rheology of high-solids biomass slurries for biorefinery applications

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Rheology of high-solids biomass slurries for biorefineryapplications

Jeffrey S. Knutsen and Matthew W. Liberatorea)

Department of Chemical Engineering, Colorado School of Mines,1613 Illinois Street, Golden, Colorado 80401

(Received 18 November 2008; final revision received 5 May 2009�

Synopsis

iomass slurries, such as dilute-acid pretreated corn stover �PCS�, will be a common processtream in biorefineries designed to convert agricultural residues into biofuels such as ethanol. Inhis work, the advantages and disadvantages of several rheological techniques are evaluated forCS suspensions. Three flow regimes were evaluated: �i� shear flow using a vane, �ii� torsionalow between parallel plates, and �iii� biaxial extensional flow between plates. The vane provided

he simplest methodology and the most reproducible results. Four experiments were conductedsing the vane: �i� transient flow, �ii� stress ramps, �iii� creep, and �iv� oscillatory shear. PCSlurries with fractions of insoluble solids �FIS� ranging from 5% to 17% by weight exhibitedoft-solid characteristics, including an apparent yield stress. Yield stresses were highly dependentn stover concentration, scaling with FIS to the sixth power, and ranged from 0.2–5000 Paetween 5% and 17% FIS. PCS suspensions were strongly shear thinning, with flow and dynamiciscosities that were highly dependent on FIS. Last, as with many concentrated suspensions, theox–Merz rule was not followed, although flow and dynamic viscosities were coincident whenlotted versus an effective shear rate.

2009 The Society of Rheology. �DOI: 10.1122/1.3143878�

. INTRODUCTION

Ethanol production from cellulosic biomass has been lauded for many years as aotential renewable, large-scale transportation fuel with numerous benefits, including themprovement of air quality, mitigation of global climate change, and improvement ofnergy security. Although non-grain feed-stock supplies, energy balances, and environ-ental impacts are generally favorable, economic analyses suggest that current biomass-

o-ethanol process designs are likely to be prohibitively expensive �Cleveland et al.2006�; Wyman �2007��. Sugars generated via enzymatic saccharification can be con-erted into value-added products, including fuel ethanol, in a new process paradigmenerally referred to as the biorefinery �Ragauskas et al. �2006��. However, to be useful,hese sugars must be manufactured at a suitably high concentration, and the economics ofaccharification must be favorable. Operation of a biorefinery at a relatively high-solidsoading is one way to meet these requirements. For example, a biomass-to-ethanol designeport compiled at the National Renewable Energy Laboratory �NREL� �Aden et al.2002�� calls for an insoluble solids content of over 10% �w/w� after pretreatment, while

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Author to whom correspondence should be addressed; electronic mail: [email protected]
2009 by The Society of Rheology, Inc.877. Rheol. 53�4�, 877-892 July/August �2009� 0148-6055/2009/53�4�/877/16/$27.00

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process developed by Iogen Corporation �Tolan �2002�� accommodates an upper-boundnsoluble solids concentration of 15%–20% �w/w�. By minimizing the amount of waterntering the process, the resulting sugar concentrations will be higher and capital andperating costs will be lower due to the decreased volume �Mohagheghi et al. �1992��. Inomparison, processes based on starch have long been run at concentrations as high as3%–37% �Thomas and Ingledew �1992��, and optimized cellulose-based designs mayventually reach similarly high concentrations.

Lignocellulosic biorefinery designs typically rely on an initial pretreatment process,uch as dilute-acid hydrolysis, to partially liquefy the raw feedstock by dissolving frac-ions of lignin and hydrolyzing hemicellulose into five-carbon sugars �Nguyen et al.1996��. Thus, pretreatment creates biomass slurries rich in cellulose and lignin that cane more readily transported to downstream processes. Unfortunately, high-solids slurriesave large viscosities and yield stresses and are notoriously difficult to process. Process-ng accounts for up to two-thirds of the energy cost per gallon of lignocellulosic fuelWyman �2007��. Compounding difficulties with design and scale up, processing costsre likely to be a very sensitive function of solids concentration. For example, Fan ando-workers �2003� observed that the necessary power requirements for mixing of paperludges increased sharply with solids concentration; a 4% increase in solids contentesulted in a fivefold increase in power consumption. Therefore, in pursuit of an optimaliorefinery design, understanding and interpretation of the slurries’ rheological propertiess of paramount importance.

Recently, investigations of high-solids lignocellulosic slurries have begun �Pimenovand Hanley �2003, 2004�; Dasari and Berson �2007�; Jørgensen et al. �2007�; Rosgaard etl. �2007�; Um and Hanley �2008�; Viamajala et al. �2009��. Accurate rheological char-cterization of these slurries is plagued by a number of difficulties, especially wall slipnd settling of particulates in dilute suspensions. Pimenova and Hanley �2003, 2004� usedhelical impeller to infer the rheological properties of corn stover suspensions through a

eries of relative measurements. Although circumventing many difficulties, the helicalmpeller induced a complex flow that did not allow direct calculation of the shear rate.ather, relative measurements were made by calibrating the tool with Newtonian andon-Newtonian fluid standards. Their work revealed that corn stover suspensions wereighly shear thinning with viscosities and yield stresses that were strongly dependent ontover concentration, increasing by orders of magnitude when the concentration of totalolids was increased from 5% to 30% �w/w� �Pimenova and Hanley �2003, 2004��.

The sensitivity of rheological properties, especially yield stress, to solids concentrations not unique to corn stover. Power-law dependences of the yield stress as a function ofoncentration �e.g., volume fraction or dry solids concentration� have been shown forentonite gels, fermentation broths, and paper pulp �Allen and Robinson �1990�; Ben-ington et al. �1990�; Alderman et al. �1991�; Mohseni and Allen �1995�; Riley et al.2000��. The power-law coefficient ranged from about 1 to 3 in these systems. Otherystems exhibit yield stresses that vary exponentially with concentration, including tita-ium dioxide suspensions, wastewater sludges, and corn stover slurries �Pimenova andanley �2004�; Mori et al. �2006�; Nguyen et al. �2006��. At low concentrations, less than.3% �w/v�, a linear correlation between yield stress and solids concentration has alsoeen observed �Laera et al. �2007��. In the current work, the concentration scaling of theretreated corn stover �PCS� slurries will be an important relationship for designingarge-scale biorefineries in the coming years.

The focus of the present work is to rheologically characterize model biomass slurriesncountered following pretreatment and preceding enzymatic saccharification. This work
ontrasts the advantages and disadvantages of several flow geometries and establishes an

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879RHEOLOGY OF HIGH-SOLIDS BIOMASS SLURRIES

ccurate, reproducible rheological methodology. Additionally, process design consider-tions demand a thorough understanding of the variability and scaling of rheologicalarameters with solids concentration. To this end, the rheology of two different batches ofCS was investigated across a broad range of solids concentrations.

I. MATERIALS AND METHODS

. Biomass

Slurries of PCS were provided by the NREL �Golden, CO�. Pretreatment at NRELonsisted of mixing corn stover with dilute sulfuric acid in an engineering-scale counter-urrent pretreatment reactor system �Nguyen et al. �1996�; Tucker et al. �1998��. Pretreat-ent hydrolyzes portions of the hemicellulose and dissolves a fraction of the lignin,

xposing cellulose fibers for subsequent enzymatic hydrolysis. The result is an aqueoususpension, or slurry, of stover particles rich in cellulose and lignin. Optical microscopyndicates that the particles are highly fibrous in nature, with length scales on the order of

icrons to millimeters and aspect ratios ranging from 1 to 20. Additionally, about 10–20arge particles of stover about 1–3 mm in diameter are present in a typical 50 mL samplef undiluted PCS. These particles are likely recalcitrant pieces of corn cob or stalk thatere not converted during pretreatment. The interstitial fluid was Newtonian and con-

isted mainly of pentose sugars and other residual solutes from pretreatment. The viscos-ty of the interstitial fluid is slightly greater than water ��=2.0�0.1 mPa s at 25 °C�.

Suspensions were characterized in terms of their fractions of total, soluble, and in-oluble solids. The weight fraction of total solids �FTS� was measured by evaporatingater and other volatile compounds from a sample of suspension. The weight fraction of

oluble solids �FSS� in the liquor was measured by centrifuging a sample of suspensionnd filtering the supernatant to remove all suspended solids. Volatile liquids in the filtrateere then evaporated, leaving only soluble solids behind. Therefore, FSS is the mass FSS

n water, not the mass FSS in the total suspension. The weight fraction of insoluble solidsFIS� represents the weight fraction of solids that do not dissolve in the suspending liquort ambient conditions. FIS was estimated by the following mass balance:

FIS =FTS − FSS

1 − FSS. �1�

IS �Eq. �1�� estimated in this manner compared well with other methodologies, includ-ng repeated washing/centrifugation and membrane diafiltration. To assess the effect ofoncentration on the rheology, the pretreated stover was diluted with de-ionized water tooncentrations as low as 5% FIS. Concentrations below 5% were not evaluated, as theseuspensions settle appreciably on the experimental time scale, resulting in measurementrtifacts. Pretreated stover was also concentrated to as much as 20% FIS by centrifugingnd removing supernatant.

. Rheometry

All rheological experiments were performed using an AR-G2 rheometer �TA Instru-ents, New Castle, DE� operated in controlled-stress or strain rate modes. Torsional and

hear flows were investigated using parallel-plate and vane-in-cup geometries, respec-ively, and biaxial extensional �squeeze� flow was examined using parallel plates. Theresence of large particles in the corn stover suspensions prevented the use of cone-and-
late geometry for shear flow experiments.

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Parallel plates used for torsional flow experiments consisted of a 40 mm diameterolished steel upper rotating plate with a larger, stationary, lower plate with Peltieremperature control. A gap height of 4 mm was maintained in torsional flow experiments.n select parallel-plate experiments, coarse-grit adhesive tape �3M, St. Paul, MN� waspplied to the upper and lower plates to mitigate fluid slip. Torsional flow experimentsere conducted by controlling the rotational speed of the upper plate and plotting the

pplied torque as a function of plate displacement.Squeeze flow circumvents many difficulties in traditional rheometers by ensuring that

he internal deformation of material occurs, especially in the presence of wall slip �Sher-ood et al. �1991�; Meeten �2000, 2002, 2004a, 2004b, 2007�; Chandler et al. �2002�;ngmann et al. �2005��. In particular, Meeten �2000, 2002� examined the utility ofqueeze flow to measure the yield stress of structured fluids. He observed that gap heightspproached limiting values hL at specific compressive forces and then compared hL toheories using the Herschel-Bulkley relation with no-slip boundary conditions. At hL, theield stress �0 can be estimated by

�0 =FhL

23�R3

, �2�

here F is the force between the plates and R is the plate radius. Thus, a plot of hL versus23�R3 /F should be linear with slope �0. Squeeze flow experiments were conducted byoading PCS preformed into a rough cylinder with an initial height of approximately 6

m, between smooth 40 mm plates. The rheometer was then programmed to maintainpecific compressive forces of 0.1, 10, 30, and 50 N by adjusting the gap height. The gaplosure rate was limited, requiring about 30 s for the compressive force to stabilize. Oncehe compressive force stabilized, the gap continued to slowly close, eventually approach-ng a limiting value after about 10 min.

Two vane geometries were used in shear flow experiments. The cup diameter for bothane geometries was 30 mm and contained Peltier temperature control. The vane diam-ters were 28 mm for the narrow gap and 15 mm for the wide gap, resulting in gap widthsf 1 and 7.5 mm, respectively. The heights of the narrow- and wide-gap vanes were 42nd 38 mm, respectively. After fully immersing the vane in the suspension, the bottom ofhe vane was 4–12 mm above the bottom of the cup. The axial force required to press theane through the slurry was monitored. Although axial forces were substantial for undi-uted PCS �17% FIS�, reaching 20–30 N in some experiments, the vane was easilyowered into dilute suspensions. Axial forces of 0.4–0.6 N for PCS at a concentration of0% FIS and 0.1 N or less at 5% FIS were measured �the latter force is on the order ofhe buoyancy force induced by the displaced volume of the vane�. To probe the magni-ude of end effects, in select experiments with undiluted PCS the vane was retracted00–300 �m to reduce the axial force below 1 N, relieving stresses at the bottom of theup. To investigate the possibility of slip at the cup wall, a serrated cup was used in selectxperiments with the narrow-gap vane. However, the presence of a few large stoverarticles or fibers with lengths of 1 to several millimeters would jam the rheometer byodging between a serration and the vane tip. Therefore, in these experiments the largestarticles were removed by sieving through a 500 �m sieve. Because large particles wereresent at a weight fraction of less than 1%, it is not likely that their removal affected theulk suspension rheology.

Several experiments were conducted with the vane, including oscillatory stress/requency sweeps in controlled-stress mode and three unidirectional experiments: tran-
ient flow, steady-state flow, and stress ramps. Oscillatory stress sweeps were used to

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dentify linear viscoelastic regions �LVRs� and were especially useful in determining theresence and magnitude of a yield stress, defined both by the crossover of G� and G� andhe maximum value of the elastic stress �G��0�. Details are described by Walls et al.2003�. Transient flow �TF� experiments were conducted by rotating the vane at a slowteady rate and plotting the applied torque as a function of vane displacement. Thesexperiments were conducted using both the narrow- and wide-gap vanes and with smoothnd serrated cups. After each experiment, the stover was removed from the cup, remixedy hand using a laboratory spatula, and re-loaded. Steady-state flow experiments wereonducted by controlling the rotational speed of the vane at several rates and monitoringhe applied torque over time. If the applied stress �torque� became relatively steady, basedn several criteria, the steady value was used as a data point in the steady-state flowxperiment. Stress ramp experiments were conducted using the narrow-gap vane byteadily increasing the applied torque and monitoring the rotational speed of the vane.ote that the first two flow experiments, based on controlled-strain rate, were generallyore reliable, as the inhomogeneous stover suspensions unpredictably jammed the rhe-

meter in controlled-stress mode. Jamming effects are mitigated in controlled-strain ratexperiments because the rheometer quickly increases the applied stress to maintain ateady rate of strain. Thus, jamming appears as small obvious stress peaks that are readilyiscounted in the data analysis. However, in controlled-stress mode, even small jams thatnly slightly exceed the applied stress will completely halt the deformation of sample.

All experiments were completed at 25 °C. Error bars represent one standard deviationf at least three replicate runs unless otherwise noted.

II. RESULTS AND DISCUSSION

. Torsional flows

Controlled-strain flow experiments using the smooth parallel-plate geometry typicallyesulted in torque versus displacement curves that rapidly approached a steady value ashe upper plate was rotated. However, bulk deformation of the stover was not observeduring these experiments, and it appeared likely that the tool was simply slipping over theuspension. Additionally, loading of the suspensions in this geometry was difficult, as thetover must be preformed into a 40 mm diameter cylinder with a height approximatelyqual to the desired gap height, typically 4 mm. Initial experiments revealed that thenitial height of the formed stover cylinder significantly affected the flow curves. Aylinder height taller than the desired gap would be compressed upon loading, resultingn a normal force between the plates and relatively high torque curves. To test whether

eaningful results could be gained from smooth plates, a set of experiments was con-ucted by changing the normal force between the plates, as shown in Fig. 1�a�. In eachxperiment, the applied torque reached a steady value after small displacements of thepper plate. Higher values were observed for higher applied normal forces. When theteady torque values are plotted against applied normal force �Fig. 1�b��, the data appearinear, with an intercept at the origin. This result suggests that the upper plate is indeedlipping, with a coefficient of dynamic friction f that is proportional to the slope of theata, as shown in Fig. 1�b�. The coefficient of dynamic friction f =FS /FN is the ratio ofhe shear or tangential force FS, acting between two sliding surfaces, to the appliedormal or compressive force FN. It is typically a constant for two given surfaces; conse-uently, the shear force is proportional to the normal force. f can be estimated using twootating circular surfaces by integrating a differential torque balance across their radii.he resulting equation shown in Fig. 1�b� relates f to the plate radius r, the torque

equired to turn the plate M, and the applied normal force �Matthews �2002��. A value of

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f �0.3 was observed in these experiments. For comparison, the coefficient of dynamicriction between PCS and the steel rotating plate is slightly higher than the publishedalue for wood-on-wood f =0.2 and roughly half the published value for steel-on-steel

f =0.57 �Serway �1995��.Controlled-strain experiments conducted with roughened parallel plates �Fig. 2� pro-

uced markedly different results than the smooth plates. In these experiments, the appliedorque grew rapidly after the initial deformation, plateaued, and then decreased as thelate continued to rotate. Early researchers have suggested that elastic deformation occursrior to yielding �e.g., Dzuy and Boger �1983��, although Meeten �2008� recently indi-ated that a viscous component is likely present in this region. As will be shown through

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IG. 1. Experiments with smooth 40 mm parallel plates using undiluted PCS �17% FIS�. �a� Torque versuslate displacement for normal forces of 5, 20, and 40 N �bottom to top�. �b� Steady torque as a function oformal force. The line is a least-squares regression of the data with an intercept at the origin. The rotationalpeed was about 1 rph.

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IG. 2. �a� Shear stress versus displacement of 40 mm parallel plates covered with rough adhesive tape forndiluted PCS �17% FIS�. The strain rate was controlled at 0.01 s−1, and the initial normal force was 2.5 N.wo example curves are shown. Photographs show PCS �1� just after loading, �2� upon yielding, and �3� afterracture and subsequent ejection from between the plates. �b� Apparent yield stress as a function of initial
ormal force upon loading. The solid line is a power-law trend to guide the eyes.

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scillatory experiments, viscous energy dissipation indeed occurs at small pre-yieldtrains. Nonetheless, in these experiments the maximum shear stress is considered anpparent yield stress. Upon yielding, the subsequent decrease in shear stress coincidedith the fracture and eventual ejection of suspension from between the plates, and there-

ore no further information is generated. The apparent yield stress did not change signifi-antly with deformation rate across three decades, from �̇=0.01 to 1 s−1. However, asith the smooth parallel plates, the normal force upon loading did affect the yield stress,

s shown in Fig. 2�b�. By qualitatively judging the contact between the roughened par-llel plates and the initial stover cylinder, useful initial normal forces were typicallyetween 1 and 3 N. Initial forces below about 1 N resulted in insufficient contact betweenhe stover and the upper plate, and the plate simply slipped over the surface of theuspension, resulting in minimal bulk deformation. Initial normal forces above about 3 Nesulted in gross biaxial deformation �squeezing� and excretion of interstitial fluid fromhe suspension �e.g., see inset photographs in Fig. 3�a��. The latter phenomenon known asadial filtration results in undesired increases in solids concentration and measurementrtifacts, especially when the interstitial fluid viscosity is low �Meeten �2007��.

. Squeeze flows

The gap height measured in several squeeze flow experiments decreased from about 6o roughly 3.5 mm when the compressive force was stepped from 0.1 to 50 N, as shownn Fig. 3�a�. The linear behavior predicted by Eq. �2� was not observed, however, whenhe limiting gap height was plotted versus 2

3�R3 /F, as shown in Fig. 3�b�. If the theoryid hold, data points would follow the shallowest line from right to left as compressionroceeded, and �0�320 Pa would be maintained throughout the duration of the experi-ent. However, radial filtration, the excretion of interstitial fluid from between the plates

s the compressive force increases, results in non-uniform solids concentrations acrosshe plate radius that increase as the experiment progresses �Meeten �2007��. As an ap-roximation, it was assumed that only interstitial fluid was squeezed radially from be-

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IG. 3. �a� Limiting gap height as a function of normal �compressive� force between 40 mm parallel plates.hotographs show the extraction of interstitial fluid from PCS due to compression �initially 17% FIS�. �b� Gapeight plotted against 2

3�R3 /F. For a yield stress fluid without slip at the interface, the data in �b� would followhe linear trend with slope �0. Deviations from linearity are possibly due to radial filtration of the interstitialuid, with predicted values of �0 increasing as shown.

eath the periphery of the upper plate, as shown by photographs in Fig. 3�a�. By this

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pproximation, FIS will increase by a predictable amount as interstitial fluid is removed,esulting in a concomitant increase in the yield stress, and strong deviations from theredicted linear behavior in Fig. 3�b�. For example, the reduction in gap height from 5400o 3300 �m results in a predicted increase in FIS from 17% to nearly 28% when aolume of fluid �V=2�r2�h� is removed. Thus, yield stresses predicted by Eq. �2� can beorrelated with FIS, as shown in Fig. 9.

. Shear flows

In general, the most successful measurements were made by inducing shear flows withhe vane geometry. Typical stress-displacement flow curves �Fig. 4� are similar to theoughened parallel-plate experiments, exhibiting a rapid increase in the shear stress atmall deformations, reaching a plateau upon yielding, and then decreasing with additionaltrain, behavior that agrees qualitatively with prior investigations �e.g., Dzuy and Boger1983��. Yield stresses measured in controlled-strain experiments were found to be inde-endent of vane rotational speeds from 0.0007 to 0.7 rpm, equivalent to shear rates of.001–1 s−1, averaged between the vane periphery and the cup wall. The independencef yield stress on rotational speed agrees with the findings of several other investigators,ho examined rotational speeds across similar ranges �Dzuy and Boger �1983, 1985�;lderman et al. �1991�; Leongpoi and Allen �1992�; Nguyen and Boger �1992�; Liddel

nd Boger �1996�; Meeten �2000��.An important consideration is the possibility of slip occurring at the inner surface of

he cup. The degree of slip was assessed qualitatively by using the narrow-gap vane withhe serrated cup and by using the wide-gap vane with the smooth cup. Twenty-eight

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IG. 4. Typical results using undiluted PCS �17% FIS� showing shear stress changing with vane displacement.ield points for each experiment are identified with circles. Numbering as follows: narrow-gap vane at 1:.0007 rpm; 2: 0.007 rpm �two example curves shown�; 3: 0.07 rpm; 4: 0.7 rpm; 5: wide-gap vane at 0.7 rpm,nd 6: narrow-gap vane using a serrated cup at �̇=0.07 rpm.

ertical serrations �2 mm deep� mitigate slip in the former geometry, and wall shear

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tresses in the latter are low enough that slip is not likely to occur. Flow curves 5 and 6n Fig. 4 measured using these geometries suggest that slip is minimal, as their respectiveield stresses are within experimental uncertainty.

After reaching maxima at the yield point, shear stresses in Fig. 4 decline with strain,ventually approaching relatively steady values that are nearly independent of shear rate.fter pre-straining the samples, the results of steady-state flow experiments across threeecades of shear rates from 0.01–10 s−1 are shown in Fig. 8. These data show thehear-thinning nature of these suspensions, as well as the strong dependency of viscosityn FIS. Due to the relative independence of the steady-state shear stress with changes inhear rate, power-law coefficients in Fig. 8 were all very close to �1 �i.e., three-orderncreases in shear rate consistently resulted in three-order decreases in the apparent vis-osity�. Additionally, viscosities at each shear rate exhibit a nearly exponential increaseith FIS; viscosities were 100–1000 times smaller when 17% FIS suspensions wereiluted to 5%.

Yield stresses were also measured by way of stress ramp experiments, in which thepplied torque was ramped up at a continuous rate. Stress ramps were performed asescribed by Meeten �2002� by plotting the shear rate versus shear stress on logarithmicxes. Two nearly linear regions were observed, and the yield stress was defined by thentersection of lines fitting each region �Fig. 5�.

The utility of creep experiments to measure yield stress was also investigated. In thesexperiments, an initial shear stress at the low end of the viscoelastic region was selected,ased on oscillatory stress sweeps, and then increased sequentially in successive steps.rior to yielding, little variation in the stover compliance is observed, and the rate oftrain decreases to nearly zero over time �Fig. 6�. The small but measurable amount oflastic deformation at low stresses corroborates the findings of Meeten �2008�. He ob-erved that stress was dissipated in step-strain experiments with dry foams at strains wellelow the yield point, suggesting pre-yield energy dissipation. Once the yield stress of

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IG. 5. Shear rate versus shear stress as measured in stress ramp experiments using the narrow-gap vane. Sheartresses were increased at rates of 100, 0.7, and 0.07 Pa/min for 17%, 10%, and 5% FIS suspensions of PCS,espectively.

CS is exceeded, compliance is highly dependent on the applied stress, and the suspen-

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ion continues to strain indefinitely. Yield stresses for 17% and 5% FIS suspensions wereound to be between 500 and 1000 Pa and 0.25 and 0.5 Pa, respectively. Creep experi-ents required long experimental times and provided only a bounding estimate of the

ield stress. Therefore, the utility of creep experiments was found to be limited, anddditional experiments were not pursued.

. Oscillatory experiments

Oscillatory stress ramps �Fig. 7� exhibit traits indicative of soft-solid behavior, com-on to both raw and diluted PCS suspensions. The presence of a LVR at low oscillatory

tresses is observed, with G� roughly one order of magnitude larger than G�, suggesting

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IG. 6. Compliance over time in creep experiments using the narrow-gap vane and smooth cup for �a� 17% andb� 5% FIS suspensions of PCS. The yield stress is between 500 and 1000 Pa in �a� and between 0.25 and 0.5a in �b�.

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IG. 7. Oscillatory stress sweep of PCS �17% FIS� using the narrow-gap vane geometry. Yield stresses areefined by �1� the crossover of G� and G� and by �2� the maximum value of the elastic stress. At low oscillatorytresses, the elastic stress equals the oscillatory stress, indicated by the diagonal line. A LVR was observed at
scillatory stresses below about 25 Pa. The oscillatory frequency was 1 Hz.

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hat deformation at low oscillatory stresses is primarily elastic, although a small viscousomponent is evident, a phenomenon observed elsewhere in dry shaving foams �Meeten2008��. As the stress is increased, G� and G� decrease and eventually cross over at theield point. At low stresses, the elastic stress is equivalent to the oscillatory stress, alsondicative of primarily elastic deformation. At moderate oscillatory stresses, however, thelastic stress falls below the oscillatory stress, indicating the onset of gross inelasticeformation. At this point, the elastic stress reaches a maximum and falls precipitouslypon yielding. The crossover of G� and G� and the maximum value of the elastic stress,ndicated by arrows �1� and �2� in Fig. 7, are two indicators of yield stress measured byscillatory experiments �Walls et al. �2003��.

Oscillatory frequency sweeps indicate strong shear-thinning behavior at all concentra-ions investigated, in qualitative agreement with steady flow experiments �Fig. 8�. Power-aw coefficients were also very similar at three stover concentrations, ranging from �0.86o �0.89. The complex viscosity varied exponentially with FIS, increasing 3 to 4 ordersf magnitude between 5% and 17% FIS.

The Cox–Merz relationship is an empirical relationship that posits similarities betweenhe shear rate dependence of the steady flow viscosity and the frequency dependence ofhe complex viscosity �Cox and Merz �1958��. A comparison of steady-state flow andscillatory experiments �Fig. 8� suggests that corn stover suspensions do not follow theox–Merz relationship at shear rates and angular frequencies tested. At all three concen-

rations, the complex viscosity is consistently higher than the flow viscosity, althoughifferences are smaller at lower stover concentrations. The relatively low flow viscositiesay result from fracturing of the suspensions and/or slip between particles �possibly in

he form of shear banding�. These results were not entirely surprising, however, as otheresearchers have noted that the Cox–Merz relationship was not observed in similarseudo-plastic fluids, such as peanut butter �Bistany and Kokini �1983a, 1983b��.

The viscosity data were re-plotted versus the oscillatory strain ��0� multiplied by thengular frequency ��, an “effective shear rate” defined by Doraiswamy et al. �1991�.alled the “modified Cox–Merz rule,” complex and flow viscosities were coincidenthen plotted versus the effective shear rate on logarithmic axes, with a slope of �1 at

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Flow:1.1920 −= γη &

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IG. 8. �a� Steady-state flow and oscillatory frequency sweeps using the narrow-gap vane and smooth cup.requency sweeps were conducted within the LVR of PCS suspensions at concentrations of 5%, 10%, and 17%IS. �b� Viscosity versus the effective shear rate ��0� for PCS at 17% FIS. Solid lines in �a� and �b� areower-law fits to the data.

ow effective shear rates, indicative of yield stress behavior. As shown in Fig. 8�b�, the

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odified Cox–Merz rule is well satisfied for undiluted PCS measured by controlled-strainrequency sweeps with �0=3% and 50%. Flow and dynamic viscosities are nearly iden-ical with a slope of about �1. The modified Cox–Merz rule was not obeyed at low stoveroncentrations, however, possibly due to low yield stresses.

. Effect of insoluble solids concentration on yield stress

Although process economics dictate that only the highest workable concentrations ofCS would be used in a biorefinery, a range of corn stover dilutions was examined toompare with the decreasing insoluble solids concentration during enzymatic hydrolysis.he effect of FIS on the yield stress measured by TF, oscillatory, and stress ramp experi-ents using the vane geometries, and by squeeze flow using parallel plates �Fig. 9�

lucidates a concentration scaling relationship for PCS suspensions. The effect of sievinghe suspensions to remove particles larger than 500 �m was also investigated and usedn combination with the serrated cup to prevent wall slip.

Yield stresses measured using the different geometries vary up to tenfold, with yieldtresses measured by TF experiments being the highest, followed by oscillatory and theny stress ramp experiments. In TF experiments, a three- to fivefold spread in yieldtresses was observed between the vane geometries. Although end effects appear signifi-ant for undiluted PCS, reducing the apparent yield stress from about 3000 to 1000 Pa

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τ 0(Pa)

Batch1 - TF - WideBatch1 - TF - NarrowBatch1 - TF - NarrowBatch1 - TF - Narrow - Serr CupBatch1 - Stress Ramp - NarrowBatch1 - SAOS G'G'' - NarrowBatch1 - SAOS G'G'' - NarrowBatch1 - SAOS MES - NarrowBatch1 - SAOS MES - NarrowBatch1 - Squeeze FlowBatch2 - TF - WideBatch2 - TF - NarrowPower Law Fit

τ 0 = 7.7×107 FIS6.0

IG. 9. Yield stresses measured using several methodologies for two batches of PCS as a function of FIS. TFnd oscillatory measurements �G�G�: crossover between moduli; MES: maximum elastic stress� were madesing wide-gap and narrow-gap vanes. Gray data points represent vane loadings with a relieved normal forcedetails in Sec. II B�. Squeeze flow measurements were made using 40 mm parallel plates.

hen the axial force is relieved after loading, the difference is within experimental

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ncertainly �Fig. 9�. End effects are insignificant for dilute suspensions, with axial forcesess than 0.5 N. At dilute stover concentrations, yield stresses measured with the wide-ap vane were generally the highest, followed by the narrow-gap vane and serrated cupusing sieved stover�, and then by the narrow-gap vane and smooth cup. However, forndiluted stover suspensions at about 17% FIS, similar yield stresses were observed forll three vane geometries, indicating that wall slip did not significantly affect yield stressalues measured via TF experiments. Yield stresses measured via oscillatory experimentsith the narrow-gap vane and smooth cup were slightly below yield stresses measured byF experiments. Values defined by the crossover of G� and G� were 20%–40% greater

han those defined by the maximum value of the elastic stress, in agreement with Wallsnd co-workers �2003�. Squeeze flow resulted in substantially lower yield stresses thanhe vane geometry. Unfortunately, radial filtration hinders the use of squeeze flow mea-urments for PCS, necessitating the assumption that only interstitial fluid is squeezedrom between the plates during the experiment. This assumption may result in over-redicted values of FIS, and actual solids concentrations are likely to be lower, resultingn better agreement with the other rheological methods.

Strong power-law correlations between the yield stresses and FIS were observed, aesult of more abundant interparticle interactions at higher concentrations. Exponentsanged from 5.2 to 6.5 for individual data sets with coefficients of variation in excess of4%. A power-law model for all rheological methods is shown in Fig. 9, with an expo-ent of 6.0�0.4 �95% confidence interval�, with a slightly lower coefficient of variationf only 90% resulting from the aggregation of methodologies. It is unclear why yieldtresses in the current study scale with solids concentration to such a large power. Priortudies, mentioned earlier, typically observed exponents of about 1–3 in several types ofarticle suspensions. One hypothesis is that the fibrous nature of the PCS particles, witharge aspect ratios and disperse size distributions, imparts more extensive interparticlenteractions. Thus, changes in the insoluble solids concentration �or particle volume frac-ion� result in changes to the yield stresses that are larger than measured in other studies.

V. CONCLUSION

The rheology of PCS slurries encountered following pretreatment and preceding en-ymatic saccharification in a biorefinery was investigated. Advantages and disadvantagesf several flow geometries were evaluated, with the goal of establishing an accurate,eproducible rheological methodology. As with other rheological studies of soft solids,liminating wall slip was a major concern. Several geometries and flow regimes wereested, including parallel-plate and vane geometries under shear, torsional, and biaxialxtensional flows. The vane was found to be the most effective geometry, easiest toeproducibly load, and reduces or eliminates wall slip. Biaxial extensional flows werelso used to ensure deformation, resulting in yield stresses that agreed reasonably wellith those measured with the vane, although radial migration of interstitial fluid must be

ccounted for. Slip was most prominent with smooth parallel-plates, and controlled-strainow experiments resulted in meaningless torques that scaled linearly with compressiveorce. Roughened parallel plates mitigated slip, but the yield stresses exhibited a greateal of variability and were especially sensitive to the compressive force upon loading.dditionally, the stover fractured and was ejected from between the plates after a few

train units.PCS suspensions at concentrations ranging from 5% to 17% FIS were found to act as

oft solids, exhibiting primarily elastic deformation at low strains prior to yielding, and
lastic moduli that were typically tenfold greater than viscous moduli at low oscillatory

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tresses. TF, oscillatory, and stress ramp experiments provided the simplest and mosteproducible estimates of the yield stresses. Other rheological experiments, such as creepnd biaxial extensional flows, proved time consuming and resulted in less reproducibleeasurements. Though the vane geometry was most reliable, a relatively large measure-ent uncertainty was still observed, with coefficients of variation of about 10%–50%,

ossibly due to the underlying heterogeneity of stover particles and batch-to-batch vari-bility. Nonetheless, yield stresses were strongly dependent on the concentration of sto-er, scaling with solids concentration to the sixth power, a dependency that is muchtronger than observed in several other studies on soft solids and concentrated suspen-ions. The difference between PCS and other soft solids may be due in part to the fibrousature of PCS. Additionally, the PCS slurries were strongly shear thinning and obeyed amodified” Cox–Merz relationship. Finally, the rheological methods described in thisork form the foundation of more extensive rheological testing, including the effect of

nzymatic hydrolysis on the stover rheology.

CKNOWLEDGMENT

Support for this research was provided by the Department of Energy and the Nationalenewable Energy Laboratory �Golden, CO� through Subcontract No. ZDJ-7-77605-02.

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rheology of high-solids biomass slurries for biorefinery applications

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modied coxmerz

newtonian

oscillatory

apparent yield

oscillatory

lamentous

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