I n d . Eng. C h e m . Res. 1990, 29, 343-349
343
Kinetic Model of Soda Black Liquor Oxidation under Pressure Alexander D. Nadezhdin* and John F. Robertson Domtar Inc., Research Centre, P.O.Box 300, Senneuille, Quebec, Canada H9X 327 Soda black liquor is a complex chemical system containing various classes of organic and inorganic compounds dissolved in water a t alkaline pH. Upon contact with oxygen under elevated temperature and pressure, a stagewise oxidative degradation of organics occurs. A simple model, based on a three-lump component, has been proposed in order to simulate the kinetic behavior of the original system in terms of total dissolved solids, chemical oxygen demand, and carbonate content of solution changes with time. Experimental data obtained in a series of batch and continuous autoclave runs have been closely approximated with three finite algebraic expressions containing two fixed reaction rate constants and one adjustable parameter. In this paper, we describe the kinetic behavior of soda black liquor subjected to oxidation by molecular oxygen under elevated pressure and temperature. A theoretical model of the system will be presented and tested by matching our own experimental data. The soda pulping of wood chips is carried out in an extruder-type reactor continuously for about 15 min at 170 "C in order to soften the wood fibers for further processing into corrugated board medium. The pulping spent liquor, called black liquor (BL), contains four major classes of components as follows: solubilized lignin and its aromatic fragments; hemicelluloses in their poly- and monosaccharide forms; highly oxidized organic matter, such as short-chain carboxylic acids; and finally, sodium carbonate both in the form of Na2C03and NaHC03 depending on the pH value. The oxidative decomposition of BL has been practiced for either complete or partial oxidation of organic matter (Morgan and Saul, 1968). In the former case, the recovery of inorganic salts is the purpose. In the latter case, however, the goal could be either disposal of partially oxidized waste or production of a usable mixture of low molecular weight components (such as salts of carboxylic acids). In order to recover the desired product mixture while minimizing problems associated with separation and disposal of high molecular weight byproducts, the destruction of the latter must be as complete as possible by the end of the oxidation process. When a black liquor is contacted with oxygen (or air) under elevated temperature and pressure, further decomposition of organic matter, eventually into C 0 2 and water, occurs. The description of such a process is usually focused on the chemical transformations of some particular compounds and/or functional groups. A number of publications have been devoted to model compound behavior, from which generalizations were drawn for the case of lignin [see Holocher-Ertl et al. (1981), Chang and Gratzl (1980),and Aoyagi et al. (1980)l and other BL components (McGinnis et al., 1984). Substantial progress has been achieved in understanding preferential routes of lignin fragment degradation in the presence of oxygen, hydrogen peroxide, and bases (Gierer and Insgard, 1980). This allows one in some cases to predict the relative yields of various products of model compound decomposition. By use of individual reaction rate constants of model compound decomposition, one could, in principle, attempt to describe the kinetic behavior of a mixture via computer simulation. However, this is a difficult task even for a small group of starting materials since it would be necessary to consider many crossover reactions between various intermediates. A kinetic description of this sort would provide many details of the system's behavior such as
individual component concentrations vs time. In some situations, it is more information than is actually needed. For example, for process analysis in engineering design terms, the concentration of solute, oxygen demand, inorganic (carbonate salts) content, and heat output per unit volume are essentially all that would be required. Finding a theoretical expression of the dependence upon time of such characteristics of the reaction will be the final goal of the following discussion.
Construction of a Model Individual BL components and/or the whole classes of BL components listed in the previous section can be characterized by the specific chemical oxygen demand value (SCOD). We define SCOD here as the stoichiometric amount of oxygen (grams) consumed per gram of a chemical or a mixture of chemicals upon its complete oxidation (under the conditions of the ASTM D1252-83 procedure). In the case of a single compound, the SCOD can be calculated by dividing the stoichiometrically required amount of oxygen in grams by the molecular weight of the compound. In the case of the unknown mixture, such as BL, SCOD equals the ratio of chemical oxygen demand (COD) to the total dissolved solids (TDS) concentration, both values being expressed in grams/liter. In a subsequent discussion, we shall combine real constituents of BL and their reactions into three classes and study their interrelations by using a well-known technique of lumping [see, for example, Coxson and Bischoff, 1987)]. Although accurate lumpability criteria and conditions have been developed for the linear systems (Wei and Kuo, 1969), in the case of nonlinear systems, such as the one presented here, the lumping procedure should heavily rely on the available chemical information about the system (Iordache et al., 1988). The three lump compounds we have choosen are designated NaPh, NaSC, and NaHC03 to represent high SCOD alkali lignin, medium SCOD carboxylic acid salts, and zero SCOD sodium bicarbonate, respectively. Let us look at the basic lump compound characteristics, the reasons for selecting those, and a meaning of the symbols used. A few of the alkali lignin and its model compositions from the literature are listed in Table I. In order to account for the high solubility of aromatic lignin fragments in BL at mildly alkaline pH around 9, it is assumed that each phenolic hydroxyl position and each carboxyl group present in lignin are in the form of a sodium salt. According to Mart,on (19711, this corresponds to 88 positions (72 phenolic plus 16 carboxylic) per 100 C6-C3 units of extracted lignin. More recent data (Mansson, 1983) indicate a slightly higher number for hardwood
08886885 f 9012629-0343$02.50/0 0 1990 American Chemical Society
344 Ind. Eng. Chem. Res., Vol. 29. No. 3, 1990 Table I. Alkali Lignin and Lignin Model Compounds. SCOD Value and Molecular Weight per Unit SCOD,g MW per compound formula“ of O?/g unit charge alkali lignin 1.7 188 Na3C27H2809 alkali spruce lignin NaC9H6.302.2(0CH3)0.9 1.7 200 kraft hardwood lignin N a C 9 H 6 , 2 0 ~ , 8 S ~ . 1 ( 0 C H 3 ~ ~ !5 1.5 205 o-hydroxypropiovanillone(model compound) NaCgH603(0CH)3 1.5 218 1.8 174 P-aryl ether lignin model (dimer 3B) Na2C,,H,,O, 1.6 174 vanillin NaC8H7O3 a One
hydrogen was replaced by sodium per each
c6-c3 or
Chargea ref Paschke, 1922 Redinger, 1961 Marton, 1971 Ekman, 1965 Dimmel and Shepard, 1982
Cs-C2 group.
Table 11. Short-Chain Carboxylate Components of Black Liquor BL solute SCOD, g MW per content O,/g unit charge (example) sodium salt formula C2H3CO2Na 0.8 82 20 acetate C,H60,Na 0.6 86 51 citrate formate CH02Na 0.5 68 rj C3H503Na 0.9 112 IF, lactate 1.1 96 10 propionate C3H502Na
lignin, 102 positions (86 phenolic) per 100 units. Accordingly one hydrogen per each C6-C3 or even c6-C2 (Ekman, 1965) unit was replaced with a sodium in the lignin formulas used in Table I. Both the SCOD value and the molecular weight of a unit associated with one anionic center are nearly constant for all alkali lignin compositions listed in Table I. The first lump cowpound, NaPh (for “phenolate”), is assigned with a SCOD value of 1.6, a molecular weight of 174, and a single anionic charge associated with the Na cation. All these characteristics are the ones of sodium vanillate (NaC8H,03), a Cs-C2 representative of lignin-derived compounds. Once again, the “representative” was chosen solely on the basis of the specific COD and the sodiumto-organic matter ratio. Many other compounds could serve as a model without affecting significantly our conclusions, as will be obvious from further deliberations. The sodium salts of short-chain carboxylic acids constitute a substantial part of the BL solute as indicated by the HPLC data. The SCOD values, normalized molecular weights, and approximate concentrations of most abundant carboxylic acid salts are listed in Table 11. While the SCOD value of alkali lignin compounds stays in the region 1.5-1.8 g of 02/g, one of the carboxylic acid salts is closer to 0.8 g of 0 2 / g on average. An intermediate position between those two groups is occupied by carbohydrates, such as hemicelluloses and individual sugars. As our analysis indicated, the total carbohydrate content of the BL solute was of the order of 10-12% (see also Simonson (1965)),while simple sugars were essentially absent from solution. The SCOD value of hemicelluloses, corresponding to the general formula C6(H20I4,equals 1.2. Considering a relatively low hemicellulose content of the BL solute, we have ignored its presence for the purpose of model construction. Nevertheless, if significant amounts of carbohydrates were present in the original solution, an obvious extention to the subsequently presented model could be introduced. The second lump compound, NaSC (for “short chain”), has characteristics of the second class of BL components: a SCOD value of 0.8 g of Oz/g and a molecular weight of 82, as in sodium acetate. In this case, the choice is additionally justified by the latter being one of the most stable and abundant components of the black liquor solute. The third component used in our formal model is sodium bicarbonate (NaHCO,), an almost always predominant form of carbonate salt in the pH range of our experiments.
Table 111. Lump Compound Characteristics comDd SCOD MW Der unit charge NaPh 1.6 174 0.8 NaSC 82 0.0 84 NaHC03 PH 10 -
7I
50
-r 150
100
TIME, MIX
Figure 1. Change of pH in the course of BL pressure oxidation a t 210 “C.
Thus far, our description deliberately (for the sake of simplicity of math procedures) neglects polyvalent short-chain acids, such as oxalic, and the presence of Na2C0, at the end of oxidation process where the pH value of solution slightly exceeds 10. The assumed “physicochemical characteristics” of the three lump compounds are summarized in Table 111.
Stoichiometry of the Model In the process of wet combustion at elevated temperature and pressure, our model components are assumed to react according to the following scheme, simulating the behavior of real BL constituents: NaHC03(H20,C02) NaPh NaSC(HSC) NaHC03(H20,C02) where HSC stands for the corresponding short-chain carboxylic acid. The production of carboxylic acids upon lignin fragment oxidation is well-known (Othmer, 1980). In our experiments, it was responsible for a drop in the solution pH at the beginning of BL oxidation, as shown in Figure 1. Gradual recovery of the pH value to its original level must correspond to the subsequent hydrolysis of acids to give, through further oxidation, carbon dioxide and water. As a result, the number of short-chain anionic charges retained in solution upon completion of alkali lignin decomposition was equal to the total number of sodium cations present in solution:
- -
NaPh
+02
NaSC
+ xH20 + yC0,
-
(1) A different situation may be created by adding labile so-
Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990 345 dium cations to the starting mixture in the form of sodium hydroxide or carbonate. Upon heating to high temperature (over 200 “C) and before the injection of oxygen, the solution pH tends to stabilize in the 9-10 interval, which corresponds to NaHCO, as a predominant form of “free” sodium. Subsequently, when free acids are formed, they will be neutralized as follows: HSC + NaHCO, NaSC + HzO + CO, (2) Accordingly, more than one carboxylic acid salt can be recovered in the product mixture per each sodium lignin unit destroyed. Equation 1 then must be replaced as follows:
-
NaPh
+ mNaHCOs
-+ +02
( m 1)NaSC + xHzO + yC0, (3) where m is determined by the difference in molecular weights between P h and SC units and by the availability of sodium ions. Within the restrictions of our model, it cannot exceed 1even if a large excess of sodium carbonate is initially present in solution. In a subsequently described computer simulation, we treat the m factor as an adjustable parameter with an allowed variation range between 0 and 1. In order to complete the picture, eq 4 is added to ac-
+Qz
+
+
NaSC NaHC0, xHzO yC0, (4) count for the destruction of short-chain anions resulting in sodium bicarbonate production. x and y are unknown stoichiometric coefficients which will not appear in the kinetic equations of the next section. We are ready now to derive molar concentrations of our lump compounds from the measured bulk solution characteristics, i.e., TDS, COD, NaHC03, and the total Na concentration by solving the following equations: TDS = [ N a P h ] M ~ ~ p+h [NaSClMNaSC + [NaHC031MNaHC03 ( 5 ) COD = [NaPhlMNaPhSCODNaPh+ [NaSClMNaSCSCODNaSC (6) [Nal = [NaPhl + [NaSC] + [NaHC03] (7) where square brackets correspond to molar concentrations and M , and SCOD, are the molecular weights and specific COD values of the individual compounds. As was already mentioned, the validity of eq 7 is based upon the additional assumption of the absence of multivalent SC anions and a low proportion of Na2C03salt. Those assumptions were invalid when the solution pH exceeded 10 and/or generally toward the end of oxidation process in our experiments when the degree of organic matter oxidation was the highest. Fortunately eqs 5 and 6 together with the known CO,’ content of solution are sufficient for deriving NaPh and NaSC concentrations. Therefore, eq 7 can be used to verify our model and its assumptions.
Experimental Results and the Behavior of Lump Compounds The initial liquor contents for the seven autoclave experiments are shown in Table IV. The experimentally determined solution characteristics in terms of TDS, COD, and COS=concentrations at 0, 10, 30, 60, and 90 min of reaction time in a batch autoclave are shown in the first three columns of Table V. In the case of continuous autoclave experiments, the solution samples were taken simultaneously from several points along the continuous autoclave length under steady-state conditions. The corresponding data are shown in Table VI. In order to relate the steady-state concentration
Table IV. Conditions of Pressure Oxidation Tests“ BL characteristics after sodium compd addition added TDS, COD, % Nain expt type of test formula g/L gfL g/L BL solute 231 255 19 1 batch 2 batch NaeCOq 44 310 300 23 305 300 21 3 batch NaHCO, 65 280 295 24 4 batch NaOH 34 208 230 17 5 continuous 6 continuous 231 280 19 290 310 22 7 continuous NaZC03 35 “All experiments were conducted at 210 “C and 600 kPa of oxygen partial pressure.
Table V. Experimental Data for Curve Fitting (Batch Tests) measured values, g/L calculated values,” reaction time, min TDS COD COS= [NaPh] [NaSC] Experiment 1 0 231 256 17 0.69 1.05 10 222 222 9.8 0.44 1.62 170 4.5 0.14 30 199 2.06 185 60 150 7.9 0.10 1.91 1.82 90 180 142 10 0.09 120 162 130 7.0 0.08 1.68
M [Na] 2.03 2.22 2.28 2.14 2.09 1.88
0 10 30 60 90 120
310 305 278 262 260 260
Experiment 2 300 37 0.73 279 25 0.50 206 15 0.04 0.00 179 22 168 27 0.00 170 22 0.00
1.60 2.22 3.03 2.80 2.63 2.66
2.94 3.14 3.33 3.16 3.08 3.02
0 10 30 60 90 120
305 273 262 233 255 260
Experiment 3 300 40 0.78 254 23 0.49 206 12 0.11 182 19 0.15 0.00 140 33 128 23 0.00
1.39 1.89 2.76 2.19 2.19 2.00
2.82 2.77 3.07 2.66 2.73 2.39
0 10 30 60 90 120
280 272 259 237 245 240
Experiment 4 295 16 0.73 244 14 0.34 223 10 0.24 189 14 0.14 0.00 160 24 134 31 0.00
1.59 2.35 2.48 2.36 2.50 2.12
2.59 2.92 2.89 2.73 2.90 2.64
Meanings of the abbreviations are given in the text
profiles to the conventional concentration vs time kinetic terms, an average residence time in a given compartment was calculated as the ratio of a compartment’s volume to solution flow rate (Levenspiel, 1972). The feed pumping rate was varied from one experiment to another, which accounts for the variability of calculated average residence times in Table VI. The lump compound concentrations, as derived from the measured solution characteristics by solving eqs 5 and 6, are shown in the fifth and sixth columns of Tables V and VI. Let us look at batch experiments first. In the last column of Table V, the amount of sodium calculated according to eq 7 is given. Apparently the calculated [Na] remains constant, in most cases within 10% of the average value. By definition of the three model compounds, a complete destruction of NaPh components must correspond to the decrease in the SCOD value of the solution from above 1.0 at the beginning to about 0.8 and further down to 0 when NaSC is being oxidized.
346 Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990 Table VI. Experimental Data for Curve Fitting (Continuous Tests)a measured values, residence g/L calculated values,* M time, min TDS COD COB= [NaPh] [NaSC] [Na] Experiment 5 229 2.9 0.51 0 208 1.40 1.96 143 2.9 0.10 48 169 1.79 1.94 4.2 0.01 112 85 147 1.69 1.78 93 12 0.00 123 136 1.45 1.65 84 11 0.00 153 123 1.31 1.49 83 9.0 0.00 163 119 1.29 1.44 0 43 74 107 140 0 43 75 108 135 143
231 177 159 153 148
Experiment 6 280 4.9 0.77 182 2.7 0.34 160 2.4 0.28 137 3.9 0.16 124 4.5 0.10
290 239 223 224 220 220
Experiment 7 310 21 0.78 208 14 0.28 169 15 0.08 165 15 0.05 0.00 153 17 160 19 0.07
1.10 1.39 1.32 1.45 1.52
1.95 1.78 1.63 1.67 1.69
- [NaPhJ X
1.51 2.07 2.28 2.36 2.37 2.19
2.65 2.59 2.62 2.66 2.67 2.59
‘The residence time was calculated as an average time required to reach a given sample point along the horizontal autoclave under the steady-state conditions. bMeanings of the abbreviations are given in the text.
- [NaSC]
__Exp.1 ------- E X ~2 .
TIME, MIN
Figure 3. Change of NaPh and NaSC concentrations with time during BL oxidation. Solid lines: experiment 1, no extra sodium added to the autoclave charge. Dotted lines: experiment 2,44 g/L Na2C03 added to the autoclave charge.
dissolved solid recovery, as is seen from Table V. A similar analysis had been applied to the data of Table VI, representing continuous autoclave experiments conducted under steady-state conditions in a horizontal flow-through reactor. Once again, increasing the sodium content of the BL solute as in experiment 7 leads to a considerable increase in NaSC production (column 6 in Table VI).
Kinetic Model Under the conditions where oxygen transfer is not a rate-limiting step and the oxygen concentration is being maintained constant, one may attempt approximating the oxidation process by a set of three first-order kinetic equations, based on the stoichiometric relations 1-4: d[NaPh]/dt = -kph[NaPh] (8)
, .. 0 8 -~
--
-\
+--.--r--7----r __--~-
0
50
r----
d[NaSCl/dt = (m + l)kph[NaPh] - ksc[NaSC] (9) d[NaHCO,]/dt = -mkph[NaPh] + ksc[NaSC]
-
___ i : 7
-
1
100
TIME, MIN
Figure 2. Change of SCOD value during BL pressure oxidation a t 210 “C.
To illustrate this point, the SCOD value of solution vs time (experiment 1 in Table V) was plotted in Figure 2. The calculated [NaPh] and [NaSC] concentrations vs time for the same experiment are shown in Figure 3 by solid lines. The increase in short-chain anion concentrations during the first 30 min was substantially undercut by their destruction during the residual 90 min necessary for the complete oxidation of alkali lignin. In the second experiment, illustrated in Figure 3 by broken lines, some sodium carbonate was added to the feed liquor, the sodium content of feed solute having been increased from 19% to 23 %. A substantial increase in the NaSC recovery upon completion of NaPh oxidation is in agreement with the stoichiometric equation (3). When sodium is added to the initial solution in the form of NaHC03 or NaOH, a similar increase in the NaSC recovery is observed in experiments 3 and 4 (Table v). Capturing produced short-chain acids in solution with labile sodium cations leads also to the increase of total
(10)
where k,, and kph are the apparent first-order rate constants. In general, those rate constants must depend on temperature and oxygen partial pressure, both parameters having been kept constant in the course of our experimental work. The initial conditions necessary in order to solve eqs 8-10 are as follows: [NaPh],=o= [NaPh], (11) [NaSC],=o= [NaSClo (12) [NaHCO,],=, = [NaHCOJo (13) The kinetic equations can be solved in finite form: [NaPh] = [NaPh], exp(-kpht) (14) [NaSC] = exp(-ksct)([NaSC]o + (n+ I)kph[NaPh]o/(kph - k s c ) ) ( l - exP(-(kph - k,c)t)) (15) [NaHCO,] = [NaHCO,lo + ([NaSCI, + ( m + l ) k ~ h [ N * h ] ~ / ( k ~-h ksc))(l exp(-ksct)) - (m[NaPhlo + ( m + l)k~c[NaPh],/(k~h ~ s c ) H- ~exp(-kpd)) (16) By substituting [NaPh], [NaSC], and [NaHCO,] values from eqs 14, 15, and 16 into eqs 5 , 6, and 7 , one finally obtains the theoretical time dependence of the measured solution characteristics, Le., TDS, COD, and COB=,expressed through a set of three zero time values. two re-
Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990 347 action rate constants, parameter, m:
kph
and ksc, and one adjustable
TDS = 174[NaPh], exp(-kpht) + 82 exp(-ksct)([NaSCIo + (m + l)ltph[NaPh]~/ (kPh - k S C ) ) ( l - exp(-(hPh - k S C ) t ) ) + 84([NaHCOJo + ([NaSC], + (m + l)kPh[NaPh],/ (kph - ksc))(l - exp(-ksct)) - (m[NaPhlo + (m + l)k~c[NaPhl,/(kph - ksc))(l - eXp(-kpht))) (17) COD = 272[NaPhIo exp(-kpht) + 64 exp(-ksct)([NaSClo + ( m + l)kph[NaPhl0/ (kPh - kSC))(l - exp(-(kph - k s c ) t ) ) (18) CO,= = 60([NaHC03], + ([NaSC], + (m + l)kph[NaPhIo/(kph - ks))(l - exP(-kpht)) (m[NaPhlo + (m + l)k~c[NaPhl,/(k~h - ~ s c ) )X (1 - exp(-k,,t))) (19) where the TDS, COD, and C03=values in the left-hand side are expressed in grams per liter, as measured experimentally. Tuning the Kinetic Equations The three adjustable parameters of eqs 17-19, Le., kph, ksc, and m, were used to tune those equations in order to obtain the best fit with the experimental kinetic data. The “best” set of parameters was defined as one that minimized the sum of absolute differences between experimental and calculated values for the three measured properties, TDS, COD, and COB=,at various times. Two sets of best parameters were evaluated by this definition: one for the 24 samples (72 numbers) of 4 batch experiments (Table V); the other for 18 samples (54 numbers) of the 3 continuous experiments (Table VI). In both cases, the procedure as described next was the same. It was necessary to obtain the approximate values of the three parameters in order to start the computer calculations. By definition, the m factor should lie between 0 and 1. A rough approximation for the kph and ksc constants can be estimated from the time scale of graphs in Figure 3. The biggest change in NaPh concentration occurred within 30 min from the start of the oxidation process, which corresponds to a reversed time constant, Le., kPh, roughly equal to 3 X min-*. The subsequent slower reduction in NaSC concentration occurs over a longer time interval of approximately 300 min, which corresponds to min-’. an apparent first-order rate constant, ksc, of 3 X It was decided, accordingly, that the three parameters, kPh, ksc, and m, would have values that would lie within the intervals of 0.02-0.04, 0.002-0.005, and 0-1, respectively. Ten values of kph and ksc and five values of m were selected at equidistant points over the given ranges. Then, for each combination of these parameters, 500 in all, and for each set of experimental conditions, the calculated values of TDS, COD, and COB=were obtained. The absolute differences between these calculated values and the experimental results were summed and reported. The values selected, then, for kph, ksc and m were those that minimized the differences. In the case of kph and ksc, a single pair was chosen for all four batch experiments and another one for the three continuous ones. In the case of the m factor, its value was chosen to minimize the differences between calculated and actual concentrations, independently for each experiment (see the discussion below). The values selected by this procedure for batch experiments were kph = 3.3 x min-’ and ksc = 2.5 x
I
Exp I m=O 6
300 -
300
*\
\
zoo-
I
’
-
0
0
IO0
100-
_--‘
h O
’
,
0
I
00 c _
0-IUS
ot’sy:,
-,
.
0-COD
-
100
Reaction Time
0
min
50
7
,100
X-C03
Figure 4. Comparison of experimental (symbols) and computed (lines) values of TDS, COD, and C 0 3 vs time. Batch autoclave tests.
min-l. For continuous experiments, the values were kph = 2.0 x min-l and ksc = 2.0 x low3min-’, i.e., within a 30% interval of the batch ones. One may conclude that the average values for k p h and ksc should be (2.7 f 0.7) X lo-* min-l and (2.3 f 0.3) X min-’, respectively. By substituting the obtained values of the reaction rate constants into eqs 17-19, while adjusting the m values individually for each experiment, we have simulated the kinetic behavior of the three characteristics of the reaction solution. The experimental values of TDS, COD, and COS’ shown vs reaction time in Figures 4 and 5 (symbols) are matched by calculated kinetic curves (lines) drawn by a computer using eqs 17-19. The corresponding m values are shown on each figure. In summary, a large number of experimental points was closely approximated using two constants and one variable parameter. An even better match would be achieved if the rate constants were allowed to change within 25% of the chosen values from test to test.
Discussion In terms of our model, one should expect a certain correlation between a computer-optimized m value and the initial sodium concentration, e.g., the sodium content of the BL solute. Indeed, a correlation does exist, as can be seen from Figure 6. Beyond the experimentally studied interval of sodium content, the m value should neither significantly exceed unity nor assume a negative value in the temperature and pressure range of our experiments, which is symbolically shown in Figure 6 by a broken line. We should remember, however, that the m factor is an artificially introduced parameter in order to make up for unknown details of the hydrolysis mechanism as well as to close the system of kinetic equations in a simple way. The value of applying the elemental reaction kinetics methods for the description of the complex reaction mixture has been demonstrated before (Wei and Kuo, 1969). Finding an adequate lumping scheme is a crucial point and the main challenge of such an approach. The essential step in the construction of our model was the replacement of bulk solution characteristics such as COD and TDS with the intrinsic parameter SCOD, which could be directly related to the individual chemical constituent properties by an appropriate summation procedure. Instead of formally persuing the latter approach in its rigorous mathematical form, we have chosen here to “invent” new compounds representing collective properties of the whole classes of real solution constituents.
348 Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990 300
Exp 5 m-0 2
Inn
A
Exp
_r -x
_- -v------
&---
,
or---
7
,*_
100
50
0
150
Lverage residence time nun 3-TDS
@,-COD
X-CO,
Figure 5. Comparison of experimental (symbols) and computed (lines) values of TDS, COD, and CO, vs average residence time. Continuous autoclave tests. m 15'
-
1
r- -
-
x
P i
,
,
x '
0 54
-- -
4
'
/x e
O$-
7
0 X
0
15
-
-
r
r-r----
20
Z Ka
1
25
in BL solute
CONTINUOUS TESTS
BATCH TFSTS
Figure 6. Correlation between sodium content of BL solute and an optimum m value.
The formal approach utilizing methods of linear algebra would not necessarily lead to immediate generalization of the developed scheme since the coupling of the kinetic equations has been based thus far on a concrete scheme of autocatalysis, i.e., NaHCO, participation in the NaPhNaSC conversion process.
Experimental Section All batch autoclave experiments were conducted using the same samples of hardwood soda black liquor obtained from one of our corrugated board medium production
mills, as a 32% w/w concentrate. Although the liquor had been thoroughly characterized by various modern analytical techniques, for the purpose of this study, only a few general characteristics of starting solution and autoclave samples are being used: total dissolved solids by drying for 24 h in forced-air oven a t 105 " C ; chemical oxygen demand by using standard ASTM D1252-83; CO,= content by measuring the carbon dioxide quantity evolved upon HC1 addition to a sample. The sodium concentration was determined by atomic absorption using a Perkin-Elmer 603 atomic absorption spectrophotometer. Batch experiments were conducted in a 4.5-L electrically heated stainless steel autoclave (Autoclave Engineers) equipped with a magnetically driven stirrer, a thermowell, oxygen sparge tube, and a sample line. The original BL solution was diluted with water to 20-24% w/w consistency, sodium salt, was added as necessary, and then, after a sealed 2.5-L charge was heated up to temperature, oxygen sparging and the retention time count were started. Periodic flash samples were obtained during the run, which lasted for 2 h. In the case of continuous autoclave experiments, a new sample of BL was obtained from the mill for each individual test. Continuous autoclave tests were run in a horizontal, five-compartment autoclave (30-L total liquid volume). The feed solution was continuously injected into the first compartment by means of a high-pressure Moyno pump, while the discharge was effected by periodic dumping of the fifth compartment, as controlled by a conductance level indicator. Each compartment was equipped with an individual sample line for simultaneous "profile'' sampling. A steady-state sample was taken after an amount of solution exceeding three autoclave volumes was passed through the system. All experiments described were run a i 210 f 5 " C under the oxygen partial pressure of 600 kPa. The starting solution was analyzed for short-chain carboxylic acid anions by using a HPLC-based procedure similar to the one described by McGinnis et al. (1984). The individual sugar and total sugar analyses of BL before and after mild acid hydrolysis were conducted for us by Pulp and Paper Institute of Canada analytical services using the modified TAPPI procedure, T249 hm-85. Simulation of the theoretical kinetic curves using optimized reaction rate constants was performed by using the Lotus 123 plotting procedure. All other computer calculations were conducted by our Data General MV/4000 superminicomputer system.
Acknowledgment We express our gratitude to Domtar Inc. for support in performing and publication of this work and to Ortech International (Mississauga, Ontario, Canada) for running our continuous autoclave experiments.
Literature Cited Aoyagi, T.; Hosoya, S.; Nakano, J. New Reaction Site in Lignin During Oxygen Alkali Treatment. In Chemistry of Delignification uith Oxygen, Ozone and Peroxides; Gratzl, J. S., Nakano, J., Singh, R. P., Eds.; UNI Publishers: Tokyo, Japan, 1980; p 165. Chang, H.-M.; Gratzl, J. S. Ring Cleavage Reactions of Lignin Models with Oxygen and Alkali. In Chemistry of Delignification with Oxygen, Ozone and Peroxides; Gratzl, J. S., Nakano, J., Singh, R. P., Eds.; UNI Publishers: Tokyo, Japan, 1980; p 151. Coxon, P. G.; Bischoff, K. B. Lumping Strategy. 1. Introductory Techniques and Applications of Cluster Analysis. Ind. Eng. ('hem. Res. 1987,26, 1239. Dimmel, D. R.; Shepard, D. Synthesis of Lignin Model Dimers by Novel Techniques. J . Wood Chem. Technol. 1982, 2. 297.
I n d . Eng. C h e m . Res. 1990,29, 349-355 Ekman, K. H. Formaldehyde Obtained by Alkaline Hydrolysis of Wood and Lignin. TAPPI 1965,48, 398. Gierer, J.; Imsgard, F. Reactions of Lignin with Oxygen and Hydrogen Peroxide in Alkaline Media. In Chemistry of Delignification with Oxygen, Ozone and Peroxides; Gratzl, J. S., Nakano, J., Singh, R. P., Eds.; UNI Publishers: Tokyo, Japan, 1980; p 137. Holocher-Ertl, M.; Fricko, P.; Kratzl, K. Oxygen Oxidations of Lignins. Ekman-Days, 1981 International Symposium on Wood Pulping Chemistry; SPCI: Stockholm, Sweden, 1981; p 83. Iordache, 0. M.; Maria, G. C.; Pop, G. L. Lumping Analysis for the Methanol Conversion to Olefins Kinetic Model. Ind. Eng. Chem. Res. 1988, 27, 2218. Levenspiel, 0. Chemical Reaction Engineering; Wiley: New York, 1972. Mansson, P. Quantitative Determination of Phenolic and Total Hydroxyl Groups in Lignins. Holzforschung 1983, 37, 143. Marton, J. Reactions in Alkaline Pulping. Lignins; Wiley: New York, 1971, p 639. McGinnis, G. D.; Prince, S. E.; Bierman, C. J.; Lowrimore, J. T. Wet
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Oxidation of Model Carbohydrate Compounds. Carbohydr. Res. 1984, 128, 51. Morgan, J. E.; Saul, S. M. The Zimmermann Process in a Soda Pulp Mill Recovery System Development of a Commercial Process. Appita 1968, 22, 60. Othmer, D. F. Wet Combustion Process. Canadian Patent 1087 129, Nov 1980. Paschke, F. Derivatives of Straw Lignin. Cellulosechem. 1922,3,19. Redinger, L. Alkali Lignin its Condensation Products with Phenols, and Preparation of Curable Resins. Monatsber. Dtsch. Akad. Wiss. Berlin 1961, 3, 571. Simonson, R. The Hemicellulose in the Sulfate Pulping Process. Suen. Papperstidn. 1965, 68, 275. Wei, J.; Kuo, J. C. W. A Lumping Analysis in Monomolecular Reaction Systems. Ind. Eng. Chem. Fundam. 1969, 8, 114.
Received for reuiew April 5, 1989 Reuised manuscript receiued October 23, 1989 Accepted November 28, 1989
Ozone and Ozone/UV Decolorization of Bleaching Waters of the Paper Industry Claustre Prat, Manuel Vicente, and Santiago Esplugas* Departamento Ingenieria Quimica y Metalurgia, Facultad de Quimica, Uniuersidad de Barcelona, Marti i FranquPs I , 08028 Barcelona, Spain
This study examines the use of ozone and ozone/ultraviolet radiation treatments of a kraft bleach plant effluent proceeding from the bleaching stages for kraft pulp decolorization. An outline for the reaction is proposed in which the oxidizable compounds of the effluent are grouped into three classes: easily degradable colored compounds, colored compounds of difficult degradation, and uncolored compounds. T h e constants for the second-order reaction rate a t 20 " C and the stoichiometric coefficient were determined. Ozonization is a widely investigated method to eliminate or reduce pollution caused by the effluents of the pulp and paper industry. This method can be economically competitive with other processes in the treatment of diluted effluents. Some research work in this topic can be found in the bibliography (Huriet and Gelly, 1970; Furgason et al., 1973; Nebel et al., 1974; Ng et al., 1978) mainly in the decolorization of the kraft black liquors, but only one (Melnyk and Netzer, 1975) made a simple kinetic study. In developing the kinetic model, effluent color was attributed to two unidentified species which differed in their reactivity to ozone. It was supposed that each one of them reacted with free ozone independently, and following first-order kinetics, it was found that the most reactive species is that which is responsible for most of the initial color. The use of ozone/UV in the treatment of refractory pollutants in wastewaters has been recently developed (Garrison et al., 1975; Kuo et al., 1977; Peyton et al., 1982a,b). The treatment of kraft bleach plant effluent proceeding from the bleaching stages using oxidant and oxidant/UV processes has been studied by Coburn et al. (1984). In their study, they modify the radiation intensity, the oxidant dosification, and the reaction time. They found that the best treatment was obtained using ozone without UV radiation. They observed a significant increase in the biodegradability.
Materials and Experimental Techniques The effluent utilized was prepared from wastewaters from the chlorination stage and from alkaline extraction (in a ratio of 2:l by volume), proceeding from kraft bleach plant pulp. The characteristics of this effluent were 2.18 0888-5885/90/2629-0349$02.50/0
for pH, 3288 mg of P t / L for color, and 1721 mg of Oz/L for COD. The analyses done were for the pH and color of the effluent and the ozone concentration in the outlet gas. Color determination of the effluent was done by the chloroplatinic method as described in the standard of the Canadian Pulp and Paper Association (1974). Ozone concentration in the gas phase was determined by measuring absorbency at 253.7 nm (Rice, 1981). Oxalate actinometry (Leighton and Forbes, 1930; De Bernardez and Cassano, 1985) was performed in order to measure the photon flux entering the reactor. The experiments were carried out in a tubular reactor capable of providing a good contact between the liquid and gas reactants and which offered the possibility of irradiating the reactant mixture with ultraviolet radiation of 253.7 nm. Figure 1 illustrates the experimental installation. The reactor is a simple bubble reactor with a cocurrent circulation of gas and liquid. A diffuser valve a t the entrance produces the mix between the liquid and gas reactants. The exit zone was designed to maintain a constant level, while at the same time permitting all the gas to exit. It consists of a spherical vessel with a 4540-cm3 capacity with exits for the gas and for recirculation of the liquid. Distilled water as thermostatic fluid circulates in a jacket. One cylindrical reflector of the circular section surrounds each lamp, giving the overall system a clover leaf shape. Table I shows the characteristics of this reacting system. The ozone used in the reaction is produced before it enters the reactor. Bottled gas (air or oxygen) passes through an SLO CONSTREMA ozonizer. The production 0 1990 American Chemical Society
