Acid hydrolysis of sunflower residue biomass

Process

Biochemistry

28 (1993) 243-247

Acid Hydrolysis

of Sunflower Residue Biomass

L. JimCnez & J.L. Bonilla Departamento de Ingenieria Quimica, Facultad de Ciencias, Universidad de Grdoba, Avda San Albert0 Magno s/n, E-14004 Ckdoba, Spain (Received 20 May 1992; accepted 29 June 1992)

The kinetics of acid hydrolysis of sunflower stalks with HCI (0.5-6% by weight) and H,SO, (2-8 % by weight) at temperatures between 1 IO and I40 “C was studied. The experimental results obtained were consistent with the following two consecutive$rst-order reactions Cellulose residue a Reducing sugars 2 Decomposition products Constants KI and Kz (in h-l) were found to be related to the temperature (in K) and the acid concentration (C, in per cent weight) through the following equations: Hydrochloric acid: KI = (10.Z8+4.53C1’53) x I01Zexp(-101~21/RT) K2 = (14,16+ 1.67C”55) x IO’exp(-677-85/RT) Sulphuric acid: K, = (964.39 + 7*69C2 ““) x lOlo exp( - 101.31 /RT) K, = (604.42 + 4.84P30) x lo7 exp( - 72*84/RT) where the activation energies are given in kJ/mol. The experimental sugar concentrations obtained on hydrolysis were reproduced with errors less than 15 and 20 % for HCI and H,SO,, respectively.

INTRODUCTION

2.8 x 10’ kg/year. These residues currently have no specific use, so they are normally piled up and burnt out on the spot. Sunflower stalks typically consist of cellulose (38.13 %), hemicelluloses (2963 Oh) and lignin (11.03 %).* Cellulose is a glucose polymer; hemicelluloses are polymers of various pentoses, hexoses and acid sugars; and lignin is a polyphenol macromolecule that prevents cellulose from being attacked by certain chemical agents.5 The high, yet incompletely exploited energy potential of these residues should be studied with a

The national Spanish sunflower seed production is estimated to be 1.2 x 10’ ton/year, of which the Andalusian region produces over 62% and the province of Seville nearly 30 %.’ Taking into account that the stalk/seed weight ratio is typically about 2*3,2,3 the national Spanish production of sunflower stalks must therefore be about Corresponding author: 218625,

ext. 217;

ProcessBiochemistry

Fax:

Dr L. Jimknez. 957-218606.

Telephone:

(957)

243 0032-9592/93/.$6.00

0

1993 Elsevier Science Publishers Ltd, England

244

L. Jidnez.

view to transforming their chemical energy into other types of more useful and direct energy via physico-chemical or biochemical processes. The biochemical synthesis of alcohols from agricultural residues entails conditioning of the starting material by physical means, hydrolysis of cellulosic components to sugars and fermentation of the sugars to alcohols to be finally concentrated for use as fuels or chemicals. Interest in application of this process to residual biomass is demonstrated by a number of recent publications.6m10 In this work, we studied the hydrolysis step involved in the synthesis of alcohols from sunflower stalks with hydrochloric and sulphuric acid at various concentrations and temperatures. Taking into account that the starting cost of these residues is zero and that the handling involved in their collection, piling and burning is unavoidable, using these residues to obtain alcohols would increase handling costs only by the sums expended to transport them to the target factory. For a relatively small area with a high concentration in these residues, such as the province of Seville, the cost of these materials at factory price would be reasonably low. In fact, Seville could use nearly 2.3 x 10’ kg of sunflower stalks per day; assuming holocellulose were transformed into sugars and then into alcohols with a yield of only 40% for both conversions, then the alcohol production of this province could be increased by 9 x IO7 kg/year.

J.L. Bonilla

apparent viscosity of the suspension. We performed two series of experiments aimed at determining the influence of the acid concentration and the temperature, respectively. The reaction was monitored by titrating reducing sugars at various times by the Somogyi-Nelson method.‘l RESULTS

AND DISCUSSION

Hydrolysis with hydrochloric acid The influence of the acid concentration on the process yield was investigated by carrying out five experiments at the same temperature (120 “C) but using various acid concentrations, namely 0.5, 1, 2, 4 and 6% by weight. Figure 1 shows the variation of the sugar concentrations obtained throughout the process. Each experiment was carried out in quadruplicate, so each point on the graph represents the average of four results. The standard deviations found were always smaller than 0.015. The particle sizes used were small enough for the acid diffusion into the particles and that of the sugars formed to the outside to be negligible. This was checked experimentally by using even smaller particles, which yielded maximum sugar concentrations of the same order as those obtained in this work. Therefore, taking into account the shape of the curves to which the experimental results can be fitted, the acid hydrolysis involved can be assumed to take place via two pseudo-homogeneous consecutive reactions :

EXPERIMENTAL Materials The sunflower stalks used were sun-dried to a moisture content of less than O-12 kg water/kg dry solid and ground to a particle size of less than 0.84 mm in a hammer mill. Apparatus We used a 150-ml Berghof reactor provided with magnetic shaking and a sample withdrawal system. The reaction temperature was monitored by means of a NiCr/Ni thermocouple introduced in the reactor, the reactor was equipped with a heating device that allowed the desired working temperature to be selected. Procedure All experiments were carried out with a suspension with a solid content of 4 %, as higher concentrations gave rise to incomplete mixing through increased

where A, B and C denote the residue (cellulose), the sugars formed and their decomposition products, respectively, and Kl and K, are the kinetic constants of the process. According to Seaman,l’ these two reactions are first order and irreversible; thus, the sugar concentration (g sugar/g potential sugars) will vary with time (in h) according to C,/C,,

= [K,I(G

-

&)I kw( - fG t>- exp(- K, 01 (1)

Where C, denotes the sugars formed concentration and C,,, the potential sugars concentration in the cellulose residue. We fitted the experimental C, and t values to eqn (1) with the aid of the program TSP (Time Series Processor, Version 4.0). The K, and K2 values (in h-l) thus obtained are listed in Table 1, which also

Acid hydrolysis of sunfIower residue biomass

245

0.3 -

$A 0.26 e Tj 5

0.2 -

z 0.16 -

p ? w s

0.1 -

Y L.7

0.06 -

0’ 0.2

0

0.4

0.6

6.6

1

1.2

1.4

0

1.0

0.2

0.4

0.6

0.a

t. h

L

I

1.2

1.4

1.6

h

Fig. 1. Variation of the sugar concentration during the hydrolysis of sunflower stalks with HCl at various concentrations and 120 “C.

Fig. 2. Variation of the sugar concentration during the hydrolysis of sunflower stalks with 2% (w/w) HCI at various temperatures.

Table 1. K, and K, Values Obtained for the Hydrolysis of Sunflower Stalks with HCl at Various Concentrations and Temperatures

similar procedure to that described above we obtained the points in Fig. 2 and the corresponding KI and Kz values, which are also given in Table 1. The K, and K, values are related to the temperature according to the Arrhenius equation:

0.5 1.0 2.0 4.0 2.0 2.0 2.0

120 120 120 120 110 130 140

0.410 0.545 0,765 1,646 0.415 1.685 4.172

1.403 1.609 1.747 2.580 1.217 2.954 5.735

0.97 o-98 0.98 0.88 O-96 0.83 0.90

193.1 284.5 198.3 46.4 163.3 24.3 36.2

15.1 20.6 17.2 19.1 14.3 9.5 25.1

11.0 171 13.6 19.4 9.6 11.4 25.0

gives the correlation coefficients (P) and Snedecor’s F and Student’s t values. The KI and K, values calculated from eqn (1) for the experiments are plotted in Fig. 1, where they are compared with the experimental C, values. The K, and K, values in Table 1 are related to the acid concentration (per cent weight) through the following equations: Ki = 0.36+ 0.16C’53 K, = 1.36+0.16C”“”

(r2 = 0.99) (r2 = 0.98)

(2) (3)

The errors made in estimating the KI and KS values from eqns (2) and (3) with respect to the experimental values were always less than 10 %. To determine the influence of the temperature on the process yield we carried out four experiments using the same HCl concentration (2%) and four temperatures, i.e. 110, 120, 130 and 140 “C. By a

KI =

2.40 x lO’“exp(-

101.21/RT)

KI =

1.99 x lO”exp(-67.85/RT)

(r” =

0.99)

(r’ =

(4) 0.99) (5)

(where the activation energies are given in kJ/mol). The errors made in estimating KI and K, from eqns (4) and (5) with respect to the experimental values (Table 1) were always less than 10 %. If the influence of the temperature on KI and K, is assumed to be independent of the acid concentration used, then eqns (2~(5) can be used to relate the constants to both the acid concentration and the temperature: KI Kz

=(10.18+4.53CV3) =(14.16+

x 101”exp(-

1.67C155)x

101*21/RT)

lO’exp(-67.85/RT)

(6) (7)

By substituting these expressions into eqn (l), we found that the experimental C, values could be reproduced with errors less than l&l 5 % (Fig. 3). The differences between the activation energies E1 and E, obtained in this work and those reported by SeamanI’ for wood (179.50 and 137.60 kJ/mol, respectively) should be attributed to the different degree of crystallinity of the cellulose present in the

L. Jimdnez, J.L. Bonilla

a.2

0.L

0

0.5

experimental

c&/c,,

Fig. 3. Sugar concentrations as calculated from eqn (1) compared with those obtained experimentally by hydrolysis of sunflower stalks with HCl. 0

0.2

0.4

0.5

0.8

1

1.2

t,

h

1.4

I.6

1.a

2

2.2

Fig. 5. Variation of the sugar concentration during the hydrolysis of sunflower stalks with 8 % (w/w) H,SO, at various temperatures.

0

0.2

0.4

0.6

0.8

1

1.2 L

1.4

1.6

1.0

2

2.2

2.4

h

Fig. 4. Variation of the sugar concentration during the hydrolysis of sunflower stalks with H,SO, at various concentrations and 120 “C.

Fig. 6. Sugar concentrations as calculated from eqn (1) compared with those obtained experimentally by hydrolysis of sunflower stalks with H,SO,.

Table 2. IT, and K2 Values Obtained for the Hydrolysis of Sunflower Stalks with H,SO, at Various Concentrations and Temperatures

materials used in each case. Thus, the higher crystallinity of cellulose in wood compared with that in sunflower stalks makes it more resistant to acid attack.

2.0 4.0 6.0 8.0 8.0 8.0 8.0 8.0

120 120 120 120 110 115 130 140

0.349 0.480 0.776 1.374 0497 0.719 2.655 4.879

1.310 1.506 1.865 2.485 1.326 1637 4,313 6.634

0.95 0.97 0.87 0.85 0.87 0.80 0.94 0.83

95.1 115.1 33.1 23.6 39.0 24.0 74.5 18.9

10.2 12.9 11.5 11.7 8.5 8.6 27.4 17.2

11.9 13.2 116 13.7 IO.9 9.9 27.8 22.1

Hydrolysis with sulphuric acid We carried out two sets of experiments: one in which four H,SO, concentrations (2, 4, 6 and 8 % by weight) and the same temperature were used; and the other in which we used the same acid concentration (8 %) at four temperatures (110, 120, 130 and 140 “C). By applying a similar treatment to that described above we obtained the results shown

Acid

in Figs 4 and equations :

5, Table

2 and

qfsunflower

hydrolysis

the following

ICI = O-33 + (2.63 x 1O-3 ,.=)

(r2 = 0.99)

K, = 1.26+(1.01

r2 = 0.99)

x 10-“CY”)

(9) 0.99)

3.

Kz = 1.13 x lOlOexp( -72,84/RT)

(10) (r2 = 0.99)

K,

=

I.

2.

(r2 =

1010exp(-101~31/RT)

(604.42 + 4.84C23”) x lo7 exp( - 72*84/RT)

(11) (12)

4.

5. 6.

(13)

The KI and K, values obtained from eqns (8k(ll) reproduced the experimental values (Table 1) with errors less than 10 % in every case. As can be seen, the activation energies found are of the same order as those obtained in the hydrolysis with HCI. Substitution of eqns (12) and (13) into eqn (1) resulted in theoretical values which reproduced the experimental C, values with errors less than 10-20 % (Fig. 6). As can be seen from the calculated C, values provided by Eqn (1) (see Figs 1, 2,4 and 5), the maximum sugar, yields obtained under similar acid concentration and temperature conditions were always higher for HCl than for H,SO,. Also, such maximum yields were reached in shorter times when using HCl, and these times decreased with increasing acid concentration and temperature.

biomass

247

REFERENCES

(8)

KI = 3.41 x 1013exp( - 101*31/RT)

KI =(964.39+7.69Ps8)x

residue

I.

8.

9.

10.

11. 12.

Anon., Anuario de Estadistica Agrario. Ministetio de Agricultura, Pesca y Alimentacion, Madrid, 1989. Gachon, L., La cinetique de I’absortion des elements nutritib majeurs chez le tourncsol. Ann. Agron., 25 (1972) 547-65. Gonzblez, P., Jurado, F. & Magallanes, M., Influencia de la fertilization fosforica y potisica en la acumulacion de materia seca de1 girasol. An. Inst. Nat. Inves. Agr. Prod. Veg., 11 (1979) 10516. Jimenez, L., Sanchez, I. & Lopez, F., Characterization of Spanish agricultural residues a view to obtained cellulose pulp. Tuppi J., 73(8) (1990) 173-6. Fan, L. T., Gharpuray, M. M. & Lee, Y. H., Cellulose Hydrolysis, Springer-Verlag, New York, 1987. Brennan, A. H., Hoagland, W. & Schell, D. J., High temperature acid hydrolysis of biomass using an engineering-scale plug flow reactor. Biotechnol. Bioeng. Symp., 17(8) (1986) 53-70. Teng, K. F. & Mutharasan, R., Kinetics of conversion of high-solids biomass slurries to glucose by acid hydrolysis. ,!%erg~? Biomass Wastes, 9 (1985) 873-94. Wright, J. D. & Power, A. J., Comparative technical evaluation of acid hydrolysis processes for conversion or cellulose to alcohol. Energ_y Biomass Wastes, 10 (1987) 949-7 1. Kim, S. B. & Lee, Y. Y., Hydrolysis of hemicellulose by solid superacid. Biocechnol. Bioeng. Symp., 15(7) (1986) 8 l-90. Gonzrilez, G., Lbpez-Santin, J., Caminal, G. & Sola, C., Dilute acid hvdrolvsis of wheat straw hemicellulose at a moderate tempera&e: a simplified kinetic model. Biotrchnol. Bioeng., D(2) (1986) 288-93. Marais, J. P., Wit, J. L. & Quiche, G. V., Analysis of lienocellulosic materials. Anal. Biochem.. 15 (1966) 373-9. S&man, J. F., Kinetics of wood saccharificakon.‘Hydrolysis of cellulose and decomposition of sugars in dilute acid at high temperature. Znd. Eng. Chem., 37(l) (1945) 43--54.