Recovery of renewable phenolic fraction from pyrolysis oil
Ljudmila Fele Žilnik*, Alma Jazbinšek
National Institute of Chemistry, Laboratory for Catalysis and Reaction Engineering,
Hajdrihova 19, POBox 660, SI-1001 Ljubljana, Slovenia, tel:+386 1 4760 220, fax: +386 1
4760 300, e-mail: ljudmila.fele@ki.si
*Corresponding author
Abstract
The aim of this work was to develope a separation process for phenolic fraction recovery from
various bio-oils, produced by fast pyrolysis process of wood and forest residues in the
framework of the EU Project BIOCOUP. Two slightly different schemes were introduced,
namely the first one starting with an aqueous extraction of pyrolysis oil and the second one
with the simultaneous use of a hydrophobic-polar solvent and antisolvent in the extraction of
bio-oil. In both cases the distribution coefficients of phenolic components between the phases
as well as extraction factors for major separation stages are presented. Different aqueous
solutions were applied and alkali solution was found to be more efficient in comparison to
water or aqueous NaHSO3 solution. From various hydrophobic-polar solvents tested, methyl
isobutyl ketone (MIBK) was shown to be the most efficient solvent for extraction of phenolics
from bio-oil in combination with 0.1 M or 0.5 M aqueous NaOH solution, followed by butyl
acetate.
Keywords: pyrolysis oil, phenolic fraction, aqueous extraction, solvent-antisolvent technique,
distribution coefficient
1. Introduction
The use of renewable energy sources is becoming increasingly important to achieve
the changes, required to address the impacts of global warming. Biomass is the most common
form of renewable energy and generates very low net greenhouse emissions [1]. Various
pretreatment processes of lignocellulosic biomass for efficient hydrolysis and biofuel
production and their advantages and disadvantages are recently discussed by Kumar et al. [1].
The energy from biomass can be obtained by various techniques, such as combustion or by
upgrading it into a more valuable fuel, gas or oil. Biomass can also be transformed into a
source of value-added products for the chemical industry by using a thermochemical method,
such as pyrolysis. Pyrolysis is one of the pretreatments of lignocellulosic material, based on
the chemical decomposition of organic materials by heating in the absence of oxygen. Often
fast pyrolysis is utilized, in which organic materials are rapidly heated to 450 - 600ºC in the
absence of air. Under these conditions, organic vapours, pyrolysis gases and charcoal are
produced. The vapours are condensed to bio-oil. Fast pyrolysis is meant to convert biomass to
a maximum quantity of liquids (bio-oil) [2]. Typically, 70-75 wt% of the feedstock is
converted into liquid bio-oil, which means that it is storable and transportable. The bio-oil
contains more than 300 compounds of different molecular sizes, mostly the degradation
products (derivatives) of three key biomass building blocks: cellulose, hemicellulose and
lignin, that are thermally and chemically unstable [3,4], with high oxygen content of about 3540 wt% and low pH.
1
Bio-oils from any waste biomass such as forestry biomass, crop residues or animal
manures contain low amounts of sulfur, are always carbon neutral and, most importantly,
unlike petroleum feedstock, are renewable [3,4,5]. Different types of biomass are utilized,
from agricultural wastes such as straw, olive pits, corncobs, tea waste and nut shells to energy
crops such as miscanthus and sorghum. In order to avoid the competition with the food
industry, an integral processing route for the conversion of non-feed biomass residues to
transportation fuels is proposed. Forestry wastes such as bark and thinnings and other solid
wastes, including sewage sludge and leather wastes, have also been studied [6]. Most of the
research work has been done on wood biomass or forestry residue.
The liquid product of biomass pyrolysis, known as bio-oil or pyrolysis oil, is a
complex mixture of several hundreds of organic compounds that exhibit a wide range of
chemical functionality. Bio-oil is a viscous, dark brown organic liquid that is comprised of
highly oxygenated compounds. It is relatively unstable and susceptible to aging. Chemically,
bio-oil is a complex mixture of water (15-30%), and the other major groups of compounds,
including hydroxyaldehydes, hydroxyketones, sugars, carboxylic acids, phenolics (phenols,
guaiacols, catechols, syringols, isoeugenol) and other oligomeric lignin derivatives. Around
35-50% of the bio-oil constituents are non-volatile [5].
A review on applied fast pyrolysis of lignocellulosic materials was given more than
ten years ago by Meier and Faix [7], where the pyrolysis reactors were described,
characteristics of bio-oil were presented and a snapshot on utilization of bio-oil was given.
The overview of applications of biomass fast pyrolysis oil was later presented by Czernik et
al. [5]. The authors summarized that research is still needed in the area of handling with biooils, on stabilization and upgrading of bio-oil to a quality of transport liquid fuel. Recent
review on pyrolysis of wood/biomass for bio-oil is given by Mohan et al. [6], where the
literature on wood/biomass fast and slow pyrolysis is surveyed together with the physical and
chemical aspects of the resulting bio-oils.
For further use of bio-oil in co-processing in existing traditional installations of the
petroleum refineries, an upgrading of oil is necessary, where oxygen is partly or totally
removed to stabilize the oil, to lower oil's acidity and viscosity, to increase the energy value of
oil [7]. Hydrotreatment (hydrogenation and/or hydrodeoxygenation, HDO) of pyrolysis
liquids can improve its properties either as fuel or as a feedstock for chemicals [8,9]. On the
other hand, the bio-oil enriched on oxygenated compounds is a valuable source for the
production of bio-chemicals [5], like alcohols, aldehydes, ketones, acids, phenolics and
sugars, either from the whole bio-oil (BioLime for SOx capturing, biodegradable slow-release
fertilizers, creosote replacement in wood preservative) or from major fractions of bio-oil. As
pointed out by the authors [5], some chemicals produced from the whole bio-oil or by its
major fractions are already commercial products, but specialty chemicals from bio-oils
require more work on developing reliable low-cost separation procedures. According to Mann
et al. [10], the lignin-derived fraction of bio-oil could be sold at half the price of phenol for
the use as a phenol replacement in phenol-formaldehyde resins. Lignin can be also used, both
as a filler and as a phenol substitute in PF resins. Phenols derived from biomass pyrolysis oils
are valuable chemicals and can be used as intermediates in the synthesis of pharmaceuticals,
for the production of adhesives and the synthesis of specialty polymers.
Phenolics in the fraction form, lignin derived, can be produced from the renewable
resources, e.g. biomass by means of fast pyrolysis process and further separation. Lignin, a
third major building block of wood, is a complex, large molecular structure containing crosslinked polymer of phenolic monomers. It is present in the primary cell wall, offering
structural support, impermeability and resistance against fungi and bacteria [7,11] an it can
generate a large amount of chemical reagents or adhesives to replace those derived from oil
2
[12]. The total amount of the phenol-guaiacolic fraction in the pyrolysis oil is approx. 4-5 wt
%, and varies depending on the type of biomass and on the process conditions (severity:
temperature, residence time, heating rate). Softwoods have the highest lignin content, ranging
from 25-35 %, which is mainly of guaiacyl type, while hardwoods contain from 16-25% of
lignin, and is guaiacyl-syringyl type. Bark produces around 29% of lignin derivatives, thus
more than the hardwood [6]. Syringols and guaiacols originate from the primary degradation
of lignin during pyrolysis. Using mild hydrotreatment as an upgrade technique for pyrolysis
oil, it was shown [8] that the water-insoluble fraction increased, but the portion of HMM
lignin was minor. An increase, compared to the feed, in proportion of phenolics and catechols
occurred and the presence of monomeric phenolics made the oil to be fluid, confirmed also by
P-NMR analysis.
During the BIOCAT project [13], funded by the European Community under the
'Energy' Programme, a separation procedure was developed in order to obtain useful
chemicals from bio-oil, based on the polarity using dichloromethane and small amounts of
acetone. Another fractionation scheme as an effective tool to characterize the bio-oil, starting
with water, was proposed by Oasmaa et al. [14]. The water insoluble fraction (low molecular
mass-LMM, high molecular mass-HMM) mainly originates from the degraded lignin,
extractives, solids [8]. Separation characteristics of biomass pyrolysis oil were studied also by
molecular distillation [15]. A critical review of separation methods and technologies related to
biorefining including pre-extracation of hemicellulose and other value-added chemicals is
presented by Huang et al. [16].
Extraction of phenols from water phase with polar organic solvents was studied by
some authors [17,18]. Distribution coefficients at high dilution at room temperature for some
phenolic solutes between water and polar organic solvents like butyl acetate and methyl
isobutyl ketone were determined by Won and Prausnitz [18]. Distribution coefficients for
solute phenol and solvent between water and solvent at different mass fractions of phenol
were measured as well. Separations of phenols from wood tar were studied and carried out by
dissolution of an oil phase in ethyl acetate and five stage alkaline extraction by Amen-Chen,
et al. [19], together with a primary conversion of the raw wood tar into a lighter oil. High pH
value was required for a complete extraction of phenols from the oil-matrix. Steam distillation
of the pyrolysis oil from birch wood and the recovery of phenols at various steam pyrolysis
oil ratios and further distillation under vacuum was studied by Murwanashyaka et al. [20].
The review on the fractionation processes toward obtaining syringols or phenolic-rich fraction
from pyrolysis oil was given by Mohan et al. [6]. Recent review by Effendi et al. [21] focuses
on the production of phenolics by fast and vacuum pyrolysis, liquifaction and phenolysis, and
covers also fractionation methods for the recovery of the concentrated fractions of phenolics.
Pyrolysis oil was often ugraded by adding water [21,22] or slight aqueous basic solutions to
neutralise the fraction or increase the pH. Quite complicated fractionation scheme for
isolation of phenolic compounds was presented by Rusell et al. [23], applying six contact
stages with different solvents in sequences, namely diethyl ether, aqueous NaHCO3, aqueous
NaOH, HCl, diethyl ether and water. Gallivan and Matschei [24] developed the fractionation
scheme comprised of several steps, namely using NaOH to reach a certain pH and using
methylene chloride or ether to obtain solvent soluble fractions, followed by distillation. The
industrial application of a proposed process is limited by the complexity of solvent extraction
routes and type of solvents used. Another separation scheme of reactive phenols and neutral
fraction in a series of liquid-liquid extraction steps was proposed by Chum et al. [25], where
the pyrolysis oil is contacted with ethyl acetate, in the second stage the solvent soluble is
contacted with water and in the third stage the water insoluble with aqueous alkali bicarbonate
solution to remove acids, followed by distillation. The yield of the phenolics and neutrals
achieved was about 30%.
3
An European integrated BIOCOUP project, supported through the sixth framework
programme for research and technological developments, started in May 2006 with the aim to
develope a strategy to converte lignocellulosic biomass residues (forestry residue, pine wood,
waste from pulp and paper industry, etc) to biofuels or value-added chemicals. It was agreed
that an introduction of biomass derived bio-fuels for co-processing in conventional refinery
units and further for transportation in the market is necessary to reduce CO2 emissions and to
achieve a biorefinery concept and sustainability.
Fig. 1. The BIOCOUP concept [26].
Hydrocarbon-rich bio-liquid
Biomass
residues
Primary
fractionation
and
liquefaction
Co-processing in
conventional
petroleum
refinery
Lignin-rich
bio-liquid
Coventional
fuels and
chemicals
De-oxygenation
Oxygenated
products
Conversion
Process
residues
Derivatives of hemicelluloses
and celluloses
Energy
production
BIOCOUP unites different universities, research centres and industrial companies in the
following activities:
- SP1: Primary Liquefaction. Improve the quality of the primary oil and reduce production
costs.
- SP2: Bio-liquid upgrading. Research and development of various upgrading-deoxygenation
technologies.
- SP3: Co-processing in standard refinery. Assessment of co-processing upgraded bio-liquids
in standard refinery equipment.
- SP4: Conversion. Identification and technology development to obtain discrete target
compounds from the bio-liquids.
- SP5: Scenario analysis. Identification and assessment of the most promising biomass-torefinery chain(s) on basis of predicted technical, economical and LCA-performances.
- SP6: Transversal activities. Coordination of the project, analytical support to all the partners
in the consortium and dissemination of the knowledge generated within BIOCOUP.
Isolation and fractionation of selected key chemicals or fractions has been extensively
investigated in the SP4 part of the BIOCOUP project. Because of thermal and chemical
instability of the bio-oil and a plethora of components present in bio-oil with similar boiling
points, the distillation can not be used as a separation technique to produce distinct
oxygenated chemicals or well defined fractions of chemicals. Solvent elution
4
chromatography, liquid-liquid extraction and fractional distillation may be used for the
separation of phenols from wood pyrolysis oil. However, the major drawback of the solvent
elution chromatography, high consumption of solvents and regeneration problem of silica gel
solid phase, was reported in the literature. Fractional distillation of the whole bio-oil to get the
desired phenolic fractions would require too much energy, and therefore, it is not considered
as a suitable technique from economic point of view for the separation of phenolic fractions.
Therefore, solvent extraction at room temperature and atmospheric pressure was suggested to
be a promising unit operation to recover the target oxygenated chemicals from the pyrolysis
oil.
The approach in this paper involves the development of an isolation and fractionation
technology for phenolic fraction from the various bio-liquids, that were produced by our
BIOCOUP partners, mainly by partners from VTT Energy, Finland. The specific objective in
the framework of the subgroup SP4 was also to produce phenolic fractions that can be further
used as bio-replacements in pilot scale processes (e.g. production of phenol-formaldehyde
synthetic resins applied in panel manufacturing, etc). In our work, we first identified possible
sources for the production of phenolics and proceeded systematically to investigate different
separation stages toward the production of the phenolic fraction. Two slightly different
schemes are introduced, namely the first one starting with an aqueous extraction of pyrolysis
oil and the second one starting with the simultaneous use of hydrophobic-polar solvent and
antisolvent in the extraction of pyrolysis oil. In both cases the distribution coefficients of
phenolic components between phases as well as extraction factors for major separation stages
are presented. Possible identified sources for the production of phenolics from pyrolysis oil
are shown on Fig. 2. To work with an apolar oil-phase, various aqueous solutions (water,
aqueous NaHSO3 solution, alkali solution) are suggested.
Fig. 2. Shematic process presentation from biomass to pyrolysis oil and identified possible
sides for phenolic extraction.
Chemicals
Raw
Biomass
Water, NaHSO3, alkali
Polar aqueous
phase
Pyrolysis
Hydrogen
BioOil
Phenolics
Apolar
oil-phase
Oxygen
removal
Phenolics
Crude
refinery
2. Conceptual process design
Phenolics can be recovered from bio-oil mainly as fractions, because of the presence
of a high number of phenolic derivatives, observed by GC-MS/FID. Since the molecular
structure of a family of components is very alike, they do not differ very much in physical
5
properties, like boiling temperature, solubility, pKa. Phenolics are weak (Lewis) acids with
small dissociation constants, whose hydrophilicity is reinforced in alkali solution. They also
have a limited solubility in water, therefore this property can be used for isolation purposes.
Proposed isolation and fractionation technology of phenolics from pyrolysis oil
obtained by fast pyrolysis, is based on extraction of phenolics from (i) an apolar oil-phase of
pyrolysis liquid obtained after an aqueous extraction of pyrolysis oil by using water, aqueous
NaHSO3 or alkali solution or from (ii) the bio-oil using simultaneously hydrophobic-polar
organic solvent and water, aqueous NaHSO3 or alkali solution that acts as an antisolvent.
2.1. Recovery of phenolics from an a-polar oil-phase of pyrolysis oil starting with an addition
of water, aqueous NaHSO3 or alkali solution to the bio-oil
The first concept, presented in Fig. 3, starts with an addition of water, aqueous
NaHSO3 or alkali solution to the bio-oil, which causes an existance of a two phases that are in
equilibrium with each other, an apolar oil phase enriched on phenolics, LMM and HMM
lignin derivatives and an aqueous phase rich on acids, sugars, alcohols, ketones, aldehydes.
As far as we know, the addition of water or slight aqueous basic solutions was used and
discussed by some authors [21,22] to upgrade the pyrolysis oil and to neutralise the fraction.
Very recently, the distribution coefficients of some compounds of interest at water to oil ratio
from 0,3 to 0,8 and from 0,4 to 0,9 for forest residue oil and for pine oil, respectively, were
presented by Vitasari et al. [27].
No extensive study was performed so far on distribution coefficients of phenolics
between an organic oily phase and raffinate phase, depending on aqueous to bio-oil ratio and
using different aqueous solutions. Aqueous NaHSO3 solution was, to our knowledge, for the
first time applied on bio-oil to produce an apolar oil phase. An enriched phase on phenolics is
further contacted with hydrophobic-polar organic solvent. The solvent soluble part is rinsed in
the next step by alkali solution to remove the rest of the aqueous soluble components,
followed by solvent removal. The phenolics still remaining in the major aqueous phase,
derived from bio-oil by aqueous extraction, can be recovered from this phase by an extraction
with hydrophobic-polar organic solvent (preferably MIBK), followed by the removal of
solvent from an organic phase
No systematic study was performed so far also on partitioning of phenolics from an
apolar oil phase to certain solvents, followed by alkali extraction. In our study, different lowboiling solvents were tested, like methyl isobutyl ketone (MIBK), isopropyl ether (IPE), ethyl
acetate (EtAc) and toluene in order to yield an effective extraction and separation. Lowboiling solvent should be recovered by means of a distillation.
6
Fig. 3. Conceptual process scheme which starts with an aqueous extraction.
Aqueous solution: 0,1 M NaOH
Solvent: MIBK
make-up
solvent
Washing agent
(water, aqueous solution)
solvent
Aqueous layer
A
AQUEOUS
Bio-oil
B
EVAP2
SOLVENT
Aq1
extract
Fraction 2
S1
EXTRACT
raffinate
S
make-up
Apolar oil phase
DISSOLVE
F
solvent
Solvent insoluble
solvent
Solvent soluble
Alkali solution
A
ALKALI
Organic phase
EVAP1
1
O
Aqueous basic solution
Main fraction
2.2. Extraction of phenolics from bio-oil using simultaneously hydrophobic-polar organic
solvent and either water, aqueous NaHSO3 or alkali solution that acts as an antisolvent
The alternative isolation route for phenolics recovery is also suggested in our work
(see Fig. 4) to simplify the recovery process scheme, by adding simultaneously hydrophobicpolar solvent and either water, aqueous NaHSO3 or alkali solution to the bio-oil to induce
phase separation. The addition of hydrophobic-polar solvent such as MIBK or ethyl acetate to
bio-oil normally does not cause a phase split, therefore an antisolvent like water, aqueous
NaHSO3 or alkali solution is added to enhance the phase separation. As far as we know, this
technique was not described in the open literature for the bio-oil application on phenolics
recovery. The addition of both solvent and antisolvent to the bio-oil causes the phase
separation and the partitioning of major phenolics and lignin derivatives to the organic phase,
and partitioning of other water soluble components to the aqueous phase. Depending on the
solvent and antisolvent used, one can expect also some partitioning of acids and aldehydes to
the organic phase. Adding aqueous NaHSO3 solution together with hydrophobic-polar solvent
(like MIBK or ethyl acetate), the aldehydes are preferably rinsed to an aqueous phase by
reactive extraction with NaHSO3. Adding alkali solution together with hydrophobic-polar
solvent, the acidic components are rinsed in the salt form to the aqueous phase. Various
hydrophobic-polar solvents are suggested in our study, namely MIBK, ethyl acetate, isopropyl
acetate and butyl acetate. The main advantages of the proposed alternative are: simplification
of the process, easier phase separation, better flowability of both phases and better handling .
7
Fig. 4. Conceptual process scheme using hydrophobic-polar solvent and an antisolvent.
Alkali solution: 0,1 M NaOH
Solvents: MIBK, EtAc, ButAc, IsopropylAc
Alkali solution
make-up
Alkali solution
solvent
solvent
S
A
Extract
Bio-oil
B
A1 ALKALI
EVAP2
Organic
stream
O
EXTRACT
Main fraction
make-up
Aqueous stream
Raffinate
SOL1
Organic phase
solvent
solvent
EVAP1
Aqueous phase
Fraction 2
3. Materials and methods
3.1. Chemicals used
Pyrolysis oils or bio-oils. Pyrolysis liquids were produced in VTT's Process
Development Unit (PDU) as described elsewhere [8,28]. In our experiments different batches
of pyrolysis oils were used, namely PDU 35-06 (pine bio-oil), PR06-27 (VTT reference pine
oil), PDU 5-07 (forest residue, bottom phase). The experiments were performed on the
original bio-oil, because of the complexity of the system. Partial GC-MS characterization of
the bio-oils used in experiments is given in the Table 1. The amount of water present in the
bio-oils normaly varies from 20 to 25 wt%. Bio-oils contain also a substantial amount of
sugars (approx. 30 wt%), that are only in part (few wt%) GC-MS detectable, as well as low
(LMM) and high (HMM) molecular mass lignin derivatives in an approximate amount of 13
wt% and 2 wt%, respectively, given for the reference pine oil.
8
Table 1
Partial characterization (GC-MS/FID) of the VTT pine bio-oil (PDU-35-06), VTT reference
pine oil (PR06-27) and VTT forest residue oil (PDU 5-07) used in our experiments.
Feedstock
PDU-35-06
VTT PR0627
PDU 5-07
bottom
phase
wt%
wt%
wt.%
wt%
wt%
wt%
wet
dry
wet
dry
wet
dry
1.64
4.04
3.04
1.42
0.27
3.48
0.04
0.16
3.05
2.19
5.39
4.05
1.89
0.36
4.64
0.06
0.21
4.07
4.3
7.36
4.06
2.55
0.84
3.38
0.05
0.07
2.89
5.6
9.72
5.36
3.37
1.10
4.42
0.06
0.09
3.82
5.63
7.35
3.47
2.11
1.14
3.57
0.07
0.16
1.04
2.56
7.54
9.85
4.65
2.82
1.52
4.78
0.09
0.22
1.39
3.43
Total
17.14
22.85
25.5
33.54
27.1
36.29
water
25
Substance group
Acids
Nonaromatic Aldehydes
Nonaromatic Ketones
Furans
Pyrans
Sugars
Catechols
Lignin derived phenols
Guaiacols (methoxy phenols)
Syringols
23.9
25.37
Source: VTI (Johann Heinrich von Thuenen Institute), Germany
VTT pine bio-oil contained around 0.2 wt% of lignin derived phenols and 4 wt% of
guaiacols, beside acids (2 wt%), nonaromatic aldehydes (5 wt%), nonaromatic ketones (4
wt%), furans (2 wt%), pyrans, sugars (30 wt%), water (25 wt%) and oligomeric lignins, all on
the dry basis. The reference pine oil PR06-27 has lower amount of lignin derived phenols,
higher amount of acids, nonaromatic aldehydes, nonaromatic ketones, furans and pyrans. VTT
forestry residue bio-oil (PDU 5-07, bottom phase) was enriched with higher amount of water
insoluble lignin material (from 35-37 wt%) in comparison to the pine bio-oil (15-20 wt%).
The bottom phase contained the same amount of lignin derived phenols as pine oil, but
smaller concentration of guaiacols (1.4 wt%) and around 3 wt% of syringols, on the dry basis.
The acid content was higher (7 wt%), as well as the concentration of nonaromatic aldehydes
(10 wt%), all on the dry basis. The water content was 25 wt%.
Standards. Fluoranthene (Aldrich, purum, 99%), 4-Hydroxy-3-Methoxyacetophenone
(Fluka, purum; ≥ 97%), 3-Ethylphenol (Fluka, purum; ≥ 95%), 2-methoxy-4-methylphenol
(Fluka, purum; ≥ 98%), 2,6 –Dimethoxyphenol (Fluka, purum; ≥ 97%), 2-Hydroxyphenol
(Fluka, purum; ≥ 99,5%), 4-Ethyl-2-methopxyphenol (Acros, purum; 98%), 2-methoxy-4propylphenol (Chemos, purum; ≥ 99%), 2-Methoxyphenol (Merck, purum; 98%), Phenol
(Fluka, purum; 99%), 4-Allyl-2,6-dimethoxyphenol (Aldrich, purum; 90+%), 4-Allyl-2methoxyphenol (Merck, purum; 99%), 4-Methylphenol (Fluka, purum; ≥ 99%), 4-Hydroxy-3methoxybenzaldehyde (Fluka, purum; ≥ 98%), 2-Methoxy-4-propenylphenol (Fluka, purum;
≥ 98%), Dimethoxy-4-hydroxycinnamaldehyde (Chromadex, purum; 98%).
Solvents. Sodium hydroxide (Merck, 1 mol/L, titrisol), Acetone (Merck, for GC
analysis), Ethyl acetate (Merck, for GC analysis), Methyl isobutyl ketone (Fluka, for GC
analysis), Toluene (Merck, pro analysis 99. 9% ), Isopropyl ether (Merck, for GC analysis),
Isopropyl acetate (SIGMA Aldrich, for GC analysis, min 99.5%), Butyl acetate (Zorka Šabac,
for GC analysis, min 99%).
9
3.2. Methods
3.2.1. Analytical method
GC analysis. The identification of the GC-eluted phenolic components in raw bio-oil
and in other samples from the extractions was carried out by gas chromatography coupled to a
mass spectrometer (Agilent GC/MS-FID). When the phenolic compounds were identified, the
analyses of GC eluted compounds (quantifications) were performed on the FOCUS Thermo
Scientific GC Instrument (GC/FID) with AS 3000, using the capillary column of the type
Thermo Scientific TR-5, 30 m*0,32mm*0.25μm. Oven initial temperature: 50˚C for 2 min,
ramp-1: 5˚C/min up to 190˚C, hold time 1 min; ramp-2: 30˚C/min up to 280˚C, hold time 10
min. Tinj=Tdet= 250˚C, split injection (10:1). All the samples were dissolved in acetone. For
quantification of GC-eluted compounds in samples internal standard (I.S.) technique was
used, using fluoranthene as I.S.
Water content. Water content of the samples was determined by Karl Fischer (KF)
coulometric titration of the sample according to the standard ASTME E-1064-05 using
Hydranal Coulomat AK for ketones.
Lignin part. For lignin part of the sample the precipitation method described
elsewhere [5] was applied, using an emulsifying process of bio-oil sample in water, and
subsequent filtration.
3.2.2. Equilibria experiments
Liquid-liquid equilibrium experiments were performed in a 50 ml tubes at room
temperature (23˚C) to prevent the polymerization of the components present in bio-oil.
Different phase ratios of solvent to feed were used. The phases were shaken for two hours
and settled overnight to reach an equilibrium. In experiments with 0.6 M aqueous NaHSO3
solution, equilibrium concentrations of phenolics were reached practically after 1 hour of
contact time, except for catechol, therefore 2 hours were chosen as time to reach an
equilibrium.
3.2.3. Solvent removal
Solvent evaporation technique under vacuum was used to remove the solvent (EtAc,
MIBK) from an organic phase. Thin film evaporation of solvent (MIBK) was also applied for
the evaporation of MIBK, using distillation traps with cooling water and dry ice.
3.2.4. Calculation method
Aspen Plus® V7 was used for solvent removal modeling. The predictive UNIFAC
group contribution method to account for the nonidealities in the liquid phase was employed.
4. Results and discussion
4.1. Recovery of phenolics from pyrolysis oil, starting with an aqueous extraction of the biooil using water, aqueous NaHSO3 or alkali solution
The proposed isolation and fractionation procedure was experimentally investigated
on the lab scale. The performance of the proposed technology on phenolics was studied on the
10
VTT pine bio-oil, labeled as PDU-35-06, containing small amounts of phenolics, mainly of
guaiacyl type, on VTT reference pine oil (PR06-27-1) and forest residue bio-oil (PDU 5-07)
containing also syringol. At this step, an extensive and systematic study was performed in our
work determining the distribution coefficients of phenolics between an apolar oil phase and an
aqueous phase, obtained by an addition of aqueous solution to bio-oil, namely water, aqueous
NaHSO3 or alkali solution in different aqueous to bio-oil ratios.
4.1.1. Effect of water to bio-oil ratio
The effect of water to bio-oil ratio in the range from 1:2.35 up to 2:1 (m/m) on
distribution coefficients of tracked phenolic components in the forest residue bio-oil (PDU-507), defined as the ratio between the mass concentration of a certain component in an aqueous
phase and its mass concentration in the organic phase, is shown in Fig. 5 (a). It can be seen,
that the distribution coefficients of all phenolic components reach high values at low water to
bio-oil ratios, and are decreasing with increasing water to bio-oil ratio and finally remain
constant below the value of 0.3 for all phenolic components, except for catechol and vanillin.
For most tracked phenolic components the minimum value of distribution coefficients is
achieved at water to bio-oil ratio between 0.65 and 1. The distribution coefficients of GCeluted phenolic components are small, because of their very limited solubility in water. The
highest partitioning to an aqueous phase is observed for catechol, which is in accordance with
its solubility in water, followed by vanillin. The partitioning of phenol and syringol to an
aqueous phase is more than 2.5 times lower than the partitioning of catechol to an aqueous
phase and 1.8 times lower than the partitioning of vanillin to an aqueous phase. The
distribution coefficients for p-cresol, acetovanillone, guaiacol, dimethylphenol, creosol are
lying nearly on the same curve and their values are in the range of 0.15-0.17 at the water to
bio-oil ratio of 1. A bit lower partitioning to an aqueous phase is observed for isoeugenol
(0.125 at water to bio-oil ratio of 1) and for methoxyeugenol, sinapinaldehyde, eugenol and 4ethylguaiacol (below 0.1 at water to phase ratio of 1), which represents half of the distribution
coefficient of guaiacol. Higher distribution coefficients of catechol and vanillin are most
likely the result of the polarity and ability to form H-bond due to the presence of 2 OH groups
on the aromatic ring for catechol and presence of OH and aldehyde group in vanillin structure,
beside the methoxy group. Nearly equal distribution coefficients are observed for phenol and
syringol, the latter with two methoxy groups attached to the phenolic ring. The presence of the
methyl group on the phenolic ring (p-cresol, dimethylphenol), or one methoxy group
(guaiacol) or methyl plus methoxy (creosol) and methoxy plus ketone group (acetovanillone)
lower the distribution coefficients of mentioned components. The propenyl group in the
structure of isoeugenol also causes a decrease in distribution coefficient in comparison to
guaiacol. A bit higher decrease is noticed when allyl group (eugenol) or ethyl group (4ethylguaiacol) is attached to the guaiacol molecule, and when allyl group (methoxyeugenol)
or propenal group (sinapinaldehyde) is attached to the syringol molecule.
11
Fig. 5. (a) Distribution coefficients and (b) Extraction factors of followed GC-eluted phenolic
components as a function of water to bio-oil ratio.
0,8
phenol
pcresol
guaiacol
dimethylphe
creosol
4ethylguaia
syringol
eugenol
vanillin
isoeugenol
acetovanill
methoxyeuge
sinapinalde
catechol
Distribution coefficients (aq/org)
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
water to bio-oil ratio (/)
8
phenol
pcresol
guaiacol
dimethylphenol
creosol
catechol
4ethylguaiacol
syringol
eugenol
vanillin
isoeugenol
acetovanillone
methoxyeugenol
sinapinaldehyde
Extraction factor (aq/org)
7
6
5
4
3
2
1
0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
water to bio-oil ratio (/)
The extraction factors, defined as Ei = Di ⋅ ( M aq M org ) , where (Maq/ Morg) denotes
the mass ratio between the phases in equilibria, are presented for each individual phenolic
component at various aqueous to bio-oil ratio, in Fig. 5 (b). It can be noticed that minimum
extraction factors are achieved at water to bio-oil ratio of around 0.7 for all tracked GC-eluted
phenolic components, since at higher water to bio-oil ratio lower organic fraction is obtained.
For instance, at water to bio-oil ratio of 0.66 the water insoluble part is approx. 17%, at ratio 1
the water insoluble part drops to 15%. The minimum value of the extraction factor for
catechol is 3.4, vanillin 2.3, for phenol and syringol 1.1, for the rest of phenolic components
from 0.3-0.9. The recover efficiency of individual GC-eluted phenolic components in an
organic phase, defined as ηi = (mi , org mi,bio − oil ) , obtained during water treatment of bio-oil
in one step at the water to bio-oil ratio of 1 is depicted on Fig. 6. An average recover
efficiency of GC eluted phenolic components present in an organic phase is around 55 %.
Since most of the HMM lignin derivatives enter the organic phase (~89%), an average
efficiency is higher than 60%.
12
Fig. 6. The recover efficiencies of GC eluted phenolic components in the first step using
water.
water to bio-oil ratio of 1
ol
cr
es
ol
g
di
m u ai a
et
hy co l
lp
he
no
cr l
eo
s
c a ol
4t
ec
et
hy
ho
lg
ua l
ia
c
sy ol
rin
g
e u ol
di
hy
ge
dr
o e n ol
ug
en
o
va l
n
i
is
oe l l i n
a c ug
e
et
o v n ol
m
a
et
ho n i l l o
x
s in ye ne
ap u ge
in
a l no l
de
hy
de
p-
ph
en
Recover efficiency (% )
water to bio-oil ratio of 0,66
80
70
60
50
40
30
20
10
0
phenolic components
4.1.2. Effect of contact time on distribution coefficients using aqueous NaHSO3 solution
Since the TU/e separation group [29] has shown, that the extraction of the carbonyls
with NaHSO3 solution from other organic mixtures than bio-oil was successful, the decision
was made to apply this solution to bio-oil to obtain an apolar oil phase and to rinse the
carbonyls to an aqueous phase by reactive extraction. Different contact times were used in the
extraction experiments with 0.6 M aqueous NaHSO3 solution at an aqueous to bio-oil ratio of
1 and at room temperature (Fig. 7). Equilibrium distribution coefficients of some phenolic
components were achieved in approx. 30 min, except for catechol, vanillin and guaiacol,
where 180 min were needed for catechol and 90 min for vanillin, guaiacol and acetovanillone.
The value of the equilibrium distribution coefficient of catechol coincides with the value
obtained by water extraction at the water to bio-oil ratio of 1, the value for vanillin is lower
(around 0.2), for guaiacol, acetovanillone, sinapinaldehyde the distribution coefficient is
comparable to that obtained by water extraction, and for the rest phenolics slighlty lower
distribution coefficients are gained. The extraction factor for catechol is comparable with the
value (3.3) from water extraction, for vanillin is lower (around 1) and for the rest the
extraction factor is in the range from 0.1-0.8. The average recover efficiency on GC-eluted
phenolic components in an organic phase, using this aqueous media is around 59%. Since
most of the HMM lignin derivatives enter the organic phase (~89%), an average recover
efficiency is higher than 60%.
13
Fig. 7. Distribution coefficients of GC-eluted phenolic components vs. contact time at an
aqueous to bio-oil ratio of 1, using aqueous NaHSO3 solution.
0,6 M NaHSO3 : bio-oil PDU 35-06 = 1:1
0,7
Distribution coefficient (/)
0,6
pcresol
guaiacol
creosol
catechol
4ethylguaiacol
eugenol
vanillin
isoeugenol
acetovanillone
cinnamaldehyde
0,5
0,4
0,3
0,2
0,1
0,0
0
20
40
60
80
100
120
140
160
180
200
equilibration time (min)
4.1.3. Effect of the alkali to bio-oil ratio and concentration
A study was performed on the extraction of phenolics from the bio-oil at different
aqueous to bio-oil ratios, using an alkali solution to rinse the acidic components to a certain
degree to an aqueous phase. During the experiments different concentrations of alkali solution
were apllied on the reference pine bio-oil PR06-27-1 for phenolics isolation at room
temperature and at three different phase ratios of alkali solution to bio-oil (Table 2). As
expected, 0.1 M NaOH solution gave the lowest partitioning of phenolic components to an
aqueous phase and easier phase separation. Good flowability of an organic phase is achieved
when aqueous to bio-oil ratio of 1 is applied.
14
Table 2
The comparison of distribution coefficients and extraction factors of GC-eluted phenolic
components at different aqueous to bio-oil ratios, using alkali solution of different
concentrations.
Distribution coefficients (/); Extraction factors (/)
0.1 M NaOH
0.5 M NaOH
2 M NaOH
Components
2:1
1:1
1:2
2:1
1:1
1:2
1:1
1:2
0.062;
0.012;
0.030;
0.159;
0.081;
0.075;
0.073;
0.103;
phenol
p-cresol
guaiacol
creosol
catechol
4-ethylguaiacol
eugenol
dihydroeugenol
vanillin
isoeugenol
acetovanillone
sinapinaldehyde
0.54
0.012;
0.11
0.083;
0.71
0.05;
0.43
0.243;
2.1
0.018;
0.16
0.006;
0.05
0.085;
0.73
0.057;
0.49
u.d*.
0.032;
0.28
u.d.
0.07
0.001;
0.008
0.093;
0.52
0.061;
0.34
0.378;
2.1
0.016;
0.09
0.005;
0.03
0.121;
0.68
0.086;
0.48
u.d.
0.040;
0.23
u.d.
0.11
0.003;
0.01
0.152;
0.59
0.115;
0.44
0.52;
1.99
0.054;
0.21
0.030;
0.12
0.303;
1.17
0.167;
0.64
u.d.
0.108;
0.42
u.d.
1.34
0.028;
0.24
0.083;
0.70
0.05;
0.43
0.312;
2.63
0.013;
0.11
0.005;
0.04
0.096;
0.81
0.090;
0.76
u.d.
0.068;
0.57
u.d.
0.46
0.014;
0.08
0.098;
0.56
0.061;
0.35
0.377;
2.16
0.010;
0.06
0.012;
0.07
0.116;
0.67
0.096;
0.55
u.d.
0.061;
0.35
u.d.
0.31
0.003;
0.01
0.13;
0.53
0.093;
0.38
0.506;
2.1
0.046;
0.19
0.022;
0.09
0.182;
0.74
0.128;
0.52
u.d.
0,077;
0,31
u.d.
0.47
0.061;
0.40
0.125;
0.81
0.068;
0.44
0.533;
3.44
0.014;
0.09
0.006;
0.04
0.292;
1.89
0.709;
4.58
u.d.
0,465;
3,01
0.237;
1.53
0.41
0.003;
0.01
0.11;
0.43
0.072;
0.29
0.630;
2.48
0.027;
0.11
0.004;
0.02
0.226;
0.89
0.157;
0.62
u.d.
0,119;
0,47
u.d.
*- under detection limit
In Table 2 the results in the form of distribution coefficients and extraction factors of
phenolic components are given, depending on the aqueous to bio-oil ratio and concentration
of alkali solution. The lowest partitioning of phenolics was reached at an aqueous to bio-oil
ratio of 1, using 0.1 M NaOH solution (Fig. 8). It is clearly seen that raising the molarity of
sodium hydroxyde solution up to 2, the partitioning of catechol, dihydroeugenol, vanillin,
acetovanillon and sinapinaldehyde to an aqueous phase is increased, especially for an aqueous
to bio-oil ratio of 1:1, where a step increase of distribution coefficients and extraction factors
of mentioned components is most pronounced. The distribution coefficients of phenol, pcresol, creosol, 4-ethylguaiacol, eugenol, isoeugenol are small, all below 0.1 at an aqueous to
bio-oil ratio of 1:1. The partitioning of sinapinaldehyde to an aqueous phase, when 0.2 or 0.5
M alkali solution was used, was negligible; low partitioning is noticed also for acetovanillone
and vanillin at lower molarity of alkali solution. Lower the alkali concentration, lower the
partitioning of the phenolic components to the aqueous phase and lower the extraction factor.
This behaviour is in accordance with the Lewis acid character of the phenolic components.
Acetovanillone posseses the lowest pKa (pKa1 of 4.08 and pKa2 of 8.54), vanillin has its pKa
value of 7.69, sinapinaldehyde of 7.5, catechol of 9.38 and guaiacol of 9.84. Syringol, not
present in this bio-oil, has its pKa value of 9.01. All other phenolic components pKa's are
above 10, therefore much higher concentration of alkali solution should be used to rinse them
into an aqueous phase as alkali salts. As already mentioned, higher partitioning of catechol at
low concentration of alkali solution or in water arises from its high solubility in water (430
g/L at 20 ˚C).
15
Fig. 8. The effect of the alkali concentration on the extraction factors of GC-eluted phenolic
components in alkali extraction of bio-oil at the initial phase ratio of 1.
5,0
4,5
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
0,1 M NaOH:bio-oil=1:1
0,5 M NaOH:bio-oil=1:1
2 M NaOH:bio-oil=1:1
ph
e
p- nol
cr
e
gu sol
ai
ac
cr o l
eo
4- c so
et ate l
hy c
h
ho lgu o l
m aia
o c co
at
ec l
h
di
hy e u ol
dr ge
oe no
ug l
en
va ol
is
ni
ac oeu llin
et g e
sin ov n
ap a n ol
i n i ll o
al
d e ne
hy
de
Extraction factor (/)
Alkali solution: bio-oil = 1:1
phenolic components
An average recover efficiency on GC-eluted phenolic components in an organic phase,
using 0.1 M NaOH solution, at an aqueous to bio-oil ratio of 1 is approx. 80%, when only
GC-detectable lignin derivatives are taken into account and the recover efficiency is higher
compared to that obtained by water addition (55%). A bit lower average recover efficiency
(Table 3) is reached by using 0.5 M NaOH solution at the same initial phase ratio and much
lower (around 58%) when 2 M NaOH solution is applied at 1:1 aqueous to bio-oil ratio.
Since most of the HMM lignin derivatives enter the organic phase, the average recover
efficiencies are higher than mentioned. It can be concluded, that an alkali extraction of bio-oil
is an appropriate method for phenolics isolation from the original bio-oil and more efficient in
comparison to water or aqueous sodium bisulphite solution.
Table 3
Average recover efficiency on GC-eluted phenolics using different concentrations of NaOH
solution and different bio-oil to alkali ratio.
Bio-oil/alkali ratio
(m/m)
2:1
1:1
1:2
Average recover efficiency (%)
0.1 M NaOH
0.5 M NaOH
2 M NaOH
77
79
81
81
79
58
78
76
As we can see from the Fig. 8, the extraction factors for vanillin, catechol,
acetovanillone and sinapinaldehyde are increased applying 2 M NaOH solution and the major
part of vanillin (80%), catechol (78%), acetovanillone (75%), dihydroeugenol (60%) and
sinapinaldehyde (60%) are rinsed into an aqueous phase.
16
4.1.4. Solvent dissolution and alkali wash
For an effective extraction of phenolics it is desirable to choose an organic solvent
which can hydrogen-bond with phenolics and which causes the miscibility gap with water.
Hydrophobic-polar solvents like ketones, esters and ethers are therefore promising solvents.
Four potential hydrophobic-polar solvents, namely methyl isobutyl ketone (MIBK), ethyl
acetate, toluene and isopropyl ether (IPE) were chosen as solvents of interest. Toluene
exhibits the lowest solubility in water, followed by isopropyl ether, MIBK and ethyl acetate.
Solubility of water in solvents is in the following order: toluene < isopropyl ether < MIBK <
ethyl acetate.
As depicted in Fig. 3 and already mentioned, an apolar oil phase was dissolved in a
solvent to improve the flowing properties of the oily phase, to decrease the viscosity and to
allow the alkali wash of some acidic components. The solvent may be the same as the solvent
used in the re-extraction of phenolic components from the water soluble phase in the first
separation step. Two bio-oils were used at this stage of the study, namely reference pine oil
PR06-27 and forest residue oil (PDU 5-07). An apolar (oil) phase of pine bio-oil was entirely
soluble in MIBK and ethyl acetate at solvent to apolar oil phase ratio from 1 to 5, lower
solubility of an apolar oil phase was observed in the other two solvents, namely in toluene and
isopropyl ether (IPE). The solvent to apolar oil phase ratio of 1:1 (m/m) was tested, with an
apolar oil phase, obtained after rinsing a bio-oil with 0,1 M NaOH solution in the mass ratio
of 1:1. Distribution coefficients and extraction factors of individual GC eluted phenolic
components between an axtract and raffinate phase for two solvents, toluene and isopropyl
ether were determined and the comparison between both solvents is shown in Fig. 9 and Fig.
10, respectively. It can be noticed that, isopropyl ether is more susceptible to phenol, cresol
(o-, p-), dimethylphenol and dihydroeugenol in comparison to toluene. The distribution
coefficient for phenol in isopropyl ether is nearly 5 times higher than in toluene, for
dihydroegenol more than 3 times higher, for (o-,p-) cresol and dimethylphenol more than 2
times higher. For the rest of phenolic components, small differences in distribution
coefficients exist between toluene and IPE. The differences in extraction factor of specific
phenolic components using toluene or IPE are smaller, since higher fraction of solvent soluble
phase is achieved by toluene as solvent than by IPE. IPE is more selective toward phenol and
toward the components with the attached alkyl groups onto the phenolic ring like cresols,
dimethylphenol, dihydroeugenol.
Fig. 9. Distribution coefficients of individual GC eluted phenolic components using toluene
or isopropyl ether in an organic phase dissolution.
17
toluene
isopropyl ether
4
3,5
3
2,5
2
1,5
1
0,5
0
ph
en
o
ocr l
es
p- ol
cr
es
ol
di gu
ai
m
a
e
co
th
6y
l
m
et lph
e
hy
lg n ol
ua
ia
c
4c r ol
et
eo
hy
so
lg
ua l
ia
c
s y ol
rin
go
di
e
hy ug l
e
dr
oe nol
ug
en
o
va l
ni
is
l li
o
ac eug n
et
e
m ova nol
et
h o nil l
o
s i xy e ne
na
pi uge
na
n
ld ol
eh
yd
e
Distribution coefficients
(extract/raffinate)
4,5
phenolic components
Fig. 10. Extraction factors for individual GC eluted phenolic components using toluene or
isopropyl ether in an organic phase dissolution.
toluene
isopropyl ether
Extraction factor (/)
5
4,5
4
3,5
3
2,5
2
1,5
1
0,5
ph
en
o
ocr l
es
p- ol
cr
es
ol
g
di
u
a
m
et i ac
6hy
ol
m
et lphe
hy
no
lg
ua l
ia
c
4c r ol
et
eo
hy
so
lg
ua l
ia
c
s y ol
rin
go
di
e
hy ug l
e
dr
oe nol
ug
en
o
va l
ni
is
l li
o
ac eug n
et
e
m ova nol
et
n
ho
il lo
s i xy e ne
na
pi uge
na
n
ld ol
eh
yd
e
0
phenolic components
The experiments have shown, that the extraction with toluene in one stage yields an
average extraction efficiency of around 47 % on GC eluted phenolics, in two stages the
efficiency of 62 % is reached. Lower average extraction efficiencies were obtained applying
isopropyl ether as solvent, namely in one stage the efficiency of only 36% was achieved, after
the second extraction stage the efficiency was increased to 53 %. From this it can be
concluded that neither toluene nor IPE are suitable as solvents for the extraction of phenolics.
In the case of the forest residue bio-oil, which contains higher amount of water
insoluble lignin material, mentioned before, an apolar (oil) phase of bio-oil was entirely
soluble in MIBK and ethyl acetate at solvent to apolar oil phase ratio of 5. When lower ratio
(2 or 3) was used, only 50% of the apolar oil phase was dissolved in MIBK and around 30%
in ethyl acetate. The dissolution dynamics of an apolar oil phase was faster in ethyl acetate
than in MIBK. The residue not soluble in both solvents represents a part of the HMM lignin
18
derivatives. Treating an organic phase, using MIBK or Ethyl acetate as solvent, with 0.5 M
NaOH aqueous solution (alkaline extraction) at the phase ratio of 1, a phase separation
occured and the acids and most acidic phenolic components like vanillin, acetovanillone and
sinapinaldehyde are removed from an organic to an aqueous phase. MIBK was found to be
more selective organic solvent towards phenol, p-cresol, creosol, guaiacols and eugenols in
comparison to ethyl acetate, as noticed after performing an alkali extraction. When MIBK was
applied, more than 90% of vanillin, acetovanillone and sinapinaldehyde were rinsed to an
aqueous phase.
4.1.5. Solvent removal
Solvent removal represents the final step of the fractionation process to yield the
phenol-guaiacolic fraction from the bio-oil, to recycle the solvent and minimize its losses.
Thin film evaporation of solvent (MIBK) was performed at pressure of 92 mbar and
temperature of 50ºC to prevent the polymerization of the components. Quickfit thin film
evaporator was employed having two distillate traps (cooling water and dry ice). It turned out
that MIBK is not so easy removable out of the sample. Decreasing an amount of MIBK below
8 wt% present in the sample, increases the losses of the phenolic components.
A study was performed to check the operating conditions where losses of the phenolic
components can be avoided. Therefore, a modeling was initiated using a predictive group
contribution UNIFAC method as a starting point to solve the separation problem. The
comparison with ethyl acetate as solvent was made. The solvent removal from a model
solution, containing 16 phenol-guaiacolic components dissolved in a solvent was studied and
it was shown that an effective separation can not be achieved without having a column with a
few stages and under a small reflux ratio. Different distillation conditions were explored in
order to minimize the losses of the fraction and to remove the solvent effectively from the
fraction. Flash calculations in AspenOneTM V7.0 using UNIFAC group contribution model at
100 mbar with MIBK and at 250 mbar with ethyl acetate and feed containing 16 phenolic
components diluted in solvents were carried out, changing the fraction of the feed that is
vaporized during the flash. The flashing results for both solvents are depicted in Figs. 11 (a)
and (b).
Fig. 11. (a) Losses of individual GC-eluted phenolic components with vapour, flash at 250
mbar, solvent- ethyl acetate, T range (42-60) ºC.
19
100
90
phenol
o-cresol
p-cresol
guaiacol
dimethylphenol
6-methylguaiacol
creosol
4-ethylguaiacol
syringol
eugenol
EtAc
isoeugenol
80
70
% loss
60
50
40
30
20
10
0
0,93
0,94
0,95
0,96
0,97
0,98
0,99
1,00
vapor fraction
Fig. 11. (b) Losses of individual GC-eluted phenolic components with vapour, flash at 100
mbar, solvent- MIBK, T range (52-65) ºC.
100
90
phenol
o-cresol
p-cresol
guaiacol
dimethylphenol
6-methylguaiacol
creosol
4-ethylguaiacol
syringol
eugenol
MIBK
isoeugenol
80
70
% loss
60
50
40
30
20
10
0
0,75
0,80
0,85
0,90
0,95
1,00
vapor fraction
From both Figs. 11 (a) and (b), it can be noticed, that increasing the vapor fraction the
components are strongly entrained by the solvent. Nearly no losses were detected for
dihydroeugenol, vanillin, acetovanillone, methoxyeugenol and sinapinaldehyde for both
solvents at all vapor fractions. Comparing both figures a considerable difference between
solvents is noticed. The vapor fraction where the loss of each component is below 6% is
0,875 for MIBK and it is higher (0,965) when ethyl acetate is used. Increasing the vapor
20
fraction above certain values, the losses of the majority of the phenol-guaiacolic components
are exponentially increasing. Higher losses of individual GC-eluted phenolic components are
noticed, when flashing the mixture containing MIBK as solvent at 100 mbar, especially at
molar vapor fraction higher than 0.95. Comparing the Figs. 11 (a) and (b) it is evident, that
there is much higher attraction between MIBK and phenolics than between ethyl acetate and
phenolics. Therefore, ethyl acetate may be easier recovered than MIBK.
The distillation column was simulated operating in a continuous manner to remove
solvent from the phenolic fraction in order to see how demanding the particular separation is.
The results of the simulation with both solvents ethyl acetate and MIBK are shown in Table 4,
where component distribution from feed (F) to distillate (D) of few most volatile components
are presented, dependent on the total number of theoretical stages (Nt), feed position (Nf) and
reflux ratio (R). It is well demostrated that good separation can be achieved for ethyl acetate
on the column having 5 stages, with the feed location on the third stage and reflux ratio of 0,2.
Slightly higher number of stages is required for the separation using MIBK as solvent, with a
bit higher amount of solvent present in the phenolic fraction in the bottom, compared to ethyl
acetate. In the real process, the combination of the flash upto certain vapour fraction and a
separator under higher vacuum should be applied to prevent the polymerization of the
temperature sensitive material. Even further fractionation should be possible by using
ultrafiltration technique [12].
Table 4
Aspen simulation of the distillation column.
Component distribution to D (from F),
Nt
Nf
R
D/F
5,49
4,18
0,71
3
4
5
2
3
3
0,5
0,2
0,2
0,995
0,995
0,9953
8,13
2,68
5
6
4
4
0,2
0,2
0,994
0,9945
Solvent
in B
fraction
(wt%)
phenol
o-cresol
0,1109
0,0016
0,00015
guaiacol
dimethylphenol
eugenol
solvent
Solvent: EtAc, P= 250 mbar, Tt= 40,2 ˚C
0,0700
0,0007
5,1E-05
0,0665
0,0006
0,00004
0,0519
0,0004
2,61E-05
0,0544
0,0011
7,79E-05
0,9995
0,9996
0,99994
Solvent: MIBK, P= 100 mbar, Tt= 51,5 ˚C
0,00228
0,00019
0,00045
2,7E-05
0,00195
0,00016
0,00013
6,21E-06
0,00082
5,97E-05
0,9993
0,9998
Tt- top temperature; N1- condenser, Nf- reboiler;
4.2. Recovery of phenolics from bio-oil using simultaneously hydrophobic-polar organic
solvent and either water, aqueous NaHSO3 or alkali solution
As already mentioned, the second alternative offers some advantages over the previous
method described, such as simplification of the process, easier phase separation, better
flowability of both phases and better handling .
In the study VTT forest residue bio-oil PDU 5-07, bottom phase was used, with lower
concentration of lignin derived guaiacols and with a presence of syringol. Experiments were
performed using several solvents, ethyl acetate, MIBK, isopropyl acetate, butyl acetate and
aqueous solutions, namely 0.6 M NaHSO3 solution and NaOH solution of different
concentrations (0.1 M; 0.5 M; 2 M). The extraction experiments were carried out at room
temperature. The Solvent: Bio-oil: Aqueous solution ratio (m/m) of 1:1:2 was applied. The
major part od the bio-oil dissolves in an aqueous phase, the minor dissolves in a solvent.
Butyl acetate exhibits the lowest solubility in water, followed by MIBK, isopropyl acetate and
21
ethyl acetate. Solubility of water in solvents is in the following order butyl acetate < MIBK
< isopropyl acetate < ethyl acetate.
The distribution coefficients of the phenolic components between an aqueous and an
organic phase are in general low, for all solvents, using either 0.6 M NaHSO3 aqueous
solution, 0.1 M or 0.5 M NaOH aqueous solution. MIBK was found to be the best solvent for
the phenolics recovery, followed by butyl acetate, ethyl acetate and isopropyl acetate.
Dihydroeugenol shows the highest partitioning to an aqueous phase with acetate solvents, and
with aqueous 0.6 M NaHSO3, 0.1 M NaOH and 0.5 M NaOH solution, having distribution
coefficients between 0.8 to 1.1 (m/m). The distribution coefficients of vanillin and
acetovanillone are in the range from 0.4 up to 0.6, with the lowest value when MIBK is
applied. Slightly lower are the distribution coefficients of sinapinaldehyde. The difference
between solvents is well presented on Figs. 12 and 13 with distribution coefficients of the
phenolics and extraction factors between an aqueous and an organic phase using 0.1 M NaOH
aqueous solution and four hydrophobic-polar solvents.
0,1 M NaOH
1,20
1,00
MIBK
0,80
Butyl acetate
0,60
Ethyl acetate
0,40
Isopropyl acetate
0,20
0,00
ph
e
o- no
cr l
e
p- sol
cr
e
gu so
ai l
a
cr co l
e
4- c os
et at ol
hy ec
lg ho
ua l
ia
sy co l
rin
di
hy e u g ol
dr ge
oe n
ug ol
en
va ol
is
n
ac oeu illin
m eto g en
et va o
h
l
sin oxy nillo
e
n
ap u e
in ge
a l no
de l
hy
de
Distribution coefficients (/)
Fig. 12. The comparison of distribution coefficients of GC-eluted phenolic components
between an aqueous and an organic phase using 0.1 M NaOH aqueous solution and different
solvents.
phenolic components
It can be seen (Fig. 13) that the extraction factors of dihydroeugenol reached the
highest value of 2.4 in isopropyl acetate and the lowest (0.7) in MIBK. The extraction factors
of acetovanillone are in the range between 0.9 up to 1.4 and vanillin from 0.85 to 1.3. Slightly
lower extraction factors were noticed for sinapinaldehyde. Significant are also the extraction
factors of catechol, especially in ethyl acetate and isopropyl acetate.
Fig. 13. Extraction factors of GC-eluted phenolic components adding solvent using 0.1 M
NaOH aqueous solution and various solvents.
22
Extraction factors (/)
0,1 M NaOH
2,5
2,0
MIBK
1,5
Butyl acetate
1,0
Ethyl acetate
0,5
Isopropyl acetate
0,0
ol ol ol o l ol o l o l ol ol ol lin ol ne o l e
en re s re s iac eos ch iac ring gen ge n nil g en illo gen hyd
h
p o- c p- c u a cr ate u a sy e u eu va eu a n eu ld e
g
c ylg
v y a
o
ro
is eto ox pin
th
yd
c
e
h
a eth in a
4di
m s
phenolic components
No significant difference in distribution coefficients and extraction factors are
observed using either 0.6 M NaHSO3 solution, 0.1 M NaOH or 0.5 M NaOH. Increasing the
concentration of alkali (NaOH) solution to 2M, the partitioning of vanillin, acetovanillone and
sinapinaldehyde to an aqueous phase as well as the extraction factors are drastically increased
when MIBK, butyl acetate or isopropyl acetate are used. Slight increase in partitioning for this
phenolic components is noticed also for ethyl acetate, but not to such extend. To rinse all the
other phenolic components to the aqueous phase as phenolates, much higher concentration of
an alkali solution should be used. The behaviour is again in accordance with the Lewis acid
character of the phenolic components. The highest overall extraction efficiency (85%) on GCeluted phenolics from the bio-oil to an organic phase in one stage is reached with MIBK,
using 0.1 M or 0.5 M NaOH aqueous solution. A bit lower average extraction efficiency is
noticed with the same solvent and 0.6 M sodium bisulphite solution. Using solvent and 2 M
sodium hydroxide, the individual component efficiencies of all GC-eluted phenolics are
decreased, thus their contributions to an aqueous phase are enhanced due to the phenolates
formation. Therefore, an average extraction efficiency in this case is around 40% for acetates
and 32% for MIBK.
5. Conclusions
The work has shown that both proposed schemes for phenolics, starting the recovery
of phenolics from pyrolysis oil by an aqueous extraction using water, aqueous NaHSO3 or
alkali solution as well as the recovery using simultaneously hydrophobic-polar organic
solvent and aqueous solution, like water, aqueous NaHSO3 or alkali solution as an antisolvent,
are feasible.
The study of the effect of water to bio-oil ratio on distribution coefficients and
extraction factors of phenolic components between an aqueous and an organic phase reveals
that the distribution coefficients of all phenolic components reach high values at low water to
bio-oil ratios, and the values are decreasing with increasing water to bio-oil ratio and finally
remain constant below the value of 0.3 for all phenolic components, except for catechol and
vanillin. Higher distribution coefficients of the latter two components are most likely due to
their polarity and ability to form H-bonds. From treating the bio-oil with an aqueous
solutions, it can be concluded, that an alkali extraction of bio-oil is an appropriate method for
23
phenolics isolation from the original bio-oil and it is more efficient in comparison to water or
aqueous sodium bisulphite solution. Four solvents were applied for the dissolution of an
apolar oil phase, namely MIBK, ethyl acetate, toluene and isopropyl ether. Neither toluene
nor isopropyl ether are suitable as solvents for the extraction of phenolics. MIBK was found
to be more selective organic solvent towards phenol, p-cresol, creosol, guaiacols and eugenols
in comparison to ethyl acetate. In solvent removal stage it has been shown that MIBK can not
be effectively removed from an organic phase to yield the phenol-guaiacolic fraction unless
having a column with a few stages and under a small reflux ratio, which was confirmed also
by modeling. The attraction interactions between MIBK and phenolics are much higher than
between ethyl acetate and phenolics. The combination of a flash unit and a separator under
higher vacuum should be used to prevent the polymerization.
The recovery of phenolics from bio-oil using simultaneously hydrophobic-polar
solvent, namely MIBK, butyl acetate, ethyl acetate and isopropyl acetate and an aqueous
solution has revealed that MIBK is the most efficient solvent for extraction of phenolics from
bio-oil in combination with aqueous NaOH solution of low molarity, followed by butyl
acetate.
The recovery of phenolics based on the extraction of phenolics from an apolar-oil
phase of pyrolysis liquid using hydrophobic/polar solvent and alkali solution was proved to be
feasible and yielded an effective fraction for resin synthesis and application in wood based
panel production during the BIOCOUP project.
Acknowledgements
The research work was carried out in the Sixth Framework Programme for Research
and Development (Contact No: 518312), within the BIOCOUP Project, supported by
European commission, and under the Programme P2-0152-0104- Chemical Reaction
Engineering financed by ARRS, Slovenian Research Agency, therefore their financial support
is gratefully acknowledged. The authors wish to acknowledge all the partners of the
BIOCOUP project, but especially VTT (Finland), TU/e, RUG, BTG (The Netherlands), VTI
(Germany), CHIMAR (Greece), ARKEMA, METEX (France).
24
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