(461d) Conceptual Design of Biorefineries Using Agrowastes, Its Decision Support and Design Rationale

This work
explores the role of OUTDO, a decision support system, in the conceptual design
of multipurpose-multiproduct plants producing lignocellulose-based biofuels and
bioproducts.  These plants (mainly
based on lignocellulose chemistry and biochemistry) are gaining importance as
part of a paradigm change in the energy and process industries.  In particular, the work addresses the
process of designing a class of plants known as biorefineries that employ
agro/industrial wastes and enzyme-based technologies to produce ethanol and
bioproducts.

The conceptual
design of this type of biorefineries is part of an inter-institutional research
project currently under execution in Mexico whose main objective is to develop
the concept of a biorefinery using agro/industrial wastes as feedstock within
the context of a mid-size economy.  Ten geographically distributed research groups participate in
the project executing work packages related to each processing stage of a
biorefinery and its design.  They
include pretreatment, saccharification, fermentation, separation, biogas,
hydrogen and electricity co-production, enzyme enhancement, synthesis of
lignine-based products, plant design and Life Cycle Assessment (LCA) of the
biorefinery concept.  As part of
the conceptual plant design work package, seven designs have been completed in
the past four years using standard techniques and tools.  Most of the process technology employed
can be found in the open literature and many of the process conditions for the
feedstock conditioning, pre-treatment, hydrolysis and fermentation steps have
been corroborated at laboratory scale by the corresponding groups participating
in the project.  Each one of the
conceptual designs has been modeled and analyzed: steady–state models
were solved using commercial simulators, the economic assessment was carried
out according to standard profitability analysis procedures, and the
environmental impact assessment is being currently carried out with LCA
employing conventional assessment methods that involve the use of heterogeneous
criteria (qualitative and quantitative).

During the
execution of the project new data has been produced that either modifies the
existing process functionality or introduces new aspects to be considered.  New stakeholders have been also
incorporated along the duration of the project.  Thus, conducting the conceptual design of the biorefinery has
become a highly dynamic and very sui
generis activity.  It is for
these reasons that the need of a flexible environment to keep track of the technical
issues associated to the design and of the design process itself was identified.

OUTDO (Oxford
University Tool for Decision Organisation) provides several functionalities
related to decision making and information and knowledge management:

a) access
to a library of multi-criteria decision analysis (MCDA) methods to improve
decisions

b) maintenance
of the design history for future analysis and re-use

c) representation
of the decision rationale; making it explicit improves communication amongst
the participants in the design and allows the exploration of the impact of
changes in the technological, environmental, economic and social conditions on
previous decisions (or even the impact of the change of a given decision in
other decisions!) through the propagation of Global Variables throughout the
network of decisions

d) integration
of probabilistic forecasting methods with the MCDA methods to evaluate alternatives
at different points in time (present and future)

e)
operates as a repository of design models, data and documents, i.e. acting as a
"glue" binding disparate pieces of information.

OUTDO provides
the basis to structure the information used in and generated during the design
process.  This includes the design
strategy and the documentation of the design process and of decision making,
e.g. the selection of a model.

To illustrate
OUTDO's functionalities we will concentrate on a single design step that
evolves one design stage (referred to as an artefact in the rest of the
document) into a more detailed design; we will discuss some of the key decision
issues and how they were resolved.

Conceptual
Design of Biorefineries using Agrowastes

The first
artefact is the block diagram drawn in continuous lines in Figure 1.  The first stage of the block diagram is
a three-step continuous thermo-chemical Pretreatment (5:1 H₂SO₄
0.75% v/v aqueous solution:feedstock) that soaks and cooks the feedstock.  Inhibitors in the liquid hidrolisates
are eliminated by overliming.  A
92% percent conversion of hemicelluloses is achieved (80% to xylose, 12% to
arabinose) and only 3% from cellulose to glucose.  The output stream is fed to the Saccharification & Fermentation
stage in which hemicelluloses and celluloses are enzimatically transformed to
sugars and then fermented.  A
2%-alcohol stream is sent to the Separation stage producing 99.5% ethanol.  Biogas is produced as a by-product in
the Waste Water Treatment stage. 
Cogeneration is considered, by burning the lignin residues.  Details of this design can be found in
Sanchez et al. (2012).

The second
artefact represents a coproduction scheme of ethanol, hydrogen as well as acetic
and butyric acids.  The Pretreatment
stage was modified to use a batch pressure cooking process (again 5:1 H₂SO₄
0.75% v/v aqueous solution:feedstock). 
The resulting liquid hidrolisates are sent to a Dark Fermentation stage
(thermophilic bacteria, 60 C, 25 hours residence time) that produces hydrogen
and CO₂ (2:1 molar ratio) (2.23 mol H₂/ mol pentoses).  Acetic and butyric acids are the
co-products sent to the Waste Water Treatment stage for biogas production.  A six-fold increment of biogas is
achieved compared against the ethanol-only production scheme (the first
artefact).  The anaerobic reactor
converts 93% of the organic contents to biogas (CH₄ and CO₂,
3:1 molar ratio) with mesophilic bacteria, at 25-45 ¼C and 6 hours residence
time.  The residual stream goes to
the aerobic reactor where almost all the organic content is removed.  Details of this flowsheet are presented
in Sanchez et al. (2011).

Some of the main
reasons for changing the Pretreatment technology were that the continuous
technology requires the payment of patent royalties, and that there was no adequate
experimental equipment available amongst the eight participant groups to test
and validate the operating conditions of the continuous operation mode.

New technology
emerged during this period with experimental evidence of an improved production
of hydrogen and biogas.  An
agreement was achieved to collaborate with another research group to
incorporate the hydrogen production stage into the design of the biorefinery.

Figure 1. 
Biorefinery Process Flowsheet.

Before proceeding with
the LCA, an economic comparison of these two artefacts was made calculating
capital investment, cost contributions per stage and total production cost of
ethanol as a function of plant capacity and feedstock price.  Polysaccharide concentration of
feedstock was established as a positive monotonic function of price and was adjusted
to local market conditions.  Typical
results are shown in Figure 2 and Table 1.  Note that the lowest production costs were obtained for
coproduction schemes using cheap feedstock (i.e. 0.42 $/l EtOH @2,100 ton/day
DB (dry basis)).  However, for
capacities below 1,000 ton/day DB using the ethanol production scheme may be
more attractive from an economic point of view under the established
conditions.  Capital investment and
patent royalties had a significant impact on the decision to discard the use of
the continuous pretreatment.  But
expected improvements in technology (not to mention the additional
consideration of other criteria such as environmental, financial or political) may
modify, in the short term, the economic conditions thus shifting this
decision. 

Figure 2.  Ethanol Total Production Costs. Enhanced coproduction (i.e.
ethanol, biogas and hydrogen) in green and ethanol-biogas in blue.         

Table 1.  Boundary Ethanol Total Production Cost for ethanol plant (columns
2 and 4) and biorefinery (columns 3 and 5).

Conclusions

The use of OUTDO is a relatively recent event in the biorefinery
design project, so part of the work has been to record an idealised decision
trail of the seven designs that have been completed in the last four years.  This exercise is exposing some decisions
that should be re-evaluated and is improving the organisation of the
accumulated information.  More
importantly, OUTDO is being used within the project to maintain its undergoing design
history, improve current decision processes (through the use of its MCDA
toolkit and the forecasting mechanisms) and foster discussion around the values
and preferences of the design participants.  It is expected that as the project unfolds the recorded
rationale will allow swift identification of those decisions affected by
changes to various design variables (costs, operation conditions, targets,
regulation limits, etc.) and the opportunity to backtrack and revise them.

References

A. Sanchez, V. Sevilla-Guitron,
G. Magana, P. Melgoza and H. Hernandez. "Co-production of ethanol, hydrogen and
biogas using agro-wastes. Conceptual plant design and NPV analysis for mid-size
agriculture sectors".  Proc. 21th
European Symposium on Computer Aided Process Engineering. Pto. Carras, Greece
(2011). Computer-Aided Chemical Engineering Vol. 29. pp. 1884-1889.

A. Sanchez, G. Magana, C. Moreno
and V. Sevilla-Guitron. "Conceptual Design and Process Economics of a
2G-Ethanol Production Plant in Medium-Scale Agriculture Sectors". Submitted to
Industrial Engineering Chemistry (2012).