Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979
draught. Rubber gloves must be used a t all times. The experiments were carried out in a polyethylene reactor under stirring with a magnetic bar. Hydrofluoric acid solution was prepared from anhydrous hydrogen fluoride and aqueous hydrofluoric acid (75% 1; 100% nitric acid and anthraquinone of reagent grade were used. Preliminary work shows that the temperature must be kept higher than 20 "C and the concentration of hydrogen fluoride above 60%. At lower temperature and hydrogen fluoride concentration the reaction rate is markedly diminished. The reaction products have been analyzed by gas chromatography (2-m column, 4% OV 1 C AW-DMCS, 100 mesh, T = 280 "C isotherm) and compared with reference (1-nitro-, 2-nitro-, 1,2-dinitroanthraquinone) of analytical purity.
Results Example 1. Under stirring, 20 g (0.096 mol) of anthraquinone was added at 15 "C to 102 g of 88% hydrofluoric acid. Then 9 g (0.14 mol) of 100% nitric acid was added dropwise over the course of 15 min and the reaction temperature was kept at 30 "C by external cooling. The reaction mass was then stirred for 1 h a t 30 "C, then cooled to 0 "C. The 1-nitroanthraquinone which had crystallized out was filtered off, rinsed with 10g of 100% hydrofluoric acid, washed with water until neutral, and dried to yield 11 g of 98% (GC) 1-nitroanthraquinone (7 = 44%). The 10 g of 100% H F used for washing was combined with the 120 g of filtrate; 12 g of this mixture was set apart (purge: 9.2%) and 118 g used again in the following nitration (example 11). The 1 2 g of isolated mother liquor was evaporated to yield 8 g of 100% hydrofluoric acid and 4 g of a residue, consisting mainly of unreacted anthraquinone. Example 11. One hundred eighteen grams of the combined mother and washing liquor according to example
127
I was re-used as the nitration medium. T o this solution was added 15.5 g (0.074 mol) of fresh anthraquinone and 4.8 g (0.076 mol) of 100% nitric acid a t 15 "C. After a reaction time of 30 to 45 min a t 30-35 "C the resulting suspension was rapidly cooled to &lo "C and the precipitated solid was filtered off and rinsed with 10 g of 100% hydrofluoric acid. There was obtained 17 g (97% GC) of 1-nitroanthraquinone (7 = 85%). After about 10 repetitions of the nitration process as in example 11, a stationary state was established and by means of the isolation procedure described and the purge ratio the 2-anthraquinone formed in one nitration cycle and water of reaction were removed from the reaction solution. The composition of the mother liquor in the stationary state was anthraquinone, 3.9% ; l-nitroanthraquinone, 2%; 2-nitroanthraquinone, 7.4% ; di3.6%; HF, 83% a (86%). nitroanthraquinone, 0.1%; The yield per cycle was about 88%. Conclusion 1-Nitroanthraquinone can be prepared in high purity by nitration in aqueous hydrogen fluoride. This simple method of preparation is much more advantageous than the previous method of preparation of pure l-nitroanthraquinone which involves distillation of the crude product obtained in the nitration of anthraquinone in concentrated H2S04. Literature Cited Coffey, Chem. Ind., 10, 1070 (1953). Comninellis,Ch., Javet, Ph., Phttner, E., Swiss Patent 11565173 (Aug IO, 1973); German Offen 2 438 210 (Feb 20, 1975). Fredenstagen, K.. German Patent 529538 (1930). Gaiinowsky, S.. Swiatly, Polish Patent Application No. 46428 (Oct 30, 1962). Simons, J. H., Passino, H. J., Archer, S., J . Am. Chem. Soc., 63, 608 (1941).
Received f o r review November 6, 1978 Accepted December 19, 1978 The authors thank CIBA-GEIGY for having partially financed this work.
Pyrolysis of a Precipitated Kraft Lignin Basil Ialridis and George
R. Gavalas"
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 9 7 725
A precipitated kraft lignin was pyrolyzed in a "captive sample" reactor at 400-700 O C for 10-120 s. Weight loss and tar were determined gravimetrically and oxides of carbon, hydrocarbons, methanol, acetone and single-ring phenols were determined by gas chromatography. The single-ring phenols (phenol, guaiacol, etc.), which are the most valuable among the products, were obtained at yields up to 3% of the original dry lignin.
Chemical pulping processes convert about one-half of the wood resource to useful product, namely cellulose fiber. The remaining organic portion of wood, largely lignin, remains in solution in the spent pulping liquor, so-called "black liquor", along with various inorganic chemicals. The black liquor is evaporated and burned to recover the inorganics with the heat of combustion just about balancing the heat of vaporization. The chemical constitution of lignin exemplified by Fruedenberg's (1971) structural model, Sarkanen and Ludwig (19711, and its subsequent refinements suggests that a variety of valuable compounds including single-ring phenols can be obtained by suitable degradation of the complex polymer. Several methods of oxidative or hydrolytic degradation of lignin to phenols and related 0019-7890/79/1218-0127$01.00/0
compounds have been reviewed by Goheen (1971). So far, however, the only commercial processes of lignin conversion to chemicals are those for vanillin and certain organic sulfur compounds. Other processes, including the catalytic liquid-phase hydrogenation of lignin to phenols (Goheen, 1966), have not so far been found economically competitive with processes based on petroleum feedstocks. Lignin degradation processes which are based on liquid-phase chemistry require the consumption of various chemicals and involve tedious separation steps. Pyrolysis or carbonization, on the other hand, requires no reagents and can be used either with the black liquor or with precipitated lignin. Its disadvantage is the relatively low yield of useful phenolic products. Previous work in lignin pyrolysis has been reviewed by Goheen (19711, Allan and 0 1979 American Chemical Society
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Figure 1. Schematic diagram of pyrolysis reactor.
Matilla (1971), and Wenzl (1970). Most of this work involved slow pyrolysis and conditions allowing extensive secondary reactions of the volatile products and hence yielding larger amounts of gases and charcoal a t the expense of tar. In a recent experimental study of Goheen et al. (1976), lignin black liquor was carbonized in a stream of steam serving to minimize secondary reactions by removing the products from the reaction zone. Under these conditions, yields of single-ring phenols as high as 4 % of the organic matter were observed. The most detailed studies of lignin pyrolysis have been reported in the Russian literature. Domburgs et al. (1970-1975) have studied the distribution of single-ring phenols from the pyrolysis of lignin a t vacuum, atmospheric pressure, in the presence of hydrogen, and in a range of temperatures. The same authors conducted experiments with model compounds to obtain mechanistic information. The purpose of the present work was to measure the product distribution from lignin pyrolysis in a broad range of temperatures and reaction times and identify the conditions that maximize the yield of single-ring phenols. Using a "captive sample" reactor, it was possible to attain high heating rates and reduce the extent of secondary reactions. The yields of the pyrolysis products were correlated empirically by first-order kinetics.
Experimental Section Materials and Reagents. The lignin studied is a Douglas fir precipitated kraft lignin in the form of a fine powder supplied by Crown Zellerbach Corporation. It was dried at 80 "C under vacuum for 3 h before use. Apparatus and Procedure. The pyrolysis was carried out by the so-called "captive sample" technique that has been used in the coal pyrolysis studies of Anthony et al. (1976). A weighted sample (-200 mg) was placed between two folds of a stainless steel wire cloth forming the resistive element clamped between two electrodes as shown in Figure 1. The electrodes were mounted in a 1200-cm3 vessel which served as a reactor and product collection vessel. After inserting the lignin sample, the reactor walls were covered with aluminum foil; the vessel was made gas-tight and flushed with helium. The final environment of pyrolysis was helium a t 1 atm. The pyrolysis was achieved by subjecting the resistive element to a current pulse of the desired amplitude and duration by means of an ac power supply (saturated core reactor) controlled by digital circuitry. Appropriate shaping of the current pulse permitted heating the screen to its final temperature within 2 s. The temperature was subsequently maintained constant for the desired reaction time (5-120 s). The heat input to the wire cloth was small relative to the thermal capacity of the vessel; therefore, the reactor walls achieved a temperature less than 50" above ambient. Thus, except in the immediate vicinity
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of the heating element, the gas contents of the vessel were maintained below 100 "C, and the secondary reactions were restricted although not completely eliminated. The pyrolysis products were distributed in three phases: the gas phase, the condensed layer on the aluminum foil lining, and the residue char in adsorbed form. The major fraction of the single-ring phenols was adsorbed on the char or the aluminum foil. Upon completion of the reaction and before analysis, the reactor was heated to about 200 "C by means of an external oven. At this temperature single-ring phenols and lighter liquids such as methanol are transferred almost completely to the gas phase. The material remaining on the aluminum foil consisted of dimers and larger fragments of lignin which will be referred to as tar. After the reactor had reached the desired temperature samples were withdrawn through a sample valve or a septum attached to the reactor walls for gas chromatographic determinations. Finally, the reactor was opened and tar and char were determined by weighing. Gas Chromatography. Each sample was subjected to three chromatographic determinations under different conditions. (i) The analysis of hydrocarbon gases and light liquids was performed with a Porapak Q column 6 ft long with temperature programming (120 s a t 70 "C, then to 150 "C with 8 "C/min). (ii) The analysis of the phenolic compounds utilized a 5.5 f t long column packed with Carbowax 20M 10% (w/w) on Chromosorb W a t a constant temperature of 150 "C. In both cases a dual flame-ionization detector was used, with nitrogen as the carrier gas. Both columns were adapted on the same gas chromatograph by means of a stainless steel six-port valve. (iii) The analysis of methane, carbon monoxide, and carbon dioxide was made with a Carbosieve B 10 f t long column and a thermal conductivity detector. Temperature programming was applied (80 s at 35 "C, then to 150 "C with 10 "C/min). Results and Discussion The results are shown in Table I and Figures 2-7. Table I shows the yield of products as percentages of the dry lignin, Among the various products in the group of light liquids analyzed by the Poropak Q column, only methanol and acetone have been listed. Acetaldehyde has been identified in smaller amounts. Other products identified
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979
129
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979
with the Poropak Q column include CH3SCH3 and CH3SSCH3both in amounts not exceeding half of the amount of acetone. Several additional peaks were observed but not identified in spite of an effort made with a GC/ mass spectrometer. The dependence of product yield on temperature and time is best discussed with the aid of Figures 2-6. Figure 2 shows the weight loss as a function of pyrolysis time a t various temperatures. At each temperature the weight loss levels off after a certain time which decreases with increasing temperature. The final weight loss increases with temperature and exceeds 65% at 750 "C. Figure 3 shows the yield of tar as a function of time. Tar has been defined operationally as the material that remains condensed on the foil lining the reactor walls a t about 200 "C. A t lower temperatures the condensed layer includes most of the single-ring phenols and smaller amounts of methanol and other compounds. The temperature of 200 "C has been found sufficient for the transfer of essentially all single-ring phenols and lower boiling compounds to the gas phase. At this temperature, however, the gas phase contains a small amount of higher boiling compounds which a t lower temperatures would be contained in the tar. The amount of compounds that remain in the gas phase depends on reactor volume and sample size. This introduces some degree of arbitrariness in the measured tar. In any case the curves of Figure 3 are similar to the overall weight loss curves. Figures 4 and 5 show the yields of methane and methanol as a function of time. The solid curves are obtained by the correlation given by eq 1below. The yields of phenols and acetone follow a similar trend with that of methanol, showing a modest temperature dependence and reaching their asymptotic value in a relatively short time. The yield of carbon monoxide follows a trend similar to that of methane which shows a strong temperature dependence and levels off in a relatively long time. Carbon dioxide occupies an intermediate position with respect to its temperature dependence. Figure 7 shows the various yields as a function of temperature for fixed pyrolysis time. As shown in Table I and Figure 7, the sum of single-ring phenols reaches a maximum of about 3.5%. Increasing the temperature beyond 650 "C does not seem to produce any significant increase in the yield. A fundamental kinetic description of lignin pyrolysis is not feasible a t present because of the large number of participating reactions and the uncertainty about the mechanisms and kinetic parameters of these reactions. The recourse to empirical kinetic modeling also presents difficulties as illustrated by Figure 4. In that figure, we have attempted to describe empirically methane production by first-order kinetics dt
= k(V* - v)
where V is the cumulative volume of methane that would evolve at the highest temperature and in a sufficiently long
time. The volume V* can be approximated by the volume measured a t the largest time a t the highest temperature ( T = 650 "C, t = 120 s), and then rate constants h can be computed for each temperature by linear regression. The results of this correlation are shown by the solid lines of Figure 4 and are clearly unsatisfactory. The main reason for the discrepancy is that the asymptotic level of the experimental yield is a function of temperature. Methane is evidently produced by the dissociation of bonds of different strengths of which the higher energy bonds make a substantial contribution to the yield only at sufficiently high temperature. A similar behavior has been observed in the case of coal pyrolysis for which reference can be made to a study by Suuberg et al. (1977) where a suitable kinetic analysis is given. Another fundamental reason for the inability of single reaction models to describe product evolution is the competition between the reactions producing tar and those producing light products. T o describe product formation in more realistic terms, we would need to include several parallel steps for each product and take into account the competition with tar formation. Unfortunately, the accuracy of the present results does not allow this refinement. An alternative but entirely empirical procedure is to correlate the yield of each product by the expression (1)but allow V* to be a function of temperature. The correlation obtained by this procedure is illustrated in Figure 6 for the case of methane. The agreement with the experimental data is good, but the results are devoid of kinetic significance. Acknowledgment The authors express their appreciation to Crown-Zellerbach Corporation for the support of this study and to Dr. D. W. Goheen of the same organization for many helpful suggestions. Literature Cited Allan, G. G., Mattila, T., "High Energy Degradation", in "Lignins", pp 575-596, V. K. Sarkanen and H. C. Ludwig, Ed., Wiley, New York, N.Y.. 1971. Anthony, D. E., Howard, J. E., Hottel, H. C., Meissner, H. P., Fuel, 55, 121 (1976). Domburgs, G., Sergeeva, N. V., Khim. Drev., 6, 133-139 (1970). Domburgs, G., Kirsbaums, I. Sergeeva. N. V., Khim. Drev., 7, 43-50 (1971). Domburgs. G., Rossinskaya. A. G., Kisliiyn, N. A,, Sergeeva, N. V., Khim. Drev., 14, 109-118 (1973). Domburgs, G., Sergeeva, N. V., Zeibe, G., Sharapova, E. T., Kirsbaums, I. Khim. Ispol'z. Lignina, 349-358 (1974). Domburgs, G., Rossinskaya, G., Sergeeva, N. V., Therm. Anal. Proc. Conf., 4th, 1974, 2, 211-220 (1975). Fruedenberg, K., "Lignins", pp 3-6, V. K. Sarkanen and H. C. Ludwig, Ed., Wiley, New York, N.Y.. 1971. Goheen, W. D., "Low Molecular Weight Chemicals", in "Lignins", pp 797-831, V. K. Sarkanen and H. C. Ludwig, Ed., Wiley, New York, N.Y., 1971. Goheen, D. W.. Adv. Chem. Ser.. "Lignin Structure and Reactions", 1966. Goheen, D. W., Orle, J. V., Wither, R. P., paper presented at the symposium, "Thermal properties, pyrolytic conversion, and combustion of carbohydrates and lignins", held during the 172nd National Meeting of the American Chemical Society, San Francisco, Calif., Sept. 1-2, 1976. Sarkanen, K. V , Ludwig, C. H., "Definition and Nomenclature", in "Lignins", pp 3-5, V. K. Sarkanen and H. C. Ludwig, Ed., Wiley, New York, N.Y., 1971. Suuberg. E. M., Peters, W. A., Howard, J. E., Ind. Eng. Chem. Process Des. Dev., 17, 37-46 (1978). Wend H. F. J., "The Chemical Technology of Wood", Academic Press, New York, N.Y., 1970.
Received f o r review October 13, 1978 Accepted January 8, 1979
