Bi-dimensional modelling of the thermal boundary layer and mass flux prediction for direct contact membrane distillation

摘要

A good knowledge on thermal boundary layer thickness and mass flux are important features in the development of membrane distillation pilot modules. The present work poses as a comprehensive approach for modelling both during the process' steady state, whilst cherishing simplicity as it does so through a simple, intuitive software - namely Calc from Libreoffice. The mass transfer model allows prediction of average mass flux, being obtained by tailoring a mass transfer equation to be used together with an overall-macroscopic energy balance for the system. Within such approach, five different convective heat transfer correlations were investigated and a mass transfer coefficient of 6.02 x 10(-7) kg . m(-2). s(-1). Pa-1 was found for the PTFE supported membrane used in the experiments. Thermal boundary layer thickness was estimated using the integral method, primarily under the assumption of constant local heat flux along the membrane extension. Thermal boundary layer thickness was coherently modelled in the middle portion of the channels, region where model results agreed with the assumptions made with 80% accuracy or higher, pointing for correspondingly low variation of heat flux with space within such region. The applicability of the temperature profile proposed in the present work in portraying the reality of permeate and feed in membrane distillation processes was discussed. (C) 2019 Elsevier Ltd. All rights reserved.