Investigating Residence Time Distribution (RTD) and Effects on Performance in 

Continuous Biomass Pretreatment Reactor Designs

David A. Sievers, Richard T. Elander, Erik M. Kuhn, Nicholas J. Nagle, Melvin P. Tucker, Noah D. Weiss contact information:

david.sievers@nrel.gov

Background and Objectives

Recent pretreatment work has focused on scaling from a batch process to a continuous process using a 1⁄4 tonne/day continuous pilot

scale horizontal pretreatment reactor. Lower xylose yields along with excessive xylose degradation in the form of furfural production were

observed when comparing the continuous reactor to the batch reactor performance data at similar reaction conditions. These results

suggest that the continuous reactor has a broader residence time distribution compared with the batch reactor producing an undesirable

variance in pretreatment severity.

The continuous reactor is a horizontal pipe with a variable speed auger. Acid-impregnated biomass and steam are continuously fed to

the pipe by a plug screw feeder and discharged at the other end to atmospheric pressure through two alternating ball valves. One

continuous reactor configuration utilizes an auger with broken flights where rectangular baffles protrude from the bottom of the pipe into

the flight breaks to facilitate mixing. Following initial experimentation, a new reactor section was designed and installed with a continuous

flight auger and anti-rotation bars in place of the baffles in attempt to achiever better plug flow and reduce severity variance.

Batch vs. Continuous Reactors

Operating Parameters and 

Results Batch Continuous

Temperature (°C) 180 175

Acid Concentration (% w/w) 0.28 0.31

Mean Residence Time (min) 1.50 2.65

Severity* 2.5 2.6

Total Xylose Yield (%) 88 69

Monomeric Xylose Yield (%) 70 62

Furfural Yield (%) 3 10

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   75.14

100 * explog ]1[ T severity t

Pretreatment Reactor

[1] Overend, R.P., and Chornet, E. (1987). Philos. Trans. R. Soc. Lond. (A). 321, 523-536

Materials and Methods

For RTD tests, the reactor was run at ambient temperature without steam addition due to pulse addition and sampled by hand.

Water sprayed corn stover (45 wt% solids) was continuously fed through the plug screw feeder at 175 g/min, and material was

manually collected as it entered the discharger section. A pulse was introduced into the reactor by pausing the plug screw

feeder for 15 seconds and manually adding 15 seconds worth of sodium chloride impregnated corn stover at 60 wt% total

solids at the outlet of the plug screw feeder. (Material normally fed through the plug screw feeder is dewatered to a level of

Broken Flight Auger Reactor with Baffles Continuous Flight Auger Reactor with Anti-Rotation Bars

g ( y g g

about 60 wt% solids where excess liquid is discarded.) Biomass exiting the reactor at the discharger was collected in 15

second sample intervals for 10 minutes following the pulse addition.

Each biomass sample was mixed with a small amount of water to enhance sodium chloride extraction (typically 25g biomass

with 15mL water). Liquor from each biomass sample was extracted using a laboratory press and sterile filtered to remove

insoluble solids. The density of the resulting clear liquor was measured and used to calculate residence time distribution

curves.

Plan View of Reactor – Showing Continuous Flight Auger Installed

sample 

collection

slurry 25g with 

15mL water to 

extract NaCl

extract liquor 

from solids with 

laboratory press

sterile filter 

liquor to remove 

insoluble solids

measure liquor 

density to determine 

relative NaCl content 

water sprayed 

biomass

NaCl sprayed 

biomass

auger pipe

pulse @ t=0

plug screw 

feeder

outlet 

screw 

feeder

Inlet 

screw 

feeder

steam injection

during normal operation

Results and Conclusions

collection extract NaCl laboratory press insoluble solids at each time interval feeder feeder

rejected liquids from cone

full speed full speed broken or continuous auger

half or full speed: 49 or 98 rpm

reaction boundary during normal pretreatment operation

discharger pretreated material to flash tank

during normal operation

alternating 

ball valves

0 7

0.8

0.9

1.0

Broken Flight

Continuous Flight

Mean Residence Time (min)

Calculated Plug Flow 

Residence Time (min)

Auger 

Speed

Broken

Flight

Continuous

Flight

49 rpm 1.31 0.62

98 rpm 1.14 0.42

 Shorter mean residence times were measured in the continuous flight reactor.

 Residence time variance was not substantially different for either reactor.

 Residence time variance and skew are decreased for both reactor types at slower auger 

speeds.

 The continuous flight reactor produced

49 rpm

Reactor RTD at Different Auger Speeds

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0123456

E(t)

time [min]

0.9

1.0

Broken Flight

Continuous Flight

Auger 

Speed

Broken

Flight

Continuous

Flight

49 rpm 2.70 2.14

98 rpm 2.65 2.04

Residence Time Variance (min2)

Auger 

Speed

Broken

Flight

Continuous

Flight

49 rpm 0.30 0.30

98 rpm 0.85 0.78

Residence Time Skew (min3)

Auger 

Speed

Broken

Flight

Continuous

Flight

49 rpm 0.12 0.15

98 rpm 0.96 1.38

g p

• Higher total xylose yield (including oligomers and monomers)

• Lower monomeric xylose yield

• Higher furfural production

 RTD measurement method does not account for rheologic changes during steam 

pretreatment and does not consider reaction time in the discharger section.

Although these results are less than ideal, this could be caused by failure to substantially reduce

back-mixing in the reactor pipe. While the baffles and flight breaks have been removed, the antirotation bars in the continuous flight version do not allow the auger to span the entire inner diameter

of the pipe. The reactor was operated with a low fill factor and this may subject a large fraction of the

particles to back-mixing underneath the auger flights. The higher speeds have a much longer tail as

shown by the higher variance and skew which is likely due to a further decrease in fill factor as auger

speed is increased. 98 rpm

Acknowledgements

Funding for this work was provided by the Office of Biomass Program in the 

Department of Energy’s Office of Energy Efficiency and Renewable Energy

We wish to thank Jonathan Stickel for his help processing the data used to create 

the RTD curves and Bill Bray, Bob Lyons, and Wes Hjelm for their assistance 

0.0 conducting the experiments.

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0123456

E(t)

time [min]

g

More work is necessary to decrease severity variance in this type of pretreatment reactor and a

rapid, non-intrusive RTD measurement technique is also of high importance to ensure accurate

reporting of residence times at pretreatment conditions. A full response surface experiment is under

way that will provide comprehensive performance data using the continuous flight reactor.

Pretreatment Performance Comparison

Operating Parameters and Results

Broken

Flight

Continuous

Flight

Temperature (°C) 175 175

Acid Concentration (% w/w) 0.31 0.28

Main Auger Speed (rpm) 98 98

Mean Residence Time (min) 2.65 2.04

Residence Time Variance (min2

) 0.85 0.78

Combined Severity* 2.6 2.5

Reactor Fill Factor (%) 1 1

Pretreated Material Solids Content 

(% solids)

28 28

Total Xylose Yield (%) 69 74

Monomeric Xylose Yield (%) 62 58

Furfural Yield (%) 10 12

p

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.

The information contained in this poster is subject to a government license. Presented at the 31st Symposium on Biotechnology for Fuels and Chemicals

InterContinental San Francisco Hotel in San Francisco, CA, May 3-6, 2009

NREL/PO-510-45809