Forced convection spinning of hollow fibre membranes: Modelling of mass transfer in the dry gap, and prediction of active layer thickness and depth of orientation

• A mass transfer model for forced convection spinning has been developed.
• The model contributes to the understanding of skin growth and the role of molecular orientation.
• The model is a general utility to help analyse any dry/wet spinning system.

A mass transfer model has been developed to predict the likely solvent evaporation rates within the forced convection chamber during hollow fibre membrane spinning. Two modes of mass transfer are considered to occur potentially: laminar forced convection describes the loss of solvent from the surface of the hollow fibre as it passes through the chamber generally, whereas transfer correlations relating to flow normal to a cylinder describe conditions when the fibre passes the point at which the forced convection gas enters the chamber – the ‘impingement zone’. Skin formation and depth of molecular orientation are discussed in the context of these two possible mass transfer regions. For ambient temperature systems, the membrane skin may be formed substantially during laminar forced convection before the impingement zone is encountered. The role of molecular orientation may be important in setting skin thickness in these systems since the depths of oriented skin predicted by the model can more closely reflect the active layer thicknesses calculated from experimental pressure-normalised fluxes and permeability coefficients. At higher temperatures, active layers tend to be much thicker. The impingement zone may account for significant increases in skin growth in these high temperature, high concentration/viscosity cases. It is hoped that the model outlined here provides a framework for better understanding of the phenomena at play within the forced convection chamber.

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