Dynamic Simulation and Optimisation of Direct Contact Membrane Distillation in Hollow Fibre Module
Direct contact membrane distillation (DCMD) in a hollow fibre module (HFM) has been of interest for concentration of thermally sensitive liquid foods when operating at low temperatures from 25 to 40°C.
Based on the developed models, a procedure to simulate the DCMD in an HFM has been set up on the basis of equating heat and mass fluxes. A double loop optimisation problem was formulated in MATLAB to solve the nonlinear equations with unknown outlet and membrane surface conditions. Steady-state simulations for a wide range of operating conditions were carried out. It was revealed that the theoretically achievable energy efficiency of DCMD within the tested range in Halar module (HL50) was about 49.3%, while it was only 20.3% for PVDF module (PV65). Overall differences between the simulated and experimental results were statistically within 2%, suggesting good performances of the models.
An additional outer loop was also implemented to accommodate the dynamic condition of a real system for concentration of 1.5kg glucose solution from 30 to 60% w/w. The pseudo-real-time dynamic optimisation of the DCMD process was performed to minimise the energy expense which accounted for the heat being exchanged between the two streams within the membrane module and the power for their pumping, while maintaining a mass flux above 0.5 kg.m-2.h-1. It was found that the optimal permeate temperature and velocity were 25.97°C and 0.41 m/s for HL50, and 23.73°C and 0.35 m/s for PV65 module, when the feed was constantly maintained at 40°C and a velocity of 0.6 m/s.