st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Sterilization of granular materials in a low-pressure plasma circulating fluidized
bed reactor and an atmospheric-pressure dielectric barrier discharge
D. Butscher1, C. Roth1, G. Oberbossel1, Ph. Rudolf von Rohr1
1
ETH Zurich, Institute of Process Engineering, Switzerland
Abstract: Plasma sterilization experiments to inactivate micro-organisms on wheat grains
have been performed in a low-pressure plasma circulating fluidized bed reactor as well as an
atmospheric pressure dielectric barrier discharge. The general feasibility of both approaches
and their potential for the treatment of granular materials could be demonstrated.
Keywords: Plasma sterilization, low-pressure, ambient pressure, granular materials
1. Motivation
The application of conventional thermal or chemical
sterilization methods is often limited, since many products
are sensitive to heat, moisture and a variety of chemicals.
A promising alternative to these methods is plasma sterilization, where the synergetic combination of electrons,
ions, reactive neutral species and UV photons can be applied to effectively inactivate microorganisms [1]. In our
research project, we focus on the plasma sterilization of
powders and granular materials from the food and pharmaceutical sector (e.g. wheat grains).
We successfully managed to combine the high gas flow
(15 slm) which is required to lift the granular materials in
the riser tube (Ø 25 mm) with the low pressure (10 mbar)
which is essential to ignite and sustain a stable and homogeneous discharge. In a first experimental investigation we demonstrated the general feasibility of our approach to effectively reduce bacillus amyloliquefaciens
spores on wheat grains.
2. Low-pressure plasma sterilization
A low-pressure plasma circulating fluidized bed reactor
(PCFBR, Fig. 1) was constructed at our institute. In this
reactor, particles are lifted by an argon/oxygen gas mixture through the riser tube and sterilized within an inductively coupled plasma. Particles are then separated from
the gas flow in a cyclone and collected in a storage tube
from where they are repeatedly conveyed to the treatment
zone. These multiple circulations enable an adequate
treatment time and at the same time limit the thermal load
emanating from the plasma to short periods.
Fig.2
Fig.1
Process flow diagram of the PCFBR
Spore reduction in PCFBR (error bars exemplarily
show standard deviation for multiple measurements)
The reduction of colony forming units (CFU) was better at longer treatment time and higher oxygen concentration (see Fig. 2). Elevating the plasma power increases the
axial extension of the plasma zone which is already reflected in the calculation of the effective treatment time.
At a plasma power of 900 Watt and an oxygen concentration of 10%, the number of CFU could be reduced by
more than 2 logarithmic units within less than 25 seconds
of effective treatment time.
Based on spectroscopic observations, we attribute the
sterilizing effect in our experiments to the mechanism of
chemical sputtering caused by the impact of argon ions
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
and the bi-radical oxygen molecule. The amount of atomic oxygen, evaluated by the O*(777.1nm) to
Ar*(750.4nm) line ratio, as well as the sterilization efficacy (log reduction per second of effective treatment time,
inverse of D-value) as a function of the molecular oxygen
admixture is shown in Fig. 3.
3. Atmospheric pressure plasma sterilization
In order to avoid the high expenses of vacuum equipment in an industrial application, we recently developed
an atmospheric pressure dielectric barrier discharge
(DBD) as an alternative to the low-pressure system. With
this DBD, we can treat granular particles (e.g. wheat
grains) in a plasma cone as shown in Fig. 4.
Fig.4
Fig.3
Atomic oxygen equivalent and spore reduction rate
as a function of the molecular oxygen admixture
(averaged for the power range from 700 to 900W).
The decrease of atomic oxygen formation with increasing molecular oxygen admixture to the process gas was
already observed and explained in literature [2,3]. The
production rate of atomic oxygen is proportional to the
molecular oxygen density, but also decreases with decreasing electron density and energy. Since oxygen is
known to have a quenching effect (molecular oxygen
causes additional collisional energy losses), a reduced
electron density and energy is expected at elevated oxygen concentrations [4].
Obviously, the amount of atomic oxygen, known for its
sterilizing effect, does not correlate with the treatment
efficiency. UV emission was emitted in the range of 280
to 330 nm (most likely emanating from the OH A-X transition), but its intensity followed the same trend as the
atomic oxygen. Other UV lines were not detected, especially not in the UV-C range, which is known to be most
efficient with respect to spore inactivation. Ozone was
also not observed since its formation requires a three body
collision which is unlikely in reduced pressure conditions.
To sum up, the inactivation efficiency in our experiments only correlates with the molecular oxygen admixture. This let us assume that the prevailing sterilization
mechanism in our experiments is chemical sputtering
caused by the impact of argon ions and the attack of
bi-radical oxygen molecules [5,6]. Energetic argon ions
impinging on the spores are able to break bonds in the
spore coat and the oxygen bi-radical (triplet ground state
with two unpaired electrons) is able to react with these
open bonds to form volatile compounds (e.g. CO, CO2
and H2O) so that the spores are etched.
Wheat grain treatment in atmospheric pressure DBD
The DBD is driven by high frequency, high voltage
pulses (3 kHz, 6 kV) and argon is used as process gas.
With this setup, we reduced the amount of CFU of
geobacillus stearothermophilus spores by 1.3 logarithmic
units within 10 minutes of treatment time. Even though
this ambient pressure inactivation process seems to be
slow, we observed similar energy consumption rates for
the spore reduction in the low-pressure PCFBR and the
atmospheric pressure DBD experiments (approx. 10
kJ/log(CFU)).
4. Conclusion
Both approaches, the low-pressure CFBR and the ambient pressure DBD, have demonstrated their potential for
the plasma inactivation of microorganisms on granular
materials. However, there is still room for improvement to
increase the spore reduction and reduce the treatment time.
Hence, we will intensify the low pressure plasma treatment by further reducing the pressure and increasing the
residence time of particles in the plasma zone. Concerning the atmospheric pressure system, we will increase the
power input and optimize the gas composition.
5. References
[1] A. Fridman, Plasma Chemisty, Cambridge University
Press (2008).
[2] J.-P. Lim, H.S. Uhm, Phys. Plasmas, 14 (2007).
[3] H. Pang, Q. Chen, B. Li, F. Fei, S. Yang, IEEE Trans.
Plasma Sci., 39, 8 (2011).
[4] A. Schwabedissen, C. Soll, A. Brockhaus, J.
Engemann, Plasma Sources Sci. Technol., 8 (1999).
[5] V. Raballand, J. Benedikt, J. Wunderlich, A. von
Keudell, J. Phys. D: Appl. Phys., 41 (2008).
[6] J. Benedikt, C. Flötgen, G. Kussel, V. Raballand, A.
von Keudell, J. Phys.: Conf. Ser., 133 (2008).
6. Acknowledgements
Supported by the Commision for Technology and Innovation (CTI) and Bühler AG, Switzerland.