Research Article
Received: 15 July 2009
Revised: 14 September 2009
Accepted: 14 September 2009
Published online in Wiley Interscience: 12 January 2010
(www.interscience.wiley.com) DOI 10.1002/jctb.2331
Silicone oil: An effective absorbent
for the removal of hydrophobic volatile organic
compounds
Guillaume Darracq,a,b∗ Annabelle Couvert,a,b Catherine Couriol,a,b
Abdeltif Amrane,a,b Diane Thomas,c Eric Dumont,d Yves Andresd and
Pierre Le Cloireca,b
Abstract
BACKGROUND: Hydrophobic volatile organic compounds (VOCs), such as toluene, dimethyl sulfide (DMS) and dimethyl disulfide
(DMDS), are poorly soluble in water and classical air treatment processes like chemical scrubbers are not efficient. An alternative
technique involving an absorption step in an organic solvent followed by a biodegradation phase was proposed. The solvent
must fulfil several characteristics, which are key factors of process efficiency, and a previous study allowed polydimethylsiloxane
(or PDMS, i.e. silicone oil) to be selected for this purpose. The aim of this paper was to determine some of its characteristics
like absorption capacity and velocity performances (Henry’s constant, diffusivity and mass transfer coefficient), and to verify
its non-biodegradability.
RESULTS: For the three targeted VOCs, Henry’s constants in silicone oil were very low compared to those in water, and solubility
was infinite. Diffusivity values were found to be in the range 10−10 to 10−11 m2 s−1 and mass transfer coefficients did not show
significant differences between the values in pure water and pure silicone oil, in the range 1.0 × 10−3 to 4.0 × 10−3 s−1 for all
the VOCs considered. Silicone oil was also found to be non-biodegradable, since its biological oxygen demand (BOD5 ) value
was zero.
CONCLUSION: Absorption performances of silicone oil towards toluene, DMS and DMDS were determined and showed that this
solvent could be used during the first step of the process. Moreover, its low biodegradability and its absence of toxicity justify
its use as an absorbent phase for the integrated process being considered.
c 2010 Society of Chemical Industry
Keywords: absorption; silicone oil (PDMS); hydrophobic VOC; mass transfer coefficient; diffusivities; biodegradation
INTRODUCTION
J Chem Technol Biotechnol 2010; 85: 309–313
low solubility in water, good chemical and thermal stabilities,
non-biodegradability, biocompatibility and, of course, cheapness.
Silicone oil (polydimethylsiloxane or PDMS) was used by several
authors4,5 as a pollutant reservoir and seemed to be suitable.
Hayachi et al.6 and Matsumoto et al.7 showed that the partitioning
coefficient octanol/water (log Kow ) allowed to check the toxicity of a
solvent towards microorganisms. Since log Kow of silicone oil is 4.25
and silicone oil is not soluble in water, this solvent could be consid-
∗
Correspondence to: Guillaume Darracq, Ecole Nationale Supérieure de Chimie
de Rennes, CNRS, UMR 6226, Avenue du Général Leclerc, CS 50837, 35708
Rennes Cedex 7, France. E-mail: [email protected]
a Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du
Général Leclerc, CS 50837, 35708 Rennes Cedex 7, France
b Université européenne de Bretagne, 35000 RENNES, France
c Faculté Polytechnique de Mons, 56, Rue de l’Epargne, B-7000 Mons, Belgium
d UMR CNRS 6144 GEPEA, Ecole des Mines de Nantes, La Chantrerie, 4 rue Alfred
Kastler, B.P. 20722, 44307 Nantes Cedex 3, France
www.soci.org
c 2010 Society of Chemical Industry
309
Since several world conferences such as New York (1997) or Kyoto
(1998), which highlighted the influence of anthropogenic VOC
emissions on the environment and the climate imbalance, air
treatment has become an important research topic. Hydrophilic
compounds can usually be removed by chemical scrubbing,
biological treatment (biofilter) or thermal oxidation. But these
processes are not really efficient for hydrophobic compounds
like toluene, dimethyl sulfide (DMS) and dimethyl disulfide
(DMDS), which have low olfactory thresholds, 8.2 mg m−3 , 1.5
and 0.1 µg m−3 , respectively, according to Hartikainen et al.,1 and
can be toxic for humans at low levels.1
An alternative process (Fig. 1), which consisted in coupling an
absorption step involving an organic solvent2 and a biodegradation step has been developed. The aim was to achieve the
consumption of the VOCs and the regeneration of the absorbent
phase. In 1991, Bruce and Daugulis set up a method to choose the
best organic phase for implementation in a multiphase bioreactor.3
The solvent must fulfil several characteristics: high absorption capacity and velocity performances (Henry’s constant, diffusivity
and mass transfer coefficient), low emulsion-forming tendency,
www.soci.org
Gas outlet
G Darracq et al.
Organic phase recycling
Organic phase inlet
Gas-liquid
contactor
Bioreactor
Settler
Organic phase
Gas inlet
Emulsion
water/solvent
Organic phase
outlet + VOCs
Aqueous phase +
Activated sludge
Air flow
Figure 1. Hybrid absorption–biodegradation process with regeneration of the organic phase.
ered as biocompatible. The aim of this work was to measure several
parameters such as dimensionless Henry’s constants (H), diffusivities (DL ) and global mass transfer coefficients (KL a) for each VOC in
order to characterise the absorption performances of silicone oil.
Table 1. Analytical conditions implemented to determine the VOC
concentration in the gas phase
Pollutant
Toluene
MATERIALS AND METHODS
Henry’s constant and global mass transfer coefficient
In 2003, Roustan described the physical absorption between a
gaseous compound and a liquid phase8 (Equation 1). Mass transfer
is governed by the driving force (i.e. the difference between the
pollutant concentrations in both phases).
dN = KL adV(CLE − CL )
(1)
310
where dN (in mol s−1 ) is the transferred amount; dV (in m3 ) is the
volume element; KL (in m s−1 ) is the global liquid mass transfer
coefficient; a (in m2 m−3 ) is the volumetric interfacial area; CL is the
pollutant liquid concentration; and CLE (in mol m−3 ) is the pollutant
liquid concentration at the equilibrium with the gas phase given
by Equation 2:
p
CG RT
(2)
CLE = =
H
H
where p (in Pa) is the VOC partial pressure; R is the gas constant
(8.314 Pa.m3 .mol−1 .K−1 ); T is temperature (K); and H Henry’s
constant (Pa.m3 .mol−1 ).
Dimensionless Henry’s constants, H, were obtained using a static
method. A known quantity of solvent (or water) was introduced
into a specific flask (vial), whose exact volume was measured.
After closing it and making it gastight, a known quantity of VOCs
was added through the septum. The vial was shaken for 3 days at
25 ◦ C by using a swivel support. Once the equilibrium was reached
(checking of the stability of the gaseous VOC concentration), the
VOC concentration in the gas phase (CG ) was determined by
gas chromatography (flame ionisation detector type focus). The
concentration in the liquid phase (CLE ) was then deduced from the
mass balance and the dimensionless constant, H, was calculated
by using Equation 2.
The gas chromatography methods used in this work are shown
in Table 1.
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Dimethyl disulfide
Dimethyl sulfide
Oven temperature, Toven
100 ◦ C (1.5 min)
20 ◦ C min−1 → 180 ◦ C
(0 min)
100 ◦ C (1.4 min)
50 ◦ C min−1 → 200 ◦ C
(3 min)
50 ◦ C (1.2 min)
50 ◦ C min−1 → 200 ◦ C
(3 min)
Carrier gas
N2 3.7 mL min−1
N2 3.3 mL min−1
N2 3.3 mL min−1
For all samples, the following equipment and conditions were used:
Thermo Focus GC with an RTx-1.15 m ×0.32 mm column and FID. The
injector temperature (Tinj ) was 150 ◦ C and the detector temperature
(Tdet ) was 250 ◦ C.
For measurements of KL a, the absorption of toluene, DMS and
DMDS in water and in silicone oil was independently studied in
laboratory-scale reactors at a constant gas flow rate (1 m3 h−1 ).
A dynamic method9 was used to determine the volumetric
mass transfer coefficients of pollutants during absorption. In this
method, a known air volume (VG = 215 L) loaded with the pollutant
was continuously flowed via a circulating loop through a water
volume (VL = 1 L) at 25 ◦ C. The operation was batchwise with
respect to the liquid system and the decrease in time course of the
pollutant concentration (toluene, DMS or DMDS) in the gas phase
was monitored.
Diffusivity
A specific mathematical model was developed to simulate the VOC
absorption into viscous solvents in a vertical wetted-wall column
with a co-current gas–liquid downflow. Considering a plane flow,
the liquid stream was considered as laminar and exempt of ripples
for Reynolds numbers lower than 4, requiring a low liquid flow
rate if the viscosity was not so high.10
In these hydrodynamic conditions, the liquid velocity profile
was parabolic, with a well-known estimation of the liquid film
c 2010 Society of Chemical Industry
J Chem Technol Biotechnol 2010; 85: 309–313
Silicone oil for removing hydrophobic VOCs
www.soci.org
thickness. The model involved a general equation describing the
steady-state diffusion of the VOC from the interface into the
diffusion falling film, with adequate boundary conditions, and
a classical mass balance between the top of the column and a
given level, used to compute the average VOC concentration in
the liquid film along the column height. More details, including
equations and assumptions, concerning the model can be found
in a previous paper.11
The simulation of the absorber required the knowledge of:
• The gas-phase mass-transfer characteristics in the column.
For sake of precision, this coefficient was not estimated
from literature correlations12 but experimentally measured
by absorption of n-pentylbenzene; a classical correction of the
value taking into account the ratio of the corresponding gas
phase diffusivities can be applied. The diffusivities of the VOC
in the gaseous phase were estimated thanks to the correlation
of Fuller et al.13
• The physicochemical properties of the silicone oil. The density was
experimentally determined using a densimeter; the viscosity
(mPa s) was experimentally measured using a falling sphere
viscometer in the range 15–30 ◦ C:
7005.3
− 19.4
(3)
T
The Henry’s constants of the absorbed VOCs.
Starting from the top, the program finally provided the VOC
outlet gas concentration. Computation of DL was made by
minimizing for each absorption test run the deviation between
experimental and calculated VOC outlet gas concentrations.
Absorption experiments were achieved in an experimental
set-up including:
A scrubber. The VOC scrubber was a small glass wetted-wall
column of inside diameter 0.02 m with an effective height
of 0.66 m, characterised by a co-current contacting mode
between liquid and gas streams entering at the top and leaving
at the bottom of the column. An external jacket flowed by a
thermostated fluid allowed to keep constant the temperature
in the absorber.
A gas supply. A precise rate of VOC was injected by a syringe
dispenser in the air stream, supplied by a compressor, whose
flow rate was measured and controlled by a gas mass-flow
controller. Valves allowed to direct the mixture toward the
by-pass; this system was useful for the determination of the
initial VOC concentration in the inlet gas or the wetted-wall
column during absorption test runs. The exit gas was vented
out through a laboratory fume hood.
A liquid supply. The liquid flow rate was fed by using a gear
pump and regulated by a mass-flow controller. The liquid
phase, namely the silicone oil, was pumped from a container,
previously ran through heat exchange coils immersed in the
thermostatic bath and flowed at the inner side of the column.
A gas sampling part. The gas analysis, upstream and downstream of the wetted-wall absorption column, was performed
by a flame ionisation detector (FID). A certified standard
(nitrogen–propane gas mixture) was used for the calibration
of the FID.
ηSO =
•
•
•
•
•
J Chem Technol Biotechnol 2010; 85: 309–313
Biodegradability
Biodegradability was deduced from measurements of the
biological oxygen demand (BOD5 ), carried out in Oxitop IS6 (WTW,
Alès, France). Activated sludge from a wastewater treatment plant
was used to inoculate samples, which consisted in a solution of
80 mg.L−1 of silicone oil in water, and the control solution. The
initial bacterial concentration was 0.5 g L−1 .
The following mineral basis was used for all experiments (g L−1 ):
MgSO4 · 7H2 O, 22.5; CaCl2 , 27.5; FeCl3 , 0.15; NH4 Cl, 2.0; Na2 HPO4 ,
6.80; KH2 PO4 , 2.80.
The BOD5 value was initially estimated based on the chemical
oxygen demand (COD) value experimentally measured by means
of a Test Nanocolor CSB 160 (Macherey–Nagel, Düren, Germany)
or calculated, BOD5 = COD/1.46. The range of expected BOD5
measurements was then deduced and hence led to the volumes of
sample, of activated sludge solution and of nitrification inhibitor
(10 mg L−l solution of N-allylthiourea) which have to be added to
the shaker flask of the Oxitop apparatus.
A similar protocol was applied for the control flask except that
it was replaced by a solution of easily biodegradable compounds,
namely glutamic acid (130 mg L−1 ) and glucose (130 mg L−1 ). Before use, KOH was added to achieve neutral pH (7.0±0.2). A similar
protocol was also considered for the blank solution, for which the
sample was replaced by water to have a negligible BOD5 value.
RESULTS AND DISCUSSION
Henry’s constants
) are
Henry’s constants in silicone oil (HSO
) and in water (Hw
reported in Table 2. The three selected VOCs were preferentially
absorbed in silicone oil. Moreover, since each VOC has infinite
solubility in silicone oil, its absorption should be improved using
silicone oil in a gas–liquid contactor. Henry’s constants were
also measured in a silicone oil/water emulsion to check for the
absence of VOC stripping in a two-phase partitioning bioreactor.
The experimental value was then compared to the theoretical
value calculated by using Equation 4:
1
1
1
= x + (1 − x)
Hmixture
HSO
Hw
(4)
is Henry’s constant of VOC in pure silicone oil; Hw
is
where HSO
Henry’s constant of VOC in pure water; and x is the volumetric
fraction of silicone oil in emulsion.
Results are reported in Fig. 2 for toluene. Since the relative
error was below 15%, Henry’s constant for toluene in an oil/water
emulsion could be estimated by Equation 4.
value appeared
very low if
In the case of toluene, the HSO
value; hence the term 1 Hw
can be neglected
comparedto the Hw
and Equation 4 led to:
towards 1 HSO
H
∼
(5)
Hmixture
= SO .
x
According to Equation 5, Hmixture
increases when the volumetric
fraction of silicone oil in emulsion decreases. This equation allows
c 2010 Society of Chemical Industry
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311
Inlet and outlet liquid and gas temperatures were registered by
temperature sensors.
The system was allowed to reach a steady-state indicated by no
significant changes of average temperatures for liquid and gas
phases, gas flow rate G, liquid flow rate L and VOC inlet and outlet
gas concentrations, cG,in and cG,out , respectively. Experimental
investigations were conducted for typical operating conditions:
L = 0.4 kg h−1 , G = 1 N m3 h−1 , temperature = 25 ◦ C and cG,in =
650–1500 ppm. Data collected were required for interpretation
and estimation of liquid diffusivities of VOC into viscous solvents.
More details on the experimental apparatus and procedure can be
found in Bourgois et al.11
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G Darracq et al.
Table 2. Henry’s constants of the selected VOCs in silicone oil and in
water
Henry’s constant (Pa m3 mol−1 )
Pollutant
Toluene
Dimethyl disulfide
Dimethyl sulfide
∗
in silicone oil, HSO
in water, Hw
1.6
2.3
41.6
609∗
119
124
Taken from Staudinger and Roberts.14
Figure 4. Henry’s constants of dimethyl sulfide in a silicone oil/water
emulsion. Comparison between experimental and calculated values. ()
H determined by experiment; () H calculated value.
Table 3. Global mass transfer coefficients of VOCs in silicone oil and
in water
Pollutant
Toluene
Dimethyl sulfide
Dimethyl disulfide
Figure 2. Henry’s constants of toluene in a silicone oil/water emulsion.
Comparison between experimental and calculated values. () H determined by experiment; () H calculated value.
KL a in pure water (s−1 ) KL a in pure silicone oil (s−1 )
1.4 × 10−3
4.0 × 10−3
1.8 × 10−3
1.6 × 10−3
4.0 × 10−3
1.6 × 10−3
of DMS in silicone oil, rather than in water, similarly to toluene and
DMDS, was demonstrated.
Global mass transfer coefficient
The results summarised in Table 3 show that there was no
significant difference between the values of KL a in pure water
and pure silicone oil. However, owing to the higher affinity of
pollutants for silicone oil than for water, higher KL a values were
expected for silicone oil than for water. The dynamic viscosity of
the silicone oil used, which was five times higher than that of water,
could account for this unexpected result. Indeed, KL a values in
bioreactors have generally to be correlated with the combination
of stirrer speed (N), superficial gas velocity (V) and viscosity of the
liquid (η) according to the equation
KL a = CV α
Figure 3. Henry’s constants of dimethyl disulfide in a silicone oil/water
emulsion. Comparison between experimental and calculated values. ()
H determined by experiment; () H calculated value.
312
an easy estimation of the minimal volumetric fraction necessary
for an efficient transfer. Results for DMDS, reported in Fig. 3, lead
to the same conclusions.
For DMS, experimental results (Fig. 4) differed from those
calculated by means of the theoretical Equation 4. A different
behaviour for DMS if compared to the other VOCs was previously
shown by Vuong et al.,15 who have calculated Henry’s constants for
DMS in other organics solvents. However, preferential absorption
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P
VL
β
ηδ
(6)
where C, α, β and δ are empirical constants depending on
both hydrodynamic conditions and geometrical parameters of
the vessel and stirrer used, and (P VL ) is the power input per liquid
volume.
Since all the operating conditions (V, P, VL ) were maintained
constant during the experiments, it should be assumed that the
benefit in using silicone oil to absorb pollutants more rapidly was
cancelled by its dynamic viscosity. Nevertheless, the use of silicone
oil enabling the absorbtion of large amounts of pollutant should
be beneficial to increase the mass transfer rate.
Modelling of VOC absorption into silicone oil for the estimation of the liquid diffusivities
The liquid diffusivities resulting from these absorption tests
achieved in the wetted wall column using the experimental
c 2010 Society of Chemical Industry
J Chem Technol Biotechnol 2010; 85: 309–313
Silicone oil for removing hydrophobic VOCs
www.soci.org
Table 4. Liquid diffusivities of VOCs in silicone oil
Sample
CG,in
CG,out
(g N m−3 ) (g N m−3 )
Toluene
Dimethyl sulfide
1.630
2.615
1.286
2.486
Dimethyl disulfide
3.277
3.026
DL (25 ◦ C) (m2 s−1 )
5.65 × 10−11
Not given because of very
high H
8.72 × 10−12
Table 5. COD and BOD5 for silicone oil in water (approx. 80 mg L−1 )
Sample
COD (mg O2 L−1 )
BOD5 (mg O2 L−1 ) with
endogenous breath
BOD5 (mg O2 L−1 ) without
endogenous breath
Endogenous breath (mg O2 L−1 )
Control flask (with
gutamic acid
Silicone Silicone
and glucose)
oil 1
oil 2
–
205
116
51
117
54
121
0
0
84
400, 53 and 3 for toluene, DMDS and DMS, respectively. Silicone
oil appeared to be a good absorbent since it efficiently solubilises
DMDS and toluene. The results obtained for DMS were a little less
interesting, most likely due to its high volatility and then to losses
during experiments. This could explain the non-significant results
recorded for various emulsion ratios, while the Henry’s constant in
pure silicone oil was high.
The results obtained for diffusivities in silicone oil showed that
DL values can be determined for two VOCs and were in the range
of 5.65 × 10−10 and 8.72 × 10−11 m2 s−1 for toluene and DMDS,
respectively. For DMS, the liquid diffusivity in silicone oil could not
be determined, owing to the high value of Henry’s constant.
Results for the global mass transfer coefficient did not show a
significant gap between the absorption in water and in silicone
oil. The rough estimate for the three selected VOCs was between
1.6 × 10−3 and 4.0 × 10−3 s−1 .
Finally, no biodegradability of silicone oil and the lack of toxicity
justify its use as an absorbent phase for the integrated process
considered.
REFERENCES
procedure described above were in the range of 10−11 to
10−10 m2 s−1 . Absorption tests results and diffusivities values are
given in Table 4.
A direct comparison with other studies was not possible, since
no details of a similar VOC–silicone oil system was found in
the available literature. The comparison of these values with
those calculated with semi-empirical formulae (Wilke and Chang,
Scheibel, Fedors)8 confirmed that they cannot be used to predict
the VOC diffusivity in viscous solvents.
Biodegradability
The results given in Table 5 are the average of two experiments
with two control flasks (glutamic acid and glucose) or two
experiments with four samples (silicone oil in water). After 5 days
of culture, a ratio of 4 was found for the BOD5 values found in the
control flask on those found in presence of silicone oil (samples).
However, the increase of the biological oxygen demand in the
flasks containing silicone oil resulted from endogenous respiration
by bacteria as shown in Table 5. At this concentration, silicone
oil was therefore non-biodegradable and non-toxic towards
microorganisms.
Moreover, during preliminary batch cultures, some analyses
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were recorded in the aqueous phase, showing that silicone oil
was not assimilated by microorganisms.
CONCLUSION
Silicone oil seems to fulfil most of the required characteristics and
will subsequently be tested for hydrophobic VOC removal in an
integrated process coupling absorption and biodegradation.
For the three VOCs, the Henry’s constants in pure water were
significantly higher than in pure silicone oil, leading to ratios of
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