Theoretical and experimental studies of laminar flow hollow fiber direct contact membrane distillation modules

摘要

The laminar flow hollow-fiber direct contact membrane distillation module for desalination to produce pure water, under both concurrent-flow and countercurrent-flow operations, is investigated theoretically and experimentally. The theoretical model is a conjugated Graetz problem and is solved analytically using the separated variable method with an orthogonal expansion technique extended in power series. The analytical solutions of water permeate flux, the temperature distribution, the local Nusselt number and the average Nusselt number are obtained. The experimental results of permeate flux under different operation conditions, including the inlet temperature and flow rate of hot saline water, are fairly close to the theoretical predictions with relative error of 2-6.1%. The temperature distributions reveal thick thermal boundary layer next to the membrane surface. The local Nusselt number distribution shows high value in the inlet part followed by low value in the rest of the module. The average Nusselt number of the module increases with both hot side and cold side Graetz numbers. The theoretical development approach can be applied to hollow fiber membrane modules for other applications and the theoretical solutions can be utilized for evaluating modules using various eddy promoting designs. (C) 2015 Elsevier B.V. All rights reserved.