Colorado School of Mines, Department of Chemical Engineering, Golden, CO 80401 2National Renewable Energy Laboratory, National Bioenergy Center, Golden, CO 80401

Presented at the 31st Symposium on Biotechnology for Fuels and Chemicals, 3-6 May 2009, San Francisco, California • NREL/PO-510-45806

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Processing of high-solids slurries will be necessary for the Vane-in-cup geometry: economic operation of lignocellulosic biorefineries. In this work, we

examine the changing yield stress and viscosity of high-solids

dilute acid pretreated corn stover (PCS) during enzymatic

saccharification. PCS slurries containing 20% (w/w) insoluble solids

were observed to liquefy to the point of being pourable after about

two days, at a biomass conversion of about 40%. Mass balance

and semi-empirical relationships were developed to connect the

progress of enzymatic hydrolysis with particle concentration and

yield stress, and the rheological properties of unsaccharified and

saccharified PCS agree well when compared at equivalent volume

fractions of insoluble particles.

• High-solids bioreactor (HSBR)

– Horizontal reactor with impellors to

mix PCS and enzymes at 48°C

• Roller bottle reactor (RBR)

– 2 L roller bottles containing 1000 g PCS

– Incubated at 48°C

– Rotated at 2 – 20 rotations / minute

– 20% (w/w) washed PCS

– 5 - 20 mg cellulase / g cellulose

• Samples of suspension

– Collected at various time points

– Boiled / refrigerated to deactivate enzyme

Prior to enzymatic saccharification...

...and after

Vane in cup geometry:

Parallel plate geometry: Liquefaction of PCS

After 1 – 2 days

30 mm

28 mm

42 mm

Because the yield stress drops with conversion, indicating when a

suspension becomes “pumpable”, this work will help the process

designer identify an optimal residence time during saccharification.

In this light, we also present preliminary results suggesting that

viscosity-modification / flow-assurance additives may significantly

reduce the yield stress and viscosity of PCS slurries, ensuring their

downstream processability. 0

 20

 40

 60

 80

 100

 0 2 4 6 8 10 12 14

Total Biomass Conversion (%)

Saccharification Time (days)

HSBR 20 mg/g

RBR 20 mg/g

RBR 10 mg/g

RBR 5 mg/g

g y

Unsaccharified

PCS

40 mm

4 mm

Rotational speed or torque

of upper plate

controlled

Stationary

25°C Peltier Plate

After Saccharification

7 days / 70% Conversion

1 mm

1 mm

Enzyme Loading:

20 mg enzyme / g cellulose HSBR

10 mg / g

5 mg / g

RBR

RBR

RBR

Pretreatment Saccharification

 0

 0.1

 0.2

 0.3

 0.4

 0.5

 0 0.05 0.1 0.15 0.2 0.25

Particle Volume Fraction (vol/vol)

Fraction Insoluble Solids (wt/wt)

RBR 20 mg/g

RBR 10 mg/g

RBR 5 mg/g

Mean

1 SD rlsp

Diluted PCS

• Weight fraction of insoluble solids: fis, estimated

by quantitatively drying

samples of the whole

suspension and interstitial

fluid.

A yield stress is the applied stress at which yield-stress fluids begin to flow. At imposed stresses

below their yield stress, such materials act much like a solid. For example, while water and honey,

both Newtonian fluids, form a smooth surface over time, mayonnaise, a yield-stress fluid, remains

deformed after mixing with a spatula.

Transient Flow experiments

– Rheometer vane rotates steadily

Mayonnaise Yield Stress = 100 Pa Honey No Yield Stress 

(Newtonian Fluid) Freshly Poured

Concrete Yield Stress = 1,000 Pa fis = Mass of Insoluble Solids

Mass of Suspension

• The particle volume fraction

is roughly proportional to the

fraction of insoluble solids.

• The fraction of insoluble

solids, and therefore the

particle volume fraction,

decrease as the biomass is

hydrolyzed. 100000 1000

G’ 360 Pa

Before 900

1000 Yield Stress:

880100 Pa

 0.25

)

HSBR 20 mg/g

RBR 20 mg/g

• Particle volume fraction:

, measured by quantifying

the fraction of liquid volume passing through a

centrifuge filter.

y

(15 min/revolution)

– Early saccharification times:

Maximum stress interpreted as yield stress

– Later saccharification times:

Yield behavior not obvious

• These trends were

independent of reactor

configuration and enzyme

Oscillatory experiments loading.

– Stress oscillated sinusoidally (1 Hz)

– Yield Stress defined by:

1. Crossover of elastic (G’) and viscous (G”) moduli

2. Maximum of the elastic stress (G’ × peak strain)

0.1

1

10

100

1000

10000

0.001 0.01 0.1 1 10 100 1000 10000

Oscillatory Stress (Pa)

G', G'' (Pa)

0.001

0.01

0.1

1

10

100

Elastic Stress (Pa)

G”

G’

G”

500 Pa

0.14 Pa

0.18 Pa

Before

Saccharification

After 7 days of

Saccharification

(90% Cellulose Conversion)

0

100

200

300

400

500

600

700

800

0 10 20 30 40 50

Strain

Stress (Pa)

Before Saccharification

After 7 days of

Saccharification

(90% Cellulose Conversion)

0.0

0.1

0.2

0.3

0.4

0.5

0 5 10 15 20 25

Strain

Stress (Pa)

 = Volume of Particles

Volume of Suspension

 0

 0.05

 0.1

 0.15

 0.2

 0 20 40 60 80 100

Fraction Insoluble Solids (wt/wt)

Total Biomass Conversion (%)

RBR 20 mg/g

RBR 10 mg/g

RBR 5 mg/g

Model

(b)

Rheology of Saccharified Corn Stover1

• A clear relationship between biomass conversion,

fraction insoluble solids, and particle volume

fraction was defined.

• At an insoluble solids concentration of 20% (w/w),

as little as 30% cellulose conversion is necessary

to achieve a “pumpable” suspension with a yield

stress below 10 Pa.

• A semi-empirical rheological model was used to

relate volume fraction with yield stress.

Viscosity Modification

Numerous studies have been conducted to assess how surfactants

and other chemical additives can be used to modify the binding

properties of cellulase enzymes, resulting in enhanced

saccharification kinetics and biomass conversions.

Additionally, “drag reducing” polymers have long been known to lower

the viscosity of water and petroleum, reducing pumping power and

transport costs in pipelines.

In this work, we explore chemical additives to both enhance

enzymatic saccharification and effect a reduction in the suspensions’

• The yield stress of PCS

suspensions decreases with

biomass conversion.

• A rheological model developed

by Wildemuth and Williams2,3

adequately correlates yield

stress with particle volume

fraction for both saccharified

and un-saccharified PCS:

 0.01

 0.1

 1

 10

 100

 1000

 10000

 0 20 40 60 80 100

Yield Stress (Pa)

Total Biomass Conversion (%)

HSBR 20 mg/g

RBR 20 mg/g

RBR 10 mg/g

RBR 5 mg/g

Model

(b)

W-W + Mass Balance

Model

 10000 y

– A single set of model parameters is capable of characterizing

yield stresses of suspensions in a number of enzymatic

hydrolysis experiments across a range of enzyme

concentrations.

– It provides a good prediction of yield stress determined from

particle volume fraction.

This work is supported by the U.S. Department of

Energy (DOE), The Office of the Biomass Program. 1Roche, CM, CJ Dibble, JS Knutsen, JJ Stickel, and MW Liberatore (2009) “Particle concentration and yield stress of

biomass slurries during enzymatic hydrolysis at high-solids loadings” In Press: Biotechnology and Bioengineering. 2Wildemuth, C. R., and M. C. Williams (1984) "Viscosity of Suspensions Modeled with a Shear-Dependent Maximum

Packing Fraction," Rheologica Acta 23:627-635. 3Wildemuth, C. R., and M. C. Williams (1985) "A New Interpretation of Viscosity and Yield Stress in Dense Slurries -

Coal and Other Irregular Particles," Rheologica Acta 24:75-91.

y

yield stress, a desirable synergy.

• Parameters with 95% CI:

– m = 1

– A = 9  5 Pa

– m0 = 0.18  0.02 Pa

– m∞ = 0.41  0.01 Pa

 

m

m

m

y A

1

0

1 



 





















   

 

 0.1

 1

 10

 100

 1000

 10000

 0.1 0.2 0.3 0.4 0.5

Yield Stress (Pa)

Particle Volume Fraction (vol/vol)

HSBR 20 mg/g

RBR 20 mg/g

RBR 10 mg/g

RBR 5 mg/g

Model

Diluted PCS

W-W Model

0

500

1000

1500

2000

Transient Flow Oscillatory

Yield Stress (Pa)

Unmodified PCSModifier 1

2

Modifiers can be used to raise

or lower the yield stress of PCS

suspensions

345

NREL/PO-510-45806