Inter-vial heat transfer heterogeneity during freeze-drying: role of vial geometry and position on the shelf - Archive ouverte HAL
Communication Dans Un Congrès Année : 2015

Inter-vial heat transfer heterogeneity during freeze-drying: role of vial geometry and position on the shelf

Résumé

Characterization of heat and mass transfer is crucial for freeze-drying optimization. Vial containing the product is directly involved in the process. It strongly influences product temperature and thus product quality and homogeneity. Vial heat transfer coefficient Kv is usually the first parameter determined when building design space of primary drying step. Kv can be defined as the sum of three components, as shown in the following Equation: where: i) Kc is the contribution resulting from conduction due to direct contact between the shelf and the vial. Its value depends on the bottom area of the vial involved in this contact; ii) Kr is the contribution from radiative heat transfer from the shelf to the vial bottom. It mainly depends on the emissivity value, characteristic of the material of the shelf; iii) Kg is the contribution of conduction through the gas between the shelf and the vial bottom. This parameter is mainly related to the chamber pressure and the bottom curvature depth of the vial. While formal relations have already been proposed in literature to define Kg and Kr (Pikal et al., 1984), Kc contribution has not completely been modelled. It has been evidenced that vials at the edges receive additional energy coming from radiative heat transfer with the walls and door of the equipment, causing a “edge vial effect” within an array of vials on the same shelf (Rambhatla et al., 2003). Our objective was to investigate in detail inter-vial heat transfer distribution within a shelf and to identify and quantify other sources of variability in heat transfer than the "edge vial effect". Measurement of vial heat transfer rates was performed using the gravimetric method in a pilot plant freezedryer (Aquafinn, GT6). Sublimation tests were carried out without stopper using pure water at 4, 6, 9, 15, 40 and 50 Pa chamber pressure and at a shelf temperature of 0°C. A number of 124 vials located at the edges and 100 vials located in the centre of the shelf was weighed. Geometrical dimensions (contact area of the vial with the shelf and bottom curvature depth of the vial) of a batch of 60 vials were measured. As expected, the “edge vial effect” phenomenon was observed but it decreased when increasing chamber pressure due to increased relative contribution of gas conduction between shelf and vials. Heterogeneity in heat transfer coefficient (Kv) was also observed among vials located in the centre of the shelf and in this case, the distribution of Kv increased when increasing chamber pressure. Normal distribution law was used to fit Kv experimental and theoretical values. Theoretical calculation of Kv distribution was performed considering (a) the distribution of the vial bottom curvature depths and (b) the distribution of the contact area of the vials with the shelf in combination with parameter (a). The results show that the contact area, through Kc contribution, has mainly an impact at low chamber pressure (roughly Pc<15 Pa), while the bottom curvature depth, through Kg contribution, has mainly an impact at high chamber pressure (roughly Pc>15 Pa). The next step of this study is the modelling of radiative phenomenon observed for the vials located at the edges of a shelf and to continue to investigate different sources of variability affecting heat transfer during the primary drying phase.
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Dates et versions

hal-01567216 , version 1 (21-07-2017)

Identifiants

  • HAL Id : hal-01567216 , version 1
  • PRODINRA : 396259

Citer

Bernadette Scutella, Stéphanie Passot, Erwan Bourlés, Fernanda Fonseca, Ioan-Cristian Trelea. Inter-vial heat transfer heterogeneity during freeze-drying: role of vial geometry and position on the shelf. 7. International Conference ISL-FD, Jul 2015, Barcelona, Spain. ⟨hal-01567216⟩
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