Effect of transient heat transfer and particle size on acid hydrolysis of hardwood cellulose

Bioresource Technology 35 ( 1991 ) 15-21

Effect of Transient Heat Transfer and Particle Size on Acid Hydrolysis of Hardwood Cellulose A. E. Abasaeed,* Y. Y. Lee:~ Department of Chemical Engineering, Auburn University, Alabama 36849, USA

& J. R. Watson Tennessee Valley Authority, Muscle Shoals, Alabama 35660, USA (Received 30 March 1989; revised version received 16 January 1990; accepted 17 January 1990)

Abstract

NOTATION

The kinetics of acid hydrolysis of hardwood cellulose were investigated covering the reaction conditions of 198-215°C and 1-3% sulfuric acid. 7he thermal diffusivities of hardwood (southern red oak) were experimentally determined. The data were incorporated into a theoretical model designed to study the effect of transient temperature variation within the solid substrate during acid catalyzed hydrolysis" of cellulose. The kinetics and the modeling results" were analyzed to assess the eff'ect of particle size on acid hydrolysis of wood celhdose, lmportant findings were that as the chip size increased the maximum attainable glucose yield decreased whereas the corresponding optimum reaction time increased as the result of transient conductive heat transfer. This effect becomes more significant as the temperature and the acid level were raised. Quantitative criteria were established to show the effect of particle size on glucose yield and optimum reaction time at various reaction conditions.

A B Bi = h(kL )-1 C

Key words: Hardwood, cellulose, acid hydrolysis, kinetics, chip size, yield, reaction time, heat transfer.

X

*Present address: Department of Chemical Engineering, King Saud's University, Riyadh, Saudi Arabia. -,+-Towhom correspondence should be addressed.

Ci Cp Ei h H i k k~ k~o L n~ R t To T L

Z

a = k ( p C p )-l 2n

P /7

Cellulose Glucose Biot number Decomposition products (including hydroxymethyl furfural) Concentration Heat capacity Activation energy Heat transfer coefficient Acid concentration (% wt) Reaction index (1 for hydrolysis and 2 for decomposition) Thermal conductivity First-order rate constant Frequency factor Half of the thickness of a slab Acid exponent Gas constant Reaction time Absolute temperature Initial temperature Steam temperature Position in a slab measured from the center xL Thermal diffusivity Eigen values defined by eqn (7) Dimensionless temperature = ( T - T,)/( ~; - L) Density Dimensionless time = a tL - 2

15 Bioresource Technology 0960-8524/91/S03.50 © 1991 Elsevier Science Publishers Ltd, England, Printed in Great Britain

16

A. E. Abasaeed, Y. Y. Lee, J. R. Watson

INTRODUCTION Acid hydrolysis of cellulosic biomass is one of a number of viable technologies being developed as biomass conversion processes. A great deal of effort has been devoted to the investigation of the kinetics (Song & Lee, 1984; Thompson & Grethlein, 1979; Teng & Muthrathan, 1985; Connor et al., 1985), reactor design (Song & Lee, 1983; Bergeron et al., 1986; Zerbe & Baker, 1987) and diffusion-related problems associated with this process (Song & Lee, 1983; Nadgir, 1985). One of the primary assumptions applied in many of the previous studies has been that of isothermal reaction conditions. However, it is an assumption that may lead to erroneous results due to the following considerations. In a typical acid hydrolysis process, the cellulosic substrate is presoaked with sulfuric acid, then exposed to steam for rapid heating. During this transient heating period, the temperature within the biomass structure changes drastically from an initial uniform low temperature to high temperature at the outer surface and low temperature at the center region, and eventually to uniform high temperature. Such a temperature variation creates a fundamental and unique problem concerning heterogeneous reaction engineering, since it alters the reaction progress from that which would be expected if the reaction were isothermal throughout. Whether this transient variation of temperature significantly affects the overall reaction depends chiefly on the relative rate of reaction to that of conduction. For example, if conduction is extremely fast compared to reaction, an isothermal temperature profile is quickly established within the solid substrate. The extent of the reaction occurring during the preheating period is then negligible, justifying the isothermal assumption. Otherwise, a significant degree of reaction may occur during the preheating period, thus making the isothermal assumption invalid. The rate of reaction and the rate of conduction strongly depend on the reaction condition and the size of the solid feed, respectively. These two parameters are keys to the understanding of the hydrolysis reaction in which biomass size imposes a non-uniform temperature distribution. Furthermore, size reduction is a cost-intensive unit process and therefore subject to both technical and economic constraints. This study was undertaken to ascertain the effect of transient heat transfer on the hydrolysis

of cellulose and to establish the quantitative criteria concerning the size of the biomass, i.e., what degree of size reduction for the biomass feed is required to prevent loss of sugar yield in the hydrolysis of hardwood cellulose for a specified reaction condition.

METHODS Pretreatment

Hardwood of southern red oak species in the form of nominal 1 in chip size chips were used as the cellulosic substrates. The wood samples were pretreated to remove the hemicellulose prior to cellulose hydrolysis. The reaction condition used in prehydrolysis was 0 . 7 5 % sulfuric acid, 184°C, and 4 rain of reaction time. This condition was found to be the optimum in the sense that the extent of hemicellulose hydrolysis is high and yet there is little degradation of cellulose. The reaction was carried out in a Masonite gun highpressure batch reactor, zirconium steel, 6 in i.d. x l5 in high ( 1 5 x 3 7 cm) The reactor was equipped with a steam jacket and direct steam injection port. An important feature of this reactor system, especially as a kinetic reactor, was that the reaction time and temperature were closely controlled by direct steam injection and rapid quenching of the reactor by abrupt release of pressure. The discharged solid residue was broken into fine particles. The composition of the prehydrolyzed wood was determined to be 61.6% cellulose, 35.7% lignin and 3-7% hemicellulose. Cellulose hydrolysis Of the residues obtained after pretreatment, only the portion with fine particle form was collected, washed with distilled water, and used in the kinetic experiments. This was done to negate the effect of particle size in the kinetics investigation. Known amounts of sulfuric acid were then added to a specified concentration and allowed to impregnate the prehydrolyzed wood particles. The reactor used in prehydrolysis was used again in cellulose hydrolysis. Duplicate runs were made for each experimental condition. The slurries from the duplicate runs were mixed together and filtered to collect the hydrolyzate to be analyzed for the sugar content. A total of 49 data points were taken, covering three levels of temperature (198, 206 and 215°C), three levels of acid concentration (1, 2, 3% wt), and 3-6 points of reaction time. Sugar analysis was done by HPLC

Acid hydrolysis of hardwood cellulose

17

(Waters Associate, with RI detector) using a BioRad ion-exchange resin column for carbohydrate. Thermal diffusivity measurement The thermal diffusivity (a = k/pCp) of the wood sample was determined experimentally by a dynamic method. In the experimental procedure, a thermocouple was inserted into the center point of the wood sample. Upon heating the sample, the variation of the center point temperature was traced and recorded into a data-logging computer (IBM PC/XT). The experimental set-up was similar to that of our previous study concerning hemicellulose hydrolysis (Tillman et al., 1989). A Sigma thermocouple thermometer (2175A), a signal amplifier/offset unit, IBM data acquisition and control board were incorporated into the experimental setup (Fig. 1 ). The samples were cut into a cylinder form with a diameter to height ratio of 6:1. The circular surface of the sample was insulated with silicone rubber thus making the heat flow unidirectional. Two different wood samples were prepared; one to take the data for heat conduction in the longitudinal direction of the wood (parallel to grain), and the other for the radial direction (perpendicular to grain). The data on variation of temperature with time at the center of the wood chip sample (Fig. 2) were then statistically analyzed using SAS non-linear regression algorithm to determine the thermal diffusivity value. This statistical parameter estimation was done in conjunction with the analytical solution tor one dimensional unsteady-state conduction equation provided in the next section.

RESULTS AND DISCUSSION Kinetics Literature information pertaining to the kinetics of acid catalyzed hydrolysis of cellulosic biomass (Thompson & Grethlein, 1979; Song & Lee, 1984; Connor et al., 1985; T e n g & Muthrathan, 1985) has shown significant disparity due to the kinetic dependence on the species of substrate, method of pretreatment and the reaction conditions. For this reason, a kinetic study specifically applicable to the substrate of this study (southern red oak, pretreated as previously described) was conducted covering a relatively narrow range of experimental conditions: 1-3% sulfuric acid, 198-215°C. These conditions were chosen in consideration of the operating conditions of the TVA biomass conversion pilot plant.

Analoq Digilalto I Convertor

I I

l

Omega

Thermometerl ]~ [Thermoc°up le ThermocoupleI

t

=-

IBM

Personal Computer - - - l " - - ~ W o o d Chip

/

",,,

Fig. 1. Schematics of the e x p e r i m e n t a l m e a s u r e m e n t of t h e r m a l d i f f u s i v i t y of w o o d .

set-up

for

84.00

68.00

[] 2.5 CM

u.T

o 2.0 CM

n¢ l<- 52.00

I.IJ 8.. u.I 36.0C 1-

20.00 0.00

35.~00

7 0 .l0 0

J 105.00

J 140.00

175.00

TIMEI SEC xlO' Fig. 2. Actual temperature profile at the center of w o o d chip in thermal diffusivity experiment: axial direction.

The hydrolysis reaction was modeled as a consecutive first-order reaction: A ~-Z~ B ~ C where A is cellulose, B is glucose, and C is sugar decomposition product. Following the concept established in the previous investigation along these lines, ~ 4 the rate constants k~ and k~ were expressed by Arrhenius equation with the addition of an acid term as follows:

k i=ki,H''exp(-EiR

IT

I)

(i)

where H is the acid concentration (in percentage by weight). The six parameters which characterize the rate constants k~ and k~ are E~, E 2, n~, n~, kin, and k20. A total of 49 data points obtained from nine batch experiments previously described were utilized in the statistical determination of these six parameters. The parameter estimation was performed by a computer program using a non-linear regression algorithm after the method of Ralston and Jennrich (1978). Following the suggestion made by Himmelblau (1970) a

18

A. E. Abasaeed, Y. Y. Lee, J. R. Watson

modified temperature scale was introduced such that ( T - J) was replaced by ( T -~ - To l) for convenience in the computation procedure. The resulting kinetic parameters thus determined (Table 1) were found to be in accordance with those previously reported for dilute acid hydrolysis of cellulosic substrates (Thompson & Grethlein, 1979; Song & Lee, 1984; Connor et al., 1985; Teng & Muthrathan, 1985) in that the activation energy and acid exponent for hydrolysis are higher than those for the decomposition reaction. As shown in a representative comparison between the predicted glucose yield from the model and the actual experimental yield (Fig. 3), the model prediction was in fair agreement with the experimental data, enough to verify that the proposed model with the provision of the statistically determined parameters is a valid representation of the acid hydrolysis of prehydrolyzed hardwood occurring under the specified conditions. More importantly, in similar comparisons made for other experimental conditions, the deviation from the model was found to be random, showing no particular trend. The kinetic parameters were further utilized in the subsequent modeling study.

Table 1. Kinetic parameters for acid hydrolysis of cellulose in prehydrolyzed southern red oak

1 2

31800(_+3300) 29800(_+2900)

1-20(_+0.12) 1.17(_+0.13)

2.85 (-+0.25)x 10 t3 2 - 7 5 ( + _ 0 ' 2 8 ) x 1 0 '2

Effect of heat transfer/particle size M o d e l development In addition to the kinetic study, three major steps (heat balance, thermal diffusivity determination and material balance) were taken in studying the effect of transient heat transfer and particle size on acid hydrolysis of hardwood cellulose. First, a heat balance within a differential segment of the chip results in an equation describing unsteadystate conduction,

OT 02T --=a-at

OX 2

with the boundary and initial conditions of OT

x=0,--=0 Ox

x=L,

0.50

[] I%

(4)

t =0, T= T,

(5)

The analytical solution to eqns (2)-(5) is obtained in the dimensionless form as follows:

,

•

2Bi(Bi2 + 2~)'/2 e x p ( - 2 2 r ) c o s ( 2 , , z ) ( 6 ) ~ = , =, 2 n ( B i 2 + 2 , , + B i )

where @ = ( T - ~)/( T - T0), r = a tL - 2, z = x L - l, B i = h ( k L ) -j

ACID

0.38 )-

t~ o9 0.25 o ....i c9 033

i

0.O0

r

i

I

15.00 30.00 45.00 60.00 REACTION TIME (MINUTES)

(7)

During the computational process, it became clear that the Bi based on the heat transfer coefficient for condensing steam (Kern, 1950; Holman, 1986) was sufficiently high to make the solution the same as that with a boundary condition of T= ~, x = L.

_J

O.O(

(3)

-kOT=h(T-T~) Ox

2 n tan(2n) = Bi, 2,, > 0 Data coverage: 198-215°C, 1 - 3 % H2SO4. 1 for hydrolysis and 2 for decomposition. Error interval is for 95% confidence level.

(2)

75.00

Fig. 3. Reaction progression in hydrolysis of hardwood cellulose at 198°C; model prediction.

Thermal diffusivity determination The thermal diffusivities of southern red oak chips presoaked with water were determined by subjecting the experimental data to eqn (6). The procedure involved a computer-aided calculation in which a best-fit thermal diffusivity value (lumped into the dimensionless parameter, r = a t L - 2) was determined by the SAS/non-linear parameter estimation program. Two different samples, raw wood and hemicellulose-free wood were tested. For the raw wood, the thermal diffusivities in the longitudinal

Acid hydrolysis of hardwood cellulose and radial directions were found to be (2-63 + 0.028) x 10 -7 m 2 s - l and (1.27 + 0.014) × l 0-7 m e s- ~, respectively. T h e error intervals are for 95% confidence level. T h e s e values were in close proximity with a literature value reported for h a r d w o o d (Stamm & Harris, 1953). For hemicellulose-free wood, the thermal diffusivity in the longitudinal and radial directions were found to be (2-73 + 0-04) x 10 -7 m 2 S-1 and (1"74 + 0"03) x 10 -7 m 2 s -I respectively. T h e difference in the thermal diffusivity between raw and hemicelluh)se-free wood may reflect physical changes occurring during prehydrolysis treatment (larger porosity) rather than changes in thermal characteristics. T h e w o o d chips are usually cut in a random direction. T h e r e f o r e the values of the thermal diffusivities (of hemicellulose-free wood) determined for the two different directions were averaged into 2-24 x 10-v m 2 s - ' and used in the subsequent simulation studies.

.~laterial balance T h e temperature profile represented by eqn (6) and the experimentally determined thermal diffusivity data were incorporated into the material balance computation which accounts for the chemical reaction occurring inside the w o o d chip under transient temperature variation. A n important point to be noted here is that the rate constants, k I and k2, are functions of temperature, and therefore change as functions of position within the w o o d chip and reaction time such that

known k~ and k 2, Runge-Kutta fourth-order m e t h o d was employed to solve the two initial value problems of eqns (8)-(10). Simpson's rule was then applied to evaluate glucose concentration (C~) at any time by integrating over the length of the chip. From the results of the computation (Figs 4 and 5 representing two of the various conditions studied) it is clearly seen that a definite trend exists correlating the chip size, sugar yield and the reaction time; as the chip size increases, the maxim u m yield of glucose decreases (loss of yield) whereas the reaction time at which the maximum occurs increases (delay in reaction). It is also noticeable that as the chip size increases, the delaying effect is seen first before loss of yield occurs. These effects are caused by the variation of temperature across the chip. One can visualize this situation as follows. At the initial stage of the reaction, the rate of hydrolysis is highest at thc external surface and progressively decreases as the location moves to the center. However, as

0.48 o

2 d U

¢~ 0 . 2 4

ILl

0.00 30.00 60.00 90.00 120.00 150.00

T h e reaction progress in the w o o d chip is described by

-

-

k IC A

0.12

o.oo

k 2 :=ke(T ) =k2(t, x)

dt

0.36

O U D ._l

k, = k , ( T ) = k l ( t , x )

dCA

19

(8)

REACTION TIME (MINUTES)

Fig. 4. Effect of chip size on glucose },ield anti reaction time at 198°C and 1.0% H2SO ~

A 0.48,

o

d CB - k , CA-- k2C, dt

(9)

,,-fi,

~ ~

2.5 cm~

'

'

24.00

48.00

0.24

with initial conditions of (A = CA0 and C~ = Cu0 = 0, @ t = 0

B 0.36

(10)

These equations were put into dimensionless forms and solved numerically. T h e p r o g r a m was set up such that it first evaluated the temperature at given values of r and z from eqn (6), then the rate constants k j and k 2 for that temperature using the p r e d e t e r m i n e d kinetic parameters. With

(~ 0.12

o.oo

0.00

12.00

60.00

7 5 .0 0

REACTION TIME (MINUTES)

Fig. 5. Effect of chip size on glucose yield and reaction time at 215°C and 3"0% HeSO a.

20

A. E. Abasaeed, Y. Y. Lee, J. R. Watson

Table 2. Comparison of maximumyield and reaction time at maximumyield Conditions

Uniform temp. 0.10 cm 0-15 cm 0.20cm 0.30cm 0.50cm 1.00cm 2'50cm

a--

198°(, 1 " 0 %

198°C, 2"0%

198°C, 3"0%

206°C, 2"0%

215°C, 1.0%

215°C 2"0%

215°C, 3"0%

a

b

a

b

a

b

a

b

a

b

a

b

a

b

1.00 1.00 0.99 0"98 0"96 0.91 0'71 0'29

1.000 1.000 1.000 1.000 1.000 1.000 0"992 0"888

1.00 1.00 0"99 0"97 0-92 0.85 0'54 0"17

1.000 1.000 1.000 1.000 1.000 0.994 0"968 0'781

1.00 1.00 0"97 0.95 0-88 0.76 0'41 0'12

1-000 1.000 1.000 1-000 1.000 0'989 0'934 0"717

1.00 1.00 0.97 0"95 0.87 0.72 0-39 0"11

1.000 1.000 1.000 1.000 1.000 0.994 0-933 0-712

1-00 1.00 0"97 0-95 0-89 0.75 0'44 0"13

1.000 1.000 1.000 1.000 1.000 1.000 0"950 0-740

1.00 0'98 0.94 0"90 0"80 0"55 0-25 0"07

1.000 1-000 1.000 1.000 1-000 0.980 0'870 0"650

1.00 0'95 0'90 0.84 0.70 0.51 0"20 0"05

1.000 1'000 0'970 0.970 0.970 0'960 0-810 0"600

Optimum reaction time under isothermal condition Optimum reaction time under non-isothermal condition

b-

Maximum yield under non-isothermal condition Maximum yield under isothermal condition

reaction time elapses, the hydrolysis reaction zone moves toward the center while sugar decomposition occurs near the surface. The net effect of this is lower yields and longer reaction times than those expected from isothermal conditions. The graphical data also indicate that the effect of particle size (non-uniform temperature distribution) becomes greater as the reaction conditions (acid concentration and/or reaction temperature) become stronger. The computational results summarized in Table 2 give the maximum glucose yield and reaction time at which maximum yield occurs for various reaction conditions and chip sizes. These values were normalized such that a value of unity indicates that there is no effect of particle size (or nonuniform temperature distribution) on either the glucose yield or the reaction time. The criteria concerning the effect of particle size can be set from the table for each experimental condition. For example, at 198°C and 1.0% H 2 S 0 4 , the nonuniform temperature distribution on the reaction time begins to take effect from the chip thickness of 0.3 cm (or L = 0"15 cm), whereas the effect on the yield is not seen until the size reaches 2.0 cm (or L = 1 cm). The results contained in this table would serve as a useful guide for feed preparation, reactor design and operation strategy.

CONCLUSIONS The kinetics of acid hydrolysis of prehydrolyzed hardwood were modeled using consecutive first-

order reactions. The rate constants were correlated by an Arrhenius equation corrected for acid effect as follows: k~ = 2.85 x 1013H 1'2° e x p [ - 31 800 (RT) k 2=

~]

2.75 x 10~2H 1~7 e x p [ - 29 800 (RT)-l]

The hydrolysis reaction was found to be more strongly dependent than the decomposition reaction upon the reaction temperature and acid concentration, as indicated by the magnitudes of the respective activation energies and the acid exponents of the Arrhenius equation. The observed maximum yield of glucose was 45% and it occurred at the highest reaction temperature applied. A theoretical model was developed to study the effects of non-uniform temperature distribution within the wood chip during the acid hydrolysis process. Thermal diffusivities for untreated hardwood in longitudinal and radial directions were experimentally determined to be 2-63 x 10-7 and 1"27 x 1 0 - 7 m 2 s-l respectively. For hemicellulose-free wood they were 2-37 x 1 0 - 7 and 1.74 x 1 0 - 7 m 2 s-J respectively. The simulation results employing these transport and kinetic parameters affirmed that increasing the particle size will result in decreased glucose yields and increased reaction time at which maximum yield occurs. The effect of particle size/heat transfer was intensified by increasing reaction temperature and/or acid concentration. Quantitative criteria were established for the effect of particle size for various reaction conditions.

A c i d hydrolysis of hardwood cellulose

ACKNOWLEDGEMENTS T h i s w o r k w a s c o n d u c t e d as a p a r t o f a c o n t r a c t r e s e a r c h p r o j e c t with T V A , M u s c l e Shoals, Alabama, USA (TVA-TV-69192A), A supplementary support was provided by the Engineering Experiment Station and the Pulp and Paper Research and Educational Center of Auburn klniversity.

REFERENCES Bergeron. P., Wright, J. D. & Werdene, P. J. (19861. Progressive-batch hydrolysis reactor single stage experiments. Biotechnol. Bioeng. A),mp., 17, 33-51. Conner, A. H., Wood, B. F., Hill, C. G. & Harris, J. F. (19851. Kinetic model for the dilute sulfuric acid saccharification of lignocellulosc. J. Wood Chem. Technol., 5, 461-89. Himmelblau, !). M. (197(I). Process Analysis by Statistical Methods. John Wiley & Sons, New York, p. 194. Ifolman, J. P. (19861. 1lear Transfer. McGraw-Hill, New York, p. 638.

21

Kern, D. (1950). l'rocess Heat Tran~l~'r. McGraw-Hill, New York, p. 164. Nadgir, Vilay M. (1985). Process developments in the acid hydrolysis of cellulose. PhD Thesis, Auburn University, USA. Ralston, M. L. & Jcnnrich, R. I. (1978). Dud, a derivative algorithm for nonlinear least squares. 7~'chnometric~, 20, 7-14. Song, S. K. & Lee, Y. Y. (1983). Counter-current reactor in acid catalyzed cellulose hydrolysis. (71era. Eng. ( "OltllHlln., 17, 23-3/). Song, S. K. & Lee, Y. Y. (1984). Acid hydrolysis of wood ccllulose under low water condition. Biomass, 6, 93-100. Stature, A. & Harris, E. (19531. ('heroical Processing q/ Wood. Chemical Publishing Co., Ncw York, p. 153. Teng, K. F. & Muthrathan, R. ( 19851. Kinetics of conversion of high-solids biomass slurries to glucose bv acid hydrolysis. Ener~o' Biomass Waste, 9,873 94. Thompson, D. & Grethlein, H. [L (1979). Design and evaluation of a plug flow reactor for acid hydrolysis of cellulose. hut. Eng. Chem. Prod. Res. l)ev., ! 8, 166--9. Tillman, L. M., Abasaeed, A. E., Lec, Y Y. & Torget, R. (199111. Effect of transient variation of temperature on acid hydrolysis of aspen hcmicellulose. Applied Biochem. Biotechnol., 20, 107-17. Zerbe, J. I. & Baker, A. J. (19871. Investigation of fundamentals of two-stage dilute sulfuric acid hydrolysis of wood. Ener~,y Biomass l~lste.s, l O, 927-47.