High-power ultrasonic system for the enhancement of mass transfer in supercritical CO2 extraction processes

Abstract

Oil is an important component of almonds and other vegetable substrates that can show an influence on human health. In this work the development and validation of an innovative, robust, stable, reliable and efficient ultrasonic system at pilot scale to assist supercritical CO(2) extraction of oils from different substrates is presented. In the extraction procedure ultrasonic energy represents an efficient way of producing deep agitation enhancing mass transfer processes because of some mechanisms (radiation pressure, streaming, agitation, high amplitude vibrations, etc.). A previous work to this research pointed out the feasibility of integrating an ultrasonic field inside a supercritical extractor without losing a significant volume fraction. This pioneer method enabled to accelerate mass transfer and then, improving supercritical extraction times. To commercially develop the new procedure fulfilling industrial requirements, a new configuration device has been designed, implemented, tested and successfully validated for supercritical fluid extraction of oil from different vegetable substrates.

Full text

High-power ultrasonic system for the enhancement of mass transfer
in supercritical CO
2
extraction processes
Enrique Riera
a,
*
, Alfonso Blanco
b
, José García
c
, José Benedito
d
, Antonio Mulet
d
, Juan A. Gallego-Juárez
a
,
Miguel Blasco
c
a
Grupo de Ultrasonidos de Potencia, Dpto. Señales, Sistemas y Tecnologías Ultrasónicas, Instituto de Acústica, CSIC, Serrano 144, E28006 Madrid, Spain
b
Instituto de Física Aplicada, CSIC, Serrano 144, E28006 Madrid, Spain
c
Centro Tecnológico AINIA, Benjamín Franklin 5-11, E46980 Paterna, Valencia, Spain
d
Dpto. Tecnología de Alimentos, Universidad Politécnica de Valencia, Camino de Vera s/n, E46022 Valencia, Spain
article info
Article history:
Received 30 June 2009
Received in revised form 16 September
2009
Accepted 16 September 2009
Available online 20 September 2009
Keywords:
Ultrasonic processes
Power ultrasound
Mass transport
Supercritical fluid extraction
Carbon dioxide
abstract
Oil is an important component of almonds and other vegetable substrates that can show an influence on
human health. In this work the development and validation of an innovative, robust, stable, reliable and
efficient ultrasonic system at pilot scale to assist supercritical CO
2
extraction of oils from different sub-
strates is presented. In the extraction procedure ultrasonic energy represents an efficient way of produc-
ing deep agitation enhancing mass transfer processes because of some mechanisms (radiation pressure,
streaming, agitation, high amplitude vibrations, etc.).
A previous work to this research pointed out the feasibility of integrating an ultrasonic field inside a
supercritical extractor without losing a significant volume fraction. This pioneer method enabled to accel-
erate mass transfer and then, improving supercritical extraction times. To commercially develop the new
procedure fulfilling industrial requirements, a new configuration device has been designed, implemented,
tested and successfully validated for supercritical fluid extraction of oil from different vegetable
substrates.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
The use of fluids, such as CO
2
, under supercritical conditions for
extraction is a current useful technique. However, it has some lim-
itations [1,2]. First of all, it requires high-pressure equipment,
which may be considered as the most serious drawback of the
technology in comparison to traditional atmospheric pressure
extraction techniques. High pressure facilities create a potential
safety hazard that has to be carefully managed. Additionally,
uncontrolled release of large quantities of carbon dioxide can
asphyxiate bystanders owing to air displacement. Strict technical
and economical requirements have to be fulfilled to perform super-
critical extraction at large scale. Special and sometimes complex
closures and joints have to be used to stand high pressures and
thus, it is only possible to extract solids under batch modus. Also,
vessel diameters are constrained by mechanical manufacture pro-
cedures and thickness requirements, which make costs increase
exponentially as diameter grows. This way, a 200–1000 l vessel
may be used for industrial purposes. On the other hand, in industry
arrangements of parallel batch extractors of more than 1 m
3
are
considered to overcome the batch operating disadvantages and at-
tain a more efficient continuous like operation. Nevertheless, these
facts are not strong enough to avoid supercritical processes or
high-pressure processes and they have been applied at industrial
scale without special problems since decades.
Taking into account that supercritical extraction may be consid-
ered environmentally friendly and a green alternative to the use of
organic solvents, it may be concluded that it is worthy to apply
supercritical fluids for extraction at industrial levels.
In this way supercritical CO
2
is considered nowadays in the food
sector as an excellent solvent in the product extraction from vege-
tables. Extraction with supercritical carbon dioxide is considered as
a technology which has gained wide acceptance as an alternative to
conventional solvent extraction because of its important advanta-
ges (non-toxic, recyclable, cheap, relatively inert and non-flamma-
ble). Nevertheless, the economics of supercritical fluid extraction
(SFE) is affected by the slow kinetics of the process. Since high pres-
sures are normally used in SFE, mechanical stirring is difficult to be
applied. The application of new techniques such as the use of high-
power ultrasound (HPU) assisting this process has proved impor-
tant benefits as a consequence of the mechanical effects produced
in the supercritical environment through the high amplitude vibra-
tions, radiation pressure, streaming, agitation, etc. [3–7].
0041-624X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultras.2009.09.015
* Corresponding author. Tel.: +34 915618806; fax: +34 914117651.
E-mail address: eriera@ia.cetef.csic.es (E. Riera).
Ultrasonics 50 (2010) 306–309
Contents lists available at ScienceDirect
Ultrasonics
journal homepage: www.elsevier.com/locate/ultras

Nowadays, high-intensity ultrasound is regarded as an emer-
gent technology in food, chemical and pharmaceutical industries
[8,9]. In particular, ultrasonic energy represents a clean way to
accelerate and improve mass transfer processes. In addition, the
use of energy would permit to obtain economically competitive
high quality food extracts. Previous studies indicate the improve-
ment of the supercritical CO
2
extraction of almond oil by using
HPU at laboratory scale [10]. Specifically, the application of HPU
to assist extraction process produced a relevant increase in the fi-
nal yield of the oil together with a noticeable reduction of the en-
ergy consumption through the increase of the extraction rate.
This paper deals with the implementation and validation at pi-
lot scale of an innovative ultrasonic system to assist supercritical
CO
2
extraction which aims to be a robust, stable, reliable and effi-
cient solution to be applied at industrial scale. The ultrasonic de-
vice was designed and built to work at 19 kHz and to overcome
the fluctuations of the specific acoustic impedance of the fluid
medium during the extraction process under the operational con-
ditions (high pressure 6400 bar, temperatures up to 80 °C, mass
flow up to 20 kg/h and density of the CO
2
6900 g/cm
3
). The ultra-
sonic applied power was maintained constant during the extrac-
tion trials of oils from different substrates. A specific hardware
and software has been developed, tested and validated to control
and monitoring the parameters involved in the supercritical
extraction process assisted by power ultrasound.
2. Experimental
2.1. Ultrasound-assisted supercritical fluid extraction (USFE) system
As showed in [11] the USFE system is made up of two units: (a)
SFE unit and (b) HPU unit. The SFE pilot plant (AINIA, Valencia) was
built to withstand high pressures up to 350 bar and temperatures
up to 80 °C (see Fig. 1). The major components of SFE include a dia-
phragm pump, a high pressure extraction vessel with 5 L capacity,
two separation units (a cyclone and a decanter) and a set of sensors
to monitor and control temperature, pressure and fluid flow rate.
High purity CO
2
(99.9%) was used as a solvent in the extractor un-
der supercritical conditions (P
c
= 73.9 bar; T
c
= 31.1 °C; D
c
=
0.468 g/mL) [12].CO
2
was used because is the most commonly
used solvent for the extraction of oils from vegetables in supercrit-
ical processes due to its excellent behavior and low cost.
In the previous USFE device [4] an operator was in charge of the
control of the operational conditions in the SFE as well as the con-
trol of the ultrasonic system. Although promising results were ob-
tained in the almond-oil extraction tests, carried out at 280 bar,
55 °C and a flow rate of 20 kg/h with an ultrasonic transducer oper-
ating at a frequency of 18 kHz and a power of 50 W (faster kinetic
curves 30% and higher extraction yields 20%), some instabilities
in its behavior were detected. Those instabilities were caused by
the changes observed in the acoustic impedance of the medium
under supercritical conditions during the extraction process. In
other words, the ultrasonic system exhibited difficulties concern-
ing stability and that made it unable for industrial use. In parallel,
resonance modes of the metallic basket placed inside the extractor
(where the substrate is deposited) were detected in the first proto-
type giving rise to modal interactions that disturb operation. In the
new system such interactions have been eliminated by separating
both basket and transducer resonant frequencies up to a value of
about 1 kHz. To achieve it, the resonant frequency and the shape
of the new transducer was established by finite element methods
(FEM) In this work special attention has been given to the trans-
ducer characteristics (impedance, phase and power spectrum of
the current) as a function of extraction parameters (pressure, tem-
perature, flow rate, density), power applied and time during the
extraction operation.
The ultrasonic system presented here is a new version of the
previous one and can be considered as the second step to scale it
up for larger operation. In fact, the new device implemented defin-
itive advantages related to the automatic control required for
industrial applications.
The present ultrasonic system basically consists of: (a) the
ultrasonic transducer to work at 19 kHz with a maximum power
of about 110 W, (b) a dynamic resonance control unit (controller),
(c) a broadband power amplifier, (d) an impedance matching unit,
(e) a specific developed software to monitor and control the
parameters of the power transducer (voltage V, current I, phase ,
impedance Z , power P and frequency f) and those of the supercrit-
ical fluid (pressure P, temperature T, flow rate F and density D), and
(f) a computer with a data acquisition hardware. To achieve auto-
matic control, the new system, also allows the power characteriza-
tion of the transducer during the extraction process in real time. To
this purpose, a virtual high-power impedance analyzer for contin-
uous operation was developed with LabView code, tested and val-
S S
US Signal
Generator
Computer
Data Acquisition
Unit
C
P
V, I, f
T, P, F, D
CO
2
supply
UST
H
PT
FT
Fig. 1. Scheme of the SFE pilot plant assisted by power ultrasound. Units: (E) extractors, (S) separators, (C) cooler, (P) high pressure pump, (H) heater, (PT) pressure meter, (FT)
flow meter, (UST) ultrasonic transducer. Electrical parameters: (V) voltage, (I) current, (f) frequency. Extraction parameters: (T) temperature, (P) pressure, (F)CO
2
flow rate,
(D) density.
E. Riera et al. / Ultrasonics 50 (2010) 306–309
307

idated experimentally [9]. In this way, the transducer behavior
during the extraction process and the enhancement by ultrasound
on the kinetic curves and the oil extraction yields of two different
products, have been analyzed.
2.2. Materials and methods
Two examples of the potential of HPU in SFE processes are pre-
sented in this work: (a) grounded almonds sieved at 3–4 mm in
size (55% oil content, wet basis), and (b) a second grounded vege-
table product ‘‘cocoa cake” with similar particle size. In both cases,
an amount of about 1500 g, were deposited in the basket placed in
the extractor of 5 L capacity. The selection of the particle size of the
samples (3–4 mm) was done based on our previous results with
grounded almonds which confirm that small particulate size favors
the ultrasonic action. In addition, experimental trials were carried
out with a second substrate cocoa cake. The time for each trial was
about 3.5–4 h. The power output applied to the ultrasonic trans-
ducer was fixed and kept constant at 85 W in all the experiments
here presented. All the experimental trials were carried out with
and without ultrasound application and replicated twice.
3. Results and discussion
The ultrasonic transducer was placed inside the extractor in-
serted on the upper part of the vessel. At low power, an impedance
analyzer was used to measure the admittance response of the
transducer in the frequency range (19–20 kHz). Only one vibration
mode was detected in air at 19,228 Hz with an impedance of 90
X
.
The same mode was detected at high excitation but at 19.1 kHz.
The transducer has an estimated power capacity of 110 W.
3.1. Stability of the prototype
The power behavior of the new transducer driven at high-exci-
tation level was studied in air and inside the extractor during the
extraction process with supercritical CO
2
. First, the stability of
the prototype was tested and validated in air at 100 W during a
long time. The new transducer showed high-stability and good
performance during 8 h of continuous operation at its maximum
power capacity. Such experiment was repeated several times up
to reach a total number of 50 h following the high-power charac-
terization procedure described in [13]. No change was detected
in its behavior during the trials. Therefore, the power characteriza-
tion of this transducer validates it for SFE. In addition, in Fig. 3 it is
shown an example of another kind of driving signals, were applied
to study the transducer behavior in air. One example consisted in
applying a power modulation between 35 W and 90 W to analyze
the transducer response. Fig. 2 shows the response that can be con-
sidered very reliable.
The transducer was also characterized at high level excitation
inside the extractor of the SFE unit in order to study the effect of
the high-pressure conditions on its behavior. Its impedance in-
creases from 90
X
up to 260
X
, and the frequency decreases from
19.2 kHz up to 18.9 kHz when the value of the pressure varies from
200 bar up to 320 bar. In Fig. 3 it is plotted the evolution of the
power applied to the transducer versus time along one extraction
trial carried out at 85 W. It is clear from the picture that the re-
sponse of the ultrasonic device is quite stable also in this case.
3.2. Almonds-oil extraction trials
In order to study the effects of USFE in almond-oil extraction,
trials were carried out at various pressures (200–320 bar), two
extraction temperatures (45 and 60 °C), times (up to 3.5 h) and
CO
2
flow rates (10–15 kg/h). The effect of the extraction pressure
was study. At 200 bar the improvement in the yield extracted
(mass transfer) was only 15% probably due to the low solubility
of the almond-oil in the supercritical CO
2
. However, the yield ex-
tracted from grounded almond at 320 bar, 45 °C and 10 kg/h gave
rise to 40% larger yields when HPU were applied as it is shown
in Figs. 4 and 5. Even larger improvements between extraction
curves with and without ultrasounds where achieved on experi-
ments carried out at 280 bar, 45 °C and 12.5 kg/h extraction yields
improvements of about 90% were obtained.
0
10
20
30
40
50
60
70
80
90
100
12:28 12:57 13:26 13:55 14:24 14:52 15:21 15:50
Time
Power (Watts)
Fig. 2. Example of power modulation applied to the transducer.
0
20
40
60
80
100
120
12:00 14:2413:12 15:36 16:48
Time
Power (Watts)
Fig. 3. Ultrasonic power versus time in an extraction trial carried out at 250 bar,
60 °C and 15 kg/h.
0
2
4
6
8
10
12
14
16
18
20
01234
Time (h)
g extract/100 g product
Fig. 4. Almond-oil extraction curve at 320 bar and 45 °C with (d) and without (s)
ultrasounds.
308 E. Riera et al. / Ultrasonics 50 (2010) 306–309

3.3. Cocoa cake-oil extraction trials
A third set of trials was carried out with a different substrate,
cocoa beans. Solid samples were prepared for the experiments as
follows: initial substrate was milled and sieved before the treat-
ment in order to have a particle size distribution between 2 and
3.5 mm. Next, the samples were placed in the basket inside the
extractor to begin with the extraction trials at 320 bar and 65 °C.
Good results were also obtained in the oil extraction with ultra-
sound. In fact, as shown in Fig. 6, the application of the ultrasonic
energy increases the extracted yield in around 43%.
4. Conclusions
An innovative system for ultrasonic application in supercritical
extraction processes at pilot plant scale was implemented and val-
idated experimentally. The system has shown to be a robust, sta-
ble, reliable and efficient solution potentially applicable at
industrial scale. The behavior of the system driven at high-power
levels showed high-stability and good performance during the tri-
als. In addition, the power ultrasonic system operates in an auto-
matic way during the extraction process, so that it is not
required any manual intervention by external operator. This fact
represents a relevant advance with respect to our previously devel-
oped ultrasonic device.
The new system confirms the high effect of the application of
ultrasonic energy in the SFE processes. In addition, the high stabil-
ity of such system gave rise to a clear enhancement of the results.
In fact, by using the previous ultrasonic system improvements of
about 20% in almond-oil extraction yields were achieved, while
with the present system the improvements reached up to 90%.
The new system has also been used with other substrates as co-
coa cake obtaining enhancements in the yield extraction of about
43%.
Acknowledgements
Work supported by the National Research Project PETRI-PTR95-
075.OP.02. The authors would like to thank V.M. Acosta for his col-
laboration in the design of the power transducers by FEM and their
development.
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0
2
4
6
8
10
12
14
16
18
20
012345
Time (h)
g extract/100 g product
Fig. 6. Cocoa cake-oil extraction curve at 320 bar and 65 °C with (d) and without
(s) ultrasounds.
0
2
4
6
8
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20
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Time (h)
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ultrasounds.
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309