Ind. Eng. Chem. Prod. Res. Dev. 1988, 2 5 , 463-469 Wang, F.; Bolognesi, A.; Immirzi, A.; Porri, L. Makromol. Chem. 1981, 182, 3617. Yang, J. H.; Tsutsui, M.; Chen, 2 . ; Bergbreiten, D. E. Macromolecules 1982, 15, 230. Yeh, H. C.; Hsieh, H. L. Polym. Prepr. (Am. Chem. SOC.,Div. Polym. Chem.) 1984, 25(2),52. Yu, G.; Chen, W.; Wang, Y. Kexue Tongbao 1983, 7 , 408.
403
Xie, D.; Zhong, C.; Yuan, N.; Sun, Y.; Xiao, S.;Ouyang, J. Gaofenzi Tongrun 1979, 4 , 233.
Received for review July 22, 1985 Revised manuscript received January 22, 1986 Accepted February 20, 1986
Delignification of Prehydrolyzed Straw by One-Step, High-Consistency Chlorination Vasslllkl Oreopoulou, Dlmllrl G. Economldes, and Emmanuel G. Kouklos" Department of Chemical Engineering, National Technical University of Athens, Athens 106 82, Greece
Barley straw, mildly prehydrolyzed by dilute mineral acid, was delignified by a chlorine-containing gas stream at high consistency. The effects of time, temperature, flow rate, chlorine concentration in the gas stream, and particle size of straw on the one-step conversion of straw lignin to alkali-soluble products were investigated. On the basis of experimentaldata, various kinetic models were tested. The rate-controlling step was found to be internal diffusion of chlorine through the layer of the reacted lignocellulose. A non-Fickean expression was used to fit the experimentally determined values of the diffusion coefficient. I n confirmation to the kinetic analysis in this work, experiments of stagewise chlorination with intermediate extraction by a highly Selective solvent resutted in increased delignification. However, one-step chlorination of prehydrolyzed material is shown to be very efficient in producing substantially delignified straw pulps.
Introduction Prehydrolysis by water and dilute mineral acid solutions has been repeatedly suggested by various research groups as a pretreatment prior to delignification of lignocellulosic materials (Koukios, 1975; Economides, 1977). As it has been shown by experimental work in this laboratory, in order to take full advantage of the effects of prehydrolysis, delignification should be characterized by high selectivity (Koukios and Nicolacopoulos, 1979). A few highly selective treatments have been recently proposed as suitable for prehydrolysis-pulping processes; examples are solvent delignification of wood (Lora and Wayman, 1978 April et al., 1982) and high-consistency chlorination of agricultural residues, e.g., straw (Valkanas et al., 1975; Koukios and Valkanas, 1982). The object of this paper is the quantitative description of the latter by a model based on kinetic data. Chlorination of low consistency (1-6'30 w/w fiber on total aqueous suspension) is an established stage in the bleaching sequences of both wood and nonwood pulps (McGovern, 1967; Reeve, 1983). Kinetic studies of this treatment have given rather contradictory results in determining its rate-controlling step: control by diffusion of chlorine through the aqueous boundary layer (Rapson and Anderson, 19661, internal diffusion of chlorine through the solid phase (Karter and Bobalek, 1971), internal diffusion of the chlorinated lignin products (Rydholm, 1965), and two parallel chemical reactions (Ackert et al., 1975) are major examples of such results. However, the importance of mass transfer has been recognized (Russell, 1966; Reeve, 1983),with a corresponding emphasis on the development of efficient mixing systems (Atkinson and Partridge, 1966; Freedman, 1968; Elliott and Farr, 1973; Paterson and Kerekes, 1984).
* Author to whom correspondence should be addressed. -.
-1
Chlorination at high consistency (20-60'30 ) was found superior to low-consistency treatment in several aspects (Hinrichs, 1962; Liebergott and Yorston, 1965): water and energy economy minimization of reactor effective volume and effluent quantity and higher chlorine and lower hypochlorite concentrations resulting in limited attack to carbohydrates. Medium-consistency (8-15 % ) bleaching by chlorination has been recently developed and industrially applied to take advantage of the benefits of processing at higher fiber-to-water ratios (Gullichsen, 1983; Torregrossa, 1983). The development of high-consistency systems can thus be viewed as a future step in that direction. In particular, due to their definitely higher selectivity the latter systems could be used for bulk delignification of low-lignin nonwood fiber like straw. Prehydrolysis, moreover, seems very promising as a pretreatment, as it removes a significant part of hemicellulose, lignin, and extraneous materials, and enhances reactions of lignin with chlorine to alkali-soluble products (Oreopoulou, 1985). Therefore, the formulation of an adequate model for high-consistency chlorination of prehydrolyzed straw could contribute to the continuing discussion on the viability of biomass refining (Koukios, 1985).
The M o d e l The reacting system in high-consistency chlorination of lignocellulosic materials consists of (1)the solid phase of lignocellulose, Le., prehydrolyzed straw; (2) the aqueous film surrounding lignocellulose; and (3) the gaseous chlorinating agent, containing chlorine gas. In modeling this system, we have made the following assumptions: 1. The geometry of straw pieces is that of a long cylinder. This assumption can be experimentally justified in any case. 2. Lignin is uniformly distributed throughout the solid phase, so that its molar density can be considered as constant. 0 1986
American Chemlcai Society
464
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
The molecular diffusion coefficient D, in this case, is expected to be a function of the thickness of the already reacted layer, z. We should note that polysubstituted and oxidized reaction products may have molar volumes greater than that of straw lignin. The correlation adopted here is therefore a non-Fickean one
-straw-,
7
D = Do( 1 L
-
The rate of chlorine diffusion at steps I and I1 can be expressed as (Figure 1)
I
=I D-dC
dr
dNIII 2xLDon(C, - C,) dt
n#O
(6)
The rate of chemical reaction of chlorine with lignin at step IV can take the following form (7) where k l is the corresponding pseudo-first-order rate constant, incorporating the effect of lignin concentration on the reaction rate (constant, according to assumption 2). When a steady state, or a quasi-steady state, is reached, the previous eq 2-7 can be combined to give an expression for the overall rate of chlorine consumption:
cw -
--
dt 2aLPy,
H
1 RHk,
-+-+
1
1 In R -In rc
Rk1
DO
+-
1
,
n=O
rckl (8)
m
--
dt
2TLPYg
=---
H
X
1
+ - 1+
1
RHk,
Rk,
\
/
+ )-l
nDo
,
+1
n
+ 0 (9)
rCkl
Chlorine consumption can be also expressed with the aid of the stoichiometry in reaction 1 b dt
1 dbVJ
b
dt
-
1 d ( r r C 2 L ) 2.rrLprCdr, = -b dt
b P d t
(10)
For the usual reaction temperature of 293-313 K, Henry's Law is valid for chlorine gas (Perry, 1973): CI = PYl/H (34 The rate of chlorine diffusion through the reacted layer at step I11 can be expressed as I
(5)
--
dt
(3)
dt
n=O
cw - 1 CW, _
ZTRL dt - kgwYg - Y1) W I
~
(44
In r, - In R '
dt
3. Both the diffusion of chlorine and the chemical reaction proceed only in the radial direction of the straw pieces. 4. The chemical reaction of lignin with chlorine is irreversible. 5. Chlorination products remain at the position of their original formation; hence, the structure and geometry of straw particles is unchanged during the reaction. 6. As it can be seen from the literature, the reaction of lignin with chlorine is very fast; therefore, we can consider a reaction front, or a thin reaction zone, moving to the center of the cylinder leaving behind it a growing layer of chlorinated material. 7 . The chemical reaction is first order with respect to chlorine concentration. 8. Due to the conditions adopted in this work, molecular chlorine is the only reactant in the chlorination medium. By taking into account the previous assumptions, we can develop a model based on the following succession of steps: (I) diffusion of chlorine through the gas boundary layer surrounding wet straw to the surface of the aqueous film; (11) dissolution of chlorine molecules in the liquid film, followed by diffusion through this phase to the surface of the straw particles; (111) penetration of chlorine in the lignocellulosic structure and diffusion through the layer of already reacted material to the surface of the unreacted core; (IV) chemical reaction of chlorine with lignin at the reaction front to form alkali-soluble products according to the general equation C1, + blignin chlorinated and oxidized products + pHCl (1)
1
i)n
- 2TLD,(C, - C,)
cwIII
--
Figure 1. Profile of chlorine concentration in the reacting system.
2arL
= Do(
where Do and n are appropriate correlation constants to be determined. Integration of (4) between r = R , C = C, and r = r,, C = C, gives
iI
1
g)n
(4)
where dNJdt gives the number of lignin moles reacting per unit of time, p is the molar density of lignin, and V , is the volume of the unreacted core. Moreover, a conversion coefficient x for lignin reaction can be defined as follows:
By combining eq 8 or 9 with eq 10 and 10a and integrating between t = 0, r, = 0 and t = t, r, = r,, we obtain
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
R 2k,H
-x
~~
R2 + -Rx + -[(l - x) In (1 - x ) + x ] + 2k1
I
40,
R
[(I - *)(1-n/2)
(b) rate control by mass transfer through the liquid film (step I n
R
bPYg
= -t
PH (c) rate control by mass transfer through the reacted layer ( s t e p 111, internal diffusion) R2
-[(1 4D0
- x) In (1- x)
bPYg
+ x ] = -t,P H
m
- 13 -
The four terms at the left-hand side of (11)and (12) represent the four types of resistance to the process. Equations 11and 12 can be simplified in the specific cases where one of the resistances becomes rate-controlling: (a) rate control by mass transfer through the gas film (step 0
-[x] 2kl
465
D = Do
D = Do(;)"
(15)
(16)
In (16), the best fit of data was found for n = 3. (d) rate control by chemical reaction ( s t e p I V )
We should note at this point that the possibility for control by chemical reaction is doubted by Froment and Bischoff (1979), since such an assumption is contradictory to the existence of a sharp boundary between reacted and unreacted zones. On the other hand, Levenspiel (1972) treats this case as possible.
Experimental Section The raw material used in this paper was barley straw of the following composition (% w/w, dry basis): acidinsoluble lignin (ash-free) 13.9, acid-soluble lignin 3.0, ash 5.7, pentosans 27.0, hexosans 43.5. The straw was ground in a laboratory hammer mill and screened, and the fraction between 0.125 and 0.630 mm was retained for further treatment. The average post-grinding length-to-diameter ratio was found to be 301. In particular, 45% of the straw particles had a ratio of 2O:l-100:1, 25% 1O:l-20:1, 15% 7:1-lO:l, 6% 5:1-lO:l, and only 9% had a ratio of less than 5:l. Therefore, the first assumption of the model, about the long cylindrical particle, is justified by microscopic measurements. Prehydrolysis was carried out in a 5-L stainless steel autoclave at 418 K for 15 min with 0.035 N H2S04and a liquid-to-solid ratio of 12.2:l. Those conditions have been
-
A . C h I o r i n s feeding vessel , B p r e s s u r e g a u g e , C f l o w m e t e r s , 0 . v a l v e s I i t h r e e w u y v a l v e s . F : g a s mixing v e s s e l , E : Nipreheater .W:warer b a t h . K chlorination reactor. M thermocouple. T , t h e r m o m e t e r s , L , wash.bottles
1
Figure 2. Experimental apparatus.
found optimal for hemicellulose hydrolysis without any undesirable effect on cellulose and lignin (Koukios, 1981). The residue from prehydrolysis (yield 68%) was extracted with ethanollbenzene 1 / 2 (v/v) for 4 h in a Soxhlet apparatus for the removal of the substances that could possibly interfere with lignin determinations in subsequent stages. Chlorination experiments were carried out with wet prehydrolyzed straw, at consistency 20% f 2 % (dry solid to total weight). The apparatus shown in Figure 2 was used. The chlorination reactor is made of glass with a porous glass bottom of G2 porosity and a cover made of PVC. Straw particles form a 1-cm-deep fixed bed at the bottom of the vessel. Chlorine gas is thoroughly mixed with nitrogen before entering the reactor, and the pressure and flow rate of each gas are measured before mixing. Reaction temperatures higher than room conditions were kept constant in the reactor with a hot foil connected to a Variac. The gaseous mixture, of the desired temperature, composition, and flow rate, passed through the lignocellulose bed for a certain period of time. The reaction was terminated by passage of a pure nitrogen stream for 1 min and then addition of a solution of 2% Na,S03. The main parameters investigated in this work were gas flow rate, mole fraction of chlorine in the gas mixture, chlorination time, and reaction temperature. A number of chlorination runs were carried out with straw of varying particle size. Another series of experimental runs consisted of repeated chlorinations with intermediate extraction of chlorolignins, first by ethanol/monoethanolamine 97/3 (v/v) and then by ethanol. A detailed analysis of the equipment and procedures for this stage is given elsewhere (Oreopoulou, 1985). Chlorinated lignocellulose was thoroughly washed with water, then extracted with 0.12 N NaOH at 323 K for 1 h (1iquid;solid 37:l) and, finally, washed again with water to give a delignified pulp. Acid-insoluble-lignin content in the pulp was determined according to TAPPI Standard Method T222, and acid-soluble-lignin content was determined as described in TAPPI Method UM 250. K number was also determined, as a measure of the oxidizable lignin content of the pulp, according to TAPPI Standard Method T 236, as modified by Berzins (1966).
Results and Discussion Fractional conversion data based on acid-insoluble lignin, total lignin (acid-insoluble plus acid-soluble) and K number (oxidizable lignin) determinations are plotted in Figures 3-5, respectively, as functions of reaction time at various mole fractions of chlorine in the gas phase. It can be seen that all three conversion expressions follow the
466
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
37-
07 06
- 05 0 C
?LOL a 0
moi f r a c t i o n o f C l z
Y I
X
03
mol
fraction of
LI2
0.2
v
99
0
v
01
0
2
5
75
10
15
20
t (mini
Figure 3. Effect of mole fraction of chlorine in gas phase on the rate of conversion of insoluble lignin at 293 K.
1
I
1
0.0I
,
0
2
5
75
10 t (mini
89 99
20
15
Figure 5. Effect of mole fraction of chlorine in gas phase on the rate of conversioii based on IC no. (oxidizable lignin) at 293 I
