although the exotherm is consistent (4) Billmeyer, F. W. Text Book of Polymer Science; Wiley: New York, 1966.
Energy &! Fuels, Vol. 5, No. 1, 1991 229
with a type of polymer cold recrystallization, we cannot rule other possibilities out a t this time. An alternative explanation for the exotherm could be recrystallization of the solvent in the coal porosity. Although the hypothesis needs further structural confirmation, all of the available evidence suggests that the low-temperature second-order phase transitions in Figures 1 and 2 are indeed glass to rubber transitions associated with a major increase in chain mobility. There are only relatively few polymers with T i s as low as those reported here. The unexpectedly low coal T 's may be attributed to the fact that the coals are in higily swollen states. Above the evaporation point of pyridine and NMP, other Tis similar to that of Lucht et a1.2were observed. It should be pointed out that the existence of two glass transitions is quite consistent because at each glass transition we are dealing with different systems. A t low temperatures the glass transition is for a highly swollen macromolecular system, and at high temperature (following solvent evaporation) the glass transition is for a nonswollen network. Following removal of the swelling solvent, a number of structural changes may occur, for example, re-formation of hydrogen bonds. Therefore, it is proposed that the rubbery behavior observed by Brenner3 is real and there exists a low-temperature Tgfor highly swollen coals. Other coals and solvents show similar behavior which will be reported in a full paper and the effects of varying solvent content will be described. Peter J. Hall, John W . Larsen* Exxon Research and Engineering Company Clinton Township, Annandale, New Jersey 08801 Received September 21, 1990 Revised Manuscript Received October 12, 1990
Book Reviews Flue Gas and Fly Ash. Edited by P. F. Sens and J. K. Wilkinson. Elsevier Applied Science, Elsevier Science Publishing Co., Inc.: New York, 1989. 173 pp. $43.25. This book presents the proceedings of a contractors' meeting organized by the Commission of European Communities, Directorate-General for Science, Research and Development, held in Brussels, June 16, 1988. It contains two main sections. The first is titled Session I: Flue gas treatment; it contains nine papers entitled Catalytic NO, reduction, Improved gas filtration systems for control of solid fuel particulate emissions, Development of a process for removal of SOz and NO, from flue gas using brown coal coke, Regenerative desulfurization in fluidized bed combustion of coal, Development of a regenerative sulfur dioxide system, Closed loop controlled integrated hot gas clean up, Electromulticyclone for coal burner exhaust gas cleaning, Development of a dust separator for hot gas cleanup, Pulse power techniques for flue gas treatment. The second section is titled Session 11: Fly ash properties, re-use and disposal, and it contains six papers entitled Characterization of fly ash from fluidized bed combustors with regard to its utilization and safe disposal, Adsorption and desorption phenomena of polycyclic aromatic hydrocarbons on fly ash (two papers had this title), Waste/soil interaction studies-the leaching of molybdenum from pulverized coal ash, Handling and conditioning of fluidized bed and pulverized coal boilers fly-ash for landfill disposal and re-use, and
The utilization of P.F.A. (pulverized fuel ash) as a filler in rigid polyurethane insulation foams. The format begins with a set of contract information (identification number, duration, level of support, principal investigator, and contractor's name and address). A Summary then follows. The headings then vary but follow the usual topics of introduction, experimental, results and discussion. Frequently, but not always, a section of conclusions, or prospects or status, comes just before the references. Figures and tables are either interspersed or come at the end of each section. A Request for Proposals issued in March 1985 led to the awarding of 66 contracts for projects on the Utilization of Solid Fuels. These papers represent a portion of that program and give reports on the progress made. Some projects had been underway for a t least a year, while others had not been active for as much as a year. Each author provided camera-ready copy for the publisher which led to a variety of presentations. The quality is generally good, but the appearance varies. The text is well illustrated with photographs of equipment and line drawings. It is also well documented in most cases with tables of data. The paper length and detail vary. Some papers are relatively brief summaries, while others are extended and quite adequately descriptive. The final parts include a brief section on conclusions and recommendations for future work, together with a list of participants and index of authors.
