Papers by Keyword: Effective Diffusivity

Abstract: The use of fresh herb is limited in the food and pharmaceutical industry thats why the dry form of the herb is the one commonly used. To save the quality of medicinal and aromatic plants it is very important to provide optimum drying and storage conditions. The aim of this study is to determine and model the drying kinetics of sage leaves. Initially the desorption isotherms are determined for different temperatures (30, 45 and 60°C). The drying experiments were carried out in a convection oven at the same temperature range. For the desorption isotherms and the drying kinetics various models reported in the literature were used and from the statistical view, the Yanniotis and Blahovec and the Fick models fit well the results of desorption isotherms and the oven drying, respectively. The net isosteric heat of desorption of the sage leaves ranged from 6.86 to 63.45 kJ/mol. The total time of oven drying reduced substantially with an increase of the drying temperature. Effective moisture diffusivity values ranged from 1.1x10^-12 to 3.7x10^-12 m²/s and significantly affected by temperature. An Arrhenius relation with an activation energy value of 66.87 kJ/mol expressed effect of temperature on the diffusivity. Keywords: Desorption isotherms, Drying kinetics, Modelling, Effective diffusivity, Sage leaves

Abstract: This paper present the results of (experiments and models) biosynthesized prodigiosin (PG) released from an implantable biomedical device on the viability of cancer cells. The implantable biomedical devices were obtained from poly-di-methyl-siloxane (PDMS) packages with well-controlled micro-channels and drug storage compartments, along with a drug storing polymer core (which contains thermosensitive Poly (N-isopropylacrylamide)(PNIPA)-based gels). The results were compared with drugs elution from devices loaded with paclitaxel^TM. The effects of localized release of PG and paclitaxel (PTx) on cell viability were elucidated via clonogenic assay testing on MDA-MB-231 breast cancer cell line. The effects of PG and PTx released were also tested over a range of temperatures (37-45 ̊C) in which localized hyperthermia is applicable. The trends in the results were analysed using statistical models before discussion their implications for localized treatment of breast cancer.

Abstract: This work applies a 3D multi-scale bottom-up approach for modeling the processes of diffusion and reaction-diffusion in porous catalyst layers. The performance of the random pore model to predict effective transport coefficients are compared with the results of the multi-scale diffusion model. The results of the 3D multi-scale diffusion model are employed in a 1D pseudo-homogeneous reaction-diffusion model with a relative good agreement with the 3D multi-scale reaction-diffusion model. Furthermore, the former multi-scale model was coupled to a full-scale reactor model with good results and high advantages in terms of computational time savings.

Abstract: An investigation of the drying of spherical food particles was performed, using peas as the model material. In the development of a mathematical model for drying curves, moisture diffusion was modelled using Fick’s second law for mass transfer. The resulting partial differential equation was solved using a forward-time central-space finite difference approximation, with the assumption of variable effective diffusivity. In order to test the model, experimental data was collected for the drying of green peas in a fluidised bed at three drying temperatures. Through fitting three equation types for effective diffusivity to the data, it was found that a linear equation form, in which diffusivity increased with decreasing moisture content, was most appropriate. The final model accurately described the drying curves of the three experimental temperatures, with an R² value greater than 98.6% for all temperatures.

Abstract: In this paper, the drying experiments of paper sludge were performed at different drying conditions. The drying kinetics and phenomena of paper sludge were investigated. The effective diffusivity and the activation energy of the paper sludge during drying had been evaluated. At the same time, seven empirical models were used to model the experimental data, such as Newton, Page, Modified Page, Henderson and Pabis, Logarithmic, Two term, Two Term exponential et al. Three statistical parameters (The coefficient of determination (R2), root mean square error (RMSE) and the residual sum of square (RSS) ) were used to evaluate goodness of fit of the tested models.

Abstract: Grape pomace is the main by-product from the wine industry. It is principally made up of grape skin and seeds. Drying this by-product is the first step for the later extraction of components with high added value like oil or antioxidants. Due to the different characteristics of the components, the study of the drying kinetics of grape pomace must be addressed taking its components into account one by one. For that purpose, grape seeds from the Spanish wine industry were dehydrated in a convective laboratory dryer at 70 °C and at 1, 2 and 3 m/s until a weight loss of 40% was reached. Drying kinetics was determined in triplicate. Modelling was carried out by means of a diffusion model without considering shrinkage and external resistance. Grape seeds were assumed to be spherical. For all the fits, the explained variance was higher than 96.9 % and the mean relative modulus was lower than 1.7 %. Between 1 and 2 m/s, effective diffusivity increased in line with air velocity, although the values of effective diffusivity calculated for 2 and 3 m/s were similar. It seems to indicate that for the experimental conditions under study, the external resistance is not negligible at 1 and 2 m/s and for higher air velocities the internal resistance to mass transfer controls the drying process. These results coincide with those found by other authors when studying different food products.

Abstract: The main aim of this work was to assess the influence of power ultrasound on mass transfer process during convective drying of a low porosity product submitted to different acoustic energy levels. Drying kinetics of carrot cubes (side 8.5 mm) were carried out at 40 °C and 1 m/s applying different electric power levels to the ultrasonic transducer: 0, 10, 20, 30, 40, 50, 60, 70, 80 and 90 W. Drying kinetics were modelled considering the diffusion theory. From the results, a significant (p<0.05) influence of power ultrasound application on drying kinetics of carrot cubes was found. Drying rate increased as the electric power applied got higher. The influence was only observed above an acoustic energy threshold, which corresponded to an electric power applied to the transducer of 20-30 W. From this threshold, a linear relationship was found between the average effective moisture diffusivity or the mass transfer coefficient and the electric power applied to the transducer.

Abstract: In this paper we propose several new mathematical models for estimating effective diffusivities of a drug released from a cylinder device to an external finite volume. These models can handle problems with ‘initial burst’ and boundary layers. Analytical solutions to the models are derived. To determine the unknown effective diffusivity, time of a initial burst and width of the effective boundary layer, a least-squares method can be used for a given experimental data set. The models were tested using experimental data and the numerical results show the usefulness and accuracy of these models.

Abstract: In this paper we perform a systematic Monte Carlo study of the effective diffusivity in a 2D composite in which the dispersed phase (squares) is arranged in square planar and brick-work patterns within the matrix phase. We focus on the commonly encountered case where the dispersed phase has a much lower diffusivity than the matrix and where the segregation of the diffusant favours the matrix phase. It is found that while the generalized Maxwell-Garnett Equation generally describes the effective diffusivity semi-quantitatively, in order to have an accurate representation of the effective diffusivity at moderate volume fractions of the dispersed phase, it is necessary to use a more refined Maxwell-Garnett Equation that takes into account the actual shape and geometry of the dispersed phase.

Abstract: Drying kinetics of carrot cubes were carried out at 1 m/s air velocity at different air drying temperatures (30, 40, 50, 60 and 70±0.1 °C) (AIR experiments), and also at the same experimental conditions but applying high power ultrasound (US experiments). Two kind of diffusion models were used to simulate the drying kinetics, according to external resistance to mass transfer being considered (ER model) or neglected (NER model) for solving the diffusion equation. Diffusion ER model was solved using a finite difference method. Drying rate increased as air temperature was higher. Ultrasound also increased drying rate at the different temperatures, but the improvement on drying rate decreased at high temperatures, and almost disappeared at 70 °C. Effective moisture diffusivities only showed an Arrhenius type relationship with temperature for AIR experiments. The NER diffusion model was not accurate to simulate the drying kinetics at any experimental conditions tested. However, diffusion ER model provided a high closeness between experimental and calculated drying data (VAR>99.80). Through the parameters identified of the ER diffusion model, effective moisture diffusivity and mass transfer coefficient, the influence of the power ultrasound application on internal and external resistance to mass transfer was shown to be significant (p<0.05).