Organic Process Research & Development 2010, 14, 1130–1139
A Facile Microwave-Mediated Drying Process of Thermally Unstable / Labile
Products
N. A. Pinchukova,*,† A. Yu. Voloshko,† O. V. Shyshkin,† V. A. Chebanov,† B. H. P. Van de Kruijs,‡ J. C. L. Arts,‡
M. H. C. L. Dressen,‡ J. Meuldijk,‡ J. A. J. M. Vekemans,‡ and L. A. Hulshof*,‡
National Academy of Sciences of Ukraine, Lenin AVenue, 60, KharkiV 61001, Ukraine, and EindhoVen UniVersity of
Technology, Den Dolech 2, 5612 AZ EindhoVen, The Netherlands
Abstract:
heat2-9 and has become a popular heating technique for various
processes including synthesis, solution concentration and drying
of food,10-14 fruits,15 chemicals,16,17 agricultural products,18
polymers,19 ceramics,20 pulp and paper,21 textiles,22 in mineral
processing23 as well as in wood processing industries.24
Although drying of pharmaceutical powders with microwave
heating has been shown to increase drying rates and product
stabilities during the drying process,25,26 application in the
pharmaceutical industry is still limited. The main property of
microwaves is to directly deliver energy from the source to the
microwave-absorbing sample. Field inhomogeneity of domestic
microwave ovens, culminating in uneven heating rates, dictate
the use of dedicated equipment for microwave processing.27
Conventionally heated drying processes are limited by the
thermal conductivity of the substances. When the thermal
conductivity is low, the drying process becomes slow, and
The drying behavior of (S)-N-acetylindoline-2-carboxylic acid,
precipitated (1a, 17 wt %) and nonprecipitated (1b, 5 wt %), and
N-acetyl-(S)-phenylalanine ((S)-2-acetamido-3-phenylpropanoic acid,
2), both pharmaceutical intermediates, and of cocarboxylase
hydrochloride (thiamine pyrophosphate, 3), a coenzyme, a bioac-
tive form of vitamin B1, being a thermolabile substance, has been
determined in straightforward drying setups. The method of
supplying energy to the system had a profound influence on the
drying rate and on the internal temperature of the samples during
drying. The drying time of (S)-N-acetylindoline-2-carboxylic acid
(1b) with the low moisture content (5 wt %) could be reduced by
a factor 4 using microwave irradiation instead of conventional
heating, while keeping the sample temperature under 35 °C.
N-Acetyl-(S)-phenylalanine (2) with a higher moisture content (22
wt %) demonstrated a decrease in drying time by a factor 2.5 to
4 depending on the applied microwave powers. A reduction in
drying time of the precipitated (S)-N-acetylindoline-2-carboxylic
acid (1a, 17 wt % moisture) by a factor 2 was demonstrated for
drying at 150 W of microwave irradiation instead of using a water
bath at 70 °C. A dramatically shorter drying time by a factor 10
was found for cocarboxylase hydrochloride (3, 15 wt % water)
on lab-scale which could be reproduced on pilot-plant scale. To
achieve with conventional heating similar drying times as under
microwave irradiation for the four examples, extremely high
energy inputs should be applied, necessitating extremely high
temperature differences between the heating source and the
sample. The results reveal that microwave irradiation is less
energy-consuming and is particularly useful for effective drying
of thermally unstable materials in short periods of time.
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Introduction
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Drying is one of the oldest and most common unit operations
in chemical engineering and is an essential procedure for
purifying and isolating products.1 Drying is one of the highest
energy-consuming and most expensive processes in the phar-
maceutical industry. Microwave irradiation has attracted much
attention in different fields dedicated to studying benefits and
drawbacks of microwaves as an unconventional source of
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* To whom correspondence should be addressed. E-mail: L.A.Hulshof@tue.nl
and pinchukova@isc.kharkov.com.
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† SSI “Institute for Single Crystals” NAS of Ukraine, Kharkiv, Ukraine.
‡ Laboratory of Macromolecular and Organic Chemistry, Applied Organic
Chemistry.
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Vol. 14, No. 5, 2010 / Organic Process Research & Development
10.1021/op100102m 2010 American Chemical Society
Published on Web 08/24/2010