Control of steady state through concentration signal

National audienc

Prediction of Intrinsic kinetics in diffusion disguised immobilized enzyme systems

When immobilized enzyme kinetics are disguised by inter- and intraparticle diffusion effects, an approximate mathematical procedure is indicated whereby experimental data obtained in the limiting ranges of first- and zeroth-order Michaelis-Menten kinetics could be used for the prediction of the kinetic constants

A Transient surface renewal model for heat transfer in bubbling fluidized beds

Abstract is not available

Ostwald Ripening of Oil Drops in a Micellar Solution

A model for Ostwald ripening of oil-in-water emulsion in the presence of small amount of surfactants is presented. It accounts for the transfer of oil from small to larger drops when the interfacial resistance is rate controlling. in this case, the continuous breakdown and reformation of the micellar structure during mass transfer through the interface, is assumed to be the rate-controlling step. a discretized population balance equation for the change in the number concentration of drops due to Ostwald ripening is solved using the method of characteristics. the third moment of the distribution (average volume) is found to increase linearly with time, consistent with available experimental observations, before reaching an abrupt equilibrium. the rate of change of drop volume is found to be strongly dependent on the solubility and is fairly insensitive to the volume fraction. the transients of the drop size distributions indicate an initial rapid decrease in the small droplets with a corresponding increase in the larger drops followed by a very slow change in the number distributions at longer times. the rate constants involved can be roughly correlated with the components of several experimental measurements, and allow comparison with the data of Kabalnov (Langmuir 10 (1994) 680), but only at small surfactant concentrations. the numerical agreement is not good but it is able to predict the anomalous direction of change with increased surfactant concentrations. the basis of the lack of numerical agreement is discussed. © 2001 Elsevier Science Ltd. All rights reserved

A Surface Renewal Model for Unsteady-State Transfer Processes

A surface renewal model is proposed for the analysis of free or fixed interfacial transport phenomena occurring in transient processes. These processes begin at a certain instant in a turbulent system in hydrodynamic equilibrium. For such a treatment a transient form of the generalized gamma residence time distribution function has been derived, from which the more relevant internal age distribution function has been obtained. Published data on four different systems, three of which pertain to entrance region transfer to fluids in turbulent flow and the fourth to transient heat transfer rates in gas fluidized beds, have been used to confirm the applicability of the dynamic model. The predictive expression relates the ratio of the transient Nusselt or Sherwood number to its respective steady-state value to the product of the ratio of the incomplete to complete gamma functions and a known function of the model parameters. The expression takes the form {A figure is presented}where ξ is given by {A figure is presented}In the limit of t tending to infinity, ψ becomes unity

Catalyst effectiveness factor for redox model kinetic equation

A generalized isothermal effectiveness factor correlation has been proposed for catalytic reactions whose intrinsic kinetics are based on the redox model. In this correlation which is exact for asymptotic values of the Thiele parameter the effect of the parameters appearing in the model, the order of the reaction and particle geometry are incorporated in a modified form of Thiele parameter. The relationship takes the usual form: Image and predicts effectiveness factor with an error of less than 2% in a range of Thiele parameter that accommodates both the kinetic and diffusion control regimes