Powder Compaction: Compression Properties of Cellulose Ethers

Effective development of matrix tablets requires a comprehensive understanding of different raw material attributes and their impact on process parameters. Cellulose ethers (CE) are the most commonly used pharmaceutical excipients in the fabrication of hydrophilic matrices. The innate good compression and binding properties of CE enable matrices to be prepared using economical direct compression (DC) techniques. However, DC is sensitive to raw material attributes, thus, impacting the compaction process. This article critically reviews prior knowledge on the mechanism of powder compaction and the compression properties of cellulose ethers, giving timely insight into new developments in this field

Computer aided analysis of viscous film flow along an inclined wavy wall

The steady, laminar flow of a Newtonian liquid along an inclined wavy wall is studied in a two-dimensional numerical experiment using the Galerkin finite element method. The dimensionless Navier-Stokes equations are solved in the whole range of the laminar flow regime. Numerical predictions are compared with available experimental data for very low Reynolds numbers. The emphasis in the discussion of results is given in the presentation of free surface profiles, streamlines, velocity, and pressure distributions along the free surface and the wall. The interaction of the dimensionless numbers of the flow is studied, criteria for flow reversal are established, and a resonance phenomenon at high Reynolds numbers is investigated. (C) 1999 Academic Press

Finite element analysis of polymeric membrane and coating formation by solvent evaporation

The solvent evaporation process from acetone-cellulose acetate solutions is studied as a numerical experiment. The process is modeled as a coupled heat and mass transfer problem with a moving boundary. The resulting non-linear system of governing equations is solved with the Galerkin finite element method. A parametric analysis is carried out and it is discussed in detail how the process conditions affect the evaporation rate, the temperature at the surface of the solution and the resulting morphology of the final product. This analysis may be applied in the modeling of polymeric membrane formation and in the drying of coatings

Transient flow and heat transfer phenomena in inclined wavy films

A finite-element numerical scheme is used to study rigorously the flow of an inclined liquid film and the heat transfer from the constant-temperature wall. Regular inlet disturbances are predicted to evolve into periodic or solitary waves depending on the frequency of the forcing. At very low disturbance frequencies parasitic crests appear and the regularity of the wavetrain is lost. The effect of a solitary wavetrain on heat transfer from the wall is studied, and it is predicted that a stationary temperature distribution develops with periodic flux variation that follows the waves. The thinning of the substrate between successive humps combines with the effect of convection at the crest and tail of the solitary humps to produce heat transfer enhancement significantly above the conduction limit. (C) 2004 Elsevier SAS. All rights reserved

Computer-aided estimation of diffusion coefficients in non-solvent/polymer systems

Full Paper: A novel method for estimating the mutual and self-diffusion coefficients of a non-solvent/polymer system is proposed in this work. The idea is to stud the evaporation process from non-solvent/solvent/polymer systems as a one-dimensional numerical experiment and to use polymer solution weight versus time data to fit the unknown parameters of the diffusion-coefficient correlations based on free-volume theory. For this purpose, the evaporation process is modeled as a coupled heat- and mass-transfer problem with a moving boundary, and the Galerkin finite-element method is used to solve simultaneously the non-linear governing equations. This method is successfully applied to the estimation of water-cellulose acetate diffusion coefficients and is valid over the whole range of temperatures and concentrations for practical applications in membrane technology. Additionally there is a detailed discussion on if water affects the morphology of the final cellulosic membrane by studying the concentration profiles of the constituents of the casting solution

Estimation of diffusion coefficients in acetone - Cellulose acetate solutions

The mutual and the self-diffusion coefficients of the acetone-cellulose acetate system are determined by using established gravimetric measurements of the acetone evaporation rate. The process is studied as a one-dimensional numerical experiment utilizing the Galerkin finite element method. The numerical technique provides simultaneous solution of the model equations and yields by comparison with gravimetric data the diffusion coefficients of acetone in cellulose acetate for a wide range of temperatures and compositions. The estimated diffusion coefficients based on free volume theory are in satisfactory agreement with the available experimental data. It is believed that this method can be applied to other systems of interest. The mutual and the self-diffusion coefficients of the acetone-cellulose acetate system are determined by using established gravimetric measurements of the acetone evaporation rate. The process is studied as a one-dimensional numerical experiment utilizing the Galerkin finite element method. The numerical technique provides simultaneous solution of the model equations and yields by comparison with gravimetric data the diffusion coefficients of acetone in cellulose acetate for a wide range of temperatures and compositions. The estimated diffusion coefficients based on free volume theory are in satisfactory agreement with the available experimental data. It is believed that this method can be applied to other systems of interest

Solitary waves on inclined films: Flow structure and binary interactions

The downstream evolution of disturbances, introduced at the inlet of a liquid film flowing along an inclined plane wall, is studied numerically by solving the full, time-dependent Navier-Stokes equation. Computational results are validated against the predictions of spatial linear stability analysis and against detailed data of the entire evolution process. The structure of the flow field below the waves is analyzed, and the results are used to test assumptions frequently invoked in the theoretical study of film flow by long-wave equations. An interesting prediction is that solitary waves exhibit strongly nonparabolic velocity profiles in front of the main hump, including a slim region of backflow. The computational scheme is subsequently used to study solitary wave interactions. It is predicted that coalescence (the inelastic collision of two humps) is not inevitable but occurs only when the waves differ appreciably in height. Waves of similar size repel monotonically, whereas for intermediate differences in height a strong oscillatory interaction between the two humps is predicted. Encouraging qualitative agreement with the limited experimental information available is noted. (C) 2002 American Institute of Physics

The Free (Open) Boundary Condition at inflow boundaries

The Free (or Open) Boundary Condition (FBC, OBC) was proposed by Papanastasiou et al. (A new outflow boundary condition, Int. J. Numer. Meth. Fluids 14 (1992) 587-608) to handle truncated domains with synthetic boundaries where the outflow conditions are unknown. In the present work, implementation of the FBC has been tested also at inflow boundaries in several test problems of viscous or viscoelastic flow. The Finite Element Method (FEM) is used to provide numerical results for both cases of planar and axisymmetric domains under laminar, isothermal or non-isothermal, steady-state conditions for Newtonian and non-Newtonian fluids. The present results extend previous ones regarding the applicability of the FBC, since they convincingly show that the FBC can be used equally well at inflow boundaries, without having to resort to artificially set inlet profiles for a given flow rate. (C) 2012 Elsevier B.V. All rights reserved

Computer-aided estimation of acetone, methyl acetate, and chloroform diffusion coefficients in poly(vinyl acetate)

The diffusion coefficients of acetone, methyl acetate, and chloroform in amorphous poly(vinyl acetate) are estimated using the solvent evaporation method. The evaporation process from polymer solutions, cast in the form of thin films, is studied as a numerical experiment. The process is modeled as a coupled heat and mass transfer problem with a moving boundary. Lattice fluid (LF) thermodynamics is used to describe polymer-solvent system volumetric properties and to derive appropriate expressions for solvents' chemical potentials. The resulting nonlinear system of governing equations is solved with the Galerkin finite element method. The estimated diffusion coefficients are in satisfactory agreement with reported data