Effects of pH, Slurry Composition, and Operating Conditions on Heat

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Effects of pH, Slurry Composition, and Operating Conditions on Heat Transfer Fouling during Evaporation of a Lignocellulosic Biomass Process Stream Raghu N. Gurram and Todd J. Menkhaus* Department of Chemical and Biological Engineering, South Dakota School of Mines and Technology, 501 East St. Joseph Street, Rapid City, South Dakota 57701, United States ABSTRACT: Fouling of stainless steel heat transfer surfaces by soluble and suspended solids within lignocellulosic biomass process streams during evaporation elevates energy consumption and cleaning costs associated with this unit operation. Efficient evaporation is important for long-term economic stability of the biorefinery industry. In this study we evaluated the effect of hydrolysate pH, pretreatment variety (dilute acid versus hot water), and shear forces at the evaporator surface on fouling, as measured by the performance parameters such as induction period, fouling rate, and final total heat transfer resistance. Using an annular fouling probe, fouling tests were conducted at pH 1.5, 3.0, and 5.0 at 120, 130, and 140 °C with 0, 10, and 20% (w/v%) total suspended solids. Effects of Reynolds number and Ca2+ ion concentration on fouling rate and induction periods were also determined. Fouling deposits were characterized using scanning electron microscopy (SEM), inductively coupled plasma (ICP), and ash analysis. Results showed that moderate mineral deposition through CaSO4 precipitation had a positive effect on heat transfer through reduced fouling resistance due to the deposition of a more porous fouling layer. Evaporation experiments also showed that the fouling rates and resistance can be greatly reduced by increasing the Reynolds number for flow within the evaporator.

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INTRODUCTION Driven by the rapid depletion of petroleum reserves, as well as environmental and political concerns associated with petrol feedstocks, there has recently been extensive research on production of biorenewable fuels such as ethanol, butanol, biodiesel, and biogasoline from nonfood biorenewable lignocellulosic feedstocks.1−3 Biochemical conversion of lignocellulosic biomass into a liquid fuel involves four main processing sections: physicochemical pretreatment, enzymatic saccharification, fermentation, and distillation for purification of liquid fuel as the primary product.4,5 Due to the limitation of solids loading (≤20%) at the first step of the biochemical conversion process (pretreatment), the final fuel titers following fermentation are relatively low (e.g., ethanol is typically 18−20 g/L), which in turn leads to an increase in energy consumption for product purification and recovery using distillation.6−9 Concentrating the sugars prior to fermentation can increase the fermentation productivity and simultaneously reduce the distillation energy requirements.10 Evaporation has been widely used in the food and dairy industries to concentrate fruit juice and milk.11−16 However, fouling of evaporator surfaces has been identified as a serious problem due to the increased heat transfer resistance leading to higher energy consumption and additional cleaning costs.17,18 Fouling is a complex phenomenon and depends on many variables such as process stream composition, flow rates and evaporator configurations, pH, and fluid temperature. For instance, pH and its relationship to the isoelectric point of dairy proteins (i.e., β-lactoglobulin) has been shown to play a key role during heat exchanger fouling within the dairy processing industry.15,16,19 At the same time, flow rate and evaporator configuration directly affect the shear rate at the heating surface © 2013 American Chemical Society

and thus influence the formation of fouling deposits. For instance, Belmar-Beiny et al. found that the amount of whey protein fouling deposited in a tubular heat exchanger decreased with increasing flow rate.20 Precipitation of inorganic salts has also been widely reported as a predominant form of fouling within evaporators and heat exchangers due to the deposition of inverse solubility salts (e.g., salts whose solubility decreases with an increase in temperature) that are originally dissolved in the process stream. Calcium sulfate is one such inverse solubility salt that has been cited in numerous studies as a common foulant during water desalination processing, as well as juice concentration within multiple-effect evaporators.21−25 Solid phase calcium sulfate has two primary stable forms, gypsum (CaSO4·2H2O), which is formed at low temperatures (120 °C).26 Fouling of heat transfer surfaces from anhydrite calcium sulfate is very difficult to mitigate because of its relatively large change in solubility at different temperatures (approximately 2.73 kg/m3 at 25 °C, while at higher temperatures it becomes more insoluble, as low as 0.34 kg/ m3 at 120 °C).27 Besides temperature, other factors affect calcium sulfate solubility, such as the solution’s pH and operating pressure of the evaporator. In general, calcium sulfate is more soluble at low pH and high pressures.28,29 While one alternative would be to remove calcium sulfate by gypsum precipitation prior to further downstream processing, this Received: Revised: Accepted: Published: 11122

April 1, 2013 July 19, 2013 July 19, 2013 July 19, 2013 dx.doi.org/10.1021/ie401039q | Ind. Eng. Chem. Res. 2013, 52, 11122−11131

Industrial & Engineering Chemistry Research

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operation adds additional complexity, increases processing time, produces additional waste, and adds significantly to the overall costs of the process. Our previous study showed significant calcium sulfate precipitation at temperatures >110 °C with the addition of 10% (w/v) total suspended solids within a dilute sulfuric acid pretreated biomass enzymatic hydrolysate at pH 5.0.30 This current study was designed to further evaluate solution properties and operating conditions that could be used to minimize evaporator surface fouling within a biorefinery process. With this in mind, the effect of hydrolysate pH (to increase the calcium sulfate solubility), the use of hot water pretreatment (to reduce calcium sulfate precipitation by removing SO42‑ ions), and different flow rates (to improve shear at the evaporator surface) were explored. In all cases pine wood was chosen as the biomass for testing and the fouling behavior and fouling deposit composition were analyzed using a custom fabricated annular fouling probe along with characterization tools such as scanning electron microscopy (SEM), inductively coupled plasma (ICP) spectroscopy, and ash analysis.

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MATERIALS AND METHODS

Raw Material, Pretreatment, and Enzymatic Hydrolysis. Ponderosa pine wood in saw dust form with particle size 120 °C, with and without the addition of suspended solids (Table 1 and Figures 2 and 3). This may be due to the presence of free SO42− ions in the acid pretreated hydrolysate at pH 5, which allowed a more porous calcium sulfate precipitate fouling layer to form on the evaporator surface compared to a fouling layer composed of only sugars and other organic compounds within the hot water sample. Our previous study confirmed this rationale by showing that the presence of SO42− ions was responsible for CaSO4 precipitation at temperatures >120 °C when 10% solids were added to the hydrolysate.30 It was also observed that the evaporation rates were higher at 10% TSS than 0% TSS. This indicated that formation of a porous mineral

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RESULTS AND DISCUSSION Effect of Hydrolysate pH and Type of Pretreatment on Evaporator Fouling. Figure 2 and Table 1 show the effects of hydrolysate pH and acid pretreatment versus hot water pretreatment on fouling resistance during evaporation at 130 and 140 °C with no suspendered solids present. At all of the temperatures tested, acid pretreated hydrolysate at pH 3.0 had a higher fouling rate and greater final fouling resistance than pH 1.5 and 5.0. A similar trend was observed at 120 °C (data not shown). The fouling was most severe and rapid at pH 3.0 and 140 °C, with a maximum fouling resistance and fouling rate of 0.98 m2·K/kW and 17.79 m2·K/kW·s, respectively (Figure 2 and Table 1). These were approximately 5 and 24 times higher, respectively, than the final resistance and rate of fouling at 120 °C, which provided the minimum fouling cases. The observed severe fouling may be due to the rapid aggregation and denaturation of the cellulase enzymes, along with the sugar concentrations reaching a critical value with an increase in temperature (e.g., previous studies have shown that above ∼75 g/L sugars, rapid and severe fouling of the 11125

dx.doi.org/10.1021/ie401039q | Ind. Eng. Chem. Res. 2013, 52, 11122−11131


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Figure 4. Effect of dissolved Ca2+ ions with total 80 g/L glucose during the evaporation at 140 °C with 10% TSS.

and facilitate the CaSO4 precipitation analysis, anhydrous Ca(OH)2 was added to the hydrolysate in such a way that total calcium ion concentration calculated was equal to 15 (∼0.37 mol/L), 20, and 25 g/L (excess calcium ion concentration). Note that even at the lowest concentration, the Ca2+ ions were double the SO42− ions in terms of a molar basis. No pH adjustment was made with the addition of different Ca levels. Similarly, anhydrous glucose was added to the hydrolysate in such a way that the total glucose concentration (including that in the original hydrolysate) resembled the concentrated liquor at 60, 80, and 100 g/L. Figure 4 shows the effect of dissolved Ca 2+ ions concentration at 80 g/L glucose, 140 °C, and with 10% TSS, on fouling resistance. As the dissolved Ca2+ ion concentration was increased, both the fouling rate and the final resistance of fouling were increased. A similar trend was observed at the other glucose concentrations tested of 60 and 100 g/L (figures not shown). Table 2 represents the actual numbers of fouling resistance and fouling rate at different calcium and glucose conditions. A maximum fouling resistance of 0.74 m2·K/kW and average fouling rate of 17.54 m2·K/kW·s was observed during the evaporation of hydrolysate with a total of 25 g/L dissolved Ca2+ ions and 80 g/L glucose, which were almost 2 and 9 times higher than the final resistance and fouling rate observed at 15 g/L Ca2+ ions, respectively (Figure 4 and Table 2). This was likely due to the availability of excess free Ca2+ ions to react with dissolved SO42− ions, thus precipitating higher amounts of salts under the nucleate boiling conditions (140 °C as described in our previous study). These findings (increased fouling with an increase in excess dissolved Ca2+ ions) indicated that more CaSO4 precipitation can be detrimental to the heat transfer characteristics but that lower fouling and increased evaporation rates can be achieved with moderate levels (as discussed above where very low calcium levels had more detrimental fouling characteristics). Jamialahmadi and Muller-Steinghagen reported similar findings with a decrease in the heat transfer coefficient (from 21 to 4 kW/ m2·K) with an increase in CaSO4 concentration (from 0.50 to 1.65 g/L).42 Lower levels were not evaluated in their study to determine if reduced heat transfer occurred at very low

deposition layer has a higher overall heat transfer coefficient than the thick nonporous gel layer from protein and sugar adsorption in the absence of inorganic precipitate. Figure 3 clearly represents the fouling severity of hot water pretreatment at 140 °C with 10% TSS. Fouling was initiated within 30 s after the evaporation process was initiated, and the fouling resistance and fouling rates were almost two times greater than the acid pretreated hydrolysate at pH 5.0 (Table 1). This confirmed that the fouling was most problematic when there were no SO42− ions and the accompanying inorganic salt precipitation was absent. Combined, these results suggest that the presence of SO42− ions in the hydrolysate is actually favorable during evaporation, in terms of reducing the severity of heat transfer surface fouling with biomass hydrolysates. In summary, both pH change (lower pH from original 5.0 to 3.0), as well as hot water pretreatment in place of dilute sulfuric acid pretreatment, which were both hypothesized to increase the solubility of CaSO4 and eliminate the salt precipitation by removing SO42− ions, actually created more adverse conditions than evaporation of the original acid pretreated hydrolysate. Combined Effect of Glucose and Ca2+ Ion Concentration. From the previous section, fouling analyses completed at different pH values and with hot water pretreated material in place of acid pretreated material indicated that CaSO4 precipitation may in fact be beneficial. Hence, we tested the CaSO4 precipitation at different dissolved Ca2+ ion levels and different concentrated glucose amounts to determine the most beneficial operating windows. These investigations were completed in a continuous mode, where feed was constantly added and vapor constantly removed, to mimic the actual industrial evaporator process (batch studies were beneficial for ease of experimentation and to provide insight into fouling mechanisms). A material balance completed from the dilute acid pretreatment provided the dissolved SO42− ions concentration in the hydrolysate to be 15 g/L or ∼0.16 mol/L (this was confirmed experimentally by Gurram et al.41). Elemental compositional analysis through ICP-MS from our previous study showed that calcium was the major soluble inorganic component in our pine wood hydrolysate, at approximately 14 g/L.30 In order to vary the dissolved Ca2+ ion concentration 11126



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elevated fermentation inhibitor concentrations that have coconcentrated with sugar (yeast inhibition from furfurals and acids can lead to low fermentation yields). Removal of potential fermentation inhibitors such as acetic acid, hydroxy-methyl furfural, and furfural prior to, or after, evaporation would be one way to reduce the yeast inhibition and at the same time increase the ethanol productivities.41,43,44 Effect of Hydrodynamics. Due to maximum fouling conditions (Figure 5 and Table 2), 25 g/L total dissolved Ca2+ ions with 80 g/L total glucose concentration and 10% TSS, at 140 °C, was chosen to evaluate the effect of Reynolds number on the fouling resistance. Viscosity for this type of biomass slurry is very difficult to measure and thus the absolute viscosity for the biomass slurry with 10% TSS at 50 °C was estimated to be 0.68 kg/m·s from the available literature.45−47 The mean density of hydrolysate with 25 g/L total dissolved Ca2+ ions, 80 g/L glucose, and 10% TSS was measured to be 1.06 kg/m3 (at 50 °C). The Reynolds number for four different flow rates of 90, 150, 220, and 300 mL/min was calculated to be 83, 138, 202, and 276, respectively. All of these Re values were in laminar flow region (Re < 2100) but did offer different levels of shear at the surface of the evaporator. It was found that the induction periods were increased from 0 to 3.25 h as the Re increased from 83 to 276, as shown in Figure 5. For tests at Re of 83, rapid fouling was observed in the first 45 min of evaporation process. As the Reynolds number increased from 83 to 276, both fouling rates and final fouling resistance were decreased by almost 2- and 6-fold, respectively (Table 2). Studies by Wilkins et al. reported the decrease in fouling rate of thin stillage from 6.9 to 3.0 m2·K/kW·min when Reynolds number was increased from 440 to 880.32 Similar findings were reported by Fahiminia et al. in a study to observe the effect of temperature and velocity on the early events of calcium sulfate precipitation fouling.48 They showed that the initial fouling rates were decreased 2-fold when the velocity of calcium sulfate solution increased from 0.3 to 1.2 m/s at an 85 °C wall temperature. Walker and Sheikholeslami also reported similar observations in a study to assess the effect of velocity and residence time in CaSO4 precipitation fouling using CFD modeling.49 It was observed that as the Reynolds number increased from 250 to 1000, the dissolved Ca2+ ions

Table 2. Effect of Glucose and Ca(OH)2 (top) and Reynolds Number (down) on Fouling Resistance and Rate of Foulinga glucose (g/L) 60

80

100

Ca(OH)2 (g/L)

fouling resistance (m2·K/kW)

15 20 25 15 20 25 15 20 25 Reynolds number (80 g/L 49 82 120 164

fouling rate (m2·K/kW·s)

0.47 8.41 0.49 8.96 0.61 23.02 0.41 1.94 0.57 9.56 0.74 17.54 0.29 10.02 0.38 10.73 0.50 13.16 glucose, 25 g/L Ca(OH)2)

0.75 0.55 0.35 0.13

17.83 19.86 12.82 7.53

a

All samples represent the average of three trials with less than 5% deviation between replicates.

concentrations. Moreover, this study was for pure calcium sulfate, so fouling by other components was not considered. Glucose concentration was found to have a mixed effect on the fouling resistance at a constant total dissolved Ca2+ ion concentration. For instance, the fouling resistance increased from 0.61 to 0.74 m 2 ·K/kW with increase in sugar concentration from 60 to 80 g/L, while at concentrated conditions of higher sugars (100 g/L) fouling resistance was decreased from 0.74 to 0.50 m2·K/kW at constant Ca2+ ion concentration of 25 g/L (Table 2). This indicated that a higher sugar concentration inhibited the calcium sulfate precipitation, even in the presence of excess Ca2+ ions, thus reducing the heat transfer surface fouling. These observed characteristics may have trade-offs during the concentration of hydrolysate to achieve higher final sugar concentrations. Higher sugar concentrations can be beneficial with respect to fouling characteristics; however, concentrating the hydrolysate to such elevated sugar levels can have processing complications in terms of increased viscosity (slurry handling difficulties) and

Figure 5. Effect of Reynolds number on fouling resistance of hydrolysate with 25 g/L Ca2+ ions, 80 g/L glucose, and 10% TSS at 140 °C. 11127



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denser fouling material). As discussed earlier, it was confirmed that the formation of porous mineral deposition improved the overall heat transfer coefficient compared to the thick bulky nonporous gel.

concentration at the boundary layer reduced from 2.0 to 0.5 g/ L, respectively, which decreased the precipitation fouling. These observations were due to the increased shear forces and decreased thermal boundary layer thickness along the heated probe surface with an increase in fluid velocity or Re.20 Increased shear forces disrupt the adsorption of fouling deposits onto the heated rod surface. A decreased boundary layer thickness reduces the volume of fluid at temperatures suitable for reactions associated with fouling to occur. From these findings, it can be inferred that the CaSO4 precipitation, along with other fouling mechanisms associated with evaporator fouling with biomass slurries, can be greatly reduced by increasing the Reynolds number. Macroscopic Appearance of Fouling Deposits. Deposits collected from the heat transfer surface of the fouling probe following all experiments had a smooth appearance and were strongly adhered to the probe, as shown in Figure 6. In all

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FOULING DEPOSIT CHARACTERIZATION SEM Analysis. Fouling deposits collected for acid pretreated hydrolysis at different pH levels of 1.5, 3.0, and 5.0, along with hot water pretreated hydrolysate, all with 10% dry (w/v) TSS operated at 140 °C internal probe temperature, were chosen to determine the effect of pH and type of pretreatment on fouling through SEM analysis. Scanning electron micrographs of these deposits are shown in Figure 7.

Figure 7. SEM images of fouling deposits collected at 140 °C 10% TSS pH 1.5 (A), pH 3.0 (B), pH 5.0 (C), hot water pretreated hydrolysate at pH 5.0 (D), and 25 g/L Ca2+ ions and 80 g/L glucose operated in continuous mode (E).

Figure 6. Photographs of fouled probe at pH 1.5 (A), pH 3.0 (B), pH 5.0 (C), and hot water pretreated hydrolysate (D) with 10% TSS at 140 °C.

Figure 7A−C shows the calcium sulfate crystallization morphology at different hydrolysate pH values of 1.5, 3.0, and 5.0, respectively. At pH 1.5, the CaSO4 structure was more of a hexagonal shape, but as the pH increased to 5.0, the single hexagonal crystals turned into more of a columnar sheet. These different structures of CaSO4 observed were remarkably similar to the reported SEM images of different calcium sulfate crystal shapes (hemibipyramidal, subhedral, hexagonal, and columnar) during the geological soil formation on gypsiferous materials.50 It was suggested that the dynamic soil environment was responsible for the observed variable calcium sulfate structures. From this study, it can be inferred that the calcium sulfate structures formed during precipitation from a biomass hydrolysate is variable and dependent on pH of the solution. The micrograph of the deposits at pH 3.0 show the restricted calcium sulfate crystal formation in a soft nonporous gel matrix (Figure 7B), which had the higher fouling characteristics. This may be due to the rapid aggregation of viscous cellulase

cases, the deposits had a brownish/black layer of what appeared to be charred sugar and burned lignin (or other residual biomass) immediately adjacent to the heating surface, as observed in our previous study.30 Fouling deposit photographs confirmed observations associated with the fouling curves and fouling resistance values for different conditions explored within Figure 3. As discussed previously, the fouling resistance was highest for the hot water pretreated hydrolysate (0.93 m2·K/kW) at pH 5.0, followed by acid pretreated hydrolystate at pH 3.0, 1.5, and 5.0 (0.72, 0.55, and 0.49 m2·K/kW, respectively Table 1). Interestingly, the fouling deposits appeared to be similar for pH 1.5 and 5.0 (Figure 6A and C) with highly porous scale formation indicating the mineral deposition, while the pH 3.0 and hot water pretreated hydrolysate (Figure 6B and D) had a similar morphology and resembled a thick bulky nonporous layer (indicating that rapid protein and sugar adsorption combined with solids to form a 11128



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Table 3. Elemental Composition and Ash Content of Fouling Deposits Collected at Different Temperatures and Solids Loadings of Acid Pretreated Hydrolysate (pH 3.0) and Hot Water Pretreated Hydrolysate (pH 5.0)a pH 3.0 120 °C Na Mg Al P S K Ca Mn Fe Zn ash a

hot water pretreated hydrolysate (pH 5.0)

130 °C

140 °C

120 °C

130 °C

140 °C

10%TSS

20%TSS

10%TSS

20%TSS

10%TSS

20%TSS

10%TSS

20%TSS

10%TSS

20%TSS

10%TSS

20%TSS

3.6 3.6 3.7 2.2 481.8 10.4 17.4 0.9 16.0 5.7 7.7

3.1 3.3 2.1 2.2 515.8 9.3 19.9 0.8 18.4 0.8 11.3

3.5 3.5 5.3 2.1 480.6 9.7 15.8 0.9 16.6 2.6 12.5

3.6 3.4 3.6 2.3 445.9 9.4 16.5 0.9 20.4 0.5 13.8

3.4 3.4 6.4 2.2 449.9 9.4 15.4 0.8 14.3 2.2 14.9

3.6 3.5 4.6 2.6 456.1 9.7 17.7 0.8 19.8 0.4 15.4

3.4 3.2 4.0 2.8 7.2 15.2 35.2 1.1 21.8 0.8 5.8

3.2 3.8 4.5 2.7 7.3 14.5 34.5 1.5 22.3 0.9 7.4

3.8 3.6 4.8 2.2 7.3 14.9 33.2 1.2 23.4 0.8 8.6

3.5 3.9 5.1 2.5 7.5 14.3 36.3 1.4 24.5 0.9 10.5

3.7 3.3 4.7 2.6 7.1 14.7 34.5 1.3 25.6 0.8 9.7

3.4 3.4 5.2 2.4 7.4 14.1 33.3 1.6 27.5 0.8 12.3

Elemental composition and ash content are represented in ppt and % dry base, respectively.

predominant elements in all the fouling deposits (except hot water pretreated hydrolysate samples) with an average concentration of 15.0 and 500.0 ppt, respectively (Table 3). This shows that the crystals observed in the SEM pictures were in fact CaSO4 precipitation. The average concentration of calcium in the fouling deposits at pH 1.5, 3.0, and 5.0 were approximately 16.0, 17.5, and 34.0 ppt, respectively, while the average concentration of sulfur were 543.0, 472.0, and 452.0 ppt, respectively. The increase in sulfur concentrations observed at lower pH values may be due to the additional H2SO4 added during the pH adjustment prior to evaporation to reduce the pH. These increasing and decreasing concentrations of calcium and sulfur from pH 1.5 to 5.0 may be responsible for the formation of different structures observed in the SEM analysis (Figure 7A−C). As expected, calcium and iron concentrations were higher in fouling deposits with higher levels of TSS at all the pH tested (Table 3) because these compounds were solubilized from the biomass during processing. All the other elements such as sodium, magnesium, phosphorus, potassium, and manganese (except aluminum and zinc) were approximately at same concentrations in 10% and 20% TSS. A corresponding increase in the ash content from fouling deposits with 20% TSS was also observed, which suggested that the ash components in fouling deposits were higher at higher levels of TSS. Higher concentrations of Al and Zn in fouling deposits at 10% TSS may have catalyzed the CaSO4 precipitation, which was in agreement with our previous study.30 Calcium (∼34.5 ppt) is the only predominant element present in all the hot water pretreated hydrolysate samples followed by sulfur, iron, potassium, magnesium, sodium, phosphorus, and trivial concentrations of manganese, and zinc (Table 3). Sulfur content was reduced by approximately 65 times when compared to acid pretreated hydrolysate deposits at pH 5.0 through the hot water pretreatment, while calcium may have come from the pine wood as observed in our previous study. With the lower amount of sulfur (or sulfate ions), calcium did not reacted and formed CaSO4 crystals, and no other inorganic salt combinations were found to precipitate. This was very likely the reason for absence of mineral deposition in the SEM images of hot water pretreated hydrolysate but also contributed to the higher fouling resistance (Figures 7D and 3).

enzymes at a pH near the isoelectric point, immobilizing the calcium sulfate crystals formed during the evaporation of hydrolysate, as described earlier. It can also be noticed that the rapid sugar and protein deposition rate may not have allowed the crystal structure to develop and organize to the extent observed in Figure 7A and C. Evidently, the nonporous gel layer with restricted crystal growth was the reason for higher fouling resistance observed when compared to the pH 1.5 and 5.0 (Figures 3 and 6A−C). With the removal of SO42− ions from the hydrolysate through hot water pretreatment, there was no inorganic salt crystal formation during the evaporation process as shown in Figure 7D. These fouling deposits were bulky, with woody material, which was presumably from the residual biomass (primarily hemicellulose and lignin). Fourier transform infrared (FTIR) spectroscopy analysis from our previous work showed that the lignin was responsible for forming the first layer of fouling when solids were added to the hydrolysate.30 These current findings indicate the high propensity for lignin to foul the heated surface under a variety of conditions. Flocculation of lignin through polyelectolyte flocculation prior to evaporation could reduce the fouling severity and simultaneously enhance the solid−liquid clarification throughput.51 Figure 7E shows the CaSO4 precipitation in continuous evaporation mode of hydrolysate with 25 g/L total dissolved Ca2+ and 80 g/L total glucose, with 10% TSS at 140 °C. The availability of excess free Ca2+ ions reacted with the dissolved SO42− ions to form elevated amounts of CaSO4 that precipitated on the heat transfer surface due to inverse solubility. The crystals were very distinctive and clustered to form a thick layer of mineral deposition on the fouling probe compared to the moderate deposition during the evaporation of acid pretreated hydrolyssate at pH 5.0 in batch mode (Figure 7C). The thick layer of CaSO4 precipitation was very likely the reason for higher fouling resistance observed with the excess dissolved Ca2+ ions compared to the evaporation of acid pretreated hydrolysate with moderate deposition. This again indicates that CaSO4 precipitation has a positive effect on heat transfer characteristics at moderate quantities, while it can be detrimental at higher depositions. ICP and Ash Analysis. ICP analysis was performed to determine the elemental composition of the different fouling deposits collected. Calcium and sulfur were the two 11129



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CONCLUSIONS Hydrolysate at pH 3.0 had a shorter induction period and a greater fouling rate than at pH 1.5 and 5.0 for all of the temperatures tested, both with and without the addition of solids. This thicker/denser fouling layer (as observed by SEM) was likely from cellulases in the hydrolysate that precipiated and adsorbed onto the evaporator surface as the pH approached the isoelectric point of the cellulases. Hot water pretreatment, which removed SO42− ions compared to the dilute acid pretreated hydrolysate in order to alleviate calcium sulfate precipitation during the evaporation process, actually had an adverse effect by increasing the rates and final resistance values of the evaporator fouling. Moderate amounts of inorganic calcium sulfate salt precipitation increased the porosity of the fouling layer and improved the evaporation process. SEM, fouling characterization, and ICP analysis showed that the moderate mineral deposition through CaSO4 precipitation had a positive effect on heat transfer through reduced fouling resistance during the evaporation at normal hydrolysate conditions. Longer induction periods with lower fouling rates and lower total resistance could also be achieved by increasing the Reynolds number. Higher calcium ion concentrations (above ∼15 g/L) caused massive salt precipitation, which resulted in adverse fouling characteristics. However, at sugar concentrations above ∼80 g/L, the higher sugar levels inhibited calcium sulfate precipitation and lowered the fouling resistance. Overall, conditions with lower fouling rates and increased induction periods would reduce the cleaning costs and improve the overall process efficiencies of lignocellulosic biorefineries. While only individual responses were explored in this study, it is probable that interactions between variables could influence performance by diminishing or exacerbating fouling effects.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (605) 394-2422. Fax: (605) 394-1232. E-mail: Todd. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for R.N.G. was provided by the USDA NIFA, AFRI Competitive Grant no. 2010-65504-20372, and the South Dakota School of Mines and Technology. In addition, the work was partially supported by funding from the U.S. Department of Energy, Office of the Biomass Program.

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Effects of pH, Slurry Composition, and Operating Conditions on Heat

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