Assessment of biorefinery process intensification by ultrasound technology

Abstract

In the present work, on the basis of literature data, correlations for ultrasound-assisted dissolution of lignin and hemicelluloses from lignocellulosic raw material were defined in Aspen Plus(A (R)) as function of applied power and duration of the ultrasound treatment. The dissolution yield of these biomass components was represented against the applied acoustic energy, taking into account the volume of solvent-solid treated in each case, making possible the calculation of ultrasonic power consumption in a simulated biorefinery pretreatment process. The proposed ultrasound-assisted process was techno-economically evaluated in terms of process yield and utility requirements. Furthermore, energy and exergy analyses were performed in order to assess the profitability of the simulated ultrasound-assisted biomass fractionation processes.

Full text

ORIGINAL PAPER
Assessment of biorefinery process intensification by ultrasound
technology
Araceli Garcı
´a•Marı
´a Gonza
´lez Alriols •
Walter Wukovits •Anton Friedl •Jalel Labidi
Received: 23 December 2013 / Accepted: 26 June 2014 / Published online: 15 July 2014
ÓSpringer-Verlag Berlin Heidelberg 2014
Abstract In the present work, on the basis of literature
data, correlations for ultrasound-assisted dissolution of
lignin and hemicelluloses from lignocellulosic raw material
were defined in Aspen Plus
Ò
as function of applied power
and duration of the ultrasound treatment. The dissolution
yield of these biomass components was represented against
the applied acoustic energy, taking into account the volume
of solvent–solid treated in each case, making possible the
calculation of ultrasonic power consumption in a simulated
biorefinery pretreatment process. The proposed ultrasound-
assisted process was techno-economically evaluated in
terms of process yield and utility requirements. Further-
more, energy and exergy analyses were performed in order
to assess the profitability of the simulated ultrasound-
assisted biomass fractionation processes.
Keywords Ultrasound Biorefinery Biomass
fractionation Process simulation Energy Exergy
Introduction
The implementation of process intensification in biorefin-
ery processes aims primarily to increase yields, to achieve
time and energy savings and to enhance product purity/
quality. Consequently, the intensification of biorefinery
processes has focused on the successful improvement of
heat/mass transfer mechanisms and separation/purification
technologies (Bulatov and Klemes
ˇ2011; Sanders et al.
2012). Heat or mass transfer intensification in biorefineries
could be reached through alternative designs of the reactors
and heat exchangers (Reay 2008). For example, the use of
microwave radiation has been proven as a powerful and
efficient tool to improve different catalytic reactions or
thermal conversions in biorefinery processes (Nikolic
´et al.
2011). Furthermore, ultrasound treatment has been suc-
cessfully applied for the lignocellulose treatment as it
achieves both heat and mass transfer improvements
(Shirsath et al. 2012).
The ultrasound-assisted treatments have been widely
investigated in order to enhance mass transfer and disper-
sion phenomena used as pretreatments (Nikolic
´et al. 2010)
or as intensification tool for different extraction processes
(Alupului et al. 2009). Among the available newer energy
sources for process intensification, the use of sound energy
can result in significant improvements because the occur-
rence of cavitational events (Garcı
´a et al. 2012b). The
acoustic cavitation in liquid media forms microbubbles that
could facilitate disintegration of solid particles, acting
superficially and providing thermo-mechanical energy to
the process. Moreover, this cavitation energy also can act at
structural level, affecting inter-molecular linkages through
the formation of restricted extremely severe conditions
(high temperatures and pressures), and therefore, enhance
yield in chemical reactions.
Moreover, the optimization of industrial processes is
becoming an important research topic in recent years,
because it allows to analyze factors that could improve the
production yield as well as to study the mechanisms and
possible changes that would achieve the reduction of
A. Garcı
´a(&)M. Gonza
´lez Alriols J. Labidi
Department of Chemical and Environmental Engineering,
University of the Basque Country UPV-EHU, Plaza Europa 1,
20018 Donostia-San Sebastia
´n, Spain
e-mail: araceli.garcia@ehu.es
W. Wukovits A. Friedl
Institute of Chemical Engineering, Vienna University of
Technology, Getreidemarkt 9/166-2, 1060 Vienna, Austria
123
Clean Techn Environ Policy (2014) 16:1403–1410
DOI 10.1007/s10098-014-0809-5

energy and water requirements and pollutant emissions
(Seider et al. 2009). In this regard, process simulation is a
very useful tool for the study and optimization of industrial
processes (Nikoo and Mahinpey 2008) and equipment
(Cardoso et al. 2009), for the assessment of the industrial
systems behavior and performance as well as to evaluate
the feasibility in the variation of existing operating con-
ditions. Process simulation has been widely used to ana-
lyze, design and optimize energy balances in chemical
technologies including those related to biomass processing,
as the pulp and paper (P&P) industry, with large raw
materials, energy and water consumptions (Hou et al.
2012). Different commercial simulation software types
(Cadsim Plus
Ò
, Aspen Plus
Ò
, Wingems
Ò
, etc.) that incor-
porate mathematic models of different unit operations and
extensive chemicals database, have been intensively used
to simulate biomass-based processes.
Exergy analysis (based on a thermodynamic second law
analysis) has proved to be a powerful tool in the study of
energy in biorefinery processes (Garcı
´a et al. 2012a) and
products (Maes and Van Passel 2014), because it allows to
analyze the impact on the environment of the energy use,
to evaluate the existing inefficiencies in the process, to
locate and quantify losses of energy quality of the process
(Ofori-Boateng and Lee 2014), and to improve the effi-
ciency of energy use (Ranjbaran and Zare 2013). In this
way, the exergy concept allows to assess the energy and
material requirements in a process considering that it
interacts with the environment, and consequently to
quantitatively evaluate the ‘‘useful’’ energy that is supplied
to a process or how it takes advantage of this supply
(Ivanovic
´et al. 2009).
In the present work, an ultrasound-assisted process for
the alkaline pretreatment of lignocellulosic biomass was
designed on the basis of experimental and literature data.
The simulation of the resulted sonoreactor could be con-
sidered as an initial step within a biorefinery scenario. Mass
and energy balances were determined in order to establish
yield and energy requirements of the simulated process.
Furthermore, exergy flows were studied for the evaluation
of energy performance and efficiency of the proposed
ultrasonic reactor.
Methodology
Correlations for the ultrasound-assisted dissolution
of hemicelluloses and lignin
The behavior and yield of ultrasound-assisted dissolution
of hemicelluloses and lignin were estimated from previous
experimental results and from literature data (Table 1). The
raw materials considered for this purpose have been widely
investigated in the literature as potential feedstock in
biorefinery processes: wheat straw (Iskalieva et al. 2012),
sugarcane bagasse (Sun et al. 2004; Brienzo et al. 2009),
and olive tree pruning (Egu
¨e
´s et al. 2013). Their cellulose,
hemicellulose, and lignin contents (30–40 %, 25–40 % and
15–25 %, respectively) represent the usual chemical com-
position of most of the agro-forestry residues that can be
considered as renewable and potential source of biorefinery
products (Garcı
´a et al. 2014).
There are different factors that affect the yield of an
ultrasound-assisted pretreatment process. Besides, the
applied sonoenergy (time and power) also the operating
conditions such as temperature, treated volume, material
size, etc. influences pretreatment results. However, in a
previous work, Velmurugan and Muthukumar (2012) stated
that the factor affecting most significantly the dissolution
of lignin and hemicelluloses was the sonication time,
whereas the particle size of the treated raw material and the
solid to liquid ratio used during the sonication experiments
resulted less influential parameters.
In the present work, for the determination of mathe-
matical correlations of ultrasound-assisted fractionation
process, the dissolution yield of biomass components was
represented against the applied sonochemical energy, tak-
ing into account the volume of solvent–solid treated in each
case, applied power and duration of the treatment. Simi-
larly to the methodology used in the literature (Hulsmans
et al. 2010), the applied volume-specific acoustic energy
(EV, kWs/L) was defined as
EV ¼Pt
V60 ð1Þ
where Pis the applied acoustic power (W), tthe sonication
time (min), and V(mL) the treated volume during the
applied ultrasound experiment. For the different US
experiments considered in this approximation, EV values
Table 1 Operating conditions reported in the literature and used for
the construction of hemicelluloses and lignin dissolution correlations
Raw
material
Solvent
(wt%)
Range of sonication
power (W) and time
(min)
Reference
Olive tree
pruning
7.5 %
NaOH
420 W, 30–120 min Garcı
´a et al. 2011
Wheat
straw
2.8 %
KOH
100 W, 5–35 min Sun and
Tomkinson
2001a
Wheat
straw
2.8 %
KOH
100 W, 5–35 min Sun and
Tomkinson
2001b
Sugarcane
bagasse
0.25-
3.25 %
NaOH
400 W, 5–50 min Velmurugan and
Muthukumar
2012
1404 A. Garcı
´a et al.
123

were calculated and represented against the obtained lignin
and hemicelluloses dissolution yields (Fig. 1). Thus, the
ultrasound treatment yield, referred to the achieved lignin
and hemicelluloses dissolution percentage for the diverse
specific ultrasonic energy applied, was statistically pro-
cessed using OriginPro 8 SR0 v8.0724 software. It was
found to successfully fit the Stirling model, a mathematical
model, represented by the following correlations:
y¼aþbekx1
kð2Þ
where, yrepresents the dissolution yield (%) and xis the
specific energy EV (kW/s L). The parameters a, b, and
kfor hemicelluloses and lignin dissolution correlation are
displayed in Fig. 1a and b, respectively.
Design and simulation of the ultrasound-assisted
treatment
Aspen Plus
Ò
2006 (Aspen Technologies Inc., 2006) was
used to design and simulate the ultrasound-assisted process
on the basis of the mathematical correlations found for
lignin and hemicelluloses dissolution from experimental
data and the open literature.
As main lignocellulosic raw material components, lignin,
cellulose, and hemicelluloses were specified in Aspen Plus
Ò
by their chemical structure and physical properties, accord-
ing to the National Renewable Energy Laboratory (NREL)
database (Wooley and Putsche 1996). They were defined as
LIGNIN
(SOLID)
, GLUCAN
(SOLID)
, and XYLAN
(SOLID)
,
respectively. The dissolved species formed during ligno-
cellulose US-assisted fractionation were named LIG-
NIN
(DISSOLVED)
and XYLOSE
(DISSOLVED)
(resulting from
lignin and hemicelluloses dissolution, respectively). These
dissolved components were considered with identical prop-
erties of their non-dissolved species, but specifying their
presence in different substream classes (SOLID or MIXED
for un-dissolved or dissolved species, respectively). Other
conventional lignocellulosic components (moisture and in-
organics) were specified from the Aspen Plus data bank
(WATER and CALCIUM-SULFATE, respectively). Also
sodium hydroxide (NAOH) was defined as component of the
aqueous media used for the sonication of the raw material.
In order to determine the thermodynamic properties of
solutions, ElecNRTL (non-random, two liquids) model was
selected due to the presence of electrolytes generated in the
dissociation of sodium hydroxide in water.
As ultrasound parameters are not contemplated in the
standard data base of Aspen Plus, and in order to
manipulate the sonication process parameters in the sim-
ulation, two fictitious ultrasound parameters were defined
as flow components for an easier access and modification
during simulation calculations: TIME and POWER. These
two ‘‘components’’ were specified and represented in a
separate stream (named US-PARAMETERS). In order to
force Aspen Plus to consider these parameters, an addi-
tional module (HEATER) was used, allowing the calcu-
lation of this fictitious specification stream and its
components. This way, sonication time and power can be
modified in the proposed design for the evaluation of
sonication processes.
For the design and simulation of the ultrasound-assisted
pretreatment of lignocellulosic material in Aspen Plus, an
RStoic module was used for the simulation of the US-
REACTOR (Fig. 2). The module was specified to operate
as an atmospheric (1 bar) and adiabatic (0 kW of heat
duty) tank allowing vapor–liquid phases. The lignin and
hemicelluloses dissolving reactions defined in the US-
REACTOR were
Fig. 1 Yields of adissolved hemicelluloses and bdissolved lignin
from the literature data (source in Table 1) and their Stirling
correlation vs. the applied specific US-energy
Assessment of biorefinery process intensification 1405
123

LIGNINðSOLIDÞ)LIGNINðDISSOLVEDÞ;ð3Þ
XYLANðSOLIDÞþWATER )XYLOSEðDISSOLVEDÞ:ð4Þ
The fractional conversion of each component (i.e., LIG-
NIN
(SOLID)
and XYLAN
(SOLID)
to LIGNIN
(DISSOLVED)
and
XYLOSE
(DISSOLVED)
, respectively) was specified accord-
ing to the previously obtained dissolution correlations
(Eq. 2and Fig. 1). For this purpose, the reactions extent for
each lignocellulosic component was defined with the aid of
the Calculator tool of Aspen Plus, using the Fortran cal-
culation method. Firstly, Eq. 1for the determination of the
supplied specific acoustic energy EV was introduced,
relating the equation variables (P and t) with the compo-
nents previously specified in the stream US-PARAME-
TERS (POWER and TIME, respectively) and defined as
import variables in the Calculator tool. Then, Stirling
correlations (Eq. 2and Fig. 1) were also introduced,
allowing to determine the lignin and hemicelluloses dis-
solution yields, that were directly related to the dissolution
extends (or fractional conversion) in the US-REACTOR
(as export variables).
As reported in the literature (Dhar et al. 2012; Gogate
et al. 2011), a significant portion of the energy applied
during the sonication process is irremediably lost as heat.
This energy lost (DUTY in kW) was established as the
20 % of the applied total acoustic energy (TOTDUTY in
kW) which was also determined in the Calculator tool as
follows:
TOTDUTY ¼EV Fð5Þ
where EV is the calculated specific acoustic energy
(kW s/L) and F is the total volumetric flow treated in the
sonoreactor (L/s). Thus, DUTY was defined as an export
variable and related with the heat duty in the US-REAC-
TOR, increasing the temperature of the sonicated slurry.
For cooling of the resulting sonicated stream (down to
40 °C), a heat exchanger module (HX) was incorporated to
the ultrasound-assisted process (Fig. 2) using water as the
cooling media.
Energy and exergy evaluation
First and second thermodynamic laws have been exten-
sively applied to establish energy and exergy balances of
processes. The energy and exergy balances of a process
may be expressed as
Energy inputs ¼energy output þenergy loss ð6Þ
Exergy inputs ¼exergy outputs þexergy destruction
ð7Þ
Exergy expresses the loss of available energy due to the
creation of entropy in irreversible processes (Ofori-Boat-
eng and Lee 2014; Hou et al. 2012). This way, for a process
stream, the specific physical exergy e
¯(kJ/mol) is deter-
mined as follows:

e¼ðhh0ÞT0ðss0Þð8Þ
where h,T, and sare the enthalpy (kJ/mol), temperature
(K), and entropy (kJ/K mol), respectively. The subscript 0
denotes the property under standard conditions (298.15 K
and 1 atm). Taking into account the molar flow of each
stream, N(mol/s), the exergy flow of a stream can be
determined:
Ex ¼N
eð9Þ
For the process equipment, general energy, and exergy
balances are defined by Eqs. 6and 7. In reactors and
exchangers, the energy balance takes into account the
energy flow of inputs or reactants, of outputs or products
and the consumption of energy due heat transfer or reaction
processes. The exergy destruction associated to a process
or equipment (Ex
dest
, kW) is determined from the balance
XExinþQ:h¼XExout þExdest ð10Þ
where the exergy input and output flows (Ex
in
and Ex
out
,
respectively, in kW) are considered. The term Qh(kW)
describes the exergy involved in reaction, and heat
exchange processes, being Qthe duty or heat exchanged
(kW) and hthe Carnot factor, that depend on the outlet
temperature from the process (T,°C) and the temperature
at standard conditions (T
0
,°C):
h¼1T0
=
Tð11Þ
The energy/exergy balances involve the analysis of
input and output streams and the simulated equipment (US-
REACTOR and HX heat exchanger). The enthalpies and
energy consumption of the process streams and equipments
were obtained using the process simulation software Aspen
Fig. 2 Flowsheet of the US-assisted pretreatment of the lignocellu-
losic material
1406 A. Garcı
´a et al.
123

Plus
Ò
. For exergy balances, the enthalpy and entropy at
reference conditions (298.15 K, 1.013 bar) for the process
streams can be also determined allowing the evaluation of
the losses due to exergy irreversibilities of the equipments.
Results And discussion
In the following lines, the results of the scale-up of the
ultrasound-assisted treatment described in the literature are
presented. For this purpose, the correlations for the ultra-
sonic dissolution of lignocellulosic material were used,
based on experimental data, obtained at the following
conditions: the ultrasound-assisted reaction was performed
at 1 bar, and the applied acoustic energy was determined as
the product of the calculated EV and the treated lignocel-
lulosic feedstock mass flow rate (1,000 kg/h, 25 °C, 45 %
cellulose, 28 % hemicelluloses, 25 % lignin and 2 %
inorganic compounds in % weight dry basis). The raw
material was sonicated in 7.5 % w/w NaOH (25 °C) at a
solid to liquid ratio of 1:6.
Results of the ultrasound-assisted treatment simulation
The simulation of the ultrasound-assisted process consisted
in the evaluation of hemicelluloses and lignin dissolving
yields, of the US-REACTOR supplied duty (kW) and of
the heat load in the heat exchanger HX for cooling the
slurry down to 40 °C, considering the mathematical cor-
relations derived for the design of this process. The soni-
cation time (min) and the sonication equipment power
(W) were considered as process variables. A sensitivity
analysis was carried out in Aspen Plus, varying the dura-
tion of the ultrasonic treatment from 5 to 120 min and the
power applied to the system by the ultrasound probe
(between 1,000 and 8,000 W). This parameter range was
chosen considering commercial specifications for this type
of equipment (Hielscher 2009). It was found that, for the
treatment of large inputs of material, 500 to 4,000 W
powered ultrasounds or immersible transducers would be
required. Therefore, for the sonication of 1,000 kg/h of
lignocellulosic feedstock with water in a solid to liquid
ratio of 1:6, between 2 and 4 ultrasound modules would be
needed.
In Fig. 3, the results of the performed sensitivity ana-
lysis are displayed. The lignin and hemicelluloses disso-
lution yield (Figs. 3a and b), with sigmoidal behavior for
low sonication parameters values and hyperbolic conduct
for the severer sonication conditions, showed maximum
values of 67.3 and 82.9 %, respectively. However, not
much different yields were observed for somewhat milder
treatment conditions (from 60 min of ultrasound irradiation
with 5,000 W of power).
The Sensitivity analysis of the simulated ultrasound-
assisted treatment also allowed the evaluation of the energy
requirement of the proposed process. For these operating
conditions, medium duty values, between 1,300 and
1,800 kW, were applied to the US-REACTOR (Fig. 4a),
and less cooling duty (1,400–2,000 kW) was required in the
HX equipment (Fig. 4b). Therefore, the mentioned ranges
should be considered as the optimum operating conditions
for the simulated ultrasound-assisted fractionation process.
The improvement in the extraction yield of an ultra-
sound-assisted laboratory-scale process has been widely
reported before (Dhar et al. 2012) leading to significant
improvements in extraction rates (Alupului et al. 2009)
using shorter treatment times and less energy requirements
(Nikolic
´et al. 2011). But, since no literature data about
scale-up of this technology is available, it is not an easy
task to evaluate the suitability and reproducibility of the
proposed sonication process applied to an industrial-scale
lignocellulosic material treatment plants. However, com-
mercial simulation software has been proved to be a very
Fig. 3 Influence of the sonication time (min) and the applied US
power (W) on athe hemicelluloses dissolution extent (%) and bthe
lignin dissolution extent (%) during the simulated ultrasound-assisted
treatment of lignocellulosic raw material
Assessment of biorefinery process intensification 1407
123

helpful tool in the evaluation of both laboratory—(Nikoo
and Mahinpey 2008) and industrial-scale chemical pro-
cesses (Cardoso et al. 2009).
Results of the exergy analysis of the ultrasound-assisted
treatment
For the evaluation of the exergy loss in the proposed
ultrasound-assisted process, the streams of raw material,
solvent, and ultrasounds treated material were considered
(see Fig. 2). The obtained results indicated that the input
streams (raw material and solvent) did not contribute to the
performed exergy balance because both streams were
introduced to the process at 25 °C and, therefore, according
to Eq. 8, no associated exergy changes occurred. Thus,
exergy input to the process only occurred due to duty and
heat requirements in the simulated modules, i.e., in US-
REACTOR and HX equipments.
By applying sensitivity analysis in Aspen Plus for
different sonication conditions (1,000–8,000 W,
5–120 min) the exergy variation in the sonicated material
stream, due to change in stream composition during the
treatment, was determined. A range of related exergy flow
of 19.25–23.88 kW was found for this output stream.
Similarly, the performing of the sensitivity analysis
allowed the establishment of exergy flows associated to
the equipment in the range of the studied process
parameters, resulting in quite low exergy values from 0.14
to 682.27 kW for the sonoreactor and from 4.16 to
157.30 kW for the simulated heat exchanger. In previous
works (Garcı
´a et al. 2012a; Garcı
´a et al. 2010), moderate
exergy destructions (less than 200 kW for pressurized
reactors) were found during the organosolv pretreatment
step of lignocellulosic materials, but higher heat exchange
requirements.
The results displayed in Fig. 4c show a clear influence
of the energy requirements during the process, resulting in
a trend quite similar to that observed for duty and heat
requirements of the simulated equipments (see Figs. 4a and
b). As the exergy concept indicates, the temperature change
of any system leads to an entropy generation (Ofori-
Boateng and Lee 2014) and the subsequent exergy
destruction of the products. Thus, because of the applica-
tion of ultrasounds implies only a moderate loss of the used
sonoenergy into heat, the ultrasound-assisted processes
should be considered as more energy efficient process than
those classical ones. In their work, Ivanovic
´et al. (2009)
found that the use of a sonication step allowed to signifi-
cantly diminish the exergy loss due to the CO
2
supercritical
treatment of aloe vera and sweet bay as well as to increase
the extracts yield of the process. Similar results were found
by Ranjbaran and Zare (2013) concerning the use of
microwaves in fluidized bed drying process of soybeans,
indicating the energy saving benefits that imply the use of
these technologies for the intensification of classical
processes.
Fig. 4 Influence of sonication time (min) and applied US power
(W) on athe acoustic energy (kW) provided to the US-reactor, bthe
energy required for cooling down the sonicated slurry (kW) and cthe
destroyed exergy (kW) during the simulated ultrasound-assisted
treatment of lignocellulosic raw material
1408 A. Garcı
´a et al.
123

Conclusions
In the present work, an ultrasonic reactor was designed and
simulated in Aspen Plus using correlations constructed
from experimental data reported in the literature. The he-
micelluloses and lignin dissolution yield, the acoustic
energy supplied to the US-reactor and the energy required
for cooling down the resulted sonicated slurry were eval-
uated under different time and power conditions of the
simulated sonication process. Subsequently, the operating
conditions for the optimum hemicelluloses and lignin dis-
solution were established (60 min and 5,000 W). Medium
energy requirements were associated to these optimum
conditions, resulting in a cooling duty of 1,048 kW in the
heat exchanger. Thus, the simulation model developed in
the present work offers the possibility of further and more
accurate studies about the use of sonication processes.
Finally, using simulation tools, an exergy assessment was
performed for the simulated process. This study indicated
an almost linear relationship between the destroyed exergy
of the process and the applied sonication power and time.
At the operating conditions established for the optimum
lignocellulose dissolution yield (60 min, 5,000 W), an ex-
ergy destruction of 178 kW was found. In this regard, the
proposed simulation study presents the basis for the design
and evaluation of ultrasound-assisted processes that can be
easily implemented in existing biorefinery facilities and
that can achieve significant energy improvements due to
the process intensification step.
Acknowledgments The authors would like to thank the Basque
Government (IT672-13 and the Postdoctoral Development Program)
for financially supporting this work.
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