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Ind. Eng. Chem. Res. 1996, 35, 590-597
Effect of Pulping Conditions and Black Liquor Composition on Newtonian Viscosity of High Solids Kraft Black Liquors A. A. Zaman and A. L. Fricke* Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611
The influence of black liquor composition and solids concentrations on the Newtonian viscosity of slash pine black liquors over wide ranges of temperature (up to 140 °C) and solids concentrations (between 50% and 83% solids) has been studied. It was found that the zero shear rate viscosity of high solids black liquors depends strongly on the cooking conditions and/or black liquor composition. Not only is high solids viscosity affected by lignin molecular weight and lignin concentration in the liquor but other organic and inorganic constituents of black liquor also make a significant contribution to viscosity. The dependency of zero shear rate viscosity on solids concentrations, and temperature is defined. The Newtonian viscosities vary over a wide range depending on temperature, solids concentrations and solids composition. The results indicate that, at fixed levels of effective alkali and sulfidity, the zero shear rate viscosities can be described as a function of both lignin concentration and lignin molecular weight. The viscosity of black liquor is an increasing function of the organics-to-inorganics ratio and is a decreasing function of the concentration of sodium and chloride ions and pH of the liquor. Introduction Rheological properties of highly concentrated black liquors are of particular interest in the recovery unit of the pulp and paper industry. Kraft black liquor, which is an aqueous solution of spent inorganic cooking chemicals and organic constituents extracted from the wood during the pulping process, is a valuable fluid which is currently being used as a source of energy for pulp and paper making. In the recovery unit, black liquor is concentrated to concentrations above 65% solids and then burned in a specially designed furnace as fuel to produce steam. After the combustion, the resulting smelt is processed to recover the cooking chemicals and recycled back to the pulping process (Empie et al., 1994). The importance of rheological properties of black liquor was realized during the past several years due to an extensive effort to increase the energy efficiency of the pulp mill by firing black liquor at higher solids concentrations (Fricke, 1985; So¨derhjelm, 1986). In fact, in the past, the pulp and paper industry has suffered from the lack of knowledge and reliable correlations for rheological properties of black liquors (especially at high solids content) as a function of the processing conditions. As the solids concentration of the liquor is increased to satisfy expectations for higher energy recovery, accurate rheological models become necessary for proper design of the components of the recovery unit. The effects of temperature and solids concentrations on rheological properties of high solids black liquors have been reported in several papers (Fricke, 1985, 1987, 1990; Zaman and Fricke, 1991, 1994, 1995a; Wight, 1985; Tiu et al., 1993; Oye et al., 1977). However, our knowledge about the effects of black liquor composition and pulping conditions on the rheological properties of black liquor is still incomplete and needs further investigation (So¨derhjelm and Sågfors, 1992; Milanova and Dorris, 1989). The rheological properties of black liquor are strongly affected by the composition of the liquor and the type of the wood species. In our earlier paper (Zaman and Fricke, 1995b) on the kinematic viscosity of low solids (e50%) slash pine kraft black liquors, we showed that, although the viscosity is influenced mainly by the lignin concentra0888-5885/96/2635-0590$12.00/0
tion and its molecular weight, other constituents of black liquor (e.g., sodium ions) make a significant contribution to viscosity. Different constituents of black liquor adversely affect the viscosity of the concentrated liquors and have an important effect on the combustion properties in the recovery furnace. Therefore, knowledge of the effects of different constituents of black liquor on the viscosity at high solids is essential (1) to avoid difficulties which may occur in the recovery unit, (2) to find procedures to decrease the viscosity of different black liquors to improve their combustion properties, and (3) to determine the variation of viscosity with the liquor composition. In this paper, the effects of the solids concentration, temperature, degree of the delignification, and chemical composition on the zero shear rate viscosity of different slash pine kraft black liquors at high solids concentrations (g50%) are presented and discussed. Background In an earlier paper (Zaman and Fricke, 1994), we reported the Newtonian viscosity data for several slash pine black liquors at high solids concentrations. By applying a combination of the absolute reaction rates and free volume concepts, we were able to develop very accurate models for Newtonian viscosity of black liquors as a function of temperature at a specific solids content. The equation that can be applied to define the zero shear rate viscosity-temperature relationship for highly concentrated black liquors over a wide range of temperature can be written as (Zaman, 1993; Zaman and Fricke, 1994):
η0 ) AT0.5 exp
{
}
CTg B + T T - 1.3Tg
(1)
where η0 is the zero shear rate viscosity (Pa‚s), T is the absolute temperature (K), Tg is the glass transition temperature (K), and A, B, and C are constants. Also, it was shown that the effects of temperature and solids concentrations can be combined into a single correlating factor and the Newtonian viscosity for a single liquor over a wide range of temperature and © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 2, 1996 591 Table 1. Pulping Conditions, Kappa Numbers, H Factors, Lignin Concentration and Lignin Molecular Weights for Slash Pine Black Liquorsa cook no.
t, min
T, K
EA, %
S, %
H
K
CL, g/g solids
M hw
ABAFX011,12 ABAFX013,14 ABAFX015,16 ABAFX017,18 ABAFX019,20 ABAFX021,22 ABAFX023,24 ABAFX025,26 ABAFX027,28 ABAFX029,30 ABAFX031,32 ABAFX033,34 ABAFX035,36 ABAFX037,38 ABAFX039,40 ABAFX041,42 ABAFX043,44 ABAFX051,52 ABAFX053,54 ABAFX055,56 ABAFX057,58 ABAFX059,60 ABAFX069,70 ABAFX073 ABAFX075,76
40.0 80.0 80.0 40.0 80.0 40.0 40.0 80.0 80.0 40.0 40.0 80.0 40.0 80.0 80.0 40.0 60.0 60.0 60.0 60.0 60.0 60.0 60.0 100.0 20.0
438.8 449.9 438.8 449.9 438.8 449.9 438.8 449.9 438.8 449.9 438.8 449.9 438.8 449.9 438.8 449.9 444.3 455.4 444.3 444.3 444.3 444.3 433.2 444.3 444.3
13.0 13.0 16.0 16.0 13.0 13.0 16.0 16.0 13.0 13.0 16.0 16.0 13.0 13.0 16.0 16.0 14.5 14.5 11.5 17.5 14.5 14.5 14.5 14.5 14.5
20.0 20.0 20.0 20.0 35.0 35.0 35.0 35.0 20.0 20.0 20.0 20.0 35.0 35.0 35.0 35.0 27.5 27.5 27.5 27.5 12.5 42.5 27.5 27.5 27.5
555.5 2731.7 893.0
106.8 40.1 61.0 34.8 90.3 53.8 53.5 18.5 77.5 74.1 88.0 26.4 76.9 43.2 48.3 35.8 51.0 24.8 83.3 29.3 81.2 42.1 92.9 35.2 108.2
0.350 0.435 0.380 0.409 0.421 0.417 0.384 0.393 0.408 0.427 0.344 0.440 0.376 0.423 0.406 0.404 0.419 0.430 0.417 0.406 0.397 0.413 0.339 0.430 0.365
6411.0 6618.0 8687.0 6422.0 6544.0 6931.5 7528.5 3910.0 7960.0 9358.0 5046.5 6023.5 5995.5 5589.0 7571.5 5693.0 9672.0 5181.0 7154.0 7149.0 9582.0 6470.0 4820.5 5002.5 6279.5
838.5 1499.5 631.2 2594.7 982.0 1401.5 473.3 2606.2 574.7 2421.7 977.1 1433.0 1270.1 3066.9 1236.8 1234.1 1248.5 1178.7 561.1 1993.3 513.66
a t, cooking time; T, cooking temperature; EA, effective alkali; S, sulfidity; K, Kappa number, H, H factor; C , lignin concentration; M hw L lignin weight-average molecular weight.
solids concentration can be described as:
[(S +S 1)T1] + c[(S +S 1)T1]
ln η0 ) a + b
2
(2)
where a, b, and c are constants that are composition dependent and S is the solids mass fraction. Equation 2 has proven to be applicable to different experimental and mill black liquors from woods of different sources. This greatly reduces the amount of the experimental data that is required to define the Newtonian viscosity behavior of a new black liquor at high solids concentrations. Although the influence of temperature, solids concentration, and shear rate on viscosity has received a lot of attention, the effects of solids composition on high solids viscosity have not been investigated extensively. The main organic components of black liquor are degradation products of lignin, hemicellulose, cellulose, and polysaccharides. Lignin, which has a spherical molecular configuration, makes the largest contribution to the viscosity of softwood kraft black liquors (Oye et al., 1977). Lignin is believed to be able to aggregate with polyphenols to form a gel that improves the combustion behavior of black liquor (Hermans and Grace, 1984). The polysaccharides present in black liquor also make a contribution to the viscosity of the liquor and cause the viscosity to increase by increasing the concentration of these components in black liquor (So¨derhjelm et al., 1992). The viscosity of black liquor also varies with the degree of delignification (Zaman and Fricke, 1995b; So¨derhjelm and Sågfors, 1994) due to the fact that the lignin concentration and lignin molecular weight distribution varies with the Kappa number of the pulp (or H factor or pulping yield). The viscosities of the liquors from high-yield pulping at low sulfidity are very high (So¨derhjelm and Sågfors, 1994) due to high lignin molecular weight in the liquor. The viscosity of the liquors from low to medium pulping yield will be decreased by increasing the degree of delignification (Kappa number), but the liquors from a high-yield pulping show a maximum in viscosity for Kappa num-
bers between 40 and 70 (So¨derhjelm and Sågfors, 1994). The carbonates at low concentration do not have a significant effect on the viscosity of black liquor at low solids content (e50%) (Zaman and Fricke, 1995b), but their effect on viscosity becomes more significant as the solids content of black liquor is increased (Milanova and Dorris, 1989; Li and Lo, 1991). In this study, the results of the study of the Newtonian viscosity of slash pine kraft black liquors at high solids concentrations are presented. The liquors are from a statistically designed pulping experiment and were made at a liquor-to-wood ratio of 4/1. Although the details of the pulping, preparation and chemical analysis of the liquors, and procedures for measurement and estimation of zero shear rate viscosities can be found in several earlier papers (Fricke, 1990; Zaman and Fricke, 1995a; Zaman et al., 1991; Schmidl et al., 1990; Schmidl, 1992; Dong and Fricke, 1990; Stoy et al., 1992; Zaman, 1993), these are briefly reviewed here. The pulping conditions, along with the Kappa number of the pulp, H factor, lignin concentration, and lignin weightaverage molecular weights, are summarized in Table 1. In this work, the effects of the solids concentration, degree of the delignification (Kappa number and H factor), and chemical composition of the liquor on the zero shear rate viscosities are discussed. Materials and Methods The liquors used in this study are from a four variable-two level rotatable composite, statistically designed experiment for pulping slash pine in a pilotscale digester operated with liquor circulation. The four cooking variables were effective alkali, sulfidity, temperature, and time at temperature. The experiments were conducted so as to include all potential commercial conditions without exhaustion of chemicals. For rheological studies, the liquors should be concentrated up to about 80-83% solids. This was done in two stages. In the first stage, the liquor was concentrated in two steps in a pilot-scale wiped film evaporator operated at about 0.45 atm. In the first step, the liquor
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was concentrated to about 30% and skimmed. It was then concentrated in the second step to 45-50% solids. In the second stage, the liquors were concentrated to about 80-83% solids in a small-scale evaporator operated at 0.2-0.4 atm. The concentrated liquors were drained into plastic bottles, sealed, cooled, and stored at 4 °C for use. Chemical analysis of the liquors and molecular weight characterization of the lignin were performed completely. Chemical analyses can be divided into six categories: (1) inorganic anions (sulfite, sulfide, sulfate, carbonate, thiosulfate, and chloride); (2) inorganic cations (Na, K, Ca, Mg); (3) low carbon number organic acids; (4) lignin; (5) lignin molecular weight; (6) TAPPI tests (Kappa number, yield, sulfated ash). The details of the methods and instrumentation are described elsewhere (Fricke, 1987, 1990). Since the rheological properties of black liquor vary over an extremely wide range depending upon temperature, solids concentration, solids composition, and shear rate, different kinds of rheological instruments were used to determine the viscosity of the liquors. At concentrations above 50% solids, a Haake RV-12 viscometer with normal open cup (for 40 °C e T e 90 °C and % solids e 65) and a custom-built pressure cell, concentric cylinder viscometer (for T g 100 °C and % solids < 75) were used to determine the viscosity of the liquors for shear rates up to 1000 s-1. An Instron capillary viscometer (Model 3211) was used for highly concentrated liquors (g65%) at temperatures up to 120 °C and shear rates up to 10 000 s-1. For conditions that the liquors showed non-Newtonian behavior, the zero shear rate viscosities were estimated by extrapolation of the shear viscosity data to zero shear rates (Zaman, 1993; Zaman and Fricke, 1994) and/or were measured directly at very low shear rates using a Rheometrics RMS-800 mechanical spectrometer with a parallel-plate setup at temperatures below 85 °C. Comparison of estimates of zero shear rate viscosities agrees with direct measurements within 7% for the liquors used in this work. Results 1. Dependence of Zero Shear Rate Viscosity on Solids Content and Temperature. Black liquors, in general, show complex rheological behavior. Their character changes from a Newtonian to a non-Newtonian fluid as the solids concentrations of the liquor is increased (Zaman and Fricke, 1991, 1994, 1995a; Tiu et al., 1993). For a Newtonian fluid, the viscosity is a material constant and does not depend on shear rate. For a non-Newtonian fluid the viscosity is not a material constant and it is a function of shear rate. At this condition, the lower limiting viscosity (viscosity at very low shear rate) is called the zero shear rate (Newtonian) viscosity. It can be expected that black liquors show Newtonian behavior at temperatures above 130 °C and concentrations up to 83% solids (Zaman, 1993; Zaman and Fricke, 1994). Therefore, it is very important to study the effects of the solids composition on Newtonian viscosity of high solids black liquors for improvement in design and efficient operation of kraft recovery systems. The Newtonian viscosities of 25 different slash pine kraft black liquors made based on a four variable-two level factorially designed experiment for pulping slash pine were determined for solids concentrations from 50% up to 85% solids and temperatures from 40 to 140 °C. The methods of determination of zero shear rate
Figure 1. Zero shear rate viscosity as a function of solids concentration for liquor ABAFX017,18 at different temperatures.
Figure 2. Zero shear rate viscosity as a function of solids concentration for liquor ABAFX019,20 at different temperatures.
viscosities have been given elsewhere (Zaman, 1993; Zaman and Fricke, 1994). Figures 1 and 2 represent the zero shear rate viscosities as a function of the solids concentrations for two of the liquors. As can be seen, the viscosity of black liquor at high solids content is a strong function of both solids concentrations and temperature. At a fixed temperature, there is at least 2-6 orders of magnitude change in viscosity as the solids content is increased from 55% to 80% solids. The effects of solids content on viscosity is more significant at lower temperatures. The plots indicate that, at any fixed temperature, the zero shear rate viscosity of black liquors can be defined accurately by a single straight line as:
ln η0 ) a + bS
(3)
where a and b are constants which are dependent on the temperature and solids composition. As an example, the zero shear rate viscosity of these liquors can be expressed as:
(a) liquor ABAFX017,18 at 120 °C ln η0 ) -15.59 + 19.95S
(4)
with R2 ) 0.998 and
(b) liquor ABAFX019,20 at 100 °C ln η0 ) -17.16 + 24.5S with R2 ) 0.997.
(5)
Ind. Eng. Chem. Res., Vol. 35, No. 2, 1996 593
Figure 3. Composite plot for zero shear rate viscosity of different black liquors.
Figure 4. Plots of zero shear rate viscosity as a function of the Kappa number at 80% solids and 120 °C.
Figure 3 is a composite plot of zero shear rate viscosities of different black liquors as a function of the combined temperature-concentration variable using the procedure which was given earlier (Zaman, 1993; Zaman and Fricke, 1995a). The data for every single liquor follows a single curve which can be defined by eq 2, which is a three-constant model. The zero shear rate viscosities of the liquors are different due to differences in the solids composition of the liquors. The zero shear rate viscosity of liquor ABAFX013,14 is considerably higher than the rest of the liquors in this plot. The lignin concentrations in liquors ABAFX013,14, ABAFX019,20, ABAFX033,34, and ABAFX051,52 are 0.435, 0.421, 0.44, and 0.43 g/g solids, respectively. The lignin weight-average molecular weights (Schmidl, 1992; by GPC) in these liquors are 6618, 6544, 6023.5, and 5181.0, respectively. These indicate that both lignin concentration and lignin molecular weight affect the viscosity of black liquor. In order to study the effects of the degree of delignification and solids composition on the Newtonian viscosity of black liquors, eq 2 was employed to fit the data for 25 different slash pine kraft black liquors and then the zero shear rate viscosities were determined at a particular solids content and temperature. The variation of the Newtonian viscosity with the pulping conditions and black liquor composition is presented and discussed here. 2. Influence of the Degree of Delignification on Zero Shear Rate Viscosity. Figure 4 is a plot of zero shear rate viscosity as a function of Kappa number
Figure 5. Plots of zero shear rate viscosity as a function of the H factor at 80% solids and 120 °C.
(Kappa number is an indication of the amount of lignin in the pulp after the pulping operation) at 80% solids and 120 °C at different levels of effective alkali and sulfidity. At a fixed level of sulfidity, the viscosity is a decreasing function of effective alkali at any Kappa number at low levels of sulfidity. At a fixed level of effective alkali, the viscosity response to sulfidity varies with the level of the Kappa number. At Kappa numbers lower than 40, the viscosity decreases by increasing the level of sulfidity at a fixed level of effective alkali. At Kappa numbers higher than 85, the viscosity increases by increasing the level of sulfidity. At any level of effective alkali and sulfidity, the viscosity increases with increasing Kappa number, reaches a maximum and then decreases with further increase in the Kappa number. This can be explained in terms of the molecular weight of lignin dissolved during the kraft pulping process. The lignin dissolved in the initial stage of the delignification has a smaller molecular weight than the lignin dissolved during the bulk delignification process. The molecular weight of lignin again decreases at the final stage of the cooking process (Fricke, 1990; So¨derhjelm, 1986; Wight, 1985). The location of the maxima varies with the level of effective alkali and sulfidity. 3. Influence of the H Factor on Zero Shear Rate Viscosity. In the pulp and paper industry, it is common to combine the effects of cooking time and cooking temperature as a single variable called the H factor (Vroom, 1957). For the liquors used in this study, H factors were determined from the temperature profiles recorded for each cook and are given in Table 1. Figure 5 is a plot of zero shear rate viscosities versus H factor at different levels of effective alkali and sulfidity for these liquors at 80% solids and 120 °C. At a fixed level of sulfidity, the viscosity response to effective alkali depends on the H factor. For the liquors used in this study, at low H factors, the viscosity increases with increasing level of effective alkali; at higher H factors (H factor ≈ 1000 for S ) 20% and H factor ) 1200 for S ) 35%), the viscosity decreases with increasing level of effective alkali. At a fixed level of effective alkali, the same response can be observed as the level of sulfidity is increased (for H factors up to 900 at EA ) 13% and for H factors up to 1100 at EA ) 16%), while at high H factors the viscosity decreases with increasing sulfidity. An increase in viscosity with an increase in sulfidity at low H factors (initial phase of delignification process) at a constant level of effective alkali indicates that, at low H factors, the presence of sulfide ions in the liquor helps the condensation reactions and formation of lignin with larger molecular weights. At high
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Figure 6. Plots of zero shear rate viscosity as a function of lignin concentration at 80% solids and 120 °C.
Figure 8. Effects of lignin molecular weight and lignin concentration on zero shear rate viscosity of black liquors at 80% solids and 120 °C. Table 2. Constants of Equation 6 along with Effective Alkali, Sulfidity, and R2 of the Fits
Figure 7. Plots of zero shear rate viscosity as a function of lignin molecular weight at 80% solids and 120 °C.
H factors, the sulfide ions in the liquor react with certain groups in lignin and prevent the formation of larger molecular weight lignins. Also, they may help the cleavage of other bonds in the lignin molecule (Rydholm, 1965; Wight, 1985) that causes the formation of smaller lignin molecules in the black liquor. The same explanation can be given for the effect of effective alkali on viscosity at a fixed level of sulfidity. At the initial stage of the delignification process (low H factors), the presence of sodium ions in the liquor leads to large lignin molecules (or larger amounts of lignin), while at high H factors (bulk delignification or final stage of delignification), sodium ions degrade lignin in solution to form smaller molecular weight lignins and also degrade carbohydrate derivatives that are dissolved in the liquor and that reduce the concentration of lignin in the black liquor. 4. Influence of Black Liquor Composition. 4.1. Effect of Lignin Concentration and Lignin Molecular Weight. To establish the effect of lignin concentration and lignin molecular weight on Newtonian viscosity of slash pine black liquors at high solids concentrations, the zero shear rate viscosity of the liquors at 80% solids and 120 °C were plotted as a function of the lignin concentration and lignin weightaverage molecular weight at different levels of effective alkali and sulfidity, as is shown in Figures 6 and 7. It can be observed that the viscosity is a strong and increasing function of the lignin concentration and the level of response varies with the levels of effective alkali and sulfidity which can be attributed to differences in
EA (%)
S (%)
a
b
R2
13 16 13 16
20 20 35 35
0.328 3.65 × 10-4 0.132 5.736 × 109
7.61 0.29 6.25 33.51
0.99 0.99 0.98 0.99
the concentration of other constituents of black liquor and lignin molecular weight. Figure 7, which is a plot of zero shear rate viscosity as a function of lignin molecular weight at different levels of effective alkali and sulfidity, indicates that, at any level of effective alkali and sulfidity, the zero shear rate viscosity is an increasing function of the lignin molecular weight. The variation of zero shear rate viscosity with molecular weight is greater at lower levels of effective alkali and sulfidity. Figures 6 and 7 indicate that, at a fixed solids content and temperature, the zero shear rate viscosities for any level of effective alkali and sulfidity can be defined by a single straight line as a function of lignin concentration and lignin molecular weight. Figure 8 shows the combined effect of lignin concentration and lignin molecular weight on zero shear rate viscosity of the liquors at different levels of effective alkali and sulfidity. At each level of effective alkali and sulfidity, the zero shear rate viscosity at a specific temperature and solids concentration can be described as:
η0 ) aM h wCLb
(6)
where M h w is the lignin weight-average molecular weight (lignin molecular weights were measured using gel permeation chromatography, GPC), CL is the lignin concentration in the liquor (g/g solids), and a and b are constants. For the liquors used in this study, at 80% solids and 120 °C, the constants a and b were determined and are shown along with R2 of the fits in Table 2. 4.2. Influence of the Organics-to-Inorganics Ratio. Figure 9 represents the zero shear rate viscosity of the liquors at 70% solids and 100 °C as a function of the organics-to-inorganics ratio in the liquors at different levels of effective alkali and sulfidity. As can be observed both at low and high levels of effective alkali and sulfidity, the viscosity increases rapidly with increasing organic-to-inorganic ratio and then there is a flat region where there is a slow variation in viscosity with a further increase in the organics-to-inorganics
Ind. Eng. Chem. Res., Vol. 35, No. 2, 1996 595
Figure 9. Plots of zero shear rate viscosity as a function of the organics-to-inorganics ratio at 70% solids and 100 °C.
Figure 10. Plots of zero shear rate viscosity as a function of the concentration of sodium ions at 80% solids and 120 °C.
ratio. At high levels of effective alkali and low levels of sulfidity and at low levels of effective alkali and high levels of sulfidity, there is almost a uniform increase in viscosity with an increase in the organic-to-inorganic ratio. The results indicate that black liquors from cooks of lower levels of effective alkali and sulfidity are higher in their organic-to-inorganic content. 4.3. Effect of Sodium Ions. The zero shear rate viscosity of the liquors used in this study show a significant decrease as the concentration of sodium ions in the liquors is increased. This is indicated in Figure 10 where zero shear rate viscosities at 80% solids and 120 °C are plotted as a function of sodium ion concentration for the liquors corresponding to cooks of different levels of effective alkali and sulfidity. The decrease in viscosity is more significant at higher concentrations of sodium ions. This is in agreement with our previous results on kinematic viscosity (Zaman and Fricke, 1995b) of black liquors and with the results that have been reported by other investigators (Milanova and Dorris, 1989; Hermans and Grace, 1984). The decrease in viscosity of black liquors by addition of NaOH and Na2SO4 has been discussed in detail by Milanova and Dorris (1989). The level of decrease in viscosity by addition of sodium ions depends on the concentration of other constituents of black liquor. For the liquors that correspond to higher levels of sulfidity, there is a slow decrease in viscosity at low concentration of sodium ions and then there will be a sharp decrease in viscosity as the concentration of sodium ions in the liquor is increased. The addition of sodium hydroxide to black
Figure 11. Plots of zero shear rate viscosity as a function of the concentration of chloride ions at 70% solids and 100 °C.
Figure 12. Plots of zero shear rate viscosity as a function of the concentration of sulfite ions at 70% solids and 100 °C.
liquors will break aggregation of lignin sols (Lindstro¨m, 1979) and therefore will reduce the viscosity of the liquor. Also, an increase in the concentration of sodium ions helps the physical dissociation of lignin by formation of ionized phenolic hydroxyl and carboxyl groups that will keep the lignin in solution (Hermans and Grace, 1984) and, as a consequence, the viscosity of the liquor will be reduced. 4.4. Influence of Chloride Ions. The influence of chloride ions on zero shear rate viscosity of the liquors at 70% solids and 100 °C is shown in Figure 11. The liquors with higher concentrations of chloride ions show less viscous behavior. This plot shows that the liquors from cooks of low levels in effective alkali and high levels in sulfidity are more viscous at any concentration of chloride ions. At high levels of effective alkali and low levels of sulfidity, there is a small decrease in viscosity of the liquor as the concentration of chloride ions is increased. At high levels of effective alkali and high levels of sulfidity, two regions of viscosity behavior can be observed. In the first region, there is a slow decrease in viscosity as the concentration of chloride ions is increased. This region is followed by a second region, where there will be a dramatic decrease in the viscosity with a further increase in the concentration of chloride ions in the liquor. 4.5. Influence of Sulfite Ions. For black liquors used in this study, the zero shear rate viscosity shows a significant increase as the concentration of sulfite ions in the liquor is increased. This is shown in Figure 12 where Newtonian viscosities of the liquors at 70% solids and 120 °C are plotted as a function of sulfite ions at
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Figure 13. Plots of zero shear rate viscosity as a function of the concentration of carbonate ions expressed as g/g Co32- at 80% solids and 120 °C.
different levels of effective alkali and sulfidity. The increase in viscosity is more significant for liquors that correspond to cooks that are either low in both levels of effective alkali and sulfidity or high in both levels of effective alkali and sulfidity. Almost no comments can be found in the literature that describe the effects of sulfite ions on the viscosity of black liquor, and there is a lack of published data in this area. However, this probably can be explained in terms of the effect of sulfur compounds on oxidation of black liquor (Herman and Grace, 1984) and the dramatic increase in the viscosity of the liquor. In black liquor oxidation, Na2S will be converted to thiosulfate, sulfite, and sulfate and therefore the alkalinity of the liquor will be decreased. Hermans and Grace (1984) have shown that, as a result of the drop in the alkalinity of the liquor, high molecular weight constituents will be degraded by oxidation to low molecular weight acid salts. As a consequence, ionized hydrophilic groups that stabilize the lignin molecule in solution will be deionized and the lignin molecules may associate to cause a dramatic increase in the viscosity of the liquor. 4.6. Effect of Carbonates. The influence of carbonates on zero shear rate viscosity can be observed from Figure 13, which is a plot of zero shear rate viscosity of the liquors at 80% solids and 120 °C at different levels of effective alkali and sulfidity. As can be observed from this figure, the viscosity response of high solids black liquors to the concentration of carbonate ions varies with the level of effective alkali and sulfidity and it is not quite the same as low solids viscosity. In our earlier work (Zaman and Fricke, 1995b) on the kinematic viscosity of black liquor at low solids content (e50%), we observed that there is a small decrease in viscosity as the concentration of carbonate ions is increased. However, these results indicate that carbonates can have both increasing and decreasing effect on the viscosity of black liquor at high solids concentrations, depending upon the level of effective alkali and sulfidity. At low levels of effective alkali (13%) the viscosity increases as the concentration of carbonate ions increases. The increase in viscosity is more significant at low levels of sulfidity. At high levels of effective alkali (16%), viscosity decreases with increasing concentration of carbonate ions. The rate of decrease in viscosity is higher at higher levels of sulfidity (35%). The effect of CaCO3 on the viscosity of a number of high solids black liquors has been studied by Milanova and Dorris (1989). They have reported that addition of CaCO3 up to a concentration of 100 g/L caused a small increase in the viscosity of the liquor. However, they concluded that if the viscosity of the
Figure 14. Plots of zero shear rate viscosity as a function of pH of the liquors at 80% solids and 120 °C.
liquors before and after addition of CaCO3 is compared at the same solids content, the effect of CaCO3 was to reduce the viscosity of the liquors. The results of our work indicate that carbonate ions can have either increasing or decreasing effects on the viscosity of high solids black liquors, depending upon the composition of the liquor. 4.7. Influence of pH. Figure 14 represents the effect of pH of the liquors on their Newtonian viscosity behavior at 80% solids and 120 °C. At any level of effective alkali and sulfidity, the liquors with lower pH are more viscous than the liquors with higher pH values. It can be observed that the effect of pH on viscosity depends upon the composition of black liquor, and the liquors corresponding to cooks of high level in effective alkali (16%) and high level in sulfidity show a large decrease in viscosity with a small increase in the level of their pH. When the pH of the liquor is increased, dissociation occurs due to the deformation of hydrogen bonds between the particles (Lindstro¨m, 1979). Also, there will be a large decrease in the apparent weight-average molecular weight of lignin (Milanova and Dorris, 1989) as the pH of the liquor is increased (up to a factor of 60 times decrease in apparent molecular weight for increasing pH from 8.5 to 13.8). These effects explain the decrease in viscosity of black liquor with increasing pH of the liquor. Summary The zero shear rate viscosity of high solids black liquors was found to depend strongly on the cooking conditions and/or composition of the liquor. The results indicate not only that high solids viscosity is affected by lignin concentration and lignin molecular weight in the liquor but also that other organic and inorganic constituents of black liquor make a significant contribution to viscosity. The viscosity of black liquor is a decreasing function of the sodium ions concentration and chloride ions concentration in the liquor. The viscosity of the liquors produced at different cooking conditions decreases as the concentration of these ions in the liquor is increased. The effect of sulfite ions on viscosity is more significant at both low levels of effective alkali (13%) and sulfidity (20%) and high levels of effective alkali (16%) and sulfidity (35%). The viscosity increases as the concentration of sulfite ions in the liquor is increased. The effect of carbonate ions on high solids viscosity varies with the cooking condition and can have either an increasing or a decreasing effect on the viscosity of high solids black liquors. The viscosity
Ind. Eng. Chem. Res., Vol. 35, No. 2, 1996 597
of the liquors that corresponds to cooks at a high level in effective alkali (16%) is an increasing function of the carbonate ions concentration (at both levels of sulfidity), while the viscosity of the liquors from cooks of low level in effective alkali (13%) and both levels of sulfidity (20% and 35%) is decreased by increasing the concentration of carbonate ions in the liquor. The viscosity of black liquor is a decreasing function of the pH of the liquor. The influence of pH on viscosity of black liquor is more significant at higher levels of effective alkali and sulfidity. These results suggest that the viscosity of high solids black liquors can be reduced by increasing the concentration of sodium and chloride ions in the liquor and also the alkalinity of the liquor. These will cause dissociation of hydrophilic groups and a decrease in the viscosity of black liquor. The viscosity of black liquor is also an increasing function of the organics-to-inorganics ratio in the liquor, which indicates that the organic compounds of black liquor, in general, do have an increasing effect on the viscosity of high solids black liquors. Comparison with our previous work on low solids viscosity (Zaman and Fricke, 1995b) indicates that the viscosity response of black liquor to liquor composition depends on the solids concentration of the liquor. The zero shear rate viscosity of a particular liquor at a specific temperature is a strong function of the solids concentration, and it may change by as much as 2-6 orders of magnitude as the concentration of the liquor changes from 50% to 80% solids. At a fixed temperature, a single straight line can describe the viscosity of the liquor as a function of the solids concentration accurately. Our previous (Zaman, 1993; Zaman and Fricke, 1994) empirical model was applied to describe the zero shear rate viscosity data of the liquors as a function of the combined temperature-concentration variable and has been shown to be successful for many different black liquors. The results of this work can be applied to correlate the constants of eq 2 to the pulping conditions and black liquor composition which will be the subject of a future paper. Acknowledgment The authors are grateful for the financial support provided by the Office of Industrial Technologies of the U.S. Department of Energy under Grant No. DE-FG0285ER40740 and by a large number of industrial firms. Nomenclature a, b, and c ) constants of eqs 2, 3, and 6 A, B, and C ) constants of eq 1 CL ) lignin concentration, g/g solids EA ) effective alkali (%) h ) H factor K ) Kappa number M h w ) lignin weight-average molecular weight S ) solids mass fraction in eqs 2-5; sulfidity elsewhere t ) time, min T ) temperature, K Tg ) glass transition temperature, K η0 ) zero shear rate viscosity, Pa‚s
Fricke, A. L. Physical Properties of Kraft Black Liquor: Interim ReportsPhase II. DOE Report on Contract No. DG-AC0282CE40606; University of Maine: Orono, ME, and University of Florida: Gainesville, FL, 1985. Fricke, A. L. Physical Properties of Kraft Black Liquor: Summary ReportsPhase I and II. DOE Report Nos. AC02-82CE50606 and FG02-85CE40740; University of Florida: Gainesville, FL, and University of Maine: Orono, ME, 1987. Fricke, A. L. A Comprehensive Program to Develop Correlations for Physical Properties of Kraft Black Liquors. Interim Report No. 2; University of Florida: Gainesville, FL, 1990. Hermans, M. A.; Grace, T. M. The Effect of Oxidation on Black Liquor Composition and Properties. Proceedings of the TAPPI Pulping Conference, 1984; pp 575-578. Li, Q.; Lo, S. N. Characteristics of Soda Wheat Straw Black Liquor and Effects of Sodium Composition on the Viscosity of its Black Liquor. Zhongguo Zaozhi, 1991, 1 0(4), 16-23. Lindstro¨m, T. Colloid Polym. Sci. 1979, 257, 277. Milanova, E.; Dorris, G. M. Effects of Residual Alkali Content on the Viscosity of Kraft Black Liquors. Proc. Int. Chem. Rec. Conf. 1989, 101-118. Oye et al. The Properties of Kraft Black Liquors from Various Eucalypts and Mixed Tropical Hardwoods. Appita J. 1977, 31 (1), 33-40. Rydholm, S. A. Pulping Processes; Interscience Publishers: New York, 1965. Schmidl, W. G., Dong, D. J.; Fricke, A. L. Molecular Weight and Molecular Weight Distribution of Kraft Lignins. Proc. Mater. Res. Symp. 1990, 197, 21. Schmidl, W. G. Molecular Weight Characterization and Rheology of Lignin for Carbon Fibers. Ph.D. Dissertation, University of Florida, Gainesville, FL, 1992. So¨derhjelm, L.; Sågfors, P. E. Factors Influencing the Viscosity of Kraft Black Liquor. J. Pulp Pap. Sci. 1994, 20 (4), 106-110. So¨derhjelm, L. Viscosity of Strong Black Liquor. Paperi ja PuusPapper Och Tra¨ 1986, 68 (9), 642. So¨derhjelm, L.; Sågfors, P. E.; Jan, J. Black Liquor Viscosity. Influence of Polysaccharides. Pap. Puu 1992, 74 (1), 56-58. So¨derhjelm, L.; Sågfors, P. E. Relationship Between the Viscosity and Composition of Black Liquor. International Chemical Recovery Conference, TAPPI Proceedings, 1992; pp 513-517. Stoy, M., Zaman, A. A.; Fricke, A. L. Vapor-Liquid Equilibrium for Black Liquors. Proceedings International Chemical Recovery Conference, Book 2, 1992; p 495. Tiu, C., Nguyen, K. L.; De Guzman, M. Non-Newtonian Behavior of Kraft Black Liquors. Appita J. 1993, 46 (3), 203-206. Vroom, K. E. Pulp Pap. Mag. Can. 1957, 58 (C), 228. Wight, M. O. An Investigation of Black Liquor Rheology Versus Pulping Conditions. Ph.D. Dissertation, University of Maine, Orono, ME, 1985. Zaman, A. A. An Investigation of the Rheological Properties of High Solids Kraft Black Liquors. Ph.D. Dissertation, University of Florida, Gainesville, FL, 1993. Zaman, A. A.; Fricke, A. L. Viscosity of Black Liquor Up to 130 °C and 84% Solids. AIChE For. Prod. Symp. Proc. 1991, 59-77. Zaman, A. A.; Fricke, A. L. Newtonian Viscosity of High Solids Kraft Black Liquors: Effects of Temperature and Solids Concentration. Ind. Eng. Chem. Res. 1994, 33 (No. 2), 428-435. Zaman, A. A.; Fricke, A. L. Shear Flow Properties of High Solids Softwood Kraft Black Liquors: Effects of Temperature, Solids Concentration, Lignin Molecular Weight and Shear Rate. Chem. Eng. Commun. 1995a, 139, 201-221. Zaman, A. A.; Fricke, A. L. Viscosity of Softwood Kraft Black Liquors at Low Solids Concentrations: Effects of Solids Content, Degree of Delignification and Liquor Composition. J. Pulp Pap. Sci. 1995b, 21 (4), April. Zaman, A. A.; Dong, D. J.; Fricke, A. L. Kraft Pulping of Slash Pine. AIChE For. Prod. Symp. Proc. 1991, 59.
Received for review March 27, 1995 Revised manuscript received July 12, 1995 Accepted July 25, 1995X
Literature Cited Dong, D. J.; Fricke, A. L. UV-Visible Response of Kraft Lignin in Softwood Black Liquor. Proc. Mater. Res. Soc. Symp. 1990, 194, 77. Empie, H. J.; Lien, S.; Samuels, D. B. Distribution of Mass Flows in Black Liquor Sprays. Presented at the AIChE Annual Meeting, Nov 13-18, 1994, San Francisco, CA.
IE950202H
Abstract published in Advance ACS Abstracts, January 1, 1996. X
