Elsevier

Biosystems Engineering

Volume 179, March 2019, Pages 49-58
Biosystems Engineering

Research Paper
CFD modelling of diffusive-reactive transport of ozone gas in rice grains

https://doi.org/10.1016/j.biosystemseng.2018.12.010Get rights and content

Highlights

  • First research on the estimating diffusion coefficient of ozone gas in rice grains.

  • CFD allows understanding the ozone gas flow in rice grains as a porous medium.

  • Diaphragm cell method permits efficient estimation of the diffusion coefficient.

The search for alternatives to the use of chemical products to control insect pests in stored grains has stimulated the development of new techniques that allow the maintenance and preservation of grain quality without posing risks to people and the environment. One of these alternatives is the use of ozone gas (O3) as a fumigant, mainly due to its oxidising and biocidal characteristics. To investigate the transport mechanisms involved in the flow of the O3 gas through rice grains, the CFD (Computational Fluid Dynamics) analysis was used, and from this evaluation, it was possible to predict the decomposition reaction constant and the O3 effective diffusion coefficient. In the experiment, grains were submitted to fumigation process in a prototype adapted from a diaphragm cell. Data on the ozone gas concentration were collected from a monitoring point immediately above the grain layer every 10 min. Parallel to the experimental procedure, the modelling of the O3 gas flow using the CFD technique was performed. The adjustment parameters input to the CFD model were the effective diffusion and the decomposition reaction constant of the O3 gas in the rice grains. The estimated diffusivity value (1.0 × 10−6 m2 s−1), and decomposition reaction constant (0.00167 s−1) are of the same order of magnitude of several other gases for agricultural grains.

Introduction

Ozone gas (O3) is an alternative to fumigation with phosphine (PH3) to control insect pests in stored grains. Ozone (O3), the triatomic form of oxygen (O2), in addition to being a powerful oxidiser with great disinfection and sterilisation capacities, was also classified in 2001 by the United States Food and Drug Administration (FDA, 2001) as a safe sanitiser for food, since its degradation product (O2) is non-toxic (Gabler, Smilanick, Mansour, & Karaca, 2010).

In the food processing industry, ozone gas has been used for decontamination of various products, including fruits and vegetables (Horvitz & Cantalejo, 2014). In the grain storage sector, O3 gas stands out as a potential tool for controlling insect pests (Isikber and Athanassiou, 2015, Kells et al., 2001, Pereira et al., 2008, Sousa et al., 2008) and fungi, and for the degradation of mycotoxins (Alencar et al., 2012, Brodowska et al., 2017, Khadre et al., 2001, Tiwari et al., 2010). It is worth noting that, provided that the recommended doses are respected, fumigation with ozone gas does not modify the quality of the treated grains and their by-products (Dubois et al., 2006, Kells et al., 2001, Mendez et al., 2003, Pereira et al., 2007, Tiwari et al., 2010).

Studies on the application of ozone gas in different storage systems are still carried out empirically, and the behaviour of this gas in the grain mass during fumigation is unknown. In general, the behaviour of fumigants in grain storage is poorly understood. To characterise the transport of fumigants through a porous medium, the decomposition rate, sorption and diffusion coefficient of the fumigants must be determined (Collins, 2010, Isa et al., 2016). This makes it possible to understand the dispersion and reaction of the fumigants in the grains, and it helps to make decisions about the time and concentration that should be used in an application.

The use of computational fluid dynamics (CFD) has proven to be an efficient technique for studying mass transport with reactions in porous media. Studies already reported the use of CFD to understand the distribution and behaviour of O3 gas flow during the pre-treatment of wheat straw (Bhattarai et al., 2015), and also to understand the air flow distribution in storage systems of rice and maize with different grain mass configurations and porosity (Lawrence and Maier, 2011, Olatunde et al., 2016). The CFD technique was also used to develop a model to describe the deep bed drying process in raw rice grains, and to visualise the temperature profile along the bed under different drying conditions (El Gamal, Kishk, & El Masry, 2017). Isa et al. (2016) have also used the CFD to comprehend the distribution of phosphine (PH3) in vertical cylindrical silos during fumigation of wheat grains. Therefore, the objective of this work was to use the CFD technique to investigate the transport mechanisms affecting the flow of the ozone gas through the diaphragm cell and, from this analysis, to predict the transport parameters of the ozone gas through the grain mass, such as the decomposition reaction constant and the effective diffusion coefficient, based on experimental data.

Section snippets

Obtaining ozone

Ozone was obtained with the ozone generator O&L3.ORM (Ozone & Life, São José dos Campos, SP, Brazil). The gas was produced by using compressed oxygen (99.99% minimum purity) as raw material, at a 0.059 g s−1 mass flow rate and 0.0022 kg m−3 concentration. In this process, the oxygen passes through a refrigerated reactor, where a dielectric barrier discharge occurs. This type of discharge is produced when a high voltage is applied between two parallel electrodes separated by a dielectric

Fluid dynamics of the flow: verification of diffusion as a governing mechanism

For the experimental procedure, the gas chamber was previously saturated with O3 gas for 30 min. After that, the connection valve was opened. The ozone injection was maintained, and both the inlet and outlet valves remained open throughout the experiment. The velocity values of O3 gas migration in the porous media (true velocity) and along the cylindrical column are of low magnitude (Fig. 2a and b). In Fig. 2a, it is possible to observe that the velocity profile has a sharp evolution in the

Conclusions

The negligible flow velocities observed in the porous medium show that the only existing mechanism of transport is the diffusion. This result shows that the adopted diaphragm cell configuration is valid for the simultaneous determination of the effective diffusion coefficient of gases and the kinetic constants that are characteristic of reactive–adsorptive processes. The effective diffusion coefficient of the O3 gas was 1.0 × 10−6 m2 s−1, and the decomposition reaction constant was 0.00167 s−1.

Acknowledgements

This work was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (Grant Number: 134505).

Nomenclature

CFD
Computational Fluid Dynamics
RMSE
Root Mean Square Error
NS
Index of agreement - Nash and Sutcliffe
M.S.Res
Mean square residual
dr
Willmott's refined index
O3
Ozone
εg
Volume fraction (porosity)
ε
Turbulent dissipation rate constant
vInlet
Velocity on inlet
t
Time (s)
ρg
Gas density (kg m−3)
M
Molar mass of air (kg kmol−1)
τ
Viscous stress tensor (kg m−2 s−2)
v
Velocity of the gas (m s−1)
T
Temperature (K)
ωO3
Ozone mass fraction
F
External forces on the gas per unit of volume (N m−3)
DeffO3
Effective diffusion coefficient

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