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 process in an HFM was set up on the basis of equating heat and mass fluxes in the process. A double loop optimisation problem was formulated in MATLAB to solve the nonlinear equations with unknown outlet and membrane surface conditions to implement the simulation procedure. Steady-state simulations for a wide range of operating conditions were carried out. It was revealed that the highest achievable energy efficiency of DCMD within the tested range in Halar module was about 49.9%. Overall differences between the simulated and experimental heat and mass fluxes under similar conditions 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 1.5 kg glucose solution from 30 to 60% w/w using Halar module HL50. A pseudo-real-time dynamic optimisation was performed to minimise the energy expense. This energy accounted for the heat being exchanged between the feed and the permeate streams within the membrane module and the power for their pumping, while maintaining a mass flux at least above 0.5 kg.m-2.h-1. It was found that the optimal permeate inlet temperature and velocity were 26.25°C and 0.49 m.s-1 when the feed solution was constantly maintained at 40°C at the inlet and a velocity of 0.6 m.s-1.