Effect of freeze-dryer design on heat transfer variability investigated using a 3D mathematical model

International audienceIn the freeze-drying process, vials located at the border of the shelf usually present higher heat flow rates which in turn result in higher product temperatures than central vials. This phenomenon, named edge vial effect, can result in product quality variability within the same batch of vials and between batches at different scales. Our objective was to investigate the effect of various freeze-dryer design features on the heat transfer variability. A 3D mathematical model previously developed in COMSOL Multiphysics and experimentally validated was used to simulate heat transfer of a set of vials located at the edge and in the centre of the shelf. The design features considered were the loading configurations of the vials, the thermal characteristics of the rail, the walls and the shelves and some relevant dimensions of the drying chamber geometry. The presence of the rail in the loading configuration and the value of the shelf emissivity strongly impacted on the heat flow rates received by the vials. Conversely, the heat transfer was not significantly influenced by modifications of the thermal conductivity of the rail, the emissivity of the walls and by the geometry of the drying chamber. The developed model revealed to be a powerful tool to predict the heat transfer variability between edge and central vials for the cycle development and scale-up and to compare various freeze-dryer design features

Characterization of evaporation rate, temperature, velocity and humidity fields in a cold chamber

3rd IIR International Conference on Sustainability and the Cold Chain, ICCC 2014, London, , 23-/06/2014 - 25/06/2014International audienceIn a food factory some pathogenic bacteria can grow even at low temperature, especially Listeria Monocytogenes which resists to temperature around 0°C. Currently, two ways are used to prevent bacterial growth; product treatment (pasteurization, pH decrease) and cleaning but it's not sufficient to eliminate all microorganisms. This work takes part of a four-year research project called EcoSec (2013-2016) in France. The practical objective is to propose an ecological and economical way to destroy Listeria Monocytogenes by humidity reduction. The principle idea is based on the dehumidification of a plant. To achieve this objective, a method to predict the evaporation rate on wet surfaces was developed using experimental and numerical approaches (CFD, Computational Fluid Dynamics). The study was performed inside a cold room (volume 29m3) in which temperature and velocity were controlled. The results show that the air humidity and its local velocity near the wet surface play a major role

Couplage écoulement, transfert thermique et dénaturation-agrégation de la béta-lactoglobuline: importance relative des termes péricinétiques et orthocinétiques.

International audienceModeling of thermal denaturation-aggregation of whey proteins can provide assistance in developing dairy products as well as in understanding the mechanisms which drive the formation of fouling deposits. Aggregation of whey proteins depends on temperature (perikinetic contribution, due to Brownian motion) and shear (orthokinetic contribution). Under realistic conditions, different temperature and shear histories are associated with fluid parcels which progress more or less quickly, far or close to the heating wall, inside the processing unit. A numerical approach is proposed for evaluating the thermal denaturation-aggregation of whey proteins, combining computational fluid dynamics and the population balance equation. Fluid flow and heat transfer are solved through the finiteelement- method in the Eulerian frame, while product transformation is evaluated along representative Lagrangian trajectories. Therefore, no assumptions are performed regarding dynamical and thermal histories. The approach is illustrated by the evolution of a suspension of betalactoglobulin, along the first section of a tubular heating exchanger (length 0.4 m, radius 0.004 m, flow rate 20 L/h, inward heat flux 13500 W/m2); at its inlet, the suspension contains 6 % of beta-lactoglobulin at 60 °C. Particle breakage is avoided (shear rate values below 125 s-1). Simulations are performed with and without the orthokinetic term of the aggregation kernel. Results put in evidence that the perikinetic and orthokinetic terms exhibit dominant role for particles sizes below and above about 1 mm, respectively

A comparison between longwave radiation codes

The reliability of two computationally-efficient, simplified radiation codes employed to evaluate the components of the budget of terrestrial radiation is assessed from comparisons with enchmarl. calculations, perforrned with higher accuracy by reference codes. A number of approximations about the longwave radiative transfer in the atmosphere was assumed in the conception ofboth simplified codes. The first one was developed at the Instituto Astronômico e Geofísico da Universidade de São Paulo (Institute of Astronomy and Geophysics, University of São Paulo (IAG-USP; São Paulo, SP); it provides irradiances throughout the atmosphere from radiances integrated over the whole terrestrial spectrum and previously computed by running the version 7 of the Low Resolution Transmittance radiative transfer code (LOWTRAN -7). The second simplified code under consideration is included in the general circulation model developed at the Center for Ocean-Land-Atmosphere Studies (COLA; USA), which runs at the Centro de Previsão do Tempo e Estudos Climáticos (CPTEC; Cachoeira Paulista, SP) for numerical weather forecasting and climate studies. Benchmark calculations employed in this study were obtained by other groups through carefulline-by-line spectral integration under hypothetical atmospheric conditions, prescribed in the scope of the program Intercomparison of Radiative Codes used in Climate Models (ICRCCM). Only the effects due to the emission I absorption by water vapor, carbon dioxide and ozone are analyzed. The effects due to the carhon dioxide are well reproduced by the code CPTEC/COLA and, excepting relatively cold atmospheric conditions, by the code developed at IAG-USP. The effects due to the ozone are underestimated by hoth simplified codes but mainly by the CPTEC/COLA, Doce the ozone bands situated outside the region 980- 1100 cm .1 were neglected in its development. Sensitivity tests conceming the water vapor continuum absorption indicare that its both components ("self', related to the water vapor partial pressure, and "foreign", related to the dry air pressure) must be considered in order to obtain reliable water vapor absorption estimares. Under hypothetical atmospheric conditions and assuming the occurrence of the three maio molecular absorbers, downward longwave irradiances at the surface can be systematically underestimated by the code CPTEC/COLA. Future studies must include not only comparisons between results provided by simplified radiation codes and the respective benchmark calculations under many atmospheric profiles obtained from soundings, but algo comparisons between these results and measurements obtained with pyrgeometers near the sounding site.Pages: 29-4

Absorption of solar radiation by water vapor in the atmosphere. Part I: a comparison between selected parameterizations and reference results.

Selected schemes conceived for providing reliable and computationally efficient estimates of the absorption of solar radiation by the atmospheric water vapor are discussed. Special attention is devoted to the scheme proposed by Lacis & Hansen, which is included in radiative codes of atmospheric models running at Brazilian centers of research and numerical weather forecasting. Additional schemes, proposed from relatively modern estimates of the effects due to the water vapor absorption, are also included in the analysis. In the goal of assessing systematic errors associated to these schemes, comparisons are performed with benchmark results available in the literature which have been obtained through careful line-by-line integration throughout the solar spectrum. Modern schemes allow more atmospheric absorption, and therefore less downward shortwave irradiance at the ground, when compared with schemes implicitly adopted at present in Brazilian centers.Pages: 275-29

Absorption of solar radiation by water vapor in the atmosphere. Part II: sensitivity tests with a general circulation model.

The impact of implementing the Chou & Lee scheme for absorption of solar radiation by water vapor in the CPTEC/COLA's General Circulation Model is analyzed in this paper. Comparison with results of the present operational version, which uses the Davies scheme, shows that Chou & Lee scheme leads to more shortwave absorption, thus providing an extra warming of the atmosphere, mainly in the upper troposphere. The changes in the meridional distribution of temperature causes weakening of the subtropical jet in the Southern Hemisphere and strengthening of the Northern Hemisphere jet, which is slightly shifted poleward. The increase of the static stability weakens the meridional circulation cells. The increasing shortwave absorption in the atmosphere decreases the downward shortwave irradiance at the surface. As a consequence, the fluxes of enthalpy and latent heat from the surface are reduced. As a result of all these features, zonal mean precipitation and evaporation are also significantly reduced. Comparison with atmospheric fields reported by the Climate Prediction Center (NCEP/NOAA) shows that the configuration of the large-scale fields is, in a general manner, improved with the use of the Chou & Lee scheme in a monthly integration for January 1997.Pages: 291-30