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Bambas, L. L., Troutman, H. D., Long, L. M., J . Am. Chsm. Soc., 77, 7 (1955). W,M., Haberkom, A., Herlinger, H., Mayer, K., Peterson, S., Ar.zmim.-forsch, 22, 1564 (1972). Brian, R . C., Chem. Ind. (London), 1965 (1955). Cutler, R. A., Stenger, R. J., Suter, C. M., J. Am. Chsm. Soc.,74, 5475 (1952). Dodd, M. C., Stillman, W. B., J. fharmacol. Exp. Ther., 82, 11 (1944). Drawbaugh, R., Bouffard, C., Strauss, M., J . Med. Chem., 19, 1342 (1976). Elion, G. B., Callahan, S. W., Hltchings, G. H., Rundles, R. W., Laszlo, J.. Cancer Chemother. Rep., 18, 197 (1962). Evans, D. D., Morris, D. S.,Smith, S. D., Tivey, D. J., J . Chem. SOC., 1687 (1954). Franklin, C. S., Morris, D. S.,Smith, S. D., J . Chem. SOC., 1683 (1954). Frei, E., Bodely, G. P., "Principles of Neoplasia", Chapter 319 in Harrison's "Principles of Internal Medicine", 7th ed, McGraw-Hill, New Yo&, N.Y., 1972. Hayes, K. J., US. Patent 2610 181 (1952). Higuchi, T., Stella, V., ACS Symp. Ser., NO. 14, (1975). Hirano, K., Hoshina, S., Okamura, K., Suzuka, I., Bull. Chem. SOC.,Jpn., 40, 2229 (1967). The MedicalLetter, 17 (13), 53 (1975). Morris, D. S.,Smith, S. D.. J . Chem. Soc., 1660 (1954).
Perlman. D., "Antibiotics", Chapter 17 in A. Burger's "Medicinal Chemistry", Wiley, New York. N.Y., 1970. Pianka, M., Browne, K. M., J. Sci. Food Agr., 18, 447 (1967). Powell, S. J., Bull. N . Y . Acad. Med., 47, 469 (1971). Rolley, R. T., Sterioff, S., Williams, G. M., Parks, L. C., Surg. Forum, 25, 266 (1974). Rollo, J. M., "Drugs Used in the Chemotherapy of Amebiasis", in Goodman and Gilman, "The Pharmacological Basis of Therapeutics", 5th ed, Macmillan, New York, N.Y., 1975. Shmidt, P., Whllhelm, M., Angew. Chem. Int. Ed. Engl., 5 , 857 (1966). Strauss, M. J., Chem. Rev., 70, 667 (1970). Suss,R., Kmzd, V., Scribner, J. D., "Cancer, Experiments and Concepts", Springer Verlag, New York, N.Y., 1973. Venulet, J., Van Etten, R. L., "Biochemistry and Pharmacology of the Nitro and Nibox, Bwps", in H. Few's, "The Cherisby of the Nibo and Nboso &a&', Wiley, New York, N.Y., 1970.
Received for review April 24, 1979 Accepted May 18, 1979
POLYMER COATINGS SECTION Design and Evaluation of a Plug Flow Reactor for Acid Hydrolysis of Cellulose Davld R. Thompson and Hans E. Grethlein" Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755
An isothermal plug flow reactor was developed to study the kinetics of acid hydrolysis of cellulosic substrates. The kinetic parameters in a model which gives the glucose formation from purified cellulose (Solka-Floc),200 mesh, were obtained over the following range of independent variables: temperature from 180 to 240 OC,sulfuric acid concentration from 0.5 to 2.0%, and slurry concentration from 5.0 to 13.5%. It was determined that the glucose formation from newsprint (a mixture of wood and pulp), 65 mesh, can be predicted from the kinetic model developed for Solka-Floc. Since at least 50% of the potential glucose can be obtained at 240 O C , 1 % acid, and 0.22min residence time, the continuous acid hydrolysis of cellulose may be a process of commercial interest.
Introduction The future need to replace petroleum has stimulated interest in the conversion of cellulose to a liquid fuel and chemical feed stocks (Wilke, 1975). Moreover, the possibility for converting cellulose from sources such as municipal refuse, agricultural wastes, and wood wastes may be a way of turning a waste disposal problem into a resource opportunity. Acid hydrolysis of cellulosic substrates followed by fermentation of the sugars to ethanol is one method for accomplishing this conversion. In the past, numerous attempts at hydrolyzing cellulose with low concentrations of sulfuric acid have been tried commercially. For example, Saeman's work in the 1940's led to the Madison Process-a semi-batch process for wood hydrolysis (Saeman, 1945). The economics of the process, however, could not compete with ethanol from petroleum, which was introduced a t that time. In 1967, Proteous (1967) proposed that municipal refuse could be disposed of at a profit to the community if the cellulose in the refuse were converted to glucose through acid hydrolysis and then fermented to ethanol. This 0019-7890/79/12 18-01 66$01.OO/O
initiated the work again on acid hydrolysis. Fagan et al. (1971), using transient batch experiments, demonstrated that the kinetic model developed by Saeman could also be used for paper. As a result, flow sheets for a new large-scale continuous process have been prepared (Grethlein, 1978) using a plug flow reactor and the kinetic data on paper hydrolysis. In this type of process analysis one has to make some reasonable but untested assumptions to evaluate the economic potential of acid hydrolysis. The major assumptions which are the focus of this study are: (1)that 50% of the potential glucose is obtained in a plug flow reactor; (2) that the slurry concentration in the range of 10 to 30% can be pumped, heated, mixed with acid, reacted, and quenched in a stable steady-state continuous process; and (3) that the kinetic model developed for dilute slurries is indeed valid for concentrated slurries. In order to obtain reliable kinetic information in the economic region of the design space of temperature, acid concentration, and slurry concentration, a laboratory scale continuous plug flow reactor was developed for acid hydrolysis (Thompson, 1977). Because of the high tem0
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FLUSHINO MOTOR AND PUMP
Figure 1. Schematic diagram of plug flow reactor.
perature and pressure and the difficulty of pumping a slurry at small flow rates, a number of compromises have been made to get the reactor to work. However, useful results have been obtained and are discussed below. Reaction Characteristics The acid-catalyzed hydrolysis of crystalline cellulose to glucose can be described as a homogeneous pseudofirst-order reaction (Wend, 1970) for reasonably small particles (20-200 mesh). Also, the acid-catalyzed decomposition of glucose is known to be first order (McKibbins et al., 1962). Since the accessible or amorphous cellulose hydrolyzes immediately to glucose, it is taken as an initial glucose fraction. The hydrolysis kinetics can be described by the model shown in reaction 1
Table I. Kinetic Model Parameters parameter
K,, min-' m E , , calimol K,, min-'
n E,, cal/mol
Douglas fir 1.73 x 1019 1.34 42900 2.38 x 10l4 1.02 32870
kraft paper 28 x 1019 1.78 45100 4.9 x 1014 0.55 32800
Solka-Floc 1.22 x 1019 1.16 42500 3.79 X 10l4 0.69 32700
experimental 170-190 "C 180-230 "C 180-240 "C conditions isothermal nonisothermal isothermal batch batch plug flow Saeman (1945) Fagan et al. (1971)
fractions of crystalline cellulose (C,(O)) and accessible cellulose equivalent to glucose (C,(O)) as shown in eq 5.
crystalline cellulose
alucose
Kg
decomposition
(1)
accessible cellulose
where dC, - = KIC, - K2Cg(net glucose formation) dt
(2)
C, is the fraction of potential glucan remaining as crystalline cellulose, C, is the fraction of potential glucan present as glucose, and K1 and K 2 can be defined by Arrhenius type equations with factors to account for the catalytic activity of the acid as shown in eq 3 and 4 K1 = K,(A)" exp(-El/RT)
(3)
K 2 = &(A)" exp(-Ez/R?3
(4)
where A is the sulfuric acid concentration in weight percent aqueous phase, T i s the temperature, and R is the universal gas constant. The activation energies El and E2, the exponents m and n, and the preexponential factors K3 and K4are the kinetic parameters which are evaluated from experimental data. Equation 2 can be integrated a t constant temperature and acid concentration for a substrate of known initial
Cg(0) exp(-Kzt) (5)
Generally the kinetic model for acid hydrolysis is applicable to many cellulosic substrates but the parameter values may be different for each. For example, the kinetic parameters for Douglas fir (Saeman, 1945) and kraft paper (Fagan et al., 1971) are given in Table I as a point for comparison with the results of this study. Experimental Equipment From past experience a laboratory scale, continuous plug flow reactor apparatus was designed and built that could hydrolyze up to 2.5 kg/ h of solids under steady-state conditions. A schematic diagram of it is given in Figure 1; it consists of seven major parts. (1) S l u r r y Tank. The slurry is prepared and held in a 16-L tank which is mounted over the pump inlet. Mixing is provided by an agitator and by the recycle flow from the pump through a 1.78-mm diameter capillary tube which maintains the required pressure differential. (2) Pump. A six-stage positive displacement moving cavity pump is used with a maximum operating pressure of 600 psi or 4.13 X lo6 Pa. The flow rate is adjustable from 1.5 to 1 2 L/min by changing the rotational speed of the pump. Since only about 300 mL/min are used in the reactor, a major part of the pump flow is recycled to the slurry tank. (3) Heating Unit. The heating unit with 6 kW capacity can raise the temperature of a slurry from 30 to 240 "C
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with a maximum flow rate of 300 mL/min. The unit is made of 26 m of concentrically coiled tubing wrapped with electrical heating tape. The entire coil is placed inside a well insulated box. The tubing has an inside diameter of 4.592 mm. With this diameter the fluid velocity is high enough to prevent any slurry particles from settling in the heater. (4) Acid Injection Unit. A controlled flow of concentrated sulfuric acid is injected into the heated slurry in a specially designed mixing tee. The tee is shaped like a Venturi tq give good turbulent flow in the throat. The acid flow is controlled by a piston pump with a pulse dampener. By adjusting the stroke length of the piston the flow rate can be changed from zero to 3.0 mL/min. In effect acid concentrations in the slurry from 0 to 2% can be obtained. (5) The Reactor. The reactor tube (Carpenter 20 cb3) with an inside diameter of 10.9 mm is mounted in a vertical position to prevent the settling of slurry particles. In order to vary the residence time, the reactor tube can be replaced with various tube lengths. The reactor, the acid mixing tee, and the connecting tubing from the heating unit are all well insulated. (6) Heat Exchanger. The hydrolysate leaves the reactor through a 1.8 m long capillary tube with an inside diameter of 0.7 mm. This small diameter is used so that the pressure differential of 500 to 600 psi or 3.44 X lo6 to 4.13 X lo6 Pa can be maintained across the reactor without having the flow exceed 300 mL/min. The hydrolysate is cooled to 50 OC by a jacketed pipe around the capillary. The residence time in the capillary is 0.15 s; consequently the time to quench the reaction is less than 0.01 of the reactor residence time which approaches instantaneous quenching. (7) Instrumentation. Four thermocouples are fastened to the outside wall of the reactor at equal intervals along the length. The temperature for each thermocouple is recorded on a strip chart. All the thermocouples were calibrated in place so that the temperature accuracy is within 0.5 OC. Because of the low heat loss from the reactor the temperature drop along the reactor is less than 0.7 "C. At present the limiting feature of the apparatus is the diameter of the capillary tubing in the heat exchanger which fixes the maximum particle diameter to 0.35 mm (42 mesh) in the slurry.
Experimental Procedure The reactor was brought on stream with water flow through the heater and reactor provided by the flushing pump (Figure 1). While waiting for the system to come to the desired temperature (-30 min), the slurry was prepared a t the desired percent solids and circulated through the slurry pump and back to the slurry storage. The combination of the circulation and mechanical agitation kept the solid suspended. When the reactor is at the desired temperature the acid pump is turned on and the slurry pump is cut in and the flush pump is cut out of the system. After a few minutes the system comes to a steady state as noted on the temperature recorder. The flow rate of the slurry is measured by collecting a fixed volume from the reactor and noting the time. When several such flow rate determinations are constant, several 50-mL samples of slurry are collected for analysis. Also, a sample of inlet slurry storage is collected. A new steady-state condition is achieved by changing the temperature or acid flow rate. Thus, several different conditions can be obtained in about 1 h of operation.
Table 11. Composition of Substrate (Dry Basis) Solka-Floc newsprint glucan mannan xylan lignin
88.2% 2.1% 7.9% 0.5%
48.6% 7.2% 9.7% 32.0% m
200
2 10
220
TEMPERATURE
230
240
('C)
Figure 2. Glucose yield vs. reactor temperature for 1% acid, fixed residence time of 0.22 min, and various slurry concentrations.
The solid concentration in the inlet and product slurry are determined as mg/mL by drying to constant weight a washed filter cake from a known slurry volume. Both solid residues are analyzed for potential glucose and xylose by the method of Scott (1976). The liquid phase is analyzed for glucose, cellobiose, and xylose by using HPLC (Palmer, 1975). The ratio of real glucose in mg/mL in the hydrolysate to the potential glucose in mg/mL in the feed slurry times 100 is reported as the per yield of glucose. Experimental Results Our experience with the reactor so far has been with Solka-Floc (Brown Paper Co.) and to a lesser degree newsprint. During the learning period of operating the reactor, Solka-Floc BW-200 ball-milled and sieved through a 0.074-mm screen (200 mesh) was used as a model substrate. Moreover, since this material was extensively studied by the Natick Laboratory for enzymatic hydrolysis, direct comparison of the glucose yield from acid and enzymatic hydrolysis can be made (Brandt et al., 1973; Mandels et al., 1974). The compositions of the Solka-Floc and newsprint are given in Table 11. First, a series of experimental runs were made a t 1% acid and a fixed residence time of 0.22 f 0.02 min for 5.0, 10.0, and 13.5% solids in the slurry (Solka-Floc). Note that the kinetic model (eq 3 and 4) depends on the acid concentration in the aqueous phase and not on the ratio of cellulose to acid. Therefore, it was important to establish if the kinetic model depended on the slurry concentration-especially at very high temperatures where the reaction rate is high and diffusional limitations may become evident. The glucose yields as a percent of potential glucose are plotted against reaction temperature in Figure 2. Within the resolution of the experimental error the slurry concentration does not affect the glucose yield over the temperature range studied. It appears that over this range of slurry concentration the continuous acid injection into the hot slurry is fast compared to the kinetic time constant of the hydrolysis. Since the viscosity of the slurry increased significantly with the percent solids, we were prepared to find different yields at the high slurry concentration than at low concentration. Additional data were obtained on Solka-Floc at 0.5, 1.5, and 2.0% acid. The experimental points are shown in
Ind. Eng.
Table 111. Observed and Predicted Glucose Yields for Newsprint
-
u 860a
x t
0
A
sample no.
2 % ACID l.S% ACID I -/.A C I D .Sol. A C I D
I
200
reaction time, min 0.274 0.275 0.279 0.268 0.223 0.231 0.228 0.218 0.222
2% A C I D
iao
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TEMPERATURE ( " C 1
24 0
Figure 3. Glucose yield vs. reactor temperature for fixed residence time of 0.22 min and various acid concentrations. Experimental data points are shown and the curves are the best fit to the kinetic model.
Figure 3. Again, the glucose yield vs. temperature is shown for Solka-Floc with a nominal residence time of 0.22. The smooth curves are determined by the model in eq 5 for 0.5, 1.0, 1.5, and 2.0% acid. The parameters P1,Ps, m, n, El, and E2 were estimated by the technique of minimizing the sum of squared deviations between the observations and the model. This set of parameter values is given in Table I in the last column. The effect of the accessible cellulose is taken into account by C&O) = 0.04 for Solka-Floc. We have also made nine runs with 5% newsprint in the slurry. Since newsprint is more like wood, it has more hemicellulose and lignin than Solka-Floc, as shown in Table 11. The newsprint was ball-milled and sieved through a 0.21-mm screen (65 mesh). Note that this is about three times larger than the Solka-Floc. However, in spite of these differences, the kinetic parameters determined for Solka-Floc appear to apply to newsprint as well. The experimental conditions, the observed glucose yields, and predicted yields are shown in Table 111. The predicted yields are from the model in eq 5 with the parameters determined for Solka-Floc and taking C,(O) = 0.04. The close agreement between the last two columns is interesting since these substrates are quite different in their lignin content. This certainly is contrary to the strong influence that lignin has on the rate of enzymatic hydrolysis in different substrates (Mandels et al., 1974). Conclusions While our tests to date are limited to Solka-Floc and newsprint, the plug flow reactor can be used to obtain the kinetics of acid hydrolysis for any cellulosic biomass at high temperature and short reaction time.
% acid
H,SO, 1.21 1.22 1.02 0.49 0.96 0.97 0.70 1.02 0.52
teomp, C 232 226 230 210 177 202 201 217 220
obsd pred. glucose glucose yield, % yield, % 51.9 49.3 17.5 7.5 2.7 11.8 7.0 25.8 18.5
49.5 48.0 16.9 8.4 2.6 10.15 6.5 25.5 17.6
In spite of the experimental error in the data (which becomes smaller with experience), it is clear that 50 to 55% glucose yield can be obtained repeatedly with 1% acid in the range of 235-240 "C with a residence time of 0.22 min. From a preliminary process design (Grethlein, 1978) we expect the hydrolysis to be economical when the yields are in this range and when the slurry is greater than 10% solids. Consequently, some of these results are in the range of commercial interest. Although newsprint and Solka-Floc are quite different in their lignin content, this fact seems to have no effect in the hydrolysis kinetics. Moreover, the high activation energy El shown in Table I means that up to 240 "C the acid hydrolysis of cellulose is kinetically controlled rather than diffusion controlled when the particles in the slurry are smaller than 0.21 mm. Acknowledgment The authors gratefully acknowledge the support of this work in part by the National Science Foundation, the Department of Energy, and American Can Company. Literature Cited Brandt, D., Hontz, L., Mandels, M., AIChf Symp. Ser., 89, No. 133, 127 (1973). Fagan, R. D., Grethlein, H. E., Converse, A. O., Porteous, A,, Envlron. Sci. Techno/., 5, 545 (1971). Grethlein, H. E., Biotechnol. Bioeng., 20, 503 (1978). Hanis,J. F., Saeman, J. F., Locke, E. G., "The chemsby of Wood", B.L. Browning, Ed. Interscience, New York, 1963. Mandels, M., Hontz, L., Nystrom, J., Biotechnol. Bioeng., 18, 1471 (1974). McKibbins, S. W., Harris, J. F., Saeman, J. F., Neill, W. K., For. Prod. J., 12, 17 (1962). Palmer, J. K.,Appl. Polym. Symp., No. 28, 237 (1975). Porteous, A., ASME Meeting, Pittsburgh, Nov 1967, Paper No. 67-WA/PID-2. Saeman, J. F.. Ind. f n g . Chem., 37, 43 (1945). Scott, R. W., Anal. Chem.. 48, 1919 (1976). Thompson, D. R., Thesis, Thayer School of Engineering, Dartmouth College, Hanover, N.H., 1977. Wenzl, H. J. F., "The Chemical Technology of Wood", Chapter I V , Academic Press, New York, 1970. Wilke, C. R., Ed., Biotechnol. Bioeng. Symp., No. 5, (1975).
Received for review December 6, 1978 Accepted June 4, 1979
