Mass Biot Number Research Articles

Analysis of moisture profiles, mass Biot number and driving forces during drying of potato slabs

The drying of shrinking bodies is analysed by numerically solving a drying model in order to obtain a deeper insight into the drying process, calculating moisture profiles and various forms of Biot number. Moisture profiles shows great differences when different units of concentration are used. Calculated Biot numbers do not satisfactorily explain the drying kinetic curves for solids with a high initial moisture content, because the equilibrium curve is not a straight line. Kinetics drying curves are satisfactorily analysed in terms of variation of total driving force with time, which differs markedly from the corresponding variation of the total driving force in terms of temperature, for a pure heat transfer process.

THE EFFECT OF SURFACTANTS ON THE GROWTH OF UNDERPRESSURIZED, ANNULAR LIQUID JETS

The effects of surfactants on the growth of isothermal, underpressurized, inviscid, annular liquid jets are studied under both steady state and transient conditions by means of an interfacial mass transfer law which depends on the equilibrium interfacial concentration and a mass resistance parameter. Equilibrium interfacial concentrations governed by both Henry's and Sievert's solubility laws are considered. It is shown that, for mass Biot numbers larger than unity, the underpressurized jet grows until the pressure of the gases enclosed by the annular liquid jet is equal to that of those surrounding the jet, while, for mass Biot numbers less than or equal to one, the pressure coefficient increases very little. It is also shown that, in some cases and contrary to the results observed in the absence of mass transfer resistance, the time required by the annular jet to reach a steady equilibrium configuration may be shorter when the equilibrium interfacial concentration is governed by Sievert's law than when it is governed by Hennry's solubility law, particularly at low Peclet numbers and for high mass transfer resistances.

The Sharp Interface Model:Zero-Order Reaction with Volume Change

The sharp interface model is examined for isothermal gas-solid reactions following zero-order kinetics with respect to the gas phase and first-order kinetics with respect to the solid phase for spherical particles either shrinking or growing during reaction. It is shown that depending on the relative magnitudes of the governing parameters which include the volume expansion factor, Damkohler number, and mass Biot number, it is possible to have up to three successive transitions from one controlling regime (external mass transfer, product layer diffusion, or chemical kinetics) to another as the reaction evolves with time. This behavior gives rise to various gas concentration and solid conversion profiles. A classification of all the possible situations is made

Diffusion of carbon dioxide in tomato fruits during cold storage in modified atmosphere

SummaryTreatments with 20% CO2 of tomato fruit produced a beneficial extension of storage life. The diffusion of CO2 in tomato (Lycopersicon esculentum Mill) cv. Platense was measured chromatographically and the concentration profiles in the internal gaseous atmosphere of the fruit were determined for non‐steady state conditions to establish minimum treatment times that can ensure adequate penetration of the gas.The effective diffusivity of CO2 in the tissue was determined by fitting concentration data, obtained from experiments with whole fruit without skin, to the non‐steady state solution of the diffusion equation, assuming negligible external mass transfer resistance. The diffusivity was modelled using Maxwell‐Eucken expressions, taking account of the heterogeneous structure of the tissue.The resistance of the fruit skin was evaluated using whole fruits with the stem scar covered with paraffin and fitting experimental data to the solution in terms of mass Biot number; the specific resistance of the peel was some 200 times greater than that of the stem scar.

A criterion of isothermality and determination of reaction boundary for pore-mouth poisoned catalyst pellet

A criterion of isothermality is derived for the inner core of a catalyst pellet deactivated by pore-mouth poisoning using the pseudo steady-state shell-progressive model. If the criterion is met, the core of the deactivated catalyst is at least as isothermal as the fresh catalyst. Knowledge of activity factor mass Biot number and the reaction boundary is required for the use of the criterion. Activity factor can be determined from the measurements of external temperature difference. For the determination of the reaction boundary, an alternate expression is derived for the activity factor for the case of reaction dominated by diffusional resistance. Determination of the reaction boundary requires knowledge of an observable, the Biot numbers and measurements of external temperature difference. Results are applied to some literature data.