b i o m a s s a n d b i o e n e r g y 4 0 ( 2 0 1 2 ) 1 1 2 e1 1 9
Available online at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Extractive fermentation with non-ionic surfactants to
enhance butanol production
Pradip B. Dhamole a,c, Zhilong Wang a,d, Yuanqin Liu a, Bin Wang a, Hao Feng a,b,*
a
Energy Biosciences Institute, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
c
Department of Biotechnology, Sinhgad College of Engineering, Pune 411041, India
d
School of Pharmacy, Shanghai Jiao Tong University, Shanghai 200240, PR China
b
article info
abstract
Article history:
One of the major limitations in butanol fermentation is the end product toxicity which
Received 29 October 2011
limits the butanol yield and increases the downstream processing costs. In this work,
Received in revised form
a range of non-ionic surfactants (Triton X 114, L64, L62LF, L61, and L62) was tested to
6 February 2012
enhance the acetoneebutanol (AB) production, and to extract and separate butanol from
Accepted 9 February 2012
the fermentation broth. In biocompatibility tests, a volume fraction of 3% L62, L62LF, and
Available online 25 February 2012
L61 did not show inhibition to AB production in 72-h fermentation using Clostridium pasteurianum. Three-percent L64 reduced the AB yield whereas Triton X 114 (3%) inhibited the
Keywords:
AB production. Further optimization with L62 at 6% resulted in a butanol yield of 225%
Butanol
higher than the control. The partition coefficient of butanol in L62-water two phase
Biofuel
systems in cloud point extraction ranged from 3 to 4. A considerable enrichment of butanol
Non-ionic surfactant
(6 times) was achieved in the surfactant-rich phase over the control. In addition, the
Extractive fermentation
downstream process volume was reduced by 4e6 times. Butanol was separated from the
Biocompatibility
surfactant-rich phase (obtained from model system) by evaporation between 120 and
130 C. The butanol was enriched in the condensate reaching a concentration of 106.8 g l1,
under which butanol automatically separated into two phases. The L62 was recovered by
evaporation and reused for 3 times without affecting the partition coefficient, volume
reduction, and butanol recovery in the surfactant-rich phase. The results demonstrated
that the L62 not only significantly enhanced the butanol production but also functioned as
a good extractant for separating butanol from the fermentation broth.
ª 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years, a renewed and growing interest in the
production of acetone, butanol and ethanol (ABE) has been
stimulated by the increasing demand for advanced biofuels.
Butanol is the main product of ABE fermentation, and has
some attractive properties. The advantages of butanol include
30% higher energy content (29.2 MJ L1) over ethanol
(19.6 MJ L1), lower vapor pressure, less volatile, less flammable, and mixable with gasoline [1e4]. However, since the
discovery of ABE fermentation a century ago, biobutanol
production has faced numerous difficulties, preventing it from
becoming a commercially viable process. There are two interrelated outstanding challenges facing the commercialization
* Corresponding author. Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, 382F-AESB, 1304
West Pennsylvania Avenue, Urbana, IL 61801, USA. Tel.: þ1 217 244 2571; fax: þ1 217 333 9329.
E-mail address: haofeng@illinois.edu (H. Feng).
0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biombioe.2012.02.007
b i o m a s s a n d b i o e n e r g y 4 0 ( 2 0 1 2 ) 1 1 2 e1 1 9
of biobutanol today. The first is the end-product toxicity to the
fermenting microorganisms. Butanol at low product concentrations (normally < 20 g L1) causes cell growth inhibition and
premature termination of ABE fermentation [5]. The toxicity of
butanol has been ascribed to passive proton flux by butanol
causing membrane leaking [6], disruption of the lipid structure in cell membranes that alters membrane-bound enzyme
activity [7], and membrane fluidity in the presence of butanol
[8]. Newly developed strains, such as Clostridium beijerinckii
P260, have shown improved butanol tolerance, but the
maximum yield of butanol is still ca. 22.27 g L1 after hydrolysate detoxification [9]. The other challenge is the high energy
cost of recovering butanol from the fermentation broth. Due
to the low concentration (w20 g L1) in the broth and the high
boiling point of butanol (117 C), removing water to obtain
purified butanol is an expensive process [10e12]. To attack the
low concentration butanol recovery problem, scientists have
explored various techniques for butanol separation, including
adsorption [13,14], pervaporation [15,16], perstraction [17,18],
liquideliquid extraction [19,20], and gas stripping [21,22].
Extractive fermentation is a potential method to eliminate
the product inhibition and thus increase the final product
concentration. The concentrated product in the extraction
phase during fermentation could save the cost in downstream process. Extractive fermentation of butanol has been
conducted in organic solvent-aqueous solution two-phase
systems [23e26] and in aqueous solution-polymer two-phase
systems formed by polymer polypropylene glycol (PPG) [27].
Kumn [28] carried out an extractive fermentation of ethanol in
polyethylene glycol (PEG)-dextrin aqueous two-phase system.
The key challenge of extractive fermentation in aqueous
solution-organic solvent two-phase systems is the biocompatibility of the organic solvent to the bacteria [29] whereas
that of the PEG-dextrin aqueous two-phase system it is the
high price of the polymer dextrin. A number of non-ionic
surfactant aqueous solution forming cloud point systems at
above a certain temperature have been developed as a novel
medium for extractive microbial fermentation [30e33]. The
main advantages of extractive fermentation in a cloud point
system include that the biocompatibility to the bacteria is
improved in comparison to that of organic solvent-aqueous
solution two-phase systems, and the cost of PEOePPOePEO
block copolymers is lower than that of dextrin in an aqueous
two-phase system. Aqueous solution of a non-ionic surfactant
at a temperature above the cloud point (the temperature at
which the copolymer solution starts to separate) forms
a surfactant rich phase (coacervate) and surfactant diluted
phase. An organic compound presenting in the non-ionic
surfactant aqueous solution should unevenly partitioned into
those two phases. Such a scheme is called cloud point
extraction (CPE). CPE has many advantages which include
mild environment, simplicity, effectiveness of operation and
easy scale up. Surfactants are amphiphilic in nature i.e.
having a polar part and an apolar part which interact with
interfaces. Amphiphilic characteristics are critically dependent on the molecular properties, such as total molecular
weight, relative block size and block sequence as well as
thermodynamic parameters, such as temperature and pressure. Water solubility of the surfactant is due to hydrophilic
group which is an ionic or highly polar group. Above critical
113
micelle concentration and/or cloud point, surfactant molecules usually assemble themselves into many kinds of structures i.e. micelles, lamellar, hexagonal, etc. These nano-sized
micellar assemblies formed facilitate removal of targeted
compound (depending on its properties).
Hence, in this work, we have explored a non-ionic surfactant aqueous solution system for overcoming the end product
(butanol) toxicity and separation of butanol from the nonioninc surfactant micelle aqueous solution by cloud point
extraction. The same hypothesis is used to relieve butanol
toxicity during fermentation in non-ionic surfactant micelle
aqueous solution, resulting in a significant increase in butanol
yield. Experiments were carried out with non-ionic surfactants to find its butanol capturing capacity and those with
high butanol capturing capacity were tested further for its
biocompatibility. The surfactant resulting in high butanol
production than control (i.e., without surfactant) was used for
concentrating and separating butanol. The separation was
carried out after fermentation by incubating the broth at
a fixed temperature. A model system was used to study
further downstream processing of butanol and recovery of
surfactant. Recovered surfactant was reused to capture
butanol.
2.
Materials and methods
2.1.
Chemicals
Glucose, yeast extract, K2HPO4, KH2PO4, ammonium acetate,
para-amino benzoic acid, thiamine, biotin, MgSO4.7H2O,
MnSO4.H2O, FeSO4.7H2O, and NaCl of analytical grade were
purchased. Non-ionic Pluronic surfactants L61, L62, L62LF, and
L64 were provided by BASF, USA as a gift sample. The letter ‘L’
in the nomenclature denotes that the surfactant is liquid.
The first number in the surfactant name indicates the
molecular weight range of the hydrophobe (i.e., PPO) whereas
the second number signifies the weight percentage of hydrophile (i.e., PEO). Thus, L61 has a weight to volume fraction of
10% hydrophile whereas L62 and L64 have 20% and 40%
hydrophile, respectively.
2.2.
Culture and cell propagation
Acetoneebutanol producing strain Clostridium pasteurianum
(NRRL B-598) obtained from the ARS culture collection centre
(NRRL, Peoria, IL, USA) was used as a model organism because
a high yield strain was not available. The strain was activated
following the procedure provided by the supplier. Stock
solutions were prepared before starting the fermentation. The
buffer stock solution consisted of K2HPO4 (50 g L1), KH2PO4
(50 g L1), and ammonium acetate (220 g L1). The vitamin
stock solution contained 0.19 g L1 para-amino benzoic acid,
0.19 g L1 thiamine, and 0.19 g L1 biotin whereas the
composition of minerals stock solution was 20 g L1
MgSO4.7H2O, 1 g L1 MnSO4.H2O, 1 g L1 FeSO4.7H2O, and
1 g L1 NaCl. Before fermentation the strain was reactivated by
incubating 1 ml of refrigerated strain into 25 mL of Difco
Infusion broth (35 g L1). 0.25 mL of the buffer stock solution
was added to the infusion broth before inoculation. The flasks
114
b i o m a s s a n d b i o e n e r g y 4 0 ( 2 0 1 2 ) 1 1 2 e1 1 9
were incubated at 32 C for 36 h and the cells thus obtained
were used for fermentation.
2.3.
micelle diameters were measured by dynamic light scattering
at a fixed scattering angle of 90 at 23 C with a NICOMP 380
ZLS Particle Sizer.
Butanol capturing capacity (BCC) of surfactants
2.7.
A dialysis cell (Scienceware, Bel-Art Products, NJ) and UF
membrane was used in determining the relative butanol
capturing capacity of a variety of surfactants. Total volume of
the cell was 18 mL which was divided into two compartments
by a membrane, with each compartment of 9 mL. One side of
the cell was filled with distilled water whereas the other side
was filled with a solution containing surfactant (30 g L1),
butanol (50 g L1), and glucose (60 g L1). The dialysis cells
were kept at 30 C throughout the study. Samples were
collected from the water side on a periodic basis till the steady
concentration of butanol was observed (data not shown). The
butanol capturing capacity was defined as amount of butanol
captured per unit amount of surfactant.
After the fermentation, cells were separated from the
fermentation broth by centrifugation at 13,130 g for 5 min to
obtain a clear fermentation broth. It was then incubated in
a water bath (Poly Science Digital Temperature Controller,
Niles, IL) at different temperatures between 35 and 70 C to
obtain a phase separation. The clear phase separation was
observed at 70 C on incubation for 30 min when a volume
fraction of 6% L62 was present. This resulted in a two phase
formation with lower phase rich in surfactant and the upper
phase being the aqueous phase. Samples were collected from
the separated phases and analyzed for butanol concentration.
2.8.
2.4.
Analysis
Acetone and butanol in fermentation was determined by
a gas chromatograph unit (GC Hewlett Packard 5890 Series II, Avondale, PA) equipped with an auto sample
injector (HP 7673A Automatic Injector) and a flame
ionization detector (FID). The column used was DB-WAX
30 m 0.250 mm 0.25 mm fused silica capillary column
(J & W scientific, Agilent Technologies, Germany). The oven
temperature was programmed from 40 C to 190 C at
20 C min1. The injector and detector temperature was set to
220 C and 250 C, respectively. The carrier gas was He at
0.72 mL min1 flow rate and acetonitrile was used as an
internal standard. Peaks, areas and percentages were calculated using Agilent Technologies GC Chemstation software
(Agilent Technologies, Germany). Glucose was estimated
using HPLC system (Waters Corporation, Milford, MA) e2695
separation module and a Waters 2414 refractive index
detector monitored by an Empower pro software version 6.2.
Aminex column (HPX-87P 300 mm 7.8 mm) equipped with
Microguard Carbo-P cartridge (30 mm 4.6 mm) from Biorad
(Hercules, CA) was used.
2.6.
1
Separation of butanol from surfactant rich phase
Fermentation
A fermentation medium consisting of glucose (60 g L1), yeast
extract (1 g L1), buffer stock solution (1 ml L1), mineral stock
solution (1 ml L1), and vitamin stock solution (1 ml L1) was
used. Nitrogen was purged after the inoculation and every
time the sample was withdrawn to maintain the anaerobic
conditions. Flasks were incubated at 32 C for butanol
fermentation. Fermentation was carried out with and without
surfactant (control). Surfactant addition was carried out after
autoclaving the fermentation medium and before inoculation.
2.5.
Extraction of butanol from fermentation broth
C. pasteurianum produces very low amount of butanol
(6e8 g L1) [34]. Due to unavailability of a high yielding strain to
the authors, fermentation was carried out with C. pasteurianum
and separation studies were conducted with model systems.
A model system consisting 50 g L1 butanol and 6% L62
(optimum for fermentation) was chosen to resemble the
process with high butanol producing strains. The model
solution (containing 50 g L1 butanol and a volume fraction of
6% L62) was incubated at 38 C to obtain the surfactant rich
phase [35]. The surfactant rich phase thus obtained was used
for further downstream processing. Butanol and water from
the surfactant rich phase was separated by evaporation
between 120 and 130 C and the vapor phase was condensed in
a condenser using tap water. The feed, condensate and
concentrate were analyzed for butanol concentration, and
their volumes were recorded for material balance calculations.
2.9.
Reuse of L62
The L62 used in extraction of butanol (50 g L1) was separated
following the above mentioned process. The concentrate
containing L62 was reused for butanol extraction. The L62
concentrate was added into a solution having a total butanol
concentration of 50 g L1 and the solution was incubated at
38 C to obtain a surfactant-rich phase and an aqueous phase.
Samples were collected from both phases for butanol analysis.
The surfactant-rich phase was again processed to recover L62
following the above procedure and was reused for butanol
extraction for the third time.
3.
Results and discussion
3.1.
Screening of surfactant
Characterization of surfactant in the model system
H NMR spectra of the L62 solutions were recorded with
a Varian Unity 400 MHz NMR spectrometer using D2O as the
solvent in 5 mm NMR tubes. The residual signal of the solvent
was used as a reference in all the spectra (D2O, d 4.8). The L62
The relative butanol capturing capacity of the selected
surfactants (non-ionic and triblock co-polymer) (Triton X 114,
L61, L62, L64, L62LF) was first examined with the dialysis cell
method at 30 C. As shown in Table 1, 1 kg of Triton X 114 and
co-polymer L62LF can capture 0.6 and 0.52 kg of butanol,
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b i o m a s s a n d b i o e n e r g y 4 0 ( 2 0 1 2 ) 1 1 2 e1 1 9
Table 1 e Butanol capturing capacity of some selected
surfactants at 30 C.
A
7
Butanol capturing
capacity, (kg kg1)
Cloud point
temperature ( C)
6
Triton X 114
L62LF
L62
L64
L61
0.6
0.52
0.32
0.06
Insoluble in water
25
28
32
58
24
5
-1
Control
L62LF
Triton X114
L43
L64
L61
L62
4
3
2
1
0
0
10
20
30
40
50
60
70
80
60
70
80
Time (hour)
B
2.0
Control
L62LF
Triton X114
L43
L64
L61
L62
1.5
-1
Acetone (g L )
respectively; much higher than that of the other 3 polymers.
Butanol captured and the cloud point of the surfactant is also
included in Table 1. It can be seen that non-ionic surfactants
having a low cloud point (than 30 C) captured high amount of
butanol at 30 C. It is assumed that micelle formation
(for Triton X 114 and L62LF) is better above cloud point. Hence,
Triton X 114 entrapped the highest amount of butanol whereas
L64 just captured very little butanol since the operating
temperature (30 C) is lower than the cloud point (58 C) of L64.
In control fermentation (i.e. without surfactant) maximum
butanol produced was 4.75 g L1 after 120 h. Fermentation was
continued till 168 h, however butanol concentration remained
unchanged. Corresponding unutilized glucose concentration
was observed to be 42.7 g L1 (Initial glucose ¼ 60 g L1) which
clearly indicated that butanol fermentation was inhibited at
4.75 g L1 of butanol. Similar studies with C. pasteurianum
(NRRL B-598) are reported in the literature however in
those studies higher inhibition levels (6e7.5 g L1) were
observed which could be attributed to different medium
composition [34].
The biocompatibility of the surfactants was evaluated with
an AB fermentation test with C. pasteurianum where 3% L62
was added in a fermentor and the butanol yield over a 72-h
period was monitored (Fig. 1). Among the 5 surfactants
examined, Triton X 114 totally inhibited AB production while
a significant reduction in butanol yield when compared to the
control (without surfactant) was observed for L64. L61 and
L62LF produced nearly the same amount of butanol as the
control. In the case of L64, the acetone and butanol produced
were about 50% of the control, respectively. Notably, the
fermentation with L62 enhanced the AB fermentation. Addition of 3% L62 increased the acetone as well as butanol
production by little over 70% (Fig. 1A and B).
The toxicity of surfactants follows the order: ionic
surfactant > non-ionic surfactant > polymeric non-ionic
surfactant [30]. All the pluronic surfactants studied in this
work are polyoxypropylene (PPO) epolyoxyethylene (PEO)
block copolymers. The toxicity of a surfactant can be correlated with its hydrophileelipophile balance (HLB) value.
Generally, an increase in HLB is accompanied by an increase
in the toxicity of the surfactant to microbes. For Triton X 114
and L64 the HLB values are 12.3 and 12 to 18, respectively and
hence were toxic to the microbes. Other three surfactants
(L61, L62 and L62LF) have HLB values in the range of 1e7 and
hence are non-toxic to the microorganisms. L61 is insoluble in
water and does not extract butanol from the fermentation;
hence no relief to the microbes from butanol. Further, the
butanol yield during fermentation with a surfactant is related
to both the surfactant toxicity and its butanol capturing
Butanol (g L )
Surfactant
1.0
0.5
0.0
0
10
20
30
40
50
Time (hour)
Fig. 1 e Biocompatiblity of different surfactants for AB
fermentation. (A) Butanol production and (B) Acetone
production. Concentration of surfactant [ 3%, volume
fraction. Temperature [ 32 C.
capacity. Therefore, for Triton X 114 having a high butanol
capturing ability, because of its high HLB values, is toxic to the
microbes. Only the L62, which had a moderate butanol
capturing capacity but a low HLB, enhanced the butanol
production (0.32 kg kg1 of surfactant). Therefore, L62 was
used in all the subsequent AB fermentation tests.
3.2.
Effect of surfactant concentration
Screening of surfactants showed that a volume fraction of 3%
of L62 enhanced the acetone and butanol production (Fig. 1).
It was desirable to find out the optimum amount of L62 which
will further increase the AB production. Since the AB yield
remained increasing at the end of 72-h fermentation, showing
the potential to further increase the AB production, if the
fermentation is continued over an extended period. It was
116
b i o m a s s a n d b i o e n e r g y 4 0 ( 2 0 1 2 ) 1 1 2 e1 1 9
3.3.
Concentration of butanol from fermentation broth
by phase separation
Butanol produced during the fermentation needs to be separated from the fermentation broth. The L62 added during the
12
-1
Butanol Produced (g L )
10
12
-1
Acetone/Butanol Produced (g L )
thus decided to study the effect of surfactant concentration on
fermentation over a period of 5 days. As can be seen from
Fig. 2, an increase in surfactant concentration from 3% to 6%
improved the acetone and butanol production by 25%.
However, a further increase in surfactant concentration (9%)
did not enhance AB production. The results suggested that 6%
L62 was the optimum for fermentation. Fig. 3 shows the time
course of AB fermentation in presence of 6% L62. It can be seen
that the AB production was higher from day 3 onwards. At the
end of 120 h, the AB produced in the presence of L62 was 225%
higher than that of the control. The fermentation was
continued till 8 days but no further increase in AB was
observed (data not shown).
It was assumed that L62 might have formed micelles and
entrapped butanol resulting in high butanol production
during fermentation. To confirm this, studies were carried out
to check the micelle formation by the surfactant in a model
system at fermentation temperature (32 C) with NMR and
dynamic light scattering. The 1H NMR spectrum (Fig. 4) clearly
shows the methyl proton signal of both L62 (d 1.1) and butanol
(d 0.8). If L62 formed micelles and entrapped butanol, the NMR
signal of butanol should decrease or disappear. However, from
the integral of the peaks the ratio of L62/butanol was similar to
the theoretical ratio (1: 0.5). The NMR data thus indicated that
butanol was free and in an un-trapped form in the model
system. The particle size measurement results (Table 2) also
demonstrated that there were no micelles detected in the
model system (L62 (6.0%) þ butanol (3 or 30 g L1)) because the
measured particle sizes (10e11 nm) were similar to that of
single L62 molecule (L62 (0.001%), 8 nm) and much smaller
than the size of L62 micelles (L62 (6%), 130 nm). It can be
concluded that no micelles formed in the model system and
the increase in AB production might be attributed to the
attachment of butanol to the monomers of L62.
Butanol (control)
Butanol (6% L62)
Acetone (control)
Acetone (6% L62)
10
8
6
4
2
0
0
20
40
60
80
100
120
Time (hour)
Fig. 3 e Time course of AB fermentation in the presence of
a volume fraction of 6% L62.
fermentation process was used to separate and concentrate
butanol from the fermentation broth. An increase in temperature above the cloud point resulted in two phases with upper
phase being aqueous and the lower one rich in L62. The
butanol was concentrated in surfactant rich phase. To separate the surfactant rich phase and to enrich butanol in the
surfactant rich phase, the entire broth was first incubated at
different temperatures to find out the phase separation
conditions. It was noted that the presence of cells increased
the turbidity and no clear phase separation was observed.
Therefore, cells were separated from the fermentation broth
by centrifugation to obtain a clear fermentation broth. The
clear supernatant was then incubated at different temperatures to separate the surfactant-rich phase from the aqueous
phase. Two phase formation was observed at 70 C after
incubating the supernatant for 30 min Table 3 shows the
partition of butanol in aqueous phase and surfactant rich
phase. The butanol was enriched in the surfactant-rich phase,
as shown by a partitioning coefficient of 3.5. Furthermore, the
volume was reduced by a factor of 6 (i.e. the initial volume/
surfactant rich phase volume ¼ 6), resulted in a significant
8
6
4
2
0
0
2
4
6
8
10
L62 Added in the Medium (%)
Fig. 2 e Effect of amount of L62 (in volume fraction) on AB
fermentation (Data after 120 h of fermentation).
Fig. 4 e 1H NMR spectrum of a model system (L62 (6.0%,
volume fraction) D butanol (3 g LL1)).
117
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Table 2 e Particle size of co-polymer L62 in the solution.
% (v/v)
L62
L62
L62
L62
(0.001%)
(6%)
(6%) þ butanol (0.3 g L1)
(6%) þ butanol (3 g L1)
Diameter at 32 C (nm)
8
130
10
11
4
38
4
3
reduction in downstream process volume and hence energy
usage in butanol recovery. It needs to be emphasized here that
in the control system (without L62) only 5 g L1 of butanol was
produced by C. pasteurianum whereas with the surfactant
system the surfactant rich phase contained 30 g L1 butanol
representing a 6 times enrichment. Therefore, the addition of
L62 during the fermentation not only increased the fermentation yield, it also reduced the process volume and enriched
the surfactant rich phase significantly.
Since C. pasteurianum used in this study produces very low
amount of butanol, to represent the butanol fermentation
with high yield strains (such as C. beijerinckii BA101), downstream processing was carried with a model system containing higher butanol concentration (50 g L1) and 6% L62
(optimum for fermentation). In the presence of 6% L62 C.
pasteurianum produced 10.71 g L1 of butanol which is 225%
higher than control. It is anticipated that high butanol
producing strains will produce butanol in presence of
biocompatible L62 (6%) in a similar manner. Thus a strain
producing 20e22 g L1 butanol may produce 45e50 g L1
butanol.
The phase diagram of L62 with 50 g L1 butanol was
reported in our earlier work [35]. It was observed that the
butanol concentration had a significant effect on phase
separation. Butanol lowers the cloud point. In the presence of
50 g L1 butanol and 6% L62, the phase separation took place at
38 C (model system). When the fermentation broth contained
10 g L1 butanol, a clear phase separation was observed at
70 C whereas with 50 g L1 butanol the phase separation
occurred at 38 C (model system) [35]. Consequently, for a high
butanol producing strain, the temperature required for
phase separation will be close to fermentation temperature.
If a phase separation takes place during fermentation,
a continuous extraction of the produced butanol can be
Fig. 5 e Separation of butanol from the surfactant rich
phase by evaporation (temperature [ 120e130 C).
achieved; this will further increase the production and reduce
the downstream separation costs.
3.4.
Separation of butanol from surfactant rich phase
and reuse of surfactant
The surfactant rich phase obtained after incubating the
aqueous solution containing butanol (50 g L1) and L62 (6%)
was used for further studies. Fig. 5 shows the material balance
over the evaporation unit and composition of the feed,
condensate and concentrate. Water and butanol were
obtained in the condensate and the high butanol concentration (106.8 g L1) resulted in two phase formation with the
upper phase being butanol. Over 95% butanol was recovered
from the surfactant rich phase. Very little butanol remained in
the surfactant (2.1 g L1). Since the entire separation system is
closed, the high boiling point L62 is retained in the concentrate. Further, the minimum amount of butanol that remained
in the L62 implies that surfactant can be reused for butanol
extraction. Hence, tests were conducted to examine the
reuse of L62 for butanol extraction. The results are included in
Table 4. The basic parameters of extractive fermentation,
i.e., the volume ratio of non-ionic surfactant-rich phase
to aqueous phase, partitioning coefficient of 1-butanol and
1-butanol concentration, were unchanged in the second and
Table 3 e Separation of L62 from the aqueous phase.
Initial concentration of butanol, g L1
Concentration of butanol in surfactant rich phase, g L1
Concentration of butanol in aqueous phase, g L1
Concentration factor
Initial volume, mL
Volume of surfactant rich phase, mL
Phase volume ratio (Aqueous phase :
Surfactant rich phase)
Partition coefficient (Butanol in surfactant
rich phase/Butanol in aqueous phase)
Fermentation broth
(after fermentation)
Model system containing
50 g L1 butanol and 6% L62
11.8
30.1
8.71
2.55
12
2
6: 1
50
95
35.18
1.9
12
3
4: 1
3.5
2.7
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Table 4 e Repeated utilization of recovered polymer non-ionic surfactant in the L62-rich phase (Butanol [ 50 g LL1,
L62 [ 6%, Incubation temperature [ 38 C).
Run
Volume ratio
Partitioning coefficient
Butanol in surfactant
phase, g L1
Butanol in aqueous
phase, g L1
4
4
4
2.6
2.7
2.87
94.82
93.3
95.8
36.47
34.55
33.38
First
Second
Third
third runs in comparison to that of first run. Therefore, the
recovered L62 can be reused to reduce the cost of butanol
enrichment in fermentation and butanol recovery in the
downstream process.
4.
[7]
Conclusions
In the present work, the effect of addition of conventional
non-ionic surfactant Triton X-114 and triblock copolymer
Pluronic surfactants on butanol fermentation and separation
has been explored. A novel L62 cloud point system has been
developed for extractive fermentation of 1-butanol. The L62
cloud point system fulfilled the basic demands of biocompatibility and doubled the butanol yield in a fermentation test.
More importantly, the product 1-butanol was concentrated in
the surfactant-rich phase after the fermentation, which is
advantageous for downstream processing. In addition, the
process volume was reduced by 4e6 times. Both would
contribute to reduce the bio-butanol production costs. 95% of
butanol was recovered from the surfactant-rich phase and the
reuse of L62 had no effect on its performance as an extractant.
The extractive fermentation with L62 is a promising method
for relieving the butanol toxicity and enhancing the butanol
production.
[8]
[9]
[10]
[11]
[12]
[13]
Acknowledgments
The project was funded by Energy Biosciences Institute grant
No. 555794-231000. We are grateful to Mr. John Jerrell for his
assistance in GC analysis.
[14]
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