Abstract
In this paper, the transport phenomena in four common membrane distillation (MD) configurations and three popular modelling approaches are introduced. The mechanism of heat transfer on the feed side of all configurations are the same but are distinctive from each other from the membrane interface to the bulk permeate in each configuration. Based on the features of MD configurations, the mechanisms of mass and heat transfers for four configurations are reviewed together from the bulk feed to the membrane interface on the permeate but reviewed separately from the interface to the bulk permeate. Since the temperature polarisation coefficient cannot be used to quantify the driving force polarisation in Sweeping Gas MD and Vacuum MD, the rate of driving force polarisation is proposed in this paper. The three popular modelling approaches introduced are modelling by conventional methods, computational fluid dynamics (CFD) and response surface methodology (RSM), which are based on classic transport mechanism, computer science and mathematical statistics, respectively. The default assumptions, area for applications, advantages and disadvantages of those modelling approaches are summarised. Assessment and comparison were also conducted based on the review. Since there are only a couple of full-scale plants operating worldwide, the modelling of operational cost of MD was only briefly reviewed. Gaps and future studies were also proposed based on the current research trends, such as the emergence of new membranes, which possess the characteristics of selectivity, anti-wetting, multilayer and incorporation of inorganic particles.
Highlights
Membrane technology has been widely studied in recent years for purification and separation
The driving force is, the vapour pressure difference between the vapour pressure PT1 of T1 and vapour pressure PT2 at the membrane surface on the permeate side and is less than the difference between vapour pressure at Tf and vapour pressure calculated with the permeate bulk temperature Tp in Direct contact MD (DCMD) and Air gap MD (AGMD)
Similar to the conventional model, response surface methodology (RSM) cannot be used to look at the local phenomena or for the optimisation of a module structure. Since it is data-based and does not rely on the characteristics shared by all Membrane distillation (MD) processes, the outcomes from RSM are only valid for the system where the data are collected, which could be a key hinderance for wide employment of RSM
Summary
Membrane technology has been widely studied in recent years for purification and separation. (CSRE) has successfully developed and commercialized a vacuum multi-effect membrane distillation (V-MEMD) process for corrosive wastewater treatment [20] It was found from the commercialization of the V-MEMD that the comprehensive cost of an MD plant using high-grade heat (steam) is lower than utilising low-grade heat source (80 ◦C) hot water, in which the cost of overall operation efficiency and treatment capability (m3/h) are included. There are hardly any reviews providing a summary of assumptions for purposes of simplifying the modelling approaches of different configurations, which were sometimes neglected or noticed by the researchers This ignorance could hinder the error analysis and possibly amplify the errors when using the models under certain conditions. Many researchers have misused temperature polarisation to assess thermal efficiency, which is clarified in this paper
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