(643g) Modeling of Solid-State Fermentation for Ethanol Production From Sweet Sorghum Stalks

Introduction

Biofuels production worldwide
continues to grow at a very rapid pace.  In 2007, China's fuel ethanol output reached 1.6 million tons, of which 80% used corn as feedstocks,
with more than four million tons of corn consumed [1]. However, corn-based
ethanol manufactures have ceased for further capacity expansion due to the new
government policies [1]. Therefore, cellulosic ethanol has become an inevitable
trend for further biofuel development. Sweet sorghum has the potential to be
used as a renewable energy crop and has become a viable candidate for ethanol
production, so it was chosen as the cellulosic feedstock for this study.
Biochemical conversion of sweet sorghum stalks to fuel ethanol can be achieved
by solid-state fermentation (SSF) or liquid-state fermentation (LSF). Only
recently, with emerging attention to biofuel demand, has SSF technology been
applied extensively in the biofuels field for simplified process design and
potential reduction of fuel production cost.  During the SSF process, the
absence of free water leads to poor heat removal characteristics, and it is not
easy to mix a bed of solid substrate particles well.  Because of this,
heat removal is a major challenge in the design and operation of large-scale
SSF bioreactors.  Therefore, to overcome these disadvantages effectively
and to solve these engineering barriers, the design of the fermenter is among
the most critical aspects for feasibility and eventual scale-up of SSF for
commercial biofuel production. Among these several types of SSF reactors,
rotating drum bioreactors (RDB) provide relatively gentle and uniform mixing by
improving baffle design, since there is no agitator within the substrate bed.
Currently, the main reason for the limited industrial application of SSF is the
lack of engineering data and knowledge about the design and scale-up of solid-state
fermenters. Fortunately, the significant improvement in understanding of how to
design, operate and scale up SSF bioreactors has been possible through
application of mathematical modeling techniques to describe the biological and
transport phenomena within the system [2]. Mathematical models are then viewed
as important tools for guiding the simulation, design, and operation of
large-scale bioreactors for optimum performance.

In this paper, we have
developed a mathematical model for discounted RDB operation, including kinetics
and heat and mass transport. The model not only describes several key variables
changing with time, i.e., biomass concentration, substrate concentration, and
main product (ethanol) concentration, but also gives the radial temperature
profile in the substrate bed. Finally, the model is validated with pilot unit
testing for a 5 m³ RDB using milled sweet sorghum stalk for the
production of bioethanol. Through our mathematical modeling approach,
significant advances in understanding the biological and mass and heat transfer
phenomena can be made toward development of quantitative scale-up strategies
for SSF bioreactors.

Methods

The yeast Saccharomyces
cerevisiae TSH-SC-1 was provided by our laboratory (New Resources Graduate School, Institute of Nuclear and New Energy Technology, Tsinghua University) and was used in the bench scale and pilot scale fermentation experiments.
The sweet sorghum stalks were collected from a local rural area of Wuyuan county of Inner Mongolia, China. Leaves were first stripped from the fresh sweet sorghum stalks
by hand, and then stalks were chopped to small particles before being loaded in
the substrate bed of the rotating drum bioreactor. Ethanol concentrations were
determined by a Shimadzu GC-14C gas chromatography system equipped with a flame
ionization detector. 

Model Development

The RDB model
includes growth kinetics of biomass, sugar consumption rate, ethanol production
rate, and mass and energy balances. Compared with aerobic solid-state fermentation,
oxygen is not needed for growth of our microorganisms in the SSF, so there is
no sterilized fresh air charged into the RDB. But for both aerobic and
anaerobic solid-state fermentation, CO2 is produced by microorganisms as a
byproduct of metabolism and will be discharged continuously from the RDB. For
anaerobic SSF, the headspace gas is mainly composed of CO₂, and the
remaining amount of CO₂ could be assumed to be a constant since RDB
usually is operated under constant pressure. The solid substrate bed was
segregated into multiple layers from the RDB wall to the center of the RDB.
Each layer was assumed to be well-mixed and therefore assumed to have a single
temperature. Each layer has a material and energy balance equation, in addition
to equations for heat transfer between the solid layers and interface with the
headspace gas. A fourth-order Runge Kutta method was used and the model was
programmed in C++ consisting of subroutine blocks for the simulation.

Pilot Plant SSF Validation

Batch fermentation
experiments were carried out in a 5 m³ stainless steel rotating drum
using yeast TSH-SC-1 to produce fuel ethanol from sweet sorghum stalks. The
experiments were conducted for 48 hours hold time without temperature control.
The drum rotated for 20 minutes every 5 hours. After each time interval and at
the end of fermentation, samples were taken for cell, sugar, and ethanol
analyses.

The cylindrical RDB
is shown in the following figure.

Results and Discussion

The models were experimentally
validated using the above pilot plant RDB experimental procedure. These
experimental and model data indicate that first, cellulosic materials such as
sweet sorghum stalks can be fermented well and converted to ethanol using yeast
Saccharomyces cerevisiae TSH-SC-1 under a relatively short fermentation
time (within 20 hours); second, cell growth, sugar reduction, and ethanol
production are synchronized well in both the experimental results and the
modeling prediction, implying good experimental work and model development in
this case; and third, further improvement of the overall process can be focused
on different cellulosic feedstocks, higher yield in biomass and ethanol
production, etc., using similar approaches. Model validation with experiments
data also is applied to the radial temperature profile.

Conclusions

Although the SSF bioreactor is a
promising option for biofuel production, scaling it up is a major challenge
because the absence of free water leads to poor heat removal characteristics,
and it is not easy to mix a bed of solid substrate particles well. The main
obstacles lie in heterogeneously distributed physiological, physical, and
chemical environments in the substrate bed. This causes difficulties in
fermentation control and heat buildup occurs due to weak heat transfer in an
SSF bioreactor, especially in an RDB. Therefore, a mathematical model is an
important tool with which to study the true nature of this complex system for
further application in bioethanol production. In this paper, we developed a
mathematical model to study the microorganism growth, sugar consumption, and
ethanol production, along with heat generation, heat removal, and heat
accumulation, in rotating drum bioreactors during the fermentation process.
Validation experiments were conducted in a 5 m³ pilot-plant
fermenter for production of fuel ethanol from chopped sweet sorghum stalks. The
mathematical model agrees with the experimental data very well, especially in
the microorganism growth period. Microorganism growth, sugar content reduction,
and ethanol production were predicted and measured in the mathematical model
and experimental results. The maximum rising temperature of the substrate bed
was also predicted and matched very well in both experimental and modeling
results, and the temperature range studied is acceptable for yeast to propagate
and grow. Since the model-predicted temperature variation with fermentation
time fits well with small scale and pilot scale experimental results, a similar
model approach can be further utilized for process design, development, and
optimization purposes in larger scale ethanol production using sweet sorghum
stalk or other cellulosic materials.

Acknowledgement.  This
work is carried out under the project 2006BAD07A01of the National Science &
Technology Pillar Program in the Eleventh Five-year Plan Period, and the
project 06YFGZSH02700 of the Science and Technology Development Program of
Tianjin, China. We also thank China Oil and Food Corporation Limited (COFCO)
for financial and other support of the pilot-plant RDB experiments

References

[1]. China Bioenergy Industry Report 2007-2008,

http://en.ec.com.cn/article/enindustry/enenergy/eneenews/200804/603886_1.html

[2]. C. Krishna, Solid-state
fermentation systems, Critical Reviews in Biotechnology. 25 (2005) 1¨C30.