Operaciones Unitarias

Chemical Engineering Journal 87 (2002) 11–19

Investigation in solid–liquid extraction: influence of ultrasound
M. Romdhane a,∗ , C. Gourdon b,1
b

a Ecole Nationale d’Ingénieurs de Gabés, Route de Medenine, 6029 Gabés, Tunisia
Ecole Nationale Supérieure d’Ingénieurs de Génie Chimique 18, Chemin de la loge, 31078 Toulouse Cedex, France

Received 1 October 2000; received in revised form 1June 2001; accepted 18 June 2001

Abstract
The extraction of active principles or oils from vegetable products by means of an appropriate solvent is a classical unit operation
in chemical engineering. The aim of the present work is to improve the yield or the selectivity of the extraction using ultrasound. Two
solid–liquid extractions have been investigated: pyrethrines from pyrethrum flowers(Chrysanthemum cineraria) and oil from woad seeds
(Isatis tinetoria). In the case of the first one, an acceleration of the kinetics and of the yield of the extraction has been obtained, probably
linked to the increase of the intraparticular diffusion of the solute, which is the limiting step of mass transfer. On the other hand, ultrasound
seems to have a weak effect in the case of woad seeds,probably because of the structure of the seeds. In order to distinguish the various
effects acting on the ultrasound activation, a thermoelectric probe has been built for measuring the ultrasonic intensity profile. By this
method, it is possible to investigate the influence of the presence of solid particles on the ultrasound wave propagation. © 2002 Elsevier
Science B.V. All rights reserved.Keywords: Ultrasound; Solid–liquid extraction; Extractor; Woad seeds; Pyrethrum flower

1. Introduction
Solid–liquid extraction with a solvent is an operation
which appears in many industrial processes. The pharmaceutical industry particularly, but also industries manufacturing perfumes or pesticides have to deal with the recovery
of active principles from plants. Vegetable materials containgenerally only a small amount of active solute, but
most of the time with high added value which justifies
the development of high-performance separation processes.
Consequently, there is a need for new activation methods,
for instance the use of ultrasound.
Ultrasound is used in different operations in chemical
engineering: waste-water treatment, drying, sonochemistry
and solid–liquid extraction[1–3]. The effect of ultrasound
on the extraction of saponin from ginseng has been studied
[4]. The extraction of saponin was examined as a function of both irradiation time and acoustic pressure. It was
observed that the yields of both total extraction and saponin
are larger with ultrasonic irradiation than without ultrasonic
irradiation. The increase in the yield of the total extractionis approximately 15%, and for saponin it is about 30%.
∗
1

Corresponding author. Tel.: +216-5-392-380; fax: +216-5-392-190.
Tel.: +33-5-62-25-23-00; fax: +33-5-62-25-23-18.

In the field of agriculture, the influence of ultrasound has
been studied on a great variety of solid substrates, showing
positive effects in some cases. Indeed, the sonication of a
seed grain increases its kineticgrowth, and consequently
the yield of the harvest. It seems also that the enzymes,
which are the key of the biochemical reaction, increase their
activity with a good proportioning of ultrasound. Primo
[5] has improved the colour, the viscosity and the yield of
orange juice. Spirov reported that sonication of grapes for
5–10 min (f = 80 000 Hz) improved the flavour of wine
and its clarification.The application of ultrasound during
short periods of maceration was effective in producing a
greater yield of alkaloids from Datura stramonium [6].
In addition to the beneficial effect of ultrasound on the
yield and on the kinetics of extraction, it allows change in
the experimental conditions, for instance decrease of temperature and pressure. Then, the quality of the solute as in
the...