I n d . Eng. Chem. Res. 1990,29, 156-162
156
of magnitude higher than that for the adsorption of the perpendicular species.
Nomenclature a = fraction of surface poisoned by adsorbed species C = concentration of thiophene in pores, mol m-3 Co = concentration of thiophene in the influent stream, mol
m-3 De = effective diffusivity, m2 s-l f = fraction of surface susceptible to adsorption k = rate constant, m3 m-2 of Ni k , = mass-transfer coefficient, m s-l q = mass of adsorbed species, mol (g of catalyst)-' r = radial position, m R = catalyst pellet radius, m s, = nickel surface area of the catalyst, m2 (g of catalyst)-' t = time on stream, s Greek Symbols a = Ni/C4H4Sadsorption stoichiometry tp
p
= catalyst void fraction = catalyst density, g of catalyst m-3
Subscripts 1 = perpendicular thiophene 2 = coplanar thiophene Registry No. Ni, 7440-02-0; thiophene, 110-02-1.
Literature Cited Ahmed, K. The mechanism and kinetics of thiophene adsorption on nickel at ambient temperatures and pressures. Ph.D. Thesis,
University of London, U.K., 1987. Ahmed, K.; Chadwick, D.; Kershenbaum, L. S . Proceedings of the 4th International Symposium on Catalyst Deactivation, Antwerp; Elsevier: Amsterdam, 1987; p 513. Ahmed, K.; Kershenbaum, L.; Chadwick, D. Chem. Eng. Sci. 1989, 44(4), 999. Bartholomew, C. H.; Pannell, R. B. J. Catal. 1980,65, 390. Bourne, K. H.; Holmes, P. D.; Pitkethly, R. C. Proceedings of the 3rd International Congress on Catalysis; North Holland: Amsterdam, 1954; Vol. 2, p 1400. Conrad, H.; Ertl, G.; Kippers, J.; Latta, E. E. Surf. Sci. 1976,57,475. Derbeneva, S . S.; Karakchiev, L. G.; Kuznetsov, B. N.; Yerlnakov, Yu. I. Kinet. Katal. 1975, 15,667. Klier, K.; Zettlemoyer,A. C.; Leidheiser, H., Jr. J. Chem. Phys. 1970, 52, 589. Lyubarskii, G. D.; Avedeeva, L. B.; Kulkova, N. B. Kinet. Katal. 1962, 3, 123. Madden, H. H.; Kippers, J.; Ertl, G. J. Chem. Phys. 1973,58, 3401. Primet, M.; Dalmon, J. A.; Martin, G. A. J. Catal. 1977, 48, 25. Rochester, C. H.; Terrell, R. J. J. Chem. Soc., Faraday Trans. 1977, 73, 596. Tracy, J. C. J. Chem. Phys. 1970,56, 2736. Viveros-Garcia,T. Adsorption of sulphur compounds on nickel catalysts: poisoning and regeneration studies. Ph.D. Thesis, University of London, U.K., 1985. Weng, H. S.;Eigenberger, G.; Butt, J. B. Chem. Eng. Sci. 1975,30, 1341. Yates, J. T.; Garland, C. W. J. Phys. Chem. 1961, 65, 617.
Received for review December 19, 1988 Revised manuscript received August 10, 1989 Accepted August 23, 1989
Organosolv Pretreatment for Enzymatic Hydrolysis of Poplars. 2. Catalyst Effects and the Combined Severity Parameter Helena L. Chum,*David K. Johnson, and Stuart K. Black Chemical Conversion Research Branch, Solar Energy Research Institute, 161 7 Cole Boulevard, Golden, Colorado 80401
The effects of added catalysts such as sulfuric acid, phosphoric acid, and their acid salts to methanol-water delignification of poplars in the 160-170 "C temperature range and 1-2.5-h residence time range are discussed in semiquantitative terms, through the modification of the severity parameter R, = t exp[(T, - Tb)/14.75], where T, is the reaction temperature and Tb is the temperature chosen for reference as a base case, by the pH of the pulping liquor at 40 "C. This function is used to explain lignin and xylan removal from poplars over a wide range of experimental conditions and trends in the rate of hydrolysis of the pretreated cellulosic residues by enzymes. Such parameters unify time, temperature, and a simple acidity function such as pH. They are a useful tool in planning experiments and increasing the data base necessary for the more complete understanding of such processes. The fractionation of lignocellulosics into their main polymeric constituents can be achieved through delignification with aqueous organic solvents. Organosolv pulping has been investigated (Kleinert and Tayenthal, 1931,1932; Kleinert, 1967, 1972, 1974a,b, 1977; Lora and Aziz, 1985; Aziz, 1987; Johansson et al., 1987) as a method for the production of pulp both in the presence and absence of catalysts, as well as in two-stage processes (Feckl and Edel, 1987). In addition, several researchers have assessed this method of fractionation for the production of cellulosic substrates that can be easily hydrolyzed by enzymes (Holzapple and Humphrey, 1984; Chum et al., 1987a,b, 1988) in biotechnological routes to fuels and chemicals. The conditions that lead to cellulosic materials that undergo fast enzymatic hydrolysis are removal of more than 50% of the original xylan fraction in the starting ligno0888-5885/90/2629-0156$02.50/0
cellulosic material and lignin removal, also found to be helpful, but the dependence of the enzymatic hydrolyzability on the xylan content of the pulp was shown to be more pronounced than that of lignin for these substrates. The fractionation of lignocellulosics by steam-aqueous pretreatments has been reviewed by Overend and Chornet (1987), who proposed an unified reaction ordinate to compare different pretreatments without the addition of inorganic or organic catalysts. Much earlier, Vroom (1957) described the use of an "H"factor as a means of expressing cooking times and temperatures as a single variable. Overend and Chornet (1987) chose a reaction ordinate based on those employed in chemical pulping processes such as prehydrolysis kraft and also kraft pulping (Brasch and Free, 1965). As observed for these processes, the temperature-time variables can be combined into a single 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990 157 reaction ordinate, which can be a practical guide to evaluate the severity of the treatment. H factors can guide cook times and allow the operator to adjust cooking cycles, when operational difficulties dictate changes from standard heating profiles. The concept assumes that the overall kinetics has a first-order dependence on the concentration of the key reaction components (e.g., hydrolysis of lignin, lignin-carbohydrate complex, or carbohydrates) with an Arrhenius-type rate constant dependence on temperature. The activation energy itself can be a function of the temperature. The use of the severity parameter R, = t exp[(Tr - Tb)/14.75], where T, is the reaction temperature and T b is the temperature chosen for reference as a base case, has been proposed. It has led to simple correlations that describe dissolution of polymers, residual polymers, and some chemical properties of these materials, regardless of the type of treatment, e.g., steam explosion in a batch or continuous operation, autohydrolysis, or solvent delignification for several lignocellulosicsubstrates in the absence of catalysts (Heitz et al., 1987,1988; Overend and Chornet, 1987; Bouchard et al., 1988; Thring et al., 1989). In this paper, we present an extension of such an unified reaction ordinate to include catalyst effects in a simple and approximate phenomenological way. Data from organosolv fractionation of poplars both uncatalyzed and with added inorganic catalysts are interpreted with the help of combined severity and acidity function parameters to explain the effects of catalyst concentration, liquor-to-wood ratio, alcohol concentration, and batch vs semicontinuous operation. The combined parameters also correlate with the extent and rate of enzymatic hydrolysis of the cellulosic residues and, thus, provide a useful indication of the effectiveness of pretreatment.
Results and Discussion Severity Parameter: log R,. The severity parameter is defined by Overend and Chornet (1987) based on the definition of the " P factor (not to be mistaken with pressure) P = exp[(Tr - Tb)/14.75] by Brasch and Free (1965) as R, = t exp[(T, - Tb)/14.75] (1) where T, is the reaction temperature (degrees Celsius), T b is the reference temperature (e.g., 100 "C), and t is the time (minutes) at temperature TI. This factor has been derived assuming that the overall kinetics follows a first-order law concentration dependence on key reaction components with an Arrhenius-type rate constant dependence on temperature; the apparent activation energy may be a function of the temperature. The application of these parameters utilizes a finite, large At, and thus, the treatment is not one of initial rates but of global rates and is necessarily just a useful approximation and not rigorous kinetics. The P factor is a relative rate function, with the reference rate being that of the reaction a t the base temperature (Tb). From 100 to 270 "C, ( T ,- 100)/14.75 is linearly related to the inverse of the absolute temperature as follows: (T, - 100)/14.75 = -(13.7 f 0.3)[1000/(T1 + 273)] (36.2 f 0.4) (2)
+
with an ? of 0.99. The right-hand-side of eq 2 corresponds to the conventional first-order kinetics and Arrhenius-type dependence: Pol
k
products
(3) (4) d[Pol]/dt = +[Pol] dt where [Pol] is the polymer concentration at time t and k is the first-order rate constant for the polymer dissolution.
Equation 4 can be expressed in an integrated way: In ([Pol]/[Pol],] = A exp[-E,/R(T, + 273)lAt (5) with an associated activation energy of about 27 kcal/mol, in which [Pol], is the initial polymer concentration. The right-hand-side exponential term in eq 5 is directly proportional to the exponential term in the severity parameter through eq 2. The organosolv delignification of poplars was shown to have an activation energy of 25 kcal/mol at 0.01 M HzSO4, which decreased with increased acid concentration (e.g., to 15.8 kcal/mol for 0.05 M HzS04);in deashed cottonwood, an activation energy of 19.2 kcal/mol was measured (Tirtowidjojo 1984; Tirtowidjojo et al., 1988). If we assume that the polymer dissolution involves the hydrogen ion activity as a first-order reaction, Pol
+ H+
k'
products
(6)
(7) d(Pol)/dt = -k'[Pol][H+] dt or in an integrated way In ([Pol]/[Pol],} = A'[H+] exp[-E,/R(T, + 273)IAt (8) we can derive a combined severity parameter that includes time and temperature parameters and the hydrogen ion activity parameters as In {[POl]/[POl],}cx [H+] eXp[(Tr - Tb)/14.75]At -t constant (9) Equation 9 can be represented as the sum of the log of the severity parameter minus the pH of the pulping liquor at temperature or another appropriate acidity function for the solutions under the pretreatment conditions. See Chum et al. (1990) for a more detailed theoretical treatment. As a first very practical approximation, the pH of the liquor can be employed as a measure of the hydrogen ion concentration for methanol-water solutions of dilute acids or well-buffered solutions. The measure of the relative hydrogen ion activity in aqueous alcoholic solutions is possible with the usual glass electrode, provided that the alcohol concentration is below 90 w t % (93.2 vol %) (Bates, 1973). The choice of scale for reference has been discussed in detail by Bates (1973);up to 65 w t % (72 vel%) alcohol, the liquid junction potential and the medium effects on the hydrogen ion are under 0.15 pH unit, and corrections can be made for near-room-temperature measurements. Such measurements were employed to generate the numbers in Table I. Alternatively, Hammett acidity functions could have been employed (Rochester, 1970; Harned and Owen, 1958) from data obtained from spectroscopic or kinetic properties of suitable systems in these media at the appropriate temperatures. Such data, for the very wide range of temperature, catalysts, and solvent systems employed in these experiments, are not available. In this paper, R, will be assumed to be based on an activation energy of 27 kcal/mol and uses 100 "C as an arbitrary reference temperature. Multiplied by the hydrogen ion activity, it gives the equivalent reaction time at 100 OC and pH 0. The activation energy was kept the same as used by previous authors (27 kcal/mol or constant 14.75 (Overend and Chornet, 1987)) for comparison with their results. Use of Combined Severity and Acidity Parameters: log R, - pH. The organosolv experiments investigated are assembled in Table I with the experimental conditions summarized as follows. Series A. This involved batch pulping of aspen (Populus tremuloides) using kilogram quantities of wood at
158 Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990 Table I. Summarv of Severitv Factors a n d DH for Selected Organosolv ExDeriments pulping conditions severity factors catalyst t: h T , "C MeOH, vol % logb (Pt) l o g Ramps lo$ R, Series A.. Liauor:Wood Ratio 4:l 70 4.09 3.92 4.31 none 2.5 165 3.92 4.66 70 4.57 none 7.5 165 3.92 4.31 70 4.09 0.05 M H3P04 2.5 165 3.92 4.31 70 4.09 0.04 M NaHS0, 2.5 165 70 4.29 3.92 4.45 0.04 M NaHSO, 4.0 165 3.92 4.31 70 4.09 2.5 165 0.10 M NaHSO, 70 3.69 3.92 4.12 1.0 0.05 M HzS04 165 70 4.09 3.92 4.31 2.5 0.05 M H2S04 165
pulp analyses,e % residual residual pentosan lignin
pH
log (R, - pH)
4.30 4.30 2.75 2.61 2.63 2.01 1.63 1.63
0.01 0.36 1.56 1.70 1.84 2.30 2.49 2.68
29.2 28.0 12.4 18.4 16.0 12.9 0. 0.
11.9 8.9 7.4 11.2 9.5 6.8 9.5 19.8
0. 0.0 0.0 13.1
5.9 0.3 13.5 2.6
1.0 1.0 1.0 1.0
170 170 170 170
40 70 30 70
Series B, Liquor:Wood Ratio 101 3.84 2.24 3.85 3.84 2.24 3.85 3.84 2.24 3.85 3.84 2.24 3.85
1.66 1.63 1.59 2.27
2.19 2.22 2.26 1.58
none 0.0005 M HZSO, 0.05 M HzS04 0.01 M H2S04
1.0 0.5 1.0 3.0
200 200 150 150
70 70 70 70
Series C, Liquor:Wood Ratio 1O:l 4.72 3.05 4.73 4.42 3.05 4.44 3.25 1.72 3.26 3.73 1.72 3.73
4.30 3.70 1.63 2.27
0.43 0.74 1.63 1.41
none none 0.025 M 0.075 M 0.075 M 0.010 M
H3PO4 H,PO, HiPo; H3P04
1.0 1.5 1.0 1.0 1.0 1.o
165 175 165 165 165 165
Series D, Continues Flow-Through Delignification 50 3.63 3.02 3.73 4.30 50 4.19 3.34 4.24 4.30 50 3.63 3.02 3.73 2.56 50 3.63 3.02 3.73 2.31 50 3.63 3.02 3.73 2.31 50 3.63 3.02 3.73 2.20
-0.57 -0.06 1.17 1.42 1.42 1.53
18.9 14.4 9.2 6.8 8.0 7.1
17.7 13.6 6.2 4.6 4.7 5.7
none 0.01 M H2S04
1.0 1.0
170 161
50 50
Series E, Static Runs for Comparison 3.76 3.23 3.87 3.63 3.02 3.73
4.43 1.53
14.5 9.9
17.0 7.2
0.05 M 0.05 M 0.05 M 0.01 M
HzS04 HzS04 HzS04 FIBS04
4.30 2.20
3.8 2.2 0.9 2.6
"Time at temperature. b P = exp[(Tr - Tb)/14.75]. cContribution to R, of the heat-up and cool down of the reactor. dlog R, = log [(Pt) e % of oven-dried weight.
+ Ramps].
a low 1iquor:wood (L:W) ratio (4:l). A very long cooling down period was used for necessary practical reasons. It allowed reprecipitation of polymers, which are more soluble a t higher than at lower liquor temperatures. Series B. This involved batch pulping of black cottonwood (Populus trichocarpa) using gram quantities of wood and a 1O:l L:W ratio carried out a t the University of Washington by Tirtowidjojo and co-workers (1984, 1988). Short heat-up and cool-down times were employed in these experiments with variable methanol concentrations a t 170 "C for 1 h. Series C. These used similar conditions to series B but variable temperatures and catalyst concentrations at 70% (volume basis) methanol concentration. Series D. Flow-through delignification was carried out on about 200 g of aspen wood. The methanol concentration was 50% (volume basis); the L:W ratio was 81 to 1 4 1 in these experiments. Washings were performed with a flow of hot aqueous methanolic solution. Series E. This was carried out in the same reactor as the D series but in batch mode; hot washings were performed under flow conditions. Although the treatment of the data using the combined severity factor (eq 9) is more rigorously valid for the batch experiments, both batch and flow experiments are plotted together. If the removal of xylan is equal to or greater than 50% of the original xylan content of the wood, hydrolysis of the resulting material by enzymes is very fast and more thorough than if more xylan remained in the substrate (Chum et al., 1988). The correlation between the percent xylan removal and the combined severity parameter is shown in Figure 1. From Table I, it is easily seen that all experiments described here involved conditions in which log R, varied between 3.2 and 4.7. Adding the
IXylan Removal *,X-c ;
I 25-
0
/'
2
,
,/'
--/.
-1
7
/'
-
-
0 1 2 Combined Severity, (log Ro-pH)
Series A
A
Series
B -Series D
3 Series
E
Figure 1. Percent xylan removal, expressed as percentage of original oven-dried weight (ODW) of the starting wood, as a function of the combined severity parameter for the organosolv delignification series: A (01, aspen, batch, 4:l L:W ratio, 70% methanol (by volume), and variable levels of catalysts at 165 "C; B (X), black cottonwood, 1O:l L:W ratio, 30-70% methanol (by volume), and variable levels of sulfuric acid at 170 "C; D (A),aspen, semicontinuous, 8:l to 141L:W ratio, 50% methanol (by volume), and variable levels of catalysts at 165 "C; and E (a), same as D except that the pulping experiment was carried out in batch mode. Details are given in Table I.
acidity function doubles the ordinate range from -0.5 to 2.7 in log units and expands the possibilities for interpretation of the results. Xylan removal increases with increased combined severity (Figure 1). The lower dashed curve of Figure 1 encompasses series A experiments, in which xylan reprecipitation onto fibers occurred, such that at the low combined severity experiments no net xylan removal from the
Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990 159 % Lignin Removal
100
Table 11. Xylan Distribution in the Liquor and Pulp for Several Combined Severity Pretreatments xylan total combined severity solubilized xylan xylan in pulp parameter
c
80
-0.57 1.17 1.42 1.53
20 c I
01 -1
A
-0.5 Series A Series D
0 0.5 1 1.5 2 Combined Severity, (log Ro-pH)
*
Series B Series E
+
2.5
3
Series C
Figure 2. Percent lignin removal, expressed as percentage of original ODW of the starting wood, as a function of the combined severity parameter for organmlv delignification series: A, B, D, and E defined above; C (+I, black cottonwood, 1 0 1 L:W ratio, 70% methanol (by volume), with variable levels of sulfuric acid at various temperatures. Details are given in Table I.
pulp was observed. Complete xylan removal was achieved at a combined severity factor of >2.3. This approximate correlation explains general trends and unifies different catalysts, at different concentration levels in a variety of conditions. In the semicontinuous experiments (series D, A),with a continuous flow of liquor at low temperatures and washings with liquor at high temperatures, it is possible to remove more effectively the lignin-hemicellulosic complex from the vicinity of the cellulosic fibers and thus avoid reprecipitation effects that were very clearly present in the experiments at 4 1 L:W ratio, in which the kilogram batch pulps were allowed to cool to room temperature in the presence of the liquor, before the materials were removed from the digester and washed. At the high combined severity parameter conditions, both 101 and 4 1 L W ratios gave 100% xylan removal. In addition, one can also note the effects of dilution and solubility. Higher xylan removal is obtained under flow than under batch conditions. Plots of lignin removal as a function of the combined severity parameter are shown in Figure 2. Series A data display a modest increasing trend of lignin removal with increased combined severity parameter. All the 1O:l L:W ratio experiments at 70% methanol concentration produced pulps with much lower residual lignin contents, again indicating the increased lignin solubility and dilution effects under these conditions (series B two data points and series C). As expected, lower methanol concentration experiments have significantly less lignin removal (series B), since the solubility of the lignins in these solvents is not as high as in the 70% methanol concentration liquors (Tirtowidjojo and co-workers, 1984, 1988). The flowthrough experiments (series D) at 50 vol % methanol concentration and higher severity displayed an intermediate lignin removal level compared to the other two trends. The trend exhibited by the two batch points of series E is similar to that exhibited in the xylan removal; at the higher combined severity, there is a significant decrease in lignin removal by batch rather than by flowthrough operation. Smaller differences are observed at lower severities; in fact, all points up to combined severity of 0.7 lie in a very good straight line (? = 0.96), suggesting that at lower severities lignin reprecipitation is less important than the xylan reprecipitation. The highest lignin removal in series A occurs at a 1.6 combined severity parameter corresponding to 0.05 M phosphoric acid and 70
33.9 74.6 81.0 81.6
77.1 25.5 19.2 27.1
111.0 100.1 100.2 108.8
Table 111. Lignin Distribution in the Liquor and Pulp for Several Combined Severity Pretreatments total solubilized lignin combined severity in pulp lignin parameter lignin -0.57 1.17 1.42 1.53
38.7 89.6 95.4 84.1
59.8 14.3 10.0 11.7
98.5 103.9 105.4 95.8
vol % methanol. With phosphoric acid as catalyst, higher yields of fractionated cellulose, lignin, and hemicellulose were observed than with the equivalent sulfuric acid concentration, indicating that this acid may have specifically reacted/associated with the polymers. Lignin isolation free from carbohydrate contamination is much more difficult from phosphoric acid catalyzed liquors than sodium bisulfate or sulfuric acid catalyzed liquors. Within the sulfuric acid series and its corresponding acid salts, the use of the combined severity parameter unifies the experimental results. The percent hexosan recovered as a function of the combined severity parameter for series A, B, D, and E appears to decrease slightly up to 1.6 combined severity parameter and then more markedly (see Figure 3a). This type of trend is very similar to that reported by Overend and Chornet (1987) for the aqueous steam treatments. The combined severity effects on the lignin-free yields are shown in Figure 3b. Again, the whole data set can be explained, in general, by two envelopes-the relatively higher yields for series A experiments, caused mainly by xylan reprecipitation, the dashed upper curve, versus the lower dashed curve, in which most of the remaining data seem to fit. Both curves exhibit decreased lignin-free yield as a function of increased combined severity. The liquors from series D experiments were reduced in volume and separated into acetone-soluble (AS) and acetone-insoluble (AI) fractions. Total mass balances of the pulp, AS, and AI fractions were 100 f 8%. The percentage of the original oven-dried weight of the sugars (xylose, mannose, and galactose) and lignins dissolved in the liquors and solubilized in acetone as a function of the combined severity parameters is shown in Figure 4. The glucose content of the acetone-soluble liquors varied from 0 for low severity to 2% of the total glucose content in the original wood for high severity parameter experiments. The solubilization increases rapidly up to a combined severity parameter of 0.5 and then increases more slowly (lignin and xylan). Galactose and arabinose determinations were not as accurate as the other sugar determinations. However, all arabinose was found to dissolve if longer reaction times were employed whether a catalyst was used or not (i.e., higher severity pretreatment conditions). Examples of the solubilization of xylan (sum of AI and AS fractions), the residual xylan in the pulp, and the total xylan are shown in Table 11. Xylan dissolution increases with the increased combined severity parameter. Concomitantly, there is a decrease in pulp xylan content as a function of the increased combined severity parameter. A very similar trend was observed for lignin dissolution (see Table 111). Inspection of these two tables seems to indicate that, at 1.5 combined severity parameters, com-
160 Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990 % Hexosan Recovered
% Hexosan Recovered 100 __
__
50
40 -
25
20 0 1 1.5 2 Combined Severity, (log Ro-pH)
-0.5
"-1
0
2.5
0.5
-
=Series A
Series B
* Series
Series D
3
Lignin-Free Yield, %
70 -
T
-
-1
-',
\,
40 -
\
-0.5
0 0.5 1 1.5 2 Combined Severity, (log Ro-pH) Series B "Series C
'Series A
A
i
2.5
Series D
3
* Series E
Figure 3. (a, top) Percent hexosan recovered, expressed as percentage of original ODW of the starting wood, as a function of the combined severity parameter for organosolv delignification series defined in Figure 1. (b, bottom) Lignin-free yield, expressed as percentage of original ODW of the starting wood, as a function of the combined severity parameter for organosolv delignification series defined in Figure 2. % Original Oven Dry Weight
1001
It
'/'
o-.
s.d
-1
-0.5
.-__
0 0.5 1 Combined Severity, (log Ro
'Xylose
' Mannose
-
1.5 pH)
Galactose
0
0.5
________- ~1.5 2 2.5 3 Combined Severity, (log Ro-pH) 1
' Series A Series B Figure 5. Product of percent hexosan recovered (W ODW) and digestability as a function of the combined severity parameter for series A and B as defined in Figure 1.
E
""
30
Digestibility _________
2
* Lignin
Figure 4. Solubilization of xylose ( O ) , mannose (X), galactose (A), and lignin ( a ) expressed , as percentage of original ODW of the starting wood component materials, as a function of the combined severity parameter for organosolv delignification series D. These materials were isolated as an acetone-soluble fraction from the liquors after solubilization and evaporation.
paratively more lignin than xylan dissolution is possible under these conditions. Obviously, on going to higher combined severity parameters, complete xylan removal could be achieved. A parallel trend was observed on going from the semicontinuous to batch delignification from the analyses of the dissolved liquor. In the AS fraction from the batch
experiment at high severity, 7% less lignin was isolated, and a smaller proportion of the sugars was dissolved under equivalent conditions, respectively 30%, 34%, 24% and 1.1% (for xylose, mannose, galactose, and glucose). These data suggest that pulping under flow conditions is more important for solubilization of sugars than for lignin removal in 50% methanol. Use of Combined Severity and Acidity Parameters: Enzymatic Hydrolysis of Organosolv Pretreated Substrates. The ultimate yield of glucose from fermentation after enzymatic hydrolysis of the organosolv pretreated residues is shown in Figure 5. The upper data curve includes the 1O:l L:W ratio and high methanol concentration. The lower data curve includes samples in which reprecipitation occurred, which decreased the accessibility of these substrates to the enzymes. The rate of enzymatic hydrolysis of pretreated lignocellulosic substrates has been phenomenologically expressed as two simultaneous pseudo-first-order reactions, in which fast and slow hydrolyses of cellulose occur; the rate constants for the fast and slow hydrolyses have been obtained for a number of organosolv pretreated substrates as well as the proportions of fast (fa) and slow (1 - fa) hydrolyzable materials in the substrates (Chum et al., 1988). Similar semiempirical models have been developed over the years for the reaction between cellulase and cellulose; they have been described as a summation of pseudo-first-order reactions (van Dyke, 1972; Brandt et al., 1973; Esterbauer and Janosi, 1984). Esterbauer and Janosi (1984) found two simultaneous pseudo-first-order reactions for pulp and newsprint but provided somewhat limited rate constant data a t the time. More recently, Sattler et al. (1989) have obtained extensive and very accurate data for Sigmacell 50 and for steam-explodedpoplar wood hydrolysis with Celluclast. The authors find a fast hydrolysis rate constant of 0.22 f0.03 h-' for the 44% fast hydrolyzable material in Sigmacell 50; the slow hydrolyzable material (56%) had an average rate constant of about 0.007 h-l. These numbers agree extremely well with the 0.2 h-' found for organosolv pretreated poplars and a-cellulose (Chum et al., 1988). The percentage of fast hydrolyzable material was found to be a function of the pretreatment severity and to correlate with the amount of residual xylan (Figure 6a). For samples that retained >50% of the starting xylan content (Le., >15% xylan content in the pulp), the amount of fast hydrolyzable material was zero. As the xylan content decreased, the rapidly hydrolyzable material increased to a maximum of 40-45%. These data can be reasonably unified by plotting
Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990 161 0.5
0.4
fa r
I---- -- -OF
Ix 0.3
\
X
k
b
0.2
i.*
\
\
\
20t
\
\o
\ 5t
0
5
10 15 20 Residual Xylan, % Series A
0.5 0.4
30
25
-
0
5
10
Series B
OSeries A
25
30
X
Series B
k2
fa
I
1
c ,,'
0
I
I
,?
X
0.3
I
3
I
30t I
20
x; ix
0.2 -
I X
) I
3
0.5
1 1.5 2 2.5 Combined Severity, (log Ro-pH) Series A
/
/
/
/
/
/
/
0
I C
0.1 -
0
20
15
YO Residual Xylan
~
3
Series B
Figure 6. Fraction of fast hydrolyzable glucan (a, top) as a function of the residual xylan content and (b, bottom) as a function of the combined severity parameter for series A and B as defined in Figure 1.
versus the combined severity factor (Figure 6b), with the exception of the phosphoric acid-catalyzed experiment in series A and the least severe treatment of series B. The mild severity pretreatments do not seem to lead to the formation of the fast hydrolyzable materials, which becomes more prominent as the severity of the treatment increases. The slow rate of hydrolyses varied in the 0.008-0.032-h-1 range, depending on the pretreatment conditions. The rate constant for the slow hydrolysis fraction is plotted as a function of the residual xylan (Figure 7a) and the combined severity parameter (Figure 7b). All data for series A were obtained with an enzyme loading of 2 mg/mL at 45 "C, and the data for series B were obtained with a 5 mg/mL enzyme loading, at 40-42 "C. Strict comparisons between the data need to take into account the different enzyme loadings and temperatures. The trend in the data is that the higher the severity of the treatment the faster the slow hydrolysis. However, the dominant term to increase the overall rate of enzymatic hydrolysis of the cellulosic substrate is the extent of production of fast hydrolyzable material induced by the pretreatment. Sattler and co-workers (1989) have also shown that these types of correlations are very important to assess objectively the effectiveness of different pretreatments and cellulase preparations relative to Celluclast.
fa
Conclusions The use of severity parameters including hydrogen ion activities can expand the ability to compare and unify different pretreatments when catalysts are present. It is
v O
0.5 1 1.5 2 2.5 Combined Severity, (log Ro-pH) OSeries A
I 3
Series B
Figure 7. Rate of slow hydrolysis of the cellulosic residue (a, top) as a function of the residual xylan content for series A and B as defined in Figure 1 and (b, bottom) as a function of the combined severity parameter for series A and B as defined in Figure 1.
a very useful tool in the design of experiments such that time, temperature, and catalyst concentration can be varied in the most meaningful way possible to minimize the total number of experiments performed. Even though pH is the simplest acidity function indicator, it has been shown to be useful in the interpretation of pretreatment results. Whether the ApK/AT of these acid salts will maintain their relative values as the temperature increases in the alcoholic media to the 130-170 "C temperature range of the reactions of interest is not known, but these data and others (see Chum et al., 1990) suggest that simple measurements can provide an effective guide for experimental design. The correlation between uncatalyzed experiments and those using sulfuric acid its corresponding acid salts with the combined severity parameters is good; however, use of phosphoric acid in methanol/water solutions leads to relatively higher recoveries of sugars and lignin removal. Additional acidity function measurements under pulping conditions could greatly expand the quantitative usefulness of these correlations.
Experimental Section The preparation of the organosolv materials discussed has been previously described (4:l liquor-to-wood ratio (Chum et al., 1988); 101 liquor-to-wood ratio (Tirtowidjojo and co-workers,1984,1988);semicontinuous results (Chum et al., 1987a,b)). The enzyme hydrolysis data are from Chum et al. (1988). The pH data were obtained with a combined glass electrode (Orion 8103) combination pH electrode using an Orion (601A) digital ionanalyzer using an acetate buffer
162 Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990
in 50% methanolic solutions as the calibration standard (10-40 "C); data were obtained at 40 "C. Because of the difference in calibration solutions and temperature, the pH values presented here differ somewhat from the values presented by Chum et al. (1988). The values in Table I are corrected for liquid-junction potentials and media effects on the hydrogen ion concentrations at low temperatures (40 "C). The calculation of the severity factors included time a t temperature and the heating and cooling ramps as shown in Table I. Series D and E analyses included Klason lignin and soluble lignin analyses according to the Tappi standard method and useful method, respectively. The individual sugars were measured after hydrolysis of the pulps or liquor residues as alditol acetates by gas chromatography, following a modification of the method described by Blakeney et al. (1983).
Acknowledgment Profitable discussions on severity parameters with Drs. E. Chornet, J. Christie, B. Krieger-Brockett, and R. Overend and on cellulose enzyme hydrolysis with Professors H. Esterbauer, K. V. Sarkanen, and W. Steiner and Dr. K. Grohmann are gratefully acknowledged. Thanks are due to Bonnie Hames for some of the pH measurements and sugar analyses performed in collaboration with Fannie Posey. The support of the Biofuels and Municipal Waste Technology Division of the US. Department of Energy (DOE) through the Biochemical Conversion/ Alcohol Fuels Program is gratefully acknowledged (FWP BF72 and 82). Finally, thanks are due to the IEA Bioenergy Agreement for allowing timely exchange of ideas in computer conferences and through special workshops, in particular Ottawa, June 1988. Dr. Don Stevens and R. D. Hayes are thanked for their efforts in making the IEA Bioenergy Agreement such a useful technical conduit for valuable information exchange. Registry No. MeOH, 67-56-1; H,PO,, 7664-38-2; NaHSO,, 7681-38-1; H2S04,7664-93-9.
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