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Multi-objective Optimisation of Pretreatment of Sugarcane Bagasse for
Bioethanol Production
Joanne Crimes
Supervisors: Prof. Duncan Fraser & Dr. Adeniyi Isafiade
Environmental & Process Systems Engineering
Chemical Engineering
University of Cape Town
Sugarcane bagasse (SCB)
Used to produce steam
3.3 Mt/year (Lynd et al., 2003)
South African Context
Lignocellulose
40% cellulose (polymer of glucose, C6)
22% lignin (aromatic polymer)
20% hemicellulose (branched polymer of C5 & C6 sugars)
Project Overview
Sugarcane bagasse Ethanol
Aim: To use modelling to optimise the process flowsheet for pretreatment and hydrolysis in terms of
both economic and environmental objectives.
Pretreatment Fermentation SeparationSCB
CO2
Bioethanol
Water
Hydrolysis
Enzymes or acid
polymers → monomeric sugars sugars → ethanol
Project Overview
Biological, physical, chemical, physicochemical
Pretreatment
Cellulose
HemicelluloseLignin
Hydrolysis
• Breaking polysaccharides into monomers using water
• Enzymatic
– pH 4.8, atm P, T of 45 – 50°C
– May require detoxification before
• Chemical
– Using acid, atm P, T of 180 - 230°C
– Produce more inhibitors to fermentation
Why modelling?
• Process design is usually uses a sequential
approach
• Modelling uses a simultaneous approach
– Interactions can be taken into account and optimised
Modelling
Superstructures
Can be used to embed many possible flowsheets into one model
Reactor 1
Reactor 2
Reactor 3a
a1
a2
b1
b2
c
b
b3
y1
y2
y3
Modelling
𝐴 → 𝐵 → 𝐶
General process synthesis problem formulation
Such that:
Mixed Integer Non-Linear Problem (MINLP)
Modelling
SimaPro using a liquid fuels database.
Environmental Impact
Account for synergistic effects between variables
Pareto curve
(Santibanez-Aguilar et al., 2011)
Multi-Objective Optimisation
0
200
400
600
800
1000
1200
1400
1600
1800
0 50 100 150 200
An
nu
al P
rofi
t (M
illio
n D
olla
rs)
Environmental Impact (10-7 Pts) A
B
C Infeasible Region
Sub-optimal Region
Max. Profit Max. EI
Min. Profit Min. EI
74% of Max. Profit 21% of Max. EI
Pretreatment: Steam explosion (acid catalysed & uncatalysed) Acid pretreatment
Delignification: Using NaOH
Hydrolysis: Acid Enzymatic
Chosen Pretreatments
Mass balance of pilot-scale pretreatment of sugarcane bagasse by steam explosion followed by alkaline delignification.
Rocha, G. J. M., Martín, C., Vinícius, F. N., Gómez, E. O., & Gonçalves, A. R.
• CTBE’s Aspen simulation based on Rocha’s paper.
– Uncatalysed, 11 barg, 190°C
– Acid catalysed, 5 barg, 150°C
• Conversions for individual reactions.
• Used in General Algebraic Modelling Software (GAMS).
Steam Explosion Model
Kinetic study of the acid hydrolysis of sugar cane bagasse. Aguilar, R., Ramırez, J., Garrote, G., & Vazquez, M.
• Concentration profiles simulated in Matlab.
– Set acid concentration (2, 4 or 6 wt%), temperature (122, 124, 128°C).
– Choose tao.
– 13 different reactors.
• Used in GAMS as black boxes.
Acid Pretreatment Model
Chemical & morphological characterization of sugarcane bagasse submitted to a delignification process for enhanced enzymatic digestibility.
Rezende, C. A., Lima, M. A. De, Maziero, P., & Ribeiro, E.
• Studied the amount of cellulose, hemicellulose and lignin remaining with different wt% NaOH.
• Used data to plot relationship.
• Used in GAMs model.
Delignification Model
y = 22.857x - 2 R² = 0.9981
y = 10.857x + 5 R² = 0.9918
y = 58.857x + 10 R² = 0.9997
0
10
20
30
40
50
60
70
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Mas
s %
Rem
ove
d
Wt% NaOH
Cellulose Hemicellulose Lignin
Dilute acid hydrolysis of sugar cane bagasse at high
temperatures: A kinetic study of cellulose saccharification and
glucose decomposition. Part I: sulfuric acid as the catalyst.
Gurgel, L. & Marabezi, K.
• Kinetic equations for cellulose reactions with Arrhenius temperature relationship.
• Conversion factors for hemicellulose to xylose and xylose to furfural.
• GAMS model optimises T (between 180 & 230°C), tao, acid percentage (0.07, 0,14 or 0.28 wt%)
Acid Hydrolysis Model
Technological Assessment Program (PAT). The Virtual Sugarcane Biorefinery (VSB).
Bonomi, A. et al.
• CTBE’s Aspen simulation.
• Conversions for individual reactions.
• Fixed T, tao and enzyme loading.
• Least flexible model.
Enzymatic Hydrolysis Model
Superstructure
Acid
Hydrolysis
Steam
Explosion
Enzymatic
Hydolysis
SCB Hydrolysate
Acid
Prehydrolysis
Delignification
with NaOH
Acid Catalysed
Steam
Explosion
Pretreatment Hydrolysis
• Creating a meaningful economic objective function
• Finding good data
– Kinetic of acids is well researched
– Steam explosion more black box approach
– Trade secrets around enzyme mixtures
• Combining models
– Acid prehydrolysis & acid hydrolysis
– Effects of pretreatment on hydrolysis
– Effects of delignification on hydrolysis
Challenges
Limitations
• Hard to quantify physical effects
• Experiments have very specific conditions
– Difficult to adjust experimental to new situations
– Little flexibility in some models (eg. fixed residence time, water to solids ratio)
• Solvers very sensitive to initialisations (local optima)
Conclusion
• Multi-objective modelling
– Economic and environmental objectives
• Pretreatment, delignification, hydrolysis
– Kinetics and simulations
• Combining models and incorporating delignification
The National Research Foundation
Laboratory of Bioethanol Science and Technology (CTBE), Campinas, Brazil
Chemical Engineering Department at Universidad Nacional, Manizales, Colombia
Acknowledgements
Thank you.
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